Mechanically Robust and Recyclable Cross-linked Fibers from Melt

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Mechanically Robust and Recyclable Cross-linked Fibers from Melt Blown Anthracene-Functionalized Commodity Polymers Kailong Jin, Aditya Banerji, David Kitto, Frank S. Bates, and Christopher J Ellison ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00209 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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ACS Applied Materials & Interfaces

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Mechanically Robust and Recyclable Cross-linked Fibers from Melt Blown Anthracene-Functionalized Commodity Polymers Kailong Jin, Aditya Banerji, David Kitto, Frank S. Bates,* and Christopher J. Ellison* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, United States *To whom correspondence should be addressed: [email protected]; [email protected] ABSTRACT: Melt blowing combines extrusion of a polymer melt through orifices and attenuation of extrudate with hot high-velocity air jets to produce nonwoven fibers in a single step. Due to its simplicity and high-throughput nature, melt blowing produces more than 10% of global nonwovens (~$50 billion market). Semi-crystalline thermoplastic feedstock, such as poly(butylene terephthalate), polyethylene, and polypropylene, have dominated the melt blowing industry because of their facile melt processability and thermal/chemical resistance; other amorphous commodity thermoplastics (e.g., styrenics, (meth)acrylates, etc.) are generally not employed because they lack one or both characteristics. Cross-linking commodity polymers could enable them to serve more demanding applications, but cross-linking is not compatible with melt processing and it must be implemented after fiber formation. Here, cross-linked fibers were fabricated by melt blowing linear anthracene-functionalized acrylic polymers into fibers, which were subsequently cross-linked via anthracene-dimerization triggered by either UV light or sunlight. The resulting fibers possessed nearly 100% gel content because of highly efficient anthracene photo-dimerization in the solid state. Compared to the linear precursors, the anthracene-dimer cross-linked acrylic fibers exhibited enhanced thermomechanical properties suggesting higher upper service temperatures (~180 oC), showing promise for replacing traditional thermoplastic-based melt blown nonwovens in certain applications. Additionally, given the dynamic nature of the anthracene-dimer cross-links at elevated temperatures (> ~180 oC,

the resulting cross-linked fibers could be effectively recycled after use, providing new

avenues towards sustainable nonwoven products. KEYWORDS: nonwoven, melt blowing, cross-linked fibers, anthracene-dimerization, reversible bonds

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INTRODUCTION Nonwovens, consisting of polymer fiber mats without stitching/weaving, are a ~$50 billion market with applications such as

disposable textiles, filtration media, and tissue

engineering, etc.1 More than 10% of global nonwovens are fabricated by melt blowing,2-7 a technique that produces fiber mats in a high-throughput manner. In the melt blowing process, a polymer melt is extruded through orifices and then attenuated by hot high-velocity air jets to form thin liquid filaments. These liquid filaments are rapidly cooled below the polymer solidification temperature4-8 (e.g., crystallization temperature, Tc, or glass transition temperature, Tg) by entrained ambient air thereby solidifying into nonwoven fibers before reaching a collector. Linear thermoplastic feedstocks (usually semi-crystalline polymers), such as polyethylene, polypropylene, and poly(butylene terephthalate), have dominated the melt blowing industry based on their low cost, melt processability, and relatively good thermal/chemical resistance. Other linear, amorphous commodity thermoplastics (e.g., (meth)acrylates) are generally not used for nonwovens applications because of relatively poor heat and solvent resistance. If processing methods can be developed that enable polymers to be cross-linked as fibers, a broader range of low-cost feedstock materials may be able to serve more demanding applications because cross-linked polymers typically exhibit superior thermal/chemical resistance over linear thermoplastic analogs. Such a process could greatly expand the palette of polymers that are suitable for melt blown nonwovens applications. Unfortunately, it has been challenging to produce cross-linked fibers by melt blowing since conventional polymer networks cannot be liquified for melt processing because of permanent/irreversible cross-links.9 To the best of our knowledge, there are only two studies to date that have demonstrated melt blown cross-linked fibers.10-11 Almost all other previously reported approaches10-23 for forming crosslinked fibers rely on either electrospinning or centrifugal spinning which are far less commonly used for producing nonwovens in industry. Generally, previous studies have formed cross-linked fibers by either in-situ UV curing of liquid monomer jets during fiber spinning12-15 or processes involving fiber spinning of functionalized polymers in a first step followed by cross-linking in a


