Silica-Based Inorganic/Organic Hybrid...

Silica-Based Inorganic/Organic Hybrid

Materials

Von der Fakultt fr Mathematik, Informatik und Naturwissenschaften der RWTH

Aachen University zur Erlangung des akademischen Grades eines Doktors der

Naturwissenschaften genehmigte Dissertation

vorgelegt von

Master of Science

Yongliang Zhao

aus Gansu, V. R. China

Berichter: Universittsprofessor Dr. rer. nat. Martin Mller

Universittsprofessor Dr. rer. nat. Regina Palkovits

Tag der mndlichen Prfung: 25. August 2016

Diese Dissertation ist auf den Internetseiten der Universittsbibliothek online verfgbar.

This research forms part of the research programme of the Dutch Polymer

Institute (DPI), project #747 NanoPA.

Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, the

Netherlands

To my family

Contents

Summary .............................................................................................................................I

Zusammenfassung .......................................................................................................... III

List of Abbreviations ........................................................................................................ V

1. Introduction and Outline of the Thesis ...................................................................... 1

1.1 Introduction ...................................................................................................... 2

1.2 Objectives of this thesis ................................................................................... 4

1.3 Outline of the thesis ......................................................................................... 5

1.4 References ........................................................................................................ 6

2. Literature Review ......................................................................................................... 9

2.1 Polymer/inorganic nanocomposites ............................................................... 10

2.1.1 Polymer/silicate nanocomposites ........................................................ 10

2.1.2 Surface modification of silica ............................................................. 11

2.1.3 Polymer nanocomposites containing ultrasmall particles ................... 12

2.1.4 MQ resins ultrasmall core-shell silicate particles ............................ 14

2.1.5 Polyamide 6 ........................................................................................ 16

2.2 Silica-based hybrid core-shell particles ......................................................... 16

2.2.1 Definition and classification of core-shell particles ............................ 16

2.2.2 Organic-silica core-shell particles ....................................................... 18

2.2.3 Design and synthetic strategy ............................................................. 19

2.2.3.1 Polymer core functionalization and silica shell coating .......... 19

2.2.3.2 Layer-by-layer (LBL) process ................................................. 21

2.2.3.3 Pickering (mini)emulsion polymerization ............................... 23

2.2.3.4 Partial extraction of polymer after sol-gel reaction ................. 25

2.2.3.5 Sol-gel microencapsulation of organic liquids......................... 26

2.3 References ...................................................................................................... 30

3. Microencapsulation of Hydrophobic Liquids in Closed All-Silica Colloidosomes(*)

........................................................................................................................................... 39

3.1 Introduction .................................................................................................... 40

3.2 Experimental Section ..................................................................................... 41

3.2.1 Materials ............................................................................................. 41

3.2.2 Preparation of partially hydrophobic silica nanoparticles .................. 42

3.2.3 Formation of all-silica colloidosomes ................................................. 42

3.2.4 Dynamic Light Scattering (DLS) measurements ................................ 42

3.2.5 Fluorescence Microscopy ................................................................... 42

3.2.6 Field-Emission - Scanning Electron Microscopy (FE-SEM) ............. 44

3.2.7 Transmission Electron Microscopy (TEM) ........................................ 44

3.2.8 Thermogravimetric Analysis (TGA) ................................................... 44

3.2.9 Fourier-Transform Infrared (FT-IR) Spectroscopy ............................. 44

3.2.10 1H NMR Spectroscopy ...................................................................... 44

3.3 Results and Discussion .................................................................................. 45

3.4 Conclusions .................................................................................................... 57

3.5 References ...................................................................................................... 58

4. Silica Nanoparticles Catalyze the Formation of Silica Nanocapsules in a

Surfactant-Free Emulsion System(*) .......................................................................... 61

4.1 Introduction .................................................................................................... 62


4.2.1 Materials ............................................................................................. 63

4.2.2 Preparation of methyl-functionalized silica nanoparticles .................. 64

4.2.3 Preparation of silica capsules .............................................................. 64

4.2.4 Characterization methods .................................................................... 64


4.4 Conclusions .................................................................................................... 82

4.5 References ...................................................................................................... 83

5. A Facile One-Step Approach towards Polymer@SiO2 Core-Shell Nanoparticles

via a Surfactant-Free Miniemulsion Polymerization Technique(*) ........................... 87

5.1 Introduction .................................................................................................... 88


5.2.1 Materials ............................................................................................. 90

5.2.2 Preparation of PS@silica composite particles .................................... 92

5.2.3 Interfacial tension (IFT) measurements .............................................. 92

5.2.4 Field-emission scanning electron microscopy (FE-SEM) .................. 92

5.2.5 Transmission electron microscopy (TEM) .......................................... 93

5.2.6 Energy-dispersive X-ray spectroscopy scanning transmission electron

microscopy (EDX-STEM) ........................................................................... 93

5.2.7 Fourier-transform infrared (FT-IR) spectroscopy ............................... 93

5.2.8 Thermogravimetric analysis (TGA) .................................................... 93

5.2.9 Dynamic light scattering (DLS) measurements .................................. 94


5.4 Conclusions .................................................................................................. 107

6. Preparation of Poly (methyl methacrylate)/SiO2 Composite Nanoparticles via a

Surfactant-Free Miniemulsion Polymerization Process ........................................... 111

6.1 Introduction .................................................................................................. 112

6.2 Experimental Section ................................................................................... 115

6.2.1 Materials ........................................................................................... 115

6.2.2 Synthesis of methylsilyl-substituted polyethoxysiloxane ................. 115

6.2.3 Preparation of PMMA/silica composite nanoparticles ..................... 115

6.2.4 NMR measurements .......................................................................... 117

6.2.5 Interfacial tension (IFT) measurements ............................................ 117

6.2.6 Field-emission - scanning electron microscopy (FE-SEM) .............. 117

6.2.7 Transmission electron microscopy (TEM) ........................................ 117

6.2.8 Fourier-transform infrared (FT-IR) spectroscopy ............................. 118

6.2.9 Thermogravimetric analysis (TGA) .................................................. 118

6.2.10 Brunauer-Emmett-Teller (BET) specific surface area measurements

.................................................................................................................... 118

6.3 Results and Discussion ................................................................................ 118

6.4 Conclusions .................................................................................................. 129

6.5 References .................................................................................................... 130

7. Synthesis and Characterization of Ultrasmall Silicate Particles - MQ Resins(*)

......................................................................................................................................... 133

7.1 Introduction .................................................................................................. 134


7.2.1 Materials ........................................................................................... 137

7.2.2 Synthesis ........................................................................................... 137

7.2.2.1 Methyl-terminated MQ resins [(Me3SiO1/2)1.0 to 1.6(SiO4/2)1.0] 137

7.2.2.2 Hydride-terminated MQ [(Me3SiO1/2)m(HMe2SiO1/2)n(SiO4/2)1.0]

............................................................................................................ 138

7.2.2.3 Hydrosilylation of hydride-terminated MQ resins ................. 139

7.2.3 Characterization Methods ................................................................. 139

7.2.3.1 NMR measurements ............................................................... 139

7.2.3.2 Gel Permeation Chromatography (GPC) ............................... 139

7.2.3.3 Fourier-Transform Infrared (FT-IR) Spectroscopy ................ 140

7.2.3.4 Dynamic Light Scattering (DLS) measurements ................... 140

7.2.3.5 Atomic Force Microscopy (AFM) ......................................... 140


7.4 Conclusions .................................................................................................. 150

7.5 References .................................................................................................... 151

8. Effect of Ultrasmall MQ Core-Shell Particles on Melt Viscosity of Polyamide 6(*)

......................................................................................................................................... 153

