Interface/Interphase in Polymer

Nanocomposites

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Interface/Interphase in Polymer

Nanocomposites

Edited by

Anil N. Netravali and K.L. Mittal

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v

Contents

Preface xiii

Part 1 Nanocomposite Interfaces/Interphases

1 Polymer Nanocomposite Interfaces: The Hidden Lever for Optimizing Performance in Spherical Nanofilled Polymers 3

Ying Li, Yanhui Huang, Timothy Krentz, Bharath Natarajan,

Tony Neely and Linda S. Schadler1.1 Introduction 4

1.1.1 Dispersion Control 51.1.2 Interface Structure 61.1.3 Interface Properties 61.1.4 Measuring and Modeling the Interface 7

1.2 Dispersion Control through Interfacial Modification 81.2.1 Introduction 81.2.2 Short Ligands 81.2.3 Polymer Brush 11

1.2.3.1 Polymer Brush Synthesis Methods 121.2.3.2 Enthalpic and Entropic Contributions

of Polymer Brushes to Dispersion Control 13

1.3 Interface Structure 161.3.1 Introduction 161.3.2 Effects of Particle Size 171.3.3 Effects of Crystallinity and Cross-linking 181.3.4 Effects of Polymer Brush Penetration 19

1.3.4.1 The Athermal Case 191.3.4.2 The Enthalpic Case 21

1.3.5 Characterizing the Interface Structure 22

vi Contents

1.4 Interface Properties and Characterization Techniques 241.4.1 Introduction 241.4.2 Molecular Mobility in Nanocomposite Interfaces 251.4.3 Thermomechanical Properties and Measurements 28

1.4.3.1 Direct Measurement 301.4.3.2 Indirect Methods 32

1.4.4 Dielectric Properties and Measurements 401.4.4.1 Effects of Nanofillers 421.4.4.2 Measurement Techniques 431.4.4.3 Indirect Measurement 441.4.4.4 Finite Element Modeling 50

1.4.5 Remarks on Characterization Methods 521.5 Summary 53Acknowledgements 54References 55

2 Interphase Engineering with Nanofillers in Fiber-Reinforced Polymer Composites 71

József Karger-Kocsis, Sándor Kéki, Haroon Mahmood

and Alessandro Pegoretti2.1 Introduction 722.2 Interphase Tailoring for Stress Transfer 74

2.2.1 Coating with Nanofillers 742.2.2 Creation of Hierarchical Fibers 80

2.2.2.1 Chemical Grafting of Nanofillers 802.2.2.2 Chemical Vapor Deposition (CVD) 812.2.2.3 Other “Grafting” Techniques 83

2.2.3 Effects of Matrix Modification with Nanofillers 852.3 Interphase Tailoring for Functionality 87

2.3.1 Sensing/Damage Detection 872.3.2 Self-healing/Repair 892.3.3 Damping 91

2.4 Outlook and Future Trends 912.5 Summary 93Acknowledgements 93Nomenclature 94References 94

Contents vii

3 Formation and Functionality of Interphase in Polymer Nanocomposites 103Peng-Cheng Ma, Bin Hao and Jang-Kyo Kim3.1 Introduction 1033.2 Formation of Interphase in Polymer Nanocomposites 1053.3 Functionality of Interphase in Polymer Nanocomposites 111

3.3.1 Load Transfer in Nanocomposites 1113.3.2 Reduction in Growth Rate of Fatigue

Cracks in Nanocomposites 1163.3.3 Controlling the Fracture Behavior

of Nanocomposites 1193.3.4 Enhancing the Damping Properties

of Nanocomposites 1213.3.5 Channels for the Transport of Ions and

Moisture in Nanocomposites 1233.3.6 Phonon Scattering in Nanocomposites 1253.3.7 Electron Transfer in Nanocomposites 128

3.4 Summary and Prospects 130Acknowledgements 133References 133

4 Impact of Crystallization on the Interface in Polymer Nanocomposites 139Nandika D’Souza, Siddhi Pendse, Laxmi Sahu, Ajit Ranade and Shailesh Vidhate4.1 Introduction 1404.2 Thermodynamics of Crystallization 1424.3 Nylon Nanocomposites 144

4.3.1 Dispersion of MLS in Nylon Nanocomposites 1454.3.2 Effect of MLS on Thermal Transitions in Nylon 1464.3.3 Permeability 149

4.4 PET Nanocomposites 1514.4.1 Dispersion of MLS in PET Nanocomposites 1514.4.2 Effect of MLS on Thermal Transitions in PET 151

4.5 PEN Nanocomposites 1564.5.1 Dispersion of MLS in PEN Nanocomposites 1564.5.2 Effect of MLS on Thermal Transitions in PEN 1574.5.3 Permeability 1624.5.4 The Role of the Interface in Permeability:

PET versus PEN 1624.6 Summary 167Acknowledgements 167References 168

viii Contents

5 Improved Nanofiller-Matrix Bonding and Distribution in GnP-reinforced Polymer Nanocomposites by Surface Plasma Treatments of GnP 171

