Enhancement of Diblock Copolymer Ordering Kinetics by Supercritical Carbon Dioxide Annealing

The morphology and periodicity of microphase-separated diblock copolymers can be precisely controlled by manipulating the chemical composition and molecular weights of the segments, respectively.1,2 Numerous applications that exploit copolymer morphology as scaffolds and templates for nanostructures with periodicities from 1 to 50 nm are emerging that make use of the unique chemical structure of each block and the ability to carry out chemistry within one domain selectively. For example, Park et al. have used copolymer morphologies as templates for pattern transfer to a substrate to generate nanodots.3 Thurn-Albrecht et al. have produced terabit arrays of nanowires in thin films using block copolymer templates.4 Stucky and co-workers have synthesized porous silica by the cooperative assembly and condensation of tetraethyl orthosilicate with nonionic block copolymer surfactants.5 Hillmyer and coworkers selectively incorporated and cross-linked an epoxy resin within the poly(ethylene oxide) (PEO) domains of a PEO-b-poly(ethylethylene) copolymer.6 Templated metal nanoclusters have been prepared by Cohen and co-workers by in situ reaction within thin block copolymer microdomains with selectively bound organometallic compounds.7-9 Recently, using supercritical carbon dioxide, Brown et al. have selectively deposited metals and semiconductors within one domain of a block copolymer template.10 Since the domain spacing in block copolymers (D) scales with the number-average degree of polymerization (N) in a known manner (D ∼ Nν where ν is /3 for lamellar morphologies in the strong segregation limit), it is easy, in principle, to design copolymer systems that exhibit microdomain spacings at any desired length scale. Xu et al. have used this approach to generate nanoporous films where the pore size was dictated by the copolymer molecular weight.11 Of particular interest are periods on the order of λ/4n, where λ is the wavelength of light and n is the index of refraction of the material. At this length scale, block copolymers can serve as templates for photonic devices.12,13 The simplest structure for photonic applications is an alternating two-component multilayer film. For low molecular weight copolymers, well-ordered lamellar structures can be obtained by simply annealing symmetric diblock copolymer films above the glass transition temperatures of both blocks. For thin films, Russell and co-workers have shown that the interactions of the blocks with the air and substrate interfaces induce near perfect orientation of the microdomains parallel to the substrate surface.14 Templates for photonic devices require periods on the order of ∼100 nm, which, in the case of a diblock copolymer of polystyrene and poly(methyl methacrylate) [P(S-b-MMA)], requires molecular weights in excess of 3 × 105 g/mol. Unfortunately, attempts have not been successful in preparing well-ordered and oriented structures from copolymers with high molecular weights by thermal annealing. The high degree of entanglement coupled with the thermodynamic barrier of diffusing one polymer block through a domain of second poses severe kinetic barriers to ordering.15-17 Consequently, alternative strategies must be used to achieve large D spacings. One route is to increase the domain size by the addition of low molecular weight homopolymers that swell each domain and enhance mobility. However, this approach has the unintended consequence of exacerbating defects by the accumulation of the homopolymer at these sites.18 Alternatively, a transient plasticizing agent can be used that enhances the mobility to allow ordering but can subsequently be removed easily. Supercritical CO2 is ideal for this purpose.19,20 It is a poor solvent for most polymers: equilibrium sorption is modest and can be controlled by pressure-mediated adjustments in fluid density.21 For example, sorption of CO2 in PS ranges from 1.3 wt % at 5 bar to 10.2 wt % at 72 bar at 35 °C21 and ranges from 0.9 wt % at 25 bar to 6.5 wt % at 174 bar at 180 °C.22 Moreover, CO2 diffusion in most systems is rapid, so the diluent reaches an equilibrium distribution rapidly in thin polymer films. Finally, upon controlled depressurization that prevents foaming, the solvent completely desorbs from the film without damaging the block copolymer morphology. Here, we show the preparation of ordered, high molecular weight copolymer thin films using CO2 as the diluent at elevated pressure. Identical films could not be ordered by thermal means alone, regardless of the temperature or length of the annealing period up to 240 h. Experimental Section. P(d-S-b-MMA), with a total molecular weight of 301 000 g/mol (denoted 301K) with a polydispersity of 1.08 and P(S-b-MMA) with molecular weight of 34 500 g/mol (34.5K) with volume fractions of PS close to 0.5, was purchased from Polymer Laboratories. These copolymers exhibit a lamellar morphology in the ordered state. Polymer thin films were spincoated from 4 wt % solutions in toluene onto polished silicon substrates to yield a film thickness of approximately 0.3 μm. Samples were either annealed in a vacuum oven at 175 °C or in the presence of CO2 (at a density of around 0.5 g/cm3) at 175 °C and 346 bar in a high-pressure vessel.23 CO2 was supplied using a computer-controlled ISCO high-pressure pump. Samples were analyzed using an Olympus BX60 optical reflection microscope (ORM), atomic force microscopy (AFM) was performed using a Digital Instruments Dimension 3000 scanning probe microscope, and X-ray photoelectron spectroscopy (XPS) was performed using a Perkin-Elmer Physical Electronics 5100 spectrometer with a Mg KR source. Results and Discussion. Ordering of thin diblock copolymer films at ambient pressure is enhanced by † Department of Chemical Engineering. ‡ Department of Polymer Science and Engineering. * Corresponding authors. E-mail watkins@ecs.umass.edu or russell@mail.pse.umass.edu. 7923 Macromolecules 2001, 34, 7923-7925