11 Search Results

Ferroelectric liquid crystalline polymers (LCPs) with a high spontaneous polarization (P_s) are of great interest for the fabrication of advanced electronic devices. However, conventional ferroelectric LCPs typically exhibit low spontaneous polarization, only around 1 mC/m². To increase the orientational polarization for ferroelectric LCPs, in this work, we designed mesogen-free comb-shaped LCPs based on isotactic polyepichlorohydrin, which contained highly dipolar sulfonyl groups (dipole moment of 4.5 Debye) in the side chains. The goal was to increase dipole density and induce ferroelectric switching. In this new series of mesogen-free LCPs, the sulfonyl groups were first moved away from the main chain tomore » mitigate strong dipole–dipole interactions. Second, methyl-branched alkyl side chains were implemented to decrease the crystallizability and induce the Smectic C (SmC) self-assembly. Intriguingly, these SmC samples, for the first time, exhibited typical paraelectric behavior (i.e., slim S-shaped single hysteresis loops) with a high dielectric constant in the range of 20–30. Finally, learning from the knowledge obtained from this study, we will be able to design new mesogen-free LCPs with tailor-made dipole–dipole interactions and realize ferroelectricity.« less

To realize advanced electrical applications for ferroelectric liquid crystalline polymers, high spontaneous polarization (P_s) is highly desired. However, current ferroelectric liquid crystalline polymers usually exhibit a low P_s. In this work, we designed and synthesized mesogen-free, chiral polyethers containing sulfonylated methyl-branched alkyl side chains with a -(CH₂)₃O- spacer between the sulfonyl and the branched alkyl groups. In contrast to the linear n-alkyl side chains, the methyl-branched alkyl side chains induced chain tilting in the smectic layers. When double chirality existed in both the main chain and the side chains, a crystalline structure was observed after mechanical stretching. Intriguingly, when singlemore » chirality existed in either the backbone or the side chains, a liquid crystalline smectic C phase was obtained. The electric displacement-electric field study, however, did not show typical ferroelectric switching, although the dielectric constants were relatively high for these liquid crystalline polymers. This is likely because the dipole-dipole interactions among neighboring sulfonyl groups along the main chain were so strong that the ferroelectric switching was hindered in the samples. For the future work, it is desired to weaken the dipole-dipole interaction to achieve ferroelectricity in these mesogen-free liquid crystalline polymers.« less

In this report, new approaches were implemented to achieve novel ferroelectric liquid crystalline self-assembly in mesogen-free comb-like isotactic polyoxypropylenes (iPOPs) bearing mono- or di-sulfonyl groups, which have potential to exhibit high spontaneous polarization for applications in advanced electronic devices. iPOPs with n-alkylsulfonylpentylthioether side chains (iPOP-SC₅SO₂C_n, where C_n is the alkyl tail with n being either 12 or 8) were prepared via a post-polymerization substitution reaction from isotactic poly[(R)-(–)-epichlorohydrin]. Here, the effects of main-chain tacticity, chiral center-to-dipole distance, number of sulfonyl dipoles per side chain, and n-alkyl tail length on the liquid crystalline self-assembly behavior were investigated by comparing iPOPs withmore » their atactic counterparts. First, when the sulfonyl dipole was placed far away from the chiral center in iPOP-SC₅SO₂C_n compared with our previously reported isotactic polyethers with n-alkylsulfonyl side chains (iPOP-SO₂C_n), the dipole–dipole interactions among the side chains decreased, which led to more liquid crystalline phases. Smectic E (SmE) and A (SmA) phases were observed after crystal melting; however, the main-chain chiral center was found to have an insignificant effect on the liquid crystalline assembly when compared to the atactic samples. Second, the longer C_n tails increased the transition temperatures as a result of stronger van der Waals interactions. Third, after the thioether linkage in the side chain was oxidized into the sulfonyl group, the chiral center–dipole and dipole–dipole interactions in the resultant polymers (i.e., iPOP-SO₂C₅SO₂C_n) enhanced. As a result, the liquid crystalline phase transitions (i.e., crystal → SmE → SmA → I) were pushed to higher temperatures. The knowledge gained in this study will help us further design and achieve the ferroelectric smectic C* phase in mesogen-free comb-like sulfonylated liquid crystalline polymers.« less

