Polymer studies rotational mobility

This investigation shows that it is indeed possible to study the flexibility of polymer chains in polymer matrices by means of excimer-forming probes and that the rotational mobility of these probes reflect the glass transition relaxation phenomena of the polymer host matrix, in agreement with the appropriate WLF equation. 

Water has an essential role in living systems and is ultimately involved in the structure and function of biological polymers such as proteins. However, in this contribution we sh tll focus primarily not on what the water does for the blopolymer but rather on the effects that the biopolymer has on the water that Interacts with it. Of Interest are alterations in the structural, energetic, and dynamic properties of the water molecules. Studies of the rotational mobility of water molecules at protein surfaces have been interpreted by dividing the solvent molecules into three groups U). The most rapidly reorienting group has a characteristic rotational reorientation time (t ) of not more than about... 

Around the glass transition temperature, the polymer is more mobile than far below Tg and the cooperative motion of polymer chains can induce rotational motion of the dye molecules embedded in the polymer matrix. The rotational motion can be studied using polarization measurements (confocal setup) or defocused wide-field imaging. 

Functional groups attached to solvent-swollen polymer chains exhibit free rotational motion as indicated by electron spin resonance rotational correlation times 132-134) These studies using nitroxide spin labels covalently bound to polystyrene matrices indicated that the mobility of the substituent is a function of the cross-link density and degree of swelling. The rotational correlation time of nitroxide within 2% cross-linked beads was about 100 times shorter in dichloromethane or benzene than in ethanol, and 2-3 times longer than nitroxide bound to non-cross-linked polystyrene. The latter observation shows that the heterogeneous reaction involving 2% cross-linked polystyrene is 2-3 times slower than the same reaction in solution. 

Photoisomerization was studied from a purely photochemical point of view in which photo-orientation effects can be disregarded. While this feature can be true in low viscosity solutions where photo-induced molecular orientation can be overcome by molecular rotational diffusion, in polymeric environments, especially in thin solid film configurations, spontaneous molecular mobility can be strongly hindered and photo-orientation effects arc appreciable. The theory that coupled photoisomerization and photo-orientation processes was also recently developed, based on the formalism of Legendre Polynomials, and more recent further theoretical developments have helped quantify coupled photoisomerization and photo-orientation processes in films of polymer. 

Another method for the study of the conformational mobility of polymers utilized the dependence of the polarization of the emitted light on the rotational diffusion of the fluorophore. This technique was pioneered by Anufrieva and her collaborators who used polymers with anthracene built into the chain backbone. They showed, for instance, that conformational mobility in polymethacrylates is much smaller than in polyacrylates. 

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