Rearrangement of inhomogeneous distribution of ionic multiplets in ethylene ionomer induced by artificial weathering Kei Nagayama a, * , Christopher D. Chan b , Dennis J. Walls b , Juan D. Londono b , Tadahisa Iwata c a Packaging & Specialty Plastics, Performance Materials Japan K.K., An Affiliated Company of the Dow Chemical Company, Tennoz Central Tower, 2-2-24, Higashi Shinagawa, Shinagawa-Ku, Tokyo,140-8617, Japan b DuPont Science and Innovation, DuPont Company, Experimental Station, 200 Powder Mill Road, Wilmington, DE, 19803, USA c Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo,1-1-1 Yayoi, Bunkyo-ku, Tokyo,113-8657, Japan article info Article history: Received 11 March 2019 Received in revised form 2 July 2019 Accepted 5 July 2019 Available online 6 July 2019 Keywords: Ionomer Ionic multiplets Low-angle upturn SAXS and xenon weatherometer abstract An effect of ultraviolet exposure over the inhomogeneous structure of ionomer was investigated by using small-angle X-ray scattering (SAXS). Ionomer sheet laminated in between two sheets of glass was arti- ficially weathered using a xenon lamp in an Atlas Ci4000 weatherometer. Though no significant changes were detected by chemical, thermal and optical analysis approach, SAXS identified changes at a low- angle upturn in the region of q < 0.01 Å 1 (where q is the magnitude of the scattering vector) after xenon light irradiation. Power law, I ~ q a , was employed to study the changes in the upturn and found a changed from 3.0 to 1.7, indicating a change in the micron-sized X-ray scatterers. A relationship between differences in the low-angle upturn and the inhomogeneous structure is interpreted as a rearrangement of cation distribution induced by xenon light irradiation. © 2019 Published by Elsevier Ltd. 1. Introduction Surlyn ® , the DuPont brand name for its ethylene-based ionomer resins, has been widely used for its unique properties, such as high transparency, good adhesion, high melt viscosity and high stiffness. One of the applications of the ionomer is as the encapsulant sheet for photovoltaic (PV) modules due to its outstanding electric insulation, mechanical stability and low moisture diffusivity [1e4]. These properties of ionomers are generally thought to be closely related with its unique structure where three phases are formed following a manner of phase-separation between the polar (ionic) and non-polar phases; crystalline lamellae of polyethylene, amor- phous matrix, and ion multiplets [5]. The ion multiplets is thought to be spherical and a few tens of angstrom in size based on small- angle X-ray scattering (SAXS) patterns [6e8] and transmission electron microscopy results [9, 10]. In the SAXS patterns of ionomer materials, a low-angle upturn is also known to appear, which in- dicates another feature of ionomer structure; long-range metal concentration variations in length scales from nanometers to microns [11e 14]. Although there are many studies on ionomer structure, the authors have found some accelerated testing studies [15, 16] on using Surlyn ® as a water vapor barrier in the PV application, no study has been found dedicated to structural changes of ionomers caused by accelerated testing. Since PV modules are expected to have long-term service lifetime to reach the grid parity, a signifi- cant requirement for encapuslant materials is long term material durability in operation to protect Si-cell from the external envi- ronment. In the PV industry, accelerated testing is usually per- formed to verify the long-term module reliability, with ultraviolet (UV)-accelerated testing attracting increasing attention [17e19]. Discoloration after UV light irradiation is one of the well-known issues for polymeric materials and generally assumed to be poly- mer degradation. In the glass laminate application, the glass is designed to filter most of the UV light, and therefore the discolor- ation phenomenon is much less an issue than direct UV exposure to the polymer. However, discoloration of ethylene vinyl acetate (EVA) family of encapsulants have been extensively studied as part of the accelerated testing of PV modules and was found to be caused primarily by the photothermal degradation of additives inside the EVA [20,21]. According to a recent report, besides additive * Corresponding author. E-mail address: Kei.Nagayama@dow.com (K. Nagayama). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab https://doi.org/10.1016/j.polymdegradstab.2019.07.002 0141-3910/© 2019 Published by Elsevier Ltd. Polymer Degradation and Stability 167 (2019) 139e145