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Molecular dynamics simulations were carried out on traditional polymers copolymerized with POSS (Polyhedral Oligomeric Silsesquioxanes) derivatives to identify the reason behind improved properties imparted to the conventional polymers with the chemical incorporation of POSS. Two classes of systems are used in the present study, namely the polystyrene and polymethyl methacrylate systems. Seven systems are studied in the polystyrene class. The effect of corner substituent groups of the POSS cage on the properties of the polymer nanocomposites was studied using the polystyrene. In addition, the effect of the type of cage structure on the properties was studied using T8, T10 and T12 POSS cage structures containing phenyl substituents on each POSS cage. Systems with polymethyl methacrylate were studied to analyze the effect of mole percent of POSS on the polymer properties, holding the corner substituents on the POSS unit constant. The corner function used was the isobutyl group. The properties analyzed using simulations include glass transition temperature, volumetric thermal expansion coefficient, X-ray scattering data, solubility parameter and mechanical properties. In both polystyrene and polymethyl methacrylate systems, simulations were also carried out on the pure parent polymers for the sake of comparison. The effect of forcefield on the predicted properties was studied using both COMPASS and PCFF forcefields. Performance analysis of the code used in the present simulation was done by analyzing the parallel run time of simulations involving pure atactic polystyrene.

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Carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) are carrying great promise as two important constituents of future multifunctional materials. Originating from their minimal defect confined nanostructure, exceptional thermal and electrical properties have been reported for these two allotropic forms of carbon. However, a brief survey of the literature reveals the fact that the incorporation of these species into a polymer matrix enhances its effective properties usually not to the degree predicted by the composite\\textquoteright s upper bound rule. To exploit their full potential, a proper understanding of the physical laws characterizing their behavior is an essential step. With emphasis on the electrical and thermal properties, the following study is an attempt to provide more realistic physical and computational models for studying the transport properties of these nanomaterials. Originated from quantum confinement effects, electron tunneling is believed to be an important phenomenon in determining the electrical properties of nanocomposites comprising CNTs and GNPs. To assess its importance, in this dissertation this phenomenon is incorporated into simulations by utilizing tools from statistical physics. A qualitative parametric study was carried out to demonstrate its dominating importance. Furthermore, a model is adopted from the literature and extended to quantify the electrical conductivity of these nanocomposite. To establish its validity, the model predictions were compared with relevant published findings in the literature. The applicability of the proposed model is confirmed for both CNTs and GNPs. To predict the thermal properties, a statistical continuum based model, originally developed for two-phase composites, is adopted and extended to describe multiphase nanocomposites with high contrast between the transport properties of the constituents. The adopted model is a third order strong-contrast expansion which directly links the thermal properties of the composite to the thermal properties of its constituents by considering the microstructural effects. In this approach, a specimen of the composite is assumed to be confined into a reference medium with known properties subjected to a temperature field in the infinity to predict its effective thermal properties. It was noticed that such approach is highly sensitive to the properties of the reference medium. To overcome this shortcoming, a technique to properly select the reference medium properties was developed. For verification purpose the proposed model predictions were compared with the corresponding finite element calculations for nanocomposites comprising cylindrical and disk-shaped nanoparticles. To shed more light on some conflicting reports about the performance of the hybrid CNT/GNP/polymer nanocomposites, an experimental study was conducted to study a hybrid ternary system. CNT/polymer, GNP/polymer and CNT/GNP/polymer nanocomposite specimens were processed and tested to evaluate their thermal and electrical conductivities. It was observed that the hybrid CNT/GNP/polymer composites outperform polymer composites loaded solely with CNTs or GNPs. Finally, the experimental findings were utilized to serve as basis to validate the models developed in this dissertation. The experimental study was utilized to reduce the modeling uncertainties and the computational predictions of the proposed models were compared with the experimental measurements. Acceptable agreements between the model predictions and experimental data were observed and explained in light of the experimental observations. The work proposed herein will enable significant advancement in understanding the physical phenomena behind the enhanced electrical and thermal conductivities of polymer nanocomposites specifically CNT/GNP/polymer nanocomposites. The dissertation results offer means to tune-up the electrical and thermal properties of the polymer nanocomposite materials to further enhance their performance. / Ph. D.

