Fabrication and characterization of poly methyl methacrylate (PMMA) matrix modified with strontium nano-rods

Abstract

In this study, strontium oxide nanorods (SrO NRs) were synthesized and incorporated into PMMA matrices at 1–5 wt% to evaluate structural, thermal, mechanical, surface, and antibacterial performance. XRD combined with FTIR confirmed the presence of residual Sr-based hydroxide and carbonate phases associated with SrO synthesis routes, contributing to composite interfacial interactions. Tensile tests revealed no statistically significant enhancement in tensile strength, despite increases in hardness and Young’s modulus, which is consistent with a shift toward higher stiffness rather than enhanced toughness. Increased stiffness was accompanied by reduced elongation-at-break, indicating SrO-induced embrittlement, especially at high loading. Surface roughness increased progressively with SrO content, correlating with the successful incorporation of nanorod fillers into the PMMA matrix, as evidenced by SEM/EDX analysis. Thermogravimetric analysis elevated thermal stability for nanofiller-loaded PMMA, with higher onset temperatures and reduced mass loss relative to neat PMMA. It may be inferred that the degradation process is influenced by the thermal dihydroxylation and decarbonation of the nanofiller itself, rather than solely by polymer degradation mechanisms, which delays the overall mass-loss kinetics. Antibacterial assays against gram-negative bacteria (E. coli), gram-positive bacteria (S. aureus), and the fungus (C. albicans) were evaluated. The combined improvements in thermal stability, density, and hardness indicate that the reinforced PMMA represents a promising material for certain functional properties except for applications requiring high toughness.

Introduction

Acrylic resins, particularly non-metallic types, are widely used in prosthetic and restorative dentistry due to their excellent aesthetics, low density, ease of manipulation, and lower cost compared to metallic materials. They also exhibit favorable properties such as biocompatibility, chemical stability, absence of odor or taste, and ease of repair after fracture. However, they suffer from certain drawbacks, including polymerization and thermal shrinkage, dimensional changes, and low impact resistance, which can result in sudden fracture during use¹. Poly(methyl methacrylate) (PMMA) has long been the preferred material for dental and orthopedic applications owing to its transparency, thermal and chemical stability, polishability, and superior biocompatibility^2–8. Despite these advantages, PMMA demonstrates poor mechanical performance and limited antimicrobial properties, which facilitate plaque accumulation and mucosal infections, ultimately compromising its clinical durability⁹. To address these challenges, reinforcement of PMMA with organic and inorganic nanomaterials—such as metal oxides, fibers, and clays—has been investigated to enhance its physicochemical and biological behavior^2,8,10–19.

Among various reinforcing agents, strontium oxide (SrO) nanofillers have recently gained considerable attention due to their multifunctional characteristics. Strontium, an alkaline earth element, plays an essential role in bone metabolism and osseointegration, with approximately 99% of body strontium located in bone tissue^20–22. SrO exhibits high basicity and excellent optical and thermal stability, making it a promising candidate for dental and biomedical composites^10,14,23,24. Nano-sized SrO possesses a cubic crystalline structure, a high melting point (~ 2531 °C), and strong corrosion resistance, which contribute to improved mechanical and biological performance in composite systems^{13,21,25–29}.

Several synthesis approaches—such as sol–gel, microwave, hydrothermal, and precipitation techniques—have been explored for producing SrO nanoparticles¹³. In the present study, SrO nanofillers were synthesized via a chemical oxidation route (wet-chemical method) using SrCl₂ in a basic medium¹³, followed by emulsion polymerization to fabricate PMMA/SrO nanocomposites. This approach aims to achieve an optimal balance between nanofiller content and enhanced structural, mechanical, thermal, antibacterial, and physical properties. The resulting materials are expected to exhibit improved durability, making them suitable candidates for use in denture base resins, acrylic prosthetics, and bone cements.

Experimental details

Materials

The following chemicals were used in this study: Strontium Chloride Hexahydrate (SrCl₂:6H₂O, Extra Pure, 98%, LOBA. CHEMIE. PVT. LTD.), Potassium Hydroxide Pellets (KOH, Extra Pure 85%, LOBA. CHEMIE. PVT. LTD.), Toluene (99.5%, AR grade), Absolute Ethanol (C₂H₅OH, 99.8%, PIOCHEM Laboratory Chemicals), Methyl Methacrylate monomer (C₅H₈O₂ MMA, Stabilized with hydroquinone, LR grade, SDFCL), Sodium Lauryl Sulfate (SLS, C₁₂H₂₅NaO₄S, Extra Pure, 85%, LOBA. CHEMIE. PVT. LTD.), Potassium Persulphate (KPS, K₂S₂O₈, Extra Pure, 98.0%, SDFCL), and Distilled Water. Prior to polymerization, the inhibitor present in the MMA monomer was removed using the adsorption technique. The monomer was purified by filtering twice through a sodium hydroxide solution to ensure the complete removal of the stabilizing agent. All chemicals were of analytical grade and used without any further purification.

Sample preparation

Synthesis of strontium oxide nanorods (SrO NRs) via the wet-chemical method

Strontium chloride hexahydrate [SrCl₂.6H₂O] and potassium hydroxide (KOH) were used as precursors for the synthesis of SrO NRs through a simple wet-chemical route³⁰. This method offers higher purity and better crystallinity. Initially, 25.0 g of KOH pellets were dissolved in a mixture of deionized water and absolute ethanol under continuous stirring. Then, 59.52 g of SrCl₂·6 H₂O were added to the solution, which was subsequently refluxed at 100 °C for 8 h. The main reactions are represented as follows²⁶:

1

2

During the reaction, KOH dissociates into K⁺ and OH^– ions, which react with the Sr^+ 2 ions to form strontium hydroxide Sr(OH)₂. The resulting precipitate was repeatedly washed with a water–ethanol mixture by centrifugation to remove excess KOH and chloride ions until a neutral pH (≈ 7) was obtained. The purified product was dried overnight at 100 °C, ground to a fine white powder, and calcined at 700 °C for 1 h at a heating rate of 10 °C·min⁻¹ to induce crystallization³¹. A slight color change from milky white to pale white was observed after calcination, indicating phase transformation²¹. The final nanopowder product consists of three crystalline phases (i.e. SrO, SrCO₃ and Sr(OH)₂H₂O), referred to as “Sr-based filler” with predominantly rod-like morphology and high yield.

