Nano- and meso-scale aggregation of poly(N-isopropylacrylamide) below the lower critical solution temperature: A...

Poly-N-isopropylacrylamide (PNIPAm), a thermorresponsive polymer, highly soluble in water below its lower critical solution temperature (LCST), is widely used in biomedical applications like drug delivery. Being able to measure PNIPAm size and aggregation state in solution quickly, inexpensively, and accurately below the LCST is critical when stoichiometric particle or molecular ratios are required. Dynamic light scattering (DLS) is probably the most widely available, and inexpensive nanoparticle sizing technique, but there are limitations with respect to sample polydispersity. Here, we first investigated factors governing the ability of DLS to accurately measure PNIPAm size in solution at 25 °C as part of a quality study of five different molecular weight, commercially sourced PNIPAm. All samples were polydisperse and accurate particle size distribution (PSD) data was only obtained from distribution fitting, being consistent and accurate down to ∼ 0.1 wt%. In water at 1 wt%, R_h, extracted from distribution fitting: 12.4 ± 0.6 nm (55 kDa), 10.0 ± 0.22 nm (38 kDa), 6.2 ± 0.15 nm (28.5 kDa), and 9.7 ± 0.14 nm (20-25 kDa) were significantly higher than that expected for single PNIPAm chains in solution. Measurements in different buffers of varying pH (7.4-5.0) yielded similar sizes (R_h of 6-15 nm) and polydispersity indicating that these were stable aggregates. These aggregates could be broken down with Triton-X but not with sodium dodecyl sulphate, ultrasound, or by heating above the LCST and then cooling. We suggest that this nanoscale aggregation and increased polydispersity was caused a variety of factors including by solid-state aging during prolonged storage (>5 years) induced by water adsorption, and/or manufacturing processes. Stirring was found to produce larger, meso-scale (R_h > 150 nm), soluble aggregates and the rate of formation of these meso-particles was linear with stirring time (with a concomitant linear decrease in the faction of original nanoscale aggregates). Meso-particle formation was not correlated with MW, but was inversely correlated to polymer concentration suggesting that aggregation was driven by adsorption at air/liquid interfaces rather than solution phase collisions. In conclusion, PNIPAm particle size and distribution was highly dependent on multiple factors including source, storage conditions, and exposure to air-water interfaces. Standard wide angle DLS is however an effective and rapid method for identifying and quantifying PNIPAm aggregation.