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

Abstract

The degree of swelling (S) of crosslinked polymers depends on several variables whose individual contributions are difficult to be distinguished. The aim of this work is to determine the contributions of the morphology to S, discriminating them from the effect of the crosslinking density. Under usual conditions of synthesis, these two characteristics vary simultaneously but here, several samples of chemically crosslinked poly(N-vinylimidazole) having the same permanent crosslinking density (determined by DSC) and different porous morphologies (determined by SEM) were prepared. It was thus found that the variation of S with porosity, keeping constant the rest of variables, is strongly solvent dependent. Swelling in methanol is almost constant whereas for ethanol, deionized water, and aqueous HCl solutions, S depends on morphological features in a different way for each medium. It was concluded that S increases with inherent porosity and solvent effects arise from the counterbalance of such increment with the dependence of the polymer-solvent interaction parameter on the polymer volume fraction.

Compression elastic modulus of neutral, ionic, and amphoteric hydrogels based on N‐vinylimidazole

“…For example, the permanent cross‐linking density determined by Tg measurements represents the covalent junctions between chains that contribute to immobilize chains or to diminish free volume. This includes the knots joined to elastically effective chains and those connected to network defects (dangling chains, intramolecular cycles) 5, 18. The same holds for ν e determined spectroscopically or chemically.…”

“…The same holds for ν e determined spectroscopically or chemically. On the contrary, the effective cross‐linking density determined through swelling measurements reflects not only permanent junctions but also morphological features such as the inherent porosity 5. Finally, the ν e determined through mechanical properties represents the density of knots joined only to elastically effective chains and therefore contributing to the elastic component of the osmotic swelling pressure.…”

“…Cross‐linked polymers exhibit interesting properties, many of which depend on the degree of crosslinking. The cross‐linking density (ν e ) can be determined by means of different techniques such as swelling measurements,1–5 stress‐strain6–8 or compression‐strain measurements,9–16 dynamic mechanic analysis,17 calorimetric measurements of the glass transition temperature,5, 18, 19 and spectroscopic studies (Raman, NMR)20 or chemical elemental analysis 21. The physical meaning of the ν e values determined through each technique is different and therefore the results obtained are also slightly different.…”

“…The correct determination of ν e is decisive to understand the role of the elastic component of the osmotic swelling pressure in the behavior of cross‐linked polymers 3, 4. Hydrogels based on N ‐vinylimidazole (VI) may be particularly useful in that respect because they have been broadly studied and the parameters necessary to perform quantitative analysis are likely known 3–5, 18, 25–34. Besides, they show many different effects with different comonomers.…”

pH and Thermo Sensitive Superabsorbent Poly (N-Hydroxymethylacrylamide-co-Itaconic Acid) Hydrogels: Synthesis, Characterization and Kinetic Studies

“…pH-sensitive hydrogels have also been used in making biosensors and permeation switches (19). Certain hydrogels which are swelling at low temperature and shrinking at high temperature have negative temperature dependence of swelling (20). The swelling of those hydrogels was reversible, responding to stepwise temperature changes.…”