Synthesis and Characterization of CdS-P (NIPAM-co-MAA) Hybrid Micro gels.
: Summary: Copolymer containing both pH and thermo sensitive properties are very much interesting due to their broad nature to various stimuli. Further, the incorporation of inorganic nanoparticles into stimuli responsive copolymers enhances their utility in different applied nature properties. In the present work such an attempt is made to synthesize copolymer of N-isopropyl acrylamide (NIPAM) and Methacrylic acid (MAA) with CdS nanoparticles. The copolymer of N-isopropyl acrylamide (NIPAM) and Methacrylic acid (MAA) was prepared through emulsion polymerization technique with various compositions and characterized by Fourier transform infrared spectroscopy (FTIR). The microspheres thus prepared were employed as micro-reactors for the deposition of semiconductor cadmium sulfide (CdS) nanoparticles. The obtained composite was characterized using optical, structural and thermal techniques. The micro gels were found to be stable up to 200 C. The crystal structure and grain size of Cadmium sulfide-poly (isopropylacrylamide-co-methacrylic acid) [CdS-P(NIPAM-co-MAA)] hybrid micro gels was studied by using X - ray Diffraction. UV Visible spectroscopy and photoluminescence spectroscopy was engaged to get the optical properties of the samples. It was found that the synthesized nanoparticles have a blue shift (higher energy) at about 360 nm which may be due to the typical quantum confinement effects.
Keywords: Cadmium sulfide, Nanoparticles, Emulsion polymerization, Photoluminescence, Optical properties.
Over the past several decades, polymer microgels have attracted much attention both in fundamental studies of colloid science and in extensive applied fields. As is well known, polymer micro gels have rapidly found important applications in materials science areas owing to their cross linked structures with three-dimensional network topologies and their ability to undergo large swelling-deswelling in reaction to a variety of stimuli (e.g., temperature, pH, ionic strength, electric and magnetic fields, etc.) [1-3]. These stimuli-responsive polymer micro gels have attracted significant interest because of their potential applications in drug delivery , chemical sensors , optical devices , catalysis , pollution control  and tissue engineering . This has led to the synthesis of new micro gels with better response upon application of stimuli like temperature , pH , solvent polarity , electric field , light irradiation , ionic strength  and the presence of some specific chemicals. Response of micro gels to these stimuli is associated with the physical and chemical nature of repeat units present in the network of the polymer matrix.
Similarly stimuli-responsive composite polymer gels containing inorganic nanoparticles are developing as an important class of materials based on the unique size-dependent properties of nanoparticles in combination with the macroscopic properties of polymer gels. Because the incorporation of nanoparticles into polymer gel provides additional functionality e.g., optical responsiveness  catalytic activity  and magnetic properties .Literature reports are also available on the production of stimuli-responsive micro gels containing metal nanoparticles , nanorods , quantum dots [21,22] and magnetic nanoparticles . Thermoresponsive [23, 24], chemical responsive [25-28] and pH-responsive hydro gel systems [29-31] are also being studied with specific interest.
Poly (methacrylic acid) (PMAA) is known to undergo a marked pH-induced conformation transition [32, 33]. At low pH, PMAA chains acquire on a highly compact form in order to minimize the hydrophobic interaction. At a high degree of ionization and in the absence of electrolytes, PMAA chains open to an expanded coil. Correspondingly, the swelling/collapsing of PMAA gels are also highly pH dependent . With increasing pH, PMAA gels can swell and show a maximum swelling in a pH range of 7-10. On the other hand, poly (N- isopropylacrylamide) P (NIPAM) is a reversible thermal-sensitive polymer. A PNIPAM single chain undergoes a coil-to-globule transition at 31.8 C  while the PNIPAM gel undergoes a volume phase transition around 33.8 C . These are explored due to their remarkable changes in physicochemical properties above their volume phase transition temperature (VPTT, -31C).