This work explores the computational modeling of electromechanical problems using peridynamics and in particular, its application in studying the potential of carbon nanotube (CNT) reinforced nanocomposites for the purpose of sensing deformation and damage in materials. Peridynamics, a non-local continuum theory which was originally formulated for modeling problems in solid mechanics, has been extended in this research to electromechanical fields and applied to study the electromechanical properties of CNT nanocomposites at multiple length scales. Piezoresistivity is the coupling between the electrical properties of a material and applied mechanical loads, more specifically the change in resistance in response to deformation. This can include both, a geometric effect due to change in dimensions as well as the change in resistivity of the material itself. Nanocomposites referred to in this work are materials which consist of CNTs dispersed in a binding polymer matrix. The origins of the extraordinary piezoresistive properties of nanocomposites lie at the nanoscale where the non-local phenomenon of electron hopping plays a significant role in establishing the properties of the nanocomposite along with CNT network formation and inherent piezoresistivity of CNTs themselves. Electron hopping or tunneling allows for a current to flow between neighboring CNTs even when they are not in contact, provided the energy barrier for electrons to hop is small enough. This phenomenon is highly nonlinear with respect to the intertube distance and is also dependent on other factors such as the potential barrier of the polymer matrix. To investigate this in more detail, peridynamic simulations are first employed to study the piezoresistivity at the CNT bundle scale by considering a nanoscale representative volume element (RVE) of CNTs within polymer matrix, and by explicitly modeling electron hopping effects. This is done by introducing electron hopping bonds and it is shown that the conductivity and the non-local length scale parameter in peridynamics (the horizon) can be derived from a purely physics based model rather than assuming an ad-hoc value. Piezoresistivity can be characterized as a function of the deformation and damage within the material and thereby used as an in-situ indicator of the structural health of the material. As such, a material system for which real time in-situ monitoring may be useful is polymer bonded explosives. While these materials are designed for detonation under conditions of a strong shock, they can be damaged or even ignited under certain low magnitude impact scenarios such as during accidental drop or transportation. Since these materials are a heterogeneous system consisting of explosive grains within a polymer matrix binder, it is proposed that CNTs can be dispersed within the binder medium leading to an inherently piezoresistive hybrid nanocomposite bonded explosive material (NCBX) material which can then be monitored for a continuous assessment of deformation and damage within the material. To explore the potential use of CNT nanocomposites for this novel application, peridynamic simulations are carried out at the microscale level, first under quasistatic conditions and subsequently under dynamic conditions to allow the propagation of elastic waves. Peridynamics equations, which can be discretized to obtain a meshless method are particularly suited to this problem as the explicit modeling of crack initiation and propagation at the microscale is essential to understanding the properties of this material. Moreover, many other parameters such as electrical conductivity of the grain and the properties of the grain-binder interface are studied to understand their effect on the piezoresistive response of the material. For example, it is found that conductivity of the grain plays a major role in the piezoresistive response since it affects the preferential pathways of current density depending on the relative ease of flow through grain vs. binder. The results of this work are promising and are two fold. Peridynamics is found to be an effective method to model such materials, both at the nanoscale and the microscale. It alleviates some of difficulties faced by traditional finite element methods in the modeling of damage in materials and can be extended to coupled fields with relative ease. Secondly, simulations presented in this work show that there is much promise in this novel application of nanocomposites in the field of structural health monitoring of polymer bonded explosives. / Ph. D. / CNT reinforced nanocomposites are known to possess extraordinary mechanical, thermal and especially piezoresistive properties. Piezoresistivity is the change in resistivity of a material in response to mechanical deformation, which can possibly be used as a tool to monitor the structural health of a material. One such set of materials are polymer bonded explosives (PBXs), a heterogeneous composite system consisting of explosive grains dispersed within a binding matrix. These materials are susceptible to mechanical insults during transportation and handling, which can damage the material at the microstructural level, decreasing the reliability and usability and may even lead to accidental detonation. It is proposed that doping the binder phase with CNTs will form inherently piezoresistive NCBX materials, whose resistivity can be monitored for microstructural changes. This may help detect and discern these damage processes that can occur on at sub-macroscale length scales, which may pass unnoticed to the naked eye or even to other non-destructive methods which may not be able to detect internal changes in the material. The current work explores the structural health monitoring (SHM) capability of NCBX materials through a recently developed computational method, peridynamics. These materials are virtually tested under various loading conditions through peridynamics simulations and compared to experimental data. The results of this work are two fold; peridynamics is found to be an effective tool to study coupled phenomena such as piezoresistivity and nancomposite piezoresistivity is well suited to monitor microstructural changes in NCBX materials. This is a first step in establishing computational models for SHM in PBX materials and can be used in various other applications ubiquitous in the engineering world such as aircrafts, spacecrafts, bridges, dams among many others.

