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مرکز اطلاعات علمی SID1
مرکز اطلاعات علمی SID
اسکوپوس
مرکز اطلاعات علمی SID
ریسرچگیت
strs
اطلاعات دوره: 
  • سال: 

    2013
  • دوره: 

    0
  • شماره: 

    20
تعامل: 
  • بازدید: 

    1260
  • دانلود: 

    0
کلیدواژه: 
چکیده: 

IT IS WELL ESTABLISHED THAT THE CHEMICAL MODIFICATION OF WOOD IS ABLE TO IMPROVE BIOLOGICAL RESISTANCE.CHEMICAL MODIFICATION REACTIONS INVOLVE THE FORMATION OF A CHEMICAL BOND BETWEEN A REAGENT AND THE CELL WALL POLYMERS OF WOOD. OFTEN, THIS REACTION OCCURS BETWEEN THE HYDROXYL GROUPS OF THESE POLYMERS AND THE REAGENT MOLECULES [1-2]...

آمار یکساله:  

بازدید 1260

دانلود 0
نویسندگان: 

SOBHAN MANESH K. | HAJIZADEH AHMAD

اطلاعات دوره: 
  • سال: 

    2005
  • دوره: 

    14
  • شماره: 

    1 (55)
  • صفحات: 

    15-21
تعامل: 
  • استنادات: 

    0
  • بازدید: 

    15488
  • دانلود: 

    19343
چکیده: 

copolymerization of ethylene with propylene is carried out semi-continuously in toluene at atmospheric pressure and with various propylene/ethylene molar ratios. The polymerization temperature and cocatalyst/catalyst molar ratio were also varied in a wide range. Cp2ZrCl2/MAO System was used as catalyst. The viscosity average molecular weights of copolymers obtained were determined by measuring the intrinsic viscosity [ h] and application of the Mark-Houwink equation. The values of molecular weights were found to be dependent on operating conditions except on polymerization time in the range 5-30 min. increasing the polymerization temperature, propylene/ethylene and cocatalyst/catalyst molar ratios caused a continuous decrease in molecular weight. An important reason for this effect was shown to be accelerated chain transfer reactions.

آمار یکساله:  

بازدید 15488

دانلود 19343 استناد 0 مرجع 0
نویسنده: 

ARABI HASSAN | BALZADEH ZAHRA

اطلاعات دوره: 
  • سال: 

    2014
  • دوره: 

    0
  • شماره: 

    11
تعامل: 
  • بازدید: 

    1470
  • دانلود: 

    0
چکیده: 

THE copolymerization OF ETHYLENE WITH TWO DISTINCT DIENE COMONOMERS, I.E. 5-ETHHYLIDENE-2-NORBORNENE (ENB) AND 2-METHYL-1, 3-BUTADIENE (ISP) WAS CARRIED OUT EMPLOYING BIS (2-PHENYLINDENYL) ZRCL2 AND METHYLALUMINOXANE (MAO) AS CATALYST AND COCATALYST, RESPECTIVELY. THEN, THE SYNTHESIZED COPOLYMERS MICROSTRUCTURE WAS STUDIED USING 1HNMR AND THERMAL ANALYSIS. IT WAS FOUND THAT, ALTHOUGH THE COMONOMER INCORPORATION IS WELL DONE IN THE CASE OF THE FORMER, VERY LOW AMOUNT OF THE LATTER IS ENTERED INTO THE COPOLYMER CHAIN.

آمار یکساله:  

بازدید 1470

دانلود 0
گارگاه ها آموزشی
نشریه: 

POLYOLEFINS JOURNALS

اطلاعات دوره: 
  • سال: 

    2018
  • دوره: 

    5
  • شماره: 

    1
  • صفحات: 

    71-84
تعامل: 
  • استنادات: 

    0
  • بازدید: 

    12511
  • دانلود: 

    7743
چکیده: 

Ethylene-norbornene copolymers were synthesized with a homogeneous catalyst system based on bis(imino) pyridine iron with the addition of diethyl zinc (DEZ) as alkyd transfer agent to promote immortal copolymerization. The addition of DEZ did not influence the catalytic activity in copolymerization with 7. 5 mmol of norbornene (NB), but in the reactions with 70 mmol, the comonomer promoted an increase of activity. We observed by thermal analysis that the norbornene inserted in the chains promoted an increase in thermal stability of the synthesized material with higher amounts of comonomer, since the temperature of initial degradation was much higher for these copolymers compared to polyethylene. In addition, for the copolymers with 7. 5 mmol of norbornene, the DEZ served as alkyd transfer agent, as shown by the gel permeation chromatography analysis, leading to a decrease in both molar mass and polydispersity. The UV-Vis spectra showed that the diethyl zinc did not change the catalytically active center, but only acted as an alkyd transfer agent.