Synthesis of PMMA and PMMA/Sr-based filler nanocomposites via emulsion polymerization

Pure PMMA (control sample) and PMMA/Sr-based filler nanocomposites were synthesized from MMA monomer using a free-radical emulsion polymerization method. The filler nanopowder content varied between 1 and 5 wt% relative to the monomer. The main difference between the composite and control syntheses was the pre-dispersion of the nanopowder filler in ethanol before starting the polymerization process. In a typical procedure, the dispersed filler suspension was transferred into a three-necked round-bottom reactor equipped with a reflux condenser, magnetic stirrer, thermometer, and dropping funnel. Sodium lauryl sulfate (SLS, 0.2 g) was added as a surfactant to promote micelle formation in the aqueous phase and to control the nano-size and dispersion³². The high surface activity of Sr-based filler facilitated sufficient adsorption of the emulsifier under vigorous stirring. Subsequently, 10 mL of purified MMA monomer was added dropwise to the reactor under continuous stirring. Potassium persulfate (KPS, 0.8 g) was introduced as a free-radical initiator, and the temperature was raised to 75 ± 2 °C for 8 h to thermally activate the initiator to generate free radicals and start the polymerization process. The temperature was controlled by circulating water through the reactor jacket. After completion, the reaction mixture was gradually cooled to room temperature. The resulting emulsion was centrifuged repeatedly with ethanol and deionized water to remove unreacted surfactant and impurities. Finally, the obtained polymeric products were dried at 60 °C to eliminate residual solvent and unconverted monomers. The final products consisted of pure PMMA and PMMA nanocomposite white powders, ready for further characterization.

Film formation

To prepare thin films, 2.0 g of the nanocomposite powder was dissolved in 50 mL of toluene under continuous stirring till clear dissolving. The homogeneous solution was cast into a Petri dish and dried at 60 °C for 10 h. The films were then obtained by rapid cooling in cold water and carefully peeled off. The resulting film thickness ranged from approximately 0.080 to 0.150 mm.

Characterization techniques

The structural, morphological, thermal, and mechanical properties of the synthesized Sr-based nanorod filler, PMMA, and PMMA nanocomposites were systematically investigated. Fourier-transform infrared spectroscopy (FTIR, Bruker Invenio S, Germany) was used in the range 400–4000 cm⁻¹ with 4 cm⁻¹ resolution to identify functional groups and polymer–filler interactions. X-ray diffraction (XRD, Bruker D8 Advance, Germany) with Cu Kα radiation (λ = 1.5405 Å, 40 kV, 40 mA) and a 2θ range of 5°–80° was employed to determine phase composition and crystallinity. Surface morphology, microstructure features, and elemental composition of the polymer nanocomposites were examined using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) with a model (JEOL JSM 6510 LV, Japan), operated at 30 kV. In addition, the transmission electron microscopy (TEM, JEOL JEM-2100, Japan, 200 kV) was used to confirm nanoparticle size and structure.

Thermal stability was analyzed via thermogravimetric analysis (TGA, SHIMADZU, Japan) from 20° to 800 °C at 10 °C·min⁻¹ under nitrogen. Film density was determined by the Archimedes principle using an AS 220-R2 PLUS balance at 17 °C with distilled water as the immersion medium. Surface hardness was tested using a Vickers microhardness tester (FM-7, Future-Tech Corp., Tokyo, Japan) at a 10 g load for 5 s. Mechanical properties, including Young’s modulus, tensile strength, and elongation at break, were measured using an INSTRON 34SC-5 machine. All tests were performed in triplicate (n = 3) and reported as mean ± standard deviation. Moreover, for biological tests, the antibacterial and antimicrobial activities of the polymer nanocomposites were tested. The antibacterial activity of the polymer nanocomposites was tested against gram-positive bacteria, Staphylococcus aureus (S. aureus), gram-negative bacteria, Escherichia coli (E. coli), and Candida albicans (C. albicans) fungus. These biological tests were obtained from the Department of Microbiology, Faculty of Medicine, Mansoura University, Mansoura.

Results and discussion

Structural analysis

Fourier transform infrared (FTIR) spectroscopy analysis

The FTIR spectra recorded in the 400–4000 cm⁻¹ range (Fig. 1) provide a comprehensive assessment of the structural evolution of Sr-based species during synthesis, calcination, and subsequent incorporation into the PMMA matrix. The as-synthesized filler sample (Fig. 1a) exhibits characteristic features associated with strontium hydroxide Sr(OH)₂ and its hydrates, which contain many small peaks throughout the spectra due to their hygroscopic nature. A broad O–H stretching envelope spanning 3500–2500 cm⁻¹, together with a sharper band near 3664 cm⁻¹, corresponds to the hydroxyl stretching vibrations of Sr(OH)₂, indicating the presence of hydrated or partially hydroxylated strontium species. The absorption band at approximately 1700 cm⁻¹ is attributed to C–O stretching arising from physisorbed CO₂ on the particle surface, which is commonly observed in alkaline-earth oxides and hydroxides^{10,24–26,33–36}. In addition, a characteristic strong absorption peak at about 1450 cm^− 1, attributed to the Sr-O bond, was drastically increased upon calcination, signifying the SrO formation^25,33. Following calcination at 700 °C (Fig. 1b), a decrease in O–H-related absorptions is observed, confirming extensive dehydroxylation and transformation into crystalline SrO. Additional sharp peaks at 580 and 525 cm^− 1 may be attributed to both Sr–O stretching and bending vibrations, respectively. The distinct bands at 1025, 859, 698, and 423 cm⁻¹ correspond to the asymmetric and symmetric stretching modes of Sr–O, further confirming the formation of SrO^{1,10,11,24–26,33,35–42}. The effectiveness of these bands is strongly dependent on the reaction conditions, and at high temperatures, a strong Sr–O–Sr molecular structure has been established¹⁰. These frequencies become stronger with an increase in temperature, thereby supporting the formation of a strong Sr-O-Sr molecular framework²⁶. In general, a strong absorption peak at about 1450 cm^− 1 may also refer to the stretching vibration of the asymmetrical C–O bond in the CO₃²⁻ group^43,44, indicating partial surface carbonation of SrO to SrCO₃ upon exposure to atmospheric CO₂. Additionally, the appearance of carbonate-related absorption peaks at 859 cm^− 1 and 698 cm^− 1 are assigned to be out-of-plane and in-plane bending vibrations, respectively^43,44. Furthermore, weak broad absorption in the 3400–3600 cm⁻¹ region suggests the presence of adsorbed moisture or minor Sr (OH)₂·H₂O formation.

Fig. 1.

FTIR spectra of (a) as-synthesized strontium hydroxide Sr(OH)₂ filler, (b) after calcination at 700 °C for 1 h, (c) pristine PMMA, and (d–g) PMMA/Sr-based filler nanocomposites with different filler concentrations of 1, 2, 3, and 5 wt%, respectively.

The FTIR spectrum of pristine PMMA⁴⁵ (Fig. 1c) exhibits the typical vibrational fingerprint of an amorphous polymer, including the strong ester carbonyl group (C = O) stretching band at ~ 1730 cm^-¹, C–H stretching modes of methyl/methylene groups between 2990 and 2840 cm^-¹, and strong C–O–C stretching bands at 1260–1150 cm^-¹^46–50. In addition, bands at 987 and 749 cm^-1 were found by C-H bending⁴⁶, whereas both bending and stretching vibrations of C = H and C = C appeared between 1330 and 740 cm^-1, respectively^48,49,51. Additional features include α-CH₂ bending at 1440–1380 cm^-¹ and characteristic backbone vibrations in the 800–600 cm^-¹ region. The absence of unexpected absorptions confirms the polymer’s chemical purity and amorphous character.

