DSC - Differential Scanning Calorimetry

DSC involves the measurement of relative changes in temperature and energy under isothermal or adiabatic conditions, i.e., the heat capacity of the sample at constant pressure. It is a technique measuring the energy necessary to establish a nearly zero temperature difference between a substance and an inert reference material because the specimen and reference are subjected to identical temperature regimens in the same environment (heated or cooled at a controlled rate). 

Two types of DSC systems are commonly used power compensation and heat-flux DSC. In the former, temperatures of the sample and reference are made identical by varying the power input to the two furnaces the energy required to do this is a measure of enthalpy or heat capacity changes in the sample relative to the reference. In heat-flux DSC, a low resistance heat flow path connects the sample and the 

DSC instruments measure the energy changes that occur in a sample with the rise of temperature, with respect to a reference material. The equipment used for such analyzes 

A DSC curve plots the heat flux versus temperature or time. The heat flow is measured in a unit of energy/mass or time, usually W-g k Generally, the heat increase in the sample is indicated as an increase of the DSC curve, which differs from DTA, where an endothermic process is displayed by a decrease of the curve. DSC curves are usually recorded in a temperature range where the sample is heated or cooled at a constant rate, similar to the DTA analysis [21, 22]. 

DSC is used extensively in the analysis of plastics, particularly those that are semi-crystalline - polyolefins, nylons, polyesters, etc. It is sensitive enough to 

Using appropriate standard materials it is also possible to use DSC to quantify the level of certain additives, e.g., peroxides, in a sample. 

DSC can also be used in thermal stability studies of the plastic compound, and to investigate the effectiveness of antidegradants and fire retardants. 

Differential scanning calorimetry (DSC) is one of the best known techniques among a group called thermal analysis methods others include differential thermal analysis, d5mamic mechanical analysis, and thermogravimetric analysis methods all of which are covered in the following sections. 

DSC is a thermal analysis technique that is used to measure the temperatures and energy flows related to transitions in materials as a function of time and temperature.These measurements provide qualitative and quantitative information about physical and chemical changes that involve endothermic or exothermic processes or changes in heat capacity. Any event, such as loss of solvent, phase transitions, crystallization temperature, melting point, and degradation temperature of the plastic sample, result in a change in the temperature of the sample. The systems available cover a wide temperature range, e g., -60°Cto l,500°C. 

Two types of systems are commonly used power compensation and heat flux DSCs. In the power compensation apparatus temperatures of the sample and the reference are controlled independently by using separate but identical furnaces. The power input to the two furnaces is adjusted to equalize the temperatures. The energy required for the temperature equalization is a measure of the enthalpy or heat capacity in the sample relative to the reference. In heat flux DSC, the sample and the reference are interconnected by a metal disk that acts as a low-resistance heat-flow path. The entire assembly is placed inside a furnace. The changes in the enthalpy or heat capacity of the sample cause a difference in its temperature compared to the reference. The resulting heat flow is small because of the thermal contact between the sample and the reference. Calibration experiments are conducted to correlate enthalpy changes with the temperature differences. In both cases, the enthalpy changes are expressed in the units of energy per unit mass. 

DSC from a temperature of Tj to T2 where the polymer becomes amorphous at a temperature of Tq prior to reaching T2 as shown by the baseline shift in Fig. 10.22. The enthalpy changes are determined according to the following procedure. 

then Eq. (10.4) can be derived to calculate the weight fraction of the crystalline phase. 

Differential scanning calorimetry (DSC) analyzes thermal transitions occurring in polymer samples when they are cooled down or heated up under inert atmosphere. Melting and glass transition temperatures can be determined as well as the various transitions in liquid crystalline mesophases. In a typical DSC experiment, two pans are placed on a pair of identically positioned platforms connected to a furnace by a common heat flow path. One pan contains the polymer, the other one is empty (reference pan). Then the two pans are heated up at a specific rate (approx. 10 K min ). The computer guarantees that the two pans heat at exactly the same rate -despite the fact that one pan contains polymer and the other one is empty. Since the polymer sample is extra material, it will take more heat to keep the temperature of the sample pan increasing at the same rate as the reference pan. A plot is created where the difference in heat flow between the sample and reference is plotted as a function of temperature. When there is no phase transition in the polymer, the plot parallels the x-axis, and the heat flow is given in units of heat, q, supplied per unit time, t  

With the heating rate defined as rise of temperamre, AT, per unit time, t 

The observation and the exact position of Tg, and especially of Tc and T are strongly dependent on the thermal history of the sample. Therefore, often several DSC temperature cycles are carried out (maximum temperature must be clearly below decomposition temperature) with analyzing the thermal transitions usually in the second and not in the first run after a defined cooling cycle. 

