Complexation of poly(acrylic acid) and poly(ethylene oxide) investigated by enhanced Rayleigh scattering method

The method of enhanced Rayleigh scattering spectroscopy (ERS) was developed to investigate the complexation of poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO) in semidilute polymer solutions. Based on the Ornstein‐Zernike equation, the relationship between macromolecular static correlation length and ERS intensity was presented. Moreover, the ERS spectra were calculated by the moving window two‐dimensional (MW2D) correlation spectroscopy to get detailed information of the polymer complexation. The results indicated that the ERS spectroscopy characteristics of the polymer mixtures have similar trend, and the ERS intensity promptly increases as the macromolecular chains contract. The increase of ERS intensity showed that the degree of complexation between PAA and PEO increases when the pH value decreases. The complexation results from the collapse of macromolecular chains, which is induced by the PAA chains contracting and the enhanced association between PAA and PEO chains because of the hydrogen bond formation. In addition, the association resulting from the complexation of PAA and PEO in solution was demonstrated by the MW2D correlation spectroscopy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1847–1852, 2010     

Binary mass diffusion in smectic C and CA phases as observed by forced Rayleigh scattering

We have observed the diffusion constants of a dye in several liquid crystals by forced Rayleigh scattering. In a liquid crystal which has a standard phase sequence of N-S_A-S_C, the diffusion anisotropy changes at the N-S_A phase transition and increases with decreasing temperature in S_A and S_C phases. The diffusion constants exhibit a rather smooth decrease with decreasing temperature except an anomaly at the S_A-S_c phase transition. In a liquid crystal which has the antiferroelectric S_CA phase, however, the diffusion constants show discontinuous increase and decrease at the S_A-S_c and the S_c-S_cA phase transition temperatures, respectively: the diffusion constant in S_C is larger than that in the higher temperature S_A phase. Anomalous signal increase and profile were observed at the phase transition temperatures, and were ascribed to the complementary transient grating due to the coexistence of two phases.     

Binary mass diffusion in smectic C and CA phases as observed by forced Rayleigh scattering

Abstract

We have observed the diffusion constants of a dye in several liquid crystals by forced Rayleigh scattering. In a liquid crystal which has a standard phase sequence of N-S_A-S_C, the diffusion anisotropy changes at the N-S_A phase transition and increases with decreasing temperature in S_A and S_C phases. The diffusion constants exhibit a rather smooth decrease with decreasing temperature except an anomaly at the S_A-S_c phase transition. In a liquid crystal which has the antiferroelectric S_CA phase, however, the diffusion constants show discontinuous increase and decrease at the S_A-S_c and the S_c-S_cA phase transition temperatures, respectively: the diffusion constant in S_C is larger than that in the higher temperature S_A phase. Anomalous signal increase and profile were observed at the phase transition temperatures, and were ascribed to the complementary transient grating due to the coexistence of two phases.     

Study of the thermal diffusion behavior of alkane/benzene mixtures by thermal diffusion forced rayleigh scattering experiments and lattice model calculations

In this work the thermal diffusion behavior of binary mixtures of linear alkanes (heptane, nonane, undecane, tridecane, pentadecane, heptadecane) in benzene has been investigated by thermal diffusion forced Rayleigh scattering (TDFRS) for a range of concentrations and temperatures. The Soret coefficient ST of the alkane was found to be negative for these n-alkane/benzene mixtures indicating that the alkanes are enriched in the warmer regions of the liquid mixtures. For the compositions investigated in this work, the magnitude of the Soret coefficient decreases with increasing chain length and increasing alkane content of the mixtures. The temperature dependence of the Soret coefficient depends on mixture composition and alkane chain length; the slope of ST versus temperature changes from positive to negative with increasing chain length at intermediate compositions. To study the influence of molecular architecture on the Soret effect, mixtures of branched alkanes (2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane, and 2,2,4-trimethylpentane) in benzene were also investigated. Our results for the Soret coefficients show that the tendency for the alkanes to move to the warmer regions of the fluid decreases with increasing degree of branching. The branching effect is so strong that for 2,2,4-trimethylpentane/benzene mixtures the Soret coefficient changes sign at high alkane content and that equimolar 2,2,3-trimethylbutane/benzene mixtures have positive Soret coefficients in the investigated temperature range. In order to investigate the effect of molecular interactions on thermal diffusion, we adapted a recently developed two-chamber lattice model to n-alkane/benzene mixtures. The model includes the effects of chain-length, compressibility, and orientation dependence of benzene-benzene interactions and yields good qualitative predictions for the Soret effect in n-alkane/benzene mixtures. For the branched isomers, we find some correlations between the moments of inertia of the molecules and the Soret coefficients. PACS numbers: 66.10.Cb, 61.25.Hq.     

