Contribution to the study of the complexation of copper by chitosan and oligomers

The complexation of copper ions by chitosan and its oligomers is investigated using potentiometric and spectrophotometric methods to study the nature of the complexes involved and the role of the degree of polymerisation. Two complexes are demonstrated. Their structure is proposed, the pH range in which they are respectively stable is determined and their stability constants calculated. Finally a degree of polymerisation of 6 appeared as the threshold value for an efficient complexation of copper ions by chitosan oligomers.     

Ratio of the glass transition temperature to the melting point in polymers

A study of the ratio of the glass temperature to the melting point of 132 polymers is described. Contrary to some other workers' observations on smaller numbers of polymers there is no sharp division between the ratios observed for symmetrical and unsymmetrical polymers. The arithmetic mean of the ratio is 0·63 and 0·69 respectively, but the combined standard deviation is 0·11. There is no correlation between the ratios and the crystalline forms of polymers.     

求解对流扩散问题的积分方程法

提出了一种求解对流扩散问题的积分方程法。在这一方法中,首先利用Laplace方程的级数形式的格林函数将对流扩散方程转化为积分方程,然后利用级数的正交性质,把积分方程进一步简化为代数方程组,求解该方程组即可得到对流扩散方程的级数形式的近似解。最后,分别利用Chebyshev多项式和Fourier级数求解了3个典型的一维和二维对流扩散问题。该方法和有限体积法、有限元法和迎风差分法相比,展现出非常高的精度并且避免了由解的不连续性造成的虚假振荡。     

Why C1 = 16–17 in the WLF equation is physical-and the fragility of polymers

From the well-recognized equivalence of the Williams-Landel-Ferry (WLF) equation and the Vogel-Tammann-Fulcher (VTF) equation, τ = τ_o exp (B/[T - T_o]), we shall show that the parameter C₁ in the former is just the number of orders of magnitude between the relaxation time at the chosen reference temperature and the pre-exponent of the VTF equation. Thus C₁^g = log(τ_g/τ_o) (a relation which is not found in the present polymer literature), measures the gap between the two characteristic time scales of the polymer liquid, microscopic and α-relaxation, at the glass transition temperature. For systems which obey these two equations over wide temperature ranges, τ_o is consistent with a quasilattice vibration period in accord with theoretical derivations of the VTF equation and also with the microscopic process of mode coupling theory. Thus for such systems, C₁^g is obliged to have the value 16–17 (depending on how T_g is defined), while C₂^g scaled by T_g will reflect the non-Arrhenius character, i.e. fragility, of the system. In fact when C₁^g has the physical value of 16–17, then (1 − C₂^g/T_g), which varies between 0 and unity, conveniently gives the ‘fragility’ of the polymer within the ‘strong/fragile’ classification scheme. This is useful because it permits prediction from the WLF parameters of other properties such as physical ageing behaviour through the now-established correlation of fragility with other canonical characteristics of glassforming behaviour. Where the best fit C₁^g is not 17 ± 2, the corresponding best fit τ_o must be unphysical, and then the range of relaxation times for which the VTF or WLF equations are valid with a single parameter set will be limited, and the predictions of other properties based on that parameter set will be unreliable. © 1997 Elsevier Science Ltd.     

New kinds of phenylethynyl-terminated polyimide oligomers with low viscosity and good hydrolytic stability

In continuing studies to develop low melt viscosity phenylethynyl end-capped imide oligomers for aerospace applications, new kinds of all-aromatic phenylethynyl-terminated imide oligomers were prepared by the reaction of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) or biphenylenetetracarboxylic dianhydride (s-BPDA) with 2,5-bis(4-aminophenoxy)-biphenyl (p-TPEQ) and 4-phenylethynylphthalic anhydride (PEPA) or 4-(1-phenylethynyl)1,8-naphthalic anhydride (PENA). The oligomers bearing pendant phenyl groups exhibit much lower melt viscosities at low temperatures, and thus provide wide processing window. The thermal curing process of the oligomers was investigated with DSC. The tensile and thermal properties of the cured films were evaluated. Oligomers derived from PENA cured at lower temperatures and the corresponding cured polymers show better hydrolytic stability than those of PEPA. Results showed that the utilization of diamine monomer (p-TPEQ) can improve the processability and solubility of phenylethynyl-terminated imide oligomers without sacrificing their thermo-oxidative stability.     