Rafael J. Zaldivar and Hyun I. Kim5.1 Introduction 1725.2 Experimental 173

5.2.1 Composite Fabrication 1735.2.2 Image Analysis 1745.2.3 Raman Spectroscopy 1745.2.4 X-ray Photoelectron Spectroscopy (XPS) 1745.2.5 Scanning Electron Microscopy (SEM) 1755.2.6 Mechanical Testing 175

5.3 Results 1755.4 Conclusions 187Acknowledgement 187References 187

6 Interfacial Effects in Polymer Nanocomposites Studied by Thermal and Dielectric Techniques 191

Panagiotis Klonos, Apostolos Kyritsis and Polycarpos Pissis6.1 Introduction 1926.2 Experimental Techniques 197

6.2.1 Differential Scanning Calorimetry (DSC) 1976.2.2 Dielectric Techniques 202

6.2.2.1 Broadband Dielectric Spectroscopy (BDS) 203

6.2.2.2 Thermally Stimulated Depolarization Current (TSDC) Techniques 207

6.3 Evaluation in Terms of Interfacial Characteristics 2096.3.1 Analysis of DSC Measurements 2096.3.2 Analysis of Dielectric Measurements 2116.3.3 Thickness of the Interfacial Layer 213

6.4 Examples 2146.4.1 DSC Measurements 2146.4.2 Dielectric Measurements 221

6.5 Prospects 2356.6 Summary 236Abbreviations 237Symbols 238Greek Symbols 239Acknowledgements 240References 241

Contents ix

Part 2 Techniques to Characterize/Control Nanoadhesion

7 Investigation of Interfacial Interactions between Nanofillers and Polymer Matrices Using a Variety of Techniques 255

Luqi Liu7.1 Introduction 2557.2 Observation of Interfacial Layer in Nanostructured

Carbon Materials-based Nanocomposites 2577.2.1 Characterization of Interfacial Layer

Around CNTs 2577.2.2 Characterization of Interfacial Layer

Around Graphene Sheets 2597.3 Interfacial Properties between Nanofiller and

Polymer Matrix 2607.3.1 Theoretical Simulations of CNT and/or

Graphene-based Nanocomposites 2607.3.1.1 Theoretical Simulation of

CNT-based Nanocomposites 2607.3.1.2 Theoretical Simulation of

Graphene-based Nanocomposites 2627.3.2 Experimental Studies to Characterize

Interfacial Behavior in CNT and/or Graphene-based Nanocomposites 2647.3.2.1 Indirect Measurement 2657.3.2.2 Direct Measurement 265

7.4 Summary 274Acknowledgements 275References 275

8 Chemical and Physical Techniques for Surface Modification of Nanocellulose Reinforcements 283

Viktoriya Pakharenko, Muhammad Pervaiz,

Hitesh Pande and Mohini Sain8.1 Introduction 2838.2 Chemical Surface Modification 285

8.2.1 Acetylation 2858.2.2 Silylation 2888.2.3 Bacterial Treatment 2898.2.4 Grafting 291

x Contents

8.2.5 Surfactant Adsorption 2938.2.6 TEMPO-mediated Oxidation 2948.2.7 Click Chemistry 296

8.3 Physical Surface Modification 2968.3.1 Plasma 2968.3.2 Corona 3018.3.3 Laser 3038.3.4 Flame 3038.3.5 Use of Ions 304

8.4 Summary 304Acknowledgments 305References 305

9 Nondestructive Sensing of Interface/Interphase Damage in Fiber/Matrix Nanocomposites 311

Zuo-Jia Wang, Dong-Jun Kwon, Jong-Hyun Kim,

Pyeong-Su Shin, K. Lawrence DeVries

and Joung-Man Park9.1 Introduction 3129.2 Experimental Specimens and Methods 315

9.2.1 Gradient Specimen Test 3159.2.2 Dual Matrix Fragmentation Test 318

9.3 Damage Sensing Using Electrical Resistance Measurements 3219.3.1 Electrical Resistance Measurement for

Strain Sensing Application 3219.3.2 Electrical Resistance Measurement for

Interface/Interphase Evaluation 3259.4 Summary 331References 331

10 Development of Polymeric Biocomposites: Particulate Incorporation, Interphase Generation and Evaluation by Nanoindentation 337

Oisik Das and Debes Bhattacharyya10.1 Introduction 33810.2 The Definitions of Composite and its Constituents 341

10.2.1 Composite 341 10.2.2 Biocomposite 341 10.2.3 The Reinforcement 341 10.2.4 The Matrix 342

Contents xi

10.3 Physical and Chemical Structures of Bio–based Reinforcements 343

10.3.1 Plant/Vegetable-based Reinforcements/Fibres 343 10.3.1.1 Physical Structure 343 10.3.1.2 Chemical Structure 343

10.3.2 Animal-based Reinforcements/Fibres 346 10.3.2.1 Physical Structure 346 10.3.2.2 Chemical Structure 347

10.4 Particulate and Short Fibre Composites 348 10.4.1 Biochar as Potential New Bio-based

Particulate Reinforcement 349 10.4.2 Properties of Particulate-based Composites:

