Nanophase separation and structural evolution of block copolymer films: A “green” and “clean” supercritical fluid approach

Abstract

Type of publicationArticle (peer-reviewed)Link to publisher's version http://dx.doi.org/10.1007/s12274-014-0616-7Access to the full text of the published version may require a subscription. Rights © Tsinghua University Press and Springer-Verlag Berlin Heidelberg2014. This is a pre-print of an article published in Nano Research. hexagonally ordered films were demonstrated using a supercritical fluid process at low temperatures and pressures. Predominant swelling of PEO domain in scCO2 induces nanophase separation. scCO2 serves as green alternative to the conventional organic solvents for the phase segregation of BCPs with complete elimination of any residual solvent in the patterned film.The depressurization rate of scCO2 following annealing was found to affect the morphology of the films. The supercritical annealing conditions could be used to define nanoporous analogues of the microphase separated films without additional processing providing a one-step route to membrane like structures without affecting the ordered surface phase segregated structure. An understanding of the BCP self-assembly mechanism can be realized in terms of the deviation in glass transition temperature, melting point, viscosity, interaction parameter and volume fraction of the constituent blocks in the scCO2 environment.

Structural Evolution of Nanophase Separated Block Copolymer Patterns in Supercritical CO2

“…The increased disparity between the CO 2 –PS and CO 2 –PEO solubility and interfacial energies at high temperatures creates larger effective interaction parameter ( χ eff ) irrespective of the film thickness, eases the process of phase separation. Previous reports suggests that the temperatures below 50 °C are more suitable to achieve phase separation under constant CO 2 pressure conditions, consistent with our finding [ 27 , 32 ]. At a temperature of 60 °C, which is just above the melting temperature of the PEO block (55 °C) for the S3 molecular weight system, PEO is in a semi-molten state which facilitates the reorientation of the PEO cylinders and the morphological change [ 35 ].…”

“…Vertical orientation represents least entropically hindered route for microphase separation, since the PEO cylinder length is limited to the thickness of the films. Previous studies have shown that the swelling of PEO in scCO 2 is between 20–25 wt%, larger than that of PS (5–6 wt%) at similar pressure of 1400 psi [ 27 , 31 , 32 , 33 ]. Thus, the swelling of the entire film is dominated by the swelling of the PEO microdomains compared to the PS block.…”

“…Microphase separation of polystyrene-b-polyethylene oxide (PS- b -PEO) BCP films into nanostructured arrangement by annealing in scCO 2 at a relatively low temperature was achieved by our group [ 27 ]. The effects of annealing temperature and depressurisation rates for one of the BCP systems were discussed previously.…”

Defect-Induced Order–Order Phase Transition in Triblock Copolymer Thin Films

“…A number of techniques, including as chemo- and grapho-epitaxy, shear flow, and electric and geometric fields have been employed to produce patterns with well-defined orientational and positional order in block copolymer thin films with different architectures. ,− Due to its simplicity and low cost, solvent annealing techniques are one of the most promising approaches to control long-range order and symmetry of block copolymer patterns. In particular, previously, we found that annealing under supercritical carbon dioxide (scCO 2 ) speeds up the mechanisms of coarsening and stabilizes hexagonal patterns with a degree of order, which is unreachable with conventional thermal annealing techniques. , Considering its low cost, wide availability, moderate critical conditions (critical temperature T C = 31 °C, critical pressure P C = 73.8 bar, and critical density ρ C = 0.468 g/cm 3 ), gas-like diffusivity, and low interfacial tension with the polymers, supercritical carbon dioxide, scCO 2 , is increasingly being used as a green solvent in different fields, including biomedical applications, polymer processing, and polymer synthesis. − …”

Order–Order Phase Transitions Induced by Supercritical Carbon Dioxide in Triblock Copolymer Thin Films

“…In general, solvent annealing techniques employ organic or halogenated solvents, even though they are usually toxic and have a negative environmental impact . As an alternative, supercritical carbon dioxide (scCO 2 ) is increasingly being used as a green solvent in different fields, including biomedical applications, polymer processing, and polymer synthesis. − In addition to its low cost, wide availability, and to the moderate critical conditions (critical temperature T C = 31 °C, critical pressure P C = 73.8 bar and critical density ρ C = 0.468 g/cm 3 ) CO 2 also offers a number of further advantages: (i) it is a poor solvent for most polymers and although its equilibrium sorption is modest, it can be controlled by pressure; (ii) it has a gas-like diffusivity, therefore the diluent rapidly reaches an equilibrium state in thin polymer films; and (iii) it has low interfacial tension with the polymers, and the gas can be eliminated completely after the termination of the process. − …”