Hybrid denture acrylic composites with nanozirconia and electrospun polystyrene fibers.

The processing and characterization of hybrid PMMA resin composites with nano-zirconia (ZrO2) and electrospun polystyrene (PS) polymer fibers were presented in this study. Reinforcement was selected with the intention to tune the physical and mechanical properties of the hybrid composite. Surface modification of inorganic particles was performed in order to improve the adhesion of reinforcement to the matrix. Fourier transform infrared spectroscopy (FTIR) provided successful modification of zirconia nanoparticles with 3-Methacryloxypropyltrimethoxysilane (MEMO) and bonding improvement between incompatible inorganic nanoparticles and PMMA matrix. Considerable deagglomeration of nanoparticles in the matrix occurred after the modification has been revealed by scanning electron microscopy (SEM). Microhardness increased with the concentration of modified nanoparticles, while the fibers were the modifier that lowers hardness and promotes toughness of hybrid composites. Impact test displayed increased absorbed energy after the PS electrospun fibers had been embedded. The optimized composition of the hybrid was determined and a good balance of thermal and mechanical properties was achieved.

Introduction

Composite materials combine the properties of their constituents offering the new material improved properties and enabling the tuning of the properties to fit predefined needs. Hybrid reinforcement composite systems are created with the aim of improving t physical and mechanical properties by a synergy of two or even more reinforcement types. In the wide area of research, hybrid reinforcements were of different combinations: particles and fibers [1-4], two different types of particles [5], particles and whiskers [6], two or three types of fibers [7]. The improvement is, in general, better with multiple rather than a single reinforcement type, so that every one of the added reinforcements improves a different material property. One of the reinforcements should be aimed at improving toughness and the other, for example, improving hardness and elastic modulus [8].

The type, shape, and dispersion of fillers in a composite significantly influence the mechanical and thermal properties of the composite [9-13]. Reducing the size of particles from micro to nano size level may lead to an enhancement of the fillers' influence on the properties of the matrix. Among the various different classes of nanocomposites that have been developed over the past two decades, biocompatible nanocomposites have gained great attention from the research community due to their high potential in saving and prolonging human lives. Along with medicine, dentistry has been focused on the design of biocompatible materials that exhibit high mechanical endurance and chemical resistance with satisfactory aesthetic standards. Particularly, denture materials should express good impact resistance, hardness, and stability in the oral environment. Therefore, the focus in their design should be on the achievement of favorable mechanical properties and durability instead of reparation and replacement [14].

Acrylic resins are the most commonly used type of polymers for dental applications [15, 16]. They possess good chemical resistance, satisfy the aesthetic requirement and are easily processed. However, low impact resistance represents a serious drawback for the acrylic resins’ use.

The idea of this work was to investigate the possibility of designing hybrid composite with specific properties—improved hardness on the surface and higher toughness in the center of the composite bulk [17, 18]. Nano-zirconia particles and electrospun non-wowen PS fibers were selected as reinforcements in the acrylic matrix. Ceramic reinforcements increase hardness and wear resistance of the composite, and the toughness can be improved with the addition of fine, continuous electrospun polymer fibers. The layered composite structure with altered layers of zirconia particles reinforced matrix and electrospun polymer fibers was prepared. Mechanical properties were tested in order to document the advantages of combining reinforcements as described.

Ceramic oxides, such as zirconia (ZrO2), have been proven to be an excellent candidate for PMMA filler, due to its high hardness values and thermal resistance. Although ceramic materials can offer improvements in mechanical and thermal properties, their incompatibilities with polymers sometimes lead to agglomeration, diminishing the reinforcing potential of the nanoparticles. Improved ceramic/polymer adhesion can be achieved by coupling agents and nanoparticles coatings [19-23] Interface properties between nanoparticles and polymer matrix are modified by attaching the nanoparticles to the matrix. All these actions result in the improved mechanical and other functional properties of nanocomposites. Modification of nanoparticles can be chosen according to the polymer matrix by silanes, tetraethyl orthosilicate (TEOS), titanium isopropoxide (TIP), etc. [24-26]. For acryl-based matrices, silanes are the most used surface modifiers. Silanes are commonly tri-alkoxysilane esters, with three alkoxy groups directly bonded to the silicon atom [27-29].

Previous studies have also introduced dental composite materials with ceramic nanoparticles only, short fibers (glass, aramid, nylon, polypropylene, polyethylene, and others), electrospun polymer and ceramic nanofibers or electrospun nanofibers doped with nanoparticles [3, 30–36]. However, separate incorporation of nanofibers and nanoparticles could enable the modification of different properties by varying the constituent concentrations.

This paper is aimed at optimizing the composition of a hybrid acrylic composite with ZrO2 nanoparticles and electrospun polystyrene (PS) fibers and to explore their influence on thermal and mechanical properties of the obtained nanocomposites. Zirconia nanoparticles were functionalized with 3-Methacryloxypropyltrimethoxysilane (MEMO) silane to improve matrix-particle bonding. It is well known that zirconia nanoparticles enhances the hardness, thermal and wear properties, while polymer fibers improve toughness [37]. These mechanical properties are very important for polymer composites in exploitation, especially in dentistry. Although the static mechanical behavior of composites in dentistry is well known and widely investigated, the dynamical loading and conditions under fracture are still developing [38, 39]. This work will aid researchers in dealing with the optimization of processing parameters for the production of composite materials with the desired advanced properties.

