Página 1 dos resultados de 266 itens digitais encontrados em 0.001 segundos

Agregação de cadeias de acetatos de celulose em LiCl/DMAc: avaliação via viscosimetria; Aggregation of chains of cellulose acetates in LiCl/DMAc: evaluation via viscometry

MORGADO, Daniella L.; MARTINS, Virginia da C. A.; PLEPIS, Ana M. de G.; FROLLINI, Elisabete
Fonte: Associação Brasileira de Polímeros Publicador: Associação Brasileira de Polímeros
Tipo: Artigo de Revista Científica
POR
Relevância na Pesquisa
289.83834%
Celulose de linter foi acetilada, visando obter acetatos de celulose com diferentes Graus de Substituição (GS) em meio homogêneo, usando cloreto de lítio/N,N-dimetilacetamida (LiCl/DMAc) como sistema de solvente, e anidrido acético como reagente acetilante. A agregação entre cadeias de celulose ou acetatos de celulose (GS 0,8, 1,5 e 2,0) em solução foi avaliada através de medidas viscosimétricas. Os resultados mostraram que a formação de agregados no sistema de solvente utilizado (LiCl/DMAc) é diferente para celulose e acetatos, e dependente da temperatura e do GS, no caso dos acetatos. Este trabalho corresponde à primeira etapa de um estudo em que se pretende preparar filmes de acetatos, assim como de acetatos reforçados com celulose, diretamente a partir de soluções destes em LiCl/DMAc. Os resultados apresentados permitem uma escolha melhor embasada do intervalo de concentração mais adequado para preparação de filmes, a partir de soluções de acetatos e celulose nesse sistema de solvente.; Linters cellulose was acetylated to obtain cellulose acetates with different degrees of substitution (DS) in homogeneous medium, using lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) as solvent system, and acetic anhydride as acetylating agent. The aggregation among chains of cellulose or cellulose acetates (DS 0.8...

Sisal cellulose acetates obtained from heterogeneous reactions

PAULA, M. Peres de; LACERDA, T. M.; FROLLINI, E.
Fonte: BUDAPEST UNIV TECHNOL & ECON Publicador: BUDAPEST UNIV TECHNOL & ECON
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
388.19496%
In the present work, cellulose obtained from sisal, which is a source of rapid growth, was used. Cellulose acetates were produced in heterogeneous medium, using acetic anhydride as esterifying agent and iodine as catalyst, to check if the procedure described in the literature for commercial cellulose also is adequate to sisal cellulose. The results indicated that iodine is an excellent catalyst to obtain sisal cellulose acetates, but the reaction is so fast as described in the literature when, instead of sisal, lower average molar weight cellulose (microcrystalline) is used. The crystallinity index (I(c)) of sisal cellulose acetates diminished compared to sisal cellulose, but there was no direct correlation between their degree of substitution (DS) and I(c). Probably acetyl groups were introduced more homogeneously along the short chains of microcrystalline cellulose, when compared to sisal cellulose, and then for microcrystalline cellulose acetates the Ic decreases as DS increases. Using the linear correlation that was found between degree of substitution (DS) and time reaction is possible to control the DS of sisal cellulose acetates, considering a large interval of degrees of substitution (0.3-2.8).; CNPq (National Council of Research...

Probing the dependence of the properties of cellulose acetates and their films on the degree of biopolymer substitution: use of solvatochromic indicators and thermal analysis

