Molecular changes on drawing isotopic blends of polyethylene and ethylene copolymers: 1. Static and time-resolved sans studies

Small angle neutron scattering (sans) has been used to observe changes in molecular orientation on deformation of isotopic, melt quenched blends of linear polyethylene and copolymers with butyl and hexyl branches, using uniaxial drawing. In no case was there evidence of significant isotopic fractionation either before or during deformation. Samples left clamped in the neutron beam following deformation showed no evidence of molecular relaxation, while significant changes in molecular orientation with time were observed for samples unclamped after deformation. This latter behaviour could be fitted adequately by a single exponential decay and the relaxation time was found to be of the order of tens of minutes. This relaxation time increased with both increasing molecular weight and the presence of branching. For samples drawn to draw ratios of up to 3.0, the anisotropy in the radius of gyration was compared with predictions for affine deformation. Only in the cases of linear labelled guest molecules in either a linear or copolymer host were significant departures from the affine model observed: no such departures were found for copolymer guest molecules. This is interpreted in terms of a delay in crystallite disruption until higher draw ratios for copolymer guest molecules with respect to linear guest.     

Effect of polymer properties on the structure of injection-molded parts

In this study, the distributions of both molecular orientation and crystallinity along the flow direction as well as across the thickness direction of injection-molded specimens of poly(ethylene terephthalate) (PET) molded at different mold temperatures were investigated. The degree of molecular orientation at the surface of the specimens was compared with that of other injected materials (polystyrene, high density polyethylene, liquid crystal polymer) showing different thermal, rheological, and crystallization characteristics. It was found that the molecular orientation at the skin layer of the molding increases with the polymer relaxation time, the rigidity of the polymer molecules, and the crystallization rate of the polymer. Moreover, in the case of PET, it was found that the crystallinity at the skin layer and in the core of the molding depends on the mold temperature. For low mold temperatures, near the gate, the maximum of crystallinity was observed at the subskin layer because of the “shear-induced crystallization” generated during the filling stage. On increasing the mold temperature, the maximum of crystallinity was found to shift to the skin layer as a result of the decrease of the thickness of this layer. For low mold temperatures, the variation of the molecular orientation in the thickness direction was found to be much the same as for the crystallinity of the polymer. This result indicates that the shear-induced crystallization process improves the degree of molecular orientation in the flow direction since it inhibits the relaxation process of the polymer molecules.     

Computer simulation of electrospinning. Part I. Effect of solvent in electrospinning

Electrospinning technique has been recognized as an efficient method to manufacture nano-fiber. At present, research focuses on the structure and morphology of the fibers, a few investigations have been reported the mechanism of electrospinning. In our experiment, appropriate polymer EVOH (ethylene/vinyl alcohol copolymer) and different solvents were chosen, the energy change in the process of molecule orientation was analyzed by computer simulation, and the morphologies of the fiber were indicated by scanning electron microscope (SEM). Results indicate that: in the process of molecule orientation, some barriers should be overcome; the height of the barrier is determined by the relaxation times (τ) of the molecule. The relaxation times vary in different solutions, when the relaxation time is short, the orientation of molecule is easy, so jet instability will be fierce, and fibers with small diameter are obtained.     

Detection of major VOCs in exhaled breath under ppb level using porous ZnO nanobelts

Atomic defects can enhance catalytic efficiency by providing coordinatively unsaturated sites in the crystal structure of metal oxides. It allows facile chemical reactions because the sites can react with other molecules with relatively lower energy. Therefore, atomic defect engineering can be a cost-effective strategy to replace novel metal catalysts for the development of ultrasensitive gas sensors. Herein, we fabricated porous ZnO nanobelts with atomic step structures for acetone, ethanol, and isoprene gas sensing under the parts-per-billion (ppb) level. Numerous atomic step structures could be formed by removing H and F atoms during the conversion process of ZnOHF at 500°C. The synthesis method of metal oxide nanomaterial by conversion from metal hydroxide fluoride will provide the atomic defects and it will be useful to prepare ultrasensitive sensing material. Furthermore, the gas selectivity of the porous ZnO nanobelt was investigated based on the appearance energy associated with the separation of the methyl group.     

