Anomalous properties of poly(γ-benzyl L-glutamate) film composed of unusual helices

Poly(γ-benzyl L-glutamate) film composed of unusual 7-residue 2-turn ( ) helices was prepared by slow casting from chloroform solution. The helical conformation was irreversibly transformed at 84°C to the normal conformation. In d.s.c. thermograms the transformation appeared as an endothermic first order transition with an enthalpy change of 3.4 cal g⁻¹. The viscoelastic and dielectric measurements indicated that the loss peak due to the side chain motion in this film was reduced to a great extent compared to the film of helices. The dielectric relaxation strength dictated that there was a motional restriction in three quarters of the side chains. By comparison to a racemic mixture of poly(γ-benzyl glutamate), these anomalous properties were attributed to the stack of terminal benzyl groups of side chains (ormed in an alternating manner between neighbouring chains). The first order transition arises from the cooperative fusion of the stack and the side chains with their benzyl groups which are restricted in thermal motion. Such stacks may be facilitated by the precise ‘knob (side chain) and hole (space between side chains)’ packing which can be attained between neighbouring helices.     

Effect of a Side Chain Length of Polymer on Both the Adhesion and Dispersibility of γ-Fe2O3

Acrylic ester-acrylic acid copolymers were synthesized as model binder of magnetic paints. The dispersibility of γ-Fe₂O₃ was investigated as a function of the content of-COOH groups and the aliphatic side chain length. The following results were obtained. The saturated absorbance of copolymers on γ-Fe₂O₃ increased with the increase in content of-COOH groups, and then the dispersibility of γ-Fe₂O₃ was improved. However, when the content of-COOH group was over the 2 mol%, the saturated adsorbance of polymers was constant and the dispersibility of γ-Fe₂O₃ decreased with increasing number of carbon atoms in the acrylic ester side groups. However, the dispersibility of γ-Fe₂O₃ was explained by the difference in conformation of adsorbed polymers.     

Molecular motion in poly(amino acid)s: 6. Molecular motion in a β-form crystal of poly(γ-methyl-d-glutamate)

The molecular motion in a β-form crystal of fully annealed poly(γ-methyl-d-glutamate) was investigated by means of an X-ray diffractometer and an infra-red spectrophotometer. The interchain periodicity (chains are bound side by side with intermolecular hydrogen bonds) increases linearly with increasing temperature, indicating ordinary thermal expansion of the lattice. The intersheet periodicty (sided chains pack into sheets) increases linearly up to 150°C and then the slope of the temperature dependence of the periodicty increases owing to the onset of active segmental motion of the side chains.It is evident from the temperature dependence of the wavenumber of the maximum of the infra-red absorption for NH stretching in the crystal that thermal expansion in the direction of the hydrogen bond is hardly affected by the segmental motion of the side chain.     

Secondary relaxation processes in polyethylene terephthalate–additive blends: 2. Dynamic mechanical and dielectric investigations

Dynamic mechanical and dielectric analysis have been used to examine the effects that low molecular weight additives have upon the secondary relaxation processes in polyethylene terephthalate. In each of the polymer/additives blends examined one main secondary relaxation peak, known as the β peak has been observed below the glass transition temperature. It would, however, appear from the dielectric and mechanical results that the β peak consists of two different relaxation processes, one on the high temperature side and one on the low temperature side of the peak. A closer examination of these relaxation processes suggests that the high temperature side of the peak is due to phenyl ring flips, whereas the low temperature side is due to the motion of the carbonyl groups. In addition, it would appear that the activation energy and the enthalpy of the phenyl ring flips are both considerably higher than that of the carbonyls. Finally, when low molecular weight additives were blended into the polymer it was observed that only the high temperature side of the peak was suppressed and that the low temperature side remained virtually unaltered. This would indicate that although the additives suppress the motion of the phenyl rings, they have little effect upon the motion of the neighbouring carbonyl groups.     

Structures of two crystal forms of poly(ethylene oxide)-sodium thiocyanate complex with molar ratios of 3:1 and 1:1

The poly(ethylene oxide) (PEO)-sodium thiocyanate system was found to exhibit polymorphism: there exist at least three crystal modifications. Among them, the crystal structures of two kinds of PEO-NaSCN complex with molar ratios (EO:NaSCN) of 3:1 (form I) and 1:1 (form II), respectively, were determined by X-ray diffraction. Crystal data are as follows: form I, monoclinic P2₁/a, a = 16.83, b = 10.64, c(chain axis)=7.19 Å^°

