Details of the Photochromic Probe Method

An unportant and generally accepted assumption is that the dye molecule does not influence its surroundings, i.e. it does not interact with the polymer matrix and it does not change the free volume itself. The polymer chain dynamics must be sufficiently slow that there is no significant relaxation of the free volume elements on the time scale required for the trans-cis transition. The latter makes this method only suitable for glassy polymers at temperatures sufficiently far below the glass transition. 

A fundamental requirement for the system to be suitable for the application of the photochromic probe method is that the polymer is sufficiently transparent to UV or visible light in the spectral range of the photochromic probe molecules, first of all to induce the photo-isomerization reaction upon irradiation of the sample, and secondly to allow the determination of the absorption spectrum of the isomers before and after the irradiation, and thus to calculate the degree of isomerization. In this respect, Hyflon AD is an excellent polymer because it is highly transparent in the visible spectrum and in most of the UV spectrum, the reason for its original application in optical devices. 

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