Influence of Crystallization on the Glass Transition of CR

Purpose

The glass transition occurs in the amorphous phases of polymers and is determined by molecular mobility in these regions. With semicrystalline polymers, crystalline and amorphous structures are both present in a material and the macromolecules can be components of both structural elements. This is the reason why crystallinity influences the glass transition of the polymer. Appropriate measurements were performed with chloroprene to demonstrate this effect. 

Sample

Unvulcanized chloroprene rubber (Baypren 210)

Conditions

Measuring cell: DSC822^e with liquid nitrogen cooling option

Pan: Aluminum standard 40 µl, pierced lid

Sample preparation: Piece of elastomer weighing 11.785 mg. The samples were cooled to the starting temperature of –100 °C at 10 K/min and then measured. For the first measurement (1st run), the sample was cooled from room temperature. After the first measurement, the sample was cooled from the final temperature (80 °C) at 10 K/min The second measurement was performed immediately afterward.

DSC measurement: Heating from 100 °C to 30 °C at 10 K/min 

Atmosphere: Nitrogen, 50 ml/min.

Prior to the first measurement (1st run), the material was semicrystalline. The glass transition can be seen at about –40 °C. The endothermic peak at about 40 °C is due to the melting of the crystallites.

Chloroprene crystallizes so slowly that the sample remains amorphous on cooling from the melt at 10 K/min (see Section 4.1.3.

Measurements of the crystallization and melting of chloroprene rubber (CR)). The second heating run therefore shows only a glass transition. 

Evaluation

The glass transition temperatures, T_g, and the step heights at the glass transition were determined using the ASTM method. To make a direct comparison of the glass transition temperatures of the sample in the amorphous and the semicrystalline state, the sample has to be cooled and heated each time at the same rates (see Section 3.1.1. Influence of sample pretreatment on the glass transition). 

The following table summarizes the evaluation of the measurement curves:  

Interpretation

The height of the glass transition step depends on the proportion of the molecular groups that participate in the cooperative rearrangements. This concerns only molecular movement that occurs in the amorphous regions. The proportion of molecules that cannot participate in molecular movement because of crystallization increases with increasing crystallinity. This is why semicrystalline samples have a smaller step Δcp, than amorphous samples. Since parts of the macromolecules can consist of both crystallites and amorphous regions, the crystallites in fact also hinder molecular mobility in the amorphous regions. The reduced molecular mobility in amorphous materials causes an increase of the glass transition temperature, as can be seen in the semicrystalline chloroprene sample (1st run).

Conclusion

Polymer crystallinity influences the temperature and the step height of the glass transition. A greater degree of crystallinity leads to a smaller step height

^{Influence of crystallization on the glass transition of CR | Thermal Analysis Handbook No.435 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2}