Deformation behaviour of poly(ether ester) thermoplastic elastomers with destroyed and regenerated structure as revealed by small-angle X-ray scattering

Abstract

The scattering behaviour of thermoplastic elastomers based on poly(ether ester) (PEE) under stress is studied. Bristles of PEE consisting of poly(butylene terephthalate) as hard segment and poly(ethylene glycol) ( M ̄ n = 1000 ) as soft segment in the ratio 50 50 wt% are drawn to five times their initial length and then annealed with fixed ends in order to create a standard initial structure. Samples with largely destroyed structure (by additional drawing) as well as with regenerated structure (by crystallization, solid-state reactions or chemical crosslinking) were prepared. Small-angle X-ray scattering (SAXS) measurements are carried out with single bristles subject to stress and with deformations up to 200%. An affine increase of the long period L with extension ϵ up to ϵ = 75% is observed in the samples with undestroyed structure. A second L 2 appears at larger ϵ. Without application of stress two discrete values, L rel 1 and L rel 2, are obtained. Qualitatively, the sample with destroyed structure behaves similarly. The deformation behaviour of samples with regenerated structure depends on the method of regeneration: (i) crystallization mostly recovers the previous deformation pattern; (ii) solid-state reactions (additional condensation and exchange reactions) result in an increase of L 1, L 2, L rel 1 and L rel 2 due to the very high number of interfibrillar contacts; and (iii) chemical crosslinking leads to the appearance of only L 1 and L rel 1. A model is proposed suggesting the existence of two types of lamellae differing in their perfection and origin. The first and more perfect lamellae refer to the starting crystalline lamellae, while the second type of less perfect lamellae are assumed to arise during the additional stretching. The latter comprises hard segments originall dispersed in the amorphous interlamellar layers or pulled out from the neighbouring crystallites. The existence of the two types of lamellae is proved by differential scanning calorimetric measurements. By variation of the number of intra- and in particular interfibrillar contacts, the predominant role of the tie molecules in the evolution of mechanical properties of these polymer materials is demonstrated. Further, two important concepts are proposed in addition to earlier studies: (i) microfibrillar chemical healing-elimination of the interfibrillar phase boundaries as a result of solid-state reactions and (ii) deformation (by slippage) of ensembles of microfibrils in the chemically crosslinked samples, almost preserving in this way the initial L value and facilitating very high deformations ( ϵ = 200%).