Glass transition and segmental dynamics in poly(dimethylsiloxane)/silica nanocomposites studied by various techniques

We report new results on segmental dynamics and glass transition in a series of poly(dimethylsiloxane) networks filled with silica nanoparticles prepared by sol-gel techniques, obtained by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), broadband dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). The nanocomposites are characterized by a fine dispersion of 10 nm silica particles and hydrogen bonding polymer/filler interactions. The first three techniques indicate, in agreement with each other, that a fraction of polymer in an interfacial layer around the silica particles with a thickness of 2–3 nm shows modified dynamics. The DSC data, in particular measurements of heat capacity jump at T_g, are analyzed in terms of immobilized polymer in the interfacial layer. The dielectric TSDC and DRS data are analyzed in terms of slower dynamics in the interfacial layer as compared to bulk dynamics. We employ a special version of TSDC, the so-called thermal sampling (TS) technique, and provide experimental evidence for a continuous distribution of glass transition temperatures (T_g) and molecular mobility of the polymer in the interfacial layer, which is consistent with the DRS data. Finally, DMA results show a moderate slowing down of segmental dynamics of the whole polymer matrix (increase of glass transition temperature by about 10 K as compared to the pure matrix).     

Redox-responsive core cross-linked micelles of poly(ethylene oxide)-b-poly(glycidyl methacrylate) by click chemistry

ABSTRACT

Redox-responsive core cross-linked polymeric micelles were prepared by using azide-alkyne click chemistry. Poly(ethylene oxide)-b-poly(glycidyl methacrylate) (PEO-b-PGMA) copolymers were prepared by the atom transfer radical polymerization, and the PGMA parts were subsequently functionalized with azido groups. The micelles of azido-functional PEO_5k-b-PGMA_1.1k-N₃ were cross-linked using dipropargyl 3,3′-dithiodipropionate as a disulfide-containing cross-linking agent. The block copolymers and polymeric micelles were characterized by using GPC, FT-IR, ¹H NMR, TEM, and DLS analyses. The nanoparticle micelles showed much improved structural stability under physiological condition, while they were rapidly dissociated in the 1,4-dithio-D,L-threitol reducing environment.     

Structure and molecular properties of (1)-Centbutindole. Comparison with Haloperidol

The structure and conformation of (1)-Centbutindole, a newly marketed neuroleptic compound, has been investigated by X-ray crystallography. It crystallizes in the monoclinic system and the non-centrosymmetric space group P2₁ (Z=2) with cell dimensionsa=8.434(6),b=6.620(3),c=18.419(9)Å, and =95.07(6)°. The chain conformation istrans extended. The embedded ³ piperidine ring exists in ahalf-chair conformation whereas the embedded piperazine ring exists in achair conformation. The propylene side chain is equatorial relative to the piperazine. A systematic conformational analysis of centbutindole and of a related molecule, Haloperidol, followed by a Monte Carlo search and a stochastic dynamics simulation, have been performed. Electronic and lipophilic properties have been computed and, molecular electrostatic potential (MEP) maps and molecular lipophilicity potential (MLP) maps, have been displayed for two selected conformers satisfying a reported pharmacophore. The two molecules exhibit very similar molecular properties.     

Mesophase design: I. Molecular structure and crystal packing motifs of 4-alkylcyanobiphenyls

A comparative study of the crystal and molecular structures of 4-alkylcyanobiphenyls (ACB-n) C _n H_{2n + 1}-C₆H₄-C₆H₄-CN (n = 2, 4–12) belonging to liquid-crystal compounds revealed no direct relationship between the molecular geometry of these compounds and their liquid-crystalline properties. Mesogenic properties are determined by the alternation of the loosely packed aliphatic and closely packed aromatic regions in the crystals. Graphs describing the crystals and mesophases were designed for ACB-n. The graph for nematic mesogens of ACB-n (n = 5−7) has one structure-forming element. The graph for smectic-nematic (n = 8 and 9) and smectic (n = 10−12) mesogens have more than one structure-forming element. If different types of secondary bonds in the smectic mesophase have equal energies, the disruption of these interactions caused by the temperature rise occurs simultaneously and the system undergoes a transition from the smectic phase to the isotropic liquid. If the energies of different types of secondary bonds are different, the destruction of the mesophase occurs in steps and the phase transitions are more complicated (smectic-nematic-isotropic transitions).     

