Molecular weight and molecular weight distribution are among the most fundamental structural parameters of polymer materials. Many physical and mechanical properties of polymers are closely related to these characteristics. High molecular weight contributes to excellent performance, such as tensile strength, impact resistance, and elasticity. However, excessively high molecular weight can negatively affect processing performance, including rheological behavior, solution stability, and overall processability.
Polymers typically have a molecular weight range between 10³ and 10â·, and their molecular weights are not uniform—this is known as polydispersity. Since polymers are composed of a mixture of homologues with different degrees of polymerization, their molecular weight is only statistically meaningful. The values obtained through experimental methods represent statistical averages. To fully describe a polymer’s molecular weight, it's essential to also provide its molecular weight distribution.
There are several types of average molecular weights used to characterize polymers: number-average molecular weight (Mn), weight-average molecular weight (Mw), Z-average molecular weight (Mz), and viscosity-average molecular weight (Mη). These averages follow a specific order: Mn < Mη < Mw < Mz. This relationship is referred to as the polydispersity index, which reflects the degree of molecular weight variation. A higher value indicates greater dispersion, while a value of 1 suggests a monodisperse system.
Various analytical techniques are used to determine the molecular weight of polymers. These include chemical methods like end-group analysis, thermodynamic methods such as freezing point depression or boiling point elevation, optical methods like light scattering, kinetic methods like ultracentrifugal sedimentation, and chromatographic techniques such as gel permeation chromatography (GPC). Each method has its own range of applicability and provides different types of average molecular weights.
Gel permeation chromatography (GPC) is widely used due to its speed, ease of operation, reliable data, and good reproducibility. The following figure shows the molecular weight distribution of PC/ABS plastic measured using GPC with tetrahydrofuran as the solvent. The resulting spectrum allows for the direct determination of four key molecular weight parameters: Mz, Mη, Mw, and Mp. Users can select the specific results they need based on their application.
MTT is an independent laboratory specializing in material and component quality inspection, identification, certification, and failure analysis.
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