Synthesis and Characterization of Poly(N-isopropylacrylamide) Hybrid Microgels with different Cross-linker Contents

Transcription

1 LUQMAN ALI SA et al., J.hem.Soc.Pak., Vol. 35, No. 6, Synthesis and haracterization of Poly(N-isopropylacrylamide) ybrid Microgels with different ross-linker ontents 1 Luqman Ali Shah, 1,2 Zahoor ussain Farooqi, 1 ina Naeem, 1 Syed Mujtaba Shah and 1 Mohammad Siddiq * 1 Department of hemistry, Quaid-I-Azam University, Islamabad 45320, Pakistan. 2 Institute of hemistry, University of the Punjab, New ampus, Lahore 54590, Pakistan. m_sidiq12@yahoo.com* (Received on 25 th October 2012, accepted in revised form 2 nd May 2013) Summary: Four compositions of thermo sensitive poly(n-isopropylacrylamide) polymer microgels were synthesized with different cross-linking contents by free radical emulsion polymerization. ybrid gels were synthesized by reducing silver ions inside the polymer chains by using NaB 4 as a reducing agent. Effects of temperature and cross-linking density on the size of microgel particles were studied. Variation in size and volume phase transition temperature (VPTT) by the introduction of silver nanoparticles in microgels was also investigated. It was found that the size of the microgel particles decreases by increasing temperature. FT-IR spectroscopy was used to confirm the presence of different functional groups in the microgels system. Dynamic Laser Light Scattering (DLLS) was used to determine the hydrodynamic radius (R h ) of the microgel particles at different temperatures. Surface Plasmon Resonance (SPR) behavior of the hybrid gels was investigated by UV-Vis spectroscopy. Key words: ybrid gels, cross linking density, dynamic laser light scattering, volume phase transition temperature. Introduction Polymer microgels are the three dimensional cross-linked colloidal particles which can swell and deswell in a suitable solvent by changing the environmental conditions such as temperature, p, electric field, magnetic field and ionic strength [1, 2]. These smart materials have gained remarkable attention in various fields, for example catalysis, drug delivery, sensors, optical devices and enzyme immobilization. Temperature sensitive microgels, are the main focus of studies and numerous reports have been published on the microgels with negative temperature sensitivity [3]. N-isopropylacrylamide (NIPAM), in particular, has attracted much attention because of the reversible volume phase transition that occurs in this system [4, 5]. Pelton and hibante reported synthesis of the first temperature-sensitive microgel obtained from NIPAM and N, N-methylene bis-acrylamide (MBA) in 1986 [6]. Poly(NIPAM) has sponge-like particles which exhibit drastic swelling/deswelling transitions with a large magnitude of volume change around volume-phase transition temperature(vptt). Presently, several studies have been carried out on the volume phase transition and swelling behavior of poly(nipam) microgels as a function of different environmental conditions [6, 7]. Recently microgels have been used as microreactors for the formation of nanoparticles (NP) owing to their simple synthesis procedure, easy functionalization, and control over particle size. This polymer matrix has the potential to control the interactions and hence ensure well-defined spatial distribution of nanoparticles. * To whom all correspondence should be addressed. Yan Lu. [8] synthesized silver nanoparticles in thermosensitive core-shell microgel particles. The core and shell consisted of polystyrene (PS) and poly (NIPAM) respectively. eating the suspension above 32 led to volume transition within the shell which was followed by a marked shrinking of network of the shell. Silver nanoparticles with diameters ranging from 6.5 to 8.5 nm were embedded into thermosensitive poly(nipam) networks with different cross-linking densities. The Ag nanoparticles did not influence the swelling and shrinking of network in the shell. The surface plasmon absorption band of the nanoparticles was shifted to longer wavelengths by increasing temperature. Shiyong Liu. [9] prepared silver nanoparticles inside the highly stable hybrid unimolecular micelles with thermosensitive poly(nipam) shells. eating of the hybrid unimolecular micellar solutions led to the shrinkage of poly(nipam) shell and allowed for tuning the relative spatial distances between the neighbouring silver nanoparticles. Pastoriza Santos et al. [10] synthesized gold nanoparticles in thermosensitive microgels and measured the VPTT. They found that VPTT was similar for both pure and hybrid microgels. The presence of a metal core did not significantly affect the swelling behavior of the poly(nipam) shell. In the present work, thermo sensitive pure poly(nipam) and (poly(nipam)-ag) hybrid microgels, with different cross-linking densities, are synthesized and the effect of various parameters (such as temperature and cross linking density) on the