Strong dipole–dipole interactions in mesogen-free aliphatic comblike polymers can induce unique liquid crystalline (LC) self-assembly. By comparing atactic and isotactic poly(oxypropylene)s with n-alkylsulfonyl side groups (aPOP-SO₂C_n and iPOP-SO₂C_n), the effect of main-chain tacticity on the ordered phase behavior was studied in this work. For aPOP-SO₂C_n, LC self-assemblies were observed when n is ≥3. Two types of LC phases were formed depending on the side chain length n: (i) a three-dimensional (3D) high-order LC structure (possibly crystal B or E) for n = 3–5 and (ii) a smectic A (SmA) structure for n = 6–12. For iPOP-SO₂C_n, crystallization was favored asmore » a result of regular main-chain configuration. Only when n ≥ 10, the isotactic comblike polymers exhibited a monotropic phase behavior with the SmA phase appearing during cooling from the isotropic melt. Regardless of atactic or isotactic main chains, these ordered LC and crystalline phases featured a double-layer structure, which was dictated by the optimal dipole–dipole interaction distance (i.e., 0.40–0.44 nm) between sulfonyl groups in the neighboring side chains. Here, this study showed that the chiral centers incorporated in the backbone could not induce chiral smectic self-assembly for the isotactic polymers. As such, we are currently implementing chiral centers in the side chains for the formation of ferroelectric structures for these mesogen-free aliphatic LC polymers.« less

Here, organic monolayers of epoxy-containing oligo(ethylene oxide)s were grafted to the surface of silicon nanoparticles via a hydrosilylation reaction. The surface functional groups suppressed the chemical and electrochemical reactivity of the as-grown and lithiated silicon nanoparticles with high material utilization. A robust Si/electrolyte interphase was formed with the participation of the grafted organic groups with facilitated Li+ transfer and was further enforced by electrode integrity via the epoxy/poly(acrylic acid) (PAA) binder reaction. The improved cycling stability and post-test analysis indicate that surface functionalization on the Si particle level is a potential method to enabling a Si anode in high-energy-density lithium-ionmore » batteries.« less

An epoxy group was successfully attached to the surface of silicon nanoparticle (SiNPs) via a silanization reaction between silanol-enriched SiNPs and functional silanes. The epoxy-functionalized SiNPs showed a much improved cell performance compared with the pristine SiNPs because of the increased stability with electrolyte and the formation of a covalent bond between the epoxy group and the polyacrylic acid binder. Furthermore, the anode laminate made from epoxy- SiNPs showed much enhanced adhesion strength. Post-test analysis shed light on how the epoxy-functional group affects the physical and electrochemical properties of the SiNP anode.

Here, this article reports a family of UCST-type thermoresponsive polymers, poly(alkyl methacrylate)s with an appropriate alkyl pendant length, in an industrially important non-volatile organic liquid polyalphaolefin (PAO). The cloud point (CP) can be readily tuned over a wide temperature range by changing the alkyl pendant length; at a concentration of 1 wt% and similar polymer molecular weights, the CP varies linearly with the (average) number of carbon atoms in the alkyl pendant. PAO solutions of ABA triblock copolymers, composed of a PAO-philic middle block and thermoresponsive outer blocks with appropriate block lengths, undergo thermoreversible sol-gel transitions at sufficiently high concentrations.more » The discovery of thermoresponsive polymers in PAO makes it possible to explore new applications by utilizing PAO’s unique characteristics such as thermal stability, non-volatility, superior lubrication properties, etc. Lastly, two examples are presented: thermoresponsive physical gels for control of optical transmittance and injectable gel lubricants.« less

