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Steady shear properties and linear and nonlinear viscoelastic behaviors of a poly(methyl methacrylate)–poly(dimethyl amino ethyl methacrylate)–poly(methyl methacrylate) polymer, (PMMA–PDMAEMA–PMMA), telechelic polymers in salt-free aqueous solution have been investigated as a function of concentration and pH. Above a critical concentration, a transient physical network is formed through an association mechanism between hydrophobic end groups, leading to a gel-like behavior. The gel-like polymer solutions were shown to exhibit a peculiar flow behavior, associated with time fluctuation of the transient first normal stress difference, attributed to orientation effects of the stiff charged polymer chains. The viscoelastic behavior was shown to be governed by two pH dependent time scales: a short time scale controlled by the lifetime of the hydrophobic associative junctions and a long time scale corresponding to the network relaxation time. All rheological results show strong evidence that Coulomb interactions, which control both macromolecular chain rigidity and inter-chain interactions, lead to specific pH-tunable properties of great potential interest.

We explore the self-assembly behavior of aqueous solutions of an amphiphilic, pH-sensitive poly(l-alanine)-b-poly(l-glutamic acid)-b-poly(l-alanine), (A5E11A5) triblock copolypeptide, end-capped by benzaldehyde through Schiff base reaction. At elevated concentrations and under physiological pH (7.4) and ionic strength (0.15M), the bare copolypeptide aqueous solutions underwent a sol-gel transition after heating and slow cooling thermal treatment, forming opaque stiff gels due to a hierarchical self-assembly that led to the formation of β-sheet-based twisted super fibers (Popescu et al. Soft Matter 2015, 11, 331-342). The conjugation of the N-termini with benzaldehyde (Bz) through a Schiff base reaction amplifies the copolypeptide pH-sensitivity within a narrow pH window relevant for in vivo applications. Specifically, the dynamic character of the imine bond allowed coupling/decoupling of the Bz upon switching pH. The presence of Bz conjugates to the N-termini of the copolypeptide resulted in enhanced packing of the elementary superfibers into thick and short piles, which inhibited the ability of the system for gelation. However, partial cleavage of Bz upon lowering pH to 6.5 prompted recovery of the hydrogel. The sol-gel transition triggered by pH was reversible, due to the coupling/decoupling of the benzoic-imine dynamic covalent bonding, endowing thus the gelling system with injectability. Undesirably, the gelation temperature window was significantly reduced, which however can be regulated at physiological temperatures by using a suitable mixture of the bare and the Bz-conjugated coplypeptide. This triblock copolypeptide gelator was investigated as a scaffold for the encapsulation of polymersome nanocarriers, loaded with a hydrophilic model drug, calcein. The polymersome/polypeptide complex system showed prolonged probe release in pH 6.5, which is relevant to extracellular tumor environment, rendering the system potentially useful for sustained delivery of anticancer drugs locally in the tumor.

Multicompartmental responsive microstructures with the capability for the pre-programmed sequential release of multiple target molecules of opposite solubility (hydrophobic and hydrophilic) in a controlled manner have been fabricated. Star block copolymers with dual-responsive blocks (temperature for poly(N-isopropylacrylamide) chains and pH for poly(acrylic acid) and poly(2-vinylpyridine) arms) and unimolecular micellar structures serve as nanocarriers for hydrophobic molecules in the microcapsule shell. The interior of the microcapsule can be loaded with water-soluble hydrophilic macromolecules. For these dual-loaded microcapsules, a programmable and sequential release of hydrophobic and hydrophilic molecules from the shell and core, respectively, can be triggered independently by temperature and pH variations. These stimuli affect the hydrophobicity and chain conformation of the star block copolymers to initiate out-of-shell release (elevated temperature), or change the overall star conformation and interlayer interactions to trigger increased permeability of the shell and out-of-core release (pH). Reversing stimulus order completely alters the release process.

We report the synthesis and characterization of a C-b-(A-co-B)-b-C triblock terpolymer gelator, named telechelic polyampholyte, bearing a P(DEAEMA-co-MAA) statistical polyampholyte as the central long block, end-capped at both ends by PMMA blocks. This associative polymer forms 3D networks in aqueous media, composed of PMMA hydrophobic nanodomains, bridged by the hydrophilic P(DEA-co-MAA) ionogenic blocks. Thanks to the weak acid/base nature of the oppositely charged ionic moieties, the ionization degree of which are controlled by pH, the formed hydrogels exhibit unique pH-responsive behavior. The network structure and in turn the hydrogel rheological properties strongly depend on the preparation conditions, i.e. swelling film or direct dissolution in aqueous media.

We investigate the influence of ionic strength on the structural properties of stimuli-responsive hydrogels from triblock polyampholytes PAA-b-P2VP-b-PAA (PAA and P2VP are negatively charged poly(acrylic acid) and positively charged poly(2-vinylpyridine)). In our previous studies, we found that the transition behavior depends on the charge asymmetry which is controlled by pH and which alters the degree of ionization of the two types of blocks [Dyakonova Macromolecules 2014, 47, 7561]. The same triblock polyampholyte, but with chemically quaternized P2VP (QP2VP) instead of P2VP as the middle block, is highly positively charged, independently of pH. In the present investigation, PAA-b-P2VP-b-PAA at pH 3 and PAA-b-PQ2VP-b-PAA at pH 5 were chosen to investigate the influence of the ionic strength on the micellar network morphology by adding NaCl at concentrations in the physiological range. Computer simulations of the latter system show that salt addition results in the formation of larger complexes due to in...

