A series of cubic network phases was obtained from the self-assembly of a single-composition lamellae (L)-forming block copolymer (BCP) polystyrene-block-polydimethylsiloxane (PS-b-PDMS) through solution casting using a PS-selective solvent. An unusual network phase in diblock copolymers, double-primitive phase (DP) with space group of Im3¯m, can be observed. With the reduction of solvent evaporation rate for solution casting, a double-diamond phase (DD) with space group of Pn3¯m can be formed. By taking advantage of thermal annealing, order–order transitions from the DP and DD phases to a double-gyroid phase (DG) with space group of Ia3¯d can be identified. The order–order transitions from DP (hexapod network) to DD (tetrapod network), and finally to DG (trigonal planar network) are attributed to the reduction of the degree of packing frustration within the junction (node), different from the predicted Bonnet transformation from DD to DG, and finally to DP based on enthalpic consideration only. This discovery suggests a new methodology to acquire various network phases from a simple diblock system by kinetically controlling self-assembling process.
Herein, we aim to examine the topological effects of block copolymer (BCP) architecture on the self-assembly of lamellae-forming star-BCPs composed of polystyrene (PS) and poly(dimethylsiloxane) (PDMS) blocks with equivalent arm length and therefore almost identical volume fraction. An interesting wet-brush-like lamellar phase with an interdigitating structure for chain packing was found in the solution-cast (PS-b-PDMS) n (n = 1, 3, or 4) samples regardless of the value of n which corresponds to the number of PS-b-PDMS arms attached. While the temperature is gradually increasing, an order−order transition from the interdigitating structure to bilayers in the self-assembled lamellar phase can be observed in the bulk state, exhibiting approximately 50% increase on d-spacing. These results implicitly indicate that it is possible to acquire the smaller spacing of microphase-separated lamellae from casting. Also, as examined by in situ temperature-resolved small-angle X-ray scattering, transformation occurs once the temperature is over the glass transition of PS and the formation of stable lamellae with bilayers is able to be expedited by increasing the arm number because of the low degree of formation of wet brushes benefited by the topological effects. Moreover, an interesting transition was found in which the forming interdigitating chain packing can be restructured after cooling down from the stable lamellae, while the thermal treatment is not able to completely disentangle the polymer chains. Such an observation is an additional evidence for the suggested mechanism and corresponding kinetics for the formation of lamellar phases with such a large variation on d-spacing. This discovery provides an insight for the transformation mechanisms of the self-assembly of BCPs; it indicates the strong dependence of the self-assembling process on the topological effects from star-block architecture, making these materials valuable for the engineering of nanostructured BCPs with temperatureresponsive d-spacing variation.
Flexible and shape-tunable features of block copolymers (BCPs) with high Flory–Huggins interaction parameters (high χ value) have drawn intensive attention due to their rich phase behaviors. Herein, this work aims to examine a fascinating superlattice structure obtained from the self-assembly of high-χ BCP, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), as evidenced by reciprocal-space imaging from small-angle X-ray scattering (SAXS) and by real-space imaging from transmission electron microscopy (TEM). Surprisingly, an interesting reversible order–order transition from superlattice structure with chain interdigitation to typical lamellae with bilayer texture can be identified by in situ temperature-resolved SAXS. In contrast to the diblock (PS-b-PDMS) n (n = 1), the forming superlattice structure will be greatly impeded in star-block (PS-b-PDMS) n (n = 3 and 4) with equivalent arm length, suggesting a topological effect on self-assembly due to their star-shaped architecture. Accordingly, a lamellae-forming PS-b-PDMS with chain interdigitation (wet-brush-like chain packing) was proposed to be the origin of the forming superlattice structure. This finding provides an insight for the possible model with ladder-like structure and corresponding transformation mechanisms of high-χ BCPs. Also, the topological effect from star-block architecture may play an important role to justify the formation of such a unique self-assembled texture. These results implicitly explore the feasibility to acquire a superlattice structure from a simple coil–coil diblock copolymer.
The synthesis of two (2) novel triblock terpolymers of the ABC type and one (1) of the BAC type, where A, B and C are chemically different segments, such as polystyrene (PS), poly(butadiene) (PB1,4) and poly(dimethylsiloxane) (PDMS), is reported; moreover, their corresponding molecular and bulk characterizations were performed. Very low dimensions are evident from the characterization in bulk from transmission electron microscopy studies, verified by small-angle X-ray data, since sub-16 nm domains are evident in all three cases. The self-assembly results justify the assumptions that the high Flory–Huggins parameter, χ, even in low molecular weights, leads to significantly well-ordered structures, despite the complexity of the systems studied. Furthermore, it is the first time that a structure/properties relationship was studied for such systems in bulk, potentially leading to prominent applications in nanotechnology and nanopatterning, for as low as sub-10 nm thin-film manipulations.
This work aims to demonstrate a facile method for the controlled orientation of nanostructures of block copolymer (BCP) thin films. A simple diblock copolymer system, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), is chosen to demonstrate vacuum-driven orientation for solving the notorious low-surface-energy problem of silicon-based BCP nanopatterning. By taking advantage of the pressure dependence of the surface tension of polymeric materials, a neutral air surface for the PS-b-PDMS thin film can be formed under a high vacuum degree (∼10–4 Pa), allowing the formation of the film-spanning perpendicular cylinders and lamellae upon thermal annealing. In contrast to perpendicular lamellae, a long-range lateral order for forming perpendicular cylinders can be efficiently achieved through the self-alignment mechanism for induced ordering from the top and bottom of the free-standing thin film.
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