Rate Of Polymer Relaxation

When a penetrant diffuses into a polymer, the perturbation will cause the polymer molecules to rearrange to a new conformational state. The rate at which this conformational adaptation occurs depends on the mobility of the polymer chains. At temperatures well above the glass transition, this occurs quite rapidly and the diffusive process resembles that in the liquid state. At temperatures near or below the glass transition, the conformational change does not take place instantaneously. Instead, there is a finite rate of polymer relaxation induced by the... 

Functionalized biopolymers have successfully taken over the era of synthetic polymers because they are cost effective, biodegradable, environmentally friendly and efficient. These biopolymers, after graft copolymerization and crosslinking, can be used for the sustained release of drugs. Based on relative rates of diffusion (Rjjff) and rate of polymer relaxation (R,, ), the diffusion of drugs depends upon the value of Diffusion Exponent (n). If n < 0.5, the R < R, . and if n a 1.0, the R. > whereas in case of n> 0.5 and < i.o, ... 

Pick s first and second laws were developed to describe the diffusion process in polymers. Fickian or case I transport is obtained when the local rate of change in the concentration of a diffusing species is controlled by the rate of diffusion of the penetrant. For most purposes, diffusion in rubbery polymers typically follows Fickian law. This is because these rubbery polymers adjust very rapidly to the presence of a penetrant. Polymer segments in their glassy states are relatively immobile, and do not respond rapidly to changes in their conditions. These glassy polymers often exhibit anomalous or non-Fickian transport. When the anomalies are due to an extremely slow diffusion rate as compared to the rate of polymer relaxation, the non-Fickian behaviour is called case II transport. Case II sorption is characterized by a discontinuous boundary between the outer layers of the polymer that are at sorption equilibrium with the penetrant, and the inner layers which are unrelaxed and unswollen. 

Star-shaped polymer molecules with long branches not only increase the viscosity in the molten state and the steady-state compliance, but the star polymers also decrease the rate of stress relaxation (and creep) compared to a linear polymer (169). The decrease in creep and relaxation rate of star-shaped molecules can be due to extra entanglements because of the many long branches, or the effect can be due to the suppression of reptation of the branches. Linear polymers can reptate, but the bulky center of the star and the different directions of the branch chains from the center make reptation difficult. 

The authors explained the observed temporary cessation of heat release by the difference between the rate of polymer formation and that of volume relaxation. According to their observations, the difference between the two phenomena leads to a situation in which the free radicals are spatially separated from the unreacted VA molecules. The subsequent spatial relaxation brings them into contact and the reaction starts again. 

Photochromic films based on phenoxynaphthacenequinone were characterized by high stability to irreversible phototransformations, in contrast to recording media with photochromic phenoxyanthraquinones. The above films allowed more than 500 cycles of rewriting of optical information at the same site of the light-sensitive layer. In addition, they were characterized by an extremely low rate of thermal relaxation from ana-quinoid into para-quinoid form. This means that the lifetime of the photoinduced form in a polymer film in darkness was equal to many years. 

Polarisation. In the event that e.e.m. takes place then the emission from the trap, excimer in the case of pure polymers, should be completely depolarised. Some time ago data was published on the emission of excimer for both polystyrene and poly(a-methyl styrene) indicating polarised emission (8) and therefore little e.e.m. More recently Phillips questioned the validity of the data and reported mesurements which suggest the excimer emission is depolarised (9 ). An experimental difference between the two sets of data is apparent - Phillips solutions were more dilute than these used in ref. 8. The range of concentrations has subsequently been extended with the results shown in Figure 1. A possible explanation for the effect of concentrations and molecular weights on the extent of polarisation of excimer emission is that the rate of rotational relaxation of this bulky entity becomes slower than the emission life-time as these two parameters increase. It is difficult to reconcile the concept of e.e.m. with polarised excimer emission. 

Thus, the rate of enthalpy relaxation for each polymer can be expressed in terms of temperature and excess enthalpy according to a relationship of the form described previously ( ), l.e.. 

These data concerning the molecular-structure dependency of the enthalpy-relaxation process suggest that the f u tors that influence the onset of segmental mobility influence enthalpy relaxation, but that the presence of mobile groups in the main chain or the ciystal-ordering tendency of polymers does not seem to have a significant effect on the rate of enthalpy relaxation. 

Case 11 diffusion The rate of penetrant diffusion is higher than the relaxation rate of polymer chains. Case 11 diffusion is characterized by a mass uptake that is proportional to the time, Q t. ... 

A tactic polystyrene has a Tg of 100°C. What are the relative rates of stress relaxation of this polymer... 

Freed et al. [3] evaluated the relaxation rate of polymer chains by the analysis of the anisotropic spectra of nitroxide spin labels. The main triplet spectrum was due to hyperfine coupling caused by the nitrogen nucleus. It narrowed with an increase in mobility of the radicals because of motional averaging of the anisotropic hyperfine interaction. The rotational correlation time of nitroxide spin labels can be estimated using the procedure of Freed et al. by taking into account the anisotropic rotational motion. The ESR line width, AHmsi of the spectrum can be expressed as follows ... 

The thermal stresses in the layers and films of the polymer can be decreased by heat treatment due to the increase in the rate of their relaxation [194],... 

Above the the rate of polymer chain relaxation is faster than the diffusion of CO2, and hence Fickian diffusion is to be expected. The diffusion of CO2 is believed to occur within the amorphous domains of the polymer matrix, and for this reason the diffusion in semi-crystalline polymers may be more complex than it in the case for glassy polymers. In the case of semi-crystalline polymers, CO2 is not soluble in the crystalline domains, and therefore the degree of crys-taUinity and hence the amorphous fraction available for CO2 molecules may influence the diffusion characteristics. Furthermore, C02-induced crystallization is likely to lead to an increase in the tortuosity factor, and thus the diffusion path length may increase as a function of time. Syndiotactic polystyrene and poly(4-methyl-l-pentene) [45] are semi-crystalline polymers which have crystalline phases (helical in the case of sPS) with lower densities than that of the amorphous phase and are exceptions, as CO2 access is not restricted to the amorphous domains, in fact CO2 diffuses faster in the helical sPS than in the amorphous polymer [46]. 

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