Apparent First‐Order Liquid–Liquid Transition with Pre‐transition Density Anomaly,in Water‐Rich Ideal Solutions

The striking increases in response functions observed during supercooling of pure water have been the source of much interest and controversy. Imminent divergences of compressibility etc. unfortunately cannot be confirmed due to pre‐emption by ice crystallization. Crystallization can be repressed by addition of second components, but these usually destroy the anomalies of interest. Here we study systems in which protic ionic liquid second components dissolve ideally in water, and ice formation is avoided without destroying the anomalies. We observe a major heat capacity spike during cooling, which is reversed during heating, and is apparently of first order. It occurs just before the glassy state is reached and is preceded by water‐like density anomalies. We propose that it is the much‐discussed liquid–liquid transition previously hidden by crystallization. Fast cooling should allow the important fluctuations/structures to be preserved in the glassy state for leisurely investigation.     

Anomalous Freezing of Nano‐Confined Water in Room‐Temperature Ionic Liquid 1‐Butyl‐3‐Methylimidazolium Nitrate

Non‐crystal formation of ice is investigated by simultaneous X‐ray diffraction and differential scanning calorimetry measurements upon cooling to ?100 °C. At room temperature, size‐tunable water confinement (≈20 Å size) in a room‐temperature ionic liquid (RTIL, 1‐butyl‐3‐methylimidazolium nitrate, [C₄mim][NO₃]) exists in a water‐rich region (70–90 mol % D₂O). The confined water (water pocket) is characterized by almost monodispersive size distribution. In [C₄mim][NO₃]‐x mol % D₂O (70<x<94), the absence of sharp Bragg reflections and a distinct exothermal peak indicate that crystallization/cold crystallization both of [C₄mim][NO₃] and D₂O is suppressed, even upon slow cooling and heating.     

“CLASSIC NMR”: An In‐Situ NMR Strategy for Mapping the Time‐Evolution of Crystallization Processes by Combined Liquid‐State and Solid‐State Measurements

A new in‐situ NMR strategy (termed CLASSIC NMR) for mapping the evolution of crystallization processes is reported, involving simultaneous measurement of both liquid‐state and solid‐state NMR spectra as a function of time. This combined strategy allows complementary information to be obtained on the evolution of both the solid and liquid phases during the crystallization process. In particular, as crystallization proceeds (monitored by solid‐state NMR), the solution state becomes more dilute, leading to changes in solution‐state speciation and the modes of molecular aggregation in solution, which are monitored by liquid‐state NMR. The CLASSIC NMR experiment is applied here to yield new insights into the crystallization of m‐aminobenzoic acid.     

Collapse Transition‐Assisted Crystallization in P3HT Solution

Understanding the multiple phase transitions such as collapse transition, phase separation, and crystallization in solutions is of fundamental importance to control the solution structure of conjugated polymers in device processing. Combining in situ synchrotron radiation small and wide‐angle X‐ray scattering, ultrasensitive differential scanning calorimetry, ultraviolet–visible absorption spectroscopy, and polarized optical microscopy, we investigate the order–disorder transitions in poly(3‐hexylthiophene)/toluene solutions during cooling and heating processes. We demonstrate the occurrence of collapse transition of polymer chains from a random coil state to a lower dimensional network prior to the onset of crystallization during cooling in solution. This conformational preordering can lead to the formation of a lyotropic liquid crystalline phase, which is of great significance to the crystallization and ordering in polymer films, and further to promote its electric performance. It is examined that the mobility of films cast from chain‐collapsed solutions can be one order of magnitude higher than that from isotropic solutions with random‐coiled conformations. Thus, the conformational preordering in solutions is proposed to be a more efficient way than the postannealing of films to improve the electric performance of conjugated polymer films. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1105–1114     

