Factors Affecting the Glass Transition Temperature

Side groups attached to the main chains have important impacts on the T of polymer fibers. Flexible side groups, including long branches, push polymer chains further apart, which in turn increases the free volume and lowers the T.  

Sources Kumar, S., Indian Journal of Fiber and Textile Research, 16, 52-64, 1991. Warner, S.B., Fiber Science, Prentice Hall, 1995.  

However, polymers with long flexible side groups or branches are difficult to form molecular orientation, and hence they are seldom used to make polymer fibers. Side groups found in polymer fibers typically are rigid and they can raise the 

Most polymer fibers are semiciystalline. The presence of the crystalline phase limits the mobility of the polymer chain segments in the amorphous region. As a result, the increases with increase in degree of ciystallinity. There also is a simple empirical relationship between the T and the melting temperature (T )  

The of polymer fibers can be reduced by the addition of low-molecular-weight organic additives, such as plasticizers. This is because these organic additives increase the intermolecular distance and weaken the intermolecular bonds between neighboring polymer chains. On the other hand, inorganic additives (e.g., TiOj and SiOj) can increase the of polymer fibers since they reduce the mobility of polymer chain segments. 

There are a number of structural features which have a bearing on the value of the glass transition temperature. Since this temperature is that at which molecular rotation about single bonds becomes restricted, it is obvious that these features are ones which influence the ease of rotation. These can be divided into two groups  

Before considering the special case of rotation about bonds in polymers it is useful to consider such rotations in simple molecules. Although reference is often made to the free rotation about a single bond, in fact rotational energies of the order of 2kcal/mole are required to overcome certain energy barriers in such simple hydrocarbons as ethane. During rotation of one part of a molecule about 

60 Relation of Structure to Thermal and Mechanical Properties AMORPHOUS,GLASS-LIKE CRYSTALLINE, FIBRE-FORMING 

This effect is also observed with some polymers. The trans form of a hydrocarbon chain requires an energy about 0.8 kcal/mole less than the gauche. The trans form leads to an extended molecule and in hydrocarbons this becomes more favoured as the temperature is lowered. Linear polyethylenes take up this conformation in the crystalline state. 

From the preceding considerations it is appreciated that the intrinsic chain flexibility is determined by the nature of the chain backbone and by the nature of groups directly attached to the backbone. 

The size of the group attached to the main chain carbon atom can influence the glass transition point. For example, in polytetrafluoroethylene, which differs from polyethylene in having fluorine instead of hydrogen atoms attached to the backbone, the size of the fluorine atoms requires the molecule to take up a twisted zigzag configuration with the fluorine atoms packed tightly around the chain. In this case steric factors affect the inherent flexibility of the chain. 

---

Controllable Factors Affecting the Glass Transition Temperature... 

The influence of factors such as chemical structure, molecular weight, cross-linking and plasticizers in the glass transition of polymers can be related to the changes that they provoke on the free volume fraction, which, as we already know, reaches a critical value at the glass transition temperature. The factors affecting the glass transition can be classified into two types (1) molecular factors, i.e., those related to the chemical structure of the polymer chain, and (2) external or controllable factors. 

The glass transition temperature, Tg, is the temperature below which the translational as well as long and short cooperative wriggling motions are frozen. In the robbery state, only the first kind of motion is frozen. The polymers that have their Tg values less than room temperature would be rubbery in nature, such as neoprene, polyisobutylene, or butyl rubbers. The factors that affect the glass transition temperatures are described in the following subsections. 

Table 1 lists the glass transition temperatures for the pertinent siloxane oligomers as a function of TFP and DP contents. The percent of each comonomer is recorded with reference to the siloxane units as well as the entire oligomer. One notes the difference that this creates between the two nominally 100% TFP siloxanes of different molecular weight. Note also the higher Tg values for the DP series at equal weight percents, a factor which limits their ease of synthesis and may affect their mobility during cure. 

Both the dipole-relaxation time and the ionic conductivity are related to the glass-transition temperature Fg. As a material is heated through its glass-transition temperature, static dipoles gain mobility and start to oscillate in an electric field. This causes an increase in permittivity and a loss-factor peak is noted. Obviously this motion is affected by frequency (lower frequencies have greater effects). This effect is shown in Figure 3.62 (Prime, 1997a), which shows the peaks in permittivity and loss factor at Tg. 

---