Macroscopic Phase Transition Research Articles

Calorimetric Analysis of Thermal Phase Transitions in Functionalized Microgels

Differential scanning calorimetry (DSC) is used to investigate the thermal phase transitions of a range of N-isopropylacrylamide (NIPAM)-based, carboxylic acid-functionalized microgels with well-defined radial and chain functional group distributions. The transition enthalpies of protonated microgels can be correlated with the hydrophobicity of the functional comonomer, while the transition enthalpies for ionized microgels can be correlated with the degree of microgel deswelling achieved across the thermal phase transition. The peak widths at half-height vary inversely with the average length of NIPAM blocks in each of the microgels, as calculated using a kinetic copolymerization model. Deconvolution of the asymmetric DSC thermograms is accomplished using a two-transition model, thought to relate to core-shell-type transitions induced by the significant local heterogeneities within the functionalized microgels. The ratio between the two transition temperatures of these deconvoluted peaks is a useful quantitative probe of the radial functional group distribution. An additional, low-temperature transition is also observed in the thermogram of the vinylacetic acid-functionalized microgel, indicative of the occurrence of local chain rearrangements prior to the macroscopic phase transition in this microgel. Complementary light scattering analysis suggests that microphase separation may account for this additional transition peak.

Acoustic emission study of phase transitions and polar nanoregions in relaxor-based systems: Application to thePbZn1∕3Nb2∕3O3family of single crystals

The restored strain energy in $\mathrm{Pb}{\mathrm{Zn}}_{1∕3}{\mathrm{Nb}}_{2∕3}{\mathrm{O}}_{3}$ (PZN) and $9%\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$-doped PZN (PZN-9%PT) relaxor crystals has been studied by means of acoustic emission (AE). Two types of AE activity signals have been recorded: (i) related to temperature- or electric-field-induced macroscopic phase transitions and (ii) associated with formation/disappearance of intrinsic polar nanoregions. Monitoring of AE under varying [001] electric fields has allowed a unique in situ observation of a low-field $(1\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm})$ irreversible orthorhombic-to-${M}_{C}$ phase transition within the morphotropic phase boundary region of PZN-9%PT.

Low Frequency Acoustic Resonance Studies

Fluid phase transitions in porous media are a powerful probe of the effect of confinement and disorder on phase transitions. Aerogel may provide a model system in which to study the effect of dilute impurities on a variety of phase transitions. In this paper we present a series of low frequency acoustic experiments on the effect of aerogel on the liquid–vapor phase transition. Acoustic resonators were used to study the liquid–vapor transition in two fluids (helium and neon) and in two different porosity aerogels (∼ 95% and ∼ 98%). While effective coexistence curves could be mapped out, the transition was sometimes difficult to pinpoint, leading to doubt as to whether this transition can be treated as an equilibrium macroscopic phase transition at all.

Fluorescence study on the mechanism of rapid shrinking of grafted poly( N-isopropylacrylamide) gels and semi-IPN gels

In order to clarify in nanometer scale the mechanism of rapid shrinking of grafted poly( N-isopropylacrylamide) (PNIPAM) hydrogels during temperature jump from room temperature to 40 °C, temperature dependences of dansyl fluorescence were studied for the rapid-shrinking grafted PNIPAM gels in which dansyl groups are attached to graft chains or network main chains. Gradual decrease in dansyl fluorescence peak wavelength, λ f, with the increase in temperature between 17 and 32 °C for grafted chains in the grafted PNIPAM gel from 540 to 500 nm showed the occurrence of coil-globule transition of graft chains during this temperature range. However, λ f of network main chains showed abrupt decrease at 30–34 °C from 540 to 510 nm reproducing the macroscopic volume phase transition of PNIPAM gels. The coil-globule transition and further aggregation of graft chains enhance the formation of heterogeneous structure during the temperature jump, providing water-expelling channels at the surface skin layer of the collapsed PNIPAM gel. The possible importance of large-scale heterogeneity with a much larger size than the mesh size of network on the rapid-shrinking is suggested, because this type of large-scale heterogeneity can be introduced by the network formation in the presence of linear macromonomer. A post-crosslinked semi-IPN gel also showed rapid shrinking and gradual decrease in λ f for the linear polymer chains inside.