Synthesis and characterization of dihydroxylammonium tetrachlorocuprate dihydrate crystals

A new hydroxylammonium compound, [(NH₃OH)₂CuCl₄], was synthesized and its crystals were grown at room temperature by slow evaporation of aqueous solutions. The crystals were characterized through powder XRD, thermogravimetry (TG), differential thermal analysis (DTA), low temperature differential scanning calorimetric (DSC) and FTIR spectra. The X‐ray powder diffraction confirms the crystallinity of the compound. A fitting decomposition pattern of the compound was formulated based on the TG and thereby confirming the formation of the compound in the stoichiometric ratio. The thermal anomalies occurring in the low temperature DSC indicate successive phase transitions. The low temperature phase transitions are attributed to the ordering of [NH₃OH]⁺ ions. While most of the phase transitions are of first order type, the one occurring at –126 °C is of second order. Two glass transitions occur when the compound was cooled between –157.9 and –136.9 °C. The characteristic vibration bands due to [NH₃OH]⁺ and CuClequation/tex2gif-stack-1.gif ions are observed in the IR spectra. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural phase transitions in the LaMnO3+λ system

Structural phase transitions in the LaMnO_3+λ system are studied at various temperatures and with different Mn⁴⁺ ion concentrations. Two structural phase transitions are established. The O′-orthorhombic – O-orthorhombic transition (O′ O transition) is due to removing the cooperative Jahn-Teller distortions and occurs in the Mn⁴⁺ concentration range from 10 to 14% at room temperature. The temperature of this transition varies from 710 K for the stoichiometric LaMnO₃ to room temperature for LaMnO_3.07. The O-orthorhombic – rhombohedral transition (O R transition) is due to the change of the rotation axis of the undistorted MnO₆ octahedra from [100] and [110] to [11 1]. The temperature of this transition varies from 1010 K for LaMnO₃ to 230 K for LaMnO_3.135. The structural phase diagram of temperature vs composition is plotted, indicating the existence of regions of the phase with different structures.     

From small molecules to polymers: Relaxation behavior of n-butyl methacrylate based systems

Calorimetric and dielectric data for a series of n-butyl methacrylate (nBMA) based systems are presented. Dynamic glass transition (α) and secondary relations (β, β′, γ) are studied for monomer, dimer, oligomers as well as polymers with a narrow molecular weight distribution. Fragility m and cooperativity N_α increase with decreasing degree of polymerization P indicating that the cooperative character of the α motions increases. All glass transition parameters change smoothly with P without clear peculiarities that would indicate a transition from small molecules to polymers. Relations between α dynamics and the different secondary relaxations in these series are discussed. The results are compared with the predictions of different approaches aimed to understand the nature of the dynamic glass transition and the interaction between localized and cooperative dynamics in glass-forming liquids.     

Effect of orientation and crystallization on dielectric and mechanical relaxations in uniaxially stretched poly(ethylene naphthalene 2,6 dicarboxylate)(PEN) films

Microstructural analysis (crystallinity, orientation) have been performed on stretched and isothermally crystallized poly(ethylene naphthalene 2,6 dicarboxylate)(PEN) films. The crystallinity ratios are higher at drawing temperatures below T_g than above T_g, and for similar draw ratios, a higher orientation can be obtained at drawing temperature below T_g than above T_g. The molecular mobility study by dielectric relaxation spectroscopy (DRS) and mechanical relaxation spectroscopy (MRS) was carried out as a function of draw ratio (λ) and drawing temperature (T_draw). Drawing in glassy state apparently splits the α-relaxation into two components: an α-low component which exhibits a strongly accelerated dynamics but this relaxation process is masked by microstructural rearrangements occurring while heating followed by an α-high relaxation process. The two sub-glass processes (β and β¹) are influenced by the drawing in the glassy state. This can be observed from the increase of relaxation amplitudes and was related to a high disorder in the amorphous phase that is induced by drawing below glass transition temperature. Drawing at 160 °C induces the opposite trend associated with crystallization and confinements effects. Differences in viscoelasticity behaviour were found by MRS in tensile mode parallel and perpendicular to stretching direction. As a result, when comparing oriented and crystallized samples with the same crystallinity ratios, a strong effect of morphology on the location and amplitude of the three relaxations of PEN can be found.     

