Polyelectrolyte microgels based on poly-N-isopropylacrylamide: influence of charge density on microgel properties, binding of poly-diallyldimethylammonium chloride, and properties of polyelectrolyte complexes
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- Kleinen, J. & Richtering, W. Colloid Polym Sci (2011) 289: 739. doi:10.1007/s00396-011-2401-4
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The influence of the charge density of microgels on the binding of oppositely charged polyelectrolytes was investigated. The charge density in the microgels was varied via the amounts of charged comonomer (as e.g., methacrylic acid) during microgel synthesis and also by changing the reaction conditions in order to influence the distribution of the charged comonomer inside the poly-N-isopropylacrylamide-co-methacrylic acid microgel. The variation in charge density was monitored by taking advantage of the polyelectrolyte effect during acid–base titration. Data of titrations of several microgels were analyzed by a modified Henderson–Hasselbalch equation to monitor the influence of the charge density. The microgels contain either different amounts of cross-linker but same amounts of charged comonomer or the microgels were synthesized with same amounts of cross-linker but different functional monomers with different reactivities yielding different spatial distributions. Charge density and spatial distribution of charges in the microgel strongly influence swelling and interaction with polyelectrolytes. As expected, a highly charged microgel binds more polyelectrolyte than a microgel with low amount of charged groups. The amount, however, does not only scale with the number of charges per microgel but also with the charge density of the microgel. The lower the charge density of the microgel, the more polyelectrolyte per negative charge can bind. In addition, the charge density determines whether and at which composition charge reversal of the microgel–polyelectrolyte complexes occur.
KeywordsCharge densityPolyelectrolytesPolycationsMicrogelsMethacrylic acid
Mixing polycations and polyanions leads to a fast formation of complexes. The formation of polyelectrolyte complexes (so-called simplexes) is phenomenologically well-known; generalized quantitative mechanistic or kinetic models are, however, not available . Adding a polyelectrolyte to a solution containing an oppositely charged polyion leads usually to a turbid solution ending in phase separation of the complexes when the charge ratio of polyanion to polycation is close to unity. The charge density (average distance between two ionisable groups) of polyelectrolytes has no influence on the simplex formation . Deviation from the ideal stoichiometry can, however, occur if one of the polyelectrolytes is branched or sterically hindered [3–5] or if an inorganic salt (as e.g., NaCl) is present. The formation of so-called “quasi-soluble” complexes can either be obtained by using very small concentrations , by mixing a high molar mass polyion with a much shorter oppositely charged polyion in non-stoichiometric ratios , or by copolymerisation of an ionic monomer with another well water-soluble but non-charged monomer as e.g., ethylene oxide  or N-isopropylacrylamide . The investigation of these self-assembling monodisperse complexes containing block copolymers with a charged and a non-charged but hydrophilic part led to many publications in the last 15 years. Different names are found in the literature to describe these structures such as block ionomer chains , polyion complex micelles , complex coacervate core micelles (C3Ms),  and (inter) polyelectrolyte complexes (I) [13, 14], and even anisotropic structures  were reported. These complexes often have no electrophoretic mobility when the ratio of negative and positive equals unity [16–18], but also systems are found which contain an excess of charged species than needed for charge compensation .
Polyelectrolytes can also interact with oppositely charged rigid colloidal particles. Polyelectrolytes can stabilize colloidal particles and prevent or promote aggregation . Especially the destabilization  of colloidal dispersions by polyelectrolytes depends on the amount of bound polyelectrolyte. A certain polyelectrolyte dose yields neutral particle–polyelectrolyte complexes and aggregation can occur. Further adsorption of polyelectrolyte leads to a restabilization because the complexes are now overcharged (charge reversal). The amounts of polyelectrolyte needed to overcharge rigid particles are typically in the range of 1 to 10 mg polyion per gram particle . Intuitively, the ratio of charges of adsorbed polyelectrolyte and surface charges of the colloidal particle should equal unity, similar to the complexes prepared from oppositely charged polymers. However, large variations can occur because polyelectrolyte charges are neutralized by counterion condensation and because the line charge density of the polylelectrolyte and the surface charge density of the particle differ [21, 22]. The amount of adsorbed polyelectrolyte thus depends on the charge density of both the particle and the polyelectrolyte and can be easily controlled by the ionic strength or by the pH value if one (or both) of the components can be protonated [23–25]. The binding of an oppositely charged species is limited by the surface charge of the rigid particle (or surface).
