Synthesis and properties of organic-inorganic hybrid P(NIPAM-<Emphasis Type="Italic">co</Emphasis>-AM-<Emphasis Type="Italic">co</Emphasis>-TMSPMA) microgels

Utilizing the hydrolysis and condensation of the methoxysilyl moieties, organic-inorganic hybrid poly(N-isopropylacrylamide-co-acrylamide-co-3-(trimethoxysilyl)propylmethacrylate) P(NIPAM-co-AM-co-TMSPMA) microgels were prepared via two different methods. The first method was that the microgels were post-fabricated from the crosslinkable linear P(NIPAM-co-AM-co-TMSPMA) terpolymer aqueous solutions above the lower critical solution temperature (LCST) of the terpolymer. For the second method, the microgels were directly synthesized by conventional surfactant free emulsion copolymerization of NIPAM, AM, and TMSPMA. The hydrodynamic diameter and stability of the resultant P(NIPAM-co-AM-co-TMSPMA) microgels strongly depend on the pH and temperature of the microgel aqueous solution. The hydrodynamic diameters of the microgels decreased with increasing the measuring temperature. The phase transition temperature of the microgels was found to be around 34°C, which was independent of the initial terpolymer concentration and shifted to lower temperature with increasing the preparation temperature. Increasing the initial amount of AM will enhance the instability of the microgels at high pH values. Moreover, the P(NIPAM-co-AM-co-TMSPMA) microgels obtained from the linear terpolymer had more homogeneous microstructures as compared with the corresponding NIPAM/AM/TMSPMA microgels prepared by one step emulsion copolymerization as revealed by light scattering measurements.     

Synthesis and physicochemical properties of cationic microgels based on poly(<Emphasis Type="Italic">N</Emphasis>-isopropylmethacrylamide)

Surfactant-free, radical precipitation copolymerization of N-isopropylmethacrylamide and the cationic co-monomer N-(3-aminopropyl) methacrylamide hydrochloride (APMH) was carried out to prepare microgels functionalized with primary amines. The morphology and hydrodynamic diameter of the microgels were characterized by atomic force microscopy and photon correlation spectroscopy, with the effect of NaCl concentration and initiator type on the microgel size and yield being investigated. When a 2,2′-azobis (2-amidinopropane) dihydrochloride (V50)-initiated reaction was carried out in pure water, relatively small microgels (∼160 nm in diameter) were obtained in low yield (∼20%). However, both the yield and size increased if the reaction was carried out in saline or by using ammonium persulfate as initiator instead of V50. Stable amine-laden microgels in the range from 160 to 950 nm in diameter with narrow size distributions were thus produced using reaction media with controlled salinity. Microgel swelling and electrophoretic mobility values as a function of pH, ionic strength, and temperature were also studied, illustrating the presence of cationic sidechains and their influence on microgel properties. Finally, the availability of the primary amine groups for post-polymerization modification was confirmed via modification with fluorescein-N-hydroxysuccinamide.     

Polypyrrole/poly(N‐isopropylacrylamide‐co‐acrylic acid) thermosensitive and electrically conductive composite microgels

The electrically conductive polypyrrole/dodecylbenzene sulfonic acid/poly(N‐isopropylacrylamide‐co‐acrylic acid) (PPy/DBSA/poly(NIPAAm‐co‐AA)) composite microgels were synthesized by a chemical oxidation of pyrrole in the presence of DBSA as the primary dopant, and poly(NIPAAm‐co‐AA) microgels as the polymeric codopant and template, in which APS was used as the oxidant. It was proposed to prepare “intelligent” polymer microgel particles containing both thermosensitive and electrically conducting properties. The polymerization of pyrrole took place directly inside the microgel networks, leading to formation of composite microgels and the morphology was observed by transmission electron microscope. PPy particles interacted strongly with microgels, as the acid groups of microgels acted as the polymeric codopant. The composite microgels thus formed showed electrically conducting behavior dependent on humidity and temperature. At temperatures lower than lower critical solution temperature, the conductivity decreased with increasing the humidity and a small hysteresis phenomenon was observed. The hysteresis became indistinct when temperature was near volume phase transition temperature. However, after the treatment of high temperature and high humidity, the conductivity increased surprisingly due to the structure reorganization inside the composite microgels. The distinctive functionality of the PPy composite microgels was expected to be utilized in many attractive applications. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1648–1659, 2006     

