Temperature dependent microphase mixing of model polyurethanes with different intersegment compatibilities Suphannee Pongkitwitoon a , Rebeca Herna ´ ndez b , Jadwiga Weksler c , Ajay Padsalgikar c , Taeyi Choi a , James Runt a, * a Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA b Instituto de Ciencia y Tecnologı ´a de Polı ´meros, CSIC, Juan de la Cierva, 328006 Madrid, Spain c AorTech Biomaterials, Dalmore Drive, Caribbean Park, Scoresby, VIC 3179, Australia article info Article history: Received 14 August 2009 Received in revised form 19 October 2009 Accepted 29 October 2009 Available online 5 November 2009 Keywords: Segmented polyurethane copolymers Microphase separation Synchrotron small-angle X-ray scattering abstract In this paper we explore the temperature dependence of segregation of hard and soft segments of selected segmented polyurethane copolymers using synchrotron small-angle X-ray scattering (SAXS). The copolymers are composed of the same hard segments but three different soft segment chemistries, of particular interest in biomedical device applications. Hard segments are formed from 4,4 0 -methyl- enediphenyl diisocyanate and 1,4-butanediol, and soft segments from an aliphatic polycarbonate [poly(1,6-hexyl 1,2-ethyl carbonate)], poly(tetramethylenoxide), or a mixed soft segment synthesized from hydroxyl-terminated poly(dimethylsiloxane) [PDMS] and poly(hexamethylenoxide) macrodiols. The changes in SAXS relative invariants and interdomain spacings are indicative of gradual dissolution of phase separated hard and soft segments with increasing temperature. All copolymers investigated herein, even those containing PDMS soft segments, transform to the single-phase state at a temperature determined by the soft segment chemistry (and hard segment content). The SAXS findings, along with those from parallel temperature-controlled Fourier Transform infrared spectroscopy measurements, also facilitate assignment of the origin of the thermal events observed in the DSC thermograms of these materials. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Segmented polyurethane (PU) copolymers are used extensively in applications ranging from foams and coatings, to a variety of important applications in life-saving biomedical devices. These polymers can be synthesized from a wide variety of isocyanates, diol or diamine chain extenders, and polyols, using convenient addition polymerization methods. However, although the polyol (i.e., the precursor for the low T g soft segments) has a relatively well-defined molecular weight, the nature of the polymerization generally yields ‘hard’ segments with a rather broad distribution of sequence lengths [1]. This creates complexity in understanding segmented PU microphase separation compared to diblock and triblock copolymers, in which the blocks have well-defined lengths and narrow polydispersity. PUs having different soft segment chemistries commonly exhibit rather incomplete hard/soft demixing when polymerized in the bulk or cast from solution, and the extent of unlike segment segregation (and resulting hard domain morphology) is particularly important in determining mechanical and other physical properties [2–4]. This arises from several factors. Hard segments having short sequence lengths (as well as so-called ‘lone’ isocyanates [5], more prevalent at lower hard segment contents), have greater solubility in the soft phase than longer sequences for entropic reasons [6]. In addition, similarity in cohesive energy density and/or the possi- bility of hydrogen bonding between hard and soft repeat units also encourage mixing. This is illustrated in some of our earlier work in which we compared degrees of hard/soft segment phase separa- tion, quantitatively determined from small-angle X-ray (SAXS) experiments, for model PUs having the same hard segments [4,4 0 -methylenediphenyl diisocyanate (MDI) chain extended with 1,4-butanediol (BDO)] and hard segment contents, and 1000 Da soft segments composed of poly(tetramethylenoxide) [PTMO], poly(1,6- hexyl 1,2-ethyl carbonate) [PHEC] or soft segments composed predominately of PDMS [7–9]. At room temperature, the non-polar PDMS segments are completely segregated from the other components in the copolymers, while less than w40% of the hard and soft segments are demixed in the PTMO soft segment PUs. * Corresponding author. E-mail address: runt@matse.psu.edu (J. Runt). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2009.10.067 Polymer 50 (2009) 6305–6311