Article pubs.acs.org/IECR
Suppression of Keto Defects and Thermal Stabilities of Polyfluorene−Kaolinite Clay Nanocomposites Chanchal Chakraborty,† Pradip K. Sukul,† Kausik Dana,*,‡ and Sudip Malik*,† †
Polymer Science Unit, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur, Kolkata700032, India ‡ Advanced Clay and Traditional Ceramics Division, Central Glass and Ceramic Research Institute, 196 Raja S. C. Mullick Road, Kolkata 700032, India ABSTRACT: A solution blending process for preparation of polymer nanocomposites composed of cationic polyfluorene (PF) and dimethyl sulfoxide (DMSO) -intercalated kaolinite (Ka) clay has been taken to evaluate the effect of Ka nanostructure on the nanocomposite structures, morphology, and properties. Composites containing 2, 5, 7.5, and 10 wt % clay have been characterized with the help of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), UV−visible spectroscopy, photoluminescence studies, etc. Additionally, the keto defect, inhibition of interchain interaction, and photostability of PF in the nanocomposites have been explored. The XRD and HRTEM studies show the exfoliation of Ka layers at lower content in composites. Intercalation of PF chains into the Ka interlayer space occurs at relatively higher clay content. Nanocomposites exhibit higher thermal stability than pristine PF due to lamination of PF into clay nanogallery through the interchange of DMSO by cationic polyfluorene. The presence of an Si−O−Si stretching band in the composites supports the formation of nanocomposites of PF with Ka. The movement of absorption maxima to higher wavelength indicates the increase of overall conjugation length of PF chains in the nanocomposites. Upon formation of nanocomposite with Ka, the keto defect sites of PF are significantly reduced. This can be attributed to the lamination of single PF chains by Ka interlayer gallery that act as a barrier to oxygen and inhibit the exciton diffusion. Current−voltage characteristics of nanocomposite films have also shown good switching behavior with low forward junction potential.
1. INTRODUCTION The design of polymer-based inorganic−organic hybrid materials at the nanoscale, combining at the molecular level organic and inorganic units, results in materials with superior physical, mechanical, and chemical properties as compared with pure polymers.1−3 This can be accomplished when the nanoscale phyllosilicate platelets are well-dispersed by delamination of the silicate-layered structure throughout the polymer matrices showing exfoliated or intercalated morphology.4 Intercalated morphology is obtained when polymer chains are located between the clay layers in a manner that increases layer spacing while attractive forces between the clay layers keep these in regularly spaced stacks. In exfoliated structures, greater delamination of the clay layers increases the basal spacing to such an extent that interlayer attractions forces cease to dominate and clay platelets are then randomly dispersed in a continuous polymer matrix.4 The interaction between polymer and layer silicate is very crucial to form nanocomposites. Proper selection of polymer structure and functionality along with layer silicate are important factors in the quality of nanocomposites. Kaolinite (Ka) is an important clay mineral of the kandite group, which also includes dickite, nacrite, and halloysite. It is a 1:1 phyllosilicate and consists of a Si2O5 sheet bound on one side to a dioctahedral gibbsite-type sheet. The gibbsite-type layer consists of aluminum cations coordinated by hydroxyl groups, with some hydroxyls replaced by the oxygens of the Si− O sheet5 with the chemical composition Al2Si2O5(OH)4, corresponding to 46.55% SiO2, 39.49% Al2O3, and 13.96% © 2013 American Chemical Society
H2O and a molecular mass of 258.10 g/mol. The structural asymmetry of Ka, due to the superposition of tetrahedral and octahedral sheets in the 1:1 layer, creates large superposed dipoles, which, in conjunction with H-bonds between the siloxane macrorings on one side and the aluminol surface on the other side, result in a large cohesive energy of the mineral.6 Due to this strong interlayer bonding and very low layer charge of Ka, the intercalation of molecular guests in the interlamellar spaces of Ka is much less developed than in the case of smectites. However, Ka preintercalated with dipolar molecules, such as dimethyl sulfoxide (DMSO),7 N-methylformamide (NMF),8 or urea9 has been used successfully as a precursor for further intercalation of other organic molecules or polymers.3,10−12 The intercalation can also lead to the chemical grafting of organic units on the interlamellar aluminol surfaces.13 Polyfluorene (PF) -based homo- and copolymers are very promising blue-light-emitting materials for electronic and optoelectronic application, such as light-emitting diodes (LEDs),14 photovoltaics,15,16 and field effect transistors17 due to high quantum yield and appropriate thermal stability.18 Polyfluorenes are also important as host polymer matrices for fluorescent and phosphorescent dyes.19 They can exhibit Received: Revised: Accepted: Published: 6722
January 3, 2013 March 13, 2013 April 23, 2013 April 24, 2013 dx.doi.org/10.1021/ie4000213 | Ind. Eng. Chem. Res. 2013, 52, 6722−6730
