Showing papers on "Differential scanning calorimetry published in 1996"

Differential scanning calorimetry

Abstract: 1 Introduction.- 2 Types of Differential Scanning Calorimeters and Modes of Operation.- 3 Theoretical Fundamentals of Differential Scanning Calorimeters.- 4 Calibration of Differential Scanning Calorimeters.- 5 DSC Curves and Further Evaluation.- 6 Applications of Differential Scanning Calorimetry.- 7 Evaluation of the Performance of a Differential Scanning Calorimeter.- Appendix 1.- Appendix 2.- References.

Synthesis and Characterization of Hydrophobic, Organically-Soluble Gold Nanocrystals Functionalized with Primary Amines

Abstract: We report on the synthesis and characterization of 25−70 A diameter amine-capped gold nanocrystals. In particular, we show how these particles can be prepared by a simple procedure and confirm the particle composition (including the identity of the amine surface passivant) through several materials characterization techniques that include infrared spectroscopy, nuclear magnetic resonance spectroscopy, ultraviolet−visible spectroscopy, mass spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray diffraction analysis, differential scanning calorimetry, thermogravimetric analysis, and elemental analysis. All physical characterizations are consistent with a charge−neutral amine/gold surface interaction described by a weak covalent bond. The stability of the particles appears to be largely kinetic, rather than thermodynamic, in nature. Comparison of these nanocrystals to amines adsorbed onto bulk Au surfaces indicates that the stability of the nanocrystal/amine system is a finite-size effect.

Mechanism and Kinetics of Epoxy-Amine Cure Studied by Differential Scanning Calorimetry

Abstract: The isoconversional kinetic analysis has been applied to nonisothermal DSC data on the cure of an epoxynovolac resin. The process reveals a dependence of the activation energy (Eα) on conversion (α). The shape of the dependence has been interpreted in the terms of the reaction mechanisms. It has been found that the model dα/dt = (k1 + αmk2)(1 − α)n used for the kinetically controlled cure gives rise to the dependence of Eα on α similar to the experimentally found one. To completely describe the diffusion-controlled cure, the effect of both T and α on the change in diffusivity has been taken into account. The equation for the specific rate constant of diffusion, kD(T,α) = Do exp(−ED/RT + Kα), has been induced. Its use allows us to obtain a model dependence of Eα on α closely matching the experimental one. A technique of predicting isothermal cure from the sole dependence of Eα on α has been considered.

Crystallization from the melt of poly(lactide)s with different optical purities and their blends

Abstract: Poly(lactide)s (PLA) with different optical purities (OP) were synthesized by copolymerization of D-lactide and L-lactide. Crystallization of D- and L-lactide-rich copolymers before and after 1 : 1 blending was studied from the melt over a wide range of annealing temperatures (Ta) using differential scanning calorimetry and polarizing microscopy. Melting temperature and crystallinity increased with an increase in OP for both the non-blended and the 1 : 1 blended PLAs and they lost their crystallizability when their OP became smaller than 76%, suggesting that the critical isotactic sequence length of PLA for crystallization was approximately 15 isotactic lactate units. Normal spherulite formation was observed for both the non-blended and the blended PLAs even when the OP of PLAs decreased to 76%, whereas the spherulitic size of the non-blended PLAs was larger than that of the 1 : 1 blended PLAs when compared at the same OP and Ta. The highest Ta for crystallization increased with increasing OP and was higher for the 1 : 1 blended PLAs than for the non-blended PLAs at a fixed OP, indicating that the crystallizable Ta range was extended to higher temperature by blending D- and L-lactide-rich PLAs. This is ascribed to stereocomplex (racemic crystallites) formation at high Ta from the 1 : 1 blended PLAs. Based on these results, the phase diagram for crystallization was depicted for the non-blended and the 1 : 1 blended PLAs.

Thermal Effects in Cellulose Pyrolysis: Relationship to Char Formation Processes

Abstract: The thermochemistry of cellulose pyrolysis has been studied by a combination of differential scanning calorimetry and thermogravimetric analysis. Additionally, the vapor pressure and heat of vaporization of levoglucosan have been determined by an effusion method. The cellulose pyrolysis has been carried out under inert gas at heating rates from 0.1 to 60 K/min. The main cellulose thermal degradation pathway is endothermic, in the absence of mass transfer limitations that promote char formation. The endothermicity is estimated to be about 538 J/g of volatiles evolved. It is concluded that this endothermicity mainly reflects a latent heat requirement for vaporizing the primary tar decomposition products. Pyrolysis can be driven in the exothermic direction by char-forming processes that compete with tar-forming processes. The formation of char is estimated to be exothermic to the extent of about 2 kJ/g of char formed. Low heating rates, in concert with mass transfer limitations, serve to drive the pyrolysis ...

