Correlating the rheological and mechanical response of polyurethane nanocomposites containing hyperbranched polymers

Cast polyurethane thermosets have been modified with randomly oriented exfoliated and intercalated montmorillonite clay usinghyperbranched polymers (HBP) as reactive additives to promote dispersion of the clay platelets. The influence of the clay content on thestiffness of the nanocomposites is discussed in terms of simplified classical micromechanical models that allow correlations to be establishedbetween the properties of the final polyurethane and the limiting high strain rate shear viscosity of the HBP or HBP/polyethylene glycolnanocomposite precursors. Such models imply certain restrictions on the potential for mechanical reinforcement with unoriented plateletsowing to crowding effects. Moreover, they appear unable to account simultaneously for the observed degrees of reinforcement in the glassyand the rubbery states, if currently accepted values for the stiffness of the MMT are assumed. Possible reasons for this are discussed.

Highly branched polymer/montmorillonite clay nanocomposites

Marlene Rodlert

The aim of this thesis has been to assess the potential of branched polymers as novel exfoliants for layered silicates and their application in polyurethanes. Three different types of –OH functional branched polymers have been studied: dendrimers, hyperbranched polymers (HBPs) and star branched polymers (SBPs), with a range of structures and molecular weights. The dendrimers and their HBP analogues were synthesized in-house, while the other HBPs and SBPs were obtained from commercial suppliers. Various types of montmorillonite (MMT) layered aluminosilicate clays have been used as modifiers. The different branched polymers were either melt processed or solution processed with the MMT, using water and THF as dispersants. Whether intercalated or exfoliated nanocomposites were obtained depended on the choice of MMT, the presence of solvent during processing as well as the characteristics of the interface between the polymer and the MMT, with exfoliation being favored when the polymer-MMT interactions were strong. Very high degrees of exfoliation were obtained from dendrimers and HBPs processed in water at loadings as high as 20 wt% unmodified Na+MMT, with intercalation only becoming dominant at higher loadings. The MMT layer spacing in the intercalated HBP/Na+MMT nanocomposites depended directly on the pseudo-generation number, that is, on the size of the HBP, at intermediate loadings, consistent with adsorption of a monolayer of unperturbed HBP molecules at the layer surface in aqueous suspension. Dispersions of the HBPs with a range of organically modified MMTs in THF, on the other hand, showed mainly intercalation on drying, with layer spacings of the order of 3.8 nm at intermediate clay contents, which were apparently independent of the pseudo-generation. The difference between the final MMT layer spacing and its value prior to mixing was found to increase significantly with the polarity of the organic modifier. The contrasting behavior of Na+MMT and the organically modified MMTs was partly attributed to the tendency of Na+MMT to exfoliate in aqueous dispersion. The HBPs and dendrimers are then able to coat the individual MMT layers and stabilize the exfoliated structure during drying. The organically modified MMTs, on the other hand, did not exfoliate in THF dispersions and intercalation consequently dominated. Predominantly intercalated nanocomposites were also obtained with the SBPs, although increasing degrees of exfoliation were observed as the strength of the interactions between the SBP and the clay increased, as a result of systematic modification of the polarity of the SBP. The rheological properties of a range of exfoliated and unexfoliated highly branched polymer/MMT nanocomposites were investigated in small strain oscillating shear. The exfoliated nanocomposites showed a sharp increase in shear viscosity at very low MMT contents, accompanied by a transition from Newtonian behavior to solid-like behavior, characterized in terms of a limiting shear modulus at low frequencies and a limiting viscosity at high frequencies, both of which were strongly dependent on MMT content. Unexfoliated nanocomposites showed a more gradual increase in viscosity with MMT content, although they showed qualitatively similar behavior to the exfoliated nanocomposites at sufficiently high loadings. The rheological behavior was analyzed in terms of models for conventional filled polymers, leading to estimates of the effective particle aspect ratio that were approximately consistent with direct morphological observations by transmission electron microscopy.To investigate the influence of the MMT on solid state properties, the HBP/MMT and SBP/MMT nanocomposites were incorporated into model polyurethane formulations. In the case of the HBP/MMT, a reactive low molar mass polyol or an un-reactive solvent (THF) was used to reduce the viscosity sufficiently for room temperature processing. Exfoliated HBP/Na+MMT led to a 60% increase in the rubbery plateau modulus relative to that of the corresponding unfilled matrix at clay contents as low as 0.5 vol%, whereas more modest relative increases were observed with unexfoliated or partly exfoliated MMT. The same materials also showed a 2-fold increase in stress at break and elongation at break in the presence of 0.5 vol% exfoliated Na+MMT, although no improvement was observed for the corresponding intercalated polyurethane nanocomposites obtained in the absence of HBP. Much higher exfoliated clay contents led to processing problems owing to their high viscosities. Polyurethanes containing unexfoliated MMT, such as obtained by using SBPs rather than HBPs as the matrix, could be prepared up to relatively high MMT contents, but the improvements in mechanical properties were modest at any given MMT content. The semi-empirical Halpin-Tsai model and Mori-Tanaka's average stress theory, which are both based on a classical approach to particle reinforcement, accounted well for the evolution of the mechanical properties of the polyurethanes as the MMT content was varied and they led to estimates of the particle aspect ratio that correlated well with those inferred from the rheological models and direct observations. This in turn allowed correlations to be established between the degree of exfoliation, the shear viscosity of the nanocomposite precursors and the mechanical properties of the final polyurethanes.

