The molecular orientation of adsorbed polyimides and model compounds with respect to highly oriented pyrolytic graphite (HOPG) surfaces was determined by reflection-absorption infrared (RAIR) spectroscopy. As the thickness of the pyromellitic diimide (PMDI) films decreased, bands near 766 and 729 cm-1, which were assigned to the CNC out-of-plane bending modes, were relatively strong in the spectra, demonstrating that PMDI was adsorbed flat-down with the imide rings parallel to the HOPG surface. As the thickness of the N,N'-diphenylpyromellitic diimide (DPPMDI) films was decreased, bands near 1785 and 1722 cm-1 which were assigned to the C=O stretching modes, were relatively strong in the RAIR spectra. Moreover, bands near 869, 841, 742, and 686 cm-1 attributed to the out-of-plane bending modes of monosubstituted benzene rings became considerably stronger. It was suggested that the DPPMDI molecules nearest to the HOPG surface were adsorbed edge-on with the carbonyl groups perpendicular to the surface and with the monosubstituted benzene rings parallel to the surface. There were significant differences in relative band intensity observed between the transmission and RAIR spectra of polyimides derived from pyromellitic dianhydride (PMDA) and oxydianiline (ODA). The band near 1725 cm-1 due to the C=O stretching mode increased its relative intensity in the RAIR spectra of PMDA/ODA polyimides, while the bands near 1499 and 1237 cm-1 due to the ODA moieties decreased their relative intensities, implying that PMDA moieties were oriented edge-on with the carbonyl groups perpendicular to the surface and that the ODA moieties were oriented mostly parallel to the surface. However, the RAIR spectra obtained from PMDA and 2,2-bis[4-(4-aminophenoxy)-phenyl]-hexafluoropropane (4-BDAF) polyimides were similar to the transmission spectra of the polyimide, indicating that PMDA/4-BDAF was randomly oriented on the HOPG surface.

The molecular structure of the interface formed by curing the polyamic acid of pyromellitic dianhydride (PMDA) and oxydianiline (ODA) against rough silver substrates was determined using surface-enhanced Raman scattering (SERS) and reflection-absorption infrared spectroscopy (RAIR). SERS spectra obtained after films of the polyamic acid were cured against rough silver substrates were independent of the thickness of the films, demonstrating that SERS was surface-selective and that the SERS signal originated from the interface, not the bulk of the films. SERS spectra of the polyimide were more similar to SERS spectra of the polyamic acid than to normal Raman spectra of the polyimide, indicating that curing of the polymer was inhibited by interaction with the substrate, probably through formation of carboxylate salts between the acid groups of the polyamic acid and silver ions of the substrate. There were significant differences in the relative intensities of the bands in the normal Raman and SERS spectra of the polyimide which were related to preferential orientation of segments of the polymer molecules. It was concluded that the PMDA moieties were adsorbed onto the silver surface edge-on with the carbonyl bonds perpendicular to the surface. Some evidence for formation of isoimide or degradation products containing C = N bonds in the interface was obtained from the SERS spectra. Results obtained from RAIR were consistent with those obtained from SERS. However, the infrared results provided additional information indicating that the ODA moieties were oriented mostly parallel to the silver surface.

Interfaces formed by curing the poly(amic acid) of pyromellitic dianhydride (PMDA) and 2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF) against silver substrates were characterized by X-ray photoelectron spectroscopy (XPS). The C(1s) spectra of poly(amic acid) films cured against thick silver films were similar to those of the neat poly(amic acid) but much different from those of the bulk, fully cured PMDA/4-BDAF polyimide, indicating that curing of the polymer was inhibited by interaction of the polyamic acid with the silver substrate. This was evidenced by observing a peak shifted upward by 3.2 eV from the main carbon peak near 284.6 eV which consisted of two overlapping peaks characteristic of the amide and carboxylate groups. Moreover, the main peak observed near 400.4 eV in the N(1s) spectra was attributed to the amides. The XPS results obtained from fracture surfaces of specimens prepared by curing thick poly(amic acid) films against polished silver substrates indicated that the silver fracture surface was characteristic of a poly(amic acid)/silver complex, while the polymer fracture surface was a PMDA/4-BDAF polyimide film. It was thus considered that a thin interfacial layer was formed between the silver surface and the cured polymer coating and that failure occurred at the interface of this interfacial layer and the bulk polyimide film. Peaks characteristic of the imides appeared in spectra of a poly(amic acid) film cured against a highly oriented pyrolytic graphite (HOPG) substrate, indicating that the poly(amic acid) was fully imidized at the HOPG surface. This was expected since no strong interaction of the poly(amic acid) with HOPG was observed. Thus, the thermal curing of poly(amic acid) for PMDA/4-BDAF at the substrate surfaces was strongly dependent on the nature of the substrates.

