Detection of surface modification of polyimide containing steroidal structure as a function of storage time using second- order nonlinear optical spectroscopy

Detection of surface modification of polyimide containing steroidal structure as a function of storage time using second- order nonlinear optical spectroscopy | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Detection of surface modification of polyimide containing steroidal structure as a function of storage time using second- order nonlinear optical spectroscopy Trinh Thi Nguyen, Wentao Du, Hien Thi Thu Khuat, Goro Mizutani, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2283463/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The modification of a polyimide surface containing a 30% fraction of steroidal structure was studied as a function of storage time by vibrational sum frequency generation (SFG) spectroscopy and optical second-harmonic generation (SHG). The variation of the symmetric and anti-symmetric stretching modes of the CH 3 isopropyl group at the end of the side chain of the rubbed polyimide was detected in the SFG spectra after 3 months storage time in a practical environment. It suggests that the isopropyl group might be reoriented. On the other hand, the SHG response of the same sample showed an insignificant storage-time dependence. As a result, the phenyl rings of the polyimide are judged to be unmodified after long-term storage time. The change is suggested to occur for two main reasons. One is the interaction between the polyimide polymers and the ambient water molecules. The other is the adsorption of unknown organic molecules on the polyimide surface. polyimide steroidal structure surface quality degradation vibrational sum frequency generation spectroscopy second harmonic generation Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Polyimide containing steroidal structure is known as a semi-aromatic polyimide. The microscopic structure of such a polyimide studied in this work is shown in Fig. 1. This polyimide possesses outstanding properties like the general polyimides, such as high thermal stability, excellent mechanical properties, and high chemical resistance [ 1 – 3 ]. Besides, the steroidal structure in this polyimide makes it possess unique properties. For example, it shows better electrical properties as the material for the alignment layers in the liquid crystal display (LCD) panel [ 4 ] than the ones with long alkyl chains or fluorine atoms. In addition, the pretilt angles of liquid crystal (LC) molecules for various LCD modes can be precisely controlled by changing the fraction of monomeric units of polyimide containing steroidal structure [ 4 ]. In principle the LC alignment on rubbed polyimide film surfaces is determined by the molecular interaction between polyimide film surface and LC molecules [ 5 , 6 ]. Hence, the quality of the polyimide film surface is crucial for high-quality LCD performance. Vibrational sum frequency generation (SFG) and second-harmonic generation (SHG) have been demonstrated as practical tools to probe polymer surfaces due to their sensitivity to asymmetric parts of materials unlike infrared or Raman spectroscopies. While vibrational SFG spectroscopy can provide information about the surface vibrational resonances and the average orientation of the molecules at the surface [ 7 – 11 ], SHG can provide information about the electronic transitions of the surface molecules [ 12 – 16 ]. In fact, we found that the SFG response of the polyimide film originates mainly from the surface and the SFG contribution from the polyimide/glass interface and the bulk can be ignored [ 17 ]. The terminal isopropyl group in the side-chain of the polyimide containing steroidal structure as shown in Fig. 1 is pointing toward the air side and the average tilt angle of the methyl group adjacent to the steroidal structure is \(40^\circ \pm 10^\circ\) in the rubbing direction. Analyzing the SHG response of the polyimide containing steroidal structure, we also figured out that both the polyimide side chains and the main chains contribute to this response [ 18 ]. In order to systematically detect surface modification behavior of polyimide films as a function of storage time, we used a combination of vibrational SFG spectroscopy to analyze the difference of vibrational modes in the CH stretching range and SHG to examine the variation of the SHG response of polyimide film surfaces, especially phenyl rings. The sample polyimide is that in Fig. 1 because we have an ample experience of this specific polyimide [ 17 ]. The purpose of this study is to establish a method to detect the change of the surface condition of an LCD alignment layer in a practical environment. 2. Materials And Methods Figure 1 illustrates the chemical structure of a polyimide containing 30% fraction of steroidal structure used in this study. The sample preparation procedure can be found in our previous paper [ 17 ]. Briefly, the polyimide films with a layer thickness of ∼ 80 nm were prepared by spin-coating on non-alkali glass substrates and then were rubbed once by a rubbing machine with a weak strength. After preparation, the polyimide films were put in plastic boxes and then stored in a dry box kept at humidity 25 ± 3% and temperature 21 ± 2 o C. The SFG spectra of the polyimide film were recorded using a narrowband SFG system. The details of the experimental setup for the SFG measurement have been published elsewhere [ 19 ]. In short, a visible beam (532 nm) and a tunable infrared beam were overlapped on the polyimide film surface at angles of incidence of 80° and 45°, respectively, to generate an SFG signal. The energies of the visible beam and infrared beam used were ∼80 µJ and from 135 to 280 µJ, respectively. After appropriate optical filtering, the SFG output was collected in the reflection direction by a monochromator and a photomultiplier tube. In this experiment, the SFG spectra were recorded in the CH-stretching region (2800 to 3000 cm − 1 ) for a PPP (P-polarized SFG, P-polarized visible, P-polarized IR) polarization combination at a 5 cm − 1 interval. For rubbed polyimide films, the SFG spectra were measured with the wave vectors of the incident beams parallel to the rubbing direction. The SFG signal was averaged over 600 pulses for each wavelength using a gated integrator to reduce noise. All the SFG spectra were fitted by the following equation[ 20 ] $$I\left({\omega }_{SFG}\right)\propto {\left|{A}_{NR}^{\left(2\right)}{e}^{i\varphi }+{\sum }_{q}\frac{{A}_{q}{\varGamma }_{q}}{{\omega }_{IR}-{\omega }_{q}+{i\varGamma }_{q}}\right|}^{2}.