Conformation transition and crystalline phase variation of long chain branched isotactic polypropylenes (LCB-iPP)

The changes of conformation and crystalline structure of long chain branched isotactic polypropylene (LCB-iPP) under different crystallization temperatures and the effects of their special molecular architecture on the crystallization behavior were investigated by a combination of Fourier transform infrared spectroscopy (FT-IR), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). In these polymers, long chain branching was introduced via in situ polymerization of polypropylene and an asymmetric diene monomer using the metallocene catalyst technology. Through the characterization of the specific IR band variation, it was proved that the conformational orders of helical sequences of LCB-iPP show great changes in different crystallization temperature ranges. In lower crystallization temperature range (100-130 °C), the intensities of all regular helical conformation bands of LCB-iPP increase with the increasing crystallization temperature and the regular helical conformation bands with more monomer units increase faster than that with less monomer units. In higher crystallization temperature range (130-150 °C), the intensities of all regular helical conformation bands of LCB-iPP decrease with the increasing crystallization temperature and the regular helical conformation bands with more monomer units decrease faster than that with less monomer units. The results of WAXD and DSC showed that LCB-iPP crystallizes from the melt as a mixture of α and γ forms. The content of the γ form increases with the increasing crystallization temperature, reaches a maximum value at 130 °C, and then decreases with a further increase of the temperature. At the same time, the crystallization of γ form is favored by the presence of the LCB structure of iPP. Moreover, the transitional temperatures of different helical conformations and crystallization structures of LCB-iPP show obvious correlations.     

Ferroelectric phase transition and electrical properties of high-TC PMN-PH-PT ceramics prepared by partial oxalate route

The high-Curie temperature (T_C) 0.15Pb(Mg_1/3Nb_2/3)O₃-0.38PbHfO₃-0.47PbTiO₃ (PMN-PH-PT) piezoelectric ceramics were prepared by the partial oxalate route via the B-site oxide mixing method. The obtained uniform nm-sized PMN-PH-PT precursor powders provide high calcining and sintering activity for synthesizing ceramics, based on which the synthesis conditions were tailored as calcining at 775 °C and sintering at 1245 °C. The partial oxalate route synthesized PMN-PH-PT ceramics are far superior to the counterparts synthesized by the columbite precursor method and exhibit excellent thermal stability of the piezoelectric properties under T_C (～292 °C), ensuring the potential application in transducers under elevated environmental temperatures. The temperature dependent Raman spectroscopy not only proves the occurrence of the ferroelectric to paraelectric phase transition around T_C, but also confirms the successive phase symmetry transitions, which correlate with the polar nanoregions (PNRs) and/or the coexistence of multiple ferroelectric phases, revealing the origin of the enhanced electrical properties in the PMN-PH-PT ceramics.     

Effect of grain size on phase transition,dielectric and pyroelectric properties of BST ceramics

Grain size effect on phase transition, dielectric and pyroelectric properties of BST ceramics under DC bias field was reported for the first time. With the grain size increased by one order of magnitude, phase transition diffuseness parameters δ and γ decreased from 282 to 63 and 1.64–1.33, respectively. This resulted in the dielectric constant maximum increased by 65%, the pyroelectric coefficient maximum and corresponding figure of merit were quadrupled and tripled, respectively. The physical essence of the great size effect can be attributed to a combination of surface effect, mechanical stress effect and extrinsic effect of grain boundary. A well understanding of the grain size effect and its mechanism will be useful for the BaTiO₃-based ceramics and other ferroelectrics.     

Chain packing and phase transition of N-hexacosylated polyethyleneimine comb-like polymer: A combined investigation by synchrotron X-ray scattering and FTIR spectroscopy

In this paper, the chain packing and phase transition of comb-like polymer has been deeply analyzed with N-hexacosylated polyethyleneimine (PEI26C) as a template, fabricated through the reaction between n-hexacosyl bromide and PEI in a homogenous solution. The effect of long alkyl groups on the chain packing and phase transition of PEI26C was systematically investigated by synchrotron X-ray scattering and variable-temperature FTIR spectroscopy. PEI26C comb-like polymer exhibited an interesting structure-evolution process, and phase transformation from orthorhombic (β_O), monoclinic (β_M), to hexagonal (α_H) phase, and finally to amorphous state was demonstrated, indicating that large alkyl domains induced the complicated structures. Size-depended phase transition behavior provides an insight into the formation of metastable structure during the early stage of polymer crystallization.     

