Effect of solute hydrophobicity on phase behaviour in solutions of thermoseparating polymers

 Two-phase systems consisting of a polymer rich phase and polymer depleted phase, where the polymer is either ethyl(hydroxy ethyl)cellulose (EHEC) or Ucon (a random copolymer of ethylene oxide and propylene oxide), have been studied. Both of these polymers can be separated from an aqueous solution by either temperature increase or addition of cosolutes. The polymers are thermoseparating and phase separate in water solutions at the cloud point temperature. Two types of EHEC have been studied: one with a cloud point at 60 °C and the other at 37 °C. The Ucon polymer used in this study has a cloud point at 50 °C. Ternary phase diagrams of polymer/water/cosolute systems have been investigated. When a strongly hydrophilic or hydrophobic cosolute is added to an EHEC- or Ucon–water solution, a phase separation occurs already at, or below, room temperature. As cosolutes, hydrophobic molecules like phenol, butyric and propionic acid, and hydrophilic molecules like glycine, ammonium acetate, sodium carboxylates (acetate to valerate), were studied. The polymer rich phase formed when mixing polymer, water and cosolute was strongly enriched or depleted with hydrophobic or hydrophilic cosolutes, respectively. The two phase region increased for propionic acid, butyric acid and phenol as a result of increased cosolute hydrophobicity. The opposite occurred in the series sodium acetate, sodium butyrate and sodium valerate. The effect of temperature on the phase behaviour has also been investigated. Model calculations based on Flory–Huggins theory of polymer solutions are presented, in form of a phase diagram, which semiquantitatively reproduce some experimental results. Received: 5 July 1996 Accepted: 4 November 1996     

Rupturing polymeric micelles with cyclodextrins

Small-angle neutron scattering has been used to investigate the associative structures formed by triblock copolymers of poly(ethylene oxide) (PEO)-polypropylene oxide (PPO)-poly(ethylene oxide) (PEO) (also known as Pluronics) and to monitor the structural changes occurring upon complexation with heptakis(2,6-di-O-methyl)-beta-cyclodextrin (hbeta-CD) over the temperature range from 5 to 70 degrees C. At low temperature, the Pluronics are dispersed as unimers. Close to ambient temperature, the hydrophobicity of PPO causes the aggregation of the polymers into spherical micelles with core sizes between 40 and 50 A and a high inclusion of solvent. The aggregation number increases with temperature as the hydrophobicity of the core is gradually enhanced. hbeta-CD spontaneously forms pseudopolyrotaxanes with the triblock copolymers either when in their unimer form or micellized. The complexation results in an increase in the effective critical micellar concentration. It is suggested that the cyclodextrins thread onto the polymer backbone to localize preferentially on the central PPO block, therefore improving its water solubility. At temperatures where the polymers exist in micellar form, complexation with hbeta-CD gives rise to a complete disruption of the aggregates. These processes are highly temperature-dependent. Above 50 degrees C, the break-up of the aggregates is inhibited, and large-scale aggregation is observed.     

Isomorphism in ternary complex: Poly(ethylene oxide), urea and thiourea

Herein,we demonstrated that poly(ethylene oxide)(PEO),urea and thiourea can crystallize into novel ternary complex with the molar ratio of guest polymer and host small molecule as 3:2,and proved that the ternary complex behaves isomorphism phenomenon by varying the ratio between urea and thiourea for the first time.This observation gives a boost to prepare co-crystals of different small molecules that cannot be obtained by direct mixing without the aid of polymer chains.     

Competitive adsorption of nonionic surfactant and nonionic polymer on silica

Competitive adsorption of the nonionic polymer poly(ethylene oxide) (PEO) and the nonionic surfactant of the type poly(ethylene oxide) alkyl ether from aqueous solutions on a silica surface is examined. From one-component solutions, both species readily adsorb onto silica and, in the bulk of mixed (two-component) solutions, polymer-surfactant complexes are not observed. Because both species bind by the same mechanism to silica, subtle differences in layer structure, or other species-specific parameters, determine whether one or both of the species will adsorb. It was found that various surfactants can displace PEO up to a certain critical molecular weight. Surfactants with a high aggregation number, in bulk and on the surface, can displace PEO with a higher molar mass than surfactants with a low aggregation number. As the molar mass of the polymer increases, the time a surfactant needs to completely displace the polymer increases. We can explain both the existence of the critical molar mass and the decrease in adsorption kinetics with a shift in the critical surface association concentration (CSAC).     