2 LUQMAN ALI SA et al., J.hem.Soc.Pak., Vol. 35, No. 6, size of microgel particles is studied. Additionally, Surface Plasmon Resonance behavior of silver nanoparticles (Ag NPs) is also investigated. Results and Discussion Physical Appearance of Poly(NIPAM) Microgels poly(nipam) microgels were synthesized via free radical precipitation polymerization (conventional emulsion polymerization) process. When the temperature of reaction mixture reached at 70, persulphate ions (S 2 O 2-8 ) of the initiator (APS) were decomposed and consequently SO 4 free radicals were produced which initiated polymerization of NIPAM. After 20 min of the polymerization/cross-linking process, the color of solution was changed from transparent to a milky dispersion and remained milky even when the solution was kept at room temperature. This indicated formation of the microgel. The milky appearance is ascribed to the change of light scattering caused by the changes in the dimensions of the microgel [11] Fourier Transform Infrared (FT-IR) Spectroscopy The FT-IR spectrum of poly(nipam) microgel shows different peaks at different positions for different functional groups which are summarized in Table-1. Notable peaks are, - asymmetric stretching (2970 ), - symmetric stretching (2874 ), =O amide group (1634 ), N- bending (1540 ), -N stretching (1366 ), 2 - and 3 bending vibrations (1460 and 1386 respectively) [12]. Table-1: Observed peaks for different microgels in FT-IR spectra. Batch (01) N- (Stret) - (Aliph) =O (Amide) N- (Bend) -N (Stret) - (Bend) FT-IR spectrum of pure NIPAM is shown in Fig. 1. The peaks characterizing double bonds are noticed both in NIPAM and the cross-linker (MBA) but the FT-IR spectrum of microgel does not show any peak characteristic of double bond in the range of (= aliphatic and aromatic) or (stretching mode of vinyl double bonds) [13]. Similarly no peaks characteristic of cis-trans and substituted groups with double bonds( ) or a peak just above 3000 which is a characteristic peak for =- could be found. Additionally the broad and intense peak near 3292 characterizes N- stretching. This peak shows hydrogen bonding due to the presence of water of hydration attached to the polymer and confirms gel formation. So, these results indicate that polymerization has taken place. FT-IR spectrum for one of the compositions is shown in Fig. 2. Temperature Sensitivity of Poly(NIPAM) Microgels Temperature sensitivity of poly(nipam) microgels was studied in terms of the changes in R h values measured at a scattering angle of θ = 90. Poly(NIPAM) microgels undergo a volume phase transition from swollen to shrunk state by increasing the temperature. The driving force for volume phase transition (temperature sensitive) was considered to be a balance between hydrophobic/ hydrophilic interactions between network chains and the water molecules [14, 15]. In the swollen state, solvation of particles increases because the hydrophilic interactions between polymer chains and solvent molecules augment which enhance the potential to absorb water and as a result microgel particles enlarge in size. The main reason behind it is the formation of hydrogen bonding between the N- group of NIPAM polymer chain and water molecules [16]. The degree of swelling is controlled by the free energy changes associated with the elasticity of network and mixing of the polymer and water molecules [17] By increasing the temperature, hydrodynamic radius of microgel particles decreases. Initially the decrease in R h value is not significant but when the temperature reaches at approximately ~34 there is a significant decrease in R h value [18]. Similar volume phase transition was also observed by Brian Vincent [19]. After the VPTT, the size becomes constant and further increase in temperature has no effect on R h value. Effect of ross-linking Density on ydrodynamic radius of Microgel Particles The tendency of particles to collapse depends on the concentration of cross-linker. As the concentration of cross-linker increases, the degree of collapsing of particles decreases [20, 21]. The hydrodynamic radius of microgel particles also undergo decrease in the swollen state but after the volume phase transition temperature (VPTT) this trend is reversed, that is the R h increases for higher concentration of cross-linker and decreases for lower concentration in the collapsed state for microgel particles [22].

