Chang Liu, PhD
PhD Thesis title:
"MECHANISTIC INSIGHTS INTO RELATIONSHIP BETWEEN NETWORK ARCHITECTURE, POLYMERIZATION KINETICS, AND MACROMOLECULAR PROPERTIES"
Bio
Chang grew up in Nanjing, China and went to Lanzhou University of Technology for his undergraduate degree in Polymer Materials and Engineering. He came to UNH in 2016 to pursue his Ph.D in Chemistry in the Tsavalas Lab.
In the Tsavalas Lab, Chang’s work was sponsored by the UNH Latex Morphology Industrial Consortium and mainly focused on mechanistic understanding of crosslinking mechanisms in both bulk polymerization and emulsion polymerization. Chang finished his Ph.D in 2021.
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Currently, Chang is working as a Senior Research Engineer for Saint-Gobain in Hoosick Falls, New York.
Project 1: Crosslinking in Semi-Batch Seeded Emulsion Polymerization: Effect of Linear and Non-Linear Monomer Feeding Rate Profiles on Gel Formation
Waterborne latex is often called a product-of-process. Here, the effect of semi-batch monomer feed rate on the kinetics and gel formation in seeded emulsion polymerization was investigated for the copolymerization of n-butyl methacrylate (n-BMA) and ethylene glycol dimethacrylate (EGDMA). Strikingly, the gel fraction was observed to be significantly influenced by monomer feed rate, even while most of the experiments were performed under so-called starve-fed conditions. More flooded conditions from faster monomer feed rates, including seeded batch reactions, counterintuitively resulted in significantly higher gel fraction. Chain transfer to polymer was intentionally suppressed here via monomer selection so as to focus mechanistic insights to relate only to the influence of a divinyl monomer, as opposed to being clouded by contributions to topology from long chain branching. Simulations revealed that the dominant influence on this phenomenon was the sensitivity of primary intramolecular cyclization to the instantaneous unreacted monomer concentration, which is directly impacted by monomer feed rate. The rate constant for cyclization for these conditions was determined to be first order and 4000 s−1, approximately 4 times that typically observed for backbiting in acrylates. This concept has been explored previously for bulk and solution polymerizations, but not for emulsified reaction environments and especially for the very low mole fraction divinyl monomer. In addition, while gel fraction could be dramatically manipulated by variations in linear monomer feed rates, it could be markedly enhanced by leveraging non-linear feed profiles built from combination sequences of flooded and starved conditions. For a 2 h total feed time, a fully linear profile resulted in 30% gel while a corresponding non-linear profile with an early fast-feed segment resulted in 80% gel.
CHANG LIU, Amit K. Tripathi, Wei Gao (DOW CHEMICAL), and John G. Tsavalas
Crosslinking in Semi-Batch Seeded Emulsion Polymerization: Effect of Linear and Non-Linear Monomer Feeding Rate Profiles on Gel Formation. Polymers, 13(4), 596. https://doi.org/10.3390/polym13040596
Project 2: Early Bird Gets the Network: The Relative Importance of Reactivity Ratios, Ψ Parameter, and Crosslinker Level on Gel Formation in FRP
Network topology can be tuned in crosslinking free-radical copolymerizations via a variety of crosslinker properties. Our prior work has focused on the impact of the reduced reactivity parameter Ψ, characteristic for each monomer/crosslinker pair, yet here we assess the relative importance of the comonomer reactivity ratios to see whether one factor can counterbalance the other. Both factors are then contrasted to simply the overall loading of crosslinker. Either n-butyl methacrylate (n-BMA) or styrene (STY) was chosen as a primary backbone monomer to copolymerize with one of three crosslinkers: 1,4-butanediol dimethacylate (BDDMA), 1,4- butanediol diacrylate (BDDA) or divinylbenzene (DVB). Both the reactivity ratio and the reduced reactivity parameter, Ψ, are dependent on the monomer-crosslinker pairs, and so the kinetics of the crosslinking reactions and resulting gel development can be significantly impacted even at low crosslinker loading. Moreover, both kinetics and gel can be most dramatically boosted when a crosslinker is applied having a reactivity ratio favoring earlier insertion than statistical incorporation. Earlier incorporation of the crosslinker into chain development leads to an earlier onset of gel formation, which ultimately results in greater final gel content. This amplification of both kinetics and gel can overcome an otherwise small Ψ (steric hinderance on pendent vinyl reactivity) due to crosslinker or main monomer choice. This experimental reactivity ratio effect was further confirmed by Monte Carlo simulations. By whatever mechanism (higher Ψ, lower rA, or higher crosslinker level), starting the gel mechanism earlier produces more gel and a tighter network topology.
Yung-Chun Lini, CHANG LIU, Amit K. Tripathi, and John G. Tsavalas
Early Bird Gets the Network: The Relative Importance of Reactivity Ratios, Ψ Parameter, and Crosslinker Level on Gel Formation in FRP, The Canadian Journal of Chemical Engineering 101 (9), 5382-5394 (2023)
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