Preparation and characterization of poly (N-isopropylacrylamide-co-dimethylaminoethyl methacrylate) microgel latexes

Monodisperse cationic thermosensitive latex microgels have been prepared by radical-initiated precipitation polymerization of N-isopropylacrylamide, methylene bisacrylamide using 2,2′-azobis(2-amidinopropane hydrochloride) as an initiator and dimethylaminoethyl methacrylate (DMAEMA) as a cationic monomer. The final microgel latexes were characterized with respect to water-soluble polymer formation, particle size and size distribution. Adding cationic monomer (DMAEMA) was found to drastically affect the particle size, but not the size distribution as observed both by transmission electron microscopy and quasielastic light scattering (QELS). However, too high a DMAEMA concentration in the feed composition led to enhanced formation of water-soluble polymer. The volume phase-transition temperature of cleaned microgels examined by QELS (particle size versus temperature) was found to be around 32 °C and was slightly dependent on the concentration of the cationic monomer. The volume phase-transition temperature range becomes broader with increasing cationic monomer concentration. In addition, the pH of the polymerization medium was found to affect the final particle size and amount of water-soluble polymer formed. Received: 29 March 2001 Accepted: 2 July 2001     

A new strategy to prepare temperature-sensitive poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) microgels

In this study, a new method was developed to prepare temperature-sensitive poly(N-isopropylacrylamide) microgels by free radical precipitation polymerization using siloxane coupling agent as the new crosslinker. Ammonium persulfate acted as the initiator for the radical copolymerization as well as the catalyst for the hydrolysis/condensation of the siloxane groups. The particle diameter and polydispersity of the microgels were measured by photon correlation spectroscopy and the results display that the microgels are monodisperse. The microgels exhibit temperature sensitivity and the phase transition temperature is approximately 31 °C. Furthermore, the diameter of the microgels changes upon heating and cooling processes. These were observed to be reversible. The novel crosslinking method described herein is the condensation of siloxane groups, which is totally different from the traditional double-vinyl crosslinkers. This innovative approach offers an alternative path to prepare functional core–shell particles and inorganic/organic hybrid materials.     

Copolymer microgels by precipitation polymerisation of N-vinylcaprolactam and N-isopropylacrylamides in aqueous medium

In this work, we designed copolymer microgels by the copolymerisation of N-vinylcaprolactam (VCL) and two acrylamides (N-isopropylacrylamide (NIPAAm) and N-isopropylmethacrylamide (NIPMAAm)) under precipitation conditions in aqueous phase. In synthesis protocols, the ratio between monomers was varied from 1:5 to 5:1 mol/mol. By NMR and Raman spectroscopy, we determined the chemical composition of PVCL/NIPAAm and PVCL/NIPMAAm copolymer microgels reflecting the initial monomer ratio in the reaction mixture. The hydrodynamic radii of PVCL/NIPAAm microgels are around 375 nm (at 25 °C) and do not vary with the copolymer composition. On the contrary, for PVCL/NIPMAAm microgels, the size decreases from 450 to 250 nm with an increase of the VCL amount in copolymer structure. The heterogeneity of the microgel structure in terms of the distribution of the monomer units was probed by ¹H transverse magnetization relaxation NMR, showing that the VCL, NIPAAm and NIPMAAm units are unorderly distributed in the colloidal networks. The investigation of volume phase transition temperature (VPTT) for copolymer microgels was performed using dynamic light scattering, NMR and differential scanning calorimetry. It has been found that PVCL/NIPAAm microgels show VPTT around 35 °C independently from the copolymer composition; however, PVCL/NIPMAAm particles exhibit a nonlinear increase of VPTT from 34 to 45 °C as the NIPMAAm fraction in copolymer structure increases.     

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.     

Synthesis,characterization, and influence of synthesis parameters on particle sizes of a new microgel family

Poly(p‐nitrophenylacrylate‐co‐methacrylamide) and poly(p‐Nitrophenylacrylate‐co‐N,N′‐isopropylacrylamide) reactive microgels were synthesized by precipitation polymerization. The process was followed qualitatively by infrared spectroscopy (ATR‐FTIR) and microgels composition was determined by nuclear magnetic resonance (¹H NMR). Scanning electron microscopy of obtained colloidal particles showed strictly spherical morphologies with a moderate polydispersity. The average hydrodynamic particle diameter and particle size distributions were measured by quasi‐elastic light scattering and the particle size distributions obtained ranged from 100 to 600 nm. Several synthetic parameters affect the particle size of these materials and thus, indirectly, their properties and future applications. In this article, we report the influence of different polymerization reaction conditions in the final microgel dimensions. For example, we observed that the different solvent‐comonomer affinity induced a significant change in swollen particle size of the copolymeric microgels. On the other hand, the crosslinking density limited the particle sizes, but an excess of crosslinker content in the reaction mixture resulted in the opposite effect. Finally, we also studied the influence of initiator content in the mean particle size. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3833–3842, 2007     

