Polymer materials stand on a very significant position in the materials industry area. The presence of organoclay nanocomposites reinforces polymer materials on many properties like strength, tensile and so on. Most previous studies on the characteristics of organoclays and polymer nanocomposites were based on the experimental approaches such as XRD (X-ray Diffraction) and NMR (Nuclear Magnetic Resonance). These methods have achieved successfully on the basic analysis of chains and layering structures of polymer nanocomposites. However, information on the molecular level cannot be provided by those approaches. MD (Molecular Dynamic) simulation method could be employed to develop further information on the molecular level about organoclays and interlayer structure polymer nanocomposites. In the research of ODTMA-MMT (Octadecyltrimethylammonium-Montmorillonite) organoclay simulation, we find that the strong layering behaviour of interlayer ODTMA molecules is present with the same minimum distance between nitrogen atoms and MMT surface in different T/O (Tetrahedral vs. Octahedral) ratio cases. Nitrogen atoms sit right above the corresponding hexagonal cavities, which is in agreement with the previous research. The interaction energy between surfactants and MMT clay will reach the lowest point when substitution ratio of tetrahedral and octahedral (T/O) is equal to 1:1. Moreover, MSD (Mean Square Displacement) and diffusion coefficient of different models under same CEC (Charge Exchange Capacity) condition are inverse ratio to the T/O proportion. In nylon6 polymer nanocomposites, sodium cations which exist originally in ensemble as charge balancer are absorbed much closer to MMT surface than the organic components in the nylon 6 ODTMA-MMT ensemble. Sodium atoms or nitrogen atoms in surfactants both have higher MSD and coefficients than those atoms in the organic-modified clays. In the exfoliated nylon 6 ODTMA-MMT nanocomposites, pair correlation has been analysed instead of density profile. Layering packing structure is also shown through this analysis, which is also consistent with previous work.

Polymer composites are fast becoming a material in the manufacturing of automotive interior and exterior parts such as facias and dashboard components. Production of rigid structural foams are ideal because they reduce the overall weight as well as reduce the amount of material used to manufacture the part. Polymer-clay nanocomposites are a classification of materials containing a blend of polymer and a small weight percentage of nanoclay. These materials are currently of interest to automotive part manufacturers because they are known to deliver improved mechanical properties and increase foamability of a polymer. The current study investigates the changes in material properties and the foamability of a thermoplastic polyolefin (TPO)-clay nanocomposite as the degree of intercalation was varied. The TPO-clay nanocomposite was produced by melt blending TPO, nanoclay and maleic anhydride grafted polypropylene (MAHgPP) in a co-rotating twin screw extruder. The material was subjected to a multi-pass process to vary the degree of intercalation. Degree of intercalation was tracked by rheology, XRD and TEM micrographs. Part density, cell density and flexural modulus measurements were performed on foamed and non-foamed injection molded bars to observe changes in the foamability of the material. Material was also processed without clay and analyzed in the same manner. Through TEM and XRD analysis it was found that the degree of intercalation and delamination was varied with increasing number of passes. Rheological measurements showed that the TPO-clay nanocomposite underwent (beta)-scission and intercalation simultaneously. The changes in intercalation had a positive effect on the foamability of the TPO-clay nanocomposite. As well, the TPO-clay nanocomposite experienced an increase in flexural properties for both unfoamed and foamed parts compared to the TPO-PPgMAH blend; TPO-clay nanocomposite experienced a 44% and 23% increase in the flexural modulus for unfoamed and foamed parts respectively. Data also showed that there was a limit to the number of times the TPO-clay nanocomposite can be recycled before the foamability of the material begins to decrease, which was attributed to material degradation. / Thesis / Master of Applied Science (MASc)

