Experimental characterisation and further molecular modelling of the conformational and elastic behaviour of poly(ethylene terephthalate) network chains

The Monte-Carlo (MC) method developed to model the elastomeric stress-strain behaviour of polyethylene (PE), poly(dimethyl siloxane) (PDMS) and poly(ethylene terephthalate) (PET) networks and the stress-optical behaviour of PE networks is now developed to investigate further the stress-strain behaviour of PET networks. Accurate infrared (IR) spectrometry measurements have been used to determine the populations of gauche conformers in the glycol residues of PET chains in melts. The proportion of gauche states was found to be 76%, consistent with the rotational energy difference of −4.16 kJ mol⁻¹ between trans and gauche states used previously.The greater conformational flexibility of the PET chain compared with the PE chain leads to lower network moduli and smaller deviations from Gaussian and affine network behaviour. Previous results are briefly reviewed and new comparisons of the elastic behaviour of PET and PE chains are made using normalised plots. Subsequent publications will apply the present results to interpreting the measured stress-strain and the stress-optical properties of entangled PET melts.     

Super soft elastomers as ionic conductors

Melts of linear brush polymers with PEO side chains attached at each repeat unit of the backbones have been doped with CF₃SO₃⁻Li⁺. Mechanical properties and ionic conductivity of such systems have been analyzed using mechanical and dielectric spectroscopies. Mechanical spectra indicated a presence of super soft states for samples with long backbones or for systems which have been slightly cross-linked (G′<10⁴ Pa). In the case of the polymer with longer crystallizing PEO side chains (MW_av=1100 g/mol), the ionic conductivity reaching the 10⁻³ S/cm level at the optimum CF₃SO₃⁻Li⁺ concentration (EO/Li⁺=10:1) have been detected at temperatures not far above the room temperature. The presence of lithium ions suppresses completely the crystallization of PEO side chains.     

Structure of polyurethane elastomers. X-ray diffraction and conformational analysis of MDI-propandiol and MDI-ethylene glycol hard segments

X-ray diffraction and conformational analysis have been used to investigate the structure of polyurethane hard segments prepared from diphenylmethane 4,4′-diisocyanate (MDI) with propandiol (PDO) and ethylene glycol (EDO) as chain extenders. The results are compared with those obtained previously for MDI-butandiol (BDO) hard segments. In the latter system, the poly(MDI-BDO) chains are fully extended with an all-trans conformation for the O(CH₂)₄O unit, and a monomer repeat of 18.95 Å. The unit cell is triclinic with a tilted base plane such that adjacent chains are staggered along the fibre axis. In contrast both poly(MDI-PDO) and poly(MDI-EDO) adopt unstaggered structures, i.e. the chains are in register and the unit cell base planes are perpendicular to the chain axis. The monomer repeats of 16.2 Å for poly(MDI-PDO) and 15.0 Å for poly(MDI-EDO) are shorter than the predicted repeats for fully-extended chains, indicating that these polymers have contracted conformations containing some gauche CH₂ groups. Conformational analysis shows that the 16.2 Å repeat for poly(MDI-PDO) can be achieved with the O(CH₂)₃O unit in the trans-gauche⁺-gauche⁺-trans or gauche⁺-trans-trans-gauche⁺ conformations. Similarly the 15.0 Å repeat for poly(MDI-EDO) is predicted for the gauche⁺-trans-gauche^? conformation for the O(CH₂)₂O unit. These conformations are of higher energy than the all-trans fully-extended chains. This may explain the higher crystalline perfection of the poly(MDI-BDO) hard segments, for which crystallization in the all-trans form will probably provide a greater driving force for phase separation.     

Estimation of crystallinity of isotactic polypropylene using Raman spectroscopy

A method to estimate the degree of crystallinity in isotactic polypropylene has been developed. The method is based on integrated intensities of the Raman bands at 808 and 841 cm⁻¹. From the observation of correlation splitting, Raman bands related to different conformational states were identified. This analysis indicates the existence of three different conformational states. The 808 cm⁻¹ band was assigned to helical chains within crystals. The 840 cm⁻¹ band was shown to be composed of a band at 840 cm⁻¹, assigned to shorter chains in helical conformation, and a broader band at 830 cm⁻¹ assigned to chains in non-helical conformation. In order to establish a quantitative relation between Raman scattering intensity and crystallinity samples subjected to different cooling rates and crystallisation temperatures were analysed. These results correlate well with those of differential scanning calorimetry.     