The work with poly acrylic acid co N- isopropylacrylamide) P (AA- co- NIPAM  is reported but there is no report on Poly (methacrylic acid co N-isopropylacrylamide) P (MAA-co- NIPAM) microgel. The PMMA is pH sensitive while P (NIPAM) is thermo sensitive, so it will be verymuch interesting to have a copolymer with both of these. The utility of these particular micro gels can be broadened by changing the range of functionalities that can be efficiently incorporated . Further, the incorporation of Cadmium sulfide, CdS particles into this copolymer is also not reported in literature. CdS nanoparticles are very important in terms of their use in semiconductor and electronic devices. Hence, the objectives of this study are to synthesize and characterize "monodisperse" submicron MAA/NIPAM copolymer micro gel particles covering a wide range of co monomer composition and their use as micro reactors for the deposition of Cadmium sulfide nanoparticles.Results and Discussion
As is shown in Fig. 1, characteristic peaks for the copolymers (G1, G2, and G3) are observed. The peak at 1728 cm-1 is for the CO, 1461 cm-1 is ascribed to amide bond CN stretching of G1, G2, and G3 while the peak around 1530 cm-1 is due to the NH stretching of the same bond. The peaks at about1366 and 1388 cm-1 are for deformation peaks of two methyl groups on -C (CH3)2. The peak at 1460 cm-1 is for the -CH3 and CH2- while 1551 cm-1 peak isfor secondary amide N-H stretching of amide II band and 1648 cm-1 peak is for amide I band. The peaks at2877 cm-1 are -CH3 symmetric stretching, 2934 cm-1 peak is for asymmetric -CH2- stretching, 2975 cm-1 peak is -CH3 asymmetric stretching while 3437 cm-1 peak is N-H stretching. These all are as described inliterature for PNIPAM .Similarities and differences are clear by simple comparision of the two spectra. With exception of PNIPAM, appearance of thecharacteristic absorption band of carboxyl group in curve G1, G2, and G3 of Fig. 1 (1728 cm-1) is a strong evidence of incorporation of MAA into the copolymer microgel.
The TG results of the copolymers under the atmosphere of nitrogen are shown in Fig. 2. This Figure clearly shows the thermal stability of microgel up to 300 C. The thermal behavior of CdS-P (NIPAM-co-MAA) composite is also compared with the thermal stability of pure P (NIPAM-co-MAA) micro gels in the same figure. In both cases, the weight loss below 300 C can be ascribed to the evaporation of physically absorbed water and residual solvent from the samples. The main weight loss shown at 335C corresponds to pyrolysis of the polymer, P (NIPAM-co-MAA). Furthermore, it should be noted that the major weight loss ended at around 370 C for CdS- P (NIPAM-co-MAA) hybrid micro gels (Fig. 2a) while it is at around 450 C for P (NIPAM-co-MAA) micro gels (Fig. 2b). These results show that the presence of CdS within the network structure of the polymer might enhance the degradation of the polymer micro gel template, indicating that CdS nanoparticles could accelerate pyrolysis of P (NIPAM-co-MAA). This phenomenon is probably related to the finding that CdS in nano size could enhance the combustion of the polymer. The observed effects are characteristic for polymers filled with inorganic particles .The X -ray Diffraction (XRD) patterns of the nanoparticle sample of CdS 1 and 2 are shown in Fig. 3. The XRD peaks are indicating very fine size of the grains of the sample. The XRD patterns of both the samples exhibits prominent, peaks at 2 values of26.60, 44.20 and 52.03 for sample 1 and respectively26.52, 43.95 and 52.30 for sample 2, these correspond to the (1 1 1), (2 2 0) and (3 1 1) planes ofthe cubic phase CdS .In order to achieve more confirmative information, the crystallite size (Dhkl) of cadmium sulfide crystals was estimated from line broadening of the (1 1 1) diffraction peak, according to the Debye-Scherrer equation .
Dhkl K/ (Bcos) (1)
where is the wavelength of Cu KA radiation ( 1.5406 A ), B is the full-width at half maximum (FMWH) intensity, is the diffraction angle (in radian) of the considered diffraction peak, and K is a Scherrer constant taken as 0.94 for the almost spherical particles. The crystallite sizes obtained from the above equation are 3.78 and 4 nm for samples 1 and 2, respectively. The nanocrystalline nature is also shown by broadening of the peaks.
