Electrospinning of nanofibers for innovative applications ...

Electrospinning of nanofibers for innovative applications: a parameter study of polyamide 6.9applications: a parameter study of polyamide 6.9 Electrospinning of nanofibers for innovative
Academiejaar 2009-2010 Faculteit Ingenieurswetenschappen Voorzitter: prof. dr. Paul Kiekens Vakgroep Textielkunde
Master in de ingenieurswetenschappen: materiaalkunde Masterproef ingediend tot het behalen van de academische graad van
Begeleider: Bert De Schoenmaker Promotor: prof. dr. ir. Karen De Clerck
Annelies Goethals
applications: a parameter study of polyamide 6.9 Electrospinning of nanofibers for innovative
Academiejaar 2009-2010 Faculteit Ingenieurswetenschappen Voorzitter: prof. dr. Paul Kiekens Vakgroep Textielkunde
Master in de ingenieurswetenschappen: materiaalkunde Masterproef ingediend tot het behalen van de academische graad van
Begeleider: Bert De Schoenmaker Promotor: prof. dr. ir. Karen De Clerck
ACKNOWLEDGMENT iv
Acknowledgment
This thesis would never have been completed without the support of many people.
First I would like to thank Bert for the countless hours of guidance. Thank you Bert, for
answering the thousands of questions I have asked you this past year. I am very grateful
for all the time and energy you have put into helping me with this thesis.
I would also like to thank Lien, Sander, Yannick, Philippe and Sabine for helping me find
my way around the department.
I especially would like to thank my supervisor prof. dr .ir. Karen De Clerck for this oppor-
tunity. Her advice was invaluable. I am also grateful for all the hours she spent helping
me to bring structure in this document.
I would like to thank Hubert and Guy for their contribution to this thesis.
Thanks to Eddy for helping with the construction of the laboratory setup.
I would also like to express my gratitude to my parents for their endless support. Without
them I would have never made it this far.
My final thanks go to my husband Zhong who inspires me to reach for the stars. I think I
just reached the first one.
Annelies Goethals
June 2010
Copyright notice
The author gives permission to make this master dissertation available for consultation
and to copy parts of this master dissertation for personal use. In the case of any other
use, the limitations of the copyright have to be respected, in particular with regard to the
obligation to state expressly the source when quoting results from this master dissertation.
Annelies Goethals
June 2010
Master dissertation submitted to obtain the academic degree of
Master of Materials Engineering
Assistant supervisor: Bert De Schoenmaker
Faculty of Engineering
Summary
This thesis will study the electrospinning of polyamide 6.9 and the properties of the formed nanofibers. The first chapter gives a short introduction to the principles of electrospinning and to the properties of PA 6.9. Chapter 2 describes the used materials and methods.
The first part of the thesis focuses on steady state electrospinning. The ranges of electrospinning parameters that result in steady state electrospinning are investigated. The second part of the thesis is a parameter study. The influence of the electrospinning parameters on the morphology and thermal behavior of the nanofibers is investigated.
Keywords
SAMENVATTING (DUTCH SUMMARY) vii
Inleiding
Elektrospinnen
Elektrospinnen is een proces dat het mogelijk maakt om nanovezels, bij definitie vezels
met een diameter kleiner dan 500 nm, te produceren. Het concept van electrospinning
werd beschreven en gepatenteerd rond 1930. Sinds 1995 geniet het elektrospinnen van
een hernieuwde belangstelling, zowel in de academisch wereld als in de industrie. Dit
omdat de nanovezels groot potentieel hebben voor gebruik in innovatieve toepassingen
zoals bijvoorbeeld medische toepassingen, filtermaterialen en nanovezelcomposieten.
De gebruikte methode wordt voorgesteld in figure 1. Bij het elektrospinnen wordt een hoge
potentiaal gebruikt om een elektrisch geladen jet van polymeeroplossing te creeren uit een
naald. De polymeeroplossing wordt vanuit de spuit (1) door een geleidende naald (2)
gepompt. Een spanningsbron wordt gebruikt om een elektrische potentiaal aan te leggen
tussen de naald en geaarde collectorplaat (7).
Door het verhogen van de elektrische potentiaal zal een druppel aan de naaldtip vervormen
tot een Taylorkegel (4). Deze kegel is het startpunt van een polymeer jet die onder invloed
van het elektrisch veld zal afbuigen en splitsen en zo nanovezels vormen (5). Voordat de
oplossing de collector bereikt, verdampt het oplosmiddel en stolt het polymeer. Op die
manier bekomt men een verbonden net van nanovezels (6) dat wordt afgezet op de geaarde
collectorplaat.
P1
2
5
6
7
Figure 1: Schema van electrospinnen - (1) spuit in pomp, (2) naald, (3) spanningsbron, (4) Taylorkegel, (5) afgelegde weg van de nanovezels, (6) non-woven nanovezelstructuur and (7) geaarde collectorplaat
Dit op eerste zicht eenvoudig proces wordt benvloed door veel parameters. Deze wor-
den opgedeeld in 3 categorieen, namelijk de oplossingsparameters, procesparameters en
omgevingsparameters.
Een belangrijke term is steady state electrospinnen. De steady state conditie houdt in dat
er voor lange tijd stabiel kan worden gesponnen. Het garandeert de reproduceerbaarheid
van de nanovezelvezelstructuren, dit is uitermate belangrijke wanneer er wordt overgegaan
naar spinnen op industriele schaal.
Polyamide 6.9
Het elektrospinnen van PA 6, PA 6.6 en PA 4.6 werd reeds bestudeerd in de Vakgroep
Textielkunde. In deze masterproef zal het elektrospinnen van PA 6.9 bestudeerd worden.
PA 6.9 werd gekozen omdat het groot potentieel heeft in het gebied van de nanovezel-
composieten. De belangrijkste eigenschappen van PA 6.9 zijn de uitstekende dimensionele
stabiliteit en lage waterabsorptie. Net deze eigenschappen zijn belangrijk in composietma-
terialen.
Doel van de masterproef
Als eerste stap zal worden onderzocht of het mogelijk is om steady state te bereiken tijdens
het elektrospinnen van PA 6.9. Als tweede stap zal onderzocht worden welke invloed de
parameters hebben op de vezelmorfologie en het smeltgedrag van de nanovezels.
Verschillende parameters zullen aan bod komen. Voor de oplossingsparameters is dit de
polymeerconcentratie en solvent verhouding. De procesparameters zijn opgelegde spanning,
afstand tussen naaldtip en collectorplaat en debiet. Tot slot zal ook de invloed van de
vochtigheid bestudeerd worden. Er wordt gebruik gemaakt van SEM en (M)DSC om hun
invloed op de eigenschappen van de nanovezels te bestuderen.
Steady state elektrospinnen
Het elektrospinproces bevindt zich in steady state als de twee volgende voorwaarden
voldaan zijn. Volgens de eerste voorwaarde moet de hoeveelheid polymeer die per tijd-
seenheid door de naald gepompt wordt gelijk zijn aan de hoeveelheid die per tijdseenheid
als nanovezels wordt afgezet op de collectorplaat. Dit impliceert dat een monster nanovezels
geen knopen of druppels mag bevatten. De tweede voorwaarde voor steady state is dat de
Taylorkegel stabiel blijft in de tijd.
Bij andere polyamides leidde het gebruik van azijnzuur:mierenzuur solvent mengsels tot
steady state elektrospinnen. Daarom zullen ook voor PA 6.9 azijnzuur:mierenzuur mengsels
gebruikt worden.
Het steady state karakter van het elektrospinproces kan worden samengevat in een steady
state tabel. Elke kolom is een solventverhouding en elke rij is een polymeerconcentratie.
Voor elke combinatie wordt nagegaan of er proces parameters zijn waarvoor steady state
mogelijk is.
Tabel 1 toont de steady state voorwaarden voor een constante afstand en een constant de-
biet. In het zwarte gebied lossen niet alle pellets op, in het grijze wel, maar deze oplossingen
bereiken nooit steady state. Het witte gebied is het steady state gebied, in de tabel wordt
de optimale aangelegde spanning weergegeven. Een eerste vaststelling is dat het steady
state gebied zeer beperkt is. De mogelijkheid om een polymeeroplossing al dan niet te kun-
nen elektrospinnen in steady state is afhankelijk van de viscositeit, de oppervlaktespanning
en de dielektrische constanten van de oplossing. Zo is er een minimale viscositeit vereist
om zonder druppels en knopen te spinnen.
Table 1: Steady state tabel voor PA 6.9, afstand: 6 cm, debiet: 1 ml h−1 - (zwart) pellets zijn niet opgelost, (grijs) geen steady state, (wit) steady state
0:100 10:90 25:75 40:60 50:50 60:40 75:25
6
8
12 25 kV 14 kV 15 kV 12 kV
14 22 kV 13 kV 14 kV
16 25 kV 17 kV 14 kV
18 27 kV 23 kV 14 kV
20 26 kV 22 kV 15 kV
22 27 kV 18 kV
24
spanning stijgt
Een andere waarneming is dat de optimale opgelegde spanning duidelijk stijgt met toene-
mende fractie aan mierenzuur. Dit is een gevolg van de hoge dielektrische constante van
mierenzuur.
