Methylsilsesquioxane-Based Aerogel Systems—Insights into the

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Methylsilsesquioxane (MSQ) Based Aerogel Systems Insights into the Role of the Formation of Molecular Clusters Ana Borba, Mauro Almangano, António Alberto Portugal, Ricardo Patricio, and Pedro Nuno Simões J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04196 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 29, 2016

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The Journal of Physical Chemistry

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Methylsilsesquioxane (MSQ) Based Aerogel Systems - Insights into the Role of the Formation of Molecular Clusters A. Borba1,*, M. Almangano1,2, A. A. Portugal1, R. Patrício2, P. N. Simões1 1

CIEPQPF - Department of Chemical Engineering, University of Coimbra, 3030-790 Coimbra, Portugal

2

Active Aerogels, IPN - Edifício C, Rua Pedro Nunes, 3030-199 Coimbra, Portugal

*

Corresponding author: Email: [email protected]; Fax: +351 239 798 703; Tel: +351 239 798 732.

1 ACS Paragon Plus Environment


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Methylsilsesquioxane (MSQ) Based Aerogel Systems - Insights into the Role of the Formation of Molecular Clusters A. Borba1,* , M. Almangano1,2, A. A. Portugal1, R. Patrício2, P. N. Simões1 1

CIEPQPF - Department of Chemical Engineering, University of Coimbra, 3030-790 Coimbra, Portugal

2

Active Aerogels, IPN - Edifício C, Rua Pedro Nunes, 3030-199 Coimbra, Portugal

Abstract Condensed clusters of hydrolyzed methyltrimethoxysilane (MTMS) were studied using two complementary approaches: (i) FTIR was applied along with the hydrolysis and condensation stages of a sol-gel process from the condensation of colloidal suspension of nanoparticles to the solid phase of bulk material; (ii) density functional theory calculations of energies, structural and vibrational data of pertinent MTMS hydrolysis products, viz. methylsilanetriol (MST) based species with different number of silicon atoms (from two to eight atoms) and different structures/conformations (linear, cyclic and cage, in a total of thirteen structures), were performed at B3LYP/6-311+G(d,p) level of theory. The calculated infrared spectra show two distinct Si-O-Si stretch vibration bands for models of caged structures. The higher frequency IR band at ca. 1120 cm-1 is derived from the antisymmetric Si-O-Si stretch vibration mode, while the lower frequency band at 1035 cm-1 is due to the symmetric Si-O-Si stretch and is characteristic of the cyclic clusters, being absent in highly symmetric cage structures. The calculated versus the experimental FTIR spectra of poly(methylsilsesquioxane) (PMSQ) dried aerogel in KBr pellet, show that cage/cyclic-like structures prevail over ladder structures (linear) in actual PMSQ.


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1. Introduction Aerogels are micro- and meso-porous networks composed of randomly interconnected nano-scale clusters of metal oxides, i.e. highly porous open cell structures, with unique physical properties. Optical transparency, low density, high porosity, high surface area, low thermal conductivity, low refractive index, and low dielectric constant are among the most important properties of this class of materials.1, 2 It is widely accepted that the macroscopic properties, such as elasticity and stiffness, strongly depend on the nanostructural features of the gel network.3 However, it is very difficult to experimentally obtain information about the formation and evolution of the first series of nano-sized clusters at the very beginning of the sol-gel reaction. In this context, molecular modeling and simulation tools applied to seeding structures presumed to be formed during the reaction can help in the identification of relevant structural and spectroscopic parameters, thus being a valuable tool to treat the problem. Provided that the starting material structure is known in detail from the computational model, structure/property relationships can be unveiled and understood.4 In general, sol-gel synthesis involves the polymerization of a metal or non-metal alkoxide resulting in the transformation of a monomer solution in colloid (sol or gel), and, finally, in a three dimensional solid network. The most common precursors are tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS), and their reaction products are a fully inorganic network of silica, widely studied.5, 6 In the last decade, due to their potential applications, there has been an increasing interest in the production of hybrid nanostructured materials made of organic and inorganic components. Organically modified

