Synthesis Of Tough And Elastic Graphite Biology Essay

Published:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

ABSTRACT:

The experiment aimed at synthesizing graphite oxide/polyAA hydrogels which possess excellent mechanical strength and elasticity. The gelation was carried out by free radical polymerization using the redox initiators, ceric sulphate and the potassium persulfate-N-[3-Dimethylamino) propyl]-methacrylamide couple. The hydrogels were tested for their mechanical strength with the aid of the Zwick Roell mechanical tester. The Young's modulus was obtained by such a test which facilitated the determination of mechanical strength. Also, the swelling ratios of the hydrogels were calculated in order to understand their swelling behaviour. Results indicated that the mechanical strength of the graphite oxide/polyacrylamide hydrogels, made using ceric sulphate, was dependent on the concentration of graphite oxide and acrylamide. Increase in concentration of the graphite oxide by two times resulted in a more than 4-fold increase in the elasticity of the hydrogels. The swelling ratios of these hydrogels were found to be inversely proportional to the acrylamide concentration. The elasticity of the graphite oxide/polyacrylamide hydrogels, made using potassium persulfate was higher in comparison to the hydrogels obtained by ceric sulphate initiator. This makes potassium persulfate, a better initiating agent for the polymerization process. Nevertheless, the experiment demonstrates a novel redox initiating system comprising of ceric ion (Ce+4) and graphite oxide. The development of this unique redox initiating system has given new insights to the field of polymer chemistry. It is postulated that the obtained hydrogels can find potential applications in the field of drug delivery and regenerative medicine, by further characterization.

TABLE OF CONTENTS

Acknowledgements……………………………………………………………………………………..2

Abstract……………………………………………………………………………………………………...3

List of Abbreviations…………………………………………………………………………………..5

Introduction……………………………………………………………………………………….6

Materials and Methods……………………………………………………………………..16

Results………………………………………………………………………………………………23

Discussion…………………………………………………………………………………………35

Conclusion…………………………………………………………………………………………42

References…………………………………………………………………………………………………43

LIST OF ABBREVIATIONS

Graphite oxide: GO

Acrylamide: AA

Cerium ammonium sulphate: CAS

Ceric sulphate: CS

Potassium persulphate: PP

SECTION 1: INTRODUCTION

Graphene

Graphene is an excellent nanomaterial consisting of a single layer of sp2-bonded carbon atoms in a two-dimensional lattice representing a honeycomb (Potts et al., 2011). It has been acclaimed as the 'miracle material of 21st century' and the 'next big thing' by the media, since its discovery and its pioneers, Andre Geim and Konstantin Novoselov from the University of Manchester, were awarded the Nobel Prize in Physics in the year 2010. They extracted graphene from a piece of graphite using adhesive tape to obtain a one-atom thick layer of carbon (Geim and Novosolev 2007). Graphene has gained immense importance in a wide range of applications due to the enormous amount of fascinating properties it possesses. These include high specific surface area, high electrical and thermal conductivity, and, internal biocompatibility, cheap and bulk production. It is known to be the thinnest, but the strongest ever material discovered due to its extraordinary mechanical strength (Xhu et al., 2010). The discovery of graphene and the resultant synthesis of graphene-based polymer composites has acquired a remarkable position in the field of nanoscience, and therefore, modern science and technology.

The first study to bring about the evolution of the science of polymer nanocomposites was done in 1909 describing the synthesis of an ion-exchange resin called as Bakelite (Alexandratos 2009). Since then, polymer nanocomposites are drawing uninterrupted attention of the researchers worldwide due to the excellent properties they exhibit such as good mechanical strength, electrical conductivity, and thermal stability (Haraguchi 2007). The primary work on polymer nanocomposites comprising of graphite fillers demonstrated the mechanism of initiation of polymerization of monomers by alkali-metal graphite intercalated composites (Potts et al., 2011). However, graphene is difficult to disperse in most of the polymer matrices due to its preference of interaction with polymers containing aromatic rings. Not many methods for the effective dispersion of graphene in polymers are devised. Some of them are melt polymerization and in-situ intercalative polymerization. Thus, functionalized derivatives of graphene are more commonly used in the studies focused on polymer nanocomposites. These functional groups conferred on graphene also impart additional properties to it (Salavagione et al., 2011). Moreover, graphite is abundant in nature. The production of graphene from graphite is a very long process and has its limitations. In this case, functionalization of graphite directly leads to easy and bulk production (Mukhopadhyay and Gupta 2011).

Figure 1.1: Preparation of Graphene Nanosheets from Graphite

Figure 1.1: Production of graphene from graphite. a) Chemical oxidation of graphite to form graphite oxide. b) Thermal exfoliation of graphite oxide to form graphene nanosheets (GNS).

The figure is adapted from Antolini E., 2012.

Figure 1.2 describes the production of graphene from graphite with the formation of intermediate product graphite oxide (GO). The red and blue coloured moieties on the GO and graphene nanosheets are indicative of the distribution of oxygen and hydrogen on the surface, respectively.

Graphite oxide (GO)

Oxidation of graphite is the principle tool to bring about chemical modification of graphene. As mentioned above, the isolation of graphene from graphite is a tedious process and involves steps like intercalation, exfoliation, etc. which are difficult to carry out in routine laboratories. Thus, GO is mostly used in the research involving graphene and/or graphite and acts as the source for graphene-based structure.

The synthesis of GO involves extensive oxidation of graphite resulting into a material with large amount of hydrophilic oxygen functional groups like carboxyl, hydroxyl, epoxy, etc. This makes it an excellent tool for the functionalization of GO (Stankovich et al, 2006). Also, the hydrophilicity and solubility of GO in various polar solvents like tetrahydrofuran, water and ethanol has drawn special attention of the chemists (Xu et al, 2009). In light of these properties, many applications of GO have been explored. With the fast-growing development of the methods for production and functionalization, graphene, alongwith GO has proved to be a potential material in various areas like nanoelectronics, energy technology associated with the field of chemistry such as supercapacitors, and fuel cells, gas sensors, and catalysis (Blake et al, 2008; Eda et al, 2009).