Colloidal copolymer microgels of N-isopropylacrylamide and acrylic acid: pH, ionic strength and temperature effects

Abstract: Aqueous colloidal microgels have been prepared, based on poly(N-isopropylacrylamide)[poly(NIPAM)], cross-linked with bisacrylamide, containing 5% w/w acrylic acid (AAc) as a comonomer. Transmission electron micrographs of the microgels show that the copolymer microgels are monodisperse spheres. The size of the microgel particles containing 5% AAc has been studied, using dynamic light scattering, as a function of pH (3–10), ionic strength (10–4–10–1M NaCl) and temperature (20–75 °C). The hydrodynamic diameter of the copolymer microgels decrease both with increasing ionic strength (at pH 6 and 25 °C) and reversibly with increasing temperature at pH values of 2.6, 3.4 and 6.5. However, under isothermal conditions in 10–3M NaCl at 25 °C, the hydrodynamic diameter increases in going from pH 3.3–9.4. In addition, the temperature-induced conformational changes in the polymer chains have been followed using high-sensitivity differential scanning calorimetry (HSDSC). A comparison is made with the behaviour of poly(NIPAM) microgel particles, not containing AAc. Explanations are offered to account for the pronounced difference in the physico-chemical properties observed.

Vitrification and Crystallization of Organic Liquids Confined to Nanoscale Pores

Abstract: The effect of finite size on the solidification of o-terphenyl and benzyl alcohol confined in model controlled pore glass (CPG) materials is described. These two organic liquids form either amorphous glasses or crystalline solids in the bulk upon cooling, depending on the rate of cooling and other factors. The solidification behavior of the liquid in the pores was studied as a function of pore diameter (4−73 nm), chemical surface treatment of the CPG and the degree of pore filling, by differential scanning calorimetry (DSC). We observe that the glass transition, Tg, shifts to a lower temperature as pore size decreases. This shift is independent of the degree of pore filling for both o-terphenyl and benzyl alcohol, suggesting that a reduction in bulk density or a negative pressure effect is not the cause of the observed shift. The crystallization behavior of o-terphenyl and benzyl alcohol is also altered by confinement and strongly depends on the pore size and degree of pore filling.

Effects of physical aging, crystallinity, and orientation on the enzymatic degradation of poly(lactic acid)

Abstract: The effects of physical aging, degree of crystallinity, and orientation of poly(lactic acid) (PLA) were studied using differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The samples of PLA with 96% [L] and 4% [D] contents were prepared by injection molding. The physical aging of PLA strongly depended on time and temperature. The change of rate of physical aging was very fast initially and slowed down as time increased. The enzymatic degradation of PLA was carried out with proteinase K at 37°C at a pH value of 8.6 in a Tris/HCl buffer solution. The enzymatic degradation rate was found to decrease as a function of physical aging (i.e., excess enthalpy relaxation). The rate of enzymatic degradation of PLA decreased with the increase in crystallinity. A threshold was observed when the heat of fusion was less than 20 J/g. The weight loss of PLA with a low level of crystallinity had no apparent change during any period of testing time. The rates of enzymatic degradation of stretched and injection-molded specimens were comparable.

Thermal properties of copolymer gels containing N-isopropylacrylamide

Abstract: The phase transition of gels containing N-isopropylacrylamide (NIPA) was investigated by differential scanning calorimetry (DSC) and swelling measurements. The enthalpy of dissociation of the hydro...

Dissolution mechanism of semicrystalline poly(vinyl alcohol) in water

Abstract: Changes occurring in the degree of crystallinity and lamellar thickness distribution of poly(vinyl alcohol) (PVA) samples during dissolution in water were investigated. PVA samples of three different molecular weights were crystallized by annealing at 90, 110, and 120°C. The initial degrees of crystallinity measured by differential scanning calorimetry (DSC) and by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) varied from 43 to 60% and the average lamellar thicknesses measured by DSC ranged from 50 to 400 A. PVA dissolution was followed at 25, 35, and 45°C from 30 s up to 195 min. Lamellar thicknesses were determined as a function of dissolution time using DSC. There was an initial drastic decrease in the degree of crystallinity, which leveled off to a fairly constant value before reaching zero by the time the polymer dissolved completely. Increase in molecular weight led to lesser number of crystals, but with larger average lamellar thickness, which were more stable in the presence of water. Increase in crystallization temperature or decrease in dissolution temperature led to larger average lamellar thickness. Based on these findings, a dissolution mechanism involving unfolding of the polymer chains of the crystal was proposed.