Industrial & Engineering Chemistry Research
Article
paints and rubber, and in cosmetics and pharmaceutical applications.37−39 Recently, Ka is effectively used to deaggregate the conjugated polymer chains by surface adsorption,40 even though there is no reported use of Ka in electronic device application yet. Although intercalated montmorillonite (MMT) has been extensively studied due to its submicrometer dimension and favorable cation-exchange properties, Ka remains relatively unexplored. Unlike MMT, the crystal structure of Ka is well-defined and it is available with higher phase purity and reproducible quality. There are a few examples of intercalated polymer−Ka nanocomposites that one can count in hand.11,41 To the best of our knowledge, the intercalation and exfoliation chemistry (Figure 1) of Ka by
thermotropic liquid crystal behavior, which allows the orientation of PFs onto suitable alignment layers for fabrication of polymer light-emitting diodes (PLEDs).20 However, polyfluorene-based PLEDs in electronic applications degrade with use when a non-pure low-energy emission at 2.2−2.3 eV appears, which alters the favored blue emission into an unwanted green emission.21 It is supposed that the low-energy emission bands are due to reordering or subsequent aggregation of polyfluorene polymer chains or excimer formation.22 Houwever, some recent reports have evidently established that the manifestation of such a low-energy green emission band in polyfluorene is solely due to keto defect23,24 formation in the polymer chain as a chemical defect during preparation. These keto defect sites can be formed during polymer synthesis as a consequence of incomplete monomer alkylation in 9-position or during polymerization by Suzuki coupling25 or as a result of photo-, electro-, or thermo-oxidative degradation processes occurring after polymer synthesis. Acting as low-energy trapping sites for singlet excitons, these keto defects, being populated by an excitation energy transfer from the polyfluorene chain, can be responsible for the low-energy emission band at 2.2−2.3 eV.25 This photo- or thermooxidative degradation of blue-emitting polymers is generated by oxygen, either residual in the polymer or released from the indium tin oxide (ITO) electrode.26 This oxygen accelerates the formation of keto defect sites within the polymer chain, causing the appearance of low-energy green emission band and degrading the PLEDs. Again, this undesired excimer formed in a polyfluorene polymer chain can easily migrate to another chain through a large π−π interaction among the polymer chains.27 Several methods have been suggested to reduce undesired excimer formation, for example, introduction of other chromophore moieties into main28 or side chains,29 crosslinking in film state via styryl end groups,30 and bulky substitution in 9,9-position.31 Introduction of these moieties or cross-linking cause a separation between polymer chains so that they cannot aggregate to form excimer. Removal of low molecular weight parts,32 triblock copolymer formation,33 and blending with a polymer possessing higher glass transition temperatures34 also can improve the spectral purity. However, in all these cases, processability of the polymers becomes very complicated, and chromophore units or other polymer units play a definite role in optical properties. Formation of poly(fluorene) polymer nanocomposites with nanocompounds like CdSe nanocrystal,35 mica,36 or MoS2- and SnS2- type27 nanocompounds is another way to suppress this undesired emission in solid polymer film. The achievement of spectral purity and stability of blue emission in the nanocomposites is attributed to the reduction of polyfluorene interchain interaction and inhibition of interchain oxygen transportation leading to keto defects. In this viewpoint, we have prepared an organic−inorganic host− guest polymer/clay nanocomposite by laminating cationic PFs in organically modified Ka phyllosilicate layer to propose an inexpensive and proficient solution process. This easy process can overwhelm the problem of spectral impurity of green emission by efficiently inhibiting the aggregation between polymer chains and reducing excimer diffusion by blocking one PF chain into 2D Ka layers effectively. Ka has several industrial applications, for example, in the paper industry as filler, in the ceramic industry to impart plasticity to ceramic formulations and confer whiteness to the sintered ceramic body, as fillers in
Figure 1. (Top) Chemical formula of polymer PF and (bottom) schematic view of pure kaolinite, Ka-DMSO, and PF-Ka nanocomposites as exfoliated and intercalated structure, derived from XRPD.
cationic polyfluorene has not been reported yet. Successful bonding and enhanced interaction of polyfluorene cation with Ka is revealed by rheological study, and subsequent production of nanocomposite with superior physical, optical, and chemical properties to as-synthesized polyfluorene polymer will be interesting and promising for potential applications.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Purified kaolinite clay (Rajmahal Quartz Sand and Kaolin Co., India) was subjected to centrifugal sedimentation in water to separate the
