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Polymer Chemistry: Factors Influencing Tg

The value of Tg depends on the mobility of the polymer chain - the more immobile the chain, the higher the value of Tg. In particular, anything that restricts rotational motion within the chain should raise Tg. A polymer chain that can move easily will change from a glass to a rubber at a low temperature. If the polymer chains don't move as easily, then it will require a relatively high temperature to change the compound into a rubbery form.

Chain Stiffness

Stiffening groups in the polymer chain reduce the flexibility of the chain and raise the value of Tg .

poly(ethylene adipate) Tg = -70 °C   poly(ethylene adipate)
poly(ethylene terephthalate) Tg = 69 °C    poly(ethylene terephthalate)

Intermolecular Forces

Stronger intermolecular forces lead to a higher Tg. PVC has stronger intermolecular forces than polypropylene because of the dipole-dipole forces from the C-Cl bond.

Atactic Polypropylene Tg = -20 °C   Atactic Polypropylene
Atactic Poly(vinyl chloride) Tg = 81 °C   chain11.gif

Pendant Groups

The influence of pendant groups on the glass transition temperature is somewhat more complicated.

Bulky pendant groups, such as a benzene ring, can catch on neighboring chains like a "fish hook" and restrict rotational freedom. This increases Tg.

Atactic Polypropylene Tg = -20 °C   Atactic Polypropylene
Atactic Polystyrene Tg = 100 °C   Atactic Polystyrene

Flexible pendant groups, such as aliphatic chains, tend to limit how close chains can pack. This increases rotational motion and lowers Tg.

Poly(methyl methacrylate) Tg = 105 °C   Poly(methyl methacrylate)
Poly(butyl methacrylate) Tg = 20 °C   Poly(butyl methacrylate)

Cross-Linking

The presence of cross-links between chains restricts rotational motion and raises Tg.

Plasticizers

Plasticizers are low molecular weight compounds added to plastics to increase their flexibility and workability. They weaken the intermolecular forces between the polymer chains and decrease Tg. Plasticizers often are added to semi-crystalline polymers to lower the value of Tg below room temperature. In this case the amorphous phase of the polymer will be rubbery at normal temperatures, reducing the brittleness of the material.

Plasticizers are added to the plastic used for automobile upholstery. In older automobiles, the plasticizer may be distilled from the upholstery during hot weather so that it becomes brittle over time. Some plasticizers have been identified as major health and environmental problems. Before 1977, Polychlorinated biphenyls (PCBs) were used as plasticizers in paints and plastics. Because they are toxic and possible endocrine disrupters, PCBs no longer are used.

Contributors

  • David Whisnant (Wofford College). Partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grants DUE #9950809 and DUE #9950296. Additional support was provided by the Camille and Henry Dreyfus Foundation.