2. Experimental

For the purpose of our work an injection molding iPP grade named "T30G" kindly supplied by Himont was employed, having the following main features: M _n = 75100; M _w = 483000; M _w/M _n = 6.4. PA6, kindly supplied by DSM, was characterized by M _w = 25000, M _n = 13000. Materials were quenched under pressure and high cooling rates according to a procedure already reported (Brucato et al., 2000; 2001). The basic idea was to set up a model experiment to couple rapid cooling histories with pressure levels up to 40 MPa under controlled conditions. The experimental apparatus consisted of a modified injection molding machine, used as a source of molten polymer supplied at a pre determinable and maintainable constant pressure (Brucato et al., 2000; La Carrubba et al., 2000). Samples were then solidified under a uniform pressure field and under a non-uniform cooling rate profile, the cooling rate depending upon the distance from the cooled surface. Thus, starting from the cooled surface, a continuous and gradually decreasing cooling rate profile was achieved, which determined a continuous variation of structure with depth (La Carrubba et al., 2000). The relationship between cooling rate and depth was taken into account in the so-called "mapping function" (Brucato et al., 2000; La Carrubba et al., 2000). Another apparatus was used for crystallizing iPP samples at low cooling rates, consisting in a confining fluid pressurized cell where the polymer was confined and maintained under...