Polyesters are an important class of polymers in many applications ranging from common-use objects—such as packaging containers, clothing, and upholstery—to more advanced applications, such as lightweight strength materials in construction, electronics, and automotive parts. Poly(ethylene terephthalate) (PET), a semicrystalline aromatic polyester, is commercially the most common and widely used polyester. However, the inability to reuse polyesters such as PET over multiple reprocessing cycles in the same application remains a challenge due to the susceptibility of the polymer to thermal, hydrolytic, and oxidative degradation during melt processing. The various degradation modes result in a drop in molecular weight, loss of key physical properties, and release of volatile compounds. Furthermore, the vast issue of plastic accumulation and pollution in diverse ecosystems, landfills, and waste streams underscores the burgeoning need to create a closed loop—responsible materials management from the cradle to the grave—through these materials' continual reuse and recycling. Additionally, most feedstock monomers used in polyester synthesis primarily come from fossil fuels. Fossil fuel extraction processes release gases and particulate matter that adversely affect health, climate, and the environment, so finding alternative sources for polyester monomers is paramount. This dissertation addresses key polyester challenges by designing and synthesizing alicyclic polyesters. First, we synthesized a series of alicyclic polyesters using various ratios of two regioisomers of a previously unexplored alicyclic monomer, bicyclohexyldimethanol (BCD). We learned from this alicyclic polyester series that we could tailor properties such as morphology and elongation while raising the glass transition temperatures (Tg) and lower melting temperatures (Tm) of the polymers based on the regioisomer composition. Furthermore, the regioisomer that led to polymers with semicrystalline morphologies inspired us to apply it to PET as a copolymer, with the goal of increasing PET's stability under melt processing conditions by lowering Tm. Next, we synthesized a series of alicyclic copolyesters with different BCD compositions in the polymer. The results showed that the presence of the alicyclic rings of BCD lowers the melting temperature and enhances the stability of the polymer in the melt compared to PET. These results directed us toward synergistically combining the benefits of alicyclic monomers with sustainable biobased monomers to enhance polyester properties, thereby decoupling fossil fuels from polymer feedstock production. Accordingly, we explored naturally ubiquitous, structurally diverse, and chemically modifiable terpenes present in the resin exudate of conifers. Specifically, we derived alicyclic diacid and diol monomers from the terpene verbenone and used them to synthesize a series of biobased alicyclic polyesters. The polymer series exhibited a range of morphologies, Tg's, as well as enhanced stabilities. The semicrystalline composition exhibited higher Tg and slightly lower Tm than PET while possessing exceptional stability in the melt over PET. / Doctor of Philosophy / Polyesters are important materials widely used today. They are very large molecules composed of a basic chemical unit linked together in a repeating fashion to make a long chain. The nature of the links between the basic units is referred to as an ester link, and materials are described as polyester when the number of these links is large. The applications of polyesters range from common-use objects—such as packaging containers, clothing, and upholstery—to more advanced applications in construction, transportation, and defense—such as body armor, seat belts, and lightweight strength materials and coatings in construction. The properties of its basic structural unit enable the wide breadth of applications of polyesters. A significant challenge that faces polyesters is the inability to reuse the material in the same application multiple times. The material must be reprocessed by melting at high temperatures to be reused. This melting breaks down the polyester chain, weakening the material and rendering it unsuitable for continued use. The need to reuse polyesters is an important area of concern because of the growing problem of plastic accumulation and pollution in diverse ecosystems and landfills. If these materials are continually reused, they will not accumulate as environmental waste. Furthermore, the basic starting unit that makes up polyesters largely comes from fossil fuels. Fossil fuel extraction processes release gases and particulate matter that adversely affect health, climate, and the environment. The issues of polyester breakdown in the melt and fossil fuel use to make the polyester can be addressed in two ways. First, reinforcing the polyester through changes to the basic structural unit can prevent the breakdown of the material when melted, thereby enabling its reuse over multiple cycles. Second, reducing the dependence on fossil fuels to make the basic structural unit of the polyester can be accomplished by using more renewable biobased sources instead. This dissertation seeks to address these two challenges. In the first approach, we investigate the effect of using a special cyclic structure in the polyester make-up to reinforce its stability when melted and enable its reuse. Next, we use plant materials to derive these unique structures to reduce the dependence on fossil fuels and mitigate the environmental, climate, and health effects of fossil fuel use.

