Journal of Sol-Gel Science and Technology

, Volume 45, Issue 1, pp 89–95

Poly (ether amide) and silica nanocomposites derived from sol–gel process

Authors

    • Department of ChemistryQuaid-i-Azam University
  • Sonia Zulfiqar
    • Department of ChemistryQuaid-i-Azam University
  • Zahoor Ahmad
    • Department of Chemistry, Faculty of ScienceKuwait University
Original Paper

DOI: 10.1007/s10971-007-1640-9

Cite this article as:
Sarwar, M.I., Zulfiqar, S. & Ahmad, Z. J Sol-Gel Sci Technol (2008) 45: 89. doi:10.1007/s10971-007-1640-9

Abstract

The sol–gel derived chemically combined organic–inorganic nanocomposites were synthesized from poly(etheramide) and tetraethoxysilane. Reaction of a mixture of 4-aminophenyl ether and 1,3-phenyldiamine with terephthaloyl chloride (TPC) in dimethylacetamide (DMAc) produced the amide chains. These chains were modified with carbonyl chloride end groups using a slight excess of diacid chloride and were then reacted with aminophenyl trimethoxysilane (APTMOS), where the amine group reacted with carbonyl chloride end groups. Hydrolysis/condensation of tetraethoxysilane (TEOS) and alkoxy groups present in APTMOS developed bonding between the polyamide chains and inorganic silica network generated in situ. By changing the relative proportions of the polymer solution and the amount of TEOS, the composition of hybrid films was varied. Thin hybrid films with various concentrations of silica network obtained after evaporation of the solvent were subjected to mechanical, dynamic mechanical thermal and morphological measurements. The results indicate a gradual increase in the modulus (3.84 GPa) and tensile strength (121 MPa) up to 15-wt.% silica relative to the pure polyamide. The elongation at break point and toughness gradually decrease with addition of silica content. These hybrids were found to be thermally stable up to a temperature of 500 °C. The weight retained above 800 °C was roughly proportional to amount of silica in the matrix. The glass transition temperature and the storage moduli increased with increasing silica concentration. The maximum increase in the Tg value (358 °C) was observed with 15-wt.% silica. Scanning electron micrographs indicated the uniform distribution of silica in the composites with an average particle size ranging from 9 to 47 nm.

Keywords

PolyamideSilicaNanocompositesSol–gel processStress–strain dataGlass transition temperatureMorphology

1 Introduction

During the last two decades, the increasing demand for high performance materials has drawn considerable interest in the field of nanocomposites. Generally, the preparation of composite materials involved the reinforcement of polymers either with fibers or with other inorganic materials, which may be in the particulate form. Inorganic components can be incorporated into organic matrices with or without interphase bonding. In such composite materials, the organic and inorganic phases may not be homogeneously dispersed at the molecular level. The physical and mechanical properties would be expected to show further improvements if the dispersion of the reinforcement in the matrix could be achieved at the nanometer level. So far, different approaches have been used to develop such hybrid materials. The potential advantages of sol–gel process; its mild processing conditions, high homogeneity and purity of resulting materials have been exploited. In this process, hydrolysis/condensation of metal alkoxide is carried out in the matrix and the most frequently used precursors are the silicates. Gels are formed in the presence of controlled amount of water using an acid or a base catalyst. Under acidic conditions the rate of hydrolysis is faster than the rate of condensation and a diffused silica network structure is formed while under basic conditions, the rate of condensation reaction is faster and dense silica particles are produced [1]. The low temperature processing of sol–gel [110] allows the in-situ generation of inorganic network structures in the polymer matrix.

