Silica silanization graft-strengthening bone cement poly(methyl methacrylate): process and dynamic mechanical properties

Poly(methyl methacrylate) (PMMA) has garnered widespread interest as a potential polymer-based bone cement (BC). However, many challenges in its mechanical properties, especially elongation behavior, need to be overcome. This study focused on PMMA reinforcement with SiO2 particles from two different perspectives, i.e., particle size (nano, submicro, and micro) and surface silanization of the SiO2 particles. Silanization improves the bonding between the additive and polymer matrix, which should improve the dynamic mechanical properties of the composite. The presence of silane bonding was confirmed through Fourier transform infrared spectroscopy, chemical titration, and x-ray photoelectron spectroscopy, and it was determined that 6000 μmol g−1 of silane was successfully coated onto the SiO2 particles. Reinforcement with silanized SiO2 nanoparticles increased elongation at break by 136%. The mechanism by which the size and silanization of the SiO2 additive affected elongation behavior was also discussed in detail.


Introduction
Many synthetic materials, such as biomaterials [1], bioceramics [2], polymers [3], and cement [4], can be used as bone substitutes [5].Bone cement (BC) plays an important role in bone-fracture treatment.The most popular BC is poly (methyl methacrylate) (PMMA) [6].However, PMMA is susceptible to potentially dangerous brittle cracks, restricting its usage.Therefore, there is a need to improve the elongation behavior of PMMA [7].Composite reinforcement is the most commonly used method to strengthen polymers, and the earliest iterations of this method involved the addition of micron-sized fillers to the polymer matrix [8,9].However, a high filler volume percentage was often required to achieve the desired elongation behavior.This is because the reinforcement must satisfy certain requirements to improve the elongation behavior of a brittle polymer [10].First, the filler must be appropriately sized (not too small or large) and have good dispersion in the polymer matrix to minimize the filler volume [11].Second, the filler must have a suitable average interparticle distance.Third, the binding between the filler and matrix must not be too strong, as it is necessary for localized bond breaking to occur between the filler and matrix when the composite is subjected to stress.
The fiber or particle fills used to reinforce polymer composites are usually inorganic materials with poor affinity with polymers.Furthermore, nanoparticles have a strong tendency to aggregate.Consequently, it is often difficult for the filler to be well-dispersed in the polymer matrix, which has negative consequences for the optical and mechanical properties of the nanocomposite material [12].Silane coupling agents are often used as a surface modifier to improve the dispersion and stability of nanoparticles in a polymer matrix.The general structure of a silane coupling agent consists of a hydrophobic head and a hydrophilic tail.When the ethoxy (EtO) groups on the silane chain are hydrolyzed, they bind to surface hydroxyls on the particle, thus modifying its surface.After silane grafting has occurred, the gaps in the nanoparticle aggregate will be filled by grafted molecular chains, which makes it disperse.Furthermore, the silane-grafted nanoparticles will have a hydrophobic surface, which improves their compatibility with the polymer matrix.Ideally, the silane chain would be grafted as a monolayer on the particle's surface hydroxyls.However, depending on the conditions of the grafting reaction, the silane coupling agent may not be fully hydrolyzed, and the presence of unhydrolyzed EtO groups will result in incomplete grafting and increases in surface C content.In a study by Howarter et al [13], x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to measure the elemental composition and thickness of silane layers, which revealed that the C/N ratio of unhydrolyzed and fully hydrolyzed silanes are 9 and 3, respectively.It was also observed that excessively high silane concentrations can cause incomplete silane hydrolysis and self-condensation, which increases the thickness of the silane layer and greatly increases the C/N ratio.
This study hypothesizes that silane grafting on SiO 2 particles will enhance their affinity to the PMMA matrix and improve the elongation behavior of PMMA-SiO 2 composites.Given the dynamic mechanical requirements for bone substitutes, our findings expand the scope of the application of PMMA-BCs.

Materials and experiment
As additives, the SiO 2 particles utilized were sourced from Aurora Applied Materials Co. Ltd The additive particles' morphology, initial size, and structure were characterized using transmission electron microscopy (TEM, JEOL JEM-2010, with an operating voltage of 200 kV).To evaluate the size distribution of these particles in MMA, dynamic light scattering (DLS, DelsaNano C model, produced by Beckman Coulter, Brea, USA) was utilized [7].The particle sizes were distributed at 22 ± 7, 234 ± 50, and 1202 ± 200 nm, leading to their classification in the study as nanoparticles, submicron particles, and microparticle additives, respectively.
The surfaces of the SiO 2 particles were modified by silanization with (3-Aminopropyl)triethoxysilane (APTES).APTES was first hydrolyzed in water for 10 min.The SiO 2 particles were then added, and a sonicator was used to disperse the particles.The solution was then heated under magnetic stirring at 80 °C for one hour.After this reaction, the solution was left to cool for 30 min before filtering with filter paper.The filtered particles were poured into anhydrous ethanol and cleaned in a sonicator bath before being recovered with filter paper.Next, the washed particles were dried in a 40 °C oven.Finally, the particles were ground in a ZrO 2 ball mill at 150 rpm for one hour to produce refined and silanized SiO 2 (s-SiO 2 ).

