Yield and plastic resistance of alpha-crystals of isotactic polypropylene

Yield behaviour and plastic resistance of the (100)[001] chain slip system in alpha-crystals of isotactic polypropylene (IPP) were studied in samples of biaxially oriented films of plain iPP and blends of iPP with hydrogenate d oligo(cyclopentadiene) (HOCP). Due to the orientation process the films i nvestigated exhibit sharp and nearly one-component texture with the (010) p lane parallel to the him surface and the chain direction parallel to the di rection of final drawing (transverse direction, TD). The films were studied in tension at various angles with respect to the orientation direction. It was found that the yield stress obeys the Coulomb yield criterion, provide d that the angle between the chain orientation axis and tensile axis is wit hin the range of 30-50 degrees. The results suggest that the (100)[001] cha in slip is active as a single deformation mechanism in this range of sample orientation. The critical resolved shear stress necessary to activate this slip, tau(c), was determined for iPP and iPP/HOCP blend samples. It was al so found that the slip process is sensitive to the stress normal to the sli p plane, similarly to the slip processes observed in linear polyethylene cr ystals. The value of tau(c) determined for plain iPP was 22.6 MPa, while fo r the 8:2 blend its value increased to 35.5 MPa. The increase is caused mos t probably by the presence of a small amount of HOCP molecules incorporated within IPP crystals, as well as by the layers of higher concentration of H OCP located at crystal-amorphous interfaces, which both cause immobilizatio n of a part of dislocations and consequently an increase of the yield stres s observed in the blend samples. The third probable cause of the increase o f yield stress in blends may be an increase of the glass transition tempera ture of the amorphous phase of iPP in the blend, as compared with plain iPP samples. (C) 1999 Elsevier Science Ltd. All rights reserved.