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Grafting thermosensitive PNIPAM onto the surface of carbon spheres
Applied Surface Science 321 (2014) 116–125
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Grafting thermosensitive PNIPAM onto the surface of carbon spheres Xingmei Guo a , Zefeng Du a , Maoning Song a , Li Qiu a , Yinghua Shen a , Yongzhen Yang b,c , Xuguang Liu a,b,∗ a
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, China c Research Center on Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China b
a r t i c l e
i n f o
Article history: Received 18 July 2014 Received in revised form 19 September 2014 Accepted 19 September 2014 Available online 28 September 2014 Keywords: Carbon spheres Poly(N-isopropylacrylamide) Thermo-responsive Composites Lower critical solution temperature
a b s t r a c t Thermosensitive polymer poly(N-isopropylacrylamine) (PNIPAM) grafted-carbon spheres (CSs) composites were synthesized and investigate the influence of synthesis parameters on phase transition temperature in order to produce thermosensitive composites with different phase transition temperatures, which may be applied to different application fields. First, vinyl groups were introduced onto the surface of CSs by mixed acid oxidation and reacting with 3-methacryloxypropyl trimethoxysilane. Then, PNIPAM was grafted onto the surface of CSs by surface free-radical polymerization. According to the differential scanning calorimetric analysis, the PNIPAM-grafted CSs composites are temperature responsive. The weight ratio of monomer to CSs and the initiator dosage had great influence on the polymer shell thickness and the lower critical solution temperature (LCST) of the composites determined by the number of grafted polymer chain, chain length and the cross-link degree. The polymer shell thickness and the LCST of the composites increased with the increase of the amount of monomer in proper range, however, first increase and then decrease with the increasing initiator dosage. The cross-linking agent content affects critically the cross-link degree, and then the LCST. Therefore, the LCST of the PNIPAMgrafted CSs composites was adjustable by changing the synthesis parameters, which lays the basis for CSs application in different fields. © 2014 Published by Elsevier B.V.
1. Introduction Recently, more and more attention has been drawn to the preparation of different kinds of organic–inorganic composite materials because the composites combine both the properties of the inorganic phase and those of organic phase and have made accessible an immense area of new functional materials [1–5]. It is worth noting that there has been of great interest in fabrication of stimuli-responsive organic–inorganic composite materials because stimuli-responsive polymer as an organic layer improves properties of composite materials. In other words, the physical–chemical properties of the composites can be adjusted by external factors to meet the demand in different areas, such as drug delivery [6,7], bionanotechnology [8,9] and sensing [10,11]. Among stimuli-responsive polymers, temperature-responsive polymers
∗ Corresponding author at: College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China. Tel.: +86 351 6014138; fax: +86 351 6014138. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.apsusc.2014.09.130 0169-4332/© 2014 Published by Elsevier B.V.
are mostly used in biomedical application because they exploit small changes of temperature of the human body as triggering agents of drug release. Poly(N-isopropylacrylamine) (PNIPAM) is the most widely studied thermoresponsive polymer, which shows a reversible volume transition at a temperature, known as the lower critical solution temperature (LCST), of 32 ◦ C in aqueous solution. It changes from hydrophilic below the LCST to hydrophobic above it owing to the reversible formation and cleavage of hydrogen bond between the amide groups and the surrounding water molecules. Therefore, with the increase of temperature, water molecules are expelled from the polymer. The reversible thermosensitivity of PNIPAM can be used in biomedical and biological field, including protein adsorption and purification [8,9,12,13], drug delivery [6,7], monolithic materials [14], catalysis [15], and so forth. Because of this extraordinary performance, PNIPAM has been extensively grafted from various substrates used in synthesizing thermoresponsive materials. In addition, the pristine PNIPAM homopolymer hydrogel has poor mechanical stability and biocompatibility, whereas the property of the composites that are composed of PNIPAM and inorganic materials with good mechanical stability and biocompatibility can be improved. Murakami
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Fig. 1. Grafting and cross-linking of PNIPAM onto the surface of CSs.
et al. [16,17] prepared thermosensitive polymer/mesoporous silica composites and investigated the influence of the crosslinking agent N,N - methylenebisacrylamide (BIS) on the structural and adsorption–desorption properties of composite. They found that the composites responded to temperature, the addition of PNIPAM and BIS hardly affected the structure of the original mesoporous silica, and the phase transition temperature shifted to higher temperature upon the addition of BIS to the PNIPAM. Chen et al. [18] described the fabrication of thermo-responsive superhydrophobic TiO2 /poly(N-isopropylacrylamide) microspheres by sol–gel and surface-initiated atom transfer radical polymerization. The wettability of the surface of the composite microspheres can switch from hydrophilic to superhydrophobic. Wang et al. [19] demonstrated the preparation of poly(N-isopropylacrylamide-co-acrylic acid)/titanium dioxide composites and studied the photocatalytic property of the composites in methyl orange solution. The experiment results suggest that degradation ratio was higher at low pH and increased with increasing temperature above volume phase transition temperature. Magnetic SBA-15/PNIPAM composites have been prepared by Zhu et al. [20]. The phase state of the temperature-responsive PNIPAM was able to control opening and closing of the pores after PNIPAM was modified onto magnetic SBA15 particles, and the drug molecules were controllably released in responsive to the environmental temperature, while the magnetic Fe2 O3 inside the channels of SBA-15 made the composite guidable by an external magnetic field. Carbon spheres (CSs), as an important member of carbon family, have attracted considerable attention because of their unique structure, such as high heat stability and chemical stability, excellent electrical, thermal conductivity and mechanical performance, which results in great potential application in many fields such as
electrode materials, reinforcing agents, lubrication, carbon films, and the support of surface molecularly imprinted polymer [21–24]. Up to now, research on surface thermosensitive functionalization of carbon materials mainly focuses on a few nanostructured carbon materials, such as carbon nanotubes and carbon black [25–28]. For the past two years, PNIPAM/graphene and PNIPAM/graphene oxide (GO) composite hydrogels were also reported [29,30]. Compared with these carbons, carbon spheres, with fullerenes-like cage structures composed of fairly concentric graphitic shells, can be seen as carbon nanotubes (CNTs) with closed ends and length-to-diameter ratio of 1:1 in shape in extreme cases, but that is the limit case to bring huge changes of characteristics of CSs from quantitative change to qualitative change, for instance, from entanglement of CNTs to discrete state of CSs, which bring about isotropy of three-dimensional distribution. In material world, properties are determined structure. The unique structure of the carbon spheres lays the foundation for synthesizing carbon-based composites with excellent properties. In recent years, our group has always devoted ourselves to the surface modification and functionalization of CSs [31–33]. Various oxygen-containing functional groups, e.g. carboxyl, hydroxyl and carbonyl were introduced onto the surface of CSs by the oxidation reaction of various oxyacids, at the same time, vinyl groups were introduced onto the surface of CSs by reacting with organics with double-bond, CSs@polymer composites were obtained by reacting with vinyl-functionalized carbon spheres. Furthermore, surface coating by silica is one of the surface modification methods of CSs. The past application research of CSs mainly concentrated in carbon-based photonic crystals, the surface molecularly imprinted materials, and so on. Better physical and chemical properties can be provided by functionalization of CSs with polymer. However, to our
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Fig. 2. FESEM images of (a1, a2) as-prepared CSs, (b1,b2) mixed acid treated-CSs and (c1, c2) KH570-treated CSs.
