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lyotropic LC systems that are usually used as templates ... we have chosen two side-chain LC copolymers, P1 and P2, that are forming nematic and ... Published on Web 11/06/2004 ... using Ni-filtered Cu KR radiation and a PSD linear position.

10868

Langmuir 2004, 20, 10868-10871

Influence of Silver Nanoparticles on the Phase Behavior of Side-Chain Liquid Crystalline Polymers Evgenii B. Barmatov,* Dmitry A. Pebalk, and Marina V. Barmatova Department of Chemistry, Moscow State University, Moscow, 119992 Russia Received June 7, 2004. In Final Form: September 24, 2004

The synthetic approach to the new class of mesomorphous nanocomposite polymer systems was developed. It is based on the in situ reduction of silver ions in the liquid crystalline (LC) polymer matrix leading to the formation of nanoparticles with typical sizes in the range of 5-30 nm. The influence of silver nanoparticles on the phase state of the LC composites, i.e., type and temperature interval of the mesophase, was studied. Regardless of chemical structure of the LC polymer matrix, an increase in the metal concentration is accompanied by a decrease of clearing temperature due to adsorption of macromolecules on the nanoparticle’s surface. In the case of an LC copolymer with cyanobiphenyl side mesogenic fragments, the complete disruption of mesophase is observed below 2 wt% content of silver. This phenomenon is, most likely, a result of chemosorption of terminal cyano groups on the nanoparticles with the formation of σ complexes that disturb packing of the mesogenic units.

Introduction Chemistry and physics of nanometer-scale objects are two of the most prospective and rapidly developing fields of modern science and technology.1 The scope of their potential applications is constantly growing and to less or more extent is including almost all out-of-the-ordinary investigations in the directions of the new materials, microelectronics, information storage and recording, pharmacy, molecular medicine, etc. Special attention of the researchers is undoubtedly attracted to polymer nanocomposites that are promising substances for all of the mentioned areas and many others. Mesomorphous polymers, which mainly consist of substances with either thermotropic or lyotropic liquid crystalline (LC) ordering,2,3 are an important class of highmolar-mass compounds. Many functional materials and composites, based on these polymers and possessing exceptional properties, have already been created and many are in the development stage. As an example, one could mention high-modulus organic fibers, nonlinear optic media, ferroelectrics, chiral photonics, and various substances for optic information recording and storage. Despite the commercial potential of the mesomorphous polymers and their value as the objects of basic studies, LC polymer metal-containing nanocomposites are studied quite poorly to date. More attention is paid to some lyotropic LC systems that are usually used as templates for the preparation and stabilization of inorganic nanoparticles.4-8 However, the role of nanoparticles in the mesophase formation, as well as their influence on liquid crystal properties, is yet to be understood. Polymer LC nanocomposites are attractive materials for various applications due to their unique properties * Corresponding author. E-mail: [email protected] Tel: (095) 939-3132. Fax: (095) 939-0174. (1) Klabunde, K. J., Ed. Nanoscale materials in chemistry; Wiley: New York, 2001. Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (2) Demus, D., Goodby, J., Grey, G. W., Spiess, H. W., Vill, V., Eds. Handbook of liquid crystals; Wiley VCH: New York, 1998, Vol. I-IV. (3) McArdle, C., Ed. Side chain liquid crystal polymers; Blackie: London, 1989.

