SEC analysis of poly(acrylic acid) and poly(methacrylic acid)
Abstract
The accurate characterization of molar mass distributions of poly(acrylic acid), PAA, and poly(methacrylic acid), PMAA, by size-exclusion chromatography (SEC) is addressed. Two methods were employed: direct aqueous-phase SEC on P(M)AA and THF-based SEC after esterification of P(M)AA to the associated methyl esters, P(M)MA. P(M)AA calibration standards, P(M)AA samples prepared by pulsed-laser polymerization (PLP), and PAA samples prepared by reversible addition-fragmentation chain transfer (RAFT) were characterized in a joint initiative of seven laboratories, with satisfactory agreement achieved between the institutions. Both SEC methods provide reliable results for PMAA. In the case of PAA, close agreement of both SEC methods was only observed for samples prepared by RAFT polymerization with weight-average molar mass between 80 000 and 145 000 g mol−1 and for standards with peak molar masses below 20 000 g mol−1. For higher molar masses of standards and for PLP-prepared PAA, the values from THF-based SEC are as much as 40 % below the molar masses determined by aqueous-phase SEC. This discrepancy may be due to branching ordegradation of branched PAA during methylation. While both SEC methods can be recommended for PMAA, aqueous-phase SEC should be used for molar mass analysis of PAA unless the sample is completely unbranched.
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