Interactions between cellulose nanocrystals and anionic and neutral polymers in aqueous solutions

Physical structures of aqueous cellulose nanocrystal (CNC) suspensions in anionic polyelectrolyte carboxymethyl cellulose (CMC) and non-ionic poly(ethylene oxide) (PEO) were investigated by studying their cross polarized, polarized optical microscope (POM) images and dynamic light scattering, zeta potential, ¹H spin–lattice relaxation nuclear magnetic resonance (NMR) data. The presence of anionic CMC and nonionic PEO in CNC suspensions led to two different kind of interactions. Semi-dilute CNC suspensions showed first gel-like behavior then phase separation by adding only semi-dilute un-entangled CMC polymer solutions, whereas the addition of PEO didn’t cause any significant change. POM images showed the phase transitions of CNC suspensions in the presence of CMC solutions from the isotropic state to nematic and chiral nematic phases. Dynamic light scattering, zeta potential and ¹H spin–lattice relaxation NMR data presented further arguments to explain polymer-CNC interactions in CMC and PEO solutions. ¹H NMR solvent relaxation technique determined the adsorption and depletion interactions between polymers and CNC. The minima in spin–spin specific relaxation rate constant showed the depletion of CNC nanoparticles in CMC. It is believed that the depletion flocculation was the case for the effects of CMC polymer chains in CNC suspensions. PEO was adsorbed on CNC surfaces and caused only weak depletion interactions due to the presence of soft particles.     

Noncovalent coatings for the separation of synthetic polypeptides by nonaqueous capillary electrophoresis

Recently, we demonstrated the possibility to extend the range of capillary electrophoresis (CE) applications to the separation of non-water-soluble synthetic polymers. This work focuses on the control of the electro-osmotic flow (EOF) and on the limitation of the solute adsorption in nonaqueous electrolytes. For these purposes, different strategies were investigated. For the initial, a viscous additive (ethylene glycol or glycerol) was used in the electrolyte in order to decrease the EOF magnitude and, possibly, to compete with solute adsorption. A second strategy was to modify, before separation, the fused-silica capillary wall by the adsorption of poly(ethylene oxide) (PEO) via hydrogen bonding. The influence of the molecular mass of the adsorbed PEO on the EOF magnitude and direction was studied in electrolytes based on methanol/acetonitrile mixtures containing ammonium ions. For PEO molecular masses above 1000 g/mol, reversed (anodic) EOF were reported in accordance with previous results obtained with PEO covalently bonded capillaries. The influence of the nature and the concentration of the background electrolyte cation on the EOF magnitude and direction were also investigated. A third strategy consisted in modifying the capillary wall by the adsorption of a cationic polyelectrolyte layer. Advantageously, this polyelectrolyte layer suppressed the adsorption of the polymer solutes onto the capillary wall. The results obtained in this work confirm the high potential and the versatility of CE for the characterization of ionizable organic polymers in nonaqueous media.     

Structural Transitions in Triblock Copolymers Based on Poly(ethylene oxide) and Polyacrylamide under the Temperature Influence

Thermal transitions in the bulk structure of triblock copolymers (PAA-b-PEO-b-PAA) based on polyacrylamide and poly(ethylene oxide) with varying molecular weight (length) of PEO block comparing with the structures of individual polymers and polymer mixtures were investigated. A lot of effects, such as the melting temperature depression, decreasing of the crystallinity degree of PEO and also appearance of the microphase separation in amorphous regions of the polymer mixtures and the triblock copolymers were found. Such investigations pointed to a strong intramolecular interaction of the polymer blocks in the triblock copolymers that is confirmed by the results of IR spectroscopy. It was shown that PEO and PAA blocks formed the system of H-bonds with participant of trans-multimers of amide groups.     

Effect of polymer architecture on the adsorption properties of a nonionic polymer

The adsorption of a linear- and bottle-brush poly(ethylene oxide (PEO))-based polymer, having comparable molecular weights, was studied by means of quartz crystal microbalance with dissipation monitoring ability (QCM-D) and AFM colloidal probe force measurements. The energy dissipation change monitored by QCM-D and the range of the steric forces obtained from force measurements demonstrated that linear PEO forms a more extended adsorption layer than the bottle-brush polymer, despite that the adsorbed mass is higher for the latter. Competitive adsorption studies revealed that linear PEO is readily displaced from the interface by the bottle-brush polymer. This was attributed to the higher surface affinity of the latter, which is governed by the number of contact points between the polymers and the interface, and the smaller loss of conformational entropy.     

