Lamellae chain extended

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

There has been a sharp debate for many years on the best description of the real macroconformation. Much experimental research has been carried out on pure polymers using different techniques (225) [small angle and intermediate angle neutron scattering (226), electron microscopy, IR, etc.]. Yoon and Flory (40, 228-231) and Gawrisch et al. (232) held the view that the probability of adjacent reentry in polymeric lamella is rather low (<50%) and does not justify the validity of such a model. The trajectory of the chain extends across numerous lamellae and its macroconformation is not far from that of the random coil. In the view of Keller and co-workers (224, 233-236) the adjacent reentry, although not complete (3 1 with respect to other possibilities) largely prevails. 

By variation of the conditions of crystallization (see Sect. 1.3.3.3) polyethylene can be obtained either as folded lamellae, as extended chain crystals (high strength fiber), or as so-called shish kebabs (fibrils with a morphology similar to cellulose). All these variants differ in properties. 

The concept Tie molecules" was introduced by Peterlin (1973), see Chap. 2. Tie molecules are part of chains or bundles of chains extending from one crystallite (or plate or lamella) to another in fibres they even constitute the core of the stretched filament. They concentrate and distribute stresses throughout the material and are therefore particularly important for the mechanical properties of semi-crystalline polymers. Small amounts of taut tie molecules may give a tremendous increase in strength and a decrease in brittleness of polymeric materials. 

Other LC-based copolymers incorporating styrene-based monomers were prepared by Ober et al. [149] who chain extended pAcOSt-TEMPO (Mn=7000, Mw/Mn=1.18) with [(4 -methoxyphenyl)4-oxybenzoate]-6-hexyl (4-vinylbenzoate) (MPVB, Fig. 10). The reactions were controlled, with molecular weights ranging from Mn=12,600-23,000 and Mw/Mn=1.19-1.44. The content of pMPVB in the copolymer determined by XH NMR increased as the molar ratio of the MPVB to pAcOSt-TEMPO increased [149]. For two out of the three copolymers prepared a smectic-isotropic transition was observed however, it was at a value lower than expected based on the composition of the copolymer, even after annealing. X-Ray diffraction patterning indicated that the copolymer was oriented in a lamellar morphology and that the smectic layers were perpendicular to the block copolymer lamellae [149]. 

Amylopectin synthesis involves chain elongation followed by branching. Branching occurs in clusters. Twenty to twenty-five chains may be included in a cluster, which is called the crystalline lamellae. About 80%-90% of the chains are involved in clusters [39]. Some of the chains in a cluster may exist as left-handed double stranded helices, which increase the stability of the clusters. Clusters are linked by long B-chains. Most B-chains extend into two to three clusters [39]. Between the clusters is a region with few branches. This region is called the amorphous lamellae. Thus, there is an alternating sequence of crystalline and amorphous lamellae within the amylopectin molecule. 

Fig. 9.22 Micrographs of two etched, chain-extended samples of HDPE with lamellae of large thicknesses (a) X — l > nm and (b) — 156 nm (from Kazmierczak et al. (2005) courtesy of Elsevier)...

Within the crystalline fibrils (see Sect. 4) the chains crystallise in lamellae as extended chains in the planar zig-zag conformation [11,12], akin to that in crys-... 

Pressure has a pronounced effect on polym - crystallization by increasing the melting point, Tm, which enables crystallization to occur at higher temperatures and so increases lamellae thickness. The thickness of the lamellae, however, appears to be controlled by thermoydnamic and kinetic effects," rather than molecular size, and these materials should be more properly called chain-extended crystals, rather than extended-chain crystals. 

The earliest concepts of crystallization of semicrystalline polymers were obtained by studies of polymers crystallized from very dilute solutions and at atmospheric pressures. Electron microscopy studies revealed that the large macromolecules formed lamellar-shaped single crystals, which had a thickness of 5-50 nm, much less than the length of the extended chain. In 1957, Keller [18] and Fischer [19] proposed that the chains fold back and forth, forming folded chain lamellar crystals, as shown in Figure 19.1. In melt-crystallized lamellae, chain folding is also observed, except there are other conformations, such as loose chain folds and tie molecules, that exit one lamella and enter an adjacent... 

Khalifa BA, Bassett DC. Morphological study of chain-extended growth in polyethylene. 3. Annealing of solution-grown lamellae. Polymer 1976 17(4) 291-7. 

The low temperature, low pressure conditions were chosen so as to prepare a lamella morphology the high temperature, high pressure conditions to give a chain-extended product (38) (Table 5). The billets were machined so that they fitted inside the nylon 11 sheath. The composite was extruded at draw ratio 5 and 120°C. 

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