8.1 Introduction .................................................................................................. 154


8.2.1 Materials ........................................................................................... 156

8.2.2 Synthesis ........................................................................................... 157

8.2.2.1 Pyrrolidinone-containing silane coupling agent .................... 157

8.2.2.2 Surface modification of SiO2 nanoparticles ........................... 157

8.2.2.3 MQ resins ............................................................................... 158

8.2.2.4 PA6/silica nanocomposites .................................................... 159

8.2.3 Characterization Methods ................................................................. 159

8.2.3.1 Fourier-Transform Infrared (FT-IR) Spectroscopy ................ 159

8.2.3.2 Transmission Electron Microscopy (TEM) ........................... 159

8.2.3.3 Rheological Characterization ................................................. 160

8.2.3.4 Gel Permeation Chromatography (GPC) ............................... 160

8.2.3.5 Dynamic Mechanical Thermal Analysis (DMTA) ................. 160

8.2.3.6 Wide-Angle X-ray Diffraction (WAXD) ............................... 161

8.2.3.7 Tensile Tests ........................................................................... 161

8.2.3.8 Differential Scanning Calorimetry (DSC) ............................. 161

8.2.3.9 Thermogravimetric Analysis (TGA) ...................................... 161


8.4 Conclusions .................................................................................................. 179

8.5 References .................................................................................................... 180

Acknowledgements ....................................................................................................... 183

Curriculum Vitae .......................................................................................................... 185

List of Publications ....................................................................................................... 186

Summary

I

Summary

This dissertation deals with two groups of silica-based composite materials,

namely organic/silica composite particles and polymer nanocomposites containing

ultrasmall silicate particles. For the synthesis of composite particles a silica precursor

polymer - hyperbranched polyethoxysiloxane (PEOS) - is employed in combination

with emulsion technique. PEOS is a highly hydrophobic liquid. Upon hydrolysis it

becomes amphiphilic and interfacial active at the oil/water interface. In an oil-in-water

Pickering emulsion stabilized by long-alkyl substituted silica nanoparticles PEOS acts

as a glue to link the particles together at the interface to form microsized all-silica

colloidosomes enclosing the oil phase. It is further demonstrated that oil-in-water

miniemulsions are formed by dispersing oil/PEOS solutions in water without any

additional particles due to high hydrophobicity and hydrolysis-induced interfacial

activity of PEOS. In such a miniemulsion system methyl-functionalized silica

nanoparticles, which are less interfacial active than the long-alkyl substituted ones, can

catalyze the conversion of PEOS to a mechanically strong silica shell at the oil/water

interface. Thus, the oil phase is encapsulated in monodispersed silica nanocapsules with

almost 100% efficiency. This process depends strongly on pH of the aqueous phase,

which controls the interfacial activity of both PEOS and silica nanoparticles as well as

PEOS hydrolysis and condensation. The catalytic effect of the silica particles is the

result of a delicate interplay between their interfacial activity that allows immersing

catalytically active silanolate groups in the PEOS-containing oil phase and the repulsion

between charged surfaces of these particles and the resulting silica nanocapsules that

splits them apart. By combining PEOS with a hydrophobic monomer, a new type of

surfactant-free miniemulsion polymerization has been developed. Thus, monodisperse

polystyrene@SiO2 nanoparticles are obtained by emulsifying a PEOS/monomer

solution in water and subsequent heating to initiate polymerization. As the

polymerization proceeds, driven by osmotic pressure and incompatibility with

polystyrene PEOS macromolecules migrate continuously towards the oil/water

interface where sol-gel reaction takes place. As soon as the polymerization is completed,

PEOS is fully expelled from the polymer phase and is converted to silica on the

Summary

II

polystyrene surface. This method allows an easy control of silica shell thickness by

varying the PEOS concentration. The particle size, on the other hand, can be regulated

not only by the shearing force, but also by pH of the aqueous medium. In the case of a

less hydrophobic monomer like methyl methacrylate, a more hydrophobic methylsilyl-

substituted PEOS should be employed to obtain stable miniemulsions and subsequently

composite nanoparticles with a narrow particle size distribution. The resulting particles

exhibit a semi-interpenetrating network structure probably because of the compatibility

of the polymer matrix with methylsilyl-substituted PEOS.

Ultrasmall silicate particles used in this work are MQ resins having a core-shell

structure with a SiO2-core and an organic shell. The synthesis is based on the acidic co-

hydrolysis and condensation of monofunctional (M, e.g. 1,1,1,3,3,3-

hexamethyldisiloxane) and tetrafunctional (Q, e.g. tetraethoxysilane) organosilicon

compounds in an organic solvent, and the size of MQ resins increases with the decrease

of M to Q ratio. The functionalized MQ resins are prepared by using 1,1,3,3-

tetramethyldisiloxane as a comonomer and subsequent catalytic hydrosilylation with

vinyl-containing compounds, and the degree of modification is defined by the ratio

between 1,1,1,3,3,3-hexamethyldisiloxane and 1,1,3,3-tetramethyldisiloxane. As

confirmed by dynamic light scattering and atomic force microscopy the MQ resin

particles synthesized in this work have a size less than 10 nm and a narrow size

distribution. MQ resins substituted with 1-vinyl-2-pyrollidinone and N,N-

dimethylallylamine are blended with polyamide 6 via a melt extrusion process. It is

shown that 2-pyrrolidinone-functionalized MQ resins are homogenously distributed in

polyamide. By adding only a small amount of such particles (0.5 wt.-%), the viscosity

of the polymer melt is significantly reduced, meanwhile the mechanical properties of

the polymer remain almost unchanged. By comparing the interparticle half-gap and

radius of gyration of the polymer chain it can be concluded that the change of

entanglement density and release of topological constraint caused by MQ resin

nanoparticles account for the viscosity reduction.

Zusammenfassung

III

Zusammenfassung

Die vorliegende Arbeit behandelt zwei Gruppen von Silica-basierten

Kompositmaterialien: SiO2-organische Kompositpartikel und Polymer Nanokomposite

mit ultrakleinen Silikat-Partikeln. Zur Synthese der Kompositpartikel wird ein SiO2-

Precursorpolymer- hochverzweigtes Polyethoxysiloxan (PEOS)- in Verbindung mit

einem Emulsionsverfahren eingesetzt. PEOS ist eine hochgradig hydrophobe

Flssigkeit. Durch die Hydrolyse wird es amphiphil und grenzflchenaktiv an der l-

Wasser Grenzflche. In einer l-in-Wasser- Pickering-Emulsion, die durch mit langen

Alkylketten substituierte SiO2-Nanopartikeln stabilisiert ist, bindet PEOS die Partikel

an der Grenzflche unter Bildung eines SiO2-Kolloidosomes im Mikrobereich, das die

lphase verkapselt. Weiterhin wird gezeigt, dass durch Dispergieren von l-PEOS-

Lsungen in Wasser l-in-Wasser Emulsionen gebildet werden, wobei wegen der

Hydrophobie und der durch Hydrolyse induzierten Grenzflchenaktivitt keine

zustzlichen Partikel zur Stabilisierung bentigt werden. In einer solchen Miniemulsion

knnen methylfunktionalisierte SiO2-Nanopartikel, die weniger grenzflchenaktiv sind

als solche mit langen Alkylresten, die Umwandlung von PEOS zu einer mechanisch

stabilen SiO2-Schale an der l-Wasser Grenzflche katalysieren. Auf diese Weise

gelingt der Einschluss der lphase in monodispersen SiO2-Nanokapseln zu fast 100%.

Der Prozess hngt stark vom pH-Wert der Wasserphase ab, der die

Grenzflchenaktivitt von PEOS und der SiO2-Nanopartikel sowie die Hydrolyse des

PEOS und die Kondensation bestimmt. Der katalytische Effekt der SiO2-Partikel beruht

auf einem Zusammenspiel ihrer Grenzflchenaktivitt, die das Einbringen katalytisch

aktiver Silanolat-Gruppen in die PEOS/lphase ermglicht, sowie der Abstoung

zwischen den geladenen Oberflchen dieser Partikel und den daraus entstehenden SiO2-

Nanokapseln, die diese trennt. Durch Kombination von PEOS und einem hydrophoben

Monomer wurde eine neuartige Tensid-freie Miniemulsionspolymerisation entwickelt.