Experimental

Materials

Commercial acryl denture material „Simgal-Acryl R", Galenika AD, Belgrade, Serbia, was used as a polymer matrix. It is a two-component system consisting of a powder and a liquid. The powder consists of a PMMA copolymer and the initiator benzoyl peroxide (BPO) in a concentration of 1.1% w/w. The liquid was made of methyl methacrylate (calc) 94.15% w/w; acid as methacrylic acid 19.8 ppm w/w; N, N-dimethyl-p-toluidine as accelerator 0.85% w/w; ethylene glycol dimethacrylate as cross-linking agent 5.00% w/w; water 27 ppm w/w. Nanopowder of ZrO2 (with a particle size ~100 nm), Sigma Aldrich, was used as particle reinforcement in the composite. 3-Methacryloxypropyltrimethoxysilane (MEMO) (Dynasylane, Evonik Industries) was used for surface modification of zirconia. Toluene and hexane (Sigma Aldrich) were used as solvents. Polystyrene (PS) used to obtain electrospun fibers was purchased as Empera®251N from Ineos Nova. Solvent for PS solution was 99.8% dimethylformamide (DMF), purchased from Sigma-Aldrich.

Modification of zirconium oxide nanoparticles

5 g of ZrO2 nanoparticles were dispersed in 150 ml of toluene in a round-bottom flask equipped with a reflux condenser under the flow of nitrogen. When the boiling point of toluene was reached, 1 g of MEMO silane was added and the resulting white suspension was stirred and refluxed for 22 h. After the completion of the reaction, the particles were filtrated and washed with hexane to remove the unreacted silane. The particles were dried at 40 °C in an oven for 12 h and then used for the preparation of nanocomposites [40].

Electrospinning of PS fibers

Electrospinning (Electrospinner CH-01, Linari Engineering) was performed with a 20 ml plastic syringe with a metallic needle of 1 mm inner diameter set vertically on the syringe pump (R-100E, RAZEL Scientific Instruments) with 15 cm distance from the needle tip to the collector, and the high-voltage power supply (Spellman High Voltage Electronics Corporation, Model: PCM50P120) set to a voltage of 28 kV at the room temperature (25°C) and the humidity of 47%. The flow rate of the polymer solution was 5.0 ml/h. The concentration of PS in DMF solution prepared for electrospinning was 22 wt. %.

Composite preparation

A neat polymer matrix was obtained by mixing a two-component system (liquid volumetric ratio of 2.5:1) for 30 seconds. After that, the paste was processed in an aluminum mold under mechanical pressure with the room temperature of polymerization for 20 minutes. All samples had the dimensions required for the impact test (60 x 60 x 3.5 mm). For the composite processing, the particles were surface-modified to obtain a good dispersion. The nanoparticles were first dispersed in a liquid monomer in an ultrasonic bath for 1 hour and then mixed with a powder to initiate the polymerization. After that, the paste was poured in a mold. The samples with PS fibers were produced (in the mold) by a modified lay-up process; alternating layers of initiated paste with nano zirconia and electrospun fibers. The compositions of a series of samples that were prepared are presented in

Characterization

The microstructure of the composites was studied by SEM microscopy using a Tescan Mira3 XMU field emission scanning electron microscope (FE-SEM) operating at 10 kV. A thin gold layer was deposited on the specimen surfaces before examination.

Image analysis was performed by Image Pro-Plus 4.0 software (Media Cybernetics) that provided the information about PS fiber diameter distribution.

Fourier transformed infrared (FTIR) analysis was performed to investigate bonding between ZrO2 nanoparticles and the matrix. FTIR spectra of the samples in KBr discs were obtained by transmission spectroscopy (Hartmann & Braun, MB-series). The FTIR spectra were recorded between 4000 and 400 cm–1 wavenumber region at a resolution of 4 cm–1.

Thermal analysis of composites was performed on a device for differential scanning calorimetry (DSC) in a temperature range from 24°C to 160°C (Q10, TA Instruments) under a dynamic nitrogen flow of 50 ml min–1. Samples of 7–9 mg were investigated. The samples were heated up at a rate of 10°C min–1. The glass transition temperature was determined at the midpoint of the step-transition for each sample. The Tg values were confirmed by the use of the derivative curve.

Mechanical characterizations of the samples were performed by Vickers microhardness (HV) tester "Leitz, Kleinharteprufer DURIMET I", using a load of 4.9 N. The loading time was 15s. Six indentations were made, yielding twelve indentation diagonal measurements, from which the average hardness could be calculated. The indentation was performed at room temperature.

Impact test was performed using Puncture Impact testing machine HYDROSHOT HITS-P10. The clamping plates with an aperture 40 mm in diameter and clamping pressure of 0.55 MPa were used. The striker with a hemispherical head, 12.7 mm in diameter, was loaded with programmable velocity, height and attained value of depth. In that manner, it was possible to control the impact energy. The data for the force, deflection, velocity and energy with time were recorded. The impact speed was set at 1 m/s and the maximum load was 10 kN. This loading regime could be considered as intermediate, which is perfectly appropriate for denture loading conditions [39, 40]. All the samples were of the same dimensions (60 x 60 x 3.5 mm). Tests were performed on five specimens according to the ASTM D 3763–15, and the results were presented as mean values with standard deviations. The data were analyzed in terms of the maximum load, energy corresponding to the maximum load and total energy.

Results and discussion

FTIR analysis

The FTIR spectra of both unmodified and modified zirconia nanoparticles and the composite with modified zirconia are presented in . All spectra have a peak at 754 cm−1 that is attributed to Zr–O stretching vibrations at ZrO2 nanoparticles. Characteristic acrylate CH3 vibration of MEMO silane was observed at 1173 cm-1 in the spectrum of modified zirconia. The peak at 1721 cm-1 that is associated with carbonyl stretching band C = O which is present in the silane coupling agent (MEMO) was observed in the spectrums of modified zirconia and the composite (shifted to 1733 cm-1) [41-43]. The presence of adsorbed water was confirmed by the Zr–H2O flexion at 1635 cm−1.

