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.
The synthesis, molecular and morphological characterization of a 3-miktoarm star terpolymer of polystyrene (PS, M¯n = 61.0 kg/mol), polybutadiene (PB, M¯n = 38.2 kg/mol) and polyisoprene (PI, M¯n = 29.2 kg/mol), corresponding to volume fractions (φ) of 0.46, 0.31 and 0.23 respectively, was studied. The major difference of the present material from previous ABC miktoarm stars (which is a star architecture bearing three different segments, all connected to a single junction point) with the same block components is the high 3,4-microstructure (55%) of the PI chains. The interaction parameter and the degree of polymerization of the two polydienes is sufficiently positive to create a three-phase microdomain structure as evidenced by differential scanning calorimetry and transmission electron microscopy (TEM). These results in combination with small-angle X-ray scattering (SAXS) and birefringence experiments suggest a cubic tricontinuous network structure, based on the I4132 space group never reported previously for such an architecture.
Novel miktoarm star copolymers of polystyrene[poly(dimethylsiloxane) n ] or PS(PDMS) n (n = 2 or 3) type as well as of the inversed sequence, namely, (polystyrene) n [poly(dimethylsiloxane)] or (PS) n PDMS (n = 2 or 3), were synthesized by combining living anionic polymerization with chlorosilane chemistry. The miktoarm star copolymers were extensively characterized through size exclusion chromatography, vapor pressure/membrane osmometry, proton nuclear magnetic resonance, and differential scanning calorimetry, in order to verify the successful synthesis. All samples with varying volume fractions and narrow dispersity indices (D̵ < 1.1) were morphologically characterized by transmission electron microscopy and small-angle X-ray scattering, in order to study their self-assembly behavior as well as to examine the effect of the complex architecture on the final adopted morphologies. For specific PS(PDMS) n (n = 2 or 3), morphologies different from those expected from theoretical predictions (self-consistent field theory or Gaussian statistics) were obtained, while for the inversed sequences, namely, (PS) n PDMS (n = 2 or 3), no discrepancies were evident. This fact further confirmed the impact of the number of arms as well as the flexibility of the segments (PS being stiffer than PDMS) on the structure/property relationship.
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.
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