Effect of poly(acrylic acid) block length distribution on polystyrene-b-poly(acrylic acid) block copolymer aggregates in solution. 2. A partial phase diagram

The first paper of the series, which focused on the effect of polydispersity on the self-assembly of block copolymer vesicles, showed that an increase in the width of the poly(acrylic acid) (PAA) block length distribution resulted in a decrease in the size of the vesicles formed. In this paper, the rest of the phase diagram is explored. For the present study, a series of polystyrene-b-poly(acrylic acid) copolymers of an identical polystyrene length of 325 units but of varying degrees of polymerization of PAA was synthesized. Mixtures of the copolymers were made to artificially broaden the molecular weight distribution of PAA at a constant number average of 48 in the polydispersity index (PDI) range of 1.1-3.3. The mixtures were dissolved in dioxane, and water was added slowly to predetermined amounts. Transmission electron microscopy was used to observe aggregate morphologies at different water contents and PAA PDIs. At low water contents, dynamic light scattering was also used to measure the sizes of the aggregates. A partial phase diagram as a function of the water content and PAA PDI was obtained. Large compound micelles and spherical micelles (average diameter of 40 nm) were found at low water contents; however, at a water content of 12% (w/w), a continuum of morphologies from spheres to rods to vesicles was found with increasing PAA PDI. In addition, each copolymer was investigated by itself under identical conditions to those used for the mixtures to determine if there was any segregation of the individual polymers into separate aggregates. No evidence for such segregation was found.     

Physicochemical properties of pH-controlled polyion complex (PIC) micelles of poly(acrylic acid)-based double hydrophilic block copolymers and various polyamines

The physicochemical properties of polyion complex (PIC) micelles were investigated in order to characterize the cores constituted of electrostatic complexes of two oppositely charged polyelectrolytes. The pH-sensitive micelles were obtained with double hydrophilic block copolymers containing a poly(acrylic acid) block linked to a modified poly(ethylene oxide) block and various polyamines (polylysine, linear and branched polyethyleneimine, polyvinylpyridine, and polyallylamine). The pH range of micellization in which both components are ionized was determined for each polyamine. The resulting PIC micelles were characterized using dynamic light scattering and small-angle X-ray scattering experiments (SAXS). The PIC micelles presented a core–corona nanostructure with variable polymer density contrasts between the core and the corona, as revealed by the analysis of the SAXS curves. It was shown that PIC micelle cores constituted by polyacrylate chains and polyamines were more or less dense depending on the nature of the polyamine. It was also determined that the density of the cores of the PIC micelles depended strongly on the nature of the polyamine. These homogeneous cores were surrounded by a large hairy corona of hydrated polyethylene oxide block chains. Auramine O (AO) was successfully entrapped in the PIC micelles, and its fluorescence properties were used to get more insight on the core properties. Fluorescence data confirmed that the cores of such micelles are quite compact and that their microviscosity depended on the nature of the polyamine. The results obtained on these core–shell micelles allow contemplating a wide range of applications in which the AO probe would be replaced by various cationic drugs or other similarly charged species to form drug nanocarriers or new functional nanodevices.     

Polystyrene-b-poly(tert-butyl acrylate) and polystyrene-b-poly(acrylic acid) dendrimer-like copolymers: two-dimensional self-assembly at the air-water interface