Microgels, however, are porous polymer networks of spherical shape and colloidal dimension. They are swollen by the solvent, and the charged groups are distributed not solely at the surface but also in the entire volume of the particle . The polymers used to synthesize microgels display often a temperature-dependent solubility in water. Changing the temperature alters the solvent quality, and the polymer can become insoluble, leading to a collapse of the microgel. Other stimuli besides temperature can be used to trigger the properties of the microgel if the microgel contains an adequate comonomer . The properties of the copolymer microgel will also depend on the nature of the comonomer, its amount, and the spatial distribution within the microgel [28–31]. Most often, charged comonomers are incorporated into microgels. These polyelectrolyte microgels can not only carry either positively or negatively charged comonomers but also polyampholyte microgel has been synthesized [32–35]. The addition of oppositely charged molecules (as e.g., surfactants  or polyelectrolytes ) to charged microgels leads to a strong interaction with the microgels and allows modification of the properties such as the size or the electrophoretic mobility. The temperature sensitivity of microgel–polyelectrolyte complexes is obtained, and the complexes show no tendency to flocculate when the electrophoretic mobility is neutral [38, 39], contrary to complexes of rigid nanoparticles and polyelectrolytes. The interaction of polyelectrolyte and microgels is mainly driven by electrostatics; non-specific interactions exist; however, only very small quantities of polyelectrolytes interact with pure poly(N-isopropylacrylamide) (PNiPAM) microgels .
The n value for monovalent acids equals 1 and becomes larger as soon as a correlation between two carboxylic acid functions (as e.g., along a polymer chain) exists. The smaller the (average) distance between two carboxylic acid functions, the higher the n value [2, 46, 47]. As a consequence, the ability of a system to keep the pH value constant at a certain degree of protonation decreases as the charge density increases. However, polyions with a high charge density cover a broad range of pH units during protonation.
The pKs value, which equals the pH value at 50% protonation, usually gives information about the chemical nature of the acid; acids can be classified in strong and weak ones. The potential (the pH value) to separate a proton from an acid characterized as weak is bigger compared to the potential needed to remove a proton from a strong acid. The pKs value depends on the chemical structure of the acids in case of monomeric acids. In multifunctional acids such as our microgels, the pKs value also depends on the cross-link density [46, 48, 49]. Deprotonating half of charged comonomers in a highly cross-linked microgel requires a higher pH value compared to a weaker cross-linked one. Deprotonation leads to a swelling of the microgel. The more the swelling is restricted by the network, the higher the potential (the pH value) to change the degree of protonation.
Nano-sized microgels have unique properties with respect to uptake and release , and the charge density of microgels will be relevant for binding of ionic species and the formation of polyelectrolyte multilayers [37, 51, 52]. In a previous publication , the penetration of polyelectrolyte into core–shell microgels was investigated. In the present study, we focus on the influence of microgels’ charge density on the binding with an oppositely charged polyelectrolyte (namely, polydiallydimethylammonium chloride (PDADMAC)), and four different microgels were synthesized for that purpose. The modified Henderson–Hasselbalch equation is used to analyze the charge density of the microgels. The ability of the Henderson–Hasselbalch equation to obtain information about the charge density of microgels is demonstrated by these sets of microgels.
The paper consists of two parts: First, the possibility to characterize the charge density of microgels by the modified Henderson–Hasselbalch equation is described. For this purpose, two series of microgels were prepared (1) PNiPAM-co-MAA with different cross-link densities and (2) PNiPAM copolymerized with different carboxylic acids but same amount of cross-linker. The second series reveals how the copolymerisation kinetics influences the distribution of the charged comonomer inside the microgel.
The second part of the manuscript is concerned with the influence of the charge density of the microgel on the binding of oppositely charged polyelectrolyte.