آمار یکساله:  

بازدید 12511

دانلود 7743 استناد 0 مرجع 0
نویسندگان: 

JONGSOMJIT B. | KHOTDEE A. | PRASERTHDAM P.

اطلاعات دوره: 
  • سال: 

    2005
  • دوره: 

    14
  • شماره: 

    6 (60)
  • صفحات: 

    559-564
تعامل: 
  • استنادات: 

    0
  • بازدید: 

    15172
  • دانلود: 

    9802
چکیده: 

In this study, behaviours of ethylene/norbornene copolymerization using zirconocene catalysts; rac-Et[Ind]2ZrCl2 (cat. A) and Cp2ZrCl2 (cat. B) were investigated. Based on activity profiles at various norbornene concentrations, it can be observed that although cat. A is more active than cat. B, it is less stable, indicating more rapid deactivation rates with time. It was found that a deactivation rate was more pronounced with high concentrations of norbornene for both catalytic systems. Asymmetric activity profiles were observed for both catalytic systems indicating higher deactivation rates compared to the activation rates. The copolymer produced by cat. A exhibited lower melting temperature (Tm), but higher incorporation of norbornene. However, both catalysts produced copolymers with random distribution.

آمار یکساله:  

بازدید 15172

دانلود 9802 استناد 0 مرجع 0
اطلاعات دوره: 
  • سال: 

    2005
  • دوره: 

    14
  • شماره: 

    3 (57)
  • صفحات: 

    235-265
تعامل: 
  • استنادات: 

    0
  • بازدید: 

    63979
  • دانلود: 

    177177
چکیده: 

Biopolymer chitin, the most abundant natural amino polysaccharide, and its most important derivative, chitosan, are recently considered as the subjects for extensive worldwide academic and industrial research. In spite of potential applications of chitin and chitosan, it is necessary to establish efficient appropriate modifications to explore fully their high potential. A variety of chemical modifications are employed to modify these carbohydrate polymers. The present article provides a comprehensive review on one of the most promising approaches to modify chitin and chitosan, i.e., graft copolymerization, with an emphasis on the synthetic aspects. Both chemically- and radiation-initiated graft copolymerization of various vinyl monomers onto the trunk polymers are investigated. Meanwhile, the limited cases of polycondensation and oxidative coupling are presented as the non-vinyl graft copolymerization methods. Then, the ring-opening graft copolymerization is described and the cases of the cyclic monomers a-aminoacid N-carboxy anhydrides and e-caprolactone are investigated. An extensive description of the “grafting onto” approach is provided. The preformed polymers discussed here for grafting onto chitin/chitosan include living poly(2-alkyl oxazolines), poly(ethylene glycol)s, block polyethers, poly(ethylene imine)s, poly(2-hydroxyalkanoate)s, polyurethanes, poly (dimethylsiloxane)s, and dendrimer-like hyperbranched polymers. Chitin/chitosan multiple modification including graft copolymerization is also investigated. Regioselective grafting using derivatives such as 6-iodo-, mercapto-, deoxy(thiosulphato)-chitins, and Ntrichloroacetyl chitosan are described as suitable approaches to achieve chitin/chitosan graft copolymers with well-known structures.

آمار یکساله:  

بازدید 63979

دانلود 177177 استناد 0 مرجع 0
strs
اطلاعات دوره: 
  • سال: 

    2010
  • دوره: 

    19
  • شماره: 

    8 (122)
  • صفحات: 

    589-598
تعامل: 
  • استنادات: 

    0
  • بازدید: 

    15345
  • دانلود: 

    14492
چکیده: 

The branching copolymerization of styrene (St) and methyl methacrylate (MMA) with divinylbenzene (DVB) as branching agent was conducted using atom transfer radical polymerization (ATRP). The kinetics and extent of branching with respect to monomer conversion were studied in details. Gas chromatography (GC), proton nuclear magnetic resonance (1H NMR) spectroscopy and dual detection size exclusion chromatography (DD-SEC) were used to monitor the reactions and analyze the resulting copolymers. The analysis of reactants conversion revealed that the conversion rate of DVB, having two double bonds, is only slightly faster than those of the monomers, with no donor/acceptor interactions between DVB molecules and the monomers. The molecular weight increases slowly which is still somehow larger than the theoretical value in the early stages; then deviates significantly from the its theoretical estimation beyond 40% conversion and increases steeply at later stages of 70% monomer conversion. These results suggest that branching takes place in the early stages of the reaction regardless of any possible donor/acceptor interactions between the monomers and DVB molecules, and the coupling reaction originates mainly from the primary chain resulting in small chain branching. Coupling reaction between the branched chains becomes significant in the later stages of the copolymerization which produces highly branched chains. The reaction system contains three components throughout the reaction: the primary chains, the slightly branched chains comprising of two primary chains, and the highly branched chains consisting of 3 or more primary chains. Although the weight fraction of the branched chains increases with monomer conversion, there is finally 20% primary chains still left in the reaction system which gives rise to limited molecular weight and high polydispersity of the branched polymers prepared through ATRP using divinyl monomer as the branching agent.

