|
Abstract
|
Oriented HDPE films having a stacked lamellar morphology were prepared to develop microporous membranes through
cast film extrusion followed by stretching. Applying higher draw ratios (DR) and annealing the cast films improved crystal alignments
remarkably. It was shown the improvement in c-axis orientation upon annealing was more significant for the films prepared with
lower DR. Furthermore, applying Raman spectroscopy, the amorphous mass fraction of the films was obtained. Subsequently, considering
a three-phase structure (i.e., crystal, amorphous and interphase), the interphase content of the films was also determined. A distinct
DR dependence of the interphase content was noticed. It was observed that in addition to crystal perfection upon annealing, the
amorphous phase fraction was also decreased due to possibly increase in free volume, promoting lamellar separation and cavitation
during the subsequent stretching step. SEM images of the membranes surfaces as well as their normalized water vapor transmission
rate (WVTR) values indicated a more uniform pore formation and increased permeability with DR. Furthermore, it was found that
lower interphase content in the films with higher DR favors lamellar separation. However, permeability in the stretched precursor
films with DR higher than a certain level (DR593) was not improved significantly. Moreover, crystallinity and crystal orientation of
produced membranes were investigated.
|