Melting under isothermal conditions

By means of synchrotron radiation it is also possible to follow melting under isothermal conditions. Such experiments were performed on Polyethyleneterephtha-late. It was shown that with oriented samples melting occured more slowly and at higher temperatures than with unoriented samples. This is in agreement with the explanation of superheating effects by means of entropy considerations. 

Figure 1 a transCrystalline layer induced by a polyethylene terephthalate (PET) fiber in a quiescent P-nudeated FP melt under isothermal conditions (crystallization temperature and time T = 124°C and = 30 min, respectively). 

The overall growth rate of the spherulites of polymers crystallized from the melt under isothermal conditions can be measured by means... 

Figure 17 shows two typical examples of well-developed spherulites of PP grown from the qiuescent melt under isothermal conditions. 

The data presented in the following discussion largely refer to the crystallization kinetics of quiescent melts under isothermal conditions—a very different situation from that pertaining to the dynamic conditions found in commercial conversion processes. In an industrial context, quantitative considerations regard-... 

The theorem also applies to a heterogeneous system, such as a liquid in presence of its saturated vapour, or in presence of the solid. In the former case, vapour is liquefied by compression and gives out its latent heat. Under isothermal conditions this would escape as fast as produced, but if the heat is compelled to remain in the system, it raises the temperature and thereby increases the pressure. If, on the other hand, a mixture of ice and water is compressed, ice melts and the mass is cooled by abstraction of heat. If heat is allowed to enter from outside, so as to restore the original temperature, more ice melts, and the pressure falls by reason of the contraction. 

It is apparent from DTA studies [1021] of the decompositions of Group IA formates in inert or oxidizing atmospheres that reaction is either preceded by or accompanied by melting. Anion breakdown leading to carbonate production may involve formation of the oxalate, through dimerization [1022] of the postulated intermediate, C02, especially during reaction of the Na and K salts in an inert atmosphere and under isothermal conditions. Oxalate production is negligible in reactions of the Li and Cs formates. Reference to oxalate formation is included here since this possibility has seldom been considered [1014] in discussions of the mechanisms of decompositions of solid formates. 

Figure 9 demonstrates the fact that the chemiluminescence process occurs predominantly in the amorphous phase of the polymer. Low molar mass dicaproyl hexamethylene diamide is a fully crystalline compound the chemiluminescence signal under isothermal conditions below the melting point (136°C) at 130°C is very low, but it becomes rather strong after the crystallites of... 

A necessary preface to a description of the procedure is that the solvent and the precipitant must be purified to exhaustion by contact with successive specimens of the acid to be purified. The acid A is dissolved in the minimum amount of solvent S. The precipitant P is then added under isothermal conditions to the solution until roughly one half to three quarters of A has been precipitated. At this stage there is a three-phase system present (vapour and two liquids) with three (or more) components (A, S, and Imp where Imp denotes an impurity), and the impurities are partitioned between A and the mixture of S and P. This mixture is separated from A by decantation or syphoning, A is redissolved in S and reprecipitated by the addition of P. At all stages of this process the mixtures must be stirred efficiently but so gently that an emulsion is not formed. It happens quite often that an acid A with a melting point near or above ambient temperature will start to crystallise after the first or second extraction. 

The melt flow under isothermal conditions, when it is described by the rheological equation for the Newtonian or power law liquid, has been studied in detail63 66). The flow of the non-Newtonian liquid in the channels of non-round cross section for the liquid obeying the Sutterby equation have also been studied 67). In particular, the flow in the channels of rectangular and trigonal cross section was studied. In the analysis of the non-isothermal flow, attention should be paid to the analysis 68) of pseudo-plastic Bingham media. 

Simultaneous heat and mass transfer plays an important role in various physical, chemical, and biological processes hence, a vast amount of published research is available in the literature. Heat and mass transfer occurs in absorption, distillation extraction, drying, melting and crystallization, evaporation, and condensation. Mass flow due to the temperature gradient is known as the thermal diffusion or Soret effect. Heat flow due to the isothermal chemical potential gradient is known as the diffusion thermoeffect or the Dufour effect. The Dufour effect is characterized by the heat of transport, which represents the heat flow due to the diffusion of component / under isothermal conditions. Soret effect and Dufour effect represent the coupled phenomena between the vectorial flows of heat and mass. Since many chemical reactions within a biological cell produce or consume heat, local temperature gradients may contribute in the transport of materials across biomembranes. 

The development of the -modification is controlled by the relative crystallization thermodynamics and kinetics of the stable a-modification and of the smectic phase towards the metastable / -phase. For PP homopolymers, it is generally accepted that under isothermal conditions, the a-phase grows more rapidly at temperatures below 105 and above 140 °C than its counterpart, which in turn is more prone to develop in between these two temperatures in the presence of selective -promoters [52,70,122]. An elegant way to get fully nucleated /3-PP specimens would consist of pressing /3-PP pellets above their melting temperature (ideally more than 250 °C to erase any a-nuclei in the system), cool the melt quickly up to a crystallization temperature in between 100 and 130 °C, let the sample crystallize, and then quench it to room temperature [70]. However, such a processing method is too time-consuming to be of industrial relevance. 

