Showing papers on "Differential scanning calorimetry published in 2003"

New Thermally Remendable Highly Cross-Linked Polymeric Materials

Abstract: Two new remendable highly cross-linked polymers, 2ME4F and 2MEP4F, were prepared without solvent. Solid-state NMR (nuclear magnetic resonance) was used to study the thermal reversibility of Diels−Alder (DA) cross-linking, and it was found that DA connections and disconnections of both polymers are thermally reversible. Differential scanning calorimeter and dynamical mechanical analysis were applied to study thermal and mechanical properties of these materials, and it is found that the glass transition temperature (Tg) of 2ME4F is about 30−40 °C and that of 2MEP4F is about 80 °C. A qualitative study of the healing efficiency of 2MEP4F showed that cracks can be healed effectively with a simple thermal healing procedure. This process can be repeated to heal cracks multiple times.

The thermal and crystallisation studies of short sisal fibre reinforced polypropylene composites

Abstract: The thermal and crystallisation behaviour of sisal/PP composites was studied by thermogravimetry (TG), differential scanning calorimetry (DSC) and polarising optical microscopy. Chemical modifications were made to sisal fibre using a urethane derivative of polypropylene glycol (PPG/TDI), maleic anhydride modified polypropylene (MAPP), and KMnO4 in order to improve the interfacial adhesion between the fibre and matrix. The thermal properties of the blends were analysed by TG analysis. The effects of fibre content and chemical treatments on the thermal properties were evaluated. It was found that treated fibre composites show superior properties compared to the untreated system. DSC measurements exhibited an increase in the crystallisation temperature and crystallinity, upon the addition of fibres to the PP matrix. This is attributed to the nucleating effects of the fibre surfaces, resulting in the formation of transcrystalline regions. On increasing the fibre content, the melting peak of the PP component was shifted to higher temperatures suggesting a constrained melting. The thickness of the transcrystalline layer formed depends on crystallisation temperature and time. The transcrystalline growth rate was slow in the quiescent state. On the other hand, upon the application of stress, transcrystallinity developed quickly. In fact, the shear stress at the polymer/fibre interface initiated the nucleation. Fibre surface modification by PPG/TDI increases the nucleating ability of sisal fibre to a very small extent.

Thermal properties of thermoplastic starch/synthetic polymer blends with potential biomedical applicability.

Abstract: Previous studies shown that thermoplastic blends of corn starch with some biodegradable synthetic polymers (poly(e-caprolactone), cellulose acetate, poly(lactic acid) and ethylene-vinyl alcohol copolymer) have good potential to be used in a series of biomedical applications. In this work the thermal behavior of these structurally complex materials is investigated by differential scanning calorimetry (DSC) and by thermogravimetric analysis (TGA). In addition, Fourier-transform infrared (FTIR) spectroscopy was used to investigate the chemical interactions between the different components. The endothermic gelatinization process (or water evaporation) observed by DSC in starch is also observed in the blends. Special attention was paid to the structural relaxation that can occur in the blends with poly(lactic acid) at body temperature that may change the physical properties of the material during its application as a biomaterial. At least three degradation mechanisms were identified in the blends by means of using TGA, being assigned to the mass loss due to the plasticizer leaching, and to the degradation of the starch and the synthetic polymer fractions. The non-isothermal kinetics of the decomposition processes was analyzed using two different integral methods. The analysis included the calculation of the activation energy of the correspondent reactions.

Determination of the crystalline phases of poly(vinylidene fluoride) under different preparation conditions using differential scanning calorimetry and infrared spectroscopy

Abstract: A method with good precision has been developed to quantitatively measure the degree of α-, β-, and γ crystallinity in poly(vinylidene fluoride) (PVDF) by means of infrared spectroscopy. The phase composition of solution-deposited PVDF films was found to be strongly influenced by the presence of hydrophilic residues on the silicon substrate, the relative humidity present at film deposition, the spatial position on the substrate, and the thermal treatment of the deposited film. Films produced on pristine surfaces gave predominantly α-phase PVDF, but when a layer of polar solvent (acetone or methanol) remained on the surface, the films produced were predominantly γ phase. Higher humidity promoted a higher fraction of γ crystallinity in the solution-deposited PVDF films. Solution-cast films had highly variable composition across the substrate, whereas spin-cast films were uniform. High-temperature annealing of PVDF films normally converts the polymer to the γ phase, but annealing the film while still attached to the silicon substrate inhibited this phase transformation. Low-temperature annealing of freestanding films led to a previously unreported thermal event in the DSC, a premelting process that is a kinetic event, assigned to a crystalline relaxation. Higher-temperature annealing gave a double endotherm, assigned to melting of different-sized crystalline domains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1093–1100, 2003

Mechanical, Physical, and Barrier Properties of Poly(Lactide) Films

Abstract: High molecular mass poly(lactide), (PLA), is an attractive polymer family because in addition to being thermoplastic, biodegradable, compostable, and produced from annually renewable resources, it shows mechanical and barrier behavior comparable to synthetic polymers like polystyrene (PS) and polyethylene terephthalate (PET). Furthermore, technology for large-scale fabrication of PLA has been fully developed. However, there is still a need to better understand the properties of PLA as this polymer is adapted to packaging applications, especially for food packaging. In this work, films from two PLA resins were studied by tensile testing; differential scanning calorimetry (DSC); and permeation of carbon dioxide, oxygen, and water vapor. The data from these two PLA film samples are compared to those of PS and PET.

