Beta relaxation-mechanism

Dyn ic mechanical analysis was discussed in terms of the nodular morphology concept in crossllnked structures. Beta relaxations in all the cured resins were bimodal in appearance. But, vAiile MPD-cured resins shewed a maximum at 25 C with a smaller shoulder at -40 C, TDA and DAEB-cured resins had maxima at -40 C with a less significant peak at 25 C. For DAIPB and DATBB-cured resins the two peaks were approximately equal in magnitude. The two overlapping peaks at -40 and 25 C were attributed to crankshaft motions in the matrix and nodules. 

Physical aging affects significantly the mechanical damping in both the alpha and beta relaxation peaks. Figure 19 shows the decrease in the beta loss peak as a func-... 

Figure 5.2. A schematic representation of typieal alpha and beta relaxations observed from mechanical and dielectric properties. The observed relaxation temperatures increase with increasing frequency.

Fig. 40. Loss tangent data from dynamic mechanical testing showing the effects of physical aging on the broad beta relaxation plateau...

Water and low temperature (20-35 C) aging influence the dynamic mechanical properties of a poly(amide-imide). At concentrations below 2 weight percent, water contributes to a low temperature relaxation between -120 and -50 C. Above 2 weight percent the water influences the beta transition. The enthalpy of activation for the beta relaxation is dependent upon aging temperature and time. Aging temperatures closer to the beta transition temperature result in higher activation enthalpies for that dispersion. 

Based on the dielectric and dynamic mechanical data, it appears that water and small polar molecules contribute to three dispersions in this poly(amide-imide). One is the low temperature relaxation between -100 and 0°C. This may be a hydrogen bonded relaxation since the activation enthalpy was 30 kJ/mol. This occurs at concentrations of water ranging between 0 to 4 weight percent. Two, the dielectric relaxation between 0 and 70 C can probably be attributed to conductive contaminants whose mobility is dependent upon a minimum amount of water. Three, at high water concentrations, greater than 2 weight percent, the water/NMP contributes to the beta relaxations observed between 50 and 150 C. 

Popli R, Glotin M, Mandelkern L and Benson R S (1984) Dynamic mechanical studies of alpha-relaxation and beta-relaxation of polyethylene, J Polym Sci Polym Phys Ed 22 407-448. 

Pharmacology These agents are synthetic adrenocortical steroids with basic glucocorticoid actions and effects. Glucocorticoids may decrease number and activity of inflammatory cells, enhance effect of beta-adrenergic drugs on cyclic AMP production, inhibit bronchoconstrictor mechanisms, or produce direct smooth muscle relaxation. Inhaler use provides effective local steroid activity with minimal systemic effect. 

Direct effects on the heart are determined largely by Bi receptors, although B2 and to a lesser extent a receptors are also involved, especially in heart failure. Beta-receptor activation results in increased calcium influx in cardiac cells. This has both electrical and mechanical consequences. Pacemaker activity—both normal (sinoatrial node) and abnormal (eg, Purkinje fibers)—is increased (positive chronotropic effect). Conduction velocity in the atrioventricular node is increased (positive dromotropic effect), and the refractory period is decreased. Intrinsic contractility is increased (positive inotropic effect), and relaxation is accelerated. As a result, the twitch response of isolated cardiac muscle is increased in tension but abbreviated in duration. In the intact heart, intraventricular pressure rises and falls more rapidly, and ejection time is decreased. These direct effects are easily demonstrated in the absence of reflexes evoked by changes in blood pressure, eg, in isolated myocardial preparations and in patients with ganglionic blockade. In the presence of normal reflex activity, the direct effects on heart rate may be dominated by a reflex response to blood pressure changes. Physiologic stimulation of the heart by catecholamines tends to increase coronary blood flow. 

Beta-adrenergic agonists are believed to induce smooth-muscle relaxation by the mechanism illustrat-... 

FIGURE 26-1 Mechanism of action of beta agonists on respiratory smooth muscle. Beta agonists facilitate bronchodilation by stimulating adenyl cyclase activity, which in turn increases intracellular cyclic AMP production. Cyclic AMP activates protein kinase, which appears to add an inhibitory phosphate group to contractile proteins, thus causing muscle relaxation and bronchodilation. 

Pinacidil is three- and tenfold more potent than hydralazine and minoxidil, respectively. It does not interact with alpha, beta, cholinergic, or histaminergic receptors, and also does not produce vasodilation via an indirect effect that is mediated by adenosine, prostaglandin, or endothelial-derived relaxant factor. Its vasodilating activity does not resemble that brought about by the conventional calcium-channel antagonists. Thus, pinacidil-induced vascular relaxation is a direct effect mediated by a novel mechanism. 

Kotlikoff MI, Kamm KE (1996) Molecular mechanisms of beta-adrenergic relaxation of airway smooth muscle. Annu Rev Physiol 58 115-141... 

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