Thermorheological Simplicity and Fragility of Azobenzene Nematogenic Side‐Chain Polymers

Abstract

The linear viscoelastic response of polymers from a nematogenic azo- benzene methacrylate and its copolymers with nonmesogenic methyl methacrylate was studied. These samples showed thermorheological simplicity in the whole investigated temperature range, even across the nematic-isotropic transition. The temperature dependence of the zero-shear viscosity confirmed a thermorheo- logically simple behavior, which allowed interpretation of h(T) in the framework of free-volume theory. The way the relaxation mechanisms tended to their asymptotic behavior was described in terms of fragility parameters, and a comparison of fragilities of the samples was carried out. The analysis of fragility and temperature dependence of viscosity provided better understanding of the way the architecture of these nematic polymers influences free-volume quantities and determines relaxation mechanisms

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

“…More specifically, in the high-temperature (isotropic) region and for the fast component of the molecular reorientation, the decoupling degree between cholestane dynamics and PMA4 random copolymer viscosities turned out to be ξ HT(F) = 0.45 for both R90 and R80 (Table ). The value is consistent with that of R70 (ξ HT(F) = 0.52), and shows a possible similarity with the homopolymer PMA4 (ξ HT(F) = 0.61) after the thermal annealing at 358 K . The relatively high value of ξ HT(F) , together with the clear sensitivity of the probe dynamics to the nematic-to-isotropic transition, located near 1.2 T g , strengthens the hypothesis that the sites experienced by the molecular tracer in these liquid crystalline polymers may be located among the mesogenic side groups.…”

“…40,41 No discontinuity was observed in the investigated temperature range for any sample, not even across T ni or T odt . 9,26,32 Moreover, the shear moduli in dynamic experiments carried out in the linear response regime did not show any heterogeneous behavior across the characteristic temperatures of the different samples. 26,32 However, the different chemical and structural arrangement of the block copolymers led to the failure of the time−temperature superposition 42 principle (TTS) in B20 and B10.…”

“…The curves of zero-shear viscosity versus temperature, obtained from steady-state viscosity (for random copolymers only), creep−recovery and dynamics experiments, are shown in Figure . Values for selected literature PMMA homopolymers are also shown in Figure for comparison. , No discontinuity was observed in the investigated temperature range for any sample, not even across T ni or T odt . ,, …”

“…40,41 No discontinuity was observed in the investigated temperature range for any sample, not even across Tni or Todt. 9,26,32 9/36 40,41 (Table 3) is also shown.…”

“…The possibility of achieving an effective high-resolution and long-term data storage deals with crucial parameters such as bit stability, homogeneity at molecular level, and working temperature. , Therefore, a full knowledge of the polymer characteristics should be gained by different spectroscopic techniques that cover several length and time scales. Accordingly, calorimetry and rheology can be adopted to investigate thermal properties of the melt and dynamics of polymer chains, − while electron spin resonance (ESR) spectroscopy can be employed to study the stability of different molecular sites and their correlation times as a function of temperature and to reveal connections between nanoscale dynamics with chain dynamics and thermal responses of the melt …”