Polymer glass formation free volume

The viscosity dependence of intramolecular excimer formation is complex. As in the case of molecular rotors (Section 8.2), most of the experimental observations can be interpreted in terms of free volume. However, compared to molecular rotors, the free volume fraction measured by intramolecular excimers is smaller. The volume swept out during the conformational change required for excimer formation is in fact larger, and consequently these probes do not respond in frozen media or polymers below the glass transition temperature. 

The method is based on the fact that the rate of conformational change required for excimer formation depends on the free volume induced by the segmental motions of the polymer occurring above the glass transition. DIPHANT (compound 3 in Figure 8.3) was used as an excimer-forming probe of three polymer samples consisting of polybutadiene, polyisoprene and poly(dimethylsiloxane).a)... 

The factors (298 Tg) and (Tg 298) are the "thermal distances" of Tg from room temperature for rubbers and glasses respectively. The influence of these "thermal distances" is probably connected with the fractional free volume of the polymer in rubbery amorphous polymers this f.f.v. increases with decreasing Tg, in glassy amorphous polymers the f.f.v. increases with increasing Tg (increasing formation of micro-voids), hence lowering of the activation energy. 

The rate Increase can be related to the sharp extension of free volume occurring within the polymer matrix at the glass-transition temperature. As a consequoice of this free-volume increase, Important molecular motions, which are required to achieve the formation of the colorless spiroform, become easier. It is, however, important to point out that, although a eharp increase of the thermoreversion rate at the glass-transition temperature can be observed, the rates are of the same order of magnitude at both sides of the Tg value. The thermoreversion remains quite appreciable at room temperature. Warming the film above Tg can only produce a more rapid decoloration. The incorporation of monomeric indollnosplropyrans in polymeric films is not sufficient to stop the thermoreversion. 

During the transformation of a monomer into a polymer, many atoms separated by van der Waals distances ( 5 A) participate in the formation of covalent bonds (1-3 A). Therefore during polymerization, an increase in the macroscopic density ensues, while on the molecular level a decrease in free volume and entropy occurs while the cooperativity of motions increase. Concomitantly the glass temperature can increase by more than 100 °C. Several important adhesive systems are based on this increase. The cyanoacrylate Super Glue starts as a monomer with a Tg <0 °C and it polymerizes to a linear soluble polymer with a Tg which is in the neighborhood of 100 °C upon appUcation under anaerobic conditions. For linear polymers the simple equation [74]... 

Among chemistry-related applications, the study of polymers is undoubtedly the most frequent. Free volume, glass transition, swelling, radiation damage, and aging are just a few examples of phenomena studied by positrons in polymers. However, radiation effects on Ps formation at low temperature bave attracted notable attention in the last few years (e.g., Hirade et al. 2000 Suzuki et al. 2000). 

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