Except of these applications quoted above, the applications of graphene in biomedical science and technology is a comparatively novel subject. The primary report by Dai et al (2008) describing the use of GO as a nanocarrier for drug delivery was the first study to document the biomedical uses of graphene. Enormous amount of research has been done, after that, to demonstrate the biomedical applications of graphene. Reinforced composite material in various systems is one amongst them (Shen et al, 2012). Some applications of graphene are described below

Drug Delivery: As stated above, the preliminary work to test the efficiency of GO as a nanocarrier for drug delivery was done by Dai et al. (2008). They used nanoscale GO for the delivery of hydrophobic aromatic anticancer drug (SN38) to tumour cells. Another work by the same group examined the use of nano GO to deliver doxorubicin or DOX (aromatic chemotherapeutic drug to treat cancer) to the cancer cells. It was found that the release of DOX from pegylated GO was pH-dependent. This suggested that this property could be employed to the acidic micro-environments of tumor cells and intracellular lysosomes for effective drug release over a certain period of time (Sun et al, 2010).

The use of multiple drugs in cancer therapy is a frequent clinical practice adopted by doctors to avoid drug resistance by cancer cells (Andersson et al, 1999; Gavrilov et al, 2005). Inspired by this, Zhang et al (2010) tested GO as a nanocarrier for controlled and targeted drug loading and delivery of multiple drugs. For this, they combined folic acid and SO3H groups with GO, which was then loaded with doxorubicin (DOX) and camptothecin (CPT). They demonstrated that the co-loading of the two drugs in GO showed specificity in targeting and greater cytotoxicity to the human breast tumor cells having folate receptors, and also significantly high cytotoxicity in comparison to the one loaded with only a single drug.

A report by Yang et al (2010) provided evidence for the design and preparation of a multi-functionalized GO as drug delivery vehicle having dual-targeting (magnetic- and bio-targeted) and pH-sensitivity. Also, there are many studies ongoing to describe the feasibility of GO as a nanocarrier for drugs of non-cancerous diseases. However, a lot of in vivo studies have to be carried out before these GO applications can be employed to humans.

Gene therapy: Gene therapy is a comparatively new and excellent technique to treat genetic diseases like cystic fibrosis, Parkinson's disease, cancer and so on. For a gene therapy to be successful, a gene vector has to be constructed which protects the DNA from attack by nucleases and enables efficient transfection of cells with the DNA.

A study reporting gene delivery using polyethylenimine-modified GO (PEI-GO) (Feng et al, 2011; Chen et al, 2011) demonstrated that graft-polymerization of PEI-GO reduces the cytotoxicity and improves the transfection efficiency of the cationic polymer indicating that PEI-GO can have potential application in gene therapies. A few more studies reporting such results have also been cited.

The history of GO can be dated back to many decades. The first study to describe GO synthesis was done by Brodie. He did this by adding potassium chlorate to the graphite slurry in fuming nitric acid (Brodie 1860). Staudenmaier modified the Brodie synthesis by adding a mixture of sulphuric acid and potassium chlorate in multiple portions to the as against the single addition done by Brodie (Staudenmaier 1898). Both these methods made use of strong oxidizing agents, nitric acid and potassium chlorate. After Staudenmaier, an alternative method of GO production was developed by Hummers and Offeman. In that, a mixture of sulphuric acid and potassium permanganate was used to carry out the oxidation process (Hummers and Offeman 1958). Since then, a number of modified versions of the Hummers and Offeman method have been evolved. The structure and properties of GO considerably change with changes in the production method (Dryer et al., 2009). As there is no specific analytical technique for the characterization of GO, exact structure of GO still remains a subject to explore. However, it is well-known now that the edge of the GO structure is localized by carboxylic functional group whereas the other functional groups, mostly hydroxyl and epoxides, are situated in the basal planes of the GO sheets (Salavagione et al., 2011). The diversity of functional groups on the GO structure facilitates its interaction with various different compounds and also, many polymeric systems.

Hydrogels

The first ever polymeric materials designed for biological use are the hydrogels. The first attempt to such a polymeric biomaterial was done by Otto Wichtele and Draholav Lim, the makers of contact lenses. Hydrogels are, classically, three-dimensional network consisting of highly hydrophilic polymers. They are able to contain a large amount of water. These hydrophilic networks are made up of homopolymers or copolymers, and are not dissolvable due to the abundance of physical and/or chemical crosslinks. The physical crosslinks give the structural and physical integrity to the network. Hydrogels show a thermodynamic compatibility with water that enables their swelling in aqueous medium. Various applications of such hydrogels have been reported, especially in the medical, biomedical and pharmaceutical areas. Hydrogels closely mimic the physiological environment of living tissues more in comparison to other kinds of synthetic biomaterials because of its water content and resulting softness in consistency. This high content of water imparts the property of biocompatibility to these hydrogels.

The two main characters of hydrogels which are superabsorbancy and permeability give them an amazing array of uses. Hydrogel of many synthetic and natural polymers have been produced with their end use mainly in tissue engineering, pharmaceutical, and biomedical fields (Syed K. H. Gulrez1 and Saphwan Al-Assaf, 2011). Currently hydrogels have applications in field of drug delivery, scaffolds for tissue engineering applications, controlled release drug delivery systems, wound dressings, cosmetic applications, bio-sensors, soil moisture retention, moisture traps, contact lenses and disposable diapers and sanitary towels(Nicodemus and Bryant, 2008 and Kopecek, 2009) . The hydrogels have also been used for stem culture research where the stiffness of gels is used to direct the stem cell differentiation (Englar and Sen, 2006). Hydrogels due to their unique biocompatibility, flexible methods of synthesis, porosity, water -retention, and desirable physical characteristics, have been the material of choice for many applications in regenerative medicine. They not only serve as scaffolds for tissue constructs but also serve as control drug and protein delivery systems to tissues and cultures (Saughter and Khurshid, 2009). Numerous strategies have been employed to improve mechanical properties of hydrogels by copolymerization, optimizing crosslinking density, or using composite hydrogel materials.

GO-based polymer nanocomposites

On account of the extraordinary characteristics discussed above, the combination of graphene and hydrogels is suggested to be a highly effective molecule. GO hydrogels are relatively, a newer subject of research in the field of polymer nanocomposites. Hence, there is lack of characterization and optimization to the process of synthesis of such hydrogels. However, the literature review gives clues to a number of parameters that can affect the synthesis and properties of GO hydrogels.

