StepScan DSC Research Articles

Thermoanalytical properties and structure of (As2Se3)100 − x(Sb2Se3)x glasses by Raman and 77Se MAS NMR using a multivariate curve resolution approach

The basic thermoanalytical characteristics and structure of (As2Se3)100−x(Sb2Se3)x bulk glasses over the whole glass-forming region (x=0–50) were studied by DSC, StepScan DSC and both Raman and 77Se MAS NMR spectroscopies. The glass transition temperature, Tg, was found to increase only slightly from 184 to 190°C with increasing Sb2Se3 content, and the changes in the isobaric heat capacity at Tg are also nearly compositionally independent. However, the isobaric heat capacity of glasses decreases because of the higher atomic mass of antimony. The undercooled melts undergo crystallization when heated; As2Se3 crystallizes for x≤20, and both stoichiometric triselenides crystallize for higher x.77Se MAS NMR spectroscopy was used for the first time in the structural study of this pseudobinary system. The results of both NMR and Raman spectroscopies were combined, and their chemometric analysis was used to elucidate the structure of the glasses over the whole glass-forming region. It was concluded that the structure is based on randomly distributed AsSe3/2 and SbSe3/2 trigonal pyramids bridged by common selenium atoms and corresponds to the transition structure between a “random covalent network model” (RCNM) and a “chains crossing model” (CCM).

Crystallization and Melting Behavior of Poly(Butylene Succinate) Nanocomposites Containing Silica-Nanotubes and Strontium Hydroxyapatite Nanorods

The multiple melting behavior of poly(butylene succinate) (PBSu) nanocomposites containing silica nanotubes (SiNTs) and strontium hydroxyapatite nanorods (SrHNRs) was studied with Step Scan DSC. In the reversing signal curves, recrystallization proved to be significant for samples crystallized at low temperature, which lead to large supercooling, favoring fast crystallization and formation of poor crystals that suffer recrystallization on subsequent heating scans. The crystallization study, under both isothermal and nonisothermal conditions, evidenced that only SiNTs act effectively as nucleating agents. Consequently, crystallization of the nanocomposite samples filled with 5 and mainly with 20 wt % SiNTs occurred faster than that of the neat polymeric matrix. The insertion of SrHNRs slightly lowered the crystallization rate of PBSu due to the formation of larger aggregates inside the PBSu matrix, which may inhibit crystallization. The size of crystals was smaller for PBSu/SiNTs samples, as it was proved by wide angle X-ray diffracrion (WAXD), whereas the addition of SrHNRs did not significantly affect the crystalline size of nanocomposites.

Competitive Crystallization of a Propylene/Ethylene Random Copolymer Filled with a β-Nucleating Agent and Multi-Walled Carbon Nanotubes. Conventional and Ultrafast DSC Study

A propylene/ethylene polymeric matrix was reinforced by the simultaneous addition of a β-nucleating agent (calcium pimelate) and multi-walled carbon nanotubes (MWCNTs) in various concentrations. The present manuscript explores the competitive crystallization tendency that is caused by the presence of the two fillers. On the one hand, calcium pimelate forces the material to crystallize predominantly in the β-crystalline form, while, on the other, the strong α-nucleating ability of MWCNTs compels the material to develop higher α-crystalline content. An in-depth study has been performed on the nanocomposite samples by means of conventional, temperature-modulated, and differential fast scanning calorimetry (DFSC) under various dynamic and isothermal conditions. The results showed that β-crystals are predominant at low MWCNT content (<2.5 wt %), while, at high MWCNT content, the material crystallizes mainly in the α-form. The recrystallization phenomenon during melting was confirmed with step-scan DSC, and the use of very high cooling rates by UFDSC made it possible to achieve and study the nucleation of the samples. The presence of MWCNTs enabled the nanocomposites to crystallize faster under both isothermal and dynamic conditions. The activation energy of the samples was also calculated according to Friedman's theory.

