Chemical Composition measurement in GPC/SEC analysis. Differentiating polyolefins with similar Molar Mass Distribution.
The simultaneous analysis of concentration and chemical composition in Gel Permeation Chromatography (GPC) or Size Exclusion Chromatography (SEC) is of significant interest for today’s complex polyolefin copolymers.
In polyethylene and copolymers, chemical composition is centered around the study of short-chain branches, created by the incorporation of comonomers into the main chain.
The measurement of short-chain branching adds a new dimension to the conventional GPC/SEC technique that may be critical when dealing with resins that have very similar Molar Mass Distribution (MMD) but different incorporation of the comonomer. Analyzing such polyolefins by standard GPC (without short-chain branching information) may make us think that they are equal, but if we add the composition signal (achieved with Infrared detection), we will see differences in their comonomer distributions that will set them apart. These differences will influence the resins’ physical properties, morphology, and ulterior performance.
Let’s look at a few examples with different types of polyolefins.
A very clear case of the importance of measuring Short Chain Branches along the MMD curve is bimodal HDPE products. When using high sensitivity Infrared Detectors, small differences in SCB in the high molar mass range can be detected, as it can be seen in Figure 1, depicting an analysis of two different bimodal HDPE products with good sensitivity for a low number of SCB.
In the next example we can see the GPC-IR analysis of a Ziegler-Natta LLDPE showing a clear drift of SCB along the molar mass separation, in contrast to the uniform comonomer incorporation of single-site catalyst products.
Another important type of polyolefins are ethylene-propylene copolymers. In Figure 3 we can see two different industrial copolymers whose Molar Mass Distributions are practically equivalent but have very different profile of ethylene incorporation along their MMD. One of them is quite homogeneous, while the other has more ethylene in the high molar mass fractions. Their properties are therefore quite different, and the online composition measurement by IR helps to identify them.