Quantifying short chain branching microstructures in ethylene 1-olefin copolymers using size exclusion chromatography and Fourier transform infrared spectroscopy (SEC–FTIR)

A rapid method for the SEC–FTIR analysis of short chain branching distribution (SCBD) across the molecular weight distribution (MWD) is described and its application demonstrated using ethylene 1-olefins copolymers. Chromatograms are generated using the root mean square absorbance over the 3000–2700 cm−1 spectral region (i.e. FTIR serves as a concentration detector). Spectra from individual time slices of the chromatogram are subsequently analyzed for comonomer branch levels using chemometric techniques. Furthermore, we are able to estimate error in the reported SCB content of each slice. Using the appropriate training sets, chemometric models can be constructed which provide SCB versus MWD profiles with sufficient precision to detect trends resulting from catalyst and process changes in LDPE and/or HDPE samples. We demonstrated the method by showing the results of a model that has enabled us to accurately quantify branching levels in polyolefins within ±0.5/1000 total carbons (i.e. ca. 0.1 mol%) in samples with relatively low levels of SCB (i.e. <10 SCB/total carbons) and mixed branch types.