STRUCTURE AND DYNAMICS OF POLY(N-DECYL METHACRYLATE) BELOW AND ABOVE THE GLASS-TRANSITION

The structure and dynamics of the ''strong'' glass of poly(n-decyl met hacrylate) (PnDMA) have been studied, respectively, with X-ray diffrac tion and dielectric spectroscopy, dynamic light scattering, and rheolo gy at temperatures below and above the glass transition temperature T- g similar to 215 K. We find three dielectrically active processes, sta rting from low temperatures: (i) the gamma-relaxation deep into the gl assy state (with an activation energy of 5.7 kcal/mol), (ii) the ''fas t'' beta-relaxation just above the calorimetric T-g, with an activatio n energy of 11.4 kcal/mol, and (iii) the mixed alpha beta-relaxation w hich has many similarities to a single alpha-process. Dynamic light sc attering also identified the last two processes. The distribution of r elaxation times for the alpha beta-relaxation exhibits a strong T depe ndence ranging from a Kohlraush-Williams-Watts (KWW) parameter of 0.25 at T-g to about 0.7 at T-g + 115 K. This strong T dependence, which i s a common feature of poly(n-alkyl methacrylates) with long side chain s, reflects the contributions from concentration fluctuations from the PMMA-like backbone and the PE-like side chain. We compare our structu ral and dynamic results with those from the other members of the poly( n-alkyl methacrylates) series. There is a pronounced dependence of the fragility or steepness index on the length of the alkyl side chain. W e found that this dynamic property has its origin on the low van der W aals peak (LVDW) of the static structure factor and reflects mainly di fferences in intersegmental distances and packing.