Synthesis of cross-linked poly(methyl methacrylate) via dispersion polymerization in supercritical carbon dioxide

Cross-linked poly(methyl methacrylate) particles were prepared via dispersion polymerization in supercritical carbon dioxide (scCO₂) using poly(heptadecafluorodecyl methacrylate) (PHDFDMA) and 2,2′-azobisisobutyronitrile as the dispersant and the initiator, respectively. The following chemicals were used as cross-linking agents: ethylene glycol dimethacrylate (EGDMA), 1,4-buthanediol di(meth)acrylate (1,4-BD(M)A), and trimethylolpropane trimethacrylate. PHDFDMA was synthesized by solution polymerization in scCO₂. We investigated the effect of the chemical structure, concentration of the cross-linking agents, reaction pressure, and CO₂ density on the morphology, the polydispersity, and the cross-linking density of polymer particles. The resulting polymer particle was characterized by field emission SEM, differential scanning calorimetry, and thermal gravimetric analysis. The cross-linked PMMA particles is more agglomerate as the cross-linking agent concentration increased and as pressure decreased at constant temperature. Glass-transition temperature (T _g) of the resulting polymer increased as the cross-linking agent increased with temperature and pressure increasing at the same CO₂ density. Decomposition temperature is slightly increased as 1,4-BDA concentration increased. From these results, we can confirm that the thermal stability of the polymer increased as the cross-linking agent and EGDMA is the best cross-linking agent in term of the thermal stability.     

Dispersion polymerization of methyl methacrylate in supercritical CO2 in the presence of non-fluorous random copolymers

A series of non-fluorous random copolymers, composed of 3-[tris(trimethylsilyloxy)silyl] propyl methacrylate and 2-dimethylaminoethyl methacrylate, poly(SiMA-co-DMAEMA) with different comonomer ratios were prepared and utilized as stabilizers for the free radical dispersion polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide (scCO₂). It was demonstrated that the composition and concentration of the stabilizer have a dramatic effect on the morphology of resulting poly methyl methacrylate (PMMA) latex. When the copolymeric stabilizer poly(SiMA-co-DMAEMA) (71:29) was employed, free-flowing spherical PMMA particles were produced in high yield. As the concentration of stabilizer increases, the resulting size of colloidal particles decreases. In addition, the monomer concentration and initial pressure affected the particle diameter of PMMA.     

Investigation of clay modifier effects on the structure and rheology of supercritical carbon dioxide‐processed polymer nanocomposites

Superior property enhancements in polymer–clay nanocomposites can be achieved if one can significantly enhance the nanoclay dispersion and polymer–clay interactions. Recent studies have shown that nanoclays can be dispersed in polymers using supercritical carbon dioxide (scCO₂). However, there is need for a better understanding of how changing the clay modifier affects the clay dispersability by scCO₂ and the resultant nanocomposite rheology. To address this, the polystyrene (PS)/clay nanocomposites with “weak” interaction (Cloisite 93A clay) and “strong” interaction (Cloisite 15A clay) have been prepared using the supercritical CO₂ method in the presence of a co‐solvent. Transmission electron microscopy images and small‐angle X‐ray diffraction illustrate that composites using 15A and 93A clays show similar magnitude of reduction in the average tactoid size, and dispersion upon processing with scCO₂. When PS and the clays are coprocessed in scCO₂, the “dispersion” of clays appears to be independent of modifier or polymer–clay interaction. However, the low‐frequency storage modulus in the scCO₂‐processed 15A nanocomposites is two orders of magnitude higher than that of 93A nanocomposites. It is postulated that below percolation (solution blended composites), the strength of polymer–clay interaction is not a significant contributor to rheological enhancement. In the scCO₂‐processed nanocomposites the enhanced dispersion passes the percolation threshold and the interactions dictate the reinforcement potential of the clay–polymer–clay network. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 823–831, 2010     

Photochemistry of 2-hydroxy-4-trifluoromethylbenzoic acid, major metabolite of the photosensitizing platelet antiaggregant drug triflusal

