Molecular weight can have a huge impact on a material’s final properties. Understanding this materials characteristic can help you better position the proper material for your application. Looking for assistance? Contact Amco Polymers today.
When we talk about molecular weight in terms of polymers, we are really talking about the length of the individual chains. Molecular weight is the average weight of the molecules that make up a polymer and gives an indication of the length of the polymer chains
The polymerization process is subject to variation so there is no single chain length, there is actually a wide range of lengths, so when we discuss molecular weight, we really mean the average molecular weight of the material. This average is found by measuring samples of the material as it is produced.
Molecular weight and MWD (or molecular weight distribution) determine the polymers properties
This table shows the effects of molecular weight on selected properties. In general, as molecular weight increase the strength, toughness and chemical stress crack resistance increase. Lower molecular weight will typically flow easier.
· A High molecular weight increases how far the material can stretch before rupturing. The higher degree of entanglement allows the material to be pulled further before the chains break
· A High molecular weight increases the impact resistance of the material. The higher degree of entanglement means that in order to rupture, more polymer bonds need to be broken, this means that the polymer can absorb more energy before failing.
· A High molecular weight increases the chemical resistance - to a point. It takes more damage to the main chains of the molecules before it will affect the strength of the material.
· A High molecular weight increases the viscosity of the material – makes it harder to process the material using conventional methods. The longer the chains, the harder it is to get them to flow because they are more tangled.
This table shows the influence of melt flow on the impact properties of polycarbonate. With increasing melt flow index, there is a significant drop off in notched Izod impact properties. Low Molecular Weight polymers have lower properties due to lack of chain entanglement and higher molecular mobility.
Shorter chains have more end groups which increase free volume and molecular mobility which makes it easier for the polymer chains to respond to stresses. This is why the strength and modulus are reduced for lower MW (higher melt flow) polymer.
Molecular weight can decrease during the course of molding. Examples could be -
· Plasticizer in rigid PVC.
· Regrind in nylon.
· Hydrolytic degradation in PC or PBT Polyester