Polymers characterisation - Transport mechanisms
The transport properties of small molecules in polymeric membranes are an important phenomenon in many applications
- food packaging (barriers to moisture and air, contamination)
- structural materials (weakening, chemical modifications)
- pharmaceuticals (drug delivery, formulation)
FTIR-ATR can be used to simultaneously obtain information about
- the kinetics of species moving in and out of the polymeric membrane
- changes in the structure of the polymeric membrane (e.g. changes in crystallinity or swelling)
- interactions between the diffusant and polymeric membrane
A good example is the diffusion of liquid methanol into PET. The single beam of the dry polymer film was used as a reference and subsequent changes of the polymer as a function of increased methanol content are monitored. The result is a growth of the v(OH) stretching band with time.
Figure 1 The v(OH) band is clearly shown to grow as the diffusion process progresses. The area of this band can be integrated and plotted against time. Fitting this plot to a model gives a calculation of diffusion coefficients. As we collect a spectrum at each time point we can also integrate this data for changes in the polymer film.
Figure 1 shows a feature at 3000cm -1 which indicates the diffusion process is accompanied by swelling.
Figure 2 Kinetic information obtained from ATR diffusion experiment.
Further details
More detailed information about the use of FTIR-ATR to follow diffusion processes is available in the following publications.
FTIR-ATR studies of the structure and dynamics of water molecules in polymeric matrices. A comparison of PET and PVC. C Sammon, C Mura, J Yarwood, N Everall, R Swart and D Hodge. Journal of Physical Chemistry B, 1998, Vol 102, No. 18, pp. 3402-3411
Vibrational spectroscopic studies of the diffusion and perturbation of water in polymeric membranes. J Yarwood, C Sammon, C Mura and M Pereira. Journal of Molecular Liquids, May 1999, Vol. 80, No. 2. pp. 93-115(23)
An FTIR-ATR study of liquid diffusion processes in PET films. Comparison of water with simple alcohols. C Sammon, J Yarwood and N Everall. Polymer, 2000, Vol.41, pp. 2521-2534
ATR-FTIR study of the diffusion and interaction of water and electrolyte solutions in polymeric membranes. C Sammon, C Mura, S Hajatdoost and J Yarwood. Journal of Molecular Liquids, 2002, 96-97, pp.305-315
FTIR-ATR studies of diffusion and perturbation of water in polyelectrolyte thin films. Part 4. Diffusion perturbation and swelling processes for ionic solutions in SPEES/PES membranes. S Hajatdoost, C Sammon and J Yarwood. Polymer 2002, 43, pp.1821-1827