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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. CHARACTERIZATION OF POLY(AAm-IA) HYDROGELS
  6. RESULTS AND DISCUSSION
  7. CONCLUSION
  8. REFERENCES

A new kind of pH and temperature responsive poly(acrylamide-co-itaconic acid) hydrogel was prepared by free radical polymerization using ammonium persulfate as initiator and different comonomer ratios. The hydrogels were characterized in terms of chemical composition, swelling-deswelling behavior, morphology, crystallographic behavior, and drug release properties. All the hydrogels showed high swelling ability in aqueous solutions, the maximum being at pH 7. Swelling decreased on either side of pH 7 (i.e., both in acidic and alkaline region) and increased with increase in temperature. The hydrogel with 10 mol% itaconic acid (IA) absorbed maximum water among the copolymer gels. The cellular structures of the hydrogels were clearly revealed by microscopic analysis and SEM pictures. Swelling of the gels in water followed non-Fickian type of diffusion principle. The hydrogel was proved to be a controlled release vehicle, for example in drug delivery by using its smart properties. The hydrogel with 10 mol% IA also absorbed maximum amount of drug (ascorbic acid) under study. Incorporation of drug in hydrogel matrix was established from XRD peak analysis. POLYM. ENG. SCI., 55:113–122, 2015. © 2014 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. CHARACTERIZATION OF POLY(AAm-IA) HYDROGELS
  6. RESULTS AND DISCUSSION
  7. CONCLUSION
  8. REFERENCES

Hydrogels are three-dimensional polymeric crosslinked networks having a solid-like appearance that do not dissolve in water but can absorb large volume of water or aqueous solution. Hydrophilic groups present in the polymer chains assist the hydrogel to swell in water. However, the mechanical strength of the gel comes from its crosslinked network [1]. Stimuli-sensitive polymers can change their structure and physical properties in response to physical or chemical changes, such as pH [2, 3], temperature [4-7], ionic strength, electric field [8], magnetic field [9], concentration, etc. They are also called intelligent or smart polymers. These smart polymers have a wide range of applications in pharmaceutical technology and the biotechnology industry such as drug delivery [10, 11], artificial implants [12], contact lenses [13], and in solving environmental problems [14]. Among the smart polymers, pH and temperature responsive polymers are most frequently studied.

pH sensitive hydrogels usually swell due to ionic networks that contain acidic or basic pendant groups. When these groups are ionized, a swelling osmotic pressure inside the gel matrix is built up, and fixed charges are trapped in the gel. As a result of the electrostatic repulsion, the diffusion of solvent in the network is increased [15, 16]; although large number of ionised groups may cause deswelling of the gel due to extreme electrostatic repulsion of the like charges, sometimes leading to gel collapse. Within the human body also, different compartments have the different pH and it can therefore be used to direct the response to a certain tissue or cellular compartment (Table 1).

Table 1. pH in various tissues and cellular compartments [17, 18].
Tissue/cellular compartmentpH
Blood7.35–7.45
Stomach1.0–3.0
Duodenum4.8–8.2
Early endosome6.0–6.5
Late endosome5.0–6.0
Colon7.0–7.5
Lysosome4.5–5.0
Golgi6.4

Temperature-sensitive polymers or hydrogels exhibit a volume phase transition at a certain temperature, which causes a sudden change in the solvation state. This is due to the presence of hydrophilic-hydrophobic interaction in the polymer chain. Polymers, which become insoluble upon heating, have a lower critical solution temperature, for example poly(N-isopropyl acrylamide), and the systems, which become soluble upon heating, have an upper critical solution temperature. For the corresponding hydrogels, they are known as lower gel transition temperature or upper gel transition temperature, respectively. The combination of a thermo-responsive monomer with one of a pH-responsive monomer yields dual-responsive copolymers or hydrogels [19]. It is more useful than the monoresponsive one.

Hydrogels can also be made as electric field responsive material by using one or more conductive compound. These hydrogels found applications in many biomedical and technological applications such as artificial implants, contact lens, pharmaceutical, and biosensors. Recently, there have been a number of publications on electrically induced phenomena in charged polyelectrolyte networks. Tang et al. prepared polyacrylamide-graphite super absorbent composite hydrogel with conductivity 0.86 mS cm−1 [20]. Polyacrylamide/Cu composite hydrogel of conductivity 1.08 mS m−1 was synthesized by Lin et al. [21]. Polyacrylate/polyaniline and poly(2-acrylamido-2-methyl propylsulfonic acid-acrylic acid)/polyaniline conducting hydrogels with an interpenetrating polymer network structure was prepared by Lin et al [22].