Schmidt and colleagues [11] prepared hybrid systems by the sol–gel process using organo-functional alkoxysilanes in TEOS. Later, they also incorporated organic molecules in these systems. In this way, both bulk [12] and coating materials [13] were obtained. The glassy inorganic materials were made more flexible by the addition of organic compounds. Mark and co-workers [14] have infused poly(dimethylsiloxane) films with TEOS and precipitated silica particles by means of sol–gel process. Wilkes and coworkers [1517] used various polymers and oligomers for such type of hybrid systems to develop abrasion resistant coatings for polymeric substrates. In these systems the organic compounds were silane functionalized and then combined with metal alkoxides. They have also reported hybrids of silica with hydroxy-terminated polydimethylsiloxane and polytetramethylene-oxide [18, 19]. Hybrids based on zirconia and titania with same polymer matrices have also been produced by the same workers [20, 21]. The introduction of titania or zirconia into the hybrid system improves the modulus and stress at break of the hybrids. The sol–gel technique has also been used to improve mechanical properties of organic materials by incorporation of various metal alkoxides, e.g., heat resistant polyamides [7, 2232], polyimides [3335], benzoxazoles [36, 37] and elastomers [3840]. In these systems, physical or chemical interactions were developed between the disparate phases.

The present work focuses on some new hybrid materials prepared by introducing silica phase into poly (etheramide) matrix at low temperature using the sol–gel process. The polymer chains used were prepared by the reaction of a mixture of 4-aminophenyl ether and 1,3-phenylenediamine with TPC. A slight excess of TPC was added at a later stage in order to have carbonyl chloride end-groups. These chain ends were then replaced with alkoxy groups using APTMOS. Silica network structure was then developed, which could chemically combine to the polymer chains through binding agent (APTMOS). The amount of silica varied from 5 to 20-wt.% and thin films obtained were characterized with regard to their mechanical, dynamic mechanical thermal and morphological measurements.

2 Experimental

2.1 Chemicals

The monomers employed for poly (etheramide) preparation include 4-aminophenyl ether 1,3-phenylenediamine and terephthaloyl chloride (TPC), were all of analytical grade. They were procured from Aldrich, and dried under vacuum at 55 °C before use. Anhydrous DMAc (99% pure) was obtained from Fluka and used as received. Aminophenyltrimethoxysilane (APTMOS) 99% and tetraethoxysilane (TEOS) 98% supplied by Gelest Inc. were used as such.

2.2 Preparation of hybrid films

A mixture of 4-aminophenyl ether and 1,3-phenylenediamine in a mole ratio of 50:50 was placed into a 250 mL flask under completely anhydrous conditions. DMAc (150 mL) was added to the flask and the solution was agitated for 30 min. The contents of the flask were cooled to 0 °C and required amount of TPC was added, still under anhydrous conditions. After 30 min reaction time, temperature of the mixture was allowed to rise to room temperature, and the stirring was continued for 24 h, after which the reaction was assumed to be complete. TPC (in slight excess) was added to endcap the polymer chains with carbonyl chloride groups. The reaction mixture was agitated for 6 h. A stoichiometric amount of APTMOS was then added to react with these end-groups, and stirring continued for a further 6 h at room temperature. The polymer solution thus prepared served as a stock solution to which various amounts of TEOS in DMAc were mixed. Each system was then agitated for 6 h at 50 °C, after which a known quantity of water in DMAc was added to carry out the hydrolysis/condensation required generating in-situ silica network. Films of the resulting nanocomposite having uniform thickness (0.17–0.20 mm) and containing various amounts of silica were prepared by evaporating the solvent in an oven at 55–60 °C. The films were soaked in distilled water to leach out any HCl produced during polymerization reaction. They were then dried under vacuum at 80 °C for 72 h. The chemical reactions leading to the synthesis of polymer chains and formation of their subsequent nanocomposites is illustrated in the Fig. 1.
https://static-content.springer.com/image/art%3A10.1007%2Fs10971-007-1640-9/MediaObjects/10971_2007_1640_Fig1_HTML.gif
Fig. 1

