Structural understanding of polyethylene crystal by retardance mapping technique

The higher-order structure of polyethylene (PE) crystal has been studied by means of retardance mapping technique. This technique allows one to visually and quantitatively examine crystal structures in polymer samples from macroscopic point of view. The retardance map obtained in the present study consists of 640 × 480 individual retardance data. The distribution of retardance in the view field or change in the retardance under an external perturbation can therefore be evaluated by mathematically treating the histograms. We obtained the second derivatives of the histograms of retardance maps that have been collected at ambient condition and detected multiple peaks in the histograms of PE specimens. The presence of shoulder peaks indicates the presence of locally developed higher-order crystalline structure in the view field. A series of retardance maps were collected during the heating of PE specimens. When temperature is increased, PE crystal starts to melt and this process can be studied based on retardance mapping technique. The difference in the melting of higher-order structure among PE samples was studied and clear difference was depicted. We performed two-dimensional (2D) correlation analysis against histogram of the retardance map to elucidate structural changes that happen before and during the melting process. As a result, we were able to conclude the disordering of higher-order structure in PE starts from temperature far below Tm.