Filler surfaces characterising

Further advantages of FMC over alternative techniqnes include the ability to measure heats of interactions of polymers in solution with solids, i.e., filler surfaces characterisation of filler surfaces by IGC is limited to reversibly adsorbed volatile molecules. Also, HPLC detectors can be connected in series with the calorimeter and used to determine quantities adsorbed, and hence provide a measure of surface coverage. 

Surface characterisation methods, both elemental (e.g. XPS, AES) and molecular (e.g. ToF-SIMS), are gaining in importance, in view of the active development of surface-modification technology to render fillers of all types more acceptable to the matrix and improve... 

H NMR transverse magnetisation relaxation experiments have been used to characterise the interactions between NR, isoprene rubber, BR, EPDM and polyethylacrylate rubbers with hydrophilic silica and silicas modified with coupling agents [124-129]. These studies showed that the physical interactions and the structures of the physical networks in rubbers filled with carbon black and rubbers filled with silicas are very similar. In both cases the principal mechanism behind the formation of the bound rubber is physical adsorption of rubber molecules onto the filler surface. 

Fractal analysis allows consideration of the surface structure of the filler particles, which are characterised by its fractal dimension (d ) and by the self-similarity interval. Because the polymer structure is also described in the framework of the fractal analysis, it becomes possible to consider the interaction between the filler surface and polymer matrix, including the interfacial layers, based on the analysis of their fractal dimensions. Application of the model of irreversible aggregation allows description of the processes of aggregation of the filler particles in a particulate filled composites. This aggregation causes changes... 

The structure of the filler particle surfaces and of the polymer surface characterised hy their fractal dimensions, affects the interfacial adhesion in composites. To explain the structural effect let us introduce the concept of the accessibility of the sites on these surfaces to form adhesion joints (physical or chemical). As a first approximation the degree of such accessibility may be defined as a difference of the fractal dimensions of two surfaces. The higher is this difference the lower is the accessibility of the surface and the less is the adhesion [21]. Suppose that the filler particle has a very rough surface with dimensions which are close to the Euclidean dimension d = 3 (for example, AI2O3 particles) [33], whereas the polymer surface is very smooth, i.e., dp = d = 2. In this case the contact between two surfaces is possible only at the apexes of the rough surface of the filler and the result could be very low adhesion. In other words, the disparity of the dimensions determines the inaccessibility of the greater part of the filler particle for the formation of adhesion bonds [21]. 

The same value of diminishing density of the cluster network of entanglements Van, which characterises the structure of a polymer matrix, was obtained for PHE-Gr-II in [44, 54] and for PHE-Gr-I in [20, 44, 54]. This fact is very important. It confirms the conclusion, made many years ago by Lipatov [1, 29, 55] that the filler surface exerts a disturbing action on the matrix structure in the interfacial layers. 

Filler surface chemistries are of more significance than the bulk ones, as they determine both the rate of wetting and the strength of interaction with polymers. They are invariably different from bulk chemistry but, unfortunately, they are poorly characterised for many fillers. Because of the interest in this very important topic, techniques for surface characterisation are covered in detail in Chapter 3. 

In this example of the rearranged equation the probe is a base with known constants Cg and Eg and is being used to determine the E and constants of the acid sites on the filler surface. But clearly, the A and B subscripts could be interchanged and an acid probe used to characterise the basic sites on the filler surface. 

This is an extremely powerful method for characterising the acidic and basic sites on filler surfaces and produces data that corresponds, at least qualitatively, with the most sophisticated quantum mechanics analyses of Mulliken. However, it is important to appreciate that the method described [12] only gives an average value for the E and constants, as mentioned earlier different active sites may have quite different characteristics. However, an adaptation to the EMC technique offers an opportunity to obtain a semi-quantitative imderstanding of this surface heterogeneity, this is described in Section 3.4.1.3. 

The choice of solvent is also important, as the enthalpy measured is essentially that of the displacement of the solvent from the filler surface by the probe molecule. For acid-base characterisation a non-polar solvent such as n-heptane is ideal. However, poor solubility of the desired probe molecule may demand the use of a polar solvent, when this is the case the enthalpy of interaction of the solvent with the filler must be considered when interpreting the results. If the probes in question are only capable of weak physical adsorption, (i.e., not via hydrogen bonding), and are only soluble in highly polar solvents such as tetrahydrofuran and dimethyl formamide, situations can arise where apparently no adsorption occurs. This is due to the solvent having equal or stronger interaction with the substrate than the probe. In such cases the value of performing FMC experiments should be questioned and the use of more soluble model compounds considered. If chemical adsorption of the probe occurs the polarity of the solvent is somewhat less critical, provided the solubility of reaction products is not enhanced. 

There are relatively few detailed studies of particulate fillers by surface analysis and space limitations preclude a review. The interested reader is encouraged to study a series of papers by Johansson and co-workers [36-39] that address the characterisation of coated Ti02 pigments. These discuss in considerable detail the relationship between data from XPS, SIMS and X-ray fluorescence as well as relating surface analytical information to other methods of pigment surface characterisation and to pigment behaviour. 

Analytical Techniques for Characterising Filler Surfaces Specular... 

This chapter has described how the most important property of a particulate filler, with regard to its reactivity, is the acidic or basic character of its surface sites. Furthermore, that the surface of a single filler particle can have sites ranging from strongly acidic to strongly basic. It has been shown that techniques for characterising filler surfaces can be broadly divided into three groups. 

As described in the earlier chapters, the filler surface plays a vital role in determining the processing behaviour and properties of composites. The main methods for characterising filler surfaces were discussed in the previous chapter. This chapter covers the use of additives to beneficially modify the surface of fillers, and thus optimise composite processing and properties. 

The main techniques for characterising the structure of filler surfaces have been covered in detail in the previous chapter. This section deals with methods for determining the amount present on a filler surface and for obtaining an indication of the amount of additive needed for mono-layer coverage. 

Overall, the in situ method remains very poorly characterised. Relying as it does on diffusion through polymer to the filler surface, where conditions must be right for hydrolysis and condensation. It sounds very haphazard, but seems to work remarkably well in practice. 

Most fillers, when produced, have a pH that arises as a consequence of their origins. The pH is rarely controlled by the production process unless a surface treatment of some sort has been applied to the filler. Nonetheless, it is useful to have information of the filler s pH as this is one of the simplest methods of characterising the filler surface. 

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