Determination of the Parameters Controlling Swelling of Chemically Cross-Linked pH-Sensitive Poly(N-vinylimidazole) Hydrogels

Abstract

The number of variables controlling the behavior of ionic gels is large and very often some of them are unknown. The aim of this work is to interpret quantitatively the swelling behavior of pH sensitive gels, with the minimum number of simplifying assumptions. With this purpose, the equilibrium degree of swelling (S) and protonation (alpha) of chemically cross-linked poly(N-vinylimidazole) (PVI) immersed in aqueous salt solutions were measured as a function of the ionic strength (mu), in the whole range of pH. In acid solutions with pH in the range 0 to 4, imidazole moieties become protonated, and PVI behaves as a polyelectrolyte gel: S decreases upon increasing mu both for NaCl and for CaCl(2), with HCl as protonating acid. In aqueous solutions with larger pH, between 4 and 12, the hydrogel is practically neutral, and S increases as mu rises, showing a salting-in effect. From the quantitative analysis of these results, the following facts emerged. Protonation induces chain stiffness (as measured by the non-Gaussian factor) and worsening of the solvent quality of the aqueous media (as measured by the polymer-solvent interaction parameter). For alpha below 33%, swelling seems to be governed by the excess of mobile counterions inside the gel with respect to the bath, with a minor but still significantly negative contribution of the osmotic swelling pressure due to polymer-solvent mixing. Above 33% protonation, it is necessary to consider Manning counterion condensation to get parameters with physical meaning. The crossover between polyelectrolyte and salting-in effects corresponds to alpha and mu values with the same ionic and mixing contributions to the osmotic swelling pressure. The formation of ionic nonpermanent cross-links, with H(2)SO(4) as the protonating acid, was discarded.

Discrimination of the roles of crosslinking density and morphology in the swelling behavior of crosslinked polymers: Poly(N‐vinylimidazole) hydrogels

“…Protonation gives place to an ionic osmotic swelling pressure that increases solvent uptake. 18,19,24 Above a certain degree of protonation equivalent to the critical charge density of Manning, the counterions condensate and the ionic contribution decreases 18 in concordance with the descent of swelling at pH i below 2.5, observed in Figure 2. It is remarkable in that plot that some curves are more symmetric than others with respect to the minimum, indicating that two independent morphological features operate in the two limits of composition, the lowest or largest [BA].…”

“…The affine model and gaussian chains were considered for P el . 22,24,31 Equation (5), with 16 v ¼ 0.496 þ 0.308v 2 , affords the m e (S) values plotted in Figure 5 for PVI swollen in deionized water.…”

“…To obtain v in acid solutions, the protonation of the imidazole rings and the subsequent ionic contribution to the osmotic swelling pressure must be considered 22,24 and thus, it results that v ¼ (0.547 AE 0.008) þ (0.01 AE 0.07)v 2 , for PVI swollen in HCl aqueous solution with pH i ¼ 4, which corresponds to a degree of protonation within the range 0.0121 AE 0.0005.…”

Synthesis and characterization of multi-responsive poly (NIPAm-co-AAc) microgels

“…In particular, they have proved very useful in the domain of material science owing to their potential applications in drug delivery, [4][5][6] sensing, [7,8], fabrication of photonic crystals, [9][10][11] template-based synthesis of inorganic nanoparticles, [10,12], separation and purification technologies [13]. The vast array of applications that microgels are suitable for arises from their stimulus-responsive nature, that is, their ability to undergo reversible volume phase transitions in response to external stimuli such as a change in pH, [14][15][16] temperature, [17,18] ionic strength of the surrounding medium, [19,20] nature of the solvent, [21,22] and the effect of an external electromagnetic field [23,24]. The swelling and deswelling transitions of stimulus-responsive microgels are governed by the imbalance between the repulsive and attractive forces acting within the molecules.…”

High resolution interferometry as a tool for characterization of swelling of weakly charged hydrogels subjected to amphiphile and cyclodextrin exposure

“…A key feature of the present work is the application of a fiber optic based interferometer to provide in-situ high resolution data of the hydrogel swelling. This monitoring tool is superior to the gravimetric and microscopic techniques commonly used [40,41] in hydrogel swelling studies. Alongside the interferometric investigation, we apply_ENREF_71 confocal laser scanning microscopy imaging to interrogate the distribution of surfactant microdomains containing the fluorescent probe perylene [42] within the hydrogel.…”