Mesogenic Structures, Lyotropic and Thermotropic Phase Transitions in Demethyl-Ionene Alkyl Sulfonate Complexes

Linear [<i>B</i>,10]-polyamines [(CH₂)_<i>B</i>N­(CH₃)­(CH₂)₁₀N­(CH₃)] were prepared (<i>B</i> = 3 or 6). Protonated by stoichiometric amounts of <i>n</i>-alkylsulfonic acids, they form demethyl-ionene sulfonate complexes, which proved thermally stable up to 220 °C. Salt free complexes were investigated by polarized microscopy, thermogravimetry, and X-ray diffraction. Except the heptyl sulfonate, which crystallized, all complexes bearing longer alkyl chains formed mesogenic phases. Being isotropic in dry state, they became optically anisotropic when exposed to humidity due to a lyotropic transition (mediated by the gas phase) to a hexagonal phase, mostly. A cubic phase containing less water was also observed. Anisotropic complexes again were converted to an isotropic state upon heating under controlled humidity. The clearing temperatures distinctly depend on humidity and rise with increasing length of the alkyl sulfonate. This may allow the use of the complexes as humidity sensors. Oriented liquid crystalline samples are formed upon fast cooling in flat capillaries

Reversibly Actuating Solid Janus Polymeric Fibers

It is commonly assumed that the substantial element of reversibly actuating soft polymeric materials is chemical cross-linking, which is needed to provide elasticity required for the reversible actuation. On the example of melt spun and three-dimensional printed Janus fibers, we demonstrate here for the first time that cross-linking is not an obligatory prerequisite for reversible actuation of solid entangled polymers, since the entanglement network itself can build elasticity during crystallization. Indeed, we show that not-cross-linked polymers, which typically demonstrate plastic deformation in melt, possess enough elastic behavior to actuate reversibly. The Janus polymeric structure bends because of contraction of the polymer and due to entanglements and formation of nanocrystallites upon cooling. Actuation upon melting is simply due to relaxation of the stressed nonfusible component. This approach opens perspectives for design of solid active materials and actuator for robotics, biotechnology, and smart textile applications. The great advantage of our principle is that it allows design of non-cross-linked self-moving materials, which are able to actuate in both water and air, which are not cross-linked. We demonstrate application of actuating fibers for design of walkers, structures with switchable length, width, and thickness, which can be used for smart textile applications

Acrylic AB and ABA Block Copolymers Based on Poly(2-ethylhexyl acrylate) (PEHA) and Poly(methyl methacrylate) (PMMA) via ATRP

Acrylic block copolymers have several advantages over conventional styrenic block copolymers, because of the presence of a saturated backbone and polar pendant groups. This investigation reports the preparation and characterization of di- and triblock copolymers (AB and ABA types) of 2-ethylhexyl acrylate (EHA) and methyl methacrylate (MMA) via atom transfer radical polymerization (ATRP). A series of block copolymers, PEHA-<i>block</i>-PMMA­(AB diblock) and PMMA-<i>block</i>-PEHA-<i>block</i>-PMMA­(ABA triblock) were prepared via ATRP at 90 °C using CuBr as catalyst in combination with N,N,N′,N″,N″-pentamethyl diethylenetriamine (PMDETA) as ligand and acetone as additive. The chemical structure of the macroinitiators and molar composition of block copolymers were characterized by ¹H NMR analysis, and molecular weights of the polymers were analyzed by GPC analysis. DSC analysis showed two glass transition temperatures (<i>T</i>_g), indicating formation of two domains, which was corroborated by AFM analysis. Small-angle X-ray scattering (SAXS) analysis of AB and ABA block copolymers showed scattering behavior inside the measuring limits indicating nanophase separation. However, SAXS pattern of AB diblock copolymers indicated general phase separation only, whereas for ABA triblock copolymer an ordered or mixed morphology could be deduced, which is assumed to be the reason for the better mechanical properties achieved with ABA block copolymers than with the AB analogues

Reversible Shape-Memory Effect in Cross-Linked Linear Poly(ε-caprolactone) under Stress and Stress-Free Conditions

The effect of cross-link density on the reversible shape-memory effect (SME) under constant load was systematically studied in cross-linked linear poly­(ε-caprolactone) (PCL). A remarkable reversible SME under stress-free conditions was observed in PCL with the highest achieved cross-link density. Thermal properties as well as morphology, size, and orientation of the nanocrystalline structure formed in covalent networks of PCL under load were compared with those in PCL crystallized under stress-free conditions. As shown, the oriented growth of crystals is the origin of both the reversible SME under and without load. Furthermore, a significant rise of crystallinity and crystal thickness was detected in PCL crystallized under constant load. The fitting curves of the temperature-dependent strain as well as the quantities of crystallinity, type of crystalline structure, size, and orientation of the crystals got by modeling the reversible SME in PCL under stress well correspond to their values obtained experimentally

Nanoporous Cathodes for High-Energy Li–S Batteries from Gyroid Block Copolymer Templates

This study reports on a facile approach to the fabrication of nanoporous carbon cathodes for lithium sulfur batteries using gyroid carbon replicas based on use of polystyrene-poly-4-vinylpyridine (PS-P4VP) block copolymers as sacrificial templates. The free-standing gyroid carbon network with a highly ordered and interconnected porous structure has been fabricated by impregnating the carbon precursor solution into the gyroid block copolymer nanotemplates and subsequently carbonizing them. A wide range of analytical tools have been employed to characterize fabricated porous carbon material. Prepared nanostructures are envisioned to have a great potential in myriad areas such as energy storage/conversion devices owing to their fascinating morphology exhibiting high surface area and uniform porosity with interconnected three-dimensional networks. The resulting carbon nanoporous structures infused with elemental sulfur have been found to work as a promising electrode for lithium sulfur batteries demonstrating a high cycling stability over more than 200 cycles

Methacrylate Copolymers with Liquid Crystalline Side Chains for Organic Gate Dielectric Applications

Polymers for all-organic field-effect transistors are under development to cope with the increasing demand for novel materials for organic electronics. Besides the semiconductor, the dielectric layer determines the efficiency of the final device. Poly­(methyl methacrylate) (PMMA) is a frequently used dielectric. In this work, the chemical structure of this material was stepwise altered by incorporation of cross-linkable and/or self-organizing comonomers to improve the chemical stability and the dielectric properties. Different types of cross-linking methods were used to prevent dissolution, swelling or intermixing of the dielectric e.g. during formation processes of top electrodes or semiconducting layers. Self-organizing comonomers were expected to influence the dielectric/semiconductor interface, and moreover, to enhance the chemical resistance of the dielectric. Random copolymers were obtained by free radical and reversible addition–fragmentation chain transfer (RAFT) polymerization. With 6-[4-(4′-cyanophenyl)­phenoxy]­alkyl side chains having hexyl or octyl spacer, thermotropic liquid crystalline (LC) behavior and nanophase separation into smectic layers was observed, while copolymerization with methyl methacrylate induced molecular disorder. In addition to chemical, thermal and structural properties, electrical characteristics like breakdown field strength (<i>E</i>_BD) and relative permittivity (<i>k</i>) were determined. The dielectric films were studied in metal–insulator–metal setups. <i>E</i>_BD appeared to be strongly dependent on the type of electrode used and especially the ink formulation. Cross-linking of PMMA yielded an increase in <i>E</i>_BD up to 4.0 MV/cm with Ag and 5.7 MV/cm with PEDOT:PSS electrodes because of the increased solvent resistance. The LC side chains reduce the ability for cross-linking resulting in decreased breakdown field strengths