Structure and Mechanical Properties of Poly(D,L-lactic acid)/Poly(ε-caprolactone) Blends

Abstract: A series of blends of the biodegradable polymers poly(D,L-lactic acid) and poly( epsilon -caprolactone) were prepared by varying mass fraction across the range of compositions. Tensile testing was performed at room temperature using an extensometer and the elastic modulus was calculated for each blend. The blends were also tested to failure, and the strain-at-failure and yield stress recorded. While the blend has been shown to have a lower critical solution temperature, the mechanical properties were insensitive to the annealing conditions. Scanning electron microscopy was used to characterize the blend microstructure and poor adhesion was observed at the interface between blend components. Differential scanning calorimetry was performed but the results were somewhat variable, indicating this blend may have complex phase behavior that depends sensitively on the method of preparation. However, nuclear magnetic resonance data indicate the two components are phase separated. A percolation model is used to explain the observed mechanical data and the results are consistent with the predictions of the Kerner-Uemura-Takayangi model. The results of these experiments demonstrate the utility of polymer blending in tuning material properties.

Nucleation ability of multiwall carbon nanotubes in polypropylene composites

Abstract: The nonisothermal crystallization of multiwall carbon nanotube (MWNT)/isotactic polypropylene (iPP) nanocomposites was investigated The results derived from the differential scanning calorimetry curves (onset temperature, melting point, supercooling, peak temperature, half-time of crystallization, and enthalpy of crystallization) were compared with those of neat iPP The data were also processed according to Ozawa's theory and Dobreva's approach These results and X-ray diffraction data showed that the MWNTs acted as α-nucleating agents in iPP Accordingly, MWNT/iPP was significantly different from neat iPP: A fibrillar morphology was observed instead of the usual spherulites © 2003 Wiley Periodicals, Inc J Polym Sci Part B: Polym Phys 41: 520–527, 2003

The Binary System Isotactic Polypropylene/Bis(3,4-dimethylbenzylidene)sorbitol: Phase Behavior, Nucleation, and Optical Properties

Abstract: The phase behavior of the binary system consisting of the commercial nucleating and clarifying agent 1,3:2,4-bis(3,4-dimethyldibenzylidene)sorbitol (DMDBS, Millad 3988) and isotactic polypropylene (i-PP) was investigated over the entire concentration range by means of differential scanning calorimetry (DSC), rheology, and optical microscopy. Experimental phase diagrams were constructed from data obtained in melting and crystallization studies, and a simple binary monotectic is advanced. Distinct regimes in the phase diagram, which apparently dictate nucleation and clarification of i-PP by DMDBS, are discussed. A maximum increase in the crystallization temperature of i-PP due to the nucleating action of DMDBS was observed in compositions containing between 0.2 and 1 wt % of the latter. Liquid−liquid phase separation was observed at elevated temperatures for i-PP/DMDBS mixtures comprising more than 2 wt % of DMDBS. A study of the optical properties of the i-PP/DMDBS system revealed that values for haze and ...

Thermodynamic Properties of 1-Butyl-3-methylimidazolium Hexafluorophosphate in the Condensed State

Abstract: Thermodynamic functions for 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) are reported in a range of temperatures from (5 to 550) K, based on new measurements by calorimetry. Heat capacities of the crystal, glass, and liquid phases for [C4mim][PF6] were measured with a pair of calorimeters. A vacuum-jacketed adiabatic calorimeter was used at temperatures between (5 and 310) K, and a heat bridge-scanning calorimeter was used from (300 to 550) K. With the adiabatic calorimeter, the fusion Tfus = 283.51 K, = 19.60 kJ·mol-1, and the glass transition Tg = 190.6 K were observed. The [C4mim][PF6] test sample was determined to have a mole fraction purity of 0.9956 by a fractional melting analysis. Densities of the liquid were measured in a range of temperatures from (298 to 353) K with a pycnometer equipped with a capillary neck. An unexpected endothermal transition, with a very small enthalpy change of 0.25 J·g-1 (0.071 kJ·mol-1), was observed in a range of temperatures from (394 to 412) K. He...

Mechanism of high-temperature CO2 sorption on lithium zirconate.