So erhlt man monodisperse Polystyrol@SiO2 Nanopartikel durch Emulgieren einer

PEOS/Monomer-Lsung in Wasser und anschlieendes Erhitzen zum Starten der

Polymerisation. Im Verlauf der Polymerisation wandern die PEOS-Makromolekle,

getrieben durch den osmotischen Druck und ihre Unvertrglichkeit mit dem Polystyrol,

stetig an die l-Wassergrenzflche, wo die Reaktion stattfindet. Sobald die

Zusammenfassung

IV

Polymerisation abgeschlossen ist, wird PEOS vollstndig aus der Polymerphase

verdrngt und an der Oberflche des Polystyrols zu SiO2 umgesetzt. Dieses Verfahren

erlaubt eine einfache Kontrolle der SiO2-Schalendicke durch Variation der PEOS

Konzentration. Die Partikelgre kann sowohl durch die Scherkrfte als auch ber den

pH-Wert des wssrigen Mediums eingestellt werden. Im Fall weniger hydrophober

Monomerer wie Methylmethacrylat sollte ein strker hydrophobes methylsilyl-

substituiertes PEOS eingesetzt werden, um eine stabile Miniemulsion und in der Folge

Kompositnanopartikel mit enger Grenverteilung zu erreichen. So erhlt man Partikel,

die vermutlich aufgrund der Kompatibilitt von Polymermatrix und dem methylsilyl-

substituierten PEOS ein halbinterpenetrierendes Netzwerk bilden.

Die im Rahmen dieser Arbeit hergestellten ultrakleinen Silikat-Partikel sind MQ-

Harze mit einem SiO2-Kern und einer organischen Schale. Die Synthese basiert auf der

saureren Co-Hydrolyse und Kondensation monofunktionaler (M, z.B. 1,1,1,3,3,3-

Hexamethyldisiloxan) und tetrafunktionaler (Q, z.B. Tetraethoxysilan)

siliziumorganischer Verbindungen in einem organischen Lsungsmittel. Die Gre der

MQ-Harze nimmt mit abnehmendem M/Q-Verhltnis zu. Zur Prparation wird 1,1,3,3-

Tettramethyldisiloxan als Co-Monomer eingesetzt, das nachfolgend einer katalytischen

Hydrosilylierung mit Vinylverbindungen unterworfen wird. Das Ausma der

Modifizierung ist durch Verhltnis von 1,1,1,3,3,3-Hexamethyldisiloxan zu 1,1,3,3-

Tetramethyldisiloxan bestimmt. Mittels dynamischer Lichtstreuung sowie AFM kann

gezeigt werden, dass der Durchmesser der Partikel bei enger Grenverteilung unter 10

nm liegt. MQ-Harze, die mit 1-Vinyl-2-Pyrrolidinon und N,N-Dimethylallylamin

substituiert sind, werden mit Polyamid-6 in einem Schmelzextrusionsprozess vermischt,

wodurch eine homogene Verteilung der mit 2-Pyrollidinon funktionalisierten MQ-

Harze im Polyamid erreicht wird. Bei Zugabe einer geringen Menge dieser Partikel (0.5

Gew.-%) wird die Viskositt der Schmelze deutlich herabgesetzt whrend die

mechanischen Eigenschaften des Polymers nahezu unverndert bleiben. Aus dem

Vergleich des interpartikulren Halbabstands mit dem Trgheitsradius des Polymers

schliet man, das die nderung der Verschlaufungsdichte und der Wegfall der

topologischen Beschrnkung verursacht durch das MQ-Harz fr die Verminderung der

Viskositt verantwortlich sind.

List of Abbreviations

V


Abbreviation Full Wording

PEOS polyethoxysiloxane

PS polystyrene

PMMA poly(methyl methacrylate)

PA6 polyamide 6

PVP polyvinylpyrrolidone

PEG poly(ethylene glycol)

PDMS polydimethylsiloxane

PP polypropylene

TEOS tetraethyl orthosilicate

CTAB cetyltrimethylammonium bromide

HMDS hexamethyldisiloxane

TMDS tetramethyldisiloxane

AIBA Azobis(isobutyl-amidine hydrochloride)

AIBN Azobisisobutyronitrile

PCMs phase change materials

DLS dynamic light scattering

FE-SEM field-emission scanning electron microscopy

TEM transmission electron microscopy

EDX-STEM energy-dispersive X-ray spectroscopy - scanning

transmission electron microscopy

AFM atomic force microscopy

TGA thermogravimetric analysis


VI

DMTA dynamic mechanical thermal analysis

WAXD wide-angle X-ray diffraction

DSC differential scanning calorimetry

FT-IR fourier-transform infrared

GPC gel permeation chromatography

IFT interfacial tension

NMR nuclear magnetic resonance

BET Brunauer-Emmett-Teller

BJH Barrett-Joyner-Halenda

NL-DFT nonlocal-density functional theory

A@B A is surrounded by B

LBL layer-by-layer

HLB hydrophile-lipophile balance

IEP isoelectric point

o/w oil-in-water

w/o water-in-oil

w/o/w water-in-oil-in-water

o/w/o oil-in-water-in-oil

Rg radius of gyration

volume fraction

m maximum random packing volume fraction

G storage modulus

G loss modulus

Tg glass transition temperature

Chapter 1: Introduction and Outline of the Thesis

1

Chapter 1

Introduction and Outline of the Thesis


2

1.1 Introduction

Composite technology combining multiple materials differing significantly in

properties to produce a composite material with characteristics different from the

constituents has progressed rapidly since the last decades.1,2 By proper design the

properties of the composites can often go far beyond that achievable with single

materials.3 The combination of inorganic and organic materials is a well-established

and widely used approach with a long history. It is still of significant academic interest

due to almost inexhaustible combinatory possibilities. The inorganic constituents

contribute not only to enhanced physical and chemical properties such as stiffness,

mechanical strength, chemical resistance, thermal stability, optical properties, and

viscoelastic behavior but also to the cost reduction. The organic parts, on the other hand,

render the composite materials flexible, ductile, processable, electrically and thermally

insulated, and possibly functional in specific application areas. However this general

summary does not apply to all events in hybrid materials, because the overall properties

are also strongly determined by the way both constituents combine and by the formed

unique composite structure.4,5

Polymer composites where inorganic fillers are dispersed in a continuous organic

polymer phase are well-established in both theory and practice.6 Especially when one

dimension of the filler falls into the nanoscale (e.g. nanoparticles, nanotubes, layered

silicates, etc.) critical phenomena and unique distinct properties can be anticipated.7 It

is generally accepted that the nanoscale inorganic building blocks may create a

substantial interfacial area among the organic matrix, leading to unique interactions

with matrix and thereby properties that are different from bulk phase even at very low

loadings and are of high benefit for industry. For instance, mechanical strength, thermal

stability, electrical conductivity, and magnetic permeability of the polymers can be

substantially altered when inorganic nanofillers are uniformly dispersed in the polymer

matrices.8-12 Although a lot of effort has been devoted to the development of polymer

nanocomposites, insight in structure-property relations is still largely absent. In most

cases, particles with tens of nanometers are used in preparation of polymer

nanocomposites. However, the situation becomes much more intriguing when the size


3

of the particles is further diminished below 10 nm, where the interparticle half-gap is

comparable or even smaller than the radius of gyration (Rg) of a polymer coil. This

produces chain confinement and distortion that may promise unusual properties. Indeed,

Mackay et al. showed that the melt viscosity of polymers can be significantly reduced

by introducing ultrasmall nanoparticles with radius of 3-5 nm.13 It is in contradiction to

the prediction of Einstein who claimed that particle addition into fluids including

polymeric ones could lead to an increase in apparent viscosity.14 This phenomenon has

been explained by an increase of the free volume upon adding nanoparticles, as

confirmed by a decrease in the glass transition temperature. It should be noted that the

viscosity reduction was observed only when the interparticle half-gap was smaller than

Rg of the polymer coil.15 There are very few papers in the literature dealing with

nanocomposites containing ultrasmall particles, largely due to the easy agglomeration

and lack of scalable synthesis of such particles.

