Structure generation in PTFE porous membranes induced by the uniaxial and biaxial stretching operations

The morphology and its formation has been studied for the steady-rate stretching operation of polytetrafluoroethylene (PTFE) porous membranes, which were prepared from PTFE fine powders with a mean diameter of approximately 2×10² nm through extruding and rolling operations prior to the stretching operation. The uniaxially stretched membranes were produced by the unique stretching operation parallel to the rolling operation, and the biaxially stretched membranes by the dual operations consisting of the parallel and the subsequent perpendicular stretches. The inversion of the stretching direction, i.e., the first stretching operation perpendicular to the rolling operation and the second one parallel has been observed to be impossible due to the occurrence of macroscopic fractures on the membrane during the first stretching operation. The uniaxially stretched porous membranes are comprised of fibrils completely oriented in the stretching direction and remaining island-like fractures of the rolled PTFE sheet. The second stretching operation provides a lattice-like porous structure by giving the island-like fractures further division along the second stretching direction and the fibrils slant from the original orientation. The stretching operation is possible within the range where the relative elongation of the whole membrane along the second stretching direction is less than 50% of that along the first one, indicating that the fibrils yielded by the first stretching operation sustain the lattice-like porous structure induced by the second stretching operation. The distribution of the slant angle of the fibrils is independent of the elongation in the second stretching operation, thus, the division of the island-like fractures linked with the fibrils steadily proceeds during the second stretching operation.     

Morphological characterization during deformation of a poly(ether ester) thermoplastic elastomer by small-angle x-ray scattering

The scattering behavior of pre-drawn and annealed bristles of a highly deformable poly(ether ester) themoplastic elastomer based on poly(butylene terephthalate) as hard segments and poly(ethylene glycol) as soft segments in a ratio of 57/43 wt.-% is studied. Small-angle x-ray seattering measurements with an area detector are carried out on bristles with and without application of stress up to 195% relative deformation. Two-dimensional scattering patterns are used for morphological characterization of the sample.At small deformations one morphology peak is found, corresponding to a periodicity that changes affinely with deformation. The morphology of the smaple represents assemblies of mutually parallel crystalline lamellae, positioned perpendicular to the stretching direction both under and without stress. When macrodeformation increases a second peak appears, and a four-point pattern is observed in the relaxed state. In this intermediate deformation range coexisting morphologies contribute to the scattering. Additional contributions arise from lamellae, which are inclined to the stretching direction, as well as from lamellae, which are again perpendicular to the stretching direction, as a result of microfibril relaxation and loss of interfibrillar contacts. At large deformations the latter morphology dominates and the 2D-scattering pattern again shows a two-point character. A morphological model for this behaviour is discussed, where the break of interfibrillar contacts during deformation and the inhomogeneous stress field in the sample play an important role.Dedicated to the 65th birthday of Prof. E.W. Fischer Prof. Fischer was always a quide and patient teacher to us, he inspired our work with his intense interest and many valuable suggestions. We want to congratulate and thank him sincerely and extend our best wishes for the future.     

利用聚乙烯醇结晶辅助制备多重交联的高强度聚丙烯酰胺水凝胶

在表面带有C=C双键的乙烯基杂化二氧化硅纳米颗粒(vinyl hybrid silica nanoparticle,VSNP)上接枝丙烯酰胺(AM),所得到的纳米刷状凝胶因子通过聚丙烯酰胺(PAM)间的氢键形成物理交联点,则多官能化的VSNP可作为拟共价交联点构筑双重交联的单一网络纳米复合物理水凝胶(nanocomposite physical hydrogel,NCP gel),表现出较高的强度和超拉伸性.为了进一步提高凝胶的强度和韧性,将少量PVA和PAM/VSNP纳米刷混合制成凝胶,通过冷冻-融化处理,使与PAM分子链相互缠绕并形成氢键作用的PVA结晶,形成新的交联点进一步交联PAM NCP gel,得到多交联的PAM NCP gel体系.通过拉曼光谱和示差扫描量热分析,证明凝胶中的PVA通过氢键既可以与PAM相互作用,又形成微晶为新交联点,大大增强了NCP gel的力学性能,与PAM NCP gel相比,凝胶的拉伸强度和断裂能分别从313 k Pa和1.41×104 J/m~2提高到了557k Pa和4.65×104 J/m~2.     