FIDALE, Ludmila C.; ISSBRUECKER, Constance; SILVA, Priscilla L.; LUCHETI, Camila M.; HEINZE, Thomas; SEOUD, Omar A. El
Fonte: SPRINGER Publicador: SPRINGER
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
383.3178%
Although cellulose acetates, CAs, are extensively employed there is scant information about the systematic dependence of their properties on their degree of substitution, DS; this is the subject of the present work. Nine CAs samples, DS from 0.83 to 3.0 were synthesized; their films were prepared. The following solvatochromic probes have been employed in order to determine the empirical polarity, E (T)(33); ""acidity, alpha""; ""basicity, beta"", and ""dipolarity/polarizability, pi*"" of the casted films: 2,6-dichloro-4-(2,4,6-triphenyl-pyridinium-1-yl) phenolate, WB; 4-nitroaniline; 4-nitroanisole; 4-nitro-N,N-dimethylaniline; 2,6-diphenyl-4-(2,4,6-triphenyl-pyridinium-1-yl)phenolate, RB. Additionally, two systems, ethanol plus ethyl acetate (EtOH-EtAc), and cellulose plus cellulose triacetate, CTA, were employed as models for CAs of different DS. Regarding the model systems, the following was observed: (i) For EtOH-EtAc, the dependence of all solvatochromic parameters on the ""equivalent-DS"" of the binary mixture was non-linear because of preferential solvation; (ii) The dependence of E (T)(33) on equivalent DS of the cellulose-CTA films is linear, but the slope is smaller than that of the corresponding plot for CAs. This is attributed to the more efficient hydrogen bonding in the model system...

Valorização da celulose de sisal: uso na preparação de acetatos e de filmes de acetatos de celulose/celulose e quitosana/celulose; Sisal cellulose valorization: utilization for prepared acetates , and cellulose acetates/cellulose and chitosan/cellulose films

Almeida, Érika Virginia Raphael de
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 04/12/2009 PT
Relevância na Pesquisa
379.4724%
Filmes foram preparados com êxito dissolvendo e misturando quitosana com celulose de sisal no sistema de solvente NaOH/tiouréia. Nenhum solvente residual foi encontrado nos filmes preparados, conforme mostrado por análise elementar (S) e absorção atômica (Na). Os filmes foram caracterizados por técnicas como espectroscopia na região do infravermelho com transformada Fourier, difração de raios-X e análise térmica (TG, DSC e DMTA). A compatibilidade de ambos biopolímeros foi avaliada por microscopia eletrônica de varredura, em cujas imagens o filme de biocompósito mostrou uma organização intermediária entre a rede de fibra de celulose e a homogeneidade do filme de quitosana. Interações com água foram estudadas por medida de ângulos de contatos de uma gota d'água com a superfície do filme, isoterma de absorção de umidade e relaxometria de RMN T2. A análise de ângulo de contato mostrou uma deformação que pode ocorrer na superfície devido a alta afinidade destes materiais com a gota d'água. O sistema de solvente NaOH/tiouréia levou a despolimerização de ambos biopolímeros, mas sem provocar perda da capacidade de formação de filme dos mesmos. Resultados de índice de cristalinidade sugeriram que as interações entre quitosana e o solvente foram mais intensas do que as interações entre celulose e o solvente. Os resultados de relaxometria de RMN T2 do biocompósitos mostrou que as características da quitosana prevaleceram sobre as da celulose no que se refere as interações com água. Acetatos de celulose de sisal com diferentes graus de substituição foram preparados em meio homogêneo (DMAc/LiCl como sistema de solvente) e caracterizados por RMN 1H (determinação do grau de substituição)...

Mecanismo da decomposição e reações com radicais em acetatos; Decomposition mecanism and radical reactions with acetates

Pradie, Noriberto Araujo
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 18/05/2011 PT
Relevância na Pesquisa
379.4724%
Estudos do mecanismo de reações unimoleculares, das moléculas de acetato de metila e etila, induzidas por absorção multifotônica e de reações bimoleculares com os radicais OH e Cl, usando cálculos ab initio e de funcional de densidade foram realizados neste trabalho. A análise dos cálculos das barreiras de energia e das constantes de velocidade microcanônicas das reações unimoleculares com o formalismo da teoria RRKM, permitiu prever algumas reações cujos produtos não foram determinados experimentalmente. Além disso, semelhanças das geometrias e dos valores de energia de algumas estruturas entre diferentes superfícies de energia, analisadas com cálculos de IRC, permitiram a explicação da viabilidade de determinada via de reação em detrimento de outra. Em outro método, relatado por Forst(1), na determinação das constantes de velocidade variacionais, comparadas com a variação da energia de Gibbs, verificou-se constantes de velocidade microcanônicas menores e a localização dos estados de transição em comprimentos de ligação, também, menores. Nas reações com o radical OH e Cl, a via predominante de reação é o ataque aos hidrogênios do grupo ligado diretamente ao oxigênio da molécula, responsável pelo valor da constante final de mais de 93% nas reações com OH e 99% nas reações com Cl...

