Colloidal Microgels Research Articles

Structural studies of poly(N‐isopropylacrylamide) microgels: Effect of SDS surfactant concentration in the microgel synthesis

AbstractThermoresponsive colloidal microgels were prepared by polymerization of N‐isopropylacrylamide (NIPAM) in the presence of a crosslinking monomer, N,N‐methylenebisacrylamide, in water with varying concentrations (<CMC) of an anionic surfactant, sodium dodecylsulphate (SDS). Volume phase transitions of the prepared microgels were studied in D2O by 1H NMR spectroscopy including the measurements of spin–lattice (T1) and spin–spin (T2) relaxation times for the protons of poly(N‐isopropylacrylamide) (PNIPAM) at temperature range 22–50 °C. In addition, microcalorimetry, turbidometry, dynamic light scattering, and electrophoretic mobility measurements were used to characterize the aqueous microgels. As expected, increasing SDS concentration in the polymerization batch decreased the hydrodynamic size of an aqueous microgel. Structures with high mobilities at temperatures above the LCST of PNIPAM were observed in the microgels prepared with small amount of SDS, as indicated by the relaxation times of different PNIPAM protons. It was concluded that the high mobility at high temperatures is in connection to a mobile surface layer with polyelectrolyte nature and with high local LCST. High SDS concentration in the synthesis was observed to prevent the formation of permanent, solid PNIPAM particles. The results from different characterization methods indicated that PNIPAM microgels prepared in high SDS concentrations appear to be more homogeneously structured than their correspondences prepared in low SDS concentration. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3305–3314, 2006

Volume phase transition and structure of poly(N-isopropylacrylamide) microgels studied with 1H-NMR spectroscopy in D2O

Thermoresponsive colloidal microgels were prepared by polymerisation of N-isopropylacrylamide (NIPAM) with varying concentration of a cross-linking monomer, N,N-methylenebisacrylamide (MBA), in water with either 0.4 or 6.7 mM concentration of an anionic surfactant, sodium dodecylsulphate (SDS). Volume phase transitions of the prepared microgels were studied in D2O by 1H-NMR spectroscopy including the measurements of spin–lattice (T1) and spin–spin (T2) relaxation times for the protons of poly(N-isopropylacrylamide) (PNIPAM) at temperature range 22–50 °C. In addition, microcalorimetry, turbidometry, dynamic light scattering and electrophoretic mobility measurements were used to characterise the aqueous microgels. The results from the different characterisation methods indicated that PNIPAM microgels prepared in 6.7 mM SDS concentration are structurally different compared to their correspondences prepared in 0.4 mM concentration. Increasing MBA concentration in the microgel synthesis appears to increase the structural heterogeneity in both cases of SDS concentration. PNIPAM structures with significantly higher molecular mobilities at temperatures above 35 °C were observed in the microgels prepared in 0.4 mM SDS concentration, as indicated by the 1H NMR relaxation times of different PNIPAM protons. We conclude that the high mobilities measured with NMR at elevated temperatures and also the clearly negative values of zeta potential are in connection to a fairly mobile surface layer with polyelectrolyte nature and a consequent high local lower critical solution temperature.

Polystyrene latex particles coated with crosslinked poly(N-isopropylacrylamide).

Thermoresponsive colloidal particles were prepared by seeded precipitation polymerization of N-isopropylacrylamide (NIPAM) in the presence of a crosslinking monomer, N,N-methylenebisacrylamide (MBA), using polystyrene latex particles (ca. 50 nm in diameter) as seeds in aqueous dispersion. Phase transitions of the prepared poly(N-isopropylacrylamide), PNIPAM, shells on polystyrene cores were studied in comparison to colloidal PNIPAM microgel particles, in H2O and/or in D2O by dynamic light scattering, microcalorimetry and by 1H NMR spectroscopy including the measurements of spin–lattice (T1) and spin–spin (T2) relaxation times for the protons of PNIPAM. As expected, the seed particles grew in hydrodynamic size during the crosslinking polymerization of NIPAM, and a larger NIPAM to seed mass ratio in the polymerization batch led to a larger increase of particle size indicating a product coated with a thicker PNIPAM shell. Broader microcalorimetric endotherms of dehydration were observed for crosslinked PNIPAM on the solid cores compared to the PNIPAM microgels and also an increase of the transition temperature was observed. The calorimetric results were complemented by the NMR spectroscopy data of the 1H-signal intensities upon heating in D2O, showing that the phase transition of crosslinked PNIPAM on polystyrene core shifts towards higher temperatures when compared to the microgels, and also that the temperature range of the transition is broader.