Verder onderzoek heeft aangetoond dat de optimale aangelegde spanning ook stijgt met
stijgend debiet en stijgende afstand. Bij een hoger debiet moet de toegenomen hoeveelheid
aan ladingen gecompenseerd worden door een hogere spanning. Een hogere afstand ver-
mindert de sterkte van het elektrisch veld, dit moet opgevangen worden door de spanning
te verhogen.
De experimenten voor de studie van de oplossingsparameters en procesparameters worden
uitgevoerd in een open set-up. De temperatuur en relatieve vochtigheid werden gereg-
istreerd. De temperatuur was 21± 2°C en de relatieve vochtige was 43± 4 %RH.
Om de invloed van vochtigheid te bepalen, werd een nieuwe opstelling gebouwd waarin de
vochtigheid kan gecontroleerd worden.
Figuur 2 toont SEM-beelden van nanovezels gemaakt met verschillende polymeerconcen-
tratie. De vezeldiameter stijgt sterk met toenemende polymeerconcentratie. Dit wordt
bevestigd door de metingen getoond in figuur 3
a. b. c. Figure 2: SEM-beelden met vergroting 50 000 - (a.) 12 m% (b.) 16 m%, (c.) 20 m%
0
50
100
150
200
250
300
350
400
450
500
G em
id de
ld e
ve ze
ld ia
m et
er [n
Figure 3: De gemiddelde vezeldiameter in functie van de polymeerconcentratie
De concentratie heeft ook een grote invloed op de kristalstructuur van de nanovezels. Dit
kan afgeleid worden uit hun smeltprofiel, zie figuur 4.
-4
-3
-2
-1
0
)
180 190 200 210 220 230 240 Temperature (°C)Exo Up Universal V4.4A TA Ins
H ea
t flo
w [W
Hoe hoger de polymeerconcentratie, hoe groter de hoeveelheid minder stabiele kristallen.
Dit resultaat werd bevestigd met XRD metingen. De DSC curves toonden ook aan dat de
kristalstructuur in de vezels verschillend is van die in het bulkmateriaal.
In de glastransitie van de nanovezels is een invloed van de polymeerconcentratie veel minder
duidelijk merkbaar. Het werd wel opgemerkt dat na een eerste opwarming de glastransi-
tietemperatuur opschuift naar een lagere temperatuur, zie figuur 5. Dit effect kan worden
toegewezen aan de interne spanningen die tijdens het elektrospin proces worden gecreeerd.
0.002
0.004
0.006
0.008
0.010
0.012
20 30 40 50 60 70 80 90 Temperature (°C)
First heating Second heating
De invloed van de solventverhouding
De experimenten tonen aan dat ook de solventverhouding een significante invloed heeft op
de vezeldiameter, er komt wel geen duidelijke trend naar voor, figuur 6. De solventver-
houding heeft geen significante invloed op de smelt en de glastransitie van de nanovezels
0
50
100
150
200
250
300
G em
id de
ld e
ve ze
ld ia
m et
er [n
Solvent verhouding azijnzuur:mierenzuur [v:v%]
Figure 6: De gemiddelde vezeldiameter in functie van de solvent verhouding
Proces parameters
De verschillende procesparameters hebben slecht een klein effect op de vezeldiameter, zie
figuur 7. De opgelegde spanning en de afstand benvloeden het elektrisch veld en ook
de elektrische veldlijnen. Vermoedelijk hebben deze veldlijnen ook een klein effect op de
eigenschappen van de nanovezels.
a. b. c. Figure 7: SEM-beelden met vergroting 100 000 - (a.) 6 cm (b.) 9 cm, (c.) 12 cm
Uit de experimenten blijkt dat het mogelijke debiet sterk gelimiteerd is. Bij een te hoog
debiet, kan het solvent niet meer verdampen voor het de collectorplaat bereikt. Dit heeft
tot gevolg dat de gevormde nanovezels opnieuw opgelossen in the solvent.
Omgevingsparameters: Vochtigheid
Recent onderzoek in de Vakgroep Textielkunde toonde aan dat de relatieve vochtigheid
een grote invloed heeft op de gemiddelde vezeldiameter, zie figuren 8, 9. PA 6.9 kan niet
worden gesponnen bij lage vochtigheden. Na enkele minuten treedt er steeds stolling op
aan de naald waardoor die uiteindelijk verstopt.
Met behulp van een zelfgebouwde laboratoriumopstelling kan de relatieve vochtigheid onder
controle worden gehouden tijdens het spinnen. Relatieve vochtigheden van 16± 4 %RH en
74± 4 %RH kunnen worden bereikt.
De diameter daalt sterk met stijgende relatieve vochtigheid. Dit is omdat het vocht in
de omgeving een weekmakend effect heeft en dus de mobiliteit van de polymeerketens
verhoogt, hierdoor kunnen de vezels meer worden gestrekt.
a. b. c. Figure 8: SEM-beelden met vergroting 100 000 - (a.) 16 %RH, (b.) 43 %RH, (c.)
74 %RH
G em
id de
ld e
ve ze
ld ia
m et
er [n
Figure 9: De gemiddelde vezeldiameter in functie van de polymeerconcentratie voor verschillende relatieve vochtigheden - 16 %RH: N , 43 %RH: , 77 %RH:
De vochtigheid heeft een minder sterke invloed op de kristalstructuur van de nanovezels,
zie figuur 10. Ook onder hoge vochtigheid neemt de hoeveelheid minder stabiele kristallen
toe. Dit kan er op wijzen dat de verhoogde mobiliteit van de vezels niet enkel meer strekken
tot gevolg heeft, maar ook meer splitsen van de jet.
-4
-2
0
2
/g )
180 200 220 240 Temperature (°C)Exo Up Universal V4.4A TA Temperatuur[°C]
H ea
a
b
c
Figure 10: DSC curves voor verschillende vochtigheden - (a) 10 m%, (b) 14 m%, (c) 20 m%
Besluit
Steady state elektrospinnen van PA 6.9 is mogelijk voor bepaalde combinaties van elek-
trospin procesparameters. Het steady state karakter van het proces wordt voornamelijk
benvloed door de viscositeit, de oppervlaktespanning en de dielectrische constanten van
de polymeeroplossingen.
De eigenschappen van de gevormde nanovezels worden hoofdzakelijk benvloed door de
polymeerconcentratie van de oplossing en door de relatieve vochtigheid van de omgev-
ing. De nanovezels worden in mindere mate benvloed door de solventverhouding en de
procesparameters.
Electrospinning of nanofibers for innovative applications: a parameter study of polyamide 6.9
Annelies Goethals
Supervisor(s): Karen De Clerck, Bert De Schoenmaker
Abstract— This article describes steady state electrospinning of polyamide 6.9 nanofibers. Afterwards a morphologic study of the steady state electrospun nanofibers is performed. The influences of different parameters on the fiber morphology are studied.
Keywords—electrospinning, polyamide 6.9, steady state, nanofibers
I. INTRODUCTION
A. Electrospinning
ELECTROSPINNING is the most promising of the processing techniques to produce polymer fibers with diameter in
nanometer scale. This technique, invented in 1934, makes use of an electric field that is applied across a polymer solution and a collector plate. As the solution jet travels, it is bend and/or split by the electric forces while the solvent evaporates. This mechanism leads to the formation of fibers which are attracted to the grounded collecting plate.
Till date, many polymers have been successfully electrospun into nanofibers. The morphology of those nanofibers is influenced by many parameters. These can be categorized in three groups: the solution, the process and the ambient parameters [1].
A key parameter for successful needle electrospinning is the steady state condition. Electrospinning reaches steady state when the amount of polymer that is transported through the needle per time unit equals the amount of polymer that is deposited as nanofibers on the collector per time unit. The second condition for steady state is a continuously stable Taylor cone as a function of time. When electrospinning is in steady state, frequent nozzle set up problems (clogging, drops and beads) can be avoided. This allows a long-term stability of the electrospinning, as is needed for industrial upscaled processes [2].
B. Polyamide 6.9
Polyamide 6.9 offers better dimensional stability and lower water absorption than the more commonly used PA 6 and PA 6.6. These properties offer great potential for use in composite materials. The use of PA 6.9 as a resin in high-end nanocomposites has been reported [3]. The electrospinning of PA 6.9 nanofibers is studied so that eventually these could be used as reinforcement in innovative nanocomposites.
II. STEADY STATE ELECTROSPINNING OF PA 6.9
A. The polymer solution parameters
A steady state study generally starts with a solvent study, however, since acetic acid : formic acid solvent mixtures have been used successfully in steady state electrospinning of other
PAs, this solvent mixture was chosen. The next step is to determine if and under which conditions steady state spinning is possible. A broad range of different polymer concentrations combined with different solvent ratios are tested. The process conditions that are considered during the study of steady state are the applied voltage, the tip-to-collector distance (TCD) and the flow rate. The steady state character of the process is summarized in a steady state table. Table 1 gives the steady state region for TCD: 8 cm and flow rate: 2 ml h−1, the applied voltage is varied to reach steady state.
0:100 10:90 25:75 40:60 50:50 60:40 75:25
6
8
C on
ce nt
ra tio
n of
P A
6. 9
[w t%
] viscosity increases
vi sc
os ity
in cr
ea se
s conductivity – surface tension increase
Fig. 1. Steady state table for PA 6.9 - (black) Non dissolvable, (gray) No steady state, (white) Steady state.
