CHAPTER 2: Experimental
2.3 Sample Characterization
2.3.7 Mechanical Properties Analysis
All hydrogels were prepared in cylindrical shapes with 10mm diameter and 4inch length before the uniaxial tensile measurement was conducted. The tensile measurement was conducted on a Test Resources universal testing machine (UTM) at ambient temperature, with the crosshead speed of 100mm/min, and initial jaws of 10 mm. The stress (from the initial cross–section (A=πr2) of 74.58mm2) and strain were recorded. The stress (σ) was calculated according to the equation,
σ = F/πr2
Where, F is the recorded load and r is the original radius of the specimen.
The strain (ε) was calculated from the change of the fracture length (l) to the initial gauge length (l0) of the measured sample, and was calculated by the equation,
ε = 𝑙 𝑙0×100
The initial modulus was calculated as the initial slope of the stress−strain curve. The fracture toughness was calculated by integrating the area underneath the stress−strain curve of each sample.
42 References:
[1] H. Tetsuka, R. Asahi, A. Nagoya et al., “Optically Tunable Amino- Functionalized Graphene Quantum Dots,” Advanced Materials, Vol. 24, pp.
5333-5338, (2012).
[2] P.-G. Su, and H.-C. Shieh, “Flexible NO2 sensors fabricated by layer-by-layer covalent anchoring and in situ reduction of graphene oxide,” Sensors and Actuators B: Chemical, Vol. 190, pp. 865-872, (2014).
[3] P.-G. Su, and Z.-M. Lu, “Flexibility and electrical and humidity-sensing properties of diamine-functionalized graphene oxide films,” Sensors and Actuators B: Chemical, Vol. 211, pp. 157-163, (2015).
CHAPTER-3
Result & Discussion
44 0.1 Synthesis of GO Based 2D Cross-linker (GOBC):
Functionalization of GO can fundamentally change the properties of GO. The resulting chemically modified graphene could then potentially become much more adaptable for many applications. There are many ways in which GO can be functionalized, depending on the desired application. NHS is used to prepare amine- reactive esters of carboxylate groups for chemical labeling, cross-linking and solid- phase immobilization applications. Carboxylates (-COOH) may be reacted to NHS in the presence of a carbodiimide such as EDC, resulting in a semi-stable NHS ester, which may then be reacted with primary amines (-NH2) to form amide cross-links (Fig. 3.1). Although NHS is not required for carbodiimide reactions, their use greatly enhances coupling efficiency.
Fig. 3.1: Schematic illustration of synthesis of GO based 2D cross-linker
+
EDC
Unstable reactive ester 0-5ºC
Semi-stable amine reactive NHS ester
A small part of GO
A small part of GO based 2D crosslinker
NHS 0-5ºC
AEM
24h at room tempeature
45 0.1.1 Functional Group Investigation by Fourier Transform Infrared
Spectroscopy (FTIR):
Evidence of successful functionalization of GO can be provided by FTIR spectra (Fig.
3.2). In the FTIR spectrum of GO, FTIR absorption peaks at 3453, 1742, 1630, 1400, 1232, and 1061cm−1 are assigned to O-H stretching mode, C=O in carboxylic acid and carbonyl groups, C=C skeleton vibrations of unoxidized graphitic domains, C-H in-plane stretching, epoxy C-O-C stretching, and alkoxy C-O stretching, respectively.
[1]
Fig. 3.2: FTIR spectra of GO & GOBC
In the case of GO based 2D cross-linker (GOBC), a peak at 3443cm-1 attributed to N–H stretching, which overlapped with a broad peak arisen from carboxylic acid stretching, can be seen. Other outstanding differences are observed in the appearance of the two peaks at 1562 and1157cm-1 attributed to N–H bending and C–N stretching.
A remarkable peak at 1640cm-1 corresponds to stretching of NHCO bond. Hence, FTIR analysis confirms the covalent functionalization of GO by 2-Aminoethyl methacrylate hydrochloride is done successfully. [2] [3] [4] [5]
Transmittance (%)
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1) 3453
1742
1630 1232
3443
1722 1562 1640
1157 GO GOBC
1400 1061
46 Table 3.1: Characteristic peaks and interpretations correspond to GO & GOBC
Wavenumber (cm-1)
Interpretations correspond to GO
Wavenumber (cm-1)
Interpretations correspond to GO 3453 O-H stretching 3443 N–H stretching 1742 C=O of carboxylic 1722 C=O of methacrylate
1630 aromatic C=C 1640 -NHCO- stretching
1400 C-H in-plane stretching 1562 N-H bending 1232 epoxy C-O-C stretching 1157 C–N stretching 1061 alkoxy C-O stretching
0.1.2 UV/Vis Absorption Spectra Analysis:
Fig. 3.3 shows the UV/Vis spectra of stable aqueous suspensions of GO and GOBC.
