Melting point depressions

PAN homopolymer melts at approximately 320 °C whereby melting refers to the endothermic breaking up of the highly ordered crystalline regions. The high melting point has been related to molecular stiffness (cf. Sect. 2.1). 

Introduction of a comonomer into the polymeric chain depresses the melting point. A copolymer AN/VA with 7% VA, for instance, melts at s277 °C. 

The depression of the melting point of a polymer by the presence of small amounts of comonomer is described by Flory and Eby Flory suggested that the comonomer is relegated to the amorphous regions of the polymer, its presence thus reducing the extension of the crystalline regions. Flory s equation for the melting point depression caused by comonomer B in polymer A reads  

Actually, a linear relationship between 1/T , and Xj is generally observed however, different comonomers result in varying values for AH for the same A. Eby s theory takes care of this discrepancy, by assuming that the comonomer B is not relegated to the amorphous phase, but instead enters the crystalline lattice as a point defect. The degree to which the comonomer disrupts the intermolecular bonds that stabilize the crystalline lattice is then, to the first approximation, proportional to the voliune it occupies within the lattice. An approximate form of Eby s equation has been given by Frushour 

Tabk 1. Meltingpointdepressionconstants, kg, for copolymers AN/B(Frushour ) and molar volume of B 

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Garbodiimide Formation. Carbodiimide formation has commercial significance in the manufacture of Hquid MDI. Heating of MDI in the presence of catalytic amounts of phosphine oxides or alkyl phosphates leads to partial conversion of isocyanate into carbodiimide (95). The carbodiimide (39) species reacts with excess isocyanate to form a 2 + 2cycloaddition product. The presence of this product in MDI leads to a melting point depression and thus a mixture which is Hquid at room temperature. 

The effect of different types of comonomers on varies. VDC—MA copolymers mote closely obey Flory s melting-point depression theory than do copolymers with VC or AN. Studies have shown that, for the copolymers of VDC with MA, Flory s theory needs modification to include both lamella thickness and surface free energy (69). The VDC—VC and VDC—AN copolymers typically have severe composition drift, therefore most of the comonomer units do not belong to crystallizing chains. Hence, they neither enter the crystal as defects nor cause lamellar thickness to decrease, so the depression of the melting temperature is less than expected. 

Batchwise fractional distillation can be used to adjust the ratio of isomers in the mixture. For example, the heat-transfer composition of Table 4 is obtained by collecting all of the ortho- some of the meta- and excluding most of the i ra-terphenyl present in the natural mixture. Economics and considerations of melting point depression favor inclusion of lower melting quaterphenyl isomers. 

A different melting point, and hence supercooling, is predicted for the strained sector. This is the basis for a different interpretation of the (200) growth rates a regime //// transition occurs on (110) but not on (200). This is despite the fact that the raw data [113] show a similar change in slope when plotted with respect to the equilibrium dissolution temperature (Fig. 3.15). It is questionable whether it is correct to extrapolate the melting point depression equation for finite crystals which is due to lattice strain caused by folds, to infinite crystal size while keeping the strain factor constant. 

A very recent application of the two-dimensional model has been to the crystallization of a random copolymer [171]. The units trying to attach to the growth face are either crystallizable A s or non-crystallizable B s with a Poisson probability based on the comonomer concentration in the melt. This means that the on rate becomes thickness dependent with the effect of a depletion of crystallizable material with increasing thickness. This leads to a maximum lamellar thickness and further to a melting point depression much larger than that obtained by the Flory [172] equilibrium treatment. 

Eq. (29) is closely related to the classical melting point depression and solubility expression for solutions of simple molecules. In the case of the ideal solution, for example, m2 m2= In N2, N2 being the... 

NMR cryoporometry relies on the melting point depression, i.e., the difference in the melting point of crystals with a finite size d, Tm(d), relative to the value of the bulk liquid Tm, which is given by the simplified Gibbs-Thomson equation [16] ... 

The identity of the dimethylketene may be determined (and an approximate check made on the concentration) by adding 35 g. of dimethylketene solution to 15 g. of aniline in 75 ml. of ether. After 2-3 minutes, the ether solution is washed with dilute hydrochloric acid, dilute potassium carbonate, then water, and the ether is evaporated. Isobutyroanilide, m.p. 102-103° (103-104° after one recrystallization, no melting-point depression in mixture... 

Floras melting-point depression theory, 25 700 Flotation... 

In this study, melting point depression was used to obtain the parameter X12, known as the Flory-Huggins interaction parameter [40] ... 

When Flory s theory (1953) of melting point depression is applied to starch gelatinization (or phase transition) in the presence of water, the situation can be described as follows. Af equilibrium state, the chemical potentials between amorphous (pu) and crystalline repeating units (p of fwo phases are equal ... 

Some soHd compounds exhibit a significant melting point depression with CO2 pressure (induced melting) [125]. For example, the melting point of the complex 5b changes from 383 K (0.1 MPa) to 301 K (6.5 MPa). This phenomenon was taken into account by the development of a saturation cell for Hquids and solids that Hquefy. 

Rast method analy chem The melting-point depression method often used for the determination of the molecular weight of organic compounds. rast. meth-od rate constant physchem Numerical constant in a rate-of-reaction equation for example, = feCaACbBCcC, where and Cc are reactant concentrations, k is the... 

The density of this form is 1.94 gm/cm . Six Sg molecules, i.e., 48 atoms, occupy the unit cell the space group is P2Jar-C2h. The ideal melting point has been recently computed to be 133°C 92), but the crystals melt around 128° because of decomposition of the Sg ring, and subsequent melting point depression by solution of the resulting rings and chains. 

Note 4 A monotropic mesophase can be stabilised in a mixture when, as a result of melting-point depression, a metastable mesophase becomes stabilised. Such a mesophase is distinct from an induced mesophase. 

Although these approaches seem to be similar, the former method has several limitations it should be recognized that intrinsic mobility (yu,A ) depends on the ionic strength of the buffer, as shown in Ref. 17. Also, the activity coefficients of the zwitterions could not be calculated via the De-bye-Hiickel equation, and other methods, such as melting point depression, should be used to obtain the activity coefficients of the zwitterions (31). On the other hand, the latter method is directly applicable to the zwitterions to obtain their acidity constants. 

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