Polymer characterization

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 which implies that the characterization should ideally also be linked to the parameters that are related to the desirable properties of the material such as strength, impermeability, toughness, optical properties and the like.

Chemical structure

The chemical structure of many polymers is rather complex because the polymerization reaction does not necessarily produce identical molecules as in the case in the DNA-coded synthesis of biopolymers. A polymeric material typically consists of a distribution of molecular sizes and sometimes also of shapes. Chromatographic methods like size exclusion chromatography often in combination with Low-angle laser light scattering (LALLS) and or viscometry can be used to determine the molecular weight distribution as well as the degree of long chain branching of a polymer, provided a suitable solvent can be found. 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 a different approach. Analytical Temperature Rising Elution Fractionation (ATREF) techniques can reveal how the short chain branches are distributed over the various molecular weights.

Thermal properties

A true workhorse for polymer characterization is thermal analysis, particularly Differential scanning calorimetry. Changes in the compositional and structural parameters of the material usually affect its melting transitions or glass transitions and these in turn can be linked to many performance parameters. For semicrystalline polymers it is an important method to measure crystallinity.

Mechanical and dielectric spectroscopy

Dynamic mechanical spectroscopy and Dielectric spectroscopy are essentially extensions of thermal analysis that can reveal more subtle transitions with temperature as they affect the complex modulus or the dielectric function of the material.

Morphology

Morphological parameters, particularly on a mesoscale (nanometers to microns) are very important for the mechanical properties of many materials. Transmission Electron Microscopy in combination with staining techniques, but also Scanning Electron Microscopy, Scanning probe microscopy and other forms of microscopy are important tools to optimize the morphology of materials like polybutadiene-polystyrene polymers and many polymer blends.

X-ray diffraction is generally not as powerful for this class of materials as they are either amorphous or poorly crystallized. The small angle range (Small Angle X-ray Scattering: SAXS can be used to measure the long periods of semicrystalline polymers.

Other techniques

• Solid state NMR
• Spectroscopic techniques: FTIR etc.
• ACOMP
• MFI
• TGA
• OIT
• Dual polarisation interferometry