FTIR spectra of: a) neat ZrO2 particles, ZrO2 particles modified with MEMO silane and a composite reinforced with ZrO2/MEMO and b) hybrid composites A-PS-ZrO2 and A-PS-ZrO2/MEMO.

FTIR spectrums of A-PS-ZrO2 and A-PS-ZrO2/MEMO are presented in . All the spectra have peaks in the region of 2995–2840 cm-1, which are assigned to the stretching of the C–H bonds contributed mostly to PMMA and PS. The peak at 749 cm-1 is attributed to Zr–O stretching vibrations from nanoparticles in all the spectrums. Double bond C = C stretch which is sensitive to ring strain vibration at 1649 cm-1, indicated conjunction with the phenyl group in PS [44– 46], emphasized with MEMO silane. An increased intensity of the signal at 960 cm-1 in A-PS-ZrO2/MEMO compared to A-PS-ZrO2 indicates the formation of Si–O–Zr bond [45].

FE-SEM analysis

Morphology and size of PS fibers were observed by FE-SEM analysis. The size distribution of PS fibers was obtained using image analysis tools and the results are presented in . PS diameter distribution with mean diameter Dmean = 1.51 μm (standard deviation = 0.52 μm) was best fitted with the Lognormal distribution curve.

(a) FE-SEM micrograph of PS fibers and (b) size distribution determined by image analysis.

FE-SEM images of cross-sections of the polymer after the impact testing are presented in Fig 3. Shows that zirconia agglomerates observed in the sample with unmodified particles had larger diameters and consisted of a larger number of individual particles, () while surface modification of nanoparticles with MEMO silane () enabled aggregates to be smaller in diameter and more evenly spaced. In the areas with fibers are presented. The modification of nano zirconia with MEMO silane produced a monolayer of silane on the surface of the particles, and promotes deagglomeration in the polymer matrix because of the steric hindrance [47].

Fig 3

FE-SEM impact fracture surface images of: a) and c) A-PS-ZrO2; b) and d)) A-PS-1% ZrO2/MEMO.

DSC analysis

Results of DSC analysis are presented in and with corresponding values of glass transition temperature (Tg). Because there is no miscibility of PMMA and PS, there is no evident change in Tg values [48]. Also, it is possible that the values of Tg for PS and PMMA could have overlapped each other with additional curing. Zirconia behave as highly functional physical cross-links, and hence reduce the overall mobility of the polymer chains, even when interactions with the polymers are only on a physical level [49, 50]. The embedding of modified nano zirconia slightly increases the Tg of the composite as a consequence of an interaction between the modified zirconia interface and acrylic resin [51]. Interfacial Si-O bond formation on the surface of zirconia enables chemical bonding with polymer matrix [50-54]. This also leads to better deagglomeration of nanoparticles. In this case, the mobility of polymer chains was suppressed even better, and Tg for this composite is the highest (Table 2). This further indicates that the thermal properties of the hybrid can also be adjusted with the optimal ratio of zirconia and PS fibers in the composite.

Table 2

DSC results for all PMMA samples.

Sample	Tg, °C
A	104.1
A-ZrO2	104.5
A- ZrO2/MEMO	108.0
A-PS	103.3
A-PS-ZrO2-1.0	103.9
A-PS-ZrO2/MEMO-1.0	104.5

Vickers hardness test

Vickers hardness test reflects the uniformity of reinforcement dispersion in the composites and its resistance to shear stresses under local volume compression. presents Vickers values for the PMMA matrix and the composites. The addition of 1 wt. % zirconia nanoparticles improved microhardness by 3%. In composites with silanized zirconia (ZrO2/MEMO) the effective dispersion and cross-linking was achieved, and thus improvement of HV value of 29% for 1 wt. % modified zirconia resulted. On the other hand, the introduction of PS fibers leads to lower hardness values. The PMMA and PS are immiscible and don't interact easily. Added PS fibers didn't interact with the PMMA paste during the preparation of the composite. But presence of PS fibers have influenced polymerization of PMMA, and resulted of phase separation in hybrid composite.

This leads to interfacial tension in those areas which is followed by attraction of nanoparticles in the vicinity of PS fibers. This influence of the composites’ mechanical behavior of leads to lower hardness and Tg [55-57].

In order to emphasize the influence of different nanoparticle concentrations in the hybrid composites, samples with 0.5 wt. % of MEMO silane functionalized ZrO2 nanoparticles were also subjected to micro Vickers test. The hardness growth trend remained the same–nanoparticles ZrO2/MEMO offer improved hardness due to higher compatibility of MEMO functional groups with PMMA matrix. The presented results revealed that the hardness could be adjusted by optimizing the content ratio of modified zirconia and PS fibers.

Microhardness testing is based on the local plastic deformation of a sample under applied stress in the vicinity of the indenter. Glass transition temperature depends of the structure and morphology of polymer chains and its behavior in higher temperatures. In glassy state critical stress for plastic deformation of amorphous polymer requires movements of macromolecule bundles against the resistance of the stiff chain segments., hardness for amorphous polymers could be higher than for semi-crystalline polymers at temperatures under Tg.

It is well known that polymer composites in the glassy state are sensitive to free volume change, and the Tg and hardness could correlate to this [58-62]. The temperature coefficients of the molar volume, free volume and enthalpy change of the glass–rubber transition are closely related to the cohesive energy density of the polymer. The CED is also the main factor determining hardness. Many analytical theories correlate with this phenomenology applied to nanocomposites [62-67]. This correlation of the composites examined in this work is presented in . There was a good linearity obtained with a correlation coefficient R2 = 0.93321. This result is in agreement with the assumptions of the influence of polymer matrix morphology on the mechanical properties of composites [58]. In an amorphous polymer matrix, the filler can be distributed freely. Composite with surface-modified nanoparticles provides stronger resistance to plastic deformation as it is chemically bound to the matrix. The addition of polymer fibers leads to certain relaxation in mechanical response to indentation, while in the hybrid composite some of the nano zirconia were constrained between PS fibers.