GO/Poly (vinyl alcohol) hydrogels: A recent report by Zhang et al., demonstrated the use of GO as a nanofiller in the polyvinyl alcohol hydrogels (Zhang et al., 2011). They observed a 132% rise in the tensile strength of the GO/poly (vinyl alcohol) hydrogels compared to hydrogels with polyvinyl alcohol alone. Also, they described an increase in the compressive strength of the hydrogels with GO nanofillers. Another study suggests the preparation and use of a GO/poly (vinyl alcohol) nanocomposite hydrogel, for drug release at normal physiological conditions (Bai et al, 2010).

GO/Poly(acrylic acid-co-acrylamide) hydrogels: GO is considered for the synthesis of new superabsorbent hydrogels due to the large number of hydrophilic functional groups on its surface. Huang et al. (2012) indicated the synthesis of a super-absorbent hydrogel containing GO and Poly (acrylic acid-co-acrylamide). They studied the effects of the GO concentration on the dispersibility and swelling behavior of the hydrogels. The results showed that low concentration of GO i.e. less than 0.30 wt% mixed properly in the polymer solution and led to the formation of a strong hydrogel. However, at higher GO concentration than this, aggregation of GO particles occurred and resulted in phase-separation. Also, swelling properties of the hydrogels were affected accordingly. The swelling properties of the hydrogels increased upto 0.30 wt% GO concentration and then decreased. This indicates that the properties of the hydrogel are dependent on the GO concentration used in it.

GO/Polyacrylamide hydrogels: Various studies reporting the swelling behavior of hydrogels, especially the ones with acrylamide (AA) backbone, have been surfaced in recent years (Shen et al., 2012; Liu et al., 2012). The investigation by Shen et al. indicates that the addition of GO into poly (acrylamide-copolymer- N, N'- methylenebisacrylamide) gel imparts better mechanical strength to the gel (Shen et al, 2012). Their results indicated that though the mechanical strength of the GO hydrogels was greater that the GO hydrogels made with Bisacrylamide, the swelling ratio was greater in Bis-gels. Liu et al. (2012) demonstrated through their study that GO nanosheets can act as cross-linkers in the GO/polyacrylamide hydrogels. They observed a high tensile strength, and a large elongation at break in the GO/polyAA hydrogels without the organic cross-linker N,N'-methylenebisacrylamide. This study suggested that the mechanical properties of such hydrogels are dependent on the underlying chain structure of the polymer. However, it is also known that the strength of the polymer network is directly related to the temperature and time conditions used for gel formation.

INTRODUCTION TO THE EXPERIMENT

The literature reviewed above highlights the importance of GO in the science of polymer nanocomposites. This experiment, thus, aimed at the synthesis of a tough and highly elastic hydrogel using GO.

GO was synthesized using a modified Hummers and Offeman method. This GO was then used for making the hydrogels. The monomer chosen to form the polymer network was AA. At this point, a brief overview of AA polymerization is necessary. Generally, the polymerization of unsaturated monomers employs chain reaction for the making of their polymer network. Chain polymerization is the process by which addition of monomers causes growth in the polymer chain. There are various active centers for the initiation of chain polymerization; free radicals are one of them. Free radical chain polymerization proceeds by the generation of free radicals due to thermal decomposition of a compound, oxidation-reduction reaction, high-energy radiations, etc. These free radicals then bring about the rest of the polymerization process.

For the generation of free radicals, the use of redox initiating system was done. Redox initiators bring about polymerization by the production of free radicals via an oxidation-reduction reaction. As this was a trial-and-error mode of research, various initiating systems were used so as to check the feasibility of the one which gives the strongest hydrogel. A novel idea to use ceric ion (Ce4+), coupled with GO in the polymer matrix, as a redox initiator has been explored.

Usually, polyacrylamide hydrogels consist of an extensive network of AA and methylene bis acrylamide (cross-linker) monomers. The strength of the gel depends on the concentration of the cross-linking monomer. It is observed that these hydrogels made by organic cross-linkers are usually brittle. The reason for this is that the cross-linking points are not evenly distributed in the AA network and also, the distribution of the cross-linked chain lengths is extensive. In order to solve this problem, the use of organic cross-linker in the experiment is avoided.

Different concentrations of GO are checked for, so as to achieve an optimum concentration at which GO can itself, act as a cross-linking agent in the polymer matrix. Also, the effect of AA content in the hydrogels was studied.

This study demonstrates the synthesis of GO hydrogels which have better mechanical properties than the conventional polyacrylamide hydrogels using the cross-linker Bisacrylamide.

It is believed that such a gel would be anti-bacterial and could have potential pharmaceutical applications like transdermal wound dressing and controlled drug release, in tissue engineering (for the purposes of scaffolding) and also in regenerative medicine.

SECTION 2: METHODS & MATERIALS

Materials:

Expanded graphite was obtained from NGS Dragon Seal (Leinburg, Germany). Acrylamide (AA), Ammonium Persulfate (APS), Potassium Persulfate (KPS), N, N'-(methylenebis)acrylamide (Bis) and cerium ammonium sulfate were supplied by Sigma-Aldrich (Loughborough, UK) and were used without further treatment. Ceric sulfate was obtained from the May & Baker Ltd (New Jersey, USA) and sulfuric acid and hydrochloric acid were supplied by the Fisher Scientific (Loughborough, UK).

Preparation of Graphite oxide:

A modified version of Hummers-Offeman method was used to prepare GO from expanded graphite powder. For this, 60ml of concentrated sulphuric acid (Fisher Scientific) was taken in a beaker and left to cool to approximately 5oC, in an ice bath. When the sulphuric acid was ice-cold, 3 grams of expanded graphite powder (Dragon Seal) was then added to it whilst stirring constantly with the aid of a mechanical stirrer. The graphite powder was added slowly to the acid so as to avoid sudden increase in temperature and maintain it below 10oC. 10 grams of potassium permanganate was then added to this mixture under constant magnetic stirring. Precautions were still taken to maintain the temperature of the mixture below 10oC. The mixture was then heated to about 35oC on a hot plate, whilst stirring, till it turns into a thick, black/brownish gray paste. Following this, 150ml of distilled water was added drip wise to this thick paste. This changed the colour of the mixture to bright brown. Further, a mixture of 10ml of hydrogen peroxide and 500ml of distilled water was added to this mixture, drip wise, thereby resulting into a change of color of the mixture from bright brown to yellow. The hydrogen peroxide added terminates the reaction by reduction of permanganate and manganese dioxide to manganese sulphate. The mixture was then left overnight to settle. The addition of peroxide completes GO formation.

