Properties of poly(vinyl alcohol)/silica hybrid gel particles

Abstract

We have been investigating the preparation of macroporous gel particles of poly(vinyl alcohol) [1-3]. These particles are obtained by the saponification of particles of poly(vinyl acetate) formed in the polymerization of vinyl acetate in a suspension process. The poly(vinyl alcohol) gel particles have been used as a column packing for aqueous separations. However, the gels have a disadvantage in pressure-resistant property because of poor mechanical strength. We expected that a gel having excellent mechanical stability might be obtained by reinforcing poly(vinyl alcohol) with an inorganic component. We report here that reinforcement using silica as an inorganic component is useful for the improvement of performance of poly(vinyl alcohol) gel particles used as a column packing for gel permeation chromatography in aqueous media. Tetraethoxy silane (TEOS) was used as the inorganic component of the hybrid gels throughout this study. The sol-gel method was applied to the preparation of hybrid gel [4-6]. The process is divided into two steps: hydrolysis of metal alkoxides to produce metal hydroxides, followed by polycondensation of hydroxyl groups. By carrying out this reaction in a poly(vinyl alcohol) gel matrix containing many hydroxyl groups, it is possible to copolymerize metal alcoxide with poly(vinyl alcohol). Thus a silica network may be incorporated into the poly (vinyl alcohol) gel matrix. The gel particles of poly(vinyl acetate) (DP = 1000) obtained by suspension polymerization were classified to give a particle size distribution with particle diameters in the range 350-400 #m. The saponification of poly(vinyl acetate) to poly(vinyl alcohol) was carried out at 30 ~ for 1 month by immersing the gel particles in a solution containing sodium hydroxide and methanol in an aqueous saturated sodium sulfate solution [7]. The gel particles of poty(vinyl alcohol) obtained above have sufficient mechanical stability in water below 55 ~ without crosslinking treatment. The gel particles (1 g) obtained above were immersed in 10 ml of ethanol for 24 h and then tetraethoxy silane and hydrochloric acid as catalyst were added to the particles. The reaction was carried out at 30 ~ For the purpose of removal of TEOS homopolymer the reaction product was then subjected to a Soxhlet extraction using tetrahydrofuran. Swelling behaviour and gel permeation chromatographic properties of the hybrid gel particles obtained above were investigated. The ratio of the volume of the particles in ethanol to that on swelling with water (swelling ratio) was evaluated from observations by optical microscopy. As shown in Fig. 1, swelling ratio decreases with increase of reaction time and levels off at about 2 h. This is due to the formation of the silica network in the poly(vinyl alcohol) gel matrix. The swelling behaviour is mainly controlled by the amount of TEOS added. The slurry of hybrid gel particles was packed into a 150 x 4 mm stainless column at a pressure of about 5 MPa. High-performance gel permeation chromatography (HP-GPC) separations were performed with a Shimazu LC-6AD, employing distilled water as eluent. Solutions (40 #1) of individual solutes were injected with an off-column syringe-septum arrangement. Detection of solutes was performed with a Shimazu Refracto Monitor Model RID-6A (cell volume = 10 ~tl, aqueous reference). The samples of poly(ethylene glycol) are designated PEG with a number which is the molecular weight provided by the suppliers (Kanto Chemicals, Tokyo). The calibration curve was established at a flow rate of 0.4cm3/min with a PEG concentration of 2.0% (w/v). The calibration curve of the hybrid gel particles established with PEG samples is shown in Fig. 2, in comparison with the poly(vinyl alcohol) gel particles. The value of the excluded molecular weight is almost the same for the hybrid gel particles and the poly(vinyl alcohol) gel particles. However, the calibration curve for the column packing containing the hybrid gel particles shifts to higher elution volumes. Since separation power is inversely proportional to the slope of the plot of molecular weight versus elution volume, it is clear that the column containing the hybrid gel particles shows better resolution separations. The shape of the slope