Compression elastic modulus of neutral and protonated poly(N‐vinylimidazole) hydrogels

Abstract

The compression modulus of poly(N‐vinylimidazole) (PVI) hydrogels synthesized by cross‐linking polymerization in aqueous solution, was measured at room temperature in several related systems: i) just after polymerization, ii) swollen at equilibrium in deionized water, iii) swollen in HCl (aq) (pH=2.5), iv) swollen in HCl (pH=2.5) and 1 M NaCl (aq) solution and v) swollen in H2SO4 (pH=2.5) (aq) solution. Samples of the first and second groups are neutral whereas hydrogels of the other three groups are ionic because of protonation of basic imidazole groups. The experimental results were fitted with the Erman‐Monnerie theory, applied to compression measurements for the first time, to determine the phantom modulus, [fph*], and the parameter κG which measures the constraining role of entanglements on the fluctuations of chains between knots.

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

“…Compression measurements were analyzed through plots of the applied stress, τ, (τ = f / A S with f being the acting force and A S the cross section of the undeformed swollen specimen) versus λ−λ −2 , where λ = L / L o is the relative deformation caused by compression of the length of the swollen sample along the direction of the stress ( L ), with respect to the initial length ( L o ) of the swollen but undistorted sample. They show the typical curvature at the smallest range of deformation that is generally assigned to imperfect geometry of the surface of the gel specimen 9, 14, 22, 35–37. The usual correction for such curvature was applied 9.…”

“…They show the typical curvature at the smallest range of deformation that is generally assigned to imperfect geometry of the surface of the gel specimen 9, 14, 22, 35–37. The usual correction for such curvature was applied 9. All corrected measurements show linear behavior and the elastic modulus, G , was determined, by linear regression, as the slope of such plots.…”

“…Uniaxial compression measurements were performed on gel cylinders by means of a Microtest (Spain) tensile testing machine at room temperature, using a load cell of ± 0.01 N sensibility and 50 N maximum load 9. The compression probe consisted of a pair of parallel Teflon plates to minimize the appearance of shear stresses on the surface of the samples under uniaxial compression.…”

“…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.…”

“…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. The cross‐linking density of hydrogels is difficult to be measured through mechanical measurements because xerogels usually are extremely rigid and swollen hydrogels very often lack of the necessary mechanical integrity 6, 9, 22–24. Even though, whenever possible, compression or stress‐strain measurements on swollen samples are preferred to determine ν e of polymer networks.…”