Construction of PS/PNIPAM core-shell particles and hollow spheres by using hydrophobic interaction and thermosensitive phase separation

This paper reports an easy and effective way to fabricate polystyrene/poly (N-isopropylacrylamide) (PS/PNIPAM) core-shell particles and PNIPAM hollow spheres. The main point of the method is to take advantage of the hydrophobic interaction between initiator and PS particles. The hydrophobic azodiisobutyronitriles automatically concentrate around the PS particles and initiate polymerization of N-isopropylacrylamide (NIPAM) and the crosslinker methylene bisacrylamide (MBA), which dissolve in the aqueous phase, at the surface of the PS nanoparticles. Then, PNIPAM adheres to the PS particles to form a core-shell structure as a result of their hydrophobic interaction. This interaction is due to the unique property of PNIPAM, namely, its ability to transition from hydrophilic to hydrophobic when the temperature rises to 32°C. Furthermore, the hollow PNIPAM spheres were obtained by etching the PS core with chloroform.     

Construction of PS/PNIPAM core-shell particles and hollow spheres by using hydrophobic interaction and thermosensitive phase separation

Abstract This paper reports an easy and effective way to fabricate polystyrene/poly (N-isopropylacrylamide) (PS/ PNIPAM) core-shell particles and PNIPAM hollow spheres. The main point of the method is to take advantage of the hydrophobic interaction between initiator and PS particles. The hydrophobic azodiisobutyronitriles automatically concentrate around the PS particles and initiate polymerization of N-isopropylacrylamide (NIPAM) and the crosslinkermethylene bisacrylamide (MBA), which dissolve in the aqueous phase, at the surface of the PS nanoparticles. Then, PNIPAM adheres to the PS particles to form a coreshell structure as a result of their hydrophobic interaction. This interaction is due to the unique property of PNIPAM, namely, its ability to transition from hydrophilic to hydrophobic when the temperature rises to 32°C. Furthermore, the hollow PNIPAM spheres were obtained by etching the PS core with chloroform. __________ Translated from Journal of Nanjing University (Natural Sciences), 2007, 43(5): 483–488 [译自: 南京大学学报(自然科学)]     

Thermosensitive poly(N-isopropylacrylamide) nanocapsules with controlled permeability

In this paper, novel thermosensitive poly(N-isopropylacrylamide) (PNIPAM) nanocapsules with temperature-tunable diameter and permeability are reported. Firstly, the core-shell composite microparticles were synthesized by precipitation polymerization with isothiocyanate fluorescein (FITC) entrapped SiO₂ as core and cross-linked PNIPAM as shell. Then, the SiO₂ core was etched by hydrofluoric acid at certain condition and the pre-trapped FITC molecules remained within the inner cavity. The FITC release profile and TEM studies clearly indicate that the release behavior of FITC could be controlled effectively by the external temperature. Above the LCST of PNIPAM (32 °C), the dehydrated PNIPAM shell inhibited the release of FITC from the internal cavity while below its LCST, the fluorophore could permeate the swollen shell easily.     

Temperature and pH effects on the stability and rheological behavior of the aqueous suspensions of smart polymers based on N‐isopropylacrylamide,chitosan, and acrylic acid

This study describes the stability and rheological behavior of suspensions of poly(N‐isopropylacrylamide) (PNIPAM), poly(N‐isopropylacrylamide)‐chitosan (PNIPAM‐CS), and poly(N‐isopropylacrylamide)‐chitosan‐poly(acrylic acid) (PNIPAM‐CS‐PAA) crosslinked particles sensitive to pH and temperature. These dual‐sensitive materials were simply obtained by one‐pot method, via free‐radical precipitation copolymerization with potassium persulfate, using N,N′‐methylenebisacrylamide as a crosslinking agent. Incorporation of the precursor materials into the chemical networks was confirmed by elementary analysis and infrared spectroscopy. The influence of external stimuli such as pH and temperature, or both, on particle behavior was investigated through rheological measurements, visual stability tests, and analytical centrifugation. The PNIPAM‐CS particles showed higher stability in acid and neutral media, whereas PNIPAM‐CS‐PAA particles were more stable in neutral and alkaline media, both below and above the lower critical solution temperature of PNIPAM (stability data). This is due to different interparticle interactions as well as those between the particles and the medium (also evidenced by rheological data), which were also influenced by the pH and temperature of the medium. Based on the results obtained, we found that the introduction of pH‐sensitive polymers to crosslinked PNIPAM particles not only produced dual‐sensitive materials but also allowed particle stability to be adjusted, making phase separation faster or slower, depending on the desired application. Thus, it is possible to adapt the material to different media. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis and thermo-responsive behavior of fluorescent labeled microgel particles based on poly(N-isopropylacrylamide) and its related polymers