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Since the discovery of carbon nanotubes (CNTs), a large interest has developed around the incorporation of these into polymeric matrices in order to introduce the excellent mechanical, thermal and electrical properties of CNTs into the resultant composites. Nanocomposites of polymer/CNT composition allow for the favourable combination of the physical properties of the polymeric matrix and of the CNT filler. The biggest existing challenge of producing such nanocomposites is presented by the tendency of CNTs to occur in bundles or aggregates which are difficult to break up and to disperse in solution which leads to non-uniform distributions within the polymeric matrix. This problem has been combated through the use of CNT surface functionalization. However, a disadvantage exists with this solution. Since covalent functionalization of the CNT surface disrupts the electronic π-electron cloud which is responsible for the excellent electronic properties which CNTs are often desired for, an alternative method of functionalization must be employed in order to maintain the excellent intrinsic properties of CNTs yet create uniform dispersion of the nanotubes upon compatibilization with the polymeric matrix. Two alternative methods for the production of noncovalent compatibilization of multi-walled carbon nanotubes (MWNTs) with polystyrene were investigated and compared. These two methods involved the synthesis of a pyrene-functional macroinitiators for reversible addition fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP). Both of these methods allow for the controlled polymerisation of pyrene functional polystyrene chains. For comparison, the direct covalently functional MWNTs were also synthesised first by oxidation of the MWNT surface and conversion of the MWNT into the multifunctional RAFT and ATRP macroinitiator in which the styrene chains were controllably directly grafted from the surface of the MWNTs. The interaction of the pyrene chains with MWCNTs was monitored by using NMR, TGA and fluorescence spectroscopy. The NMR results showed the broadening and weakening of the pyrene protons as well as the polystyrene (PS) protons. TGA showed the loss of the pyrene-functional PS portion throughout the heating process. Fluorescence provided the conclusive result that the noncovalent compatibilization had occurred through the quenching of the emission and excitation signals as a result of electron transfer being facilitated by the π-stacking interactions. Finally, the MWNT nanocomposite polymer nanofibres are produced via the electrospinning technique with the various covalent and non-covalent compatibilized MWNT. The fibre morphology for the different compatibilization methodologies is compared as a function of the MWNT content. Distinct differences are observed for the different composites. / AFRIKAANSE OPSOMMING: Sedert die ontdekking van koolstof-nanobuisies (KNBs), het ʼn groot belangstelling ontwikkel rondom die betrekking van KNBs in polimeriese matrikse om samestellings met uitstekende meganiese, termiese en elektriese eienskappe te vervaardig. Nanosamestellings van polimeer/KNB komposisie laat toe dat gunstige kombinasies van fisiese eienskappe van die polimeer en die KNB vuller gerealiseer kan word. Die grootste uitdaging van die vervaardiging van sulke nanosamestellings is die neiging van KNBs om gebondelde formasie te vorm wat baie moeilik is om op te breek. Dit maak hulle verspreiding in oplossings en in polimeer matrikse oneweredig. Hierdie probleem word deur funksionalisering opgelos. Nogtans, ʼn nadeel van hierdie oplossing is dat kovalente funksionalisering verander die elektroniese struktuur van die KNB oppervlakte deur die ontwrigting van die π-elektron wolk wat vir die uitstekende elektroniese eienskappe verantwoordelik is. Dus moet ʼn alternatiewe funksionalisering metode gebruik word om die inherente eienskappe van die KNBs te behou en terselfde tyd ʼn uniforme verspreiding te bewerkstellig gedurende die vermenging met die polimeer matriks. Twee alternatiewe metodes vir die vervaardiging van nie-kovalente gefunksionaliserde multiommuurde koolstof-nanobuisies (Eng: MWNTs) met polistireen (PS) was ondersoek en vergelyk. Hierdie twee metodes was uitgevoer deur die sintese van ʼn pyreen-funksionele omkeerbare addisie-fragmentasie-kettingoordrag (OAFO) en atoomorrdragradikaaladdisie (AORA) makromiddel. Al twee van hierdie metodes lei tot ʼn gekontrollerde polimerisasie van pyreen-gefunksionaliserde stireen. Vir vergelyking was ʼn kovalente- gefunksionaliserde MWNT vervaardig deur die oksidasie van die MWNT oppervlakte en die daaropvolgende immobilisasie van dieselfde AORA en OAFO middel aan hierde aktiewe punte. Daarvan af was stireen gekontroleerd gepolimeriseer deur middel van die AORA en OAFO middel. Die interaksie was gekarakteriseer deur TGO, KMR en fluoressensie spektroskopie. Die KMR resultate het seine gewys van die verspreiding en verswakking van die pyreen en PS protone. TGO het die verlies van die pyreen-funksionele PS deel van die nie-kovalente produk gewys. Fluoressensie het beslissende bewyse gelewer dat die nie-kovalente funksionalisiering plaas gevind het deur die onderdrukking van die stralende en opwekkings seine as ʼn gevolg van die elektron oordrag wat deur die π-stapel interaksies gefasiliteer word. Uiteindelik was die nanosamestellings vermeng met PS en geelektrospin. Die vesel morfologie vir die verskillende gefunksionaliserde MWNT nanosamestellings metodes was vergelyk as ʼn funksie van MWNT inhoud. Duidelike verskille is waargeneem vir die verskillende samestellings.

