The use of a tetrafunctional epoxy‐based additive to modify the molecular structure of poly(ethylene terephthalate) (PET) was investigated with the aim of producing PET foams by an extrusion process. The molecular structure analysis and shear and elongation rheological characterization showed that branched PET is obtained for 0.2, 0.3 and 0.4 wt% of a tetrafunctional epoxy additive. Gel permeation chromatography (GPC) analysis led to the conclusion that a randomly branched structure is obtained, the structure being independent of the modifier concentration. The evolution of shear and extensional behavior as a function of molecular weight (Mw), degree of branching, and molecular weight distribution (MWD) were studied, and it is shown that an increase in the degree of branching and Mw and the broadening of the MWD induce an increase in Newtonian viscosity, relaxation time, flow activation energy and transient extensional viscosity, while the shear thinning onset and the Hencky strain at the fiber break decrease markedly. 相似文献
Summary: The molecular weight distribution (MWD), formed in emulsion polymerization that involves the polymer transfer reaction during Interval II, may approach the power‐law distribution as polymerization proceeds. The power exponent, α, of the weight fraction distribution W(M) = M?α conforms to the relationship, α = 1/Pb, where Pb is the probability that the chain end is connected to a backbone chain. The MWD of emulsion‐polymerized polyethylene reported in literature agrees reasonably well with the relationship, W(M) = M?α with α = 1/Pb. This simple relationship could be used to estimate the Pb value from the MWD data, possibly leading to determining the polymer transfer constant under well‐designed experimental conditions. Because α > 1, the number‐average MW always approaches a finite value, but the weight‐ and higher order‐averages of MWD may continue to increase as the particle grows without limit depending on the magnitude of Pb. The power‐law distributions are self‐similar, possessing the nature of fractals and lacking a characteristic scale. The i‐th moment of the MWD for the present reaction system continues to increase without limit during Interval II for Pb ≥ 1/i.
Molecular weight distribution of the emulsion‐polymerized polyethylene. 相似文献
The rheological and processing behavior (melt fracture performance) of linear lowdensity polyethylenes (LLDPEs) is studied as a function of both the weight average molecular weight (Mw) and its distribution (MWD). A number of LLDPE resins having different molecular characteristics were tested, with essentially one characteristic (Mw or MWD) changing at a time. The first series of resins consisted of nine samples having a wide range of polydispersities (3.3–12.7) and nearly constant Mw and short chain branching. The second series had six resins with varying Mw (51,000–110,000) but fixed MWD (about 4). The influence of Mw and MWD on the viscosity profiles, linear viscoelastic moduli as expressed by means of a discrete spectrum of relaxation times, extrudate swell, and melt fracture behavior for these resins is reported. Correlations between the molecular characteristics of the resins and their rheological and processing behavior are also reported. It is found that for a given molecular weight, the optimum melt fracture performance is obtained at a specific polydispersity value, and it is characterized by a minimum relaxation time for the resin defined in terms of recoverable shear. 相似文献
The rheological properties of a semi‐dilute ultra‐high molecular weight polyethylene (UHMw‐PE)/paraffin wax solution were investigated by mainly focusing on the influence of its concentration on the shear flow viscosity. It was found that the UHMw‐PE solution exhibits a shear‐thinning behavior at a very wide shear rate range from 10?4 to higher than 103 sec?1. Furthermore, this typical non‐Newtonian behavior was more obvious with a concentration increase. From the concentration dependence of the zero‐shear creep compliance or other rheological factor, it was found that the extremely large Me value of the system gives rise to various kinds of non‐Newtonian behaviors, especially those highly elastic in nature. Finally, the origin of the abnormal stress fluctuation during the steady shear measurement was found to be related to the shear‐induced structural development of the solution. 相似文献