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second step (e.g., by UV cross-linking).10, 16-20, 22 For example, Montgomery et al.10 utilized an linear amorphous polyester having UV-cross-linkable stilbene groups that was first melt blown into fibers, followed by UV irradiation to trigger irreversible cross-linking between stilbene groups in a second step. In fact, all these previous studies10,

12-23

have generated

irreversibly/permanently cross-linked fibers, which cannot be effectively recycled after use. Therefore, if permanently cross-linked fiber products are not properly recycled, they could become “persistent” microfiber/microplastic waste with potential for detrimental impact on the environment.24-25 Of greater utility would be fibers that can remain in the cross-linked state during use but also decross-link and flow as viscous liquids during reprocessing. In a recent study,11 we introduced a one-step strategy for producing reversibly cross-linked fibers by melt blowing thermally reversible Diels-Alder polymer networks26-38 comprised of methacrylate-based backbones covalently bonded by dynamic furan-maleimide linkages. These thermoreversible networks can decross-link and behave like viscous liquids at melt blowing temperatures (since furan-maleimide cross-links can decross-link into free furan and maleimide groups upon heating), then undergo cooling-induced cross-linking during/after melt blowing to form a network again. The resulting cross-linked fibers possessed not only good chemical resistance because of their network structure but also sustainable recyclability due to the reversible nature of the furan-maleimide cross-links.11 One potential limitation of these cross-linked fibers is that they have relatively poor heat resistance since furan-maleimide linkages begin to cleave at ~100 oC

as driven by thermodynamic equilibrium; in turn, this decreases the cross-link density and

degrades

material

properties,

which

is

amplified

upon

further

heating.11

Other

reversible/dynamic chemistries with higher dissociation temperatures are desired for cross-linked fiber mats with good heat resistance and sustainable recyclability. Among the existing dynamic chemistries in the literature,26-43 reversible anthracene-dimerization is an attractive potential candidate.43 Anthracene (Figure 1a) consists of three linearly fused aromatic rings with an extended



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π-system43 that presents remarkable photo-sensitivity.44 When irradiated with UV light (λ > 300 nm) or sunlight,44 anthracene groups can undergo efficient [4+4]-cycloaddition (Figure 1a) to form anthracene-dimers.44 These dimers can cleave and revert to free anthracenes by annealing at relatively high temperatures45 or irradiating with high-energy UV light (λ < 300 nm).46 The reversible nature of anthracene-dimerization is highly attractive in the context of stimuliresponsive materials47-54 and self-healing/reprocessable polymer networks.43-45,

55-57

Of our

particular interest is that the anthracene-dimers have been demonstrated to possess a relatively high dissociation temperature.45 A recent study on reversible polymer networks45 by Du Prez and coworkers showed that the anthracene-dimer cross-links remained intact at temperatures up to ~170 oC, then started to cleave and revert to free anthracenes upon further heating. We hypothesize that the incorporation of the reversible anthracene-dimerization chemistry in the context of fiber manufacturing could potentially lead to cross-linked and recyclable nonwoven fiber mats with relatively higher upper service temperatures (Tu). In this study, cross-linked fibers were formed by processing a linear, amorphous acrylic polymer containing pendant anthracene groups (Figure 1b) into fibers via melt blowing in a first step, followed by irradiation with either UV light or sunlight to trigger reversible cross-linking between anthracene groups in a second step. Here, an acrylic polymer (Figure 1c) was chosen because of its synthetic convenience and acrylate-based backbone, characteristics that are potentially attractive to the nonwovens industry. In addition, the amorphous nature of acrylics could be beneficial for light penetration into the interior of the fiber, enabling cross-linking throughout the entire sample.10

RESULTS AND DISCUSSION An acrylic random copolymer (Figure 1c) comprised of 55 mol% methyl acrylate (MA) and 45 mol% n-butyl acrylate (nBA) (characterized by proton nuclear magnetic resonance, 1H NMR; Figures S1) was synthesized by free radical polymerization, a technique commonly used in industry for commodity poly(meth)acrylates. (See Supporting Information for more


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experimental details.) According to size exclusion chromatography (SEC) analysis, the numberaveraged molecular weight, Mn, = 16.0 kg/mol and molecular weight dispersity, Ð, = 2.1 (Figure 2 and Table S1) for the obtained linear MA-nBA copolymer.

