Polyethylene lamellae

The resulting PE-fr-PMMA was purified by soxhlet extraction with THF and characterized by NMR, DSC, and TEM micrography (Table 1). The TEM of the obtained PE-fr-PMMA revealed unique morphological features which depended on the content of the PMMA segment. The block copolymer possessing 75 wt % PMMA contained 50-100 nm spherical polyethylene lamellae uniformly dispersed in the PMMA matrix (Fig. 12). Moreover, the PE-b-PMMA block copolymers effectively compatibilized homo-PE and homo-PMMA at a nanometer level (Fig. 13). 

Polymers are unique in the extent of the detail of their history which they retain in their morphology, essentially because of the restricted mobility of long molecules once added to a lamella. Indeed polymer morphology has driven almost all advances in understanding the fundamental nature of polymeric self-organization, not least chainfolding. In the present case it demonstrates clearly that polyethylene lamellae crystallized at atmospheric pressure did not have a hexagonal precursor. 

Insufficiently precise data do not allow the theoretical possibility to be decided whether thin polyethylene lamellae are more stable at atmospheric pressure in the hexagonal phase rather than the orthorhombic though it appears not to be the case. 

The single crystalline nature of orthorhombic polyethylene lamellae shows simply and clearly that they did not have a hexagonal precursor. Had they done so they would have been threefold twins. 

Fig. 3 Epitaxial growth of high-density polyethylene on oriented polypropylene. Polyethylene lamellae aligns at a 40° angle with the direction of orientation of the polypropylene. The orientation direction of polypropylene in the micrograph is along the horizontal. (From Ref...

Annealing of Polyethylene Lamellae Grown Epitaxially on Extended-chain Crystals... 

Fig. 7.3. Polyethylene lamella, crystallised from solution, (from R. A. Fava in Macro-mol Rev. 5 (1971), p. 2)...

There are two other principal nonplanar habits of polyethylene lamellae chair and ridged crystals. Chair crystals (30) form simultaneously with, and are related to, hollow pyramids their shape may be described as resulting from a hollow pyramid divided in two along the short diagonal, the two halves rotated 180° aroimd their long diagonal and then constrained to rejoin. The proportion of chair to hollow pyramidal crystals appears to vary with crystallization conditions. 

Early experiments on solution-grown lamellae revealed not only the earlier melting of the 100 sectors in polyethylene lamellae (25) [which Atomic Force Microscopy (AFM) has confirmed (33) to be thinner than 110 sectors in collapsed, and presumably as-grown, crystals] but also the presence of holes (43) in lamellae that had been heated in contact with a carbon film (Fig. 2). While providing clear evidence of lamellar thickening, the presence of holes is a response to the carbon film, which restrains lateral contraction holes do not appear on thickened upper layers of the same sample that are not so restrained. 

Fig. 4. Melt-crystallized polyethylene lamellae (a) linear polymer crystallized at 130°C as a planar crystal in which successive layers spiraling around the central (etched-out) giant screw dislocation are not in contact (b) ridging along 6 in a 17,000 mass fraction of linear poljrmer crystallized at 129°C (c) an ethyl-branched copolymer crystallized at 123°C showing a central S-profile, asymmetrically placed screw dislocations and new layers diverging therefrom. From Ref. 66.

LAM spectroscopy gained much popularity in polymer characterization and morphology since it has been possible to measure from Raman spectra the length of the iranj-planar stems in polyethylene lamellae and in many other relevant cases. 

Tracz A, Kucinska I, Jeszka JK. Formation of highly ordered, unusually broad polyethylene lamellae in contact with atomically flat solid surfaces. Macromolecules 2003 36 10130-10132. 

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