Tough combinatorial poly(urethane-isocyanurate) polymer networks and hydrogels synthesized by the trimerization of mixtures of NCO-prepolymers

Abstract

The development of tough hydrogels is an essential but challenging topic in biomaterials research that has received much attention over the past years. By the combinatorial synthesis of polymer networks and hydrogels based on prepolymers with different properties, new materials with widely varying characteristics and unexpected properties may be identified. In this paper, we report on the properties of combinatorial poly(urethane-isocyanurate) (PUI) type polymer networks that were synthesized by the trimerization of mixtures of NCO-functionalized poly(ethylene glycol) (PEG), poly(propylene gylcol) (PPG), poly( ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC) prepolymers in solution. The resulting polymer networks showed widely varying material properties.Combinatorial PUI networks containing at least one hydrophilic PEG component showed high water uptakes of > 100 wt%. The resulting hydrogels demonstrated elastic moduli of up to 10.1 MPa, ultimate tensile strengths of up to 9.8 MPa, elongation at break values of up to 624.0% and toughness values of up to 53.4 MJ m −3 . These values are exceptionally high and show that combinatorial PUI hydrogels are among the toughest hydrogels reported in the literature. Also, the simple two-step synthesis and wide range of suitable starting materials make this synthesis method more versatile and widely applicable than the existing methods for synthesizing tough hydrogels. An important finding of this work is that the presence of a hydrophobic network component significantly enhances the toughness and tensile strength of the combinatorial PUI hydrogels in the hydrated state. This enhancement is the largest when the hydrophobic network component is crystallizable in nature. In fact, the PUI hydrogels containing a crystallizable hydrophobic network component are shown to be semi-crystalline in the water-swollen state. Due to their high toughness values in the water-swollen state together with their water uptake values, elastic moduli and ultimate tensile strengths, the developed hydrogels are expected to be promising materials for biomedical coating-and adhesive applications, as well as for tissue-engineering.

Stiff, Strong, Tough, and Highly Stretchable Hydrogels Based on Dual Stimuli-Responsive Semicrystalline Poly(urethane–urea) Copolymers

“…The measured swelling ratios in tough hydrogels are highly sensitive to their chemical nature and environmental conditions (pH, temperature, etc) . Nonetheless, the values found in this study are consistent with those found for physically cross-linked tough polyurethane …”

“…67 Nonetheless, the values found in this study are consistent with those found for physically cross-linked tough polyurethane. 10 DSC analyses on the specimen immersed in water for different times were performed to investigate the thermal stability of the switching domains, namely, the POE crystal. In the dry state, the copolymers show endothermic peaks at 46.4, 47.6, and 44.1 °C for PU-5-30, PU-8-30, and PU-5-40, respectively, which are attributed to the melting of PEO SS crystals ( PEO T m ) (Figure 8b, Supporting Information Figure S7).…”

“…Hydrogels have the property to show network retention upon hydration, providing them with outstanding mechanical properties. − Several molecular origins of tough hydrogels are presented in the literature. For instance, physical bonds can be used to act as reversible sacrificial bonds, allowing to undergo huge dissipation of energy before fracture. , …”

Role of Acetate Anions in the Catalytic Formation of Isocyanurates from Aromatic Isocyanates

“…Isocyanurates, heterocyclic structures of 1,3,5-triazine-2,4,6-trione, formed via cyclotrimerization of isocyanates are widely used to enhance the physical properties of a variety of polyurethanes. − Whereas the controlled trimerization of aliphatic isocyanates is used for the preparation of isocyanurate cross-linkers for high-performance polyurethane coatings, − the cyclotrimerization of aromatic isocyanates into polyisocyanurate (PIR) structures is used to improve the thermal stability and flame retardancy of polyurethane rigid foams. − The initial molar ratio of functional groups and especially the type of catalyst are the main factors influencing the formation of PIR structures. The most commonly used catalysts in industrial applications are based on carboxylates, such as potassium acetate, potassium 2-ethylhexanoate, trimethyl hydroxypropyl ammonium formate, or phenolate such as 2,4,6-tris­(dimethylaminomethyl)­phenol. ,− The various industrial and commercial applications of isocyanurate-based polyurethane materials have attracted much attention in developing more effective catalysts for isocyanate trimerization. A number of catalysts for the trimerization of isocyanates have been extensively studied and reported in academic literature, including among others tetrabutylammonium fluoride (TBAF), 2-phosphaethynolate anion (OCP – ), sodium p -toluenesulfinate ( p -TolSO 2 Na)/tetrabutylammonium iodide (TBAI), , tetrakis­(dimethylamino)­ethylene (TDAE), proazaphosphatrane, − N-heterocyclic carbenes, olefins, , etc.…”

Reaction kinetics and properties of MDI base poly (urethane-isocyanurate) network polymers

“…Since the trimerization of isocyanate occurs easily and controllably to form a clear trifunctional isocyanate ring, this reaction is an ideal candidate for the synthesis of a clear polymer network [ 3–5 ]. The trimer reaction of isocyanate can be used to prepare many kinds of materials, such as resin [ 6 ], foams [ 7 ], aerogel [ 8 , 9 ], hydrogel [ 10 , 11 ], polyurethane gradient material [ 12 , 13 ], shape memory material [ 14 ], etc. and it has been applied to aerospace, aviation, biology, medicine, shipbuilding and other fields.…”