Optical,X-ray,and calorimetric studies of phase transitions in ND4BeF3

Phase transitions in ND₄BeF₃ crystals at T₁ = 330 and T₂ = 245 K were studied by specific heat (DSC) and X-ray lattice parameters measurements in function of temperature. These ferroelastic transitions are accompanied by anomalous heat absorption, sudden changes in the unit cell parameters and crystal symmetry. Formation of domain structure in particular phases examined under a polarizing microscope is also described. Domain walls of the type (100), (010), and (001) observable at room temperature indicate on the triclinic symmetry of the Phase II. Above T₁ the crystal becomes orthorhombic. First-order phase transition at T₂ causes the symmetry of the crystal in the Phase III to be monoclinic with the c-axis unique. The phase transition diagram is proposed and compared with the phase situation in nondeuterated NH₄BeF₃ crystals.     

Growth and characterization of titanium suboxide crystals

The titanium and vanadium suboxides MO_x (where 1.5 < x < 2 and M is Ti or V) are particularly interesting compounds because many show metal-insulator transitions which result in interesting electrical, magnetic and spectroscopic phenomena. We have grown high purity crystals of the congruently melting compounds Ti₂O₃, Ti₃O₅ and V₂O₃ and mixed compounds using the tri-arc melting technique. Crystals were grown at speeds of 1-1.5 cm/h. The Magneli phases M_nO_2n-1 (n?4) do not melt congruently and therefore we have grown a number of these compounds using chemical vapor transport with TeC1₄ or NH₄C1. Twinned needles typically larger than 1 cm long are grown by pulling transport in 3–5 days. The crystals have been characterized by X-ray diffraction, atomic absorption and gamma-ray activation analysis. In addition, the use of a scanning microcalorimeter made it possible to determine phase transitions on needles weighing as little as 2–10 mg. The stoichiometry was determined using X-ray diffraction, pycnometric density measurements and thermogravimetry.     

Crystal Structure and Thermal Phase Behaviour of 2,3-Dicyano-4-n-hexyloxyphenyl-trans-4-n-heptyleyelohexanoate

The title compound with the formula C₇H₁₅ C₆H₁₀ COO C₆H₂(CN)₂ OC₆H₁₃ (DHPHC) crystallizes in the monoclinic space group P 21/n with a = 5.633(1), b = 14.254(1), c = 33.435(7) Å, β = 91.56(2)°, and four molecules per unit cell. The structure has been solved by direct methods and refined to R = 0.082. The DHPHC molecules adopt a banana-shaped but considerably stretched form and show a parallel arrangement of their long axes in the crystal lattice. The phase transitions of DHPHC have been investigated by DSC and thermal microscopy. The compound shows a rather complicated melting behaviour and forms two solid phases and a monotropic smectic A phase.     

Mechanisms of the self-organization of star-shaped polymers with a varied structure of branching center based on fullerene C<Subscript>60</Subscript> in solutions

The self-organization of star-shaped polymers in toluene has been studied by small-angle neutron scattering. Polystyrene stars with a mono-C₆₀ branching center are ordered into globular clusters (∼1700 nm in diameter), whereas stars with a double (C₆₀-C₆₀) center are ordered into anisotropic structures (superchains), which are linked (depending on the concentration) into triads (chain clusters ∼2500 nm in diameter). On the contrary, heteroarm polystyrene and poly-2-vinylpyridine stars with a C₆₀ center are weakly associated into dimers. Moderately polar stars with arms composed of polystyrene and diblock copolymer (poly-2-vinylpyridine-poly-tret-butyl methacrylate) form short chains composed of four macromolecules, while stars of higher polarity based on polystyrene and poly-tret-butyl methacrylate form clusters containing ∼12 macromolecules ∼50 nm in diameter. Thus, by varying the structure of the center and the arm polarity, one can control the modes of star structuring.     

