Crystalline polymers hierarchical structure

Fig. 3.5 Structural hierarchy in liquid-crystalline fibers. The mechanical performance of highly oriented polymers can approach the ultimate theoretical properties at high degrees of elongation. Anisotropic, rod-like macromolecules, like aromatic copolyesters composed of 2,6-naphthyl and 1,4 phenyl units, often form oriented structures, which can exhibit liquid crystallinity. Extensive structural studies of fibers of these oriented copolyesters showed a hierarchical structure like the one depicted in this Figure. In aramids (Kevlar or Twaron) similar structures may exist. Adopted with permission from [17]...

First, the lyotropic phase is used as a template for the preparation of a bicontinuous silica structure, from which the polymer is removed by calcination or extraction. In the second step the porous inorganic structure is filled with monomer and crosslinker which is polymerized to form a bicontinuous organic polymer network from which the silica template is removed by treatment with hydrofluoric acid. An example for the preparation of hierarchical structures is the synthesis of bicontinuous pore structures by using two templates simultaneously [115]. In this case a liquid crystalline lyotropic phase of an amphiphilic block copolymer is used as a template together with suspended latex particles. The sol-gel process with subsequent calcination leads to a bicontinuous open pore structure with pores of 300 nm and 3 nm. 

Containing himdreds and thousands of monomeric units, the hierarchical structure of polymers also gives rise to their diversity in aggregations—polymers are not distinctly amorphous or crystalline. The aggregation of polymers often has amorphous subdivisions and crystalline parts which make them exhibit combined features of amorphisms as well as crystals. 

Fig. 10.7 Illustration of multi-scale hierarchical structure of crystalline polymers (For instance, polyethylene) forming folded-chain lamellae and spherulites...

Most of crystalline polymer materials exhibit multi-scale hierarchical structures. At the scale of 0.1 nm, the polymer chains contain regular sequences. At the scale of 0.5 nm, they form stable helical conformations, which then pack together in a compact parallel fashion to make the periodic lattice structure, with the unit cell at the scale of 1 nm. At the scale of 10 run, the folded-chain lamellar crystals are formed for the flexible polymer chains. At the scale of micrometers or larger, the lamellae further assemble into spherulites. Such hierarchical structural characteristics at varying length scales of polymer morphologies are illustrated in Fig. 10.7. 

When a crystallizable polymer is cooled below its equilibrium melting temperature, the hierarchical structure formed can be probed by in situ scattering. Crystallization mechanisms can be determined by comparing the time evolution of the degi ee of crystallinity (wc) determined from WAXS and the total integrated scattering intensity or invariant during crystallization [27,38] from SAXS. 

Structural differences, together with differences in the synthesis, result in considerable variations of the physical properties of the PiPAAm structural isomers. Thus, PiPOz is a crystalline polymer [389] and is able to crystallize from water as a fibrous material when its solution is annealed for 24 h above Tip = 65°C [373, 387]. Coagulated PiPOz particles exhibit hierarchical structures with two levels of ordering that are micron-sized spherical particles consisting of fibrils wifh a cross-sectional diameter of about 30-50 nm and a length of several microns [373]. The densely packed microspheres formed in dilute solutions are uniform in size and shape and resemble a ball made of rattan. 

The hierarchical morphology of semicrystalline polymers includes structural details in the range from nanometers to millimeters. The smallest ordered structures are crystalline blocks or domains. Figure 2.7 shows a branched PE (LDPE) with many crystalline blocks with defect layers in between. The blocks are arranged in long, narrow lamellar bands. The defect layers between the blocks, as well as the boundary at the lamellae-like structures, appear with an improved, clear contrast (black). 

Semicrystalline polymers contain liquid-like amorphous and ordered crystalline phases. When solidified from the pure melt, these polymers show a spherulitic structure in which crystalline lamellae composed of folded chain crystallites radiate from the center of the spherulite in such a way that a constant long period or crystallinity is apvproximately maintained. The amorphous regions reside in the interlamellar regions in the form of tie chains, whose ends are attached to adjacent lamellae loop chains, whose ends are attached to the same lamella cilia chains with only one end attached to a lamella (or dangling chain ends), and floating chains which are not attached to any lamellae. This hierarchical structure is illustrated in Figure 1. 

The SEM images of PBO fibers show fibrillar structure. A hierarchical structure model [81] was proposed for oriented liquid crystalline polymers, in which a fiber is made up of macrofibrils. 

The crystallization of homopolymers yields a hierarchical structure in polymer materials, which substantially controls their physical properties. Therefore, the crystalline morphology of homopolymers has been one of the important research subjects in polymer science. In addition, the crystallization of homopolymers spatially confined in various nanodomains, such as micelles, AAO, or microdomain structures, may bring new information on crystallization mechanisms of homopolymers, because it will be possible to highlight a specific crystallization mechanism (e.g., nucleation or crystal growth) in the overall crystallization process consisting of several combined mechanisms. Furthermore, the crystallization in nanodomains has the possibility of providing new polymer materials, and their physical properties should be unique as compared with usual polymer materials. This is because the substantial control of nano-ordered structures formed in polymer materials will be possible by this crystallization, which is never achieved by the crystallization of neat homopolymers. 

Pig. 1.6 Hierarchical structures are common in semi-crystalline polymers. By varying the sample-detector distance one can observe different structural entities... 

One may now ask whether natural systems have the necessary structural evolution needed to incorporate high-performance properties. An attempt is made here to compare the structure of some of the advanced polymers with a few natural polymers. Figure 1 gives the cross-sectional microstructure of a liquid crystalline (LC) copolyester, an advanced polymer with high-performance applications [33]. A hierarchically ordered arrangement of fibrils can be seen. This is compared with the microstructure of a tendon [5] (Fig. 2). The complexity and higher order of molecular arrangement of natural materi-... 

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