An experimental technique for the characterization of molecular orientation through the thickness of plastic products



An experimental technique has been developed for the characterization of the molecular orientation through the thickness of plastic products. The technique consists of milling the specimen to the depth where the molecular orientation is to be measured, polishing the exposed surface, and characterizing the polished surface by external reflection infrared spectroscopy. The technique was first tested on both unoriented and oriented poly(ethylene terephthalate). It was found that the intermediate steps of the process change the apparent surface orientation, but carrying the process to completion (to a finish of 0.05 μm) removes the altered material and leaves a surface whose orientation corresponds to that of the original sample. The technique was also tested on polyetheretherketone (PEEK), high density polyethylene (HDPE), and liquid crystal polymer (LCP). For polymers with a high transition temperature, like PEEK and LCP, the conclusions are the same as for PET. However, for polymers like HDPE, with a low glass transition temperature, the milling and polishing process used to prepare the samples can alter the orientation to a greater extent. This effect can be reduced by cooling the sample with liquid nitrogen during preparation.