Thermoreversible Ion Gel with Tunable Modulus Self-Assembled by a Liquid Crystalline Triblock Copolymer in Ionic Liquid

Ion gels with tunable storage moduli are prepared through the self-assembly of an ABA triblock copolymer AOA-12 comprised of a thermotropic liquid crystalline (LC) polymer as the end-block A and poly­(ethylene oxide) (PEO) as the mid-block B in a room-temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium bis­(trifluoro­methyl­sufonyl)­imide ([EMIM]­[TFSI]). The PEO mid-block is well-solvated by this ionic liquid, whereas the LC polymer end-blocks aggregate and serve as the physical cross-linkers. For comparison, a triblock copolymer AOA-0 containing a non-LC side-chain polymer with the same mesogen is also synthesized, and its corresponding ion gel is prepared. The ion gels with relatively high concentrations of the LC triblock copolymer have hierarchical structures with different microphase-separated nanostructures and the LC arrangement of the LC blocks. By incorporating the azobenzene mesogen in the side chains, transparent AOA-<i>n</i>/[EMIM]­[TFSI] (where <i>n</i> is the number of carbon atoms in the spacer between the azobenzene mesogen and the polymethacrylate backbone, and <i>n</i> = 0, 12), ion gels are obtained with concentrations of the polymer as low as around 2 wt %. The ion gel obtained has a storage modulus as high as ∼10 kPa, while its conductivity is close to that of the pure IL mainly because of the high IL concentration of the ion gel. Furthermore, the storage modulus of the AOA-12/IL ion gel can be tuned by temperature because of the thermotropic phase behavior of the LC block. These ion gels are potentially useful as high-temperature ionic membranes or thermal-responsive soft actuators

Remarkably Rich Variety of Nanostructures and Order–Order Transitions in a Rod–Coil Diblock Copolymer

A remarkably rich variety of nanophase-separated structures and various order–order transitions are observed in a series of low-molecular weight (MW) rod–coil block copolymers (BCPs) with the rod blocks of different lengths (<i>L</i>_Rod’s). The rod–coil diblock copolymer studied herein is poly­(dimethylsiloxane)-<i>b</i>-poly­{2,5-bis­[(4-methoxyphenyl)­oxycarbonyl]­styrene} (PDMS-<i>b</i>-PMPCS), in which PMPCS is a rod-like polymer and exhibits an MW-dependent liquid crystalline (LC) phase behavior. When the polymerization degree of the PMPCS rod block (<i>N</i>_Rod) is less than 32 (<i>L</i>_Rod < 8 nm), the PMPCS block is always amorphous in the entire temperature range. And the corresponding PDMS-<i>b</i>-PMPCS BCPs with <i>N</i>_Rod from 11 to 29 and the volume fraction of the PMPCS rod (<i>f</i>_Rod) from 43% to 67% self-assemble into various equilibrium nanostructures after annealed at temperatures above the glass transition temperatures of the PMPCS blocks. When <i>N</i>_Rod = 11 and <i>f</i>_Rod = 43%, the BCP forms a lamellar structure (LAM); when <i>N</i>_Rod = 15 and <i>f</i>_Rod = 51%, the BCP forms a double gyriod structure (GYR) ; when <i>N</i>_Rod = 20 and <i>f</i>_Rod = 57%, the BCP forms a GYR structure after annealed below 180 °C and transforms to the <i>Fddd</i> structure after annealed above 180 °C; when <i>N</i>_Rod = 29 and <i>f</i>_Rod = 67%, the nanostructure of the BCP after annealed below 180 °C is hexagonally packed cylinders (HEX) and changes to a body centered cubic structure (BCC) after annealed above 180 °C. When <i>N</i>_Rod > 32 (<i>L</i>_Rod > 8 nm), the PMPCS rod block is amorphous at low temperatures and transforms to a stable columnar LC phase after annealed at high temperatures. Correspondingly, the PDMS-<i>b</i>-PMPCS BCP with <i>N</i>_Rod = 44 and <i>f</i>_Rod = 75% forms a HEX structure after annealed at lower temperatures at which the PMPCS block is amorphous, and the nanostructure transforms to LAM after the sample is annealed at higher temperatures at which the PMPCS block enters into the LC phase. Therefore, only by a small change of the rod length in the low-MW PDMS-<i>b</i>-PMPCS rod–coil BCPs, various nanostructures including LAM, GYR, <i>Fddd</i>, HEX, and BCC are obtained. In addition, by increasing annealing temperatures, GYR-to-<i>Fddd</i> and HEX-to-BCC transitions are observed in the BCPs with the amorphous PMPCS, and a HEX-to-LAM transition occurs in the BCP when the LC PMPCS block undergoes an isotropic-to-LC phase transformation