A series of random and amorphous copolyesters containing different cycloaliphatic rings within the polymer chains were prepared by melt polycondensaton of difunctional monomers (diesters and diols) in the presence of a catalyst. These polyesters were characterized by nuclear magnetic resonance (NMR), size exclusion chromatography (SEC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile tests and/or dynamic mechanical analysis (DMA). The copolyester based on dimethyl bicyclo[2.2.2]octane-1,4-dicarboxylate (DMCD-2) was observed to have a higher Tg, about 115ºC, than the other copolyesters with the same compositions in this study. For copolyesters containing different compositions of dimethyl-1,4-cyclohexane dicarboxylate (DMCD) and DMCD-2, the Tg increased linearly with the increase of DMCD-2 mole content. DMA showed that all of the cycloaliphatic copolyesters had secondary relaxations, resulting from conformational transitions of the cyclohexylene rings. The polyester based on DMCD-3 in the hydrolytic tests underwent the fastest hydrolytic degradation among these samples. A new triptycene diol (TD) was synthesized and incorporated into a series of cycloaliphatic copolyester backbones by melt condensation polymerization. Straight chain aliphatic spacers, including ethylene glycol (EG), 1,4-butanediol (BD) and 1,6-hexanediol (HD), were used as co-diols to explore their effects on polyester properties. An analogous series of non-triptycene copolyesters based on various hydroxyethylated bisphenols were also prepared for comparison. The results revealed that the TD-containing polymers had higher thermal stability and higher Tg's than the corresponding non-TD analogs. For TD-containing copolyesters, the mechanical properties were found to be dependent on the types and compositions of the co-diols. A 1,4-butanediol-based triptycene copolyester was observed to have a significantly increased Tg and modulus while maintaining high elongation at ambient temperature. Furthermore, it was demonstrated that the triptycene polyester exhibited higher Tg and modulus than those containing bisphenol derivatives. However, all of the 1,4-butanediol based copolyesters were brittle and had comparable moduli at low temperatures (-25°C or -40 °C). Melt polycondensation was also used to prepare a series of all-aliphatic block and random copolyesters including the following aliphatic monomers: trans-DMCD, DMCD-2, neopentyl glycol (NPG), diethylene glycol (DEG) and dimethyl succinate (DMS). The polymer compositions were determined by 1H NMR, and the molecular weights were determined using SEC. The polyesters were also characterized by TGA, DSC, DMA and tensile tests. Phase separation was not observed in these block copolyesters. However, the block copolyester containing DMCD-2 and NPG was observed to have a higher Tg than the block copolyester based on trans-DMCD and NPG. In addition, these block copolyesters were found to have better mechanical properties than the corresponding random copolyesters. / Ph. D.

