Conventional Differential Scanning Calorimetry Research Articles

Synthesis of the bio-based polyester poly(propylene 2,5-furan dicarboxylate). Comparison of thermal behavior and solid state structure with its terephthalate and naphthalate homologues

Poly(propylene-2,5-furan dicarboxylate) (PPF) is a new alipharomatic polyester which can be prepared from monomers derived from renewable resources and efforts are currently being made in order to use PPF as a replacement for its terephthalate (PPT) and naphthalate (PPN) homologues. For this reason, an in-depth comparative study of its thermal behavior and solid state structure was performed in the current manuscript. Conventional and temperature-modulated differential scanning calorimetry were utilized for the evaluation of the different characteristics and behavior of the samples under study. Regarding the structural characteristics, it was found that the different structural rings and the ultimate chain rigidity of the samples play a major role on their final properties. On this basis, the more rigid PPN exhibited higher glass transition temperature and thermal stability than PPF and PPT, as the results suggest. The equilibrium melting point of PPF was found equal to 199 °C, while the heat of fusion of pure crystalline PPF was 141.75 J/g or 27.8 kJ/mol. The spherulitic morphology was also investigated by means of polarized optical microscopy and it was found that PPF forms smaller spherulites than its other two homologues.

Glass transition in a marginal glass-forming alloy studied by dynamic mechanical analysis

For marginal amorphous-forming alloys, the glass transition is difficult to detect using conventional differential scanning calorimetry. Since the mechanical response is more sensitive than the thermal signal when the amorphous solid transforms to the supercooled liquid, dynamic mechanical analysis (DMA) can be used to detect the glass transition temperature in marginal amorphous-forming alloys. In the present study, DMA was successfully used to measure the glass transition in Al90Tb10 metallic glass. A glass transition temperature of 443 K was estimated from the crossover region of the storage modulus, where the crossover region corresponds to the transition from an amorphous solid to a supercooled liquid. The activation energies of 63 and 221 kJ/mol for the low temperature relaxation and viscous flow were calculated using the time-temperature superposition principle.

Photo- and thermo-stimulated structural relaxation in As–S glasses

Homogeneous AsxS100−x glasses (25≤x≤42) are investigated with conventional differential scanning calorimetry. Enthalpy losses during structural relaxation are determined for the glasses aged at different temperatures in the dark and under the exposure with 620nm light. It is shown that light exposure accelerates structural relaxation in chalcogen-rich glasses, but does not modify the kinetics of As40S60 and As42S58 compositions. The results are analyzed from the temperature-dependent constraint theory point of view.

Vitrification in a wide cooling rate range: the relations between cooling rate, relaxation time, transition width, and fragility.

The cooling rate dependence of the thermal glass transition of polystyrene (PS) is measured in a range between 0.2 K/min (0.003 K/s) and 4000 K/s using conventional differential scanning calorimetry (DSC) and Fast Scanning Calorimetry (Flash DSC 1). The cooling rate dependence of the thermal glass transition can be described in an analogy to the frequency dependence of the dynamic glass transition. The relation between cooling rate, β(c), and frequency, ω, is usually described by the Frenkel-Kobeko-Reiner-(FKR) hypothesis β(c)/ω = C, where C is a constant. We have introduced a new property to describe the kinetics of the vitrification process; the vitrification function, κ. This function is the ratio between the width of the thermal and dynamic glass transition. The validity of the FKR hypothesis is analyzed by two independent methods, the analysis of the activation diagram using the Vogel-Fulcher-Tammann-Hesse equation and the analysis of the temperature dependence of the transition width. We derived a relation for the FKR-constant, which indicates the validity range of the FKR hypotheses. This hypothesis is valid if the logarithmic width of the vitrified and the non-vitrified relaxation spectrum is temperature invariant. This condition is fulfilled for polystyrene in the measured cooling rate range. Furthermore we discuss the relation between the vitrification function, the transition width, the FKR constant, and the fragility.