Thermally induced sign change of Soret coefficient for dilute and semidilute solutions of poly(N-isopropylacrylamide) in ethanol

We studied the thermal diffusion behavior of poly(N-isopropylacrylamide) (PNiPAM) in ethanol in a temperature range from T = 14.0 degrees C to T = 40.0 degrees C by means of thermal diffusion forced Rayleigh scattering. The obtained Soret coefficient S(T) of PNiPAM was positive for lower temperatures (T < 34 degrees C), while S(T) showed a negative value for higher temperatures (T > 34 degrees C). This means PNiPAM molecules move to the cold side for temperatures T < 34 degrees C, whereas they move to the warm side for T > 34 degrees C. This is the first nonaqueous polymeric system for which a sign change with temperature has been observed. We performed static and dynamic light scattering experiments in the same temperature range. The second virial coefficient determined from dilute solutions by static light scattering (SLS) was positive in the comparable temperature range. The results of SLS for the semidilute solution showed a strong repulsion among PNiPAM chains which was enhanced by increasing temperature. These results imply that the observed thermally induced sign change of S(T) does not depend on the intermolecular interactions among PNiPAM chains.     

Melting of water/poly(ethylene oxide) systems

The transition temperatures of poly(ethylene oxide) (PEO)/water systems were studied over the temperature range from ?90 to 110°C by differential scanning calorimetry. The composition of the systems and the average molecular weight of the PEO were varied over a wide range. The systems under study tend to supercool and become partially or completely glassy depending on the cooling rate, composition, and PEO molecular weight. The glassy phases crystallize on being heated above their glass transition temperature. Systems containing about 60 wt % of PEO are most readily super cooled without crystallization. The dependence of melting temperatures and enthalpies on composition was studied and their isobar phase diagrams were drawn. They show that at a definite polymer content (about 50 wt %), which is molecular weight dependent, the systems behave as binary eutectic mixtures. The results confirm the idea that PEO forms quite stable molecular complexes with water.     

Static light scattering from poly(ethylene oxide) in methanol

Static light scattering measurements were performed on dilute solutions of monodisperse poly(ethylene oxide) (PEO) in methanol at 25°C. PEOs of five different molecular weights ranging from nominal M_w = 8.6 × 10⁴ to 9.13 × 10⁵ were used. Linear Zimm plots were obtained for all the PEO samples: no downturn was observed at small angles, indicating that no large aggregates of PEO molecules exist in the solution. From the plots, values of the weight-average molecular weight, M_w, the radius gyration, R_G, and the second virial coefficient, A₂, were successfully determined for respective PEOs. Observed relationship between R_G and M_w indicates that methanol is certainly a good solvent for the polymer. © 1996 John Wiley & Sons, Inc.     

Clustering of poly(ethylene oxide) in water revisited

The dynamic light scattering results presented in this letter demonstrate that the clustering of poly(ethylene oxide) (PEO) can be observed even in ultrapure, freshly double‐distilled and filtered deionized water. It is confirmed that the filtration of solutions removes the clustering structure and that a steady‐state amount of PEO in clusters is reformed in filtered solutions within 24 h. Adding a drop of chloroform to unfiltered aqueous solutions of PEO temporarily alters the clustering structure, but it prevents the clustering of PEO in filtered solutions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 135–138, 2003     

Structuring effects and hydration phenomena in poly(ethylene glycol)/water mixtures investigated by brillouin scattering

