Development of a multiple fractionation protocol for the comprehensive analysis of low density polyethylene

Eselem Bungu, Paul Severin (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.


ENGLISH ABSTRACT: Low density polyethylene (LDPE) is a branched synthetic thermoplastic with a complex microstructure. In addition to the broad molar mass distribution (MMD), LDPE exhibits long chain branching (LCB) and short chain branching (SCB). Both LCB and SCB are statistically distributed across different polymer chains of varying molar masses, thus providing broad distributions in molar mass and branching. This interrelated MMD and branching distribution (BD) influences the end-use properties and, therefore, the applications. To be able to design new materials, comprehensive structure-property relationships must be established. To this aim, the microstructural parameters (MMD and BD) must be measured quantitatively and related to the thermophysical properties. In the current study, the microstructure of different branched polyethylenes was investigated. The quantification and determination of MMD was achieved by multidetector size exclusion chromatography (SEC). The branch types and branching contents were quantitatively measured using carbon-13 nuclear magnetic resonance spectroscopy (13C NMR). As an alternative and complementary method, SEC was coupled to an infrared detector to measure total branching and BD as a function of molar mass. LCB distributions were measured as a function of molar mass by SEC coupled to multiangle laser light scattering (MALLS). Branching information as a function of crystallizability was obtained by crystallization analysis fractionation (CRYSTAF), solution crystallization analysis by laser light scattering (SCALLS) and differential scanning calorimetry (DSC). Chain branching was also measured by high-temperature solvent gradient interactive chromatography (HT-SGIC). The effects of BD and MMD on the thermophysical behaviour were investigated by observing the crystallization and melting behaviour using DSC. Thermal properties to a large extent determine the processing properties of a given material. In the first part of this work, a multiple preparative fractionation concept was developed and used for the comprehensive characterization of LDPE. Narrowly dispersed molar mass and branching fractions were obtained using preparative molar mass fractionation (pMMF) and preparative temperature rising elution fractionation (pTREF) techniques, respectively. The molar mass and branching information were obtained by analysing the separated fractions using advanced analytical techniques. Cross-correlation of molar mass and branching was obtained by combining pTREF and pMMF results with SEC and CRYSTAF to construct 2D images of molar mass vs. branching. In the second part of the work, the multiple fractionation concept was used to investigate the microstructural differences between different LDPE samples. With the help of pTREF and pMMF, fractions with narrow molar mass and branching were generated. The fractions were analysed for branching and molar mass and cross-fractionation plots highlighted the microstructural differences between the samples. From the preparative fractions having broad molar mass and branching ranges, libraries were obtained with samples (1) having similar molar masses but different degrees of branching and, alternatively, (2) having different molar masses but similar degrees of branching. These library samples were analysed by CRYSTAF, SGIC, and multidetector SEC to investigate the effects of branching and molar masses on thermal properties. In the third part of this study, the multiple preparative fractionation concept was used to generate samples with similar molar mass/varying branching (pTREF) and similar branching/varying molar mass (pMMF). The library samples and bulk resins were analysed by DSC and thermal fractionation by successive self-nucleation and annealing (SSA) to provide information regarding crystal sizes and crystal size distributions. From the SSA results, methylene sequence length distribution (MSLD) plots were constructed providing information that was directly related to the branching and branching heterogeneity of these samples. In the last part of this study, the molecular structure of novel grafted polymers HDPE-g-LDPE and their linear and branched PE references was investigated. Fractionations were conducted by pTREF to generate fractions with varying degrees of branching and/or grafting. The cross-correlation techniques (TREF-SEC and TREF-CRYSTAF) were used to compare the grafting products. In addition, 2D- LC experiments were conducted to correlate branching/grafting to molar mass.

