•Lozenge shape PS-b-PLLA single crystals with uniform size are prepared.•Isotropic–nematic transition occurs at low volume fraction for crystal suspensions.•The orientation of crystals in nematic phase varies with crystal size.•Ethanol can block the lateral attraction.•Composite single crystals lose the highly orientation-dependant lateral attraction.We have studied the influence of the particle size and the tunable lateral interactions on the isotropic–nematic (I–N) phase transition of a plate-like colloidal system. The particles are single crystals of a block copolymer PS-b-PLLA (BCSC) prepared using a self-seeding procedure. These lozenge shape crystals have a uniform thickness and a narrowly distributed lateral size. The equilibrium phase behavior and I–N phase transition have been characterized using crossed polarizers at the room temperature. A nematic phase exists for all systems with size ranging from 700 to 4000 nm. For smaller crystals (<1200 nm), the I–N phase transition follows a process of slow sedimentation and subsequent macroscopic phase separation, resulting in a highly oriented nematic phase with a sharp I–N interface. For larger crystals (⩾1200 nm), the I–N phase transition follows a process of nucleation and subsequent sedimentation, resulting in a random orientation of crystals in the nematic phase and a rough I–N interface. The I–N transition occurs at a very low volume fraction (<0.2%) for all systems, which is at least one order of magnitude lower than the theoretical prediction (2–7%). However, addition of a small amount of ethanol into the solution, the I–N transition can be significantly suppressed. These results demonstrate the existence of a lateral attraction between crystals, which is due to the polar attraction between the uncovered PLLA crystalline domains. Polar ethanol molecules can adsorb to the PLLA crystalline surface and screen the attraction. The attraction exhibits highly orientation-dependent. To further demonstrate this highly directional attraction, we have prepared two composite single crystal suspensions with PLLA homopolymer, which have a much wider open angle for the polar attraction. Indeed, the resulting liquid crystalline phases show much less horizontal ordering.

Despite the extensive study of periodic precipitation and rhythmic crystal growth into ringed patterns, the detail of the evolution process remains unclear, and thus the explanation is rather elusive. Herein, we focus on monitoring the detailed growth process and dynamic, elucidating the underlying mechanism, and exploring key factors for the generation of poly(ε-caprolactone) (PCL) concentric ringed spherulites in evaporating droplets. In situ observation exhibits that accompanying the rhythmic evolution of the crystal, the region ahead the growth face changes periodically and the radial growth within each period is non-linear. It shows that having an evaporation-driven convection that carries liquid to the growth face drives the periodic dimple generation and rupture that leads to the rhythmic growth into discrete ringed spherulites. The non-linear growth is attributed to the coupling of evaporation and crystallization. We find that there are two key factors in the drying process that ensure the occurrence of the evaporation-driven flow and then the periodic crystal pattern. The structure formation reveals a complex interplay among solvent extraction, solution flow, solute diffusion, and crystal growth.

Confined crystallization of the micromolded poly(butadiene)-block-poly(ε-caprolactone) (PB-b-PCL) diblock copolymer thin film was studied in this work. The long-range regular ordering of the PCL crystal with crystallographic b-axis parallel to the long-axis of the channel was detected, as indicated by the electron diffraction and grazing-incidence X-ray diffraction experimental results. This preferential crystallographic orientation is mainly because that PCL block crystallization was readily influenced by the geometric effect, then, the fast-growth direction (crystallographic b-axis) was forced to extend along the long-axis of the channel to grow long. Moreover, the substrate induced ordering of the block copolymer restricted the “in-plane” molecular diffusion in the residual layer, and cross-channel crystallization was precluded. Hence, micromolding seems to be a promising method for tailoring the nanoscale crystallization of block copolymer in thin films.

Previously, by modulating the interplay of chain diffusion and crystal growth rate in evaporating solution-cast poly(ε-caprolactone) (PCL) thin films, discrete banded crystals were obtained when diffusion and growth are competitive. In this study, we further investigated the effect of diffusion and growth, here mainly the growth rate on the crystallization of PCL. We found that there is a threshold (or range of) growth rate above which the banding cannot develop, and thus a band-to-nonband transition leads to the emergence of a novel crystal pattern. Accompanied by the band-to-nonband transition, the lamellar orientation also changes from flat-on to mainly tilted or even edge-on. The result reveals the significant impact of the diffusion and growth, especially the growth rate on the crystal construction and then the crystal morphology.

Previously, poly(ε-caprolactone) (PCL) classical ring-banded spherulites with periodic twisted lamellae and nonclassical concentric ringed structures induced by rhythmic growth were obtained by modulating the competition between diffusion flux and spherulitic growth. In this study, the modulation of diffusion flux and spherulitic growth on ring-banded structures is further studied, and hierarchical nested ring-banded patterns with a banded structure nested in the ridge rings of the other concentric ringed structure are prepared during slow solvent evaporation in poly(ethylene adipate) (PEA) solution films. The structural characterizations reveal that the big concentric ringed structure derives from periodic variation of thicknesses and that the small inner banded structure consists of periodic twisted lamellae. A diffusion-induced rhythmic growth mechanism and an unbalanced surface stress induced lamellar twisting model are proposed to illustrate the formation of the big concentric and the inner banded structures, respectively.

