Minimum Molecular Weight and Tie Molecule Content for Ductility in Polyethylenes of Varying Crystallinity

Abstract

Semicrystalline polymers of low glass transition temperature, such as polyethylene (PE), can be either brittle or ductile depending on their content of intercrystallite stress transmitterssuch as tie molecules (TMs), chains that directly bridge the intercrystalline amorphous layer. TM content will increase with increasing molecular weight (M) or with the fraction of high-M chains in a disperse polymer and with decreasing intercrystallite repeat spacing d, which can be manipulated through thermal history and the incorporation of comonomer. The present work examines the failure mode of model narrow-distribution linear PEs (LPEs) of high crystallinity, where d is varied through crystallization history (either quenching or slowly cooling), and ethylene-butene copolymers (hydrogenated polybutadienes (hPBs)) of moderate crystallinity, where d is limited by the short-branch content. For each series (LPEs with different thermal histories and quenched hPBs), a rather sharp brittle-to-ductile transition (BDT) is observed with increasing M, at a value M BDT. However, across the three series, the value of M BDT does not depend solely on the value of d; indeed, a higher M is required to achieve ductility in quenched samples of hPB than in LPE, despite the much lower values of d for hPB. Consequently, the calculated value of TM fraction at the BDT increases strongly as crystallinity decreases, by a factor of ∼50 from slow-cooled LPE to quenched hPB. This strong dependence is explained by considering the influence of TMs on the brittle fracture stress (σb), with the BDT occurring when there are sufficient TMs for σb to exceed the yield stress (σ y ), which is strongly dependent on crystallinity but independent of TM content.

Minute-Scale Evolution of Free-Volume Holes in Polyethylenes during the Continuous Stretching Process Observed by In Situ Positron Annihilation Lifetime Experiments

“…For both I 3 /(1 – X c ) and FFV r /(1 – X c ), there exists a sequence of LDPE ≅ LLDPE < MDPE < HDPE, and their variation tendencies with increasing crystallinity are similar. This is attributed to the decrease in the densities of entanglement and tie chains with increasing crystallinity, which is reported in many previous works. − …”

Origin of Lamellar Stack Structures in Dilute Semicrystalline Polymers: Role of Entanglements and Tie-Molecules

“…The selection of L crit is still under debate. Here, we use L crit = 2 d c + d a following the arguments by Cho and Register. , For bimodal distributed semicrystalline polymer mixtures with narrowly distributed fractions, the averaged tie chain fraction for the whole sample is given by P normala normalv normalg = n ( normalP normalC normalL 200 ) P ( L c r i t , normalP normalC normalL 200 ) + n ( normalP normalC normalL 2 ) P ( L c r i t , normalP normalC normalL 2 ) where n (PCL200) and n (PCL2) are mole fractions of PCL200 and PCL2, respectively. Considering that the oligomer used in this study is extremely short, the second term of eq equals zero.…”

Elucidations of Structure and Molecular Dynamics of Complex Polymers by State-of-the-Art Solid-State NMR Spectroscopy

“…The molecular weight (entanglement) significantly affects the viscosity of the polymer melt, crystallization kinetics, crystallinity, , morphology, and the mechanical property of semicrystalline polymers. However, it is not well understood how entanglement influences folding structure of semicrystalline polymer.…”