Molecular weights of 2,000, 6,000 and 10,000 of silane functionalized atactic polystyrene (aPS) and α,ω-divinyl functionalized polydimethylsiloxane (PDMS) were prepared via living anionic polymerization and bulk anionic ring opening polymerization respectively. Functionalization of the homopolymers was confirmed by FT-IR and 1H-NMR spectroscopy and their molecular weights were determined via 1H-NMR end group analysis. A hydrosilylation reaction between the functionalized homopolymers of different molecular weights produced nine polystyrene-block-polydimethylsiloxane-block-polystyrene (aPS-b-PDMS-b-aPS) triblock copolymers. Field emission scanning electron microscopy observations revealed the copolymers self-assemble into supramolecular structures. Dynamic Light Scattering measurements show only small increase in the order of nanometers of its hydrodynamic radius as the individual molecular weights of the homopolymers were increased. Nanocomposites of the copolymers were prepared by incorporating 1% of oxidized single walled carbon nanotubes (SWNTs) within the aPS-PDMS-aPS matrices via coagulation precipitation. Differential scanning calorimetry (DSC) thermal analysis shows the SWNT interacting with both aPS and PDMS constituting blocks. SWNTs interaction with aPS block either increases the polymer glass transition temperature (Tg) by restricting its segmental motion or decreases the Tg by a plasticization effect. Within the PDMS block the SWNTs act as nucleating sites accelerating the crystallization rate of the polymer. This is evident by the appearance of single and double melting endotherms in the DSC thermograms.

The processing and properties of novel polymer-based nano-composite (PNC) dielectrics for capacitor applications has been studied. PNCs were fabricated via a vacuum based deposition technique and their micro/nano-structure, chemical and dielectric properties investigated. After process development and optimisation, co-deposited Al and nylon-6 PNCs had a dielectric constant k∼7 at an approximate Al volume fraction of 0.3 that agreed with analytical predictions if it was assumed that the Al transformed to an oxide in-situ and/or after deposition. The significant effect of absorbed water vapour and temperature on PNC dielectric properties was revealed using different types of post-deposition heat treatment. Alternately-deposited PNCs consisting of Al or Ag 2-20 nm layers sandwiched between nylon-6 layers were fabricated in which the overall PNC Al or Ag volume fraction was controlled by varying the nominal Al or Ag layer thickness. Ag layers comprised of discrete nano-islands that produced a nano-capacitor network effect that increased k to ∼11. In the case of Al layers, when the layer thickness was ≥ 5 nm, corresponding to a nominal volume fraction of 0.1, Al (core)-oxide (shell) nanoparticles were formed and the PNC dielectric constant increased to ∼19. The detailed nano-structure of the core-shell particles was studied using various types of transmission electron microscopy (TEM), and the elevations in dielectric constant ascribed to multiple-interface polarisation effects dependent on the formation of the core-shell structure. PNCs based on alternate deposition of Ti sandwiched in nylon-6, and then both Ti and Ag in nylon-6 were also fabricated, with k reaching ∼73 for Ag+Ti/nylon-6 PNCs. As well as Ti-based core (metal)-shell (oxide) particles, the Ag volume fraction was sufficiently high in the 10 nm nylon-6 layers to again form a nano-capacitor network that contributed to the overall device capacitance and effective dielectric constant. Again, various types of high magnification TEM were critical in resolving the Ti-based core-shell structure and its role in high-k behaviour. The vacuum-based alternate deposition technique has been developed to offer ease of operation, reliability, flexibility and applicability to chemically different filler and matrix systems in the fabrication of high-k PNC based capacitors, in which high-k performance relies critically on the formation of core (metal)-shell (oxide) particles in both Al and Ti based systems.