Poly(Methyl Methacrylate)-Based Core-Shell Electrospun Fibers: Structural and Morphological Analysis

Abstract

Dicyclopentadiene (DCPD)–poly(methyl methacrylate) (PMMA) core–shell fibers were fabricated via coaxial electrospinning to develop a self-healing polymer composite. A PMMA shell containing a first-generation Grubbs catalyst was co-spun with a DCPD core at 0.5 mL h⁻¹ and 28 kV, yielding smooth, cylindrical fibers. The diameter range of nanofibers was 300–900 nm, with 95% below 800 nm, as confirmed by FESEM image analysis. FTIR spectroscopy monitored shell integrity via the PMMA C=O stretch and core polymerization via the trans-C=C bands. The high presence of the 970 cm⁻¹ band in the healed nanofiber mat and the minor appearance in the uncut core–shell mat demonstrated successful DCPD polymerization mostly where the intended damage was. The optical clarity of PMMA enabled the direct monitoring of healing progress via optical microscopy. The presented findings demonstrate that PMMA can retain a liquid active core and catalyst to form a polymer layer on a damaged site and could be used as a model material for other self-healing systems that require healing monitoring.

1. Introduction

Cracking in polymer matrix composites leads to significant economic losses because the repair and replacement of damaged components incur high costs. Unlike traditional materials that require manual intervention whenever damage occurs, self-healing polymers can autonomously restore their structural integrity, extending service life, enhancing safety, and reducing maintenance expenses. Drawing inspiration from biological systems, self-healing strategies have evolved rapidly over the past two decades, yielding a variety of formulations and healing mechanisms [1,2,3,4,5]. Early self-healing composites commonly relied on microcapsule-based architectures. In these systems, healing agents such as dicyclopentadiene (DCPD) or isophorone diisocyanate are encapsulated within polymer shells and mechanically mixed into epoxy matrices reinforced with glass or carbon fibers [6,7]. When a crack propagates, it ruptures nearby capsules, releasing monomer that polymerizes upon contact with an embedded catalyst, often a Grubbs ruthenium-based complex, thereby sealing the damage. Alternative approaches have filled hollow capillaries with healing monomers and crosslinkers, providing a continuous supply of repair agents but posing challenges in capillary integration [8].

Electrospinning has emerged as an attractive, versatile alternative for fabricating fiber mats with self-healing potential. In a typical electrospinning setup, a high-voltage electric field draws a charged polymer solution or melts into a fine jet, which thins into nanofibers as the solvent evaporates or the polymer cools [9]. The resulting nonwoven mats boast exceptionally high surface-area-to-volume ratios, tunable porosity, and customizable fiber alignment, making them ideal candidates for tissue engineering scaffolds, drug delivery vehicles, sensor substrates, and protective textiles [10,11]. By varying solution concentration, applied voltage, flow rate, needle–collector distance, and collector design, one can precisely tailor fiber diameter, orientation, and mat porosity to meet the demands of specific applications. Functionalization is straightforward: nanoparticles, drugs, or catalysts can be blended into the spinning solution to produce one-step functional fibers. Despite the successes of single-fluid electrospinning, encapsulating sensitive agents and isolating incompatible components within a single fiber often prove challenging. Coaxial electrospinning allows the encapsulation of sensitive or incompatible components, such as healing monomers and catalysts, within distinct core and shell domains of the same fiber [12,13]. This architecture has proven particularly useful for localized chemical activation, including applications in catalysis, damage-triggered polymerization, and responsive membranes [14]. Furthermore, while many materials are vulnerable to crack propagation, the advantage of electrospun core–shell fibers lies not only in their ability to localize mechanical failure but also in their potential to deliver site-specific healing agents upon damage, thus actively mitigating crack growth and structural degradation [15]. Recent studies illustrate the power of coaxial designs. Wang and colleagues fabricated polyetherimide@polyaniline core–shell membranes showing enhanced charge storage in flexible supercapacitors [16]. Fan et al. produced polyurethane/carbon-nanotube cores with polyurethane/Fe₃O₄ shells that combine EMI shielding, electrical heating, and biomotion sensing while retaining durability and washability [17]. Ortega et al. incorporated Aloe vera mucilage into core–shell scaffolds to achieve excellent biocompatibility and mechanical performance for tissue engineering [18]. Shao et al. used a dual-nozzle spinneret to create hollow PVDF fibers with tunable wall thickness, optimizing their piezoelectric output [19]. Chen and co-workers applied coaxial electrospinning to integrate epoxy healing agents into PAN fibers, yielding composites that regained 93% flexural strength after healing at 130 °C for 20 min and showed a 16% increase in bending strength with fiber incorporation [15].