1 Å=10⁻¹ nm, γ=125.5° (c-unique), N=12 EO units (2 chains) and 4 NaSCN ions; form II, monoclinic P2₁/c, a=7.55, b=12.10, c(chain axis)=5.83 Å, β=97.5° (b-unique), N=4 EO units (2 chains) and 4 NaSCN ions. Form I has a crystal structure resembling that of the PEO-NaI complex. The polymer chains have a twofold helical structure of conformation, the chain repeat comprising six EO units. The helical polymer chain coils around an array of Na ions and each Na ion is coordinated by four polymer O atoms and two N of the SCN ions (the coordination number is six). In form II, which exists only under high tension, the polymer chains have a glide structure of conformation, the chain repeat comprising two EO units. Since the PEO chain in form II takes a rather stretched conformation, the Na ions are not wrapped by the polymer chain. The coordination number is again six, but each Na ion is coordinated by two polymer O atoms, two N and two S of the SCN ions. Form II is transformed into form I when the tension is released.     

Ferroelectric Ceramics in the PbMg1/3Nb2/3O3-PbCd1/3Nb2/3O3 System

Solid solutions (1-x)PbMg_1/3Nb_2/3O₃ + xPbCd_1/3Nb_2/3O₃ with x = 0-0.30 are investigated with purpose to work out a capacitor ceramics with good dielectric properties and low sintering temperature. It is found that the perovskite phase forms at sintering near to 980°C and begins to decompose at higher temperatures. When x grows from 0 to 0.30, the Curie temperature linearly grows from -10°C to +25°C, the dielectric permittivity ε_m in the Curie point T_C decreases from 18000 to 6800 and the phase transition becomes more diffused. The dielectric permittivity at room temperature is rather high and the temperature stability is improved. The system is of interest, because it can serve as a base for working out some ceramic materials for capacitors with low sintering temperature, which needs of no special atmosphere at burning.     

Computational Study on the Conformation and Vibration Frequencies of β-Sheet of ?-Polylysine in Vacuum

Two oligomers, each containing 3 l-lysine residues, were used as model molecules for the simulation of the β-sheet conformation of ɛ-polylysine (ɛ-PLL) chains. Their C terminals were capped with ethylamine and N terminals were capped with α-l-aminobutanoic acid, respectively. The calculations were carried out with the hybrid two-level ONOIM (B3LYP/6-31G:PM3) computational chemistry method. The optimized conformation was obtained and IR frequencies were compared with experimental data. The result indicated that the two chains were winded around each other to form a distinct cyclohepta structure through bifurcated hydrogen bonds. The groups of amide and α-amidocyanogen coming from one chain and the carbonyl group from the other chain were involved in the cyclohepta structure. The bond angle of the bifurcated hydrogen bonds was 66.6°. The frequency analysis at ONIOM [B3LYP/6-31G (d):PM3] level showed the IR absorbances of the main groups, such as the amide and amidocyanogen groups, were in accordance with the experimental data.     

Dielectric relaxation and the molecular motion of poly(vinylidene fluoride) crystal form II under high pressure

The α dielectric relaxation of poly(vinylidene fluoride) crystal form II is studied under pressures up to 7 kbar at temperatures from 100° to 200°C. The isotherm for variation of the dielectric increment with pressure shows a maximum. This behaviour is examined on the basis of models of molecular motion for the α relaxation previously proposed; longitudinal disorder exists in the crystalline chains. The calculations reproduce the experimental results except for the pressure coefficient of the dielectric increment. The metastable conformation exists together with the most stable conformation in one chain, and dipole reversal parallel to the molecular axis occurs throughout the whole chain.     

Insight into the liquid crystalline behavior of poly-{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene}: a typical thermotropic mesogen-jacketed liquid crystalline polymer

Based on the thermodynamic and kinetic analyses, the thermotropic hexagonal columnar liquid crystalline (LC) phase of poly-{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene} (PBPCS) at high temperature is self-assembled by driving forces of the conformation entropy and macromolecular interaction, respectively. Because of the strong ??coupling effect?? between flexible backbone and rigid side chain, the molecular structure of PBPCS can be regarded as ??dual chains?? model that a flexible backbone and a rigid side chain are in parallel, the backbone and side chain are corresponded to different molecular thermal motion characteristics. When the orientation requirements of the side mesogenic chain are dominant, LC phase appears; when the random motions of flexible backbone are dominant, LC phase disappears. PBPCS demonstrates various types of metastable phases in the LC phase transition. Above LC phase transition temperature, molecular motion of rigid side chain is dominant; therefore, LC phase is formed. In the temperature range from glass transition to LC phase transition, molecular motion of flexible backbone is dominant; therefore, metastable LC phase may be relaxed partly and even disappear. The stability of LC phase can be determined by the kinetic factors, it is related to the residence time for LC phase formation and the interaction among the aggregated columnar macromolecular chains. Metastable LC phase disappeared in the subsequent cooling procedure, however, the stable LC phase was not.     