Crystallization behavior and morphological characterization of poly(ether ether ketone)

The crystal structure and morphology of poly(ether ether ketone) (PEEK) was investigated using standard differential scanning calorimetry (DSC), flash DSC, optical microscopy, atomic force microscopy, and small angle X-ray scattering tools. The flash DSC results suggested that the double melting peaks phenomenon observed in conventional DSC work originated from the reorganization of PEEK crystals, which was due to the much faster recrystallization rate of PEEK than the DSC heating and cooling rate. A refined crystallization model to describe PEEK crystal structure formation was proposed. The refined crystallization model could help reconcile the discrepancy found between the bulk crystallinity measured by DSC and the linear crystallinity obtained from SAXS experiments by taking into account possible variation in crystal perfection within the lamellar structure. Simplified molecular dynamic modeling was carried out to support this model. Implications of the above findings to the fundamental understanding of structure–property relationships in PEEK were discussed.

Measurement of the limiting fictive temperature over five decades of cooling and heating rates

The fictive temperature (Tf) was defined by Tool in the 1940s as a measure of glassy structure. Tf is generally measured on heating and can be calculated from the enthalpy overshoot in calorimetric studies using a method developed by Moynihan. Prior work has demonstrated that the limiting fictive temperature (Tf′) is similar to Tg (measured on cooling) and depends on the cooling rate in a manner consistent with the Williams–Landel–Ferry (WLF) relationship. Theoretically, the limiting fictive temperature should not depend on heating rate, but this has been experimentally verified only for a very limited range of heating rates. Here, rapid-scanning chip calorimetry and conventional differential scanning calorimetry (DSC) are combined to investigate Tf′ for polystyrene over a broad range of heating rates ranging from 0.017 to 3000K/s after cooling at different rates. The results show that Tf′ depends on cooling rate following the WLF equation. On the other hand, Tf′ is not a function of heating rate, consistent with theoretical predictions, in spite of the change in the magnitude and placement of the enthalpy overshoot.

Formation of a Rigid Amorphous Fraction in Poly(3-(2'-ethyl)hexylthiophene).

Herein, we detail the formation of a rigid amorphous fraction in poly(3-(2'-ethyl)hexylthiophene) (P3EHT) at high relative crystallinity, yielding a more complete picture of the solid-state structure. In the differential scanning calorimetry (DSC) heating scans of isothermally crystallized P3EHT a distinct endothermic peak appears slightly above the crystallization temperature. This previously undescribed endothermic feature of P3EHT's thermal behavior is observed consistently ∼20 °C above the crystallization temperature-shifting to higher temperatures with increasing crystallization temperature-and increases in magnitude with both time and crystallization temperature. Here, we determine the origins of this endothermic peak with DSC and temperature-modulated DSC (TMDSC). TMDSC reveals that the annealing peak observed in the total heat flow (THF)-heat flow equivalent to that of conventional DSC-is a consequence of an enthalpic relaxation observable as an endothermic peak in the nonreversible heat flow (NHF) and a glass transition evident as a step increase in the reversible heat flow (RHF). In conjunction with conventional DSC observations, these results indicate that the observed annealing peak is a consequence of the formation of distinct amorphous regions-a mobile amorphous fraction (MAF) and a rigid amorphous fraction (RAF)-during the isothermal crystallization process and not the melting of a distinct crystallite population or melt recrystallization.

Detection of Phase Transition in Free-Cutting Al-Mg-Si Alloys by Resonant Ultrasound Spectroscopy

The transition temperatures of small amounts of eutectic particles contained in free-cutting aluminum alloys (namely AA6262 and AA6023) are investigated in this paper. Detection of the transition temperatures by conventional differential scanning calorimetry (DSC) measurements is difficult because of the small volume fraction (~ 1wt.%) of the eutectic. On the other hand, the melting of the particles induces small changes of elastic moduli of the alloy, which can be sensitively measured by resonant acoustic methods, for example by resonant ultrasound spectroscopy (RUS). It is shown that the phase transitions of the particles correspond to well-detectable changes of the resonant spectrum, which enables a more detailed characterization of the transition process. A significant thermal hysteresis is observed between the cooling and the heating runs, and also the widths of the temperature ranges in which the transitions occur exhibit a strong melting/freezing asymmetry.

