Accepted Manuscript
Title: The facile preparation for temperature sensitive Silica/PNIPAAm composite microspheres
Authors: Xiumei Tai, Jing hong Ma, Zhiping Du, Wanxu Wang
PII: S0169-4332(12)02318-5 DOI: doi:10.1016/j.apsusc.2012.12.153 Reference: APSUSC 24912
To appear in: APSUSC
Received date: 17-9-2012 Revised date: 10-12-2012 Accepted date: 28-12-2012
Please cite this article as: X. Tai, J. Ma, Z. Du, W. Wang, The facile preparation for temperature sensitive Silica/PNIPAAm composite microspheres, Applied Surface Science (2010), doi:10.1016/j.apsusc.2012.12.153
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. *Highlights (for review)
Highlights
1) The optimum reaction conditions of temperature sensitive SiO2/
PNIPAAm composites are supported.
2) The products prepared have obvious temperature sensitive property.
3) The products show excellent monomodel microsphere.
Accepted Manuscript
Page 1 of 25 Graphical Abstract (for review)
Graphic abstract
75 (A) 0.7:1 1:1 70 2:1
) 4:1
。 (
65
60 Contactangel
55
298 300 302 304 306 308 Temperature (K)
75 (B) 300 K 306 K
70
)
。 ( 65
60 Contactangel
55
0.7:1 1:1 2:1 4:1 The different molar ratio
Accepted Manuscript
Page 2 of 25 *Manuscript
The facile preparation for temperature sensitive Silica/PNIPAAm composite microspheres
Xiumei Tai†,‡, Jing hong Ma†,*, Zhiping Du ‡,§,**, Wanxu Wang ‡
† Key Laboratory of Coal Science and Technology, Ministry of Education, Institution of
Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, P. R. China
‡ China Research Institute of Daily Chemical Industry, Taiyuan 030001, P. R. China
§ School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006,
P. R. China
* Correspondence to: J.H. Ma, Key Laboratory of Coal Science and Technology, Ministry
of Education, Institution of Special Chemicals, Taiyuan University of Technology, 79
Yingze Street, Taiyuan, Shanxi Province, 030024, PR China. ** CorrespondenceAccepted to: Z. Du, China Research InstituteManuscript of Daily Chemical Industry, 34 Wenyuan Street, Taiyuan, Shanxi Province, 030001, PR China. Tel.: +86 13209805149;
fax: +86 3514040802.
E-mail addresses: [email protected], [email protected], [email protected].
1
Page 3 of 25 Abstract: The temperature sensitive SiO2-PNIPAAm composite microspheres were designed and successfully synthesized. The influences of monomer concentration, reaction time, reaction temperature and initiator amount on water contact angel of the prepared microspheres were investigated. The structure, morphology, surface composition were characterized by Fourier Transform Infrared Spectroscopy (FTIR) and Transmission
Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS). The temperature sensitive properties in aqueous solution were studied by water contact angle measurement instrument and dynamic light scatter (DLS). The results show that the low critical solution temperature (LCST) of SiO2/PNIPAAmcomposites is around 304 K and the particle is monodisperse microsphere.
Key words: Temperature sensitive material; Silica/PNIPAAm; monodisperse; composite microsphere
Accepted Manuscript
2
Page 4 of 25 1. Introduction
Environmentally responsive polymers have been focused in recent years for their practical and potential applications in drug controlled release, heterogeneous catalysis, immobilization of enzyme and so on [1-4]. Environmentally responsive polymers are usually synthetic polymers that exhibit volume or phase transitions in response to slight
environmental changes, such as temperature [5], pH [6], ionic strength [7], light [8], electric
[9] and magnetic fields [10]. Among them, temperature sensitive polymers have a lower critical solution temperature (LCST) around which there is the obvious molecular transition from a hydrophilic structure to a hydrophobic structure. A representative example is poly-
(N-isopropylacrylamide) (PNIPAAm), which shows a LCST around 305 K [11]. Since
Heskins and Guillet reported the preparation of PNIPAAm in 1968, PNIPAAm became a polymer of much interest due to its novel reversible, temperature-sensitive property [12] and have been widely applied in several different fields, such as extraction [13, 14], controlled release [15, 16], enzyme activity control [17], hydrophobic adsorption [18] and biotechnology [19-21].
