CROSSLINKED POLYACRYLAMIDE TEMPERATURE-SENSITIVE HYDROGEL. SYNTHESIS AND CHARACTERIZATION

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1 CROSSLINKED POLYACRYLAMIDE TEMPERATURE-SENSITIVE HYDROGEL. SYNTHESIS AND CHARACTERIZATION I. Neamtu, A.P. Chiriac, L.E.Nita, O.Paduraru, M.C.Popescu, C.Grigoras Petru Poni Institute of Macromolecular Chemistry Aleea Grigore Ghica Voda no. 41-A, Iasi Romania Abstract: A polyacrylamide hydrogel having a thermoresponsive character was prepared by simultaneous crosslinking solution copolymerization method. Different thermoresponsive properties were obtained by changing the initial ratio of acrylamide and N,N -methylenebisacrylamide as crosslinker. The swelling behaviour in water until equilibrium was evaluated for all the hydrogel systems. Keywords: temperature-sensitive gels, polyacrylamide hydrogel, crosslinking copolymerization, magnetic field influence, swelling behaviour 1. INTRODUCTION Polymer gels belong to the privileged class of the most actively studied polymer systems. The main immediate motivations for the most of gel studies are various technological applications [1-9]. Hydrogels are hydrophilic, three-dimensional polymer networks that are capable of imbibing large quantities of water. The integrity of the hydrogel network is maintained by either physical or chemical crosslinks. Physical crosslinks include chain entanglements, crystallites and weak associations such as hydrogen bonding or van der Waals forces. Hydrogels can also be formed by chemical crosslinking of bifunctional monomers using trifunctional or tetrafunctional crosslinker molecules [10 17]. Numerous studies have been conducted on the stimuli-sensitive hydrogels that are able to respond to the changes in environmental conditions such as ph, temperature, ionic strength, and electromagnetic field. Due to their drastic swelling and syneresis in response to environmental stimuli, these polymeric hydrogels have been investigated for many biomedical and pharmaceutical applications, including carriers for controlled drug delivery, membranes for molecular separation, tissue culture substrates, and systems to control enzymatic activity. Among these intelligent polymers, temperature and ph-sensitive hydrogels are the most widely investigated. Work has also been done to combine the ph and temperature sensitivities by copolymerizing two monomers or forming interpenetrating polymer networks (IPNs) [18-19]. Temperature-sensitive hydrogels exhibit reversible volume phase transition upon decreasing/increasing of the temperature in aqueous solutions, i.e. reversible swelling and

2 deswelling below and above a lower critical solution temperature (LCST), respectively. At low temperatures the polymer shows full-hydrated and extended conformation. Over LCST it extensively dehydrates and changes to compact chain conformation. Such phase transitions is accompanied by the changes in hydrophilic/hydrophobic properties: the gel is hydrophilic in character below LCST, but becomes hydrophobic as the temperature rises above LCST. Hydrogels can be prepared by simultaneous copolymerization and crosslinking of one or more monofunctional and one multifunctional monomer or by crosslinking of a homopolymer or copolymer in solution. The latter involves two steps. In the first step, the linear polymer is synthesizeded in the absence of a crosslinking agent, and in the second step, the synthesized polymer is crosslinked using either chemical reagents or irradiation. In recent years, considerable research has been done on the characterization and swelling behaviour [20-26] of hydrogels prepared by simultaneous free radical copolymerization and crosslinking in the presence of an initiator and a crosslinking agent. The gelation of acrylamide based hydrogels by free radical polymerization has been reported in many studies. Acrylamide can be polymerized easily, its polymer and copolymer derivates are biocompatible and the most preferred members in the field of hydrogels [27-30]. The swelling properties of hydrogels will affect its usability as a biomaterial in medicine, pharmacy and veterinary practice. There are a wide variety of biomaterials used in contact with biological fluids. Since the biologic environment is mainly composed of water, the wettability and sorption are two important properties of biomaterials. In particular, low or high water sorption properties of hydrogels may be important for the use of sorbents in many immobilization techniques for biomolecules and cells and in many applications of biomaterials in biotechnology. The aim of this study was to investigate the synthesis of polyacrylamide hydrogels with the addition of a multifunctional crosslinker, such as N,N -methylenebisacrylamide, suitable for the immobilization of bioactive substances or as material for cold therapy packs. It was followed the influence of the presence during the reaction of a magnetic field with the intensity of 1100 Gs. The effect of the content of N,N -methylenebisacrylamide on the swelling behaviour of prepared hydrogel systems was examined, too. 2. EXPERIMENTAL Acrylamide (Merck, Darmstadt, Germany) (AAm) as monomer, N, N'-methylenebisacrylamide (Sigma, St. Louis, US) (Bis) as crosslinker, potassium peroxodisulphate (Merck, Darmstadt, Germany) (KPS) as initiator, ascorbic acid (Sigma, St. Louis, US)(AAsc) as catalyst, were analytical reagent grade and used as received. The polyacrylamide gels were prepared by redox solution copolymerization of acrylamide and N,N -methylenebisacrylamide at a concentration of monomers during gel preparation of 6 wt%. in distilled water. The reaction was conducted at room temperature (+20 o C) for 24 hrs in glass ampoules with 12 mm diameter and 130 mm length, as polymerization reactors. The nonconventional method was realized by placing the ampoules between the poles of a magnet of 1100 Gs. Feed composition and the reaction parameters are shown in Table 1. A typical procedure for the polymerization can be described as follows: acrylamide (AAm) and N,N -methylenebisacrylamide (Bis) were dissolved in distilled water. Potassium peroxodisulphate (KPS) and ascorbic acid (AAsc) solutions were added to the monomers solution mixture, respectively. The polymerization medium was purged in the glass ampoules and the polymerization was performed. Fresh hydrogels obtained in long cylindrical shapes were washed and thoroughly rinsed with large amounts of distilled water to remove any unreacted monomers and initiators

