Use graphene coated silica core shell particles to provide rapid, precise, and equivalent chemical composition distribution analysis for polyolefin materials by high temperature thermal gradient interaction chromatography.

High temperature thermal gradient interaction chromatography (HT-TGIC) has been widely used to measure chemical composition distribution due to its applicability to separate crystalline and non-crystalline amorphous polyolefin materials. The compatibility of HT-TGIC with various detectors (infrared (IR), light scattering (LS), and viscometer) has also allowed a comprehensive analysis of molecular architecture of polyolefin and recycled plastics. The introduction of an easy-to-fabricate graphene coated onto non-porous silica particles as HT-TGIC column in 2020 showed a superior chromatographic performance over the traditional graphite column. A reduction in peak broadness (∼47 %) under identical experimental conditions was demonstrated in that research. This paper similarly uses a graphene column but with the focus on optimization of experimental parameters (concentration, and thermal cooling and heating rates etc.). Equivalent chemical composition distribution (CCD) data to that obtained by the incumbent graphite column over a wide range of polyolefins products was achieved, in addition to a shortened analysis time from 120 min down to 88 min per sample. The materials studied included semicrystalline linear low-density polyethylene (LLDPE), elastomers, terpolymers, model blends to mimic recycled plastics. The results also suggest that the elimination of substrate pores enable a better HT-TGIC separation. Coupling the ease and reproducibility of the graphene column fabrication process enables long term chromatographic robustness. This not only results in equivalent CCD data compared to the traditional graphite column but also a 27 % reduction in analysis time. These results demonstrate a substantial advancement of technology in the high throughput industrial laboratory setting where fast testing turnaround time is critical. In addition, simple fabrication with commercially available silica particles and graphene nanopowder provides a cost-effective approach to make HT-TGIC columns reproducibly.

Development of a robust calibration method for improving the long-term precision of polyethylene comonomer content distribution using crystallization elution fractionation.

Comonomer content distribution (CCD), also commonly known as chemical composition distribution (CCD) and short chain branching distribution (SCBD), describes the variation of short chain branching composition between individual polymer chains in polyolefin materials. It is of particular importance for controlling polyolefin performance. Crystallization-based separation methods have evolved over the past four decades aiming at resolution, speed, precision, and accuracy. Two of the commonly used techniques are Crystallization Elution Fractionation (CEF) and Temperature Rising Elution Fractionation (TREF), where polymer chains are physically separated along the column or on the surface of the support based on their crystallinity, respectively. CEF analysis takes much less time than TREF. There is a critical need for precise temperature calibrations for data repeatability. This report demonstrates a novel CEF methodology using a two-point technique to consistently and conveniently calibrate the comonomer composition and column temperature. This column temperature calibration methodology was adopted in a study by tracking the reproducibility over a period of 8 years, using multiple instruments located in different laboratories and in different geographies. The results exhibited superior repeatability, with less than 0.3% of the relative error calculated from 3000 data points of the eluting peak temperature, thus demonstrating this as a robust method for industrial labs that require good quality controls.

Polyolefin Analyses with a 10 mm Multinuclear NMR Cryoprobe.