Nano-silica have been produced on an industrial scale and used as additives to cosmetics, drugs, printer toners, varnishes, and food. Moreover, nano-silica is being extended a lot of biomedical andAccepted biotechnological applications suchManuscript as cancer therapy, DNA transfection, drug delivery, and enzyme immobilization [22-27]. Compared with the easily agglomerated nano-silica [28], the polymer/SiO2 composites attracted much attention owing to their excellent mechanical properties, thermal stability, biodegradation and abrasion resistance
[29-32]. It is reported that there are several methods for the synthesis of the polymer/SiO2
3
Page 5 of 25 composites, such as surface-initiated atom transfer radical polymerization, sol-gel method and emulsion polymerization [14-16]. These methods have both advantages and disadvantages. For example, surface-initiated graft polymerization requires multiple synthetic steps [33].
To achieve the synergic advantages of silica and PNIPAAm materials, the silica/PNIPAAm membrane composites or porous hydrides were much concentrated in recent years [34, 35,
36]. The monodisperse nanoparticles has been widely applied for their technological and fundamental scientific importance, many novel properties and potential purposes have emerged from monodisperse materials with small dimensions [37-41]. However, there are few reports about the monodisperse Silica/PNIPAAm composite microspheres.
In this work, a flexible and facile approach has been developed for the preparation of the monodisperse Silica/PNIPAAm composite microspheres in the system of absolute ethanol as the solvent. The effects of reactant molar ratio, reaction temperature, reaction duration and initiator on the temperature sensitive properties of the composites were investigated by the water contact angle measurement. Dynamic light scatter (DLS) is used to further evaluate the temperature sensitive properties of the composites.
2. ExperimentalAccepted methods Manuscript 2.1 Materials
The materials required are described as follows. Tetraethoxysilane (TEOS) and
N-Isopropylacrylamide (NIPAAm) were obtained from Aladdin Reagent Co. and used as received. Absolute alcohol and ammonia were of analytical grade from Beijing Chemical
4
Page 6 of 25 works. 3-methacryloxypropyl -trimethoxysilane (MPS) was commercial from Jiangsu
Chenguang Coincident Dose Co. Ltd. 2, 2’-Azobisisobutyronitrile (AIBN) was purchased from Merck and recrystallized twice in ethanol.
2.2 Polymerization procedure
Preparation of PNIPAAm/MPS compounds [42]. The compound precursor,
PNIPAAm-co-MPS, was synthesized by free radical polymerization of NIPAAm and MPS initiated by AIBN. In a typical synthesis procedure, NIPAAm, MPS and AIBN were dissolved in absolute ethanol at room temperature for 0.5 h under the protection of N2. The mixture was stirred vigorously for a scheduled time to form the compound precursor to get solution A.
Preparation of temperature sensitive SiO2/PNIPAAm composite microsphere.
Monodisperse Silica nanoparticles were prepared in ethanol according to the Stöber method
[43, 44]. In the experiment, 4 mL TEOS was added in 100 mL absolute ethanol, followed by the addition of 12 mL ammonia drop by drop under vigorous stirring. After stirring at room temperature for 24 h, the obtained SiO2 suspension solution was called as the solution
B. The solution A was mixed with the solution B by mechanical stirring at ambient temperature.Accepted Then the mixture was heated up toManuscript 333±2 K under stirring continuously. After 12 h of reaction, the solution was cooled to room temperature and the final white powders were collected by filtering, washed with absolute alcohol for several times to purify. The filter cake were vacuum dried at 333±2 K for 12 h.
