Microscale mechanochemical characterization of drying oil films by in situ correlative Brillouin and Raman spectroscopy

Correlative Brillouin and Raman microspectroscopy (BRaMS) is applied for the in situ monitoring of the chemical and physical changes of linseed oil during polymerization. The viscoelastic properties of the drying oil throughout the phase transition were determined by Brillouin light scattering (BLS) and joined to the Raman spectroscopic information about the chemical process responsible for the oil hardening. A comparative study was then performed on an oil mock-up containing ZnO, one of the most common white pigments used in cultural heritage. The intriguing outcomes open new research perspectives for a deeper comprehension of the processes leading to the conversion of a fluid binder into a dry adhering film. The description of both chemical and structural properties of the polymeric network and their evolution are the basis for a better understanding of oil painting degradation. Last, as a feasibility test, BRaMS was applied to study a precious microfragment from J. Pollock’s masterpiece Alchemy.

multiple scattering contributions were considered using the method described in Ref. (52). Furthermore, the finite back-scattering collection angle induces a distribution of exchanged wave number ( ), which also contributes to the broadening of the Brillouin peak. The calculation was therefore corrected by considering that the numerical aperture of the lens used (NA=0. 42), corresponding to a minimum scattering angle = 155.2°, causes a widening in , which can be approximated to about 2.3% of the frequency position of the peak itself.
The adiabatic (zero-frequency) longitudinal modulus, 0 , is related to the sound velocity through the relation: 0 = 0 2 , where is the density of the sample. For a system subject to relaxing phenomena, a complex frequency-dependent modulus has to be used: * ( ) = ′ ( ) + ′′ ( ), with the real and imaginary part given by Eqs. S2, S3: Within the GHz frequency range, these two quantities can be derived in a unique way from Brillouin measurements. The imaginary part ′′ ( ), is a measure of the energy dissipation of the sound waves within the medium, while the real part represents its dispersive behaviour. A decrease of ′ ( ), for example as a function of temperature, is simply indicating that the material is becoming softer; 0 can be considered as the zero-frequency limit of the real part. All these physical quantities are called "apparent" because of their frequency-dependency (13,52).

Refractive Index (n) and Density ( )
In order to derive , ′ ( ) and ′′ ( ), we need to know the refractive index and the mass density of the sample. The calculations were made considering the values of ( 0 )=1.48 (53)(54)(55) and ( * ) 1 =1.51 at the initial time of the reaction and at the time when the Brillouin shift no longer showed noticeable changes, respectively, corresponding to an n-variation during desiccation of about 2%, in agreement with other polymerization experiments (56). We based on a linear behaviour of n(t) between 0 and * then followed by a constant trend for the remaining time (57). To have more safety margin on our data, considering that we also made calculations using ( * ) 2 =1.54, and ( * ) 3 =1.57, as suggested in Refs (55,(58)(59), which can be referred to as the maximum variation of n in our experiment. The ρ values were obtained through the Clausius-Mossotti law Eq. S4: where ρ is the effective optical polarizability, calculated from the values of n and ρ at 0 and considered almost constant during the reaction (Fig. S2).
To give an idea of how much different refractive index estimates affect the calculation of the longitudinal elastic modulus, Figure S3 shows the results for ′ ( ) as a function of the annealing time: we found that the different n values at * provide similar values for the physical quantities, reaching a maximum difference of 2%.

Raman Data
This technique has been widely used to classify oils of different origins and applications (food, environmental, biomaterials, and cultural heritage among others (23,24), a typical Raman spectrum of the pure linseed oil is reported in Figure S4 together with its main peak assignments (60)(61).
In our analysis, each Raman spectrum was baseline corrected and normalized to CH2 signal at 1440 cm -1 before other analysis. Three different peaks of the Raman spectrum were chosen as markers of linseed oil polymerization progression, namely the i) stretching and the ii) bending modes of (=C-H)cis vibration at 3014 and 1265 cm -1 , respectively, and iii) the stretching band of (C=C)cis at 1660 cm -1 (34) i) The (=C-H)cis vibration is part of a system of bands between 2800-3150 cm -1 ( Figure S4), composed of several peaks originating from stretching of CH2 and CH3 bonds, which make it difficult to isolate the contribution of the solely (=C-H)cis vibration. Therefore, the peak intensity estimation was performed using a three-component multiple Lorentzian fit (the behaviour of this band with polymerization time is shown in the main paper where it is also directly compared with the variation of the BLS peak shift ( Fig. 2A). ii) In a similar way, to follow the diminishing of the bending mode of (=C-H)cis vibration at 1265 cm -1 we recurred to a two-component multiple Lorentzian fit including the in-phase methylene twist at 1305 cm -1 which instead increases with the polymerization time (61). The spectral closeness as well as the opposite behaviour of the two bands is probably the reason for the larger dispersion observed in the diminishing of the 1265 cm -1 band (Fig.  S5).

Fig. S4. Micro-Raman spectrum and band assignments of fresh linseed oil
Micro-Raman spectrum recorded on a fresh oil sample with the main spectral bands whose assignment is reported in the table.