Cross-Linked Nonwoven Fibers by Room-Temperature Cure Blowing

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Cross-Linked Nonwoven Fibers by Room-Temperature Cure Blowing and in Situ Photopolymerization Aditya Banerji, Kailong Jin, Kunwei Liu, Mahesh K. Mahanthappa, and Christopher J. Ellison* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States

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S Supporting Information *

ABSTRACT: Current synthetic nonwoven fiber production methods typically require transforming preformed polymers into a processable melt or solution state by heating or adding organic solvents, respectively, to facilitate fiber spinning. The significant energy demands and the use of volatile organic compounds render these processes suboptimal. Furthermore, conventional synthetic fiber manufacturing processes are limited to thermoplastics because cross-linked thermosets do not flow; however, the superior thermal and chemical resistance of cross-linked fibers render them attractive targets. In this study, we describe a “cure blowing” process that addresses these limitations by producing cross-linked fibers at room temperature with little or no solvent, using a lab-scale spinning die resembling those used for commercial melt blowing, an approach that currently produces >10% of global nonwovens. Specifically, a photocurable liquid mixture of thiol and acrylate monomers was extruded through an orifice and drawn by high-velocity air jets at ambient temperature into liquid fibers which were cross-linked into solid fibers by in situ photopolymerization during flight toward the collector. The effect of process parameters on the fiber diameter and morphology was investigated to understand the fundamental principles of cure blowing. Two intrinsic process limitations were identified in the drive to produce smaller yet uniform fibers, and strategies to circumvent them were identified. We anticipate that cure blowing may be an industrially relevant and environmentally friendly method for producing cross-linked polymeric nonwovens for a wide range of applications.

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INTRODUCTION Nonwovens are broadly defined as randomly oriented fibers held together by physical entanglements and contact forces without weaving or knitting.1 The global nonwoven industry has grown progressively over the past decade at a rate of 6−7% annually with a projected worth of $50.8 billion in 2020.2 Because of their light-weight, high specific surface area, and porous characteristics,3−6 nonwovens are used in a variety of applications, including disposable medical textiles and fabrics,7,8 catalysis,9,10 filtration and separation,11,12 tissue engineering,13,14 and drug delivery.15 Typical synthetic nonwoven fiber production methods involve spinning a thermoplastic that is transformed into a processable melt or solution state either by heating for melt spinning and melt blowing, or by adding organic solvents for electrospinning, respectively. The processable polymer feed is then drawn in an extensional flow to form a liquid fiber which solidifies on cooling below either the polymer glass transition temperature (Tg) or crystallization temperature (Tc), or by rapid solvent evaporation. These processes are notably distinct from wet spinning, which employs a coagulation bath of poor solvents to solidify an extruded polymer solution.1,7 Generally, the heat- and solvent-based processing approaches largely limit industrial manufacture of nonwovens to thermoplastic resins.16−18 While cross-linked thermoset fibers are attractive targets because of their superior thermal and chemical resistance and tailorable mechanical properties, thermosets cannot be processed in the same manner as thermoplastics. © XXXX American Chemical Society

Previously reported approaches to produce cross-linked fibers19−26 often require specialized materials that are not commercially available or they rely on volatile solvents, heat, or other chemical agents to process precursors that are crosslinked in secondary post-spinning steps. An increased focus on sustainable and environmentally friendly industrial manufacturing processes has driven attempts to minimize both overall energy consumption from heating and volatile organic compound (VOC) emissions in fiber manufacturing processes.27 To address similar concerns, the polymer coating industry has widely adopted ultraviolet (UV) radiation curing to tackle stringent global regulations on VOC emissions.28−32 These “greener” processes typically involve rapidly coating a viscous mixture of oligomers containing UVreactive groups on rapidly moving substrates and photopolymerizing them into cross-linked solid coatings. The major advantages of cured coatings include formulations with nearly 100% reactive components and thus low VOC content, low energy requirements due to reduced external heating requirements, rapid rates of polymerization, tunable properties, and chemical stability.31−35 UV photopolymerization has also been extensively applied in other applications, such as 3D printing,36−41 lithography,42−45 and dental restorative compoReceived: May 15, 2019 Revised: July 29, 2019

