Lin Wang

Northwestern University, Evanston, IL, United States

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Publications (7)4.61 Total impact

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    ABSTRACT: Conventional continuum mechanistic models for polymer degradation typically involve thousands of coupled differential-algebraic equations, requiring an efficient solver to solve the complex set of stiff model equations. This can be overcome by formulating the problem in terms of a stochastic simulation procedure, requiring only iterative operations to solve the model. The present work describes the detailed mechanistic modeling of pyrolysis of poly(styrene peroxide) (PSP) using kinetic Monte Carlo (KMC) simulation to predict the product yields and gain a better understanding of the product evolution pathways. The traditionally accepted mechanism of PSP pyrolysis proposed by Mayo and Miller, which involves the key reaction steps of peroxide bond fission, alkoxy radical recombination and disproportionation, and end chain β-scission, was initially tested using the KMC model to predict the peroxide concentration profile and the product yields. This model was only qualitatively able to predict the major products, benzaldehyde and formaldehyde, while the formation of minor products like α-hydroxy acetophenone, phenyl glycol, and phenyl glyoxal was not captured at all. Hence, a new mechanism that also incorporated hydrogen abstraction and β-scission was proposed and implemented in KMC. The final model tracked 949 reactions of 83 species. The rate coefficients for all the reaction steps were based on the existing literature reports, and hence no parameter estimation was done to fit the model against the experimental data. The revised model was quantitatively able to predict all the products of PSP pyrolysis, which was attributed to the stabilization of the alkoxy radicals by hydrogen abstraction, and the subsequent generation of additional alkoxy radicals by β-scission. KMC allowed the dominant pathways for the formation of minor products and dimers to be identified explicitly. Finally, the implications of this study in understanding the effect of trace peroxide bonds on poly(styrene) pyrolysis are outlined.
    Chemical Engineering Science. 02/2012; 69(1):456–471.
  • Lin Wang, Linda J. Broadbelt
    Macromolecular Theory and Simulations 02/2011; 20(3):191 - 204. · 1.61 Impact Factor
  • Lin Wang, Linda J. Broadbelt
    Macromolecular Theory and Simulations 10/2010; 20(1):54 - 64. · 1.61 Impact Factor
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    ABSTRACT: The nitroxide-mediated controlled radical polymerization (NM-CRP) of 4-acetoxystyrene with an alkoxyamine was analyzed by a combined experimental and modeling approach. At low nitroxide concentrations, thermal initiation was significant, and control of the polydispersity was poor, as was observed previously for styrene. A continuum model based on the method of moments was used to regress the parameters for the reversible nitroxide uncoupling/coupling reactions (activation energy of uncoupling), thermal initiation (activation energy of initiation), and termination (frequency factor of recombination). The model was able to capture the molecular weight averages and the polydispersity index as a function of time and the nitroxide concentration qualitatively and quantitatively. Using this mechanistic framework, we developed kinetic Monte Carlo models that allowed the molecular weight distributions to be predicted explicitly in good agreement with experimental data. A comparison of the NM-CRP of 4-acetoxystyrene and styrene is provided to illustrate the effect of the acetoxy substituent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
    Journal of Applied Polymer Science 05/2010; 118(2):740 - 750. · 1.40 Impact Factor
  • Lin Wang, Linda J. Broadbelt
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    ABSTRACT: The kinetic details of segment formation in living radical polymerization (LRP) were investigated by studying styrene (S)/methyl methacrylate (MMA) copolymerization via nitroxide-mediated controlled radical polymerization (NM-CRP) using kinetic Monte Carlo (KMC) simulations. It was demonstrated that the classic theory describing the distribution of instantaneous segment lengths developed for conventional free radical polymerization (FRP) does not always hold in LRP. Segment formation can be controlled by deactivation through coupling of growing radicals and nitroxide radicals instead of selective preference during propagation due to different reactivity ratios and composition of competing monomers if the transient lifetime is too short. Mathematical equations were proposed for determining in a facile way whether the segment growth is controlled by deactivation under given reaction conditions. The uniformity of segment growth was analyzed as well. It takes a surprisingly long time to allow the majority of the chains to be reactivated at least once in an S-rich reaction system, which leads to high polydispersity in monomer sequences formed on different chains. It was also demonstrated that there inevitably exists a distribution of segment lengths at a given monomer composition no matter how “ideal” the livingness is. This distribution is determined by the “intrinsic” kinetics of copolymerization and is not manipulated by the features of LRP.
    Macromolecules. 02/2010; 43(5).
  • Lin Wang, Linda J. Broadbelt
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    ABSTRACT: The impact of the synthesis conditions on the formation of the monomer sequences of styrene (S) and methyl methacrylate (MMA) gradient copolymers synthesized by forced gradient copolymerization with nitroxide-mediated controlled radical polymerization (NM-CRP) was investigated using kinetic Monte Carlo (KMC) simulations. The factors affecting the formation of the individual segments, arrangement of these segments along the chain, and the uniformity of monomer sequences were investigated. It was shown that instantaneous segment lengths increase exponentially as a function of monomer composition. The concentration of nitroxyl radicals also plays a key role in the formation of segment lengths. In addition, the arrangement of the segments mainly depends on the feed profile of the second monomer. A constant feed profile, which is widely used in current syntheses of gradient copolymers, is shown to not be suitable to make a structural gradient along the chain. It was also demonstrated that the uniformity of sequence patterns can be affected by the entire growth history of the chains, and thus strong control over the uniformity of chain growth is required throughout the reaction in order to achieve sequences in different chains that resemble one another.
    Macromolecules. 11/2009;
  • Lin Wang, Linda J. Broadbelt
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    ABSTRACT: In spite of many efforts made to study gradient copolymers, the monomer-by-monomer sequence along the chain is still obscure. A general computational framework based on kinetic Monte Carlo simulations was developed to predict the explicit sequence of copolymers. We demonstrate our approach using styrene (S)/methyl methacrylate (MMA) gradient copolymers synthesized by a semibatch process with nitroxide-mediated controlled radical polymerization (NM-CRP). It was found that the variation in the average segment length as a function of chain length does not resemble that of the instantaneous composition. Our findings indicate that copolymers with compositional gradients may have monomer-by-monomer sequences resembling those of statistical copolymers. It was also found that the explicit sequence can significantly deviate from that of the instantaneous composition as a function of chain length when combination is the favored termination mode. These details of explicit sequence revealed by KMC simulations are obscured by only considering fractional composition measures to characterize the gradient shape.
    Macromolecules. 10/2009; 42(20).