From a Hyperbranched Polyethoxysiloxane Toward Molecular Forms

Chapter 34

From a Hyperbranched Polyethoxysiloxane Toward Molecular Forms of Silica: A Polymer-Based Approach to the Monitoring of Silica Properties 1

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Downloaded by UNIV OF ARIZONA on December 24, 2012 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch034

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V. V . Kazakova , E . A . Rebrov , V . B. Myakushev , T . V . Strelkova , A . N. Ozerin , L . A . Ozerina , T. B. Chenskaya , S. S. Sheiko , E . Y u . Sharipov , and A . M. Muzafarov 1

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1Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 117393 Profsoyuznaya Ul., 70, Moscow, Russia Organische Chemie-III, Universität Ulm, Albert-Einstein-Allee 11, Postfach 4066, D-89069, Ulm, Germany 2

Synthesis of a new modification of silica soluble in THF is described. At the first synthetic step, a hyperbranched polyethoxysiloxane (HBPES) is synthesized by heterofunctional condensation using triethoxysilanol previously generated in reaction mixture by neutralization of correspondent sodium salt with acetic acid. At this step, the process was monitored by IR spectroscopy, SEC, and 29Si N M R spectroscopy. At the second step, hydrolysis and intramolecular condensation involving silanol groups is carried out to yield silica sol macromolecules. A SAXS method was used to determine the size and fractal coefficient of trimethylsilated derivatives and silica sols obtained. A n atomic-force microscopy imaging of silica sol supported on a mica substrate showed the silica sol particles to be predominantly spherical in shape. Prospects for theoretical, experimental and practical applications of silica sols are discussed.

Complexity of the silica chemistry stems from the high functionality of starting reagents in the reaction mixture involving a large variety of chemical processes: hydrolysis, homo- and heterofunctional condensation, cyclization, chemical aggregation as well as a purely physical aggregation of the particles formed. The process can be further complicated by the formation, at an early step, of a solid-state phase, with the resulting loss of solubility capable of changing the course of chemical reactions. Traditionally, the control over chemical reactions for preparation of silica with tailored properties is exerted empirically by choosing appropriate conditions. On the whole, this approach has led to remarkable results. Researchers have gained expertise in preparing a wide range of silicas - from ultradisperse particles (lj to mesoporous silica with controllable pore structure (2-5).

© 2000 American Chemical Society

In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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The polymer chemistry and silica chemistry, at first sight so basically unlike in many aspects, in recent decades have been sharing spheres of common interests in an everincreasing number. One reason for this is that various types of silica are used traditionally as fillers for polymer composites. This is to say that commercial properties of a range of polymers depend on the structural properties of silica, its specific surface, concentration of functional groups, and size, shape and degree of ordering of silica particles. Interpenetration of these two chemical disciplines was prompted by the emergence of sol -gel processes and associated therewith interest in hybrid composite materials (4-5). However, the formative processes of silica particles continued, as before, to be monitored on an empirical basis. Main factors involved in the control over the reactive medium were temperature, concentration, proportion of components, and the type of the polymer template used; in fact, the only response to the effect of these factors was a specified property (or a set of them) of the materials formed. Even the latest achievements in the preparation of controllable mesoporous silicas (3) have produced no change in the actual state-ofthe-art. The use of surface-active substances (SASs) has brought about merely a change in the shape and size of the template on which hydrolysis and condensation of orthosilisic acid esters were carried out. Despite the significant results obtained in the field, the experimental techniques were in fact indiscriminate in their effect on the structure of silica species formed and provided no means to monitor the silica structure at a molecular level. An alternative approach to the synthesis of silicas with tailored properties is based on a strategy used in the synthesis of hyperbranched polymers. Extension of this strategy to the chemistry of silica precursors will make it possible to obtain end products with the desired properties, which can be achieved by the structural monitoring of silica macromolecules. The progress that has been accomplished over the last decade in the chemistry of dendrimers, or cascade polymers (6-8), and their irregular analogs, hyperbranched polymers (9-12), in particular, organosilicon polymers (13-16) provides reason to believe in such a possibility. In the present work, the approaches commonly employed in the molecular structural monitoring of hyperbranched polymeric systems have been extended to polyethoxysiloxanes and products of their hydrolytic polycondensation. Experimental 1. Materials. Methods. Organic solvents: toluene, hexane, tetrahydrofuran, ethanol (analytical-reagent grade) were dehydrated by boiling and distilling over calcium hydroxide. The gaseous N H 3 was dried by passing through an alkali-packed column. Tetraethoxysilane and hexamethyldisilazan(high-purity grade) were used as-received. Nuclear magnetic resonance (NMR) spectra were measured on a "Bruker WP-200SY" specrometer, with a working frequency of 200.13 M H z for *H nuclei. 29siNMR-spectra were taken using a proton pulse suppression technique, with no account for the Overhauser effect, with a delay of 30 s. Kaicou fl-pacTBopuTejib Hcnojib30BajiCH^H N M R spectra of the products studied were measured using 20% solutions in CCI4, and ^ S i NMR-spectra, using 50% solutions in THF or toluene. Tetramethylsilane was used as the reference. Gas-liquid chromatographic (GLC) analysis was done on a 3700 chromatograph (Russia). A thermal conductivity cell was used for detecting signals; the carrier gas