N,N′-methylenebisacrylamide (BIS) and potassium peroxodisulfate (KPS) were ordered from Merck. Methacrylic acid (MAA), acrylic acid (AA) and 3-butenoic acid (BA) were obtained by ABCR, and N-isopropylacrylamide (NiPAM) was purchased from Acros. PDADMAC, with high MW (400–500 kg mol−1), and polystyrenesulfonate (PSS), with a molecular weight of 70–100 kg mol−1, were delivered by Aldrich. Toluidine O was purchased from Serva Chemicals. All solutions were prepared with double-distilled water.
The microgels were synthesized by free radical polymerisation in water. Water was heated to 70 °C and flushed with nitrogen for 1 h. The monomers were dissolved, and the reaction was started by KPS. The synthesis was done in water, and the monomer concentration was around 2 mass%.
After 6 h, the solutions were allowed to cool down overnight, and the solution was centrifuged in a Sorvall Discovery 90 ultracentrifuge for 45 min. Between each centrifugation, the supernatant was removed and replaced by water to redisperse. After 3 cycles of centrifugation, the solution was freeze-dried.
Microgels with different amounts of cross-linker: Amounts of NiPAM were replaced by cross-linker (BIS). The concentrations of the cross-linker were 1.7, 3.5, 5.0, and 10.6 mass%. The amount of methacrylic acid (MAA) in the synthesis was around 8 mass% each. The microgels are named after the amount of cross-linker in the synthesis.
Microgels with different comonomers: The amounts of NiPAM, BIS, and charged comonomer were kept constant in all syntheses. The amount of charged co-monomer was 17.1 mmol. MAA, AA, and 3-BA were used. The microgels are named according to the comonomer.
Microgels used for the formation of microgel–polyelectrolyte complexes: The microgels contain methacrylic acid (MAA) as charged comonomer. Letters were used to name the microgels.
One hundred milligrams of microgel was dissolved in 50-mL water and transferred to a tempered titration cell equipped with a nitrogen inlet. HCl (0.1 M) was added until the pH value of the solution was around 3. After allowing the solution to equilibrate for 30 min, portions of 2-μL 0.1 M NaOH were added by a Methrohm 665 autotitrator. The titrations were performed at 25 °C. Conductivity and pH value were measured. Titrations were conducted from acidic to basic pH, and after 1 cycle, a second titration run was run from basic to acidic, yielding the same results.
The amount of pH-sensitive comonomer is obtained as absolute value from the buffering region. Information about the sequence of the monomer is obtained by making use of the polyelectrolyte effect.
Measurements of the hydrodynamic radius and electrophoretic mobility were performed on a Nano ZS zetasizer (Malvern). The values of the electrophoretic mobility were not converted to a zeta potential. The usual hard-sphere model may not be used to convert the mobility to a zeta potential since the microgels are porous, soft, and swollen by the solvent . The standard deviation of the measurements was less than 10%. For the sake of clarity, error bars are given in the plots for few data points only.
Dynamic light scattering
Dynamic light scattering measurements were done on an ALV goniometer with a programmable cyrostat to control the temperature of the sample. Laser wavelength was 633 nm, and scattering angle was 60°. The samples were highly diluted to avoid multiple scattering. The samples were measured at different angles at room temperature prior to temperature-dependent runs. Samples were measured for 180 s. The reproducibility of this method is 5% or better. Error bars with 5% deviation are given for the first and last data point of temperature-dependent measurements. The DLS measurements were done at pH values corresponding to full protonation and deprotonation, respectively. The pH values were determined by acid–base titration.
The indication of a 1:1 complex between polycation and polyanion by a dye responding to an excess of one polyion was first described in 1952 . We used a fototitrator which was a gift by BASF and described by Horn . A PSS solution was added by an autotitrator to a sample of PDADMAC, which was diluted with approximately 100 mL water. Twenty microliters of a 2-mM Toluidine O solution was added as indicator. Every PDADMAC solution obtained from the supernatant of microgel–polyelectrolyte mixtures was titrated at least three times to evaluate the concentration.
Prior to the determination of the amount of PDADMAC in the supernatant, calibration experiments were conducted: The concentration of the PDADMAC stock solutions was estimated by polyelectrolyte and argentometric titration: AgNO3 solution of known concentration was added to determine the concentration of the chloride counterions of the polycation by potentiometric titration.
