Introduction to Polymer Science and Technology

Behaviour of polymers

Figure 5.18Dynamic elastic modulus and tan5 vs. temperature

One of the basic methods of measuring glass transition temperature is dilatometry,where volume is measured as a function of temperature. A small piece of polymer sample of known weight is immersed in mercury in a small cylindrical glass bulb, which is attached to a graduated capillary tube. The mercury, which has a constant coefficient of thermal expansion over the temperature ranges of interest, fills part of the capillary tube as well. The dilatometer, prepared in this manner, is then placed in a water or an oil bath and heated. The rise of mercury in the capillary with temperature is recorded, which is a reflection of the expansion of the material. From this information, with calibration, the specific volume of the material can be obtained and plotted as a function of temperature. As shown in Figure 5.19, the T , which represents a second-order transition, is therefore the temperature where the volume-temperature curve changes slope.

Melting, however, is a first-order transition and produces a clear step/discontinuity change in specific volume at the melting point (T_m). Sometimes the change in slope is not very distinctive and, therefore, to identify the transition temperature more clearly a plot of the coefficient of thermal expansion, which is the derivative of the specific volume with respect to temperature, against temperature is plotted as in Figure 5.20 and produces a discontinuity or an abrupt change at T .

Figure 5.19A plot of specific volume (V) vs. temperature (T) for a semi-crystalline polymer

Introduction to Polymer Science and Technology

Behaviour of polymers

Figure 5.20A plot of (dV/dT) vs.T

5.3.1 Dependence of T and T on chemical structure

G m

In general any structural feature, which encourages intra molecular (i.e., within a single molecule) and inter molecular (in between molecules) motions yields low T and T_m and, conversely, structural features that hinder molecular motion cause an increase in T and T .

G m

5.3.1.1 Molecular weight

Molecular weight influences both the molecular chain length and the number of molecule chains. A polymer with shorter chains will have more chain ends per unit volume, so there will be more free volumeto accommodate molecular motion. Hence the T for a thermoplastic with shorter molecular chains will be lower than if the chains were longer as illustrated in Figure 5.21.

Date: 2015-12-11; view: 785