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Surface Modification of Colloidal Silica Nanoparticles Bull. Korean Chem. Soc. 2013, Vol. 34, No. 9 2747 Surface Modification of Colloidal Silica Nanoparticles: Controlling the size and Grafting Process Wentao He, Danhua Wu, Juan Li, Kai Zhang, Yushu Xiang, Lijuan Long, Shuhao Qin, * Jie Yu, * and Qin Zhang National Engineering Research Center for Compounding and Modification of Polymeric Materials, and Institute of Mining Technology, GuiZ
  Surface Modification of Colloidal Silica Nanoparticles Bull. Korean Chem. Soc.   2013 , Vol. 34, No. 9 2747 Surface Modification of Colloidal Silica Nanoparticles: Controlling the size and Grafting Process Wentao He, Danhua Wu, Juan Li, Kai Zhang, Yushu Xiang, Lijuan Long, Shuhao Qin, *  Jie Yu, * and Qin Zhang  National Engineering Research Center for Compounding and Modification of Polymeric Materials, and Institute of Mining Technology, GuiZhou University, GuiYang 550003, China *  E-mail: (J. Y.); (S. Q.) Received May 10, 2013, Accepted June 25, 2013 Surface modification of colloidal silica nanoparticles without disrupting the electric double layer of nanoparticles is a major challenge. In the work, silane was employed to modify colloidal silica nanoparticleswithout inducing bridge flocculation obviously. The effect of pH value of the silica sol, the amount of silane infeed, and reaction temperature on the graft amount and the final size of modified particles was investigated.The increased weight loss by TG and the appearance of T 2  and T 3  except for Q 2 and Q 3 signals by CP/MAS 29 Si NMR of the modified samples verified the successful grafting of silane. The graft amount reached 0.57 mmol/ g, which was slightly lower than theory value, and the particle size remained nearly the same as unmodifiedparticles for acidic silica sol at the optimum condition. For alkaline silica sol after modification, aggregatescomposed of several nanoparticles connected together with silane moleculars as the bridge appeared. Key Words :  Silica sol, Modification, Silane, Graft amount, Particle size Introduction Inorganic fillers, specially silica nanoparticles, have beenwidely used in polymers to improve the mechanical pro- perties, conductivity and so on. 1-4  However, the weak com- patibility between organic matrix and inorganic nanoparticles,which usually results in unevenly dispersion of inorganicnanoparticles in the matrix, is the main problems in practicalapplications. The covalent grafting of silane molecular oninorganic surfaces has been attempted to improve the dis- persion and received some effect. 5,6  Also Liu et al.  proposed a new method to organically functionalize nano-scaled silica  particles through the reaction between the silanol group of silica and the oxirane ring of epoxy compounds. 7  However,in most studies the grafting of silane was performed inorganic solvent, which is undesired due to environmentalconcerns. 5-8   Now, the treatment with modified inorganic sols could bean advantageous alternative, but there has been relativelylittle work on colloidal polymer nanocomposites. 9,10  Recent-ly, some groups reported the synthesis of polymer/silica nanocomposites by polymerization of unsatured monomersin the presence of silica sol. 11-13  Wu et al. 12  prepared poly( L -lactic acid)/SiO 2  nanocomposites via in situ  melt poly-condensation of L -lactic acid in the presence of acidic silica sol. The results demonstrated some OLLA chains weregrafted onto the surface of silica nanoparticles and provided extra steric stabilization for the nanoparticles, which ensured satisfactory nano-scale dispersion in the final nanocomposites.Therefore surface modification of colloidal silica nano- particles instead of powder silica nanoparticles will have thegreater applications.Colloidal silica nanoparticles, namely silica sols, are aque-ous suspension of silica particles with particle size rangingfrom 1 nm to 1 µm. The stability of silica sol is related toelectrolyte, silica concentration, pH value and so on. Silica sol is usually considered as stable due to the lack of signi-ficant rate of sedimentation or agglomeration. However,surface modification of silica sol with silane molecularswithout disrupting the electric double layer of nanoparticlesis difficult because the silica nanoparticles are in the inter-mediate stable state and minimal change of surface