Preparation of s-SiO 2 /PMMA composites
The PMMA composites were synthesized using a solution casting technique.Initially, 0.081 g of benzoyl peroxide (BPO) was dissolved in 270 ml of MMA monomer and incubated at 85 °C for 30 min.After cooling the monomer solution, filler particles (s-SiO 2 and SiO 2 ).The mixture underwent ultrasonication at 600 W for 5 min.Subsequently, it was transferred to a constant temperature environment for polymerization at 85 °C for another 30 min.The blend was then poured into a glass mold and placed in a thermostatic environment at 70 °C for 3 h, followed by a resting period of 12 h.Finally, the mold was subjected to an additional 3 h curing phase in an oven set at 70 °C.

Characterization of the silane-grafted structure
The surface structure of the s-SiO 2 particles was characterized using Fourier-transform infrared (FTIR) spectroscopy.To this end, 2 mg of s-SiO 2 was ground and mixed with 150 mg of KBr in an agate mortar, and 60 mg of the mixture was pressed into a transparent pellet, which was then analyzed using an FTIR spectrometer.Silane grafting was confirmed by thermogravimetric analysis (TGA).Because APTES has a boiling point of 217 °C, silane chains that are physically adsorbed onto the s-SiO 2 particles are fully desorbed before 300 °C.Conversely, the C-Si chains formed by grafted silane chains will only be cleaved at 450 °C-510 °C.Hence, TGA allowed us to determine whether the SiO 2 particles were successfully functionalized with the silane groups [14].

Determining the amount of silane grafting
The number of surface NH 2 groups on the s-SiO 2 particles was determined via chemical titration.First, an HCl solution of known concentration was mixed with s-SiO 2 , as HCl reacts with surface NH 2 groups to form NH 3 Cl (see figures 3-5).Next, the solution was centrifuged, and the supernatant was titrated with NaOH to determine the concentration of HCl remaining in the solution.Subtracting this HCl concentration from the initial HCl concentration gives the number of surface NH 2 groups that reacted with HCl.The procedure details are as follows: 0.1 g of s-SiO 2 powder was first added to 20 ml of 0.2 M HCl, and the mixture was ultrasonicated for 10 min.The mixture was then centrifuged, and three drops of phenolphthalein were added to 10 ml of the supernatant.Titration was then performed with 0.2 M NaOH, and the amount of NaOH added at the point of discoloration was recorded.
The NH 2 content of the s-SiO 2 particles was calculated using the following equation: W is the NH 2 content of the s-SiO 2 particles, in μmol/g; V 1 is the initial volume of the HCl solution, in L; C 1 is the initial concentration of the HCl solution, in M; V 2 is the volume of NaOH that was titrated into the supernatant, in L; C 2 is the concentration of the NaOH solution, in M; M is the weight of the sample, in g.

Characterizing the mode of silane grafting
The surface elemental composition of the s-SiO 2 particles was analyzed using XPS (PHI VersaProbe 4) before and after silanization.As the literature provides that the ideal C/N ratios for fully hydrolyzed and unhydrolyzed APTES are 3 and 9, respectively [13,15], the C/N ratio of the s-SiO 2 particles was calculated using the elemental ratios obtained by XPS, minus their initial C content.These C/N ratios were then used to gauge the mode of silane grafting for each APTES concentration.

Characterization of PMMA composites
The ductility behavior of composite materials was assessed using a universal tensile tester (AGS-X, SHIMADZU, Japan), adhering to the ASTM D638 standard.The surface morphology of the cross-section of the composite material after the tensile test was observed using an Ultra-high Resolution Field Emission Scanning Electron Microscope (UHRFE-SEM: AURIGA, Carl Zeiss, Oberkochen, Germany).Photoelastic measurements involved positioning the stretched specimen between two polarizing plates and photographing it with illumination from a light source below.Dynamic Mechanical Analysis (DMA) aimed to characterize the thermomechanical properties of s-SiO 2 -PMMA composites with different amounts of s-SiO 2 additive.To this end, rectangular samples were fabricated using these materials, and strain sweep tests were performed at a fixed frequency at various temperatures (30 °C-180 °C).The temperature-dependent mechanical properties of the material were determined by measuring the stress in each experiment.Finally, the recorded storage moduli were plotted against tan δ.The experiments were performed at a heating rate of 10 °C min −1 and frequency of 1 Hz, and the length, width, and thickness dimensions of the specimens were 5 cm, 0.8 mm, and 4 mm, respectively.

