Shear-Induced Nucleation and Growth of Long Helices in Supercooled Isotactic Polypropylene

Abstract

Flow-induced conformational ordering in the supercooled isotactic polypropylene (iPP) is studied with in situ Fourier transform infrared spectroscopy (FTIR) coupled to an extrusion slit die. At temperature around the normal melting point of iPP, helices with monomer numbers up to 12 and 14 can be induced by shear. A window of shear strength exists to induce helices with different lengths, which increases with temperature. After the cessation of shear, the intensity of the 841 cm -1 band in FTIR spectra, corresponding to helices with 12 monomers, increases sharply in the first stage, which is followed by a slow growth process, whereas a reduction of shorter helices is observed after the cessation of shear. The different trends of intensity evolutions of the long and short helical bands suggest that a coupling between coil-helix transition and intermolecular ordering occurs with helices with a length of 12 monomers, which eventually leads to an isotropic-liquid-crystal transition. Therefore, the flow-induced precursor may be a liquid crystal phase, which will transform into crystal and initiate crystal growth later.

Flow Memory and Stability of Shear-Induced Nucleation Precursors in Isotactic Polypropylene

“…Further studies suggest that the stability of helical conformations also depends on the number of repeating units in the sequence. 33 Size-dependent stability of shish structures was also found by Fu et al who observed longer lifetime for the larger shishes produced at higher shear rates. 62 Recently, Balzano et al reported that the aspect ratio of i-PP shishes and their stability in the melt increase with the shear stress.…”

Fourier Transform Infrared Spectroscopy study of polymorphism in propylene-co-1-pentene copolymers: Trigonal form identification

“…The thermal properties of the different specimens were analyzed in a Perkin-Elmer DSC-7 calorimeter connected to a cooling system (Intracooler 2P from Perkin Elmer) and calibrated with different standards (indium, zinc and Table 1 Characteristic iPP infrared regularity bands and isotactic helical sequence length [38]. n-dodecane).…”

“…These latest vibrations involve, then, conformational arrangements and are commonly designated as regularity bands or helix bands. The majority of the absorption bands that appear at frequencies below 1400 cm À1 are regularity bands [32] in the case of iPP, these bands being very important because the frequency at which vibrational coupling is observed depends on the helical sequence length [32][33][34][35][36][37][38] (see Table 1). Its significance is also related to the absence of true crystalline absorptions (resulting from intermolecular vibrational coupling of atoms within a crystal lattice) in the iPP infrared spectrum since the TGTGTG (3 1 ) helical conformation of its macromolecular chains is identical in the long-established polymorphs (including the mesomorphic modification [39]).…”

Flow-Induced Helical Coiling of Semiflexible Polymers in Structured Microchannels

“…For macromolecular and biological filaments with typical length scales of nano-to micrometers, which we are interested in, thermal fluctuations broaden the buckling transition and generate a smooth crossover from the unbuckled to the buckled state [1][2][3]. However, very little attention has been payed to conformational instabilities far from equilibrium [5,6], which exhibit qualitatively new features compared to equilibrium ones. Here, the transport of semiflexible polymers in microchannels and capillaries presents a new opportunity to study the non-equilibrium behavior of such filaments [7][8][9].…”