The steady state region is mainly determined by the viscosity, the surface tension and the dielectric constants of the polymer solutions. A minimum viscosity is required to reach steady state. When the viscosity is too low, drops fall from the needle. When it is too high, the solution solidifies at the needle tip, eventually clogging the needle. The surface tension needs to be overcome before fibers can be formed, thus a decreasing surface tension facilitates steady state. The conductivity increases with increasing fraction of formic acid causing the electric field to stretch the polymer solution faster towards the collecting plate. If this is not compensated by a higher viscosity, steady state cannot be reached. It was observed that the optimal applied voltage increases with increasing fraction of formic acid. This is also the result of the increased conductivity.
B. The tip-to-collector distance
To maintain a constant magnitude of the electric field, the optimal applied voltage must increase with increasing the TCD. The steady state region becomes smaller with increasing TCD.
C. The flow rate
Increasing the flow rate also results in higher optimal voltages. When the flow rate is increased, more charges flow out of the needle at one time. Similar to the increase in conductivity, this requires a higher electric field. The steady state region becomes smaller with increasing flow rate
III. MORPHOLOGIC STUDY
A. The polymer concentration
Figure 2 gives the average fiber diameter as a function of the polymer concentration. A 50:50 v:v% solvent mixture was used. The process parameters are kept constant.
The fiber diameters increase with increasing concentration of PA 6.9. This is the result of the increased viscosity which causes faster solidification of the fibers and thus less time for bending and splitting is available. This results in thicker fibers.
0
50
100
150
200
250
300
350
400
450
500
Av er
ag e
fib er
d ia
m et
er [n
Concentration of polyamide 6.9 [wt%]
Fig. 2. The average fiber diameter as a function of the polymer concentration
Figure 3 shows the melting peaks for different polymer concentrations. A shoulder appears left of the dominant peak when the polymer concentration is increased. This indicates that the fraction of less stable crystals increases. This is also the result of the lesser time available for crystallization.
-4
-3
-2
-1
0
1
/g )
H ea
t flo
w [W
Temperature [°C]
Increasing concentration
Fig. 3. DSC heating curves for polymer concentrations varying from 10 to 20 wt%
B. The solvent ratio
The solvent ratio has a significant effect on the diameter, however the variations in average fiber diameter appear to be ran-
dom. The melt behavior of the nanofibers is not influenced by the
solvent ratio.
C. The process parameters
The applied voltage and the TCD have practically no effect on the morphology of the nanofibers. The flow rate has a minor effect on the diameter and the crystal structure of the nanofibers.
D. The relative humidity
Recent studies in the Department of Textiles have shown that the relative humidity has a major influence on the fiber morphology. An in-house build setup which allows the control of humidity was used for the experiments. The different humidities that are compared are 16 %RH, 43 %RH and 74 %RH.
During electrospinning at low humidity, clogging at the tip of the needle was observed. Thus at low humidity, steady state electrospinning is not possible, however a small sample suited for SEM observation and DSC analysis could be produced.
Figure 4 shows the average fiber diameter as a function of the polymer concentration for different relative humidities.
0
50
100
150
200
250
300
350
400
450
Av er
ag e
fib er
d ia
m et
er [n
Fig. 4. The average fiber diameter as a function of the polymer concentration for different relative humidities - 16 %RH: N , 43 %RH: , 77 %RH:
With increasing relative humidity the average fiber diameter decreases. At high humidity, the polymer chains in the solution have increased mobility because the water works as a plasticizer. Therefore the bending and/or splitting of the nanofibers also increases. This results in finer fibers.
The relative humidity does not affect the melting peaks greatly. This indicates that the increase in relative humidity will mainly increase the splitting of the nanofibers.
IV. CONCLUSION
Steady state electrospinning of PA 6.9 is possible. The properties of the electrospun nanofibers are mainly influenced by the polymer concentration and the relative humidity.
REFERENCES
[1] S.H. Tan et Al. , Systematic parameter study for ultra-fine fiber fabrication via electrospinning process, Journal of Applied Polymer Science, 108:308- 319, 2008.
[2] S. De Vrieze et Al. , Solvent System for Steady State Electrospinning of Polyamide 6.6, Journal of Applied Polymer Science, 115:837-842, 2009.
[3] C. Sender et Al. , Dynamic Mechanical Properties of a Biomimetic Hydrox- yapatite/Polyamide 6,9 Nanocomposite, Journal of Biomedical Materials Research Part B: Applied Biomaterials, 83B:628-635, 2007.
CONTENTS xix
1.1.4 The influence of parameters on electrospinning . . . . . . . . . . . . 4
1.1.5 Steady state electrospinning . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Nanofibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Polyamide 6.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
CONTENTS xx
2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3.2 (Modulated) Differential Scanning Calorimetry ((M)DSC) . . . . . 14
2.3.3 X-Ray Diffraction (XRD) . . . . . . . . . . . . . . . . . . . . . . . 14
3 The electrospinning of PA 6.9 nanofibers under steady state conditions 15
3.1 General methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.1 Steady state tables: a combined effect of the polymer
concentration and the solvent ratio . . . . . . . . . . . . . . . . . . 17
3.2.2 The effect of the TCD on steady state electrospinning . . . . . . . . 21
3.2.3 The effect of the flow rate on steady state electrospinning . . . . . . 23
3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3 The effect of the polymer concentration . . . . . . . . . . . . . . . . . . . . 26
4.3.1 The effect of the polymer concentration on the average fiber diameter 27
4.3.2 Thermal analysis of PA 6.9 via conventional and modulated DSC. . 29
4.3.3 The effect of the polymer concentration on the thermal behavior . . 34
4.3.4 The effect of the polymer concentration on the glass transition . . . 37
4.4 The effect of the solvent ratio . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.4.1 The effect of the solvent ratio on the average fiber diameter . . . . 38
4.4.2 The effect of the solvent ratio on the thermal behavior . . . . . . . 41
4.4.3 The effect of the solvent ratio on the glass transition . . . . . . . . 42
4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.3 The effect of the applied voltage . . . . . . . . . . . . . . . . . . . . . . . . 45
5.3.1 Visual observation of the electrospinning process . . . . . . . . . . . 45
5.3.2 The effect of the applied voltage on the average fiber
diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
CONTENTS xxi
5.3.3 The effect of the applied voltage on the thermal behavior . . . . . . 48
5.3.4 The effect of the applied voltage on the glass transition . . . . . . . 49
5.4 The effect of the tip-to-collector distance . . . . . . . . . . . . . . . . . . . 49
5.4.2 The effect of the TCD on the average fiber diameter . . . . . . . . . 50
5.4.3 The effect of the TCD on the thermal behavior . . . . . . . . . . . 55
5.4.4 The effect of the TCD on the glass transition . . . . . . . . . . . . 56
5.5 The effect of the flow rate . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.5.2 The effect of the flow rate on the average fiber
diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.5.3 The effect of the flow rate on the thermal behavior . . . . . . . . . 61
5.5.4 The effect of the flow rate on the glass transition . . . . . . . . . . 62
5.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
6.2 Study of the polymer concentration . . . . . . . . . . . . . . . . . . . . . . 67
6.2.1 Observation of the electrospinning process at high relative humidity 67
6.2.2 Observation of the electrospinning process at low relative humidity 67
6.2.3 The effect of the relative humidity on the average fiber diameter . . 67
6.2.4 The effect of the relative humidity on the melt behavior . . . . . . . 70
6.2.5 The effect of humidity on the glass transition . . . . . . . . . . . . 72
6.3 Study of the solvent ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.3.1 Observation of the electrospinning process at high relative humidity 72
6.3.2 Observation of the electrospinning process at low relative humidity 73
6.3.3 The effect of the relative humidity on the average fiber diameter . . 73
6.3.4 The effect of the relative humidity on the melt behavior . . . . . . . 74
6.3.5 The effect of the humidity on the glass transition . . . . . . . . . . 75
6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Bibliography 79
XRD X-Ray Diffraction
RH Relative Humidity
TCD Tip-to-collector distance
1.1.1 Introduction
The fundamental idea of electrostatic spinning dates back to the 1930s. From 1934 to 1944,
Formals published a series of patents, describing an experimental setup for the production
of polymer filaments using an electrostatic force [1]. However those patents did not result
in industrially manufactured fibrous materials, and unfortunately, at that time, further
development and application of the principle for electrospinning was not popular in the
academic and industrial world. Electrostatic spinning never reached industrial application
in spite of the fact that the invention had high commercial potential. The probable rea-
sons for that might be the lack of appropriate equipments that should have enabled the
researchers to discover the ’nanodimension’ of electrospun fibers, since the first prototype
electron microscope came into existence only in 1931. The other reason could be the ab-
sence of industrial initiative and interest to manufacture electrospun materials until the
1980s. Briefly speaking, the fields of tissue engineering, electronics and ultrafiltration, that
use such nanomaterials, were developed only in recent times [2].
The growing interest in ’nanomaterials’ resulted in a renewed interest in the electrospinning
process. This process has now become the preferred method for making nanofibers. Other
techniques for producing nanofibers are available, however the electrospinning process has
the greatest potential when it comes to large scale industrial production of nanofibers [3].