In this study, an attempt has been made to synthesis a pH responsive and temperature responsive (both characteristic being present in one) smart hydrogel by using copolymers made from acrylamide (AAm) as nonionic monomer; and an anionic monomer; itaconic acid (IA). The gels have been studied in terms of its swelling behavior, microscopic study, and drug release behavior. The effects of pH on swelling and shrinking behaviors of the prepared hydrogels are studied. Release kinetics of the hydrogels has also been investigated. Structures of the monomers and crosslinker used are as in Scheme 1.

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Scheme 1. Structures of the monomers and the crosslinker.

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EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. CHARACTERIZATION OF POLY(AAm-IA) HYDROGELS
  6. RESULTS AND DISCUSSION
  7. CONCLUSION
  8. REFERENCES

Materials

AAm (Merck, Darmstadt, Germany), IA (Aldrich), ammonium persulfate (APS) were used as received. N,N′-methylenebisacrylamide (MBA) was used without further purification. Buffer capsules were used as supplied by Merck. Distilled water was used to synthesis gels and for swelling studies.

Preparation of Gels

All the polymerization reactions were carried out in glass tubes. In all the cases AAm, itaconic acid, MBA and APS were dissolved in water separately, and then mixed. The total monomer concentration was kept constant in each case. Then the solution was kept at 70°C for 8 h under constant stirring. The details of the monomers taken are given in Table 2.

Table 2. Composition of the hydrogels.
HydrogelAAm (mol%)IA (mol%)MBA (mol%)APS (mol%)
AI0100020.1
AI595520.1
AI10901020.1
AI20802020.1
AI30703020.1

CHARACTERIZATION OF POLY(AAm-IA) HYDROGELS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. CHARACTERIZATION OF POLY(AAm-IA) HYDROGELS
  6. RESULTS AND DISCUSSION
  7. CONCLUSION
  8. REFERENCES

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectra of the dried gel samples were recorded by making KBr pellets on a Perkin–Elmer BX II spectrophotometer. The FTIR spectra were recorded in the range of 4000–400 cm−1. The FTIR spectra of the copolymer gels are given in Fig. 1.

Swelling Measurement

Dried hydrogel samples were allowed to swell in distilled water at different temperatures and in different pH solutions. The swelled gels were taken out at certain time intervals, wiped on its surface with tissue paper, and then weighed and the process is repeated until the constant weight was reached.Equilibrium swelling was measured by the equation:

  • display math(1)

where Ws is the weight of the swollen gel and Wd is the weight of the dry gel.

Deswelling Measurement

The gel samples were allowed to swell in distilled water at room temperature (27°C) and then dropped in pH 4 medium to deswell (as all the hydrogels under study swell in pH 4 medium in much lesser amount compared to that in pH 6.5). At regular time intervals, gel samples were taken out, wiped with tissue paper on its surface, and weighed again.

Water retention was measured by the equation:

  • display math(2)

where Wt is the weight of the gel sample at time t, Wd is the weight of the dry gel, and W is the weight of the water absorbed by the gel in fully swollen condition.

Drug Encapsulation and Release Study

Previously weighed dry hydrogels were loaded with ascorbic acid by immersing them in an aqueous solution of a drug (ascorbic acid in this case) until equilibrium was reached. Swelling was carried out by keeping the electrode of a pH meter immersed in it. Amount of ascorbic acid uptake was determined from the decrease of conductivity of the solution using Conductivity vs. ascorbic acid concentration calibration curve, which was obtained using standard ascorbic acid solutions at 27°C.

The loaded hydrogels were then dried at room temperature for 3–4 days and weighed to obtain the concentration of drug in the hydrogels. To study the drug delivery kinetics, the loaded hydrogels were immersed in 25 ml of distilled water at 27°C, which were continuously stirred. To follow the delivery kinetics, the conductivity of the solution was measured at certain time interval. The amount of drug released at any given time was obtained from the calibration curve.

Optical Microscopic Studies

The internal or cross-sectional morphology of the swollen gels were determined by using optical microscope, Kruss, A. Kruss Optronic, Germany, at different magnification. The dried gels were first swollen to equilibrium and then thin strips of the gel sample were placed under microscope.

