The Effect Of Processing Conditions On The Morphology, Mechanical And Electrical Properties Of Batio3-Polymer Composites

Ferroelectric ceramic – polymer composites have a unique mix of electrical and mechanical properties. Piezoelectric and pyroelectric activity, a wide range of dielectric constants and high breakdown strength are combined with mechanical flexibility, formability and low acoustic impedance. Furthermore their properties may be tailored by the judicious choice of the polymeric matrix and ceramic filler, of their volume fraction, and of the type of connectivity, making these materials very attractive for applications as embedded capacitors, sensors and actuators. Composites with 0-3 connectivity, i.e. a continuous polymer matrix filled with a ceramic phase discontinuous in all directions, are particularly interesting because of their relative ease of fabrication and processing. The functionality of these composite materials is mostly due to the dielectric, ferroelectric and piezoelectric properties of the ceramic phase, which therefore often needs to constitute more than 50% in volume. On the other hand the possibility of integrating these composites as embedded devices in host structures, which are often polymer based, or using them as efficient actuators depends greatly on their thermomechanical properties. Most of the studies on ferroelectric ceramic-polymer composites, however, focus on their electrical properties, and relatively few systematic studies on their thermal and mechanical behaviour have been published so far. Furthermore the ceramic volume fractions considered are often much lower than those used in most applications [1, 2], studies of highly filled composites mainly being limited to lead-based ferroelectric materials [3]. In the present study 0-3 BaTiO3 – polymer composites were prepared, containing up to 60% volume fraction of ceramic phase. BaTiO3 was chosen as the ceramic filler as it is a common piezoelectric material with high dielectric constant, having the additional benefit from an environmental point of view of being lead-free. For the matrix, a low viscosity epoxy resin and a polyvinylidene fluoride based polymer were chosen. The first is a thermoset resin commonly used for several applications, and the second is a thermoplastic semicrystalline polymer which displays piezoelectric activity. A common route for achieving very high loading of ceramic filler is the use of solvents to reduce the viscosity during processing. However, as the materials prepared via the solvent route were found to have poor mechanical properties, attributed to the presence of residual solvent, most of the effort focused on obtaining high ceramic volume fractions via a solvent free process. The thermomechanical, dielectric, ferroelectric and piezoelectric properties of the obtained composites were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. X-ray diffraction was carried out to determine the crystal structure of the BaTiO3 in the pure powder and in the composites, confirming that processing did not alter the crystallographic structure of the ceramic phase. Scanning electron microscope observation of fracture surfaces of the composites evidenced that a satisfactorily homogeneous dispersion of BaTiO3 in the matrix could be obtained, although aggregates with diameter of some micrometers and some porosity were detected. The thermal and mechanical behaviour of the composites was studied by differential scanning calorimetry and dynamic mechanical analysis. The glass transition temperature was found not to significantly depend on the amount of filler, while both the storage and loss moduli increased upon addition of BaTiO3. Furthermore, all the composites displayed a region of linear stress-strain behaviour smaller than the pure matrices. Finally, experimental results also evidenced the possibility to obtain low leakage current and high breakdown voltage.