\left(1\right)$$ Here \({A}_{NR}^{\left(2\right)}\) and \(\varphi\) denote the amplitude and phase of the non-resonant SFG signal, ω IR is the infrared frequency. A q , ω q , and Γ q denote the amplitude, the resonant frequency, and the line width of the q th vibrational mode, respectively. The details of the experimental setup for the SHG measurement can be found elsewhere [ 21 ]. In short, a fundamental beam (532 nm) with energy at ∼ 20 µJ irradiated the polyimide film at an angle of 45° to generate an SHG signal (266 nm). The SHG output was collected in the same way as the SFG output. In the SHG experiment, we mounted the polyimide film on a 360° rotational stage. Then its SHG response at sample rotation angle intervals of 10° for a Pin/Pout (P-polarized fundamental beam and P-polarized SHG) polarization combination was recorded. The SHG signals of 1000 pulses were accumulated for each data point. 3. Results 3.1 SFG spectra of polyimide films as a function of storage time The polyimide films were carefully kept in a dry box to reduce the impact of the environment unless they were under SFG observation. Still, the polyimide surface was modified after long-term storage. To check the quality of the polyimide film surfaces at different storage times, we collected the SFG spectra of the polyimide films in the CO/CH stretching regions and above 3000 cm − 1 . Herein we just show the SFG spectra obtained in the CH stretching region because the spectra obtained in the CO stretching region and above 3000 cm − 1 were ambiguous. Figure 2 shows SFG spectra of rubbed polyimide films for the PPP polarization combination as a function of the storage time. The detailed assignments of the vibrational modes at the bottom are based on those for fresh polyimide in our previous paper [ 17 ]. The fitting parameters used for the theoretical curves can be found in Supporting Information. While the polyimide spectra observed at and before 2 months 11 days storage time show only a slight change from the bottom spectrum, those after longer storage time show significant changes. As shown in Fig. S1 (Supporting Information), the amplitude ratios of the \({r}_{2}^{+}\) peak (CH 3 symmetric stretching mode of the isopropyl group) and the \({r}_{2}^{-}\) peak (CH 3 anti-symmetric stretching mode of the isopropyl group) within 2 months 11 days storage time are similar whereas those ratios of the \({r}_{2}^{+}\) peak and the \({r}_{2}^{-}\) peak significantly change after longer storage time. Moreover, a considerable variation of the peak position and the shape of the \({r}_{2}^{-}\) peak is observed as a function of the storage time. Indeed, the \({r}_{2}^{-}\) peak shifts from 2965 cm -1 to 2961 cm -1 after 3 months 12 days storage time. Vibrational SFG is extremely sensitive to order and conformation of molecules at surfaces, the variations of the \({r}_{2}^{+}\) and \({r}_{2}^{-}\) peaks therefore suggest that the structures of the isopropyl groups on the polyimide surface changed somehow after a long-term storage time. In a similar way we investigated SFG response of unrubbed polyimide film surfaces after different storage times (Fig. S2, Supporting Information). We found that the modification of polyimide surface occurs in the unrubbed polyimide film as well as in the rubbed polyimide film. To check the impact of ambient environment factors such as moisture and oxygen on the polyimide surface, we recorded SFG spectra of the rubbed polyimide film kept in a quartz cell filled with dry N 2 gas as a function of storage time (Fig. S3, Supporting Information). Although the number of observed peaks in the SFG spectrum of the sample after 10 months storage time are like those at 7 days, the relative peak intensities and the peak positions are quite different. The results indicate that the polyimide surface in dry N 2 gas also might be modified after 10 months storage time. 3.2 SHG intensity patterns of polyimide films as a function of time As previously mentioned, we cannot extract molecular information of the polyimide main chain, namely the imide group and phenyl ring, from the SFG spectra. This information is crucial to determine the origin causing the polyimide surface modification after long-term storage. Hence, we use optical SHG as a supplementary method to check the orientation of the phenyl rings in the surface molecular layer of the polyimide. Figure 3 shows SHG intensity patterns of rubbed polyimide films possessing a 30% fraction of steroidal structure as a function of the storage time. Due to the rubbing process, an anisotropy of SHG intensity, namely a larger SHG response around 0 deg or in the rubbing direction, is observed for the fresh sample [Fig. 3(a)]. The SHG anisotropy was also obtained for the rubbed samples at 5 months and even at 13 months storage time [Figs. 3(b) and 3(c)]. The results indicate that the SHG response is independent of the storage time. Similarly, we recorded SHG responses of unrubbed polyimide films, as shown in Fig. 4. As it is natural, no essential anisotropy is seen in the SHG response of the samples measured at either 28 days [Fig. 4(a)], 8 months [Fig. 4(b)] or 13 months [Fig. 4(c)] storage time. The SHG response of both rubbed and unrubbed samples show negligible dependence on the storage time. Interestingly, these SHG results look inconsistent with the SFG ones of the same samples. 