Influence of n-alkyl substituents on the Thermal behaviour of Polyhedral Oligomeric Silsesquioxanes (POSSs) with different cage's periphery

Polyhedral Oligomeric Silsesquioxanes (POSSs) of general formula R7R’(SiO1.5)8, where R = isobutyl, cyclopentyl or phenyl and R’ = -(CH2)5-CH3, -(CH2)7- CH3 or -(CH2)9-CH3-, were prepared by corner capping reaction of trisilanols with suitable alkyltriethoxysilane. The compounds obtained were characterized by elemental analysis and 1H NMR spectroscopy, and the results were in very good agreement with those of expected products. The prepared POSSs were degraded in dynamic heating conditions from r.t. to 700 °C, in both inert and oxidative atmospheres, and the temperatures at 5% weight loss (T5%) were determined and compared with each other, in order to investigate how the various groups affect the resistance to thermal degradation. The results did not evidence any substantial difference between the two investigated atmospheres, but indicated clearly an increasing resistance to the thermal degradation according to the following order: isobutyl POSSs < cyclopentyl POSSs < phenyl POSSs. Also, T5% values were linearly increasing as a function of the number of methylene groups in R’ alkyl chain. The results were discussed and suggested a group contribution to thermal stability. Graphical abstract Image for unlabelled figure Figure options Keywords POSS; Group additivity; Thermal stability; Cage's periphery; Thermogravimetry. 1. Introduction Silsesquioxanes are an extremely interesting class of inorganic/organic hybrid compounds, whose chemical structure corresponds to the composition (RSiO1.5)n, where n can assume the values 6, 8, 10, 12. These compounds can be classified in two categories: 1) silsesquioxanes, having un-caged (random, ladder or partially caged) structure and 2) silsesquioxanes having caged structure. These last compounds are characterized by the presence of a of cage silicon and oxygen atoms, with the silicon atoms at the vertices and linked to organic R groups by covalent bonds. Caged silsesquioxanes, usually called Polyhedral Oligomeric Silsesquioxanes (POSSs), have 8 as the most common value of n and are highly symmetric nanosized compounds with a diameter usually falling in the 1.5–3 nm range [1], the R groups of vertices included. Also, the hybrid organic-inorganic framework renders POSSs thermally and chemically very stable. Due to their peculiar characteristics, their use spread in various fields, as, for example: low dielectric constant materials [2], resists for electron beam lithography materials [2], organic thin-film transistor (O-TFT) [3], catalysts [4], materials for cardiovascular implants and nanomedicine [5] and [6]. Moreover, it has been widely reported in literature that the reinforcement of polymer matrices with silsesquioxanes gives rise usually to the improvement of thermal [7], mechanical [8] and flammability [9] properties of obtained nanocomposites in respect to neat polymer. So, in the last years, POSSs received further considerable attention also as fillers for polymer based nanocomposites. The most common molecular formulas of POSSs used in this field are (RSiO1.5)8 or R7 R’1 (SiO1.5)8. The properties of POSSs depend on the organic substituents in their structure: in particular, the nature of R- and R’- affects the solubility in conventional solvents, the compatibility with polymers and the capability to undergo nanometric dispersion in host polymer matrices, and then the thermal, physical and mechanical behaviour of the resulting nanocomposites. On considering that to obtain materials with good thermal and mechanical properties filler must be well miscible with polymer matrix, research would be driven towards the preparation of POSSs having substituents that enhance the resistance to thermal degradation but do not decrease the miscibility with polymer. Studies on homo-substituted POSSs of type (RSiO1.5)8, where R is an alkyl (C2-C10) chain, suggested that the presence of aliphatic groups increase solubility but worsen thermal properties [10] and [11] whilst the presence of aromatic groups seems to act in the opposite direction, considerably enhancing thermal stability but decreasing solubility and compatibility [2]. Finally, it is important to remember, from this viewpoint, that highly symmetric POSSs are more difficult to mix with polymers and to solve in the most common organic solvents, and then this aspect must be considered in preparing novel POSSs to be used as fillers for polymers [12]. In recent years our group has been engaged in the synthesis, characterization and study of novel POSSs [13], [14] and [15]. In particular we studied the effect of the introduction of a phenyl group (or variously substituted phenyl groups) into a POSS molecule containing seven isobutyl [16] or cyclopentyl [17] groups on the thermal properties of POSSs and of the corresponding Polystyrene (PS) based nanocomposites. The results obtained showed a good increment of the resistance to thermal degradation due to the introduction of aromatic groups for both POSSs and nanocomposites [18] and [19]. Continuing our work in this field, and on the basis of our preceding experiences, in this work