Crystallization and melting of propylene–ethylene random copolymers. Homogeneous nucleation and β-nucleating agents

The different crystallization behavior of two propylene–ethylene random copolymers, (PP-R and PP-RCT), one of which (PP-RCT) contains a β-nucleating agent is investigated in the present paper. Isothermal and non-isothermal crystallization behavior was thoroughly studied. The subsequent melting behavior after isothermal crystallization was studied by means of standard and Step-Scan DSC. The equilibrium melting point of α crystals determined using the Hoffman–Weeks extrapolation was 11.8°C higher than that for β crystals. The Lauritzen–Hoffman theory was applied and the nucleation constant Kg and the surface free energy σe was calculated. From the isothermal experiments it was concluded that the nucleating agent increased drastically the crystallization rate of the copolymer, also promoting the generation of β crystals. On cooling PP-RCT crystallized at about 8°C higher temperatures than PP-R. The non-isothermal crystallization of the materials was examined and for the kinetics analysis the Avrami, Ozawa and Mo models were elaborated. Nucleation activity was calculated with the use of Dobreva’s method. The effective activation energy was calculated for both polymers according to the iso-conversional method of Friedman and lower values resulted as expected in case of the PP copolymer with nucleating agent.

Synthesis, Crystallization, and Enzymatic Degradation of the Biodegradable Polyester Poly(ethylene azelate)

AbstractPoly(ethylene azelate) (PEAz) was synthesized and its melting behavior was studied with DSC and Step Scan DSC. The equilibrium melting point of PEAz was found 62 °C and the heat of fusion of 100% crystalline polymer was estimated at 160 J · g−1. Polarized light microscopy showed mixed birefringence. From isothermal crystallization DSC study, after self‐nucleation, crystallization rates were estimated. The Lauritzen–Hoffman plots showed crystallization regime I to II and II to III transition at 34 and 19 °C respectively. Non‐isothermal crystallization was studied. The activation energy was calculated applying the isoconversional method of Friedman. Chemical hydrolysis of PEAz was very slow, while enzymatic hydrolysis showed comparable rates with PCL. magnified image

Nanoscale cooperativity on a series of statistical methacrylates copolymers with electron donor–acceptor pendant groups

Standard and StepScan DSC studies have been performed on a series of statistical methacrylate copolymers with electron-donor and electron-acceptor pendant groups that form intramolecular electron transfers. From standard DSC analysis we concluded that glass transition temperature slowly increased with increasing electron-acceptor monomeric moiety ratio up to 0.5 in the main chain. Using StepScan DSC method we calculated the size and volume of cooperative rearranging region as well mean temperature fluctuation at glass transition temperature. It was estimated also the average number of monomer units in the cooperative rearranging region. All parameters were calculated according to the method proposed by Donth based on Heat Capacity Spectroscopy. The results show that the presence of intermonomeric electron transfers decreased the chain mobility, as well as the cooperativity of relaxation processes of these structures in the glass transition range. This is reflected by minimal values of these parameters around 0.4 ratio of copolymer composition. Such behavior is similar to that of crosslinked or confined systems (e.g., nanocomposites, thin films) that have reduced chain mobility.

New features of the glass transition revealed by the StepScan® DSC

Abstract The results of StepScan DSC obtained for various oxides, chalcogenides, and organic glasses are discussed in connection with the commonly accepted theory of the glass transition. The new experimental features supporting the apparent idea of a reversible equilibrium being a part of the glass transition that is commonly interpreted as purely kinetic-relaxation phenomenon are discussed. Two alternative methods of the description of the reversible part of StepScan DSC record are compared:the empirical one using the exponential-power function [1 − exp(T/T g) n ], and the second one based on the van’t Hoff’s equation describing the temperature dependence of equilibrium constant in terms of reaction enthalpy, ΔH. The adequacy of the empirical description is rationalized in the framework of the Tool–Narayanaswamy–Moynihan’s relaxation theory.