Schematic synthesis of poly (etheramide) and formation of its nanocomposites

Samples thus obtained were characterized with regards to their mechanical, thermal and morphological measurements. Mechanical properties of rectangular strips of the hybrid films were studied at 25 °C with strain rate of 0.5 cm min−1 by the method depicted under ASTM D882 using Instron Universal Testing Instrument Model TM-SM 1102 UK. The thermal stability of the nanocomposites were determined by Seiko Instrument SSC/5200 using 1–5 mg of the sample in Al2O3 crucible heated from 25 to 1,000 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere with a gas flow rate of 30 mL min−1. Dynamic mechanical thermal analysis was carried out with a Rheometric Scientific DMTA III in the temperature range 50–500 °C using 10 Hz frequency. SEM micrographs were taken on a LEO Gemini 1530 scanning electron microscope at an accelerating voltage of 5.80 kV. The samples were fractured in liquid nitrogen and the fractured surfaces etched with a suitable solvent to partially remove the polymer matrix surface. The specimens were dried in a vacuum oven to remove remaining solvent. The particle size was measured using the software program Image J.

3 Results and discussion

The pure poly (etheramide) film was transparent and gave slight tinge of greenish color. All the composite films were found to be semitransparent. Increased amount of silica reduced the transparency of the films to opaqueness. The transparency of hybrid material depends on the size, size distribution and homogeneity of the inorganic particles in the organic phase. The silica particles tend to agglomerate [41] and possibly the distribution in the matrix become irregular as the percentage of silica is increased and this increases the magnitude of the scattering and the associated opaqueness. Mechanical properties of the neat polymer and hybrid films were measured at 25 °C and average values of 5–7 samples were reported for each concentration. DMTA and TGA measurements were carried out to determine the glass transition temperature, storage moduli and thermal stability of these nanocomposites respectively. SEM analysis was also performed to study the morphology of these materials.

3.1 Mechanical properties of nanocomposites

Stress–strain data of pure poly (etheramide) and composite films with 5 to 20-wt.% silica are presented in Table 1 and Fig. 2. The results show an increase in the tensile strength of the composite materials relative to the pure polymer. The incorporation of a silica network structure in the matrix enhances the tensile strength of the polymer. The variation in maximum stress (ultimate strength) as a function of silica content can be seen from Table 1. A gradual increase in ultimate strength was observed. A maximum value of tensile strength (121 MPa) was obtained with 15-wt.% silica relative to pure polymer (105.2 MPa) that indicates an improvement in the mechanical strength of material. Tensile modulus was calculated from the initial slopes of the stress–strain data. The values of the modulus increase with addition of silica contents up to 15-wt.% as the maximum stress (Table 1). The elongation at break point (maximum strain) was found to decrease gradually with increase of silica content. Chemical bonding between the matrix and silica network provides reinforcement to the hybrid materials. The results indicate that if silica contents are increased beyond 15-wt.%, the excess silica particles may not be linked with polymer chains and it also increases the tendency towards particle growth. For sample containing 20-wt.% silica the inorganic particles agglomerate [41], their distribution becomes irregular and non-homogeneous making the samples more porous and brittle and as a result mechanical properties of the composite materials reduce at higher concentration of inorganic phase. Toughness of these nanocomposites measured by calculating the area under the stress–strain curves, which shows that the values of toughness decrease with addition of silica in the matrix, as does the length at break point. The results obtained on the tensile strength indicate improvement in the mechanical properties of materials up to 15-wt.% silica, presumably due to bonding between the polymer chains and the silica, beyond which properties undergo deterioration.
Table 1

Mechanical and glass transition data of poly (ether amide)-silica nanocomposites

Silica content (wt.%)

Max. stress (MPa) 

±0.30

Max. strain (%)

±0.02

Initial modulus (GPa)

±0.03

Toughness (MPa)

±0.50

Tg (°C)