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was helium; columns 2 m ¥ 3 mm were used; the stationary phase was SE-30 (5%) supported on a "Chromaton-H-AW" sorbent. Chromatograms were processed using a "CI-100A" computer-controlled integrator (Czechia). Analysis of polyethoxysiloxanes and silica sols by the gel-permeation chromatography (GPC) method was done on a GPC chromatograph (Czechia). The detector was a refractometer; a 4x250 mm column was used; the sorbent was Silasorb-600 (7.5 um) treated with hexamethyldisilazan; the eluents were toluene and THF. Infrared (IR) absorption spectra of the products were measured using a "Bruker IFS110" IR spectrometer. SAXS - measurements. SAXS curves were measured using a home-made KRM-1 diffractometer with a slit scheme for the primary beam collimation (CuKaline, Ni filter, scintillation detector). The scattering coordinate was measured in terms of the scattering vector modulus s = 4n sinO/A,, where 0 is the difraction angle, and X - 0.1542 nm is the radiation wavelength. The scattering intensity was measured in the range of s = 0.07 to 4.26 n m . The experimental data were processed as described in (17-18). Structural studies of silica sols films were carried out using atomic force microscopy (AFM) method. The films, were prepared by casting solutions on a rapidly spinned substrate - mica. The film surface was scanned at room temperature and atmospheric pressure using a Nanoscope III atomic-force microscope (Digital Instruments, California). Silicon tips with a constant of elasticity of 50 N/m and a tip radius smaller that 10 nm, vibrated at a resonance frequency of about 320 kHz, were used. - 1

2. Synthesis of a hyperbranched polyethoxysiloxane. Using a method described in (in print.), a specimen of hyperbranched polyethoxysiloxane was prepared: the solution of 25.4 g (0.125 mole) of sodiumoxytriethoxysilane (preliminary prepared by interaction of 5.0 g of sodium hydroxide and 26.1 g of tetraethoxysilane) was added dropwise to the solution of 7.9g (0.132 mole ) of acetic acid in 160 ml of dry toluene with stirring at 0 °C. Reaction mixture was stirred for another two hours at room temperature. The precipitated CHaCOONa was filtered and washed with dry toluene on the filter. The solvent was evacuated at room temperature. 21.7 (82%) of triethoxysilanol was prepared and used for the hyperbranched polyethoxysiloxane preparation. Probe of prepared triethoxysilanol was dissolved in dry T H F and analized by S i N M R (THF,39.76 MHz): 5 = -78.49 (s,lSi). 17.2 g(1.01 mole) of dry liquid ammonia was added to solution preliminary cooled at -30°C of 20.0g (0,111 mole) of triethoxysilanol in 15 ml of absolute ethanol. The temperature of reaction mixture was slowly increased to room condition. After three hours of stirring volatile products were evacuated at 133.3 Pa) at 25°C, 14.0 g (94%) of transparent colorless oil like liquid was obtained: n o 1.4116, d 4 1.2030. Found, %: Si 23.50, C 30.41, H 6.42. Si N M R (toluene, 39.76 MHz): 8 -88.42 . (OSi(OEt) ), 8 -96.12. (0 Si(OEt) ), 8 -103.02 (0 Si(OEt)), 8 -105 - 106 (0 Si). Trimethylsilylated derivatives were prepared by hyperbranched polyethoxysiloxane probe treatment with excess of trimethyltrifluoroacetoxysilane. Mixture was reflaxed 29

