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Influence of curing protocol on selected properties of light-curing polymers: Degree of conversion, volume contraction, elastic modulus, and glass transition temperature

DEWAELE, Magali; ASMUSSEN, Erik; PEUTZFELDT, Anne; MUNKSGAARD, E. Christian; BENETTI, Ana R.; FINNE, Gauthier; LELOUP, Gaetane; DEVAUX, Jacques
Fonte: ELSEVIER SCI LTD Publicador: ELSEVIER SCI LTD
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
66.12%
Objectives. The purpose of this study was to investigate the effect of light-curing protocol on degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition temperature (T(g)) as measured on a model polymer. It was a further aim to correlate the measured values with each other. Methods. Different light-curing protocols were used in order to investigate the influence of energy density (ED), power density (PD), and mode of cure on the properties. The modes of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman micro-spectroscopy. C was determined by pycnometry and a density column. E was measured by a dynamic mechanical analyzer (DMA), and T(g) was measured by differential scanning calorimetry (DSC). Data were submitted to two-and three-way ANOVA, and linear regression analyses. Results. ED, PD, and mode of cure influenced DC, C, E, and T(g) of the polymer. A significant positive correlation was found between ED and DC (r = 0.58), ED and E (r = 0.51), and ED and T(g) (r = 0.44). Taken together, ED and PD were significantly related to DC and E. The regression coefficient was positive for ED and negative for PD. Significant positive correlations were detected between DC and C (r = 0.54)...

Glass transition and degree of conversion of a light-cured orthodontic composite

Sostena, Michela M. D. S.; Nogueira, Renata A.; Grandini, Carlos Roberto; Moraes, João Carlos Silos
Fonte: Universidade de São Paulo (USP), Faculdade de Odontologia de Bauru Publicador: Universidade de São Paulo (USP), Faculdade de Odontologia de Bauru
Tipo: Artigo de Revista Científica Formato: 570-573
ENG
Relevância na Pesquisa
66.12%
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Objective: This study evaluated the glass transition temperature (T(g)) and degree of conversion (DC) of a light-cured (Fill Magic) versus a chemically cured (Concise) orthodontic composite. Material and Methods: Anelastic relaxation spectroscopy was used for the first time to determine the T(g) of a dental composite, while the DC was evaluated by infrared spectroscopy. The light-cured composite specimens were irradiated with a commercial LED light-curing unit using different exposure times (40, 90 and 120 s). Results: Fill Magic presented lower T(g) than Concise (35-84 degrees C versus 135 degrees C), but reached a higher DC. Conclusions: The results of this study suggest that Fill Magic has lower T(g) than Concise due to its higher organic phase content, and that when this light-cured composite is used to bond orthodontic brackets, a minimum energy density of 7.8 J/cm(2) is necessary to reach adequate conversion level and obtain satisfactory adhesion.

Inversion in the temperature coefficient of the optical path length close to the glass transition temperature in tellurite glasses

Lima, S. M.; Andrade, L. H. C.; Falcao, E. A.; Steimacher, A.; Astrath, N. G. C.; Medina, A. N.; Baesso, M. L.; Oliveira, R. C.; Moraes, João Carlos Silos; Yukimitu, K.; Araujo, E. B.
Fonte: American Institute of Physics (AIP) Publicador: American Institute of Physics (AIP)
Tipo: Artigo de Revista Científica Formato: 3
ENG
Relevância na Pesquisa
66.26%
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); In this study, thermal lens spectrometry was applied to determine the thermo-optical properties of fragile tellurite glasses as a function of temperature, close to the glass transition region. The results showed an inversion from positive to negative values in the temperature coefficient of the optical path length occurring after the glass transition temperature, which is the region where structural changes from the TeO(4) trigonal bipyramidal unit to a TeO(3) trigonal pyramid containing nonbridging oxygen take place. In addition, the thermal diffusivity values as a function of temperature exhibited behaviors that were related to thermodynamic and kinetic structural changes in the glass.

