Physical Phenomena Related to Free Volumes in Rubber and Blends A. J. Marzocca, W. Salgueiro and A. Somoza Abstract In the present chapter, different aspects related to free volumes and the physical phenomena involving free volumes in rubbers and blends are discussed. Experimental results were obtained using conventional experimental techniques (e.g. dynamic mechanical tests, differential scanning calorymetry and swelling) and principally a non-conventional one (positron annihilation lifetime spectros- copy—PALS). PALS has demonstrated a high capability to give direct information on free volumes. Due to its significant role in the study of nanoscopic effects in molecular systems (among them polymers), the physical grounds of the technique are explained. It is also illustrated how PALS detects free nanohole volumes and gives information on their changes as a consequence of different reactions induced in polymers. Based on the latest experience of the authors, some examples of PALS studies on NR and SBR rubbers and NR/SBR blends are presented. The results obtained are discussed using a modern scientific approach to the study of physicals processes in these elastomers; i.e. the analysis of the experimental information is given into the frame of well recognized theoretical models. A. J. Marzocca (&) LP&MC, Facultad de Ciencias Exactas y Naturales, Departamento de Física, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón I C1428EGA Buenos Aires, Argentina e-mail: marzo@df.uba.ar W. Salgueiro  A. Somoza IFIMAT, Universidad Nacional del Centro de la Provincia de Buenos Aires, Pinto 399 B7000GHG Tandil, Argentina W. Salgueiro  A. Somoza Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Buenos Aires, Argentina P. M. Visakh et al. (eds.), Advances in Elastomers II, Advanced Structured Materials 12, DOI: 10.1007/978-3-642-20928-4_10, Ó Springer-Verlag Berlin Heidelberg 2013 399