An application of Monte Carlo method to simulate disorders in polymer crystals

The Monte Carlo method is applied to polymer crystals of idealized linear chain molecules of 30 carbon atoms, and the unharmonic, large-amplitude, oscillations and the subsequent conformational disorders of the chains are investigated. A crystalline field that confines the chain is treated by the molecular field approximation, and assumed to be cylindrical in this work. A production type simulation is adopted taking into account rigorous statistical weights for each sample conformation. Both the rotational isomeric model and the continuous rotation model of chain conformation are considered. By averaging over 10⁴–8 × 10⁴ chains, mean-square end-to-end distance, fractions of gauche and trans states and a detailed distribution of internal rotation angle are obtained. The effects of temperature and pressure on the conformation of the chain in the crystals are also simulated.     

Hybrid inorganic-organic polymer electrolytes: synthesis, FT-Raman studies and conductivity of {Zr[(CH2CH2O)8.7]ρ/(LiClO4)z}n network complexes

This paper describes the synthesis and characterization of three-dimensional hybrid inorganic-organic networks prepared by a polycondensation reaction between Zr(O(CH₂)₃CH₃)₄ and polyethylene glycol 400 (PEG400). Eleven hybrid networks doped with varying concentrations of LiClO₄ salt were prepared. On the basis of analytical data and FT-Raman studies it was concluded that these polymer electrolytes consist of inorganic-organic networks with zirconium atoms bonded together by PEG400 bridges. These polymers are transparent with a solid rubber consistency and are very stable under inert atmosphere. Scanning electron microscopy revealed a smooth glassy surface. X-ray fluorescence microanalysis with energy dispersive spectroscopy demonstrated that all the constituent elements are homogeneously distributed in the materials. Thermogravimetric measurements revealed that these materials are thermally stable up to 262 °C. Differential Scanning Calorimetry measurements indicated that the glass transition temperature T_g of these inorganic-organic hybrids varies from −43 to −15 °C with increasing LiClO₄ concentration. FT-Raman investigations revealed the TGT (T=trans, G=gauche) conformation of polyether chains and allowed characterization of the types of ion-ion and ion-polymer host interactions in the bulk materials. The conductivity of the materials at different temperatures was determined by impedance spectroscopy over the 20 Hz-1 MHz frequency range. Results indicated that the materials conduct ionically and that their ionic conductivity is strongly influenced by the segmental motion of the polymer network and the type of ionic species distributed in the bulk material. Finally, it is to be highlighted that the hybrid network with a n_Li/n_O molar ratio of 0.0223 shows a conductivity of ca. 1×10⁻⁵ S cm⁻¹ at 40 °C.     

Theory for double potential step chronoamperometry for any potential values at spherical electrodes: Simultaneous determination of the diffusion coefficients of the electroactive species

We present a general and explicit analytical solution for double potential step chronoamperometry with any applied potential values (E₁, E₂) corresponding to a reversible charge transfer process at spherical electrode. This solution is essential to analyze double pulse electrochemical techniques such as RPV and DPV. We consider unequal diffusion coefficients, initial presence of both electroactive species and that the reaction product can dissolve in the electrolytic solution or in the electrode.From the analytical equation obtained it is possible to deduce interesting simplified expressions for some particular cases: both species soluble in the electrolytic solution with equal diffusion coefficients, planar electrodes, ultramicroelectrodes when both species are soluble in the electrolytic solution, and double potential step chronoamperometry with limit current potentials (E₁−E^°′→−∞, E₂−E^°′→+∞). In this last case, when reaction product is not initially present it is pointed that planar electrodes and ultramicroelectrodes cannot be used for determining both diffusion coefficients. This interesting practical consequence can be demonstrated by means of the analytical expression deduced here, which represents a notable advantage in front of numerical results.     