New Route to Smart Core‐Shell Polymeric Microgels: Synthesis and Properties

Summary: A new method has been developed to prepare smart polymeric microgels that consist of well‐defined temperature‐sensitive cores with pH‐sensitive shells. The microgels were obtained directly from aqueous graft copolymerizations of N‐isopropylacrylamide and N,N‐methylenebisacrylamide from water‐soluble polymers containing amino groups such as poly(ethyleneimine) and chitosan. The gel diameters ranged from 300 to 400 nm. The unique core‐shell nanostructures, which had narrow size distributions, exhibited tuneable responses to pH and temperature.

Transmission electron micrograph of the poly(N‐isopropylacrylamide)/chitosan core‐shell microgels.     

Preparation and properties of thermo-sensitive organic/inorganic hybrid microgels

By utilizing the hydrolysis and condensation of the methoxysilyl groups, thermo-sensitive organic/inorganic hybrid poly[ N-isopropylacrylamide- co-3-(trimethoxysilyl)propylmethacrylate] [P(NIPAm- co-TMSPMA)] microgels were successfully prepared via two different methods without addition of any surfactant. First, the microgels were obtained by a two-step method; that is, the linear copolymer P(NIPAm- co-TMSPMA) was first synthesized by free radical copolymerization, and the aqueous solution of the copolymer was then heated above its low critical solution temperature (LCST) to give colloid particles, which were subsequently cross-linked via the hydrolysis and condensation of the methoxysilyl groups to form the microgels. Second, the microgels were also prepared via conventional surfactant-free emulsion polymerization (SFEP) of the monomers NIPAm and TMSPMA. TMSPMA can act as the cross-linkable monomer. No surfactant was involved in the preparation of the hybrid microgels. The obtained microgels were rather spherical and exhibited reversible thermo-sensitive behavior. The size, morphology, swellability, and phase transition behavior of the microgels were dependent on the initial copolymer or monomer concentration, preparation temperature, and the content of TMSPMA. The size of microgels obtained by SFEP was found to be more uniform than that by the two-step method. The hybrid microgels obtained by these two methods had more homogeneous microstructures than those prepared via conventional emulsion polymerization with chemical cross-linker N, N'-methylene-bisacrylamide.     

Novel Synthesis and Properties of Smart Core-Shell Microgels

A new method has been developed to prepare smart microgels that consist of well-defined temperature-sensitive cores with pH-sensitive shells. The microgels were obtained directly from aqueous graft copolymerization of N-isopropylacrylamide and N,N′-methylenebisacrylamide from water-soluble polymers containing amino groups such as poly(ethyleneimine) and chitosan. The gel diameters ranged from 300 to 400 nm with narrow size distribution. The unique core-shell nanostructures exhibited tuneable responses to pH and temperature.     

Facile synthesis of thermo- and pH-responsive biodegradable microgels

A novel and facile strategy has been designed to prepare biodegradable microgels with thermo- and pH-responsive property. The microgels were synthesized by the crosslinking of N-isopropylacrylamide with vinyl groups functionalized poly(L-glutamic acid) (PGA). The resultant microgels exhibited pH-dependent phase transition behaviors in aqueous solutions and underwent abrupt lower critical solution temperature decrease when the pH was reduced below the pK _a of PGA. Dynamic light scattering measurement revealed that the microgels exhibited shrinkage as the temperature increased or the pH decreased.     

Novel siloxane‐carrying dithiol derived from 5‐membered cyclic dithiocarbonate and its curing reactions for coating application

A new efficient and straightforward method to convert amines into siloxane‐thiol hybrid molecules was developed. The method relies on the nucleophilic addition of amines to a cyclic dithiocarbonate having siloxane moiety (DTC‐Si), and the successive ring‐opening reaction of the dithiocarbonate moiety to give the corresponding acyclic thiourethane having a thiol moiety. Based on this method, amine‐terminated poly(propylene glycol) was successfully transformed into the corresponding polyether having thiol‐terminals and siloxane groups. In the presence of moisture, the alkoxysilyl moiety underwent condensation reaction to make the polyether cured into a transparent resin having solvent‐resistance. Addition of bisphenol A diglycidyl ether (Bis A‐DGE) to the curing process resulted in two simultaneous reactions, i.e., (1) condensation of siloxane part and (2) addition reaction of the thiol terminal and the epoxide group. When this curing process was carried out on a glass surface, the siloxane part reacted with silanol group on the surface, forming a coating layer having excellent mechanical toughness graded as maximum 7H by pencil toughness test (JIS‐K5400). © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5119–5126, 2005     

