Porosity Inherent to Chemically Crosslinked Polymers. Poly(N-vinylimidazole) Hydrogels

Abstract

Swollen polymer networks exhibit multiscale pores filled with solvent. Such porosity, inherent to cross-linked polymers, determines some of their most relevant physical properties and applications. In this research, several samples of chemically crosslinked poly(N-vinylimidazole) were synthesized with the same permanent crosslinking density at two different conversions, and their inherent porosity was characterized on freeze-dried specimens by SEM, TEM and nitrogen physisorption. It was thus found that all of the samples showed pores, both on the nanometer and the micrometer scales, whose dimensions were mostly equal to or larger than the mesh size of the primary polymer network (22 nm) and whose volume and specific surface decreased with increasing conversion. Micropores have, in all cases, a very minor contribution. Samples synthesized with the largest comonomer concentrations show quasi-spherical mesopores (90 nm average diameter at any conversion) and macropores (from 5 to 10 microm with increasing conversion), whereas the mesopores of samples synthesized with the largest crosslinker ratios were channel-like (150 nm) and the macropores were interconnected contiguous voids (3 microm). Samples with intermediate compositions exhibit the lowest porosity due, mostly, to interconnected mesopores. The differences in shape were ascribed to the mechanism of phase separation, taking place during polymerization, even for samples that are transparent following polymerization. The inherent porosity is a significant source of spatial inhomogeneity, which contributes to the increase in turbidity. Light scattering decreases with increasing ionization when the degree of protonation is greater than 10%. An important consequence of the inherent porosity is that the degrees of swelling determined either gravimetrically or through size measurements are not equivalent.

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

“…3 The porous morphology depends on several variables such as the porogen and crosslinker used in feed, or the previous thermal history of the sample. [3][4][5][6][7][8][9][10][11][12] Sponges and macroporous polymers are prepared with porogen agents 9,10 or other porogenic methods. 4,11,13,14 Polymer networks synthesized without using porogens, exhibit pores (inherent porosity 12 ) that in the swollen state are filled with solvent and contribute significantly to the degree of swelling.…”

“…[3][4][5][6][7][8][9][10][11][12] Sponges and macroporous polymers are prepared with porogen agents 9,10 or other porogenic methods. 4,11,13,14 Polymer networks synthesized without using porogens, exhibit pores (inherent porosity 12 ) that in the swollen state are filled with solvent and contribute significantly to the degree of swelling. 5,8,11,12 The roles of morphology and crosslinking in determining polymer properties are difficult to be distinguished because upon changing crosslinking, the morphology changes too.…”

Effect of calcium neutralization on elastic and swelling properties of crosslinked poly(acrylic acid) - correlation to inhomogeneities and phase behaviour

“…Opacity is frequently considered to be the clearest sign of heterogeneity or inhomogeniety in samples [26]. Furthermore, it has previously been shown that phase separation is a phenomenon commonly occurring in crosslinking polymerization [18,25,26] and that increasing crosslinker content is an inducing factor [21,22].…”

“…Furthermore, it has previously been shown that phase separation is a phenomenon commonly occurring in crosslinking polymerization [18,25,26] and that increasing crosslinker content is an inducing factor [21,22]. In the literature it has been suggested that the solubility for PAA gels in Ca 2+ containing solution is reduced [10], and that precipitation of PAA occurs in presence of high Ca 2+ concentration [14].…”

“…The degree of heterogeneity is known to increase with the amount of crosslinker [21,22]. It has been shown that the swelling of hydrogels is strongly dependent on the network structure [23,24] and that the elastic modulus of hydrogels decreases with increasing inhomogeniety [21,25]. The decrease in elastic modulus is explained by the formation of areas of high or low crosslink density during the polymerization.…”