Microhardness under strain. III. Microhardness behavior during stress-induced polymorphic transition in blends of poly(butylene terephthalate) and its block copolymers

Abstract

ABSTRACT: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 transition. In the present study, these investigations are extended to blends of PBT and PEE. For this purpose, drawn and annealed with fixed ends at 170ЊC for 6 h in vacuum bristles of PBT-PEE, blends were characterized with respect to their microhardness at various stages of tensile deformation. H was measured under stress, with each step of deformation amounting 5%. The variation of H with strain (1) shows 2 sharp stepwise decreasing values (by 40%). Each step is defined in a relatively narrow deformation (1) range (2-5%) due to the stress-induced a r b polymorphic transitions arising in PBT crystallites. The first polymorphic transition (at 1 Å 2-3%) is assigned to the PBT crystallites of the homopolymer (homoPBT). The second transition (at 1 Å 25%) is associated to those crystals within the PEE copolymer. From the observation of two distinct transitions, separated by a deformation interval of 1 Å 20% it is concluded that (1) homoPBT and the PBT segments from PEE crystallize separately (no cocrystallization takes place), and (2) the 2 species of PBT crystallites are subject to the external mechanical loading, not in a simultaneous manner, but in a two-stage process. In the deformation range between the 2 transitions ( 1 Å 2-3% and 25%), it is pointed out that conformational changes are induced through stretching, mainly in the amorphous regions.

Damage measurements in epoxy structural adhesives using microhardness

“…Stress-induced changes in microhardness measurements have been reported in the literature, and demonstrate that the material hardness decreases with increasing levels of strain [19,29]. However, it is possible to observe a reversal in this trend, depending on the specific polymer.…”

Polymorphism of Isotactic Poly(1-butene) as Revealed by Microindentation Hardness. 1. Kinetics of the Transformation

“…It has been shown that the amount of the α and β phases in an isotactic polypropylene (iPP) sample can be successfully distinguished using the microhardness technique . Moreover, microindentation hardness has been applied with success to gain information on the stress-induced polymorphic transition in poly(butylene terephthalate), its block copolymers and blends, and iPP. − …”

CRYSTALLIZATION IN PARTIALLY MOLTEN ORIENTED BLENDS OF POLYCONDENSATES AS REVEALED BY X-RAY STUDIES*

“…We did not observe the mentioned type of cocrystallization, but quite the opposite-formation of continuous crystals comprising two, possibly nonspatially separated (i.e., having common crystal surfaces), crystallographically identical populations of crystallites, differing in size, perfection, origin, and time of appearance [24]. This phenomenon was originally [24] called "partial cocrystallization" and proven by smallangle X-ray scattering (SAXS) [24], DSC [24], and microhardness measurements [32]. Thermograms of the oriented blend show two melting peaks, thus proving that the blend comprises two populations of crystallites, probably the more perfect ones arising from the homopolymer PBT and the less-perfect crystallites made from the PBT blocks of the PEE block copolymer [24].…”

“…In the case of the first type of cocrystallization, as reported by Gallagher et al [30] for their system, only a single population of crystallites would exist, and consequently only one polymorphic transition would be found with the progress of deformation. Recently, on the other hand, we described two distinct polymorphic alpha-beta transitions of PBT in the microhardness versus external deformation dependencies for the PBT/PEE blend with lower PBT content [32]. It was concluded that homo-PBT and the PBT segments from PEE segregate in different crystallites [32].…”

“…Recently, on the other hand, we described two distinct polymorphic alpha-beta transitions of PBT in the microhardness versus external deformation dependencies for the PBT/PEE blend with lower PBT content [32]. It was concluded that homo-PBT and the PBT segments from PEE segregate in different crystallites [32]. A clear support for the occurrence of segregated crystallization was found in a series of 2D WAXS flat-film patterns taken at room temperature after different thermal treatment of the predrawn blend [24].…”