Results and discussion
Characterization of the silanized structure FTIR was used to compare the chemical structures of the SiO 2 particles before and after silane grafting, whereas TGA was used to confirm silane grafting.The FTIR spectra of the s-SiO 2 and SiO 2 particles are shown in figure 1(a).The peaks at 1100-1200 cm −1 and 467 cm −1 are characteristic peaks corresponding to the asymmetric stretching and bending modes of Si-O-Si.The peaks at 3400-3500 cm −1 , 1637 cm −1 , and 800 cm −1 , on the other hand, correspond to the stretching vibrations of adsorbed waters and the H-OH bending mode of residual waters.In the s-SiO 2 particles, the 3450 cm −1 , 1100 cm −1 , and 950 cm −1 absorptions correspond to the stretching, asymmetric stretching, and bending modes of the surface Si-OH groups.The s-SiO 2 particles also exhibited a C-H stretching mode at 2950 cm −1 and an N-H bending mode at 1650 cm −1 [16].The TGA results are shown in figure 1(b).As weight losses that occur between 100 °C and 200 °C correspond to water loss, while weight losses between 400 °C and 600 °C correspond to the breaking of grafted silane bonds, the TGA data confirms that silane grafting has occurred on the s-SiO 2 particles.

Quantification of silane grafting
In [8], APTES was used to deposit a silane layer on a silicon substrate, and an ellipsometer was used to measure the thickness of the silane layer.Increasing the APTES concentration decreased the hydrolysis, which resulted in self-condensation between the silane molecules.This increased the thickness of the silane layer and induced aggregation [8).In this study, the effects of APTES concentration on the number of surface NH 2 groups on the s-SiO 2 particles were determined via acid-base back titration, where the amount of HCl that reacted with the s-SiO 2 particles was used to calculate the number of surface NH 2 groups.As shown in figure 2, increasing the APTES concentration from 1% to 2% increases the number of surface NH 2 groups.However, a further increase in APTES concentration led to incomplete hydrolysis, silane self-condensation, and aggregation, which reduced the number of NH 2 groups available to react with HCl.Hence, APTES concentrations greater than 2% decreased the number of surface NH 2 groups.
Discerning the mode of silane grafting Figure 3 shows the XPS spectra (i.e., surface elemental compositions) of s-SiO 2 particles prepared with different APTES concentrations.The unmodified SiO 2 particles (0% APTES) had no N atoms on their surface, and their C atoms (9.5%) can be attributed to those derived from the environment and solvent residue.Increasing the APTES concentration from 1% to 2% increased the surface N content of the particles, and the lowest C/N ratio was attained with an APTES concentration of 2%.Further increases in APTES concentration led to incomplete silane hydrolysis, which increased C content and dramatically increased the C/N ratio.The surface element compositions of the s-SiO 2 particles prepared at each APTES concentration are shown in table 1.