environ-ment will lead to aggregation. Most of previous researchemployed  γ -aminopropyltrimethoxysilane (KH550) to modifythe surface of silica nanoparticles, however, amino groups ishydrophilic and alkaline, which will very likely disrupt theelectric double layer of nanoparticle in silica sol. Recentlysome effort 14-17  has been made to modify the surface of silica sol with different silanes to suit different needs. Still, syste-matic investigations about the influencing factors of the graftamount and the final size of modified silica sol are notreported.One objective of this study was to modify colloidal silica nanoparticles with  γ -glycidoxypropyltrimethoxysilane (KH560)without obviously disturbing the colloidal stability. In orderto achieve this purpose, silica sol with different pH value and silane moleculars with different end functional groups wereemployed. Another objective was to study the effect of reac-tion conditions, including silane concentration, reactiontemperature and dialysis time on the graft amount of silanemoleculars and physicochemical properties of final products. ExperimentalMaterials.  Acidic silica sol (pH 2.6; particle size, 30-40nm; the solid content, 25 wt%) was purchased from Qingdao  2748  Bull. Korean Chem. Soc . 2013 , Vol. 34, No. 9 Wentao He et al. yumingyuan silica-gel reagent factory, China. Alkaline silica sol (pH 9.0; particle size, 22 nm; the solid content, 50 wt %)was purchased from Adrich, USA. KH550 and KH560 were provided by Hubei Debang Chemical Co. Ltd (China). Allother reagents and solvents were used as received withoutfurther purification Preparation of the Surface-modified Silica Sol.  In a typical procedure, 10 g of silica sol was added to an ap- propriate amount of ethanol in a round flask equipped withan electric stirring rod and spherical condenser. After stirred for 30 min, the solution was heated to a predetermined temperature and a certain amount of KH560 was added. Thereaction was maintained for 6 h at set temperature. The pro-duct was purified by dialysis against pure water for differenttimes using a cellulose membrane Mw cutoff 3000 withwater changed every 4 h. The grafted products are designat-ed as SKHm/(-n), where m represents the amount of KH560in feed (mmol/g SiO 2 ) and n represents the dialysis time(day). Characterization.  Thermogravimetric analysis of the un-treated and treated MMTs was performed on a TA instru-ments Q50 to quantitatively determine the grafting amountas well as the thermal stability. Samples were heated from100 o C to 750 o C at the rate of 10  o C/min under a nitrogenflow (60 mL/min). An organic Elemental Analyer (Elementar,Germany) was used to determine the carbon content of thebare and the functionalized MMT. And the results are com- pared with that of TG analysis to survey the degree of mea-surement data reliability. The average size of unmodified and modified silica sol was measured by a dynamic lightscattering instrument (DLS, Malvern). For TEM, a drop of diluted solution was dried on a grid with 300 mesh for 2 minand analyzed using a JEM 100 CX TEM with an accele-rating voltage of 100 kV. The CP/MAS 29 Si spectrum wererecorded on a Bruker AVANCE III 400 WB spectrometerequipped with a 7 mm standard bore CP/MAS probehead whose X channel was tuned to 79.50 MHz, using a magneticfield of 9.39 T at 297 K. The dried and finely powdered samples were packed in the ZrO 2  rotor closed with Kel-Fcap which were spun at 5 kHz rate. A total of 3000 scanswere recorded with 3 s recycle delay for each sample. AllCP/MAS  29 Si chemical shifts are referenced to the reson-ances of 3-(trimethylsilyl)-1-propanesulfonic acid sodiumsalt (DSS) standard. Results and Discussion For the purpose of surface modification of silica sol, a commonly used silane KH550 was first employed. After thesilane was added to silica sol (pH 2.6 or 9.0) and remained for some period, the solution changed from light blue tomilky and some aggregates appeared, indicating the irrever-sible agglomeration of nanoparticles. In contrast, addition of KH560 to the above silica sol did not induce the same phenomenon; instead, the solution remained light blue. The probable causes include: (1) the alkalinity of KH550 dis-turbs the intermediate stable state of silica sol; (2) thehomocondensation is faster than the grafting process for thehydrolyzed KH550 and the silane moleculars react with eachother, forming aggregates