Coagulation and flocculation measurements by photon correlation spectroscopy — colloidal SiO2 bare and covered by polyethylene oxide

With photon correlation spectrometry (PCS) the diffusion coefficients, average diameters and polydispersities of colloidal particles can be determined in dilute aqueous suspensions. In this study PCS is used to follow the coagulation and flocculation of silica particles. Electrolyte solution added to suspensions of bare particles and of particles covered with adsorbed polyethylene oxide layers induces aggregation. The rate constants of aggregation are evaluated by the second-order Smoluchowski theory with the assumptions of spherical aggregated particles and volume proportional light-scattering amplitude. Adsorbed PEO layers of molar mass lower thanM _w=160000 decrease the critical flocculation concentration and the flocculation states and rate constants for bare and covered particles are the same at high electrolyte concentrations. Polymer layers of high molar mass (M _w=325000, 900000) reducved at full coverage the rate constants and stabilize the suspensions even at high electrolyte concentrations. At low coverage adsorption of high molar mass polymers results in the same values as of low molar mass PEO. The correlation between rate constants and hydrodynamic PEO layer thicknesses demonstrates the steric influence of the tails of the adsorbed macromolecules on stability and flocculation.Dedicated to Prof. Dr. Joachim Klein on the occasion of his 60th birthday     

Polystyrene‐graft‐poly(ethylene oxide) copolymers prepared by macromonomer technique in dispersion. I. Liquid chromatographic separation of product mixtures

Products of the radical dispersion copolymerization of methacryloyl‐terminated poly(ethylene oxide) (PEO) macromonomer and styrene were separated and characterized by size exclusion chromatography (SEC), full adsorption‐desorption (FAD)/SEC coupling and eluent gradient liquid adsorption chromatography (LAC). In dimethylformamide, which is a good solvent for PEO side chains but a poor solvent for polystyrene (PS), amphiphilic PS‐graft‐PEO copolymers formed aggregates, which were very stable at room temperature even upon substantial dilution. The aggregates disappeared at high temperature or in tetrahydrofuran (THF), which is a good solvent for both homopolymers and for PS‐graft‐PEO. FAD/SEC procedure allowed separation of homo‐PS from graft‐copolymer and determination of both its amount and molar mass. Effective molar mass of graft‐copolymer was estimated directly from the SEC calibration curve determined with PS standards. Presence of larger amount of the homo‐PS in the final graft‐copolymer products was also confirmed with LAC measurements. The results indicate that there are at least two or maybe three polymerization loci; namely the continuous phase, the particle surface layer and the particle core. The graft copolymers are produced mainly in the continuous phase while PS or copolymer rich in styrene units is formed mostly in the core of monomer‐swollen particles. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2284–2291, 2000     

Block copolymers of styrene,isoprene, and ethylene oxide prepared by anionic polymerization. I. Synthesis and characterization

Anionic polymerization has been used as a technique for the synthesis of five-block copolymers of polystyrene (PS), polyisoprene (PI), and poly(ethylene oxide) (PEO). Two types of such polymers, PEO-PI-PS-PI-PEO and PEO-PS-PI-PS-PEO with varying PEO block length, have been prepared, using potassium naphthalene as the initiator and tetrahydrofuran as the solvent. The polymers were purified by extraction with ethyl acetate, diethyl ether, and water. After the addition of each monomer, a sample from the living polymer solution was taken and analyzed by spectroscopy (infared (IR) and proton magnetic resonance (PMR)), osmometry, and gel-permeation chromatography (GPC) to obtain information about composition, molecular weight and molecular weight distribution of the intermediate polymers. The five-block copolymers have also been characterized by the same techniques and by elemental analysis.     

PEO flocculation with phenolic microparticles

Polystyrene latex and precipitated calcium carbonate (PCC) with and without dextran sulfate pretreatment were flocculated by the consecutive addition of high-molecular-weight poly(ethylene oxide) and a novel composite latex microparticle consisting of a polystyrene core and a poly(p-vinylphenol) shell. Good flocculation of polystyrene latex and PCC was obtained, whereas the PCC coated with dextran sulfate was not flocculated. The interaction of the composite microparticle with the target colloids was governed by electrostatic forces, whereas hydrogen bonding and hydrophobic interactions drove the PEO adsorption onto the composite particles.     

Preparation of thin surface layers by grafting of PEO

We have developed a two‐stage process to graft poly(ethylene oxide) (PEO) onto a silica surface. In the first stage the adsorption of an anchor reactive polymer to the surface is carried out, and in the second stage the grafting of compatibilizing macromolecular tails is performed via the reactions of functional groups of the polymer anchored. Random copolymers of styrene and maleic anhydride (SM) were chosen as reactive anchoring polymers. The kinetics of adsorption of SM from dilute solutions onto the silica surface as well as the grafting of PEO to SM macromolecules adsorbed was experimentally investigated by null ellipsometry. A model of the structure at the surface is proposed.     

