Factors Affecting Tg

As Tg marks the onset of molecular motion, a number of factors that affect rotation about links (necessary for movement of polymer chains) will also influence the Tg of a polymer. These include (a) chain flexibility, (b) molecular structure (steric effects), (c) molecular weight, and (d) branching and crosslinking. 

For symmetrical polymers, the chemical nature of the backbone chain is the important factor determining the chain flexibility and hence Tg. Chains made up of bond sequences which are able to rotate easily are flexible, and hence polymers containing -(-CH2-CH2-)-, -(-CH2-0- H2-)-, or -(-Si-O-Si-)- links will have correspondingly low values of Tg. For example, poly(dimethyl siloxane) has one of the lowest Tg values known (-123°C) presumably because the Si-0 bonds have considerable torsional mobility. 

The value of Tg is raised markedly by the insertion of groups which stiffen the chain by impeding rotation, so that more thermal energy is required to set the chain in motion. Particularly effective in this respect is the p-phenylene ring. Thus, a chain consisting entirely of p-phenylene rings, namely, poly(p-phenylene) (XVI) 

When the polymer chains are unsymmetricai, with repeat units of the type -(-CH2-CHX-)-, an additional restriction is imposed by steric effects depending on the size of the pendant group X. Bulky pendant groups hinder the rotation about the backbone and cause Tg to increase. 

Problem 2.21 Presence of flexible pendant groups reduces the glass transition of the polymer, whereas bulky or stiff side groups increase it. Why  

Some examples of such polymers are poly(xylylene), -( (g) )jr, 

Some examples of such polymers are poly(xylylene), -(-CH2-C6H4-CH2 7r, Tg 280°C and poly(phenylene oxide), Tg = 83°C, as compared to polyethylene -(-CH2CH2 V, 

Another factor is the flexibility of the chemical bonds in the molecule itself. Materials with flexible bonds such as Si-0 or CH2-CH2 have low-temperature glass transitions whereas more rigid structures have higher temperature transitions. 

The glass transition is not a single point and occurs over a region in temperature, in some cases many tens of degrees, therefore care is required when quoting a value for 7. A choice can be made from the following calculations  

Midpoint Endpoint Inflection point Fictive temperature 

This is a specific value and comes from the definition of the fictive temperature as being the temperature of the frozen glassy state at which a material is in equilibrium (see Section 1.5.6). The calculation is taken from the enthalpy curve and is the intersection of tangents taken from above and below the Tg. The same value can be obtained by drawing tangents to the integral of the heat flow trace. 

In practice, one of the most often used values is the midpoint calculation but there is no rule that says that this or any other value should be used, so the method of calculation 

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This long list of factors affecting Tg demonstrates that many factors not related either to the composition or to the structure of a polymer can significantly affect Tg. Some internal... 

Another geometric factor affecting Tg is cis-trms configuration. Double bonds in the cis form reduce the energy barrier for rotation of adjacent bonds, soften the chain, and hence reduce Tg (Table 4.5). 

This long list of factors affecting Tg demonstrates that many factors not related either to the composition or to the structure of a polymer can significantly affect Tg. Some internal inconsistency, and the need to exercise judgment and to make choices, is therefore inherent in preparing any data set collected from different sources for use in developing or validating any correlative or predictive scheme for Tg. In the best of all possible worlds, one would synthesize all of the polymers which will be used in the dataset, characterize them very carefully, and... 

External factors affecting Tg, such as the cooling rate and the frequency of the mechanical stress, depend on the service conditions of the material considered. With respect to the structural or compositional parameters, they can be modified at will by the chemist or the formulator in order to adjust Tg and thus meet the requirements of a given application. 

Though the factors that govern Tg have been known for some years, there is still a wide variation in values for particular polymers. Polymer Tg s are sensitive to parameters which may or may not have been evaluated by the authors. Published values should be reviewed considering all the factors which affect T,. The main factors affecting Tg values are polymer structure, sample crystallinity, diluent types and concentrations, molecular weight distributions, previous thermal history of the sample, and system pressure. More detailed treatments are given in reviews (6,48,49,1241-1249). 

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