The curve of GO dispersion exhibits a maximum at 230nm and a tiny shoulder at 300nm, which are attributed to π–π* transitions of aromatic C-C bonds and n–π* transitions of C=O bonds, respectively. [1] [6] [7] Amino functionalization causes a clear bathochromic red-shift of the GO. Due to the formation C-N bond, an n–π* transitions occurs which corresponds at 211nm in the spectra. [8] [3]
Fig. 3.3: UV/Vis spectra of GO & GOBC
0.7
0.6
0.5
0.4 0.3
0.2
0.1 0.0
Absorbance (a.u.)
800 700
600 500
400 300 200
Wavelength (nm)
GO GOBC 230nm
211nm
300nm
47 0.1.3 Study of Surface Morphology of GOBC Using Field Emission Scanning
Electron Microscopy (SEM):
Surface morphology of prepared GOBC is further studied from the SEM analysis. The SEM images of GOBC are shown in Fig. 3.4 a) and b) respectively. GOBC sheets are informally folded on each other with many ripples and seem to form the tangled patches. Between the crosswise edges of the multi-layer of GOBC a porous network with a size ranging from 100 to 500 nm, is clearly observable. These specifications could be related to the hydrogen bending of amino surface functional groups. [2] [9]
[10]
Fig. 3.4: SEM image of a) GOBC with magnification×30,000 and b) GOBC with magnification×50,000
0.1.4 Energy-Dispersive X-ray Spectroscopy (EDS/EDX) Analysis of GOBC:
The EDS elemental mapping (Fig. 3.5) of carbon, oxygen and nitrogen of a large area of GOBC dispersed powder illustrate distribution of N functionalities on the graphene structure. The EDS elemental analysis, which is given in Table 3.2, reveals 58.61, 11.20, 30.18 mass % and 64.50, 10.70, 24.93 atom% for C, N and O respectively.
This analysis confirms the presence of nitrogen containing functional group on the surface of graphene oxide. [2] [9]
a) b)
48 Fig. 3.5: EDS spectra of GOBC
Element (keV) mass % atom %
C 0.277 58.61 64.50
N 0.392 11.20 10.70
O 0.525 30.18 24.93
Table 3.2: EDS elemental analysis of GOBC
0.2 Synthesis of GO Based 2D Cross-linked Poly(acrylic acid) (PAA-GOBC):
To prepare polymer composite, the in situ polymerization technique is attractive since it enables control over both the polymer architecture and the final structure of the composites. The key point of this approach relies on good dispersion of cross-linker in solvent, followed by in situ polymerization initiated by heat. Since cross-linker can be thoroughly exfoliated in water, it is very suitable to be used in preparation of hydrogel composites. In this research, GO based 2D cross-linked polymer composite is prepared through an in situ polymerization technique between acrylic acid (AA) monomer and GO based 2D cross-linker (GOBC) in presence of potassium persulfate
49 (KPS) at 55ºC temperature. The schematic illustration of preparation of 2D cross- linked poly(acrylic acid) is given below:
Fig 3.6: Schematic illustration of preparation of 2D cross-linked poly(acrylic acid) hydrogel (PAA-GOBC)
0.2.1 Study of Surface Morphology of PAA-GOBC Using Field Emission Scanning Electron Microscopy (SEM):
Morphological characterization on PAA-GOBC hydrogels was done through SEM. It can be seen that the morphology of the hydrogel is quite uniform in large scale (Fig.