0

105.2

12.63

2.82

105.1

326

5

107.3

10.93

2.96

88.8

341

7.5

109.2

9.33

3.20

76.8

347

10

113.3

8.38

3.39

70.9

355

15

121.0

8.13

3.84

72.0

358

20

77.3

4.88

3.09

25.0

https://static-content.springer.com/image/art%3A10.1007%2Fs10971-007-1640-9/MediaObjects/10971_2007_1640_Fig2_HTML.gif
Fig. 2

Tensile strength of poly (ether amide)-silica nanocomposites with various silica wt.% in the matrix; 0 (●), 5 (○), 7.5 (▲), 10 (△), 15 (■), 20 (□)

3.2 Thermogravimetric analysis of nanocomposites

The thermal stability of composite materials, as measured by TGA under nitrogen atmosphere, is described in Fig. 3. Poly (etheramide) had thermal decomposition temperature in the range of 250 to 500 °C that may be mainly due to decomposition of the polymer. The network structure introduced was expected to increase this temperature. Considerable increase in thermal stability of the nanocomposites was observed presumably because the silica network was extensively cross-linked. As expected, the mass of residue obtained at 800 °C was almost proportional to the silica content in the nanocomposites.
https://static-content.springer.com/image/art%3A10.1007%2Fs10971-007-1640-9/MediaObjects/10971_2007_1640_Fig3_HTML.gif
Fig. 3

TGA curves for poly (ether amide)-silica nanocomposites with different silica contents obtained at a heating rate of 10 °C min−1 in nitrogen; silica wt.% in matrix:0 (—––), 5 (- - - - -), 7.5 (········), 10 (-·-·-·-), 15 (–··–··)

3.3 Dynamic mechanical thermal analysis of nanocomposites

DMTA was also performed on the samples containing various proportions of silica in the poly (etheramide) in the temperature range of 50–500 °C. The variation of the loss tanδ as a function of temperature for neat polyamide and its composites containing various amounts of silica are shown in Fig. 4. It is apparent from the figure that with an increase in temperature, a stage is reached when the onset of segmental motion starts. At this temperature a sharp increase in tanδ is observed. In case of pure sample a sharp peak is observed at 326 °C, while in case of composites the peak shifts to a higher temperature; in addition it splits up into two parts, showing a maxima and a shoulder at still higher temperature. These peaks correspond to the α-relaxation temperatures associated with glass transition temperatures for these materials. The first maxima correspond to the neat polymer chains while the second maxima correspond to the polymer chains linked with the inorganic network. The introduction of the inorganic network reduces the segmental motion of the polymer chains, and as the proportion of the inorganic network increases the position of the second maximum shifts towards higher temperature. The greater interactions between the polymeric and inorganic phases generally result in an increase in Tg. The intensity of the peaks decreases and become broader with higher amount of silica content restricting the motion of the polymer chains, thereby resulting in higher Tg. The change in Tg verses silica content is given in Table 1. These values change from 326 °C for pure polyamide to 358 °C for nanocomposite with 15-wt.% silica content. The change of glass transition temperature of the polymer-composites relative to the pure polymer is attributed to the interaction between the fillers and the matrix at interfacial zones. The mobility of polymer molecules was suppressed due to the existence of such interaction zone that results in the increase of Tg. The intensity of peak decreases with higher amounts of silica and the shift in the peak position is due to restricted motion of the polymer chain resulting in high Tg values.
https://static-content.springer.com/image/art%3A10.1007%2Fs10971-007-1640-9/MediaObjects/10971_2007_1640_Fig4_HTML.gif
Fig. 4

Variation of the loss tangent (tanδ) with temperature for poly (ether amide)-silica nanocomposites at 10 Hz; silica wt.% in the matrix: 0 (—––), 5 (– – – –), 7.5 (- - - - - -), 10 (-·-·-·-), 15 (–··–··)

