Polyméthacrylate de méthyle | EPFL Graph Search

Le poly(méthacrylate de méthyle) (souvent abrégé en PMMA, de l'anglais poly(methyl methacrylate)) est un polymère thermoplastique transparent obtenu par polymérisation en chaîne dont le monomère est le méthacrylate de méthyle (MMA). Ce polymère est plus connu sous son premier nom commercial de Plexiglas, marque déposée utilisée dans le langage courant comme antonomase, même si le en mondial du PMMA est Altuglas International du groupe Arkema, sous le nom commercial Altuglas. Il est également vendu sous les noms commerciaux Lucite, Crystalite, Hésalite, Perspex et Nudec. Synthèse Il se polymérise à l'aide de radicaux qui amorcent une polymérisation radicalaire en chaîne. Également possible, la polymérisation anionique, au moyen d'amorceurs nucléophiles de type carbanion. La polymérisation anionique doit se faire en milieu anhydre ; en effet, dans ce type de réaction, il n y a pas d'étapes de terminaison, la chaîne en construction continue à absorber des monomères jusqu

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

Polymer - Carbon Nanostructures Composites

Maryam Majidian

Synthesis and characterization of polymer-based composites containing carbon nanostructures is the focus of this dissertation. The main polymer which has been used for this research is SU8. The motivation of the work is to overcome the drawbacks of SU8 e.g. (high electrical and thermal resistance, brittleness) by addition of the carbon-based nanofillers. In this thesis, three classes of carbon nanostructures have been used as composite fillers: 2 dimentional graphene, 1 dimentional carbon nanotubes and 0 dimentional onion like carbons. The idea behind this material selection is to study the influence of dimensionality of the fillers on the electrical transport properties of the composites. In addition, we have performed comprehensive characterizations of graphene composites and addressed some questions about carbon nanotubes and nano onions composites. SU8 graphene composites were prepared using solution mixing method. The microstructure analysis of the composites showed a homogeneous dispersion of the graphene flakes. The study of electrical properties of the composite as function of the filler loading exhibited superior electrical conductivity compared to other graphene-base polymer composites. The study of the viscoelastic behavior of the composites showed that the rheological percolation is very close to zero, which we attribute to the polymer chain restriction due to high aspect ratio graphene fillers. The mechanical properties were evaluated with nanoindentation technique. 67 percent enhancement for Youngâs modulus and 75 percent enhancement for hardness were acquired. The possibility of the linkage between the filler and the matrix was investigated by spectroscopy techniques including Photoluminescence, Raman and Fourier Transform Infrared spectroscopy. Our findings suggest that covalent bonds are formed between SU8 and the functional groups on the surface of the graphene flakes. SU8-CNT composites were synthesized using both randomly dispersed and well-aligned tubes. For composites with randomly dispersed CNTs, the effect of nanotube length and polydispersity was investigated with experimental approach, for the first time. We have shown that the conductivity in such composites is proportional to mean length of CNTs, Ln, rather than weighted average length, Lw, which is predicted by theory. For case of aligned CNTs, we have measured the thermal conductivity, kappa, parallel and perpendicular to the orientation of the tubes, which exhibited an anisotropy close to 20 . The study was motivated by thermal management applications. From the same composite, lamellas with thickness ranged 20-100 nm were prepared using ultramicrotomy technique, for proton channeling applications. The successful sample preparation and pioneering channeling experiments give an encouraging outlook for future investigations in this field. Composites containing Poly methyl methacrylate as matrix and onion-like carbon as fillers were prepared for transport studies. The temperature and pressure dependence of the conductivity were measured. Due to the complexities associated to non homogeneous structure of these composites, we do not have a unified model to describe the dependence of conductivity upon concentration, temperature and pressure, and this question has remained open.

EPFL2016

Fiber bragg gratings in polymer optical fibers

Nico Gérard Harbach

Polymer optical fibers (POF) have received increased attention in recent years in the fields of data communication and sensing applications. The lower cost and higher flexibility are the main advantages of POF compared to silica fibers and make them interesting candidates for Fiber Bragg grating (FBG) sensor applications. FBG are convenient measurement devices for strain and temperature measurements, as they can be multiplexed within one fiber yielding a sensor array and the fiber can be embedded in structures. This work investigated the possibility of producing FBG in polymer fibers and their use as sensor units. It could be shown that using excimer laser irradiation at 308 nm, it is possible to write FBG in single-mode POF employing a standard phase mask, side writing technique. Index changes of up to 1.7 × 10-4 and reflectivies of up to 87% could be reached. The induced refractive index change due to pulsed UV irradiation was shown to be negative. The polymer material of the core (Polystyrene (PS) / Polymethyl methacrylate (PMMA)) was not sensitized prior to irradiation. During the grating formation an irradiation induced insertion loss of up to 11 dB/cm was observed. Excimer laser written FBG showed stability of over 9 months for approximately 40% of written FBG. Results of FBG writing using femto second laser irradiation showed FBG with reflectivities of up to 1.2 × 10-4, however these POFBG were not stable. The POFBG were characterized using optical low coherence reflectometry (OLCR) which enables to calculate the FBG location and length as well as the induced amplitude Δnac and mean refractive index change Δndc. Taking into account fiber and insertion losses, good agreement of these calculations and measurements were found. The results show that large variations of the induced index change result from irradiation. Local index change peaks of up to 4 times the mean value were observed, indicating inhomogeneity of the fiber material. Birefringence in the core of the POF (≈ 1.2 × 10-3) is up to a factor 3 higher than in the cladding which is due to the PS content within the core. The birefringence is due to inelastic strain and stress induced in the drawing process. Annealing, uniform irradiation and FBG writing using 308 nm excimer laser light induces a decrease of the absolute birefringence value. The changes upon irradiation are confined to the core of the fiber. Large variations in the initial and final birefringence before and after irradiation support the findings of the OLCR measurements indicating material inhomogeneities in the fiber. POFBG were found to be sensitive to relative humidity, temperature and strain. This is in contrast to glass fiber FBG which do not show humidity sensitivity. POFBG relative humidity sensitivity is non-linear with a change of up to 8 nm for a RH change of ≈ 100%. The non-linearity is introduced by a non-linear water sorption process. The POF grating response to temperature changes under dry conditions (1.5±1 % RH) is –10±0.5 pm/°C. The temperature response of the FBG submerged in water is –36±2 pm/°C due to an increased thermal expansion coefficient and a change in polarizability. Under ambient conditions the grating response to heating is typically ≈ –138 pm/°C, predominantly due to a change in POF swelling, i.e changes in relative humidity and POF water content. The diffusion coefficient of water in this POF at 23.5°C is 6.7 × 10-9cm2/s for sorption and 10 × 10-9cm2/s for desorption. Equilibrium of water content within the fiber and the surrounding air is typically reached after approximately one hour. Calculation showed that a reduction of the fiber diameter can increase this humidity sensitivity response time down to approximately 5 minutes for a fiber diameter of 25 µm.

EPFL2008