3.7a). The image shows the presence of small sheets of GOBC uniformly dispersed in the polymer matrix of PAA-GOBC. Closer observation (Fig. 3.7b) shows that the hydrogels exhibit a three-dimensional network of randomly oriented sheet-like structures with wrinkled texture. The SEM images suggest that the GOBC sheets indeed behaved as a cross-linker. The covalent bonds of GOBC took part in the radical polymerization of AA. The bonding of multiple sites on the surface of GOBC sheets hence led to cross-link reactions during the polymerization. In other words, the GOBC sheets are like chemical cross-linkers leading to the successful formation of hydrogels. SEM images of the freeze-dried supramolecular hydrogels demonstrated that both supramolecular native and hybrid hydrogels exhibited a loose sponge-like Acrylic acid (AA)
GOBC
KPS initiator 55ºC, 4h
PAA-GOBC
+
50 structure indicating the leakage or removal of the trapped water molecules during freeze-drying. [11] [12]
Fig. 3.7: SEM image of a) PAA-GOBC with magnification×5,000 and b) PAA-GOBC with magnification×10,000
0.2.2 Energy-Dispersive X-ray Spectroscopy (EDS/EDX) Analysis of PAA- GOBC:
Elemental analysis of carbon, oxygen and nitrogen in a large area of PAA-GOBC which is given in Table 3.3, reveals 62.23, 11.46, 26.32 mass% and 67.75, 10.73, 21.52 atom% for C, N and O respectively.
Fig. 3.8: EDS spectra of PAA-GOBC-0.05%
a) b)
51 Element (keV) mass % atom %
C 0.277 57.28 63.21
N 0.392 11.81 11.17
O 0.525 30.92 25.62
Table 3.3: EDS elemental analysis of PAA-GOBC-0.05%
0.2.3 Thermogravimetric Analysis (TGA) of PAA-GOBC hydrogel:
A thermogravimetric analysis of the PAA-GOBC-0.05% hydrogel and PAA- BIS-0.05% hydrogel was carried out provided some useful information on thermal properties of these materials. Fig. 3.9 illustrates the TGA curves of PAA-GOBC- 0.05% hydrogel and PAA-BIS-0.05% hydrogel and the data are summarized in Table 3.4. A small first loss of mass is observed in the range of 100-200°C temperature and is attributed to the removal of water molecules trapped inside the hydrogel structure.
The first obvious weight loss for both hydrogels were observed in the range of 200- 300°C, indicating that the pyrolysis of oxygen containing functional groups of hydrogels. Another major degradation observed in 300-450°C temperature range corresponds to decomposition of cross-linked polymer networks. TGA analysis of PAA-GOBC-0.05% and PAA-BIS-0.05% is confirmed that both hydrogels have similar thermal stability.[12][13]
52 Fig. 3.9: TGA analysis of PAA-GOBC-0.05% & PAA-BIS-0.05%
Hydrogel
loss of weight (%) at temperature (°C)
200 300 400 500 600
PAA-GOBC-0.05% 5.39 34.55 61.72 83.34 85.96
PAA-BIS-0.05% 6.37 36.82 60.23 83.92 86.01
Table 3.4: Thermogravimetric data of PAA-GOBC-0.05% & PAA-BIS-0.05%
0.2.4 Differential Scanning Calorimetric Analysis of PAA-GOBC:
But DSC heating thermograms of PAA-GOBC-0.05% and PAA-BIS-0.05% are summarized in the Fig. 3.10. The curve of PAA-GOBC hydrogel exhibits higher Tg and Tm than PAA-BIS which is attributed to the presence of greater intermolecular force and cross-linking in PAA-GOBC hydrogel. So the DSC graphical representations are clearly illustrated better thermal stability of PAA-GOBC over PAA-BIS.
100
80
60
40
20
0
Mass (%)
800 700 600 500 400 300 200 100
Temperature (°C)
PAA-BIS-0.05%
PAA-GOBC-0.05%
53 Fig. 3.10: DSC thermograms of PAA-GOBC-0.05% & PAA-BIS-0.05%
0.2.5 Study of Swelling Kinetics of PAA-GOBC Hydrogel
The swelling behavior is one of the important characteristic parameters for any hydrogel and is depending on the nature of the polymers and the environmental conditions (pH, temperature etc. of the medium). When a polymer network is in contact with an aqueous solution, the network starts to swell due to the thermodynamic compatibility of the polymer chains and water. Fig. 3.11 shows the swelling ratio (SR) as a function of time for the PAA-GOBC composite hydrogels in distilled water at 25ºC. Initially the rates of swelling increased rapidly. The swelling ratio increased to 192.56 within initial 24h and then it approaches swelling equilibrium around 32h.
We continuously observed the hydrogel swelling behavior for more than 2 days and it was found that there is no apparent dissolving phenomenon; this result could be used to conclude that there is some kind of force maintaining the hydrogel network integrity. Because of chemical cross-linking agent is used during the hydrogel preparation process, the entanglement of polymer chains constructed the hydrogel network and the GOBC could be regarded as the „analogous cross-linking points‟.
