Effect of Thermal Degradation on Polymer Thermal Properties

Thermal Degradation of Nylon 6,6

TGA was used to determine the moisture content and degradation profile of the Nylon 6,6. As shown in Figure 1, the moisture content of the Nylon 6,6 sample was approximately 2.2%. The degradation onset temperature was around 413 °C and the weight loss at the onset temperature was 96.1%.

Control the Degree of Degradation with TGA

Even below the degradation onset temperature, polymers could still be slightly degraded (<5 wt% degradation). To study the effects of small degrees of degradation of polymer on their thermal properties, TGA was to precisely control the degrees of degradation. Nylon 6,6 was heated to above 300 °C to remove the ~2% moisture. With the Abort segment, TGA can be used to generate desired thermal degraded sample with 1% to 4% of degradation. The procedure of the abort segment was set to be between 97-94% as stated in Table 1 to account for the ~2% moisture content from the Nylon 6,6. The degree of degradation was determined from the wt% loss from 300 °C to maximum temperature as shown in Figure 2. As it can be seen, TGA enabled a precise control of degrees of degradation.

Table 1. TGA Controlled Degrees of Degradation

Degradation Impact on Thermal Properties

During thermal degradation, random-chain scission, end-chain scission and side-chain cleavage can occur.[2] Chain scission will reduce the molecular weight and change the molecular weight distribution of the polymer and hence will significantly affect the viscosity and flow properties; and further affecting on the quality of the polymer processes. The thermal properties of a polymer (such as glass transition and melting) are highly sensitive to the molecular weight and molecular weight distribution.[3] Hence DSC can be a simple way to characterize the degree of degradation of a polymer. The degraded Nylon 6,6 samples were tested on a DSC 2500 to study the thermal properties. A heat-cool-heat experiment was performed from 0 °C to 300 °C with 10 °C/min heating and cooling rates. The first heating removes any thermal history of the samples. To give all the samples the same thermal history, 10 °C/min cooling rate was used for cooling and the second heating curves were used for all the data analysis and comparisons.

Glass transition and melting were observed in the DSC thermograph. Data analysis was performed using “Glass Transition” and “Peak Integration (enthalpy)” analysis functions in TA Instruments TRIOS software. Figure 3 shows the analysis results of the untreated Nylon 6,6 sample. The Nylon 6,6 shows a glass transition at 61.30 °C and a melting peak at 262.6 °C.

Nylon 6,6 samples with removed moisture and controlled degrees of degradation using TGA were also tested using DSC. All the data was used the same analysis for comparison and the results are shown in Table 2.

Table 2. Degree of Degradation Impacts on the Glass Transition and Melting of Nylon 6,6

The glass transitions of the degraded Nylon 6,6 samples shifted to lower temperatures with increasing degrees of degradation (Figure 4). The decrease in glass transition may be due to the chain scission and the reduced molecular weight during the thermal degradation. Straus et. al. showed random chain scission was predominate in Nylon decomposition.[4] Random chain scission resulted in lower molecular weight species and increase molecular weight distribution. The molecular weight reduction enhanced the molecular mobility of the polymer chain and therefore reduced the glass transition temperature. The same trend was also observed in the melting temperature. The melting onset and peak temperatures decrease with increasing degrees of thermal degradation (Figure 5). The Nylon 6,6 melting data can be fitted into a linear fitting (Figure 6) and used as a calibration curve to further quantify the degree of degradation of an unknown samples. The correlation between the thermal properties and degrees of degradation can be studied for different materials with TGA which provided a precisive controlled degradation.