Microhardness under strain, 4: Reversible microhardness in polyblock thermoplastic elastomers with poly(butylene terephthalate) as hard segments

4 pags., 2 figs.The microhardness (H) technique was recently applied to poly(butylene terephthalate) (PBT) and its multiblock copolymers for examination of the stress-induced polymorphic transition. Following these investigations the present study attempts to observe the reversible variation of microhardness under strain. For this purpose bristles of drawn and annealed (at 160°C in vacuum) poly(ether ester) (PEE) were characterized with respect to their microhardness at various stages of tensile deformation. H was measured under and after loading (σ) in deformation steps of 5% each. In accordance with previous results on PBT and PEE, H values at σ ≠ 0 show a sharp drop (by 40%) in a relatively narrow deformation interval (ε = 10-15%), owing to the stress-induced α ⇔ β polymorphic transition. The hardness measurements at σ = 0 show a continuous decrease of H with a remaining strain. H values at σ = 0, corresponding to plastic deformation up to 5%, are much higher than the corresponding ones taken under stress at an overall deformation between 10 and 25%. The higher H values are explained by the regeneration of the starting α modification. Results reveal that in materials characterized by high and reversible deformability it is possible to observe a reversible microhardness behaviour, provided the strain-induced structural changes are reversible.Grateful acknowledgement is due to DGICYT (Grant PB94-0049), Spain, for the support of this investigation. The partial support of NATO (Grant No. CRG 920985 and No. NIG 951394) is also highly appreciated. It is a pleasure for one of us (S. F.) to acknowledge the tenure of a sabatical grant from DGICYT, Spain