Flow Induced Crystallization in Isotactic Polypropylene-1,3:2,4-Bis(3,4-dimethylbenzylidene)sorbitol Blends: Implications on Morphology of Shear and Phase Separation Luigi Balzano, ²,‡ Sanjay Rastogi,* ,²,‡,§ and Gerrit W. M. Peters ‡,| Department of Chemical Engineering and Department of Mechanical Engineering, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, Institute of Polymer Technology and Materials Engineering (IPTME), Loughborough UniVersity, Loughborough, LE11 3TU, United Kingdom, and Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX EindhoVen, The Netherlands ReceiVed July 2, 2007; ReVised Manuscript ReceiVed NoVember 4, 2007 ABSTRACT: Nucleation is the limiting stage in the kinetics of polymer crystallization. In many applications of polymer processing, nucleation is enhanced with the addition of nucleating agents. 1,3:2,4-Bis(3,4-dimethylben- zylidene)sorbitol or DMDBS is a nucleating agent tailored for isotactic polypropylene (iPP). The presence of DMDBS changes the phase behavior of the polymer. For high enough temperatures, the system iPP-DMDBS forms a homogeneous solution. However, in the range of concentration spanning from 0 to 1 wt % of DMDBS, the additive can phase separate/crystallize above the crystallization temperature of the polymer, forming a percolated network of fibrils. The surface of these fibrils hosts a large number of sites tailored for the nucleation of iPP. The aim of this paper is to investigate the combined effect of flow and DMDBS phase separation on the morphology of iPP. To this end, we studied the rheology of phase separated iPP-DMDBS systems and its morphology with time-resolved small-angle X-ray scattering (SAXS). The effect of flow is studied combining rheology, SAXS, and a short-term shear protocol. We found that, with phase separation, DMDBS forms fibrils whose radius (∼5 nm) does not depend on the DMDBS concentration. The growth of these fibrils leads to a percolated network with a mesh size depending on DMDBS concentration. Compared to the polymer, the relaxation time of the network is quite long. A shear flow, of 60 s -1 for 3 s, is sufficient to deform the network and to produce a long-lasting alignment of the fibrils. By design, lateral growth of iPP lamellae occurs orthogonally to the fibril axis. Therefore, with crystallization, the preorientation of DMDBS fibrils is transformed into the orientation of the lamellae. This peculiarity is used here to design thermomechanical histories for obtaining highly oriented iPP morphologies after shearing well above the melting point of the polymer (i.e., without any undercooling). In contrast, when shear flow is applied prior to DMDBS crystallization, SAXS showed that iPP crystallization occurs with isotropic morphologies. 1. Introduction Morphology control is an important issue in polymer process- ing as it influences a broad range of properties of the final products. For instance, mechanical, optical, and transport properties of polymeric materials depend on the size and shape of the crystallites. 1,2 It is well-known that thermal and mechan- ical histories do play an important role in the creation of these morphological features 3,4 and that additives can also have a remarkable influence. 2,5-8 Nucleating agents are a family of additives used to speed up processing rates of polymers. In the case of isotactic polypropylene (iPP), a common nucleating agent is a sorbitol derivative: 1,3:2,4-bis(3,4-dimethylben- zylidene)sorbitol or DMDBS. DMDBS is a chiral molecule that, driven by hydrogen bonding, can self-assemble into fibrillar structures. Crystallization of DMDBS within the iPP matrix corresponds to a liquid-solid phase separation, in the following, referred to as DMDBS crystallization or DMDBS phase separation. The DMDBS molecule has a special “butterfly” configuration, see Figure 1. The “wings” of the molecule (phenyl rings with two methyl groups attached) enable dissolution in the polymer and, at the same time, are tailored nucleation sites for iPP, while the “body” comprises two moieties: one dictates the geometry of the molecule and the other bears the polar groups (hydroxyls) for hydrogen bond formation. 9 Polarity is, therefore, one of the main features of DMDBS. In contrast, iPP is a fully apolar molecule. This difference becomes clear and leads to a rich phase diagram when iPP and DMDBS are compounded together. Kristiansen et al. 10 proposed a monotectic model for this phase diagram where the eutectic point lies around 0.15 wt % of the additive. In their model, miscibility of the two molecules is always possible at high temperatures. They define four con- centration regimes based on different phase transitions occurring during the cooling of a homogeneous mixture. From the application point of view, the most interesting concentration regime is where the additive plays the role of clarifier enhancing * To whom correspondence should be addressed. ² Department of Chemical Engineering, Eindhoven University of Tech- nology. ‡ Dutch Polymer Institute. § Loughborough University. | Department of Mechanical Engineering, Eindhoven University of Technology. Figure 1. Chemical structure of DMDBS. 399 Macromolecules 2008, 41, 399-408 10.1021/ma071460g CCC: $40.75 © 2008 American Chemical Society Published on Web 12/19/2007