Water-in-oil emulsions stabilized by water-dispersible poly(N-isopropylacrylamide) microgels: understanding anti-Finkle behavior

Emulsions were prepared using poly(N-isopropylacrylamide) microgels as thermoresponsive stabilizers. The latter are well-known for their sensitivity to temperature: they are swollen by water below the so-called volume phase transition temperature (VPTT = 33 °C) and shrink when heated above it. Most of the studies reported in the literature reveal that the corresponding emulsions are of the oil-in-water type (O/W) and undergo fast destabilization upon warming above the VPTT. In the present study, whereas O/W emulsions were obtained with a wide panel of oils of variable polarity and were all thermoresponsive, water-in-oil (W/O) emulsions were found only in the presence of fatty alcohols and did not exhibit any thermal sensitivity. To understand the peculiar behavior of emulsions based on fatty alcohols, we investigated the organization of microgels at the oil-water interface and we studied the interactions of pNIPAM microgels with octanol. By combining several microscopy methods and by exploiting the limited coalescence process, we provided evidence that W/O emulsions are stabilized by multilayers of nondeformed microgels located inside the aqueous drops. Such behavior is in contradiction with the empirical Finkle rule stating that the continuous phase of the preferred emulsion is the one in which the stabilizer is preferentially dispersed. The study of microgels in nonemulsified binary water/octanol systems revealed that octanol diffused through the aqueous phase and was incorporated in the microgels. Thus, W/O emulsions were stabilized by microgels whose properties were substantially different from the native ones. In particular, after octanol uptake, they were no longer thermoresponsive, which explained the loss of responsiveness of the corresponding W/O emulsions. Finally, we showed that the incorporation of octanol modified the interfacial properties of the microgels: the higher the octanol uptake before emulsification, the lower the amount of particles in direct contact with the interface. The multilayer arrangement was thus necessary to ensure efficient stabilization against coalescence, as it increased interface cohesiveness. We discussed the origin of this counterexample of the Finkle's rule.     

Functional group distributions in carboxylic acid containing poly(N-isopropylacrylamide) microgels

Control of the functional group distribution is of fundamental importance in the design of functional polymer particles, particularly in biological applications. Surface-functionalized particles are useful for bioconjugation and medical diagnostics, while internally functionalized particles may have applications in drug delivery. We have prepared a series oftemperature-sensitive poly(N-isopropylacrylamide) (PNIPAM)-based microgels containing carboxylic acid functional groups via copolymerization with methacrylic acid and acrylamide, which was selectively hydrolyzed under optimized conditions to generate the carboxylic acid functionality. The resulting microgels were analyzed using conductometric and potentiometric titration, dynamic light scattering, and electrophoresis. Acrylamide-containing microgels hydrolyzed below the volume phase transition temperature (VPTT) show broad particle size versus temperature profiles, relatively low electrophoretic mobilities at basic pH, and time-dependent base titration profiles, suggesting the presence of internal functional groups whose titration is diffusion-controlled. Methacrylic acid containing microgels show sharper particle size versus temperature profiles, higher electrophoretic mobilities at basic pH, and time-independent base titration profiles, suggesting the presence of a "core-shell" structure with primarily surface functionalization. Similar results were obtained when acrylamide-containing microgels were hydrolyzed at temperatures above the VPTT. Thus, through selection of comonomer and hydrolysis conditions, we have developed strategies to control and characterize the number and distribution ofcarboxylic acid functional groups in PNIPAM-based microgels.     

Temperature-sensitive swelling of poly(N-isopropylacrylamide) brushes with low molecular weight and grafting density

