Selective inclusion of chromophore molecules into poly(styrene-bmethylmethacrylate)

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1 Selective inclusion of chromophore molecules into poly(styrene-bmethylmethacrylate) block copolymer nanodomains: a study of morphological, optical and electrical properties Claudia Diletto, Pasquale Morvillo, Rocco Di Girolamo, Finizia Auriemma & Claudio De Rosa Journal of Sol-Gel Science and Technology ISSN DOI /s

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3 DOI /s ORIGINAL PAPER Selective inclusion of chromophore molecules into poly(styrene-bmethylmethacrylate) block copolymer nanodomains: a study of morphological, optical and electrical properties Claudia Diletto Pasquale Morvillo Rocco Di Girolamo Finizia Auriemma Claudio De Rosa Received: 13 June 2014 / Accepted: 5 November 2014 Ó Springer Science+Business Media New York 2014 Abstract Innovative nanocomposites based on a nanostructured block copolymer (BCP) matrix whose lamellar nanodomains are selectively loaded with organic molecules, were prepared. A symmetric poly(styrene-b-methylmethacrylate) (PS-b-PMMA) amorphous BCP showing a lamellar morphology was employed. Thin films of PS-b- PMMA were deposited by spin-coating on indium thin oxide (ITO) substrate in order to achieve orientation of lamellae with the lamellar surface perpendicular to the substrate. Nanocomposites were then prepared by selective incorporation of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) molecules into the PS lamellar domain. The self-assembly of the BCP generated the nanotemplate to selectively control the spatial location of the PCBM molecules, in which the apolar properties of PS block provided the physical stabilization for achieving uniform PCBM distribution. These innovative approaches can be utilized as a tool to realize memory devices. Keywords molecules Nanocomposites Block copolymers PCBM C. Diletto (&) R. Di Girolamo F. Auriemma C. De Rosa Department of Chemical Sciences, University of Napoli Federico II, Complesso Universitario Monte S. Angelo, via Cintia, Naples, Italy claudia.diletto@unina.it P. Morvillo ENEA Italian National Agency for New Technologies, Energy and Sustainable Development, UTTP-NANO, Piazzale E. Fermi 1, Portici, Italy 1 Introduction Flexible electronic circuits, displays, sensors and photovoltaic cells based on organic active materials will enable future generations of electronics products that may eventually enter the mainstream electronics market. The motivations in using organic active materials come from their ease in tuning electronic and processing properties by chemical design and synthesis, low cost processing based on low temperature processes and reel-to-reel printing methods, mechanical flexibility, and compatibility with flexible substrates [1, 2]. However the performance of these systems relies crucially on understanding and controlling the morphology on the nanometer scale. For example, optimization of organic photovoltaics and light emitting devices relies on controlling the nanoscale morphology of at least two semiconducting materials with different energy level alignments to maximize charge separation, recombination, and transfer [3, 4]. In fact, within these devices, the presence of continuous pathways for charge carriers is necessary to allow for efficient charge transport and collection. It has been postulated that the patterning length scale necessary to affect charge separation and overall efficiency in photovoltaics must be 10 nm that is difficult to achieve via conventional lithographic patterning techniques or by phase separation of polymer blends. Recently, the expansion of our understanding of block copolymer (BCP) self-assembly to materials appropriate for organic electronics has received a lot of interest. Block copolymers (BCPs) for organic optoelectronics must contain at least two distinct electronic functionalities in the chain. For OLEDs, polymers generally must contain at least electron and hole transporting (and, occasionally, emitting) functionalities while block copolymers for

