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 and characterization of thermo-sensitive poly (N-isopropylacrylamide) hydrogel with fast response rate

Thermo-sensitive poly (N-isopropylacrylamide) (PNIPA) hydrogel with fast response rate was prepared by polymerizing N-isopropylacrylamide (NIPA) in an aqueous hydroxyl-propyl-methyl cellulose solution. The volume phase transition temperature of PNIPA hydrogels was characterized by differential scanning calorimetry (DSC), and the surface morphology was observed by scanning electron microscopy (SEM). The swelling ratios of the hydrogels at different temperatures were measured. Furthermore, the deswelling kinetics of the hydrogels was also studied by measuring their water retention capacity. In comparison with a conventional PNIPA hydrogel prepared in water, the hydrogel synthesized in aqueous hydroxyl-propyl-methyl cellulose solution has higher swelling ratios at temperatures below the lower critical solution temperature and exhibits a much faster response rate to temperature changes. For example, the hydrogel made in aqueous hydroxyl-propyl-methyl cellulose solution lost 89% water within 1 min and about 93% water in 4 min, whereas the conventional hydrogel lost only about 66% water in 15 min from the deswelling measurement in similar conditions. Translated from Chinese Journal of Applied Chemistry, 2006, 23(6): 581–585 (in Chinese)     

Thermoresponsive nanostructured poly(N-isopropylacrylamide) hydrogels made via inverse microemulsion polymerization

The synthesis of nanostructured poly(N-isopropylacrylamide) (polyNIPA) hydrogels by a two-stage polymerization process is reported here. The process involves the synthesis of slightly crosslinked polyNIPA nanoparticles by inverse (w/o) microemulsion polymerization; then, these particles are dried, cleaned and dispersed in an aqueous solution of NIPA and a crosslinking agent (N,N-methylene-bis-acrylamide or NMBA) and polymerized to produce the nanostructured hydrogels. Their swelling and de-swelling kinetics, volume phase transition temperatures (T _VPT) and mechanical properties at the equilibrium swollen state are investigated as a function of the weight ratio of polyNIPA particles to monomer (NIPA). The nanostructured gels exhibit larger equilibrium water uptake, faster swelling and de-swelling rates and similar T _VPT than those of the conventional ones; moreover, the elastic and Young moduli are larger than those of the conventional hydrogels at similar swelling ratios. The fast swelling and de-swelling kinetics are explained in terms of the controlled inhomogeneities introduced by the method of synthesis.     

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.     

Rapid deswelling of sodium alginate/poly(N-isopropylacrylamide) semi-interpenetrating polymer network hydrogels in response to temperature and pH changes

In this study, temperature-/pH-responsive semi-interpenetrating polymer network (semi-IPN) hydrogels based on linear sodium alginate (SA) and cross-linked poly(N-isopropylacrylamide) (PNIPAAm) were prepared. The semi-IPN hydrogels reached an equilibrium deswelling state within 6 h in response to temperature or pH stimuli. Compared with the conventional PNIPAAm hydrogel, their dewelling rate in response to temperature was improved significantly, owing to the formation of a porous structure within the hydrogels in the presence of ionized SA during the polymerization process. Moreover, the deswelling process could be well described with a first-order kinetics equation and it is possible to design any hydrogel with the desired deswelling behavior through the control of the SA content in the semi-IPN hydrogels.     

The electrophoresis of poly(N-isopropylacrylamide) microgel particles

 The electrophoretic mobility of a poly(N-isopropylacrylamide) microgel containing carboxylic groups has been measured as a function of the ionic strength, between 0.1 and 100 mM NaCl, over the temperature range 2545 ^∘C. The mobility data obtained have been evaluated using different models, including the porous-sphere, the soft-plate and the soft-sphere models as well as the hard-sphere model developed by Henry and later refined by O'Brien and White. The “porous” or “soft” behaviour is evident at lower temperatures, whereas at higher temperatures none of the models can fully explain the observed behaviour. It is suggested that the discrepancies at higher temperatures can be partly ascribed to the neglect of the relaxation effect in the “soft” models. Received: 30 June 1999/Accepted in revised form: 12 October 1999     