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) brushes with different molecular weights M(n) and grafting densities σ were prepared by the "grafting-to" method. Changes in their physicochemical properties according to temperature were investigated with the help of in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Brush criteria indicate a transition between a brush conformation below the lower critical solution temperature (LCST) and an intermediate to mushroom conformation above the LCST. By in situ ellipsometry distinct changes in the brush layer parameters (wet thickness, refractive index, buffer content) were observed. A broadening of the temperature region with maximum deswelling occurred with decreasing grafting density. The brush layer properties were independent of the grafting density below the LCST, but showed a virtually monotonic behavior above the LCST. The midtemperature ?(half) of the deswelling process increased with increasing grafting density. Thus grafting density-dependent design parameters for such functional films were presented. For the first time, ATR-FTIR spectroscopy was used to monitor segment density and hydrogen bonding changes of these very thin PNIPAAm brushes as a function of temperature based on significant variations of the methyl stretching, Amide I, as well as Amide II bands with respect to intensity and wavenumber position. No dependence on M(n) and σ in the wavenumber shift of these bands above the LCST was found. The temperature profile of these band intensities and thus segment density was found to be rather step-like, exceeding temperatures around the LCST, while the respective profile of their wavenumber positions suggested continuous structural and hydration processes. Remaining buffer amounts and residual intermolecular segment/water interaction in the collapsed brushes above the LCST could be confirmed by both in situ methods.     

Viscoelastic properties of amorphous polymers 3: low molecular weight poly(methylphenylsiloxane)

By making creep and recoverable creep measurements of a nearly monodisperse low molecular weight poly(methyl phenyl siloxane) sample, we have found on decreasing temperature towardsT _g that there is continuously a change in the viscoelastic spectrum concomitant with a decrease of the steadystate recoverable compliance. This behavior is exactly the same as previously observed in low molecular weight poly(styrene), proving that this spectacular anomaly in the viscoelasticity of low molecular weight polymers is general and deserves an explanation. Photon correlation spectroscopic measurements performed on the same sample have extended the observation of the viscoelastic response to shorter times and the result corroborates the trend of variation established by the creep data.Dedicated to Prof.Dr. E. W. Fischer on his 65th Birthday. Prof.Dr. Fischer is known for his valuable contribution to fosterine, international collaboration of research in polymer science. This work is an example of his contribution because it would not be possible without him bringing us together. One of us (KLN) would like to take this opportunity to thank Prof. Dr. Fischer for his unwaiving support of the 1st (Crete) and the 2nd (Alicante) International Discussion Meeting on Relaxations in Complex Systems     

Polystyrene and polystyrene-butadiene latexes stabilized by poly(N-isopropylacrylamide)

Latexes stabilized by poly(N-isopropylacrylamide) (polyNIPAM) were prepared by polymerizing NIPAM in the presence of polystyrene and polystyrene-butadiene latex or by styrene emulsion polymerization in the presence of NIPAM. In 0.01 M CaCl₂ polyNIPAM stabilized latexes exhibited critical flocculation temperatures in the range 32–35°C, which is approximately equal to the lower critical solution temperature of polyNIPAM in water. Partial substitution of NIPAM with some acrylamide (AM) gave higher flocculation temperatures. Coagulation studies with cleaned latex indicated that the polyNIPAM or polyNIPAM-co-AM polymer chains were anchored to the latex particle surfaces.     

Thermosensitive pickering emulsion stabilized by poly(N-isopropylacrylamide)-carrying particles

Poly(N-isopropylacrylamide) (PNIPAM)-carrying particles were characterized as thermosensitive Pickering emulsifiers. Emulsions were prepared from various oils, such as heptane, hexadecane, trichloroethylene, and toluene, with PNIPAM-carrying particles. PNIPAM-carrying particles preferentially formed oil-in-water (O/W)-type emulsions with a variety of oils. All the emulsions stabilized by PNIPAM-carrying particles were stable for more than 3 months as long as they were stored at room temperature. However, when the emulsions were heated from room temperature to 40 degrees C, at which point the PNIPAM layer caused a coil-to-globule transition, phase separation occurred. Thus, by using thermosensitive PNIPAM-carrying particles as emulsifiers, the stability of the Pickering emulsions could be controlled by a slight change in temperature.     

