Effect of soft segment structure on properties of polyurethane elastomers
【Abstract】Using four different soft segments, i.e., polytetrahydrofuran diol (PTMG), polycaprolactone diol (PCL), hydroxyl-terminated polybutadiene with high cis-1,4 content (HTPB), and commercial hydroxyl-terminated polybutadiene produced by free radical polymerization (FHTPB), four kinds of polyurethane elastomers (PUEs) were prepared by two-step solution polymerization. The effects of soft segment structure on mechanical properties and thermal properties at room temperature and low temperature were investigated by tensile test, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). It is found that the tensile strength and elongation at break of the four PUEs at low temperature (−30 °C) are superior to those at room temperature. This is not only related to the autofrettage effect induced by crystallization of soft segment at low temperature, but also associated with the increase in the degree of micro-phase separation between the soft and hard segments. Compared with the other three kinds of PUE, the HTPB-PUE has the soft segment with the lowest polarity and glass transition temperature (Tg), so the micro-phase separation is in the highest degree, and the elastomer has the most excellent flexibility. Even at −30 °C, the elongation at break of the HTPB-PUE is up to 660%. PCL-PUE and PTMG-PUE are rigid because their soft segments are easy to crystallize and their degree of micro-phase separation between soft and hard segments is low. The cyclic tensile testing indicates that HTPB-PUE and FHTPB-PUE still have excellent elasticity at −30 °C. The results from DSC and DMA show that the Tg of HTPB-PUE is much lower than those of the other three PUEs, and the value from DSC is about −103 °C. The initial thermal decomposition temperatures of the four PUEs are very similar, and are all around 270 °C.
【Keywords】 polyurethane; elastomer; soft segment; structure; properties;
 Shanxi Provincial Institute of Chemical Industry. Polyurethane Elastomers Manual [M]. Beijing: Chemical Industry Press, 2001: 7 (in Chinese).
 Liu J, Li F. New progress of polyurethane elastomer [J]. Chemical Propellants & Polymeric Materials, 2018, 16 (5): 26–31, 43 (in Chinese).
 Li S X, Liu Y J. Polyurethane Resin and Application [M]. Beijing: Chemical Industry Press, 2002: 45–47 (in Chinese).
 Meng F N, Hu K F. Study on mechanical properties of HTPB type PU elastomers [J]. Adhesion, 2010, 31 (9): 71–74 (in Chinese).
 Xiang P, Zhang C B, Zhang J, et al. Synthesis and property of polyurethane elastomer with different isocyanate [J]. New Chemical Materials, 2017, (4): 70–72 (in Chinese).
 Liu T, Ma M, Luo X, et al. The synthesis and property research of polycaprolactone diol and the polyurethane elastomer [J]. Polyurethane Industry, 2015, (5): 10–13 (in Chinese).
 Zhang X Y, Liu J C. Study on synthesis and properties of polycaprolactone polyurethane elastomer [J]. Special Purpose Rubber Products, 2012, 33 (1): 27–30 (in Chinese).
 Bagdi K, Molnár K, Pukánszky B, et al. Thermal analysis of the structure of segmented polyurethane elastomers [J]. Journal of Thermal Analysis and Calorimetry, 2009, 98 (3): 825–832.
 Eceiza A, Larrañaga M, de la Caba K, et al. Structure–property relationships of thermoplastic polyurethane elastomers based on polycarbonate diols [J]. Journal of Applied Polymer Science, 2008, 108 (5): 3092–3103.
 Li J, Chen S H, Song X N, et al. Performance effect study on thermoplastic polyurethane elastomer with polyester molecular weight [C] //Proceedings of the 2015 Annual Meeting of the Elastomer Professional Committee of China Polyurethane Industry Association. Taiyuan: Shanxi Provincial Institute of Chemical Industry, 2015: 176–181 (in Chinese).
 Klinedinst D B, Yilgör I, Yilgör E, et al. The effect of varying soft and hard segment length on the structure–property relationships of segmented polyurethanes based on a linear symmetric diisocyanate, 1,4-butanediol and PTMO soft segments [J]. Polymer, 2012, 53 (23): 5358–5366.
 Liu L B, Liu H M, Jia L C. Effect of soft-segment of polyester on mechanical properties of polyurethane elastomer [J]. Polyurethane Industry, 2007, (4): 20–23 (in Chinese).
 Mao K, Xia M, Luo Y. Thermal and mechanical properties of two kinds of hydroxyl-terminated polyether prepolymers and the corresponding polyurethane elastomers [J]. Journal of Elastomers & Plastics, 2016, 48 (6): 546–560.
 Kébir N, Campistron I, Laguerre A, et al. Use of hydroxytelechelic cis-1,4-polyisoprene (HTPI) in the synthesis of polyurethanes (PUs) (Ⅰ): Influence of molecular weight and chemical modification of HTPI on the mechanical and thermal properties of PUs [J]. Polymer, 2005, 46 (18): 6869–6877.
 Wen Y, Wang H F, Liu C Y, et al. Mechanical properties and hydrolysis resistance comparison of H-HTPB and HTPB based polyurethane elastomer [J]. Polyurethane Industry, 2018, 33 (1): 31–33 (in Chinese).
