Melt-Blown Cross-Linked Fibers from Thermally Reversible Diels

Oct 17, 2018 - Melt blowing is a process in which liquid polymer is extruded through orifices and then drawn by hot air jets to produce nonwoven fiber...
0 downloads 0 Views 424KB Size
Download PDF
Letter Cite This: ACS Macro Lett. 2018, 7, 1339−1345

pubs.acs.org/macroletters

Melt-Blown Cross-Linked Fibers from Thermally Reversible Diels− Alder Polymer Networks Kailong Jin, Sung-soo Kim, Jun Xu, Frank S. Bates,* and Christopher J. Ellison* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States

Downloaded via UNIV OF TEXAS AT EL PASO on October 17, 2018 at 13:56:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Melt blowing is a process in which liquid polymer is extruded through orifices and then drawn by hot air jets to produce nonwoven fibers with average diameters typically greater than one micron. Melt-blown nonwoven fiber products constitute a significant fraction (i.e., more than 10%) of the $50 billion global nonwovens market. Thermoplastic feedstocks, such as polyethylene, polypropylene, poly(phenylene sulfide), and poly(butylene terephthalate), have dominated melt-blown nonwovens because of their combined cost, good chemical resistance, and high-temperature performance. Cross-linked nonwovens from other commodity polymers (e.g., (meth)acrylates, styrenics, silicones, etc.) could be attractive alternatives; however, no commercial cross-linked nonwovens currently exist. Here, crosslinked fibers were produced via one-step melt blowing of thermoreversible Diels−Alder polymer networks comprised of furanand maleimide-functional methacrylate-based polymer backbones. These dynamic networks de-cross-link and flow like viscous liquids under melt-blowing conditions and then revert to a network via cooling-induced cross-linking during/after melt blowing. Finally, the resulting cross-linked fibers can be recycled after use because of their reversible dynamic nature, which may help address microfiber waste as a significant source of microplastic pollution.

N

suitable for melt blowing/processing because permanent covalent bonds obviate flow. Generally, reactive monomer mixtures, e.g., amine and epoxy monomers,12 etc., may also react during processing which could damage processing equipment. Moreover, most reactions are too slow relative to characteristic processing speeds (i.e., fiber flight time) to be successfully implemented. Despite these challenges, researchers have made some progress toward producing cross-linked fibers. One approach by Montgomery et al.13 involved a thermoplastic polyester containing UV-cross-linkable stilbene groups first processed into fibers by melt blowing, followed by UV irradiation in a second step to trigger cross-linking between stilbene groups.13 To the best of our knowledge, this is the only report describing cross-linked melt-blown fibers.13 A potential limitation of this strategy is that UV-induced cross-linking could suffer lightshadowing effects near crossing points of nearby fibers, an effect expected to be amplified in “thick” or multilayer nonwoven mats. Furthermore, adequate light penetration into the fiber interior could be challenging if light-activated species are too concentrated or light scattering from crystalline domains in semicrystalline fiber is significant; both scenarios may predominantly cross-link the surface, leaving a significant amount of solvent-soluble material in the interior.13 A number

onwovens consist of randomly, or occasionally directionally oriented, polymer fiber mats. Nonwoven products are a $50 billion industry with applications ranging from disposable wipes to filtration media.1 Melt blowing2−6 can produce nonwovens rapidly and economically in a single step due to its solvent-free and high-throughput characteristics; as a result, more than 10% of global nonwovens are produced by this process. Melt blowing combines extrusion of a polymer through small orifices in a die with drawing of the extrudate by hot high-velocity air jets to form fine liquid polymer filaments. Within a few centimeters of exiting the die, the liquid filaments are cooled below their solidification temperature4−7 (e.g., glass transition temperature, Tg, or crystallization temperature, Tc) by entrained ambient air, and then solid fibers are deposited onto a collector. Importantly, an appropriate viscosity is needed for successful processing (Supporting Information describes typical melt-blowing conditions),1 and linear thermoplastic polymers (e.g., poly(butylene terephthalate), poly(phenylene sulfide), polyethylene, and polypropylene, etc.) with relatively low melt viscosities are typical.2−6 Cross-linked fibers are highly desirable because of their tailorable properties (e.g., modulus, elastic recovery, etc.) and superior thermal/chemical resistance over linear thermoplastics.8,9 For example, cross-linked fibers are attractive for both harsh thermal/chemical filtration and applications seeking soft materials, such as biological tissue scaffolds and hydrogels;10,11 the latter additionally requires biocompatibility and toxicity considerations. However, conventional cross-linked thermosets8 (e.g., epoxies, vulcanizates, etc.) are generally not © XXXX American Chemical Society

