Crystallization in the thin and ultrathin films of poly(ethylene-vinyl acetate) and linear low-density polyethylene

The crystallization of poly(ethylene-vinyl acetate) and linear low-density polyethylene (LLDPE) films spun-cast from the polymer/toluene solutions with as-cast thickness from 460 to 10 nm was studied. The lamellar thickness was measured using small-angle X-ray scattering (SAXS) and found to increase from 14 to 21 nm for films thinner than 100 nm. The morphology of LLDPE measured by scanning probe microscopy (SPM) showed an edge-on lamellae for the films thicker than 30 nm and flat-on lamellae for the films thinner than 15 nm. A pseudo-"shish-kebab" tiny crystal structure was observed in between the larger lamellae. Crystallinity was confirmed using attenuated total reflectance-Fourier transformed infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The shear modulation force microscopy technique (SMFM) was used to measure the melting point, T _m, which was found to decrease for films thinner than 100 nm. The rate of decrease was a function of the annealing protocol, but in all cases for films approximately 20 nm thick Tm was depressed by 35-40°C. This large value cannot be predicted from the classical Gibbs-Thomson relation, unless a change in the effective heat of fusion is assumed due to surface interactions.