Deswelling comparison of temperature‐sensitive poly(N‐isopropylacrylamide) microgels containing functional ?OH groups with different hydrophilic long side chains

Two monomers containing functional ? OH groups with different hydrophilic long side chains (viz., triethyleneglycol methacrylate (TREGMA) and polyethyleneglycol methacrylate (PEGMA)) were selected to modify the swelling/deswelling behavior of poly(N‐isopropylacrylamide) (pNIPAM) microgels. Dynamic scattering technique, turbidimetric method, and differential scanning calorimetry (DSC) were employed to investigate the deswelling behavior of the microgels. Experimental results show that the two series of microgels are identical in that incorporation of hydrophilic chains containing ? OH groups causes the volume‐phase transition temperature (VPTT) of pNIPAM microgels to shift to higher temperature; the more hydrophilic the side chains, the more the VPTTs shift. Although PEGMA are more effective in elevating the VPTTs of pNIPAM microgels than TREGMA, p(NIPAM‐co‐TREGMA) microgels show better deswelling properties than p(NIPAM‐co‐PEGMA) microgels, i.e., they have much larger deswelling ratios (α) and display less continuous volume‐phase transition. The VPTTs of the modified microgels can be modulated to well close to the normal body temperature of human beings. These characteristics along with the functional ? OH groups they contain make the microgels competitive candidates for biomaterials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3575–3583, 2005     

Catalytic reduction of 4‐nitrophenol using silver nanoparticles‐engineered poly(N‐isopropylacrylamide‐co‐acrylamide) hybrid microgels

Nearly monodisperse poly(N ‐isopropylacrylamide‐co ‐acrylamide) [P(NIPAM‐co‐AAm)] microgels were synthesized using precipitation polymerization in aqueous medium. These microgels were used as microreactors to fabricate silver nanoparticles by chemical reduction of silver ions inside the polymer network. The pure and hybrid microgels were characterized using Fourier transform infrared and UV–visible spectroscopies, dynamic light scattering, X‐ray diffraction, thermogravimetric analysis, differential scanning calorimetry and transmission electron microscopy. Results revealed that spherical silver nanoparticles having diameter of 10–20 nm were successfully fabricated in the poly(N ‐isopropylacrylamide‐co ‐acrylamide) microgels with hydrodynamic diameter of 250 ± 50 nm. The uniformly loaded silver nanoparticles were found to be stable for long time due to donor–acceptor interaction between amide groups of polymer network and silver nanoparticles. Catalytic activity of the hybrid system was tested by choosing the catalytic reduction of 4‐nitrophenol as a model reaction under various conditions of catalyst dose and concentration of NaBH₄ at room temperature in aqueous medium to explore the catalytic process. The progress of the reaction was monitored using UV–visible spectrophotometry. The pseudo first‐order kinetic model was employed to evaluate the apparent rate constant of the reaction. It was found that the apparent rate constant increased with increasing catalyst dose due to an increase of surface area as a result of an increase in the number of nanoparticles.     

Dual stimuli‐responsive microgels based on photolabile crosslinker: Temperature sensitivity and light‐induced degradation

The synthesis and characterization of a new photocleavable crosslinker is presented here. Dual stimuli‐responsive P(VCL‐co‐NHMA) microgels were prepared by precipitation polymerization of vinylcaprolactam (VCL) with N‐hydroxymethyl acrylamide (NHMA) and the new crosslinker. The microgels had distinct temperature sensitivity as observed in the case of PVCL‐based particles and their volume phase transition temperature (VPTT) shifted to higher temperature with increasing NHMA content. Photolytic degradation experiments were investigated by irradiation with UV light, which led to microgel disintegration caused by cleavage of the photolabile crosslinking points. The degradation behavior of the microgels was conducted with respect to degradation rates by means of the relative turbidity changes. Hence, the microgels could totally degrade into short linear polymers by UV light, thus representing a great potential as new light and temperature dual responsive nanoscale materials. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1676–1685     

