Studies Of Dynamic Heterogeneities Of Polymers At Glass Transition Determined By Variable-temperature Proton Wide-line NMR

Amorphous polymer systems exhibit complex dynamic heterogeneities which are not related with their structures when close to the glass transition temperature. For most glass systems this results in highly non exponential a relaxation. Its time behavior is related to a superposition of relaxation processes, which have different rates so giving rise to the notion of dynamic heterogeneities. Studies on dynamic heterogeneities are important for understanding the essence of glass transition of amorphous polymer systems.Proton Wide-line NMR has been widely used in studying the dynamics of polymer systems. Combining dipolar filter with spin diffusion, we developed a new pulse sequence, which could provide information about dynamic heterogeneities of amorphous polymers at temperatures around glass transition. Comparing with previously reported methods, the new pulse sequence takes much less experimental time and is much easier in experiment setup. Using the method, we carried out an in-depth study on the dynamic heterogeneities of Poly-vinyl acetate (PVAc) at different temperature. By an empirical formula, we estimated the length scale of the less mobile regions and calculated the percentage of the more and less mobile regions in PVAc.In the Proton Wide-line NMR experiments, the ringing noise which could last as long as 60 us in our NMR spectrometer is one of the major barriers in acquiring reliable signal. Combined with the "Solid Echo", "Windowless Solid Echo" and "phase cycles", three novel pulse sequences which can be used to successfully eliminate ringing effects (REESE) have been introduced. By employing these novel sequences, a proton wide-line spectrum with an echo delay of only 1.1 us was obtained. These novel pulse sequences could be applied not only to the studies on the dynamic heterogeneities, but also to other quantitative analysis.