Network formation of catanionic vesicles and oppositely charged polyelectrolytes. Effect of polymer charge density and hydrophobic modification

In nonequimolar solutions of a cationic and an anionic surfactant, vesicles bearing a net charge can be spontaneously formed and apparently exist as thermodynamically stable aggregates. These vesicles can associate strongly with polymers in solution by means of hydrophobic and/or electrostatic interactions. In the current work, we have investigated the rheological and microstructural properties of mixtures of cationic polyelectrolytes and net anionic sodium dodecyl sulfate/didodecyldimethylammonium bromide vesicles. The polyelectrolytes consist of two cationic cellulose derivatives with different charge densities; the lowest charge density polymer contains also hydrophobic grafts, with the number of charges equal to the number of grafts. For both systems, polymer-vesicle association leads to a major increase in viscosity and to gel-like behavior, but the viscosity effects are more pronounced for the less charged, hydrophobically modified polymer. Evaluation of the frequency dependence of the storage and loss moduli for the two systems shows further differences in behavior: while the more long-lived cross-links occur for the more highly charged hydrophilic polymer, the number of cross-links is higher for the hydrophobically modified polymer. Microstructure studies by cryogenic transmission electron microscopy indicate that the two polymers affect the vesicle stability in different ways. With the hydrophobically modified polymer, the aggregates remain largely in the form of globular vesicles and faceted vesicles (polygon-shaped vesicles with largely planar regions). For the hydrophilic polycation, on the other hand, the surfactant aggregate structure is more extensively modified: first, the vesicles change from a globular to a faceted shape; second, there is opening of the bilayers leading to holey vesicles and ultimately to considerable vesicle disruption leading to planar bilayer, disklike aggregates. The faceted shape is tentatively attributed to a crystallization of the surfactant film in the vesicles. It is inferred that a hydrophobically modified polyion with relatively low charge density can better stabilize vesicles due to formation of molecularly mixed aggregates, while a hydrophilic polyion with relatively high charge density associates so strongly to the surfactant films, due to strong electrostatic interactions, that the vesicles are more perturbed and even disrupted.     

Altering associations in aqueous solutions of a hydrophobically modified alginate in the presence of beta-cyclodextrin monomers

The formation of associative networks in semidilute aqueous solutions of hydrophobically modified alginate (HM-alginate) is dependent on intermolecular hydrophobic interactions. Addition of beta-cyclodextrin (beta-CD) monomers to the system provides decoupling of these associations via inclusion complex formation with the polymer hydrophobic tails. This results in a dramatic decrease in the viscoelastic response of the system and a more extended local structure of the polymer chains, as shown by small-angle neutron scattering (SANS) measurements. The zero-shear viscosity decreases about an order of magnitude when the beta-CD concentration is increased from 0 to 12 mm. The lifetime of the associative network decreases strongly with increasing levels of beta-CD addition. These findings clearly demonstrate that the hydrophobic association effect is efficiently reduced as the amount of beta-CD is increased. In the framework of drug delivery, this effect may be useful to improve the release of therapeutic molecules that can be entrapped in the polymer matrix.     

Thermodynamic behavior of polyelectrolytes with the lower critical solution temperature (LCST) phenomenon

A series of vinyl polymers with L-valine and L-leucine residues, and related copolymers with N-isopropylacrylamide, were studied in aqueous solution at different temperatures (25, 30 and 35°C) and at two ionic strengths (0.01 M and 0.1 M NaCl). The protonation behavior revealed great differences between the polymers that were attributed to the size of the hydrophobic lateral group. Macromolecular shrinkage, occurring above a critical degree of protonation β, was related to hydrophobic forces outweighing the electrostatic repulsions between COO – groups. Low salt concentrations increased the electrostatic potential while high temperatures increased the hydrophobic interaction at lower β. The release of fewer water molecules structured around the polymer chain, responsible for the lower critical solution temperature phenomenon, revealed lower entropy changes at higher temperatures. The reversible configuration of graft polymer chains instantly responded to changes in pH and temperature, modifying the water filtration rates through the pores of cellulose membranes.     