Glass transitions and state diagram for freeze-dried pineapple

Telis, VRN; Sobral, PJA
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 199-205
ENG
Relevância na Pesquisa
56.28%
Glass transition temperature of freeze-dried pineapple conditioned by adsorption at various water activities at 25 degreesC was determined by differential scanning calorimetry (DSC). High moisture content samples corresponding to water activities higher than 0.9, obtained by liquid water addition, were also analysed. The DSC traces showed a well-visible shift in baseline at the glass transition temperature (T(g)). Besides, no ice formation was observed until water activity was equal to 0.75. For water activities lower than 0.88, the glass transition curve showed that T(g) decreased with increasing moisture content and the experimental data could be well-correlated by the Gordon-Taylor equation. For higher water activities, this curve exhibited a discontinuity, with suddenly increasing glass transition temperatures approaching a constant value that corresponds to the T(g) of the maximally freeze-concentrated amorphous matrix. The unfreezable water content was determined through melting enthalpy dependence on the sample moisture content.

Glass transitions for freeze-dried and air-dried tomato

Telis, VRN; Sobral, PJA
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 435-443
ENG
Relevância na Pesquisa
56.26%
Glass transition temperatures of freeze-dried tomato conditioned at various water activities at 25 C were determined by differential scanning calorimetry (DSC). Air-dried tomato with and without osmotic pre-treatment in sucrose/NaCl solutions was also analyzed. Thermograms corresponding to the low water activity domain (0.11 less than or equal to a(w) less than or equal to 0.75) revealed the existence of two glass transitions, which were attributed to separated phases formed by sugars and water and other natural macromolecules present in the vegetable. Both transitions were plasticized by water and experimental data could be well correlated by the Gordon-Taylor equation in the low-temperature domain, and by the Kwei model in the high-temperature domain. For higher water activities, the low-temperature glass transition curve exhibited a discontinuity, with suddenly increased glass transition temperatures approaching a constant value that corresponds to the T-g of the maximally freeze-concentrated amorphous matrix. The unfreezable water content was determined through the melting enthalpy dependence on the moisture content. (C) 2002 Elsevier B.V. Ltd. All rights reserved.

Collapse and Color Changes in Grapefruit Juice Powder as Affected by Water Activity, Glass Transition, and Addition of Carbohydrate Polymers

Telis, V. R. N.; Martinez-Navarrete, N.
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica Formato: 83-93
ENG
Relevância na Pesquisa
66.12%
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Physicochemical and structural properties of grapefruit juice powder were investigated as affected by the addition of maltodextrins of two dextrose equivalent (DE) and gum arabic. Freeze-dried powdered juices were equilibrated at different vapor pressure atmospheres, giving samples with water activity between zero and 0.84. The mechanical properties of the powders were assessed by confined compression, and the compressed samples were subjected to color analysis. The maximum force attained during the compression and the color coordinates were related to water activity and glass transition temperature, and a single value of Delta T = T -aEuro parts per thousand T (g) could be taken as the critical limit to the safe storage of the powders, regardless of their composition. The results indicated that from the perspective of the time at which deleterious changes would take place in powders stored at certain ambient conditions and exposed to certain rate of water uptake, the collapse of the powder would precede browning development.

Estudo da influencia da transição vitrea sobre a instantaneização de maltodextrinas por processo de aglomeração umida.; Influence of the glass transition temperature on the maltodextrin agglomeration process.

