Supramolecular thermogels from branched PCL-containing polyurethanes

Abstract

Thermogels are temperature-responsive hydrogels which are most commonly formed by supramolecular self-assembly of polymer amphiphiles comprising of both hydrophobic and hydrophilic segments. Although polyurethane thermogels have shown great promise as biomaterials, their synthesis by step-growth polymerisation of diols and diisocyanates can also result in formation of allophanate branches, which arise from the reaction between free isocyanate groups and urethane linkages along the polymer backbone. In this paper, we investigate the effects of different synthetic conditions on the degree of allophanate branching on polyurethane amphiphiles, and explore the influences of these branches on the polymers' critical micelle concentration (CMC), thermodynamics of micellization and subsequent thermogel properties. Our findings offer new insights into the relationship between polymer structure, micelle and gel properties. These results highlight the importance of taking polymer branching into account for understanding the hierarchical self-assembly of polymer amphiphiles and the resulting thermogel properties and behaviour.

Scheme 1. Formation of allophanate branch points along a polyurethane by reaction of isocyanates with pre-formed urethane groups.

Scheme 2. Synthesis of branched thermogelling polyurethanes.

Fig. 1. ¹H NMR spectrum of random triblock thermogelling polyurethane copolymer containing PEG : PPG and PCL in a 4.00 : 1.00 : 0.01 wt/wt/wt ratio (CDCl₃).

Fig. 2. Changes in PPG-to-HMDI molar ratio of the growing polymer chain at different reaction durations. Synthesis of polyurethanes were performed using a 2 : 1 : 3 molar ratio of PEG, PPG and HMDI respectively at 110 °C in anhydrous toluene, catalysed by DBTL (see Table S1, ESI, for further details).

Fig. 3. (A) UV-Vis absorbance spectra of DPH dye at different concentrations of polymer 1P30-1h at 25 °C; (B) difference in DPH absorbance at 378 and 400 nm as a function of polymer concentration of 1P30-1h at 25 °C. The CMC was determined by the intersection of the extrapolated linear best fit lines at low and high concentrations of polymer.

Fig. 4. Arrhenius plot of ln(X_CMC) versus T⁻¹/K⁻¹ for polyurethane 1P30-1h. The CMC (in mole fractions), or X_CMC, was determined at 15, 25, 35 and 45 °C. A best fit line is drawn across and the standard enthalpy of micellisation (ΔH) is given by the product of the gas constant and the gradient.

Fig. 5. Temperature-sweep rheology for a 7 wt/v% solution of 1P30-1h in deionised water. Gelation occurs at the temperature when the storage modulus first becomes larger than the loss modulus and in this case it is 25.7 °C.

Fig. 6. (A) Plot of T_gel of different polyurethanes as a function of polymer molecular weight (M_n); (B) plot of T_gel of different polyurethanes as a function of increasing degrees of branching.

Fig. 7. (A) Micelle formation and gelation via micelle packing (or jamming) for amphiphilic copolymers with hydrophilic–hydrophobic–hydrophilic structure. (B) Micelle formation and gelation via micelle bridging for amphiphilic copolymers with hydrophobic–hydrophilic–hydrophobic structure. Reprinted with permission from ref. 64 (C) micelle formation and gelation via aggregation of semi-bald micelles to form a percolated micelle network. Reprinted with permission from ref. 45. Copyright 2020 American Chemical Society.

Fig. 8. Competing effects of polymer amphiphile branching on micelle formation and self-assembly to form thermogels.

Fig. 9. Schematic of poly(PEG/PPG/PCL urethane) polymer structure with crosslinks.

Scheme 3. The working principle of the titration experiments with acetic anhydride to determine quantity of OH present.