Figure 1. (a) Schematic of reversible anthracene photo-dimerization. (b) Linear anthracenefunctionalized polymers can undergo UV-cross-linking to form polymer networks comprised of dynamic anthracene-dimer cross-links, which can reversibly cleave after heating at elevated temperatures. (c) Synthesis of linear anthracene-functionalized acrylic copolymers by transesterification reactions in the bulk state.

The anthracene-functionalized acrylic copolymer was then synthesized via bulk transesterification reactions58 between the linear MA-nBA copolymer and commerciallyavailable 9-anthracenemethanol (AN) (# of acrylate units : # of AN units = 2 : 1 for the reaction feed ratio) in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as an organocatalyst (Figure 1c; see Supporting Information for more details). After reacting at 140 oC in a twin-



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screw batch mixer for 25 min, ~80% of the AN units in the feed were covalently attached to the acrylic backbone, resulting in a functionalized acrylic copolymer with ~40 mol% pendant anthracene groups (noted as linear MA-nBA-AN; the copolymer composition was 25-35-40 mol%, measured by 1H NMR in Figure S2). The linear MA-nBA-AN copolymer was isolated from the unreacted AN units and TBD catalysts by repeated dissolution/precipitation processes (Supporting Information). SEC results showed that both Mn (= 32.0 kg/mol) and Ð (= 2.6) of the linear MA-nBA-AN are higher than those of the linear MA-nBA (Figure 2 and Table S1), consistent with anthracene-functionalization of the acrylic copolymer through transesterification reactions in the bulk state. (We note that ~40 mol% anthracene groups were incorporated to achieve a linear MA-nBA-AN copolymer with good cross-linking performance as well as a Tg that is suitable for melt blowing (i.e., Tg > room temperature (RT)). In comparison, a lower anthracene-functionalization degree, e.g., ~6 mol%, led to equally good cross-linking performance but failed to provide a copolymer Tg above RT. Future studies are warranted to fine tune the copolymer composition to achieve a linear MA-nBA-AN with a minimal AN content ( ≤ ~6 mol%) and a Tg above RT.) Decross-linked MA-nBA-AN Linear MA-nBA-AN Linear MA-nBA

Light scattering detector signal

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12

14

16

18

Elution time (min)

20

Figure 2. SEC data collected using a multi-angle light scatting detector of linear MA-nBA, linear MA-nBA-AN (anthracene-functionalized acrylic copolymer), and decross-linked MAnBA-AN. The SEC data are intentionally normalized to the peak value for better clarity.


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It is noteworthy that almost all previous reports of anthracene-functionalized polymers have involved synthesis of specifically designed anthracene monomers54-55,

59-61

and a second

copolymerization step to incorporate them into polymer backbones. In comparison, this postpolymerization functionalization strategy could be more straightforward synthetically and more relevant industrially. Without any optimization, a relatively high transesterification efficiency (i.e., ~80% of the AN units reacted) was obtained in the bulk state. We envision that this organocatalyzed transesterification approach could provide a platform suitable for large-scale production of functional polymers (including anthracene-functionalized polymers) from commodity (meth)acrylates and (meth)acrylate-based copolymer feedstocks. Next, we investigated the anthracene photo-dimerization of the linear MA-nBA-AN copolymer using a film geometry. Such an understanding can be translated into other formats, e.g., fibers. To trigger the photo-reaction, a ~250-µm-thick melt-pressed film of the linear MAnBA-AN copolymer was exposed to UV light (> 300 nm) with an intensity of ~0.2 W/cm2 at RT for 10 min on each side of the film. (Note: The UV light source used in this study has a maximum intensity at ~365 nm.) Before UV irradiation, the unreacted linear MA-nBA-AN was completely soluble in tetrahydrofuran, indicating a gel fraction of 0%. After UV irradiation, the resulting film was insoluble in tetrahydrofuran and had a gel content of ~100% (determined by swelling tests), indicating the formation of a cross-linked network that reached the full gel state within experimental error (noted as cross-linked MA-nBA-AN). In contrast, a ~250-µm-thick film of the linear MA-nBA copolymer with no anthracene groups, exhibited no observable changes in its physical properties (e.g., solubility in tetrahydrofuran and Tg; Figure S3) after UV irradiation under the same conditions. Based on these results, cross-linking of the linear MAnBA-AN copolymer can be attributed to the anthracene photo-dimerization. The anthracene photo-dimerization was further confirmed by UV-Vis spectroscopy measurements on a ~3-µm-thick, spin-coated film of the linear MA-nBA-AN copolymer supported on a quartz substrate.54-55, 57 (Thicker films of the linear MA-nBA-AN copolymer led to low signals beyond instrument limits.) As shown in Figure 3a, the absorption peaks of the