Polyolefins are important and broadly used materials. Their molecular microstructures have direct impact on macroscopic properties and dictate end-use applications. 13C NMR is a powerful analytical technique used to characterize polyolefin microstructures, such as long-chain branching (LCB), but it suffers from low sensitivity. Although the 13C sensitivity of polyolefin samples can be increased by about 5.5 times with a cryoprobe, when compared with a conventional broadband observe (BBO) probe, further sensitivity enhancement is in high demand for studying increasingly complex polyolefin microstructures. Toward this goal, distortionless enhancement by polarization transfer (DEPT) and refocused insensitive nuclei enhanced by polarization transfer (RINEPT) are explored. The use of hard, regular, and new short adiabatic 180° 13C pulses in DEPT and RINEPT is investigated. It is found that RINEPTs perform better than DEPTs and a sensitivity enhancement of 3.1 can be achieved with RINEPTs. The results of RINEPTs are further analyzed with statistics software JMP and recommendations for optimal usage of RINEPTs are suggested. An example of analyzing saturated chain ends in an ethylene-octene copolymer sample with a hard 180° 13C RINEPT pulse is demonstrated. It is shown that the experimental time can be further reduced in half because of faster proton relaxation, where the total experimental time is about 580 times shorter when compared to using a conventional method and a 10 mm BBO probe. A naturally abundant nitrogen-containing polyolefin is analyzed using 1H-15N HMBC and, to our knowledge, is the first 1H-15N HMBC presented in the field of polyolefin characterization. The relative amount of similar nitrogen-containing structures is quantified by two-dimensional integration of 1H-15N HMBC. Two pragmatic technical challenges related to using high-sensitivity NMR cryoprobes are also addressed: (1) A new 1H decoupling sequence Bi_Waltz_65_256pl is proposed to address decoupling artifacts in 13C{1H} NMR spectra which contain a strong 13C signal with a high signal-to-noise ratio (S/N). (2) A simple pulse sequence that affords zero-slope spectral baselines and quantitative results is presented to address acoustic ringing that is often associated with high-sensitivity cryoprobe use.

Analyses of Short Chain Branches in Polyolefins with Improved 1H NMR Spectroscopy.

Polyolefin microstructures, for example, short chain branching (SCB) and short chain branch distribution (SCBD), have a direct impact on properties and thus ultimately influence end-use applications. The 1H NMR approach to analyze SCB and SCBD is particularly useful when only a limited amount of sample is available, for example, polyolefin film layers or the fractions from polyolefin separation techniques, such as gel permeation chromatography (GPC), crystallization elution fractionation (CEF), high temperature liquid chromatography (HTLC), and thermal gradient interaction chromatography (TGIC). In this paper, we discuss the best approach to find a good decoupling frequency and propose an improved 1H pulse sequence with homonuclear decoupling for better measuring SCB. With this new pulse it is possible to reach a S/N of 10 (level of quantification) for the methyl signal from SCB in an ethylene-hexene copolymer (EH, 3.6 mol % H) in 3.5 min with 0.5 µg of sample. We also show an easy method to calculate SCB/1000C and demonstrate the proper use of heteronuclear single quantum coherence (HSQC) to measure SCB in a complicated system. A very quick approach to examine the presence of a small amount of LDPE in a polyolefin sample is also suggested, which can reduce NMR acquisition time from a couple of days to a few minutes.

Separating effective high density polyethylene segments from olefin block copolymers using high temperature liquid chromatography with a preloaded discrete adsorption promoting solvent barrier.

Recent advances in catalyst technology have enabled the synthesis of olefin block copolymers (OBC). One type is a "hard-soft" OBC with a high density polyethylene (HDPE) block and a relatively low density polyethylene (VLDPE) block targeted as thermoplastic elastomers. Presently, one of the major challenges is to fractionate HDPE segments from the other components in an experimental OBC sample (block copolymers and VLDPE segments). Interactive high temperature liquid chromatography (HTLC) is ineffective for OBC separation as the HDPE segments and block copolymer chains experience nearly identical enthalpic interactions with the stationary phase and co-elute. In this work we have overcome this challenge by using liquid chromatography under the limiting conditions of desorption (LC LCD). A solvent plug (discrete barrier) is introduced in front of the sample which specifically promotes the adsorption of HDPE segments on the stationary phase (porous graphitic carbon). Under selected thermodynamic conditions, VLDPE segments and block copolymer chains crossed the barrier while HDPE segments followed the pore-included barrier solvent and thus enabled separation. The barrier solvent composition was optimized and the chemical composition of fractionated polymer chains was investigated as a function of barrier solvent strength using an online Fourier-transform infrared (FTIR) detector. Our study revealed that both the HDPE segments as well as asymmetric block copolymer chains (HDPE block length≫VLDPE block length) are retained in the separation and the barrier strength can be tailored to retain a particular composition. At the optimum barrier solvent composition, this method can be applied to separate effective HDPE segments from the other components, which has been demonstrated using an experimental OBC sample.

Toward absolute chemical composition distribution measurement of polyolefins by high-temperature liquid chromatography hyphenated with infrared absorbance and light scattering detectors.

