Polymer Chemistry: Morphology

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Polymer morphology is the overall form of polymer structure, including crystallinity, branching, molecular weight, cross-linking, and so on. Small molecules usually have crystalline solids, which are highly-ordered 3-dimensional arrays of the molecules. Solid polymers can be crystalline or amorphous (disordered arrangements of randomly coiled and entangled chains). Thermoplastics usually are semicrystalline - a combination of crystalline and amorphous regions. The properties of thermoplastics are strongly influenced by their morphology.

Polymer Chemistry: Classification of Polymers
The most common way of classifying polymers is to separate them into three groups - thermoplastics, thermosets, and elastomers. The thermoplastics can be divided into two types - those that are crystalline and those that are amorphous.
Polymer Chemistry: Crystallization Tendency
Some polymers form more crystalline solids than others. It will be useful for us to relate the tendency to crystallize to the chemical composition and structural details of particular polymers. Six factors favor a polymer with a high percent crystallinity: a regular and symmetrical linear chain, a low degree of polymerization, strong intermolecular forces, small and regular pendant groups, a slow rate of cooling, and oriented molecules.
Polymer Chemistry: Polymer Crystallinity
Most small molecule behavior can be understood in terms of three states: gas, liquid, and solid. Polymers are large molecules with strong intermolecular forces and tangled chains, and do not have a vapor phase - they decompose at high temperature before forming a vapor. The length of polymer molecules also makes it difficult for the large crystals found in the solid phases of most small molecules to form. Instead solid polymers can be modeled in terms of two phases - crystalline and amorphous.

Contributors and Attributions

David Whisnant (Wofford College). 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.