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Macromolecules sites46−49 because of the ease of use and mild reaction conditions. At the intersection of photopolymerization and scalable yet environmentally friendly polymer processing, our group previously reported methods for producing cross-linked nonwoven fibers by integrating UV-initiated thiol−ene chemistry with fiber-spinning techniques such as electrospinning50−53 and centrifugal spinning.54−56 In these prior reports, photocurable mixtures of multifunctional liquid monomers (thiol and acrylate/ene monomers) and a freeradical photoinitiator were formed into liquid fibers and rapidly UV cross-linked into solid fibers during flight toward the fiber collector. Because the reactive components were low vapor pressure liquids (mol wt ≈ 500 g/mol), the nonvolatile monomer mixture did not require organic solvents or heat for rapid fiber production. Furthermore, the use of cross-linkable monomer mixtures enabled tunably cross-linked networks with excellent thermal and chemical stabilities. Building from this concept, Boyd et al. demonstrated that hydrodynamic shaping in a microfluidic channel could be used to produce thiol−ene and thiol−yne fibers with different cross-sectional geometries.57,58 However, the concept of scalable simultaneous extrusion and photopolymerization as a route to cross-linked photopolymerized nonwovens has yet to be demonstrated. The majority of the previous research on photopolymerized fibers is based on either electrospinning or centrifugal spinning, both of which are not widely industrially applicable.50−56 In comparison, melt blown synthetic nonwovens with fiber diameters as low as ∼1−2 μm presently constitute >10% of the global nonwoven market.18,59 The prospect of integrating ambient temperature UV photopolymerization with conventional melt blowing thus presents an attractive high-throughput strategy for producing cross-linked fibers. Notably, reactive spinning has been widely employed in commercial production of thermoplastic polyurethane fibers by spinning a polyurethane prepolymer solution.60,61 To the best of our knowledge, the thermally triggered reaction during spinning causes chain extension of the linear prepolymer and serves to improve the crystallinity of the fibers. However, organic solvents are still required for processing and only linear thermoplastic polyurethane fibers are produced. Herein, we describe a “cure blowing” process that integrates thiol−acrylate photopolymerization with a fiber-spinning process resembling melt blowing that operates at ambient conditions. In this process, multifunctional thiol and acrylate monomer mixtures are drawn by room-temperature air jets into liquid fibers that are cross-linked into solid, continuous, and fully amorphous fibers by rapid UV photopolymerization during flight toward the collector. Our study qualitatively and quantitatively identifies the roles of various processing parameters such as air flow rate, monomer delivery rate, and monomer mixture viscoelasticity in specifying the ultimate fiber diameter and morphology.

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phosphine oxide) was provided by IGM Resins. All chemicals were used as received. The chemical structures of the monomers and photoinitiator are provided in Figure 1.

Figure 1. Chemical structures of the reactive monomers and photoinitiator. (a) DPPA, (b) PETT, (c) Omnirad 2100: composed of 90−95 wt % of ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (left) and 5−10 wt % of phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (right). Monomer Feed Mixture Preparation. A specified amount of PEO was first dissolved in PETT at 80 °C for 4 h. PETT was chosen because its lower viscosity favors complete PEO mixing and dissolution. This mixture was then added to predetermined amounts of DPPA (maintaining a constant molar ratio between thiol and acrylate functional groups), ethyl acetate, and photoinitiator (Omnirad 2100) followed by vortex mixing for 5 min. A typical monomer mixture comprised 64 wt % DPPA, 19 wt % PETT, 5 wt % Omnirad 2100, and 12 wt % ethyl acetate, with the PEO content maintained at 500 ppm (based on the total mass of DPPA and PETT) unless otherwise noted. The prepared mixture was then loaded into a 5 mL syringe that was masked with a black tape to block out stray UV irradiation during fiber spinning to avoid premature cross-linking. All these steps were performed in a room with overhead light filtering to minimize effects of ambient light during sample preparation. Cure Blowing Apparatus Components and Experimental Conditions. A typical cure blowing apparatus configuration is shown in Figure 2a. The cure blowing die design is similar to the commonly used “Exxon” die for melt blowing.62 Figure 2b−d shows 3D crosssectional and top views of the single-orifice cure blowing die. The center hole was designed to encase blunt tip needles to control the spinning orifice diameter. The die has an air-knife gap dg = 0.015″, an air-gap width dw = 0.4″, and the die tip is set out from the die-face by a distance ds = 0.015″. The air jets are directed to the die-face through a v-slot with an angle of 60°. All die components were machined from stainless steel (grade 316). The experimental setup was designed based on a die geometry and orifice diameter similar to that used extensively in commercial melt blowing equipment with the following modifications. The monomer mixture was delivered at specific flow rates by a syringe pump (KD Scientific Inc. Legato 100) via a 1.5″ luer-lock blunt tip needle with a 210 μm inner diameter attached to the light-masked syringe (Air-Tite All-Plastic NORM-JECT syringe, 5 mL). Note that the spinning orifice diameter may be readily controlled by selecting needles with different inner diameters, for example, 160 or 260 μm. The UV irradiation system consisted of a light source (OmniCure S1500 Spot UV Light Curing System, Excelitas Technologies) supplemented with a UV reflective mirror (Thorlabs). In some experiments, a second light source (OmniCure S2000 spot UV light curing system, Excelitas Technologies) was used in addition to the first one. Both light sources feature a 200 W broad-spectrum Hg vapor arc lamp with an emission spectrum ranging from 320 to 500 nm. Each light source was

EXPERIMENTAL SECTION

Materials. Penta-functional acrylate, dipentaerythritol pentaacrylate (DPPA, contains ≤650 ppm MEHQ inhibitor), tetra-functional thiol, pentaerythritol tetrakis (3-mercaptopropionate) (PETT, >95%), poly(ethylene oxide) (PEO) (Mv = 106 g/mol), and ethyl acetate (anhydrous, 99.8%) were purchased from MilliporeSigma. Omnirad 2100 (a mixture of 90−95 wt % ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate and 5−10 wt % phenyl bis(2,4,6-trimethylbenzoyl)B

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Figure 2. (a) Schematic representation of the complete cure blowing apparatus and (b) a 3D representation of the cure blowing die along with detailed (c) cross-sectional view and (d) end-on views.

