Significant Improvement of Mechanical Properties for Polyvinyl Alcohol Film Prepared from Freeze/Thaw Cycled Gel

Abstract

The mechanical properties of polyvinyl alcohol (PVA) films prepared by evaporating water from freeze/thaw cycled gel were investigated as a function of the number of freeze/thaw cycles. The maximum stress of the PVA film prepared by freeze/thaw cycling was larger than that prepared without the freeze/thaw cycle process. The largest maximum stress was 46.2 MPa for a film prepared with 10 freeze/thaw cycles, which was twice as large as that for a cast PVA film with-out freeze/thaw cycling (22.3 MPa). This is due to the formation of small crystallites during the freeze/thaw cycle proc-ess. Furthermore, when the film was annealed at 130˚C, the maximum stress was as high as 181 MPa which was com-parable to that for PVA films prepared using additives. The crystallinity is not the main factor that determines the maxi-mum stress for either the non-annealed or annealed freeze/thaw cycled films, but the glass transition temperature is well correlated with the maximum stress, irrespective of the annealing process. This is due to the different molecular mor-phology; the non-annealed freeze/thaw cycled film consists of many small crystallites, but the annealed film consists of larger crystallites formed during the annealing process.