Polyesters are an attractive class of polymer that can be readily modified with a wide range of different comonomers, during polymerization or with melt blending, to achieve a wide variety of physical properties. This research primarily focuses on polyesters that incorporate rigid aromatic structures that have excellent potential to enhance thermal and mechanical properties. Copolyesters were prepared through melt polycondensation of diesters and diols in the presence of an exchange catalyst. Monomer incorporation was verified with nuclear magnetic resonance (NMR) and molecular weights were obtained by measuring inherent viscosity (ninh). Physical properties were assessed with thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and rheology. Mechanical properties were assessed with tensile and impact testing. Copolyesters of poly(ethylene terephthalate) (PET) were synthesized by substituting dimethyl terephthalate (DMT) with dimethyl 4,4'-biphenyldicarboxylate (4,4'BB) resulting in enhanced glass transition (Tg) temperatures relative to PET while affording melting temperatures (Tm) low enough to allow facile melt processing. Further modification with dimethyl isophthalate (DMI) or dimethyl 3,4'-biphenyldicarboxylate (3,4'BB) slowed crystallization sufficiently to allow biaxial orientation, leading to further studies assessing the permeability of oriented films. Novel amorphous polyesters were synthesized with 3,4'BB or 4,4'BB in combination with neopentyl glycol (NPG), 1,4-cyclohexandimethanol (CHDM) and ethylene glycol (EG). Use of multiple diols produced clear, amorphous copolyesters with Tgs as high at 129 C. A series of novel high temperature(Tm) copolyesters were synthesized from dimethyl 2,6-naphthalenedicarboxylate (DMN) and 4,4'BB combined with CHDM. Studies were performed with standard DSC and thin film calorimetry to show the convergence of multiples melting endotherms in an effort to determine their origin. Preliminary work was performed on the modification of poly(1,4-cyclohexylenedimethylene terephthalate) (PCT), poly(1,4-cyclohexylenedimethylene 2,6-naphthalate) (PCN) and poly(1,4-cyclohexylenedimethylene 4,4'-bibenzoate) (PCB) with dimethyl p-terphenyl-4,4''-dicarboxylate. / PHD / Polyesters have a unique balance of properties that sets them apart from other polymers formed by step growth reactions. The transesterification reaction that forms polyesters occurs continually at reaction temperatures, making it easy to randomly distribute a mixture of different comonomers along the backbone during the polymerization process, or even when blending two different polyesters. Poly(ethylene terephthalate), commonly referred to as PET, is the most important polyester currently in production, and is prized for its transparency, chemical resistance, and recyclability. PET was first made by John Whinfield and James Dickson at Calico Printers’ Association of Mansfield, in 1941 and was eventually licensed to DuPont in the 1970s. It has since become a valuable resource for producing synthetic textiles and replacing heavier materials, such as glass and metal, to produce lightweight containers, especially for food storage. Many of the polyesters, such as PET, that we see on a daily basis are actually copolyesters that contain low levels of additional comonomers that have been added to improve some property of the final polymer or to facilitate processing. In research, modification of polyesters with different comonomers broadens our understanding in how the molecular structure of comonomers affects polyester properties. This makes it possible to tune a copolyester’s physical properties in a way that can enhance its suitability for a wide range of applications. The research described in this dissertation is focused on exploring how rigid monomer structures containing multiple aromatic rings might be used to produce polyesters with improved performance relative to current commercial polyesters. Materials that demonstrate good barrier to gases such as CO₂ and O₂ are important for packaging that can seal in and preserve food and beverages. In our research, we modified PET with bibenzoate structures to produce films that showed improved gas barrier when stretched in a manner that imitates the stretch blow molding process used to produce bottles. These materials showed good promise for packaging capable of preserving food for longer periods of time. Clear, food safe plastics that do not deform at the boiling temperature of water are important for baby bottles and durable dishwasher safe containers, which are commonly sterilized with boiling water. Until recently, such materials were produced from bisphenol-A polycarbonate (BPA PC), which fell out of favor for food safe applications over concerns that BPA, believed to have endocrine disrupting activity, may leach into food and beverages. Bibenzoate monomers, which increases the application temperature of many polyesters, were combined with different combinations of diol monomers to produce transparent copolyesters that are usable at higher temperatures. These materials demonstrated excellent potential as food safe alternatives to BPA containing materials. Crystalline plastics that resist distorting at high temperatures are important for applications in the electronics and automotive industry. Semicrystalline polyesters provide less expensive alternatives to the costly liquid crystalline polymers commonly used for high temperature applications. We explored the properties of a number of semicrystalline copolyester compositions capable of exceeding the application temperature of semicrystalline polyesters currently on the market.