	Macromolecules are found everywhere in our daily lives. They are produced from monomer units, small building blocks that can be polymerized to large molecules with different lengths (and, therefore, molar masses), structures, compositions, and end groups. If a macromolecule is synthesized using only one type of monomer with the same chemical structure, it is considered a homopolymer. If it comprises two different types of monomers, it is called a copolymer. In addition, terpolymers are polymers composed of three different monomers.

	Since all technical polymerizations are statistical processes, a common feature of the vast majority of macromolecules is heterogeneity. The molar mass distribution is a well-known heterogeneity, and GPC/SEC is the method of choice to measure it. 

	In the case of copolymers, chemical composition is an additional heterogeneity that also governs the macroscopic product properties. Not only does the average percentage of comonomers present in the final product affect the material’s properties, but the distribution of the monomers along the chain also affects performance.

Articles

Articles in this Issue

Comonomer Identification: Strategies and Detection Options

	GPC/SEC analysis reveals a multitude of molar mass information about unknown samples that are chemically simple or homogenous. However, results for copolymers may be compromised by the fact that fractions with different composition and different molar mass may co-elute.

	There are different methods on how to characterize such samples:

	• chemical composition analysis studied in GPC/ SEC mode by concentration detectors

	• chemical heterogeneity analysis investigated by HPLC based on retention differences of samples with different composition

	• comprehensive 2D analysis that allows samples to be separated by composition and molar mass simultaneously.

	This article will discuss the chemical composition analysis with multiple detection, while the latter two will be discussed in the next two articles in this eBook.

Compositional Analysis by GPC/ SEC With Multiple Detection

	Copolymer analysis is challenging because at least two distributions can be present simultaneously: the molar mass distribution (MMD) and the chemical composition distribution (CCD).

	While there are molar mass sensitive detectors for the determination of the MMD of homopolymers, the applicability of these detectors for copolymers suffers from potential co-elution in GPC/SEC and (in case of lightscattering) from the dependence of the dn/dc on composition. In addition, these detectors do not provide any insight into the CCD.

	The CCD can be determined using several techniques including dual detection, which was discussed in the first chapter of this eBook. In this case, GPC/SEC separation based on the size (hydrodynamic volume) is applied and the CCD of, for example, block copolymers is determined using at least two different detectors.
	 

Polymer Liquid Adsorption Chromatography

	Copolymer analysis is challenging because at least two distributions can be present simultaneously: the molar mass distribution (MMD) and the chemical composition distribution (CCD). A problem in GPC/SEC, that cannot be solved by using any detection technique, is that the method may suffer from limited chromatographic resolution.

	A combination of different separation techniques into a single experiment (multidimensional chromatography (also called 2D chromatography, orthogonal chromatography and cross-fractionation) can help to overcome this. The corresponding peak capacity in an n-dimensional separation is substantially higher because each dimension contributes to the total peak capacity as a factor and not as an additive term.

	For copolymer analysis (e.g., block copolymers, graft copolymers or others), the most common approach is to combine LAC (Polymer-HPLC, separation according to CCD) or LAC under critical conditions (separation according to CCD or end groups) with GPC/SEC in the second dimension to investigate the MMD. 

LCGC E-Books-06-01-2017

Comonomer Identification: Strategies and Detection Options

Compositional Analysis by GPC/ SEC With Multiple Detection

Polymer Liquid Adsorption Chromatography

Two-Dimensional Chromatography of Copolymers