Multifunctional star-graft quarterpolymers PSn[P2VP-b-(PAA-g-PNIPAM)]n with two different arm types, shorter PS arms and longer P2VP-b-PAA block copolymer arms with grafted PNIPAM chains, were studied in terms of their ability to form micellar structures at the air/water and air/solid interfaces. Because of the pH-dependent ionization of P2VP and PAA blocks, as well as thermoresponsiveness of PNIPAM chains, these multifunctional stars have multiple responsive properties to pH, temperature, and ionic strength. We observed that the molecular surface area of the stars is the largest at basic pH, when the PAA blocks are strongly charged and extended, and PNIPAM chains are spread at the interface. At acidic conditions, the molecular surface area is the smallest because the P2VP blocks submerge into the water subphase and the PAA blocks are contracted and form hydrogen bonding with grafted PNIPAM chains. The molecular surface area of the stars at the air/water interface gradually increases at elevated temperatu...

In this work, the self-assembly behavior and pH responsiveness of a triblock copolypeptide in aqueous media are demonstrated. The copolypeptide was composed of a central pH responsive poly(l-glutamic acid) (PGA), flanked by two hydrophobic poly(l-alanine) blocks (PAla) (PAla5-PGA11-PAla5). This system showed a pH-responsive transition from short tapes to spherical aggregates by increasing the pH, as a result of deprotonation of the PGA block and a conformational change from α-helix to random coil. Increasing the ionic strength to physiological conditions (0.15 M) has triggered fibrillar self-assembly through intermolecular hydrogen bonding of PAla end-blocks that form β-sheet nanostructures, in conjunction with charge screening of the central random coil PGA segments. At elevated concentrations a thermo-responsive free supporting hydrogel was obtained, consisting of rigid β-sheet based twisted superfibers, resulting from hierarchical self-assembly of the copolypeptide. Yet, morphological transformation of this nanostructure was observed upon switching the pH from physiological conditions to pH 4. An unexpected morphology constituted of α-helix-based giant nanobelts was observed as a consequence of the secondary peptide transitions.

We investigate the influence of pH on the rheological and structural properties of hydrogels formed by hydrophobic association of the sticky ends of the triblock terpolymer poly(methyl methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate-co-methacrylic acid)-b-poly(methyl methacrylate) (PMMA-b-P(DEA-co-MAA)-b-PMMA). The middle block is a weak polyampholyte having a pH dependent charge density and sign, which enables tuning of the rheological and structural properties by pH variation. Small-angle neutron scattering (SANS) studies of solutions in D2O at 0.05 wt% and pH 3.0 reveal clusters of interconnected spherical micelles having PMMA cores, stabilized by repulsive ionic interactions in the middle polyampholyte block. With increasing pH, the degree of ionization of the DEA units decreases, whereas the one of the MAA units increases, resulting in a complete loss of the correlation between these micelles. At a concentration of 3 wt% at low pH values, the system forms a gel with charged fuzzy spheres from PMMA interacting via a screened Coulomb potential. With increasing pH, the gel disintegrates due to the decrease in the effective charge on the micelles. At both concentrations, the hydrophobic aggregation of micelles is observed near the isoelectric point. At pH 3.0–7.4, the autocorrelation functions measured by rotational dynamic light scattering at 3 wt% exhibit a decay steeper than single exponential, which confirms that the gels are frozen, presumably due to the glassy PMMA cores and hydrophobic interpolyelectrolyte complexes. At pH 11, the diffusion of single micelles is observed in addition to the frozen dynamics.

We investigate the conformational properties of stimuli-responsive hydrogels from triblock polyelectrolytes PtBA-b-P2VP-b-PtBA (PtBA and P2VP are poly(tert-butyl acrylate) and poly(2-vinylpyridine)) and the corresponding polyampholytes PAA-b-P2VP-b-PAA (PAA is poly(acrylic acid)), the latter with nonquaternized or quaternized P2VP blocks. The block lengths are the same in all three polymers with relatively short end blocks and long middle blocks. The mechanical properties of the hydrogels have previously been found to depend strongly on the pH value and on the nature of the blocks (Polymer 2008, 49, 1249). Here, we present results from rheological studies and small-angle neutron scattering revealing the underlying hydrogel structures. The hydrogel structure of the polyampholyte depends on the charge asymmetry, controlled by the pH value, and reveals several transitions with increasing charge ratio. A low charge asymmetry causes the collapse of the chains into large globular structures due to the fluctuati...

The self-assembly of an amphiphilic pentablock terpolymer, consisting of a pH-sensitive poly(2-vinylpyridine) (P2VP) central block, bearing at both ends hydrophilic poly(ethylene oxide) (PEO) end-capped with hydrophobic poly(e-caprolactone) (PCL) blocks was investigated in aqueous media. When the polymer was transferred to aqueous medium through solvent exchange technique, micellar assemblies were obtained, with a morphological switch from “flower-like” crew-cut micelles to large compound vesicles upon partial quaternization of 2VP block. More importantly, well-defined polymer vesicles were formed when thin film hydration method was employed. Such polymersomes presented “flower-like” morphology, with a membrane composed of self-assembled P2VP hydrophobic central blocks shielded by PCL hydrophobic ends and surrounded by PEO looping chains at the inner and outer surfaces. The potential of the pentablock vesicles as drug delivery carriers was explored by encapsulation of calcein, a model hydrophilic drug, in the polymersomes and monitoring its release at different pH values simulating physiological environments. It is shown that the release profile of the nanoparticle formulations could be modulated by tuning the membrane permeability either through chemical (i.e. partial quaternization) and/or physical (i.e. pH variations) pathways.

