Segregation Processes in Segmented Polyurethaneurea Solutions Observed by Fluorescence Depolarization

Abstract

ABSTRACT:The onset of a segregation process in segmented polyurethaneurea (SPUU) solutions was studied by fluorescence spectroscopy using free fluorescein as a probe. For SPUUs with a given molecular weight and a constant ratio of the hard to soft segments, the time-resolved fluorescence measurements reveal the environmental inhomogeneity on the excited lifetime of fluorescein. It is shown in this work that the critical concentration for the segregation process detected by fluorescence spectroscopy through the transition of the anisotropy ratio of the fluorescein occurs at a concentration lower than that observed by small angle X-ray scattering (SAXS). These experimental results indicate that fluorescence depolarization can be used as an alternative method to investigate the structural transition in SPUU solutions.KEY WORDS Polyurethaneurea I Lifetime I Depolarization I Microdomain ISegmented polyurethaneurea (SPUU), are thermoplastic block copolymers which have mechanical properties similar to those of conventional elastomers. They belong to a class of (A-B)y type multi-block copolymers composed of two chemically dissimilar chain segments called hard and soft segments. 1 The hard segments act as multifunctional junctions, playing a role of both physical crosslinks and reinforcing fillers. These segments are formed by reacting an aromatic diisocyanate with a low molecular weight diol and/or diamine. On the other hand, the soft segments impart the elastomeric properties which are usually the aliphatic polyethers or polyester macroglycols. At low temperature, the low melting soft segments are incompatible with the hard segments of high melting point, leading to a microphase separation.Similar to multiphase polymeric materials, it is expected that the physical properties of SPUUs critically depend on the phase separation between the hard and soft segment components. In the past decade, a large number of studies were carried out to establish the correlation between the morphology and the mechanical as well as the thermal properties of polyurethanes. 2 • 3 Therefore, understanding of microphase separation processes is important for the control of the physical properties of these materials. Microphase separation of SPUUs has been studied by a number of experimental techniques including small angle X-ray scattering (SAXS). 4 However, SAXS requires a certain threshold for the contrast in electron density difference to be observable and therefore, it would be interesting to observe these segregation processes by some other alternative methods which are sensitive to the structural transition in solutions of SPUU. Fluorescence depolarization is a promising candidate because transitions in the local structures can be detected through the changes in transport coefficients such as rotational relaxation time. This provides an alternative method to observe the microphase separation from the dynamic point of view. Furthermore, the rotational relaxation time is directly t To whom correspondence should be addressed. related to th...

Effect of Soft Segment Length and Chain Extender Structure on Phase Separation and Morphology in Poly(urethane urea)s

“…Phase separation has been studied using a number of techniques, most notably small‐angle X‐ray scattering (SAXS)1,7,9–20 and differential scanning calorimetry (DSC) 16,20–27…”

Structure and properties of segmented poly(urethaneurea)s with relatively short hard-segment chains

“…Moreover, it has been reported that the SPUU samples microphaseseparate at polymer concentrations as low as 15 wt %. 21 In the past, fundamental researches on structures in SPUs and SPUUs have been directed to reveal crystalline structures of hard segments and higher order structures of hard-segment domains, such as spherulites, as well as microdomain structures. [1][2][3][4][5][6][7][8][9][10][11][13][14][15][16][17][18][19][20] The hard segments of MDI-butanediol (BDO) were reported to give some diffraction arcs in a wide-angle X-ray diffraction pattern.…”