UV-Visible absorption spectra of the CdS-P (NIPAM-co-MAA) nano-composites with different CdS nanoparticles, and bulk CdS are shown in Fig. 4. The spectra of samples with CdS i.e., CdS-P (NIPAM-co-MAA) were found significantly blue- shifted as compared to bulk CdS. In case of CdS nanoparticles prepared via cadmium acetate/ thioacetamide the absorption edge is located at about362 nm while the same prepared from cadmium chloride/ sodium sulfide shows it at 375nm. The maximum absorption for CdS as a bulk material is around 450nm. The blue-shift of the absorption onset indicates that this sample and other CdS clusters are quantum-confined due to decreasing particle size.The photoluminescence (PL) spectrum (Fig.5) of CdS nanoparticles was obtained by dispersing 1 mL sample in 10 mL of ethanol. It is shown to emit blue light in the range of 510520 nm under photoluminescence excitation at 360 nm which is also the maximum absorption peak observed in UV- VIS spectra. The luminescence at 520 nm may be due to a higher level transition in CdS crystallites. It may also be as a result of the small dimensions of nanoparticles along with quantum confinement effects. This is in agreement with UVvisible spectra where the band gap absorption was at 360 nm (Fig.4). It was difficult to observe the band-edge emission peak because of the low excitation binding energy of CdS, especially when the crystal size is very small and atomic concentration on the surface is very high. However, the bandband transition of CdS nanocrystals may be observed on the PL spectra .The band gap energy of the synthesized CdS, determined from the absorption spectra is found to be 2.65 eV and the band gap of Bulk CdS was2.42eV. The band gap of the corresponding nanoparticles was calculated by plotting (Ah)2 vs h (shown in Fig. 6) using Tauc relation .
Ah const (h-Eg)1/2 (2) The value of A is obtained from relationA 2.3026(A/ x) (3)
where A is absorption and x is the thickness of the sample.The relation between (Ah)2 and h satisfies the equation (2) for n 0.5. It is a known fact that n 0.5 for direct transitions (Crystalline materials) and 2for an indirect ones (amorphous materials), [44, 45] therefore, since CdS is crystalline material, so its nanoparticles are behaving like a direct band gapsemiconductor. The intercept of photon energy axisin Fig. 6 shows the band gap energy of the CdSnanoparticles is 2.65eV.
N-isopropylacrylamide (NIPAM monomer) was obtained from Sigma (99%), which was recrystallized in hexane-acetone (1:1 volume ratio) mixture and was dried under vacuum before use. Methacrylic acid (MAA, 99.5%), N, N-methylene bis-acrylamide (MBA,), and hydrochloric acid (HCl) were purchased from Sigma, while ammonium per sulfate (APS), ammonium hydroxide (25%), sodiumdodecyl sulphate (SDS) were from Merck. All ofthese were used as such. Deionized water (resistance of 18 M) (Milli-Q Reagent Water System, Millipore Corporation) filtered through a 0.2 m filter was used in all experiments. Filtration was done to remove any particulate matter.
For preparation of Cadmium sulfide (CdS) nanoparticles, cadmium chloride (CdCl2 99%, Sigma) and Sodium Sulfide (Na2S 99%, Merck) were used as received.
Preparation of P(NIPAM-co-MAA) micro gels
The micro gel particles were prepared by emulsion copolymerization in aqueous solution. Appropriate amounts of NIPAM, MAA, MBA, and SDS (see Table 1) were added to 100 mL water. A250 mL glass reactor fitted with nitrogen bubbling tube, stirring glass rod containing a 6 cm Teflon paddle and a reflux condenser was used. In order to remove oxygen, the solution was heated to 70 C in an oil bath, with continuous stirring for 40 min at 400 rpm while being purged with nitrogen. Then, 0.0022g APS dissolved in 5 mL of water was added. The reaction was allowed to proceed for 5 h. A portion of the polymerized lattices was purified to remove SDS, free monomers and other impurities by four successive centrifugations, decantations, and redispersions in deionized water. The resultant micro gel was further purified by dialysis for one weekthrough Spectra/Por(R) molecular porous membranetubing with a cutoff of 12000-14000, while changing water very frequently at room temperature (25 C). The other part of the lattices from the reaction mixture was centrifuged, decanted, and vacuum-dried to constant weight at 45 C.
Table-1: Feed composition of p(NIPAM-co-MAA) micro gel particles.
