Thiirane functional spherosilicate as epoxy resin modifier

Epoxy resin-based composites, also those containing POSS compounds, have been the subject of extensive research because of their attractive properties and broad practical applications. However, in some applications they suffer from too long curing times or not strong enough adhesion to metals. A solution to this problem is the use of episulfide (thiirane) derivatives or their combination with epoxies. In this paper, the synthesis of new octakis[(3-(thiiran-2-yloxy)propyl)dimethylsiloxy]octasilesquioxane from its 3-glycidoxypropyldimethylsiloxy analog is described. Results of study of its influence on thermal properties as well as curing processes of modified with this compound epoxy resins hardened with maleic, phthalic, and pyromellitic anhydrides are also presented. It is shown that addition of this new thiirane functional POSS compound can significantly increase thermal stability of obtained epoxy resin composites and decrease its curing temperatures.


Introduction
Polyhedral oligomeric silsesquioxanes (POSS), especially well defined T 8 derivatives and spherosilicates which are specific class of POSS, have nowadays become one of the most extensively explored as modifiers and nanobuilding blocks group of compounds. Their popularity and multifarious applications stem from their useful physicochemical properties, ease of chemical modification, reactivity, and nanometric-size [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. From the point of view of chemical synthesis, particularly important are those with reactive functional groups e.g., hydrogen atoms, vinyl, amino, methacryloxy, or chloralkyl ones. Besides the above mentioned, also very important and broadly explored are epoxy-functional silsesquioxanes and spherosilicates. They can be used as starting material for further transformations and synthesis of new compounds [15] as well as components of epoxy nanocomposites influencing and determining their unique parameters like crystallinity, structure, curing temperatures, mechanical, and thermomechanical properties [16][17][18][19][20].
Epoxy resins have been subjects of extensive research, because of their broad practical applications and attractive properties [16][17][18][19][20][21][22][23][24]. However, in some applications they suffer from too long curing times or not strong enough adhesion to metals. A solution to this problem is the use of episulfide (thiirane) resins or their combination with epoxides. Episulfides should be suitable for low-temperature rapid curing and can react not only with epoxy resin but also with many kinds of metals, because of the formation of thiol (-SH) groups able to form sulfur-metal bonds resulting in improved adhesion and corrosion resistance [21,25].
As a natural continuation of our earlier work on synthesis of functional POSS compounds exploiting octakis[(3-glycidoxypropyl)dimethylsiloxy]octasilsesquioxane as substrate, we decided to prepare its thiirane analog and use it as a modifier of anhydride cured epoxy resin systems of improved thermal stability. To our best knowledge, this compound has not yet been described in the literature.

Experimental
All chemicals were used as received from suppliers (Sigma-Aldrich, Gelest, Organika Sa_ zyna S.A.) without any further purification. Water used in the experiments was redistilled freshly before use. Octakis(hydridodimethylsiloxy)octasilsesquioxane was synthesised according to the procedure described elsewhere [30].