In the past two decades polymer/clay nanocomposites (PCNs) have emerged as promising materials that exhibit remarkably improved properties when compared to conventional composites and pristine polymers. Such improvements strongly depend on the dispersion of clay nanoparticles in the polymer matrix. In spite of great efforts expended in characterizing clay dispersion, effective, simple and quantitative techniques are still needed. This work addresses this challenge by presenting new aspects of 1H solid-state NMR for quantifying clay dispersion in PCNs filled with clay containing paramagnetic ions. Employing these 1H solid-state NMR methods, some structure-processing-deformation relationships of PCNs were derived, and basic insights into nuclear relaxation and spin diffusion in PCNs were gained as well. Detailed models and analyses were described for 1H spin-lattice relaxation in the presence of paramagnetic clays in PCNs. Relaxation recovery was analytically correlated to clay dispersion in two ways: one is the initial relaxation recovery which is related to clay surface area, and the other is the spin-lattice relaxation time which is related to interparticle spacing. These two NMR observables were employed to quantitatively observe the evolution of clay morphology in poly(propylene)/clay (PP/MMT) nanocomposites upon equibiaxial stretching, as well as upon in situ uniaxial deformation. The initial relaxation recovery was independently utilized to determine the polymer-clay interfacial surface area and the degree of clay exfoliation. We demonstrated the capabilities of our models in quantitatively characterizing several materials, including poly(vinyl alcohol), nylon 6, poly(å-caprolactone) (PCL), poly(lactic acid) (PLA) and PP nanocomposites. These results were used to examine the dependence of clay morphology upon processing (strain ratio, strain rate, temperature), deformation (extension), component characteristics (polymer molecular weight, clay surface modification) and clay content. Effects of paramagnetic Fe3+ concentration and external magnetic field strength on 1H spin-lattice relaxation in PCNs were also investigated and discussed. In particular, low field separates the initial relaxation recovery into two stages: one related to clay content and the other related to the polymer-clay interfacial surface area. The low field was observed to enhance the paramagnetic contribution to the spin-lattice relaxation rate, increasing its sensitivity to clay morphology. In addition, measurements of long-distance spin diffusion coefficients for a variety of polymers and paramagnetic characteristics of organically modified clay were explored. Overall, the utility of NMR relaxometry in characterizing PCNs has been significantly expanded and successfully demonstrated in this dissertation.

Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2009. / Synthesis and structural characterization of two novel cationic and three new neutral reversible addition–fragmentation chain transfer (RAFT) agents is described. The cationic RAFT agents bear a quaternary ammonium group: N,N-dimethyl-N-(4- (((phenylcarbonothionyl)thio)methyl)benzyl)ethanammonium bromide (PCDBAB) and N-(4-((((dodecylthio)carbonothioyl)thio)methyl)benzyl)-N,N-dimethylethanammonium bromide (DCTBAB). The three neutral RAFT agents synthesized are 1,4- phenylenebis(methylene)dibenzene carbodithioate (PCDBDCP), didodecyl-1,4- phenylenebis(methyllene)bistrithiocarbonate (DCTBTCD) and 11-(((benzylthio)carbonothioyl) thio)undecanoic acid (BCTUA). The self-assembly behaviour in diluted aqueous solutions of the cationic RAFT agents, PCDBAB and DCTBAB, is described. The self-assembly behaviour was promoted by the presence of the thiocarbonyl- thio group on the RAFT agents, in addition to the overall chemical structure of the surfactant that also influence self-assembly. The RAFT agents were used for the bulk or miniemulsion RAFT-mediated controlled free-radical polymerization in the presence of clay to yield polymer–clay nanocomposites (PCNs). Bulk polymerization resulted in PCNs with better control of molar mass and polydispersity index (PDI) values when compared to PCNs prepared by miniemulsion polymerization. In both bulk and miniemulsion polymerizations the molar masses and PDI values were dependent on the amount of clay and RAFT agent present in the system. Free-radical bulk neutral RAFT agent-mediated polymerization resulted in PCNs with predominantly intercalated morphology. This was attributed to radical–radical coupling of the initiator anchored onto the clay galleries on which polymerization took place. On the other hand, when the cationic RAFT agent anchored onto clay, i.e. RAFT-modified clay was used, bulk polymerization resulted in predominantly exfoliated PCNs. However, miniemulsion polymerization carried out in the presence of the RAFT-modified clays resulted in PCNs with a morphology that ranged from partially exfoliated to intercalated morphology, as the clay loading was increased. The changing morphology for miniemulsion-based PCNs was attributed to the decreasing molar mass as the clay loading was increased. The PCNs obtained had enhanced thermo-mechanical properties as a result of the presence of clay. The thermo-mechanical properties depended on the molar mass, PDI, clay loading, and the morphology of the PCNs.