The following day, some of the liquid was discarded without disturbing the settled matter. It was then stirred to form an evenly dispersed solution. This solution was equally divided and transferred to two Beckman 250mL Centrifuge tubes. Using an AVANTI J-301 centrifuge, the solution was centrifuged for 10 minutes at 19oC at a speed of 6000rpm. The supernatant was removed and was replaced by 10% hydrochloric acid (HCl). This was done in order to wash the solution with acid. This solution was again centrifuged (at the same parameters as above). Five such washes were done.

To neutralize the pH, the mixture was then washed with deionized water. The procedure was repeated as for the acid washes. However, the centrifuge parameters were changed to a speed of 12000rpm and for 30 minutes. Seven such washes were given to the solution. After each wash, the pH of the supernatent was recorded and the final pH of the solution was 5.5.

This mixture was then freeze-dried and the final product i.e dried GO was obtained.

Preparation of GO/AA nanocomposite hydrogels:

For the initial preparation of hydrogels, the concentration of GO taken was high i.e. 1wt% stock solution of GO was prepared. Accordingly, 0.1g of GO was first dissolved in 10ml of distilled water and was then sonicated using Misonix Ultrasonic Liquid Processor Model CL5 for 6 minutes (twice for 3 minutes each, at an interval of 5 minutes) to obtain even dispersion of GO in aqueous solution. Acrylamide stock solution (17.2wt%) was prepared, which was mixed with GO solution in equal quantities (2ml each), before the addition of the initiator. 0.1g Cerium ammonium sulphate (CAS) was dissolved in 5ml of 0.1 M sulphuric acid to make the CAS stock solution and was further diluted to give the desired concentration in the samples. Table 2.1 below gives the exact concentration of initiator CAS to the solutions for polymerization.

Table 2.1:

SOLUTION

GO (2ml)

Acrylamide(2ml)

Ceric ammonium sulfate

Total volume of mixture

A

1wt%

17.2wt%

0.4wt%

5ml

B

1wt%

17.2wt%

0.33wt%

5ml

C

1wt%

17.2wt%

0.26wt%

5ml

D

1wt%

17.2wt%

0.18wt%

5ml

E

1wt%

17.2wt%

0.095wt%

5ml

Further, depending on the results, changes in the constituents and their concentration were done for the preparation of better hydrogels. For that, GO was first dissolved in distilled water and was then sonicated for 6 minutes (twice for 3 minutes each, at an interval of 5 minutes). This solution was then purged with nitrogen gas for 30 minutes. This step ensured removal of oxygen which is a potent inhibitor of the polymerization process. The obtained GO solution was then used to prepare hydrogels. As the behavior of GO is not very stable in water, the GO solution was prepared fresh each time. Acrylamide (Sigma Life Science) monomer was used as the hydrogel backbone. The acrylamide concentration varied between the hydrogels. For the hydrogel preparation, AA was directly added to the GO solution and was allowed to mix thoroughly by keeping on an ice bath. The initiator ceric sulfate (CS) was then added to this ice-cold GO/AA solution. The final reaction mixture contained GO solution, Acrylamide and the initiator CS. Experiments were carried out by changing the acrylamide and GO concentrations. The table 2.2, 2.3 and 2.4 indicate the concentration of the constituents in the reaction mixture. The polymerization was allowed to occur at 60oC in an oven, for 24 hours.

Table 2.2: Changing acrylamide concentration

SOLUTIONS

GO (4g)

Acrylamide

Ceric sulfate(1g)

Total volume of mixture

A

0.06wt%

15wt%

0.4wt%

5ml

B

0.06wt%

20wt%

0.4wt%

5ml

C

0.06wt%

25wt%

0.4wt%

5ml

D

0.06wt%

30wt%

0.4wt%

5ml

E

0.06wt%

34.2wt%

0.4wt%

5ml

Table 2.3: Lowering GO concentration

SOLUTIONS

GO (4g)

Acrylamide

Ceric sulfate(1g)

Total volume

A

0.03wt%

15wt%

0.4wt%

5ml

B

0.03wt%

20wt%

0.4wt%

5ml

C

0.03wt%

25wt%

0.4wt%

5ml

D

0.03wt%

30wt%

0.4wt%

5ml

E

0.03wt%

34.2wt%

0.4wt%

5ml

Table 2.4: Changing GO concentrations

SOLUTIONS

GO (4g)

Acrylamide

Ceric sulfate(1g)

Total volume

A

0.012wt%

17.4wt%

0.4wt%

5ml

B

0.03wt%

17.4wt%

0.4wt%

5ml

C

0.06wt%

17.4wt%

0.4wt%

5ml

D

0.09wt%

17.4wt%

0.4wt%

5ml

E

0.12wt%

17.4wt%

0.4wt%

5ml

For the synthesis of hydrogels using the potassium persulphate (PP) initiator, GO and AA were used as described in section ii. The stock solution of PP was made by dissolving 0.31g of PP in 10ml of distilled water. The monomer amine used was N-[3-(Dimethylamino)propyl]-methacrylamide. The monomer amine stock was prepared 0.259g in 10ml of distilled water. The constituents were then added as per table 2.5. Polymerization took place at room temperature for 24 hours.

Table 2.5:

SOLUTI-ONS

GO (4g)

(in wt%)

AA (in wt%)

PP(1g) (in wt%)

Monomer amine(1g) (in wt%)

Total volume

A

0.0144

17.4wt%

0.52

0.43

6ml

B

0.036

17.4wt%

0.52

0.43

6ml

C

0.072

17.4wt%

0.52

0.43

6ml

D

0.108

17.4wt%

0.52

0.43

6ml

E

0.144

17.4wt%

0.52

0.43

6ml

Standardization of GO solution for the hydrogels

In order to optimize the conditions for polymerization, an experiment, to determine the age of GO solution to be used for hydrogels, was carried out. For that, 10 days old GO solution and freshly prepared GO solution were used to prepare hydrogels, according to the following table 2.6.

Table 2.6:

GO solution (4g) Old/Fresh

Acrylamide

Ceric ammonium sulfate(1g)

Total volume

0.06wt%

34.2wt%

0.4wt%

5ml

0.06wt%

34.2wt%

0.26wt%

5ml

0.06wt%

17.2wt%

0.4wt%

5ml

0.06wt%

17.2wt%

0.26wt%

5ml

*keep the vials in oven at 60oC for 24 hours.

