Crystallization in films of polar vinyl polymers—I. Crystallinity of polyvinyl alcohol films prepared by evaporation of aqueous solutions

Abstract

The development of crystallinity in polyvinyl alcohol films prepared by controlled evaporation of aqueous solutions has been studied. Simultaneous nucleation and crystallization starts as the weight fraction of polymer solids is increased beyond the sol-gel transition composition, but crystallization only becomes rapid after a prolonged induction period ( t ̄ ). t ̄ varies with rate of evaporation ( R e ) according to the relation, t ̄ = t ̄ 1/R e 0.7 hr . The rate of crystallization is expressed by Avrami relation, while the rate of growth ( R g ) during the main growth surge (crystallinity, w x = 0·1–0·3) varies with R e according to the relation, R g ≏ 2–3 R e hr −1 . The final crystallinity, near the gel-glass transition composition, remains practically independent ( w x = 0·35–0·39 at 23°) of evaporation rate, but increases to some extent with increased temperature of evaporation. The high degree of crystallinity and sub-microscopic crystal size is mainly determined by a high degree of nucleation with low rates of growth onto nuclei. Addition of 2–20% high b.p. solvent (plasticizer) leads to higher final crystallinity ( w xf > 0·45).