knowledge, few work so far has been reported on the preparation of thermal-responsive polymer functionalized CSs. PNIPAM-modified surfaces have been extensively applied to several bio-medical areas, such as gene delivery systems and controlled drugs [6,7,34], microfluidics [35], cell-culture substrate [36]. Inspired by these studies, in the present paper, we grafted thermosensitive polymer PNIPAM onto the surface of the CSs. However, because as-prepared CSs are of chemically inert surface and short of effective functional groups [31–33], first of all, mixed acid was used as oxidant to oxidize CSs to introduce OH functional groups onto the surface of CSs. Then vinyl-functionalized CSs were obtained via the reaction of OH with silane coupling agent with double bond, which is a chemical that functions at the interface to create a chemical bridge between organic and inorganic materials. The thermosensitive monomer was graft-polymerized and crosslinked on the surface of CSs under the action of initiator and cross-linking agent. The structure and properties of the resultant PNIPAM composites were
characterized by field-emission scanning electron microscopy (FESEM), Fourier-transform IR spectra (FT-IR), differential scanning calorimetric (DSC), X-ray photoelectron spectroscopic (XPS) and dynamic light scattering (DLS). The combination of CSs and thermosensitive polymer can simultaneously help to improve mechanical stability of polymer and obtain thermosensitive CSs, which can expand the application scope of CSs. This study opens up novel prospects for the application of CSs in stimuli-responsive materials. 2. Materials and methods 2.1. Materials Carbon spheres were prepared by the pyrolysis of acetylene at atmospheric pressure in a tubular furnace [32,33]. Concentrated nitric acid (HNO3 , 65%), sulfuric acid (H2 SO4 , 98%)
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and absolute alcohol (EtOH) were obtained from Beijing Chemical Works. 3-Methacryloxypropyl trimethoxysilane (KH-570, 95 wt%), ammonium persulfate (NH4 )2 S2 O8 (APS, 99.5 wt%) and N,Nmethylenebisacrylamide (MBA, 98%) were obtained from Shanghai Jingchun Reagent Co., Ltd. N-isopropylacrylamide (NIPAM, 97%) was purchased from Sigma–Aldrich and purified by recrystallization from n-hexane. All other chemicals were used as received. Deionized water was prepared with an ion exchange system. 2.2. Oxdization modification of CSs surface The oxidation process was the same as Ref. [32]. One gram of CSs was dispersed in 120 ml of a 3:1 (v/v) mixture of concentrated H2 SO4 /HNO3 under ultrasonication treatment and then refluxed at 80 ◦ C for 20 min. The resulting black solid was collected by filtration and washed with deionized water until the pH of the filtrate reached neutral. The mixed acid treated-CSs were dried at 60 ◦ C in a vacuum oven overnight. 2.3. Preparation of vinyl-functionalized carbon spheres Vinyl-functionalized carbon spheres were prepared by silanization with coupling agent KH-570. A three-necked flask was charged with 0.5 g of mixed acid treated-CSs and 50 ml of EtOH and 30 ml of H2 O. After sonicated at ambient temperature for 40 min, the mixture was heated to 60 ◦ C in water bath. Then, 1.5 ml of KH570 dissolved in 40 ml of deionized water was added dropwise into the above mixture by dropping funnel under electromagnetic stirring. The vinyl functionalization process proceeded for 4 h at 60 ◦ C. Finally, vinyl-functionalized CSs (defined as KH570 treated-CSs) were obtained by filtering, washing with ethanol, and drying in vacuum. 2.4. Grafting thermosensitive PNIPAM onto the surface of carbon spheres Thermosensitive polymer functionalized carbon spheres were prepared by means of surface free radical reaction. KH570 treatedCSs (0.3 g) were dispersed evenly in 45 ml of H2 O under the ultrasonic conditions. A certain amount of NIPAM and MBA were added and the mixture was bubbled with nitrogen for 30 min under magnetic stirring to remove the oxygen in the reaction system. When the reaction temperature was reached, the solution of APS dissolved in 5 ml of H2 O was injected. The reaction continued at 70 ◦ C for 4 h to obtain thermosensitive polymer functionalized carbon spheres, defined as PNIPAM-grafted CSs. The schematic illustration for grafting and cross-linking of thermosensitive PNIPAM onto the surface of carbon spheres is shown in Fig. 1. 2.5. Characterization of materials The morphology and size of the parent CSs and the functionalized CSs were observed by field emission scanning electron microscopy (FESEM JSM-6700). The samples were sputtered with gold in vacuum for scanning electron microscopy characterization. The particle size was estimated from the measurement of the diameter of 100 particles in photographs. Fourier-transform IR (FT-IR) spectra were recorded on a Japan MODEL-8400s FT-IR spectrometer using KBr pellet, which were used to prove the introduction and formation of various functional groups on the surface of carbon spheres. X-ray photoelectron spectroscopic (XPS) analysis was performed by PHI5000 Versaprobe spectrometer with
Fig. 3. FT-IR spectra of (a) as-prepared CSs, (b) mixed acid treated-CSs and (c) KH570-treated CSs.
Al (K␣) X-ray source (1486.3 eV). Differential scanning calorimetric (DSC) curves were recorded on a DSC Q100 V9.4 Build 287. Dynamic light scattering (DLS) measurements were conducted on Malvern Zetasizer ZS90, and the scattering angle was 90◦ .
3. Results and discussion 3.1. Oxidization and vinyl functionalization of CSs In order to graft polymer, it is extremely important to introduce double bond onto the surface of CSs. Therefore, as-prepared CSs were treated first by mixed acid, and then reacted with coupling agent KH-570. Fig. 2 shows FESEM images of as-prepared CSs, mixed acid treated-CSs and KH570 treated-CSs. It can be seen that the difference between the surface modified CSs and as-prepared CSs lay in surface morphology and the particle size. The average diameter of as-prepared CSs with smooth surface was about 300 nm (Fig. 2a1 and a2). After oxidation modification, the size of CSs remained almost unchanged, but the surface of mixed acid-treated CSs became slightly rougher as a result of mixed-acid etching (Fig. 2b2). However, the average size of KH570 treated-CSs increased slightly to about 310 nm. The difference in size was due to the thickness of KH570 grafted on the surface of CSs. At a higher magnification (Fig. 2c2), it is clearly seen that a thin grafting layer covered the surface of CSs. In order to detect the change of surface functional groups at different modification stages, the FT-IR spectra of as-prepared CSs, mixed acid treated-CSs and KH570 treated-CSs are shown in Fig. 3. Compared with as-prepared CSs (Fig. 3a), mixed acid treatedCSs showed the characteristic bands of carboxyl and hydroxyl at 1248,1731 and 3458 cm−1 , among which the band at 3458 cm−1 is attributed to hydroxyl groups (O H). The result provides clear evidence that oxygen-containing functional groups were introduced onto the surface of CSs after treatment by mixed acid. As shown in Fig. 2c, after treated by KH570, the band at 3458 cm−1 due to hydroxyl groups is dramatically decreased, which demonstrates the reaction of hydroxyl on the surface of mixed acid treated-CSs with KH570. At the same time, one new band appears at 1564 cm−1 , corresponding to the C C. According to the FT-IR analysis, the C C double bonds were introduced onto the surface of the CSs, which lays the foundation for the subsequent grafting and cross-linking of NIPAM.
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Fig. 4. FESEM images of PNIPAM-grafted CSs composites prepared with different weight ratio of monomer to CSs (a1,a2) 1:1, (b1,b2) 3:1 and (c) 5:1. The other reaction conditions are: 1 wt% APS dosage, 15 wt% MBA content.