Shape memory polymers with controlled time-dependent shape recovery

Charly David Azra

The aim of this thesis has been to investigate the use of heat-activated shape memory polymers (SMPs) as novel smart actuators for the controlled delivery of fluids and their suitability for applications such as low-cost drug delivery. On heating to temperatures in the vicinity of their glass transition temperature, Tg, amorphous SMPs are able to recover their equilibrium shape from a “programmed” temporary shape generated by mechanical deformation, and fixed by cooling to below Tg. This phenomenon is linked to the substantial increase in molecular mobility on heating through the glass transition, and the memory of an equilibrium shape associated a crosslinking network. While SMPs have received considerable attention in the literature, there has to date been relatively little focus on the time-dependence of shape recovery in these materials. There are nevertheless many potential biomedical applications, for example, in which it is necessary to control the actuation rate in SMP-based devices activated by natural heat sources such as the human body, whose temperature may be subject to significant fluctuations. The present work has therefore focused on the time-dependent shape memory effect under isothermal conditions and its sensitivity to temperature variations. The first part of the study involved commercial thermoplastic (physically cross-linked) and thermoset (chemically cross-linked) shape memory polyurethanes (SMPUs). This provided insight into the physical mechanisms underlying the time-dependence of the shape memory effect, which was found to correlate strongly with the viscoelastic behaviour of a given material and the thermomechanical variables associated with the shape memory cycle. It also revealed the importance of the width of the α-transition, ∆Tα , as determined by dynamic mechanical analysis (DMA), and its position on the temperature axis relative to the recovery temperature, Tr. In particular, it was shown that controlled recovery rates could be obtained if Tr corresponded to the onset of the α-transition, and a broad transition led to a reduction in the sensitivity of the shape recovery rate to thermal fluctuations about a given Tr. The next part of the study was devoted to the development of materials optimized for controlled shape recovery under well-defined conditions. The commercial formulations were found to be unsuitable for this purpose, because they offered relatively limited possibilities for tailoring Tg and ∆Tα . Thermoset SMPUs with tunable Tg and ∆Tα were therefore produced in house, based on polytetrahudrofuran (PTHF), 4,4’-diphenylmethane diisocyanate (MDI) and 1,1,1-trimethylolpropane (TMP). The starting point was a literature formulation, which was used to optimize a reactive injection molding (RIM) set-up designed to produce void-free parts with a molding time of less than 3 minutes. These SMPUs were characterized by a PTHF network linked by rigid domains made up of crosslinked MDI and TMP. This made it possible to vary Tg from -60 to 80 °C and ∆Tα from 35 to 100 °C, by varying the molecular weight of the PTHF from 2000 to 650 g/mol, and hence the cross-link density for a given stoichiometric ratio PTHF:MDI:TMP. In order to adjust ∆Tα independently of Tg, flexible segments were introduced to the rigid domains and to the junctions between the rigid domains and the PTHF, using a fourth component, 1,4-butanediol (BDO), or by replacing the TMP with ethoxylated TMP (eTMP). This provided a highly flexible platform for optimizing the shape memory performance. Moreover, the resulting SMPUs showed equilibrium moduli, Ee, of up to 13 MPa, which is relatively high for unreinforced thermosets, and clearly beneficial for fluid displacement where high recovery stresses may be important. To increase Ee further, alumina nanoparticles were dispersed in the SMPUs. This was achieved through ultrasonic, solvent-assisted dispersion of the alumina in PTHF and subsequent curing with the MDI and TMP. Reproducible, void-free, SMPU nanocomposite sheets with alumina contents as high as 15 wt% could be produced in this way within the 3 minute pot-life of the reactive mixtures. These showed a homogeneous dispersion of the alumina (aggregates less than 50 nm in diameter), as confirmed by transmission electron microscopy, leading to a modest increase in Ee to about 21 MPa. This indicated reinforcement to be essentially a geometrical effect (strain amplification), suggesting relatively limited matrix/filler interactions. On the other hand, Rr remained high in these nanocomposites (a decrease of only 5 % on addition of 15 wt% alumina), in contrast with reports in the literature of a significant degradation in shape memory performance for SMPUs containing highly interacting fillers. A linear thermo-viscoelastic model was used to simulate the small strain shape memory effect. Continuous relaxation and retardation spectra, H(τ) and L(τ) respectively, were used to model the time-dependence, and a shift factor, aT(T), was used to model the temperature dependence. To streamline the procedure, a novel characterization method was developed for the determination of aT (T ) from a constant frequency DMA temperature sweep, based on the establishment of an approximate relationship between the storage modulus, the loss modulus and aT (T ). It was hence possible to generate continuous data for aT (T ) from a single DMA run, avoiding the time-consuming and operator-dependent construction of a mastercurve from multiple experiments. Using the Havriliak-Negami parametric functions, H(τ) and L(τ) could be subsequently obtained over a large number of decades of time from the modulus and compliance data. Comparison with results from experimental shape memory data for different model SMPUs, showed this approach to provide a reliable indication of the long-term shape recovery rate. Moreover, this method has great potential for providing rapid insight into the temperature-dependent viscoelastic properties of thermorheologically simple polymers in general, regardless of whether they are being considered specifically for shape memory applications. In the final part of the study, a tailored SMPU nanocomposite formulation was used to produce demonstrators in the form of active shape memory reservoirs, capable of expelling a liquid content under isothermal conditions. The reservoirs consisted of circular membranes (the equilibrium shape), inflated at elevated temperature to form 5 ml blisters and subsequently stabilized in this form by cooling (the programmed shape), using a biaxial deformation set-up incorporating temperature control. This innovative programming procedure made it possible to generate active SMPU reservoirs that could be activated at the skin temperature (32 °C), resulting in low average flow rates ranging between 1 and 5 μL/min over several hours. Such as a device has important potential applications, e.g. skin-temperature activated, low-cost transdermal drug delivery patches.