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles labeled with 3-(2-propenyl)-9-(4-N,N-dimethylaminophenyl)phenanthrene (VDP) as an intramolecular fluorescent probe were prepared by emulsion polymerization. The thermo-responsive behavior of the VDP-labeled PNIPAM microgel particles dispersed in water was studied by turbidimetric and fluorescence analyses. The transition temperature of the VDP-labeled PNIPAM microgel particles in water determined by turbidimetric analysis was ca. 32.5 °C. The wavelength at the maximum fluorescence intensity of the VDP units linked directly to the microgel particles dramatically blue-shifted around the transition temperature. In addition it gradually blue-shifted even below the transition temperature where there was no change observed in the turbidity. These findings suggest that the gradual shrinking of microgel particles occurs with increasing temperature and the subsequent dramatic shrinking results in the increasing in the turbidity. The transition temperatures of VDP-labeled poly(N-n-propylacrylamide) and poly(N-isopropylmethacrylamide) microgel particles determined by turbidimetric analysis were ca. 23 and ca. 42.5 °C, respectively, and their thermo-responsive behavior was similar to that for the VDP-labeled PNIPAM system. In these three systems the microenvironments around the fluorescent probes above the transition temperatures became more hydrophobic than those below the transition temperature, and the estimated values of microenvionmental polarity around the VDP units on their collapsed states were almost the same.     

Introduction of silica into thermo-responsive poly(N-isopropyl acrylamide) hydrogels: A novel approach to improve response rates

The response rates of novel thermo-responsive poly(N-isopropyl acrylamide) (PNIPAM) hybrid hydrogels are compared to those of conventional chemically crosslinked PNIPAM hydrogels. The former materials were obtained by applying the sol-gel technology, in which the inorganic silica particles act as physical crosslinks for the organic polymer chains, leading to a semi-interpenetrating polymer network structure. In situ modulated temperature DSC shows that the introduction of hydrophilic silica improves the thermal response rate of the hybrid hydrogels to a great extent as compared to aqueous PNIPAM solutions and conventional PNIPAM hydrogels. Ex situ gravimetrical measurements also illustrate that the shrinking/swelling rate of the hybrid hydrogels is largely improved. It is assumed that the uniform distribution of the SiO₂ units, as demonstrated by cryo-field emission scanning electron microscopy, causes the silica to act as nano-sized water reservoirs, which reduce the characteristic diffusion length of water in the PNIPAM matrix so that it can be transported faster within the hybrid PNIPAM nano-composite.     

Fabrication of an asymmetric hollow particle with a thermo-sensitive PNIPAM inside corona

Asymmetric hollow particles were fabricated from the self-assembly of block copolymers poly(N-isopropylacrylamide)-block-poly(4-vinylpyridine) (PNIPAM-b-P4VP) and poly(ethylene glycol)-block-poly(acrylic acid) (PEG-b-PAA) in water. The shell of the asymmetric hollow particle was a polyion complex layer which acted as a semipermeable membrane. Outside the shell were PEG hydrophilic layers and inside were thermo-sensitive PNIPAM chains. When temperature was higher than the lower critical solution temperature (LCST) of PNIPAM, it became insoluble and collapsed onto the shrinking shell to form a hydrophobic lumen. The whole process was characterized by dynamic light scattering (DLS), static light scattering (SLS), transmission electron microscopy (TEM), atom force microscopy (AFM) and nuclear magnetic resonance (NMR).     

