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Activators generated by electron transfer for atom transfer radical polymerization of styrene in the presence of mesoporous silica nanoparticles
Materials Research Bulletin 59 (2014) 241–248
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Activators generated by electron transfer for atom transfer radical polymerization of styrene in the presence of mesoporous silica nanoparticles Khezrollah Khezri a, *, Hossein Roghani-Mamaqani b a b
School of Chemistry, University College of Science, University of Tehran, PO Box 14155-6455, Tehran, Iran Department of Polymer Engineering, Sahand University of Technology, PO Box 51335-1996, Tabriz, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 January 2014 Received in revised form 12 July 2014 Accepted 14 July 2014 Available online 21 July 2014
Activator generated by electron transfer for atom transfer radical polymerization was employed to synthesize well-defined mesoporous silica nanoparticles/polystyrene composites. Inherent features of spherical mesoporous silica nanoparticles were evaluated by nitrogen adsorption/desorption isotherm, X-ray diffraction and scanning electron microscopy analysis techniques. Conversion and molecular weight evaluations were carried out using gas and size exclusion chromatography respectively. By the addition of only 3 wt% mesoporous silica nanoparticles, conversion decreases from 81 to 58%. Similarly, number average molecular weight decreases from 17,116 to 12,798 g mol1. However, polydispersity index (PDI) values increases from 1.24 to 1.58. A peak around 4.1–4.2 ppm at proton nuclear magnetic resonance spectroscopy results clearly confirms the living nature of the polymerization. Thermogravimetric analysis shows that thermal stability of the nanocomposites increases by adding nanoparticles content. Decrease of glass transition temperature is also demonstrated by the addition of 3 wt% of silica nanoparticles according to the differential scanning calorimetry results. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Polystyrene Mesoporous silica nanoparticles AGET ATRP Nanocomposite
1. Introduction Combination of nanoparticles with polymer matrices results in a new class of materials which is commonly known as polymer nanocomposites [1,2]. These materials combine the different properties of the constituents such as rigidity and thermal stability from inorganic parts and flexibility and ductility from the organic polymers [3,4]. Therefore, addition of nanofillers results in considerable enhancement in several properties of nanocomposites in comparison with the conventionally filled composites [5,6]. Titanium dioxide, magnetic iron oxide, montmorillonite, silica, and mesoporous silica are some of the most important nanofillers in polymer nanocomposites [7–10]. Although polymer matrix and nanofiller characteristics dictate the nanocomposites properties, processing condition is an effective parameter. Physical mixing and polymerization in the presence of nanofillers are the two general procedures for nanocomposite production [11,12]. After the discovery announcement of novel class of mesoporous M41S molecular sieves by the scientist from the Mobil Oil Company in 1992, a great attention is shown to apply this family
* Corresponding author. E-mail address: [email protected] (K. Khezri). http://dx.doi.org/10.1016/j.materresbull.2014.07.021 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
of materials in various applications [13–15]. By using templating method and also phase transformation approach, various morphologies of mesoporous silica can be obtained. Therefore, controlling morphology of mesoporous materials for different purposes is an important factor which needs to be considered. Tubules-within-tubule (TWT), pillar-within sphere (PWS), spherical, rod-like, and top-like morphologies are reported for MCM-41 nanoparticles [16–18]. The M41S family has been classified into four main groups: disordered rods and three well-defined structures of MCM-41, MCM-48 with a three-dimensional cubic pore, and MCM-50 with an unstable lamellar structure [19]. Narrow pore size distribution with size controllable pores, large pore openings, high surface area, and ownership of hexagonal arrangement of uniformly sized parallel channel pores are some unique characteristics of MCM-41 nanoparticles. According to the unique features of MCM-41 nanoparticles, it can be used in many different applications such as separation of proteins, selective adsorption of large molecules, and heterogeneous catalysis [16,20–22]. Weakness of free radical polymerization (FRP) to synthesize pure block copolymers, low PDI polymers, and precise control over the molecular weight of polymers is a driving force to explore and developing new polymerization methods. Controlled radical polymerization (CRP) is a robust route to synthesize polymers
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Table 1 Designation of the samples. Sample
Preparation method
Content of MCM-41 nanoparticles (wt%)
Time of MCM-41 nanoparticles dispersion in monomer (h)
NPS PSN 1
AGET ATRP In situ AGET ATRP In situ AGET ATRP In situ AGET ATRP
0 1
– 15
2
15
3
15
PSN 2 PSN 3
Fig. 1. General mechanism of AGET initiation process in ATRP.
with well-defined structures and low PDI values. CRP methods are generally based on establishing a rapid dynamic equilibrium between the growing free radicals and dormant species. Atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addition–fragmentation chain transfer (RAFT) are three main categories of CRP. Among these methods, ATRP has attracted more attention because it provides some unique benefits such as its application for a large range of monomers, various polymerization media, and commercial availability of its reagents [23–27]. Application of CRP techniques for the preparation of mesoporous silica nanoparticles/polymer composites has been reported rarely. Hong et al prepared core–shell nanostructure with mesoporous silica nanoparticle core and polymer shell around the exterior surface of mesoporous silica nanoparticles synthesized by surface RAFT polymerization [28,29]. Bals et al employed surface initiated NMP for grafting polystyrene chains in both the inner and outer surface of mesoporous silica nanoparticles. They used various types of mesoporous silica nanoparticles with different morphologies and pore sizes [30]. Pasetto et al synthesized hybrid materials by grafting polymer chains on the surface of mesoporous silica nanoparticles via surface-initiated ATRP (SIATRP) [31]. They used various shapes of mesoporous silica nanoparticles as substrates such as micrometric particles, submicrometric polydisperes spherical particles, and monodisperse core–shell particles They compared the macromolecular features of the free and attached polymer chains and therefore their study has highlighted the effect of mesoporous confined space and channel length on the results of the polymerization. Also, core– shell nanostructure with mesoporous silica nanoparticles core and hyperbranched polymer shell has been prepared by Li et al via surface-initiated self condensing atom transfer radical polymerization. Their results indicate that hybrid nanoparticles showed appropriate dispersibility in organic solvents [32]. Meer et al investigated the effect of mesoporosity on thermal and mechanical properties of silica/polystyrene composites [11]. They incorporated mesoporous and colloidal silica particles into polymer matrices via
two different methods of melt blending and SI-ATRP. Their results indicate that both the composites have similar grafted polymer characteristics. In addition, Liu et al synthesized MCM-41/poly (acrylic acid) composites by using FRP. They introduced acrylic acid and initiator into the channels of MCM-41 nanoparticles by using supercritical carbon dioxide as solvent at low temperature followed by polymerization at a higher temperature [33]. In this study, benefits of ATRP were employed to synthesize predetermined molecular weight polystyrene chains in the presence of MCM-41 nanoparticles. Among different initiation techniques of ATRP, normal ATRP involves lower oxidation state metal complexes in which special handling procedures will be required. Reverse ATRP (RATRP), applies more stable metal species and therefore its process is generally more convenient. Disability to produce pure block copolymers and independently reduction of complex concentration are two main drawbacks of RATRP. Overcoming to drawbacks of normal ATRP and RATRP can be achieved by AGET initiation technique. Therefore, in this research, AGET ATRP as an interesting initiation process was selected since it applies less oxygen sensitive component at the start of reaction (general mechanism of AGET ATRP is illustrated in Fig. 1). In addition, by AGET ATRP, polymerization rate can be directly controlled by the amount of added reducing agent. Participation of mesoporous silica nanoparticles in the polymer matrix was performed by using in situ polymerization technique. Synthesis and characterization of mesoporous silica nanoparticles, nanocomposites preparation method, effect of mesoporous silica nanoparticles on the AGET ATRP, and thermal properties of the products are discussed in detail. 2. Experimental 2.1. Materials Styrene (Aldrich, 99%) was passed through an alumina-filled column to remove inhibitors. The compounds copper(II) bromide
Fig. 2. General procedure for preparation of polystyrene/MCM-41 nanocomposites via in situ AGET ATRP.
