Proton spin relaxation and molecular motion in a bulk polycarbonate

Abstract

Proton spin-lattice relaxation and proton spin-lattice relaxation in the rotating frame were observed in the solid polycarbonate of l,l-dichloro-2,2-bis(4-hydroxyphenyl)ethylene from 150 to 400 K. This polymer is suited to this type of study since it contains only a single type of proton. The relaxation times are quantitatively interpreted as a function of temperature using a correlation function-spectral density approach. Two types of correlation functions that are somewhat successful result from the one-dimensional defect diffusion model (DDM) and the correlated states model (CSM). Both can adequately simulate the proton relaxation data but the CSM also successfully accounts for the temperature dependence of the second moment and accurately predicts the location and much of the width of the low-temperature mechanical loss peak. The activation energy of the fundamental process in the CSM is also close to that observed in dilute solutions of the same polycarbonate and close to that calculated by MNDO (modified neglect of diatomic overlap) applied to model compounds. The Hartree-Fock calculations in conjunction with the earlier proton line width study are used to consider the repeat unit level geometry of the motion causing both spin relaxation and mechanical loss.

On the suitability of semiempirical calculations as sources of force field parameters

“…5), which take values of a few tenths of kcal/mol (see Table 9). From the results presented above, it can be stated that semiempirical methods (especially the AM 1), even though they minimize the torsional energy barriers, can be good alternatives to highly sophisticated ab initio calculations, since results reported here show deviations between semiempirical and empirical derived parameters that are within the narrow range of variability of experimental values, and in good agreement with results reported by other authors [4,[35][36][37][38][39]. Nevertheless, it has been also reported [4] that semiempirical methods lead to bad representations of some torsions, like those of conjugate bonds, for which sophisticated ab initio calculations, including correlation effects, are sometimes necessary.…”

Physical basis of fragility

“…Detailed studies of the polycarbonate molecule suggest that a mobile carbonate unit (CO 3 ) is one defect whose motion contributes to the mechanical, dielectric and nuclear magnetic relaxation in the glassy state. [23][24][25] Other types of defects have not been identified to date. However, it is interesting to speculate on the nature of the defects in other materials.…”

Molecular dynamics of solid polymers as revealed by deuteron NMR

“…Considering the importance of the subject, experimental methods capable of elucidating the details of motional mechanisms are surprisingly scarce. Among the various techniques employed in this area [4] nuclear magnetic resonance (NMR) is well established although conventional proton wide line data [5] can quantitatively be analyzed in exceptional cases only [6].…”