The selenium based chalcogenide glasses with low content of As and Sb: DSC, StepScan DSC and Raman spectroscopy study

Thermal properties and structure of As x Se 100− x and Sb x Se 100− x glass-forming systems ( x = 0, 1, 2, 4, 8 and 16) were studied by conventional and StepScan DSCs and Raman spectroscopy. Compositional dependence of the glass transition temperature, T g, was determined from reversible part of StepScan DSC records and discussed. The attention was also focused on the crystallization of undercooled melts of these systems. It was found that only selenium crystallizes from undercooled melts of As–Se system and its tendency to crystallize decreases markedly with increasing As content, for arsenic content higher than 4 at.% no crystallization was observed. In the case of Sb–Se system Sb 2Se 3 crystallizes in the first step followed by trigonal selenium crystallization from non-stoichiometric undercooled melt. Sb 2Se 3 crystallizes from incongruent melt with crystallization enthalpy Δ H c(Sb 2Se 3) = −52 ± 2 J/(g of Sb 2Se 3), Johnson–Mehl–Avrami kinetics of crystallization and kinetic exponent close to 3 was found. Raman spectra were measured to obtain basic information on the structure of both glassy systems.

Isothermal Crystallization Kinetics and Melting Behavior of Poly(ethylene terephthalate)/Attapulgite Nanocomposites Studied by Step-scan DSC

The crystallization kinetics of poly(ethylene terephthalate)/attapulgite (AT) nanocomposites and their melting behaviors after isothermal crystallization from the melt were investigated by DSC and analyzed using the Avrami method. The isothermal crystallization kinetics showed that the addition of AT increased both the crystallization rate and the isothermal Avrami exponent of PET. Step-scan differential scanning calorimetry was used to study the influence of AT on the crystallization and subsequent melting behavior in conjunction with conventional DSC. The results revealed that PET and PET/AT nanocomposites experience multiple melting and secondary crystallization processes during heating. The melting behaviors of PET and PET/AT nanocomposites varied in accordance with the crystallization temperature and shifted to higher temperature with the increase of AT content and isothermal crystallization temperature. The main effect of AT nanoparticles on the crystallization of PET was to improve the perfection of PET crystals and weaken its recrystallization behavior.

Chitosan nanoparticles loaded with dorzolamide and pramipexole

Chitosan (CS) nanoparticles of dorzolamide hydrochloride (Dorzo) and pramipexole hydrochloride (Prami) were prepared by the ionic gelation method and their in vitro properties were studied. The long-term objective is the development of efficient ocular formulations for Dorzo and efficient oral formulations for Prami. The particle size of nanoparticles was affected by the CS/drug ratio whereas it was not affected by the type of drug (Dorzo or Prami). Drug association efficiency to the nanoparticles did not appear to correlate with the drug/CS ratio whereas the loading capacity tended to fall with increasing drug proportion. Based on WAXD data, Dorzo was dispersed in the nanoparticles in crystalline form, probably due to the weak interaction developed between Dorzo and CS/TPP matrix as FT-IR data indicated. In contrast, WAXD and step-scan DSC data indicated that Prami formed a molecular dispersion within the nanoparticles. This was probably due to the potent interactions developed between Prami and CS/TPP matrix, as FT-IR data revealed. The nanoparticles exhibited mucoadhesive properties which diminished with increasing drug content. Sustained in vitro drug release was observed with the Dorzo-loaded CS nanoparticles in PBS (pH 7.4) and with the Prami-loaded CS nanoparticles in simulated intestinal fluid. The results obtained in this study suggest that the Dorzo-loaded CS nanoparticles and the Prami-loaded CS nanoparticles could be further evaluated for the controlled ocular delivery of Dorzo and the controlled oral delivery of Prami, respectively.

Effect of molecular weight on the cold-crystallization of biodegradable poly(ethylene succinate)

A series of poly(ethylene succinate) (PESu) samples with varying molecular weight were synthesized by the melt polycondensation method. The effect of the molecular weight on the cold-crystallization and the subsequent melting behaviour of melt-quenched PESu was studied by means of standard DSC and Step Scan DSC. Recrystallization and multiple melting followed the cold-crystallization of the samples. In general a melting–recrystallization–remelting scheme was supposed. The phenomenon is more pronounced for the low molecular weight samples. Higher degree of crystallinity is achieved during heating of low molecular weight samples. Cold-crystallization kinetics was also studied using microscopic and macroscopic models. The modified Avrami and the Ozawa methods seem to describe well the experimental data in contrast to the Tobin model. The spherulite growth rates during isothermal crystallization from the melt were studied using polarizing light microscopy (PLM). They were found to decrease with increasing molecular weight. A regime transition was observed at about 70 °C and the K g I I I / K g I I ratio was close to 1.8. Furthermore, the nucleation constant K g of the Lauritzen–Hoffmann equation was estimated using either isothermal PLM measurements or non-isothermal DSC cold-crystallization data and the assumption that the growth rate is inversely proportional to the half crystallization time. No clear trend of K g with the polymer molecular weight was observed. Finally, the effective activation energy at different relative degrees of crystallinity was estimated using the differential isoconversional method of Friedman. Based on these values, both Lauritzen–Hoffman parameters ( U * and K g) were evaluated using the overall rates of non-isothermal cold-crystallization according to the method of Vyazovkin and Sbirrazzuoli. A clear increase of U * with the molecular weight was observed meaning that the effect of diffusion is more pronounced in polymers having higher average molecular weight