Impact test

The position of a sample in the impact machine and samples before and after the impact test is presented in

The results of the controlled energy impact test are presented in . The impact behavior of the hybrid nanocomposites with PS fibers and modified particles was significantly improved, compared to the pure PMMA. presents the absorbed energy values. Absorbed energy is defined as the energy difference between the total energy and the energy at peak load. As the composite materials are brittle, it was assumed that energy up to the peak load was due to the elastic deformation of the sample and that beyond the peak load, energy was spent on creation and propagation of cracks.

As expected, sample A-PS showed the highest ability to absorb energy during the impact, almost 92% higher than the pure PMMA.

FE-SEM analysis showed the difference in the observed matrix-PS fiber interface after the impact test. Acrylic resin with PS fibers has a clear and smooth surface that indicates poor contact between the two polymers (. In the case of a matrix with unmodified ZrO2 particles, it could be seen that particles built some agglomerates () at PS surface and slightly improved contact between the fibers and the matrix. Surface modification of ZrO2 particles with MEMO silane significantly improved the compatibility of the interfaces in a hybrid composite A-PS-ZrO2/MEMO (). Modified composite matrix (A-ZrO2/MEMO) filled the space between the fibers and improved the contact between the matrix and the fibers. This indicated that the interactions between the fibers and the matrix were strong enough to allow load transfer from the matrix to the fibers, which should ensure better mechanical properties of the processed hybrid nanocomposite [41, 42]. The interfacial adhesion strength between the matrix and the fibers affected the impact property of composites and resistance to crack generation and propagation [65-69]. These failure mechanisms led to the high absorption of impact energy as a consequence of dissipation during crack propagation. FE-SEM analysis indicates a brittle fracture tendency in specimens with PS fibers and hybrids with untreated zirconia (river pattern associated with crack propagation).

FE-SEM images of fracture surfaces: a) A-PS, b) A-PS-ZrO2, c) A-PS-ZrO2/MEMO and d) corresponding Force-time curves from impact test.

For the hybrid composites with modified zirconia, , with the presence of flexible − O−Si−O− bond [24-26] consequently provide better adhesion to the matrix and cracks found difficult to propagate. Brittle to ductile transformation of failure mechanism was obtained [65-67].

Results of impact behavior show the possibility of composite mechanical properties modulation, [70]. Incorporation of pure brittle zirconia, prone to agglomeration, reduced absorbed energy, while it was successfully compensated latter with the combination of modified particles and the electrospun polymer fibers. Therefore, sample A-PS-ZrO2/MEMO-1.0 shows the value of total absorbed energy close to the one of A-PS, and the shape of failure mode which is, following the assumption, based on FE-SEM analysis.

A comparison of results presented in this paper and the literature is presented in Table 5. It is evident that for some systems of hybrid reinforcements design of mechanical properties is possible with the proper combination of constituents according to exploitation requirements.

Table 5

Performance parameters comparison of the given system with the literature data.

	Matrix	Reinforcements	Mechanical properties	Reference
1.	Acrylic Denture Base Material	1% Silanized zirconium oxide (ZrO2) nano filler and 2.5%electrospun PS fibers	Absorbed Impact energy improvement 70%,Hardness improvement 10%	This paper
2.	Acrylic Denture Base Material	Silanized zirconium oxide (ZrO2) nano filler and plasma treated polypropelene (PP) fibers	Tensile strength, improvement 44%	[1]
3.	PMMA	PP fibers 2.5 t.%)/Al2O3 nanoparticles (1 wt.%)	Impact strength improvement 119%Surface hardness improvement 4.2%	[2]
4.	Denture base PMMA	ZrO2-Al2O3	Tensile strength decrease 17.0% (highest value for 100% ZrO2)Fracture toughness improvement 32.5% (ZrO2/Al2O3 = 20/80)	[5]
5.	Denture base PMMA	ZrO2/Aluminum borate whiskers	Surface hardness improvement 26.4% (reinforcement ratio 1:2, 3% ZrO2)Flexural strength improvement 52.3% (reinforcement ratio 1:2, 2% ZrO2)	[6]
6.	Denture base PMMA	Glass/ UHMW PE fibers in form of fabrics5.3%	Flexural strength 90% and modulus 76%	[7]

Conclusion

Properties of novel hybrid resin composites with nano-zirconia and electrospun PS polymer fibers were presented in this study. The surface modification of ceramic reinforcement was introduced to improve the interaction of particles and the matrix. FTIR analysis confirmed the successful modification of nanoparticles with coupling agents, which provided better matrix-particle bonding and resulted in better mechanical properties. Furthermore, the silane modification of zirconia ensured more favorable dispersion, which further improved the stiffness of the composites. The research was carried out in order to determine the change in microhardness and impact behavior with different content of the constituents and their content ratio. Electrospun non-woven PS fibers should be a promising solution for the compensation of increased brittleness brought on by the incorporation of ceramic nanoparticles (that are) prone to agglomeration in the polymer matrix. DSC analysis was useful in the determination of the obtained composites’ thermal properties. The slight change in Tg was detected as an influence of different hybrid compositions. A nearly linear correlation between Tg and microhardness was obtained, which could be explained by a change in cohesive energy density. Microhardness and impact test revealed that the optimal results were achieved by a combination of PS fibers and ZrO2/MEMO in an acrylate matrix, where the fibers were able to compensate for brittleness caused by the ceramic nanoparticles. Moreover, the introduction of modified particles as reinforcement in the matrix improved the contact of the matrix to PS fibers and realized the optimal combination of materials’ reinforcement in this composite. Ceramic nanoparticles, which possess appropriate surface modification; and non-woven polymer fibers are proven to be good candidates for incorporation in the acrylic matrix to successfully modulate of mechanical and thermal properties of the hybrid nanocomposite material.

