Letter pubs.acs.org/macroletters
Unfolding of Isotactic Polypropylene under Uniaxial Stretching Jia Kang,† Shichen Yuan,† You-lee Hong,† Wei Chen,† Akihiro Kamimura,‡ Akihiro Otsubo,‡ and Toshikazu Miyoshi*,† †
Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States SunAllomer, Ltd., 3-2 Yako 2-chome, Kawasaki-ku Kawasaki 210-0863, Japan
‡
S Supporting Information *
ABSTRACT: Despite numerous investigations on polymer processing, understanding the deformation mechanisms of semicrystalline polymer under uniaxial stretching is still challenging. In this work, 13C−13C Double Quantum (DQ) NMR was applied to trace the structural evolution of 13C-labeled isotactic polypropylene (iPP) chains inside the crystallites stretched to an engineering strain (e) of 21 at 100 °C. DQ NMR based on spatial proximity of 13C labeled nuclei proved conformational changes from the folded chains to the locally extended chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays a critical role to achieve large deformability of iPP.
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from folded to locally extended chains inside the crystallites, is required. Recently, our group employed 13C−13C dipolar-based double quantum (DQ) NMR and selective 13C isotope labeling to investigate chain-folding (CF) structures of solution- and meltgrown iPB121−24 and iPP crystals.25,26 Dipolar interaction is inversely proportional to the third power of internuclear distance. Thus, this interaction can be used to investigate shortrange distances between 13C labeled nuclei. Such spatial sensitivity allows us to evaluate only adjacent reentry structures of polymer chains among heterogeneous structures. As a result, it was demonstrated that flexible polymer chains adopt significant amounts of adjacent re-entry structures in the melt- and solution-grown crystals.21−26 The purpose of this work is to understand structural evolution of folded polymer chains inside the crystallites under uniaxial stretching via DQ NMR. To achieve our goal, iPP was chosen as our model system due to its large drawability and αc mobile classification. Our experimental strategy is illustrated in Figure 1. The blue and black lines represent methyl-13C labeled and nonlabeled chains, respectively. Red dots represent 13C atoms. The labeled chains were mixed with nonlabeled chains at a blend ratio of 1:9. The mixed blends in melt state (Figure 1a) were cocrystallized (Figure 1b). DQ NMR was used to evaluate the ensemble average of successive adjacent re-entry folding number ⟨n⟩ and the adjacent re-entry fraction ⟨F⟩ (see Details
niaxial stretching of semicrystalline polymers has been extensively studied for decades because of its fundamental impact on the understanding of polymer deformation behavior and its implications in processing.1−15 Various characterization tools such as transmission electron microscopy (TEM),2 atomic force microscopy (AFM),3 and small- and wide-angle X-ray scattering (SAXS and WAXS)4 have successfully detected lamellae fragmentation,4 orientation of fragmented crystal blocks, fibril formation,2,3 and fibril sliding1 in stretched semicrystalline polymers. Neutron scattering (NS) was applied to investigate the evolution of radius of gyration (Rg) of 2Hlabeled polyethylene (PE) under uniaxial stretching.5−10 It was found that the molecules affinely deform with fibers and suggested that folded chains are extended under large strains.9 Semicrystalline polymers show largely different defromabilities. For example, ultrahigh molecular weight PE and isotactic polypropylene (iPP) show very high draw ratios of >200 and 66, respectively.11,12 In spite of similar chemical structures, syndiotactic PP only shows a maximum draw ratio of 5−9,13 and isotactic poly(1-butene) (iPB1) can only be stretched to draw ratio