3 LUQMAN ALI SA et al., J.hem.Soc.Pak., Vol. 35, No. 6, PURE NIPAM 95 %T Wavenumbers (cm-1) PG01 Fig. 1: FT-IR spectrum of pure NIPAM %T cm Fig. 2: FT-IR spectrum of microgels with 2mol % cross-linker (PG01). This is interpreted as that at low concentration of cross-linker, polymer network is more elastic and hence has large mesh size which shows maximum swelling by allowing water molecules to enter inside. In other words the water molecules can enter easily in the polymer network of the microgel particles having low concentration of cross linker in the swollen state, and as a result particle size increases. On the other hand, in the collapsed state the size of microgel particles having higher cross-linker content is larger as compared to lower cross-linker content microgel particles, because at low cross-linker composition the amount of water in polymer network is large which is forced out by increasing the temperature. This leads to reduction in the size of microgel particles in the collapsed state [23].

4 LUQMAN ALI SA et al., J.hem.Soc.Pak., Vol. 35, No. 6, With the increase in cross-linker content up to 30 or 40% the colloidal property of microgel vanishes out and we get a precipitate. Fig. 3 shows the temperature sensitivity of poly(nipam) microgels with different cross-linking densities PG01 PG02 PG03 PG04 R h (nm) Temperature ( o ) Fig. 3: Temperature dependence of average R h values of poly(nipam) microgels with different cross-linking content. The deswelling process may be described in terms of the deswelling ratio, which is worked out by using the following equation. Rh( T ) α= Rh( T ) 3 where α is the deswelling ratio, R h (T) is the hydrodynamic radius at high temperature and R h (T ) is the hydrodynamic radius at low temperature. In the fully collapsed state, microgels with the lowest crosslinker density (2% for PG01) show a deswelling ratio of As we increase the cross-linker concentration upto 15% for PG04 the deswelling ratio increases to which is shown by the reduction in the degree of collapsing (Table-2). Table-2: De-swelling ratios of microgels with various cross-linker content. Mol% of ross-linker De-swelling ratio (α) Fig. 4 shows variation of hydrodynamic radius of microgels particles with mol% of crosslinker in two different states. Fig. 4: ydrodynamic radius of microgels vs mol% of cross-linker at two different states. Effect of ross-linking Density on Transition Temperature The VPTT is independent of the cross-linker concentration for low cross-linked microgels, but increases slightly for higher concentration. For each 10% increase in the cross-linking density, the transition temperature rises by about 2.5. This increase indicates that interaction between the polymer chains and the solvent molecules changes when the amount of cross-linking agent is high enough. This could be related to the solubility of the MBA moieties in the polymer network [23]. The extent of swelling of microgels is mainly controlled by the elasticity of the polymer network. It also shifts the transition range of collapsing of the microgel particles. The lower the cross-linker content the sharper the transition and vice versa. owever the transition temperature keeps practically constant and consequently the thermodynamics associated with the swelling process do not change significantly. Temperature Sensitivity of ybrid Microgels Fig. 5 shows comparative graphs of pure and hybrid microgels. The size of hybrid microgel particles is smaller than pure microgel particles. By increasing temperature the hybrid microgels particles shrink slightly as compared to the pure microgel particles. This could be ascribed to the strong complexation of silver ion (Ag + ) with the nitrogen atoms of poly(nipam) chains of microgels. This complexation leads to stabilization of nanoparticles within the microgels. The difference in hydrodynamic radius could be associated with the ionic strength of the hybrid system [24] and physical cross-linking of the microgel particles with silver nanoparticles [25, 26].