The present review article highlights a specific class of responsive polymer-based hydrogels which are formed through association of oppositely charged polyion segments. The underpinning temporary three-dimensional network is constituted of hydrophilic chains (either ionic or neutral) physically crosslinked by ion pair formation arising from intermolecular polyionic complexation of oppositely charged repeating units (polyacid/polybase ionic interactions). Two types of hydrogels are presented: (i) hydrogels formed by triblock copolymers bearing oppositely charged blocks (block copolyampholytes), forming self-assembled networks; and (ii) hydrogels formed by co-assembly of oppositely charged polyelectrolyte segments belonging to different macromolecules (either block copolymers or homopolyelectrolytes). Due to the weak nature of the involved polyions, these hydrogels respond to pH and are sensitive to the presence of salts. Discussing and evaluating their solution, rheological and structural properties in dependence on pH and ionic strength, it comes out that the hydrogel properties are tunable towards potential applications.

We present the layer-by-layer (LbL) assembly of amphiphilic heteroarm pH-sensitive star-shaped polystyrene-poly(2- pyridine) (PSnP2VPn) block copolymers to fabricate porous and multicompartmental microcapsules. Pyridine-containing star molecules forming a hydrophobic core/hydrophilic corona unimolecular micelle in acidic solution (pH 3) were alternately deposited with oppositely charged linear sulfonated polystyrene (PSS), yielding microcapsules with LbL shells containing hydrophobic micelles. The surface morphology and internal nanopore structure of the hollow microcapsules were comparatively investigated for shells formed from star polymers with a different numbers of arms (9 versus 22) and varied shell thickness (5, 8, and 11 bilayers). The successful integration of star unimers into the LbL shells was demonstrated by probing their buildup, surface segregation behavior, and porosity. The larger arm star copolymer (22 arms) with stretched conformation showed a higher increment in shell thickness due to the effective ionic complexation whereas a compact, uniform grainy morphology was observed regardless of the number of deposition cycles and arm numbers. Small-angle neutron scattering (SANS) revealed that microcapsules with hydrophobic domains showed different fractal properties depending upon the number of bilayers with a surface fractal morphology observed for the thinnest shells and a mass fractal morphology for the completed shells formed with the larger number of bilayers. Moreover, SANS provides support for the presence of relatively large pores (about 25 nm across) for the thinnest shells as suggested from permeability experiments. The formation of robust microcapsules with nanoporous shells composed of a hydrophilic polyelectrolyte with a densely packed hydrophobic core based on star amphiphiles represents an intriguing and novel case of compartmentalized microcapsules with an ability to simultaneously store different hydrophilic, charged, and hydrophobic components within shells.

We report the pH-controlled surface behavior of amphiphilic PS n (P2VP- b -PAA) n heteroarm star block terpolymers, ( n = number of arms, 9 and 22), bearing amphoteric diblock arms with varying polyvalent ionic charges (i.e. negative, positive, and zwitterionic) at the air–water and air–solid interfaces. We investigated the assembly of these pH-sensitive star terpolymers in Langmuir and Langmuir–Blodgett monolayers under different pH conditions of the subphase. The π-A isotherms acquired at variable pH conditions revealed a distinct aggregation behavior of surface micelles which is dependent on the ionization of the polyelectrolyte blocks and the number of arms. The star block copolymer with a small number of arms ( n = 9) was found to exhibit a strong pH-dependent phase transformation under compression. The pH responsive (zwitterionic) behavior results in changes in surface morphologies from circular micelles to complex labyrinth structures. In contrast, star polymers with a larger number of arms (22) and a crowded branched architecture show stable circular domain morphology without the internal reorganization under variable conditions. The observed variety of surface behaviors is attributed to the fine balance of intra molecular interactions caused by the highly branched chain architecture composed of both acidic and basic blocks. This study proposes a novel strategy for pH induced tunable surface assembly of responsive ultrathin films with multicompartments. We indeed demonstrated that the enhanced responsiveness of star polymers to wide pH range and their diverse morphology at the interface is enabled by coupling amphoteric concept and tailored molecular architecture.

We present a unique multisegmented, heteroarm star block terpolymer that can be used as an effective dispersing agent for multiwalled carbon nanotubes in aqueous media. The (polystyrene)22(poly(2-vinylpyridine)-b-poly(acrylic acid))22, PS22(P2VP-b-PAA)22, star terpolymer exhibits a pH responsive self-assembly behavior in aqueous solution, due to the ampholytic nature of its diblock copolymer arms. Depending on the pH of the medium, two different self-assemblies were observed. At a low pH (pH 2), core–shell unimolecular micelles, with a hydrophobic PS core and P2VP, PAA segments in the shell, as well as multicore large compound micelles were formed. At pH 8.5, the star was transformed from a bis-hydrophilic into a bis-hydrophobic state, leading to a network-like intermicellar assembly. This star terpolymer was capable of dispersing MWCNTs, through non-covalent interactions, thus leading to stable MWCNT–star nanohybrids in water. More importantly, the resulted MWCNT–star nanohybrids adopted the “smart” pH responsive properties of the physisorbed stars, i.e. the nanohybrids can be positively or negatively charged at low and high pH respectively, while at an intermediate pH window, phase separation occurs due to neutralization of the oppositely charged segments.