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10 hours, then it was cooled and volatile products was removed in vacuum. Rest was dissolved in toluene with addition of a few drops of hexamethyldisilazan and reflaxed another 5 hours. Solution was cooled, volatile products evacuated, resin like substance was dissolved in toluene or THF and used for analysis of molecular mass distribution. 3. Hydrolysis of poiyethoxysiloxane. A mixture of 10 g (0.555 mole) of water and 2 drops of concentrated HC1 was added dropwise to a mixture of 5 g (0.037 mole) of poiyethoxysiloxane and 557 ml of THF with stirring at 20 °C. The course of the reaction was monitored by sampling portions of the mixture for analysis. The solvent was removed by evaporation in vacuum, and the mixture was inspected over time by IR spectroscopy until the absorption bands n = 2950 c m * (corresponding to the stretching C - H mode of the ethoxy group) and d = 1480 cnW (bending C - H mode of the same group) disappeared completely. The conversion of the ethoxy groups having been completed, the reaction solution was colourless and transparent. -

4. Condensation of the poiyethoxysiloxane hydrolysate. 1% solution of the poiyethoxysiloxane hydrolysate in THF was boiled at 65 °C for 8 hours. To remove ethanol and water (formed as side products of condensation), 200 ml of an aseotropic mixture THF + ethanol + water was removed by distillation in three successive runs (each time, a fresh portion of THF was added). Next, 386 ml of solvent was driven off from the mixture to reach a 3% concentration of the reaction product in THF. The IR spectrum of the product exhibited an absorption band at n = 3400 cm~l corresponding to the O - H stretching mode of the hydroxyl group. Elemental analysis of a 9.3% solution of poiyethoxysiloxane hydrolysate in THF gives (%): Si 2.57; O H 0.94. Results and Discussion Synthesis of the hyperbranched poiyethoxysiloxane. Viewed from a standpoint of the chemistry of dendrimers and hyperbranched polymers, triethoxysilanol (regarded in (20) as a primary product of hydrolysis) is no more than a reactant A B 3 according to the Flory condition. This signifies that, by generating this product under the conditions of a heterofunctional condensation, one can direct the reaction such that a hyperbranched poiyethoxysiloxane is formed, that is, to make the process structurally selective. It is known that, in hyperbranched polymers, cyclization is a minor contributor to the molecular structuring because of the paucity of A-type functionalities. In other words, with allowance made for structural imperfection of the hyperbranched polymer and for the fact that proportions of the dentritic, linear and end chains depend on a number of factors, it is possible in principle to obtain an end product with desired properties by monitoring structure, rather than process parameters, of the polymer formed. It was shown in (14) that tetraethoxysilane derivatives can be used as building blocks in the buildup of a hyperbranched structure; successive steps in the synthesis of such a polymer have been discussed in (19). The main steps of this process are shown in Scheme 1. It was also discussed that scheme 1 illustrate an idealized development of


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the processes while real process include a number of side reactions. At the same time the obtained product characterization gave support to consider this scheme as a main. Scheme 1 BO

EtO EtO-Si-OH

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EtO-Si-OH

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-EtOH

EtO

EtO

Et

f ? EtO-Si-O-Si-OH ' ' EtO OEt

(EtO) S»OH 3

-EtOH


OEt EtOk^OEt EtO OEt EtO-Si-O-^i-OH Etd

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(EtO) SiOH

n(EtO) SiOH

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-EtOH *

-nEtOH*

Hyperbranched Polyethoxysiloxanes

Etcri OEt OEt ,%

The process in question can be monitored conveniently by following changes in the intensity of the absorption bands of the hydroxyl stretching mode in the IR spectra.

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O

CO CD

From a Hyperbranched Polyethoxysiloxane Toward Molecular Forms

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