Global Minimum‐Energy Structure and Spectroscopic Properties of I2.−⋅n H2O Clusters: A Monte Carlo Simulated Annealing Study

The vibrational (IR and Raman) and photoelectron spectral properties of hydrated iodine‐dimer radical‐anion clusters, I₂^.? ? n H₂O (n=1–10), are presented. Several initial guess structures are considered for each size of cluster to locate the global minimum‐energy structure by applying a Monte Carlo simulated annealing procedure including spin–orbit interaction. In the Raman spectrum, hydration reduces the intensity of the I? I stretching band but enhances the intensity of the O? H stretching band of water. Raman spectra of more highly hydrated clusters appear to be simpler than the corresponding IR spectra. Vibrational bands due to simultaneous stretching vibrations of O? H bonds in a cyclic water network are observed for I₂^.? ? n H₂O clusters with n≥3. The vertical detachment energy (VDE) profile shows stepwise saturation that indicates closing of the geometrical shell in the hydrated clusters on addition of every four water molecules. The calculated VDE of finite‐size small hydrated clusters is extrapolated to evaluate the bulk VDE value of I₂^.? in aqueous solution as 7.6 eV at the CCSD(T) level of theory. Structure and spectroscopic properties of these hydrated clusters are compared with those of hydrated clusters of Cl₂^.? and Br₂^.?.     

Plastic deformation behavior of polypropylene sheet with transversal orientation

The deformation behavior of a polypropylene (PP) sheet, in which c‐axis of β‐form PP orients perpendicular to the flow direction in the sheet [transverse direction (TD)], is studied focusing on the mechanical anisotropy. The deformation mechanism of this sample is found to be strongly dependent on the stretching direction. Shear yielding is dominant in TD stretching, indicating that reorganization and phase transformation of crystalline form occur easily with low yield stress. On the contrary, in machine direction (MD) stretching, numerous microvoids appear with the transformation of crystalline form as well as the rotation of c‐axis. Moreover, the crazing process, initiated at the orientation defects, occurs locally with remaining undeformed region in which β‐form PP orients to TD. The deformation behavior is applicable to a microporous film. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 897–906     

Nematic and smectic liquid single crystal elastomers: Influence of external stress parallel and perpendicular to the director

The influence of uniaxial mechanical stress applied parallel or perpendicular to the director axis (optical axis) of nematic and smectic-A elastomers having a permanent macroscopically ordered monodomain structure is investigated. Due to the different phase structures applying an external mechanical stress leads to completely different responses with respect to the young moduli and reorientation processes of the director. Nematic elastomers exhibit a weak anisotropy of the moduli parallel (μ₁₁ and perpendicular (μ) to the optical axis with μ₁₁ / μ = 1.5. The moduli are mainly determined by the rubber elasticity of the poly(siloxane) network. Applying the mechanical stress perpendicular to the initial director axis causes a rotation of the director axis that is connected with a periodic pattern formation and a ‘soft elastic’ response. For smectic-A elastomers μ₁₁/μ is in the order of 10². While μ reflects a rubber elastic response similar to that of the nematic system, μ₁₁ indicates an enthalpy-elastic behaviour of the one-dimensional long range order of the smectic layers. In this direction reorientation of the phase structure with a deformation of the smectic layers occurs above a threshold elongation. In contrast to the nematic networks, a deformation perpendicular to the optical axis causes no director reorientation, displaying the liquid-like properties within the smectic layers.     

Stretching temperature and direction dependency of uniaxial deformation mechanism in overstretched polyethylene

In order to elucidate microscopic deformation behavior at different locations in isotropic semicrystalline polymers, the structural evolution of a preoriented high‐density polyethylene sample during tensile deformation at different temperatures and along different directions with respect to the preorientation was investigated by means of combined in situ synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques. For samples stretched along preorientation, two situations were found: (1) at 30 °C, the sample broke after a moderate deformation, which is accomplished by the slippage of the microfibrils; (2) at 80 and 100 °C, fragmentation of original lamellae followed by recrystallization process was observed resulting in new lamellar crystals of different thickness depending on stretching temperature. For samples stretched perpendicular or 45° with respect to the preorientation, the samples always end up with a new oriented lamellar structure with the normal along the stretching direction via a stress‐induced fragmentation and recrystallization route. The thickness of the final achieved lamellae depends only on stretching temperature in this case. Compared to samples stretched along the preorientation direction, samples stretched perpendicular and 45° with respect to the preorientation direction showed at least several times of maxima achievable stress before macroscopic failure possibly due to the favorable occurrence and development of microdefects in those lamellar stacks with their normal parallel to the stretching direction. This result might have significant consequence in designing optimal procedure to produce high performance polyethylene products from solid state. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 716–726     

