Free volume probing methods

Free Volume and Free Volume Probing Methods for Polymers... 

Mechanical and chemical methods for qualitative and quantitative measurement of polymer structure, properties, and their respective processes during interrelation with their environment on a microscopic scale exist. Bosch et al. [83] briefly discuss these techniques and point out that most conventional techniques are destructive because they require sampling, may lack accuracy, and are generally not suited for in situ testing. However, the process of polymerization, that is, the creation of a rigid structure from the initial viscous fluid, is associated with changes in the microenvironment on a molecular scale and can be observed with free-volume probes [83, 84]. 

A review of the application of ESR to the study of free radical polymerisation is given by Yamada and co-workers [146]. A survey of the application ESR spectroscopy spin label/probe methods in heterogeneous polymer systems is provided by Veksli and co-workers [147]. Spin probe methods allow the study of the MD of the polymer, its free volume, phase separation and phase morphology. 

The method is based on the fact that the rate of conformational change required for excimer formation depends on the free volume induced by the segmental motions of the polymer occurring above the glass transition. DIPHANT (compound 3 in Figure 8.3) was used as an excimer-forming probe of three polymer samples consisting of polybutadiene, polyisoprene and poly(dimethylsiloxane).a)... 

A number of techniques have been employed to examine free volume properties of polymers. These include small angle x-ray scattering and neutron diffraction that have been used to determine denisty fluctuations to deduce free volume size distributions [4-7]. Photochromic labelling techniques by site specific probes have been developed to monitor the rate of photoisomerizations of the probes and from this deduce free volume distributions [8-11]. Additional probing methods used to probe voids and defects in materials such as scanning tunneling microscopy (STM) and... 

The scope of this chapter is to determine the average free volume size and size distribution for Hyflon AD (Figure 4.1) by the photochromic probe method and to study the transport of organic vapours in membranes of this polymer. The final aim is to correlate the transport data in the polymer with the free volume element (FVE) size distribution and to gain a better understanding of structure-property relationships in perfluoropolymers. 

By definition, the free volume cannot be measured directly it is a vacuum which can neither be seen, nor sensed in any way. The only possible way for the determination of the free volume is by sensing of its surroundings, from which we can deduce the part of the volume that is not occupied. Several probe methods have been developed for probing the occupied volume in a polymeric sample, each of them based on different physical properties and on different measuranent principles [1,2]. 

Positron annihilation lifetime spectroscopy (PALS) studies the lifetime spectrum of ortho-positrons after being injected into the sample [3,4]. This lifetime depends on the probability of the ortho-positronium (o-Ps) particle (a hydrogen-like bound state formed by a positron-electron pair) to be quenched and annihilate. This probabihty is higher in condensed matter than in vacuum. Of all the probe methods PALS is nowadays probably the most versatile one and the most widely used. The o-Ps particle is the smallest probe available and can thus detect the smallest free volume elements furthermore, the method furnishes information on the average free volume size and on the FV size distribution. 

The spin probe method was one of the first methods used to evaluate the free volume in polymers [1,7]. It is based on the principle that the rotational frequency of spin probes, usually stable nitroxyl radicals such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is sensitive to the free volume. The relatively complex correlation between spectral data and FV makes this method more suitable for qualitative comparison of different polymers than for quantitative analysis of the FV [1,8]. 

The free volume (FV) in polymer systems is of great interest because the size and concentration of its elements (holes) affect numerous transport and other physicochemical properties of polymers. Positron annihilation lifetime (PAL) spectroscopy is now one of the most efficient approaches for investigations of FV. The foundations of this method for probing polymers were based in particular on Walker-Brandt-Berko s free volume model (7). According to this model, Positronium, Ps, (a bound atomic system, which consists of an electron and the positron) tends to be localized or trapped before its annihilation in FV or, in other words, in areas with reduced electron density. Accordingly, annihilation characteristics (lifetimes and intensities of longer lifetime components of annihilation radiation) provide information regarding the concentration and sizes of FV elements. (2-5)... 

In another type of application a low molecular fluorescent probe is added to a system containing macromolecules. As would be expected, the rotation of a small species is insensitive to the molecular weight of high polymers, but depends on the "microscopic viscosity" which is a function of free volume. For instance, Nishijima has shown that the microscopic viscosity of liquid paraffin hydrocarbons levels off for molecular weights above 1000 and that the microscopic viscosity of polystyrene containing 10 volume"/ benzene is only 200 times as high as that of benzene (15). Nishijima also showed that the emission anisotropy is a useful index of molecular orientation. Since both the excitation and the emission are anisotropic, the method yields the fourth moment of the distribution function of orientations, while other optical properties (dichroism, birefringence) depend on the second moment (15). 

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