Tadele Daniel Mekuria ,Lei Wang ,Chunhong Zhang,,Ming Yang ,Qingtao Lv ,Diaa Eldin Fouad

1 Polymer Materials Research Center,Key Laboratory of Superlight Materials and Surface Technology,Ministry of Education,College of Materials Science and Chemical Engineering,Harbin Engineering University,Harbin 150001,China

2 Chemistry Department,College of Natural and Computational Sciences,Assosa University,Ethiopia

Keywords:Silicon nitride (Si3N4) nanoparticles Polyimide composites Mechanical properties Thermal properties Hydrophobicity

ABSTRACT Polyimide (PI) composite films were synthesized incorporating amino modified silicon nitride (Si3N4)nanoparticles into PI matrix via in situ polymerization technique.The mechanical and thermal performances as well as the hydrophobic properties of the as prepared composite films were investigated with respect to the dosage of the filler in the PI matrix.According to Thermogravimetric(TGA)analysis,meaningful improvements were achieved in T5(5%weight loss temperature)and T10(10%weight loss temperature) up to 54.1 °C and 52.4 °C,respectively when amino functionalized nano-Si3N4 particles were introduced into the PI matrix.The differential scanning calorimetry (DSC) results revealed that the glass transition temperature(Tg)of the composites was considerably enhanced up to 49.7°C when amino functionalized Si3N4 nanoparticles were incorporated in the PI matrix.Compared to the neat PI,the PI/Si3N4 nanocomposites exhibited very high improvement in the tensile strength as well as Young’s modulus up to 105.4% and 138.3%,respectively.Compared to the neat PI,the composites demonstrated highly decreased water absorption behavior which showed about 68.1% enhancement as the content of the nanoparticles was increased to 10 wt%.The SEM (Scanning electron microscope) images confirmed that the enhanced thermal,mechanical and water proof properties are essentially attributed to the improved compatibility of the filler with the matrix and hence,enhanced distribution inside the matrix because of the amino groups on the surface of Si3N4 nanoparticles obtained from surface functionalization.

1.Introduction

The outstanding material performances of polyimides such as excellent thermal resistance and thermal stabilities,high chemicals resistance,outstanding corrosion resistance,good mechanical performances and superb adhesive properties made them to be highly researched and used in high-tech fields[1].The main application areas of PIs are aerospace and automotive industries,railways,food packaging,liquid crystal displays,optoelectronic and microelectronic devices in the form of foams,films,fibers and matrix materials [2–4].Nevertheless,further improvements are needed in the mechanical and thermal performances as well as the hydrophobic properties of PIs in order to meet the fast growing and extreme demands in some specific fields such as aerospace industries aeronautics and microelectronic applications.Some of the various approaches for improving the key properties of PIs are modification of the chemical structure by introducing some functional groups into the polymer chains,replacing the symmetrical aromatic ring structures by the asymmetrical ones and incorporation of suitable inorganic reinforcements in the PI matrix [5–10].The incorporation of inorganic nanoparticles in the polyimide matrix is the most commonly used approach to achieve polyimide composites with enhanced material properties [11,12].

Organic–inorganic nanocomposites are a class of fast-growing engineering materials with extensive applications because of their exceptionally improved properties pertaining to the combined effects of the properties of the organic as well as inorganic constituents.Polymer nanocomposites are employed in a range of applications for instance food packaging,medicine,cosmetics,agriculture,construction,aerospace,optics,textiles and semiconductor devices [13–16].Recently,nanoparticles and corresponding nanomaterials have attracted extensive attention in research as well as industry for several applications in the form of reinforcements,sorbents,luminophores,quantum dots or catalysts because of their unique characteristics pertaining to their nano size and superior specific surface area [17].

