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.