Electronic structure and optical properties of germanium-bridged platinum(II)-containing diethynylfluorene monomer and oligomers: A theoretical investigation

We apply quantum-chemical techniques to investigate the structural, electronic, and optical properties of some platinum-containing fluorenyleneethynylenegermylene-derived monomers and oligomers. The aim of our quantum-chemical calculations is to investigate the role of the transition metal centers in the organometallic system in terms of electronic structure and to estimate the influence of metal on the optical properties of the monomer and oligomers. The results indicate that there is a weak electronic interaction between the metal-based fragment and the π-conjugated organic segments, and consequently the photophysical properties are mainly based on the fluorenyleneethynylene (TFT) π-conjugated fragment with little contribution from the metal center. The role of metal center can be described as weak delocalization coupled with strong localization characteristic along the organometallic backbone. The introduction of platinum ions into the π-conjugated structure leads to bathochromic shifts in the absorption features as compared to those for the free ligands.     

Dynamics and rheology of entangled linear polymers

In order to describe the dynamics of the linear polymer in monodisperse melts, we present the so-called reaction-diffusion type equation that contains contributions from reptation and contour-length fluctuation based on the tube model. The relaxation function calculated from this equation is in good agreement with the experimental results of the dielectric relaxation function and also gives the stress relaxation function in binary blends by use of semi-empirical scheme called double reptation. We add advection term to this equation in order to predict the nonlinear rheological properties of steady shear. The calculated shear stress and the first normal stress difference at steady state agree well with the measurements.     

Cyclodepolymerization as a method for the synthesis of macrocyclic oligomers

Macrocyclic oligomers (MCOs) are important as starting materials for Entropically-driven Ring-opening Polymerizations (ED-ROPs). This article reviews the preparation of MCOs by the cyclodepolymerization (CDP) of condensation polymers. Many MCOs have been prepared successfully this way and in numerous cases individual macrocycles have been isolated. This approach can provide one-step syntheses of many macrocycles. The combination of CDP plus ED-ROP is an attractive potential method for recycling (‘Ring-chain Recycling’) many condensation polymers.     

Hot-melt adhesion and wettability between polyethylene and other polymers in the vicinity of the adherend melting point

The effect of bonding temperature on the bonding strength of polyethylene to polypropylene or other polymers was investigated. It became clear that the interfacial bonding strength reached maximum strength at a bonding temperature in the vicinity of the melting point of the adherend (i.e., the polymer having the higher melting point). The effect of temperature on the contact angle of the molten polymer having the lower melting point on the adherend surface was also investigated. The wettability of the adherend by the molten polymer drop was considerably reduced at the temperature near the melting point of the adherend. The relationship of increasing adhesiveness to decreasing wettability was found out.     

Electrochemical, spectroscopic and electrogravimetric detection of oligomers occluded in electrochemically synthesized poly(3,4-ethylenedioxythiophene) films

This paper shows that oligomers are retained inside the polymer film during the electrosynthesis of poly(3,4-ethylenedioxythiophene), PEDOT, in aqueous media. The behavior of the electrochemically generated PEDOT film is highly dependent on the presence of these oligomers. A detailed study of the release of oligomers trapped in the PEDOT film has been carried out using bidimensional spectroelectrochemistry (BSEC), spectroelectrochemical quartz crystal microbalance (SEQCM) and scanning electrochemical microscopy (SECM). These multiresponse techniques have allowed us to determine when these EDOT oligomers are released into solution and to investigate their electrochromic properties. Mass spectroscopy measurements revealed that most of these oligomers consist of four or six monomer units, which seem to be the most stable species in aqueous solution.     

Coordination and Organometallic Polymers and Oligomers of Upper-Rim Functionalized Calix[4]arenes by Transition Metals

This short review focuses on recent advances in the syntheses, characterization and structures of coordination and organometallic complexes of upper-rim functionalized calix[4]arenes that form either small oligomers or polymers. This field is very limited, presumably due to the lack of X-ray data or reliable characterization that demonstrates the presence of oligomers or polymers. Nonetheless, the few published works already clearly demonstrate the immense versatility of the calix[4]arene macrocycle as it forms polymeric materials via coordination bondings with transition metals.     