Silicon nitride (Si3N4),an essential ceramic material has been used in many fields due to its outstanding material properties such as low density,excellent thermal stability and resistance,high mechanical strength,exceptional abrasion and wear resistance,superior thermal shock resistance,good fracture toughness and good oxidation resistance [18–20].It has been proven that Si3N4nanoparticles can be used as efficient ceramic fillers to synthesize polymer composites with improved tribological properties,wear resistance,mechanical performances and thermal stabilities [21–23].For instance,Ramdani and co-workers [22] reported that the mechanical performances and thermal stabilities of polybenzoxazine nanocomposites were highly enhanced up on the incorporation of surface modified Si3N4nanoparticles.Likely,no research works had been reported concerning the incorporation of Si3N4nanoparticles in PI matrix and its influence on the key properties of PI/Si3N4nanocomposite films.

In our current work,a series of novel PI/Si3N4nanocomposite films with various dosages of amino modified Si3N4nanoparticles were successfully fabricated through in situ polymerization technique and successive thermal treatment to achieve complete imidization.The as prepared composite films with various loadings of amino functionalized Si3N4nanoparticles were characterized to explore their mechanical performance,thermal properties and hydrophobic behavior.

2.Experimental

2.1.Materials

Pyromellitic dianyhydride (PMDA,≥ 98%),4,4′-oxydianiline(ODA,98%) and Si3N4nanoparticles (99.9%) with a mean particle size of 20 nm were purchased from Energy Chemicals,China.3-Aminopropyltriethoxysilane (APTES,99%) obtained from Aladdin Co.Ltd,China.Absolute ethanol (99.9%)and N,Ndimethylacetamide (DMAc,99.5%) were purchased from Tianji Fuyu Fine Chemicals Co.Ltd.,China.DMAc was dried by a distillation technique over calcium hydride before utilization.PMDA was refined and vacuum-dried at 80 °C prior to use.The nano-Si3N4particles were dried in a vacuum oven at 80°C.All the other reagents were used without any further treatment.

2.2.Surface functionalization of the nanoparticles

The as received Si3N4nanoparticles were modified using the coupling agent,APTES as diagrammatically described in Fig.1.Specifically,Si3N4nanoparticles were absolutely dispersed in a 250 ml three-necked round bottom flask in ethanol under magnetic stirring at 25 °C.After ultrasonic treatment for 0.5 h,APTES(5 wt%of Si3N4)was gradually added into the uniform Si3N4/ethanol suspension.The suspension was subjected to a reflux condensation at 80°C and kept for 5 h.Finally,the subsequent suspension was washed with ethanol on a centrifuge after being cooled to room temperature.The amino-functionalized Si3N4nanoparticles obtained after drying in an oven overnight at 80 °C were labelled as Si3N4-NH2.

2.3.Preparation of pure PI and PI/Si3N4 nanocomposites

A series of PI nanocomposites with different contents of homogeneously distributed Si3N4nanoparticles were prepared through in situ polymerization and successive thermal treatment to attain complete imidization.Amino functionalized Si3N4nanoparticles were first dispersed in a fresh purified DMAc under ultrasonication for 0.5 h and magnetic stirring for 3 h.Then,a calculated amount of ODA was added to the aforementioned suspension and agitated for additional 0.5 h.Finally,PMDA with equal molar ratio was added into the suspension in four portions and stirred for extra 24 h under nitrogen at room temperature.

Fig.1.Diagrammatic illustration of surface functionalization of Si3N4 nanoparticles.

Fig.2.Representative pictorial elaboration of the preparation of neat PI and its composites films.

As elaborated in Fig.2,the viscous and homogenous PAA/Si3N4suspensions obtained were spread onto clean glass slides and vacuum-dried at 80 °C for 12 h.Subsequently,the samples were subjected to a thermal treatment from 100 to 150 °C for 1 h then 180 to 200 °C for 0.5 h and finally at 260 and 300 °C each for 1 h to ensure complete imidization [2,24,25].After cooling to room,the resulting films were soaked in deionized water and gently detached from the glass slides.The as obtained films were labelled as PIx,where x designates the mass content of amino functionalized Si3N4in the matrix.Similarly,PI composite films with untreated Si3N4nanoparticles were prepared to compare the findings with those of the amino functionalized nanoparticles.Neat PI films were also fabricated following the same procedure,except the incorporation of Si3N4nanoparticles.