Atom transfer radical polymerized MR fluids

A novel magnetorheological fluid, in which the surface of iron particles is coated with poly(butyl acrylate) by surface-initiated atom transfer radical polymerization (ATRP), is investigated. The polymer coating procedure includes two steps, which are immobilization of initiator: 2-(4-chlorosulfonylphenyl)-ethyltrichlorosilane (CTCS) on the iron particles' surface and graft polymerization of butyl acrylate from the surface. The surface coating is characterized by FTIR and SEM. This magnetorheological fluid has controllable off-state viscosity and high shear yield stress. Coating polymer on the iron particles' surface by ATRP can significantly reduce iron particles' settling and improve stability of the MR fluid. Glass transition temperature is obtained using the step-scan DSC method. The molecular weight and conversion can be controlled by the molar ratio of monomer to initiator, reaction temperature and time. The reaction is first order determined by the plot of ln[M] 0/[M] against polymerization time.

Quantification of amorphous content in maltitol by

A method was developed for the quantification of low levels of amorphous content in maltitol with StepScan DSC. The method was based on the fact that the change of specific heat at the glass transition is linearly proportional to the amorphous content. The influence of different measurement parameters of StepScan DSC was evaluated and two different calibration heating rates were tested. Synthetic mixtures with various proportions of crystalline and amorphous maltitol were prepared. Two different measurement methods were compared and the linear regression between ΔCp and amorphous content was obtained. The limit of detection (LOD) and the limit of quantification (LOQ) values were for the fictive temperature 0.24% (amorphous content) and 0.81% and for the half point temperature 0.27 and 0.92%, respectively, (method 1) and for the fictive temperature 0.18 (amorphous content) and 0.61% and for the half point temperature 0.16 and 0.52%, respectively (method 2). Very low determination limits for the quantification of amorphous content could be attained with the StepScan DSC method. However, the realistic limit of quantification was somewhat higher (about 3%) because of noise in the StepScan measurement. The main advantage of the StepScan DSC method for quantification of amorphous content was that the glass transition and relaxation peaks are separated into different curves and the interpretation becomes easier.

Melting behaviour and evolved gas analysis of xylose

Two enantiomeric forms of xylose were identified as α-D-xylopyranose and α-L-xylopyranose by powder diffraction. Their melting behaviour was studied with conventional DSC and StepScan DSC method, the decomposition was studied with TG and evolved gases were analyzed with combined TG-FTIR technique. The measurements were performed at different heating rates. The decomposition of xylose samples took place in four steps and the main evolved gases were H2O, CO2 and furans. The initial temperature of TG measurements and the onset and peak temperatures of DSC measurements were moved to higher temperatures as heating rates were increased. The decomposition of L-xylose started at slightly higher temperatures than that of D-xylose and L-xylose melted at higher temperatures than D-xylose. The differences were more obvious at low heating rates. There were also differences in the melting temperatures among different samples of the same sugar. The StepScan measurements showed that the kinetic part of melting was considerable. The melting of xylose was anomalous because, besides the melting, also partial thermal decomposition and mutarotation occurred. The melting points are affected by both the method of determination and the origin and quality of samples. Melting point analysis with a standardized method appears to be a good measure of the quality of crystalline xylose. However, the melting point alone cannot be used for the identification of xylose samples in all cases.

