Summary: Crystallization analysis fractionation (Crystaf) is a polymer characterization technique based on differences in chain crystallizabilities in a dilute solution during non-isothermal crystallization. Crystaf profiles, a weight distribution function of chains crystallized at each temperature, can be used to infer the chemical composition distribution (CCD) of copolymers when a Crystaf calibration curve, a relationship between peak crystallization temperature and average comonomer content, is known. In this investigation, the effect of the number average molecular weight, comonomer type, and cooling rate on Crystaf calibration curves were experimentally investigated. It was found that the cooling rate and comonomer type may strongly affect Crystaf calibration curves, while the influence of molecular weight is relatively subtle. 相似文献
Crystallization analysis fractionation (Crystaf) is a new technique used to estimate the chemical composition distribution (CCD) of semi-crystalline copolymers. In this study, the effect of chain microstructure and operation parameters on Crystaf profiles was investigated using a series of ethylene/1-hexene copolymers and their blends. The Crystaf profiles were also modeled via stochastic simulation based on the distribution of average ethylene sequence lengths. 相似文献
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
Summary: Ethylene/1-olefin copolymers synthesized with multiple-site-type catalytic systems typically exhibit broad molecular weight distribution (MWD) and chemical composition distribution (CCD). These microstrucral characteristics can be described by the presence of several active site types, each of which produces chains with distinct chain microstructures. In this work, a new approach to identify the number of active site types and chain microstructures produced on each active site type was developed based on simultaneous deconvolution of the bivariate MWD/CCD information. Chain microstructures produced on each active site type are assumed to follow Stockmayer's bivariate distribution. The proposed approach was validated with simulated data of model ethylene/1-butene and ethylene/1-octene copolymers. 相似文献
Summary: An artificial neural network (ANN) with a 4-3-3-1 architecture was developed to estimate average comonomer content of ethylene/1-olefin copolymers from crystallization analysis fractionation (Crystaf) results. The ANN was trained with a back propagation algorithm. It was found that average comonomer contents predicted from ANN agree well with experimental results for both training and testing data sets. The developed ANN was also used to systematically investigate the effects of chain microstructures and Crystaf operating conditions on Crystaf calibration curves. 相似文献
Summary: As known, the pore structure of the catalytic support plays a decisive role during the polymerization reactions determining intra-particle mass and heat transport phenomena. In this work, several ethylene polymerizations have been carried out by using as catalytic support different mesostructured materials with unimodal and bimodal pore size distributions in order to evaluate the influence of this pore size distribution on the catalytic behavior. Calcined mesoporous materials were impregnated with the catalytic system MAO/(nBuCp)2ZrCl2 and used for ethylene polymerization and ethylene/1-hexene copolymerizations, at 70 °C and 5 bar of ethylene pressure. Polyethylenes obtained were characterized by GPC, DSC and Crystaf. Results indicate that porous structure of the support has a significant influence on polymerization activity and polymer properties. Despite the catalyst bimodal pore size distribution, only ethylene/1-hexene copolymers presented a bimodal chemical composition distribution. 相似文献
Summary: Linear olefin block copolymers (OBCs) have microstructures that are unique among polyolefins and exhibit properties that are different from those of other polyolefin elastomers. Characterizing their chain microstructures is a challenging task, as conventional characterization techniques cannot probe directly block length distribution or composition. In this work, we used a Monte Carlo model to predict the microstructure details of OBCs and a modified version of the Crystaf model previously developed in our groups to describe theoretical Crystaf profiles for model OBCs. This model can be used as a tool to interpret Crystaf results of these interesting new polyolefins and to relate them to OBC microstructures. Effects of polymerization parameters on OBC microstructure and Crystaf profiles were also discussed. 相似文献
The effects of branch content (BC) and copolymer composition distribution (CCD) on the non-isothermal crystallization kinetics of metallocene m-LLDPEs were studied using modified Avrami analysis, modulated differential scanning calorimetry (MDSC), and Crystaf. Several m-LLDPEs and an m-HDPE - all having comparable Mw and PDI - were experimented. In addition, a ZN-LLDPE was used for comparison purposes. The branch content, unlike the used cooling rates (2-6 °C/min), significantly affected the crystallization behavior. Crystallization peak temperature, , decreased linearly with increasing BC. All the m-LLDPEs showed primary and secondary crystallizations. The secondary crystallization showed to be more pronounced at high BC. The primary crystallization Avrami parameter n for m-HDPE ranged between 3.72 and 4.50, indicating spherulitic crystal growth whereas that for the m-LLDPEs, varied from 2.02 to 5.70. The ZN-LLDPE (having broader composition distribution) offered higher values of and than the m-LLDPEs with similar BC, Mw, and PDI. It, unlike the m-LLDPEs and m-HDPE, fairly agreed with the crystallization kinetic model proposed by Liu et al. The lamella thickness of the m-LLDPEs, L, calculated as per Gibbs-Thomson equation, showed to be in the range 2-16 nm, depending on BC and it decreased approximately following the relationship: L (nm) = 15.0 e(−0.0498BC). 相似文献
A comprehensive mathematical model is developed for the dynamic calculation of the molecular distributed properties (i.e. MWD and CCD) in a gas‐phase, catalytic, ethylene‐1‐butene copolymerization, FBR, taking into account the various kinetic, micro‐ and macroscopic phenomena in the reactor. The effects of the two single‐site catalyst mass fractions and reactor operating conditions on the production of polyolefins with ‘tailor‐made’ bimodal molecular properties are investigated. It is shown that PE grades with either a bimodal MWD or CCD can be produced in a single FBR, using a mixture of two single‐site catalysts under properly selected operating conditions.
Polyethylenes with bimodal molecular weight distribution (MWD) were synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) combined with different activators, which were prepared from alkylaluminium. It is found that the molecular weight (MW) and MWD was influenced by not only iron complexes but activators as well. The activator plays key important role in determination of the MW and MWD of final polymer and the MWD shape could be regulated by selection of various activators and polymerization conditions. The study on the variation of the MWD with the polymerization time and fitting of bimodal MWD with two Flory distributions suggests that bimodal MWD is caused by chain transfer reaction to activator or two active sites. 相似文献