elongates the extruded fiber melt while simultaneously attenuating its diameter. During flight toward a screen-based collector, the fiber rapidly cools in ambient air and solidifies as the fiber temperature drops below the Tg or Tc of the polymer. The magnitude of the drag force, which is controlled by the air flow rate and polymer mass flow rate, crucially determines the extent of fiber drawing.18,63 Additionally, the viscoelasticity of the polymer melt, mainly regulated by the polymer molecular weight and extrusion temperature, significantly influences the drawing dynamics and liquid fiber stability against surface tension-driven break-up.18,64 Adaptation of conventional melt blowing concepts into a cure blowing process requires recognition of a few key differences that mandate modifications to integrate UV irradiation and photopolymerization steps. In contrast to polymer melts, liquid mixtures of monomers, photoinitiator, and rheology modifier serve as the feed to the die. The cure blowing die employed here is a vital component of the design, and it is geometrically nearly identical to that used for melt blowing. While the die may be heated as needed to maintain feed homogeneity, we maintain the die at room temperature throughout this study. The liquid monomer mixture is first extruded through the die orifice to form a liquid fiber, after which intersecting high-velocity sheet-like, ambient temperature air jets causes rapid drawing and diameter attenuation (the draw down ratio ranged from 50 to 1250). After extrusion through the orifice, the liquid monomer fiber properties and process history mirror melt blowing. In other words, monomer mixture viscoelasticity, the air flow rate, and monomer delivery rate dictate the extent of fiber drawing and diameter attenuation while mitigating surface tension-driven fiber break-up prior to solidification. Solidification of the liquid monomer fiber is achieved in flight by rapid UV photopolymerization and cross-linking of the monomers under a Hg vapor arc lamp combined with a UV reflective mirror, liquid light guide, and collimating lens to enable cross-linking over a path length of 4.5 cm. Beyond the rheology of the feed material, which is equally important for both melt blowing and cure blowing, the photopolymerization reaction kinetics and the evolution of a cross-linked network are additional factors that must be carefully considered. Notably, Figure S3 shows that a ∼45 μm thick film of the monomer mixture exhibits the

connected to a liquid light guide with a collimator (OmniCure Adjustable Spot Collimating Adaptor) attached at the end to ensure uniform light intensity distribution and uniform spot size. A UV-grade mirror with average reflectance >90% over the range λ = 250−450 nm was used so that the reflected light can further promote photopolymerization reactions. The incident light intensity was maintained at 1.4 W/cm2 while the reflected beam had an intensity of 0.6 W/cm2, as measured by a radiometer (Coherent FieldMaxII) in the same plane as the fiber path. The composite light spot (combination of incident and reflected light spots), formed at the plane of the fiber path, was approximately 4.5 cm in diameter. Solid fibers were subsequently collected onto a 0.25″ metal mesh screen located 25 cm from the die tip. In this study, the monomer delivery rate was modulated from 0.03 to 0.25 mL/min while the air flow rate ranged from 0.5 to 2 standard cubic feet per minute (SCFM), which translates to an air velocity of 32−155 m/s at the die-face (the crosssectional area for air flow was calculated as 2dgdw). Characterization. Viscosities of the monomers and their mixtures were measured with a stress-controlled AR-G2 rheometer (TA Instruments). Steady-state shear experiments were performed using a parallel plate geometry with a 40 mm diameter upper plate on a Peltier lower stage at 25 °C. Steady-state shear viscosities were measured over the range of shear rates γ̇ = 0.1−100 s−1. Photopolymerization reaction kinetics (see Figures S1 and S2) were monitored using real-time Fourier-transform infrared (FTIR) spectroscopy with a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific) equipped with a KBr beam splitter, mercury cadmium telluride-A (MCT-A) detector, and a customized horizontal transmission accessory. UV−visible absorption spectra of the monomer mixtures were obtained using a Thermo Scientific Evolution 220 UV− visible spectrometer. The fiber mats were imaged with a visible light microscope (Nikon Optiphot equipped with a Canon SL1 digital camera) and a field-emission scanning electron microscope (SEM, JEOL 6500). Additional details about the characterization techniques are provided in the Supporting Information.

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RESULTS AND DISCUSSION Development of the Cure Blowing Process. We designed a cure blowing process based on a conventional melt blowing apparatus by integrating fiber spinning and in situ photopolymerization. To understand the rationale behind the design of the apparatus (Figure 2a), it is important to first understand the concept and operation of melt blowing systems. In melt blowing, a polymer melt is extruded through orifice(s) and drawn into liquid fiber(s) by high velocity hot air jets in a single step. The drag force exerted by the air jets C

