Gas solvation strategies control polymerization kinetics and material properties

On February 9, 2023, Professor Rong Yang’s team of Cornell University and Professor Jingjie Yang published an article entitled “Engineering solvation in initiated chemical vapour deposition for control over polymerization kinetics and material” in the journal Nature Synthesis properties”.

The research team proposed a gas solvation strategy, which expanded the polymerization kinetics and controllability of material properties of vapor-deposited polymers in a new dimension. This achievement proposes a theoretical kinetic model of gas solvation effect, which provides a theoretical basis for understanding and designing gas phase recombination, interfacial adsorption and surface polymerization processes at the molecular level, and accelerates the development of advanced polymer thin film materials.

The corresponding authors of the paper are Yang Rong and Yang Jingjie; The first author is Chen Pengyu.

Due to its flexible design and rich functions, polymer films are attracting attention in many fields such as chip manufacturing, biosensing, clean energy, and advanced coatings. For a long time, polymer synthesis and film processing based on liquid phase method have been widely used in the preparation of various polymer films, but the separation of liquid phase method is difficult and the film is difficult to control, which greatly restricts the development and application of polymer films.

Since the 21st century, initiated Chemical Vapour Deposition (iCVD) has developed rapidly and is widely used in the controllable preparation of nanopolymer films. Compared with the traditional liquid phase method, iCVD realizes the synthesis and processing of monomer to high-purity polymer film in one step, shortening the manufacturing time and reducing the negative impact of polymer synthesis on the environment. At the same time, vapor deposition technology provides powerful in-situ real-time monitoring and precise material transportation, enabling the control of film thickness and composition at the nanoscale. Professor Yang Rong’s team is committed to developing the methodology of gas-phase synthetic polymers and exploring the application of polymer films in the fields of biological interfaces and clean energy, and has made a series of research progress (such as Macromolecules 2020, 53 (24), 10699-10710; Front. Bioeng. Biotechnol. 2021, 309; Adv. Mater. Interfaces 2021, 8 (5), 2001791; ACS Biomater. Sci. Eng. 2021; Biomacromolecules 2021, 22 (12), 4933-4944; Matter 2021, 4 (11), 3753-3773; J. Mater. Chem. B 2022, 10 (14), 2728-2739; Sci. Adv. 2022, 8 (2), eabl8812; J. Vac. Sci. Technol. 2022, 40 (3), 33-406; Chem. Mater. 2022, 34 (13), 5960-5970; J. Am. Chem. Soc. 2022, 144 (42), 19344-19352; ACS Sustain. Chem. Eng. 2022, 10 (48), 15699-15713; Langmuir 2023, 39 (3), 1215-1226.)。

Recently, in view of the mechanism defect of high coupling between material properties and monomer chemistry in iCVD technology, the research team proposed a new, simple and powerful gas phase solvation strategy. This method forms a molecular complex through the solvation behavior between the monomer and the solvent, which affects the chemical properties and interfacial adsorption of the monomer, and then regulates the polymerization reaction kinetics of the interface. In this study, the above process is deconstructed in detail and rigorously, and a theoretical kinetic model of gas solvation effect is proposed to accurately predict the deposition kinetics under gas solvation effect. In addition, unlike traditional monomer pressure and temperature parameters, the gas solvation strategy brings a new degree of freedom, showing extensive control over deposition kinetics, molecular weight, and mechanical properties. At the same time, through in-situ annealing of the gas phase solvent, the gas phase solvation strategy gives new characteristics to the surface morphology of the film under the control of traditional iCVD. This research lays the foundation for expanding the application of polymer films in many fields.

In this work, the authors used the hydrogen bond interaction force between vinylpyridine (4VP) and hexafluoroisopropanol (HFIP) to establish a model system, and first studied the differences between deposition kinetics, molecular weight and mechanical properties of thin films using inert carrier gas or active solvents under the same deposition conditions, and preliminarily demonstrated the efficient and widely controllable effect of gas phase solvation strategy on material synthesis and material properties.

Figure 1: Gas phase solvation enhancement process and material property controllability.

Subsequently, the authors studied the vapor phase solvation behavior of monomers and solvents by varying the feed ratio and pressure of monomers and solvents. The results suggest that monomers and solvents rapidly form chemical equilibrium and exist as molecular complexes. In order to further reveal the influence of gas solvation behavior on deposition kinetics, the authors systematically changed the reaction conditions and found that: (1) after the introduction of gas solvation, the deposition kinetics deviated from the classical model; (2) The bias pressure of the solvent independently affects the deposition kinetics. Based on this, the authors establish a kinetic model of the gas phase solvation effect.

Figure 2: Chemical equilibrium dominated deposition kinetics of gas-phase solvation.

The gas-phase solvation effect has been extended to a variety of solvent systems, and the strength of the interaction between the monomer and the solvent is a key factor determining the strength of the solvation effect.

Figure 3: Solvent versatility of gas-phase solvation strategies.

The type of solvent affects where solvation occurs. Combining experimental, simulation and theoretical models, the authors found that the solvation of HFIP and 4VP mainly occurs in the gas phase, while the solvation of acetic acid and 4VP occurs at the interface. The regulation of solvation sites by solvent types is expected to further enhance the control of film properties, such as temperature sensitivity of deposition kinetics. In situ solvent vapor annealing process strength, etc.

Figure 4: Gas phase solvation or interface solvation?

Finally, this work takes HFIP as an example to expand the universality of solvation strategies for polar monomers, and analyzes the advantages and disadvantages of iCVD based on gas phase solvation enhancement compared with traditional iCVD and liquid phase methods in the synthesis of polymer films. This work provides a novel, simple and efficient strategy to improve the controllability of iCVD on material synthesis and properties, which is expected to accelerate the discovery and wide application of polymer thin film materials in many fields.

Figure 5: Mocell versatility and technical advantages of gas phase solvation strategies.(Source: Science Network)

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