Multidimensional analytical approach for the characterization of complex ethylene-propylene copolymers

Phiri, Justice Mohau (2016-03)

Thesis (PhD)--Stellenbosch University, 2016

Thesis

ENGLISH ABSTRACT : Impact polypropylene copolymers (IPC) are complex multiphase materials having ethylenepropylene rubber particles incorporated in a semi-crystalline isotactic polypropylene (iPP) matrix. The ethylene-propylene rubber (EPR) particles form the most critical part of IPC. They are continuously dispersed in the iPP matrix and are supposed to prevent crack propagation under mechanical stress. The EPR phase decreases the crystal size of the crystalline iPP phase, and as a result an improvement in the impact strength of IPC is usually observed. IPC materials have much improved impact resistance as compared to conventional iPP and, thus, they can be used in a wide temperature range. IPCs usually have low melt flow rates (MFR) that limit their application in a number of moulding applications. One way to increase the MFR is the vis-breaking process that decreases the molar mass of the polymer. Details regarding the molecular structures of the vis-broken IPC and the molecular background of the vis-breaking process are not completely available up to date. Therefore, it was the main aim of the present study to investigate the compositional heterogeneities of EPR and vis-broken IPC materials using multidimensional analytical approaches. A set of EPR copolymers prepared by Ziegler-Natta and metallocene catalyst systems were used in the present study as model systems to mimic the rubber phase in IPC. The EPR samples had high comonomer contents in the range of 30 – 70 mol %. For another part of the work, a set of vis-broken IPC samples with different MFR values were prepared by degradation with an organic peroxide. Bulk samples from the two sets of samples were analysed by SEC, DSC, FT-IR, 13C NMR, CRYSTAF and HT-HPLC. The bulk samples were fractionated by preparative TREF procedures to obtain fractions at different TREF elution temperatures which could be further analysed to obtain comprehensive information on the molecular compositions of the samples. For the EPR copolymer samples, HT-SEC showed that the bulk samples and their TREF fractions have broader MMDs as compared to similar metallocene-based samples. The bulk sample analyses showed that the EPR samples consist mainly of random EP copolymers making about 65 % of the bulk samples. DSC and CRYSTAF were able to analyse only the semi- and highly crystalline TREF fractions. Both techniques did not fully characterise the 30 °C TREF fractions since these fractions contained random EP copolymers which were completely amorphous. 13C NMR was used to determine the comonomer contents of the fractions as well as their monomer sequence distributions. NMR analyses of comparable metallocene-based copolymer fractions showed that these had remarkably high chemical heterogeneities. The separation of the samples and their TREF fractions by HT-HPLC was found to be sensitive to the ethylene content of the samples. The exact chemical compositions of the fractions were determined by coupling HT-HPLC to FT-IR spectroscopy. This hyphenated technique provided information on the ethylene and propylene contents of the fractions as a function of the elution volume in HPLC separation. The late eluting TREF fractions were found to contain PE and iPP homopolymers in addition to the expected EP copolymers. These homopolymer fractions indicated that the EPR samples were not completely amorphous as it was suggested by previous studies. Vis-broken IPC samples were analysed using a similar multidimensional analytical approach. HT-SEC showed that there was a decrease in molar masses of the bulk samples with the largest amount of the peroxide used in the degradation process. DSC and CRYSTAF showed that the 60 and 90 °C TREF fractions contain the segmented EP copolymers with complex molecular compositions. FT-IR was also useful in providing the chemical composition of the vis-broken IPC samples by detecting the presence of carbonyl groups in the degraded samples. The 30 °C TREF fractions were found to contain the highest concentration of carbonyl groups and the number of these groups increased with an increase of the amount of the peroxide. HPLC was able to analyse both amorphous and crystalline fractions since the HPLC separation depends mainly on the chemical composition of the samples and not the crystallizability. There was a significant shift to lower HPLC elution volumes of the samples with the most degraded sample showing the strongest shift of the elution volume peak maximum. This suggests that EP copolymers with longer ethylene sequences are affected most by action of an organic peroxide. To summarize, the present study proved that a single technique approach is not sufficient to obtain comprehensive information on the molecular compositions of the vis-broken IPC and EPR copolymers. As a result, the present study established a number of multidimensional analytical approaches to fractionate and characterize these complex IPC polymer systems.

