Temperature and pH dual-responsive coatings of oligoperoxide-graft-poly(N-isopropylacrylamide): Wettability, morphology, and protein adsorption

Poly(N-isopropylacrylamide) (PNIPAM) coatings attached to glass with novel approach involving polymerization from oligoperoxide grafted to surface with (3-aminopropyl)triethoxysilane exhibit not only temperature- but also pH-dependence of wettability and protein adsorption. Wettability and composition of coatings, fabricated with different polymerization times, were determined using contact angle measurements and Time Of Flight-Secondary Ion Mass Spectrometry (TOF-SIMS), respectively. Thermal response of wettability, measured between 20 and 40 °C, was prominent at pH 9 and 7 and diminished or absent at pH 5 and 3. This indicates a transition between hydrated loose coils and hydrophobic collapsed chains that is blocked at low pH. Higher surface roughness and dramatically increased adsorption of model protein (lentil lectin labeled with fluorescein isothiocyanate) were observed with AFM and fluorescence microscopy to occur in hydrophobic phases (at pH 3, for pH varied at constant temperature of 22 °C and at ∼33 °C, for temperature varied at constant pH 9). Protein adsorption response to pH was confirmed by TOF-SIMS and Principal Component Analysis.     

Controlled free-radical polymerization of methyl acrylate in the presence of a cyclic trithiocarbonate under γ-ray irradiation at low temperature

Controlled radical polymerization of MA has been achieved in the presence of a cyclic trithiocarbonate, 1,5-dihydro-2,4-benzodiehiepine-3-thione, under γ-ray irradiation (60 Gy/min) at low temperature. The narrow molecular weight distributions and the linear kinetics curve indicate that the polymerization is a controlled free-radical process at low temperature (especially at −76 °C). The structures of resultant polymers were characterized by matrix assisted laser desorption/ionization time-of-flight mass spectrometry, and the results show that cyclic polymers can be formed at −76 °C, which may result from the reduced diffusion rate and the suppressed chain-transfer reaction at the lower temperature. It is further evidenced that the good control of the polymerization at the lower temperature may be associated with the suppressed chain-transfer reaction, not like reversible addition-fragmentation transfer polymerization. The linear polymers probably result from the polymer chain radicals reacting with the radicals produced by the interaction of the irradiation and the monomer.     

Effects of the polymerization temperature on the structure,morphology and conductivity of polyaniline prepared with ammonium peroxodisulfate

Polyaniline (PANI) samples were prepared by the oxidation of aniline with ammonium peroxodisulfate in a reaction vessel placed in a bath thermostated to particular temperature, T_b, from −20 °C to 40 °C. Temperature–time profiles of reaction mixtures were monitored except for the reaction at −20 °C that proceeded in the solid state. The temperature regime was found to influence the molecular structure, morphology, crystallinity and electrical conductivity of PANI. The increase in T_b results in an increased content of meanwhile unspecified structure defects in the formed PANI chains (the presence of attached self-doping groups is improbable), decreased crystallinity, toughness and compactness of PANI microparticles and increased steepness of the temperature dependence of PANI conductivity. The PANI prepared in the solid-state polymerization at −20 °C shows, besides a rather high crystallinity, the unusually high position of the quinonoid band maximum: 643 nm, which suggests a high regularity of its chains. A correlation between the temperature dependence of PANI conductivity at low temperatures (range from 13 to 318 K) on one hand and the temperature regime of PANI preparation on the other hand, is reported for the first time. The dependences obtained only poorly meet the variable random hopping model.     

Multihollow structured poly(methyl methacrylate)/silver nanocomposite microspheres prepared by suspension polymerization in the presence of dual dispersion agents

Poly(methyl methacrylate) (PMMA)/silver nanocomposite microspheres with unique multihollow structures were prepared by suspension polymerization in the presence of dual dispersion agents. The addition of a lipophilic emulsifier, polyethylene glycol (30EO) dipolyhydroxystearate (Arlacel P135), not only stabilized water-in-oil (W/O) emulsion, but also converted silver nanoparticles from hydrophilic to lipophilic. When a suspension polymerization dispersion agent, poly(vinyl alcohol), was added to the above W/O emulsion system, a water-in-oil-in-water suspension was formed with silver nanoparticles dispersed in the oil phase. The suspension polymerization was carried out at low temperature with 2,2’-azobis(2,4-dimethylvaleronitrile) as the initiator. When modified silver nanoparticles were added, the rate of polymerization increased slightly. High monomer conversion (about 85%) was obtained in spite of low polymerization temperature of 30 °C. Under controlled conditions, PMMA/silver microspheres with various hollow structures were synthesized. The PMMA/silver microspheres with multihollow structure showed high antibacterial ability.     

