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Macromolecules 2009, 42, 3138-3146
Ordered Microphase Separation in Thin Films of PMMA-PBA Synthesized by RAFT: Effect of Block Polydispersity Wilasinee Sriprom,† Michael James,‡,§ Se´bastien Perrier,† and Chiara Neto*,† School of Chemistry and Key Centre for Polymers & Colloids, The UniVersity of Sydney, NSW 2006, Australia; School of Chemistry, The UniVersity of Sydney, NSW 2006, Australia; and Bragg Institute, Australian Nuclear Science and Technology Organisation (ANSTO), PMB 1, Menai, NSW 2234, Australia ReceiVed NoVember 24, 2008; ReVised Manuscript ReceiVed March 5, 2009
ABSTRACT: The microphase separation of diblock copolymers synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, containing one monodisperse block (poly(methyl methacrylate), PMMA) and one polydisperse block (poly(butyl acrylate), PBA), was investigated in thin films ( 0.7. The shift of phase boundaries resulting in breaking of the phase diagram symmetry was previously observed in simulations. The second interesting effect of polydispersity is that a larger range of film thicknesses can accommodate a flat film without displaying hole/island morphologies. This means again that the system comprising chains of variable length is more flexible to adapt to certain morphologies, especially in the case of the lamellar morphology. This was observed in particular for PMMA77-PBA23 and PMMA69-PBA31, in which thin films are flat and uniform and microphase separate in complete parallel lamellae for thickness that are t ) (nL0 + b) ( xL0 nm, where x can be as high as 0.4-0.5. This observation seems to be consistent with the simulation work by Burger et al. which observed an increase in the area of the stability regions on the phase diagram for copolymers with increased PDI.31 Also, this observation is compatible with the general impression in simulations that polydispersed copolymers have larger domain spacing as compared to monodisperse systems. The mechanism of the increase of domain spacing is based on the existence of different chain lengths in a system leading to a decrease of the stretching energy.51,77-79 The presence of a large distribution of chains within the same system seems to allow for greater adaptability and flexibility of polydisperse chains to morphologies which would be metastable or unstable in a monodisperse systems. Our third important observation is the stabilization of the hexagonally perforated lamella morphology, which is assumed to be metastable in monodisperse copolymers. This result is in agreement with recent simulation work by Matsen,36 who predicted the formation of a mixed lamellar and cylindrical region due to increased PDI, and work by Listak et al., who observed the formation of an hexagonally perforated lamella on thick solvent cast films.75 However, in our thin films, the perforated lamellae occur only in certain thickness regimes and for a composition where the polydisperse block is the majority block (fPBA ) 0.6). As previously suggested, the polydispersity mitigates the chain packing frustration and stabilizes structures with high standard deviation of mean curvature, such as the perforated lamella. By studying thin films of PMMA-PBA block copolymers, we were able to investigate the interplay between polydispersity and wetting effects. This interplay was particularly evident in copolymers with fPBA ) 0.23, 0.31, and 0.6. At fPBA ) 0.60, as depicted in Figure 7F, the perforated lamellae are only stable in a specific thickness regime. At fPBA ) 0.60, as the film thickness increases, we observe two transitions: the first between parallel cylinders and perforated lamellae and the second back to parallel cylinders. These transitions are likely to be caused by the preference of the system for the structure that presents
Thin Films of PMMA-PBA 3145
the least degree of frustration of its characteristic period. As previously observed for thin films of copolymers,22,27 the frustration of the lamellar period is avoided by changing the orientation of the lamella from parallel to perpendicular to the surface or better by forming a combination of the two morphologies, such as in the perforated lamella. This transition is particularly likely in a system, such as ours, where the two substrate/film and film/air interfaces are almost nonselective toward the two blocks, given their chemical similarity. Our study confirms that simple synthetic techniques such as RAFT polymerization have the potential to be valid alternatives to more complicated polymerization techniques and can produce block copolymers of well-controlled architecture, polydispersity, and composition. For the first time thin films of RAFTpolymerized block copolymers are shown to be ideal model systems to confirm existing and future simulation results on the interplay between interfacial wetting, microphase separation, and polydispersity in confined geometries.31,36,72,77 Contrary to what observed for block copolymers synthesized by nitroxidemediated polymerization,51 block copolymers made by RAFT with one polydisperse block and with low degree of incompatibility χN microphase separate with high reproducibility and with excellent degree of long-range order. Block copolymers synthesized by RAFT are therefore demonstrated to be valid candidates in numerous advanced materials applications that rely on the formations of ordered patterns, such as suboptical largescale patterns, photonic materials, and high-density magnetic recording devices. Conclusions We studied the phase separation of PMMA-PBA block copolymers synthesized by RAFT polymerization, containing one block with PDI ) 1.3-1.4, and spin-cast in films thinner than 100 nm. Thermal characterization of the bulk samples studied by DSC and TGA highlighted the presence of two glass transition temperatures (Tg), which suggests the occurrence of microphase separation in bulk, and showed that the block copolymers are not degraded at a temperature of 180 °C. Annealing of the thin films at 180 °C induced microphase separation of the blocks. As volume fraction fPBA increases from 0.23 to 0.60, the microphase separation changes from lamellar to cylindrical domains and then to spherical at a volume fraction of 0.79. Remarkably, at a volume fraction fPBA of 0.23-0.31 we observe lamellae parallel to the substrate, as predicted in simulations for polydisperse systems. At a PBA volume fraction of 0.6, domain transitions from parallel cylinders to perforated lamellae to parallel cylinders were observed with increasing film thickness. A wetting layer of a half-lamella PMMA-PBA brush is present underneath all films due to the preference of the PMMA block for the substrate. It was shown that RAFT polymerization produces block copolymers that are suitable for controlled and reproducible studies of microphase separation. These blocks, with a low degree of incompatibility χN, microphase separate into regularly ordered domains and confirm the trends expected from simulations on similar polydisperse block copolymers. The main difference in domain formation with respect to monodisperse systems is the shift of the domain boundaries to more asymmetric volume fractions, the ability to accommodate domains in layers different from exact multiples of a layer period, and the stabilization of hexagonally perforated lamellae. Supporting Information Available: Neutron reflectivity data. This material is available free of charge via the Internet at http:// pubs.acs.org.
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References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40)
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