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 methacrylate) (PnDMA) have been studied, respectively, with X-ray diffraction and dielectric spectroscopy, dynamic light scattering, and rheology at temperatures below and above the glass transition temperature T g ∼ 215 K. We find three dielectrically active processes, starting from low temperatures: (i) the γ-relaxation deep into the glassy state (with an activation energy of 5.7 kcal/mol), (ii) the “fast” β-relaxation just above the calorimetric T g, with an activation energy of 11.4 kcal/mol, and (iii) the mixed αβ-relaxation which has many similarities to a single α-process. Dynamic light scattering also identified the last two processes. The distribution of relaxation times for the αβ-relaxation exhibits a strong T dependence 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 is a common feature of poly( n -alkyl methacrylates) with long side chains, reflects the contributions from concentration fluctuations from the PMMA-like backbone and the PE-like side chain. We compare our structural 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. We found that this dynamic property has its origin on the low van der Waals peak (LVDW) of the static structure factor and reflects mainly differences in intersegmental distances and packing.