Commercial PUF was obtained from Comercial del Caucho (Daplasca, Sabadell, Spain), PA (Nylon 6.6, type 200, DuPont) and PAN fibers (type 75, DuPont) from woven fabrics were used (Figure ). Organics and metal salts (acetone, 4-np, NaOH, HCl, NaBH4, HNO3, and AgNO3) from Panreac Company (Castellar del Vallès, Barcelona, Spain) were used as received.
Structural units of the polymeric matrices (a) PA, (b) PAN, and (c) PUF.
Pretreatment of the PUFs
The pretreatment of PUFs was investigated to activate the material. First, foams were washed with acetone and then with distilled water to eliminate the possible commercial treatments applied to the material. Different pretreatments were applied to 1 cm3 of foam samples, which were immersed in 25 ml of the pretreatment reagent solution (1 M HNO3, 3 M HNO3, 1 M NaOH, and 3 M NaOH) for 2 h under agitation. Afterwards, the samples were washed several times with distilled water.
In order to determine the possible effect of the pretreatments in the chemical structure of the PUFs, attenuated total reflectance Fourier transform infrared (FTIR-ATR) spectra were recorded with a Perkin Elmer Spectrum GX spectrometer (Norwalk, CT, USA). Moreover, for determining the concentration of the functional groups before and after the pretreatment of the matrix, two titration methods were applied to calculate IEC (in meq/g) of the material [16
1 For determining cation exchange groups: 1 cm3 of PUF was immersed in 100 ml of NaOH 0.1 M and shaked at room temperature for 48 h, time enough to ensure a complete neutralization of the acidic groups. Then, an aliquot of 10 ml was titrated with standardized HCl 0.1 M (3 replicates).
2 For determining anion exchange groups a similar procedure was used, but immersing the sample in 100 ml of HCl 0.1 M, and using standardized NaOH 0.1 M to titrate the 3 aliquots of 10ml.
Synthesis of AgNPs
The synthesis of AgNPs in the polymeric matrices by the IMS methodology consisted of the following: (1) loading of the material with the metal ions (AgNO3 0.4 M solution) and (2) reduction of metal ions to zero-valent MNPs through reaction (by using NaBH4 0.5M solution). The reactions involved are as follows:
Although equations depict a pure ion exchange mechanism, the generation of coordination bonds between species may also result in the immobilization of the ionic species in the polymeric matrix. In addition, the entry of metal ions into the matrix could be significantly affected by the synthetic conditions (i.e., temperature) which can affect the structural organization of the polymer matrices thus making the matrix temporarily accessible to the metal ions by opening their structure; after the synthesis, the fibers revert back to their closely packed state thus trapping the MNPs within the polymer structure. For the PUFs, the procedure described above was performed at room temperature; whereas in the case of the textile fibers, synthesis using different temperatures (25°C, 40°C, and 80°C) were applied.
In order to determine the exact metal content in the prepared nanocomposites, samples of known weight were digested with concentrated HNO3. The resulting solutions (two replicates) were diluted and analyzed by inductively coupled plasma mass spectrometry (ICP-MS).
With the aim of characterizing the size and structure of the obtained AgNPs, transmission electron microscopy (TEM) was performed by a JEOL JEM-2011 HR-TEM (JEOL Ltd., Tokyo, Japan). Before observation, the samples were deposited between two plastic sheets in an epoxy resin, and ultra-thin slices were obtained using an ultra-microtome.
Catalytic properties evaluation
The catalytic performance of nanocomposites was evaluated by using the reduction of 4-np to 4-ap by NaBH4 as a model reaction, which was considered to follow a pseudo-first-order kinetics, and the apparent rate constant (kapp) was calculated. In a typical run, a piece of nanocomposite (1 cm2 for textile fibers and 1 cm3 for PUFs) was added to a vessel of 50 ml solution containing 4-np (0.5 mM) and NaBH4 (500 mM). The process was monitored at 390 nm by a Pharmacia LKB Novaspec II spectrometer (Biochrom Ltd., Cambridge, UK).