Showing papers on "Differential scanning calorimetry published in 2000"

Synthesis and Characterization of pH- and Temperature-Sensitive Poly(methacrylic acid)/Poly(N-isopropylacrylamide) Interpenetrating Polymeric Networks

Abstract: Hydrogels of an interpenetrating polymeric network (IPN) composed of the temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) and the pH-sensitive poly(methacrylic acid) (PMAA) were prepared by a sequential UV polymerization method. The IPN hydrogels were characterized for their temperature- and pH-responsive behavior by equilibrium swelling studies, oscillatory swelling studies, and differential scanning calorimetry. The permeability of these IPNs has been investigated under various pH and temperature conditions. The results showed that these hydrogels exhibited a combined pH- and temperature-sensitivity at a temperature range of 31−32 °C and a pH value of approximately 5.5. Permeation study results indicate a significant size exclusion behavior while model drugs with different sizes permeate through the IPN membranes. The permeability of the IPN membrane can be significantly affected by varying the pH and temperature conditions.

Differential scanning calorimetric analysis of edible oils: Comparison of thermal properties and chemical composition

Abstract: The thermal profiles of 17 edible oil samples from different plant origins were examined by differential scanning calorimetry (DSC). Two other confirmatory analytical techniques, namely gas-liquid chromatography (GLC) and high-performance liquid chromatography (HPLC), were used to determine fatty acid (FA) and triacylglycerol (TAG) compositions. The FA and TAG compositions were used to complement the DSC data. Iodine value (IV) analysis was carried out to measure the degree of unsaturation in these oil samples. The DSC melting and crystallization curves of the oil samples are reported. The contrasting DSC thermal curves provide a way of distinguishing among these oil samples. Generally, the oil samples with a high degree of saturation (IV 65). Each thermal curve was used to determine three DSC parameters, namely, onset temperature (T o ), offset temperature (T f ) and temperature range (difference between T o and T f ). Reproducibility of DSC curves was evaluated based on these parameters. Satisfactory reproducibility was achieved for quantitation of these DSC parameters. The results show that T o of the crystallization curve and T f of the melting curve differed significantly (P<0.01) in all oil samples. Our observations strengthen the premise that DSC is an efficient and accurate method for characterizing edible oils.

Activation energy and crystallization kinetics of untreated and treated oil palm fibre reinforced phenol formaldehyde composites

Abstract: In this paper, oil palm fiber reinforced phenol formaldehyde (PF) treated, as well as untreated, composites have been taken for the study. The untreated sample (sample 1) contains oil palm fiber reinforced in the PF matrix, and the same fiber is treated with silane (sample 2) and with alkali (sample 3) to produce two types of treated fibers. These treated fibers were then reinforced in the matrix to produce two treated samples. Differential scanning calorimetry has been employed to study the crystallization kinetics and the energy of crystallization for all the samples. All the samples show the well-defined peaks of crystallization. In the case of silane-treated sample, double crystallization is observed. The crystallization data are analyzed in terms of a modified Kissinger’s equation to determine the activation energy. The activation energy and other crystallization parameters have also been determined using Matusita’s equation and are compared with the values obtained from other equations. It has also been found that various treatments have improved the thermal stability of the composites to different extents.

Lipid Coating on Polyelectrolyte Surface Modified Colloidal Particles and Polyelectrolyte Capsules

Abstract: Dipalmitoyldiphosphatidic acid (DPPA), dipalmitoyldiphosphatidylcholine (DPPC), and sphingosine were adsorbed onto polyelectrolyte coated colloids and capsules forming composite lipid−polyelectrolyte layers. The stepwise coating was performed either by adsorption of preformed vesicles onto the capsule surface or by a solvent exchange protocol. The lipid assembly was monitored by ζ-potential measurements. Single particle light scattering, flow cytometry, and fluorescence studies of Forster energy transfer have been used to quantify the lipid coating. Confocal microscopy images of capsules coated with fluorescent lipids demonstrated a homogeneous coverage of the capsule surface. Differential scanning calorimetry shows a phase transition temperature characteristic for lipid layer structures. It was concluded that the adsorbed DPPA form bilayers while DPPC may form multilayers. It was further shown that on top of the lipid layers further polyelectrolyte layers could be assembled. The permeability of 6-carboxy...