Methylation-GC-MS analysis of arabinofuranose- and galactofuranose-containing structures: rapid synthesis of partially O-methylated alditol acetate standards

Sassaki,Guilherme L.; Iacomini,Marcello; Gorin,Philip A.J.
Fonte: Academia Brasileira de Ciências Publicador: Academia Brasileira de Ciências
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2005 EN
Relevância na Pesquisa
264.5713%
Arabinose and galactose were treated with MeOH containing traces of H2SO4 or HCl at 25ºC to give mixtures of their methyl alpha- and beta-furanosides, as shown by 1D and 2D nuclear magnetic resonance (NMR). Oxidation of the Me alpha,beta-Araf mixture with NaIO4 preferentially oxidised the beta-isomer, to give pure Me alpha-Araf . Each product was progressively O-methylated using the Purdie reagent (MeI/Ag2O) at 25ºC and resulting mixtures of partially methylated glycosides (PMGs) were rapidly assayed by thin layer chromatography (TLC) first to favour higher yields of mono-O-methyl derivatives and later for products with higher degrees of methylation. The products were converted to complex mixtures of partially O-methylated alditol acetate derivatives (PMAAs) by successive hydrolysis, reduction with NaBD4, and acetylation. These can be used as gas chromatography-mass spectrometry (GC-MS) standards in methylation analysis of complex carbohydrates containing arabinofuranosyl and galactofuranosyl units. Of particular interest were the retention times and electron impact MS of the difficult to prepare alditol acetates of 5,6-Me2Gal, 2,5-Me2Gal, 2,5,6-Me3Gal, 3,5,6-Me3Gal, 5-MeAra, 2,5-Me2Ara, and 3,5-Me2Ara. The relative reactivities of hydroxyl groups for mixtures of Me alpha- and Me beta-Galf were HO-2 > HO-3 > HO-6 > HO-5...

Identification of acetates in elasmopalpulus lignosellus pheromone glands using a newly created mass spectral database and kóvats retention indices

Jham,Gulab N.; Silva,Alexsandro A. da; Lima,Eraldo R.; Viana,Paulo A.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/08/2007 EN
Relevância na Pesquisa
373.7958%
Based on a specially created mass spectral database utilizing 23 tetradecenyl and 22 hexadecenyl acetate standards along with Kóvats retention indices obtained on a very polar stationary phase [poly (biscyanopropyl siloxane)] (SP 2340), (Z)-9-hexadecenyl acetate, (Z)-11-hexadecenyl acetate and (E)-8-hexadecenyl acetate were identified in active pheromone extracts of Elasmopalpus lignosellus. This identification was more efficient than our previous study using gas chromatography/mass spectrometry with a dimethyl disulfide derivative where we could only identify the first two acetates. The acetate composition of the pheromone gland differed from region to region in Brazil and from that from the Tifton (GA, USA) population, suggesting polymorphism or a different sub-species.

Kováts retention indexes of monounsaturated C12, C14, and C16 alcohols, acetates and aldehydes commonly found in lepidopteran pheromone blends.

Marques,Francisco de A.; McElfresh,J. S.; Millar,Jocelyn G.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2000 EN
Relevância na Pesquisa
273.79578%
Many lepidopteran pheromones consist of blends of monounsaturated alcohols, aldehydes and acetates. These compounds frequently are obtained in only nanogram quantities from pheromone gland extracts, prohibiting use of most standard spectroscopic methods. However, use of GC retention indexes, particularly in combination with mass spectrometry and electroantennograms, can provide substantial information about trace amounts of unknowns. Comparison of Kováts indexes (KIs) of an unknown with those of standards can provide an unambiguous identification, or at least, limit the number of possible structures to a few compounds. Furthermore, because KIs are highly reproducible, a full set of standards is not necessary; comparisons can be made using tables of KIs determined in another laboratory. We provide here tables of the KIs of almost all possible isomers of monounsaturated alcohols, aldehydes, and acetates with 12, 14, or 16 carbon chain lengths, measured on nonpolar (DB-5) and polar (DB-WAX) stationary phases.

