Folded chain segments

Figure 7.9 Mechanisms of laying down chain segments on the growing face of a crystallite a) tight folds with adjacent re-entry, b) independent deposition of chain segments... 

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

While the lamellar structures present in spherulites are similar to those present in polymer single crystals, the folding of chains in spherulites is less organized. Further, the structures that exist between these lamellar structures are generally occupied by amorphous structures including atactic chain segments, low molecular weight chains, and impurities. 

LLDPE rapidly crystallizes from the melt with the formation of sphemlites, small spherical objects 1-5 im in diameter visible only in a microscope. The elementary structural blocks in spheruliles are lamellae, small flat crystallites formed by folded linear segments in LLDPE chains, which are interconnected by polymer chains that pass from one lamella to another (lie molecules). Crystalline lamellae within sphemlites give LLDPE articles necessary rigidity, whereas the large amorphous regions between lamellae, constituting over 60% of tile splierulile volume, provide flexibility. 

Fig. 18 The model of elementary steps as used in the rate equation and Monte Carlo simulation treatments that reproduced the self-poisoning minimum. A cross-section (row of stems) normal to the growth face is shown. There are three elementary steps differing in their barrier and driving force attachment (rate A) and detachment (rate B) of segments equal to half the chain length, and partial detachment of an extended chain (rate C). The key self-poisoning condition is that attachment of the second half of an extended chain is allowed only if m = 1, i.e. an extended chain cannot deposit onto a folded chain (from [49] by permission of the American Institute of Physics)...

The orientation functions for a 75/25 PVC/PCL blend are presented in Figure 5, which shows that the PCL chains in the 75% PVC blend orient in essentially the same way as the isotactic segments and the other folded-chain PVC segments represented by the 693 cm 1 peak. This peak is shifted from 691 cm 1 for pure PVC. The shape and magnitude of the orientation functions for the carbonyl and 693-cm"1 C-Cl peak are similar to the orientation functions shown for the amorphous 50% NC blend. 

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