In situ PET microfibrils are created by drawing melt‐blended PP and PET. The drawn blend is used to prepare polymer/polymer MFCs, and isolated PET microfibrils are used for the manufacturing of MF‐SPCs. Samples are prepared with different fibril orientations to determine the effect of orientation on the mechanical properties of the two types of composites. The resulting composites show improvements in stiffness of 77% for uniaxial MFCs, and 125% for uniaxial MF‐SPCs, with the highest recorded modulus of 8.57 GPa for a uniaxial MF‐SPC sample. SEM observations confirm that the fibrillar structure and excellent alignment is maintained. The changes in the reinforcement effect with orientation are very similar to those predicted by the rule of mixtures for the crossply.

Summary In‐situ poly (ethylene terephthalate) (PET) nanofibrils were created by drawing of melt blended polypropylene and PET. The drawn blend was used to prepare polymer‐polymer nanofibrillar composites (NFCs), in order to compare the effect of different fibril orientations on the mechanical properties. The resulting composites showed excellent improvements in stiffness of 116% vs. the isotropic PP matrix for uniaxial NFCs, with the highest recorded modulus of 4.16 GPa for a uniaxial NFC sample, and 3.41 GPa for a crossply sample. Scanning electron microscopy observations confirmed that the nanofibrillar structure and excellent alignment was maintained. Comparisons were also drawn between the composites created in this study, and previous work on microfibrillar composites.

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