The effects of weight-average molecular weight (M
W) and short and long chain branching
on the linear viscoelastic behavior of polyethylene (and ethylene−α-olefin copolymers) are described. Short
chain branching had no effect up to a comonomer (butene) content of 21.2 wt %. The zero shear viscosity
of the linear polyethylenes scaled in the expected manner with M
W. Using a high molecular weight, narrow
molecular weight distribution (MWD), linear polyethylene, an estimate of the plateau modulus and
molecular weight between entanglements (M
e) was obtained. A solution property based technique for
quantifying levels of long chain branching well below 1 LCB/104
C in polyethylene is presented. Also, the
applicability of 13C NMR for measuring such LCB levels is demonstrated. For metallocene polyethylene,
long chain branching (LCB) increased the zero shear viscosity as compared to that of a linear material
of the same molecular weight. LCB also broadened the relaxation spectrum by adding a long time
relaxation mode that was not present for the linear polyethylene with the same MWD.
Crystallization analysis fractionation and temperature rising elution fractionation are two techniques used to estimate the chemical composition distributions of semicrystalline copolymers. This study investigates the cooling rate and cocrystallization effects for both techniques with a series of ethylene/1-olefin copolymers and their blends. Ideally, both techniques should operate in the vicinity of thermodynamic equilibrium so that crystallization kinetic effects are avoided. The results show that, in fact, crystallization kinetic effects play an important role at the typical cooling rate used with both techniques. Cocrystallization is significant when fast cooling rates are used.
The independent and combined effects of long and short chain branching on the thermorheological behavior of polyethylene are described. The activation energy of linear poly(ethylenebutene) increases with butene content to approximately 33-34 kJ/mol and then levels off for butene contents between 7 and 25 wt %. Long chain branched homo-polyethylene is thermorheologically complex and is most sensitive to temperature at low frequency. A technique for determining the activation energy spectra of thermorheologically complex materials is proposed. Short and long branches in the same system synergistically increase the zero-shear rate activation energy.
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