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We report a high-resolution electrospray ionization mass spectrometric (HR ESI-MS) access route leading to in-depth insights into the spontaneous zwitterionic copolymerization mechanism between cyclic imino ethers (i.e., 2-methyl-2-oxazoline (MeOx), 2-ethyl-2-oxazoline (EtOx), or 2-ethyl-2-oxazine (EtOz)) with acrylic acid (AA), exploiting the characteristic species accumulating during the copolymerization as well as tandem mass spectrometry (MS/MS). We demonstrate preferences in α,ω-end group formation by screening various feed ratios of cyclic imino ethers and acrylic acid (e.g., MeOx:AA = 1:1; MeOx:AA = 2:1; and MeOx:AA = 1:2). Critically, a calibration curve--based on AA-MeOx-AA dimer--was established allowing for the semiquantitative determination of the end-group ratios with different feed ratios of acrylic acid. The formation of previously suggested alternating copolymers was confirmed by MS/MS experiments. Deviations from an ideal alternating composition were found to decrease from MeOx to EtOx to EtOz. The results of (semiquantitative) HR ESI-MS and MS/MS measurements suggest, for the first time presented in such precision, a polymerization mechanism for the spontaneous zwitterionic (alternating) copolymerization indicating optimal monomer ratios and pairings. [ABSTRACT FROM AUTHOR]

It is challenging to develop hydrogels with a combination of excellent mechanical properties, versatile functions, and good processability. Here we report a physical hydrogel of poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) cross-linked by carboxyl-Fe3+ coordination complexes that possesses high stiffness and toughness, fatigue resistance, and stimulation-triggered healing along with shape memory and processing abilities. The copolymers have randomly dispersed AAm and AAc repeat units, making the physical cross-links with different strength. The strong coordination bonds and their associations serve as permanent cross-links, imparting the elasticity, whereas the weak ones reversibly rupture and re-form, dissipating the energy. Furthermore, a stick-slip instability is observed during the tearing test, which should be associated with the specific nature of metal-coordination bonds that are strong yet fragile. Because of the dynamic nature of coordination bonds, both tensile and tearing mechanical properties are rate dependent. By tuning the bond strength via pH, the gels show distinct mechanical properties, shape memory ability, and even reversible sol-gel transition. The system also shows good processability; the copolymer solutions can be processed into tough gels with different structures by three-dimensional printing technology. These versatile, tough, yet processable hydrogels should be a promising material as structural elements in various applications. [ABSTRACT FROM AUTHOR]

Melt state dynamics for a series of strictly linear polyethylenes with precisely spaced associating functional groups were investigated. The periodic pendant acrylic acid groups form hydrogen-bonded acid aggregates within the polyethylene (PE) matrix. The dynamics of these nanoscale heterogeneous morphologies were investigated from picosecond to nanosecond timescales by both quasi-elastic neutron scattering (QENS) measurements and fully atomistic molecular dynamics (MD) simulations. Two dynamic processes were observed. The faster dynamic processes which occur at the picosecond timescales are compositionally insensitive and indicative of spatially restricted local motions. The slower dynamic processes are highly composition dependent and indicate the structural relaxation of the polymer backbone. Higher acid contents, or shorter PE spacers between pendant acid groups, slow the structural relaxation timescale and increase the stretching parameter (β) of the structural relaxation. Additionally, the dynamics of specific hydrogen atom positions along the backbone correlate structural heterogeneity imposed by the associating acid groups with a mobility gradient along the polymer backbone. At time intervals (<2 ns), the mean-squared displacements for the four methylene groups closest to the acid groups are up to 10 times smaller than those of methylene groups further from the acid groups. At longer timescales acid aggregates rearrange and the chain dynamics of the slow, near-aggregate regions and the faster bridge regions converge, implying a characteristic timescale for the passage of chains between aggregates. The characterization of the nanoscale chain dynamics in these associating polymer systems both provides validation of simulation force fields and provides understanding of heterogeneous chain dynamics in associating polymers. [ABSTRACT FROM AUTHOR]

Iron oxide (IO)-based composite latex particles with a patchy IO overlayer were successfully prepared by Pickering emulsion polymerization of acrylic monomers and/or styrene using acrylic acid or methacrylic acid as auxiliary comonomers. The ultrasmall IO particles adsorbed at the latex surface and played the role of a solid stabilizer. The influence of the synthesis conditions on the composite particle size, polymerization kinetics, IO incorporation efficiency, and particle morphology was studied. Thermogravimetric analysis indicated that the efficiency of iron oxide incorporation was improved with increasing the suspension pH, the amount of auxiliary comonomer, or the IO content and reached ca. 90% under optimized conditions. Reducing the initial iron oxide concentration at constant monomer concentration led to an increased particle size and a reduced colloid stability, indicating that the magnetic nanoparticles stabilized the colloidal nanocomposites. Transmission electron microscopy studies confirmed the presence of iron oxide at the particle surface. In light of these results, a tentative mechanism for Pickering emulsion polymerization using IO nanoparticles as solid stabilizer in the presence of (meth)acrylic acid auxiliary comonomers was proposed. [ABSTRACT FROM AUTHOR]

