Goran Ungar

Poly(vinylidene fluoride) (PVDF) was crystallized under a combination of shear rate (0–31 s–1), pressure (50–600 MPa), and temperature (150–200 °C), producing flexible pressure sensor films with a high piezoelectric stress coefficient g33 of up to 1.2 V·m·N–1. This matches the highest value reported in the literature, but is achieved more quickly, at lower temperatures, and at significantly lower pressures. The key was to overcome the entropically favored gauche-rich centrosymmetric alpha form and obtain almost exclusively the trans-rich piezoelectric β and γ forms. This was achieved by steering the system through the highly supercooled hexagonal mesophase, normally achievable only at high pressures, but here, aided by shear, was transiently reached at only 1 kbar (100 MPa). A further advantage of the highly oriented shish-kebab structure that results from this process is that the favorable molecular dipole orientation is intrinsically higher compared with that in the normal spherulitic structure. A qualitative, thermodynamics-based mechanism is proposed, explaining the synergism of flow and pressure. The films were obviously poled, although no special poling procedure was applied. The “self-poling” was attributed to the temperature gradient in the pressure-shear cell. Global symmetry breaking is assumed to have occurred at the transition from the hexagonal mesophase to the crystalline beta form.

A custom-built experimental setup combining in situ fast scanning chip calorimetry (FSC) with small-angle X-ray scattering (SAXS) is presented. The setup enables synchronized calorimetric and structural measurements with millisecond acquisition times and heating or cooling rates of up to several thousand kelvin per second. Its capabilities were demonstrated using the ultra-long monodisperse n -alkane n -C ₃₉₀ H ₇₈₂ , a model compound for linear polyethylene. Varying the cooling rate from the melt over nearly five orders of magnitude produces pronounced changes in lamellar thickness, corresponding to different non-integer-folded chain configurations, from approximately the F ₅ form after rapid quenching to F ₂ –F ₃ configurations after slow cooling. During subsequent in situ heating at 1000 K s ⁻¹ , SAXS revealed a pronounced increase in the average crystalline lamellar thickness from values characteristic of the F ₅ form toward those expected for the F ₄ form, consistent with rapid structural reorganization during the onset of melting. These results demonstrate that significant structural reorganization can occur even under ultrafast heating conditions and highlight the potential of the combined FSC–SAXS approach for probing transient and kinetically controlled structural transformations in polymers and other soft-matter systems.

We report a new 3D‐ordered liquid‐crystal (LC) phase where alkyl and fluoroalkyl side‐chains are confined within separate stretched octahedral cages of a tetragonal framework with struts of aromatic rods held together by their hydrogen‐bonded ends acting as flexible hinges. This is the first example of a multicolor micellar LC heralding the development of soft and dynamic dual‐compartment frameworks, also capable of a martensitic‐type phase transition.

We report a new 3D‐ordered liquid‐crystal (LC) phase where alkyl and fluoroalkyl side‐chains are confined within separate stretched octahedral cages of a tetragonal framework with struts of aromatic rods held together by their hydrogen‐bonded ends acting as flexible hinges. This is the first example of a multicolor micellar LC heralding the development of soft and dynamic dual‐compartment frameworks, also capable of a martensitic‐type phase transition.

Self-poisoning is ubiquitous in polymer crystallization but has so far manifested itself visibly only as minima in growth rate vs temperature in either monodisperse systems where, e.g., unstable folded chains obstruct crystallization of stable extended chains, or in periodically segmented chains where unstable stems with n-1 segments disturb deposition of stable stems with n segments. Here, we report a new type of self-poisoning found in poly(lactic acid), where a less stable crystal form ( α ′ ) disturbs growth of the stable form ( α ). While α requires strict up-down order of the polar chains, α’ does not, hence is kinetically favored. Unexpectedly, below the temperature of the growth rate minimum, the lamellar thickness increases rather than drops, as in all other reported cases of polymer crystallization with decreasing temperature. A growth rate equation model is developed, giving good match with experiments, but revealing an unexpectedly low fold surface free energy of α ′ form. Delayed crystallization due to self-poisoning of α in practical fast-cooling processing not only gives the low-modulus α ′ form, but also leads to an increased glassy amorphous fraction that results in embrittlement of the biofriendly poly(lactic acid) through physical aging.

A liquid crystal (LC) polymethylsiloxane (PMS) with rod‐like aromatic side‐groups attached via an alkylene spacer and bearing three n‐dodecyl end‐tails is found to form an unusual cubic structure. In a normal LC double gyroid (DG), the two chiral subspaces, one each side of the G‐surface, are occupied by one network each. Here each such network is split into two aromatic strands that wind around the central polysiloxane bundle, forming a double helix, resulting in a four‐network gyroid (4NG). While in previous normal LC DGs the network twist was assumed to follow that of the subspace, in 4NG the twist sense of the double‐helix is opposite to that of the subspace., i.e., while a right‐handed subspace twists by +70.5° between junctions, the double‐helix “supertwists” by −109.5°, and the opposite is true for the left‐handed subspace. Detailed analysis by X‐ray diffraction, DSC, and depolarized fluorescence (DF) shows a gradual but significant reversible change in the degree of mixing between the aromatic side groups and the polysiloxane backbones at 120 °C–130 °C in 4NG. Also, a significant increase in the system mobility starts only at ∼40 °C above the melting point, indicating persistence of local double‐helical segments even in the melt.

FePt nanoparticles (NPs) with thermally responsive array structures were prepared by surface modification with a liquid-crystalline phenylethyl ether-type dendron. Dendron modification was carried out in two steps. In the first step, COOH moieties were introduced on the surface of oleyl-modified FePt NPs by ligand exchange using terminal COOH-substituted n-alkyl phosphonic acid. This was followed by the amidation reaction between COOH moieties and the amino-substituted dendron. Infrared spectroscopic measurement and thermogravimetric analysis confirmed that the dendrons were successfully grafted onto the surface of FePt NPs. Transmission electron microscopy also showed an increase in interparticle distance due to dendron modification. Small-angle X-ray scattering (SAXS) revealed that the dendronized NPs pack on an FCC lattice, which melts above 100 °C in thermal response, consistent with the endothermic peak observed in differential scanning calorimetry (DSC). Electron density maps offer detailed insights into the structural organization of the dendronized FePt NP array. These results indicate that the formation of an FCC phase through self-assembly and its thermal transition into a disordered phase enabled dynamic control over magnetic NP arrays. Finally, changes in magnetic properties during thermal phase transition were investigated by a superconducting quantum interference device. The results show certain changes in magnetization behavior introduced by the thermal phase transition. These results demonstrate that dendron modification of FePt NPs enables the introduction of thermal responsiveness in their supramolecular structure and magnetic behavior. The flexible control of magnetic properties is expected to expand the application potential of the magnetic NPs used as building blocks for tunable smart magnetic devices.

A polysiloxane with tris-alkoxy-ended rod-like mesogenic side groups forms an unusually low-symmetry antiferrochiral liquid crystal, space group P1̅, consisting of distorted left- and right-handed double-helices containing alternating splay and twist sections. The unit cell contains two double-helical columns. On faster cooling, a closely related metastable orthorhombic structure is formed, symmetry Fddd, with 8 double helices per cell.