The mechanical robustness imparted by shell polymers, such as poly(methyl methacrylate) (PMMA), can be combined with the reactivity of core materials like DCPD to achieve autonomous, chemically controlled self-healing. PMMA in particular has garnered attention not only for its transparency and mechanical strength but also for its resistance to environmental degradation, making it highly suitable as a coating matrix. In protective and functional coatings, PMMA contributes to high weather ability, potential for UV resistance, and chemical stability [20]. UV-resistant PMMA, such as the one described by Martínez-García et al., retains 90% UVA transmittance and 45–88% UVB transmittance, with no significant degradation after 900 h of accelerated aging or 9 months in natural sunlight [21]. Compared to other polymers like PET (52% UVA transmission) or polycarbonate (33% UVA), UV-resistant PMMA offers superior photostability and longevity, making it particularly suitable for optical or protective applications requiring long-term exposure to sunlight. Electrospun PMMA nanofibers for energy-harvesting applications showed improved surface potential, triboelectric performance, and tunable surface chemistry via polarity-controlled electrospinning [22]. Additionally, PMMA-based scaffolds have shown promising cytocompatibility and cell-integration behavior for biomedical coatings on metallic implants, highlighting their biocompatibility and versatility [23,24]. In anticorrosive barrier coatings, hybrid PMMA–silica formulations demonstrate exceptional thermal stability and impedance properties, making them ideal for long-term protection in saline or harsh environments [25,26]. Although these advances have validated the promise of coaxial fibers, most studies rely on qualitative assessments of core–shell integrity by observing intact or ruptured fibers via electron microscopy or noting macroscopic healing outcomes. The quantitative relationship between fiber structure and chemical performance remains poorly understood.

In this work, we focus on PMMA shells containing a first-generation Grubbs catalyst and DCPD monomer core, a combination that offers robust mechanical support, optical clarity, and ring-opening metathesis polymerization-driven self-healing. We introduce a dual-analysis framework that quantitatively links morphology and chemistry, moving beyond qualitative characterizations. Field-emission scanning electron microscopy (FESEM) gives insight into diameter distributions, representing morphological dispersity. Fourier-transform infrared (FTIR) spectroscopy, employing spatial chemical mapping, monitors PMMA shell integrity and DCPD polymerization. Our integrated approach provides a rapid, non-destructive quality-control metric for coaxial electrospinning. By targeting specific diameter distributions, researchers can predict and ensure shell integrity, precluding premature core release and enhancing self-healing efficacy.

2. Materials and Methods

Polymethyl methacrylate Acryrex^® CM205 (Chi Mei Corp., Rende, Tainan, Taiwan, MW ≈ 90,400 g mol⁻¹) pellets were used as a shell material. The first-generation Grubbs catalyst (Bis(tricyclohexylphosphine)benzylidine ruthenium(IV) dichloride) and dimethylformamide 99.8% (DMF) were purchased from Sigma-Aldrich, St. Louis, MO, USA. DCPD was purchased from Acros Organics, Geel, Belgium.

