Chemical Clusters in Polyurethane Networks. SAXS, Photoelastic and Dynamic Mechanical Behavior of Networks from Poly(Oxypropylene)Diol, Diisocyanate, and Trimethylolpropane Prepared One‐Stage and Two‐Stage Process

Abstract

The effect of the initial ratio of components and of the one-or two-stage process of formation of polyurethane networks prepared from poly(oxypropylene)diol (PD, M n ¼ 2100), 4,4 0 -diisocyanatodiphenylmethane (MDI) and trimethylolpropane (TMP) on their extraction, swelling, small-angle X-ray spectroscopy (SAXS), photoelastic, and dynamic mechanical behavior was investigated. The networks were prepared at various ratios of hydroxy (OH) to isocyanate (NCO) groups, r HT ¼ [OH] TMP /(0.5[NCO]) ¼ 1 -1.5, by a one-or two-stage process (in the first step [OH] PD /[NCO] ¼ 1/2) up to the full conversion of NCO groups. Due to decreasing reactivity of OH groups on TMP a nonuniform distribution of hard (TMP-MDI-TMP) segments (chemical clusters) was developed in both network series. The SAXS experiments proved that one-stage networks exhibit a more heterogeneous structure than two-stage ones. As expected, the weight fraction of the gel, w g , and equilibrium modulus, G e , decrease with increasing deviation from stoichiometry; the G e and w g values of one-stage networks at constant r HT are always higher than those of twostage networks. The frequency-temperature ( f-T ) superposition of both components of the complex compliance J Ã (¼J 0 -iJ 00 ) was performed and the horizontal shift factor, log a T , satisfied the WLF equation for all networks. From the dependence of superimposed storage, J 0 p , and loss, J 00 p compliances on reduced frequency f.a T , three contributions could be determined from the Cole-Cole distribution; each process was characterized by the relaxation strength, DJ i , relaxation time, t i and distribution broadness, b i . The broadness b and strength DJ of the high and medium frequency process (i ¼ 1,2), which correspond to the glass-rubber transition, are roughly independent of r HT and the preparation method. The t 3 and DJ 3 values of

Relationship between Photoelasticity of Polyurethane and Dielectric Anisotropy of Diisocyanate, and Application of High-Photoelasticity Polyurethane to Tactile Sensor for Robot Hands

“…Scheme 1 shows how a polyurethane sheet produces photoelasticity when subjected to a tensile force. As shown in this figure, polyurethane forms a network by polymerizing a bifunctional polyol, a bifunctional diisocyanate, and a small amount of trifunctional triol as a crosslinker [ 16 ]. The diisocyanate is bound to the polyol along the long axis of its molecule, and when the sheet is stretched in the vertical direction, the long axis of the diisocyanate is partially oriented in the vertical direction.…”

“…It is known that the photoelasticity of polyurethane is derived from aromatic diisocyanate [ 16 ]. We assumed that the photoelasticity of polyurethane depended on the concentration of the aromatic ring of diisocyanate, and in an unpublished study, we investigated the relationship between the 4,4′-methylenediphenyl diisocyanate (MDI) content in polyurethane and photoelasticity using polybutadiene polyol and MDI ( Figure 1 ).…”