Investigation of heat of biomass pyrolysis and secondary reactions by simultaneous thermogravimetry and differential scanning calorimetry

The heat of pyrolysis reactions is of great importance not only for its impact on biomass pyrolysis process but as a fundamental parameter for modelling biomass thermochemical conversion. Previously frequently-used technique that integrates DSC heat flow curves to determine the heat of pyrolysis actually presented the enthalpy change of pyrolysis within a temperature range. This enthalpy change in effect took no account of the sensible heat of the volatiles leaving the crucible before reaching the upper limit of the temperature range. This work was attempted to determine both the heat of biomass pyrolysis as a function of conversion ratios or temperatures and the heat of secondary reactions. To this end, the conventional simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC) technique was re-examined to investigate thermal behaviour of cellulose, xylan, lignin and four lignocellulosic biomass samples using a TG–DSC analyser. Selected samples around 3.3mg were thermally decomposed in a crucible either with or without a pierced lid at a heating rate of 10°C/min to a maximum temperature of 500°C. Specific heat capacities of all the original biomass and char samples were measured through DSC analyses. Temperature-dependent formulas were developed for extrapolating the specific heat of both biomass and char samples at high temperatures. The heat of pyrolysis as a function of conversion ratios was characterised by the derivative of the enthalpy change versus mass changes during pyrolysis. Thus-obtained heat of biomass pyrolysis represented the difference between the enthalpies of formation of the products and reactants at a specific temperature or conversion ratio instead of a temperature range. The results showed that the heat of pyrolysis reactions changed from high endothermic values, via moderately endothermic values and finally rapidly to high exothermic values as the conversion ratios increased. Linear relations were established between the heat of biomass pyrolysis and conversion ratios ranging from 0.2 to 0.8. The heat of secondary reactions was about 80–280J/g for cellulose and increased with conversion ratios up to 2500–4000J/g for the lignocellulosic biomass samples.

Phase separation in coamorphous systems: in silico prediction and the experimental challenge of detection.

Combinatorial chemistry has enabled the production of very potent drugs that might otherwise suffer from poor solubility and low oral bioavailability. One approach to increase solubility is to make the drug amorphous, which leads to problems associated with drug stability. To improve stability, one option is to molecularly disperse the drug in a matrix. However, the primary reason for the failed stabilization with this approach is phase separation, which has been carefully studied in polymeric systems. Nevertheless, the amorphous-amorphous phase separation in coamorphous small molecule mixtures has not yet been reported. The goal of the present study was to experimentally detect the amorphous-amorphous phase separation between two small molecules. A modified in silico method for predicting miscibility by the Flory-Huggins interaction parameter is presented, where conformational variations of the studied molecules were taken into account. A series of drug-drug mixtures, with different mixture ratios, were analyzed by conventional differential scanning calorimetry (DSC(conv)) to detect possible amorphous-amorphous phase separations. The phase separation of coamorphous drug-drug mixtures was also monitored by temperature modulated DSC (MDSC) and Fourier transform infrared (FT-IR) imaging at temperatures above Tg for prolonged time periods. Amorphous-amorphous phase separation was not detected with DSC(conv), probably due to the slow kinetics of phase separation. However, the melting of the separated and subsequently crystallized phases was detected by MDSC. Furthermore, FT-IR imaging was able to detect the separation of the two amorphous phases, which demonstrates the ability of this method to detect small molecule phase separations.

