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HPLC Application to Polymer Analysis
Mie University, Tsu, Mie, Japan
A polymer. especially a synthetic polymer, is a large molecule built up by the repetition of small, simple chemical units. The repeating units of the polymer are usually equivalent to the monomer or the starting material composing the polymer. The length of the polymer chain is specified by the number of the repeating units in the chain, which is called the degree of polymerization. The molecular weight (MW) of the polymer is the product of the MW of the repeating unit and the degree of polymerization. A synthetic polymer contains molecules having many different chain lengths and, unlike many small molecules and biopolymers, it has a distribution of the different chain lengths (the different MW). A biopolymer, such as protein, is an assembly of several amino acids connected by peptide linkage and consists of molecules having the same chain length (the same MW).
For a given polymer composition, molecular weight averages and a molecular weight distribution (MWD) of a synthetic polymer are some of molecular characteristics that deter-mine its properties. In copolymers, the chemical composition and the chemical composition distribution are other molecular characteristics of the copolymers. Several other molecular characteristics can be considered, and they are measured by several techniques of instrumental analysis. Molecular characteristics measured by high-performance liquid chromatography (HPLC) are listed in Table 1.
Size-exclusion chromatography (SEC) is a technique that separates materials according to the differences in their molecular sizes (hydrodynamic volumes in a solution). Nonexclusion liquid chromatography (NELC) is a technique that separates molecules by separation modes (adsorption and partition) other than size exclusion. Hyphenated techniques such as SEC-viscometry can give the information on branching and a branching distribution of synthetic polymers. Microgels in polymers eluting at or earlier than the exclusion limit of an SEC column system are, because of a small amount of microgels in the polymers compared with the main component, almost impossible to detect with a refractomer, which is usually used as detector for SEC. SEC-LS (light scattering) can detect such small amounts of microgels. Temperature-rising elution fraction (TREF) is a technique to fractionate crystalline pol-ymers or stereo regular polymers, such as polyethylene and polypropylene by raising the column temperature to increase the solubility of the polymers adsorbed on the surface of packings in a column. Field-flow fractionation (FFF) is a technique to separate polymers and particles in the order of increasing MW or size and is different from other HPLC modes on