Governing Factors 355 10.4.2.1 Particulate Properties 355 10.4.2.2 Particulate Structure 359

10.5 Nanoindentation Technique to Determine Interphase and Composite Properties 362

10.5.1 The Technique and Theory of Nanoindentation 362 10.5.1.1 Different Types of Indenter Tips 364 10.5.1.2 Nanoindentation Theory 366 10.5.1.3 Nanoindentation Instrument 368

10.5.2 Nanoindentation on Polymeric Composites and their Interphase 368

10.6 Concluding Remarks 373References 374

11 Perspectives on the Use of Molecular Dynamics Simulations to Characterize Filler-Matrix Adhesion and Nanocomposite Mechanical Properties 379

Sanket A. Deshmukh, Benjamin J. Hanson,

Qian Jiang and Melissa A. Pasquinelli11.1 Introduction 38011.2 Overview of Molecular Dynamics (MD) Simulations 38111.3 Characterization of Interfacial Adhesion with

MD Simulations 385 11.3.1 Quantifying Adhesion Strength 385 11.3.2 Effect of the Strength of Matrix-filler

Interactions 387 11.3.3 Effect of Filler Geometry 390 11.3.4 Effect of Ordering and Cross-linking within

the Polymer Matrix 392

xii Contents

11.4 Characterization of Mechanical Properties with MD Simulations 395

11.4.1 Predicting Static Mechanical Properties 396 11.4.2 Predicting Dynamic Mechanical Properties 399

11.5 Prospects 40311.6 Summary 404Acknowledgements 404References 404

Index 413

Preface

Polymer composites are made of two components: polymer or matrix (continuous phase) and filler or reinforcement (discontinuous phase) to obtain properties that cannot be achieved by a single component alone. The specific tensile properties of fiber reinforced composites are excellent because of their low density and high mechanical properties. Over the past few decades such composites have replaced metals in many applications from aerospace to sports gears, from automobiles to wind turbines, and from circuit boards to civil structures such as bridges and buildings. With composites impacting every part of our lives they have become ubiquitous. Over the past 2-3 decades the fillers or reinforcing elements used in com-posites have become smaller and smaller to their current nanosize. Using nanoparticles or nanofibrils in polymers or resins provides significant advantages. Hundreds of studies have shown that only a small weight per-cent (loading) of nanoparticles can significantly alter the stiffness, strength and fracture strain as well as electrical, thermal or other functional prop-erties of polymers because of their high surface-to-volume ratio. However, such benefits can be derived only if the particle dispersion is uniform and no clustering occurs. It is also universally accepted that the nanoparticle/resin interface and the interphase region in nanocomposites play a critical role in enhancing their properties. With better understanding of interface and interphase characteristics it should be possible to predict as well as design polymer nanocomposites with desired properties and performance. This book brings together several experts and leading researchers in this field to present their cutting edge research in understanding, modifying and controlling interfacial interactions between various nanofillers and a host of polymer matrices.

The book is divided into two parts; Part 1: Nanocomposite Interfaces/Interphases with 6 chapters and Part 2: Techniques to Characterize/Control Nanoadhesion with 5 chapters. In chapter 1 Schadler and coworkers define and discuss the two phases of polymer nanocomposites: polymeric matrix phase and inorganic nanofiller phase. Efforts have been made to improve the intrinsic properties of both the matrix and the nanofiller to enhance

xiii

xiv Preface

the overall performance of polymer nanocomposites. Accordingly, this chapter discusses the thermodynamic mechanisms governing nanofiller dispersion. The thermodynamic matrix/filler interactions also influence the structure and properties of the interfacial region, which can be sig-nificantly different from the bulk material. Examples of such structural modifications in semicrystalline and thermoset polymer nanocomposites are presented. In chapter 2 Pegoretti and colleagues discuss engineering of interphase with nanofillers in fiber-reinforced polymer composites. The first part of the chapter surveys recent advancements in the inter-phase engineering of fiber-reinforced polymer composites using different nanofillers. The second part of the chapter discusses strategies followed for stress transfer improvement or adding functionality to the interphase. The chapter also includes state-of-the-art knowledge on the characterization and modelling of the interphase. In the last ‘Outlook’ section some chal-lenges and perspectives in the engineering of fiber/matrix interphase are summarized. The third chapter by Kim and colleagues discusses formation and functionality of interphase, a distinct region between the two phases in polymer nanocomposites. This chapter presents fundamental issues on the formation of interphase between carbon-based nanofillers, such as carbon nanotubes, graphene, carbon black, and polymer matrices. Special empha-sis is placed on illustrating the role of interphase in governing the mechan-ical, electrical, thermal and other functional properties of nanocomposites. Based on the progress made so far, some suggestions are proposed for designing the interphase with specific structures for intended applica-tions of nanocomposites. In chapter 4 D’Souza and colleagues examine the effects of crystallization on the interface in polymer nanocomposites. Crystallization in polymer nanocomposites is influenced by the nature of the polymer, the percentage of nanoparticles present and their dispersion and interparticle distance. This chapter presents the effect of montmoril-lonite nanoclay on the interfacial crystallization in three polymers: nylon, poly (ethylene terephthalate) and poly (ethylene naphthalate). The effect of crystallization on the permeability of all three systems is also exam-ined. Chapter 5 by Zaldivar and Kim discusses a new class of Graphite Nanoplatelets (GnPs) based nanocomposites that have unique electri-cal and thermal properties. To obtain the highest possible properties, the nanoparticle/resin bonding needs to be improved. The chapter discusses how the nanoparticle surface can be optimally functionalized using Split Plasma Method. The sixth and the final chapter of Part 1 by Pissis and asso-ciates is devoted to the experimental investigation of interfacial effects in polymer nanocomposites using calorimetric studies for the glass transition and dielectric techniques for the segmental dynamics. After discussing the