Novel chemosensory materials based on polyfluorenes with 2-(2′-pyridyl)-benzimidazole and 5-methyl-3-(pyridin-2-yl)-1,2,4-triazole groups in the side chain

Polyfluorenes with 2-(2′-pyridyl)-benzimidazole (P1, P2 and P4) and 5-methyl-3-(pyridin-2-yl)-1,2,4-triazole (P3) groups in the side chain were synthesized by Suzuki polycondensation. The responsive properties of polymers on metal ions and H⁺ were investigated by absorption and emission spectra. The fluorescences of polymers (P1-P4) were completely quenched upon the transition metal ions such as Co²⁺, Ni²⁺, Fe³⁺ and Ag⁺ due to the enhanced electronic communication properties of conjugated polymers. The obvious differences to Ni²⁺ ion responsive sensitivity were observed between P1 and P4 polymers. The fluorescences of P1 and P4 were quenched to 50 (I₀/I) and to 22 (I₀/I) upon the addition of a Ni²⁺ solution of 3.2 × 10⁻⁶ M, as well as 5.0 × 10⁻⁶ M, respectively, owing to the different conjugated backbone. The fluorescences of P2 and P3 were completely and hardly quenched upon the addition of a Al³⁺ solution of 1.0 × 10⁻⁴ M, respectively, owing to the different receptors in the side chain. P2 showed good selectivity to Ni²⁺ ion in the range of quencher concentration as low as 5 ppm, owing to the different chelating abilities of receptor with ions. Cu²⁺ and Mn²⁺ ions hardly quenched the fluorescences of polymers (P1-P4), which were different from the oligopyridyl-functionalized conjugated polymers. The results further opened the opportunities to develop the tailored sensory materials through the appropriate alteration of receptors in the side chain and the conjugated backbone.     

Polymerization of vinyl ethers using titanium catalysts containing tridentate triamine ligand of the type N[CH2CH(Ph)(Ts)N]2

Titanium complexes having tridentate triamine of the type N[CH₂CH(Ph)(Ts)N]₂²⁻ in combination with methylaluminoxane (MAO) was able to polymerize ethyl vinyl ether in good yields. The polymers obtained in general were having molecular weight in the order of 10⁵ with narrow molecular weight distributions. Polymerization conditions had an impact on the molecular weight and the polydispersity index (PDI). Using chlorobenzene as the solvent the polymer had an M_n of 350?000 and PDI of 1.21, where as under neat conditions the M_n was 255?000 with PDI of 1.21. The type of solvent and the temperature dictated the polymerization rate and the polymer stereo regularity. The molecular weight of the polymer is distinctly governed by the polymerization temperature. Temperature ranging between −50 and ambient (30 °C) resulted in high molecular weight polymers and vice versa at a temperature of 60-70 °C resulted in low molecular weight polymers in moderate yields. The polymers obtained below 30 °C are highly stereo-regular compared to that of the ones produced at and above ambient temperature. The polymerization of iso-butyl vinyl ether (IBVE) was faster than that of linearly substituted n-butyl vinyl ether (BVE) and less bulky ethyl vinyl ether (EVE). The order of isotacticities of the polymers obtained are polyIBVE > polyBVE > polyEVE. The use of borate cocatalyst for activation generated narrow molecular weight polymers with a linear increase in the yield and molecular weight over time suggesting the living nature of the catalyst system.     

The local structure of molten polyethylene

Wide-angle X-ray scattering provides a direct link to the local structure of non-crystalline materials. Methods of obtaining structural parameters from the scattering functions are described. Comparison of both scattering and real space functions calculated from a wide range of models with those obtained by experiment shows that the chains in molten polyethylene are irregular. The measurements confirm that the chain conformation comprises 60% trans bonds and 40% gauche, the sequences being random apart from the exclusion of pairs of gauche of opposite sign, and that the energy difference between the trans and gauche states is between 500–700 cal mol^?1. The packing of the irregular chains is modelled using a novel approach based on the geometry of randomly packed spheres. There is no evidence for significant correlation of segmental orientation.     