The molecular structure of interphases formed by curing the polyamic acid of pyromellitic dianhydride (PMDA) and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF) against silver substrates was determined by surface-enhanced Raman scattering (SERS) and reflection-absorption infrared spectroscopy (RAIR). Bands characteristic of amide, imide, and carboxylate groups were observed in the SERS spectra of the polyamic acid after curing, indicating that curing of the polyamic acid to the polyimide was inhibited by interaction of acid groups with the silver substrate. These conclusions were substantiated by results obtained from RAIR spectra of thin films of the polyamic acid cured against silver substrates. As the thickness of the polymer films decreased, bands characteristic of imide groups and C6H4 rings became weaker and bands characteristic of carboxylate groups appeared. For films having a thickness of approximately 15 angstrom, bands related to the imide groups almost completely disappeared, while those related to the carboxylate groups became relatively strong. A band assigned to the ether COC asymmetric stretching mode was strong regardless of the film thickness. Changes in the RAIR spectra as a function of film thickness were related to chemical interaction between the polyamic acid and the substrate and to preferential orientation of polymer segments near the interface. It was concluded that acid groups in the PMDA moieties formed carboxylate sets with silver ions from the substrate, thus inhibiting curing of the polymer adjacent to the substrate. It was also concluded that the planes of the PMDA moieties were parallel to the surface and that the planes of the carboxylate groups were perpendicular to those of the PMDA moieties and the silver surface. Approximately half of the C6H4 rings in the 4-BDAF moieties were parallel to the surface, but the planes of the COC groups were perpendicular to the surface.

Surface-enhanced Raman scattering (SERS) was used for the non-destructive characterization of interphases between epoxy/anhydride adhesive systems and silver substrates. The normal raman spectrum of benzophenone tetracarboxylic dianhydride (BTDA) was characterized by strong bands near 1785 and 1860 cm−1 that were assigned to the anhydride groups, a strong band near 1690 cm−1 that was assigned to the benzophenone C─O stretching mode, and by a strong band near 1620 cm−1 that was attributed to vibration v(8b) of the benzene rings. The bands due to v(8b) and to the benzophenone C─O stretching mode were prominent in the SERS spectrum of BTDA but the bands related to the anhydride group were missing, indicating that the anhydride groups were hydrolyzed at the silver surface to form carboxylate groups. A band related to a CH out-of-plane bending mode which was absent from the normal Raman spectrum of BTDA was strong in the SERS spectrum, indicating that the molecules were adsorbed onto silver with a flat configuration. SERS spectra obtained from a diglycidyl ether of bisphenol-A (DGEBA) epoxy cured against a silver substrate using BTDA as the curing agent were identical to SERS spectra of BTDA and were independent of the epoxy/BTDA ratio, indicating that the spectra were characteristic of the interface rather than the bulk adhesive and that BTDA was preferentially adsorbed onto the silver substrate. SERS spectra obtained from thin films of BTDA adsorbed onto silver and then overcoated with thick films of epoxy were identical to the spectrum of BTDA and showed no evidence of bands related to DGEBA, supporting the conclusion that the SERS spectrum obtained from the DGEBA/BTDA adhesive was characteristic of the interfacial region. Key Words: Raman scatteringsurface-enhancedadhesivesepoxy/anhydridebenzophenone tetracarboxylic dianhydridediglycidyl ether of bisphenol-Ainterphasesdurability

Surface-enhanced Raman scattering (SERS) was used to investigate the adsorption of model compounds for polyimides onto silver island films and onto highly oriented pyrolytic graphite (HOPG). The intensity of the SERS spectra of pyromellitic diimide (PMDI) adsorbed onto silver island films was mostly independent of the thickness of the PMDI films, confirming that SERS is an interfacial rather than bulk effect and that SERS can be used to examine the interface between an adsorbed film and a metal substrate without interference from normal Raman scattering by the bulk of the film. However, the relative intensities of several bands in the SERS spectra of PMDI adsorbed onto silver island films did depend weakly on the thickness of the films because of orientation effects. It was concluded that most molecules were physisorbed with a random orientation. However, a strong band near 690 cm-1 in the SERS spectra was assigned to PMDI chemisorbed with a vertical configuration in which the plane of the molecules was perpendicular to the surface and one imide group was adjacent to the surface. Similar conclusions were reached for phthalimide (PIMH) adsorbed onto silver island films. When PMDI was deposited onto an HOPG substrate and silver island films were deposited on top of the PMDI, the SERS spectra depended strongly on the thickness of the PMDI films. For thick PMDI films, the SERs spectra were similar to Raman spectra of bulk PMDI, but for thin films, the SERS spectra were similar to those obtained from PMDI deposited onto silver. It was concluded that the molecules in thick films of PMDI were also mostly randomly oriented but those closest to the substrate had a vertical configuration. SERS spectra of N-phenylphthalimide and N,N'-diphenylpyromellitic diimide adsorbed onto silver were very similar to the normal Raman spectra. It was concluded that both of these compounds were adsorbed edge-on with one (N-phenylphthalimide) or two (N,N'-diphenylpyromellitic diimide) carbonyl groups adjacent to the surface.