Chemical composition distribution (CCD) is a fundamental metric for representing molecular structures of copolymers in addition to molecular weight distribution (MWD). Solvent gradient interaction chromatography (SGIC) is commonly used to separate copolymers by chemical composition in order to obtain CCD. The separation of polymer in SGIC is, however, not only affected by chemical composition but also by molecular weight and architecture. The ability to measure composition and MW simultaneously after separation would be beneficial for understanding the impact of different factors and deriving true CCD. In this study, comprehensive two-dimensional chromatography (2D) was coupled with infrared absorbance (IR5) and light scattering (LS) detectors for characterization of ethylene-propylene copolymers. Polymers were first separated by SGIC as the first dimension chromatography (D1). The separated fractions were then characterized by the second dimension (D2) size exclusion chromatography (SEC) with IR5 and LS detectors. The concentrations and compositions of the separated fractions were measured online using the IR5 detector. The MWs of the fractions were measured by the ratio of LS to IR5 signals. A metric was derived from online concentration and composition data to represent CCD breadth. The metric was shown to be independent of separation gradients for an "absolute" measurement of CCD breadth. By combining online composition and MW data, the relationship of MW as a function of chemical composition was obtained. This relationship was qualitatively consistent with the results by SEC coupled to IR5, which measures chemical composition as a function of logMW. The simultaneous measurements of composition and MW give the opportunity to study the SGIC separation mechanism and derive chain architectural characteristics of polymer chains.

Carbonaceous sorbents for high-temperature interactive liquid chromatography of polyolefins.

The elution behavior of polyethylene (PE) and the three stereoisomers of polypropylene (PP) was studied on porous graphite along with three other carbon-based sorbents, carbon-clad zirconia particles, activated carbon, and exfoliated graphite in a systematic way in this work. Decahydronaphthalene, 1,2,3,4-tetrahydronaphthalene, 1,3,5-trimethylbenzene, tetrachloroethylene, xylene and p-xylene were used as mobile phases. While PE is adsorbed to various extents on all the tested carbonaceous sorbents from the majority of the solvents, PP is fully adsorbed only in selected cases. Testing alcohols (C7-C9) as mobile phase with Hypercarb™ indicates that all stereoisomers of PP are selectively adsorbed and desorbed when a solvent gradient alcohol→1,2,4-trichlorobenzene is used at 160°C. The retention of all stereoisomers of PP increases with the polarity of the alcohol. Linear PE is retained on Hypercarb™ even from 1,2-dichloro- and 1,2,4-trichlorobenzene, when a temperature below 120°C is applied, while it is not retained from these solvents at higher temperatures. All stereoisomeric forms of PP are not adsorbed under the same conditions. Some of the tested new sorbent/solvent systems have potential to be applied in routine analysis of industrially synthesised polyolefins.

A high-throughput three-capillary noise-cancelling viscometer for chromatographic systems.

This paper describes an analytical three-capillary viscometer detector that eliminates the traditional viscometer "hold-up" volume (commonly found in four-capillary designs) while maintaining cancellation of short-term pump noise and long-term baseline drift of temperature and solvent flow rate that are inherent in chromatography systems. This improvement allows a staggered sample injection approach in chromatography, yielding a significant increase in sample throughput by cutting down the chromatographic run time. It also provides a more robust design as it does not require capillary rebalancing, complex purging, flushing or changing the hold-up volume to accommodate long-term chromatographic use.

Determination of antioxidants in polyolefins using total dissolution methodology followed by RPLC.

Antioxidants are added to polyolefins to improve the stability of the resin from oxidation and degradation during processing of the finished article and to increase product lifetime. Without antioxidants, polyolefins would quickly degrade during and after the extrusion or thermoforming process, which would cause inferior appearance and physical properties. The proper level must be added to protect the polymer and to minimize cost. Antioxidants are usually extracted from the resin and the extract is analyzed by RPLC, GC, or Fourier transform infrared spectroscopy. Unfortunately, many of these procedures require significant manual labor, time, and solvent, rendering them impractical for high-throughput work processes. In addition, they may not provide complete extraction of the additives depending upon the type of resin. A validated analytical method was needed for the determination of three common antioxidants, Irganox(®) 1010, Irganox(®) 1076, and Irgafos(®) 168 in polyolefin resins. This paper shows the determination of these antioxidants using dissolution followed by precipitation with o-xylene and methanol. Direct analysis of the solution is achieved in 8 min using RPLC.

