Crystalline polymers lamellar crystallites

Well-annealed polymeric solids tend to be stiffer and more brittle than unannealed solids. For crystalline polymers annealing increases the degree of crystallinity and crystallite sizes (lamellar thickening). These changes are defined and measured thermodynamically as the decrease in enthalpy and entropy. 

Finally, it should be emphasized that the tendency of polyimides containing aliphatic spacers to form stable layer structures of various dimensions and degrees of order can be utilized for an even wider variety of studies. A recent example is a study of the epitaxial growth of polyethylene on smectic crystallites of PEI [140]. Most likely smectic crystallites adopt a lamellar form with the large surface covered by loops of the aliphatic spacers. This hypothesis still need detailed studies and confirmation. Anyway, smectic crystalline polymers are interesting substrates for studies of epitaxial crystallizations. Furthermore, layer structures derived from long aliphatic spacers (alkanes or oligoethers) may play... 

Strobl G (2000) From the melt via mesomorphic and granular crystalline layers to lamellar crystallites A major route followed in polymer crystsallization Eur Phys JE 3 165-183... 

The break-up of crystallites and the reformation of the lamellar fragments into microfibrUs is the basis of a theory for the cold-drawing of isotropic semi-crystalline polymers due to Peterlin. " (See also Hosemann et and Robertson .) Both Peterlin and Hosemann assert that the main mechanism is the break-up of each crystallite into approximately twenty smaller units which lie like pearls on a string with their chain axes parallel to the IDD. Many aspects of these theories would seem to be relevant to the deformation of oriented polymers of modest draw ratios. 

In semi-crystalline polymers such as polyolefins, initial oxidation occurs in the amorphous tie molecules between crystallites and sometimes even in the inter-lamellar amorphous chains within crystallites. This allows a relatively low levels scission to cause a disproportionately large changes in mechanical properties due to brittle failure in the amorphous regions (Celina, 2013). In PP for instance, beginning stages of bulk embrittlement corresponded to only 0.01% of oxidation (Fayolle et al., 2004). 

Isotropic samples of a partially crystalline polymer essentially correspond to an ensemble of densely packed, isotropically distributed stacks of parallel lamellar crystallites. If the extensions of the stacks parallel and normal to the lamellar surfaces are large compared to the interlamellar distance, the scattering behavior can be related to the electron density distribution Pe z) measured along a trajectory normal to the surfaces. This trajectory will pass through amorphous regions with density /c e,a stnd crystallites with a core density pe,c The average density (pe) lies between these two limits. 

At present, there are two points of view on the network structure of macromolecular entanglements in semi-crystalline polymers. One of these [101] supposes that the nodes of macromolecular entanglement networks in polyethylene are crystalline areas, which are lamellar crystallites. Another point of view [111] presumes that the nodes are concentrated in non-crystalline regions of semi-crystalline polymer. It is quite obvious that the condition

volume fraction of polymer non-crystalline regions, i.e., about 0.3 for HDPE and 0.5 for LDPE [86]. 

It is now well established that similar lamellar crystallites exist in bulk polymer samples crystallized from the melt, although the lamellae may be up to 1 /an thick. Recent results support the presence of a third, interfacial region between the crystalline lamellae and the amorphous phase. This interfacial phase can make up some 10 to 20% of the material Furthermore, there doesn t seem to be much, if any, direct-reentry folding of chains in bulk-crystallized lamellae. This is illustrated in Fig. 5.3. Orientation of the lamellae along with additional orientation and crystallization in the interlamellar amorphous re-... 

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