EPFL2005

Highly filled polymer nanocomposite films derived from novel nanostructured latexes

Riccardo Ruggerone

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.

EPFL2010

Light trapping polymer-based coatings for cost effective flexible thin film photovoltaics

Marina Aymara González Lazo

Thin film silicon solar cells benefit from a lower fabrication cost than crystalline silicon cells, but they have a lower efficiency. The aim of this thesis was to explore the light-management capabilities of sub-micron polymer-based textures, both for the back reflector and for the front encapsulation of thin film silicon solar cells. The thin active layers of these devices require some form of photon path length enhancement to increase their efficiency. For this purpose, light trapping textures were replicated using transparent low shrinkage acrylate hyperbranched polymer nanocomposites. To enable low pressure replication of textures with enhanced mechanical performance, silica nanoparticles and silicon-based sol-gel precursors were combined to create novel low viscosity hybrid nanocomposite resins. Indeed, the viscosity of these hybrid suspensions was found to be one to two orders of magnitude lower than that of particulate suspensions with an equivalent silica loading. The hybrid resins were cured by combined UV polymerization and condensation, and the mechanical performance of the resulting nanocomposites was characterized in terms of their Vickers microhardness. The Vickers microhardness increased from 112 MPa for the unmodified polymer matrix to 190 MPa and 148 MPa for the hybrid nanocomposites and particulate nanocomposites, respectively, at 20 vol% silica, and reached as much as 287 MPa for the hybrid nanocomposites at 30 vol% silica. For the back reflector, light scattering (LS) textures in the form of random sub-micron pyramidal features were replicated in the nanocomposites using a nickel template and UV-nanoimprint lithography. The influence of nanoparticle content and pressure on the morphology and light scattering properties of the replicas was studied using scanning electron microscopy and optical analysis. The roughness and coherence length of the textures were similar to those of the template for all the compositions and process pressures investigated. After optimization of the dual-cure process, very high replication fidelity could be obtained in each case, leading to haze greater than 99 % over the whole of the visible light spectrum and very effective light scattering performance for a broad range of angles. A durable antireflective (AR) front encapsulation is also key to enhancing the photovoltaic performance of thin film silicon solar cells. The AR performance of hyperbranched polymer nanocomposite textures replicated from a moth eye pattern using UV-nanoimprint lithography was established both experimentally and through simulations. Optimum patterns were found on the basis of effective medium theory (EMT) to be arrays of paraboloids with a periodicity of 340 nm, which provided stable AR performance for aspect ratios in the range of 0.6 to 1.75. Good agreement was obtained between the simulated and measured optical behavior of such AR arrays, with a normal reflectance in the visible range of around 4 % when using a glass substrate. The simulations also predicted a reduction in the hemispherical reflectance from 15 % to 6 % for this texture. A hierarchical texture was replicated from a lotus leaf into the hyperbranched polymer modified with a perfluorinated acrylate, and the same material was also used to replicate the AR moth eye texture. In both cases, the replication fidelity was again found to be excellent and the textures exhibited improved hydrophobicity. [...]

EPFL2015