Melting behaviors of polyethylene crystals: An application of fast-scan DSC

Melting behaviors of linear polyethylene (PE) have been experimentally examined by fast-scan and conventional differential scanning calorimetry (DSC). By fast-scan heating up to 104 K s−1, the melting peak temperature, Tpeak, is free from the influences of re-organization and re-crystallization and follows the shift only with superheating, which has been evaluated by a power-law behavior. The melting point, TM, of the chain-folded crystals has been determined as Tpeak extrapolated to zero heating rate without the influences of non-equilibrium effects. The Hoffman–Weeks plot of TM against crystallization temperature, Tc, suggests the equilibrium melting point TM0=141.1°C of the linear PE having Mw = 52.0 × 103 and the doubling of lamellar thickness concluded from the linear relationship between TM and Tc with the slope close to 1/2. While the lamellar doubling is a well-known behavior of PE crystallized from the melt, the present results with Tc down to ∼100 °C suggest the doubling completed in a quite short time interval. The value of TM0=140.4°C has also been obtained by the Gibbs–Thomson plot of TM against the inverse of lamellar thickness, (dc)−1, determined by small angle X-ray scattering. The agreement of the results within ±1 °C suggests the Hoffman–Weeks plot as an appropriate method for the melting point determination with fast-scan heating, only with which the determination procedure is free from re-organization and re-crystallization. The consistent behavior has also been confirmed in terms of the end-surface free energy, σe, determined by the Gibbs–Thomson plot and by the growth kinetics of crystallization.

Study of melting and crystallization behavior of polyacrylonitrile using ultrafast differential scanning calorimetry

The thermal behavior of polyacrylonitrile (PAN) has been investigated using X-ray diffraction, differential scanning calorimetry (DSC), and ultrafast DSC. In conventional DSC, it is difficult to prevent the concurrent occurrence of the exothermic reactions of PAN with melting. However, in the ultrafast DSC curve, the exothermic peak due to these reactions disappears over the temperature range 0–400 °C at heating rates above 250 °C s−1. Alternatively, the glass transition and the melting of PAN are observed over the temperature range 109–129 °C and 335–362 °C, respectively. Moreover, upon cooling from the molten state at a rate of −7500 °C s−1, PAN crystallization is observed at 204 °C. These findings were observed repeatedly during heating and cooling measurements. From the extrapolation analysis, the zero-entropy-production melting temperature of PAN is found to lie in the temperature range 320–350 °C. Finally, the equilibrium melting temperature of PAN is estimated to be ca. 465 °C.

Influence of crosslinker and ionic comonomer concentration on glass transition and demixing/mixing transition of copolymers poly(N-isopropylacrylamide) and poly(sodium acrylate) hydrogels

Hydrogels based on N-isopropylacrylamide and sodium acrylate as ionic comonomer were synthesized by free radical polymerization in water using N,N′-methylenebisacrylamide as crosslinker and ammonium persulfate as initiator. The glass transition of dried copolymers poly(N-isopropylacrylamide) (PNIPA) and poly(sodium acrylate) (SA) gels and demixing/mixing transition of PNIPA-SA hydrogels swollen with increasing amounts of water were studied using conventional differential scanning calorimetry. In the crosslinked polymers, the glass transition linearly increases, and the transition range becomes broader, with increasing crosslinker content. Increasing content of ionic comonomer also produces an increase of glass transition temperature, which moves to higher temperatures with higher sodium acrylate fraction. The influence of chemical structure of PNIPA-SA hydrogels on the lower critical solution temperature (LCST) of PNIPA-SA/water mixtures during heating and cooling was quantified as function of the content of the crosslinker and the ionic comonomer, as well as water content of the hydrogel in the range from 95 to 70 wt%. At parity of water content, the LCST occurs at higher temperatures for gels containing higher amounts of sodium acrylate. Similarly, the introduction of N,N′-methylenebisacrylamide causes an increase of the LCST, which grows with increasing of crosslinking degree of the hydrogel.