4 photovoltaics characteristically contain both electron donating and accepting functionalities. These semiconducting properties in polymers can be derived either by conjugation of the polymeric backbone [5] or by attaching semiconducting molecules pendant to the backbone [6]. Block copolymers consist of covalently linked chemically distinct macromolecules that tend to segregate into different microdomains due to their mutual repulsion, resulting in the spontaneous formation of periodic nanostructures by self assembly [7, 8]. In order to minimize the contact area of nanodomains, different types of nanostructures are obtained. In fact depending on the relative lengths of the polymer blocks several morphologies can be obtained. For instance, in asymmetric diblock copolymers, the short polymer blocks segregate into nanometer-sized spheres or cylinders embedded in a matrix composed of the longer polymer blocks. Symmetric BCPs composed of polymer blocks of similar lengths, instead, form alternating layers or lamellae [7, 8]. Self-assembled microdomains of BCPs nanostructures (sphere, cylinders or lamellae) may act as hosts for sequestering nanofillers of appropriate chemical affinity according to prefixed periodic geometries, producing special hybrid nanocomposites characterized by long-range order in the positioning of nanoparticles [9]. The key for using the BCP nanostructures as scaffolds for engineering of new nanocomposites, is the ability to control the final morphology of BCP nanostructures and to achieve a selective infiltration of nanoparticles in the target microdomains. The segregation of nanoparticles into one of the domains of a self-assembled copolymer offers an attractive method to obtain a well-defined spatial arrangement of nanoparticles. Block copolymers offer interesting possibilities for examining the role of device architecture on optoelectronic performance [10, 11]. A suitably designed block copolymer with chromophores and/or hole transport and electron transport groups would therefore be suitable for constructing, from the same starting materials, devices containing either homogeneously mixed or spatially segregated components. For example, a ordered block copolymer structure can be used to create two distinct zones, arranged perpendicular to the device electrodes, thanks to the self organization of the block copolymer, and ideally, contain selectively electron transport groups and hole transport groups or molecules. This paper describes a simple method to obtain a special nanocomposite by using poly(styrene-b-methylmethacrylate) (PS-b-PMMA) BCP characterized by selective inclusion of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in specific nanodomains. We have selected a BCP with lamellar morphology exploiting our ability to obtain a perpendicular orientation of PS and PMMA lamellar Scheme 1 Structure of [6,6]-phenyl-C61-butyric acid methyl ester nanodomains in a single step using a ITO substrate [12]. We report morphology, properties and device characteristics of self-assembled symmetric PS-b-PMMA BCP and their hybrids with different amounts of PCBM. This composite system served as an excellent archetype; the selfassembly of BCP generated the nanotemplate to selectively control the spatial locations of the PCBM molecules in PS block, in which the apolar properties of PS block provided the physical stabilization for preventing the self-aggregation of PCBM molecules. It is worth noting that previous studies have shown that PCBM molecules are highly miscible with polystyrene. In PS mixtures the formation of PCBM aggregates and PCBM crystallization is prevented probably due to the favorable p p interactions of PS phenyl groups with the C61 ball-like units [13]. The PS-b-PMMA:PCBM nanocomposites were prepared as thin films on indium thin oxide (ITO) quartz substrate by spin coating technique. The characterization of these materials were performed by UV Vis spectroscopy, I V dark (Current Voltage) measurements and transmission electron microscopy (TEM). 1.1 Experimental We used a poly(styrene-b-methylmethacrylate) block copolymer (PS-b-PMMA) (Polymer Source Inc.) with molecular masses M n of the PS and PMMA blocks equal to 25.0 and 26.0 kg mol -1, respectively, and mass distribution M w /M n = The volume fraction of the PS block of 0.52 was selected in order to obtain a lamellar microphase separated morphology. Thin films of PS-b-PMMA 73 nm thick were obtained at room temperature by spin coating after deposition of a slight excess of toluene BCP solutions on quartz/ito substrate. The substrate has been cleaned using a 20 wt% solution of ethanolamine in deionized water at 80 C in an ultrasonic bath.

