Toward a Tunable Fibrous Scaffold: Structural Development during Uniaxial Drawing of Coextruded Poly(ε-caprolactone) Fibers

A continuous fibrous composite tape of poly­(ethylene oxide) (PEO) and poly­(ε-caprolactone) (PCL) was produced using novel multilayer coextrusion fiber manufacturing. A three-step washing process was utilized to remove the PEO matrix, resulting in a PCL fiber mat (>99 wt %). Synchrotron X-ray radiation was utilized to determine the optimized postprocessing uniaxial drawing conditions to achieve efficient crystalline orientation. An examination of small-/wide-angle X-ray scattering (SAXS/WAXS) revealed two regimes in the uniaxial drawing process; at DR < 5, crystalline orientation kinetics were dominant, while at DR > 5, amorphous chain alignment kinetics were dominant. Uniaxial drawing was shown to be an effective tool for tuning individual fiber size from 2.6 ± 0.6 μm by 1.6 ± 0.4 μm in the as-extruded state to 0.31 ± 0.05 μm by 0.13 ± 0.02 μm in the oriented state, while increasing specific surface area 3.5-fold. The elastic modulus and tensile strength of the PCL fiber mat were also increased by a factor of 30 and ∼10, respectively, through uniaxial drawing. Compared to electrospun PCL fiber systems produced with individual fiber dimensions similar to those of the as-extruded and oriented PCL fiber mats, the melt-processed PCL fibers exhibit a 6-fold increase in specific surface area over the corresponding circular, electrospun PCL fibers while maintaining similar thermomechanical properties. The elastic modulus of the oriented, coextruded PCL fiber mat was increased by a factor of 50 compared to the corresponding electrospun PCL fiber mat, while exhibiting a 2.5-fold increase in specific surface area. The ability to melt-process and utilize uniaxial drawing to produce PCL fibers in high volume with a consistent, tunable range of properties that are similar or enhanced when compared to traditional electrospun fibers provides a unique advantage in the field of tissue engineering, surface modification, and drug delivery

Exploring the Role of Supramolecular Associations in Mechanical Toughening of Interpenetrating Polymer Networks

Model “supramolecular IPNs” were developed via the formation of a hydrogen-bonded, supramolecular network of 2-ureido-4-[1<i>H</i>]-pyrimidinone (UPy) telechelic poly­(ethylene-<i>co</i>-1-butene) (SPEB) in the presence of photopolymerizable, hydroxyl-terminated polybutadiene (HTPB). The role of a supramolecular elastomeric phase in mechanical toughening of IPNs was explored through (1) dynamic dissociation and reassociation of the noncovalent, UPy supramolecular associations, and (2) interphase formation. While an ∼4× increase in tensile toughness of the HTPB matrix was observed through incorporation of 10 wt % ethylene–propylene rubber (EPR)as a conventional elastomeric toughening agentinto HTPB, it was shown that adding the same amount of supramolecular elastomer SPEB to HTPB led to ∼600× enhancement in tensile toughness. Strain rate-dependent mechanical response and fractography studies revealed that this dramatic toughness enhancement was due to dissociation/reassociation of the dynamic UPy linkages in the elastomeric phase that facilitated dilatational yielding of the IPN. This toughness enhancement was only observed in combination with the existence of strong interfacial coupling between the matrix and supramolecular phase as revealed by transmission electron microscopy and dynamic mechanical analysis. By exploiting noncovalent dynamics and interfacial control in interpenetrating networks, pathways are envisioned toward a new class of tough materials

Molecular Design: Network Architecture and Its Impact on the Organization and Mechanics of Peptide-Polyurea Hybrids

