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Raman Spectroscopy
CHAPTER 2
Raman Spectroscopy P.S. Goh, A.F. Ismail, B.C. Ng Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia
Chapter Outline 1. 2. 3. 4.
Introduction 31 Principle of Raman Spectroscopy 33 Raman Spectroscopy for Polymer Characterization 35 Raman Spectroscopy for Polymeric Membrane Characterization 4.1 4.2 4.3 4.4
Polymeric Polymeric Polymeric Polymeric
Membrane Formation 37 Fuel Cell Membranes 38 Composite Membrane With Additives/Fillers Membrane Antifouling Strategy 40
5. Conclusion 42 List of Abbreviation References 44
36
40
44
1. Introduction Raman spectroscopy is a technique used to observe vibrational, rotational, and other low-frequency modes. It is a complimentary method to infrared spectroscopy to obtain information on crystalline structure of the macromolecules and change of polymeric structure in membrane. In brief, this technique relies on the inelastic Raman scattering of monochromatic laser light that ranges from visible to near ultraviolet. The laser light interacts with molecular vibrations, phonons, or other excitations to result in energy changes of the laser photons where the shift up or down in the energy gives information about the vibrational modes. Raman spectroscopy has been commonly used in the field of material sciences and chemistry to provide the fingerprint to allow direct identification and intepretation of different molecules. In fact, this technique can be suitably applied for nondestructive, microscopic, chemical analysis, and imaging characterizations. One of the most significant advantages of Raman spectroscopy is the ability to provide key information easily and quickly with minimum sampling handling issues. Membrane Characterization. http://dx.doi.org/10.1016/B978-0-444-63776-5.00002-4 Copyright © 2017 Elsevier B.V. All rights reserved.
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32 Chapter 2 Raman spectroscopy is known to provide valuable information in polymer characterization. This technique has been widely used to determine the functional groups and group structure of the polymers; conformation and orientation of the macromolecule chains and to monitor the changes in the polymer structural properties upon exposure to environmental or mechanical stresses [1,2]. Study of the crystalline structure of polymers will add to the existing knowledge of intramolecular forces in crystals and their effect on stable polymer structures. The main advantage in polymer characterization using Raman spectroscopy is the capability of the Raman bands to relate to chemical structure of polymeric materials based on the fundamental modes measurement. Raman spectroscopy is very useful to determine the functional groups, and group structure, conformation and orientation of the chains and to follow changes in the structural parameters as the polymers are exposed to environmental or mechanical stresses. In addition, modification to the surrounding field of the molecular unit can also be effectively studied through Raman spectroscopy. For instance, a polyvinylidene fluoride (PVDF) based materials can present many types of molecular and crystal structures depending on their preparation conditions. As such, Raman spectra can be used to distinguish the different crystalline forms of PVDF and the presence of polymer chain defects. In term of polymeric membrane characterization, Raman spectra provide information, in both qualitative and quantitative, about the (i) inter and intramolecular interactions among the functional groups that present in the polymer macromolecules, (ii) crystalline structure of the macromolecules in the membrane, (iii) change of polymer structure, during membrane formation, (iv) structural change of the polymeric membrane upon heating, and (v) interfacial properties of the composite or coated membrane. Besides that, this vibrational technique also offers a reliable solution in tackling qualitative and quantitative analytical problems present in the studies of asymmetric polymeric membranes. It has been applied to study the composition and distribution of the particles of composite membranes that consist of inorganic particles incorporated in a polymer matrix. Interestingly, when Raman spectroscopy is combined with electron microscopy, it can be used to offer better understanding of the complex mechanism involved in the inversion phase process as well as to determine the polymer chain structure in different areas across the membrane [2]. Over the last decades, remarkable technical improvement has been done in Raman spectroscopy to tackle issues such as fluorescence, poor sensitivity, or reproducibility to enhance the reliability of this technique. Additionally, the further improvements in Raman instruments such as lasers, monochromators, and detectors are helpful to reduce the complexity and cost of this technique as well as broadening the application of Raman spectroscopy for a wide range of applications. This chapter presents the principle of Raman spectroscopy and followed by the applications of Raman spectroscopy for the characterization of polymer materials, particularly polymeric membranes. It is expected that, the use of Raman spectroscopy will help to understand the relationship between
Raman Spectroscopy 33 structure and transport properties, which could have an important impact on designing membranes for specific scientific problems.
2. Principle of Raman Spectroscopy Raman spectroscopy has been used to study the interactions of the vibrational energies of atoms or groups of atoms within molecules. The Raman effect is produced through the exchange of energy between incident photons and the vibrational energy levels of the molecule [3]. Raman scattering is the inelastic scattering of laser light impinged onto the sample and is scattered with a transfer of energy between the excitation light and the sample. Raman scattering occurs when the molecular motion produces a change in the polarizability of the molecule. In principle, the scattered light can be categorized as antiStokes or Stokes depending on their energy level: The former has higher energy, whereas the latter has lower energy compared to the illumination light. In practice, Raman microscopy is almost exclusively based on the analysis of Stokes-scattered light that exhibits much higher intensity. The light scattered by the molecules contains frequencies different from the incident monochromatic light that correspond to the normal vibrational frequencies of the molecules. When a molecule absorbs radiations, its energy increases in proportion to the photon. The increased energy may be at the level of the electronic, vibrational, or rotational energy of the molecule [4]. The Raman spectra are then generated from the vibrational motions that resulted from the change in a source-induced molecular dipole moment. Fig. 2.1 illustrates the process involved in collecting Raman spectra [3]. As depicted, Raman spectrometer is typically composed of light source, monochromator, sample holder, and detector. The Raman scattered light is dispersed according to wavelength and processed as Raman spectra. The factors that affect the quality of Raman spectra are high signal-to-noise ratio, instrument stability, and sufficient resolution. Recently, the advancement in near infrared or red excitation lasers could tackle this issue by eliminating the fluorescence that detrimentally affect the Raman signals. Normally, the magnitudes of the Raman shifts in frequency can be corresponded to those of infrared absorptions. However, some vibrational modes appear only in the Raman spectrum. Particularly, the more symmetric the molecule is, the greater the differences that can be observed between its Raman and infrared spectra. Generally, for chain-like polymer molecules, vibrations of the carbon chain are easily studied by using Raman spectroscopy, whereas the vibrations of the branches are more feasibly characterized by using infrared spectroscopy [5]. Also, it is found that, infrared spectra demonstrate weak absorptions for molecular vibrations that result in intense Raman peaks and vice versa. The uniqueness of the vibrational patterns of Raman and IR spectroscopy can be complementarily used in the measurement of the structure of molecules to the greatest extent. Although it is well acknowledged that, the infrared and Raman spectra of a molecule complement each other
34 Chapter 2 Dispersive Raman Spectroscopy
Wavenumber Selector
Sample
Detector (PMT/CCD)
Radiation Source
Laser Source (NIR)
F-T Raman Spectroscopy
Sample Beam splitters Moving Mirror
Detector
Stationary Mirror
Figure 2.1 Schematic for process involved in Raman spectra collection [3].
where the complete information regarding the vibrational spectrum of a molecule always requires both infrared and Raman vibration, however, when compared to infrared technique, Raman spectroscopy possesses some advantages: Raman spectroscopy involves a scattering process where it does not have the problems associated with light transmission hence samples of any size or shape can be examined at low frequency region. Besides that, Raman spectra also offers greater information about the skeletal movements of the molecule. In terms of sample preparation, Raman spectra can be obtained directly from a wide range of materials in the form of bulk solids, liquids, tablets, polymers, and paper with minimum or no sample preparation. Several modes of Raman spectroscopy have been established to improve the spatial resolution, enhance sensitivity, and acquire more specific information. One of the advancement in Raman spectroscopy is the development of surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy that have significantly increased the measured signal. Another interesting feature of Raman spectroscopy is that it can be readily adapted to microscopy as the excitation light can be chosen to be in the visible or near infrared spectral range [6]. This advantage allows the usage of high quality objectives. Some of the commonly adapted analysis methods are scanning probe microscopy, scanning electron
Raman Spectroscopy 35 microscope and confocal laser scanning microscopy. In a natural extension of the Raman technique, the laser beam is focused to provide sample illumination on the micron scale and Raman scattering is collected with the instrument hardware to produce spectra of samples with a spatial resolution approaching 1e10 mm. The discrete sample sites can be examined for multiple Raman spectra collection to accomplish the mapping of a sample surface to provide structural data on a microscopic scale. The advancement of Raman microscope in terms of the designs and microscopic approaches enables the utilization of imaging and microscopy for the investigation of polymer surface structure, or the defect and contaminant analysis of coatings, polymer films, and laminates.