Cristina Yoshie Takeiti
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 28/02/2007 PT
Relevância na Pesquisa
66.2%
Maltodextrinas são oligossacarídeos solúveis em água, provenientes da hidrólise de amidos. As condições da hidrólise permitem definir a distribuição de massa molecular das maltodextrinas e, conseqüentemente, suas características funcionais podem ser ajustadas para aplicações distintas e específicas. Estes ingredientes são empregados em formulações de pós alimentícios prontos para o consumo e, portanto, o conhecimento de atributos que afetam sua reconstituição em água é determinante para o desenvolvimento desses produtos. Maltodextrinas comerciais com diferentes graus de dextrose-equivalente (DE) e provenientes de três fábricas nacionais distintas foram avaliadas quanto à: morfologia das partículas, teor de umidade, massa molecular, diâmetro médio, densidade aparente e real, porosidade, tempo de molhamento, tempo de dissolução, grau de cristalinidade, isotermas de sorção de umidade e temperatura de transição vítrea (Tg). Os pós comerciais apresentaram características morfológicas distintas, o que influenciou o seu desempenho quanto às propriedades de reconstituição em água. Os produtos que apresentaram características intermediárias de reconstituição foram submetidos a um processo de aglomeração por jato a vapor em mini-instantaneizador para melhorar suas qualidades tecnológicas. As condições de operação foram otimizadas por metodologia de superfície de resposta. O produto aglomerado de maltodextrina com DE igual a 10 apresentou o melhor rendimento e excelentes propriedades instantâneas...

Glass transition of semi-crystalline PLLA with different morphologies as studied by dynamic mechanical analysis

Picchioci, R.; Yang, Y.; Rodriguez-Perez, M. A.; Alves, N. M.; Mano, J. F.
Fonte: Universidade do Minho Publicador: Universidade do Minho
Tipo: Artigo de Revista Científica
Publicado em //2007 ENG
Relevância na Pesquisa
66.2%
Poly(L-lactic acid) was crystallized from the glassy state at different temperatures to produce fully transformed semi-crystalline specimens exhibiting different lamellar morphologies. The materials were tested by dynamic mechanical analysis, where a Tg decrease was found with an increasing crystallization temperature. Considering a three-phase model, this tendency was related to the corresponding increase in the thickness of the rigid amorphous phase. It is suggested that this phase could, in some extent, accommodate through local translational/ rotational motions the cooperative motions taking place within the mobile amorphous phase. This could be due to the non-compact structure of the cooperatively rearranging regions, which can present a string-like or fractal structure in their edges. The width of the loss factor peak associated to the glass transition increases with increasing crystallization temperature, suggesting an increase in the broadness of the distribution of relaxation times. The drop in the storage modulus across Tg varies systematically with the crystallization temperature in the different materials and could be correlated with the crystalline content. Above Tg, the loss factor exhibits a plateau-like behaviour at significantly high values...

Glass transition dynamics and structural relaxation of PLLA studied by DSC : Influence of crystallinity

Mano, J. F.; Gómez Ribelles, J. L.; Alves, N. M.; Salmerón-Sánchez, M.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em /09/2005 ENG
Relevância na Pesquisa
66.2%
Poly(L-lactic acid), PLLA, in amorphous and semi-crystalline forms were studied by DSC, in order to investigate both molecular dynamics and structural relaxation features, and to understand the influence of the crystalline confinement on the segmental mobility of the intraspherulitic amorphous phase. Experimental data were generated after submitting the materials to different thermal histories below Tg and were treated with a model based on the configurational entropy concept, allowing to extract a series of physical-meaningful parameters and to obtain the temperature dependence of the relaxation times. The main features of the relaxation process in the glassy state (activation energy of the glassy state and x) and the fragility index were found to be apparently insensitive to crystallinity. Significant differences between the two materials were detected in both the position of the glass transition temperature and the width of the distribution of relaxation times. In the framework of the Adam and Gibbs theory, it is suggested that for the semi-cristalline PLLA the mobility of the amorphous chains is more or less restricted, depending on their distance to the rigid lamellar walls or on the intra-lamellar thickness; this will imply that their conformational motions will take place at different temperatures...

