Structural relaxation processes in Poly(ethylene glycol) methacrylate macromonomers

The investigation of the structural relaxation properties of poly(ethylene glycol) methacrylates are of practical interest, due to their ability to work as the inert backbone to which poly(ethylene oxide) oligomers can be attached to obtain highly amorphous polymeric matrices.

Two poly(ethylene glycol) methacrylate, PEGMA, macromonomers, with different side chain lengths, are investigated by the dielectric technique, in the frequency range 0.3-300 kHz, and by Brillouin scattering. The analysis of the Brillouin spectra gives evidence for the existence of a relaxation process in both the systems. Furthermore, a comparison of the normalized absorption data with their corresponding classical values (deduced from the shear viscosity data) suggests that what the Brillouin scattering experiment detects is a dissipative relaxational process in which the shear viscosity plays the main role. The comparison of the Brillouin scattering results with the dielectric data shows that what we are observing, in both the systems, is a single relaxation process. The temperature dependences of the relaxation times, observed on more than 10 decades, fail to follow simple Arrhenian behaviours. Both the systems can be interpreted as intermediate between strong and fragile liquids, following the Angell classification, and appear characterized by the existence of a wide variety of local structural environments, triggered by some relaxation process. Such a situation is more clearly evidenced by the macromonomer with longer chain. From the whole body of experimental data, it can be deduced that shorter-side-chain PEGMA macromonomers are better candidates for the formation of highly amorphous comb-branched polymers.