Local structure study of phase transition behavior in Ba(Ti,Sn)O3 perovskite by X-ray absorption fine structure

In this work, BaTi_1−xSn_xO₃ (BST) powders (x=0–0.95) were synthesized by a conventional mixed-oxide method. The phase information was investigated by a combination of X-ray diffraction and X-ray absorption spectroscopy techniques. The XRD measurements indicated the global phase transition from tetragonal to cubic perovskite structure. From the synchrotron X-ray Absorption Near-Edge Structure (XANES) measurements at the Ti K-edge and Sn L₃-edge, it was seen that an increase of Sn content in BaTiO₃ affected the phase transition behavior and local structure of BST. In addition, the local structure of Ba(Ti,Sn)O₃ materials were experimentally determined and compared with that obtained from the simulation. The local structure obtained from the XANES technique provided additional structural information unavailable from XRD investigation. Finally, the phase transition behavior from relaxor ferroelectric to polar cluster in BST system was discussed and attributed mainly to the change in the local structure.     

Kinetics of the ordered phase growth at the phase transition from the isotropic to cholesteric state in a main chain liquid crystalline polymer

The optical microscope investigation of the isotropic- ordered phase transition in a main chain LC polymer and subsequent statistical treatment of the microscopic images allowed recognition of three regimes of the ordering: non-stationary regime which is characterized by increasing rate of the droplets growth; stationary regime; within it the ordered phase grows with a constant rate, and the coalescence regime for which the rate of the droplets growth deceases. Linear interpolation of the time dependence of the mean droplets diameter in log–log scales allowed conclude that within the stationary regime of the cholesteric phase growth, log <d> is a linear function of log t, whereas within the coalescence regime, <d> is proportional to t ^1/2 as expected from the theory of phase separation binary liquids. The time of beginning and duration of both growth regimes of the phase transition for the main chain LC polymer exceed those for low molecular weight LC compounds. This is caused by lower mobility of macromolecules in comparison with that for low molecular weight LC compounds.     

Role of Cu and Y in sintering,phase transition,and electrical properties of BCZT lead-free piezoceramics

Lead-free (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃-xwt%CuOywt%Y₂O₃ (BCZT-Cu_xY_y) ceramics with high piezoelectricity were synthesized by the conventional solid-state reaction method. The role of Cu and Y (Cu/Y) in sintering, phase transition, and electrical properties of such ceramics was systematically studied. The results indicated that the sintering temperatures of BCZT-Cu_xY_y decreased by at least 100?°C due to the low melting point of CuO. The promotion effect of Cu/Y on phase transition lied in the improvement of T_C by 5–15?°C and the coexistence of O+T phase near room temperature. The contribution of Cu/Y to electrical properties was mainly ascribed to the grains growth, the formed oxygen vacancies and lattice distortions, and the donor doping effect of Y³⁺. Adding 0.10?wt% Cu²⁺ and 0.06?wt% Y³⁺ into BCZT dramatically improved the electrical properties as following: d₃₃ =?552 pC/N, ε_m =?10175, ε_r =?4546, tanδ =?0.016, T_C =?100?°C, k_p =?0.475, Q_m =?157.2, P_r =?10.82 μC/cm² and E_C =?2.33?kV/cm. A plausible mechanism was obtained to explain the reaction process and the favorable performances of BCZT-Cu_xY_y. Co-doping Cu²⁺ and Y³⁺ into BCZT could be a promising method to improve and balance the sintering, phase transition, and electrical properties for potential practical applications of lead-free piezoceramics.     