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Table 2 Extracted data from nitrogen adsorption/desorption isotherm of the calcined MCM41 nanoparticles. BET surface area
Langmuir surface area
992.95 m2/g
1497.05 m2/g 21.93 Å
Average pore diameter (V/A by BET)
BJH adsorption average pore diameter (4 V/A)
BJH desorption average pore diameter (4 V/A)
26.29 Å
25.60 Å
2.2. Synthesis of spherical MCM-41 nanoparticles At first, CTAB (5.01 g, 13.7 mmol) was dissolved in 100 mL of deionized water and vigorously stirred for 5 min until the solution became clear. Then, 37.4 mL of ammonium solution was slowly added to the CTAB solution under stirring to form gel like mixture. Then, the solution was left under stirring for 10 min. Afterward, ethanol (152 mL) was added to the solution and stirred for 20 min. Subsequently, TEOS (10.2 mL, 0.03 mmol) was added dropwise during 30 min and left under stirring for 3 h. The white powder was precipitated, filtered, and washed with deionized water several times and then dried in vacuum oven at 110 C for 48 h. To remove CTAB, the white powder was calcinated at 550 C for 6 h with the heating rate of 10 C/min. 2.3. General procedure for AGET ATRP of styrene in the presence of the MCM-41 nanoparticles Fig. 3. XRD graph of the synthesized MCM-41 nanoparticles.
(CuBr2, Fluka, 99%), N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), ethyl alpha-bromoisobutyrate (EBiB, Aldrich, 97%), tin(II) 2-ethylhexanoate ([Sn(EH)2], Sigma, 95%), anisole (Aldrich, 99%), tetrahydrofuran (THF, Merck, 99%), neutral aluminum oxide (Aldrich, 99%), ammonium solution (Merck, 25 wt % in water), ethanol (Merck, 99%), cetyltrimethylammonium bromide (CTAB, Merck, 97%), and tetraethoxysilane (TEOS, Merck) were used as received.
AGET ATRP was performed in a 100-ml three-neck lab reactor which was equipped with a refluxcondenser, nitrogen inlet valve, and a magnetic stir bar that was placed in an oil bath thermostated at desired temperature. A typical batch of polymerization was run at 110 C with the molar ratio of 200:1:1:1:05 for [Styrene]:[EBiB]: [CuBr2]:[PMDETA]:[Sn(EH)2] giving a theoretical polymer molecular weight of 20,833 g mol1 at the final conversion. Styrene (20 ml), anisole (7 mL), CuBr2 (0.194 g, 0.87 mmol) and PMDETA (0.18 mL, 0.87 mmol) were added into the reactor and the reactor was degassed and back-filled with nitrogen three times and stirring was continued at room temperature. The solution turned green color since the CuBr2/PMDETA complex was formed. Subsequently, predeoxygenated solutions of reducing agent [Sn(EH)2, 0.14 mL, 0.43 mmol] and initiator [EBiB, 0.12 ml, 0.87 mmol] were injected into the reactor and the reactor temperature was increased to 110 C during 5 min. After 8 h, polymerization process was stopped by opening the reactor and exposing the catalyst to air. For preparation of nanocomposites, a desired amount of modified MCM-41 nanoparticles was dispersed in 10 ml of styrene and the mixture was stirred for 15 h. Then, the remained 10 ml of styrene was added to the mixture. Subsequently, polymerization procedure was applied accordingly. Designation of the samples with the content of MCM-41 nanoparticles is summarized in Table 1. 2.4. Separation of polymer chains from mesoporous silica nanoparticles and catalyst removal For separating polymer chains from mesoporous silica nanoparticles, nanocomposites were dissolved in THF. By high-speed ultracentrifugation (10,000 rpm) and then passing the solution through a 0.2 Micrometer Filter, polymer chains were separated from MCM-41 nanoparticles. Subsequently, polymer solutions passed through an alumina column to remove catalyst. 2.5. Characterization
Fig. 4. Nitrogen adsorption/desorption isotherms of the calcined MCM-41 nanoparticles.
