3D Micromachined Polyimide Mixing Devices for in Situ X‑ray Imaging of Solution-Based Block Copolymer Phase Transitions

Advances in modern
interface- and material sciences often rely
on the understanding of a system’s structure–function
relationship. Designing reproducible experiments that yield in situ
time-resolved structural information at fast time scales is therefore
of great interest, e.g., for better understanding the early stages
of self-assembly or other phase transitions. However, it can be challenging
to accurately control experimental conditions, especially when samples
are only available in small amounts, prone to agglomeration, or if
X-ray compatibility is required. We address these challenges by presenting
a microfluidic chip for triggering dynamics via rapid diffusive mixing
for in situ time-resolved X-ray investigations. This polyimide/Kapton-only-based
device can be used to study the structural dynamics and phase transitions
of a wide range of colloidal and soft matter samples down to millisecond
time scales. The novel multiangle laser ablation three-dimensional
(3D) microstructuring approach combines, for the first time, the highly
desirable characteristics of Kapton (high X-ray stability with low
background, organic solvent compatibility) with a 3D flow-focusing
geometry that minimizes mixing dispersion and wall agglomeration.
As a model system, to demonstrate the performance of these 3D Kapton
microfluidic devices, we selected the non-solvent-induced self-assembly
of biocompatible and amphiphilic diblock copolymers. We then followed
their structural evolution in situ at millisecond time scales using
on-the-chip time-resolved small-angle X-ray scattering under continuous-flow
conditions. Combined with complementary results from 3D finite-element
method computational fluid dynamics simulations, we find that the
nonsolvent mixing is mostly complete within a few tens of milliseconds,
which triggers initial spherical micelle formation, while structural
transitions into micelle lattices and their deswelling only occur
on the hundreds of milliseconds to second time scale. These results
could have an important implication for the design and formulation
of amphiphilic polymer nanoparticles for industrial applications and
their use as drug-delivery systems in medicine.