Temperature dependence of birefringence and stress for natural rubber vulcanizates at strained state

The temperature dependence of stress and birefringence for natural rubber vulcanizates under medium and large deformation was measured for the processes of cooling, heating and re-cooling. In order to investigate the relation between the stress and crystal phase, the observed birefringence, Δt, was converted into the crystallinity, X_v, by the following equation:

$\text{X}{}_{\mathrm{<mtext>v</mtext>}}\text{=}\text{Δt?Δn}{}_{\mathrm{<mtext>a</mtext>}}\text{°f}{}_{\mathrm{<mtext>a</mtext>}}\text{Δn}{}_{\mathrm{<mtext>c</mtext>}}\text{°f}{}_{\mathrm{<mtext>c</mtext>}}\text{?Δn}{}_{\mathrm{<mtext>a</mtext>}}\text{°f}{}_{\mathrm{<mtext>a</mtext>}}$

where Δn⁰_c, Δn⁰_a, f_a and f_c are the intrinsic birefringence of the crystal, that of the amorphous phase, the orientation factor of crystallites, and that of amorphous phase, respectively. The fusion of crystallites induced by the thermal crystallization resulted in the increasing contractile force, while the fusion of strain-induced crystallites induced the reduction of contractile force.