In the PMMA/Sr-based filler nanocomposites (Fig. 1d–g), the appearance of new peaks at 605 and 480 cm^− 1, related to the stretching and bending vibrations of the Sr–O, respectively^10,25,40,42, was attributed to the successful incorporation of Sr-based filler into the polymer matrix during the polymerization process. In addition, subtle variations—including minor shifts and intensity changes in the C = O and C–O–C stretching regions—are observed with increasing filler concentration. These changes are commonly attributed to interfacial interactions between polar carbonyl groups and the Sr²⁺-rich surface of filler nanorods, which can influence dispersion behavior and contribute to modifications in the thermal and mechanical properties of the composite. The metal–oxygen absorption near 1450 cm⁻¹ suggests the physical entrapment and integration of Sr-based nanofiller within the PMMA matrix³³. Furthermore, the bands in the 605–480 cm⁻¹ region become progressively more pronounced as the filler loading increases from 1 to 5 wt%, corresponding to Sr–O stretching and bending vibrations, respectively^10,25,40,42. Another new peak at 650 cm^− 1, only appeared at higher filler content (3& 5 wt%), attributed to the stretching vibration of the Sr–O bond. The increasing intensity of these bands reflects the higher fraction of inorganic filler in the composites and supports the presence of uniformly dispersed filler nanorods. These spectral trends collectively indicate strong interfacial interactions and effective intermolecular association between PMMA chains and the embedded filler⁵², consistent with expected structural and property enhancements in polymer–oxide nanocomposite systems.

Transmission electron microscopy (TEM)

To further investigate the morphology and size of the synthesized powder via the wet chemical route, a transmission electron microscopy (TEM) analysis was carried out. As shown in Fig. 2a, the TEM micrographs reveal that the powder possesses a nanometric rod-like morphology, which are arranged in channel-like structures. Most of the nanorods appear nearly similar in length and diameter. The nanorods tend to form “bundles of spaghetti” due to their high surface energy and tendency toward self-assembly. Using the TEM micrographs, the particle size distribution histogram of the filler nanorods is shown in Fig. 2b. The average diameter of these nanorods is 34.42 ± 10.26 nm, confirming their nanostructure.

Fig. 2.

(a) TEM images of the synthesized filler and (b) histogram of nanorods size distribution.

Overall, the TEM observations confirm the successful formation of nanorod filler with well-defined morphology and uniform nanoscale dimensions that are expected to enhance interfacial interactions and stability when incorporated into the PMMA matrix.

X-ray diffractometer (XRD) analysis

The structural crystallinity and phase composition of the synthesized powder nanorods and PMMA/Sr-based filler nanocomposites were examined using XRD analysis (Fig. 3). As expected for a highly hygroscopic material, the as-prepared powder spectrum (Fig. 3a) exhibits numerous broad and low-intensity reflections corresponding to Sr(OH)₂ and its hydrate forms⁵³. The principal diffraction peaks at 2θ = 14.38°, 19.54°, 26.64°, 28.47°, 31.92°, 36.50°, 39.36°, 40.62°, 50.19°, and 56.78° were indexed to the (010), (110), (200), (011), (120), (201), (211), (121), (320), and (410) planes of orthorhombic strontium hydroxide hydrate Sr(OH)₂ (H₂O)⁵⁴, in agreement with PDF card 01-072-0057. Additional reflections at 23.08°, 28.47°, 29.06°, 31.92°, 36.50°, 46.75°, and 50.19° matched the (210), (111), (201), (211), (400), (321), and (202) planes of strontium hydroxide Sr(OH)₂ (PDF 00-027-0847), confirming the presence of mixed hydroxide phases^{24,31,33,54–56}. The sharp and well-defined Bragg peaks indicated a high degree of crystallinity in the as-synthesized powders²¹.

Fig. 3.

XRD spectra of (a) dried strontium hydroxide Sr(OH)₂ filler, (b) after calcination at 700 °C for 1 h, (c) neat PMMA, and (d–g) PMMA/Sr-based filler nanocomposites loaded with different ratios of the filler range 1–5 wt%, respectively.

Upon calcination at 700 °C for 1 h (Fig. 3b), notable increases in peak intensities were observed alongside the emergence of new reflections at 25.84° and 35.18°, and further enhancement of peaks at 25.27° and 44.18°. Although the calcination treatment did not significantly shift peak positions, the improved intensities suggest enhanced crystallite growth and partial phase transformation. The diffractogram contained contributions from both Sr(OH)₂·H₂O, with reflections at 14.33°, 19.55°, 24.41°, 26.65°, 28.43°, 30.31°, 31.86°, 36.44°, 39.31°, 40.51°, 50.08°, and 60.51° corresponding to the (100), (110), (001), (020), (101), (120), (210), (021), (121), (211), (230), and (212) planes (PDF 00-028-1222), and strontianite (SrCO₃), with peaks at 25.21°, 25.84°, 35.24°, 36.44°, 44.18°, and 59.76° corresponding to the (111), (021), (031), (130), (032), and (151) planes (PDF 01-078-4340). The presence of strontium hydroxide with a cubic structure was due to the powder being synthesized using a wet process. It is proposed that, following calcination and the consequent removal of hydride ions, electrostatic interactions arise as a result of ionization potential effects, ultimately leading to the formation of the orthorhombic phase²⁶. Furthermore, the diffraction peaks observed at 2θ values of 25.29°, 30.37°, 35.25°, 40.57°, 50.02°, 59.70°, 60.44°, and 63.95° were due to cubic SrO^10,33,35,38. Therefore, the sharp peaks indicated the formation of high crystalline powders of three crystalline phases (i.e., SrO, SrCO₃ and Sr(OH)₂H₂O)²⁴, reflecting them as “Sr-based filler.”

The low electron-negativity of strontium facilitates the reaction of Sr²⁺ ions to react with oxygen and H₂O in air. Hydration of SrO to form Sr(OH)₂ is predicted to chemically stabilize the metal oxide. Further stabilization is observed when Sr(OH)₂ is further hydrated to Sr(OH)₂·1H₂O and Sr(OH)₂·8H₂O. The predicted stability trend is as follows: SrO < Sr(OH)₂ < Sr(OH)₂·1H₂O < Sr(OH)₂·8H₂O, indicating that stability increases with increasing hydration of SrO⁵⁷. In a previous work⁵⁸, it was shown that the hydrated Sr has a strong tendency to absorb CO₂ in air to form SrCO₃ whereas non-hydrated Sr does not, as illustrated in Eq. 3. Such a phenomenon may occur in other alkaline-earth-metal compounds as well.