Only MNTS, among the ten powdery ehemicals of the TD type tested herein, is powdery at room temperature but self-heats as a liquid at temperatures higher than its melting point. It thus follows that it is the Semenov equation, not the F-K equation, that should be applied to calculate the Ty for an arbitrary volume of molten MNTS confined in an arbitrary closed container of the corresponding shape and size, and placed in the atmosphere under isothermal conditions. It is, therefore, useless to measure the value of atr of a powdery specimen of MNTS at a temperature near room temperature. 

At all events, however, once a powdery chemical of the quasi-AC type confined in the closed cell finishes melting, the chemical starts the quasi-autocatalytic reaction suddenly, even if the test is performed under isothermal conditions. It is, therefore, possible also in the isothermal storage test to locate without much difficulty the exothermic onset point, i.e., the point of time b , such as indicated in Fig. 7 in Section 3.3, on the strip chart of the temperature recorder in particular, it is possible and easy to do so, if the test is performed at a Ti, on the high temperature side, corresponding to a relatively small value of At vl Eq. (59), nAt = a/T, + b, holding for the induction period of the quasi-autocatalytic reaction of 2 cm of a powdery chemical of the quasi-AC type confined in the closed cell and subjected to the isothermal storage test. 

The dynamic mechanical analysis gives detailed information about the viscoelastic properties of a sample when heated, cooled, or held under isothermal conditions. The three a, (3, and y peaks displayed by the material before melting can be used to evaluate the effects on the PE molecular structure of additives... 

Considerable supercoolings are realized in small liquid drops. Water drops from 500 to 20 pm in diameter in oil were located on the junction of a differential thermocouple. Every drop was melted down and crystallized several tens of times. Measurements at the same temperature were made on 5-10 drops similar in size. The distribution of crystallization events of isolated drops was studied in repeated experiments under isothermal conditions and continuous supercooling. ... 

Rubber particle size or ligament thickness has no effect on the onset of plastic deformation but may affect the extent of plastic deformation (i.e., the maximum draw ratio). A small change in draw ratio is of little consequence under isothermal conditions, but under adiabatic conditions it may result in enough heat generation for local melting and, hence, melt blunting. 

FIGURE 5 Time-temperature plot of CFLPE under isothermal conditions Tm - silicon melting point,... 

Figure 6.11 SEM micrographs of iPP/PB-l/HOCP blends (a) 50/10/40, crystaUized from the melt at room temperature (b) 50/30/20, crystallized from the melt at room temperature (c) 50/30/20, crystallized under isothermal condition up to complete crystallization of the iPP component and eventually quenched in liquid nitrogen. (From Reference 8 with permission from John Wiley Son, Inc.)...

We start the discussion with crystallization under isothermal conditions. When the temperature drops below the melting temperature, density fluctuations increase to critical size. Below critical size, fluctuations decay and above critical size, they become crystallization nuclei that are able to grow to the new phase by attaching stems of adjacent chains (so-called nucleation and growth mechanism). The dynamics of crystallization is governed by the dynamic law (see Chapter 7, Equation (9)) ... 

Samples were melt pressed in a vacuum laboratory hot press (Carver Press, Model C) at 160°C for 30 min. The molded films were then allowed to cool to room temperature under vacuum. A dual temperature chamber for the melt crystallization experiments consists of two large thermal chambers maintained at the melt temperature (Ti = 160°C) and the crystallization temperature (Ts = 81°C, 83°C, 86°C, 89°C, 92°C or 96°C). After 5-10 min at Ti, the copper sample cell was transferred rapidly ( 2 s) to the other chamber by means of a metal rod connected to a pneumatic device. A detailed description of the arrangement of the sample and of the two detectors used to measure WAXS and SAXS simultaneously has been provided previously [32]. Each polymer sample within the copper cell was 1.5 mm thick and 7 mm in diameter and was contained between two 25 im thick Kapton films. The actual sample temperature during crystallization (T2) and melting (Ti) was monitored by means of a thermocouple inserted into the sample cell. The crystallization temperature was usually reached 120 s after transfer without overshooting. Under isothermal conditions the fluctuations in the sample temperature are less than 0.5°C. Unless stated otherwise, all references to time are times elapsed after transferring the sample to the crystaUization chamber. 

The dimensionless temperature and down-channel distance in Figs. 7.112 and 7.113 are related to the average fluid velocity. In comparing Fig. 7.112 to 7.113, it is seen that increased pseudo-plasticity reduces the temperature build-up in the polymer melt at equal volumetric flow rates. It is also quite apparent that the temperature build-up under adiabatic conditions is substantially higher than under isothermal conditions. 

Figure 20.5. Polarized optical microscopic picture showing the development of transcrystalline layer on alpha-phase PP tape in a PP matrix. Note the tape was placed in PP melt that crystallized under isothermal conditions...

The question addressed here is whether the components crystallize separately or together. DSC is probably the most popular method. In this case the melting-point distribution w TjJ is measured and is transformed into a crystal thickness distribution w L ) using the Thompson-Gibbs equation. Let us select a sample with a broad, continuous molar mass distribution. The sample is crystallized under isothermal conditions for different periods of time. Each isothermal treatment is followed by a rapid cooling of the sample to a temperature at which crystallization is no longer active. The melting endotherm is recorded after each cycle of isothermal... 

Figure 8.31 Fraction of low temperature peak as a function of isothermal crystallization temperature for a sample with a broad molar mass distribution. Insert melting trace of sample crystallized under isothermal conditions followed by rapid cooling.

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