Segmented block copolymers were synthesized using monodisperse diaramide (TΦT) as hard segments and PTMO with a molecular weight of 2 900 g · mol?1 as soft segments. The aramide: PTMO segment ratio was increased from 1:1 to 2:1 thereby changing the structure from a high molecular weight multi‐block copolymer to a low molecular weight end‐block copolymer. The thermal and thermal‐mechanical properties were studied by DSC and DMA. Also studied were the tensile and the elastic properties as well as the melt rheological behavior as a function of frequency. The crystallinity of aramide in the end‐block copolymers was found to be higher than in the multi‐block copolymer.
Summary: A convenient method for the preparation of well‐defined core‐shell alumina/polystyrene composite nanoparticles (50 nm) through emulsion polymerization from needle‐shaped alumina is reported. The pretreatment of alumina with 3‐methacryloxypropyltrimethoxysilane (MPTMS) is essential for successful encapsulation and for creating strong interfacial interaction between alumina and polystyrene (PS). High yield and binding efficiency (both higher than 90%) are achieved by a proper selection of the reaction conditions. As revealed by transmission electron microscopy (TEM) and atomic force microscopy (AFM), an ideal core‐shell structure is obtained with one alumina needle as the core and PS as the shell when the amount of alumina is appropriate. However, when more alumina is used, the PS latexes contain an agglomerate of a few alumina needles arranging in a parallel fashion without any precipitation of alumina. The analysis of the grafted and free PS has shown that the grafted PS has bigger molecular weight (MW) and larger molecular weight distribution (MWD) presumably due to some condensation between the methoxyl groups of the silane moiety. The polymerization kinetics and the encapsulation mechanism have also been discussed.
Schematic representation of the encapsulation procedure. 相似文献
pH‐sensitive micelles formed by interchain hydrogen bonding of poly(methacrylic acid)‐block‐poly(ethylene oxide) copolymers were prepared and investigated at pH < 5. Both and Rh of the micelles increase with decreasing pH of the solution, displaying an asymptotic tendency at low pH values. The observed micelles are well‐defined nanoparticles with narrow size distributions (polydispersity ΔRh/Rh ≤ 0.05) comparable with regular diblock copolymer micelles. The CMCs occur slightly below c = 1 × 10?4 g · mL?1. The micelles are negatively charged and their time stability is lower than that of regular copolymer micelles based purely on hydrophobic interactions.
The polymerization of (+)‐catechin with various aldehydes has been performed in the presence of an acid catalyst to produce catechin‐aldehyde polycondensates in high yields. NMR analysis of the product showed that the polymerization regioselectively proceeded to form the polymer linked by an ethyl bridge through C‐6 and C‐8 positions of catechin A ring. Specific rotation and CD measurements of the polymer showed the change of chemical bonding and conformation at near chiral center. The direction of specific rotation was changed from dextrorotatory (+) to levorotatory (?) and a new chiral center was formed on the bridge moiety between the catechin units. Computer simulation suggests the coiled interactions in the polymer by hydrogen bonds between hydroxyl groups and pyranic oxygens.
Alternating free‐radical copolymerization of vinylbenzyl‐ and methacryloyl‐terminated macromonomers in the presence of Lewis acid was applied to the synthesis of prototype copolymer brushes composed of polystyrene/poly(ethylene oxide) (PS/PEO) alternating structure. Random copolymer brushes were also prepared by radical copolymerization of both macromonomers in the absence of Lewis acid. It was found from dilute solution properties that both copolymer brushes composed of short aspect ratio formed an ellipsoid‐like single molecule in solution. To discuss the intramolecular phase separation of PS/PEO brushes in solution, we determined the radius of gyration (Rg) and cross‐sectional radius of gyration (Rg,c) of copolymer brushes by small‐angle X‐ray scattering (SAXS) using Guinier's plots in DMF and styrene. We used styrene as a solvent to cancel each other out with the electron density of PS side chains. We made also clear the effect of branching topology on polymer crystallinity to be examined by comparing the copolymer brushes with corresponding linear PEO or PEO‐block‐PS block copolymer.
The degree of coverage and the structure and thickness of interphases in composite latex particles consisting of a poly(butyl acrylate) (PBA) core incorporated with poly(methyl methacrylate) (PMMA) macromonomer of different molecular weight and a PMMA shell was analyzed by using solid‐state NMR spin‐diffusion measurements. By changing the temperature at a given filter strength or by adjusting the filter strength at a given temperature, the spin‐diffusion experiment could be performed in such a way that the overall structure could be detected. At filter strengths softer than necessary for the detection of the overall structure, information regarding larger structures can be obtained, whereas for stronger filters detailed information about small structures in the interphase region can be extracted. The data obtained by NMR were related to previous TEM and dynamic mechanical studies on these particles.
Summary: Stabilized nanoparticles functionalized with carboxy groups were synthesized directly from dextran and acrylic acid (AA), without using any organic solvent and surfactant. The composition and morphology of these dextran‐based nanoparticles, as well as the mechanism of this one‐pot synthesis, were also investigated in this paper. This approach is anticipated to be applicable to various water‐soluble polysaccharides to fabricate nanoparticles facilely.