Conformational changes in novel thermotropic liquid crystalline polymer without conventional mesogens: A Raman spectroscopic investigation

Raman spectroscopy was applied to study the conformational changes of poly[di(alkyl) vinylterephthalate] (alkyl = n-butyl and sec-butyl). Spectral regions assigned to C-H, C-C and ester carbonyl stretching modes were utilized to provide information on the molecular motion, the relative content of trans and gauche conformers, and the specific interactions existed in the mesophase formation process of these novel thermotropic liquid crystalline polymers. Both the experimental results and the conformational analysis suggested that the aliphatic side groups do have a significant impact on the formation of stable liquid crystalline phase. Based on the van’t Hoff relation, the thermodynamic parameters in the phase transition of poly[di(n-butyl) vinylterephthalate] were estimated (ΔH = 5.52 kJ mol⁻¹, ΔS = 14.02 J mol⁻¹ K⁻¹), which were much smaller than the typical values generally obtained in first-order phase transition. Based on generalized two-dimensional correlation Raman analysis, specific interaction among the phenyl-ring in the mesophase development process was elucidated.     

Conformational transition characterization of glass transition behavior of polymers

The conformational transition behavior of polymer in the amorphous state has been investigated through molecular dynamics simulations across the glass transition temperature (T_g). We find that the conformational transition, a localized and short time dynamics feature, crosses over different barrier heights when the system transforms from the molten state into the glass state and the barrier height in the glass state is markedly lower than that above T_g. In addition to the overall transition behavior, the specific transitions between the rotational isomeric states (RIS) g⁺, t⁻, t⁺ and g⁻ are also investigated in detail. The populations of these specific transitions undergo considerable changes when the temperature decreases; meanwhile, the larger transition rates of the ending torsions get diminished. Besides the rate, the rotation degrees of the dihedrals during the transitions also change their distributions tremendously through T_g, below which most of the larger transition angles (50-100°) were inhibited remaining those sharply around 30°. This possibly explains why below T_g the conformational transition process has a lower effective barrier.     

Forced translocation of polymer chains through a nanotube: A case of ultrafiltration

Translocation of polymer chains under the application of an external force has been studied through coarse-grained Monte Carlo simulations. The chains are pulled through a nanotube of finite length and diameter and their translocation times measured. The average translocation time, τ follows a scaling relation involving the chain length, N and applied force, F as, τ ∼ N^ν′F^−μ, where ν′ and μ are two different exponents (ν′ = 0.674, and μ = 0.95 ± 0.05). The scaling law is closely similar to the nanopore translocation scaling law reported by Milchev et al. [Ann N Y Acad Sci 2009;1161:95]. Characteristic signatures of the chain escape time have been exhibited by the square of end-to-end distance R², axial radius of gyration R_g−x and other constituent properties. The behavior of the linear polymers under the application of a pulling force has been exploited to gain insights into the ultrafiltration process of unentangled polymers in dilute solution. The generic pulling force-translocation time (F, τ) data obtained through simulation can be matched reasonably well with the hydrodynamic force-critical macroscopic flow time (f_h, Q_c⁻¹) data and also with the hydrodynamic force-reduced critical microscopic flow time (f_h, q_c⁻¹) data obtained in the ultrafiltration experiment on long linear polystyrene chains in cyclohexane, as recently reported by Ge et al. [Macromolecules 2009;42:4400] The simulation technique reported here may be extended to study biomolecular transports occurring in long protein channels, as studied experimentally through current-time or voltage-time traces.     

Diffusion of rigid rodlike polymer in isotropic solutions studied by dissipative particle dynamics simulation

Dissipative particle dynamics (DPD) was employed to simulate the diffusion of rigid rodlike polymers in isotropic solutions. In a dilute solution range, the simulated diffusion behavior is in good agreement with that as described by the Kirkwood theory. In a semi-dilute range, the simulation shows that the DPD model adopting soft repulsive interactions can effectively reproduce the entanglement effect on both rotational and translational diffusions. The rotational diffusion coefficient D_r obeys the asymptotic scaling law D_r ∼ (νL³)⁻² (ν is the number of polymers per volume and L is the polymer length) for the large νL³, which corresponds to formation of a completely enclosed tube in the Doi-Edwards theory. The parallel translational diffusion coefficient D_‖ decreases with ν increase, which can be attributed to the friction effect of surrounding medium. The perpendicular translational diffusion coefficient D_⊥ decays more drastically with ν increase, which is caused by the topological constraint.     