X–ray diffraction spectra were collected on an X–ray diffraction instrument (Siemens D5000) with a Cu target (l = 0.1540 nm). The
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Fig. 5. SEM images of the calcined MCM-41 nanoparticles with two different magnifications.
system consists of a rotating anode generator operated at 35 kV and 20 mA current. The samples were scanned from 2u = 2 to 10 at the step scan mode, and the diffraction pattern was recorded using a scintillation counter detector. The basal spacing or d001-spacing of the samples were calculated using Bragg's law of diffraction (l = 2dsinu). Materials porosity was characterized by N2 adsorption/desorption curves obtained with a Quntasurb QS18 (Quntachrom) apparatus. The surface area and pore size distribution values were obtained with the corrected BET equation and Broekhoff and de Boermodels, respectively. Surface morphology of powder samples was examined by scanning electron microscope (SEM: Philips XL30) with acceleration voltage of 20 kV. Also, transmission electron microscope (TEM), FEG Philips CM, with an accelerating voltage of 200 kV was used. Gas chromatography (GC) is a simple and highly sensitive characterization method and does not require removal of the metal catalyst particles. GC was performed on an Agilent-6890 N with a split/splitless injector and flame ionization detector, using a 60 m HP–INNOWAX capillary column for the separation. The GC temperature profile included an initial steady heating at 60 C for 10 min and a 10 C/min ramp from
60 to 160 C. The samples were also diluted with acetone. The ratio of monomer to anisole was measured by GC to calculate monomer conversion throughout the reaction. Size exclusion chromatography (SEC) was used to measure the molecular weight and molecular weight distribution. A Waters 2000 ALLIANCE with a set of three columns of pore sizes of 10,000, 1000, and 500 Å was utilized to determine polymer average molecular weight and polydispersity index (PDI). THF was used as the eluent at a flow rate of 10 mL/min, and calibration was carried out using low polydispersity polystyrene standards. Proton nuclear magnetic resonance spectroscopy (1H NMR) spectra were recorded on a Bruker 300-MHz 1H NMR instrument with CDCl3 as the solvent and tetramethylsilane as the internal standard. Thermal gravimetric analysis (TGA) was carried out with a PL thermo-gravimetric analyzer (Polymer Laboratories, TGA 1000, UK). Thermograms were obtained from ambient temperature to 700 C at a heating rate of 10 C/min. Thermal analysis were carried out using a differential scanning calorimetry (DSC) instrument (NETZSCH DSC 200 F3, Netzsch Co, Selb/Bavaria, Germany). Nitrogen at a rate of 50 mL/min was used as purging gas. Aluminum pans containing
Fig. 6. TEM images of the synthesized MCM-41 nanoparticles in two different magnifications.
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Fig. 7. SEC traces of polystyrene and its nanocomposites with different MCM-41 content.
2–3 mg of the samples were sealed using DSC sample press. The samples were heated from ambient temperature to 220 C at a heating rate of 10 C/min.
Fig. 8. 1H NMR spectra of the neat polystyrene and its nanocomposites.
in the vicinity of around 2u = 3.5–5 can be attributed to the wellhexagonal structure of the synthesized MCM-41 nanoparticles [37]. Nitrogen adsorption and desorption isotherms of the calcined MCM-41 nanoparticles are shown in Fig. 4. The isotherm can be classified as IV type of isotherm, which is characteristic of MCM-41 materials according to the International Union of Pure and Applied Chemistry (IUPAC) classification [38,39]. Therefore, at low p/p0 values, the isotherm presents a linear increase of adsorbed volume which can be attributed to the monolayer adsorption of N2 on the wall of the mesoporous silica nanoparticles. Then, a sudden uptake of the adsorbed amount of N2 was shown over a narrow range of relative pressure by capillary condensation of nitrogen inside the mesopores (narrow pore size distribution can be recognized). And finally, the long plateau at higher relative pressures indicates saturation step (low adsorption of N2 on the external surface of calcined MCM-41) [38,39]. Table 2 summarized the extracted data form nitrogen adsorption and desorption isotherms SEM is employed to evaluate surface morphology and size distribution of the synthesized nanoparticles. SEM images of the calcined MCM-41 nanoparticles in two different magnifications are displayed in Fig. 5. According to these images, nanopaticles with spherical shapes are observed. Although particles size is mainly ranged from 500 to 800 nm, some larger particles are also observed (around 1000 nm).
3. Results and discussion Investigating different initiation processes of ATRP is an interesting subject for synthesis of favorite polymers and polymeric nanocomposites. AGET initiation technique can be considered as a suitable route to circumvent on oxidation problems of normal ATRP since it applies less oxygen sensitive reactants. In AGET ATRP, reducing agents, which are unable to initiate new chains, are used to reduce transitional metal complex in the higher oxidation state (Mtn+1/ligand). Then, generated activators (Mtn/ ligand) participate in ordinary ATRP equilibrium. By this initiation technique, polymerization rate can be directly controlled by the amount of added reducing agent and so, polymer chains with more narrow molecular weight distribution in comparison with the other ATRP initiation techniques can be achieved. Some reducing agents such as Cu, [Sn(EH)2], hydrazine, and ascorbic acid have been used in AGET ATRP [34–36]. Schematic presentation of AGET ATRP in the presence of MCM-41 nanoparticles is shown in Fig. 2. Inherent characteristics and morphology of the MCM-41 nanoparticles are two important factors which can affect the polymerization kinetic and also properties of the produced nanocomposites. Specific structure of the mesoporous silica nanoparticles was examined by XRD (Fig. 3). A sharp and intense peak in the diffraction angles of about 2.5 and also few weak peaks
Table 3 Molecular weights and PDI values of the extracted polymers resulted from SEC traces. Sample
Reaction time (h)
Conversion (%)
NPS PSN 1 PSN 2 PSN 3
8 8 8 8
81 72 66 58
Mn (g mol1) Exp.
Theo.
17,116 14,243 13,061 12,798
16,875 15,000 13,750 12,083
Mw (g mol1)
PDI
21,392 19,848 19,245 20,221
1.24 1.39 1.47 1.58
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Fig. 9. (A) TGA and (B) DTG thermograms of the neat polystyrene and its nanocomposites.
