Flow-induced Precursors Research Articles

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

The dissolution of flow-induced nucleation precursors in isotactic polypropylene is investigated indirectly by means of in situ rheo-SAXS measurements. The progress of crystallization and the evolution of crystal orientation are recorded in isothermal conditions after a controlled shear step followed by an annealing step of different duration at various melt temperatures. The results confirm that the survival time of shear-induced nucleation precursors is extremely large compared to typical rheological relaxation times and it is longer for the precursors originated at higher shear rate. Most important, we show that the effect of flow on the development of oriented morphologies is lost much earlier than that on the overall crystallization kinetics. A schematic model for precursors’ dissolution involving gradual transformation from row into point-like nuclei is proposed.

A Study on the Chain−Particle Interaction and Aspect Ratio of Nanoparticles on Structure Development of a Linear Polymer

The stability of metastable flow-induced precursor (FIPs) in the polymer melts in presence of nanoparticles, viz. single-walled carbon nanotube (SWCNT) and zirconia nanoparticles, is studied at, 142 °C, close to the equilibrium melting point of unconstrained extended chain crystals of linear polyethylene (PE). The results conclusively demonstrate the influence of chain−particle interactions, between PE and the nanoparticles, on the stretch of the long chains. With the applied flow, SWCNTs together with PE chains are observed to align along the flow direction, giving rise a strong streak like pattern along the equator. At the initial stages, intensity of the observed streak in the presence of SWCNTs is stronger than that for the neat polyethylene. The streak intensity stabilizes with time, where the time required for the stabilization depends on the amount of the dispersed nanotubes in the polymer matrix. On the contrary, in the presence of zirconia nanoparticles, where the chain−particle interactions between PE and the nanoparticles are weak the initially observed streak tends to disappear with time, where the time required is strongly dependent on the concentration of the nanoparticles in the polymer matrix. Thus, compared to the neat polymer, the presence of zirconia nanoparticles destabilizes the shish formation. The chain orientation along the flow direction is determined using Herman’s orientation function and the length of the oriented chains (shish) by Ruland’s streak analysis. On cooling, with the crystallization of the polymer, scattering develops along the meridian, indicating the development of folded chain crystals, where the oriented chains present along the flow direction provide the epitaxy matching thus suppressing the nucleation barrier. The meridional intensity (arising with the formation of crystals, called kebabs) at room temperature, shows strong dependence on the stable streak intensity (chain orientation along the flow direction, called shish) along the equator prior to cooling.

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

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.

Crystallization and Precursors during Fast Short-Term Shear

This paper deals with structural and morphological developments during flow-induced crystallization of molten isotactic polypropylene (iPP). Several authors have invoked the formation of precursors in the early stages of this process. However, it is not clear whether these precursors can be generated and can crystallize already during flow. We address this issue using X-ray scattering (SAXS and WAXD) with a high image capturing rate during and immediately after a strong shear pulse to the undercooled melt. Eventually, we provide the first in situ evidence of formation of flow-induced precursors (FIPs) of crystallization generated applying shear to a fast crystallizing melt of flexible macromolecules, like iPP. Moreover, it is shown that a rheological classification can be used to define the flow conditions promoting FIPs formation. In fact, when molecular stretch is achieved, we found that shear rate is the parameter dominating the formation of structures during shear. When the shear rate is high enough, ...

Simultaneous birefringence, small- and wide-angle X-ray scattering to detect precursors and characterize morphology development during flow-induced crystallization of polymers

An experimental configuration that combines the powerful capabilities of a short-term shearing apparatus with simultaneous optical and X-ray scattering techniques is demonstrated, connecting the earliest events that occur during shear-induced crystallization of a polymer melt with the subsequent kinetics and morphology development. Oriented precursors are at the heart of the great effects that flow can produce on polymer crystallization (strongly enhanced kinetics and formation of highly oriented crystallites), and their creation is highly dependent on material properties and the level of stress applied. The sensitivity of rheo-optics enables the detection of these dilute shear-induced precursors as they form during flow, before X-ray techniques are able to reveal them. Then, as crystallization occurs from these precursors, X-ray scattering allows detailed quantification of the characteristics and kinetics of growth of the crystallites nucleated by the flow-induced precursors. This simultaneous combination of techniques allows unambiguous correlation between the early events that occur during shear and the evolution of crystallization after flow has stopped, eliminating uncertainties that result from the extreme sensitivity of flow-induced crystallization to small changes in the imposed stress and the material. Experimental data on a bimodal blend of isotactic polypropylenes are presented.