2.1. Coaxial Electrospinning

The shell of the electrospun coaxial fibers was made of PMMA combined with the first-generation Grubbs catalyst, while the core contained a mixture of DMF and DCPD in a 90:10 volumetric ratio. The concentration of PMMA in the DMF solution prepared for electrospinning was 22 wt% [27]. The solid Grubbs catalyst was added after the homogenization of the solution. The concentration of the catalyst in the PMMA shell was 5 wt%. The solution of the catalyst had a purple color during preparation and electrospinning, indicating that the catalyst remained active in DMF due to protection by PMMA. Electrospinning (Electrospinner CH-01, Linari Engineering, Pisa, Italy) was performed with the following setup: 20 mL plastic syringes were connected to coaxial needles; the inner needles (D = 0.45/0.85 mm) were concentrically placed in the outer needles (D = 1.37/1.83 mm), with a 15 cm distance to the needle tip from the collector; and the high-voltage power supply (Spellman High Voltage Electronics Corporation, Hauppauge, NY, USA, Model: PCM50P120) was set to a voltage of 28 kV at room temperature (25 °C) and a humidity of 48%. Fibers were collected using a rotating cylinder at a speed of 300 rpm, to obtain a multilayered mat. The flow rate of both PMMA solutions with Grubbs’ catalyst and the DCPD/DMF core was kept constant at 0.5 mL h⁻¹. The setup for coaxial electrospinning is presented in Figure 1.

2.2. Characterization of Samples

For the morphological analysis of samples, field emission scanning electron microscopy (FESEM) was utilized with Tescan Mira 3 instruments (Brno, Czech Republic), where gold was sputtered before imaging. Particle size was determined by analysis of three FESEM images using the software Image-Pro Plus 6.0 (Rockville, MD, USA). For the structural analysis of PMMA and composites, Fourier transform infrared spectroscopy (FTIR) was performed using a Thermo Scientific Nicolet iS10 spectrometer, Waltham, MA, USA, in the range from 4000 to 500 cm⁻¹, with a resolution of 4 cm⁻¹.

3. Results and Discussion

3.1. Morphology of Electrospun Fibers

Under 100k× magnification, FESEM revealed the cylindrical morphology of the coaxial PMMA fibers, with no visible pores (Figure 2a). The absence of the catalyst’s agglomerates suggested fine dispersion in the PMMA fibers. As shown in Figure 2b, the evaporation of DCPD/DMF was captured, proving that the core remained intact during electrospinning, which is of great importance for showing the suitability of the processing technique. Furthermore, bead formation was observed, but in small amounts compared to the fiber’s regular cylindrical shape.

Statistical analysis of the FESEM images showed that the fiber diameter range was from 0.3 µm to around 0.9 µm (Figure 2c), and around 95% of the fibers had diameters below 800 nm, showing that the coaxial electrospinning of PMMA can produce fine-textured nanofibers. The SEM image presented in Figure 3a shows the stacking of nanofiber layers in a fiber mat, while Figure 3b represents the edge of the cut core–shell mat. As the figure shows, fibers attached during the evaporation of the solvent were ‘glued’ by the PDCPD, formed from DCPD in the presence of Grubbs’ catalyst after cutting off the mat, indicating the successful self-healing of the core–shell nanofiber coating.

3.2. FTIR of Coaxial Composite Fibers

The polymerization of DCPD occurs via the ring-opening metathesis polymerization mechanism, in the presence of ruthenium-based catalysts. Figure 4 presents the expected trans-PDCPD formation with the use of the first-generation Grubbs catalyst.

FTIR spectroscopy (Figure 5) was employed to investigate the structural changes in DCPD/PMMA core–shell nanofibers before and after mechanical damage. For the uncut mat, the spectrum is characterized by a strong C=O peak (1730 cm⁻¹), signifying the dominance of intact PMMA shell material [28]. Several other characteristic PMMA bands are also clearly evident. The asymmetric C–H stretching of –CH₃ groups manifests at 2995 cm⁻¹, while asymmetric and symmetric –CH₂ stretches appear at 2925 cm⁻¹ and 2856 cm⁻¹, respectively [29]. The strong peak at 1430 cm⁻¹ originates from the asymmetric bending of C–CH₃, while C–O stretching vibrations are represented by doublets at 1267/1241 cm⁻¹ and 1183/1143 cm⁻¹. Additional features include the O–CH₃ bending at 987 cm⁻¹, skeletal chain vibration at 1197 cm⁻¹, and out-of-plane C–H bending between 900 and 720 cm⁻¹ [30]. The absence of cyclic C=C vibrations around 1600–1570 cm⁻¹ confirms that unreacted DCPD was not present in detectable quantities at the fiber surface, indicating that the core remained encapsulated [31]. In contrast, the spectrum of the freshly cut mat exhibits a broad but unresolved band near 970 cm⁻¹, alongside a severely diminished C=O peak. More importantly, distinct bands at 1609 cm⁻¹ and 1582 cm⁻¹ emerge, which correspond to the cyclic double bonds in unreacted DCPD, specifically from the cyclopentene and norbornene rings. This confirms that the shell rupture allowed the monomer to diffuse outward, yet polymerization had not yet been triggered due to insufficient time. The chemical profile of this region represents a pre-polymerization state, where the self-healing reaction is pending activation.

