Glassy polyethylene terephthalate

Jabarin, S. A., Crystallization kinetics of polyethylene terephthalate. III. Effect of moisture content on the crystallization behaviour of PET from the glassy state, J. Appl. Polym. Sci., 34, 103-108 (1987). 

Recently Moore and Petrie (5) have demonstrated that control of sample thermal history can result in transition from ductile to brittle behavior for polyethylene terephthalate. This transition in behavior was related to volume relaxation of the glassy state. 

In the solid phase, support for a similar contention has come from the drawing of polyethylene terephthalate, combined with stress optical measurements, and also for the behaviour of craze fibrils in a range of glassy polymers. 

Many crystallizable polymers can be prepared in the amorphous glassy state by rapid quenching as films. Measurements of Aglass transition temperature determined. Such results are shown for amorphous polyethylene terephthalate (PET) in Figure 13 (17). The Brillouin splittings change slope at 70°C. If both Aa>(i) and Awt can be measured, the Poisson ratio (T can be determined according to ... 

Crystallinity is important in determining optical properties because the refiaetive index of the crystalline region is always higher than that of the amorphous component irrespeetive of whether the amorphous component is in the glassy or rubbery state. This difference in refractive indices of the component phases leads to high scattering and consequently, the translucency or haziness of semicrystalline polymers. For a purely amorphous polymer, this does not occur, and hence amorphous polymers are usually transparent. Therefore the state of polyethylene terephthalate can be explained as follows ... 

A noncrystalline polymeric material that has no definite order or crystallinity. A polymer in which the macromo-lecular chain has a random conformation in solid (glassy or rubbery) state. On the one hand, an amorphous polymer may show a short range order, while on the other, a crystalline polymer may be quenched to the amorphous state (viz., polyethylene terephthalate (PET)). The maximum value of a periodically varying function, e.g., used to describe the energy transmitted from the ultrasonic welding horn to the weld joint. 

Not aU polymers can crystallize. Polyethylene does, but atactic polystyrene (the common form) does not. Though the speciahzed syndiotactic and the more common isotactic forms do, albeit into completely different crystal forms (de Rosa and Auriemma 2013). Isotactic polypropylene and many nylons commonly crystallize into more than one crystal type, and polyethylene terephthalate (PET), a polyester, is commonly found in a glassy form in soft drinks bottles but crystallizes readily on appropriate thermal treatment. 

Amorphous polyethylene terephthalate is a glassy polymer. During exposure to temperatures below glass transition temperature, molecular rearrangements facilitate equilibrium in the material. This process changes the free volume and causes a reduction in relaxation enthalpy. 

Normally, one would not usually consider that the change from an amorphous or glassy state to a crystalline or ordered state is readily discerned by infrared absorption. With some polymers, such as polyethylene terephthalate (PET), this change in structure gives rise to diffefrences in the infrared spectrum that can be used for quantitative analysis of samples for crystalline content. With PET, a band at 10.2 was shown by Cobbs and Burton [ ] to be a function of crystallinity and variously annealed samples were run for infrared absorption and density. The density of amorphous PET is 1.33, while the density for crystalline polymer is 1.47 according to x-ray unit cell measurements. From the density-infrared absorbance samples it was then possible to construct a calibration curve of absorbance versus percent crystallinity. 

Fully developed crystallinity is hampered by kinetic reasons. Partial crystallinity is typical for many stereo regular flexible polymers as polyethylene (PE), poly(ethylene terephthalate), and poly(propylene). We may say, semi-crystalline polymers are a composite of small crystalline regions in a matrix of dramatically less ordered material. The amorphous material can be glassy as in PET or more rubbery as in PE. Partial crystallinity is caused by... 

Fully amorphous polyethylene is rather difficult to make because of its fast crystallization. By superfast quenching it was, however, possible to make small amounts of polyethylene glass of phase area 11 [6]. Glassy polymers are much more easily available for macromolecules that crystallize slowly, such as poly(ethylene terephthalate) and poly(oxy-l, 4-phenyleneoxy-1,4-phenylenecarboxy-1,4-phenylene) or macromolecules that have structural irregularities which prohibit crystallization completely, such as atactic polystyrene andpoly(methyl methacrylate). More details about the special properties of the transition 11— are discussed in Sect. 6.3. 

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