Polyethylene terephthalate molecular orientation

In this review the definition of orientation and orientation functions or orientation averages will be considered in detail. This will be followed by a comprehensive account of the information which can be obtained by three spectroscopic techniques, infra-red and Raman spectroscopy and broad line nuclear magnetic resonance. The use of polarized fluorescence will not be discussed here, but is the subject of a contemporary review article by the author and J. H. Nobbs 1. The present review will be completed by consideration of the information which has been obtained on the development of molecular orientation in polyethylene terephthalate and poly(tetramethylene terephthalate) where there are also clearly defined changes in the conformation of the molecule. In this paper, particular attention will be given to the characterization of biaxially oriented films. Previous reviews of this subject have been given by the author and his colleagues, but have been concerned with discussion of results for uniaxially oriented systems only2,3). 

In this review recent theoretical developments which enable quantitative measures of molecular orientation in polymers to be obtained from infra-red and Raman spectroscopy and nuclear magnetic resonance have been discussed in some detail. Although this is clearly a subject of some complexity, it has been possible to show that the systematic application of these techniques to polyethylene terephthalate and polytetramethylene terephthalate can provide unique information of considerable value. This information can be used on the one hand to gain an understanding of the mechanisms of deformation, and on the other to provide a structural understanding of physical properties, especially mechanical properties. 

Sloane, Boerio and Koenig, and McGraw have described the sampling and other instrumental considerations for Raman spectra of polymers (144). Other reports on Raman investigations of polymers include molecular orientation in bulk polyethylene terephthalate (145). crystallinity of ethylene-propylene rubber (146). and the structure of unsaturated polyester resins cross-linked with styrene (147). 

The variation of compliances with draw ratio for cold drawn polypropylene filaments examined at 20°C appeared very similar to that of high density polyethylene, with an increase in all compliances but Sii, which was insensitive to draw ratio. Ward aggregate theory was not applicable except for low draw ratios, implying that other processes intervened in addition to an orientation of pre-existing units. It was probable that even above the glass transition temperature increasing orientation led to a reduction in molecular mobility, as was known to occur in polyethylene terephthalate. ... 

The use of a fibre-coupled confocal Raman microscope and an infrared microscope for both point mapping and global imaging in the study of spatial variations in polymer chemistry and morphology is illustrated by studies of the curing of the UV-cured acrylate coatings, crystallinity in drawn polyethylene terephthalate (PET) film, molecular orientation in PET bottles, and the analysis of a PES/PEES copolymer blended with epoxy resin and cured at elevated temperature. 8 refs. 

A tightly structured polymer such as a crystalline polyester or a highly crosslinked polymer where the polar elements can be directly affected by the field by rotation but cannot move far from the equilibrium position does absorb energy from the field and heat to some extent. The restricted motion reduces the effect and these materials show much lower dielectric losses and generally are quite resistant to dielectric breakdown in the A.C. fields. The polyethylene terephthalate polymer films (some of the most resistant to dielectric breakdown of the commonly used plastics insulating materials), are used in a highly oriented and crystalline form which has the tightest structure and restricts molecular freedom to the maximum extent. 

In polyethylene terephthalate, however, stretching produces both molecular orientation and small regions of three-dimensional order, termed crystallites, because the orientation processes have brought the molecules into adequate juxtaposition for regions of three-dimensional order to form. 

In addition to the discrete reflections from the crystallites, the diffraction pattern of a polymer shows diffuse scattering attributed to amorphous regions. Such polymers are said to be semicrystalline, with the crystalline fraction being controlled by molecular regularity. By comparing the relative amounts of crystalline and amorphous scattering of X-rays the crystallinity has been found to vary from more than 90 per cent for linear polyethylene to about 30 per cent for oriented polyethylene terephthalate. 

Table 7.6 compares the measured compliances for isotropic samples of five polymers with the Reuss and Voigt average compliances calculated from measurements on highly oriented specimens. For polyethylene terephthalate and low-density polyethylene the measured isotropic compliances fall between the calculated boimds, suggesting that here molecular orientation could well be the principal factor that determines mechanical anisotropy. For nylon 6 6 the... 

Polymers that do not crystallise, such as polymethyl methacrylate, show a good correlation between the (low) degree of mechanical anisotropy and molecular orientation determined from birefringence. There is so much disorder that it seems unlikely that a significant proportion of the chains can achieve the high alignment of a crystalline polymer such as polyethylene. Other polymers such as polyethylene terephthalate, which have a... 

The natural draw ratio for amorphous polymers is very sensitive to the degree of preorientation, i.e. the molecular orientation in the polymer before cold-drawing. This was reported for polyethylene terephthalate by Marshall and Thompson [20] and for PMMA and polystyrene by Whitney and Andrews [26]. 

Unlike nylon, which in the as-spun state contains a high amount of crystalline component, polyethylene terephthalate fibers are essentially amorphous as spun. In order to secure a usable textile yarn or staple fiber, this product must be drawn under conditions that will result in an increase in both molecular orientation and crystallinity. This is done by drawing at a temperature well above the glass transition point, 7, which is about 80 C. Conditions of rale and temperature must be... 

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