Surface-enhanced Raman scattering (SERS) is an analytical technique in which the Raman scattering cross-sections of molecules adsorbed onto the roughened surfaces of certain metals are enhanced by as much as six orders of magnitude compared to their value for normal Raman scattering. The mechanisms are associated with the large electric fields that can be induced at the surfaces of metal particles having small radii of curvature and with the formation of charge-transfer complexes between the adsorbed molecules and the substrate. Enhancement due to the charge-transfer mechanism is obtained only for molecules immediately adjacent to the surface, but enhancement due to the electromagnetic mechanism may extend several monolayers away from the surface. Since normal Raman scattering by polymers is weak and scattering by molecules adjacent to the substrate is strongly enhanced, SERS can be used for non-destructive characterization of interfaces between polymer films and metals as long as the films are not so thick that scattering by the bulk of the film is comparable in intensity to SERS from the interface.We have used SERS to examine the interface between silver and an acrylic adhesive system consisting of an acrylic monomer and a cure system comprising cumene hydroperoxide (CHP), acetylphenylhydrazine (APH) and saccharin. SERS spectra obtained from films of the adhesive spin-coated onto silver island films were similar to normal Raman spectra of salts of saccharin, indicating that saccharin had adsorbed on the silver dissociatively. The spectra were independent of the thickness of the adhesive films, indicating that the SERS signal was characteristic of the interface and not of the bulk adhesive.SERS has been used also to characterize the interface between silver and pyromellitic diimide (PMDI), a model compound for polyimides. SERS spectra of PMDI adsorbed onto silver were considerably different from normal Raman spectra. The strongest band in the normal spectra, a carbonyl stretching mode near 1770 cm-1, was absent from the SERS spectra and a band near 700 cm-1, which was very weak in normal Raman spectra, was strong in SERS spectra. It was concluded that PMDI was adsorbed in a vertical conformation with one imide group adjacent to the surface.

Surface-enhanced Raman scattering (SERS) was used to determine the conformation of 2-vinylpyridine/styrene (2VP/S) block copolymers adsorbed onto silver surfaces. The SERS spectra were also obtained from polystyrene (PS) and poly(2-vinylpyridine) (P2VP) to confirm band assignments of 2VP/S diblock copolymers. The bands near 1615 and 1041 cm-1 related to the styrene block were only weakly observed in the SERS spectra of 2VP/S diblock copolymers and the bands due to the 2VP block observed near 1073, 1161, 1336, and 1585 cm-1 were relatively strong in intensity in the SERS spectra, indicating that the SERS spectra observed for 2VP/S diblock copolymers were mostly contributed by the 2VP block. Since SERS has previously been shown to be a surface-selective effect, the results obtained here confirmed that the 2VP block preferentially adsorbed to the surfaces while the styrene block was positioned away from the surfaces. The results obtained from angle-resolved X-ray photoelectron spectroscopy (XPS) were consistent with those obtained from SERS. The ratio of nitrogen to carbon (N/C) in the interface region of the adsorbed copolymer was larger than in the bulk copolymer. The N/C ratio was also found to increase with increasing "takeoff" angle in the near-interface region. Since nitrogen is a characteristic element of the 2VP block and the N/C ratio can be used to determine the composition profiles, it was again confirmed that the 2VP block preferentially adsorbed onto the silver surface and that the 2VP layers were enriched in the interface region. Differences in the relative intensities and frequencies of several bands in the normal Raman and SERS spectra of P2VP adsorbed onto the silver surfaces were related to orientation effects. It was concluded that P2VP and the 2VP block of diblock copolymers were adsorbed by sigma-bonding through the pyridine nitrogen atoms with a vertical conformation.

