Blending cocrystallization

First of all it is worth to remind that there are not well documented cases of cocrystallizations between different components of a polymer blend. Hence, also... 

For instance, also the homopolymers PVF and PVDF have been described to crystallize in separate crystals in their blends [99] (Though constituted by isomorphous monomeric units which can cocrystallize in the copolymers in the whole range of composition, as seen in Sect. 4.1). Moreover, at least for the studied conditions, the polymorphic behavior of PVDF is not altered by the presence of PVF [99]. 

Blends of polybutylene terephthalate and polyethylene terephthalate are believed to be compatible in the amorphous phase as judged from (a) the existence of a single glass-transition temperature intermediate between those of the pure components and (b) the observation that the crystallization kinetics of the blend may be understood on the basis of this intermediate Tg. While trans esterification occurs in the melt, it is possible to make Tg and crystallization kinetics measurements under conditions where it is not significant. When the melted blend crystallizes, crystals of each of the components form, as judged from x-ray diffraction, IR absorption, and DSC. There is no evidence for cocrystallization. There is a slight mutual melting point depression. 

Figure 6.10. Schematic representation of the two types of cocrystallization of homo-PBT and PEE (with different PBT contents) in drawn blends of the two polymers (a) complete cocrystallization (PBT in PEE is 75-91 wt%), (b) partial cocrystallization (PBT in PEE is below 75 wt%, in the present case it is 50 wt%). (c = lamella thickness, Tc = crystallization temperamre, = length of the hard segments (PBT) in PEE. (From Apostolov et ai, 1994.)...

In summarizing the results from the last three sections, one can conclude that the systematic variation of microhardness under strain performed on (a) homo-PBT (Section 6.2.1), (b) its multiblock copolymer PEE (Section 6.2.2) and (c) on blends of both of these (this section) is characterized by the ability of these systems to undergo a strain-induced polymorphic transition. The ability to accurately follow the strain-induced polymorphic transition even in complex systems such as polymer blends allows one also to draw conclusions about such basic phenomena as cocrystallization. In the present study of a PBT/PEE blend two distinct well separated (with respect to the deformation range) strain-induced polymorphic transitions arising from the two species of PBT crystallites are observed. From this observation it is concluded that (i) homo-PBT and the PBT segments from the PEE copolymer crystallize separately, i.e. no cocrystallization takes place, and (ii) the two types of crystallites are not subjected to the external load simultaneously but in a sequential manner. 

A typical cooling curve for a blend sample is shown in Figure 6. There is typically an undercooling of about forty degrees which is observed for these samples. In all cases, only a single crystallization exotherm is noted, suggesting that cocrystallization is occurring. 

Very little work has appeared in the literature which deals with blends in which the component materials can cocrystallize. It is generally believed (16.17) that a requirement for cocrystallization is that there must be a close matching of the polymer chain conformations and of crystalline dimensions. Also, some level of miscibility should exist between the two polymers and the crstallization kinetics cannot be very different. Certainly, in the case of liquid crystalline polymers, in general, these requirements would be expected to be met. Some of our recent work (8) has suggested, however, that not all liquid crystal polymers do cocrystallize. The present work suggests that in certain cases it may be possible to achieve this effect. 

The presence of one phase In the xnelt for PB/EMA or PE/EMA-salt blends is not inconsistent with the thermal analysis results presented previously (17). It has been determined that there is separate crystallization of PE and EMA or EMA-salt in PE/EMA of PE/EMA-salt blends. In fact, even for two polymers which form one phase in the melt, co-crystallization is quite difficult to Induce. Kyu et al. (42) have shown that blends of linear and branched polyethylenes do not have the ability to cocrystallize and Prud homme (43) has shown that it suffices to have a small difference in molecular weight between two poly(ethylene oxides) to have separate crystallization. Therefore, homogeneity in the melt does not hinder the separate crystallization of the two conqponents of the mixture. 

