General Method for the Synthesis of Hierarchical Nanocrystal-Based Mesoporous Materials

Block copolymer templating of inorg. materials is a robust method for the prodn. of nanoporous materials. The method is limited, however, by the fact that the mol. inorg. precursors commonly used generally form amorphous porous materials that often cannot be crystd. with retention of porosity. To overcome this issue, the authors present a general method for the prodn. of templated mesoporous materials from preformed nanocrystal building blocks. The work takes advantage of recent synthetic advances that allow org. ligands to be stripped off of the surface of nanocrystals to produce sol., charge-stabilized colloids. Nanocrystals then undergo evapn.-induced co-assembly with amphiphilic diblock copolymers to form a nanostructured inorg./org. composite. Thermal degrdn. of the polymer template results in nanocrystal-based mesoporous materials. This method can be applied to nanocrystals with a broad range of compns. and sizes, and the assembly of nanocrystals can be carried out using a broad family of polymer templates. The resultant materials show disordered but homogeneous mesoporosity that can be tuned through the choice of template. The materials also show significant microporosity, formed by the agglomerated nanocrystals, and this porosity can be tuned by the nanocrystal size. The authors demonstrate through careful selection of the synthetic components that specifically designed nanostructured materials can be constructed. Because of the combination of open and interconnected porosity, high surface area, and compositional tunability, these materials are likely to find uses in a broad range of applications. For example, enhanced charge storage kinetics in nanoporous Mn3O4 is demonstrated here.

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

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

Couches minces nanoporeuses pour la réalisation de dispositifs optiques

Gaëtan Wicht

The physical-chemical properties of polymer films can be modified by embedding inorganic materials (e. g. inorganic nanoparticles) into the polymer matrix. Depending both on the material properties (e. g. nanoparticles concentration) and on the fabrication procedure, air pores can appear into such hybrid organic-inorganic matrices, thus creating nanoporous layers. At the end of the 90's, Ilford Imaging Switzerland GmbH was a pionner in the field with the development of a unique "roll-to-roll" process to coat such layers on large surfaces and on an industrial scale. First used as absorbing layers in the ink-jet printing of high quality photographs, the interest for such nanoporous films as optical films has been rapidly growing. In particular, the possibility of introducing air nanopores on demand in a polymer matrix make these layers the perfect candidates for the fabrication of optical films with a very low refractive index (n < 1.2). In optical devices based on thin film multilayers, such nanoporous materials can guarantee an excellent index matching between the air and a plastic substrate, thus acting as very good antireflection coatings. On the other hand, in a different approach, these nanoporous layers cans also yield a good index contrast between a guiding layer and its claddings, thus allowing the fabrication of good polymer waveguides. The goal of this thesis was then to demonstrate the optical potential of the industrial nanoporous layers fabricated by Ilford on flexible plastic substrates. First, both the morphology and the optical response of different nanoporous layers were thoroughly characterised with respect to the fabrication parameters. The impact of the fabrication process as well as of the nanoparticle type (silica (SiO2), bohemite (AlOOH) and titania oxyde (TiO2)), size and concentration were studied. While reflectance and transmittance measurements revealed the highly transparent nature of all these layers, ellipsometry measurements showed the large range of achievable refractive indices (1.15 < n < 2.0). With the possibility of coating several nanoporous layers one on top of the other, with independently adjusted thicknesses and indices, we designed and fabricated antireflection multilayers with graded-index, that, once coated on a plastic substrate successfully reduce its surface reflectance from 10 % to few 0.1 %. Besides the measurements, a theoretical model based on the transfer matrix method allowed us to analyse in details the response of our samples. We could thus show that the residual interface roughness is responsible for the loss of coherence of the light reflected at each interface, thus reducing the amplitude of the interference fringes in the reflectance spectrum and yielding an optical response almost constant over the entire visible spectrum. Finally, the nanoporous layers were also used as optically isolating layers for the fabrication of polymer optical waveguides. An original measurement method, based on the emission of a fluorescent dye dispersed inside the guiding layer, allowed us to clearly demonstrate the feasability of such guides. Propagation loss in the order of 1.5 dB/cm were measured, a very promising value that, after optimisation of both the material and the optical design, is likely to be further reduced in order to reach better performances than standard polymer waveguides and, eventually, to approach those of inorganic devices. Having highlighted the optical properties of our polymer nanoporous layers with the fabrication of two complementary optical devices based on polymer-nanoporous multilayers, this thesis clearly demonstrates their potential that, combined with their industrial nature, can guarantee them a very promising future, even brighter than that for similar prototype layers fabricated at a lab scale.