Multiple melting behaviour of poly(3-hydroxybutyrate-co-hydroxyvalerate) using step-scan DSC

Melting behaviour and crystal morphology of poly(3-hydroxybutyrate) (PHB) and its copolymer of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with various hydroxyvalerate (HV) contents [5 wt.% (PHB5V), 8 wt.% (PHB8V) and 12 wt.% (PHB12V)] have been investigated by conventional DSC, step-scan differential scanning calorimetry (SDSC), wide angle X-ray diffraction (WAXRD) and hot stage polarised optical microscopy (HSPOM). Crystallisation behaviour of PHB and its copolymers were investigated by isothermal crystallisation kinetics. Thermal properties were investigated after isothermal crystallisation treatment. Multiple melting peak behaviour was observed for all polymers. SDSC data revealed that PHB and its copolymers undergo melting–recrystallisation–remelting during heating, as evidenced by exothermic peaks in the IsoK baseline ( C p,IsoK , non-reversing signal). An increase in degree of crystallinity due to significant melt–recrystallisation was observed for isothermally crystallised polymers. SDSC proved a convenient and precise method for measurement of the apparent thermodynamic specific heat ( C p,ATD , reversing signal). PHB and PHBV showed different crystal morphologies for similar crystallisation condition. HSPOM results showed that the crystallisation rates reduced and sizes of spherulites were significantly increased as HV content increased.

Melting and thermal history of poly(hydroxybutyrate-co-hydroxyvalerate) using step-scan DSC

Melting behaviour and crystal morphology of poly(3-hydroxybutyrate) (PHB) and its copolymer of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with various hydroxyvalerate (HV) contents [5 wt.% (PHB5HV), 8 wt.% (PHB8HV) and 12 wt.% (PHB12HV)] have been investigated by conventional DSC, step-scan differential scanning calorimetry (SDSC) and hot-stage polarised optical microscopy (HSPOM). Crystallisation behaviour of PHB and its copolymers were investigated by SDSC. Thermal properties were investigated after different crystallisation treatments, fast, medium and slow cooling. Multiple melting peak behaviour was observed for all polymers. SDSC data revealed that PHB and its copolymers undergo melting–recrystallisation–remelting during heating, as evidenced by exothermic peaks in the IsoK baseline (non-reversing signal). An increase in degree of crystallinity due to significant melt–recrystallisation was observed for slow-cooled copolymers. PHB5HV showed different crystal morphologies for various crystallisation conditions. SDSC proved a convenient and precise method for measurement of the apparent thermodynamic specific heat (reversing signal) HSPOM results showed that the crystallisation rates and sizes of spherulites were significantly reduced as crystallisation rate increased.

High-temperature dynamic thermomechanical analysis of NBS 711 glassy fiber

The complex modulus in bending of NBS 711 glassy fibers was investigated using dynamic thermomechanical analysis (DTMA) in a wide temperature range from room temperature to the supercooled liquid region through the glass transition region. Temperature dependencies of both real part, E′, and imaginary part, E″, of the dynamic modulus as well as the loss angle, tanδ, were studied as a function of heating rate and frequency. Experimental data were compared with results of the static thermomechanical analysis and one kind of dynamic DSC (StepScan DSC), viscosity data of NBS 711 were also used. Good agreement of DTMA and StepScan DSC results was found. Activation energy of plastic deformation calculated from frequency dependence of E″ and activation energy of viscous flow were found close to each other.

Separation of gelation from vitrification in curing of a fiber‐reinforced epoxy composite

AbstractPrepregs of a mixture of the tetrafunctional epoxy tetraglycidyl 4,4‐diaminodiphenyl methane (TGDDM) and the tetrafunctional amine 4,4'‐diaminodiphenylsulfone (DDS) were characterized with temperature‐modulated DSC (TMDSC) as well as dynamic mechanical analysis (DMA). The baseline shift of the glass transition was separated from the curing exotherm by using temperature‐modulated and step scan DSC temperature scans. Likewise, the baseline shift in heat capacity due to vitrification was isolated using TMDSC isotherms. Using the TMDSC glass transition temperature, degree of conversion, and vitrification results, combined with the gelation data generated from DMA, a time‐temperature‐transformation (TTT) diagram was constructed, providing information necessary for optimization of industrial processing of the epoxy prepreg. Thus, effects of storage, preprocessing, and postprocessing on the overall curing process are taken into account.