ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия А, 2011, том 53, № 8, с. 1394-1401
ПОЛИМЕРНЫЕ СЕТКИ
УДК 541.64:532.77
SYNTHESIS, CHARACTERIZATION AND SWELLING KINETICS OF THERMORESPONSIVE PAM-g-PVA/PVP SEMI-IPN HYDROGELS1
© 2011 г. Qing-Bo Wei", Yan-Ling LuoA, Lou-Jun Gao", Qiao Wang", and Dan-Jun Wang"
a Key Laboratory of Chemical Reaction Engineering of Shaanxi Province, College of Chemistry & Chemical Engineering,
Yan'an University, Yan'an, 716000 China b Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Materials Science,
Shaanxi Normal University, Xi'an, 710062 China e-mail: luoyanl@snnu.edu.cn, luoyl0401@yahoo.com.cn (Y.L. Luo) Received September 5, 2010 Revised Manuscript Received February 10, 2011
Abstract—Polyacrylamide grafted poly(vinyl alcohol)/polyvinylpyrrolidone (PAM-g-PVA/PVP) semi-interpenetrating polymer network (semi-IPN) hydrogels were designed and prepared via a simple free radical polymerization route initiated by a PVA-(NH4)2Ce(NO3)6 redox reaction technique. The structure of the PAM-g-PVA/PVP hydrogels was characterized by a Fourier transform infrared spectroscope (FTIR), and the morphologies were observed by a scanning electron microscopy (SEM). The swelling kinetics investigations demonstrated that the equilibrium swelling (Qe) of the (PAM-g-PVA/PVP) semi-IPN hydrogels depended on PVP compositional ratios and temperature. The Qe values were reduced with increasing the PVP contents, which was in agreement with theoretical water contents (Sx) fitted by swelling kinetic data, and the swelling mechanism belonged to a non-Fickian mode for the PAM-g-PVA/PVP hydrogels. These hydrogels displayed thermosensitivities different from the common thermoresponsive gels that have a lower critical solution temperature. The swelling is enhanced with increasing the temperature of the media before 42°C, and later the equilibrium swelling is contrarily reduced. Therefore, the swelling behavior of the PAM-g-PVA/PVP hydrogels may be controlled and modulated by means of the compositional ratios of PVP to PAM-g-PVA and temperature.
INTRODUCTION
In recent years, hydrogels have been extensively investigated for their unique properties and for providing important building blocks for the construction of functional structures [1]. They are cross-linked polymer networks that swell but do not dissolve in water. The water content in the hydrogels at equilibrium is one of their basic properties, which depends on various stimuli, such as temperature, pH, solvent- or ionic composition, electric field, light intensity as well as introduction of specific ions [2—8]. They have been demonstrated to induce abrupt changes in degree of swelling (phase transition). Due to their drastic change of properties in response to environmental stimuli, these hydrogels may be used in controlled drug delivery, tissue culture substrates, molecular separation and several technical applications [9—14]. K. Disek, I. Galaev, and Y. Osada et al. [15—17] have detailedly reviewed the essential studies on the smart polymer hydrogels, including synthesis, various stimuli response, and technological applications, etc. It is much significant for us to deeply understand the volume phase transition from the viewpoints of structure (including conformation), dynamics, equilibrium
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thermodynamics, and the relation between the transition and biological interactions.
Poly(vinyl alcohol) (PVA) has been used in a wide variety of fields because of its desirable properties such as nontoxicity and noncarcinogenicity. It is a highly polar, water-soluble and biodegradable polymer [18— 20]. It finds extensive applications as biomaterials, such as artificial blood vessels, artificial intestines, contact lenses and artificial kidneys [21]. Polyvinylpyrrolidone (PVP) has good biocompatibility and for many years has been applied as a biomaterial or additive to drug compositions, e.g. as a blood plasma expander and as vitreous humor substitute [22]. On the other hand, it is well known that polyacrylamide (PAM) is typically a kind of linear-type watersoluble polymer, whose hydrogels possess pH response after hydrolyzation [23, 24], temperature sensitivities in the presence of N,N'-methylene-bis(acrylamide) [25] and good biocompatibility. Their high swelling capacity makes them an extremely important carrier for drug controlled release. These characteristics are originated from the charge formation in their interiors [26]. Considering the above-mentioned description, we make an attempt to combine these materials via the following approach so as to obtain a drug release carrier material with excellent properties. PVA and ceric ammonium nitrate are employed as a redox system to
initiate the cross-link polymerization reaction of acry-lamide monomer in the presence of N,N-methyle-nebisacrylamide, while PVP macromolecular chains are interpenetrated inside the PAM-g-PVA network obtained to mediate the properties of the hydrogels. Therefore, the objective of this work is to design and fabricate a PAM-g-PVA/PVP semi-interpenetrating polymer network (semi-IPN) hydrogels, and to elucidate its swelling kinetic behavior. They are expected to be found wide applications in specific biomedical fields like drug control release etc.
EXPERIMENTAL
Materials and Reagents
The acrylamide (AM), analytical grade (A.G), was supplied by the Shantou Xianhua Chemicals Factory, China. The PVA, provided by the Xi'an Chemicals & Glasses Factory (China), served as both a reactant and
a reducer, with an average degree of polymerization of 1750. Ceric ammonium nitrate (NH4)2Ce(NO3)6 (CAN) was obtained from the Shanghai Chemical Factory (China), which constructed a redox system as an oxidant, together with the PVA. The cross-linker, N,N-methylenebisacrylamide (BIS or NNMBA, A.G), was purchased by the Tianjin Kermol Chemical Regent Developing Center (China). The PVP, pharmaceutical grade, was from the Shanghai Welltone Material Technology Co., Ltd (China). All chemicals were used without further purification.
Preparation ofPAM-g-PVA/PVP Semi-IPN Hydrogels
PAM-g-PVA/PVP semi-IPN hydrogels were synthesized by using a free radical polymerization approach, and the synthesizing strategy was displayed in Scheme.
Scheme. A schematic reaction route demonstrating the synthesis of PAM-g-PVA/PVP semi-IPN hydrogel involved multi-reactions of grafting and crosslinking.
Typically, 0.2 g PVA was dissolved in 10 ml deion-ized water to form a viscous solution, and then 2 g AM monomer and a stoichiometric PVP (the mass ratios of PVP to PAM-g-PVA are 0, 9 and 23%) were added into the abovementioned PVA solution. After stirring at 50°C for about 10 min, 1 ml 0.2 mol l-1 CAN solution and 0.02 g 1 wt% NNMBA (the total weight of the monomer) were added into the reaction mixture with rapid stirring. The reaction proceeded at 50°C for about 5 h. The products were immersed in deionized water for 7 days to remove residues of the unreacted monomers, initiators and cross-linking agents, as well as the homopolymers that were not involved in the for-
mation of the semi-IPN structure. The water was changed twice a day during the 7-day immersion. The resultant hydrogels were dried in an oven at 35°C until constant weight.
Characterization and Measurements
The structural characterization of PAM-g-PVA/PVP hydrogels was carried out on an EQUINX55 Fourier transform infrared spectroscope (FTIR) manufactured by the Bruker Corp, Germany, using the method of the potassium bromide tableting. The morphological characterization of the samples
(A) (B)
Fig. 1. FTIR spectra of (a) AM monomer, (b) PVA polymer, (c) PAM-g-PVA, and (d) PAM-g-PVA/PVP hydrogel.
was conducted on a Quanta 200 scanning electron microscopy (SEM) produced by the Philips-FEI Corp, the Netherlands, with an operating voltage of 20 kV. To understand swelling of the PAM-g-PVA/PVP hydrogels, the dried hydrogels (with weight of W0) were immersed in an excess amount of deionized water at room temperature (25 °C) until swelling equilibrium was attained. The wet weight of the sample (Wt) was determined after removing the surface water by blotting with filter paper, and the equilibrium water content was designated (Wm). The swelling ratio (SR) and equilibrium degree of swelling (Qe) of the samples were calculated from the following equations:
Sr = (W - W.)/Wo
Qe = W - Wo)/W.
RESULTS AND DISCUSSION
Scheme shows the synthesizing strategy of PAM-g-PVA/PVP semi-IPN hydrogels, of which the PAM grafting reaction on PVA was carried out by the ceric ion induced grafting from PVA. According to the well-accepted mechanism [27-30], the rate-determining step of the oxidation is the unimolecular dispropor-tionation of the complex C to yield cerous ion, a proton, and a free radical on the alcohol substrate:
Ce4+ + RCH2OH —- Ce3+ + H+ + RC ■ HOH (1)
The carbon free radicals can initiate the polymerization of vinyl monomers. Many literatures reported the implementation of some of the graft reactions in aqueous medium initiated by ceric ammonium nitrate [31-33]. Since there is a small amount of 1,2-diols in the PVA structure, the carbon-carbon bonds between
the hydroxyls yield an aldehyde or ketone and a free radical:
OH OH OH O
-C-C-+ Ce4+-^ -C. +C-+ Ce3+ + H+ (2)
II II
The free radicals in both cases would supposedly be terminated by a fast oxidation by another ceric ion:
OH O
_O. + Ce4+ -^-C- + Ce3+ + H+ (3)
I
Therefore, the ceric ion induced grafting from PVA should be accompanied by slight degradation of the PVA backbone. The possible initiation sites along the PVA backbone are correlated with the competition for Ce4+ between 1,2-diols and isolated hydroxyls, as well as the mole percent 1,2-diol contents and the initial concentrations of Ce4+. Changing Ce4+ concentrations one can obtain products with a few block and numerous grafted segments. Based on the above description, the mechan
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