5
Page 7 of 25 2.3 Characterization
A schematic of the apparatus for measuring contact angles is presented in Fig. 1. The glass slide was placed on a movable stage in front of a microscope, which was connected to a
CCD camera. Drops of 5μL in volume were deposited on the solid substrates
(SiO2/PNIPAAm pellets) using a microsyringe. The entire process was captured using a high speed video camera at a rate of 30 frames per second. The samples temperature was controlled by an electrical heated thermostat water bath. For each sample, the experiment was repeated to produce at least 5 sets of data at the same temperature and the water contact angel reported is averaged over 5 experimental points. The samples were equilibrated at every specified temperature for 20 min before the contact angle values were measured. The contact angles were estimated with an on-screen protractor software
(Powereach Co., Shanghai, China).
Fourier Transform Infrared Spectra (FTIR) were recorded with a Shimadzu IRAffinity-1 spectrometer in the wavenumber range from 400 to 4000 cm-1. The solid powder was incorporated with KBr into hybrid particles pellets.
Transmission Electron Microscopic (TEM) observation was made on a JEM-1100 (JEOL,
Tokyo, Japan) at 100 kV. Samples were dispersed in water in an ultrasonic bath for 15 min, and then depositedAccepted on a copper grid covered withManuscript a perforated carbon film, and the grid dried at room temperature.
X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using a
THERMO VG ESCALAB250 with the PHI quantum ESCA micro-probe system to study the chemical composition of the composite surfaces. The Al Kα (1486.6 eV) X-ray source
6
Page 8 of 25 was operated. The fitting XPS curves were analyzed with Multipeak 6.0 A software.
The hydrodynamic diameters (DH) of the particles were measured via a Zeta Plus Particle
Size Analysis Instrument (Brookhaven, USA), which was equipped with a Spectra-physics
127 Helium Neon laser (633 nm, power 35 mW). The light was collected at a scattering angle of 90 º. The CONTIN statistical method was used to convert the measured correlation data into particle size distribution. The samples temperature was controlled by a thermostat bath. All particles were dispersed and diluted highly. The aqueous solutions equilibrated at
298 K for 8 h prior to each experiment. At each temperature, the sample was kept for 15 min to reach equilibrium.
Fig. 1. Optical set-up to study droplet spreading contact angle (1- light source, 2- filter
plate, 3- microsyringe, 4- sample stage, 5- CCD camera, 6- computer)
3. Results andAccepted discussion Manuscript
3.1 Water contact angle measurement
Water contact angle measurement is the fundamental, accurate and most sensitive tool for determining the surface hydrophilic or hydrophobic property. Water contact angle measurement is used to examine the temperature switching behavior of the SiO2/PNIPAAm
7
Page 9 of 25 composite in this paper. If there is a discontinuity in contact angle around designed temperature, this point might be ascribed to the hydrophilic-hydrophobic transition temperature of the thermal responsive materials.
3.1.1 Effect of the different monomer concentration (NIPAAm to MPS)
Fig. 2 presents the measurement result of water contact angles – temperature of composites derived from the different monomer concentration (NIPAAm to MPS). It can be seen from
Fig. 2A that all the prepared SiO2/PNIPAAm composites prepared with different molar ratios of NIPAAm and MPS are temperature sensitive materials. The LCST of composites is around 304 K. As shown in Fig. 2B, the hydrophobicity of the samples enhances with the increasing of the monomer concentration up to the molar ratio of NIPAAm and MPS 1:1, and then decreases with further increase in NIPAAm concentration. Thus, the optimum molar ratio of NIPAAm and MPS is 1:1, with which the water contact angles is 73º at 306
K. The initial hydrophobicity increase may be attributed to that a mass of the monomer is utilized by the available free radical sites in the polymerization process. Moreover, the extent of homopolymerization of the monomer is very small at the lower concentration.
The hydrophobicity reaches a maximum value and subsequently decreases, which might be due to the Accepted number of free radical sites not Manuscript enough to the formation of preferential homopolymer. Therefore, the optimum monomer concentration of NIPAAm to MPS is 1:1.