16 Sep 2019

PONE-D-19-20656

Hybrid denture acrylic composites with nanozirconia and electrospun polystyrene fibers

PLOS ONE

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Reviewer #1: This manuscript reports the preparation of a hybrid nano-composites composed of nano-zirconia and electrospum polystyrene.

Results of the studies are very interesting an

1) first three sentences of the abstract are controversial and do not present manuscript well.

2) over all English needs careful proof reading

3) uniqueness aspects of the proposed work is not discussed well in introduction.

4) compare the performance parameters of this system with literature.

5) what are the prospects of this research.

Reviewer #2: The research work presented in the manuscript is scientifically sound and well written.

However, before accepting the same please do the following corrections:

1. Commercial acryl denture material „Akril R“, do the needful correction.

2. Fe-SEM images are not very clear and donot satisfy the claimed findings. Kindly elaborate further. Also, for comparison with the effect of PS, the image 3c should also be of 500 nm scale.

3. No rationale for better dispersion of ZrO2 with surface modification and presence of PS fibre is given. Explanation required.

4. Are the DSC images plotted (not original). Provide the original DSC images. Further, how much weight of each sample was taken for the analysis. The variation in the thermal stability and changes with different composition need to be further elaborated in the section, for better understanding of the science behind the same. Use suitable reference for the same.

5. The improvement in hardeness with modified ZrO2 was 29%, however, the change is TG is not of similar extent. Why? Further, with addition of PS fibre the hardness decreases, does PS lead to poor dispersability of the nanoparticles?

6. A-PS and A-PS-ZrO2/MEMO have similar hardness. Elaborate. What is the significance of using nanoparticles if only PS can improve the overall mechanical property of the matrix.

7. Indicate in the Fig 8 (mark or highlight the same in the Fig), how FE-SEM can indicate the brittle fracture tendency and its propagation.

8. A comparison table of other reported modification of the same matrix using nanoparticles with the present reported system would likely improve the quality of work.

9. The author could also cite the following articles at suitable position (for the improvement in impact strength with nanoparticle dispersions).

i. NiO nanofiller dispersed hybrid Soy epoxy anticorrosive coatings

ii. High performance anti-corrosive epoxy–titania hybrid nanocomposite coatings

iii. High-Performance Soya Polyurethane Networked Silica Hybrid Nanocomposite Coatings

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Reviewer #1: No

Reviewer #2: No

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28 Nov 2019

Dear Editor and reviewers,

Thank you for your comments and suggestion that had helped to improve our paper.

We hope that our enclosed corrections will be full field and satisfied the requirements.

Best regards,

Autors

Reviewers' comments:

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Reviewer #2: Yes

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Reviewer #1: This manuscript reports the preparation of a hybrid nano-composites composed of nano-zirconia and electrospum polystyrene.

Results of the studies are very interesting an

1) First three sentences of the abstract are controversial and do not present manuscript well.

Thank you for this remark. We have corrected the first sentences in Abstract:

The processing and characterization of hybrid PMMA resin composites with nano-zirconia (ZrO2) and electrospun polystyrene (PS) polymer fibers were presented in this study. Reinforcement was selected with the intention to tune the physical and mechanical properties of the hybrid composite. Surface modification of inorganic particles was performed in order to improve the adhesion of reinforcement to the matrix.

2) Over all English needs careful proof reading

We consulted the native English lector and correction of English was done.

3) Uniqueness aspects of the proposed work is not discussed well in introduction.

Thank you for this remark. It was very helpful for better impact of our work. We have made reorganization of Introduction with all suggested corrections.

4) Compare the performance parameters of this system with literature.

We have done suggested comparison in Table 5:

A comparison of results presented in this paper and the literature is presented in Table 5. It is evident that for some systems of hybrid reinforcements design of mechanical properties is possible with the proper combination of constituents according to exploitation requirements.

Table 5. Performance parameters comparison of the given system with the literature data

Matrix Reinforcements Mechanical properties Reference

1. Acrylic Denture Base Material 1% Silanized zirconium oxide (ZrO2) nano filler and 2.5 %electrospun PS fibers

Absorbed Impact energy improvement 70 %,

Hardness improvement 10 % This paper

2.

Acrylic Denture Base Material

Silanized zirconium oxide (ZrO2) nano filler and plasma treated polypropelene (PP) fibers tensile strength, improvement 44% 1

3. PMMA PP fibers 2.5 t.%)/Al2O3 nanoparticles (1 wt.%) Impact strength improvement 119%

Surface hardness improvement 4.2% 2

4. Denture base PMMA ZrO2-Al2O3 Tensile strength decrease 17.0% (highest value for 100% ZrO2)

Fracture toughness improvement 32.5% (ZrO2/Al2O3=20/80) 5

5. Denture base PMMA ZrO2/Aluminum borate whiskers Surface hardness improvement 26.4% (reinforcement ratio 1:2, 3% ZrO2)

Felxural strength improvement 52.3% (reinforcement ratio 1:2, 2% ZrO2) 6

6. Denture base PMMA Glass/ UHMW PE fibers in form of fabrics

5.3% flexural strength 90% and modulus 76% 7

5) What are the prospects of this research?

