Influence of the structure of soft and stiff chain fragments on propertiesof segmented polyurethanes. I. Phase morphology

Model segmented polyurethanes (SPUs) prepared from either oxypropylene glyc ol oligomer or butylene adipate glycol oligomer, both of molar mass 2 kg/mo l (soft fragments, SFT), and three different diisocyanates (all-trans 4.4'- dicyclohexylmethane diisocyanate, t, t-HMDI-1.0; HMDI with 20% of trans iso mers, t, t-HMDI-0.2; and 4, 4'-diphenylmethane diisocyanate, MDI) (stiff fr agments, STF) were characterized by specific heat capacity measurements in the temperature interval 140-540 K, and by wide-angle and small-angle X-ray scattering at room temperature. Limited miscibility of SFT and STF chain c omponents resulted in incomplete separation into a regular three-dimensiona l macrolattice of STF-rich microdomains and SFT-rich microphases. The compo sition of STF-rich microdomains was estimated by fitting the softening temp eratures to the Couchman's equation, whereas the relative contents of SFT-r ich and STF-rich microphases were assessed by comparing the specific heat c apacity change at the glass transition temperatures to corresponding additi ve values. The overall degree of microphase separation, as well as the mean macrolattice spacings between STF microdomains decreased in the order, MDI > t, t-HMDI-1.0 much greater than t, t-HMDI-0.2. The conformation of STF f ragments within the STF-rich microdomains changed h-om nearly extended (for MDI) through slightly contracted (for t, t-HMDI-1.0) to strongly contracte d (for t, t-HMDI-0.2).