The two-dimensional self-assembly at the air/water (A/W) interface of two dendrimer-like copolymers based on polystyrene and poly(tert-butyl acrylate) (PS-b-PtBA) or poly(acrylic acid) (PS-b-PAA) was investigated through surface pressure measurements (isotherms, isochores, and compression-expansion hysteresis experiments) and atomic force microscopy (AFM) imaging. The two dendrimer-like block copolymers have an 8-arm PS core (Mn = 10 000 g/mol, approximately 12 styrene repeat units per arm) with a 16-arm PtBA (Mn = 230 000 g/mol, approximately 112 tert-butyl acrylate repeat units per arm) or PAA (Mn = 129 000 g/mol, approximately 112 acrylic acid repeat units per arm) corona. The PS-b-PtBA sample forms stable Langmuir monolayers and aggregates into circular surface micelles up to a plateau observed in the corresponding isotherm around 24 mN/m. Beyond this threshold, the monolayers collapse above the interface, resulting in the formation of large and irregular desorbed aggregates. The PS-b-PAA sample has ionizable carboxylic acid groups, and its A/W interfacial self-assembly was therefore investigated for various subphase pH values. Under basic conditions (pH = 11), the carboxylic acid groups are deprotonated, and the PS-b-PAA sample is therefore highly water-soluble and does not form stable monolayers, instead irreversibly dissolving in the aqueous subphase. Under acidic conditions (pH = 2.5), the PS-b-PAA sample is less water-soluble and becomes surface-active. The pseudoplateau observed in the isotherm around 5 mN/m corresponds to a pancake-to-brush transition with the PAA chains dissolving in the water subphase and stretching underneath the anchoring PS cores. AFM imaging revealed the presence of circular surface micelles for low surface pressures, whereas the biphasic nature of the pseudoplateau region was confirmed with the gradual aggregation of the micellar PS cores above the PAA chains. The aggregation numbers for both samples were estimated around 3-5 dendrimer-like copolymers per circular surface micelle. These rather low values confirmed the tremendous influence of molecular architecture on the two-dimensional self-assembly of block copolymers.     

Studies on Micellization of a Polystyrene-b-poly(acrylic acid) Copolymer in Aqueous Media by Pyrene Fluorescence

The aggregation of amphiphilic block copolymers is a crucial phenomenon that controls various applications, and has been extensively treated in the literature1-3. Pyrene is a widely used fluorescence probe for the relative intensities of the vibrational fine structure (five bands) of its fluorescence spectra have been found to be sensitive to the polarity of its environment, the so-called Ham effect4. We report the studies of the aggregation properties of a PS-b-PAA copolymer using pyrene …     

Synthesis and pH-sensitive micellization of doubly hydrophilic poly(acrylic acid)-b-poly(ethylene oxide)-b-poly(acrylic acid) triblock copolymer in aqueous solutions

A doubly hydrophilic triblock copolymer poly(acrylic acid)-b-poly(ethylene glycol)-b-poly(acrylic acid) (PAA-b-PEO-b-PAA) with M _w/M _n = 1.15 was synthesized by atom transfer radical polymerization of t-butyl acrylate (tBA), followed by acidolysis of the PtBA blocks. The pH-sensitive micellization of PAA-b-PEO-b-PAA in acidic solution was investigated by potentiometric titration, fluorescence spectrum, dynamic light scattering and zeta potential. The pK _a was 6.6 and 6.0 in deionized water and in 0.1 mol/L NaCl solution, respectively. The copolymer formed micelles composed of a weakly hydrophobic core of complexed PAA and PEO and a hydrophilic PEO shell in 1 mg/mL solution at pH < 5.5 due to hydrogen bonding. The critical micelle concentration was 0.168 mg/mL at pH 2.0. At pH < 4.5, steady and narrow distributed micelles were formed. Increasing pH to 5.0, unsteady and broad distributed micelles were observed. At pH > 5.5, the micelle was destroyed owing to the ionization of the PAA blocks.     

Synthesis of associating poly(acrylic acid) in supercritical carbon dioxide and its solution properties

Hydrophobically associating polymers have been synthesized in supercritical carbon dioxide by copolymerization of acrylic acid with different amounts of acrylate with hydrocarbon or fluorocarbon groups. It was found that conversion of hydrocarbon comonomers was above 95% whereas that for fluorocarbon comonomers was only about 50%. In addition, large amounts of hydrophobic groups could be easily introduced to poly(acrylic acid) by reaction in supercritical carbon dioxide. The solution properties were investigated by rheology. The results indicated that intermolecular association of the copolymer was strong and viscosity was maximum under acidic conditions. In aqueous solutions fluorocarbon hydrophobes associated much more strongly than the hydrocarbon variety, but the viscosifying effect of PAAC-18 series copolymers in 2% (w/w) solution was more pronounced than that of the PAAF series, results which did not agree with the conclusions of Ravey and Stébé. It was also found that the thixotropy behavior of copolymer solution at pH 3.2 was more complex than that at pH 5.0, at which pseudoplasticity only was observed for solutions of all copolymers. Contact angles of copolymer solutions on a glass sheet were measured. The data indicated that contact angles of hydrocarbon-modified polymers were smaller than those of fluorocarbon analogues. As time passed the contact angle became smaller and smaller. Fluorocarbon analogues were better than hydrocarbon analogues, and longer hydrophobic chains were better than shorter chains, at maintaining the hydrophobic character of the surface.     