AFRIKAANSE OPSOMMING : Impak polipropileen ko-polimere (IPC) is komplekse multifase materiale met etileen-propileen rubber partikels, bevat in 'n semi-kristallyne isotaktiese polipropileen (iPP) matriks. Die etileenpropileen rubber (EPR) partikels vorm die mees kritieke deel van IPC. Hulle is dwarsdeur die IPP matriks versprei en is veronderstel om kraak verspreiding onder meganiese spanning te voorkom. Die EPR fase verminder die kristal grootte van die kristallyne IPP fase en as gevolg word 'n verbetering in die impak sterkte van IPC gewoonlik waargeneem. IPC materiale het baie beter impak weerstand in vergelyking met konvensionele IPP en dus kan hulle gebruik word oor 'n wye temperatuur reeks. IPCs het gewoonlik ‘n lae smelt vloeitempo (MFR) wat hul toepassing in 'n aantal giettoepassings beperk. Een manier om die MFR te verhoog is die vis-breking proses wat die molêre massa van die polimeer verlaag. Inligting oor die molekulêre strukture van die visgebreekte IPC en die molekulêre agtergrond van die vis-breking proses is tot op hede nog nie heeltemal beskikbaar nie. Daarom was dit die hoofdoel van die huidige studie om die komposisionele heterogeniteit van EPR en vis-gebreekte IPC materiale met behulp van multidimensionele analitiese benaderings te ondersoek. 'n Stel EPR ko-polimere, voorberei deur Ziegler-Natta en metalloseen katalisator sisteme, was gebruik in die huidige studie as model stelsels om die rubber fase in IPC na te boots. Die EPR monsters het hoë ko-monomeer inhoud in die reeks van 30-70 mol%. Vir 'n ander deel van die werk, was 'n stel vis-gebreekte IPC monsters met verskillende MFR waardes voorberei deur degradasie met 'n organiese peroksied. Ongefraksioneerde monsters van die twee stelle was ontleed deur SEC, DSC, FT-IR, 13C KMR, CRYSTAF en HT-HPLC. Die monsters was gefraksioneer deur preparatiewe TREF prosedure om fraksies by verskillende TREF eluering temperature te verkry wat verder ontleed kon word om omvattende inligting oor die molekulêre samestelling van die monsters te bekom. Vir die EPR ko-polimeer monsters, het HT-SEC getoon dat die ongefraksioneerde monsters en hul TREF fraksies breër MMDs in vergelyking met soortgelyke metalloseen-gebaseerde monsters het. Die ongefraksioneerde monster ontledings het getoon dat die EPR monsters hoofsaaklik uit ewekansige EP ko-polimere bestaan wat ongeveer 65% van die ongefraksioneerde monster opmaak. DSC en CRYSTAF was in staat om net die semi- en hoogs kristallyne TREF fraksies te ontleed. Beide tegnieke kon nie ten volle die 30 °C TREF fraksie karakteriseer nie aangesien hierdie fraksies ewekansige EP ko-polimere bevat wat heeltemal amorfe was. 13C KMR was gebruik om die inhoud van die ko-monomeer in die fraksies, asook hul monomeer kombinasie verspreiding te bepaal. KMR ontleding van vergelykbare metalloseen-gebaseerde ko-polimeer fraksies het getoon dat hul merkwaardig hoë chemiese heterogeniteit het. Die skeiding van die monsters en hul TREF fraksies deur HT-HPLC was gevind om sensitief teenoor die etileen inhoud van die monsters te wees. Die presiese chemiese samestelling van die fraksies was bepaal deur die koppeling van HT-HPLC en FT-IR spektroskopie. Hierdie gekoppelde tegniek het inligting oor die etileen en propileen inhoud van die fraksies verskaf as 'n funksie van die eluering volume in HPLC skeiding. Dit was gevind dat die TREF fraksies wat laat elueer, PE en iPP homopolimere bevat tesame met die verwagte EP ko-polimere. Hierdie homopolimeer fraksies het aangedui dat die EPR monsters nie heeltemal amorfe was, soos voorgestel deur vorige studies nie. Vis-gebreekte IPC monsters was ontleed met behulp van 'n soortgelyke multidimensionele analitiese benadering. HT-SEC het getoon dat daar 'n afname in molêre massas van die ongefraksioneerde monsters was waar die grootste hoeveelheid van die peroksied gebruik was in die degradasie proses. DSC en CRYSTAF het getoon dat die 60 en 90 °C TREF fraksies, die gesegmenteerde EP ko-polimere met komplekse molekulêre komposisies bevat. FT-IR was ook nuttig in die verskaffing van die chemiese samestelling van die vis-gebreekte IPC monsters deur die opsporing van die teenwoordigheid van karbonielgroepe in die gedegradeerde monsters. Dit was gevind dat die 30 °C TREF fraksies die hoogste konsentrasie karbonielgroepe bevat en die aantal van hierdie groepe het toegeneem met 'n toename in die hoeveelheid peroksied. HPLC was in staat om beide amorfe en kristallyne fraksies te analiseer aangesien die HPLC skeiding hoofsaaklik afhang van die chemiese samestelling van die monsters en nie die kristalliseerbaarheid nie. Daar was 'n beduidende verskuiwing na laer HPLC eluering volumes van die monsters met die mees gedegradeerde monster wat die sterkste verskuiwing van die eluering volume piek maksimum gewys het. Dit dui daarop dat EP ko-polimere met langer etileen reekse, die meeste geaffekteer word deur die aksie van ‘n organiese peroksied. Om op te som, die huidige studie het bewys dat 'n enkel tegniek benadering nie voldoende is om omvattende inligting oor die molekulêre samestelling van die vis-gebreekte IPC en EPR ko-polimere te bekom nie. As gevolg hiervan het die huidige studie 'n aantal multidimensionele analitiese benaderings gevestig om hierdie komplekse IPC polimeer sisteme te fraksioneer en karakteriseer.

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