3. Raman Spectroscopy for Polymer Characterization Raman spectroscopy is a powerful qualitative and quantitative tool with some particular advantages for the analysis of polymers. One of the main advantages in polymer characterization using Raman spectroscopy is the ease of obtaining a good spectrum with minimum sample handling. It finds numerous applications in polymer chemistry, polymer material, and composite, for example, in applications such as monitoring of polymerization reaction, prediction of physical properties, and mapping of polymeric phases [7e9]. Table 2.1 summarizes the important information obtained from Raman spectra for polymeric materials characterization. Raman spectroscopy exhibits high sensitivity for the measurement of localized polymer structures in which it allows the analysis of extremely small samples with individual segmental orientations and provides detailed interpretation of the structural conformational distribution in both ordered and disordered forms. Disordered polymers possess vibrational modes that can be easily detected by Raman [10]. An important area of research in polymer studies involves the investigation of the influences on the molecular orientation Table 2.1: Important information obtained from Raman spectra for polymeric materials characterization. Properties Chemical structure and composition Stereo-order Conformational order
State of order Orientation
Surface and interface structures
Details Structural units, end groups, type, and degree of branching, additives cisetrans isomerism and stereoregularity Physical arrangement of polymer chains both in the solid and in the melt states, e.g., planar zigzag or helix conformations. Crystalline, mesomorphous, and amorphous phases; intermolecular forces; lamellar thickness Raman depolarization and infrared dichroism of preferential polymer chains and side group alignments in anisotropic polymers Interfacial properties of composite materials
36 Chapter 2 induced by external forces, drawing on the physical and mechanical properties of a polymer. Local structural effects that can be determined through Raman spectroscopy include the amount of remaining amorphous content and type of order induced by processing, defects in structure, and in some cases tacticity [4]. It is known that polymer can exist in a number of configurations, i.e., syndiotactic and isotactic or helical and planar. Sometimes, they can be atactic with an ordered conformation. The feasibility of Raman spectroscopy for polymer characterization was first explored to investigate the skeletal bending modes of polypropylene (PP), which appeared in the low frequency [11]. The syndiotactic and isotactic sequences in characterizing the overall chain configuration of PP can be distinguished from the Raman spectra. High temperature Raman spectroscopy has been carried out to observe the structural and morphological changes occurring during the deformation of PP at elevating temperature [12]. Since then, it has been widely used to investigate the regularity and local structures in various kinds of polymers including polyethylene, polyethylene terephthalate, polystyrene, and polytetrafluoroethylene [13]. Raman spectroscopy is particularly sensitive toward the modification to the surrounding field of the molecular unit such as hydrogen bonding, crystal field splitting, chain packing, and salvation. Additionally, it is also very sensitive to the thermal history of the polymer. It is known that hydrogen bonding and the changes in the chain conformation directly affect the electronic structure of the molecular unit meanwhile crystal field splitting, chain packing, and salvation interact with the molecular unit by coupling external electric fields with electronic structure of the molecular unit. Both types of interactions affect the spring constants and the effective masses of the molecular unit, thus leading to shift in the normal mode frequencies and line shapes [14]. Raman polarization studies offer detailed information about the orientation distribution of structural units for both crystalline and noncrystalline regions. Polymers can be classified based on their crystallinity, i.e., amorphous and crystalline states that depend on the molecular weight of the polymer. The time taken for the polymer to crystallize from solution dictates the formation of a crystalline region. In general, polymer with low molecular weight polymer contains higher content of crystallites compared to that of high molecular weight polymer. Various morphologies are possible between a completely crystalline and completely amorphous conformation. Crystalline polymers have a number of morphological features that can be studied by Raman spectroscopy. For instance, many phase transitions can be monitored by following the changes in the Raman spectra to associate with the crystalline structure [4].
4. Raman Spectroscopy for Polymeric Membrane Characterization Raman spectroscopy is a promising technique to obtain information on crystalline structure of the macromolecules and observe the change of structure in a polymeric
Raman Spectroscopy 37 membrane. There are three important structural levels in the polymeric membrane, i.e., (i) the molecular level that represents the chemical nature of the polymer. The molecular level responsible for the microcrystalline nature and it is characterized by polar, steric, and ionic factors; (ii) the microcrystalline, which affects both the transport and the mechanical properties of membrane; and (iii) the colloidal level that is related to the aggregation of macromolecules. It governs the statistics of pores that includes the pore size, pore size distribution, pore density, and void volume. Raman spectroscopy is applicable for the investigation at each level to achieve more rigorous understanding of the polymeric structure in the membrane [4].
4.1 Polymeric Membrane Formation As polymeric membranes can be processed through the casting of solutions by doctor blade to produce the flat sheet membrane or via spinning to produce the hollow fiber membranes, it is generally agreed that some orientations, either in plane or out of plane, will be induced during the processes. The changes in the molecular environment may trigger the Raman effects in several ways. As the bulk properties of a polymer can be directly related to its morphology, the understanding and controls of the morphology are technologically essential. It is also expected that by establishing the relationship between membrane morphology and its transport behavior the mechanism of mass transport in the membrane can be revealed and understood. Particularly, the polymer morphology in the selective skin layer has been carefully controlled and fine tuned during the design of a synthetic polymeric membrane. Many attempts have been made to establish the cause and effect relationship between membrane preparation, polymer morphology, and membrane performance. When the membrane is subjected to Raman spectroscopy analysis, the orientation of polymer chain and the degree of crystallinity of the polymer in the membrane, which represent the morphology of the membrane, can be systematically studied. Besides that, the dynamic features of Raman spectroscopy also enable the monitoring of the morphology change during membrane formation [15]. Micro-Raman spectroscopy with high spatial resolution has been used to locally investigate and determine the chemical composition [16]. The technique has also been applied to membrane characterization and membrane formation. Raman confocal spectroscopy has been applied to provide quantitative information on the formation of a surface liquid layer on the top of the membrane made of poly(ether-imide) (PEI) and Nmethylpyrrolidone (NMP) during the formation via water vaporeinduced phase inversion [17]. Changes in the film thickness and in the NMP/PEI ratio at the film surface were determined by using Raman confocal spectroscopy during membrane forming in dried and humid air (relative humidityw50%). The results obtained showed that when the cast film was exposed to humid air, the polymer concentration at the film surface first decreased and subsequently increased. It was found that Raman spectroscopy not only provided evidence
38 Chapter 2 supporting the existence of a surface liquid layer but also provided quantitative information about the composition in the layer. Very recently, the chemical changes and oxidative degradation mechanisms of PP microfiltration membranes were examined at the micron scale by using cross-sectional analysis with micro-Raman spectroscopy [18]. New peaks were observed at w1740 cm 1 and w3450 cm 1 that corresponded to carbonyl eC]O aldehyde stretching groups and eOH bond, respectively, for all the degraded samples.