Within the inorganic-organic hybrid material family, considerable effort is devoted

to the development of colloidal inorganic-organic nanocomposites due to their

promising applications in catalysis, chromatography, coating, biotechnology, etc.4,16,17

Thanks to the huge progress in synthetic techniques in both organic and inorganic

chemistry (e.g. layer-by-layer deposition, in-situ polymerization, in-situ sol-gel

reaction, synthesis of functional inorganic particles, etc.) a wide variety of composite

particles were developed during the last decades.18 Since the properties of such kind of

particles depend not only on the chemical nature of both constitutes, but also on the

designed micro-/nanostructure, the possibility of tailoring the subsequent performance

properties, in principle, is not exhaustive. Composite particles can be designed with

various morphologies, such as raspberry-like, currant bun-like, core-shell, snowman-

like, dumbbell-like, and daisy-shaped structures.19 Among all these configurations,

core-shell particles are the simplest examples that have been extensively studied and

widely applied.20 As typical core-shell particles, organic materials are surrounded by

inorganic materials or vice versa. Coating inorganic particles with organic shells can

protect the inorganic core from chemical corrosion and introduce functional groups to

improve the affinity towards organic environment. On the contrary, if the organic phase

is surrounded by inorganic materials, colloidal stability in aqueous phase and further


4

surface functionalization can be promised. The preparation strategy for core-shell

composite particles and design of specific structures are of particular interest in

encapsulation technologies, drug delivery and release, catalyst, and so forth.20

Silica as one of the most abundant material on the earth is the most frequently used

inorganic constituent for the preparation of inorganic-organic composite materials.

Being chemically inert, mechanically stable, low toxic, biocompatible, and optically

transparent, silica-based materials are of high demand in various fields of material

science.21 Importantly, silica nanomaterials can be prepared using sol-gel technology

that combines the control over composition and microstructure at the molecular level

with the ability to shape the material to particles of a wide size range, fibers, or thin

films under mild and low-energy conditions.22,23

1.2 Objectives of this thesis

One of the objectives of this thesis is to modify polymer matrices with ultrasmall

silica-based nanoparticels with a size less than 10 nm. The main focus will be laid on

the influence of these particles on the processability of the polymer matrix. For this

purpose, ultrasmall MQ resin particles with a SiO2 (Q) core and an organic shell

consisting of monofunctional components (M) in the form of R3SiO1/2 will be prepared,

and their surface will be substituted with groups that are compatible with the polymer

matrix. The influence of these ultrasmall MQ particles on the rheological behavior of

resulting polymer nanocomposites will be systematically investigated to establish the

structure-property relations by comparing the interparticle half-gap and radius of

gyration of polymer coils. Another goal is to develop a novel and facile approach for

the synthesis of different kinds of silica-based composite particles with the focus on

core-shell structures using a silica precursor polymer - hyperbranched

polyethoxysiloxane (PEOS) - in combination with emulsion technique. PEOS is a

highly hydrophobic liquid synthesized via condensation of tetraethoxysilane with acetic

anhydride.24 It was successfully used as an interfacial glue in water-in-oil Pickering


5

emulsions to link the stabilizing silica nanoparticles at the interface to obtain all-silica

colloidosomes enclosing an aqueous phase.25,26

1.3 Outline of the thesis

This thesis is structured into 8 chapters.

Chapter 2 provides a literature survey on two kinds of organic-inorganic hybrid

materials, namely nanoparticle-filled bulk composite materials and polymer/silica

hybrid particles. For the bulk nanocomposites the focus is laid on the ultrasmall filler

particles with a size less than 10 nm. In case of hybrid particles organic@SiO2 core-

shell structures are mainly discussed. In Chapter 3, microsized all-silica colloidosomes

enclosing a hydrophobic liquid are prepared by gluing the stabilizing long-alkyl

functionalized silica nanoparticles with PEOS at the interface in an oil-in-water

Pickering emulsion. It is shown that different hydrophobic liquids can be

microencapsulated by this means and the silica shell acts as a good barrier to retard the

release of encapsulated materials. In Chapter 4 we report a new way to prepare

monodisperse silica nanocapsules. PEOS is shown to stabilize solely oil-in-water

miniemulsions. In this system, methylated silica nanoparticles that are less interfacial

active may catalyze the conversion of PEOS into a mechanically stable silica shell at

the oil/water interface instead of being building blocks of the capsule shell. Moreover,

different silica capsular structures ranging from particle-free microcapsules,

colloidosomes and nanocapsules are obtained depending on the pH of the aqueous

phase. In Chapter 5, a new type of surfactant-free miniemulsion polymerization has

been developed by using PEOS as the only stabilizer for a hydrophobic monomer. Thus,

monodisperse polystyrene@SiO2 core-shell nanoparticles are obtained by emulsifying

a PEOS/monomer solution in water and subsequent heating to initiate polymerization.

This method allows an easy control of silica shell thickness by varying the PEOS

concentration. The particle size, on the other hand, can be regulated not only by the

shearing force, but also by pH of the aqueous medium. Chapter 6 extends the

miniemulsion polymerization method to a less hydrophobic monomer, methyl


6

methacrylate. In this case, a more hydrophobic methylsilyl-substituted PEOS should be

employed to obtain stable miniemulsions and subsequently composite nanoparticles

with narrow particle size distribution. The resulting particles exhibit a semi-

interpenetrating network structure probably because of the compatibility of the polymer

matrix with methylsilyl-substituted PEOS. Chapter 7 describes the synthesis and

characterization of ultrasmall MQ resin particles with different molecular weight (size).

The synthesis is based on acidic co-hydrolysis and condensation of monofunctional (M)

and tetrafunctional (Q) organosilicon components. Further functionalization is carried

out by hydrosilylation of SiH-functionalized MQ resin particles with vinyl-containing

compounds. In Chapter 8, ultrasmall MQ resin particles are blended with polyamide 6

in a melt extrusion process and the resulting nanocomposites are characterized by

different physical techniques. It is shown that 2-pyrrolidinone-functionalized MQ resins

are homogenously distributed in polyamide. By adding only a small amount of such

particles (0.5 wt.-%), the viscosity of the polymer melt is significantly reduced,

meanwhile the mechanical properties of the polymer remain almost unchanged. By

comparing the interparticle half-gap and radius of gyration of the polymer it can be

concluded that the change of entanglement density and release of topological constraint

caused by MQ resin nanoparticles are responsible for the viscosity reduction.

1.4 References

[1] Chung, D. D. L. Composite Materials: Functional Materials for Modern

Technologies; Springer: New York, 2003.

[2] Chung, D. D. L. Composite Materials: Science and Applications, 2nd ed.; Springer:

New York, 2010.


7

[3] Gay, D. Composite Materials: Design and Applications, 3rd ed.; CRC Press: Boca

Raton, 2015.

[4] Bourgeat-Lami, E.; Lansalot, M. In Hybrid Latex Particles: Preparation With

(Mini)emulsion Polymerization; Springer-Verlag Berlin: Berlin, 2010.

[5] Zou, H.; Wu, S. S.; Shen, J. Chem. Rev. 2008, 108, 3893-3957.

[6] Friedrich, K.; Fakirov, S.; Zhang, Z. Polymer composites: from nano-to-macro-scale;

Springer: New York, 2005.

[7] Koo, J. H. Polymer nanocomposites: processing, characterization, and applications;

McGraw-Hill: London, 2006.

[8] Balazs, A. C.; Emrick, T.; Russell, T. P. Science 2006, 314, 1107-1110.

[9] Caseri, W. In Hybrid Materials: Synthesis, Characterization, and Applications;

Wiley-VCH: Weinheim: Germany, 2004.

[10] Caseri, W. R. Mater. Sci. Technol. 2006, 22, 807-817.

[11] Ajayan, P. M.; Schadler, L. S.; Braun, P. In Nanocomposite Science and Technology;

Wiley-VCH: Weinheim, 2003.

[12] Schaefer, D. W.; Justice, R. S. Macromolecules 2007, 40, 8501-8517.

[13] Mackay, M. E.; Dao, T. T.; Tuteja, A.; Ho, D. L.; Van Horn, B.; Kim, H. C.; Hawker,

C. J. Nat. Mater. 2003, 2, 762-766.

[14] Albert, E. Ann. Phys. 1906, 19, 371-381.

[15] Tuteja, A.; Mackay, M. E.; Hawker, C. J.; Van Horn, B. Macromolecules 2005, 38,

8000-8011.

[16] Chauhan, B. P. Hybrid nanomaterials: synthesis, characterization, and

applications; Wiley: Oxford, 2011.

[17] Kalia, S.; Haldorai, Y. Organic-inorganic hybrid nanomaterials; Springer: London,

2015.

[18] Guozhong, C.; Ying, W. In Nanostructures and Nanomaterials: Synthesis,

Properties, and Applications; Imperial College Press: London, 2004.


8

[19] Percy, M. J.; Amalvy, J. I.; Barthet, C.; Armes, S. P.; Greaves, S. J.; Watts, J. F.;

Wiese, H. J. Mater. Chem. 2002, 12, 697-702.

