# Spectroscopic Ellipsometry and Optical Modelling of Structurally Colored Opaline Thin-Films
## 2 Materials and Methods  
### Samples  
The structurally colored polymeric opal materials used in this paper are based on ensembles of core-interlayer-shell (CIS) particles, as synthesized at Aberystwyth University  
Figure 1: The core-interlayer-shell structure of polymer precursor particles is illustrated in (**a**), with the polystyrene (PS), poly(methyl-methacrylate) (PMMA) and poly(ethyl-acrylate) (PEA) components as indicated. In schematic (**b**), the processing of particles into ordered arrays using the bending-induced oscillatory shear (BIOS) is illustrated, as bending shear is applied to the material encapsulated within rigid PET layers. In (**c**), a photographic image of a resultant thin-film polymer opal sample is shown, which the shear-processing direction indicated by arrow. The scale bar is approximately 1 cm. In (**d**), the multilayer pseudo-model used to evaluate ellipsometric constants at incident angle \(\theta\) is illustrated, where a random disorder of \(\delta\) nm (Gaussian distribution) is introduced into \(\lambda/4\) layers of alternating higher and lower refractive index (\(n_{1}\), \(n_{2}\)).  
and Varichem Ltd., via a multi-stage emulsion polymerization process [18; 27; 28]. As illustrated in Figure 1, the CIS particle precursors have a total diameter of around 220 to 250 nm, and consist of a relatively rigid cross-linked polystyrene (PS) core, coated with a thin polymer layer containing poly(methyl-methacrylate) (PMMA) as a grafting agent, and a soft poly(ethyl-acrylate) (PEA) outer-shell. The particular batch of \"green opal\" used here has been widely characterized previously as having a mean CIS diameter of 223 nm with a poly-dispersity of better than 1% size variation [17; 28]. The fractional refractive index contrast between core and shell material (\(\Delta n/n\)) is ascertained to be \(\approx\)0.07, with the volume fraction of PS core particles in the material set at about 50% by the initial reaction mix composition. The average index (\(\overline{n}\)) is independently determined as being 1.52, by sample refractometry [27]. A _MiniLab_ mini-extruder (Haake, Technik, Vreden, Germany) was used at a temperature of 150 \({}^{\circ}\)C to melt-homogenize the material and to produce extrudate ribbons of thickness 1 mm.  
Film samples for ellipsometric studies are then produced via the BIOS process. The ribbons are roller-laminated between two pre-cleaned PET sheets at 80 \({}^{\circ}\)C, giving a polymer opal thin film of \(\approx\)150 um thickness in a 'Timoshenko sandwich' arrangement between the sheets (Figure 1b). The sandwich beam was then drawn over rollers of 2 cm diameter, at a stabilized temperature of 85 \({}^{\circ}\)C and shear-rate \(\approx\) 1 cm s\({}^{-1}\). Multiple samples were generated varying the number of BIOS shear passes between 0 and 40, in order to investigate the effects of improving particle ordering. After processing, the opals have a characteristically brilliant blue-green iridescence (Figure 1c), and can then be readily peeled away from the PET tapes to produce free-standing films as required. Experimental and micrographic determinations of the physical mechanisms of BIOS shear-ordering, and the resultant structural ordering metrics, have been extensively reported on [17; 28; 29] and theoretically elucidated elsewhere [30], and these issues are thus not a focus of the present paper.