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Factors Affecting the Glass Transition Temperature, Melt Temperature and Viscous Flow Temperature of Polymers

2022-07-18 Page view : 15 views

The glass transition temperature (Tg), melt temperature (Tm) (for crystalline polymers) and viscous flow temperature (Tf) (for non-crystalline polymers) of polymers are important temperature parameters. Tg determines the service temperature of the polymer, while Tm and Tf determine the processing temperature of the polymer. There are many factors that influence the Tg, Tm and Tf values of a polymer.

Influence of polymer structure and properties

I. Chain structure

(1) Main chain. The introduction of rigid groups such as phenyl, biphenyl groups and conjugated double bonds on the main chain increases the rigidity of the chain, while Tg, Tm and Tf all increase; the introduction of ether bonds and isolated double bonds on the main chain makes the chain flexible, and Tg, Tm and Tf all decrease.

(2) Side groups. When the side group is a rigid group, the flexibility of the chain decreases as the volume of the side group increases, and Tg, Tm and Tf all increase; when the side group (or side chain) is a flexible group (or flexible chain), the larger the side group (chain), the better the flexibility, then the flexibility of the whole molecular chain is better, and Tg, Tm and Tf all decrease.

The combination of (1) and (2) shows that all chain structure factors that increase chain stiffness can increase Tg, Tm and Tf values, and all chain structure factors that increase chain flexibility can decrease Tg, Tm and Tf values.

II. Intermolecular forces
Polar polymers, with strong interactions between the polar groups on the molecular chain and strong intermolecular forces, have larger Tg, Tm and Tf values than the corresponding values for non-polar polymers; and Tg, Tm and Tf values all increase with the increase in intermolecular forces.

III. Molecular weight
As Tm is related to crystallization, in general, molecular weight has little effect on Tm and both Tg and Tf increase with increasing molecular weight. For Tg, this trend is more pronounced at low molecular weights, while Tg changes extremely slowly when the molecular weight increases to a certain level.

The effect of molecular weight on Tf is much more significant than that on Tg. This is because the effect of molecular weight on Tg is attributed to the chain end effect, which can only be shown when the chain end content of the system is relatively high, i.e. the molecular weight is relatively low; after the molecular weight is high to a certain extent and the weight of the chain end is small to an almost negligible extent, its effect on Tg is not significant.

Tf is the critical temperature at which the centre of mass of the entire polymer chain is displaced, and the movement of the entire chain is achieved by the concerted movement of all chain segments. The higher the molecular weight, the greater the number of chain segments that need to move in concert, and the greater the frictional forces that need to be overcome during the movement, and the higher the Tf. The Tf value is therefore strongly dependent on the molecular weight.

Influence of external factors

I. Small molecule soluble additives

Plasticisers or other soluble additives are sometimes added to the ingredients in order to improve processing properties or to impart a certain aspect to the product during polymer forming and processing. In the case of polymers, these small molecules are equivalent to diluents and they reduce the Tg, Tm and Tf of the polymer.

II. External forces

Unidirectional external forces have a pushing effect on the chain segments, so increasing external forces can lower Tg and Tf. Increasing the duration of the external force also favours the movement of the molecules in the direction of the external force, which also lowers Tf. An increase in pressure reduces the free volume and increases Tg and Tf.

The effect of external forces on Tm is as follows: when a polymer crystallizes under tensile forces, the ability to crystallize increases, raising the degree of crystallinity and also the melting point of the crystals, i.e. Tm increases; crystallizing under pressure increases the thickness of the wafer, which increases the perfection of the crystal, also making Tm higher.

III. Test rate

This is in terms of the magnitude of the test value obtained in terms of the temperature test. Since the movement of polymer chains is a relaxation process and is time-dependent, there is a relationship between the Tg test value and the experimental time scale: increasing the rate of warming or the frequency of dynamic experiments will increase Tg. The same is true for Tf, and the opposite is true for Tm. When testing Tm values, if the temperature is increased slowly, imperfect grains can be melted first and then recrystallised into more perfect and stable crystals at a slightly higher temperature. The final so-called “melting point” is the temperature at which all the more perfect crystals melt and will be higher than the value measured at a rapid temperature rise.

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