T.H. Kalantar, D.M. Meunier, J.W. Lyons, J.J. Kiefer, Q.J. Niu, Y. Li, K.J. Bouck, C.J. Tucker, P.S. Russo, R. Cueto
Dow Chemical Company,
Keywords: polymer, nanaoparticle, size exclusion chromatography, differential viscometry, intrinsic viscosity, Mark-Houwink
Summary:Conventional size-exclusion chromatography (SEC) and size-exclusion chromatography coupled to differential viscometry (SEC/DV) methodologies have been developed and applied to the dilute suspension characterization of solvent-swollen crosslinked unimolecular polymeric nanoparticles (PNPs) in the 5 ~ 50 nm weight-average diameter (dw) range. The crosslinked nanoparticles were prepared by batch and semi-batch microemulsion polymerization techniques and isolated. SEC and SEC/DV characterization techniques yield, based on the principle of universal calibration, a wealth of information regarding the structural attributes of PNPs, including apparent and absolute molecular weight distributions (MWD), apparent and absolute molecular weight averages, peak and weight-average particle diameters, particle size distributions in both the solvent-swollen and solvent-free states, particle conformation (shape), and an estimate of the volumetric swell factor. The particle VSF was found to decrease and the density to increase with increasing molecular weight within the MWD of any given particle. VSFs of ~2.0 have been observed, indicating that extremely compact PNPs can be made at very small particle diameters. Using this method, narrowly dispersed, PNPs containing adequate crosslinker and having a high parent chain molecular weight were observed to exhibit, across each of their respective MWDs, conformations consistent with spherical architecture (i.e., Mark – Houwink power law exponent, α ~ 0) (Figure 1, right). The densities of the swollen particles calculated from the MALS data were consistent with measured VSF and the known density of bulk PS. The Mark – Houwink plot obtained via SEC/DV was shown to be a good diagnostic of conformation in suspension. Based on the excellent agreement between SEC/DV derived sizes and sizes measured by other techniques, one would conclude that PNP’s obey universal calibration under the separation conditions used here, and that SEC methods are suitable for the rapid characterization of PNPs. The approach for extracting size and volumetric swelling parameters from SEC/DV data will be presented, as will solute conformation gleaned from Mark-Houwink plots of the nanoparticle samples, and, SEC/DV-based sizes will be compared to those obtained by other techniques.