An ESR Study on the Heterogeneity of Dynamics in a Nematic Polymer Induced by Thermal Annealing in the Isotropic Melt

Abstract

Heterogeneities induced in a nematic polymethacrylate by thermal annealing in the isotropic phase were investigated by studying the dynamics of the cholestane spin probe dissolved in the host matrix by electron spin resonance. The molecular site distribution was well described by a two δ-like distribution function. The temperature dependences of the dynamics of the slow and fast components were fully characterized in going from the isotropic state to the glassy state through the nematic phase. It was found that the behaviors of the spinning correlation times with temperature in the isotropic and nematic regions were well represented by the Vogel-Fulcher law. By comparing the temperature dependence of the probe dynamics with the R relaxation process of the polymer, we estimated the cooperativity degrees in the dynamics of the molecular probe in the different sites. The relative populations of the slow and fast sites were determined over the whole temperature range investigated. It was shown that such population was sensitive to conformational change of the polymer backbone.

Probing the cooperative dynamics varying the side-chain length of poly(alkyl acrylate)s: ESR experiments

“…that also were found to work in other ESR studies by us [14][15][16][17], and also with other kinds of probe spectroscopy [11].…”

“…The above result on PAAs intrigues and proposes the ESR as a major spectroscopy to investigate the crossover dynamics. Actually, similar dynamic anomalies, related to the occurrence of a larger decoupling from the structural relaxation of the host matrix, were found for the cholestane rotation in other polymers [14][15][16] whereas, with the smaller paramagnetic probe By inspection of figure 6, one can appreciate the sharpness of the dynamic crossover seen by ESR spectroscopy. It also appears that the relation between guest rotational dynamics and host viscous flow follows fractional laws:…”

“…They can be expressed as the ratio of the pseudo-activation energy of regions II or III over the one obtained from viscosity, and represent the coupling degree between probe dynamics and structural relaxation of the host [15]. The decoupling (ξ < 1) found in the region II has to be ascribed to steric hindrance effects of the guest-host geometry and interaction [15].…”

“…They can be expressed as the ratio of the pseudo-activation energy of regions II or III over the one obtained from viscosity, and represent the coupling degree between probe dynamics and structural relaxation of the host [15]. The decoupling (ξ < 1) found in the region II has to be ascribed to steric hindrance effects of the guest-host geometry and interaction [15]. This statement seems to be supported by the results in PEA polymers [16], where ξ was found to be independent of the polymer repeating unit, and in the present case of PAAs where the relation between the ξ value and the side-chain length is consistent with a linear decreasing proportionality (figure 7a).…”

“…ESR investigation of rotational dynamics of different paramagnetic tracers dissolved in low molecular weight glass formers and polymers evidenced the existence, in the supercooled region, of a dynamic crossover at about 1.2 g T [13][14][15][16] and of partial coupling between probe rotation and shear viscosity [14][15][16][17].…”