Search

Most nonwoven fiber mats are produced with a uniform, narrow fiber diameter distribution. However, building evidence suggests that a bimodal diameter distribution (i.e., comprised of two populations of fibers, one with a smaller average diameter (dav), d, and the other with a larger dav, D, where D≥ 5d), has certain advantages in applications such as filtration media. To the best of our knowledge, all previous reports describing production of bimodal fiber diameter distributions have relied on solution-based electrospinning, a much less common fiber-spinning technique, compared to melt blowing, which currently produces more than 10% of nonwovens globally (an approximately $50 billion market). In this study, we demonstrate a facile method for producing bimodal fiber diameter distributions by melt blowing immiscible ternary polymer blends, with the two minority blend components being randomly dispersed as isolated, bimodally sized particles within the continuous matrix. Such a ternary blend can be obtained by selecting a matrix phase that preferentially wets/encapsulates both dispersed phases having vastly different viscosity ratios. Specifically, two model immiscible ternary blends comprised of polystyrene/polyethylene/Nylon-6 (PS/PE/Nylon) and poly(ethylene oxide)/polyethylene/Nylon-6 (PEO/PE/Nylon) with the desired morphologies and PS or PEO as the matrix were examined. During melt blowing of the blends, the PE minority domains (∼8 μm in diameter) and Nylon minority domains (∼70 μm in diameter) dispersed within the matrix were transformed to PE nanofibers (dav≈ 560 nm) and Nylon microfibers (dav≈ 8 μm) embedded in the elongated PS matrix fibers, and similarly for fibers made with a PEO matrix. Subsequent removal of the matrix polymer with an appropriate solvent (tetrahydrofuran for PS or water for PEO) produced a macroscopic mat of randomly distributed, bimodal nanofibers and microfibers. The nanofiber and microfiber compositions of the bimodal-diameter fiber mats were effectively tuned by varying the composition of the minority components of the original ternary polymer blends. Interestingly, the resulting bimodal-diameter fiber mats possessed few nanofiber bundles; we hypothesize that this is due to the presence of Nylon microfibers that are intrinsically intermixed among the PE nanofibers that physically restrict the formation of nanofiber bundles. Overall, this versatile method could provide a high-throughput route to scalable quantities of fiber mats with a bimodal distribution of fiber diameters, thus promoting the application of hierarchically structured nonwovens.

Poly(lactide) (PLA) is currently the most successful bioderived synthetic polymer. However, applications of PLA are limited due to brittleness, which develops after about 1 day of physical aging at room temperature following melt processing. In this study, we investigate the ability of low molecular weight poly(isoprene)-block-poly(ethylene oxide) (IO) diblock copolymers to toughen glassy PLA. Melt blending IO with PLA leads to submicron diameter macrophase separated IO domains in the PLA matrix, stabilized by surfactant-like behavior associated with compatibility between the poly(ethylene oxide) (O) blocks and PLA. IO molecular weights that bracket the room temperature order-to-disorder transition temperature, Mn= 1.7–3.3 kg/mol, produce outstanding toughness with strains at break εb> 150%, which persists through 9 days of aging at room temperature. Higher molecular weight IO diblocks do not perform as well. We hypothesize that this behavior reflects two unique characteristics: (1) optimal particle sizes leading to cavitation and craze initiation when subjected to triaxial strain, and (2) draining of IO into the advancing crazes thereby stabilizing the porous craze structures. Experiments with melt mixed poly(isoprene) (I) homopolymer in PLA containing controlled particle sizes support this mechanism. These results demonstrate a general strategy for designing block copolymer additives that toughen otherwise brittle plastics.

The structure, stability, and reorganization of lamella-forming block copolymer thin film surface topography (“islands” and “holes”) were studied under boundary conditions driving the formation of 0.5 L0thick structures at short thermal annealing times. Self-consistent field theory predicts that the presence of one perfectly neutral surface renders 0.5 L0topography thermodynamically stable relative to 1 L0thick features, in agreement with previous experimental observations. The calculated through-film structures match cross-sectional scanning electron micrographs, collectively demonstrating the pinning of edge dislocations at the neutral surface. Remarkably, near-neutral surface compositions exhibit 0.5 L0topography metastability upon extended thermal treatment, slowly transitioning to 1 L0islands or holes as evidenced by optical and atomic force microscopy. Surface restructuring is rationalized by invoking commensurability effects imposed by slightly preferential surfaces. The results described herein clarify the impact of interfacial interactions on block copolymer self-assembly and solidify an understanding of 0.5 L0topography, which is frequently used to determine neutral surface compositions of considerable importance to contemporary technological applications.

Two high-χ block copolymers, lamella-forming poly(styrene-block-[isoprene-random-epoxyisoprene]) (PS-PEI78, with 78 mol % epoxidation) and lamella-forming poly(4-trimethylsilylstyrene-block-d,l-lactide) (PTMSS-PLA), were used to study three combinations of interfacial neutrality involving at least one neutral interface. PS-PEI78 annealed on a nonpreferential polymer mat (SMG) produced perpendicular lamellae independent of film thickness, indicating a neutral substrate and neutral free surface. In contrast, the presence of only one neutral interface results in the formation of surface topography (“islands” and “holes”) with 0.5L0step heights. PS-PEI78 (neutral free surface) annealed on PS brush (PS block preferential) forms “half” islands and holes. The inverse experiment, PTMSS-PLA (with a PTMSS preferential free surface) annealed on a neutral (or near neutral) substrate surface, also generates 0.5L0topography. These “half” island and hole structures are stable to extended thermal annealing. PS-PEI78 exposes both blocks at the free surface in contrast to PTMSS-PLA, which exposes just one. All three combinations of interfacial neutrality are explained by the precise balancing of the wetting tendencies of the two blocks. Evolution of the 0.5L0motifs appears to be facilitated by a preference to form half-period thick nuclei in the initial stages of morphological development.

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.