D.I. Bower

Characterization of biaxial orientation in poly(ethylene terephthalate) by means of refractive index measurements and Raman and infra-red spectroscopies

Abstract The three principal refractive indices of a film of poly(ethylene terephthalate), prepared by drawing an amorphous melt-extruded sheet at constant width between rollers whose speeds were set to give a nominal draw ratio of 3.5:1, were found to be 1.645, 1.573 and 1.541. Thirty-six measurements of the scattered intensity for the 1616 cm−1 Raman line were made for various combinations of orientations of the electric vectors of the incident and scattered light with respect to the sample and six absorbances were determined for each of the 1017 and 875 cm−1 infra-red peaks with the polarization vector of the radiation in different orientations with respect to the sample. From the Raman measurements five parameters Pr200, Pr220, Pr400, Pr420, and Pr440 which characterize the distribution of orientations of the benzene rings were deduced. Values of Pr200 and Pr220, in good agreement with the Raman values, were also deduced from the infra-red measurements, and a further two parameters, Pr202 and Pr222 were deduced from infra-red and Raman results combined. The four quantities Pr200, Pr220, Pr202 and Pr222 are shown to be consistent with the refractive index measurements using either of two alternative models for the chain conformation. The results indicate that there is slight preferential orientation of the chain axes out of the plane of the film, an unexpected result, but that there is also some preferential orientation of the planes of the benzene rings towards the plane of the film, in agreement with earlier X-ray measurements on similar films.

Molecular orientation and conformational changes due to uniaxial-planar deformation of poly(ethylene terephthalate) films

Abstract Conformational changes and chain axis orientation in poly(ethylene terephthalate) due to uniaxial-planar deformation (at constant width) at constant engineering stress in the range 1.2–5.5 MPa at temperatures in the range 85–105°C have been studied by means of infrared spectroscopy. For all samples the state of orientation was frozen just after the deformation reached the equilibrium value at a draw ratio λ p . For a given temperature there are two stress regimes, a lower one where λ p decreases with increasing stress, corresponding to ‘stretching with flow’, and a higher one where λ p increases with increasing stress, corresponding to ‘crystallizing stretching’. It is shown that the crystallinity of the samples is uniquely related to the content of trans conformers and that the overall chain axis orientation may be determined, to a good approximation, by determining the orientation of the trans conformers; the chain axes associated with gauche conformers are effectively randomly oriented. Under conditions of ‘crystallizing stretching’ the content of trans conformers and the chain axis orientation can be understood in terms of a rubber-like network extension if the drawing temperature is between 80°C (the glass transition temperature) and 95°C. At higher temperatures of drawing, relaxation occurs in the amorphous phase. For all samples there is an excellent correlation between the trans concentrations and the overall orientation, independent of whether relaxation or crystallization occurs.

Development of molecular orientation in sequentially drawn PET films

A study has been made of the transverse stretching of poly(ethylene terephthalate) film previously drawn longitudinally at constant force and width. The crystalline phase has been characterized using wide-angle X-ray diffraction: the orientation of the chain axis and the normal to the phenylene ring have been determined, as well as the crystallite size. Infra-red spectroscopy and refractive index measurements have been used to characterize the orientation of the material averaged over the crystalline and amorphous phases. The results showed that the crystals, which were preferentially aligned with their chain axes along the first stretching (longitudinal) direction, break down during the second drawing and that recrystallization takes place to produce crystallites with their c-axes aligned along the transverse stretching direction. The observed increase in the tendency for the normal to the phenylene ring to be normal to the film plane correlates with the increase in crystallite size along the chain axis for crystallites oriented with their chain axes in the transverse direction.

Raman spectroscopy of stressed samples of oriented poly(ethylene terephthalate)

Abstract Measurements of the shift and change of shape of the 1616 cm−1 Raman scattering peak of two moderately oriented samples of poly(ethylene terephthalate) (PET) under tensile loads of up to 0.2 GPa are reported, together with the corresponding strains. To obtain reproducible results load cycling procedures were adopted similar to those established for the study of viscoelastic behaviour. The Raman scattering was observed with polarized incident and scattered light, with the polarization directions either both parallel or both perpendicular to the draw direction in the samples. The results showed that for both samples the Raman shift was linearly related to the applied stress below the yield point. Up to the yield point very little change of line width was observed but above the yield point the width increased significantly. Differences in both widths and shifts were observed for the two polarization directions at the same stress level. The results are discussed in terms of the usual assumptions that the shift of the line gives a measure of the average stress in those chains which predominantly contribute to the peak and that the width and shape of the line give information about the spread of stresses. It is concluded that the technique can give useful information about the molecular stress distribution in thick samples of moderately oriented PET under load, including information about the different stress distributions on chains at different angles to the draw direction.

A study of the CCl stretching region of the Raman spectrum of PVC

Abstract The C-Cl stretching regions of the Raman spectra of a number of samples of PVC prepared by various methods which lead to differences in tacticity, conformational content and crystallinity have been studied. It is shown that the spectra of all samples can be fitted well with a set of nine Lorentzian peaks whose positions and widths are the same for each spectrum. Two of these peaks are attributable to the A g and B 3 g species vibrations of crystalline material and the remaining seven may be assigned with reasonable confidence to specific structures in the amorphous regions, three of which are associated with isotactic placements. The area under the A g and B 3 g peaks may be used to estimate the crystallinity of the samples and the area under the peaks assigned to isotactic material may be used to estimate the tacticity of the polymer.

