Structure‐property relationships in thermoplastic elastomers: I. Segmented polyether‐polyurethanes

Abstract

AbstractSegmented poly(ether‐b‐urethanes) have been synthesized with 2000 MW polypropylene oxide coupled with diisocyanates and diol type chain extenders. The diisocyanates used were symmetric rigid 4, 4′‐diphenylmethane diisocyanate (MDI), linear aliphatic hexamethylene diisocyanate (HDI), and unsymmetric rigid toluene‐2, 4‐diisocyanate (TDI). The chain extenders were symmetric N, N′‐bis(2‐hydroxyethyl) terephthalamide (BT) and N, N′‐bis(2‐hydroxyethyl)‐hydroquinone (BH) unsymmetric N, N′‐bis(2‐hydroxyethyl)isophthalamide, and linear aliphatic 1, 4‐butanediol (B). Hard segment contents ranged from 20 to 40 wt percent. The thermal behavior of these materials is consistent with phase separation into separate hard and soft domains, In order of increasing temperature above the soft segment Tg, there are transitions which occur in the regions −56 to −36°C (Ta), 70 to 90°C (Tb), and 138 to 168°C (Tm). The former is probably associated with soft segment change from a viscoelastic to an elastomeric state. Values of Ta are ∼ −51 C and −56°C for the MDI‐BT and HDI‐BT polymers, respectively, and are independent of hard segment content. Microscopy showed that the former polymers have spherulitic morphology, so these materials have good microphase separation and exhibit crosslinked elastomeric properties. The TDI‐BT or BI and MDI‐B polyurethane have composition‐independent Ta values of −41 and −36°C, respectively. These materials probably have considerable “domain‐bound‐ary‐mixing”. At low hard segment content the MDI‐B polymers behave as non‐crosslinked elastomers. Only the MDI‐BI polymers have Ta values, which are strongly affected by composition, increasing in magnitude with increasing of hard segment content. This is interpreted as significant “mixing‐in‐domains” and is supported by morphology observed by microscopy. The next higher transition, Tb, probably involves dissociation of interdomain hydrogen bonding. In the case of the MDI‐BT polyurethanes, the spherulites associated with the hard domains had disappeared at 141°C and the few small spherulites in the MDI‐BI polymers disappeared at 130°C. The Tb values are 70, 83 to 90, and 100°C for the MDI‐B, HDI‐BT, and HDI‐BI polymers, respectively. The melting transitions occurred between 138 to 168°C for the various polyurethanes except for the MDI‐BT systems which decompose before melting. Thermal decomposition is a two‐stage process. Hard segments decompose between 200 and 300°C. The initial decomposition temperatures are lowered in the presence of strong acid. Soft segments decompose at higher temperatures. The mechanical properties of the MDI‐BI polyurethanes are charateristic of crosslinked elastomer, the results of which will be presented in a subsequent paper.