4. Discussion In the previous section the storage-time dependence of the anisotropy of the SHG response of the rubbed polyimide containing steroidal structures was not found to be remarkable, while the change of the vibrational SFG spectrum of the same sample after 3 months storage time was found to be clear. These observations are not necessarily inconsistent because SFG and SHG can generally originate from different parts of the polyimide film surface [ 12 , 22 ]. Namely, the SFG signal in Fig. 2 is assigned as molecular vibrations of the CH 2 groups or the CH 3 groups in the surface side chains, while the SHG signal in Figs. 3 and 4 are suggested to come from the aromatic rings. Here, we only focus on interpreting the variations of the symmetric and anti-symmetric stretching modes of the CH 3 isopropyl group ( \({r}_{2}^{+}\) , \({r}_{2}^{-}\) ) in the polyimide SFG spectra as a function of storage time because these variations were systematically observed in Fig. 2. The variation of the \({r}_{2}^{+}\) and \({r}_{2}^{-}\) peaks of the isopropyl group after 3 months storage time as reported in Section 3.1 can be due to six candidate origins as: Change of Fresnel factors, Breaking down or oxidation in air of the isopropyl group, Breaking down of the main chain, Breaking down of the steroidal structure, Effect of the environmental moisture on the polyimide film and a resultant modification of the isopropyl group’s orientation. Polyimide surface contamination Fresnel factor is used to correct the SFG intensity for its modification by linear optical effect of input and output radiations at surfaces or interfaces [ 23 ]. In order to compare the Fresnel factors for the polyimide/air interface for the fresh rubbed polyimide film and the one after 10 months storage time, the refractive indices of these samples were measured using spectroscopic ellipsometry (Supporting Information). We found no significant variation between the refractive indices of the fresh sample and those of the sample after 10 months storage time. Hence, the change of the peak intensity as a function of the storage time in Fig. 2 is judged to be not due to the modification of the Fresnel factors, and we can exclude candidate (1). Now we look at candidate (2), namely breaking down or oxidation by oxygen in air of the isopropyl group. The isopropyl group is the simplest structure group containing a tertiary carbon - a carbon atom directly bound to three other carbon atoms. Generally, this group is extremely stable in normal room temperature conditions [ 24 , 25 ]. Since the storage of our samples was carried out in a very mild condition, we can exclude this candidate (2). Next, we consider candidate (3), namely breaking down of the main chain. In general, chemical bonds between C and N atoms in the imide group or C and C atoms are the two most easily broken bonds in polyimide [ 26 – 29 ]. Since the strength of the C-N bond is lower than that of the C-C bond [ 28 ], the C-N breakage might most likely occur after 3 months storage time in this polyimide. On the other hand, the SHG intensity patterns of the polyimide films show slight difference as a function of storage time. One can see in Fig. 1 that if the C-N breakage occurred, it would affect the configuration of the aromatic rings. The SHG results indicate that it is not the case. Hence candidate (3) may not have caused the variations of the \({r}_{2}^{+}\) and \({r}_{2}^{-}\) peaks in the SFG spectra as a function of storage time. We can clearly exclude candidate (4), namely breaking down of the steroidal structure. Due to the ‘ trans’ configuration at all three fusions, the steroidal structure is commonly rigid and remains semi-flat [ 30 ]. That is why cholesterol, the most abundant steroid in animals, is so stable in animal’s body [ 31 ]. Hence, this structure is not easily broken in normal environmental conditions. Here, we discuss candidate (5), namely the effect of moisture on the polyimide and a resultant modification of the surface isopropyl group’s orientation. In our previous study [ 17 ], we suggested that the isopropyl group points toward the air side from the rubbed polyimide surface when the film is fresh. Water molecules can affect the isopropyl group in two ways: Firstly, moisture in the air may drive the hydrophobic isopropyl group into the bulk. Secondly, water molecules in the polyimide bulk may drive the isopropyl group already in the bulk back toward the air side. Then, to minimize the total energy of the film surface, the hydrophobic isopropyl group may most likely take more disordered configuration due to its interaction with water molecules after a long-term storage time than when the film is fresh [ 32 , 33 ]. This view also explains the fact that the SFG spectra of unrubbed polyimide film changes as a function of time as seen in Fig. S2 (Supporting Information). Lastly, we discuss candidate (6), namely polyimide surface contamination. The surface contamination might occur due to the adsorption of unknown organic components on the polyimide surfaces. Then, the interaction between hydrophobic side chains of polyimide and hydrophobic organic contaminants might affect the orientation of the isopropyl group. As expected, the modification of the polyimide surface was also observed in the polyimide kept in the quartz cell filled dry N 2 gas (Fig. S3, Supporting Information). Candidates (5) and (6) have opposite behaviors, and one or both of them may be the reason for the variations of the \({r}_{2}^{+}\) and \({r}_{2}^{-}\) peaks in the SFG spectra as a function of storage time. Further study is necessary for determining which one is intense. 5. Conclusions Vibrational SFG spectroscopy and SHG were used to detect the modification of polyimide film surfaces containing a 30% fraction of steroidal structure as a function of storage time. The \({r}_{2}^{+}\) and \({r}_{2}^{-}\) peaks in the SFG spectra of the rubbed polyimide film showed systematic storage time dependence whereas the SHG response of the same sample just showed slight variation. Disorder of the orientation of the isopropyl groups in the polyimide side chain due to the interaction with moisture or surface contamination might be the main reason for these observed facts. The phenyl rings of the polyimide were found not to be reoriented because the SHG response of the same samples showed an insignificant storage-time dependence. The experimental results prove that the combination of vibrational SFG spectroscopy and optical SHG can be a useful method to detect the change of the polymer film surface in a practical environment. Declarations Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT author statement Trinh Thi Nguyen: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing-original draft. Wentao Du : Investigation, Formal analysis. Hien Thi Thu Khuat : Methodology, Supervision. Goro Mizutani : Conceptualization, Methodology, Writing - Review & Editing, Supervision. Yoshitaka Murakami : Conceptualization, Resources. Takashi Okada : Resources, Supervision. References M. Fahim, J. Bijwe, and H. S. Nalwa, Polyimides for Microelectronics and Tribology Applications (2001). T. Matsumoto, High Perform Polym 13 , (2001). A. S. Mathews, I. Kim, and C. S. Ha, Macromol Res 15 , 114 (2007). M. 