DSC measurements can be used, moreover, to determine how much of a polymer sample is crystalline. For that purpose, the area of the melting peak has to be measured the DSC plot gives the heat flow per gram of material as a function of temperature. Heat flow is heat given off per second. So the area of the peak is given as  

When the peak area is divided by the heating rate 

Physical and chemical changes may often be induced by raising or lowering the temperature of a substance. Typical examples are phase transitions, such as fusion, or chemical reactions, such as the solid state polymerization of sodium chloroacetate, which has an onset at 471 K [227]  

Differential scanning calorimetry (DSC) was designed to obtain the enthalpy or the internal energy of those processes and also to measure temperature-dependent properties of substances, such as the heat capacity. This is done by monitoring the change of the difference between the heat flow rate or power to a sample (S) and to a reference material (R), A P = Ps - = (dQ/dt)s - (dQ/dt)r, as 

The prospects of DSC, have been reviewed in a special issue of Thermochimica Acta, which includes a collection of articles on advances of thermal analysis in the twentieth century and expected future developments [232,235,236]. This journal and the Journal of Thermal Analysis and Calorimetry, where research articles about DSC and its applications are often published, are very useful sources of information on the technique. Although relatively old, the reviews by McNaughton and Mortimer [237] and by Mortimer [238] contain excellent examples of applications of DSC to molecular thermochemistry studies. The analytical uses of DSC, which are outside the scope of this book, can be surveyed, for example, in biannual reviews that appear in the journal Analytical Chemistry [239], 

In a power compensation DSC (figure 12.3), the sample and the reference crucible holders consist of two small furnaces, As and Ar, each one equipped with a temperature sensor, Bs or Br, and a heat source, Cs or Cr. The furnaces 

When the glass transition temperature of the polymer sample is reached in the DSC experiment, the plot will show an incline. It is obvious that the heat capacity increases at T, and therefore DSC can monitor the of a polymer. Usually the middle of the incline is taken to be the Tg. Above Tg, the polymer chains are much more mobile and thus might move into a more ordered arrangement they may assume crystalline or liquid-crystalline order. When polymers self-or-ganize in that way, they give off heat which can be seen as an exothermal peak in 

Na-cefazolin is unstable in its amorphous state. Takeda [1.32] described a method to ensure complete crystallization in which microcrystalhne Na-cefazolin was added to supersaturated Na-cefazolin solution at 0 °C, frozen and freeze-dried. The product did not contain amorphous or quasi-crystalline components. 

If in certain substances no crystallization or eutectic mixtures can be found by DSC (cephalosporin [1.35]) with the experimental conditions used, one has to seek different conditions [1.32]. 

Van Winden et al. [1.161] used MTDSC in lyoprotected liposomes to detect the glass transition in samples in which it overlaps with the bilayer melting endotherm. 

Kett et al. [1.162] studied Tg in freeze-dried formulations containing sucrose as a function of relative humidity and temperature during storage by TMDSC and ther-mogravimetric analysis. Craig et al. [1.163] found it helpful to asses the relaxation behavior of freeze-dried amorphous lactose by MTDSC. Relaxation times were calculated from measurements of Tg, and the magnitude of the relaxation endotherm. Scannnig was performed at 2°C/min with a modulation amplitude of 0.3 °C and a period of 60 s. 

The amount of heat absorbed or given out by a sample of polyurethane is measured as the sample is heated over a range of temperatures. Changes in the state of the polyurethane occur at various temperatures. Changes occur 

Samples are normally heated to just above softening point and then allowed to cool down before a test is carried out. With castable polyurethanes, decomposition takes place when the sample is softened, so either the annealing cycle must not be used or the temperatures must not be taken up as high. 

Melting temperatures were measured at the peak maximum and glass transition temperatures were measured at the inflection point. Crystallinity in the composite matrix was estimated by following the equation used by Roman and Winter [11]  

Where Wp is the weight fraction of the reinforcement in the composite and is the heat of fusion of the matrix polymer at 100% crystallinity. As an approximation, was taken as 34 J/g, which is the heat of fusion of 100% crystalline cellulose tributyrate (CTB) . 

Reprinted from [a.21] with permission from Elsevier 

5 Matrix-Assisted Laser Desorption/lonisation Mass Spectrometry (MALDI) 

TGA is a thermal method that measures the weight loss as a function of temperature or time. Polypropylene decomposes at a lower temperature than polyethylene because of the substitution of a methyl group. Some works showed an increase of the degradation temperature of composites with the addition of carbon nanotubes and other synthetic fibers [52]. PE with natural fiber composites show two steps degradation processes because of cellulose, Thermal stability of PE/cellulosic fiber composites decreases with increase in fiber loading, showing two degradation processes. However, 

NaOH treated composites had higher stability than that of untreated cellulose ones [53]. 

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The crystal stmcture of glycerides may be unambiguously determined by x-ray diffraction of powdered samples. However, the dynamic crystallization may also be readily studied by differential scanning calorimetry (dsc). Crystallization, remelting, and recrystallization to a more stable form may be observed when Hquid fat is solidified at a carefully controlled rate ia the iastmment. Enthalpy values and melting poiats for the various crystal forms are shown ia Table 3 (52). 

The process known as transimidization has been employed to functionalize polyimide oligomers, which were subsequentiy used to produce polyimide—titania hybrids (59). This technique resulted in the successhil synthesis of transparent hybrids composed of 18, 37, and 54% titania. The effect of metal alkoxide quantity, as well as the oligomer molecular weight and cure temperature, were evaluated using differential scanning calorimetry (dsc), thermogravimetric analysis (tga) and saxs. 

