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1.
Fabio Fabri  Wanda de Oliveira 《Polymer》2006,47(13):4544-4548
Half-sandwich samarium(III) diketiminate bromide was successfully synthesized and was shown to be active in methyl methacrylate (MMA) polymerization. The effects of temperature, polymerization time and catalyst concentration were studied. Activities of ca. 18 kg of polymethacrylate (PMMA) per mol of samarium per hour were obtained under optimum conditions (0 °C and a MMA/catalyst molar ratio of 100/1), giving a polymer with a molar mass Mn>24,000 g mol−1 and a molar mass distribution (Mw/Mn)<1.4. After 1 h of polymerization, conversions of MMA as high as 96% were observed.  相似文献   

2.
A well-defined, amphiphilic poly(styrene-co-acrylic acid) copolymer was synthesized in a single step by nitroxide-mediated controlled free-radical copolymerization of styrene and acrylic acid, without protection of the acid groups: Mn=6500 g mol−1, Mw/Mn=1.5 and a composition of FAA=0.70±0.03 in acrylic acid. In addition to the good control over molar mass and molar mass distribution, the copolymer exhibited a narrow composition distribution with a slight gradient. Such copolymer was an efficient stabilizer for the emulsion polymerizations of styrene and of mixtures of methyl methacrylate and n-butyl acrylate, until 45 wt% solids. A low amount (typically 3-4 wt% based on the monomer(s)) was needed for a good stabilization. This is approximately a decade lower than the required amount of random, amphiphilic copolymers prepared via conventional free-radical polymerization. The performances were, however, below those of analogous diblock copolymers, but the great advantage is the very easy synthetic procedure.  相似文献   

3.
The radiochemical degradation of polytetrafluoroethylene (PTFE) samples has been studied in air at dose rate 100 Gy/h for doses up to 5000 Gy, at ambient temperature. The polymer degradation has been monitored by DSC, tensile testing and Essential Work of Fracture (EWF) testing. Some fractured samples have been observed by scanning electron microscopy. The polymer undergoes a fast chain scission, its number average molar mass is divided by about 20 for a dose of 1000 Gy and tends towards a pseudo asymptotic value of ∼20 kg mol−1 (against 6200 kg mol−1 initial value). The modulus and yield characteristics seem to be almost unaffected whereas ultimate properties undergo strong variations. The ultimate elongation εR and the EWF plastic work characteristic βwp first increase and then decrease. The ultimate stress decreases and tends towards a pseudo asymptotic value. The mechanisms of radiation induced ultimate property changes are discussed. The first stage could be due to the destruction of non-extended tie molecules (due to the presence of very long chains) responsible for interfibrillar bridging during fracture. The (more classical) second stage is a progressive embrittlement due to the destruction of the entanglement network. The critical molar mass Mc for embrittlement is such as Mc∼50Me, Me being the molar mass between entanglements in the melt. This relationship could be a general characteristic of high crystallinity non-polar polymers.  相似文献   

4.
LaFeO3 were synthesized via a sol-gel route based on polyvinyl alcohol (PVA). Differential scanning calorimetry (DSC), Thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) techniques were used to characterize precursors and derived oxide powders. The effect of the ratios of positively charged valences to hydroxyl groups of PVA (Mn+/-OH) on the formation of LaFeO3 was investigated. XRD analysis showed that single-phase and well-crystallized LaFeO3 was obtained from the Mn+/-OH = 4:1 molar ratio precursor at 700 °C. For the precursor with Mn+/-OH = 2:1, nanocrystalline LaFeO3 with average particle size of ∼50 nm was formed directly in the charring procedure. With increase of PVA content to Mn+/-OH = 1:1, phase pure LaFeO3 was obtained at 500 °C.  相似文献   

