The influence of confinement and curvature on the morphology of block copolymers

Abstract

In the bulk, at equilibrium, diblock copolymers microphase separated into nanoscopic morphologies ranging from body-centered cubic arrays of spheres to hexagonally packed cylinders to alternating lamellae, depending on the volume fraction of the components. However, when the block copolymers are forced into cylindrical pores, where the diameter of the pores are only several repeat periods of the copolymer morphology or less, then commensurability of the copolymer period and the pore diameter can impose a frustration on the microdomain morphology. In addition, due to the small pore diameter, a curvature is forced on the microdomain morphology. In combination with interfacial interactions between the blocks of the copolymer and the pore walls, the preferential segregation of one component to the walls, spatial confinement and forced curvature are shown to induce transitions in the fundamental morphology of the copolymers seen in the bulk. Lamellar morphologies transformed into torus-type morphologies, cylinders are forced into helices, and body-centered cubic arrays of spheres are force into helical arrays of spheres due to these restraints. The novel morphologies, not accesssible in the bulk, open a large array of nanoscopic structures that can be used as templates and scaffolds for the fabrication of inorganic nanostructured materials.

Gyroid-Forming Diblock Copolymers Confined in Cylindrical Geometry: A Case of Extreme Makeover for Domain Morphology

“…When the pore diameter is reduced to a size comparable to the bulk period (D/L 0 =0.92~2.2), a rich variety of morphologies, not seen in the bulk, were observed that included stacked torus-like morphologies and single-, double-, and triple-helical morphologies [10,11]. Similarly, for the sphere-forming PS-b-PB, one observes that large pores (D/L 0 > 3.2) lead to spherical PB domains aligned along the pore axis, whereas in pores with smaller diameters (D/L 0 < 3.2), core-shell cylindrical morphologies, single columns of spherical microdomains, and spirals of doubly and triply paired spherical microdomains are observed [9,10]. Helical and stacked toroidal structures have also been observed by Wang and coworkers in the study of asymmetric poly(styrene-b-2-vinylpyridine) DBCPs confined in anodic aluminum oxide nanopores [12].…”

“…[6] Various novel and sometimes complex structures have been observed or predicted for block copolymers confined in cylindrical pores. Experimentally, Russell and coworkers carried out a series of studies on symmetric (bulk lamella-forming) [7][8][9][10] and asymmetric (bulk cylinder-and sphere-forming) [8][9][10][11] poly(styrene-b-butadiene) (PS-b-PB) DBCPs confined in cylindrical nanopores in alumina membranes. For the lamellaforming symmetric PS-b-PB, a set of curved, cylindrical lamellae parallel to the pore axis (called "concentric lamellae", CL) were observed when the pore diameter D is much larger than the bulk equilibrium period L 0 .…”

Self-assembly of semicrystalline PE-b-PS diblock copolymers within AAO nanoporous templates

“…For the infiltrated samples, the total mass (block copolymer plus AAO template) was 25 mg, while the block copolymer mass was approximately 0.5 mg according to TGA experiments. The results were compared with those obtained with PE-b-PS neat samples at the same scanning rates and employing a mass of 2 mg. 40 60 Figure 5: DSC cooling scans at 30 ºC/min for the indicated samples before and after infiltration within AAO 60 nm templates.…”

Electrospun polymer nanofibers: Mechanical and thermodynamic perspectives

“…15,16 Similarly, thickness and surface interactions of ultrathin polymer films (the film thickness is in the order of 2R g of a polymer chain, or less) highly influence their glass transition and melting temperatures, [17][18][19] polymer dynamics in the glassy state, 20 crystallization kinetics and degree of crystallinity, [21][22][23] phase behavior, 24 and morphology. 25,26 For all the above examples, the portion of ''surface material'' is comparable with the bulk, due to extremely small object size, therefore, the ansatz claiming ''nano-object ¼ bulk þ surface'' seems relevant for such systems from a physical point of view. However, in certain experimental systems, 27,28 the size-dependent behavior of nanofibers bearing considerably higher transversal size ($500 nm) refute the size-dependence ansatz and require additional expressions to explain their behavior.…”