Aqueous mixtures of pH-sensitive block random BAB triblock copolymers with different hydrophobic B blocks connected to the same hydrophilic A block were studied in order to investigate comicellization and the impact on the dynamic mechanical properties. The B blocks were statistical copolymers of acrylic acid (AA) and n-butyl acrylate (nBA) with varying AA contents, whereas the A block was a pure PAA. Neat triblocks self-assembled into transient networks for which the mechanical relaxation time depended both on the AA content within the B blocks and on the pH, which affected the ionization of the AA units. Static and dynamic light scattering measurements were done on mixtures of equivalent AB diblock copolymers that showed that comicellization occurred only at conditions at which both copolymers considered separately self-assemble. When comicellization occurred, the characteristic escape time of both types of B blocks from the mixed hydrophobic cores impacted the rheological properties of the binary triblock mixture. Using binary mixtures of BAB triblock copolymers exhibiting pH-controlled dynamics thus allows control and fine-tuning of the viscoelastic properties at constant pH by formulation without the need to synthesize a large number of different polymers. Moreover, the more dynamic B blocks were slowed down in the presence of the less dynamic ones, and vice versa, so that a frozen network could be transformed into a transient one by coassembly with very dynamic chains. [ABSTRACT FROM AUTHOR]

Poly(acrylic acid) with grafted TEMPO moieties, PAA-T, phase separates over the pH range 2-4, whereas at lower and higher pH, the polymer is water-soluble. The pH range of the two-phase region increases linearly with the content of grafted TEMPO moieties. As evidenced by surface tension measurements, the PAA-T molecules have little hydrophobic character. We propose that at low pH PAA-T is an amphoteric polymer that, in addition the anionic carboxyl groups, contains cationic species resulting from the disproportion of TEMPO at low pH. Furthermore, we propose that two-phase regions occur near the isoelectric points of the polymer, suggesting that phase separation is due to electrostatically driven association of polymer molecules. [ABSTRACT FROM AUTHOR]

An in situ NMR technique is used to measure the relative monomer consumption rates of acrylamide (AM) and acrylic acid (AA) over a broad range of initial comonomer compositions, initial monomer concentrations (5-40 wt % in aqueous solution), and the complete range of AA ionization. The composition drift with conversion was found not only to be a strong function of AA ionization but also to vary systematically with monomer concentration, with the latter influence increasing at increased degrees of ionization. The coupled effect is captured by expressions developed to represent the monomer reactivity ratios as a function of these two major variables. The proposed correlation, easily implemented into any modeling framework, is based on experimentally measured composition over a much broader range of conditions than examined in previous studies and accurately captures the effect of monomer concentration at low and high degrees of ionization. [ABSTRACT FROM AUTHOR]

Poly(vinylphosphonic acid-co-acrylic acid) (PVPA-co-AA) has recently been identified as a possible candidate for use in bone tissue engineering. It is hypothesized that the strong binding of PVPA-co-AA to calcium in natural bone inhibits osteoclast activity. The free radical polymerization of acrylic acid (AA) with vinylphosphonic acid (VPA) has been investigated with varying experimental conditions. A range of copolymers were successfully produced and their compositions were determined quantitatively using 31P NMR spectroscopy. Monomer conversions were calculated using 1H NMR spectroscopy and a general decrease was found with increasing VPA content. Titration studies demonstrated an increase in the degree of dissociation as a function of VPA in the copolymer. However, a VPA content ca. 30 mol % was found to be the optimum for calcium chelation, suggesting that this composition is the most promising for biomaterials applications. Assessment of cell metabolic activity showed that PVPA-co-AA has no detrimental effect on cells, regardless of copolymer composition. [ABSTRACT FROM AUTHOR]

Inorganic sulfites as reducing agents were successfully used in combination with typical reversible addition-fragmentation chain transfer (RAFT) agents for the controlled DET-RAFT (DET: dissociative electron transfer) of methacrylates, methyl acrylate (MA), and styrene (Sty) near room temperature (30 °C). The polymerizations were first-order with respect to monomer conversion and polymers with narrow molecular weight distributions (Đ 1.2), and "living" features were obtained. MALDI-TOF experiments demonstrated the integrity of the chain-ends and clearly showed the absence of SO2 in the polymer chains. Kinetic studies revealed that an increase of either temperature or concentration of sulfites provided faster reactions, without loss of control. Ab initio quantum chemistry calculations suggested that in the presence of the reducing agent the RAFT agent undergoes one-electron reduction to a stable radical anion that can then undergo fragmentation to yield the initiating carbon-centered radical. The new metal-free DET-RAFT developed proved to be versatile and robust, as it could be also used for the polymerization of different relevant monomers, such as glycidyl methacrylate (GMA), 2-(diisopropylamino)ethyl methacrylate (DPA), and 2-(dimethylamino)ethyl methacrylate (DMAEMA). [ABSTRACT FROM AUTHOR]