silicon

alkoxides

are

the

usual

precursors

to

these

organic-inorganic hybrid materials. The incorporation of organics into nano- or molecular-scale hybrids is enabled by the room temperature conditions of the sol-gel method. The organic group is distributed throughout the network and becomes an integral component of the framework. Poly(methylsilsesquioxane) (PMSQ) chains are composed of the R-SiO1.5 monomeric repeating units, with a single silicon atom attached to other repeating units in the polymer through up to three siloxane bonds. The remaining substituent is an alkyl group attached to the silicon through a silicon-carbon single bond (Figure 1). The knowledge of the structures, stabilities, and reactions of small silicate clusters is of crucial importance for understanding the molecular processes occurring in both sol-gel synthesis of ceramics and hydrothermal synthesis of 3 ACS Paragon Plus Environment


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micro-porous materials. In the literature,7-14 there are both theoretical and experimental evidences regarding the network evolution of TEOS aerogel. On the contrary, studies on methyltrimethoxysilane (MTMS) products are still lacking. The aerogel hybrid material under investigation on in this work has an inorganic backbone with an organic part (methyl groups) inside. This aerogel is obtained after sol-gel sol gel synthesis of the MTMS precursor for the production of flexible exible structures with a high cross-linked cross linked network analogous to fully inorganic gels.7, 8

(a)

(b)

Figure 1 – (a) Basic unit and (b) representative network of poly(methylsilsesquioxane) p oxane).

Several theoretical results based on density functional theory (DFT)) calculations10-12 have shown that primary particles of the TEOS aerogel system have an important role in the hierarchical evolution of the network. network Despite the relevance of PMSQ compounds, its microstructure during the sol-gel sol gel process is not well understood. The knowledge of such a microstructure with molecular detail, i.e. whether the inaugural gural PMSQ molecules are comprised of closed cages, partially open cages, double chain ladders, or random network structures, is very important for understanding the formation process of the aerogels and concomitantly their final properties. The ability to obtain such information from experimental analysis, e.g.. infrared (IR) spectroscopy, is very important for understanding the structure-property structure property relationships of poly(silsesquioxane) based materials in view of the improvement of the desired physical properties properties by modifying the monomer structures and incorporating co-monomers. co monomers. The IR spectroscopy study of microstructures of poly(silsesquioxane) was reported by Park et al.15 The authors concluded

that

poly(methylsilsesquioxane),

poly(isobutylsilsesquioxane)

and

poly(phenylsilsesquioxane) in as-polymerized as samples exhibit a cage-like cage structures, rather than ladder structures. Hydrogen bonding and dipole-dipole dipole dipole interactions between silica and solvent often maintain the metastability of silica sols in the case of TEOS.16 The interactions of 4 ACS Paragon Plus Environment

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MTMS-derived oligomers and polymers with polar solvents are, however, much weaker because of the presence of hydrophobic Si-CH3 groups and the associated van der Waals forces, which reduces the miscibility in the solvent. Considering this behavior, we addressed the effect of cluster structures on growing process of trifunctional sites of PMSQ by predicting the most probable structures formed in the early stage of the condensation. Considering the importance of PMSQ for a variety of applications and the scarce information available on its structural and spectroscopic features, we report here the predicted IR frequencies for various microstructure models obtained from DFT calculations and compare the calculated spectra with experiments. This unified experimental and computational analysis allowed to elucidate the microstructures of PMSQ, thus providing new insights into the condensation stage of sol-gel polymerization.

2. Materials and methods 2.1. Experimental details Pure (spectroscopic grade) MTMS (CH3Si(OCH3) 98%, Aldrich), methanol (CH3OH, 99.8%, Riedel-de-Haän), oxalic acid (C2O4H2, 99%, Fluka) and ammonium hydroxide (NH4OH, 25% in water, Fluka) were used as precursor, solvent, acid and basic catalysts, respectively. The applied sol-gel route is a two-step acid-base catalyzed sol-gel process followed by ageing and drying stages as described by Durães et al.17 The IR spectra (4000-500 cm-1) were collected with 2.0 cm-1 spectral resolution using a Nicolet 6700 Fourier transform infrared spectrometer equipped with a deuterated triglycine sulphate detector and a Ge/KBr beamsplitter. The acquisition started at the second step of the condensation reaction, with the addition of the base, and terminated at the gel point (see Figure 2). The reaction evolution was analyzed by the changes in the individual absorption peaks by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The experimental FTIR spectra of the PMSQ dried aerogel, in KBr pellet at room temperature were investigated in the same equipment.