Solutions of microgels were prepared by dissolving 100 mg microgel in 10 mL water. Different quantities of aqueous solution of PDADMAC solution were added to 1 mL of the microgel solution. Water was added to give a total volume of 10 mL. After 3 days of gentle stirring, the mixture was centrifuged for 30 min at 50,000 rpm in a Sorvall Discovery 90SE. The supernatant was removed carefully and kept for further experiments. Five milliliters of water was added to the precipitate in the centrifugation tube to redisperse the microgel–polyelectrolyte complex. After 2 days, the redispersed complex was investigated by different techniques. Prior to use, samples of all polyelectrolyte solutions were centrifuged at 50,000 rpm for 30 min. No precipitate was formed, and the supernatant had the same polyelectrolyte concentration as the starting solution.
Results and discussion
First of all, the possibility to characterize the charge density of microgels by the modified Henderson–Hasselbalch equation needs to be investigated. Therefore, the titration data of several sets of microgels were characterized. As a first set, four microgels with different amounts of cross-linker are discussed. The second set consists of three microgels with different comonomers. Finally, four microgels are characterized, and the amount of binding polyelectrolyte to the microgel is explained by the different charge densities of these microgels.
Influence of cross-link density
Properties of the PNiPAM-co-MAA microgels with different amounts of cross-linker
Electrophoretic mobility pH 9 20 °C/m2 V−1 s−1
Rh pH 3 20 °C/nm
Rh pH 3 50 °C/nm
Rh pH 9 20 °C/nm
Results from literature [41, 42] as well as the results in the second part of this publication prove that MAA is mainly incorporated near the cross-linked core region. Thus, deprotonation of the COOH groups requires a higher potential the stronger the cross-links restrict the swelling of the microgel. The slope of the titration curves gives information about the sequence of COOH groups along the polymer chains. The sequence stays constant, as indicated by the similar n values at around 1.7 (see Table 1). The microgel with 5.0 mass% BIS contains only 9.2 mass% MAA as compared to the other microgels with 11 mass%, and thus, the n value is only around n = 1.6.
The size of the PNiPAM-based microgels depends on temperature and pH value. The swelling ratio of the particles comparing different temperature and pH conditions will be described with two parameters:
Influence of comonomer
Three different microgels with different charged comonomers were prepared. These comonomers were methacrylic acid, acrylic acid, and 3-butenoic acid. For this reason, the amount of comonomer in the microgel is not given in milligrams per gram but in millimoles per gram to allow comparison of the three microgels.
Properties of the PNiPAM microgels with different functional comonomers
Incorporated comonomer/mmol g−1
Electrophoretic mobility pH 9 20 °C/m2 V−1 s−1
Rh pH 3 20 °C/nm
Rh pH 3 50 °C/nm
Rh pH 9 20 °C/nm
The pH-dependent measurements of the electrophoretic mobilities of the microgels with different comonomers are shown in Fig. 3, right side. The microgel containing AA as comonomer exhibits the highest electrophoretic mobility, although the PNiPAM-co-MAA microgel contains almost twice the amount of functional groups. This behavior can be explained by the different spatial distributions of the comonomers. The MAA groups are incorporated close to the core region, while AA groups are homogenously distributed within the particle. The βpH values also indicate the incorporation of the MAA groups near the core region.
The titration data discussed for these two series of different microgel samples reveal that data analysis with the modified Henderson–Hasselbalch equation provides interesting information on the properties of polyelectrolyte microgels. The findings about the spatial distribution of different comonomers in PNiPAM-based microgels agree nicely with the results reported by Hoare and Pelton who investigated the distribution of the charged groups also by analyzing titration data. All microgels were prepared in a way that these microgels contain the same amount of charged comonomer . In addition, we could show that the cross-link density of the microgel influences the pKs value but not the n value. In conclusion, the n value was found to be a sensitive parameter to describe the charge density of microgels [40, 56–58].
Characterization of the microgels used for the formation of microgel–polyelectrolyte complexes
Properties of the four different PNiPAM-co-MAA microgels
MAA/mass% in microgel
Rh pH 4 20 °C/nm
Rh pH 9 20 °C/nm
MAA/mass% in feed
The highly charged core–shell microgel possesses the highest n value of 1.9, which is close to the value of pure poly(methacrylic acid) of 2 . Microgel C (11.5 mass% MAA) is characterized with n = 1.7 and microgel A (3.1 mass% MAA) with n = 1.5. Microgel B (The 5.4 mass% MAA), however, has the lowest charge density of the four microgels because its n value is only 1.3.