Learning intentional behavior in the K-model of the amygdala and entorhinal cortex with the cortico-hyppocampal formation

The interaction between entorhinal cortex (EC), amygdala, and hippocampal and cortical areas in vertebrate brains is studied using the dynamical K model approach. Special emphasis is given to the role of EC in decision making under the influence of sensory, orientation, and motivational clues. We introduce a simplified KIV model with positive and negative reinforcement learning in the hippocampus and the cortex. The developed model is implemented in a 2D computational environment for multi-sensory control of the movement of a simulated animal. Our results support the interpretation of recent EEG measurements with instantaneous macroscopic phase transitions.

Fusion and fission of fluid amphiphilic bilayers

The system water-oil (n-decane)-nonionic surfactant (C12E5) forms bilayer phases in a large concentration region, but, for a given oil-to-surfactant ratio, only in a narrow temperature range. In addition to the anisotropic lamellar phase (Lalpha) there is also, at slightly higher temperature, a sponge or L3-phase where the bilayers build up an isotropic structure extending macroscopically in three dimensions. In this phase the bilayer mid-surface has a mean curvature close to zero and a negative Euler characteristic. In this paper we study how the bilayers in the lamellar and the sponge phase respond dynamically to sudden temperature changes. The monolayer spontaneous curvature depends sensitively on temperature and a change of temperature thus provides a driving force for a change in bilayer topology. The equilibration therefore involves kinetic steps of fusion/fission of bilayers. Such dynamic processes have previously been monitored by temperature jump experiments using light scattering in the sponge phase. These experiments revealed an extraordinarily strong dependence of the relaxation time on the bilayer volume fraction phi. At phi < 0.1 the relaxation times are so slow that experiments using deuterium nuclear magnetic resonance (2H-NMR) appear feasible. We here report on the first experiments concerned with the dynamics of the macroscopic phase transition sponge-lamellae by 2H-NMR. We find that the sponge-to-lamellae transition occurs through a nucleation process followed by domain growth involving bilayer fission at domain boundaries. In contrast, the lamellae-to-sponge transformation apparently occurs through a succession of uncorrelated bilayer fusion events.

Anisotropy of inter-exciton interaction and pattern formation dynamics in photoinduced structural phase transitions

We study theoretically nonlinear nonequilibrium real-time dynamics of an optically excited many-exciton system in a two-dimensional insulator, and search for conditions under which excitons can proliferate efficiently to produce a macroscopic photoinduced structural phase transition (PIPT). To study this problem theoretically, we take a many-exciton system coupling strongly with Einstein-phonons, and numerically calculate the proliferation of excitons and the resultant domain patterns.In particular, we focus on the anisotropy of inter-exciton interaction, and compare a strong anisotropic case to a weak one. We have concluded that the anisotropic case leads to very efficient proliferation, and makes the final PIPT extensive and successful compared to isotropic cases.

Multiple-phase behavior and memory effect of polymer gel

A poly(4-acrylamidosalicylic acid) gel (PASA gel) exhibits multiple phases as characterized by distinct degrees of swelling; the gel can take one of four different swelling values, but none of the intermediate values. The gel has remarkable memory: the phase behavior of the gel depends on whether the gel has experienced the most swollen phase or the most collapsed phase in the immediate past. The information is stored and reversibly erased in the form of a macroscopic phase transition behavior. The structure factors corresponding to these four phases were obtained by SANS, which indicated the presence of characteristic structures depending on pH and temperature, particularly in the shrunken state.

Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms.

For a system at a temperature of absolute zero, all thermal fluctuations are frozen out, while quantum fluctuations prevail. These microscopic quantum fluctuations can induce a macroscopic phase transition in the ground state of a many-body system when the relative strength of two competing energy terms is varied across a critical value. Here we observe such a quantum phase transition in a Bose-Einstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. As the potential depth of the lattice is increased, a transition is observed from a superfluid to a Mott insulator phase. In the superfluid phase, each atom is spread out over the entire lattice, with long-range phase coherence. But in the insulating phase, exact numbers of atoms are localized at individual lattice sites, with no phase coherence across the lattice; this phase is characterized by a gap in the excitation spectrum. We can induce reversible changes between the two ground states of the system.

Intrinsic fluorescence investigation on the change in conformation of cross-linked gelatin gel during volume phase transition

Gelatin gel cross-linked with glutaraldehyde was prepared. Its volume is sensitive to the acetone–water composition. Near 50% acetone content, a discontinuous macroscopic volume phase transition (VPT) occurred. The gel was found to emit intrinsic blue fluorescence at 415nm, which is attributed to the formation of dimeric species of tyrosine (bityrosine) in the gelatin network. The ratio of intensity of bityrosine to that of tyrosine monomer, IB/IM, was used to examine the change in conformation of gelatin during volume phase transition (VPT). With the increasing acetone concentration, IB/IM continuously increased till VPT, and then dropped little, revealing a high yield of bityrosine species, which is considered to stem from the closer contact of inter-coil, inter-helix, intra-helix and coil-helix in the collapsed network. The continuous IB/IM change suggests the non-discontinuous volume phase transition of gel occurring in microscale.