Influence of charge density on rheological properties and dehydration dynamics of weakly charged poly(N-isopropylacrylamide) during phase transition

It is well-known that introduction of charged groups to poly(N-isopropylacrylamide) (PNIPAM) raises its phase transition temperature. However, the influence of charged groups on structural evolution and dehydration dynamics of weakly charged PNIPAM during phase transition still lacks systematic investigation. In the current study, armed with rheometer and two-dimensional Fourier transform infrared spectrometer (2D-FTIR), we investigated on mesoscopic and microscopic scales the phase transition of sodium poly(N-isopropylacrylamide-co-2-acrylamido-2-methylpropanesulfonate), abbreviated as poly(NIPAM-co-NaAMPS), with charge density of 1–10%. At ambient temperature, scaling exponent of poly(NIPAM-co-NaAMPS) varies from that of neutral polymer to polyelectrolytes as charge density increases. Above phase transition temperature, mesoscopic structure of poly(NIPAM-co-NaAMPS) varies from network of physical gel to viscoelastic liquid containing branched aggregates with increase of charge density, indicating increasing hindrance to intra/inter-chain association due to electrostatic repulsion. On a molecular level, poly(NIPAM-co-NaAMPS) exhibits distinctive microdynamic sequence of dehydration during phase transition, in contrast to neutral PNIPAM. In particular, sulfonate groups decouple the cooperative dehydration of alkyl and carbonyl groups, resulting in their distinctive phase transition temperature as well as temperature range. In analogy to hydration of proteins, it is proposed that the microdynamic sequence, implying the hydration stability of each group, is closely related to the density of hydration layer as well as influence of electrostatic field generated by charged groups. For poly(NIPAM-co-AMPS) with charge density of 3%, there still remains 72.3% of hydrogen bonds between carbonyl group and water at 60 °C, meanwhile a highly hydrated network forms with network strands 1–2 times as long as the copolymer chain length.     

Preparation and characterization of HfO2/VO2/HfO2 sandwich structures with low phase transition temperature,excellent thermochromic properties,and superior durability

In the last few decades, smart windows made from VO₂-based thermochromic films have attracted extensive attention, but their actual commercial applications are limited by low luminous transmittance (T_lum), low solar modulation ability (ΔT_sol), high phase transition temperature (T_c), and poor durability. In this study, glass/HfO₂/VO₂/HfO₂ tri-layer films were designed and deposited on glass substrates by pulse laser deposition. Crystal structures, surface morphology, surface roughness, electrical properties, and optical properties of as-prepared sandwich structure films were analyzed. Results showed that both HfO₂ buffer layer and antireflection layer (ARL) were monoclinic phase and grew along the (020) and (?111) crystal planes, respectively. HfO₂ buffer layer not only reduced T_c of VO₂ film by about 20 °C, but also played an important role in regulating crystal quality and surface morphology of VO₂ films. More importantly, by covering films with HfO₂ ARL, T_lum and ΔT_sol of VO₂ film were greatly improved. In particular, when the thicknesses of HfO₂ buffer layer and ARL were 80 nm and 120 nm, the obtained HfO₂/VO₂/HfO₂ tri-layer film reached a balance between high T_lum (～47.2%), high ΔT_sol (～9.1%) and low T_c (～49.1 °C). In addition, after 216 h of boiling water treatment, T_lum and ΔT_sol of HfO₂/VO₂/HfO₂ film covered with 120 nm thick ARL still remained at 49.3% and 7.0%, showing excellent durability. This research provides a new strategy for designing VO₂-based smart windows with high performance and good durability.     