Hydrogel microspheres III. Temperature-dependent adsorption of proteins on poly-N-isopropylacrylamide hydrogel microspheres

Precipitation polymerization of N-isopropylacrylamide (NIPAM) with methylenebisacrylamide (MBAAm) in water at 70°C gave thermosensitive hydrogel microspheres. The adsorbability of proteins on the poly-NIPAM microspheres was found to depend on temperature. Below the lower critical solution temperature (LCST) of poly-NIPAM in an aqueous medium, that is, around 32°C, the microspheres hold a large amount of water inside and their surface is hydrophilic enough to suppress the adsorption of proteins. On the contrary, above 32°C, the micropheres deswell and their surface becomes hydrophobic and, consequently, susceptible to adsorption of a large amount of proteins. Proteins once adsorbed on the microspheres at a high temperature could be desorbed more or less by lowering the temperature to below 32°C. The extent of desorption at low temperatures was found to depend on the incubation time for adsorption at high temperatures.     

Dielectric behavior of polyaniline synthesized by different techniques

Polyaniline doped with dodecylbenzenesulfonic acid (Pani.DBSA) was synthesized by different procedures: by a dedoping-redoping process, by one step inverted emulsion polymerization and by one step aqueous dispersion polymerization. The effect of these different techniques on the electric properties (dielectric constant, dielectric losses, and complex electric modulus) of the corresponding emeraldine base has been studied by thermal dielectric analyzer (DETA) in the temperature range −130 °C to 200 °C and in frequency range 0.03-10⁵ Hz. It was found that the preparation technique has significant influence on the dielectric properties of Pani. The different synthetic routes give rise to polyaniline with different distribution of electric relaxation process, indicating different chain structure. Emeraldine base from Pani.DBSA prepared by one step aqueous dispersion polymerization exhibits one single relaxation peak with narrow distribution whereas that prepared by inverted emulsion polymerization exhibits two relaxation peaks, indicating two-phase structure as indicated by a bimodal distribution of relaxation process. Emeraldine base from Pani.DBSA prepared by dedoping-redoping process presents an intermediary behavior. Percentage crystallinity of Pani.DBSA samples has also been investigated using wide-angle X-ray diffraction analysis. Pani.DBSA prepared by aqueous dispersion exhibited higher crystallinity degree, which agrees with the higher conductivity.     

NMR linewidth study of a latex interpenetrating network

Polymer structure developed in latex particles has been investigated by measuring the temperature dependence of the carbon magnetic resonance (CMR) peak linewidths. Interpenetrating polymer networks (IPN) were formed by a continuous-addition emulsion polymerization process, in which a crosslinked seed particle was the site for formation of a linear second-stage polymer. Morphology was controlled by the level of crosslinking in the seed stage. The two polymers of the composite particle differ in their glass-transition temperatures by 100°C. By heating the particles to 50°C above the glass-transition temperature of the softer polymer, which formed the continuous network phase of this composite, it was possible to observe its CMR spectrum independently of the other polymer. The temperature dependence of the linewidth varied with the structure of the network. This was characterized by limiting linewidth behavior at both low and high temperature, which systematically varied as a function of the degree of mixing the two phases. Differential scanning calorimetry and electron microscopy corroborated the morphology change observed with the linewidth data. The influence of a diluent, present during formation of the network stage, was also examined with this CMR approach.     

