Large-scale molecular motions

Selective Quenching of Large-Scale Molecular Motions by Cross-Linking in the Strained State... 

The large scale molecular motions which take place in the rubber plateau and terminal zones of an uncross-linked linear polymer give rise to stress relaxation and thereby energy dissipation. For narrow molecular weight distribution elastomers non-catastrophic rupture of the material is caused by the disentanglement processes which occur in the terminal zone, e.g., by the reptation process. In practical terms it means that the green strength of the elastomer is poor. 

In liquid crystals or LC-glasses one looks for orientational order and an absence of three-dimensional, long-range, positional order. In liquid crystals, large scale molecular motion is possible. In LC-glasses the molecules are fixed in position. The orientational order can be molecular or supermolecular. If the order rests with a supermolecular structure, as in soap micelles and certain microphase separated block copolymers, the molecular motion and geometry have only an indirect influence on the overall structure of the material. 

At the glass-transition temperature, the specific heat increases abruptly because the previously frozen large-scale molecular motions are now available to the chains for the uptake of thermal energy. The subsequent sharp downturn is due to crystallization... 

In most polymeric as well as non-polymeric amorphous materials, the ability to undergo large-scale molecular motions implies the freedom to flow, so that the material becomes a fluid above Tg. However, in the special class of polymers commonly described as thermosets , covalent crosslinks limit the ability to undergo large-scale deformation. Consequently, above Tg, thennosets become elastomers (also known as crosslinked rubbers ). 

In the glassy state, below Tg, when large scale molecular motion does not take place, crosslinking and chain scission are expected to occur with a fairly low yield because only those tertiary hydrogen atoms that are close enough to an excited acetate group can be abstracted. However, suitable orientation of the reactants can be reached through rotation of... 

In general, the rotational and vibrational motions are limited in the amorphous glassy state. In the rubbery state, on the other hand, large-scale molecular motion, such as translational motion, is possible (Ubbink and Schoonman, 2003). Therefore, the encapsulated flavors or oils exist stably in the amorphous glassy state, but in the rubbery state some deterioration may take place. Since an amorphous state is not an equilibrium state, a thermodynamic driving force tends to shift the amorphous state to a more stable crystal state, resulting in a time-dependent crystallization, solidification of powders, and caking. 

Clarke and Shanks have examined the influence of sample thickness on the benzoin photoinitiated polymerization of butyl acrylate. They found that as the photoinitiator concentration increases so the extent of polymerization become less susceptible to changes in sample thickness. Grauchak et al. have successfully photopolymerized acrylic monomers in polyamide matrices with aromatic carbonyl compounds. In the photocycloaddition of olefins to poly(4,-vinylbenzo-phenone) and its copolymers with styrene, the rate of addition was found to be independent of the glass transition temperature suggesting that large-scale molecular motion is unimportant in this photoreaction. 

Liquid crystals are materials characterized by long-range orientational order of molecules as in crystals but absence of three-dimensional positional order as in liquids. In these positionally disordered crystals or orientationally ordered liquids, large-scale molecular motion is possible [3-5]. The name liquid crystals was given by Lehman in 1907 [6] because of their optical anisotropy and the hquid-like flow of these materials. 

The term mesophase also includes ordered liquids (nematic, smectic, cholesteric and discotics), which present long-range orientational order like in a solid, but positional disorder like in a liquid [2]. In these materials, large-scale molecular motion is possible, which is a characteristic of the liquid state rather than of the solid state. The term liquid crystals is conventionally used to address them. This sub-class of mesophases will not be treated in this context. 

Polymers are the only class of materials that possess this combination of properties. Because of their extended, chain-like molecular structure they form networks (which may or may not be cross-linked) that resist large-scale molecular motion, and therefore provide solid-like behavior. The portions of the polymer molecule in between network junctions, however, can undergo small-scale motion that is similar to that of a liquid when viewed on a short time scale. An important feature of all PSAs is the glass transition temperature Tg of its polymeric components. If the PSA is cooled below its Tg, then it will behave like a glassy solid, since even the small-scale, between-network motions will be frozen. Above its Tg, though, the... 

The GT is due to the freezing of large-scale molecular motions without changes in structure. The GT temperature is the main characteristic of both amorphous solid and liquid states. By DSC, however, one can observe the GT upon both freezing and heating analysis (see Fig. 25). Such behavior results from the considerable temperature dependence of the relaxation times of large-scale molecular motions. In other words, the glass-liquid transition occurs at an easily... 

We assign the sharp rise in friction at elevated RH to the glass-to-rubber transition. It is well known that both PVOH and gelatin are plasticized by absorbed water. ] At approximately 60% RH a moisture content of 8% is expected in PVOH. This would shift the glass transition from around 80 C in anhydrous PVOH to 20-22°C, i.e. room temperature, as used in the present experiment. Thus above 60% RH, large-scale molecular motions in response to shear and compressive stresses (tip) will dissipate strain energy. Enhanced energy dissipation is measurable as an abrupt... 

Kramer O (1988) Selective quenching of large-scale molecular motions by cross-linking in the strained state. ACS Symp Ser 367 48-58... 

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