We apply a molecular dynamics (MD) simulation technique to complex molecules at interfaces. Partitioning of dendritic surfactants into clay gallery and Ab protein behavior near hydrated lipids are chosen for the purpose. Using a full atomistic model of dendritic surfactants, the confinement force profiles featuring oscillatory fashion at moderate layer separation of 10 to 25 Å were observed. Integration of the confinement forces led to free energy profiles, which, in turn, were used to determine the final morphology of the nanocomposite. From the free energy profiles, smaller and linear surfactants (G1 and G2L) are expected to intercalate into the clay comfortably, while larger surfactants (G2 and G3) are expected to form frustrated intercalated structures due to the location and depth of the free energy minima. This would agree with the previous observations. As primary steps to understand the Ab protein behavior under biological conditions, simulations of bulk water and hydrated lipids were performed and the results were compared with the literature. Hydrated lipids were simulated using a full atomistic model of lipids (dipalmitoylphosphatidylcholine) and water with a cvff force-field and it was found that structural properties such as the molecular head group area and membrane thickness were accurately produced with MD simulation. Systems of the protein Ab(1-42) in bulk water were simulated and some secondary structural change, with loss of part of the a-helical structure, occurred during the 1 ns of simulation time at 323K. The fragment Ab(31-42) with b-sheet conformation was also simulated in bulk water, and the extended b-sheet structure became a bent structure. Simulations of Ab(1- 42) or Ab(31-42) near lipid bilayers have been performed to investigate the structural property changes under biological conditions. The different nature of structural change was observed from the simulations of the protein or fragment in water and near lipid bilayers due to the different solvent environment. The protein has close contacts with the membrane surface. It was impossible to observe the conformational change to b-sheet and protein entrance into the lipid bilayer within 1 ns simulations.

Polymer-nanoplatelet hybrid membranes show promise as the next generation of membranes, but in order to make these realizable, methods to produce these materials on a large scale are necessary. Some authors have successfully produced these types of gas separation membranes. Typically these reports have utilized melt blending and in situ polymerization. Few, however, have utilized solution blending for creating membranes via phase inversion (asymmetric membranes). And to date, there have not been any reports regarding the fabrication of asymmetric membranes containing nanoplatelet filler materials. In this work we have developed a solution-based procedure for the formation of hybrid polymer-nanoplatelet dopes for dense film and asymmetric hollow fiber membrane formation. Dense film membrane studies were used to prove the effectiveness of our exfoliation and dispersion process developed for this work. Permeation measurements showed the hybrid membranes have desirable transport properties that are on par with mathematical model predictions. Additionally, TEM characterization provided strong evidence supporting the efficacy of our preparation procedures to produce an exfoliated system of nanoplatelets. We also showed that these procedures are applicable to different polymer systems (cellulose acetate and Torlon) of commercial relevance. Demonstrating the successful production of dense films set the stage for asymmetric hollow fiber membrane formation. We report the first production of asymmetric hollow fiber membranes containing nanoplatelet fillers; indicating that the process can be applied in a realistic membrane formation platform. These accomplishments serve as the groundwork for future nanocomposite formation.

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Yes / The mechanical property enhancement of polymer reinforced with nano-thin clay platelets (of high aspect ratio) is associated with a high polymer-filler interfacial area per unit volume. The ideal case of fully separated (exfoliated) platelets is generally difficult to achieve in practice: a typical nanocomposite also contains multilayer stacks of intercalated platelets. Here we use numerical modelling to investigate how the platelet properties affect the overall mechanical properties. The configuration of platelets is modelled using a statistical interpretation of the Representative Volume Element (RVE) approach, in which an ensemble of "sample" heterogeneous material is generated (with periodic boundary conditions). A simple Monte Carlo algorithm is used to place non-intersecting platelets in the RVE according to a specified set of statistical distributions. The effective stiffness of the platelet-matrix system is determined by measuring the stress (using standard Finite Element analysis) produced as a result of applying a small deformation to the boundaries, and averaging over the entire statistical ensemble. In this work we determine the way in which the platelet properties (curvature, filling fraction, stiffness, aspect ratio) and the number of layers in the stack affect the overall stiffness enhancement of the nanocomposite. Thus, we bridge the gap between behaviour on the macroscopic scale with that on the scale of the nano-reinforcement, forming part of a multi-scale modelling framework.