Linear and non-linear viscoelastic behavior of poly(methyl methacrylate) in the hard region

The relaxation behavior of poly(methyl methacrylate) in the hard region is caused by hindered rotations of the methoxycarbonyl side groups of the molecules. Measurements of the Young's modulus and the shear modulus at different frequencies, temperatures and strains lead to the following conclusions: each moving side group is surrounded by the elastic matrix of mean chains, The elasticity of this matrix depends on volume and temperature. Also the activation energy of the moving side group depends on volume and temperature, because the activation energy is the sum of an intramolecular and an intermolecular component. Due to the statistical entanglement of the molecules, the intermolecular contribution to the activation energy has a statistical distribution depending on volume. The reaction rate theory is of value for interpreting the viscoelastic behavior in the linear and non-linear range if several considerations are followed.     

Thermal behaviour of TiO2-encapsulating polymers

Comparing the thermal properties of TiO₂ encapsulating polystyrene and poly(methyl methacrylate) with those of TiO₂ dispersion polymers it was found that the encapsulating polymers have two thermal relaxation regions. The activation energy of those thermal relaxation regions was determined using the Wunderlich method and it was found that the values are similar to the activation energy for the dynamic dispersion. It is suggested that the low-temperature thermal relaxation is caused by the local change of conformation of molecular chains, while the high-temperature thermal relaxation is similar to that of the normal glass transition temperature including the interaction with TiO₂. In addition, the thermal behaviour near the degradation point in different atmospheres indicates that the encapsulating polymer has a specific structure for adsorbing a large amount of oxygen.     

Phase transition of LCP fluids confined in nanochannels through MD simulation

Manipulation of molecular orientation alignment in MCTLCPs (main-chain thermotropic liquid crystalline polymers) by pure shear at nano scale has been investigated for the first time using molecular dynamics (MD) simulation. Results indicate that high planar shear induces long-range uniform orientation ordering (liquid crystalline phase) of initially randomly orientated molecules of MCTLCP fluid confined in a nanochannel, which is confirmed by analyzing the orientation order parameter and the snapshots of MCTLCP liquid in a nanochannel under different shear rates. Insights into the origin of the phase transition phenomena are given at molecular level through investigating the thermodynamic density distribution of MCTLCP molecules in the nanochannel, suggesting that the energy shift due to a radical jump of system density affects both the magnitude and the orientation of the molecular ordering. Simulation results also show that there is a critical shear rate for transforming isotropic phase into liquid crystalline phase. The critical shear rate is dependent on the temperature of the MCTLCP system. Findings in this paper may present useful information for processing TLCP molecules at nano scale and the understanding of nanoflow.     

Microwave dielectric relaxation study of poly(propylene glycol) in dilute solution

Dielectric relaxation studies of poly(propylene glycol), average molecular weight 2000 g mol⁻¹, in dilute solution of cyclohexane, decaline, benzene and carbon tetrachloride have been carried out at 10.10 GHz and 35 °C. Average relaxation time τ₀, and relaxation times corresponding to segmental motion τ₁, group rotations τ₂ and dipole moment µ have been determined. It is found that τ₀, τ₁ and µ are influenced by the solvent environment while the τ₂ value is solvent‐independent. A comparison has been made with the dielectric behaviour of poly(ethylene glycol), average molecular weight 1500 g mol⁻¹, in dilute solutions of benzene and carbon tetrachloride because both systems have an equal number of monomer units. The effect of methyl side‐groups on dielectric relaxation in poly(propylene glycol) molecules is discussed. The Kirkwood correlation factor is also evaluated in dilute solutions with concentration variation and it is found that these molecules exist in cluster form due to intermolecular hydrogen bonding. © 2000 Society of Chemical Industry     