8
Page 10 of 25 75 (A) 75 (B) 0.7:1 300 K 1:1 306 K
70 2:1 70 )
) 4:1
。
。
( (
65 65
60 60
Contactangel Contactangel
55 55
298 300 302 304 306 308 0.7:1 1:1 2:1 4:1 Temperature (K) The different molar ratio
Fig. 2. (A) Water contact angles – temperature plots of composites derived from the
different molar ratio (NIPAAm to MPS), (B) Water contact angles of composites derived from different molar ratio (NIPAAm to MPS) at 300 K and 306 K, respectively. (Reaction
conditions: AIBN amount 1.0 wt. %; 333 K; 8 h)
3.1.2 Effect of the different reaction time
The composites prepared with 5 h, 8 h and 11 h are temperature-sensitive materials and have the LCST of 304 K, but the composite prepared with 3 h is not thermal responsive in
Fig. 3A. It suggested that the copolymer of NIPAAm and MPS can be not produced in short reaction duration, which might lead to the failure of the formation of SiO2/PNIPAAm composite. As seen in Fig. 3B, the optimum reaction time for the co-polymerization is 8 h. The decreaseAccepteds of active radicals for generating Manuscriptactive sites on polymeric backbone lead to the decomposition of the monomer, so the hydrophobicity of the sample decreases after 8 h.
9
Page 11 of 25 80 75 (A) (B) 75
300 K ) ) 70 3h 306 K
。 70
。 ( ( 5h 65 8h 65 11h 60 60
Contactangel 55 Contactangel 55 50 50 45 45 298 300 302 304 306 308 2 4 6 8 10 12 Time (h) Temperature (K)
Fig. 3. (A) Water contact angles – temperature plots of composites derived from different reaction time; (B) Water contact angles of composites derived from different reaction time at 300 K and 306 K, respectively. (Reaction conditions: mol (NIPAAm to MPS) 2:1; AIBN
amount 1.0 wt. %; 333K)
3.1.3 Effect of the different reaction temperature
Temperature is one of very important factors in polymerization process. The decomposition rate and initiation of AIBN are determined as function of temperature. Fig. 4A illustrates water contact angles versus temperature plots of composites derived from 323 K to 351 K.
All the samples obtained from 323 K to 351 K are temperature sensitive materials and have the LCST of around 304 K. Fig. 4B shows that the hydrophobicity increased with the increase of Accepted temperature up to 333 K. This mayManuscript be on account of the increasing of the initiation rates of graft copolymerization. However, higher combination rates of monomer are obtained increasing homopolymerization reactions at higher temperatures, which might result in a weaker hydrophobicity.
10
Page 12 of 25 65 (A) (B)
) 64 。 ( 60 60 300 K
) 306 K 。
55 ( 56
52 Contactangel 50 323K 48 45 333K 343K Contactangel 44 351K 40 40
298 300 302 304 306 308 Temperature (K) 320 325 330 335 340 345 350 355 Temperature (K)
Fig. 4. (A) Water contact angles – temperature plots of composites derived from different
reaction temperature; (B) Water contact angles of composites derived from different
reaction temperature at 300 K and 306 K, respectively. (Reaction conditions: mol
(NIPAAm to MPS) 2:1; AIBN amount 1.0 wt. %; 8 h)
3.1.4 Effect of the initiator amount
The common initiator is AIBN in free radical polymerizations process. The influences of the AIBN amount over the range of 0.5-3.0 wt. % with respect to the monomers (NIPAAm and MPS) on the water contact angles of SiO2/PNIPAAm composites were studied, and the measurement results are exhibited in Fig. 5. As seen in Fig. 5 (A, B), when the AIBN amount is 0.5 wt. %, the water contact angels keep constant with temperature increasing from 299 K Acceptedto 308 K, and the average contact angelManuscript is 75 º. This might be ascribed to the low initiator amounts not enough to produce free radicals for initiating all the monomers to polymerize. The bigger contact angel may be owing to the reaction between SiO2 and MPS.