The structures of imogolite-poly(acrylic acid) hydrogels were investigated using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) to determine the effects of particle concentration and the magnitude of deformation. The imogolite-poly(acrylic acid) hydrogel was synthesized by using γ-ray radiation as a nanocomposite gel with chemical bonds between the particles and polymers. The SAXS measurements revealed that the imogolite network was composed of particle overlaps. Under deformation, the gel structure was rearranged to increase the dimensionality of the network, forming a relaxed structure of overlaps by changing the orientation of the imogolite. The structural response of the deformed gel depended on the imogolite concentration, which influenced the changes in dimensionality of the network and the number of overlaps. The SANS patterns indicated that the polymers wrapped the imogolite aggregates, allowing polymers to follow the imogolite behavior. These observations demonstrate that the behavior of the imogolite can contribute to both the relaxation of stress and maintenance of the structure during an applied strain. [ABSTRACT FROM AUTHOR]

For some thermosensitive polymers, the absence of an upper critical solution temperature (UCST) in water in an accessible temperature range (say, 5-90 °C) can be attributed to the influence of charged groups in the polymer structure. This property was exploited in the present study to achieve ultrasensitive pH-induced water solubility switch of UCST polymers in physiological medium. By incorporating either acrylic acid (AAc) or 4-vinylpyridine (4VP) comonomer units in the random copolymer of acrylamide and acrylonitrile (P(AAm-co-AN-co-AAc) or P(AAm-co-AN-co-4VP)), the pH-induced shift of UCST was investigated by monitoring the solution cloud point. The results revealed an unusually large shift of the cloud point upon pH variation over a small range. In particular, one P(AAm-co-AN-co-4VP) sample exhibited a cloud point drop from 72 °C at pH 4.75 to 15 °C at pH 4.50 (i.e., 57 K shift over 0.25 pH units), and its transition from soluble to insoluble state at room temperature was visually observable over a pH change as little as 0.05 unit. Using this sample as macromolecular chain transfer agent to polymerize dimethylacrylamide (DMA) through RAFT, an ABA-type triblock copolymer of P(AAm-co-AN-co-4VP)-b-PDMA-b-P(AAm-co-AN-co-4VP) was obtained, and it showed an even larger cloud point switch from 71 to 10 °C with pH decreasing from 4.75 to 4.50. Consequently, the micelle formed by this block copolymer was stable at 37 °C with pH from 7.00 down to 4.75 but abruptly dissolved at pH 4.50 due to the water solubility switch. This study demonstrates a new UCST polymer-based approach to polymer assemblies that can sense a very small pH change by undergoing straightforward water solubility switch. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations areperformed for two model pressure-sensitive adhesive (PSA) materials,atactic poly(n-butyl acrylate) [poly(n-BA)] and atactic poly(n-butyl acrylate-co-acrylic acid) [poly(n-BA-co-AA)] at a very low concentration in acrylic acid (one acrylic acidmonomer per fifty butyl acrylate monomers plus three acrylic acids at each one of the two chain ends) in the bulk and confined between three crystallinesubstrates, silica (SiO2) represented as α-quartz,α-ferric oxide (α-Fe2O3), and metallicα-ferrite (α-Fe), over a range of temperatures. The simulationsare carried out with the accurate, all-atom Dreiding force-field andprovide important information for the distribution of local mass densityof the two polymers at the three crystalline substrates, and theiradsorption in conformations that lead to the formation of loop, train,and tail structures. By analyzing potential energy interactions betweenthe two polymers and each one of the three substrates through theirvan der Waals contact area or by computing the diagonal elements ofthe local stress tensor in the vicinity of the substrate, we havecalculated the effect of the type of substrate on the work of adhesionof the two polymers. Our calculations reveal a considerably strongeradsorption on α-quartz and α-ferric oxide compared toα-ferrite, which we attribute to strong attractive oxygen interactionsof the two polymers with the oxygen atoms of SiO2and Fe2O3. Detailed calculations of the local number densitydistribution of individual atomic species above the three substrates,of the local variation of the bond order parameter for skeletal andside group bonds or chords, of the distribution of dihedral angles,and of the histograms of chain ends support that the two PSAs adsorbon the three substrates with both their skeletal and pendant C–Cand C–O groups lying practically parallel to the surface andin such a way that the CO bond of the pendant butyl acrylategroups points toward the surface. The detailed MD simulations indicatethat adding a small amount (7.1% wt. here) of acrylic acid to poly(n-BA) significantly improves its wetting characteristicson oxygen-containing substrates. This is in full support of the findingsof Kisin et al. [Chem. Mater.2007, 19, 903−907] that introducing oxygen atoms to a metallicsurface or to polymer molecules increases considerably the work ofadhesion. [ABSTRACT FROM AUTHOR]