Electrochromic WO3−x films with reduced lattice deformation stress and fast response time

Electrochromic properties of electrochemically deposited and etched (EDE) WO_3−x films have been investigated using voltammetry and nanogravimetry to elucidate the amount of residual stress associated with lattice polarization and deformation in WO_3−x nanoparticles. The cathodic WO_3−x deposition from pertungstic acid solution and unusual properties of the cathodic electroetching of the oxide in tetraethyl ammonium chloride solution are reported and elucidated on the basis of Electrochemical Quartz Crystal Nanogravimetry (EQCN) measurements. The stress enhanced resonant frequency shift was observed upon WO_3−x film coloration. However, the stress enhancement appeared to be much lower (up to 4-6 times) than that measured for films synthesized by other methods. The stress reduction in WO_3−x films under study has been attributed to the stress relaxing propensity of EDE film to suppress the compressive stress wave. A considerable isotopic effect has been observed in nanogravimetry of the H⁺ and D⁺ ion intercalation into WO_3−x films. We have found that the isotopic effect is primarily due to the true mass loading difference between hydrogen and deuterium ions, for the same concentration of color centers (2.65 × 10²¹ cm⁻³), since EQCN frequency shifts associated with stress in the film for H⁺ and D⁺ are very close to each other.     

Novel PVA-based polymers showing an anti-Hofmeister Series property

A series of poly(vinyl alcohol)-trimellitate (PVA-T) polymers with different esterification degrees (ED: 82, 61, and 32 mol%) were prepared through the esterification of PVA and trimellitic anhydride (TA). Solubilities of these polymers and the swelling properties of PVA-T hydrogels, which were prepared by crosslinking with ethylene glycol diglycidyl ether (EGDGE), were investigated in various salt solutions comprising of SO₄²⁻, Cl⁻, SCN⁻ and Li⁺, Na⁺, K⁺, Cs⁺. The PVA-T polymers proved to have larger solubilities in salt solutions (1 M) than in pure water, and the ionic “salting-in” effect was significant in order of SO₄²⁻ > SCN⁻ > Cl⁻ for anions, and Li⁺ > Na⁺ > K⁺ > Cs⁺ for cations, regardless of their ED values. The PVA-T hydrogels also showed corresponding swelling properties; they significantly swelled in sulfate solutions of medium concentration (0.1-1 M), while in other salt solutions no appreciable swelling occurred. The marked salting-in effects exerted by sulfate anion, which is otherwise a typical “salting-out” agent, means that PVA-T polymers have an “anti-Hofmeister Series” (anti-HS) property. This is naturally ascribed to the trimellitic acid group because the degree of swelling in sulfate solutions was more significant for PVA-T with higher ED values; the combination of π-electron system and acidic protons seems to be essential to endow polymers with the anti-HS property.     

Influence of chain microstructure on the conformational behavior of ethylene-1-olefin copolymers. Impact of the comonomeric mole content and the catalytic inversion ratio

The rotational isomeric state model was employed to provide a better understanding of the role of chain microstructure on the conformational behavior of homogeneous ethylene-1-olefin copolymers. The chain microstructure was assembled in accordance with the copolymerization theory using a set of conditional probabilities in direct relation to the reactivity ratios and the feed compositions of the comonomers. The catalytic inversion influence on the tacticity of the polymeric microstructure was also taken into account by considering the replication probability during the Monte Carlo simulation. Statistical weight factors of the rotational isomeric states were evaluated using molecular dynamics runs of the various homopolymers according to the earlier work of Mattice et al. Probability distribution surfaces constructed by the integration of the molecular dynamics trajectories of sufficient length to sample all of the conformational space indicated the increase of the probability of g^±t joint states at the expense of g^±g^± pairs with the increase in the side chain length of the 1-olefin comonomers. It was also indicated that this behavior had a maximum around poly(1-butene)/poly(1-hexene) with an apparent reversal in case of poly(1-octene) due to the side chain crowding, which forces the chain to favor more of the g^±g^± joint states. The characteristic ratios calculated for the copolymers on the basis of the rotational isomeric state model also indicated the increased extension of the polymer backbone with the increase in the side chain length. The lower characteristic ratio calculated for the octene polymers may, in fact, explain the experimental observation that poly(1-octene) has a lower melting point than those of other poly(1-olefin)s of shorter side chains. A complete account of the role of tacticity on the characteristic ratio and the radial distribution function is also given.     

