Summary and Terms

Polymers comprise one of the fundamental classes of materials and have structures which fundamentally differ from typical materials that might assemble into highly ordered unit cells or disordered glasses. This unique structure gives polymers distinct properties which make them extremely useful to modern society. Polymers typically exhibit low melting temperatures, low thermal and electrical conductivity, modest mechanical properties, and often low densities. Furthermore, individual polymer chains can occupy specific conformations and architectures which significantly impact their properties.

Polymers can be thought of as very large molecules consisting of many smaller subunits linked together. How different subunits can assemble depends on their chemistry, with the resulting polymer often having lower potential energy than the monomers that were used to produce it. This means polymers are often very stable - something that is rapidly becoming a problem for us and our planet.

The way a polymer chain arranges itself in 3d space is its conformation. The conformations a polymer's chains are most likely to occupy determine that polymer's physical properties - think about how a rubber band might act if its chains tended to stretch out straight instead of scrunch themselves up. In the simple (but very useful) polyethylene, conformation can be described through the angle that each repeat unit is "twisted" relative to its neighboring unit while keeping the angle between them constant. Many polymers can be modeled in this way - simply by specifying which angles the chain is free to occupy while constraining others. Many polymers tend towards "scrunched up" conformations, something we saw emerge from our random walk model. One could think of this tendency as the most statistically likely outcome or the chain increasing its entropy by moving towards the most disordered state. In reality, both are the same argument!

Polymer Architectures can be divided into four main categories: linear, branched, crosslinked, and Network. Linear and branched polymers consist of long chains like the polymers we started with, but can have "offshoots" from the main chain in the case of branched polymers. The amount of branching a polymer exhibits can drastically change its properties by altering the secondary interactions that occur between chains. These two architectures are often called thermoplastics due to the fact that they can be thermally reprocessed through melting and reforming. Network and crosslinked polymers on the other hand consist of huge webs of interlinked molecules, either built from the ground up in the case of network, or by binding chains together through crosslinks. These two architectures are called thermosets because they cannot be thermally reprocessed, and have very different properties from thermoplastics.

There are a variety of ways that monomers can be assembled into monomers. In step growth, monomers can chemically bind together, releasing a byproduct such as water in the process. Step growth polymerizations happen in series throughout the reaction - because the groups will spontaneously react with one another there is no activator needed.