4.2 Polymeric Fuel Cell Membranes Raman spectroscopy is a powerful tool for investigation of molecular interactions at a functional group level hence it has been widely used to characterize Nafion membrane for fuel cell applications [19e21]. It has been pointed out that water molecules in the polymer electrolyte membrane play important roles in the proton transfer for the proton exchange fuel cell (PEFC) operation since the existence of hydrated proton during the proton transfer in the membrane [16]. Hydration structure of a Nafion membrane in a PEFC was explored at a functional group level to study the eOH stretching modes and the intensity variation of sulfonate symmetric stretching mode as a function of the alkali ions. It has also been used to characterize the nature of the proton exchange sites during treatment in acidic/alkali environment that plays a crucial role in the performance of fuel cells. The protonation of commercial Fumapem and Fumasep membranes was confirmed through the new vibration peaks that appeared in the regions 3700e3800 and 1600e1700 cm 1 and the disappearance of peak at 1679 cm 1 after protonation process [22]. It is known that the membrane electrode assembly (MEA) durability in a fuel cell is closely dependent on the operating conditions such as temperature, gas humidity, and load dynamics. The harsh working environment could result in both chemical and mechanical degradation of the ion-conducting membrane. Chemical degradation of the membrane can decrease the proton conductivity and weaken the mechanical stability of the membrane and subsequently lead to the operation failure. Thus, avoiding degradation is important for reaching the goals in terms of life-time for wide commercialization. When coupled with a confocal microscope, Raman spectroscopy has the advantage of measuring molecular composition and the measurement can be localized to a micrometer size volume. By using confocal Raman spectroscopy they observe a lower concentration of sulfonic acid groups. Measurements can be applied to MEA in any size and shape [23]. In general, the Raman active bonds in the membrane are either due to the backbone; consisting of CeC and CeF bonds, or the active side chain groups; also this part with CeC and CeF bonds, but furthermore also having Raman signals arising from CeOeC, CeS, and SeO vibrations [20]. There are mainly two different molecular degradation mechanisms of interest; one where the hydrophilic sulfonic acid end groups (branches) are broken away from the
Raman Spectroscopy 39 backbone and a second where the backbone is broken into shorter chains. In the study conducted by Holber et al. to study the membrane aging, the CeF vibrational mode at 731 cm 1 was used as the standard for both the pristine reference and the aged membrane due to the strong nature of this vibration and the large amount of CeF bonds in the material [23]. From the spectra obtained, the loss of sulfonic acid end groups was observed as a decrease of intensity in the CeOeC, CeS, and SeO signals compared to the pristine reference. A conventional high-temperature membrane electrode assembly primarily consists of a polybenzimidazole (PBI)-type membrane containing phosphoric acid. Raman spectroscopy has been used to study the effects of acid doping on PBI-type polymers due to its high sensitivity toward molecular structural changes that occur during the acidebase proton exchange reaction [24]. Additionally, it is also sensitive to molecular interactions between membrane components. Raman spectra of pristine and acid-doped PBI materials with different degrees of doping have been obtained for further band assignments as depicted in Fig. 2.2(a) [25e27]. The Raman band at 1000 cm 1 was assigned to the meta-benzene ring vibration that showed minimum changes despite the increasing acid content in the membrane. In contrast, the band at 1539 cm 1, which is associated with the symmetric stretch of the imidazole group, became stronger and shifted toward 1570 cm 1 with increasing acid content. The acid doping of PBI polymer resulted in the protonation of the imidazole group, which caused this Raman blue shift and increase of intensity. As shown in Fig. 2.2(b), once the protonation reached saturation (two per repeating unit of PBI), the intensity of the band became constant as confirmed by plotting the ratio of the band intensity of 1570 cm 1 to that of 1000 cm 1 against the acid-doping level [27].
Figure 2.2 (a) Raman spectra of pristine and phosphoric acid-doped polybenzimidazole and (b) ratio of relative intensity versus acid-doping level [27].
40 Chapter 2
4.3 Polymeric Composite Membrane With Additives/Fillers Over the last decade, composite membranes consist of organic or inorganic fillers/additives presenting an interesting approach for improving the physical and chemical properties of the polymeric membranes. Many scientific efforts have also been undertaken for the fabrication of multifunctional polymer nanocomposites that take advantage of the unique combination of mechanical, electrical, and thermal properties of the fillers such as carbon nanotubes and zeolites. During the composite membrane preparation, the routine determination of the quantitative cross-sectional additive/fillers concentration profile is of great importance as this information is especially valuable to adjust formulation and processing parameters to obtain reproducible, reliable, and efficient performance. To date, a substantial number of studies have been devoted to study the secondary effects of the additive or fillers introduced to the polymer dope on the membrane structure. The characteristics in terms of morphology, surface properties, pore size, and porosity can be eventually related to the membrane performance such as permeability/flux and fouling resistance. The study on the distribution of additive across the whole thickness of a polymeric membrane should is essential for the design of a membrane with desired properties. Dufour et al. [15] studied the polyvinylpyrrolidone (PVP) concentration profile along the cross-section radial axis of an UF hollow fiber made from a blend of PVDF as polymer matrix and PVP as additive using the analytical method by the spectra peak intensity ratio calculation. It was found that, Raman spectroscopy technique easily enabled the achievement of point-by-point mapping along the cross-section of PVDF hollow fiber membrane owing its high spatial resolution. As shown in Fig. 2.3, two regions were chosen: 920e955 cm 1 and 783e805 cm 1 for PVP and PVDF, respectively. A straightforward dependence of the concentration with the peak intensity ratio was found with R2 value of 0.950. The PVP mass fraction for each point of the membrane crosssection was then determined from the calibration curve. For the characterization of fillers in composite polymeric membranes, Raman imaging [28] allows the direct characterization of the filler dispersion and loading in the polymer matrix by observation of the distribution of the intensity of the filler bands on the composite’s surface. Raman spectroscopy is also known to provide information on the structure, crystallinity, and aging of both components of a composite material by evaluation of the associated vibrational features (band frequencies and widths). The changes observed in the band spectral features of the fillers can serve as indicators of their dispersion/loading characterization.
4.4 Polymeric Membrane Antifouling Strategy SERS has been applied to study the chemical variation in single-species biofilms in different growth phases, from initial attachment to mature biofilms [29,30]. Chen et al.