Double-responsive core-shell-corona micelles from self-assembly of diblock copolymer of poly(t-butyl acrylate-co-acrylic acid)-b-poly(N-isopropylacrylamide)

Self-assembly of poly(t-butyl acrylate-co-acrylic acid)-b-poly(N-isopropylacrylamide) [P(tBA-co-AA)-b-PNIPAM], which was obtained from part hydrolysis of PtBA-b-PNIPAM synthesized by sequential atom transfer radical polymerization (ATRP) was studied. Thermo- and pH-responsive core-shell-corona (CSC) micelles with different structures were formed from (PtBA-co-PAA)-b-PNIPAM in aqueous solution. At pH 5.8 and 25 °C, the block copolymer self-assembled into spherical core-shell micelles with hydrophobic PtBA segments as the core, hydrophilic PAA/PNIPAM segments as the mixed shell. Increasing temperatures, core-shell micelles converted into CSC micelles with PtBA as the core, collapsed PNIPAM as the shell and soluble PAA as the corona. Moreover, decreasing pH at 25 °C, PAA chains collapsed onto the core resulting in CSC micelles with PtBA as the core, PAA as the shell and PNIPAM as the corona.     

Synthesis and micellization of thermo- and pH-responsive block copolymer of poly(N-isopropylacrylamide)-block-poly(4-vinylpyridine)

A novel double-hydrophilic block copolymer poly(N-isopropylacrylamide)-block-poly(4-vinylpyridine) (PNIPAM-b-P4VP) with low polydispersity which could respond to both temperature and pH stimuli in aqueous solution was synthesized by atom transfer radical polymerization. Micellization of the copolymer in aqueous solution was characterized by dynamic and static laser scattering, ¹H NMR and transmission electron microscopy. In aqueous solution, the copolymer existed as unimer at pH 2.8 at 25 °C. When the temperature was raised to 50 °C at pH 2.8, the copolymer associated into spherical core-shell micelles with the PNIPAM block forming the core and the P4VP block forming the shell. On the other hand, when pH was increased from 2.8 to 6.5 at 25 °C, the copolymer associated into spherical core-shell micelles with the core formed by the P4VP block and the shell formed by the PNIPAM block. The process was reversible. The critical aggregation temperature of the block copolymer is 36 °C, and the critical aggregation pH value is 4.7.     

Doubly thermo-responsive brush-linear diblock copolymers and formation of core-shell-corona micelles

Doubly thermo-responsive brush-linear diblock copolymer of poly[poly(ethylene glycol) methyl ether vinylphenyl]-block-poly(N-isopropylacrylamide) (PmPEGV-b-PNIPAM) is prepared by RAFT polymerization. The obtained brush-linear diblock copolymer exhibits two lower critical solution temperatures (LCSTs) corresponding to the linear poly(N-isopropylacrylamide) (PNIPAM) block and the brush poly[poly(ethylene glycol) methyl ether vinylphenyl] (PmPEGV) block in water. This brush-linear diblock copolymer undergoes a two-step temperature sensitive micellization. At temperature above the first LCST, the brush-linear diblock copolymer self-assembles into core-corona micelles with the dehydrated PNIPAM block forming the core and the solvated brush PmPEGV block forming the corona. When temperature increases above the second LCST, the polystyrene backbone in the brush PmPEGV block collapses onto the dehydrated PNIPAM core to form core-shell-corona micelles, in which the dehydrated PNIPAM block forms the core, the collapsed polystyrene backbone in the brush PmPEGV block forms the shell and the solvated poly(ethylene glycol) side-chains forms the corona. The effect of the length of the PNIPAM block and the length of the poly(ethylene glycol) side-chains on the thermo-responsive micellization and the size of core-shell-corona micelles is investigated.     