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Macromolecules 74% transmission at λ = 365 nm. This observation indicates that for the range of fiber diameters involved in this study (∼5−35 μm), light transmission issues are not expected to arise nor affect the extent of cross-linking throughout the fiber cross-section. To summarize, cure blowing is essentially comprised of the following process steps: (i) preparation of the reactive monomer feed mixture, which may be accomplished either batch-wise or continuously using a selectable multicomponent feed system and an in-line static mixer in place of the melt extruders used for melt blowing, (ii) extrusion of the monomer mixture through the die orifice and liquid fiber drawing by air jets at room temperature, (iii) in-flight UV photopolymerization and cross-linking of the monomer-based fiber to generate the solid, cured fiber, and (iv) accumulation of the fiber product on a screen-based collector. Parameters Governing the Cure Blowing Process. From the discussion above, it is apparent that there are several combinations of parameters that may specify the final fiber characteristics, including: (i) the chemistry and composition of the reactive monomer feed mixture that determines the photopolymerization kinetics, (ii) the viscoelastic properties of the feed that govern the formation and stability of liquid fibers, and (iii) process parameters, such as air flow rate and monomer delivery rate that directly influence the extent of fiber attenuation and flight time from the orifice to the collector. Photopolymerization kinetics can be modified by the monomer type and functionality, composition of the reactive functional groups, and the amount of photoinitiator. In this study, the reactive components of the feed include a pentafunctional acrylate (DPPA) and a tetra-functional thiol (PETT). The high degree of monomer functionality promotes rapid gelation of the monomer mixture in the short UV exposure time available to the liquid fiber during flight. In principle, gelation fixes the fiber diameter, thereby preventing further development of surface tension-driven fiber instabilities. For the monomer mixtures used in this study, we maintained the acrylate to thiol functional group ratio at [C C]:[−SH] = 4:1. Previous reports have demonstrated that the thiol content of this composition substantially reduces oxygen inhibition without appreciably slowing the overall photopolymerization kinetics, conditions ideal for obtaining wellcured, defect-free fibers.50,65 Omnirad 2100 was used as the free radical photoinitiator, primarily due to its 365 nm peak absorbance wavelength that matches the output spectrum of the light source. To obtain a homogenous monomer mixture and aid in processing, ethyl acetate (12 wt % in all cases) was used as a non-reactive diluent to modify the final monomer mixture viscosity. Note that ethyl acetate could be replaced with nonvolatile reactive diluents, such as 1,6-hexanediol diacrylate, to eliminate volatile organic components. Measurements of the viscosities of the monomers and typical monomer mixtures by steady-shear experiments (Figure S4) revealed their Newtonian fluid behavior with negligible elasticity. The latter feature is problematic for liquid fiber drawing because elongated liquids are susceptible to surface tension-driven Rayleigh instabilities that drive fiber break-up (e.g., water dripping from a faucet).66,67 The Rayleigh instability typically begins with the development of surface undulations and periodic necking along the liquid fiber axis, which ultimately results in fiber break-up into droplets of characteristic sizes.68−70 This instability can be effectively

suppressed if its growth is opposed by imparting sufficient elasticity to the liquid fiber.70−72 Therefore, a trace amount of high molecular weight PEO (M v = 106 g/mol) was incorporated into the monomer mixture to enhance its elastic properties without significantly altering the viscosity.54,73,74 The aforementioned three categories of parameters form a convoluted and interdependent variable space that crucially governs the fundamental principles of the cure blowing process. To simplify the analysis in this initial study, the monomer feed composition and UV intensity were held constant to ensure that the intrinsic photopolymerization kinetics remain unchanged. Note that these specific conditions were chosen to produce well-cured smooth fibers with uniform diameters comparable to commercial melt blown fibers, establishing an attractive base condition by which process or material changes may be evaluated. Figure S7 shows an example of a cure blown fiber mat, qualitatively demonstrating their flexibility. The differential scanning calorimetry (DSC) thermogram in Figure S8 highlights the amorphous nature of the cure blown fibers. This study primarily focuses on the relative importances of air flow rate, monomer delivery rate, and PEO content in determining the fiber diameter and morphology. Other outcomes of this parametric study include establishing conditions for producing a range of fiber diameters and morphologies and provision of an initial understanding of the challenges and limitations of cure blowing along with strategies to overcome them. Dependence of the Fiber Diameter and Morphology on Operating Conditions. The ability to control the fiber diameter and morphology ultimately dictates the properties of the fiber mat and its potential utility. For example, micron and submicron fiber diameters provide high specific surface areas and small average pore sizes that are useful for filtration and separation applications.11,12 As described earlier, fibers are formed as high velocity air jets exert a drag force on the extruded liquid, and the magnitude of this force along with the viscoelastic properties of the extruded liquid govern the extent of attenuation and therefore the final fiber diameter.18,64 The drag force (Fd) acting on the liquid fiber is quantified by the drag equation Fd =

1 C Dρu 2A f 2

(1)

where CD is the drag coefficient, ρ is the mass density of air, u is the air velocity relative to the fiber jet velocity, and Af is the surface area of the fiber jet. Thus, the magnitude of the drag force can be increased by increasing the air velocity (Fd ∝ u2) or surface area of the fiber (Fd ∝ Af). Figure 3a−d shows that increasing the air flow rate from 0.5 to 1.25 SCFM (corresponds to u = 32−88 m/s) at a constant monomer delivery rate of 0.25 mL/min results in a progressive reduction in the average fiber diameter from dav = 26.3 ± 1.9 to 10.0 ± 2.1 μm. In addition to the air flow rate, the monomer delivery rate can also be used to tailor the fiber diameter. Figure 3e−h shows that the dav reduces from 23.4 ± 2.5 to 9.2 ± 1.9 μm as the monomer delivery rate was decreased from 0.25 to 0.04 mL/min at a constant air flow rate of 0.5 SCFM (u = 32 m/s). When the monomer delivery rate is reduced at a constant air flow rate, the same Fd acts on a reduced extrudate mass, enhancing fiber drawing and decreasing the final fiber diameter. Essentially, a combination of higher air flow rate and lower monomer delivery rate promotes formation of fibers with smaller average diameters. Notably, the average diameters D