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Figure 2 – Schematic representation of sol-gel synthesis.

2.2. Computational details We based our approach on an integrated vibrational frequency prediction within the DFT/B3LYP framework and its experimental counterpart. Bennett et al.18 also used this approach to study SiO/SiC stretch assignments of Raman spectroscopy of series of methyl-methoxysilanes

(tetramethoxysilane,

methyltrimethoxysilane,

dimethyldimethoxysilane, and trimethylmethoxysilane). The knowledge of the conformational preferences of these small PMSQ chains paves the way for assessing a larger scale, viz. the constitutive morphology of the network. It is not easy to experimentally detect conformations of the first series of clusters in this fast polymerization process, as the sol-gel reaction. The computational approach is a valuable tool to understand what happens in the first stage of that process. Different PMSQ clusters of increasing size were studied theoretically by quantum chemical calculations aiming at establishing a possible framework for the very beginning of the MTMS polycondensation process. Due to computational constrains, the calculations were restricted to the conformations, energies, and IR vibration spectra of key silica clusters containing up to eight Si atoms. Thirteen structures of methylsilanetriol (MST) based species, from 2 to 8 silicon atoms, with different structures (linear, cyclic and cage) and different conformations, were studied at B3LYP/6-311+G(d,p) level of theory, in vacuum, with the G03 package.19 Harmonic frequency calculations were performed to make sure that the computed structures corresponded to the minimum on the potential energy surface and thus to support the assignment of the observed experimental vibrational features.


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2.3. Results and discussion 2.3.1. Geometries and energies of PMSQ clusters The cluster molecules were divided in three types: A, linear; B, cyclic; and C, cage/open-cage. The discussion will focus their energetic, geometric and spectroscopic properties. Silica clusters are classified in this work as described elsewhere,10,

11

,

where n represents the number of silicons that are bonded to m bridging oxygens. Figure 3 shows the molecular structures of the monomeric form (methylsilanetriol) and the selected MTMS poly-condensed molecules. The relevant optimized parameters, bond lengths and angles, for the most stable conformations of the MSQ based clusters are presented in Table SI-1 at Supporting Information - SI. The total energy (relative electronic energies, including the zero-point vibrational energy), the Gibbs free energy (sum of electronic and thermal free energies), dipole moment and rotational constants for all silica clusters discussed in this work are presented in Table 1. For each cluster, the different conformations are ordered by their energies.



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Monomer (methylsilanetriol) - C1

Clusters Type A - Linear

- C1

- C2

- C1

- C2

- C3

- CS

- C2

Type B - Cyclic

- CS

- CS

- C4

- CS

Type C - Cage and Open Cage

- CS

- Oh

Figure 3 - B3LYP/6-311+G(d,p) optimized structures of the MSQ based clusters, including a schematic representation. The symmetry point groups of the most stable conformers are also showed. , where n represents the number of silicons that are bonded to m bridging oxygens, according to references10, 11.


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Table 1: Total energy, Gibbs free energy, dipole moment and rotational constants for different conformations of several silicate clusters, calculated at the B3LYP/6-311+G(d,p) B3LYP/6 level.a Symc