Triflusal is a platelet antiaggregant drug with photoallergic side effects. However, it is considered a prodrug since it is metabolized to 2-hydroxy-4-trifluoromethylbenzoic acid (HTB)--the pharmacologically active form. HTB was found to be photolabile under various conditions. Its major photodegradation pathway appears to be the nucleophilic attack at the trifluoromethyl moiety. The involvement of the triplet state in the photodegradation has been unequivocally proved by direct detection of this transient in laser flash photolysis and by quenching experiments with oxygen, cyclohexadiene and naphthalene. Finally, the photobinding of HTB to proteins such as bovine serum albumin has been demonstrated using ultraviolet-visible (UV-Vis) and fluorescence spectroscopy. Nucleophilic groups present in the protein appear to be responsible for the formation of covalent drug photoadducts, which is the first step involved in the photoallergy shown by triflusal.     

Influence of structure and solubility of chain transfer agents on the RAFT control of dispersion polymerisation in scCO2

Reversible addition–fragmentation chain transfer (RAFT) dispersion polymerisation of methyl methacrylate (MMA) is performed in supercritical carbon dioxide (scCO₂) with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. Kinetic studies of the polymerisation in scCO₂ also confirm these data. By contrast, only poor control of MMA polymerisation is obtained in toluene solution, as would be expected for this CTA which is better suited for acrylates. In this regard, we select a range of CTAs and use them to determine the parameters that must be considered for good control in dispersion polymerisation in scCO₂. A thorough investigation of the nucleation stage during the dispersion polymerisation reveals an unexpected “in situ two-stage” mechanism that strongly determines how the CTA works. Finally, using a novel computational solvation model, we identify a correlation between polymerisation control and degree of solubility of the CTAs. All of this ultimately gives rise to a simple, elegant and counterintuitive guideline to select the best CTA for RAFT dispersion polymerisation in scCO₂.

RAFT dispersion polymerisation of methyl methacrylate is performed in scCO₂ with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight.     

傅立叶变换红外光谱技术在超临界CO2作用聚合物体系中的应用

超临界CO₂（scCO₂）作为一种物理化学性质优良、具有高扩散速率及优良溶解性能的溶剂,在科学研究及工业生产中广受青睐。将scCO₂应用于聚合物体系中,CO₂ 与聚合物间特殊的相互作用有利于CO₂分子在聚合物中的吸附与扩散。同时通过CO₂的吸附及其对聚合物的溶胀和塑化作用,聚合物所处微观化学环境以及整体结构性质会发生一定的变化。由于傅立叶变换红外光谱（FTIR）技术能够有效地考察化学环境变化对分子结构造成的影响,这一表征技术在超临界CO₂作用体系中广为应用。本文主要选取了近年来利用FTIR技术考察scCO₂作用于聚合物体系的一些实例,从CO₂-聚合物相互作用机理,scCO₂对聚合物或生物大分子的加工过程的影响两方面,阐述了利用红外光谱技术在scCO₂作用体系中的应用以及前景。     

Investigation of structural,rheological and thermal properties of PMMA/ONi-Al LDH nanocomposites synthesized <Emphasis Type="Italic">via</Emphasis> solvent blending method: Effect of LDH loading