5.
Poly(ether sulfone)s were prepared by polycondensation of silylated 4-tert-butylcatechol and 4,4′-difluorodiphenylsulfone in N-methylpyrrolidone. The feed ratio and the reaction time were varied to study the influence of stoichiometry and conversion on molecular weight and extent of cyclization. Molecular weights and molecular weight distributions (MWD)s were characterized by SEC measurements calibrated with polystyrene. Light scattering confirmed that calibration with polystyrene gives reasonable results and revealed a tendency towards a bimodal MWD for the samples rich in cycles. The MALDI-TOF mass spectrometry indicated that the extent of cyclization increased with higher conversion and with optimization of the stoichiometry. This interpretation was confirmed by 1H NMR endgroup analyses. For the samples with the highest molar masses only mass peaks of cycles were found, which were detectable up to 20 000 Da before and up to 27 000 Da after fractionation. Via the pseudo-high dilution method low molar mass poly(ether sulfone) containing more than 95 mol% of cycles were prepared, and even these low molar mass samples had broad MWDs. DSC measurements indicated that the glass transition temperatures depend on the structure of the endgroups and increase with higher fractions of cycles.  相似文献   

6.
The electroreduction of Fe(II) and Nd(III) in MClx-acetamide-urea-NaBr-KBr were studied by cyclic voltammetry and chronoamperometry. The reduction of Fe(II) to Fe is an irreversible process, the value of αnα of the electrode reaction was calculated to be 0.31 and the diffusion coefficient of Fe(II) was calculated to be 9.53 × 10−7 cm2 s−1 at 343 K. Nd(III) cannot be reduced alone in urea melt, but Nd-Fe can be codeposited by induced codeposition. The composition of Nd-Fe film varies with the Nd(III)/Fe(II) molar ratio, at the potential of −1.25 V the maximum content of Nd in Nd-Fe film is 60.4 wt%. The morphology of Nd-Fe film was investigated by SEM and AFM. Nd-Fe film comprises of nanoparticles with the size about 100-200 nm. X-ray diffraction (XRD) shows it is amorphous. After heat-treatment at 1173 K the crystal Nd2Fe17 phase can be formed. The magnetic properties of the Nd-Fe films were determined using hysteresis loops, at 5 K the coercive field Hc of Nd (62.6 wt%)-Fe amorphous film is 1225 Oe, the remanent magnetization MR and the saturation magnetization MS are 5.15 and 15.80 emu g−1, respectively.  相似文献   

7.
Weihui Xie 《Polymer》2007,48(23):6791-6798
Amphiphilic biodegradable mPEG-PCL diblock copolymers have been synthesized using rare earth catalyst: yttrium tris(2,6-di-tert-butyl-4-methylphenolate) [Y(DBMP)3] in the presence of monomethoxy poly(ethylene glycol) (mPEG, Mn = 5000) as macro-initiator. The diblock architecture of the copolymers was thoroughly characterized by 1H NMR, 13C NMR and SEC. The molecular weights of mPEG-PCLs can be well controlled by adjusting the feeding molar ratio of ?-CL to mPEG. Thermal and crystallization behaviors of the diblock copolymers were investigated by DSC and POM (polarized optical microscope). The crystallization property of mPEG-PCL block copolymers depends on the length of PCL blocks. As the molecular weight of PCL block increased, the crystallization ability of mPEG block was visibly restrained. Aqueous micelles were prepared by dialysis method. The critical micelle concentration of the copolymers, which was determined to be 0.9-6.9 mg/L by fluorescence technique, increased with the decreasing of PCL block length. The particle sizes determined by DLS were 30-80 nm increasing with the PCL block length. TEM images showed that these micelles were regularly spherical in shape.  相似文献   

8.
Kathrin Harre 《Polymer》2006,47(20):7312-7317
Freshly prepared solutions of poly(2,5-di-n-dodecyl-1,4-phenylene) (PPP 12) in toluene are metastable at room temperature with regard to a process which leads to the formation of aggregates composed of up to 100 individual macromolecules. This aggregation process has an induction period of more than 10 h at room temperature. The kinetics of aggregation was investigated by making use of a fast capillary membrane osmometer. Aggregation follows an Avrami-Evans type formalism and suggests that clusters of a lyotropic liquid crystalline phase of the polymer are formed of the same type as observed in the melt. The long induction period of aggregate formation in dilute solution in toluene allows to apply conventional techniques of molar mass determination like membrane osmometry and size-exclusion chromatography (SEC). A relationship [η] = 1.94 × 10−3 M0.94 was found for PPP 12 in toluene at 20 °C and a persistence length of 15.6 nm was derived applying the Bohdanecky-formalism. This gives evidence of the worm-like nature of the non-aggregated PPP 12 in dilute solution.  相似文献   