The use of colloidal microgels as a (trans)dermal drug delivery system

A co-polymer of poly( N-isopropylacrylamide) (85%) co-butyl acrylate (10%) co-methacrylic acid (5%) (NIPAM/BA/MAA) (85/10/5) microgel was synthesised and investigated as a potential pH and temperature sensitive transdermal delivery device. Three compounds having different octanol/water partition coefficients and solubilities were incorporated into the microgel, namely: salicylamide (SA), methyl paraben (MP) and propyl paraben (PP). Physico-chemical characterisation of these microgel–drug complexes showed that microgels incorporating MP and SA have smaller volumes after changing environmental pH or temperature when compared with the co-polymer NIPAM/BA/MAA (85/10/5) alone. This reduction in volume could be attributed to the incorporation of the compounds into the microgel particles, having a shielding effect on the charged groups present within the network. Diffusion studies, across human skin, were performed at 305 K in the range of pH 3–7 for saturated solutions of SA, MP and PP, and for microgel particles incorporating the three compounds. The transport rate for these microgels incorporating MP was reduced by 2/3-fold compared to the saturated solution, by one order of magnitude for PP, meanwhile the transport rate for these microgels incorporating SA is the same order of magnitude as that for the corresponding saturated solutions. Transdermal release studies of the saturated colloidal dispersions indicated that pH control of the drug release was marginal. The incorporation of compounds into the pH/temperature sensitive co-polymer NIPAM/BA/MAA (85/10/5) and the subsequent release depends on the octanol/water partition coefficient and solubility of the respective compound.

Glass transition dynamics of hard sphere like microgel colloids with short-rangedattractions

We have studied by dynamic light scattering the glass transition dynamicsof a binary mixture of polystyrene microgel particles with a size ratio ofRsmall/Rlarge = 0.81 and a number ratio of , where a short-ranged depletion attraction () was induced by addition of linear polystyrene. This system shows a reentrantglass transition. We have determined the glass transition lines of this system inthe experimental control parameter space given by the colloid volume fractionφ and the polymerconcentration cp by employing the power law of mode coupling theory (MCT) for theα-relaxation times. We find a reentry region which is much larger thanpredicted by theory and reported for another colloidal system with depletionattractions. Analysing the fluid dynamics along the transition lines with theβ-scaling law ofMCT we extract the cp dependence of the exponent parameterλ and the non-ergodicityparameter fqc. The results are in qualitative agreement with the prediction ofMCT for increasing attraction strength. The observed ‘jump’ offqc isindicative of a change of the close by glass phase from a packing-driven to a bonding-driven glass. Theincrease of λ to 0.91 and its subsequent decrease may indicate the neighbourhood of anA4 singularity.

Thermodynamic considerations of microgel swelling behavior.

A simple but novel thermodynamic model is presented, based upon van't Hoff analysis, for the reversible swelling behavior of colloidal microgels. The swelling, as a function of temperature, of poly(N-isopropylacrylamide/N,N'-methylenebisacrylamide) as well as poly(N-isopropylacrylamide/vinylpyridine/N,N'-methylenebisacrylamide) and poly(N-isopropylacrylamide/acrylic acid/N,N'-methylenebisacrylamide) microgel dispersions in H2O and D2O has been studied by photon correlation spectroscopy (PCS). PCS data was used to obtain the hydrodynamic diameter and hence the volume of the microgels (before and after reconstitution following freeze-drying) as a function of temperature. The choice of standard reference states, for analyzing the data attained, is discussed, and the one selected is that of the volume of the microgels at 333 K in H2O. For all microgels examined the volume, at this temperature, is shown to be independent of solvent (H2O, D2O). The derived data has allowed the exploration of a novel thermodynamic approach to the study of the swelling behavior of the microgels. The constant volume, at 333 K, for each of the polymer systems constituting the microgels is suggested to be an intrinsic property of the polymers themselves.

Small-angle neutron scattering study of structural changes in temperature sensitive microgel colloids.