This article addresses the synthesis of organically tailored Ni-Al layered double hydroxide (ONi-Al LDH) and its use in the fabrication of exfoliated poly(methyl methacrylate) (PMMA) nanocomposites. The pristine Ni-Al LDH was initially synthesized by co-precipitation method and subsequently modified using sodium dodecyl sulfate to obtain ONi-Al LDH. Nanocomposites of PMMA containing various amounts of modified Ni-Al LDH (3 wt%-7 wt%) were synthesized via solvent blending method to investigate the influence of LDH content on the properties of PMMA matrix. Several characterization methods such as X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), rheological analysis, differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA), were employed to examine the structural, viscoelastic and thermal properties of PMMA/OLDH nanocomposites. The results of XRD and TEM examination confirm the formation of partially exfoliated PMMA/OLDH nanocomposites. The FTIR results elucidate that the characteristic bands for both pure PMMA and modified LDH are present in the spectra of PMMA/OLDH nanocomposites. Rheological analyses were carried out to examine the adhesion between polymer matrix and fillers present in the nanocomposite sample. The TGA data indicate that the PMMA nanocomposites exhibit higher thermal stability when compared to pure PMMA. The thermal decomposition temperature of PMMA/OLDH nanocomposites increases by 28 K compared to that of pure PMMA at 15% weight loss as a point of reference. In comparison with pure PMMA, the PMMA nanocomposite containing 7 wt% LDH demonstrates improved glass transition temperature (T _g) of around 3 K. The activation energy (E _a), reaction orders (n) and reaction mechanism of thermal degradation of PMMA/OLDH nanocomposites were evaluated using different kinetic models. Water uptake capacity of the PMMA/OLDH nanocomposites is less than that of the pure PMMA.     

Phase separation of polymer‐functionalized SWNTs within a PMMA/polystyrene blend

Phase separation of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends was used as a means to segregate PS‐ or PMMA‐functionalized single‐walled carbon nanotubes (SWNTs) in thin films. Dilute solutions (5 wt % in THF) of 1:1 PS/PMMA blends containing the functionalized nanotubes were spin cast and annealed at 180 °C for 12 h. Two different polymer molecular weights were used (M_n = 8000 or M_n = 22,000), and were of approximately equivalent molecular weight to those attached to the surface of the nanotubes. Nanotube functionalization was accomplished using the Cu(I)‐catalyzed [3 + 2] Huisgen cycloaddition, in which alkyne‐decorated nanotubes were coupled with azide‐terminated polymers, resulting in polymer‐SWNT conjugates that were soluble in THF. Characterization of the annealed films by scanning Raman spectroscopy, which utilized the unique Raman fingerprint of carbon nanotubes, enabled accurate mapping of the functionalized SWNTs within the films relative to the two phase‐separated polymers. It was found that nanotube localization within the phase‐separated polymer films was influenced by the type of polymer attached to the nanotube surface, as well as its molecular weight. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 450–458, 2009     

Spectroscopic analysis and in situ monitoring of impregnation and extraction of polymer films and powders using supercritical fluids

A range of vibrational spectroscopic techniques are used to monitor supercritical fluid extraction and impregnation of polymers. Impregnation processes of this type show potential as alternative apporaches to the synthesis of polymer based catalysts and new materials. Methods have been devised using conventional Fourier transform infrared spectroscopy for real time monitoring of the extraction and impregnation of polyethylene films using an organometallic complex, CpMn(CO)₃ (Cp = η⁵ ? C₅ H₅) as a spectroscopic probe. Both low and high density powdered polyethylene may be impregnated using supercritical carbon dioxide. The resulting materials are analysed using FTIR photoacoustic spectroscopy, diffuse reflectance infrared spectroscopy and FT Raman spectroscopy to probe both the impregnated bulk of the polymer and surface-coated material. © 1994 John Wiley & Sons, Inc.     

Effect of the polymer matrix on the formation of silver nanoparticles in polymer–silver salt complex membranes

