Origin of multiple melting endotherms in a high hard block content polyurethane: Effect of annealing temperature

A. Saiani, A. Novak, L. Rodier, G. Eeckhaut, J. W. Leenslag, J. S. Higgins

    Research output: Contribution to journalArticlepeer-review

    Abstract

    We have investigated the thermal behavior of a set of model linear thermoplastic polyurethanes (TPU) with a relatively high content of hard segments: from 50 to 100 wt %. The soft segment of these samples consists of poly(propylene oxide), end-capped with ethylene oxide (EO-PPO-EO), while the hard segment is composed of a 4,4'-methylenediphenylene isocyanate (4,4'-MDI) chain extended by a short diol chain, 2-methyl1,3-propanediol (MP-Diol). In the present article, we have investigated the origin of the endotherms observed when samples are annealed below the glass transition of the hard segments T gHs. The work was carried out using mainly differential scanning calorimetry (DSC) and small-angle and wide-angle X-ray scattering (SAXS and WAXS). The so-called "annealing endotherm", TA, was observed 20-30 °C above the annealing temperature. The temperature and enthalpy of TA were found to increase linearly with the logarithm of the annealing time. This endotherm was assigned to the relaxation (physical aging) of the interfacial materials. With increasing annealing temperature a change in the appearance of the hard phase glass transition, TgHP, was observed. For long annealing times TgHP is observed as an endotherm on die DSC thermographs. It is suggested that some of the hard segments undergo relaxation below TgHS resulting in an enthalpy relaxation endotherm being present below or around TgHP. An additional endotherm, TM; was observed as a shoulder at high temperature, just below the microphase mixing transition, TMMT. This endotherm is thought to be due to the ordering during the phase separation process of the hard segment present in the mixed phase. Finally TTMM was observed at all annealing temperature used suggesting that even at low annealing temperatures phase separation occurs. The delay time before phase separation starts and the maximum absolute degree of phase separation reached are found to increase with increasing annealing temperature. Our results suggest that a "thermodynamic equilibrium" is reached for each annealing temperature at long enough annealing times. © 2007 American Chemical Society.
    Original languageEnglish
    Pages (from-to)7252-7262
    Number of pages10
    JournalMacromolecules
    Volume40
    Issue number20
    DOIs
    Publication statusPublished - 2 Oct 2007

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