Interplay of the polymer stiffness and the permeability behavior of silane and siloxane polymers

The effect of the modification of the molecular structure on the stiffness of the polymeric backbones in relation to the diffusion coefficients of typical rubbery and glassy silane and siloxane polymers at different temperatures was investigated. The inflexibility in the polymeric chains as deduced by higher values for the persistence length was shown to correspond consistently to lower values for the self-diffusion coefficients. Increasing the temperature resulted in decreasing the persistence length of the silane polymers and increasing it in the case of the siloxane polymers. This was found to be due to the fact that the bond angle about the oxygen atoms is approximately 144° and rotations about the oxygen atoms for the trans isomeric states will bring the side groups on the neighboring Si-atoms in close proximity to each other thus increasing the torsional energies of the trans isomeric states considerably. The obtained simulation results showed an excellent agreement to those determined experimentally.     

Correlation between equation of state and temperature and pressure dependence of self-diffusion coefficient of polymers and simple liquids

Correlation between the equation of state and the temperature dependence of the self-diffusion coefficient D for polymers such as polystyrene (PS) and polydimethyl siloxane (PDMS) and simple liquids such as argon, methane and benzene and the pressure dependence of D for oligomers such as dimethyl siloxane (DMS) and simple liquids such as cyclohexane and methanol has been examined based on the equation of state derived previously. The experimental data used were published by Antonietti et al. and McCall et al. for polymers, by McCall for linear dimethylsiloxanes and by Jonas et al. and Woolf et al. for simple liquids. The expression for D in this work is given by

where A₁(M) is a function of molecular weight M_w, C₁(T) and P₁(T) are functions of temperature and B₁, n₁ and m₁ are constants determined experimentally. For simple liquids, the values of n₁ obtained range from 0.3 to 1.2, with an average , and m₁ is in the range 0.5–1.2, with . For polymers, values of n₁ are in the range 2.5–7.0 for PS and 0.5–1.3 for PDMS and m₁ for DMS is in the range 0.8–1.0. The relation Dη/T = f(M) is found to be useful for simple liquids over a wide range of temperature including the critical region and for pressures up to ≈5 kbar

1 kbar = 100 MPa There is a close correlation between ln(D/T) and _p and β_T through ln(D/T)ln D_c−⁻¹_p−β⁻¹_T, where D_c is D at the critical temperature and _p and β_T are the thermal expansion coefficient and compressibility, respectively. The molecular weight dependence of D for polymers and simple liquids is discussed based on the experimental data and recent theory of Doi and Edwards. A new model for the mechanism of self-diffusion in the liquid state is proposed.     

Thermoanalytical and infrared spectroscopy investigations of some mineral pastes containing organic polymers

The interaction in the presence of water between organic polymers [two polyvinyl alcohol acetate (PVA1 and PVA2)-, one polyvinyl acetate (PVAc)-, and one acryloethyl-metacrylomethyl (R)-type copolymers] and inorganic materials (3CaO·Al₂O₃+CaSO₄·2H₂O, 12CaO·7Al₂O₃, and CaO·Al₂O₃) was investigated using complex thermal analyses and infrared (IR) spectroscopy. The DTA curves of the 1-to 28-day-hydrated organo-mineral composite samples showed some significant differences in comparison with those of the hydrated inorganic materials. The most important differences consist in the presence of some exothermic peaks that are not attributable to the pure organic polymers. More than that, none of the specific effects of the pure organic polymers are evidenced on the DTA curves of the hydrated organo-mineral composite materials. This, in connection with their IR spectra, which clearly evidence the disappearance of some investigated polymers specific IR bands, can be considered as an indirect evidence of the cross-linking of polymer chains via metal ions.     

Synthesis and characterization of some polyazomethine conducting polymers and oligomers

Polyazomethine polymers were prepared from the polycondensation of terephthaldehyde with different diamines, namely, benzidine, 2,3-diaminopyridine, 1,4-diaminoanthraquinone and 3,5-diamino-1,2,4-triazole. The electrical conductivities of the polymers were studied with doping with H₂SO₄ and 5% I₂ together with the undoped polymers. The crystallinity and morphology of the polymers were studied using XRD and scanning electron microscopy. A comparative study of the electrical conductivities of oligomers of polyazomethines and those of their corresponding polymers was performed. Received: 8 January 1997/Accepted: 10 May 1997     

A novel analytical method to estimate molar mass and virial coefficients of polymers from osmometry

In the routine analysis of osmometric data for polymer-solvent systems, a graphical method has been used to estimate the virial coefficients and number average molar mass. However, it is realized that the graphical method has the disadvantage due to inaccuracies involved in the estimation of the intercept and slope. The least-squares method is certainly a better alternative, but it also has certain limitations. A new analytical method is suggested which is no less effective than the least-squares method, but also provides valuable information about the behaviour of the observed points in the experimental data. A comparative study of the different methods is presented here to indicate the advantages of the new method over the conventional treatments. © 1997 Elsevier Science Ltd.     