Raman Spectroscopy 41
Figure 2.3 Raman spectra of polyvinylpyrrolidone (PVP) and polyvinylidene fluoride (PVDF). Regions of 920e955 cm 1 and 783e805 cm 1 were chosen for PVP and PVDF, respectively to obtain the peak intensity ratio [15].
monitored the development of a dual species biofilm formed by two model bacteria (Brevundimonas diminuta or BD and Staphylococcus aureus or SA) on a mixed cellulose ester membrane surface [31]. The extent of membrane biofouling was also monitored by plotting SERS peak intensity against culture time as shown in Fig. 2.4. SERS is a promising nondestructive technique for characterization of interfaces between polymers and active substrate without interference by normal Raman scattering from the bulk of the polymer. SERS, combined with multivariate analysis, has been successfully used to discriminate bacteria down to species and even strain levels [32]. The dynamic changes in dominant species within the biofilm as the function of culture time can be characterized based on the highly distinguishable SERS features of BD and SA as well as a semiquantitative analysis of SERS. Furthermore, bacterial attachment on the membrane as early as 1 h was detected by SERS, demonstrating its high sensitivity and capability for early diagnosis of biofouling. The findings implied that SERS can be applied to provide insights into interspecies interactions in biofouling development and help the development of antifouling strategies. The use of SERS for possible foulant identification offers a rapid, cost-effective, and portable tool for onsite analysis of membrane surfaces. Lately, Lamsal et al. used SERS
42 Chapter 2
Figure 2.4 Surface-enhanced Raman spectroscopy spectra of biofilms formed by BD and SA and their mixture (BDSA) as the function of culture times [31].
and normal Raman spectroscopy to examine fouling caused by natural organic matter on a nanofiltration membrane [33]. It was observed that, the virgin, fouled, and cleaned membranes exhibited markedly different SERS spectra (Fig. 2.5), which indicated that the foulant layer on the polyamide membrane could be monitored. The observed SERS peak of fouled membrane at wavenumber 1543 cm 1 suggested the presence of protein meanwhile the peaks in the range of 1200e1400 cm 1 indicated the possible presence of carbohydrate on the membrane surface. These results demonstrated that SERS is capable of identifying the functional groups of organics involved in NF membrane fouling in water treatment that can favorably lead to strategies for membrane fouling mitigation.
5. Conclusion Raman spectroscopy is a powerful nondestructive technique that has found promising application for polymeric materials analysis. Although the installation of Raman spectroscopy equipment might require a higher initial investment, as discussed earlier, it is
Raman Spectroscopy 43
Figure 2.5 Surface-enhanced Raman spectroscopy spectra of virgin, fouled, and cleaned membrane [33].
increasingly useful due to the ease of sample preparation, less susceptibility to aqueous solutions, and generation of easily interpreted spectra for identification of the polymeric materials. When polymeric membrane is concerned, it has been well agreed that Raman spectroscopy is a feasible approach to provide further insight into the polymer structure and interface properties. The characterization results can in turn contribute to valuable understanding of chemical and physical properties for further development of advanced and novel polymeric membranes. The recent advancement made in the field of Raman spectroscopy in both techniques and instrumentations is expected to create more scientific interest in this characterization tool. In summary, several key factors are contributing to promising future of Raman, i.e., (i) recent advances in instrumentation such as compact diode lasers can possibly reduce the cost of the instrument; (ii) recent publications that dealt with Raman spectroscopy could promote its potential for the characterization of organic compounds; and (iii) advanced and enhanced modes of Raman spectroscopy such as SERS and micro-Raman are expected to close the gaps to large scale, reproducible fabrication, opening the door to industrial applications. Despite the advantages demonstrated, Raman spectroscopy technique also suffers from several limitations and drawbacks. Optical clarity and fluorescence can be one of the major issues, especially with impure and chemically complex samples. Another problem that is specific to polarization studies is the scrambling caused by multiple scattering in a
44 Chapter 2 heterogeneous system. Besides that, it is also found that Raman method is the difficulty of performing quantitative measurements as the Raman spectrum is usually sensitive only to chain conformation and insensitive to the lateral order of the crystalline phase. Put that aside, the continuing improvements made in the context of automated computerized system, advancements in lasers and detector, and the extensive spectral libraries are expected to increase the reliability, applicability, and convenience of Raman investigation. It is clear that Raman spectroscopy is extremely versatile and can tackle a diverse range of analytical problems. In the future, there will be a massive expansion of the applications of the Raman method for the analysis of polymeric materials, particularly polymeric membranes.
List of Abbreviation BD MEA NMP PBI PEFC PEI PP PVDF PVP SA SERS TERS
Brevundimonas diminuta Membrane electrode assembly N-methylpyrrolidone Polybenzimidazole Proton exchange fuel cell Poly(ether-imide) Polypropylene Polyvinylidene fluoride Polyvinylpyrrolidone Staphylococcus aureus Surface-enhanced Raman spectroscopy Tip-enhanced Raman spectroscopy
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Raman Spectroscopy 45 [5] Xue G. Fourier transform Raman spectroscopy and its application for the analysis of polymeric materials. Prog Polym Sci 1997;22:313e406. [6] Mauricio-Iglesias M, Guillard V, Gontard N, Peyron S. Raman depth-profiling characterization of a migrant diffusion in a polymer. J Membr Sci 2011;375:165e71. [7] Brun N, Chevrel M-C, Falk L, Hoppe S, Durand A, Chapron D, Bourson P. In situ monitoring of styrene polymerization using Raman spectroscopy. Chem Eng Technol 2014;37:275e82. [8] Wook Jo B, Hyun Ahn K, Wook Jo B, Hyun Ahn K. J Polym Sci Part B: Polym Phys 2014;52:848e57. [9] Brenner TJK, Li Z, McNeill CR. Phase-dependent photocurrent generation in polymer/fullerene bulk heterojunction solar cells. J Phys Chem C 2011;115(44):22075e83. [10] Lyon LA, Keating CD, Fox AP, Baker BE, He L, Nicewarner SR, Mulvaney SP, Natan MJ. Raman spectroscopy. Anal Chem 1998;70:341Re61R. [11] Khafagy RM. In situ FT-Raman spectroscopic study of the conformational changes occurring in isotactic polypropylene during its melting and crystallization processes. J Polym Sci Part B Polym Phys 2006;44:2173e82. [12] Brookes A, Dyke JM, Hendra PJ, Meehan S. The FT-Raman spectroscopic study of polymers at temperatures in excess of 200 C. Spectrochim Acta A 1997;53:2313e21. [13] Nielsen AS, Batchelder DN, Pyrz R. Estimation of crystallinity of isotactic polypropylene using Raman spectroscopy. Polymer 2002;43:2671e6. [14] Khulbe KC, Kruczek B, Chowdhury G, Gagn S, Matsuura T, Verma SP. Characterization of membranes prepared from PPO by Raman scattering and atomic force microscopy. J Membr Sci 1996;111:57e70. [15] Dufour E, Gassara S, Petit E, Pochat-Bohatier C, Deratani A. Quantitative PVP mapping in PVDF hollow fiber membranes by using Raman spectroscopy coupled with spectral chemiometrics analysis. Eur Phys J Spec Top 2015;224:1911e9. [16] Morita S, Kitagawa K. Hydration structure of a Nafion membrane in a polymer electrolyte fuel cell, 46th AIAA/ASME/SAE/ASEE, joint propulsion conference & exhibit. July 25e28, 2010. Nashville, TN. [17] Menut P, Su YS, Chinpa W, Pochat-Bohatier C. A top surface liquid layer during membrane formation using vapor-induced phase separation (VIPS)devidence and mechanism. J Membr Sci 2008;310:278e88. [18] Lu X, Shah P, Maruf S, Ortiz S, Hoffard T, Pellegrino J. Forensic analysis of degraded polypropylene hollow fibers utilized in microfiltration. J Appl Polym Sci 2015;132:41553e64. [19] Ovstrovskii D, Torell LM, Paronen M, Hietala S, Sundholm F. State of water in sulfonated poly(vinyl fluoride) membranes: an FTIR study. Solid State Ionics 1997;97:315e21. [20] Gruger A, Regis A, Schmatko T, Colomban P. Nanostructure of Nafion membrane at different state of hydratin. An IR and Raman study. Vib Spectrosc 2001;26:215e25. [21] Innocent C, Huguet P, Bribes JL, Pourcelly G, Kameche M. Characterisation of cation exchange membrane in hydro-organic media by electrochemistry and Raman spectroscopy. Phys Chem Chem Phys 2001;3:1481e5. [22] Chikvaidze G, Gabrusenoks J, Kleperis J, Vaivars G. Application of micro Raman spectroscopy to industrial FC membranes. J Phys Conf Ser 2007;93:012026. [23] Holber M, Johansson P, Jacobsson P. Raman spectroscopy of an aged low temperature polymer electrolyte fuel cell membrane. Fuel Cells 2011;11:459e64. [24] Zeis R. Materials and characterization techniques for high-temperature polymer electrolyte membrane fuel cells. Beilstein J Nanotechnol 2015;6:68e83. [25] Conti F, Majerus A, Di Noto V, Korte C, Lehnert W, Stolten D. Raman study of the polybenzimidazoleephosphoric acid interactions in membranes for fuel cells. Phys Chem Chem Phys 2012;14:10022e6. [26] Giffin GA, Conti F, Lavina S, Majerus A, Pace G, Korte C, Lehnert W, Di Noto V. Int J Hydrogen Energy 2014;39:2776e84. [27] Daletou MK, Geormezi M, Vogli E, Voyiatzis GA, Neophytides SG. The interaction of H3PO4 and steam with PBI and TPS polymeric membranes. A TGA and Raman study. J Mater. Chem A 2014;2:1117e27.