Crystallization Behaviour of Polytetrafluoroethylene over very Large Cooling Rate Domains

Polytetrafluoroethylene (PTFE) is a semi-crystalline polymer that demonstrates a very fast crystallization process on cooling. This study investigates the nonisothermal PTFE ultra-fast crystallization over a wide range of cooling rates via conventional Differential Scanning Calorimetry (DSC), Fast Scanning Calorimetry (FSC) and Ultra-Fast Scanning Calorimetry (UFSC). A new knowledge about crystallization kinetics of PTFE is obtained from the data obtained under very fast cooling rates. The shift of the melting peak to lower temperature shows that the crystals formed under fast cooling rates are slightly less stable than those produced under slower cooling rates. SEM analysis allows to observe these differences in crystal morphologies. According to the results, the crystallization is still present even for the fastest cooling rate employed and in consequences it is impossible to reach a metastable glassy state. The effective activation energy (Eα) displays a variation with the relative extent of crystallization (α) that is characteristic of a transition of PTFE crystallization from regime II to regime III around 312°C. Following the Hoffman-Lauritzen theory the Eα dependency obtained from the crystallizations under the different cooling rates was fitted in order to study the theoretical dependence of the growth rate.

Thermoanalytical study and structure of stoichiometric As–Sb–S glasses

The glass-forming system (As2S3)100−x(Sb2S3)x was studied by thermal analysis (conventional and StepScan differential scanning calorimetry) and Raman spectroscopy. It was found that the bulk glasses are homogeneous up to x = 60, while supercooled melts are unstable and when x ≥ 40, Sb2S3 (stibnite) crystallizes during heating. Depending on the chemical composition, the glass transition temperature initially increases as the Sb2S3 concentration is increased from 0 to 5 %, decreases to a minimum at ~20 %, and then gradually increases as the concentration is further increased and the main Raman peak also shifts non-monotonically. Combining these results with chemometric analysis of the Raman spectra showed that the image of the structure of the studied glasses can be described by the linear combination of three chemically different stable clusters, rather than by the chains crossing model, CCM, and that the properties of the glasses are controlled by medium-range order.

Spinnability and Characteristics of Polyvinylidene Fluoride (PVDF)-based Bicomponent Fibers with a Carbon Nanotube (CNT) Modified Polypropylene Core for Piezoelectric Applications.

This research explains the melt spinning of bicomponent fibers, consisting of a conductive polypropylene (PP) core and a piezoelectric sheath (polyvinylidene fluoride). Previously analyzed piezoelectric capabilities of polyvinylidene fluoride (PVDF) are to be exploited in sensor filaments. The PP compound contains a 10 wt % carbon nanotubes (CNTs) and 2 wt % sodium stearate (NaSt). The sodium stearate is added to lower the viscosity of the melt. The compound constitutes the fiber core that is conductive due to a percolation CNT network. The PVDF sheath’s piezoelectric effect is based on the formation of an all-trans conformation β phase, caused by draw-winding of the fibers. The core and sheath materials, as well as the bicomponent fibers, are characterized through different analytical methods. These include wide-angle X-ray diffraction (WAXD) to analyze crucial parameters for the development of a crystalline β phase. The distribution of CNTs in the polymer matrix, which affects the conductivity of the core, was investigated by transmission electron microscopy (TEM). Thermal characterization is carried out by conventional differential scanning calorimetry (DSC). Optical microscopy is used to determine the fibers’ diameter regularity (core and sheath). The materials’ viscosity is determined by rheometry. Eventually, an LCR tester is used to determine the core’s specific resistance.

Effect of surface interactions on the glass transition temperature behavior of amorphous polystyrene

The glass transition temperatures (T g) of nanosized thin polystyrene (PS) films were determined using FTIR (or, more specifically, the absorbance peak at 797 cm−1). This novel method was used to verify the presence of surface interactions between polymer and substrate. As surface interactions may lead to changes in T g, various amounts of nanoparticles of TiO2 and of ZnSe were uniformly deposited on the surface of free-standing PS film to simulate increases in surface interactions. FTIR spectra of thin PS films coated with various amounts of these nanoparticles were recorded as a function of temperature, and the T g values obtained using this method were compared to results obtained using conventional differential scanning calorimetry (DSC). The addition of TiO2 nanoparticles leads to an increase in T g, while the addition of ZnSe depresses the glass transition temperature. Both effects are linear with respect to the amount of nanoparticles. These results support the argument that increasing surface interactions leads to changes in chain mobility, as determined from variations in T g.

