Composites and methods for their production

Intercalates and exfoliates formed from layered materials and dendritic polymers are disclosed, together with the uses of these materials, e.g. in producing composite materials. The dispersion of layered materials in a dendritic polymer matrix (hyperbranched polymers, star shaped polymers or star branched polymers) or has not been disclosed in the prior art. The high number of end-groups per molecule of dendritic polymer combined with their unique globular architectures leads to large intergallery spacings when intercalated in layered silicates and particularly facile exfoliation in spite of the high molar mass of the dendritic polymer. The excellent processability and potential high reactivity of dendritic polymers makes them extremely promising for thermoset applications, and coatings.

Low stress acrylated hyperbranched polymers

Lars Schmidt

The objective of this study was to investigate the behavior of highly functional acrylates, during isothermal ultraviolet (UV) curing. The materials included a pentafunctional acrylate and two acrylated hyperbranched polymers, one with a stiff polyester core and one with a more flexible polyether core. In particular, the influence of UV intensity and reactive blend composition on structural transitions, such as gelation and vitrification, and on the dynamics of internal stress was considered. Curing kinetics were studied with photo differential scanning calorimetry. The chemical conversion was analyzed using an autocatalytic model and a criterion for identifying vitrification directly from photocalorimetric experiments was proposed. It was observed that reactive blends containing HBPs had a higher conversion at vitrification, compared to the pure penta-functional acrylate. Strong intensity dependence of the maximum conversion rate and a weak intensity dependence of the ultimate conversion were observed. The latter was found to be controlled by the conversion at vitrification. The structural transitions and the modulus build-up during UV polymerization were determined by photorheology. A refined data processing algorithm was developed, that allows monitoring the shear modulus over 5 orders of magnitude within a short experimental time scale, with millisecond time resolution. Gelation – the liquid-solid transition – was found to be below 5 % conversion for all acrylates investigated. In contrast, the conversion at vitrification was strongly dependent on the actual monomer and increased with increasing UV intensity. The results of the photo DSC and the photorheology study were synthesized in the form of timeintensity- transformation diagrams. The dynamics of internal stress and cure shrinkage were studied using beambending and an interferometry-based method, respectively. The internal stress of the acrylated HBPs was largely reduced compared to the standard highly functional acrylate monomer. Moreover, in the case of one HBP with a polyester core and a reactive blend of the HBP with the standard highly functional acrylate, the stress reduction was obtained with a combined increase of Young's modulus, which was attributed to retarded modulus build-up and a higher final conversion. It was found that curing at a lower UV intensity led to earlier vitrification, hence earlier internal stress build-up, but limited maximum conversion thus limited final stress. Curing at a higher intensity led to later stress build-up but higher final stresses. Polymer microstructures were fabricated from the different acrylates in a photolithographic process and compared to SU-8, an epoxy frequently used for this kind of application. It was shown that the shape accuracy is linked to the processinduced internal stresses: the best result for thick and high aspect ratio microstructures – as used for example for microfluidic devices – was obtained for the acrylated HBP with a polyether core.

EPFL2006

Structure et mécanismes de microdéformation de polymethylmethacrylates renforcés au choc

Laure Dupuits Lalande

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.

EPFL2007

Low-Stress UV-Curable Hyperbranched Polymer Nanocomposites for High-Precision Devices

Valérie Geiser

The aim of this thesis was to investigate the process-structure-property relationships of UV-curable hyperbranched polymer (HBP)/silica nanocomposites. Special attention was paid to the interplay between photo-conversion, rheological behavior, shrinkage, and stress dynamics. This knowledge was used to maximize the shape fidelity and dimensional stability of imprinted nano-patterns made with these nanocomposites. Two different processing routes, resulting in different nanocomposite morphologies, were compared. The first and more conventional approach was ultrasonic, solvent-assisted mixing with solid silica nanoparticles followed by UV curing, which led to a discrete dispersion of silica particles in the matrix. The second and more novel approach was a dual-cure process combining UV and sol-gel processing with liquid tetraethyl orthosilicate. The sol-gel process, using a low-viscosity organometallic precursor, overcame the potential processing issue of the highly viscous particulate nanocomposites and led to a hybrid material with a homogeneous silica network at nanometer scale. Rheological analysis of silica nanoparticle suspensions up to the concentrated regime in the HBP showed an exponential increase of the viscosity with the particle fraction for well-dispersed discrete particle systems. A liquid-to-solid transition occurred in the 5 to 10 vol% range, which was correlated with an immobilized layer of polymer hydrogen-bonded to the silanol groups on the surface of the particles, as was confirmed by calorimetric analysis. Polymerization kinetics of HBP nanocomposites and hybrids were analyzed by means of photo differential scanning calorimetry using an autocatalytic model. A time-intensity-superposition principle with power-law dependence was established, which invalidated the classic radiation dose equivalence principle. Gelation and modulus build-up were monitored using photo-rheology. The calorimetric and rheological data were combined in the form of time-intensity-transformation diagrams. It was found that gelation was delayed with respect to conversion up to 36% when lower UV intensities were used, which favored reduce polymerization stresses. The dynamics of polymerization shrinkage and internal stress build-up were investigated using photo-hyphenated interferometry and beam-bending methods. The linear shrinkage of the HBP was as low as 4.5%, which was further reduced to 3.3% with the addition of 20 vol% nanoparticles. The residual stress of HBP/SiO2 nanocomposites was below 5.5 MPa. That is well below the level of standard non-reinforced resins, in spite of an increased stiffness. For the sol-gel hybrids with 20 vol% SiO2, stress reduction by 50% with simultaneous stiffness increase by 20% with respect to corresponding particulate composites was demonstrated. The coefficient of thermal expansion was also lowered by 30%. Finally, nanogratings with a period of 360 nm and a step height of 12 nm were fabricated using UV-nanoimprint lithography with a glass or nickel master in a low-pressure (max. 6 bar) and rapid (≈1 min) process. Stable gratings were imprinted in the composite material containing up to 25 vol% of silica nanoparticles, despite the high viscosity of such compositions. Thanks to the exudation of a HBP-rich surface layer, the shape fidelity in terms of period of the replicated patterns was within 98% of the master, with shape distortion increasing with internal stress. The obtained nanogratings were used as a substrate for grated high refractive index TiO2 waveguides and then applied as wavelength-interrogated optical sensors (WIOS). An immunoassay with the polymer WIOS showed the same ultrahigh detection sensitivity as the standard glass-based devices. Therefore, the novel polymer-based sensor should be useful to probe contaminants in liquids with concentrations as low as a few ppb.

EPFL2010