EPFL2013

Time dependence of material properties of FRP composites in fire

Thomas Keller

Temperature dependence of physical and mechanical properties is considered in most thermal and mechanical response models for composite materials. Based on thermogravimetry analysis, differential scanning calorimetry, and dynamic mechanical analysis conducted at different heating rates, it is demonstrated that thermophysical and thermomechanical properties are not univariate functions of temperature, but also functions of time. Temperature- and time-dependent models for physical and mechanical properties at elevated and high temperatures are proposed, which show good accordance with experimental results. Based on a finite difference method, complex realistic thermal loading programs such as the ISO fire curve can be taken into account. © 2009 SAGE Publications

2009

Processing of Sustainable Cellular Biocomposites

Carole Boissard

Petroleum-based polymers and composites are commonly used in a wide variety of application fields. However ecological concerns, increasing oil prices and dwindling natural resources explain why nowadays industrial and public interest is more than ever focused on new bio-based – “greener”- products. In order to find viable substitutes for petroleum-based materials, major efforts are devoted to the development of sustainable solutions combining fully compostable matrices and fibres from renewable resources. Polylactide (or polylactic acid) (PLA) is currently one of the most versatile biodegradable polymers derived from renewable resources such as non-food crops of corn or sugar cane. Its transformation into foamed cellular structures would decrease its density and at the same time may improve its specific mechanical properties. This could provide cost-effective and sustainable products for packaging and construction. In the context of developing “green” foams, the focus in the present work has been on a solvent-free process based on physical foaming with supercritical carbon dioxide (ScCO2) as a foaming agent. This foaming process includes thermally driven but kinetically controlled phenomenon. Solubility and diffusion of the gas in the molten polymer, cell nucleation and growth, as well as stabilization of the final foam structures were studied for neat and composite materials. At low processing temperatures and low depressurization rates, homogeneous structures were obtained thanks to a balance between cell growth mechanisms and cell stabilization, thus limiting uncontrolled cell coalescence. The density of the resulting polylactide foams ranged from 0.12 to 0.3 g/cm3 with corresponding compression moduli of 6 to 73 MPa respectively. The processing windows were strongly dependent on the rheological melt properties of the PLAs. PLA is generally considered to be a high strength, high modulus thermoplastic but its brittleness has limited its industrial use. PLA was therefore combined with wood fibres (WF) and microfibrillated cellulose (MFC) in order to improve its mechanical performance without compromising its environmental impact. Two compounding methods, a wet mixing papermaking-like process and a solvent casting process were investigated for the production of WF/PLA and MFC/PLA biocomposites. Degradation of PLA occured during wet-mixing, due to the water used as a dispersant, and to several heating steps applied during compounding. Improvement of the drying and processing conditions and/or the replacement of water by isopropanol limited this degradation. The presence of cellulose fibres considerably modified foaming mechanisms; gas solubility was decreased and the coefficient of diffusion increased, leading to lower quantities of CO2 available for foaming. In addition, stiffness of the fibre network impeded full expansion of the polymer matrix. Thus smaller cells were created. Densities from 0.15 (neat PLA) to 0.37 and 0.55 g/cm3 and moduli ranging from 35 (neat PLA) to 125.5 and 373 MPa were obtained by the addition of 5wt% WF and MFC respectively. Addition of a chain extender enabled viscosity of the matrix to be increased causing increased orientation of the fibre network during flow and hence fibre alignment in the cell walls. Density was thus decreased by 95 % to 0.017 g/cm3 and compression modulus decreased to 0.32 MPa for 5wt% WF composite. Cell wall shrinkage was observed in the neat PLA foams obtained with the chain extender. This was reduced by adding wood fibres while maintaining similar density and modulus. Cellulose fibres act as an extra scaffold in the cellular structures. Industries are currently considering the use of these materials for applications in packaging e.g. as cushioning materials, or in architecture as display panels, or as sandwich core materials for the replacement of expanded polystyrene, polyvinyl chloride or polyurethane foams.