3

Collectively, these results explain the predominance of hydrated hydroxide (~ 83.6%) and confirm partial conversion to carbonate species during exposure to ambient conditions. This is consistent with the FTIR results. Using to the Debye-Scherrer equation (Eq. 4), the average crystallite size (D) of the synthesized powder was determined:

4

where β is the full width at half maximum (FWHM, in radians), θ is the Bragg diffraction angle (radians), λ is the X-ray wavelength (1.540 Å), and K is the shape factor, taken as 0.9^11,27. Calcination temperature had a notable influence on crystallinity and crystallite growth¹¹. The average crystallite size increased from 29.63 nm in the as-synthesized powder to 36.48 nm after calcination. This enlargement is attributed to thermally induced coalescence and grain growth, driven by the relief of internal stresses and enhanced atomic mobility during heat treatment. Additionally, the removal of surface impurities and the transition to a more stable crystalline phase contribute to the formation of larger, well-defined crystallites^21,26.

The diffraction profile of pure PMMA (Fig. 3c) exhibits a broad, low-intensity halo centered near 2θ ≈ 14.6°, accompanied by two weak, diffuse features around 30.8° and 43.2°, as previously shown in⁴⁵. These broad amorphous reflections are characteristic of atactic PMMA and result from the absence of long-range structural order, which is attributed to steric hindrance imposed by the pendant ester groups along the polymer backbone—consistent with previously reported findings^51,59,60.

Upon incorporation of nanofillers into the PMMA matrix (Fig. 3d–g), the XRD patterns reveal the simultaneous presence of the amorphous PMMA halo and the sharp diffraction peaks associated with crystalline SrO-derived phases. The appearance of these reflections confirms the successful embedding of nanofillers within the polymer matrix. As the Sr-based nanofiller loading increases from 1 to 5 wt%, the intensity and sharpness of SrO-related peaks become progressively more pronounced, indicating a higher volume fraction of the inorganic phase and improved nanorod dispersion. A similar trend—enhancement of crystalline peak intensity with filler concentration—has been reported in metal-oxide/PMMA systems and is associated with increased structural ordering induced by the filler. Quantitative crystallinity analysis shows a gradual increase from that of pure PMMA to 3.32%, 5.51%, 6.75%, and 7.24% for the 1, 2, 3, and 5 wt% nanocomposites, respectively. This enhancement in apparent crystallinity is attributed to the contribution of the metal–oxygen framework (Sr–O–Sr) of the nanofillers, which is influenced by synthesis parameters such as reaction time, temperature, and preparation method¹¹. The presence of SrO nanofillers promotes localized structural ordering within the PMMA matrix. These observations are consistent with the FTIR results, which indicate strong interfacial interactions—likely involving coordination between PMMA carbonyl groups (C = O) and Sr²⁺ surface sites—facilitating efficient dispersion and polymer–filler adhesion⁶¹. Overall, the XRD results demonstrate the coexistence of an amorphous PMMA phase with crystalline SrO structures, confirm the successful incorporation and distribution of Sr-based nanorod filler, and reveal a systematic increase in microstructural ordering with higher filler content. These structural insights, supported by spectroscopic analysis, validate the effective formation of PMMA/Sr-based filler nanocomposites with enhanced structural characteristics.

Morphological and elemental analysis (SEM/EDX)

Scanning electron microscopy (SEM) was employed to visualize the surface morphology of the prepared nanocomposites at the micro- and nanoscale, allowing assessment of Sr-based nanorod filler distribution within the polymer matrix after polymerization. Figure 4a–e shows SEM micrographs of pure PMMA and PMMA/Sr-based filler nanocomposites containing different filler loadings (1–5 wt%) at the same magnification.

Fig. 4.

SEM images of (a) pure PMMA and (b–e) PMMA/Sr-based filler nanocomposites loaded with different ratios of the filler, 1, 2, 3, and 5 wt%, respectively.

As seen in Fig. 4a, the pristine PMMA surface appears relatively smooth and featureless, consistent with its amorphous nature^51,62. With the incorporation of nanorod fillers, notable morphological changes are observed. The smooth clusters of PMMA progressively transform into smaller, rougher, and more uniformly distributed spherical-like domains embedded within the polymer matrix (Fig. 4b-e). Strontium-based fillers act as nucleating agents, making the PMMA matrix surround them. The homogeneous contrast in the micrographs suggests a well-dispersed of nanorod filler within the PMMA matrix, confirming strong interfacial interaction between the two components and the absence of significant particle agglomeration. At higher nanofiller loadings (up to 5 wt%), the nanocomposites appear more uniformly distributed. This behavior is attributed to enhanced van der Waals forces and the high surface energy of nanofillers, which promote particle coalescence and growth into larger grains⁵³.

Overall, the SEM observations confirm successful incorporation and good dispersion and embedding of the filler nanorods within the PMMA matrix. The Sr-based nanorod fillers act as effective nucleating and reinforcing agents, restricting polymer chain mobility and thereby expected to improve the physical properties of the resulting PMMA nanocomposites.

Energy dispersive X-ray spectroscopy (EDX)

The EDX analysis was used to determine the elemental composition and confirm the addition of Sr-based nanofiller in PMMA nanocomposites. The performed results let us determine the distribution of strontium (Sr), together with carbon (C) and oxygen (O), as represented in Fig. 5. Therefore, these specific chemical elements in polymer nanocomposites were identified in all the EDX spectra of the PMMA nanocomposites, as illustrated in Table 1. It was observed that the Sr peak is present and increased with increasing the filler content, indicating the formation of strontium/polymer nanocomposites. The increase in oxygen content is attributed to the presence of Sr–O bonds arising from strontium oxide/hydroxide species dispersed in the matrix, which causes the carbon content to decrease along with Sr loading. This phenomenon indicates enhanced incorporation and better dispersion of strontium-containing oxide/hydroxide filler within the polymer matrix. Therefore, the strontium-rich sample with 3 wt% content showed the highest strontium peak, with 0.72 weight% value. On the other hand, the increase in carbon content (5 wt%) may arise from strontium carbonate species due to atmospheric CO₂ exposure. These results are in good qualitative agreement with the composition of the synthesized nanocomposites and SEM observations.

Fig. 5.

EDX images of (a) pure PMMA and (b–e) PMMA/Sr-based filler nanocomposites loaded with different filler ratios, 1, 2, 3, and 5 wt%, respectively.

Table 1.

EDX composition of PMMA and PMMA/Sr-based filler nanocomposites with 1–5 wt% filler content.

Sample	Elements	Weight (%)	Atomic (%)
PMMA	Carbon (C), K Oxygen (O), K	70.09 29.91	75.73 24.27
PMMA/ 1 wt% Sr-based filler	Carbon (C), K Oxygen (O) K Strontium (Sr), K	64.14 35.58 0.28	70.57 29.39 0.04
PMMA/ 2 wt% Sr-based filler	Carbon (C), K Oxygen (O) K Strontium (Sr), K	65.94 33.67 0.39	72.24 24.70 0.06
PMMA/ 3 wt% Sr-based filler	Carbon (C), K Oxygen (O) K Strontium (Sr), K	64.85 34.43 0.72	71.42 28.47 0.11
PMMA/5 wt% Sr-based filler	Carbon (C), K Oxygen (O) K Strontium (Sr), K	68.02 31.30 0.68	74.25 25.65 0.10
Total	100

K= Spectroscopic term related to EDX.

