Fluorine-Enriched Melt-Blown Fibers from Polymer Blends of Poly(butylene terephthalate) and a Fluorinated Multiblock Copolyester

Melt-blown fibers (<i>d</i>_av ∼1 μm) were produced from blends of poly­(butylene terephthalate) (PBT) and a partially fluorinated random multiblock copolyester (PFCE) leading to enhanced hydrophobicity and even superhydrophobicity (static water contact angle = 157 ± 3°) of the associated fiber mats. XPS measurements demonstrated quantitatively that the surface fluorine content increased systematically with the bulk loading of PFCE, rising to nearly 20 atom %, which corresponds to 41 wt % PFCE at a bulk loading of 10 wt %. The PBT/PFCE fibers exhibit greater fluorine surface segregation than either melt-blown PBT/poly­(ethylene-<i>co</i>-chlorotrifluoroethylene) (PBT/PECTFE) fibers or electrospun fibers obtained from blends of poly­(styrene) and fluoroalkyl end-capped polystyrene (PS/PSCF). Dynamic contact angle measurements further demonstrated decreased surface adhesion energy of the melt-blown PBT/PFCE fiber mats due to the blooming of PFCE to the surface

Synthesis and Rheology of Branched Multiblock Polymers Based on Polylactide

To improve the toughness and processability of poly­(lactic acid) (PLA), a branched multiblock polymer was prepared from d,l-lactide and ε-decalactone. A hydroxy telechelic four-arm star poly­(ε-decalactone)–poly­(d,l-lactide) diblock was synthesized using sequential ring-opening transesterfication polymerization (ROTEP) and coupled using a substoichiometric amount of sebacoyl chloride to obtain a segmented multiblock with a comb-like architecture. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed that this branched multiblock was microphase separated but lacked long-range order. Unlike a linear multiblock of similar mass, the branched material demonstrated significant extensional hardening in the disordered state, suggesting much improved processability in polymer processing methods that require fast elongational flows. Additionally, the branched multiblock material exhibited remarkable tensile toughness. This simple synthetic approach allows for simultaneous control of mechanical and rheological properties using a single macromolecular architecture to address key practical issues with PLA

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Controlling the Morphology of Immiscible Cocontinuous Polymer Blends via Silica Nanoparticles Jammed at the Interface

Cocontinuous polymer blends have wide applications. They can form conductive plastics with improved mechanical properties. When one phase is extracted, they yield porous polymer sheets, which can be used as filters or membrane supports. However, the cocontinuous morphology is intrinsically unstable due to coarsening during static annealing. In this study, silica nanoparticles, ∼100 nm diameter, with different wetting properties were melt compounded in polyethylene/poly­(ethylene oxide) blends. Calculated wetting coefficients of these particles match well with their phase contact angles and their locations in the blends. We demonstrated that a monolayer of particles jamming at interfaces can effectively suppress coarsening and stabilize the cocontinuous morphology. We also correlated the wettability of individual particles at interface to their coarsening suppression ability and found that the most hydrophobic silica nanoparticle is the most effective to arrest coarsening. Moreover, during annealing, we used the rheological dynamic time sweep, a facial but sensitive method, to relate the morphology change with particle dispersion on the interface. We further corroborated these measurements by scanning electron microscopy and confocal microscopy imaging