We report on the growth of gold nanoparticles in polystyrene/poly(2-vinyl pyridine) (PS/P2VP) star-shaped block copolymer monolayers. These amphiphilic PS(n)P2VP(n) heteroarm star copolymers differ in molecular weight (149,000 and 529,000 Da) and the number of arms (9 and 28). Langmuir-Blodgett (LB) deposition was utilized to control the spatial arrangement of P2VP arms and their ability to reduce gold nanoparticles. The PS(n)P2VP(n) monolayer acted as a template for gold nanoparticle growth because of the monolayer's high micellar stability at the liquid-solid interface, uniform domain morphology, and ability to adsorb Au ions from the water subphase. UV-vis spectra and AFM and TEM images confirmed the formation of individual gold nanoparticles with an average size of 6 ± 1 nm in the P2VP-rich outer phase. This facile strategy is critical to the formation of ultrathin polymer-gold nanocomposite layers over large surface areas with confined, one-sided positioning of gold nanoparticles in an outer P2VP phase at polymer-silicon interfaces.

This article is dealing with the design of novel segmented polymers comprising homopolymer and random copolymer building blocks designated as block-random. This type of polymeric materials can be prepared through macromolecular engineering by using controlled polymerization methods. By replacing a homopolymer block with a random one, in block copolymer topologies, further tuning of the copolymer properties can be achieved. The present article highlights the recent developments on block-random segmented macromolecules, bearing building blocks of tunable properties (e.g. thermo-sensitivity (LCST), hydrophobicity) and exhibiting responsive behavior in aqueous environments. Furthermore, preliminary novel results regarding pH-sensitive segmented macromolecules of various topologies, bearing random polyampholyte blocks among others, are also demonstrated and discussed.

In this communication an extended ‘‘in–out’’ polymerization method is presented, which leads to the synthesis of novel heteroarm star block terpolymers of the type An(B-b-C)n. A four step/one-pot synthetic procedure is pursued using anionic polymerization under an inert atmosphere. The resulted star-shaped terpolymer consists of a divinyl benzene nodule bearing pure polystyrene and poly(hexyl methacrylate)-block-poly(methyl methacrylate) diblock copolymer arms. It is shown that this kind of star terpolymers can self-assemble in the bulk forming lamellae mesophase by arm and block segregation. The mechanical properties of the terpolymer have been examined in detail. Finally, the proposed synthetic procedure can be easily employed in other controlled polymerization methods.

We report the surface behavior and morphologies of two series of pH-responsive amphiphilic heteroarm star block copolymers. In this respect, we studied polystyrene/poly(2-vinylpyridine)/poly(tert-butylacrylate), PSn(P2VP-PtBA)n, heteroarm star block terpolymers and their precursors, PSnP2VPn, star copolymers. These star block polymers differ in architecture (copolymer vs terpolymer), block topology, arm length (molecular weight of PtBA segments varies from 8 900 up to 15 250 Da), and number of arms (n = 9, 22, and 28). The π−A isotherms at different subphase pH (pH 5.8 and 2.0) exhibited strong pH dependence, leading to different limiting molecular area and surface micelle stability. Because of the pH-induced ionization of the P2VP block, the surface morphology of star copolymers bearing the free P2VP arms was strongly dependent on the pH of the subphase, whereas the star terpolymers containing the protonated hydrophilic P2VP block as midblocks maintained the same circular morphology at low pH and high pr...

This work is focused on the self-organization of an heteroarm star copolymer consisting of 5 polystyrene and 5 poly(2-vinylpyridine) (P2VP) arms emanated from a poly(divinyl benzene) core and the chemical stabilization of the resulting supramolecular nano-objects in the bulk and in solution. To tune various morphologies from the same star copolymer, selective and nonselective solvent media were used. Thepyridine moieties, forming distinct P2VP nanodomains in the copolymer nanostructures, were selectively crosslinked using 1,4-dibromobutane under mild conditions to yield stabilized polymeric “hairy” nano-objects, dispersible in hot tetrahydrofuran. The morphology of the resulting nanostructures was studied using scanning electron microscopy and was found to be strongly dependent on various factors, such as the self-assembly/casting conditions, the total time of the crosslinking reaction, and the dispersion procedure. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1636–1641, 2010

We report on the gelation capability of polystyrene/poly(2-vinyl pyridine) amphiphilic heteroarm polyelectrolyte stars in acidic salt-free aqueous media. The star polymers associate through hydrophobic interactions, by retraction of the stretched arms under no interdigitation conditions, in the dilute regime forming colloidal soft nanoparticles comprising about 6 stars, At concentrations significantly higher than the hydrodynamic overlap concentration (c > 40c∗), the crowding of the colloidal nanoparticles drives a jamming transition, leading to a colloidal gel. The intermediate overlap regime (c∗

The self-organization of the double hydrophilic triblock copolymer poly(ethylene oxide)-b-poly(2-vinylpyridine)-b-poly(ethylene oxide), PEO-b-P2VP-b-PEO, was investigated in dilute aqueous solution under several experimental conditions using turbidimetry, as well as static and dynamic light scattering. As a result of the temperature-sensitive properties of the end PEO blocks and the p H-responsive properties of the middle P2VP block, the formation of large star-like micellar nanostructures is observed at high p H, while at low p H, but in the presence of salt and at high temperature, flower-like micelles are formed. Moreover, the viscosimetric and dynamic light scattering studies at low p H revealed that micelle-like nanostructures are formed upon mixing the triblock copolymer with poly(acrylic acid), PAA, due to hydrogen bonding interpolymer complexation.

