Influence of Stress Whitening Pretreatment on Cell Structure, Foaming Behavior, and Mechanical Properties of AN/MAA Copolymer Foam

Abstract

Stress whitening pretreatment on expandable acrylonitrile (AN)/methacrylic acid (MAA) copolymer was adopted to reduce the cell size of high-performance AN/MAA copolymer foam. The article studied the influence of stress whitening on cell structure and mechanical properties of AN/MAA copolymer foam, observed foaming behavior of stress- whitened copolymer by hot stage optical microscopy, and discussed its bubble nucleation mechanism. The results indicate that stress-whitening pretreatment makes the cell size of corresponding copolymer foam reduce sharply when stress whitening occurs. The cell size of copolymer foam with the density of 32 kg/m3 and 75 kg/m3 reduces from 1.07 mm to 0.37 mm and from 0.59 mm to 0.076 mm, respectively. It also causes residual fragmental films in cells. The defects created by stress whitening work first as a bubble nucleus, then expand and combine together as cells. Stress whitening creates new interface between gas and polymer phase and new volume of gas phase, reduces the change of interface free energy and volume free energy during bubble nucleation, and improves the bubble nucleation rate. The foaming phenomenon of stress whitened copolymer is in line with the defect nucleation mechanism. However, stress whitening pretreatment reduces the mechanical properties of final foam because of residual fragmental films.