Melting temperature depression

For PVDF/PMMA blends prepared by a melt process without additional thermal treatment, the melting point of crystalline PVDF cannot be observed for 

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The melting temperature depression or enhancement may now be expressed in terms of the melting temperature and the entropy of fusion of component A, the activity coefficients of impurity B in the liquid and solid solutions at infinite dilution, and the total concentration of the impurity B, xb [12] ... 

The blend of a crystalline polymer undergoes a sharp melting-temperature depression following certain thermodynamic rules. Figure 7.6 shows the results of... 

DSC results showed a melting temperature depression of HOPE caused by the dilution effect of the non-crystalline EVA and the probable co-crystallization of some EVA chains with HD PE chains [38, 40]. Changes in the crystallization and melting temperatures of EVA were determined mainly by the nucleation effect of HDPE crystals and the effect of partial miscibility between these polymers. Crystallization kinetics results showed that the addition of more HDPE into an EVA matrix caused more heterogeneous nucleation, while the addition of EVA would delay the nucleation of HDPE at the beginning of the cooling process. Intermolecular interaction in the melt facilitated the crystallization of both EVA and HDPE. 

Papkov (1978, 1981) presented different versions of P+LMWL state diagrams where LMWL crystallization is important. For instance, a water melting temperature depression to —60°C has been observed in the system dextraiie+water at a 50% dextrane content (loclovich ct al., 1975). 

If the steric structures of both comonomer imits in random copolymers are similar, the melting temperature depression equation will be the same as Eq. 1, with the interaction parameter calculated with Eq. 4. For a given copolymer, the crystallizabilities of copolymer chains in dilute solution strongly depend on the chain composition. From thermodynanuc considerations, this can be explained from the fact that changes in copolymer composition alter the value of the interaction parameter de ed by Eq. 4. For copolymers with two chemically similar comonomers, xia will be very dose to xiB, ind Xab will approach zero, hi this system, one can simply use Eq. 1 with Xl = XiA Xib-... 

Equation (2.39) represents the relation between the melting temperature and crystallite thickness f for different chain lengths. The crystallite thickness is not constrained to its equilibrium value and cTec is characteristic of the particular interface that is developed in the crystallite under the specific set of crystallization conditions. The melting temperature depression, Eq. (2.39), is calculated from the equilibrium melting temperature of the infinite chain, T. For high molecular weights Eq. (2.39) reduces to... 

Typical phase diagrams that illustrate these principles are presented in Fig. 3.8 for isotactic poly(propylene)-alky phenol mixtures (24) and in Fig. 3.9 for poly (N,N -sebacoyl piperazine) (23) with various diluents. At the higher isotactic poly (propylene) compositions only liquid-solid curves are observed that result in typical melting temperature depression. Although at lower polymer concentrations... 

The values of determined from the melting temperature depression by diluent are given in Table 6.1 along with related quantities of interest. The values listed have been selected as representing the best for the given polymer, after carefully examining all the available data. To allow for different sizes the value... 

It is well-known that nylons have poor compatibility with other polymers because of their strong hydrogen bonding characteristics. The compatibility of nylon-6 with poly(vinyl-acetate) (PVAc) and poly(vinyl alcohol) (PVA) has been studied [203). Compatibility was judged from the melting temperature depression. The results indicate that nylon-6/poly vinyl... 

Using DSC, Giannotti and Valvassori [96] studied the melting enthalpy of SPS crystals in the presence of diluents by a melting-temperature depression approach. A A// value of 82.4 J/g was obtained, based on Flory s theory of polymer solutions. In contrast, a relatively low value A// = 53.2J/g was... 

T and T are equilibriiun melting temperatures of the polymer blend and the pure polymer respectively, and are the molar volumes of the repeat units of crystalline and amorphous polymer, respectively AH is the enthalpy of fusion of the perfect crystal ( ). is the volume fraction of the amorphous polymer and x is the polymer-polymer interaction parameter and is dependent on the heat of mixing but independent of combined entropy of mixing. Hence, melting temperature depression is only dependent on the degree of interaction between the polymer, provided the samples are crystallized and melted in the same manner [65]. To determine the interaction parameter, an accurate measurement of the equilibrium melting temperature is needed. However, for comparison purpose the non-equilibrium melting temperature depression can be used to yield a qualitative, rather than a quantitative, estimation of the level of interaction between the components. 

It was pointed out in the discussion of melting temperature depressions that the actual concentration of the chain structural defects needs to be specified. Specification of the pentad fraction is not adequate. Analysis of the crystallization kinetics could be affected in a significant manner, depending on the sequence distribution. 

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