3.2. Grafting thermosensitive PNIPAM onto the surface of carbon spheres 3.2.1. Influence of monomer dosage The PNIPAM-grafted CSs composites were prepared with different weight ratio of monomer to CSs (1:1, 3:1and 5:1) while initiator dosage and cross-linking agent content remained unchanged, that is, 1 wt% and 15 wt% (relative to mass of NIPAM), respectively. As shown in Fig. 4, when the weight ratio of monomer to CSs was 1:1, the composite spheres with a mean diameter of 332 nm and a shell thickness of ca. 16 nm were observed (Fig. 4a). When the weight ratio was increased to 3:1, the composites was about 400 nm in size, that is, the thickness of the polymer shell was 50 nm (Fig. 4b). Further increasing the weight ratio of monomer to CSs to 5:1, the composite morphology changed a lot (Fig. 4c). The composites did not show the regular sphericity but became
massive, a few CSs were faintly visible, which embedded inside the polymer matrix. It shows that when the weight ratio reached 5:1, the self-polymerization of a substantial portion of monomer formed pure PNIPAM microgel. The results show that the amount of monomer had great influence on the PNIPAM grafted on the surface of CSs. The diameter and the corresponding shell thickness of the composites increased with the increase of the amount of NIPAM monomer in proper range. The optimum weight ratio of monomer to CSs did not exceed 3:1. The thickness of grafted polymer shell can be modulated by adjusting monomer-to-CSs ratio, that is, controlling the amount of NIPAM. The monomer polymerized under action of initiator, the double bonds introduced onto the surface of CSs participated in chain polymerization, linear PNIPAM was grafted onto the surface of CSs. In the meantime, linear PNIPAM molecules were interlinked to form crosslinked PNIPAM coating layer because of the presence of crosslinking
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Fig. 5. DCS curves of PNIPAM-grafted CSs composites prepared with different weight ratio of monomer to CSs (a) 1:1 and (b) 3:1. The other reaction conditions are: 1 wt% APS dosage, 15 wt% MBA content.
agent MBA. Because the increase of monomer dosage led to the increase of polymerization rate and chain length, the thickness of coating layer increased. However, too much of monomer caused the formation of bulk PNIPAM as a result of the self-polymerization rather than grafting polymerization of monomer. It is important to evaluate the thermosensitive property, especially phase transition behavior, of a material with surface grafted thermosensitive polymer for actual application. DSC is usually employed to measure the phase transition temperature of the material. The onset point of the endothermal peak on the DSC curve is more commonly used to represent the phase transition point (LCST). The onset temperature Tonset is obtained by the intersection of the baseline and the leading edge of the endotherm [37,38]. In the current work, the phase transition temperature was defined as the onset temperature. The DSC curves of the composites prepared at different weight ratio are shown in Fig. 5. The existence of the endothermal peaks indicates the thermosensitivity of the composites. It can be seen from Fig. 5a, the composites had an LCST of 34.0 ◦ C at the monomer-to-CSs weight ratio of 1:1. When the weight ratio was increased to 3:1, the LCST of the composites was about 40.8 ◦ C. The LCST of PNIPAM-grafted CSs composites increased with the increase of monomer dosage and exceeded that of pure PNIPAM hydrogel, which is typically about 32 ◦ C. On one hand, the existence of CSs probably hinders the collapse and aggregation of the hydrophobic polymer segments that takes place above LCST, resulting in a higher temperature required to overcome the obstacles [39]. On the other hand, the increase of monomer dosage leads to the increase of polymerization rate and chain length, therefore more physical and chemical crosslinking occurs, that is to say, crosslinking degree increase. In general, the increase of crosslinking degree leads to the increase of LCST because crosslinking prohibits the formation of inter- and intra-chain hydrogen bonds and the aggregation of hydrophobic isopropyl groups [37]. The above two factors work together and influence the LCST of the composites. 3.2.2. Influence of initiator content The effect of initiator content on surface grafting of CSs with PNIPAM was also investigated from 0.5 wt% to 2 wt% (relative to mass of NIPAM) while other reaction parameters were kept constant at the monomer-to-CSs weight ratio of 3:1 and cross-linking agent content of 15 wt%. As shown in Fig. 6, the composites kept spherical morphologies within the scope of investigation. Similar
Fig. 6. FESEM images of PNIPAM-grafted CSs composites prepared with different initiator content (a) 0.5 wt%, (b) 1 wt% and (c) 2 wt%. The other reaction conditions are: the weight ratio of monomer to CSs: 3:1, 15 wt% MBA content.
to the effect of monomer amount, the size of the grafted composites changed. In comparison with as-prepared CSs (Fig. 2a1), the composites were nearly 50 nm larger in size than as-prepared CSs at APS content of 0.5 wt%, indicating the thickness of the grafted PNIPAM shell was about 25 nm. When APS content was 1 wt%, the thickness of grafted PNIPAM shell was 50 nm. With further increased initiator content up to 2 wt%, the thickness of grafted shell decreased, which was about 31 nm. The characterization results show that the conversion of monomer and grafting degree of monomer increased
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Fig. 7. DCS curves of PNIPAM-grafted CSs composites prepared with different initiator content (a) 0.5 wt%, (b) 1 wt% and (c) 2 wt%. The other reaction conditions are: the weight ratio of monomer to CSs: 3:1, 15 wt% MBA content.
when the amounts of APS changed from 0.5 wt% to 1 wt%, so the thickness of the grafted shell increased. However, further increase of initiator content to 2 wt% resulted in the reduction of the thickness of the shell. According to general principle of polymerization reaction, the function of the initiator is to produce primary radical, which initiates monomer polymerization or grafting polymerization. If excessive amount of initiator is used, it would generate lots of radicals as active center, the number of monomer linked with each active center would be reduced, resulting in a decrease of grafted chain length. Besides, excessive amount of initiator can easily lead to coupling with the grafted chain active end, which can speed up the chain termination reaction. In this way, the conversion of monomer would be reduced. Based on the above analysis, we can come to the conclusion that the reduction of the thickness of the shell at the initiator content of 2 wt% was the result of the reduction of conversion of monomer and the grafting short polymer chains. Thus, the appropriate content of initiator was determined as 1 wt%. The DSC analysis results for PNIPAM-grafted CSs composites with different initiator content were shown in Fig. 7. When APS contents were 0.5 wt%, 1 wt% and 2 wt%, the LCST was 36.0 ◦ C, 40.3 ◦ C and 39.0 ◦ C, respectively. As was mentioned above, the increase of initiator content leads to the formation of more but shorter polymer chains. However, both more chains and longer chains are in favor of crosslinking. In a proper initiator content range, more polymer chains with suitable chain length would lead to an increasing number of active chains participating in crosslinking, and hence the increase of crosslinking degree. The increase of crosslinking degree leads to the increase of LCST [37]. Therefore, when the APS content increased from 0.5 wt% to 1 wt%, the LCST of the composites increased from 36.0 ◦ C to 40.3 ◦ C. Further increasing the initiator content to 2 wt%, the decline of the LCST may be the result of the decrease of crosslinking degree. Too much initiator would give rise to shorter grafted chain onto the surface of CSs, which could not effectively crosslink, hence crosslinking degree decreased. From the above analysis we can see the monomer dosage and the initiator content had significant effect on the polymer shell thickness and the LCST of the composites determined by the number of grafted polymer chain, chain length and the cross-link degree. Therefore, the LCST of the PNIPAM-grafted CSs composites can be adjustable by changing the monomer dosage and the initiator content.
