4: Polymer Properties
- Page ID
- 190665
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- 4.1: Molecular Weight of Polymers
- It might seem obvious that molecular weight is an essential property of any molecular compound. In polymers, molecular weight takes on added significance. That's because a polymer is a large molecule made up of repeating units, but how many repeating units? Thirty? A thousand? A million? Any of those possibilities might still be considered a representative of the same material, but their molecular weights would be very different, and so would their properties.
- 4.2: Viscosity of Polymers
- Viscosity is a term we use to describe the "thickness" of different liquids.
- 4.3: Rheology
- Viscosity measurements are the realm of a field of science called rheology. Rheology is, literally, the study of flow.
- 4.4: Glass Transition
- The glass transition is probably the most commonly-cited characteristic of a polymeric material. At the glass transition temperature, the physical nature of the material changes subtly. It goes from being a rubbery, flexible material at higher temperature, above the glass transition, to a glassy, harder material at a lower temperature. The material is still a solid either way, but there is a definite change in how it responds to stimuli.
- 4.5: Crystallinity in Polymers
- Polymers are different from many other such solids. Metals, for instance, have crystalline structures, in which atoms form a regularly repeating pattern, row upon row. Polymers, in contrast, are generally somewhat amorphous. Think of a strainer filled with cooked spaghetti, the long chains of pasta looping over each other randomly. That's how the molecules of amorphous or "shapeless" polymers tend to distribute themselves.
- 4.6: Microphase Separation
- Sometimes, more rigid segments of a polymer are deliberately built into the structure. If the block lengths are the right size, the two segments are able to separate into two phases. As a result of stronger intermolecular attractions, lengths of chains containing hard segments cluster together, pushing out the soft segments that would otherwise get in the way of these intermolecular attractions. This phenomenon is called microphase separation.