Potentiometric measurements and impedance characteristics of Li0.30La0.57TiO3 membrane in lithium anhydrous solutions

The potentiometric response of the Li⁺ ion-selective electrode based on the fast ion conductor Li_3xLa_2/3−xTiO₃ (x = 0.10) membrane (named LLTO) as well as the impedance of the LLTO membrane/Li⁺ solution in either anhydrous or hydrated PC solvent have been carried out. A four-electrode configuration has been used for the investigation of the interfacial phenomenon. It has been shown that the LLTO membrane can be used to detect the Li⁺ activity in anhydrous solutions through a Li⁺ ion exchange mechanism. The potentiometric response shows a Nernstian behavior with a Li⁺ sensitivity of −72 mV/decade at 25 °C. This high sensitivity can be correlated to a localised hydroxylation of the oxide surface with the residual water present in the solution in combination to the Li⁺ exchange reaction. An apparent standard current density of 12 μA/cm² and a charge-transfer coefficient of 0.29 have been determined. However, as water content in the electrolyte increases, the activity domain of the detection decreases to lead to the disappearance of the Li⁺ ion exchange mechanism in Li⁺ aqueous solution. This annihilation of the exchange process may be due to the predominant catalytic reaction of [Ti-O]⁻ with H₂O and/or to the formation of a water layer on the oxide surface.     

Spectroelectrochemical characterisation of copper salen-based polymer-modified electrodes

Electrogenerated polymers based on copper salen-type complexes were characterised electrochemically and by in situ UV-vis and ex situ EPR spectroscopy. The films, poly[Cu(salen)] and poly[Cu(saltMe)], exhibit reversible oxidative electrochemical behaviour in a wide potential range (0.0-1.5 V). Different regimes for charge transport behaviour are accessed by manipulation of film thickness and experimental time scale: thin films (surface concentration, Γ < ca. 80 nmol cm⁻²) show thin-layer/surface behaviour in the scan rate range used (0.020-2.0 V s⁻¹), whereas thicker polymers (Γ > ca. 90 nmol cm⁻²) exhibit a changeover from thin-layer to diffusion control regime at a critical scan rate that depends on polymer and film thickness: 0.15-0.20 V s⁻¹ for poly[Cu(salen)], 90 < Γ < 130 nmol cm⁻² and 0.20-0.30 V s⁻¹ for poly[Cu(saltMe)], 170 < Γ < 230 nmol cm⁻².UV-vis and EPR spectroscopies have allowed the characterisation of electronic states in the reduced and oxidised forms. The role of the copper atom during film oxidation was probed by combining UV-vis data with EPR on copolymers of the copper and nickel complexes. Data from both techniques are consistent and indicate that polymerisation and redox switching are associated with ligand-based processes. EPR of Ni-doped Cu polymers provided evidence for the non-involvement of the metal centre in polymer oxidation; like the analogous nickel polymers, copper polymers behave like delocalised π-system (‘conducting’) rather than discrete site (‘redox’) polymers.     

Specific membrane transport of silver and copper as Ag(CN)3 and Cu(CN)4 ions through a supported liquid membrane using K-crown ether as a carrier

Facilitated transport of silver and copper from cyanide solutions through a supported liquid membrane (SLM) containing K⁺-crown ether as a carrier is described. The SLM used is a thin porous polypropylene (Celgard 2500, 2400) membrane impregnated with dibenzo-18-crown-6 (DB18C6), diaza-18-crown-6 (DA18C6), hexathia-18- crown-6 (HT18C6) and hexaaza-18-crown-6 (HA18C6) dissolved in a mixture of ethanol/chloroform (v/v). K₋⁺-crown ether showed a high efficiency to carry silver and copper as Ag(CN)₃²⁻ and Cu(CN)₄³⁻ species through the SLM. However, the mass flux of both silver and copper ions decreases when concentration of cyanide ions in the feed phase increases due to the difference in stability of the complexes M(CN)_nⁿ⁻(M=Ag, Cu) when n increases from 2 to 4. This was related to the partition of the species in the aqueous phase using a theoretical model.