Infoscience

Thesis

Highly filled polymer nanocomposite films derived from novel nanostructured latexes

The overall aim of this thesis has been to assess the potential of latex-based technologies for the preparation of polymer/clay nanocomposites. The key feature of latex-based technologies is that they offer the possibility of improved control of the final nanocomposite morphology at significantly higher clay loadings than can be obtained with more conventional processing techniques, such as melt blending or in situ polymerization. The idea is to exploit swelling of the clay in either the aqueous or the monomer phase of a water-based latex, depending on the clay surface functionalization, to produce hybrid polymer/clay latex particles with controlled diameters of the order of 100 nm, which may then be consolidated to produce solid nanocomposite films. The materials considered in this work were based on styrenic matrices, considered to be a model system, and acrylics, which are of more interest for commercial coating applications. Two different polymerization techniques were investigated, namely conventional emulsion polymerization and miniemulsion polymerization. The thermal and mechanical properties of films produced from the resulting latexes were then studied in detail. Conventional emulsion polymerization was found to be particularly suitable for the preparation of particles with a well-defined "armoured" morphology, in which the clay formed a more or less complete shell around a matrix core, providing the focus for the remainder of the project. Clay contents of up to about 50 wt % were obtained for both the styrenic and the acrylic latexes using this approach, with excellent degrees of dispersion, the average clay aggregate thickness not exceeding 10 nm. The armoured morphology of the latex particles resulted in a cellular arrangement of the clay in the consolidated films, which became better defined as the clay content increased. The reinforcing effect of the clay on mechanical properties varied according to the physical state of the matrix. Increases in Young's modulus by a factor of 3 to 4 were observed in styrenic films with the cellular morphology in the glassy state, and the degree of exfoliation of the clay was found to be a critical parameter under these conditions, samples containing 5 to 7 wt % of clay showing increased moduli with respect to those obtained at somewhat higher clay contents, for which aggregation was more apparent. In the rubbery state, on the other hand, the Young's modulus increased by more than 2 orders of magnitude for clay contents above 20 wt % and was strongly correlated with the overall filler content. Thermal analysis showed that a significant proportion of the matrix remained immobilized in the rubbery state, i.e. did not contribute to the glass transition. This was argued to be due to strong physical confinement of regions of the matrix intercalated in the clay aggregates. While the increases in Young's modulus in the glassy state could be accounted for in terms of classical micromechanical models, such as those of Halpin-Tsai and Mori-Tanaka, the same models failed to predict the behaviour in the rubbery state. Models based on foam mechanics were therefore developed incorporating an immobilized matrix fraction in the cell walls, whose elastic properties were treated as fitting parameters. Although somewhat different values of the Young's modulus for this immobilized matrix fraction were required to fit the data, depending on the details of the model, they were consistently found to be two to three orders of magnitude greater than that of the neat matrix in the rubbery state (but not to exceed the Young's modulus of the matrix in the glassy state), providing direct evidence for the importance of this interphase for the overall nanocomposite properties. The importance of the cellular arrangement of the clay was also confirmed through comparison with nanocomposites containing non-cellular morphologies resulting from other preparation techniques or the use of mechanical deformation to break-up the initial cellular structure. Finally, it was demonstrated that the results obtained for the styrenic systems could be extended to acrylic-based nanocomposites with comparable morphologies, underlining their broader significance for formulations of potential commercial interest. A further goal of this thesis was to study the effect of the clay on the microdeformation and fracture mechanisms of the nanocomposites, and it was also of interest to compare these results with those obtained for conventional isotactic polypropylene (PP)/clay nanocomposites prepared by melt blending, whose macroscopic properties have been studied previously in our institute. In situ TEM investigation of deformation in glassy styrenic nanocomposite films of about 200 nm in thickness containing the cellular structure revealed a decrease in local matrix drawability at clay contents above 10 wt %, and extensive coarse cavitation, thought to be associated with the particle cores, which replaced crazing as the dominant deformation mechanism, accounting for the observed decrease in macroscopic tensile strength at intermediate clay contents. Moreover, at the highest clay contents, these mechanisms were replaced by failure of the particle-particle interfaces, leading to an extremely brittle macroscopic response. Above Tg, localized deformation zones were also observed, indicating the network formed by the clay and the immobilized regions of matrix to show yielding behaviour, again consistent with the macroscopic response. The conventional PP/clay nanocomposites also showed a decrease in matrix ductility and an increase in coarse cavitation with increasing clay content. In this latter case, however, the zones of cavitation were clearly identified with breakdown of the clay aggregates or the interfaces between the clay and the matrix, underlining the important role of the clay-matrix interface for the properties of the styrenics, and by inference, those of the acrylics. To provide further insight into the role of the cellular structure of the styrenics, finite element (FE) simulations were used to investigate the distribution in hydrostatic stress in the nanocomposite films under finite deformations. These showed the principal stress concentrations to appear in the clay aggregates aligned in the direction of the applied deformation, while the hydrostatic stress in the matrix remained relatively uniform, suggesting that as long as the interface and clay aggregates remain stable, cavitation may initiate anywhere within the matrix, as in the unmodified polymer. The FE simulations were also used to model the elastic properties in the rubbery state, confirming the need to assume the presence of an immobilized matrix fraction in order to account for the observed Young's moduli. The results of these calculations were found to be consistent with those obtained from the foam-based micromechanical models, confirming the applicability of these latter, and suggesting the local anisotropy of the clay aggregates to play a limited role in the overall low strain elastic properties. The main outcome of this thesis has been the establishment of a general physical basis for predicting structure-mechanical property relationships in a new range of latex-based nanocomposite materials with exceptional properties, thanks to the possibility of incorporating very high clay loadings without compromising processability, and vast potential for fine tuning of stiffness and stiffness related properties. Moreover, the work has highlighted the important role of the "nano" effect, particularly at temperatures above the glass transition of the matrix, where matrix immobilization at the clay-matrix interface is clearly demonstrated to contribute to both thermal and mechanical properties. It is consequently expected to provide solid guidelines for the choice of morphological and materials parameters for the optimization of basic mechanical properties in such materials via controlled synthesis. Moreover, with further advances in synthesis and processing and hence in film quality, it should ultimately be possible to extend the basic physical model developed here to account for the structure dependence of other important applicative properties, such as permeability and fire resistance.

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