The interaction of polymers with supercritical fluids (SCFs) offers unique opportunities for preparing advanced materials and markedly enhancing the efficiency of polymer processing strategies that are limited by mass transfer control. The behavior of these polymer/SCF systems, the dynamics of associated processes, and the properties of materials prepared in this environment are linked directly to mobility, structure development, phase behavior and interfacial interaction. Using a suite of experimental methods, including neutron reflectivity (NR), small angle neutron scattering (SANS) and fluorescence spectroscopy under high-pressure conditions, this IRG will fill a critical gap in the characterization of heterogeneous polymer/SCF systems. These include measurement of chain dynamics, reaction kinetics in melts and at polymer/polymer interfaces, and block copolymer ordering in SCF-dilated films.
The properties of SCFs will also be used to probe issues of broad relevance to polymer science. These include a new closed-loop phase behavior discovered in block copolymers and exploration of the region of validity for the mean field dilution approximation. These fundamental studies will complement the second objective of the IRG: the preparation of functional, structured materials in supercritical CO2, including the application of CO2-dilated block copolymers as platform materials for photonic assemblies and oriented mesoporous silica films. Here, highly ordered and oriented copolymer templates are first prepared in an SCF-mediated environment, then patterned lithographically. One or more of the domains are then modified, without disrupting long-rang order, via phase selective chemical reactions in the dilated template. The versatility and capabilities of this approach are intimately tied to the dynamics and phase behavior of SCF-dilated systems. Complementary expertise to define fully the fundamental behaviors that characterize polymer/SCF systems and to design, synthesize and characterize the functional materials prepared by exploiting this environment will be provided through interactions with Junhan Cho (HOMRC), Keith Johnston (UT-Austin), Jin Kon Kim (Pohang, Korea), Jimmy Mays (U. Tennessee) and Gregory Smith and William Tumas (Los Alamos National Laboratory).
Fundamentals
Enhanced Macroscopic Ordering
The high degree of structural control over block copolymer morphology has fostered intense interest in the use of these materials as templates or scaffolds for structured devices, including photonic and mesoporous materials.48-50 Practical application of block copolymers as devices or device templates is hindered by a number of factors. Well-ordered structures of high molecular weight cannot be obtained directly by thermal annealing due to kinetic barriers that arise from chain entanglement and the thermodynamic barriers to diffusion of one block through the domains of another. Moreover, as discussed in IRG-I, self-assembly is not sufficient for most applications. Rather, precise control over the orientation and ordering of microdomains is critical. In particular, for high molecular weight copolymers, the competition between microphase separation and diffusion of chains over large distances precludes achieving highly oriented, ordered structures. With few exceptions, the chemical composition of copolymers does not give rise to a desired functionality, rather, copolymers are envisioned as templates or scaffolds to direct organization of the desired components. Most applications will require selective modification of one or both domains to impart specific functionality or contrast between the phases. Template integrity must be maintained through this modification process. As illustrated throughout the IRG, SCFs provide a direct solution for each of these issues.
Watkins and Russell have recently demonstrated a unique route around barriers to ordering and orientation using supercritical CO2.51 An as-cast high molecular weight P(dS-b-MMA) symmetric diblock copolymer film (Mw=3.01x105) shows no evidence of microdomain ordering. With extensive annealing at 170˚C under vacuum, the copolymer is partially ordered, but neutron reflectivity profiles indicate no long-range ordering. Annealing the copolymer in the presence of supercritical CO2, however, produces a much higher degree of order and orientation. The enhanced ordering is seen in the optical micrographs in the figure below showing (from left to right) interference colors of as-cast, thermally-annealed, and SCF-annealed films (the width of each micrograph is 280 m).
For both as-cast and thermally annealed samples, the changes in the interference colors, arising from the film thickness, are gradual, indicative of no long-range orientation. However, in the presence of SCF CO2, discrete changes in the interference colors are seen. AFM measurements reveal that each step is 93 nm in height, which corresponds to the lamellar period. Thus, for the first time, block copolymer templates with periodicities suitable for photonic assemblies and other applications are directly accessible by self-assembly.
Penelle, Gido, and Mays will determine bulk and thin film ordering kinetics for a series of high molecular weight model copolymer systems, including block copolymers of styrene with methylmethacrylate P(S-b-MMA), isoprene P(S-b-I), butadiene P(S-b-BD), ethylene oxide P(S-b-PEO) and 2-vinylpyridine P(S-b-2-VP), where the segmental interactions vary from weak to strong. Specific modifications to these and other copolymer systems will be made to facilitate ordering by enhancing CO2 sorption and by introducing reactive handles for subsequent chemical modification. For example, CO2 sorption in the isoprene, butadiene and acrylate blocks is enhanced by fluorination. Films will be annealed in CO2 using high-pressure cells equipped with resistively heated stages capable of achieving uniform surface temperatures in excess of 250°C and lateral temperature gradients.
Lateral gradients will be applied to enhance ordering and orientation. Analysis will be performed both in situ and ex situ by means of SANS and optical birefringence for bulk specimens and NR and SANS for thin films. Both Russell and Watkins have gained substantial experience in the execution of these experiments and with Smith (Los Alamos) have tested the limits of time-resolved NR to intervals of less than a minute. Ordering kinetics will be performed on copolymers having spherical, cylindrical and lamellar microdomain morphologies where the size scales of the microdomains and the repeat period are suitable for the fabrication of optical and photonic bandgap structures (Dinsmore).