3 physically entrapped within the polymer gel, blot dried with filter paper, dried in air and vacuum, and stored for analyses and swelling studies. Table 1. Synthesis of polyacrylamide hydrogels Sample 1 I 2 II 3 III 4 IV 5 V 6 VI Reaction parameters AAm / Bis 0.56/ / / / / / Feeding molar ratio KPS a), M x10-3 AAsc b), M x10-3 Temperature, o C Time, hrs Classical method x) x - x - x - x - x - x - Nonconventional method xx) - xx - xx - xx - xx - xx - xx a) moles, with respect to monomer concentration b) moles, with respect to monomer concentration x) - classical method xx) in the presence of a magnetic field of 1100 Gs The polymer composition was characterized using Fourier Transform Infrared Spectroscopy (FTIR) experiment [31] on a spectrophotometer DIGILAB Scimitar Series- USA, with 4 cm -1 resolution; it was kept constant the amount of polymer (5 mg) in the KBr tablet (500 mg). The glass transition temperature T g was recorded on a Mettler DSC-12E Instrument at a heating rate of 15 C/min under inert atmosphere of N 2, and C temperature range, under dynamic conditions. The dynamic swelling behaviour of dried hydrogels was folllowed by immersion in distilled water at 20 0 C in a water bath. The water absorbed was determined by weighing the samples, at various time intervals, after wiping with filter paper. Swollen gels were weighed by a precision electronic balance (A&D Co. Ltd. HR 200). 3. RESULTS AND DISCUSSION 3.1.Synthesis of hydrogels In this study, six hydrogels were prepared by redox polymerization in aqueous solutions of acrylamide and different amounts of crosslinker N, N'-methylenebisacrylamide. The reaction was accomplished comparatively, classical and in the presence of a magnetic field of 1100 Gs. In the polymerization, the polymer can continue growing indeffinitely, with the active center being continually shifted to the free end of the chain. Crosslinker molecules can be incorporated into chains simultaneously and form a permanent link between them. Polymerization and crosslinking process has been taken 3-4 hours in AAm gelation in the studied conditions, but for all hydrogel systems, it has been waited for 24 hrs for good gelation. The crosslinked copolymers are colourless and transparent. They are soft and elastic, with a slippery or slimly surface. For a polymerization process, the magnetic field effects are mainly observed during the initiation step. Radical pairs, generated through known methods, have a spin correlation according to the multiplicity of their direct precursors. The modification of the electronic spin

4 alignment to the triplet state as a result of the magnetic field presence, determines the decreasing of the radicals recombination processes. This phenomenon of micellar supercage effect was evidenced and mentioned by Turro [32]. Studies on the polymerization reactions made in a continuous external magnetic field specify a lessening of the activation energy thinking up that through the application of an external magnetic field the entropy is modified [33]. As a consequence, the reactions performed in a magnetic field record significant increases in rate and conversion. Beside the modifications brought to the evolution of the polymerization process, the magnetic field influences the properties of the resulting polymers obtained through nonconventional method [34]. FTIR spectra (Fig. 1 a,b) of investigated samples 1 and I (Table 1) show that in the chosen conditions, there are presented the three principal absorbtions in the three regions of spectrum for the primary amides: (owed to symmetrical and asymmetrical valence vibrations of the N-H links in NH 2 groups), (the main contribution for the characteristic bands belongs to the valence vibration of C=O link and to the deformation vibration of N-H link) and cm -1 (owed to a C-N vibration). Hydrogen bonds formation between C=O and N-H groups provokes great displacements of bands to the lower frequencies. 0, Wavenumber (cm -1 ) a I 1 absorbance I - absorbance 1 0,15 0,10 0,05 0,00-0, wavenumber (cm -1 ) Figure 1. FTIR spectra of polyacrylamide crosslinked gel (AAm/Bis molar ratio 0.56/0.0065): a-synthesized by classical (1) and nonconventional (I) method; b-difference spectrum between the two methods It is underlined the increase of the characteristic bands intensity in the spectrum of the sample synthesized in the presence of the magnetic field (figure 1 b). Thus, it is evidenced the relation: absorbance I absorbance 1 > Dynamic swelling study of the hydrogels The extent to which a gel can swell is limited by the presence of crosslinks. As can be expected, the swelling is decreased with increasing of the crosslinking component content. Generally, when crosslinker is added to hydrogel systems, its molecules are placed between the chains of monomer. Then, the hydrophilic groups number is decreased and swelling ratio is decreased. A fundamental relationship exists between the swelling of a polymer in a solvent and the natures of the polymer and the solvent. The percentage swelling (%S) was calculated from the following equation (1) [23]: b