1.1 Solvent electrospinning 2
The electrospinning process creates nanofibers through an electrically charged jet of poly-
mer solution. Its fundamental principle is similar to electrospraying. One can imagine a
spherical, electrically charged polymer drop of low molecular weight. When an electric field
is applied, two forces will act on the drop. An electrostatic repulsive force E that enforces
the drop to disintegrate due to long range repulsive Coulombic forces between ions of the
same signs. The second is a capillary force that causes liquid particles to flock together to
minimize the liquid surface tension γ, resulting from short distance intermolecular inter-
actions at quantum level. At equilibrium, both forces have the same magnitude. This is
illustrated in equation 1.1 [2, 4].
1
8πε0
Q2
R2 = 2πε0γsR (1.1)
Q (Coulomb) is the electrostatic charge on the droplet’s surface, R (meter) is the radius of
the droplet ε0 is the dielectric permeability in vacuum [8.854 1012 F m−1] and γs (N m−1)
is the surface tension.
When the magnitude of the electric field increases, the surface charge of the drop also
increases. Until the repulsive force surpasses the surface tension. At this critical point, the
drop will split into smaller drops, as illustrated in figure 1.1.
Figure 1.1: Scheme of electrospraying
This theory can also be applied on high molecular weight polymer solutions. Due to the
higher viscosity, the solution will disintegrate in long tiny liquid columns. The enormous
concentration of charged particles of similar nature, forces them to be stretched longitu-
dinally. This stretching tendency, along with jet inertia and rheology, results in a random
lateral jet motion and enormous elongation leading to quick decrease of the jet radius,
typically down to several hundreds or tens of nanometers [2].
1.1.3 The method of electrospinning
The basic setup of the electrospinning process is sketched in figure 1.2. The process requires
three main components: a container (1) for the polymer solution, a high voltage source
(3) and a conductive collector plate (7), which must be grounded for safety reasons. The
container, typically a syringe with a conductive needle (2), is placed in a pump.
P1
2
5
6
7
Figure 1.2: Basic scheme of electrospinning setup - (1) pump, (2) needle, (3) HV source, (4) Taylor cone, (5) nanofiber path, (6) nonwoven and (7) grounded collector plate
The polymer solution is pumped out of the syringe through the needle and a drop of
solution is formed at the needle tip. This drop is charged by applying a high voltage onto
the needle. By increasing the applied voltage, the Coulombic forces will counteract the
surface tension and the solution drop is distorted into the so-called Taylor cone (4). When
the electric field surpasses a certain threshold value, a charged fluid jet is ejected from the
tip of the Taylor cone toward the collector plate (5). Other than initiating the jet flow,
the electric field and the Coulombic forces tend to stretch the jet, thereby contributing to
the thinning effect of the resulting fibers. These fibers are deposited on the collector plate
in the form of a nonwoven (6).
Depending on the different parameters of the polymer, the solution, the setup and the
surrounding atmosphere, different modes can occur. The two main modes are described
below [4, 5].
In the first mode, the repulsive forces that act on the jet are still too large. Because of
the excessive repulsive forces, which are larger than the surface tension, the jet will further
split into many thinner jets. The divided jets repel each other, thereby acquiring lateral
velocities and chaotic trajectories, which gives a bush-like appearance in the region beyond
the point at which the jet first splits [6, 7].
In the second mode, the outflowing jet will first run in a straight line over a certain distance,
following the direction of the applied electric field. Then the beam will bend in the electric
field due to an inhomogeneous charge distribution. This is caused by rapid evaporation of
the solvent. The jet follows a spiral path. The repulsive forces can cause the jet to extend
thousands of times so that a very fine stream of fibers is obtained. Thus in this mode, the
nanofibers are obtained by extensive stretching of the polymer jet. These nanofibers are
often thicker than the ones obtained through the first mode.
Once the nanofibers are formed, it can no longer be determined by which mode they were
formed. Most theoretical models of electrospinning are based on a bending jet and neglect
the splitting mode [8, 9]. However it can be assumed that both modes occur simultaneously
during electrospinning.
1.1.4 The influence of parameters on electrospinning
Research has proven that the morphology of the electrospun fibers is influenced by a large
number of different parameters. These parameters can be divided in three main categories:
solution properties, processing conditions and ambient conditions [10]. An overview of the
parameters is given in table 1.1.
Table 1.1: Electrospinning parameters
Especially the solution properties such as the polymer concentration, molecular weight of
the polymer and the electric conductivity of the solution have a major effect on the fiber
morphology. Recent studies in the Department of Textiles have shown that the humidity
also has an important effect on the fiber morphology of the nanofibers [11].
1.1.5 Steady state electrospinning
If nanofibers are to be produced at a larger, industrial scale, then it is an absolute neces-
sity that the properties of the nanofibrous material are guaranteed. This requires stable
electrospinning of nanofibers for longer periods of time, without any changes in properties.
The key factor for this is the steady state condition [12].
Electrospinning reaches steady state when the amount of polymer that is transported
through the needle per time unit equals the amount of polymer that is deposited as
nanofibers on the collector per time unit. The second condition for steady state is a
continuously stable Taylor cone as a function of time. When electrospinning is in steady
state, frequent nozzle problems, such as clogging, droplets, and beads, can be avoided.
This allows a long-term stability of the electrospinning, as is needed for industrial upscaled
1.2 Nanofibers 6
processes. A solvent mixture of formic acid and acetic acid was already the key for steady
state electrospinning of polyamide 6 [11], polyamide 6.6 [13] and polyamide 4.6 [14].
1.2 Nanofibers
1.2.1 Properties of nanofibers
Through electrospinning, fibers with diameters of a few hundred nanometer can be ob-
tained. Figure 1.3 shows a fiber classification based on the diameter. When the diameter
is smaller than 500 nm, the fibers are classified as nanofibers. This makes them 100 to
1 000 times finer than a human hair.
Nanofiber: <500 nm
Textile fiber: 50 – 200 μm
Figure 1.3: Classification of fibers
Nanofibrous structures have a high specific surface area, a small pore size and high porosity,
compared to other conventional textile fiber structures [15]. The ratio of the fiber surface
to the fiber volume can be a thousand times greater than in microfibers [16]. Nanofibers
have superior mechanical properties such as tensile strength and stiffness, compared to
other forms of material. They also show great affinity for the implementation of functional
groups on the fiber surface.
1.3 Polyamide 6.9 7
1.2.2 Applications of nanofibers
Figure 1.4 gives an overview of the potential applications for nanofibers [16, 17]. Most
patents are applied for medical applications [18], followed by filtration [19]. It is important
to note that most of these patents are not yet found in commercial applications. They are
still on the level of laboratory research and development.
Polymer nanofibers
medicine
• Porous membrane for skin • Tubular shapes for blood vessels and
nerve regeneration • Three dimensional scaffolds for bone and
cartilage regenerations
Military protective clothing
aerosol particles • Anti-bio-chemical weapons
Filter media
Applications in life science
Figure 1.4: Potential applications of nanofibers
1.3 Polyamide 6.9
1.3.1 Introduction
Polyamides, also known as nylons, are a very important group of fiber materials. There are
many types of nylons, the most common used are PA 6 and PA 6.6. Polyamide fibers are
produced using traditional methods such as melt spinning, wet and dry spinning. The most
widely used are multifilament, monofilament or staple fibers. Depending on the production
method, the fiber diameter can vary from 10 to 500 µm. These diameters are very well
suited for most applications, but some high performance applications require even finer
fibers. These fibers can be obtained by solvent electrospinning of a polyamide solution.
1.3 Polyamide 6.9 8
PA 6 and PA 6.6 are most often electrospun, references to electrospinning of PA 6.9 however
seem to be nonexistent also references to the use of PA 6.9 for conventional fibers were
not found. In general the scientific literature on PA 6.9 is very limited. The few studies
reported deal with PA 6.9 as a blend with other polyamides or as a component in composite
materials. Only one report dedicated to the properties and behavior of PA 6.9 could be
found [20].
1.3.2 Properties of polyamide 6.9
Compared to PA 6 and PA 6.6, PA 6.9 is prepared in much smaller quantities, but the
preparation manner is similar [21]. The nylon salts are synthesized from hexamethylene-
diamine and dicarboxilic azaleic acid. The subsequent polycondensation is carried out as
a discontinuous process.
Figure 1.5: Repeat unit of PA 6.9
PA 6.9 (see figure 1.5) has a diamine section with a length of 6 carbons and a diacid that
is reasonable long at 9 carbons. This means that the amide density is lower than the
industry-standard polyamide 6 and polyamide 6.6 which should give more flexibility of the
chains. Because of the extra methylene groups, PA 6.9 will have better water resistance,
dimensional stability and electrical properties, but the degree of crystallinity and mechan-
ical properties are lower [22]. A list of bulk properties of polyamide 6.9 is given in table
1.2.
1.3 Polyamide 6.9 9
Table 1.2: List of typical bulk properties of polyamide 6.9 [23]
Property value unit
Melt temperature 210 °C
Mould shrinkage 1.8 %
Dissipation Factor 0.02 kHz
Dielectric Constant 3.2 kHz
The crystallization is not only influenced by the length of the repeat unit, but also by
the fact that PA 6.9 is an even-odd polyamide. The even-odd status will play its part in
the ability of the chains to form hydrogen bonds as they crystallize [20]. Polyamides of
the n-type with even number crystallize generally in the γ phase for n>8. PA 4 and PA
6 can crystallize in either the α or the γ phase. However their predominant structure is
the α phase. Polyamides of the m,n-type with even-even carbon atom numbers crystallize
mainly in the α phase, whereas the odd-odd, odd-even and even-odd numbers crystallize
generally in the γ phase. The α phase can occur as a result of high distortions during the
production process [24, 25, 26].
