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Journal of Colloid and Interface Science 308 (2007) 142���156 www.elsevier.com/locate/jcis Comparative characterization of polymethylsiloxane hydrogel and silylated fumed silica and silica gel V.M. Gun���ko a,���, V.V. Turov a, V.I. Zarko a, E.V. Goncharuk a, I.I. Gerashchenko a, A.A. Turova a, I.F. Mironyuk b, R. Leboda c, J. Skubiszewska-Zi�� eba c, W. Janusz c a Institute of Surface Chemistry, 17 General Naumov Street, Kiev 03164, Ukraine b Pricarpatsky Stefanyk University, 57 Shevchenko Street, Ivano-Frankovsk, Ukraine c Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland Received 10 October 2006 accepted 16 December 2006 Available online 25 January 2007 Abstract Polymethylsiloxane (PMS) hydrogel (CPMS = 10 wt%, soft paste-like hydrogel), diluted aqueous suspensions, and dried/wetted xerogel (pow- der) 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 char- acterized by a higher viscosity (at concentration CPMS = 3���5 wt%) 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 ��H ��� 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. �� 2007 Elsevier Inc. All rights reserved. Keywords: Polymethylsiloxane hydrogel Silylated fumed silica Silica gel Aqueous suspension Structured water Morphology Structural characteristics 1H NMR TSDC QELS Rheometry Protein adsorption 1. Introduction Polymethylsiloxane (PMS) materials have drawn consider- able fundamental and technological interest because of their ap- plications as components of nanocomposites [1���3], copolymers for synthesis of ion-conducting polymeric materials [4���11]. PMS is used in chromatography [12���14], e.g., for coating of a silica surface , and in medicine as a component of medic- inal preparations (e.g., Cleocin, Universal Washaid, USA), im- plants, adjuvant Capsil (Aquatrols, USA Scotts, USA), a vac- cine adjuvant , etc. Additionally, PMS in the form of soft * Corresponding author. Fax: +380 44 424 3567. E-mail address: email@example.com (V.M. Gun���ko). pastelike hydrogel (CPMS ��� 10 wt%) 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  or as polymer backbones for preparation of liquid crystalline polymers [24,25]. NMR inves- tigations and dielectric relaxation measurements show a strong dependence of the mobility of polysiloxanes on the structure of side groups . This structure determines the shear elas- ticity 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 poly- dimethylsiloxanes, PDMS), the behavior of interfacial water in xerogels, hydrogels, and aqueous suspensions of branched PMS 0021-9797/$ ��� see front matter �� 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.12.053
V.M. Gun���ko et al. / Journal of Colloid and Interface Science 308 (2007) 142���156 143 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 dif- ferent particle morphology. Branched PMS can be synthesized using different precur- sors 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 materi- als 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 (���Si���O)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 Si���O���Si bonds on condensation of hydrophilic silanols . Heated PMS xerogel with the maximal cross- linking degree is practically hydrophobic and not wetted by water (contact angle 95���). However, PMS hydrogel can re- tain 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 wa- ter (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 mo- bility . There are several water structures (types) adsorbed on PDMS/silica composite, desorption of which can be sepa- rated depending on temperature of desorption . The hydrophilic���hydrophobic properties of the PMS hydro- gel and xerogel surfaces can be close to those of partially and completely silylated fumed silica, respectively [31���36] because these materials have close specific surface area (200���300 m2/g) and the same surface functionalities (���SiCH3, ���SiOH, and ���Si���O���Si���) and are characterized by textural porosity (i.e., voids between particles nondensely packed in secondary struc- tures). The hydrophobic functionalities (trimethylsilyl, TMS, or dimethylsilyl, DMS) on a silylated silica surface inhibit