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

Abstract

The rotational dynamics of the tracer cholestane dissolved in non-entangled nearly monodisperse poly(alkyl acrylate) melts has been investigated by means of electron spin resonance spectroscopy, Three samples with almost the same molecular weight were selected, poly(methyl acrylate), poly(ethyl acrylate) and poly(n-butyl acrylate) and their linear viscoelastic properties were also characterized. Large temperature intervals were found with power laws relating shear flow relaxation and probe rotational diffusion. The main aim of this paper was the study of the sharp crossover between two different fractional regimes that for all the samples was observed at a temperature T-C = 1.15 - 1.25 T-g. The decoupling from Structural relaxation at T < T-C was ascribed to the onset of cooperative effects on the length scale probed by the tracer. These results fit in the general scenario regarding the dynamical information extractable from ESR on cholestane, a probe with its long-axis dimension greater than the typical length characterizing the rigid segment of polymers

Chain-Length Dependence of Relaxation and Dynamics in Poly(methyl methacrylate) from Oligomers to Polymers

“…15,58 In the samples with the highest masses, the diffusion anisotropy ratio was found constant with temperature but with values increasing with the mass of the sample. The interval ranged from 15 to 18, with values similar to the ones reported in previous ESR studies in poly(alkyl acrylate)s (range of the ratio 12−20), 37,47 and liquid crystalline polymers and copolymers exhibiting an anisotropy ratio of 15. 17,19 The spinning correlation times τ ∥ of the cholestane dissolved in PMMAs are shown in Figure 4 in the overall investigated temperature ranges.…”

“…25,40,41 The rotation of cholestane in PMMAs is described in terms of a diffusion stochastic model under cylindrical symmetry, 42−44 characterized by a spinning diffusion coefficient D ∥ and a tumbling diffusion coefficient D ⊥ . 45,46 For comparison purposes with the results of rotational dynamics reported in the literature, 17,19,25,37,47 in this work the reorientation of cholestane is discussed in terms of correlation times τ ∥ and τ ⊥ defined as τ ∥ = 1/(6D ∥ ) and τ ⊥ = 1/(6D ⊥ ). 48,49 More details regarding ESR spectroscopy, the theory, and the simulation program of ESR line shapes that provides dynamic information in slow motion regime are reported elsewhere.…”

“…The fractional exponent ξ is the ratio of the pseudoactivation temperature T b of the VFT law relevant to the rotational dynamics over T b pertaining to the viscosity. In particular, ξ = 1 signals a complete coupling of the molecular tracer dynamics to 58 poly(alkyl acrylate)s, 37,47 other polymers, 25 and azobenzene methacrylates. 19 The dashed area is a guide for the eye.…”

Heterogeneity and Dynamics in Azobenzene Methacrylate Random and Block Copolymers: A Nanometer–Nanosecond Study by Electron Spin Resonance Spectroscopy

“…12 The cholestane molecule is quite asymmetric in shape and can be sketched as a prolate ellipsoid with semiaxes of about 0.99 and 0.29 nm. 16,34 Its size is just across the nanometer length-scale that 7,21,22 characterizes the dynamic heterogeneity and cooperativity in polymers and supercooled liquids on approaching the glass transition. The cholestane rotational dynamics is described in terms of the diffusion model under cylindrical symmetry.…”

“…63 A different scenario is available with the dynamics of R10 (Figure 6), where the mesogenic unit percentage is low and the polymer matrix does not evidence any liquid crystalline feature, both at a macroscopic length scale, as detected by DSC and rheological measurements, and at nanoscale, as detected by ESR spectroscopy. The rotational dynamics of the molecular tracer appears homogeneous as for molecular glass formers, low molecular weight polymer glass formers 16,17,22,24,34,57,63,64 and non mesogenic polymer [18][19][20][21]23 matrices. Three dynamics regions are identified: a high temperature VF region down to the crossover at 1.22Tg, another VF region at intermediate temperatures ranging from 1.22Tg to 1.10Tg, and finally an Arrhenius regime, below Tg.…”