pH-sensitiverandom P(nBA1–x-stat-AAx)100(MHx) and block–random P(nBA1–x-stat-AAx)100-b-PAA100(DHx) amphiphiliccopolymers have been synthesized, wherex stands for the molar ratios of pH-sensitive hydrophilic acrylicacid (AA) units statistically distributed with hydrophobic n-butyl acrylate (nBA) ones within therandom block. Static and dynamic light scattering revealed that self-assemblyof the random associating block (MHx) and block–random (DHx)copolymers is strongly affected by the pH and ionic strength of thesolution and also by the amount of AA units within the MHx blocks.Below a characteristic pH, MHx self-assembles into finite size sphericalparticles that grow in size with decreasing pH until they eventuallybecome insoluble. DHx self-assembles into similar spherical particles,but the hydrophilic PAA100corona surrounding the MHx coreprevents insolubility at low pH. Self-assembly of DHx at higher pHis fully correlated to that of the neat MHx blocks, indicating thatit is possible to control precisely the extent of self-assembly ofdiblock copolymers by tuning the hydrophobic character of their associatingblock. Here this was done by controlling the fraction of charged unitswithin the random associating block. [ABSTRACT FROM AUTHOR]

Wereport tensile testing and in situX-ray scatteringmeasurements of a homologous series of precise poly(ethylene-co-acrylic acid) copolymers (pxAA). Thenumber of backbone carbons (x) between pendant acrylicacid groups along the polyethylene chain (x= 9,15, 21) has a pronounced effect on both their tensile properties aswell as their morphologies during deformation. The semicrystallineprecise copolymer (p21AA) displays yielding behavior similar to polyethylene.Also, strain hardening in p21AA coincides with the originally isotropicacid-rich layered structures strongly aligning with acid layers perpendicularto the strain direction, demonstrating the facile nature of the H-bondingwithin the acid aggregates. When the alkyl spacer is only nine carbons(p9AA), the precise copolymer withstands strains of >1000% withoutfailing, because the liquid-like assembly of acid aggregates permitsthe acid groups to exchange without developing substantial anisotropyin the structure. Both p21AA and p9AA maintain their morphology typeduring deformation with considerable plastic deformation and onlymodest increase in their interaggregate distances. In contrast, p15AAexhibits a structural transformation from a nominally spherical toa layered aggregate morphology during tensile deformation as evidencedby higher order peaks at intermediate scattering angles and largerinteraggregate spacing, coinciding with substantial strain hardening.The structural changes in p15AA are particularly sensitive to thestrain rate, because the relaxation times of the PE segments and theacid aggregates are accessible. Commensurate with this structuraltransformation, p15AA has the highest tensile strength of the precisepoly(ethylene-co-acrylic acid) copolymers. [ABSTRACT FROM AUTHOR]

Inthis article, we report the manipulation of block terpolymermorphology through control of the segment distribution. We considera model system comprised of three acrylic monomers: hydrophilic poly(hydroxyethylacrylate) (H), hydrophobic poly(octyl acrylate) (O), and polar poly(methyl acrylate) (M). Foreach of four chemical compositions, we altered the Msegmentdistribution in four terpolymer architectures with reversible addition–fragmentationchain transfer (RAFT) polymerization to yield: two triblock terpolymerarchitectures, HOMand HMO, and two diblockterpolymer architectures, HM/Oand H/MO, where the Msegmentsare statistically distributed in the Oor Hblocks, respectively. Using a combination of small-angle X-ray scatteringand dynamic shear rheology, we illustrate how the monomer distributioncan be used to manipulate the thermodynamic behavior of terpolymersat constant chemical composition. These results will be of use tothose wishing to partially decouple the formulation of a block copolymerfrom its morphology. [ABSTRACT FROM AUTHOR]

Triblockcopolymers of BAB type were synthesized by ATRP with apoly(acrylic acid) central block and random copolymer end blocks containingboth acrylic acid (AA) and hydrophobic n-butyl acrylate(nBA) units with varied composition of the B-blocks.Visco-elastic properties of these amphiphilic polymers were investigatedby oscillatory shear measurements as a function of composition, concentration,temperature and pH. Whatever the composition for each polymer theterminal relaxation time and thus the viscosity increased sharplywith decreasing degree of ionization of the AA units (α) andat low values self-supporting hydrogels were formed. However, theα- (respectively pH-) dependence of the terminal relaxationtime as well as the critical range of α (respectively pH) wherea transition between visco-elastic solutions and self-supporting hydrogelsoccurs strongly depend on the composition of the B-block. [ABSTRACT FROM AUTHOR]