Surface roughness

The surface roughness of the prepared samples was evaluated from SEM micrographs using Gwyddion software for statistical surface analysis. The roughness parameters were analyzed as a function of filler content incorporated into the PMMA polymer matrix⁷. Among the various roughness descriptors, the most commonly used are the average roughness (Ra) and the root mean square roughness (Rq)⁵. Additional parameters such as the average maximum height of the profile (Rz) and skewness (Rsk) were also considered to provide a more comprehensive surface characterization.

Table 2 presents the roughness parameter values (Ra and Rq) for pristine PMMA and PMMA nanocomposites. As shown, the neat PMMA exhibits the lowest roughness values (Ra = 18.6 nm, Rq = 24.2 nm), whereas the composite containing 5 wt% nanofillers shows the highest values (Ra = 28.4 nm, Rq = 38.2 nm). The progressive increase in surface roughness with filler content indicates that the incorporation of inorganic nanofillers modifies the surface topography of the polymer, leading to a rougher and more compact morphology. This behavior is attributed to the uniform dispersion of nanofillers within the PMMA matrix, which enhances the interfacial interaction between the two phases⁴⁶. The areal (3D) surface profiles (Fig. 6) further confirm that the surface of the nanocomposites becomes increasingly uneven with higher filler loading, reaching a maximum roughness at 5 wt% nanofiller. These morphological changes suggest some chemical interactions and adhesion between the nanorod fillers and PMMA chains, consistent with the FTIR and SEM/EDX findings that indicate successful incorporation of the inorganic filler into the organic polymer matrix.

Table 2.

Values of different surface roughness parameters in (nm).

Sample	Roughness average (Ra):	Root mean square roughness (Rq):	Average maximum height of the profile (Rz):	Skewness (Rsk):
Neat PMMA	18.589	24.178	172.749	− 0.0496
PMMA/1 wt% Sr-based filler	21.584	27.428	182.287	8.148 × 10⁻³
PMMA/2 wt% Sr-based filler	23.274	30.149	224.075	− 0.0722
PMMA/3 wt% Sr-based filler	26.374	34.649	254.076	0.1374
PMMA/5 wt% Sr-based filler	28.440	38.191	294.143	0.1233

Ra: Roughness average, Rq: Root mean square roughness and Rz: Average maximum height of the profile.

Fig. 6.

Typical areal (3D surface profilometic) roughness graphs of (a) pristine PMMA and (b–e) the nanocomposites loaded with different nanorod filler ratios, 1–5 wt%, respectively.

Thermal properties

Thermogravimetric analysis (TGA& DTG)

Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) were employed to evaluate the thermal stability of pure PMMA and PMMA nanocomposites^26,63. Measurements were performed under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹ from room temperature (~ 25 °C) to 800 °C, recording weight loss as a function of temperature^63,64. The enhancement of polymer thermal stability generally depends on the type, amount, and dispersion of the inorganic nanofiller, as well as the composite’s structural characteristics^65,66. Figure 7(a, b) represents the TGA/DTG curves of the as-synthesized pure PMMA and PMMA/Sr-based filler nanocomposites with different nanofiller loadings (1–5 wt%). As shown in Fig. 7a, the neat PMMA (as previously illustrated in⁴⁵ exhibits a single major degradation stage between 258.5 and 420 °C, with complete decomposition at approximately 372.5 °C and a total weight loss of about 98% (Table 3). A minor mass loss (~ 1.56%) occurs below 258 °C, associated with moisture removal, initiator fragments, and early depolymerization at weak head-to-head linkages^5,51,52,67. Above 420 °C, nearly complete thermal decomposition of the polymer matrix occurs. This decomposition is attributed to the degradation at which a random scission of the polymer backbone (carbonyl group) was initiated⁷. Contrarily, upon increasing filler content, the nanocomposites have different stages of weight loss to be recognized.

Fig. 7.

Thermogravimetric analysis curves (TGA/DTG) of the as-synthesized (a) neat PMMA and (b–e) PMMA/Sr-based filler nanocomposites with different nanofiller loadings (1–5 wt%), respectively.

Table 3.

Thermal parameters obtained from TGA/DTG curves for pure PMMA and PMMA nanocomposites.

Sample	Stage I		Stage II		Stage III		Stage IV
	Temperature (start-end) First Stage	Weight loss (%)	Temperature (start-end) Second Stage	Weight loss (%)	Temperature (start-end) Third Stage	Weight loss (%)	Temperature (start-end) Fourth Stage	Weight loss (%)	Temperature at maximum process rate (Tm ) ⁰C	Residual mass (%)
Pure PMMA	30–258.54	1.56	258.54–420	97.71	–	–	–	–	372.494	1.434
PMMA/1 wt% Sr-based filler	19.11 -151.77	1.62	151.77-263.51	6.05	263.51-336.42	39.18	336.42-417.93	51.7	302.64 361.21	2.135
PMMA/2 wt% Sr-based filler	20.97-213.53	3.47	213.53-263.17	2.64	263.17-328.69	28.3	328.69-424.59	57.46	294.16 374.94	9.153
PMMA/3 wt% Sr-based filler	19.93-258.08	3.51	258.08-318.04	34.02	318.04-420.98	54.86	–	–	291.66 3621.28	9.428
PMMA/5 wt% Sr-based filler	28.16-215.97	3.52	279.09-320.09	9.26	215.97-279.09	18.98	320.09-427.43	60.68	377	8.257

Upon Sr-based nanofiller incorporation, the nanocomposites show multiple degradation steps with reduced weight loss and higher residual mass, indicating improved thermal resistance. For instance, the 1 wt% Sr-based filler composite exhibits four degradation stages, with the final decomposition occurring at 361 °C and a total mass loss of 98.55%. Thermal stability is enhanced by a reduction in weight loss and an increase in residual mass, which corresponds to the estimated nanofiller amount in the nanocomposites⁷. Increasing the nanofiller content to 2 wt% markedly enhances stability, reducing total mass loss to 91.87% (⁓ 7.5% enhancement), confirming the protective effect of SrO nanofillers. Further nanofiller addition (3&5 wt%) slightly increases the mass loss (to ⁓ 92.4%) but remains more stable than pure PMMA. These results are more effective in polymer reinforcement compared to our previous work in studying PMMA/ZnO nanocomposite reinforcement⁴⁵.

