| In this study, we developed a pH-induced shape-memory polymer nanocomposites by blending poly(ethylene glycol)-poly(s-caprolactone)-based polyurethane (PECU) with functionalized cellulose nanocrystals (CNCs). The pH-sensitive CNCs percolation network in polymer matrix served as the switch units of shape-memory polymers (SMPs). Furthermore, the modified CNCs percolation network and polymer molecular chains had strong hydrogen bonding interactions among hydroxyl, carboxyl, pyridine moieties and isocyanate groups, which could be formed or destroyed through changing pH value. The shape memory function of the nanocomposite network was only dependent on the pH variation of environment. Therefore, this pH-induced shape-memory nanomposites could be potentially developed into a new smart polymer material.Firstly, we prepared CNCs from microcrystalline cellulose by sulfuric acid hydrolysis method. A detailed study of the dimensions, structure, morphology and so on were performed about CNCs. The results showed that we successfully prepared the morphologies of CNCs were rod-like with length ranging from 200 nm to 300 run and width of 5-20 nm. CNCs were functionalized with pyridine moieties (CNC-C6H4NO2) through hydroxyl substitution of CNCs with pyridine-4-carbonyl chloride. Meanwhile, the CNC-C6H4NO2 conducted on the dimensions, structure, morphology, pH sensitivity analysis and so on. The results show that the success through the surface chemical modification method, making the surface of CNCs with pyridine group. At high pH value the CNC-C6H4NO2 had attractive interactions from the hydrogen bonding between pyridine groups and hydroxyl moieties; at low pH value the interactions reduced or disappeared due to the protonation of pyridine groups which are a Lewis base. The hydrogen bonding interactions of CNC-C6H4NO2 can be readily disassociated by altering pH values, endowing the pH-responsiveness of CNCs.Secondly, we use 2,2,6,6-Tetramethyl-l-piperidinyloxy (TEMPO), NaBr, NaOCl mediated surface oxidation of hydroxyl on the surface of CNCs. A detailed study of the dimensions, structure, morphology and so on were performed. The results showed that the CNC-CO2H could be expected to respond to pH variation in an opposite manner about the CNC-C6H4NO2. In an aqueous environment of high pH, the CNC-CO2H were negatively charged, and therefore, electrostatic repulsions should significantly reduce the attractive interactions between the CNCs. Conversely, at low pH the CNC-CO2H had attractive interactions, which resulted from hydrogen bonding of the carboxylic acid and hydroxyl moieties.Finally, we developed a biocompatible materials by blending poly(ethylene glycol)-poly(ε-caprolactone)-based polyurethane (PECU), which was obtained by, first, the synthesis of poly(ethylene glycol)(PEG)-poly(ε-caprolactone)(PCL) by the ring-opening polymerization of ε-caprolactone (ε-CL) with PEG; second, getting the resultant polymer with PCL-PEG-PCL as the soft segment of polyurethane (PU) matrix materials by the reaction of PCL-PEG-PCL with 4,4-Diphenylmethane diisocyanate (MDI) in the presence of chain extender 2,2-Bis(hydroxymethyl) propionic acid (DMPA). The PECU called as (PCL-PEG-PCL)70%-MDI-DMPA. When these functionalized CNCs were added in PECU polymer matrix to form nanocomposite network which was confirmed with rheological measurements, the mechanical properties of PECU were not only obviously improved but also the pH-responsiveness of CNCs could be transferred to the nanocomposite network. We also analyzed the microstructure, water contact angle, thermal properties, swelling degree and static mechanical properties, pH-induced shape-memory properties of the nanocomposites. We found that the pH-sensitive CNCs percolation network in polymer matrix served as the switch units of SMPs. In this nanocomposite system, the pH-induced shape-memory effect (SME) is realized through the association and dissociation of the hydrogen bonding interactions between the modified CNCs percolation network and matrix materials. |
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