The Synthesis And Characterization Of Fast Responsive Thermosensitive Poly(N-isopropylacrylamide)/Clay Nanocomposite Hydrogels

Poly(N-isopropylacrylamide)(abbreviated as PNIPAAm) has been one of the intensively studied thermosensitive polymers and exhibits a volume phase transition temperature (VPTT) around 32℃. Below this temperature, the PNIPAAm polymer is well soluble in aqueous media but precipitates from the solution as the temperature is increased above the VPTT. The properties of PNIPAAm hydrogel have been of great interest for a wide variety of applications, for instance, devices for controlled drug release or immobilization of enzymes and recyclable absorbents. Conventional PNIPAAm hydrogels, prepared using organic cross-linkers, had some serious disadvantages such as the lack of mechanical toughness, low swelling/deswelling rates and transparency which limited their applications greatly. In this paper, thermosensitive PNIPAAm/Clay nanocomposite hydrogels with fast responsive rate and improved mechanical properties were synthesized by using different pore-foaming agents and freeze-drying. The structure and morphology, the swelling and deswelling behaviors and the mechanical properties of these novel hydrogels were investigated in detail. The main results obtained are as follows:1. The exploring experiments show that the interaction between KPS and clay platelets is much stronger than those between Na₂CO₃. or CaCO₃ with clay platelets, sothe clay still acts as an-effective multi-functional cross-linker and the stable PNIPAAm/Clay nanocomposite hydrogels could be formed. Poly(ethylene glycol), (PEG) chains can absorbs onto clay platelets and form a polymer layer around the particles, which prevented the clay to be a cross-linker, and the hydrogels couldn't to be prepared.2. Fast responsive thermosensitive PNIPAAm/Clay nanocomposite hydrogels (NCx/y(z) hydrogels) were synthesized by using CaCO₃ particles as a pore-foaming agent. The effects of meshes and content of CaCO₃ particles on responsive rates and tensile mechanical properties of these hydrogels were investigated. It was found that compared with the PNIPAAm/Clay hydrogels without introduction of CaCO₃, the deswelling rate of NCx/y(z) hydrogels in response to temperature was improved significantly, owing to the formation of a porous structure within these hydrogels. And the porous structure was determined by the meshes and content of CaCO₃ particles. The smaller CaCO₃ particles can get smaller and denser pores which can lead to faster deswelling rates of the hydrogel. With increasing of the CaCO₃ content, the deswelling rate of hydrogels was improved more significantly. The mechanical properties of NCx/y(z) hydrogels were weakened with the increasing of the CaCO₃ content, which is due to the formation of a porous structure within these hydrogels. However the tensile strength and toughness of NCx/y(z) hydrogels were improved significantly compared with the conventional PNIPAAm hydrogels crosslinked by chemical crosslinker.3. Fast responsive thermosensitive PNIPAAm/Clay nanocomposite hydrogels (NCx/y hydrogels) were synthesized by using Na₂CO₃ as a pore-foaming agent. The effects of Na₂CO₃ content and clay content on responsive rates of hydrogels were syudited. It was found that compared with the PNIPAAm/Clay hydrogels the deswelling rate of NCx/y hydrogels in response to temperature was improved significantly compared with the PNIPAAm/Clay hydrogels, and the variety of contents of clay or Na₂CO₃ has tiny influence on the deswelling rates of NCx/y hydrogels.4. PNIPAAm/Clay hydrogels treated by freeze-drying (FD hydrogels) exhibit fast deswelling rates. The size of pores, responsive rates and tensile mechanical properties of FD hydrogels were investigated, especially the effect of water content in PNIPAAm/Clay hydrogels during the freeze-drying process. It was found that water content in the process of freezing gel was an important factor to control the pore size, and the deswelling rate was accelerated with increasing water content on the FD treatment. The formation of a porous structure within FD hydrogels also leaded to the decrease of tensile strength and elongation at break.