High temperature multidimensional chromatography of complex and functionalized polyolefins

Ndiripo, Anthony (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Polyolefins like all other synthetic polymers have complex microstructures that must be understood in order to predict product performance and adjust catalyst and reactor technologies. Between synthesis and polymer application, characterization techniques act as the visual aid for polymer microstructure. In further conversion processes, products can be modified e.g. via oxidation or grafting which introduces functionality and functionality type distributions (FTD), in addition to the molar mass and chemical composition distributions (MMD and CCD, respectively). Furthermore, the use of multiple reactor and catalyst technologies to produce a given product results in complex mixtures or alloys of polymers with superior properties, but very complex microstructures. These microstructural properties must be defined and if possible, libraries of microstructural data and suitable comprehensive techniques should be established for each type of material. High-temperature chromatographic techniques have emerged as fast and reliable techniques for polyolefin characterization. High-temperature liquid chromatography (HT-LC) which can be operated using solvent gradient and thermal gradient interaction modes (SGIC and TGIC, respectively) provides interesting alternatives to gas chromatography (GC) for the analysis of low molar mass oxidized waxes. In addition, non-crystallizing elastomers that cannot be characterized using crystallization-based methods such as temperature rising elution fractionation (TREF) and crystallization analysis fractionation (CRYSTAF) can be analysed using the mentioned modes of HT-LC. Polyolefin characterization is enhanced by the hyphenation of multiple techniques as no one method is able to fully define the multiple microstructural distributions. Preparative fractionation has emerged as an indispensable tool that, when coupled to other separation and fractionation techniques, yields a wealth of information on polyolefin microstructure. The aim of the thesis is to utilize comprehensive HT-LC methods for the separation of complex polyolefin materials such low molar mass oxidized waxes, elastomers, and bimodal high density polyethylene (HDPE). Each type of selected polyolefin material presents a set of specific characterization challenges that must be addressed by HT-LC and/or hyphenation to a suitable preparative fraction technique. Work presented in this study demonstrates the separation of oligomers on porous graphitic carbon (PGC) of oxidized waxes under tailored HT-LC conditions irrespective of oxidation level. However, oxidation levels are shown to affect detector response. As a preparative fractionation technique, solution crystallization fractionation (SCF) is also shown to enhance the separation and identification of smaller oligomers. For the first time, the separation according to polarity of oxidized low molar mass waxes using HT-SGIC and HT-TGIC on silica as the stationary phase is demonstrated under tailored conditions. In addition, the oxidation level is shown to influence the quantities of retained fractions. Using a set of propylene-ethylene copolymers with increasing comonomer contents, the separation ranges of HT-SGIC and HT-TGIC are compared. In HT-SGIC a linear dependency of elution volume on the ethylene content is obtained for the entire average chemical composition (ethylene content) range. However, with HT-TGIC a linear dependency only is obtained within certain ethylene content limits. From this set of samples, it is also shown that HT-SGIC has a greater separation range (26.8 – 100 mol% ethylene) as compared to that of HT-TGIC (50 – 100 mol% ethylene). Lastly, preparative TREF is coupled offline to HT-SGIC for the characterization of bimodal ethylene-1-hexene HDPEs. Two model bimodal HDPE samples with similar comonomer contents are compared to a HDPE homopolymer produced with the same catalyst technology. The presence of copolymer fractions in the two samples introduces complexity in the microstructure of the bimodal HDPE. For a detailed analysis of the resins, the use of multiple techniques is shown to provide more information on the bimodal HDPE microstructure. It is also shown that the separation and selectivity of HT-SGIC for the analysis of bimodal HDPEs improves when coupled to a preparative fractionation technique such p-TREF as using this approach the complexity of the materials is reduced. However, the co-elution of low molar mass PE with the copolymer remains an obstacle, which can be overcome.