forma- tion of a continuous layer of adsorbed water . Therefore, one can expect clusterization of interfacial water in PMS hy- drogel or water adsorbed on PMS xerogel. Comparison of the adsorption characteristics of unmodified and differently mod- ified silicas [33���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 ((���Si���O)3SiCH3 and (���Si���O)2Si(OH)CH3 for PMS and ���Si���OSi(CH3)3 and (���Si���O)2Si(CH3)2 for TMS- and DMS-nanosilica, respectively), can lead to certain differ- ences in the behavior of bound water, as well as in the physic- ochemical characteristics of concentrated and diluted aqueous suspensions, hydrogels, and dry powders with these materi- als. Wetting/drying of nanosilica, which is composed of rigid primary particles forming nonrigid aggregates and agglomer- ates 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���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 com- bination 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 ad- sorption characteristics of PMS in the forms of suspension, hydrogel, and xerogel compared with those of aqueous suspen- sions 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. 2. Materials and techniques 2.1. Materials Commercial polymethylsiloxane hydrogel (Kreoma-Pharm, Kiev, Ukraine) synthesized using methyltrichlorsilane [17,18, 28] including 10 wt% of PMS and 90 wt% of water (ho- mogeneous soft dough, paste-like hydrogel in which all wa- ter is bound in pores) was used as the initial material. For NMR investigations, the aqueous suspensions of PMS (CPMS = 1.25, 2.5, and 5 wt%) were prepared by dilution of the initial PMS hydrogel with distilled water. Dry PMS xerogel (CPMS ��� 97 wt%) was obtained from the initial material dried in air at 300 K for five days. PMS xerogel wetted by ethanol and then ethanol/water and repeatedly washed off with water contains approximately 72 wt% of water. Fumed silicas A-300 and A-380 (pilot plant of the Institute of Surface Chemistry, Kalush, Ukraine) at the specific surface areas SBET ��� 337 and 378 m2/g, respectively, were silylated by dymethyldichlorosilane (CDMS = 0.24, 0.57, 0.84, 1.12, and 1.21 mmol/g) and hexamethyldisilazane, HMDS (CTMS = 0.09, 0.14, 0.12, 0.23, 0.42, and 0.79 mmol/g), respectively. The structural and other characteristics of silylated A-380 sam- ples were described in detail elsewhere . Structural characteristics of unmodified silica gels Si-40, Si- 60, and Si-100 (Merck) and silylated Si-60 (approximately 30% OH groups in silanols were replaced by TMS groups in reaction of silica with HMDS) used here in comparative investigations and the behavior of pore water in these silica gels are described in detail elsewhere [33,42���44]. 2.2. 1H NMR For recording 1H NMR spectra of water bound to the sil- ica and PMS surfaces, a high-resolution WP-100 SY (Bruker)
144 V.M. Gun���ko et al. / Journal of Colloid and Interface Science 308 (2007) 142���156 NMR spectrometer with a bandwidth of 50 kHz was used. Rel- ative mean errors were ��10% for signal intensity and ��1 K for temperature. The characteristics of bound (structured, un- frozen) water were determined from the intensity of the 1H NMR spectra recorded at T 273 K. Concentration of un- frozen water as a function of temperature (Cuw(T )) was deter- mined by comparison of the integral intensities of the 1H NMR signals of unfrozen water at given temperatures and liquid water at 285 K . The 1H NMR signals of water molecules from ice, functionalities of PMS, surface TMS, DMS, and silanols do not contribute to the 1H NMR spectra because of features of the measurement technique and the short time (���10���6 s) of transverse relaxation of protons in immobile structures, which is shorter by several orders of magnitude than that of mobile water molecules. This method and its applications to different materials are described in detail elsewhere . Water can be frozen in narrower pores at lower temperatures that can be described by the Gibbs���Thomson (GT) relation for freezing point depression for pore liquids [39���41], (1) Tm = Tm,��� ��� Tm(R) = ��� 2��slTm,��� Hf��R = k R , where Tm(R) is the melting temperature of ice in