Quasiperiodicity and the nanoscopic morphology of some polyurethanes

Abstract

When straining polyurethane elastomers (PUEs), it is often observed that the long-period peak of the small-angle X-ray scattering (SAXS) does not shift normally. An explanation is indicated for some PUEs in the real-space chord distribution. It exhibits a sequence of constant long-period bands. The band positions form a Fibonacci sequence. This relates to the underlying chemical synthesis by polyaddition of hard and soft modules, indicating a nearly quasiperiodic setup in sequences of stringed hard domains. These sequences appear to be the probes provided by SAXS for the study of morphology evolution in such PUEs. Should a regular-as-possible arrangement of physical crosslinks optimize a property of the material, then in the synthesis the mole fraction n H of hard modules should be chosen to be n H = /(1 + ) ' 0.62, where is the golden ratio. research papers J. Appl. Cryst. (2015). 48, 313-317 Norbert Stribeck et al. Quasiperiodicity and polyurethanes 317

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

“… varies only slightly with ϵ and has a value of about 8 nm. 8 nm is also the height of hard domains that has been extracted from the data of machine‐cast materials containing MDI hard segments, as well. has been identified from a peculiar quasi‐periodic signature of the CDF peaks and related to a sequence of 2 MDI segments along the chain of the copolymer.…”

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

“…Thus, the interpretation as Fibonacci sequence requires additional considerations that we find by inspection of Figure (the band positions do not appear to be equidistantly spaced), in the unusual response to the macroscopic strain, and in the synthesis path of the studied polyadducts. The corresponding relationships have been discussed in a spin‐off manuscript . The main points shall be briefly sketched here.…”

“…The result is a random copolymer with some restrictions concerning the sequence of the three components (MDI, BD, and PTHF). A detailed discussion and sketches are presented in a previous theoretical paper of some of us.…”

“…Let us introduce a term, the “module” in two variants, namely “soft module” (S) and “hard module” (H). In this study, the soft module is PTHF1000, and the hard module of an infinitely long hard block is 4,4′‐methylene diphenyl diisocyanate (MDI) + 1,4‐butane diol (BD).…”

“…With these materials a peculiar effect is observed which is described in a separate paper. 4 The strain probes in these materials appear to be quasi-periodic sequences of hard and soft modules.…”