Dielectric behaviour and relaxation in poly(propylene glycol)–water mixtures studied by time domain reflectometry

Dielectric behaviour of poly(propylene glycol) (PPG) of number‐average molecular weight 2000 g mol^?1 and binary mixtures of PPG with water (PPG–W) of various concentrations were carried out in the frequency range 10 MHz to 4 GHz at 25 °C. The dielectric dispersion and absorption curves related to the orientational motion of these molecules in the binary mixtures are described by a single relaxation time using Debye's model. The values of static dielectric constant ε_o, high frequency limiting dielectric constant ε_∞, and dielectric relaxation time τ_o were determined for PPG and PPG–W mixtures. The values of the dielectric parameters were used to explore the nature of homogeneous and heterogeneous dynamic networks formed through hydrogen bonding in the binary mixtures of PPG and water molecules with concentration variation. The dielectric studies of PPG molecules were also carried out in the same frequency range at four temperatures, namely 25, 35, 45 and 55 °C. The temperature‐dependent relaxation times were used to evaluate the thermodynamical parameters for the dielectric relaxation processes. The dielectric relaxation free energy of activation ΔF_τ for PPG molecules was found in the range ～4.5 to 4.7 kcal mol^?1, which corresponds to the activation energy needed for the breakage of hydrogen bonds. Furthermore, the large negative value of the entropy ΔS_τ of PPG molecules confirms that the configuration involved in dipolar orientation has an activated state, which is more ordered than in the normal state. Copyright © 2004 Society of Chemical Industry     

Molecular Dynamics Simulation of the Structure and Hydroxylation of Silica Glass Surfaces

The surface structure of silica glasses has been simulated using molecular dynamics. The surface hydroxyl concentration was estimated to be 4.5/nm², based on surface defect statistics. Hydroxyl-silica potentials were developed and used to study the hydroxylation of silica surface. It is found that the energy of chemisorption of water declines in the sequence: three coordinated silicon (Si³) and non-bridging oxygen (NBO) on separate sites, Si³ and NBO on combined sites, two- and three-membered rings. Partial hydroxylation of the most reactive sites, which leads to an OH coverage of 2.5/nm², was studied. Structural relaxation after hydroxylation was observed.     

Computer simulation and molecular theory of self-organization and mechanical properties of polymers

In this contribution, we report on a study of the self-organization of extended linear polymer chains into condensed globules using molecular dynamics methods. We have found that condensed chains (globules) are amorphous or crystalline depending on the interaction potential between constituents. Any structure is formed in several stages. The structures obtained were used in further computer simulations to investigate such processes as compression, decompression, fracture, and flow. We have found that there are different types of fracture and plastic behavior with their own energy, fluctuation, and defect characteristics. On the basis of the computer simulation results obtained, the equation of motion is derived for the system of chain macromolecules slipping relative to each other. The equation takes into account the relaxation and friction in the system. The solution obtained gives the general law that connects stress, strain rate, molecular mass, potential relief, and temperature. It also gives sound physical grounds for some empirical relations that are used in polymer technology.     

Interaction of molecules with molybdenum disulphide catalysts: An experimental and molecular graphics study. Isoprene

The interaction of isoprene with molybdenum disulphide has been studied by computer-assisted modelling of the binding of the molecule at active sites. Possible structures of intermediate-active-site complexes were evaluated by the method of molecular mechanics. For isoprene hydrogenation the preferred catalytic sites, according to energy minimisation calculations, proved to be corner molybdenum atoms having threefold unsaturation. This conclusion agrees with deductions from experimental studies of isoprene hydrogenation. The modelling studies are valuable for revealing the steric relationships between active sites and substrate molecules, for calculating the most likely structures, and for revealing non-bonded interactions.     