Preparation of the individual compact single-chain globular particulates of Poly(N-isopropylacrylamide)

The compact single-chain (SC) particulates of Poly(N-isopropylacrylamide) (PNIPAM), which have been formed above its lower critical solution temperature in an aqueous solution containing the surfactant of sodium n-dodecyl sulfate (SDS), were recovered from the solution by freeze-drying. Under scanning electron microscopy, the compact particulate appears as a spherical or elliptical particulate individually dispersed in SDS, which acts as a solid solvent to prevent agglomeration. The conformation of the compact SC particulates of PNIPAM dispersed in SDS had been studied by the solid-state high-resolution ¹³C NMR spectroscopy. The ¹³C spin-lattice relaxation time T ₁ of the SC sample was determined in comparison with that of the original one. It was found that the T ₁ of the methyl carbon in the isopropyl group of the SC sample was about 45% higher than that in the original multichain sample, which revealed the differences in the motion of the methyl group in the different condensed states and illuminated the characteristic conformation of the compact SC globular particulates of PNIPAM.     

Synthesis of thermoresponsive polystyrene-based comb-type grafted poly( N -isopropylacrylamide)

Narrowly distributed polystyrene-g-p(N-isopropylacrylamide) (PSt-g-PNIPAM) was prepared by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide using the brominated polystyrene as macroinitiator and CuCl combined with hexamethyltriethylenetetramine as catalyst. Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy confirmed the structure of PSt-g-PNIPAM. The gel permeation chromatography (GPC) showed that the graft copolymer had a single distribution peak with molecular weight, M _n (g/mol) of 19815 g/mol (using polystyrene as the standard). Differential scanning calorimetry (DSC) revealed that due to both effects of hydrophobic isopropyl groups and hydrogen bonds in the amide group, the glass transition temperature (T _g) of PSt-g-PNIPAM enhanced 16.0°C compared to the T _g of the polystyrene.     

DNA mutation analysis based on capillary electrochromatography using colloidal poly(N-isopropylacrylamide) particles as pseudostationary phase

Poly(N-isopropylacrylamide) (PNIPAM) is an interesting class of temperature sensitive, water soluble polymer that has a lower critical solution temperature (LCST) of 32 °C. Above the LCST, PNIPAM gets phase-separated and precipitates out from water. The fascinating temperature-sensitive property of PNIPAM has led to a growing interest in diverse fields of applications. Recently, capillary electrochromatography (CEC) has gained attention due to the wide range of applications based on the use of open tubular capillaries. In this paper, the use of phase-separated PNIPAM as a pseudostationary phase for CEC is demonstrated for the detection of single nucleotide polymorphisms (SNPs). Owing to the dynamic coating, the phase-separated PNIPAM particles did not require any immobilization technique and could exist as a mobile stationary phase in the open tubular capillary. The heteroduplex analyses of mutation samples could be successfully performed based on the phase-separated PNIPAM particles in the constructed CEC system. The CEC system, based on PNIPAM particles capable of having a narrow size distribution, shows great potential as an alternative to conventional DNA mutation systems.     

The thermal degradation kinetics of poly(di-n-alkylitaconates)

The non-oxidative thermal degradation kinetics of poly(di-n-alkyl itaconates), ranging from the methyl to then-octyl derivatives, were studied by non-isothermal and isothermal TG. The thermal degradation activation energy and characteristic mass loss temperatures were found to decrease with increasing substituent size. The shapes of the DTG curves were dependent on the size of the alkyl substituent. The different DTG maxima were ascribed to various modes of initiation of depolymerisation. The thermal stability of poly(di-n-hexyl itaconate) was found to be independent of the initial molar mass of the sample in the range ofM _w from 10⁴ to 10⁷ g/mol.