When polymer–silver salt complex membranes were exposed to UV irradiation, the separation performances of both the permeance and selectivity for propylene–propane decreased, which was primarily attributed to the reduction of the silver ions in the membranes to silver nanoparticles. Here, the effect of the polymer matrix on the formation of silver nanoparticles in the polymer–silver salt complex membranes was investigated. This effect was assessed for the complexes of two kinds of silver salts (AgBF₄ and AgCF₃SO₃) with several polymeric ligands containing three different carbonyl groups, including poly(vinyl pyrrolidone) (PVP) with an amide group, poly(vinyl methyl ketone) (PVMK) with a ketone group, and poly(methyl methacrylate) (PMMA) with an ester group. UV–vis spectra and transmission electron microscopy (TEM) images clearly indicated that the reduction rate of the silver ions has the following order in the various polymer matrices: PVP > PVMK > PMMA, whereas the size and the distribution of the nanoparticles exhibited the reverse order. The tendency to form silver nanoparticles was explained in terms of the differences between the comparative strengths of the interactions of the silver ions with the different carbonyl oxygens in the matrices, as well as that of the silver ions with counteranions, which was characterized by X‐ray photoelectron spectroscopy (XPS) and FT‐Raman spectroscopy. It was concluded that when the concentration of free silver ions was low due to weak polymer–silver ion and strong silver ion–anion interactions, as found with PMMA, the reduction rate of silver ions to silver nanoparticles was slow. Therefore, the PMMA–silver complex membranes were less sensitive to decreases in separation performance upon UV irradiation than compared to the PVP membranes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1168–1178, 2006     

Microscopic spacial effect on the dispersion polymerization in scCO2

We report for the first time the microscopic spacial effect on the dispersion polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide (scCO₂). A variety of different-sized high-pressure vessels including microstructured holey optical fiber were employed to conduct the polymerization reactions. The molecular weights of the polymer products indicate that the function of the stabilizer and the process of chain growth are not significantly influenced. However, the SEM images show a gradual loss of the controlled morphology for the polymer products in reactors of dimension less than 1 mm under the same reaction conditions. This study provides a better understanding of the mechanism of the dispersion polymerization progress and gives a very important caution on the performance of microreactors.     

In situ radical polymerization of methyl methacrylate in a solution of surface modified TiO2 and nanoparticles

Surface modified TiO₂ nanoparticles dissolved in toluene were encapsulated in PMMA by in situ radical polymerization of methyl methacrylate initiated by 2,2′-azobisisobutyronitrile. The surface modification of the TiO₂ nanoparticles (average diameter of 4.5 nm) was achieved by the formation of a charge transfer complex between TiO₂ nanoparticles and 6-palmitate ascorbic acid. The surface modified TiO₂/nanoparticles were characterized using UV−Vis and FTIR spectroscopy, while the obtained polymer nanocomposites were characterized using reflection and ¹H NMR spectroscopy, as well as gel permeation chromatography. The influence of the TiO₂ nanoparticles on the thermal properties of the PMMA matrix was investigated using thermo-gravimetric analysis and differential scanning calorimetry. The glass transition temperature of the polymer was not influenced by the presence of the nanoparticles while the thermal stability was significantly improved.     

Synthesis of highly dispersed,block copolymer‐grafted TiO2 nanoparticles within neat block copolymer films

The objective of the study is to formulate exclusive block copolymer (BCP) nanocomposites by dispersing bcp end‐grafted nanoparticles (bcp‐g‐nps) of PMMA‐b‐PS‐g‐TiO₂ within PS‐b‐PMMA matrix. PMMA‐b‐PS‐g‐TiO₂ is synthesized using a “grafting‐to” approach and characterized by XPS and TGA to establish that the copolymer chains were bonded to NPs. Good dispersion of bcp‐g‐nps in PMMA and PS‐PMMA bcp films is observed, in contrast to poor dispersion in PS films. In PS‐PMMA films, the compatible and identical bcp nature of the end‐grafted polymer, and large NP size caused it to span across entire PS‐PMMA domains. Poor and good dispersion in PS and PMMA matrices, respectively, can be rationalized by the fact that NPs interactions are driven by the PMMA at the outer corona of the bcp‐g‐nps. Developing bcp‐g‐nps as a strategic route to preparation of highly dispersed high permittivity NPs like titanium dioxide (TiO₂) in bcp matrix can have important ramifications for energy storage devices. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 468–478     

Dispersion polymerization of MMA in supercritical CO2 stabilized by random copolymers of 1H,1H–perfluorooctyl methacrylate and 2‐(dimethylaminoethyl methacrylate)

