Investigating mechanical responses to structural changes in crystalline materials

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dc.contributor.advisorBarbour, Leonard Jamesen_ZA
dc.contributor.advisorLoots, Leigh-Anneen_ZA
dc.contributor.authorvan Rijn, Raymond Michaelen_ZA
dc.contributor.otherStellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.en_ZA
dc.date.accessioned2023-11-29T08:51:39Zen_ZA
dc.date.accessioned2024-01-08T22:59:22Zen_ZA
dc.date.available2023-11-29T08:51:39Zen_ZA
dc.date.available2024-01-08T22:59:22Zen_ZA
dc.date.issued2023-11en_ZA
dc.descriptionThesis (MSc)--Stellenbosch University, 2023.en_ZA
dc.description.abstractENGLISH ABSTRACT: Previous work has established that the compound 2,7-dimethylocta-3,5-diyne-2,7-diol crystallizes to produce a range of inclusion compounds – the crystals of which exhibit elastic flexibility. In an attempt to tailor the Young’s modulus of the system, substitution of guest species, using both polar and apolar compounds, was carried out. Ultimately, little difference in elasticity was observed, regardless of the included guest species. Analysis of crystal structures revealed that little interaction takes place between guest molecules, or between host and guest. Thus, hydrogen bonding in the host framework is concluded to be the determining factor in crystal flexibility. Several other elastically flexible crystals were subsequently investigated to determine how their elastic moduli would change when temperature was varied. A relationship was identified between the change in elasticity and thermal expansion of the crystals. As the bending axis of a crystal expands in length, the Young’s modulus decreases. Thus, for crystals exhibiting positive thermal expansion, elasticity is reduced as temperature is decreased. Conversely, for a material displaying negative thermal expansion, decrease in temperature was found to produce an increase in elasticity. Greater intermolecular spacing likely allows for a greater degree of molecular reorientation that must occur to facilitate mechanical bending of crystals. Two of the compounds subjected to variable-temperature flexibility studies, Pd(acac)2 and Cu(acac)2, are isostructural and isomorphous, yet exhibit opposite thermal expansion characteristics along their crystallographic b axes. As such, they were identified as promising candidates to form solid solutions, whose thermal expansion behaviours could be tuned. A mixed crystal displaying near-zero thermal expansion was successfully produced, demonstrating the applicability of solid solutions in tailoring the thermal properties of molecular compounds. A reaction between 2,7-dimethylocta-3,5-diyne-2,7-diol and iodine was observed to produce a novel cumulene-type compound in high yield. It was established that light is required for the reaction to proceed, and can also be used to isomerize the cumulene in a cis to trans manner. Several other diyne compounds were found to react in analogous ways under the novel reaction conditions, provided they featured hydroxyl functional groups. Thus, it is proposed that formation of hydrogen-bonded adducts is responsible for halting the halogenation reaction upon formation of a cumulene.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Vorige werk het vasgestel dat die verbinding 2,7-dimethylokta-3,5-diyne-2,7-diol kristalliseer om ‘n reeks insluitingsverbindings te produseer – waarvan die kristalle elastiese buigsaamheid vertoon. In ‘n poging om die Young’s Modulus van die stelsel aan te pas, is die vervanging van gasspesies, met beide polêre en apolêre verbindings, uitgevoer. Uiteindelik is min verskil in elastisiteit waargeneem, ongeag die ingeslote gas-spesies. Ontleding van kristalstrukture het aan die lig gebring dat min interaksie tussen gas-molekules, of tussen gasheer en gas plaasvind. Dus word tot die gevolgtrekking gekom dat waterstofbinding in die gasheerraamwerk die bepalende faktor in kristalbuigsaamheid is. Verskeie ander elastiese, buigsame kristalle is daarna ondersoek om te bepaal hoe hul elastiese module sou verander wanneer temperatuur gevariëer word. ‘n Verband is geïdentifiseer tussen die verandering in elastisiteit en termiese uitsetting van die kristalle. Namate die buig-as van ‘n kristal in lengte uitbrei, verminder die Young’s Modulus. Dus, vir kristalle wat positiewe termiese uitsetting vertoon, word elastisiteit verminder as die temperatuur verlaag word. Omgekeerd, vir 'n materiaal wat negatiewe termiese uitsetting vertoon, is gevind dat afname in temperatuur ‘n toename in elastisiteit veroorsaak. Groter intermolekulêre spasiëring maak waarskynlik ‘n groter mate van molekulêre heroriëntasie moontlik wat moet plaasvind om meganiese buiging van kristalle te vergemaklik. Twee van die verbindings wat aan veranderlike-temperatuur buigsaamheidstudies onderwerp is, Pd(acac)2 en Cu(acac)2, is isostruktureel en isomorfies, maar vertoon egter teenoorgestelde termiese uitsetting-eienskappe langs hul kristallografiese b-asse. Dus is hulle geïdentifiseer as belowende kandidate om soliede oplossings te vorm, waarvan die termiese uitsettingsgedrag aangepas kon word. ‘n Gemengde kristal wat amper-nul termiese uitsetting vertoon is suksesvol vervaardig, wat die toegepaslikheid van soliede oplossings demonstreer om die termiese eienskappe van molekulêre verbindings aan te pas. ‘n Reaksie tussen 2,7-dimethylokta-3,5-diyne-2,7-diol en jodium is waargeneem om ‘n nuwe kumuleen-tipe verbinding in hoë opbrengs te produseer. Daar is vasgestel dat lig benodig word vir die reaksie om voort te gaan, en dit kan ook gebruik word om die kumuleen op ‘n cis tot trans manier te isomeriseer. Daar is gevind dat verskeie ander diyne-verbindings op analoog maniere reageer onder die nuwe reaksietoestande, mits hulle hidroksiel funksionele groepe bevat. Daar word dus voorgestel dat vorming van waterstofgebonde addukte verantwoordelik is vir die beëindeging van die halogeneringsreaksie wanneer ‘n kumuleen vorm.af_ZA
dc.description.versionMastersen_ZA
dc.format.extent118 pages : illustrationsen_ZA
dc.identifier.urihttps://scholar.sun.ac.za/handle/10019.1/129116en_ZA
dc.language.isoen_ZAen_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.subject.lcshCrystals -- Elastic propertiesen_ZA
dc.subject.lcshExpansion (Heat)en_ZA
dc.subject.lcshMaterials -- Mechanical propertiesen_ZA
dc.subject.lcshHydrogen bondingen_ZA
dc.titleInvestigating mechanical responses to structural changes in crystalline materialsen_ZA
dc.typeThesisen_ZA
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