Synthesis and optical properties of poly(N-isopropylacrylamide) nanogel containing silver nanoparticles

Composite nanoparticles representing silver nanoparticle-containing polymer gels have been synthesized. The synthesis comprises two main stages. Initially, monodisperse hydrogel particles with a controlled diameter of approximately 500 nm are obtained by N-isopropylacrylamide polymerization. Then, silver ions are reduced on the surface of the polymer network. Variations in the concentration ratio between reductants and silver nitrate make it possible to produce silver nanoparticles with sizes in a range of 10–30 nm and different packing densities on the gel particle surface. The resultant nanocomposites have been studied by transmission electron microscopy, spectrophotometry, and dynamic light scattering. Depending on the size and packing density of the silver nanoparticles on the polymer particle surface, the plasmon resonance of the nanocomposites varies in a range of 420–750 nm, which determines variations in the color of the colloid from yellow, orange, and red to blue and blue-green. After the inclusion of silver nanoparticles, nanogels of poly(N-isopropylacrylamide) retain their capability for thermosensitive phase transition with a lower critical mixing temperature of 31°C.     

Evidences of a hydrolysis process in the synthesis of N-vinylcaprolactam-based microgels

The hydrolysis process suffered by vinylcaprolactam (VCL) monomer during polymerization and the way of avoiding it is a matter of considerable interest when the final material is used in biomedical applications. In this work, in order to study the hydrolysis process in the synthesis of VCL-based microgels, VCL was polymerized by emulsion polymerization in batch and semi-batch reactors by using N,N′-methylenebisacrylamide (BA) as cross-linker and potassium persulfate (KPS) as initiator. Moreover, the partial conversion evolutions of VCL and BA were analyzed by ¹H NMR. The colloidal characterization was carried out by measuring the final average diameters as a function of temperature. The results showed that there were two competitive reactions (hydrolysis and propagation) in the polymerizations carried out in the absence of a buffer. However, VCL did not suffer hydrolysis by adding a buffer in the recipes. A new reliable method for the determination of the partial conversions was developed using the quantitative NMR method. The final cross-linked colloidal particles (microgel particles) obtained in the buffered reaction presented the conventional swelling de-swelling behavior corresponding to temperature sensitive microgels.     

Preparation, characterization and enzyme inhibition of methylmethacrylate copolymer nanoparticles with different hydrophilic polymeric chains

Methylmethacrylate copolymer nanoparticles with different hydrophilic chains were prepared by the free radical polymerization of methylmethacrylate with N-isopropylacrylamide (NIPAAm), N-methacrylic acid (MAA), N-trimethylaminoethylmethacrylate chloride (TMAEMC) or N-dimethylaminoethylmethacrylate hydrochloride (DMAEMC). These particles were characterized by particle size and zeta potential. The polymerization conditions were shown to influence the particle size and surface charge. Particle sizes of MMA-NIPAAm nanoparticles after 3 h of reaction reached constant level at 180 nm. An increasing amount of total monomer (0.5-5%) would result in the nanoparticles of particle size of 115-204 nm for 30% NIPAAm of the total monomer. In the same range of 5-40% NIPAAm of the total monomer, the particle size decreased from 280 to 170 nm. The concentration of the initiator APS up to a concentration of 0.2% for MMA-TMAEMC and 0.1% for MMA-NIPAAm showed no effect on the particle size of the final nanoparticle suspensions, while higher concentration would lead to aggregation in the polymerization process. MMA-NIPAAm nanoparticles were pH-dependent in zeta potential at pH 1-12 values reducing from 12.2 mV to −16.8 mV, respectively. Nanoparticles were incubated with pepsin and trypsin at 37 °C for 20 min and their enzyme inhibition was determined. The activity of pepsin decreased to 27% in the presence of MMA-NIPAAm nanoparticles, and MMA-MAA nanoparticles reduced the activity of trypsin to 39%, respectively.     