pores of ra- dius R, Tm,��� is the bulk melting temperature, �� is the den- sity of the solid, and ��sl the energy of solid���liquid interac- tion. Equation (1) can be transformed into the integral equation (IGT) [45,46] (2) Cuw(Tm) = A Rmax Rmin k (Tm,��� ��� Tm(R))R 2 fV(R) dR, where Rmax and Rmin are the maximal and minimal pore radii (or sizes of unfrozen liquid structures), respectively, and A is a normalization factor. The IGT equation was solved using modified CONTIN  or CONTIN/maximum entropy method (MEM) [45,46,48] procedures. The pore size distribution (pores filled by structured water) can be calculated directly with Eq. (1) using the dependence of the amount of unfrozen water on temperature (Cuw as a function of T determined as changes in the 1H NMR signal of unfrozen water at this temperature) and pore size (R) as a function of Tm(R) that gives changes in Cuw as a function of R. Calcula- tions with the integral equation (2) give the distribution function f (R), which can differ from that calculated directly, because solution of Eq. (2) is a well-known ill-posed problem due to the impact of noise on measured data, and the regularization pro- cedure, especially in the case of application of the maximum entropy principle to this function, gives a clean, more exact so- lution than that on direct calculations with Eq. (1) because noise effects are reduced. The f (R) distribution was used to calculate the contribu- tions of micropores (R 1 nm), mesopores (1 R 25 nm) and macropores (R 25 nm) to the total porosity, (3) V = Rmax Rmin f (R) dR, where Rmin and Rmax correspond to the integration ranges for the mentioned types of pores. Additionally, this f (R) function related to pore volume (fV(R)) can be converted to fS(R) re- lated to the specific surface area using the corresponding model of the pore shape, (4) fS(R) = w R fV(R) ��� V (R) R , where w = 1, 2, and 1.36 for slit shaped and cylindrical pores and voids between spherical particles packed in the cubic lat- tice, respectively. Integration of the fS(R) function gives the specific surface area (SIGT) of the studied materials in contact with structured water, which can be divided into the corre- sponding pore ranges as was done for the pore volume. 2.3. QELS Quasielastic light scattering (QELS) measurements of the particle size distributions [37,38] were performed using a Ze- tasizer 3000 (Malvern Ins., �� = 633 nm, scattering angle 90���, software version 1.4). A PMS suspension at CPMS = 1 wt% was prepared with the initial commercial hydrogel (CPMS = 10 wt%) diluted with twice-distilled deionized water (pH 6.72, conductivity 2 ��Scm���1) and sonicated by a Sonicator Mis- onix (USA) ultrasonic disperser (22 kHz, 500 W) for 5 min. PMS suspensions with lower concentrations of PMS were prepared by dilution of this suspension (CPMS = 1 wt%) to CPMS = 0.25, 0.0625, 0.0156, and 0.0078 wt% and then son- icated for 1 min. The particle size distributions are shown here with respect to the scattered light intensity, which was also re- calculated to the size distribution related to the particle number and the particle volume using the Malvern software. The value of the effective diameter of particles (Deff) is the average value, which can be used to characterize changes in the particle size distribution of the studied systems. 2.4. Rheometry The rheological behavior of the aqueous suspensions of PMS prepared using the initial commercial hydrogel (CPMS = 10 wt%) diluted by twice-distilled deionized water to CPMS = 1, 3, and 5 wt% was studied with increasing shear rate from 0.1 to 1312 s���1 and then its decrease to 0.1 s���1 at 293 K using a Rheotest 2.1 (VEB MLW Prufgerate-Werk Medingen Sitz Ftreital, Germany) rotational viscometer with a cylindrical measuring system. The distribution function of activation energy f (E) of the shear viscosity was determined using an integral equation de- rived for concentrated systems , (5) ��(T , �� �� ) = ����� + (��0 ��� �����) Emax Emin 1 z z 0 dx ��� 1 + x2 f (E) dE, where ��0 and ����� are the viscosity at the shear rates �� �� ��� 0 and �� �� ��� ���, respectively, Emin and Emax are the limits of integra- tion, and