Aggregation behaviour of well defined amphiphilic diblock copolymers with poly(N-isopropylacrylamide) and hydrophobic blocks

Series of amphiphilic diblock copolymers with poly(N-isopropylacrylamide) as a hydrophilic block and a hydrophobic block consisting of either polystyrene or poly(tert-butyl methacrylate) were synthesised using RAFT polymerisations. Differential scanning calorimetry showed the chemically different blocks being phase separated in dry polymers. Light scattering and microcalorimetry studies were performed on aqueous solutions to investigate the phase behavior of the diblock copolymers. By carefully transferring the polymers from an organic solvent to water, either micellar particles or large aggregates were obtained depending on the relative lengths of the blocks. Large aggregates collapsed upon heating, whereas collapse occurred slowly within a broad temperature range in the case of micelle like structures. However, microcalorimetrically the collapse of the PNIPAM chains was observed to take place in all samples, suggesting that the shells of the micellar particles are crowded in a way which hinders the compression of the poly(N-isopropylacrylamide) chains.     

聚（N-异丙基丙烯酰胺）/聚丙烯酸半互穿网络微凝胶的合成及表征

利用IPN技术合成了一种具有温度和pH双重敏感性的聚（N-异丙基丙烯酰胺）/聚丙烯酸半互穿网络微凝胶（PNIPAM/PAAc semi-IPN）。这种微凝胶在酸性条件下发生典型的体积相转变;而在弱碱性条件下,当温度低于聚（N-异丙基丙烯酰胺）（PNIPAM）微凝胶的体积相转变温度（VPTT）时,微凝胶的粒径随着温度的上升而增大,当温度达到VPTT后,粒径突然急剧减小,并随着温度的逐渐上升而减小,最终趋向平衡。     

Synthesis and characterization of thermo-responsive supramolecular diblock copolymers

In this paper, we report the synthesis and characterization of a new thermo-responsive supramolecular amphiphiles diblock copolymer, methoxy poly (ethylene glycol)-block-Poly(N-isopropylacrylamide) (mPEG-b-PNIPAM). The mPEG with β-cyclodextrin (β-CD) group and the PNIPAM with adamantine (AD) group were synthesized, respectively, and mPEG-b-PNIPAM was then obtained by the host-guest inclusion between AD and β-CD. The structure and molecular weight of mPEG-b-PNIPAM was confirmed by ¹H NMR and GPC measurements. At low temperature (T?<?33 °C), the PNIPAM block is hydrophilic. With increasing temperature, the hydrophobicity the of PNIPAM block increases. This molecular feature leads to interesting aggregation behavior of micelles in aqueous solutions at different temperatures. The self-assembly was revealed by UV, DLS measurements, and TEM observations.     

Synthesis of cationic poly(methyl methacrylate)-poly(N-isopropyl acrylamide) core-shell latexes via two-stage emulsion copolymerization

Thermally sensitive poly(methyl methacrylate (MMA))-poly(N-isopropylacrylamide (NIPAM)) core-shell particles were prepared via a two-stage emulsion copolymerization process. Methylene bisacrylamide (MBA), 2,2′-azobis (2-amidinopropane) dihydrochloride (V50) and dodecylethyl dimethyl ammonium bromide (DEDAB) were used as crosslinker, cationic initiator and surfactant, respectively. Functional core-shell particles were prepared using aminoethyl methacrylate hydrochloride (AEMH) as cationic co-monomer to increase the surface charge density. The influences of the crosslinker and co-monomer concentrations on the thickness and swelling capacity of the PNIPAM-based shell layer were studied. The latex particle size and particle size distribution were determined both by dynamic light scattering (DLS) and scanning electron microscopy (SEM). Monodisperse particles were produced with diameters between 150-250 nm (at 25 °C) and 140-190 nm (at 50 °C). The surface charge density was determined by chemical titration and higher values (∼10 μmol/g) were obtained for the functional core-shell particles. The electrokinetic properties of the dispersions at several pH and temperature values confirm the presence of the shell layer and cationic surface charges.     