5 LUQMAN ALI SA et al., J.hem.Soc.Pak., Vol. 35, No. 6, Fig. 6 shows the temperature sensitive UV- Vis absorption properties of poly(nipam)-ag hybrid gels. Fig. 5: Variation of average hydrodynamic radius, R h of poly(nipam) pure microgels and hybrid gels (A) PG01 and G01; (B) PG02 and G02; () PG03 and G03; (D) PG04 and G04 Temperature Sensitive UV-Vis Absorption properties of Poly(NIPAM)-Ag ybrid gels Fig. 6: Temperature sensitive UV-Vis absorption properties of poly(nipam)-ag hybrid gels: (A) G01; (B) G02; () G03; (D) G04. At nanometer scale the electron cloud oscillates on the surface of particles and absorbs electromagnetic radiation at a particular energy. This resonance is known as surface plasmon resonance (SPR). The shape of SPR spectrum is determined by the relative dimensions of the particle to the wavelength of the incident light. Variation in the refractive index of the surrounding medium can also

6 LUQMAN ALI SA et al., J.hem.Soc.Pak., Vol. 35, No. 6, change the position and intensity of the resonance peaks. UV-Vis absorption spectrum shows an absorption peak at ~400 nm, which is characteristic of the metallic silver metal colloids. The peak at ~400 nm indicates that silver nanoparticles (Ag NPs) with almost narrow size distribution have been formed inside the polymer microgels. By increasing temperature the size of thermo sensitive hybrid microgel particles decreases. As a result nanoparticles come close to each other inside the microgel. This leads to increase in concentration per unit volume ratio, consequently the intensity of absorption peak increases. Another reason could be the changes in the refractive indices of the microgel and the solvent [27]. This increase in absorption is more significant from 30 to 35 because in this temperature range volume phase transition takes place. Experimental Materials N, N-methylene bis-acrylamide (MBA) was obtained from Acros hemicals and all other chemicals were purchased from Aldrich. NIPAM was recrystallized from hexane-toluene (1:1 volume ratio) mixture and dried in vacuum. N, N-methylene bisacrylamide (MBA), ammonium persulphate (APS), sodium dodecyl sulphate (SDS), silver nitrate (AgNO 3 ) and sodium borohydride (NaB 4 ) were used as recieved without further purification. Deionized water was used for all reactions and preparation of solutions. Synthesis of Pure Microgels Poly(NIPAM) microgel dispersions were prepared by the free radical emulsion polymerization of NIPAM and MBA using APS as an initiator. Four samples of microgels were prepared with different contents of cross-linker and NIPAM. Feed compositions of the materials used in the synthesis of microgels are given in Table-3. For each composition NIPAM and MBA were mixed with 0.057g SDS in 95mL of deionized water in a three necked round bottom flask equipped with a condenser, nitrogen inlet and a thermometer. The flask was kept in silicon oil. The reaction mixture was continuously stirred and heated at 70 for one hour with a continuous supply of N 2 to remove oxygen from the reaction mixture. Thereafter 5mL of 0.06M APS solution was added to the reaction mixture to start polymerization reaction. The reaction was carried out for six h at 70 with continuous stirring and purging of N 2 gas. Then the reaction was stopped and microgel dispersion was allowed to cool. The resultant microgel was purified by centrifugation, decantation and washing with water. Finally, each microgel sample was refined by dialysis for one week (Spectra/Por molecular porous membrane tubing, cutoff, ) against frequently changing pure water at room temperature to remove unreacted monomers, surfactants and other impurities. Proposed mechanism for the synthesis of microgels is shown in Fig O N N-isopropyacrylamide N O n n O N 2 N O + APS 2 SDS / 70 2 n n O N O 3 3 N 2 N O 2 N,N-methylene-bis acrylamide Poly(N-isopropyl acrylamide) microgel Fig. 7: Synthesis scheme of temperature sensitive poly(nipam) microgels initiated/catalyzed by APS at 70. Table-3: Feed composition of poly(nipam) microgel particles. Batch NIPAM (%) MBA (%) APS (M) SDS (g) Water (ml) PG PG PG PG In-situ Synthesis of Ag NPs in Poly(NIPAM) Microgels ybrid microgels with silver nanoparticles (AgNPs) immobilized inside, were synthesized from poly(nipam) microgels. 15mL of poly(nipam) microgel was diluted up to 45mL with deionized water. It was kept in ice cold water under constant magnetic stirring for an hour to increase the size of