The structure of temperature-sensitive poly(N-isopropylacrylamide) microgels in dilute suspension was investigated by means of small-angle neutron scattering. A direct modeling expression for the scattering intensity distribution was derived which describes very well the experimental data at all temperatures over an extensive q range. The overall particle form as well as the internal structure of the microgel network is described by the model. The influence of temperature, cross-linking density, and particle size on the structure was revealed by radial density profiles and clearly showed that the segment density in the swollen state is not homogeneous, but gradually decays at the surface. The density profile reveals a box profile only when the particles are collapsed at elevated temperatures. An increase of the cross-linking density resulted in both an increase of the polymer volume fraction in the inner region of the particle and a reduction of the smearing of the surface. The polymer volume fraction inside the colloid decreased with increasing particle size. The structural changes are in good agreement with the kinetics of the emulsion copolymerization used to prepare the microgel colloids.

Colloidal microgels as transdermal delivery systems

This paper presents a report of the synthesis of temperature-sensitive microgels based on a copolymer of butyl acrylate (10%) co-polyNIPAM (90%), in the presence of and in the absence of ibuprofen (IBU), methyl paraben (MP) and propyl paraben (PP), by a surfactant-free emulsion polymerisation in water. N ′, N ′-methylenebisacrylamide was used as a cross-linking agent and potassium persulphate as an initiator. Physicochemical properties of the microgels were determined using different techniques including dynamic light scattering and transmission electron microscopy. It is speculated that the microgel appearance is similar to a core-shell microgel, having in the core the complex IBU or MP or PP-butyl acrylate and in the shell poly(NIPAM). Permeation across a model silicone membrane and human skin was investigated over a range of temperatures (292–313 K). The transport rate of IBU and, PP from these poly(NIPAM) microgels is significantly reduced by two and one orders of magnitude, respectively, compared with the transport rate from saturated solutions. Such a reduction in flux was not however observed for MP.

Unresolved issues in the preparation and characterization of thermoresponsive microgels

AbstractThe preparation and properties of colloidal thermal responsive microgels based on poly(N‐isopropylacrylamide) are described with an emphasis on the unresolved issues which will impair commercial implementation. The main difficulties are: 1) removal of sol, surfactants and other impurities; 2) obtaining colloidally stable particles less than 50 nm in diameter; 3) synthesis of concentrated, colloidally stable microgel dispersions; 4) accurate measurement of water content; and, 5) control of microgel particle morphology.

Isothermal titration calorimetric studies of the acid–base properties of poly( N-isopropylacrylamide-co-4-vinylpyridine) cationic polyelectrolyte colloidal microgels

Isothermal titration calorimetry (ITC) and potentiometric titration were used to study the protonation properties of the 4-vinylpyridine (4-VP) moiety in cationic poly( N-isopropylacrylamide-co-4-vinylpyridine) colloidal microgels [poly(NIPAM-co-4-VP)]. Calorimetric pH titrations were performed using microgels of different 4-VP content and the influence of ionic strength and counter ions have been examined. The calorimetric titration output consists of several thermal contributions reflecting the complex nature of the interactions in the aqueous microgel dispersions. In contrast to the potentiometric results, the calorimetric titration data could not be completely described by a theoretical model solely taking into account protonation equilibria. Deviations from the proposed model correlate with swelling or shrinking of the gel particles. The calorimetric results also reveal a pronounced counter-ion effect of perchlorate compared to chloride ions. In the presence of perchlorate ions, small secondary thermal effects accompany protonation of the 4-VP moiety due, in part, to kinetically limited conformational changes in the co-polymer microgel.

Structural study of poly (N-isopropylacrylamide) microgels interpenetrated with polypyrrole

Thermosensitive cross-linked poly(N-isopropylacrylamide)[poly(NIPAM)] microgels in D2O present a continuous volume phase transition from swollen to deswollen states at a temperature within 32–34°C and the process is thermoreversible without hysteresis. The poly(NIPAM) microgels with 0.25% (w/w) cross-linker content were used as a matrix to entrap polypyrrole. Pyrrole was dissolved in the colloidal microgel dispersion and polymerisation was performed at three selected temperatures: 20°C in the swollen state of the microgel, at 37°C having the microgel collapsed and at 32.5°C corresponding to the beginning of the volume phase transition of the microgel. Dynamic light scattering, differential scanning calorimetry, small angle neutron scattering and transmission electron microscopy have been used to study the structural modifications induced by polypyrrole in these microgels. The small angle neutron scattering pattern was analysed using an expression that consists of solution-like concentration fluctuations given by an Orstein–Zernike type function, a solid-like concentration fluctuations described by a Gauss function and a contribution arising from the surface of the colloidal microgel. It seems that polymerisation of pyrrole, when the microgel particles are in the swollen state (20°C), results in a uniform distribution of polypyrrole within the particles. However, when the polymerisation is carried out at the beginning of the volume phase transition (32.5°C) an outer shell of polypyrrole could be formed. Finally, polymerisation of pyrrole at 37°C produced non-stable colloidal particles.

