  Citation: Erartsın, O.; Zanjani, J.S.M.; Baran, I. Thermoset/Thermoplastic Interphases: The Role of Initiator Concentration in Polymer Interdiffusion. Polymers 2022, 14, 1493. https://doi.org/10.3390/ polym14071493 Academic Editors: Somen K. Bhudolia and Sunil Chandrakant Joshi Received: 23 February 2022 Accepted: 4 April 2022 Published: 6 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Communication Thermoset/Thermoplastic Interphases: The Role of Initiator Concentration in Polymer Interdiffusion Ozan Erartsın *, Jamal Sayyed Monfared Zanjani and Ismet Baran * Faculty of Engineering Technology, University of Twente, 7500 AE Enschede, The Netherlands; j.seyyedmonfaredzanjani@utwente.nl * Correspondence: o.erartsin@utwente.nl (O.E.); i.baran@utwente.nl (I.B.) Abstract: In the co-bonding of thermoset and thermoplastic polymers, the interdiffusion of the poly- mers results in the formation of an interphase between them. Understanding the factors influencing the interdiffusion and the resulting interphase is crucial in order to optimize the mechanical perfor- mance of the bond. Herein, for the first time, the effect of the initiator concentration of the thermoset resin-initiator mixture on the interphase thickness of co-bonded thermoset-thermoplastic polymers is investigated. The dependence of the gelation time on the initiator concentration is determined by rheometer measurements. Differential scanning calorimetry measurements are carried out to determine the speed of cure. To co-bond the polymers, pieces of already-manufactured thermoplastic plates are embedded in a resin-initiator mixture. The interphase thickness of the co-bonded poly- mers is measured with an optical microscope. The results of this study show that the gelation time decreases as the initiator concentration increases. This decrease leads to a significant reduction in both interphase thickness and diffusivity. For instance, increasing the initiator/resin weight ratio from 1% to 3% reduces the gelation time by 74% and the interphase thickness by 63%. Keywords: thermosetting resins; thermoplastics; co-bonding; interphase; diffusion; adhesion 1. Introduction Fiber-reinforced polymer composites (FRPC) provide not only a high strength-to- weight ratio but also exceptional properties such as high durability, stiffness, and corrosion resistance. While knowledge of the manufacturing of individual FRPC parts has reached a level of maturity, the integration and assembly of different FRPC parts is far less developed, particularly considering the co-bonding process. Co-bonding is a bonding technique in which a prefabricated part (in this case, a thermoplastic (TP) polymer) is bonded with a (neat or fiber-reinforced) thermoset polymer through a curing reaction of the thermoset resin [1–4]. The areas of application of this technique involve the bond between the pultruded profiles at the blade root, spar cap, and leading edge protection (LEP) layer and the over-infused main body of the wind turbine blade [5,6]. Although co-bonding may refer to the bonding of two parts with or without an adhesive between them [1–4,7], in this work we will focus on co-bonding without adhesives, where bonding takes place by the interdiffusion of polymers that are in contact as the curing takes place. The interdiffusion of the bonded polymers, and, subsequently, the curing of the resin result in the formation of an interphase [2,4,8–12]. The size and morphology of the interphase have been shown to depend on the gelation time and viscosity of the resin, the thermodynamic affinity between the polymers, and the physical state of the thermoplastic [2,4,8–10]. For instance, high levels of thermodynamic affinity may promote homogeneous mixing, whereas phase separation may take place at lower levels of affinity [9]. The gelation time and viscosity of the resin have competing effects on the interphase thickness [2]. Higher gelation times allow more time for the interdiffusion to take place, which eventually promotes an increase in the interphase thickness. Conversely, an increase in the resin viscosity hampers the diffusion of resin into the thermoplastic, leading to a lower interphase thickness. Polymers 2022, 14, 1493. https://doi.org/10.3390/polym14071493 https://www.mdpi.com/journal/polymers