Glass transition time-temperature correspondence

In this case, an apparent activation energy is determined, and it has higher values than secondary relaxations 100-300 kJ/mol for urethane-soybean oil networks (Cristea et al. 2013), 200-300 kJ/mol for polyurethane-epoxy interpenetrating polymer networks (Cristea et al. 2009), more than 400 kJ/mol for semicrystalline poly(ethylene terephtalate) (Cristea et al. 2010), and more than 600 kJ/mol for polyimides (Cristea et al. 2008, 2011). The glass transition temperature is the most appropriate reference temperature when applying the time-temperature correspondence in a multifrequency experiment. The procedure allows estimation of the viscoelastic behavior of a polymer in time, in certain conditions, and is based on the fact that the viscoelastic properties at a certain tanperature can be shifted along the frequency scale to obtain the variation on an extended time scale (Brostow 2007 Williams et al. 1955). The shift factor is described by the Williams-Landell-Ferry (WLF) equation ... 

Lastly, it was demonstrated with PPO substituted with a series of alkyl side-chains as we have here, that the glass transition temperature decreases with an increase in the side-chain length (28). At the same time, the Tg s of the more flexible side-chain liquid crystalline polymers investigated to date are always much higher than those of the corresponding polymers without the mesogenic side-chains (3). Therefore, it is quite likely that we may obtain side-chain liquid crystalline polymers of approximately the same Tg from PPO and PECH. 

The glass transition temperature is generally measured- by experiments that correspond to a time scale of seconds or minutes. If the experiments are done more rapidly, so that the time scale is shortened, the apparent Tg value is raised. If the time scale is lengthened to hours or days, the apparent Tg value is lowered. Thus, as generally measured, Tg is not a hue constant but shifts with the time scale of the experiment or observation. Moreover, Tg is masked by experimental difficulties, compounded by multiple and often inaccurate definitions of Tg in the literature. The least... 

The transition from ideal elastic to plastic behaviour is described by the change in relaxation time as shown by the stress relaxation in Fig. 66. The immediate or plastic decrease of the stress after an initial stress cr0 is described by a relaxation time equal to zero, whereas a pure elastic response corresponds with an infinite relaxation time. The relaxation time becomes suddenly very short as the shear stress increases to a value equal to ry. Thus, in an experiment at a constant stress rate, all transitions occur almost immediately at the shear yield stress. This critical behaviour closely resembles the ideal plastic behaviour. This can be expected for a polymer well below the glass transition temperature where the mobility of the chains is low. At a high temperature the transition is a... 

The glass-transition temperatures and the corresponding specific heats were measured three times for each sample in order to enable the calculation of the standard deviations, which were in the range of 3% or lower. Apparently, the kind of substituent greatly influences the Tg values, and rigid substituents (phenyl or methyl) or flexible substituents (ethyl or nonyl) cause an increase or decrease in corresponding Tg values, respectively. The measured Tg values are plotted in Fig. 19. 

The glass transition is usually characterized as a second-order thermodynamic transition. It corresponds to a discontinuity on the first derivative of a thermodynamic function such as enthalpy (dH/dT) or volume (dV/dT) (A first-order thermodynamic transition, like melting, involves the discontinuity of a thermodynamic function such as FI or V). However, Tg cannot be considered as a true thermodynamic transition, because the glassy state is out of equilibrium. It may be better regarded as a boundary surface in a tridimensional space defined by temperature, time, and stress, separating the glassy and rubbery (or liquid) domains. 

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