Fourier transform infrared spectrometry for the analysis of polymers

TWI Bulletin, September/October 1988

Sheila Stevens, C Chem, MRSC, is a Senior Research Chemist in the Materials Department at The Welding Institute.

A Fourier transform infrared-microspectroscopy facility has recently been installed in the Materials Department for plastics analysis. Its use for investigating problems connected with the properties and behaviour of polymeric materials is described here.

To an increasing extent, engineering thermoplastics are seen as competitive with metals in a variety of applications. Their use has necessitated development work on joining processes for various types of thermoplastics. Individual polymers, such as polyethylene and polypropylene homopolymers are themselves identifiable chemical compounds, with particular properties which may be modified by copolymerisation or blending with other polymers, or by the use of additives (such as plasticisers, fillers, stabilisers, etc). Joining (welding) processes involve heating and deformation of the polymer, and a study of the effect of these two influences upon the properties of the polymer is essential if a full understanding of the design and use of welded joints in polymers is to be developed.

Microscopic examination will reveal the topography of weld areas in thermoplastics, but is unable to provide information on any chemical (and allied physical) changes which may have occurred as a result of the joining process. So to foster a greater understanding of polymer behaviour during welding, it is necessary to undertake suitable chemical analysis, and to a large degree this can be accomplished by Fourier transform infrared spectrometry (FTIR). This may be done in conjunction with an infrared microscope attachment which enables the examination and analysis of areas as small as 10µm square, and is thus ideally suited to the study of weld areas.

Principles of FTIR

The technique uses the principle that when infrared radiation is passed through a polymer sample, some frequencies (wavenumbers) are absorbed and others transmitted. The transitions involved in infrared absorption are associated with vibrational changes within the molecule, different bonds absorbing different frequencies.

Conventional dispersive infrared spectrometers consist of an infrared source, a monochromator to disperse the radiation and isolate the required frequencies, and a detector to measure the intensity of the dispersed radiation at each frequency. Dispersive instruments scan the entire frequency range of the infrared spectrum and sequentially measure the transmission at each frequency, typically taking about 10min per scan of each sample. These instruments have been largely superseded by FTIR spectrometers in which the monochromator is replaced by an interferometer. A digitized interferogram is produced, which is converted into an infrared spectrum by a computerised Fourier transformation. Since slits are not needed to improve resolution, the entire energy from the source passes through the sample, and FTIR instruments analyse the entire spectrum simultaneously. Thus FTIR spectrometers have the advantages of speed and greater sensitivity. Also, the use of a HeNe laser provides an internal frequency calibration standard to within 0.01cm ^-1, whereas dispersive instruments require calibration against reference spectra. Spectra are recorded as either wavelength (µm) or frequency (wavenumber) (cm ^-1) against absorbance or percent transmission.

The importance of FTIR analysis has been described as follows: ^[1] 'Infrared spectrometry is indispensable for the analysis of polymers: for control of raw materials, for control of processing (blending, orientation, etc), for analysis of failures, and for developing an understanding of polymer physics and chemistry. It is virtually the only method which can readily study the chemical functionality of both the crystalline and non-crystalline components of polymer systems.'

Instrumentation

The Welding Institute facility ( Fig.1) comprises a Mattson Instruments' Polaris/Icon spectrometer system with a Spectratech IR Plan microscope which has an additional external detector. Specification details for these are given in the Table.

The spectrometer can accept samples in the form of solids, liquids, or gases, but the most convenient form for plastics is a thin film or microtomed section of 10-200µm thickness (depending on the analytical information required). Such films or sections are also suitable for microspectroscopic analysis by transmission methods, although the microscope can also carry out reflectance measurements on sample surfaces. An additional side port for the Reflachromat objective allows the analysis of large objects in situ.

Applications

Fourier transform infrared spectrometry is capable of producing both qualitative and quantitative information on many aspects of polymer chemistry. The type of information obtainable includes:

Composition

Identification of materials for quality control purposes, identification of additives, and detection of the presence of copolymers and blends can be routinely carried out.

Molecular structure

Information can be obtained on the steric structure of the polymer, for instance stereo-regularity, and differentiation between cis-trans isomeric forms, and also on polymer conformation, such as tacticity and the physical arrangement of the polymer chain ( i.e. as a planar zig-zag or 3D helix).

Crystallinity

Some polymers, such as polyethylene and polyamide-6, can consist of crystalline and amorphous phases, and it is often possible to use FTIR to distinguish between these phases and to determine the degree of crystallinity, which will affect the mechanical properties of the material.

Molecular degradation mechanisms

When a stress is applied to a polymeric material, degradation may occur as a result of chain scission (mechanical degradation) or by activation of other processes such as thermal oxidation or photo-oxidation. FTIR can in principle be used to study such polymer degradation, whether during welding or in service.

Compatibility studies of polymer blends

Polymer blends can be subdivided into a number of different categories, one of which is incompatible and compatible blends. Incompatible blends consist of two or more phases, with two or more glass transition temperatures (T _g) and comprise the large majority of all blend systems. Compatible blends contain only one phase and have one T _g, such as polyphenylene oxide/polystyrene (but only if the polystyrene is not impact modified). Comparison of the blend spectrum with those of the individual polymers enables differentiation between the two types of blend.

Orientation

The mechanical properties of thermoplastics, such as polypropylene, are influenced by the degree, type, and homogeneity of orientation of macromolecular chains, and FTIR can be used to study the effects of injection moulding and welding on the orientation.

Small sample analysis

The microscope allows the above properties to be measured in small amounts of material, or in small areas of bulk material. Obvious examples of this application would be for inclusion and weld area analysis.

Other uses

Fourier transform infrared spectrometry is not limited to polymers, and can be extended to other areas such as the analysis of adhesives, polymer coatings on metals, metal oxide films, corrosion products on metals, contaminants in microjoining, and painted surfaces.

Practical examples of use

The following examples of ways in which the technique is used at TWI illustrate the usefulness of FTIR analysis, in addition to routine quality control.

In Fig.2 the spectrum of a polyethylene hot plate weld is compared with that of parent polyethylene and this illustrates spectral changes following welding, in bands sensitive to the molecular orientation of the material. Samples can be identified by comparison with library spectra, as shown in Fig.3a, which confirms that an unknown sample is amorphous atactic polystyrene. If the spectral region between 800 and 1400 cm ^-1 is expanded ( Fig.3b) an additional band at 966 is evident, which arises from butadiene, meaning that the sample is high impact polystyrene. The usefulness of the technique for identifying polymer blends is illustrated in Fig.4, which compares the spectra of Noryl 731 and polystyrene, and clearly identifies the Noryl as a blend of polyphenylene oxide and polystyrene, the latter again modified with butadiene (band at 960 cm ^-1).

The equipment has also been used to establish the presence of ethylene in a sample of polypropylene; the latter is often copolymerised with 5-15% of ethylene to improve toughness, since polypropylene homopolymer is brittle at 0°C. In Fig.5 the spectrum of isotactic polypropylene is compared with that of a sample, in which the presence of the band at 720 cm ^-1 is due to ethylene, thus the material is actually a propylene-ethylene copolymer.

Summary

The FTIR spectrometer has already proved essential for quality control of materials studied in the TWI plastics joining research programme, and has proved useful for investigating problems connected with the properties and behaviour of polymeric materials. In common with other analytical facilities at TWI, this instrument is available for use in studying Members' polymer applications and the author would be pleased to discuss individual problems or regular QC requirements.

Reference