Facile synthesis of stabilized nanoparticles functionalized with carboxy groups from dextran and AA. 相似文献
The morphological behavior of PS‐containing diblock copolymer blends is reported. Three sets of PS/PLA and PS/PEO blends are prepared and evaluated by SAXS, WAXS, DSC, and DMA. No macrophase separation is observed, and the formation of PLA/PEO composite cylinders is identified by SAXS for all compositions. The PLA and PEO segments in these blends have been found to be miscible and the crystallization of the confined PEO is suppressed in the ordered nanostructures. The domain spacing, diameter of composite cylinders, and order/disorder transition temperature could be tuned by varying the blend composition in the binary blends.
The polycondensation of trialkoxysilanes to PSSQs in a microreactor setup is demonstrated. While continuous‐flow processes involving microreactors found various applications in chain growth polymerization, their influence on step‐growth polymerization is less explored, and the polycondensation of multifunctional monomers has not been studied in detail. We found significantly increased yields and a decreased polydispersity of the obtained polymers in comparison to the batch process. By variation of the residence time molecular weights could be adjusted reproducibly ranging from = 1 900 to 11 000 g · mol?1. Thus, the microreactor setup offers for the first time the possibility to synthesize PSSQ with adjustable properties.
Summary: Porous poly(N‐isopropylacrylamide) (PNIPAAm) hydrogels were successfully prepared in the presence of nanosized silica particles with network morphology and subsequent silica extraction by HF treatment. The nanosized pores induced very rapid deswelling of the hydrogels, accompanied by volume phase transitions from swelling to deswelling states at approximately 30 °C. Deswelling, which was analyzed by a quick change in temperature of the aqueous media from 20 to 40 °C, was facilitated by increased silica particle content. The deswelling rate constant of the hydrogel prepared using silica nanoparticles with a network morphology (NT gel) was about 1 900‐fold greater than that of nonporous hydrogels and 13‐fold larger than that of porous hydrogels perforated using spherical silica nanoparticles (SP gel). The releasing rate of the low‐molecular weight chromophore from the NT gel was higher than that from the SP gel. In the chromophore‐labeled macromolecules, the network morphology was effective for increasing the releasing rate based on the transfer inside the hydrogel, as well as for increasing the amount released based on the surface pore morphology.
Polypyrrole‐silica nanoparticles were prepared by in situ polymerization of pyrrole monomer in the presence of specially modified silica nanoparticles having vinyl functional groups on the surface. The presence of polypyrrole (PPy) on the surface of silica nanoparticles was confirmed using FT‐IR spectroscopy. The core‐shell morphology was analyzed by TEM. A uniform and smooth shell of PPy on the silica nanoparticles and absence of free PPy homopolymer were the main features of the proposed method that employs modified silica nanoparticles for the synthesis of the PPy‐coated silica nanoparticles. SEM investigation of the particles deposited on a glass surface showed a narrow size distribution with an average diameter of 240 nm for the particles. The weight percentage of PPy was estimated using TGA and DSC to be approximately 10 wt.‐%. Further investigations a by two‐probe conductivity method of glass slides coated with the nanoparticles showed an electrical conductivity of 7 S · cm?1. The static water contact angle was as high as 169 °.
Zirconium pyrophosphate (ZPP)/poly(2,5‐benzimidazole) composites were prepared by polymerization of 3,4‐diaminobenzoic acid with zirconium hydrogen phosphate in polyphosphoric acid. The composite membranes for polymer electrolyte membranes were prepared by casting the polymerization solutions directly onto stainless steel plates. Membranes doped in 60 wt.‐% phosphoric acid solution had high proton conductivity values of more than 0.12 S · cm?1 (at 180 °C and 1% RH). Physical properties of the doped membranes, including the mechanical strength and dimensional stability, improved as ZPP content in the composites increased to 10–20 wt.‐%.
Summary: The fragmentation of supports, a key issue for the polymerization of olefins via supported metallocene catalysts, was investigated by laser scanning confocal fluorescence microscopy (LSCFM). Metallocenes were immobilized on silica and on two organic supports, namely polystyrene latex particles and a polystyrene resin, which were stained by fluorescent perylene dyes. All supported catalysts showed different activities in the ethylene polymerization under the same reaction conditions. Recording fluorescence images of the polyolefin product beads at different polymerization times by LCSFM proved different fragmentation processes for the catalyst. While for the µm‐sized polystyrene resins almost no fragmentation was observed, supports based on aggregated latex particles fragmented throughout the bead from the beginning, corresponding to the multi‐grain model established for Ziegler‐catalysts. For the silica particles a fragmentation starting from the outer spheres to the core was detected, confirming in a fast and easy way the layer‐by‐layer model developed by electron microscopy studies.