1.3.3 Applications of PA 6.9
PA 6.9 is not a very commonly used polyamide in applications. It is suited for the same
applications as PA 6 and PA 6.6, but because of the higher cost it is chosen far less. Mainly
when a low water absorption or a good dimensional stability is required, it may be the
preferred choice. It has been reported in composite applications. Indeed in composites the
dimensional stability is of prime importance. Moreover composites on PA 6.9 may focus
towards high-end applications for which the cost is less important. PA 6.9 has been used
as a resin in biomimetic nanocomposites [27]. In another applications a copolyamide-6/6,9
1.4 Objective of the thesis 10
was used as a base material for semi-conductive polymers [28].
In stead of using PA 6.9 as a resin, it could be used as nanofibers in composite materi-
als. Other applications which are often associated with nanofibers are filter materials and
wound dressings. Also in this area, PA 6.9 nanofibers are very promising. Because of
the low water absorption, there will be only a minimum influence of the moisture on the
structural integrity of the nanofiber’s filter or wound dressing.
1.4 Objective of the thesis
As PA 6.9 may offer potentials as nanofibers for applications such as filter and compos-
ite materials, the main objective of this thesis is to determine whether PA 6.9 can be
electrospun. Especially steady state electrospinning will be studied, because the steady
state guarantees reproducibility of the results. This is reported in chapter 3.
To allow for a generic understanding of the electrospinning potentials, the influence of
the electrospinning parameters on the fiber morphology will be studied. The polymer
solution parameters such as the polymer concentration and the solvent ratio are studied
in chapter 4. The studied processing conditions in chapter 5 are the applied voltage,
the tip-to-collector distance and the flow rate. Finally chapter 6 focuses on the effect
of humidity. The parameter values will be varied within the limits of the steady state
conditions. Characterization will focus on the fiber diameter, the crystallinity and the
thermal behavior of the nanofibers.
MATERIALS AND METHODS 11
2.1 Materials
Polyamide 6.9 was obtained from Scientific Polymer Products, Inc and used as received.
All the pellets used during the experiments came from the same lot. GPC measurements
were performed to determine the molecular weight showing that the PA 6.9 pellets have a
molecular weight Mw of 60 000 g mol−1.
98 - 100 v% formic acid and 99.8 v% acetic acid were both obtained from Sigma-Aldrich.
The solutions for electrospinning were prepared by dissolving PA 6.9 pellets in various acetic
acid:formic acid solvent mixtures. The solutions were slightly stirred with a magnetic stir
bar until all pellets were dissolved.
Silicagel orange was obtained from Sigma-Aldrich and 98 wt% potassium nitrate (KNO3)
was obtained from Merck Eurolab.
2.2 Electrospinning equipment
The influence of polymer solution parameters and processing parameters is researched in an
open electrospinning setup. Under normal atmospheric pressure temperature and relative
humidity were monitored during all experiment resulting in a temperature of 21± 2 °C and
a relative humidity of 43± 5 %RH.
2.2 Electrospinning equipment 12
A scheme of the electrospinning setup is illustrated in figure 2.1. An infusion pump (KD
Scientific Syringe Pump Series 100, 1) is used to control the flow rate. The pump allows a
flow rate from 0.1 ml h−1 to 99.9 ml h−1 with an accuracy of 0.1 ml h−1. The polymer solu-
tion is pumped from a syringe (20 ml Norm-jet of Henke SassWolf) through a 15.24 cm long
needle (2) with an internal diameter of 1.024 mm. The pump is installed on a laboratory
jack which allows adjustment of the tip-to-collector distance.
In order to obtain a high potential difference, the needle is connected to a high voltage
source (Glassman High Voltage Series EH, 3). The high voltage source can deliver an
output voltage that is continuously adjustable over the range from 0 to 30 kV, with an
accuracy of 0.06 kV. The non-woven formed by the nanofibers is collected on aluminum
foil (4), which was placed on top of the grounded collector plate.
Figure 2.1: Scheme of the open electrospinning setup - (1) KD Scientific Syringe Pump Series 100, (2) Needle, (3) Norm-jet of Henke SassWolf syringe and (4) Aluminum foil collector
2.2.2 Closed electrospinning setup
In order to study the influence of humidity, an in-house build closed electrospinning setup
is used. A scheme of this setup is shown in figure 2.2. The infusion pump (1), syringe,
needle (2), collector plate (4) and high voltage source (3) are of the same type as described
in section 2.2.1. A plexiglass and PVC chamber (5) was built to fit around the needle and
the collector plate.
Different baths can be used to control the relative humidity inside the chamber [29]. The
chosen baths are listed in table 2.1. In order to have a homogeneous environment, a fan (6)
2.3 Characterization 13
is used between the bath (7) and the chamber. The inlet (8) and outlet (9) of the airflow
are also positioned at far ends of the chamber to increase the homogeneity. A humidity
sensor (Vaisala HMI 41 indicator, 10) is placed between the outlet of the chamber and the
bath, allowing continuous measurements of the relative humidity in the chamber.
Table 2.1: The baths used to control relative humidity
Baths Preparation Relative Humidity
Silicagel 200 g dried at 105°C for 3 hours 16± 4 %RH
KNO3 72 g dissolved in 200 ml distilled water 74± 4 %RH
Figure 2.2: Scheme of humidity controlled electrospinning setup - (1) KD Scientific Syringe Pump Series 100, (2) Needle, (3) Norm-jet of Henke SassWolf syringe, (4) aluminum foil collector, (5) closed chamber, (6) fan, (7) bath, (8) inlet airflow, (9) outlet airflow and (10) humidity sensor
2.3 Characterization
2.3.1 Scanning Electron Microscopy (SEM)
The morphology of the electrospun nanofibers was examined using a scanning electron
microscope (Jeol Quanta 200 F FE-SEM) at an accelerating voltage of 20 kV. Prior to
SEM analysis, the sample was coated with gold using a sputter coater (Balzers Union
SKD 030). This coating is responsible for the cracks that appear on the fibers in the SEM
images. The nanofiber diameters are measured using Cellˆ D software from Olympus. The
2.3 Characterization 14
average fiber diameters and their standard deviations are based on 50 measurements per
sample.
2.3.2 (Modulated) Differential Scanning Calorimetry ((M)DSC)
The analysis of the glass transition and the melting is performed using a Q2000 MDSC
from TA instruments. Samples of 3±0.3 mg were placed in appropriate sealed standard
Tzero aluminum pans. Conventional DSC experiments were performed from 0 to 250°C,
with a heating rate of 10°C min−1, under a constant nitrogen flow of 50 ml h−1. MDSC
experiments were performed from -30 to 120°C, with a heating rate of 2°C min−1 and a
modulation of ±2°C min−1. After staying for 5 min at 120°C, the pans are cooled to -
30°C and reheated to 250°C. All with the same heating rate and modulation and under a
constant nitrogen flow of 50 ml h−1. The DSC and MDSC results are analyzed using TA
Universal Analysis software package.
2.3.3 X-Ray Diffraction (XRD)
XRD measurements are performed in a D5000 Diffractometer from Siemens at the Depart-
ment of Solid State Sciences. A complete range of the 2 - Θ - scale is measured at room
temperature.
THE ELECTROSPINNING OF PA 6.9 NANOFIBERS UNDER STEADY STATE CONDITIONS 15
Chapter 3
nanofibers under steady state
3.1 General methodology
Steady state conditions are essential in needle electrospinning to generate a stable process
which fabricates reproducible material. Electrospinning reaches steady state when the
amount of polymer that is transported through the needle per time unit equals the amount
of polymer that is deposited as nanofibers on the collector per time unit. The second
condition for steady state is a continuously stable Taylor cone as a function of time. When
electrospinning is in steady state, frequent nozzle problems, such as clogging, drops, and
beads, can be avoided.
Whether the steady state conditions are fulfilled, is determined by visual assessment of
the Taylor cone and observations with a SEM. When the Taylor cone is stable and SEM
images show no drops or beads, the system is in steady state. The nanofibers in figure 3.1a
are spun under steady state conditions. For the nanofibers in figure 3.1b, the steady state
condition is not fulfilled. Both samples were produced in the open setup.
3.1 General methodology 16
a. b. Figure 3.1: SEM-images of different nanofiber samples, the magnification is 2 000 -
(a.) Nanofibers spun under steady state conditions, (b.) Nanofibers not spun under steady state conditions
A steady state study generally starts with a solvent study to determine which solvents result
in steady state electrospinning. In this case the complete solvent study is not performed.
A solvent mixture of formic acid and acetic acid was already the key for steady state
electrospinning of PA 6 [11], PA 6.6 [13] and PA 4.6 [14]. This mixture will also be used
for electrospinning of PA 6.9 (Mw: 60 000 g mol−1). Properties of the solvents are listed in
table 3.1.
Density Boiling point Dielectric constant Surface tension Viscosity
Solvents [g cm−3] [°C] [kHz] [mN m−1] [mPa s]
Acetic acid 1.049 118 6.19 26.9 1.1
Formic acid 1.022 101 58.5 37.7 1.8
Electrospinning solutions of different polymer concentrations and solvent ratios will be
tested. A broad study on the different process parameters is performed, resulting in the
knowledge of the steady state interval. All experiments are conducted in the open setup,
as described in section 2.2.1, at a temperature of 21± 2 °C, and a relative humidity of
43± 5 %RH.