A series of random copolymers, composed of 1H,1H‐perfluorooctyl methacrylate (FOMA) and 2‐dimethylaminoethyl methacrylate (DMAEMA) were prepared as stabilizers for the dispersion polymerization of methyl methacrylate in supercritical CO₂ (scCO₂). Free‐flowing, spherical poly(methyl methacrylate) (PMMA) particles were produced in high yield by the effective stabilization of poly(FOMA‐co‐DMAEMA) containing 34–67 w/w % (15–41 m/m %) FOMA structural units. Less stabilized but micron‐sized discrete particles could be obtained even with 25 w/w % (10 m/m %) FOMA stabilizer. The result showed that the composition of copolymeric stabilizers had a dramatic effect on the size and morphology of PMMA. The particle size was controllable with the surfactant concentration. The effect of the monomer concentration and the initial pressure on the polymerization was also investigated. The dry polymer powder obtained from dispersion polymerization could be redispersed to form stable aqueous latexes in an acidic buffered solution (pH = 2.1) by an electrostatic stabilization mechanism due to the ionization of DMAEMA units in the stabilizer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1365–1375, 2008     

Comparison of GO and polymer microcapacitors prepared by ion beam writing

Carbon beam writing was employed as a method for maskless production of microscale capacitors in both insulating graphene oxide (GO) and poly(methyl methacrylate) (PMMA) matrix. The GO and PMMA foils were irradiated using a 5-MeV C³⁺ beam with micrometer scale resolution. As follows, the shape of the created microstructures and compositional changes was studied using the scanning electron microscopy/energy-dispersive X-ray spectroscopy method (SEM/EDS). The structural and compositional progression was characterized by Raman spectroscopy, Rutherford backscattering spectroscopy (RBS), and elastic recoil detection analysis (ERDA) spectroscopy. The improvement of the prepared structures' electrical properties was also studied, and it can be concluded that carbon irradiation leads to the removal of oxygen and hydrogen and to growth of the carbon domains, which is connected with the conductivity increase of the irradiated parts and capacitance of the final products in the order of pF.     

Crystallization of titania ultra-fine particles from peroxotitanic acid in aqueous solution in the present of polymer and incorporation into poly(methyl methacylate) via dispersion in organic solvent

We report a novel strategy for incorporation of titanium dioxide (TiO₂) particles, which were crystallized from peroxotitanic acid in the presence of hydrophilic polymer by hydrothermal treatment in aqueous solution, into poly(methyl methacrylate) (PMMA) via dispersion into chloroform. Dispersion of TiO₂ particles into chloroform was achieved by solvent change from water to chloroform in aid of amphiphilic polymer dispersant, poly(N-vinyl pyrrolidone) (PVP), poly(N-vinyl pyrrolidone-co-methyl methacrylate) (PVP-co-PMMA), poly(N-vinyl pyrrolidone-block-methyl methacrylate) (PVP-b-PMMA) through azeotropical removal of water. Incorporation of TiO₂ particles into PMMA was carried out by a casting process of a mixture of TiO₂ particles dispersed with PVP₁₅₄-b-PMMA₁₅₆ in chloroform and PMMA on a glass substrate. Resultant hybrid film containing TiO₂ less than 10 wt.% showed high transparency in visible region attributable to homogeneous dispersion into PMMA matrix. The refractive index of the hybrid films increased with TiO₂ content and agreed with the calculated values.     

Novel Color-Shifting Mobility Sensitive Fluorescent Probes for Polymer Characterization

1-Methyl-7-dimethylamino quinolinium tetrafluoroborate, a highly stable, highly fluorescent color-shifting mobility sensitive fluorescent probe was employed for detecting the glass transition and phase transitions, notably crystallization, in polymers and polymer blends. Glass transitions in amorphous and semi-crystalline polymers were detected by a change in gradient in emission wavelength λ_max versus T plots. Crystallization resulted in discrete blue shifts in λ_max versus T plots. Selective probing of PMMA in a PS/PMMA blend, down to a PMMA content of 1%, was demonstrated. Dielectric relaxation spectroscopy has established a clear link between the mobility of dipoles in PMMA and the emission wavelength of the fluorescent probe.     