Crystallization and Dissolution of Flow-Induced Precursors

We make use of a specially synthesized linear high density polyethylene with a bimodal molecular weight distribution (MWD) to demonstrate that it is possible to produce a suspension of extended-chain (shish) crystals only. Such a suspension can be generated at high temperatures, above but close to the equilibrium melting temperature of the unconstrained extended-chain crystals (T(m)(0)=141.2 degrees C) and requires stretch of the longest chains of the MWD. After the application of a shear flow of 120 s(-1) for 1 s at 142 degrees C, x-ray scattering suggests the presence of a large number of metastable needlelike precursors with limited or no crystallinity. Precursors that are too small dissolve on a timescale that correlates perfectly with the reptation time of the longest polymer molecules. Whereas, precursors that exceed a critical size crystallize forming extended-chain shishes.

Thermal Stability of Shear-Induced Shish-Kebab Precursor Structure from High Molecular Weight Polyethylene Chains

In-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques were used to investigate repeated melting and re-formation of the flow-induced shish-kebab precursor structure in a once-sheared polyethylene (PE) bimodal blend at the confined quiescent state. The blend consisted of a noncrystallizing low molecular weight PE matrix (LMWPE, Mw = 53 000 g/mol, polydispersity = 2.2) and a small amount (2 wt %) of crystallizing high molecular weight PE (HMWPE, Mw = 1 500 000 g/mol, polydispersity = 1.1) under the chosen experimental temperature. After a step shear (shear rate = 125 s-1, shear duration = 20 s, temperature = 126.5 °C), combined SAXS and WAXD results confirmed that the shish-kebab structure was developed mainly from HMWPE chains, following a diffusion-controlled-like process. Although shish formed first followed by microkebabs and then macrokebabs, shish and microkebabs were melted simultaneously as an integrated entity after the macrokebab melting. Upon c...

Probing Flow-Induced Precursor Structures in Blown Polyethylene Films by Synchrotron X-rays during Constrained Melting

Structural changes during constrained melting of blown polyethylene (PE) films were followed by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Results indicated that thermally stable flow-induced crystallization precursor structures (i.e., shish-kebabs) can be revealed above the nominal melting point determined by differential scanning calorimetry (DSC). Two samples were investigated: (1) linear low-density polyethylene (LLDPE) with Mw of 116 kg/mol and (2) a binary blend of 95 wt % LLDPE and 5 wt % high-density polyethylene (HDPE). HDPE had a bimodal molecular weight distribution, containing 80 wt % of low molecular weight (LMW) component (Mw = 99 kg/mol) and high molecular weight (HMW) component (Mw = 1100 kg/mol). Even though the blend contained only a low concentration of HMW−HDPE chains (c = 1 wt %), above its overlap concentration (c* = 0.5 wt %), the initial film showed significantly higher lamellar orientation, and the high-temperatu...

Spatially Resolved Flow-Induced Crystallization Precursors in Isotactic Polystyrene by Simultaneous Small- and Wide-Angle X-ray Microdiffraction

The structural and morphological evolution of shear-induced crystallization precursors generated by pulling a single fiber through a thin layer of molten isotactic polystyrene (i-PS) has been spatially resolved by simultaneous small- and wide-angle X-ray microdiffraction in samples with different thermomechanical histories. Frozen precursor structures with shish-kebab morphology have been detected at different levels of evolution during early stages of their formation, the first step being the development of bundles of few parallel chains oriented along the flow direction (shish) followed by the growth of layered lamellar assemblies (kebabs) simultaneously to the appearance of a very weak wide-angle X-ray diffraction (WAXS) signal corresponding to a crystallinity index lower than 1%. A high degree of orientation of the ordered precursors has been found in the polymer matrix at very close distance from the fiber surface when the sample is quenched to room temperature immediately after cessation of flow; mu...