"To Be or Not to Be" of a Polymer Nanogel-Unravelling the Relationship of Product Properties vs. Synthesis Conditions Governing the Radiation Crosslinking of Poly(acrylic acid) Using GPC/SEC-MALLS.

In this paper, a state-of-the-art multi-detection gel permeation chromatography/size exclusion chromatography (GPC/SEC) system including multi-angle laser light scattering (MALLS) is applied to monitor radiation-induced synthesis of internally crosslinked nanostructures from poly(acrylic acid) (PAA). The aim is to demonstrate that this modern tool yields a more detailed picture of reaction mechanism and product structure than the techniques used to date. The prevailing intramolecular crosslinking narrows the molecular weight distribution from M/M = 3.0 to 1.6 for internally crosslinked structures. A clear trend from over 0.7 to 0.5 in the Mark-Houwink exponent and a decrease in R/R from 1.7 to 1.0 point to the formation of nanogels, more rigid and less permeable than the starting coils. Changes in the coil contraction factor (' = []/[]) as a function of the radical density revealed the existence of two modes in intramolecular crosslinking, the initial one (up to 0.075 radicals per monomer unit) where the compactness of products changes strongly with progressing crosslinking and a second one where further compacting is suppressed by the lower flexibility of the partially crosslinked chain segments. This indicates a transition from soft, still internally crosslinkable nanogels to more rigid structures, less prone to further intramolecular loop formation. Our findings provide means for the tailored design of new PAA nanomaterials.