Biaxial orientation/sample

Normal incidence measurements are sufficient for uniaxially oriented samples, but a third spectrum along the ND (Y) is necessary to describe the orientation in biaxially oriented samples or in the case of uniaxial anisotropy in the thickness... 

In the case of PET the (100) and (—105) net planes are of special interest. The normals of the (100) planes are perpendicular to the planes of the benzene rings, while the normals of the (—105) planes are nearly parallel to the chain direction. The corrected pole figures of the two net planes are presented in Fig. 29. From these figures it can be seen immediately, that in the case of the investigated biaxially oriented sample the benzene rings are mostly aligned parallel to the film surface (see Fig. 29 a), while the chains exhibit a relatively broad distribution around the first drawing direction, MD (see Fig. 29 b). 

A biaxially oriented sample has three independent principal refractive indices, so the method can be adapted to give information about the degree of orientation with respect to the three principal axes of the sample. 

Biaxially oriented samples can be characterised by making tilted-film measurements somewhat similar to those made for the infrared characterisation of biaxial samples. 

The theory of the method is rather complicated, because the amplitude of Raman scattering is described by a second-rank tensor, so it will not be discussed here. Just like for fluorescence, P2 cosff)) and (P4(cosd)), or cos 6) and (cos" 6), can, at least in principle, be obtained for the simplest type of uniaxial orientation distribution and these values now refer directly to the molecules of the polymer itself. In practice it is often necessary to make various simplifying assumptions. For biaxially oriented samples several other averages can be obtained. 

For studies of biaxially oriented samples, the so-called XYZ technique is available where samples are studied for an incident ray propagating parallel to one of the three principal axes. For this purpose, it is necessary to use thin sections of the samples cut perpendicular to these axes. 

The above general analysis is readily extended to the case of biaxially oriented samples, as shown by Nomura et al If the orientation distribution is symmetrical with respect to the X1X2, XjJ and X2 3 planes, then biaxial orientation fimctions of the form sin cos2 may be obtained from measurements of A2—Aj)l A3+A2+Ai). Assuming that the distributions over 6 and are independent, then functions such as cos2 = 2cos —1 can subsequently be obtained, as shown by Stein. Owing to experimental difficulties, however, no applications of these analyses seem to have yet been made. For both uniaxial and biaxial orientation. 

One approach to characterising the molecular orientation in both uniaxially and biaxially oriented samples of PET makes use of the ratio of the absorption bands near 1250 and 1725 cm, the first of which shows parallel dichroism and the second perpendicular dichroism. An equation is developed that relates this ratio to the molecular orientation with respect to the direction of measurement. Thus, it is possible to determine individually the orientation functions with respect to the machine and transverse directions (131). 

The remanent polarization of the same 20 samples was also measured by measuring the amount of charge released by the sample when it was suddenly heated to above the melting temperature, about 175 C The remanent polarization of the 10 uniaxially oriented samples obuined this way was 0.04S 0.003 Om and for the 10 biaxially oriented samples it was 0.0S7 0.001 C/m ... 

In contrast to the above cases, the spectra in the four geometries are necessary for samples showing biaxial orientation. The information along the ND is accessible because under p-polarization, the electric field has components both in-plane (Z or X) and out-of-plane (Y) with respect to the sample. Although the details are beyond the scope of this chapter, Everall and Bibby have shown how to determine directly the different (P2mn) values (m, n — 0 or 2) by measuring the four absorbances of Equation (28) for two bands with different [j angles [32],... 

Specular reflection IR spectroscopy has been used by Cole and coworkers to study the orientation and structure in PET films [36,37]. It has allowed characterizing directly very highly absorbing bands in thick samples, in particular the carbonyl band that can show saturation in transmission spectra for thickness as low as 2 pm. The orientation of different conformers could be determined independently. Specular reflection is normally limited to uniaxial samples because the near-normal incident light does not allow measuring Ay. However, it was shown that the orientation parameter along the ND can be indirectly determined for PET by using the ratio of specifically selected bands [38]. This approach was applied to the study of biaxially oriented PET bottles [39]. 

Table IV gives the essential parameters which were involved in successful biaxial orientation of this copolymer sample. The most im-...

