Temperatur aggregation

Figure 6.13 The low-temperature (77 K) absorption spectrum of a F-colored NaCl crystal after room temperature aggregation under illumination with light lying in the F absorption band. Bands associated with different color centers can be clearly seen (reproduced with permission from Lifante, 1989).

Suppose AHr and ASR are both negative. In this case ASR opposes aggregation while AHr favors it. Since the resistance to aggregation decreases with decreasing temperature, aggregation is expected as T is lowered. Poly(12-hydroxystearic acid) adsorbed from n-heptane and polyoxyethylene adsorbed from methanol are examples of systems that display a CFT with decreasing temperature. Since ASR is the source of the stabilization in these cases, this mechanism is called entropic stabilization. 

An intricate diffusion limited scenario was observed in the low-temperature aggregation of PVBA and the related species PEBA (4-[(pyrid-4-yl-ethynyl)]-benzoic acid) on Ag(lll) [78,79]. The operation of anisotropic hydrogen bonding between the molecular endgroups in conjunction with the smoothness of the substrate leads to the formation of molecular networks, as demonstrated by the STM data in Fig. 6. At higher... 

Availability of the critical thin layer favorable for radiation processing can be associated with the surface tension caused by the interactions in the system of low-temperature aggregates on bitumen surface. The breaks of the bonds between aggregates lying on the surface create an unbalanced shear that probably prevents dissipation of radiation energy through the system of aggregates. 

Proper mixing temperature is vital and is determined by the type of bituminous binder used. To achieve the required mixing temperature, aggregates and bitumen should have the appropriate temperature when entering the mixer. 

On the other hand, polyfpropylene oxide) (PPO) is hydrophilic at low temperatures and hydrophobic at high temperatures. Thus, water is a good solvent for PPO below approximately 15 °C, whereas at higher temperatures aggregates are formed in aqueous solutions. With blocks of PEO and blocks of PPO combined into single polymer chains, one can therefore expect amphiphilic characteristics and aggregation phenomena. It has to be emphasized that, for this triblock copolymer, the outer PEO blocks are more water soluble than the inner PPO blocks, and thus the mechanism of gel formation will be different from the above mentioned triblock copolymers and is still a matter of discussion. 

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