Dissolution and re-emergence of flow-induced shish in polyethylene with a broad molecular weight distribution

Flow-induced shish formation in semicrystalline polymers is an important phenomenon since shish can strongly influence final material properties. The formation, dissolution, and re-emergence of flow-induced shish were studied for a polyethylene system with a broad molecular weight distribution using time-resolved synchrotron small-angle X-ray scattering (SAXS). The results show that shish are generated for a critical strain of 100 at various shear rates, ranging from 25 to 200 s–1. The shish formed at 140 °C are not stable and decay during a subsequent isothermal process. The specific shish dissolution dynamics is in contrast with the shish formation, affected by the shear rate applied. Unexpectedly, when the polymer melt with dissolved shish was cooled down, shish re-emerges. It demonstrates that the shish created did not dissolve completely into relaxed random coils but relaxed only partially into some ordered precursors that are below the limit of SAXS detection. Moreover, shish dissolution dynamics was significantly slowed down by lowering the isothermal annealing temperature from 140 to 138 or 135 °C, indicating that the shish dissolution involves more than chain mobility at low temperatures

Flow induced crystallization in iPP-DMDBS blends: implications on morphology of shear and phase separation

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-di- methylbenzylidene) 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 1wt% 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, DMDBSforms fibrils whose radius (~5nm) 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 60s-1 for 3s, 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 pre-orientation ofDMDBS fibrils is transformed into 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 under cooling). In contrast, when shear flow is applied prior to DMDBS crystallization, SAXS showed that iPP crystallization occurs with isotropic morphologies

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 ofnanoparticles, 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 thenanoparticles, on the stretch of the long chains. With the applied flow, SWCNTs together with PE chains areobserved to align along the flow direction, giving rise a strong streak like pattern along the equator. At theinitial stages, intensity of the observed streak in the presence of SWCNTs is stronger than that for the neatpolyethylene. The streak intensity stabilizes with time, where the time required for the stabilization dependson the amount of the dispersed nanotubes in the polymer matrix. On the contrary, in the presence of zirconiananoparticles, where the chain-particle interactions between PE and the nanoparticles are weak the initiallyobserved streak tends to disappear with time, where the time required is strongly dependent on theconcentration of the nanoparticles in the polymer matrix. Thus, compared to the neat polymer, the presenceof zirconia nanoparticles destabilizes the shish formation. The chain orientation along the flow direction isdetermined using Herman’s orientation function and the length of the oriented chains (shish) by Ruland’sstreak 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 directionprovide the epitaxy matching thus suppressing the nucleation barrier. The meridional intensity (arising withthe formation of crystals, called kebabs) at room temperature, shows strong dependence on the stable streakintensity (chain orientation along the flow direction, called shish) along the equator prior to cooling

Short-term flow induced crystallization in isotactic polypropylene : how short is short?

The so-called short-term flow protocol is widely applied in experimental flow-induced crystallization studies on polymers in order to separate the nucleation and subsequent growth processes [Liedauer et al. Int. Polym. Proc. 1993, 8, 236–244]. The basis of this protocol is the assumption that structure development during flow can be minimized and the rheological behavior, i.e., the viscosity, does not change noticeably. In this work we explore the validity of this assumption for short but strong flows and reveal the structure formation during the early stages of crystallization. Viscosity and structure evolution of an isotactic polypropylene (iPP, Mw ˜ 365 kg/mol and Mw/Mn = 5.4) melt at 145 °C are measured during the short-flow period (0.2–0.25 s) using the combination of a slit rheometer and fast X-ray scattering measurements. For high enough (apparent) shear rates (=240 s–1) a viscosity rise during flow is observed; i.e., the condition for short-term flow is not satisfied. With a time delay with respect to the viscosity rise, the development of shish is observed at a position halfway the length of slit, along the flow direction, by means of ultrafast time-resolved SAXS measurements. Depending on the shear rate, these shish are detected during (shear rates = 400 s–1) or after flow (240 s–1 = shear rates <400 s–1). For even lower shear rates of 160 and 80 s–1, the viscosity does not change significantly, and instead of shish, oriented row nuclei (X-ray undetectable) are generated. These two shear conditions qualify as short-term flow. A full understanding of the coupled flow and crystallization phenomena requires that the transient and nonhomogeneous behaviors, both in flow and in flow gradient direction, have to be taken into account. This can only be done by a full numerical model, and therefore, the results presented in this paper also provide a valuable data set for future numerical studies

Influence of shear in the crystallization of polyethylene in the presence of SWCNTs

It is well accepted that due to epitaxy matching, carbon nanotubes are good nucleatingagent for linear polyethylene. We demonstrate that not only in the quiescent conditionsbut also at the relatively low shear rates the presence of single-walled carbon nanotube(SWCNT) accelerates the crystallization kinetics of polyethylene (PE). The influence ofSWCNTs on the crystallization kinetics in the quiescent condition is followed with the helpof rheological and differential scanning calorimetry studies. The influence of flow on thestretch of the polymer chain is probed using time-resolved small-angle X-ray scattering(SAXS) and is verified with the Deborah number. SAXS data indicates that the strong shearingconditions (shear rate > 50/s for 1 s) are requisite to form shish-kebab structure in theneat polymer. However, for the low shear (shear rate <50/s for 1 s), the shish-kebab structurethat arises due to chain orientation is enhanced in the presence of SWCNTs. Thedevelopment of oriented structures in SWCNT/PE composites and their absence in the neatpolymer under low shear rate indicates that the presence of SWCNTs plays a significantrole in the chain orientation. Overall, the results manifest the influence of SWCNTs onchain relaxation of the polymer

Bimodal polyethylene : synthesis and study of shear induced oriented structures generated from high molecular weight polyethylene chains

Abstract onl

Effect of cooling rate on the crystal/mesophase polymorphism of polyamide 6

The solidification of the quiescent polyamide 6 (PA 6) melt has been analyzed as a functionof the cooling rate in wide range between about 100 and 10^4 K min^1, by means of differentialscanning calorimetry, by recording of continuous cooling curves, and by time-resolved X-raydiffraction. The performed experiments allowed for the first time to establish the relationshipbetween the cooling rate, the crystallization temperature and the X-ray structure of PA 6.Exclusive formation of monoclinic alpha-crystals is only detected if the crystallizationtemperature is higher than about 430 K or if the cooling rate is slower than about 300 K min^1,respectively. The formation of alpha-crystals is increasingly replaced by development ofmesophase with increasing cooling rate, accompanied with a decrease of the temperature ofcrystallization/ordering. Finally, completely amorphous samples were obtained on coolingfaster than (510) 10^3 K min^1. The continuous decrease of the temperature ofcrystallization with increasing cooling rate, regardless the specific structure formed, precludesa primary effect of the nucleation mechanism on the alpha-crystal/mesophase polymorphism ofPA 6. A preliminary discussion of the effect of molar mass of PA 6 on the cooling-ratedependent polymorphism is also included

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 (Tm0=141.2¿°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¿°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