Porosity of structure and pore condition of the MCM-41 nanoparticles was evaluated by TEM. Fig. 6 depicts TEM images of pristine MCM-41 naoparticles in two different magnifications. According to these images, porous structure with regular hexagonal channels of the synthesized MCM-41 naoparticles can be clearly demonstrated. SEC is employed to examine chain characteristics of the synthesized polymer chains (e.g. number and weight average molecular weights and PDI values) and also to investigate the effect of MCM-41 loading on these parameters. According to Fig. 7, neat polystyrene and its nanocomposites show monomodal peak with narrow distribution. By adding MCM41 nanoparticles content, noticeable increase in PDI values is observed. Broadening of these traces by increasing MCM-41 nanoparticles content (variation of PDI values from 1.24 to 1.58 by adding only 3 wt% of MCM-41 nanoparticles), indicates that MCM41 nanoparticles as a filler disturbs the equilibrium of ATRP reaction and results in higher PDI values [27,34,40]. Extracted results of SEC are represented in Table 3. Appropriate correlation between the theoretical and experimental molecular weights can be considered as an evidence for controlled nature of the polymerization. Moreover, color change of the reaction media from green (at the beginning of the reaction) to brown is also appropriate evidence for establishment of ATRP equilibrium [34,41]. Theoretical molecular weight is calculated by using Eq. (1): ¼ MTheo n
½St0 Conversion Mmonomer ½EBiB0
According to the Table 3, by adding nanofillers content, a clear shift in conversion and molecular weight toward lower values is occurred. ATRP equilibrium can be affected by the MCM-41 nanoparticles as an impurity and also nature of the nanoparticles: irreversible reactions between the prepared radicals during ATRP and abundant hydroxyl groups on the surface of MCM-41 nanoparticles can affect the concentration of radicals and kinetics of ATRP and therefore lower conversion and molecular weights are obtained [42–44]. It is clear that by addition of MCM-41 loading, the chance of these irreversible reactions increases and more reduction in conversion and molecular weight can be occurred. Moreover, mobility of macroradicals in the solution can be restricted by the MCM-41 nanoparticles (particularly at higher loading) and therefore rate of polymerization and conversion value tend to decrease by increasing MCM-41 nanoparticles loading [45].
(1)
where [St]0, [EBiB]0 are initial concentration of the monomer and initiator respectively and Mmonomer represents molecular weight of the monomer, which in the case of styrene is equal 104.15 g mol1.
Table 4 Thermal characteristics of the polystyrene and its nanocomposites. Sample
NPS PSN 1 PSN 2 PSN 3
TGA
DTG ( C)
Char (%) at 600 ( C)
Start point
Peak point
End point
2.03 3.04 4.29 5.30
335 346 349 356
412 414 419 421
461 463 461 465 Fig. 10. Graphical illustration of temperature and degradation relationship.
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Fig. 11. (A) Heating and (B) cooling path thermogram of DSC for polystyrene and its nanocomposites.
1 H NMR spectroscopy was employed to evaluate composition of the polymer chains 1H NMR spectra of the neat polymer and its various nanocomposites are shown in Fig. 8. The signal at chemical shift of 41 ppm which originates from hydrogen atom of the terminal units ( CHBr) provides a suitable evidence for living nature of the polymerization. In addition, inherent peaks of polystyrene chains such as benzene rings, methylene ( CH2), and tertiary carbon hydrogen [ CH(C6H5) ] appears in the range of around 7.40–6.25, 1.71–1.15, and 2.31–1.71 ppm, respectively [44]. An improvement in thermal stabilities of all the nanocomposites in comparision with the neat polystyrene can be easily observed in Fig. 9(A). Moreover, by increasing MCM-41 loading, more improvement in thermal stabilities of nanocomposites is observed. Except the main degradation step, other degradations can be ascribed to desorption of chemisorbed water and the silanol groups on the surface of MCM-41 nanoparticles which are removed at the range of around 100 C. Also, degradation of volatile materials such as residual monomer, MCM-41 functionalities, and low molecular weight oligomers occurred at the range of around 100–300 C [33,46]. Char values of the samples at 600 C are shown in Table 4. The percentage of char values increases by increasing MCM-41 nanoparticles content. Also, MCM-41 nanoparticles leaves 96.32% char after complete degradation at 600 C. Extracted data from TGA are graphically illustrated in Fig. 10. In this graph, degradation temperature against the amount of degradation is drown to prove the fact that by adding MCM-41 nanoparticles loading in the polymer matrix, thermal stability improves (TX: temperature threshold at which X% of the polymer and its nanocomposites degradation is occurred). Improving thermal stability of the nanocomposites by increasing MCM-41 nanoparticles content can be attributed to several reasons: (i) high thermal stability of nanofillers (MCM-41 nanoparticles), (ii) interaction between MCM-41 nanoparticles and polymer matrix [47], (iii) physical interaction between polystyrene chains and surface of MCM-41 nanoparticles and finally
Table 5 Glass transition temperatures of the neat polystyrene and its nanocomposites. Sample
Mn (g mol1)
PDI
NPS PSN 1 PSN 2 PSN 3
17,116 14,243 13,061 12,798
1.24 1.39 1.47 1.58
Tg ( C) Heating path
Cooling path
100.1 97.6 94.9 91.4
100.4 97.2 94.5 91.7
degradation from outer surface inward can be considered as an important factor for increasing thermal stability of the nanocomposites [11]. Extracted data from TGA (char values) and DTG curves which indicates an improvement in thermal stability of the nanocomposites by increasing MCM-41 nanoparticles content are summarized in Table 4. Thermal behavior of the neat polystyrene and its nanocomposites was evaluated by DSC. Fig. 11 depicts some information on thermal behavior of the neat polystyrene and its nanocomposites. Temperature range of 40–180 and 170–40 C are used to describe DSC results in heating and cooling path, respectively. Since MCM-41 nanoparticles do not bear any transitions in this range of temperature, therefore only thermal transitions of polymer chains is observed. Firstly, samples are heated from room temperature to 220 C to remove their thermal history. Then, they are cooled to room temperature to distinguish the phase conversion and other irreversible thermal behaviors. Finally, samples are heated from room temperature to 220 C to obtain the more appropriate Tg values. According to the Table 5, Tg value of the neat polystyrene is higher than the all of the nanocomposites and by increasing MCM41 nanoparticles content, a decrease in Tg value occurs. Reduction of Tg values by increasing MCM-41 content may be ascribed to the weak interaction between polymer chains and OH-containing MCM-41 nanoparticles. Therefore, MCM-41 nanoparticles can reduce the packing polystyrene chains and increase the segments mobilities which in turn results in Tg reduction. Moreover, other parameters such as decrease in molecular weights and increment of PDI values need to be considered since decrease of molecular weight results in decrease of Tg and high PDI values may cause reduction of Tg values. This is mainly on account of the fact that in very high PDI values, chains with low molecular weights can act as viscosity reducer like plactisizers. 4. Conclusions Dispersion of MCM-41 nanoparticles in the well-defined polystyrene matrix were carried out by in situ AGET ATRP. The synthesized MCM-41 nanoparticles reveal spherical morphology, hexagonal structure, and high surface area with regular pores. Decreases in final conversion from 81 to 58% and from 17,116 to 12,798 g mol1 for number average molecular weight are resulted by 3 wt% addition of MCM-41 nanoparticles. However, PDI values increases from 1.24 to 1.58. Low intensity peak in the range of around 41 ppm in 1H NMR spectra demonstrates the living nature of polymerization. Improvement in thermal stability of the
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[31]
[32]
[33]
[34]
[35]
[36] [37] [38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Activators generated by electron transfer for atom transfer radical polymerization of styrene in the presence of mesoporous silica nanoparticles Khezrollah Khezri a, *, Hossein Roghani-Mamaqani b a b
School of Chemistry, University College of Science, University of Tehran, PO Box 14155-6455, Tehran, Iran Department of Polymer Engineering, Sahand University of Technology, PO Box 51335-1996, Tabriz, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 January 2014 Received in revised form 12 July 2014 Accepted 14 July 2014 Available online 21 July 2014