Most hydrocarbon resins are composed of a mixture of monomers and are rather difficult to hiUy characterize on a molecular level. The characteristics of resins are typically defined by physical properties such as softening point, color, molecular weight, melt viscosity, and solubiHty parameter. These properties predict performance characteristics and are essential in designing resins for specific appHcations. Actual characterization techniques used to define the broad molecular properties of hydrocarbon resins are Fourier transform infrared spectroscopy (ftir), nuclear magnetic resonance spectroscopy (nmr), and differential scanning calorimetry (dsc). 

The compositional distribution of ethylene copolymers represents relative contributions of macromolecules with different comonomer contents to a given resin. Compositional distributions of PE resins, however, are measured either by temperature-rising elution fractionation (tref) or, semiquantitatively, by differential scanning calorimetry (dsc). Table 2 shows some correlations between the commercially used PE characterization parameters and the stmctural properties of ethylene polymers used in polymer chemistry. 

The cure of novolaks with hexa has been studied with differential scanning calorimetry (dsc) and torsional braid analysis (tba) (46) both a high ortho novolak and a conventional acid-cataly2ed system were included. The dsc showed an exothermic peak indicating a novolak—hexa reaction ca 20°C higher than the gelation peak observed in tba. Activation energies were also calculated. 

Thermal analysis iavolves techniques ia which a physical property of a material is measured agaiast temperature at the same time the material is exposed to a coatroUed temperature program. A wide range of thermal analysis techniques have been developed siace the commercial development of automated thermal equipment as Hsted ia Table 1. Of these the best known and most often used for polymers are thermogravimetry (tg), differential thermal analysis (dta), differential scanning calorimetry (dsc), and dynamic mechanical analysis (dma). 

Transitions such as T and are rapidly and conveniently studied using differential scanning calorimetry (dsc). This technique monitors changes in... 

The glass-tiansition tempeiatuiesfoi solution-polymeiized SBR as well as ESBR aie loutinely determined by nuclear magnetic resonance (nmr), differential thermal analysis (dta), or differential scanning calorimetry (dsc). 

From differential scanning calorimetry (dsc) data from Technochemie GmbH—Verfahrenstechnik. Heating rate = 10° C/min ... 

Differential scanning calorimetry (DSC) Onset temperature of exotherms, heat of reaction... 

What are the consequences What is the maximum pressure Vapor pressure of solvent as a function of temperature Gas evolution Differential Thermal Analysis (DTA) / Differential Scanning Calorimetry (DSC) Dewar flask experiments... 

Difl erential thermal analysis (DTA) and differential scanning calorimetry (DSC) are the other mainline thermal techniques. These are methods to identify temperatures at which specific heat changes suddenly or a latent heat is evolved or absorbed by the specimen. DTA is an early technique, invented by Le Chatelier in France in 1887 and improved at the turn of the century by Roberts-Austen (Section 4.2.2). A... 

The solid-liquid transition temperatures of ionic liquids can (ideally) be below ambient and as low as -100 °C. The most efficient method for measuring the transition temperatures is differential scanning calorimetry (DSC). Other methods that have been used include cold-stage polarizing microscopy, NMR, and X-ray scattering. 

In general, X-ray data are used in conjunction with other techniques to obtain as full a picture as possible. For liquid-crystalline materials, differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) are conventionally used. 

Differential scanning calorimetry (DSC) is fast, sensitive, simple, and only needs a small amount of a sample, therefore it is widely used to analyze the system. For example, a polyester-based TPU, 892024TPU, made in our lab, was blended with a commercial PVC resin in different ratios. The glass transition temperature (Tg) values of these systems were determined by DSC and the results are shown in Table 1. 

The SCB distribution (SCBD) has been extensively studied by fractionation based on compositional difference as well as molecular size. The analysis by cross fractionation, which involves stepwise separation of the molecules on the basis of composition and molecular size, has provided information of inter- and intramolecular SCBD in much detail. The temperature-rising elution fractionation (TREE) method, which separates polymer molecules according to their composition, has been used for HP LDPE it has been found that SCB composition is more or less uniform [24,25]. It can be observed from the appearance of only one melt endotherm peak in the analysis by differential scanning calorimetry (DSC) (Fig. 1) [26]. Wild et al. [27] reported that HP LDPE prepared by tubular reactor exhibits broader SCBD than that prepared by an autoclave reactor. The SCBD can also be varied by changing the polymerization conditions. From the cross fractionation of commercial HP LDPE samples, it has been found that low-MW species generally have more SCBs [13,24]. 

The various terms appearing in these equations are self-evident. The differential heat release, dkidt, data are computed from differential scanning calorimetry (DSC). A typical DSC isotherm for a polyurethane reactive system appears in Fig. 11. Energetic composite processing is normally conducted under isothermal conditions so that Eq. (15) is more applicable. 

Barton, J. M. The Application of Differential Scanning Calorimetry (DSC) to the Study of Epoxy Resins Curing Reactions. Vol. 72, pp. 111 — 154. 

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