Characterization by SEC Combined with Polymerization Model Predictions: Determination of Polymer Chain Branching

MEIRA, G. R.; CASIS, N.; RODRIGUEZ, V. I.; GUGLIOTTA, L. M.; VEGA, J. R.; ESTENOZ, D. A.

Primosten, Croacia

Simposio; 14th. International Symposium on Separation Sciences: New Achievements in Chromatography; 2008

Croatian Soc. of Chem. Eng.

Some polymerization models enable to predict the detailed molecular structure of the produced polymer. Unfortunately however, many of such predictions cannot be verified with the present characterization technology. Thus, the determination of the distribution of long chain branching (DB) by SEC-viscometry is subject to large propagation errors; and requires to input the value of the empirical exponent e that interrelates the g and g’ contraction parameters. This paper deals with improving SEC estimates with the help of polymerization model predictions. The first example deals with the determination of: a) branching efficiency of high impact polystyrene (HIPS) produced through a bulk free-radical process; and b) the DB of the styrene (St)-butadiene (Bd) graft copolymer present in such material. A bulk polymerization model was developed that predicts the bivariate chain length distributions of each of the generated topologies; with each topology characterized by the number branches per molecule. The branching efficiency was obtained by deconvolution of the UV chromatogram (at 256 nm) of the total polymer. The sensor “sees” the St repetitive units from the free PS and grafted PS branches, but not the Bd repeating units. The UV chromatogram was simulated assuming ideal SEC fractionation. For a St-Bd graft copolymer in tetrahydrofuran at 25 C, the branching exponent was adjusted to e = 2, by combining the model predictions with SEC measurements, and with an independent determination of the average branching by solvent extraction-gravimetry.2 Finally, the DB was indirectly estimated from the adjusted polymerization model. The second example corresponds to a (moderately branched) polyisoprene obtained by a free-radical emulsion polymerization. The polymer was analyzed by SEC/viscometry; and the adjusted polymerization model predicts the average molecular weights and degree of branching. The branching exponent in tetrahydrofuran at 25 C was adjusted to e = 2.5 through an iterative procedure that minimized the difference between the average branching predicted by the model and the average branching obtained by SEC with a different possible values of e.