Aqueous solutions of poly(ethylene glycol) (PEG) of mean molecular mass of 600 g/mol (PEG600) are investigated by Brillouin scattering technique. At high PEG content, a relaxation phenomenon is observed, which is related to a local rearrangement of the polymer structure where the interaction, via hydrogen bonding, with the solvent molecules plays a role. The obtained values of the relaxation times match the literature data very well for a fast relaxation time revealed by dielectric relaxation measurements in very similar mixtures. The calculated concentration behaviors of the excess adiabatic compressibility turns out in good agreement with the previous findings from ultrasonic measurements at 3 MHz. The observed minimum in the adiabatic compressibility is interpreted as the result of the interaction between water and the EO units of the PEG chain, which results in a structure tighter then that typical of bulk water and of pure PEG600. Such a hypothesis is supported by the observation that volume fraction value of about 0.3 coincides with the concentration value at which full hydration of EO units takes place. The observation that at the same concentration, the polymer coils start to overlap each other further supports the idea that the adiabatic compressibility behavior is monitoring the structural evolution of the mixture. However, similar results are obtained for largely different binary mixture which suggests caution in taking this conclusion too literally. In particular, the hypothesis that the occurrence of an extreme in the excess adiabatic compressibility could be simply originated by statistical effects and that further work is required for disentangling entropic contribution from effects of hetero-association and self-aggregation of one or both the components.     

Solubility, diffusion coefficient and ionic conductivity of alkyl viologens in poly(ethylene oxide) and its derivatives

A series of alkyl viologens RV (R denotes ethyl, butyl, hexyl, heptyl, and dodecyl) was dissolved in poly(ethylene oxide) (PEO) oligomers (average molar masses of 200, 300, 400, 600 and 1000 g mol⁻¹). The solubility of RV in PEO oligomers decreased with increasing alkyl chain length of RV and the molar mass of PEO. Cyclic voltammograms of RV in PEO containing 0.50 M LiClO₄ clearly show two redox waves. The ionic conductivity of PEO oligomers containing RV decreased with increasing alkyl chain length, suggesting the migration of RV itself in the PEO oligomers. Potential step chronoamperometry was used to obtain the apparent diffusion coefficient of RV in the PEO oligomers. The ionic conductivity has a linear relationship with the apparent diffusion coefficient regardless of the RV species, the PEO molar mass and the temperature. RV was shown to act as a redox mediator in PEO oligomers as long as the ionic conductivity of the PEO was high. Poly(oligo(oxyethylene) methacrylate) (PMEO) was used as a solid solvent for a series of alkyl viologens. Since PMEO is an excellent ion-conducting polymer, RV was confirmed to be an effective redox mediator in this PMEO. It was concluded in this study that ionic conductivity in the polymer matrix could be used as an effective parameter for prediction of the diffusion coefficient of charged organic molecules.     

Interaction between poly(ethylene glycol) and two surfactants investigated by diffusion coefficient measurements

Dynamic light scattering (DLS) and fluorescence recovery after pattern photobleaching (FRAPP) were used to study the interaction of low molecular weight poly(ethylene glycol) (PEG) with micelles of two different surfactants: tetradecyldimethyl aminoxide (C(14)DMAO, zwitterionic) and pentaethylene glycol n-dodecyl monoether (C(12)E(5), non-ionic). By using an amphiphilic fluorescent probe or a fluorescent-labeled PEG molecule, FRAPP experiments allowed to follow the diffusion of the surfactant-polymer complex either by looking at the micelle diffusion or at the polymer diffusion. Experiments performed with both fluorescent probes gave the same diffusion coefficient showing that the micelles and the polymer form a complex in dilute solutions. Similar experiments showed that PEG interacts as well with pentaethylene glycol n-dodecyl monoether (C(12)E(5)).     

Micellization of polystyrene-graft-poly(ethylene oxide) and its mixtures with polystyrene homopolymer in ethanol

Micellization behaviors of polystyrene-graft-poly(ethylene oxide) (PS-graft-PEO) and its mixtures with PS homopolymer in ethanol were investigated by means of nuclear magnetic resonance (¹H NMR) spectroscopy, transmission electron microscopy (TEM), and viscometry. It was revealed that PS-graft-PEO could self-assemble to form polymeric micelles with a core-shell structure in the shape of spherical. Micelle formation of PS-graft-PEO in ethanol was strongly dependent on the concentration, temperature, and the PS chain contents in PS-graft-PEO. The introduction of PS homopolymer not only decreases the critical micelle concentration, but also changes the morphology of the micelles.     

