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.
To crystallize a polymer chain must be linear, although limited crystallization can take place if a small number of branches are present. Crystallization is favored by a regular arrangement along the polymer chain giving the structure a high degree of symmetry.
|Linear polyethylene for example can form a solid with over 90% crystallinity in some cases. This is made possible by the planar zig-zag structure easily assumed by the molecule.|
Normal polystyrene is atactic with no regular order in the position of the benzene rings along the chain. The irregularity prevents the chains from packing closely to each other.
Atactic polystyrene, is amorphous. It is comparatively soft, low melting, and becomes swollen in solvents.
In syndiotactic polystyrene the benzene rings are on alternate sides of the chain. This allows the chains to pack into crystals.
Syndiotactic polystyrene is crystalline. It is rigid, high melting, and not penetrated readily by solvents.
Degree of Polymerization
Relatively short polymer chains form crystals more readily than long chains, because the long chains tend to be more tangled. High crystallinity generally means a stronger material, but low molecular weight polymers usually are weaker in strength even if they are highly crystalline. Low molecular weight polymers have a low degree of chain entanglement, so the polymer chains can slide by each other and cause a break in the material.
Crystallinity is favored by strong interchain forces. The presence of polar and hydrogen bonding groups favors crystallinity because they make possible dipole-dipole and hydrogen bonding intermolecular forces. A polyester, such as poly(ethylene terephalate), contains polar ester groups. Dipole-dipole forces between the polar groups hold the PET molecules in strong crystals.
Crystallinity in poly(ethylene terephalate) also is favored by the structural regularity of the benzene rings in the chain24. The benzene rings stack together in an orderly fashion.
Regular polymers with small pendant groups crystallize more readily than do polymers with large, bulky pendant groups. Poly(vinyl alcohol) (PVA) is made by the hydrolysis of poly(vinyl acetate) (PVAc).
PVA crystallizes more readily than PVAc because of the bulky acetate groups in PVAc. The -OH groups in PVA also form strong hydrogen bonds.
A major difference between small molecules and polymers is that the morphology of a polymer is dependent on its thermal history. The crystallinity of a polymer can be changed by cooling the polymer melt slowly or quickly, and by "pulling" the bulk material either during its synthesis or during its processing
A. Cooling Rate
When they are processed industrially, polymers often are cooled rapidly from the melt31. In this situation, crystallization is controlled by kinetics rather than thermodynamics. There may not be time for the chains, which are entangled in the melt, to separate enough to form crystals, so the amorphous nature of the melt is "frozen into" the solid. A polymer is more likely to have a higher percent crystallinity if it is cooled slowly from the melt.
Crystallinity can be enhanced by pulling the bulk material either when it is synthesized or during its processing. This is common for both films and fibers. When a film is formed the small crystallites tend to be randomly oriented relative to each other. Drawing (stretching) the film pulls the individual chains into a roughly parallel organization as is shown in the schematic diagram at the right. Films can either be uniaxially oriented (oriented in only one direction) or biaxially oriented (oriented in two directions).
Fibers normally are drawn so that they are oriented in one direction. Unstretched nylon fibers are brittle, for example, when the fibers are stretched the oriented fibers are strong and tough. Polyethylene can be unentangled by forming a gel with a low molecular weight solvent. When the gel is drawn, the resulting fibers are highly oriented. Ultra-oriented PE formed in this way is used in bullet-proof vests.
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.