46 Chapter 2 [28] Bounos G, Andrikopoulos KS. Evaluation of multi-walled carbon nanotube concentrations in polymer nanocomposites by Raman spectroscopy. Carbon 2014;76. [29] Chao YQ, Zhang T. Surface-enhanced Raman scattering (SERS) revealing chemical variation during biofilm formation: from initial attachment to mature biofilm. Anal Bioanal Chem 2012;404:1465e75. [30] Ivleva NP, Wagner M, Szkola A, Horn H, Niessner R, Haisch C. Label-free in situ SERS imaging of biofilms. J Phys Chem B 2010;114:10184e94. [31] Chen P, Cui L, Zhang K. Surface-enhanced Raman spectroscopy monitoring the development of dualspecies biofouling on membrane surfaces. J Membr Sci 2015;473:36e44. [32] Patel IS, Premasiri WR, Moir DT, Ziegler LD. Barcoding bacterial cells: a SERS-based methodology for pathogen identification. J Raman Spectrosc 2008;39:1660e72. [33] Lamsal R. Use of surface enhanced Raman spectroscopy for studying fouling on nanofiltration membrane. Sep Purif Technol 2012;96:7e11.
Raman Spectroscopy P.S. Goh, A.F. Ismail, B.C. Ng Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia
Chapter Outline 1. 2. 3. 4.
Introduction 31 Principle of Raman Spectroscopy 33 Raman Spectroscopy for Polymer Characterization 35 Raman Spectroscopy for Polymeric Membrane Characterization 4.1 4.2 4.3 4.4
Polymeric Polymeric Polymeric Polymeric
Membrane Formation 37 Fuel Cell Membranes 38 Composite Membrane With Additives/Fillers Membrane Antifouling Strategy 40
5. Conclusion 42 List of Abbreviation References 44
36
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1. Introduction Raman spectroscopy is a technique used to observe vibrational, rotational, and other low-frequency modes. It is a complimentary method to infrared spectroscopy to obtain information on crystalline structure of the macromolecules and change of polymeric structure in membrane. In brief, this technique relies on the inelastic Raman scattering of monochromatic laser light that ranges from visible to near ultraviolet. The laser light interacts with molecular vibrations, phonons, or other excitations to result in energy changes of the laser photons where the shift up or down in the energy gives information about the vibrational modes. Raman spectroscopy has been commonly used in the field of material sciences and chemistry to provide the fingerprint to allow direct identification and intepretation of different molecules. In fact, this technique can be suitably applied for nondestructive, microscopic, chemical analysis, and imaging characterizations. One of the most significant advantages of Raman spectroscopy is the ability to provide key information easily and quickly with minimum sampling handling issues. Membrane Characterization. http://dx.doi.org/10.1016/B978-0-444-63776-5.00002-4 Copyright © 2017 Elsevier B.V. All rights reserved.
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32 Chapter 2 Raman spectroscopy is known to provide valuable information in polymer characterization. This technique has been widely used to determine the functional groups and group structure of the polymers; conformation and orientation of the macromolecule chains and to monitor the changes in the polymer structural properties upon exposure to environmental or mechanical stresses [1,2]. Study of the crystalline structure of polymers will add to the existing knowledge of intramolecular forces in crystals and their effect on stable polymer structures. The main advantage in polymer characterization using Raman spectroscopy is the capability of the Raman bands to relate to chemical structure of polymeric materials based on the fundamental modes measurement. Raman spectroscopy is very useful to determine the functional groups, and group structure, conformation and orientation of the chains and to follow changes in the structural parameters as the polymers are exposed to environmental or mechanical stresses. In addition, modification to the surrounding field of the molecular unit can also be effectively studied through Raman spectroscopy. For instance, a polyvinylidene fluoride (PVDF) based materials can present many types of molecular and crystal structures depending on their preparation conditions. As such, Raman spectra can be used to distinguish the different crystalline forms of PVDF and the presence of polymer chain defects. In term of polymeric membrane characterization, Raman spectra provide information, in both qualitative and quantitative, about the (i) inter and intramolecular interactions among the functional groups that present in the polymer macromolecules, (ii) crystalline structure of the macromolecules in the membrane, (iii) change of polymer structure, during membrane formation, (iv) structural change of the polymeric membrane upon heating, and (v) interfacial properties of the composite or coated membrane. Besides that, this vibrational technique also offers a reliable solution in tackling qualitative and quantitative analytical problems present in the studies of asymmetric polymeric membranes. It has been applied to study the composition and distribution of the particles of composite membranes that consist of inorganic particles incorporated in a polymer matrix. Interestingly, when Raman spectroscopy is combined with electron microscopy, it can be used to offer better understanding of the complex mechanism involved in the inversion phase process as well as to determine the polymer chain structure in different areas across the membrane [2]. Over the last decades, remarkable technical improvement has been done in Raman spectroscopy to tackle issues such as fluorescence, poor sensitivity, or reproducibility to enhance the reliability of this technique. Additionally, the further improvements in Raman instruments such as lasers, monochromators, and detectors are helpful to reduce the complexity and cost of this technique as well as broadening the application of Raman spectroscopy for a wide range of applications. This chapter presents the principle of Raman spectroscopy and followed by the applications of Raman spectroscopy for the characterization of polymer materials, particularly polymeric membranes. It is expected that, the use of Raman spectroscopy will help to understand the relationship between
Raman Spectroscopy 33 structure and transport properties, which could have an important impact on designing membranes for specific scientific problems.
