Comparative characterization of polymethylsiloxane hydrogel and silylated fumed silica and silica gel

Abstract

Polymethylsiloxane (PMS) hydrogel ($C_{\mathrm{PMS}}=10\text{wt}\mathrm{\%}$, soft paste-like hydrogel), diluted aqueous suspensions, and dried/wetted xerogel (powder) were studied in comparison with suspensions and dry powders of unmodified and silylated nanosilicas and silica gels using ¹H NMR, thermally stimulated depolarization current (TSDC), quasielastic light scattering (QELS), rheometry, and adsorption methods. Nanosized primary PMS particles, which are softer and less dense than silica ones because of the presence of CH₃ groups attached to each Si atom and residual silanols, form soft secondary particles (soft paste-like hydrogel) that can be completely decomposed to nanoparticles with sizes smaller than 10 nm on sonication of the aqueous suspensions. Despite the soft character of the secondary particles, the aqueous suspensions of PMS are characterized by a higher viscosity (at concentration $C_{\mathrm{PMS}}=3\text{–}5\text{wt}\mathrm{\%}$) than the suspension of fumed silica at a higher concentration. Three types of structured water are observed in dry PMS xerogel (adsorbed water of 3 wt%). These structures, characterized by the chemical shift of the proton resonance at ${\delta }_{\mathrm{H}}\approx 1.7,3.7$, and 5 ppm, correspond to weakly associated but strongly bound water and to strongly associated but weakly or strongly bound waters, respectively. NMR cryoporometry and QELS results suggest that PMS is a mesoporous–macroporous material with the textural porosity caused by voids between primary particles forming aggregates and agglomerates of aggregates. PMS is characterized by a much smaller adsorption capacity with respect to proteins (gelatin, ovalbumin) than unmodified fumed silica A-300.

Introduction

Polymethylsiloxane (PMS) materials have drawn considerable fundamental and technological interest because of their applications as components of nanocomposites [1], [2], [3], copolymers for synthesis of ion-conducting polymeric materials [4], [5], [6], [7], [8], [9], [10], [11]. PMS is used in chromatography [12], [13], [14], e.g., for coating of a silica surface [15], and in medicine as a component of medicinal preparations (e.g., Cleocin, Universal Washaid, USA), implants, adjuvant Capsil (Aquatrols, USA; Scotts, USA), a vaccine adjuvant [16], etc. Additionally, PMS in the form of soft pastelike hydrogel ($C_{\mathrm{PMS}}\approx 10\text{wt}\mathrm{\%}$) is utilized as a medicinal enterosorbent, Enterosgel (Kreoma-Pharm, Ukraine) [17], [18]. Functionalized PMSs [19], [20] are used for modification and functionalization of solid surfaces [21], [22]. They are also used as supports for catalysts [23] or as polymer backbones for preparation of liquid crystalline polymers [24], [25]. NMR investigations and dielectric relaxation measurements show a strong dependence of the mobility of polysiloxanes on the structure of side groups [26]. This structure determines the shear elasticity of polymers [17], [18], [27] and other characteristics. Despite numerous investigations of polysiloxane materials (the lion's share of them are related to linear polysiloxanes such as polydimethylsiloxanes, PDMS), the behavior of interfacial water in xerogels, hydrogels, and aqueous suspensions of branched PMS is practically not studied, especially in comparison with that in silylated nanosilica possessing close specific surface area, the same surface functionalities but differently composed, and different particle morphology.

Branched PMS can be synthesized using different precursors X₃SiCH₃, where X = Cl, OR, etc. Three reactive X groups hydrolyzed provide cross-linking, in contrast to linear PDMS synthesized using X₂Si(CH₃)₂. The properties of PMS materials in different forms (hydrogels, xerogels, dry powders, etc.) depend on the cross-linking degree, which is maximal in dry xerogel (hydrophobic), in which all the Si atoms are bonded by three siloxane bonds (SiO)₃SiCH₃, and on the hydration degree. Heating/drying of PMS hydrogels can lead to loss of the hydrophilic properties because of enhancement of cross-linking by the SiOSi bonds on condensation of hydrophilic silanols [28]. Heated PMS xerogel with the maximal cross-linking degree is practically hydrophobic and not wetted by water (contact angle >95°). However, PMS hydrogel can retain significant amounts of water and its aqueous suspension is stable for a long time because of incomplete cross-linking and the presence of residual hydrophilic silanols [17], [18], [28]. The PDMS-silica contact distance is decreased as the level of water (both chemisorbed and physisorbed) in the interfacial region is decreased. In addition, water in the interfacial region seems to screen the long-range interactions, mediating the polymer relaxation dynamics and ultimately increasing the polymer mobility [29]. There are several water structures (types) adsorbed on PDMS/silica composite, desorption of which can be separated depending on temperature of desorption [30].

The hydrophilic–hydrophobic properties of the PMS hydrogel and xerogel surfaces can be close to those of partially and completely silylated fumed silica, respectively [31], [32], [33], [34], [35], [36] because these materials have close specific surface area (200–300 m²/g) and the same surface functionalities (SiCH₃, SiOH, and SiOSi) and are characterized by textural porosity (i.e., voids between particles nondensely packed in secondary structures). The hydrophobic functionalities (trimethylsilyl, TMS, or dimethylsilyl, DMS) on a silylated silica surface inhibit formation of a continuous layer of adsorbed water [33]. Therefore, one can expect clusterization of interfacial water in PMS hydrogel or water adsorbed on PMS xerogel. Comparison of the adsorption characteristics of unmodified and differently modified silicas [33], [34], [35], [36] shows that even partial silylation of the silica surface reduces the adsorption of water by several times. Nevertheless, partially or completely silylated nanosilica placed in an aqueous medium can disturb a relatively thick layer of interfacial water [33], [35], [36]. A similar effect can be expected for PMS hydrogel and suspension. However, the differences in the structure of primary particles of PMS [17], [18], [28] and silylated fumed silica [31], [37], as well as in the composition of surface sites ((SiO)₃SiCH₃ and (SiO)₂Si(OH)CH₃ for PMS and SiOSi(CH₃)₃ and (SiO)₂Si(CH₃)₂ for TMS- and DMS-nanosilica, respectively), can lead to certain differences in the behavior of bound water, as well as in the physicochemical characteristics of concentrated and diluted aqueous suspensions, hydrogels, and dry powders with these materials. Wetting/drying of nanosilica, which is composed of rigid primary particles forming nonrigid aggregates and agglomerates of aggregates, changes both structural and morphological characteristics of the powder [37], [38]. Similar effects caused by transformation of PMS hydrogel (composed of soft primary and secondary particles) into xerogel can be stronger than those for fumed silica because of the enhancement of the cross-linking degree in dried PMS particles.

To analyze the effects of the liquid media on rigid and soft materials, such techniques as NMR cryoporometry [33], [39], [40], [41], thermally stimulated depolarization current (TSDC) [33], [42], and quasielastic light scattering (QELS) can be used without removal of the liquids (i.e., as nondestructive methods) in combination with standard adsorption [33], [42] and other methods applied to both aqueous dispersions and dry powders. The aim of this work is to analyze regularities in the structural and adsorption characteristics of PMS in the forms of suspension, hydrogel, and xerogel compared with those of aqueous suspensions and dry powders of unmodified and silylated fumed silica and silica gel using ¹H NMR and TSDC with layer-by-layer freezing-out of bulk and interfacial water, QELS, rheometry, and adsorption of proteins.