Crystalline blocks

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. 

Another type of fibril substructure in PET fibers, besides the microfibrillar type already discussed, is the lamellar substructure, also referred to as the lateral substructure. The basic structural unit of this kind of substructure is the crystalline lamella. Formation of crystalline lamellae is a result of lateral adjustment of crystalline blocks occurring in neighboring microfibrils on the same level. Particular lamellae are placed laterally in relation to the axis of the fibrils, which explains the name—lateral substructure. The principle of the lamellar substructure is shown in Fig. 2. 

Figure 2 The lamellar substructure of a fibril. (a) Reciprocal positions of crystalline lamellae as a result of fiber annealing. (b) The situation after relaxation of stress affecting TTM. ai.2 - average angle of orientation of TTM CL - crystalline lamellae CB - crystalline blocks (crystallites) mF -border of microfibrils and F - fibril. In order to simplify it was assumed that (1) there are the taut tie molecules (TTM) only in the separating layers, (2) the axis of the fibril is parallel to the fiber axis.

Two independent routes to magnesium amidinates containing very bulky terphenyl substituents have been developed, yielding both mono- and bis(amidinate) magnesium complexes. As shown in Scheme 17, the free amidine reacted cleanly with 0.5 equivalents of dibutylmagnesium in toluene to form the bis (amidinate) in moderate yield. The highly soluble compounds was recrystallized from hexanes as clear, colorless crystalline blocks. ... 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

Silk proteins (spidroins in spiders and fibroins in Lepidoptera insects) are assembled into well-defined nanofibrillar architectures (Craig and Riekel, 2002 Eby et al., 1999 Inoue et al., 2000b, 2001 Li et al., 1994 Putthanarat et al, 2000 Vollrath et al., 1996). Spidroins and fibroins are largely constructed from two chemically distinct repetitive motifs or blocks (Table I), an insoluble crystalline block and a soluble less-crystalline block (Craig, 2003 Fedic et al., 2002 Hayashi and Lewis, 2000 Hayashi et al., 1999). The crystalline blocks are composed of short side-chained amino acids in highly repetitive sequences that give rise to /1-sheet structures. 

Unbridged metallocenes rarely achieve highly stereoselective polymerizations because free rotation of the r 5-ligands results in achiral environments at the active sites. An exception occurs when there is an appreciable barrier to free rotation of the r 5-ligands. Fluxional (con-formationally dynamic) metallocenes are initiators that can exist in different conformations during propagation. Stereoblock copolymers are possible when the conformations differ in stereoselectivity and each conformation has a sufficient lifetime for monomer insertion to occur prior to conversion to the other conformation(s). Isotactic-atactic stereoblock polymers would result if one conformation were isoselective and the other, aselective. An isotactic-atactic stereoblock polymer has potential utility as a thermoplastic elastomer in which the isotactic crystalline blocks act as physical crosslinks. 

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. 

In this chapter, structure formation in semicrystalline diblocks containing PE, PEO and other crystalline blocks is discussed in Section 5.2. Section 5.3 is concerned with theories for the equilibrium crystallization of block copolymers, whilst Section 5.4 summarizes recent experimental work on the kinetics of crystallization. There have been few studies of crystallization in thin block copolymer films, and consequently Section 5.5 is correspondingly short. Finally, structure formation in glassy diblocks is considered in Section 5,6. 

The synthesis of block copolycondensates by condensation reactions has also been described very often indeed the ineluctable presence of reactive end groups makes these molecules especially suitable for reactions with dibasic acids, diisocyanates, diacid chlorides, diamines, diols, etc. Using this method it was for example possible to synthesize polycondensates in which crystalline blocks alternate with amorphous blocks similarly it makes possible the synthesis of high molecular weight polymers from polycondensates of relatively low degree of polymerization. 

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... 

Discovery of thermoplastic elastomers by block-copolymerisation (rubbery blocks flanked by glassy or crystalline blocks in one chain)... 

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

III. The third step of direct longitudinal transmission of strain onto connected crystalline blocks leads to a perfect stretching of these fibrils. Because of the alignment of the molecules the fibers in this condition should possess a strength about 1 to 2 orders of magnitude higher than the yield stress of randomly distributed folded polycrystals. As the fibrils are able to stabilize the enhanced micro-void volume between them, a lateral coalescence of these voids finally provides a local deformation zone in the shape of a craze as known from amorphous polymers. 

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