Received: September 7, 2018 Accepted: October 12, 2018

1339

DOI: 10.1021/acsmacrolett.8b00685 ACS Macro Lett. 2018, 7, 1339−1345

Letter

ACS Macro Letters of other approaches10,11,14−23 for cross-linked fibers have also been reported that are based on either electrospinning or centrifugal spinning which are much less common in industrial production of nonwovens. Examples include fibers formed by either in situ, simultaneous UV curing of liquid monomer jets at room temperature during fiber spinning,11,14−16 or processes which incorporate a second cross-linking step (e.g., by thermal or UV curing) after fiber spinning functionalized materials.10,17−21 In this study, we introduce a one-step approach for producing cross-linked fibers by melt blowing a thermoreversible polymer network9,24−39 with dynamic cross-links. Unlike conventional thermosets, reversible networks can undergo dynamic molecular rearrangement to achieve macroscopic flow in response to external stimuli (e.g., heat), exhibiting selfhealing capability and reprocessability/recyclability.9,24−39 Here, a thermoreversible network comprised of methacrylatebased backbones with functional pendants that can participate in covalent cross-linking by forming Diels−Alder linkages was selected. A Diels−Alder reaction9,24−35 is defined as a [4 + 2] cycloaddition between a conjugated diene (e.g., furan) and a dienophile (e.g., maleimide) (Figure 1A). Below a certain

they possess an appropriate viscosity for melt blowing, while upon cooling during/after melt blowing, the Diels−Alder reaction again produces cross-linked fibers. Reversibly crosslinked fibers can be recycled because of their dynamic nature, potentially promoting sustainability for nonwoven fiber products. The need for research on sustainable fibers has been highlighted in a recent article in C&EN news that has stated “microfibers are the biggest source of plastic pollution you haven’t heard of yet”.40 The thermoreversible furan−maleimide Diels−Alder network was synthesized by mixing a linear random copolymer, poly[(furfuryl methacrylate)-co-(butyl methacrylate)] (FMABMA; Figures 1B and S1 and Table 1), prepared by free radical Table 1. FMA-BMA Copolymer Synthesized by Free Radical Copolymerization synthesized copolymer

Mna (kg/mol)

Đa (Mw/Mn)

FMAb mol %

Tgc (±1 °C)

FMA-BMA

17.0

1.5

14.5

27.5

a

Measured by SEC coupled with a multiangle light-scattering detector. bMeasured by 1H NMR. cTg,1/2ΔCp during second heating by DSC.

polymerization having pendant furan groups, with a commercially available small-molecule bismaleimide (M2; Figure 1B). This model system was synthetically convenient, and the methacrylate-based backbone is potentially attractive to the nonwovens industry. We expect that a nearly unlimited number of material variations on this theme could be successfully implemented. To provide brief experimental details, FMA-BMA and M2 were codissolved in dichloromethane, followed by the removal of solvent under vacuum and curing at room temperature (RT) (Figure 1C). The FMABMA/M2 bulk mixture hereafter consists of 14.5−85.5 mol % of FMA-BMA random copolymer (number-average molecular weight, Mn = 17 kg/mol) with a stoichiometric equivalent amount of furan and M2 maleimide functional groups (confirmed by proton nuclear magnetic resonance, 1H NMR;33 Figure S3). Figure 2 shows the evolution of gel fraction (from swelling tests) and Tg (Tg,1/2ΔCp from differential scanning calorimetry, DSC) with annealing time at RT for the bulk FMA-BMA/M2 mixture. (While Figures 2

