The long‐range order of spherical block copolymer films is greatly improved if the underlying surface has been etched to form wells and mesas. Epitaxial growth of polystyrene/poly(2‐vinylpyridine) block copolymers on SiO2 with structures 1–10 μm wide yields single‐crystal‐like structures on top of the mesas (see Figure), and in the wells, provided the height of the mesas exceeds one layer of copolymer spheres (27 nm). (See also inside front cover.)

Most tough hydrogels are reinforced by introducing sacrificial structures that can dissipate input energy. However, because the sacrificial damage cannot rapidly recover, the toughness of these gels drops substantially during consecutive cyclic loadings. We propose a damageless reinforcement strategy for hydrogels using strain-induced crystallization. For slide-ring gels in which polyethylene glycol chains are highly oriented and mutually exposed under large deformation, crystallinity forms and melts with elongation and retraction, resulting both in almost 100% rapid recovery of extension energy and excellent toughness of 6.6 to 22 megajoules per square meter, which is one order of magnitude larger than the toughness of covalently cross-linked homogeneous gels of polyethylene glycol.

Fundamental oscillations up to 1.04 THz were achieved in resonant tunneling diodes at room temperature. A graded emitter and thin barriers were introduced in GaInAs/AlAs double-barrier resonant tunneling diodes for reductions of the transit time in the collector depletion region and the resonant tunneling time, respectively. Output powers were 7 μW at 1.04 THz and around 10 μW in 0.9–1 THz region. A change in oscillation frequency of about 4% with bias voltage was also obtained.

The structure of asymmetric poly(styrene-b-2-vinylpyridine) (PS-PVP) diblock copolymers allowed to order in a thin film is observed by a combination of secondary ion mass spectrometry and scanning force microscopy. The surface/interface-induced ordering persists over a surprisingly long range (more than 1 μm). The 2-D structure in the layer parallel to a surface is mainly a distorted hexagonal structure similar to that of the (110) plane of a body-centered cubic structure. In contrast to the long-range order in the direction perpendicular to the surfaces, the in-plane structure shows only short-range order. The surface has a very strong effect on the structure only in the direction perpendicular to it. The development of the layered structure induced by the surfaces was also investigated as a function of temperature. When the molecular weight of PS-PVP is low but the copolymer still forms the spherical structure, the layered structure decays more rapidly with distance from the free surface as the temperature is increased. We find that the layered structure changes from an order that is nearly liquid-like near the surface to a long-range-ordered structure continuously as the temperature is decreased, which suggests that the ultimate range of order of the layered structure is governed by thermodynamics. When the molecular weight of PS-PVP is high, the propagation of the layered structure is kinetically limited due to slow diffusion. Similar to symmetric diblock copolymers with a lamellar structure, asymmetric diblock copolymers with a layered spherical domain structure form islands or holes at the free surface when the film is thin and the film thickness does not correspond to an integral number of layers of spheres. As the film becomes thick, the shape of the edge of the island or hole structure changes from a step function to a tanh function, and finally these islands or holes disappear.

The temperature and molecular weight dependence of the self-diffusion coefficient of asymmetric diblock copolymers (polystyrene-b-2-vinylpyridine) (PS-PVP) with a spherical PVP domain structure has been measured by forward recoil spectrometry. The self-diffusion coefficient D is decreased by up to a factor of 10 -4 by the existence of the ordered spherical microstructure. The normalized diffusion coefficient D/D0, where D0 is the diffusion coefficient of homopolystyrene with same molecular weight, depends strongly on the product of the interaction parameter between PS and PVP and NPVP, the number of segments in the shorter PVP block, scaling approximately as exp(-1.2 NPVP). Our results suggest that the diffusion process is governed by the thermodynamic barrier for moving PVP blocks from one spherical domain to another in contrast to the normal reptation mechanism seen for homopolymer selfdiffusion.

Gels with high mechanical performance have attracted great interest because of their potential biomedical applications. Tough gels reported thus far usually contain sacrificial species to dissipate energy, thus compromising the fatigue resistance. In this study, highly stretchable and recoverable gels can be achieved by cross-linking cyclodextrin (CD)-based polyrotaxane with a low host coverage, synthesized via a one-pot enzymatic end-capping reaction with 90% yield and ∼2% CD coverage (PR02). The low coverage allows the CD cross-links to freely slip on the axis over large distance (∼2/3 of the axis length) and thus allows the PR02 slide-ring network keep intact under large deformation via the pulley effect. The PR02-hydrogel can be stretched up to ∼1600% long, withstand ∼1 MPa stress, and fully recover instantly. PR02-DMSO gels exhibit a shape memory behavior that withstood large deformation. As the first research to control the final property of the network by precisely controlling the slide distance of the cross-links, this work not only pushes the performance envelope of soft matters but also opens new opportunities for designing tough materials.

No abstract

Supercritical (SC) carbon dioxide is of significant interest as an environmentally innocuous physical blowing agent that is used to introduce microscopic cells into polymers.[1±7] Conventional SC CO 2 processes, in which CO 2 is introduced into polymeric monoliths and depressurized quickly to foam, have never produced closed nanoscopic cells (nanocells), that is, cells less than 100 nm in diameter, in polymers.[1±7] Herein, we report a novel, environmentally benign method using CO 2 to fabricate optically transparent nanocellular polymeric materials using a fluorinated block copolymer as the template. A block copolymer, self-assembling into a nanoscopic ordered structure, with a fluorinated block, is saturated with SC CO 2 . The CO 2 -soluble fluorinated blocks effectively localize CO 2 in the nanoscopic domains. After temperature quenching followed by depressurization, the block copolymer exhibits nanocells with an average diameter of 10±30 nm and a density of the order of 10 16 cells cm ±3 . Furthermore, the size and spacing of such nanocells can be fine-tuned by changing the saturation pressure of SC CO 2 . Saturating polymers with SC CO 2 followed by rapid depressurization is a well-established technique for introducing microcellular structures into polymers.[1±7] Recently, the gelation of CO 2 -soluble compounds has been proven to be a new route to cellular and porous polymeric materials. [8,9] The solubility of compounds in CO 2 during the crosslinking reaction is the key to controlling cellular or porous structures. Such environmentally friendly CO 2 -processes usually produce cells or pores with a broad distribution. On the other hand, there have been several successful approaches for making open nanoporous polymeric materials by decomposing particular domains of block copolymers.[10±14] Chemical decomposition approaches require that decomposed fragments be removed from domains through channels; therefore, it takes an extremely long time, if it is not impossible, to fabricate a closed cellular structure with a narrow distribution of cells or pores. Thermal decomposition methods of introducing closed cells tend to end up with the cells vulnerable to collapse, due to the combination of the high processing temperature for decomposition, the high surface energy of small cells, and the plasticized matrix with decomposed products.[10] Herein, we report a nondestructive, environmentally harmless method for producing monodisperse nanocellular polymeric monoliths using fluorinated block copolymers as templates. In SC CO 2 , a diblock copolymer with a CO 2 -soluble fluorinated block localizes the CO 2 in an array of highly swollen, fluorinated block domains surrounded by a less swollen polystyrene (PS) continuous domain. By isobaric temperature reduction to 0 C and following depressurization, CO 2 dropletsÐsurrounded and stabilized by the fluorinated blockÐare fixed in the copolymer matrix and then non-destructively removed. Consequently, we successfully fabricated more than 10 16 closed nanocells in 1 cm 3 . A pol...