Assembly And Controlled Dissolution Of Multilayer Films Of Temperature-Responsive Polymers

Layer-by-layer (LbL) assembly is a powerful means for fabricating multilayer thin filmswith controlled architecture and composition. Scientists have fabricated multilayer films withvarious kinds of functions by using LbL assembly. In recent years, some groups focused theirresearch interest on fabricating temperature-responsive films through LbL assembly. Suchfilms allow control of surface wettability, adhesion properties and film permeability, etc. bychanges in temperature.Two approaches are usually applied for preparing temperature-responsive multilayerfilms.(1) The second monomer with assembly capability is introduced into atemperature-responsive polymer through copolymerization, and a temperature-responsive LbLfilm is then assembled via covalent bonds or electrostatic interactions. This approach is notconvenient, and a copolymer has to be synthesized.(2) LbL films are assembled usingtemperature-responsive homopolymers via hydrogen bonding. Hydrogen-bonded LbL films ofneutral temperature-responsive homopolymers were usually assembled with poly(carboxylicacid)s at low pH, and this limits the application of the films at physiological conditions. In thisdissertation, we measured the cloud points of mixed solutions of every two polymers amongseveral temperature-responsive polymers. It was found that the cloud point of some mixedsolutions significantly decreased, which indicated interpolymer interaction. Thus, we selectedfive temperature-responsive polymers, poly(N-vinylcaprolactam)(PVCL),poly(2-hydroxypropyl acrylate)(PHPA), poly(N-L-(1-hydroxymethylpropyl)methacrylamide)(P(L-HMPMAm)), poly(N-isopropylmethacrylamide)(PNIPMAm) and poly(N-propyl-methacrylamide)(PnPMAm). Multilayer films were assembled at neutral pH. Dissolution ofthe multilayer film was studied in purified water with step-by-step decrease of temperature.Furthermore, the effects of tetramethylurea and several salts on temperature-responsivedissolution of multilayer films were studied.In Chapter2, the cloud point of some mixed solutions of temperature-responsivepolymers is lower than both cloud points of the corresponding pure polymers, such asPVCL/PHPA, PVCL/P(L-HMPMAm), and PNIPAAm/PHPA. The significant decrease of cloud points of mixed solutions of temperature-responsive polymers suggested that theintermolecular interaction between two polymer chains of different kinds was stronger thanthat between two polymer chains of the same kind. Strong intermolecular interaction betweentwo polymer chains of different kinds is a prerequisite for LbL assembly. Thus, a LbL film oftwo temperature-responsive polymers, PVCL and PHPA, was fabricated at neutral pH at aconstant temperature. The film can be partially dissolved when immersed in purified water at alower temperature. In contrast, the LbL film is relatively stable in buffer solutions nearphysiological pH at the assembly temperature. Additionally, a PVCL/P(L-HMPMAm)multilayer film and a PNIPAAm/PHPA multilayer film were also successfully assembled atneutral pH. Successful fabrication of these LbL films indicates that phase behavior of mixedsolutions of temperature-responsive polymers provides an important guide for the selection ofpolymers suited for hydrogen-bonded LbL assembly at neutral pH. To clarify the effect oftemperature on dissolution of films, the dissolution process of films at a constant temperaturein purified water can be described by a stretched exponential kinetics. As the temperaturedecreased, the dissolution rate increased and final remaining fraction decreased. Watertemperature can be used to control the remaining fraction of the PVCL/PHPA multilayer film.In Chapter3, a LbL film of two neutral temperature-responsive polymers, PnPMAm andPHPA, was fabricated at28°C. The film can be dissolved when immersed in purified water ata lower temperature. The dissolution temperature of the film increased in tetramethylureaaqueous solutions. In tetramethylurea aqueous solutions with different concentration, as thecloud point of mixed solutions of temperature-responsive polymers increased, the dissolutiontemperature of films also increased. In other words, the effect of tetramethylurea ondissolution temperature of films and that on cloud point of mixed solutions are positivelycorrelated. The remaining fraction of temperature-responsive films could be controlled bychanging the concentration of tetramethylurea aqueous solutions.In Chapter4, a LbL film of two neutral temperature-responsive polymers, PVCL andP(L-HMPMAm), was fabricated at a constant temperature. The assembly amount of polymersdecreased with the decrease in assembly temperature. The film can be dissolved when immersed in purified water at a lower temperature. The cloud points of mixed solutions ofPVCL and P(L-HMPMAm) decreased when several chloride salts with different concentrationwere added. The dissolution temperature of multilayer films in chloride salts solutions is lowerthan in purified water. The effect of these chloride salts on dissolution temperature ofmultilayer films and that on cloud point of mixed solutions are positively correlated.PVCL/P(L-HMPMAm), PVCL/PHPA and PnPMAm/PHPA multilayer films in NaCl solutionsare more stable than in purified water. These phenomena could be attributed to salting-out oftemperature-responsive polymers.