3.2 Steady state electrospinning
3.2.1 Steady state tables: a combined effect of the polymer
concentration and the solvent ratio
The steady state character of the electrospinning process is summarized in a steady state
table as shown in table 3.2. The columns represent different solvent ratios and the rows
represent different polymer concentrations. For each combination it is determined whether
steady state is possible or not. The process parameters are varied over a broad range of val-
ues to detect the steady state. The combinations that result in steady state electrospinning
are registered.
voltage, tip-to-collector distance (TCD) and flow rate are considered when studying the
steady state. The result of all these parameters cannot be combined in one steady state
table. For this reason, one steady state table summarizes the steady state character of the
process for two constant process parameters, only one process parameter can vary within
a certain steady state table. As an example table 3.2 gives the steady state table for a set
value of TCD at 8 cm and flow rate 1 ml h−1. The voltage is adjusted to achieve steady
state.
The polymer concentration range is limited from 6 to 24 wt%. This because the polymer
concentrations below 6 wt% only result in drops, not in the formation of nanofibers and
because above 24 wt% PA 6.9 no longer dissolves in any of the solvent mixtures.
3.2 Steady state electrospinning 18
Table 3.2: Steady state table of PA 6.9 - (black) pellets do not dissolve, (gray) no steady state, (white) steady state
0:100 10:90 25:75 40:60 50:50 60:40 75:25
6
8
C on
ce nt
ra tio
n of
P A
6. 9
[w t%
conductivity – surface tension increase
The steady state table generally consists of three regions. The black region represents
the combinations of polymer concentration and solvent ratio that result in solutions for
which the pellets are only partially dissolved. In the gray region, polymer solutions are
formed, however the solution cannot be electrospun under steady state conditions. The
white region represents all the polymer solutions that can be electrospun under steady
state conditions.
These regions of the steady state table are the combined result of different parameters that
play a role in the electrospinning process. The surface tension, the viscosity, the dielectric
constant of the solvent mixture, the solidification process, and the solubility of the PA in
the solvent mixture determine the borders of the regions [12].
Table 3.2 illustrates how the viscosity, conductivity and surface tension of the polymer
solutions vary within the steady state table. The lower dielectric constant and the lower
surface tension values of acetic acid in comparison with those of formic acid results in the
respective reduction in the conductivity and the surface tension values of the resulting
solutions with increasing acetic acid content [30].
The viscosity increases with increasing polymer concentration and increasing fraction of
acetic acid. The viscosity of a polymer solution is described in equation 3.1.
(ηv)sol = φpMw
Mv
(3.1)
(ηv)sol is the solution viscosity, φp is a measure for the polymer concentration, Mw is the
weight-average molecular weight and Mv is the solution entanglement molecular weight.
Since both molecular weights are constant, the viscosity will increase with increasing poly-
mer concentration [31]. With increasing fraction of acetic acid, the PA will be dissolved
in a smaller amount of formic acid, thus actually the polymer concentration in formic acid
increases, resulting in an increase in viscosity.
The white region is the one of interest and will be further addressed. In this region a
voltage could be found that resulted in steady state for the set values of TCD and flow
rate.
The presence of the black region is the result of the high volume fraction of the acetic acid,
which does not dissolve polyamides. The higher the polymer concentration, the sooner the
maximum volume fraction of acetic acid is reached.
The gray region is the region where although the PA 6.9 dissolves completely in the solvent
mixture, no steady state electrospinning is possible. This region can be divided roughly in
three different regions: left, below and above the (white) steady state region.
In the case of table 3.2, the region below the steady state region is very limited. It consists
of solutions with more than 22 wt%. During electrospinning the PA 6.9 is deposited onto
the tip of the needle, eventually blocking it. No steady state is obtained because of the
fast solidification.
In the region above the steady state region, steady state electrospinning is not possible
because of the combined result of the viscosity, the surface tension and the conductivity.
When the polymer concentration is less than the border value the viscosity is too low to
electrospin in steady state. The higher the formic acid content, the higher the dielectric
constant and the more the electric field pulls at the polymeric solution. If this is not
compensated by a higher viscosity of the polymeric solution, the polymeric jet will break
and end up in polymeric droplets at the collector. As mentioned, the surface tension
decreases with decreasing fraction of formic acid. A decreasing surface tension facilitates
the formation of steady Taylor cones at lower viscosity or thus lower polymer concentration
[12].
The region left of the steady state region consists of solutions with high fractions of formic
acid. Again the viscosity, surface tension and high dielectric constant of formic acid are
responsible for this effect. The electrospinning solutions with high formic acid content have
a low viscosity and high surface tension and conductivity. These properties prevent the
formation of a stable Taylor cone.
The steady state region shows that a minimum polymer concentration of 10 wt% is needed
for this particular solvent mixture. However adding acetic acid clearly broadens the range
of polymer solutions in the steady state region. This is because the increase in acetic acid
facilitates steady state electrospinning.
For the chosen TCD and flow rate of respectively 8 cm and 1 ml h−1, table 3.3 shows the
minimum applied voltage required for steady state electrospinning. Generally the optimal
applied voltage is a range of 3 to 4 kV above the value in the table. However this is not
always the case, therefore some examples of voltage ranges are also shown in the table.
Table 3.3: Steady state table for TCD 8 cm and flow rate 1 ml h−1 - (black) pellets do not dissolve, (gray) no steady state, (white) steady state
0:100 10:90 25:75 40:60 50:50 60:40 75:25
6
8
12 16 - 18 kV 18 kV 16 kV
14 26 - 27 kV 18 - 21 kV 17 kV
16 27 - 30 kV 24 kV 16 kV
18 26 kV 15 - 22 kV
20 26 - 28 kV 26 kV 22 - 23 kV
22 30 kV 22 - 26 kV
24
C on
ce nt
ra tio
n of
P A
6. 9
[w t%
Vo lta
ge in
cr ea
se s
Voltage increases
Looking at a certain row in the steady state region, the optimal applied voltage decreases
with increasing fraction acetic acid. This is mainly explained by electric properties of the
electrospinning solution. The higher acetic acid content causes a decrease of the solution’s
dielectric constant, therefore the optimal applied voltage will be lower. This has as con-
sequence that the results do not necessarily mean that that it is impossible to spin any
100 v% formic acid solutions. The high voltage source is limited to 30 kV. It is likely that
that with a higher voltage some 100 v% can be electrospun under steady state conditions.
When looking at a certain column in the steady state region, the trend of the optimal
applied voltage is less obvious. In each column the optimal applied voltage appears to
reach a minimum in the middle. However the variation within voltages is much smaller
than the variation in voltages caused by the solvent ratio. This effect could be related to a
combination of the viscosity, surface tension of dielectric properties of the electrospinning
solutions.
3.2.2 The effect of the TCD on steady state electrospinning
One steady state table only summarizes the results for a certain fixed TCD and flow rate.
It is likely that changing one of these processing parameters will effect the steady state
conditions. The investigated parameter space is illustrated in figure 3.2. The white area
was investigated in the previous section, the white area is table 3.4 which shows the steady
state conditions for a TCD fixed at 6 cm.
Polymer concentration
Solvent Ratio
TCD
Figure 3.2: The parameter space - study of the effect of the TCD
0:100 10:90 25:75 40:60 50:50 60:40 75:25
6
8
10 17 kV 16 kV
12 25 kV 14 - 15 kV 15 kV 12 - 13 kV
14 22 - 25 kV 13 - 17 kV 14 kV
16 25 - 27 kV 17 kV 14 kV
18 27 kV 23 - 25 kV 14 kV
20 26 kV 22 kV 15 kV
22 27 kV 18 - 22 kV
24
C on
ce nt
ra tio
n of
P A
6. 9
[w t%
]
Decreasing the TCD result in a lower optimal applied voltage. The steady state region
is also larger. With the current process conditions, it is possible to electrospin a 100 v%
formic acid solution under steady state conditions.
The decrease in optimal can be explained by considering the electric field. The electric
field can be simplified to the ratio of the applied voltage to the TCD. In order to keep
the electric field constant when decreasing the TCD, the applied voltage should also be
increased.
When decreasing the TCD to less than 6 cm, some solutions can no longer be electrospun.
The tip of the needle and the plate are too close together causing the nanofibers to float
between the needle and collector. This interrupts the stable process.
When increasing the TCD to more than 8 cm, the optimal voltages also increases and the
steady state region becomes smaller. Eventually, not one solution can be electrospun under
steady state conditions. Thus steady state electrospinning is only possible for a limited
range of TCDs.
3.2.3 The effect of the flow rate on steady state electrospinning
Similar to the TCD, the flow rate can be varied. In the parameter space (figure 3.3) the
white region represents table 3.5 which summarizes the steady state condition for a TCD
of 6 cm and a flow rate of 2 ml h−1. The increase in flow rate means that there is more
polymer solution and thus more charges flowing out of the needle at one time. This increase
in electric charges needs to be compensated by an increase in applied voltage.
