Crystalline block copolymers

Block copolymers can contain crystalline or amorphous hard blocks. Examples of crystalline block copolymers are polyurethanes (e.g. B.F. Goodrich s Estane line), polyether esters (e.g. Dupont s Hytrel polymers), polyether amides (e.g. Atofina s Pebax grades). Polyurethanes have enjoyed limited utility due to their relatively low thermal stability use temperatures must be kept below 275°F, due to the reversibility of the urethane linkage. Recently, polyurethanes with stability at 350°F for nearly 100 h have been claimed [2]. Polyether esters and polyether amides have been explored for PSA applications where their heat and plasticizer resistance is a benefit [3]. However, the high price of these materials and their multiblock architecture have limited their use. All of these crystalline block copolymers consist of multiblocks with relatively short, amorphous, polyether or polyester mid-blocks. Consequently they can not be diluted as extensively with tackifiers and diluents as styrenic triblock copolymers. Thereby it is more difficult to obtain strong, yet soft adhesives — the primary goals of adding rubber to hot melts. 

Chao CY, Li X, Ober CK, Osuji C, Thomas EL. Orientational switching of mesogens and microdomains in hydrogen bonded side-chain hquid-crystalline block copolymers using AC electric fields. Adv Funct Mater 2004 14 364-370. 

Lee KM, Han CD. Microphase separation transition and rheology of side-chain liquid-crystalline block copolymers. Macromolecules 2002b 35 3145-3156. 

Osuji C, Chao CY, Bita 1, Ober CK, Thomas EL. Temperature-dependent photonic bandgap in a self-assembled hydrogen bonded liquid-crystalline block copolymer. Adv Funct Mater... 

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

Light microscopy has been used in a number of contexts to characterize block copolymer morphology. For crystalline block copolymers, spherulitic structures that result from organization of crystalline lamellae can be examined using microscopy. In solutions, polarized light microscopy can reveal the presence of lamellar and hexagonal-packed cylindrical micellar phases. Cubic micellar phases are optically isotropic, and consequently cannot be distinguished from sols only on the basis of microscopy. 

Due to the lack of vinyl monomers giving rise to crystalline segment by cationic polymerization, amorphous/crystalline block copolymers have not been prepared by living cationic sequential block copolymerization. Although site-transformation has been utilized extensively for the synthesis of block copolymers, only a few PIB/crystalline block copolymers such as poly(L-lactide-fc-IB-fc-L-lactide) [92], poly(IB-fr- -caprolactone( -CL)) [93] diblock and poly( -CL-fr-IB-fr- -CL) [94] triblock copolymers with relatively short PIB block segment (Mn< 10,000 g/mol) were reported. This is most likely due to difficulties in quantitative end-functionalization of high molecular weight PIB. 

Keywords Coil-crystalline block copolymers Micelles in solution Light scattering Transmission electron microscopy Cross-linkable micelles... 

Scheme 14 Chemical structure of hydrogen-bonded side-chain liquid-crystalline block copolymer [28]...

ADAMS GRONSKI Amorphous—Liquid-Crystalline Block Copolymers 175... 

The critical thicknesses are thus in the range of the dimensions of lamellar, cylindrical or spherical mesophases in block copolymers with ordered morphologies. The question is whether the phase boundary between the amorphous and the liquid-crystalline phase in a block copolymer will exert an ordering effect as assumed in the original theory or rather a disordering influence. The latter case and transitions between the two cases have also been treated recently by an extension of the theory (5). Therefore a theoretical framework exists, within which the transition behaviour of amorphous / liquid-crystalline block copolymers can be described. 

ADAMS GRONSKI Amorphous—Liquid- Crystalline Block Copolymers... 

The cosolvent method, also known as solvent injection method permits vesicle formation for glassy or crystalline block copolymers. As a rule, the amphiphilic copolymer is dissolved in an appropriate organic solvent or solvent mixture, the role of which is to lower the Tg below room temperature next the solution is added dropwise to an aqueous buffer under vigorous stirring. Originally this method has been employed for PS-PAA and PS-PEO copolymers that self-assembled in vesicular structures by adding water to DMF or dioxane polymer solution [61,105] and then further applied to many other polymer systems [165],... 

