Glass transition main-chain structure

Several factors related to chemical structure are known to affect the glass transition tempera lure. The most important factor is chain stiffness or flexibility of the polymer. Main-chain aliphatic groups, ether linkages, and dimethylsiloxane groups build flexibility into a polymer and lower Tg Aliphatic side chains also lower Tg, (he effect of the length of aliphatic groups is illustrated by the methacrylate series (4,38) ... 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

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 glass transition results from large scale conformational motion of the polymer chain backbone all moieties making up the structural unit of the polymer contribute to it. The main chain motions also satisfy the De Gennes reptation model (1971), where the chains move back and forth in snakelike motions. 

Observed Tg s vary from -123°C for polyfdimelhyl siloxane) (1-43) to 273°C for polyhydantoin (11-2) polymers used as wire enamels and to even higher temperatures for other polymers in which the main chain consists largely of aromatic structures. This range of behavior can be rationalized, and the effects of polymer structure on Tg can be predicted qualitatively. Since the glass-to-rubber transition... 

The effect of the chemical nature of the main chain of the polymer on the glass transition temperature is similar to the effect that it has on the melting temperature, T. The chemical structure has a determining influence on the flexibility of the chain. For example, polymers such as polyethylene, (—CH2 — CH2—) , and polyoxyethylene, (—CH2—CH2 — O—) , have relatively flexible chains as a result of the ease of rotation around their chain bonds. Thus they have low values of Tg and as can be seen in Table 2.3. The incorporation into the main chain of units that hinder rotation and consequently increase the rigidity of the chain clearly causes a large increase in Tg. For example, the incorporation of a p-phenylene ring (Ph) into the monomeric unit of polyethylene gives poly(p-xylylene), which has a Tg of around 353 K (see Table 2.3). 

---