Polymer Molecular Weight Characterisation

Smithers Rapra provides a high quality Gel Permeation Chromatography (GPC) service for molecular weight characterisation of plastics & other polymers. As one of the world leading suppliers of GPC Characterisation, considerable investment has been made in the latest technological advancements & the service now operates from a recently refurbished laboratory with vastly experienced consultants.

Molecular weight distribution is one of the most important properties of polymers, influencing the strength of solid plastics & rubber, & affecting the viscosity of molten polymers. GPC is the most widely used & cost effective method of comparing the molecular weight distribution of rubber, plastics & other polymers.

The Benefits of utilising GPC

GPC is recognised as a useful aid in the study of polymer degradation or as part of a quality control system of polymer production or processing. The GPC process can also provide information to assist in material or product failure analysis & can establish if the polymer molecular weight was suitable to the processing & the application of the material.

Problems associated with polymer processing can occur if there is a change in the molecular weight distribution of the raw material. A “new” or “similar” material may offer a fundamental improvement but the processing conditions may need to match the change in raw material. Failure analysis of polymers can benefit from the comparison of molecular weight distribution for a “Good product” compared to a “Failed product” & likewise “Raw material for a proven product” compared to a “Raw material for a failed product”.

A standard GPC process is usually the best approach to comparing the molecular weight distribution of polymers known to be of the same chemical type & structure (chain branching). If the samples are believed to be of a different chemical type or a comparison of chain branching is required, Triple Detection GPC or combined GPC-Viscosity may deliver greater accuracy. Your Smithers Rapra consultant will discuss with you the best approach to your issue & the type of GPC analysis required.

A variety of approaches to GPC exist & through consultation Smithers Rapra will advise you on the method most applicable to your individual needs. The options are;

• Conventional or Triple detection GPC with solvents such as THF & Chloroform.
• Conventional or Triple detection GPC with aqueous based eluents
• High temperature conventional GPC or combined GPC-Viscosity of Polyolefins
• High temperature GPC with a range of phenolic & polar solvents
• GPC with HFIP as the solvent

Conventional GPC Analysis

Conventional GPC is available using all systems & uses the response from the refractive index (concentration) detector alone & the comparative molecular weight data is obtained via a log molecular weight versus retention time calibration. The various organic solvent systems are normally calibrated with poly(styrene) or poly(methyl methacrylate) & the results are expressed as PS/PMMA equivalent molecular weights. For some common polymer types, a mathematical correction might be applied to express the results as for a specific polymer. For aqueous eluent systems, various calibrant polymer types are used.

GPC with HFIP as the solvent is aimed specifically at the molecular weight characterisation of polyamides & polyesters such as PET or PBT. Utilising HFIP has been found to be appropriate for other difficult & complex polymer types including PEBAX & acetals.

Triple Detection & GPC Viscosity

Triple detection (light scattering, viscosity & refractive index) is available for near ambient GPC with tetrahydrofuran & chloroform, & for aqueous based eluents. Other solvents suitable for use at near ambient conditions can also be considered. GPC-viscosity is available for the high temperature GPC analysis of polyolefins.

Light scattering & or viscosity detection are often used to provide the true molecular weight data of plastic & other polymer samples as opposed to expressing them as “equivalent” molecular weights or applying a possible questionable mathematical correction. These approaches are valuable in providing information on the differences in structure or composition between samples.

Multiple detection GPC approaches are not usually appropriate for low molecular weight polymer.

High Temperature GPC

Conventional or combined GPC-viscosity is carried out in conditions applicable to polyolefins (1,2,4-trichlorobenzene at 160 °C) & conventional high temperature GPC is carried out using a range of polar solvents for polar polymer types or phenolic solvents for various engineering polymers. One of the most applicable applications is using phenol/trichlorobenzene at 115°C for the analysis of PEEK.