Fig. 8. FESEM images of PNIPAM-grafted CSs composites prepared with different cross-linking agent content. The other reaction conditions are: the weight ratio of monomer to CSs: 3:1, 1 wt% APS dosage.
3.2.3. Influence of cross-linking agent content Fig. 8 shows the effect of cross-linking agent content (relative to mass of NIPAM) on the morphology of the composites. As shown in Fig. 8, the size of the composites with spherical morphology was about 370 nm and 400 nm when cross-linking agent content was 7 wt% and 15 wt%, respectively, which illustrates that the thickness of PNIPAM shell was 35 nm and 50 nm, respectively (Fig. 8a and b). When the cross-linking agent content was increased to 34 wt%, the mean diameter of the composites was about 410 nm, that is, the
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contents. When MBA contents were 7 wt%, 15 wt% and 34 wt%, the LCST was 33.0 ◦ C, 40.1 ◦ C and 48.5 ◦ C, respectively. The LCST shifted toward higher temperature along with increasing MBA concentration. Obviously, when the content of the cross-linking agent increased, the cross-link degree became higher, resulting in higher LCST. It follows that the content of cross-linking agent shows obvious influence on the cross-link degree, and then the LCST.
Fig. 9. DCS curves of PNIPAM-grafted CSs composites prepared with different cross-linking agent content. The other reaction conditions are: the weight ratio of monomer to CSs: 3:1, 1 wt% APS content.
thickness of the shell was 55 nm (Fig. 8c). All samples were already dried before SEM observation, therefore, these SEM images are reflecting the shrinking state of the composites. The crosslinking degree increased with increasing content of cross-linking agent, which resulted in the reduction of the composites shrinkage degree during drying process, so the diameter increased accordingly. But there were some differences in surface appearance. From Fig. 8a, the grafted polymer layer prepared with the mass percentage ratio of MBA to NIPAM of 7 wt% was loose, probably, because of the low extent of crosslinking caused by less content of cross-linking agent. Inefficient crosslinking occurred at the cross-linking agent content of 7 wt%, that is to say, only a portion of the grafted chains participated in chemical and physical crosslinking, consequently the grafted polymer shell was loose rather than compact. When the content of cross-linking agent increased to 15 wt%, the thermosensitive polymer shell became denser. The content of the cross-linking agent was further raised to 34 wt%, the surface of the composites was not only dense but also filiform, because crosslinking not only existed in adjacent grafted chains but also nonadjacent grafted chains. The influence of the content of the cross-linking agent on LCST was also evaluated. Fig. 9 presents the DCS curves of the composites prepared with different cross-linking agent
3.2.4. FT-IR and XPS analysis The FT-IR spectra and XPS spectra of PNIPAM-grafted CSs composites prepared at preferable parameters (the weight ratio of monomer to CSs: 3:1, APS: 1 wt%, MBA: 15 wt%) are shown in Fig. 10. Compared with the spectra of KH570 treated-CSs, the two new bands absorption peak at 1641 and 1539 cm−1 appear, which are due to C O stretching (amide I) and N H stretching (amide II) of the CONH groups of PNIPAM. The characteristic band found at 1370 cm−1 is C H stretching of isopropyl groups of PNIPAM (Fig. 10a). The absorption band at 1564 cm−1 due to C C disappears as a result of surface polymerization reaction. The bands at 2870 and 2970 cm−1 correspond to the characteristic stretching and bending vibrations of methylene groups. The broad band between 3200 and 3400 comes from the N H stretching of amide. The presence of the grafted PNIPAM on the surface of CSs was also confirmed by XPS analysis. The wide scan spectra show the N 1s (C N) peak at about 400.8 eV (Fig. 10b). These results indicate that PNIPAM was successfully grafted on the surface of CSs. 3.3. Temperature responsive property of the composites The thermoresponsiveness of the composites (the weight ratio of monomer to CSs: 3:1, APS: 1 wt%, MBA: 15 wt%) was studied by DLS at temperature increasing from 25 to 52 ◦ C. Fig. 11 shows the DLS curve of PNIPAM-grafted CSs composites. It is clearly seen that the hydrodynamic diameter (Dh) of the composites decreases when the temperature is raised, which suggests that the composites are sensitive to temperature. The diameter decreased from 585 nm to 471 nm when environmental temperature increased from 25 ◦ C to 52 ◦ C. Especially from 40 ◦ C to 41 ◦ C, the diameter decreased drastically from 562 nm to 473 nm, indicating the temperature of phase transition was about 40 ◦ C. The LCST measured by DLS is consistent with that of DCS. At room temperature (25 ◦ C), the size of the composite spheres from DLS (585 nm) was bigger than that of observed by SEM (400 nm) in the same conditions. This is because the composite spheres are swollen in water, however
Fig. 10. FT-IR spectra (a) and XPS spectra (b) of PNIPAM-grafted CSs composites.
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temperature of human body. This novel composite would open up prospects for CSs application in different fields. Acknowledgements The authors acknowledge financial support from Program for Changjiang Scholar and Innovative Research Team in University (IRT0972), National Natural Science Foundation of China (20971094, 21176169, 51152001), Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011), Natural Science Foundation of Shanxi Province (2012011020-03), Research Project Supported by Shanxi Scholarship Council of China (2012-038), Taiyuan S&T Program (120247-25). References
Fig. 11. Hydrodynamic diameter (Dh) of the PNIPAM-grafted CSs composites as a function of temperature.
SEM observation shows the diameters of the dried sample. The temperature responsive property of the composites comes from the thermosensitivity of the PNIPAM shell. As is known to all, the PNIPAM has temperature stimuli response and a LCST of about 32 ◦ C in water. When the temperature is below the LCST, it is water-soluble as a result of the formation of hydrogen bonding between water molecules and amide group within the polymer chains, adopts an extended forms, swells and expands because of water uptake. However, it becomes water-insoluble as a result of the breakage of hydrogen bonding and hydrophobic interactions between polymer chains become stronger when the temperature of the solution exceeds the LCST. Consequently, it collapses and shrinks by expelling its contents. In this case, as the temperature of the solution increased to higher the LCST. The shrinkage of the cross-linked PNIPAM shell on the surface of CSs brought about the decrease of the shell thickness of the composite with a corresponding decrease of the diameter of the composites.
