Supercritical impregnation of polymers - ZyXEL NSA210

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402 I. Kikic, F. Vecchione / Current Opinion in Solid State and Materials Science 7 (2003) 399–405 only be dyed in SCCO 2 when a small amount of water is dissolved in the SCCO 2 and in the silk. Cotton was dyed poorly under all conditions. Aminated cotton was dyed better than cotton but not as well as silk and wool. Fixation on aminated cotton was 87%. It was concluded that dichlorotriazine dye can react in moist SCCO 2 with amino groups of protein textile (e.g. silk and wool) but not with hydroxyl groups of cellulosic textiles (e.g. cotton) [15]. 3.3. Organic metallic complexes impregnation Synthesis of metallopolymer nanocomposites has attracted much attention recently. Owing to their small size, metal particles exhibit some unusual structural, magnetic, catalytic and biological properties untypical for large particles. The process consists in the diffusion of an organometallic compound from the supercritical solution into the polymer matrix followed by its reduction/decomposition, achieved by application of heat, chemical reagents or radiations. The method of impregnation from solution in SCCO 2 has several advantages: 1. The matrix tends to prevent an agglomeration of metal particles. 2. SCCO 2 shows high penetrability into polymers. 3. It is possible to control the solvent power and the impregnation rate and, therefore, to control the composition and morphology of the obtained composite. 4. The final product does not require special drying. 5. Low surface tension allows impregnation even of such barrier polymers as Teflon or obtaining of a continuous metal layer on their surface [16]. Platinum (Pt) nanoparticles in poly(4-methyl pentene) and poly(tetrafluoroethylene), silver (Ag) nanoparticles on the surface of polyimide film, Ag nanoparticles in poly(ethylether ketone) and poly(styleneme-divinyl benzene) were investigated [17]. Preparation of noble metal (Pt, Pd) particle-dispersed polyimide films as precursors of metal-doped carbon molecular sieve membranes (CMS) was performed by Yoda et al. [17] using impregnation of an acetyl-acetonate complex dissolved in supercritical CO 2 . Carbon molecular sieve membranes are known as good gas separation membranes even at high temperature. The gas separation properties of CMS membranes can be controlled by doping metal particles having an affinity to the relevant gas. Therefore, the hydrogen separation performance of CMS is expected to be improved by noble metals having an affinity to hydrogen. Significant increase of the Pt content will depend on the decomposition of polyimide catalyzed by Pt particles during thermal treatment, and on the temperature. Since the solubility of the Pt(II)(acac) 2 or Pd(II)(acac) 2 is independent of the temperature, the increase of the metal content should be ascribable to polyimide films, probably to thermal relaxation of the polymer network. Both Pt(II)(acac) 2 and Pd(II)(acac) 2 have similar solubility in SCCO 2 . The high dispersion of Pd particles could be related to lower decomposition temperature (>453 K in air) than Pt(II)(acac) 2 (its melting point is 523 K) and to chemical interactions with carboxyl structures of polyimide. Nazem et al. [18] investigated the preparation of reflective poly(etherether) ketone (PEEK) thin silvercoated films via supercritical fluid impregnation with CO 2 as the carrier and subsequent thermal reduction of (1,5-cyclooctadiene-1,1,1,5,5,5-hexafluoroacetylacetonato)silver(I) dimer, [Ag-(COD)HFA] 2 . No chemical reducing agent was needed to effect silver reduction. The resulting films were (a) reflective, (b) maintained their flexible nature and (c) had a surface electrical resistance higher than 10–11 X cm 1 , in spite of their metallic appearance. The reflectivity of the surface depends on the impregnation and cure conditions. At constant CO 2 pressure (15 MPa) and impregnation time (120 min) at 20% additive, silver atomic concentration increased from 7.5% to 18.5% as the impregnation temperature increased from 80 to 110 °C. At 10 wt.% additive and constant temperature and pressure (110 °C and 15 MPa) in another set of preliminary experiments, the amount of silver increased from 9.6% to 15.5% in going from 60 to 120 min impregnation time. At constant impregnation conditions (e.g. 110 °C, 15 MPa, 120 min) the amount of silver on the surface decreased as the weight percent of additive changed from 20% to 10% and 5%, as might have been expected (e.g. 18.5%, 15.5%, 3.9%). At constant CO 2 pressure, the reflectivity of the surface increased increasing the impregnation temperature and the impregnation time. Thermal cure parameters at fixed impregnation conditions suggested that lower cure temperatures require longer cure times in order to obtain high reflectivity. Ten nanometer diameter silver particles appeared in the film bulk near the surface. Different sized silver clusters on the surface could be grown by controlling the impregnation and cure conditions. The silver on the surface strongly adhered to the polymer surface [18]. New chelate complexes of copper diiminate (Cu(II)– HFDI) and iron diiminate (Fe(III)–HFDI) were synthesized by Said-Galiyev et al. [16]. The polymers impregnated via supercritical CO 2 were polyarylate (PAR) films with thicknesses of 12and 39 mm. The equilibrium degree of polyarylate swelling in SCCO 2 at 45 °C and 8 MPa is 10%; this establishes the impregnation operating conditions. In the case of Cu(II)–HFDI the impregnation can be as large as 11–51% while the content of pure copper in film is as large as 1.3–6.3 wt.%;
