Crossover region and entanglement in nearly monodisperse poly(ethyl acrylates) studied with electron spin resonance spectroscopy

Abstract

The topic of dynamic changes undergone by glass-forming materials in the supercooled region is addressed in this study. Crossover regions and temperatures are generally considered as key features in order for the glass transition phenomenon to be understood. The attention is here focused on the crossover region of polymers and its dependence on the polymeric entangled dynamics. To avoid the superposition of possible dependence on the polymeric polydispersity, nearly monodisperse syntheses of poly(ethyl acrylates) have been used. Rotational dynamics have been investigated with electron spin resonance spectroscopy, dissolving the cholestane molecular tracer in a poly( ethyl acrylate) (M-n = 7500 amu). Comparison is carried out with the findings obtained in the case of an almost monodisperse poly(ethyl acrylate) with M-n = 58 200 amu. Different dynamic regimes and crossover regions were recognized in the temperature dependence of the molecular rotation. The crossover temperatures T-c were found to be dependent on the molecular weight. Moreover, the dynamics in non-Arrhenius regions were satisfactorily described as a fractionary law of structural relaxation

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

“…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).…”

“…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:…”

“…The PMA ( 0 = n ) and the PnBA ( 3 = n ) samples were sealed by Polymer Source Inc. (Montreal, Canada), which provided molar mass data from size exclusion chromatography (SEC) using polystyrene standards. The PEA sample ( 1 = n ) was synthesized as described in [16]. Linear viscoelastic measurements on the PAA samples were carried out with a Haake RS150H…”

“…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].…”