Thermorheological simplicity,

Generally, the rheology of polymer melts depends strongly on the temperature at which the measurement is carried out. It is well known that for thermorheological simplicity, isotherms of storage modulus (G (co)), loss modulus (G"(complex viscosity (r (co)) can be superimposed by horizontal shifts along the frequency axis ... 

The time-temperature superposition method can be also applied to viscosity data (Ferry, 1980). For any viscoelastic parameter, exact matching of the adjacent curves is an important criterion for the applicability of the method. In addition, when possible, the same values of oy must superpose all the viscoelastic parameters and the temperature dependence of ar should have a reasonable form based on experience. One advantage of the method is that the range of frequencies are extended beyond those available experimentally. The time-temperature method has been also referred to as thermorheological simplicity (Plazek, 1996). 

Plazek, D. J. 1996. 1995 Bingham medal address Oh, thermorheological simplicity, wherefore art thou J. Rheology 40 987-1014,... 

For low molecular weight fractions, the variation in the values of the compliance function increases as either the chain length or the temperature increases. The changes observed in the compliance with temperature for very low molecular weight fractions are illustrated in Figure 8.17 (16). This lack of thermorheological simplicity was also observed for other amorphous polymers, specifically poly(ethyl methacrylate) (21), poly( -butyl methacrylate) (22), poly( -hexyl methacrylate) (23), and low molecular weight poly(methylphenyl siloxane) (24). 

Breakdown of Thermorheological Simplicity of Low Molecular Weight Polymer... 

The most common means to extend the frequency scale is to invoke time-temperature superpositioning (Ferry, 1980). If all motions of a polymer contributing to a particular viscoelastic response are affected the same by temperature, then changes in temperature only alter the overall time scale such a material is thermorheologically simple. Thermorheological simplicity means conformance to the time-temperature superposition principle, whereby lower and higher strain rate data can be obtained from measurements at higher and lower temperatures, respectively. 

Below Tg (4,5) co-operative molecular processes are usually assumed to be inactive. However, physical ageing implies that conformational changes may be still able to occur if rather infrequently. Structural relaxation processes are observed to be non-exponential and are represented by a continuous distribution or stretched exponential form (d). Thermorheologically simplicity (TRS) implies that the molecular relaxation process has the same form at different temperatures (7) and the validity of this assumption is addressed in this paper. Isobaric volume recovery (8,9) has been described by a single parameter mc el, however all fi ee volume models (10,11) have limitations and a distribution of hole sizes and relaxation times leading to a pseudo-linear theory is a more realistic model(72). Comparison of data fi om various techniques should throw light on the molecular nature of physical agdng. 

The disparity between the temperature dependences of t and % [Eqs. (41) and (42)] depends on the size of the couphng parameter n. Polystyrene has a larger n (= 0.63) than polyisobutylene (= 0.45) [19, 96, 97]. Hence we expect the degree of breakdown of thermorheological simplicity is lesser in polyisobutylene (PIB) than polystyrene (PS).This expectation, as well as other predicted differences in viscoelastic properties of PIB and PS, were confirmed... 

See the discussion on thermorheological simplicity in Chapter 26 on Viscoelastic Behavior. 

Viscoelastic functions depend on both temperature and time. For many polymers, the logarithmic plot of a viscoelastic function at the temperature T may be obtained from that at the temperature Tq by shifting the curve along the logarithmic time axis by the amount of log (T)- This procedure is called time-temperature superposition. The ability to superpose viscoelastic data is known as thermorheological simplicity. Thermorheological simplicity demands that all the molecular mechanisms involved in the relaxation process have the same temperature dependencies. 

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