Thermal field-flow fractionation and the advanced analysis of complex polymers

Thesis (PhD)--Stellenbosch University, 2019.ENGLISH ABSTRACT: Self-assemblies (SAs) and composites designed from block copolymers (BCPs) have found an important niche in nanotechnology. In order to establish commercially viable and sustainable polymeric applications for such materials in industry, there is a need to develop advanced analytical methods for their characterization. SAs being polymeric in nature inherently exhibit molecular heterogeneity, and thus their molecular properties are not specific but rather exist as distributions, which immensely impact on their ultimate performance. As such, there is a critical need to move away from the traditional batch mode analytical techniques which only yield average property values. Instead, it is imperative to embrace separation techniques such as size exclusion chromatography (SEC) and field-flow fractionation (FFF) which have been applied for the analysis of molecular properties and the corresponding distributions. In particular, the advent of thermal field-flow fractionation (ThFFF) as a sub-technique of FFF has enabled unique separations of SAs according to both hydrodynamic size (Dh) and chemical composition (CC). Herein, the focus of the research was directed at developing multidimensional analytical techniques for the analysis of SAs in three separate but mutually related studies. In the first case, the focus was on investigating the impact of microstructural and molecular packing differences of the core domains on the behaviour of SAs. The core microstructure and molecular density are cumulatively shown to impact on the morphology, critical micelle concentration (CMC), hydrodynamic diameter (Dh), molar mass (Mw), aggregation number (Z), elastic behaviour and thermal stability. Secondly, ThFFF was successfully applied in the analysis of nanoreactors. Specific SAs are shown to be capable of functioning as micelle nanoreactors (MNRs) by triggering phase separated and thermally induced physical transitions within the micelle core to produce new SAs with unique properties. In essence, the MNRs are shown to circumvent the inherent precipitations associated with stereocomplexation (SC) of polymers, thus providing a vital route for studying SCs in solution. Finally, a ThFFF-quintuple detection system (ThFFF-QD) was presented for the comprehensive analysis of SAs and other complex polymers. This novel approach used the so-called triple detection (TD), that is, differential viscometer (dVis), refractive index and (dRI) and static light scattering (SLS) detectors. TD was complimented by a second concentration detector (ultraviolet, UV) and a dynamic light scattering (DLS) detector. The method development process for ThFF-QD was based on linear, branched and isotopic polystyrenes (PSs) as model compounds. In particular, ThFFF of isotopic polystyrene analogues was fundamentally shown to exhibit gravimetric and density sensitivities. Structural information for SAs was elucidated from the Mark-Houwink (MH) and confirmation plots and correlated with chemical composition, structure, compactness, molar mass, aggregation number (Z), size and intrinsic viscosity (IV).AFRIKAANSE OPSOMMING: Selfversamelings (SAs) en samestellings wat uit blok ko-polimere (BCPs) ontwerp is, het ‘n belangrike nis in nanotegnologie gevind. Ten einde kommersieel lewensvatbare en volhoubare polimeertoepassings vir sulke materiale in die industie te vestig, is daar ‘n behoefte om gevorderde analitiese metodes vir hul karakterisering te ontwikkel. SAs se polimeriese aard toon inherent molekulêre heterogeniteit, en dus is hul molekulêre einskappe nie spesifiek nie, maar bestaan dit eerder as verspreidings, wat ‘n geweldige impak op hul uiteindelike gedrag het. As sodanig is daar ‘n kritiese behoefte om weg te beweeg van die tradisionale bondel-modus analitiese tegnieke wat slegs gemiddelde eienskapwaardes lewer. In plaas daarvan is dit noodsaaklik om skeidingstegnieke soos grootte-uitsluitingschromatografie (SEC) en veld-vloeifraksionering (FFF) te gebruik wat aangewend is vir die analise van molekulêre eienskappe en die ooreenstemmende verspreidings. In die besonder het die koms van termiese veld- vloeifraksionering (ThFFF) as ‘n subtegniek van FFF, unieke en gesamentlike skeidings van SAs volgens beide hidrodinamiese grootte (Dh) en chemiese samestelling (CC) toegelaat. Die fokus van die navorsing is gerig op die ontwikkeling van multidimensionele analitiese tegnieke vir die analise van SAs in drie afsonderlike, maar onderling verwante, studies. In die eerste plek is die fokus op die ondersoek van die impak van mikrostruktuur- en molekulêre pakkingsverskille van die kernarea, op die gedrag van SAs. Daar word aangedui dat die kernmikrostruktuur en molekulêre digtheid ‘n kumulatiewe impak op die morfologie, kritiese miselkonsentrasie (CMC), hidrodinamiese deursnee (Dh), molekulêre massa (Mw), samevoegingsnommer (Z), elastiese gedrag en termiese stabiliteit het. Tweedens, is ThFFF suksesvol toegepas in die analise van nanoreaktore. Daar word aangedui dat spesifieke SAs in staat is om as miselnanoreaktore (MNRs) te funksioneer deur fase-geskeide en termies ge deur fase-geskeide en termies geïnduseerde fisiese oorgange binne die miselkern te aktiveer om nuwe SAs met unieke eienskappe te produseer. In wese word daar aangedui dat die MNRs die inherente neerslae, wat verband hou met stereokompleksering (SC) van polimere, omseil en dus ‘n belangrike roete bied vir die bestudering van SCs in oplossing. ‘n ThFFF-vyfvoudinge-opsporingstelsel (ThFFF-QD) vir die omvattende analise van SAs en ander komplekse polimere is voorgestel. Hierdie nuwe benadering gebruik die sogenaamde drievoudige opsporing (TD); wat differensiële viscometer- (dVis), brekingsindeks- (dRI) en statiese ligverstrooiings- (SLS) detektors is. TD is gekomplimenteer deur ‘n tweede konsentrasie detector (ultraviolet, UV) en ‘n dinamiese ligverstrooiings- (DLS) detector. Die metode-ontwikkelingsproses vir (ThFFF-QD) is gebaseer op ‘n lineêre, vertakte en isotopiese polistireen (PS) as modelverbindings. In die besonder, is daar fundamenteel aangedui dat ThFFF van isotopiese polistireenanaloë, gravimetriese- en digtheidsensitiwiteite uitbeeld. Strukturele inlgting vir SAs is uit die Mark-Houwink (MH) en konformasiegrafieke verklaar en gekorreleer met chemiese samestelling, struktuur, kompaktheid, molêre massa, samevoegingsnommer (Z), grootte en intrinsieke viskositeit (IV).Doctora