Preface xv

experimental techniques briefly, the authors focus on proper evaluation of the measurements to extract maximum information from the data. The authors also present methods and equations used to evaluate the results in terms of interfacial characteristics, in particular polymer fraction in the interfacial layer (the fraction of polymer with modified properties) and thickness of the interfacial layer. The chapter provides an overview of the state-of-the-art in the field from the materials point of view simply by using various methods to characterize several selected polymer nanocomposites.

Part 2 of the book spans chapter 7 to chapter 11. In chapter 7 Liu describes the recent progress in theoretical and experimental aspects of interfacial adhesion in nanostructured carbon materials based polymer nanocomposites and summarizes the common methods utilized to char-acterize the interfacial properties in nanocomposites. The next chapter by Sain and colleagues discusses chemical and physical techniques for surface modification of nanocellulose reinforcements. The polarity of cellulose fibers due to the abundance of hydroxyl groups is responsible for poor wet-ting of natural fibers by non-polar resins. Furthermore, a large difference in surface free energy levels between resins and natural fiber reinforce-ments is responsible for poor interfacial bonding. The chapter discusses the most recent surface treatment techniques being employed to develop high-performance nanocomposites. In chapter 9 Park and colleagues dis-cuss a unique electro-micromechanical technique developed as an efficient nondestructive evaluation (NDE) method for sensing and determination of micro-damage at the filler/epoxy interface in nanocomposites. This ‘self-sensing’ method has also been used to evaluate interfacial damage in fiber reinforced polymer matrix nanocomposites. Among the advantages of this new NDE method, compared to other evaluation methods, include better stability, lower cost and its relative simplicity. Bhattacharyya and colleague discuss particulate incorporation, interphase generation and evaluation by nanoindentation in polymeric biocomposites in chapter 10. This chapter provides an overall perspective on the development of composites contain-ing bio-based reinforcements, e.g., biochar. The properties and governing factors of the biochar composites are explained, which is followed by a discussion of the suitability of nanoindentation technique for determin-ing nano-sized particle/resin interfacial properties. Finally, several studies involving nanoindentation on the nano-sized interfacial regions of com-posites are reviewed and critically discussed. In the 11th and the final chap-ter Pasquinelli and colleagues demonstrate the use of molecular dynamics (MD) simulations to quantify filler-matrix adhesion and nanocomposite mechanical properties. They also illustrate how MD simulations can pre-dict the mechanical properties of polymer nanocomposites as a function

xvi Preface

of the chemical and structural composition of these materials. Other pros-pects for MD simulations include calculating other physical properties, improving the structure-property prediction through advancements in hardware architecture and software development, and connecting through multiscale modeling the nanoscale/microscale details from MD simula-tions to the macroscale characteristics.

The book should be of interest to researchers in academia, in government research labs and R&D personnel in a host of industries (e.g. aerospace, automotive, biomedical, composites, dentistry, fibers, medical, microelec-tronics, packaging, plastics, textiles) who are interested in designing and improving the properties of polymers and composites by the addition of nanoparticles. Industries such as aerospace and automotive where light-weighting of each component is critical and an ongoing effort, improved properties through scientific understanding of nanocomposites could be very advantageous. Anyone working in plastics/polymers and composites industries should find this book of great interest and very useful.

It is our great pleasure to thank those who made this book possible. First and foremost, we are profusely thankful to the contributing authors for their sustained interest, enthusiasm and cooperation and for investing their valuable time in sharing their knowledge and cutting edge research (in the form of chapters) with the interested community. This book would not have been possible without their hard work. The unwavering interest and support of Martin Scrivener (Scrivener Publishing) in this book proj-ect and for giving this book a body form is also very much appreciated.