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Zusammenfassung Mittels nichtisothermer und auch isothermer TG wurde die nichtoxidative thermische Zersetzungskinetik von Poly(di-n-alkylitakonaten) untersucht, dabei bewegt sich die Länge der Alkylkette von Methyl bisn-Oktyl. Die Aktivierungsenergie der thermischen Zersetzung und die charakteristischen Masseverlusttemperaturen nehmen mit steigender Substituentlänge ab. Aussehen der DTG-Kurven hängt von der Größe der Alkylkette ab. Die verschiedenen DTG-Maxima wurden verschiedenen Initiierungs schritten der Depolymerisierung zugeschrieben. Man fand, daß im Intervall M_w zwischen 10⁴ und 10⁷ g·mol die thermische Stabilität der Poly(di-n-hexylitakonate) unabhängig von der anfänglichen molaren Masse der Probe ist.

     

Synthesis and phase behavior of aqueous poly(N-isopropylacrylamide-co-acrylamide), poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) and poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate)

Poly(N-isopropylacrylamide) (PNIPAM) and random copolymers of Poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (PNIPAM-HEMA), poly(N-isopropylacrylamide-co-acrylamide) (PNIPAM-AAm), and poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (PNIPAM-DMAA) with various volume fractions γ of NIPAM were synthesized by radical polymerization. The phase behavior of the polymers in water was investigated by means of optical transmittance and dynamic light scattering. With decreasing γ, the cloud point temperature T _cp for PNIPAM-HEMA decreased whereas the T _cp for both PNIPAM-AAm and PNIPAM-DMAA increased. Increase of hydrodynamic radius around T _cp, which resulted from the aggregation of the globules of each polymer, was observed from dynamic light scattering. The relationships between the reciprocal of T _cp of the polymer solutions and 1-γ were linear for the three copolymers in the experimental range of 0.65<γ<1. The results are discussed from the aspect of the interaction parameters of copolymer solutions.     

Uptake and release of surfactants from polyampholyte microgel particles

Polyampholyte microgel particles, containing both methacrylic acid and 2-(dimethylamino) ethyl methacrylate (a weak base), in a mainly N-isopropyl acrylamide network, have been prepared by free-radical dispersion polymerisation. The swelling properties of the particles have been shown to be pH and temperature dependent and to exhibit a minimum in size at the iso-electric point. The uptake and release of cetylpyridinium chloride and Triton X-100, into and from, the polyampholyte microgel particles have been investigated as a function of pH. The absorbed amounts at different pH values have been related to various specific interactions between the surfactant and the microgel network.     

Bistable photoswitching in poly(N-isopropylacrylamide) with spironaphthoxazine hydrogel for optical data storage

A bistable switching photochromic poly(N-isopropylacrylamide) with spironaphthoxazine hydrogel copolymer (PNIPA-SPO-BIS) has been designed and synthesized by radical polymerization. The PNIPA-SPO-BIS copolymer is identified by ¹H NMR spectroscopy, FIT-IR spectroscopy and gel permeation chromatography (GPC). The morphology of the internal microstructures of the PNIPA-SPO-BIS hydrogels was observed by scanning electron microscopy (SEM). The PNIPA-SPO-BIS copolymer showed excellent photochromic behavior in water solution and in gel state. In addition, erasable and rewritable (EARW) photoimaging on the PNIPA-SPO-BIS hydrogel was successfully demonstrated. A novel optical data storage materials based on photochromic hydrogel was developed. These developments are crucial for fundamental studies and eventual technical application for all-photo mode high-density optical data storage.     

Colloidal crystals made of poly(N-isopropylacrylamide) microgel particles

Poly (N-isopropylacrylamide) microgel particles are found to form colloidal crystals similar to those occurring in typical hard-sphere colloids like poly(methylmethacrylate) beads. Samples made of particles with different cross-linker concentrations are investigated and their deswelling ratio is determined using dynamic light scattering. Small-angle neutron scattering data are also presented and analysed in terms of a face-centred-cubic crystal structure. The characteristic length, a, of the elementary cell is found to be 535 ± 16 and 495 ± 15 nm for the two systems investigated. This leads to particle radii of 189 ± 6 and 175 ± 5 nm, respectively. These values compare well to the radii determined using several different methods. Received: 26 July 1999/Accepted: 21 March 2000     

