A structural study of epoxidized natural rubber (ENR-50) ring opening under mild acidic condition Rosniza Hamzah AQ5 , 1 Mohamad Abu Bakar, 2 Omar Sabbar Dahham, 1 Nik Noriman Zulkepli, 1 Saad Sabbar Dahham 3 1 Center of Excellence Geopolymer and Green Technology, Faculty of Engineering Technology, Universiti Malaysia Perlis, Perlis, Malaysia 2 Nanoscience Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, Minden Penang 11800, Malaysia 3 EMAN Research and Testing Laboratory, School of Pharmaceutical Sciences, Universiti Sains Malaysia, Minden Pulau Pinang 11800, Malaysia Correspondence to: R. Hamzah (E - mail: rosnizahamzah@unimap.edu.my) ABSTRACT: A structural study of ring opening reaction of purified epoxidized natural rubber (ENR) with acetic acid was conducted using the NMR techniques and its thermal characteristic was evaluated with TG/DTG and DSC analyses. 1 H-NMR revealed that 19.56% of epoxide was ring-opened from the total amount of the epoxide unit in ENR-50 and this was supported by FTIR spectros- copy. 13 C-NMR suggests the fixation of alkyl (R) i.e. acetate group to the epoxide carbon via ester linkage and formation of hydroxyl groups in the polymer chains. The attachment location of R occurred at both most (") and least (#) hindered carbons of the epoxide. The TG/DTG results of acid treated ENR-50 showed three decomposition steps at 235–338, 338–523, 523–627 8C due to the presence of the polymer chains mixture, i.e. ring-opened and intact epoxide of ENR-50. This increases the T g value of acid treated ENR-50 at 24.6 8C as compared to purified ENR-50 at 217.7 8C. V C 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44123. KEYWORDS: inorganic polymers; ring-opening polymerization; rubber; structure–property relations; thermal properties Received 28 February 2016; accepted 25 June 2016 DOI: 10.1002/app.44123 INTRODUCTION Epoxidized natural rubber (ENR) is a commercial polymer pro- duced from the epoxidation process of natural rubber (NR) with formic peroxide or acetic peroxide in a reactor. 1,2 ENR contains both epoxidized isoprene (E) and isoprene (C) units in the polymer chains. 3,4 Epoxide ring is the reactive site of the E unit while the double bond is the reactive site for C unit, respectively. The epoxide at E unit is involved with the ring opening reaction while the double bond at C unit is involved with crosslinking. 5–8 Epoxide or oxirane is an ether in which the oxygen atom is incorporated into the C@C bond to form a three-membered ring. Ethylene oxide is the simplest form of epoxide. The carbons in an epoxide are very reactive electro- phile thus able to react with a variety of nucleophile. The ring strain in the three-membered ring of epoxide is able to relieve through the ring opening reaction. 9 The majority of epoxide reactions are studied in solution and most of the reaction involves the epoxide ring opening and addition of a molecule of reagent. 10 In practice, the oxirane ROR is a common reaction during the preparation of ENR. The ring opening products vary the properties and application of ENR. 11 Thus, this reaction offers opportunities to functionalize ENR with other reactive substances, fillers, or polymers. The oxirane ring is reactive toward nucleophilic reagents such as amine, carboxylic acid, alcohol, dibutylphosphate, and phosphoric acid derivatives. 12–14 ENR is a pH sensitive polymer and favors neutral pH. 15,16 The exposure of ENR to pH below than 7 (acidic) is able to catalyst the ring opening reaction of an epoxide and provides crosslink- ing amongst the polymer chains through the ether groups. 15–17 The excessive reaction temperature and too low pH contribute to the secondary ring opening reaction of ENR-50 such as diol and furans. 18,19 Typically, acidity and temperature are the main factors for the formation of ring opening products, i.e. hydroxyl-acetates, diols, and intermolecular ethers. 11 The cleav- age of epoxide ring under acidic condition involves the forma- tion of oxonium ion. Then this oxonium ion is attacked by the nucleophile in a S N 2 displacement that produced b-alkoxy alco- hols. However, the addition of nucleophile onto epoxide ring is difficult to obtain under acidic condition due to the undesirable side reactions obtained. 13 The ring opening reaction of epoxide using acid treatments is preferred due to their simplicity and low cost. 4,10,20 The ring opening reaction of ENR-50 using V C 2016 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.44123 44123 (1 of 13) J_ID: z8e Customer A_ID: APP44123 Cadmus Art: APP44123 Ed. Ref. No.: APP-2016-02-0688.R1 Date: 13-July-16 Stage: Page: 1 ID: pachiyappanm Time: 23:16 I Path: //chenas03/Cenpro/ApplicationFiles/Journals/Wiley/APP#/Vol00000/160635/Comp/APPFile/JW-APP#160635