Nature has achieved controlled and tunable mechanics via hierarchical organization driven by physical and covalent interactions. Polymer–peptide hybrids have been designed to mimic natural materials utilizing these architectural strategies, obtaining diverse mechanical properties, stimuli responsiveness, and bioactivity. Here, utilizing a molecular design pathway, peptide–polyurea hybrid networks were synthesized to investigate the role of architecture and structural interplay on peptide hydrogen bonding, assembly, and mechanics. Networks formed from poly­(β-benzyl-l-aspartate)–poly­(dimethylsiloxane) copolymers covalently cross-linked with a triisocyanate yielded polyurea films with a globular-like morphology and parallel β-sheet secondary structures. The geometrical constraints imposed by the network led to an increase in peptide loading and ∼7x increase in Young’s modulus while maintaining extensibility (∼160%). Thus, the interplay of physical and chemical bonds allowed for the modulation of resulting mechanical properties. This investigation provides a framework for the utilization of structural interplay and mechanical tuning in polymer–peptide hybrids, which offers a pathway for the design of future hybrid biomaterial systems

Reducing Environmental Impact: Solvent and PEO Reclamation During Production of Melt-Extruded PCL Nanofibers

An improved subtractive manufacturing process for fabrication of rectangular, high-surface-area poly­(ε-caprolactone) (PCL) fibers is presented. PCL fibers were derived from continuous coextruded tapes of poly­(ethylene oxide) (PEO)/PCL with 75% reduction in washing time, while still achieving >99 wt % PCL purity with a quantitative yield of PCL fibers. The fabricated PCL fiber mat had a measured surface area of 3.27 ± 0.53 m²/g. A two-stage distillation process was used to recover methanol and water used in composite solvation to remove PEO. Both methanol and water were recovered at ∼100% purity with a fractional recovery of 87 ± 2% and 95 ± 2%, respectively. Solvated PEO was also recovered at a fractional recovery of 94 ± 4% at ∼100% purity. Gel permeation chromatography and thermal analysis revealed no chain scission, thermal degradation, or cross-linking within the recovered PEO, which suggested the possibility of reincorporating recovered PEO to the multilayer coextrusion process for future composite coextrusion. These waste reduction figures represent recovery on the laboratory-scale process with substantial room for improvement in a fully automated, large-scale industrial process. By reducing overall waste generation >90%, fibers derived from multilayer coextrusion may become an industrially viable alternative for nanofiber manufacturing

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)-<i>block</i>-poly­(dimethylsiloxane)-<i>block</i>-poly­(benzyl-l-glutamate) (PBLG-<i>b</i>-PDMS-<i>b</i>-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

Toward Anisotropic Materials via Forced Assembly Coextrusion

Multilayer coextrusion offers a diverse platform to examine layer dependent confinement effects on self-assembling nanomaterials via conventional extrusion technology. A triblock copolymer (BCP) with a cylindrical microstructure was processed via “forced assembly” to elucidate the effect of microdomain orientation on the mechanical behavior of multilayer films. The mechanical response was investigated in both the extrusion (ED) and transverse directions (TD) of the multilayer systems, revealing an influence of both cylinder-orientation and the interface on the mechanical response with decreasing layer thickness. The stress–strain curves for samples with the stress field along the cylinder axis revealed a sharp yielding phenomenon, while curves for specimens with the stress field applied perpendicular to the axis exhibited weak yielding behavior. The extensibility of the multilayer films stressed in the ED increases with decreasing layer thickness, but remains constant when deformed along the TD. Coextrusion technology allows for tunable mechanical toughness in industrial grade polymers via a continuous process. By altering the layer thickness of the two polymeric materials, we can tune the mechanics from strong, brittle behavior to a tough, ductile response by manipulation of the hierarchical structure. The enabling technology provides a unique platform to couple the inherent mechanical response of dissimilar polymers and allows for the design of composite materials with tailored mechanics

