Polyethylene nucleation density

In the case of linear polymers, this process has been demonstrated to be highly dependent on the molecular weight distribution, since it is the longer molecules which are most prone to entanglement. However, in crystallizable cross-linked systems where a molecular network already exists, the above nucleation process should be of considerable consequence, since in such materials extended molecular conformation may occur without the prerequisite of entanglement. Indeed in the case of cross-linked polyethylene, recent results demonstrate that the nucleation density does indeed increase with increased cross-linking. ... 

Fig. 1,10 DSC traces showing the effect of crystallization temperature on the melting behaviour ofa nucleated polyethylene blend (20% high-density and 80% low-density polyethylene). In this case, all the peaks represent specific lamellar populations within each system. 

A blend of low-density polyethylene (LDPE) polyethylene (LDPE) with the terpolymer ethylene-propylene-diene monomer rubber (EPDM) exhibits a synergistic effect on tensile strength if EPDM is partially crystalline, but a nonsynergistic effect if the EPDM is amorphous [65]. This example shows the dramatic effect that morphology can have on properties of polymer blends. The synergism apparently arises from a tendency for crystallites in the LDPE to nucleate crystalli2ation of ethylene segments in the EPDM. 

Polypropylene has a crystal growth rate some one hundred times lower than that of high density polyethylene for example. It therefore responds well to heterogeneous nucleation by well dispersed additives present in the crystallizing melt. 

X. Sun, G. Shen, H. Shen, B. Xie, W. Yang, M. Yang, Co-crystallization of blends of high-density polyethylene with linear low-density polyethylene An investigation with successive self-nucleation and annealing (SSA) technique. J. Macromol. Sci. Part B Phys. 52, 1372-1387 (2013)... 

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