Abstract: In this paper, the phenomenon that stress whitening of expandable acrylonitrile/ methacrylic acid copolymer could accelerate the bubble nucleation of the copolymer was discovered. We studied the influence of stress whitening on the bubble size, used hot stage polarized optical microscopy to observe the foaming action of stress-whitened copolymer, and discussed the mechanism of such phenomenon. The results show that if the expandable acrylonitrile/ methacylic acid copolymer is stress whitened before foaming, the cell size of copolymer foam can be reduced greatly, and the cell size of 75 kg/m3 foam reduces from 0.59 mm to 0.076 mm. The cell size saltates when stress whitening happens. The mechanism is explained as follows: the stress whitening in the expandable copolymer creates new interface between gas phase and polymer phase. So, it reduces the transformation of interface free energy during bubble nucleating, reduces the transformation of system Gibbs free energy, and improves the quotiety of bubble nucleation.

Key words: acrylonitrile-methacrylic acid copolymer, stress-whitening, bubble nucleation