Piroxicam Loading onto Mesoporous Silicas by Supercritical CO2 Impregnation

Piroxicam (PRX) is a commonly prescribed nonsteroidal anti-inflammatory drug. Its efficacy, however, is partially limited by its low water solubility. In recent years, different studies have tackled this problem and have suggested delivering PRX through solid dispersions. All these strategies, however, involve the use of potentially harmful solvents for the loading procedure. Since piroxicam is soluble in supercritical CO₂ (scCO₂), the present study aims, for the first time, to adsorb PRX onto mesoporous silica using scCO₂, which is known to be a safer and greener technique compared to the organic solvent-based ones. For comparison, PRX is also loaded by adsorption from solution and incipient wetness impregnation using ethanol as solvent. Two different commercial mesoporous silicas are used (SBA-15 and Grace Syloid^® XDP), which differ in porosity order and surface silanol population. Physico-chemical analyses show that the most promising results are obtained through scCO₂, which yields the amorphization of PRX, whereas some crystallization occurs in the case of adsorption from solution and IWI. The highest loading of PRX by scCO₂ is obtained in SBA-15 (15 wt.%), where molecule distribution appears homogeneous, with very limited pore blocking.     

Electrical and rheological percolation of PMMA/MWCNT nanocomposites as a function of CNT geometry and functionality

Composites of poly(methyl methacrylate) (PMMA) with multi-walled carbon nanotubes (MWCNT) of varying aspect ratio and carboxylic acid functionality were prepared using melt mixing. The extent of dispersion and distribution of the MWCNTs in the PMMA matrix was investigated using a combination of high-resolution transmission electron microscopy (HRTEM), wide-angle X-ray diffraction (XRD) and Raman spectroscopy. The electrical resistivity and oscillatory shear rheological properties of the composites were measured as a function of MWCNT geometry, functionality, and concentration. The fundamental ballistic conductance of the pristine free-standing MWCNTs was investigated using a mechanically controlled break-junction method. The electrical conductivity of PMMA was enhanced by up to 11 orders of magnitude for MWCNT concentrations below 0.5 wt.%. MWCNTs having higher aspect ratio, above 500, or functionalized with carboxylic acid groups readily formed rheological percolated networks with thresholds, determined from a power law relationship, of 1.52 and 2.06 wt.%, respectively. The onset of pseudo-solid-like behaviour and network formation is observed as G′, η∗, and tan δ⁻¹ are independent of frequency as MWCNT loading increased. Sufficiently long and/or functionalized tubes are required to physically bridge or provide interfacial interactions with PMMA to alter polymer chain dynamics. Carboxylic acid functionalization disrupts the crystalline order of MWCNTs due to a loss of π-conjugation and electron de-localisation of sp² C-C bonds resulting in non-ballistic electron transport in these tubes, irrespective of how highly dispersed they are in the PMMA matrix.     

固相浸渍法制备NiO/CeO2催化剂及其在CO氧化反应中的应用

采用固相浸渍法制备了一系列NiO/CeO2催化剂,并通过与常规湿浸渍法比较,考察了制备方法对催化剂和CO氧化反应性能的影响.同时结合X射线衍射(XRD),N2吸附-脱附(BET),透射电镜(TEM),氢气-程序升温还原(H2-TPR),拉曼(Raman)光谱,X射线光电子能谱(XPS)等手段对催化剂的结构和表面物种分散状态进行了表征.CO氧化活性测试结果表明,当镍负载量相同时,固相浸渍法制备的催化剂相比于湿浸渍法表现出更好的催化性能.TEM、XPS、H2-TPR结果表明,固相浸渍法更有利于加强镍铈间的相互作用和得到高分散的镍物种,从而促进镍物种的还原.Raman结果表明固相浸渍法相比于湿浸渍法能产生更多氧空位,这有利于氧气在催化剂表面的活化,使得CO氧化反应更容易进行.