Microstructure evolution during tensile loading histories of a polyurea

Abstract

The evolution in the hard/soft domain microstructure of an elastomeric‐like polyurea during different tensile loading histories was studied using in situ small‐ and wide‐angle X‐ray scattering (SAXS/WAXS). The nonlinear stress–strain behavior is initially stiff with a rollover yield to a more compliant response; unloading is highly nonlinear showing substantial hysteresis while also exhibiting significant recovery. Reloading reveals a substantially more compliant “softened” behavior and dramatically reduced hysteresis. WAXS peaks monitor characteristic dimensions of regular features within the hard domains; the peak location remains unchanged with tensile deformation indicating no separation of the internal structure within a domain, but the peak intensity becomes anisotropic with deformation evolving in a reversible manner consistent with orientation due to stretch. The SAXS profiles provide information between major hard domains. SAXS peaks are found to shift with tensile loading in a relatively affine manner up to a tensile true strain of ∼0.4, which, using a Bragg reduction to aid interpretation, reveals an axial increase and a transverse decrease in interdomain spacings; this evolution is reversible for strains less than ∼0.4. Increasing axial strain beyond a true strain of ∼0.4 is accompanied by a dramatic, progressive, and irreversible reduction in axial Bragg spacing, indicating a breakdown in the hard domain aggregate network structure. A four‐point pattern is seen to develop during stretching. The breakdown in networked structure during a first load cycle gives a new structure for subsequent load cycles, which is seen to evolve in a reversible manner for strains less than or equal to the prior maximum strain. However, for strains exceeding the prior maximum strain excursion, additional breakdown is found. These SAXS results show that a breakdown in the hard domain aggregate network structure is a governing mechanism for the large dissipation (hysteresis) loops of the first load cycle and are also responsible for the softened reloading response. The absence of structure breakdown during subsequent load cycles corresponds to the substantially reduced hysteresis loops as well as the stable softened behavior. DMA data on pristine and previously deformed samples show a more compliant storage modulus in the predeformed sample, supporting the softened cyclic stress–strain data and the structural breakdown observed in the SAXS; the loss modulus was unchanged with deformation, which correlates with the lossy features measured in DMA with time‐dependent viscosity rather than losses due to structural breakdown. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Microstructure and Segmental Dynamics of Polyurea under Uniaxial Deformation

“…At 100% strain, a four-point intensity pattern along with two weaker reflections are observed, the scattering behavior arising due to shearing at the microdomain level, induced by the local torque exerted by the strained soft segments. 21−25 A similar four-point pattern was observed in deformed polyurea by Rinaldi et al 18 The four and two-point patterns are seen clearly in Figure 6, which displays the angular intensity dependence vs azimuthal angle. At 100% strain, we observe a four point pattern with maxima at 55°, 125°, 235°, and 305°, and less intense reflections at 90°and 180°.…”

“…The phaseseparated microstructure of polyurea is crucial in determining molecular dynamics and mechanical properties, and its evolution during mechanical loading and unloading was recently studied by Rinaldi et al 18 up to moderate strain. The purpose of the present work was to determine how the structure of PU, its orientation, and the interaction of the phase domains, are affected by mechanical deformation.…”

Experimentally-based relaxation modulus of polyurea and its composites

“…The hard domains of polyurea are nanometer size [Castagna et al, 2012], and the T g of the hard domain is significantly higher than the soft domain [Rinaldi et al, 2011], we expect most of the relaxation happens in the soft domain for most of the frequencies, while for the extremely high frequency above 10 9 Hz, some energy may go into the local resonance due to the existence of the hard domains. For most applications, 10 9 Hz is high enough.…”

“…In this study, polyurea is synthesized using Versalink P-1000 (Air Products and Chemicals, Inc.) and Isonate 143L (Dow Chemicals Co.), and the resultant material is a segmented copolymer which has hard domains dispersed in a soft matrix [Castagna et al, 2012, Rinaldi et al, 2011. The T g of the soft domain is around −60 • C, which also is regarded as the T g of polyurea, and the T g of the hard domain is above 100 • C [Rinaldi et al, 2011].…”

“…The T g of the soft domain is around −60 • C, which also is regarded as the T g of polyurea, and the T g of the hard domain is above 100 • C [Rinaldi et al, 2011]. Due to the difference of the T g of the soft and hard segments, polyurea's behavior in the majority of its applications' conditions (temperature and frequencies of interest) may be likened to a lightly cross-linked elastomer reinforced by the nano-size hard domains [Castagna et al, 2012], which also serve as chemical and physical cross-link sites.…”