Glass transition, polymer film

Thermal characterization of an emulsion polymer essentially means the measurement of the glass transition temperature Tg, that is the temperature above which the hard, glass-like polymer film becomes viscous or rubber-like. Polymers whose Tg lies well above room temperature are designated as hard , those with a Tg much lower than room temperature as soft . Normally Tg is measured by differential scanning calorimetry (DSC [25]). In this technique, the difference between the heat absorbed per unit time by the polymer film to that absorbed by a thermally inert reference material is recorded during a linear temperature ramp. The sample and the reference are placed on a sensor plate of defined thermal resistance R, and the temperature difference AT between the sample and the reference is then recorded over the temperature ramp. Usually, the heat flow difference, which is the negative quotient of AT and R, is plotted as a function of temperature (Fig. 3-11). 

T and are the glass-transition temperatures in K of the homopolymers and are the weight fractions of the comonomers (49). Because the glass-transition temperature is directly related to many other material properties, changes in T by copolymerization cause changes in other properties too. Polymer properties that depend on the glass-transition temperature include physical state, rate of thermal expansion, thermal properties, torsional modulus, refractive index, dissipation factor, brittle impact resistance, flow and heat distortion properties, and minimum film-forming temperature of polymer latex... 

Polymeric binder can be added to the network either as an aqueous latex dispersion or as a solution that should be dried prior to lamination in this process. In either case, the polymer should form a film and join adjacent fibers together and thus improve the stress transfer characteristics of the fibrous network. Provided that the proper film forming conditions are available, the property profile of the bonded network is determined to a significant degree by the properties of the polymeric binder at the temperature of use [20,22]. For example, if a softer type of product is desired, a binder with a relatively low glass transition temperature Tg) is often chosen. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

Grohens, Y, Hamon, L., Reiter, G., Soldera, A. and Holl, Y. (2002) Some relevant parameters affecting the glass transition of supported ultra-thin polymer films. Eur. Phys. J. E, 8, 217-224. 

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. 

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. 

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