Glass transition and crystallization of Al–Ni–La based metallic glasses studied by temperature modulated DSC

Crystallization behavior and glass transition phenomenon of Al89Ni6La5, Al87Ni6La7 and Al87Ni5La7M1 (M=Ag, Cu) metallic glasses have been studied by temperature modulated differential scanning calorimetry (TMDSC). A clear glass transition of these glasses, which could not be detected in conventional differential scanning calorimetry (DSC), even at higher heating rates, was observed through TMDSC measurements. All these glasses undergo two-stages of crystallization on heating. The precipitation and growth of fcc-Al in Al89Ni6La5 and bcc-metastable phase in other alloys are responsible for the first stage of crystallization. The glass transition phenomenon in these alloys except Al87Ni6La7, could not be detected in DSC because the signals are overlapped with the relatively large heat release from the primary crystallization. This was confirmed from the TMDSC measurements. These measurements were necessary for the development of new nanocrystalline/amorphous composite materials.

Amphiphilic polymers bearing gluconolactone moieties: Synthesis and long side-chain crystalline behavior

The synthesis and characterization of amphiphilic polymers bearing gluconolactone moieties has been described. In a first step, an unprotected glycomonomer 2-[({[4-(d-gluconamid-N-yl)butyl]amino}carbonyl)oxy]ethyl acrylate, HEAG, has been synthesized. Posterior, this glycomonomer has been copolymerized with methyl methacrylate at different compositions and the kinetic behavior has been also studied calculating the monomer reactivity ratios by Kelen–Tüdös extended equation. In addition, the long side-chain crystalline behavior of these carbohydrate-based copolymers with high composition of glycomonomer has been examined by using conventional and modulated differential scanning calorimetry and X-ray diffraction measurements. At the same time, the phase separation behavior of carbohydrate-based copolymers with lower HEAG content has been determined by their glass transition temperature measurements. Finally, the thermal stability of all these amphiphilic copolymers has been evaluated by thermogravimetric analysis.

Contributions of Tzero™ technology to temperature modulated Differential scanning calorimetry

Differential Scanning Calorimetry (DSC) is the most used of thermal analysis techniques. Temperature Modulated DSC is a technique which has extended a lot the applications of DSC because of new possibilities like improved separation of overlapped transitions, a higher sensitivity and an improved accuracy of heat capacity measurements. Basic principle of Modulated DSC is to apply two heating rates simultaneously and consequently to measure the heat flow obtained by this solicitation. The average underlying heating rate gives the same heat flow than conventional DSC (called total heat flow), while the sinusoidal instantaneous heating rate allows the separation of this total heat flow into two components, one heat flow linearly dependent of the heating rate change, and one heat flow dependent on time and temperature, and not dependent of the heating rate change. This technique allows a direct measurement of heat capacity, and the separation of thermal events due to a heat capacity change (for instance glass transition) from kinetic events (polymerization, dehydration) because of the purely sinusoidal thermal solicitation. Tzero™ technology has been specifically developed to quantify the instrument contribution to the DSC measurement; and offers a much more accurate representation of the absorbed or evolved heat flow by the analyzed material. This technique directly measures the heat transfers through the thermal sensors (thermal resistances and thermal capacitances of the sample and reference platforms) and consequently two independent measurements of sample and reference heat flows. The result is the improved resolution of thermal peaks and a quick return to baseline after a transition. Advanced Tzero™ technology takes in account heat transfers through sample and reference pans in the heat flow measurement. This allows Modulated DSC experiments at average heating rates similar to conventional DSC, typically 10K/min.