Structure and Properties of Fatigued Segmented Poly(urethaneurea) I. Segment Orientation Mechanism due to Fatigue

Abstract

ABSTRACT:Changes of structure and properties of segmented poly(urethaneura)s (SPUU) subjected to a sinusoidal strain were studied as a function of the duration of the load (the fatigue time). Mechanical experiments as well as infrared dichroism experiments indicate that destruction of hard segment domains and phase mixing between hard and soft segment domains proceed with increasing fatigue time. Infrared dichroism shows that the hard segments orient negatively, then positively, with increasing elongation. Negative orientation results from the behavior of the hard segment domain as a filler having an anisotropic shape ("domain orientation"). Subsequent positive orientation is explained by the hard segment orientation coupled with fragmentation of the hard segment domains ("segment orientation"). In the fatigue process, on the other hand, proceeding of phase mixing is dominant followed by phase demixing related to rearrangement and reorientation of the hard segments, which would finally lead to fracture. It is found that the fatigue process consists of three stages; the domain orientation stage, the phase-mixing stage, and the segment orientation stage.KEY WORDS Poly(urethaneurea) I Fatigue Mechanism I Infrared Dichroism I Spherulite Deformation I Segmented poly(urethaneurea) (SPUU) block copolymers are thermoplastic copolymers which have mechanical properties similar to those of conventional crosslinked rubbers. These materials belong to a class of (A-B)n type multiblock copolymers 1 -u consisting of alternately joined blocks of two chemically dissimilar segments along the polymer backbone, which are called hard and soft segments.Physical crosslinks are provided by the hard segments, which are generally formed from an extension of an aromatic diisocyanate with a low molecular weight diol or diamine. Elastomeric properties are imparted by the soft segments, usually formed from an aliphatic polyether or polyester macroglycol. Such SPUU is extensively hydrogen bonded. 3 -10 In all cases, the NH and C=O groups of each urethane or urea linkage can act as a proton donor and a proton acceptor. Although hydrogen bonding is an important structural feature of these materials, its influence on their properties and morphologies is not clear. In addition, two-phase microdomain structure 7 -10 • 12 • 13 has not been well understood due predominantly to the chemical complexity of the materials themselves. Charactensttc mechanical properties of SPUU, such as, high extensibility, and resiliency,

Segregation Processes in Segmented Polyurethaneurea Solutions Observed by Fluorescence Depolarization

“…Similar critical behavior of SAXS has been reported previously for a number of analog SPUUs in DMAc. 4 These SAXS results, indicate that the microphase separation in SPUU solutions takes place in the vicinity of 25 wt% for the SPUU used in this study. This critical concentration is slightly higher than the concentration 15 wt% observed by fluorescence depolarization of Fl for the same solutions.…”

“…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.…”

Effect of strain and strain rate on fatigue‐accelerated biodegradation of polyurethane

“…8 Although no direct correlation to the rate of degradation has yet been demonstrated, phase mixing of hard-and soft-segment domains, as has been observed during uniaxial fatigue of a polyurethane in air, [13][14][15] potentially increases the rate of degradation by increasing the diffusion rate of reactive species. If and how these effects contribute to chemical degradation depends on the specific loading conditions.…”

Structure and Properties of Fatigued Segmented Poly(urethaneurea)s II. Structural Analyses of Fatigue Mechanism

“…The details of the fatigue test are described in ref 6. The fatigued sample code is designated by attaching the fatigue time in thousand seconds after the as-cast sample code as we employed in ref 6.…”