Fourier transform infrared spectrometry

Fourier transform infrared spectrometry was done on the CAS, AA, hydrogel, and white particles found in the hydrogel using a Perkin Elmer Frontier 1600 FTIR. Transmittance (T%) values were obtained by that and a graph was accordingly plotted against the wavenumber (cm-1)

Swelling properties of hydrogels

For the determination of the swelling behaviour of the hydrogels, they were immersed in equal amounts of distilled water at room temperature. Excess water was wiped off and the gels were weighed out everyday, tilll a constant weight was achieved. This was considered as the final swollen weight of the gels (W1). Following this, the gels were removed from the water and left to dry completely at room temperature. The dried weights of the gel was determined (W2). These weights were then used to calculate the swelling ratios (SR) of the gels by the formula:

SR= W1/W2

Mechanical testing of hydrogels

The hydrogels were tested for their mechanical strength using Zwicki Line-testing machine. The hydrogels were prepared in small vials for this test and compresson test was conducted on them. The machine was programmed to give the upper force limit of 6N and the maximum deformation of gels was set to 2.5mm. The data obtained by this testxpertII software was used to determine the Young's modulus of all the hydrogels. The Young's modulus of the gels were compared in order to determine the strength of the gels.

Statistical Analysis

The data obtained by the mechanical testing of the hydrogels were analyzed using the Microsoft Excel 2010 software and SPSS and appropriate graphs were made.

SECTION 3- RESULTS

Effect of presence of initiator on the synthesis of GO/AA nanocomposite hydrogels:

FIGURE 3.1:

Figure 3.1: Effect of initiators on the synthesis of GO/AA nanocomposite hydrogels. A) 1.71g of AA dissolved in 4ml of distilled water and heated at 50oC for 24 hours (without initiator and GO). B) 1.71g of AA dissolved in 4ml of 1wt% GO solution and heated at 50oC for 24 hours (without initiator). C) 1.0456g of AA added to 4ml of 0.144wt% GO solution. Further, 1ml of 3.1wt% PP solution and 1ml of 2.59g/g monomer amine solution were added to initiate the polymerization reaction. This solution was allowed to react at room temperature. D) 1.71g of AA dissolved in 4ml of 1wt% GO solution. 1ml of 0.4wt% CAS was then added to initiate the polymerization reaction. Polymerization occurred at 60oC for 24 hours.

Figure 3.1 indicates the synthesis of GO/AA hydrogels in the presence and absence of initiators. In the figure, C and D are inverted to show solid state of the obtained hydrogels. It was seen that GO solution was completely miscible with the acrylamide solution. Also, acrylamide alone dissolved completely in the GO solution. However, in the absence of an initiator, polymerization did not take place and thus, no hydrogel was formed (A & B). When PP was added along with a monomer amine solution, polymerization occurred at room temperature (C). Also, the oxidizing agent CAS initiated polymerization reaction and led to the synthesis of GO/AA nanocomposite hydrogel (D).

Effect of CAS concentration on the synthesis of GO/AA nanocomposite hydrogel:

FIGURE 3.2:

Figure 3.2: Effect of CAS concentration on the synthesis of GO/AA nanocomposite hydrogels. A-E: Each vial contains 1.71g of AA added to 4ml of 0.06wt% GO solution. The polymerization reaction was initiated by adding 1ml of different CAS concentration solution to the mixture. The concentration of CAS in the mixture, from A to E is as follows: 0.4wt%, 0.333wt%, 0.26wt%, 0.18wt% and 0.095wt%. The control 1 (G) contained GO solution and acrylamide, without CAS. Control 2 (H) contained acrylamide and CAS. The polymerization reaction took place at 50oC for 24 hours.

Figure 3.2 shows that the synthesis of GO/AA hydrogel is dependent on the concentration of the initiator CAS. The gels with a higher concentration of CAS (0.4wt%, 0.333wt%, and 0.26wt%) were comparatively stronger than the ones having lower concentration of CAS (0.18wt% and 0.095wt%). The gels formed were tested manually. Also, some white particles were seen in the hydrogels formed. The formation of white particles was inversely proportional to the CAS concentration i.e more white particles were seen in the gel having 0.095wt% CAS, whilst no white particles were seen in the one with 0.4wt% CAS. This gradient was easily visible and evident.

The initiation of polymerization by CAS indicated a redox reaction between CAS and GO. Control experiments suggested that CAS alone, in the absence of GO, could not initiate polymerization in acrylamide. Thus, the data points out that GO is involved in the redox reaction with CAS to produce free radicals needed for polymerization.

Determination of unknown white particles formed in the GO/AA hydrogels using the CAS initiator, by Fourier transform infrared spectrometry(FTIR):

FIGURE 3.3: FTIR

Figure 3.3: Determination of the white particles in the GO/AA/CAS hydrogels by FTIR. The picture shows the spectra for the CAS crystals, AA crystals, white particles extracted from the hydrogel and the hydrogel.

FTIR was done to determine the identity of white particles formed in the GO/AA nanocomposite hydrogels described in Figure 3.2.

Figure 3.3 is a graph of the spectra observed by FTIR for the four substances: CAS, AA, white particles and the hydrogel. It can be seen from the graph that the absorbance peaks of white particles are similar to the peaks of the hydrogel indicating certain degree of resemblance in the structure of the two substances. For the white particles strong and extended absorption peak is seen around 3368 cm-1 and 3227 cm-1. Strong peaks were also seen around 1663 cm-1 and 1616 cm-1.

The repeated prevalence of the white particles in the hydrogels made them unfit for further experiments. Hence, experiments were conducted to check polymerization initiation by ceric sulfate (CS).

Effect of acrylamide concentration on the mechanical strength of GO/AA nanocomposite hydrogel using the initiator CS and having lower concentration of GO:

FIGURE 3.4: Effect of [AA] on the hydrogel elasticity

Figure 3.4: Effect of acrylamide concentration on the mechanical strength of the GO/AA hydrogels is represented. The graph indicates the Young's moduli of the gels at different acrylamide concentrations i.e. 15wt%, 20wt%, 25wt%, 30wt% and 34.2wt%. The GO and CS concentration for all the samples in the series is 0.03wt% and 0.4wt% respectively. Polymerization took place at 60oC for 24 hours.