9.
Isotactic polypropylene (iPP)-polystyrene (PS) and iPP-poly(methyl methacrylate) (PMMA) multiblock copolymers were synthesized by atom transfer radical coupling (ATRC) of PS-iPP-PS and PMMA-iPP-PMMA triblock copolymers obtained by atom transfer radical polymerization (ATRP) of styrene (St) and methyl methacrylate (MMA), respectively, using α,ω-dibromoisobutyrateoligopropylene (iPP-Br) as a bifunctional macroinitiator. The iPP-Br was prepared by hydroxylation and subsequent esterification of telechelic oligopropylene having terminal vinylidene double bonds at both ends obtained by controlled thermal degradation of iPP. ATRP of St and (meth) acrylic monomers using iPP-Br formed the corresponding triblock copolymers. It was confirmed that the PMMA-iPP-PMMA triblock copolymer was effective as the compatibilizer for the iPP/PMMA blend. An iPP-PS multiblock copolymer (Mn: 25?000 g/mol and Mw/Mn: 4.1) was prepared by ATRC of PS-iPP-PS triblock copolymer (Mn: 8900 g/mol and Mw/Mn: 1.3). ATRC with St of PMMA-iPP-PMMA triblock copolymer (Mn: 13?000 g/mol and Mw/Mn: 1.4) provided an iPP-PMMA multiblock copolymer containing St chains (Mn: 39?000 g/mol and Mw/Mn: 2.8).  相似文献   

10.
The synthesis of 3-arm star polymers from reversible addition-fragmentation chain transfer (RAFT)-prepared precursor homopolymers in combination with thiol-ene click chemistry is described. Homopolymers of n-butyl acrylate and N,N-diethylacrylamide were prepared with 1-cyano-1-methylethyl dithiobenzoate and 2,2′-azobis(2-methylpropionitrile) yielding materials with polydispersity indices (Mw/Mn) ≤ 1.18 and controlled molecular weights as determined by a combination of NMR spectroscopy, size exclusion chromatography (SEC), and matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Subsequent one-pot reaction of homopolymer, hexylamine (HexAM), dimethylphenylphosphine (DMPP), and trimethylolpropane triacrylate (TMPTA) results in cleavage of the thiocarbonylthiol end-group (by HexAM) of the homopolymer yielding a macromolecular thiol that undergoes DMPP-initiated thiol-Michael addition to TMPTA yielding 3-arm star polymers. The presence of DMPP is demonstrated to serve an important second role in effectively suppressing the presence of any polymeric disulfide as determined by SEC. Such phosphine-mediated thiol-ene reactions are shown to be extremely rapid, as verified by a combination of FTIR and NMR spectroscopies, with complete consumption of the CC bonds occurring in a matter of min. MALDI-TOF MS and SEC were used to verify the formation of 3-arm stars. A broadening in the molecular weight distribution (Mw/Mn ∼ 1.35) was observed by SEC that was attributed to the presence of residual homopolymer and possibly 2-arm stars formed from trimethylolpropane diacrylate impurity. Interestingly, the MALDI analysis also indicated the presence of 1- and 2-arm species most likely formed from the fragmentation of the parent 3-arm star during analysis. Finally, a control experiment verified that the consumption of CC bonds does not occur via a radical pathway.  相似文献   

