Polymer characterisation - Transport mechanisms
The competitive movement of small molecules through polymers, membranes and constitutes 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 dynamics of species moving in and out of the polymeric membrane
- changes in the structure of the polymeric membrane (eg changes in crystallinity or swelling)
- interactions between the diffusant and polymeric membrane
A good example is the diffusion of water and acetone into PVOH nanocomposites (Figure 1) where the spectrum of the dry nanocomposite film has been subtracted from each spectrum. Subsequent changes of the PVOH spectrum as a function of increased acetone:water content in the film are monitored over time. The result is a growth of both the v(C=O) stretching band (1700 cm-1) and the δ(OH) band (1620 cm-1) with time combined with a loss of intensity of the bands for PVOH as the polymer swells.
Advantages of using the FTIR approach
An important advantage of using FTIR to follow the movement of molecules into polymers and nanocomposites is that the spectra also provide information on the changes in other parameters. For example, the data in Figure 2 compare and contrast the speed at which water and acetone diffuse through a PVOH film without (Figure 2a) and with 5 wt% of clay (Figure 2b). The presence of clay delays the uptake of the two molecule by a considerable margin but the data also provide information about the clay content, swelling and crystallinity of the sample under scrutiny.
More detailed information about the use of FTIR-ATR to follow diffusion processes is available in the following publications.
FTIR-ATR Studies of the sorption and diffusion of water/acetone mixtures in poly(vinyl alcohol)-clay nanocomposites. A.F. Breen, C. Breen*, F. Clegg, L.M. Doeppers, Khairuddin, M. Labet, C. Sammon and J. Yarwood. Polymer. 2012, 53, 4420-4428.