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We conducted dielectric and viscoelastic relaxation measurements for polystyrene (PS)/4-pentyl-4′-cyanobiphenyl (5CB) and PS/4-pentyl-4′′-cyanoterphenyl (5CT) mixtures in the miscible state with the weight fractions of 5CB or 5CT from 0.04 to 0.27 to examine the relationship between two-component dynamics in the mixture. Dielectric relaxation measurements mainly probed the dynamics of low-mass molecule (LM), 5CB or 5CT, because of the much larger dipole moment of LMs compared to that of PS. PS/5CB mixtures exhibited bimodal dielectric relaxation, while PS/5CT showed unimodal relaxation. The observed two relaxation modes (slow and fast modes) in PS/5CB were attributed to the full rotational motion and partial rotational motion of 5CB, respectively. The latter motion is allowed in the restricted space surrounded by less mobile PS matrix. In the case of 5CT, the fast mode was suppressed due to the larger size of 5CT molecule, and only the slow mode (rotational mode) appeared. The relaxation times of slow mode for 5CB and 5CT had approximately the same (non-Arrhenius-type) temperature dependence with the viscoelastic relaxation time for the terminal flow zone of PS at high temperatures (at least 20–30 K higher than the glass transition temperature). This result suggested that the slow modes of 5CB and 5CT are cooperative with a global motion of PS-chain. In contrast, relaxation time for the fast mode observed in PS/5CB had weaker (Arrhenius-type) temperature dependence than the slow mode, which indicated that the fast mode of 5CB is decoupled with the PS chain motion. [ABSTRACT FROM AUTHOR]

Theeffect of chain packing on tensile properties was studied employingamorphous poly(lactic acid) PLA. It was found that the samples cooledin the temperature range from 60 to 80 °C, that is, slightlyhigher than the glass transition temperature Tg, showed ductile behavior with a low brittle-ductile transitiontemperature. Furthermore, the samples obtained by prolonged coolingat 56 °C also showed ductile behavior, whereas a shorter coolingtime at the same temperature provided a brittle product. Even forthe samples quenched at 40 °C, they showed ductile behavior afterthe exposure to postprocessing annealing operation at 60 °C;that is, the strain at break is larger than 3. This is an anomalousphenomenon for a glassy polymer. The dynamic mechanical analysis andthermal characterization revealed that the ductile samples show slightlyhigher Tgthan the brittle ones, presumablydue to high packing density of polymer chains. Moreover, it was foundfrom infrared spectroscopy that the ductile samples show strong absorbanceat 1267 cm–1, ascribed to high energy gauche–gauche ggconformers. Following the classic Robertson’sdescriptions of plastic flow, it is concluded that the increase inthe gauche–gauche ggconformers, which showsthe conformation change under a low stress level, reduces the criticalonset stress for shear yielding. The results demonstrated that themechanical toughness of PLA can be controlled by the cooling conditionsduring processing and the postprocessing annealing operation. [ABSTRACT FROM AUTHOR]

Effects of solvents on segmental dynamics of polystyrene (PS) at the chain end have been studied by the site-specific spin-label method of electron spin resonance (ESR). PS’s having similar number-average molecular weight (ca. 25 kDa) but labeled with a nitroxde at the chain end (E-PS) or midchain segments (M-PS) individually were used for the measurements. Toluene and dioctyl phthalate (DOP) which are good and theta solvents, respectively, for PS were selected as solvents. The temperature variation of ESR spectra was simulated based on the “macroscopic order with microscopic disorder” model to determine the segmental mobility of the labeled segment. The E-PS showed obviously and slightly higher mobility compared to the M-PS in the bulk PS and in the concentrated PS/DOP (50/50) solution, respectively. On the other hand, the E-PS showed the same mobility with the M-PS within experimental uncertainties in the dilute PS/toluene (1/99), concentrated PS/toluene (60/40), and dilute PS/DOP (3/97) solutions. Namely, the chain end showed higher segmental mobility than midchain segments in high viscous media where large intermolecular hindrances such as frictional resistance, spatial restriction for conformational transition, and intermolecular coupling are expected. From this result, we conclude that lesser intermolecular hindrances around chain ends are the origin of the higher mobility at the chain end segments. [ABSTRACT FROM AUTHOR]