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free anthracenes between 340 and 400 nm decreased gradually with increasing UV-irradiation time and essentially disappeared after ~5 min of irradiation. This is consistent with the fact that the formation of anthracene-dimers disrupt the extended π-system of the free anthracene groups and lead to changes in the UV absorption spectrum.55 An important feature of these anthracenefunctionalized polymers after UV-irradiation is that they exhibit almost no absorbance of light between 340 and 400 nm, i.e., the UV-irradiated sample becomes nearly transparent to the irradiating UV light. This should enable the UV light to effectively penetrate the interior of the film to activate the cross-linking process throughout the entire sample. Consistently, a ~250-µmthick film of the linear MA-nBA-AN copolymer was able to reach the full gel state (within experimental error) after UV irradiation, as discussed earlier. (Note: It is not necessary that all the anthracene groups should react and form dimers for the linear MA-nBA-AN film to reach the full gel state. Any linear MA-nBA-AN copolymer chain could be incorporated into the network to contribute to the gel fraction with only one intermolecular anthracene-dimer linkage between the chain and the network.) Figures 3b and 3c show the T-dependences of elastic modulus (G'), viscous modulus (G''), and tan δ (G''/G') (measured on a shear rheometer in the linear viscoelastic region of the polymer; 5 oC/min heating scan) for a ~250-µm-thick film of the linear MA-nBA-AN copolymer before and after UV irradiation, respectively. According to Figures 3b and 3c, the tan δ peak, representative of the glass transition region,42 for the cross-linked MA-nBA-AN sample (from ~60 to ~120 oC) is broader than that of its linear precursor (from ~60 to ~90 oC). Similarly, differential scanning calorimetry (DSC) characterizations (Figure S3) showed that the Tg value of the linear MA-nBA-AN film increased after UV-cross-linking. These results are consistent with notion that the UV-cross-linking should lead to a decrease in the segmental mobility and an increase in the heterogeneity of the system.62-63 It is noteworthy that the anthracene photodimerization in the linear MA-nBA-AN film was highly effective, even when the system was in the glassy state (i.e., Tg, linear MA-nBA-AN > RT). This is consistent with a previous study by Kondo et al.49 where they observed that the pendant anthracene groups on a linear methacrylate-based


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backbone (Tg ≈ 120 oC) underwent nearly complete photo-dimerization after UV irradiation at ambient conditions.49 The highly effective anthracene-dimerization in the glassy state has been attributed to efficient π-π stacking44 between neighboring anthracene groups, presumably established during melt or solution processing, which facilitates the photo-dimerization even if the mobility of the polymer backbones is restricted.49 According to Figure 3b, both G' and G'' decreased continuously with increasing T for the linear MA-nBA-AN sample when T > 90 oC (above Tg), with G'' being consistently higher than G'. These are typical characteristics of a liquid-like solvent-soluble (sol) linear polymer above Tg.28,

37, 64

(Additional frequency sweep experiments at 140 oC showed that G' ~  at low

frequency, indicating liquid-like behavior well above Tg;28, 37, 64 Figure S4) In stark contrast, the G' of the UV-irradiated film (Figure 3c) decreased initially with increasing temperature, then entered a plateau region between ~130 to ~180 oC, and decreased again upon further heating. The presence of a G' plateau above Tg is a typical characteristic of a solid-like elastic material.28, 37, 64

(Additional frequency sweep experiments at 175 oC further demonstrated that G' and G''

exhibited solid-like elastic scaling at low frequency well above Tg;28,

37, 64

Figure S4) These

results are consistent with the linear MA-nBA-AN sample being effectively cross-linked via anthracene photo-dimerization. In addition, UV-cross-linking led to an enhancement in the dynamic mechanical properties, e.g., the cross-linked MA-nBA-NA sample showed G' ≈ 105 Pa, which is three-orders-of-magnitude higher than that of its linear precursor (G' < 102 Pa) at 180 oC.