Figure 1. (A) Thermoreversible furan−maleimide Diels−Alder reaction. (B) FMA-BMA copolymer and M2 monomer structures. (C) FMA-BMA/M2 networks synthesized through a Diels−Alder reaction can undergo de-cross-linking through a retro-Diels−Alder reaction upon heating.

temperature (usually ∼100 °C), furan−maleimide linkages remain connected, and the Diels−Alder network behaves like a thermoset. At elevated temperatures (>100 °C), furan− maleimide linkages cleave and revert to free furan and maleimide functionalities through a retro-Diels−Alder reaction, forming de-cross-linked materials with thermoplastic characteristics.9,24−35 Upon heating to a certain temperature,

Figure 2. Gel fraction and Tg versus annealing time at RT for bulk FMA-BMA/M2 and corresponding fibers. Error bars are standard deviations from three independent measurements. 1340

DOI: 10.1021/acsmacrolett.8b00685 ACS Macro Lett. 2018, 7, 1339−1345

Letter

ACS Macro Letters and 3 both contain bulk and fiber data, the current discussion will be focused on the bulk data with comparisons between the

maleimide networks.26,27,31 The final conversion is mainly dictated by the thermodynamic equilibrium of the Diels−Alder reaction.27 However, cross-linking may also limit chain mobility and topologically hinder furan and maleimide groups from collocating to undergo further reaction. The thermoreversibility of the fully cured, bulk FMA-BMA/ M2 networks (after annealing at RT for 168 h) was examined further by DSC and FTIR. Figure 3B shows DSC thermograms, collected on the second heating after quenching from a specific Tannealing to a lower starting temperature at 20 °C/min. When the fully cured FMA-BMA/M2 bulk sample was heated (after holding at Tannealing = 50 °C for 5 min to remove thermal history), the sample (thermogram 1 in Figure 3B) exhibited a Tg at ∼39 °C and an endotherm starting at ∼90 °C with a peak at ∼145 °C, corresponding to the dissociation of furan− maleimide linkages (or decross-linking) through the retroDiels−Alder reaction.9,30 Thermogram 2 was obtained on a previously fully cured FMA-BMA/M2 bulk network after holding at Tannealing = 162 °C for 15 min, which showed a decreased Tg of ∼5 °C. This provides supporting evidence that the annealed sample underwent de-cross-linking; the full recovery of furan and maleimide moieties was also confirmed by FTIR (Figure 3A).29,32 Additionally, thermogram 2 showed a small exothermic peak starting at ∼70 °C (heat flow rises above the red dashed line, prior to the endothermic dissociation process); this thermal feature is likely due to the reconnection of individual furan and maleimide groups as they gain mobility upon heating in analogy to cold-crystallization.9 Thermogram 3 was obtained on a cured FMA-BMA/M2 network after annealing at 162 °C for 15 min, followed by quenching to RT at 20 °C/min and maintaining at RT for 120 h. This sample (thermogram 3 in Figure 3B) exhibited the same Tg as the originally fully cured FMA-BMA/M2 bulk network shown as thermogram 1, confirming the robust thermoreversibility of the furan−maleimide network. Such excellent reversibility can be attributed to the selection of furan and maleimide which allows the retro-Diels−Alder reaction to occur without significant side reactions.34,43−46 Thermograms 4 and 5 in Figure 3B show the FMA-BMA and M2 pure components, respectively, confirming that they are thermally stable over the temperature range of interest. Dynamic rheological measurements were performed to verify the melt processability of the FMA-BMA/M2 networks. Figures 4A and 4B show the elastic (G′) and viscous (G″) moduli versus temperature when cooling from ∼160 °C at 5 °C/min for the FMA-BMA/M2 bulk mixture and neat FMABMA. (M2 is a liquid at RT and cannot generate enough torque for proper rheological measurements at higher temperatures.) According to Figure 4A, G″ > G′ for the FMA-BMA/M2 mixture at higher temperatures (> ∼152 °C), characteristic of a liquid-like solvent-soluble material; additionally, frequency sweep experiments at 160 °C confirmed that the moduli exhibited liquid-like scaling at low frequency (Figure S6).27,35,47 Thus, at higher temperatures, the bulk FMA-BMA/M2 sample is in the de-cross-linked state, allowing for melt processing. Upon cooling, G′ increased faster than G″, and a crossover in G′ and G″ was observed at ∼152 °C. The crossover temperature, Tcrossover, is usually taken as the gel point.27,35,47 We note that Tcrossover is different from Tonset (∼100 °C) for the dissociation of furan−maleimide linkages. Tcrossover is probably the most important factor that dictates the upper use temperature of these materials in fiber applications. Tcrossover