Scanning Electron Microscope (SEM)

The SEM of the dried gels were taken by using SEM apparatus, S-3400N Hitachi, Japan. The samples were gold coated with coating time of 45 s using gold coater Hitachi E1010.

X-ray Diffraction (XRD)

Identification of crystalline planes in itaconic acid, pure hydrogel and drug loaded hydrogel were investigated by wide angle x-ray scattering study by using XPERT-PRO Diffractometer system, Goniometer PW3050/60 (Theta/Theta; XRD PANalytical, Netherland). Studies were undertaken from 5° to 70° 2Theta using continuous scan with 0.0330° 2Theta step size.

Differential Scanning Calorimetry (DSC)

The DSC studies were performed on a Perkin–Elmer DSC 7. The samples were heated from 50 to 300°C, with a heating rate of 20°C per min in an inert condition.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. CHARACTERIZATION OF POLY(AAm-IA) HYDROGELS
  6. RESULTS AND DISCUSSION
  7. CONCLUSION
  8. REFERENCES

FTIR Spectra of the Gel

The FTIR analysis of the different gel samples (Fig. 1) showed the presence of peaks corresponding to the functional groups of the monomeric units used in preparing the copolymeric gels. The characteristic absorption peaks of AAm and itaconic acid units appear at their usual wave numbers. The peaks at 1650 and 1710 cm−1 are assumed to be due to the C[DOUBLE BOND]O group of AAm and itaconic acid part respectively [23]. The peak corresponding to O[BOND]H bond of [BOND]COOH group of itaconic acid part was observed around 2929 cm−1 and N[BOND]H peak of AAm part was observed at around 3445 cm−1. The broad nature of N[BOND]H peak reveals that the H-atom is involved in making hydrogen bond. The absence of peak at 1610–1640 cm−1 (for conjugated C[DOUBLE BOND]C) and at 3000–3100 cm−1 (for [DOUBLE BOND]C[BOND]H) ensures complete polymerization with the formation of gels of copolymers and/or blends of two homopolymers [24]. The above IR analysis indicates the presence of all monomeric units in the crosslinked network, and as such no functional groups are lost during crosslinking.

Swelling Properties of the Gels

Swelling in Water

Poly(AAm-co-IA) hydrogels absorb large amount of water compared to the conventional polyacrylamide hydrogel. On increasing the itaconic acid content in the copolymer network swelling increases. In case of hydrogel with 10% itaconic acid (sample AI10) swelling is maximum. The % swelling then falls with further increase in itaconic acid content (Fig. 2). Introduction of polar [BOND]COOH group in the polymer network increases the hydrophilicity of the gel. Carboxylic acid ([BOND]COOH) group of the itaconic acid part ionizes partially in contact with water to form [BOND]COO ion, which attracts the polar water molecules in the crosslinked network and thus imparting development in swelling properties of the gel [25]. The behavior of the two [BOND]COOH group present in the IA is not identical; the primary one being the most reactive readily ionizes whereas the secondary [BOND]COOH group undergoes dissociation rather slowly. However under any circumstances the dissociation of the [BOND]COOH group is never completed. Beside, there is a probability of formation of inter molecular H-bonding between the carbonyl oxygen atom of [BOND]CONH2 (amide) group of the AAm unit with the hydrogen atom of the [BOND]COOH (carboxylic acid) functionality of the itaconic acid unit, which results in cage like structure that can hold large amount of water in it (Scheme 2). With increase in content of IA beyond 10% the surface area available for cage formation due to crosslinking of the poly AAm chains (the content of which decreases with increase in IA) also decreases. Consequently, the volume accessible for swelling gradually diminishes.

image

Scheme 2. Structure of the hydrogen bonded crosslinked network developed in the gel matrix.

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It is assumed that with increase in IA content during the copolymer formation the statistical probability of intramolecular and intermolecular crosslinking amongst the IA moieties and between the IA and poly AAm increases. As a result of which the cage structure undergoes a substantial reduction in sizes. The reduction in swelling with increasing IA beyond 10% may possibly be explained by the following schematic representation.