2.4.Characterization techniques

The Fourier transform infrared(FTIR)peaks for the surface treated and bare Si3N4nanoparticles were obtained from Perkin–Elmer spectrometer from 4000 to 400 cm-1to confirm the successful functionalization of Si3N4nanoparticles with APTES.Similarly,the IR spectra of pure PI and PI/Si3N4composite films with various dosages of modified Si3N4nanoparticles were obtained.DSC (differential scanning calorimetry) measurements were done on TA instrument(Q200)from 20 to 400°C at 20°C﹒min-1under nitrogen flow at the rate of 50 ml﹒min-1.The Thermogravimetric Analysis(TGA) was performed using TA instrument (Q50) from 20 to 800°C at 20°C﹒min-1under nitrogen environment at 50 ml﹒min-1ate 25 °C.The tensile tests of PI without filler and its composites with pristine and modified Si3N4nanoparticles were carried out on a universal tensile testing apparatus (Instron 3365) with 2 mm﹒min-1of crosshead movement speed.The cross-sectional surface morphologies of virgin PI and the composites with pristine and functionalized Si3N4nanoparticles were investigated by a SEM(scanning electron microscope,model FEI QANTA 200) at 20 kV after coating the samples with a thin conductive layer of platinum.The water surface contact angle test for the bare PI and the composites was done on a goniometer(JYSP-360,Japan)equipped with a digital camera to take photos of the droplet of water on the film surfaces in four replicates at 25 °C.The water absorption features of neat PI films and PI/Si3N4composites were assessed based on the standard test method of ASTM D570-81.Initially,surface moisture of the specimens was eliminated by heating the films in a heating oven at a moderate temperature for 1 h.Then a portion of the films was weighed out on an analytical balance to acquire Wd(mass of dry film) of the films.The aforementioned films were soaked into deionized water at room temperature.After 24 h,the films were taken out of the water and weighed rapidly after wiping away the surface water to obtain Ww(mass of wet film) and the process was repeated every 24 h until the films attained a stable weight.The percentage of water absorption (Wa) of the films was estimated using Eq.(1) and the average of four measurements was reported.

3.Results and Discussion

3.1.Amino functionalization of Si3N4 nanoparticles

The surface of Si3N4nanoparticles was modified with a typical silane coupling agent,APTES as diagrammatically elaborated in Fig.1,to achieve an improved interfacial interaction with the PI matrix.The FT-IR spectra of functionalized and pristine Si3N4nanoparticles are revealed in Fig.3.

As clearly shown in Fig.3,a broad band was detected at 3411 cm-1which is ascribed to the stretching vibration of OH bonds due to the moisture absorbed on the surface of Si3N4nanoparticles [26].Both pristine and modified Si3N4samples exhibited strong peak at 931 cm-1which is basically ascribed to the backbone vibration mode of Si—N—Si [27].Compared to the untreated Si3N4nanoparticles,the surface modified Si3N4displayed new absorption peaks around 3000–2800 cm-1,which are ascribed to the CH3and CH2stretching vibrations.The intense peaks detected at around 2923 and 1606 cm-1are resulted from the NH stretching and bending vibrations,indicating successful amino grafting on Si3N4nanoparticles [28].

3.2.FT-IR study of PI and PI/Si3N4 composites

FTIR spectroscopy was used to prove the chemical structure of PI and PI/Si3N4composites upon different loadings of modified Si3N4nanoparticles as displayed in Fig.4.

Fig.3.FT-IR spectra of pristine and functionalized Si3N4 nanoparticles.

Fig.4.FT-IR spectra of neat PI and its composites with various contents of pristine and functionalized Si3N4 nanoparticles.

It is clearly depicted in Fig.4 that all the films possess infrared spectra which are characteristic to PI imide groups.For example,imide ring the absorption peaks around 1712 and 1776 cm-1corresponding to C=O symmetric and asymmetric stretching,respectively were seen in all the films.Furthermore,the bands at 721 and 1368 cm-1related to C=O bending and C—N stretching,respectively were observed in all of the films regardless of the nanoparticles incorporated.Due to successful thermal imidization of the film samples,a specific absorption peak of PAA around 1660 cm-1(C=O amide) was not seen in all of the films that confirms complete imidization [3].A characteristic absorption peak was observed around 931 cm-1in all of the composites which is assigned to Si-N-Si stretching vibration [29].Moreover,the peak strength was increased as the doping content of the nanoparticles was increased in the PI system.