4. Conclusion Thermosensitive polymer PNIPAM-grafted CSs composites were prepared by surface free-radical polymerization. First, as-prepared CSs were modified by mixed acid oxidation and reacting with KH570 to obtain vinyl-functionalized CSs. Then, PNIPAM was grafted and crosslinked on the surface of CSs using APS as initiator, MBA as cross-linking agent. Effect of grafting conditions on surface grafting of CSs with PNIPAM, including the weight ratio of monomer to CSs, the initiator dosage and the cross-linking agent content, was emphasized. The results show that the PNIPAM-grafted CSs composites were temperature responsive, as confirmed by DSC and DLS analysis. The weight ratio of monomer to CSs and the initiator dosage had great influence on the number of grafted polymer chain, chain length and the cross-link degree, and then the polymer shell thickness and the LCST of the composites. The polymer shell thickness and the LCST of the composites increased with the increase of the amount of monomer in proper range, however, first increase and then decrease with the increasing initiator dosage. The crosslinking agent content critically affects the cross-link degree, and then the LCST. Therefore, the LCST of the PNIPAM-grafted CSs composites was adjustable by changing the weight ratio of monomer to CSs, the initiator dosage and the cross-linking agent content in a certain range, which would meet its application in different fields, such as drug delivery, which requires that the LCST be close to the
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Grafting thermosensitive PNIPAM onto the surface of carbon spheres Xingmei Guo a , Zefeng Du a , Maoning Song a , Li Qiu a , Yinghua Shen a , Yongzhen Yang b,c , Xuguang Liu a,b,∗ a
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, China c Research Center on Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China b
a r t i c l e
i n f o
Article history: Received 18 July 2014 Received in revised form 19 September 2014 Accepted 19 September 2014 Available online 28 September 2014 Keywords: Carbon spheres Poly(N-isopropylacrylamide) Thermo-responsive Composites Lower critical solution temperature
a b s t r a c t Thermosensitive polymer poly(N-isopropylacrylamine) (PNIPAM) grafted-carbon spheres (CSs) composites were synthesized and investigate the influence of synthesis parameters on phase transition temperature in order to produce thermosensitive composites with different phase transition temperatures, which may be applied to different application fields. First, vinyl groups were introduced onto the surface of CSs by mixed acid oxidation and reacting with 3-methacryloxypropyl trimethoxysilane. Then, PNIPAM was grafted onto the surface of CSs by surface free-radical polymerization. According to the differential scanning calorimetric analysis, the PNIPAM-grafted CSs composites are temperature responsive. The weight ratio of monomer to CSs and the initiator dosage had great influence on the polymer shell thickness and the lower critical solution temperature (LCST) of the composites determined by the number of grafted polymer chain, chain length and the cross-link degree. The polymer shell thickness and the LCST of the composites increased with the increase of the amount of monomer in proper range, however, first increase and then decrease with the increasing initiator dosage. The cross-linking agent content affects critically the cross-link degree, and then the LCST. Therefore, the LCST of the PNIPAMgrafted CSs composites was adjustable by changing the synthesis parameters, which lays the basis for CSs application in different fields. © 2014 Published by Elsevier B.V.
1. Introduction Recently, more and more attention has been drawn to the preparation of different kinds of organic–inorganic composite materials because the composites combine both the properties of the inorganic phase and those of organic phase and have made accessible an immense area of new functional materials [1–5]. It is worth noting that there has been of great interest in fabrication of stimuli-responsive organic–inorganic composite materials because stimuli-responsive polymer as an organic layer improves properties of composite materials. In other words, the physical–chemical properties of the composites can be adjusted by external factors to meet the demand in different areas, such as drug delivery [6,7], bionanotechnology [8,9] and sensing [10,11]. Among stimuli-responsive polymers, temperature-responsive polymers
∗ Corresponding author at: College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China. Tel.: +86 351 6014138; fax: +86 351 6014138. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.apsusc.2014.09.130 0169-4332/© 2014 Published by Elsevier B.V.
are mostly used in biomedical application because they exploit small changes of temperature of the human body as triggering agents of drug release. Poly(N-isopropylacrylamine) (PNIPAM) is the most widely studied thermoresponsive polymer, which shows a reversible volume transition at a temperature, known as the lower critical solution temperature (LCST), of 32 ◦ C in aqueous solution. It changes from hydrophilic below the LCST to hydrophobic above it owing to the reversible formation and cleavage of hydrogen bond between the amide groups and the surrounding water molecules. Therefore, with the increase of temperature, water molecules are expelled from the polymer. The reversible thermosensitivity of PNIPAM can be used in biomedical and biological field, including protein adsorption and purification [8,9,12,13], drug delivery [6,7], monolithic materials [14], catalysis [15], and so forth. Because of this extraordinary performance, PNIPAM has been extensively grafted from various substrates used in synthesizing thermoresponsive materials. In addition, the pristine PNIPAM homopolymer hydrogel has poor mechanical stability and biocompatibility, whereas the property of the composites that are composed of PNIPAM and inorganic materials with good mechanical stability and biocompatibility can be improved. Murakami
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Fig. 1. Grafting and cross-linking of PNIPAM onto the surface of CSs.
et al. [16,17] prepared thermosensitive polymer/mesoporous silica composites and investigated the influence of the crosslinking agent N,N - methylenebisacrylamide (BIS) on the structural and adsorption–desorption properties of composite. They found that the composites responded to temperature, the addition of PNIPAM and BIS hardly affected the structure of the original mesoporous silica, and the phase transition temperature shifted to higher temperature upon the addition of BIS to the PNIPAM. Chen et al. [18] described the fabrication of thermo-responsive superhydrophobic TiO2 /poly(N-isopropylacrylamide) microspheres by sol–gel and surface-initiated atom transfer radical polymerization. The wettability of the surface of the composite microspheres can switch from hydrophilic to superhydrophobic. Wang et al. [19] demonstrated the preparation of poly(N-isopropylacrylamide-co-acrylic acid)/titanium dioxide composites and studied the photocatalytic property of the composites in methyl orange solution. The experiment results suggest that degradation ratio was higher at low pH and increased with increasing temperature above volume phase transition temperature. Magnetic SBA-15/PNIPAM composites have been prepared by Zhu et al. [20]. The phase state of the temperature-responsive PNIPAM was able to control opening and closing of the pores after PNIPAM was modified onto magnetic SBA15 particles, and the drug molecules were controllably released in responsive to the environmental temperature, while the magnetic Fe2 O3 inside the channels of SBA-15 made the composite guidable by an external magnetic field. Carbon spheres (CSs), as an important member of carbon family, have attracted considerable attention because of their unique structure, such as high heat stability and chemical stability, excellent electrical, thermal conductivity and mechanical performance, which results in great potential application in many fields such as
electrode materials, reinforcing agents, lubrication, carbon films, and the support of surface molecularly imprinted polymer [21–24]. Up to now, research on surface thermosensitive functionalization of carbon materials mainly focuses on a few nanostructured carbon materials, such as carbon nanotubes and carbon black [25–28]. For the past two years, PNIPAM/graphene and PNIPAM/graphene oxide (GO) composite hydrogels were also reported [29,30]. Compared with these carbons, carbon spheres, with fullerenes-like cage structures composed of fairly concentric graphitic shells, can be seen as carbon nanotubes (CNTs) with closed ends and length-to-diameter ratio of 1:1 in shape in extreme cases, but that is the limit case to bring huge changes of characteristics of CSs from quantitative change to qualitative change, for instance, from entanglement of CNTs to discrete state of CSs, which bring about isotropy of three-dimensional distribution. In material world, properties are determined structure. The unique structure of the carbon spheres lays the foundation for synthesizing carbon-based composites with excellent properties. In recent years, our group has always devoted ourselves to the surface modification and functionalization of CSs [31–33]. Various oxygen-containing functional groups, e.g. carboxyl, hydroxyl and carbonyl were introduced onto the surface of CSs by the oxidation reaction of various oxyacids, at the same time, vinyl groups were introduced onto the surface of CSs by reacting with organics with double-bond, CSs@polymer composites were obtained by reacting with vinyl-functionalized carbon spheres. Furthermore, surface coating by silica is one of the surface modification methods of CSs. The past application research of CSs mainly concentrated in carbon-based photonic crystals, the surface molecularly imprinted materials, and so on. Better physical and chemical properties can be provided by functionalization of CSs with polymer. However, to our
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Fig. 2. FESEM images of (a1, a2) as-prepared CSs, (b1,b2) mixed acid treated-CSs and (c1, c2) KH570-treated CSs.