Influence of oriented β‐lamellae on deformation and pore formation in β‐nucleated polypropylene

Four β‐nucleated polypropylene samples with increasing die draw ratio (DDR) were prepared to modify lamellae arrangement. The DSC, SEM, and 2D‐XRD results show that all four cast films had similar crystallinity, high contents of β‐crystal but lowering stability of β‐lamellae with ascending DDR. Meanwhile, the anisotropy of β‐lamellae distribution strengthens gently and the stacked lamellae structure perpendicular to the machine direction (MD) predominates dramatically. Tensile testing at 25 °C and 90 °C were conducted along MD and transverse direction (TD), respectively. The markedly expanding difference of deformation indicates the anisotropy highlighted significantly. Additionally, when the samples stretched along MD, a more homogeneous deformation occurs with ascending anisotropy, which is completely opposite to the β‐lamellae stability. But samples deformed more heterogeneous when stretched along TD. The characterization of morphological evolutions during stretching shows that the stacked lamellae debonds uniformly and abundant microvoids formed when the sample stretched along MD with higher anisotropy, resulting in evenly dispersion of stress, consequently making a more uniform distribution of defects and a better isotropic deformation. Moreover, the microfibrils and defects distributed uniformly within higher orientation sample after longitudinal stretching stretched along MD, leading to the dramatic improvement of pore size distribution of the membrane after biaxial stretching. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1745–1759     

A clear-cut experimental method to discriminate between in-plane and out-of-plane molecular transition moments

A new method to orient organic molecules, based on their absorption as guest of the crystalline nanoporous delta phase of uniaxially stretched syndiotactic polystyrene films, is presented. This molecular alignment method not only allows high degrees of guest orientation to be attained but also orients the planar guest molecules with their smallest cross section nearly parallel to the stretching direction (B) rather than perpendicular (A, as usual for absorption in amorphous polymeric phases). As a consequence, in-plane and out-of-plane transition moment vectors maximize their absorption intensities for light polarization nearly perpendicular and parallel to the stretching direction, respectively. Hence, simple linear dichroism measurements by polarized spectra can allow an easy and clear-cut discrimination between in-plane and out-of-plane guest transition moment vectors.     

Polymesomorphism and orientation in liquid crystalline poly(triethylene glycol p,p′-bibenzoate)

The phase transitions and the orientational behavior of liquid crystalline poly(triethylene glycol p,p′-bibenzoate) have been studied. The real-time synchrotron diffraction results indicate that, on cooling from the isotropic melt, an orthogonal SmA mesophase is formed first, and later it is transformed into a tilted SmC mesophase. However, the SmA mesophase is stable in a rather wide temperature interval, and the transformation into the SmC phase occurs at temperatures close to the glass transition, so that not very high tilting angles are attained. The uniaxial deformation of the SmC mesophase indicates that usual parallel orientation of the molecular axes in relation to the stretching direction is obtained at high strain rates, while anomalous perpendicular orientation occurs at low deformation rates, with the smectic layers aligned with the stretching direction and the molecular axes almost perpendicular. A mixture of the two types of orientation is observed at intermediate rates, with rather interesting features.     

X-ray diffraction by liquid-crystalline elastomers

A new series of mesomorphic side chain polysiloxane networks has been recently synthesized in which the chemical nature of the linkage and the mesogenic group have been varied and the gelation conditions during the chemical reaction have been studied. This paper presents an X-ray diffraction study of the mesogenic group orientation in stretched samples of these networks. The angular extension of the so-called wide angle diffuse ring is used to estimate the orientational order of the mesogenic group versus strain. To perform these experiments, a special stretching device was developed and a new two-dimensional X-ray detector was used which allowed us to collect the data in a few minutes. On stretching, it was observed that the mesogenic groups orient themselves perpendicular to the stress direction for all of the samples but for one for which the parallel orientation prevailed. This prevents the establishment of a simple general law. From another point of view, the polymer concentration during the chemical reaction, which controls the gelation, is shown to be an important parameter with which to understand the physical properties: the networks synthesized below the gel point do not display reproducible and reversible behaviour, rather they flow when they are stretched. Conversely, all of the networks synthesized above the gel point really show the same well-defined behaviour independent of the sample history. Their orientational order increases regularly with the strain, first quickly, then moderately until it eventually saturates. This saturation value of the mesogenic group orientational order does not reach the nematic order parameter of the same (uncross-linked) mesomorphic side chain polymers. This suggests that the cross-links may create local tensions which disturb the nematic field.     

