Using a Multiscale Modeling Approach to Correlate Reaction Conditions with Polymer Microstructure and Rheology

Abstract

are created-with molecular weight (MW), branching, and comonomer distribution being the most important parameters. This is the reason for the incredibly broad spectrum of polymer properties that can be tuned by adjusting a polymeric microstructure on a molecular level. As "polymers are products by process," their microstructure is-to a certain extent-directly controllable via process conditions. [1] This opens up a huge potential of process optimizations as well as aimed product designs for systems with interesting polymeric microstructures that are industrially relevant and fairly well understood. One of those processes is the high-pressure free-radical polymerization of ethylene to low-density polyethylene (LDPE) under supercritical conditions. It is a remarkable example not only because of its high industrial relevance, but mainly because of its complex random branching distribution. The short-and especially long-chain branches (LCBs) impact material properties drastically. [2,3] The LCBs are introduced by an intermolecular transfer reaction of a propagation macroradical to a polymer molecule. The resulting midchain radical can then either undergo a β-scission reaction or propagate further by monomer addition, which forms LCBs. This is shown schematically in Figure 1. Full kinetic schemes of the LDPE polymerization can be found in the literature. [4,5] LCBs are essential when it comes to polymer properties and processing behavior. They lead to effective size reduction of the polymer coil, which can be seen both analytically in light-scattering experiments [6] as well as by studying flow behavior. Rheological experiments show reduced viscosities for LDPE compared to linear high-density polyethylenes. [3,7] However, when the technically relevant strongly nonlinear flows in extension are investigated, LCBs lead to an increased network connectivity in the polymer melt and consequently reduce the rate of disentanglement, if an external force is applied. Thus, a pronounced increase of elongational viscosity with time is observed in extensional flows, which is referred to as strain hardening behavior and very beneficial in terms of processing operations such as blow molding, film blowing, foaming, or fiber spinning. [8] Following this argumentation, understanding structure-property relationships and how they can be manipulated by choosing appropriate reaction conditions is extremely beneficial in the LDPE context. A three-step multi-scale modeling strategy was recently introduced by Pflug and Busch, [4] which demonstrates Reaction conditions have a huge impact on the resulting polymer properties, but capturing this requires understanding the correlation of the underlying kinetics, the polymer architecture, and polymer flow behavior. Long-chain branched polymers created randomly by free-radical polymerization, such as low-density polyethylene (LDPE), show complex rheological behavior and are thus interesting in this context. A study applying a multiscale modeling approach is used to simulate varying reaction condition...

Translating Simulated Chain Length and Molar Mass Distributions in Chain‐Growth Polymerization for Experimental Comparison and Mechanistic Insight

“…[1][2][3] The MMD has a direct impact on macroscopic properties such as the polymer strength and flowability under melt/solution conditions, explaining the huge interest in this distribution in the polymer science and engineering community. [4][5][6][7][8][9][10][11][12][13][14] to y-axis values larger than 1 compared to linear normalization (vide infra). Superposed on that SEC broadening [42][43][44][45] takes place in practice, disguising the measurement of an absolute log-MMD.…”

“…in which f n (i) is the number fraction of (dead polymer) chains with a length i and A is a constant equal to (8/[(2−α) (4−α)]) C, in which C is the reciprocal of the low chain length kinetic chain length formally evaluated with k t, 11 .…”

Linking molecular structure to plant conditions: advanced analysis of a systematic set of mini-plant scale low density polyethylenes

“…SCBs affect mainly the density and crystallizability of the material, LCBs lead to a reduced coil radius and influence the flow properties of the material. 6,7 The heterogeneity of LDPE-type materials is particularly challenging from an analytical point of view. Attempts have been made to comprehensively analyze LDPE and quite a few complementary techniques have been developed and used.…”

Molecular Weight Distribution in Ring‐Opening Polymerization of Propylene Oxide Catalyzed by Double Metal Complex: A Model Simulation

“…[1][2][3][4][5][6][7][8][9][10] MWD of produced polymer usually depends on the polymerization mechanism and kinetics. [11][12][13][14][15][16][17][18][19][20] In general, living anionic polymerization produces chains with narrow MWD, while traditional free radical polymerization yields broadly distributed chains. [20] To better understand the connection between kinetics and MWD as well as to clarify which theory is correct or more proper, many efforts have been made to simulate MWD.…”

“…[20] To better understand the connection between kinetics and MWD as well as to clarify which theory is correct or more proper, many efforts have been made to simulate MWD. [11,12,16,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Three methods, i.e., moments, Monte Carlo, and solving ordinary differential equations directly, have been developed. [14,16] Among them, the most used is the method of moments because it is not only based on clear reaction mechanism and kinetic equation, but also ratio (RR) of chain propagation to chain transfer can give an impact on the degree of uniformity of chain length.…”