Crystalline polymers superstructures

Polytetrafluoroethylene (PTFE) is an attractive model substance for understanding the relationships between structure and properties among crystalline polymers. The crystallinity of PTFE (based on X-ray data) can be controlled by solidification and heat treatments. The crystals are large and one is relieved of the complexity of a spherulitic superstructure because, with rare exceptions, spherulites are absent from PTFE. What is present are lamellar crystals (XL) and a noncrystalline phase (NXL) both of which have important effects on mechanical behavior. 

The precise function of microfibrils in hard elastic polymers is not well understood. Their importance in various models of crystalline polymers has ranged from mere tie points [4,5] to the fundamental element [9]. The recent disclosure of hard elastic behavior in crazed HIPS [11] clearly revealed the connection between microfibrillar superstructure and hard elastic behavior, since no lamellae exists in this material. Polymers with a fibrillar structure are not necessarily hard elastic,however. Gore-Tex has an extensive fibrillar domain as shown in Figure 2c. Its loading cycle (Figure 5), however, clearly reveals an inelastic material. Hence, the necessary criterion of microfibrillar structure for a hard elastic polymer must be established. 

Zentel, R., Untwisting of the helical superstructure in the cholesteric and chiral smectic C phases of cross-linked liquid-crystalline polymers by strain, Liq. Crvst., 3, 531-536 (1988). 

Under certain conditions, the lamellae of the TPEEs organize into a spherulitic structure, which is characteristic structure of the common semi-crystalline polymers. Based on the results of different methods, it was concluded that crystallization occurs by chain folding through which a spherulitic superstructure is formed. A well developed spherulitic crystalline superstructures, with diameters of about 5-20 pm, can be formed depending on the crystallization conditions [33]. Also, the soft, amorphous phase is embedded between radial crystalline fibrils of the hard segment spherulites. Under some conditions, other structures such as dendrites are developed. 

Figure 1.17. Supramolecular structure of a liquid crystalline polymer. Top Molecular structure. Lower left Triple helical superstructure derived from X-ray data. Lower right Helical textures observed by electron microscopy (A) L-enantiomers, (B) D-enantiomers, and (C) mesoform. Courtesy by J.-M. Lehn (from [82]).

Small-angle X-ray diffraction provides information about the period of the lamella stacking in semicrystalline polymers and about the layer thickness of smectic liquid-crystalline polymers. The azimuthal angle dependence of the small-angle pattern provides information about the orientation of these superstructures (Fig. 9.12). 

The semi-crystalline structure of polymers is characterized by special features. Firstly, the crystallites are embedded in an amorphous matrix, resulting in a two-phase morphology. Secondly, most polymers form folded-chain crystals in which the chains fold back into the same crystallite. Thirdly, several crystallites stack up and form superstructures known as sphemlites [8], Fig. 1.2. Finally, chains wander from one crystallite to the next one, thereby connecting them to each other. The chain segment between two adjacent crystals is known as tie-molecule [9], Tie-molecules act as stress-transmitters [10-12]. Thus they play an important role in mechanical properties of semi-crystalline polymers. 

Fig. 1.2 Upon cooling from the melt some polymers crystallize and form folded-chain lamellae. Several lemeUae stack up and form superstructures known as spheruhtes. Chains wander from one lamellae to the next one. Parts of the chains shared by two neighboring lamellae are called tie-molecules. Tie-molecules play an important role in the mechanical behavior of semi-crystalline polymers...

As we have seen, the phase behaviour of block copolymers consisting of flexible polymer coils is remarkably rich. If one of the blocks is rigid, the copolymer would be expected to exhibit even more complex phase behaviour. For example, the rigid block could be mesogenic. This leads to the possibility of self-assembly of structures consisting of domains of liquid crystalline material within a microphase-separated block copolymer superstructure. Diblock copo-... 

Studies have been conducted on poly (tetramethylene oxide )-poly-(tetramethylene terephthalate) -segmented copolymers that are identical in all respects except for their crystalline superstructure (66,67,68). Four types of structures—type I, II, and III spherulites (with their major optical axis at an angle of 45°, 90°, and 0° to the radial direction, respectively), and no spherulitic structure—were produced in one segmented polymer by varying the sample-preparation method. Figures 10 and 11 show the stress-strain and IR dichroism results for these samples, respec-... 

Finally, there is considerable interest in polymeric assemblies both in solution and in liquid crystalline phases [87]. In a seminal report, Meijer and co-workers [49] have synthesized dimers of module 75 (e.g. 101) and shown that its solutions have rheological properties similar to those shown by normal polymer solutions (Fig. 25). In this regard, the high dimerization constant of 75 allows a high degree of polymerization at accessible concentrations. Likewise, Lehn has shown that 1 1 mixtures of 102 103 and 33 104 form supramolecular, polymeric, liquid crystalline phases (Fig. 25). The structure of 102 103 is believed to contain a triple helical superstructure [88], whereas rigid assembly 33 104 forms a lyotropic mesophase [89]. 

The term "supramolecular isomerism" was first used by Zaworotko to describe distinct forms of highly related coordination polymer materials. This is complicated by the observation that supramolecular isomerism for a given network system is commonly combined with a variation in guest solvent molecules within the extended structure. Variation of guest molecules within a framework does not, of course, define new supramolecular isomers of the framework if the latter is unchanged. In a recent review, Zaworotko et stated that supramolecular isomerism is closely related to the well-documented subject of polymorphism in crystalline solids." Zaworotko defined supramolecular isomerism in this context as "the existence of more than one type of network superstructure for the same molecular building blocks" and related the phenomenon "to structural isomerism at the molecular level."... 

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