Preparation of ellipsoidal hematite/polymer hybrid materials and the corresponding hollow polymer ellipsoids

Ellipsoidal hematite/poly(ethyleneglycol dimethacrylate) core-shell hybrid materials were prepared by distillation precipitation polymerization of ethyleneglycol dimethacrylate (EGDMA) in the presence of 3-(methacryloxy)propyl trimethoxysilane (MPS)-modified hematite (α-Fe₂O₃) particles as the seeds. The polymerization of EGDMA was performed in neat acetonitrile with 2,2′-azobisisobutyronitrile (AIBN) as initiator to coat MPS-modified hematite seeds through the capture of EGDMA oligomer radicals with the aid of vinyl groups on the surface of the MPS-modified hematite particles in absence of any stabilizer or surfactant. The shell-thickness of the core-shell hybrid particles was controlled by the feed of EGDMA monomer during the polymerization. Other hematite/polymer core-shell hybrid particles, such as hematite/polydivinylbenzene (α-Fe₂O₃/PDVB) and hematite/poly(divinylbenzene-co- methacrylic acid) (α-Fe₂O₃/P(DVB-co-MAA)) were also prepared by this procedure. Hematite/poly(N,N′-methylenebisacrylamide-co-methacrylic acid) (α-Fe₂O₃/P(MBAAm-co-MAA)) were synthesized with unmodified hematite particles as the seeds. Hollow polymer ellipsoids were subsequently developed after the selective removal of the hematite core with hydrochloric acid (HCl) from hematite/polymer core-shell hybrids. The resultant core-shell hybrid particles and hollow polymer ellipsoids were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectra (FT-IR) and vibrating sample magnetometer (VSM).     

Preparation and characterization of composite resin by vinyl chloride grafted onto poly(BA-EHA)/poly(MMA-St)

Crosslinked poly(butyl acrylate-co-2-ethylhexyl acrylate)/poly(methyl methacrylate-co-styrene) (ACR I) latex was synthesized by multi-stage emulsion polymerization. A series of grafting vinyl chloride (VC) composite latices were prepared by emulsion copolymerization in the presence of core-shell ACR I latex. The effects of ACR I amount and its core/shell ratio on particle diameters of the composite latices and mechanical properties of the prepared materials were investigated. The grafting efficiency (GE) of VC grafted onto ACR I increases with an increasing ACR I content. Transmission electron microscope (TEM) study indicates that ACR I latex particles have a regular core-shell structure obviously. However, when styrene content in the shell of ACR I is more than 70 percent of the shell by weight, ACR I latex particles have an irregular core-shell morphology like sandwich. The composite latex particles synthesized by core-shell ACR I latex grafting VC have a clear three-layered core-shell structure. Dynamic mechanical analysis (DMA) study reveals that the compatibility between ACR I and PVC is well improved. With increasing ACR I content, the loss peak in low temperature range for every composite sample becomes stronger and stronger and gradually shifts to a higher temperature. Scanning electron microscope (SEM) graphs showed that the fractured surface of the composite sample exhibited better toughness of the material. TEM graphs showed that ACR I was uniformly dispersed in the PVC matrix.     

Complex micelles with a responsive shell for controlling of enzymatic degradation

Complex polymeric micelles with a PLA core and a mixed PEG/PNIPAM shell were prepared by self-assembly of two block copolymers: poly(ethylene glycol)-b-poly(lactic acid) (PEG-b-PLA) and poly(N-isopropylacrylamide)-b-poly(lactic acid) (PNIPAM-b-PLA). Using ¹H NMR spectroscopy and dynamic light scattering, the micellization and the enzymatic degradation status were characterized. At 25 °C, the PNIPAM block is hydrophilic and the PLA core is prone to the enzymatic degradation, resulting in the disassembly of the micelles. While increasing the temperature to 45 °C, the PNIPAM collapsed onto the PLA core, protecting the PLA core from the attack by the enzyme, and the micelles exhibit a resistance to the enzymatic degradation. Furthermore, the enzymatic degradation rate of the micelles can also be tuned by changing the ratio of PEG to PNIPAM. With increasing content of PNIPAM, the conformation of the collapsed PNIPAM changes from patchy domains to a continuous and dense layer, and the enzyme accessibility to the PLA core is changed.     