Wereport atomistic molecular dynamics (MD) simulations of modelionomers with precise spacing between charged groups (polyethylene-co-acrylic acid). We explore different counterion types,neutralization levels, and spacer lengths between acid groups andprovide a thorough analysis of the resulting ionic aggregate morphologies.Structure factors computed from the simulations are in good agreementwith previous experimental X-ray scattering data, which provides strongvalidation of the simulation methods. Aggregate morphologies rangefrom small spherical aggregates to string-like shapes and large percolatednetworks. The unexpected morphologies of the ionic aggregates suggestthe need for a novel interpretation of scattering data for these materials.We quantify cation–anion and oxygen–hydrogen association,the two interactions primarily responsible for aggregate formation,and report detailed information pertaining to local structures aroundcations. This information is difficult to obtain experimentally andmay have important consequences for ion transport. [ABSTRACT FROM AUTHOR]

A localglass transition temperature, Tg, aroundionic aggregates in poly(ethylene-random-methacrylicacid) (E-MAA) ionomers neutralized with sodium has been studied bythe combination of the position-selective spin probing and microwavepower saturation method of electron spin resonance (ESR). 5DSA, 7DSA,10DSA, and 12DSA spin probes based on stearic acid allowed us to determinethe local Tgat different distances fromthe center of the ionic core (multiplet), whereas 10DND probe basedon a nonadecane backbone reflected the Tgat the polyethylene (PE) matrix. The local Tgs at the ionic multiplet itself and matrix region were determinedto be 320 and 281 K, respectively, in the quenched E-MAA ionomer fromthe melt. Moreover, it was found that the region around the multipletsis dynamically restricted, and the thickness of the restricted regionis ca. 10 Å. In the restricted region, the Tggradually decreased with receding from the ionic multiplet.The thickness of the dynamically restricted region was in good agreementwith the persistence length of the PE at the theta condition. [ABSTRACT FROM AUTHOR]

A novelway for the production of shape memory hydrogels containingcrystalline domains is described. Hydrogels were prepared by micellarcopolymerization of acrylic acid with the hydrophobic comonomer n-octadecyl acrylate (C18) in an aqueous NaCl solution ofsodium dodecyl sulfate (SDS). The presence of NaCl causes the SDSmicelles to grow and thus enables solubilization of large amounts(16% w/v) of C18 in the micellar solution. DSC measurements show thatthe swollen hydrogels, possessing 61–84% water, melt and crystallizewith a change in temperature. Independent of the hydrophobe levelbetween 20 and 50 mol %, the melting and crystallization temperaturesof the hydrogels are 48 ± 2 and 43 ± 2 °C, respectively.The hydrogels exhibit 3 orders of magnitude change in the elasticmodulus when the temperature changes between below and above the meltingtemperature of the crystalline domains. The blocky structure of thenetwork chains formed by micellar polymerization is responsible forthe drastic change in their mechanical properties and significantshape memory effect. [ABSTRACT FROM AUTHOR]

Toobtain a better understanding of how lanthanide-encoded polystyrenemicrobeads are formed during their synthesis, we carried out kineticstudies of two-stage dispersion copolymerization of styrene and acrylicacid (AA, 2 wt % based on styrene), with or without added TmCl3, in ethanol in the presence of poly(N-vinyl-2-pyrrolidone)(PVP) as a polymeric stabilizer. Particles of narrow size distributionand micrometer diameters were obtained, allowing us to compare ratesof monomer conversion, Tm ion incorporation, and growth in particlesize. In these reactions, AA or (AA + TmCl3) were addedin a second stage (t= 1 h). Much of the AA incorporationinto the particles occurred in the latter half of the reaction. Inthe presence of small amounts of TmCl3there was some retardationof the polymerization reaction. The most striking result was thatat the higher levels of TmCl3most of the metal ion incorporationoccurred late in the reaction. [ABSTRACT FROM AUTHOR]

pH and temperature sensitive poly(N-isopropylacrylamide)(pNIPAm) microgel-based etalons were fabricated by sandwiching a “monolithic”microgel layer between two semitransparent, 15 nm Au layers. The devicesexhibit visual color and multipeak reflectance spectra, both of whichprimarily depend on the distance between the Au surfaces (mediatedby the microgel diameter). In this submission, we fabricate etalonsfrom either pNIPAm-co-acrylic acid (pNIPAm-co-AAc) or pNIPAm-co-N-(3-aminopropyl)methacrylamide hydrochloride (pNIPAm-co-APMAH) microgels and investigate their response to the presenceof polycations or polyanions. We show that when the etalon is at apH that renders the microgels multiply charged, the microgel layerof the etalon deswells in the presence of the oppositely charged polyelectrolyte;it is unresponsive to the presence of the like charged polyelectrolyte.Furthermore, the etalon’s response depended on the thicknessof the Au overlayer. For example, low molecular weight (MW) polyelectrolytecould penetrate all Au overlayer thicknesses, while high MW polyelectrolytescould only penetrate the etalons fabricated from thin Au overlayers.We hypothesize that this is due to a decrease in the Au pore sizewith increasing thickness, which excludes the high MW polyelectrolytesfrom penetrating the microgel-based layer. These devices show promiseas MW selective sensors and biosensors. [ABSTRACT FROM AUTHOR]

