X-ray photoelectron and static secondary-ion mass spectroscopic studies of segmented block copoly(ether-ester)s

Abstract

X-ray photoelectron spectroscopy (x.p.s.) and static secondary-ion mass spectroscopy have been applied to study the surface composition and structure of copoly(ether-ester) elastomers. These materials are multiblock copolymers containing repeat units that are capable of crystallization (hard segments) and amorphous blocks (soft segments). The hard segments in the samples examined were based on poly(butylene terephthalate) (PBT) and the soft block was poly(tetramethylene oxide) (PTMO). The results indicate that the surfaces of these copoly(ether-ester)s are always enriched in the soft segment and this enrichment is driven by the surface energy difference between the hard and soft segments. The surface enrichment is shown to vary with composition and molecular weight. For a copoly(ether-ester) containing 60% w/w PTMO in the bulk, average surface composition in the top 40 Å is seen to change from ∼ 66% w/w for PTMO of M n = 1000, to ∼ 85% for PTMO of M n = 2900. Differential scanning calorimetry studies in conjunction with x.p.s. studies suggest that the surface enrichment is reduced by an increase in the bulk crystallinity of the material. Addition of poly(hexamethylene terephthalate) to the PBT hard segment reduces the bulk crystallinity of the copoly(ether-ester)s and results in a surface that is more enriched in the PTMO soft segment. The process by which the sample is prepared is also shown to affect the extent of surface enrichment. The angle-dependent x.p.s. data have been fitted by a continuous profile, which shows an enriched soft-segment region at the air/copolymer interface and a depleted region adjacent to the preferentially enriched surface layer.