AFRIKAANSE OPSOMMING: Poliolefiene, soos alle ander sintetiese polimere, het 'n komplekse mikrostruktuur wat verstaan moet word om produkprestasie te voorspel en om katalisator- en reaktortegnologieë aan te pas. Tussen sintese en die toepassing van polimeer word karakteriseringstegnieke gebruik as visuele hulpmiddel vir die identifisering van polimeer mikrostruktuur. As 'n na-sintese-proses kan produkte deur middel van oksidasie of enting aangepas word. Hierdie prosesse beïnvloed nie net die funksionaliteit en funksionaliteitsverdelings (FTD) van ʼn materiaal nie maar ook die molêre massa en chemiese samestellingverdelings (MMD en CCD onderskeidelik). Die gebruik van meervoudige reaktor- en katalisator-tegnologieë om 'n enkele produk te produseer lei dikwels na komplekse mengsels van polimere met superieure eienskappe en komplekse mikrostrukture. Hierdie ingewikkelde mikrostruktuur eienskappe moet gedefinieer word en, indien moontlik, moet biblioteke van mikrostruktuurdata en geskikte, omvattende tegnieke vir elke tipe materiaal opgestel word. Hoëtemperatuur-chromatografiese tegnieke het vinnige bekend geword as betroubare tegnieke vir die karakterisering van poliolefien. Hoë-temperatuur vloeistof chromatografie (HT-LC), wat gebruik kan word in die oplosmiddel-gradiënt en temperatuur-gradiënt interaksie modes (SGIC en TGIC onderskeidelik), bied interessante alternatiewe vir gaschromatografie (GC) vir die analise van lae molêre massa geoksideerde wasse. Boonop kan nie-gekristalliseerde elastomere, wat nie gekarakteriseer kan word deur kristallisasie-gebaseerde metodes soos temperatuurstygingseluksiefraksionering (TREF) en kristallisasie analise fraksionering (CRYSTAF) nie, gekarakteriseer word met behulp van die genoemde modi van HT-LC. Poliolefien karakterisering word verder versterk deur die inkorporering van verskeie tegnieke aangesien geen enkele metode die volledige mikrostruktuurverdelings volledig kan definieer nie. Preparatiewe fraksionering het ook na vore gekom as 'n noodsaaklik hulpmiddel wat, wanneer gekoppel aan ander skeiding en fraksioneringstegnieke, 'n rykdom van inligting oor poliolefienmikrostruktuur lewer. Die doel van hierdie proefskrif is om omvattende HT-LC metodes te gebruik vir die skeiding van komplekse poliolefienmateriale soos lae molêre massa geoksideerde wasse, elastomere en bimodale hoëdigtheid poliëtileen (HDPE). Elke tipe geselekteerde poliolefienmateriaal bied 'n stel spesifieke karakteriseringsuitdagings wat aangepak moet word deur HT-LC en / of die koppeling aan 'n geskikte preparatiewe fraksioneringstegniek. Werk wat in hierdie proefskrif aangebied word, wys die skeiding van oligomere op poreuse grafitiese koolstof (PGC) van geoksideerde wasse onder geselekteerde HT-LC toestande, ongeag oksidasievlak. Dit word ook gewys dat oksidasievlakke wel die detektorreaksie beïnvloed. As 'n preparatiewe fraksioneringstegniek word kristallisasie-fractionasie (SCF) ook gebruik om die skeiding en identifikasie van kleiner oligomere te verbeter. Vir die eerste keer word die skeiding van geoksideerde lae molêre massa wasse volgens polariteit gedemonstreer deur middel van HT-SGIC en HT-TGIC met silika as die stasionêre fase en geselekteerde toestande te gebruik. Dit word ook gedemonstreer dat die oksidasie-vlak beïnvloed die hoeveelheid materiaal in elke fraksie. Met behulp van 'n stel etileen-propileen kopolimere, met toenemende komonomer inhoud, word die skeidingsgrense van HT-SGIC en HT-TGIC vergelyk. In HT-SGIC word 'n lineêre afhanklikheid van die elusie-volume op die etileeninhoud verkry vir die totale gemiddelde chemiese samestelling (etileeninhoud) van die reeks. By HT-TGIC word egter slegs lineêre afhanklikheid binne sekere grense van etileeninhoud verkry. Uit hierdie stel monsters word ook getoon dat HT-SGIC 'n groter skeidingsgrens het (26.8 - 100 mol% etileen) in vergelyking met HT-TGIC (50 – 100% etileen). Ten slotte word preparatiewe TREF gekoppel aan HT-SGIC vir die karakterisering van bimodale etileen-1-heseen HDPE. Twee model bimodale HDPE monsters met soortgelyke komonomer inhoud word vergelyk met 'n HDPE homopolymeer wat met dieselfde katalisatortegnologie vervaardig was. Die teenwoordigheid van kopolimeer fraksies in die twee monsters verhoog die kompleksiteit in die mikrostruktuur van die bimodale HDPE. Vir 'n gedetailleerde analise van die harse, word die gebruik van verskeie tegnieke getoon om meer inligting oor die bimodale HDPE mikrostruktuur te verkry. Daar word ook getoon dat die skeiding en selektiwiteit van HT-SGIC vir die analise van bimodale HDPE verbeter wanneer dit gekoppel word aan 'n preparatiewe fraksioneringstegniek soos p-TREF aangesien die kompleksiteit van die materiale verminder word. Die ko-elusie van lae molêre massa PE met die kopolimeer bly egter 'n hindernis, maar dit kan oorkom word.

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