Kinetics of recovery of biaxially oriented styrene-acrylonitrile copolymer films

Styrene-acrylonitrile copolymers were biaxially oriented to obtain a draw ratio of about 6.9 in the machine direction (length) and 2.9 in the transverse direction (width). The prepared films, 28 μm thick, were annealed at elevated temperatures for different time periods. The length and width of the film gradually decreased, accompanied by an increase of the film thickness during recovery. The variations of the film length, width and area with time were all found to follow second-order kinetics for the later part of the recovery process. The second-order rate constant for area recovery, ka, was found to be related to k1 and kw, the rate constants for length and width recovery, as 1ka = l∞kw + w∞k1. The fitted final area A∞ was equal to l∞w∞, obtained individually from the fitting of second-order kinetics. The Arrhenius plots of the rate constants showed activation enthalpies in the range from 120 to 127 kcal mol−1. Based on these observations, the kinetics of volume recovery were also shown to be second-order at long recovery times.