Importantly, the formed anthracene-dimer cross-links remained intact at temperatures up to ~180 oC during the heating ramp, as indicated by the G' plateau of the cross-linked MA-nBAAN sample between approximately 130 and 180 oC (Figure 3c). This suggests that these acrylic polymer networks containing anthracene-dimer cross-links should possess a relatively high upper service temperature (Tu). For example, the cross-linked film exhibited no observable change in its modulus with increasing annealing time at 150 oC (Figure S5), indicating that the anthracenedimer cross-linked acrylics possess a Tu > 150 oC. This is higher than the Tu values of some



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thermoplastics that are commonly used in the nonwovens industry. For traditional melt blown thermoplastic nonwovens, Tu is determined by either melting temperature (Tm) or Tg of the polymer (e.g., Tu ≈ Tm ≈ 130 oC for polyethylene). Therefore, the anthracene-dimer cross-linked acrylics could potentially replace some traditional thermoplastic feedstocks for certain nonwoven applications.

Figure 3. (a) UV-Vis spectra of a ~3-µm-thick film of linear MA-nBA-AN after UV-irradiation for 0, 1, 3, 5, and 10 min. G' and G'' versus temperature obtained upon heating a ~250-µm-thick film of (b) linear MA-nBA-AN, (c) cross-linked MA-nBA-AN (i.e., linear MA-nBA-AN in (a)


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11 after UV-irradiation at RT for 10 min on each side of the film), (d) decross-linked MA-nBA-AN (i.e., cross-linked MA-nBA-AN in (c) after heating at 225 oC for 10 min), and (e) recross-linked MA-nBA-AN (i.e., decross-linked MA-nBA-AN in (d) after UV-irradiation at RT for 10 min on each side of the film). All samples were annealed at 100 °C for 10 min before rheological measurements. All experiments were performed in the linear viscoelastic region.

Of equal importance is that the anthracene-dimer cross-links possess intrinsic thermoreversibility.45-46 Figure 3c shows that both G' and G'' of the cross-linked MA-nBA-AN film decreased dramatically above 180 oC, with G' evolving faster than G''. G' and G'' crossed at ~205 °C, at which the solid-like elastic material (G' > G'') reverted to a liquid-like material (G'' > G').28, 37, 64 Correspondingly, the DSC thermogram of the cross-linked MA-nBA-AN exhibited an exotherm54,

65

starting at ~175 °C with a peak at ~210 °C (Figure S3). (Note:

thermogravimetric analysis showed that the cross-linked MA-nBA-AN sample was thermally stable up to ~250 °C; Figure S6.) These results can be attributed to cleavage of the anthracenedimers producing free anthracene groups upon heating at elevated temperatures (> 180 oC),45, 54 thus resulting in a decrease in cross-link density (or network connectivity) and ultimately the complete loss of the network structure. The reversibility of the anthracene-dimers was confirmed by the reappearance of the free anthracene absorption peaks54-55, 57 on the UV-Vis spectrum (Figure S7) of the sample obtained by heating the cross-linked MA-nBA-AN at 225 oC for 10 min under nitrogen gas purge. In addition, the resulting sample was soluble in tetrahydrofuran, confirming the loss of network structures. (Thus, the sample obtained by heating the cross-linked MA-nBA-AN at 225 oC for 10 min is noted as decross-linked MA-nBA-AN). It is noteworthy that both Mn (68 kg/mol) and Ð



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12 (4.6) of the decross-linked MA-nBA-AN sample are higher than those of the original linear MAnBA-AN (Figure 2 and Table S1). This could be attributed to the presence of branched (partially dissociated) MA-nBA-AN chains with one or more anthracene-dimer linkages in the decross-linked MA-nBA-AN sample. This is consistent with the anthracene absorbance peaks on the UV-Vis spectrum of the decross-linked MA-nBA-AN film that failed to fully return to their original state as in the linear MA-nBA-AN (Figures 3a and S7). Figure 3d displays the dynamic rheological measurements of the decross-linked MAnBA-AN film. A comparison between Figures 3b and 3d indicates that the linear and decrosslinked