Abstract: Lithium zirconate (Li2ZrO3) is one of the most promising materials for CO2 separation from flue gas at high temperature. This material is known to be able to absorb a large amount of CO2 at around 400-700 degrees C. However, the mechanism of the CO2 sorption/desorption process on Li2ZrO3 is not known yet. In this study, we examined the CO2 sorption/desorption mechanism on Li2ZrO3 by analyzing the phase and microstructure change of Li2ZrO3 during the CO2 sorption/desorption process with the help of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses. Li2ZrO3 powders were prepared from lithium carbonate (Li2CO3) and zirconium oxide (ZrO2) by the solid-state method, and the CO2 sorption/desorption property was examined by TGA. It was shown that pure Li2ZrO3 absorbs a large amount of CO2 at high temperature with a slow sorption rate. Addition of potassium carbonate (K2CO3) and Li2CO3 in the Li2ZrO3 remarkably improves the CO2 sorption rate of the Li2ZrO3 materials. DSC analysis for the CO2 sorption process indicates that doped lithium/potassium carbonate is in the liquid state during the CO2 sorption process and plays an important role in improving the CO2 uptake rate. XRD analysis for phase and structure change during the sorption/desorption process shows that the reaction between Li2ZrO3 and CO2 is reversible. Considering all data obtained in this study, we proposed a double-shell model to describe the mechanism of the CO2 sorption/desorption on both pure and modified Li2ZrO3.

Role of Bound Water and Hydrophobic Interaction in Phase Transition of Poly(N-isopropylacrylamide) Aqueous Solution

Abstract: It has been suggested that the phase transition of the poly(N-isopropylacrylamide) (PNiPAM) aqueous solution consists of at least two different processes: (1) a rearrangement of bound water around either hydrophobic or hydrophobic and hydrophilic group of PNiPAM and (2) a hydrophobic association of the hydrophobic groups in the PNiPAM chains or a residual interaction between the PNiPAM side chain residues. However, a single endothermic peak does not provide direct evidence for this suggestion. This study shows that the phase transition in PNiPAM aqueous solution comprises at least two processes by using temperature modulated differential scanning calorimetry. The single endothermic peak is separated into an endothermic and exothermic peak, and the heats from these peaks depend on the applied frequency. Curve fitting to experimental data interprets the transition mechanism of the PNiPAM aqueous solution in more detail, and the results and the limitation of the curve fitting are discussed.

Swelling behavior of interpenetrating polymer network hydrogels composed of poly(vinyl alcohol) and chitosan

Abstract: Interpenetrating polymer network (IPN) hydrogels composed of poly(vinyl alcohol) (PVA) and chitosan were prepared by UV irradiation. The characteristics of IPN hydrogels were investigated by swelling experiments, Fourier transform infrared (FT-IR) spectroscopy and differential scanning calorimetry (DSC). IPN hydrogels exhibited a relatively high swelling ratio in the range of 210–350% at 35 °C. The swelling ratio of PVA/chitosan IPN hydrogels depended on pH and temperature. DSC was used for the quantitative determination of the amounts of freezing and non-freezing water. Free water contents in IPN1, IPN2 and IPN3 were 62.0, 69.6 and 72.3% in pure water, respectively.

Restricted Relaxation in Polymer Nanocomposites near the Glass Transition

Abstract: The relaxation behavior of organically modified layered silicate−epoxy nanocomposites was studied using a combination of standard and temperature-modulated differential scanning calorimetry. For such nanocomposites, the silicate layers were intercalated in the matrix resin and epoxy networks were grafted onto the silicate layer surfaces. Enthalpy recovery that occurred during physical aging was used as a probe to detect restricted relaxation behavior in the nanocomposites. Addition of the intercalated nanoparticles resulted in a slower overall relaxation rate and a wider distribution of relaxation times. The nanocomposites also showed a higher glass transition temperature compared to that of the unreinforced resin. To explain the observed results, a domain relaxation model was proposed that included three possible relaxation modes. On the basis of this model, the restricted relaxation arising from intercalated and exfoliated layered silicates can be understood.

Glass transition in ultrathin polymer films: calorimetric study.

Abstract: The ultrasensitive differential scanning calorimetry is used to observe the glass transition in thin (1-400 nm) spin-cast films of polystyrene, poly (2-vinyl pyridine) and poly (methyl methacrylate) on a platinum surface. A pronounced glass transition is observed even at a thickness as small as 1-3 nm. Using the high heating (20-200 K/ms) and cooling (1-2 K/ms in glass transition region) rates which are typical for this technique, we do not observe appreciable dependence of the glass transition temperature over the thickness range from hundreds of nanometers down to 3 nm thick films. The evolution of calorimetric data with film thickness is discussed in terms of broadening of transition dynamics and loss of transition contrast.