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Figure 3. (a−d) Representative SEM micrographs of cure blown fibers produced at a constant monomer delivery rate of 0.25 mL/min with air flow rates of (a) 0.5, (b) 0.75, (c) 1, and (d) 1.25 SCFM. (e−h) Representative SEM micrographs of cure blown fibers produced at a constant air flow rate of 0.5 SCFM with monomer delivery rates of (e) 0.25, (f) 0.1, (g) 0.075, and (h) 0.04 mL/min (inset: surface undulations developed at low monomer delivery rates).

decreased (see Figure 3h inset for representative examples). Smaller fiber diameters possess increased surface area-tovolume ratios (A/V) because A/V increases as the inverse of the diameter (i.e., A/V = 4D−1). The increased A/V amplifies the destabilizing surface tension stresses present on the free surface of the liquid fiber. In cases where smooth fibers with uniform diameters were observed, the instability is presumably suppressed by the buildup of extensional stress during fiber drawing because of the presence of high molecular weight PEO in the monomer feed mixture. As the magnitude of the surface tension stress increases at progressively higher A/V or smaller fiber diameters, the extensional stress eventually becomes insufficient to suppress the growth of the instability, and the free surface of the liquid jet develops undulations. Provided enough fluid mobility, the periodic surface undulations can further evolve into a beads-on-string type of morphology or, in the most severe case, droplets with characteristic sizes.54,71,72 Presumably, the undulations observed in the inset of Figure 3h are covalently arrested by fiber photopolymerization. Therefore, our ability to obtain cure blown fibers with smaller fiber diameters is limited by the onset of (i) fiber fusion that reflects a kinetic limit of fiber photopolymerization, and (ii) surface undulation stemming from the rheological and surface tension-driven fluid stability limit. In the following sections, we describe semi-quantitative analyses that illuminate the origins of these constraints. Onset of Fiber Fusion: A Kinetic Limit. As the fiber velocity increases (directly related to diameter decreases), the time the fiber spends in the UV-illuminated region during flight also decreases. Accordingly, the system approaches a photopolymerization kinetic limit beyond which well-cured, non-fused fibers with smaller diameters no longer form. This limit may be better understood and quantified by defining characteristic timescales for comparison. A convenient timescale for characterizing the time evolution of polymer networks during polymerization is the gel time. The gel point is defined as the critical fractional conversion of monomer functional groups at which a continuous network first appears; the reaction time required to reach the gel point is called the gel time. Because our cure blowing feed mixture comprises a thiol and an acrylate, photopolymerization proceeds by a combination of fast acrylate homopolymerization in conjunction with slower thiol−acrylate chain transfer reactions. For this mixed

reported as part of Figure 3 are similar to those produced by commercial melt blowing (∼2−20 μm). Statistical analyses of the fiber diameters from Figure 3 are provided in the Supporting Information (Figure S5). The average and standard deviation of the fiber diameters given in these figures were obtained by fitting the diameter data to a normal distribution. However, the fiber mats in Figure 3c,d produced with air flow rates ≥1 SCFM (u ≥ 68 m/s) exhibit fused junctions where the fibers contact one another, and the extent of fiber fusion increases with increasing air flow rate. Similarly, Figure 3g,h shows that lower monomer delivery rates foster more fused fiber junctions. Fibers that are fused at their junction points but retain their fibrillar state are likely photopolymerized beyond the gelation point yet lack sufficiently high conversion to have vitrified into solids. Consequently, the fiber surfaces are incompletely cured and are susceptible to fiber fusion during collection. Importantly, both the fiber diameter reduction and development of fiber fusion at higher air flow rate or lower feed delivery rate may be explained by increases in the fiber velocity. Higher air velocities (u) increase Fd that enhances fiber drawing to produce smaller diameters and, in turn, smaller diameter fibers must adopt higher velocities because of conservation of mass. On the other hand, the higher surface area to volume ratios of smaller diameter fibers formed at lower monomer delivery rates increase the drag force acting on the fiber per unit mass; this more aggressively reduces the fiber diameter and increases its velocity. Consequently, fiber fusion is amplified at higher fiber velocities because of the reduced time that the liquid fiber spends in the irradiation region (lower UV dose), which lowers overall monomer conversion. In extreme cases where air flow rates ≥1.25 SCFM (u ≥ 88 m/s), the UV dose is low enough to incur extensive fiber fusion, and the mat no longer resembles a conventional nonwoven. Note that intentionally forming fibers with fused junctions in a single step may be advantageous for applications where conventional nonwoven fiber mats are treated with heat or liquid binders to fuse nearby fibers in a second processing step called bonding.1 Additionally, nonuniform fiber diameters with surface undulations were observed at monomer delivery rates ≤0.075 mL/min. While the extent of surface undulation is difficult to quantify, it qualitatively increased as the monomer delivery rate was reduced and the average fiber diameter E