Energy Total b Gibbs Absolute Absolute ∆E

∆G

Rotational constants

Dipole Moment

A

B

C

Monomer

C1

-349577.90 349577.90

-

-349567.43

-

1.50

3922.14

3592.86

3473.95

C2 C1 C1

-651197.95 651197.95

0.00 0.54 0.83

-651162.31

1.30 0.00 0.03

1.06 1.46 3.46

1952.20 1897.47 1888.82

742.86 717.54 709.35

738.78 705.35 688.28

C1 C1

-952821.34 952821.34

0.00 2.55

-952757.13

0.00 0.44

1.19 1.81

790.67 795.67

391.27 394.90

326.72 328.11

C3 C1

-1254440.95 1254440.95

0.00 1.15

-1254351.91

0.00 1.59

5.05 3.41

319.54 357.08

319.54 290.54

228.86 226.86

C2 C2 C1

-1254442.35 1254442.35

0.00 0.39 0.84

1.78 0.00 6.97

0.32 0.46 6.02

528.58 503.88 379.18

185.37 178.32 289.22

172.59 171.22 236.06

C1 C1

-1556064.44 1556064.44

0.00 3.69

-1555947.13

0.00 1.15

4.86 2.75

229.42 245.25

183.44 163.22

168.84 138.38

CS C1 C1 CS

-1857682.04 1857682.04

0.00 2.99 3.02 6.37

-1857547.09

0.00 2.32 1.44 2.56

3.82 4.35 3.62 4.92

158.5 .52 149.86 146.67 148.48

126.10 131.98 134.69 128.35

83.32 83.04 83.29 80.19

C2

-2460921.89 2460921.89

0.00

-2460741.80

0.00

1.32

136.67

54.81

50.79

1.05

2.11

108..43

60.30

54.09

Cluster Type A - Linear

C1 Cluster Type B - Cyclic

-1254356.48

0.34

CS C1

-904848.44 904848.44

0.00 0.40

-904798.80

0.00 0.80

0.13 2.19

644.93 657.28

643.69 641.29

438.83 444.78

CS C1

-1158496.57 1158496.57

0.00 0.03

-1158428.36

0.00 0.01

1.83 1.73

543.25 550.43

342.80 341.95

326.27 323.00

C1

-1206473.47 1206473.47

0.00

1.41

3.86

458.80

304.70

278.88

C1

C4 C1

-1206475.48 1206475.48

Cluster Type C - Cage and Open Cage Oh -2221105.59 2221105.59 C1

CS C1

-2317030.54 2317030.54

1.94

-1206394.16

0.00

1.67

465.63

274.91

249.91

0.00 1.08

-1206397.42

0.39 0.00

1.03 2.41

403.27 389.02

403.27 378.92

250.08 239.84

0.00 0.07

-2220957.86

0.00 0.62

0.00 0.01

132.09 132.91

132.09 132.64

132.09 132.51

0.00 1.57

-2316865.98

0.00 0.48

3.44 2.95

131.77 134.40

99.47 92.03

93.85 89.14

a

Energies in kcal mol-1, dipole moment in Debye and Rotational constants in MHz. Total energy is the relative elative electronic energies, energie including the zero-point vibrational energy. c Sym, symmetry point group b



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In type A, three conformations were considered for the simplest linear cluster, , with point symmetry C2 and C1. Considering the total energy, the C2 conformation is the global minimum, being more stable than C1 conformations by 0.54 to 0.83 kcal mol-1. On the other hand, taking into account the Gibbs free energy, the C1 conformations are more stable than C2 (1.30 kcal mol-1). For the cluster, type B, the trend is identical. Thus, in both cases ( and ) the stability depends on thermal free energies. For Q Q

cluster (cyclic) and cluster (cage) two conformations were investigated, C4/C1 and Oh/C1, respectively. In both cases, the conformers are essentially isoenergetic, meaning that here the symmetry is not relevant in terms of stability. For the other clusters the conformations with the lowest energy (global minima) are those with the highest symmetry (see Table 1). In summary, as expected, the energetic analysis confirmed that the stablest conformations are the high symmetric structures. The

Gibbs

free

energy

change

(∆RG)

of

the

condensation

reaction

(Monomer ↔ Cluster + H2O) was used for comparing the stabilities of the different clusters. The results are summarized in Table 2 for the stablest conformations only (see also Table 1). Despite not accounting for solvent and/or other environmental effects, this approach give us clues to predict the most probable structures formed in the condensation process. In order to consider an index that allows comparing different polycondensed species, the calculated energies of the PMSQ clusters were corrected by the number (s) of silicon atoms in the cluster, according to a procedure reported by Pereira et. al.10, 11 The condensation reactions for the investigated clusters are spontaneous processes (∆RG

Methylsilsesquioxane-Based Aerogel Systems—Insights into the

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