Structural Relaxation Processes in Polyethylene Glycol/CCl₄ Solutions by Brillouin Scattering

Abstract

We present results of a Brillouin scattering experiment on solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in CCl4. The relaxation process detected has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. The concentration dependencies of the hypersound velocity and normalized absorption are compared against the indications from several models proposed in the literature. The concentration evolution of the system is described in terms of two distinct regimes. At high polymer content, the system is dominated by the structure of the dense polymer, where polymer-polymer interactions, together with excluded volume effects, induce the existence of a preferred local arrangement resulting in a narrow distribution of the relaxation times, with the average value of the relaxation time following a simple Arrhenius temperature dependence. As the concentration decreases, the original structure of the hydrogen bonded polymer network is destroyed, and a number of different local configuration coexist, giving rise to a wider distribution of relaxation times or to a multiple relaxation. At low concentrations, the experimental data are well fitted assuming a Vogel-Fulker-Tammon behavior for the average relaxation time. In addition, the observed deviation from the ideal behavior for the refractive index and the density suggests that CCl4 does not behave as an inert solvent, and due to polarization effects, it can develop local hetero-associated structures via electrostatic interaction with the O-H end groups of the polymeric chains. The hypothesis has been successfully tested by fitting the concentration behavior of the hypersonic velocity to a recent three-component model, suitable to describe the concentration dependence of sound velocity in moderately interacting fluids. The indication of the model furnishes a very high value for the association constant of the PEG600, confirming the literature indication that, in polymeric systems capable of developing long liner aggregates via hydrogen bonding interaction, the Brillouin probe is insensitive to the true length of the polymeric chains. The Brillouin scattering experiment just sees an effective hydrogen bonded aggregate that is huge relative to the length of the single polymeric chain and becomes sensitive only to the density fluctuations of the local segmental motions.

Lattice Dynamics of a Protein Crystal

“…This difference is due, in part, to the temperature dependence of v. Proteins in general [19,20], and RNase in particular [21], exhibit a dynamical transition with temperature resembling the glass transition. For other polymer systems exhibiting a glass transition, v has been found to decrease approximately linearly with increasing temperature [22 -25], with slopes ranging from ÿ4:3 to ÿ2:2 ms ÿ1 K ÿ1 for polyethylene glycol (PEG) in CCl 4 [25] and poly(4-methyl-1-pentene) (P4MP1) [22], respectively, and in particular ÿ2:5 ms ÿ1 K ÿ1 for crystals of lysozyme [hen egg-white lysozyme (HEWL) [24] ]. Moreover, whereas an earlier measurement (using laser-generated ultrasound, the same technique as in Ref.…”

Spectroscopic and photopolymerization studies of benzyl methacrylate/poly(benzyl methacrylate) two‐component system

“…As aforementioned, it is difficult to find general formulae for the relaxation behavior in a wide temperature range. However, sometimes in the high temperature range the relaxation is well described by assigning a simple relaxation time to it 17, 32, 33. The simple viscoelastic theory based on a single relaxation time approximation was adopted to estimate the relaxation parameters for BzMA.…”

High‐Frequency Dynamical Behavior of Poly(ethylene glycol)+H2O Mixtures by Brillouin Spectroscopy

“…At high concentrations the values of the hypersonic velocity are definitely higher than those measured by the ultrasonic probe confirming a viscoelastic behavior already observed in similar com-pounds. [10,17] The concentration characteristic of normalized absorption (presented in Figure 2) shows a quite complicated nonmonotonic behavior, suggesting the existence of concentration dependent relaxation process.…”

“…Also in earlier Brillouin spectroscopy investigations on PEG200 and PEG400 [5][6][7] a single relaxation process was revealed taking place on a time-scale of the order of 20ps, very close to that observed in our recent Brillouin scattering experiments in PEG600 and its solutions in few organic solvents. [8,9] The time-scale separation between processes revealed by these two highfrequency spectroscopies suggests that dielectric and Brillouin methods analyze two different relaxation processes. This suggestion has been used in interpretation of the dynamical results obtained in our recent Brillouin scattering experiment in PEG600/H 2 O mixtures.…”

“…This suggestion has been used in interpretation of the dynamical results obtained in our recent Brillouin scattering experiment in PEG600/H 2 O mixtures. [10] The fast process revealed by Brillouin scattering has been interpreted as a consequence of the crankshaft motion of C-O-C groups, whereas the slower process, observed in dielectric experiment, has been assigned to the dynamics of the ether oxygens involved as acceptors in hydrogen bonds. In view of those considerations, the absence of the fast process in dielectric spectrum has been interpreted as the result of a quite weak dipole moment fluctuation connected with the crankshaft motion.…”