As Macromolecules celebrates its 50 th anniversary, we reflect on the impact of polymer chemistry and engineering on the advancement of synthetic polymer fibers. In this Perspective, we focus on two exemplary cases: i) high performance fibers and ii) ultrafine electrospun fibers. High performance in this context refers to fibers like Kevlar ® and Spectra ® , which emerged as a consequence of novel chemistry and processing innovations to convert synthetic polymers into fibers with exceptional specific stiffness and strength. More recently, the development of "ultrafine" (i.e. submicron diameter) fibers by technologies such as electrospinning has advanced dramatically, resulting in interesting, emergent structures and properties, such as surface and internal morphologies, electrical and mechanical properties, and growth applications like tissue engineering and sensors, which are subjects of current research. In both cases, challenges and

Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene-b-isoprene), PS-b-PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post-fabrication annealing of the fibers to obtain equilibrium self-assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self-assembled structures. Various confined assemblies such as co-existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self-assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self-assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block-copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS-b-PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS-b-PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self-assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high-end applications such as drug delivery.

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Our recent experimental study on electrospinning of block copolymer (BCP)-nanoparticle (NP) nanocomposites has revealed the formation of unique self-assembling structures in submicron scale fibers. In this paper, we use coarse-grained molecular dynamics (MD) simulations to investigate the effect of cylindrical confinement on self-assembly of model asymmetrical BCPs with and without NPs with the aim to understand and control our experimentally found structures. First, the effects of the ratio of the cylindrical confinement diameter to the BCP domain spacing, D/L 0 , the total polymer chain length, and the polymer-wall interactions on the confined assembly were thoroughly investigated. We examined the core assembled structures along the cylinder axis and constructed a phase diagram for asymmetrical BCP. The structures are categorized by three features: the number of layers of domains, radially interconnected domains, and the number of axially perforated domains. Secondly, NPs with selective attraction towards the (i) minor domain (A) and (ii) major domain (B) were incorporated into asymmetric BCPs. We found that swelling of either domain caused by the inclusion of selective NPs yields different morphologies when compared with a pure BCP with the same effective volume ratio. Interestingly, the effect of confinement on nanoparticle placement was prominently seen if nanoparticles were selectively placed into the minor domain that preferentially wets the confining wall. Finally, the predicted BCP-NP structures are validated by those observed in electrospun BCP-NP nanofibers. The current study demonstrates that coarse-grained MD simulation can offer a useful tool to elucidate, predict and tailor self-assembled structures in electrospun BCP-NP nanofibers.

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