ToF-SIMS and principal component analysis: Effect of film thickness on crystal surfaces of polymers

The effect of film thickness on the structural conformation of the surfaces of the amorphous state, edge-on lamellae and flat-on lamellae of a semiflexible polymer, poly(bisphenol-A-etheroctane), was investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS) with the aid of principal component analysis (PCA). PCA results empirically indicate that a structurally regular polymer surface was obtained with the formation of the flat-on lamellae from the amorphous state at a low degree of supercooling. A higher concentration of end group and cilium ion fragments, which are indicative of free chain ends, was observed on the edge-on lamellar surfaces than on the amorphous and the flat-on lamellar surfaces. This finding was attributed to the fact that the lateral surface of the edge-on lamellae contains many growth fronts, leaving behind a large number of uncrystallized chain remnants on the surfaces. Structural disorder was facilitated on both edge-on and flat-on lamellar surfaces as the film thickness decreased. Hence, this PCA study offers new insights into the nonequilibrium nature of polymer crystals and the mechanism of polymer crystallization in thin and ultrathin films.