section%2050.3-5.4%20-%20Polymer%20Crystals%20and%20Tg

section%2050.3-5.4%20-%20Polymer%20Crystals%20and%20Tg -...

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5.3 Polymer Crystals Objectives Describe the morphology of polymers. Predict trends in crystallinity. When we talked about the structure of metals we learned that they are crystalline. Polymers are different. Many polymers are completely amorphous , or glassy , meaning they don’t have a regular crystalline structure. Many people will say that a glass is a frozen liquid. This does not mean it will ever flow like a liquid. What it means is that if you took a snapshot of a liquid, it would have the same random structure as a glass. Think about long polymer molecules, and how difficult it would be to arrange them into a regular, repeating crystal structure. The tangled polymer chains can’t easily move around and form a crystalline lattice. Some polymers are semicrystalline , which means that a sample of the polymer has two phases, an amorphous phase and a crystalline phase. However there are no polymers that are 100% crystalline; it’s just too hard to get those long molecules arranged in that way. Figure 5.3.1 shows a schematic of the morphology of a semicrystalline polymer. How can a long polymer chain even fit into a unit cell at all? It doesn’t; one chain will pass through many unit cells. Figure 5.3.2 shows the unit cell of polyethylene, which has 5 different molecules passing through it. You should also notice that for the crystalline phases in Figure 5.3.1 the chains are folded back and forth within a single crystal. Fortunately the properties of polymers don’t depend on the crystal structure like for metals, so we can pretty much ignore the specific crystalline lattice structures of polymers.
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section%2050.3-5.4%20-%20Polymer%20Crystals%20and%20Tg -...

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