Synthesis of bulk and alumina-supported γ-Mo2N catalysts by a single-step complex decomposition method

A single-step complex decomposition method for the synthesis of bulk and alumina-supported γ-Mo₂N catalysts is described. The complex precursor (HMT)₂(NH₄)₄Mo₇O₂₄·2H₂O (HMT: hexamethylenetetramine) is converted to γ-Mo₂N under a flow of Ar in a temperature range of 823–1023 K. Furthermore, decomposition of the precursor in a NH₃ flow forms γ-Mo₂N in a temperature range of 723–923 K. Compared with direct decomposition of the precursor in Ar, the reaction in NH₃ shows obvious advantages that the nitride forms at a lower temperature. In addition, alumina-supported γ-Mo₂N catalysts with different nitride loadings can be prepared from the alumina-supported complex precursor in the Ar or NH₃ flow. The resultant catalysts exhibit good dibenzothiophene HDS activities, which are similar to the γ-Mo₂N/γ-Al₂O₃ prepared by traditional TPR method. The catalyst prepared by decomposition in an Ar flow exhibits highest activity. It proves that such a single-step complex decomposition method possesses the potential to be a general route for the preparation of molybdenum nitride catalysts.     

An investigation on the molecular mobility through the glass transition of chlorinated butyl rubber

In this paper, dynamic mechanical analysis (DMA), dielectric spectroscopy (DS) and positron annihilation lifetime spectroscopy (PALS) were used to study chlorinated butyl rubber (CIIR), in order to shed light on its unique relaxation behaviors. The dynamic mechanical loss tangent of CIIR reveals an asymmetrical broad structure with a maximum peak on the high-temperature side and a shoulder peak on the low-temperature side. DS clarifies that the shoulder peak, which exactly corresponds to the ″ peak, is the α process originating from the local segmental motion. While the maximum peak is assigned to the slow process arising from the motion of longer chain segments. The slow process exhibits stronger frequency dependence than the α process. The PALS analysis also shows the two processes; moreover, it suggests that CIIR exhibits very effective chain packing. It is due to the effective chain packing that the motion of longer chain segments is retarded and separates from the local segmental motion in time scale. This effect is another reason for the two-peak structure of CIIR, besides the low intermolecular co-operativity.     

Monte Carlo simulation of molecular motion and phase transition in poly(ethylene) crystal

Conformational properties of a chain and a phase transition in poly(ethylene) crystal are studied by extending a previous Monte Carlo calculation for a chain in a cylindrical crystalline potential. The crystalline potential is here estimated from van der Waals interactions between one CH₂ group of a chain and its six neighbouring chains. Firstly, conformational disorders of the chains of various chain length are examined, and a definite dependence of the chain conformation on its length is demonstrated. Secondly, the behaviour of the chain at atmospheric pressure is simulated, where the modes of molecular motions and the associated disorders in conformation are clarified as a function of temperature. Thirdly, the phase transition at high temperature and pressure, from orthorhombic phase to hexagonal one, is simulated by assuming a proper molecular field for the chain. All these calculations show the present Monte Carlo calculation to have a wide variety of applications in the studies of polymer crystals.     

Critical exponents of the dielectric constants in diffused-phase-transition crystals

The critical exponent γ in the relation between the dielectric constant and temperature (1/ε - 1/ε_m = C'^-1×(T - T_m)^γ) has been determined precisely for relaxor ferroelectrics Pb(Mg_1/3 Nb_2/3)O₃, Pb(Zn_1/3 Nb_2/3)O₃ and a related solid solution 0.88Pb(Zn_1/3 Nb_2/3)O₃-0.12PbTiO₃, as well as for normal ferroelectrics BaTiO₃ and K(Ta_0.55Nb_0.45)O₃. A high correlation of the γ value with the phase transition diffuseness has been found empirically. Moreover, this γ value is very close to another critical exponent γ^* which is defined in the relation between the dielectric constant and hydrostatic pressure (1/ε - 1/ε_m = C^*-1(p - p_m)^γε).     