Hybrid reinforcement systems have been created with aim to improve physic mechanical properties by synergy of dual reinforcements. This improvement should e better than adding them separately. So the contribution of this paper is developing of hybrid composites with designed properties according exploitation conditions. We have corrected the Introduction and added some references.

Composite materials combine the properties of their constituents offering the new material improved properties and enabling the tuning of the properties to fit predefined needs. Hybrid reinforcement composite systems are created with the aim of improving t physical and mechanical properties by a synergy of two or even more reinforcement types. In the wide area of research, hybrid reinforcements were of different combinations: particles and fibers [1- 4], two different types of particles [5], particles and whiskers [6], two or three types of fibers [7]. The improvement is, in general, better with multiple rather than a single reinforcement type, so that every one of the added reinforcements improves a different material property. One of the reinforcements should be aimed at improving toughness and the other, for example, improving hardness and elastic modulus [8].

The type, shape, and dispersion of fillers in a composite significantly influence the mechanical and thermal properties of the composite [9-13].

1. Ismail IJ, “Development and Performance of Composite from Modified Nano Filler with Plasma Treated Fiber and Heat Cured Acrylic Denture Base Material on Some of Its Properties – In Vitro Study“, International Journal of Science and Research (IJSR) Volume 6 Issue 3, March 2017

2. Muklif OR, Ismail IJ, “Studying the effect of addition a composite of silanized nano-Al2O3 and plasma treated polypropylene fibers on some physical and mechanical properties of heat cured PMMA denture base material,” Journal of Baghdad College of Dentistry, vol. 27, no. 3, pp. 22–27, 2015.

3. Gad MM, Al-Thobity AM, Rahoma A., Abualsaud R, Al-Harbi AF, Akhtar S, “Reinforcement of PMMA Denture Base Material with a Mixture of ZrO2 Nanoparticles and Glass Fibers“, Hindawi, International Journal of Dentistry, Volume 2019, Article ID 2489393, 11 pages, https://doi.org/10.1155/2019/2489393

4. Gad M, Fouda S, Al-Harbi F, Napankangas R, Raustia A, “PMMA denture base material enhancement: a review of fiber, filler, and nanofiller addition,” International Journal of Nanomedicine, vol. 12, pp. 3801–3812, 2017.

5. Alhareb AO, Ahmad ZA, “Effect of Al2O3/ZrO2 reinforcement on the mechanical properties of PMMA denture base,” Journal of Reinforced Plastics and Composites, vol. 30, pp. 1–8, 2011.

6. Zhang XY, Zhang XJ, Huang ZL, Zhu B., Chen RR, “Hybrid effects of zirconia nanoparticles with aluminum borate whiskers on mechanical properties of denture base resin PMMA,” Dental Materials Journal, vol. 33, no. 1, pp. 141–146, 2014.

7. Yu SH, Lee Y, Oh S, Cho HW, Oda Y, Bae JM, “Reinforcing effects of different fibers on denture base resin based on the fiber type, concentration, and combination,” Den. Mat. Journal, vol. 31, no. 6, pp. 1039–1046, 2012.

8. Lazouzi G, Vuksanović M, Tomić N. Z, Mitrić M, Petrović M, Radojević V, Jančić Hainemann R, Optimized preparation of alumina based fillers for tuning composite properties,Ceramics International, 7442-7449, 2018,

9. Salih SI, Oleiwi JK, Hamad QA, “Investigation of fatigue and compression strength for the PMMA reinforced by different system for denture applications,” International Journal of Biomedical Materials Research, vol. 3, no. 1, pp. 5–13, 2015.

10. Chen S, Liang W, “Effects of fillers on fiber reinforced acrylic denture base resins,” Mid-Taiwan Journal of Medicine, vol. 9, pp. 203–210, 2004.

Reviewer #2: The research work presented in the manuscript is scientifically sound and well written.

However, before accepting the same please do the following corrections:

1. Commercial acryl denture material ``Akril R`` do the needful correction.

The correction was done:

„Simgal-Acryl R“, Galenika AD, Belgrade,

2. Fe-SEM images are not very clear and do not satisfy the claimed findings. Kindly elaborate further. Also, for comparison with the effect of PS, the image 3c should also be of 500 nm scale.

Thank you for this suggestion. In Manuscript were presented samples A-PS-ZrO2; and A-PS-3% ZrO2/MEMO with 500 nm scale and only A-PS-3% ZrO2/MEMO with 5 �m scale. Now we added the photo of A-PS-ZrO2 sample with same magnification (5 �m scale). We also have corrected the omitted marks of samples.

Fig. 3. FE-SEM images of impact fracture surface of: a) and c) A-PS-ZrO2; b) and d)) A-PS-1% ZrO2/MEMO

3. No rationale for better dispersion of ZrO2 with surface modification and presence of PS fibre is given. Explanation required.

The followed text of FESEM description is corrected:

FE-SEM images of cross-sections of the polymer after the impact testing are presented in Fig. 3. Shows that zirconia agglomerates observed in the sample with unmodified particles had larger diameters and consisted of a larger number of individual particles, (Fig. 3a) while surface modification of nanoparticles with MEMO silane (Fig. 3b) enabled aggregates to be smaller in diameter and more evenly spaced. In Fig 3c) and 3d) the areas with fibers are presented. The modification of nano zirconia with MEMO silane produced a monolayer of silane on the surface of the particles, and promotes deagglomeration in the polymer matrix because of the steric hindrance [47].

47. Mohammadnezhad G, Dinari M, Soltani R., Bozorgmehr Z, “Thermal and mechanical properties of novel nanocomposites from modified ordered mesoporous carbon FDU-15 and poly(methyl methacrylate) “, Appl. Surf. Sci. 346 (2015) 182-188.

