(Invited) Force-Responsive Hydrogel Made of Block Polyrotaxane

Stimuli-responsive polymers that are sensitive to certain triggers from the external environment have been attracting many researchers’ interests. Stimulus that can be used to trigger these materials have been varied from temperature, light, electrical or magnetic fields, to chemicals. However, some applications require new triggers such as force. We thought that polyrotaxane-based gel is a new candidate of force-responsive material. Polyrotaxane is a type of mechanically interlocked supramolecule consisting of polymer axle and ring molecules, in which multiple rings are threaded onto a polymer axle and prevented from dethreading by bulky end groups. When a force is applied to polyrotaxane gels, in which polyrotaxanes are crosslinked through ring molecules, ring molecules slide along an axle polymer to evenly distribute the applied force. Recently we created block polyrotaxane whose axle is a triblock copolymer poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-PPO-PEO).[1] β-Cyclodextrin (β-CD) form inclusion complex with PPO but not with PEO in water. We hypothesized that chemical property of the hydrogels obtained from this block polyrotaxane would be converted to hydrophobic by force. β-CDs should stay on PPO segment without loading, while β-CDs would slide from PPO to PEO segment to expose hydrophobic PPO segment when a force is applied. In order to confirm this concept, we studied hydrogels based on polyrotaxane made of β-CDs and PEO-PPO-PEO with several molecular weight and PEO/PPO ratio. Three polyrotaxanes compared in this study are followings; PEO60-PPO30-PEO60 with 17CDs (PPR_7k), PEO80-PPO70-PEO80 with 13CDs (PPR_11k), PEO110-PPO55-PEO110 with 12CDs (PPR_13k) (subscripts denote a number of repeating unit of each segment). 1H NMR measurements were conducted to investigate a position of β-CDs of a series of polyrotaxanes in water. It was found that β-CDs in the PPR_13k shuttles both on PEO and PPO segment but β-CDs in PR_7k and PR_11k stayed on PPO. Small-angle X-ray scattering (SAXS) was conducted to the hydrogels obtained from cross-linked polyrotaxane. Figure shows SAXS 1D profiles of the hydrogel obtained from PPR_11k. Before stretching, no peak was observed based on a periodic structure. Then this hydrogel was extended to 1.8 times of the original state to show a peak corresponding to a periodic distance of 29 nm. These results indicate PPO segments were exposed after loading to form hydrophobic periodic domains in PPR_11k. Other hydrogels obtained from PPR_7k and PPR_13k did not show any periodic structures even after loading. We are hoping that this new force-responsive block polyrotaxane system can be applied to the materials that can alter materials permeability by an applied force. [1] S. Uenuma et al. Chem. lett., 45 (8), 991-993 (2016) Figure 1