11.
Lihui Cao  Weimin Dong  Xuequan Zhang 《Polymer》2007,48(9):2475-2480
The oxovanadium phosphonates (VO(P204)2 and VO(P507)2) activated by various alkylaluminums (AlR3, R = Et, i-Bu, n-Oct; HAlR2, R = Et, i-Bu) were examined in butadiene (Bd) polymerization. Both VO(P204)2 and VO(P507)2 showed higher activity than those of classical vanadium-based catalysts (e.g. VOCl3, V(acac)3). Among the examined catalysts, the VO(P204)2/Al(Oct)3 system (I) revealed the highest catalytic activity, giving the poly(Bd) bearing Mn of 3.76 × 104 g/mol, and Mw/Mn ratio of 2.9, when the [Al]/[V] molar ratio was 4.0 at 40 °C. The polymerization rate for I is of the first order with respect to the concentration of monomer. High thermal stability of I was found, since a fairly good catalytic activity was achieved even at 70 °C (polymer yield > 33%); the Mn value and Mw/Mn ratio were independent of polymerization temperature in the range of 40-70 °C. By IR and DSC, the poly(Bd)s obtained had high 1,2-unit content (>65%) with atactic configuration. The 1,2-unit content of the polymers obtained by I was nearly unchanged, regardless of variation of reaction conditions, i.e. [Al]/[V], ageing time, and reaction temperature, indicating the high stability of stereospecificity of the active sites.  相似文献   

12.
Jing Quan 《Polymer》2007,48(9):2595-2604
A facile and regioselective enzymatic synthesis approach to prepare polymerizable lipophilic chlorphenesin vinyl esters was developed in this research. The influence of different organic solvents, enzyme sources, reaction time and the acylation reagent on the synthesis of chlorphenesin vinyl esters was investigated. Then the polymerizable monomers 1-O-vinylsuccinyl-chlorphenesin (OVSC) and 1-O-vinyladipoyl-chlorphenesin (OVAC) were homopolymerized using AIBN as the initiator. The obtained polymeric prodrugs were characterized with IR, NMR and GPC analyzes. The poly-OVSC has Mn of 1.35 × 104 and Mw/Mn of 1.95, and the poly-OVAC has Mn of 2.37 × 104 and Mw/Mn of 4.30. Moreover, 6-O-vinyladipoyl-d-glucose (OVAG), a biocompatible monomer, was copolymerized with OVSC and OVAC. Polymeric prodrugs of chlorphenesin with saccharide branches were successfully obtained with high molecular weight.  相似文献   

13.
C. Triebel  M. Blankenburg 《Polymer》2011,52(16):3621-5236
It has recently been shown that the linear elastic steady-state compliance Je0 reacts very sensitively on the addition of nanoparticles to a polymer melt. This effect can be attributed to an interaction between the particles and the matrix molecules. Creep-recovery experiments have evolved as a very suitable tool to measure Je0, because the time window can be extended wide enough to detect the processes underlying the interactions. Whereas the effect of the particle geometry on Je0 has already been investigated to some extent, the influence of the molecular structure of the matrix is still an open question. Therefore, in this study investigations of two polystyrenes with different molar masses and distributions and composites with 1 vol.% silica nanoparticles each are reported. One polystyrene is an anionic product (aPS) with a narrow molar mass distribution, the other a radically polymerized sample (PS 158K) with a broader molar mass distribution. Due to their molecular structures the unfilled polymers already differ significantly in their rheological properties. The linear steady-state elastic compliance is found to be Je0 = 1.9 × 10−5 Pa−1 for the aPS and Je0 = 2.5 × 10−4 Pa−1 for the PS 158K, which is in agreement with the literature.Investigations on nanocomposites with a poly (methyl methacrylate) of Mw/Mn = 1.5 as the matrix have shown that the elasticity, measured by Je0 in the creep-recovery experiment, strongly increases with the specific surface area of the nanoparticles added. Also for the PS composites an increase of Je0 was found by adding 1 vol.% of silica nanoparticles. However, the relative increase strongly depends on the elasticity of the unfilled matrix. Whereas for the PS 158K an increase of Je0 of only 70% is found, it is much larger, namely 470%, in the case of the anionic PS.  相似文献   