Mechanisms behind the faceting of catanionic vesicles by polycations: chain crystallization and segregation

Vesicles composed of an anionic and a cationic surfactant, with a net negative charge, associate strongly with a hydrophobically modified polycation (LM200) and with an unmodified polycation with higher charge density (JR400), forming viscoelastic gel-like structures. Calorimetric results show that in these gels, LM200 induces a rise of the chain melting temperature (Tm) of the vesicles, whereas JR400 has the opposite effect. For both polymer-vesicle systems, the shear viscosity exhibits an inflection point at Tm, and for the LM200 system the measured relaxation times are significantly higher below Tm. The neat vesicles and the polycation-bound vesicles have a polygonal-like faceted shape when the surfactant chains in the bilayer are crystallized, as probed by cryo-transmission electron microscopy. Above Tm, the neat and the LM200-bound vesicles regain a spheroidal shape, whereas those in the JR400 system remain with a deformed faceted shape even above Tm. These shape changes are interpreted in terms of different mechanisms for the polymer-vesicle interaction, which seem to be highly dependent on polymer architecture, namely charge density and hydrophobic modification. A crystallization-segregation mechanism is proposed for the LM200-vesicle system, while, for the JR400-vesicle one, charge polarization-lateral segregation effects induced by the polycation in the catanionic bilayer are envisaged.     

Polyelectrolyte-surfactant complexes with long range order

Mixtures of carboxymethyl cellulose (CMC) or hydrophobically modified CMC with an oppositely charged surfactant (benzyldimethyltetradecylammonium chloride) in water were prepared. When the global polymer concentration is 0.18% by weight and the surfactant content is high enough, a precipitate with hexagonal order is formed. The precipitate composition shows practically constancy in its water content and a slight diminution in polymer concentration when the global surfactant content is varied between 0.9 and 23 wt%. The lattice parameter in this phase decreases when the polymer/surfactant ratio in the phase increases; this variation is faster with CMC than with the hydrophobically modified CMC. In this way electrostatic and hydrophobic interactions are far from being additive. From the extrapolation to infinite dilution, the global interaction seems to depend on the substitution degree in the polymer. Additionally, the comparison between the radius at the polar-apolar interface in the cylinders and the lattice parameter as a function of polymer/surfactant ratio in the hexagonal phase is compatible with some of the alkyl chains belonging to the hydrophobically modified CMC being present in the aqueous zone.     

Effects of added surfactants on thermoreversible gelation of associating polymer solutions

The influence of added surfactants on physical properties of associating polymer solutions was examined by a new statistical‐mechanical theory of associating polymer solutions with multiple junctions and by computer simulation. The sol–gel transition line, the spinodal line, and the number of elastically effective chains in the mixed networks were calculated as functions of the concentration of added surfactants. All of them exhibited nonmonotonic behavior as a result of the following two competing mechanisms. One was the formation of new mixed micelles by binding surfactants onto the polymer associative groups. These micelles serve as crosslink junctions and promote gelation. The other was the replacement of polymer associative groups in the already formed network junctions by added surfactants. Such replacement lowers connectivity of junctions and destroys networks. The critical micelle concentration was also calculated. The results are compared with the reported experimental data on poly(ethylene oxide)‐based associating polymers and hydrophobically modified cellulose derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 733–751, 2004     

Gelation of spontaneously formed catanionic vesicles by water soluble polymers

The gelation of two spontaneously formed charged catanionic vesicles by four water soluble polymers was systematically studied by tube inversion method and rheology. Eight phase maps were successfully documented for the catanionic vesicle–polymer mixtures. The experimental results, as represented by the relaxation time and the storage modulus at 1 Hz, revealed that the catanionic vesicle–polymer interactions at play were of electrostatic and hydrophobic origin. Firstly, no association between charged catanionic vesicles and the polymer without charge/hydrophobic modification was observed due to lack of both electrostatic and hydrophobic effects. Secondly, hydrophobic interactions accounted for the association between the hydrophobically modified polymer without charge and charged catanionic vesicles with hydrophobic grafts of the polymer inserting in the catanionic vesicle bilayer. Thirdly, the positively charged polymer without hydrophobic modification could interact with negatively charged catanionic vesicles through electrostatic force on one hand but could not interact with positively charged catanionic vesicles on the other hand. Finally, the positively charged polymer with hydrophobic modification could interact both electrostatically and hydrophobically with negatively charged catanionic vesicles, resulting in the formation of strong gels. The hydrophobic interaction might even overcome the unfavorable electrostatic interaction between the positively charged vesicles and the polymer with positive charge/hydrophobic modification.     