Benzyl acetates as attractants for the male oriental fruit fly, Dacus dorsalis, and the male melon fly, Dacus cucurbitae

Metcalf, Robert L.; Metcalf, Esther R.; Mitchell, W. C.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /03/1986 EN
Relevância na Pesquisa
273.79578%
Fifty compounds related to benzyl acetate were evaluated quantitatively as attractants to the male oriental fruit fly (Dacus dorsalis) and the male melon fly (Dacus cucurbitae). Thienylmethyl acetate was nearly as attractive as benzyl acetate to both species, but cyclohexylmethyl acetate was completely unattractive, emphasizing the role of the planar aromatic ring in receptor interaction. Although benzyl acetate was equally attractive to both species, para substituents invariably reduced attraction to the oriental fruit fly. A number of derivatives, including p-hydroxy-, p-methoxy-, p-acetoxy-, and p-cyanobenzyl acetates were highly attractive to the melon fly. Selective fluorination indicated that the polarizability of the carbonyl carbon is important in receptor interaction with the melon fly but is not specifically involved in the oriental fruit fly. Attraction to the melon fly was lost with ortho, meta, or dimethoxy substitution of benzyl acetate, but the 3,4- and 2,5-dimethoxybenzyl acetates were attractive to the oriental fruit fly. These results are interpreted in terms of specific olfactory receptor interactions for the two species of fruit flies.

Remarkable rate acceleration of SmI3-mediated iodination of acetates of Baylis-Hillman adducts in ionic liquid: facile synthesis of (Z)-allyl iodides*

Liu, Yun-Kui; Zheng, Hui; Xu, Dan-Qian; Xu, Zhen-Yuan; Zhang, Yong-Min
Fonte: Zhejiang University Press Publicador: Zhejiang University Press
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
273.79578%
Stereoselective transformation of Baylis-Hillman acetates 1 into corresponding (Z)-allyl iodides 2 has been achieved by treatment of 1 with samarium triiodide in THF. Remarkable rate acceleration of samarium triiodide-mediated iodination of 1 was found when ionic liquid 1-n-butyl-3-methyl-imidazolium tetrafluroborate ([bmim]BF4) was used as reaction media in stead of THF. This novel approach proceeds readily at 50 °C within a few minutes to afford (Z)-allyl iodides 2 in excellent yields. A mechanism involving stereoselective iodination of the acetates of Baylis-Hillman adducts by samarium triiodide is described, in which a six-membered ring transition state played a key role in the stereoselective formation of 2.

Occurrence and Sources of Triterpenoid Methyl Ethers and Acetates in Sediments of the Cross-River System, Southeast Nigeria

Oyo-Ita, Orok E.; Ekpo, Bassey O.; Oros, Daniel R.; Simoneit, Bernd R. T.
Fonte: Hindawi Publishing Corporation Publicador: Hindawi Publishing Corporation
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
264.5713%
Pentacyclic triterpenol methyl ethers (PTMEs), germanicol methyl ether (miliacin), 3-methoxyfern-9(11)-ene (arundoin), β-amyrin methyl ether (iso-sawamilletin), and 3-methoxytaraxer-14-ene (sawamilletin or crusgallin) were characterized in surface sediments of the Cross-River system using gas chromatography-mass spectrometry (GC-MS). Triterpenol esters (mainly α- and β-amyrinyl acetates and hexanoates, and lupeyl acetate and hexanoate) were also found. These distinct compounds are useful for assessing diagenesis that can occur during river transport of organic detritus. Poaceae, mainly Gramineae and Elaeis guineensis higher plant species, are proposed as primary sources for the PTMEs and esters in the sediments. PTMEs are biomarkers of specific higher plant subspecies, while the triterpenol esters are indicators of early diagenetic alteration of higher plant detritus.

