Structure et mécanismes de microdéformation de polymethylmethacrylates renforcés au choc

Polymethylmethacrylate (PMMA) is used in many applications requiring good optical properties, resistance to ageing and resistance to UV irradiation, in spite of its relatively poor mechanical response to impact loading. In order to extend the range of application of this transparent but fragile material, and improve its competitiveness with engineering polymers such as polycarbonate, it is necessary to improve its fracture resistance. Several generations of toughened PMMA reinforced by addition of rubber particles have therefore been developed by Arkema over the last decade ("rubber toughened PMMA" (RTPMMA)). However, there remains a need for substantial improvement even in these relatively fracture resistant materials, providing the overall motivation for this project. The first type of material considered is based on RTPMMA in which the matrix ductility has been improved by copolymerization with up to 25 % ethyl acrylate (EA). The reinforcing particles used in these new modified RTPMMA grades comprise a PMMA core, either one (conventional 3L particles), or two rubbery layers (experimental 4L particles), and finally an outer grafted PMMA shell. A second, entirely new type of rubber modified PMMA, based on Poly(MMABA-MMA) copolymers (MAM) prepared by controlled radical polymerization (PRC) is then considered. PRC is carried out using nitroxide initiators, making it possible to polymerize the blocks of PMMA from the blocks of PBA in a continuous mass process. Although little was known about their mechanical response prior to the present work, the MAM copolymers represent an attractive alternative to RTPMMA from an economic point of view since their production involves essentially only one step. The scientific aim of this study is to understand the relationship between the mechanical properties, the crack propagation mechanisms and the microstructure of these various impact reinforced PMMA materials, and, if possible, to use this knowledge to identify strategies for improving their performance. It is first shown that there is an increase in the strain to break from 10 % to up to 30 % on addition of 25 % EA to the PMMA matrix, and a corresponding reduction in the yield stress from 95 to 60 MPa. The TEM observations of thin films deformed in-situ show these changes to be associated with a change in microdeformation mechanism from crazing in the pure PMMA to homogeneous plastic deformation. However, they are not accompanied by any substantial improvement in the high speed fracture resistance, the value of the maximum stress intensity factor, KImax, at 1 m/s remaining at about 2 MPa.m1/2 in all the matrices investigated. The ductility improves significantly on addition of the 3L and 4L particles, and this improvement is particularly marked with the 4L particles, the strain to break reaching 80 % and the yield stress dropping to about 40 MPa, regardless of the matrix EA content. The fracture resistance also increases over the full range of speeds investigated, and optimum performance is obtained in the matrix containing 25 % EA combined with 4L particles, KImax ranging from 3.2 MPa.m1/2 at 10-4 m/s to 3.5 MPa.m1/2 at 1 m/s. At low speeds, both the 3L and 4L particles deform by cavitation of the rubbery layer and, as the EA content is raised, the matrix microdeformation mechanism at the crack tip changes from crazing to simple shear, consistent with the observations for the matrix. Nevertheless, at very high speeds, at which crazing dominates in all the materials, the rubbery phase of the particles is argued to become relatively rigid and hence increasingly ineffective in decreasing the crack propagation energy. Thus, the overall improvements in mechanical behaviour obtained on EA addition are not reflected by an improvement in the notched Charpy impact toughness, which does not exceed 5.5 kJ/m2 in any of the RTPMMA. There may be scope for further optimizing the particle size at high EA contents owing to an observed decrease in entanglement molar mass, as demonstrated in previous work by Béguelin et al., but this is not thought to be viable from an economic point of view. MAM copolymers provide an alternative, low cost means of introducing a rubbery phase with controlled dimensions and morphology into a PMMA matrix. These MAM copolymers show similar ductility in tension to the RTPMMA (strain to break around 70 %) and an even lower yield stress (around 25 MPa depending on the grade). MAM is found to show a lamellar morphology for PMMA contents, ∫PMMA < 75%, and deformation of this morphology results in extension of the PBA phase and formation of chevron-like regions of shear deformation. Even so, the fracture resistance remains relatively low in the first generation of these materials, KImax not exceeding 2 MPa.m1/2 at 1 m/s, and Charpy impact tests on notched samples showing relatively little improvement between MAM and the PMMA homopolymer (notched Charpy impact toughness of up to 2.25 kJ/m2 compared with 1 kJ/m2 for PMMA and 5.5 kJ/m2 for RTPMMA). The morphological observations and mechanical test results for MAM nevertheless point to the important role of the molar mass and ∫PMMA, and also to the interest of stripping, i.e. removal of residual BA monomer prior to polymerization of the MMA block. It has hence been possible to design and produce a second generation of stripped MAM copolymers with higher molar masses (Mn > 100 kg/mol) and an optimum ∫PMMA (< 65 %). These are found not only to show ductile behaviour in tension and excellent optical properties, but also notched Charpy impact toughnesses approaching 6 kJ/m2. Although this is still significantly less than for polycarbonate (12 kJ/m2), it demonstrates the promise of MAM for expanding the range of applications of MMA-based systems beyond that currently accessible to conventional RTPMMA.

