Boehmite Nanofillers in Epoxy Oligosiloxane Resins: Influencing the Curing Process by Complex Physical and Chemical Interactions

In this work, a novel boehmite (BA)-embedded organic/inorganic nanocomposite coating based on cycloaliphatic epoxy oligosiloxane (CEOS) resin was fabricated applying UV-induced cationic polymerization. The main changes of the material behavior caused by the nanofiller were investigated with regard to its photocuring kinetics, thermal stability, and glass transition. The role of the particle surface was of particular interest, thus, unmodified nanoparticles (HP14) and particles modified with p-toluenesulfonic acid (OS1) were incorporated into a CEOS matrix in the concentration range of 1–10 wt.%. Resulting nanocomposites exhibited improved thermal properties, with the glass transition temperature (Tg) being shifted from 30 °C for unfilled CEOS to 54 °C (2 wt.% HP14) and 73 °C (2 wt.% OS1) for filled CEOS. Additionally, TGA analysis showed increased thermal stability of samples filled with nanoparticles. An attractive interaction between boehmite and CEOS matrix influenced the curing. Real-time infrared spectroscopy (RT-IR) experiments demonstrated that the epoxide conversion rate of nanocomposites was slightly increased compared to neat resin. The beneficial role of the BA can be explained by the participation of hydroxyl groups at the particle surface in photopolymerization processes and by the complementary contribution of p-toluenesulfonic acid surface modifier and water molecules introduced into the system with nanoparticles.


Materials and film curing
. Structures of (a) unmodified and (b) p-toluenesulfonic acid modified Boehmite (adapted from [1]).  The characteristic IR bands of CEOs and Boehmite are summarized in Table S1.

Determination of Glass transition temperature (Tg)
Glass transition of neat CEOS and different nanocomposite formulation was evaluated using DSC ( Figure S4). Tg was detected as the temperature at half-height of change in heat capacity. The scaled heat flow signal with fitting example of all studied compositions are given in Figure S5 - Figure  S7.

Thermal gravimetric analysis
TGA of Boehmite nanoparticles was carried out in order to determine the present degradation steps in used fillers ( Figure S8). Two degradation steps in unmodified Boehmite (HP14) are detectable. The first is observed at temperatures below 100 o C and can be associated with physically absorbed water. The second step, appearing as a main weight loss, take place between 300 o C and 470 o C with the DTG maximum rate at 445 o C. It represents the dihydroxylation of Boehmite leading to formation of Al2O3 and H2O [6].
Four degradation stages can be noticed for organo-modified Boehmite OS1. The first step appears between 30 and 150 o C with the DTG curve maximum at 65 o C. The second step begins at around 300 o C with DTG peak at 445 o C. The third degradation step exhibits shallow DTG peak at 470 o C and partially overlaps with the second. The fourth step begins at 550 o C. The first and second degradation steps is believed to correspond to the same processes as in the unmodified Boehmite, i.e. physical water desorption and dihydroxylation processes. It was suggested that the presence of PTSA in OS1 increases water absorption/desorption rate compared to HP14 what results in a decrease of temperature at which the first two degradation steps appear [7]. In our case higher content of physically absorbed water for OS1 is detected from weight loss at the first step. The third degradation step, present only in o-Boehmite, is attributed to the decomposition of surface modifier, resulting in sulfates formation [7]. The last fourth step represents decomposition of aluminum sulfate formed in PTSA degradation. The overall weight loss during heating up to 750 o C of Boehmite powders were 27% and 24% for HP14 and OS1, respectively.

Photoinitiator structure and decomposition
The chemical formula of used photoinitiator and simplified representation of photolysis are given below: Scheme S1. (a) Chemical structure and (b) simplified scheme of photodecomposition of arylsulfonium hexafluorophosphate salt.

Real-Time Infrared spectroscopy: curing kinetics
The evolutions of band absorptions associated with the decrease of C-H stretching in oxirane ring and the formation of hydroxyl groups located at 2980 cm -1 and 3450 cm -1 , respectively, are given in Figure S9 - Figure S10.  First derivative of peak area changes is given in Figure S11. The deceleration at the beginning of UV irradiation is observed for CEOS/BA nanocomposites, which becomes more pronounced with an increase of Boehmite concentration.

The effect of moisture on epoxy conversion
It is believed that water has no significant influence on the photodecomposition of triarylsulfonium salts. Water might play a solvating role, but it is assumed that it does not impact the number of active sites. Two competitive mechanisms involving water molecules as a chain-transfer agent should be considered during cationic ring-opening polymerization, so-called active-chain end (ACE) and active monomer (AM) mechanism [8] [9] [10]. The mechanisms of cationic ring-opening involving water are illustrated in the following Scheme.S 2.
Absence of a chain transfer agent (in our case water molecules) leads to the ACE reaction mechanism resulting in the subsequent isolation of active sites and therefore gradual decrease of polymerization rate. This phenomenon is observed for CEOS photocuring. The addition of water, for instance, during moisture post-annealing, into reaction-limited polymer matrix results in reaction between H2O molecule and tertiary oxonium ion yielding a hydroxyl end group and an acidic proton that reinitiates a neutral epoxide. On the other side, an ample amount of H2O leads to polymerization followed by AM mechanism leading to significant increase of polymerization events as it was found for moisture post-treatment of cycloaliphatic epoxy [10].