Polymeric materials undergo dramatic changes in orientational order in response to dynamic processes, such as flow. Their rich cascade of dynamics presents opportunities to create and combine distinct alignments of polymeric nanostructures through processing. In situ rheo-optical measurements complemented by ex situ x-ray scattering reveal the physics of three different trajectories to macroscopic alignment of lamellar diblock copolymers during oscillatory shearing. At the highest frequencies, symmetry arguments explain the transient development of a bimodal texture en route to the alignment of layers parallel to the planes of shear. At lower frequencies, larger-scale relaxations introduce rearrangements out of the deformation plane that permit the formation of lamellae perpendicular to the shear plane. These explain the change in the character of the pathway to parallel alignment and the emergence of perpendicular alignment as the frequency decreases.

Rheo-optical methods are used to examine the combined effect of shear frequency, strain amplitude, and temperature on the direction and kinetics of flow-induced alignment in lamellar block copolymers. The development of shear-induced alignment in a nearly symmetric polystyrene−polyisoprene diblock (ODT ≃ 164 °C) is recorded in real time using flow birefringence as a probe of the transient lamellar orientation distribution. As alignment progresses during large amplitude oscillatory shearing, the birefringence shows an initial “fast” and a later “slow” change. While increasing strain amplitude (γo) generally speeds both the fast and the slow processes, below a critical γo the slow process is not observed and a well-aligned state is not achieved. The transient birefringence observed at a particular frequency and temperature, but different strain amplitudes, can be partially superposed by scaling time with However, the “fast” and “slow” processes require different values of n(ω). Estimates of n(ω) show that effects of strain are highly nonlinear and stronger than the simple rescaling of time in terms of either cumulative strain (∼tγo) or cumulative flow energy The effect of temperature enters most strongly through the shift of time scale of molecular relaxations (aT ).

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In-situ rheo-optical methods are used to guide electron microscopy (TEM) and X-ray scattering (SAXS) studies of structure development during flow-induced alignment in a lamellar block copolymer melt (nearly symmetric polystyrene-polyisoprene diblock, ODT = 172 °C). The progress of shear-induced alignment is recorded in real-time using flow birefringence; at selected points during alignment samples are taken for ex-situ characterization by TEM and SAXS along all three axes (v, ∇v, ∇ × v) of the flow geometry. Three different trajectories are examined: perpendicular alignment and two qualitatively different routes to parallel alignment in the high-frequency regime (ω > ω′ c ). In general, the initial "fast" process not only enhances the projection of the orientation distribution that corresponds to the final state but also increases other projections of the distribution; the late-stage "slow" process eliminates these other projections and perfects a single alignment. For example, the highest frequency path to parallel alignment begins by transforming poorly organized regions into layers that are predominantly oriented along the parallel and transverse directions. The transition to the slow process is marked by the development of a characteristic texture in which tilt wall boundaries normal to the flow direction separate bands that form a repeating "chevron" pattern (layers tilted up and down about the ∇×v axis). The coarsening of this pattern dominates the slow process, during which the transverse projection is also eliminated.