Investigating the effect of hydrophobic structural parameters on the thickening properties of HEUR associative copolymers

Hydrophobically modified ethoxylated urethanes (HEUR) associative copolymers are prepared through the hydrophobic modification of polyethylene glycol based polyurethanes. These types of thickeners are categorized as anionic associative thickeners. To investigate the effect of structure of the hydrophobic groups on the thickening properties, three different hydrophobic groups were selected. These groups were comprised of cetyl alcohol (16 carbons), dodecyl alcohol (12 carbons) and a cyclic group such as cyclo hexanol. These functional groups were substituted on the identical prepolymers made from H₁₂MDI and polyethylene glycol. Here, three different urethane associative polymers containing hydrophobic segments and different hydrophobic groups were synthesized. The viscoelastic characteristics of all the samples were determined using a cone and plate rheometer. The viscosities of the examined HEUR samples showed both Newtonian and non-Newtonian behaviours (shear thinning and thickening) for the explored shear rates window. The steady shear viscosity results were interpreted using the theories by Raspaud [Macromolecules 27 (1994) 2956] and Semenov [Macromolecules 28 (1995) 1066]. The Cox-Merz rule happened to be applicable to these systems for the various hydrophobic ends and concentrations at lower shear rates indicating typical associative polymer behaviour. The cyclic end groups did not show viscoelasticity for the frequencies window explored. The zero shear viscosity increased by almost two orders of magnitude from HEUR-cyc to HEUR-dod and by four orders of magnitude from HEUR-cyc to HEUR-cet. The intermediate shear thickening was only observed for the concentrated HEUR-cet samples.     

Structural and rheological properties of hydrophobically modified polysaccharide associative networks

The phase behavior of hydrophobically modified chitosans (HMCs) in aqueous solution has been investigated using scattering and rheology experiments. We observed four regions on the phase diagram of the associative polymer: (i) a supernatant phase (unimers phase) at low polymer concentration; (ii) a dilute solution of intermolecularly bridged flowerlike micelles at intermediate concentration; (iii) an associative gel phase at high polymer content; and (iv) a phase separation. In the present paper, we discuss the structural and dynamical properties of the HMC associative networks (c > c*) at a fixed hydrophobic degree of substitution of 2% and fixed alkyl side chains (stickers) length C8 (domains iii and iv of the phase diagram). As the polymer concentration is increasing, a connecting network is formed from the percolation of bridges between micellar aggregates. In this regime, small-angle neutron scattering and light scattering measurements show that -50-nm flower aggregates are acting like junction points in the network. The effect of the concentration, the stress, and the shear on the structure of the network is discussed. In particular, we observe bridge-to-loop transitions and then the formation of microgels or a low-connected network under shear. Therefore, our results are compared to recent theoretical models and to the results reported for telechelic systems.     

Vesicle--biopolymer gels: networks of surfactant vesicles connected by associating biopolymers

The effect of adding an associating biopolymer to surfactant vesicles and micelles is studied using rheology and small-angle neutron scattering (SANS). The associating polymer is obtained by randomly tethering hydrophobic alkyl chains to the backbone of the polysaccharide, chitosan. Adding this polymer to surfactant vesicles results in a gel; that is, the sample transforms from a Newtonian liquid to an elastic solid having frequency-independent dynamic shear moduli. SANS shows that the vesicles remain intact within the gel. The results suggest a gel structure in which the vesicles are connected by polymer chains into a three-dimensional network. Vesicle-polymer binding is expected to occur via the insertion of polymer hydrophobes into the vesicle bilayer. Each vesicle thus acts as a multifunctional junction in the network structure. Significantly, gel formation does not occur with the native chitosan that has no hydrophobes. Moreover, adding the hydrophobically modified chitosan to a viscous sample containing wormlike micelles increases the viscosity further but does not give rise to a gel-like response. Thus, the formation of a robust gel network requires both the presence of hydrophobes on the polymer and vesicles in solution.     

Dual cross-linked networks hydrogels with unique swelling behavior and high mechanical strength: based on silica nanoparticle and hydrophobic association

A series of physically cross-linked hydrogels composed poly(acrylic acid) and octylphenol polyoxyethylene acrylate with high mechanical strength are reported here with dual cross-linked networks that formed by silica nanoparticles (SNs) and hydrophobic association micro-domains (HAMDs). Acrylic acid (AA) and octylphenol polyoxyethylene acrylate with 10 ethoxyl units (OP-10-AC) as basic monomers in situ graft from the SNs surface to build poly(acrylic acid) hydrophilic backbone chains with randomly distributed OP-10-AC hydrophobic side chains. The entanglements among grafted backbone polymer chains and hydrophobic branch architecture lead to the SNs and HAMDs play the role of physical cross-links for the hydrogels network structure. The rheological behavior and polymer concentration for gelation process are measured to examine the critical gelation conditions. The correlation of the polymer dual cross-linked networks with hydrogels swelling behavior, gel-to-sol phase transition, and mechanical strength are addressed, and the results imply that the unique dual cross-linking networks contribute the hydrogels distinctive swelling behavior and excellent tensile strength. The effects of SNs content, molecular weight of polymer backbone, and temperature on hydrogels properties are studied, and the results indicate that the physical hydrogel network integrity is depended on the SNs and HAMDs concentration.     