Anil N. Netravali K. L. MittalCornell University Hopewell Junction, NYIthaca, NY usharmittal@gmail.comann2@cornell.edu October 2016

Part 1

NANOCOMPOSITE

INTERFACES/INTERPHASES

3

Anil N. Netravali and K.L. Mittal (eds.) Interface/Interphase in Polymer Nanocomposites, (3–70)

© 2017 Scrivener Publishing LLC

1

Polymer Nanocomposite Interfaces: The Hidden Lever for Optimizing

Performance in Spherical Nanofilled Polymers

Ying Li1, Yanhui Huang2, Timothy Krentz2, Bharath Natarajan2,

Tony Neely3 and Linda S. Schadler2*

1Performance Indicator LLC, Lowell, MA, USA 2Department of Materials Science and Engineering,

Rensselaer Polytechnic Institute, Troy, NY, USA 3Department of Chemistry and Biochemistry,

University of South Carolina, Columbia, SC, USA

AbstractPolymer nanocomposites consist of at least two phases: a polymeric matrix phase

and an inorganic nanofiller phase. To enhance the overall performance of polymer

nanocomposites, efforts have been made to improve the intrinsic properties of

both the matrix and the nanofiller. A hidden lever for performance optimization,

however, lies in understanding and tailoring the matrix/filler interface. Depending

on the dispersion state of the nanofiller and the interface area, the resulting inter-

facial region can be a critical component in polymer nanocomposites. Generally,

uniform nanofiller dispersion, which maximizes interface area and therefore the

volume of the interfacial region, is desirable. This chapter will first briefly discuss

the thermodynamic mechanisms governing nanofiller dispersion. The thermo-

dynamic matrix/filler interactions also influence the structure and properties of

the interfacial region, which can be significantly different from the bulk material.

Examples of such structural modification in semicystalline and thermoset poly-

mer nanocomposites will be given. Deviations in polymer properties, such as the

*Corresponding author: schadl@rpi.edu

4 Interface/Interphase in Polymer Nanocomposites

change in molecular mobility in the interfacial region, influence the macroscopic

mechanical and dielectric properties of the polymer nanocomposite. However,

measuring these properties is a challenge because of the hidden nature of the

interfacial region. The measurement techniques can be broadly divided into direct

and indirect methods. The direct methods rely on the use of probes to directly

measure the local properties at the interface while indirect methods deduce the

interface properties by analyzing differences in the measurements from bulk com-

posites and the neat matrix.

Keywords: Nanocomposite, interface, thermodynamics, dielectric relaxation,

viscoelasticity

1.1 Introduction

Understanding the structure and properties of the 2D nanofiller/matrix interface and the resulting 3D interfacial region or “interphase” (used interchangeably in this chapter) that develops is the lynchpin to controlling and optimizing the properties of polymer nanocomposites (Figure 1.1). The 2D interactions are critical because:

1. The nanofiller/matrix interfacial interactions determine the dispersion state of the filler particles and the amount of interfacial area.

2. These 2D interactions impact the structure and properties of the 3D interfacial region.

Matrix 3D Interfacial region

2D Interface

Nanoparticle

Figure 1.1 An illustration of a 2D nanofiller/matrix interface and the resulting 3D

interfacial region.

Polymer Nanocomposite Interfaces 5

The 3D interfacial region is a region with properties different from the bulk polymer matrix. Due to the large interfacial area to volume ratio in nanocomposites, this region may constitute a significant portion of the bulk. For example, in a polymer nanocomposite filled with 5 vol% mono-disperse 10 nm spherical nanoparticles, even if the thickness of the interfa-cial region is as small as 5 nm, the volume fraction of this interfacial region can be as high as 35 vol%. As the nanofiller loading level increases, this becomes an even more significant volume fraction of the composite. The 3D region controls nanocomposite performance in two key ways:

1. The changes in structure or chemistry in this region can drastically change bulk composite structure (e.g. average cross-link density, crystalline morphology).

2. The changes in mobility (e.g. changes in relaxation time spectra) strongly impact the composite bulk properties, in particular, the mechanical and dielectric properties.

To model and/or predict the properties of a nanocomposite, it is essential to include the properties of the interfacial region. This inclusion needs to occur in a spatially specific way and not as a volume averaged property because of the role of percolation, stress concentrations, and defects on composite bulk properties. However, we do not yet have a full understand-ing of how to control the structure and properties of the interfacial region, which reduces the impact of polymer nanocomposites in high-end appli-cations. This lack of understanding and control of the interfacial region is attributable, in part, to the challenge of measuring the structure and proper-ties of the interfacial region because of its small size and embedded nature.

This chapter will address the impact of the 2D interface on the propen-sity for nanofiller dispersion in quasi-equilibrium conditions in detail, briefly touch upon its role under kinetic processing conditions, and pro-vide a summary of what is currently known about the structure of the 3D interfacial region. Further we discuss three key approaches to determining interface properties including: dynamic mechanical analysis to understand thermomechanical and relaxation processes, dielectric spectroscopy to understand permittivity changes as well as dielectric relaxation processes, and the role materials informatics plays in developing an in-depth quanti-tative understanding of interfacial interactions.

