Smectic C Self-Assembly in Mesogen-Free Liquid Crystalline Chiral Polyethers with Sulfonylated Methyl-Branched Side Chains.

To realize advanced electrical applications for ferroelectric liquid crystalline polymers, high spontaneous polarization (Ps ) is highly desired. However, current ferroelectric liquid crystalline polymers usually exhibit a low Ps . In this work, mesogen-free, chiral polyethers containing sulfonylated methyl-branched alkyl side chains with a (CH2 )3 O spacer between the sulfonyl and the branched alkyl groups are designed and synthesized. In contrast to the linear n-alkyl side chains, the methyl-branched alkyl side chains induce chain tilting in the smectic layers. When double chirality exists in both the main chain and the side chains, a crystalline structure is observed after mechanical stretching. Intriguingly, when single chirality exists in either the backbone or the side chains, a liquid crystalline smectic C phase is obtained. The electric displacement-electric field study, however, does not show typical ferroelectric switching, although the dielectric constants are relatively high for these liquid crystalline polymers. This is likely because the dipole-dipole interactions among neighboring sulfonyl groups along the main chain are so strong that the ferroelectric switching is 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.

Nanometer-precision non-local deformation reconstruction using nanodiamond sensing.

Spatially resolved information about material deformation upon loading is critical to evaluating mechanical properties of materials, and to understanding mechano-response of live systems. Existing techniques may access local properties of materials at nanoscale, but not at locations away from the force-loading positions. Moreover, interpretation of the local measurement relies on correct modeling, the validation of which is not straightforward. Here we demonstrate an approach to evaluating non-local material deformation based on the integration of nanodiamond orientation sensing and atomic force microscopy nanoindentation. This approach features a 5 nm precision in the loading direction and a sub-hundred nanometer lateral resolution, high enough to disclose the surface/interface effects in the material deformation. The non-local deformation profile can validate the models needed for mechanical property determination. The non-local nanometer-precision sensing of deformation facilitates studying mechanical response of complex material systems ranging from impact transfer in nanocomposites to mechano-response of live systems.

Correlating the effect of co-monomer content with responsiveness and interfacial activity of soft particles with stability of corresponding smart emulsions.

HYPOTHESIS: This study presents the synthesis and characterization of Poly(N-isopropylacrylamide)-co-methacrylic acid (PNIPAM-co-MAA) based multi-responsive soft microgel particles employed as "smart emulsifiers" for controlled stabilization and breakage of the decane-in-water Pickering emulsions. These soft microgel particles can act as reversible stabilizers, i.e. they can either stay at the oil-water interface by supporting emulsion formation or preventing aggregates; while triggering demulsification can be controlled by varying the temperature, pH or ionic strength of the microgel system. EXPERIMENTS: Dynamic light scattering was applied to observe the variation in hydrodynamic radius of the particles as a function of temperature and pH of the multi-responsive microgel system. Microgel composition was varied in terms of MAA-content and influence of this variation on their thermo-sensitivity and pH responsiveness as well as on the stability of corresponding emulsions was evaluated. FINDINGS: The microgel particles with highest MAA content showed a significant impact on multi-responsive behaviour. Thermal sensitivity is pH dependent under acidic conditions but this dependence is gradually reduced as the pH increases above 7.5. On the other hand, pH-responsiveness is enhanced with the rise in temperature and stable emulsions were formed under highly alkaline conditions even the temperature was far above the volume phase transition temperature (VPTT). Understanding the correlation of stimuli responsiveness at interface with the emulsion stability would help to fabricate and design novel smart Pickering emulsions with better control over desired properties.

Microgel Particles at Interfaces: Phenomena, Principles, and Opportunities in Food Sciences.

The use of soft microgel particles for stabilizing emulsions has captured increasing attention across a wide range of disciplines in the past decades. Being soft, the nanoparticles, which are spherical in solution, undergo a structure change when adsorbed at the oil-water interface. This morphology change leads to the special dynamic properties of interface layers and packing structures, which then alter the interfacial tension and rheological properties of the interface. In addition, emulsions stabilized by these particles, known as Pickering emulsions, can be triggered by changing a variety of environmental conditions, which is especially desirable in industrial applications such as oil transportation processes and biphasic catalysis, where the emulsions can be stabilized and destabilized on demand. Although many studies of the behavior of soft microgel nanoparticles at interfaces have been reported, there are still many challenges in gaining a full understanding of the structure, dynamics, and effective interactions between microgels at the interface. In this Feature Article, we address some of the most important findings and problems in the field. They include the adsorption kinetics of soft microgel particles, particle conformation at the interface, pH and thermal responsiveness, and the interfacial rheological properties of soft-particle-occupied interfaces. We also discuss some potential benefits of using emulsions stabilized by soft particles for food applications as an alternative to conventional surfactant-based systems. We hope to encourage further investigation of these problems, which would be very beneficial to extending this knowledge to all other related soft matter systems.

Hybrid nanodiamond quantum sensors enabled by volume phase transitions of hydrogels.

