Structure‐Property Relationships of Linear and Long‐Chain Branched Metallocene High‐Density Polyethylenes Characterized by Shear Rheology and SEC‐MALLS

Abstract

Summary: Linear and long‐chain branched high‐density polyethylenes with a molar mass $\overline M _{\rm w}$ between 1 700 and 1 150 000 g · mol−1 were synthesized using metallocene catalyst systems. Depending on the polymerization parameters the molar mass distribution reached values ranging from 2 to 12. The resins were characterized with various analytical methods. The branch detection took place via two independent methods, melt rheology and SEC‐MALLS. New relationships between catalyst structure, polymerization conditions, and the branching content of polyethylenes were established. Besides the branched materials strictly linear polymers are presented; for those no long‐chain branches were detected either by light scattering or by rheology. The viscosity function was observed to be strongly influenced by the molar mass distribution and the degree of long‐chain branching. The molar mass distribution was affected by the catalyst type and the polymerization conditions. A dependence of the melting point and the melting enthalpy on the molar mass was observed.

Lattice sizes, crystallinities, and spacing between amorphous chains - characterization of ethene-/ɑ-olefin copolymers with various comonomers and comonomer contents measured by wide angle X-ray scattering

“…This depends on the chain mobility which is dominated by the viscosity at very low shear rates close to the zero shear-rate viscosity  0 . The zero shear-rate viscosity  0 is proportional to the weight-average molar mass M w for linear molecules [27], while this relation is usually not fulfilled for long-chain branched samples [12,33,37,41]. The strong effect of the rheological behavior has been shown for example by Piel et al [12] for the crystallinity of HDPE with molar masses M w between 3,000 g/mol and 1,200,000 g/mol varying between 80% and 50% for a relatively fast cooling rate of the DSC of 20 K/min.…”

“…One parameter influencing the lattice dimensions is the weight average molar mass M w (which strongly influences the rheological properties [27] and thus the chain mobility (and also long-chain branches [12,16,34,41])). For this purpose, several HDPEs (defined by the absence of sizable numbers of short-chain branches) -some slightly long-chain branched-were characterized.…”

“…Another advantage of the single-site catalysts are their tendency to produce samples with a very homogeneous comonomer incorporation, which is not given for materials synthesized with ZN-catalysts or via the high pressure process [9][10][11]. By choosing a proper single-site catalyst and synthesis conditions, it is also possible to adjust the molar mass to almost any desired level with a narrow molar mass distribution (M w /M n =2) [12]. This affinity of metallocene catalysts to incorporate longer -olefins easily also means that they are able to incorporate vinyl terminated chains, thus producing longchain branched materials [7,[12][13][14][15].…”

“…By choosing a proper single-site catalyst and synthesis conditions, it is also possible to adjust the molar mass to almost any desired level with a narrow molar mass distribution (M w /M n =2) [12]. This affinity of metallocene catalysts to incorporate longer -olefins easily also means that they are able to incorporate vinyl terminated chains, thus producing longchain branched materials [7,[12][13][14][15]. Because of the higher probability of a chain termination upon incorporation of a longer -olefin the production of highly SCB LCBmPE proves difficult [16].…”

“…Polyethylene is a semi-crystalline material, whose crystallinity depends on the molar mass [12,17,18] and the comonomer content [3,10,18]. Under normal conditions the maximum crystallinity is limited to about 0.8.…”