Both comonomers crystallize

Another example of confined crystallization is found in the isothermal overall crystallization kinetics of di- and triblock copolymers of poly(e-caprolactone)- 

Random type copolymers, where both comonomers can crystallize, are also known. Here a distinction must be made between co-crystallization with iso-morphous or isodimorphic replacement, and when the comonomers do not cocrystallize. In either case the crystallization kinetics is directly influenced by the appropriate phase diagram. 

An example of isodimorphism, and the related crystallization kinetics, is typified by the random copolymers of 3-hydroxy butyrate-3-hydroxy valerate.(82) The melting temperature-composition relation of this copolymer was given in Fig. 5.17 (Volume 1). For this copolymer, depending on the composition, either of the comonomers can co-crystallize in the other s lattice. When the concentration of the 3-hydroxy butyrate is less than 40 mol percent, crystallization occurs in the poly(3-hydroxy butyrate) lattice. When its concentration is greater than 40 mol percent, crystallization takes place in the poly(3-hydroxy valerate) lattice. The copolymer that contains 41 mol percent of 3-hydroxy valerate reflects the coexistence of both crystal phases and corresponds to a pseudo-eutectic composition. 

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In the case of copolymer solutions, the melting temperature also depends on interactions between the different monomeric imits and the solvent. Considering the case in which the crystalline phase is pure (i.e., only monomeric units of a single type crystallize and no solvent is present in the lattice), the decrease in the melting temperature can be derived in a similar manner as for the homopolymer solution case using the Flory-Huggins theory with an appropriate modification [15]. To take into accoimt the interactions between both comonomers and solvent, the net interaction parameter for binary copolymers should be calculated as follows ... 

In a copolymer system where both comonomer units are crystaUizable, co-crystallization may be observed if the comonomer units are similar in chemical structure, repeat-unit length, and/or crystal chain conformation [80]. Furthermore, a copolymer is said to be isodimor-phic if co-crystallization of the different comonomer sequences leads to the observation of two distinct crystalline phases depending on copolymer composition [80]. 

Besides its effects on morphology, comonomer sequence distribution also affects copolymer crystallization kinetics. In statistical copolymers, due to the broad distribution of crystaUizable sequence lengths, bimodal melting endotherms are typically observed. In block copolymers, the dynamics of crystallization have features characteristic of both homopolymer crystallization and microphase separation in amorphous block copolymers. In addition, the presence of order in the melt, even if the segregation strength is weak, hinders the development of the equihbrium spacing in the block copolymer solid-state structure. 

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. 

In spite of the similarity of the structure of the monomer units the two corresponding isotactic polymers crystallize in two different chain conformations tiie helix of poly-3-methyl-l-butene contains four monomer units per turn (4/1) with a chain repeat of 6.85 A the helix of poly-4-methyl-l-pentene contains 3.5 units per turn (7/2) and has a repeat of 13.85 A. The copolymers tend to crystallize. Their chain conformation and cross sectional area in the crystal lattice are analogous to those of the homopolymer corresponding to the predominant comonomer. For 4-methyl-l-pentene contents higher than 50% some evidence exists that the system simultaneously contains both chain conformations. 

As to the B/branched a-olefin copolymers, the degree of cocrystallization falls progressively with the size of the comonomer units. Apart from the B/3MB system, already discussed, in the B/4-methyl-pentene-l copolymers partial isomorphous replacement of monomer units in the two homopolymer crystal phases is observed, with lattice dimensions changes (Table 1). With 4,4 -dimethyl-pentene-l both homopolymer phases occur, physically separated, without lattice dimension changes. In each case, for high butene contents, the PB II phase is observed, i.e. the phase with larger CSA, which indicates that at least some degree of cocrystallization is always present. 

The Tg of copolymers of e-CL and DXO was in the range from -64 to -39 °C [131]. Both crystallinity and Tm decrease with increasing amount of DXO and about 40% of DXO comonomer units of poly(e-CL-co-DXO) are incorporated into the poly(e-CL) crystals. Some inclusion of DXO in the crystalline lattice of poly(S-VL) was also indicated on the basis of crystallinity data [138]. 

The maximum rates of crystallization of the more common crystalline copolymers occur at 80—120°C. In many cases, these copolymers have broad composition distributions containing both fractions of high VDC content that crystallize rapidly and other fractions that do not crystallize at all. Poly(vinylidene chloride) probably crystallizes at a maximum rate at 140—150°C, but the process is difficult to follow because of severe polymer degradation. The copolymers may remain amorphous for a considerable period of time if quenched to room temperature. The induction time before the onset of crystallization depends on both the type and amount of comonomer PVDC crystallizes within minutes at 25°C. 

Alternating ethylene-tetrafluoroethylene copolymer (ETFE) having a 50/50 comonomer composition crystallizes in two polymorphic forms. An orthorhombic form, stable at room temperature, and a disordered hexagonal form, stable at high temperatures. The crystal structures of both forms have been studied and the structural disorder present in both forms have been analyzed as a function of the composition [250-256]. 

The CCD is the second most important microstructural distribution in polyolefins. Differently from the MWD, the CCD carmot be determined directly only the distribution of crystallization temperatures (CTD) in solution can be measured and one can try to relate this distribution to the CCD using a calibration curve. Two techniques are commonly used to determine the CTD or CCD of polyolefins TREF and Crystaf. Both operate based on the same principle chains with more defects (more comonomer molecules or stereo-and/or regioirregularities) have lower crystallization temperatures than chains with fewer defects. Figure 2.11 compares the TREF and Crystaf profiles of an ethylene/1-butene copolymer made with a heterogeneous Ziegler-Natta catalyst. Notice that they have very similar shapes the Crystaf curve is shifted toward lower temperatures because it is measured as the polymer chains crystallize, while the TREF curve is determined as the polymer chains dissolve (melt) and are eluted from the TREF column, as explained in the next few paragraphs. 

The subject of the crystallization of copolymers can be quite complex, dependent on the comonomer. It should also be recognized that the effects of variations in tacticity are very similar to the effects of comonomer inclusion, since both are effectively the insertion of defects into the polymer chain. The earliest treatment [26] recognized this fact, and is applicable to any defect, whether tactic, head-to-head link or comonomer, when measured as a defect content. This approach makes the assumption that all defects are excluded from the crystal. On this basis the probability of forming a critical secondary nucleus is dependent on the distribution of the defects throughout the polymer chain. The formulation of the probabilities leads to the logarithm of the rate of linear growth of a spherulite being dependent on the defect concentration. In practice, the behavior of most copolymers... 

Polymers are always polydisperse with a distribution in molar mass and often contain chain branches, either introduced specifically during synthesis or as a consequence of synthetic defects, and both these effects will influence the observed morphology. As we shall see later, copolymers are a special case however, the introduction of low levels of comonomers can lead to behaviour which is rather like that of random branched chains. Different molecular species crystallize in different stages indicating the thermodynamic control on the overall process, i.e. they are incorporated into the crystal structure at different temperatures and times. The intermediate and high molar mass component crystallizes early in the stacks of thick dominant crystals. Small pockets of rejected molten low molar mass material remain after crystallization... 

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