Activator generated by electron transfer for atom transfer radical polymerization was employed to synthesize well-defined mesoporous silica nanoparticles/polystyrene composites. Inherent features of spherical mesoporous silica nanoparticles were evaluated by nitrogen adsorption/desorption isotherm, X-ray diffraction and scanning electron microscopy analysis techniques. Conversion and molecular weight evaluations were carried out using gas and size exclusion chromatography respectively. By the addition of only 3 wt% mesoporous silica nanoparticles, conversion decreases from 81 to 58%. Similarly, number average molecular weight decreases from 17,116 to 12,798 g mol1. However, polydispersity index (PDI) values increases from 1.24 to 1.58. A peak around 4.1–4.2 ppm at proton nuclear magnetic resonance spectroscopy results clearly confirms the living nature of the polymerization. Thermogravimetric analysis shows that thermal stability of the nanocomposites increases by adding nanoparticles content. Decrease of glass transition temperature is also demonstrated by the addition of 3 wt% of silica nanoparticles according to the differential scanning calorimetry results. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Polystyrene Mesoporous silica nanoparticles AGET ATRP Nanocomposite
1. Introduction Combination of nanoparticles with polymer matrices results in a new class of materials which is commonly known as polymer nanocomposites [1,2]. These materials combine the different properties of the constituents such as rigidity and thermal stability from inorganic parts and flexibility and ductility from the organic polymers [3,4]. Therefore, addition of nanofillers results in considerable enhancement in several properties of nanocomposites in comparison with the conventionally filled composites [5,6]. Titanium dioxide, magnetic iron oxide, montmorillonite, silica, and mesoporous silica are some of the most important nanofillers in polymer nanocomposites [7–10]. Although polymer matrix and nanofiller characteristics dictate the nanocomposites properties, processing condition is an effective parameter. Physical mixing and polymerization in the presence of nanofillers are the two general procedures for nanocomposite production [11,12]. After the discovery announcement of novel class of mesoporous M41S molecular sieves by the scientist from the Mobil Oil Company in 1992, a great attention is shown to apply this family
* Corresponding author. E-mail address: [email protected] (K. Khezri). http://dx.doi.org/10.1016/j.materresbull.2014.07.021 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
of materials in various applications [13–15]. By using templating method and also phase transformation approach, various morphologies of mesoporous silica can be obtained. Therefore, controlling morphology of mesoporous materials for different purposes is an important factor which needs to be considered. Tubules-within-tubule (TWT), pillar-within sphere (PWS), spherical, rod-like, and top-like morphologies are reported for MCM-41 nanoparticles [16–18]. The M41S family has been classified into four main groups: disordered rods and three well-defined structures of MCM-41, MCM-48 with a three-dimensional cubic pore, and MCM-50 with an unstable lamellar structure [19]. Narrow pore size distribution with size controllable pores, large pore openings, high surface area, and ownership of hexagonal arrangement of uniformly sized parallel channel pores are some unique characteristics of MCM-41 nanoparticles. According to the unique features of MCM-41 nanoparticles, it can be used in many different applications such as separation of proteins, selective adsorption of large molecules, and heterogeneous catalysis [16,20–22]. Weakness of free radical polymerization (FRP) to synthesize pure block copolymers, low PDI polymers, and precise control over the molecular weight of polymers is a driving force to explore and developing new polymerization methods. Controlled radical polymerization (CRP) is a robust route to synthesize polymers
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Table 1 Designation of the samples. Sample
Preparation method
Content of MCM-41 nanoparticles (wt%)
Time of MCM-41 nanoparticles dispersion in monomer (h)
NPS PSN 1
AGET ATRP In situ AGET ATRP In situ AGET ATRP In situ AGET ATRP
0 1
– 15
2
15
3
15
PSN 2 PSN 3
Fig. 1. General mechanism of AGET initiation process in ATRP.
with well-defined structures and low PDI values. CRP methods are generally based on establishing a rapid dynamic equilibrium between the growing free radicals and dormant species. Atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addition–fragmentation chain transfer (RAFT) are three main categories of CRP. Among these methods, ATRP has attracted more attention because it provides some unique benefits such as its application for a large range of monomers, various polymerization media, and commercial availability of its reagents [23–27]. Application of CRP techniques for the preparation of mesoporous silica nanoparticles/polymer composites has been reported rarely. Hong et al prepared core–shell nanostructure with mesoporous silica nanoparticle core and polymer shell around the exterior surface of mesoporous silica nanoparticles synthesized by surface RAFT polymerization [28,29]. Bals et al employed surface initiated NMP for grafting polystyrene chains in both the inner and outer surface of mesoporous silica nanoparticles. They used various types of mesoporous silica nanoparticles with different morphologies and pore sizes [30]. Pasetto et al synthesized hybrid materials by grafting polymer chains on the surface of mesoporous silica nanoparticles via surface-initiated ATRP (SIATRP) [31]. They used various shapes of mesoporous silica nanoparticles as substrates such as micrometric particles, submicrometric polydisperes spherical particles, and monodisperse core–shell particles They compared the macromolecular features of the free and attached polymer chains and therefore their study has highlighted the effect of mesoporous confined space and channel length on the results of the polymerization. Also, core– shell nanostructure with mesoporous silica nanoparticles core and hyperbranched polymer shell has been prepared by Li et al via surface-initiated self condensing atom transfer radical polymerization. Their results indicate that hybrid nanoparticles showed appropriate dispersibility in organic solvents [32]. Meer et al investigated the effect of mesoporosity on thermal and mechanical properties of silica/polystyrene composites [11]. They incorporated mesoporous and colloidal silica particles into polymer matrices via
two different methods of melt blending and SI-ATRP. Their results indicate that both the composites have similar grafted polymer characteristics. In addition, Liu et al synthesized MCM-41/poly (acrylic acid) composites by using FRP. They introduced acrylic acid and initiator into the channels of MCM-41 nanoparticles by using supercritical carbon dioxide as solvent at low temperature followed by polymerization at a higher temperature [33]. In this study, benefits of ATRP were employed to synthesize predetermined molecular weight polystyrene chains in the presence of MCM-41 nanoparticles. Among different initiation techniques of ATRP, normal ATRP involves lower oxidation state metal complexes in which special handling procedures will be required. Reverse ATRP (RATRP), applies more stable metal species and therefore its process is generally more convenient. Disability to produce pure block copolymers and independently reduction of complex concentration are two main drawbacks of RATRP. Overcoming to drawbacks of normal ATRP and RATRP can be achieved by AGET initiation technique. Therefore, in this research, AGET ATRP as an interesting initiation process was selected since it applies less oxygen sensitive component at the start of reaction (general mechanism of AGET ATRP is illustrated in Fig. 1). In addition, by AGET ATRP, polymerization rate can be directly controlled by the amount of added reducing agent. Participation of mesoporous silica nanoparticles in the polymer matrix was performed by using in situ polymerization technique. Synthesis and characterization of mesoporous silica nanoparticles, nanocomposites preparation method, effect of mesoporous silica nanoparticles on the AGET ATRP, and thermal properties of the products are discussed in detail. 2. Experimental 2.1. Materials Styrene (Aldrich, 99%) was passed through an alumina-filled column to remove inhibitors. The compounds copper(II) bromide
Fig. 2. General procedure for preparation of polystyrene/MCM-41 nanocomposites via in situ AGET ATRP.
