ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия Б, 2012, том 54, № 3, с. 483-490


УДК 541.64:539.2:542.952



© 2012 г. Hossein Roghani-Mamaqani", V&hid Haddadi-Asl", Mohammad Najafi4, and Mehdi Salami-Kalajahic

a Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, 15875-4413 Tehran, Iran b Polymer Engineering Division, Research Institute of Petroleum Industry (RIPI), 1485733111 Tehran, Iran cDepartment of Polymer Engineering, Sahand University of Technology, 51335-1996 Tabriz, Iran e-mail: r.mamaghani@aut.ac.ir, haddadi@aut.ac.ir, najafim@ripi.ir, mskalajahi@aut.ac.ir

Received August 10, 2011 Revised Manuscript Received September 25, 2011

Abstract—Polystyrene nanocomposites, being a combination of nanoclay-attached and free polystyrene chains were prepared using in situ atom transfer radical polymerization. Subsequently, they were electrospun to form fibers with diameter varying from 450—700 nm according to scanning electron microscopy data; in addition, the transmission electron microscopy and x-ray diffraction analysis revealed that nanoclay layers were oriented along the nanofiber axis during the electrospinning process. Molecular weight of the extracted free polymer chains from the nanocomposites is higher than the attached chains. However, Anchored chains are characterized by higher polydispersity index in comparison with the free ones. Polydispersity index of polymer chains increases by the addition of nanoclay. Thermogravimetric analysis results shows that increasing clay content leads to a decrease in the quantity of polymer chains attached to the clay surface.


Within the past decades, polymer clay nanocomposites have been considered as a prominent field of study. Significant improvements in mechanical properties, thermal stability and flame retardancy, magnetic and electric properties, and enhanced modulus of the neat polymer matrix can be obtained by adding nanoclay into the polymer matrix [1—3]. Nanoclay because of the high aspect ratio and nanometric thickness of its platelets together with the inorganic nature of the layers could be considered as the resource of these improvements. The degree of dispersion of the clay platelets into the polymer matrix determines the structure of the resultant nanocomposites and affects the above-mentioned properties. Therefore, in situ polymerization method, which results in high dispersion of exfoliated clay platelets within a polymer matrix, has been considered as a powerful tool of nanocom-posite preparation.

Considering the variety of polymerization techniques, one cannot ignore the prominent role of controlled free-radical polymerization (CRP) in the synthesis of polymers with well-defined topology. CRP has attracted much attention in recent years for providing simple and robust routes to the synthesis of well-defined, low dispersity polymers. In this context, nitroxide-mediated polymerization [4, 5], reversible addition-fragmentation chain transfer polymerization (RAFT) [6, 7], and atom transfer radical polymeriza-

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tion (ATRP) [8—13] have extensively been studied. Some advantages ofATRP over other CRP systems are the applicability to a wide variety of monomers and systems of polymerization and its less sensitivity to impurities [14].

The electrospun nanofibers have been employed in wide variety of applications, such as scaffolds for tissue engineering, sensors, molecular electronics, and protective clothing [15—17]. Electrospinning is known to be a useful way to prepare nanowebs comparing of ul-trathin fibers having a diameter of about few hundred nanometer. This is a versatile route which can utilize various polymers, polymer blends, sol-gels, and composites to produce nanofibers. Since nanowebs have a high specific surface area and their pore sizes are in the range of nanometer, it is easy to functionalize their surface; therefore, they could provide a wide range of physical and chemical properties. By electrospinning, a filament is electrospun from a polymer solution [18, 19] or polymer melt in a high electrical field [20, 21]. The possibility of dispersing nanofillers within the polymer matrix has made this process attractive as a route to production of composite nanofibers.

Preparation of nanocomposites using atom transfer free-radical polymerization (ATRP) has been reported extensively [22—27]. In this polymerization technique, as monomer penetrates into the intergallery of clay platelets and polymerization initiates via providing appropriate conditions, the silicate layers could be gradually pushed apart. However, electrospinning in combination with CRP methods has been employed