An asymmetric, amphiphilic, A n (B-b-C) n heteroarm star block terpolymer bearing polystyrene and poly(2-vinyl pyridine)-block-poly(acrylic acid) arms, was synthesized by anionic polymerization, using an extending "in-out" method and a post polymerization deprotection reaction. Due to the pH-dependent protonation/deprotonation equilibrium of the P2VP/PAA blocks, a rich phase behavior was observed as a function of pH. At pH = 2, the star terpolymers form a physical hydrogel through a solvent-induced sol/gel transition in a DMF/water solvent mixture. The gelation mechanism was attributed to a jamming effect mediated by increasing the dielectric permittivity of the medium.

A poly(methyl methacrylate)–poly(acrylic acid)–poly(2-vinyl pyridine)–poly(acrylic acid)–poly(methyl methacrylate) (PMMA–PAA–P2VP–PAA–PMMA), ABCBA pentablock terpolymer was synthesized by “living” anionic polymerization and was studied in aqueous media at different pH conditions in the presence of MeOH. By tuning the pH and/or the solvent selectivity and dielectric constant of the medium, reversible hydrogels of different nature were formed. At low pH the hydrogel is based on a three dimensional network comprising PMMA hydrophobic cores (physical crosslinks) interconnected by complex bridging (elastically active) chains constituted of positively charged P2VP and non-ionic PAA segments. At high pH the hydrogel is transformed reversibly to a negatively charged network the bridging chains of which comprise ionized PAA segments interrupted by hydrophobic P2VP blocks, swellable on MeOH addition. Furthermore, we found conditions for the formation of flower-like micelles of different topologies and nature like core–shell–corona micelles with positively charged corona (pH

A poly(methyl methacrylate)-block-poly(acrylic acid)-block-poly(2-vinyl pyridine)-block-poly (acrylic acid)-block-poly(methyl methacrylate) (PMMA-PAA-P2VP-PAA-PMMA), pentablock terpolymer has been synthesized by anionic polymerization with sequential addition of monomers and studied in aqueous media at low pH. The system exhibits combined properties and adopts the behavior of 'telechelic' polyelectrolytes and that of double hydrophilic polyampholytes. This complex behavior leads to the pentablock terpolymer forming a pH and temperature sensitive reversible hydrogel at very low polymer concentration.

We report on the diverse self-assembly of the poly(2-vinyl pyridine)-b-poly(acrylic acid)-b-poly(n-butyl methacrylate) (P2VP−PAA−PnBMA) ABC terpolymer in aqueous solutions. This has allowed us to fabricate a range of hierarchically organized nanostructured particles and hydrogels just by tuning both the pH and the concentration of polymer solutions. The multiresponsive molecule demonstrates a unique diversity of structural organizations caused by the combination of the P2VP and PAA building blocks. We found conditions for the formation of thermosensitive centrosymmetric core−shell−corona micelles, compact spheres, polyelectrolyte flowerlike micelles, a charged pH-sensitive 3D network, toroidal nanostructures, and finite size clusters (microgels). The transition between different micelles and the hydrogel was induced via the regulation of electrostatic and hydrophobic interactions in the system.

Double-hydrophilic poly[(oligo(ethylene glycol) methacrylate)-co-methyl methacrylate]-b-poly(2-(diethylamino)ethyl methacrylate), P(EGMA-co-MMA)-b-PDEA, diblock terpolymers were designed and explored in aqueous media. Thanks to the thermosensitivity of the P(EGMA-co-MMA) statistical block and the pH sensitivity of the PDEA block, these terpolymers form two distinct micellar self-assemblies at different conditions of pH and temperature. The thermosensitivity of these terpolymers can be tuned by controlling the LCST of the statistical block through its monomer unit composition.

A novel double hydrophilic poly(N,N-diethyl-acrylamide)-poly(acrylic acid)-poly(N,N-diethylacrylamide) (PDEAAm-PAA-PDEAAm) triblock copolymer was synthesized by sequential anionic polymerization and modification of the poly(tert-butyl acrylate) middle block by selective hydrolysis and neutralization to its ionic functions. Due to the pH-sensitivity of the PAA central block and the thermo-sensitivity of the PDEAAm end-blocks, it exhibits responsive behavior in aqueous media. At low temperatures and high pH, it is molecularly dissolved while at temperatures above the LCST of the PDEAAm end-blocks, a sol-gel transition was observed which should be ascribed to the formation of a three-dimensional transient network comprising PDEAAm hydrophobic physical crosslinks interconnected by PAA negatively charged elastic chains. The sol-gel transition and the rheological properties of the physical gel are strongly influenced by the presence of salt.

Diblock terpolymers that consist of homopolymer and statistical copolymer (polyampholyte) building blocks are synthesized by group transfer polymerization. Two types of block tepolymers are explored in aqueous media: the amphiphilic poly{[(diethylamino)ethyl methacrylate]-co-(methacrylic acid)}-block-poly(methyl methacrylate) and the double hydrophilic poly[oligo(ethylene glycol) methacrylate]-Modk-poly{[(diethylamino)ethyl methacrylate]-co(methacrylic acid)}. The first terpolymer self-assembles in aqueous media to form responsive micelles that change their corona charge sign upon switching pH. The second terpolymer exhibits a multi-responsive behavior. It forms neutral, positive, or negative micelles depending on a combination of different environmental conditions such as temperature, pH, and ionic strength.