A simultaneous (or concurrent) crystallization can only occur when the crystallization temperature ranges overlap and if the crystallizability of both blend components is similar. Cocrystallization is only possible when the components are isomorphic or miscible in the amorphous as well as in the crystalline phase. In both cases mixed crystals can result, but in the case of concurrent crystallization no changes in crystal strucmre may be induced. Cocrystallization requires chemical compatibihty, close matching of the chain conformations, lattice symmetry and comparable lattice dimensions [Olabisi et al., 1979]. Some examples of miscible polymer blends with two crystalline components are given in Table 3.3 together with the type of crystalhzation. 

Depending on the blend preparation cocrystallization when sequentially mixed and separate crystallization when simultaneously mixed... 

Since both components cocrystallize [Edward, 1986 Hu et al., 1987 Gupta et al., 1994] the crystalline growth can be considered to be identical. As a consequence, the differences seen in the Avrami exponent, n, in the blends must be due to a difference in the nucleation behavior that depends on the blend composition. A value varying from 0 (instantaneous nucleation) to 1 (sporadic nucleation) was attributed to the contribution of the nucleation of LLDPE and HOPE, respectively, to the Avrami constant, n. The remaining part of n (e.g., 1.94 for HOPE... 

Linear polyethylene (LPE or HOPE) has been blended with LLDPE with varying branch distributions. The critical branch content for phase separation was lower for mPE, where the branches were more evenly distributed, than for Ziegler-Natta LLDPE, where the branches are heterogeneously distributed. Where cocrystallization occurred, the unit cell of the crystals showed marked expansion (4). Crystallization conditions were shown to be important for the cocrystallization of blends of linear and branched polyethylenes, low isothermal temperatures promote cocrystallization as did quench cooling (5). 

Within a BPE with a distribution of branches, there will be a distribution of lamella thickness. This will result in a broad melting range. BPE with a bimodal distribution of branches will have a bimodal distribution of lamella thickness and a corresponding melting temperature range. When two polyethylenes are blended, assuming they are miscible, they will cocrystallize only where they have common MSL. Some molecular segments in each BPE will crystallize independently of... 

Datta and Birley (61) first reported on the crystallization behavior of HDPE/LLDPE blends using DSC and wide-angle X-ray diffraction (WAXD). In the DSC measurements, the blends revealed only one endothermic peak for both fast and slow cooling conditions, indicating that the blend components were cocrystallized. The X-ray... 

The effect of branch distribution of LLDPE on the crystallization of HDPE/ LLDPE blends has also been investigated. The DSC and scanning electron microscopy (SEM) studies of Lee et al. (68) found that aZiegler-Natta-catalyzed LLDPE-B having a relatively heterogeneous branch distribution (179,000 Mw, 4.2 PDl, and 16 ethyl branches per 1000 backbone carbons) cocrystallizes with HDPE (147,000 Mw and 3.7 PDl) in a wide range of blend compositions, and at the same time. 

From the results discussed above, it is concluded that the crystallization behavior of HDPE/LLDPE blends depends on the number, length, and distribution of branches in the LLDPE component. In general, it was found that increases in the number and degree of branch distribution in the LLDPE component reduce the tendency of cocrystallization in the LLDPE blend with HDPE. 

Microscopically Viewed Structural Characteristics of Polyethylene Blends Between Deuterated and Hydrogenated Species Cocrystallization and Phase Separation... 

From all these data analyses, we can definitely say that the D and H chain stems are distributed statistically randomly in the crystalline lamellae of the D/H cocrystallized blend. This conclusion is quite important in relation with the chain-folding problem, a controversial research theme that had been discussed for a long time (30). The random distribution of the D and H chain stems naturally supports the idea that the D and H chains reenter randomly into and out of the crystalline lamellae as shown in Fig. 5.7. The regular adjacent reentry model is impossible to apply at all as for as the melt-crystallized sample is concerned. 

In order to investigate the cocrystallization and phase segregation behaviors of the above-mentioned PE blend samples, we performed the experiments for both the isothermal and nonisothermal crystallizations. In the nonisothermal crystallization, the temperature is changed gradually and the WAXD, SAXS, or infrared spectra are collected as a function of temperature. In the isothermal crystallization, the... 

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