Mechanical responsiveness in many plants is produced by helical organizations of cellulose microfibrils. However, simple mimicry of these naturally occurring helical structures does not produce artificial materials with the desired tunable actuations. Here, we show that actuating fibres that respond to solvent and vapour stimuli can be created through the hierarchical and helical assembly of aligned carbon nanotubes. Primary fibres consisting of helical assemblies of multiwalled carbon nanotubes are twisted together to form the helical actuating fibres. The nanoscale gaps between the nanotubes and micrometre-scale gaps among the primary fibres contribute to the rapid response and large actuation stroke of the actuating fibres. The compact coils allow the actuating fibre to rotate reversibly. We show that these fibres, which are lightweight, flexible and strong, are suitable for a variety of applications such as energy-harvesting generators, deformable sensing springs and smart textiles.

Chromatic materials such as polydiacetylene change colour in response to a wide variety of environmental stimuli, including changes in temperature, pH and chemical or mechanical stress, and have been extensively explored as sensing devices. Here, we report the facile synthesis of carbon nanotube/polydiacetylene nanocomposite fibres that rapidly and reversibly respond to electrical current, with the resulting colour change being readily observable with the naked eye. These composite fibres also chromatically respond to a broad spectrum of other stimulations. For example, they exhibit rapid and reversible stress-induced chromatism with negligible elongation. These electrochromatic nanocomposite fibres could have various applications in sensing.

The investigation on the formation mechanism of helical structures and the synthesis of helical materials is attractive for scientists in different fields. Here we report the synthesis of helical mesoporous materials with chiral channels in the presence of achiral surfactants. More importantly, we suggest a simple and purely interfacial interaction mechanism to explain the spontaneous formation of helical mesostructures. Unlike the proposed model for the formation of helical molecular chains or surpramolecular packing based on the geometrically motivated model or the entropically driven model, the origin of the helical mesostructured materials may be attributed to a morphological transformation accompanied by a reduction in surface free energy. After the helical morphology is formed, the increase in bending energy together with the derivation from a perfect hexagonal mesostructure may limit the curvature of helices. Our model may be general and important in the designed synthesis of helical mesoporous materials.

We succeeded recently in developing a series of new pathways to polymeric micelles and hollow spheres via intermolecular specific interactions. A new micellization mechanism of block copolymers was realized by using the specific interaction between a low molecular weight compound and one of the blocks in low-polarity solvents. Many more successes have been achieved by our "block copolymer-free" strategies. We are now able to use homopolymers, random copolymers, oligomers, etc. as building blocks to construct noncovalently connected micelles (NCCM), in which the core and shell are connected by hydrogen bonding. Some of such NCCMs are readily converted further into hollow spheres by cross-linking the shell and then switching the medium to one that dissolves the core. Rigid polymer chains and their complementary homopolymers can directly assemble into large hollow spheres thanks to the propensity to parallel packing of the rigid chains. In addition, some of the NCCMs show perfect stimuli-responsive properties. pH-dependent micellization and pH-dependent micelle-hollow-sphere transition are realized in water-soluble graft copolymers driven by complexation and decomplexation between the main chain and grafts.

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Double hydrophilic block copolymers, poly(N-isopropylacrylamide)-b-poly(N-vinylimidazole) (PNIPAM-b-PVim), were successfully prepared with good control via reversible addition−fragmentation chain transfer (RAFT) using PNIPAM-based macromolecular xanthate agents (i.e., MADIX, macromolecular design via the interchange of xanthates). This represents the first preparation of well-defined block copolymers based on PVim, which has been well-known to be able to catalyze esterolysis reactions. The imidazole-containing diblock copolymers molecularly dissolve at low temperatures in water. Above the phase transition temperatures of PNIPAM or in a proper mixture of methanol/water (cononsolvency), the PNIPAM block becomes hydrophobic and stable micelles form with a dense core consisting of a hydrophobic PNIPAM block and a polar PVim shell. The catalytic activities of PNIPAM44 -b-PVim51 and PNIPAM44 -b-PVim21 toward the hydrolysis of p-nitrophenyl acetate (NPA) at different temperatures or methanol/water compositions were then determined using a stopped-flow apparatus and compared to that of PVim homopolymer. The Arrhenius plot for the PVim-based diblock copolymers exhibited a pronounced upward curvature above the critical micellization temperature (cmt). Moreover, in the methanol/water mixture, the catalytic activities of PNIPAM-b-PVim diblock copolymers evolved discontinuously as a function of solvent composition and exhibited a maximum in the range of volume fraction of methanol, φmethanol, between 0.3 and 0.5, corresponding to the solvent composition range where cononsolvency-induced micellization took place. We thus observed for the first time that double hydrophilic block copolymer micelles of PNIPAM-b-PVim can serve as self-catalyzing nanoreactors. Most importantly, the catalytic activities can be well-tuned with external temperature or solvent compositions.

Polymeric micelles have attracted much interest in both theoretical and applied research fields. [1][2][3][4][5][6] In most cases the polymeric micelles are produced from selfassembly of block copolymers in selective solvents although some different approaches to the micelles have been suggested. [7][8][9][10][11][12][13][14][15][16] Usually, the self-assembly can only be performed in very dilute solutions; the preparation efficiency is low. Extensive studies [17][18][19][20][21][22][23] have also been made to stabilize the micellar structure by cross-linking the core or the shell of the micelles formed in selective solvents leading to stabilized nanoparticles. Here we report a new approach to the preparation of such stabilized nanoparticles by directly cross-linking one of the blocks in the copolymer in its nonselective solvent (common solvent) (Scheme 1). Furthermore, using this one-step route, we are able to produce the nanoparticles at high concentrations up to 0.2 g/mL.We used two samples of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP), S1, PS 658 -b-P4VP 336 (the subscripts are the average number of the repeat units of the PS and the P4VP blocks, respectively) with M w / M n 1.09 prepared in our laboratory by living anionic polymerization, and S2, PS 84 -b-P4VP 112 with M w /M n 1.09 produced by Polymer Source Inc. The cross-linking reaction took place in solutions of the block copolymers in DMF, in which both the PS and P4VP blocks were solvated, at concentrations ranging from 0.01 to 0.2 g/mL at room temperature. 1,4-Dibromobutane was used as the cross-linker. The molar ratio of 1,4-dibromobutane to the pyridyl groups in the block copolymer to be cross-linked was 2:1. The cross-linking reaction was continued for 5 days, which was proven by 1 H NMR measurements to be long enough for the completion of the cross-linking reaction. In all the cases, nanoparticles rather than gelation were obtained. The resulting solutions and particles were then characterized by DLS, TEM, and SEM. The characterization data of the nanoparticles formed at different concentrations of S1 and S2 measured by DLS are presented in Table 1.According to Flory's gelation theory, 25 the proportion of cross-linkage is remarkably small for the onset of gelation when cross-linking a polymer. However, in the present case of cross-linking PS-b-P4VP under the conditions mentioned above, with the average numbers of cross-linkable units per polymer chain of S1 and S2 being as high as 336 and 112, respectively, nanoparticles rather than macrogel were obtained. This can obviously be attributed to the presence of the solvated non-crosslinkable PS block, which plays the key role in preventing gelation.