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

Approche expérimentale du comportement mécanique des polymères en sollicitation rapide

In the more recent applications of technical polymers, they are frequently submitted to high strain rates. Being viscoelastic by nature, their mechanical properties are rate dependent. It is therefore important to investigate their properties over a wide range of strain rates. The aims of this study are twofold: i) to propose an experimental technique applicable to a large domain of strain rates, included those covering impact tests, and ii) to apply this technique to the study of the rate dependence of the mechanical properties of some technical polymers. In the first section, the deformation mechanisms of polymers, and the way in which they are influenced by strain rate, are reviewed. Some basics of fracture mechanics are also briefly presented. A detailed study of our original approach to the high rates tests then follows. This technique is based on the reduction of dynamic effects which often affect the measurements at high loading rates. Rather than impact tests, our technique is a quasi-static tests at high rates. This results from the use of a damper as coupling element between the servohydraulic piston and the specimen, together with the specific design of load and displacement detectors. The transient acceleration, responsible for the dynamic effects is decreased, but the high loading rates are maintained. The second section is devoted to the investigation of the rate effect on the tensile and rupture properties of polyoxymethylene (POM) and impact modified polymethylmethacrylate (PMMA). It is shown that high molecular weight POM is less sensitive to the increase of the loading rates than for lower molecular weights. The physical structure, which depends on the processing conditions, plays an important role as well. Results show greater embrittlement by increasing the loading rate than by lowering the temperature. Finally, a molecular model of fracture is proposed. PMMA modified by different fractions and morphologies of the rubbery phase are submitted to tensile and fracture tests at room temperature. Several stages in the micromechanisms of deformation are identified by transmission electron microscopy. The progressive disappearance of some of these stages leads to ductile to brittle transitions in some of the materials studied. The velocity at which these transitions occur depends on the fraction and on the morphology of the modifying phase. It is also shown that a spherical morphology of the rubbery phase is not necessarily required to improve the toughness, but that this latter depends on the interactions between the phases.

EPFL1996

The Influence of Matrix Modification on Fracture Mechanisms in Rubber Toughened Polymethylmethacrylate

Laure Dupuits Lalande, Jan-Anders Månson, John Christopher Plummer

The fracture behavior of composite rubber particle-toughened polymethylmethacrylate has been investigated over a wide range of test speeds, encompassing impact conditions. When the entanglement density of the matrix was increased and its glass transition temperature reduced by copolymerization, there were significant increases in the crack initiation and propagation resistance of the particle-toughened materials at low to intermediate speeds. At impact speeds, on the other hand, where crazing became the dominant matrix microdeformation mechanism in all the materials investigated, the fracture response of the copolymer matrix was closer to that of the polymethylmethacrylate homopolymer, and the toughening effect of the rubber particles was no longer effective in either case. This is discussed in terms of the onset of the matrix b transition, associated with the transition from shear to crazing, and the a transition of the rubber domains, both of which occurred in the temperature range immediately below room temperature in low frequency dynamic torsion measurements.

2006

Approche expérimentale du comportement mécanique des polymères en sollicitation rapide

EPFL1996

Highly filled polymer nanocomposite films derived from novel nanostructured latexes

Riccardo Ruggerone

EPFL2010