Structural properties of thermoresponsive poly(N-isopropylacrylamide)-poly(ethyleneglycol) microgels

We present investigations of the structural properties of thermoresponsive poly(N-isopropylacrylamide) (PNiPAM) microgels dispersed in an aqueous solvent. In this particular work poly(ethyleneglycol) (PEG) units flanked with acrylate groups are employed as cross-linkers, providing an architecture designed to resist protein fouling. Dynamic light scattering (DLS), static light scattering (SLS), and small angle neutron scattering (SANS) are employed to study the microgels as a function of temperature over the range 10 °C ≤ T ≤ 40 °C. DLS and SLS measurements are simultaneously performed and, respectively, allow determination of the particle hydrodynamic radius, R(h), and radius of gyration, R(g), at each temperature. The thermal variation of these magnitudes reveals the microgel deswelling at the PNiPAM lower critical solution temperature (LCST). However, the hydrodynamic radius displays a second transition to larger radii at temperatures T ≤ 20 °C. This feature is atypical in standard PNiPAM microgels and suggests a structural reconfiguration within the polymer network at those temperatures. To better understand this behavior we perform neutron scattering measurements at different temperatures. In striking contrast to the scattering profile of soft sphere microgels, the SANS profiles for T ≤ LCST of our PNiPAM-PEG suspensions indicate that the particles exhibit structural properties characteristic of star polymer configurations. The star polymer radius of gyration and correlation length gradually decrease with increasing temperature despite maintenance of the star polymer configuration. At temperatures above the LCST, the scattered SANS intensity is typical of soft sphere systems.     

PGSE-NMR studies of solvent diffusion in poly(N-isopropylacrylamide) colloidal microgels

The solvent self-diffusion coefficient has been studied in thermoshrinking poly(N-isopropyl acrylamide) microgel dispersions by the pulsed-gradient spin-echo PGSE-NMR technique, as a function of temperature and mass fraction. After suitable corrections for the temperature, the H₂O/D₂O ratio and the relative volume fractions, all the self-diffusion data obtained over a temperature range of approximately 40 °C and mass fraction (2–12 % wt/wt) could be superimposed with the volume fraction as the universal factor. The observed reduction in the solvent self-diffusion coefficient with volume fraction was greater than that predicted by simple obstruction theory. After correction for-, and the subsequent removal of the obstruction effect, the diffusion of the solvent through the core of the particle is elucidated. As found for other polymer-solvent systems, there were no specific binding effects. The diffusion of the solvent in these dispersions over such temperature and mass fraction ranges could be rationalised assuming a constant solvent self-diffusion coefficient in the core of the particles.     

Template-guided synthesis and individual characterization of poly(N-isopropylacrylamide)-based microgels

We have succeeded in making cylindrical PNIPA-based microgels by a novel strategy in which template-guided synthesis and photochemical polymerization were combined. Cylindrical PNIPA-based microgels of relatively uniform sizes were easily obtained corresponding to the pore size of the template PC membranes. We have also individually characterized our PNIPA-based microgels by a laser trapping/Raman spectroscopy technique. On the basis of these results, the individual microscopic objects were confirmed to be PNIPA-based microgels.     

Layer-by-layer polyelectrolyte coating of low molecular weight poly(lactic acid) nanoparticles

Low molecular weight (M(w)) poly(L-lactic acid) (PLA) nanoparticles were coated with polyelectrolytes (PEs) by layer-by-layer (LbL) technique using a filtration approach. Poly(allylamine hydrochloride) and poly(sodium 4-styrenesulfonate) were applied as PEs in coating. LbL coating is aimed to use in producing (nano)particulate drug delivery systems with improved biocompatibility and sustained or targeted release of drug substances. Nanoparticles of rapidly biodegradable polymers, like the low M(w) PLA, open up a possibility to control the release of the encapsulated substance by the coating, but set challenges to the coating process due to increased aggregation tendency and degradation rate of the polymer. When the core PLA nanoparticles were prepared by nanoprecipitation, surface properties of the nanoparticles were affected by solvent selection. Successful LbL coating of the PLA nanoparticles was obtained only with chloroform, but not with dichloromethane as the solvent during nanoprecipitation. Reason for this was found to be the more charged surface of the nanoparticles prepared with chloroform compared to the nanoparticles prepared with dichloromethane.     