Glass transition and degree of conversion of a light-cured orthodontic composite

Sostena,Michela M. D. S.; Nogueira,Renata A.; Grandini,Carlos R.; Moraes,João Carlos Silos
Fonte: Faculdade De Odontologia De Bauru - USP Publicador: Faculdade De Odontologia De Bauru - USP
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2009 EN
Relevância na Pesquisa
66.12%
OBJECTIVE: This study evaluated the glass transition temperature (Tg) and degree of conversion (DC) of a light-cured (Fill Magic) versus a chemically cured (Concise) orthodontic composite. MATERIAL AND METHODS: Anelastic relaxation spectroscopy was used for the first time to determine the Tg of a dental composite, while the DC was evaluated by infrared spectroscopy. The light-cured composite specimens were irradiated with a commercial LED light-curing unit using different exposure times (40, 90 and 120 s). RESULTS: Fill Magic presented lower Tg than Concise (35-84ºC versus 135ºC), but reached a higher DC. CONCLUSIONS: The results of this study suggest that Fill Magic has lower Tg than Concise due to its higher organic phase content, and that when this light-cured composite is used to bond orthodontic brackets, a minimum energy density of 7.8 J/cm² is necessary to reach adequate conversion level and obtain satisfactory adhesion.

Thermorheological Properties Near the Glass Transition of Oligomeric Poly(methyl methacrylate) Blended with Acrylic Polyhedral Oligomeric Silsesquioxane Nanocages

Kopesky, Edward T.; Boyes, Stephen G.; Treat, Neil; Cohen, Robert E.; McKinley, Gareth H.
Fonte: MIT - Massachusetts Institute of Technology Publicador: MIT - Massachusetts Institute of Technology
Tipo: Pré-impressão Formato: 598728 bytes; application/pdf
EN_US
Relevância na Pesquisa
66.3%
Two distinct oligomeric species of similar mass and chemical functionality (Mw ≈ 2,000 g/mol), one a linear methyl methacrylate oligomer (radius of gyration Rg ≈ 1.1 nm) and the other a hybrid organic-inorganic polyhedral silsesquioxane nanocage (methacryl-POSS, r ≈ 1.0 nm), were subjected to thermal and rheological tests to compare the behaviors of these geometrically dissimilar molecules over the entire composition range. The glass transition temperatures of the blends varied monotonically between the glass transition temperatures of the pure oligomer (Tg = −47.3°C) and the pure POSS (Tg = −61.0°C). Blends containing high POSS contents (with volume fraction φ_POSS ≥ 0.90) exhibited enhanced enthalpy relaxation in DSC measurements, and the degree of enthalpy relaxation was used to calculate the kinetic fragility indices m of the oligomeric MMA (m = 59) and the POSS (m = 74). The temperature dependences of the viscosities were fitted by the free volume-based WLF-VFT framework and a dynamic scaling relation. The calculated values of the fragility from the WLF-VFT fits were similar for the POSS (m = 82) and for the oligomer (m = 76), and the dynamic scaling exponent was similar for the oligomeric MMA and the POSS. Within the range of known fragilities for glass-forming liquids...

Investigation of the capacity of low glass transition temperature excipients to minimise amorphisation of sulfadimidine on co-milling