Optimum conditions for preparing micron-sized PMMA beads in the dispersion polymerization using PVA

The optimum conditions for preparing micron-sized monodisperse polymethylmethacrylate (PMMA) beads by dispersion polymerization in a methanol/water mixture were proposed. PMMA forming microspheres having an average molecular weight of 55,300 g/mol, 2.6 μm weight-average diameter, with a 5.3% coefficient of variation and 91% conversion, were successfully obtained in the presence of 15 wt.% of polyvinylalcohol (PVA), 100/50 (g/g) of MeOH/water mixtures at 70°C; the reaction lasted for 8 h. Compared to dispersion polymerization using polyvinylpirrolydone, PVA proved to be an extremely stable steric stabilizer in the dispersion polymerization of methylmethacrylate.     

Curing conditions of polyarylacetylene prepolymers to obtain thermally resistant materials

Prepolymers of polyarylacetylene (PAA) were synthesized from 1,4-diethynylbenzene using nickel catalyst (C20, C25, and C30) or by direct thermal polymerization (T48). Their curing behaviors were investigated in detail to determine the proper curing conditions that lead to high char yields in cured PAA resins. Dynamic and isothermal differential scanning calorimetry (DSC) measurements were employed to investigate the curing conditions of the prepolymers. Dynamic DSC study reveals that exothermic heat starts at about 120 °C, reaches to a maximum at 210 °C, and ends around 300 °C. Moreover, step isothermal DSC investigation (at 120, 160, 200, 250, and 300 °C; 1 h for each temperature) shows that the major curing occurs at 160 °C, 200 °C and 250 °C, with more than 85% of the acetylene groups reacted. Using this step-curing conditions, very high thermal resistance is realized on C30, with thermal decomposition temperature (at 10% weight loss) and char yield (at 800 °C) being 686 °C and 86%, respectively. Current results indicate that highly thermal resistant PAA resins are obtainable using step curing of PAA prepolymers synthesized by Ni-catalyzed reaction.     

Thermally reversible light scattering films based on the melting/crystallization of organic crystals dispersed in an epoxy matrix

Films that can be reversibly switched from opaque to transparent states by varying temperature (TRLS films), have potential applications in thermal sensors, optical devices, recording media, etc. A dispersion of organic crystals in a thermoset may be used for these purposes provided that at temperatures higher than the melting point there is a matching of refractive indices of both phases. A model system consisting on a dispersion of diphenyl (DP) crystals in an epoxy matrix based on diglycidyl ether of bisphenol A and m-xylylenediamine, was analyzed as a possible TRLS film encapsulated between transparent covers to avoid sublimation of DP. To obtain a uniform dispersion of DP-rich domains in the epoxy matrix by polymerization-induced phase separation, it was necessary to add 5 wt% of polystyrene (PS) to the initial formulation. Phase separation induced by polymerization at 80 °C led to a dispersion of PS/DP domains in the epoxy matrix due to the low compatibility of PS with the epoxy and its high compatibility with DP. Crystallization and melting processes were confined to the interior of dispersed domains leading to an excellent reproducibility of the optical properties of TRLS films in the course of successive heating-cooling cycles.     

Optimising cell temperature and dispersion field strength for the screening for putrescine and cadaverine with thermal desorption-gas chromatography-differential mobility spectrometry

Biogenic amines, and putrescine and cadaverine in particular, have significant importance in the area of food quality monitoring, and are also potentially important markers of infection, for cancer, diabetes, arthritis and cystic fibrosis. A thermal desorption-gas chromatograph-heated differential mobility spectrometer was constructed and the significant effect of interactions between cell temperature and dispersion field strength on the observed responses studied. The experiment design was a Box-Wilson central composite design (CCD) over the levels of 10-24 kV cm⁻¹ for dispersion field strength and 100-130 °C for cell temperature. The optimum values were estimated to be 16.22 kV cm⁻¹ and 116 °C for putrescine and 14.78 kV cm⁻¹ and 112 °C for cadaverine, respectively with an ammonia dopant at 19 mg m⁻³.An amine test atmosphere generator was constructed and produced stable concentrations of putrescine (7 mg m⁻³) and cadaverine (4 mg m⁻³) vapours at 50 ± 0.5 °C. Tenax TA-Carbotrap adsorbent tubes were used to sample putrescine and cadaverine vapour standards and a linear response function over the range of sample masses 5-20 ng was obtained at 15.0 kV cm⁻¹ 115 °C, with a R² of 0.99 for both putrescine and cadaverine. The sample mass at the limit of detection was estimated to be 3 ng for putrescine and cadaverine. Preliminary data from sampling the headspace of chicken meat revealed a 62% increase in the recovered masses of putrescine from 0.84 to 1.36 ng in the sampled air.     

