Miscible blend both components crystallize

The temperature interval for crystallization of most of the blends is small so that a quantitative analysis of the data according to Eq. (11.8) is precluded. 

In contrast to the spherulite growth rates, the overall crystallization of both components can be resolved in these blends.(33) Typical isotherms are observed for the crystallization of poly(vinylidene fluoride). They can be fitted with an Avrami = 3 for a significant portion of the transformation. There is a progressive shift of the isotherms to longer times with dilution. These results are thus consistent with the reduction in spherulite growth rates with the addition of poly(butylene 

---

Chlorinated polymers/Copolyester-aniides Recent studies (5) of blends of chlorinated polyeAylenes with caprolactam(LA)-caprolactone(LO) copolymers have been able to establish a correlation between miscibiUty and chemical structure within the framework of a binary interaction model. In some of the blends, both components have the ability to crystallize. When one or both of the components can crystallize, the situation becomes rather more complicated. Miscible, cystallizable blends may also undergo segregation as a result of the crystallization with the formation of two separate amorphous phases. Accordingly, it is preferable to investigate thermal properties of vitrified blends. Subsequent thermal analysis also produces exothermic crystallization processes that can obscure transitions and interfere with determination of phase behavior. In these instances T-m.d.s.c has the ability to separate the individual processes and establish phase behavior. 

This has been observed for blends of poly(1,4 cyclohexanedimethanol terephthalate) with PC (29) and for PVF2 blends with poly(vinyl acetate) (10). An even more intriguing situation is case (e), where polymer 2 does not crystallize at all in the neat state but through the plasticizing action of the miscible diluent, it does so in blends. This has been observed for PC, which does not crystallize during normal melt processing, but does so when blended with poly-(e-caprolactone) (31) and other low Tg polyesters (13) as illustrated in Fig. 5. This situation can result when the transition behavior follows the pattern shown on the right in Fig. 2. Other systems show minor variations of those patterns illustrated in Fig. 4 and combinations of them when both components crystallize. 

The blend composition, the crystallization conditions, the degree of miscibility and the mobility of both blend components, the nucleation activity of the amorphous component are important factors with respect to the crystallization kinetics (3.3.4). 

When dealing with miscible blends containing two crystalline components, several modes of crystallization are possible separate crystallization, concurrent crystallization, co-crystallization, etc. Only those blends in which both components are miscible in the melt are considered here (Table 3.3). PET/PBT blends were reported to be an example of separate crystallization [Escala and Stein, 1979 Stein et al., 1981]. A spherulitic crystallization was observed for the neat components as well as for blends with small amounts of one component, and the crystals of the minor component were included within the spherulites of the major component, which results in a coarsening of the spherulitic texture. Transesterihcation is, however, the reason for the homogenous amorphous phase. 

For some miscible blends, a melting point elevation has been reported with respect to that of the neat crystallizable component, both crystallized at the same temperature [Eshuis et al, 1982 Rim and Runt, 1983 1984]. These observations may originate from recrystallization, enhanced crystal perfection, and increased crystal size. 

The crystallization of blends tends to depend on the level of mutual miscibility of the components. In miscible blends, the general result is that suppression or otherwise of crystallization with miscibility is dependent on the relative glass transition temperatures of both phases [33, 34]. For example, in a blend of an amorphous and semicrystalline polymer, if the amorphous material has the higher Tg, the miscible blend will also have a higher Tg than that of the semicrystalline homopolymer and, at a given temperature, the mobility and thus the efficacy of the semicrystalline phase molecules to crystallize is reduced. The converse is often true if the amorphous phase has a lower glass transition. Effects such as chemical interactions and other thermodynamic considerations also play a role and the depression of the melting point in a miscible blend can be used to determine the Flory interaction parameter x [40]. 

Until a few years ago, most of the recognized examples of miscible pol3nner blend pairs involved only amorphous components. However, recently numerous blend systems have been identified in which one or both components are crystallizable. The systems of interest here are miscible in the melt state, but upon cooling one or more of the components separates from the mixture as a pure crystalline phase. This form of solid-liquid phase separation represents a different situation than a liquid-liquid miscibility gap since complete miscibility may still exist in the remaining amorphous phase. The objective of this paper is to review some of the pertinent fundamental issues and recent results for miscible blends where crystallization is possible. 

Important morphological questions about miscible blends in which one or both components have crystallized concern the nature of the overall crystalline texture and the location of the amorphous material. A number of recent publications have sought answers to these questions for particular systems. Early microscopy studies showed that noncrystallizable diluents increased the coarseness of the spherulites of the crystallizable components (21,22) however, more recent scattering experiments have been able to determine more detailed information. 

When dealing with miscible blends containing two crystalline components, several modes of crystallization are possible separate crystallization, concurrent crystallization, cocrystallization, etc. Only those blends in which both components are miscible in the melt are considered here (Table 3.3). PET/PBT blends were reported to be an example of separate crystallization (Escala and Stein 1979 ... 

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. 

The former discussion deals with liquid-liquid phase behavior however, one or both components of the blend can sometimes crystallize. For a polymer pair that is miscible in the melt, cooling well below the melting point of pure ciystallizable component leads to a pure crystalline phase of that component. Far below the melting point, the free energy of crystallization is considerably larger than that of mixing- Because polymers never become 100% crystalline, the pin-e crystals coexist with a mixed amorphous phase consisting of the material that did not crystalhze (6,7). 

Crystallization of blended polymers leads to their phase separation, because the formation of isomorphic crystals occurs very rarely. Blends of poly(ethylene terephthalate) and poly(butylene terephthalate) remain miscible in the amorphous phase after crystallization of both components. Unlimited mutual solubility of polymers is exceptional. It can be achieved under certain conditions, for example, in blends of poly(vinyl chloride) and butadiene-nitrile rubber or poly(vinyl acetate) and cellulose nitrate. 

---