Waterlike glass polyamorphism in a monoatomic isotropic Jagla model

We perform discrete-event molecular dynamics simulations of a system of particles interacting with a spherically-symmetric (isotropic) two-scale Jagla pair potential characterized by a hard inner core, a linear repulsion at intermediate separations, and a weak attractive interaction at larger separations. This model system has been extensively studied due to its ability to reproduce many thermodynamic, dynamic, and structural anomalies of liquid water. The model is also interesting because: (i) it is very simple, being composed of isotropically interacting particles, (ii) it exhibits polyamorphism in the liquid phase, and (iii) its slow crystallization kinetics facilitate the study of glassy states. There is interest in the degree to which the known polyamorphism in glassy water may have parallels in liquid water. Motivated by parallels between the properties of the Jagla potential and those of water in the liquid state, we study the metastable phase diagram in the glass state. Specifically, we perform the computational analog of the protocols followed in the experimental studies of glassy water. We find that the Jagla potential calculations reproduce three key experimental features of glassy water: (i) the crystal-to-high-density amorphous solid (HDA) transformation upon isothermal compression, (ii) the low-density amorphous solid (LDA)-to-HDA transformation upon isothermal compression, and (iii) the HDA-to-very-high-density amorphous solid (VHDA) transformation upon isobaric annealing at high pressure. In addition, the HDA-to-LDA transformation upon isobaric heating, observed in water experiments, can only be reproduced in the Jagla model if a free surface is introduced in the simulation box. The HDA configurations obtained in cases (i) and (ii) are structurally indistinguishable, suggesting that both processes result in the same glass. With the present parametrization, the evolution of density with pressure or temperature is remarkably similar to the corresponding experimental measurements on water. Our simulations also suggest that the Jagla potential may reproduce features of the HDA-VHDA transformations observed in glassy water upon compression and decompression. Snapshots of the system during the HDA-VHDA and HDA-LDA transformations reveal a clear segregation between LDA and HDA but not between HDA and VHDA, consistent with the possibility that LDA and HDA are separated by a first order transformation as found experimentally, whereas HDA and VHDA are not. Our results demonstrate that a system of particles with simple isotropic pair interactions, a Jagla potential with two characteristic length scales, can present polyamorphism in the glass state as well as reproducing many of the distinguishing properties of liquid water. While most isotropic pair potential models crystallize readily on simulation time scales at the low temperatures investigated here, the Jagla potential is an exception, and is therefore a promising model system for the study of glass phenomenology.     

Cocrystallization of ethylene/1‐octene copolymer blends during crystallization analysis fractionation and crystallization elution fractionation

Blending of ethylene/1‐octene copolymers can be used to achieve a well‐controlled broad chemical composition distribution (CCD) required in several polyolefin applications. The CCD of copolymer blends can be estimated using crystallization analysis fractionation (CRYSTAF) or crystallization elution fractionation (CEF). Unfortunately, both techniques may be affected by the cocrystallization of chains with different compositions, leading to profiles that do not truly reflect the actual CCD of the polymer. Therefore, understanding how the polymer microstructure and the analytical conditions influence copolymer cocrystallization is critical for the proper interpretation of CRYSTAF and CEF curves. In this investigation, we studied the effect of chain crystallizabilities, blend compositions, and cooling rates on cocrystallization during CEF and CRYSTAF analysis. Cocrystallization is more prevalent when the copolymer blend has components with similar crystallizabilities, one of the components is present in much higher amount, and fast cooling rates are used. CEF was found to provide better CCD estimates than CRYSTAF in a much shorter analysis time. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Glycine phases formed from frozen aqueous solutions: Revisited

Glycine phases formed when aqueous solutions were frozen and subsequently heated under different conditions were studied by Raman scattering, x-ray diffraction, and differential scanning calorimetry (DSC) techniques. Crystallization of ice I(h) was observed in all the cases. On cooling at the rates of 0.5 K∕min and 5 K∕min, glassy glycine was formed as an intermediate phase which lived about 1 min or less only, and then transformed into β-polymorph of glycine. Quench cooling of glycine solutions (15% w∕w) in liquid nitrogen resulted in the formation of a mixture of crystalline water ice I(h) and a glassy glycine, which could be preserved at cryogenic temperatures (80 K) for an indefinitely long time. This mixture remained also quite stable for some time after heating above the cryogenic temperature. Subsequent heating under various conditions resulted in the transformation of the glycine glass into an unknown crystalline phase (glycine "X-phase") at 209-216 K, which at 218-226 K transformed into β-polymorph of glycine. The "X-phase" was characterized by Raman spectroscopy; it could be obtained in noticeable amounts using a special preparation technique and tentatively characterized by x-ray powder diffraction (P2, a = 6.648 A?, b = 25.867 A?, c = 5.610 A?, β = 113.12[ordinal indicator, masculine]); the formation of "X-phase" from the glycine glassy phase and its transformation into β-polymorph were followed by DSC. Raman scattering technique with its power for unambiguous identification of the crystalline and glassy polymorphs without limitation on the crystallite size helped us to follow the phase transformations during quenching, heating, and annealing. The experimental findings are considered in relation to the problem of control of glycine polymorphism on crystallization.     

