Stimuli responsive dynamic polymer networks with tunable toughness and elasticity through the regulation of covalent crosslinking and reversible hydrogen bonds

Abstract

Constructing dual crosslinking networks consisting of both covalent crosslinkers and dynamic hydrogen bonds (H-bonds) and regulating their compositions are essential for adjusting the mechanical properties and responsiveness in the field of stimuli responsive materials. In this study, a styrene–isoprene–styrene triblock copolymer (SIS) was modified with varying amounts of epoxy (E) and hydroxyl (–OH) groups to create ESIS and SIS-OH. Subsequently, the synthetic covalent crosslinkers, i.e., spiropyran (SP) derivatives with diisocyanate terminals (SPM₁-NCO and SPM₂-NCO), and the dynamic crosslinker, i.e., 2(6-isocyanatohexy-laminocarbonyl-amino)-6-methyl-4[1H]-pyrimidinone (UPy-NCO) with quadruple H-bonds, were incorporated into the resulting SIS-OH through a one-pot reaction to construct dual crosslinking networks (DPNs), i.e., SIS-SP₁/SP₂-UPy. The degree of epoxidation/hydroxylation (DE/DH) and the density of covalent crosslinking/H-bonds were regulated. The mechanical properties, recyclability and stimuli responsive behaviors were studied. The results demonstrated that the elongations at break of all ESIS and SIS-OH with different DE/DH were over 1200%. Although the quadruple H-bonding UPy did not notably enhance the stress at break of the resulting SIS-UPy, the high toughness and elasticity showed a tendency of first increasing and then decreasing with the increase of the UPy content due to the cooperative intermolecular effect of quadruple H-bonds and the large amount of remaining –OH units, particularly evident in SIS-UPy-20% which exhibited the highest toughness (15.9 MJ m⁻³) and elasticity values (1467%). Additionally, the tensile properties of DPNs were significantly enhanced, in which the toughness and elasticity values could reach up to 18.5 MJ m⁻³ and 1505%, respectively. SPM₂-NCO showed photoinduced solvatochromic behaviors, which further endowed DPNs with ultrasound-induced mechanochromic behaviors. This study would allow us to increase the comprehension of intrinsic mechanisms.