Mixture Toxicity of SN2-Reactive Soft Electrophiles: 2—Evaluation of Mixtures Containing Ethyl α-Halogenated Acetates

Dawson, D. A.; Mooneyham, T.; Jeyaratnam, J.; Schultz, T. W.; Pöch, G.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
264.5713%
Four ethyl α-halogenated acetates were tested in (1) sham and (2) nonsham combinations and (3) with a nonreactive nonpolar narcotic. Ethyl iodoacetate (EIAC), ethyl bromoacetate (EBAC), ethyl chloroacetate (ECAC), and ethyl fluoroacetate (EFAC), each considered to be an SN2-H-polar soft electrophile, were selected for testing based on their differences in electro(nucleo)philic reactivity and time-dependent toxicity (TDT). Agent reactivity was assessed using the model nucleophile glutathione, with EIAC and EBAC showing rapid reactivity, ECAC being less reactive, and EFAC lacking reactivity at ≤250 mM. The model nonpolar narcotic, 3-methyl-2-butanone (3M2B), was not reactive. Toxicity of the agents alone and in mixture was assessed using the Microtox acute toxicity test at three exposure durations: 15, 30 and 45 min. Two of the agents alone (EIAC and EBAC) had TDT values >100%. In contrast, ECAC (74 to 99%) and EFAC (9 to 12%) had partial TDT, whereas 3M2B completely lacked TDT (<0%). In mixture testing, sham combinations of each agent showed a combined effect consistent with predicted effects for dose-addition at each time point, as judged by EC50 dose-addition quotient values. Mixture toxicity results for nonsham ethyl acetate combinations were variable...

Evaluation of curcumin acetates and amino acid conjugates as proteasome inhibitors

Wan, Sheng Biao; Yang, Huanjie; Zhou, Zhongyuan; Cui, Qiuzhi Cindy; Chen, Di; Kanwar, Jyoti; Mohammad, Imthiyaz; Dou, Q. Ping; Chan, Tak Hang
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /10/2010 EN
Relevância na Pesquisa
264.5713%
Curcumin (diferuloylmethane) is the main active ingredient of turmeric, a traditional herbal medicine and food of south Asia. Curcumin has been found to have a wide range of biological activities, including antioxidant, anti-inflammatory, chemopreventive and chemotherapeutic activities. Curcumin is currently being tested in clinical trials for treatment of various types of cancers, including multiple myeloma, pancreatic cancer and colon cancer. Although no toxicity associated with curcumin (even at very high doses) has been observed, the effects of curcumin in other solid tumors have been modest, primarily due to poor water solubility and poor bioavailability in tissues remote from the gastrointestinal tract. Therefore, there is a need for the discovery of curcumin analogs with better water solubility or greater bioavailability for the treatment of solid tumors such as prostate cancer. In this study, curcumin acetates and amino acid conjugates of curcumin were studied in terms of their proteasome inhibitory and antiproliferative effects against several human cancer cell lines. It was found that the water soluble amino acid conjugates of curcumin showed a potent antiproliferative effect and are potent proteasome inhibitors. Docking studies of the curcumin amino acid conjugates for proteasome inhibition were carried out to explain their biological activities. It is suggested that they may serve as the water soluble analogs of curcumin.

Palladium-catalyzed, pyrrolidine-mediated arylmethylation of ketones and aldehydes with coumarinyl(methyl) acetates†

Cattopadhyay, Kalicharan; Recio, Antonio; Tunge, Jon A.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
264.5713%
We report the palladium-catalyzed, pyrrolidine-mediated α-benzylation of enamines generated from aldehydes and ketones. The method allows for direct coupling of medicinally relevant coumarin moieties with aldehydes and ketones in good yield under mild conditions. The reaction is believed to proceed via a Pd-π-benzyl complex generated from (coumarinyl)methyl acetates.

