Secondary relaxation effects

Simplified Nonequilibrium Theory of Secondary Relaxation Effects in Programmed Field-Flow Fractionation, J. C. Giddings, Anal. Chem., 58, 735 (1986). Feasibility Study of Dielectrical Field-Flow Fractionation, J. M. Davis and J. C. 

In the-preceding chapters we showed that several thermo-analytical techniques are available to study thermal transitions in polymers. The thermal transitions of new, polymeric systems are usually first investigated by DSC measurements (small amount of sample needed/high scanning speed). The DSC technique is, however, often not sensitive enough to detect weak and/or secondary relaxation effects. DMA (rigid polymers) or dielectric (rigid/rubbery/viscous systems) experiments are then necessary. The sensitivity of the dielectric measurements depends on the polymer s polarisability (see 5.1.2). Besides, DC conduction effects can seriously hamper the detection of the relaxation effects studied. The TSD analysis technique offers in such a case an attractive and sensitive alternative. 

Giddings, J.C. Simplified nonequiUbrium theory of secondary relaxation effects in programmed field flow fractionation. Anal. Chem. 1986, 58, 735-740. 

The time-dependent retention R(t) is still expressed by Eq. 9, if the changing rate of the field is slow so that each particle can move to the equilibrium position which is determined by the force of the field and the Brownian motion of the particle, that is the "secondary relaxation effect" can be neglected. 

The nonequilibrium effect on the zone broadening should be evaluated by the computer simulation using Eqs. 1 to 3. In the present study, the simplification by using Eq. 8 instead of Eq. 3 was employed, because the zone broadening due to the longitudinal diffusion is of minor effect. For the simulation without the nonequilibrium and the secondary relaxation effects, Eq. 7 was used... 

Effect of Diluents and the Diluent-Induced Secondary Relaxation. 146... 

It has also been established for the copolymers HEMA-AAm (Fig. 7) that the temperature and intensity of the secondary dispersion do not change systematically with the copolymer composition. Thus, molecular motions underlying the ji or /S dispersions in polymethacrylates or polyacrylates differ from each other. This may be ascribed to a different cooperation of the backbone, but the results obtained so far do not suffice for a more precise interpretation. Likewise, it is difficult to explain156) that PAAc, in contrast to PMAAc, does not exhibit any secondary relaxation above the liquid nitrogen temperature. It is to be noted that the effect of stereoregularity and diluents168) on the /8 relaxation is not easy to estimate because the concomitant decrease in T accounts for the overlapping of the /3 and a transitions. 

On the other hand, these compounds have a general depressant effect on the CNS that is, they cause a global decrease in CNS excitability that results in generalized sedation. It therefore seems possible that some of their muscle relaxant effects are caused by their sedative powers rather than a selective effect on specific neuronal reflex pathways.11,92 This observation is not to say that they are ineffective, because clinical research has shown that these drugs can be superior to a placebo in producing subjective muscle relaxation.8 20 80 97 However, the specific ability of these drugs to relax skeletal muscle remains doubtful, and it is generally believed that their muscle relaxant properties are secondary to a nonspecific CNS sedation. 

To summarize, most of the experimental results on the yielding of thermosets may be interpreted by stating that the structure affects chemical structure, secondary relaxations, physical aging, etc., on the proportionality constant are still to be explored. 

The results reported for these systems, indicate that the subglass relaxation in P3M2NBM is indeed a broad and weak relaxational process [95], The low strength of the observed secondary processes in these systems, has been attributed to the bulkiness of the side chain. In one case, the low strength of the secondary relaxation prevents the analysis of this peak in terms of an empirical relaxation. The effect of the methyl substituent on the norbornyl ring is to lower the position of the value of the peak of the a -relaxation about 50 K. Another effect that can be observed in the low frequency side of the spectra in both polymers is the conductive one. 

The P zone extend over a large temperature range. This is a characteristic of a secondary process which involve local motions of the lateral groups [155], They are more diversified movements with a large spectrum of relaxation times. Therefore, thermal cleaning of the t.s.c. global spectra is used to study the broad relaxation peaks of the low temperature secondary relaxation [42], This is effective because it allows one to excite only the specific transition of interest [155],... 

The effect of diluents on the viscoelastic behavior of amorphous polymers is more complex at temperatures below T, i.e., in the range of secondary relaxation processes. Mechanical, dielectric and NMR measurements have been performed to study the molecular mobility of polymer-diluent systems in this temperature range (see e.g. From extensive studies on polymers such as polycarbonate, polysulfone and polyvinylchloride, it is well known that diluents may suppress secondary relaxation processes. Because of the resulting increase in stiffness, these diluents are called antiplasticizers . Jackson and Caldwell have discussed characteristic properties... 

The effect of temperature on microhardness at low temperatures (for PE between —60 and 25 °C and for isotactic polypropylene (i-PP) between -20 and 25 °C) has also been studied (Perena et al, 1989 Martin et ah, 1986). While for i-PP it is possible to detect accurately the glass transition temperature, for PE it is concluded that the use of only H for recognizing the secondary relaxations in PE does not allow their precise temperature location. Here the joint use of dynamic mechanical techniques and microhardness is recommended (Perena etal, 1989). 

---