New Developments in Thermoplastic Elastomers: The Hard Segment Inelastic Effects OnThe Mechanical Performance of Polyurethane Elastomers Cristina. Prisacariu, Elena Scortanu, and Paul C. Buckley Abstract— A study was made of a family of polyurethane copolymers, in which the chemical components were: a hard segment (giving, on phase separation, hard nano-scale reinforcing particles); a soft segment (giving, on phase separation, an elastomeric matrix), and a diol chain extender. The chemical compositions of all three components were varied systematically and independently, and their mechanical responses were measured in cyclic tensile tests at room temperature, up to stretches in the range 5-6. Particular attention was paid to characterizing the inelastic features – hysteresis, and stress relaxation in interrupted tests – and their variations between the materials. The same materials were also studied by wide-angle X-ray scattering (WAXS), to determine levels of crystallinity. Results showed that hysteresis was increased by increasing hard phase crystallinity. This was the case for polyurethanes based on the novel diisocyanate 4,4’-dibenzyl diiscyanate (DBDI). The extent of stress relaxation in interrupted tests was found to increase with hysteresis and hence with hard segment crystallinity, reflecting the higher flow stress of the reinforcing hard domains. Polymers based on DBDI hard segments, displayed higher stiffness and strength than did the conventional materials. Both features of the response were attributed to differences in hard phase plastic flow stress, resulting from crystallinity in the DBDI phase. Index Terms—flexible hard segments, physical –mechanical properties, polyurethanes. . I. INTRODUCTION A study has been made of inelastic effects in the deformation of thermoplastic copolyurethane elastomers, where there is potential for formation of a two-phase Manuscript received October 9, 2007. This work was supported in part by an ongoing NATO Collaborative Linkage Grant, 2005 – 2008, Project nr: PDD(CBP.EAP.CLG 981805/ 22.07.2005). F.A. Author (presenting author), C. Prisacariu is with the Institute of Macromolecular Chemistry Petru Poni, Iasi, Romania, (phone: 40-232-217454; fax: 40-232-211299; e-mail: crispris@icmpp.ro ). S. B. Author, E. Scortan,u is with the Institute of Macromolecular Chemistry Petru Poni, Iasi, Romania, (phone: 40-232-217454; fax: 40-232-211299; e-mail: nutiscor@icmpp.ro ). T. C. Author, C. P. Buckley, is with The Department of Engineering Science, University of Oxford, OX1 3 PJ, Oxford, UK; e-mail: paul.buckley@eng.ox.ac.uk microstructure (hard reinforcing particles embedded in an elastomeric matrix), but where the nature of the hard phase and the degree of phase separation can be controlled via the chemical structure and preparation conditions. Numerous block copolyurethanes (PUs) were investigated, based on several diisocyanates, macrodiols and chain extenders, with the aim of improving understanding of the relationship between molecular/supramolecular architecture at the nm-scale and macroscopic mechanical properties in such systems. A novel diisocyanate, DBDI, (Figure 1) and a triol chain extender (1,1,1-trimethylol propane (TMP)) were included as well as more widely-used components, in order to widen the range of structures achievable beyond those normally available. Figure 1. Schematic of isocyanate DBDI. Ref: [1]. It has been shown already that these chemical changes, and variation of thermal history, lead to varying degrees of phase segregation and crystallization in the hard segment, particularly when using DBDI which displays a variable geometry due to which there result PUs with increased degrees of crystallinity. Rotation around the central –CH 2 -CH 2 - bond in DBDI allows alignment of successive aromatic rings thus favoring the tendency of crystallization which involves the DBDI hard phase [1-5]. As previously shown, in the case of PUs with DBDI which were synthesized with ethylene glycol (EG) as a chain extender, we observed a remarkable tendency of the EG-DBDI segments to crystallize. As shown, the origin of these effects was intimately related to the nano-scale structure of the elastomers: the degree of phase segregation and the size and perfection of the hard domains [5] Proceedings of the World Congress on Engineering 2008 Vol II WCE 2008, July 2 - 4, 2008, London, U.K. ISBN:978-988-17012-3-7 WCE 2008