Star polymers composed of equal numbers of poly(ethylene oxide) (PEO) and polystyrene (PS) arms with variable lengths and a large (up to 38 total) number of arms, PEO(n)PS(n), have been examined for their ability to form domain nanostructures at the air-water and air-solid interfaces. All PEO(n)PS(n) star polymers formed stable Langmuir-Blodgett (LB) monolayers transferable to a solid substrate. A range of nanoscale surface morphologies have been observed, ranging from cylindrical to circular domains to bicontinuous structures as the weight fraction of the PEO block varied from 19% to 88% and n from 8 to 19. For the PS-rich stars and at elevated surface pressure, a two-dimensional supramolecular netlike nanostructure was formed. In contrast, in the PEO-rich star polymer with the highest PEO content, we observed peculiar dendritic superstructures caused by intramolecular segregation of nonspherical core-shell micellar structures. On the basis of Langmuir isotherms and observed monolayer morphologies, three different models of possible surface behavior of the star polymers at the interfaces were proposed.

The solution behavior and self-assembly of a ‘hairy’ heteroarm star copolymer polystyrene/poly(2-vinylpyridine) containing 20 fairly short polystyrene and 20 long poly(2-vinylpyridine) arms was studied in 1,4-dioxane–methanol mixtures and in acidic aqueous solutions. The copolymer forms reversible micelles in 1,4-dioxane–methanol mixtures. Since the conformation of the unimolecular heteroarm star (unimer) with collapsed insoluble PS arms and stretched soluble PVP arms resembles the spherical core/shell micelle, the solubility of the copolymer in the selective solvent is fairly high and the multimolecular micelles are formed by only few heteroarm stars. Both heteroarm stars (in pure 1,4-dioxane) and micellar systems (composed of both micelles and unimolecular heteroarm stars (unimers) in 1,4-dioxane–methanol mixtures) may be transferred in acidic aqueous media by dialysis. Polyelectrolyte behavior of unimer stars was studied in detail in acidic aqueous media. Static and dynamic light scattering measurements show that heteroarm stars interact electrostatically over long distances in low ionic strength solutions. Experimental data indicate that the charged stars may form temporary shell interacting (electrostatically stabilized) aggregates at elevated concentrations.

A highly asymmetric P2VP 58 -PAA 924 -PBMA 48 double hydrophilic block terpolymer exhibits a rich phase behavior as a function of pH. In acidic media (pH 1) three compartment micelles with a positively charged outer coronas are formed. At high pH, the above structure is transformed into a three-dimensional transient network constituted of hydrophobic domains interconnected with negatively charged bridging chains (PAA chains). At even higher pH (14) the network is disrupted and finally exhibits a closed loop sol-gel-sol behavior.

A novel thermothickening phenomenon exhibited by a water-soluble triblock copolymer in salt-free aqueous solutions has been investigated through rheological measurements and supported by dynamic li...

The self-organization of a mono hydrophilic ABC terpolymer, which is constituted of a central long polyelectrolyte end-capped by two different hydrophobic short blocks (i.e., polystyrene-b-poly(acrylic acid)-b-poly(n-butyl methacrylate), PS-PAA-PnBMA), was examined in aqueous salt-free dilute solutions by static and dynamic light scattering, scanning electron microscopy, and rheology. The neutralized PS-PAA-PnBMA terpolymer is self-organized hierarchically from flowerlike micelles to dendronlike micellar aggregates and eventually to a three-dimensional physical network. The rheological properties of the resulting physical gel were also investigated and discussed.

We report on the phase behavior and micellization phenomena of a poly(2-vinylpyridine-b-poly(methyl methacrylate)-b-(poly(acrylic acid) block terpolymer (P2VP−PMMA−PAA) in aqueous dilute solutions. This double hydrophilic terpolymer exhibits different self-assemblies depending on the pH of the solution. At low pH (1−2) heteroarm starlike micelles are formed bearing interactive P2VP and PAA arms in the corona. Aging phenomena due to intramicelleral interactions lead to morphology changes from Janus (segregated arms) to shell cross-linked micelles (mixed arms). As the pH increases from 2 to 3, electrostatic intermicellar attractive interactions rise leading to the formation of nonregular aggregates. An insoluble two-phase region around the isoelectric point of the ampholitic chains was observed between pH 3.2 and 5.8. Finally, at higher pH, an unexpected probably out-of-equilibrium morphology of aggregated amphiphilic heteroarm starlike micelles was observed after deposition on the mica substrate.

The crystallization behavior and kinetics of poly-(ethylene oxide) in polystyrene/poly(ethylene oxide) hetero-arm star copolymers were studied by differential scanning calorimetry and optical microscopy. A comparison between star and linear amorphous-crystalline block copolymers showed that the macromolecular architecture is an important factor affecting crystallinity. The following points were observed: the equilibrium melting point is higher in the star copolymers, the crystallinity reduces as the number of arms increases, leading to smaller and ill-defined spherulites, and crystallization proceeds faster in linear copolymers at low supercooling.