Tunable Mechanics in Electrospun Composites via Hierarchical Organization

Design strategies from nature provide vital clues for the development of synthetic materials with tunable mechanical properties. Employing the concept of hierarchy and controlled percolation, a new class of polymer nanocomposites containing a montmorillonite (MMT)-reinforced electrospun poly­(vinyl alcohol) (PVA) filler embedded within a polymeric matrix of either poly­(vinyl acetate) (PVAc) or ethylene oxide–epichlorohydrin copolymer (EO–EPI) were developed to achieve a tunable mechanical response upon exposure to specific stimuli. Mechanical response and switching times upon hydration were shown to be dependent on the weight-fraction of MMT in the PVA electrospun fibers and type of composite matrix. PVA/MMT.PVAc composite films retained excellent two-way switchability for all MMT fractions; however, the switching time upon hydration was decreased dramatically as the MMT content was increased due to the highly hydrophilic nature of MMT. Additionally, for the first time, significant two-way switchability of PVA/MMT.EO-EPI composites was achieved for higher weight fractions (12 wt %) of MMT. An extensive investigation into the effects of fiber diameter, crystallinity, and MMT content revealed that inherent rigidity of MMT platelets plays an important role in controlling the mechanical response of these hierarchical electrospun composites

Probing the Interplay of Ultraviolet Cross-Linking and Noncovalent Interactions in Supramolecular Elastomers

Ultraviolet (UV) irradiated supramolecular polybutadienes (PBs) containing 2-ureido-4-[1<i>H</i>]-pyrimidone (UPy) linkages were examined as a simple model for curable supramolecular elastomers. Via precise control of UV exposure, the cure and the degradation of the vinyl groups within the PB elastomeric core were investigated. The combination of UPy binding and covalent cross-linking by UV irradiation dramatically enhanced mechanical properties of these UPy-functionalized elastomers, yielding toughness enhancement up to ∼200× at the 5 min UV cure. UV-initiated cross-linking dominated the curing process up to ∼50 min exposure time. Beyond this cure time, dominant degradation of the vinyl linkages was observed. Control of this UV-initiated process yielded supramolecular elastomers with a covalently cross-linked phase induced by UV irradiation combined with a noncovalent UPy cross-linked phase induced by secondary hydrogen bonding interactions. Of particular note, it was determined that the presence of UPy hydrogen-bonded aggregates accelerated the UV cross-linking process during the initial stage of exposure. This observation was attributed to microphase-separated structure of UV-irradiated supramolecular elastomer, where UPy aggregation increased the probability of interaction between the pendant vinyls responsible for UV cross-linking. The systematic study of uniaxial tensile behavior of the UV-irradiated supramolecular elastomers offers new insight into the design and architecture of mechanically tunable supramolecular elastomers

Stimuli-Responsive and Mechanically-Switchable Electrospun Composites

We report on a family of electrospun nanocomposites, which are capable of altering their stiffness upon hydration. An electrospun mat of poly­(vinyl alcohol) (PVA) was incorporated as the filler in a polymeric matrix consisting of either poly­(vinyl acetate) (PVAc) or ethylene oxide–epicholorohydrin copolymer (EO–EPI). The tensile modulus of the EO–EPI-based composites was found to increase significantly upon incorporation of the PVA filler mat, while PVAc-based composites exhibited modulus enhancement only above the matrix glass transition. Materials based on the PVAc matrix and PVA electrospun filler exhibited a reversible reduction of the tensile modulus by a factor of 280 upon exposure to water. In contrast, composites comprised of a rubbery EO–EPI matrix and PVA filler showed a reduction of tensile modulus upon water uptake, but with incomplete restoration when dried. A systematic investigation revealed that the underlying mechanism of mechanical response is related to the matrix–filler interactions and filler crystallinity. The robust technique of electrospinning allows the tailoring of matrix–filler interactions in a new series of all-organic composites to achieve desired mechanical response upon exposure to various stimuli

Structural Evolution during Mechanical Deformation in High-Barrier PVDF-TFE/PET Multilayer Films Using in Situ X‑ray Techniques

Poly­(vinylidene fluoride-<i>co</i>-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