Polymer Chemistry
Factors Influencing Tg
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Polymer ChemistryFactors Influencing Tg |
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The value of Tg depends on the mobility of the polymer chain - the more immobile the chain, the higher the value of Tg34. 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. |
Stiffening groups in the polymer chain reduce the flexibility of the chain and raise the value of Tg .
| poly(ethylene adipate) | Tg = -70oC |  |
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| poly(ethylene terephthalate) | Tg = 69oC |  |
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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 = -20oC |  |
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| Atactic Poly(vinyl chloride) | Tg = 81oC |  |
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The influence of pendant groups on the glass transition temperature is somewhat more complicated.
1. Bulky pendant groups, such as a benzene ring, can catch on neighboring chains like a "fish hook" and restrict rotational freedom34. This increases Tg.
| Atactic Polypropylene | Tg = -20oC | |
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| Atactic Polystyrene | Tg = 100oC |  |
2. 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 = 105oC |  |
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| Poly(butyl methacrylate) | Tg = 20oC |  |
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The presence of cross-links between chains restricts rotational motion and raises Tg.
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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 disruptors, PCBs no longer are used.
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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. PCOL faculty also acknowledge the National Science Teachers Association which awarded the PCOL Faculty Consortium the 1998 Gustav Ohaus Award for Innovation in College Science Teaching.
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This page was last updated on July 11, 2000
whisnantdm@wofford.edu