Gas permeability of copolypeptide membranes composed of γ-methyl L-glutamate and γ-benzyl L-glutamate

Membranes of copoly(γ-methyl L -glutamate, γ-benzyl L -glutamate) (PMBLG) as well as the related homopolymer membranes were prepared, and permeabilities of oxygen, nitrogen, argon, and carbon dioxide were measured in the 0–70°C temperature range. The values of permeability coefficients and solubility coefficients of the copolymers were smaller than those of permeability coefficients and solubility coefficients of the two homopolymers for every gas studied. The diffusion coefficients of the gases showed a minimum at around 25 mole % benzyl glutamate. The temperature at a maximum of tan δ spectra for the membranes showed a maximum at around 25 mole % benzyl glutamate. The Arrhenius plots of diffusion coefficients and solubility coefficients for PMBLG, which contained 50 and 71 mole % benzyl glutamate, showed a break at about 50 and 40°C, respectively. This permeation behavior is explained by assuming a unique interaction between methyl glutamate and benzyl glutamate side chains.     

Nanosized γ-Al2O3 protective film for fluorescent lamps

Nanosized γ-Al₂O₃ particles were prepared by the sol–gel method with aluminum ion hydrolysis control performed by nitric acid. The as-prepared particles were mixed with deionized-water and stabilizer, and cycled in a high speed sand mill to form a stable γ-Al₂O₃ suspended slurry, which was then coated on the surface of the glass substrate to form a γ-Al₂O₃ protective film. Observations of SEM and visible transmission spectra show that a well-dispersed γ-Al₂O₃ slurry could be obtained after three-cycle grinding suitable to coat fluorescent lamp glass with a dense and uniform film of visible light transmission up to 95%.     

η-Al2O3与γ-Al2O3在CS2催化水解反应中的性能比较

通过制备高纯度的前驱体湃铝石获得了η-Al₂O₃材料,采用XRD验证了η-Al₂O₃与γ-Al₂O₃在晶相结构上的差异,比较了两者的表面形貌、织构及酸碱性能,结果显示,η-Al₂O₃与γ-Al₂O₃的比表面积相当,但η-Al₂O₃具有更弱的弱碱位和较少的强碱位,并拥有丰富的中等强度酸性位。将η-Al₂O₃与γ-Al₂O₃作为催化剂应用于CS₂水解反应,结果表明,在(200~450) ℃测试温度范围内,η-Al₂O₃催化剂对CS₂的水解活性始终优于γ-Al₂O₃,两种催化剂上CS₂反应的浓度效应也明显不同,推测与它们的酸碱性质影响了对CS₂的吸附能力有关,导致两者催化CS₂水解反应遵循了不同的机制。     

Phase transitions and polytypes in β TlInS2 ferroelectric-semiconductor

The temperature dependences of the dielectric properties, conductivity, spontaneous polarization as well as the effect of hydrostatic pressure on the anomalies of these properties have been investigated. The β-TlInS₂ crystals are shown to be often a mixture of the polytypes. Transition into a ferroelectric state for some of the β-TlInS₂ polytypes is not preceded by an incommensurats phase transition.     

Dieletric and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg_1/3Nb_2/3O₃)_0.67(PbTiO₃)_0.33 (PMN-33%PT). A sharp ferroelectric phase transition was observed near 425 K and 429 K for cooling and heating processes, respectively. As temperature decreases, a diffuse phase transition (which begins near 330 K upon cooling) was detected. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 330 and 425 K upon cooling) are diffuse first-order and first-order, respectively. The frequency-dependent dielectric data ε'₁₁₁ (ƒ, T) prove the existence of an electric dipolar relaxation process between 350 and 400 K. The activation energy, the Vogel-Fulcher temperature and attempt frequency corresponding to this relaxation process are also calculated.     

High-resolution solid-state C nuclear magnetic resonance study of isotactic polypropylene polymorphs

High-resolution ¹³C nuclear magnetic resonance spectra are reported for three solid samples of isotactic polypropylene (i-PP): (i) the -crystalline form (monoclinic), (ii) the β-crystalline form (hexagonal), and (iii) the mesomorphic or smectic form of i-PP. These spectra were obtained using magic angle spinning, high-power proton dipolar-decoupling and cross-polarization techniques, and for the β- and smectic forms of i-PP constitute the first reported high-resolution solid-state spectra. The spectrum of the -crystalline form shows well resolved splittings (1 ppm) of the methyl and methylene carbon resonances, as was reported previously by Bunn et al. These splittings are absent in the spectra observed for the β- and smectic forms of i-PP. Based on a comparison of the chemical shifts and T₁ relaxation times observed for the carbon resonances in these three forms of solid i-PP, we reach the following conclusions: (i) i-PP adopts the same 3₁ helical conformation in each crystalline polymorph, (ii) the packing of i-PP helices in the β-form crystal is closer to the more distant of the two interhelical packings in the -crystalline form, and (iii) the local packings of 3₁ helices are very similar in the β- and smectic forms of i-PP.     

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.