Mechanical behaviors of s-SiO 2 -reinforced PMMA composites
Figure 4(a) shows the effects of reinforcement with SiO 2 and s-SiO 2 nanoparticles in terms of the elongation behavior of the PMMA composite.As the silanization of SiO 2 nanoparticles reduced the size of the aggregates, decreased concentrations of stress, it provided more space for void growth and thus gave a dramatic improvement in elongation behavior.Figure 4(b) shows that sub-micron SiO 2 particles are well-dispersed at additive concentrations of 0.05 vol.% and 0.1 vol.%, which is why silanization did not significantly improve elongation behavior.However, large aggregates formed at 0.2 vol.%, which reduced elongation; in this case, silanization caused a significant improvement in elongation behavior.Figure 4(c) shows that micron-sized SiO 2 particles are well-dispersed at all additive concentrations, and they significantly improve elongation behavior as  the debonding of these particles creates several voids.In this case, silanization decreased elongation behavior as the s-SiO 2 particles became more strongly bonded to the polymer matrix, which decreased debonding.
Fracture mechanism of PMMA composites reinforced with s-SiO 2 particles Figure 5 shows the fracture surface of PMMA composites reinforced with varying amounts of s-SiO 2 nanoparticles.Nanoparticles were dispersed as 1-3 μm aggregates enveloped by the polymer matrix, which were plastically deformed by tensile stress.It may be observed that voids formed in the shear band, crack pinning zones, and aggregates and that the number of aggregates increased with increasing s-SiO 2 addition.Nonetheless, large aggregates causing stress concentrations were not formed in any of these cases.Figure 6 shows the fracture surface of PMMA composites reinforced with s-SiO 2 submicroparticles, and it is shown that tensile deformation led to the formation of shear bands and crack pinning zones in the matrix.Some of the s-SiO 2 particles were still enveloped by the polymer matrix, decreasing the void quantity created by particle debonding.Figure 7 shows the   fracture surface of PMMA composites reinforced with s-SiO 2 microparticles, and it is shown that tensile deformation created arc-shaped crack pinning zones and shear bands around the s-SiO 2 particles.As the particles were still enveloped by the polymer matrix, a relatively small quantity of voids were formed by particle debonding.Small matrix-enveloped aggregates may be observed in the 0.2 vol.% specimen.The silanization process significantly enhances the compatibility of these particles with the matrix material, leading to increased interfacial strength and a consequent rise in the storage modulus in the glassy state over samples without any fillers.Despite this enhancement, the glass transition temperature of the silanized composites does not show a marked shift, suggesting that at lower filler concentrations, the interparticle and matrix bonding is not strong enough to hinder the mobility of the polymer chains significantly.On the other hand, inorganic particles that have not undergone silanization exhibit poor compatibility with polymethyl methacrylate (PMMA), resulting in lower interfacial strength and storage modulus compared to PMMA without fillers.Post-silanization, however, the particles' interfacial bonding with PMMA improves due to cross-linking with silane chains, and the composite's storage modulus increases with higher particle loadings.Fibers or particulate fillers, typically inorganic, are utilized to enhance the strength of polymer composites.However, dispersing these nanoparticles within a polymer matrix presents a challenge due to their inherent incompatibility and propensity to aggregate, negatively impacting the nanocomposites' optical and mechanical properties.To address this, silane coupling agents and effective surface modifiers stabilize nanoparticle dispersion.These agents feature a dual-structured chemical composition with hydrophobic heads and hydrophilic tails.Interaction with water triggers the hydrolysis of ethoxy groups on the silane chain into hydroxyl groups, which then bind to the particle surface's  hydroxyl groups, achieving surface modification.Post-silane grafting, these molecular chains fill the voids in the aggregates, promoting their separation.Consequently, the particle surface becomes more hydrophobic, enhancing its compatibility with the polymer matrix.

Conclusion
This study successfully prepared a PMMA composite containing silane-grafted SiO 2 (s-SiO 2 ) additive.Adding an appropriate amount of s-SiO 2 and silane grafting significantly improved composite elongation behavior.The grafting of polymer chains to s-SiO improved their dispersion in the monomer, reducing aggregation.Nanoparticle addition improves elongation behavior through the elongation of pores inside the nanoparticle aggregates.However, nanoparticles tend to form large aggregates, creating stress concentrations that reduce their effectiveness in improving elongation behavior.Silanization reduced the formation of large aggregates, greatly improving the toughening effect of nanoparticle addition.The addition of submicron particles improves elongation behavior through voids formed by particle debonding and the internal pores of their aggregates.However, these particles also form large aggregates that induce stress concentrations.As silanization reduced the formation of large aggregates, an adequate toughening effect was attained even when a large amount of submicron s-SiO 2 particles were added.
In the case of micron-sized particles, the primary mechanism by which they improve elongation behavior is the generation of voids through particle debonding.As silanization improves affinity to the polymer matrix, it reduces the number of voids formed by particle debonding and thus reduces the toughening effect associated with microparticle addition.In summary, silanization improves interfacial cohesion with the polymer matrix, and s-SiO 2 -reinforced composites consistently have higher storage moduli than pure PMMA.Furthermore, the storage modulus increases with s-SiO 2 addition up to 0.1 vol.%.However, as adding these particles does not restrict the movement of PMMA molecular chains, s-SiO 2 did not significantly affect the glass transition temperature.The newly developed s-SiO 2 /PMMA composite material in this study maintains the inherent benefits of PMMA, including its high transparency and low density.These characteristics make it ideal for use in various applications such as lampshades, vehicle windows, advertising billboards, and as a foundational denture material.

Figure 2 .
Figure 2. Number of surface NH 2 groups on s-SiO 2 versus APTES concentration.

Table 1 .
XPS-derived surface element compositions of the silane-modified SiO 2 particles.