Primary Chemical Sequence Ultimately Determines Crystal Thickness in Segmented All-Aliphatic Copolymersстатья

Статья опубликована в высокорейтинговом журнале

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Дата последнего поиска статьи во внешних источниках: 20 января 2015 г.

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[1] Primary chemical sequence ultimately determines crystal thickness in segmented all-aliphatic copolymers / Y. I. Odarchenko, D. V. Anokhin, D. David et al. // Macromolecules. — 2014. — Vol. 47, no. 22. — P. 7890–7899. Thermoplastic elastomers (TPEs) are known to exhibit a phase-separated morphology which depends on their chemical structure and processing. The design of novel TPEs with predefined properties which are also independent of the material thermal history has so far remained a challenge. The focus of this work is on the semicrystalline morphology of all-aliphatic thermoplastic elastomers consisting of alternating polytetrahydrofuran (PTHF) segments and uniform glycine or β-alanine bisoxalamide units. The thickness of the hard segment crystals was found to be highly monodisperse and independent of the sample thermal history. Using Nanocalorimetry, we observed that at cooling rates as high as 12 000 oC s–1 the bisoxalamide segments can still crystallize although the crystallization temperature decreases by ca. 26 oC. The surface free energy of the hard block crystals is found to be extremely low (∼18 mJ·m−2), which is likely due to the entropic contribution of soft segments forming tie chains bridging the neighboring crystals. To investigate the combined effect of crystal orientation and phase transitions, simultaneous time-resolved X-ray scattering and mechanical tensile tests were performed. Upon stretching, elastomeric PTHF segments with lengths above 1000 g mol–1 crystallize at ambient temperatures. Under these conditions two main morphologies were observed: at low strains the long axes of the fibril-like crystals were oriented parallel to the flow direction, whereas higher strains caused bisoxalamide crystal fragmentation and changed their preferential direction to the one perpendicular to the drawing direction. The chain tilts in the bisoxalamide crystals were calculated from the characteristic four-spot SAXS patterns and were ∼5o–16o in the case of glycine end-groups and 24o for alanine and propyl terminal groups. To our knowledge, this is the first attempt to determine the chain tilt for the nonlamellar crystals in block copolymers. [ DOI ]

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