The healed mat shows a strong and sharp peak at 970 cm⁻¹, indicative of trans-double bonds in PDCPD and a retained but slightly attenuated C=O peak from PMMA. The absence of the cyclic C=C signals in this case confirms that DCPD underwent ring-opening metathesis polymerization [32]. This confirms that core–shell rupture was followed by successful healing, where DCPD came into effective contact with the embedded Grubbs catalyst. To gain deeper insight into the chemical evolution and self-healing behavior of PMMA/DCPD core–shell nanofibers, we calculated intensity ratios from FTIR spectra (Table 1). These ratios help quantify the relative contributions of PMMA (shell) and DCPD (core) signals and enable us to distinguish between intact, unreacted, and polymerized states.

Ratios relative to unreacted DCPD

Comparisons using the 1582 cm⁻¹ and 1609 cm⁻¹ bands, attributed to the cyclic double bonds in unpolymerized DCPD (norbornene and cyclopentene), support the interpretation of monomer distribution. In the freshly cut nanofibers, where unreacted DCPD is visibly present, the 970/1582 ratio is moderate, confirming the dominance of monomeric species with emerging polymer signals. The 970/1609 ratio follows a similar trend. In contrast, both the uncut and healed mat show higher 970/1582 values, indicating monomer absence due to either intact encapsulation or completed polymerization. In the uncut nanofiber mat, polymer presence could be the consequence of solution dropping or rupture of thin walls in low-diameter fibers.

Shell integrity and core polymerization

The inverse ratio of the PMMA carbonyl stretch at 1730 cm⁻¹ to the PDCPD trans-C=C stretch at 970 cm⁻¹, expressed as 970/1730, acts as a direct indicator of core accessibility relative to shell dominance. In the uncut mat, the low 970/1730 value reflects the preservation of the PMMA shell and minimal core diffusion. This value increases in the healed mat, consistent with DCPD polymerization and residual shell material presence at the damage interface. The freshly cut region, with the highest value, highlights a fully ruptured shell and exposed monomer core.

DCPD conversion efficiency

To assess the extent of polymerization, the 970/1582 ratio served as a relative measure for DCPD conversion, comparing polymerized (970 cm⁻¹) and unreacted (1582 cm⁻¹) signals. This ratio was lowest in the freshly cut mat, where DCPD was released but not yet polymerized, and highest in the uncut mat, due to minimal monomer presence and weak polymer peaks from incidental diffusion. The healed mat displayed an intermediate ratio, marking the coexistence of formed polymer and unreacted fragments, indicating that the healing was not complete.

The quantification of structural changes enables monitoring of transitions from intact to ruptured and healed regions, reinforcing the value of FTIR mapping as a diagnostic tool for quantifying shell performance and healing progression in smart coating applications.

3.3. Optical Microscopy and FTIR Analysis of Uncut and Cut Nanofiber Mats

The spectrum collected from the translucent, unhealed fibers shows a minor trans-C=C signal, which could be caused by the localized microdamage (Figure 6). PMMA bands dominate, indicating shell integrity and little core exposure. Microvoids can be observed, indicating that further adjustments of electrospinning parameters could result in a denser nanofiber network.