3.3.Mechanical properties

Mechanical property is one of the basic features of highperformance engineering materials such as PI composites.Compared to the neat PI,the tensile strength as well as tensile modulus of PI/Si3N4composites was greatly improved when surface modified nano-Si3N4particles were loaded in the PI matrix.As can be clearly seen in Fig.5(b) and (c),the tensile strength as well as the Yong’s modulus of the nanocomposite films were enhanced approximately up to105.4% and 138.3%,respectively when 2%–10% Si3N4nanoparticles were loaded in the matrix.The composites comprising of 2 wt% functionalized Si3N4showed the tensile strength and modulus of about 152.1 MPa and 4.89 GPa,respectively.However,the composites with 2 wt% pristine Si3N4exhibited very small increment in tensile strength and modulus(125.1 MPa and 4.3 GPa)respectively.Highly enhanced mechanical properties of the composites are largely ascribed to the following factors:

(1) the homogeneous dispersion of Si3N4nanoparticles in PI medium because of the surface amino functionalization.(2) the improved compatibility of nanofiller which could lead to the excellent interfacial interaction within Si3N4and the matrix to achieve effective stress transfer from PI matrix to the filler nanoparticles.

3.4.Thermal properties

Additionally,much consideration has been given to the thermal properties of polyimides for high-tech application areas such as aircraft and spacecraft industries.The thermal stabilities and heat resistances of the nanocomposites prepared were investigated by TGA and DSC techniques,respectively.As high performance industrial materials,thermal stability is one of the strategic performances of PI nanocomposites for application in elevated temperature environments [28].

Fig.5.Characteristic Stress-strain curves (a) Tensile strength (b) and tensile modulus (c) of neat PI and its composite films with pristine Si3N4 and various contents of functionalized Si3N4 nanoparticles.

Fig.6.TG curves of pure PI and,its composites with pristine Si3N4 and various contents of functionalized Si3N4 nanoparticles at a heating rate of 20 °C﹒min-1.

Fig.6 and Table 1 exhibit the TGA results of PI and PI/Si3N4composites with pristine Si3N4and various loadings of amino functionalized Si3N4.Fig.6 clearly shows that the T5and T10of the nanocomposites with amino functionalized nanoparticles were noticeably enhanced compared to PI films without filler.However,the enhancement was very low when 2 wt% pristine Si3N4was loaded in the PI system compared to that of the amino-treated Si3N4nanoparticles.As depicted in Table 1,gradual increase in T5from 513.4 °C(for neat PI)to 549.7 °C was observed upon the addition 2 wt% functionalized Si3N4nanoparticles into the PI matrix while it was 534.3 °C for the same dosage of pristine Si3N4.The T5of the composites showed an increase as high as 567.5 °C when the content of functionalized nanoparticles was increased to 10 wt%in the PI matrix which shows an approximate increment by 54.1 °C in comparison with that of neat PI.Similarly,the T10of the nano composite films was also improved from 533.2 (neat PI) to 569.5 °C when 2 wt% functionalized Si3N4was loaded into the PI matrix while this value was 558.5 °C for the loading of 2 wt% pristine Si3N4nanoparticles into the matrix.The improvement in T10reached up to 585.6 °C as the content of the filler was increased to 10 wt%which is about 52.4 °C increment when compared to that of the neat PI films.

Highly enhanced thermal stabilities of PI/Si3N4nanocomposites may be ascribed to the following factors:(1) a uniformly distributed Si3N4which is resulted from the enhanced compatibility of the filler with the PI matrix.(2) A high thermal conductivity of Si3N4nanoparticles,which enhanced the heat dissipation in the PI nanocomposite films [30].(3) The surface modification could improve the dispersion of Si3N4in the PI matrix leading to an increased thermal stability.(4) A strong covalent interfacial adhesion of Si3N4nanoparticles with PI which decreased the mobility of PI chains around the interfaces,and the rigidity of the PI chins could increase the thermal stability of the composite films.Generally,incorporation of suitable contents of Si3N4restricted the decomposition of PI by deferring the leakage of decomposition products from the nanocomposites at high temperatures [10].