Crystalline structure and phase transition of syndiotactic styrene‐based copolymers

Crystalline structure and phase transition of clathrate phase of syndiotactic poly[styrene‐co‐(p‐R‐styrene)] (St/RSt; R = CH₃ (Me), tert‐butyl (Bu), HO), prepared from solutions, have been investigated with wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry. The copolymers containing less than 67.2 mol% of MeSt or 9.4 mol% of BuSt formed δ phase crystallized from toluene solution. The Bragg distance of 010 and $\stackrel{￣}{2}10$ reflections of the St/MeSt copolymer slightly increased with increasing of the MeSt content over 10 mol%. The St/HOSt copolymer containing 16.1 mol% of HOSt crystallized from dioxane solution also formed δ phase crystal. The orientation of the crystalline phase of the copolymers in films, which were prepared by cast‐crystallization from o‐xylene or dioxane solution, was evaluated using WAXD. The degrees of orientation were almost independent of RSt content in the copolymers. Phase transition from the δ phase crystal to the α or γ phase crystal of the copolymers in the heating process was observed using WAXD. The copolymers containing less than 14.8 mol% of MeSt or 8.4 mol% of BuSt showed a transition from the δ phase to the γ phase at more than 150 °C. Crystallinity of St/BuSt in the γ phase was lower than that in the δ phase. © 2018 Society of Chemical Industry     

Structural investigation on water-induced phase transitions of poly(ethylene imine), Part IV: Changes of intra- and intermolecular hydrogen bonds in the hydration processes as revealed by time-resolved Raman spectral measurements

Poly(ethylene imine) (PEI) exhibits water-induced phase transitions among four kinds of crystalline hydrates; anhydrate (EI monomer unit/water molecule = 1/0), hemihydrate (1/0.5), sesquihydrate (1/1.5) and dihydrate (1/2). The chain conformation changes from a double helix in the anhydrate to a planar-zigzag form in the three types of hydrates. Time-resolved Raman spectral measurements have been successfully performed for the first time in the hydration processes of PEI using light and heavy waters. Raman spectral profiles characteristic of each crystal form were obtained in the frequency region of 30–3500 cm⁻¹. Exchange of water from H₂O to D₂O was helpful for shifting the many overlapped bands to identify the characteristic bands. Details of the change in intra- and intermolecular hydrogen bonds have been clarified by quantitative interpretation of the observed Raman spectral data. The conformational disordering occurring in the transition process from the double-stranded helices to the planar-zigzag chains was also discussed.     

Phase transition,microstructure and electrical properties of Fe doped Ba0.70Ca0.30TiO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics of Ba_0.70Ca_0.30Ti_1?xFe_xO₃ (x=0–0.03) have been synthesized by a conventional solid state reaction method. The influence of Fe content on the microstructure, phase transition, dielectric, ferroelectric, and piezoelectric properties is investigated systematically. The ceramics with x≤0.02 are diphasic composites of tetragonal Ba_0.80Ca_0.20TiO₃:Fe and orthorhombic Ba_0.07Ca_0.93TiO₃:Fe solid solutions. The tetragonal phase is gradually suppressed as x increases, the ceramic with x=0.03 is found to have diphasic pseudocubic and orthorhombic phases. And the grain size is dependent on Fe content significantly. Introduction of Fe at B-sites improves the densification and decreases the sintering temperature. As x increases from 0 to 0.03, the room temperature relative dielectric permittivity enhances, dielectric loss decreases, and the Curie temperature decreases monotonically from 128 °C to 58 °C. However, the ferroelectricity enhances slightly and reaches the maximum near x=0.005, and then weakens with increasing x. On the other hand, the piezoelectric coefficient (d₃₃) and the electromechanical coupling coefficient (k_p) decrease simultaneously with increasing x, whereas the mechanical quality factor (Q_m) increases significantly. The structure–electrical properties relationship is discussed intensively to give more information on (Ba,Ca)TiO₃-based lead-free piezoelectric ceramics.     