[20] Chaudhuri, R. G.; Paria, S. Chem. Rev. 2012, 112, 2373-2433.

[21] Pagliaro, M. Silica-based materials for advanced chemical applications; RSC Pub.:

Cambridge, 2009.

[22] Brinker, C. J.; Scherer, G. W. Sol-gel science: the physics and chemistry of sol-gel

processing; Academic Press: London, 1990.

[23] Wright, J. D.; Sommerdijk, N. A. J. M. Sol-gel materials: chemistry and

applications; Gordon and Breach Science Publishers: Australia, 2001.

[24] Zhu, X. M.; Jaumann, M.; Peter, K.; Mller, M.; Melian, C.; Adams-Buda, A.;

Demco, D. E.; Blmich, B. Macromolecules 2006, 39, 1701-1708.

[25] Wang, H. L.; Zhu, X. M.; Tsarkova, L.; Pich, A.; Mller, M. Acs Nano 2011, 5,

3937-3942.

[26] Zhang, C.; Hu, C. Y.; Zhao, Y. L.; Mller, M.; Yan, K. L.; Zhu, X. M. Langmuir

2013, 29, 15457-15462.

Chapter 2: Literature Review

9

Chapter 2

Literature Review


10

2.1 Polymer/inorganic nanocomposites

The incorporation of inorganic nanoparticles into polymer matrices to form

nanocomposites is a well-established but still fast growing approach for the

improvement of material properties.1,2 The promise of nanocomposites lies in their

multifunctionality and the possibility of obtaining unique combinations of properties

that are unachievable with traditional materials. In comparison to composites with

micron-sized fillers, nanocomposites exhibit significantly enhanced performance

properties including mechanical strength, stiffness, barrier properties, heat resistance,

flame retardancy, wear resistance and dielectric properties at even low filler volume

fraction.3 The performance properties of the polymer nanocomposites are achieved on

the basis of three major characteristics: a nano-scopically confined polymer matrix, a

nanoscale inorganic constituent, and nanoscale arrangement of these constituents.4 The

list of inorganic nano-fillers includes carbon nanotubes or nanofibers, layered silicates,

layered chalcogenides, graphitic layer, nanoparticles of metals, metal oxides, quantum

dots, and so forth.4,5

2.1.1 Polymer/silicate nanocomposites

Among all these fillers, silica is considered as the most important and widely used

inorganic materials, and polymer/silica nanocomposites have attracted more attention

in both industrial and academic communities, which can be evidenced from the fast

increasing of practical applications and scientific publications.5 Nanoscale colloidal

silica particles and layered silicates are the most widely used reinforcing fillers.

Hectorite (Mx(Mg6-xLix)Si8O20(OH,F)4), saponite (MxMg6(Si8-xAlx)Si8O20(OH)4), and

montmorillonite (Mx(Al4-xMgx)Si8O20(OH)4), where M is an exchangeable monovalent

cation and x is the degree of isomorphous substitution, are among the most commonly

employed smectite-type layered silicates for the preparation of nanocomposites.6

Nanoscale colloidal silica particles of different sizes is commercially available in both

aqueous and organic solvents. There are three principal procedures commonly

employed to disperse nanoparticles as homogeneous as possible in a polymeric matrix:


11

(a) blending in solution; (b) blending in melt; (c) in-situ processing. Notably, the in-situ

processing can be further divided into two routes: in-situ polymerization in the presence

of silica and in-situ synthesis of silica in polymer matrix.

Direct blending the silica and polymer solution provides a convenient route to form

polymer/silica hybrid materials after solvent evaporation or fast precipitation of

polymer nanocomposites.7,8 The melt blending method is environmentally benign due

to the absence of organic solvents. Furthermore, it is compatible with current industrial

processes, such as extrusion and injection moulding. Melt blending for nanocomposites

has been developed successfully when the silica particle surface is substituted with

organic groups.9,10 In the in-situ polymerization method, the silica fillers are first

dispersed in the monomer(s) and then the polymerization is triggered.11 A

functionalized silica surface that promises a uniform distribution in organic monomer

is also essential. In a classical in-situ sol-gel process, in-situ formation of silica is based

on co-hydrolysis and then polycondensation of silica precursor molecules under acid or

basic conditions in the solution of polymers.12,13 A reactive extrusion process where the

sol-gel process is carried out in a polymer melt has been developed by DWI, where a

non-volatile silica precursor polymer, hyperbranched polyethoxysiloxane, has been

employed.14,15

2.1.2 Surface modification of silica

Homogenous dispersion of nanoscale silica can lead to an extremely large

interfacial area between polymer matrix and fillers and may promise a dramatic

improvement of macroscopic performance of final materials even at a low volume

fraction.4,5 However, due to the intrinsic chemical difference between organic polymers

and inorganic silica that often cause phase separation, enhancing the interfacial

interaction between two phases of nanocomposites is the most decisive factor that

significantly affects the performance properties of the materials.16 Surface modification

of silica is the most frequently used method to realize a strong interaction and a

subsequent uniform dispersion state. Generally, this modification can be performed in


12

either a chemical or a physical way.17 Chemical methods mainly involve modification

with either modifiers (e.g. silane coupling agents) or with grafting polymers through

covalent bonding of end-functionalized polymers with the surface (grafting to

method), or in-situ polymerization of monomer initiated from silica surface (grafting

from approach). Modification by physical interaction is much weaker and is realized

by absorbing surfactant molecules (e.g. CTAB, oleic acid) or macromolecules (e.g.

chitosan) on the silica surface through electrostatic interaction18 or hydrogen bonds.19

2.1.3 Polymer nanocomposites containing ultrasmall particles

In most studies on polymer nanocomposites, particles of tens of nanometers are

used. However, the situation becomes much more intriguing when the size of the

particles is further diminished below 10 nm. In this case, molecular dimensions are

approached and the interparticle half-gap (h) is comparable or even smaller than the

radius of gyration (Rg) of a polymer coil (Figure 1 and Table 1). This produces chain

confinement and distortion that can result in depletion driven phase segregation, or in

the case segregation can be avoided, may promise unusual properties.20 From Table 1,

one can expect quite a small h when r is small enough even at a very low .

Figure 1. A typical polymer composite structure, where freely mobile bulk polymer

between nanoparticles and immobilized polymer on the nanoparticle surface are

considered as two main polymer phase.21


13

Table 1. Interparticle half-gap (h) vs particle radius (r) and volume fraction (). h can

be estimated by the simple relation: = [ ]1 3 1 , where = 0.638

(maximum random packing fraction).22

, %

r, nm

0.5 1 2 5 10 20 40

h, nm

3 12.1 9.0 6.5 4.0 2.6 1.4 0.5

5 20.2 15.0 10.9 6.9 4.3 2.4 0.8

10 40.3 30.0 21.7 13.4 8.5 4.7 1.7

20 80.7 59.9 43.4 26.7 17.1 9.4 3.4

Indeed, Mackay et al. reported on a continuous drop in the viscosity of linear

polystyrene (PS) by blending with intramolecular cross-linked PS nanoparticles with a

radius of 3-5 nm over a broad mass fraction range.22 The viscosity decrease is explained

by the increase of excluded free volume produced around nanoparticles according to

fact that an accompanying drop in glass transition temperature of PS is observed. It was

shown in later publications that the viscosity decrease can only be observed when the

linear PS molecule is entangled and confined by nanoparticles. They attributed the

viscosity decrease to the constraint release caused by addition of nanoparticles,

although they admitted it is a very complicated phenomenon which also involves the

introduction of free volume by the nanoparticles.8,23 This observation is contrary to the

expression proposed by Einstein predicting the increase of the viscosity is solely a

function of the particle volume fraction () and the viscosity of dispersion liquids.24 In

another report, ultrasmall silicate clusters with a radius of 0.35 nm were blended with

linear polydimethylsiloxane (PDMS) and a decrease in viscosity was observed whereas

no viscosity was found by addition of larger silicate particles, again pointing to the

unusual nanoscale effect that nanoparticles may provide.25 They state that the small

clusters appear to solvate the PDMS since the particle size approaches the length

scale of a monomer unit, leading to a decrease in viscosity. Jain et al. reported on a

dramatic viscosity decrease of polypropylene (PP) filled with in-situ prepared silica


14

nanoparticles with a radius of 7.5-15 nm in a narrow concentration window (up to

approx. 0.5 wt%).26 A reduction in entanglement density caused by adsorption of high

molar mass chains on the surface of silica nanoparticles was eventually thought to

reduce the viscosity. Although the manner of viscosity decrease and mechanism to

elucidate viscosity decrease are not completely consistent in these cases, at least there

is a certain nanoscale effect which cannot be observed with larger nanoparticles.

