A dynamic supramolecular polyurethane network whose mechanical properties are kinetically controlled

Abstract

We report the synthesis and characterization of a kinetically controlled, thermoreversible supramolecular polyurethane whose mechanical properties depend unusually strongly on the processing history. Materials were prepared by solution casting, quenching and annealing of quenched material, allowing pronounced micro-structural evolution, which leads to rapid increases in modulus as determined by rheological analysis. Tensile tests showed that the quenched material is soft, weak and ductile (shear modulus ~ 5 MPa, elongation ~ 250 %), but after annealing, at 70 °C for one hour, it becomes stiffer, stronger and more brittle (~ 20 MPa, ~ 20 %). FTIR and NMR spectroscopic analysis, coupled with MDSC and SAXS, were performed to investigate the network's dynamic structural changes. SAXS results suggest the presence of a lamellar structure in the sample when solution cast at high temperature, or annealed. This ordering is unique when compared to structurally-related supramolecular bisurethane and bisurea polymers, and may be the cause of the observed path dependence. These mechanical properties, which can be switched repeatedly by simple thermal treatments, coupled with its adhesion properties as determined from peel and tack tests, make it an excellent candidate as a recyclable material for adhesives and coatings.

Synthesis and analysis of a healable, poly(propylene glycol)-based supramolecular network

“…6 Fundamental studies into supramolecular polymer networks which self-assemble through hydrogen bonds have revealed the key design principles required for successful network formation. [7][8][9][10][11][12][13][14][15][16][17][18] One approach has utilized molecular motifs with high association constants resulting from well-defined, multiple hydrogen bonding (Ka > 10 6 M -1 ), covalently attached to a variety of polymeric backbones, which self-associate very effectively in the apolar polymer matrix to yield robust elastomeric and dynamic materials. 7,9 An alternative approach to the creation of supramolecular networks has employed a combination of phase-separation between apolar polymer blocks and self-assembling polar chain-ends.…”

“…Such systems generally possess less well-defined and far weaker (Ka < 250 M -1 ) or more dynamic hydrogen bonds. [10][11][12][13][14][15][16][17][18] The networks formed in both approaches can be addressed thermally and importantly exhibit reversible character. These studies have shown that the physical properties of these networks are dependent upon (i) the degree of association of the receptor units, (ii) the 'supramolecular valency', 19 and (iii) the dynamics of the non-covalent assemblies, [15][16][17][18] in addition to (iv) phase separation between the recognition motifs and the polymer segments.…”

“…[10][11][12][13][14][15][16][17][18] The networks formed in both approaches can be addressed thermally and importantly exhibit reversible character. These studies have shown that the physical properties of these networks are dependent upon (i) the degree of association of the receptor units, (ii) the 'supramolecular valency', 19 and (iii) the dynamics of the non-covalent assemblies, [15][16][17][18] in addition to (iv) phase separation between the recognition motifs and the polymer segments. [12][13][14][15][16][17][18] Variation of these structural factors has enabled detailed investigation of the processing temperatures and strength of these supramolecular materials.…”

“…These studies have shown that the physical properties of these networks are dependent upon (i) the degree of association of the receptor units, (ii) the 'supramolecular valency', 19 and (iii) the dynamics of the non-covalent assemblies, [15][16][17][18] in addition to (iv) phase separation between the recognition motifs and the polymer segments. [12][13][14][15][16][17][18] Variation of these structural factors has enabled detailed investigation of the processing temperatures and strength of these supramolecular materials. [17][18][19][20][21][22][23][24] Developments in understandings of such networks have provided numerous routes to materials that are both mechanically robust as well as healable.…”

“…[17][18][19][20][21][22][23][24] Developments in understandings of such networks have provided numerous routes to materials that are both mechanically robust as well as healable. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] A recent example reported by Gooch et al 22 describes polyurethane-based elastomers which assemble through triple hydrogen bonding interactions. Gao et al 23 have reported healable polyurethane 'cationomers' with a modulus of toughness of approximately 180 MPa, suitable for protective roles such as packaging and casing.…”