Synthesis strategies to incorporate acrylic acid into N‐vinylcaprolactam‐based microgels

Two different synthesis strategies were used to synthesize N‐vinylcaprolactam (VCL)‐acrylic acid (AA)‐based temperature‐ and pH‐sensitive microgels under the adequate conditions to avoid possible hydrolysis of VCL due to the presence of carboxylic groups provided by AA. Polymeric and colloidal features of the microgels were analyzed: the partial conversion evolutions of each comonomer were determined by ¹H NMR and the swelling/deswelling behavior by means of Photon Correlation Spectroscopy. Considering that microgels are porous soft nanoparticles, conductimetric titrations at the swollen state were carried out to calculate the volumetric charge density. The results indicate that the addition of AA after 30 minutes of reaction time helped to incorporate higher amounts of AA into microgels and as a result, to obtain both temperature‐ and pH‐sensitive nanoparticles. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Thermosensitive and control release behavior of poly (N‐isopropylacrylamide‐co‐acrylic acid) latex particles

In this work, poly(N‐isopropylacrylamide‐co‐acrylic acid) (poly(NIPAAm‐AA)) copolymer latex particles (microgels) were synthesized by the method of soapless emulsion polymerization. Poly(NIPAAm‐AA) copolymer microgels have the property of being thermosensitive. The concentration of acrylic acid (AA) and crosslinking agent N,N′‐methylenebisacrylamide were important factors to influence the lower critical solution temperature (LCST) of poly(NIPAAm‐AA) microgels. The effects of AA and crosslinking agent on the swelling behavior of poly(NIPAAm‐AA) microgels were also studied. The poly(NIPAAm‐AA) copolymer microgels were then used as a thermosensitive drug carrier to load caffeine. The effects of concentration of AA and crosslinking agent on the control release of caffeine were investigated. How the AA content and crosslinking agent influenced the morphology and LCST of the microgels was discussed in detail. The relationship of morphology, swelling, and control release behavior of these thermosensitive microgels was established. A new scheme was proposed to interpret the control release of the microgels with different morphological structures. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5734–5741, 2008     

Si and Zr based NBBS for hybrid O/I macromolecular materials starting by preformed zirconium oxo-clusters

In this study surface-functionalized zirconium oxoclusters (ZrNBB), formed by reaction between vinylacetic acid and zirconium propoxide, were chemically modified through condensation with siloxane groups, so that the core integrity of ZrNBB was preserved. The direct coupling between zirconium oxo-clusters and siloxane moiety was performed by co-polymerisation with vinyl trimethoxysilane (VTMS) with Zr:Si molar ratio 1:2 in air at room temperature and benzoyl peroxide (BPO) was used as an initiator to start radical polymerization. The polymerisation was studied by differential scanning calorimetry (DSC) and Fourier Transform Infra Red (FTIR) spectroscopy. ²⁹Si and ¹³C liquid and solid state NMR analyses were performed in order to study the microstructure of the hybrid material and the role of the clusters in polymerization and condensation. Beyond this, shear storage modulus (G′) and loss modulus (G′′), were investigated by dynamical mechanical spectroscopy (DMS). The results indicated a great stability at high temperature without any viscous flow during glass transition.     

Preparation and Chiral Recognition of Thermosensitive Poly(N-isopropylacrylamide) Microgels Containing Modified L-Phenylalanine Groups

Thermosensitive poly[N-isopropylacrylamide(NIPAM)-co-N-acryloyl-L-phenylalanine ethyl ester (NALPE)] microgels were prepared by the free radical polymerization of NIPAM and chiral monomer, NALPE. Such microgels exhibited spherical shape and favorable monodispersity. Increasing the content of NALPE units would enhance the average diameter, but decrease the thermosensitivity and volume-phase transition temperatures of the microgels. Compared with PNIPAM microgels, the microgels containing NALPE units performed chiral recognozable capacities for D-phenylalanine and D-tartaric acid, and the enantioselectivity and adsorption capacity of the microgels improved with increasing the temperature and/or the content of NALPE units.     

