Change of the Glass Transition on Vulcanization

Purpose

With some elastomers, the glass transition temperature depends on the degree of vulcanization (or conversion). The relationship between the conversion and the glass transition temperature is shown using an NBR as an example. 

Sample

Unvulcanized NBR with 50% polymer content  

Conditions

Measuring cell: DSC822^e

Pan: Aluminum 40 µl, with pierced lid 

Sample preparation: Cubes of approx. 25 mg were cut from the starting material. Before measurement, the samples were vulcanized for different periods of time at 130 °C. Additional measurements were also performed at 70 °C and 150 °C. The conversion was determined by measuring the glass temperature as previously described in Section 3.1.3. Measurement of Isothermal Vulcanization Kinetics.

DSC measurement: Heating from –60 °C to –10 °C at 10 K/min

Atmosphere: Nitrogen, 50 ml/min

Evaluation

The increasing degree of cross-linking (conversion) shifts the glass transition temperature of the sample to higher temperatures. The temperature shift between the uncross-linked sample and the sample after maximum vulcanization is about 4 K. For the reliable measurement of such small differences, it is important to make sure that the tangents used to evaluate the glass transition are parallel. 

The following diagram shows the variation of the glass transition temperature as a function of conversion. The measurements were performed after different periods of vulcanization at 70 °C, 130 °C and 150 °C. 

It can be seen that the glass transition temperature increases linearly with conversion. The extent to which the glass transition temperature depends on the conversion is influenced by the cross-linking system. A larger maximum degree of crosslinking, for example, leads to a greater change in the glass transition temperature. 

Conclusions

During the cross-linking reaction of the NBR sample, the glass transition temperature changes linearly with conversion. This has to do with the change in molecular mobility due to chain cross-linking. 

^{Change of Glass Transition on Vulcanization | Thermal Analysis Application No. HB440 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2}