Melting point depression, polymer crystal

Chain ends and branches can be thought of as impurities which depress the melting points of polymer crystals. The behaviour can be analysed in terms of the chemical potentials (Section 3.2.3) per mole of the polymer repeat units in the crystalline state and in the pure liquid //H (the standard state). For the pure polymer, which is a single-component system... 

If the value of this probability parameter is a <= 1, an essentially iso tactic structure is obtained. If, on the other hand a 0, almost all the monomer units in the chain are in syndiotactic placements. If the polymer is capable of crystallization and the crystallization takes place under equilibrium conditions, then the limitation of this model is that a small melting point depression implies also a high degree of percent crystallinity. Although there are a number of systems, for example stereoregular methyl methacrylate (2, 8), in which this is true and this model is valid, this is not the case for polymers of propylene oxide from different catalysts that we discuss in another chapter (1). 

The melting temperature depends on water content, as depicted in Figure 6.24b. This phenomenon is comparable to the melting point depression commonly observed for impure solid materials (e.g., imperfect crystals). Flory has derived an equation for the melting temperature Tm as a function of the volume fraction of polymer q> in concentrated polymer-... 

The DSC results for HyPPS03Na materials demonstrate the existence of low-melting crystals that form when these polymers are maintained at room temperature. The volume fraction of such crystals increases as the degree of sulfonation is increased, but the melting temperature is unaffected by the level of sulfonation. The higher-melting crystallites qualitatively obey the Flory equation for melting-point depression in random copolymers. 

The principles of polymer fractionation by solubility or crystallization in solution have been extensively reviewed on the basis of Hory-Huggins statistical thermodynamic treatment [58,59], which accounts for melting point depression by the presence of solvents. For random copolymers the classical Flory equation [60] applies ... 

Miscible CTystaUine/amorphous polymer blends such as PLA/PVC blends have been widely investigated, and oriented crystallization has also been applied to some miscible ciystalline/amorphous polymer blends. For miscible blends containing semicrystaUine polymers, analysis of the melting point depression is widely used to estimate the Flory-Huggins interaction parameter (x). 

For polymer blends in which one component is crystalline the melting behaviour depends on circumstances. For immiscible blends, where the components are phase separated (prior to crystallisation) and act independently, the crystal melting temperature will be that of the homopolymer. In miscible blends, where the amorphous phase contains both components, the melting temperature will be lower than the equilibrium melting temperature for the crystallisable homopolymer, i.e. the crystalline polymer exhibits a melting point depression as discussed above. The Nishi and Wang approach (Sect 3.2) has been used to estimate the magnitude of the interaction parameters in a niunber of blends (Sect. 7). Poly(e-caprolactone) blends are often semi-crystalline and the above considerations, therefore, apply to many PCL blends. 

In the previous sections it was shown that the formation of lamellae with folded chains was essentially a kinetically controlled phenomenon. This section treats the free energy of polymer crystallization and melting point depression. 

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