Crystallization of hard segments in MDI/BD-based polyurethanes deformed at elevated temperature and their dependence on the MDI/BD content

• Published in: European polymer journal Vol. 97; pp. 423 - 436
• Main Authors: Li, Xuke, Lu, Ying, Wang, Hai, Pöselt, Elmar, Eling, Berend, Men, Yongfeng
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.12.2017; Elsevier BV
• Subjects: Butanediol; Computation; Crystal form transformation; Crystallization; Deformation; Diphenyl methane diisocyanate; Distribution functions; Hard domain elongation; Hard segment content; High temperature; Periodic microfibrils; Polyurethane; Polyurethane resins; Scattering; Small angle X ray scattering; Strain-induced crystallization; Temperature dependence; Thermoplastic polyurethane; Thermoplastics; Urethane thermoplastic elastomers; Strain-induced crystallization; Thermoplastic polyurethane; Hard segment content; Hard domain elongation; Periodic microfibrils; Crystal form transformation
• ISSN: 0014-3057; 1873-1945
• DOI: 10.1016/j.eurpolymj.2017.10.014

[Display omitted] •Strain-induced crystallization of amorphous hard segments of TPU with low HSC is proposed.•Para-crystalline hard segments of TPU with high HSC transform into form III of MDI/BD modifications.•Formed microfibrils exhibit lateral periodic correlation.•Thickness of hard domain can be elongated at 100 °C. Morphological variation of thermoplastic polyurethanes (TPUs) with different hard segment contents (HSCs) stretched at 100 °C higher than the Tg of hard domain was monitored by in-situ wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) techniques. WAXS patterns reveal that the crystalline modification of TPUs with high HSCs transforms from paracrystal (termed as form I) to form III of 4,4′-methylene diphenyl diisocyanate/1,4-butanediol (MDI/BD) segment, and TPUs with low HSCs show the direct transition from amorphous MDI/BD packing to form III packing. The form III crystals are fibrillar hard domains. Chord distribution functions (CDFs) computed from the SAXS patterns were employed to illustrate the domain topology. The streak-like CDF peaks off the meridian indicate the existence of cylindrical microfibrils which are composed of very low inhomogeneities between the destructured hard domains and stretched soft domains along the straining direction. These microfibrils of TPUs with low HSCs present a periodically lateral correlation. Interface distribution function (IDF) and chord length distribution (CLD) were computed from Bonart's longitudinal and transverse projection to quantify the domain stacking and microfibril arrangement respectively. The average size of hard domains increases up to a constant size with the increasing macroscopic strain which may be related to the maximal sequence length of hard segment. The average size of soft domain also reaches up to a constant size at large strain that is smaller than the fully-extended length of soft segment. The variation of domain size is different from deformation at room temperature. The HSC affects the behaviors of morphological evolution significantly.