This article reports on improved lubricating performance by combining oil-soluble poly(lauryl methacrylate) brush-grafted silica nanoparticles (hairy NPs or HNP) and an oil-miscible phosphonium-phosphate ionic liquid (IL) as a friction-reducing additive for a polyalphaolefin (PAO) oil. The HNP was synthesized by surface-initiated reversible addition–fragmentation chain transfer polymerization. At a total concentration of 2% and sufficiently high individual concentrations for HNP and IL in PAO, high-contact stress, ball-on-flat reciprocating tribological tests showed that the friction decreased by up to 23% compared with 2% HNP alone in PAO and by up to 35% compared to the PAO mixed with 2% IL. Scanning electronmore » microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS) analysis revealed that the tribofilm formed from the PAO containing 1% HNP + 1% IL was enriched with both Si and P, indicating that both hairy NPs and IL were involved in the tribochemical reactions. In addition, the O 1s and Si 2p peaks in the core-level XPS spectra exhibited significant shifts for the mixture of 1% HNP + 1% IL compared to those for 2% HNP, suggesting the possible formation of new covalent bonds. These results indicated that HNP and IL reacted with each other and also with the metal substrate during the rubbing process, which likely strengthened the tribofilm and its bonding with the substrate and thus further improved the lubrication.« less

This paper reports on the synthesis of a series of poly(alkyl methacrylate) brush-grafted, 23 nm silica nanoparticles (hairy NPs) and the study of the effect of alkyl pendant length on their use as oil lubricant additives for friction and wear reduction. The hairy NPs were prepared by surface-initiated reversible addition–fragmentation chain transfer polymerization from trithiocarbonate chain transfer agent (CTA)-functionalized silica NPs in the presence of a free CTA. We found that hairy NPs with sufficiently long alkyl pendant groups (containing >8 carbon atoms, such as 12, 13, 16, and 18 in this study) could be readily dispersed in poly(alphaolefin) (PAO),more » forming clear, homogeneous dispersions, and exhibited excellent stability at low and high temperatures as revealed by visual inspection and dynamic light scattering studies. Whereas poly(n-hexyl methacrylate) hairy NPs cannot be dispersed in PAO under ambient conditions or at 80 °C, interestingly, poly(2-ethylhexyl methacrylate) hairy NPs can be dispersed in PAO at 80 °C but not at room temperature, with a reversible clear-to-cloudy transition observed upon cooling. High-contact-stress ball-on-flat reciprocating sliding tribological tests at 100 °C showed significant reductions in both the coefficient of friction (up to 38%) and wear volume (up to 90% for iron flat) for transparent, homogeneous dispersions of hairy NPs in PAO at a concentration of 1.0 wt % compared with neat PAO. Finally, the formation of a load-bearing tribofilm at the rubbing interface was confirmed using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy.« less

The development of high performance lubricants has been driven by increasingly growing industrial demands and environmental concerns. Here we demonstrate oil-soluble polymer brush-grafted inorganic nanoparticles (hairy NPs) as highly effective lubricant additives for friction and wear reduction. A series of oil-miscible poly(lauryl methacrylate) brush-grafted silica and titania NPs were synthesized by surface-initiated atom transfer radical polymerization. These hairy NPs showed exceptional stability in polyalphaolefin (PAO) base oil; no change in transparency was observed after being kept at -20, 22, and 100 °C for at least 55 days. High contact stress ball-on-flat reciprocating sliding tribological tests at 100 °C showed thatmore » addition of 1 wt% of hairy NPs into PAO led to significant reductions in coefficient of friction (up to ~ 40%) and wear volume (up to ~ 90%). The excellent lubricating properties of hairy NPs were further elucidated by the characterization of the tribofilm formed on the flat. These hairy NPs represent a new type of lubricating oil additives with high efficiency in friction and wear reduction.« less