Benzene ring orientation in uniaxialplanar poly(ethylene terephthalate) films

Abstract A study is presented of the benzene ring orientation in a series of eight films of poly(ethylene terephthalate) drawn at constant width under constant force above the glass transition temperature. For each sample, seven orientation moment averages, Prlmn, for the overall benzene ring distribution were determined from a combination of Raman and infrared spectroscopic measurements. Comparison of orientation moment averages calculated with and without the assumption of a uniaxial Raman tensor, and with and without a refractive index correction to the Raman intensities, leads to the conclusion that the seven Prlmn are given, to a good approximation, by the simplest calculation assuming a uniaxial Raman tensor and neglecting the refractive index correction. The seven Prlmn were used to calculate the ‘most probable’ distribution function and the isometric projections of this distribution have been produced. The results indicate that the C1–C4 axis shows an approximately uniaxial overall distribution with a slight tendency to be more oriented out of the plane of the film and that the benzene rings tend to lie in the plane of the film. From a combination of these results with those of earlier X-ray measurements, the amorphous and crystalline contributions to the overall molecular orientation were obtained. The results show that the benzene ring planes are preferentially oriented towards the plane of the film only in the crystalline phase and have an almost uniaxial distribution around the draw direction in the amorphous phase. It has been shown that, in the amorphous phase as well as in the crystalline phase, the benzene rings tend to lie in the plane defined by the draw direction and the C1–C4 axis.

Molecular orientation in uniaxial, one-way and two-way drawn poly(vinyl chloride) films

Abstract Uniaxially oriented poly(vinyl chloride) films drawn at 90, 100 and 110°C to draw ratios in the range 1.5–3.5 have been investigated using i.r. spectroscopy and refractive index measurements. Their 10 s isochronal extensional creep moduli and dynamic tensile moduli have also been measured. One-way drawn films with draw ratios in the range 1.56 × 1–2.56 × 1 and simultaneously and sequentially two-way drawn films with draw ratios in the range 1.46 × 1.46–1.79 × 1.79 have been investigated using refractive index measurements and i.r. spectroscopy. These films were drawn at 85 and 95°C. Thermal analysis of the drawn samples showed that the uniaxially drawn films were drawn at temperatures well above their final glass transition temperatures, whereas the one-way and two-way drawn films were drawn close to their glass transition temperatures. The results show that the crystallites are more highly oriented than the amorphous chains and that one-way drawn samples depart slightly from uniaxiality. No significant difference in orientation was found between simultaneously and sequentially two-way drawn samples. It was shown that the i.r. peak at 613 cm −1 , attributed to short syndiotactic segments, could be used to characterize the orientation of the amorphous chains. The elastic moduli of the uniaxially drawn samples were found to depend on orientation in a similar way to that previously reported.

The determination of molecular orientation in oriented polypropylene by wide angle X-ray diffraction, polarized fluorescence and refractive index measurements

Abstract Orientation studies have been undertaken in uniaxially oriented drawn tapes of isotactic polypropylene, using the techniques of wide angle X-ray diffraction, polarized fluorescence and optical birefringence. The results have been interpreted on the basis of a simple two-phase model for orientation of the crystalline and noncrystalline regions. The rise in the orientation of the crystalline regions with increasing draw ratio is extremely rapid, even compared with the pseudo-affine deformation scheme. This result, taken in conjunction with the discontinuous change in the long period, supports the view that the plastic deformation involves a complete breakdown of the original lamellar structure and subsequent recrystallization of the highly oriented drawn material. Further support for this conclusion is obtained from the observation that the orientation of the long fluorescent probes, which might be expected to be constrained to the more highly oriented material, is similar to that measured for the crystalline material by X-ray diffraction.

Molecular orientation in uniaxially drawn poly(aryl ether ether ketone): 1. Refractive index and X-ray measurements

Abstract The development of orientation has been studied in samples of poly(aryl ether ether ketone) (PEEK) drawn at 150°C from initially amorphous material at an initial strain rate of 40% min−1 to draw ratios λ = 1–3.24. Two sets of samples were made starting from film of thickness 7 or 300 μm. Refractive index measurements were made on both sets and detailed X-ray measurements were made on the thick set, with more limited studies of the thin set. The principal polarizabilities and refractive indices of the unit cell were calculated and used to derive the overall orientation average 〈P2(cos ϑ)〉 for the chains, where ϑ is the angle between the chain axis and the draw direction. X-ray data for the (110), (200) and (002) reflections confirmed that the crystallites were randomly oriented around the chain axis and provided values of 〈P2(cos ϑc)〉, fc and the fractional crystallinity determined from density measurements were used to calculate values of 〈P2(cos ϑam)〉=fam for the amorphous regions. The two sets of samples oriented in an identical manner, with 〈P2(cos ϑ)〉 rising to 0.6 for λ = 3.24. For λ>1.75, fc was constant at ∼0.97. At λ = 1.75 the crystallinity was approximately 4% and rose to a maximum of ∼30% at λ = 3.24. The thin sample drawn to λ = 2.92 was annealed at 300°C and developed a crystallinity of 45% and a 〈P2(cos ϑ)〉 of 0.78. The development of molecular orientation in PEEK is not described well by either of the commonly used deformation schemes but is probably basically a pseudo-affine process modified by crystallization.

Orientation distribution functions for biaxially oriented polymers

Abstract The use of the ‘most probable’ distribution function previously introduced for the prediction of orientation distribution functions for uniaxially oriented polymers is extended to biaxial polymers. Isometric projections of plots of the functions for model distributions are given, together with the application to a set of experimental data.