Figure 3. (A) FTIR spectra of nonreacted and fully cured FMABMA/M2 bulk samples, fully cured fiber, and the bulk sample after annealing at 162 °C for 15 min. (B) DSC thermograms for cured FMA-BMA/M2 bulk mixtures after (1) annealing at 50 °C for 5 min, (2) annealing at 162 °C for 15 min, and (3) annealing at 162 °C for 15 min, followed by quenching to RT at 20 °C/min and maintaining at RT for 5 days. Other thermograms are (4) neat FMA-BMA and (5) neat M2.

two coming later.) At 0 h (immediately after solvent removal), the nearly unreacted FMA-BMA/M2 mixture was soluble in dichloromethane, indicating a gel fraction of 0%. At 16 h of annealing time at RT, the partially reacted sample was insoluble and had a gel fraction of 82(±7)%, indicative of cross-linked network formation.41 After ∼120 h, the gel fraction reached a plateau value of 97(±3)%, indicating that the bulk FMA-BMA/M2 mixture reached the full gel state (within experimental error). According to Figure 2, the Tg of the bulk FMA-BMA/M2 mixture initially increased over time due to the formation of furan−maleimide linkages which inhibit chain mobility. After ∼100 h at RT, the Tg reached a plateau value of ∼39 °C, ∼10 °C higher than that of the linear FMA-BMA precursor (Tg ≈ 28 °C) because of cross-linking. This indicates that the bulk FMA-BMA/M2 mixture achieved an equilibrated state at RT,27 consistent with the gel fraction results. (Note: the Diels−Alder reaction rate can be accelerated by optimizing curing temperature,30,37 i.e., the time required to reach equilibrium decreases from ∼100 h at RT to less than 1 h at 60 °C.30) Additionally, the Tg of (partially) cross-linked FMABMA/M2 is relatively broad, indicative of heterogeneous dynamics within the network.42 Note that the Tg breadth is characterized by the temperature range of the first derivative peak in the DSC thermogram as shown in Figure S4; Tg breadth values are also summarized in Table S2. The Diels−Alder reaction was also confirmed by Fourier transform infrared spectroscopy (FT-IR; Figure 3A). A comparison between the FTIR spectra of nonreacted and cured FMA-BMA/M2 bulk samples shows that after curing both furan (∼1015 cm−1; ring breathing) and maleimide (∼695 cm−1, C−H bending) peaks decreased, whereas a new peak (∼1775 cm−1) specific to furan−maleimide adducts appeared.26−29 The final conversion of the stoichiometric furan−maleimide reaction at RT was determined to be ∼85% (Supporting Information), close to previously reported furan− 1341