Very recently, self-healing was demonstrated in a multi-layered nanocomposite

polymer structure, i.e. the nanoparticles dispersed in a polymer matrix migrated to a

crack generated at the interface between the polymer and a glassy layer.27 According to

results of computer simulations, nanoparticles in a polymer fluid can segregate to the

surfaces and into the cracks due to a polymer-induced depletion attraction between

the particles and the surface.20 In this case, only particles comparable in size to the Rg

of were driven from the matrix to the surface in the crack area. Importantly, a

homogeneous dispersion of the nanoparticles in the polymer matrix is a prerequisite for

achieving the above-mentioned properties.

2.1.4 MQ resins ultrasmall core-shell silicate particles

The synthesis of ultrasmall silica particles that are compatible with organic

polymers is a big challenge. On the other hand, MQ resins that are well-known in the

organosilicon industry are three-dimensional silicone polymers composed of a

monofunctional component (M) in the form of R3SiO1/2 and a tetrafunctional unit (Q)

in the form of SiO2, as shown in Table 2. They are widely used in pressure-sensitive

adhesives, silicone rubbers, coatings and so on.28-34 A MQ resin particle can be

considered as an ultrasmall particle with a SiO2 core and an organic alkylsilyl shell. The

particle size ranges from 0.5 to 3 nm and can be simply adjusted by varying the Q/M

ratio. The most frequently used method for MQ synthesis is on the basis of hydrolysis

and condensation of Me3SiCl in the presence of aqueous sodium silicate. Alternatively,

they can be prepared by co-hydrolysis and condensation of tetraethoxysilane (TEOS)

and Me3SiCl or hexamethyldisiloxane with addition of water in an organic solvent.


15

However, in the first case, the structure control of MQ resins is not easy and gels are

often produced due to a poor control of the rate of silicic acid polymerization.

Furthermore, this method always yields a broad molecular weight distribution of

resulting products.35 In the second method, the reaction takes place under homogeneous

conditions, thus a much better structure control and narrow molecular weight

distribution can be achieved. However, a part of alkoxy groups often remain unreacted

in the products due to incomplete hydrolysis when Me3SiCl is used. This situation

cannot be significantly improved even when more readily hydrolysable alkoxy groups,

for example, tetramethoxysilane, are used as starting material. However, when a

disiloxane (e.g. hexamethyldisiloxane) is used to replace Me3SiCl, MQ particles grow

in a controlled manner and all the alkoxy groups are completely consumed.35

Importantly, by this means 1,1,3,3-tetramethyldisiloxane can be used to synthesize MQ

resins with terminal SiH groups, which can be further modified via a catalytic

hydrosilylation reaction to introduce different groups for improving their compatibility

with polymer matrices.

Table 2. Nomenclature of MDTQ units in organosilicon materials.

Basic monomers Functionality Structure in polymer R/Si Symbol

SiX4 4 Si OOO

O

SiO2 0 Q

R-SiX3 3 Si OCO

O

RSiO3/2 1 T

R2-SiX2 2 Si OCO

C

R2SiO 2 D

R3-SiX 1 Si OCC

C

R3SiO1/2 3 M


16

2.1.5 Polyamide 6

It is a target of the present thesis to modify polyamide 6 (PA6) by blending with

ultrasmall particles. PAs are one class of the most widely used commodity polymers.

Within the PA family, PA6 is the most widely used one. PAs possess good material

properties, in particular a high impact resistance, stiffness, abrasion resistance,

elongation at break, modulus of elasticity, resiliency, dimensional stability on heating,

barrier properties and puncture resistance. In order to further improve their mechanical

properties, PAs are often mixed with inorganic fillers such as glass fibers, impact

modifiers or nanoparticulate clay minerals. Silica nanoparticles are the most widely

used filler, and PA/silica nanocomposites have been the subject of extensive research

during the past decades. Although plenty of work has been devoted to the development

of PA-based nanocomposites, very limited work on the precessability of PA6 by

addition of small nanoparticles has been reported,36 though this is a very important topic.

Thermal degradation of PA chains often occurs during melt processing under

particularly high temperature (above 250 oC), thus resulting in a loss in performance

properties. It is notable that the research group of Voit has performed a tremendous

amount of work on modifying the melt viscosity of PAs by blending with

hyperbranched polymers. The reduction of the polymer melt viscosity is explained by

the plasticization effect of the hyperbranched polymers.37-39

2.2 Silica-based hybrid core-shell particles

2.2.1 Definition and classification of core-shell particles

Core-shell particles are probably the most common colloidal composites. As the

name implies, core-shell composite particles, which are composed of two or more

materials, can be generally defined as particles comprising a core (inner material) and

a shell (outer layer material).40 To date, different geometries of core-shell particles have

been synthesized and summarized in several reviews, as schematically shown in Figure


17

2.40 Concentric core-shell particles by far are most commonly synthesized and applied,

where a spherical core is fully surrounded or coated by a continuous shell of a different

material (Figure 2a). Recently, core-shell particles with different shapes (e.g. hexagon-

like, shuttle-like structure) are also of high research interest due to their novel

performance properties. Generally, they are synthesized by templating against a

nonspherical core (Figure 2b). When one single core is replaced with a cluster of several

small particles, further deposition of shell materials leads to a typical core-shell particle

with multiple cores (Figure 2c). One spherical core can also be closely encapsulated via

alternative coating of different materials, thereby forming a concentric multilayer shell

around the core (matryoshka-like particles, Figure 2d). After the nearest layer around

core particles is removed either by calcination or chemical etching, hollow particles

with movable core particles are synthesized (Figure 2e). In fact, plenty of core-shell

particles have been reported in the literatures and some are already available in

industrial applications. As a result, the classification of all core-shell particles in one

fixed criterion, such as industrial application, unique property, or assembly method,

seems difficult but essential on a certain occasion. Classification by the combination of

constituent materials (inorganic or organic), so far, is broadly adopted. Thus the core-

shell particles can be mainly divided into four groups: (i) inorganic-inorganic; (ii)

organic-organic; (iii) inorganic-organic; (iv) organic-inorganic.

Figure 2. Different shaped core-shell particles: (a) spherical core-shell particles, (b)

hexagonal core-shell particles, (c) multiple cores coated by single shell materials, (d)

matryoshka material, and (e) movable core within hollow shell material.


18

2.2.2 Organic-silica core-shell particles

For the sake of topic concerned in this thesis, organic-inorganic core-shell particles,

in particular organic-silica core-shell particles will be discussed in detail. In this class

of core-shell particles, the core can be hard polymers, organic liquids, and also can be

organic phase change materials. A large variety of homopolymers, such as PS,41-46

poly(ethylene glycol),47 polyurethane,48 poly(vinyl benzyl chloride),49 poly (vinyl

pyrrolidone),50 dextrose,51 and some of copolymers like poly(acrylonitrile-butadiene-

styrene),52 poly(styrene-acrylic acid),53 poly(styrene-methyl methacrylate),54 poly(N-

isopropylacrylamide-co-acrylic acid),55 have already been used as cores to synthesize

core-shell particles. Organic liquids, to some extent can be regarded as soft templates

(or soft cores) for coating shell materials and then be encapsulated to form core-shell

particles.56-63 Phase change materials (PCMs) are a special group of chemicals (e.g. wax,

fatty acids, and fatty acid esters) that are liquid droplets for shell coating above phase

transition temperature and then keep the shape after cooling.64-69 The inorganic shell

can also be made of different materials, such as metals,42,49,70 metal oxides,41,47,51,54,71

metal chalcogenides,53 and silica.41,72-86 The inorganic shell, especially a metal oxide

coating on an organic core can greatly increase overall mechanical strength of particles,

resistance to oxidation, thermal and colloidal stability, and abrasion resistance.87-91

Among these inorganic materials, silica is a promising shell material owing to some

basic advantages over other inorganic (metals or metal oxide) or organic coatings.