Proposed solution for the Flory-Charlesby equation for crosslinked polymers and application for 1,2-polybutadiene crosslinked with AIBN and aryl diazide

The derivation and properties of a tractable function that approximates well the solution of the Flory-Charlesby equation for crosslinked polymers with logarithmic normal primary molecular size distribution is presented. This function gives the possibility to determine the crosslinking density, ρ, from measurements of insoluble fraction, what is the only quantitative method that can be applied for postsynthesis crosslinked polymers. Applications to 1,2-polybutadiene crosslinked with AIBN and 2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone by heating allowed to obtain information about the gel point as well as about the crosslinking yield and mechanism. In addition, it was found that the temperature where the damping factor in DMA curves is maximum increases with ρ following a second order dependence.     

Transition of polymers from the fluid to the forced high-elastic and leathery states at temperatures above the glass transition temperature

The transition from the fluid to the forced high-elastic (rubber-like or quasi-vulkanizate) state and then to the leathery state (that is between rubbery and glassy states⁷) has been systematically studied by increasing stresses and deformation rates, using an uncured high molecular-mass 1, 2-polybutadiene characterized by a rather high glass transition temperature (?18°C). Investigation of polymers in simple shear is possible up to critical stresses corresponding to the transition of fluid polymers to FHES. Uniaxial extension offers wide possibilities for characterizing polymers in FHES and FLS since it covers the range of 4–6 decades of deformation rates over the range of simple shear. The deformability of polymers is limited by their fracture even in FHES, and the fracture process is determined only by recoverable deformation. The failure envelope covering FHES and transition to FLS has been constructed and the long-term durability in the range of 7 decades of time measured. The maximum extensibility corresponding to the transition of the polymer to FLS is determined by the ultimate extensibility of macromolecular coils. Over a wide range of stresses the polymer behaves like a linear viscoelastic body. This makes it possible to correlate the data obtained for uniaxial extension and low-amplitude sinusoidal shear deformation, which is important for the prediction of fracture phenomena by means of the low amplitude non-destructive dynamic method.     

Self-assembly of conjugated oligomers and polymers at the interface: structure and properties

In this review, we give a brief account on the recent scanning tunneling microscopy investigation of interfacial structures and properties of π-conjugated semiconducting oligomers and polymers, either at the solid-air (including solid-vacuum) or at the solid-liquid interface. The structural aspects of the self-assembly of both oligomers and polymers are highlighted. Conjugated oligomers can form well ordered supramolecular assemblies either at the air-solid or liquid-solid interface, thanks to the relatively high mobility and structural uniformity in comparison with polymers. The backbone structure, substitution of side chains and functional groups can affect the assembling behavior significantly, which offers the opportunity to tune the supramolecular structure of these conjugated oligomers at the interface. For conjugated polymers, the large molecular weight limits the mobility on the surface and the distribution in size also prevents the formation of long range ordered supramolecular assembly. The submolecular resolution obtained on the assembling monolayers enables a detailed investigation of the chain folding at the interface, both the structural details and the effect on electronic properties. Besides the ability in studying the assembling structures at the interfaces, STM also provides a reasonable way to evaluate the distribution of the molecular weight of conjugated polymers by statistic of the contour length of the adsorbed polymer chains. Both conjugated oligomers and polymers can form composite assemblies with other materials. The ordered assembly of oligomers can act as a template to controllably disperse other molecules such as coronene or fullerene. These investigations open a new avenue to fine tune the assembling structure at the interface and in turn the properties of the composite materials. To summarize scanning tunneling microscopy has demonstrated its surprising ability in the investigation of the assembling structures and properties of conjugated oligomers and polymers. The information obtained could benefit the understanding of the elements affecting the film morphology and helps the optimization of device performance.