The pH-dependent electrophoretic mobility of microgels B and C (5.4 mass% MAA and 11.5 mass% MAA) behaves very similarly at similar pH values. Microgel A (3.1 mass% MAA) shows only a very small change in the electrophotic mobility, while the mobility of microgel D (31.7 mass% MAA) is strongly influenced by the pH value of the solution.
The same comonomer (MAA) was used in all syntheses, but the synthesis conditions were different. Microgel B was synthesized at pH = 6, while the other microgels were synthesized at acidic conditions of pH = 3.5. The amounts of MAA added in the feed and obtained in the microgel are listed in Table 3. The 3.1 mass% and the 11.5 mass% microgels (microgels A and C) incorporate roughly the 1.3-fold amount of offered MAA, indicating a higher charge density compared to the 5.4 mass% microgel (microgel B). The amount of offered and incorporated amount of MAA are equal for the 5.4 mass% microgel, indicating a homogeneous charge distribution compared to the other microgels. This difference is also displayed by the low n value of only 1.3 and in a similar behavior of the pH-dependent electrophoretic mobility of microgels B and C. Microgel B contains a smaller quantity of charged comonomer, but the MAA is homogenously distributed in the microgel, while the microgel C is characterized by a high charge density in the core region of the microgel.
Important parameters as, e.g., size and number of charged comonomers per microgel can be controlled by the amount of cross-linker and comonomer. More important than these parameters, however, is the local charge density which can be controlled by the chemical nature of the comonomer or by the architecture of the microgel. The charge density of microgels can be easily probed by the modified Henderson–Hasselbalch equation as shown above. The charge density of microgels will have influence on the ability to bind oppositely charged polyelectrolytes. The ability of charged microgels with different charge densities to bind polyelectrolyte will be thus discussed in the following.
Complexes of the different microgels with PDADMAC
Microgels A–D were used to prepare complexes with the strong polycation (namely poly-diallyldimethylammonium chloride (PDADMAC)). The amount of bound polyelectrolyte will not be discussed as mass polyelectrolyte per mass microgel but as positive charges (of the polycation) per negative charge (of the microgel) to compare the different microgels.
The amount of negative charges in the microgels at a certain concentration is known from titrations as discussed above. Furthermore, polyelectrolyte and argentiometric titration enable us to determine the concentration of PDADMAC solutions. Then, the ratio between positive charges from the PDADMAC and the negative charges of the microgel when both species are mixed can be calculated and is named initial charge ratio (icr).
Adding a certain amount of PDADMAC to a microgel does not necessarily lead to full binding of PDADMAC, so the true composition of the microgel–polyelectrolyte complex can vary from the icr.
The composition of the complexes was experimentally determined by separating the non-attached PDADMAC from the redispersable complex by centrifugation. The amount of PDADMAC in the supernatant was quantified, allowing for the calculation of the amount of PDADMAC bound to the microgel. In the following, the composition of the complexes will be characterized by the nominal charge ratio (ncr). The ncr cannot be bigger than the icr. If the ncr = icr, all PDADMACs are bound to the microgel. The use of icr and ncr allows comparing the binding polyelectrolytes to the different microgels.
Compositions at charge reversal and maximum composition of the complexes prepared from PDADMAC and the four different microgels
MAA/mass% in microgel
ncr at charge reversal
Milligrams PDADMAC/g microgel at charge reversal
Maximal mg PDADMAC/g microgel
No charge reversal
The charge density of a microgel determines the composition at which charge reversal of the complexes occurs. The core–shell microgel possesses a weakly cross-linked but highly charged shell, and therefore, PDADMAC can bind in a way that all counterions of both the methacrylic acid as well as of the PDADMAC are released. This leads to a perfect arrangement of polycation and negatively charged network chains. That prevents extra binding of polycation beyond icr = 1 and charge reversal. Thus, binding of PDADMAC to microgel D is similar to the formation of C3Ms  between a polyelectrolyte and a charged but hydrophilic diblock polymer.