SEM images of the catalyst supported on (A) microsized PS beads, (B) nanosized PS beads and (C) silica gel. 相似文献
Summary: Multishell particles were prepared by γ‐radiation‐induced polymerization of methyl methacrylate (MMA) in polystyrene‐block‐poly(methacrylic acid) (PS‐b‐PMA) aqueous micellar solution and their structure was studied by small‐angle neutron scattering (SANS). Before polymerization, almost all MMA molecules are distributed in aqueous phase and only 1% of MMA is accumulated inside the micelles. The newly formed polymer (PMMA) is deposited on the surface of PS cores of the original micelles. The effect of the MMA concentration, micelle concentration, absorbed radiation dose, and absorbed dose rate on the characteristics of the resulting particles was examined. The thickness of the PMMA shell (20–218 Å for the presented series of samples) can be easily controlled by variation of monomer and/or micelle concentration. Universal plots of the core volume (PS+PMMA) and SANS curve were presented. These plots facilitate choosing proper monomer and micelle concentrations and detecting possible irregularities in the parameters of the resulting particles.
Summary: Matching macrocyclic and linear polystyrenes (PS) were synthesized by the initiation of styrene with 2,7‐dimethyl‐3,6‐diphenyloctane dianion lithium salt followed by high dilution coupling with 1,4‐bis(bromomethyl)benzene or protonation. Liquid chromatography at the critical condition shows the presence of less than 4% of linear PS impurities in the fractionated cycles. SEC studies confirm that the ratios of apparent MWs of cyclic and linear PS increase from about 0.7 to more than 0.9 as MWs decrease. Fluorescence studies show that the monomer emissions at 285 nm strongly increase with decreasing MW whereas those of the linear polymers are not significantly affected. This may be due in part to the increased rigidity of the smaller cycles that decreases the rate of radiation‐less deactivation.
A series of anthracene‐based conjugated copolymers containing 9,10‐bis(6‐bromonaphthalen‐2‐yl)‐2‐tert‐butylanthracene (BNA) and 2,7‐diphenyl substituted fluorene (DPPF) moieties are prepared via a palladium‐catalyzed Suzuki polymerization. All of the synthesized polymers emit blue light at around 450 nm and show good thermal and color stability. Their electroluminescence spectra remain unchanged at high driving voltage. The double‐layer polymer light‐emitting diode (PLED) fabricated with ITO/PEDOT:PSS/DPPFBNA3/CsF/Al, produces a maximum brightness of 1 650 cd · m?2 and has a luminance efficiency of 0.39 cd · A?1. The ITO/PEDOT:PSS/TFB/DPPFBNA3/CsF/Al multilayer PLED, incorporating a TFB layer to facilitate hole transportation, produces a maximum brightness of 5 371 cd · m?2 and a luminance efficiency of 1.18 cd · A?1.
Summary: A new kind of degradable gel was prepared by radical copolymerization of vinyl sorbate (VS) with molecular oxygen. A soluble and linear copolymer was produced at the initial stage of the polymerization and the ratio of the gel fraction was rapidly increased in the process of polymerization. An analysis of the polymerization by near infrared (NIR) spectroscopy strongly suggests that the conjugated diene group is much more reactive than the vinyl ester group. The degradation of an insoluble part (VS‐gel) was investigated by thermogravimetric (TG) and differential thermal analyses (DTA). Thermal degradation exothermically occurred around 110 °C. The weight loss of the gel was simultaneously observed. The VS‐gels obtained from polymerizations at various monomer concentrations had different physical properties, but similar degradation properties. The higher the VS monomer concentration in the feed, the lower the swelling ratio of the obtained gels due to an increase in a crosslinking density. The degradation of the VS‐gel under several stimuli other than heating was also investigated.
A series of substituted fluorenes with vinyl groups as reactive sites were synthesized and characterized. Fluorene‐based polymers with vinyl groups as end chains and side chains functionalities were also prepared. In both cases, successful preparation of functionalized Parylene C films (modylayers) was achieved. The set of spectral methods used has allowed us to conclude that such functionalization is based on the chemical reaction of double bonds with xylylene radicals during deposition process. Parylene C films chemically modified with specially synthesized polyfluorenes showed PL quantum efficiency comparable with neat polyfluorenes.
A simple, economical and high yielding method to prepare poly(1,4‐dihexyloxybenzene), an alkylated derivative of poly(para‐phenylene) (PPP), is reported. We further prepared a composite of poly(1,4‐dihexyloxybenzene) and CdS nanoparticles and studied their structural, optical, stability and transport properties. It was observed that the conductivity of poly(1,4‐dihexyloxybenzene) increased by several orders of magnitude when doped with CdS nanoparticles. Similarly, CdS nanoparticle‐doped PPP showed higher thermal stability, when compared to the neat polymer. As these composites could be processed in the same way as organic polymers, they would find applications in many low‐cost optoelectronic devices.
The synthesis of a polymeric, photoresponsive drug‐delivery system on the basis of 4‐methylcoumarin‐side‐chain‐functionalized methacrylic copolymers with different degrees of functionalization is reported. Drug‐release experiments in solution indicated a maximum release of 22 µg of 5‐fluorouracil per 1 mg of polymer. Polymer analogous photochemical drug immobilization does not lead to observable intermolecular cross‐linking. Miscibility experiments with PMMA suggest that the bulk polymerization of homogeneous, transparent polymer sheets for intraocular lens fabrication, using MMA as the matrix monomer and incorporating the polymer‐drug conjugates as a functional component, is possible.