Dynamics of supramolecular associative polymer networks at the interplay of chain entanglement,transient chain association,and chain‐sticker clustering

The dynamic mechanical properties of supramolecular associative polymer networks depend on the average number of entanglements along the network‐forming chains, N_e, and on their content of associative groups, f . In addition, there may be further influence by aggregation of the associative groups into clusters, which, in turn, is influenced by the chemical structure of these groups, and again by N_e and f of the polymer. Therefore, the effects of these parameters are interdependent. To conceptually understand this interdependency, we study model networks in which (a) N_e, (b) f , and (c) the chemical structure of the associative groups are varied systematically. Each network is probed by rheology. The clustering of the associative groups is assessed by analyzing the rheological data at the end range of frequency covered and by comparison of the number of supramolecular network junctions with the maximum possible number of binary transient bonds. We find that if the total number of the network junctions, which can be formed either by interchain entanglement or by interchain transient associations, is greater than a threshold of 13, then the likelihood of cluster formation is high and the dynamics of supramolecular associative polymer networks is mainly controlled by this phenomenon. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1209–1223     

Dynamics of entangled supramolecular polymer networks in presence of high‐order associations of strong hydrogen bonding groups

Dynamics of entangled polymer chains in the melt state are deliberately excluded in most of the studies on supramolecular polymer networks by utilizing nonentangled precursor chains. Relaxation of the system mainly depends on the dissociation of the associative groups in latter case and nonentangled chains deliver nothing to resist afterward. Conversely, in an entangled system, relaxation of polymer chains and dissociation of associative groups can occurred parallel. Supramolecular networks based on an entangled precursor polymer with different levels of strong associating ureidopyrimidinone (UPy) groups are synthesized to screen the corresponding effects on the dynamics of the system. Binary‐associated UPy groups phase separate into collective assemblies by stacking and form high‐order, needle‐like domains at higher UPy contents. Relaxation of polymer chains is significantly hindered due to the trapping of polymer segments between UPy stacks. Above a certain threshold of UPy content (~4 mol%), the plateau level and final relaxation time of networks show a significant jump, which is attributed to the onset of high‐order association of UPy groups.     

Superchaotropic Nano-ion Binding as a Gelation Motif in Cellulose Ether Solutions

Nanometer-sized anions (nano-ions) like polyoxometalates and boron clusters exhibit so-called superchaotropic behavior, which describes their strong binding to hydrated non-ionic matter in water. We show here that nano-ions, at millimolar concentrations, dramatically enhance the viscosity and induce gelation of aqueous solutions of non-ionic cellulose ethers (CEs), a class of widely utilized polymers known for their thickening and gel-forming ability. These phenomena arise from an interplay of attractive forces and repulsive electrostatic forces between CE-chains upon nano-ion binding. The attractive forces manifest themselves as aggregation of CE-chains into a physically crosslinked polymer network (gel). In turn, the electrostatic repulsions hamper the viscosity increase and gelation. Superchaotropic nano-ion binding emerges as a novel and general physical crosslinking motif for CE-solutions and exceeds by far the conventional thickening effects of classical salts and ionic surfactants.     

Water-Soluble Magnetic Nanocomposites Based on Carboxymethyl Cellulose and Iron(III) Oxide

The one-step synthesis of water-soluble composites from maghemite (γ-Fe₂O₃) nanoparticles with a diameter of 12 ± 4 nm and a biocompatible polysaccharide, namely, sodium salt of carboxymethyl cellulose, is described. The role of the polymer matrix consists in stabilization of the resulting nanoparticles by the electrostatic interaction of polymer carboxyl groups with the surface atoms of iron in the (3+) oxidation state. The dissolution of the composites in water affords aggregatively stable dispersions responding to the external magnetic field. The content of the magnetic phase (iron oxide) in the formulation of the maghemite–carboxymethyl cellulose composite is defined by the ratio of components during the synthesis.     

Microviscosity of hydrophobically modified hydroxyethyl cellulose aqueous solutions

The microviscosity of hydrophobically modified hydroxyethyl cellulose (HMHEC) aqueous solutions is experimentally determined by conductometry with added ions as probe. Compared to its bulk viscosity, the microviscosity of HMHEC solution could be lower by four orders of magnitude. Since the electric conductivity reduction of added NaCl is almost the same for HMHEC and its unmodified counterpart at an equal weight concentration, one can conclude that the hydrophobic modification for the polymer hardly has any effect on the solution's microviscosity.     

