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Dynamics of polyurethane elastomeric polymers by solid state nuclear magnetic resonance
Abstract
2D solid-state exchange NMR has been used to investigate the motion of a cross-link unit in a model network. The $\sp{31}$P chemical shift anisotropy provides an orientational probe of the tris(4-isocyanatophenyl)thiophosphate (TIPTP) cross-link in the network interconnected with telechelic poly(propylene glycol). 2D spectra were taken at two temperatures near the glass transition and at a series of mix times. From the variation of the 2D NMR pattern with temperature and mix time, the motion of the cross-link is identified as Brownian reorientational diffusion. Good simulations of the spectra were obtained using a distribution of the Williams-Watts stretched exponential correlation times. The temperature dependence of the cross-link motion follows the WLF equation. The parameters derived from the NMR data are sufficient to describe the temperature dependence and breadth of both the dielectric and mechanical loss associated with the glass transition. Solid $\sp{31}$P spin-lattice relaxation times (T$\sb1\sp{\rm P})$ have been measured over a wide range of temperatures for a series of network polymers with molecular weights between cross-links (M$\sb{\rm c})$ ranging from 250 to 2900. The networks were formed from poly(tetrahydrofuran) and TIPTP. The chain length dependence of the T$\sb{\rm g}$ obeys the Fox-Loshaek relation. The dominant mechanism for T$\sb1\sp{\rm P}$ relaxation was found to be chemical shift anisotropy. The data fitted equally well to the Cole-Cole or the Williams-Watts relaxation functions. The motion of cross-links can be described quantitatively by the activation energies and the coupling parameters. The effect of increasing M$\sb{\rm c}$ is to reduce the width of the distributions and the co-operative motional mode in the cross-link point. $\sp{13}$C NMR spectra and relaxation times have been measured as a function of extension on a segmented polyester and polyether-polyurethane elastomer by both cross-polarization and Bloch decay techniques. A sample spooling technique was developed for magic angle spinning of elastomers under extension. Analysis of cross-polarization dynamics allows semiquantitative analysis of three soft segment motional regimes over the full extension range: bulk, strained, and crystalline. The characteristic times for dipolar dephasing (T$\sb2\sp*),$ spin-lattice relaxation (T$\sb1\sp{\rm C}$ and T$\sb1\sp{\rm H}),$ and rotating-frame relaxation (T$\sb{1\rho}\sp{\rm H}$) were sensitive to the extension on thermoplastic elastomers.
Subject Area
Polymer chemistry|Physical chemistry|Chemistry
Recommended Citation
Shi, Jie-Feng, "Dynamics of polyurethane elastomeric polymers by solid state nuclear magnetic resonance" (1993). Doctoral Dissertations Available from Proquest. AAI9408347.
https://scholarworks.umass.edu/dissertations/AAI9408347