Polyethylene thin lamellae

JS A polymer spherulite growing into the melt. In polyethylene the ciystalline fibrils are thin lamellae. The molecules crystallize most rapidly on to the (010) plane the b axis is therefore the direction of most rapid growth and is p lel to the spherulite radius R. The a and c axes are randomly distributed around R. If the solidification is isothermal, the lamellae are all of the same thickness. In order to fill space the raoiating lamellae must branch and give birth to daughter lamellae as they grow out into the melt. Amorphous polymer is left trapped between the crystals. 

Fig. 11.14 TEM micrograph of polyethylene thin film with a particle of CaCOa (seen as a continuous black region in the left-hand side). Crystalline lamellae seen as black ribbons when oriented edge on against interface (From Chacko et al. (1982) reproduced with permission of Wiley)...

Fig. 3. Two populations of lamellae in a linear polyethylene (Rigidex 140/60) crystallized for 3 h at 128°C, then quenched into cold water. The thicker layers formed at 128°C and have established a framework within which the thin lamellae formed on quenching. Replica of etched cut surface.

Fig. 4. Two populations of lamellae in a linear low-density polyethylene (T-SQHs/lOOOC atoms) which was crystallized for 9 min at 118°C, then quenched into dry ice/acetone. The thin lamellae comprising the matrix contain a higher proportion of branches than do the thick dominants. Replica...

The random arrangement of adjacent crystals in the fringed-micelle model (see Fig. 2.4), does not accord with evidence obtained from microscopy. The electron microscope shows the crystals in polyethylene to be very thin twisted lamellae laid one upon another (Fig. 1.7). The lamellae are too small to be observed with the light microscope. Light microscopic examination of thin films or sections between crossed polarizers reveals complex polyhedral objects known as spherulites (see Fig. 2.5). They are in fact a complex ordered aggregation of the sub-microscopic crystals. In polyethylene the crystal lamellae are about 10 nm thick. They are separated one from another by thin lamellae of amorphous polymer of about the same thickness. The lateral... 

Under conditions of applied stress and elevated temperature, thin crystallites are disrupted. Some of these can be melted by elevated temperature alone, while others require the combination of stress and elevated temperature to melt them. Thin lamellae are more susceptible to disruption by imposed stress because they are inherently less stable than thicker ones. As the stiffness of a polyethylene sample is directly related to its degree of crystallinity, any reduction in the volume of the ciystalline phase due to melting results in increased flexibility. The linear chain sequences that comprise a crystallite are not fixed permanently in place. Under applied stress, which is transmitted by taut tie chains, individual linear... 

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. 

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