From figure 3.4, it is evident that the Young's modulus of the GO/AA nanocomposite hydrogel increases with an increase in acrylamide concentration. Thus, the strongest gel was obtained at a concentration of 34.2wt%and the Young's modulus for the hydrogel containing 15wt% acrylamide was negligible, indicating poor strength.

Effect of GO concentration on the mechanical strength of GO/AA nanocomposite hydrogels using the initiator CS:

FIGURE 3.5: Effect of [GO] on the hydrogel elasticity

Figure 3.5: Effect of GO concentration on the synthesis and mechanical properties of the GO/AA hydrogels using initiator CS are demonstrated. The graph represents Young's moduli for the hydrogels. Different GO concentrations were used to prepare the gels i.e. 0.012wt%, 0.03wt%, 0.06wt%, 0.09wt% and 0.12wt%. The acrylamide and CS concentration was 17.4wt% and 0.4wt% respectively. The polymerization took place at 60oC for 24 hours.

Figure 3.5 indicated that the change in GO concentration greatly influences the mechanical properties of the GO/AA hydrogels made using CS. There was a rapid increase in the strength of the hydrogels with an increase in GO content. The highest concentration of GO in the series yielded a gel with better mechanical strength and elasticity and at the lowest GO concentration, poorly polymerized hydrogel was obtained.

Effect of GO concentration on mechanical strength of GO/AA nanocomposite hydrogels using the redox initiator PP and monomer amine:

FIGURE 3.6: Effect of [GO] on hydrogel elasticity

Figure 3.6: Effect of GO concentration on the synthesis and mechanical properties of the GO/AA hydrogels using the initiator PP along with monomer amine is indicated. Different GO concentrations were used to prepare the gels i.e. 0.0144wt%, 0.036wt%, 0.072wt%, 0.108wt%, 0.144wt%. The polymerization took place at room temperature for 24 hours. AA, monomer amine, and PP concentration were kept constant for the series at 17.4wt%, 2.59wt% and 3.1wt% respectively.

Highly elastic and strong GO/AA hydrogels were obtained using the initiator PP, as is obvious from figure 3.6. The data reflects the dependency of the mechanical strength of the hydrogels on the GO concentration. 3 times higher increase in the Young's modulus was seen when the GO concentration was elevated from 0.0144wt% to 0.036wt%. After that, however, the increase in mechanical strength was only negligible. This suggests that the mechanical strength is improved by the addition of GO upto a concentration of 0.36wt%. Increasing the concentration after this point, has no great effect on the mechanical properties of the hydrogels.

In comparison with the GO/AA hydrogels obtained by the CS initiator (Figure 3.5), the gels obtained using the PP initiator has higher Young's modulus indicating better mechanical properties.

Swelling behavior of the hydrogels:

FIGURE 3.7: Effect of [AA] on the swelling ratio of hydrogels

Figure 3.7: Swelling behavior of the GO/AA nanocomposite hydrogels using the CS initiator. Different acrylamide concentrations were checked for in the experiment. The graph indicates swelling ratio of the hydrogels having acrylamide concentrations of 20wt%, 25wt%, 30wt%, and 34.2wt%. The acrylamide and CS concentration was 17.4wt% and 0.4wt% respectively and was constant for the series. The polymerization took place at 60oC for 24 hours.

Figure 3.7 shows that the swelling ratio of the GO/AA hydrogels is dependent on the concentration of the acrylamide. The swelling ratio decreases with an increase in the acrylamide concentration, as is demonstrated by the graph.

FIGURE 3.8:Effect of [GO]on the swelling ratio of hydrogel

Figure 3.8: Swelling behavior of the GO/AA hydrogels made with the PP initiator. Swelling behavior of the gels at different GO concentration i.e. 0.0144wt%, 0.036wt%, 0.072wt%, and 0.108wt%. The polymerization took place at room temperature for 24 hours. AA, monomer amine, and PP concentration were kept constant for the series at 17.4wt%, 2.59wt% and 3.1wt% respectively.

Figure 3.8 describes the swelling ratio of the GO/AA hydrogels polymerizaed using the PP initiator. It is seen from the figure that, increase in concentration of GO results into decrease in the swelling ratio of the hydrogels. However, this reduction in the swelling ratio is not very significant indicating that it is almost independent of the GO concentration.

SECTION 4- DISCUSSION

Previous studies on tissue engineering have provided sufficient data on the poor mechanical properties exhibited by hydrogels (Orwin et al., 2003; Petrini et al., 2003). The mechanical properties are central to the hydrogels needed for tissue engineering. The reason for this is that, the synthesis of tissues is dependent on the mechanical strength of the hydrogel. Hydrogels with poor strength and elasticity lead to generation of mechanically weak tissues, in comparison to biological tissues (Ahearne et al., 2008). To overcome this issue, a need for very strong hydrogel has arisen.

This problem was addressed in this project which aimed to synthesize a tough and strong hydrogel using the acrylamide monomer. It was hypothesized that addition of GO to the hydrogel structure would enhance its mechanical properties. Also, the hypothesis was that at a certain optimum concentration, GO nanosheets can act as the cross-linker in the polymer structure. So GO hydrogels were prepared and their mechanical and swelling properties were determined.

The preparation of GO was done by slight modifications to the Hummers' method. For this, strong oxidizers viz. sulphuric acid and potassium permanganate were used to treat graphite. The reaction takes place as follows:

KMnO4 + 3H2SO4 → K+ + Mno3+ + H3O+ + 3 HSO4-

MnO3+ + MnO4- → Mn2O7

Mn2O7 i.e. Dimanganese heptoxide is a very strong oxidizing agent. Oxidation of graphite results into the functionalization of the graphite surface structure with carbonyl, hydroxyl, and epoxide groups. The carbonyl groups are mostly situated on the edges of the GO structure while the hydroxyl and epoxide groups are abundant in the middle of the GO sheets. However, the structure of GO varies with the method of synthesis, and so are the properties (Dryer et al., 2009). The GO produced in this experiment was acidic in nature (pH 5.5). On completion of the oxidation process, yellow ochre coloured product was formed.