Coating an organic core with a silica layer to form a core-shell particle is an important

technique to impart hydrophilicity, biocompatibility and modifiability that are intrinsic

properties of silica to the organic material.92-94 Furthermore, it also reduces the bulk

conductivity and increases the colloidal stability in aqueous phase. Due to its chemical

inertness, silica shell can protect the core from environmental chemical reactions that

may induce deterioration of core materials. The unique biocompatibility and optical

transparency make silica be widely applied in biotechnology and optical coatings.

Moreover, the silica surface is easily functionalized with other molecules, implying

enormous potential in various application areas.95,96


19

2.2.3 Design and synthetic strategy

The formation of organic-silica core-shell particles has been described in a large

number of patents and scientific publications, however this list is not exhaustive. In

general, the core-shell particles can be synthesized by either a two-step process

including first core preparation and subsequent shell synthesis or a one-step reaction

where the core and shell are formed simultaneously. The two-step synthesis technique

can be summarized into two categories regarding the availability of core particles: (i)

the core particles are synthesized independently and then introduced to a new system

with appropriate surface treatment for further silica coating; (ii) the core particles are

prepared in-situ optionally with help of surface-active materials, and then followed by

a shell material coating. The one-step process means that the core is formed in the

presence of preformed shell and vice versa. For example, the core monomer is

polymerized in the presence of silica nanoparticles, namely Pickering polymerization.

The nanoparticles are thus immobilized on the surface of the polymer core, forming a

unique silica shell with permeation properties. Conversely, the continuous silica shell

can be formed around a liquid core by a one-step sol-gel reaction of silica precursor,

which is a typical technique in sol-gel microencapsulation. Upon different synthetic

routes, the morphology of the silica shell can either be a continuous film that closely

surrounds the organic core, or be discrete silica particles that are immobilized on the

surface of the inner core (so-called colloidosome97-99), or possess a composite structure

that contains both silica film and silica nanoparticle.100-102 Correspondingly, the organic

core encapsulated can be hard polymer particles, liquid droplets, or the combination of

both.

2.2.3.1 Polymer core functionalization and silica shell coating

Thanks to the huge progress in polymerization techniques, polymer particles,

especially PS particles are commercially available and widely used as core templates

for synthesis of core-shell materials.76 However, the intrinsically hydrophobic property

of PS makes the dispersion in aqueous phase and surface coating by silica difficult due


20

to a lack of effective interaction. So functionalization of polymer surface plays a vital

role in determining the subsequent silica coating and final overall morphology of core-

shell particles. Normally, the surface functionalization of polymer core can be divided

into post-treatment and prior-treatment approaches. The post-treatment method is based

on introduction of functional groups or active molecules onto preformed polymer

surface via chemical or physical routes. These surface-active groups or molecules will

structurally direct the further silica coating, forming dense, porous or highly ordered

shell structure. Han et al. introduced sulfonate groups onto the surface of PS by means

of sulfonating reaction (Figure 3).83 Kim et al. developed a plasma technique to

hydroxylate the surface of PS surface in order to increase the compatibility between

polymer core and silica shell (Figure 3).80 In both methods, it is required to transfer

these functionalized polymer particles into a mixture of alcohol and ammonia solution

for subsequent shell formation on polymer surface according to the well-established

Stber method.103

Figure 3. Schematic illustration of polymer surface treatment via sulfonating reaction

and plasma and further silica coating to prepare a core-shell particle.

The prior-treatment method, which incorporates functional groups by co-

polymerization of styrene with other functional monomers, from synthetic view of point,

is more general and flexible due to the large range of available functional groups. For

example, Bourgeat-Lami et al. and Wu et al. introduced silanol groups to the PS surface

by co-polymerization of methacryloxypropyltrimethoxysilane with styrene, allowing


21

further silica precipitation by condensation of the surface silanol groups with TEOS

(Figure 4).45,104-106 4-Vinylpyridine is another functional monomer used by Zou et al. to

prepare PS latex particles bearing basic groups for further silica coating (Figure 4).107

In most cases surfactant molecules, like polyvinylpyrrolidone (PVP),77,81,108-110

cetyltrimethylammonium bromide (CTAB),72,86 or poly-L-lysine46 are essential in

stabilizing emulsion droplets during the polymerization step and directing silica coating

on polymer core in a controllable, homogeneous way.

Figure 4. Schematic representation of the copolymerization step and sol-gel reaction

of TEOS involved in preparation of polymer-silica core-shell particles.

2.2.3.2 Layer-by-layer (LBL) process

In addition to chemical bonds, noncovalent interactions between polymer core and

silica shell can also be utilized to prepare core-shell type particles. Layer-by-layer

method for the preparation of polymer@SiO2 core-shell particles is based on the

principle of alternate depositing of oppositely charged polyelectrolytes and silica onto

a surface of polymer particle through electrostatic self-assembly. It is worthwhile to

note that the adsorption of particles onto a solid substrate in LBL manner was firstly

proposed by Iler in the middle 1960s.111 Then, Decher et al. put forward this LBL

method by combination of polycations and polyanions in the early 1990s.112,113 In both


22

cases, macroscopically flat, charged surfaces were employed as substrates. Caruso et al.

have made a huge progress in applying LBL method to coat colloidal particles, thus

allowing to form composite core-shell particles.41,71,92,114,115 Negatively charged

polystyrene latex with a diameter of 640 nm were used as templates for sequential

electrostatic deposition of a linear cationic polymer poly(diallyldimethylammonium

chloride) (PDADMAC) and 25 nm silica nanoparticles with negative charge (Figure

5).71,116 The thickness of the composite multilayers could be adjusted by controlling the

number of deposition cycles. After removal of PS core particles via calcination or

solvent dissolution, submicron-sized hollow particles with wall thickness ranging from

tens to hundreds of nanometers were successfully produced. The LBL process also

makes the alternating deposition of oppositely charged polyelectrolytes onto core

particles through electrostatic interaction possible, thus forming a composite organic

shell around the core materials.92

Figure 5. Illustration of procedures for preparing hybrid core-shell particles and hollow

particles by LBL method.71

Besides, The LBL self-assembly technique is also an effective way to tune the

surface property of core particles by adsorbing polymers. Chiu et al. showed the ability

to grow silica directly on negatively charged surface of PS particles or latex

nanocapsules (Figure 6).117 The surface is first functionalized with alternately deposited,

oppositely charged polyelectrolytes, namely cationic PDADMAC and anionic

poly(styrenesulfonate). After the surface is finally capped with PDADMAC, the sol-gel


23

reaction of TEOS takes place to form a silica layer. Upon increasing cycling times, the

thickness of the shell, which has a sandwiched architecture, can be well controlled and

the shell exhibits a low permeability and high mechanical strength after the core is

removed.

Figure 6. Schematic illustration of strategy for coating polyelectrolyte and silica onto

PS particles.

2.2.3.3 Pickering (mini)emulsion polymerization

Instead of organic surfactant molecules, solid particles or 2D layers have also been

reported to stabilize emulsion droplets, which are often referred to Pickering

emulsions.118,119 It is generally accepted that the stability of Pickering emulsions is due

to the coherent particle layer around the droplets that acts as a steric barrier against

coalescence.120,121 It has also been reported that repulsive colloidal particles, which give

dilute planar monolayers on the droplet surface, can serve likewise as efficient emulsion

stabilizers, and the stabilization is explained by particle bridges.122,123 At the very

beginning of this century, the emulsion polymerization in the presence of solid particles

has been investigated by many research groups to prepare core-shell particles.100,124-134

In such processes, solids are thought to act as stabilizer for emulsion droplets or

polymer nuclei during polymerization and as building blocks of the resulting shell.

Notably, the particulate shell composed of closely packed particles is quite different

from the continuous and homogeneous shell obtained from sol-gel reaction of TEOS.

Bon et al. proposed a concept of Pickering miniemulsion polymerization where

nanoparticles or nanoclays were used to stabilize the initial emulsion droplets of

monomer. After polymerization these particles became irreversibly attached onto the

surface, thus forming composite particles with a core-shell structure (Figure 7).127


24

Figure 7. Synthesis of laponite RD armored polystyrene latex particles via Pickering

miniemulsion polymerization.