13C NMR of polyolefins with a new high temperature 10 mm cryoprobe.

Recently, a high temperature 10 mm cryoprobe was developed. This probe provides a significant sensitivity enhancement for (13)C NMR of polyolefins at a sample temperature of 120-135 degrees C, as compared to conventional probes. This greatly increases the speed of NMR studies of comonomer content, sequence distribution, stereo- and regioerrors, saturated chain end, unsaturation, and diffusion of polymers. In this contribution, we first compare the (13)C NMR sensitivity of this probe with conventional probes. Then, we demonstrate one of the advantages of this probe in its ability to perform 2D Incredible Natural Abundance Double Quantum Transfer Experiment (2D INADEQUATE) in a relatively short period of time. The 2D INADEQUATE has been rarely used for polymer studies because of its inherently very low sensitivity. It becomes even more challenging for studying infrequent polyolefin microstructures, as low probability microstructures represent a small fraction of carbons in the sample. Here, the 2D INADEQUATE experiment was used to assign the (13)C NMR peaks of 2,1-insertion regioerrors in a poly(propylene-co-1-octene) copolymer.

A new decoupling method for accurate quantification of polyethylene copolymer composition and triad sequence distribution with 13C NMR.

(13)C NMR is a powerful analytical tool for characterizing polyethylene copolymer composition and sequence distribution. Accurate characterization of the composition and sequence distribution is critical for researchers in industry and academia. Some common composite pulse decoupling (CPD) sequences used in polyethylene copolymer (13)C NMR can lead to artifacts such as modulations of the decoupled (13)C NMR signals (decoupling sidebands) resulting in systematic errors in quantitative analysis. A new CPD method was developed, which suppresses decoupling sidebands below the limit of detection (less than 1:40,000 compared to the intensity of the decoupled signal). This new CPD sequence consists of an improved Waltz-16 CPD, implemented as a bilevel method. Compared with other conventional CPD programs this new decoupling method produced the cleanest (13)C NMR spectra for polyethylene copolymer composition and triad sequence distribution analyses.

The influence of PEO/poly(vinyl phenol-co-styrene sulfonate) aqueous complex structure on flocculation.

Colloidal suspensions were flocculated with complexes formed from high molecular weight polyethylene oxide (PEO) and a cofactor. Poly(vinyl phenol-co-potassium styrene sulfate) (PKS) or poly(styrene-co-styrene sulfonate) (PS-co-SSS) copolymers were used as the cofactors for this work. The larger the PEO/cofactor complex species, the better the initial flocculation. Factors such as increasing temperature or ionic strength that gave smaller complexes also gave poorer flocculation. Cofactor performance was sensitive to the balance of hydrophobic phenolic groups and hydrophilic styrene sulfonates. If there are too few phenolic groups, the PEO/PSK complexes are large but are too weak to give shear-resistant flocs, whereas complexes formed with high phenolic content PSK are relatively small, giving poorer flocculation but more shear-resistant flocs. Both phenyl and phenol groups are effective as the hydrophobic component in the cofactor. The hydrogen-bonding potential of phenolic cofactors does not seem to offer much advantage relative to phenyl groups. A crucial step in the flocculation is the adsorption of PEO/cofactor complex onto the target colloids. Thus, flocculation is sensitive to the target colloid surface chemistry. Positively charged precipitated calcium carbonate and surfactant-free polystyrene latex are particularly easy to flocculate because adsorption is driven by electrostatic and hydrophobic interactions, respectively. By contrast, the latex coated with hydrophilic poly(N-isopropylacrylamide) (PNIPAM) does not flocculate because the PEO/cofactor complex does not bind to PNIPAM. Finally, the flocculation of highly negatively charged, dextran sulfate coated calcium carbonate seems to be stimulated by the presence of soluble calcium ions that make the complex less soluble and more likely to adsorb.