Synthesis and solution properties of amphiphilic cycloterpolymers of 1,1-diallyl-4-formylpiperizinium chloride, diallyloctadecylammonium chloride and sulfur dioxide

The cycloterpolymerizations of hydrophilic monomer (1,1-diallyl-4-formylpiperizinium chloride) with hydrophobic monomer (diallyloctadecylammonium chloride) and sulfur dioxide in dimethyl sulfoxide using azobisisobutyronitrile (AIBN) as the initiator afforded a series of water-soluble monocationic polyelectrolytes (MCPEs) in well over 90% yields. Acidic (HCl) hydrolysis of the N-formyl groups in MCPEs to NH₂⁺Cl⁻ groups gave the dicationic polyelectrolytes (DCPEs), which on treatment with one equivalent of NaOH furnished the basic cationic polyelectrolytes (BCPEs) containing basic as well as quaternary nitrogens. The solution properties of the resultant series of interconverting polyelectrolytes (MCPE → DCPE ↔ BCPE) were investigated by viscometric techniques. The polymer concentration C∗_HA of 0.2 g/dL was required for the manifestation of hydrophobic associations in a MCPE containing 4.5 mol% octadecyl pendents. The DCPEs exhibited sharp increase in viscosity in salt (NaCl)-added solution as compared to salt-free water. The pH-responsive BCPE is shown to demonstrate better associative properties in the absence of HCl; upon addition of HCl, the charge density increases in the polymer chains thereby resulting in increased electrostatic repulsions and decreased associations. Polymer-surfactant interactions were investigated using cationic surfactant cetyltrimethylammonium chloride (CTAC); tremendous increase in the viscosity values of the DCPE was observed in the presence of the surfactant.     

Rheological properties of hydrophobically modified alkali-soluble polymers—effects of ethylene–oxide chain length

The rheological properties of hydrophobic alkali-soluble associative polymers (HASE) were studied using controlled rate (Mettler LS40) and controlled stress (TA CSL 500) rheometers. The effects of pH and polymer concentrations on the rheological properties of three HASE model polymer systems (i.e., HASE 5141, 5134, and 5142, with a degree of ethoxylation of 2.5, 10, and 40 mol, respectively) and a reference polymer without associative hydrophobes (MAAEA) were examined. As the pH is increased by addition of ammonia to greater than 5–6, the carboxyl groups ionize to carboxylate ions and the polymers become water soluble. The HASE polymers thicken mainly by hydrophobic association. Viscosity can increase by two to three orders of magnitude as pH is raised to 9. The degree of ethoxylation in the macromonomer controls the nature of the hydrophobic association junctions by altering the flexibility and hydrophobicity of the macromonomer. Optimum thickening efficiency is observed in the system with approximately 10 mol of an ethylene–oxide spacer between the polymer backbone and the macromonomer. Viscoelastic study shows that the maximum thickening efficiency also corresponds to the dominant elastic property observed in the system with 10 mol of EO. All the model systems except the control system without hydrophobe exhibit strain thickening of the viscous and elastic components. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2275–2290, 1998     

Macromolecules in ordered media. II. A fluorescence study of the polymer-liposome association

The interactions of poly (4-vinyl pyridine) with dimyristoylphosphatidic acid-based liposomes were studied in heterogeneous solution systems. The polymer was dispersed in aqueous buffer solutions of small unilamellar vesicles and after a suitable period of incubation to raise the equilibrium, the experiments has been conducted as a function of polymer concentration and temperature. According to the measurements of the steady-state fluorescence spectra, changes in the intensities as well as shifts in the emission peak wavelength, has served as raw data to accounting for the polymer-vesicle interactions. Quantitative evaluation of the interaction has been done using a simple associating model with some approximations. The key parameters evaluated were the amount of polymer bound and the number of charged lipids on a membrane site as well as the number of effective charges of poly (4-vinyl pyridine) chains at the liposome interface.     

阴离子疏水缔合共聚物/阳离子蠕虫状胶束协同增粘自组装网络

将阴离子疏水缔合丙烯酰胺共聚物P(NaAMC14S-b-AM)与阳离子蠕虫状胶束十六烷基三甲基溴化铵/水杨酸钠(CTAB/NaSal)在水溶液中自组装制备了新型的缔合增粘体. 由稳态剪切和动态流变实验结果得出: 自组装体系在80 ℃下仍具有显著的协同增粘效应, 其流变行为符合Maxwell模型. 同蠕虫状胶束相比, 自组装体系的稳态模量G0、力学松弛时间τR和缠结点密度ν都有增加, 由此分析缔合体系中两组分间形成了相互缠结的网络结构, 在链缠结处共聚物主链上的疏水侧链嵌入到了蠕虫状胶束的内核.