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

Research Department, Union Carbide Chemicals Company, South Charleston, West Virginia

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.

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