Phase behavior of thermally responsive microgel colloids.

The phase behavior of poly-N-isopropylacrylamide (PNIPAM) nanoparticles dispersed in water is investigated using a thermodynamic perturbation theory combined with light-scattering and spectrometer measurements. It is shown how the volume transition of PNIPAM particles affects the interaction potential and determines a novel phase diagram that has not been observed in conventional colloids. Because both particle size and attractive potential depend on temperature, PNIPAM aqueous dispersion exhibits phase transitions at a fixed particle number density by either increasing or decreasing temperature.

The effect of free polymer on the interactions and the glass transition dynamics of microgel colloids

By studying the phase behaviour and the short range order of polystyrene microgel colloids in a good solvent we demonstrated that these particles can be considered as hard spheres or nearly hard spheres if the internal cross link density is made sufficiently high. A binary mixture of 1:50 crosslinked (1 crosslink per 50 monomers) particles with a size ratio R(small)/R(large) = 0.83 and a number ratio N(small)/N(large) = 2.7 was characterized with respect to the short range order and glass transition dynamics. We found that this mixture can be mapped onto a one-component hard sphere system with an effective polydispersity of 11%, even though for the particle interactions an inverse power potential u(r) alpha r(-35) was established. The only significant difference observed was a shift of the glass transition to a higher volume fraction of 0.595. Thus, the binary mixture was used as a non-crystallizing reference system to study the effect of free polymer on interactions and glass transition dynamics. Addition of linear polystyrene with a size ratio delta = R(g,polymer)/R(colloid) = 0.054 induced short-ranged depletion attractions between microgel particles. In the case of a glassy sample already a rather small amount of polymer leads to melting of the glass and a re-entrant glass transition is observed at high polymer content. From the difference of the line shapes of the density autocorrelation functions on approaching the different transition lines. measured over the full dynamic range, a change of the freezing mechanism from a packing-driven glass transition at low polymer content to a bonding-driven glass transition at high polymer content was deduced.

Novel Gelling Behavior of Poly(N-isopropylacrylamide-co-vinyl laurate) Microgel Dispersions

Aqueous colloidal microgel particles prepared from N-isopropylacrylamide (NIPAM) co-polymerized with different ratios of the comonomer vinyl laurate have been investigated with respect to their phy...

The effect of free polymer on the glass transition dynamics of microgel colloids

From the phase behavior we establish that 1:10 crosslinked polystyrene microgel colloids in a good solvent can be considered as hard spheres whereas 1:50 crosslinked particles are slightly softer. Nevertheless, the glass transition dynamics of 1:10 microgels and a binary mixtures of 1:50 microgels can be described in both cases within the mode coupling theory for hard spheres. The only difference is that the increase of the polydispersity from about 7% to about 13% when going from the one-component system to the mixture is accompanied by a shift of the glass transition from ϕ g≈0.56 to ϕ g≈0.595. On addition of linear polystyrene to the 1:50 binary mixture we find a tremendous acceleration of the density fluctuations and the melting of a colloidal glass. This is accompanied by a significant decrease of the first peak of the static structure factor and its shift to higher scattering vectors. The presence of free polymer in colloidal dispersions is known to act as an effective attraction via the `depletion effect'. Thus, our observations are consistent with the predictions of recent extensions of mode coupling theory [Phys. Rev. E 63 (2001) 1401] where the inclusion of short-ranged attractions has been found to shift the glass transition line to significantly higher volume fractions.

Internal dynamics in colloidal PNIPAM microgel particles immobilised in mesoscopic crystals

Abstract Internal dynamics of colloidal microgels based on N -isopropyl-acrylamide is investigated by means of neutron spin-echo spectroscopy. To avoid interference of the relaxation of interest with the centre of mass diffusion, the microgels were immobilised in a colloidal crystal. Due to the q -range used in these quasi-elastic neutron scattering experiments, it is possible to obtain nearly completely decaying ensemble averaged intermediate scattering functions 〈ISF〉 E . These can be fitted by single exponential forms, simplifying largely the data analysis compared with photon correlation spectroscopy. This is due to the fact that for light q 2 〈 ξ 2 〉→∞ can not be reached, whereas for neutrons this condition is approximately fulfilled. The obtained network diffusion coefficients decrease linearly with increasing cross-linker concentration. This is in contrast to previous results from photon correlation spectroscopy obtained for macroscopic poly( N -isopropyl-acrylamide) gels [Macromolecules, 29 (1996) 8746].