The relief structure formation associated with microphase separation and dewetting of the nearly symmetric poly(styrene)-block-poly(ε-caprolactone) diblock copolymer thin film was studied in this work, for which a suite of complementary methods, namely atomic force microscopy, optical microscopy, and X-ray photoelectron spectroscopy combined with hot stages, were applied. Through control of the microphase separation strength, indicated by the χN (where χ and N indicate the Flory–Huggins interaction parameter and the total degree of polymerization, respectively), varied relief structures were observed. On one side, when there was no microphase separation (χN = 3.9), typical droplets resulting from autophobic dewetting were revealed. On the other side, when there was intermediate microphase separation (χN ≥ 14.0), superimposed lamellae except for droplets was discerned. Moreover, with continual heating, the formation of the superimposed lamellae and its dynamic transition to ordered droplets were first revealed with the in situ AFM scanning. On the basis of these findings, we conclude that the superimposed lamella is a metastable structure, resulting from the coupling of dewetting and microphase separation, and it finally reaches the equilibrium droplets. The formation of superimposed lamellae was attributed to that the microphase separation strength was forced to yield to the minimization tendency of surface tension.

Despite its wide occurrence in soft confined block co-polymers, breakout crystallization remains poorly understood and is difficult to control. In this work, thin films of cylinder-forming poly(butadiene)-block-poly(ε-caprolactone) (PB-b-PCL) diblock co-polymers, with PCL being the minority block, have been chosen as the study subject. We demonstrate a new route to study the breakout crystallization by obtaining the microphase separation structure within terraced lamellae first and then in situ tracking down the lamellar coalescence, resulting from the development of the crystal growth front. We find that the crystal growth front has sucked materials from the surrounding amorphous lamellae, which lead to the decrease of the lamellar zones and coalescence of the microphase separation structure. Dividing the breakout crystallization into parallel breakout and vertical breakout, we illustrate that it is the crystallization-driven molecular diffusion that make the molecules overcome the topography constraint and grow into large-scale spherulite. Moreover, the results show that the polymer microphase separation structure has a significant influence on the crystal nucleation and greatly retarded the crystal growth rate. With a well-designed microphase separation structure within terraces and an easily tunable atomic force microscopy in situ imaging technique, an intensive study of the breakout crystallization and concomitant microdomain coalescence has been offered.

A large number of lozenge-shaped and sandwiched polystyrene-block-poly(l-lactide) (PS-b-PLLA) single crystals were prepared by the self-seeding technique. The single crystals were nearly monodispersed in both thickness and diameter. They are well-dispersed because of the steric stabilization offered by tethered PS in p-xylene, which is a good solvent for PS. The suspensions were observed to separate into a transparent upper phase and a turbid lower phase. The lower phase showed uniform iridescent stripes extending over the whole tube between crossed polarizers. The birefringence demonstrates the liquid crystal order, and the uniform stripes reveal that the phase is a well-oriented single domain. The phase-transition concentration is rather low. Polarizing light microscopy (PLM) images show Schlieren texture and thread-like texture. Small-angle X-ray scattering (SAXS) results showed that the single crystals in the liquid crystal phase oriented horizontally with a vertical repeat distance of about 70 nm. Additionally, the possible structure of the liquid crystal phase is being discussed. The novel disclike colloidal particle might be useful for anisotropic photonic materials.

We have studied the coupling behavior of microphase separation and autophobic dewetting in weakly segregated poly(ε-caprolactone)-block-poly(l-lactide) (PCL-b-PLLA) diblock co-polymer ultrathin films on carbon-coated mica substrates. At temperatures higher than the melting point of the PLLA block, the co-polymer forms a lamellar structure in bulk with a long period of L ∼ 20 nm, as determined using small-angle X-ray scattering. The relaxation procedure of ultrathin films with an initial film thickness of h = 10 nm during annealing has been followed by atomic force microscopy (AFM). In the experimental temperature range (100–140 °C), the co-polymer dewets to an ultrathin film of itself at about 5 nm because of the strong attraction of both blocks with the substrate. Moreover, the dewetting velocity increases with decreasing annealing temperatures. This novel dewetting kinetics can be explained by a competition effect of the composition fluctuation driven by the microphase separation with the dominated dewetting process during the early stage of the annealing process. While dewetting dominates the relaxation procedure and leads to the rupture of the ultrathin films, the composition fluctuation induced by the microphase separation attempts to stabilize them because of the matching of h to the long period (h ∼ 1/2L). The temperature dependence of these two processes leads to this novel relaxation kinetics of co-polymer thin films.