Different deswelling behavior of temperature-sensitive microgels of poly(N-isopropylacrylamide) crosslinked by polyethyleneglycol dimethacrylates

Polymerization of N-isopropylacrylamide (NIPAM) with polyethyleneglycol dimethacrylates (n G, n representing the number of --CH2CH2O-- units in polyethyleneglycol dimethacrylates) through surfactant-free radical polymerization was used to prepare the temperature-sensitive microgels. The morphology, dispersity, and deswelling behavior of the microgels were investigated by means of transmission electron microscopy (TEM), ultraviolet-visible spectroscopy, differential scanning calorimetry (DSC), and dynamic light scattering (DLS) techniques. TEM micrographs revealed that it was feasible to obtain regular spherical microgels for crosslinking agents with short chain. Turbidity, DSC, and DLS analysis showed that in marked contrast to 1G and 3G crosslinked microgels, the collapse of microgels crosslinked by 9G, 14G, and 23G proceeded in a two-step mechanism. The amide groups dehydrated at the lower temperature leading to the first-step transition. In the transition, the hydrophilic long --(--CH2CH2O--)n-- segments could be enriched on the surface of the microgels, which was further verified by variable temperature 1H NMR spectroscopy. The hydrophilic long --(--CH2CH2O--)n-- segments can be dehydrated at the higher temperature.     

A new route to poly(N-isopropylacrylamide) microgels supporting a polyvinylamine corona

Poly(N-isopropylacrylamide) thermoresponsive microgel particles with an amine-rich corona were prepared by the copolymerization of N-isopropylacrylamide with N-vinylformamide, NVF. Hydrolysis above the volume phase transition temperature converted the surface formamide moieties to the corresponding amine. The surface amine concentration was enriched by coupling iodine-terminated polyNVF oligomers (DP=7) to the microgel amines, followed by a second hydrolysis to give the corresponding polyvinylamine. Microgel swelling and electrophoretic mobility values as functions of pH and temperature were consistent with published results for amine-containing microgels.     

Preparation and characterization of poly (N-isopropylacrylamide)/polyvinylamine core-shell microgels

In this paper, well-defined temperature- and pH-sensitive core-shell microgels were synthesized by graft copolymerization in the absence of surfactant and stabilizer. The microgel particles consisted of poly (N-isopropylacrylamide (NIPAm)) core crosslinked with N, N′-methylene-bisacrylamide (MBA) and polyvinylamine (PVAm) shell. The effect of MBA content and NIPAm/PVAm ratio on microgel size was investigated. SEM showed that the microgels were spherical and had narrow particle-size distribution. TEM images of the microgels clearly displayed well-defined core-shell morphologies. Zeta-potential measurement further elucidated that the microgels possessed positively charged PVAm molecules on the microgel surface. Turbidity measurement and ¹H-nuclear magnetic resonance (NMR) experiments indicated that the VPTT of microgels was the same as the LCST of PNIPAm. ¹H-NMR experiments also inferred that the methyl proton of N-isopropylacrylamide appeared three peaks and responded to hydrogen-bonding interaction including polymer chain with water molecular, intramolecular interaction and intermolecular interaction, respectively.     

Melting behaviour of low molecular weight poly (ethylene-oxide) fractions

Summary Melting temperatureT _m and enthalpy of fusion have been measured, by DSC, for folded chain crystals of low molecular weight poly(ethylene-oxide) fractions ranging from 3000 to 10000. These crystals are formed by molecules folded a small integer number,n, of times and show unusual thermal stability on heating. The rates of chain unfolding during isothermal crystal growth and subsequent heating were measured and a reliable stability criterion could be defined for folded chain polymer crystals.Extending the theoretical treatment ofFlory andVrij to folded chain crystals and usingT _m data, a reasonable estimate was derived for the respective surface free energy contributions of chain ends and chain folds. The results suggest considerable hydrogen bonding between OH end groups, with a bonding energy of 3.1 Kcal/mole, when the crystal surface contains only chain ends. Hydrogen bonding is essentially destroyed by chain folding. Further analysis leads to an estimate of the contour length of cilia, associated with chain ends and to that of chain folds containing, on average, 2.8 and 3.5 monomer units respectively. Chain folds must thus be sharp involving adjacent re-entry. Finally, an analytical expression is derived showing the separate dependence ofT _m on chain length andn, parameters which determine the crystal thicknessL. Critical comparison of this relationship to that commonly used for determining surface free energies from linearT _m vs 1/L plots shows that the latter only applies accurately to chains of infinite length and to crystals of thickness larger than a critical valueL*.

