Stress-induced Solid-Solid Crystal Transition in Trans-1,4-polyisoprene

The polymorphic transition of trans-1,4-polyisoprene(TPI) during stretching was investigated by in situ wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. The influences of the initial structure, stretching temperature, and strain rate on the contents of different crystal modifications(α, β) were explored. The results confirm that the α-β transition occurs during stretching of TPI that only contains αcrystal(α-TPI). When the stress is relaxed, the β crystal formed during stretching remains, which indicates that the transition is irreversible. On the other hand, stretching of TPI that only contains β crystal(β-TPI) results in orientated β crystal. No β-α transition occurs during stretching. The different structures of stretched α-TPI and β-TPI exclude the previously proposed "melting-recrystallization mechanism". The α-β transition depends significantly on temperature and strain rate, indicating the transition is governed both by thermodynamics and kinetics. Our results support a solid-solid transition mechanism rather than a melting-recrystallization mechanism. The irreversible nature of the transition is attributed to the metastability of the β phase in the unstretched state. Different from the "β phases" that appear in polymers with stress-induced reversible transitions, e.g. poly(butylene terephthalate) and poly(butylene succinate), the stability of β phase in TPI is high that can be long-lived.The strain rate dependence of α-β transition hinders the determination of critical stress for the transition. It further indicates that the local stress within the sample is more heterogeneous at higher strain rates.

Stress-induced Solid-Solid Crystal Transition in Trans-1,4-polyisoprene

The polymorphic transition of trans-1,4-polyisoprene (TPI) during stretching was investigated by in situ wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. The influences of the initial structure, stretching temperature, and strain rate on the contents of different crystal modifications (α, β) were explored. The results confirm that the α-β transition occurs during stretching of TPI that only contains α crystal (α-TPI). When the stress is relaxed, the β crystal formed during stretching remains, which indicates that the transition is irreversible. On the other hand, stretching of TPI that only contains β crystal (β-TPI) results in orientated β crystal. No β-α transition occurs during stretching. The different structures of stretched α-TPI and β-TPI exclude the previously proposed “melting-recrystallization mechanism”. The α-β transition depends significantly on temperature and strain rate, indicating the transition is governed both by thermodynamics and kinetics. Our results support a solid-solid transition mechanism rather than a melting-recrystallization mechanism. The irreversible nature of the transition is attributed to the metastability of the β phase in the unstretched state. Different from the “β phases” that appear in polymers with stress-induced reversible transitions, e.g. poly(butylene terephthalate) and poly(butylene succinate), the stability of β phase in TPI is high that can be long-lived. The strain rate dependence of α-β transition hinders the determination of critical stress for the transition. It further indicates that the local stress within the sample is more heterogeneous at higher strain rates.