Hydrophobically modified polyelectrolytes I. Dilute solution properties of fluorocarbon-containing poly (acrylic acid)

Dilute solution viscosity of fluorocarbon‐containing hydrophobically modified poly (acrylic add) was measured in aqueous solutions of various NaCl concentrations. The intrinsic viscosity ([η]) and Huggins coefficient (k_H) were evaluated using Huggins equations. It is found that, at low Nacl concentration, the modified polymers exhibit values of intrinsic viscosity ([η]) and Huggins coefficient (k_H) similar to those of unmodified polymers. For both of the modified and unmodified polymers, the intrinsic viscosity decreases with increase of NaCl concentration, while the Huggins coefficient increases upon addition of NaCl. But the variation of [η] and k_H is more significant for the modified polymers, which reflects the enhanced intra‐ and intermolecular hydrophobic association at higher Nacl concentration.     

Thermoresponsive micelles from Jeffamine-b-poly(L-glutamic acid) double hydrophilic block copolymers

Double hydrophilic block copolymers (DHBC) consisting of a Jeffamine block, a statistical copolymer based on ethylene oxide and propylene oxide units possessing a lower critical solution temperature (LCST) of 30 degrees C in water, and poly(L-glutamic acid) as a pH-responsive block were synthesized by ring-opening polymerization of gamma-benzyl-L-glutamate N-carboxyanhydride using an amino-terminated Jeffamine macroinitiator, followed by hydrolysis. This DHBC proved thermoresponsive as evidenced by dynamic light scattering and small-angle neutron scattering experiments. Spherical micelles with a Jeffamine core and a poly(L-glutamic acid) corona were formed above the LCST of Jeffamine. The size of the core of such micelles decreased with increasing temperature, with complete core dehydration being achieved at 66 degrees C. Such behavior, commonly observed for thermosensitive homopolymers forming mesoglobules, is thus demonstrated here for a DHBC that self-assembles to generate thermoresponsive micelles of high colloidal stability.     

Calorimetric investigation of poly(methacrylic acid) and poly(acrylic acid) in aqueous solution

The enthalpy of dissociation of poly(acrylic acid) and of poly(methacrylic acid) in water and in 0.5N NaCl at 25°C has been measured over a wide range of degrees of neutralization of the polyelectrolytes. In the case of poly(methacrylic acid) the calorimetric data permit the direct evaluation of the enthalpy of conformational transition of the polymer. For this transition, with the aid of standard free energy data derived from potentiometric titrations, the change in entropy was also estimated. The relative accuracy of the thermodynamic data, and the possibility of deriving therefrom information on the mechanism of transitions of the type, globular coils → expanded coils for partially hydrophobic synthetic polyelectrolytes in aqueous solution are discussed.     

Pulse radiolysis of poly(acrylic acid) in deoxygenated aqueous solution

Absorption spectra of macroradicals formed by irradiation of poly (acrylic acid) in aqueous solution are presented and the role of intermediates of water radiolysis in their formation is discussed. Both the rate constant of ·OH reaction with the polymer and the process of macroradicals decay are shown to be strongly dependent on pH. The macroradicals formed in dissociated polymer molecules have a lifetime of several minutes.     

The surface tension of poly(acrylic acid) in aqueous solution

Summary Surface tension has been measured of polyacrylic acidwater solutions with pH 3–11. Until approximately pH 4.6 corresponding to 10% neutralization, PAA-solutions are surface active and inactive at higher pH, time dependence of equlibrium surface tension is analysed. Non-ionized neutral chain segments are surface active, their diffusion to surface is rate determining.

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Zusammenfassung Es wurde die Oberflächenspannung von wäßrigen Polyacrylsäure-Lösungen der pH-Werte von 3–11 gemessen. Bis zu einem pH-Wert von etwa 4.6, d. h. 10% Neutralisation, sind die Lösungen oberflächenaktiv, bei höheren pH-Werten inaktiv. Es wird die Zeitabhängigkeit der Einstellung der Gleichgewichtsoberflächenspannung analysiert. Nicht ionisierte, neutrale Kettensegmente sind oberflächenaktiv. Diffusion zur Oberfläche ist der zeitbestimmende Faktor.