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Résumé La températureT _m et l'enthalpie de fusion ont été mesurées sur des cristaux à chaines repliées de fractions de polyoxyéthyléne, de masse moléculaire variant entre 3000 et 10000. Ces cristaux sont constitués par des molécules repliées un nombren entier de fois et ils ont une stabilité thermique inhabituelle. La vitesse de dépliement des chaines a été mesurée lots de la croissance isotherme des cristaux et du chauffage consécutif et l'on a pu définir un critère de stabilité pour des cristaux à chaines repliées.En étendant la théorie deFlory etVrij aux cristaux à chaines repliées et en utilisant les valeurs deT _m on a pu estimer les contributions respectives des bouts de chaine et des repliements à l'énergie libre superficielle. Les résultats suggèrent un couplage important, par liaison hydrogène entre les groupes terminaux OH lorsque la surface des cristaux est constituée entièrement de bouts de chaine, l'énergie de couplage étant de 3,1 Kcal/mole. Ce couplage disparait pratiquement par le rephement des chaines. L'analyse des résultats permet aussi d'estimer la longueur des bouts de chaines et celle des repliements qui émergent du réseau cristallin: ils sont formés, respectivement de 2,8 et 3,5 unités de monomère, en moyenne. Les repliements sont serrés et relient des positions adjacentes du réseau. On déduit finalement une expression deT _m en fonction de la longueur des chaines et den, paramètres qui déterminent l'épaisseurL du cristal. La comparaison critique de cette relation avec celle généralement utilisée pour déterminer l'énergie libre superficielle, impliquant une variation linéaire deT _m avec 1/L, montre que cette dernière ne s'applique en toute rigueur qu'aux chaines de longueur infinie et aux cristaux d'épaisseur supérieure à une valeur critiqueL*.

     

The impact of the cononsolvency effect on poly (N-isopropylacrylamide) based microgels at interfaces

The paper addresses the effect of solid interfaces on the cononsolvency effect for poly(N-iso propylacrylamide) based microgels containing different contents of the co-monomer allyl acetic acid (AAA). The cononsolvency effect is studied by dynamic light scattering (DLS) in solution and with atomic force microscopy (AFM) at surfaces against different mixtures of water and organic solvent (ethanol, iso-propanol, and tetrahydrofuran). For the studies at interfaces, the microgels are spin coated on silicon wafers that are precoated with poly(allylamine hydrochloride) (PAH). The minimum in particle volume due to cononsolvency shows a pronounced shift from 10–20 % of organic solvent to 40–50 % after deposition at the Si/PAH wafer. The strong shift indicates an increase of water to organic solvent ratio within the gel at the surface with respect to the bulk solution. In order to understand the increase of water to organic solvent ratio, shrinking/reswelling AFM experiments for different spin-coating conditions and under ambient conditions are carried out. Spin coating from water instead from different solvent mixtures has no effect on the cononsolvency. In ambient conditions, the cononsolvency effect disappears     

Effects of Hofmeister anions on the flocculation behavior of temperature-responsive poly(N-isopropylacrylamide) microgels

Effects of some sodium salts (NaCl, NaClO₃, and NaSCN) in the Hofmeister series on deswelling and temperature-induced aggregation behavior of microgels of poly(N-isopropylacrylamide) (PNIPAAM) and PNIPAAM-co-PAA with attached poly(acrylic acid) moieties were investigated with the aid of turbidimetry and dynamic light scattering. Addition of salt in the concentration range 0.1–0.5?M generated aggregation of the PNIPAAM microgel particles at elevated temperatures, but it was no distinct difference between chaotropic and kosmotropic anions. In contrast, the flocculation behavior at high temperatures for PNIPAAM-co-PAA revealed a prominent influence of salinity and type of anion on the formation of aggregates. The aggregation transition was shifted to the highest temperature for the most chaotropic anion (SCN^?), and the aggregation transition at the same salt concentration is consistent with the typical Hofmeister series. The turbidity results from the PNIPAAM-co-PAA microgels disclosed a two-step transition for the considered anions, and both a low and high temperature change in the turbidity data was observed. The high-temperature transition followed the Hofmeister series.     