Synthesis and Characterization of α,ω‐bi[2,4‐dinitrophenyl (DNP)] poly(2‐methoxystyrene) Functional Polymers. Initial Evaluation of the Interaction of the Functional Polymers with RBL Mast Cells

Two series of functional polymers, α,ω‐bi[2,4‐dinitrophenyl][poly(ethylene oxide)‐b‐poly(2‐methoxystyrene)‐b‐poly(ethylene oxide)] (DNP‐PEO‐P2MS‐PEO‐DNP) and α,ω‐bi[2,4‐dinitrophenyl caproic][poly(ethylene oxide)‐b‐poly(2‐methoxystyrene)‐b‐poly(ethylene oxide)] (CDNP‐PEO‐P2MS‐PEO‐CDNP), were synthesized by anionic living polymerization. The polymers were characterized by FT‐IR, ¹H‐NMR and Gel Permeation Chromatography (GPC). The molecular weight distributions for the lower molecular weight functional polymers were slightly broad (1.3–1.5). However, the molecular weight distributions for higher molecular weight polymers were narrower (1.1–1.2). Differential scanning calorimetry (DSC) studies showed thermal transitions indicative of the presence of microphases in the polymer solid state. The polymers were white powders and soluble in tetrahydrofuran. The binding affinity of DNP‐PEO‐P2MS‐PEO‐DNP ligands towards anti DNP IgE was determined by titrations with fluorescently labeled FITC‐IgE. A water soluble CDNP‐PEO‐P2MS‐PEO‐CDNP/DMEG (dimethoxyethylene glycol) complex binds and achieves steady state binding with solution IgE within a few seconds. This strongly suggests that CDNP functional polymers with improved water solubility have potential in therapeutics. Higher molecular weight (water insoluble) CDNP‐PEO‐P2MS‐PEO‐CDNP polymers were electrosprayed as fibers (500 nm) on silicon surface. Fluorescence spectroscopy clearly showed that RBL mast cells were interacting with the fibers suggesting that the cell‐surface receptors were clustered along the fiber surface. These observations suggest that the functional polymers hold promise for developing an antibody detection device.     

Effects of dissolved polymer on the transport of colloidal particles through a microcapillary

The effect of water-soluble polymer on the transport of latex particles through a microcapillary was investigated. Capillary hydrodynamic fractionation (CHDF) experiments were performed using polystyrene (PS) particles and poly(ethylene oxide) (PEO) solutions as the eluant. Generally, the average particle velocities were greater than those corresponding to a polymer-free eluant. A decrease in the sample axial dispersion was also observed using the PEO solutions. In addition, increasing the polymer molecular weight resulted in lower particle residence times in the capillary tube. The enhanced particle transport arises primarily from an increase in the particle diameter resulting from the adsorption of PEO onto the PS surfaces, and, more importantly, from the migration of particles toward the capillary axis due to the normal stress of the PEO solution.     

Polymer bilayer formation due to specific interactions between beta-cyclodextrin and adamantane: a surface force study

The purposes of this study are to utilize the interactions between an adamantane end-capped poly(ethylene oxide) (PEO) and a cationic polymer of beta-cyclodextrin to build polymer bilayers on negatively charged surfaces, and to investigate the interactions between such layers. The association of this system in solution has been studied by rheology, light scattering, and fluorescence measurements. It was found that the adamantane-terminated PEO (PEO-Ad) mixed with the beta-cyclodextrin polymer gives complexes where the interpolymer links are formed by specific inclusion of the adamantane groups in the beta-cyclodextrin cavities. This results in a higher viscosity of the solution and growth of intermolecular clusters. The interactions between surfaces coated with a cationized beta-cyclodextrin polymer across a water solution containing PEO-Ad polymers were studied by employing the interferometric surface force apparatus (SFA). In the first step, the interaction between mica surfaces coated with the cationized beta-cyclodextrin polymer in pure water was investigated. It was found that the beta-cyclodextrin polymer adsorbs onto mica and almost neutralizes the surface charge. The adsorbed layers of the beta-cyclodextrin polymer are rather compact, with a layer thickness of about 60 A (30 A per surface). Upon separation, a very weak attractive force is observed. The beta-cyclodextrin solution was then diluted by pure water by a factor of 3000 and a PEO-Ad polymer was introduced into the solution. Two different architectures of the PEO-Ad polymer were investigated: a four-arm structure and a linear structure. After the adsorption of the PEO polymer onto the beta-cyclodextrin layer reached equilibrium, the forces were measured again. It was found that the weak repulsive long-range force had disappeared and an attractive force caused the surfaces to jump into contact, and that the compressed layer thickness had increased. The attractive force is interpreted as being due to a specific recognition between the hydrophobic adamantane groups on the PEO-Ad polymer and the hydrophobic cavity in the beta-cyclodextrin molecules. Furthermore, the attractive force observed on separation has increased significantly, which is a further indication of a specific interaction between the beta-cyclodextrin polymer and the adamantane groups.     