1.1.1 Dispersion Control

To optimize the amount of interfacial area, the nanoparticles need to be well distributed and well dispersed [1, 2]. It is well known that even in the

6 Interface/Interphase in Polymer Nanocomposites

athermal limit (no attraction or repulsion between matrix and particle), there could be entropically driven depletion forces that encourage the agglomeration of nanoparticles [3]. As the particle-particle attraction increases, this driving force increases. This interaction can be mitigated by attaching surface ligands to the nanoparticles that alter the enthalpic and entropic interfacial interactions [4–6]. The attached surface ligands can be broadly categorized into short molecules or polymer brushes. Short molecules primarily impact the enthalpic interactions with the matrix [7], while polymer brush ligands introduce a large entropic component and therefore complicate the thermodynamic interaction between the filler and matrix. The approaches to ligand engineering for dispersion control and the prediction of nanofiller dispersion based on interfacial thermody-namics models will be presented in Section 1.2.

1.1.2 Interface Structure

As the thermodynamic interactions are tuned between the nanofiller and the matrix, the structure of the resulting interfacial region changes. For example, in thermosetting and elastomeric matrices, the degree of chemical/ physical cross-linking can be impacted by the presence of nanoparticles with a modified surface [8, 9]. In semicrystalline thermoplastic polymer matrices, the particle surface can impact matrix crystallization [10–12], and during crystallization the alignment of matrix polymer chains can cause or pre-vent agglomeration of nanoparticles [13, 14]. In the case of polymer brush grafted nanoparticles dispersed within amorphous polymers, depending on the graft density and the length of the brush, the matrix can penetrate the brush (wet brush) or be repelled from the brush (dry brush) [15], which, in turn, determines the structure of the interfacial region. Section 1.3 will focus on the current understanding of interface structure.

1.1.3 Interface Properties

The scientific and technological significance of interface structure studies primarily lies in their implications for tailoring interface properties. To be noted, the mobility of the matrix and/or brush polymer chains in the inter-facial region with a defined structure has led to significant discussion in the literature [16–18]. Short molecules, typically used to create compatibility, can contribute to changes in properties [19]. For example, short ligands can repel, attract, or bond with matrix chains, and modify matrix chain mobil-ity [20]. The morphology of grafted long polymer brushes also impacts the polymer chain mobility in the vicinity of particle surface as well as the

Polymer Nanocomposite Interfaces 7

ability to transfer stress from the matrix to the particle. In addition, adding a highly polar molecule can lead to significant changes in dielectric per-mittivity or dielectric breakdown strength [21–23]. Other surface ligands can introduce ions to the interface, alter the band structure, act as traps or scattering sites for charge carriers, etc. [24, 25]. The first part of Section 4 will focus on changes in mobility due to the 2D interface.

1.1.4 Measuring and Modeling the Interface

A key to understanding interfacial structure and properties is the ability to measure them. The second part of Section 1.4 will focus on measuring the dielectric and viscoelastic properties of the 3D interfacial region. There are two broad methods for determining the properties of the interface: direct and indirect measurements. Direct measurements use a local probe with nanoscale resolution to “see” the interface or measure the properties directly. For example, nano Dynamic Mechanical Analysis (nano DMA) has been used to measure local viscoelastic properties with nanometer res-olution [26]. The challenge with probes such as Atomic Force Microscopy (AFM) and intermodulated AFM is that the impact of the surface on the measured properties is difficult to separate out [27]. Indirect methods can also be a powerful approach for estimating interface properties. In this case, the changes in bulk properties are monitored as a function of particle loading or systematic change to the nanoparticle surface ligands, and the changes in properties are ascribed to the interfacial region. For example, photoluminescence spectroscopy can provide useful information on charge trapping and transfer at the interface [28]. Thermally stimulated depolarization current measurements are useful for characterizing slow relaxation processes especially those associated with space charge separa-tion and propagation [29, 30]. Using an inverse problem approach, finite element models that explicitly include the matrix and filler properties can be used to handle nanofiller dispersion explicitly, and then tune the inter-face properties to match experimental results. Using this indirect approach, interface properties can be inferred. One powerful approach that is being developed for understanding the interface and designing nanocomposites is materials informatics. Informatics combines both empirical and first principles models, data mining, targeted experimental validation, and ultimately processing parameters. By combining informatics with finite element models (FEMs), interface properties may be inferred. Once the relationships between interface chemistry and properties are developed, informatics can be used to create a design loop that should lead to faster introduction of polymer nanocomposite materials into the marketplace.

8 Interface/Interphase in Polymer Nanocomposites

1.2 Dispersion Control through Interfacial Modification

1.2.1 Introduction

Nanosized inorganic particles possess unique features compared to chemi-cally identical materials on a larger size scale, and can be used to significantly alter the properties of polymers. However, the dispersion of nanoparticles (NPs) into polymeric matrices, which determines the distribution and amount of interface present in the nanocomposite, is a significant challenge that requires an in-depth understanding of both entropic and enthalpic driving forces [7, 31, 32]. For the case of zero enthalpic mismatch, even though the ideal translational entropy favors dispersion of NPs, excluded-volume effects and depletion attractions between the NPs can lead to NP aggregation [33, 34]. When the NPs are sufficiently small, the enthalpic driving force for inorganic NP agglomeration is mainly determined by van der Waals (vdW) core-core attractions [33]. It is well-known that a strongly bound surface layer of matrix-compatible ligands markedly diminishes the interfacial tension between modified NPs and the matrix and suppresses agglomeration. These ligands can be classified into two types based on their size. Short ligands are found to tailor the enthalpic compatibility at the interface, and long polymer chains tune the enthalpic as well as entropic interactions [7, 35]. While this functionalization has been found to alter the proclivity to aggregate, it also simultaneously alters the interface character-istics (Section 1.3). In this section, we focus primarily on the effect of inter-facial thermodynamics on NP dispersion. The aforementioned approaches to ligand engineering are discussed in further detail. Predictive thermody-namic models are then introduced to shed light on the unique morphol-ogy-structure-property relations of inorganic/organic nanocomposites.