Invar effect in PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics

High-density lead ferroniobate PbFe_1/2Nb_1/2O₃ (PFN) is prepared by conventional ceramic technology. Its structural properties are studied in a wide temperature range (293 ≤ T ≤ 973 K). The following chain of phase transitions is established in the vicinity of the transition to the polar phase: Rh (rhombohedral phase) (T < 363 K) → Psc (pseudo-cubic phase) (363 < T < 387 K) → C (cubic phase) (T > 387 K). The paraelectric range contains five ranges of constant unit-cell volume (invar effect): I (387 ≤ T ≤ 413 K), II (433 ≤ T ≤ 463 K), III (553 ≤ T ≤ 613 K), IV (743 ≤ T ≤ 773 K), and V (798 ≤ T ≤ 823 K). It is shown that the anomalous behavior of the PFN dielectric characteristics above the Curie temperature, which was revealed previously, is associated with the specific features of its real (defect) structure, which is caused by the crystal-chemical specificity of the main structure-forming agents: α-Fe₂O₃ and α_ht-Nb₂O₅.     

Thermal Investigations of nBABAs

The phase transitions occurring in nBABAs (n = 1 to 5), especially those involving the nematic phase, have been studied by DSC. Schemes of transition are presented and the chain length (n) dependences of the the transition temperatures and entropies are discussed.     

Study of phase transitions and the incommensurate phase in ACBX 4 crystals by the Monte Carlo method

The statistical model of successive phase transitions in the family of crystals described by the general formula ACBX ₄ has been studied. It is assumed that tetrahedral BX ₄ groups in the disordered tetragonal phase can be located in four equally probable equilibrium positions. Five representatives with different sequences of phase transitions in this family of crystals are considered. The effective equilibrium constants of interactions between tetrahedral groups are calculated within the framework of the electrostatic model. The corresponding phase diagrams and the thermodynamic characteristics of the phase transitions are studied by the Monte Carlo method. The experimental data provided the establishment of the sequence of phase transitions in Rb₂ZnCl₄ and K₂SeO₄ crystals with the formation of an intermediate incommensurate phase. The character of the structure modulation in this phase is established. The calculated thermodynamic characteristics of the phase transitions agree quite well with the experimental data for all the crystals under consideration.     

On the Electrical Switching Properties of Polymeric Liquid Crystals

The properties of side chain polymeric liquid crystals in an electric field are described. The polymers investigated are (A) a polysiloxan-copolymer with Cl- and OCH₃-end groups in the mesogenic side chains and (B) a polyacrylic ester homopolymer with a CN-group in the mesogenic side chain.

Applying the Fréedericksz-effect, the rise times and the passive decay times are determined. With the two-frequency technique, the active decay time is also obtained. The order of magnitude for the active decay time is about one second. The active decay time is approximately constant in a temperature region about 15 K below the clearing point.     

Switchable control of hydrophilicity and hydrophobicity in conjugated polymer nanoparticles by carbon dioxide

Abstract

We synthesized yellow-emissive, fluorine-based conjugated polymer to fabricate CO₂-responsive conjugated polymer nanoparticles (CPNPs). The CPNPs were functionalized with tertiary amine to have responsiveness to CO₂. The amine-functionalized CPNPs became hydrophilic by CO₂, bubbling because the bubbling led to formation of cationic ammonium ions at the side chains of the hydrophobic CPNPs. This resulted in high dispersion stability in aqueous phase even after vigorous mixing in the presence of organic phase (1-octanol). Subsequent N₂ bubbling was done to remove CO₂ present in water, leading to deprotonation of the side chains of CPNPs. The CPNPs became hydrophobic and moved to the organic phase. The CO₂-responsive property was based on the amine groups in the side chain of polymer that reversibly interacted with bicarbonate ion (HCO₃^-), formed by dissolving CO₂ in water, generating switchable hydrophilicity and hydrophobicity.     