pH Dependent Swelling of the Gel

To determine the pH sensitivity of the poly(AAm-IA) hydrogels, the dried hydrogel samples were allowed to swell to equilibrium in buffer solutions (by using standard buffer capsules) having different pH at room temperature (25°C). Equilibrium swelling of poly AAm hydrogel is independent on pH. However, equilibrium swelling of all the poly(AAm-IA) hydrogels were strongly dependent on the pH of the medium as revealed in Fig. 3a. Swelling of hydrogel first increases with increasing pH upto pH 7, then it falls on further increase of pH of the medium. That is the hydrogels swell to its maximum at pH 7 solution in all the cases. Itaconic acid being a weak acid the [BOND]COOH groups introduced in polymer backbone ionizes little forming [BOND]COO (carboxylate) ions. These carboxylate ions attract the polar water molecules to enter into the crosslinked network. The cage structure formed by the intermolecular H-bonding between carbonyl group of the amide functionality with the acid functionality of the itaconic acid unit can hold the large amount of water molecules. Ionization of the [BOND]COOH group is arrested in acidic medium (pH < 7) as the medium contains excess H+ ions. Even amide group may undergo protonation in this medium, which results on the whole breakage of H-bonding structure and decrease in equilibrium swelling. In basic medium ionization of the acid group increases with the formation of large amount of [BOND]COO ions, which repel each other resulting disruption of cage structure and decrease in swelling.

image

Figure 1. FTIR spectra of copolymer gels (AI0-AI30).

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image

Figure 2. Variation of swelling with itaconic acid content in the crosslinked network.

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image

Figure 3. (a) Swelling of the copolymer gels in different pH medium (AI0-AI30) and (b) Swelling of the gels at different temperatures (AI0-AI30). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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It can be assumed that at the high acid concentration the ionization is somewhat arrested. The unionized –COOH group, which may remain in equilibrium with the ionized [BOND]COO group may form a sheathing layer or screen through which the diffusion of water molecule towards the cage structure is inhibited to a great extent.

Temperature Dependent Swelling of the Gel

It is quite apparent that like the insensitive behavior exhibited by virgin poly AAm gel in different pH media the said gel is also almost insensitive to changes in temperature. The copolymer gels are however very much sensitive to changes in temperature as can be seen from the Fig. 3b. Here also we can find that the copolymer gel with IA content of 10 mol% gives the highest swelling at all temperatures under study. Moreover, this gel is observed to undergo a drastic jump in swelling efficiency.

To determine the temperature sensitivity of the poly(AAm-IA) gel, the samples were subjected to swell in distilled water, kept at different temperatures. Swelling of the gel increases with temperature in all the cases (Fig 4). This can be explained by considering the effect of temperature on the ionization of weak acids. According to the equation:

  • display math(3)

where ΔG0 is standard Gibbs free energy change, R the universal gas constant, T the temperature (kelvin), and Ka the ionization constant. With the increase in temperature the ionization constant increases, that is, more [BOND]COOH group ionizes to form [BOND]COO (carboxylate) ions, which drag larger number of water molecules in network causing higher rate of diffusion at elevated temperatures.

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Figure 4. (a) Extent of swelling of the gels (AI0-AI30) with time and (b) Swelling kinetics plot according to Fick's law equation. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Swelling Kinetics

The rate of swelling is very fast initially for all the poly(AAm-IA) gels, then it slows down with time and reaches equilibrium (Fig. 4a). The rate and extent of swelling is highest in case of hydrogel AI10. To determine the nature of water diffusion into hydrogels, the equilibrium swelling data were fitted to the following exponential equation [26, 27]:

  • display math(4)

where, F is the fraction of water uptake at time t, inline image, and inline image represent the weight of the swelled hydrogels at time t and at equilibrium respectively, k is a characteristic constant of the hydrogel, and n is the diffusional exponent, which is indicative of the transport mechanism; n = 0.5, represents a Fickian diffusion, when the rate of diffusion is much slower than the rate of relaxation. In case of n = 1, it is case II transport, the diffusion is very fast, contrary to the rate of relaxation, and the third case corresponds to an anomalous diffusion (or non-Fickian diffusion) with n values lying between 0.5 and 1. A representative plot of log t versus log F is shown in the Fig. 4b for all the samples. The n exponents are calculated from slopes of the lines and are given in Table 3. In case of poly AAm gel (sample AI0), n is close to 0.5 though they are not exactly equal to 0.5. It can be said that the process is quasi-Fickian diffusion with partly anomalous behavior, that is, chain relaxation controlled process. However, the swelling of copolymer gels (samples AI5-AI30) is anomalous having n value between 0.5 and 1. The strong interchain interactions via the formation of intermolecular H-bonding leading to a compact structure may play a significant role in the anomalous diffusion behavior.