As the AIBN amount enhances up to 1.0 and 2.0 wt. %, the samples show the obvious thermal responsive property, the LCST of composites is around 304 K. When more initiator
11
Page 13 of 25 (3.0 wt. %) added leads to self–polymerization of NIPAAm and the difficulty in grafting to the surface of SiO2, the sample is not temperature sensitive material. This indicates that this optimum concentration is sufficient to inhibit the possible homopolymerization. If the concentration of initiator further increase, ions of AIBN prohibit the growing macroradicals of grafted part and the initiator is not efficiently used. The initiator amount of 1.0 wt. % account for the monomers is chosen as the optimum for the free radical polymerization.
(A)
) 75 (B)
70 。 70
) 300 K 。 65 ( 306 K 65
60 60 Contact angel(
55 55 0.5 wt.% Contactangel 1.0 wt.% 50 50 2.0 wt.% 3.0 wt.% 45 45 298 300 302 304 306 308 0.5 1.0 1.5 2.0 2.5 3.0 Temperature (K) The initiator amount (%)
Fig. 5. (A) Water contact angles – temperature plots of composites derived from different
AIBN amount; (B) Water contact angles of composites derived from different AIBN amount at 300 K and 306 K, respectively. (Reaction conditions: mol (NIPAAm to MPS) 2:1;
333 K; 8 h) Accepted Manuscript 3.2 FTIR spectra
FTIR is certainly one of the most important analytical techniques to examine molecules that attached to the particle surface. Fig. 6a represents the FTIR spectra of bare SiO2. The bands at 3480 and 1630 cm−1 are due to the stretching and bending vibration of the -OH
12
Page 14 of 25 group. The band at 1100 cm-1 might be ascribed to the stretching vibration of Si-O, the band at 465 cm-1 s to the bending vibrations of Si-O and band at 795 cm-1 to the bending vibrations of Si-O-Si. The spectrum of SiO2/PNIPAAm composite (Fig. 6b) show the bands at 1650 cm-1 and 1540 cm-1 attributing to C=O stretching and N-C=O symmetric stretching
−1 of amide groups of PNIPAAm and SiO2/PNIPAAm, respectively. The band at 1730 cm shows the –C=O stretching vibration of MPS. The bands at 2980 and 2850 cm−1 in the spectra are ascribed to the –C-H antisymmetric stretching vibration and the –CH2 stretching vibration. The bands at 650~850 cm-1 are the absorbance bands of Si-C stretching vibration.
The results indicate that SiO2/PNIPAAm has been prepared successfully.
Fig. 6. FTIR spectra of (a) bare SiO2, (b) SiO2/PNIPAAm composite
Accepted Manuscript
3.3 TEM analysis
The morphology (size, shape and distribution) of the nanoparticle is observed by TEM. The
TEM images of bare SiO2 and SiO2/PNIPAAm composites are showed in Fig. 7. It can be seen that the two samples are monodisperse sphere and the diameter of SiO2 and
13
Page 15 of 25 SiO2/PNIPAAm are 189 nm and 204 nm, respectively. After grafted with NIPAAm (Fig.
8b), the diameter of SiO2/PNIPAAm composites is larger and shows coarsened surfaces comparing with the bare SiO2 (Fig. 7a). It can be derived that the polymer chains have been grafted with SiO2.
