| The nature of hydrogen bonding, phase segregation, and hard segment organization is studied in a series of segmented polyurethanes based on hydroxy terminated polybutadiene (HTPBD) soft segments and hard segments consisting of either 2,4- or 2,6-toluene diisocyanate (TDI) and 1,4-butanediol (BDL). The existence of two phase morphology is deduced from dynamic mechanical relaxation and thermal analysis. All polymers exhibit a soft segment glass transition very close to Tg of free HTPBD homopolymer and independent of hard segment structure and concentration. The high temperature transitions, corresponding to the glass transition in 2,4-TDI polymers and melting in the 2,6-TDI polymers are strongly dependent upon hard segment concentration and average hard segment length. Thermal history is found to have negligible effect on the position of the soft segment glass transition and, therefore, in the degree of phase segregation for all materials studied. The time-dependence in dynamic mechanical and thermal properties is related to physical changes within the hard segment domains, independent of the soft segment matrix.; An analysis of hydrogen bonding in a selection of HTPBD polyurethanes and model compounds based on 2,4- and 2,6-TDI and p,p'-diphenylmethane diisocyanate (MDI) and BDL hard segments is made using Fourier Transform Infrared Spectroscopy (FTIR). Changes in the frequency, half-width and intensity of the bonded N-H absorption band resulting from thermal treatment are correlated to structural changes as evidenced by differential scanning calorimetry (DSC); variations in behavior are related to differences in packing and the ability and ease at which crystallization and reorganization of the hard segment repeat unit occurs. Resolution of the N-H band into its hydrogen bonded and non-bonded components indicates 80 to 90 percent of the N-H groups are hydrogen bonded at room temperature. Since hydrogen bonded interactions are limited to the hard segment components in PBD-containing polyurethanes, the results provide a quantitative measure of the extent of microphase segregation. Temperature-dependent studies indicate that the onset temperature for hydrogen bond dissociation occurs at 25 to 75(DEGREES)C in the 2,4-TDI polymers and at about 130(DEGREES)C in the 2,6-TDI polymers, close to the transition temperatures for the amorphous and crystalline domain structure, respectively. Heat of dissociation is about 4 kcal/mole for the former series and 7-8 kcal/mole for the latter. Overall, the results indicate that the thermal behavior of hydrogen bonding is sensitive to structure organization and correlate well with other properties studied via calorimetry and dynamic mechanical relaxation. |
