C60 fullerene inclusions in low-molecular-weight polystyrene–poly(dimethylsiloxane) diblock copolymers
Graphical abstract
Introduction
Incorporation of fullerenes with polymers often produces dramatic improvements to the polymer properties [1], [2] including gas barrier properties [3], microhardness of films [4], [5], inhibition of thermo-oxidative degradation [6], photovoltaic efficiency in solar cell applications [7], [8], [9], [10], [11], [12], [13], and electro- and photoluminescence [14], [15]. However, in most cases, the limited solubility and miscibility of fullerenes in both solvents and polymers [16], coupled with a lack of understanding of how to control these interactions is often a severe restriction to the general utility and wider applicability of using fullerenes. Therefore a majority of studies of polymer–fullerene systems have concentrated on covalently bonding the C60 onto the polymer chains thereby avoiding the problems associated with limited solubility [17], [18], [19], [20], [21], [22], [23]. Whilst considerable synthetic capability is required to obtain C60-functionalized polymers, simple blending of unfunctionalized fullerenes, in particular C60, with homopolymers has also shown significant modification of a number of the polymer properties including photoluminescence [23], [24], heat capacity [25], thin film dewetting behavior [26], [27], permeability [28], and thermal stability [29], [30]. Functionalization of the fullerene cage, such as [6]-phenyl C61 butyric acid methyl ester (PCBM), which is widely utilized for solubilizing C60 with a range of conjugated polymers, has been extensively used for incorporating the fullerenes into macromolecular structures [31]. For a recent review of polymers containing fullerenes, see reference [2].
The incorporation of fullerene molecules into block copolymers has been much less widely reported than for metallic or ceramic nanoparticles [32], [33]. Due to their limited solubility in most polymers [34], [35], functionalization of the fullerene by grafting polymer brushes to the C60 cage can promote preferential miscibility of the fullerene into only one block of a diblock copolymer [36], [37], [38], [39]. However, this approach adds unnecessary complexity to fullerene synthesis, detracting from the general applicability of using much more widely available unmodified C60. In systems without functionalized fullerenes, hydrogen bonding and acid-base interactions have been used to enhance incorporation of fullerenes into polymers [40], [41]. For example, when C60 fullerene was added to a PS-P4VP block copolymer, a cylindrical to spherical morphology was induced [42]. In this case, the morphology change was attributed to the electron-accepting C60 forming charge-transfer complexes with electron-donating pyridine groups from different P4VP chains [10], [42], [43]. Despite these studies there remains a limited understanding of polymer–fullerene miscibility and indeed no tool for its prediction.
We are currently investigating inclusion of fullerenes in block copolymers as part of a wider effort to investigate materials for global quantum information processing (QIP) [44]. In this paper, we discuss the potential use of low-molecular-weight block copolymers as templates for ordering C60 fullerenes into cylindrical domains of the block copolymer. The use of low-molecular weight polymers are essential for the length scales of fullerene ordering required for the functionality of the QIP materials.
Section snippets
Experimental
Polystyrene-b-poly(dimethylsiloxane) (PS-PDMS) diblock copolymers were synthesized by sequential anionic polymerization of styrene and hexamethylcyclotrisiloxane (HMCTS) [45], [46], [47]. Cyclohexane and tetrahydrofuran (THF) were distilled from diphenylhexyllithium and sodium benzophenone, respectively, and styrene monomer was purified by distilling from dibutylmagnesium. Polymerizations were performed at room temperature under a nitrogen atmosphere in an Innovative Technologies glovebox (<1
Results and discussion
Two different PS-PDMS copolymers were synthesized with molecular characteristics as shown in Table 1. The molecular weights of the PS blocks of the copolymers were determined using GPC on an aliquot of PS extracted from the reaction mixtures before HMCTS was added. The molecular weight of the PDMS blocks were determined from solution NMR measurements of the molar ratio of PDMS to PS using the methyl protons for the PDMS and the aromatic protons for the PS. These NMR-determined mole fractions
Conclusions
Low-molecular-weight PS-PDMS diblock copolymers have been synthesized with total molecular weights of 9.4 and 12.1 kg/mol and weight fractions of the PS blocks of 0.17 and 0.24. Despite the low-molecular weights of these copolymers, they phase separate to give a cylindrical morphology in the bulk as determined by SAXS, with a PS cylinder repeat spacing of ∼13 nm. The morphological behavior of the pure PS-PDMS copolymers and its physical blends with increasing loadings of C60 has been
Acknowledgements
The authors are extremely grateful to Prof Tony Ryan and Dr. S. Mykhaylyk at University of Sheffield for fruitful discussions as well as use of their SAXS equipment, Dr Andrew Watt at University of Oxford for assisting in the TEM studies, and Mrs B. Gurun for WAXS measurements. We thank Sasha Myers at Princeton University for assistance in the diblock synthesis and characterization. J.H.W. thanks the Oxford-Princeton Partnership and the Princeton Center for Complex Materials, funded by the
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Present address: Laboratory of Composite and Polymer Technology, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland.