ABSTRACT
The molecular structure elucidation of complex ethylene-propylene copolymers (EPCs) has benefited tremendously from the ability to combine preparative temperature rising elution fractionation (prep TREF) with various conventional analytical techniques. Recently reported, prep TREF-high-temperature solvent gradient interaction chromatography (HT-SGIC) (Cheruthazhekatt et. al, Macromolecules 45:2025-2034, 2012) is one of the most effective and highly useful coupled methods that allow for the exact measurement of the chemical composition distribution (CCD) present in various components of EPCs. The major drawback of prep TREF involving slow crystallization and elution steps is the long time per experiment. Here, we present a new and by far the simplest and fastest preparative fractionation method for complex polyolefins-preparative solution crystallization fractionation (prep SCF). The scope of the present study was to achieve a fast fractionation of complex bulk samples into an amorphous, semicrystalline and highly crystalline fraction, in sufficient amounts for the subsequent detailed compositional analysis. The effects of two different solvents, xylene and trichlorobenzene (TCB), and their influence on the solution crystallization of chemically different components of EPC were systematically investigated by combining prep SCF with crystallization analysis fractionation (CRYSTAF), FTIR, differential scanning calorimetry (DSC) and HT-SGIC analyses. Significant differences in the chemical composition of similar SCF fractions obtained from xylene and TCB were observed indicating the strong influence of the solvent on solution crystallization. Prep SCF-HT-SGIC results showed that, under similar experimental conditions, TCB as the fractionation solvent provides superior separation of complex semicrystalline ethylene-propylene (EP) components. Very interestingly, for the first time, separation of soluble fractions (30 °C) of iPP, EPC and PE homopolymer components in complex EPC was achieved by prep SCF in TCB. On the other hand, SCF fractionation in xylene provides a soluble fraction that is perfectly amorphous as has been shown by DSC and CRYSTAF. Based on these results, the present SCF approach and an updated method of the combination of prep SCF-HT-SGIC hold significant promise for the fractionation and characterization of similar complex EPCs in a simple way within a short analysis time, by using significantly smaller amounts of solvent compared to the previously reported, rather time-consuming, prep TREF-HT-SGIC combination. No similarly selective solution crystallization fractionations in preparative scale have been reported before.
ABSTRACT
High-temperature solvent gradient interaction chromatography (HT-SGIC) is a fast and efficient fractionation technique for the chemical composition analysis of olefin copolymers. The separation of ethylene-propylene random copolymers (EPRs) was achieved on a graphitic stationary phase, Hypercarb, at 160 °C by using linear solvent gradient elution from 1-decanol to 1,2,4-trichlorobenzene (TCB). In the present work, the solvent gradient profile was modified to improve the chromatographic separation of EPRs. With the aim to obtain a better resolution in separation, a slow increase in the volume fraction of TCB was applied. This allowed for a relatively large retention region for linear polyethylene (PE) chains on the column; thereby, a broader elution volume zone between the start of the gradient and the PE elution was achieved. The efficiency of this new gradient profile was demonstrated by analysing two fully amorphous EPR samples. Clear differences in the chemical composition of these EPR samples with similar ethylene contents have been proven by using this modified solvent gradient. The comprehensive chemical composition and microstructure analysis of the SGIC-separated fractions by FTIR revealed that ethylene/propylene (EP) copolymer chains were eluted according to their ethylene/propylene contents and E or P sequence lengths, even though they are distributed in a random manner. These results showed that the solvent composition is an important factor to affect the interactive adsorption or desorption behaviour of EP chains on Hypercarb. In this way, for the first time, the determination of the complex composition and chain structure of EPR samples was achieved within short analysis time, which is not possible till now using other fractionation techniques reported.
ABSTRACT
For the first time, the solution crystallization and dissolution behavior of polyolefins in a variety of solvents was investigated by using a recently developed crystallization based analysis technique, solution crystallization analysis by laser light scattering (SCALLS). SCALLS results provide clear evidence that crystallization and dissolution of linear polyethylene (PE) and isotactic polypropylene (iPP) are greatly influenced by the type of solvent used. It was demonstrated for a blend of PE and iPP that cocrystallization effects are minimal in solvents such as TCB and o-DCB and are significantly more pronounced in xylene and decalin. Surprisingly, in xylene, individual dissolution curves (bimodal SCALLS profile) for both PE and iPP with minimal codissolution effects were observed while in TCB, o-DCB, and decalin both components dissolve simultaneously. These findings provide a novel and facile approach to understand the effect of solvents on cocrystallization and codissolution of chemically dissimilar components in preparative fractionations such as prep TREF (which normally uses xylene), by using TCB as the crystallization solvent and xylene as the eluent.
ABSTRACT
A novel, powerful analytical technique, preparative temperature rising elution fractionation (prep TREF)/high-temperature (HT)-HPLC/Fourier transform infrared spectroscopy (FTIR)/high-performance differential scanning calorimetry (HPer DSC)), has been introduced to study the correlation between the polymer chain microstructure and the thermal behaviour of various components in a complex impact polypropylene copolymer (IPC). For the comprehensive analysis of this complex material, in a first step, prep TREF is used to produce less complex but still heterogeneous fractions. These chemically heterogeneous fractions are completely separated by using a highly selective chromatographic separation method--high-temperature solvent gradient HPLC. The detailed structural and thermal analysis of the HPLC fractions was conducted by offline coupling of HT-HPLC with FTIR spectroscopy and a novel DSC method--HPer DSC. Three chemically different components were identified in the mid-elution temperature TREF fractions. For the first component, identified as isotactic polypropylene homopolymer by FTIR, the macromolecular chain length is found to be an important factor affecting the melting and crystallisation behaviour. The second component relates to ethylene-propylene copolymer molecules with varying ethylene monomer distributions and propylene tacticity distributions. For the polyethylene component (last eluting component in all semi-crystalline TREF fractions), it was found that branching produced defects in the long crystallisable ethylene sequences that affected the thermal properties. The different species exhibit distinctively different melting and crystallisation behaviour, as documented by HPer DSC. Using this novel approach of hyphenated techniques, the chain structure and melting and crystallisation behaviour of different components in a complex copolymer were investigated systematically.
ABSTRACT
Impact polypropylene copolymers (IPC) are extremely complex materials that can only be effectively analysed by multidimensional analytical approaches. IPC consists of isotactic polypropylene (iPP) as the major phase, ethylene-propylene (EP) copolymers of various compositions and small amounts of polyethylene. The molecular heterogeneity of two IPC samples having different ethylene contents was studied by using a novel cross-fractionation technique, developed from a combination of various analytical separation methods into an effective characterisation tool for complex polyolefins. The initial step involves the fractionation of the sample into EP rubber, EP segmented copolymer, and iPP, by preparative temperature rising elution fractionation (TREF). The resulting fractions are still distributed with regards to chemical composition and molar mass. The separation with respect to these parameters is conducted by comprehensive HT 2D-LC. This is the first time that the individual components in all TREF fractions of an IPC are separated and analysed mutidimensionally, by both SEC-FTIR, high-temperature (HT) HPLC-FTIR, and HT 2D-LC. Molar mass analysis of the chemically homogeneous fractions from HT HPLC is accomplished by HT SEC in the second dimension of HT 2D-LC. The chemical composition of all species is determined by coupling FTIR spectroscopy to HT HPLC via an LC-transform interface. This novel approach reveals the capability of this hyphenated technique to determine the exact chemical composition of the individual components in the complex TREF fractions of IPCs. The HT HPLC-FTIR results confirm the separation mechanism in the given chromatographic system using a 1-decanol to TCB solvent gradient and a Hypercarb stationary phase. The components of differing chemical composition are separated according to the nature and length of the propylene/ethylene segments, with their arrangement in the chains strongly affecting their adsorption/desorption on the stationary phase. FTIR analysis provides information on the ethylene and propylene contents of the fractions as well as on the ethylene and propylene crystallinities.