     

Synthesis and solution properties of comblike polymers from octadecyl methacrylate and acrylic acid

Amphiphilic polymers consisting of a statistical distribution of octadecyl methacrylate (ODMA) and acrylic acid in respective molar ratios of 83-22 and 17-78 mol% and in a molecular-weight range of 2.35-4.70᎒⁴ gmol^-1 have been synthesized. The series of polymers consisting of various mole fractions of ODMA and acrylic acid are expected to exhibit unique characteristics resembling ionomer to hydrophobically modified polyelectrolytes. The changes in the I₃/I₁ emission intensity ratios of pyrene, occurring in the presence of tetrahydrofuran (THF) solutions of the polymers have been taken as the main basis for inferring solution structures. The polymers are found to form random-coil to collapsed-coil/aggregated structures in THF solvent depending on the copolymer compositions. The polymer consisting of 83 mol% ODMA and 17 mol% acrylic acid behaves as an ionomer, capable of forming collapsed-coil structures at concentrations of 0.02 gml^-1 and above as shown by a very high I₃/I₁ of 1.20 (I₃/I₁ of pyrene in THF is 0.85). In contrast, the poly(octadecyl methacrylate) homopolymer and the sets of copolymers consisting of a very high proportion of acrylic acid to an extent of 73 mol% and above contribute to almost negligible or very small changes in I₃/I₁ similar to the homopolymer, poly(octadecyl methacrylate), suggesting the formation of random-coil structures.     

Poly(N-isopropylacrylamide)-block-poly(acrylic acid) hydrogels: synthesis and rapid thermoresponsive properties

In this contribution, we reported the synthesis of poly(N-isopropylacrylamide)-block-poly(acrylic acid) (PNIPAAm-b-PAA) copolymer networks via sequential reversible addition-fragmentation chain transfer (RAFT) polymerization. The PNIPAAm-b-PAA block copolymer networks were characterized by means of Fourier transform infrared spectroscopy (FTIR) and small angle X-ray scattering (SAXS). The volume phase transition (VPT) temperatures of the PNIPAAm-b-PAA hydrogels were measured by means of micro-differential scanning calorimetry (micro-DSC). It was found that the block copolymer hydrogels displayed the VPT temperatures lower than the control PNIPAAm hydrogel. Compared to the control PNIPAAm hydrogel, the deswelling and reswelling properties of the block copolymer hydrogels were significantly improved. The improved thermoresponsive properties of the PNIPAAm-b-PAA hydrogels have been interpreted on the basis of the formation of the architecture of the block copolymer networks.     

Preparation and properties of poly(acrylic acid) oligomer stabilized superparamagnetic ferrofluid

Ferrofluids, which are stable dispersions of magnetic particles, behave as liquids that have strong magnetic properties. Nanoparticles of magnetite with a mean diameter of 10-15 nm, which are in the range of superparamagnetism, are usually prepared by the traditional method of co-precipitation from ferrous and ferric electrolyte solution. When diluted, the ferrofluid dispersions are not stable if anionic or cationic surfactants are used as the stabilizer. This work presents an efficient way to prepare a stable aqueous nanomagnetite dispersion. A stable ferrofluid containing Fe3O4 nanoparticles was synthesized via co-precipitation in the presence of poly(acrylic acid) oligomer. The mechanism, microstructure, and properties of the ferrofluid were investigated. The results indicate that the PAA oligomers promoted the nucleation and inhibited the growth of the magnetic iron oxide, and the average diameter of each individual Fe3O4 particle was smaller than 10 nm. In addition, the PAA oligomers provided both electrostatic and steric repulsion against particle aggregation, and the stability of dispersions could be controlled by adjusting the pH value of solution. A small amount of Fe2O3 was found in the nanoparticles but the superparamagnetic behavior of the nanoparticles was not affected.     

