Polymers relaxation times

For these first experiments, a temperature relatively close to Tg, T=123°C, was chosen with the intention of minimizing the relaxation of stress and chain orientation during the quenching the weight-average relaxation time of sample SI at 123°C is calculated from that at 140°C and the thermal shift factor between 123°C and 140°C Xw(123°C) 380s. On the other hand the cooling time of the stretched specimens can be estimated to a few seconds [19], which is very small compared to the polymer relaxation time at the temperature of the experiments. 

The Rouse model is the earliest and simplest molecular model that predicts a nontrivial distribution of polymer relaxation times. As described below, real polymeric liquids do in fact show many relaxation modes. However, in most polymer liquids, the relaxation modes observed do not correspond very well to the mode distribution predicted by the Rouse theory. For polymer solutions that are dilute, there are hydrodynamic interactions that affect the viscoelastic properties of the solution and that are unaccounted for in the Rouse theory. These are discussed below in Section 3.6.1.2. In most concentrated solutions or melts, entanglements between long polymer molecules greatly slow polymer relaxation, and, again, this is not accounted for in the Rouse theory. Reptation theories for entangled... 

Another criterion for predicting if the transport in polymeric gels is controlled by diffusion (Fickian) or by relaxation, is to determine the diffusional Deborah number De), which is a ratio between the characteristic polymer relaxation time of the polymer (2) when it is subject to a swelling stress and a characteristic diffusion time (6), defined as the coefficient between the square of the sample thickness (h) and the coefficient of water diffusion in the polymeric gel... 

Polymer relaxation time for simple laminar flow Polymer relaxation time for turbulent flow Kinematic viscosity of solution... 

The analysis of mass transfer from a small cylindrical wire or from a rotating disk in a dilute polymer solution by using an electrochemical technique is reported here. Steady state measurements showed a large decrease of the mass transfer rate with respect to the newtonian behaviour, starting from critical velocity values. This phenomenon was ascribed to the fast stretching of the polymer coils which occurs when the elongational gradient equals the reciprocal of the polymer relaxation time. 

Fig. 5 The effect of the polymer relaxation time on the extent of c3rag reciuction...

Various polymers, relaxation times of free radicals... 

Note, however, that these authors only consider a reaction to be diftusion controlled , if a system cannot reach an equilibrinm on the typical time scale of reaction and local depletion of reactants occurs. For termination reactions the typical time scale of reaction is severely decreased by intermolecular excluded volume effects, the mean time needed for reaction is larger than typical polymer relaxation times and consequently this reaction is not drSiision controlled. It is important to realize that this definition differs from the one used in this thesis. 

FIGURE 3.2 The relationship between elasticity modulus E and time up to fracture, corresponding to the Eq. (3.8), in double logarithmic coordinates for samples HDPE with sharp notch (t = t is polymer relaxation time) [7],... 

Trends in interlayer adhesion in coextruded structures have been correlated to the nonlinear viscoelastic normal stresses in the interface vicinity using an ordering parameter called Viscosity Normalized Nonlinearity Ratio . Larger deviation of this parameter from a value of 1.0 (large disparities in melt extensional configuration across the interface) corresponded to poorer observed peel strength. This parameter is of particular importance in the region where the melt layers come into contact for the first time, in scenarios where polymer relaxation times are of a similar order as the flow timescales. 

---