A series of sub-100 nm superparamagnetic iron oxide nanoparticleswith amphiphilic poly(acrylic acid-b-butylacrylate);PAA-b-PBA) copolymer shells were synthesized andcharacterized by NMR spectroscopy, dynamic light scattering (DLS),transmission electron microscopy (TEM), thermogravimetric analysis(TGA), and a superconducting quantum interference device (SQUID) toinvestigate the effect of the polymer structure on the interfacialtension for nanoparticles adsorbed at the dodecane-water interface.Large reductions in interfacial tension of up to 27.6 mN/m were measuredat nanoparticle concentrations of 0.27 wt %, indicating significantnanoparticle adsorption and interaction between the oil and watermolecules at the interface. The adsorption energy of the polymer-coatednanoparticles at the dodecane/water interface was determined fromthe interfacial tension and nanoparticle radius, and analyzed in termsof the structure of the polymer stabilizer. Furthermore, the equilibriumadsorption of amphiphilic copolymer-functionalized iron oxide nanoclustersat the oil–water interface was determined by material balancefrom the concentration in the excess water phase and the known overalloil/water interfacial area. The formation and stabilization of oildroplets were on the order of 10 μm in water with unusuallylow nanoparticle concentrations was explained in terms of the highinterfacial activity of the particles. [ABSTRACT FROM AUTHOR]

Polymerization of nonionized acrylic acid (AA) in aqueoussolution has been studied via single pulse–pulsed laser polymerizationin conjunction with electron paramagnetic resonance spectroscopy.Termination of two types of radicals, secondary chain-end radicals(SPRs) and midchain radicals (MCRs), as well as intramolecular chaintransfer (backbiting) of SPRs have been studied between 5 and 40 °Cat initial AA concentrations of 10 and 50 wt % in the presence of15 wt % poly(AA). Predici modeling of the measured SPR and MCR concentrationvs time traces after single-pulse initiation at t= 0 yields rate coefficients for backbiting, kbb, and for propagation from an MCR, kpt. These rate coefficients increase toward lowerAA-in-water concentration. Estimates for termination of two SPRs, kts,s, and of an SPR and an MCR, kts,thave been obtained by assumingthe reported composite-model behavior for acrylates to also hold foracrylic acid polymerization. [ABSTRACT FROM AUTHOR]

pH and thermoresponsive polymers have been prepared by copolymerizing N-isopropylacrylamide (NIPAM) with various amounts of ionizable comonomers, either acrylic acid (AA) or N-[3-(dimethylamino)propyl]methacrylamide (MADAP). In aqueous solution, the LCST-type phase transition of these copolymers studied by differential scanning calorimetry is strongly influenced by the comonomer ratio. Under un-ionized conditions, the phase transition temperature progressively increases with MADAP content at pH 12 while it remains unchanged or slightly decreases with AA at pH 3 due to the formation of hydrogen bonds between AA and NIPAM units. When the copolymer chains are progressively charged by tuning the pH, the phase transition of PNIPAM-AA and PNIPAM-MADAP is shifted at higher temperature and is no longer observable below 60 °C when the ionic content exceeds 10%. By comparison with these single systems, where the association properties can be finely adjusted by coupling hydrophobic attractions and electrostatic repulsions, we also investigate the possibility to couple hydrophobic interactions with electrostatic attractions by mixing oppositely charged copolymers: PNIPAM-AA and PNIPAM-MADAP. This study was carried out with the copolymer pair containing 10 mol % of ionizable groups (A10 and M10) which was the most adaptable one from the point of view of responsivity. At pH 7, when AA and MADAP units are ionized, the copolymer chains are separately soluble in water in the whole temperature range while their mixture, also soluble at room temperature, phase separates upon heating. The original feature highlighted in this work is that the phase transition proceeds through a selective mechanism between complementary chains (formation of a reversible interpolyelectrolyte complex) and that this selectivity can be switched with the pH. Indeed, starting at room temperature with the copolymer mixture (A10 and M10), which remains homogeneous at all pH, we demonstrate that the association process can be switched by increasing the temperature and that the pH can be used to specifically address these associations: from A10/A10 at low pH to A10/M10 at pH 7 up to M10/M10 at high pH. Finally, the responsive precursors A10 were grafted onto a polyacrylamide backbone, and the viscoelastic properties of graft copolymers were studied in the semidilute regime and compared with calorimetric data. In this way, we show that the same set of interactions can be readily applied to more sophisticated macromolecular assemblies with the responsive formation of physical gels under pH and temperature control. [ABSTRACT FROM AUTHOR]