MA-nBA-AN

samples

exhibited

almost

the

same

T-dependences

of

the

rheological/mechanical properties. DSC characterizations also showed that the linear and decross-linked samples exhibited nearly identical Tg values (Figure S3). These results indicate that the cross-linked MA-nBA-AN sample can be reverted to a decross-linked polymer that resembles the original linear MA-nBA-AN. The decross-linked MA-nBA-AN sample can undergo effective anthracene photo-dimerization at RT, resulting in a cross-linked MA-nBA-AN again (Figures S4 and S7). Figure 3e displays the dynamic rheological measurements of the recross-linked MA-nBA-AN film (i.e., the decross-linked film after UV irradiation under similar conditions described above). A comparison between Figures 3c and 3e indicates that the recrosslinked and the original cross-linked MA-nBA-AN samples exhibited similar T-dependent rheological/mechanical properties, both demonstrating characteristics of a cross-linked polymer at lower temperatures (< 180 oC) as well as the ability to reversibly decross-link at elevated temperatures (> 180 oC). Therefore, these anthracene-dimer cross-linked acrylics possess relatively robust thermoreversibility and can be recycled after use because of the reversible nature of the anthracene-dimer cross-links. Future studies are warranted to examine the evolution


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13 of material properties with increasing number of cross-linking/decross-linking cycles. After demonstrating that the linear MA-nBA-AN can undergo effective anthracene photo-dimerization to from cross-linked acrylics in films, we employed the linear MA-nBA-AN samples as the feedstock for producing fiber mats by melt blowing.2-6 Melt blowing is chosen over other fiber spinning techniques (e.g., electrospinning66 or centrifugal spinning23) because of its solvent-free and high-throughput characteristics combined with its industrial relevance. Melt blowing experiments were performed at 175 oC using an experimental set-up based on commercial melt blowing equipment employing a single-orifice with a diameter of 200 μm. (polymer flow rate = 0.15 g/(min-hole); see additional details in Ref. [4] and Supporting Information.) At 175 oC, the zero-shear rate viscosity was determined to be ~100 Pa∙s as shown in Figure S8, which is suitable for melt blowing; typical zero-shear rate viscosities for melt blowing range from ~10 to 500 Pa∙s. The melt blown fiber mats (Figure 4A inset) showed a relatively uniform fiber morphology, as revealed by the scanning electron microscopy (SEM) image in Figure 4A. By applying a log-normal fit4-5 to the fiber diameter distribution, the average diameter of the obtained linear MA-nBA-AN fiber mats was determined to be dav = 6.1 µm (Figure 4b). (We note that these samples are close to the dav of typical melt blown fiber mats,4 which usually range from ~1 to ~20 μm.67) Consistently, the linear MA-nBA-AN fiber mats were completely soluble in tetrahydrofuran and exhibited within error the same Tg values (Figure S9) as those of the linear films.



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Figure 4. Representative SEM images of (a) linear and (c) cross-linked MA-nBA-AN fiber mats as well as (e) cross-linked fibers after the gel fraction test. Statistical analyses of fiber diameters are provided in (b), (d), and (f). The inset in (a) is a representative photograph of the melt blown linear MA-nBA-AN fiber mats. The inset in (b) is a photograph showing cross-linked MA-nBAAN fibers which are insoluble in tetrahydrofuran.

The melt blown linear MA-nBA-AN fiber mats were then subjected to UV-irradiation, following the same procedure (i.e., 10 min exposure on each side of the fiber mats at RT) for cross-linking the linear MA-nBA-AN film. The UV-irradiated fibers were no longer soluble (Figure 4c inset) in tetrahydrofuran (a good solvent for the linear MA-nBA-AN fiber) and


