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Positron lifetimes in polytetrafluroethylene (PTFE) and polyethylene (PE) have been measured as a function of residual deformation [MATH]. For PTFE, the variation of o-Ps intensity shows a three-stage processes : the elastic region ([MATH]), plastic-f low region ([MATH]) and strain-strengthening region ([MATH]). The changes of free-volume hole size and concentration suggest the separation, scission and orientation of segments. For PE, the o-Ps parameters are sensitive to the elastic and plastic regions. Particularly, the hole concentration decreases linearly with [MATH], which indicates the crystallization during deformation

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