Synthesis of octakis[(3-glycidoxypropyl)dimethylsiloxy] octasilsesquioxane (1)
Octakis(hydridodimethylsiloxy)octasilsesquioxane (20 g, 19.6 mmol) with 20 % excess of allyl-glycidyl ether (22.4 mL, 188 mmol) together with 200 mL of toluene as solvent was placed in a three-necked round bottom flask equipped with a thermometer, a condenser and a magnetic bar. Then Karstedt catalyst (85.6 mg, 2.35 9 10 -6 mol Pt) was added at room temperature and the solution was heated to 110°C and kept at this temperature for 8 h. After the reaction mixture cooled down, the solvent and the excess of olefin were evaporated under vacuum and the residue was filtered off to give a product as an viscous oil (36.8 g, 97 % of theoretical yield). The results of spectroscopic analysis of the product confirmed its structure. 1
In a three-necked round bottom flask, quipped with a thermometer, condenser, magnetic stirrer, and an addition funnel 16 g (164.8 mmol) of potassium thiocyanate was dissolved in 24 mL of distilled water. The additional funnel was filled up with a solution of 20 g (10.4 mmol) of octakis[(3-glycidoxypropyl)dimethylsiloxy]octasilsesquioxane in 120 mL of 2-propanol. Then, the solution of epoxy spherosilicate solution was slowly added dropwise to the thiocyanate solution on vigorous stirring. The reaction mixture was heated up to 60°C and kept at this temperature for 24 h. After cooling down, the solution was transferred to the separation funnel and 120 mL of methylene chloride was added. The organic phase was collected, washed with distilled water three times and dried over anhydrous sodium sulfate. The product was isolated by vacuum evaporation of the solvent and characterized with NMR and FT-IR spectroscopies. 1 H NMR (CDCl 3 , 298 K, 300 MHz) d = 0.14 (SiCH 3 ); 0.47 (SiCH 2 ); 1.51 (CH 2 ); 2.18 (SCH 2 ); 2.97 (SCH); 3.38 (H 2 CO); 3.55, 388 (OCH 2 ) ppm. 13  Curing procedure All cured samples were prepared in the same way and cured in the same conditions. To a portion of Epidian 6 (E6) epoxy resin of the epoxide index in the range from 0.51 to 0.54, a portion of octakis[(3-glycidoxypropyl)dimethylsiloxy]octasilsesquioxane (POSS-O) or octakis[(3-(thiiran-2-yloxy)propyl)dimethylsiloxy]octasilsesquioxane (POSS-S) making 10 mass% of the epoxide resin was added at room temperature and was stirred together for several minutes. Then, the appropriate amount of phthalic (PHT), maleic (MAL), or pyromellitic (PYR) anhydride was added to the mixture, and the contents were stirred again. The samples prepared were then cured for 1 h at 200°C and left for 24 h at room temperature. The amounts of hardener (anhydride) were chosen to ensure the anhydride to oxirane and/or thiirane groups ratio of 0.5. According to the procedure described, three sets of samples were prepared differing in the type of curing agent and modifier used. The amounts of substrates used are listed in Table 1. The cured samples were subjected to TG analysis. 1 H NMR (300 MHz), 13 C NMR (75 MHz), and 29 Si MNR (59 MHz) spectra were recorded on a Varian XL 300 spectrometer at room temperature using CDCl 3 as solvent. FT-IR spectra were recorded on a Bruker Tensor 27 Fourier Transform spectrometer equipped with a SPECAC Golden Gate diamond ATR unit. For all spectra, 16 scans at a resolution of 2 cm -1 were collected. Thermal stabilities of the samples prepared were measured on a Q50-TGA thermogravimetric analyzer (TA Instruments, Inc.) under nitrogen at flow rate of 60 mL min -1 . Samples (10-15 mg) loaded in a platinum pan were heated from RT to 800°C at a rate of 20°C min -1 . DSC measurements for the determination of curing profiles were carried out on a differential scanning calorimeter DSC-1 (Mettler-Toledo) under air atmosphere at a heating rate of 10°C min -1 . Samples for DSC experiment of about 20 mg were prepared according to the curing procedure given above (see Table 1) and placed in 40-lL aluminum pans with a pierced lid and subjected to the analysis immediately after preparation.