Comparison of the Structural Evolution of β Polypropylene during the Sequential and Simultaneous Biaxial Stretching Process

In this work, the lamellar structural evolution and microvoids variations of β polypropylene(β-PP) during the processing of two different stretching methods, sequential biaxial stretching and simultaneous biaxial stretching, were investigated in detail. It was found that different stretching methods led to significantly different lamellae deformation modes, and the microporous membranes obtained from the simultaneous biaxial stretching exhibited better mechanical properties. For the sequential biaxial stretching, abundant coarse fibers originated from the tight accumulation of the lamellae parallel to the longitudinal stretching direction, whereas the lamellae perpendicular to the stretching direction were easily deformed and separated. Those coarse fibers were difficult to be separated to form micropores during the subsequent transverse stretching process, resulting in a poor micropores distribution. However, for the simultaneous biaxial stretching, the β crystal had the same deformation mode, that is, the lamellae distributed in different directions were all destroyed, forming abundant microvoids and little coarse fibers.     

Stress‐Induced Stabilization of Crystals in Shape Memory Natural Rubber

In contrast to all known shape memory polymers, the melting temperature of crystals in shape memory natural rubber (SMNR) can be greatly manipulated by the application of external mechanical stress. As shown previously, stress perpendicular to the prior programming direction decreases the melting temperature by up to 40 K. In this study, we investigated the influence of mechanical stress parallel to prior stretching direction during programming on the stability of the elongation‐stabilizing crystals. It was found that parallel stress stabilizes the crystals, which is indicated by linear increase of the trigger temperature by up to 17 K. The crystal melting temperature can be increased up to 126.5 °C under constrained conditions as shown by X‐ray diffraction measurements.     

Tightening of gelatin chemically crosslinked networks assisted by physical gelation

Developing the use of polymers from renewable sources to build hydrogels with tailored mechanical properties has become an increasing focus of research. The impact of the thermo‐reversible physical networks of gelatin (arising from the formation of triple‐helices) on the structure formation of a chemical network, obtained by crosslinking with glutaraldehyde (a non‐catalytic crosslinker), was studied using optical rotation, oscillatory rheology, and large strain mechanical deformation. We observed a direct correlation between the storage shear modulus of the chemical network grown in the gel state (i.e., simultaneously with the physical network) and the amount of gelatin residues in the triple‐helix conformation (χ). Since χ is directly affected by temperature, the value of the storage modulus is also sensitive to changes in the temperature of gel formation. χ values as low as 12% lead to an increase of the shear storage modulus of the crosslinked gel by a factor of 2.7, when compared to a chemical network obtained in the sol state (i.e., in the absence of a physical network). Our results show that the physical network acts as a template, which leads to a greater density of the chemical crosslinks and a corresponding higher elastic modulus, beyond what is otherwise achieved in the absence of a physical network. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1850–1858     

3D Motions of Iron in Six‐Coordinate {FeNO}7 Hemes by Nuclear Resonance Vibration Spectroscopy

The vibrational spectrum of a six‐coordinate nitrosyl iron porphyrinate, monoclinic [Fe(TpFPP)(1‐MeIm)(NO)] (TpFPP=tetra‐para‐fluorophenylporphyrin; 1‐MeIm=1‐methylimidazole), has been studied by oriented single‐crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in‐plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in‐plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in‐plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out‐of‐plane Fe‐N‐O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v₅₀ as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe‐X‐O (X=N, C, and O) complexes is correlated with the Fe?XO bond lengths. The nature of highest frequency band at ≈560 cm^?1 has also been examined in two additional new derivatives. Previously assigned as the Fe?NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.     

Density Functional Theory Study of HCN Adsorption on Small Gold Clusters

A theoretical study was carried out on the adsorption of hydrocyanic acid on small Aun （n ≤ 7） clusters using density functional methods. For HCN adsorption on gold clusters, no dependence was found with respect to the even-odd alternation in relation to the number of gold atoms in the cluster. The HCN molecule is adsorbed at simple adsorption sites （1-fold coordination）, perpendicular to the adsorption site. The largest adsorption energy is only about 74.61 kJ·mol^-1, which indicates that the HCN molecule does not decompose and the C-N bond retains triple bond, and that the C-H and C-N stretching frequencies are only weakly perturbed. The adsorbed C-N and C-H stretching frequencies are blue- and red-shifted compared with the values of free HCN, respectively.     