2. Principle of Raman Spectroscopy Raman spectroscopy has been used to study the interactions of the vibrational energies of atoms or groups of atoms within molecules. The Raman effect is produced through the exchange of energy between incident photons and the vibrational energy levels of the molecule [3]. Raman scattering is the inelastic scattering of laser light impinged onto the sample and is scattered with a transfer of energy between the excitation light and the sample. Raman scattering occurs when the molecular motion produces a change in the polarizability of the molecule. In principle, the scattered light can be categorized as antiStokes or Stokes depending on their energy level: The former has higher energy, whereas the latter has lower energy compared to the illumination light. In practice, Raman microscopy is almost exclusively based on the analysis of Stokes-scattered light that exhibits much higher intensity. The light scattered by the molecules contains frequencies different from the incident monochromatic light that correspond to the normal vibrational frequencies of the molecules. When a molecule absorbs radiations, its energy increases in proportion to the photon. The increased energy may be at the level of the electronic, vibrational, or rotational energy of the molecule [4]. The Raman spectra are then generated from the vibrational motions that resulted from the change in a source-induced molecular dipole moment. Fig. 2.1 illustrates the process involved in collecting Raman spectra [3]. As depicted, Raman spectrometer is typically composed of light source, monochromator, sample holder, and detector. The Raman scattered light is dispersed according to wavelength and processed as Raman spectra. The factors that affect the quality of Raman spectra are high signal-to-noise ratio, instrument stability, and sufficient resolution. Recently, the advancement in near infrared or red excitation lasers could tackle this issue by eliminating the fluorescence that detrimentally affect the Raman signals. Normally, the magnitudes of the Raman shifts in frequency can be corresponded to those of infrared absorptions. However, some vibrational modes appear only in the Raman spectrum. Particularly, the more symmetric the molecule is, the greater the differences that can be observed between its Raman and infrared spectra. Generally, for chain-like polymer molecules, vibrations of the carbon chain are easily studied by using Raman spectroscopy, whereas the vibrations of the branches are more feasibly characterized by using infrared spectroscopy [5]. Also, it is found that, infrared spectra demonstrate weak absorptions for molecular vibrations that result in intense Raman peaks and vice versa. The uniqueness of the vibrational patterns of Raman and IR spectroscopy can be complementarily used in the measurement of the structure of molecules to the greatest extent. Although it is well acknowledged that, the infrared and Raman spectra of a molecule complement each other
34 Chapter 2 Dispersive Raman Spectroscopy
Wavenumber Selector
Sample
Detector (PMT/CCD)
Radiation Source
Laser Source (NIR)
F-T Raman Spectroscopy
Sample Beam splitters Moving Mirror
Detector
Stationary Mirror
Figure 2.1 Schematic for process involved in Raman spectra collection [3].
where the complete information regarding the vibrational spectrum of a molecule always requires both infrared and Raman vibration, however, when compared to infrared technique, Raman spectroscopy possesses some advantages: Raman spectroscopy involves a scattering process where it does not have the problems associated with light transmission hence samples of any size or shape can be examined at low frequency region. Besides that, Raman spectra also offers greater information about the skeletal movements of the molecule. In terms of sample preparation, Raman spectra can be obtained directly from a wide range of materials in the form of bulk solids, liquids, tablets, polymers, and paper with minimum or no sample preparation. Several modes of Raman spectroscopy have been established to improve the spatial resolution, enhance sensitivity, and acquire more specific information. One of the advancement in Raman spectroscopy is the development of surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy that have significantly increased the measured signal. Another interesting feature of Raman spectroscopy is that it can be readily adapted to microscopy as the excitation light can be chosen to be in the visible or near infrared spectral range [6]. This advantage allows the usage of high quality objectives. Some of the commonly adapted analysis methods are scanning probe microscopy, scanning electron
Raman Spectroscopy 35 microscope and confocal laser scanning microscopy. In a natural extension of the Raman technique, the laser beam is focused to provide sample illumination on the micron scale and Raman scattering is collected with the instrument hardware to produce spectra of samples with a spatial resolution approaching 1e10 mm. The discrete sample sites can be examined for multiple Raman spectra collection to accomplish the mapping of a sample surface to provide structural data on a microscopic scale. The advancement of Raman microscope in terms of the designs and microscopic approaches enables the utilization of imaging and microscopy for the investigation of polymer surface structure, or the defect and contaminant analysis of coatings, polymer films, and laminates.
3. Raman Spectroscopy for Polymer Characterization Raman spectroscopy is a powerful qualitative and quantitative tool with some particular advantages for the analysis of polymers. One of the main advantages in polymer characterization using Raman spectroscopy is the ease of obtaining a good spectrum with minimum sample handling. It finds numerous applications in polymer chemistry, polymer material, and composite, for example, in applications such as monitoring of polymerization reaction, prediction of physical properties, and mapping of polymeric phases [7e9]. Table 2.1 summarizes the important information obtained from Raman spectra for polymeric materials characterization. Raman spectroscopy exhibits high sensitivity for the measurement of localized polymer structures in which it allows the analysis of extremely small samples with individual segmental orientations and provides detailed interpretation of the structural conformational distribution in both ordered and disordered forms. Disordered polymers possess vibrational modes that can be easily detected by Raman [10]. An important area of research in polymer studies involves the investigation of the influences on the molecular orientation Table 2.1: Important information obtained from Raman spectra for polymeric materials characterization. Properties Chemical structure and composition Stereo-order Conformational order
State of order Orientation
Surface and interface structures
Details Structural units, end groups, type, and degree of branching, additives cisetrans isomerism and stereoregularity Physical arrangement of polymer chains both in the solid and in the melt states, e.g., planar zigzag or helix conformations. Crystalline, mesomorphous, and amorphous phases; intermolecular forces; lamellar thickness Raman depolarization and infrared dichroism of preferential polymer chains and side group alignments in anisotropic polymers Interfacial properties of composite materials
36 Chapter 2 induced by external forces, drawing on the physical and mechanical properties of a polymer. Local structural effects that can be determined through Raman spectroscopy include the amount of remaining amorphous content and type of order induced by processing, defects in structure, and in some cases tacticity [4]. It is known that polymer can exist in a number of configurations, i.e., syndiotactic and isotactic or helical and planar. Sometimes, they can be atactic with an ordered conformation. The feasibility of Raman spectroscopy for polymer characterization was first explored to investigate the skeletal bending modes of polypropylene (PP), which appeared in the low frequency [11]. The syndiotactic and isotactic sequences in characterizing the overall chain configuration of PP can be distinguished from the Raman spectra. High temperature Raman spectroscopy has been carried out to observe the structural and morphological changes occurring during the deformation of PP at elevating temperature [12]. Since then, it has been widely used to investigate the regularity and local structures in various kinds of polymers including polyethylene, polyethylene terephthalate, polystyrene, and polytetrafluoroethylene [13]. Raman spectroscopy is particularly sensitive toward the modification to the surrounding field of the molecular unit such as hydrogen bonding, crystal field splitting, chain packing, and salvation. Additionally, it is also very sensitive to the thermal history of the polymer. It is known that hydrogen bonding and the changes in the chain conformation directly affect the electronic structure of the molecular unit meanwhile crystal field splitting, chain packing, and salvation interact with the molecular unit by coupling external electric fields with electronic structure of the molecular unit. Both types of interactions affect the spring constants and the effective masses of the molecular unit, thus leading to shift in the normal mode frequencies and line shapes [14]. Raman polarization studies offer detailed information about the orientation distribution of structural units for both crystalline and noncrystalline regions. Polymers can be classified based on their crystallinity, i.e., amorphous and crystalline states that depend on the molecular weight of the polymer. The time taken for the polymer to crystallize from solution dictates the formation of a crystalline region. In general, polymer with low molecular weight polymer contains higher content of crystallites compared to that of high molecular weight polymer. Various morphologies are possible between a completely crystalline and completely amorphous conformation. Crystalline polymers have a number of morphological features that can be studied by Raman spectroscopy. For instance, many phase transitions can be monitored by following the changes in the Raman spectra to associate with the crystalline structure [4].