In the regions of the mat that underwent mechanical cutting (Figure 7), optical microscopy revealed two markedly different textures: a darker, opaque zone directly at the cut interface and an adjacent translucent area where the original fiber network remained intact. The darker regions corresponded to in situ polymerization of dicyclopentadiene (DCPD), which, upon release from the fiber core, underwent ring-opening metathesis and filled voids created by the cut. Under polarized light, these healed zones exhibited a subtle birefringence consistent with the formation of semi-crystalline PDCPD domains, whereas the surrounding uncut fibers showed the familiar smooth, glassy appearance of PMMA. Notably, closer examination of the transparent mat segments uncovered occasional microvoids (“holes”) where individual fibers were displaced or absent; these defects did not display any evidence of new polymer deposition, suggesting that healing was most efficient where core material could directly contact the damaged interface.

The FTIR measurements corroborate visual observations; the spectrum acquired from the dark healing zones shows a pronounced band at 970 cm⁻¹, confirming that DCPD polymerization occurred precisely at the damage site.

Although the present study focused on morphological characteristics and spectroscopic indicators of healing, it is important to consider the mechanical behavior of the resulting material system in future research. The electrospun PMMA shell, with a reported Young’s modulus of around 10 MPa depending on porosity and fiber orientation, provides structural flexibility while maintaining integrity [33]. Upon the damage and release of the core, the in situ polymerization of DCPD into poly(dicyclopentadiene) (PDCPD) results in a significant local increase in stiffness due to the thermoset nature of PDCPD, whose modulus ranges from 1.4 to 2.6 GPa [34]. This transformation from a soft core to a rigid polymer upon healing suggests that the system not only seals the damage but also reinforces the affected region. Future experimental work will be aimed at quantifying this mechanical transition using tensile or indentation-based techniques to better correlate healing efficacy with stiffness recovery.

4. Conclusions

This study presents a morphological and structural investigation of the self-healing process in electrospun DCPD-PMMA (with Grubbs catalyst) core–shell fibers. Dark, polymer-filled healing zones are readily distinguished using optical microscopy, as well as a trans-C=C signature at 970 cm⁻¹, identified with FTIR analysis, whereas unhealed areas preserve a dominant PMMA shell fingerprint, characterized by a strong carbonyl band at 1730 cm⁻¹. Occasional microvoids, identified visually, indicate limitations imposed by fiber displacement and highlight opportunities for further refining fiber alignment and mat compaction. This dual-analysis approach not only confirms the occurrence of DCPD polymerization at targeted sites but also provides a predictive framework for engineering next-generation self-healing fiber mats with minimal unhealed defects. An additional advantage of the PMMA shell lies in its intrinsic optical transparency, which facilitates the direct visualization of fiber morphology and healing progression. This property enables the real-time monitoring of damage sites and polymerization events via optical microscopy, making PMMA particularly valuable in the development of smart coatings where non-invasive inspection is essential. Future work will include mechanical testing, such as tensile, nanoindentation, or modulus mapping, to quantify stiffness recovery after healing and assess elastic performance before and after damage.

Author Contributions

Conceptualization, I.S., V.R. and A.S.; methodology, I.S.; validation, I.S. and V.R.; formal analysis, A.S., Z.L., M.C. and B.H.; investigation I.S. and A.K.; writing—original draft preparation, I.S. and V.R.; writing—review and editing, A.S. and Z.L.; visualization, A.S.; supervision, V.R.; funding acquisition, B.H. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1 Experimental setup for coaxial electrospinning.

Figure 2 (a) SEM of DCPD-PMMA core–shell fibers. (b) DCPD core release. (c) Fiber diameter distribution.

Figure 3 (a) Nanofiber multilayered mat; (b) healed fibers after cut.

Figure 4 The polymerization of DCPD in the presence of the first-generation Grubbs catalyst.

Figure 5 FTIR spectra of uncut, freshly cut, and healed DCPD-PMMA core–shell nanofiber mats.

Figure 6 Optical microscopy and FTIR spectrum of uncut nanofiber mat.

Figure 7 Optical microscopy and FTIR spectrum of healed nanofiber mat.

Table 1

Intensity ratio of crucial FTIR bands.

Wavenumbers	1730/1609	1730/1582	970/1582	970/1730
Uncut Mat	-	-	-	0.43
Freshly cut mat	0.43	0.63	0.83	1.32
Healed mat	1.27	1.47	1.28	0.86

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