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is much smaller than the gel time, the fiber mat transitions from a non-fused to a fused morphology. In other words, insufficient UV exposure time limits monomer conversion and fails to prevent fusing at the collector. Qualitatively, the extent of fiber fusion increases with a greater imbalance between the gel time and the UV exposure time, evidenced by the final entry in Table 1 that exhibits the greatest amount of fusion. The comparison between these two characteristic timescales reveals the source of the kinetic limit. We hypothesized that the kinetic limit for producing smaller diameter, non-fused fibers using this particular monomer system may be partially circumvented or extended by using higher fiber velocities (higher air flow and/or lower monomer delivery rates) in conjunction with a longer UV illuminated path length. As a result, the UV exposure times should increase (Table S1) to enable sufficient monomer conversion in the liquid fiber jet so as to avoid fiber fusion. To test this hypothesis, a second, identical UV light source along with a mirror assembly was added to the existing light arrangement (Figure S6) to increase UV exposure time. Comparison between Figure 4a (replicate of Figure 3d; fused fibers with a

chain and step growth polymerized network, eq 2 developed by Reddy et al. enables calculation of the gel point in terms of fractional conversion of the acrylate bonds pα as51,75 k 2 (facrylate − 1) CC pα + (facrylate − 1)(fSH − 1) r k CS ij y jj1 + 2 k CC zzzp 2 = 1 jj r k CS zz{ α k

(2)

where r is the [thiol]/[acrylate] stoichiometric ratio, facrylate and f SH are the acrylate and thiol monomer functionalities, respectively, and kCC/kCS is the ratio of the propagation constant for the acrylate−acrylate reaction to the chain transfer constant for the thiol−acrylate reaction. According to a previous report, kCC/kCS = 1.5.76 Based on the monomer composition used in this study, pα ≈ 2% acrylate double bond conversion. Note that high monomer functionality ensures formation of an infinite network at low monomer conversions, which is a distinct advantage of this particular thiol−acrylate reaction scheme. The thiol−acrylate reaction kinetics were monitored using real-time (RT)-FTIR spectroscopy using the same light source as that used for cure blowing experiments. Reduction in the acrylate double bond (1630 cm−1) peak area was used to track the fractional conversion of acrylate double bonds with UV exposure time.50,77 Using the calculated pα, the gel time at the fiber processing UV intensity was estimated from the conversion-time data to be 2.6 ms. The detailed procedure for estimation of gel time using RT-FTIR is provided in the Supporting Information. Intuitively, one way to define the kinetic limit is to compare the gel time to the UV exposure time. The UV exposure time is primarily governed by the fiber velocity in the UV-illuminated region and can thus be estimated as the total flight time in this area of the cure blowing apparatus. Experiments on and simulations of melt blowing have quantified the air velocity and fiber velocity profiles as a function of distance from the dieface.78−83 In the first few centimeters from the die-face, fiber velocity increases quickly as the fiber is rapidly drawn by the air jets, after which it decays at a similar rate as the air velocity.78 In this latter regime, experimental data obtained using a similar die configuration indicated that the air velocity decays with distance from the die, according to a power law model.84 To the best of our knowledge, an analytical model of the fiber velocity profile capturing the full behavior has yet to be reported. Therefore, we have used the power law decay model for the air velocity profile from Moore et al. to estimate the monomer fiber velocity in the UV-illuminated region.84 Table 1 summarizes the approximate UV exposure times estimated at different air flow rates and compares them with the calculated gel time for the monomer mixture composition. From these data, we observe that when the UV exposure time

Figure 4. Representative SEM micrographs showing the effect of increased UV exposure time on the morphology of the fibers produced at (a,b) 0.25 mL/min monomer delivery rate and 1.25 SCFM air flow rate; (c,d) 0.04 mL/min monomer delivery rate and 0.5 SCFM air flow rate. (a,b) are replicates of Figure 3d,h respectively.

shorter UV exposure time) and Figure 4b demonstrates that a longer UV exposure time considerably reduced the extent of fiber fusion. Similar reduction in fiber fusion is also observed on comparing Figure 4c (replicate of Figure 3h) and Figure 4d. These results indicate that a prolonged UV exposure time can extend the kinetic limit to enable production of smaller fiber diameters without significant fusion. Additionally, we anticipate that this kinetic limit may also be delayed by decreasing the gel time through modification of the monomer mixture composition with higher functionality (macro)monomers or higher photoinitiator content. Onset of Surface Undulations: A Fluid Stability Limit. The development of fibers with nonuniform diameters and periodic surface undulations (see Figure 3h inset) represents a surface tension-driven fluid stability limit. On approaching this limit, pushing the system to produce smaller diameter fibers by increasing air flow and/or decreasing monomer delivery rates qualitatively increases the prevalence of surface undulations. Further decreasing the fiber diameter eventually leads to the dominance of surface tension stresses over the opposing

Table 1. Summary of the Estimated UV Exposure Times, Gel Time of the Monomer Mixture, and Corresponding Fiber Morphology at Different Air Flow Rates air flow rate (SCFM)

UV exposure time (ms)

gel time (ms)

fiber mat morphology

0.5 0.75 1 1.25

3.7 2.4 1.7 1.3

2.6 2.6 2.6 2.6

non-fused non-fused fused fused F

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Macromolecules extensional stresses in the liquid fiber, leading to the growth of periodic surface undulations and nonuniform “beads-on-string” or droplet morphologies. While it is tempting to characterize these features as undesirable defects, they may be exploited for certain applications.85,86 The onset of the fluid stability limit at smaller fiber diameters may be rationalized based on the analysis presented by Regev et al. on the relative importance of the two competing stresses in terms of a nondimensional Boussinesq number expressed as the ratio of the surface and extensional stresses in a viscoelastic liquid jet.87 The critical liquid fiber diameter (Dcrit) obtained from the balance of the stresses is given by Dcrit =

4σσ σE

ωmax =

(3)