The enhancement in the nanocomposite’s thermal behavior can be attributed to the barrier and insulating effects of the well-dispersed nanofillers, which can act as a nucleating agent to hinder heat transfer, restrict PMMA chain mobility, and suppress random scission of the polymer backbone, leading to the delayed thermal decomposition process⁶⁸. Additionally, strong interfacial interactions and increased physicochemical bonding density between the nanofillers and the PMMA matrix^7,52 contribute to delaying the onset of the thermal decomposition process. Overall, the TGA/DTG results confirm that SrO nanofillers significantly enhance the thermal stability of PMMA, with the optimum improvement observed at approximately 2 wt% filler loading. Beyond this concentration, a slight reduction in thermal performance is detected. This demonstrates that thermal stability is directly dependent on the amount of the added nanofiller⁶⁶. XRD and FTIR analyses further revealed that the SrO nanofillers undergo partial transformation to Sr(OH)₂·H₂O and SrCO₃, owing to the hygroscopic nature of SrO and its reactivity toward atmospheric moisture and CO₂. These analyses also indicated improved interfacial compatibility and increased structural order upon incorporation of the SrO nanofillers. These crystalline domains produce a harder and more rigid nanocomposite structure, thereby raising the decomposition temperature and improving the overall thermal stability of the samples. It may therefore be inferred that the degradation process is influenced by the thermal dihydroxylation and partial decarbonation of the filler itself, rather than solely by polymer degradation mechanisms.

Figure 7b also illustrates the DTG curves of pure PMMA and PMMA nanocomposites loaded with different filler contents. It reveals that a maximum thermal degradation temperature (T_max) can be detected⁶⁹. For pure PMMA, a complete decomposition with a broad endothermic peak was at ~ 400 °C, where the highest thermal decomposition (T_max) was recorded at 372.5 °C. No weight loss was observed after this temperature. On the other hand, upon increasing Sr-based nanorod filler, T_max was observed to be divided into two main endothermic peaks, depending on the added amount of the nanofiller, as illustrated in Table 3. At the highest filler addition, the decomposition temperature increases around 5 °C, from 372 °C to 377 °C for pure PMMA and 5 wt% nanofiller, respectively.

Physical and mechanical properties

Density measurement

Figure 8 represents the average density values of pure PMMA and PMMA nanocomposites containing various Sr-based nanofiller loadings (1, 2, 3, and 5 wt%, respectively). Using Archimedes’ principle, the densities of the nanocomposite films were measured by the AS 220-R2 PLUS balance device instrument, which was a high-precision digital weighing balance to record the mass directly. Distilled water was used as the immersion fluid; measurements were taken at room temperature (17 °C). Calculated density values for each specimen were obtained by dividing the mass of the sample in air by the weight loss in water, which equaled the difference between the readings of the mass before and after immersion in water⁷⁰. All measurements were taken in triplicate (n = 3) for each concentration, and the mean density value and standard deviation (mean ± SD) were calculated as represented in Fig. 8. A clear positive correlation is observed between density and filler concentration. The density increased from 1.16 g/cm³ for pure PMMA to 1.21 g/cm³ for the composite containing 5 wt% nanofiller, representing a ⁓4.5% enhancement. This increase is attributed to the higher intrinsic density of SrO nanofillers (4.70 g/cm³) compared to PMMA (1.18 g/cm³)^51,70–72. The incorporation of SrO nanofillers reduces porosity and enhances compactness, confirming the formation of dense structures with improved liquid diffusion resistance.

Fig. 8.

The average density of pure PMMA and PMMA/Sr-based filler nanocomposites with varying nanofiller content (1–5 wt%), respectively.

For higher nanofiller loadings (particularly 5 wt%), the nanocomposites exhibited further improvement in density, suggesting uniform dispersion and strong interfacial interaction with the PMMA matrix, as observed in SEM micrographs. The homogeneous distribution of the nanofillers within the polymer chains during the polymerization process minimizes void formation, resulting in better packing efficiency. This densification correlates with the enhanced thermal stability and stiffness, as demonstrated by the TGA results. These results confirm that the nanocomposites possess a more compact and less porous morphology compared to neat PMMA⁷⁰.

Vickers microhardness test

The Vickers microhardness test was employed to assess the resistance of the nanocomposites to plastic deformation and surface indentation^73,74. Hardness is a critical indicator of a material’s mechanical strength and correlates closely with properties such as yield stress and elastic modulus⁷⁴. The static indentation test is the most popular and simplest method for measuring hardness behavior of polymer nanocomposite films, where a steady load is applied to an indenter (diamond pyramid indenter)⁷⁴. Measurements were performed using a digital Vickers microhardness tester (Model FM-7, Future-Tech Corp., Tokyo, Japan) under a constant load of 10 g force applied for 5 s at room temperature. By measuring the correctional area or depth of indentation and knowing the applied load, the hardness number is automatically evaluated. All measurements were taken in triplicate (n = 3) for each concentration, and three indentations were made at different locations for each one. The mean hardness value and standard deviation (mean ± SD) were calculated and represented in Fig. 9, illustrating the change in the hardness number of PMMA polymer upon different additions of the nanofiller (1–5 wt%), respectively. The microhardness number (Hv) was calculated according to the following equation:

5

Fig. 9.

Hardness as a function of Sr-based filler content for PMMA nanocomposite films.

where F is the applied load on the indenter (gf) and d is the mean diagonal length of the indentation (mm)⁷⁵. As illustrated in Fig. 9, the hardness of PMMA increased with Sr-based filler content, reaching a maximum of 207.55 MPa at 3 wt%, approximately 39% higher than that of pure PMMA (149.55 MPa), which is a much higher value than ZnO reinforcement, as in our previous work⁴⁵. This improvement is attributed to nanofiller homogeneous dispersion and strong interfacial adhesion, which facilitate effective load transfer from the polymer matrix to the rigid nanofillers^51,73. However, with the further addition of Sr-based filler (5 wt%), a minor decrease in hardness value occurred, but it was still higher than the pure PMMA. This may be because of the increase of SrCO₃ phase in the Sr-based filler, as previously illustrated in XRD and EDX analyses. These findings are consistent with the observed density trends, confirming that optimal filler loading (3 wt%) significantly enhances the mechanical performance of PMMA nanocomposites^5,76,77.

Tensile strength

The tensile properties, including tensile strength, Young’s modulus, and elongation at break, of pure PMMA and PMMA/Sr-based filler nanocomposites were evaluated. Each composition was measured in triplicate (n = 3) and the mean values with standard deviations (mean ± S.D.) were calculated. Only one sample can be used to represent each concentration in the stress–strain curve, as displayed in Fig. 10-a. Table 4 summarizes the ultimate strength, toughness, and fracture strain values, which were obtained from the stress-strain relation. Pure PMMA exhibited a maximum tensile strength of approximately 50 MPa. Upon the addition of 1 wt% Sr-based filler, the tensile strength decreased to about 37 MPa, followed by a general decline with increasing the filler content. Interestingly, at 3 wt% Sr-based filler, the tensile strength increased to around 40 MPa, though still below that of pure PMMA. This concentration has an optimum value relative to other concentrations, which was in good agreement with hardness number data. This improvement may be attributed to the high SrO content in the nanofiller with uniform dispersion within the polymer matrix (as confirmed by SEM/EDX analyses) and the interfacial bonding between the two phases—likely through hydrogen bonding—which enhances crystallinity (as supported by XRD analysis)^78–80. Overall, the observed reduction in tensile strength of the nanocomposites upon Sr-based filler incorporation can be attributed to the presence of mixed phases of strontium hydroxide/carbonate, as in the XRD/EDX illustrations, resulting in lowering the mechanical performance compared to the pure PMMA matrix. The increased SrCO₃ content in the nanocomposites has been shown to adversely affect their mechanical properties, as previously reported in the literature⁸¹.

