Plastic deformation of polypropylene in relation to crystalline structure

The tensile drawing behavior of quenched and annealed films of isotactic po lypropylene is investigated as a function of draw temperature and strain ra te. A strain-induced structural change from the smectic to the monoclinic f orm is observed for the quenched films. A kinetic interpretation is propose d for the phenomenon. Data of thermal activation volume at the yield point indicate two regimes of plastic flow fell the quenched sample, between 25 a nd 60 degrees C, but only one regime for the annealed sample. Homogeneous a nd heterogeneous crystal slip processes are proposed to account for these r egimes in relation to the nucleation and propagation of screw dislocations. The basic mechanism of molecular motion in the polypropylene crystal is su ggested to be a wormlike motion of conformational defects along the 3/1 hel ix chains that allows a 120 degrees rotation and a c/3 translation. The occ urrence of the smectic form as a transitory state in the deformation pathwa y is discussed in terms of plasticity defect generation. (C) 1999 John Wile y & Sons, Inc.