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Relationships between Structure and Properties

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What makes polymers hard or soft?

Any polymer may be either hard or soft; it is all a function of the temperature. Unlike simple solids, which have only one phase transition (from solid to liquid), synthetic polymers typically undergo a phase transition from a glassy to a rubbery state at a temperature well below their melting point. With some polymers, we are more familiar with the transition in the other direction, since they are rubbery at room temperatures. For example, if you put a rubber band in liquid nitrogen, it goes brittle and can be crushed into powder. Polymer scientists (being particularly imaginative people) call this temperature of transition from a rubber to a glass the glass transition temperature, Tg.

Some glassy polymers we have met:
approx Tg (°C)
poly(styrene) 100
poly(acrylonitrile) 100
poly(vinyl chloride) 80

Some rubbery polymers are:
approx Tg (°C)
natural rubber (poly(cis-isoprene)) -70
low density poly(ethene) -80
poly(butyl acrylate) -20

There are a number of factors that go into determining the glass transition temperature of a polymer. From the numbers above, you can probably guess that the more side-groups a polymer has, the higher the glass transition temperature - the bulkier a segment of polymer is, the more energy it takes to move it, so its glass transition will be at a higher temperature. If there is the possibility of good intermolecular binding between the polymer chains, this effect will be more pronounced.

Glass transition temperature will rise with the molecular weight of a polymer, but this effect plateaus above a molecular weight of about 10,000.

Poly(butyl acrylate) may look rather anomalous - it has a long side-group; why doesn't it have a Tg up there with poly(styrene) and poly(vinyl chloride)?

The side-group in poly(butyl acrylate) is long and flexible, and acts like a "molecular ball bearing", enabling the polymer chains to slip past one another more freely.

When polymer scientists talk about polymer chains, terms such as intertwining and entangling are often used. Sometimes, polymers are said to be "loosely coiled" however that is not a formal term. What is meant by "loose coiling and intertwining of polymer strands" is a random conformation: lots of different chains randomly intertwined like spaghetti. Many polymers display this random conformation.

Polymer chains become entangled like strands in a bowl of spaghetti.