Fig. 10.

(a) Stress-strain curves, (b) tensile Strength and Young’s (or elasticity) modulus, and (c) percentage elongation, and toughness of PMMA and PMMA/Sr-based filler nanocomposites synthesized with varying loading of Sr-based filler nanorods: 1, 2, 3, and 5 wt%, respectively.

Table 4.

The mechanical properties of PMMA and PMMA nanocomposites with different Sr-based filler loadings, 1–5 wt%, respectively.

Sample	Young Modulus (MPa)	Mean (GPa)	Toughness (MPa)	Mean (MPa)	Ultimate strength (MPa)	Mean (MPa)	Tensile strain at break (%)	Mean (%)
Pure PMMA	1115.934 ± 8.42 1039.523 ± 5.58 1237.686 ± 7.31	1.131 ± 0.09	1.900 2.050 1.634	1.861 ± 0.21	53.39 48.37 50.05	50.603 ± 2.55	5.62 6.31 5.07	5.666 ± 0.62
PMMA/ 1 wt% Sr-based filler	1479.819 ± 5.67 1750.428 ± 11.04 955.957 ± 4.76	1.395 ± 0.40	0.866 1.093 0.823	0.927 ± 0.14	36.25 34.24 41.71	37.4 ± 3.86	3.64 4.93 3.14	3.903 ± 0.92
PMMA/ 2 wt% Sr-based filler	1299.252 ± 38.14 1350.333 ± 17.34 1258.733 ± 16.55	1.303 ± 0.04	0.378 0.430 0.239	0.349 ± 0.09	25.58 27.98 21.69	25.083 ± 3.17	2.64 2.65 2.00	2.43 ± 0.37
PMMA/ 3 wt% Sr-based filler	1806.130 ± 7.12 1437.919 ± 8.82 1443.348 ± 5.71	1.562 ± 0.21	0.851 1.089 1.688	1.209 ± 0.43	41.17 39.16 40.07	40.133± 1.00	3.30 4.03 4.91	4.08 ± 0.80
PMMA/ 5 wt% Sr-based filler	921.692 ± 4.64 797.654 ± 3.63 735.596 ± 3.59	0.818 ± 0.09	0.842 0.706 0.821	0.790 ± 0.07	34.79 29.58 24.91	29.76 ± 4.94	4.23 4.03 3.93	4.063 ± 0.15

Overall, the incorporation of SrO nanofillers into the PMMA matrix produced a non-linear influence on tensile strength, showing both reinforcement and deterioration depending on the type and content of the nanofiller. The mechanism governing tensile strength enhancement in such nanocomposites remains complex and not yet fully understood. Future studies should therefore investigate a broader range of SrO loadings and dispersion techniques to further optimize these materials for dental applications.

Young’s modulus

Young’s modulus exhibited a distinct trend compared to tensile strength. Figure 10-b shows the calculated Young’s modulus values for pure PMMA and PMMA nanocomposites with varying Sr-based filler content (1–5 wt%, respectively). An increase in Young’s modulus was observed with increasing the filler content, reaching a maximum of approximately 1.56 GPa at 3 wt% Sr-based filler—significantly higher than that of neat PMMA (1.13 GPa), with about a 38% enhancement. The enhancement in Young’s modulus at optimal filler concentration can be attributed to the high SrO in filler content with uniform dispersion within the polymer matrix (as confirmed by SEM/EDX analysis) and the interfacial bonding between the two phases—likely through hydrogen bonding—which enhances crystallinity (as in XRD illustration)^78–80. Therefore, the nanofillers act to restrict polymer chain mobility and increase the polymer stiffness. The slight reduction at 2 wt% Sr-based filler may be due to less uniform nanoparticle dispersion within the matrix during synthesis. At higher loadings (5 wt%), Young’s modulus decreased to around 0.8 GPa, likely as a result of increased SrCO₃ content in the nanocomposites, acting as defect points upon application of tensional loads⁸¹.

Percentage elongation

The variation in percentage elongation at break is shown in Fig. 10-c and generally mirrors the trend observed in tensile strength. Pure PMMA exhibited the highest elongation value (~ 5.66%), reflecting the unrestricted mobility of its polymer chains and consistent with its ductile polymeric nature. Upon incorporation of Sr-based filler, elongation decreased, except for the 3 and 5 wt% nanofillers, which showed a slight improvement to approximately 4.08 and 4.06, respectively. Although these values remain lower than that of pure PMMA, the modest improvement in ductility can be attributed to higher Sr-filler content with optimal dispersion, allowing the polymer chains to deform more effectively under applied stress and absorb additional strain energy prior to fracture⁷⁹.

Fracture toughness

The fracture toughness of a material is defined as the energy absorbed by the material prior to its failure through crack propagation and is quantified in terms of the energy absorbed per unit crack extension. Therefore, any process that absorbs energy at the crack tip can give rise to an increase in fracture toughness⁹. Additionally, it is a critical property for evaluating whether the material is suitable for certain specific applications without failure. Unlike static properties of strength and stiffness, the fracture mechanics and toughness of nanocomposites are independent of the effective stress transfer mechanism. Figure 10c illustrates the fracture toughness values of the nanocomposite samples reinforced with 1, 2, 3, and 5 wt% of Sr-based filler, along with that of the unfilled PMMA matrix as a control sample. It was observed that the fracture toughness values start decreasing upon Sr-based filler being loaded. It is clear from the tabulated data that the nanocomposite with 3 wt% nanofiller exhibits the highest fracture toughness value, which is still lower than that of the unfilled PMMA matrix. The key factor affecting toughness is mostly influenced by the degree of interfacial adhesion and interaction between the filler materials and matrix, which determines the effectiveness of the transfer of interfacial shear stress from the filler to the matrix. These interactions are so highly energetic and, as a result, play a dominant role in controlling the fracture process⁸². The load transfer efficiency within the composites increased with a higher degree of cross-linking and network formation⁹.

