Thermal behavior of sulfur/organic copolymers

Abstract

It is well known that elemental sulfur during heating, at the temperature of about 159C under atmospheric pressure, exhibit an ability to undergo spontaneous ring-opening polymerization. Eight-membered rings (S8), which are the basic form of sulfur under ambient conditions, cleave results in creation of linear sulfur diradicals which polymerize into polymeric form of sulfur. However, sulfur-polymer material obtained from the reaction consists predominantly of unreacted sulfur in form of S8 and contains merely few percent of the polymeric phase. What’s more, in time the content of the polymeric phase slowly decrease due to its instability under ambient conditions. Hence, to obtain a polymeric product it is crucial to stabilize the polymeric phase of sulfur [1-4]. Stabilization of the polymeric phase may be achieved by copolymerization of elemental sulfur with unsaturated organic compounds i.e. dicyclopentadiene, styrene, limonene or 1-3 diisopropenylbenzene via bulk copolymerization process [1,4]. This process was recently described in the literature as “inverse vulcanization”, because of its contrary nature to classic vulcanization of rubber [1]. By following this approach the content of the polymeric phase significantly increases. After extraction of unreacted sulfur, amorphous sulfur/organic copolymers can be used as curing agents for rubber [3], asphalt modifiers [5], sulfur concretes [6] or in various advanced material applications (i.e. Li-S batteries) [1,7,8]. Investigation of thermal properties of sulfur/organic copolymers is essential to investigate the ability to copolymerization between elemental sulfur and particular organic comonomers, as well as to predict their further behavior during processing (i.e. vulcanization of rubber or modification of asphalts at temperatures above 160C). In this work differential scanning calorimetry (DSC) was used to investigate the phase structure of elemental sulfur and sulfur/organic copolymers as well as to track its purification process (extraction of elemental (soluble) sulfur with toluene). Thermogravimetric analysis (TG) was used to determine thermal stability of sulfur/organic copolymers and to confirm that the purification process of sulfur/organic copolymers was performed successfully. [1] W. I. Chung et al., Nat. Chem., 5 (2013) 518 [2] S. Oae, „Organic Chemistry of Sulfur”, Plenum Press, (1977) 33-38 [3] H. Colvin, C. Bull, Jr., Rubb. Chem. Technol., 68 (1995) 746 [4] B. R. Currell et al., „New Uses of Sulfur - Advances in Chemistry”, American Chemical Society, (1975) 1-17 [5] M. Al-Ansary, Innovative solutions for sulphur in Quatar, Proceedings of Sulphur World Symposium in Qatar (2010) [6] N. Ciak, J. Harasymiuk, Sulphur, Technical Sciences, 16 (2013) 323 [7] J. Lim, J. Pyun, K. Char, Angew. Chem. Int. Ed., 54 (2015) 2 [8] A. G. Simmonds et al., ACS Macro Lett., 3 (2014) 229