On Confocal Raman Spectroscopy of Semicrystalline Polymers: The Effect of Optical Scattering

Abstract

A series of semicrystalline polymers has been prepared through morphological control. Each of these has an identical refractive index but a different, well-defined, scattering behavior. From existing geometrical optical theories of confocal Raman spectroscopy, these materials should behave identically. Initially, the extent of scattering in each system was assessed quantitatively, from the near-infrared through the visible wavelength range, by UV/visible spectroscopy. The effect of optical scattering on the variation of intensity of the Raman scattered radiation with subsurface position was then examined in all four materials; the effect of surface roughness was also considered in the highest clarity system. Where surface effects are removed through careful sample preparation and the materials are interrogated using identical optical systems to mitigate against the impact of refractive index mismatch and other optical effects, the Raman response is strongly affected by the scattering characteristics of each material. A simple empirical relationship has been determined that adequately described all our specimens.

Application of confocal Raman spectroscopy to thin polymer layers on highly scattering substrates: a case study of synthetic adhesives on historic textiles

“…35 Comparison with peak assignments made for a poly(ethyl acrylate) system indicate that the 1450 cm 1 peak may be assigned to CH 2 wag and 1100 cm 1 to the C-C asymmetric stretch. 37 The spectrum of Lascaux 360HV is significantly different from that of Paraloid 72B and comparison with the spectrum from a model sample of poly(butyl methacrylate) (PBMA) 36 confirms it to be, as described in the literature, a 'poly(butyl methacrylate) dispersion'. 24 In summary, we have shown that Raman spectroscopy of synthetic polymeric adhesives used in textile conservation can discriminate between chemically similar EVA formulations and identify unambiguously the precise constituents of acrylic copolymers.…”

“…However, Tabaksblat et al 43 predicted that, within a thick transparent sample, 'more than 20% of the total amount of light passing through the (confocal) pinhole comes from planes more than 5 µm away from focus'. Other authors 29,40 -46 have also noted that useful Raman scattered photons are obtained from an extended region (named variously the confocal volume, 30 the illuminated region, 40,41 the laser field, 41 the focal spread, 42 the scattering volume or sampled volume 44 and the focal volume 44 -46 ). Progressively immersing this extended region into a transparent sample is reported as giving a linear increase in Raman peak intensity; that is, if the focal point were, initially, located a distance z d above the surface, the detected signal would, presumably, then correspond to the integral of all the Raman radiation that could contribute to the final signal from z D z d to z D C1 in the coordinate system defined above.…”

“…From Fig. 6, it is evident that, despite the fact that maximum peak intensity of a homogeneous transparent material is obtained when focused within the sample, 30 in this case the relative intensity of the vinyl acetate peaks to the silk peaks is maximized when focused at or slightly above (0-5 µm) the polymer surface. This is, of course, dependent on the absolute scattering intensity upon defocusing for both the polymer and the underlying silk.…”

“…For analysis of the adhesives, the laser was focused on the perfectly flat surface of the polymer and then the microscope stage was raised 20 µm; this has been shown to provide maximum Raman scattered signal from these transparent materials. 30 One hundred accumulations of 10 s extended scans were used to provide high-quality spectra of the solid polymeric adhesives.…”

“…20,21 We have recently developed a protocol for applying Raman spectroscopy to the identification of pigments on historic painted textiles, in order to aid conservation of these culturally important artifacts. 22 Raman spectroscopy has also been used to identify degradation and decomposition products on historic artifacts to inform chosen conservation procedures. 13 -15 This paper considers another contribution that Raman spectroscopy can make to aid the work of textile conservators, that of identifying synthetic materials applied during previous conservation treatments.…”