Effects of addition of anionic and cationic surfactants to poly(N-isopropylacrylamide) microgels with and without acrylic acid groups

The interaction of the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactant hexadecyl trimethyl ammonium bromide with poly(N-isopropylacrylamide) (PNIPAAM) microgels with and without poly(acrylic acid) (PAA) was investigated by means of dynamic light scattering (DLS), zeta potential, and turbidimetry measurements. The DLS results show that the PNIPAAM microgels with PAA will contract when an anionic or cationic surfactant is added to the suspension, while the PNIPAAM microgels without PAA expand in the presence of an ionic surfactant. A collapse of the PNIPAAM microgels is observed when the temperature is increased. From the zeta potential measurements, it is observed that the charge density of PNIPAAM microgels in the presence of an ionic surfactant is significantly affected by temperature and the attachment of the negatively charged PAA groups. The turbidity measurements clearly indicate that the interaction between PNIPAAM and SDS is more pronounced than that of the cationic surfactant.     

Surface interaction forces mediated by poly(N-isopropylacrylamide) (PNIPAM) polymers: effects of concentration and temperature

Total internal reflection microscopy was used to directly measure the interaction potentials between a micron-sized silica sphere and a flat silica surface in the presence of a linear poly(N-isopropylacrylamide) (PNIPAM) aqueous solution. When the PNIPAM concentration was low, no discernible forces were detected. A further increase in PNIPAM concentration resulted in a long-range attraction which was likely due to a combined of the reduced electrostatic interaction between the silica particle and the flat surface after the polymer adsorption and polymer bridges formation. On the other hand, for a fixed PNIPAM concentration, the interaction potential profiles between the particle and flat surface were once again characterized by attraction as temperature was increased. This attractive force can be explained in terms of the conformational changes of PNIPAM chains at the surfaces, which subsequently affected the polymer adsorption and enhanced the segment–segment interaction among the adsorbed polymer chains.     

One-step synthesis of low polydispersity, biotinylated poly(N-isopropylacrylamide) by ATRP

Low polydispersity poly(N-isopropylacrylamide) with a biotin end-group was obtained in one step from a biotinylated initiator for atom transfer radical polymerization and interacted with streptavidin to generate the thermosensitive polymer-protein conjugate.     

Thermodynamics of temperature-sensitive polyether-modified poly(acrylic acid) microgels

The temperature-induced structural changes and thermodynamics of ionic microgels based on poly(acrylic acid) (PAA) networks bonded with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) (Pluronic) copolymers have been studied by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), differential scanning calorimetry (DSC), and equilibrium swelling techniques. Aggregation within microgels based on PAA and either the hydrophobic Pluronic L92 (average composition, EO8PO52EO8; PPO content, 80%) or the hydrophilic Pluronic F127 (average composition, EO99PO67EO99; PPO content, 30%) was studied and compared to that in the solutions of the parent Pluronic. The neutron scattering results indicate the formation of micelle-like aggregates within the F127-based microgel particles, while the L92-based microgels formed fractal structures of dense nanoparticles. The microgels exhibit thermodynamically favorable volume phase transitions within certain temperature ranges due to reversible aggregation of the PPO chains, which occurs because of hydrophobic associations. The values of the apparent standard enthalpy of aggregation in the microgel suspensions indicate aggregation of hydrophobic clusters that are more hydrophobic than the un-cross-linked PPO chains in the Pluronic. Differences in the PPO content in Pluronics L92 and F127 result in a higher hydrophobicity of the resulting L92-PAA-EGDMAmicrogels and a larger presence of hydrophobic, densely cross-linked clusters that aggregate into supramolecular structures rather than micelle-like aggregates such as those formed in the F127-PAA-EGDMA microgels.