On the structure of poly(N-isopropylacrylamide) microgel particles

This investigation presents a study of the internal structure of poly(NIPAM/xBA) microgel particles (NIPAM and BA are N-isopropylacrylamide and N,N'-methylene bisacrylamide, respectively). In this study, x is the wt % of BA used during microgel synthesis. Two values of x were used to prepare the microgels, 1 and 10. The microgel dispersions were investigated using photon correlation spectroscopy (PCS) and small-angle neutron scattering (SANS). These measurements were made as a function of temperature in the range 30-50 degrees C. Scattering maxima were observed for the microgels when the dispersion temperatures were less than their volume phase transition temperatures. The SANS data were fitted using a model which consisted of Porod and Ornstein-Zernike form factors. The analysis showed that the macroscopic hydrodynamic diameter of the microgel particles and the submicroscopic mesh size of the network are linearly related. This is the first study to demonstrate affine swelling for poly(NIPAM/xBA) microgels. Furthermore, the mesh size does not appear to be strongly affected by x. The data suggest that the swollen particles have a mostly homogeneous structure, although evidence for a thin, low segment density shell is presented. The study confirms that poly(NIPAM/xBA) microgel particles have a core-shell structure. The shell has an average thickness of approximately 20 nm for poly(NIPAM/1BA) particles which appears to be independent of temperature over the range studied. The analysis suggests that the particles contained approximately 50 vol % water at 50 degrees C. The molar mass of the poly(NIPAM/1BA) microgel particles was estimated as 6 x 10(9) g mol(-1).     

Solvent concentrations of dimethylsulfoxide-water and 1-propanol-water solutions inside and outside poly (N-isopropylacrylamide) gel

The volumes of poly (N-isopropylacrylamide) (NIPA) gel in both dimethylsulfoxide (DMSO)-water and 1-propanol-water solutions were measured at 25°C. The solvent concentrations inside and outside the NIPA gel were also measured. The gel was swollen in water, shrunk according to increase in concentration of organic solvent, and reswollen in pure organic solvent. This phenomenon is typical reentrant swelling behavior. The DMSO concentrations inside the gel were almost equal to those outside the gel in the whole concentration range. On the other hand, the 1-propanol concentrations between inside and outside the gel were much different from each other in the shrunk state, though they were almost the same in the swollen state.     

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.     

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.     

pH sensitivities of egg phosphatidylcholine liposomes and dioleoylphosphatidylethanolamine liposomes triggered by poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide-co-methacrylic acid-co-octadecylacrylate)

Egg phosphatidylcholine (PC) liposomes bearing pH-sensitive polymers and dioleoylphosphatidylethanolamine (DOPE) liposomes including the same polymers were prepared by a sonication method. As pH-sensitive polymers, copolymers of N-isopropylacrylamide, methacrylic acid, and octadecylacrylate were used. The liposomes were stable in neutral pH ranges in terms of release. But the release became marked at pH 5.5, and it was accelerated as pH further decreased. For example, the degree of release from egg PC liposomes (polymer/lipid ratio is 3:10, w/w) for 120 s increased from 2% to 63% as pH decreased from 7.5 to 4.5. Under the same condition, the degree of release from DOPE liposomes increased from 4% to 80%. These results indicate that DOPE liposome is more pH-sensitive than egg PC liposome.     

Glucose-triggered release from liposomes incorporating poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide-co-methacrylic acid-co-octadecylacrylate) and glucose oxidase

In order to design liposomes which release their contents in a glucose-sensitive manner, the surfaces of egg phosphatidylcholine (egg PC) liposomes or dioleoylphosphatidylethanolamine (DOPE) liposomes were modified with the copolymer of N-isopropylacrylamide/methacrylic acid/octadecylacrylate and hydrophobically modified glucose oxidase (EC 1.1.3.4.). Whichever the liposomes were prepared with egg PC or DOPE, an extensive release of calcein was observed at acidic conditions. And DOPE liposomes were more pH sensitive than egg PC liposomes in terms of the release. In glucose-dependent calcein release experiment, there was no release for 180 min when the suspension of liposome was free of glucose. When the glucose concentration was 50 mg/dl, no appreciable amount of calcein was released for the first 50 min, but a significant release was observed for the last 130 min. At glucose concentration of 200 mg/dl, calcein release became more extensive and the releases for 180 min from egg PC and DOPE liposome were 84% and 98%, respectively.