Research Article pubs.acs.org/journal/ascecg
Network from Dihydrocoumarin via Solvent-Free Metal-Mediated Pathway: A Potential Structure for Substantial Toughness Improvement of Epoxidized Plant Oil Materials Cong Li,† Jonggeun Sung,† and Xiuzhi Susan Sun*,†,‡ †
Biomaterials and Technology Lab, Department of Grain Science and Industry, Kansas State University, 1979 Agriculture Road, BIVAP Innovation Center, Manhattan, Kansas 66506, United States ‡ Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, Kansas 66506, United States ABSTRACT: The main challenge in converting polymerized epoxidized plant oils (P-EPO) to high performance materials is the limitation of their short cross-link structures and brittle properties. Herein, a network (N-2) fabricated from a renewable material, dihydrocoumarin (DHC), demonstrated the potential to overcome substantially the brittleness of P-EPO material. The fused ring in DHC was successfully opened via a solvent-free chromium(III) salen-mediated pathway, and a novel network, termed N-2, was created through an alternating copolymerization of DHC and diglycidyl ether epoxides. Repeating ether units in N-2 offered the capacity to sustain large deformation, and phenoxide end groups provided the reactivity to attack epoxides. Once a double network was built between N-2 and EPO derived network (N-1), a substantial performance improvement of P-EPO could be achieved. We targeted epoxidized soybean oil (ESO) as the model material, which showed a significant toughness enhancement (9-fold improvement in elongation at break combined with 85% tensile strength retention compared to the control P-ESO) after the introduction of N-2 network. Creep behaviors revealed the introduction of N-2 in P-ESO matrix lengthened the chains between cross-links, prolonged the response process and transferred the stress from N-1 segments to N-2 segments to retard P-ESO network fracture. KEYWORDS: Epoxidized soybean oil, Dihydrocoumarin, Bionetwork, Alternating copolymerization, Toughness improvement, Renewable materials
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INTRODUCTION Use of plant oils as renewable raw materials is necessary in terms of developing alternative resources to petroleum chemicals.1,2 The main constituents of plant oils are triglycerides, which are products of esterification of glycerol with three fatty acids.3 Epoxidation of unsaturated groups in triglycerides produces plant oil-derived materials, and the resulting epoxidized plant oils (EPOs) can be conveniently polymerized, thereby showing promise for various applications, such as plastic, coating, and adhesive.4,5 In general, polymerization is based on cationic reactions of epoxy groups or an epoxy group and an active hydrogen-containing group through photoinitiation or heat initiation.5−8 However, applications of polymerized epoxidized plant oil (P-EPO) materials often pertain to adhesives. This is attributed to the low flexibility of P-EPO because of the relatively short chains and brittle backbones formed in the polymerization process.6−8 Toughness enhancement would improve P-EPO material performance and expand material use in packaging, coating, and tissue engineering. Therefore, an effective pathway of the chain skeleton modification must be identified and utilized in order to obtain desired toughness for P-EPO materials. Epoxy groups in EPO © XXXX American Chemical Society
can change the material’s chain structures by engaging in a ringopening reaction with other epoxide-based flexible materials. Approaches to ring-opening reactions of epoxides based on cationic, anionic, and coordination-catalyzed methods have been studied extensively in recent decades.9−11 The internal location of epoxides on carbon chains in EPOs causes low efficiency ring-opening via anionic reaction.12 Although a cationic reaction can realize the ring-opening effectively, the chain transfer characteristic causes a side reaction and low molecular weight propagation.13 Coordination-catalyzed polymerization yields high molecular weight polymers via attacks of coordinating catalysts on the primary carbon atom of monosubstituted epoxides.14 However, these reactions are typically carried out at a very low temperature (e.g., −80 °C).14,15 To our best knowledge, no chemical pathway has been available to toughen significantly P-EPO materials. As a renewable material, dihydrocoumarin (DHC) is an appealing monomer because it is inexpensive (