The negative charges in microgel C are mainly located near the core of the microgel. Thus, PDADMAC has to penetrate into the microgel. This steric hindrance leads to charge reversal, though the amount of charges of the polycation and the negative groups in the microgel does not equal unity (ncr < 1). Though charge reversal is observed at ncr < 1, PDADMAC binds quantitative up to ncr ≈ 1.2. More PDADMAC per negative charge can bind to the microgel since the charge density of the charged groups within the microgel is smaller compared to microgel D or to linear polyanions .
Decreasing the charge density of the microgels even further as in microgels A and B requires a higher amount of PDADMAC to overcharge the complexes. More PDADMAC per negative charge is needed since the low charge density cannot be balanced as the charged monomers in the microgel which are confined in the microgel network are different to polyelectrolytes, with different charge densities.
Surprisingly, microgel B (5.4 mass% MAA) and microgel C (11.5 mass% MAA) bind almost the same quantity of polyelectrolyte per mass microgel before charge reversal occurs. Charge reversal occurs at 183 and 173 mg PDADMAC per gram microgel, respectively. These masses correspond to nominal charge ratios of 1.8 and 0.8, respectively. More PDADMAC have to bind to microgel B before charge reversal occurs though the microgel C contains more than twice the amount of MAA. This behavior can be explained with the different charge densities of the microgels.
The composition and the measurements of the electrophoretic mobility of the complexes clearly show that the charge density of the microgels has an influence on both the bound amount of polyelectrolyte and the composition at which charge reversal occurs. The lower the charge density of the microgel, the more polyelectrolyte is needed per negative charge in the microgel in order to overcharge the microgel–polyelectrolyte complex. The influence of the charge density on the composition at which charge reversal occurs is also found for rigid nanoparticles as silica and latex  but not for polyelectrolyte simplexes . Linear polyelectrolytes with a low charge density can change their conformation, thus changing the distance between the charged groups, leading to precipitating complexes with an oppositely charged polyion of high charge density at 1:1 composition. Rigid nanoparticles cannot change their charge density, and bound polyelectrolyte of high charge density leads to overcharged complexes at charge ratios bigger than unity . The concept of the mismatch of the charge densities of polyelectrolyte and substrate is also valid for the investigated PNiPAM-co-MAA microgels and PDADMAC. The charge density of microgels can be increased, and a 1:1 complex is formed just as in simplex formation. This 1:1 complex is not charged but does not flocculate, contrary to complexes of polyelectrolyte and rigid nanoparticles  but similar to C3Ms . Both, microgel–polyelectrolyte complexes and C3Ms, are still stabilized by hydrophilic groups.
This study demonstrates the prominent properties of multisensitve microgels. The temperature- and pH-sensitive PNiPAM-co-MAA microgels merge qualities of (linear) polyelectrolytes and nanoparticles. It was found that the charge density of the microgel can be probed by titration and employing the modified Henderson–Hasselbalch equation. The charge density is a key parameter in determining the amount of polyelectrolyte binding to a microgel. Stable and soluble complexes of microgels and oppositely charged polyelectrolytes are formed over the investigated range of composition, similar to interpolyelectrolyte complexes. The amount of bound polyelectrolyte can be used to manipulate the electrophoretic mobility similar to rigid nanoparticles.
The study helps to understand and to predict the amount of binding polylelectrolyte to charged microgels. It correlates both the binding amount and the composition at which charge reversal occurs with the charge density of the microgels. The charge density of microgels can be influenced by parameters during the synthesis as, e.g., the pH value but also by the architecture of the microgel.
The results will help to tune the properties of microgels to optimize, e.g., the take-up of enzymes or DNA since the quantity of the binding species can be influenced by the amount and the distribution of charges in the microgel. The distribution of the charges thus allows adjustment of the composition at which charge reversal occurs. In addition, the charge density of microgel and microgel–polyelectrolyte complexes can be influenced by temperature. This potential trigger will be investigated to change the composition of microgel–polyelectrolyte complexes.
We thank Sebastian Wanders, Michael Kather, Christian Plum, and Manuel Noack for help with the microgel synthesis and complex formation, respectively. This work was supported by the Deutsche Forschungsgemeinschaft.