Date of Award

Spring 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

School

Polymer Science and Engineering

Committee Chair

Sergei Nazarenko

Committee Chair School

Polymer Science and Engineering

Committee Member 2

Jeffrey S. Wiggins

Committee Member 2 School

Polymer Science and Engineering

Committee Member 3

Robert Y. Lochhead

Committee Member 3 School

Polymer Science and Engineering

Committee Member 4

Sarah E. Morgan

Committee Member 4 School

Polymer Science and Engineering

Committee Member 5

Derek L. Patton

Committee Member 5 School

Polymer Science and Engineering

Abstract

Crosslinked polymers are widely used due to its several advantages not limited to high mechanical strength combined with the easy processability. Despite of its popular usage, the fundamental understanding of polymer structure affecting the desired properties is still lacking. This PhD thesis is in two parts, the first part is devoted to the design and developing a basic understanding of structure and chemical composition dependencies of gas transport, whereas in the second part a fundamental relationship between structure to the fire-retardant properties is established.

Membrane based gas separation technique has attracted interest of selective removal of carbon dioxide gas from mixture of light gases such as H2, O2, N2 and CH4. Polyethylene glycol (PEG) has been employed to improve the solubility of acidic gases such as CO2 to improve the selective permeation. While conventional research on solubility selective membranes focuses on the strategies to prepare amorphous membrane while incorporating maximum PEG content, to the best of our knowledge, no studies have been focused in determining the effect of increasing PEG units on solubility/ selectivity of CO2/light gases. This research aims to determine the increasing effect of PEG units in solubility selectivity of UV curable thiol-ene based membranes. We determined the threshold amount of PEG units to achieve maximum CO2 gas solubility/ selectivity. We also examined the effect of network architecture on solubility when PEG units are placed in the backbone or as a dangling chain. The results indicated that CO2 solubility / selectivity saturated at 10 weight percentage of PEG for these elastomeric networks despite of the placement of the PEG units. Knowing that the required amount of PEG to achieve maximum selectivity is around 10 wt%, several other moieties that incorporate flexibility such as PDMS can be incorporated to further increase the permeability without compromising the selectivity, thus improving the overall membrane separation process.

Crosslink density affects several properties of a crosslinked network. The effect of network crosslink density on fire retardant performance was examine via cone calorimeter using thiol-ene model networks in chapter 3. A series of network was designed to vary the rigidity and crosslink density. Rigidity was tuned by using different types of ene monomer from aliphatic to aromatic nature. By crosslinking trifunctional ene with thiol with varying functionality from 2 through 4, it was possible to increase the crosslink density without changing the chemical nature of the network. We determined that fire retardant properties improved with increasing crosslink density and rigidity within the series of networks examined. Pyrolysis behavior was examined via scanning electron microscope on networks constructed with two structurally similar ene monomers, allyl triazine (TOT) and an allyl isocyanurate (TTT). The combustion process was interrupted by quenching in liquid nitrogen at increasing times, and the cross section was examined via SEM. SEM images revealed that the isomers undergo distinct pyrolysis behavior. Networks containing ether linkage had faster bulk pyrolysis, while the monomers with allyl linkage underwent surface pyrolysis. Through cross section elemental analysis, we were able to quantify the composition different zone and were able to trace the extend of degradation at various time of combustion. This could be an important tool in enhancing the fire-resistant properties of the neat polymeric systems.

Since epoxy networks are another class of polymers widely used in aerospace, electrical insulation and construction, special emphasis was given in understanding and correlating the epoxy resin structure with several fire-retardant properties determined via cone calorimeter in chapter 4. TGA analysis was used to calculate the activation energy of decomposition via fitting in Ozawa plot. The main emphasis was given to relate structural parameters such as glass transition temperature, network crosslink density to the fire performance of these networks. The presence of aromatic content in the networks influenced the char formation which reduced the heat release rate. For the first time, FR properties determined via cone calorimeter was corelated with numerically calculated heat release and heat capacity values predicted via molar group contribution method. Comparative studies of structurally similar isomers, 3,3’-DDS and 4,4’-DDS, revealed the differences in properties arising solely from the differences in configurational entropy between these monomers. Network containing 4,4’-DDS possess higher onset temperature and higher Tg, but interestingly, the peak heat release rate determined via cone calorimeter was inferior as compared to 3,3’-DDS containing networks. This is mainly due to the higher configurational entropy of 3,3’-DDS making the chain to pack better at elevated temperature during the combustion process.

Following this basic understanding of structure fire retardant properties of epoxy amine networks, a comprehensive comparison of graphene oxide (GO) modified with a phosphorous based compound, DOPO-V and a polysiloxane (PMDA) flame retardant was studied. This was accomplished by two step process: in the first step, GO synthesis via Hummer’s method, while in the second step, the GO was functionalized by addition of DOPO and PMDA which were synthesized separately. The chemical modification of GO with DOPO-V and PMDA was verified using FTIR, XPS and AFM. Two separate FR additive, GO-DOPO-V and GO-PMDA was added to a standard DGEBA based epoxy resin which was cured with a polyether diamine (Jeffamine D230) to form a composite. Thermal stability of the composites were examined using TGA. DSC results showed no change in Tg which indicated that the added FR additive did not affect the epoxy amine matrix properties. Cone calorimetry was used as a tool to evaluate the flame-retardant properties of composites prepared using GO-DOPO-V and GO-PMDA. The cone results were compared with standard epoxy-amine matrix along with composites made by mixing GO, DOPO-V and PMDA separately which revealed that the presence of grafted GO, ie GO-DOPO-V and GO-PMDA, improved the flame retardancy. The char morphology analyzed via SEM revealed that the presence of GO along with the FR additives led to a honeycomb type morphology. We hypothesize that the modified GO improved the dispersion within the matrix which improved the FR properties.

In the last session of this thesis, a new approach of using dissolved metal in improving flame retardant properties of several polymers including epoxy-amine (EP), polyurethane (PU), polystyrene (PS) and polyethylene oxide (PEO) is presented. Cone calorimeter was used as a standard tool to evaluate the flame retardancy of these metal dissolved composites. It was discovered unexpectedly that dissolution of a divalent metal, which is capable of forming an oxide layer upon combustion, improves the flame retardancy of a polymer matrix by suppressing the smoke formation and reducing the heat release rate. It was discovered that the presence of primary or secondary amine aids in metal dissolution of certain metal salts such as zinc acrylate, but metal salts containing long organic tail such as zinc stearate was readily dissolved upon heating. The dissolution was evidenced from formation of transparent composites and through the loss of crystal structure of metal salt detected through wide angle X-ray analysis. We discovered that the improvement in flame retardancy was greatly enhanced when the metal was dissolved rather than dispersed in the polymer matrix. A two-step additive approach was followed where in the first step an additive containing dissolved metal in amine was prepared which was subsequently added in to the desired polymer matrix in the second step. The solubility of the additive to a common solvent was chosen as a criterion to disperse in the desired polymer matrix, ie., the nature of the amine in additive manufacturing was selected such that it solubilizes with a common solvent of the polymer. For instance, a water-soluble additive was prepared using ethylene diamine which was subsequently added to a polyethylene oxide (PEO) which had water as a common solvent. The choice of amine made it possible to add this additive to several polymers which made this approach more versatile in nature. This approach can be a termed as “green” technique due to the absence of halogenated, phosphorous or boron containing compounds which releases toxic smoke during suppressing the fire.

ORCID ID

https://orcid.org/0000-0001-5883-986X

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