Glass transition temperature thin polymer films

Tsui, O. K. C. and Zhang, H. F. (2001) Effects of chain ends and chain entanglement on the glass transition temperature of polymer thin films. Macromolecules, 34, 9139—9142. 

The last few years have witnessed a high level of activity pertaining to the research and development of all-solid, thin-film polymer electrolyte batteries most of these use lithium as the active anode material, polymer-based matrices as solid electrolytes, and insertion compounds as active cathode materials. High-performance prototypes of such batteries stand currently under research, whose trends are expected to include the development of amorphous polymers with very low glass-transition temperatures, mixed polymer electrolytes, and fast-ion conductors in which the cationic transport number approaches unity. 

Our research is using high glass transition temperature amorphous polymers in which substituted azobenzenes are bound on the side chain or in the main chain, as thin films. The films are subjected to polarized light, which induces trans-cis-trans isomerization cycles and subsequent rearrangement of the azobenzene groups preferentially perpendicular to the polarization direction. This preferential director of the azobenzene groups creates a dichroism... 

Keddie, J. L., Jones, R. A. L. and Cory, R. A. (1994) Interface and Surface Effects on the Glass-Transition Temperature in Thin Polymer-Films. Faraday Discuss., 98, 219-230. 

Amorphous polymers characteristically possess excellent optical properties. Unlike all the other commercially available fluoropolymers, which are semicrystalline, Teflon AF is quite clear and has optical transmission greater than 90% throughout most of the UV, visible, and near-IR spectrum. A spectrum of a 2.77-mm-thick slab of AF-1600 is shown in Figure 2.5. Note the absence of any absorption peak. Thin films of Teflon AF have UV transmission greater Ilian 95% at 200 mm and are unaffected by radiation from UV lasers. The refractive indexes of Teflon AF copolymers are shown in Figure 2.6 and decrease with increasing FDD content. These are the lowest refractive indexes of any polymer family. It should be noted that the abscissa could also be labeled as glass transition temperature, Tg, since Tg is a function of the FDD content of the AF copolymer. Abbe numbers are low 92 and 113 for AF-1600 and AF-2400. 

When the polymer wets the solid (0 = 0), polymer films are thermodynamically stable. For 0 > 0 the films are only metastable. When a thin, metastable film is heated above the glass transition temperature, holes start to form spontaneously, usually at small defects. The holes increase in size until only a network of polymer lines is formed which eventually breaks up into individual droplets (Fig. 7.19) [150], The film stability of films with a thickness of 1-100 nm is determined by long-range surface forces, mainly van der Waals forces [151,268, 294,295],... 

Addition of a thin-film heater to the back of a metal sample holder allowed studies of thin polymer films by GRAS at up to 200 °C (121). In a study of an ultrathin film of polymethylmethacrylate, the two doublets near 1240/1270 and 1150/1190 cm"1 exhibited changes in relative intensity above the glass transition temperature of 100 °C, indicating that the polymer glass structure was maintained even in such thin films. 

Havens and Bell performed an electron beam irradiation of methylene-bridged aromatic polyesters, which were synthesized by polycondensation between two pairs of aromatic diacid chlorides and 3,3 -methylenediphenol or 4,4 -methylenediphenol [54]. They found that irradiation of thin films of these polyesters at room temperature resulted in some chain extension and cross-linking, and that irradiation at a temperature near or above the glass transition temperatures of the polymers greatly enhanced the tendency for the polymers to crosslink. 

In terms of their dielectric response, thin polymer films fumed out to be stable thermally in a pure nitrogen atmosphere, even when kept for a long time (i.e. 24 hours) at temperatures well-above the glass transition. An example is given in Fig. 7 for a thin PS film of 89 nm after 24 hours at 180 °C under a flow of pure nitrogen the sample was measured again and no changes in the dielectric response were detected. 

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