5 M t M o % S = 100 (1) M o where M t is the mass of the swollen gel at time t and M o is the mass of the dry gel at time 0. The water intake of initially dry hydrogels was followed for 120 hours, until equilibrium. Swelling isotherms of PAAm hydrogel systems by using Bis crosslinker are plotted in Figure 2 a,b. Swelling degree, % Classical synthesis of PAAm crosslinked gel Time, hours a Swelling degree, % Nonconventional synthesis in a magnetic field of PAAm crosslinked gel Time, hours b V II IV VI I III Figure 2. Swelling degree of the PAAm hydrogel samples (in Table 1) obtained by classical (a) and nonconventional (b) method It can be seen that percentage swelling increases with time until a certain point, when it becomes constant. This value of percentage swelling may be named equilibrium (percentage) swelling. The values of equilibrium (percentage) swelling of PAAm hydrogels are used for the calculation of equilibrium water content (EWC) - the water absorbed by hydrogel - quantitatively represented by the relation (2) [23]: EWC M M t o = (2) M t where M t is the mass of the swollen gel at time t (equilibrium) and M o is the mass of the dry gel at time 0. The values of EWC are given in Table 2. Table 2. EWC and T g for the PAAm hydrogels EWC T g, o C Sample Classical Nonconventional Classical Nonconventional PAAm Hydrogel 1/I 0,909 0, Hydrogel 2/II 0,916 0, Hydrogel 3/III 0,942 0, Hydrogel 4/IV 0,958 0, Hydrogel 5/V 0,959 0, Hydrogel 6/VI 0,972 0,

6 The values of EWC for PAAm hydrogels are in direct relation with the Bis content and the synthesis method. Thus, the absorbed water increases with the reducing of the crosslinker amount and for all the samples prepared in the presence of the magnetic field. The preordered structure of the hydrogel due to the presence of the magnetic field makes the easier access of water in the network than in a disorderly structure with the random disposal of loops. At the same time, all EWC values of the hydrogels (~0.9) are greater than the percent water content values of the human body (about 0.6). Thus, the PAAm hydrogels were exhibit similarity of the fluid contents with those of living tissues. It can be said that PAAm hydrogel systems crosslinked by Bis, with the condition of purity and medical agreement, would be used as a biomaterial in medicine, pharmacy, veterinary and other applications Differential Scanning Calorimetry From the DSC thermograms, the glass-transition temperature Tg given in Table 2, related to a variation in the heat capacity, is influenced by the synthesis method and the amount of crosslinker. The magnetic field presence causes the increase of Tg, evidenced for polyacrylamide samples (from 142 to 157 o C) and are according to the literature data [34]. At the same time, the inclusions of small molecular compounds in the gel network, due to their plastifying effect between the macromolecular chains, determine the decrease of T g approximately with 24% for gels realized by classical method and with 19% for those by nonconventional method. CONCLUSIONS PAAm hydrogels were prepared by redox crosslinking aqueous solution polymerization of acrylamide with N,N -methylenebisacrylamide as crosslinker at different monomer/ crosslinker ratios, at ambient temperature. The hydrogels were swollen to equilibrium in water and equilibrium swelling data were used for the determination of equilibrium water contents. These results showed that polyacrylamide hydrogels could be used as water retainer for carrying some substances in aquatic fields in pharmaceutical, agricultural, environmental and biomedical applications, or for immobilization of biologically active molecules. This work is supported by the Romanian Ministry of Education and Research through INVENT project 2507/2004. REFERENCES 1. F.Ganachaud, M.J.Monteiro, R.G. Gilbert, M.A.Dourges, S.H. Thang, E.Rizzardo, Macromolecules 2000, 33, M.Miura, C.A.Cole, N.Monji, A.S.Hoffman, J.Biomater.Sci. Polym.Ed. 1994,5, Y.Okahata, H.Noguchi, T.Seti, Macromolecules 1986,19, A.Kaschewsky, E.D.Rekai, E.Wischerhoff, Macromol.Chem.Phys. 2001,202, E.Wischerhoff, E.Zacher, A.Laschewsky, E.D.Rekai, Angew.Chem. Int.Ed. 2000,39, H.Iwata, I.Hirata, Y.Ikada, Langmuir 1997, 13, M.V.Sussman, A.Katchalsky, Science 1970, 167,

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