Results and discussion
The above-described methodology of thiirane-POSS derivative synthesis, starting from the appropriate epoxy-POSS derivative seems to be optimal (Fig. 1). The use of KSCN as a sulfur donor permitted obtaining the target compound in high yield. Attempts at replacement of potassium thiocyanate with thiourea without changing any other reaction parameter resulted in a significant decrease in the final product yield (from 70 to 45 %).
The FT-IR product spectra reveal significant changes in the range from 500 to 1,000 cm -1 (Fig. 2). The bands at 547, 839, and 902 cm -1 present in the substrate spectrum and coming from oxirane ring disappear, while new bands appear at 617, 694, 729, and 800 cm -1 , and according to the literature, they can be assigned to thiirane group [31][32][33].
Analysis of TG curves (Fig. 3) confirmed the influence of either of the two additives (POSS-O and POSS-S) on thermal properties of epoxy resins. The effect of the additives was described in terms of the temperatures of 1, 5, and 10 % mass loss and DT values for each sample series were calculated. All data obtained are presented in Table 2.
The influence of octakis[(3-(thiiran-2-yloxy)propyl)dimethylsiloxy] octasilsesquioxane (POSS-S) addition on the thermal properties of epoxy resin is more pronounced than that of its oxirane analog. It is especially noticeable for Epidian 6 samples hardened with maleic anhydride. The difference between the temperature of 1 % mass loss of the sample without any modifier and the sample containing 10 % of POSS-S is higher than 100°C. For the temperatures of 5 and 10 % mass loss, the difference in DT is not as high, but still exceeds 50 or 60°C. The differences in thermal stability of Epidian 6 cured with pyromellitic anhydride modified with either of the two spherosilicates are not so remarkable but still considerable. Unfortunately, it is not possible to analyze the influence of additives used on the thermal stability of the system cured with phthalic anhydride, because only the sample modified with thiirane derivative was hardened. However, this result confirms higher reactivity of thiirane groups than that of oxirane ones. Analysis of TG curves also indicated a higher ash residue in the samples containing silsesquioxane.
Performed DSC measurements also revealed influence of POSS additives on epoxy resin properties-their curing  processes in this case. It is especially significant for octakis [(3-(thiiran-2-yloxy)propyl)dimethylsiloxy]octasilsesquioxane modified compositions regardless of the hardener used (see Table 3). For all samples prepared with thiirane additive, curing peak was narrower than for samples prepared with epoxy POSS or without any modifier. It is also important that peak temperatures of curing processes were significantly decreased for samples hardened with pyromellitic and maleic anhydrides and containing thiirane or epoxy POSS additives. The type of additive also influences the enthalpy of curing process. For samples hardened with pyromellitic anhydride, enthalpy decreases from 201.7 J g -1 for sample without any POSS additive to 175.5 J g -1 for sa mple modified with 10 mass% of epoxy POSS derivative and down to 117.2 J g -1 for sample modified with 10 mass% of thiirane compound. Also, there is a significant change of curing enthalpy of epoxy resin in the presence of phthalic anhydride. No reaction was observed in this case for sample containing no additive, curing enthalpy of sample   modified with epoxy POSS derivative was relatively low (11.6 J g -1 ) and raised up to 167.2 J g -1 for sample containing 10 mass% of octakis[(3-(thiiran-2-yloxy)propyl)dimethylsiloxy] octasilsesquioxane. Those results are in good compliance with macroscopic observations and fact that only last sample from this series was hardened under applied curing procedure. Curves of curing recorded for samples hardened with maleic and pyromellitic anhydrides are shown on below figure (Fig. 4).

Conclusions
Exploring the known reaction mechanism, a new, not reported up to date octakis[(3-(thiiran-2-yloxy)propyl)dimethylsiloxy]octasilsesquioxane was synthesized in good yield and applied as epoxy resin modifier, enhancing its thermal stability and influencing the curing process. The materials obtained exhibited temperatures of 1 % mass loss even by 100°C higher than that of raw materials. Improved thermal stability of epoxies modified with octakis[(3-(thiiran-2-yloxy)propyl)dimethylsiloxy]octasilsesquioxane can be attributed to the possibility of formation of sulfur-containing bridges, additionally crosslinking the polymer. Obtained DSC experiment results have shown that application of octakis[(3-(thiiran-2-yloxy)propyl)dimethylsiloxy]octasilsesquioxane as modifier in described compositions can cause significant decrease of curing temperature and narrowing of curing temperature range.