Similarly,the thermal resistance of PI and its composites with of pristine Si3N4and different contents of functionalized Si3N4nanoparticles was assessed and the corresponding results are demonstrated in Table 1 and Fig.7.

Table 1 Summary of thermal properties of neat PI and its composites incorporating pristine Si3N4 and different contents of functionalized Si3N4 nanoparticles.

The Tgof PI composites with functionalized Si3N4exhibited a meaningful increase from 298.3 °C (neat PI) to 343.5 °C when 2wt% of the nanoparticles were loaded into the matrix,which is approximately 45.2 °C enhancement.Though,the dosage 2 wt%pristine Si3N4in the matrix displayed improvement of 36.9 °C.The Tgvalue reached up to 348.0 °C when 10 wt% functionalized Si3N4was incorporated in the matrix.Generally,improvement of 49.7 °C in Tgwas achieved upon the incorporation of maximum dosage of amino treated Si3N4in the PI matrix.Essentially,this may be attributed to the prevention of the mobility of the PI chain segments near the loaded Si3N4surfaces by the strong covalent interactions within Si3N4and the PI due to the surface modification of nano-Si3N4particles with APTES which enhanced the dispersion of the nanoparticles in the matrix.Furthermore,the PI molecules were cross-linked with the nanoparticles incorporated and formed networked arrangements and more energy absorption was required to make a transition to glassy states [28].

3.5.Water absorption and water surface contact angle

Resistance to water permeation is a key subject of polymer materials for microelectronics and food packaging applications.As most polymers usually distort in harsh situations such as high moisture areas,the water resistance of the polymers needs to be modified [31].The water absorption behavior of bare PI and composite films with pristine Si3N4and various amounts of functionalized Si3N4nanoparticles were investigated as revealed in Fig.8.

Fig.8.Water absorption property of neat PI and its composite films incorporating pristine Si3N4 and various contents of functionalized Si3N4 nanoparticles.

It can be clearly seen in Fig.8 that the percentage of water absorption (Wa) of the composites was decreased from 1.9% (for neat PI) to 1.7% when 2 wt% pristine Si3N4nanoparticles were loaded into the matrix,while this value was even decreased to 1.4% when equal amount of amino functionalized Si3N4was added into the PI matrix.The water uptake characteristics of PI composites with amino functionalized Si3N4nanoparticles showed a significant decrease as the content of the nanoparticles in the PI matrix was increased and it reached 0.6% when the dosage gets 10 wt%.In general,the hydrophobicity of the composites showed approximately 68.1% enhancement compared to the neat PI matrix.

The enhanced hydrophobic properties may be attributed to the possible interfacial interaction of Si3N4with PI due to the amino groups on the surface of Si3N4particles which was confirmed by SEM analysis in Fig.9.The interfacial interaction of the homogeneously dispersed Si3N4nanoparticles could decrease the hydrophilic components which may rather absorb water by hydrogen bonding with water molecules and this results in the aggregation of water drop on the surface of the composite films as stated by Lu et al.[24].Additionally,some hydrophilic constituents on amino functionalized Si3N4surfaces have been thermally removed during the composite film fabrication step [32,33].Moreover,the interlocked structure of PI-Si3N4system might increase the compactness of the composites,which could improve the water penetration resistance of the composites [3].Additionally,the water surface contact angle (CA) tests were carried out for pure PI and its composites films to explore the effect of Si3N4on the hydrophobic behavior the composites and the resulting digital images and the contact angles are revealed in Fig.10.

Fig.9.SEM images of fracture surfaces of neat PI(a),PI with pristine Si3N4(b)and PI composites with functionalized Si3N4 contents of 2 wt%(c),5 wt%(d),7 wt%(e)and 10 wt% (f).

