Molecular weight in polymer processing

>> Monday, 15 September 2008

A polymer is a set of molecules with very high molecular weight called macromolecules. Macromolecules are composed of basic units called monomers. Monomers are groups of the same linked atoms. Molecular weight is a sum of atoms weight for molecule.
Molecular weight impacts everything, viscosity, strength, thermal resistance, light transmition, but not density. Macromolecules are different, different in length, start from monomers and end up with “huge” macromolecules. This is the statistical (e.g. normal distribution) process of production, every lot (even bag!) from the same plant, from the same method, can be different in molecular weight distribution and average molecular weight.

Different fractions have different impact on processing and application properties. Short fractions, or single monomers are gaseous substances, waxes and the advantages are problematic (easy degradation, there are responsible for bad odor) With molecular weight grow, viscosity will grow, mechanical properties will grow, etc. chains will be longer.
Average molecular weight and the range have to be consider during choosing the grade for processing methods, e.g. extrusion blow molding one of the most important is melt strength which protect against sagging. It will be possible for grades with grater viscosity, high molecular weight, etc. for low shear rate.
Technique still benefits natural polymers, e.g. caoutchouc from elastomers, from polymers cellulose and proteins and it derivative.

PS. Picture shows: molecular chain of polyethylene and comes from: University of Florida, USA

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