Recrystallization of Cold Drawn Polypropylene

Abstract

An a-axis orientation mixed with c-axis in polypropylene fiber structure has been observed in the following cases: crystallization under molecular orientation, and annealing at high temperatures in the absence of tension after cold drawing. This study elucidates the latter case, i. e. the structural changes during annealing of cold drawn polypropylene films.
This study involves the application of the following physical techniques: wide angle X-ray diffraction, small angle X-ray diffraction, differential scanning calorimetry and dilatometry. Cold drawn films are mainly used for the experiments. Irradiated cold drawn films and drawn samples treated with fuming nitric acid are also used supplementally.
The results obtained are as follows:
1) Annealing of cold drawn polypropylene films is accompanied with lamella thickening and produces 1-8% fractions of a-axis oriented crystallites.
2) Cold drawn films are treated with fuming nitric acid at 90°C for varing times. The films treated for 5hr whose crystallinity is estimated to be more than 90%, show a-axis orientation when annealed at 134°C for 5 min. This suggests that tie chains or amorphous regions between lamella are not changed into a-axis oriented crystallites by annealing.
3) Annealing of irradiated films after cold drawing produces much more amounts of aaxis oriented crystallites than that observed in annealing cold drawn polypropylene.
4) DSC thermograms of annealed films after cold drawing suggest that high temperature peak is governed by crystallization kinetics, whereas low temperature peak is thermodynamically preferred. Especially in the case that both the double peaks and a-axis orientationare observed, low temperature peak corresponds to the melting peak of a-axis orientedCrystallites.
5) The melting point of a-axis oriented crystallites in annealed cold drawn films varieswith the dose of irradiation.