Amorphous fold surface

The exact location of the plasticizer is frequently evaluated to better understand the structure of plasticized polymers. In poly[(vinylidene fluoride)-co-hexafluoropropene], VDF/HFP plasticized with dibutyl phthalate, SAXS measurements indicate that DBP resides in the amorphous zone outside the lamellar stacks. If ciystallization is slow the inclusion of DBP inside the lamellar stacks is also possible. In another contribution for the same copolymer, plasticizer was also found in amorphous phase close to the interface with crystalline stmctures which was evidenced by almost constant spacing in the range of tricresyl phosphate studied.In the plasticized PVC, the plasticizer molecules were found in the amorphous area but were also present in the interlamellar, intefibrillar, inter-spheralitic regions and in the amorphous fold surfaces. ... 

All the aspects and processes discussed above apply also to the crystallisation of polymers [18-31]. However, in addition, the connectivity of the monomers causes several restrictions. One of the most obvious ones is represented by the quasi-two-dimensional lamellar shape of polymer crystals. A lamella is formed by crystalline segments of the chain (the stems) arranged vertically and limited (on top and below) by amorphous (fold) surfaces. Therefore, polymer crystals grow essentially only in two dimensions. Growth in the third dimension is rather difficult, in particular when the polymer contains non-crystallisable units which will segregate to the surfaces of the crystals. Growth in the third dimension necessitates deviations from the perfect lamellar structure, e.g. screw dislocations. The topic of polymer crystallisation has been the subject of a tremendous amount of studies over the last 60 years [5-8,10-65],... 

There is ample scope for future research. For example, new monodisperse oligomers are currently being contemplated, notably aromatic and model biopolymers. Deuterium-labeled long alkanes are being studied by neutron scattering and NMR. Fold surfaces of IF crystals, devoid of amorphous overlayer, may be amenable to studies by different... 

In Table 6.2 it can be noted that the two series of values, so obtained, have a similar dependence on blend composition (except for the PB-l/HOCP 70/30 blend in the series of values obtained from Ai ). The interlamellar regions inducing an increase in the surface entropy of folding. As a consequence, PB-1 lamellar crystals with a less regular fold surface are obtained when the PB-l/HOCP blends are isothermally crystallized from the melt in the examined range of values. 

Crystallization in miscible blends can occur with rejection of the noncrystallizing component, so that its concentration in the amorphous phase increases. Alternatively, if it can be accommodated in the unit cell, it may be entrapped, with consequent alteration in the mean unit cell volume (Tomlin and Roland, 1993). In NR, there is also a shift to formation from a-lamellae to the )3-lamellar form (Zemel and Roland, 1992b) (Figure 3.25). These crystal structures have the same unit cell, but the latter has a greater fold-surface free energy. Thus, the noncrystallizing blend component is more readily accommodated into the fold plane at the crystal surface. 

Finally, to more specifically explore the nature of the amorphous component of spherulitic polyethylene and the nature of the fold surfaces of the lamellae, Hoffman and collaborators (Guttman et al. 1981 Guttman and di Marzio 1982) performed a probabilistic choice computation on whether molecules in the amorphous component undergo adjacent reentry, undergo non-adjacent reentry... 

Guttman, C. M., DiMarzio, E. A., and Hoffman, J. D. (1981) Modeling the amorphous phase and the fold surface of a semi-crystalline polymer - the Gambler s Ruin Method ,... 

Important experiments have been carried out with isolated lamellae, also called crystal cores, of which the chain ends could be removed chemically GPC analysis has shown that the cross-links mainly occur at the folds and that the G(cl) in the crystal cores is only ca 0.2 (lOOeV)" The fact that cross-links are formed with such a low yield in the crystal is attributed to the fact that the carbon atoms on the adjacent chains are too far apart (> 0.41 nm) for interchain C—C bonds (0.15 nm) to be formed. It is generally accepted now that the cross-linking in polyethylene mainly takes place at the fold surfaces and in the amorphous regions. 

At the extreme low-frequency end of the Raman spectrum, the so-called longitudinal acoustic modes (LAM), probe the lamellar structure, including the fold surfaces. These lattice modes correspond to accordionlike deformations of the chain. The lamella thickness is related to the frequency of these modes. A quantitative expression has not been found for helical PP, but an inverse relation has been proved. LAM bands in the range 10-20 cm have been used to compare PP samples in terms of lamella thicknesses. A similar mode exists in the amorphous phase the so-called disoriented LAM (DLAM). A DLAM doublet observed near 220 cm in iPP melts has been associated with 3, helix segments. 

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