Fig.10.Plots of water surface contact angle(a)and respective images of neat PI and its composites with pristine Si3N4 and different contents of functionalized Si3N4 nanoparticles.

Neat PI displayed a CA of 58.4° and the composites showed increased CA values from 11.0° to 25.2° when Si3N4content in the PI medium was increased from 2 wt% to 10 wt%.

The high CA values of the nanocomposite films approves that the hydrophobic properties of the composites with functionalized Si3N4nanoparticles are much higher than that of the neat PI,and this may be attributed to the hydrophobic nature of Si3N4nanoparticles and their uniform distribution in the matrix due to the amino groups grafted to nano-Si3N4surfaces [34].

Thus,the amino groups on the surface of the filler could utilize the hydrophilic groups on the matrix and this might improve the hydrophobicity of the composites [35].

3.6.Morphological study

The rupture surface of PI and PI/Si3N4composite films were spotted out by SEM to investigate the dispersion state of pristine and amino functionalized Si3N4in the PI medium as revealed in Fig.9.As shown in Fig.9(a),pure PI film exhibited a homogeneous and smooth faces without any pleats [36].Nevertheless,PI composite films consisting of pristine Si3N4and various contents of functionalized Si3N4nanoparticles displayed bumpy surfaces due to the existence of the fillers in the matrix as portrayed in Fig.9(b)–(f).As the content of the nanoparticles in the PI system increased from 2 wt% to 10 wt%,the pleats became more visible and the roughness of the film surfaces became clearer.Furthermore,all composites except those containing pristine Si3N4nanoparticles exhibited homogeneous surface which confirms uniform distribution of Si3N4in the PI medium.Generally,the amino functionalized Si3N4nanoparticles were well dispersed and evenly distributed in the matrix which may be attributed to the amino groups on nano-Si3N4surfaces which resulted in an enhanced interfacial interaction between Si3N4and PI and prohibited the formation of aggregation of nanoparticles in the matrix system [4].Nonetheless,pristine Si3N4nanoparticles exhibited poor dispersion in the matrix due to the limited interfacial interactions with the PI and consequently,clear aggregates were formed in the PI/Si3N4composites as clearly seen in Fig.9(b).In general,the NH2groups attached to the surface of nano-Si3N4particles could form strong covalent bonds with the PI matrix and hence improve the dispersion state of the nanoparticles in the entire.

4.Conclusions

Unique PI-Si3N4nanocomposite films were successfully synthesized using amino group functionalized Si3N4nanoparticles through in situ polymerization and successive thermal treatment to ensure complete imidization.The surface amino-treated Si3N4nanoparticles exhibited prominent compatibility with PI and improved dispersion in matrix because of the strong interactions within the interfaces.Compared to the neat PI,the composite films displayed greatly improved tensile strength and Young’s modulus which reached up to 105.4% and 138.3%,respectively when 2–10 wt% of amino functionalized Si3N4nanoparticles were loaded into the PI matrix.Likewise,the thermal stabilities and the Tgvalues were significantly enhanced as functionalized Si3N4nanocomposites were loaded into PI matrix as a result of the unique thermal properties of Si3N4nanoparticles.The composites showed increased T5and T10values up to 54.1 °C and 52.4 °C,respectively when functionalized Si3N4nanoparticles were doped into the PI matrix.The Tgof PI/Si3N4composites was also increased up to 49.7 °C when 2 wt%–10 wt% of functionalized Si3N4nanoparticles were loaded into the PI matrix.A significant enhancement in the hydrophobic properties was observed for PI/Si3N4composite films as the dosage of modified Si3N4nanoparticles in the matrix was increased.The percentage of water absorption of the composites was decreased by about 68.1%as the content of Si3N4nanoparticles in the matrix reached to 10 wt%.Therefore,the incorporation of modified Si3N4nanoparticles in the polymer matrix systems is an effective approach to attain high strength materials with outstanding thermal resistance and stability as well as enhanced waterproof properties for high-tech applications.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors greatly appreciate the financial supports from the National Natural Science Foundation of China(51373044)and Natural Science Foundation of Heilongjiang Province of China(E2017018).