Semicrystalline block copolymers

The morphology of the solids formed from block copolymers in which one component can crystallise often depends strongly on whether they are 

An interesting question arises for those copolymers that crystallise with lamellar structures. Do the polyethylene chains fold so that the chain axes are perpendicular to the lamellar surface, as they do in the homopolymer, or parallel to it The answer need not be the same in all cases, even for a single type of copolymer, but there is certainly evidence that the chains fold so that their axes are perpendicular to the lamellar planes in some cases. Such evidence has, for instance, been obtained for PE-PEP-PE triblocks by studying the development of WAXS and SAXS patterns as samples were oriented by drawing. 

These results show that the crystalline block copolymers constitute an interesting class of materials, but their study is a relatively new area of polymer science and much work remains to be done before the various morphologies can be reprodudbly obtained and fully understood. 

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The technique of self-nucleation [75] can be very useful to study the nucleation and crystallization of block copolymer components, as already mentioned in previous sections. In block copolymers, factors like the volumetric fraction and the degree of segregation affect the type of confinement and therefore modify the self-nucleation behavior. In the case of semicrystalline block copolymers, several works have reported the self-nucleation of either one or both crystallizable components in PS-fc-PCL, PS-b-PB-b-PCL, PS-b-PE-b-PCL, PB-fr-PIB-fr-PEO, PE-fr-PEP-fr-PEO, PS-fc-PEO, PS-h-PEO-h-PCL, PB-b-PEO, PB/PB-fc-PEO and PPDX-fc-PCL [29,92,98,99,101-103,134] and three different kinds of behavior have been observed. Specific examples of these three cases are given in the following and in Table 5 ... 

Fig. 5.10 Schematic of perpendicular and parallel chain folding in semicrystalline block copolymers.

Combination of Living Cationic and Anionic Ring-Opening Polymerization for the Synthesis of Semicrystalline Block Copolymers... 

Crystallization in block copolymers has a profound effect on their structure. This review article focusses on the morphology of semicrystalline block copolymers, and those containing two crystallizable blocks. The effect of crystallization on mechanical properties is briefly considered. The extent of chain folding upon crystallization is discussed, as is the orientation of crystal stems with respect to the microstructure. The effect of selective solvent on solution crystallization is also highlighted. Recent work on crystallization kinetics is summarized and finally the theories for crystallization in block copolymers are outlined. 

Crystallization has been investigated for other block copolymers, in particular those containing poly(e-caprolactone) (Tm=57 °C). Experimental results for these materials are also summarized here. The morphology in block copolymers where both blocks are crystallizable is also discussed. However, except in section 7, the remainder of this review concerns semicrystalline block copolymers. Elements of this review have appeared in an extended form elsewhere [ 1 ]. 

Crystallization in semicrystalline block copolymers can have a dramatic impact on mechanical properties, and hence is important to end use. There have been numerous studies, in particular of PE-containing block copolymers including industrial materials. A comprehensive overview of this work is outside the scope of this review. Instead, an illustrative example serves to illustrate the essential physics. 

The sonic technique suffers from one of the same difficulties as the birefringence method in that only an average orientation of the total system is obtained. Thus, if one has a multicomponent system (e.g. semicrystalline, block copolymer. etc.X one cannot separate component orientation by this technique alone. It is noteworthy that the orientation measured by the sonic method can be correlated directly with birefringence data on the same material.This ease of correlation has been suggested to lead to values of the intrinsic birefringence by extrapolation of the sonic data to perfect orientation. 

De Rosa, C., Park, C., Lotz, B., Fetters, L.J., Wittman, J.C., Thomas, E.L. Control of molecular and microdmnain mientation in a semicrystalline block copolymer thin film by epitaxy. Macromolecules 33,4871 (2000)... 

The crystallinity index, determined by various methods, on a semicrystalline block copolymer system with values ranging from 30—75%, has highlighted the problems in defining the index. In particular, the thermal analysis method gave lower values than did density determination. A better correlation was obtained between X-ray and density determined crystallinity, but no reason was offered for these observations. 

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... 

He WN, Xu JT. Crystallization assisted self-assembly of semicrystalline block copolymers. Prog Polym Sci 2012 37 1350-1400. 

Shiomi T, Tsukada H, Takeshita H, Takenaka K, Tezuka Y. Crystallization of semicrystalline block copolymers containing a glass amorphons component. Polymer 2001 42 4997-5004. 

In semicrystalline block copolymers, the crystallization behavior is often more complex than that observed in statistical copolymers because the solid-state morphology adopted by block copolymers can be driven either by block incompatibility or by crystallization of one or more blocks [5-8]. In this chapter, we will cover only block copolymers with homogeneous or weakly segregated melts, such that crystallization is always the dominant factor in determining solid-state morphology. Crystallization of block copolymers from strongly segregated melts is covered in Chapter 12. Furthermore, the... 

Figure 11.12 Schematic of the equilibrium model for a semicrystalline block copolymer, where d represents the characteristic lamellar microdomain spacing. Reprinted with permission from Reference [102]. Copyright 1980 American Chemical Society.

In semicrystalline block copolymers, the presence of a noncrystalline block enables modification of the mechanical and structural properties compared to a crystalline homopolymer, through introduction of a rubbery or glassy component. Crystallization in homopolymers leads to an extended conformation, or to kinetically controlled chain folding. In block copolymers, on the other hand, equilibrium chain folding can occur, the equilibrium number of folds being controlled by the size of the second, noncrystallizable block. The structure of block copolymers following crystallization has been reviewed [1,145]. 

Theories for semicrystalline block copolymers are able to provide predictions for the scaling of amorphous and crystal layer thickness with chain length... 

Crystallization of a semicrystalline block copolymer quenched from the melt will also be briefly reviewed. Chu and Hsiao [68] comprehensively reviewed recent developments in SAXS where they discussed simultaneous measurements with other techniques. Among recently developed techniques, we will focus on simultaneous SAXSAVAXS (wide-angle X-ray scattering) and/or Hv-SALS (depolarized small-angle light scattering) measurements [69,70] because these are powerful techniques to study crystallization and spherulitic higher-order hierarchical structures in semicrystalline block polymers [71,72]. Current developments will also be reviewed later in the subsection on semicrystalline block polymers. 

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