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.     

Effects of hectorite content on the temperature-sensitivity of PNIPAM microgels

A new method, adopting inorganic clay (synthetic hectorite) as a physical cross-linker, was used to prepare poly(N-isopropylacrylamide) (PNIPAM) microgels via surfactant-free emulsion polymerization. The effect of hectorite content on the temperature-sensitivity of PNIPAM microgels was investigated by means of DLS, UV/Vis and DSC. It was found that, in the absence of surfactant, with increasing hectorite content, the particle size tends to decrease to 300 nm at room temperature, while increases as weight ratio (WR) of hectorite and N-isopropylacrylamide (NIPAM) exceeds 21%. Furthermore, with increasing WR from 7% to 21%, the volume phase transition temperature of PNIPAM microgels has little shift, while decreases slightly when WR increases up to 28%.     

Fabrication of free-standing multilayer films by using pH-responsive microgels as sacrificial layers

A facile way to prepare free-standing polyelectrolyte multilayer films of poly(sodium 4-styrenesulfonate)(PSS)/poly(diallyldimethylammonium)(PDDA) was developed by applying a new pH-dependent sacrificial system based on cross-linked poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) microgels. The tertiary amine groups of PDMAEMA microgels can be protonated in acidic environment, and the protonated microgels were deposited by layer-by-layer (LbL) technique with PSS. PSS/PDDA multilayer films were constructed on the top of the PSS/microgels sacrificial layers. The LbL assembly process was investigated by UV–vis spectroscopy. Further study shows that the free-standing PSS/PDDA multilayer films can be obtained within 3 min by treating the as-prepared films in alkali aqueous solution with a pH of 12.0. The pH-triggered exfoliation of PSS/PDDA multilayer films provides a simple and facile way to prepare LbL assembled free-standing multilayer films.     

Multi‐responsive methylcellulose/Fe‐alginate‐g‐PVA/PVA/Fe3O4 microgels for immobilizing enzyme

To take advantage of the respective character of methylcellulose (MC), poly(vinyl alcohol) (PVA), and alginate, novel physically cross‐linked methylcellulose/Fe‐alginate‐g‐PVA/PVA (MC/Fe‐Alg‐g‐PVA/PVA) microgels were prepared by emulsification/thermal gelation/freezing–thawing/ionic cross‐linking technique. Subsequently, some ferrous ions bound in the microgels were transformed into magnetite via in situ self oxidation. A model enzyme α‐amylase was immobilized into microgels by direct adsorption. The release behavior of α‐amylase indicated that the obtained complex microgels were magnetic‐, temperature‐, and pH‐ triple sensitive. Copyright © 2010 John Wiley & Sons, Ltd.     

Copolymeric nano/microgels of N-isopropylacrylamide and carboxyalkyl methacrylamides: Effect of methylene chains and the ionization state of the weak acids on size and sensitivity to pH and temperature

ABSTRACT

We prepared nano/microgels by precipitation copolymerization of N-isopropylacrylamide (NIPAAm), and one of three different carboxyalkyl methacrylamides [methacryloylamido hexanoic acid (M₅), 8-methacryloylamido octanoic acid (M₇), and 11-methacryloylamido undecanoic acid (M₁₀)], either in the acid forms or as carboxylates (potassium salts). The hydrodynamic diameter (Dh) of the nano/microgels prepared with the carboxylates was smaller (≈100 nm for M₁₀ copolymers), compared to the size of homopolymeric NIPAAm microgels prepared by dispersion polymerization (around 600 nm), indicating that the carboxylates act as surfactants reducing the size of the seeds during the polymerization process. These materials presented a swollen-shrunken transition temperature (T _tr) similar to the T _tr of the homopolymeric NIPAAm microgels, without pH sensitivity. On the other hand, the copolymeric microgels prepared from the acid form of the comonomers have a similar or bigger size than NIPAAm microgels. For these copolymers, the T _tr can be tuned by the type and proportion of acid comonomer used and present pH sensitivity. This is important for biomedical applications such as positive temperature control release. Polyelectrolyte titration demonstrates that the nano/microgels prepared with the carboxylates behave as hard spheres, while the microgels prepared with the weak acid behave as porous materials.