Atomic‐Level Mechanisms of Nucleation of Pure Liquid Metals during Rapid Cooling

To obtain a material with the desired performance, the atomic‐level mechanisms of nucleation from the liquid to solid phase must be understood. Although this transition has been investigated experimentally and theoretically, its atomic‐level mechanisms remain debatable. In this work, the nucleation mechanisms of pure Fe under rapid cooling conditions are investigated. The local atomic packing stability and liquid‐to‐solid transition‐energy pathways of Fe are studied using molecular dynamics simulations and first‐principle calculations. The results are expressed as functions of cluster size in units of amorphous clusters (ACs) and body‐centered cubic crystalline clusters (BCC‐CCs). We found the prototypes of ACs in supercooled liquids and successfully divided these ACs to three categories according to their transition‐energy pathways. The information obtained in this study could contribute to our current understanding of the crystallization of metallic melts during rapid cooling.     

Isothermal crystallization of P3HT:PCBM blends studied by RHC

Defining appropriate annealing temperatures and times is vitally important for increasing the efficiency of bulk heterojunction solar cells by favoring the crystallinity of the polymer-fullerene blend components. In order to better understand the annealing process, the isothermal crystallization of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend investigated by means of rapid heating cooling calorimetry (RHC). Isothermal crystallization experiments at temperatures in between the glass transition and melting, within the temperature range of 70–150 °C, can successfully be performed since RHC permits cooling at a sufficiently high rate in order to prevent crystallization during cooling. Crystallization isotherms were determined from the subsequent melting behavior of the blend. They were measured for a wide set of annealing temperatures and times, and the evolution of the crystallization rate with temperature is compared for annealing from the glassy state and from the melt state.     

Oxygen barrier properties of crystallized and talc‐filled poly(ethylene terephthalate)

The improvement in oxygen barrier properties of poly(ethylene terephthalate) (PET) by incorporation of an impermeable phase such as crystallinity or talc platelets was examined. Crystallinity was induced by crystallization from the glassy state (cold crystallization). Microlayering was used to create talc‐filled structures with controlled layer architecture. The reduction of permeability in crystallized and talc‐filled PET was well described by Nielsen's model. Changes in permeability of crystalline PET could not be ascribed to the filler effect of crystallites only. Our data on solubility, obtained on the basis of measurements of the oxygen transport coefficients, confirmed a previous finding that the amorphous phase density of PET decreases upon crystallization. The data were amenable to interpretation by free volume theory. Talc‐filled materials processed by different methods showed the same permeability; however, much better mechanical properties were achieved by microlayering. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 847–857, 1999     

Crystallization and stereocomplexation behavior of poly(D‐ and L‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) block copolymers

The thermal properties, crystallization, and morphology of amphiphilic poly(D ‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) (PDLA‐b‐PDMAEMA) and poly (L ‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) (PLLA‐b‐PDMAEMA) copolymers were studied and compared to those of the corresponding poly(lactide) homopolymers. Additionally, stereocomplexation of these copolymers was studied. The crystallization kinetics of the PLA blocks was retarded by the presence of the PDMAEMA block. The studied copolymers were found to be miscible in the melt and the glassy state. The Avrami theory was able to predict the entire crystallization range of the PLA isothermal overall crystallization. The melting points of PLDA/PLLA and PLA/PLA‐b‐PDMAEMA stereocomplexes were higher than those formed by copolymer mixtures. This indicates that the PDMAEMA block is influencing the stability of the stereocomplex structures. For the low molecular weight samples, the stereocomplexes particles exhibited a conventional disk‐shape structure and, for high molecular weight samples, the particles displayed unusual star‐like shape morphology. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1397–1409, 2011     

Dilatometric behavior and glass transition in a styrene‐acrylonitrile copolymer

Equilibrium and glass transition behavior of a styrene‐acrylonitrile copolymer (SAN) under different thermobaric histories were studied by means of a PVT dilatometer. Equilibrium behavior, as determined by isothermal and isobaric measurements, could be satisfactorily described using the Simha‐Somcynsky and Tait equations of state. Glass transition behavior depended upon the applied transformation path from the liquid‐equilibrium state to the glassy state. From isobaric cooling ramps performed at constant rate and at several pressures, it was possible to determine the glass transition temperature and its dependence upon pressure; whereas from isothermal compressions at various temperatures, it was possible to determine a glass transition pressure and its temperature dependence. Both the dependences were linear, and a correlation was observed between the slopes of the fitting lines. A possible interpretation of this correlation is provided in terms of free volume determined at the glass transition point by applying the Simha‐Somcynsky theory. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1904–1913, 2005     

