Abstract
In order to gather information on viscoelastic properties of crystalline high polymers, stress relaxation of polyvinyl alcohol (PVA) films were studied. The original films were casted from concentrated solutions of PVA having the viscosity average degree of polymerization (DP) of 2180, 1600 and 600, and had no orientation in the plane of the film. Some of them were heated at 160°and 200°C for 10 minutes to vary the crystallinity from about 20% of the original film to 36.0% and 47.3%, respectively. The degree of crystallinity were evaluated from density measurements. The measurements of the stress relaxation were carried out by employing an Instron Tensile Tester of table model. In order to measure at various temperatures (20°∼120°C) and relative humidities (0%∼75%), a particular cell was inserted between two jaws of the tester and the sample film was tested in this cell at controlled temperature and humidity.
The results obtained are summerized as follows:
1) Original films having DP 2180 and 1600 showed quite the same behavior at 0% and 60% R.H. at 20°C, while the film having DP 600 did somewhat different behavior from the others, though it had almost the same degree of crystallinity. This discrepancy may be attributed to the difference in its fine structure from the others.
2) Original films of DP 2180 and 600 were tested at 20°C over a wide range of relative humidity, and a notable dispersion was found near 50% R.H. for both of the films. Relaxation modulus against logarithm of time at various R.H. can be superposed by horizontal shift of the curves along the log time axis to give a composite curve at any reference humidity. The relaxation spectrum calculated from the composite curve referred to 0% R.H. ranged from 0 to 1015sec, and showed good agreement with that obtained by Kawai et al. from the dynamic data over a range of 103 to 108sec. Although our spectrum ranged to very long time, any flow region was not observed.
3) Heat-treated film having DP 2180 and degree of crystanity 47.3% was tested in a temperature range from 20°C to 120°C. The time-temperature superposition principle was not valid for the result. Qualitatively speaking, it seemed that for PVA film there appeared many dispersions in glassy state differing from the cases of amorphous polymers.
4) The effect of crystallinity on sterss relaxation were studied with two samples having the degree of crystallinity of 36.0% and 47.3%, respectively. Although the phehomena observed were too complicated to discuss quantitatively, it was clear that the effect appeared only in the dispersion regions.
