Measurement Methods, Polymer Characterization, Compositions

In conclusion one can say that SEC is a very powerful method for polymer characterization, especially in combination with other composition sensitive or absolute calibration methods. A big advantage is also that the sample amount is fairly small, typically 10 mg. For more complex polymers, such as polyelectrolytes, enthalpic effects often become dominant and also for rather high molecular weight polymers chromatographic methods such as field-flow fraction (FFF) techniques might be more suitable. For fast routine measurements linear columns are often used. 

The fundamental difficulty is that polymeric substances cannot be obtained in a stmcturally and molecularly uniform state, unlike low-molecular-weight compounds. Thus, macromolecular materials of the same analytical composition may differ not only in their structure and configuration (see Sect. 1.2) but also in molecular size and molecular weight distribution they are polydisperse, i.e., they consist of mixtures of molecules of different size. Hence, it is understandable that the expression identical is not, in practice, applicable to macromolecules. Up to the present time, there is no possibility of preparing macromolecules of absolutely uniform stmcture and size. It follows, therefore, that physical measurements on polymers can only yield average values. The aforementioned peculiarities of macromolecular substances mean that the methods of characterization suitable for low-molecular-weight compounds are frequently not applicable or only applicable in a substantially modified form often completely new methods of investigation must be employed. 

First and paramount method of nanocomposites characterization is, undoubtedly, transmission dectron microscopy (TEM), which allows one to observe the particle shape, to determine particle sizes, to control the homogeneity of the composite and to obtain the histogram of nanoparticle sizes (Figs. 1, 2). TEM (especially axial bright fidd microscopy) is the most informative method of characterization with the resolution up to units of angstroms allowing visualization of the lattice plane and measurement of interplanar distances of nano-partides. However, its application to polymer composites is hindered due to charge instability of some polymers (such as PVA [39], or PVDF [35]) and by the difficulty of sample preparation. 

Nevertheless, historically a differentiation exists between "dielectric" and "impedance" spectroscopies. Traditional dielectric analysis has been applied primarily to the analysis of bulk "dielectric" properties of polymers, plastics, composites, and nonaqueous fluids with very high bulk material resistance. The dielectric method is characterized by using higher AC voltage amplitudes, temperature modulation as an independent variable, lack of DC voltage perturbation, and often operating frequencies above 1 kHz or measurements at several selected discrete frequencies [2, p. 33]. 

Solution Polymers. Acryflc solution polymers are usually characterized by their composition, solids content, viscosity, molecular weight, glass-transition temperature, and solvent. The compositions of acryflc polymers are most readily determined by physicochemical methods such as spectroscopy, pyrolytic gas—liquid chromatography, and refractive index measurements (97,158). The solids content of acryflc polymers is determined by dilution followed by solvent evaporation to constant weight. Viscosities are most conveniently determined with a Brookfield viscometer, molecular weight by intrinsic viscosity (158), and glass-transition temperature by calorimetry. 

Being sensitive to the chemical composition of the outermost layers of the surface, contact angle measurement is widely used for characterizing polymer surfaces. Surface characterization for polymers using contact angle measurement and XPS will now be described in detail, as these are the most widely used methods. 

Experimental variables such as temperature, flow rate, sample concentration and mobile phase composition can cause changes in the elution volume of a polymer [439,457,460-464]. Chromatographic measurements made with modem equipment are limited more by the errors in the absolute methods used to characterize the molecular weight of the calibration standards than any errors Inherent in the measurements themselves, since the determination of molecular weights by SEC is not an absolute method and is dependent on calibration [462]. The Influence of temperature on retention in SEC is not very great, since no strong sorptive interactions are involved in the retention mechanism. Temperature differences between the column and solvent delivery... 

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