The first treatment done to GO was sonication. Sonication is the application of ultrasound radiations to produce strong agitation in a solution. The dispersibility of GO in solution, central to further processing, depends both on the solvent and the diversity of functional groups acquired during oxidation process. Sonication of GO facilitated breakage of intermolecular bonds leading to a fine dispersion of GO sheets in distilled water. This accelerated the process of dissolution and also resulted in exfoliation of GO. It was also seen that the GO aqueous solution maintain their stability for a longer time, when sonicated for a longer period of time. However, sonication for 6 minutes led to thickening of GO solution after a week. Hence, freshly prepared GO solutions were used for the preparation of hydrogels.

Also, experiment to determine the feasibility of old GO solutions (10 days old) was done. The results indicated that stronger hydrogels are obtained using freshly prepared GO solution as against old GO solution.

Control experiments to check the interaction between GO and acrylamide at high temperatures were conducted. From figure 3.1-B, it was found that GO alone was not capable of polymerizing acrylamide. Also, acrylamide did not polymerize on its own at a temperature of 60oC (figure 3.1-A). Hence, this prompted the need for an initiating system for the purpose of hydrogel preparation.

CAS is used as an initiator in redox polymerization (Hussain & Gupta, 1977; Chowdhary et al, 2001). In this experiment, 1ml of 0.4 wt% CAS was used to initiate polymerization reaction. It was seen that, when CAS was added to the GO/AA solution at room temperature, polymerization occurred immediately and hydrogel was obtained within a period of two to five minutes. However, phase separation in the form of white, cloudy patterns was observed in the gels. This phase separation in the gels may be attributed to improper or uneven dispersion of CAS in the GO/AA solution due to fast onset of polymerization. Hence, to overcome this problem, the GO/AA solution was first kept on an ice bath. When it became ice-cold, CAS was added to it to ensure thorough mixing of the two solutions. This also avoided immediate initiation of polymerization and phase separation. The solution was, therefore, kept at 60oC in a drying oven for polymerization to take place. This method resulted into a very tough and stretchable hydrogel, on manual testing, as described by figure 3.1-D.

The polymerization of AA by CAS, in the presence of GO, led to the hypothesis that GO was an active participant in the redox reaction involving CAS. Hence, control experiment to check whether CAS initiates polymerization in AA alone was conducted and a negative result was obtained (Figure 3.2-G). This confirmed the hypothesis and it was concluded that GO was a component of the redox system and the hydrogel obtained was a result of the GO/CAS initiating system. This is a novel redox initiating system demonstrated in the experiment. CAS is a strong oxidizing agent and thus, GO participates in the reaction by being oxidized.

Figure 3.2 shows the effect of CAS concentration on the synthesis of hydrogels. The strongest hydrogel was obtained at a CAS concentration of 0.4 wt%. A strange occurrence of white particles took place in the hydrogels having lower concentration of CAS. Repeated experiments indicated the same results, thereby, eliminating the possibility of contamination. It was also observed that the amount of white particles in the GO/AA nanocomposite hydrogels was dependent on the CAS concentration i.e. maximum number seen in the hydrogel containing 0.095 wt% CAS, while fewer seen in the one with 0.18 wt% CAS and no inclusions seen in the hydrogel with 0.4 wt% CAS.

These white bodies were isolated from the hydrogel and analysed using FTIR.  FTIR enables emission of infrared rays on a sample. The sample absorbs certain wavelength of the infrared light and the rest is transmitted. The intensity of the emitted light is then recorded by a detector/receiver which produces a spectrum of the absorption by the sample. Certain specific bond types absorb infrared light (causing a drop in %T) in certain regions causing bond vibrations. Specific bonds absorb specific regions of wavelengths so that there can be a comparison and it can then be predicted as to what bonds have caused this and the absorption intensity gives clues as to the abundance of those bonds. FTIR is generally used to identify the chemistry of unknown samples and determine the quality of materials.

For the analyses, CAS, AA, white inclusions and the hydrogel were taken and their transmittance (T %) values were plotted, as shown in figure 3.3. As can be seen in figure 3.3, CAS and acrylamide spectra are visibly different from the white particle and hydrogel spectrum. For the white particle, strong absorption peak extending from 3368 cm-1 to 3227 cm-1 indicates stretching of NH2, which is similar to the FTIR spectrum of polyacrylamide. The peak at 1663 cm-1 is representative of the C-O stretching vibration bands and the strong absorption peak at 1616 cm-1 indicates NH2 bending. The medium intensity peaks at 1457 cm-1 and 1357 cm-1 is characteristic of CH2 bending and CH2 wagging respectively. The smaller peaks around 1213 cm-1 and 1133 cm-1 are due to C-C stretching. This structure is fairly similar to the polyacrylamide structure. The figure 4.1 below indicates a FTIR spectrum of polyacrylamide from Yu et al, 2011.

FIGURE 4.1: FTIR spectrum of polyacrylamide

Thus, it can be concluded that the white particle was polyacrylamide. Hence, its resemblance to the hydrogel structure is not surprising (Figure 3.3). A possible explanation for the occurrence of white particles in the hydrogels may be the uneven dispersion of constituents in the solution. It may also be that the evaporation of the solution at high temperature, caused its condensation on the walls of the vial and consequently bringing about phase separation in the polymer network.

The occurrence of white bodies in the hydrogel demanded a need for the change in the initiator. Hence a related compound of CAS, called as ceric sulfate (CS) was then used for further experiments.

Initial experiments with CS showed no signs of the occurrence of white inclusions in the GO/AA hydrogels and so CS was fit for the synthesis of hydrogels. The hydrogels obtained by using CS initiator had comparable strength to that obtained by CAS. This proved that the ceric ion (Ce4+) was the active member of the redox system for the initiation of AA polymerization with GO.