Mapping Polymer Heterogeneity Using Atomic Force Microscopy Phase Imaging and Nanoscale Indentation

Abstract: Polymer coatings often contain degradation-susceptible regions, and corrosion of the metallic substrate can occur directly underneath these regions. In this paper, the microstructure of model coating materials is investigated using atomic force microscopy (AFM). Specifically, AFM is used to study heterogeneity in thin film blends of polystyrene (PS) and polybutadiene (PB) as a function of annealing time at 80 °C. PS/PB blend films with thicknesses of approximately 250 nm are prepared by spin casting from solutions onto silicon substrates. Both topographic and phase imaging in tapping mode AFM are performed on these films under ambient conditions and at different force levels using a silicon tip. For certain force levels, phase imaging provides good contrast between the phase-separated PS and PB regions, primarily because of the large compliance difference between the two materials. This contrast decreases with increasing annealing time because thermal oxidation causes cross-linking in PB, and thus, the compliance of the PB region increases toward that of PS. Nanoscale indentation measurements are then made on the observed phase-separated regions to identify these regions as PS- and PB-rich and to better understand the influence of relative surface stiffness on the phase images. Cast and free-standing films of pure PS and pure PB are also studied as a function of annealing time using AFM, contact angle measurements, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Results from studies of the individual PS and PB films are related to the AFM results for the blend films. The use of phase imaging for cure monitoring of polymers and for studies of chemically heterogeneous polymer systems is also discussed.

Structure‐property relationships of degradable polyurethane elastomers containing an amino acid‐based chain extender

Abstract: A series of degradable polyurethanes of variable soft segment chemistry and content were synthesized and characterized. An amino acid-based diester chain extender was used to confer degradability and both polycaprolactone diol (PCL) and polyethylene oxide (PEO) were used as soft segments. In addition, the diisocyanate component was a potentially nontoxic diisocyanate (2,6-diisocyanato methyl caproate, LDI). The physicochemical properties of these unique series of polyurethanes were investigated. It was found that the PEO containing polyurethanes were generally weak, tacky amorphous materials. In contrast, the PCL polyurethanes were relatively strong, elastomeric materials which ranged from completely amorphous to semicrystalline as noted by differential scanning calorimetry. The PCL containing polyurethanes exhibited increasing tensile strength, modulus, and ultimate strain with increasing PCL molecular weight because of increasing phase separation and increasing soft segment crystallinity. Fourier transform infrared analysis showed significant hard segment urea and urethane hydrogen bonding which increased with hard segment content, although interphase bonding is believed to be significant for the PCL polyurethanes. Surface characterization carried out by contact angle analysis and X-ray photoelectron spectroscopy indicated soft segment surface enrichment for all of the polyurethanes. The PEO-based polymers were very hydrophilic whereas the PCL-based polymers displayed significantly higher contact angles, indicating greater surface hydrophobicity. The observed diversity in material properties suggests that these polyurethanes may be useful for a wide range of applications. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1522–1534, 2000

Phase transitions of sea-salt/water mixtures at low temperatures: Implications for ozone chemistry in the polar marine boundary layer

Abstract: We present laboratory experiments employing differential scanning calorimetry as well as flow cell microscopy to study the microphysics of aqueous NaCl and sea-salt solutions and droplets at temperatures below 273 K. The freezing and melting points of ice and other precipitates were determined in NaCl and sea-salt bulk samples as well as in emulsion samples. Using flow cell microscopy, we have determined the deliquescence and efflorescence relative humidities of NaCl and sea-salt droplets at temperatures between 249 and 273 K, extending the existing room temperature data to polar conditions. Our measurements suggest that sea-salt aerosols will most likely be liquid most of the time under polar marine boundary conditions. In addition, we show that sea-salt aerosols or seawater spray deposited on the polar ice pack will remain partly liquid down to 230 K, with concentrations of Cl− and Br− increasing by more than an order of magnitude upon cooling when compared to normal seawater concentrations. This is likely to enhance the rate at which heterogeneous bromine activation reactions occur in the sea-salt deposits. Such reaction rate enhancements with decreasing temperatures are currently not implemented in chemical models, and might help explain the fast bromine activation and subsequent ozone destruction observed during ozone depletion events in the polar marine boundary layer in spring.

