Alex M. Jordan

Surface Modification of Melt Extruded Poly(ε-caprolactone) Nanofibers: Toward a New Scalable Biomaterial Scaffold

A photochemical modification of melt-extruded polymeric nanofibers is described. A bioorthogonal functional group is used to decorate fibers made exclusively from commodity polymers, covalently attach fluorophores and peptides, and direct cell growth. Our process begins by using a layered coextrusion method, where poly(ε-caprolactone) (PCL) nanofibers are incorporated within a macroscopic poly(ethylene oxide) (PEO) tape through a series of die multipliers within the extrusion line. The PEO layer is then removed with a water wash to yield rectangular PCL nanofibers with controlled cross-sectional dimensions. The fibers can be subsequently modified using photochemistry to yield a “clickable” handle for performing the copper-catalyzed azide–alkyne cycloaddition (CuAAC) reaction on their surface. We have attached fluorophores, which exhibit dense surface coverage when using ligand-accelerated CuAAC reaction conditions. In addition, an RGD peptide motif was coupled to the surface of the fibers. Subsequent cell-based studies have shown that the RGD peptide is biologically accessible at the surface, leading to increased cellular adhesion and spreading versus PCL control surfaces. This functionalized coextruded fiber has the advantages of modularity and scalability, opening a potentially new avenue for biomaterials fabrication.

Semibatch monomer addition as a general method to tune and enhance the mechanics of polymer networks via loop-defect control

Significance We demonstrate that slow monomer addition during step-growth polymer network formation changes the fraction of loop defects within the network, thus providing materials with tunable and significantly improved mechanical properties. This phenomenon is general to a range of network-forming reactions and offers a powerful method for tuning the mechanics of materials without changing their composition. Controlling the molecular structure of amorphous cross-linked polymeric materials is a longstanding challenge. Herein, we disclose a general strategy for precise tuning of loop defects in covalent polymer gel networks. This “loop control” is achieved through a simple semibatch monomer addition protocol that can be applied to a broad range of network-forming reactions. By controlling loop defects, we demonstrate that with the same set of material precursors it is possible to tune and in several cases substantially improve network connectivity and mechanical properties (e.g., ∼600% increase in shear storage modulus). We believe that the concept of loop control via continuous reagent addition could find broad application in the synthesis of academically and industrially important cross-linked polymeric materials, such as resins and gels.

Structural evolution during mechanical deformation in high-barrier PVDF-TFE/PET multilayer films using in situ X-ray techniques.

Poly(vinylidene fluoride-co-tetrafluoroethylene) (PVDF-TFE) is confined between alternating layers of poly(ethylene terephthalate) (PET) utilizing a unique multilayer processing technology, in which PVDF-TFE and PET are melt-processed in a continuous fashion. Postprocessing techniques including biaxial orientation and melt recrystallization were used to tune the crystal orientation of the PVDF-TFE layers, as well as achieve crystallinity in the PET layers through strain-induced crystallization and thermal annealing during the melt recrystallization step. A volume additive model was used to extract the effect of crystal orientation within the PVDF-TFE layers and revealed a significant enhancement in the modulus from 730 MPa in the as-extruded state (isotropic) to 840 MPa in the biaxially oriented state (on-edge) to 2230 MPa in the melt-recrystallized state (in-plane). Subsequently, in situ wide-angle X-ray scattering was used to observe the crystal structure evolution during uniaxial deformation in both the as-extruded and melt-recrystallized states. It is observed that the low-temperature ferroelectric PVDF-TFE crystal phase in the as-extruded state exhibits equatorial sharpening of the 110 and 200 crystal peaks during deformation, quantified using the Hermans orientation function, while in the melt-recrystallized state, an overall increase in the crystallinity occurs during deformation. Thus, we correlated the mechanical response (strain hardening) of the films to these respective evolved crystal structures and highlighted the ability to tailor mechanical response. With a better understanding of the structural evolution during deformation, it is possible to more fully characterize the structural response to handling during use of the high-barrier PVDF-TFE/PET multilayer films as commercial dielectrics and packaging materials.

Thin film confinement of a spherical block copolymer via forced assembly co-extrusion

A spherical, block copolymer (BCP) with a statistical mid-block was investigated under confinement via forced assembly co-extrusion. This approach provided a continuous methodology to investigate the effect of layer thickness and substrate on the morphology of a unique self-assembling material. It was demonstrated that substrate interactions and layer thickness can promote long-range ordering for use in highly diverse nanotechnology applications.

Drawing in poly(ε-caprolactone) fibers: tuning mechanics, fiber dimensions and surface-modification density

Uniaxial drawing of melt-coextruded poly(ε-caprolactone) (PCL) microfibers was investigated to understand impact on topological, mechanical, and chemical properties of the fibrous scaffolds. Fibers were uniaxially elongated up to 7-fold to observe polymer chain orientation and crystal structure. Crystallinity and orientation of crystal domains were investigated by DSC and X-ray scattering. Polymer physical properties were directly correlated to bulk fiber properties. Furthermore, the drawn fibers were modified photochemically with functionalized benzophenones. The results of these studies allowed for comparison between fiber dimension/surface area, mechanical properties, and photochemical reaction yield for surface modification. As drawing increased, the modulus and tensile strength of the fibers increased as did the surface area of the scaffolds. By contrast, increased drawing led to a decrease in the ability to undergo photochemical reaction at the polymer surface. This fundamental investigation provides a predictive framework to understand how post-processing impacts three critical parameters for coextruded fibrous biomaterial scaffolds.