Structure,equilibrium and viscoelastic mechanical behaviour of polyurethane networks based on triisocyanate and poly(oxypropylene)diols

Using the theory of branching processes, the detailed structure of polyurethane networks from poly(oxypropylene)diols (PPDs), monofunctional alcohol (cyclohexanol) and triisocyanate prepared at various initial ratios of the reactive groups r_HD ≡ OH]_PPD/NCO] = 0·5−1·7 was characterized in terms of the number, size and structure of elastically active network chains (EANCs), backbone and dangling chains. From an analysis of the dependence of the critical molar ratio at gelation r_HD^c on dilution it follows that PPD samples are composed of molecules bearing primary and secondary hydroxy groups. The branching theory, in which the presence of both primary and secondary hydroxy groups in PPDs is accounted for, adequately describes the dependence of the mass fraction of the sol w_s on r_HD when no side reactions occur in the system (networks in the range r_HD ≥ 1 or networks with monofunctional alcohols). The equilibrium photoelastic behaviour can be described by the junction-fluctuation theory with front factor A = 1 without entanglement contribution. The frequency-temperature superposition can be performed for all networks; the temperature dependence of the horizontal shift factor satisfies the WLF equation. While the position of the main transition region of viscoelastic functions on temperature or frequency depends on the content of the polar triisocyanate component, the shape of these functions at the end of the transition is determined predominantly by the concentration of EANCs.