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Salvati, and I. Yilgor, Macromolecules 27 , 2409 (2002). K.-W. Lee, S.-H. Paek, A. Lien, C. Durning, and H. Fukuro, Microscopic Molecular Reorientation of Alignment Layer Polymer Surfaces Induced by Rubbing and Its Effects on LC Pretilt Angles (1996). Additional Declarations No competing interests reported. Supplementary Files SupportingInformation.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2283463","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":153122561,"identity":"d4ab8bea-59cd-46ac-a75d-8a08a181dff3","order_by":0,"name":"Trinh Thi Nguyen","email":"","orcid":"","institution":"Japan Advanced Institute of Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Trinh","middleName":"Thi","lastName":"Nguyen","suffix":""},{"id":153122564,"identity":"c61a40f8-7bf4-4758-9f88-50efb38c9147","order_by":1,"name":"Wentao Du","email":"","orcid":"","institution":"Japan Advanced Institute of Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Wentao","middleName":"","lastName":"Du","suffix":""},{"id":153122566,"identity":"32e1146e-9ad0-4b29-b876-45fe06b67ad7","order_by":2,"name":"Hien Thi Thu Khuat","email":"","orcid":"","institution":"Japan Advanced Institute of Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Hien","middleName":"Thi Thu","lastName":"Khuat","suffix":""},{"id":153122567,"identity":"74f7cd69-9818-4865-b5b7-5c0ff5f87db6","order_by":3,"name":"Goro Mizutani","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA50lEQVRIie3RsQrCMBCA4ZNCXQ66RhR8AiEiiIPiq1SEujo6OHTSRXTtYwi+wJWDdlFcHRx06ayL1EWMVnBrMwrmDyRLvpAQAJPpF7OyBR0gNbLcYqK2YsV/EVeHQEZAkuZmaMzLSfUyPdZaIZ/Dewp1x4fklEfaDJ6gKME2RZLRhWZAMJIFJBJksyIkWV2stAZ1SD4pzQQ9GFt+fAlTF/oaxLJFOGOUsJWkLjbQILbV2S0ZBW3HjJ4YBlz0ln18Pkxu3HeCeHNNu93ear7wTnnkm6D3rD4XPT0B6j8+lSNNYjKZTH/SE4gKTwbQL3WJAAAAAElFTkSuQmCC","orcid":"","institution":"Japan Advanced Institute of Science and Technology","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Goro","middleName":"","lastName":"Mizutani","suffix":""},{"id":153122568,"identity":"a4162d1a-3dc9-4af8-87fb-7073d8483c5d","order_by":4,"name":"Yoshitaka Murakami","email":"","orcid":"","institution":"JSR Corporation","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Yoshitaka","middleName":"","lastName":"Murakami","suffix":""},{"id":153122570,"identity":"93307631-f829-4c2c-b595-d684c0670502","order_by":5,"name":"Takashi Okada","email":"","orcid":"","institution":"JSR Corporation","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Takashi","middleName":"","lastName":"Okada","suffix":""}],"badges":[],"createdAt":"2022-11-17 08:59:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2283463/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2283463/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":29404098,"identity":"7060a8bd-8f68-4c77-afbb-f8d786a33332","added_by":"auto","created_at":"2022-11-22 18:38:02","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":140659,"visible":true,"origin":"","legend":"\u003cp\u003eChemical structure of polyimide containing steroidal structure used in this study.\u003c/p\u003e","description":"","filename":"Fig1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2283463/v1/180d5181779c628d09c82731.jpg"},{"id":29404099,"identity":"865a777b-874a-4148-a908-279e48b0350b","added_by":"auto","created_at":"2022-11-22 18:38:03","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":65926,"visible":true,"origin":"","legend":"\u003cp\u003eSFG spectra of the rubbed polyimides for the PPP polarization combination as a function of the storage time. The solid lines are the fitting curves obtained using Eq. (1) in the text.\u003c/p\u003e","description":"","filename":"Fig2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2283463/v1/e4e22d420888f2feb08fa9cf.jpg"},{"id":29404580,"identity":"65d4af90-4fff-4517-b690-8a990c7377fd","added_by":"auto","created_at":"2022-11-22 18:46:03","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":306736,"visible":true,"origin":"","legend":"\u003cp\u003eSHG intensity pattern of the rubbed polyimide film as a function of sample rotation angle for Pin/Pout polarization combination measured at (a) 20 days, (b) 5 months, and (c) 13 months storage time. 0 degrees means that the beam propagation direction is in the rubbing direction.\u003c/p\u003e","description":"","filename":"Fig3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2283463/v1/b0abda828059ba2b131a9b54.jpg"},{"id":29404100,"identity":"2acd7b52-b63d-4dec-bf12-9dba7f84cef5","added_by":"auto","created_at":"2022-11-22 18:38:03","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":335287,"visible":true,"origin":"","legend":"\u003cp\u003eSHG intensity pattern of the unrubbed polyimide film as a function of sample rotation angle in as Pin/Pout polarization combination measured at (a) 28 days, (b) 8 months, and (c) 13 months storage time.\u003c/p\u003e","description":"","filename":"Fig4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-2283463/v1/003ef04a13a29b294813bb93.jpg"},{"id":34974915,"identity":"2c343a9a-319e-4c0c-b918-d1631475f4ce","added_by":"auto","created_at":"2023-03-29 11:29:45","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":568711,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2283463/v1/91b34b7b-c9df-4d3f-aece-524db4ec3005.pdf"},{"id":29404102,"identity":"d808e6d7-a7d7-42a0-8699-678539040350","added_by":"auto","created_at":"2022-11-22 18:38:03","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":1476954,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-2283463/v1/dfe7996ef67a954d2b8b9e07.