The effect of molecular architecture on the dynamic viscoelastic properties of new metallocene high density polyethylenes has been analyzed. Bimodal molecular weight distribution metallocene polyethylenes show features different from conventional polydisperse and bimodal polyethylenes. Higher values of Newtonian viscosity (ηo) at the same values of weight average molecular weight (Mw) and stronger frequency dependence of dynamic viscosity (η′) than in conventional HDPE-s have been observed; this leads to lower values of the characteristic frequency for the onset of non-Newtonian behavior (ωo) and higher values of the power law index (α). These features are probably due to the presence of very small amounts of long chain branching (LCB). The implications of these results in polymer processing are analyzed comparing extrusion rheometer data, which leads to the conclusion that extrusion difficulties in metallocene catalyzed polyethylenes can be overcome with bimodal molecular weight distributions. 相似文献
The effect of molecular weight distribution (MWD) on diffusion at symmetric polymer/polymer interfaces is investigated by rheological tools. A new model allowing the determination of a self‐diffusion coefficient of polydisperse polymer systems is presented. The model is based on the double reptation theory and Doi and Edwards' molecular dynamics applied to A/A polymers brought into intimate contact in the molten state. The material parameters for the model are obtained from linear oscillatory shear experiments, in which the dynamic shear modulus is measured in parallel plate geometry under a small amplitude of deformation as a function of time and frequency for a sandwich‐like assembly. The experiments were conducted on polystyrene (PS) blends with constant weight average molecular weight (Mw) but with variable number average molecular weights (Mn). The measured self‐diffusion coefficients showed that the presence of short molecules in the blend increases the mean value of the self‐diffusion coefficient and the magnitude of such increase can be quantitatively evaluated by the proposed model. 相似文献
Linear low density polyethylenes are manufactured by copolymerizing ethylene with 1-alkenes, yielding a linear polyethylene backbone with short side chains. Due to the nature of the catalyst used in the polymerizaton, multimodal branching distributions are typically obtained. In this report, we have investigated the processability of four 1-octene linear low density polyethylenes as a function of the short chain branching distribution. Analytical techniques such as 13C nuclear resonance spectroscopy, size exclusion chromatography, differential scanning calorimetry, and temperature rising elution fractionation, in particular were used to elucidate the molecular structure. Processability measurements were made using various extrusion techniques and dynamic mechanical analyses.It was determined that in the absence of any variations in molecular weight, the polymers with the higher proportions of linear polyethylene showed inferior processability In terms of onset of surface imperfections at lower extrusion rates. Polymers with worse processability characteristics also exhibited higher zero shear viscosities. 相似文献
This paper reviews a new family of olefin polymerization catalysts. The catalysts, named FI catalysts, are based on non‐symmetrical phenoxyimine chelate ligands combined with group 4 transition metals and were developed using “ligand‐oriented catalyst design”. FI catalysts display very high ethylene polymerization activities under mild conditions. The highest activity exhibited by a zirconium FI catalyst reached an astonishing catalyst turnover frequency (TOF) of 64,900 s –1 atm –1, which is two orders of magnitude greater than that seen with Cp2ZrCl2 under the same conditions. In addition, titanium FI catalysts with fluorinated ligands promote exceptionally high‐speed, living ethylene polymerization and can produce monodisperse high molecular weight polyethylenes (Mw/Mn<1.2, max. Mn>400,000) at 50 °C. The maximum TOF, 24,500 min –1 atm –1, is three orders of magnitude greater than those for known living ethylene polymerization catalysts. Moreover, the fluorinated FI catalysts promote stereospecific room‐temperature living polymerization of propylene to provide highly syndiotactic monodisperse polypropylene (max. [rr] 98%). The versatility of the FI catalysts allows for the creation of new polymers which are difficult or impossible to prepare using group 4 metallocene catalysts. For example, it is possible to prepare low molecular weight (Mv∼103) polyethylene or poly(ethylene‐co‐propylene) with olefinic end groups, ultra‐high molecular weight polyethylene or poly(ethylene‐co‐propylene), high molecular weight poly(1‐hexene) with atactic structures including frequent regioerrors, monodisperse poly(ethylene‐co‐propylene) with various propylene contents, and a number of polyolefin block copolymers [e.g., polyethylene‐b‐poly(ethylene‐co‐propylene), syndiotactic polypropylene‐b‐poly(ethylene‐co‐propylene), polyethylene‐b‐poly(ethylene‐co‐propylene)‐b‐syndiotactic polypropylene]. These unique polymers are anticipated to possess novel material properties and uses. 相似文献