Curtin, Vincent; Amharar, Youness; Hu, Yun; Erxleben, Andrea; McArdle, Patrick; Caron, Vincent; Tajber, Lidia; Corrigan, Owen I; Healy, Anne Marie
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: info:eu-repo/semantics/article; all_ul_research; ul_published_reviewed
ENG
Relevância na Pesquisa
66.12%
peer-reviewed; The coprocessing of active pharmaceutical ingredient (API) with an excipient which has a high glass transition temperature (Tg) is a recognized strategy to stabilize the amorphous form of a drug. This work investigates whether coprocessing a model API, sulfadimidine (SDM) with a series of low Tg excipients, prevents or reduces amorphization of the crystalline drug. It was hypothesized that these excipients could exert a Tg lowering effect, resulting in composite Tg values lower than that of the API alone and promote crystallization of the drug. Milled SDM and comilled SDM with glutaric acid (GA), adipic acid (AA), succinic acid (SA), and malic acid (MA) were characterized with respect to their thermal, X-ray diffraction, spectroscopic, and vapor sorption properties. SDM was predominantly amorphous when milled alone, with an amorphous content of 82%. No amorphous content was detected by dynamic vapor sorption (DVS) on comilling SDM with 50% w/w GA, and amorphous content of the API was reduced by almost 30%, relative to the API milled alone, on comilling with 50% w/w AA. In contrast, amorphization of SDM was promoted on comilling with 50% w/w SA and MA, as indicated by near-infrared (NIR) spectroscopy. Results indicated that the API was completely amorphized in the SDM:MA comilled composite. The saturated solubility of GA and AA in the amorphous API was estimated by thermal methods. It was observed that the Tg of the comelt quenched composites reached a minimum and leveled out at this solubility concentration. Maximum crystallinity of API on comilling was reached at excipient concentrations comparable to the saturated concentration solubility of excipient in the API. Moreover...

Estrutura, cristalização e transição vítrea de líquidos iônicos; Structure, crystallization, and glass transition of ionic liquids

Faria, Luiz Felipe de Oliveira
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 30/07/2015 PT
Relevância na Pesquisa
66.12%
Esta tese consiste na investigação estrutural de sais fundidos em baixas temperaturas, os chamados líquidos iônicos (LIs). Estes sais foram estudados nas fases líquida, líquido super-resfriado, cristalina e vítrea através de mudanças nas variáveis termodinâmicas temperatura e pressão. Devido ao complexo comportamento de fases de LIs, este trabalho também contempla o estudo da competição entre os processos de transição vítrea e cristalização com ênfase no processo de nucleação frustrada. Uma grande variedade de LIs, formados por diferentes espécies de cátions e ânions, foram investigados por uma conjunção de metodologias experimentais e simulações computacionais. Os resultados obtidos das simulações de dinâmica molecular (MD) foram confrontados com medidas de espalhamento de raios X de amplo ângulo (WAXS) utilizando radiação síncrotron. Desta forma, a organização estrutural local na fase líquida de uma série de LIs pôde ser elucidada. A espectroscopia Raman permitiu o estudo de aspectos estruturais das fases formadas a partir das diferenças em frequência vibracional, intensidades relativas e forma de banda resultante da modificação da conformação ou do ambiente local experimentado pelos íons. A região de baixas frequências dos espectros Raman...

Use of glass transition temperature for stabilization of board's cracks of Eucalyptus grandis

Calonego,Fred W.; Severo,Elias T.D.; Cunha,Antonio R.; Gaia,Daiane C.
Fonte: Academia Brasileira de Ciências Publicador: Academia Brasileira de Ciências
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/09/2010 EN
Relevância na Pesquisa
66.12%
The Eucalyptus grandis logs temperatures were determined and correlated with the board's cracks during steaming. Thermocouples were inserted in the logs center, registering their temperatures during steaming at 90"C. The logs were sawed and the board's cracks measured. It was concluded that: (1) the logistic S-shaped curve explains the logs temperature variation; (2) the logs with diameter of 20 to <25, 25 to <30 and 30 to <35 cm presented, respectively, 84.2"C, 73.1"C and 45.8"C in the steaming; and (3) the cracks lengths significantly decreased in logs that reached the glass transition temperature.

Influence of the polyhedral oligomeric silsesquioxane n-phenylaminopropyl: POSS in the thermal stability and the glass transition temperature of epoxy resin