AFRIKAANSE OPSOMMING: Lae digtheid poliëtileen (LDPE) is 'n vertakte sintetiese termoplastiek met 'n komplekse mikrostruktuur. Benewens die breë molêre massa verspreiding (MMD), vertoon LDPE lang ketting vertakking (LCB) en kort ketting vertakking (SCB). Beide LCB en SCB word statisties versprei oor verskillende polimeerkettings van wisselende molêre massas, wat sodoende breë verspreidings in molêre massa en vertakking bied. Hierdie interafhanklike MMD en vertakkingsverspreiding (BD) beïnvloed die finale eienskappe en dus die toepassings. Om nuwe materiale te kan ontwerp, moet omvattende struktuur-eienskap verhoudings gevestig word. Vir hierdie doel moet die mikrostruktuur parameters (MMD en BD) kwantitatief gemeet word en verband hou met die termofisiese eienskappe. In die huidige studie was die mikrostruktuur van verskillende vertakte poliëtileen ondersoek. Die kwantifisering en bepaling van MMD is gedoen deur multidetektor grootte-uitsluitings chromatografie (SEC). Die vertakkingstipe en vertakkingsinhoud is kwantitatief gemeet met behulp van koolstof-13 kern magnetiese resonans spektroskopie (13C NMR). As 'n alternatiewe en komplementêre metode was SEC gekoppel aan 'n infrarooi detektor om totale vertakking en BD as 'n funksie van molêre massa te meet. LCB verspreidings was gemeet as 'n funksie van molêre massa deur SEC te koppel aan multihoek laser lig verstrooiing (MALLS). Vertakkingsinligting as 'n funksie van kristallisasie was verkry deur kristallisasie analise fraksionering (CRYSTAF), oplossing kristallisasie-analise deur laserligverstrooiing (SCALLS) en differensiële skanderingskalorimetrie (DSC). Kettingvertakking was ook gemeet deur hoë-temperatuur oplosmiddel-gradiënt interaktiewe chromatografie (HT-SGIC). Die effek van BD en MMD op die termofisiese gedrag was ondersoek deur die kristallisasie- en smeltgedrag met DSC waar te neem. Termiese eienskappe bepaal tot ‘n groot mate die verwerkingseienskappe van 'n gegewe materiaal. In die eerste deel van hierdie werk was 'n veelvoudige preparatiewe fraksionerings konsep ontwikkel en gebruik vir die omvattende karakterisering van LDPE. Smal verspreide molêre massa en vertakkingsfraksies was verkry deur onderskeidelik preparatiewe molêre massa fraksionering (pMMF) en preparatiewe temperatuurstyging elueringsfraksionering (pTREF) tegnieke. Die molêre massa en vertakkingsinligting was verkry deur die geskeide fraksies te analiseer met gebruik van gevorderde analitiese tegnieke. Kruiskorrelasie van molêre massa en vertakking was verkry deur pTREF en pMMF resultate te kombineer met SEC en CRYSTAF om 2D-beelde van molêre massa teen vertakking te konstrueer. In die tweede deel van die werk was die veelvoudige fraksioneringskonsep gebruik om die mikrostruktuurverskille tussen verskillende LDPE monsters te ondersoek. Met behulp van pTREF en pMMF was fraksies met nou molêre massa en vertakking verkry. Die fraksies was geanaliseer vir vertakking en molêre massa en kruis-fraksioneringskurwes het die mikrostruktuurverskille tussen die monsters uitgewys. Uit die preparatiewe fraksies met breë molêre massa en vertakkingsreekse was biblioteke verkry van monsters (1) met soortgelyke molêre massas, maar verskillende grade van vertakking en alternatiewelik (2) met verskillende molêre massas, maar soortgelyke grade van vertakking. Hierdie biblioteekmonsters was met behulp van CRYSTAF, SGIC en multidetektor SEC ontleed om die effek van vertakking en molêre massas op termiese eienskappe te ondersoek. In die derde deel van hierdie studie was die veelvoudige preparatiewe fraksioneringskonsep gebruik om monsters met soortgelyke molêre massa/wisselende vertakking (pTREF) en soortgelyke vertakking/wisselende molêre massa (pMMF) te verkry. Die biblioteekmonsters en grootskaalmonsters was geanaliseer deur DSC en termiese fraksionering deur opeenvolgende selfkernvorming en “annealing” (SSA) om inligting oor kristalgroottes en kristalgrootte verspreiding te verskaf. Metileen-reekslengte distribusiekurwes (MSLD) was opgestel vanuit die SSA resultate wat inligting verskaf het wat direk verband hou met die vertakking en vertakking heterogeniteit van hierdie monsters. In die laaste gedeelte van hierdie studie was die molekulêre struktuur van nuwe “ent” polimere HDPE-g-LDPE en hul lineêre en vertakte PE verwysings ondersoek. Fraksionering was met pTREF uitgevoer om fraksies met verskillende grade van vertakking en/of “enting” te genereer. Die kruis-korrelasietegnieke (TREF-SEC en TREF-CRYSTAF) was gebruik om die “entings” produkte te vergelyk. Daarbenewens was 2D-LC eksperimente uitgevoer om die vertakking/“enting” met molêre massa te korreleer.

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