We have studied the microstructures of the solvent-cast films of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer with PMMA cylinder-forming composition. The casting solvents, 1,1,2-trichloroethane (TCE), toluene (Tol) and their binary mixtures, were controlled to evaporate at a certain rate R ≈ 0.01 mL/h, and the effect of the preferential affinity of the solvents for a certain block P2VPng microstructures is investigated. 1,1,2-Trichloroethane and toluene, with similar volatility, are good solvents for both PS and PMMA blocks but having opposite preferential affinity for each block (TCE has preferential affinity for the minority PMMA block and Tol has preferential affinity for the majority PS block), and the preferential affinity of the solvents is modulated by mixing the two solvents. As the preferential affinity of the solvents for the majority PS block increases, a series of microstructures including perforated lamella with ringlike morphologies, PMMA cylinders, PMMA spheres, and PMMA cylinders covered with a layer of PS microdomain have been observed. The results are discussed in view of the effective volume fraction of each block induced by the solvent preferential affinity and the special mechanical strain field brought by the solvent evaporation.

Single crystals of poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) have been prepared by tetrahydrofuran vapor annealing and controlling solvent evaporation, respectively. The morphology and structure of the single crystals are characterized using optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and wide-angle X-ray diffraction. It is observed that in P3HT single crystals, the molecules are packed with π−π stacking direction perpendicular to the length axis of the crystals and main chains parallel to the substrate, whereas in P3OT single crystals, the molecules are packed with π−π stacking direction parallel to the length axis of the crystal and main chains parallel to the substrate. In the field effect transistors, the current flow is parallel to the length axis of the single crystals, and the mobility is 1.57 × 10−3 cm2/Vs for a P3HT single crystal and 0.62 cm2/Vs for a P3OT single crystal. The single crystals of P3HT and P3OT showed high anisotropic electrical properties. The influences of molecular conformation and alkyl chain length on the electrical properties of P3ATs are discussed.

A series of binary SB blend samples with various overall volume fraction of PS (ΦPS) and different discrete distribution of the block length (denoted as dPS or dPB) were prepared by mixing various asymmetric poly(styrene)-block-poly(butadiene) (SB) block copolymers with a symmetric SB block copolymer. The influences of the external solvent field, composition, and the block length distribution on the morphologies of the blends in the thin films were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The experimental results revealed that after solvent annealing, the interface of the blend thin films depended mainly on the cooperative effects of the annealing solvent and the inherently interfacial curvature of the blends. Upon exposure to the saturated vapor of cyclohexane, which has preferential affinity for the PB block, a “threshold” of ΦPS (approximate 0.635∼0.707) was found. Below such threshold, the influence of the annealing solvent played an important role on the interfacial curvature of the blend thin film. The morphologies of the thin films and the long-range order of the structures were related to the value of dPS, regardless of the change of dPB.

Needle-like single crystals of poly(3-octylthiophene) (P3OT) have been prepared by tetrahydrofuran-vapor annealing. The morphology and structure of the crystals were characterized with optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and wide-angle X-ray diffraction. It is observed that the P3OT molecules are packed with the backbones parallel to the length axis of the crystal and the alkyl side chains perpendicular to the substrate. The field effect transistor based on the P3OT single crystal exhibited a charge carrier mobility of 1.54 × 10−4 cm2/(Vs) and on/off current ratio of 37, and the molecular orientation of the crystal is ascribed to account for the device performance. The time-dependent morphological evolution demonstrated that the crystals underwent Ostwald ripening when annealed.

Birefringent ring-banded spherulites with radial periodic variation of thicknesses were grown from poly(ε-caprolactone) (PCL) solutions under conditions for which the solution concentration was held constant during the whole development of the morphology. The as-grown ring-banded spherulites were investigated by optical (OM) and atomic force (AFM) microscopies, by transmission electron microscopy (TEM) of samples sectioned parallel to the plane of film, and also by electron diffraction (ED) and grazing incidence X-ray diffraction (GIXD) techniques. The results indicate that the concentric rings in the birefringent ring-banded spherulites, as well as those in the nonbirefringent ring-banded spherulites, are a manifestation of periodic variation of thicknesses along the radius. The development of the ring-banded spherulites with radial periodic variation of thicknesses is due to periodic diffusion-induced rhythmic growth associated with periodical change in the concentration gradient in polymer solution with constant concentration. The morphological features reflect the competition between the diffusion flux of polymer chains in solution, J, and spherulitic growth with radial growth velocity, V, which can be characterized by the parameter δ = J/V. The effects of crystallization conditions, including polymer molecular weight, initial solution concentration, and solvent evaporation rate, on the formation of ring-banded spherulites with radial periodic variation of thicknesses were studied.