Formation of nanocrystals based on decomposition in the amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy

Abstract: Primary crystallization and decomposition in the bulk amorphous alloy Ar41.2Ti13.8Cu12.5Ni10Be22.5 have been studied by small angle neutron scattering (SANS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). SANS data of samples annealed isothermally at 623 K exhibit an interference peak centered at q=0.46 nm(^-1) after an incubation time of about 100 min. TEM and DSC investigations confirm that the respective periodic variation in the scattering length density is due to the formation of nanocrystals embedded in the amorphous matrix. These observations suggest that during the incubation time a chemical decomposition process occurs and the related periodic composition fluctuations give rise to the observed periodic arrangement of the nanocrystals.

Incorporation of poly(acrylic acid), poly(vinylsulfonate) and poly(styrenesulfonate) within layered double hydroxides

Abstract: Poly(acrylic acid), poly(vinylsulfonate) and poly(styrenesulfonate) have been incorporated between the positively charged sheets of layered double hydroxides (LDHs)M1–xAlx(OH)2+(M = Mg, Ca, Co) and Zn1–xM′x(OH)2+(M′= Al, Cr) to form layered nanocomposites. The resulting nanocomposites contained the LDH sheet structures separated by 7.6–16.0 A, which is sufficient to accommodate polymer bilayers between the LDH sheets. Preparations were carried out in deaerated aqueous base by a template reaction, involving the formation and precipitation of nanocomposites from metal nitrate-salt precursors in the presence of the dissolved polymer. Structural and compositional details were provided by X-ray diffraction (XRD), FTIR spectroscopy, elemental analysis, differential scanning calorimetry (DSC) and thermogravimetry (TG). Scanning electron microscopy (SEM) indicates that the nanocomposition of LDHs with ionomers significantly alters the particle microstructure from that of the LDH carbonates derived from aqueous precipitation.

N-Palmitoyl Sphingomyelin Bilayers: Structure and Interactions with Cholesterol and Dipalmitoylphosphatidylcholine†

Abstract: The structure and thermotropic properties of N-palmitoyl sphingomyelin (C16:0-SM) and its interaction with cholesterol and dipalmitoylphosphatidylcholine (DPPC) have been studied by differential scanning calorimetry (DSC) and X-ray diffraction methods. DSC of hydrated multi-bilayers of C16:0-SM shows reversible chain-melting transitions. On heating, anhydrous C16:0-SM exhibits an endothermic transition at 75 degrees C (delta H = 4.0 kcal/mol). Increasing hydration progressively lowers the transition temperature (TM) and increases the transition enthalpy (delta H), until limiting values (TM = 41 degrees C, delta H = 7.5 kcal/mol) are observed for hydration values > 25 wt % H2O. X-ray diffraction at temperatures below (29 degrees C) TM show a bilayer gel structure (d = 73.5 A, sharp 4.2 A reflection) for C16:0-SM at full hydration; above TM, at 55 degrees C, a bilayer liquid-crystal phase is present (d = 66.6 A, diffuse 4.6 A reflection). Addition of cholesterol to C16:0-SM bilayers results in a progressive decrease in the enthalpy of the transition at 41 degrees C, and no cooperative transition is detected at > 50 mol % cholesterol. X-ray diffraction shows no difference in the bilayer periodicity, position/width of the wide-angle reflections, or electron density profiles at 29 and 55 degrees C when 50 mol % cholesterol is present. Thus, cholesterol inserts into C16:0-SM bilayers progressively removing the chain-melting transition and changing the structural characteristics of the bilayer. DSC and X-ray diffraction data show that DPPC is completely miscible with C16:0-SM bilayers in both the gel and liquid-crystalline phases; however, 30 mol % C16:0-SM removes the pre-transition exhibited by DPPC.

Host‐dependent optical transitions of Er3+ ions in lead–germanate and lead‐tellurium‐germanate glasses

Abstract: Differential scanning calorimetry, Raman scattering, optical absorption, and upconversion of infrared to green luminescence have been studied for Er3+‐doped lead‐germanate glass 55GeO2⋅20PbO⋅10BaO⋅10ZnO⋅5K2O (GPBZK) and lead‐tellurium‐germanate glass 30GeO2⋅30PbO⋅30TeO2⋅10CaO (GPTC). Judd–Ofelt intensity parameters of Er3+ in the two host glasses were determined and used to calculate radiative transition rates and lifetimes. Values of the radiative quantum yield of the 4S3/2→4I15/2 transition and the infrared (797 nm) to green (547 nm) upconversion efficiency of Er3+ were obtained. It has been found that the 4S3/2→4I15/2 radiative transition rate of Er3+ in GPTC glass is about twice that in the GPBZK glass and the upconversion efficiency in the GPTC glass is about four times larger than that in the GPBZK glass. These host‐dependent properties are mainly attributed to the enhanced local field and the reduced multiphonon rate in lead‐tellurium‐germanate glass compared to lead‐germanate glass.