4. Raman Spectroscopy for Polymeric Membrane Characterization Raman spectroscopy is a promising technique to obtain information on crystalline structure of the macromolecules and observe the change of structure in a polymeric
Raman Spectroscopy 37 membrane. There are three important structural levels in the polymeric membrane, i.e., (i) the molecular level that represents the chemical nature of the polymer. The molecular level responsible for the microcrystalline nature and it is characterized by polar, steric, and ionic factors; (ii) the microcrystalline, which affects both the transport and the mechanical properties of membrane; and (iii) the colloidal level that is related to the aggregation of macromolecules. It governs the statistics of pores that includes the pore size, pore size distribution, pore density, and void volume. Raman spectroscopy is applicable for the investigation at each level to achieve more rigorous understanding of the polymeric structure in the membrane [4].
4.1 Polymeric Membrane Formation As polymeric membranes can be processed through the casting of solutions by doctor blade to produce the flat sheet membrane or via spinning to produce the hollow fiber membranes, it is generally agreed that some orientations, either in plane or out of plane, will be induced during the processes. The changes in the molecular environment may trigger the Raman effects in several ways. As the bulk properties of a polymer can be directly related to its morphology, the understanding and controls of the morphology are technologically essential. It is also expected that by establishing the relationship between membrane morphology and its transport behavior the mechanism of mass transport in the membrane can be revealed and understood. Particularly, the polymer morphology in the selective skin layer has been carefully controlled and fine tuned during the design of a synthetic polymeric membrane. Many attempts have been made to establish the cause and effect relationship between membrane preparation, polymer morphology, and membrane performance. When the membrane is subjected to Raman spectroscopy analysis, the orientation of polymer chain and the degree of crystallinity of the polymer in the membrane, which represent the morphology of the membrane, can be systematically studied. Besides that, the dynamic features of Raman spectroscopy also enable the monitoring of the morphology change during membrane formation [15]. Micro-Raman spectroscopy with high spatial resolution has been used to locally investigate and determine the chemical composition [16]. The technique has also been applied to membrane characterization and membrane formation. Raman confocal spectroscopy has been applied to provide quantitative information on the formation of a surface liquid layer on the top of the membrane made of poly(ether-imide) (PEI) and Nmethylpyrrolidone (NMP) during the formation via water vaporeinduced phase inversion [17]. Changes in the film thickness and in the NMP/PEI ratio at the film surface were determined by using Raman confocal spectroscopy during membrane forming in dried and humid air (relative humidityw50%). The results obtained showed that when the cast film was exposed to humid air, the polymer concentration at the film surface first decreased and subsequently increased. It was found that Raman spectroscopy not only provided evidence
38 Chapter 2 supporting the existence of a surface liquid layer but also provided quantitative information about the composition in the layer. Very recently, the chemical changes and oxidative degradation mechanisms of PP microfiltration membranes were examined at the micron scale by using cross-sectional analysis with micro-Raman spectroscopy [18]. New peaks were observed at w1740 cm 1 and w3450 cm 1 that corresponded to carbonyl eC]O aldehyde stretching groups and eOH bond, respectively, for all the degraded samples.
4.2 Polymeric Fuel Cell Membranes Raman spectroscopy is a powerful tool for investigation of molecular interactions at a functional group level hence it has been widely used to characterize Nafion membrane for fuel cell applications [19e21]. It has been pointed out that water molecules in the polymer electrolyte membrane play important roles in the proton transfer for the proton exchange fuel cell (PEFC) operation since the existence of hydrated proton during the proton transfer in the membrane [16]. Hydration structure of a Nafion membrane in a PEFC was explored at a functional group level to study the eOH stretching modes and the intensity variation of sulfonate symmetric stretching mode as a function of the alkali ions. It has also been used to characterize the nature of the proton exchange sites during treatment in acidic/alkali environment that plays a crucial role in the performance of fuel cells. The protonation of commercial Fumapem and Fumasep membranes was confirmed through the new vibration peaks that appeared in the regions 3700e3800 and 1600e1700 cm 1 and the disappearance of peak at 1679 cm 1 after protonation process [22]. It is known that the membrane electrode assembly (MEA) durability in a fuel cell is closely dependent on the operating conditions such as temperature, gas humidity, and load dynamics. The harsh working environment could result in both chemical and mechanical degradation of the ion-conducting membrane. Chemical degradation of the membrane can decrease the proton conductivity and weaken the mechanical stability of the membrane and subsequently lead to the operation failure. Thus, avoiding degradation is important for reaching the goals in terms of life-time for wide commercialization. When coupled with a confocal microscope, Raman spectroscopy has the advantage of measuring molecular composition and the measurement can be localized to a micrometer size volume. By using confocal Raman spectroscopy they observe a lower concentration of sulfonic acid groups. Measurements can be applied to MEA in any size and shape [23]. In general, the Raman active bonds in the membrane are either due to the backbone; consisting of CeC and CeF bonds, or the active side chain groups; also this part with CeC and CeF bonds, but furthermore also having Raman signals arising from CeOeC, CeS, and SeO vibrations [20]. There are mainly two different molecular degradation mechanisms of interest; one where the hydrophilic sulfonic acid end groups (branches) are broken away from the
Raman Spectroscopy 39 backbone and a second where the backbone is broken into shorter chains. In the study conducted by Holber et al. to study the membrane aging, the CeF vibrational mode at 731 cm 1 was used as the standard for both the pristine reference and the aged membrane due to the strong nature of this vibration and the large amount of CeF bonds in the material [23]. From the spectra obtained, the loss of sulfonic acid end groups was observed as a decrease of intensity in the CeOeC, CeS, and SeO signals compared to the pristine reference. A conventional high-temperature membrane electrode assembly primarily consists of a polybenzimidazole (PBI)-type membrane containing phosphoric acid. Raman spectroscopy has been used to study the effects of acid doping on PBI-type polymers due to its high sensitivity toward molecular structural changes that occur during the acidebase proton exchange reaction [24]. Additionally, it is also sensitive to molecular interactions between membrane components. Raman spectra of pristine and acid-doped PBI materials with different degrees of doping have been obtained for further band assignments as depicted in Fig. 2.2(a) [25e27]. The Raman band at 1000 cm 1 was assigned to the meta-benzene ring vibration that showed minimum changes despite the increasing acid content in the membrane. In contrast, the band at 1539 cm 1, which is associated with the symmetric stretch of the imidazole group, became stronger and shifted toward 1570 cm 1 with increasing acid content. The acid doping of PBI polymer resulted in the protonation of the imidazole group, which caused this Raman blue shift and increase of intensity. As shown in Fig. 2.2(b), once the protonation reached saturation (two per repeating unit of PBI), the intensity of the band became constant as confirmed by plotting the ratio of the band intensity of 1570 cm 1 to that of 1000 cm 1 against the acid-doping level [27].
Figure 2.2 (a) Raman spectra of pristine and phosphoric acid-doped polybenzimidazole and (b) ratio of relative intensity versus acid-doping level [27].