Thermoresponsive core-shell-corona micelles of poly(ethyleneglycol)-b-poly(N-isopropylacrylamide)-b-polystyrene

Core-shell-corona micelles with a thermoresponsive shell self-assembled by triblock copolymer of poly(ethyleneglycol)-b-poly(N-isopropylacrylamide)-b-polystyrene (PEG₄₅-b-PNIPAM₁₆₈-b-PS₄₆) are studied by ¹H NMR, light scattering and atomic force microscopy. The thermoresponsive triblock copolymer, which has a relatively short hydrophobic PS block, can disperse in water at room temperature to form core-shell-corona micelles with the hydrophobic PS block as core, the thermoresponsive PNIPAM block as shell and the hydrophilic PEG block as corona. At temperature above lower critical solution temperature (LCST) of the PNIPAM block, the PNIPAM chains gradually collapse on the PS core to shrink the size and change the structure of the resultant core-shell-corona micelles with temperature increasing. It is found that there possibly exists an interface between the PNIPAM shell and PEG corona of the core-shell-corona micelles at temperature above LCST of the PNIPAM block.     

多重响应壳聚糖微凝胶的制备及溶胀性

采用无皂乳液聚合法制备聚(N-异丙基丙烯酰胺)/壳聚糖微凝胶(PNIPAM/CS),透射电镜和动态光散射研究了微凝胶外貌形态及刺激响应性。结果显示,微凝胶颗粒呈球形,具有核、壳结构形态。加入壳聚糖对PNIPAM的体积相转变温度(VPTT)有影响,微凝胶VPTT随壳聚糖用量的增加向高温迁移,此结果与示差量热法(DSC)测定一致。不同pH条件下微凝胶粒径变化表明,颗粒直径随pH增大逐渐减小,至碱性又增大,显示明显的pH敏感性;相应颗粒Zeta电位逐渐减小,接近中性达到等电点,至碱性反转为负值,这一变化能对微凝胶pH敏感性进行合理解释。     

Influence of charge density on rheological properties and dehydration dynamics of weakly charged poly(N-isopropylacrylamide) during phase transition

It is well-known that introduction of charged groups to poly(N-isopropylacrylamide) (PNIPAM) raises its phase transition temperature. However, the influence of charged groups on structural evolution and dehydration dynamics of weakly charged PNIPAM during phase transition still lacks systematic investigation. In the current study, armed with rheometer and two-dimensional Fourier transform infrared spectrometer (2D-FTIR), we investigated on mesoscopic and microscopic scales the phase transition of sodium poly(N-isopropylacrylamide-co-2-acrylamido-2-methylpropanesulfonate), abbreviated as poly(NIPAM-co-NaAMPS), with charge density of 1–10%. At ambient temperature, scaling exponent of poly(NIPAM-co-NaAMPS) varies from that of neutral polymer to polyelectrolytes as charge density increases. Above phase transition temperature, mesoscopic structure of poly(NIPAM-co-NaAMPS) varies from network of physical gel to viscoelastic liquid containing branched aggregates with increase of charge density, indicating increasing hindrance to intra/inter-chain association due to electrostatic repulsion. On a molecular level, poly(NIPAM-co-NaAMPS) exhibits distinctive microdynamic sequence of dehydration during phase transition, in contrast to neutral PNIPAM. In particular, sulfonate groups decouple the cooperative dehydration of alkyl and carbonyl groups, resulting in their distinctive phase transition temperature as well as temperature range. In analogy to hydration of proteins, it is proposed that the microdynamic sequence, implying the hydration stability of each group, is closely related to the density of hydration layer as well as influence of electrostatic field generated by charged groups. For poly(NIPAM-co-AMPS) with charge density of 3%, there still remains 72.3% of hydrogen bonds between carbonyl group and water at 60 °C, meanwhile a highly hydrated network forms with network strands 1–2 times as long as the copolymer chain length.