N‐vinylcaprolactam‐based microgels: Synthesis and characterization

Poly(N‐vinylcaprolactam) (PVCL) is well known for its thermoresponsive behavior in aqueous solutions. PVCL combines useful and important properties; it is biocompatible polymer with the phase transition in the region of physiological temperature (32–38 °C). This combination of properties allows consideration of PVCL as a material for design biomedical devices and use in drug delivery systems. In this work, PVCL based temperature‐sensitive crosslinked particles (microgels) were synthesized in a batch reactor to analyze the effect of the crosslinker, initiator, surfactant, temperature, and VCL concentration on polymerization process and final microgels characteristics. The mean particle diameters at different temperatures and the volume phase‐transition temperature of the final product were analyzed. To obtain information about the inner structure of microgel particles, semicontinuous polymerizations were carried out and the evolution of the hydrodynamic average particle diameters at different temperatures of the microgel synthesized was investigated. In the case of microgel particles obtained in a batch reactor the size and the swelling‐de‐swelling behavior as a function of the temperature of the medium can be tuned by modulating the reaction variables. When the microgel particles were synthesized in a semibatch reactor different swelling‐de‐swelling behaviors were observed depending on particles inner structure. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2510–2524, 2008     

Poly(methyl methacrylate)/poly(N-isopropylacrylamide) core-shell particles prepared by seeded precipitation polymerization: Unusual morphology and thermo-sensitivity of zeta potential

Poly（methyl methacrylate）/poly（N-isopropylacrylamide） （PMMA/PNIPAM） core-shell particles were synthesized by seeded precipitation polymerization of N-isopropylacrylamide （NIPAM） in the presence of PMMA seed particles. The anionic potassium persulfate was used as initiator, and acrylic acid as functional comonomer. It was shown that the weight ratio of the PNIPAM shell to the PMMA core can be greatly increased through continuous addition of NIPAM monomer at a relatively slow rate. PMMA/PNIPAM particles with different shell thickness were obtained by varying the amount of charged NIPAM monomers. These particles exhibited unique nonspherical core-shell morphology. PMMA core was partially coated by dense hair-like or antler-like PNIPAM shell depending on the shell thickness. The measurement of these particles＇ zeta potential at different temperatures showed that the absolute value of zeta potential unusually decreased as the particle size decreased with temperature.     

Temperature-programmed synthesis of micron-sized multi-responsive microgels

A new synthetic protocol for the synthesis of large diameter (2.5 to 5 μm), temperature-, and pH-responsive microgels via aqueous surfactant-free radical precipitation copolymerization is presented. We have found that in this size range, which is not typically attainable using traditional dispersion polymerization approaches, excellent monodispersity and size control are achieved when the synthesis employs a programmed temperature ramp from 45 to 65 °C during the nucleation stage of the polymerization. A combined kinetic and thermodynamic hypothesis for large particle formation under these conditions is described. Particle sizes, volume phase transition temperatures, and pH responsivity were characterized by particle tracking and photon correlation spectroscopy to illustrate their similar behavior to particles made via more traditional routes. These particles have been enabling for various studies in our group where microscopic visualization of the particles is required.     

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.     

Poly (N-isopropylacrylamide) microgel based assemblies for organic dye removal from water: microgel diameter effects

Poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel based assemblies (aggregates) were synthesized from microgels of various diameters via polymerization of the crosslinker N,N′-methylenebisacrylamide (BIS) in the presence of microgels in solution. We investigated the ability of the respective aggregates to remove the organic, azo dye molecule 4-(2-hydroxy-1-napthylazo) benzenesulfonic acid sodium salt (Orange II) from water at both room and elevated temperatures. The results from the microgel aggregates made from 1.1-μm-diameter [Parasuraman and Serpe. ACS Applied Materials & Interfaces, 2011] microgels were compared to aggregates synthesized from 321-nm and 1.43-μm-diameter microgels. Aggregates made from the same size microgels showed increased uptake efficiency as the concentration of BIS in the aggregates was increased, while for a given BIS concentration, the uptake efficiency increased with increasing microgel size in the aggregate. We attribute this to the “nature” of the aggregates; aggregates have void space between the microgels that can serve as reservoirs for Orange II uptake—the void spaces are hypothesized to increase with larger diameter microgels. By exploiting the thermoresponsive nature of the microgels, and microgel based aggregates, 85.3 % removal efficiencies can be achieved. Finally, all uptake trends for the aggregates, at room temperature, were fit with a Langmuir sorption isotherm model.     

The kinetics of poly(N-isopropylacrylamide) microgel latex formation

Conversion versus time curves were measured for poly(N-isopropylacrylamide) microgel latexes prepared by polymerization in water with sodium dodecyl sulfate, SDS. Polymerization rates increased with temperature with methylenebisacrylamide crosslinking monomer consumed faster thanN-isopropylacrylamide. The particle diameter decreased with increasing concentrations of SDS in the polymerization recipe and there was evidence that the rate of polymerization increased somewhat with SDS concentration. Particle formation occurred by homogeneous nucleation as micelles were absent.Comparison of particle size distributions from dynamic light scattering to those from a centrifugal sizer led to the conclusion that larger particles within a specific latex were less swollen with acetonitrile than were the smaller ones. This was interpreted as evidence for the polymer in larger particles having a higher crosslink density. Particle swelling was estimated from swelling ratios defined as the particle volume at 25 °C divided by the volume at 50 °C. In the absence of crosslinking poly(N-isopropylacrylamide) linear chains would disolve at 25 °C. The swelling results indicated that the average crosslink density in the particles decreased with conversion. This was explained by the observation that the methylenebisacrylamide was consumed more quickly and is typical of crosslinking in emulsion polymerization where polymer particles have high polymer concentrations at their birth.     