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Table 2 Extracted data from nitrogen adsorption/desorption isotherm of the calcined MCM41 nanoparticles. BET surface area
Langmuir surface area
992.95 m2/g
1497.05 m2/g 21.93 Å
Average pore diameter (V/A by BET)
BJH adsorption average pore diameter (4 V/A)
BJH desorption average pore diameter (4 V/A)
26.29 Å
25.60 Å
2.2. Synthesis of spherical MCM-41 nanoparticles At first, CTAB (5.01 g, 13.7 mmol) was dissolved in 100 mL of deionized water and vigorously stirred for 5 min until the solution became clear. Then, 37.4 mL of ammonium solution was slowly added to the CTAB solution under stirring to form gel like mixture. Then, the solution was left under stirring for 10 min. Afterward, ethanol (152 mL) was added to the solution and stirred for 20 min. Subsequently, TEOS (10.2 mL, 0.03 mmol) was added dropwise during 30 min and left under stirring for 3 h. The white powder was precipitated, filtered, and washed with deionized water several times and then dried in vacuum oven at 110 C for 48 h. To remove CTAB, the white powder was calcinated at 550 C for 6 h with the heating rate of 10 C/min. 2.3. General procedure for AGET ATRP of styrene in the presence of the MCM-41 nanoparticles Fig. 3. XRD graph of the synthesized MCM-41 nanoparticles.
(CuBr2, Fluka, 99%), N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), ethyl alpha-bromoisobutyrate (EBiB, Aldrich, 97%), tin(II) 2-ethylhexanoate ([Sn(EH)2], Sigma, 95%), anisole (Aldrich, 99%), tetrahydrofuran (THF, Merck, 99%), neutral aluminum oxide (Aldrich, 99%), ammonium solution (Merck, 25 wt % in water), ethanol (Merck, 99%), cetyltrimethylammonium bromide (CTAB, Merck, 97%), and tetraethoxysilane (TEOS, Merck) were used as received.
AGET ATRP was performed in a 100-ml three-neck lab reactor which was equipped with a refluxcondenser, nitrogen inlet valve, and a magnetic stir bar that was placed in an oil bath thermostated at desired temperature. A typical batch of polymerization was run at 110 C with the molar ratio of 200:1:1:1:05 for [Styrene]:[EBiB]: [CuBr2]:[PMDETA]:[Sn(EH)2] giving a theoretical polymer molecular weight of 20,833 g mol1 at the final conversion. Styrene (20 ml), anisole (7 mL), CuBr2 (0.194 g, 0.87 mmol) and PMDETA (0.18 mL, 0.87 mmol) were added into the reactor and the reactor was degassed and back-filled with nitrogen three times and stirring was continued at room temperature. The solution turned green color since the CuBr2/PMDETA complex was formed. Subsequently, predeoxygenated solutions of reducing agent [Sn(EH)2, 0.14 mL, 0.43 mmol] and initiator [EBiB, 0.12 ml, 0.87 mmol] were injected into the reactor and the reactor temperature was increased to 110 C during 5 min. After 8 h, polymerization process was stopped by opening the reactor and exposing the catalyst to air. For preparation of nanocomposites, a desired amount of modified MCM-41 nanoparticles was dispersed in 10 ml of styrene and the mixture was stirred for 15 h. Then, the remained 10 ml of styrene was added to the mixture. Subsequently, polymerization procedure was applied accordingly. Designation of the samples with the content of MCM-41 nanoparticles is summarized in Table 1. 2.4. Separation of polymer chains from mesoporous silica nanoparticles and catalyst removal For separating polymer chains from mesoporous silica nanoparticles, nanocomposites were dissolved in THF. By high-speed ultracentrifugation (10,000 rpm) and then passing the solution through a 0.2 Micrometer Filter, polymer chains were separated from MCM-41 nanoparticles. Subsequently, polymer solutions passed through an alumina column to remove catalyst. 2.5. Characterization
Fig. 4. Nitrogen adsorption/desorption isotherms of the calcined MCM-41 nanoparticles.