Novel cellulose derivatives. IV. Preparation and thermal analysis of waxy esters of cellulose

Abstract: Cellulose esters with linear aliphatic acyl substituents ranging in size from C12 (lauric acid) to C20 (eicosanoic acid) were prepared in homogeneous solution (DMAc/LiCl) using a novel synthetic method based on the use of a mixed p-toluenesulfonic/carboxylic acid anhydride. The resulting waxy cellulose esters had a high degree of substitution (DS), between 2.8 and 2.9, and showed little degradation. Thermal analysis of these cellulose derivatives by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) revealed a series of transitions that represented motion by both ester substituents and cellulosic main chain. Broad crystallization and melting transitions attributed to side-chain crystallinity were observed in the range between −19 and +55°C; these side-chain Tm and Tc transition temperatures increased by 10°C per carbon atom of the ester substituent. The Tg of these derivatives increased linearly with increasing substituent size from 94°C for C12 (cellulose laurate) to 134°C for C20 (cellulose eicosanoate). Evidence of “main-chain” crystallization was not observed for these samples, except in the case of peracetylated C12 and C14 esters, which had Tm values of 96°C and 107°C, respectively. © 1996 John Wiley & Sons, Inc.

Development and characterization of high-performance polybenzoxazine composites

Abstract: To develop high-performance composites with high temperature resistance, two newly synthesized polybenzoxazines were successfully cured with carbon fiber to obtain composites with 60 vol% fiber. Results from differential scanning calorimetry studies were used to modify the benzoxazine monomers to improve processability in terms of melting point and solubility. The density and void content of these composites were measured. Dynamic mechanical tests were performed to determine the glass transition temperature (T g ) and the activation enthalpy of the glass transition process for these two composites. The effect of cure temperature on the T g of the composites was investigated. Thermal characteristics were studied by means of dynamic mechanical analysis in terms of isothermal aging, decomposition temperature from thermogravimetric analysis, and storage moduli change at high temperatures. Mechanical evaluations of these composites were conducted by flexural and interlaminar shear tests. The mechanical and thermal properties of these two composites exceed bismaleimide composites and compete with polyimide composites, while exhibiting easier processability than polyimides.

How does “phase transformation toughening” work in semicrystalline polymers?

Abstract: The possibility of phase transformation toughening is demonstrated by the example of the β-modification of isotactic polypropylene (β-iPP), which undergoes βα-transformation (i.e., from hexagonal to monoclinic) during mechanical loading. The resulting α-iPP exhibits a higher crystalline density than the initial β-modification. That, along with the exothermic character of the βα-recrystallization, is responsible for the improvement in toughness that occurs. The occurrence of this βα-transformation is evidenced by differential scanning calorimetry (DSC). Toughness of the α- and β-iPP is studied and compared with the “essential work of fracture” concept by using static-loaded deeply double-edge-notched tensile (DDEN-T) specimens. The main effect of the βα-transformation is a large increase in the specific plastic work consumed in the necked zone. Light microscopic (LM) and infrared thermographic (IT) pictures reveal that the plastic zone becomes larger and its shape more circular when βα-transformation takes place. It is suggested that the principle of mechanical stress-induced phase transformation from a less toward a more dense crystalline state may be a universal tool for toughness upgrading in semicrystalline polymers.

Characteristic length of the glass transition

Abstract: The definition of molecular cooperativity is discussed. The characteristic length of the glass transition describes the size of this cooperativity. Differential scanning calorimetry (DSC) and heat capacity spectroscopy (HCS) results of a series of poly(n-alkyl methacrylates) (alkyl = methyl, ethyl, propyl, butyl, pentyl, hexyl, and octyl) and a series of statistical copolymers poly(n-butylmethacrylate-stat-styrene) are discussed in terms of molecular cooperativity in the αβ splitting region, where a high-frequency dispersion zone a splits off into the main transition zone α and a Goldstein Johari process β at lower frequencies. The characteristic length tends to small values of order one monomer diameter in the splitting region for scenarios with an α relaxation onset. The statements about the size scale of cooperativity are conditional upon certain assumptions leading to the equation used for calculation of this size from HCS and DSC data. The step height of heat capacity (Δcp) and, with less certainty, the square root of the cooperativity volume or number (V1/2α or 1/2α) are proportional to the temperature distance from the cooperativity onset, T = Tons. © 1996 John Wiley & Sons, Inc.