Bly et al. (4) used the near-IR bands at 2.312 and 2.275 p, and stated that these bands may be preferable to the fundamental ones for crystalline block copolymers and homopolymer mixtures. 

Scheme 2 Various donor and acceptor monomers can be combined to obtain block copolymers with amorphous or crystalline segments. In the left box, the polymerizable monomers are shown. On the right, the architectures of the resulting two main classes of D-A block copolymers are depicted amorphous-crystalline and crystalline-crystalline block copolymers...

However, another approach for improving the charge carrier mobility is to employ conjugated, semi-crystalline polymers. Here, a further advantage is the extended absorption in the visible range. These issues are addressed in the next chapter which is concerned with crystalline-crystalline block copolymers comprised of poly(3-hexythiophene) and PPerAcr. 

Scheme 6 One-pot synthesis of P3HT-MI macroinitiators for NMRP using the McCullough method followed by in situ endcapping with the Grignard derivative of a common alkoxyamine initiator. Starting from these macroinitiators, the acceptor monomer PerAcr is polymerized to give fully functionalized, double-crystalline block copolymers P3HT-/ -PPerAcr...

Xiang, M.L. et al., Surface stability in liquid-crystalline block copolymers with semifluorinated monodendron side groups, Macromolecules 33, 6106-6119, 2000. 

The results are presented in Table III. These values for the volume fractions agree well with those calculated from the mole % PS. The same conclusion was reached for non-crystalline block copolymers. Therefore, the agreement also supports the theory that the domain is formed first (8). 

Figure 2. Structure of liquid crystalline block copolymers (LC-BCPs) (A) rod-coil diblock copolymer (B) rod-coil diblock copolymer with flexible spacer in the rod block (C) side group liquid crystal-coil (SGLC- coil) diblock copolymers (D) coil -rod-coil ABC triblock copolymers (predicted to be novel ferroelectric fluid by R. G. Petschek and K. M. Wiefling, Phys. Rev. Lett., 1987, 59(3), 343-346) (E) rod-rod diblock copolymer (one example of well-defined po-ly(n-hexyl isocyanate-fc-n-butyl isocyanate) rod-rod diblock copolymer was given by Novak et al. [68], however, no morphology studies were reported) (F) dendritic liquid crystal-coil (DLC-coil) diblock copolymer (not reported).

Table 3. Comparison of different synthetic methods for liquid crystalline block copolymers.

The field of liquid crystalline block copolymers has undergone great development in the last 10 years showing increasing attraction for both polymer chemists and polymer physicists. More work needs to be carried out in order to fully explore this class of materials. Novel architectures such as LC-BCP with a dendritic LC block, rod-rod block with different rod diameters, and other novel structures will prove to be very interesting. 

However, understanding the thermodynamics of phase separation in liquid crystalline block copolymers is in its infancy. The morphology of such block copolymers will be influenced by the competition between... 

Block copolymers of butadiene and styrene are therefore readily synthesized an-ionically, with either of the two monomers polymerized first. Precursor copolymers of poly(styrene-h/oc -butadiene) have been used to prepare well-defined liquid crystalline block copolymers by the same polymer analogous reaction described in Sec. 2.5 of this chapter (Scheme 21) [201-203]. Following anionic polymerization by sequential monomer addition, the polymer analogous reactions of the cholesterol (PS-PBCh) [201] and azobenzene (PS-PBAz) [203] derivatives were essentially quantitative, while that of the phenyl benzoate (PS -PBBz) block went to up to 94% conversion [202]. The polydispersities of the liquid crystalline copolymers (pdi= 1.13-1.23) were nearly as narrow as those of their precursor copolymers (pdi = 1.08 -1.21, M =8.09-9.2xl0 ) [201, 203]. 

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