4. Are the DSC images plotted (not original). Provide the original DSC images. Further, how much weight of each sample was taken for the analysis. The variation in the thermal stability and changes with different composition need to be further elaborated in the section, for better understanding of the science behind the same. Use suitable reference for the same.

Thank you for this remark. We have paid attention to correct experimental part, and discussion after that. Also, we included original DSC image. Following inserted text and Literature are presented below:

Thermal analysis of composites was performed on a device for differential scanning calorimetry (DSC) in a temperature range from 24 ºC to 160 ºC (Q10, TA Instruments) under a dynamic nitrogen flow of 50 ml min–1. Samples of 7-9 mg were investigated. The samples were heated up at a rate of 10 °C min–1. The glass transition temperature was determined at the midpoint of the step-transition for each sample. The Tg values were confirmed by the use of the derivative curve.

***

Zirconia behave as highly functional physical cross-links, and hence reduce the overall mobility of the polymer chains, even when interactions with the polymers are only on a physical level [49, 50]. The embedding of modified nano zirconia slightly increases the Tg of the composite as a consequence of an interaction between the modified zirconia interface and acrylic resin [51]. Interfacial Si-O bond formation on the surface of zirconia enables chemical bonding with polymer matrix [50-53]. This also leads to better deagglomeration of nanoparticles. In this case, the mobility of polymer chains was suppressed even better, and Tg for this composite is the highest (Table 2).

References:

49. Vacatello M, “Monte Carlo simulations of polymer melts filled with solid nanoparticles“, Macromolecules, 34(6) (2001) 1946–1952.

50. Thomas P, Dakshayini BS, Kushwaha HS, Vaish R, Effect of Sr2TiMnO6 fillers on mechanical, dielectric and thermal behaviour of PMMA polymer, J. Adv. Dielect. 5(2) (2015) 1550018 (11 pages) DOI: 10.1142/S2010135X15500186

51. Tommasini FJ, Cunha Ferreira L, Pimenta Tienne LG , de Oliveira Aguiar V, Prado da Silva MH, da Mota Rocha LF, de Fátima Vieira Marques M, Poly (Methyl Methacrylate)-SiC Nanocomposites Prepared Through in Situ Polymerization, Materials Research. 2018; 21(6): e20180086

52. Abboud, M, Turner M, Duguet E, Fontanille M, PMMA-based composite materials with reactive ceramic fillers Part 1.—Chemical modification and characterisation of ceramic particles. J. Mater. Chem. 7, 1527–1532 (1997).

53. Turner M, Duguet E, Labrugere C, Characterization of silane-modified ZrO2 powder surfaces. Surf. Interface Anal. 25, 917–923 (1997).

54. Otsuka T, Chujo Y, Poly(methyl methacrylate) (PMMA)-based hybrid materials with reactive zirconium oxide nanocrystals, Polymer Journal (2010) 42, 58–65

5. The improvement in hardeness with modified ZrO2 was 29%, however, the change is TG is not of similar extent. Why?

This suggestion for discussion is very usefull for paper, thank you. We added our text in Manuscript:

Microhardness testing is based on the local plastic deformation of a sample under applied stress in the vicinity of the indenter. Glass transition temperature depends of the structure and morphology of polymer chains and its behavior in higher temperatures. In glassy state critical stress for plastic deformation of amorphous polymer requires movements of macromolecule bundles against the resistance of the stiff chain segments. , hardness for amorphous polymers could be higher than for semi-crystalline polymers at temperatures under Tg.

Further, with addition of PS fibbers the hardness decreases, does PS lead to poor dispersability of the nanoparticles?

The PMMA and PS are immiscible and don't interact easily. Added PS fibers didn't interact with the PMMA paste during the preparation of the composite. But presence of PS fibers have influenced polymerization of PMMA, and resulted of phase separation in hybrid composite.

This leads to interfacial tension in those areas which is followed by attraction of nanoparticles in the vicinity of PS fibers. This influence of the composites’ mechanical behavior of leads to lower hardness and Tg [55-57].

***

Composite with surface-modified nanoparticles provides stronger resistance to plastic deformation as it is chemically bound to the matrix.

55. Chuai C, Almdal K, Lyngaae-Jørgensen J, Thermal Behavior and Properties of Polystyrene/Poly(methyl methacrylate) Blends, Journal of Applied Polymer Science, Vol. 91, 609–620 (2004)

56. Ton-That C,. Shard AG, Teare DOH, . Bradley H, XPS and AFM surface studies of solvent-cast PS/PMMA blends, Polymer, Volume 42, Issue 3, February 2001, Pages 1121-1129

57. Lee J.Y, Zhang Q, Emrick T, Crosby A.J, Nanoparticle Alignment and Repulsion during Failure of Glassy Polymer Nanocomposites, Macromolecules 2006, 39, 7392-7396

6. A-PS and A-PS-ZrO2/MEMO have similar hardness. Elaborate. What is the significance of using nanoparticles if only PS can improve the overall mechanical property of the matrix?

Hardness of A-PS was 211MPa while hardness of A-PS-1% ZrO2/MEMO was 269 MPa. Relative to pure PMMA (243 MPa) adding of PS fibers decrease hardness by 13%, while adding of nanoparticles increase by 10%.

The idea of this work was that properties of hybrid composite could be moderate by proper ratio of fibers and nanoparticles in accordance of exploitation requirements. Addition of zirconia nanoparticles should increase the hardness, while addition of nanofibers will improve toughness.

7. Indicate in the Fig 8 (mark or highlight the same in the Fig), how FE-SEM can indicate the brittle fracture tendency and its propagation.

Thank you for this remark. We added explanation of figures 8a and 8b and also for diagram at 8d which is responsible for failure modes detection. The picture from literature data is enclosed which explain the brittle to ductile modes transition.