Dynamics of Capillary-Driven Flow in 3D Printed Open Microchannels

Microchannels have applications in microfluidic devices, patterns for micromolding, and even flexible electronic devices. Three-dimensional (3D) printing presents a promising alternative manufacturing route for these microchannels due to the technology’s relative speed and the design freedom it affords its users. However, the roughness of 3D printed surfaces can significantly influence flow dynamics inside of a microchannel. In this work, open microchannels are fabricated using four different 3D printing techniques: fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering, and multi jet modeling. Microchannels printed with each technology are evaluated with respect to their surface roughness, morphology, and how conducive they are to spontaneous capillary filling. Based on this initial assessment, microchannels printed with FDM and SLA are chosen as models to study spontaneous, capillary-driven flow dynamics in 3D printed microchannels. Flow dynamics are investigated over short (∼10^–3 s), intermediate (∼1 s), and long (∼10² s) time scales. Surface roughness causes a start–stop motion down the channel due to contact line pinning, while the cross-sectional shape imparted onto the channels during the printing process is shown to reduce the expected filling velocity. A significant delay in the onset of Lucas-Washburn dynamics (a long-time equilibrium state where meniscus position advances proportionally to the square root of time) is also observed. Flow dynamics are assessed as a function of printing technology, print orientation, channel dimensions, and liquid properties. This study provides the first in-depth investigation of the effect of 3D printing on microchannel flow dynamics as well as a set of rules on how to account for these effects in practice. The extension of these effects to closed microchannels and microchannels fabricated with other 3D printing technologies is also discussed

Effects of Inorganic Fillers on Toughening of Vinyl Ester Resins by Modified Graphene Oxide

Graphene-based nanomaterials show great potential as tougheners for thermosetting resins because they offer effective toughening of resins at an extremely low loading level with minimal change in <i>T</i>_g or modulus. However, commercial resin formulations sometimes contain inorganic fillers that could affect the toughening behavior of graphene-based additives. In this study, the effects of combining modified graphene oxide (mGO) with several widely used filler materials (fumed silica, kaolin clay, and calcium carbonate) on toughening of vinyl ester resins was evaluated. The relative size of additional filler particles compared to mGO particles strongly influences the toughening effects. mGO can toughen the resin if the other filler particles are much smaller and well dispersed. In contrast, the addition of mGO to filler-containing vinyl ester resin may decrease the fracture toughness if it reduces the size of the filler inclusion. Therefore, it is important to know the behavior of a filler in a resin before applying graphene-based toughener to it

Porous Films via PE/PEO Cocontinuous Blends

The design of a porous membrane support layer derived from cocontinuous polymer blends is presented. We investigate the effect of blend composition, shear rate, residence time, and annealing time on the cocontinuous morphology of polyethylene (PE)/poly­(ethylene oxide) (PEO) blends. Porous PE sheets were generated by water extraction of PEO and used as a support layer for gas separation membranes. The PE/PEO blends using nonfunctional and maleic anhydride functional PE (PE-<i>g</i>-MA) were mixed in a batch microcompounder and in a pilot plant scale corotating twin-screw extruder. Using PE-<i>g</i>-MA resulted in pore size reduction from 10 to 2 μm and suppression of coarsening of the morphology during further annealing of the blends due to formation of PE–PEO graft copolymers. Equilibrium interfacial tension, estimated by fitting the rheology of droplet blends to the Palierne viscoelastic droplet model, was 3 and 0.4 mN/m for PE/PEO and PE-<i>g</i>-MA/PEO systems, respectively. The specific interfacial area and phase size distribution were calculated from 3D images acquired by laser scanning electron microscopy (LSCM). We prepared gas separation membranes by solvent casting an acetone solution of ionic gel into porous PE sheets and discussed the effect of type of processing, average pore size, pore size distribution, and pore wall functionality on their performance

