The Interphase in Lamellar Semicrystalline Polymers

A lattice theory is developed to describe in detail the transition from perfect crystalline order to the isotropy of the adjoining amorphous state in a lamellar semicrystalline polymer. The required dissipation of chain flux causes a majority, ca. 70%, of the chains to reenter the same lamellae from which they emerge. However, even in the most favorable circumstance in which the interface is normal to the stems of chains in the crystal, fewer than 20% of these chains are engaged in adjacent folds with immediate reentry. Most of the reentry in the first three lattice layers adjoining the crystal surface occurs at sites that are nearby, but nonadjacent, to the site of exit. The interphase in which order persists is predicted to be 10-12 A in thickness, in agreement with experiments. The interfacial free energy, 55-70 erg cm^-2, calculated from the persistence of order beyond the crystal interface is compared with deductions from observed depressions of melting temperatures due to the finite thickness of the lamellae.