14.
Wallace W. Yau 《Polymer》2007,48(8):2362-2370
Model calculations were performed to investigate the sensitivity of zero-shear melt viscosity (η0 or Eta0) on the molecular weight (MW) polydispersity of linear polymers. Simulated MW distributions (MWD) were generated with the generalized exponential (GEX) distribution function for various levels of polydispersity Mw/Mn and Mz/Mw. For linear entangled polymeric chains in the melt, the linear viscoelastic properties were predicted by using the double reptation blending rule and the so-called BSW relaxation time spectrum, named after the authors: Baumgaertel, Schausberger and Winter [Baumgaertel M, Schausberger A, Winter HH. Rheol Acta 1990;29:400-8]. Published rheological parameters appropriate for polyethylene were used in the calculations. It was found that Eta0 depended mostly on Mw, but it also significantly depended on the extent of high-MW polydispersity Mz/Mw. A revision to the fundamental MW dependency of Eta0 was proposed to compensate for this polydispersity effect. To offset the polymer polydispersity differences, we propose a new MW average (MHV or Mx with x = 1.5) to replace Mw in the historical rheological power-law equation of Eta0 ∝ Mwa, where the literature value of exponent “a” ranges from 3.2 to 3.6. The use of MHV instead of Mw in the power-law equation made the calculated Eta0 independent of the sample high-MW polydispersity. With the removal of the complication from polydispersity effect, the new Eta0 power law can now provide a more robust base for studying polymer long-chain branching (LCB). A new LCB index is thus proposed based on this new melt-viscosity power law. The values of MHV in the new power law can be calculated for polymer samples from the conventional gel permeation chromatographic (GPC) slice data.  相似文献   

15.
I.A. Zucchi 《Polymer》2005,46(8):2603-2609
Polystyrene (PS, Mn=28,400, PI=1.07), poly(methyl methacrylate) (PMMA, Mn=88,600, PI=1.03), and PS (50,000)-b-PMMA (54,000) (PI=1.04), were used as modifiers of an epoxy formulation based on diglycidyl ether of bisphenol A (DGEBA) and m-xylylene diamine (MXDA). Both PS and PMMA were initially miscible in the stoichiometric mixture of DGEBA and MXDA at 80 °C, but were phase separated in the course of polymerization. Solutions containing 5 wt% of each one of both linear polymers exhibited a double phase separation. A PS-rich phase was segregated at a conversion close to 0.02 and a PMMA rich phase was phase separated at a conversion close to 0.2. Final morphologies, observed by scanning electron microscopy (SEM), consisted on a separate dispersion of PS and PMMA domains. A completely different morphology was observed when employing 10 wt% of PS-b-PMMA as modifier. PS blocks with Mn=50,000 were not soluble in the initial formulation. However, they were dispersed as micelles stabilized by the miscible PMMA blocks, leading to a transparent solution up to the conversion where PMMA blocks began to phase separate. A coalescence of the micellar structure into a continuous thermoplastic phase percolating the epoxy matrix was observed. The elastic modulus and yield stress of the cured blend modified by both PS and PMMA were 2.64 GPa and 97.2 MPa, respectively. For the blend modified by an equivalent amount of block copolymer these values were reduced to 2.14 GPa and 90.0 MPa. Therefore, using a block copolymer instead of the mixture of individual homopolymers and selecting an appropriate epoxy-amine formulation to provoke phase separation of the miscible block well before gelation, enables to transform a micellar structure into a bicontinuous thermoplastic/thermoset structure that exhibits the desired decrease in yield stress necessary for toughening purposes.  相似文献   

16.
Narrow molecular weight distribution polyisoprenes (PI), polybutadienes (PBD), poly(isobuty1enes) (PIB), poly(methylmethacry1ates) (PMMA), polystyrenes (PS), and poly(octadecy1 methacrylates) (PODMA) have been studied by size exclusion chromatography (SEC) in tetrahydrofuran (THF) with light scattering and viscometric detectors. The molecular weight (M) dependence of the intrinsic viscosity, [η], and radius of gyration, Rg, is reported for THF solutions of these polymers, in many instances for the first time. The availability of these data for a series of chains of varying flexibility allows a test of the universal calibration principle in SEC. Furthermore, an apparent dependence of the hydrodynamic parameter in good solvents, Φ, on chain stiffness is observed. All chains appear to exhibit hydrodynamic draining in THF. © 1996 John Wiley & Sons, Inc.  相似文献   