Effect of monomer concentration was checked in the GO/AA hydrogels by using the CS initiator. Their mechanical strength was tested using the Zwick mechanical tester. The graphical data obtained from the mechanical testing of the samples was analysed to calculate the Young's modulus of the GO/AA hydrogels. Figure 3.4 shows the effect of AA concentration on the mechanical strength of the hydrogels. From the figure, it was observed that the strength of the hydrogel increased with an increase in the AA concentration. This increase in the mechanical strength was significant and in accordance with a study by Patil et al, 1996. Data was not available for the hydrogel with 15wt% AA. The hydrogel with the maximum AA was the strongest while with the minimum AA was the weakest. Thus, this study reports the dependency of the mechanical strength of the nanocomposite hydrogels on the initial concentration of the monomer. Also, from figure 3.7, it can be seen that the swelling behavior of the GO/AA hydrogels obtained using the CS initiator are affected by the change in monomer concentration. The effect of AA concentration on the swelling behavior of the hydrogel is inversely proportional i.e an increase in the AA concentration leads to a decrease in the swelling ratio. This phenomenon can be justified by the fact that increase in the monomer concentration in the hydrogel leads to an increase in the mesh network of the polymer, which potentially acts as a cross-linkage between acrylamide structures. Due to this, the entry of additional water in the hydrogel network becomes difficult, thereby, affecting their swelling behavior. This shows that the GO/AA hydrogels prepared using CS possessed good mechanical strength but poor swelling properties. The poor swelling properties may be due to inadequate cross-linking in the acrylamide backbone. Another possible reason for this may also be that the presence of hydrophilic groups on the GO structure may lead to water-retention in the molecule.

The effect of GO concentration on the mechanical strength of the GO/AA hydrogels is also demonstrated in the experiment. From figure 3.5, it is seen that concentration of GO directly enhances the strength of the hydrogels made using CS. This data is also reflected in a report by Liu et al, 2012. However, in the initial experiments with 1wt% GO, the hydrogels obtained were weak. The reason for this can be that, at such a high concentration, the GO nanosheets aggregate due to their high surface activation energy. This causes a decrease in the specific surface area and causes sliding between the layers of GO nanosheets (Zhang et al, 2011). This, in turn, hampers the bonding between GO and polyacrylamide and reduces the mechanical strength of the hydrogels. Nevertheless, as shown in figure 3.5, up till a certain optimum concentration of 0.12wt%, no such aggregation of GO nanosheets occurs. The strength of the hydrogel with 0.12wt% GO is 3 times of the one with 0.09wt% GO.

The data obtained for the GO/AA hydrogels made using PP initiator was similar to the hydrogels made with CS. The mechanical properties of those hydrogels increased with increase in GO concentration, as is evident from figure 3.6. From the figure, we see that the mechanical properties increased when the GO concentration was elevated from 0.0144wt% to 0.036 wt%. After that, the effect of higher GO concentration on the Young's modulus was negligible. Also, the change in the swelling behavior due to increase in GO concentration is not significant. This is described in figure 3.8. A possible cause for such a behavior of the GO/AA hydrogels may be higher amount of cross-linking between AA molecules due to the potent initiator PP.

A few experiments with other initiators like ascorbic acid, hydrazine solution, sodium borohydride, 2,2'-Azobis[2-methylpropionamidine] dihydrochloride, and ammonium persulphate were also done. The results yielded showed either less or no signs of polymerization. Hence, they were not considered for future analysis (Data not available). Also, a lot of studies have already been documented on the use of these initiators. Hence, more work was focused on the novel initiating system with ceric ion and GO.

The results obtained in the experiment were adequately supported by the literature in this field. However, certain drawbacks can be identified in the research. The first major limitation to the experiment was that the data obtained was not replicated due to time-restraints. Hence, the credibility of the results is affected. Repititions of the experiments are needed for their validation. This could, however, be neglected at this point because of the correspondence of the data to that of the literature. Also, additional experiments to characterize the hydrogel structure would have helped the understanding of the behavior of GO in the obtained hydrogels. This can be done by rheology, FTIR, X-ray diffraction, transmission electron microscopy, differential scanning calorimetry. Also, optimum concentration of GO could not be defined through this experiment.

Immediate future research can be done on the GO/AA hydrogels obtained by this experiment to characterize the structure of hydrogel and validate the data proposed. Optimization of the GO, AA and CS concentrations is necessary in order to obtain a tough and strong hydrogel with exceptional mechanical strength. A lot of future research is needed to unravel the true potential of GO and GO-reinforced nanocomposite materials. Experiments to characterize the antimicrobial and cytotoxicity properties of the GO hydrogels will be useful to understand their biocompatibility. Hydrogels are gaining recurring importance in the field of regenerative medicine and tissue engineering. Further research to study the behavior of GO with other polymers should be done so as to acquire the hydrogel which has the greatest suitability in biological systems. GO produces irreversible modifications to the graphene structure. Many studies have been concentrated on the use of GO in polymeric systems (Potts et al., 2011). Methods to incorporate graphene, in its original form, should be developed to harness its properties to polymeric systems. Chemists, in collaboration with biologists, can develop GO hydrogels useful for transdermal drug delivery and wound healing.

SECTION 5- CONCLUSION

In this experiment, a novel redox initiating system consisting of GO and ceric ion (Ce+4) is demonstrated for the polymerization process. Also, GO/AA hydrogels using CS as the initiator, have been synthesized which exhibit good mechanical properties. The results indicate the successful synthesis of tough and elastic hydrogels. The concentration of GO and acrylamide actively affects the polymerization process and mechanical properties of the hydrogels. The swelling ratios are not affected to a great extent with a change in GO content, as is demonstrated by the experiment. It was seen that the swelling behavior of the hydrogels made using PP initiator was better than that of the ones with the CS initiator. Also, the mechanical strength of the PP hydrogels was greater than that of the CS hydrogels. The novel initiating system consisting of ceric ion and GO has the potential to bring about high degree of polymerization. Further optimization and replication of data is needed for its validation.

Writing Services

Essay Writing
Service

Find out how the very best essay writing service can help you accomplish more and achieve higher marks today.

Assignment Writing Service

From complicated assignments to tricky tasks, our experts can tackle virtually any question thrown at them.

Dissertation Writing Service

A dissertation (also known as a thesis or research project) is probably the most important piece of work for any student! From full dissertations to individual chapters, we’re on hand to support you.

Coursework Writing Service

Our expert qualified writers can help you get your coursework right first time, every time.

Dissertation Proposal Service

The first step to completing a dissertation is to create a proposal that talks about what you wish to do. Our experts can design suitable methodologies - perfect to help you get started with a dissertation.

Report Writing
Service

Reports for any audience. Perfectly structured, professionally written, and tailored to suit your exact requirements.

Essay Skeleton Answer Service

If you’re just looking for some help to get started on an essay, our outline service provides you with a perfect essay plan.

Marking & Proofreading Service

Not sure if your work is hitting the mark? Struggling to get feedback from your lecturer? Our premium marking service was created just for you - get the feedback you deserve now.

Exam Revision
Service

Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.