7 LUQMAN ALI SA et al., J.hem.Soc.Pak., Vol. 35, No. 6, microgel particles. Then 5mL of 1mM solution of AgNO 3 was added to make the total volume up to 50mL and the solution was further stirred for two h continuously. Then the sample was transferred to 100mL round bottom flask with continuous stirring and N 2 purging for 30 minutes. Thereafter freshly prepared NaB 4 solution (0.44g in 5mL of water) was added drop wise to the reaction mixture with continuous stirring at room temperature for two h. The resulting microgels incorporated with Ag NPs were purified by decantation and dialyzed against frequently changing deionized water at room temperature for 30 minutes. The hybrid gels were coded as G01, G02, G03 and G04. haracterization FT-IR spectroscopy was used to identify different functional groups present in the microgels. The FTIR spectra of dried microgels were recorded with Fourier transform infrared spectrometer. The UV-Visible absorption spectra were taken with Shimadzu 1601 UV-Vis spectrometer at different temperature conditions. Dynamic laser light scattering experiments were performed with a standard laser light scattering spectrometer (Brookhaven Instruments) at an angle of 90. e-ne laser (35mW, 637nm) was used as a light source. All the microgels and hybrid gels were passed through Millipore Millex V filters with a pore size of 0.45µm to remove any dust particles before performing measurements. The correlation functions were analyzed by constrained regularized ONTIN method to obtain distribution decay rates. These decay rates gave us the apparent diffusion coefficients. The apparent diffusion coefficient is given by D app =Γ/q 2 with scattering vector q = 4πn/λ sinθ/2 where n, λ and θ being the solvent refractive index, the wavelength of incident light and the scattering angle respectively. The hydrodynamic radius was calculated by using Stokes-Einstein equation, R h = k B T/6πηD where T, k B, and η are the absolute temperature, the Boltzmann constant, and the solvent viscosity respectively. onclusion FT-IR data evidenced the formation of polymer microgels. The hydrodynamic radius of microgel particles was found to decrease with increase in temperature (showing the thermo sensitive behavior of microgels) as well as with each incremental addition of cross-linker in swollen state but in the shrunk state the trend was reversed. It was found that the degree of collapsing decreases with the addition of cross-linker but volume phase transition temperature (VPTT) is independent of the cross-linking density. ence both the pure and hybrid microgels have the same volume phase transition temperatures (VPTT). ydrodynamic radius of hybrid microgel was found smaller than that of the pure microgel. This decrease is assigned to the complexation of nanoparticles with the microgel network. In case of hybrid microgel, the characteristic peak at ~400 nm revealed the formation of silver nanoparticles (Ag NPs). UV-Vis absorption studies of hybrid gels showed an increase in absorbance by increasing the temperature. Acknowledgment We are thankful to igher Education ommission (E) Pakistan for the financial support under indigenous scholarship programme. References 1. B. R. Saunders and B. Vincent, Journal of Advances in olloid and Interface Science, 80, 1 (1999). 2. M. J. Murray and M. J. Snowden, Journal of Advances in olloid and Interface Science, 54, 73 (1995). 3. D. Duracher, A. Elaissari and. Pichot, Journal of olloid Polymer Science, 277, 905 (1999). 4. B. R. Saunders, Langmuir, 20, 3925 (2004). 5. K. Kratz, T. ellweg and W. Eimer, Polymer, 42, 6631 (2001). 6. R.. Pelton and P. hibante, Journal of olloids and Surfaces, 20, 247 (1986). 7. M. J. Snowden, B. Z. howolhry, B. Vincent and G. E. Morris, Journal of the hemical Society Faraday Transaction, 92, 5013 (1996). 8. Y. Lu, Y. Mei and M. Ballauff, Journal of Physical hemistry B, 110, 3930 (2006). 9.. Xu, J. Xu, Z. Zhu,. Liu and S. Liu, Macromolecules, 39, 8451 (2006). 10. R.. aceres, A. S. Iglesias, M. Karg, I. P. Santos, J. P. Juste, J. Pacifico, T. ellweg, A. F. Barbero and L. M. L. Marzan, Journal of Advanced Materials, 20, 1666 (2008). 11. J. T. Zhang, L. X. Liu, A. Fahr and D. K. Jandt, Journal of olloid Polymer Science, 286, 1209 (2008). 12. M.. Kim, J.. Kim,. Y. Lee, J. D. Kim and J.. Yang, Journal of olloids and Surfaces B, 46, 57 (2005). 13. A. Khan, Journal of olloid and Interface Science, 313, 697 (2007).

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