Figure 5. Representative SEM micrographs showing the effect of increased PEO contents in the monomer mixture on the morphology of the fibers produced at 0.04 mL/min monomer delivery rate and 1 SCFM air flow rate with (a) 500 ppm PEO content and (b) 1000 ppm PEO content. Some typical examples of surface undulations are circled in the images.

respectively. Upon inclusion of 1000 ppm PEO to increase λE, the extent of surface undulations was noticeably reduced at an average fiber diameter of 6.1 μm. This effect arises from the larger extensional stress present at higher PEO content, which stabilizes smaller diameter liquid fibers more effectively. Furthermore, a smaller limiting diameter was observed at the 1000 ppm PEO content as compared to the 500 ppm PEO content (data not shown), which may be rationalized by eq 3 that shows that the Dcrit ≈ σE−1. We anticipate that the onset of surface undulations may be further delayed to fiber diameters as small as ∼1−3 μm by further enhancing the elastic properties of the monomer mixture by increasing the PEO content or its Mv. Smallest Fibers without Significant Fusion or Surface Undulation. The previous discussions suggest that extending the kinetic limit requires a concomitant delay in the fluid stability limit to obtain the smallest fiber diameters with minimal fusion or surface undulations, through the combination of a modified light arrangement and higher PEO content. Figure 6 shows the smallest fiber diameters obtained at different conditions after making simple modifications to the light arrangement (using only the UV light sources and mirrors available in our laboratory) and PEO content. A few instances of fused junctions and surface undulations can be identified in some of these cases, however, they are not representative of the majority of the fibers. The average diameters range from 3.6 to

λE ρR3/σ

(5)

where S = 1/(1 + GλE//μS) is the retardation number, Ca = (ρν2)/(σr02) is the capillary number, λE is the fluid relaxation time, μS is the fluid viscosity, r0 is the characteristic radius, ν is the characteristic viscosity (ν = μS/Sρ), ρ is the density, σ is the surface tension, and G is the elastic modulus of the viscoelastic fluid jet. Simplification of eq 5 shows that ωmax is inversely proportional to a linear function of λE (see Supporting Information for detailed analysis). Both of these analyses establish that the growth rate of the surface instability may be arrested with an increase in the fluid relaxation time of the monomer feed mixture, i.e., the fluid stability limit can be delayed. Increasing the high molecular weight PEO elasticity modifier content in the monomer feed mixture presents a facile means of precisely tailoring the fluid relaxation time to meet process needs.54,71 Figure 5a,b shows the morphology of fibers produced with monomer mixtures containing 500 and 1000 ppm of PEO,

where σσ is the surface tension-driven stress and σE is the extensional stress. Equation 3 establishes a stability criterion whereby a specific extensional stress in the viscoelastic monomer fiber, which may be tuned by the specific PEO Mv and content in the monomer feed mixture, specifies the critical diameter below which the surface tension-driven instability begins to dominate and surface undulations appear. Our experimental results qualitatively agree with this analysis because surface undulations typically developed and intensified below a certain average diameter. Furthermore, eq 3 also suggests that with an increase in the extensional stress in the monomer jet, the critical diameter is expected to decrease. This indicates a potential opportunity to suppress the appearance of surface undulations at smaller fiber diameters by suitably enhancing the elastic properties of the monomer mixture. Fundamentally, the balance between the σσ driving the instability and the opposing σE due to drawing of the viscoelastic feed mixture can be evaluated by the Deborah number (De). De correlates the two competing stresses in terms of a ratio of characteristic timescales for fluid relaxation (a measure of the elastic response of the fluid) and jet break-up due to the surface tension instability as shown in eq 488,89 De =

1 2 2Ca (1 + 3S Ca /2 )

(4)

where λE is the characteristic fluid relaxation time, R is the initial diameter of the monomer fiber, ρ is the density, and σ is the surface tension of the monomer feed mixture. Equation 4 indicates that a De ≫ 1 (i.e., larger λE compared to the surface tension instability timescale) favors complete suppression of the surface instability and promotes formation of fibers with uniform diameters. Fang et al.54 and Yu et al.71 experimentally observed centrifugal spinning and electrospinning, respectively, in which a critical De ≈ 6−7 demarcates the transition from fibers with uniform diameters to nonuniform diameters; for smaller De, fibers were obtained with a beads-on-string morphology, representative of further growth of the surface instability. On this basis, delaying this fluid stability limit of the cure blowing process requires a monomer mixture with a longer fluid relaxation time. The role of a higher fluid relaxation time is further highlighted by the expression derived by Yu et al. for a maximum dimensionless surface instability growth rate (ωmax) for a viscoelastic fluid jet.71 This expression was obtained using a dispersion relation formulated by Chang et al. that describes the axisymmetric instability growth in a cylindrical viscoelastic fluid jet as72 G

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Figure 7. Variation of the average fiber diameter and morphology with Γ (air mass flux/monomer mixture mass flux) obtained using both the initial (1 UV source) and modified (2 UV sources) light arrangements. The equations provided are obtained by power law fits on the corresponding fiber diameter data. The shaded areas denote the intermediate region over which the transition of fiber morphology occurs.