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Detection of surface modification of polyimide containing steroidal structure as a function of storage time using second- order nonlinear optical spectroscopy","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePolyimide containing steroidal structure is known as a semi-aromatic polyimide. The microscopic structure of such a polyimide studied in this work is shown in Fig. 1. This polyimide possesses outstanding properties like the general polyimides, such as high thermal stability, excellent mechanical properties, and high chemical resistance [\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e]. Besides, the steroidal structure in this polyimide makes it possess unique properties. For example, it shows better electrical properties as the material for the alignment layers in the liquid crystal display (LCD) panel [\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e] than the ones with long alkyl chains or fluorine atoms. In addition, the pretilt angles of liquid crystal (LC) molecules for various LCD modes can be precisely controlled by changing the fraction of monomeric units of polyimide containing steroidal structure [\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e]. In principle the LC alignment on\u0026nbsp;rubbed polyimide film surfaces is determined by the molecular interaction between polyimide film surface and LC molecules [\u003cspan class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e]. Hence, the quality of the polyimide film surface is crucial for high-quality LCD performance.\u003c/p\u003e\n\u003cp\u003eVibrational sum frequency generation (SFG) and second-harmonic generation (SHG) have been demonstrated as practical tools to probe polymer surfaces due to their sensitivity to asymmetric parts of materials unlike infrared or Raman spectroscopies. While vibrational SFG spectroscopy can provide information about the surface vibrational resonances and the average orientation of the molecules at the surface [\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e11\u003c/span\u003e], SHG can provide information about the electronic transitions of the surface molecules [\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]. In fact, we found that the SFG response of the polyimide film originates mainly from the surface and the SFG contribution from the polyimide/glass interface and the bulk can be ignored [\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]. The terminal isopropyl group in the side-chain of the polyimide containing steroidal structure as shown in Fig.\u0026nbsp;1 is pointing toward the air side and the average tilt angle of the methyl group adjacent to the steroidal structure is \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(40^\\circ \\pm 10^\\circ\\)\u003c/span\u003e\u003c/span\u003e in the rubbing direction. Analyzing the SHG response of the polyimide containing steroidal structure, we also figured out that both the polyimide side chains and the main chains contribute to this response [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eIn order to systematically detect surface modification behavior of polyimide films as a function of storage time, we used a combination of vibrational SFG spectroscopy to analyze the difference of vibrational modes in the CH stretching range and SHG to examine the variation of the SHG response of polyimide film surfaces, especially phenyl rings. The sample polyimide is that in Fig.\u0026nbsp;1 because we have an ample experience of this specific polyimide [\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]. The purpose of this study is to establish a method to detect the change of the surface condition of an LCD alignment layer in a practical environment.\u003c/p\u003e"},{"header":"2. Materials And Methods","content":"\u003cp\u003eFigure 1 illustrates the chemical structure of a polyimide containing 30% fraction of steroidal structure used in this study. The sample preparation procedure can be found in our previous paper [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Briefly, the polyimide films with a layer thickness of \u0026sim; 80 nm were prepared by spin-coating on non-alkali glass substrates and then were rubbed once by a rubbing machine with a weak strength. After preparation, the polyimide films were put in plastic boxes and then stored in a dry box kept at humidity 25\u0026thinsp;\u0026plusmn;\u0026thinsp;3% and temperature 21\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003csup\u003eo\u003c/sup\u003eC.\u003c/p\u003e \u003cp\u003eThe SFG spectra of the polyimide film were recorded using a narrowband SFG system. The details of the experimental setup for the SFG measurement have been published elsewhere [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. In short, a visible beam (532 nm) and a tunable infrared beam were overlapped on the polyimide film surface at angles of incidence of 80\u0026deg; and 45\u0026deg;, respectively, to generate an SFG signal. The energies of the visible beam and infrared beam used were \u0026sim;80 \u0026micro;J and from 135 to 280 \u0026micro;J, respectively. After appropriate optical filtering, the SFG output was collected in the reflection direction by a monochromator and a photomultiplier tube. In this experiment, the SFG spectra were recorded in the CH-stretching region (2800 to 3000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) for a PPP (P-polarized SFG, P-polarized visible, P-polarized IR) polarization combination at a 5 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e interval. For rubbed polyimide films, the SFG spectra were measured with the wave vectors of the incident beams parallel to the rubbing direction. The SFG signal was averaged over 600 pulses for each wavelength using a gated integrator to reduce noise. All the SFG spectra were fitted by the following equation[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$I\\left({\\omega }_{SFG}\\right)\\propto {\\left|{A}_{NR}^{\\left(2\\right)}{e}^{i\\varphi }+{\\sum }_{q}\\frac{{A}_{q}{\\varGamma }_{q}}{{\\omega }_{IR}-{\\omega }_{q}+{i\\varGamma }_{q}}\\right|}^{2}.