Effect of Elongation and Temperature on the Recovery and Apparent Glass Transition Behavior of an Experimental Modacrylic Fiber

Abstract

The shape of the stress-strain curve of the modacrylic fiber is correlated in a definite manner with the stress-relaxation and recovery behavior. At the yield point, where the stress-strain curve has minimum slope, the tensile recovery decreases most rapidly with increasing elongation, and the rate of stress relaxation passes through a maximum. Above the normal (low-strain) glass transition temperature of the fiber (90° C.), the stress-strain curve no longer has a yield point, and the recovery and stress-relaxation behavior become relatively independent of elongation. The tensile- and work-recovery values show a definite minimum (permanent set shows a maximum), occurring at the glass transition temperature at low, 1%, strain, and shifting to lower temperatures with increasing elongation. This shifting of the minimum in the recovery-temperature curves is interpreted to indicate a lowering of the glass transition temperature with stretching. At temperatures of 25° and 60° C., the yield strain approximates that elongation required to reduce the minimum in the recovery-temperature curves to that temperature. These results lead to a fundamental definition of the yield point as the strain level at which the glass transition temperature is lowered to the experimental temperature. A free volume increase accompanying stretching is postulated as the underlying mechanism whereby the glass transition temperature is reduced. The equations of Ferry [13, 14] indicate that a quite reasonable value of 0.35 for Poisson's ratio could lead to an increase in free volume sufficient to speed up the molecular response by a factor of 105 at the yield strain.