Transparent chiral polymers for optical applications

Optically active chiral polymers and copolymers of cholesteryl methacrylate have been studied for use in optical applications including plastic optical fibers. Homopolymers of chiral cholesteryl methacrylate with differing molecular weights and copolymers with methyl methacrylate were synthesized by free‐radical copolymerization in tetrahydrofuran using azobisisobutyronitrile at 67°C for 26 h. All polymers were characterized for molecular weight, glass‐transition temperature, optical rotation, transparency, and refractive index and solution blended to test for compatibility with poly(methyl methacrylate). Such chiral materials are of particular interest because they offer useful polarization properties without requiring bulk orientation of the molecules. This makes it possible to produce low cost optical elements such as circularly birefringent or circularly polarizing optical elements with potential applications in polarization manipulations and sensing. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 58–65, 2005     

Intermolecular segregation of siloxane in P3HT: surface quantification and molecular surface-structure

Surfaces of spin-coated and solution-cast poly(3-hexylthiophene) (P3HT) films are analysed by X-ray Photoelectron Spectroscopy and Low Energy Ion Scattering. Here, we use the P3HT-siloxane system with only 2% siloxane monomers in the bulk as a model system to study segregation and surface orientation of molecules in polymers. The surfaces are enriched in siloxane due to the intermolecular segregation of the siloxanes present in P3HT. The siloxane coverage fraction was found to depend on the preparation parameters such as spinspeed and solution-concentration, and ranges from 25 to 100%.The molecular orientation of segregated siloxanes on P3HT was found to resemble that on pure PDMS. Furthermore, siloxane molecules prefer specific sites on P3HT, such that sulphur atoms are screened from being at the outermost surface to lower the surface free energy. The results presented here demonstrate clearly the unique ability of LEIS to quantify the composition of the outermost atomic layer, and to obtain detailed information on the surface structure.     

Correlation of dielectric and visco-elastic relaxation in polymer solutions

The interpretation of studies of dielectric relaxation in polymers is often restricted by ignorance of the mode of motion responsible for dipole orientation in an applied field. Relevant information can be drawn from studies of visco-elastic relaxation, since the visco-elastic relaxation time is comparable with the dielectric relaxation time when the latter requires some ‘whole molecule’ mode of motion involving simultaneous movement of many chain segments. The dielectric and visco-elastic relaxation times are expected to differ when the former involves a localised segmental motion. The dielectric relaxation process is shown to involve a localised segmental motion in acrylic polymers, but involves a ‘whole molecule’ rotation in low molecular weight poly(N-vinylcarbazole). Poly(p-substituted phenyl acetylenes) exhibit two dielectric relaxation processes. A low frequency relaxation comparable with the visco-elastic relaxation is ascribed to the backbone double bonds being polarised unidirectionally along the chain, and a high frequency process is ascribed to a localised segmental motion of the substituted phenyl moiety.     

微孔发泡塑料动态成核机理的研究

分析动态条件对分子取向、出口膨胀中分子链松驰过程的影响，进而指出对泡沫塑料气泡成核行为的影响。     

A molecular simulations study of the miscibility in binary mixtures of polymers and low molecular weight molecules

The miscibility behavior of binary mixtures of polymeric and low molecular weight molecules was studied using a combination of modified Flory-Huggins theory and molecular simulation techniques. Three different atomistic approaches were used to investigate the phase behavior and χ parameters of binary mixtures consisting of polymethyl methacrylate (PMMA) and 4-n-pentyl-4′-cyanobiphenyl (5CB). Binary mixtures of methyl methacrylate monomer/5CB and methyl methacrylate oligomer/5CB were also studied. As a first approach, a fast method that calculates the local interaction between a fragment of the polymer and the organic molecule and then extends it to determine the energy of mixing using an estimated coordination number was used. By using modified coordination numbers, we were able to extend this method to include cases where the polymer segment and the small molecules are slightly dissimilar in size. More detailed studies which take into account bulk effects were also carried out where the cohesive energies of the pure compounds were derived from molecular dynamics simulations and the interaction parameters were determined from the differences in the cohesive energies. The concentration and temperature dependence of the χ parameters was evaluated by calculating the energy of mixing from the differences in the cohesive energy densities of the mixed and demixed systems. The present study provides a detailed understanding of the miscibility of PMMA and 5CB as PMMA polymerizes from its monomer, and the results indicate that although methyl methacrylate and 5CB are completely miscible, 5CB is not miscible in PMMA even in small quantities.     