Determination of Substrate Preferences for Desaturases and Elongases for Production of Docosahexaenoic Acid from Oleic Acid in Engineered Canola

Production of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in plant seed oils has been pursued to improve availability of these omega‐3 fatty acids that provide important human health benefits. Canola (Brassica napus), through the introduction of 10 enzymes, can convert oleic acid (OLA) into EPA and ultimately DHA through a pathway consisting of two elongation and five desaturation steps. Herein we present an assessment of the substrate specificity of the seven desaturases and three elongases that were introduced into canola by expressing individual proteins in yeast. In vivo feeding experiments were conducted with 14 potential fatty acid intermediates in an OLA to DHA pathway to determine the fatty acid substrate profiles for each enzyme. Membrane fractions were prepared from yeast expression strains and shown to contain active enzymes. The elongases, as expected, extended acyl‐CoA substrates in the presence of malonyl‐CoA. To distinguish between enzymes that desaturate CoA‐ and phosphatidylcholine‐linked fatty acid substrates, we developed a novel in vitro method. We show that a delta‐12 desaturase from Phytophthora sojae, an omega‐3 desaturase from Phytophthora infestans and a delta‐4 desaturase from Thraustochytrium sp., all prefer phosphatidylcholine‐linked acyl substrates with comparatively low use of acyl‐CoA substrates. To further validate our method, a delta‐9 desaturase from Saccharomyces cerevisiae was confirmed to use acyl‐CoA as substrate, but could not use phosphatidylcholine‐linked substrates. The results and the assay methods presented herein will be useful in efforts to improve modeling of fatty acid metabolism and production of EPA and DHA in plants.     

Phase transition,microstructure, and dielectric properties of Li/Ta/Sb co-doped (K,Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.93−xTa_xSb_0.07)O₃ (abbreviated KNLNST_x) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130 °C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNST_x ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TTB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375<x<0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST_0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d₃₃=313 pC/N, k_p=47%, and ε_r=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d₃₃=200 pC/N and ε_r=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics.     

The evolution of structure,properties and polar domains in rare earth and PbTiO3 co-substituted BiFeO3 ferroelectric ceramics

BiFeO₃ (BFO) based ferroelectric solid solutions attract long-lasting research interests due to their multi-functionalities including electric/multiferroic/energy-storage properties. However, achievement of large ferroelectric polarization is still highly challenging in BFO based bulk ceramics due to large leakage. In this work, the structure and electrical properties of rare earth Nd- and PbTiO₃ co-modified BFO ceramics have been explored. Based on high temperature in-situ X-ray diffraction and dielectric measurements, a preliminary ferroelectric phase diagram is established, depicting the morphotropic phase boundaries (MPB) and a critical temperature that cannot be correlated to any macroscopic phase transition. The effects of rare earth substitution on structure evolution have been investigated by comparing the results in this work and literature. The accomplishment of ferroelectric switching with giant ferroelectric polarization above 65 μC/cm² is successfully achieved without resorting to quenching treatment. The MPB compositions demonstrate the maximum piezoelectric coefficients and the lowest coercive field, suggesting the “softening” effects. The domain evolutions suggest two coexisting phases in MPB composition distribute separately in different grains.     

Phase transition of Eu2Ti2O7 under high pressure and a new ferroelectric phase with perovskite‐like layered structure

Ferroelectrics with perovskite‐like layered (PL) structure are well‐known for their high T_c and the application prospect of high‐temperature‐piezoelectric sensing. In this study, the PL‐structure Eu₂Ti₂O₇ was prepared by 1‐step high‐pressure sintering, which show the pyrochlore structure of Eu₂Ti₂O₇ would change into PL structure at 11 GPa, 1300°C. The PL‐structure Eu₂Ti₂O₇ is metastable, which will change back to pyrochlore structure at about 900°C in the air. The PL‐structure Eu₂Ti₂O₇ was confirmed as a high‐temperature ferroelectric material for the first time. The ferroelectric domain switching was directly observed using piezoelectric force microscope. The piezoelectric constant of the PL Eu₂Ti₂O₇ ceramic was measured as 0.7‐0.9 pC/N and its thermal depoling temperature (T_d) was determined as 800°C, which is associated with the PL‐pyrochlore transition.     