Heat flow (mW/mg)
400 350 300 250 200 150 100 50
Temperature (°C) Tg=54.01°C
Tg=61.26°C
Tm=290.26°C Tm=280.01°C PAA-BIS-0.05%
PAA-GOBC-0.05%
54 Fig. 3.11: Swelling kinetic curve of PAA-GOBC hydrogel
0.2.6 Mechanical Properties of PAA hydrogels:
PAA-GOBC-gels and PAA-BIS-gels are differed markedly with regard to their mechanical properties. Typical strain–stress curves for different weight (%) of cross- linker are given in Fig. 3.12. As can be clearly seen, the tensile strength and modulus of PAA-GOBC are very low compared with the mechanically toughened PAA-BIS.
But PAA-BIS readily breaks at lower deformation in elongation and its elongation at break are in the range of 141% to 474%. On the other hand PAA-GOBC hydrogels show totally different tensile behaviour from those of PAA-BIS hydrogels, suggesting that a strong interaction exists among GO sheets and polymer matrix even in such a water-abundant hydrogel. The PAA-BIS hydrogels exhibit a brittle fraction, on the other hand, PAA-GOBC hydrogel exhibit rubber-like behaviour. Elongations of PAA- GOBC hydrogels are much higher (1492% to 1919%) than those of PAA-BIS hydrogels. As shown in Fig. 3.13, at constant water content, the elongations at break of PAA-GOBC hydrogels are completely dependent on GOBC content, while strength, modulus and elongation of PAA-GOBC hydrogels change with increasing GOBC content. At constant AA and water content content, the elongations at break of PAA-GOBC-0.025% was found the highest but the tensile strength and modulus of
250
200
150
100
50
0
Swelling ratio (g/g)
3000 2500 2000 1500 1000 500
0
Time (min)
PAA-GOBC-0.05%
55 this hydrogel were found comparatively lower than other two compositions. That means PAA-GOBC fracture readily on elongation increases with increase of GOBC content which indicates that higher content of cross-linker increases brittleness of hydrogel. Such a simultaneous improvement in elongation for polymer composites with a low amount of GO sheets is most likely caused by the strong interaction between GOBC sheets and PAA matrix. It may be attributed to the different network nature between the two types of hydrogels. [11]
Fig. 3.12: Stress-strain curves of PAA-GOBC-0.05% & PAA-BIS-0.05%
Fig. 3.13: Comparison of mechanical properties of PAA-GOBC & PAA-BIS hydrogels
0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
Stress (MPa)
2000 1500
1000 500
0
Strain (%)
PAA-BIS-0.01%
PAA-BIS-0.025%
PAA-BIS-0.05%
PAA-GOBC-0.01%
PAA-GOBC-0.025%
PAA-GOBC-0.05%
0.4
0.3
0.2
0.1
0.0
a) b) c) d) e) f)
Hydrogels
Tensile Strength (MPa) Young's Modulus (MPa)
2000
1500
1000
500
0
Elongation at Break (%)
a) b) c) d) e) f)
Hydrogels
56 Hydrogel Composite Tensile Strength
(MPa)
Young's Modulus (MPa)
Elongation at Break (%)
a) PAA-BIS-0.01% 0.155 0.274 337
b) PAA-BIS-0.025% 0.134 0.226 474
c) PAA-BIS-0.05% 0.152 0.289 141
d) PAA-GOBC-0.01% 0.066 0.192 1882
e) PAA-GOBC-0.025% 0.046 0.138 1919
f) PAA-GOBC-0.05% 0.054 0.139 1492
Table 3.5: Table of mechanical properties of PAA-GOBC & PAA-BIS hydrogels Since PAA-GOBC hydrogels show extraordinarily large deformation, and the polymer chains in the swollen state at ambient temperature could be regarded as flexible polymer chains just like those in the rubbery state of a solid polymer, the effective cross-link chain density of PAA-GOBC hydrogels can be evaluated by the kinetic theory. The polymer chains in PAA-BIS hydrogels have a broad distribution in length between the cross-link points and are restricted due to the random nature of cross-link reaction. Moreover, the friction between the chains is small in PAA-BIS hydrogel. These two factors result in the inhomogeneous distribution of tension among the polymers. The scission of the polymer chains occurs when the sample is under tension. However, the situation is different in the PAA-GOBC hydrogels. The network of the PAA-GOBC hydrogels is composed of rigid GO sheets and flexible polymer chains, which increases the friction between the chains by sliding of rigid GO sheets under the external tensile force. The rigid GO sheets are not only able to orient along the direction of tension, but also prevent effectively crack propagation by suppressing the stress concentration during the tensile measurement.