Effect of thiol-containing monomer on the preparation of temperature-sensitive hydrogel microspheres

The main objective of this study is to prepare, thermally, sensitive microgel particles bearing thiol groups via precipitation polymerization of N-isopropylacrylamide (NIPAM), methylenebisacrylamide (MBA) and vinylbenzylisothiouronium chloride (VBIC) using 2-2′-azobis(2-amidinopropane)-dihydrochloride (V50) as initiator. The influence of various parameters has been investigated as a systematic study to point out the role of each reactant on polymerization conversion, and consequently, on particles and water-soluble polymer formation. The final microgel particles were characterized with respect to particle size and swelling ability. The aim of this paper is to complete our first short communication; Macromolecular symposia, 2000. 150: p. 283–290.     

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.     

Thermosensitive Au-PNIPA yolk-shell particles as “nanoreactors” with tunable optical properties

We demonstrate that Au-silica-poly(N-isopropylacrylamide) (PNIPA) trilayer composite particles with controllable thickness of PNIPA out-layer can be developed via free radical polymerization using Au-silica core-shell particles as seed. The presence of Au-silica particles does not influence of thermosensitivity of PNIPA shell, which exhibits a similar swelling behavior as pure PNIPA microgels. The etching of silica midlayer by NaOH treatment leads to Au-PNIPA particles with yolk-shell structure. The obtained yolk-shell particles can work as “nanoreactors” for the further growth of Au core within the PNIPA shell via seeded growth approach, which is followed with interesting optical properties. In addition, the optical properties of the Au cores can be modulated by the volume transition of the PNIPA shell.     

Fabrication of silver nanoparticles in pH responsive polymer microgel dispersion for catalytic reduction of nitrobenzene in aqueous medium

Copolymer microgels based on N-isopropylacrylamide (NIPAM) and methacrylic acid (MAA) have been synthesized by free radical emulsion polymerization using N,N-methylenebisacrylamide (BIS) as a cross-linker. Synthesized microgels were characterized by Fourier transform infrared spectroscopy (FTIR). Then silver nanoparticles were fabricated in the synthesized microgels by in-situ reduction of AgNO₃ with NaBH₄. The formation of silver nanoparticles was confirmed by UV–Vis spectroscopy. The pH sensitivity of the copolymer microgels was investigated using dynamic light scattering technique (DLS). Hydrodynamic radius of P (NIPAM–MAA) microgels increases with increase in pH of the medium at 25°C. Surface plasmon resonance wavelength (λ_SPR) of silver nanoparticles increases with increase in hydrodynamic radius due to change in pH of the medium. The catalytic activity for the reduction of nitrobenzene (NB), an environmental pollutant, into aniline was investigated by UV–Vis spectroscopy in excess of NaBH₄ using hybrid microgels as catalyst. The value of apparent rate constant (k_app) of the reaction was calculated using pseudo first order kinetic model and it was found to be linearly related to the amount of catalyst. The results were compared with literature data. The system was found to be an effective catalyst for conversion of NB into aniline.     

Synthesis of multifunctional nanogels using a protected macromonomer approach

Nanoparticles possessing multiple functionalities provide synthetic handles for varied surface chemistries, making them useful for a range of applications such as biotargeting and drug delivery. However, the combination of interfering functionalities on the same particle is often challenging. We have employed a synthetic scheme involving chemical protection/deprotection to combine interfering functional groups on the same hydrogel nanoparticle. The synthesis of amine-containing poly(N-isopropylacrylamide) nanogels was carried out via free radical precipitation polymerization by incorporating a Fmoc-protected amine poly(ethylene glycol) (PEG) macromonomer. The Fmoc group was then removed to obtain free amines, which were shown to be available for conjugation. We further explored pNIPAm-co-acrylic acid nanogels with a protected amine-PEG, yielding zwitterionic particles. With careful attention to the order of the chemoligation and deprotection steps, these interfering functional groups can be forced to behave in a pseudo-orthogonal fashion, allowing for multiple chemoligation steps that employ both the amine and carboxylic acid groups.