| Transmission electron microscopy (TEM) methods have been developed and applied to structural investigations of oriented semicrystalline poly(ethylene terephthalate) (PET) systems. The focus of this undertaking has been directed towards the development of TEM specimen preparation techniques from the bulk states and imaging without artificial contrast enhancement.;For producing satisfactory electron-transparent specimens, angular ultramicrotomy methods have been explored and procedures have been established for the material systems under investigation. Thin-sectioning was carried out along various angular directions from major material's axes and the microstructure-sensitivity of the sectioning angle was demonstrated. This ultramicrotomy investigation has provided further evidence supporting the concept of generic "easy sectioning paths," i.e., thin-sectioning is likely to be successful along these structural routes.;For TEM imaging with sufficient contrast, the technique of "defocus microscopy" has been used and satisfactory results have been obtained. Deliberate defocus of the objective lens was applied to shift the first pass-band of the microscope contrast transfer function to the desired resolution range, thus producing object images of best possible contrast. Special considerations for the better utilization of defocus microscopy are discussed.;Three oriented PET systems have been studied by TEM, viz., solution-drawn uniaxial films, commercial biaxially processed melt-cast films, and commercial melt-spun fibers subjected to post-spinning treatments. The fast crystallization kinetics and the effect of biaxial drawing have been successfully revealed by TEM imaging. For the highly crystallized biaxial films, the original unidirectional lamellar structure was seen to be destroyed as transverse draw increases, and eventually developed into a biaxial structure with principal structural repetition in the transverse direction.;In the earliest stage of structural development of a uniaxial system, direct observation showed an embryonic microfibrillar structure, which is believed to consist of crystalline microfibrils separated by intermicrofibrillar regions. As further crystallization occurs, the original microfibrillar structure perfects and lateral alignment of the crystallites was seen to take place, particularly in extensively crystallized materials. Based on the TEM observations, a microfibrillar model was proposed to describe early crystallization behavior. |
