T. Jawhari

Structural analysis of injection-moulded semicrystalline polymers by Fourier transform infra-red spectroscopy with photoacoustic detection and differential scanning calorimetry: 1. Poly(ethylene terephthalate)

Abstract Fourier transform infra-red spectroscopy coupled with photoacoustic detection (PA FT i.r.) has proved to be a useful tool for finding out about quantitative structural changes in solid materials. Poly(ethylene terephthalate) (PET) is a well known semicrystalline polymer that shows important changes on annealing. In order to obtain a complete picture of isomer distributions in industrial PET samples, spectroscopic measurements were correlated with differential scanning calorimetry (d.s.c.). The results obtained indicate that the structural characteristics of the thermally treated samples are related to the fabrication process. Two different strata in the plates can be distinguished: a skin layer and the core. The correlation between the apparent degree of crystallinity of the surface obtained by d.s.c. and the percentage of trans isomer obtained by PA FT i.r. allows the latter parameter to be separated into crystalline and amorphous trans isomer and to follow its evolution with the annealing process. Amorphous trans isomer vanishes at the primary isomerization (∼ 100°C) while crystalline trans and gauche conformations show sigmoidal evolution. At higher annealing temperatures (> 140°C) the ordered trans conformation shows an approximately linear increase at the expense of the gauche conformation. Finally, a direct correlation between the ordered trans isomer and the apparent degree of surface crystallinity can be made.

Structural analysis of poly(ethylene terephthalate) reinforced with glass fibre: 1. A photoacoustic Fourier transform infra-red study

Abstract Fourier transform infra-red spectroscopy coupled with photoacoustic detection (PA-FTi.r.) has proved to be a useful tool for finding out about quantitative structural changes in glass-fibre-reinforced poly(ethylene terephthalate) (PET) obtained by compression moulding and subsequently annealed. Analysis of the photoacoustic parameters indicates that the photoacoustic characteristics of the composite are very similar to those of the isolated matrix. It was demonstrated that glass-fibre-reinforced PET satisfies the same two-phase conformational model as the matrix. In contrast to the annealed matrix, the primary and secondary isomerizations cannot be detected in the composite. Therefore, the thermal treatment on the composite samples does not show further conformational changes. As a result, the manufacturing process seems to determine the composite microstructure, and its effect is quite similar to that observed in annealed matrix at the highest temperatures. Consequently, it can be considered that the trans isomer content in the amorphous phase is negligible in the composite.

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.

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.

Micro-Raman spectroscopy study of the process of microindentation in polymers

The residual deformation produced by Vickers microindentations in poly(3,3-dimethyl oxetane) (PDMO) was analysed by Raman microspectroscopy. Microstructural variations were observed as the Raman spectra were recorded at different positions along and out of the microindentation. A gradual distribution of the monoclinic and orthorhombic structures arises as the spectra were scanned from the centre to the edge of the impression area. This crystalline variation in PDMO, which reflects the degree of stress present on the surface of the sample, was utilized to obtain information on the distribution of pressure in and around the microindentation. In addition to the clear variation of pressure inside the permanently deformed region, no crystalline transformation was noted out of the impression area. Similar features were observed when the Raman spectra were recorded along one of the diagonals of the indentation.

Micro-Raman mapping of the transition region in the neck region of stretched poly(vinylidene fluoride)

A micro-Raman mapping of the transition region corresponding to the non-oriented to oriented transformation zone in the neck was obtained for stretched poly(vinylidene fluoride) (PVF2). Since Raman spectroscopy is a technique sensitive to the change in the crystalline structure in PVF2 when this polymer is submitted to a stress, the distribution of the crystalline modification along the neck was obtained from the analysis of the micro-Raman mapping. The results show a gradual increase in the degree of crystalline phase transformation along ≈200 μm in the stretching direction.

Raman mapping of the microdeformed zone produced by Vickers and Knoop microindentation techniques in poly(vinylidene fluoride)

Abstract Raman microspectroscopy was used to follow the crystalline phase transformation produced when Knoop and Vickers microindentations are made in poly(vinylidene fluoride). An evolution of the degree of crystalline phase transformation was observed along the microindentations which can be schematized into three regions. A high gradient in the degree of crystalline transformation was detected near the centre of the indentation, whereas Raman microanalysis of the region near the edges of the impression did not indicate a significant variation in the crystalline phase modification. An explanation for these observations is proposed.

Damage of polymers studied by micro-Fourier Transform Raman spectroscopy

SummaryThe potential of the micro-Fourier Transform Raman tool in examining specific localized regions in polymeric materials with some degree of fluorescence when analyzed by conventional Raman spectroscopy is examined. Analysis of characteristic bands of the vibrational spectra obtained in a small area damaged by a visible and NIR laser beam in commercial Polyethylene Terephthalate (PET) shows a different conformer ratio than that observed in a non irradiated zone.

Application of a new system controlled by computer to measure in real time microhardness on LLDPEs

Abstract A new microhardness testing technique is used on polymers which enables the indentation process to be continously examined in real time, during the on-load period and following the removal of the load. Time-dependent viscoelastic behaviour on different linear low density polyethylenes (LLDPEs) is analysed and the results obtained are correlated to the structural characteristics of the different samples studied.