to prepare smart solvent-resistant nanofibers by Fu et al [28]. They obtained poly(vinylbenzyl chloride-co-glycidyl methacrylate) nanofibers with diameters in the range of 0.4—1.5 ^m. Spinning core-sheath nanofibers using ATRP and electrospinning were also reported by this group; they prepared nanofibers of polystyrene with diameters in the range of 10— 1000 nm [28]. Subsequently, polystyrene nanofibers were used as a core to surface initiation of acrylamide sheath [29]. The effect of solvent on the preparation of styrene nanofibers has been reported by Uyar et al. [30]. They reported that the reproducibility of uniform polystyrene fibers by electrospinning was strongly dependent on the solution conductivity. Also, there are some reports on the electrospinning of various polymers in the presence of clay. Wang et al. have reported the use of montmorillonite to enhance the thermal properties of poly(MMA-co-MAA) electrospun nanofibers [31]. They found that dispersion of clay within the nanocomposites improved the electrospin-nability of the nanocomposites. Polyurethane nano-composite nanofibers have been prepared by electro-spinning as recently reported by Hong et al. [32]. They have also reported that by electrospinning, the exfoliated clay layers oriented along the fibers. However, there are not any clear reports on the preparation of tailor-made polystyrene nanocomposite nanofibers. Meanwhile, polystyrene nanocomposite with an exfoliated structure could be an appropriate resource of clay-dispersed polystyrene nanofiber.

In this study, polystyrene nanocomposite was synthesized by ATRP; afterwards, the nanocomposite was used as a precursor to prepare polystyrene nanocom-posite nanofiber. The ATRP process was applied to the mixture of monomer and modified montmorillonite having a double bond on its modifier structure. The platelets of nanoclay pushed apart as the ATRP process continued. Finally, nanoclay containing polystyrene nanofiber was obtained by electrospinning of the nanocomposite which prepared by the grafting through in situ ATRP.



Styrene (Aldrich, 99%) was passed through an alumina-filled column, dried over calcium hydride, and distilled under reduced pressure (60°C, 40 mmHg). Na-montmorillonite (Southern clay product) was stirred in de-ionized water for a day, and then was separated by centrifugation, filtered, dried, and finally stored under vacuum oven (50°C, 40 mmHg). Copper (I) bromide (CuBr, Aldrich, 98%) was washed by glacial acetic acid, filtrated, washed again by ethanol, dried in vacuum oven (50°C, 40 mmHg), and then stored under a nitrogen blanket, N,N,N',N'',N"-pen-tamethyldiethylenetriamine (PMDETA, Aldrich, 99%), ethyl a-bromoisobutyrate (EBiB, Aldrich, 97%), anisole (Aldrich, 99%), vinylbenzyl trimethy-

lammonium chloride (Aldrich, 99%), and neutral aluminum oxide (Aldrich) were used as received.

Montmorillonite Modification Process

A quaternary alkylammonium cation having a double bound in its structure was employed as the pristine clay modifier. A suspension of 4.5 g of Na- montmorillonite with a CEC value of 92 mEq/100 g in 200 ml of distilled water was stirred in a round-bottom flask over 5 h. In another flask, 1.05 g of intercalating agent and 200 ml ethanol were stirred for 5 h. The modifier solution in ethanol was added drop-wise to the clay suspension. After the solution was stirred overnight, the white precipitated part was filtered, washed with 80/20 (v/v) methanol/water mixture several times until no chloride ion could be detected by an AgNO3 solution.

Preparation of Polystyrene/Clay Nanocomposite

The ATRP polymerizations were performed in a 250 ml reactor, which was placed in an oil bath ther-mostated at the desired temperature. A number of batch polymerizations were run at 110° C in bulk with the molar ratio of 140:1:1:1 for [styrene]:[EBiB]:[Cu-Br]:[PMDETA]. The reactor was degassed and backfilled with nitrogen gas three times, and then left under N2. The batch experiments were run by orderly adding deoxygenated monomer (styrene, 32.2 ml, 0.02 mol), modified montmorillonite, catalyst (CuBr, 0.287 g, 0.002 mol), ligand (PMDETA, 0.417 ml, 0.002 mol), and 0.50 ml of deoxygenated anisole as an internal standard to the reactor and then increasing the reaction temperature to 110° C (within about 15 min.). The solution turned light green as the CuBr/PMDETA complex formed. Finally, after the majority of the metal complex had formed, initiator (EBiB, 0.293 ml, 0.002 mol) was added to the system to start the styrene ATRP. A sample was taken before the reaction started and used as a comparison reference for the samples later taken at the different stages of the reaction so as to measure the monomer conversion.

Separation of Polymer Chains from Clay

The polymer samples were dissolved in THF and passed through a neutral aluminum oxide column to remove catalyst particles. By high-speed ultracentrifu-gation and then passing the solution through a 0.2 mm filter, the unattached polymer chains were separated from the anchored ones by passing through the filter pores [33]. The remained tethered chains were cleaved from the layer

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