17.
Phenanthrene α-end-labeled poly(N-decylacrylamide-b-N,N-diethylacrylamide) (PDcAn-b-PDEAm) block copolymers consisting in a highly hydrophobic block (n = 11) and a thermoresponsive block with variable length (79 ≤ m ≤ 468) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. A new phenanthrene-labeled chain transfer agent (CTA) was synthesized and used to control the RAFT polymerization of a hydrophobic acrylamide derivative, N-decylacrylamide (DcA). This first block was further used as macroCTA to polymerize N,N-diethylacrylamide (DEA) in order to prepare diblock copolymers with the same hydrophobic block of PDcA (number average molecular weight: Mn = 2720 g mol−1, polydispersity index: Mw/Mn = 1.13) and various PDEA blocks of several lengths (Mn = 10,000-60,000 g mol−1) with a very high blocking efficiency. The resulting copolymers self-assemble in water forming thermoresponsive micelles. The critical micelle concentration (CMC) was determined using Förster resonance energy transfer (FRET) between phenanthrene linked at the end of the PDcA block and anthracene added to the solution at a low concentration (10−5 M), based on the fact that energy transfer only occurs when phenanthrene and anthracene are located in the core of the micelle. The CMC (∼2 μM) was obtained at the polymer concentration where the anthracene fluorescence intensity starts to increase. The size of the polymer micelles decreases with temperature increase around the lower critical solution temperature of PDEA in water (LCST ∼ 32 °C) owing to the thermoresponsiveness of the PDEA shell.  相似文献   

18.
In this work, the reversible addition-fragmentation chain transfer (RAFT) polymerization of vinyl acetate (VAc) was successfully performed at room temperature using 60Co γ-irradiation as the initiation source. Under the dose rate of 10 Gy/min irradiation, the polymerization proceeded smoothly and converted approximately 90% of the monomer within 7 h. The molecular weight distribution (Mw/Mn) remained narrow (Mw/Mn < 1.35) up to 90% conversion. Compared to AIBN-initiated RAFT polymerization at 60 °C, 60Co γ-irradiation-initiated RAFT polymerization is a technique that can better control the molecular weight, especially at high conversion. The 1H NMR spectra and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry confirmed that most of the chain ends of poly(VAc) (PVAc) from γ-irradiated RAFT polymerization were living and can be reactivated for chain-extension reactions. The microstructures of PVAc from 60Co γ-irradiated RAFT polymerization (almost head-to-tail addition) and AIBN-initiated RAFT polymerization (5% tail-to-tail addition) were different, as revealed by the 13C NMR spectra. For the first time, 60Co γ-irradiation was used as an initiation source for RAFT polymerization of VAc at room temperature.  相似文献   

19.
Md. Abdul Mannan 《Polymer》2007,48(3):743-749
A new cyclic nitroxide 1 and the corresponding alkoxyamines 9 and 10 were synthesized and the polymerization of styrene (St) initiated with 10 was investigated. The NO-C bond of 9 is very weak, cleaving at room temperature. On the other hand, alkoxyamine 10 is stable at room temperature and the Aact and Eact for the NO-C bond homolysis were determined to be 1.4 × 1015 s−1 and 124.5 kJ mol−1, respectively. When the polymerization of St was carried out at 70 °C, the resultant poly(St) showed narrow polydispersities below 1.25. In the polymerization at 90 °C, the resulting poly(St) showed narrow polydispersity until 60% conversion, but Mw/Mn was rapidly increased above 60% conversion. On the other hand, the polymerization at 120 °C gave poly(St) with broad polydispersities. The unusual polymerization behavior was discussed on the basis of the SEC and ESR results.  相似文献   

20.
The alkane n-C198H398 has been crystallised in both extended chain and once-folded forms and annealed to produce materials with low concentrations of gauche bonds. The concentrations of the specific conformers detected by FTIR spectroscopy at −173 °C are calculated, using measurements on liquid n-hexadecane for calibration: values are all generally less than 2.0 per 100 carbon atoms, with extended chain samples showing values less than 1.0 per 100 carbon atoms. A subtraction spectrum (Once-folded chain sample minus Extended chain sample) shows positive bands at 1298, 1340, 1347 and 1369 cm−1, which are predicted in earlier calculations for a (110) fold, while additional positive bands at 1353 and 1363 cm−1 are assigned, respectively, to gg conformers and (tentatively) to strained gtg or gtg′ conformations.  相似文献   

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