Growth,structural, thermal and optical studies on L‐glutamic acid hydrobromide – A new semiorganic NLO material

A new semiorganic crystal, L‐glutamic acid hydrobromide, C₅H₁₀NO₄Br (GHB) has been grown from aqueous solution. The single crystal X‐ray analysis of the crystal showed that it belongs to the non‐centrosymmetric P2₁2₁2₁ space group with protonated glutamic acid as cation and bromine as anion. The back‐bone conformations of the amino acid are in cis and trans form. The side‐chain conformations are observed to be in gauche I / trans / cis / trans forms. The characteristic ‘head‐to‐tail’ hydrogen bonding interaction is observed through a chain C(5) motif. Further, the crystal structure is stabilized by an intricate three‐dimensional hydrogen bonding network. TGA/DTA showed that the grown crystals are thermally stable upto 219 °C without any phase transition. The functional groups responsible for the various modes of vibrations were identified by using FTIR spectroscopy. UV‐Vis‐NIR spectra showed that the crystals have excellent transparency in the visible and infrared regions. The second harmonic generation (SHG) conversion efficiency was investigated to explore the NLO characteristics of the material. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New chiral liquid crystal materials based on menthol: Synthesis and phase behavior

To study structure-mesomorphism relationships of chiral materials based on menthol, a series of chiral compounds containing menthyl group were synthesized. Their chemical structures, formula, and the purity were characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (¹H NMR), and elemental analyses. The corresponding phase behavior was investigated with differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The effect of the mesogenic core, flexible spacer, and terminal groups on the mesophase formation and type was discussed. The compound 4-menthyloxyacetoxybenzoyloxy-4′- (6-bromohexyloxy)biphenyl showed the smectic A (S_A) and cholesteric phases on heating cycle, and a cubic Blue Phase, chiral smectic C (S_C^*), S_A and cholesteric phases on cooling cycle. However, the compound 4-menthyloxyacetoxybenzoyloxy-4′-(4-bromobutoxy)biphenyl showed enantiotropic S_C^*, S_A and cholesteric phases, moreover, the platelet texture of a cubic Blue Phase was also observed on cooling cycle. The methacrylate monomer with six flexible methylene units and three phenyl rings as mesogenic core only showed cholesteric phase, while the methacrylate monomer with four methylene unit and three phenyl rings showed the S_C^* and cholesteric phases. However, when the terminal methacryloyloxy groups in the monomers were substituted by thiophenylacryloyloxy groups, the corresponding monomers all show no mesophase. With increasing the rigidity of mesogenic core or decreasing the flexible methylene spacer length, the corresponding melting temperature (T_m) or isotropic temperature (T_i) increased.     

Dielectric studies of segmental dynamics in poly(dimethylsiloxane)/titania nanocomposites

Differential scanning calorimetry, thermally stimulated depolarization currents and dielectric relaxation spectroscopy techniques, covering together a broad frequency range of 10⁻⁴ to 10⁶ Hz, were employed to investigate the effects of in situ synthesized titania nanoparticles on thermal transitions, segmental dynamics and interfacial interactions in poly(dimethylsiloxane)/titania nanocomposites. Titania particles (TiO₂, 20-40 nm in diameter) were prepared and well dispersed into the polymer network through sol-gel technique, aiming at stable and mechanically reinforced systems. The interactions between polymer and fillers were found to be strong, supressing crystallinity and affecting the temperature development of the glass transition. The segmental relaxation associated with the glass transition consists of three contributions, arising, in the order of decreasing mobility, from the bulk (unaffected) amorphous polymer fraction (α relaxation), from polymer chains restricted between condensed crystal regions (α_c relaxation), and from the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface (α? relaxation). The thickness of the interfacial layer was estimated to be in the range of 3-5 nm. Measurements using different thermal protocols proved very effective in analyzing the origin of each relaxation and the respective effects of filler addition.