Effect of high pressure on the physical properties of barley starch

Abstract: The kinetics of the high-pressure gelatinization of barley starch suspensions at two different concentrations (10 and 25%) were studied in the pressure range 400–550 MPa. The pressure-treated samples were examined using rheological methods, microscopy and differential scanning calorimetry (DSC). The rheological properties, the microstructure and loss of birefringence, as well as the melting of amylopectin crystals as determined by DSC, were all both pressure- and time-dependent. For the 10% suspension, only a slight increase in viscosity was observed, while a strong paste with a creamy texture and a maximum storage modulus of approximately 23 kPa was formed during pressurisation of the 25% suspension. The rheological properties and the microstructure of the pressure-treated samples were different from those of the heat-treated samples. The starch granules remained intact after the pressure treatment and no leaching of amylose was observed. The retrogradation of pressure-induced gels was investigated by DSC and was shown to be similar to that of a heat-induced gel.

Thermodynamic Properties of the NaCl + H2O System. 4. Heat Capacities of H2O and NaCl(aq) in Cold-Stable and Supercooled States

Abstract: The heat capacities of water and NaCl(aq) from 0.05 mol·kg-1 to 6 mol·kg-1 were measured from 285 K to a lower temperature of 202 K < T < 236 K, at 0.1 MPa, dependent on composition. These measurements were performed with a differential scanning calorimeter (DSC). A cooling scan method permitted supercooling of water and aqueous solutions well below their normal freezing points. Calibration methods for the DSC and measurement methods for the heat capacity using cooling scans are described. The uncertainties of temperature and heat-flow calibration coefficient for the DSC were ±0.08 K and ±0.3%, respectively. The heat capacity of dilute NaCl(aq) increases with decreasing temperature below 270 K, qualitatively similar to the effect observed for water. At larger concentrations of NaCl(aq), this behavior reverses and the heat capacity only decreases with decreasing temperature. An equation of state for the NaCl + H2O system was generated on the basis of the present measurements as well as other experimental r...

Cold crystallization of water in hydrated poly(2-methoxyethyl acrylate) (PMEA)

Abstract: The structure of water associated with poly(2-methoxyethyl acrylate) (PMEA), which is known to exhibit excellent blood compatibility, has been investigated using differential scanning calorimetry (DSC). The total equilibrium water content (EWC) of PMEA was 9.0 wt%. Water in the PMEA could be classified into three types: non-freezing, freezing-bound and free water. Cold crystallization of water was clearly observed at about −42 °C on heating when the water content was more than 3.0 wt%. Cold crystallization is attributed to the phase transition from the amorphous ice to the crystal ice in PMEA. The relative proportions of freezing-bound water at the EWC is 48 % of all the water in hydrated PMEA. © 2000 Society of Chemical Industry

Effect of Particle Size on Colloidal Solid Triglycerides

Abstract: The size-dependent behavior of different crystalline, colloidal monoacid and complex triglycerides in aqueous dispersion is investigated, mainly by differential scanning calorimetry and X-ray diffraction. With decreasing particle size the melting range of the triglycerides broadens and shifts to lower temperatures. The melting of particles in small-size dispersions (e.g., with mean particle sizes around 100 nm) of even, saturated monoacid triglycerides such as tripalmitin and trimyristin is characterized by a sequence of discrete sharp transitions which are not due to polymorphism. This complex melting process is observed with different types of stabilizers. The sequence of melting events originates from the shape and layered structure of the triglyceride nanocrystals leading to successive discrete transitions corresponding to the melting of particle fractions with different thickness. In dispersions of complex triglycerides, the individual transitions are less sharp or even undetectable. The heat of fusion and the crystallization temperature of the nanoparticles decrease slightly with decreasing particle size whereas the rate of polymorphic transitions increases. Moreover, dispersions containing extremely small nanoparticles display an X-ray diffraction pattern indicating the presence of a crystal form that seems not to have been described previously for these triglycerides.

Crystallization mechanism and properties of a blast furnace slag glass

Abstract: The complex crystallization process of a Brazilian blast-furnace slag glass was investigated using differential scanning calorimetry (DSC), X-ray diffraction, optical microscopy, transmission electron microscopy (TEM), selected area diffraction (SAD), energy dispersive spectroscopy (EDS) and micro-Raman spectroscopy. Three crystalline phases (merwinite, melilite and larnite) were identified after heat treatment between Tg (742°C) and the DSC crystallization peak (T=1000°C). Merwinite was identified as a metastable phase. A small amount (0.004 wt%) of metallic platinum was found in the glass composition. Particles of Pt3Fe, detected by EDS and SAD–TEM, were the starting points of crystallization acting, therefore, as heterogeneous nucleating sites. Only melilite and larnite precipitated in a glass sample heat-treated at 1000°C for 1 h. The flexural strength of this crystallized sample was less than that of the glass, probably due the allotropic phase transformation of larnite.