Fillers, analysis DRIFTS

Further insight into the nature of the interaction between irreversibly adsorbed species and the filler surface can be gained from DRIFTS analysis of the filler sample taken from the FMC cell after completion of the adsorption - desorption cycle. DRIFTS is described in more detail in Section 3.5.4, but in summary, it is an infrared spectroscopic technique, that by virtue of a significant proportion of glancing angle reflections, affords enhanced resolution of filler surface functional groups. The authors have found this technique particularly useful when studying competitive adsorption of polymer stabilisers and carboxylic acids onto silica and metal hydroxides, respectively. 

If the small molecule has a lower refractive index than the carrier solvent and the probe a higher refractive index than the carrier solvent, the calculated level of amount of probe adsorbed will be greater than it really is. If however, both the probe and the small molecule have lower refractive index than the carrier solvent, the amount of probe adsorbed will be smaller than it really is. The extent to which this problem occurs also depends upon the solubility of the small molecule in the carrier solvent. This effect has cansed problems in a recent study [13] of adsorption of various carboxylic acids on to metal hydroxides. Figure 3.4 shows both the heat of adsorption and amount of carboxylic acid adsorbed. It is evident that adsorption of stearic acid (octadecanoic acid) and isostearic acid (16-methyl heptadecanoic acid) from toluene affords apparently reduced levels of adsorption, relative to when adsorbed from heptane. This is due to both the probe and the water having lower refractive indices than toluene. DRIFTS analysis of the respective filler samples retrieved from the FMC cell, however, indicates similar levels of adsorption. Only in the case of adsorption from heptane is the level of adsorption in concordance with theoretical values. This is due to reduced solubility of water in heptane, relative to toluene. 

FTIR data analysis software enables spectral data for the imcoated filler to be subtracted from that of the coated specimen, thus creating a difference spectrum that enhances absorption bands associated only with the surface treatment. It is important to point out that subtraction does not always yield additional information, especially when a small absorption associated with a surface treatment is obscured by a large absorption associated with the filler. Spectral changes relative to the unhound surface treatment/additive provide insight into the natnre of interaction with the surface. Examples of use of DRIFTS are described in Section 3.5.4.4. DRIFTS spectra can be quantified provided sample preparation/presentation and representation of the data is correct. 

The following examples illustrate the power of the DRIFTS technique for analysis of interaction between fillers and surface treatments or polymer additives. 

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