knowledge, few work so far has been reported on the preparation of thermal-responsive polymer functionalized CSs. PNIPAM-modified surfaces have been extensively applied to several bio-medical areas, such as gene delivery systems and controlled drugs [6,7,34], microfluidics [35], cell-culture substrate [36]. Inspired by these studies, in the present paper, we grafted thermosensitive polymer PNIPAM onto the surface of the CSs. However, because as-prepared CSs are of chemically inert surface and short of effective functional groups [31–33], first of all, mixed acid was used as oxidant to oxidize CSs to introduce OH functional groups onto the surface of CSs. Then vinyl-functionalized CSs were obtained via the reaction of OH with silane coupling agent with double bond, which is a chemical that functions at the interface to create a chemical bridge between organic and inorganic materials. The thermosensitive monomer was graft-polymerized and crosslinked on the surface of CSs under the action of initiator and cross-linking agent. The structure and properties of the resultant PNIPAM composites were
characterized by field-emission scanning electron microscopy (FESEM), Fourier-transform IR spectra (FT-IR), differential scanning calorimetric (DSC), X-ray photoelectron spectroscopic (XPS) and dynamic light scattering (DLS). The combination of CSs and thermosensitive polymer can simultaneously help to improve mechanical stability of polymer and obtain thermosensitive CSs, which can expand the application scope of CSs. This study opens up novel prospects for the application of CSs in stimuli-responsive materials. 2. Materials and methods 2.1. Materials Carbon spheres were prepared by the pyrolysis of acetylene at atmospheric pressure in a tubular furnace [32,33]. Concentrated nitric acid (HNO3 , 65%), sulfuric acid (H2 SO4 , 98%)
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and absolute alcohol (EtOH) were obtained from Beijing Chemical Works. 3-Methacryloxypropyl trimethoxysilane (KH-570, 95 wt%), ammonium persulfate (NH4 )2 S2 O8 (APS, 99.5 wt%) and N,Nmethylenebisacrylamide (MBA, 98%) were obtained from Shanghai Jingchun Reagent Co., Ltd. N-isopropylacrylamide (NIPAM, 97%) was purchased from Sigma–Aldrich and purified by recrystallization from n-hexane. All other chemicals were used as received. Deionized water was prepared with an ion exchange system. 2.2. Oxdization modification of CSs surface The oxidation process was the same as Ref. [32]. One gram of CSs was dispersed in 120 ml of a 3:1 (v/v) mixture of concentrated H2 SO4 /HNO3 under ultrasonication treatment and then refluxed at 80 ◦ C for 20 min. The resulting black solid was collected by filtration and washed with deionized water until the pH of the filtrate reached neutral. The mixed acid treated-CSs were dried at 60 ◦ C in a vacuum oven overnight. 2.3. Preparation of vinyl-functionalized carbon spheres Vinyl-functionalized carbon spheres were prepared by silanization with coupling agent KH-570. A three-necked flask was charged with 0.5 g of mixed acid treated-CSs and 50 ml of EtOH and 30 ml of H2 O. After sonicated at ambient temperature for 40 min, the mixture was heated to 60 ◦ C in water bath. Then, 1.5 ml of KH570 dissolved in 40 ml of deionized water was added dropwise into the above mixture by dropping funnel under electromagnetic stirring. The vinyl functionalization process proceeded for 4 h at 60 ◦ C. Finally, vinyl-functionalized CSs (defined as KH570 treated-CSs) were obtained by filtering, washing with ethanol, and drying in vacuum. 2.4. Grafting thermosensitive PNIPAM onto the surface of carbon spheres Thermosensitive polymer functionalized carbon spheres were prepared by means of surface free radical reaction. KH570 treatedCSs (0.3 g) were dispersed evenly in 45 ml of H2 O under the ultrasonic conditions. A certain amount of NIPAM and MBA were added and the mixture was bubbled with nitrogen for 30 min under magnetic stirring to remove the oxygen in the reaction system. When the reaction temperature was reached, the solution of APS dissolved in 5 ml of H2 O was injected. The reaction continued at 70 ◦ C for 4 h to obtain thermosensitive polymer functionalized carbon spheres, defined as PNIPAM-grafted CSs. The schematic illustration for grafting and cross-linking of thermosensitive PNIPAM onto the surface of carbon spheres is shown in Fig. 1. 2.5. Characterization of materials The morphology and size of the parent CSs and the functionalized CSs were observed by field emission scanning electron microscopy (FESEM JSM-6700). The samples were sputtered with gold in vacuum for scanning electron microscopy characterization. The particle size was estimated from the measurement of the diameter of 100 particles in photographs. Fourier-transform IR (FT-IR) spectra were recorded on a Japan MODEL-8400s FT-IR spectrometer using KBr pellet, which were used to prove the introduction and formation of various functional groups on the surface of carbon spheres. X-ray photoelectron spectroscopic (XPS) analysis was performed by PHI5000 Versaprobe spectrometer with
Fig. 3. FT-IR spectra of (a) as-prepared CSs, (b) mixed acid treated-CSs and (c) KH570-treated CSs.
Al (K␣) X-ray source (1486.3 eV). Differential scanning calorimetric (DSC) curves were recorded on a DSC Q100 V9.4 Build 287. Dynamic light scattering (DLS) measurements were conducted on Malvern Zetasizer ZS90, and the scattering angle was 90◦ .
3. Results and discussion 3.1. Oxidization and vinyl functionalization of CSs In order to graft polymer, it is extremely important to introduce double bond onto the surface of CSs. Therefore, as-prepared CSs were treated first by mixed acid, and then reacted with coupling agent KH-570. Fig. 2 shows FESEM images of as-prepared CSs, mixed acid treated-CSs and KH570 treated-CSs. It can be seen that the difference between the surface modified CSs and as-prepared CSs lay in surface morphology and the particle size. The average diameter of as-prepared CSs with smooth surface was about 300 nm (Fig. 2a1 and a2). After oxidation modification, the size of CSs remained almost unchanged, but the surface of mixed acid-treated CSs became slightly rougher as a result of mixed-acid etching (Fig. 2b2). However, the average size of KH570 treated-CSs increased slightly to about 310 nm. The difference in size was due to the thickness of KH570 grafted on the surface of CSs. At a higher magnification (Fig. 2c2), it is clearly seen that a thin grafting layer covered the surface of CSs. In order to detect the change of surface functional groups at different modification stages, the FT-IR spectra of as-prepared CSs, mixed acid treated-CSs and KH570 treated-CSs are shown in Fig. 3. Compared with as-prepared CSs (Fig. 3a), mixed acid treatedCSs showed the characteristic bands of carboxyl and hydroxyl at 1248,1731 and 3458 cm−1 , among which the band at 3458 cm−1 is attributed to hydroxyl groups (O H). The result provides clear evidence that oxygen-containing functional groups were introduced onto the surface of CSs after treatment by mixed acid. As shown in Fig. 2c, after treated by KH570, the band at 3458 cm−1 due to hydroxyl groups is dramatically decreased, which demonstrates the reaction of hydroxyl on the surface of mixed acid treated-CSs with KH570. At the same time, one new band appears at 1564 cm−1 , corresponding to the C C. According to the FT-IR analysis, the C C double bonds were introduced onto the surface of the CSs, which lays the foundation for the subsequent grafting and cross-linking of NIPAM.
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Fig. 4. FESEM images of PNIPAM-grafted CSs composites prepared with different weight ratio of monomer to CSs (a1,a2) 1:1, (b1,b2) 3:1 and (c) 5:1. The other reaction conditions are: 1 wt% APS dosage, 15 wt% MBA content.
