Structural Investigation on Thermoresponsive PVA/Poly(methacrylate-co-N-isopropylacrylamide) Microgels across the Volume Phase Transition

Abstract

Characterization of switchable microgels is a major task in drug delivery science. The study of soft polymeric devices requires a combined use of spectroscopy, microscopy, and scattering approaches enabling the characterization of nanostructured features across a volume phase transition. In this work the structural changes of poly(vinyl alcohol) based thermoreversible microgel particles which incorporate p(NiPAAm-co-methacrylate) chains across the transition temperature occurring at 33°C have been addressed by utilizing reciprocal and direct space approaches such as small angle neutron scattering, SANS, dynamic light scattering, DLS, soft transmittance X-ray microscopy, STXM, and confocal laser scanning microscopy, CLSM, respectively. The comparison between the results obtained from those approaches allows an evaluation of the driving forces acting in the transition and reveals the changes in the microgel structure at nanoscale level. The structure of the poly(vinyl alcohol) based microgel particles, incorporating p(NiPAAm) sequences, consist of a hydrogel core and of a crown of polymer chains projected, at room temperature, in aqueous medium. An increase of the temperature above 33°C causes a volume phase transition of the system characterized by the collapse of the particle core and of the chains grafted at the particle surface. This transition is accompanied by a massive release of water and an increase of the interface with the dispersing aqueous medium, causing the passage from a permeable to a semipermeable structure.

Quantitative X-ray microscopic analysis of individual thermoresponsive microgel particles in aqueous solution

“…Considering the overall particle diameter, these ndings are in accordance with a previous study based on scattering data (dynamic light scattering, neutron scattering) and confocal laser scanning microscopy. 52 However, according to our results, the particle shrinkage is mainly governed by a deswelling of the highly cross-linked core network. The diameter of the shell (R 1 À R 0 ) is found to be relatively constant, while this regime also shows a strong increase of polymer density that goes along with solvent release.…”

“…This behaviour goes along with a smoother particle surface and a decrease of pore sizes as it has been detected by SANS, DLS and confocal laser scanning microscopy. 29,52,58 Moreover, our results evidence that the diameter decrease above VPTT is mainly due to the core shrinking more than to the shell. Such details are accessible only to STXM analysis, in which the core and shell regions are directly distinguished.…”

“…51,52 The effective amount of NIPAM incorporated in the network was 22% (w/w) and the average diameter of the particles was 1.8 mm according to scanning electron microscopy (dry state), DLS and confocal scanning laser microscopy (CLSM) in solution. The incorporation of NIPAM induces a thermoresponsive deswelling of the PVA-based microgel network above a VPTT of about 33 C to achieve a new type of switchable microgels for drug transport.…”

Critical fluctuations and static inhomogeneities in polymer gel volume phase transitions

“…Analytical expressions for both summands have been published for different experimental conditions such as variations in the solvent quality and the degree of gel swelling. [30][31][32][33] In yet another experimental approach, Paradossi and coworkers used SANS to probe colloidal-scale pNIPAAm-based microgels and fitted their data to a sum of two Ornstein-Zernike functions to deduce the static polymer-network correlation length n and the static microgel heterogeneity on larger lenthscales N. 34 Whereas n decreases during the collapse of the polymer network, as also found in an older study by Saunders, 35 N increases during the volume phase transition.…”

“…[30][31][32][33] The resulting scattering function can be approximated by a sum of . 34 After that, we fit the original, non-linearized data to eq 3, using n and I OZ (0) from the preceding determination as fixed parameters while determining N and I SL (0) as free fitting parameters, as shown for our most homogeneous precursor-polymer photogelled sample in Figure 4(C) and for the more heterogeneous free-radically gelled sample prepared at T prep 5 10 8C in Figure 4(D).…”