Armes et al. have contributed a lot to the emulsion polymerization of monomer(s)

in the presence of silica nanoparticles to prepare polymer/silica nanocomposite core-

shell particles (Figure 8).100 The strategy is based on an initial electrostatic adsorption

of cationic initiator azobis(isobutyl-amidine hydrochloride) (AIBA) onto the surface of

silica nanoparticles and a subsequent surface initiation of polymerization. As the

nascent particles grow in size, the silica coverage on polymer surface is reduced and

free anionic silica nanoparticles that remain in water are eventually adsorbed onto the

polymer surface, leading to typical core-shell particles. Though it is argued that

polymerization in this case is unlike the traditional Pickering emulsion polymerization

because the initial emulsion droplets have a larger size than that of the final

nanocomposite particles, the silica nanoparticles indeed participate in the stabilization

process during polymerization and in building up the shell. Other research groups, such

as Bourgeat-Lami et al., Wu et al. and Wang et al. also extend this topic by synthesizing

novel core-shell particles in their own way.125,126,128,133,134 Despite the diversity and

distinction in experimental manipulation and mechanism clarification, polymerization

in the presence of solid particles or 2D layers is a facile, general and effective way to

synthesize special core-shell particles.


25

Figure 8. Schematic representation of polymer@silica nanocomposite particles by

emulsion (co)polymerization in the presence of a glycerol-functionalized ultrafine silica

sol.

2.2.3.4 Partial extraction of polymer after sol-gel reaction

In the early 2000s the group of Avnir developed a very unique and novel way to

prepare core-shell particles, which is based on performing sol-gel reaction under basic

alcohol medium within surfactant-stabilized droplets containing PS (not latex particles)

in TEOS.84 As the sol-gel reaction proceeds fast at the interface, the hydrophobic TEOS

molecules are converted to a hydrophilic rigid shell that entraps the host PS without

permitting them to accumulate into a large PS phase, thereby forming bicontinuous

PS/silica composite particles (Figure 9a). When placing these composite particles in

methylene chloride, the entrapped PS molecules can be extracted from the particle

interior. After the polymer is removed by partial extraction, a core-shell structure is

formed, in which the shell consists of porous silica and the core comprises a PS/silica

composite (Figure 9b). The shell thickness can be controlled simply by the duration of

the extraction process. If PS is completely removed by calcination or solvent extraction,

mesoporous rather than hollow silica particles are obtained. This synthetic procedure

involves a one-step sol-gel reaction, and promises an efficient way to prepare

monodisperse submicron-sized composite particles.


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Figure 9. TEM images of (a) composite sol-gel particles of PS and silica; (b) SiO2-

shell/(PS+SiO2)-core particles achieved by partial extraction of hydrophobic phase.84

2.2.3.5 Sol-gel microencapsulation of organic liquids

The synthetic strategies discussed above mainly focus on forming silica shell on

polymer templates which are prepared either before or during the formation of core-

shell particles. In some cases, the hard polymer cores also sustain the silica shell with

enhanced mechanical integrity or immobilize the shell particles together on the polymer

surface. Apart from hard polymer cores, soft liquids can also be used as core templates

for preparation of core-shell particles. It is well described in several reviews that

capsules with liquid cores have been widely used in industry ranging from food and

cosmetics to pharmacy and medicine, implementing protection and controlled release

of active substances.40,59,60,135 Organic polymers are the most widely used shell

materials and silica is another promising alternative. Sol-gel microencapsulation of

organic liquids is considered as the most efficient, simple and economic approach to

synthesize silica-shell/liquid core particles. In brief, the emulsion droplets, which can

be oil-in-water (o/w) or water-in-oil (w/o) type, provide a templating interface for the

hydrolysis and condensation of silica precursors. Surfactants (e.g. cationic, anionic, and

nonionic surfactants) are essential not only for stabilizing emulsion droplets but also

for directing further silica precipitation.

The type of emulsion used as templates, in the final analysis, is determined by the

property of target molecules to be encapsulated. In w/o emulsions, droplets of an


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aqueous phase, which may contain hydrophilic active substances, are dispersed into a

hydrophobic oil with aid of surfactants. The encapsulation starts with the hydrolysis

and condensation of the silica precursor around the hydrophilic droplet, which in turn

plays a role of template. Once the precursor is fully converted into silica, hydrophilic

materials are encapsulated within the silica shell (Figure 10).136 Hence, by changing the

emulsion parameters, such as hydrophile-lipophile balance (HLB) value of surfactant,

water/oil ratio, aqueous pH, amount of silica precursor, one can delicately control the

overall particle size and the shell thickness as well as the core size. TEOS, one of the

mostly used silica precursors, can be initially located in the oil phase or water droplets.

With help of ethanol, TEOS dissolves in water and forms sol droplets in the oil phase.

Since the nonpolar alkyl chains of TEOS partly hydrolyzed prefer the nonpolar paraffin

oil, the hydrolysis and later condensation take place at the water/oil interface, leading

to a thickness growth of silica shell from oil/water interface toward the core of water

droplets.137

Figure 10. Procedure for direct microencapsulation of aqueous phase within silica shell

through a sol-gel reaction.

In principle, the microencapsulation of hydrophobic substances within a silica

shell is not possible in the case of w/o emulsion except the use of w/o/w or o/w/o double

emulsions, avoiding fast migration of lipophilic materials in the oil phase.60 The most

facile way is to directly encapsulate the hydrophobic compounds in an o/w emulsion

combined with a directed interfacial sol-gel reaction. Hydrophobic substances, such as

flavors, proteins, polymers and other chemicals are first dissolved in appropriate oils

and the homogeneous mixture is then dispersed into an aqueous phase with the aid of


28

surfactants and/or high shear force in some cases, yielding stable o/w type emulsions.

The silica precursor initially located in the dispersed hydrophobic phase continuously

diffuses from the droplet interior to the water/oil interface until complete consumption.

The resulting particles have a silica shell and a hydrophobic liquid core, which can also

be controllably liberated through the porous shell under external stimulus to produce

silica hollow particles. As another representative of organic core materials, phase

change materials which have a characteristic transition temperature can be coated with

silica in a similar o/w way mentioned above. Remarkably, they serve as soft liquid cores

which can be easily shaped and emulsified during preparation, and as hard solid cores

which in turn sustain the silica shell without collapse after cooling.

In both o/w and w/o cases, corresponding surfactants are essential to stabilize the

emulsion droplets and in some cases to direct the formation of unique shell structure.

In recent years, it has been reported that amphiphilic silica precursor can self-assemble

into different structures or morphologies without any surfactants, and can be eventually

converted into silica after hydrolysis and condensation. The amphiphilicity of silica

precursor reported so far is generally produced from the fact that hydrophobic

alkoxysilane can be hydrolyzed into hydrophilic silanol groups under catalysis of acid

or base.138,139 For example, well-defined siloxane oligomers with three alkoxysilane

functionalities and a chemically bonded alkyl chains have been synthesized by

Shimojima et al.139 Various hybrid mesostructures, namely lamellar, 2D hexagonal, and

2D monoclinic structures, are formed from these compounds without any other

structure-directing agents (Figure 11).

Figure 11. Schematic presentation of self-assembly of the silica precursor into three

types of hybrid mesostructures.139


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Another novel way to render the silica precursor amphiphilic is to introduce

hydrophilic groups (e.g. ethylene glycols). However, the monomeric silanes that

partially substituted with ethylene glycols cannot effectively self-assemble into

sophisticated structures probably due to their high solubility and fast hydrolysis in water.

To avoid this problem, a highly hydrophobic silica precursor polymer, hyperbranched

polyethoxysiloxane (PEOS), is partly substituted with poly(ethylene glycol) (PEG) and

then used for guiding the formation of nanostructured silica materials via self-

templating in water (Figure 12).140 Mesoporous particles, hollow nanocapsules, and

ultrasmall solid particles are prepared simply by controlling substitution degree of

PEOS with PEG. This self-assembly technique of functionalized silica precursor

polymers without any surfactants can certainly become a new approach towards silica-

shell/organic-core hybrid materials.

Figure 12. Schematic illustration of self-assembly of different PEG-PEOS molecules

in water and their conversion to silica particles of different morphologies.140


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