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reached nearly 100% gel content (determined by swelling tests). These results are consistent with the observation in the linear MA-nBA-AN film, i.e., that high levels of cross-linking can be achieved by efficient anthracene photo-dimerization. Compared to the cross-linked MA-nBAAN film (~250-µm-thick), the cross-linked fibers exhibited a slightly higher Tg (Figure S9), indicative of a higher cross-link density in the cross-linked MA-nBA-AN fibers. This difference could be attributed to the fact that the specific irradiation energy (i.e., irradiation energy per unit mass) in the fibers is higher than that in the film (e.g., in a typical UV-cross-linking experiment, ~100 and ~15 mg of the linear MA-nBA-AN film and fibers were used, respectively). Figure 4c shows a representative SEM image of the cross-linked MA-nBA-AN fiber mats. A comparison between Figures 4a and 4c indicates that UV-cross-linking exerted very little effect on the fiber morphology. This is reasonable since significant movement of the polymer backbones (requirement for morphological change) is suppressed or prohibited in the glassy state during the UV-cross-linking process. As shown in Figure 4d, the cross-linked fiber mats exhibit within error the same dav value (5.6 µm) as the linear precursors. The fiber morphology of the cross-linked MA-nBA-AN fiber mats displayed no change after swelling tests and drying (Figure 4e). Additionally, within experimental error the same dav values (Figures 4d and 4f) were obtained for the cross-linked fiber mats before and after swelling tests, consistent with the ~100% gel fraction in the cross-linked fibers. The fiber morphology of the cross-linked MA-nBA-AN fiber mats was nearly unchanged after annealing at 170 oC for 24 h in a convection oven (Figure S9), whereas their linear precursors were converted into a droplet morphology immediately after reaching 170 oC. Therefore, UV-cross-linking of the linear acrylic fiber mats enabled them to be used at higher temperatures. Similarly, the cross-linked MA-nBA-AN fibers exhibited thermoreversibility at elevated temperatures because of the dynamic nature of the anthracene-dimer cross-links, e.g., after annealing at 225 oC for 10 min, the Tg of the cross-linked MA-nBA-AN fibers decreased to a value close to that of their linear precursors, as shown by DSC characterizations (Figure S10).



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Figure 5 demonstrates that these reversibly cross-linked fiber mats can be recycled into bulk films (by melt pressing at 205 oC for 3 min), or secondary fibers, because of this dynamic nature. The recycled bulk materials (e.g., the recycled film in Figure 5) can undergo effective crosslinking again upon UV irradiation, resulting in a recross-linked sample with nearly the same rheological/mechanical properties as those of the original cross-linked sample (Figures S4 and S11).

Figure 5. Robust fiber/film recycling of the anthracene-dimer cross-linked MA-nBA-AN. Crosslinked fibers can be melt pressed at 205 oC into a bulk film, which could be processed into fibers again by melt blowing.

Finally, in place of relatively more energy-intensive UV light, we further demonstrate that sunlight can also effectively trigger the anthracene photo-dimerization to cross-link the linear acrylic copolymer fibers. After exposure to sunlight for a prolonged time (e.g., ~10 days), cross-linked MA-nBA-AN fibers were obtained. These fibers exhibited nearly the same physical properties (e.g., gel fraction, Tg, and thermoreversibility) as those for the UV-cross-linked fibers discussed earlier. Future studies will examine the full development of the cross-link density and related materials properties with increasing irradiation time under either sunlight or UV light. Therefore, these anthracene-functionalized acrylic fibers with high surface-to-volume ratio can effectively “harvest” solar energy to trigger anthracene photo-dimerization, resulting in cross-


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linked fibers with enhanced material properties and robust thermoreversibility.

CONCLUSIONS In conclusion, a high-throughput route to cross-linked fibers was demonstrated using an amorphous, linear anthracene-functionalized acrylic copolymer that was first melt blown into fibers, followed by irradiation with either UV light or sunlight to induce the anthracenedimerization cross-linking reaction. The photo-dimerization between pendant anthracene groups is highly effective even in the glassy state, resulting in anthracene-dimer cross-linked fibers with nearly 100% gel content. On one hand, the obtained cross-linked fibers exhibit enhanced thermomechanical properties and higher upper service temperatures relative to their linear precursors, introducing potential replacement of traditional thermoplastic nonwovens for certain applications. The approach outlined here could be applied to numerous other commodity thermoplastics (e.g., styrenics, silicones, etc.) to achieve cross-linked fibers for desired applications. On the other hand, the cross-linked fibers obtained using this strategy are intrinsically reprocessable/recyclable because of the dynamic nature of the anthracene-dimer cross-links. This strategy could promote sustainability for nonwoven products, potentially alleviating the building microplastic pollution problem.

EXPERIMENTAL SECTION Materials and detailed experimental procedures are provided in the Supporting Information.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: xxx/xxx.xxx. Experimental methods as well as Figures S1-S12 and Table S1 illustrating additional



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characterizations.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge Cummins Filtration for funding. Partial support was provided by the Center for Sustainable Polymers, a National Science Foundation (NSF)-supported Center for Chemical Innovation (CHE-1413862). SEM 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 (NSF-MRSEC) program.

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