Polymerization of Low-Entangled Ultrahigh Molecular Weight Polyethylene: Analytical Model and Computer Simulations

Abstract

We developed a theoretical model of linear ultrahigh molecular weight polyethylene (UHMWPE) homogeneous polymerization. We considered polymerization to be living and occurring in a poor solvent. We derived the dependency of the entanglement length on the chain length during this process. We assessed how the rate of polymerization and the concentration of initiators affect entanglement of chains. Theoretical predictions were supported by the molecular dynamics computer simulations of the coarse-grained model of polyethylene. The computer model implemented living polymerization of linear chains, poor solvent conditions, formation of entanglements, and crystallization of growing chains. Polyethylene chains were modeled by using specific angular potential with the three energy minima. Our theory and the simulation results pointed out that there are two stages of homogeneous polymerization. At first, the chains grow independently. When the chains become long enough, they start to entangle, and the entanglement length starts to decrease. We also observed in simulations the existence of the third stage, when the entanglement length grows with the chain length at the very high conversion degrees. Theoretical predictions and the simulation results showed that a decrease of the concentration of initiators leads to a decrease in the entanglement density in the resulting semicrystalline UHMWPE sample. Our theory also predicted the existence of an optimal reaction rate for obtaining semicrystalline UHMWPE samples with the highest possible entanglement length at a given concentration of initiators. Our work could be a guide how to obtain low-entangled UHMWPE samples by tailoring the reaction conditions.

Optimal Entanglement of Polymers Promotes Formation of Highly Oriented Fibers

“…A distinctive feature of our work was the method of variation of entanglement density in initial conformations. We used relatively "natural" way of controlling the entanglement density by varying the reaction rate of polymerization with simultaneous crystallization, see the details in Methods and our previous work [42]. The properties of the systems under study are summarized in Table I.…”

“…There is no spot-like intensity maxima demonstrating the presence of crystalline domains oriented perpendicularly to the deformation axis ("kebab"). This phenomenon might occur due to the low temperature at which we carried out the deformation simulations: T = 0.5 roughly corresponded to 2C [42], which is well below the usual temperatures used in tensile deformation experiments. We propose that, unlike the classical hot-stretching, the low-temperature solid-state deformation of high molecular weight polymers (the so-called "cold drawing") does not proceed via the "shish-kebab" structure formation.…”

“…First, we performed CG MD simulations of polyethylene homogeneous polymerization in poor solvent below the crystallization temperature using the same model as in ref. [42]. Polymerization reaction was modeled using the stochastic "mesoscale chemistry" model [48][49][50] in NPT ensemble with pressure P = 8.0 (analogous to 1 atm) and temperature T = 0.5 (above the temperature of glass transition, but below the temperature of crystallization) up to conversion degree f ≈ 0.9.…”

“…These systems were polymerized at various reaction rates, which directly affected the resulting entanglement length N e . Thus we obtained three semicrystalline systems composed of chains of length N ≈ 9 × 10 4 beads and N e from 50 (close to N e in equilibrium polyethylene melt [42], obtained by a procedure of fast polymerization, Supplementary Material, Section 1) to 158 (see Table I). The fourth system was a reference one and consisted of relatively short chains.…”

“…We simulated the process of volume-conserving tensile deformation by using the standard procedure implemented in LAMMPS software. Temperature, volume interaction parameters, and bond potentials were identical to those used during polymerization [42].…”