MICROHARDNESS UNDER STRAIN - III - MICROHARDNESS BEHAVIOR DURING STRESS-INDUCED POLYMORPHIC TRANSITION IN BLENDS OF POLY(BUTYLENE TEREPHTHALATE) AND ITS BLOCK-COPOLYMERS

The microhardness (H) technique was recently applied to poly(butylene terephthalate) (PBT) and its multiblock copolymer of poly( ether ester ) (PEE) type for examination of the stress-induced polymorphic transit ion. In the present study, these investigations are extended to blends of PBT and PEE. For this purpose, drawn and annealed with fixed ends at 170 degrees C for 6 h in vacuum bristles of PBT-PEE, blends were ch aracterized with respect to their microhardness at various stages of t ensile deformation. H was measured under stress, with each step of def ormation amounting 5%. The variation of H with strain (epsilon) shows 2 sharp stepwise decreasing values (by 40%). Each step is defined in a relatively narrow deformation (epsilon) range (2-5%) due to the stres s-induced alpha --> beta polymorphic transitions arising in PET crysta llites. The first polymorphic transition (at epsilon = 2-3%) is assign ed to the PBT crystallites of the homopolymer (homoPBT). The second tr ansition (at epsilon = 25%) is associated to those crystals within the PEE copolymer. From the observation of two distinct transitions, sepa rated by a deformation interval of epsilon = 20% it is concluded that (1) homoPBT and the PET segments from PEE crystallize separately (no c ocrystallization takes place), and (2) the 2 species of PET crystallit es are subject to the external mechanical loading, not in a simultaneo us manner, but in a two-stage process. In the deformation range betwee n the 2 transitions (epsilon = 2-3% and 25%), it is pointed out that c onformational changes are induced through stretching mainly in the amo rphous regions. (C) 1998 John Wiley & Sons, Inc.