Local modification of solvent dynamics by polymeric solutes

The Journal of Chemical Physics

Santangelo

Roland

2002

Dynamic mechanical spectroscopy, dielectric spectroscopy, and viscosity measurements were carried out on Aroclor 1254 ͑PCB54͒, a chlorinated biphenyl. The temperature dependences of both the dielectric relaxation times and the viscosity depart from a single Vogel-Fulcher behavior, at a temperature, T B , equal to 294 K. The ratio of this characteristic temperature to T g (ϭ246 K) is close to that found previously for other Aroclors, having different chlorine levels. The shapes of the dielectric and mechanical relaxation functions were equivalent, yielding a value for the Kohlrausch stretch exponent equal to 0.62 5 at T g. However, the relaxation times for the dielectric modulus were substantially larger than the viscosity and mechanical relaxation times. The respective temperature dependences of the viscosity and the dielectric relaxation times, although similar, deviated from the Debye-Stokes relation with decreasing temperature. The temperature associated with this deviation is somewhat higher than T B , and coincides with the temperature at which the relaxation function approaches Debye behavior. These phenomena all reflect the alleviation at high temperature of intermolecular constraints on the dynamics. The addition of high molecular weight polystyrene ͑PS-90k͒ to the PCB54 had an almost negligible effect on the dielectric relaxation times, notwithstanding that the polystyrene's glass temperature is 130 degrees higher than T g for PCB54. The PS-90k exerted a more significant effect on the mechanical relaxation times of the PCB54. However, the modification of the Aroclor dynamics is still orders of magnitude less than the effect of the dissolved PS-90k on the relaxation times of an oligomeric PS ͑PS-o͒. This low molecular weight PS has a T g identical to that of the PCB54. The dramatic difference between the effect of added polymer on the dynamics of Aroclor versus PS-o can be ascribed to differences in their capacity for intermolecular coupling with the PS-90k solute.

Section: Mixtures With Polystyrenementioning

confidence: 99%

“…There has been long-standing interest in modifying the dynamics of small molecules by the addition of polymers. [1][2][3] Both the sign and the magnitude of the change in the solvent's viscosity and relaxation times reflect changes in the motion of solvent molecules residing in the vicinity of dissolved polymer chains.…”

Section: Introductionmentioning

confidence: 99%

Dynamics of aroclor and its modification by dissolved polystyrene

The Journal of Chemical Physics

Santangelo

Roland

2002

“…Many investigations of PCB have been reported, with early work exploiting their solvent power. Experiments on dilute PCB solutions were the first to show that dissolved polymer chains modify the local motions of the solvent molecules 27,28,29,30 . A very anomalous form of this modification was observed in mixtures of PCB with polybutadiene.…”

Section: Introductionmentioning

confidence: 99%

Effect of chemical structure on the isobaric and isochoric fragility in polychlorinated biphenyls

Roland

The Journal of Chemical Physics

2005

Pressure-volume-temperature data, along with dielectric relaxation measurements, are reported for a series of polychlorinated biphenyls (PCB), differing in the number of chlorine atoms on their phenyl rings. Analysis of the results reveals that with increasing chlorine content, the relaxation times of the PCB become governed to a greater degree by density rho relative to the effect of temperature T. This result is consistent with the respective magnitudes of the scaling exponent gamma yielding superpositioning of the relaxation times measured at various temperatures and pressures, when plotted versus rho(gamma)/T. While at constant (atmospheric) pressure, fragilities for the various PCB are equivalent, the fragility at constant volume varies inversely with chlorine content. Evidently, the presence of bulkier chlorine atoms on the phenyl rings magnifies the effect which the density has on the relaxation dynamics.

“…Small‐molecule diluents can alleviate the severity of constraints on local motion, thereby reducing the degree of intermolecular cooperativity associated with the local motion 6–8. Alteration of the local friction, for example, through changes in the available free volume or the configurational entropy, 9–12 can change the dynamics of the chain molecules, by what might be called a “ T g ‐effect”. In this article we describe dielectric spectroscopy measurements of the primary and secondary relaxations in a polybutadiene (1,4‐PB), of primarily 1,4‐microstructure, both neat and containing a nonpolar diluent.…”

Section: Introductionmentioning

confidence: 99%

?- and ?-Relaxations in neat and antiplasticized polybutadiene

J. Polym. Sci. B Polym. Phys.

Ngai

Robertson

et al. 2000

Dielectric spectroscopy was carried out to measure the α‐relaxation (local segmental motion) and the higher frequency, secondary relaxation (β‐mode) in 1,4‐polybutadiene, both neat and containing a nonpolar diluent, mineral oil. The α‐relaxation shifted to lower frequencies (antiplasticization) in the presence of the diluent, suggesting the glass temperature of the latter is higher than the Tg of the polymer (i.e., >187K). The Tg of neat mineral oil cannot be determined directly, due to crystallization. While the diluent increased the magnitude of the α‐relaxation times, it had no effect on the β‐relaxation. Moreover, neither the shape of the α‐relaxation function nor its temperature dependence was influenced by the diluent. From this we conclude that the main effect of the mineral oil was to increase the local friction, without changing the degree of intermolecular cooperativity of the molecular motions. We also find that near the glass temperature, there is rough agreement between the time scale of the secondary relaxation process and the value of a noncooperative relaxation time estimated from theory. This approximate correspondence between the two relaxation times also holds for 1,2 polybutadiene. However, the β‐process cannot be identified with the noncooperative α‐relaxation, and the relationship between them is not quantitative. © 2000 John Wiley & Sons, Inc.* J Polym Sci B: Polym Phys 38: 1841–1847, 2000