**12. Acknowledgement**

This work was supported by eni SpA under contract N.o 3500007596. The authors wish to thank W O Ho and A Burnley, Alphasense Ltd., for many helpful comments and clarifications concerning the PID operation, S Zampoli and G Cardinali, IMM CNR Bologna, for many discussions on PID characterisation and E Benvenuti, Netsens Srl, for his valuable technical support.

Assistance and support by the Management and technical Staff of Polimeri Europa Mantova is also gratefully acknowledged.
