Polymer Fundamentals and Classification

Monomers of common commercial polymers

Knowing which monomers build a commercial polymer is a frequent test point, so it helps to group common materials by the reaction that joins their units. Addition polymers such as polyethylene and polystyrene come from olefin or vinyl monomers without losing any small molecule. Condensation polymers release a small molecule, usually water, at each linkage, and polyurethanes are formed by polyaddition with no byproduct. Cellulose is a linear homopolymer of \beta-D-glucose units joined by 1,4-glycosidic linkages, so glucose is its only monomer. Fructose and galactose are other hexoses that occur in sucrose, inulin, galactans and hemicelluloses, and lactic acid is the monomer of polylactic acid, not of cellulose. Nitrile rubber (NBR) is an elastomer made by emulsion copolymerization of 1,3-butadiene and acrylonitrile, and the acrylonitrile fraction controls resistance to oils and fuels. Styrene-butadiene rubber and styrene-acrylonitrile resin share one comonomer with it but are different materials. Melamine-formaldehyde resin is a thermosetting condensation polymer. Melamine carries three amino groups, and each reacts with formaldehyde to form methylol groups that condense into N-CH_2-N bridges with loss of water. Because the triazine core is trifunctional, the product is a three-dimensional network that cannot be remelted. Aromatic aldehydes, ketones such as acetone, and monofunctional alcohols such as ethanol cannot form this bridge pattern. Other standard pairs are epichlorohydrin with bisphenol A for epoxy resin, a dianhydride such as pyromellitic anhydride with an aromatic diamine for polyimide (formed through a poly(amic acid) precursor that is then cyclized), a diisocyanate such as hexamethylene diisocyanate with a diol such as tetramethylene glycol for polyurethane, and maleic acid with propylene glycol for unsaturated polyester.

Chain conformation in solvents

A linear polymer in dilute solution adopts a random coil, and its size is usually described by the root-mean-square end-to-end distance \langle r^2\rangle^{1/2}. The coil dimension depends on solvent quality because it reflects the balance between polymer-solvent contacts and segment-segment contacts. In a good solvent, polymer-solvent contacts are favoured, excluded-volume repulsion swells the coil, and the chain is larger than its ideal size. In a theta (ideal) solvent, the excluded-volume effect is cancelled and the chain takes its unperturbed dimensions. In a poor solvent, segment-segment attraction dominates and the coil contracts below the ideal size. The ordering of sizes is therefore R_{\text{good}} > R_{\theta} > R_{\text{poor}}. The unperturbed dimension of an ideal linear chain follows \langle r_0^2\rangle^{1/2} \propto \bar{l}\,n^{1/2}, where \bar{l} is the segment length and n is the number of segments, so it depends only on chain architecture through these two quantities. Branch points fold the molecule and reduce its spatial extent, so a branched chain of the same molecular weight and number of segments has a smaller mean-square end-to-end distance than the linear chain. The root-mean-square end-to-end distance is therefore lower, and the ratio is less than unity but not a fixed constant.