The temperature variation of storage moduli for this system is shown in Fig. 5. The storage modulus initially increases with the addition of silica content and this increase may be due to the completion of the condensation process leading to nearly complete network formation. The storage moduli increase with the addition of silica at a given temperature. The increase in moduli can be due to inorganic network, which has less free volume and is less flexible as compared to organic phase. However, beyond a certain limit the inorganic structure tends to agglomerate into larger particles. This porous structure has less cohesion with the organic matrix, which results in a low modulus. The sharp decreases in modulus with onset of thermal motions are seen to occur at higher temperatures. The value of the modulus beyond 400 °C is seen to increase again with temperature, possibly due to increases in cross-linking [26]. The TGA results presented in the Fig. 3 show that the polymer chains begin to decompose around 250 °C, and this may produce free radicals which could result in increased cross-linking. At constant temperature, increasing the amounts of the silica generally increases the storage modulus, because of silica’s hardness and small free volume. This simple dependence can be complicated, however, by the fact that beyond a relatively low concentration of silica, there are no longer any reactive polymer ends left for the desired interfacial bonding.
https://static-content.springer.com/image/art%3A10.1007%2Fs10971-007-1640-9/MediaObjects/10971_2007_1640_Fig5_HTML.gif
Fig. 5

Temperature dependence of storage modulus for poly (ether amide)-silica nanocomposites at 10 Hz; silica wt.% in the matrix: 0 (—––), 5 (– – – –), 7.5 (- - - - - -), 10 (-·-·-·-), 15 (–··–··)

3.4 Scanning electron microscopy of nanocomposites

The microstructure of the fractured samples of nanocomposite films was investigated using SEM and the particle distribution of silica in the hybrid materials was recorded. The micrographs of the hybrid materials with 5, 10, 15 and 20-wt.% of silica in the matrix are illustrated in Fig. 6. The particle sizes within the hybrid films prepared by sol–gel process are ranging from 9–47 nm. This exhibits that nanocomposites can be prepared using the same technique. The micrographs clearly show a fine interconnected or co-continuous morphology. This phenomenon also reveals that the silica network have rough surfaces and diffused boundaries, giving co-continuous morphology with a better interfacial cohesion that improve the efficiency of stress transfer mechanism between the two components. In spite of the enhanced cohesion, the elongation at break decrease since the interconnected silica phase hinder the plastic flow of the polyamide phase preventing large deformation to occur prior to fracture. Also the better compatibility between smaller silica nanoparticles and the poly (etheramide) in the nanocomposite films result in improved tensile strength. The morphological data indicate that particle size of the hybrid films increases with increasing silica concentrations and these large domain sizes of silica can result in scattering of light giving opaqueness to the hybrids. Therefore, these results are in accordance with the physical properties measured with different silica concentration of composite films.
https://static-content.springer.com/image/art%3A10.1007%2Fs10971-007-1640-9/MediaObjects/10971_2007_1640_Fig6_HTML.jpg
Fig. 6

Scanning electron micrographs for nanocomposites, silica wt.% in the matrix: (a) 5 (b) 10 (c) 15 and (d) 20

4 Conclusions

Mechanically strong and thermally stable poly (etheramide)/silica nanocomposite materials were successfully prepared through the sol–gel process. The chemical bonding between the organic and inorganic phases provides reinforcement to the materials and is reflected in the mechanical properties. However, only appropriate amounts of both the phases give better interactions and in the present case an optimum tensile strength observed (121 MPa) with 15-wt.% silica content in the matrix. The glass transition temperature and the storage moduli measurements show the better cohesion between the two disparate phases. The shifts in glass transition imply the interaction between the two phases. The morphological investigations indicate a narrow size distribution of silica particles in the polymer matrix.

Acknowledgements

Special thanks are due to Professor Dr. Gerhard Wegner and Dr. Ingo Lieberwirth of Max Planck Institute for Polymer Research, Mainz, Germany, for providing the SEM measurement facility.

Copyright information

© Springer Science+Business Media, LLC 2007