pH-Responsive
Shape Memory Poly(ethylene glycol)–Poly(ε-caprolactone)-based
Polyurethane/Cellulose Nanocrystals Nanocomposite
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Abstract
In this study, we developed a pH-responsive
shape-memory polymer nanocomposite by blending poly(ethylene glycol)–poly(ε-caprolactone)-based
polyurethane (PECU) with functionalized cellulose nanocrystals (CNCs).
CNCs were functionalized with pyridine moieties (CNC–C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub>) through hydroxyl substitution
of CNCs with pyridine-4-carbonyl chloride and with carboxyl groups
(CNC–CO<sub>2</sub>H) via 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) mediated surface oxidation, respectively. At a high pH value,
the CNC–C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub> had attractive
interactions from the hydrogen bonding between pyridine groups and
hydroxyl moieties; at a low pH value, the interactions reduced or
disappeared due to the protonation of pyridine groups, which are a
Lewis base. The CNC–CO<sub>2</sub>H responded to pH variation
in an opposite manner. The hydrogen bonding interactions of both CNC–C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub> and CNC–CO<sub>2</sub>H
can be readily disassociated by altering pH values, endowing the pH-responsiveness
of CNCs. 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. The pH-sensitive CNC percolation network
in polymer matrix served as the switch units of shape-memory polymers
(SMPs). Furthermore, the modified CNC percolation network and polymer
molecular chains also 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 the environment. Therefore, this pH-responsive
shape-memory nancomposite could be potentially developed into a new
smart polymer material