5 a Intensity θ (deg) b DSC endotherm heating T g (PS) Cooling T g (PMMA) Temperature ( C) Fig. 1 X-ray powder diffraction profile (a) and DSC heating and cooling curves (b) of the sample of PS-b-PMMA [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) (scheme 1) sample was purchased from Solenne BV. Toluene solutions of the PS-b-PMMA/PCBM mixtures were prepared. BCP and PCBM were dissolved in toluene using sonication, leading to a brown solution. Thin films of PS-b- PMMA:PCBM nanocomposites containing 5, 7, and 10 wt% of PCBM molecules were prepared by spin-coating of the corresponding solutions on quartz/ito substrates, dried in vacuum (10-3 mbar) and annealed at 150 C for 6 h. Thin films of pristine PS-b-PMMA and PS-b- PMMA:PCBM nanocomposites ones were backed with a carbon film, floated off on water with the help of a poly(acrylic acid) backing, mounted on copper grids and analyzed by TEM. The films were stained with RuO 4 in order to achieve a good contrast between PS and PMMA domains. The X-ray powder diffraction profile of pristine PS-b- PMMA was obtained with a Philips automatic powder diffractometer, using the Cu Ka (nickel-filtered) radiation. The diffraction patterns were registered scanning continuously the diffraction angle 2h at a rate of 0.01 s -1. DSC curves were obtained by a DSC Mettler-822 calorimeter. TEM images of pristine BCP and nanocomposites were obtained with a Philips EM 208S microscope operating at a Fig. 2 TEM bright-field image of a thin film of PS-b-PMMA deposited on a ITO substrate by spin coating (3000 rpm, 30 s) from a 2 wt% toluene solution and stained with RuO 4 voltage of 100 kv. TEM analysis was repeated on different regions of the specimens, in order to check that the morphology was uniform over the macroscopic area of the ITO substrate (2 cm 9 2 cm). The results were confirmed also by repeating the measurements on at least three independent samples. The thickness L of the resulting films was measured using a KLA Tencor P-10 model-surface Profile Measuring System. UV Vis spectra were recorded on a UV Vis NIR spectrophotometer Perkin Elmer Lambda 900 equipped with an integrating sphere allowed to measure the fraction of incident light that is reflected and transmitted by the sample. The appropriate positioning of the sample within utilized instrument allows obtaining the desired property. Dark I V measurements were carried out using a Keithley 2400 source meter. 2 Results and discussion The X-ray powder diffraction profile and the DSC cooling and heating curves recorded at 10 C min -1 of the sample of PS-b-PMMA are reported in Fig. 1a, b respectively. The BCP sample is amorphous with glass transition temperatures of the PS and PMMA blocks of 108 and 126 C, respectively. The bright field TEM image of thin film of PS-b-PMMA deposited on a quartz/ito substrate by spin coating from a toluene solution of the BCP, annealed at temperature above the glass transition temperatures (150 C) in vacuum (10-3 mbar) for 6 h and stained with RuO 4 is shown in Fig. 2. The dark regions correspond to the stained PS lamellar microdomains while the lighter regions are the PMMA domains. The images exhibit a striped surface structure that can be interpreted as the presence of PS and PMMA lamellae oriented with the lamellar surface

6 perpendicular to the film substrate. For this film an average lamellar spacing of 13.2 nm for PS and 18.8 nm for PMMA has been evaluated (Fig. 2). In a previous work [12] we have studied the influence of substrate on BCP morphology and we have established that ITO substrate allows obtaining a perpendicular orientation of lamellae thanks to the neutral surface with non-preferential interactions with the PS and PMMA domains. This affords a fine control of the surface behavior of the block copolymer and of the relative surface affinity of two components, which is very useful for the design of a particular nanostructure that will be used as host for selectively sequestering inorganic metallic nanoparticles or organic functional molecules. Conversely, thin films of PS-b-PMMA deposited on glass substrate are characterized by lamellar microdomains oriented with their surface parallel to the substrate [12]. This orientation is probably induced by the non-neutral interactions of the substrate with the PS and PMMA domains and is the result of the competition between the scarce interactions occurring at bottom (polymer/glass substrate) and top interface (polymer/air). We have investigated the possibility to use the lamellar nanolayers of Fig. 2 as matrix of nanocomposites through selective inclusions of organic molecules in specific layers. In general, the selective inclusion of functional molecules and/or nanoparticles can be achieved by three general strategies: surface decoration techniques involving the preferential segregation of deposited material to favored domains on a pre-oriented BCP film surface [14, 15], in situ synthesis of surface modified nanoparticles or ex situ synthesis and co-assembly of nanoparticles and BCP [9]. In this last strategy the surface of the metal nanoparticles are generally modified to improve chemical affinity with the target microdomains of the BCP nanostructure, for instance by attaching oligomer chains to the particle surface that will favorably interact with the more affine target domains during the self-assembly process [9]. We have prepared BCP nanocomposites incorporating PCBM molecules into the lamellar nanostructure of PS-b- PMMA. BCP and PCBM were dissolved in toluene using sonication. Brown toluene solutions of concentration 5, 7, and 10 wt% of PBCM were prepared. Films of PS-b- PMMA:PCBM nanocomposites containing 5, 7, and 10 wt% of PCBM molecules were prepared by spin-coating of the corresponding solutions on quartz/ito substrates. The films were dried in vacuum (10-3 mbar), annealed at 150 C for 6 h and analyzed at TEM. The dispersion of the fullerene derivatives in the microphase-separated morphologies is controlled by the physical interactions after mixing. The TEM images of the PS-b- PMMA:PCBM nanocomposites with 5 and 10 % of PCBM, stained with RuO 4 are shown in Fig. 3a, b, respectively. Fig. 3 TEM bright-field images of thin films of PS-b-PMMA:PCBM nanocomposite with 5 wt% (a) and 10 wt% (b) of PCBM obtained by spin coating at room temperature from toluene solutions on a ITO substrate, stained with RuO 4 and submitted to thermal annealing at 150 C for 6 h The dark regions correspond to the stained PS lamellar domains containing PCBM, assuming that also the benzene ring of the PCBM molecules are marked with RuO 4. The presence of light regions, corresponding to PMMA regions indicates that the PCBM molecules are selectively included only in the PS domains. The self-assembly of the BCP generate the nanotemplate to selectively control the spatial location of the PCBM molecules in PS block, where the apolar properties of PS block provide the physical stabilization for preventing the self-aggregation of PCBM molecules. However, the presence of PCBM transforms the lamellar nanostructure of neat PS-b-PMMA (Fig. 2) into a more anomalous lamellar morphology. This is more evident for the sample of Fig. 3b with higher content of PCBM. Thin film of the BCP without PCBM exhibits clear lamellar nanodomains with short range order in the parallel arrangement of PS and PMMA lamellae and with a measurable lamellar spacing (Fig. 2). After inclusion of PCBM molecules into PS, the domains are larger and rather random, and for high PCBM concentration, the lamellar shape of the nanodomains is almost lost (Fig. 3b). Inclusions of

7 Optical Density 0,2 0,1 a b c d (10 6 m -1 ) 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,0 e Wavelength (nm) 0,5 0, wt% of PCBM Fig. 4 Optical absorption spectra of spin-coated thin films of pure PCBM (a), PS-b-PMMA:PCBM nanocomposites with 10 wt% (b), 7 wt% (c) and 5 wt% (d) of PCBM, and pure PS-b-PMMA (e) Fig. 5 Values of absorption coefficients a, determined from the measured optical densities and thicknesses of PS-b-PMMA:PCBM films, as a function of PCBM concentration Table 1 Optical density (OD), linear absorption coefficient (a) determined at 328 nm of thin films of BCP/PCBM nanocomposites of thickness L Sample (wt% PCBM) L (nm) OD (328 nm) a (10 6 m -1 ) PCBM molecules produces enlargement of PS domains with deformation of their initial lamellar shape. We have performed different indirect tests to further prove the selective inclusions of PCBM molecules into PS domains. Firstly, we carried out spectrophotometric measurements in order to determine the fraction of incident light reflected F R and transmitted F T by the samples. These values were used to determine the optical density OD. In the case of thin layers of solid materials, the OD is related to the linear absorption coefficient a by: OD ln 10 a ¼ ¼ ec ln 10 L where e is the extinction coefficient of the absorbing entity, c is its molar concentration and L is the thickness of the sample. The optical absorption spectra of thin films of PSb-PMMA:PCBM with 10, 7 and 5 wt% of PCBM and pure PS-b-PMMA and PCBM are reported in Fig. 4. The peak around k = 328 nm is attributed to PCBM absorption [16]. The optical densities at this particular wavelength were used to determine the absorption coefficients of each composition at 328 nm. The values of optical densities and linear absorption coefficient a determined at 328 nm are given for different films of BCP:PCBM nanocomposites in Table 1. We obtained a good linear dependence of a