Applied Mechanics of Polymers: Properties, Processing, and Behavior

Book Preface

Polymers are one of the primary classes of materials rivaling metals and ceramics while being an integral part of the remarkable and transformative hybrid materials of polymer matrix composites. Polymers are commonly referred to as plastics, but we ought to formalize this nomenclature since the classification of polymers is important for the fundamental understanding of their physical properties and mechanical behaviors, as will be discussed later. Polymers are ubiquitous in many applications ranging from aerospace to auto-motive, from consumer goods to household goods, and from sports gear to bio-medical devices. In fact, we daily encounter polymers, intentionally or unintentionally, in even some of the most mundane activities. For example, an observant eye would notice polymers everywhere in the interior of an aircraft as soon as you step inside the fuselage cabin, where the structural cabin window system and the accompanying dust cover that we often are asked to lower down during takeoff and landing, are made of different polymers. Another common transportation vehicle that encompasses a large percentage of polymers is the passenger automobile, which has nearly 30% of all its parts made of different types of polymers depending on the design requirements. The dashboard, instru-ment panel, interior trim, seating, interlayer in the front and rear windshields, and carpet fibers are just a few examples of interior parts in the car that are made of polymers. Many automotive exterior parts are also made of polymers where they are subjected to aggressive loading and environmental conditions such as the tires (being the most obvious), the bumpers, the undercarriage, the wheel housing, the radiator support, and the many parts in the fuel system, again just to list a few examples.
Evident from the diversity of the applications mentioned above is the broad range of operating and environmental conditions. Polymers that are used under the hood of a car are required to endure combined thermal and mechanical load-ing for a long duration, while those that are used in the fuel system are required to resist aggressive chemicals, e.g., the fuel, while supporting a working pres-sure of ?345 kPa (mechanical loads). Moreover, some polymers have to bear substantial mechanical loads while being abrasive resistant, e.g., tires help support a sizable portion of the car weight (?25%) while enduring abrasion dur-ing contact with the road. In all, the class of polymeric materials itself is as diverse as the applications they are integrated into. Therefore, the goal of this textbook is to provide the fundamental background to assist engineers in per-forming meaningful stress analyses based on material constitutive models derived from the theory of continuum mechanics. These material models rep-resent and describe the mechanical behavior of the polymer used in the design.
With this background in mind, it is now fitting to briefly define polymers noting that the term polymer is commonly used interchangeably with the word macromolecule, within the polymer science community. The basic definition of polymers comes from the Greek roots of the word, where the origin of ‘poly’ in Greek is either “polus’ meaning “much’ or ‘polloi” meaning “many.” Polymers are then materials of many repeating parts, units, or “mers” (from the Greek word “meros”). Polymers are compounds consisting of repeating long-chain units that are connected, where each single chain may have thousands or even millions of the repeating mers. Moreover, polymers are materials with interdig-itated molecules with different length scales. Generally, polymers are either hydrocarbons (covalently-bonded carbon-hydrogen backbone) or silicones (silicon-oxygen backbone). This molecular structure is responsible for the over-all properties of polymers. Polymers can either be natural (e.g., human DNA and hair) or engineered (such as nylon and polyvinyl chloride). Some of the overarching advantages and limitations of polymers are succinctly summarized in Table 1.1, which is not intended to be comprehensive or applicably inclusive to all types of polymers. While time-dependent properties are included in the table as an example of general disadvantage, this might be a desirable attributes for polymers used in dynamic or impact situations.
Before delving into more details, there are three questions that we need to ask and answer to not only motivate the whole field of study of polymer mechanics but also to gain an insightful perspective about this interesting class of materials. This perspective will be a running theme throughout the book. It is worth noting that these questions are listed from the perspective of a skeptical engineer who may be concerned about leaning towards a polymer for the design of a new part or component.