Poly(acrylic acid) (PAA) covalently immobilized on glass substrates was made fluorescent by grafting a BODIPY derivative (PMOH) via an ester linkage. Although only nanograms/square centimeter of polymer are understood to be immobilized onto the SiO2substrate, the fluorophore-tagged polymer was readily visible to the naked eye and its fluorescence was easily detected. The characteristic BODIPY emission, centered at 550 nm, was used to follow the degrafting of PAA from the glass substrates in aqueous solution in real-time using total internal reflection fluorescence (TIRF) microscopy. The substrate−initiator bond hydrolysis and the conditions at which the PAA degrafting occurred were unequivocally confirmed in real-time by TIRF microscopy. No cleavage of the polymer occurred between pH 6.5 and 10.5 in the absence of NaCl. In contrast, polymer degrafting from the substrate occurred at pH ≥ 9.5 when 10 mM NaCl was added to the buffer solution. [ABSTRACT FROM AUTHOR]

A series of novel amphiphilic 21-arm, starlike diblock copolymers, poly(acrylic acid)-b-poly(3-hexylthiophene) (PAA-b-P3HT), based on β-cyclodextrin (β-CD) with well-defined molecular architectures and ratio of two chemically distinct blocks were prepared, for the first time, via a combination of quasi-living Grignard metathesis method (GRIM), click reaction, and atom transfer radical polymerization (ATRP). The starlike PAA-b-P3HT diblock copolymers consist of hydrophilic coil-like PAA cores and hydrophobic rodlike P3HT shells with narrow molecular weight distribution and well-defined molecular weight of each block. Owing to the compact structure, the amphiphilic starlike PAA-b-P3HT formed a unimolecular micelle. Emulsion based on these novel amphiphilic starlike, coil–rod diblock copolymers were readily produced by cross-linking hydrophilic coil-like PAA cores with a bifunctional cross-linker, ethylenediamine. [ABSTRACT FROM AUTHOR]

Triblock copolymers were synthesized by ATRP with a poly(acrylic acid) (PAA) central block and random copolymer end blocks containing both AA and n-butyl acrylate (nBA) units. Self-assembly in aqueous solution was investigated over a wide range of concentrations. The degree of ionization of the AA units (α) was varied between 0.1 and 0.9 by varying the pH. The dynamic mechanical properties of the systems were investigated using oscillatory shear measurements. With increasing pH a transition from frozen hydrogels to dynamic networks was observed at pH = 5.2 (α = 0.30). The dynamic polymer networks behaved as viscoelastic liquids with a terminal relaxation time that decreased over 7 decades with increasing pH. This remarkable feature enables fine-tuning and control of the rheology. [ABSTRACT FROM AUTHOR]

Thin film assemblies of branched polyethylenimine (PEI) and poly(acrylic acid) (PAA), deposited using the layer-by-layer technique, were studied in an effort to produce all-polymer thin films with low oxygen permeability. Altering the pH of PEI and PAA results in large thickness variations (from 90 nm to 4.74 μm for 30-bilayer films). Cross-linking these films with glutaraldehyde (GA) create an inhibition of polymer interdiffusion, causing exponential film growth to be reset. AFM images show the surface morphology of PEI/PAA assemblies can be controlled by the pH and the final polymer layer deposited, with surface roughness ranging from 2.1 to 49.1 nm. Most pH combinations failed to produce a film with high oxygen barrier, but 8 bilayers of PEI at pH 10 and PAA at pH 4 produce a 305 nm thick film with an oxygen transmission rate below 0.005 cm3/(m2day). This unique thin film barrier (PO2< 3.2 × 10−21cm3(STP) cm/(cm2s Pa)) is a promising alternative to current polymeric membranes, flexible electronics, and food packaging materials. [ABSTRACT FROM AUTHOR]

The combination of click chemistry and layer-by-layer (LbL) assembly provides a useful and convenient means of preparing functional, covalently stabilized films and capsules. Herein, we examine various parameters that affect the buildup of click-LbL assembled multilayers of azide- and alkyne-modified poly(acrylic acid) (PAAAzand PAAAlk, respectively). We demonstrate that film thickness and morphology can be tailored by varying the assembly conditions. The thicknesses of multilayers assembled from PAAAzand PAAAlk[(PAAAz/PAAAlk)5] on planar substrates varied from ∼70 nm when assembled at pH 2.5 to 10 nm when prepared at pH 4. Increasing the ionic strength of the adsorption solution resulted in an increase in the (PAAAz/PAAAlk)5film thickness, with a maximum of ∼30 nm observed for solutions with ionic strengths of 150 mM and greater. A deposition time of 5 min was found to give close to saturated adsorbed layer amounts. Additionally, the influence of the click moieties on multilayer assembly was investigated. By altering the azide content of PAAAzand maintaining the alkyne content of PAAAlkat ∼15%, the thicknesses of (PAAAz/PAAAlk)5films were shown to increase exponentially from about 20 nm at 5% azide functionalization of PAAAzto 90 nm at 30% azide functionalization of PAAAz. Furthermore, atomic force microscopy measurements showed distinct morphological changes (i.e., enhanced porosity and/or creases and folds) for (PAAAz/PAAAlk)5films prepared from PAAAzwith different azide contents at pH 3.5 when subjected to basic conditions (pH 10). This was attributed to the different cross-linking degree between the multilayers. The current study was extended to the assembly of hollow polymer capsules to determine an optimum range of 10−20% azide functionalization of PAAAzfor the assembly of click polymer capsules. [ABSTRACT FROM AUTHOR]

