Depression of glass transition temperature

Ten Brinke, G., Karasz, F.E., and Ellis, T.S. (1983). Depression of glass transition temperatures of polymer networks by diluents. Macromolecules, 16, 244-299. 

The transport of caibon dioxide in polymers has historically been analyzed in the same manner as other simple gases (1). Recent studies have shown, however, that the effects of CX>2 on polymers include scrnie features commonly associated with organic solvents, including swelling (2-5). and depression of glass transition temperatures, i.e., plasticization (6-8). Moreover, CO2 can be handled as a liquid at room temperature under ratho moderate pressures its critical temperature is 31 0 and its saturated vapor pressure at 25" C is 64.6 atm (950 psi). For these reasons it seems appropriate to consider near-critical CX>2 as a highly volatile solvent, rather than as a gas, in its interactions with polymers. 

The authors found that the depression of glass transition temperature (Tg) due to the addition of plasticiser is substantially reduced by the loading of wood flour. In addition, various wood-plastic composites were compounded into different colours, and several pairs of the compounds with different rheological properties were extruded in single and twin-screw extruders to see whether any wood-patterns are developed. When the differences in the shear viscosity and the Tg of the two compounds were too large, the incomplete plasticisation of the higher viscosity component was observed due to the lower viscosity component. It was found also that distinct wood-patterns were only developed both inside and on the snrface of the extruded prodncts for the pairs of the composites with optimal differences in both viscosity and plasticiser content. 

Care has to be taken when utilising these types of additive because some of the final cured properties may be degraded. Possible examples would be depression of glass transition temperatures and an increase in thermal expansion coefficients. However, by careful formulation, the advantages offered by CTBN-type additives can be made to far outweigh the disadvantages. 

F. E. Karasz, T.S. Ellis Interaction of epoxy resins with water the depression of glass transition temperature. Polymer, 25 (1984), p. 664-669... 

Depression of Glass Transition Temperature of Amorphous Polymer by the Addition of Low-Molecular-Weight Soluble Diluent... 

Gibbs and DiMarzio (1958) appear to have been the first to offer a molecular interpretation, via statistical mechanics, of the depression of glass transition temperature (Tg) of an amorphous polymer by the addition of low-molecular-weight soluble diluents, in terms of the size and stiffness of diluents and the diluent concentration. Subsequently, using both classical and statistical thermodynamics, Chow (1980) derived the following expression, which predicts the T depression of polymer/diluent mixtures ... 

Also, as might be expected, the use of plasticiser has a similar effect to that of increasing the hydroxyvalerate content. It also has a more marked effect on depressing the glass transition temperature and therefore improves properties such as impact strength and ductility at lower temperatures. 

As a consequence, the overall penetrant uptake cannot be used to get direct informations on the degree of plasticization, due to the multiplicity of the polymer-diluent interactions. The same amount of sorbed water may differently depress the glass transition temperature of systems having different thermal expansion coefficients, hydrogen bond capacity or characterized by a nodular structure that can be easily crazed in presence of sorbed water. The sorption modes, the models used to describe them and the mechanisms of plasticization are presented in the following discussion. 

Another example involved a batch of isocyanate crosslinker which was too tacky. Upon comparing the HPGPC trace of this sample with that of a control as shown in Figure 9, it is seen that the major difference between these two samples was the level of free caprolactam. The high content of free caprolactam in sample CX-006 depressed the glass transition temperature (Tg) of the sample to such an extent that CX-006 became too tacky. This method of analysis has proved to be a reliable and useful technique for detecting low levels of free caprolactam in this type of oligomeric crosslinker. 

Since each of these polycarbonates had exceptionally high glass transition temperatures—256° and 290°C., respectively—it was possible to add appreciable amounts of antiplasticizers without depressing the glass transition temperatures to room temperature or lower. In addition, since the bisphenol II polycarbonate already had a relatively high tensile modulus (4.7 X 105 p.s.i.), it was of interest to determine how much this modulus could be increased. 

Measurements of glass transition temperatures at high pressure were made indirectly using, in particular, creep compliance [95, 96] or directly using differential scanning calorimetric techniques [97, 98]. The measured depression reaches values as high as 60°C for poly(methyl methacrylate) and polystyrene. 

Both lactose and sucrose have been shown to crystallize in an amorphous form at moisture contents close to the glass transition temperature (Roos and Karel 1991a,b Roos and Karel 1992). When amorphous lactose is held at constant water content, crystallization releases water to the remaining amorphous material, which depresses the glass transition temperature and accelerates crystallization. These authors have done extensive studies on the glass transition of amorphous carbohydrate solutions (Roos 1993 Roos and Karel 1991d). 

Figure 11.36 shows that the glass transition depression is a linear function of plasticizer concentration. Two ester plasticizers depress the glass transition temperature more extensively than paraffinic oil. Dioctyl sebacate deerease the flexural modulus more rapidly with smaller concentrations of plasticizer (up 10 wt%) then the flexural modulus levels... 

Blends of PES and PEI were also investigated to exploit the combined action of glass transition temperature depression after blowing agent sorption and to the interfaces between the different polymer matrices. The result was the production of foams with nanocellular cells, but very high foam density (Krause, Sijbesma et al., 2001). 

Plasticizers and oils, which can be incorporated in extremely large quantities into certain elastomer compounds, are commercially important for two main reasons. They depress the glass transition temperature and hence improve the working temperature range of an elastomer selected on the basis of other properties. Secondly, they allow the incorporation of more diluent filler than the surface adsorption properties of the polymer would normally allow, hence reducing the volumetric cost of the compound. 

The dynamic mechanical properties of VDC—VC copolymers have been studied in detail. The incorporation of VC units in the polymer results in a drop in dynamic modulus because of the reduction in crystallinity. However, the glass-transition temperature is raised therefore, the softening effect observed at room temperature is accompanied by increased brittleness at lower temperatures. These copolymers are normally plasticized in order to avoid this. Small amounts of plasticizer (2—10 wt %) depress T significantly without loss of strength at room temperature. At higher levels of VC, the T of the copolymer is above room temperature and the modulus rises again. A minimum in modulus or maximum in softness is usually observed in copolymers in which T is above room temperature. A thermomechanical analysis of VDC—AN (acrylonitrile) and VDC—MMA (methyl methacrylate) copolymer systems shows a minimum in softening point at 79.4 and 68.1 mol % VDC, respectively (86). 

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