on PCBM concentration as shown in Fig. 5, that proves the existence of PCBM in the thin films [17]. The presence of PCBM in PS microdomains was also demonstrated by TEM analysis. As discussed above, RuO 4 is a selective reagent for double aromatic bonds of PS and we have supposed that it is also selective for double aromatic bonds of PCBM, especially for bonds of phenyl ring. We have demonstrated that PCBM molecules are marked with RuO 4 by analysis of TEM images of thin films of nanocomposites based on PMMA homopolymer and PCBM. Thin films of PMMA/PCBM nanocomposites have been prepared by spin coating of toluene solutions in the same conditions used for the preparation of PS-b-PMMA/ PCBM nanocomposites. The TEM images of this PMMA/ PCBM nanocomposite before and after staining with RuO 4 are reported in Fig. 6a, b, respectively. No contrast is visible in the image without staining of Fig. 6a, whereas the visible presence of dark spots after the staining (Fig. 6b), clearly demonstrates that RuO 4 is a selective reagent for double aromatic bonds of PCBM. The dark spots corresponds, indeed, to the stained PCBM molecules dispersed into the light PMMA homopolymer matrix. The electrical characteristics of these nanocomposites were evaluated by current/voltage (I V) dark measurements. The electrical behaviors were tested on ITO/PS-b- PMMA:PCBM/Al sandwiched architecture. Aluminium top electrode was deposited by evaporation and successive condensation onto the polymer nanocomposite surface under high vacuum (10-7 mbar). The electrical properties are reported in Fig. 7. The I V measurements show that the current density increases as the content of PCBM increases up to 5 wt% (with respect to the BCP weight), and decreases for PCBM concentration of 10 wt%. The

8 3 Conclusions Fig. 6 TEM bright-field images of thin films of PMMA/PCBM composite with 5 wt% of PCBM without staining (a) and after staining with RuO 4 (b) In this paper we have studied the morphology, the properties and device characteristics of nanocomposites based on the use of block copolymers nanostructures as matrix for the selective infiltration of organic molecules. A symmetric PS-b-PMMA block copolymers was employed with the aim to obtain a self-assembled lamellar nanostructure, characterized by alternating PS and PMMA lamellae. The ideal morphology characterized by PS and PMMA lamellae oriented with the lamellar surfaces perpendicular to the polymer film surface was obtained by employing a ITO substrate on which thin films of the BCP were deposited by spin coating. Starting from this nanostructure we have prepared nanocomposites by selective incorporation of PCBM molecules into the PS lamellar domain. This nanocomposite system served as an excellent archetype; the self-assembly of BCP generated the nanotemplate to selectively control the spatial location of the PCBM molecules in PS block, where the apolar properties of PS block provided the physical stabilization for preventing the selfaggregation of PCBM molecules. The prepared thin films of these nanocomposites containing PCBM molecules dispersed in PS lamellar domains have shown promising I V properties. These results confirms that strategies based on the use of block copolymer nanostructures may be very promising in optimizing optoelectronic device performance by controlling the morphology of the active layer. Different devices can be realized depending on the physical properties of the nanostructured host materials and nanospecific characteristics of the sequestered components. References Fig. 7 I-V dark characteristics of ITO/PS-b-PMMA:PCBM/Al devices with (a) 0,(b) 5 and (c) 10 wt% of PCBM. The peak around 4 V for the sample with 5 wt% of PCBM is due to a temporary electrical instability decrease of conductivity with increasing of PCBM content is probably due to the partial breaking of the phase-separated morphology (Fig. 3b) associated with tendency of PCBM to self-aggregate. 1. Bao Z, Locklin, J (2007) Taylor and Francis Group LLC 2. Ling M-M, Bao Z-N (2004) Chem Mater 16: Yu G, Gao J, Hummelen J-C, Wudl F, Heeger A-J (1995) Science 270: Greenham N-C, Moratti S-C, Bradley D-D-C, Friend R-H, Holmes A-B (1993) Nature 365: Su W-P, Schrieffer J-R, Heeger A-J (1980) Phys Rev B 22(4): Hoegl H-J (1965) Phys Chem 69(3): Bates F-S, Fredrickson G-H (1990) Annu Rev Phys Chem 41: Leibler L (1980) Macromolecules 13: Bockstaller M, Mickiewicz R-A, Thomas E-L (2005) Adv Mater 17: Gratt J-A (1999) Ph.D. Thesis, Massachusetts Institute of Technology 11. De Boer B, Stalmach U, Van Hutten P-F, Melzer C, Krasnikov V-V, Hadziioannou G (2001) Polymer 42: Diletto C, Morvillo P, Di Girolamo R, Auriemma F, De Rosa C (2013) J Nanosci Nanotechnol 13(7):

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