Effect of crystallization and liquid–liquid phase separation on phase‐separation kinetics in poly(ethylene‐co‐vinyl alcohol)/glycerol solution

Liquid–liquid thermally induced phase separation of the polymer‐diluent system of poly(ethylene‐co‐vinyl alcohol) (EVOH)‐glycerol was examined under light scattering. For EVOH with an ethylene content of 38 mol % (EVOH38), maxima of the scattered light intensity were observed that indicated that phase separation occurred by the spinodal decomposition (SD). The growth of the structures formed by the general liquid–liquid phase separation obeyed a power‐law scaling relationship in SD. For EVOH with an ethylene content of 32 mol % (EVOH32), the liquid–liquid phase separation resulted from the polymer crystallization. In this case, the structure growth showed the characteristic behavior in which the crystalline particles were initially formed, and then the droplets formed by the liquid–liquid phase separation induced by the crystallization grew rapidly. Furthermore, the growth of the droplet by the phase separation was followed by an optical microscope measurement at a constant cooling rate. The phase‐separated structure formed after the crystallization can grow faster than that formed by the normal liquid–liquid phase separation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 194–201, 2003     

A novel preparation method for microspheres by glycerol modified solid‐in‐oil‐in‐water multi‐emulsion

Biodegradable material poly(D, L ‐lactic‐co‐glycolic) acid (PLGA) plays an important role in drug‐sustained release systems. Here, we describe a glycerol modified solid‐in‐oil‐in‐water (m‐S/O/W) emulsion method for PLGA microspheres, in order to encapsulate proteins in PLGA by utilizing dextran glassy particles to protect the proteins from denaturing, unfolding, and aggregation during preparation and new external water phase to prevent the inner dextran glassy particles from leaking into the external water phase. External water phase containing 20, 40, 60, 80% glycerol showed that proteins released faster and more completely with increased glycerol content. According to their varied release profiles, microspheres of different formulations could be used to encapsulate vaccines or for delivering proteins over long‐term. Copyright © 2009 John Wiley & Sons, Ltd.     

Use of Aggregation‐Induced Emission for Selective Detection of Phase Transformation during Evaporative Crystallization of Hexaphenylsilole

Crystallization of organic molecules is quite complicated because the crystallization process is governed by weak intermolecular interactions. By exploiting aggregation‐induced emission (AIE), we attempted to realize the selective detection of phase transformation during the evaporative crystallization of hexaphenylsilole (HPS), which shows different fluorescent colors in the amorphous and crystalline phases. No fluorescence emission was observed in the HPS solution immediately after dropping on the glass substrate due to the non‐radiative deactivation induced by intramolecular rotational or vibrational motion, suggesting that HPS exists as a monomer in solution. As time elapsed after dropping, green emission first appeared, which changed to blue after solvent evaporation, because of phase transformation from the amorphous state to the crystalline state. This phenomenon supports not only the two‐step nucleation model involving an intermediate such as a liquid‐like cluster prior to nucleation but also the real‐time detection of Ostwald's rule of stages during evaporative crystallization.     

Collective motion artifacts arising in long‐duration molecular dynamics simulations

We tested a variety of molecular dynamics simulation strategies in long‐duration (up to several nanoseconds) constant‐temperature simulations of liquid water under periodic boundary conditions. Such long durations are necessary to achieve adequate conformational sampling in simulations of membrane assemblies and other large biomolecular systems. Under a variety of circumstances, serious artifacts arise in the form of spurious collective behavior that becomes obvious only after the simulation has gone at least several hundred picoseconds. The potential energy of the system drops and the system changes from a liquid to an icy or glassy state. The underlying cause is accumulated center‐of‐mass motion of the system, coupled with velocity rescaling associated with constant‐temperature control. The velocity rescaling in the constant‐temperature algorithm reduces the thermal velocity as the net center‐of‐mass velocity grows, effectively causing the kinetic energy of the system to drain from thermal motions into coordinated motions. We found that the incidence and magnitude of the underlying artifactual motion leading to the spurious transition is mediated by: choice of method for computing electrostatic interactions; choice of ensemble; size of the simulation cell; SHAKE tolerance; frequency of nonbonded pairlist updating; and closeness of coupling to the temperature bath. The appearance of the spurious transition can be avoided by periodically subtracting net center‐of‐mass motion during the dynamics, or by improving the accuracy of the simulation by means of tightening SHAKE tolerance and updating nonbonded pairlists every timestep. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 121–131, 2000     

