Comparison of glass transition dynamics between fluorophore-labeled and -doped flexible Poly(vinyl chloride) plasticized by ultra-small branched star Poly(epsilon-caprolactone)

Glass transition dynamics are one of the most important characteristics with regards to governing the properties of polymers. Herein, we prepare pyrene-labeled and pyrene-doped flexible poly(vinyl chloride)s plasticized by ultra-small branched star poly(epsilon-caprolactone) (L-PVC/USB-SPCL and D-PVC/USB-SPCL, respectively), and observe the changes in their fluorescence intensities with temperature to ascertain the actual glass transition behaviors of the polymer chains. Fluorescence spectroscopy measurements show that the D-PVC/USB-SPCLs have a single Tg, which is comparable to the Tg measured by differential scanning calorimetry (DSC). However, the L-PVC/USB-SPCLs exhibit a broad glass transition from the onset of Tg for the L-PVC/USB-SPCLs to the endset of Tg for the PVC chain itself (i.e., in the absence of USB-SPCL). In addition, the L-PVC/USB-SPCLs show a relatively low nonradiative decay degree compared to the D-PVC/USB-SPCLs. These results indicate that the labeled pyrene moieties along the PVC chains exhibited limited movement due to the cooperative reptation motion of the chains, even when the space for the polymer chain to move sufficiently is created by an increase in the free volume. As the USB-SPCL content increase in the flexible PVC, the fluorescence intensity reduction degrees of the various D- and L-PVC/USB-SPCLs decrease due to the polarity effect of USB-SPCL. However, in the case of the L-PVC/USB-SPCLs, when the USB-SPCL loading reach 40 phr, the fluorescence intensity reduction degree increases once again due to the dominance of the decrease in caging effect, which reveals that the labeled system, in which a fluorescent substance is directly attached to a polymer chain, allows identification of the motion of the polymer chain in a direct and sensitive manner.