Silane-Crosslinked Ethylene–Octene Copolymer Blends: Thermal Aging and Crystallization Study Kalyanee Sirisinha, Sirinya Chimdist Department of Chemistry, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400, Thailand Received 16 May 2007; accepted 3 December 2007 DOI 10.1002/app.28236 Published online 9 May 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Thermally stable materials can be achieved by crosslinking. This article presents the thermal aging and thermal energy storage properties of ethylene–octene copolymer (EOR) and low-density polyethylene (LDPE) blends as affected by silane crosslinking. Fourier transform infrared spectroscopy revealed a similar degree of silane grafting among the various blend compositions. However, the highest crosslink content was observed in EOR, whereas the lowest was found for LDPE. From melting temperature and heat of fusion data, a linear relationship between the amount of the crystalline component and the crosslink content was found. The decrease in crystallinity due to crosslinking was very limited, which implied a high thermal energy storage capacity of the silane-cross- linked products and their good mechanical properties at room temperature. Furthermore, a strong ability to retain the properties after thermal aging indicated good thermal stability of the materials. Ó 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109: 2522–2528, 2008 Key words: ageing; blends; crosslinking; crystallization; structure INTRODUCTION Thermally stable materials are generally achieved by crosslinking. Crosslinked polymers, especially polyeth- ylene (PE), have become commonly used for a num- ber of industrial applications, including wire and cable coating, hot-water piping insulation, and heat-shrink- able products. Different procedures may be used for the initiation of crosslinking. One of them is based on the formation of silane-crosslinkable materials via the free-radical grafting of vinyl silane in the presence of peroxide. The silane-grafted polymer is then hydro- lyzed and crosslinked in the solid state by the forma- tion of siloxane (SiÀÀOÀÀSi) linkages. The chemistry of the reaction is shown in Figure 1. Because of the tech- nological importance, a number of research studies have been carried out on this topic. 1–17 Substantial changes in the polymer structure and properties can occur during the crosslinking process. However, extensive studies on this area are few, and the understanding of how the siloxane-crosslinked net- work affects the crystallization and thermal properties of a polymer remains matters of controversy. Transmis- sion electron microscopy studies of silane-grafted PE, including high-density PE, linear low-density polyethyl- ene (LLDPE), and low-density polyethylene (LDPE), were carried out by Wong and Varrall. 1 Their results reveal dramatic changes in the lamellar distribution and length of the silane-grafted polymers. The molecu- lar structure change varied according to PE type. 1 The studies of Celina and George 2 showed that silane cross- linking did not affect the properties of LDPE in the same manner as did peroxide crosslinking. The melting temperature (T m ) and enthalpy of silane-crosslinked polymers remained nearly constant, whereas a drastic decrease in T m and enthalpy was observed in the per- oxide-crosslinked samples. This result seemed to be in contrast with the findings of Shieh and Hsiao, 3 who reported multiple melting behavior of silane-cross- linked LDPE. Two melting points were observed at about 107.0 and 94.08C and were reported to be due to two crystalline structures, including sol (107.08C) and gel (94.08C) fractions in the crosslinked material. As the crosslinking time increased, the melting point of the gel fraction decreased, whereas the melting point of the sol fraction increased. In their study, peroxide-induced crosslinking also occurred simultaneously during the grafting reactions. This complicates the assessment of crystallization occurring in silane-crosslinked material. Kuan et al. 4 reported an improvement in the mechani- cal properties of LLDPE after silane–water crosslinking. However, the increase in the mechanical properties observed was found to be mainly due to the self-cross- linking of LLDPE during the silane-grafting process. 4 In this study, the characteristics of silane-crosslinked ethylene–octene copolymer (EOR) and LDPE blends were investigated. The LDPE was incorporated into the system to improve the blend stiffness and thermal properties. The silane-grafted samples were prepared Correspondence to: K. Sirisinha (sckpr@mahidol.ac.th). Contract grant sponsor: Thailand Research Fund. Contract grant sponsor: Commission on Higher Educa- tion. Journal of Applied Polymer Science, Vol. 109, 2522–2528 (2008) V V C 2008 Wiley Periodicals, Inc.