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Solution for Molecular weight distribution and additives analyses with a single injection

In order to relate polymer properties to end-use properties, various tests are performed at development to manufacturing stage, such as physical property and thermal analyses, molecular weight distribution and additive analyses using separation techniques. In general, each analysis requires each dedicated system. Especially in analysis based on chromatography, not only the system but also suitable sample preparation is required for each analysis. When these molecular weight distribution measurement and additive quantitation are properly performed with a single injection of sample with same preparation, material development and manufacturing can be accelerated.

Molecular weight (MW) distribution and polymer additives analyses

Since the synthetic polymer substances are mixture of molecules with a distribution of molecular weight and MW distribution, both of which directly affect physical property of polymer materials such as strength, toughness and flexibility. Characterization of MW distribution is therefore necessary to control end-use properties and quality standards. On the other hand, polymer additives contribute to product life-time prolongation (that is, maintain the physical properties of the polymer) and provide new functionality to the end products.

In general, analyses of these substances require different sample preparation and dedicated system for each as following figure.

To analyze polymer additives that are small molecule, although complicated sample preparation is required, there are several techniques to directly analyze additives.

  1. EGA(Evolved Gas Analysis)-GC/MS
    Technique that uses GC/MS to directly analyze additives that are thermally extracted from solid sample by heating. MW and MW distribution cannot be measured in this technique. (refer to Pyrolyzer-APGC/MS system)
  2. GPC/RI/MS
    Technique that uses size exclusion chromatography as separation and measures MW distribution with RI detector, and quantifies polymer additives with Mass spectrometer. There is a probability to be able to measure both substances with a single injection.

Challenge for GPC / RI / MS

Dissolved polymer material is directly injected into GPC system and polymer substances are separated based on molecular size. Eluent from column is split to both RI and MS detector, polymer components with fast elution time are detected with RI detector and additive with late elution time are detected with MS detector.

However, there are challenges on separation and ionization when SEC is used.

  1. Separation capability on SEC
    – Separation of large molecule and small molecule
    – Chromatographic resolution at low molecular weight region for additive analysis
  2. Ionization and sensitivity
    – Mobile phase commonly used for GPC is not usually suitable for ionization on MS

These challenges are achieved by adopting ACQUITY APC (Advanced Polymer Chromatography) system that is higher resolution GPC system due to a low dispersion system and detector, a rigid and high pore volume hybrid particle column, and diluting the eluent in appropriate solvent for ionization using a make-up pump. (refer to Polymer characterization with APC)

MW distribution and polymer additive analyses with a single injection using APC / RI / QDa

The system consists of APC (p-ISM)/RI detector used for advanced MW distribution measurement, make-up pump for effective ionization and ACQUITY QDa that is easy to use mass detector. (refer to Material quality control utilizing mass detector)

Eluent from column is split into RI and QDa (split ratio 100 : 1). The eluent to QDa is diluted by the make-up pump before being introduced into QDa. During times when polymer elutes, the eluent is not introduced into QDa by using a diverter valve.

Using APC with QDa system, the same sample preparation as conventional GPC analysis, that is, just dissolving the polymer material, enables a single injection to perform molecular weight distribution measurement of polymer and quantitative analysis of additives in a shorter time with higher quality than before.

The mixture of polystyrene and polymer additives (1000ppm of additive for each in polystyrene) as a model sample was dissolved into THF at the concentration of 0.1% (this solution contains approximately 0.1% of polystyrene and 1 ppm of each polymer additive. Since polymer and additives and between additives can be separated due to the high separation capability of APC, MW distribution measurement and additives quantitation can be performed simultaneously by using two detectors.

RI chromatogram for molecular weight distribution measurement(upper) and SIR chromatogram for polymer additives quantitation (bottom)

Polymer products are routinely analyzed for both molecular weight distribution and quantitation of polymer additives in order to understand polymer product performance. However, in conventional methods, appropriate sample preparation and a dedicated system for each analysis are required. When APC/RI/QDa is used, both the separation of polymer and additives, and between the additives improves. As a result, both analyses can be performed at the same time with a single injection of dissolved sample without any other sample preparation. And as with conventional SEC, analyses can be performed according to target molecular weight of polymer and additives by connecting columns with appropriate exclusion limits.

Polymer material development and its quality control can be accelerated though better efficiency of analyses.

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