Sample MAA NIPAM###BIS###SDS###APS###Total###H2O
G0###0###1.9554 0.12336 0.055 1 A- 10-3###0.018###100
G1###0.08298 1.87393 0.06168 0.055 1 A- 10-3###0.018###100
G2###0.09957 1.83319 0.12336 0.055 1 A- 10-3###0.018###100
G3###0.13276 1.7721 0.1542 0.055 1 A- 10-3###0.018###100
Preparation of CdS (NIPAM-co-MAA) hybrid micro gels
The hybrid polymer micro gel containing cadmium sulfide nanoparticles were prepared by taking 10 mL of pure polymer micro gel (G2) in a250mL round bottom flask.
Synthesis of CdS (1) (NIPAM-co-MAA)Firstly stock solutions of Cd(CH COO) 2H O (5 mmoles, 1.33 g) andthioacetamide (10 mmols, 0.75 g) were prepared freshly in appropriate solvent (water). Then 10mL ofCd (CH3COO) 22H2O stock solution and 0.4326g of SDS were added to 10mL of pure polymer micro gel in the corresponding solvent. Stirring and purgingwas done for 1 h so as to complete the dispersion of Cadmium ions (Cd+) in the polymer micro gel network at 30 C. After an hour of continuous stirring and purging, 10mL of thioacetamide was added. Stirring and purging was continued for 20 h giving yellowish orange color of Cadmium Sulfide nanoparticles. After the reaction, the synthesized samples were purified from Cadmium free ions bydialysis and successive centrifugation.
Synthesis of CdS (2) (NIPAM-co-MAA)
First, 2mM stock solutions of cadmium chloride and sodium sulfide in appropriate solvent (water) were prepared freshly. Then, 5mL of portion of CdCl2 stock solution was added to 10mL of pure polymer micro gel containing 0.4326g of SDS in the corresponding solvent. Stirring and purging was done for 1 h so as to complete the dispersion of Cadmium ions (Cd+) in the polymer micro gel network at 30 C. After an hour of continuous stirring and purging,5mL of Na2S was added. Stirring and purging was continued for 20 h giving yellow color of Cadmium Sulfide nanoparticles. After the reaction, the synthesized samples were purified from Cadmium free ions by dialysis and successive centrifugation.
The chemical structures of the pure micro gels and hybrid composite polymer micro gel were analyzed by Fourier-transform infrared spectroscopy (FTIR) using an FTIR Shimadzu (IR Prestige) Japan on a resolution of 4 cm-1 with 100 accumulations. The FTIR samples were prepared after drying the emulsions at 40C for 12 h and were measured usingthe standard KBr method.
To determine the CdS contents in composite particles, the thermo gravimetric analysis (TGA) of NIPAM-co- MAA microgel and hybrid polymer containing CdS nanoparticles was carried out on a Schimadzu DT-40 TG-DTA Analyzer in nitrogen atmosphere. The microspheres used as sample for TGA were dried at 45C for 48 h before measuring. The samples were analyzed in closed aluminum cups in the temperature range 30-600 C at a heating rate of 5 C/min to examine the thermal properties of the polymers.
X-ray powder diffraction (XRD) spectra of the hybrid micro gel were recorded by using a JDX-3532, JEOL, Japan, with Cu KA (Wavelength1.540A) radiation at 35 kV and 40 mA. A scan rate of 0.02/s in the 2 range of 1070 was applied.
UVVIS absorption spectrum of the sample was recorded employing a double beam, Perkin Elmer Lambda 50 UV/VIS spectrophotometer, with quartz cuvettes of 1 cm optical path length.
The Photoluminescence spectrum was obtained using Perkin Elmer LS-50 fluorescence spectrophotometer with an excitation wavelength of360 nm. The excitation and emission bandwidths were both 1 nm.
In present paper we report the preparation of hybrid micro gels [CdS-P(NIPAM-co-MAA)] which consists of polymeric matrix and CdS nanoparticles. Incorporation of CdS was performed by treatment of cadmium acetate and thioacetamide in the presence of P(NIPAM-co-MAA) micro gel particles. In this way hybrid micro gels with CdS contents have been prepared. Obtained hybrid micro gels were quite stable in aqueous solution above 200 C. The prepared CdS nanoparticles have a blue-shift to higher energy which shows a typical quantum confinement effect. Band gap values also confirm the quantum confinement effect.
One of us (AK) acknowledges the financial support of Higher Education Commission, Islamabad, Pakistan under the IPFP program.
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|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Apr 30, 2014|
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