Materials. Biaxially oriented polypropylene (PP) films of 50 um thickness were obtained from 3M and have been described (2). PMDA-ODA (PI) was Kapton H polyimide from Dupont. Copper-plated PTFE films were obtained from Spire Corporation (Bedford, MA). They were prepared using the Ion Beam Enhanced Deposition (IBED) process in which a 100 nm thick Cu film was vapor-deposited onto a PTFE substrate in the presence of a beam of 400 eV Ar+ ions of 25 uA/cm2 (IQ). Shortly before SIMS analysis, the Cu film was removed slowly by peeling at 90° in ambient conditions. Metal-coated PI films were prepared by sputtering 50 nm Cr and 1 um Cu onto a 50 um thick Kapton film on both sides. Thermal annealing was performed in a vacuum chamber at 2xl0 6 torr using a quartz lamp as the heating source. The samples were held for 15 min at the desired temperature and then cooled down to ambient temperature inside the chamber for about 2 hours. Just prior to SIMS analysis, the metal films were peeled slowly at 90° and then immediately introduced into the vacuum chamber of the instrument. 

Samples. Polyethylene terephthalate (PET) samples are industrial (Mylar) biaxially oriented films (10 pm thick). Polymethyl-methacrylate (PMMA) has been spin coated on microscope glasses from a PMMA (1) / CHjClj (9) solution, giving thin layers of a few hundred A thickness. 

From Fig. 2a it is evident that as the test temperature raises from 0 up to 70 °C the resulting load-displacement curves display a decrease of the maximum load and an increase of the elongation at failure. On the other hand, as reported in Fig. 2b, when the displacement rate increases the load-displacement curves evidence an increase of the maximum load and, quite surprisingly, also the elongation at break increases. It is worth noting, that this rather unexpected rate effect on the load-displacement curves of notched samples has been already reported by Karger-Kocsis and co-workers for both biaxially oriented filled PET [7] and amorphous copolyester [9], and by Plummer et al. for polyoxymethylene tested at high temperature [14]. 

Commerically available samples of biaxially oriented polystyrene and SMA copolymer sheet material, having a thickness of 0.0381cm, were used in this investigation. It is generally recognized that crystallization under stress can enhance the tensile properties of a semi-crystalline polymer through a special arrangement of the crystalline portion ( 23). Therefore, the physical properties of the styrene-maleic anhydride copolymers chosen were compared to those of polystyrene produced in the same manner and are shown in Table I (24). 

Legrand et al. [100] reported F wideline NMR measurements on a 70/30 mol% VDF-TrFE random copolymer, made in order to study molecular motion both below and above the ferroelectric transition temperature, Tc. The samples consisted of semicrystalline copolymer films of 0.51 mm thickness, with biaxial orientation of the crystalline axis. The samples were rolled (without poling) at 70°C, with a draw ratio of 300%. The F resonance was chosen, rather than the proton resonance, because the abundance ratio of F to nuclei is 1.4. In addition, the F free-induction decay (FID) lasts longer than that of the proton, which decreases the influence of spectrometer deadtime. FID analyses were made assuming a simple superposition of two... 

In biaxial orientation a eylindrically symmetric orientation distribution no longer exists about a reference axis. Therefore, one needs to tilt the sample in order to obtain the orientation function with respect to a second reference axis such as the normal to the plane of a film or the normal to the transverse direction. Knowing two such functions for two orthogonal axes, one can calculate the third function for the third orthogonal axis by use of the direction cosines [Eq. (4)]. 

In Figure 6, one can see, as with amorphous materials, that the more unbalanced the biaxial orientation is the greater are the reductions in permeability. Interpretations of the transport data for biaxially and uniaxially drawn PET samples can be explained by observing conformational changes in the polymer backbone itself. Apparently, the chain packing efficiency of the amorphous phase improves as the number of trans isomers in the ethylene glycol unit increases. Polarized infrared analysis of uniaxially and biaxially oriented systems indicates that the fraction of... 

In another example, unoriented PVDF films were stretched at 150°C in the longitudinal direction at a stretch ratio of 3.5 to 1. A sample of the longitudinally oriented film was stretched in the transverse direction at 60°C at a stretch ratio of 3.4 to 1, thus producing a biaxially oriented film. Dielectric constant and loss factor values were measured for these films (Table 6.10). 

The absorption correction (correcting for the effects arising from variations in irradiated volume as well as in beam path length within the sample) is important, and one way to ensure its accuracy is to obtain a reference sample having the same constitution and shape as the test sample but known to be isotropic and to make sure that the intensity measured with this reference sample is truly constant after the absorption correction is applied. From the intensity /( ) or /( , ) obtained after the absorption correction, the pole distribution r( ) or t( , d>), for a sample of uniaxial or biaxial orientation, respectively, is evaluated according to... 

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