Shrinkage features,microstructure evolution and properties of Gd2O3-MgO optical composite ceramics with Zr as phase stabilizer

A novel composite ceramic, composed of equal-volumetric Zr-stabilized Gd₂O₃ and MgO phases, was prepared to be transparent in mid-wave infrared range. Zr stabilized Gd₂O₃ is proved to have a lower lattice parameter (10.7516 Å) using XRD refinement. Pressureless sintering behavior of Gd₂O₃-MgO with/without 2 at% Zr-doping (naming ZGM and GM) was studied via the real-time observation technique. The shrinkage of ZGM green body proceeds steadily up to 1400 °C while that of the undoped one shrinks sharply at 1250 °C due to Gd₂O₃ phase transition. The segregation of Zr element along the grain boundaries of Zr-Gd₂O₃ creates a synergized effect on the grain refinement with pinning effect. Dense ZGM ceramics exhibit superior transmittance of 78.3 %‐85.6 % at 3–5 µm, which show good consistency with the calculated values. The refractive index of Zr- Gd₂O₃ varies from 1.87 at 3 µm to 1.80 at 5 µm, which is smaller than those of monoclinic Gd₂O₃.     

Gas inclusion crystal between 1-D coordination polymer and nitrous oxide: occurrence of crystal phase transition induced by a slight amount of fluid guests inside host

A novel inclusion crystal, {[Rh(II)₂(bza)₄(pyz)]_n · 3n(N₂O)} (1), was prepared and characterized by single-crystal X-ray diffraction analysis; linear alignment (N₂O)_n coherently generates in the channels of host rhodium benzoate pyrazine and there is a high possibility of the existence of an intermediate phase induced by a slight amount of adsorbed N₂O inside the crystal even at room temperature.     

Structural phase transition and high temperature phase structure of Friedels salt, 3CaO · Al2O3 · CaCl2 · 10H2O

Friedels salt, the chlorinated compound 3CaO · Al₂O₃ · CaCl₂ · 10H₂O (AFm phase), presents a structural phase transition at about 30°C from a monoclinic to a rhombohedral phase. It has been studied by X-ray powder diffraction and optical microscopy in transmitted light with crossed polarisers on single crystals prepared by hydrothermal synthesis. The high temperature phase was determined at 37°C from X-ray single crystal diffraction data. The compound crystallises in the space group R c with lattice parameters of a = 5.7358(6)Å and c = 46.849(9)Å (Z = 3 and D_x = 2.111 g/cm³). The refinement of 498 independent reflections with I > 2σ(I) led to a residual factor of 7.1%. The Friedels salt can be described as a layered structure with positively charged main layers of composition [Ca₂Al(OH)₆]⁺ and negatively charged layers of composition [Cl⁻,2H₂O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and four to water molecules. The structural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molecules.     

Structural phase transition,electrical and photoluminescent properties of Pr3+-doped (1-x)Na0.5Bi0.5TiO3-xSrTiO3 lead-free ferroelectric thin films

Lead-free ferroelectric Pr³⁺-doped (1-x)Na_0.5Bi_0.5TiO₃-xSrTiO₃ (x?=?0–0.5) (hereafter abbreviated as Pr-NBT-xSTO) thin films were prepared on Pt/Ti/SiO₂/Si and fused silica substrates by a chemical solution deposition method combined with a rapid thermal annealing process at 700?°C, and their structural phase transition, dielectric, ferroelectric, and photoluminescent properties were investigated as a function of STO content. Raman analysis shows that with increasing STO content, the phase structures evolve from rhombohedral phase to coexistence of rhombohedral and tetragonal phases (i.e. morphotropic phase boundary), and then to tetragonal phase. The structural phase transition behavior has been well confirmed by temperature- and frequency- dependent dielectric measurements. Meanwhile, the variation in photoluminescence intensity of Pr³⁺ ions with different STO content in the NBT-xSTO thin films also indicates that there exists a clear structural phase transition when the film composition is close to the morphotropic phase boundary. Superior dielectric and ferroelectric properties are obtained in the Pr-NBT-0.24STO thin films due to the formation of morphotropic phase boundary. Our study suggests that Pr-NBT-xSTO thin films be promising multifunctional materials for optoelectronic device applications.