Infrared spectra and ab initio calculations for the F- -(CH4)n (n = 1-8) anion clusters

Infrared spectra of mass-selected F- -(CH4)n (n = 1-8) clusters are recorded in the CH stretching region (2500-3100 cm-1). Spectra for the n = 1-3 clusters are interpreted with the aid of ab initio calculations at the MP2/6-311++G(2df 2p) level, which suggest that the CH4 ligands bind to F- by equivalent, linear hydrogen bonds. Anharmonic frequencies for CH4 and F--CH4 are determined using the vibrational self-consistent field method with second-order perturbation theory correction. The n = 1 complex is predicted to have a C3v structure with a single CH group hydrogen bonded to F-. Its spectrum exhibits a parallel band associated with a stretching vibration of the hydrogen-bonded CH group that is red-shifted by 380 cm-1 from the nu1 band of free CH4 and a perpendicular band associated with the asymmetric stretching motion of the nonbonded CH groups, slightly red-shifted from the nu3 band of free CH4. As n increases, additional vibrational bands appear as a result of Fermi resonances between the hydrogen-bonded CH stretching vibrational mode and the 2nu4 overtone and nu2+nu4 combination levels of the methane solvent molecules. For clusters with n < or = 8, it appears that the CH4 molecules are accommodated in the first solvation shell, each being attached to the F- anion by equivalent hydrogen bonds.     

Anisotropic heat transport in nanoconfined polyamide-6,6 oligomers: atomistic reverse nonequilibrium molecular dynamics simulation

While polymers are known as thermal insulators, recent studies show that stretched single chains of polymers have a very high thermal conductivity. In this work, our new simulation scheme for simulation of heat flow in nanoconfined fluids [H. Eslami, L. Mohammadzadeh, and N. Mehdipour, J. Chem. Phys. 135, 064703 (2011)] is employed to study the effect of chain ordering (stretching) on the rate of heat transfer in polyamide-6,6 nanoconfined between graphene surfaces. Our results for the heat flow in the parallel direction (the plane of surfaces) show that the coefficient of thermal conductivity depends on the intersurface distance and is much higher than that of the bulk polymer. A comparison of results in this work with our former findings on the heat flow in the perpendicular direction, with the coefficient of heat conductivity less than the bulk sample, reveal that well-organized polymer layers between the confining surfaces show an anisotropic heat conduction; the heat conduction in the direction parallel to the surfaces is much higher than that in the perpendicular direction. The origin of such anisotropy in nanometric heat flow is shown to be the dramatic anisotropy in chain conformations (chain stretching) beside the confining surfaces. The results indicate that the coefficients of heat conductivity in both directions, normal and parallel to the surfaces, depend on the degree of polymer layering between the surfaces and the pore width.     

Variation of the ℏω with the particle number and the appearance of “kinks” for atomic clusters

The dependence of the harmonic oscillator (HO) energy level spacing ?ω on the particle number N is studied analytically for atomic (metal) clusters on the basis of their electronic densities, parametrizing Ekardt's results (for sodium clusters) by means of a Fermi distribution. An interesting feature of such an approach is that it leads, under the assumptions made, to “kinks,” that is, to “marked discontinuities in the slope” of ?ω at the closed shells. These discontinuities diminish as N increases. For large N, ?ω becomes simply: ?ω?c₁N^?1/3+c₂N^?1. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Liesegang pattern formation in kappa-carrageenan gel

We report a new class of the spatial pattern formation process in which the gel plays essential roles. The system studied here is the solution of kappa-carrageenan in which potassium chloride is diffused. The solution transforms into the gel state with the diffusion of potassium chloride. Then the stripe pattern, which is perpendicular to the direction of the diffusion of potassium chloride, appears within the gel. The pattern thus formed in the gel is studied as a function of the concentration of the solution of potassium chloride. We find that the dense region of the stripe pattern consists of the liquid crystalline gel, whereas the dilute region is the amorphous gel. The transition from the amorphous gel to the liquid crystalline gel, hence, occurs in the gel state of kappa-carrageenan. The gel behaves as a pattern-forming substance as well as the supporting medium of the pattern in this system. The period and the thickness of the layers of liquid crystalline gel are analyzed. Both the period and the thickness of the layers are found to depend strongly on the concentration of the solution of potassium chloride.