Summary: New crystallization procedures have been developed for the analysis of the chemical composition distribution in polyolefins by pumping a small flow of solvent during the crystallization cycle. One of the new techniques, crystallization elution fractionation (CEF) combines the separation power of TREF and CRYSTAF and has been shown to provide very fast analysis of the composition distribution. 相似文献
Summary: Crystallization Elution Fractionation has been optimized to improve resolution power and reduce co-crystallization. Most important step to be optimized is the first separation cycle of Dynamic Crystallization which can be investigated with the help of Crystallization Analysis Fractionation experiments. A new approach to reduce co-crystallization and improve resolution is presented by applying successive cooling and heating cycles in a long column. The new process known as Multiple Crystallization Elution Fractionation adds extended separation at each cooling and heating cycles. 相似文献
Summary: A series of poly(propylenes) (PPs) were prepared by slurry polymerization using a MgCl2-supported transition metal catalyst. Two different external donors (EDs) were used: diphenyl dimethoxysilane (DPDMS) and methylphenyl dimethoxysilane (MPDMS). The molecular weight (MW) of the PPs was controlled using molecular hydrogen that was used as a transfer agent. To obtain materials with differing molecular weight and similar tacticities, polymers were fractionated with prep-TREF. DSC analyses of blends of TREF fractions showed that the crystallization behaviour of the polymer blends are strongly affected by the configuration (tacticity) and MW of the PP. 相似文献
Complete polymer compositional information of polyolefins is critically valuable in understanding the polymer structure‐processing‐property relationship, thus helping tailor‐design polymers with desired end‐use properties. In this paper, a robust on‐line 2D analytical TREF combined with high‐throughput SEC technique (2D aTREF‐hSEC) is described that can rapidly determine the bivariate distribution of chemical composition and molecular weight of polyethylene resins. Unlike Nakano and Goto's pioneering cross fractionation apparatus and the commercial CFC instrument, this 2D aTREF‐hSEC system has the following unique characteristics: First of all, the GPC injection volume is not limited by the dead volume of the full TREF column. It can be set at any desired values to ensure optimized separation in SEC. Second, the flow rates for the TREF and SEC unit are independently controlled, which add operational flexibility and help optimize the separation efficiency in both the TREF and the SEC dimension. Third, the 2D aTREF‐hSEC system runs continuously instead of stepwise. Once started, the TREF column elutes continuously without stopping to wait for the completion of downstream SEC elution before the next SEC injection, increasing the resolution in TREF dimension. Fourth, experimental time is significantly reduced thanks to the employment of high throughput SEC column. And finally, this 2D aTREF‐hSEC technique in principle can be applied equally well to characterize other semi‐crystalline polymers and potentially any polymers whose solubility is a function of temperature. 相似文献
A commercially available gel permeation chromatograph with triple detection (GPC3D) was coupled with a gas chromatograph to obtain a fast analytical temperature rising fractionation system with triple detection (ATREF3D). On this hybrid instrument we developed a fast method for evaluating in one run, and with less than 0.5 mg of sample, the density, molecular weight, and the type of polyethylene. The method was evaluated with excellent results on commercial polyethylene resins typically used for film and packaging applications. 相似文献
Summary: Temperature rising elution fractionation (TREF) and crystallization analysis fractionation (CRYSTAF) fractionate semicrystalline polymers according to their crystallizabilities from dilute solution and have been widely used to measure the CCD of LLDPE. A new fractionation technique, known as crystallization elution fractionation (CEF), has been developed recently. The main difference between CEF and TREF and CRYSTAF is that the crystallization cycle in CEF is performed dynamically under solvent flow in a long column that contains an inert support material. In this paper, several metallocene-LLDPE resins have been analyzed by CEF to investigate the effect of cooling cycle parameters, comonomer fraction, polymer molecular weight, and blend cocrystallization on the fractionation. This new technique can be used to obtain CCDs with better resolution and in shorter times than TREF and CRYSTAF. 相似文献
A high temperature gel permeation chromatograph (GPC) was coupled with a gas chromatograph (GC) oven to obtain an analytical temperature rising elution fractionation (TREF) system with evaporative light scattering detection. On this instrument, a new column partially filled with pristine carbon nanotubes (CNT) was tested by evaluating the elution profiles in function of temperature (thermograms) of different polyethylene (PE) types. By comparing these thermograms with those obtained with a traditional TREF column filled with metallic wires, the adsorption of polymer chains on the pristine CNT was clearly evidenced. The thermograms given by the column filled with CNT are similar with those provided by literature when chromatographic columns filled with porous graphitic carbon are used for this application, usually described as high temperature thermal gradient interaction chromatography (HT-TGIC). 相似文献
Low-frequency Raman spectra were recorded along the transition front of uniaxially cold drawn High Density Polyethylene (HDPE) samples with a Raman microscope instrument. The high spatial resolution capacity of the technique allows to analyze the variation of the Longitudinal Acoustic Mode (LAM) along the transition zone between the isotropic structure and the oriented one. The evolution of the LAM over the transition zone was followed by measuring the LAM intensity, bandwidth and position as a function of the distance along the transition front. The LAMs are affected by the stretching process before the transition zone is reached and the lamellar structure was found to be totally disrupted before the fibrillar structure is formed. Whereas the LAM intensity decreases gradually along the transition region, the bandshape and peak frequency do not change significantly, indicating that the distribution of fully planar zigzag segments in the remaining lamella, structure is not really affected during the deformation process. The concentration of lamella, which still remain dimensionally intact, decreases gradually along the transition zone up to the stage where the structure is getting fibrillar. 相似文献
Using a layer‐multiplying coextrusion process to fabricate films with thousands of alternating polymer nanolayers, we report here a new crystalline morphology in confined polymer nanolayers and an abrupt transition in the crystallization habit. At higher temperatures, poly(ethylene oxide) crystallizes as large, in‐plane lamellae. A 5 °C change in the crystallization temperature produces an on‐edge lamellar orientation. The results point to a transition from heterogeneous nucleation to substrate‐assisted nucleation. This may be a general phenomenon that accounts for previously unexplained differences in the preferred chain alignment of confined polymer crystals.
Heterogeneous Ziegler-Natta catalysts produce polyolefins that have broad distributions of molecular weight (MWD) and chemical composition (CCD). For such broad distributions, mathematical models are useful to quantify the information provided by polyolefin analytical techniques such as high-temperature gel permeation chromatography (GPC), temperature rising elution fractionation (TREF), and crystallization analysis fractionation (CRYSTAF). In this paper, we developed a mathematical model to deconvolute the MWD and CCD of polyolefins simultaneously, using Flory's most probable distribution and the cumulative CCD component of Stockmayer's distribution. We have applied this procedure to “model” polyolefin resins and to one industrial linear low-density polyethylene (LLDPE) resin. The proposed methodology is able to deconvolute theoretical distributions even when random noise is added to the MWDs and CCDs, and it can be used to calculate the minimum number of active site types on heterogeneous Ziegler-Natta catalysts. 相似文献
