SAXS investigation of structure–property relationship of polypropylene/montmorillonite composites during load cycling

Abstract

Polypropylene (PP) can hardly be reinforced by the layered silicate montmorillonite (MMT), but the material fatigue appears somewhat reduced. The probable reason is amplified competitive nucleation of the PP by MMT component. Utilizing small‐angle X‐ray scattering (SAXS) from synchrotron, we investigate the nanostructure evolution of the PP in straining experiments from neat PP and compatibilized composite materials. The compatibilizer is a styrene–ethylene/butylene–styrene copolymer (SEBS). Oriented injection‐molded test bars are studied. The discrete SAXS probes variations of sizes and distances among those crystalline domains that are not placed at random. Crystallite dimensions and distances are documented for modeling purposes. The nanoscopic strain is computed from the distance variation and compared with the macroscopic strain. Differences between macroscopic and nanoscopic strain are observed. They require postulating regions with statistical placement of crystallites (poorly arranged region, PAR) in addition to the SAXS‐probed well‐arranged semi‐crystalline entities (WAE). The extensibility of WAEs must be different from that of the PARs. In neat PP, the observed WAEs are well developed and stronger than the PARs. In the composites, the WAEs are made from thin and less extended crystalline domains. They are weaker than the PARs that appear reinforced. Thus, enclosing each MMT layer a PAR is formed, and the WAEs generated farther away remain imperfect. Consequently, in the composites, the narrow crystalline domains from the WAEs do not break into even smaller pieces, and the fatigue of the composites is lower than that of the neat PP. Copyright © 2013 John Wiley & Sons, Ltd.

Studying nanostructure gradients in injection-molded polypropylene/montmorillonite composites by microbeam small-angle x-ray scattering

“…This scaling has been chosen to simultaneously visualize features of strongly differing intensity. In a previous paper [28] dedicated to the investigation of fatigue under mechanical load-cycling we have used a macrobeam and thus recorded SAXS patterns averaged over the complete cross-section of the rod-shaped sample. The corresponding patterns look similar to the new microbeam patterns from the skin region.…”

“…Again, the isotropic component may be explained by some grainy fragments. In our load-cycling study [28] we have postulated such material and have called it ‘dust’. Now the isotropic scattering feature in the NC structure gradient supports our earlier explanation.…”

“…Our group has published two SAXS studies [25, 28] on different injection-molded PP/MMT materials under mechanical load. The mechanical properties of the composites and of the neat PP have been similar, but the semicrystalline structure of the PP and its response to load have been different.…”