آمار یکساله:  

بازدید 15345

دانلود 14492 استناد 0 مرجع 0
نویسندگان: 

SHAARI MOGHADAM SH. | HAJIR BAHRAMI S.

اطلاعات دوره: 
  • سال: 

    2005
  • دوره: 

    14
  • شماره: 

    12 (66)
  • صفحات: 

    1032-1041
تعامل: 
  • استنادات: 

    0
  • بازدید: 

    30668
  • دانلود: 

    29173
چکیده: 

copolymerization of acrylonitrile (AN) with acrylic acid (AA) carried out by solvent (DMF)-water suspension method, at 60 oC, using azobisisobutyronitrile (AIBN) as an initiator. The effects of mole fraction variation of acrylic acid and concentration variation of water in reaction medium, on the properties of synthesized copolymers were investigated by FTIR, viscometry, and DSC techniques. The reactivity ratios were calculated using Fineman-Ross and Kelen-Tudos methods. The results obtained show that by increasing the mole fraction of acrylic acid in the feed up to 0.0756, the conversion percentage decreases. However, the molecular weight increases in this range of feed ratio. In the samples prepared by keeping the feed ratio of comonomers constant and varying the ratio of the solvent/nonsolvent, the molecular weight increases by increasing nonsolvent amount. The reactivity ratio of AA is 6.5 times greater than that of AN. DSC Exothermic peak of homopolymer shows a single peak whereas, in copolymers it appears as doublets, due to more distinction of dehydrogenation, oxidation, and cyclization reactions.

آمار یکساله:  

بازدید 30668

دانلود 29173 استناد 0 مرجع 0
نویسندگان: 

NAGUIB H.F.

اطلاعات دوره: 
  • سال: 

    2002
  • دوره: 

    9
  • شماره: 

    -
  • صفحات: 

    207-211
تعامل: 
  • استنادات: 

    315
  • بازدید: 

    4921
  • دانلود: 

    9195
کلیدواژه: 
چکیده: 

آمار یکساله:  

بازدید 4921

دانلود 9195 استناد 315 مرجع 0
نویسندگان: 

SINGHA AMAR S. | RANA ASHVINDER K.

اطلاعات دوره: 
  • سال: 

    2011
  • دوره: 

    20
  • شماره: 

    11 (137)
  • صفحات: 

    913-929
تعامل: 
  • استنادات: 

    0
  • بازدید: 

    22012
  • دانلود: 

    38459
چکیده: 

Natural fibres have received vast attention because of their combustible, non-toxic, low cost, hydrophilic and biodegradable properties. In this study, functionalization ofcannabis indica fibre has been carried out by its grafting with acrylic acid (AAc) using a Ce3+-HNO3 redox initiator in an aqueous medium. Different reaction parameters such as reaction time, pH, reaction temperature, concentrations of initiator, monomer and nitric acid were optimized to achieve the maximum percentage of grafting (21.08%). The low graft yield obtained with acrylic acid was because of its high affinity towards water. A suitable mechanism to explain initiation, propagation and termination of graft copolymerization has also been proposed. The kinetics of the graft copolymerization of acrylic acid ontocannabis indica fibre has also been studied. It has been found that the rate expression for graft polymerization is (Rg) =k [I]0.63 [AAc]1.22.The overall activation energy for the graft copolymerization of AAc ontocannabis indicafibre was found to be 15.25 kJ/mol within the temperature range of 25-45°C.Besides kinetic study the grafted samples were also evaluated for their physicochemical properties such as swelling behaviour in different polar and non-polar solvents along with resistance towards acid and base. Graft copolymerized samples have been found to show less resistance towards chemicals as compared to the raw fibre.Furthermore, the grafted samples were characterized by different techniques such as Fourier transform infrared (FTIR), scanning electron microscope (SEM), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) studies. Thermal stability of the fibre has been found to decrease after graft copolymerization which is also supported by activation energy (Ea) values calculated both for raw and grafted fibres.

آمار یکساله:  

بازدید 22012

دانلود 38459 استناد 0 مرجع 0
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