Poly (ether amide) and silica nanocomposites derived from sol–gel process
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- Sarwar, M.I., Zulfiqar, S. & Ahmad, Z. J Sol-Gel Sci Technol (2008) 45: 89. doi:10.1007/s10971-007-1640-9
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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.
KeywordsPolyamideSilicaNanocompositesSol–gel processStress–strain dataGlass transition temperatureMorphology
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 . The low temperature processing of sol–gel [1–10] allows the in-situ generation of inorganic network structures in the polymer matrix.
Schmidt and colleagues  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  and coating materials  were obtained. The glassy inorganic materials were made more flexible by the addition of organic compounds. Mark and co-workers  have infused poly(dimethylsiloxane) films with TEOS and precipitated silica particles by means of sol–gel process. Wilkes and coworkers [15–17] 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, 22–32], polyimides [33–35], benzoxazoles [36, 37] and elastomers [38–40]. 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.
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
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  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
Mechanical and glass transition data of poly (ether amide)-silica nanocomposites
Silica content (wt.%)
Max. stress (MPa)
Max. strain (%)
Initial modulus (GPa)
3.2 Thermogravimetric analysis of nanocomposites
3.3 Dynamic mechanical thermal analysis of nanocomposites
3.4 Scanning electron microscopy of nanocomposites
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.
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.