1.2.2 Short Ligands

The short organic compounds used for modifying NPs include thiols, car-boxylic acids, amines, silanes, and phosphonates, which can react with NP surface atoms via covalent, electrostatic, or hydrogen bonding interactions and act as reactive anchors (Figure 1.2) [36]. Dithioesters or trithiocarbon-ates have been reported to directly attach to gold substrates as anchoring groups [36]. Carboxylic acids are routinely used to stabilize metal oxide NPs upon their synthesis, with oleic acid being the most commonly utilized ligand. The oleic acid prevents surface oxidation of the metal oxide and due to its long alkyl chain, improves the dispersibility of the nanoparticles

Polymer Nanocomposite Interfaces 9

in nonpolar solvents. However, carboxylic acids cannot serve as strong anchoring groups for surface ligands on nanoparticle surfaces, and many procedures use ligand exchange reactions to replace oleic acid with silanes or phosphonic acid. A variety of commercially available silanes, such as alkoxysilane, hydrogenosilane (hydrogenated monosilane), or chlorosi-lane, can be used for SiO

2, ZrO

2, TiO

2, or HfO

2 modification, and can also

bear numerous functionalities, such as amino, cyano, carboxylic acid, and epoxy groups [36]. This method works best for silicon- containing inorganic phases such as: silica, silicate glasses, or silicon nitride. As a complemen-tary method, phosphonate groups including organophosphorus acid and its derivatives are best suited for the anchoring of organic ligands to metal oxide NPs such as: TiO

2, SnO

2, and Al

2O

3 [36, 37].

In short-ligand modified filler-polymer systems, mean-field enthalpic effects dominate the matrix/particle interactions [2, 7]. The relative strength of these interactions is found to determine the propensity to aggregate. In weakly interacting (athermal) mixtures, a balance between the loss of con-formational entropy of the matrix chains due to distortion, excluded vol-ume interactions between particles, and the gain in translational entropy is expected to determine the dispersion state [33, 38, 39]. If interparticle vdW

R

R

R R R R

R S

S

O

R

R

OOH

OH OO

O O

P

R

O

P

R

O

P

SiO OH

OH OH

metal oxide

OH OH OH OH

O

Si Si Si

Si SiSi

SiSi

Si

Si

O O O O

OO O OO

O

O

O

O

O O

O

OH OH OH OH

HO

HO

HO

CI

CI

CI

R R R R

R

R

R RR

R

O O

OO O O O

S S SR

(a)

(b)

(c)

(d)

Figure 1.2 Schematic representation of the surface modification of a metal oxide surface

showing the substitution of native hydroxyl groups by (a) organosulfur, (b) carboxylic

acid/carboxylate, (c) octadecyltrichlorosilane, and (d) phosphonate ligands, with

increasing degree of chemisorption going from left to right [36, 37].

10 Interface/Interphase in Polymer Nanocomposites

energy is considered, the mechanism of “contact aggregation” was found to dominate [40]. Hooper and Schweizer [40] found through the polymer reference interaction site model (PRISM) that increasing the interparticle enthalpic attraction led to a lower probability of miscibility. Molecular dynamics (MD) simulations of particle clustering in polymeric matrices [41, 42], through Lennard-Jones (dispersion) forces, determined that the dominant energetic factor was the ratio of the interaction strength of matrix and particle to the interaction strength of particle and particle. The degree of agglomeration was found to increase non-linearly as this ratio decreased (higher interparticle attraction), as shown in Figure 1.3.

Validating the models that describe the impact of interface energetics on dispersion has received limited attention in systematic experimental stud-ies. The challenge is that nanocomposites are not in equilibrium. They can be homogenized by a number of mixing techniques [43], but this first mix-ing step during which the polymer-particle interface is formed is primarily a kinetic process. In two studies, one of which tried to reach equilibrium [20], and the other which studied the kinetically trapped state [44], the inter-face formation was found to be dictated by the relative interaction strengths between NPs and between the NPs and the matrix. Both studies represented the interface energetics as the ratio of the experimentally measured work of cohesion of the NPs and the work of adhesion between the matrix and the NPs (W

pp/W

mp) [20, 44]. These results, including the non-linear rela-

tionships, were in agreement with the simulation studies discussed earlier. Figure 1.4 shows the effect of W

pp/W

mp and the relative work of adhesion,

–5

–5.5

–6

–6.5

–7

60

50

40

30

20

10

0

2000 4000

Time = t/

mp

u pp

Dispersed

Nu

mb

er

of

ne

igh

bo

uri

ng

fill

ers

Clustered

(a) (b)

