Distribution of Phenols in Thermoresponsive Hydrogels

Abstract

The effect of three hydrotropic molecules, phenol, resorcinol and phloroglucinol, on the thermodynamic equilibrium and the dynamics of poly(N-isopropylacrylamide) (PNIPA) hydrogels is reported. With increasing hydroxyl number of this molecular sequence both the depression of the temperature and the width of the volume phase transition (VPT) increase, reflecting the nonuniform distribution of these molecules inside the gel. Small-angle X-ray scattering detects no notable change in the local structure of the polymer chains in the presence of phenol. Dynamic light scattering observations, however, show that phloroglucinol, unlike phenol and resorcinol, produces a small but significant decrease in the mobility of the polymer in the solvent. Isothermal microcalorimetry reveals an anomalous increase in the exothermic enthalpy of mixing just below the transition, both with phenol and phloroglucinol, which indicates a pretransition state in the gel in which the aromatic molecules partly replace the bound water. This finding is corroborated by small-angle neutron scattering measurements under contrast matched conditions showing that at low phenol concentration the aromatic molecules are uniformly dispersed in the solvent, but in pretransition conditions a sparsely populated layer of phenol forms close to the polymer chains.

Interactions in aromatic probe molecule loaded poly(N-isopropylacrylamide) hydrogels and implications for drug delivery

“…Molecules with phenolic OH can influence the swelling behaviour of the PNIPA in various ways [12][13][14][15][16]. While some molecules have only a slight effect or no effect at all, other guest molecules can change the transition temperature even at low additive concentrations [16,17].…”

“…The temperature shift depends on the concentration and chemical structure of the additive. Several molecules and ions reduce the LCST [12][13][14][15][16].…”

“…In our previous publications the influence of phenol, resorcinol and phloroglucinol, studied by scattering, calorimetric and later by NMR techniques were reported in detail (12)(13)(14)19). It was found earlier that they exhibit an interaction with the polymer chains that depends on the number of the OH-groups [19].…”

“…Binding of phenol to PNIPA is mediated by hydrogen bonds between the amide group of the NIPA chain and the hydroxyl group of the phenol molecule [20]. The associative behaviour between phenol and PNIPA was also demonstrated by smallangle neutron scattering (SANS) and solid state NMR techniques [12][13][14]. By contrast, 1 H solid-state NMR results showed that dopamine does not associate with the polymer chains [15].…”

Host–Guest Interactions in Poly(N-isopropylacrylamide) Hydrogels

“…Phenol, resorcinol, and phloroglucinol, for example, reduce T VPT . 4 At temperatures T < T VPT , the gels deswell with increasing aromatic content, rapidly expelling the solvent. Although the effect of such aromatic pollutants on the swelling properties may be described with a certain degree of success by mean field theories in terms of the average quality of the solvent, 57 and even by theories involving cooperative effects, 8 detailed knowledge of the role played on a local scale by the foreign molecules is lacking.…”