Properties of thermoplastic polyurethane elastomers in solution

Fundamental correlations between the molar mass and the solution viscosity (dimethylformamide (DMF)/1% di-n-butylamine (DBA)) are acquired and discussed on selected polyesterurethanes (PUREs) and polyetherurethanes (PUR-Et) with NCO/OH ratios ranging from 955 to 1030. Static light scattering measurements reveal mass average molar masses in the range 20 000-95 000 g mol(-1) membrane osmometry reveals number average molar masses in the range 15 000-50 000 g mol(-1). Comparative tests with the analytical ultracentrifuge reveal the agreement with respect to the molar mass and the At-values; additionally this measurements confirm unimodal Schulz-Zimm molar mass distributions with an average sedimentation constant of S = 1.1 sved (1 sved = 10(-13) s). The results of light scattering und ultracentrifuge measurements show that the solvent mixture DMF/1 wt.-% DBA is well suitable for molar mass measurements and solution viscosity measurements. The linear light scattering Zimm diagram and the sedimentation run exclude microgels and higher molecular associates (for example allophanate and biuret crosslinkage). Because of the good correlation between the determined molar masses and the intrinsic viscosities [eta] and the viscosities of the concentrated solutions it was possible to establish a calibration function [eta] = K M-a. The exponent a = 0.6-0.9 of the Mark-Houwink-relation indicates that the polymer chains, in diluted solution, are isolated coiled molecules. In concentrated solutions the chains are partly crosslinked, depending on the molar mass. The correlation between the molar mass and the viscosity (Fox-relation) shows an unsteadiness at M-w = 55 000 g mol(-1) (critical molar mass). Above the critical molar mass the solution viscosity increases with the molar mass with an exponent of epsilon = 3.4. Below the critical molar mass of M-w = 55 000 g mol(-1) the solution viscosity increases straight proportional with the molar mass (epsilon = 1).