Amorphous polymers vitrification

When an amorphous polymer is gradually cooled from above the glass transition temperature Tg its volume decreases (see Fig. 13.32) according to its thermal expansion coefficient aj. In the region around the Tg the volume decrease will lag behind, starting at temperature Tel because the rate of reorganisation process becomes too small. The polymer starts to vitrify and a temperature Tel will be reached where the reorganisation completely stops and where the vitrification process is completed. Decrease of volume is only the result of normal volume contraction with expansion coefficient ag. The relationship between both thermal expansion coefficients is... 

Conventional models or theories consider only the segmental relaxation of amorphous polymers and the primary relaxation of nonpolymeric glass-formers in the change of molecular mobility with temperature and pressure (and concomitant changes in free volume and/or configurational entropy) leading to vitrification. Here we wish to recognise two different kinds of secondary relaxation processes. There... 

In case of glassy amorphous polymers the melting line may be replaced by a vitrification line. This concept may be applied to various systems and table III - 4 summarises. some examples of this thennally induced phase separation (TIPS) process. 

Coatings derived from cholesteric liquid crystalline polymers are used commercially as reflective sheets and polarisers. The liquid crystal is cooled below the vitrification temperature resulting in a solid polymer that is amorphous but contains large regions of frozen liquid crystalline order. Such structures are also found in nature in the iridescent, almost metallic colours of beetles and other insects, which result from helical cholesteric structures in the outer layer of the carapace. 

The glass transition temperature Tg is one of the most important structural and technical characteristics of amorphous solids. The correlations of Tg of linear polymers with their chemical composition, molecular weight, rigidity and symmetry of chains, as well as some other characteristics of macromolecules are well documented 57,58) Thg information on networks is much poorer. At present, for networks there exists mainly one parameter in structure-Tg correlations. It is the concentration of crosslinks — a parameter which is very insufficient, since in networks there are chemical crosslinks of different functionality (connectivity) which are distinguished by their molecular mobility. This means that the topological aspect of the network structure should be taken into account in the Tg analysis. Another difficulty connected with Tg determination of polymers lies in vitrification occurring during polymer formation (Sect. 6). 

The amorphous state is the characteristic of all polymers at temperatures above their melting points (except under special circumstances where liquid crystals may form). If a molten polymer retains its amorphous nature on cooling to the solid state, the process is called vitrification. In the vitrified amorphous state, the polymer resembles a glass. It is characteristic of those polymers in the solid state that, for reasons of structure, exhibit no tendency toward crystallization. The amorphous solid state is characterized by glass transition (Tg), which is described in a later section. We consider below only the behavior of polymer melt. 

Transport Properties. Sorption and transport properties are highly dependent on the post-vitrification history of glassy polymers (77) hence one would expect parameters such as physical aging, antiplasticization and amorphous orientation to affect transport properties. The reduction in diffusivity and permeability due to aging, orientation, and antiplasticization can be modeled via entropy or fi ee volume arguments (77). In addition, diffusive jumps of penetrant molecules in glassy polymers can be affected by (facilitated by) the segmental mobility that is manifested in sub-Tg relaxations 78),... 

Schick C, Wurm A, Mohammed A (2001) Vitrification and Devitrification of the Rigid Amorphous Fraction of Semicrystalline Polymers Revealed from Frequency-dependent Heat Capacity. Colloid Polymer Sci 279 800-806. 

If the temperature of a supercooled liquid can be reduced far enough below the normal freezing point without crystallization occurring, the specific heat and thermal expansivity may be observed to decrease abrupdy over a narrow range of temperature. Below this temperature range the material is in a vitreous or glassy state and possesses many of the physical properties of solids while retaining the amorphous X-ray diffraction pattern of a liquid. Vitrification has been observed in all types of liquids and is an important aspect of the thermal behavior of polymers. 

If another solvent is used to dehydrate flie polymer hydrogel to observe the internal structures, it often causes shrinkage and oflier structural changes. Thus, if possible, it is desirable when observing internal structures to use water in the sample. One of flie methods is flie rapid fi%ezing method [66]. Due to the rapid cooling, flie formation of microcrystals of ice can be suppressed and amorphous vitrification takes place. The cooling rate... 

---