I. Kikic, F. Vecchione / Current Opinion in Solid State and Materials Science 7 (2003) 399–405 403 in the case of Fe(III)–HFDI it arrives at 57.6% while the content of iron is 1.5–4.5%. It was shown that after thermal heating the copper atom is in a nearly univalent state. The size of metal-containing particles ranges from 15 to 60 nm (with a maximum at 34 nm), which corresponds, for example, to the size range of catalytic particles used in heterogeneous catalysis (2–50 nm) [16]. 3.4. Monomer and initiator impregnation (polymer blends) One of the most interesting applications of supercritical fluid impregnation is the modification of polymers via the infusion of a monomer and an initiator into aCO 2 -swollen polymer matrix with subsequent polymerization of a monomer within the polymer matrix for the preparation of polymer blends [19]. With SCCO 2 it is possible to obtain polymer modifications by avoiding thermal stresses. The impregnation of styrene and acrylic acid into a series of polyamide products (nylon1212, nylon1010, nylon66, nylon6) using supercritical CO 2 as additive carrier and substrate-swelling agent was performed by Xu and Chang [20]. The impregnation efficiency of additives into substrates is attributed to complicated interactions within the systems. In the examined pressure range from 8 to 16 MPa, acrylic acid always has a higher impregnation uptake than styrene. It was found that the relative solubility of the additive in the polymer substrate and CO 2 is a major factor governing the incorporated amount; however swelling of the substrate and CO 2 -induced crystallization also contribute to the value [20]. Li et al. [21] studied the preparation of nanometer dispersed polypropylene/polystyrene (PP/PS) interpenetrating networks (IPNs) by the radical polymerization and crosslinking of styrene (St) within supercritical CO 2 - swollen PP substrates. In this method, monomer styrene (St), crosslinking agent divinyl benzene (DVB), and the initiator benzoyl peroxide (BPO) were first impregnated into PP matrix using SCCO 2 as a solvent and swelling agent at 35.0 °C, and then the polymerization and crosslinking were carried out at 120 °C. The PP/PS IPNs mass uptake increases initially with soaking time, and it is independent of soaking time after about 13 h when equilibrium conditions are reached. A maximum of PP/PS IPNs mass uptake is shown at about 140 bar. This is due to the competition between two opposite effects with increasing pressure: the degree of swelling of the polymeric matrix and the solubility of the monomers in the CO 2 -rich phase. The composition of the IPNs can be controlled by SCCO 2 pressure and concentrations of St and DVB in the fluid phase. The synthesis of conducting polymers has become an important research area since its discovery in the past 20 years. Most conducting polymers, such as polypyrrole (PPy), poly(3-octyl thiophene), polyaniline, were synthesized by oxidative or electro-chemical polymerization [21]. One way to overcome the poor mechanical properties of these conductive polymers is to blend them with another insulating polymer. Tang et al. [22,23] investigated the preparation of electrically conductive polypyrrole–polystyrene composites by supercritical carbon dioxide impregnation. These authors studied: 1. The effect of the blending conditions; 2. The effect of doping conditions. The host polymer was blended with pyrrole monomer using either supercritical carbon dioxide or high-pressure liquid carbon dioxide (HPLCO 2 ) near the supercritical conditions as the carrying solvent. After the blending process, the blended host polymer was soaked in an oxidant metallic salt solution. For blending the host polymer with pyrrole monomer, SCCO 2 provides better conditions than HPLCO 2 at the same density. The maximum conductivity of the polymer composites also increases with temperature and pressure at the same SCCO 2 density and it is about one order of magnitude higher in SCCO 2 than in HPLCO 2 . This result can be explained by the swelling efficiency in the blending period, which is a controlling factor for the structure and conductivity of the resulting composites. Referring to the doping conditions, acetonitrile and water were used as the doping solvents and iron perchlorate, and iron nitrate were selected as oxidants at temperatures between 15 and 45 °C. The maximum conductivity of the composites with iron compounds as oxidants decreases in the following order of anions: chloride > sulfate > perchloride > nitrate in aqueous solutions [22,23]. The modification of the polymeric substrates bisphenol A poly(carbonate) (PC), poly(vinyl chloride) (PVC) and poly(tetrafluoro ethylene) (PTFE) by the vinylic monomers styrene (S), methyl methacrylate (MMA) and methacrylic acid (MAA) under supercritical conditions was performed by Muth et al. [24]. The monomers solubility in SCCO 2 and their phase behaviour have been investigated in order to achieve impregnation in homogeneous phase and to prevent the extraction of the impregnated monomers during the polymerization. At 40 °C pressure conditions must be higher than 11 MPa, while at 80 °C pressures below 11 MPa must be chosen. In a first step, the polymers were impregnated with the monomers and a radical initiator (azoisobutyrodinitrile (AIBN) for all cases), followed by a polymerization
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