Hierarchical Structures in Thin Films of Macrophase- and Microphase-Separated AB/AC Diblock Copolymer Blends

Interesting hierarchical structures are generated in thin films of the AB/AC diblock copolymer (diBCP) blends of poly­(dimethylsiloxane)-<i>b</i>-poly­{2,5-bis­[(4-methoxyphenyl)­oxycarbonyl]­styrene} (PDMS-<i>b</i>-PMPCS, DMPCS) rod–coil diBCP and poly­(dimethylsiloxane)-<i>b</i>-poly­(methyl methacrylate) (PDMS-<i>b</i>-PMMA, DMMA) coil–coil diBCP with the common block as the minor component in both diBCPs. The macrophase separation and microphase separation occur in the DMPCS/DMMA BCP blends in bulk, confirmed by small-angle X-ray scattering (SAXS) results. Moreover, the macrophase- and microphase-separated morphologies in thin films of the DMPCS/DMMA BCP blends are directly observed by transmission electron microscopy experiments owing to the different electron densities among the three different blocks. For the blends of DMPCS and DMMA, both of which have the nanostructures of hexagonally packed cylinders (HEX) (DMPCS_HEX/DMMA_HEX), when the blend contains 75 wt % of one diBCP, subordered macrophase-separated structures with ordered nanostructures in the macrodomains develop in the thin film. When the matrix of the macrophase is the coil–coil DMMA_HEX diBCP which has the nanostructure of vertically oriented cylinders in the thin film, the macrophase-separated submicrometer structures become more ordered, and the interfaces of the macrodomains become more smooth. For the blends of the lamellar DMPCS and the HEX-structured DMMA having similar volume fractions of PDMS (DMPCS_LAM/DMMA_HEX), with 75 wt % of lamellar DMPCS in the blend, hamburger-like structures form in the DMPCS_LAM macromatrix of the thin film, which is ascribed to the solubility of DMMA in the lamellar DMPCS on the segmental length scale. When the weight fraction of the lamellar DMPCS in the blend is 25%, the short DMPCS lamellae with a few layers are uniformly dispersed in the HEX-structured DMMA macromatrix

How Big Is Big Enough? Effect of Length and Shape of Side Chains on the Single-Chain Enthalpic Elasticity of a Macromolecule

Polymers with a carbon–carbon (C–C) backbone are an important class of polymers, which can be regarded as the derivatives of polyethylene (PE). To investigate the effect of side chains on the single-chain enthalpic elasticity (SCEE) of polymers with a C–C backbone, several polymers with pendants or side chains of different lengths and shapes have been studied by single-molecule AFM. We find that both length and shape of the side chains count: only the side chains that are both long and bulky (i.e., bulky dendrons of second or higher generation as side chains) affect the SCEE. Thus, only rare polymers have special SCEE. For the vast majority of polymers, the SCEE is identical to that of PE, which means that the SCEE is determined by the nature of the C–C backbone. It is expected that this conclusion can also be popularized to all polymers with various backbones. This study is an important update to the understanding of polymers at the single-chain level

Synthesis and Properties of a Coil‑<i>g</i>‑Rod Polymer Brush by Combination of ATRP and Alternating Copolymerization

