Dynamic Properties of Materials for Alpine Skis

The aim of the present research has been to quantify the influence of different materials on the global dynamic response of a ski. The properties of the individual constituent materials were characterized as a function of temperature and frequency using dynamic mechanical analysis. At the same time, the dynamic behavior of skis with different designs was investigated in a cold room at between -15 and 25 °C using specially developed apparatus. The results indicated the overall behavior to be influenced significantly by the polymeric topsheet, which showed a strong damping peak at about 0 °C. Elasticity-based FEA accounted well for the experimental results for the two first flexural vibrational modes of the skis. For higher modes, however, the viscoelastic nature of the polymeric components led to significant discrepancies between the predicted and observed behaviors.

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

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

Sara Dalle Vacche, Yves Leterrier, Jan-Anders Månson, Véronique Michaud

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.

2010

Adaptive composite structures for tailored human-materials interaction

Christian Fischer

Controlling the static and dynamic properties of sports equipment has since long been of great concern in the sports engineering industry. More recently there has been interest in better understanding and improving the interplay between such properties and the perception of the athlete using the equipment. The aim of this thesis was first to develop a new approach to identifying the corresponding requirements for a particular type of sports equipment. The tailoring of a novel material with the potential to fulfill these requirements, and the integration of this material into composite structures was then investigated. Alpine skiing was used as a case study for this thesis. During this first project phase, the importance of specific ski properties was identified. Perceived characteristics were linked to physically measurable properties, and the level of differences in mechanical properties discernable by skiers was examined. For this purpose, four pairs of slalom skis that differed in flexural rigidity by a maximum of 20 % and in torsional rigidity by 60 % were tested by advanced and expert skiers under specific snow conditions. Among other results, Spearman correlations showed that expert skiers considered a high torsional rigidity to have a positive effect on characteristics such as quietness at high speed or energy restitution in carved turns, and advanced skiers judged stiff skis in torsion to positively influence the grip during short turns. Furthermore, it was also shown that athletes are able to discern small differences in damping and vibration of the skis. These results demonstrated that a quantitative approach to perceived properties is possible, and that it should be considered for the future design of sports equipment. Stiffness and damping usually counter-balance one another. The need for high values of both properties, as for example in skis and other types of sports gear where shocks induce large deformations, presents a considerable challenge for materials scientists. Shear-thickening fluids (STFs) are exceptional in this perspective, because they reversibly increase their moduli by 2-3 orders of magnitude, and simultaneously increase their energy absorption, when a critical shear stress or strain is reached. The dynamic shear-thickening behaviour of two different types of STFs was investigated, with the aim of integrating them into dynamically loaded structures. Dynamic frequency sweeps showed that the complex viscosity at different imposed strain amplitudes followed a unique power-law type behaviour up to the onset of strain thickening. Similar behaviour was also observed in the post-transition state, i.e. the viscosities again superimposed at frequencies beyond the transition frequency. Furthermore, the shear-thickening effect considerably decreased with increasing frequency. Viscosity increased by 3 orders of magnitude at 0.1 rad/s, and by 1 order of magnitude at 100 rad/s, suggesting that shear-thickening would totally vanish at frequencies beyond those measurable by conventional methods. Depending on the composition of the STFs, they either thickened at a constant shear stress throughout the frequency domain (monodisperse STFs) or required a minimum shear strain to be activated at high frequencies (polydisperse STFs). Results from dynamic rheological measurements for the STFs were correlated with results from vibrating beam tests (VBT) on model sandwich structures containing layers of the same STF sandwiched between solid beams. At the highest STF concentration, continuously increasing the excitation amplitude led to a 9-fold increase of the damping of the 1st resonance frequency. Since the increased viscosity of the STF interlayer resulted in improved stress transfer between the stiff polymer layers, the flexural rigidity simultaneously increased by over 20 %. Also, it was shown that the extent of damping and stiffening of the structure could be tailored by varying the type of STF. Finally, methods to integrate STFs into composite structures at different scales have been developed. At macro-scale, a STF damping device showed that the latter could strongly affect the dynamic behaviour of a ski. At meso-scale, the most promising design was based on glass fibre-epoxy rods integrated into the structure such that they function as small pistons when the structure is bent. For a tailorable deformation and frequency domain, the STF at the rod-matrix interface thickened, increasing the stress transfer between the rods and the matrix, and thus leading to changes in the structure's dynamic response. Dynamic mechanical analysis of soft matrix composites, containing STF at the rod-matrix interface, demonstrated that STFs could be integrated at the micro-scale of composite materials, which was confirmed by a micro-mechanical model. In summary, this thesis has provided a tool to include human perception into the design of sports equipment, and has built up a broad knowledge of the dynamic behaviour of an adaptive material, which can be included into a material-structure-athlete loop with the aim of evaluating its potential for any specific application in sports where comfort, feel and control of the performance should be improved.

EPFL2007