Thermal diffusion and soret feedback of gold-doped polyorganosiloxane nanospheres in toluene

We have investigated diffusion and thermal diffusion properties of light-absorbing colloidal polyorganosiloxane microgels containing tiny nanometer-sized gold clusters dispersed in toluene. Transient holographic gratings allow for very subtle perturbations in the linear regime where Soret feedback is negligible. Gold-doped colloids of different size and crosslink ratios show different Soret coefficients but identical thermal diffusion coefficients D(T). Undoped colloids tend to aggregate, but a consistent interpretation is obtained if an identical D(T) is assumed for the doped, the undoped, and the aggregated particles. Previously reported Soret feedback measurements on similar systems incidentally yielded comparable Soret coefficients. We show, however, that they suffer from strong convective perturbations.

Influence of charge density on the swelling of colloidal poly( N-isopropylacrylamide-co-acrylic acid) microgels

The volume phase transition of colloidal poly( N-isopropylacrylamide-co-acrylic acid) microgels depends in a complex way on the effective charge density within the polymer network. A series of monodisperse PNIPAM/AAc microgels with different content of acrylic acid were synthesized by surfactant-free emulsion polymerization employing sonication instead of a conventional stirring technique. Subsequently, the colloids were characterized by dynamic light scattering and electron microscopy. Potentiometric titrations provided the amount of carboxyl groups incorporated into the copolymer. The effective charge density was systematically controlled by the content of acrylic acid monomers, the pH value of the suspension, and the salt concentration. The hydrodynamic dimensions of the microgels have been measured by dynamic light scattering. The swelling/deswelling behavior is determined by the delicate balance between hydrophobic attraction of NIPAM and the repulsive electrostatic interactions of the carboxylate group of the acrylic acid moieties. Compared to their macroscopic counterparts the charged microgel particles show a significantly different swelling/deswelling behavior. This manifests in the occurrence of a two-step volume phase-transition process with increasing acrylic acid content. Hydrogen bonding has to be considered to understand this two step volume phase transition uniquely observed for colloidal microgels. Another interesting phenomenon presented here is the reversible formation of well-defined aggregates at low pH and under high salt conditions.

Effect of connectivity and charge density on the swelling and local structural and dynamic properties of colloidal PNIPAM microgels

AbstractThe structural and dynamic properties of thermosensitive neutral and partially charged colloidal microgels of poly(N‐isopropylacrylamide‐co‐acrylic acid) (PNIPAM/AAc) have been investigated by neutron and light scattering techniques. Introduction of cross‐linker induces topological constraints into the polymer network, and hence reduces the swelling capability of the particles as expressed in the variation of their hydrodynamic radii. Small‐angle neutron scattering experiments reveal that below the transition temperature the inhomogeneous structure of the microgel consists of local solid‐like frozen regions and solution‐like regions where the polymer chain behaves as in semidilute polymer solution. The temperature‐induced collapse leads to a strongly segregated micro‐phase separated particle. The swelling of charged microgels is determined by their charge density, controlled through the amount of acrylic acid comonomer, the pH, and the salt concentration.

Spherical microgel colloids — Hard spheres from soft matter

AbstractWhile gels are usually considered to be soft materials, we demonstrate that it is possible to model hard sphere behaviour when the gel structure is confined to spherical objects of sizes in the colloidal range. We have measured the static structure factor of microgel spheres dispersed in good solvents, differing in size, crosslink density and swelling behaviour, by light scattering and small angle neutron scattering. Comparing with theoretical calculations for polydisperse hard spheres we show how the interactions in highly concentrated dispersions of spherical microgel colloids are determined by an interplay of the relative length of dangling polymer ends at the surface (determined by the average crosslink density and the particle size) and the swelling ratio (determined by the average crosslink density and the solvent quality). In addition the swelling ratio controls the volume fraction range in which colloidal behaviour is not superposed by the gel character. The latter leads to a loss of structural order due to the deformability of the microgel spheres, an effect that occurs at increasingly lower volume fractions if the swelling ratio increases. To achieve hard sphere behaviour up to very high volume fractions, thus, requires high crosslink densities and small swelling ratios.