Polyurea Segmented Multi-Block Copolymers: Morphology, Mechanics and Rheology

Jai A. Pathak, Naval Research Lab

While Polyurea (PU) segmented multi-block copolymers are widely used impact-resistant coatings, molecular underpinnings of this application are poorly understood, thus motivating this study. Due to inter-segmental repulsion, PU copolymers micro-phase separate, comprising domains of .hard. (immobile/rigid) and .soft. (mobile/flexible) segments. We studied the mechanical properties and rheology of a PU (containing 19.8 % hard segments by mass) by measuring its stress-strain behavior and by oscillatory shear rheometry, respectively. PU shows elastomeric mechanical response over a wide temperature range, as the rigid domains physically cross-link the soft domains. The secant modulus increases, while the residual strain in specimens stretched to failure decreases, with increasing strain rate. Linear viscoelasticity (LVE) of Polyurea violates time-temperature superposition, and the shear modulus is very sensitive to annealing history. To elucidate how morphology determines mechanical behavior and LVE, we investigated the morphology of undeformed PU by temperature-resolved Small-Angle X-Ray Scattering (SAXS). At room temperature PU has two scattering intensity peaks that arise from crystallinity in the hard domains (at larger angles) and micro-phase separated domains (at smaller angles). As temperature increases, PU shows micro-phase mixing and then melting of hard segment crystallites. To determine the effects of deformation on morphology, we performed SAXS on PU specimens stretched to failure. Quasi-statically failed specimens show anisotropic scattering (orientation imparted to hard domains), while samples rapidly stretched to failure at large strain rates show isotropic scattering (no molecular orientation). Deformation effects on morphology are controlled by rigid (hard) domain response over the deformation time-scale.

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