\\left(1\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eHere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({A}_{NR}^{\\left(2\\right)}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\varphi\\)\u003c/span\u003e\u003c/span\u003e denote the amplitude and phase of the non-resonant SFG signal, ω\u003csub\u003eIR\u003c/sub\u003e is the infrared frequency. A\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e, ω\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e, and Γ\u003csub\u003eq\u003c/sub\u003e denote the amplitude, the resonant frequency, and the line width of the \u003cem\u003eq\u003c/em\u003e\u003csup\u003eth\u003c/sup\u003e vibrational mode, respectively.\u003c/p\u003e \u003cp\u003eThe details of the experimental setup for the SHG measurement can be found elsewhere [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. In short, a fundamental beam (532 nm) with energy at \u0026sim; 20 \u0026micro;J irradiated the polyimide film at an angle of 45\u0026deg; to generate an SHG signal (266 nm). The SHG output was collected in the same way as the SFG output. In the SHG experiment, we mounted the polyimide film on a 360\u0026deg; rotational stage. Then its SHG response at sample rotation angle intervals of 10\u0026deg; for a Pin/Pout (P-polarized fundamental beam and P-polarized SHG) polarization combination was recorded. The SHG signals of 1000 pulses were accumulated for each data point.\u003c/p\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1 SFG spectra of polyimide films as a function of storage time\u003c/h2\u003e\n\u003cp\u003eThe polyimide films were carefully kept in a dry box to reduce the impact of the environment unless they were under SFG observation. Still, the polyimide surface was modified after long-term storage. To check the quality of the polyimide film surfaces at different storage times, we collected the SFG spectra of the polyimide films in the CO/CH stretching regions and above 3000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Herein we just show the SFG spectra obtained in the CH stretching region because the spectra obtained in the CO stretching region and above 3000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were ambiguous.\u003c/p\u003e\n\u003cp\u003eFigure 2 shows SFG spectra of rubbed polyimide films for the PPP polarization combination as a function of the storage time. The detailed assignments of the vibrational modes at the bottom are based on those for fresh polyimide in our previous paper [\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]. The fitting parameters used for the theoretical curves can be found in Supporting Information.\u003c/p\u003e\n\u003cp\u003eWhile the polyimide spectra observed at and before 2 months 11 days storage time show only a slight change from the bottom spectrum, those after longer storage time show significant changes. As shown in Fig. S1 (Supporting Information), the amplitude ratios of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{+}\\)\u003c/span\u003e\u003c/span\u003e peak (CH\u003csub\u003e3\u003c/sub\u003e symmetric stretching mode of the isopropyl group) and the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peak (CH\u003csub\u003e3\u003c/sub\u003e anti-symmetric stretching mode of the isopropyl group) within 2 months 11 days storage time are similar whereas those ratios of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{+}\\)\u003c/span\u003e\u003c/span\u003e peak and the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peak significantly change after longer storage time. Moreover, a considerable variation of the peak position and the shape of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peak is observed as a function of the storage time. Indeed, the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peak shifts from 2965 cm\u003csup\u003e-1\u003c/sup\u003e to 2961 cm\u003csup\u003e-1\u003c/sup\u003e after 3 months 12 days storage time. Vibrational SFG is extremely sensitive to order and conformation of molecules at surfaces, the variations of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{+}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peaks therefore suggest that the structures of the isopropyl groups on the polyimide surface changed somehow after a\u0026nbsp;long-term storage time.\u003c/p\u003e\n\u003cp\u003eIn a similar way we investigated SFG response of \u003cem\u003eunrubbed\u003c/em\u003e polyimide film surfaces after different storage times (Fig. S2, Supporting Information). We found that the modification of polyimide surface occurs in the unrubbed polyimide film as well as in the rubbed polyimide film. To check the impact of ambient environment factors such as moisture and oxygen on the polyimide surface, we recorded SFG spectra of the rubbed polyimide film kept in a quartz cell filled with dry N\u003csub\u003e2\u003c/sub\u003e gas as a function of storage time (Fig. S3, Supporting Information). Although the number of observed peaks in the SFG spectrum of the sample after 10 months storage time are like those at 7 days, the relative peak intensities and the peak positions are quite different. The results indicate that the polyimide surface in dry N\u003csub\u003e2\u003c/sub\u003e gas also might be modified after 10 months storage time.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2 SHG intensity patterns of polyimide films as a function of time\u003c/h2\u003e\n\u003cp\u003eAs previously mentioned, we cannot extract molecular information of the polyimide main chain, namely the imide group and phenyl ring, from the SFG spectra. This information is crucial to determine the origin causing the polyimide surface modification after long-term storage. Hence, we use optical SHG as a supplementary method to check the orientation of the phenyl rings in the surface molecular layer of the polyimide.\u003c/p\u003e\n\u003cp\u003eFigure 3 shows SHG intensity patterns of rubbed polyimide films possessing a 30% fraction of steroidal structure as a function of the storage time. Due to the rubbing process, an anisotropy of SHG intensity, namely a larger SHG response around 0 deg or in the rubbing direction, is observed for the fresh sample [Fig.\u0026nbsp;3(a)]. The SHG anisotropy was also obtained for the rubbed samples at 5 months and even at 13 months storage time [Figs.\u0026nbsp;3(b) and 3(c)]. The results indicate that the SHG response is independent of the storage time. Similarly, we recorded SHG responses of unrubbed polyimide films, as shown in Fig.