AbstractSurface‐enhanced Raman scattering (SERS) was observed from thin films formed by 1,4‐ and 1,3‐dinitrobenzene (DNB) spin‐coated onto silver island films from dilute solutions. During laser irradiation of 1,4‐DNB films on silver substrates at relatively high laser powers, new bands appeared near 1152, 1350, 1412 and 1460 cm−1 and bands near 1118, 1358 and 1372 cm−1 decreased in inensity. A band near 1598 cm−1 shifted to near 1605 cm−1. The bands near 1152 and 1412 cm−1 were assigned to stretching modes of ϕN andNN bonds of azo compounds, respectively, indicating that polymerization of 1,4‐DNB had occurred. The extent of polymerization was greatest when the surface coverage of adsorbed 1,4‐DNB was greatest and when the SERS experiments were carried out in a nitrogen atmosphere, indicating that reducing conditions were required for the polymerization. No polymerization was observed for 1,3‐DNB under similar circumstances. The difference was attributed to orientation of the adsorbed species. It was concluded that 1,4‐DNB was adsorbed with a horizontal configuration in which the aromatic rings were parallel to the substrate surface and both nitro groups were in contact with the surface while 1,3‐DNB was adsorbed with a vertical configuration in which the rings were perpendicular to the surface and only one nitro group was in contact with the surface.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSelective oxidation of para-substituted polystyrenes during surface-enhanced Raman scatteringF. J. Boerio, W. H. Tsai, P. P. Hong, and Giorgio MontaudoCite this: Macromolecules 1989, 22, 10, 3955–3960Publication Date (Print):October 1, 1989Publication History Published online1 May 2002Published inissue 1 October 1989https://pubs.acs.org/doi/10.1021/ma00200a024https://doi.org/10.1021/ma00200a024research-articleACS PublicationsRequest reuse permissionsArticle Views113Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

AbstractSurface‐enhanced Raman scattering (SERS) was used to characterize thin films of poly(α‐methylstyrene) (PMS) spin‐coated onto silver island films supported by glass substrates. At laser powers of a few tens of milliwatts, SERS spectra of thin films of PMS (about 100 Å) were weak, and only the bands near 1010 and 1040 cm−1 were observed. At laser powers of about 100 mW, additional bands characteristic of PMS were observed near 720 and 1610 cm−1. However, oxidative degradation of the PMS films to form graphite‐like substances was also observed at the higher laser powers. When the thickness of the PMS films was increased to several hundred angstroms, degradation of the films was inhibited, but the intensity of the Raman scattering remained constant, indicating that the observed SERS was an interfacial rather than bulk effect. Degradation during SERS experiments was also inhibited by overcoating PMS films with much thicker films of poly(4‐styrene sulfonate) (PSS). Scattering from the PSS overlayers was not observed as long as the thickness of the PMS films was greater than several tens of angstroms, again showing that the SERS was an interfacial effect. Oxidative degradation of the PMS films was also inhibited by adding a few percent of the antioxidant 2,6‐di‐tert‐butyl‐4‐methylphenol to the polymer. Bands related to sulfite contaminants adsorbed onto the silver island films were observed near 640 and 940 cm−1. These bands disappeared when PSS, but not PMS, was spin‐coated onto the SERS substrates, indicating a strong interaction between PSS and silver.

AbstractSurface‐enhanced Raman scattering (SERS) has been observed from thin films of polystyrene (PS), diglycidyl ether of bisphenol‐A (DGEBA), and poly(4‐vinyl pyridine) (PVP) deposited on silver island films Degradation of the polymers occurred rapidly during laser irradiation and was accompanied by the appearance of strong bands near 1375 and 1575 cm‐1. These bands were attributed to the formation of graphite‐like species by the silver‐catalyzed thermal oxidation of the polymers induced by localized laser heating of the substrate. When the thin films of PS, DGEBA, or PVP were overcoated with much thicker films of a second polymer such as polystyrene sulfonate (PSS), the degradation was greatly reduced, and excellent SERS spectra of the PS, DGEBA, and PVP films were obtained. Overlayers reduced degradation within the first films deposited on silver island films by restricting the availability of oxygen at the interface to its solubility in the overlayer polymer or by altering the adsorption of oxygen onto the substrate. SERS was observed for the PS, DGEBA, and PVP films and the PSS overlayers when the films were deposited from relatively dilute solutions. When the PS, DGEBA, and PVP films were deposited from more concentrated solutions, SERS was not observed from the PSS overlayers. It was suggested that most of the SERS was due to a short‐range, charge‐transfer mechanism associated with sites of atomic scale roughness and that SERS was observed from the overlayer when the first film failed to occupy all of the sites.