Synthesis and characterization of poly(ethylene oxide)/polylactide/poly(ethylene oxide) triblock copolymer

Poly(ethylene oxide/polylactide/poly(ethylene oxide) (PEO/PL/PEO) triblock copolymers, in which each block is connected by an ester bond, were synthesized by a coupling reaction between PL and PEO. Hydroxyl‐terminated PLs with various molecular weights were synthesized and used as hard segments. Hydroxyl‐terminated PEOs were converted to the corresponding acid halides via their acid group and used as a soft segment. Triblock copolymers were identified by Fourier transform infrared spectroscopy, ¹H NMR, and gel permeation chromatography. Differential scanning calorimetry (DSC) and X‐ray diffractometry of PEO/PL/PEO triblock copolymers suggested that PL and PEO blocks were phase‐separated and that the crystallization behavior of the PL block was markedly affected by the presence of the PEO block. PEO/PL/PEO triblock copolymers with PEO 0.75k had two exothermic peaks (by DSC), and both peaks were related to the crystallization of PL. According to thermogravimetric analysis, PEO/PL/PEO triblock copolymer showed a higher thermal stability than PL or PEO. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2545–2555, 2002     

Crystallization behavior and toughening mechanism of poly(ethylene oxide) in polyoxymethylene/poly(ethylene oxide) crystalline/crystalline blends

In this study, the unique crystallization behavior of poly(ethylene oxide) (PEO) in polyoxymethylene (POM)/PEO crystalline/crystalline blends was examined in detail. This study was the first to report the typical fractionated crystallization of PEO in POM/PEO blends when PEO is fewer than 30 wt.%. The delayed crystallization temperature of PEO was confirmed at about 5°C to 14°C by using differential scanning calorimetry and perturbation–correlation moving‐window 2D correlation IR spectroscopy. Wide‐angle X‐ray diffraction indicates that no new crystal structures or co‐crystals were generated in POM/PEO. The statistical calculations of scanning electron microscopy photos show that the average diameter of PEO particles is 0.227 µm to 1.235 µm and that the number of small particles is as many as 10⁹ magnitudes per cm³. Theory analysis establishes that the delayed crystallization of PEO is a heterogeneous nucleation process and not a homogeneous nucleation process. A significant toughening effect of PEO to POM was also observed. The impact strength of POM/PEO acquires a maximum of 10.5 kJ/m² when PEO content is 5%. The impact strength of the blend increases by 78.0% compared with pure POM. To improve the toughening effect, the best particle size is established between 0.352 and 0.718 µm, with a PEO particle spacing of 0.351 µm to 0.323 µm. The number of corresponding particles was 0.887 × 10⁹ per cm3 to 3.240 × 10⁹ per cm³. A PEO toughening model for POM was proposed to provide a new and effective way to solve the problem of POM toughening. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of water on the photodegradation of poly(ethylene oxide)

The UV-light degradation of polyethylene oxide (PEO) in aqueous solution was investigated operating under long wavelengths (λ > 300 nm) at 20 °C in different pH conditions varying from 2.3 to 12.0 and at two different concentrations. Thermo-oxidation experiments on PEO aqueous solution at 50 °C are also reported and compared to photo-oxidation results. The formation of oxidation products was followed by infrared analysis of deposits obtained by evaporation of aliquots of irradiated polymer solution. Photo-oxidation led to formates and esters but a third product was also identified, formic acid ions formed by partial hydrolysis of formates. The degradation of PEO in water led to the acidification of the aquatic medium. Size exclusion chromatography (SEC) was used to monitor the changes in molar weight and intrinsic viscosity with irradiation time. It was shown that the photo-oxidation produced a dramatic decrease of the average molar weights which is more important in acidic medium. Total organic carbon (TOC) measurements of the aged aqueous solutions showed that the mineralization of PEO could not be achieved in these photo-oxidative conditions.     

