Influence of diisocyanate reactivity and water solubility on the formation and the mechanical properties of gelatin-based networks in water

Abstract

Gelatin can be covalently crosslinked in aqueous solution by application of diisocyanates like L-lysine diisocyanate ethyl ester in order to form hydrogels. Reaction of isocyanate groups with water is however a limiting factor in hydrogel network formation and can strongly influence the outcome of the crosslinking process. Here, diisocyanates with different water solubility and reactivity were applied for the formation of gelatin-based hydrogel networks and the mechanical properties of the hydrogels were investigated to gain a better understanding of starting material/ hydrogel property relations. L-Lysin diisocyanate ethyl ester (LDI), 2,4-toluene diisocyanate (TDI), 1,4-butane diisocyanate (BDI), and isophorone diisocyanate (IPDI) were selected, having different solubility in water ranging from 10 -4 to 10 -2 mol·L -1 . BDI and LDI were estimated to have average reactive isocyanates groups, whereas TDI is highly reactive and IPDI has low reactivity. Formed hydrogels showed different morphologies and were partially very inhomogeneous. Gelation time (1 to 50 minutes), water uptake (300 to 900 wt.-%), and mechanical properties determined by tensile tests (E-moduli 35 to 370 kPa) and rheology (Shear moduli 4.5 to 19.5 kPa) showed that high water solubility as well as high reactivity leads to the formation of poorly crosslinked or inhomogeneous materials. Nevertheless, diisocyanates with lower solubility in water and low reactivity are able to form stable, homogeneous hydrogel networks with gelatin in water.

One step preparation of porous polyurea by reaction of toluene diisocyanate with water and its characterization

“…New peaks around 152.75 appeared, which have been oen assigned as characteristic resonance of polyurea. 65,66 These are strong indication that all -NCO (TDI) were effectively reacted.…”

“…It is well known that TDI has a very limited solubility in water, and the reactivity of its two isocyanate groups is quite different. 65,66 It was questionable that polyurea was effectively formed by dispersing TDI in the binary solvent. To this end, PPU as-prepared was undergone 13 C and 1 H NMR tests.…”

“…5), respectively. 21,50,65 As to PPU spectra, it is to note that all the resonance signals were very weak because of limited solubility of PPU in DMSO. However, with extended scanning time and enlarged spectra (inserted on PPU spectra), one can see quite clearly that, the peaks assigned to the carbon atoms attached to -NCO, seen in TDI spectra (C 2 , C 4 , C 2 0 and C 6 0 , at 132.55 & 132.83), all disappeared.…”

Renewable Polyurethanes from Sustainable Biological Precursors

“…As result, the primary ε-NCO has slightly higher reactivity than the secondary α-NCO. , The combination of the primary ε-NCO and the secondary α-NCO determines the overall reactivity of l -lysine diisocyanates. This was compared to commercial diisocyanates in recent studies, and it is estimated to be equal to the reactivity of HDI (hexamethylene diisocyanate) and higher than that of IPDI, but lower than that of TDI . Less steric hindrance from the planar structure, as well as delocalized electrons into the π-ring system, results in the highest reactivity of aromatic NCO groups such as MDI or TDI, while the lowest reactivity of IPDI comes from high steric hindrance and weak electrophilicity .…”

“…This was compared to commercial diisocyanates in recent studies, 108 and it is estimated to be equal to the reactivity of HDI (hexamethylene diisocyanate) 109 and higher than that of IPDI, 106 but lower than that of TDI. 110 Less steric hindrance from the planar structure, as well as delocalized electrons into the π-ring system, results in the highest reactivity of aromatic NCO groups such as MDI or TDI, while the lowest reactivity of IPDI comes from high steric hindrance and weak electrophilicity. 111 Studies 106,111 also indicated that the steric hindrance is more effective than the electron withdrawing effect on the final property of the NCO group.…”