Quantification of competitive hydrogen bonding between hard and soft segments in polyurethanes: quantum mechanical calculations and experimental results

摘要

Segmented polyurethanes and polyurethaneureas are probably the most widely investigated copolymer systems both by academic and industrial researchers. This is mainly because of the availability of a wide range of starting materials leading to the preparation of polyurethane systems with properties ranging from rubberlike elastomers to tough strong thermoplastic resins. It has been demonstrated that interesting solid-state properties of segmented polyurethanes and polyureas are due to the phase separation between hard (urethane, urea) and soft (polyether, polyester, polysiloxane) segments. Microphase morphologies of elastomeric systems consist of a continuous soft segment interconnected with hard segment domains. Although there is a general agreement on the two phase morphologies of polyurethanes, there is veery limited information on the actual composition (purity) of the hard and soft segment phases. It is well documented that various factors strongly influence the phase separation and morphology of segmented polyurethanes. These include (i) the structure and average molecular weight of hard and soft segments, (ii) inherent solubility of hard and soft segments, (iii) crystallization of hard and soft segments, (iv) extent of competitive hydrogen bonding between hard and soft segments, and (v) nature of interdomain interface. Although there have been numerous studies to understand the influence of each of these factors on the morphological behavior of polyurethanes, the actual picture is still not very clear. It has been demonstrated that when the hard and soft segments are not inherently soluble in each other, or do not form any hydrogen bonding, such as siloxane-urea copolymers, they display well separated microphase morphologies and excellent thermal and mechanical properties. When the free energy of mixing (ΔGmix = ΔHmix - TΔSmix) is considered, two factors seem to have the most important contribution under ambient conditions (at room temperature). Both of these are included in the enthalpy (ΔHmix) term. These are (i) the hydrogen bonding between hard-hard and hard-soft segments and the relative magnitudes of H-bond energies (ΔHhb-hh and ΔHhb-hs), where ΔHhb-hh and ΔHhb-hs indicate hydrogen bond energies between hard-hard and hard-soft segments respectively, and, (ii) crystallization of the hard and soft segments and their enthalpy of fusion (ΔHfus). In addition to the crystallization of pure hard and soft segments, another possibility that should be considered is the co-crystallization of mixed (hard+soft) segments. It has already been reported that blends of amorphous polyethers and amorphous polyurethane hard segments show complete miscibility over any composition range. It has also been demonstrated that when stress-strain behavior of polyurethanes with identical hard/soft segment compositions are compared, copolymers with crystallizable soft segments display higher modulus and ultimate tensile strengths.