The structure and elasticity of polyurethane networks: 1. Model networks of poly(oxypropylene) triols and diisocyanate

The equilibrium mechanical and optical behaviour of networks prepared from poly(oxypropylene) triols (PPT) and 4,4′-diphenylmethane diisocyanate (MDI) at various initial molar ratios of reactive groups, $\text{r}{}_{\mathrm{<mtext>H</mtext>}}\text{=}\text{OH}\text{]}\text{NCO}\text{]}$, in the range 0.6 < r_H < 1.75 have been investigated. The reaction proceeded at 80°C and in the presence of an organotin catalyst to a high conversion of minotirty groups (0.96–1.0). A comparison between experimental and theoretical (based on the theory of branching processes) dependences of the equilibrium modulus, G_e, on r_H or on the sol content, w_s, led to the following conclusions: (1) In the range r_H ? 1 the theory adequately describes experimental dependences, while in the range r_H < 1 excess crosslinking takes place, obviously due to the formation of allophanate groups. (2) In PPT networks with a higher molecular weight (M_n = 2630), G_e is about twice the theoretical value for the front factor A = 1, which is interpreted as a contribution due to permanent interchain interactions. (3) In networks of lower PPT (M_n = 708), experimental G_e values lie between the theoretical ones calculated for A = 1 (affine deformation crosslinks) and $\text{A =}\text{1}\text{3}$ (phantom network). (4) The difference between the eperimental and theoretical moduli for $\text{A =}\text{1}\text{3}$ may be adequately described by using Langley's concept of trapped entanglement contribution (for networks of the longer triol quite satisfactorily, for those of shorter PPT with some systematic deviation) with the same proportionality constant. (5) For correlations between the experiment and theory a generalized plot of the reduced modulus of the weight fraction of the gel proved to be useful.