Capillary Coatings: Flow and Drying Dynamics in Open Microchannels

Capillary flow and drying of polymer solutions in open microchannels are explored over time scales spanning seven orders of magnitude: from capillary filling (10^–3–10 s) to the formation of a dry thin film (a “capillary coating”; 10²–10³ s). During capillary filling, drying-induced changes (increased solids content and viscosity) generate microscale pinning events that impede contact line motion. Three unique types of pinning are identified and characterized, each defined by the specific location(s) along the contact line at which pinning is induced. Drying is shown to ultimately pin the contact line permanently, and the associated total flow distances and times are revealed to be strong functions of channel width and drying rate. In general, lower drying rates coupled with intermediate channel widths are found to be most conducive to longer flow distances and times. After the advancing contact line permanently pins, internal flows driven by uneven evaporation rates continue to drive polymer to the contact line. This phenomenon promotes a local accumulation of solids and persists until all motion is arrested by drying. The effects of channel width and drying rate are investigated at each stage of this capillary coating process. These results are then applied to case studies of two functional inks commonly used in printed electronics fabrication: a PEDOT:PSS (poly­(3,4-ethylenedioxythiophene)-poly­(styrenesulfonate)) ink and a graphene ink. Although drying is shown to permanently arrest flow in both inks, both systems exhibit an increased resistance to pinning unexplained by mechanisms identified in aqueous polymer systems. Instead, arguments based on chemistry, particle size, and rheology are used to explain their novel behavior. These case studies provide insight into how functional inks can be better designed to optimize flow distances and maximize overall dry film uniformity in capillary coatings

Capillary Coatings: Flow and Drying Dynamics in Open Microchannels

Capillary flow and drying of polymer solutions in open microchannels are explored over time scales spanning seven orders of magnitude: from capillary filling (10^–3–10 s) to the formation of a dry thin film (a “capillary coating”; 10²–10³ s). During capillary filling, drying-induced changes (increased solids content and viscosity) generate microscale pinning events that impede contact line motion. Three unique types of pinning are identified and characterized, each defined by the specific location(s) along the contact line at which pinning is induced. Drying is shown to ultimately pin the contact line permanently, and the associated total flow distances and times are revealed to be strong functions of channel width and drying rate. In general, lower drying rates coupled with intermediate channel widths are found to be most conducive to longer flow distances and times. After the advancing contact line permanently pins, internal flows driven by uneven evaporation rates continue to drive polymer to the contact line. This phenomenon promotes a local accumulation of solids and persists until all motion is arrested by drying. The effects of channel width and drying rate are investigated at each stage of this capillary coating process. These results are then applied to case studies of two functional inks commonly used in printed electronics fabrication: a PEDOT:PSS (poly­(3,4-ethylenedioxythiophene)-poly­(styrenesulfonate)) ink and a graphene ink. Although drying is shown to permanently arrest flow in both inks, both systems exhibit an increased resistance to pinning unexplained by mechanisms identified in aqueous polymer systems. Instead, arguments based on chemistry, particle size, and rheology are used to explain their novel behavior. These case studies provide insight into how functional inks can be better designed to optimize flow distances and maximize overall dry film uniformity in capillary coatings

Capillary Coatings: Flow and Drying Dynamics in Open Microchannels

Capillary flow and drying of polymer solutions in open microchannels are explored over time scales spanning seven orders of magnitude: from capillary filling (10^–3–10 s) to the formation of a dry thin film (a “capillary coating”; 10²–10³ s). During capillary filling, drying-induced changes (increased solids content and viscosity) generate microscale pinning events that impede contact line motion. Three unique types of pinning are identified and characterized, each defined by the specific location(s) along the contact line at which pinning is induced. Drying is shown to ultimately pin the contact line permanently, and the associated total flow distances and times are revealed to be strong functions of channel width and drying rate. In general, lower drying rates coupled with intermediate channel widths are found to be most conducive to longer flow distances and times. After the advancing contact line permanently pins, internal flows driven by uneven evaporation rates continue to drive polymer to the contact line. This phenomenon promotes a local accumulation of solids and persists until all motion is arrested by drying. The effects of channel width and drying rate are investigated at each stage of this capillary coating process. These results are then applied to case studies of two functional inks commonly used in printed electronics fabrication: a PEDOT:PSS (poly­(3,4-ethylenedioxythiophene)-poly­(styrenesulfonate)) ink and a graphene ink. Although drying is shown to permanently arrest flow in both inks, both systems exhibit an increased resistance to pinning unexplained by mechanisms identified in aqueous polymer systems. Instead, arguments based on chemistry, particle size, and rheology are used to explain their novel behavior. These case studies provide insight into how functional inks can be better designed to optimize flow distances and maximize overall dry film uniformity in capillary coatings