 Cao Z, Jie S Y, Li B G. Preparation and properties of epoxided hydroxyl-terminated polybutadiene based polyurethane elastomers [J]. Acta Polymerica Sinica, 2017, (8): 1350–1357 (in Chinese).
 Yi J J. Synthesis and property investigation of polybutadiene-based polyurethane elastomer [D]. Jinan: Shandong University, 2014 (in Chinese).
 Cao Z, Zhou Q Z, Jie S Y, et al. High cis-1,4 hydroxyl-terminated polybutadiene-based polyurethanes with extremely low glass transition temperature and excellent mechanical properties [J]. Industrial & Engineering Chemistry Research, 2016, 55 (6): 1582–1589.
 Wingborg N. Increasing the tensile strength of HTPB with different isocyanates and chain extenders [J]. Polymer Testing, 2002, 21 (3): 283–287.
 Sekkar V, Gopalakrisham S, Devi K A. Studies on allophanate–urethane networks based on hydroxyl terminated polybutadiene: effect of isocyanate type on the network characteristics [J]. European Polymer Journal, 2003, 39 (6): 1281–1290.
 Kulkarni P, Ojha U, Wei X, et al. Thermal and mechanical properties of polyisobutylene-based thermoplastic polyurethanes [J]. Journal of Applied Polymer Science, 2013, 130 (2): 891–897.
 Ahmad N, Khan M B, Ma X, et al. The influence of cross-linking/chain extension structures on mechanical properties of HTPB-based polyurethane elastomers [J]. Arabian Journal for Science and Engineering, 2014, 39 (1): 43–51.
 Yi J J, Li Y, Chen J M, et al. Relationships between crosslinked network homogeneity and mechanical properties of hydroxyl terminated polybutadiene based polyurethane elastomer [J]. China Synthetic Rubber Industry, 2015, 38 (4): 289–293 (in Chinese).
 Zhou Q Z, Cao Z, Jie S Y, et al. Synthesis of HTPB with high cis-1,4 content from PBD original solution [J]. Acta Polymerica Sinica, 2016, (3): 293–299 (in Chinese).
 Ai Q S, Xu D M, Fang X L, et al. Analysis on formula raw materials and their proportions of polyurethane elastomer by IR and 1H-NMR [J]. Plastic Science and Technology, 2016, 44 (1): 39–43 (in Chinese).
 Liu T. Synthesis, modifined and property research of polycaprolactone polyurethane elastomer [D]. Hangzhou: Zhejiang University of Technology, 2015 (in Chinese).
 Zhou Q Z. Controllable synthesis of hydroxyl functional polybutadiene liquid rubber via oxidolysis of butadiene rubber [D]. Hangzhou: Zhejiang University, 2016 (in Chinese).
 Li Z F, Li D H, Yang G H. The application of FTIR in polyurethane chemistry [J]. China Elastomerics, 1997, (4): 30–35 (in Chinese).
 Zhou Q Z, Jie S Y, Li B G. Preparation of hydroxyl-terminated polybutadiene with high cis-1,4 content [J]. Industrial & Engineering Chemistry Research, 2014, 53 (46): 17884–17893.
 Zhou Q Z, Jie S Y, Li B G. Facile synthesis of novel HTPBs and EHTPBs with high cis-1,4 content and extremely low glass transition temperature [J]. Polymer, 2015, 67: 208–215.
 Holden G, Legge N R, Quirk R P, et al. Thermoplastic Elastomers [M]. Fu Z F, trans. Beijing: Chemical Industry Press, 2000: 21–22 (in Chinese).
 Zou Y C, Zhang Y L, Zhai J X, et al. Effect of sequence structure and crystallization behavior of the prepolymer on mechanical properties of the polyurethane elastomers [J]. Polymer Materials Science and Engineering, 2018, 34 (3): 22–29 (in Chinese).
 Pang A M. Investigation on the dynamic mechanical property of HTPB polyurethane elastomer [J]. Journal of solid rocket Technology, 1999, (1): 42–45 (in Chinese).
 Rocco J A F F, Lima J E S, Lourenço V L, et al. Dynamic mechanical properties for polyurethane elastomers applied in elastomeric mortar [J]. Journal of Applied Polymer Science, 2012, 126 (4): 1461–1467.
 Li J, Wang J C, Han Y, et al. Study on microphase separation of H12MDI based polyurethane elastomers [J]. China Plastics Industry, 2010, 38 (6): 47–51 (in Chinese).
 Liu L B. Effects of chemical structure on thermal degradation of polyurethane elastomer [C] //Proceedings of the 14th Annual Meeting of China Polyurethane Industry Association. Shanghai: China Polyurethane Industry Association, 2008: 257–261 (in Chinese).
 Cui X, Liu B L, He C H, et al. Research on thermal degradation mechanism and thermal stability of aliphatic polyetherpolyurethane elastomer [J]. Chemical Industry and Engineering Progress, 2016, 35 (11): 3585–3589 (in Chinese).