Influence of Crystallization on the Glass Transition of a Silicone Elastomer
Purpose
In Section 3.1.2. Cold crystallization using silicone rubber as an example it was shown that with appropriate cooling the silicone rubber used was almost completely amorphous, and that on heating cold crystallization occurred. With such samples, the degree of crystallinity can be changed by varying the cooling conditions. A more detailed discussion of the relationship between the step height at the glass transition and the degree of crystallization is then possible.
Sample
Vulcanized, unfilled silicone rubber
Conditions
Measuring cell: DSC822e with liquid nitrogen cooling option
Pan: Aluminum standard 40 µl, pierced lid
Sample preparation: Piece of elastomer weighing 34.146 mg. Before the first measurement, the sample was placed in the cold measuring cell (at 140 °C) and then measured. Before the other measurements, the sample was cooled at different rates (20 K/min, 10 K/m and 5 K/min). The sample crystallized during cooling.
DSC measurement: Heating from 140 °C to 20 °C at 10 K/min
Atmosphere: Nitrogen, 50 ml/min
The glass transition can be seen as a step at –125 °C after the initial deflection. The step height decreases with decreasing cooling rate. The sample crystallizes In the range –100 °C to – 80 °C. The melting peak follows directly afterward with a maximum at about –40 °C. The sample that was cooled most slowly shows no crystallization effect. This sample has already almost completely crystallized during cooling.
Evaluation
The degree of crystallinity of the sample before the measurement is proportional to the enthalpy of fusion determined from the peak area. If the sample crystallizes during measurement, the crystallization peak must be taken into account in the determination of the enthalpy of fusion. As shown in the diagram, this is done by a suitable choice of the integration limits. To discuss the evaluation of the glass transition in more detail, the relevant part of the curve is shown scale-expanded in the following diagram.
The arrow in the diagram denotes increasing crystallinity. At lower cooling rates the step at the glass transition is smaller due to the larger crystallinity. The specific enthalpy of fusion determined, 'h, the degree of crystallinity below the glass transition, αc, the step heights of the glass transitions, Δcp, and the glass transition temperatures, Tg, are listed in the following table.
As has already been shown in Section 3.1.2 the degree of crystallinity, Dc, is calculated from the equation
The value for polydimethylsiloxane (Δhc= 38.2 J/g) was used for the enthalpy of fusion of the 100% crystalline material, Δhc. Even if the value Δhc is not known, the measured enthalpy of fusion is proportional to the crystallinity. The relationship between crystallinity and the step height at the glass transition can therefore be determined from the DSC measurements:
In this diagram, the measured absolute values are plotted on the axes on the left and below. In comparison, the corresponding relative heat capacity change at the glass transition and the degree of crystallinity are shown on the axes on the right and above respectively.
In the region of crystallinity considered, Δcp decreases linearly with the enthalpy of fusion (crystallinity) for the sample measured. From the linear fit
a value of Δcp,amorph of 0.308 J/gK is obtained for the step height at the glass transition for the amorphous material. The slope, α, is 1.354*10-2 K-1 . From this equation the enthalpy of fusion of a material can be estimated that is so crystalline that a glass transition no longer occurs (Δcp= 0). This Δh0, is 22.75 J/g. For this material the value for the maximum degree of crystallization, αmax, is 60%. The value of αmax is less than 100% because with polymers an other phase is also present besides the crystalline and mobile amorphous phases. This third phase is referred to as the rigid amorphous phase. It is actually amorphous but nevertheless cannot participate in the glass transition due to the hindrance caused by the crystallites. With the maximum degree of crystallinity the contribution of the rigid amorphous phase amounts to 40%.
Interpretation
The silicone rubber crystallizes on cooling in the temperature range between the melting temperature of about –40 °C and the glass transition at about –120 °C. The resulting degree of crystallinity depends on the cooling rate and can be estimated from the peak areas measured in the heating run afterward. The higher the cooling rate, the lower the crystallinity after cooling. The continuous decrease of Δcp with increasing crystallinity is a measure of the proportion of the mobile amorphous phase αa:
Conclusion
The step height at the glass transition and the enthalpy of fusion yield additional information on the structure of semicrystalline polymers. For comparison purposes, it is not necessary to know the enthalpy of fusion of the fully crystalline material.
Influence of Crystallization on The Glass Transition of Silicone Rubber | Thermal Analysis Handbook No.HB436 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2