Table 3. Diffusion constant (n) values of the swelling process of poly (AAm-IA) gels.
Hydrogeln-ValueType of swelling
AI00.49Quasi-Fickian diffusion
AI50.73Non-Fickian diffusion
AI100.85Non-Fickian diffusion
AI200.698Non-Fickian diffusion
AI300.563Non-Fickian diffusion
Swelling in Salt Solutions

The effect of salt concentration on the swelling capacity of the hydrogel samples was examined with sodium chloride solution (Fig. 5). Swelling of all the hydrogels in salt solutions are lower compared with that of pure water. This is because, the swelling of polyelectrolyte hydrogels ismainly due to the electrostatic interactions among the charge centers present on the polymer chain, so the extent of swelling is influenced by any change in physical parameters, such as pH, ionic strength, and type of counter-ions present in the swelling medium, which may alter these electrostatic interaction [28]. Thus, with the increase in concentration of counter-ions, that is, salt concentration in the external media, the osmotic swelling pressure responsible for swelling decreases, which ultimately results in a decrease in the water uptake of the hydrogel samples. This is supported by the fact that the swelling of the hydrogel AI0 with no itaconic acid is not at all affected by salt concentration.

image

Figure 5. Swelling of the copolymer gels (AI0-AI30) in NaCl salt solutions of different strengths. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Deswelling Kinetics of the Gel with pH Changes

Deswelling of the gel was studied by using its pH sensitive behavior. The hydrogel samples were first allowed to swell to reach equilibrium in pH 8 solutions and subsequently dropped in medium of pH 4 to deswell. The nature of water retention kinetics was more or less same for all the cases as shown in Fig. 6 irrespective of compositions of the copolymer gels.

image

Figure 6. Deswelling of the swelled copolymer gels (AI5-AI30) with time. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Drug Encapsulation and Release Study

We are choosing ascorbic acid as a representative of water soluble molecule to characterize the effectivity of the delivery vehicle. Ascorbic acid or vitamin C is a weak acid. The aqueous solution of ascorbic acid is conductive of electricity and its conductivity value is linearly related with its concentration.

During swelling of poly(AAm-IA) hydrogel in aqueous ascorbic acid solution the polar acid molecule along with water enters into the crosslinked cage like structure formed by the interchain H-bonding in copolymer gel structure and gets trapped by electrostatic attraction of the polar [BOND]COOH and [BOND]COO group of the itaconic acid unit. With increase of itaconic acid content in the polymer network, the polar environment in the network as well as possibility of formation of H-bonded cage structure increases. This is evidenced by the increase in ascorbic acid uptake by the gel with increase in itaconic acid content and gets maxima for sample AI10 (Fig. 7a). Further increase of itaconic acid content, reduces the size of the cage structure resulting lesser amount of ascorbic acid uptake.

image

Figure 7. (a) Absorbtion of ascorbic acid by copolymer gels (AI5-AI30) with time and (b) Release kinetics of ascorbic acid by the copolymer gels (AI5-AI30). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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This is again reflected in ascorbic acid release behavior. Rate and extent of release is slowest in case of gel AI10, that is, hydrogel with 10% itaconic acid (Fig. 7b).

Microscopic Study

Figure 8 shows the porous cross sectional structure of the gel AI0, AI5, AI10, and AI20 in swollen condition. In Fig. 8a, some surface porosity is seen, but these pores are considerably smaller at the same magnification (10×). With the incorporation of IA in the crosslinked network, the cellular structure becomes more prominent and ordered. There seems an orientation and elongation of voids at the surfacein case of gel AI10. Figure 10c indicating the cellular structure of AI10 shows that many of the cells remain no longer nonintercommunicating and sometimes they reveal an intercommunicating type of cellular structure. Moreover, the large proportion of the cellular body in the swelled condition appears to be tattered and sometimes the polymers walls contain a large number of minute cells and thus enabling it to absorb maximum amount of water.

image

Figure 8. Optical microscopic pictures of the gel (a) AI0, (b) AI5, (c) AI10, and (d) AI20. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Scanning Electron Microscope