Linear and nonlinear viscoelastic properties of an asymmetric triblock copolymer poly(acrylic acid)−poly(2-vinylpyridine)−poly(acrylic acid) (PAA−P2VP−PAA) in salt-free aqueous solutions have been investigated. At pH 3.4, long-range electrostatic interactions prevail, due to protonated P2VP units and negative PAA end groups. Above a critical Cg = 2.5% w/w, a transient network is formed through intermolecular electrostatic interactions between negatively charged groups located at the end PAA blocks and positively charged protonated pyridine groups located at the middle long P2VP block. The so-formed network exhibits some atypical rheological behavior characterized by a strain hardening of storage modulus in intermediate strain amplitudes and a pronounced shear thickening in moderated shear stresses. The shear-induced changes in the structure of the network have been attributed to enhancement of the number of elastically active bridges through association of free dangling ends and a transition from intra- t...

Single molecule conformations and associate morphologies for polystyrene/poly(2-vinylpyridine) (PS7−P2VP7) heteroarm star copolymer, formed in controlled environment and deposited onto mica or Si wafers, have been studied by atomic force microscopy (AFM) with molecular resolution. At concentrations below 0.01 mg/mL PS7−P2VP7 exists in the molecularly dissolved state in both selective (acidic water, toluene) and common good (chloroform, tetrahydrofuran) solvents. In acid conditions PS7−P2VP7 forms either unimolecular or multimolecular micelles depending on concentration and pH. The core of the micelles is constituted of the collapsed PS arms surrounded by protonated P2VP shell. PS7−P2VP7 undergoes inverse intramolecular segregation upon addition of toluene. In this case, P2VP arms form the dense core of the micelles embedded in the swollen PS shell. The transition between those two inverse types of micelles is strongly modified by interaction with the mica substrate. The micelles deposited onto mica from a...

An asymmetric model triblock copolymer of poly(acrylic acid)-b-poly(vinylpyridine)-b-poly(acrylic acid) (PAA-b-P2VP-b-PAA) was synthesized through anionic polymerization and explored in aqueous media. This block polyampholyte exhibits a rich phase behavior responding to pH changes. At low pH and low concentrations the polymer is molecularly dissolved. At pH close to 3.4 and above a critical concentration, a three-dimensional transient network is formed. The physical cross-links arise from electrostatic interactions among protonated positively charged P2VP units and negatively charged PAA units. In the intermediate pH region (4 < pH < 7) and around the isoelectric point, the polyampholyte is precipitated. The polymer is redissolved at higher pH, forming compact micelles with P2VP hydrophobic cores and extended PAA chains in the corona. The phase behavior of the system is reversible.

Unimolecular micelles constituted from the star-shaped PS7−P2VP7 polysterene/poly(2-vinylpyridine) block copolymer single molecules were studied with molecular resolution using AFM. In selective solvent the unimers show core−shell structures with very pronounced segregation into the collapsed core and the extended shell formed by stretched polymer arms of P2VP and PS in water and toluene, respectively. Metallization of P2VP arms of the copolymer leads to the organic−inorganic nanocomposite with 1−3 nm in diameter palladium clusters localized in the outer shell of unimers, forming starlike structures with metallized arms and improving the contrast to resolve the fine molecular morphology.

We report on the viscoelastic response of a physical gel formed in aqueous solutions of poly(sodium acrylate) end-capped with short polystyrene (PS) blocks. Above a percolation threshold (cgel = 0.2 wt %) a transient network is formed constituted of PS hydrophobic domains (cross-links) interconnected via fully charged polyelctrolyte chains. Oscillatory shear and step-strain experiments were employed to study the viscoelastic properties. The stretched conformation of the charged middle blocks (bridges) and the high hydrophobicity of the PS ends have fundamental consequences on the various viscoelastic parameters: (1) The linear viscoelastic behavior is only observed at very small strains, indicating that the telechelic polyelectolytes form weak gels. (2) The magnitude of the plateau modulus, in the unentangled regime, is very high due to the stretched conformation of the polyelectrolyte chains that limits the formation of elastically inactive loops. (3) The lifetimes of the physical cross-links are influe...

Coil-rod-coil block copolymers composed from luminescent rigid units and acrylate flexible blocks have been synthesized using atom transfer radical polymerization. α,ω-Difunctionalized oligophenylenes properly modified to act as ATRP initiators have been used for the polymerization of the various acrylates. Copolymers with controlled shape and in some cases, relatively low polydispersities have been obtained as proved by size exclusion chromatography and NMR. In cases, where t-butyl acrylate blocks have been used as the flexible part, selective hydrolysis resulted in coil-rod-coil copolymers containing poly(acrylic acid) blocks. The solution behavior of the synthesized copolymers was explored in various solvents. The poly(acrylic acid) copolymers in aqueous solutions form large aggregates, while in organic selective solvents for the flexible block, monomolecular micelles seem to be formed.

A diversity of pH-responsive amphoteric self-assemblies formed by a multisegmented, multiarm star-shaped (PS33)9(P2VP126-b-PAA69)9 terpolymer in aqueous media is reported.

