Synthesis And Characterization Of Thermo-and PH-Sensitive Poly[(N, N-diethylacrylamide)-co-(acrylic Acid)] Hydrogels With Fast Response Rate

In this thesis, thermo-and pH-sensitive poly(N,N-diethylacrylamide-co-acrylic acid) hydrogels with rapid response rates were prepared by radical cross-linked copolymerization. Several strategies, including physical and chemical, were adopted to improve the response rates. Meanwhile, functionalized multi-walled carbon nanotubes (MWCNTs) with vinyl groups were introduced to the gel networks to enhance its mechanical strength. The stimuli responsive properties, the swelling/deswelling kinetics as well as the mechanical properties were investigated in detail. Furthermore, a series of poly(N,N-diethylacrylamide) samples with different molecular weights was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The reaction conditions such as reaction temperature, reaction time, and [CTA]o/[/]o were studied systematically. The main results are shown as following:1. Thermo-and pH-sensitive P(DEA-co-AA) hydrogels with fast response rates were obtained by using PEG-6000 as a pore-forming agent during polymerization. PEG-6000 can be removed completely by sequential immersed in alkaline buffer solutions and distilled water. The swelling studies show that (1) the volume phase transition temperature (VPTT) of the hydrogels can be modulated to 37.5℃, which is close to human physiological temperature, by changing the amount of AA in the initial total monomers. (2) The hydrogels respond to the pH change by introducing AA into the copolymers. (3) The shrinking process at 60℃(above VPTT) is remarkably influenced by combination of the addition of PEG-6000 and the amount of hydrophilic comonomer (AA). The shrinking behavior in buffer solutions of pH= 3.0 has two stages. (4) The pulsatile swelling behavior in distilled water at temperatures alternating between 20℃and 60℃is remarkably reversible. However, the reversibility behavior of pH-responsive is dependent on the AA amount, and also the network structure of the hydrogel. 2. A series of porous P(DEA-co-AA) hydrogels with thermo-and pH-response was successfully prepared by using NaCl aqueous solution as the reaction medium. In comparison with the conventional P(DEA-co-AA) hydrogels, the P(DEA-co-AA) hydrogels prepared in NaCl solution have the similar temperature and pH sensitivity, but exhibit much faster deswelling response rates to both temperature and pH changes, and the deswelling rates depend on the concentration of NaCl solution. These improved properties are attributed to the porous network structure formed by using NaCl solution as the polymerization/crosslinking medium, a finding that is further confirmed by SEM micrographs.3. Thermo-and pH-sensitive P(DEA-co-AA) hydrogels with fast swelling/deswelling rates were obtained by a two-step copolymerization. The following significant facts are found:(1) the VPTTs of the hydrogels are not dependent on the synthesis procedure of the hydrogels, but the composition of the hydrogels; (2) the hydrogels respond to pH change in a wide range after introducing AA into the copolymers, and the response rates are improved by using the two-step copolymerization; (3) the hydrogels produced by the two-step copolymerization have faster swelling/deswelling rates compared with that by conventional method, and both the two steps are necessary for the fast response rates; (4) the deswelling rates are much faster than the swelling rates, because the diffusion resistance of water is considered to be smaller in the shrinking process than that in the swelling process.4. Thermo-and pH-sensitive comb-type grafted P(DEA-co-AA) hydrogels with different compositions were synthesized by free radical copolymerization. The swelling capacities and the deswelling kinetics as well as their reversibility to temperature and pH changes were examined. At room temperature, hydrogels with comb-type grafted chains exhibited higher equilibrium swelling ratios in both distilled water and higher pH buffer solutions (pH> 5). The deswelling behavior of the hydrogels was dependent on the test temperature. In addition, the pulsatile stimuli-responsive studies showed that the hydrogels with comb-type grafted chains had better thermo-reversibility, which was attributed to the freely mobile grafted chains. Furthermore, the mechanism of the response behavior was studied by using steady state fluorescence anisotropy.5. First, multi-walled carbon nanotubes (MWCNTs) with vinyl groups were prepared and characterized by Ramman and FTIR. Second, dual-stimuli sensitive MWCNT-polymer composite hydrogel was synthesized by copolymerizing DEA, AA and MWCNTs with vinyl groups. The results showed that the composite hydrogel (NCG) had both temperature and pH sensitivity. Particularly, comparing with the normal type hydrogel (NTG), NCG exhibited better mechanical strength, and relatively faster response rates to temperature changes. Moreover, the swelling results showed that both NCG and NTG adopt the same diffusion mechanism (Fickian diffusion mechanism).6. A series of poly(N,N-diethylacrylamide) samples with narrow molecular weight distribution was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The reaction conditions such as reaction temperature, reaction time, and [CTA]o/[I]o were studied systematically. It was proved that well-defined PDEA could be successfully synthesized by RAFT polymerization, and this work provided a theory basis for the synthesis of well-defined PDEA-based hydrogels with rapid response rates.