Palladium-catalyzed substitution of (coumarinyl)methyl acetates with C-, N-, and S-nucleophiles

Chattopadhyay, Kalicharan; Fenster, Erik; Grenning, Alexander J; Tunge, Jon A
Fonte: Beilstein-Institut Publicador: Beilstein-Institut
Tipo: Artigo de Revista Científica
Publicado em 27/07/2012 EN
Relevância na Pesquisa
264.5713%
The palladium-catalyzed nucleophilic substitution of (coumarinyl)methyl acetates is described. The reaction proceeds though a palladium π-benzyl-like complex and allows for many different types of C-, N-, and S-nucleophiles to be regioselectively added to the biologically active coumarin motif. This new method was utilized to prepare a 128-membered library of aminated coumarins for biological screening.

Mixture Toxicity of SN2-Reactive Soft Electrophiles: 3. Evaluation of Ethyl α-Halogenated Acetates with α-Halogenated Acetonitriles

Dawson, D. A.; Pöch, G.; Schultz, T. W.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
264.5713%
Mixture toxicity for each of four ethyl α-halogenated acetates (ExACs) with each of three α-halogenated acetonitriles (xANs) was assessed. Inhibition of bioluminescence in Vibrio fischeri was measured after 15, 30 and 45-min of exposure. Concentration-response curves were developed for each chemical at each exposure duration and used to develop predicted concentration-response curves for the dose-addition and independence models of combined effect. Concentration-response curves for each mixture and each exposure duration were then evaluated against the predicted curves, using three metrics per model: 1) EC50-based additivity quotient (AQ) or independence quotient (IQ) values, 2) mean AQ (mAQ) or mean IQ (mIQ) values, calculated by averaging the EC25, EC50 and EC75 AQ or IQ values, and 3) deviation values from additivity (DV-A) or independence (DV-I). Mixture toxicity for ethyl iodoacetate (EIAC) was dose-additive with each of the xANs at all exposure durations and was often consistent with independence as well. The same was true for mixture toxicity of ethyl bromoacetate (EBAC) with each xAN. However, for the two more slowly reactive chemicals ethyl chloroacetate (ECAC) and ethyl fluoroacetate (EFAC) mixture toxicity with each xAN only became consistent with dose-addition upon increasing exposure duration. Consistency with independence for both ECAC and EFAC with the xANs was essentially limited to the EC50-IQ metric; thereby demonstrating the utility of calculating the mean quotient (mAQ...

Identification of acetates in elasmopalpulus lignosellus pheromone glands using a newly created mass spectral database and kóvats retention indices.

JHAM, G. N.; SILVA, A. A. da; LIMA, E. R.; VIANA, P. A.
Fonte: Química Nova, São Paulo, v. 30, n. 4, p. 916-919, 2007. Publicador: Química Nova, São Paulo, v. 30, n. 4, p. 916-919, 2007.
Tipo: Artigo em periódico indexado (ALICE)
EN
Relevância na Pesquisa
373.7958%
Based on a specially created mass spectral database utilizing 23 tetradecenyl and 22 hexadecenyl acetate standards along with Kóvats retention indices obtained on a very polar stationary phase [poly (biscyanopropyl siloxane)] (SP 2340), (Z)-9-hexadecenyl acetate, (Z)-11-hexadecenyl acetate and (E)-8-hexadecenyl acetate were identified in active pheromone extracts of Elasmopalpus lignosellus. This identification was more efficient than our previous study using gas chromatography/mass spectrometry with a dimethyl disulfide derivative where we could only identify the first two acetates. The acetate composition of the pheromone gland differed from region to region in Brazil and from that from the Tifton (GA, USA) population, suggesting polymorphism or a different sub-species.; 2007

Thermal decomposition of mercerized linter cellulose and its acetates obtained from a homogeneous reaction; Decomposição térmica de celulose de linter mercerizado e seus acetatos obtidos a partir de reação homogênea