we synthesized nine novel POSSs, with general formula R7R’(SiO1.5)8where: R= isobutyl; cyclopentyl; phenyl R’ = -(CH2)5-CH3-; -(CH2)7-CH3-; -(CH2)9-CH3-and carried out a comparative study on their thermal degradation in both inert and oxidative atmospheres, aiming to study organically the effect of the introduction of some selected groups into the same structure. The studied compounds were the following: n hexyl hepta isobutyl POSS 1 n octyl hepta isobutyl POSS 2 n decyl hepta isobutyl POSS 3 n hexyl hepta cyclopentyl POSS 4 n octyl hepta cyclopentyl POSS 5 n decyl hepta cyclopentyl POSS 6 n hexyl hepta phenyl POSS 7 n octyl hepta phenyl POSS 8 n decyl hepta phenyl POSS 9 Table options Various samples will be indicated in the text by the corresponding numbers and the structures of synthesized POSSs are reported in Table 1Table 2Table 3 Table 1. Molecular structure of the various mono alkyl hepta isobutyl POSS. chain. n hexyl hepta isobutyl POSS (1) n octyl hepta isobutyl POSS (2) Full-size image (13 K) Full-size image (13 K) n decyl hepta isobutyl POSS (3) Full-size image (12 K) Table options Table 2. Molecular structure of the various mono alkyl hepta cyclopentyl POSS. n hexyl hepta cyclopentyl POSS (4) n octyl hepta cyclopentyl POSS (5) Full-size image (15 K) Full-size image (14 K) n decyl hepta cyclopentyl POSS (6) Full-size image (13 K) Table options Table 3. Molecular structure of the various mono alkyl hepta phenyl POSS. n hexyl hepta phenyl POSS (7) n octyl hepta phenyl POSS (8) Full-size image (15 K) Full-size image (15 K) n decyl hepta phenyl POSS (9) Full-size image (14 K) Table options These compounds have been projected according to the following criteria: 1) the R’ substituent in a vertex is different than those (R) in the other seven vertices, to low the symmetry of molecule, and then to improve the miscibility with polymers; 2) in order to verify if different chain length affects thermal stability, R’ substituents are various linear aliphatic groups; 3) the R substituents were chosen to check how much the substitution of aliphatic iso-butyl groups with cyclo-aliphatic and, then, with phenyl groups, affects thermal parameters. POSSs were prepared by a simple and inexpensive method we set-up in our preceding works [17] and were then characterized by elemental analysis and 1H NMR. The evaluation of the resistance to thermal degradation of various POSSs was made by thermogravimetric degradation experiments in the scanning mode from 25 to 700 °C, in both flowing nitrogen and static air atmosphere, and the temperatures at 5% weight loss (T5%) were determined as well as the residues at 700 °C, which were also analysed by FT-IR spectra. 2. Experimental 2.1. Materials Tetraethoxysilane (TEOS), 1-bromohexane, 1-bromooctane and 1-bromodecane have been purchased from Aldrich Co. and used as received. Tetrahydrofuran (THF) was distilled over a Na-benzophenone mixture. Ethanol was dried with Na and distilled from NaOEt. n-Hexyltriethoxysilane, n-Octyltriethoxysilane and n-Decyltriethoxysilane were prepared from the appropriate Grignard reagent and TEOS [20], [21], [22] and [23]. Trisilanolisobutyl POSS (iC4H9)7–Si7O9 (OH)3 was prepared according to literature method [24]. Trisilanolcyclopentyl POSS(c C5H9)7–Si7O9 (OH)3 was prepared according to literature methods [25] and [26]. Trisilanolphenyl POSS has been purchased from Hybrid Plastics co, and used as received. All compounds 1–9 were prepared by Corner Capping Reaction of trisilanols with the suitable alkyltriethoxysilane, which was conducted for compounds 1-6differently for compounds < rk-bold> 7-9. The procedure used for compounds 1-6 is the following, which is here described in details for compound 5: 4.37 g (5 mmol) of trisilanolcyclopentyl POSS were dissolved in 100 mL of anhydrous ethanol, and 5.5 mL (1.37 g, 15 mmol of a 25% methanol solution) of tetramethylammonium hydroxide were added under stirring. The clear solution was cooled at 0–5 °C in an ice bath, and 1.38 g (5 mmol) of n-Octyltriethoxysilane were added. The solution was stirred and temperature was maintained at 0–5 °C for 24 h. The resulting solid was filtered and washed with two portions (10 mL) of anhydrous ethanol. After drying under reduced pressure, the white solid obtained was crystallized from THF/MeCN mixture to give 4.18 g of desired compound (72.8% yield). The procedure used for Trisilanolphenyl POSS derivatives 7-9 is given, as an example, for compound 8. 4.65 g (5 mmol) of Trisilanolphenyl POSS were dissolved in dry THF (40 ml) followed by the addition of n-Octyltriethoxysilane (1.38 g, 5 mmol) and 10 drops of methanesulfonic acid under stirring. The solution was maintained at room temperature for 24 hours and then heated at 50-60 °C for 4 days. The reactant mixture was cooled and solvent was reduced under vacuum; methanol was added and the white solid obtained was crystallized from THF/MeOH (3.60 g, 59.7% yield). Yields and elemental analysis data of the obtained compounds are reported in Table 4. Table 5 Note to users: Accepted manuscripts are Articles in Press that have been peer reviewed and accepted for publication by the Editorial Board of this publication. 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