Thermoresponsive nanocomposites from multilayers of nanofibrillated cellulose and specially designed N-isopropylacrylamide based polymers

Abstract

In this work positively charged polymers based on N-isopropylacrylamide (NIPAAm) have been synthesised and investigated in solution, on surfaces and in polyelectrolyte multilayers (PEMs) in combination with nanofibrillated cellulose (NFC). Polymers having thermoresponsive properties at low salt concentrations in solution and when adsorbed onto surfaces were obtained by separating the charged groups from the thermoresponsive part in different blocks within the polymer. The polymers have been synthesised using atom transfer radical polymerisation (ATRP) of NIPAAm and (3-acrylamidopropyl)trimethylammonium chloride (APTAC), which is the cationic monomer. All the block copolymers exhibited a lower critical solution temperature (LCST) in water between 35 and 39 °C and a positive correlation was found between the LCST and the charge densities of the polymers. It is shown that electrostatic interactions control the adsorption of the prepared polyelectrolytes to SiO₂ surfaces and that the block copolymers are thermoresponsive when adsorbed at the solid–liquid interface. PEMs were also assembled with the synthesised polymers and NFC which is a renewable, fibrillar nanomaterial with interesting strength and biocompatibility properties. Even more interestingly it has been found that the formation of the layer-by-layer (LbL) structures of NFC and the block copolymers were demonstrated to have thermal responsivity. This type of thermoresponsive nanocomposite could be used as nano-containers for controlled release or for example in membranes where the permeability could be controlled by the temperature.