MORGADO, Daniella L.; FROLLINI, Elisabete
Fonte: Associação Brasileira de Polímeros Publicador: Associação Brasileira de Polímeros
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
386.09508%
Cellulose acetates with different degrees of substitution (DS, from 0.6 to 1.9) were prepared from previously mercerized linter cellulose, in a homogeneous medium, using N,N-dimethylacetamide/lithium chloride as a solvent system. The influence of different degrees of substitution on the properties of cellulose acetates was investigated using thermogravimetric analyses (TGA). Quantitative methods were applied to the thermogravimetric curves in order to determine the apparent activation energy (Ea) related to the thermal decomposition of untreated and mercerized celluloses and cellulose acetates. Ea values were calculated using Broido's method and considering dynamic conditions. Ea values of 158 and 187 kJ mol-1 were obtained for untreated and mercerized cellulose, respectively. A previous study showed that C6OH is the most reactive site for acetylation, probably due to the steric hindrance of C2 and C3. The C6OH takes part in the first step of cellulose decomposition, leading to the formation of levoglucosan and, when it is changed to C6OCOCH3, the results indicate that the mechanism of thermal decomposition changes to one with a lower Ea. A linear correlation between Ea and the DS of the acetates prepared in the present work was identified.; Acetatos de celulose com graus de substituição...

Thermal decomposition of mercerized linter cellulose and its acetates obtained from a homogeneous reaction

Morgado,Daniella L.; Frollini,Elisabete
Fonte: Associação Brasileira de Polímeros Publicador: Associação Brasileira de Polímeros
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2011 EN
Relevância na Pesquisa
386.09508%
Cellulose acetates with different degrees of substitution (DS, from 0.6 to 1.9) were prepared from previously mercerized linter cellulose, in a homogeneous medium, using N,N-dimethylacetamide/lithium chloride as a solvent system. The influence of different degrees of substitution on the properties of cellulose acetates was investigated using thermogravimetric analyses (TGA). Quantitative methods were applied to the thermogravimetric curves in order to determine the apparent activation energy (Ea) related to the thermal decomposition of untreated and mercerized celluloses and cellulose acetates. Ea values were calculated using Broido's method and considering dynamic conditions. Ea values of 158 and 187 kJ mol-1 were obtained for untreated and mercerized cellulose, respectively. A previous study showed that C6OH is the most reactive site for acetylation, probably due to the steric hindrance of C2 and C3. The C6OH takes part in the first step of cellulose decomposition, leading to the formation of levoglucosan and, when it is changed to C6OCOCH3, the results indicate that the mechanism of thermal decomposition changes to one with a lower Ea. A linear correlation between Ea and the DS of the acetates prepared in the present work was identified.

Potentialities of two solventless extraction approaches—Stir bar sorptive extraction and headspace solid-phase microextraction for determination of higher alcohol acetates, isoamyl esters and ethyl esters in wines

Perestrelo, R.; Nogueira, J. M. F.; Câmara, J. S.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em /12/2009 ENG
Relevância na Pesquisa
379.4724%
A stir bar sorptive extraction with liquid desorption followed by large volume injection coupled to gas chromatography–quadrupole mass spectrometry (SBSE-LD/LVI-GC–qMS) was evaluated for the simultaneous determination of higher alcohol acetates (HAA), isoamyl esters (IsoE) and ethyl esters (EE) of fatty acids. The method performance was assessed and compared with other solventless technique, the solid-phase microextraction (SPME) in headspace mode (HS). For both techniques, influential experimental parameters were optimised to provide sensitive and robust methods. The SBSE-LD/LVI methodology was previously optimised in terms of extraction time, influence of ethanol in the matrix, liquid desorption (LD) conditions and instrumental settings. Higher extraction efficiency was obtained using 60 min of extraction time, 10% ethanol content, n-pentane as desorption solvent, 15 min for the back-extraction period, 10 mL min−1 for the solvent vent flow rate and 10 °C for the inlet temperature. For HS-SPME, the fibre coated with 50/30 μm divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) afforded highest extraction efficiency, providing the best sensitivity for the target volatiles, particularly when the samples were extracted at 25 °C for 60 min under continuous stirring in the presence of sodium chloride (10% (w/v)). Both methodologies showed good linearity over the concentration range tested...