Fig. 7. TEM images of (a) bare SiO2 (189nm), (b) SiO2/PNIPAAm composites (204nm)
3.4 XPS analysis
XPS could be used to determine the composition of samples and to identify the valence states of various species. Fig. 8 depict the XPS spectra of SiO2/PNIPAAm composite and bare SiO2. As shown in Fig. 8a, it can be seen that a new N1s peak appears at 400.14 eV, which is labeled by N atom in C-N bond of SiO2/PNIPAAm composite. The C1s scan of
SiO2/PNIPAAAcceptedm (Fig. 8b) could be curve fitted Manuscriptinto three peak components attributable to hydrocarbon (C-H), carbon adjacent to an amide group (C-N), and carbon attached to oxygen (C=O) corresponding to binding energies at 284.83, 286.12, and 288.65 eV, respectively. For O1s spectra of bare SiO2 (Fig. 8c), there is only one peak at 532.68 eV (O atom in Si-O bond), while a more peak with binding energy of 533.66 eV (O atom in C=O
14
Page 16 of 25 bond) appears in O1s spectra of SiO2/PNIPAAm composite (Fig. 8d). The accurate agreement of carbon components appeared in XPS C1s high resolution scan with that of chemical structure of respective polymers affirmed the pure and contamination free surfaces. This was further supported by a close agreement between theoretical and experimental atomic concentrations revealed by XPS (Tab. 1). Compared with the bare
SiO2, the content of the C and N elements of the SiO2/PNIPAAm composite increase from
1.46% to 29.85% and from 0.00% to 1.85% respectively, while the content of O element decrease from 63.59% to 44.03%. Thus, the results above can verify the success preparation of SiO2/PNIPAAm composites.
Accepted Manuscript
Fig. 8. XPS spectra of (a) N1s of SiO2/PNIPAAm, (b) C1s of SiO2/PNIPAAm, (c) O1s
15
Page 17 of 25 of bare SiO2, (d) O1s of SiO2/PNIPAAm
Tab. 1 XPS surface atomic concentrations
Sample XPS surface atomic concentration (%)
Si O C N
Bare SiO2 63.59 34.94
SiO2/PNIPAAm 24.92 44.03 28.85 1.85
3.5 DLS analysis
DLS is used to further evaluate the temperature sensitive property of composite microsphere. Fig. 9 shows the temperature-dependent size changes of SiO2/PNIPAAm composites microsphere in aqueous solution. It is observed that the particle diameter abruptly decreases from about 270 nm to 240 nm when the temperature increases from 304
K to 305 K, indicating that the composite is temperature sensitive. There are consistent with the results of water contact angle. Fig. 9B and 9C exhibit the particle size distribution at 302 K and 309 K, respectively. It can be seen that the distribution of the particle size is monodisperse, which is in agreement with the result of the TEM. Accepted Manuscript
16
Page 18 of 25 280 (A)
270
260
250 Diameter(nm)
240
230 298 300 302 304 306 308 310 312 Temperature (K)
100 (B)
80
60
40 T=302K Intensity
20
0
245 250 255 260 265 270 275 280 285 Diameter (nm)
100 (C)
80
60
Intensity T=309K 40 Accepted20 Manuscript 0
220 225 230 235 240 245 250 255 260 Diameter (nm)
Fig. 9. The diameter as the function of temperature for SiO2/PNIPAAm in aqueous solution by DLS (A), the particle size distribution of the SiO2/PNIPAAm at 302 K (B), 309 K (C),
respectively.
17
Page 19 of 25
4. Conclusions
The temperature sensitive SiO2-PNIPAAm composites were synthesized successfully by a simple and effective method. There are different influences of the monomer concentration, reaction time, reaction temperature and initiator amount on the preparation of thermal responsive composite microsphere. The LCST of SiO2-PNIPAAm composites are around
304 K. This work might provide a new sight for the synthesis of organic-inorganic composites.
Acknowledgments
The authors gratefully acknowledge the support of China National Petroleum & gas
Corporation science and technology development project ―Nano intelligent chemical flooding agent‖ (No. 2011A-1001), the National Natural Science Foundation of China (No.
21073234, 41171250), Shanxi Province Science Foundation for Youths (No.2011021010-2) and Shanxi Province Graduate Outstanding Innovation Project (No. 0113127). The authors also express their gratitude to Guojin Li (China Research Institute of Daily Chemical
Industry) for TEM, and to Yan Wang (Taiyuan University of Technology) for FTIR. Accepted Manuscript References
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