Glass transition temperature depression

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... 

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). 

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. 

Glass transition temperature (Tg), measured by means of dynamic mechanical analysis (DMA) of E-plastomers has been measured in binary blends of iPP and E-plastomer. These studies indicate some depression in the Tg in the binary, but incompatible, blends compared to the Tg of the corresponding neat E-plastomer. This is attributed to thermally induced internal stress resulting from differential volume contraction of the two phases during cooling from the melt. The temperature dependence of the specific volume of the blend components was determined by PVT measurement of temperatures between 30°C and 270°C and extrapolated to the elastomer Tg at —50°C. 

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. 

Keddie, J. L, Jones, R. A. L. and Cory, R. A. (1994) Size-Dependent Depression of the Glass-Transition Temperature in Polymer-Films. Europhys. Lett., 27, 59-64. 

By incorporating acrylonitrile into polystyrene we can depress the copolymer s glass transition temperature below that of pure polystyrene. When sufficient acrylonitrile is present, the copolymer s glass transition temperature falls below room temperature. The resulting copolymer is tough at room temperature and at higher temperatures. 

On the other hand, the presence of the salt, LiPEe, assists the occurrence of supercooling by increasing the solution viscosity and by depressing the liquidus temperature. At practical concentrations of LiPFe ( 1.0 M), even the solidus temperature can be circumvented, since there is no crystallization process observed for LiPFe/EC/EMC solution down to —120 °C, while the glass transition occurs at —103 °C. In such concentrated solutions, even the presence of MCMB cannot initiate crystallization, and the supercooling is completely suppressed at the cooling rate of 10 °C/min. 

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. 

The so called plasticization effect, i.e., the depression of the glass transition temperature is also an important feature the sorbed gas acts as a kind of lubricant , making it easier for chain molecules to slip over one another, and thus causing polymer softening. 

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. 

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