Polymer characterization

Polymer characterization is the analytical branch of polymer science. Polymer characterization is the analytical branch of polymer science. The discipline is concerned with the characterization of polymeric materials on a variety of levels. The characterization typically has as a goal to improve the performance of the material. As such, many characterization techniques should ideally be linked to the desirable properties of the material such as strength, impermeability, thermal stability, and optical properties. Characterization techniques are typically used to determine molecular mass, molecular structure, morphology, thermal properties, and mechanical properties. The molecular mass of a polymer differs from typical molecules, in that polymerization reactions produce a distribution of molecular weights and shapes. The distribution of molecular masses can be summarized by the number average molecular weight, weight average molecular weight, and polydispersity. Some of the most common methods for determining these parameters are colligative property measurements, static light scattering techniques, viscometry, and size exclusion chromatography. Gel permeation chromatography, a type of size exclusion chromatography, is an especially useful technique used to directly determine the molecular weight distribution parameters based on the polymer's hydrodynamic volume. Gel permeation chromatography is often used in combination with multi-angle light scattering (MALS), Low-angle laser light scattering (LALLS) and/or viscometry for an absolute determination (i.e., independent of the chromatographic separation details) of the molecular weight distribution as well as the branching ratio and degree of long chain branching of a polymer, provided a suitable solvent can be found. Molar mass determination of copolymers is a much more complicated procedure. The complications arise from the effect of solvent on the homopolymers and how this can affect the copolymer morphology. Analysis of copolymers typically requires multiple characterization methods. For instance, copolymers with short chain branching such as linear low-density polyethylene (a copolymer of ethylene and a higher alkene such as hexene or octene) require the use of Analytical Temperature Rising Elution Fractionation (ATREF) techniques. These techniques can reveal how the short chain branches are distributed over the various molecular weights. A more efficient analysis of copolymer molecular mass and composition is possible using GPC combined with a triple-detection system comprising multi-angle light scattering, UV absorption and differential refractometry, if the copolymer is composed of two base polymers that provide different responses to UV and/or refractive index. Many of the analytical techniques used to determine the molecular structure of unknown organic compounds are also used in polymer characterization. Spectroscopic techniques such as ultraviolet-visible spectroscopy, infrared spectroscopy, Raman spectroscopy, nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, X-ray diffraction, and mass spectrometry are used to identify common functional groups. Polymer morphology is a microscale property that is largely dictated by the amorphous or crystalline portions of the polymer chains and their influence on each other. Microscopy techniques are especially useful in determining these microscale properties, as the domains created by the polymer morphology are large enough to be viewed using modern microscopy instruments. Some of the most common microscopy techniques used are X-ray diffraction, Transmission Electron Microscopy, Scanning Transmission Electron Microscopy, Scanning Electron Microscopy, and Atomic Force Microscopy. Polymer morphology on a mesoscale (nanometers to micrometers) is particularly important for the mechanical properties of many materials. Transmission Electron Microscopy in combination with staining techniques, but also Scanning Electron Microscopy, Scanning probe microscopy are important tools to optimize the morphology of materials like polybutadiene-polystyrene polymers and many polymer blends.