\u0026nbsp;4. As it is natural, no essential anisotropy is seen in the SHG response of the samples measured at either 28 days [Fig.\u0026nbsp;4(a)], 8 months [Fig.\u0026nbsp;4(b)] or 13 months [Fig.\u0026nbsp;4(c)] storage time. The SHG response of both rubbed and unrubbed samples show negligible dependence on the storage time. Interestingly, these SHG results look inconsistent with the SFG ones of the same samples.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eIn the previous section the storage-time dependence of the anisotropy of the SHG response of the rubbed polyimide containing steroidal structures was not found to be remarkable, while the change of the vibrational SFG spectrum of the same sample after 3 months storage time was found to be clear. These observations are not necessarily inconsistent because SFG and SHG can generally originate from different parts of the polyimide film surface [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Namely, the SFG signal in Fig.\u0026nbsp;2 is assigned as molecular vibrations of the CH\u003csub\u003e2\u003c/sub\u003e groups or the CH\u003csub\u003e3\u003c/sub\u003e groups in the surface side chains, while the SHG signal in Figs.\u0026nbsp;3 and 4 are suggested to come from the aromatic rings. Here, we only focus on interpreting the variations of the symmetric and anti-symmetric stretching modes of the CH\u003csub\u003e3\u003c/sub\u003e isopropyl group (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{+}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e) in the polyimide SFG spectra as a function of storage time because these variations were systematically observed in Fig.\u0026nbsp;2.\u003c/p\u003e \u003cp\u003eThe variation of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{+}\\)\u003c/span\u003e\u003c/span\u003eand \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peaks of the isopropyl group after 3 months storage time as reported in Section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e can be due to six candidate origins as:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eChange of Fresnel factors,\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eBreaking down or oxidation in air of the isopropyl group,\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eBreaking down of the main chain,\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eBreaking down of the steroidal structure,\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eEffect of the environmental moisture on the polyimide film and a resultant modification of the isopropyl group\u0026rsquo;s orientation.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003ePolyimide surface contamination\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eFresnel factor is used to correct the SFG intensity for its modification by linear optical effect of input and output radiations at surfaces or interfaces [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. In order to compare the Fresnel factors for the polyimide/air interface for the fresh rubbed polyimide film and the one after 10 months storage time, the refractive indices of these samples were measured using spectroscopic ellipsometry (Supporting Information). We found no significant variation between the refractive indices of the fresh sample and those of the sample after 10 months storage time. Hence, the change of the peak intensity as a function of the storage time in Fig.\u0026nbsp;2 is judged to be not due to the modification of the Fresnel factors, and we can exclude candidate (1).\u003c/p\u003e \u003cp\u003eNow we look at candidate (2), namely breaking down or oxidation by oxygen in air of the isopropyl group. The isopropyl group is the simplest structure group containing a tertiary carbon - a carbon atom directly bound to three other carbon atoms. Generally, this group is extremely stable in normal room temperature conditions [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Since the storage of our samples was carried out in a very mild condition, we can exclude this candidate (2).\u003c/p\u003e \u003cp\u003eNext, we consider candidate (3), namely breaking down of the main chain. In general, chemical bonds between C and N atoms in the imide group or C and C atoms are the two most easily broken bonds in polyimide [\u003cspan additionalcitationids=\"CR27 CR28\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Since the strength of the C-N bond is lower than that of the C-C bond [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], the C-N breakage might most likely occur after 3 months storage time in this polyimide. On the other hand, the SHG intensity patterns of the polyimide films show slight difference as a function of storage time. One can see in Fig.\u0026nbsp;1 that if the C-N breakage occurred, it would affect the configuration of the aromatic rings. The SHG results indicate that it is not the case. Hence candidate (3) may not have caused the variations of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{+}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peaks in the SFG spectra as a function of storage time.\u003c/p\u003e \u003cp\u003eWe can clearly exclude candidate (4), namely breaking down of the steroidal structure. Due to the \u0026lsquo;\u003cem\u003etrans\u0026rsquo;\u003c/em\u003e configuration at all three fusions, the steroidal structure is commonly rigid and remains semi-flat [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. That is why cholesterol, the most abundant steroid in animals, is so stable in animal\u0026rsquo;s body [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Hence, this structure is not easily broken in normal environmental conditions.