Pistor,Vinícius; Soares,Bluma Guenther; Mauler,Raquel Santos
Fonte: Associação Brasileira de Polímeros Publicador: Associação Brasileira de Polímeros
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2013 EN
Relevância na Pesquisa
66.12%
In this study, epoxy nanocomposites containing different fractions of n-phenylaminopropyl (POSS) were prepared. The nanocomposites were studied by transmission electron microscopy (TEM), gel content, dynamic-mechanical analysis (DMA) and thermogravimetric analysis (TGA). The parameters for Avrami's equation were calculated from the degradation curves. The dispersions used to form the nanocomposites were effective above 5 wt % of POSS, and the gel content increased with the addition of POSS. The DMA analysis exhibited an increase in the storage modulus (E') and a shifting of Tg to higher temperatures upon POSS incorporation. The weight loss indicated that the POSS promoted an increase in thermal stability of the epoxy resin. The Avrami parameters demonstrated that the addition of POSS decreased the Avrami constant (k'), increased the half-life (t1/2) of degradation and promoted changes in the Avrami exponent (n). These results suggest that the increase in the glass transition temperature and thermal stability depend on the reactive groups in the POSS nanoparticles.

Thermal stability of Hibiscus sabdariffa L. anthocyanins in solution and in solid state: effects of copigmentation and glass transition

Gradinaru, G.; Biliaderis, C. G.; Kallithraka, S.; Kefalas, P.; García-Viguera, Cristina
Fonte: Elsevier Publicador: Elsevier
Tipo: Artículo Formato: 259768 bytes; application/pdf
ENG
Relevância na Pesquisa
66.2%
14 pages, 7 figures, 3 tables.; Kinetic studies on thermal stability of anthocyanins isolated from the dry calyces of Hibiscus sabdariffa L. (roselle) were carried out in aqueous solutions (55–98 °C), either as free or copigmented anthocyanins with chlorogenic acid, and in the dry state as free anthocyanins or co-lyophilized with an amorphous polysaccharide (pullulan) and stored in different relative humidity environments (water activities 0.33, 0.53, 0.75 and 0.84) at 40 °C. The rate constants for degradation were obtained from first-order reaction kinetic plots. The degradation kinetics of individual anthocyanin components in solution, as assessed by HPLC, followed an Arrhenius-type response with respect to temperature; activation energies, Ea, varied between 13.3 and 15.1 kcal/mol. Copigmentation of anthocyanins with chlorogenic acid did not seem to improve their stability in solution. In the dry state, the degradation rate constants increased with the water activity, particularly above 0.53. In the freeze-dried pullulan–anthocyanin mixtures, the polysaccharide matrix delayed colour degradation compared to the free anthocyanin preparations by 1.5–1.8 times. The degradation kinetics of anthocyanins did not show any dependence on the molecular mobility of the system...

Glass transition and degree of conversion of a light-cured orthodontic composite

Sostena, Michela M. D. S.; Nogueira, Renata A.; Grandini, Carlos R.; Moraes, João Carlos Silos
Fonte: Universidade de São Paulo. Faculdade de Odontologia de Bauru Publicador: Universidade de São Paulo. Faculdade de Odontologia de Bauru
Tipo: info:eu-repo/semantics/article; info:eu-repo/semantics/publishedVersion; ; Formato: application/pdf
Publicado em 01/12/2009 ENG
Relevância na Pesquisa
66.12%
OBJECTIVE: This study evaluated the glass transition temperature (Tg) and degree of conversion (DC) of a light-cured (Fill Magic) versus a chemically cured (Concise) orthodontic composite. MATERIAL AND METHODS: Anelastic relaxation spectroscopy was used for the first time to determine the Tg of a dental composite, while the DC was evaluated by infrared spectroscopy. The light-cured composite specimens were irradiated with a commercial LED light-curing unit using different exposure times (40, 90 and 120 s). RESULTS: Fill Magic presented lower Tg than Concise (35-84ºC versus 135ºC), but reached a higher DC. CONCLUSIONS: The results of this study suggest that Fill Magic has lower Tg than Concise due to its higher organic phase content, and that when this light-cured composite is used to bond orthodontic brackets, a minimum energy density of 7.8 J/cm² is necessary to reach adequate conversion level and obtain satisfactory adhesion.