Figure 6. Representative SEM micrographs of the fibers with the lowest average fiber diameters produced to date at (a) 0.04 mL/min monomer delivery rate and 1 SCFM air flow rate, (b) 0.04 mL/min monomer delivery rate and 1.25 SCFM air flow rate, (c) 0.03 mL/min monomer delivery rate and 0.5 SCFM air flow rate, and (d) 0.04 mL/ min monomer delivery rate and 1 SCFM air flow rate. (a,b) had 1000 ppm PEO content while (c,d) had 500 ppm PEO content in the monomer mixtures.

fused to fused morphology does not occur at a specific Γ, rather there is a broader transition region (the shaded regions). For example, both non-fused and fused morphologies were observed at Γ ≈ 1.5 in the case of two UV sources. Figure 7 also shows that the transition region clearly shifts to a greater Γ on switching from one to two light sources, consistent with our observation that smaller fibers without fused junctions can be obtained by increasing UV exposure time. The combination of these two observations indicates that Γ fails to account for the competition between the gel time and the UV exposure time, which triggers this particular fiber morphology change. This finding is not unexpected, because Γ only accounts for the air and monomer mixture mass flux without considering any other process features. A more comprehensive analysis which introduces additional parameters is necessary to successfully capture the different competing features of the cure blowing process.

6.9 μm, and standard deviations (≤16% of the mean) of the fiber diameter distributions were obtained by fitting the diameter data to a normal distribution; these data are superposed on the representative SEM images in Figure 6. Recalling that melt blown nonwovens typically possess average diameters ∼2−20 μm, we anticipate that significantly smaller diameter fibers < 3.6 μm may be obtained by cure blowing with further optimization of the apparatus and/or modifying the monomer chemistry, among other possibilities. Quantitative Analysis of the Effects of Process Parameters on the Fiber Diameter and Morphology. To supplement our qualitative understanding of the effects of process parameters on the fiber diameter and morphology presented above, we pursued an initial quantitative analysis. Previous studies have reported that the variations in melt blown fiber diameters can be adequately described by the ratio of the air to polymer mass fluxes (Γ) over a wide range of processing conditions.63,90 The dimensionless parameter Γ, which we redefine here as the ratio of air to monomer mixture mass flux, is evaluated in Figure 7 and summarizes results obtained from fiber mats produced by varying the air flow rate and monomer delivery rate at a constant monomer feed composition with the two different illumination conditions. According to the data, the average fiber diameter exhibits a power law dependence on Γ (with R2 = 0.89 and 0.95 for one or two light sources, respectively), consistent with previous observations for melt blowing where the average fiber diameters also exhibited a similar power law dependence.90,91 This validates the similarities between the underlying mechanism of fiber diameter attenuation in both cure blowing of a monomer feed mixture and thermoplastic melt blowing. We anticipate that the power law model may serve as a predictive tool to estimate the expected average fiber diameter for specific operating parameters. Additionally, non-fused and fused fibers are marked distinctly in Figure 7 to assess whether Γ accounts for the transition in the fiber morphology (in this figure we only focus on fiber fusion). Figure 7 shows that the transition from a non-

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CONCLUSIONS Cross-linked nonwoven fibers were produced by a new “cure blowing” process, which integrates in situ photopolymerization with an ambient temperature extrusion process that mimics conventional melt blowing. Specifically, a mixture of thiol and acrylate monomers loaded with a small amount of an elasticity enhancing additive was drawn into liquid fibers by high velocity air jets and rapidly photopolymerized into solid crosslinked fibers upon UV irradiation during flight. Our systematic studies revealed that process efficacy and its intrinsic limitations are dictated by a convolution of interdependent parameters that stem from the polymerization kinetics, material properties, and process operating conditions. Analysis of the interplay between air flow rate, monomer delivery rate, and monomer mixture elasticity led to the identification of a kinetic limit and a fluid stability limit at smaller fiber diameters; the onsets of which manifest as the appearance of fused fibers or fibers with surface undulations. Potential strategies to circumvent/delay these process limits, including increasing the UV exposure time and modifying the monomer mixture elasticity, were also experimentally investigated. These studies lay a foundation for the optimization of cure blowing into an industrially relevant technique for producing useful crossH

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linked polymeric nonwovens for a wide range of applications. Ongoing studies seek to develop a more thorough and quantitative understanding of the fundamentals of this technique.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01002.

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Details of the characterization methods used, conversion-time data at different UV intensities, gel times of the monomer mixture as a function of the UV intensities obtained from polymerization kinetics studies using RT-FTIR, UV-visible absorption spectra obtained from a ∼45 μm thick film of the monomer mixture before and after photopolymerization, viscosities of the DPPA, PETT, and the monomer mixture, statistical analyses of the fiber diameter, schematic representation of the cure blowing apparatus with the modified light arrangement, photograph of a fiber mat, DSC thermogram of as-obtained cure blown fibers, and summary of the estimated UV exposure times for the initial and modified light arrangements (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aditya Banerji: 0000-0003-0654-9195 Kailong Jin: 0000-0001-5428-3227 Mahesh K. Mahanthappa: 0000-0002-9871-804X Christopher J. Ellison: 0000-0002-0393-2941 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge 3M and the National Science Foundation (grant # CBET-1659989) for financial support. Parts of this work (SEM and OM) were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the National Science Foundation through the Materials Research Science and Engineering Center program (DMR-1420013).

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REFERENCES

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DOI: 10.1021/acs.macromol.9b01002 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b01002 Macromolecules XXXX, XXX, XXX−XXX