\u003c/p\u003e \u003cp\u003eHere, we discuss candidate (5), namely the effect of moisture on the polyimide and a resultant modification of the surface isopropyl group\u0026rsquo;s orientation. In our previous study [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], we suggested that the isopropyl group points toward the air side from the rubbed polyimide surface when the film is fresh. Water molecules can affect the isopropyl group in two ways: Firstly, moisture in the air may drive the hydrophobic isopropyl group into the bulk. Secondly, water molecules in the polyimide bulk may drive the isopropyl group already in the bulk back toward the air side. Then, to minimize the total energy of the film surface, the hydrophobic isopropyl group may most likely take more disordered configuration due to its interaction with water molecules after a long-term storage time than when the film is fresh [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. This view also explains the fact that the SFG spectra of unrubbed polyimide film changes as a function of time as seen in Fig. S2 (Supporting Information).\u003c/p\u003e \u003cp\u003eLastly, we discuss candidate (6), namely polyimide surface contamination. The surface contamination might occur due to the adsorption of unknown organic components on the polyimide surfaces. Then, the interaction between hydrophobic side chains of polyimide and hydrophobic organic contaminants might affect the orientation of the isopropyl group. As expected, the modification of the polyimide surface was also observed in the polyimide kept in the quartz cell filled dry N\u003csub\u003e2\u003c/sub\u003e gas (Fig. S3, Supporting Information). Candidates (5) and (6) have opposite behaviors, and one or both of them may be the reason for the variations of the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{+}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peaks in the SFG spectra as a function of storage time. Further study is necessary for determining which one is intense.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eVibrational SFG spectroscopy and SHG were used to detect the modification of polyimide film surfaces containing a 30% fraction of steroidal structure as a function of storage time. The \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{+}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{2}^{-}\\)\u003c/span\u003e\u003c/span\u003e peaks in the SFG spectra of the rubbed polyimide film showed systematic storage time dependence whereas the SHG response of the same sample just showed slight variation. Disorder of the orientation of the isopropyl groups in the polyimide side chain due to the interaction with moisture or surface contamination might be the main reason for these observed facts. The phenyl rings of the polyimide were found not to be reoriented because the SHG response of the same samples showed an insignificant storage-time dependence. The experimental results prove that the combination of vibrational SFG spectroscopy and optical SHG can be a useful method to detect the change of the polymer film surface in a practical environment.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCRediT author statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTrinh Thi Nguyen:\u003c/strong\u003e Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing-original draft. \u003cstrong\u003eWentao Du\u003c/strong\u003e: Investigation, Formal analysis. \u003cstrong\u003eHien Thi Thu Khuat\u003c/strong\u003e: Methodology, Supervision. \u003cstrong\u003eGoro Mizutani\u003c/strong\u003e:\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eConceptualization, Methodology, Writing - Review \u0026amp; Editing, Supervision. \u003cstrong\u003eYoshitaka Murakami\u003c/strong\u003e: Conceptualization, Resources. \u003cstrong\u003eTakashi Okada\u003c/strong\u003e: Resources, Supervision.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eM. 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Pratt, \u003cem\u003eFundamentals of Biochemistry\u003c/em\u003e (John Wiley \u0026amp; Sons, New York, 1999).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eC. J. Jalbert, J. T. Koberstein, R. Balaji, Q. Bhatia, L. Salvati, and I. Yilgor, Macromolecules \u003cb\u003e27\u003c/b\u003e, 2409 (2002).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eK.-W. Lee, S.-H. Paek, A. Lien, C. Durning, and H. Fukuro, \u003cem\u003eMicroscopic Molecular Reorientation of Alignment Layer Polymer Surfaces Induced by Rubbing and Its Effects on LC Pretilt Angles\u003c/em\u003e (1996).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"polyimide, steroidal structure, surface quality degradation, vibrational sum frequency generation spectroscopy, second harmonic generation","lastPublishedDoi":"10.21203/rs.3.rs-2283463/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2283463/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe modification of a polyimide surface containing a 30% fraction of steroidal structure was studied as a function of storage time by vibrational sum frequency generation (SFG) spectroscopy and optical second-harmonic generation (SHG). The variation of the symmetric and anti-symmetric stretching modes of the CH\u003csub\u003e3\u003c/sub\u003e isopropyl group at the end of the side chain of the rubbed polyimide was detected in the SFG spectra after 3 months storage time in a practical environment. It suggests that the isopropyl group might be reoriented. On the other hand, the SHG response of the same sample showed an insignificant storage-time dependence. As a result, the phenyl rings of the polyimide are judged to be unmodified after long-term storage time. The change is suggested to occur for two main reasons. One is the interaction between the polyimide polymers and the ambient water molecules. The other is the adsorption of unknown organic molecules on the polyimide surface.\u003c/p\u003e","manuscriptTitle":"Detection of surface modification of polyimide containing steroidal structure as a function of storage time using second- order nonlinear optical spectroscopy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-11-22 18:37:58","doi":"10.21203/rs.3.rs-2283463/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"de8a6d82-1640-4b4f-abae-5fd097197e28","owner":[],"postedDate":"November 22nd, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2023-03-29T11:29:36+00:00","versionOfRecord":[],"versionCreatedAt":"2022-11-22 18:37:58","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2283463","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2283463","identity":"rs-2283463","version":["v1"]},"buildId":"cBFmMYwuxLRRLfASyISRj","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}