Polyethylene crystallized from dilute solution

Linear Polyethylene Crystallized from Dilute Solution. 61... 

The y relaxation of polyethylene, expressed in terms of tan 5, is centered in the vicinity of — 120°C at 1 Hz. This relaxation is believed to be caused by molecular motions occurring in the amorphous phase as indicated by the fact that the relaxation is very weak in highly crystalline polyethylene crystallized from dilute solutions (39,40). The relatively high intensity and universality of the y relaxation in polyethylenes, independent of whether they do or do not have branches in their structure, seems to suggest that the y relaxation may be associated with the glass transition. According to this interpretation, the glass transition temperature of polyethylene would be located in the vicinity of — 120°C. 

It has been noted that the transformation is almost complete for linear polyethylene crystallized from dilute solution. For example, the crystallinity level that is attained at the isothermal crystallization temperature varies from about 85-90% at the lower molecular weights, < 1 x 10, to about 75-80% at higher molecular weights, including M = 3.1 x 10 .(28,38) These results can be contrasted with those for bulk crystallization. At the isothermal crystallization temperatures the... 

Schreiber and co-workers have noted very persistent history effects in linear polyethylenes (69). Fractions which have been crystallized from dilute solution required times of the order of hours in the melt state at 190° C in order to attain a constant die swell behavior upon subsequent extrusion. The viscosity on the other hand reached its ultimate value almost immediately. The authors concluded from this result that different types of molecular interactions were responsible for elastic and viscous response. However, other less specific explanations might also suffice, since apparent viscosity might be relatively intensitive to the presence of incompletely healed domain surfaces, while die swell, requiring a coordinated motion of the entire extrudate, might be affected by planes of weakness. It would... 

Fig. 5.3 Electron micrographs of single crystals of polyethylene crystallised from dilute solution in xylene (a) diamond-shaped crystals and (b) truncated crystals. (Reprinted by permission of John Wiley Sons, Inc.)...

Fig. 4.30. A typical electron micrograph of lamellae formed by homopolymers crystallized from dilute solution. The example illustrated is for linear polyethylene.

Single crystals of linear polyethylene prepared from dilute solutions in xylene and similar solvents provided the evidence in favour of chain folding. The large surfaces which contain the chain folds are commonly referred to as the fold surfaces. The single crystals shown in Fig. 7.11 exhibit planar, lateral... 

Figure 7.11 Single crystals of linear polyethylene (BP Rigidex 140 60) crystallized from dilute solution in xylene at 74 C. Transmission electron micrograph by A. M. Hodge and D. C. Bassett, University of Reading, Reading, UK.

Figure 7.15 Lateral shape of crystals of linear polyethylene crystallized in dilute solutions with different solvents at different temperatures, as indicated in the figure. Drawn after data from Organ and Keller (1985) and Bassett, Olley and A1 Reheil (1988).

Figure 33 Dendritic single crystals of high density polyethylene grown from dilute solution in xylene. (From Ref. 105.)...

Fig. 13.16 Plotof time in minutes for 10% of the transformation to occur, ro.i, as a function of molecular weight for fraction of linear polyethylene crystallizing from dilute n-hexadecane solution. Crystallization temperatures are indicated. (From Chu (29))...

Another mechanism for the role of cilia, in the crystallization from dilute solution, has been postulated by Toda and coworkers.(62) This theory is based on the works of Seto (47,63) and on Frank s theory,(64) which was discussed in Chapter 9. In essence, two steps traveling in opposite directions approach one another and are allowed to collide. As a consequence of the contact a pair of cilia is generated. Each can nucleate on the next layer and travel in both directions. The major conclusions from the analysis of this model are as follows. When the chain length is relatively small, nucleation by cilia can be neglected. For linear polyethylenes the cut-off molecular weight is approximately 3.0 x lO . Below this molecular weight cilia nucleation is not important. In Regime I, irrespective of whether or not cilia nucleation is taken into account... 

---