X–ray diffraction spectra were collected on an X–ray diffraction instrument (Siemens D5000) with a Cu target (l = 0.1540 nm). The
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Fig. 5. SEM images of the calcined MCM-41 nanoparticles with two different magnifications.
system consists of a rotating anode generator operated at 35 kV and 20 mA current. The samples were scanned from 2u = 2 to 10 at the step scan mode, and the diffraction pattern was recorded using a scintillation counter detector. The basal spacing or d001-spacing of the samples were calculated using Bragg's law of diffraction (l = 2dsinu). Materials porosity was characterized by N2 adsorption/desorption curves obtained with a Quntasurb QS18 (Quntachrom) apparatus. The surface area and pore size distribution values were obtained with the corrected BET equation and Broekhoff and de Boermodels, respectively. Surface morphology of powder samples was examined by scanning electron microscope (SEM: Philips XL30) with acceleration voltage of 20 kV. Also, transmission electron microscope (TEM), FEG Philips CM, with an accelerating voltage of 200 kV was used. Gas chromatography (GC) is a simple and highly sensitive characterization method and does not require removal of the metal catalyst particles. GC was performed on an Agilent-6890 N with a split/splitless injector and flame ionization detector, using a 60 m HP–INNOWAX capillary column for the separation. The GC temperature profile included an initial steady heating at 60 C for 10 min and a 10 C/min ramp from
60 to 160 C. The samples were also diluted with acetone. The ratio of monomer to anisole was measured by GC to calculate monomer conversion throughout the reaction. Size exclusion chromatography (SEC) was used to measure the molecular weight and molecular weight distribution. A Waters 2000 ALLIANCE with a set of three columns of pore sizes of 10,000, 1000, and 500 Å was utilized to determine polymer average molecular weight and polydispersity index (PDI). THF was used as the eluent at a flow rate of 10 mL/min, and calibration was carried out using low polydispersity polystyrene standards. Proton nuclear magnetic resonance spectroscopy (1H NMR) spectra were recorded on a Bruker 300-MHz 1H NMR instrument with CDCl3 as the solvent and tetramethylsilane as the internal standard. Thermal gravimetric analysis (TGA) was carried out with a PL thermo-gravimetric analyzer (Polymer Laboratories, TGA 1000, UK). Thermograms were obtained from ambient temperature to 700 C at a heating rate of 10 C/min. Thermal analysis were carried out using a differential scanning calorimetry (DSC) instrument (NETZSCH DSC 200 F3, Netzsch Co, Selb/Bavaria, Germany). Nitrogen at a rate of 50 mL/min was used as purging gas. Aluminum pans containing
Fig. 6. TEM images of the synthesized MCM-41 nanoparticles in two different magnifications.
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Fig. 7. SEC traces of polystyrene and its nanocomposites with different MCM-41 content.
2–3 mg of the samples were sealed using DSC sample press. The samples were heated from ambient temperature to 220 C at a heating rate of 10 C/min.
Fig. 8. 1H NMR spectra of the neat polystyrene and its nanocomposites.
in the vicinity of around 2u = 3.5–5 can be attributed to the wellhexagonal structure of the synthesized MCM-41 nanoparticles [37]. Nitrogen adsorption and desorption isotherms of the calcined MCM-41 nanoparticles are shown in Fig. 4. The isotherm can be classified as IV type of isotherm, which is characteristic of MCM-41 materials according to the International Union of Pure and Applied Chemistry (IUPAC) classification [38,39]. Therefore, at low p/p0 values, the isotherm presents a linear increase of adsorbed volume which can be attributed to the monolayer adsorption of N2 on the wall of the mesoporous silica nanoparticles. Then, a sudden uptake of the adsorbed amount of N2 was shown over a narrow range of relative pressure by capillary condensation of nitrogen inside the mesopores (narrow pore size distribution can be recognized). And finally, the long plateau at higher relative pressures indicates saturation step (low adsorption of N2 on the external surface of calcined MCM-41) [38,39]. Table 2 summarized the extracted data form nitrogen adsorption and desorption isotherms SEM is employed to evaluate surface morphology and size distribution of the synthesized nanoparticles. SEM images of the calcined MCM-41 nanoparticles in two different magnifications are displayed in Fig. 5. According to these images, nanopaticles with spherical shapes are observed. Although particles size is mainly ranged from 500 to 800 nm, some larger particles are also observed (around 1000 nm).
3. Results and discussion Investigating different initiation processes of ATRP is an interesting subject for synthesis of favorite polymers and polymeric nanocomposites. AGET initiation technique can be considered as a suitable route to circumvent on oxidation problems of normal ATRP since it applies less oxygen sensitive reactants. In AGET ATRP, reducing agents, which are unable to initiate new chains, are used to reduce transitional metal complex in the higher oxidation state (Mtn+1/ligand). Then, generated activators (Mtn/ ligand) participate in ordinary ATRP equilibrium. By this initiation technique, polymerization rate can be directly controlled by the amount of added reducing agent and so, polymer chains with more narrow molecular weight distribution in comparison with the other ATRP initiation techniques can be achieved. Some reducing agents such as Cu, [Sn(EH)2], hydrazine, and ascorbic acid have been used in AGET ATRP [34–36]. Schematic presentation of AGET ATRP in the presence of MCM-41 nanoparticles is shown in Fig. 2. Inherent characteristics and morphology of the MCM-41 nanoparticles are two important factors which can affect the polymerization kinetic and also properties of the produced nanocomposites. Specific structure of the mesoporous silica nanoparticles was examined by XRD (Fig. 3). A sharp and intense peak in the diffraction angles of about 2.5 and also few weak peaks
Table 3 Molecular weights and PDI values of the extracted polymers resulted from SEC traces. Sample
Reaction time (h)
Conversion (%)
NPS PSN 1 PSN 2 PSN 3
8 8 8 8
81 72 66 58
Mn (g mol1) Exp.
Theo.
17,116 14,243 13,061 12,798
16,875 15,000 13,750 12,083
Mw (g mol1)
PDI
21,392 19,848 19,245 20,221
1.24 1.39 1.47 1.58
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Fig. 9. (A) TGA and (B) DTG thermograms of the neat polystyrene and its nanocomposites.
Porosity of structure and pore condition of the MCM-41 nanoparticles was evaluated by TEM. Fig. 6 depicts TEM images of pristine MCM-41 naoparticles in two different magnifications. According to these images, porous structure with regular hexagonal channels of the synthesized MCM-41 naoparticles can be clearly demonstrated. SEC is employed to examine chain characteristics of the synthesized polymer chains (e.g. number and weight average molecular weights and PDI values) and also to investigate the effect of MCM-41 loading on these parameters. According to Fig. 7, neat polystyrene and its nanocomposites show monomodal peak with narrow distribution. By adding MCM41 nanoparticles content, noticeable increase in PDI values is observed. Broadening of these traces by increasing MCM-41 nanoparticles content (variation of PDI values from 1.24 to 1.58 by adding only 3 wt% of MCM-41 nanoparticles), indicates that MCM41 nanoparticles as a filler disturbs the equilibrium of ATRP reaction and results in higher PDI values [27,34,40]. Extracted results of SEC are represented in Table 3. Appropriate correlation between the theoretical and experimental molecular weights can be considered as an evidence for controlled nature of the polymerization. Moreover, color change of the reaction media from green (at the beginning of the reaction) to brown is also appropriate evidence for establishment of ATRP equilibrium [34,41]. Theoretical molecular weight is calculated by using Eq. (1): ¼ MTheo n
½St0 Conversion Mmonomer ½EBiB0
According to the Table 3, by adding nanofillers content, a clear shift in conversion and molecular weight toward lower values is occurred. ATRP equilibrium can be affected by the MCM-41 nanoparticles as an impurity and also nature of the nanoparticles: irreversible reactions between the prepared radicals during ATRP and abundant hydroxyl groups on the surface of MCM-41 nanoparticles can affect the concentration of radicals and kinetics of ATRP and therefore lower conversion and molecular weights are obtained [42–44]. It is clear that by addition of MCM-41 loading, the chance of these irreversible reactions increases and more reduction in conversion and molecular weight can be occurred. Moreover, mobility of macroradicals in the solution can be restricted by the MCM-41 nanoparticles (particularly at higher loading) and therefore rate of polymerization and conversion value tend to decrease by increasing MCM-41 nanoparticles loading [45].