6000 80000

1 1.1 1.2 1.3 1.4 1.5

mp = 4mp = 10

mp = 2

mp = 1

Figure 1.3 (a) A plot of particle-particle potential (upp

) vs. particle-matrix polymer

interaction (εmp

) illustrating a smooth transition from a clustered (low upp

) to dispersed

(high upp

) state with increasing εmp

[41]. (b) A plot of the total number of neighboring

fillers as a function of aggregation time (t) normalized by Lennard Jones time (τ). Over

time, a higher εmp

leads to fewer nearest neighbors i.e., better dispersion [42].

Polymer Nanocomposite Interfaces 11

ΔWa, on a representative set of samples [20]. The dispersion quality is found

to decrease with increasing Wpp

/Wmp

and ΔWa. Figure 1.4 also shows the

abrupt non-linear change in dispersion quality from Wpp

/Wmp

< 1 (well dis-persed amino poly(2-vinyl pyridine) (P2VP))) to W

pp/W

mp > 1 (aggregated

amino poly(methyl methacrylate)).In the extreme case of strongly attracting particles, dispersion is dictated

by physical bridging effects [46]. Simulations on high loadings of highly attractive systems found that the matrix polymer chains were adsorbed onto the surfaces of neighboring particles, leading to the formation of polymer bridges. This bridging was observed to cause polymer–mediated agglomeration [34].

1.2.3 Polymer Brush

The effects of entropic interactions become even more relevant for polymer-brush-grafted NPs, whose dispersion states within polymer

Figure 1.4 A representative schematic showing the effect of Wpp

/Wmp

and ΔWa on

dispersion quality of 3 and 8 wt% functionalized silica (amino-, chloro- and octyl- silane

modified) in various matrices, such as polystyrene (PS), poly(methyl methacrylate)

(PMMA), and poly(ethyl methacrylate) (PEMA), using TEM micrographs obtained at a

magnification of 20000 ×. The scale bars represent 1 μm. An increasing Wpp

/Wmp

is shown

to significantly decrease the quality of dispersion. In the systems where the matrix-particle

attraction is stronger than particle-particle attraction (Wpp

/Wmp

< 1) an increasing ΔWa

is found to cause re-agglomeration. We note that the Wpp

/Wmp

and ΔWa values indicated

are calculated from the literature and experimentally measured surface energy values for

polymer and particles [45].

8 wt%

3 wt%Amino PEMA1.11

Amino PMMA1.04

Wpp/WmpWa

Wpp/Wmp< 1 systems

Amino P2VP0.92

Chloro PMMA

0.62 mJ/m2

Octyl PS

3.48 mJ/m2

Chloro PS

6.16 mJ/m2

12 Interface/Interphase in Polymer Nanocomposites

matrices are determined by the complex interplay between enthalpic nanoparticle core-core attraction and entropic brush/matrix entanglement [2, 47–52]. Compared to short ligands, the synthesis of polymer brushes requires more sophisticated schemes and techniques.

1.2.3.1 Polymer Brush Synthesis Methods

The attachment of a polymer brush can be carried out using either a “ grafting to” or “grafting from” technique. The “grafting to” method involves the attachment of a pre-formed and end-functionalized polymer chain via organic reactions such as the alkyne-azide “click” reaction or direct coupling reactions using organo-phosphate or carboxylate ligands (Figure 1.5) [53]. The advantage of this method is that well-defined poly-mer brushes can be readily obtained [54, 55]. However, in this case, the diffusion of a polymer chain to a functionalized surface suffers from steric repulsion between the diffusing chain and those chains already attached [56]. A “grafting from” technique, on the other hand, offers much higher graft density. The particle surface is functionalized with the appropriate initiator or chain transfer agent (CTA), and the polymer chain is then grown from the surface using surface-inititiated atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), or reversible-addition fragmentation chain transfer (RAFT). For example, Hojjati and coworkers have attached carboxylic acid functionalized initia-tor agents to titania (TiO

2) nanoparticles [57, 58]. Subsequently, polymers

including poly(methyl methacrylate) (PMMA) and poly(acrylic acid) (PAA) were grafted from the surface to obtain dispersed titania (TiO

2)

nanocomposites.The most popular method for the synthesis of polymer brushes is

ATRP. First described in 1995, ATRP controls the polymerization through

Figure 1.5 Schematic representation of the synthesis procedure for the grafting of

polymer chains onto the surface of silica particles [53].

Cleavable

group

Anchoring

group

A

II*

Initiator

HOHO

HO OHC1

C1C1

BrSi

Si

RO

OROR

NaN3

OH

OH

OH

SiO2

SiO2

SiO2

Grafting-from

Grafting-to

Click reactionN3

N3

N3

N3

N3

N3

N3

I*

I*

I*

I*

I*I

II*

I*IA

A

AA

A

A

I

I

II

A