The design of photoresponsive macromolecules has opened the route to many applications, in particular to trigger macroscopic responses induced by light irradiation in complex fluids and polymer−surfactant formulations. In this report, we studied the association of three sets of azobenzene modified polymer (AMPs) derived from poly(acrylic)acid with varying integration levels of azobenzene and various azobenzene hydrophobic moieties, with the neutral surfactant Triton X 100 (TX 100). Binding isotherms in dilute aqueous solutions were determined by spectrophotometry (to measure the fraction of bound azobenzene) and capillary electrophoresis (to measure the amount of bound TX 100). The degree of binding of TX 100 to AMPs increases markedly with increasing azobenzene hydrophobicity and density in AMPs. A noticeable and reversible photoresponse of the associates was observed upon exposure to UV/visible lights, although the magnitude of the UV-triggered photodissociation and blue-triggered association depends on the chemical structure of both the azobenzene and AMPs. We introduce a critical distance lcthat accounts as single parameter for the balance between energy gain of hydrophobic binding and energy loss due to chain conformational constraints. Only segments of chains flanked with two azobenzene groups at their ends and shorter than lcare assumed to bind tightly. lcis used to fit both the maximum fraction of azobenzene transferred into TX 100 micelles (with saturation well below 100% despite the presence of excess free TX 100) and the amount of bound TX 100 as a function of the density of azobenzene in the chains. The model includes the effect of random distribution of azobenzene moieties along the chains. From this analysis, we find criterions for optimization of the photoresponse as a function of the azobenzene hydrophobicity and density in the chain, and the chain length. [ABSTRACT FROM AUTHOR]

A novel type of hydrophobic association hydrogels (HA-gels) was prepared through micellar copolymerization of acrylic acid (AA), acrylamide (AAm) as basic monomers and a small amount of octylphenol polyoxyethylene ether acrylate with seven ethoxyl units (OP7-AC) as hydrophobic association monomer. The HA-gels exhibited desirable mechanical property and stably reversible phase transition between opaque and transparency. The influences of adding urea and varying AA:AAm molar ratio on the phase transition behavior were discussed, which indicated that the phase transition was introduced by forming or dissociating of hydrogen bonding between amide and carboxyl groups. The introduction of hydrophobic units (OP7-AC) to poly(acrylic acid-co-acrylamide) (P(AA-AAm)) copolymer would result in the adulterating and cross-linking effects on the transition temperature. The former sharply reduced the transition temperature while the later gradually raised it. The transition temperature became linearly dropping with the increasing sodium dodecyl sulfate (SDS) content in the HA-gels. Therefore, the phase transition temperature can be finely adjusted by means of changing AA:AAm ratio, concentration, OP7-AC and/or SDS dosages in the synthesis of HA-gels. [ABSTRACT FROM AUTHOR]

We report some new and interesting observations on the abrupt and large shear-induced thickening of aqueous solutions of hydrophobically modified poly(N,N′-dimethylacrylamide-co-acrylic acid). High molecular weight copolymer was prepared by free radical copolymerization of N,N′-dimethylacrylamide [DMA] and acrylic acid [AA] and was subsequently modified to different extents using a hydrophobic compound, namely, 3-pentadecylcyclohexylamine [3-PDCA], which is derived from a renewable resource material, cashew nutshell liquid [CNSL]. The structural elucidation of the base copolymer and the hydrophobically modified copolymers was performed by 1H and 13C NMR spectroscopy. The zero shear viscosities [η0] of the hydrophobically modified polymers were lower than that of the precursor poly(N,N′-dimethylacrylamide-co-acrylic acid) until some critical polymer concentration, which increased with increase in hydrophobic modification. Above the critical concentrations, the η0of the hydrophobically modified copolymers surpassed that of the precursor at the same concentration. At moderate shear rates some of these hydrophobically modified copolymers exhibited an abrupt shear-induced thickening in which the viscosity of the samples increased severalfold. We show here from creep experiments that thickening occurs only when the shear rate reaches a critical value, γ̇crit, and that the thickened samples can be trapped in different metastable states by controlling the applied stress. Interestingly, the shear thickened samples showed further thickening upon decreasing the applied stress. Eventually, the metastable samples revert to their equilibrium states at characteristic time that depends on (small) probe stress. [ABSTRACT FROM AUTHOR]