(1)
where [St]0, [EBiB]0 are initial concentration of the monomer and initiator respectively and Mmonomer represents molecular weight of the monomer, which in the case of styrene is equal 104.15 g mol1.
Table 4 Thermal characteristics of the polystyrene and its nanocomposites. Sample
NPS PSN 1 PSN 2 PSN 3
TGA
DTG ( C)
Char (%) at 600 ( C)
Start point
Peak point
End point
2.03 3.04 4.29 5.30
335 346 349 356
412 414 419 421
461 463 461 465 Fig. 10. Graphical illustration of temperature and degradation relationship.
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Fig. 11. (A) Heating and (B) cooling path thermogram of DSC for polystyrene and its nanocomposites.
1 H NMR spectroscopy was employed to evaluate composition of the polymer chains 1H NMR spectra of the neat polymer and its various nanocomposites are shown in Fig. 8. The signal at chemical shift of 41 ppm which originates from hydrogen atom of the terminal units ( CHBr) provides a suitable evidence for living nature of the polymerization. In addition, inherent peaks of polystyrene chains such as benzene rings, methylene ( CH2), and tertiary carbon hydrogen [ CH(C6H5) ] appears in the range of around 7.40–6.25, 1.71–1.15, and 2.31–1.71 ppm, respectively [44]. An improvement in thermal stabilities of all the nanocomposites in comparision with the neat polystyrene can be easily observed in Fig. 9(A). Moreover, by increasing MCM-41 loading, more improvement in thermal stabilities of nanocomposites is observed. Except the main degradation step, other degradations can be ascribed to desorption of chemisorbed water and the silanol groups on the surface of MCM-41 nanoparticles which are removed at the range of around 100 C. Also, degradation of volatile materials such as residual monomer, MCM-41 functionalities, and low molecular weight oligomers occurred at the range of around 100–300 C [33,46]. Char values of the samples at 600 C are shown in Table 4. The percentage of char values increases by increasing MCM-41 nanoparticles content. Also, MCM-41 nanoparticles leaves 96.32% char after complete degradation at 600 C. Extracted data from TGA are graphically illustrated in Fig. 10. In this graph, degradation temperature against the amount of degradation is drown to prove the fact that by adding MCM-41 nanoparticles loading in the polymer matrix, thermal stability improves (TX: temperature threshold at which X% of the polymer and its nanocomposites degradation is occurred). Improving thermal stability of the nanocomposites by increasing MCM-41 nanoparticles content can be attributed to several reasons: (i) high thermal stability of nanofillers (MCM-41 nanoparticles), (ii) interaction between MCM-41 nanoparticles and polymer matrix [47], (iii) physical interaction between polystyrene chains and surface of MCM-41 nanoparticles and finally
Table 5 Glass transition temperatures of the neat polystyrene and its nanocomposites. Sample
Mn (g mol1)
PDI
NPS PSN 1 PSN 2 PSN 3
17,116 14,243 13,061 12,798
1.24 1.39 1.47 1.58
Tg ( C) Heating path
Cooling path
100.1 97.6 94.9 91.4
100.4 97.2 94.5 91.7
degradation from outer surface inward can be considered as an important factor for increasing thermal stability of the nanocomposites [11]. Extracted data from TGA (char values) and DTG curves which indicates an improvement in thermal stability of the nanocomposites by increasing MCM-41 nanoparticles content are summarized in Table 4. Thermal behavior of the neat polystyrene and its nanocomposites was evaluated by DSC. Fig. 11 depicts some information on thermal behavior of the neat polystyrene and its nanocomposites. Temperature range of 40–180 and 170–40 C are used to describe DSC results in heating and cooling path, respectively. Since MCM-41 nanoparticles do not bear any transitions in this range of temperature, therefore only thermal transitions of polymer chains is observed. Firstly, samples are heated from room temperature to 220 C to remove their thermal history. Then, they are cooled to room temperature to distinguish the phase conversion and other irreversible thermal behaviors. Finally, samples are heated from room temperature to 220 C to obtain the more appropriate Tg values. According to the Table 5, Tg value of the neat polystyrene is higher than the all of the nanocomposites and by increasing MCM41 nanoparticles content, a decrease in Tg value occurs. Reduction of Tg values by increasing MCM-41 content may be ascribed to the weak interaction between polymer chains and OH-containing MCM-41 nanoparticles. Therefore, MCM-41 nanoparticles can reduce the packing polystyrene chains and increase the segments mobilities which in turn results in Tg reduction. Moreover, other parameters such as decrease in molecular weights and increment of PDI values need to be considered since decrease of molecular weight results in decrease of Tg and high PDI values may cause reduction of Tg values. This is mainly on account of the fact that in very high PDI values, chains with low molecular weights can act as viscosity reducer like plactisizers. 4. Conclusions Dispersion of MCM-41 nanoparticles in the well-defined polystyrene matrix were carried out by in situ AGET ATRP. The synthesized MCM-41 nanoparticles reveal spherical morphology, hexagonal structure, and high surface area with regular pores. Decreases in final conversion from 81 to 58% and from 17,116 to 12,798 g mol1 for number average molecular weight are resulted by 3 wt% addition of MCM-41 nanoparticles. However, PDI values increases from 1.24 to 1.58. Low intensity peak in the range of around 41 ppm in 1H NMR spectra demonstrates the living nature of polymerization. Improvement in thermal stability of the
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