Notes Suppression of Skin-Core Structure in Injection-Molded Polymer Parts by in Situ Incorporation of a Microfibrillar Network Gan-Ji Zhong, ² Liangbin Li, ‡ Eduardo Mendes, § Dmytro Byelov, ⊥ Qiang Fu, ² and Zhong-Ming Li* ,² College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan UniVersity, Chengdu, 610065, P. R. China, National Synchrotron Radiation Laboratory and Department of Polymer Science and Engineering, UniVersity of Science and Technology of China, Hefei, 230026, China, Section Nanostructured Materials, Delft UniVersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands, and FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands ReceiVed March 4, 2006 ReVised Manuscript ReceiVed June 16, 2006 Introduction The achievement of high performance in polymers, especially commodity polymers, such as polyethylene (PE), polypropylene (PP), etc., is still an important open research subject. 1 The mechanical properties of polymers can be enhanced through orientation of polymer chains during processing, such as spinning, hot stretch, and especially newly developed shear controlled orientation in injection molding (SCORIM). 2,3 For semicrystalline polymers, shear flow at the mold filling stage during injection molding may induce high molecular orientation. In the extreme case, the oriented crystals can form an interlocked shish-kebab structure, which results in dramatic enhancement of mechanical properties in the flow direction of injection- molded parts. 4,5 However, the parts usually exhibit an inhomo- geneous structure, namely a skin-core structure. During injection molding, the hot polymer melt contacting cold mold walls experiences high strain, high stress and large cooling rate, and thus a skin layer with high orientation is formed near the walls. This indeed is an intrinsic problem of normal injection molding because these boundary conditions create large gradi- ents of temperature, shear rate and stress fields. This heterogeneous structure is not favorable to the improve- ment of mechanical properties due to the residual stress produced by different levels of crystal orientation in the thickness direction. Moreover, the different levels of crystal orientation may cause a deterioration of macroscopic properties. 6 From a practical point of view, elimination of the skin-core structure is expected to improve mechanical properties. One approach to deal with the problem, as has been previously reported, 7 is the use of a nucleating agent that can effectively suppress the crystal orientation. The presence of nucleating agent can eliminate the skin-core structure even with high shearing imposed on the polymer flow. However, in this case, the crystal orientation is generally low. 7-9 This is not always desirable since some level of orientation in the injection molded parts is of great benefit to the improvement of mechanical properties. In isotactic polypropylene/poly(ethylene terephthalate) (iPP/ PET) blends, PET microfibrils with diameter of 1-10 μm can be produced through in situ hot-stretch, which constructs a microfibrillar network in the iPP matrix. 10,11 It is expected that the iPP melt would flow through the microchannels or pores formed by the network during injection molding of such blends. In addition to the effect of redefining the flow field, microfibril networks also promote the nucleation of matrix polymers. In the in situ microfibrillar blend of iPP/PET, three origins for crystal nucleation in shear flow field were identified: (a) the shear induced row nucleation; (b) classical fibril nuclei; (c) nuclei induced by fibril-assisted alignment. 11 One can therefore speculate that the in situ microfibrillar network can suppress the formation of the skin-core structure of the injection-molded parts since the network can: (1) help to homogenize the flow rate of the fluid across the thickness of the part, (2) act as an efficient nucleating agent to generate a typical transcrystalline layers with the c-axis deflected from the flow direction, and (3) assist shear flow to form nuclei. In this Note, the objective is to study the effect of an in situ microfibrillar network on the crystal orientation distribution of injection molded parts. Analyses of orientational parameters of different positions within the injection molded parts show that the presence of the microfibrillar network suppresses the skin- core structure effectively. Injection molded parts with high and homogeneous orientation were obtained with a combination of SCORIM and microfibrillar network. Experimental Section The materials used in this study were poly(ethylene terephthalate) (PET) and isotactic polypropylene (iPP). The PET as the mi- crofibrillar candidate was a commercial grade of textile polyester and was supplied in pellets by LuoYang Petroleum Chemical Co. (China) with M h n of about 2.3 × 10 4 g/mol. The iPP used as the matrix was F401, a commercial product of Lanzhou Petroleum Chemical Co. (China) with M h n of about 11.0 × 10 4 g/mol, and its melt flow index (MFI) was 2.5 g/10 min (190 °C, 21.6 N). To avoid hydrolysis, the PET was dried in a vacuum oven at 100 °C for at least 12 h prior to processing. Preparation of in situ microfibrils had been reported in detail elsewhere. 11,12 The microfibrillar iPP/PET blend in this work consists of 15 wt % PET microfibrils of 1-10 μm in diameter, and iPP as the matrix. The representative structure of the PET microfibrils is shown in Figure 1, where the matrix iPP was etched away for clear observation. The neat iPP has also undergone the same processing for comparison purposes. The blend and neat iPP were injected into a mold using SZ 100 g injection molding machine at 200 °C and about 90 MPa. Then, SCORIM technology invented * Correspondence author. Telephone and Fax: +86-28-8540-5324. E-mail: zm_li@263.net.cn. ² College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University. ‡ National Synchrotron Radiation Laboratory and Department of Polymer Science and Engineering, University of Science and Technology of China. § Section Nanostructured Materials, Delft University of Technology. ⊥ FOM-Institute for Atomic and Molecular Physics. 6771 Macromolecules 2006, 39, 6771-6775 10.1021/ma0604845 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/25/2006