We synthesized a coil-<i>g</i>-rod polymer brush, poly­{styrene-<i>alt</i>-(maleimide-<i>g</i>-poly­{2,5-bis­[(4-methoxyphenyl)­oxy­carbonyl]­styrene})}­(P­{St-<i>alt</i>-(MI-<i>g</i>-PMPCS)}), by alternating copolymerization of styrene (St) and maleimide-terminated poly­{2,5-bis­[(4-methoxyphenyl)­oxycarbonyl]­styrene} (MI-PMPCS) with the “grafting through” strategy. MI-PMPCS was synthesized by using the protection strategy in which the initiator protected by Diels–Alder reaction with furan was used to initiate atom transfer radical polymerization of 2,5-bis­[(4-methoxyphenyl)­oxycarbonyl]­styrene (MPCS), and then furan was deprotected by retro-Diels–Alder reaction. ¹H NMR, gel permeation chromatography (GPC), and GPC coupled with multiangle laser light scattering were used to determine the chemical structures and molecular weights of the polymer brushes. The highest degree of polymerization (DP) of the main chain is 103, which is significantly large for polymer brushes with rigid side chains. The main-chain length increases with increasing feeding ratio and decreases with increasing side-chain length. The thermal properties and transitions of all samples were studied by thermogravimetric analysis and differential scanning calorimetry. Finally, polarized light microscopy and one-/two-dimensional wide-angle X-ray diffraction were used to examine the phase structures of the polymer brushes. To our surprise, when the DP of the side chain is below the critical value of PMPCS for forming liquid crystalline (LC) phases, the polymer brush can form the LC phase. On the one hand, with the longer main chain, the LC phase of the polymer brush becomes less ordered. On the other hand, with the longer side chain, the LC phase of the polymer brush becomes more ordered. For polymer brushes with LC side chains, the LC phase of the polymer brush may be less ordered than that of the LC PMPCS side chain

Hierarchically Self-Assembled Amphiphilic Alternating Copolymer Brush Containing Side-Chain Cholesteryl Units

We synthesized a novel amphiphilic alternating copolymer brush (AACPB), poly­{maleimide-<i>g</i>-poly­[10-(cholesteryl­oxycarbonyl)­decyl methacrylate]}-<i>alt</i>-(styrene-<i>g</i>-poly­(ethylene oxide)) (P­(MI-<i>g</i>-PCholMA)-<i>alt</i>-(St-<i>g</i>-PEO)), by copolymerization of maleimide-terminated poly­[10-(cholesteryl­oxycarbonyl)­decyl methacrylate] (MI-PCholMA) and styrene-terminated poly­(ethylene oxide) (St-PEO). The thermal properties of the polymer brushes were investigated by thermogravimetric analysis and differential scanning calorimetry. After solvent and thermal annealing, the AACPB self-assembles into a hierarchically ordered nanostructure. One is the microphase-separated lamellar nanostructure with a 9.66 nm scale. The other is the cholesteryl double-layer smectic A phase (SmA_d) with a 5.46 nm scale. The order–disorder transition of the AACPB is associated with the SmA_d–isotropic transition. It is the first report on the microphase separation of AACPBs. We can construct ordered nanostructures with a sub-10 nm length scale with AACPBs. After the doping of 0.2 equiv of LiCF₃SO₃, the <i>d</i>-spacing of the lamellar structure formed by the PCholMA₈-<i>alt</i>-PEO₂₅/LiCF₃SO₃ complex increases because the interaction between Li⁺ and oxygen atom makes the PEO chains more stretched. Such a structure offers lithium salt-doped PEO nanochannels which can act as pathways for the transport of lithium ion

Persistent Formation of Self-Assembled Cylindrical Structure in a Liquid Crystalline Block Copolymer Constructed by Hydrogen Bonding

A series of supramolecular liquid crystalline block copolymers (SLCBCPs) were prepared by hydrogen-bonding interaction between poly­(dimethyl­siloxane)-<i>b</i>-poly­(2-vinyl­terephthalic acid) (PDMS-<i>b</i>-PM1H) and [4-(4′-hexyloxy)­styryl]­pyridine (NC6). PDMS-<i>b</i>-PM1H serves as the hydrogen-bonding donor and NC6 as the hydrogen-bonding acceptor. The SLCBCPs are obtained by mixing the hydrogen-bonding acceptor and donor in pyridine. Through increasing the molar ratio of pyridine to carboxyl, the SLCBCPs can transform from coil–coil block copolymers (BCPs) to rod–coil ones. When the ratio of pyridine to carboxylic acid is 0.50 or lower, the SLCBCPs are coil–coil-like. However, when the ratio exceeds 0.50, the SLCBCPs behave like rod–coil BCPs because the supramolecular block PM1H­(NC6) exhibits liquid crystalline (LC) behavior owing to the “jacketing” effect. Small-angle X-ray scattering and transmission electron microscopy experiments were employed to characterize the microphase-separated nanostructures of the SLCBCPs. Interestingly, when the weight fraction of the supramolecular block PM1H­(NC6) ranges from 51% to 92%, hexagonally packed cylinders (HEX) are always obtained. Compared to conventional BCPs, the SLCBCPs prepared can more easily self-assemble into the HEX nanostructures that may potentially serve as nanotemplates and porous materials after selective etching. In addition, the SLCBCPs can form hierarchically ordered nanostructures, including the HEX nanostructure of the SLCBCP and the LC phase of the supramolecular block