溶胶-凝胶伴随相分离制备SiO2多孔块体

以正硅酸甲酯(TMOS)为前驱体, 0.01 mol·L^-1盐酸(HCl)为催化剂, 环氧丙烷(PO)为凝胶促进剂, 粘均分子量(M_v)为10000的聚氧化乙烯(PEO)为相分离诱导剂, 采用溶胶-凝胶伴随相分离制备SiO₂多孔块体材料,利用差热分析(DTA)、傅里叶变换红外(FT-IR)光谱、扫描电镜(SEM)、X射线衍射(XRD)、汞压、N₂吸附/脱附等测试技术对所制得的SiO₂多孔块体进行了表征, 探讨了环氧化物调控溶胶-凝胶以及PEO诱导相分离机理. 结果表明, 加入PEO能诱导SiO₂凝胶发生相分离, 当PEO/TMOS摩尔比为0.0018时, 可以获得共连续多孔结构的SiO₂块体材料, 其大孔孔径分布在1-3 μm之间, 比表面积达719 m²·g^-1, 孔体积为0.48 m³·g^-1. 环氧丙烷因其环氧原子的强亲核性和不可逆的开环反应, 促进溶胶-凝胶转换, 同时借助吸附在SiO₂低聚物上的PEO诱导SiO₂凝胶相分离, 从而制备共连续大孔及骨架结构的多孔块体.     

Adsorption of polyethylene oxide on surface modified silica – stability of bare and covered particles in suspension

 The adsorption of poly-ethylene oxide (PEO) on modified colloidal silica and the stability of the aqueous suspension was investigated. With octanol some silanol groups at the silica surface were replaced by octylgroups. The size of the modified silica particles and the charge and chemical groups on the surface were charaterized by ultracentrifugation, photon correlation spectrometry, polyelectrolyte titration and IR spectrometry. The adsorbed amounts of polyethylene oxides of different molar mass were determined on the modified silica in water. With photon correlation spectrometry (PCS) the hydrodynamic layer thickness of the PEO layers on the particles were measured. The dependences of the layer thicknesses on molar mass of the PEO, polymer concentration and adsorption time were determined. The aggregation of the suspended PEO coated and uncoated modified silica particles was examined with PCS by the time dependence of the diffusion coefficient at different salt concentrations. The influence of molar mass and concentration of PEO as well as of the age of the dispersion was explored. The measured dependences are discussed and compared with the behavior of unmodified silica- and latex-particles. Received: 6 April 1998 Accepted: 27 May 1998     

Particles trapped at the droplet interface in water-in-water emulsions

Water-in-water emulsions were formed by mixing incompatible aqueous solutions of dextran and poly(ethylene oxide) (PEO) in the presence of latex or protein particles. It was found that particles with a radius as small as 0.1 μm become trapped at the interface between the PEO- and dextran-rich phases with interfacial tensions down to 10(-6) N/m. The particles were visualized at the interface of the emulsion droplets using confocal laser scanning microscopy (CLSM) allowing determination of the contact angle. Various degrees of coverage with particles could be observed. On densely covered droplets, the particles had a hexagonal crystalline order. At intermediate coverage, transient clustering of the particles was observed. The diffusion coefficient of the particles at the interface was determined using multiparticle tracking. Fusion of droplets was observed in all cases leading eventually to macroscopic phase separation.     

Small-angle neutron scattering study of adsorbed pluronic tri-block copolymers on laponite

The adsorption of selected poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) tri-block copolymers on synthetic clay particles (laponite) has been investigated. The adsorbed amount and distribution of polymer was determined as a function of relative block composition and size, using the technique of contrast variation small-angle neutron scattering. The pluronic molecules appear to adsorb via a preferential segregation of hydrophobic PPO segments at the surface, with hydrophilic PEO segments dangling into solution. The effect of the PPO segments is substantial with large increases in adsorbed amount and layer thickness as the anchor fraction decreases/PEO chain length increases. This is in direct contrast to the behavior observed for PEO homopolymer adsorption (of much higher molecular weights) where the adsorbed amount and layer thickness are smaller and change little with molecular weight.