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

Abstract

The effects of the different architecture on the dynamic response at nanometers and nanoseconds was investigated by means of electron spin resonance spectroscopy in a series of random and block copolymers of an azobenzene methacrylate (MA4) and methyl methacrylate (MMA) counits. The study evidenced the presence of fast and slow molecular sites for the molecular reorientation at nanoscale, modulated by the amount of MA4 counits in the random copolymers, and by the self-assembly in supramolecular structures in the block copolymers. A series of dynamics features was found, such as the presence of memory effects, the existence and the degree of coupling of the rotational dynamics to the structural relaxation of the matrix or to its viscosity, and the population of the different dynamic sites, that provided a detailed description and characterization of the different mechanisms leading the dynamic response of the structurally different copolymer systems.

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

“…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.…”

“…15,36 Its stiffness and cylindrical shape make it suitable to ESR studies in anisotropic media and polymers. 17,19,22 Its size selects the length scale of the experiment, which delivers the relaxation time of the spontaneous fluctuation of the system. Regarding the nanometer length scale pertinent to cholestane, its suitability in ESR experiments on polymers and glass-formers comes from the finding 26 that it characterizes dynamic heterogeneity and cooperativity in polymers and supercooled liquids on approaching the glass transition.…”

“…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. All the sets of data exhibit a similar qualitative behavior with three different dynamics regimes, that onset at specific temperatures.…”

“…In this respect, the observation of crossover phenomena in our experiments by using ESR spectroscopy dates back to almost 20 years ago and is a recurrent result, on both molecular glass-formers and polymeric materials. 15,16,19,25,47,50,79,80 The specific value of the crossover temperature depends on the material and may depend also on the guest molecule used [see ref 25 and references therein]; however, the range of obtained values by ESR experiments spans over the values that different spectroscopies provide for T C 73 and for the critical temperature of the mode coupling theory T cr , in the case of both small molecules and polymers. 70 More specifically, in the case of the investigated PMMAs, the high-temperature crossover is located in the range 1.22T g −1.6T g (Table 4).…”

“…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. The mean value of the T C /T g ratio calculated over the plateau values is 1.19 ± 0.1.…”