Thermal analysis studies of doping effects on the conformational motions of polymer chains in solid solutions with lasing molecules

Abstract

The advancement of the solid-state dye laser performance largely depends on the systematic study of the dye-matrix interactions at the nanoscopic scale. The current work deals with blends of a comparatively inert dye host, poly(methyl methacrylate) (PMMA), with nonionic/apolar (substituted perylenes) and ionic/polar (rhodamine 6G, pyrromethene 567) dyes at ≈10−4 mol L−1 loading. Differential scanning calorimetry (DSC) and thermally stimulated currents (TSC) were used to explore the relative strength of inter- and intramolecular guest-host interactions by monitoring blending-induced modifications of the high-temperature signals: the segmental relaxation, the space-charge relaxation, and the liquid-liquid transition. Both techniques revealed the antiplasticizing role of the oligomeric organics on the relaxation dynamics of polymer segments, evidenced by clear glass-transition temperature upshifts. It becomes apparent that this effect is independent of the size, polarity, and ionicity of the dopant, signifying a common mechanism underway. It is suggested that, at least for the dyes under investigation, the chromophores simply fill the voids within the matrix, imposing strong steric hindrances on the rearrangement of the long-range structure. A comparison between the present results and earlier low-temperature dielectric data reveals that the large-scale relaxation dynamics show stronger perturbations due to blending, in comparison to the localized rotational motion of the pendant groups. DSC provided estimates for the unconverted monomer percentages in the solid blends. These were also determined more accurately by nuclear magnetic resonance (NMR), which additionally confirmed that the tacticity of the chains is not affected by the presence of the dye.

Desorption-Limited Mechanism of Release from Polymer Nanofibers

“…Rhodamine dye forms nonaggregated (isolated) molecular dispersions in such polymer matrices as PMMA 50 (which was confirmed in the present work using individual fiber images obtained using an optical fluorescence microscope). Therefore, in the present case, dissolution of dye crystals prior to the desorption-driven release stage cannot delay or decelerate the release process.…”

Influence of Functionalized Graphene Sheets on Modulus and Glass Transition of PMMA

“…These two compounds were extracted using THF/methanol procedure described in ref. The low molecular weight additives can act as plasticizers and decrease the T g of PMMA. , Hence, the removal of PMMA oligomer and additives is likely the reason for the ∼20 °C increase in T g between R-PMMA and PMMA.…”

Dielectric Analysis (DEA)

“…For example, by increasing the size of the ester sidegroup R 2 (from methyl, to ethyl, n -propyl, n -butyl, etc. The signal shows a narrow relaxation time distribution and is dependent on the chain structure [e.g., by showing internal plasticization with increasing alkyl group length (Dudognon et al 2003(Dudognon et al , 2004] or dopants [e.g., by being antiplasticized -i.e., appearing at higher temperatures -in blends with dye molecules (Kalogeras et al 2006b(Kalogeras et al , 2007a]. At the same time, the α transitions are shifted to lower temperatures ( " internal plasticization " effect) and merge with the secondary signals (Hedvig 1977 ; Dudognon et al 2004 ).…”

“…Several relaxations have been isolated and comprehensively studied by dielectric as well as dynamic mechanical spectroscopy, and subsequently attributed (not always in a straightforward manner) to the amorphous or crystalline microphases in the material [e.g., see Sauer et al (1997) for information on the relaxation activity of POM]. Changes in the local molecular environment are possible by a number of means, including the use of highor low -molecular -weight diluents, crystallization (McGrum et al 1967 ), or interpenetrating network formation (Kalogeras et al 2006b(Kalogeras et al , 2007a. At somewhat lower temperatures, and only in selected polymers, the spectra show a normal -mode (n) relaxation (see Section 6.2.4.3 ).…”