The interfacial adhesion strength between the matrix and the fibers affected the impact property of composites and resistance to crack generation and propagation [65-69]. These failure mechanisms led to the high absorption of impact energy as a consequence of dissipation during crack propagation. FE-SEM analysis Fig. 8a, b) indicates a brittle fracture tendency in specimens with PS fibers and hybrids with untreated zirconia (river pattern associated with crack propagation).

*****

For the hybrid composites with modified zirconia, Fig. 8c, with the presence of flexible − O−Si−O− bond [24-26] consequently provide better adhesion to the matrix and cracks found difficult to propagate. Brittle to ductile transformation of failure mechanism was obtained [65-67].

Results of impact behavior show the possibility of composite mechanical properties modulation, Fig. 8d [70]. Incorporation of pure brittle zirconia, prone to agglomeration, reduced absorbed energy, while it was successfully compensated latter with the combination of modified particles and the electrospun polymer fibers.

70. Shah V. Handbook of Plastics Testing and Failure Analysis; John Wiley & Sons: New Jersey. 2007.

8. A comparison table of other reported modification of the same matrix using nanoparticles with the present reported system would likely improve the quality of work.

We have done suggested comparison in Table 5:

Table 5. Comparsion the performance parameters of this system with literature

Matrix Reinforcements Mechanical properties Reference

7. Acrylic Denture Base Material 1% Silanized zirconium oxide (ZrO2) nano filler and 2.5 %electrospun PS fibers

Absorbed Impact energy improvement 70 %,

Hardness improvement 10 % This paper

8.

Acrylic Denture Base Material

Silanized zirconium oxide (ZrO2) nano filler and plasma treated polypropelene (PP) fibers tensile strength, improvement 44% [1]

9. PMMA PP fibers 2.5 t.%)/Al2O3 nanoparticles (1 wt.%) Impact strength improvement 119%

Surface hardness improvement 4.2% [2]

10. Denture base PMMA ZrO2-Al2O3 Tensile strength decrease 17.0% (highest value for 100% ZrO2)

Fracture toughness improvement 32.5% (ZrO2/Al2O3=20/80) [5]

11. Denture base PMMA ZrO2/Aluminum borate whiskers Surface hardness improvement 26.4% (reinforcement ratio 1:2, 3% ZrO2)

Felxural strength improvement 52.3% (reinforcement ratio 1:2, 2% ZrO2) [6]

12. Denture base PMMA Glass/ UHMW PE fibers in form of fabrics

5.3% flexural strength 90% and modulus 76% [7]

Comparison of results presented in this paper and literature is presented on Table 5. It could be seen that for some system of hybrid reinforcements design of mechanical properties is possible with proper combination of constituents according to exploitation requirements.

9. The author could also cite the following articles at suitable position (for the improvement in impact strength with nanoparticle dispersions).

i. NiO nanofiller dispersed hybrid Soy epoxy anticorrosive coatings

ii. High performance anti-corrosive epoxy–titania hybrid nanocomposite coatings

iii. High-Performance Soya Polyurethane Networked Silica Hybrid Nanocomposite Coatings

The suggested articles were involved in Introduction for the surface modification and discussion about the improvement in impact strength with nanoparticle dispersions:

Improved ceramic/polymer adhesion can be achieved with coupling agents and nano paticle coatings [18-22]. They modify the interface properties between nanoparticles and polymer matrix by attaching the modified nanoparticles to the matrix resulting in the improved mechanical and other functional properties of nanocomposites. Modification of nanoparticles could be chosen according to the polymer matrix by silanes, tetraethyl orthosilicate (TEOS), titanium isopropoxide (TIP) etc. [23-25].

****

For the hybrid composite with modified zirconia, Fig. 8c, with the presence of flexible − O−Si−O− bond [23-25] consequently have better adhesion to the matrix and cracks found difficult to propagate. Brittle to ductile transformation of failure mechanism was obtained [64-66].

Submitted filename: Answers to reviewers.docx

Click here for additional data file.

3 Dec 2019

Hybrid denture acrylic composites with nanozirconia and electrospun polystyrene fibers

PONE-D-19-20656R1

Dear Dr. Tomic,

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Reviewers' comments:

10 Dec 2019

PONE-D-19-20656R1

Hybrid denture acrylic composites with nanozirconia and electrospun polystyrene fibers

Dear Dr. Tomić:

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Table 1

Samples used for the comparison of thermal and mechanical properties.

Sample	ZrO2, wt. %	PS fibers, wt. %	ZrO2/MEMO, wt. %
A (pure PMMA)	0	0	0
A-ZrO2	1.0	0	0
A- ZrO2/MEMO	0	0	1.0
A-PS	0	2.5	0
A-PS-ZrO2	1	2.5	0
A-PS-ZrO2/MEMO-0.5	0	2.5	0.5
A-PS-ZrO2/MEMO-1.0	0	2.5	1.0

Table 3

Results of Vickers hardness test.

Sample	HV, MPa	St.dev., MPa
A	243	±1
A-ZrO2	250	±5
A- ZrO2/MEMO	313	±8
A-PS	211	±5
A-PS-ZrO2-1.0	229	±8
A-PS-ZrO2/MEMO-0.5	232	±1
A-PS-ZrO2/MEMO-1.0	269	±3

Table 4

Absorbed energy during impact.

Sample	Eabs, J	SD, J
A	0.46	±0.01
A-PS	0.84	±0.03
A-ZrO2	0.28	±0.01
A-ZrO2/MEMO	0.55	±0.02
A-PS-ZrO2	0.51	±0.01
A-PS-ZrO2/MEMO-0.5	0.74	±0.04
A-PS-ZrO2/MEMO-1.0	0.79	±0.05