Photoinduced Disordering Dynamics of a Cyanobiphenyl Liquid Crystal Containing Quinizarin

Photoinduced changes in a liquid crystal (LC) alignment were measured for 4-octyloxy-4′-cyanobiphenyl containing quinizarin. Kinetic analyses were carried out for both photoinduced response and thermal relaxation of the LC alignment in the temperature range from 50°C to 78°C. Thermal relaxation curves of birefringence after termination of Ar+ laser irradiation show only one component in the smectic A phase, and two components in the nematic phase, revealing the microscopic environment of dye molecules in LC solvents. This change does not exactly correspond to the macroscopic phase transition from smectic to nematic, which suggests interaction between LC mesogens and dye molecules due to photoexcitation. An unrecoverable portion of birefringence for thermal ordering/relaxation processes after termination of photoirradiation was observed in the smectic phase measured in the course of heating process.

Study of the spontaneous relaxor to normal ferroelectric phase transition in la-doped lead titanate

Highly lanthanum doped lead titanate ceramics exhibit relaxor properties and undergo on cooling a subsequent spontaneous macroscopic phase transition. The dielectric response is strongly affected by chemical (copper) doping and is found to become independent of the applied ac field for temperatures lower than the phase transition one.

The gel that memorizes phases

A poly(4-acrylamidosalicylic acid) gel exhibits multiple phases as characterized by distinct degrees of swelling; the gel can take one of four different swelling values, but none of the intermediate values. The multiple phase behavior appears as a result of the combination of hydrogen bonding and hydrophobic interaction between polymer segments. The gel has remarkable memory: The phase behavior of the gel depends on whether the gel has experienced the most swollen phase or the most collapsed phase in the immediate past. The information is stored and reversibly erased in the form of a macroscopic phase transition behavior. These phenomena are explained by a mean field theory where the number of hydrogen bonds is added to the equation of the state of a gel.

AFM observation of polymer gels incorporated with microspheres

Atomic force microscopy (AFM) has been used to visualize the gel microspheres immobilized in the polymer networks. The gel microspheres with submicron diameter were made of poly N-isopropylacrylamide (NIPA) by conventional emulsion polymerization, which were subsequently incorporated into poly NIPA networks at gelation. The microspheres can swell and shrink under mechanical constraint in response to temperature change. The macroscopic volume change associated with the change in the hydrophobic interaction of both the host chains and the guest microgels strongly depends on the amount of microspheres: in the case of a small amount, the phase transition is discontinuous, in contrast to a continuous phase transition observed for larger concentration than a threshold. We discuss the distribution of microspheres and its effect on the macroscopic phase transition behavior with references to the emergence of large domain structures in the host homogeneous networks, which consist of the aggregated guest microspheres.

Change of Free Volume in Polymer Gels As Studied by Positron Annihilation Lifetimes

Variations of a free volume size of a poly(N-isopropylacrylamide) gel and a polyacrylamide gel during a volume phase transition induced by a change of external environment — temperature, solvent composition, pH, and ion strength, are described. The free volume size is probed by positron annihilation lifetime technique. Variations of the free volume size suggest that a nanoscopic environment of the gels is affected by the free volume of the solvent phase and an interaction between the polymer chain and the solvent molecule in the swollen state, and by the aggregation of the polymer chain and a balance of the interaction among the polymer chain and the solvent molecules in the collapsed state. It is revealed that the free volume size reflects the essential change of the nanoscopic environment in the gels which could induce the macroscopic volume phase transition. PACS numbers: 82.70.Gg