Recrystallization of water in a non-water-soluble polymer examined by Fourier transform infrared spectroscopy: poly(2-methoxyethylacrylate) with low water content

Crystallization of water during heating, the so-called "recrystallization of water", in poly(2-methoxyethylacrylate) (PMEA) was investigated by temperature-variable Fourier transform infrared spectroscopy. Recrystallization in a polymer-water system is generally understood to be a phase transition from glassy water (condensed water) to crystalline water. However, infrared spectral changes of the PMEA-water system with low water content indicated that the formation of ice I h during heating occurred by a vapor deposition process rather than by a crystallization process.     

Novel method to detect the volumetric glass --> liquid transition at high pressures: glycerol as a test case

We report a novel method of detecting the glass --> liquid transition at high pressures, which comprises measuring the relative volume change incurred upon heating glassy samples into the liquid state. We show data on glycerol in the pressure range 0.050-1.00 GPa to demonstrate the viability of the method. The reversible glass --> liquid transition is observed by means of a kink in the relative volume change on heating the sample isobarically, which is attributed to the glass --> liquid transition temperature Tg. This kink can only be observed in the second and subsequent heating cycles since it is superposed by a compaction in the first heating cycle. The isobaric thermal expansivity beta, which is closely related to the first derivative of this curve, shows the features expected for a glass --> liquid transition, including a sharp rise of beta(glass) in a narrow temperature interval to beta(viscous liquid) and an accompanying overshoot effect. Both Tg and the size of the overshoot effect vary in accordance with theory upon changing the ratio of cooling to heating rates. From the shape of this curve the onset, inflection, overshoot peak, and endpoint of the glass --> liquid transition can be extracted, which can be employed to calculate the reduced glass transition width as a measure for the fragility of the liquid. Comparison with literature data allows quantifying the accuracy of the liquid's thermal expansivity beta to be at least +/-10%, while the error in beta is significantly larger for the expansivity of the glassy state. The reproducibility of the glass --> liquid transition temperature Tg is better than +/-2 K. Our glycerol data confirms literature studies showing a nonlinear increase of Tg with increasing pressure (approximately 35 K/GPa on average), which is accompanied by an increase in fragility.     

Heat‐Triggered Crystallization of Liquid Crystalline Macrocycles Allowing for Conductance Switching through Hysteretic Thermal Phase Transitions

A polymesomorphic thermal phase‐transition of a macrocyclic amphiphile consisting of aromatic groups and oligoethylene glycol (OEG) chains is reported. The macrocyclic amphiphile exists in a highly‐ordered liquid crystal (LC) phase at room temperature. Upon heating, this macrocycle shows phase‐transition from columnar‐lamellar to nematic LC phases followed by crystallization before melting. Spectroscopic studies suggest that the thermally induced crystallization is triggered by a conformational change at the OEG chains. Interestingly, while the macrocycle returns to the columnar‐lamellar phase after cooling from the isotropic liquid, it retains the crystallinity after cooling from the thermally‐induced crystal. Thanks to this bistability, conductance switching was successfully demonstrated. A different macrocyclic amphiphile also shows an analogous phase‐transition behavior, suggesting that this molecular design is universal for developing switchable and memorizable materials, by means of hysteretic phase‐transition processes.     

Structures and properties of liquid‐crystalline polymers based on laterally attached oligo p‐phenylenes

The structures and properties of liquid‐crystalline polymers containing laterally attached p‐terphenyl and p‐pentaphenyl have been studied. In contrast to their mesogenic groups, that is, p‐terphenyl and p‐pentaphenyl, the polymers have much lower crystallinity and also lower nematic‐to‐isotropic transition temperatures. The significant depression in crystallinity can be attributed to flexible chain segments laterally attached to the oligo p‐phenylene rods, which prevent close packing of the rods and thus disrupt the crystallization. The destabilization of the liquid‐crystalline phase is due to the diluting effect of the flexible polymer backbones; that is, the concentration of the mesogenic groups is reduced. The polymer containing p‐pentaphenyl can still exhibit good solubility in common solvents and emit light at about 402 nm in the solvent tetrahydrofuran. In the solid state, the emission redshifts to 418 nm, which is fairly close to the blue‐light emission. An interdigitated packing structure of mesogenic groups has been proposed to represent the structure of the polymer in the oriented state. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3394–3402, 2005