New Morphologies and Phase Transitions of Rod–Coil Dendritic–Linear Block Copolymers Depending on Dendron Generation and Preparation Procedure

Amphiphilic rod–coil dendritic–linear block copolymers PEG­(G_<i>m</i>)-<i>b</i>-PMPCS (where <i>m</i> is the number of dendron generation, and <i>m</i> = 1, 2, 3) composed of a semirigid Percec-type dendron with hydrophilic poly­(ethylene glycol) (PEG) tails and a rod-like mesogen-jacketed liquid crystalline polymer, poly­{2,5-bis­[(4′-methoxy-phenyl)­oxycarbonyl]­styrene} (PMPCS), were successfully prepared. The self-assembled structures undergo a transition from vesicles through large compound vesicles (LCVs) to short cylindrical micelles with increasing dendron generation. PEG­(G₂)-<i>b</i>-PMPCS forms stable LCVs with porous surfaces of a narrow size distribution in a mixed solvent of tetrahydrofuran and water. The formation mechanism of the supramolecular structure with nano- and microsized scales is studied through changing the rate of water addition. It is composed of two steps: morphological transformation and vesicles fusion or differentiation. Vesicles are precursors for LCVs regardless of what the initial morphology is. However, the final LCV structures are different. Slow addition of water produces spherical LCVs, while those formed during fast water addition are irregular (like deformed spherical) LCVs

Microphase Separation and High Ionic Conductivity at High Temperatures of Lithium Salt-Doped Amphiphilic Alternating Copolymer Brush with Rigid Side Chains

An amphiphilic alternating copolymer brush (AACPB), poly­{(styrene-<i>g</i>-poly­(ethylene oxide))-<i>alt</i>-(maleimide-<i>g</i>-poly­{2,5-bis­[(4-methoxy­phenyl)­oxycarbonyl]­styrene})}­(P­{(St-<i>g</i>-PEO)-<i>alt</i>-(MI-<i>g</i>-PMPCS)}), was synthesized by alternating copolymerization of styrene-terminated poly­(ethylene oxide) (St-PEO) and maleimide-terminated poly­{2,5-bis­[(4-methoxy­phenyl)-oxy­carbonyl]­styrene} (MI-PMPCS) macromonomers using the “grafting through” strategy. ¹H NMR and gel permeation chromatography coupled with multiangle laser light scattering were used to determine the molecular characteristics of AACPBs. Although these AACPBs cannot microphase separate with thermal and solvent annealing methods, they can form lamellar structures by doping a lithium salt. This is a first report on lithium salt-induced microphase separation of AACPBs, and the lithium salt-doped AACPBs can serve as solid electrolytes for the transport of lithium ion. For the same AACPB, the ionic conductivity (σ) increases with increasing doping ratio. In addition, σ values of different AACPBs with the same doping ratio become higher for shorter PMPCS side chains. The σ value of the lithium salt-doped AACPB increases with increasing temperature in the range of 25–240 °C, and σ is 1.79 × 10^–4 S/cm at 240 °C. The relatively high σ values of the lithium-doped AACPBs at high temperatures benefit from the rigid PMPCS side chain and the AACPB architecture. The lithium salt-doped AACPBs have the potential to serve as solid electrolytes in high-temperature lithium ion batteries

Hierarchical Self-Assembly of Disc-Rod Shape Amphiphiles Having Hexa-peri-hexabenzocoronene and a Relatively Long Rod

Two disc-rod shape amphiphiles consisting of hexa-peri-hexabenzocoronene (HBC) and a nanosized rodlike mesogen were designed and synthesized. Thermotropic phase behaviors were carefully studied. Despite significant steric mismatch between the discs and rods, hierarchical structures were observed for both disc-rod shape amphiphiles at ambient temperature and upon heating. Molecular packing schemes were proposed and confirmed using the reconstructed electron density maps, molecular dynamics simulation, and direct observation using transmission electron microscope. The results demonstrate that the shape effect is of great importance in the self-assembly of shape amphiphiles