Effect of microwave energy on chain propagation of poly(ε-caprolactone) in benzoic acid-initiated ring opening polymerization of ε-caprolactone

Microwave-assisted ring opening polymerization of ε-caprolactone (ε-CL) initiated by benzoic acid was investigated. The molar ratio of ε-CL to benzoic acid was 5, 15 and 25. The mixtures of ε-CL/benzoic acid were heated under microwave irradiation and the temperatures were self-regulated to equilibrium from 204 to 240 °C with microwave power ranging from 340 to 680 W. The polymer chain propagated fast between 160 and 230 °C, within which the higher the temperature, the faster the propagation. However, when the temperature was over 230 °C, the resultant poly-(ε-caprolactone) (PCL) degraded. The advantage of microwave-assisted polymerization was that the propagation of PCL chain was significantly enhanced but the formation of growing center at the beginning stage of the polymerization was greatly inhibited. With this metal-free method, PCL with weight-average molar mass (M_w) over 4×10⁴ g/mol was prepared.     

N,N′-Diaminoethane linked bis-TEMPO-mediated free radical polymerization of styrene

The N,N′-diaminoethane linked bis-TEMPO nitroxide (C2)-mediated free radical polymerization of styrene at 135 °C in bulk was studied. It was found that under comparable conditions a single nitroxide group of C2 biradical retards the polymerization more than TEMPO. The results were discussed in terms of through-space interactions between two TEMPO moieties of C2 biradical and diffusion effects. According to experimental results analyzed by means of statistical methods, the polymerization system displays a bimodal molecular-weight distribution (MWD) from the beginning of the polymerization process, most probably by undergoing decomposition side reactions leading to irreversible polymer arm (P) separation from PC2P to PC2 and PC2H alkoxyamines. The scale of the decomposition depends rather on the time the system is maintained at the polymerization temperature than on conversion of monomer. Generally, the contribution of low molecular weight chains to overall MWD increases with time of polymerization whereas the contribution of high molecular weight chains to MWD increases for less controlled polymerization systems. For polymers obtained at high [dinitroxide]/[initiator] ratio, the thermal treatment of polystyrene in mass at 135 °C unexpectedly revealed an increase of M_n, which can probably be ascribed to post-polymerization effects involving polystyrene with unsaturated chains end groups.     

Monodisperse and isolated microspheres of poly(N-methylaniline) prepared by dispersion polymerization

Monodisperse and isolated microspheres of poly(N-methylaniline) were successfully prepared through chemical polymerization of N-methylaniline by in adipic acid containing poly(vinylpyrrolidone) (PVP). Mean diameters of the microspheres with smooth surfaces changed from 320 to 100 nm by increasing the reaction temperature from 25 to 75 °C. The concentration of PVP did not affect much the size of microspheres, but the increased PVP concentration led to longer induction times for the onset of dispersion polymerization.     

Morphology of micron-sized monodispersed poly(butyl methacrylate)/polystyrene composite particles produced by seeded dispersion polymerization

In order to develop the seeded dispersion polymerization technique for the production of micron-sized monodispersed core/shell composite polymer particles the effect of polymerization temperature on the core/shell morphology was examined. Micron-sized monodispersed composite particles were produced by seeded dispersion polymerizations of styrene with about 1.4-μm-sized monodispersed poly(n-butyl methacrylate) (Pn-BMA) and poly(i-butyl methacrylate) (Pi-BMA) particles in a methanol/water (4/1, w/w) medium in the temperature range from 20 to 90 °C. The composite particles, PBMA/polystyrene (PS) (2/1, w/w), consisting of a PBMA core and a PS shell were produced with 2,2′-azobis(4-methoxy-2,4-dimethyl valeronitrile) initiator at 30 °C for Pn-BMA seed and with 2,2′-azobis(isobutyronitrile) initiator at 60 °C for Pi-BMA seed. The polymerization temperatures were a little above the glass-transition temperatures (T _g) of both Pn-BMA (20 °C) and Pi-BMA (40 °C). On the other hand, when the seeded dispersion polymerizations were carried out at much higher temperatures than the T _g of the seed polymers, composite particles having a polymeric oil-in-oil structure were produced. Received: 14 October 1998 Accepted in revised form: 2 June 1999     