3.2. Grafting thermosensitive PNIPAM onto the surface of carbon spheres 3.2.1. Influence of monomer dosage The PNIPAM-grafted CSs composites were prepared with different weight ratio of monomer to CSs (1:1, 3:1and 5:1) while initiator dosage and cross-linking agent content remained unchanged, that is, 1 wt% and 15 wt% (relative to mass of NIPAM), respectively. As shown in Fig. 4, when the weight ratio of monomer to CSs was 1:1, the composite spheres with a mean diameter of 332 nm and a shell thickness of ca. 16 nm were observed (Fig. 4a). When the weight ratio was increased to 3:1, the composites was about 400 nm in size, that is, the thickness of the polymer shell was 50 nm (Fig. 4b). Further increasing the weight ratio of monomer to CSs to 5:1, the composite morphology changed a lot (Fig. 4c). The composites did not show the regular sphericity but became
massive, a few CSs were faintly visible, which embedded inside the polymer matrix. It shows that when the weight ratio reached 5:1, the self-polymerization of a substantial portion of monomer formed pure PNIPAM microgel. The results show that the amount of monomer had great influence on the PNIPAM grafted on the surface of CSs. The diameter and the corresponding shell thickness of the composites increased with the increase of the amount of NIPAM monomer in proper range. The optimum weight ratio of monomer to CSs did not exceed 3:1. The thickness of grafted polymer shell can be modulated by adjusting monomer-to-CSs ratio, that is, controlling the amount of NIPAM. The monomer polymerized under action of initiator, the double bonds introduced onto the surface of CSs participated in chain polymerization, linear PNIPAM was grafted onto the surface of CSs. In the meantime, linear PNIPAM molecules were interlinked to form crosslinked PNIPAM coating layer because of the presence of crosslinking
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Fig. 5. DCS curves of PNIPAM-grafted CSs composites prepared with different weight ratio of monomer to CSs (a) 1:1 and (b) 3:1. The other reaction conditions are: 1 wt% APS dosage, 15 wt% MBA content.
agent MBA. Because the increase of monomer dosage led to the increase of polymerization rate and chain length, the thickness of coating layer increased. However, too much of monomer caused the formation of bulk PNIPAM as a result of the self-polymerization rather than grafting polymerization of monomer. It is important to evaluate the thermosensitive property, especially phase transition behavior, of a material with surface grafted thermosensitive polymer for actual application. DSC is usually employed to measure the phase transition temperature of the material. The onset point of the endothermal peak on the DSC curve is more commonly used to represent the phase transition point (LCST). The onset temperature Tonset is obtained by the intersection of the baseline and the leading edge of the endotherm [37,38]. In the current work, the phase transition temperature was defined as the onset temperature. The DSC curves of the composites prepared at different weight ratio are shown in Fig. 5. The existence of the endothermal peaks indicates the thermosensitivity of the composites. It can be seen from Fig. 5a, the composites had an LCST of 34.0 ◦ C at the monomer-to-CSs weight ratio of 1:1. When the weight ratio was increased to 3:1, the LCST of the composites was about 40.8 ◦ C. The LCST of PNIPAM-grafted CSs composites increased with the increase of monomer dosage and exceeded that of pure PNIPAM hydrogel, which is typically about 32 ◦ C. On one hand, the existence of CSs probably hinders the collapse and aggregation of the hydrophobic polymer segments that takes place above LCST, resulting in a higher temperature required to overcome the obstacles [39]. On the other hand, the increase of monomer dosage leads to the increase of polymerization rate and chain length, therefore more physical and chemical crosslinking occurs, that is to say, crosslinking degree increase. In general, the increase of crosslinking degree leads to the increase of LCST because crosslinking prohibits the formation of inter- and intra-chain hydrogen bonds and the aggregation of hydrophobic isopropyl groups [37]. The above two factors work together and influence the LCST of the composites. 3.2.2. Influence of initiator content The effect of initiator content on surface grafting of CSs with PNIPAM was also investigated from 0.5 wt% to 2 wt% (relative to mass of NIPAM) while other reaction parameters were kept constant at the monomer-to-CSs weight ratio of 3:1 and cross-linking agent content of 15 wt%. As shown in Fig. 6, the composites kept spherical morphologies within the scope of investigation. Similar
Fig. 6. FESEM images of PNIPAM-grafted CSs composites prepared with different initiator content (a) 0.5 wt%, (b) 1 wt% and (c) 2 wt%. The other reaction conditions are: the weight ratio of monomer to CSs: 3:1, 15 wt% MBA content.
to the effect of monomer amount, the size of the grafted composites changed. In comparison with as-prepared CSs (Fig. 2a1), the composites were nearly 50 nm larger in size than as-prepared CSs at APS content of 0.5 wt%, indicating the thickness of the grafted PNIPAM shell was about 25 nm. When APS content was 1 wt%, the thickness of grafted PNIPAM shell was 50 nm. With further increased initiator content up to 2 wt%, the thickness of grafted shell decreased, which was about 31 nm. The characterization results show that the conversion of monomer and grafting degree of monomer increased
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Fig. 7. DCS curves of PNIPAM-grafted CSs composites prepared with different initiator content (a) 0.5 wt%, (b) 1 wt% and (c) 2 wt%. The other reaction conditions are: the weight ratio of monomer to CSs: 3:1, 15 wt% MBA content.
when the amounts of APS changed from 0.5 wt% to 1 wt%, so the thickness of the grafted shell increased. However, further increase of initiator content to 2 wt% resulted in the reduction of the thickness of the shell. According to general principle of polymerization reaction, the function of the initiator is to produce primary radical, which initiates monomer polymerization or grafting polymerization. If excessive amount of initiator is used, it would generate lots of radicals as active center, the number of monomer linked with each active center would be reduced, resulting in a decrease of grafted chain length. Besides, excessive amount of initiator can easily lead to coupling with the grafted chain active end, which can speed up the chain termination reaction. In this way, the conversion of monomer would be reduced. Based on the above analysis, we can come to the conclusion that the reduction of the thickness of the shell at the initiator content of 2 wt% was the result of the reduction of conversion of monomer and the grafting short polymer chains. Thus, the appropriate content of initiator was determined as 1 wt%. The DSC analysis results for PNIPAM-grafted CSs composites with different initiator content were shown in Fig. 7. When APS contents were 0.5 wt%, 1 wt% and 2 wt%, the LCST was 36.0 ◦ C, 40.3 ◦ C and 39.0 ◦ C, respectively. As was mentioned above, the increase of initiator content leads to the formation of more but shorter polymer chains. However, both more chains and longer chains are in favor of crosslinking. In a proper initiator content range, more polymer chains with suitable chain length would lead to an increasing number of active chains participating in crosslinking, and hence the increase of crosslinking degree. The increase of crosslinking degree leads to the increase of LCST [37]. Therefore, when the APS content increased from 0.5 wt% to 1 wt%, the LCST of the composites increased from 36.0 ◦ C to 40.3 ◦ C. Further increasing the initiator content to 2 wt%, the decline of the LCST may be the result of the decrease of crosslinking degree. Too much initiator would give rise to shorter grafted chain onto the surface of CSs, which could not effectively crosslink, hence crosslinking degree decreased. From the above analysis we can see the monomer dosage and the initiator content had significant effect on the polymer shell thickness and the LCST of the composites determined by the number of grafted polymer chain, chain length and the cross-link degree. Therefore, the LCST of the PNIPAM-grafted CSs composites can be adjustable by changing the monomer dosage and the initiator content.