Polymer concentration
Solvent Ratio
Flow rate
Figure 3.3: The parameter space - study of the effect of the flow rate
3.3 Conclusion 24
0:100 10:90 25:75 40:60 50:50 60:40 75:25
6
8
12 22 - 23 kV 17 - 18 kV 17 kV
14 27 kV 18 - 20 kV 22 kV
16 19 kV 17 kV
18 17 kV
22 20 kV
C on
ce nt
ra tio
n of
P A
6. 9
[w t%
]
The increase in optimal voltage was observed for several higher flow rates. The steady state
region also becomes smaller with increasing flow rate until eventually not one combination
of parameters can be found that results in steady state electrospinning. When decreasing
the flow rate, the steady state region first becomes larger, until the flow rate is so low that
nanofibers can no longer be formed. Thus the flow rate is also limited.
3.3 Conclusion
Steady state electrospinning of polyamide 6.9 is possible. The acetic acid : formic acid sol-
vent mixtures have proven to be suitable for steady state electrospinning. Only a limited
range of polymer concentration, solvent ratio and process parameters result in steady state.
The combinations of those parameters that result in steady state electrospinning is deter-
mined by the viscosity, the surface tension and the electric properties of the electrospinning
solutions.
The polymer solution parameters include molecular weight, polymer concentration, solvent
type and more, see table 1.1. This chapter will focus on the influence of the polymer
concentration and the solvent ratio (acetic acid:formic acid) on the fiber morphology and
thermal behavior. The molecular weight of the polyamide used in the electrospinning
solutions is 60 000 g mol−1.
In chapter 3 it became clear that PA 6.9 can be electrospun under steady state conditions
for a broad range of concentrations. The first concern is that the nanofibers are spun
under steady state conditions so that the reproducibility of the results is guaranteed. The
electrospinning process parameters are kept constant when possible.
4.2 Materials and methods
PA 6.9 with a molecular weight of 60 000 g mol−1 and acetic acid:formic acid solvent mix-
tures are used.
The flow rate is set at 2 ml h−1, this is chosen because more nanofiber material can be
collected at one time. The other parameter values are then chosen based on steady state
table 3.5. A tip-to-collector distance (TCD) of 6 cm is selected, since this allows the use
of lower voltages. The applied voltage is chosen depending on which polymer solution
4.3 The effect of the polymer concentration 26
parameter is studied. The ambient parameters are monitored during electrospinning, the
temperature is 21± 2°C and the relative humidity is 43± 5 %RH.
The thermal behavior is analyzed using (M)DSC. Standard Tzero aluminum pans were
filled with 3± 0.3 grams of nanofibers. A heat-cool-heat profile from 0 to 250°C with a
heating rate of 10°C min−1 was applied. The glass transition is studied using a heat-
cool-heat profile from -30 to 120°C with a heating rate of 2°C min−1 and a modulation of
2°C min−1.
4.3 The effect of the polymer concentration
Nanofibers are made with different concentrations of PA 6.9, varying from 8 wt% to 20 wt%.
Steady state table 3.5 shows that most of those polymer concentration can be electrospun
under steady state conditions when using a 50:50 v% acetic acid:formic acid solvent mixture
and an applied voltage of 18 kV.
As the polymer concentration of the solutions is increased, the time necessary to dissolve
all the pellets increases. The lower concentrations result in a clear liquid, whereas the
higher concentrations result in a more opaque and more viscous solution.
The steady state condition is not fulfilled for 8 wt% PA 6.9, drops cannot be avoided.
However a small sample suited for SEM observation with a minimum of drops can be
produced. Although the applied voltage of 18 kV is too low to spin the 10 wt% solution
under steady state conditions for a long time, it was possible to produce a drop free sample
for the first minutes. For both solutions the obtained nanofiber structures are deposited
as a ringshape.
For the 12 to 20 wt% solutions the steady state condition is fulfilled. These nanofibers are
deposited as a circle. The diameters of the circular deposition area decreases with increasing
concentration. The results are summerazid in table 4.1. The smaller deposition area
may be attributed to the increased viscosity which discourages the bending and splitting
instabilities to set for a longer distance as it merges from the tip of the needle. As a result,
the jet path is reduced and the bending instability stretches over a smaller area [32].
Table 4.1: Deposition area of nanofibers [*ringshaped deposition area]
Concentration [wt%] Diameter [cm]
4.3.1 The effect of the polymer concentration on the average
fiber diameter
Figure 4.1 shows SEM images of the nanofibers made with different concentrations of PA
6.9. Since they all have the same magnification, it is clear that with increasing polymer
concentration, the fiber diameter increases.
This may be attributed to increased viscosity of the electrospinning solutions. Also Also
during electrospinning, the solvents evaporate. At certain moment, the jet will reach a
critical amount of formic acid that remains in the liquid phase to keep the polyamide
dissolved. This critical amount will occur much faster in the higher polymer concentration
solutions. The faster the critical amount is reached, the faster solidification of the PA 6.9
occurs, generating thicker nanofibers. When solutions with a lower polymer concentrations
are used, the polyamide is kept longer in dissolved form, resulting in a longer time of
stretching and/or splitting of the system and thus thinner nanofibers [12].
a. b.
c. d.
e. f.
g. Figure 4.1: SEM-images of nanofibers made with different concentrations of PA 6.9,
the magnification is 50 000 (a.) 8 wt%, (b.) 10 wt%, (c.) 12 wt%, (d.) 14 wt%, (e.) 16 wt%, (f.) 18 wt%, and (g.) 20 wt%
The measured average fiber diameters and their standard deviations are summarized in
figure 4.2. These standard deviation are a measure for the uniformity of the fibers. As the
polymer concentration increases, the average fiber diameter increases exponentially.
0
50
100
150
200
250
300
350
400
450
500
Av er
ag e
fib er
d ia
m et
er [n
Figure 4.2: The average fiber diameter as a function of the concentration of PA 6.9
4.3.2 Thermal analysis of PA 6.9 via conventional and modulated
DSC.
Figure 4.3 shows the heat flow as a function of the temperature for electrospun nanofibers.
The profile of the curve in the first heating is different from the profile in the second
heating.
During the first heating an endotherm between 40 and 100°C is observed in the nanofiber
samples. In the second heating, this endotherm is no longer visible. At about 150°C
a second endotherm appears and at 195°C an exotherm is observed. This is the result
of a melting and recrystallization process. In the second heating, this exotherm is even
more prominent. The irregular shape of the exotherm in the second heating has also been
observed for other polyamides [33].
Two effects can explain the appearance of the first endotherm. It could be caused by
moisture that is trapped in the sample. Even when PA 6.9 does not absorb a lot of water,
the porous structure of the nanofibrous material can still trap water molecules. Or it could
be the result of the electrospinning process. Nanofibers are stretched during electrospinning
which causes internal stresses. Once the fibers are formed and stored, relaxation occurs to
release some of the internal stresses. This is possible because room temperature is only a
little below the glass transition temperature, thus the polymer chains have some mobility.
When the fibers are heated above the glass transition temperature an enthalpy relaxation
may be visible as an endotherm in the heat flow curve.
-3
-2
-1
0
1
2
Exo Up Universal V4.4A TA I
H ea
Figure 4.3: Example of a DSC heating curve
It should be noted that the measurements of the melt enthalpy are only taken from 195 to
245°C. This because of the melting and recrystallization that takes place below 195°C. The
top of the exotherm is higher than the baseline of the heating curve, making it difficult to
measure the melt enthalpy. In order to minimize melting and recrystallization, rapid heat
DSC could be used in future research.
Solution casted bulk and nanofibers made from a 14 wt% solutions are compared in fig-
ure 4.4. Curves (a) and (b) represent the first heating of respectivily the bulk and the
nanofibers.
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
/g )
40 60 80 100 120 140 160 180 200 220 240 Temperature (°C)Exo Up Universal V4.4A TA Temperature [°C]
H ea
g-1 ]
a
b
Figure 4.4: DSC heating curves - a: solution casted bulk, b: nanofibers
It is obvious that the melt behavior of the nanofibers differs from the behavior of the bulk
material. The bulk material shows no endotherm between 40 and 100°C. This is because
bulk material does not have such high internal stresses and it only absorbs very small
amounts of water. Another distinct difference is the peak at 220°C that appears in the
melting peaks of the nanofibers indicating that the crystal structure in the fibers differs
from the one in the bulk material.
It is known that the γ phase is the dominant crystal phase in PA 6.9. However due to high
distortion, the α phase can be formed. The peak at 220°C could therefore be attributed
to the α phase.
Different samples of nanofibrous material which were produced with exactly the same
electrospinning parameters are analyzed with the DSC. The only difference between the
samples is that the solutions were made with different pellets. The melt profiles show some
variation which is the result of the variation of properties within the pellets. The difference
in melt temperature is 0.9°C and the difference in melt enthalpy is 2 J g−1.
Some of those samples were also analyzed in the MDSC. Figure 4.5 shows small but at
random variations in glass transition temperature. Probably due to a moisture content or
non-reproducible stresses in the sample.
0.004
0.006
0.008
0.010
0.012
0.014
))
20 30 40 50 60 70 80 90 Temperature (°C) Universal V4.4A TA
D er
iv . R
Temperature [°C]
Figure 4.5: MDSC curves for nanofibers made from different pellets - first heating
To minimize the influence of moisture and internal stresses, a second heating is applied.
The glass transitions observed in the second heating are shown in figure 4.6.