Compressive fracture characteristics of a Zr-based bulk metallic glass at high test temperatures

Wang, Gang; Shen, J; Sun, J F; Lu, Z P; Stachurski, Zbigniew; Zhou, B D
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.27%
This paper presents the fracture characteristics of Zr41.25 Ti13.75Ni10Cu12.5Be22.5 bulk metallic glass (BMG) subjected to compression tests at the calorimetric glass transition temperature and in the supercooled liquid region. Under compressive load, the catastrophic fracture is not a pure shear process and the normal stress played a critical role. Based on the Mohr-Coulomb criterion, it was found that the increase in cohesive strength due to the increase of strain rates leads to the change of fracture angles from 40° to 56°. At the calorimetric glass transition temperature, the fracture characteristics are similar to those observed at room temperature, while at higher test temperatures, different fracture features including the flow layer and round cores that are regarded as the unique features for tensile fractures of BMGs, were observed.

Determination of the minimun integral entropy, water sorption and glass transition temperature to establishing critical storage condition of beetroot juice microcapsules by spray drying

Guadarrama-Lezama,A.Y.; Cruz-Olivares,J.; Martínez-Vargas,S.L.; Carrillo-Navas,H.; Román-Guerrero,A.; Perez-Alonso,C.
Fonte: UAM, Unidad Iztapalapa, División de Ciencias Básicas e Ingeniería Publicador: UAM, Unidad Iztapalapa, División de Ciencias Básicas e Ingeniería
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2014 EN
Relevância na Pesquisa
66.12%
The aim of this work was to microencapsulate beetroot juice (BJ) (Beta vulgaris L.) by spray-drying using as protective colloid gum Arabic. The adsorption isotherms of the microcapsules and the minimum integral entropy (ΔSint)T were determined at 25, 35 and 40 °C. The glass transition temperature (Tg) was measured by differential scanning calorimetry and modeled by Gordon-Taylor equation. The water contents-water activity (M-aW) sets obtained from (ΔSint)T, and critical water content (CWC) and critical water activity (CWA) from the Tg were similar, being in the range of water content of 5.11-7.5 kg H2O/100 kg d.s. and in the water activity range of 0.532-0.590. These critical storage conditions were considered as the best conditions for increase the stability of the microcapsules, where the percentage of retention Betanin in the microcapsules was higher compared with other storage conditions in the temperature and a w range studied.

A Study on Physical Aging of Semicrystalline Polyethylene Terephthalate below the Glass Transition Point

Farhoodi,M.; Mousavi,S. M.; Sotudeh-Gharebagh,R.; Emam-Djomeh,Z.; Oromiehie,A.; Mansour,H.
Fonte: UNAM, Centro de Ciencias Aplicadas y Desarrollo Tecnológico Publicador: UNAM, Centro de Ciencias Aplicadas y Desarrollo Tecnológico
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/10/2012 EN
Relevância na Pesquisa
66.2%
Physical aging of semicrystalline polyethylene terephthalate was studied using differential scanning calorimetry (DSC). PET samples with crystallinity content of 0.28 were aged at two different temperatures, 25 and 45°C. The samples were stored for several days and periodically tested using DSC method. The glass transition temperature for the samples aged at 25°C was about 73-74°C, and the position and intensity of endothermic peaks were approximately constant. Higher glass transition of the samples aged at 45°C, 73-86°C, was attributed to the enthalpy relaxation process of amorphous regions of semicrystalline PET. For the samples aged at 45°C, the endothermic peaks shifted to higher temperatures with increasing aging time. The position of the endothermic peaks determined by the temperature of the maximum, Tmax, tended to increase with aging time for samples aged at 45°C, and the intensity of the peaks continuously increased with time; however, the results showed that the aging of PET samples at 45°C even after 120 days continued the enthalpic relaxation of semicrystalline PET and that the process could be studied by DSC method. The results also showed that the aging process could affect the final degree of crystallinity of c-PET samples and the samples stored at 45°C showed higher degree of crystallinity than the samples aged at 25°C.