Diamond nitrogen-vacancy (NV) center-based magnetometry provides a unique opportunity for quantum bio-sensing. However, NV centers are not sensitive to parameters such as temperature and pressure, and immune to many biochemical parameters such as pH and non-magnetic biomolecules. Here, we propose a scheme that can potentially enable the measurement of various biochemical parameters using diamond quantum sensing, by employing stimulus-responsive hydrogels as a spacing transducer in-between a nanodiamond (ND, with NV centers) and magnetic nanoparticles (MNPs). The volume phase transition of hydrogel upon stimulation leads to sharp variation in the separation distance between the MNPs and the ND. This in turn changes the magnetic field that the NV centers can detect sensitively. We construct a temperature sensor under this hybrid scheme and show the proof-of-the-principle demonstration of reversible temperature sensing. Applications in the detection of other bio-relevant parameters are envisioned if appropriate types of hydrogels can be engineered.

Comparing the Relative Interfacial Affinity of Soft Colloids With Different Crosslinking Densities in Pickering Emulsions.

Pickering emulsions prepared by various kinds of soft colloids such as the poly(N-isopropylacrylamide) (PNIPAM)-based microgels, have been studied for decades in order to fabricate stimuli-responsive emulsions. It has been generally viewed that the interfacial properties of the microgel monolayers and the emulsion stability are dominated by the softness or deformability of the microgel particles. However, there is still no convenient way to characterize the adsorption/desorption energy of the microgels at the interface although this is an essential topic for microgel-stabilized emulsions. This paper presents a novel method for directly comparing the relative interfacial affinity of microgel particles with comparable size but different crosslinking densities, therefore, different softness at the oil/water interface. Typical micron-sized PNIPAM-based microgels were synthesized and used in this study. With advanced fluorescent labeling techniques, we are capable of distinguishing different kinds of microgels in a Pickering emulsion. During vigorous agitation, particles with higher adsorption energy are more likely to be found at the oil/water interface instead of the loosely adsorbed counterparts. By counting the ratio of interfacial area occupied by two microgels, the interfacial affinity of them can be compared. It is found that interfacial affinity of microgels is not only dependent on the softness but also strongly correlated with the core-shell morphology of the microgels, especially the outmost collapsed polymer layer at the interface. This result is consistent with the interfacial morphology model proposed by other researchers. The understanding of the stabilization of such Pickering emulsions can help us to design and develop responsive Pickering emulsions with better controlled stability.

A confocal microscopy study of micron-sized poly(N-isopropylacrylamide) microgel particles at the oil-water interface and anisotopic flattening of highly swollen microgel.

HYPOTHESIS: Responsive poly(N-isopropylacrylamide) microgel (PNIPAM microgel) stabilized Pickering emulsions were investigated in this study. A recent theoretical study of other researchers has suggested that large soft particles at the oil/water interface are less deformable than their small counterparts. Therefore, we expected that our micron-sized microgel particles might not significantly deform at the oil/water interface. EXPERIMENTS: We applied confocal laser scanning microscopy (CLSM) to examine the structure of soft PNIPAM-based microgel particles at the decane-water interface in a microgel-stabilized emulsion. Using micron-sized microgel particles with better labelling techniques, we could compensate the weakness in resolution of using CLSM. Seven PNIPAM-based microgel samples with various softness values and morphologies were examined at different pH values. FINDINGS: Our results demonstrate that the deformation of ordinary micron-sized microgel samples was not significant if they were not in the pH-swollen state. Nevertheless, the soft, pH-swollen microgel particles exhibited anisotropic deformation at the decane-water interface. Such flattening was not reported in previous studies. The studies of microgel particles at the oil-water interface with different imaging techniques and their comparison are valuable to help to elucidate the particles' roles in stabilizing the Pickering emulsions.

Controlling the synthesis and characterization of micrometer-sized PNIPAM microgels with tailored morphologies.

This Article presents the controlling synthesis and characterization of micrometer-sized, multiresponsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-MAA) microgel particles. By combining semibatch and temperature-programmed surfactant-free precipitation polymerization, we have successfully developed a novel approach to the preparation of temperature- and pH-responsive PNIPAM microgels with a dense-shell (DS), dense-core (DC), or homogeneous (HOMO) structure. We then investigated the interaction between the synthesized microgels and some fluorescent dye molecules using confocal laser scanning microscopy (CLSM). Our results have qualitatively revealed that the cross-linkers and the functional carboxylic groups (-COOH) could be homogeneously distributed, predominately localized inside the core, or concentrated near the surface of the synthesized microgels. Moreover, pH-responsive swelling behaviors of the microgels were investigated and discussed with titration and CLSM data. We found that the swelling capability is strongly dependent on the morphology of the PNIPAM microgel. Besides the absorption of fluorescent molecules, the synthesized microgels also showed a strong affinity for fluorescently labeled polypeptide, even at a relatively high salt concentration.

Preparation of responsive micrometer-sized microgel particles with a highly functionalized shell.

We describe a facile approach for the synthesis of micrometer-sized (∼3.5 µm), pH-responsive microgel particles, which have functional carboxylic acid groups concentrated in the shell. The large size offers the possibility to directly study the interactions between individual, isolated microgel particles with active ingredients by optical microscopy. Our results show that the synthesized microgel particles can load and release active ingredients via changing pH values. The complexation of Ca(2+) with the -COOH functional groups located at the microgel surfaces not only regulates the active ingredient's uptake efficiency, but also provides a novel way to reveal the spatial distribution of the functional groups inside the microgel particles.