Highly efficient monolithic silica capillary columns modified with poly(acrylic acid) for hydrophilic interaction chromatography

Monolithic silica capillary columns for hydrophilic interaction liquid chromatography (HILIC) were prepared by on-column polymerization of acrylic acid on monolithic silica in a fused silica capillary modified with anchor groups. The products maintained the high permeability (K=5 x 10(-14)m(2)) and provided a plate height (H) of less than 10 microm at optimum linear velocity (u) and H below 20 microm at u=6mm/s for polar solutes including nucleosides and carbohydrates. The HILIC mode monolithic silica capillary column was able to produce 10000 theoretical plates (N) with column dead time (t(0)) of 20s at a pressure drop of 20 MPa or lower. The total performance was much higher than conventional particle-packed HILIC columns currently available. The gradient separations of peptides by a capillary LC-electrospray mass spectrometry system resulted in very different retention selectivity between reversed-phase mode separations and the HILIC mode separations with a peak capacity of ca. 100 in a 10 min gradient time in either mode. The high performance observed with the monolithic silica capillary column modified with poly(acrylic acid) suggests that the HILIC mode can be an alternative to the reversed-phase mode for a wide range of compounds, especially for those of high polarity in isocratic as well as gradient elution.     

Association of octyl-modified poly(acrylic acid) onto unilamellar vesicles of lipids and kinetics of vesicle disruption

Water-soluble polymers containing a few hydrophobic anchors are known to bind onto lipid vesicles and are used as stabilizers of liposome-based formulas. In contrast, polymers with high hydrophobicity destabilize the lipid bilayers. With macromolecules of intermediate hydrophobic/hydrophilic balance, a gradual sweep of the stabilization-destabilization capacity can be achieved and is considered as promising triggered systems for drug release, although the mechanism of permeabilization and membrane breakage using polymers is essentially conjectural to date. As a model system, we used short octyl-modified poly(acrylic acid)s (MW 8000 g/mol) sensitive to pH, temperature, and ionic strength in conjunction with small unilamellar vesicles mainly comprised of DPPC or egg-PC. Kinetics of vesicle fragmentation was followed using static and dynamic light scattering. Polymer adsorption was studied by nonradiative energy transfer between pyrene-labeled lipids and a naphthalene-modified polymer. The permeability of the vesicles was characterized by calcein leakage experiments. The key findings were (i) the lack of coupling between the density of bound polymer and the rate of disruption and (ii) the qualitative difference depending on whether the polymer contains or not isopropyl side groups. Point i relates to the increase of the rate of polymer adsorption with increasing bulk polymer concentration, while the breakage is essentially unaffected. Point ii relates to the stabilization of large membrane fragments (Stokes radius ca. 40 nm) in the presence of a polymer with no isopropyl side groups, while micelle-like assemblies (Stokes radius 8 nm) containing the lipids are obtained with an isopropyl-containing polymer of similar hydrophobicity. Both polymers display similar efficiency at disrupting small vesicles. The mechanism of polymer-induced disruption appears to differ markedly from the disruption steps now recognized for conventional (molecular) surfactant and is discussed on the basis of data obtained with different membrane fluidity, polymer structure, concentration, and hydrophilicity.     

Rheology and phase behavior of poly(n-butyl acrylate)-block-poly(acrylic acid) in aqueous solution

The surface activity and the rheological properties of aqueous solutions of the amphiphilic block copolymer poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBA-b-PAA) were studied as a function of the degree of neutralization, alpha, of the poly(acrylic acid) block. Although the block copolymer spontaneously forms spherical micelles having a stretched PAA corona and a collapsed PnBA core in water for alpha > 0.1, the solutions do not exhibit any surface activity at this degree of neutralization. Cryo-TEM micrographs show that the radii of the hydrophobic core of the largest micelles are as long as the length of the hydrophobic chain. The micelles, however, have a broad size distribution, and on average, as shown by SANS, the micelles are only about half as long. At concentrations as low as 1 wt %, the solutions exhibit highly viscoelastic behavior and have a yield stress value depending on alpha. The globular micelles are highly ordered in the bulk phase, and the viscoelastic properties are a result of the dense packing of the micelles. The addition of salt or cationic surfactants dramatically decreases the viscosity of the solution. The observed properties seem to be due to electrostatic interactions between the PAA chains of the micelles.