A Simple Micromechanical Model for Pseudoelastic Behavior of CuZnAl Alloy

A simple micromechanical model for thermoelastic martensitic phase transitions (PT) is developed. It is deduced from the local description of PT in transforming particles with subsequent usage of average procedure, based on a model for elastic three-phase materials (austenite, martensite and new infinitesimal nucleus) under assumption of homogeneity of stresses in each phase. In contrast to known approaches, a new local PT criterion and a corresponding extremum principle for PT with dissipation are used. The macroscopic PT criterion obtained is split into two different equations for description of temperature-induced PT and stress-induced PT. To identify the material parameters of the model and to check its validity, simple one-dimensional experiments were carried out for CuZnAl alloy. The experimental values of martensite start and finish temperatures and austenite finish temperature for temperature-induced PT and the stress-strain diagram for stress-induced direct PT at any fixed temperature have allowed to determine six material parameters of the model for the simplest one-dimensional case. Then model prediction is compared with other independent tests. A good agreement is obtained of the calculated stress-strain curves for reverse PT (martensite-austenite) at 01 = 20C and for direct PT at temperature range of 30—80C with experimental data. Finally, the formula for determination of the transformation heat during temperature-induced PT for the given model is derived. It is shown that the predicted transformation heat is close to the experimental one.

Nanosecond dynamics of phase transitions in chromophoredoped ferroelectric crystals

The microscopic dynamics of phase transitions in ferroelectric crystals are studied using time-resolved fluorescence spectroscopy. Fluorescence decay times of the molecular probe tryptophan, embedded in the ferroelectric crystals potassium dihydrogen phosphate (KDP) and triglycine sulfate (TGS), are measured over a large temperature range. The dependence of the lifetime data on temperature were different between the two crystals. The magnitude of the fluorescence lifetime shows a rapid change around the phase transition temperature in KDP, while the changes in the lifetime data of the TGS crystal have been smooth. The data indicate that the order of the phase transition in solids (first or second) can be recovered using this technique. Since fluorescence lifetime is related to the dynamic interactions between the chromophore and the environment, insight into the emergence of macroscopic phase transition behavior from microscopic fluctuations near the phase transition region may be realized.

Theory of diffuse phase transition in complex perovskite relaxor ferroelectrics

A Landau type mean field theory of diffuse phase transition in complex perovskite relaxor ferroelectrics considering a gradual condensation of local clusters of polarizations was proposed. Numerical calculations for Pb(Mg1/3Nb2/3)O3 (PMN) and highly B-site ordered Pb(Sc1/2Ta1/2)O3 (PST) were conducted with appropriate phenomenological parameters. It was shown that, no macroscopic phase transition occurs in PMN in a short observation time scale, whereas in the case of highly ordered PST, a macroscopic first order transition occurs with the existence of the local clusters of frozen polarizations. These results confirm a picture of diffuse phase transition in relaxor ferroelectrics, the existence of local clusters and a gradual condensation of local polarizations in these clusters.

Structural phase transitions of carbon observed with photoelectron spectroscopy

We employed photoelectron spectroscopy, namely ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), to study (1) the structural phase transition from a crystalline to an amorphous phase initiated by low-energy ion irradiation, and (2) the bias-enhanced nucleation (BEN) and subsequent deposition of polycrystalline diamond films. Both experiments are performed in-situ, the samples are transferred from the preparation to the analysis chamber without breaking the vacuum, thus avoiding surface contamination. At least partial amorphisation of the surface of a polycrystalline CVD-diamond film and the destruction of graphitic inclusions in an amorphous carbon (a-C) film by ion irradiation (1 keV Ar + ions) are examples for a loss of crystallinity. These structural changes are observed as a gradual function of ion dose and the results of the UPS analysis are discussed in some detail. The bias-enhanced nucleation, on the other hand, does not constitute a macroscopic structural phase transition, but the co-deposition of an amorphous carbon and a crystalline diamond phase as well as the formation of a thin SiC interface are observed. The temporal evolution of the Cls core level peak shows that the amorphous phase is mostly removed again by etching when switching to deposition parameters appropriate for the subsequent diamond growth. The experimental results are discussed with respect to the nucleation mechanism when bias-enhanced nucleation is employed.

Pair-distribution functions of ferroelectric perovskites: Direct observation of structural ground states

Experimental information about the microscopic structure and the changes in this structure at macroscopic phase transitions for the ferroelectric perovskites provide direct tests of theoretical descriptions of ferroelectricity. Unfortunately, the local ferroelectric displacements can differ from the average periodic structure and may not be correctly described using crystallographic techniques; however, the use of real-space pair-distribution functions, which can now be obtained from scattering data with unprecedented accuracy, provides an excellent opportunity for studying the local structure. We compare crystallographic and local structures for BaTiO3, and KNbO3 above and below the low-temperature phase transitions and for PbTiO3, at low temperatures, both derived from the same powder neutron scattering data. These results elucidate the nature of the ground-state structures in each of these materials and provide insight into the mechanisms for the phase transitions.