Three samples of sodium alginate grafted by amino-terminated PNIPAM (low and high molecular weight poly(N-isopropylacrylamide)) and a random P(NIPAM-co-NtBAM) (NtBAM: N-tertiary butyl acrylamide) copolymer, were synthesized via the carbodiimide chemistry and their temperature-induced hydrogelation capability was evaluated by rheology. All the samples showed thermothickening behaviour depending on concentration, ionic strength and hydrophobic comonomer content, incorporated to the PNIPAM thermo-sensitive pendant chains. The main result of this study was that a slight hydrophobic enrichment (just 15 mol% NtBAM) of PNIPAM was sufficient to shift the gelation temperature well below the physiological temperature at 32 ºC and at the lowest polymer concentration studied (i.e. 10 wt%). The consequence of that was that the ALG-g- P(NIPAM-co-NtBAM) graft copolymer hydrogel exhibited the best rheological thermo- responsive properties and might be used as injectable hydrogel for potential applications in biomedicine. Fil: Lencina, María Malvina Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Planta Piloto de Ingeniería Química. Universidad Nacional del Sur. Planta Piloto de Ingeniería Química; Argentina. University of Patras. Department of Chemical Engineering; Grecia Fil: Iatridi, Zacharoula. University of Patras. Department of Chemical Engineering; Grecia Fil: Villar, Marcelo Armando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Planta Piloto de Ingeniería Química. Universidad Nacional del Sur. Planta Piloto de Ingeniería Química; Argentina Fil: Tsitsilianis, Constantinos. University of Patras. Department of Chemical Engineering; Grecia

Ionic heteroarm star copolymers bearing polystyrene (PS) and poly(acrylic acid) (PAA) arms (PSnPAAn) were prepared by quantitative hydrolysis of the poly(tert-butyl acrylate) (PtBA) arms of the corresponding PSnPtBAn star copolymer. The aggregation properties of these copolymers were studied in various solvents. In 1,4-dioxane PS12PAA12 (with nearly symmetrical PS and PAA arms) forms reverse micelles of low aggregation number (Nagg) and spherical morphology. In an 80 : 20 (v/v) 1,4-dioxane/water mixture these micelles are transformed to regular micelles with an unexpectedly high Nagg and an elongated rod-like structure. An abnormal behavior was observed in aqueous solutions of charged PS24PANa24 (with asymmetrical PS and PAA arms, WPS = 19 wt.-%) at low concentrations. A non-equilibrium physical gel is formed, characterized by a very high viscosity and an elastic response upon oscillatory shearing.

Polystyrene/poly(ethylene oxide) heteroarm star copolymers (i.e., star-shaped polymers consisting of a central poly(divinylbenzene) core bearing an equal number of short polystyrene and long poly(ethylene oxide) arms, PS10PEO10) were synthesized by anionic polymerization, and their phase behavior was investigated in aqueous solutions as well as in water−oil solvent mixtures. Because of their amphiphilic character and despite their star architecture, these polymeric species self-assemble under equilibrium conditions into lyotropic liquid crystalline structures.

Polystyrene-block-poly(2-vinyl pyridine)block-poly(methyl methacrylate) ABC triblock copolymers were synthesized by sequential living anionic polymerization. Their solution properties were investigated in toluene, which is a bad solvent for the middle block. Spherical micelles are formed, which consist of a poly(2-vinyl pyridine) dense core bearing polystytene and poly(methyl methacrylate) soluble chains at the corona. These micelles exhibit the architecture of heteroarm star copolymers obtained by living polymerization methods. The aggregation numbers strongly depend on the length of the insoluble P2VP middle block thus remarkably attecting the size of the micelles.

A polystyrene−poly(sodium acrylate)−poly(n-butyl methacrylate) (PS−PANa−PnBMA) ABC triblock copolymer was synthesized by sequential anionic polymerization and modification of the poly(tert-butyl acrylate) middle block by selective hydrolysis and neutralization to its ionic functions. Because of the fact that both outer hydrophobic blocks are very short comparable to the middle one, this polymer can be viewed as a “heterotelechelic” polyelectrolyte. The rheological behavior of the PS−PANa−PnBMA aqueous solutions was explored by steady state and oscillatory shear flow measurements. Dilute solutions (c < 1 wt %) exhibit very high viscosity, showing that these kinds of polymers can be used as effective thickeners. This behavior is attributed to the formation of a transient network constituted from PS and/or PnBMA physical cross-links interconnected by stretched polyelectrolyte chains. An unusual observation is that the viscosity of the system seems to be invariant over a wide temperature range. Finally, the d...

A two step synthetic procedure towards model coil-rod-coil triblock copolymers was developed. In the first step, a,co-bromo-functionalized oligopheny lenes (rod part) were modified to anionic bifunctional initiators. In the second step, flexible chains were grown from both ends of the rod part by an anionic polymerization procedure leacing to polystyrene-rod-polystyrene and/or poly(2-vinyl pyridine)-rod-poly(2-vinyl pyridine) triblock copolymers. Characterization of the final products by gel permeation chromatography, NMR spectro scopy, light scattering, Raman spectroscopy and differential scanning calorimetry demonstrated that well-defined coil-rod-coilblock copolymers can be synthesized.

Polystyrene (PS) end-capped polyelectrolytes were prepared by modification of polystyrene−poly(tert-butyl acrylate)−polystyrene triblock copolymers synthesized via anionic polymerization. These amphiphilic polymers are associated in dilute aqueous solutions forming loose multimicellar clusters. At higher polymer concentrations an extensive intermicellar bridging occurs, leading to the formation of a transient network as revealed by the viscoelastic response of the system. The so-formed gel is characterized by low gel concentration, i.e., 0.2 wt %, yield stress, complex steady-shear viscosity profile, high plateau modules, and long relaxation times. These characteristics can be attributed to the highly hydrophobic character of the PS junctions and the stretched conformation of the polyelectrolyte bridges.