Crystalline-amorphous diblock copolymers

While EVA wax crystal modihers have a rather mixed performance, including rather significant fractions of inactive materials, the novel line of wax crystal modifiers based on relatively low molecular weight crystalline-amorphous diblock copolymers have proven to exhibit excellent activity as CFPP suppressors even in unruly fuels [77]. This included a number of middle distillate fuels that resisted any treatment or formulations based on the EVA additives. In the case of the EVA copolymers, the application evaluations were based on an Edisonian approach. In contrast, the development of the novel diblock copolymer additives was based on a microscopic scientific approach, where quantitative knowledge of the aggregation behavior guided the optimization of the additive activity. 

We will first describe the structure of the diblock copolymer additives. Then, from the structural data the aggregate free energy of formation is derived. Microscopic investigations also served in order to directly observe the wax additive interaction showing that the available aggregate surface is the decisive parameter. This aggregate surface is then derived on the basis of the free energy of formation and finally compared with experimental measiue-ments of the CFPP. 

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As we shall see, the hairy platelets formed by the crystalline-amorphous diblock copolymers consist of an inner crystalline core and an outer brush. This lateral profile gives rise fo a form factor that modulates the profiles of the Bragg rods. The form factor relates directly to the volume fractions b(z) of the brush and coordinate perpendicular to the platelet surface. In terms of these quantities the core brush form factor is given by ... 

Table 3 Representative molecular characteristics of PE-PEP crystalline-amorphous diblock copolymers...

It is widely recognized that amorphous-amorphous diblock copolymers form a variety of microdomain structures when the segregation strength between different blocks is moderately large. When one block is crystalline and the other is amorphous (i.e., crystalline-amorphous diblock copolymers), it is easily supposed that the morphology formation at low temperatures is driven by a close interplay between... 

Figure 10.7 Schematic illustration showing the possible morphology formation in crystalline-amorphous diblock copolymers by the crystallization of constituent blocks. The upper route represents break-out crystallization, that is, the microdomain structure is completely replaced with the lamellar morphology, whereas the lower route shows confined crystallization, where the microdomain structure is preserved after crystallization, a-d indicate driving factors for the morphology formation.

DlMarzio et al. [78] and Whitmore and Noolandi [79] theoretically predicted an equihbrium lamellar morphology formed in crystalline-amorphous diblock copolymers. The long period of the lamellar morphology L, that is, a sum of crystalline layer thickness and amorphous layer thickness, is expressed by a scaling form ... 

Figure 10.9 Illustration showing possible conformations of crystalline and amorphous blocks in the lamellar morphology of crystalline-amorphous diblock copolymers, (a) n. = 1, (b) n. = 2, and (c) n = i, where represents the chain-folding number of crystalline blocks.

When of crystalline-amorphous diblock copolymers is sufficiently large, the soft microdomain structure is stable against the subsequent crystallization. Therefore, this structure is preserved through the crystalUzation process, that is, constituent blocks crystallize within the soft microdomain stmcture, to yield a crystalline microdomain structure (lower route in Fig. 10.7). Amorphous domains in the crystalline microdomain structure are not hard in this case, so that crystalline domains can deform moderately during crystalUzation in order to get a larger crystalUnity and/or favorable crystal orientation, which is criticaUy different from the crystallization of block copolymers with high-T amorphous blocks, as described in Section 10.3.2. 

Model crystalline-amorphous diblock copolymers with a sufficiently large are not easy to synthesize. Therefore, experimental studies on the confined crystallization within soft nanodomains are very limited, and a relationship between the detailed crystalline morphology and the deformation of soft nanodomains is unclear. 

The crystalline morphology formed in crystalline-crystalline diblock copolymers is more complicated as compared with that in crystalline-amorphous diblock copolymers, because two kinds of crystallization start from some microdomain structure existing in the melt. It is useful to classify this crystallization into two cases in terms of the crystallizable temperature of both blocks (Fig. 10.8) two-step crystallization when of one block is significantly higher than that of the other, and simultaneous crystallization when both values are sufficiently close. 

The basic research on the crystallization in more complicated systems started recently to find ouf unique morphologies formed in polymer systems. The crystallization of block copolymers is a striking example of such crystallization, which is intimately dependent on the molecular characteristics of crystalline block copolymers. For example, the crystallization of crystalline-amorphous diblock copolymers yields the lamellar morphology or crystalline microdomain structure depending on xN of block copolymers, Tg of amorphous blocks, crystallization conditions, and so on. These kinds of crystallization have the possibility of developing new crystalline polymer materials. Therefore, we strongly anticipate future advances in this research field. 

Zhu L, Calhoun BH, Ge Q, Quirk RP, Cheng SZD, Thomas EL, Hsiao BS, Yeh F, Liu L, Lotz B. Initial-stage growth controlled crystal orientations in nanoconfined lamellae of a self-assembled crystalline-amorphous diblock copolymer. Macromolecules 2001 34 1244-1251. 

Chen WC et al. Self-assembly structures through competitive interactions of crystalline-amorphous diblock copolymer/ homopolymer blends Poly(epsilon-caprolactone-b-4-vinyl pyridine)/poly(vinyl phenol). Macromolecules 2009 42(10) 3580-3590. 

Hong S, Yang L, MacKnight WJ, Gido SP. Morphology of a crystalline/amorphous diblock copolymer poly((ethylene oxide)-b-butadiene). Macromolecules 2001 34(20) 7009-16. 

Spatial confinement provided by microphase separation should play a crucial role in the crystallization of crystalline/amorphous diblock copolymers. Register and coworkers [126 130] extensively studied crystallization kinetics and clarified how crystallization is influenced by the segregation strength, the domain structure, and the mobility of the amorphous chains. Readers who are interested in those general issues could refer to Chapter 6 or ref. [11]. This... 

It has been clarified that the crystallization temperature of crystalline/amorph-ous diblock copolymers strongly affects orientation of the crystallite. Such preferential orientation of the crystallite is also observed when crystalline/ amorphous diblock copolymers are crystallized in a thin film [142,143]. These studies suggest that the morphology of crystalline/amorphous block copolymers can be controlled at the nanometer scale by combining spatial confinement and the appropriate crystallization temperature. 

Hong S., MacKnight W. J., Russell T. P., and Gido S. P. (2001b) Structural evolution of multilayered, crystalline-amorphous diblock copolymer thin films. Macromolecules 34 2876-2883. 

Zhu L., Cheng S. Z. D., Calhoun B., Ge Q., Quirk R. P., Thomas E. L., Hsiao B. S., Yeh F., and Lotz B. (2000) Crystallization temperature-dependent crystal orientations within nanoscale confined lamellae of a self-assembled crystalline-amorphous diblock copolymer. J. Am. Chem. Soc. 122 5957-5967. 

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