instead of grafting to the surfaceof silica nanoparticles. So the surface modification was performed with KH560 in the following experiments.To ensure complete removal of ungrafted silane mole-culars and gain more insight into the size change after dia-lysis, the dialysis process was monitored by TG and DLS atintervals. The weight loss of samples gradually reduced as Figure 1. The TG curve (a) and weight loss (b) of SKH2 withdifferent dialysis time. Table 1. The weight loss and average size of modified silica solSKH2 with different dialysis time Samples Dialysis time (day) size (nm) Weight loss(%)SKH2 a 0 34.9 17.0SKH2-0.7 0.7 29.2 8.5SKH2-1 1 38.9 7.6SKH2-2 2 37.2 6.6SKH2-3 3 28.1 6.1SKH2-4 4 40.8 6.1 a For SKH2, the reaction was performed at 50  o C for 6 h with acidic silica sol as pristine sol.  Surface Modification of Colloidal Silica Nanoparticles Bull. Korean Chem. Soc.   2013 , Vol. 34, No. 9 2749 dialysis time increased, and eventually reached a constantvalue, indicating the dialysis process can effectively removethe unreacted silane coupling agent KH560 in the product.The increased weight loss compared to unmodified silica solalso verified the successful grafting of silane moleculars. ForSKH2, the weight loss reached 6.1% and the particle sizeremained below 45 nm after 4 days dialysis (Table 1 and Figure 1). As the amount of KH560 in feed increased, thedialysis time increased. For SKH6-0 and SKH10-0, thedialysis time was 11 days and 13 days, respectively (support-ing information).As mentioned above, after addition of KH560 to the silica sol, whether acidic or alkaline, the appearance of silica soldid not change obviously. The effect of pH on the grafting process was investigated and shown in Table 2. Though theweight loss of modified silica sol in the case of pH 9.9reached 8.7%, higher than that at pH 2.6, the average sizeincreased from 22 nm to 199.7 nm. After modification, theaverage size of acidic silica sol slightly increased from 30nm to 40 nm and the weight loss was 6.1%. The differencein average size and weight loss was mainly duo to the differ-ent grafting routes (Scheme 1). To condense an alkoxysilaneto the silica surface the alkoxy groups of the silane first mustbe hydrolyzed to produce silanol groups. This is a fastreaction and the reaction rate is significantly increased whenthe pH is increased. 18  After hydrolysis, the condensationfalls into three categories. (1) Small part of silane molecularsreact with each other and form oligomers. (2) Some silanemoleculars are grafted to the surface of silica nanoparticles.(3) Some silane moleculars react with each other and aregrafted to the surface of nanoparticles simultaneously, form-ing large aggregates, which are composed of several nano- particles connected together with silane moleculars as thebridge. Similarly, the condensation reaction is, within the pHrange 1-10, significantly faster at high pH than low pH and occurs before the silanes are completely hydrolyzed. 19 Therefore the hydrolysis and condensation of silane at high pH is more difficult to control. It is probable that in acidicsol reaction (II) is the main reaction whether reaction (III)dominates in alkaline sol.TEM results confirmed our speculation. From TEM, pri-stine acidic silica sol presented irregular spherical particlesand small aggregates could be observed due to hydrogenbonds between the surface hydroxyl groups of silica nano- particles. After silane grafting, the silica sol showed regularspherical structure. The modified silica sol was monodis- perse and no large aggregate was observed, indicating thatsilane grafting can prevent aggregation of silica nanoparticlesto some extent. For the alkaline sol, the nanoparticles sticked to each other and formed large aggregations after silanemodification (Figure 2). This results support our proposalabout the different grafting routes on silica surface based onthe DLS analysis.CP/MAS 29 Si NMR spectrums of unmodified and modi-fied silica sol are shown in Figure 3. The peaks at − 93.3 ppm, − 102.8 ppm, − 112 ppm are ascribed to Q 3 [Si(OSi) 3 OM](M stands for Al, Mg, etc.), Q 3 [(SiO) 3 SiOH], and Q 4 [(SiO) 4 Si](Q represents the tetrafunctional) for unmodified acidicsilica sol. The Q 3 [Si(OSi) 3 OM] is characteristic of Si atomsin acidic silica sol since Al 3+  is known as counterions. TheQ 3 [Si(OSi) 3 OH] and Q 4 [(SiO) 4 Si] are attributed to isolated silanol groups present at the surface of silica nanoparticlesand Si-O-Si bond in the bulk of particles respectively. After Table 2. The effect of pH on the grafting process pH a  Size beforemodification (nm)Size aftermodification (nm)Weightloss (%)2.6 34.9 40.8 6.19.9 22 199.7 8.7 a Both reaction was performed at 50  o C for 6 h, with the amount of KH560 fixed in 2 mmol/g for both silica sol. Scheme 1.  