LaShanda T. J. Korley

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.

Utilizing Peptidic Ordering in the Design of Hierarchical Polyurethane/Ureas

One of the key design components of nature is the utilization of hierarchical arrangements to fabricate materials with outstanding mechanical properties. Employing the concept of hierarchy, a new class of segmented polyurethane/ureas (PUUs) was synthesized containing either a peptidic, triblock soft segment, or an amorphous, nonpeptidic homoblock block soft segment with either an amorphous or a crystalline hard segment to investigate the effects of bioinspired, multiple levels of organization on thermal and mechanical properties. The peptidic soft segment was composed of poly(benzyl-l-glutamate)-block-poly(dimethylsiloxane)-block-poly(benzyl-l-glutamate) (PBLG-b-PDMS-b-PBLG), restricted to the β-sheet conformation by limiting the peptide segment length to <10 residues, whereas the amorphous soft segment was poly(dimethylsiloxane) (PDMS). The hard segment consisted of either 1,6-hexamethylene diisocyanate (crystalline) or isophorone diisocyanate (amorphous) and chain extended with 1,4-butanediol. Thermal and morphological characterization indicated microphase separation in these hierarchically assembled PUUs; furthermore, inclusion of the peptidic segment significantly increased the average long spacing between domains, whereas the peptide domain retained its β-sheet conformation regardless of the hard segment chemistry. Mechanical analysis revealed an enhanced dynamic modulus for the peptidic polymers over a broader temperature range as compared with the nonpeptidic PUUs as well as an over three-fold increase in tensile modulus. However, the elongation-at-break was dramatically reduced, which was attributed to a shift from a flexible, continuous domain morphology to a rigid, continuous matrix in which the peptide, in conjunction with the hard segment, acts as a stiff reinforcing element.

Confinement of elastomeric block copolymers via forced assembly coextrusion.

Forced assembly processing provides a unique opportunity to examine the effects of confinement on block copolymers (BCPs) via conventional melt processing techniques. The microlayering process was utilized to produce novel materials with enhanced mechanical properties through selective manipulation of layer thickness. Multilayer films consisting of an elastomeric, symmetric block copolymer confined between rigid polystyrene (PS) layers were produced with layer thicknesses ranging from 100 to 600 nm. Deformation studies of the confined BCP showed an increase in ductility as the layer thickness decreased to 190 nm due to a shift in the mode of deformation from crazing to shear yielding. Postextrusion annealing was performed on the multilayer films to investigate the impact of a highly ordered morphology on the mechanical properties. The annealed multilayer films exhibited increased toughness with decreasing layer thickness and resulted in homogeneous deformation compared to the as-extruded films. Multilayer coextrusion proved to be an advantageous method for producing continuous films with tunable mechanical response.

Nanomanufacturing of continuous composite nanofibers with confinement-induced morphologies

Continuous core-shell nanofibers with poly(isoprene-block-dimethylaminoethyl methacrylate) (PI-b-PDMAEMA) block copolymer/polymer derived ceramic (PDC) precursor nanocomposites as cores enveloped in rigid polyacrylonitrile (PAN) shells were nanomanufactured using coaxial electrospinning. The cylindrical confinement imposed by the rigid shell led to ordered morphologies in the core not observed in bulk block copolymer nanocomposites.

Deformation of Confined Poly(ethylene oxide) in Multilayer Films

The effect of confinement on the deformation behavior of poly(ethylene oxide) (PEO) was studied using melt processed coextruded poly(ethylene-co-acrylic acid) (EAA) and PEO multilayer films with varying PEO layer thicknesses from 3600 to 25 nm. The deformation mechanism was found to shift as layer thickness was decreased between 510 and 125 nm, from typical axial alignment of the crystalline fraction, as seen in bulk materials, to nonuniform micronecking mechanisms found in solution-grown single crystals. This change was evaluated via tensile testing, wide-angle X-ray diffraction (WAXD), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). With the commercially relevant method of melt coextrusion, we were able to overcome the limitations to the testing of solution-grown single crystals, and the artifacts that occur from their handling, and bridged the gap in knowledge between thick bulk materials and thin single crystals.

Mechanical enhancement via self-assembled nanostructures in polymer nanocomposites

This article reports the synthesis of a diacetylene containing organogelator for the use as a nanoscale filler for mechanical enhancement in a polymer nanocomposite. The tensile storage modulus increased when the self-assembled nanostructures were incorporated into a polymer matrix. Below the glass transition temperature, a modest increase of the tensile storage modulus was observed, but above the glass transition temperature, almost a two order of magnitude increase at the highest filler loadings was found. Additionally, the presence of the self-assembled nanofibers, increased the tensile storage modulus greater than a similar filler that does not form 1D nanofibers highlighting the importance of self-assembly to the mechanical reinforcement.

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.

Enhanced mechanical pathways through nature's building blocks: amino acids

Amino acids are the core building blocks of natures mechanically robust proteins. Their innate ability to self-assemble into well-ordered secondary structures, such as the α-helix and β-sheet, coupled with unique load-bearing characteristics, has sparked considerable interest in their use in innovative engineering materials. Biomimickry and bioinspired approaches to materials design can be utilized to facilitate the conception of these peptidic-based materials by introducing principles proven by the demanding conditions of nature. In this review, we will explore the design process of tailored mechanics through the examination of research that has employed amino acid sequences inspired by silks, elastin, and resilin to construct hybrid functional polymeric materials as well as polymeric materials exploiting non-canonical or non-native amino acids as building blocks. We foresee the next generation of nature-inspired materials finding widespread use, not only in biomedical and bioengineering applications, but also in roles that require tailored and functional coatings, films and fibers.

Influence of secondary structure and hydrogen-bonding arrangement on the mechanical properties of peptidic-polyurea hybrids

Bio-inspired materials design is an important strategy used in the fabrication of tunable and mechanically enhanced polymeric systems. An important aspect of bio-inspiration is to understand how components, such as hierarchy and self-assembly, affect the properties of the designed materials. In this investigation, we explore the use of polypeptide secondary structure and hydrogen bonding arrangement, in order to determine their effects on the thermal and mechanical properties of fully synthetic peptidic polyureas. Specifically, we incorporate either short β-sheet forming peptide blocks of poly(β-benzyl-l-aspartate)5 or poly(ε-carbobenzyloxy-l-lysine)5 or longer peptide blocks of poly(β-benzyl-l-aspartate)20 or poly(ε-carbobenzyloxy-l-lysine)20 as α-helix forming domains into non-chain extended polyureas based on 1,6-hexamethylene diisocyanate and poly(dimethysiloxane). Secondary structure was found to be influenced by the weight fraction of peptide, e.g. increasing peptide weight fractions increased sheet or helical ordering. Additionally, the polyurea microstructure was comprised of nanofibrils with a secondary structure dependent fiber width, attributed to the peptidic motif alignment within the nanothreads. Analysis of the thermomechanical and tensile response revealed multiple trends, such as increased toughness attributed to β-sheet ordering and increased modulus with increased peptide weight fraction. It is anticipated that this observed interplay between peptide organization and mechanics will be applicable to engineering and biomaterial development due to the simplicity of the synthetic protocol and the promising mechanical tunability guided by the peptide segment.

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.