Figure 9 shows the SEM pictures of the gel AI5, AI10, and AI20. The SEM picture (Fig. 9d) of the gel AI5 (i.e., copolymer gel with 5% IA) is almost a reflection of its microscopic picture as given in Fig. 8b. The bulk of the surface of the particular copolymer gel contains swollen blisters distributed almost throughout the network. On increasing the proportion of IA in the copolymer, the number of cells and the shape of the cells also undergo a tremendous change. The blister sides appear to be substituted by alarge number of holes and pit marks distributed throughout the surface in an irregular manner (Fig. 9b). Thus, this gel appears to be capable of swelling to a much larger extent than the previous one. In the SEM image of AI20, we can find a blend of blisters and closed swelled holes distributed in a random manner (Fig. 9c). The sizes of both undergo a decrease remarkably.

image

Figure 9. SEM image of hydrogels (a) AI5, (b) AI10, (c) AI20, and cry SEM images of hydrogels (d) AI5 and (e) AI10. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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From the cryo SEM (Fig. 9d and e), the reticulum of the cell structure, which consists of random combinations of cells of different sizes is being distinctly observed. Moreover, the gel with 15% of IA exhibits cells having greater depth than those of the gel with 10% of IA.

X-Ray Diffraction

Ascorbic acid is a crystalline organic compound. When bound to the copolymer, the crystalline structure will disappear because ascorbic acid molecule gets entrapped into the hydrophilic cage structure formed by the intermolecular H-bonding between the amide group and the carboxylic acid group. This has been confirmed by the wide angle X-ray diffraction (WAXD). Figure 10 shows the WAXD spectrum of pure ascorbic acid, dried hydrogel sample and the dried hydrogel containing ascorbic acid. The sharp peaks in the spectrum of pure IA indicate its crystalline structure. No peaks were visible in the WAXD spectrum of the dried hydrogel sample containing ascorbic acid. This result suggests that the ascorbic acid is absorbed by the hydrogel.

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Figure 10. XRD pictures of the samples (a) ascorbic acid, (b) pure hydrogel AI10, (c) hydrogel AI10 imbibed with vitamin C, and (d) DSC thermogram of gel AI10.

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Differential Scanning Calorimetry

The poly AAm gel has glass transition temperature Tg in the region 179–190°C [29]. As copolymerization is said to be a process of internal plasticization, it leads to a drustic reduction in Tg. This phenomenon has been observed with the gel discussed in this work. The modification of poly AAm with itaconic acid has reduced the Tg to about 118°C (Fig. 10c) due to the incorporation, which prove copolymer formation during the formation of the gel.

Evidences on Copolymerization

  • (a) Itaconic acid is a monomer, which does not undergo homopolymerization [24]. It seems that the reactivity of the radical formed from the monomer towards its own monomer is far less than the reactivity with the other olefinic monomers. This leads to the possibility of complete copolymer formation with remote possibility of any homopolymer.
  • (b) The possibility of entrapment of monomeric itaconic acid with the network of poly AAm can be considered negligible as itaconic acid is highly soluble in water and the complete removal of the acid monomer has been ensured through repeated washing of the gel.
  • (c) The % swelling ability of the gel under study has been remarkably enhanced with reference to the neat poly AAm gel. This great enhancement can only be explained by a suitable copolymer formation.
  • (d) The poly AAm gel is neither pH sensitive nor temperature sensitive. The copolymer gel under investigation is however strongly pH sensitive as has been discussed in our present manuscript.
  • (e) The poly AAm gel has glass transition temperature Tg in the region 179–190°C [29]. As copolymerization is said to be a process of internal plasticization it leads to a drustic reduction in Tg. This phenomenon has been observed with the gel discussed in this work. The modification of poly AAm with itaconic acid has reduced the Tg to about 118°C due to the incorporation.

These substantially prove copolymer formation.

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. CHARACTERIZATION OF POLY(AAm-IA) HYDROGELS
  6. RESULTS AND DISCUSSION
  7. CONCLUSION
  8. REFERENCES

pH responsive and temperature sensitive poly(AAm-IA) hydrogels were prepared successfully by free radical polymerization using APS as initiator and MBA as crosslinker. Swelling of the gel increased with increasing itaconic acid content in the polymer network upto 10 mol%. Swelling of the hydrogel was strongly pH dependent as it contains an anionic monomer. Swelling was increased with pH up to pH 7 and then it falls. Therefore, the gel can be used as controlled release vehicle and pH responsive switches by using its smart properties.

ABBREVIATIONS
AAm

Acrylamide

IA

Itaconic acid

MBA

N,N′-Methylenebisacrylamide

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. CHARACTERIZATION OF POLY(AAm-IA) HYDROGELS
  6. RESULTS AND DISCUSSION
  7. CONCLUSION
  8. REFERENCES