The reaction mechanism of grafting process.  2750  Bull. Korean Chem. Soc . 2013 , Vol. 34, No. 9 Wentao He et al. modification, the signal of Q 3 [Si(OSi) 3 OH] became weakrelative to Q 4  as a portion of the surface (SiO) 3 SiOH groupshad reacted with KH560 and was converted to (SiO) 3 SiOSigroups. Two new peaks at − 58.5 ppm, − 68.3 ppm appeared,ascribing to T 2  [Si(OSi) 2 (OH)R'] (R' = CH 2 CH 2 CH 2 OCH 2 -CHOCH 2 ) and T 3  [Si(OSi) 3 R'] (T represents the trifunctional)respectively. 20,21  This provides supporting evidence for thegrafting of silane onto the silica nanoparticles. The spectrumof alkaline sol was similar to that of acidic sol except that the peak at − 93.3 ppm disappeared, which was due to the ab-sence of metal ions in sol. A major difference betweenmodified acidic and alkaline sol is that the signal of T 3  isobviously stronger than T 2  in alkaline sol. The higher signalintensities of the T 3  [Si(OSi) 3 R'] unit suggests that the graft-ing or the condensation is faster than the hydrolysis.TG and elemental analysis were both employed to cal-culate the grafted amount of silane on the surface of silica nanoparticles. Acidic silica sol is modeled for the next study.The following Eq. (1) is used to calculate the amount of grafted silane based on the weight loss between 100 o C and 750  o C, W 100-750 , taking into account that the weight loss of dried silica sol in same temperature range is below 1 wt%.Grafted amount (mmol/g) = (1)where M (g/mol) is the molecular weight of the grafted silane molecules.The grafted amount (expressed in mmol of grafted silane per g of silica nanoparticles as mentioned above) is alsocalculated from the Eq. (2) based on the difference ∆  (wt%)of carbon content after and before grafting, where N C  and M(g/mol) designate the number of carbon atoms and themolecular weight of the grafted silane molecule:Grafted amount (mmol/g) (2)For SKH2, the content of carbon ∆ C is 3.77%. The grafted amount calculated from TG and elemental analysis is 0.57mmol/g and 0.56 mmol/g, respectively. These results verifythe successful grafting of silane moleculars onto the nano- particles and demonstrate that the calculation method of thegrafted amount based on TG is reliable.It can be anticipated, there is a restriction in the amount of silane for silica surface modification. In order to calculatemaximum coverage of silica surface by silanes, an approxi-mation depending on some known values can be made. Thefollowing Eq. (3) is used to calculate maximum coverage of silica surface by silane moleculars, with some approximatevalues adopted. 22  S represents specific surface area of silica nanoparticles, ϕ  represents Si-OH groups/m 2  on silica surface,A represents the surface area occupied by a silane molecular,and N theory  represents the maximum number of mol of grafted silane/g silica nanoparticles in theory: N theory  = = 0.675 mmol/g(3) which corresponds to 5.1 µ mol/m 2  based on the approximatevalue S. The percentage of consumed Si-OH group formaximum coverage can be calculated by Eq. (4):P consumed  = N/(S × ϕ ) = = 65%(4)For SKH2, the grafted amount is 0.57 mmol/g (4.4 µ mol/ m 2 ) and the percentage of consumed Si-OH group is 54.8%,slightly smaller than theoretical calculation, but is believableaccording to similar researches. In previous study, theexperimentally determined maximum values for the surfacecoverage of oxides by 3-methacryloxypropyltrimethoxy-silane ranged from 2.8 to 7.5 µmol/m 2 , depending on the10 3 W 100750 –  M100W 100750 –   –  ( ) -------------------------------------------10 3 ∆ C12N C  ∆ C ( )  M – -----------------------------------SA6.022 × 10 23 ×( ) -----------------------------------------6.7510 4 × 1308 × 10 6 –  × ------------------------------- Figure 2.  TEM of unmodified (a, c) and modified (b, d) acidic andalkaline silica sol. Figure 3. CP/MAS 29 Si NMR spectrum of unmodified (a, c) andmodified (b, d) acidic and alkaline silica sol.
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