Compression-Molded Polyurethane Block copolymers. 2. Evaluation of Microphase Compositions

Two general methods are presented for the determination of microphase compositions of pseudo-two-phase segmented block copolymers. The development is based upon combined differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) analyses. The first method employs an extension of the Couchman equations to relate microphase compositions to glass transition temperatures. The second procedure uses a modified Fox formalism for this purpose. Microphase compositions are estimated by these methods for a series of polyurethane segmented copolymers synthesized from 4,4′-diphenylmethane diisocyanate (MDI), 1,4-butanediol (BDO), and poly(oxypropylene glycol). The self-consistency of the calculations is checked by comparing measured and estimated electron density variances. The results of the extended Couchman analysis are found to be inconsistent with experimental data. The modified Fox treatment leads to results which are qualitatively self-consistent. Calculated microphase compositions are used to estimate the critical hard-segment sequence length below which hard sequences dissolve within the soft microphase. The estimated critical lengths fall in the range of 5-7 (MDI residues per sequence). The overall phase mixing behavior and morphology are shown to be in qualitative agreement with the predictions of the Koberstein-Stein model for hard microdomain structure.