On the Molecular Motions Originating from the Dielectric γ-Relaxation of Bisphenol-A Polycarbonate

Abstract

We have investigated the molecular motions responsible for the dielectric γ-relaxation of bisphenol A polycarbonate (BPA-PC) by taking advantage of the unbalanced influence of cold-drawing in the dielectric losses. The direct comparison of the dielectric relaxation curves measured on the BPA-PC samples uniaxially stretched below the glass transition temperature with those corresponding to the nonstretched samples allowed us to resolved two main components of the dielectric γ-relaxation. The characteristic times of each of these components nearly coincide with those derived by neutron scattering and deuterium nuclear magnetic resonance for two welldefined jumps of the phenylene rings, evidencing the cooperativity among the motions of the different molecular groups forming the polycarbonate repeating unit. A more detailed analysis of the orientation effects on the dielectric γ-relaxation reveals a weaker third component with characteristic time scales close to those reported in the literature for the fast phenylene ring oscillations. On the basis of these three components we have been able to accurately describe the relaxation behavior of both oriented and nonoriented samples over an extremely wide temperature interval (50-350 K). From these results, we conclude that the low temperature dielectric behavior is mainly driven by the phenylene π-flips, but at temperatures above 200 K, the dielectric relaxation becomes increasingly governed by the concurrent rotation of the carbonate/phenylene units, being these motions those more strongly hindered by chain orientation. Some implications of these finding on the mechanical properties of polycarbonate are discussed.

Thermal Expansion Behavior of Antiplasticized Polycarbonate

“…This is known as β-relaxation and can be ascribed to the local motion of the main chain. [28][29][30] In the temperature region beyond the β-relaxation, E' keeps almost a constant value until the glassto-rubber transition, although there is a weak peak around at 75 ºC. Then, it falls off sharply at 160 ºC.…”

“…3,[16][17][18]20,22) The addition of an antiplasticizer enhances the modulus and reduces the β -relaxation mode located around at -100 ºC, which is attributed to mechanisms such as ring-flip process of phenyl groups 28,29) and rotation of the phenylene rings. 30) Although numerous researches have been carried out on antiplasticization, to the best of our knowledge, the thermal Thermal expansion behavior and viscoelastic properties of antiplasticized polycarbonate (PC) are studied employing p-terphenyl (p-tPh) as an antiplasticizer. The rheological characterization reveals that the free volume fraction at the glass transition temperature and thermal expansion coefficient of the free volume in the rubbery region are unchanged by the p-tPh addition.…”

Dielectric secondary relaxation and phenylene ring dynamics in bisphenol-A polycarbonate

“…The characteristics of the BPA-PC samples used in this work (sheets 0.175 mm thick) can be found elsewhere [3]. Before any measurement or mechanical treatment, the sheets were dried above T g for several hours to remove any trace of moisture.…”

“…[3] for details). The sample capacitor was formed with 20 mm diameter gold-plated electrodes and the sample cell was placed in a cryostat that uses a nitrogen jet temperature control system.…”

“…Despite of the huge effort, the microscopic origin of the secondary relaxation process of BPA-PC has not been completely elucidated. Recently, we have shown [3] that the dielectric c-relaxation of BPA-PC can be analyzed in terms of three components, each reflecting a distinct phenylene ring motion. By means of this picture the experimental dielectric data were accounted for over a very broad frequency range (10 À2 -10 9 Hz) and over an extremely wide temperature interval (50-350 K).…”