Barrier properties and structure of liquid crystalline epoxy and its nanocomposites YuanQiao Rao a, * , Andong Liu b , Kathleen O'Connell c a The Dow Chemical Company, Core R&D, 400 Arcola Rd, Collegeville, PA, 19426, USA b Core R&D, 936 Zhangheng Rd, Shanghai, China c EM, 455 Forest St, Marlborough, MA, 01752, USA article info Article history: Received 5 December 2017 Received in revised form 9 March 2018 Accepted 14 March 2018 Available online 15 March 2018 Keywords: Nanocomposites Liquid crystalline epoxy Barrier Free volume Mechanical properties abstract There is ever increasing demand for a suitable sealant material to protect electronic devices, in particular, different displays. We present a new approach to generate a liquid processible material that possesses excellent barrier properties and flexibility that can be used for electronic encapsulation. The liquid processable material is based on liquid crystalline (LC) epoxy diglycidyl ether of 4, 4 0 -dihydroxy-a- methylstilbene (DGE-DHAMS or EDHAMS) and its nanocomposite comprising organo-modified mont- morillonite clay. The best-achieved water vapor transmission rate is 4.5 g-mil/m 2 -day at ambient pres- sure, which is more than 20 times better than that of a conventional bisphenol A epoxy. The improved barrier performance comes from the reduction in the water diffusivity. The diffusivity in the liquid crystalline epoxy is 1.25  10 9 cm 2 /s, one order of magnitude lower than that of bisphenol A epoxy. The addition of 1 vol% of organoclay further reduces the diffusivity by 2 times. The cavity size of the free volume of the liquid crystalline epoxy is much smaller than that of conventional bisphenol A epoxy as measured by positron annihilation lifetime spectroscopy, suggesting that the improved barrier property originates from this reduced free volume cavity size. The free volume cavity size of the film with an addition of 1 vol% of an organically modified montmorillonite clay is similar to that of the liquid crys- talline epoxy. We hypothesize that the additional barrier performance improvement by the nanoclay is due to its effect on the liquid crystalline phase morphology (reduced LC phase domain sizes) or the tortuosity effect of the high aspect ratio particles. The material also has excellent adhesion, optical transparency, and thermal stability. © 2018 Published by Elsevier Ltd. 1. Introduction Electronic devices are typically sensitive to moisture and oxy- gen. [1] Edge sealant is usually applied to the frame of a device to protect it and prolong its lifetime. Using an OLED display as an example, Fig. 1 shows advantages and disadvantages of different encapsulation technologies. [2e4] Desired properties of edge sealant materials include high barrier properties, hight flexiblility, sufficient adhesion and are easy to process. In addition to the need for better water barrier properties, current sealants often lack sufficient flexibility and crack when the device is bent. It is there- fore important to develop new materials that improve on these deficiences. The existing sealants for rigid middle size displays are mainly glass frits, which need higher processing temperature and are not flexible, and therefore are not suitable for flexible or large size devices. Another commonly used sealant material, epoxides, can provide the flexibility, but do not have sufficient barrier prop- erties. For instance, according to Table 1 bisphenol A based epoxy (BPA) has a reported water vapor transmission rate of about 100g- mil/m 2 -day-atm [5], which is about ten times worse than poly(- vinylidene chloride) [6], a well-known polymer with excellent water barrier properties. Epoxides are important materials in constructing electronic devices, and extensive work was carried out to further improve the mechanical and electric properties of ep- oxides including the use of bifillers of core-shell SiO 2 @MWNTs and montmorillonite, polymer modified graphene oxide, and layer-by- layer carbon nanotube grafting to carbon fibers. [7e9] This work aims to improve the overall performance of sealants by developing liquid processable materials that have improved water barrier properties and flexibility. * Corresponding author. E-mail address: yrao@dow.com (Y. Rao). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer https://doi.org/10.1016/j.polymer.2018.03.027 0032-3861/© 2018 Published by Elsevier Ltd. Polymer 142 (2018) 109e118