Microwave dielectric relaxation and molecular dynamics in binary mixtures of poly(propylene glycol) 2000 and poly(ethylene glycol)s of varying molecular weight in dilute solution

Molecular dynamics of binary mixtures of poly(propylene glycol) (PPG) and poly(ethylene glycol)s (PEGs) of varying molecular weight due to molecular interactions, chain coiling and elongation in dilute solution under various conditions, ie varying number of monomer units of PEG, method of mixing of polymers and solvent environment, has been explored using microwave dielectric relaxation times. The average relaxation time τ_o, relaxation time corresponding to segmental motion τ₁ and group rotations τ₂, of a series of binary mixtures of poly(propylene glycol) 2000 and poly(ethylene glycol) of varying molecular weight (ie PPG 2000 + PEG 200, PPG 2000 + PEG 300, PPG 2000 + PEG 400, and PPG 2000 + PEG 600 mixed by equal volume in the pure liquid states, and PPG 2000 + PEG 1500, PPG 2000 + PEG 4000 and PPG 2000 + PEG 6000 mixed equal weights in solvent) have been determined in dilute solution in benzene and carbon tetrachloride at 10.10 GHz and 35 °C. A comparison of the results of these binary systems of highly associating molecules shows that the molecular dynamics corresponding to rotation of a molecule as a whole and segmental motion in dilute solutions are governed by the solvent density when the solutes are mixed in their pure liquid state. Furthermore, the molecular motion is independent of solvent environment when the polymers are added separately in the solvent for the preparation of binary mixtures. It has also been observed that there is a systematic elongation of the dynamic network of the species formed during mixing of pure liquid polymers in lighter environment of solvent with increasing PEG monomer units, while the elongation behaviour of the same species in the heavier environment of carbon tetrachloride solvent is in contrast to the elongation behaviour of the polymeric species formed in pure PEG. The role of rotating methyl side‐groups in the PPG molecular chain has been discussed in term of the breaking and reforming of hydrogen bonds in complex polymeric species for the segmental motion. In all these mixtures, the relaxation time corresponding to group rotations is independent of the solvent environment and constituents of the binary mixtures. The effect of chain flexibility and coiling in these binary mixtures is discussed by comparing the relaxation times of the mixtures with their individual relaxation times in dilute solutions measured earlier in this laboratory. © 2001 Society of Chemical Industry     

Molecular dynamics of 4-n-octyl-4′-cyanobiphenyl in partially filled nanoporous SBA-type molecular sieves

The molecular dynamics of 4-n-octyl-4′-cyanobiphenyl (8CB) confined to the nanopores of new SBA-type molecular sieves was investigated in a wide temperature range using broadband dielectric spectroscopy (10⁻²–10⁹ Hz). One molecular sieve has a hexagonal structure of the pores while the other is a cellular nanoporous material. To explore the extent of surface interaction effects a high and a low filling degree were considered.

For the molecular sieves with a high filling degree two relaxation regions were observed: a bulk-like relaxation process related to molecules, which behave as mesophase, located in the centre of the pores. The second relaxation process has a much lower relaxation rate than the former and is assigned to molecules located in a surface layer. The temperature dependence of its relaxation rates follows the Vogel–Fulcher–Tammann law, characteristic for glassy dynamics.

For samples with a low filling degree only one relaxation process due to the surface layer was observed. Moreover, especially at the temperatures lower than the melting point of bulk 8CB, its relaxation rate is situated between the characteristic frequencies of the two relaxation processes observed for the pores with a high filling degree. This behaviour gives a measure of the extension of the influence of the wall on the neighbouring 8CB molecules. In addition, the differences revealed by the molecule dynamics inside the two types of nanoporous materials are related to both surface interactions and geometrical constraints.