Restricted self-diffusion in an aqueous solution of poly(ethylene oxide) poly(propylene oxide) poly(ethylene oxide) triblock copolymer

 The self-diffusion behavior of a triblock copolymer (PEO–b– PPO–b–PEO) in an aqueous solution of 20% (m/m) was investigated during a temperature-induced phase transition from liquid to gel state using pulsed field gradient NMR and static light scattering. The measured self-diffusivity shows a strong dependence on the observation time in the gel phase indicating the existence of diffusion barriers in the size range of about 0.6 μm. Additional static light-scattering measurements show a structure in the same size range of several hundred nanometers, which is far above molecular or micellar sizes and thus, has to be caused by larger clusters. The similarity in the space scales suggests that the restriction of molecular propagation is correlated with the grain boundaries between the domains of the poly-crystalline structure formed by the arranged micelles. Received: 28 October 1996 Accepted: 21 March 1997     

Conformational changes of poly(ethylene oxide) in poly(ethylene oxide)/poly(methyl methacrylate) blends by supercritical carbon dioxide

The effects of supercritical carbon dioxide (SC CO₂) fluids on the morphology and/or conformation of poly(ethylene oxide) (PEO) in PEO/poly(methyl methacrylate) (PMMA) blends were investigated by means of differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and Fourier transform infrared (FTIR). According to DSC data for a given blend, the melting enthalpy and, therefore, degree of crystallinity of PEO were increased, whereas the melting temperature of PEO was decreased, with SC CO₂ treatment. The enhancement of PEO crystallization with SC CO₂ treatment, as demonstrated by DSC data, was supported by WAXD data. According to FTIR quantitative analyses, before SC CO₂ treatments, the conformation of PEO was transformed from helix to trans planar zigzag via blending with PMMA. This helix‐to‐trans transformation of PEO increased proportionally with increasing PMMA content, with around 0.7% helix‐to‐trans transformation per 1% PMMA incorporation into the blend. For a given blend upon SC CO₂ treatments, the conformation of PEO was transformed from trans to helix. This trans‐to‐helix transformation of PEO decreased with increasing PMMA contents in the blends because of the presence of interactions between the two polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2479–2489, 2004     

Small-angle neutron scattering and theoretical investigation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) stabilized oil-in-water microemulsions

The aim of this study is to determine the effects of oil solutes and alcohol cosolvents on the structure of oil-in-water microemulsions stabilized by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers. The systems investigated involved the solubilization of 1,3,5-trimethylbenzene or 1,2-dichlorobenzene by P123 (EO(20)-PO(70)-EO(20)) pluronic surfactant micelles in water and water + ethanol solvents. The structures of these swollen micelles were determined by small-angle neutron scattering (SANS). A thermodynamic model was employed to interpret the characterization data. The results of the thermodynamic model for micellization agreed well with the SANS data from samples of micelles swollen by both oils. The model predicted the size of the micelles within 5% accuracy using only one fitting parameter, the micelle polydispersity. Ethanol had significantly different effects on the polymer micelles that contained solubilized oil compared to pure polymer micelles. For pure polymer micelles, the addition of ethanol increased the solubility of the polymer and, therefore, decreased the total volume fraction of micelles, while for polymer-oil aggregates, ethanol tended to have a positive effect on the volume fraction of micelles. SANS results showed that the greatest divergence from pure aqueous solvent results occurred at oil concentrations above the microemulsion stability limit.     

Unusual phase behavior in mixtures of poly(ethylene oxide) and ethyl alcohol

Poly(ethylene oxide) (PEO), soluble in both aqueous and organic solvents, is one of the most intriguing polymers. PEO solution properties have been extensively studied for decades; however, many of the studies have focused on specific properties, such as clustering, of PEO in aqueous solutions, and the behavior of PEO in organic solvents has not been adequately explored. The results presented here demonstrate that PEO crystallizes into a lamellar structure in ethyl alcohol after the mixture is quenched to room temperature from a temperature above the crystal melting point. Above the melting temperature, PEO completely dissolves in ethyl alcohol, and the mixture exhibits regular polymer solution thermodynamic behavior with an upper critical solution temperature (UCST) phase diagram. Remarkably, the UCST phase boundary is significantly below the melting temperature, and this indicates that the system undergoes a crystallization process before the phase separation can occur upon cooling and, therefore, possesses an unusual phase transition. The phase transition from the crystalline state to the miscible solution state is reversible upon heating or cooling and can be induced by the addition of a small amount of water. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 557–564, 2006