DOI: 10.1021/acsmacrolett.8b00685 ACS Macro Lett. 2018, 7, 1339−1345

Letter

ACS Macro Letters

could be decreased by reducing the FMA-BMA copolymer molecular weight (see Table S1 and Figure S7). Below Tcrossover, G′ entered a rubbery plateau between ∼110 and ∼70 °C. Correspondingly, a frequency sweep of the FMA-BMA/ M2 sample at 90 °C revealed a G′ plateau at low frequency, characteristic of a solid-like elastic material (Figure S6). In contrast, the neat FMA-BMA copolymer showed liquid-like behavior with G″ > G′ above Tg (∼30 °C; Figure 4B). These data indicate that, upon cooling, the forward Diels−Alder reaction occurred in the FMA-BMA/M2 bulk mixture, resulting in a rheological transition when sufficient crosslinks form in the FMA-BMA/M2 sample.8,27 During the cross-linking transition, the complex viscosity (η*) of the FMA-BMA/M2 sample exhibited a dramatic (>3 orders of magnitude) increase within a relatively narrow temperature range (e.g., η* ≈ 10 Pa s at ∼160 °C and η* > 104 Pa s at ∼125 °C; Figure 4C); in contrast, the neat FMA-BMA copolymer showed a gradual η* increase. The dramatic increase in η* for the bulk FMA-BMA/M2 sample is due to the formation of network structures upon cooling, which greatly hinder chain motion. This is analogous to crystallization in semicrystalline polymers,7,8 during which the formation of immobile crystalline regions greatly reduces chain mobility, increasing the viscosity of the system. (Also note that crystallization is a common solidification mechanism for melt blown fibers.4−7) In general, the data in Figures 4A−4C suggest that thermoreversible furan−maleimide networks are rheologically suitable for melt blowing. At higher temperatures, the de-cross-linked state with a relatively low viscosity allows for extrusion and fiber attenuation; upon cooling during/after melt blowing, the viscosity increases dramatically, and fiber solidification occurs. This supports the

Figure 4. G′ and G″ versus temperature taken upon cooling for (A) the bulk FMA-BMA/M2 mixture and (B) neat FMA-BMA copolymer. (C) η* versus temperature taken upon cooling for the bulk FMA-BMA/M2 mixture and neat FMA-BMA copolymer. (D) η* versus frequency at various temperatures for the bulk FMA-BMA/M2. All samples were annealed at 162 °C for 15 min (except those in D, which were preannealed for 15 min at the indicated temperatures) before rheological measurements in the linear viscoelastic region.

Figure 5. (A) Representative SEM images of melt-blown FMA-BMA/M2 fibers with a polymer flow rate of 0.2 g/(min hole). The inset in (A) is a representative photograph of the fiber mat. (B) Statistical analyses of fiber diameters. (C) Demonstration of robust film/fiber recycling for the FMA-BMA/M2 samples: as-obtained FMA-BMA/M2 films were melt blown into cross-linked fibers, which were recycled into films again by pressing at 165 °C for 5 min. This recycling process can be repeated because of the dynamic nature of the FMA-BMA/M2 samples. 1342

DOI: 10.1021/acsmacrolett.8b00685 ACS Macro Lett. 2018, 7, 1339−1345

Letter

ACS Macro Letters idea of a new solidification mechanism for melt-blown fibers via cooling-induced cross-linking, distinguishing itself from conventional solidification mechanisms caused by fiber cooling below Tg or Tc.4−7 To identify suitable melt-blowing conditions, frequency sweeps were performed at different temperatures. Figure 4D shows η* versus frequency (equivalent to steady shear viscosity by Cox−Merz rule48) for the bulk FMA-BMA/M2 sample. Zero-shear rate viscosity (η0) at 162 °C is estimated to be ∼100 Pa s, a viscosity suitable for melt blowing.2−6 Additionally, η* exhibited a minor increase with time at 162 °C (∼8% increase after ∼15 min; Figure S8) indicating the viscosity could be maintained during melt blowing (