Metastable copolymer crystals

Figure 5.27 seems to verify Eq. (4). A more careful analysis, however, shows that copolymer crystals are never close enough to equilibrium to follow Eq. (4). For example, if one extrapolates the curve described by Eq. (4) to concentration equal to 1, one never reaches the equilibrium melting temperature. The temperature observed is typically 5 to 10 K below the equilibrium melting temperature, indicating that even the largest crystals of the copolymer are metastable and must be treated as thin lamellae, as is discussed in Fig. 4.28. This raises the question How well can a metastable equilibrium be expressed by an equilibrium equation At best one can hope that the measured free enthalpy is parallel to the equilibrium curve (see Figs. 4.29 and 4.30). Then, one can introduce ATj j, the measured lowering of melting point from the zero-entropy-production melting temperature of the homopolymer, into Eq. (4) for the approximate evaluation of the influence of...

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

Conformational disorder and kink-bands structures have recently been found also in random copolymers of syndiotactic polypropylene with small amounts of ethylene.192 193 The ethylene units are included in the crystalline regions193 and induce the crystallization of the metastable form II of sPP with conformationally disordered chains characterized by kink bands. Portions of chains containing the ethylene units tend, indeed, to assume a trans planar conformation, producing the kink-band defects in chains in the prevailing twofold helical conformation.192193... 

Liquid crystals with a large molecular mass are able to form a glassy state with mesomorphic behaviour. Yitzchaik and coworkers38 report about a new class of copolymers with non-linear optical properties, and a monomeric representative with a glassy metastable... 

In the copolymers described in Sect. 3.4, the multiple components of the system are joined by chemical bonds and demixing, needed for complete phase separation of the components, is strongly hindered and may lead to partial or complete decoupling from crystallization. The resulting product is then a metastable micro- or nanophase-separated system with arrested, local equilibria. In some cases, however, it is possible to change the copolymer composition during the crystallization or melting by chemical reactions, such as trans-esterification or -amidation. In this case, the chemical and physical equilibrium must both be considered and a phase separation of the copolymer into either crystalline homopolymers or block copolymers is possible. 

Keller A, Hikosaka M, Rastogi S, Toda A, Barham PJ, Goldbeck-Wood G (1994) An approach to the formation and growth of new phases with application to polymer crystallization effect of finite size, metastability, and Ostwald s rule of stages. J Mater Sci 29(10) 2579-2604 Kim G, Han CC, Libera M, Jackson CL (2001) Crystallization within melt ordered semicrystalline block copolymers exploring the coexistence of microphase-separated and sphtmilitic morphologies. Macromolecules 34(21) 7336—7342... 

This portion of the chapter can be summarized by noting that there is a substantial body of evidence demonstrating that formal phase-equilibrium thermodynamics can be successfully applied to the fusion of homopolymers, copolymers, and polymer-diluent mixtures. This conclusion has many far-reaching consequences. It has also been found that the same principles of phase equilibrium can be applied to the analysis of the influence of hydrostratic pressure and various types of deformation on the process of fusion [11], However, equilibrium conditions are rarely obtained in crystalline polymer systems. Usually, one is dealing with a metastable state, in which the crystallization is not complete and the crystallite sizes are restricted. Consequently, the actual molecular stmcture and related morphology that is involved determines properties. Information that leads to an understanding of the structure in the crystalline state comes from studying the kinetics and mechanism of crystallization. This is the subject matter of the next section. 

On cooling a typical copolymer melt, one observes, after the customary supercooling, crystallization of pure A. The melt must thus increase to some degree in concentration B as predicted by the liquidus line of Fig. 4.23. But in copolymer systems, one neither reaches the liquidus concentration, nor observes the eutectic point. The system freezes to a metastable state before the eutectic temperature is reached. Usually only one component crystallizes in random copolymers. All of the component B and a large fraction of A remain in the amorphous portion of the semicrystalline sample. For a more extensive discussion of the irreversible melting of homopolymers and copolymers see Ref. 57, Chapters IX and X. 

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