resulting from a combination of characteristic features of macromolecular LC (self-ordering, optic anisotropy, ability to orient under the action of external magnetic or electric fields, freezing structure below glass transition temperature) and nanopatricles (tremendous active surface, high reactivity, quantum size effects). The growing need in such substances may appear soon in the area of microelectronics, information storage, and especially optical devices due to the interest of manufacturers in further miniaturization of their key elements to the nanometer-scale level. Taking this into consideration, one could formulate the following general tasks in this promising field located between polymers and colloid chemistry, materials science, and nanochemistry. First, it is crucial to work out various synthetic approaches to the mesomorphic polymer nanocomposites and to study their regularities. Second, it is important to study structure and physicochemical properties of such substances. Finally, the influence of nanoparticles on the ability of the polymer matrix to form mesophases, their structure, degree of molecular ordering, and thermodynamic stability should be investigated. It is evident that practical usage of such mesomorphous polymer nanocomposites necessarily requires further broadening of these research directions. In the current work, we developed an approach to the synthesis of the new class of hybrid mesomorphic polymer systems by in situ reduction of silver atoms with the formation of metal nanoparticles dispersed in LC polymer. The main goal of the research was to study the influence of silver nanoparticles on the phase state of the mesomorphous nanocomposites. As the objects of investigation, we have chosen two side-chain LC copolymers, P1 and P2, that are forming nematic and SmE phases and are (4) Patakfalvi, R.; Dekany, I. Colloid Polym. Sci. 2002, 280, 461. (5) Qi, L.; Gao, Y.; Ma, J. Colloids Surf., A 1999, 157, 285. (6) Bouchama, F.; Thathagar, M. B.; Rothenberg, G.; Turkenburg, D. H.; Eiser, E. Langmuir 2004, 20, 477. (7) Andersson, M.; Alfredsson, V.; Kjellin, P.; Palmqvist, A. E. C. Nano Lett. 2002, 2, 1403. (8) Jiang, X.; Xie, Y.; Lu, J.; Zhu, L.; He, W.; Qian, Y. Chem. Mater. 2001, 13, 1213.

10.1021/la048601h CCC: $27.50 © 2004 American Chemical Society Published on Web 11/06/2004

Influence of Silver Nanoparticles on Phase Behavior

Langmuir, Vol. 20, No. 25, 2004 10869

Table 1. The Molecular Mass Characteristics and Temperature of Phase Transitions for Polymers P1 and P2 sample

Mw

Mw/Mn

phase transitions (°C)

P1 P2

9300 11800

1.4 1.5

glass 37 N 96 I SmE 141 I

different by the chemical structure and polarity of the terminal group, R, in their mesogenic fragments.

Figure 1. UV-visible absorption spectra of P1-Ag and P2Ag composites and representative TEM image of silver nanoparticle composite P1-Ag-4.64. Table 2. The Composition and the Average Silver Particle Size as Determined by TEM for P1-Ag and P2-Ag Nanocomposites

Experimental Section The synthesis and phase behavior of LC copolymers P1 and P2 (Table 1) that were used in this work as matrixes for the silver nanocomposite synthesis were described earlier.9,10 LC nanocomposites were obtained using the following procedure. To the solution of copolymer P1 or P2 in freshly distillated anhydrous tetrahydrofurane (THF) (1 wt%) the calculated amount of (1,5-cyclooctadiene)-(hexafluoroacetylacetonato) silver (I) (purchased from Aldrich) was added. The mixture was mixed on the magnetic stirrer for 24 h at room temperature. After that, the solvent was evaporated on a rotorary evaporator. For in situ reduction of metal ions, the obtained composite was kept at 95 °C for 7 days at normal pressure and then at 12 h under vacuum. The morphology of synthesized nanoparticles was observed on a LEO 912AB OMEGA transmission electron microscopy (TEM). The samples were prepared by dropping THF solution of the silver nanoparticles on the Cu grids and were observed at an accelerating voltage of 60-120 kV. Microcalorimetry studies were performed with a Mettler differential scanning calorimeter TA4000. The heating rate was 10 K/min. Polarizing optical microscopy observations were made with a Zeiss polarizing microscope equipped with a Mettler FP82HT hot stage controlled by a Mettler FP90 unit. X-ray diffraction (XRD) patterns were obtained from a 2 mm diameter capillary sample with a modified STOE STADI 2 diffractometer using Ni-filtered Cu KR radiation and a PSD linear position scanning detector. The UV spectra were recorded by a Unicam UV 500 spectrometer in the region 250-900 nm at a spectral resolution of 2 nm. The IR spectra were recorded by a FTIR spectrometer (Biorad FTS 6000) in the region 400-4000 cm-1 at a spectral resolution of 1-4 cm-1 and an uncertainty

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