Deuteron NMR measurements of order and mobility in the hard segments of a model polyurethane

Abstract

Pulsed deuteron NMR results are reported for a series of segmented model polyurethanes with monodisperse hard segments containing specifically labeled sites. The hard segments consist of five piperazine rings ( 1-5) separated by carbonyloxytetramethyleneoxycarbonyl spacers; three labeled derivatives are used, with piperazine-d_8 at the 1,5-, 2,4-, and 3-rings. The soft segments are polydisperse poly(tetramethylene oxide) (M_n ≈ 2000). To facilitate interpretation of the polyurethane spectra, measurements are also made on model compounds of the hard segment oligomer, and a model polymer consisting of soft segments chain extended by individual piperazine-d_8 rings. The results indicate that 85% of the hard segments exist in the hard phase, with low mobility at temperatures below 410 K (≈T_m - 10 K). The rest of the hard segments appear to be dispersed in the soft phase, with high mobility at temperatures above 260 K (≈T_g + 60 K). Molecular mobility is much greater at the exterior than the center of hard segments in the hard phase at temperatures between 300 and 410 K. Comparison of changes in molecular mobility and in the storage modulus with temperature (T) shows that (1) as T increases from 200 to 280 K, increase in mobility of the soft phase due to the glass-to-rubber transition and crystallite melting correlates with the 100-fold decrease in modulus in this range, (2) as T increases to 400 K, increase in mobility at the exterior of the hard phase correlates with the gradual decrease in modulus, and (3) at T ≈ 410 K, onset of high mobility at the center of hard segments in the hard phase correlates with the loss of mechanical integrity.