MICROHARDNESS UNDER STRAIN - 2 - MICROHARDNESS BEHAVIOR DURING STRESS-INDUCED POLYMORPHIC TRANSITION IN BLOCK-COPOLYMERS OF POLY(BUTYLENE TEREPHTHALATE)

In addition to homopoly(butylene terephthalate) (homo-PBT), the microh ardness technique was applied to its multiblock copolymers for examina tion of the stress-induced polymorphic transition. Drawn and annealed at 170 degrees C for 6 h with fixed ends in a vacuum, bristles of poly (ether ester) (PEE) having PET as hard segments and poly(ethylene oxid e) (PEO) with molecular weight 1000 as soft segments (PBT/PEO = 57/43 wt%) were characterized with respect to their microhardness H at vario us stages of tensile deformation. A sharp H decrease (by 20%) in a nar row deformation interval (2-4%) due to the stress-induced alpha double left right arrow beta polymorphic transition is observed followed by an increase and decrease of H. The transition is registered at much hi gher relative tensile deformations (between 25% and 30%) compared with the case of homo-PBT. This is explained by the presence of a very sof t amorphous phase of PEG, which deforms first. The same phase, disting uished by low viscosity in which the PET crystallites are ''floating,' ' is the reason for the very low H values measured (between 25 and 35 MPa, depending on the amount of alpha- and beta-modifications). The st ress-induced polymorphic transition in copolymers of PET with PEO is d emonstrated for the first time, also supporting the recently proposed concept of the microhardness depression effect.