0.004
0.006
0.008
0.010
0.012
0.014
))
20 30 40 50 60 70 80 90 Temperature (°C) Universal V4.4A TA
D er
iv . R
Temperature [°C]
Figure 4.6: MDSC curves for nanofibers made from different pellets - second heating
After the first heating the moisture will have evaporated and the stresses will be reduced
due to relaxation. The glass transition in the second heating shows less variation. It has
also shifted to lower temperatures. This is clearly illustrated in figure 4.7.
0.002
0.004
0.006
0.008
0.010
0.012
20 30 40 50 60 70 80 90 Temperature (°C)
First heating Second heating
Eerste opwarming Tweede opwarming
Figure 4.7: Comparing the glass transition of first and second heating
The shift to lower temperature indicates that the endotherm in the first heating can mainly
be attributed to the internal stresses within the fibers. Internal stresses generally shift
the glass transition to higher temperatures, because they increase the orientation of the
polymer chains within the fibers. If it was the result of moisture, the glass transition
temperature in the second heating should be higher, because water works as a plasticizer
that reduces the glass transition temperature. Once it has evaporated, the polyamide
would show a higher glass transition temperature.
For further experiments both the first and second heating were recorded. The first heating
shows indeed always an at random distribution similar to figure 4.6, thus the results are
limited to the second heating.
4.3.3 The effect of the polymer concentration on the thermal
behavior
Nanofibers made out of polymer solutions with a polymer concentration varying from
10 wt% to 20 wt% are studied with DSC. The 8 wt% polymer solution was not analyzed
with DSC because it was not spun under steady state conditions. SEM images showed that
these nanofiber materials contain mostly drops, therefore DSC would mostly be analyzing
the bulk properties of the polyamides instead of those of the nanofibers.
Figure 4.8 shows the melting peaks for nanofibers produced from different polymer concen-
trations. For the lower polymer concentrations, the DSC heating curves shows two different
peaks. A dominant peak at about 210°C and a smaller peak at 220°C. As the polymer
concentration increases, the peak temperatures of both peaks remain unchanged, but the
peak at 220°C becomes less explicit. Another observation that can be made, is that with
increasing concentration, an additional shoulder at about 208°C appears in the heating
curve.
-4
-3
-2
-1
0
1
/g )
H ea
t flo
w [W
Temperature [°C]
Increasing concentration
Figure 4.8: DSC curves for nanofibers spun out of solutions with polymer concentrations increasing from 10 to 20 wt%
In contradiction to PA 6 and PA 6.6, where many references exist to assign the different
melting peaks to the α or γ phase, this is not the case for PA 6.9. However the small peak
at 220°C may be attributed to the α phase, but this cannot be confirmed by literature.
Even when the phases cannot be identified, it is obvious that less stable crystals are formed
when higher polymer concentrations are used.
That these different crystal phases are not only the result of a heating and recrystallization
process in the DSC, can be confirmed by XRD diffractograms. These are measured at room
temperature, therefore recrystallization cannot occur. When comparing the diffractograms
of a 10 wt% and an 18 wt% polymer solution, see figures 4.9, 4.10, it is clear that additional
peaks appear in the 18 wt% diffractogram. These peaks indicate that depending on the
polymer concentration an additional phase is formed during electrospinning.
Figure 4.9: XRD diffractogram for nanofibers spun out a 10 wt% polymer solution
Figure 4.10: XRD diffractogram for nanofibers spun out a 18 wt% polymer solution
The presence of the less stable phase crystals can be explained by the faster jet solidification
caused by the higher initial polymer concentration. The solidification is faster, thus there
is less time for crystallization. This results in less stable phase crystals with a lower melting
temperature.
Table 4.2 summarizes the melt temperature of the dominant peak and the total melt
enthalpy. The table confirms that the melt temperature of the dominant peak remains
unaltered. The variation in melt enthalpy is smaller than the variation caused by the
variation withing the pellets and therefore can be considered to be insignificant.
Table 4.2: The melt temperature and melt enthalpy for varying polymer concentrations
Concentration [wt%] Melt temperature [°C] Melt enthalpy [J g−1]
10 210.6 58
12 210.8 58
14 210.9 61
16 210.8 60
18 211.2 60
20 210.6 60
4.3.4 The effect of the polymer concentration on the glass tran-
sition
The glass transition observed in the second heating is shown in figure 4.11. The curve
for 10 wt% clearly differs from the other curves. The glass transition temperature appears
to decrease with increasing polymer concentration, see also table 4.3. Thus the polymer
concentration may influence the glass transition, it is however important to stress that the
trend is very small.
20 30 40 50 60 70 80 90 Temperature (°C)
10 wt% 12 wt% 14 wt% 16 wt% 18 wt% 20 wt%
Universal V4.4A TA
Figure 4.11: Glass transition of nanofibers produced from different polymer concentrations
Table 4.3: The glass transition temperature Tg
Concentration [wt%] Tg [°C]
4.4 The effect of the solvent ratio
The properties of the acetic acid : formic acid solvent mixture such as viscosity, surface
tension and polarity are influenced by the solvent ratio. The solvent ratio could therefore
have a significant effect on the morphology and thermal behavior of the nanofibers.
In this section, the influence of solvent ratios from 30:70 to 60:40 acetic acid:formic acid will
be investigated using a 14 wt% polymer solution. This concentration was chosen because
it will result in fine fibers and still allow a broad range of different solvent ratios to be
electrospun under steady state conditions. As mentioned before, the TCD is 6 cm and the
flow rate is 2 ml h−1. When looking at steady state table 3.5 it is clear that not one value
for the optimal applied voltage can be chosen that will allow steady state electrospinning
of the complete range of solvent ratios. Therefore the applied voltage will vary during the
experments as described in table 4.4.
Table 4.4: The average fiber diameter and the standard deviation for different polymer concentrations
Acetic acid:formic acid [v:v%] Applied voltage [kV]
30:70 26
35:65 26
40:60 22
45:55 21
50:50 21
55:45 21
60:40 21
The variation in applied voltage is kept minimal, however it could have an additional
influence on the results. All these combinations result in a stable Taylor cone and a
constant circular deposition area and thus steady state spinning.
4.4.1 The effect of the solvent ratio on the average fiber diameter
In the SEM image of the nanofibers produced with 35:75 v:v% acetic acid acid : formic acid
the fibers loook as if they are fused together. This may be attributed to the solvent that
has not yet completely evaporated when the fibers are deposited.
4.4 The effect of the solvent ratio 39
a. b.
c. d.
e. f. Figure 4.12: SEM-images of nanofibers made with different acetic acid:formic acid sol-
vent ratios, the magnification is 100 000 (a.) 35:65 v:v%, (b.) 40:60 v:v%, (c.) 45:55 v:v%, (d.) 50:50 v:v%, (e.) 55:45 v:v% and (f.) 60:40 v:v%
0
50
100
150
200
250
300
Av er
ag e
fib er
d ia
m et
er [n
Acetic acid:formic acid solvent ratio [v:v%]
Figure 4.13: The average fiber diameter as a function of the concentration of PA 6.9
Figure 4.13 shows that the solvent ratio affects the average fiber diameter, however no
trend is recognized. There is some at random variation. To ensure that this variation
is not caused by the varying applied voltage, a Duncan test was performed, see table
4.5. The Duncan test is used to determine the significant differences between averages.
It categorizes the averages in different subsets, within one subset, the averages are not
significantly different.
Since the subsets are not divided according the applied voltages, this will not cause the
variation. Since the solvent ratios are randomly divided over the subsets, the test also
confirms that there is no trend.
The solvent ratio influences the viscosity as well as the surface tension and the dielectric
constants of the solution. This may explain why no trend appears. In literature on PA 6
some similar at random variation were reported [4], others cite a trend [30].
Table 4.5: Results of Duncan test performed on diameter measurements of nanofibers made with different solvent ratios
acetic acid:formic acid Applied voltage Subset for α= 0.5
[v:v%] [kV] N 1 2 3
60:40 21 50 170.2
45:55 21 50 171.3
50:50 21 50 172.8
35:65 26 50 163.3
55:45 21 50 192.6
40:60 22 50 212.1
Significance 0.71 0.28 0.14
4.4.2 The effect of the solvent ratio on the thermal behavior
The melting peaks (figure 4.14) show small, at random variations. This is similar to what
was found for the average fiber diameter.
-4
-2
0
2
/g )
180 190 200 210 220 230 240 Temperature (°C)Exo Up Universal V4.4A TA InTemperature [°C]
H ea
Increasing acetic acid v%
Figure 4.14: Melting peaks for nanofibers formed out of electrospinning solutions with different solvent ratios - The fraction acetic acid increases from 35 to 60 v%
Table 4.6: The melt temperature and melt enthalpy for varying solvent ratio
Solvent ratio [v:v%] Melt temperature [°C] Melt enthalpy [J g−1]
35:65 210.9 57
40:60 210.4 58
45:55 210.7 60
50:50 211.1 61
55:45 211.4 59
60:40 210.6 58
4.4.3 The effect of the solvent ratio on the glass transition
Again no trend is observed in the glass transition of nanofibers spun out of solutions with
different solvent ratios.
Universal V4.4A TA
Figure 4.15: The glass transition for different solvent ratios
4.5 Conclusion 43
4.5 Conclusion
Both the polymer concentration and the solvent ratio have a significant effect on the
morphology and thermal behavior of the nanofibers. The polymer concentration clearly has
a major effect on the properties of the nanofibers, mainly caused by t

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