This paper was originally published and presented at The Society of Plastics Engineers’ ANTEC in 2018
Three different thermoplastic polyester materials were evaluated to investigate the connection between the structure of the materials and their properties. Three materials representing distinct characteristic structures were selected to contrast the results. The resins evaluated included polycarbonate, with carbonate ester functionality; poly(ethylene –co- 1,4-cyclohexanedimethylene terephthalate), a poly(ethylene terephthalate) copolymer; and poly(ethylene naphthalate), with two condensed aromatic rings. The characteristics tested as part of this work included tensile properties to illustrate the short-term mechanical attributes, glass transition temperatures to represent the thermal response of the materials, and creep modulus to demonstrate the time dependency.
The goal of this paper is to explore the relationship between the structure of different polyesters and their thermal and mechanical properties. The characteristic performance of individual plastics is the direct result of their unique molecular structure. Inherently, polymeric materials have a distinctive structure, as compared with other materials, which results in their distinguishing short-term and long-term properties. This structure includes a) a relatively high molecular weight resulting from chains formed by a great number of covalently bonded repeating units, b) the entanglement of these individual polymer chains, and c) the presence of weak intermolecular forces, such as hydrogen bonding and Vander Wal forces, between the polymer molecules that allow movement and disentanglement of the individual polymer chains. While plastics can be characterized by their collective attributes, the variation within the properties demonstrated by different plastics arises from diversity in their structure and molecular weight. Additionally, they often contain additives, such as fillers and reinforcements, anti-degradants and stabilizers, flame retardants, and plasticizers to enhance their properties. However, the underlying attributes of a plastic material are determined by the polymer. It is common to focus on the thermal and mechanical properties of plastics to explain their performance. However, the polymer chemistry, particularly the structure, drives the thermal and mechanical response, and is at the core of performance.
It is the conclusion of this work that a direct correlation was evident between the thermal and mechanical properties of the three polyesters evaluated and their respective structures. In particular, structure influences polymer chain stiffness, molecular mobility, and intermolecular bonding between the chains. This manifests as thermal and mechanical properties. The relationship between structure and performance has been evaluated and demonstrated for many years.14, 15
Further, the work appears to support a correlation between the glass transition temperature and long-term creep performance. Higher glass transition temperatures are accompanied by enhanced creep resistance, as measured by the time required to produce the apparent modulus to 10% of its original value. Further work, including polymers with significantly different structures and functional groups would need to be performed to verify this relationship.