Machine prepared thermoplastic polyurethanes of varying hard segment content: Morphology and its evolution in tensile tests

Abstract

Machine‐cast thermoplastic polyurethanes are strained and monitored by small‐angle X‐ray scattering (SAXS). They are prepared from 4,4′‐methylene diphenyl diisocyanate, 1,4‐butane diol, and polytetrahydrofuran. Upon stretching hard domains are destroyed. Most stable are the domains of materials with a hard‐domain content (HSC) of 30%. Domain stability decreases with increasing HSC and crosslinking. Most materials show stability up to a strain 0.6. At higher strain, the apparent long period decreases for the materials with HSC = 30%. Correlated hard domains, the “strain probes” relax as others are destroyed. The fraction of relaxing probes and their ultimate relaxation decrease with increasing HSC. Chord distribution functions computed from the SAXS exhibit the same sequence of static long‐period bands. The band positions form a Fibonacci series, related to the underlying polyaddition process. This indicates a nearly quasicrystalline arrangement of stringed hard domains, identified as the strain probes of the discrete SAXS. At strains <0.6, the probes experience half of the macroscopic strain, which reflects hard‐domain rigidity. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1213–1223

Morphological Changes under Strain for Different Thermoplastic Polyurethanes Monitored by SAXS Related to Strain at Break

“…Only mqt shows peculiarities that have previously been found with machine‐cast materials . It is the softest‐but‐one material (Figure ).…”

“…Only the material mqt exhibits the relaxation behavior known from machine‐cast materials that has already been discussed qualitatively. With the other hand‐cast materials the known slow increase of the nanoscopic strain ϵ n (ϵ) ≈ 0.6 ϵ continues up to the end of the experiment. We explain the low slope (0.6 < 1) by the rigidity of the hard domain (average height: ) that is part of the long period .…”

“…In each row the strain ϵ increases from top to bottom. Compared to respective machine‐cast materials the SAXS patterns look similar, but the CDFs are fundamentally different. With the hand‐cast materials the long‐period peaks are broad (in vertical direction).…”

“…To capture the nanostructure change quantitatively by means of the long period peak, we track its position and shape during the tensile tests. In previous work on polyurethanes we have done this as well . Figure presents the evolution of the nanoscopic strain ϵ n (ϵ) computed from the most‐frequent long period.…”

“…Nevertheless, an analysis that may yield deeper insight is only possible, if the IDF can be fitted by a suitable model. In the case of machine‐processed materials a peculiar band structure has been found. This complex topology requires either the construction of an involved morphological model based on quasi‐periodicity or the development of appropriate approximations.…”