Synthesis and characterization of silica-poly(N-isopropylacrylamide) hybrid membranes:: Switchable molecular filters

Hybrid materials consisting of stimuli-responsive polymers (SRPs) in an inorganic matrix have the potential for use in a variety of applications as switchable or tunable membranes, coatings, adsorbents, and solute reservoirs. Poly(N-isopropylacrylamide) (PNIPAAM) is an SRP that exhibits inverse solubility behavior in water upon an increase in temperature through a transition from a hydrophilic to a hydrophobic state. In an earlier communication, we demonstrated that when PNIPAAM is encapsulated in a silica matrix by a sol-gel process, the resultant hybrid material can function as a switchable molecular filter. In this paper, we present detailed investigation and characterization of these membranes, which exhibit reversible on/off permeation behavior, above and below the transition temperature of PNIPAAM, respectively. Scanning and transmission electron microscopic studies show that the membranes are highly homogeneous, and phase separation of the polymers is not observed. Differential scanning calorimetry, contact angle, and permeation experiments indicate that the PNIPAAM in the hybrid materials retains its aqueous critical solubility transition. Nitrogen porosimetry measurements indicated that the dried hybrid materials are mesoporous with an average pore size of similar to30 Angstrom. Permeation of solutions of poly(ethylene glycols) (PEGs) varying in molecular weight through the hybrid membranes clearly demonstrates that they can function as switchable molecular filters. Under controlled pressure, the membranes are impermeable to water below the transition temperature. At the same pressures, above the transition temperature, membranes are permeable to PEGs with molecular weights below a threshold value. In the membranes studied, the molecular weight of PNIPAAM encapsulated in the silica did not dramatically affect the PEG molecular weight threshold for permeation.