Investigation Of Hydrogels With Incorporated Polyaniline

Polymeric hydrogels with stimuli-responsive swelling behavior have broad potential applications, such as chemical mechanics, artificial Muscle, controlled drug delivery, separation/filtration, and flux-controlling. On the aspect of either pure research or application, it is surely significant to give intensive attention to the field of hydrogels. We have found in the experiments that two hydrogel systems, with incorporated polyaniline or polyaniline derivative, have multiple stimuli-responsive swelling behaviors. In this paper, the study will focus on two aspects: 1. The preparation, characterization and investigation of stimuli- responsive behaviors of polyaniline-poly (acrylic acid) blend hydrogel; 2. Theoretical simulation of pH-responsive behavior of polyelectrolyte hydrogel; 3. The preparation, characterization of polyacrylamide-sulfonated polyaniline semi-interpenetrating network hydrogel; 4. release of sulfonated polyaniline from polyacrylamide-sulfonated polyaniline semi-IPN hydrogel. The work in this paper is summarized in the following.1. Polyaniline-poly(acrylic acid) blend hydrogels with different composition were prepared and characterized. The temperature-, pH-, ionic strength-, and electric field- responsive behaviors were investigated. The hydrogels exhibited low-degree swelling at low pH values/low temperature, and high degree swelling at moderate high pH values/high temperature, at absolutely high pH range the hydrogels would return to low-degree swelling state. The increasing of relative molar fraction of polyaniline in hydrogels caused the interaction between polyaniline and poly (acrylic acid) more intense, thus significantly decreased the degree of stimuli-responsive behaviors, when the relative molar fraction of aniline repeat units to acrylic acid repeat unit in matrices was above 6/10 mol/mol, the interaction in network become intense enough that the hydrogel would loss electric field- response. The pH- and electric field-responsive behaviors had common mechanism, that is, there were two kinds of interplaying factors within the hydrogel: one was electrostatic repulsion between dissociated carboxyl groups which favored swelling; the others were attractive such as hydrophobic property of polyaniline, hydrogen bonding and polar-polar interaction, et al, which favored deswelling. The swelling behavior was determined by the shift of balance between these opposite aspects in response to varying external factors. From the fact that the hydrogels exhibited reversible triggering between high swelling state and low swelling state, we could deduce that there are strong interaction between PANI chains and PAAc chains, and interaction between PANI chains was relative weak in compared with that in HCl-doped PANI.The PANI-PAAc hydrogels in as-prepared state contained a few amount of sodium cations. After dipped in diluted HC1 aqueous solution and rinsed with large amount of water, the sodium cations in gel could be almost wholly removed, and there left only one kind of counter ion, hydrogen ion, in gel system. It was found that the pH-responsive swelling behavior in the presence of sodium ions was distinguishable with that in the absence of sodium ions, mainly in the range of pH 6~8. Ricka and Tanaka had pointed out that, the kinds of counter ions in the gels had tremendous influence to the Donnan contribution of mobile ions, especially hydrogen ions which determined the ionization degree of -COOH groups. According to their opinion, if there existed other counter ions than hydrogen ions, the hydrogen ions would be exchanged out from the gel, this caused the ionization degree of -COOH increasing to compensate the loss of hydrogen ions, then imposed significant influence on the hydrogel's swelling degree.2. Using poly(acrylic acid) as a model polymer, thermodynamic simulation of pH-responsive swelling behavior of polyelectrolyte hydrogels was carried out in this section. At first, every factor that determined the swelling behavior of polyelectrolyte hydrogels was introduced briefly, including polymer-solvent mixing, elastic retraction of deformed network, osmotic pressure of mobile ion species, and electrostatic interaction. The result of thermodynamic simulation primely reflected the essential characterization of ph-responsive behavior of polyelectrolyte hydrogels. In the section, based on Katchalsky's equation of electrostatic free energy, a new equation for calculation of elecrostatic swelling pressure was put forward according to a reasonable assumption. The new formulation can give out fairly good simulating results.The simulating results exhibited that, the formulation, usually used before in many literatures, to calculate the osmotic pressure due to Donnan contribution of mobile ion species, based on the assumption that this osmotic pressure was only proportional to the concentration of pendent ionic groups, could not give good simulating results. If the ionic strength of the medium is taken into account in calculating the osmotic pressure, as Flory had pointed out, fairly good results were obtained. The simulating results also exhibited that, the inherent difference between dissociating equilibrium of polyelectrolyte and that of low-molecular-weight electrolyte must be taken into account, otherwise reasonable results could not be obtained.3. In the third part of this paper, a new type of semi-interpenetrating network system consisting of a flexible polyacrylamide hydrogel with embedded linear polyelectrolyte, sulfonated polyaniline (SPANI), is successfully prepared. Incorporation of polyelectrolyte SPANI into the uncharged network results in pH-responsive swelling degree. It can be deduced that variation of pH could impose significant influence upon the interaction and aggregating state of SPANI chains. In acidic and neutral medium, molecules ofSPANI is almost all in the state of aggregation, seldom SPANI chains is dissociative; in moderately alkali medium, the electrostatic repulsion between anions on the SPANI chains lead to the collapse of aggregates of SPANI; in strong alkali medium, the electrostatic shielding favored the formation of aggregates of SPANI. From FTIR analysis, in acidic and neutral medium, there were considerably intense hydrogen bonding in gel matrices, on alkali medium the hydrogen bonding was largely weakened. The pH-dependent swelling behavior resulted from the interplay of two main force between SPANI chains: repulsive electrostatic interactions which separate polyelectrolyte chains from each other, and attractive interactions of pole-pole interaction, hydrogen bonding, and poor solution condition which tend to make them condensate. From the fact that uncharged network can responsive to pH value, it is obviously implied that SAPNI chains interact strongly with network chains, although they are not covalently attached.4. Release of SPANI from PAAm-SPANI semi-IPN hydrogels was investigated. The influence of pH of medium on the release rate was discussed from aspect of inter-molecular interaction and aggregating state of SPANI. In acidic or neutral medium SPANI chains were effectively retained by the gel, this implied that SPANI in aggregating state could not releasae due to intense inter-molecular interaction; in alkali medium of 0.1M NaOH, there was considerable electrostatic repulsion between undoped SPANI molecules, leading to dissociation of the aggregates of SPANI chains, the swelling degree of semi-IPN hydrogel increased, then the entrapped single SPANI chains were able to release gradually.The release rate of SPANI in medium of 0.1 M NaOH aqueous solution was far more greater than that in 0.01M NaOH aqueous solution, and the diffusion coefficient differed as much as several magnitudes. This implied that in 0.1 M NaOH aqueous solution, the inter- molecular interaction between two network components was eliminated more completely and the aggregates of SPANI were dissociated more efficiently. Besides, in medium of 0.1 M NaOH aqueous solution the diffusion coefficient of SPANI at 29 "C was 1 ~ 2 magnitudes greater than that at 15°C; but in 0.1M NaOH aqueous solution, temperature did not exert so great influence on diffusion coefficient.In alkali medium there was a induction period, to, for the release of SPANI, this implying that in initial period the attractive interaction in gel matrices remained prevailing, resulting in retaining of SPANI in hydrogel. Only after swelling degree of the gel increased and the attractive interaction was weakened, SPANI chains were able to release from the gel. After induction period the concentration of SPANI in surrounding medium had a fairly good linear relation to (t-to)1/2. Due to change of swelling degree of the gel and the intense interaction, the diffusion coefficient of SPANI in gel did not obey the prediction of free volume theory. The electrostatic repulsion was the predominant reason for the release of SPANI.