57 Fig. 3.14: a) photographs of the PAA-GOBC-0.01% hydrogel, b) highly stretched PAA-GOBC-0.01% hydrogel, c) knotted PAA-GOBC-0.01% hydrogel, d) highly stretched PAA-GOBC-0.01% hydrogel even with a knot
Fig.3.15: Comparison of toughness of PAA-GOBC & PAA-BIS hydrogels
120
100
80
60
40
20
0 Toughness (MJm-3 )
a) b) c) d) e) f)
Hydrogel
58 Hydrogel Composite Toughness (MJm-3)
a) PAA-BIS-0.01% 4.23
b) PAA-BIS-0.025% 5.30
c) PAA-BIS-0.05% 1.65
d) PAA-GOBC-0.01% 13.46 e) PAA-GOBC-0.025% 9.87
f) PAA-GOBC-0.05% 7.17
Table 3.6: Table of toughness of PAA-GOBC & PAA-BIS hydrogels
The stress, strain and toughness of the PAA gels increase when the cross-linker content is lower, and decrease when the cross-linker content is higher (Fig. 3.16 and 3.17). It perhaps results from the interplay between the covalent cross-linking and the molecular weight of the PAA network chains among the adjacent cross-linking points.
Under deformation, the covalently cross-linked network remains intact to absorb and redistribute the applied stress and thus delay the crack propagation via the PAA network chains within the covalent cross-linking, consequently maintaining the original configuration. A higher cross-linker content means more cross-linking, and it can effectively absorb and redistribute more applied stress. However, further increasing cross-linker content may induce degraded mechanical properties. The degraded stress, toughness and stretchability of the PAA hydrogels at high cross- linker content are attributed to a highly dense covalently cross-linked network within homogeneous cross-linking points and short network chains of the polymer. [14]
59 Fig. 3.16: Stress-strain curves of PAA-BIS & PAA-GOBC hydrogels
Fig. 3.17: Toughness curves PAA-BIS & PAA-GOBC hydrogels
0.3 Conclusion:
In this research, oxidized graphene oxide with a good no. of –COOH functional group is synthesized following modified Hummers‟ method and then it is successfully functionalized by 2-aminoethylmethacrylate to prepare acrylate functionalized graphene oxide (AFGO) following EDC coupling reaction process. Formation of acrylate functionalized graphene oxide (AFGO) is confirmed by FT-IR and UV/Vis specta analysis. Surface morphology of prepared AFGO is studied from SEM analysis and presence of N containing functional group is confirmed by EDS analysis.
160
150
140
130
120
Stress (kPa)
50x10-3 40
30 20
10
BIS content (wt%)
500
400
300
200
100
Strain (%)
70
65
60
55
50
45
40
Stress (kPa)
50x10-3 40
30 20
10
GOBC content (wt%)
2000
1900
1800
1700
1600
1500
1400
Strain (%)
60
50
40
30
20
10
0 Toughness (Jm-3 )
50x10-3 40
30 20
10
BIS content (%)
120
110
100
90
80
70
60 Toughness (Jm-3 )
50x10-3 40
30 20
10
GOBC content (%)
60 Poly(acrylic) acid hydrogel cross-linked by graphene oxide based 2D cross-linker (acrylate functionalized graphene oxide) and N,N-methylenebisacrylamide (BIS) were prepared successfully through in-situ free radical polymerization. Surface morphology of prepared PAA-GOBC hydrogel is studied from SEM analysis and presence of N containing functional group is confirmed by EDS analysis. Thermal stability of PAA- GOBC and PAA-BIS hydrogels were studied by TGA and DSC analysis which confirmed better thermal stability of PAA-GOBC over PAA-BIS. Remarkable elongation, ductility and toughness of PAA-GOBC hydrogel were found in the study of mechanical properties using UTM. Elongation at break (%) of PAA-GOBC-0.025 (wt%) was 1919% where PAA-BIS-0.025 (wt%) can be stretched only 474%. So, it can be summarized as fabricated PAA-GOBC hydrogel is highly stretchable, ductile and tough hydrogel.
61 References:
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59874-59880, (2015).
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117, pp. 6516-6524, (2013).