J. C. Merino

Deformation‐induced conformational changes in stretched samples of amorphous poly(ethylene terephthalate)

The combination of Fourier transform Raman spectroscopy and thermal analysis has been proved to be adequate for the study of the quantitative structural changes which take place in amorphous poly(ethylene terephthalate) on stretching. The two-phase conformational model previously proposed for annealed samples was applied to uniaxially stretched samples. The samples were deformed at different temperatures and speeds. Different conformer contents were found in the stretched sample depending on the stretching conditions. In general, deformation induces a conformational transition of gauche to trans. This behavior was lost at a narrow range of temperatures above the glass transition and at the lowest speeds, where the stretching seemed not to induce conformational changes. This different behavior has been associated to the high mobility of the polymer chains under these conditions, which imposes a different process of deformation marked by a high degree of chain slipping. This kind of deformation is present all along the sample and with the absence of necking in the uniaxial drawing process.

Microdomain structure and chain orientation in polypropylene/polyethylene blends investigated by micro-Raman confocal imaging spectroscopy

Compositional domain structure of blends of isotactic polypropylene with linear polyethylene and chain orientation of neat polymers and of the blends were assessed by micro-Raman confocal imaging spectroscopy and FT Raman spectroscopy. The results were correlated with polarised photoacoustic FTIR and with DSC. The surface and inner parts of compression-moulded, injection-moulded and drawn specimens were compared. Polymer domains in the blends and domains of different orientation were identified. The larger size of the polymer domains and lower chain orientation in the core of the specimens prepared by injection-moulding were explained by different cooling conditions of the melt. Possibilities and limitations of the micro-Raman confocal spectroscopy method were discussed.

Thermally induced structural changes in low-shrinkage poly(ethylene terephthalate) fibers

This work studies the behavior of low shrinkage PET fibers during free-ends thermal annealing. The interest in this type of sample stems from the fact that it possesses an interesting structure characterized by the presence of crystalline and amorphous domains both in a highly extended and oriented state. Furthermore, thermal annealing is not able to produce a significant increase in the crystalline content. Thus, the lack of crystallization allows to isolate the effect of chain recoiling on the observed phenomena. To follow changes at molecular and microstructural levels, Fourier transform infrared spectroscopy with photoacoustic detection, differential scanning calorimetry, wide-angle x-ray diffraction and scanning electron microscopy techniques were employed. By their use, substantial structural changes in the amorphous and crystalline domains were found which, finally, were related to the macroscopical behavior of the material, mainly the observed shrinkage and the mechanical properties.

Rheo-optical Raman study of chain deformation in uniaxially stretched bulk polyethylene

Abstract A rheo-optical study has been developed to examine the nature of the structural changes, other than the well known chain orientation, that can appear during uniaxial drawing of polyethylene samples. Stretching was performed at room temperature in samples of melt-crystallized high-and low-density polyethylene. Raman spectra were simultaneously collected along with stress and strain data in order to gain insight into the molecular basis of the properties resulting from the mechanical process. The need for real-time studies was to detect possible transient occurrences that cannot be observed with non-rheo-optical studies. These facts originate from the rapid relaxation phenomena of this polymer. By these means, a band shift and asymmetric band broadening were observed on the CC asymmetric stretching Raman mode (1064 cm −1 ), which has been attributed to some degree of backbone chain deformation. The band shift and, accordingly, the rate of bond deformation did not show a linear stress dependence. There is no substantial band shift during the first moments of the stretching. That initial zone was followed by a stage, during formation of the transition fronts, where the band shift begins to be appreciable. Finally, a third stage was observed when the observation zone falls into the neck. In that stage, a linear dependence was found between the band shift and the applied load. Qualitatively, the observed trend can be thought of as the consequence of the initial isotropic nature of the stretched sample. For that kind of sample, the transference of the load on the polymer backbone is not effective until the sample shows a high degree of orientation in the stretching direction. The band shifts were reversible as the load was removed.