Copolymerization of ethylene and non-conjugated diene using homogenous catalyst

Ethylene and different amounts of 1,7-octadiene were copolymerized using the metallocene catalyst system ethylidene-bis(fluorenyl) zirconium dichloride and methylaluminoxane (MAO) at both 50 and 90 °C. The catalyst activity has slightly increased with the addition of low amounts of the diene in relation to the homopolymerization of ethylene. The obtained polymers were characterized according to their melting temperature (T_m) and crystallinity degree (x_c) by differential scanning calorimetry (DSC). Weight-average molecular weight (M_w) and polydispersity were determined by gel permeation chromatography (GPC). Diene contents in the copolymer were obtained through the FTIR spectroscopy. The results indicated that at polymerization temperature of 90 °C, crosslinking bonds in the obtained copolymers were low, differently from what was observed at 50 °C. The diene content in the copolymer achieved more than 3 mol% and the comonomer conversion was around 15%. Moreover, the obtained copolymers have M_w around 100,000 and large polydispersity.     

Synthesis and thermal properties of the thermosetting resin based on cyano functionalized benzoxazine

A novel thermosetting resin based on cyano functionalized benzoxazine (BZCN) has been synthesized from 2,6-bis(4-diaminobenzoxy)benzonitrile phenol and formaldehyde by solution reaction. The structure of the monomer is supported by FTIR, ¹H NMR and ¹³C NMR spectra, which have exhibited that the reactive benzoxazine rings and cyano group exist in the molecular structure of BZCN. The curing reactions of BZCN are monitored by the disappearance of the nitrile peak and the tri-substituted benzene ring that is attached with oxazine ring peak at 2231 and 930 cm⁻¹, respectively. The complete cured materials could achieve char yields up to 70% at 800 °C in nitrogen atmosphere, above 64% at 600 °C in air (20% oxygen) environments and the glass transition temperature up to 250 °C. The thermally activated curing polymerization reaction of BZCN follows multiple polymerization mechanisms via the ring-opening polymerization of oxazine rings and the triazine ring-formation of cyano groups, which contribute to the stability of the polymer.     

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

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

Ring-opening polymerization of ε-caprolactone initiated by diphenylzinc

In the present work the polymerization of ε-caprolactone (ε-CL) using Ph₂Zn as initiator is reported. The effects of reaction temperature, molar ratio of monomer/initiator and reaction time on the yield and the molecular weight are investigated. The temperature is varied between 20 and 120 °C and the molar ratio of monomer to initiator between 200 and 800 mol/mol. The results indicate that the Ph₂Zn induces the polymerization of ε-CL to high conversion and produces polymer with high molecular weight at temperatures around 40-60 °C.     

Kinetic studies on the syndiotactic propylene polymerization catalyzed over iPr(Cp)(Flu)ZrCl2

Kinetic studies on the syndiospecific polymerizations of propylene with iPr(Cp)(Flu)ZrCl₂/methylaluminoxane (MAO) were performed at 20, 40 and 70 °C and at 5 atm with various Al/Zr molar ratios. The average polymerization activity for 60 min decreased, and the time to reach a maximum activity (t_max) decreased as Al/Zr molar ratio increased. However, at Al/Zr molar ratio of 10,000, catalytic activity decreased rapidly and became the smallest among any other Al/Zr molar ratios after 20 min of polymerization. At higher Al/Zr molar ratio, methylation and cationization progress rapidly, but its polymerization rate decayed quickly due to strong interaction between MAO and metallocene, resulting in less active species. Regardless of change in polymerization temperature, t_max was maintained around 15 min. Stereoregularity was strongly dependent on the polymerization temperature, and active site isomerization was dominant source for stereoirregularity, and it was strongly influenced by polymerization temperature.