Polystyrene, FTIR spectrum

Such a theoretical model has been quite successfully checked by Fourier transform infra-red spectroscopy (FTIR) on polystyrene [11,12]. Indeed, the FTIR spectrum of polystyrene contains vibrational lines associated with out-of-plane modes of the phenyl ring, whose position depends on the gauche or trans conformation of the main-chain C - C bonds. By recording FTIR spectra during sample stretching, it is possible to determine the amount of gauche or trans conformation as a function of strain. 

Figure 2.3 Effect of sample shape on PA-FTIR spectrum of polystyrene...

Figure 11.12. (a) Py-rGC trace for polystyrene, (b) FTIR spectrum of peak A identifying it as styrene (courtesy of P. J. Haines, Kingston University)... 

Soiirce of sample Polystyrene standard for FTIR spectroscopy Sample preparation None. ATR accessory used to run FTIR spectrum... 

Figure 4.1 FTIR spectrum of a thin film of polystyrene, The y-axis unit is %T the x-axis is in wavenumbers (cm- ).

FIGURE 4.19 An example of an FTIR spectrum for three different example polymers poly(vinyl pyridine) (P4VP), polystyrene, and a poly(vinyl pyridine)-block-polystyrene (PS-b-P4VP). In the data shown, we can clearly see that the block copolymer has characteristics of both of its individual polymer block components. The C=C stretch mode is visible at about 1500 cm" for all three molecules. 

Successful combination of a chromatographic procedure for separating and isolating additive components with an on-line method for obtaining the IR spectrum enables detailed compositional and structural information to be obtained in a relatively short time frame, as shown in the case of additives in PP [501], and of a plasticiser (DEHP) and an aromatic phenyl phosphate flame retardant in a PVC fabric [502], RPLC-TSP-FTIR with diffuse reflectance detection has been used for dye analysis [512], The HPLC-separated components were deposited as a series of concentrated spots on a moving tape. HPLC-TSP-FTIR has analysed polystyrene samples [513,514], The LC Transform has also been employed for the identification of a stain in carpet yarn [515] and a contaminant in a multiwire cable [516], HPLC-FTIR can be used to maintain consistency of raw materials or to characterise a performance difference. 

When only spectroscopic methods are used, they are able to identify polymer components with respect to their chemical nature. However, in many cases, they are unable to answer the question whether two chemical structures are combined to yield a copolymer or a blend or both. For example, analyzing a rubber mixture one is able to identify styrene and butadiene as the monomer units. However, using FTIR or NMR it is impossible to decide if the sample is a mixture of polystyrene (PS) and polybutadiene (PB),or a copolymer of styrene and butadiene, or a blend of a styrene-butadiene copolymer and PB. For the latter case, even the copolymer composition cannot be determined just by running a FTIR or NMR spectrum. 

IR spectrometers must be calibrated for wavelength accuracy. FTIRs are usually calibrated by the manufacturer and checked on installation. Wavelength calibration can be checked by the analyst by taking a spectrum of a thin film of polystyrene, which has well-defined absorption bands across the entire mid-IR region, as seen in Fig. 4.1. Polystyrene calibration standard films are generally supplied with an IR instrument or can be purchased from any instmment manufacturer. Recalibration of the spectrometer should be left to the instmment service engineer if required. 

FTIR Studies of Polymers For the most part, the techniques used in the measme-ment of FTIR are the same as those applied to conventional samples. Spectral features of a component in a polymer are isolated from the solvent bands and from aU other compounds present in the system. Analysis is carried out by computer programs. Figure 17.26 shows the spectrum of polystyrene with an IR microscope through a 10-pm aperture. We see that the baseline is flat even below 1000 cm . This is very important because the frequency range of the IR spectrum below 1300 cm especially that below 1000 cm , is known as the fingerprint region. Changes in frequency and intensity may be used to determine the microscopic characterization of the polymer to see if there exists any defect due to stress or the environment. 

Fig. 3. FTIR spectra, (a) Interferogram (b) FT spectrum of polystyrene plus background (c) spectrum of polystyrene after subtraction of background.

In rare instances the laser may fail to do its job, and incorrect wavenumbers will be measured. There is then a need for a calibration check on the wavenumbers to ensure that the laser is functioning properly. This is often done by measuring the infrared spectrum of a standard material such as polystyrene. In fact, when you buy an FTIR you are frequently provided with a polystyrene film mounted in a cardboard frame for this purpose. In many labs the position of the polystyrene peak at 1601 cm is used as a standard peak position. This is seen in the spectrum of polystyrene featured in Figure 2.29. 

Once the solvent has evaporated, the infrared transparent window and cast film are placed in a holder in the sample compartment of an FTIR, and the spectrum is measured. Holders such as the ones shown in Figure 4.7 can be used for this purpose. Ideally, the background spectrum should be run on the same window holding the cast film. This technique could work on powders because they can be dissolved and evaporated on a window. However, once the window is held vertically to place it into the IR beam, the powder will fall off the window. This technique works with polymers because of their ability to form films and adhere to flat surfaces. A spectrum of polystyrene cast from methyl ethyl ketone solution onto a KBr window is shown in Figure 4.14. Note that the SNR is good, the baseline is flat, the sample peaks are well resolved and on scale, and there is no offset. 

Figure 1 shows the FTIR spectra, in part, of a sample of atactic polystyrene of nominal MW 4,000 run at 30°C and at 230°C. The lower trace in Fig. 1 is the difference spectrum obtained by subtracting the 230°C spectmm from the 30°C spectrum. 

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