Rotational motions, amorphous glass

When polymer melts, rubbers, or elastomers are cooled down below Tg, they may freeze to glasses (noncrystalline amorphous phases). The rotations motions of the chain segments (micro-Brownian motions) are almost stopped now, and the transparent materials become stiff and (in most cases) brittle. 

So, if the interfacial regions have to be considered, we must differentiate the amorphous/crystal polymer interface from the amorphous polymer/mineral interface (29). By DMA measurements, a decrease in Tg matrix from 7°C for the PP/talc composites and also for the neat PP processed under similar conditions while a decrease upto 13°C was found for PP/mica composites. A higher fraction of free amorphous phase on the PP/mica system than on the PP/talc composites was evidenced. This free amorphous phase appeared to participate in the cooperative segmental free-rotation motion, well accepted (30) to be responsible for glass transition for the polymer matrix as fully discussed in Reference 29. 

This is the temperature below which an amorphous rubbery polymer becomes brittle. These changes are completely reversible and depend on the molecular motion within the polymer chain. In the rubbery state of polymer melt, the chains are in rapid rotational motion (many rotations per second), but as the temperature is lowered, this movement is slowed down until it eventually stops and the polymer behaves like a frozen liquid with a completely random structure. Although the value of Tg is useful when considering polymers, any glass-forming liquid will have a similar transition, e.g. TgS for quartz 1200 °C, B2O3 250 °C, sulphur 75 °C, polyphosphoric acid — 10 °C, glycerol — 90 °C, toluene — 170°C. 

In general, the rotational and vibrational motions of molecules are limited in the amorphous glass state. In the rubbery state, large-scale molecular motion, such as translational motion, is possible (Ubbink and Schoonman, 2003). Therefore, the encapsulated flavor or oil exists stably in the amorphous glassy state, but in the rubbery state some deterioration takes place. Since amorphous states are nonequilibrium states, thermodynamic driving forces tend to shift the amorphous state into a more stable crystalline state, resulting in time-dependent crystallization, solidification of powders, and caking. 

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

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