Control Algorithm Development for Independent Wheel Torque Distribution with 4 In-wheel Electric Motors

Electronic Stability Control (ESC) devices are going to become mandatory in the near future. Moreover, the high price of the electronic sensors makes engineers to build its active safety systems based on the knowledge of a reduced number of real vehicle states, while the rest must be estimated from the real ones. In this case, the new possibilities offered by an independently controlled wheel traction system are explored. The advantages enable to easily implement anti-lock braking and traction control systems, chassis motion control like Direct Yaw Control (DYC) and an estimation of road surface condition. Hence, yaw moment reference signal is possible to be followed not only by braking, but also by giving an increasing torque to wheels in order not to lose any global vehicle velocity. Fault-tolerant strategies are also covered for the dynamic behaviour to be appropriately adapted to an electrical fault. An electronic differential strategy is implemented in both axles in order to develop an active help to curve performance.

Temperature dependence of microhardness indentations and dynamic mechanical moduli of polyesters in the vicinity of the glass transition

Etude de la similarite entre la brusque augmentation de la microdurete de vecteurs et le module de perte maximal a la temperature de transition vitreuse

Limited-supply diffusion in the liquid polystyrene–glassy poly(phenylene oxide) pair. Further results in extended times scale

Diffusion between a liquid polystyrene and a glassy poly(phenylene oxide) matrix is experimentally studied over a wide range of temperatures and diffusion times, using confocal Raman microspectroscopy. A specially designed experimental setup allows precise direct following of time evolution of the chemical composition profiles along the diffusion path. A direct and precise quantification is made for the experimental errors involved in two methods used for Raman measurements. An already proposed diffusion model is used to predict the time evolution of the advancing composition profiles along the diffusion path, and gives precise results. Experimental thermodynamic and kinetic data taken from literature are used for the model calculations, and excellent agreement with experimental results is obtained. Diffusion slow down is confirmed at the lowest diffusion temperature used, and probable causes are discussed.

Rheo‐Optical Raman Study of Chain Deformation in Uniaxially Stretched Bulk Isotactic Polypropylene

A rheo-optical system was used to analyse the nature of the structural changes, other than the well known chain orientation, which can appear during uniaxial drawing of bulk isotactic polypropylene samples with a spherullitic structure. The stretching was performed at room temperature and Raman spectra were simultaneously collected along with stress and strain data to gain insight into the molecular basis of the properties involved in the mechanical process. Some degree of backbone chain deformation is suggested based on observations of the wavenumber shift of some Raman bands, especially those with high contributions of skeletal vibrations. The dependence of the band shift on the applied stress was not linear and this is interpreted in terms of the different average orientation that the polymer shows during the stretching. The band shift and its associated phenomena were reversible as the load was removed.

Scanning electron microscopy and differential scanning calorimetry study of the transition front in uniaxially stretched isotactic polypropylene

This work discusses the potential of calorimetric (DSC) measurements to study the deformation process of semicrystalline polymer samples. It has been pointed out that parameters such as lamellar fragmentation and the parameter of intrachain melting cooperativity suffered significant changes, showing that the stretching process strongly modifies the structure of the isotatic polypropylene (iPP) sample. Furthermore, the observed changes are restricted to a small region between the isotropic and fibrillar regions which is called the transition front. By observing the evolution of the parameter of intrachain melting cooperativity in comparison with lamellar thickness, it has been possible to characterize, at least in a qualitative way, the origin and further evolution of the double-fold molecules ; totally extended polymer chain connecting two adjacent crystallites in the fibrillar structures-tie molecules. Thus, the procedure described here allows, in a simple way, the observation of these molecules, being a possible alternative to TEM methods. Finally, scanning electron microscopy helped in the interpretation of the calorimetric results, revealing that the intermediate values of the different measurements made in this work must be understood as the macroscopic average of the inhomogeneous mixture of the two different structures coexisting in the transition front, i.e., newly formed fibrillar structures and the remains of the original spherulitic structure.

Variable temperature and stretching cell for Raman spectroscopy studies of polymers

Abstract A variable temperature cell for use in laser spectroscopy studies of polymers and for the drawing of compounds is described. ‘In situ’ polarized Raman spectra of polyethylene at different draw ratios and different temperatures are recorded. The depolarization factors allow assignment of the vibrational types to the Raman active modes and the chain orientation study.