Fig. 8. FESEM images of PNIPAM-grafted CSs composites prepared with different cross-linking agent content. The other reaction conditions are: the weight ratio of monomer to CSs: 3:1, 1 wt% APS dosage.
3.2.3. Influence of cross-linking agent content Fig. 8 shows the effect of cross-linking agent content (relative to mass of NIPAM) on the morphology of the composites. As shown in Fig. 8, the size of the composites with spherical morphology was about 370 nm and 400 nm when cross-linking agent content was 7 wt% and 15 wt%, respectively, which illustrates that the thickness of PNIPAM shell was 35 nm and 50 nm, respectively (Fig. 8a and b). When the cross-linking agent content was increased to 34 wt%, the mean diameter of the composites was about 410 nm, that is, the
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contents. When MBA contents were 7 wt%, 15 wt% and 34 wt%, the LCST was 33.0 ◦ C, 40.1 ◦ C and 48.5 ◦ C, respectively. The LCST shifted toward higher temperature along with increasing MBA concentration. Obviously, when the content of the cross-linking agent increased, the cross-link degree became higher, resulting in higher LCST. It follows that the content of cross-linking agent shows obvious influence on the cross-link degree, and then the LCST.
Fig. 9. DCS curves of PNIPAM-grafted CSs composites prepared with different cross-linking agent content. The other reaction conditions are: the weight ratio of monomer to CSs: 3:1, 1 wt% APS content.
thickness of the shell was 55 nm (Fig. 8c). All samples were already dried before SEM observation, therefore, these SEM images are reflecting the shrinking state of the composites. The crosslinking degree increased with increasing content of cross-linking agent, which resulted in the reduction of the composites shrinkage degree during drying process, so the diameter increased accordingly. But there were some differences in surface appearance. From Fig. 8a, the grafted polymer layer prepared with the mass percentage ratio of MBA to NIPAM of 7 wt% was loose, probably, because of the low extent of crosslinking caused by less content of cross-linking agent. Inefficient crosslinking occurred at the cross-linking agent content of 7 wt%, that is to say, only a portion of the grafted chains participated in chemical and physical crosslinking, consequently the grafted polymer shell was loose rather than compact. When the content of cross-linking agent increased to 15 wt%, the thermosensitive polymer shell became denser. The content of the cross-linking agent was further raised to 34 wt%, the surface of the composites was not only dense but also filiform, because crosslinking not only existed in adjacent grafted chains but also nonadjacent grafted chains. The influence of the content of the cross-linking agent on LCST was also evaluated. Fig. 9 presents the DCS curves of the composites prepared with different cross-linking agent
3.2.4. FT-IR and XPS analysis The FT-IR spectra and XPS spectra of PNIPAM-grafted CSs composites prepared at preferable parameters (the weight ratio of monomer to CSs: 3:1, APS: 1 wt%, MBA: 15 wt%) are shown in Fig. 10. Compared with the spectra of KH570 treated-CSs, the two new bands absorption peak at 1641 and 1539 cm−1 appear, which are due to C O stretching (amide I) and N H stretching (amide II) of the CONH groups of PNIPAM. The characteristic band found at 1370 cm−1 is C H stretching of isopropyl groups of PNIPAM (Fig. 10a). The absorption band at 1564 cm−1 due to C C disappears as a result of surface polymerization reaction. The bands at 2870 and 2970 cm−1 correspond to the characteristic stretching and bending vibrations of methylene groups. The broad band between 3200 and 3400 comes from the N H stretching of amide. The presence of the grafted PNIPAM on the surface of CSs was also confirmed by XPS analysis. The wide scan spectra show the N 1s (C N) peak at about 400.8 eV (Fig. 10b). These results indicate that PNIPAM was successfully grafted on the surface of CSs. 3.3. Temperature responsive property of the composites The thermoresponsiveness of the composites (the weight ratio of monomer to CSs: 3:1, APS: 1 wt%, MBA: 15 wt%) was studied by DLS at temperature increasing from 25 to 52 ◦ C. Fig. 11 shows the DLS curve of PNIPAM-grafted CSs composites. It is clearly seen that the hydrodynamic diameter (Dh) of the composites decreases when the temperature is raised, which suggests that the composites are sensitive to temperature. The diameter decreased from 585 nm to 471 nm when environmental temperature increased from 25 ◦ C to 52 ◦ C. Especially from 40 ◦ C to 41 ◦ C, the diameter decreased drastically from 562 nm to 473 nm, indicating the temperature of phase transition was about 40 ◦ C. The LCST measured by DLS is consistent with that of DCS. At room temperature (25 ◦ C), the size of the composite spheres from DLS (585 nm) was bigger than that of observed by SEM (400 nm) in the same conditions. This is because the composite spheres are swollen in water, however
Fig. 10. FT-IR spectra (a) and XPS spectra (b) of PNIPAM-grafted CSs composites.
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temperature of human body. This novel composite would open up prospects for CSs application in different fields. Acknowledgements The authors acknowledge financial support from Program for Changjiang Scholar and Innovative Research Team in University (IRT0972), National Natural Science Foundation of China (20971094, 21176169, 51152001), Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011), Natural Science Foundation of Shanxi Province (2012011020-03), Research Project Supported by Shanxi Scholarship Council of China (2012-038), Taiyuan S&T Program (120247-25). References
Fig. 11. Hydrodynamic diameter (Dh) of the PNIPAM-grafted CSs composites as a function of temperature.
SEM observation shows the diameters of the dried sample. The temperature responsive property of the composites comes from the thermosensitivity of the PNIPAM shell. As is known to all, the PNIPAM has temperature stimuli response and a LCST of about 32 ◦ C in water. When the temperature is below the LCST, it is water-soluble as a result of the formation of hydrogen bonding between water molecules and amide group within the polymer chains, adopts an extended forms, swells and expands because of water uptake. However, it becomes water-insoluble as a result of the breakage of hydrogen bonding and hydrophobic interactions between polymer chains become stronger when the temperature of the solution exceeds the LCST. Consequently, it collapses and shrinks by expelling its contents. In this case, as the temperature of the solution increased to higher the LCST. The shrinkage of the cross-linked PNIPAM shell on the surface of CSs brought about the decrease of the shell thickness of the composite with a corresponding decrease of the diameter of the composites.
4. Conclusion Thermosensitive polymer PNIPAM-grafted CSs composites were prepared by surface free-radical polymerization. First, as-prepared CSs were modified by mixed acid oxidation and reacting with KH570 to obtain vinyl-functionalized CSs. Then, PNIPAM was grafted and crosslinked on the surface of CSs using APS as initiator, MBA as cross-linking agent. Effect of grafting conditions on surface grafting of CSs with PNIPAM, including the weight ratio of monomer to CSs, the initiator dosage and the cross-linking agent content, was emphasized. The results show that the PNIPAM-grafted CSs composites were temperature responsive, as confirmed by DSC and DLS analysis. The weight ratio of monomer to CSs and the initiator dosage had great influence on the number of grafted polymer chain, chain length and the cross-link degree, and then the polymer shell thickness and the LCST of the composites. The polymer shell thickness and the LCST of the composites increased with the increase of the amount of monomer in proper range, however, first increase and then decrease with the increasing initiator dosage. The crosslinking agent content critically affects the cross-link degree, and then the LCST. Therefore, the LCST of the PNIPAM-grafted CSs composites was adjustable by changing the weight ratio of monomer to CSs, the initiator dosage and the cross-linking agent content in a certain range, which would meet its application in different fields, such as drug delivery, which requires that the LCST be close to the
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