40 Chapter 2
4.3 Polymeric Composite Membrane With Additives/Fillers Over the last decade, composite membranes consist of organic or inorganic fillers/additives presenting an interesting approach for improving the physical and chemical properties of the polymeric membranes. Many scientific efforts have also been undertaken for the fabrication of multifunctional polymer nanocomposites that take advantage of the unique combination of mechanical, electrical, and thermal properties of the fillers such as carbon nanotubes and zeolites. During the composite membrane preparation, the routine determination of the quantitative cross-sectional additive/fillers concentration profile is of great importance as this information is especially valuable to adjust formulation and processing parameters to obtain reproducible, reliable, and efficient performance. To date, a substantial number of studies have been devoted to study the secondary effects of the additive or fillers introduced to the polymer dope on the membrane structure. The characteristics in terms of morphology, surface properties, pore size, and porosity can be eventually related to the membrane performance such as permeability/flux and fouling resistance. The study on the distribution of additive across the whole thickness of a polymeric membrane should is essential for the design of a membrane with desired properties. Dufour et al. [15] studied the polyvinylpyrrolidone (PVP) concentration profile along the cross-section radial axis of an UF hollow fiber made from a blend of PVDF as polymer matrix and PVP as additive using the analytical method by the spectra peak intensity ratio calculation. It was found that, Raman spectroscopy technique easily enabled the achievement of point-by-point mapping along the cross-section of PVDF hollow fiber membrane owing its high spatial resolution. As shown in Fig. 2.3, two regions were chosen: 920e955 cm 1 and 783e805 cm 1 for PVP and PVDF, respectively. A straightforward dependence of the concentration with the peak intensity ratio was found with R2 value of 0.950. The PVP mass fraction for each point of the membrane crosssection was then determined from the calibration curve. For the characterization of fillers in composite polymeric membranes, Raman imaging [28] allows the direct characterization of the filler dispersion and loading in the polymer matrix by observation of the distribution of the intensity of the filler bands on the composite’s surface. Raman spectroscopy is also known to provide information on the structure, crystallinity, and aging of both components of a composite material by evaluation of the associated vibrational features (band frequencies and widths). The changes observed in the band spectral features of the fillers can serve as indicators of their dispersion/loading characterization.
4.4 Polymeric Membrane Antifouling Strategy SERS has been applied to study the chemical variation in single-species biofilms in different growth phases, from initial attachment to mature biofilms [29,30]. Chen et al.
Raman Spectroscopy 41
Figure 2.3 Raman spectra of polyvinylpyrrolidone (PVP) and polyvinylidene fluoride (PVDF). Regions of 920e955 cm 1 and 783e805 cm 1 were chosen for PVP and PVDF, respectively to obtain the peak intensity ratio [15].
monitored the development of a dual species biofilm formed by two model bacteria (Brevundimonas diminuta or BD and Staphylococcus aureus or SA) on a mixed cellulose ester membrane surface [31]. The extent of membrane biofouling was also monitored by plotting SERS peak intensity against culture time as shown in Fig. 2.4. SERS is a promising nondestructive technique for characterization of interfaces between polymers and active substrate without interference by normal Raman scattering from the bulk of the polymer. SERS, combined with multivariate analysis, has been successfully used to discriminate bacteria down to species and even strain levels [32]. The dynamic changes in dominant species within the biofilm as the function of culture time can be characterized based on the highly distinguishable SERS features of BD and SA as well as a semiquantitative analysis of SERS. Furthermore, bacterial attachment on the membrane as early as 1 h was detected by SERS, demonstrating its high sensitivity and capability for early diagnosis of biofouling. The findings implied that SERS can be applied to provide insights into interspecies interactions in biofouling development and help the development of antifouling strategies. The use of SERS for possible foulant identification offers a rapid, cost-effective, and portable tool for onsite analysis of membrane surfaces. Lately, Lamsal et al. used SERS
42 Chapter 2
Figure 2.4 Surface-enhanced Raman spectroscopy spectra of biofilms formed by BD and SA and their mixture (BDSA) as the function of culture times [31].
and normal Raman spectroscopy to examine fouling caused by natural organic matter on a nanofiltration membrane [33]. It was observed that, the virgin, fouled, and cleaned membranes exhibited markedly different SERS spectra (Fig. 2.5), which indicated that the foulant layer on the polyamide membrane could be monitored. The observed SERS peak of fouled membrane at wavenumber 1543 cm 1 suggested the presence of protein meanwhile the peaks in the range of 1200e1400 cm 1 indicated the possible presence of carbohydrate on the membrane surface. These results demonstrated that SERS is capable of identifying the functional groups of organics involved in NF membrane fouling in water treatment that can favorably lead to strategies for membrane fouling mitigation.
5. Conclusion Raman spectroscopy is a powerful nondestructive technique that has found promising application for polymeric materials analysis. Although the installation of Raman spectroscopy equipment might require a higher initial investment, as discussed earlier, it is
Raman Spectroscopy 43
Figure 2.5 Surface-enhanced Raman spectroscopy spectra of virgin, fouled, and cleaned membrane [33].
increasingly useful due to the ease of sample preparation, less susceptibility to aqueous solutions, and generation of easily interpreted spectra for identification of the polymeric materials. When polymeric membrane is concerned, it has been well agreed that Raman spectroscopy is a feasible approach to provide further insight into the polymer structure and interface properties. The characterization results can in turn contribute to valuable understanding of chemical and physical properties for further development of advanced and novel polymeric membranes. The recent advancement made in the field of Raman spectroscopy in both techniques and instrumentations is expected to create more scientific interest in this characterization tool. In summary, several key factors are contributing to promising future of Raman, i.e., (i) recent advances in instrumentation such as compact diode lasers can possibly reduce the cost of the instrument; (ii) recent publications that dealt with Raman spectroscopy could promote its potential for the characterization of organic compounds; and (iii) advanced and enhanced modes of Raman spectroscopy such as SERS and micro-Raman are expected to close the gaps to large scale, reproducible fabrication, opening the door to industrial applications. Despite the advantages demonstrated, Raman spectroscopy technique also suffers from several limitations and drawbacks. Optical clarity and fluorescence can be one of the major issues, especially with impure and chemically complex samples. Another problem that is specific to polarization studies is the scrambling caused by multiple scattering in a
44 Chapter 2 heterogeneous system. Besides that, it is also found that Raman method is the difficulty of performing quantitative measurements as the Raman spectrum is usually sensitive only to chain conformation and insensitive to the lateral order of the crystalline phase. Put that aside, the continuing improvements made in the context of automated computerized system, advancements in lasers and detector, and the extensive spectral libraries are expected to increase the reliability, applicability, and convenience of Raman investigation. It is clear that Raman spectroscopy is extremely versatile and can tackle a diverse range of analytical problems. In the future, there will be a massive expansion of the applications of the Raman method for the analysis of polymeric materials, particularly polymeric membranes.
List of Abbreviation BD MEA NMP PBI PEFC PEI PP PVDF PVP SA SERS TERS
Brevundimonas diminuta Membrane electrode assembly N-methylpyrrolidone Polybenzimidazole Proton exchange fuel cell Poly(ether-imide) Polypropylene Polyvinylidene fluoride Polyvinylpyrrolidone Staphylococcus aureus Surface-enhanced Raman spectroscopy Tip-enhanced Raman spectroscopy
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46 Chapter 2 [28] Bounos G, Andrikopoulos KS. Evaluation of multi-walled carbon nanotube concentrations in polymer nanocomposites by Raman spectroscopy. Carbon 2014;76. [29] Chao YQ, Zhang T. Surface-enhanced Raman scattering (SERS) revealing chemical variation during biofilm formation: from initial attachment to mature biofilm. Anal Bioanal Chem 2012;404:1465e75. [30] Ivleva NP, Wagner M, Szkola A, Horn H, Niessner R, Haisch C. Label-free in situ SERS imaging of biofilms. J Phys Chem B 2010;114:10184e94. [31] Chen P, Cui L, Zhang K. Surface-enhanced Raman spectroscopy monitoring the development of dualspecies biofouling on membrane surfaces. J Membr Sci 2015;473:36e44. [32] Patel IS, Premasiri WR, Moir DT, Ziegler LD. Barcoding bacterial cells: a SERS-based methodology for pathogen identification. J Raman Spectrosc 2008;39:1660e72. [33] Lamsal R. Use of surface enhanced Raman spectroscopy for studying fouling on nanofiltration membrane. Sep Purif Technol 2012;96:7e11.