In the general interpretation for the mechanical properties, and regarding the changes in ductility and fracture toughness upon Sr-based filler incorporation (Fig. 10-c), the pristine PMMA exhibits the highest elongation at break, consistent with its ductile polymeric nature. The addition of rigid nanofillers restricts polymer chain mobility, leading to a reduction in elongation and fracture toughness, which is a well-established behavior in ceramic-reinforced PMMA systems. However, among the reinforced samples, the nanocomposite containing 3 wt% nanofiller shows a relative recovery in elongation and the highest fracture toughness, although both remain lower than those of unfilled PMMA. This behavior suggests high SrO content rather than hydroxide/carbonate ratios, with improved dispersion and more effective interfacial bonding, allowing limited plastic deformation and partial energy dissipation prior to failure.

It is important to emphasize that denture base materials are primarily subjected to compressive and flexural stresses under masticatory loading, where elastic modulus, dimensional stability, and surface hardness play a more critical role than resistance to pure tensile fracture. Accordingly, the substantial increases in Young’s modulus (~ 38%) and hardness (~ 39%) at 3 wt% nanofiller provide significant functional advantages by limiting deformation and improving wear resistance. Although a reduction in ductility and fracture toughness is observed, the measured values at 3 wt% nanofiller remain within the range reported for clinically acceptable PMMA-based denture materials. For 5 wt% nanofiller contents, there is an increased degree of carbonation of SrO to SrCO₃, which adversely affects their mechanical properties. Therefore, 3 wt% nanofiller represents a compromise composition, where reinforcing benefits are maximized while brittleness is partially mitigated relative to other filled systems. Consequently, the term “optimal” refers to the optimization of the functional performance for denture applications, rather than the maximization of fracture-related properties alone.

Antibacterial and antimicrobial activity evaluation

For biological tests, the as-synthesized PMMA and nanocomposites of PMMA/Sr-based filler were evaluated using the agar well diffusion approach to evaluate their potential as antimicrobial agents. The antimicrobial agents (i.e., SrO) are important to be added to the polymer matrix to improve the antibacterial capacity of these materials, due to their cariostatic effects^83–85. Furthermore, the antibacterial activity of metal oxide NRs is dependent on various parameters such as particle size, surface area, morphology, concentration/dosage, the nature of the microorganisms, time of exposure to bacteria, etc. Different types of microorganisms were used in this study, such as Escherichia coli (E. coli, Gram -ve), Staphylococcus aureus (Gram +ve), and fungus (Candida albicans), as shown in Fig. 11. Different strains have been preserved in nutrient agar plates seeded with the tested bacteria or fungi, and after incubating the synthesized liquid samples at 38 °C for 24 h, their antimicrobial activity was evaluated through the inhibition zone (mm) calculation, as presented in Table 5. The evaluation of the nanocomposites’ antibacterial activity adds to understanding their potential applications in combating bacterial infections. The results provide insights into the nanocomposites’ inhibition zones against gram-positive (S. aureus) compounds, which were smaller than their inhibition zones against gram-negative (E. coli) compounds.

Fig. 11.

Images of antibacterial and antimicrobial behavior of pure PMMA and PMMA/strontium-based filler nanocomposites for (a) E. coli, (b) S. aureus, and (c) C. albicans, respectively.

Table 5.

Inhibition zone values of the PMMA and PMMA nanocomposites with different ratios of Sr-based nanofiller (1–5 wt%).

Concentration in weight% (wt%)	E. coli	S. aureus	C. albicans
	Diameter of inhibition zone (mm)
Pure PMMA	25	12	28
PMMA/1 wt% Sr-based filler	–	13	21
PMMA/2 wt% Sr-based filler	19	20	20
PMMA/3 wt% Sr-based filler	25	16	22
PMMA/5 wt% Sr-based filler	20	16	20

PMMA (control sample) is generally considered biologically inert and not intrinsically antimicrobial. The measured zones are therefore more likely due to experimental artifacts, such as residual monomer, incomplete polymerization, or leaching of additives, as previously shown in literature⁴⁵. The increasing SrO content in the nanofiller has an effective antibacterial behavior through a slight increase in the inhibition zone diameter (in S. aureus)^86–88. The effective antibacterial agents can be explained based on the reactive oxygen species (ROS), such as hydroxyl radicals (OH⁻), superoxide (O^− 2), and hydrogen peroxide (H₂O₂), damaging the DNA and the cell membrane^45,89,90. In general, the lower concentrations of Sr-based nanofiller did not show an increase in antimicrobial behavior. In addition, the presence of a mixed filler effect of Sr(OH)₂ and SrCO₃, explains why there was no observed increase in the inhibition zone in some of the tested nanocomposites. In a previous study⁹¹, a negligible antibacterial effect of Sr(OH)₂ was observed even with the high concentrations tested. This was most likely due to the fact that Sr—especially the high concentrations—tended to precipitate at the bottom of the agar plates. The higher concentrations of Sr logically also had higher pH, which, in turn, could explain the antimicrobial effect on the various bacterial strains, particularly at 3 wt%. Therefore, this concentration shows a minor increase in inhibition zones due to the increase of SrO/ Sr(OH)₂ content, as previously illustrated in the EDX analysis. For higher filler content (5 wt%), the increased SrCO₃ ratio causes the inhibition zones to decrease again.

Conclusion

SrO nanofillers were successfully synthesized and incorporated into PMMA matrices up to 5 wt% to improve their functional performance for dental and biomedical applications. Structural characterization (XRD, FTIR) confirmed the presence of SrO alongside residual hydroxide and carbonate phases related to synthesis, which contributed to interfacial interactions within the polymer network. Mechanical testing showed that the nanofiller addition enhanced hardness and stiffness but did not significantly increase tensile strength, indicating that the fillers primarily restricted chain mobility rather than promoting effective stress transfer. The reduction in elongation at break with filler loadings further demonstrated increased brittleness, consistent with a stiffness–toughness trade-off commonly reported in ceramic-filled polymer systems. On the other hand, surface roughness progressively increased with the nanofiller content. Thermal analysis revealed improved stability for the nanofiller-loaded PMMA, where delayed mass loss was attributed not only to polymer degradation but also to filler dihydroxylation and decarbonation processes. Importantly, the incorporation of nanofillers imparted antibacterial activity against S. aureus, providing a functional advantage absent in neat PMMA. Overall, Sr-based nanofiller-reinforced PMMA exhibits enhanced thermal stability with the trade-off of increased brittleness, hardness, and surface roughness at higher loadings. Further optimization of dispersion and interfacial design is recommended to balance mechanical integrity with antimicrobial function for dental applications.

Author contributions

O. N. Megahed did the experimental part. M. I. Abdelhamid, N. A. Elwassefy and Ahmed M. Youssef prepared figures and Tables. G- El-Damrawi and N. A. Bakr reviewed the introduction and experimental part analysis. M. I. Abdelhamid, N. A. Elwassefy, G- El-Damrawi, and N. A. Bakr analyze and discuss the main results. All authors wrote and reviewed the manuscript.

Funding

Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). Mansoura University provided financial support for this work through a grant (Grant No. Mu-SCi − 23–32).

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request. [olanagy@mans.edu.eg](mailto: olanagy@mans.edu.eg).

Declarations

Competing interests

The authors declare no competing interests.