OVERVIEW OF PROPERTIES OF POLYMERIC MATERIALS
This post is based on the discussion of polymer properties by J. Bicerano, Prediction of Polymer Properties, third edition, Marcel Dekker, New York, 2002.
The properties of a polymer fall into two general classes. Material properties are mainly related to the nature of the polymer itself. Specimen properties are primarily consequences of the size, shape and layout of the finished specimens prepared from that polymer, and the process used to prepare these specimens.
Morphology at the microscale and/or the nanoscale plays an especially important role in determining the properties of more complex polymeric materials; such as block copolymers, polymer blends, and polymer matrix composites. Kinetic factors (determined by fabrication process conditions) can have a major influence on the morphology. Hence the separation of the properties of polymers into these two classes is unfortunately not always straightforward. Material properties affect the specimen properties, and fabrication conditions affect the observed material properties. The following are two examples:
- The density is essentially a material property. The density of a given specimen, however, can be affected by the nature of the specimen. For example, many polymers can be prepared with different percent crystallinities, and hence with different densities, by changing the preparation method or annealing the specimens after preparation.
- The amount of directional variation ("anisotropy") induced in various physical properties by uniaxial or biaxial orientation ("drawing") is a process-related specimen property. The response of a specimen to a given set of drawing conditions, however, depends on the material properties of the polymer from which it is made.
Another imprecise (since it depends somewhat on one’s perspective) but somewhat helpful distinction is whether a material property is of a fundamental or derived nature.
- Fundamental properties, such as the van der Waals volume, cohesive energy, heat capacity, molar refraction and molar dielectric polarization, are directly related to some very basic physical factors; namely, that: (a) Materials are constructed from assemblies of atoms with certain sizes and electronic structures. (b) These atoms are subject to the laws of quantum mechanics. (c) These atoms interact with each other via electrical forces arising from their electronic structures. (d) The sizes, electronic structures and interactions of atoms determine their spatial arrangement. (e) Finally, the interatomic interactions and the resulting spatial arrangements determine the quantity and the modes of absorption of thermal energy.
- Derived properties, such as the glass transition temperature, density, solubility parameter and modulus, are more complex manifestations of the fundamental properties, and can be expressed in terms of combinations of them.
The following is a list of polymer properties. Different properties are more important for different applications.
- Volumetric properties; such as van der Waals volume, molar volume, and density.
- Thermodynamic properties; such as heat capacity and properties that can be calculated from it (enthalpy, entropy, and Gibbs free energy).
- Cohesive properties; such as cohesive energy density and solubility parameter.
- Transition and relaxation temperatures; such as glass transition temperature (Tg), melting temperature (Tm), crystallization temperature (Tc), heat distortion temperature (HDT), and softening temperature.
- Interfacial properties; such as surface tension, interfacial tension, adhesion, and coefficients of friction when in contact with other surfaces.
- Optical properties; such as refractive index, optical losses, and stress-optic coefficient.
- Electrical properties; such as dielectric constant, dissipation factor, and dielectric strength.
- Magnetic properties; such as diamagnetic susceptibility.
- Mechanical properties:
- Small-deformation properties; such as moduli (tensile, compressive, flexural, shear, and bulk) and compliances under various modes of deformation, and surface hardness.
- Abrasion (scratch, mar) resistance.
- Large-deformation properties; such as yield stress, fracture stress, and fracture toughness.
- Residual stresses due to thermal expansion coefficient mismatch, such as stresses caused by shrinkage during curing.
- Long-term properties affecting durability; such as creep resistance, stress relaxation, dynamic fatigue resistance, chemical resistance, ultraviolet (UV) light resistance, and resistance to microorganisms. It should be noted that the preference for a given type of long-term behavior may depend on the application. For example, one would want a polymer to have high resistance to microorganisms if it is expected to continue to retain its mechanical properties for a very long time in an application where it is buried under soil. On the other hand, if a compostable polymer is desired, one would want it to undergo relatively rapid biodegradation when exposed to microorganisms in a compost pile.
- Dilute solution properties; such as steric hindrance parameter, characteristic ratio, persistence length, radius of gyration, statistical chain segment length, and intrinsic viscosity.
- Rheological properties; such as viscoelastic properties under shear or extension, shear viscosity, and extensional viscosity.
- Heat transport properties; such as thermal conductivity and thermal diffusivity.
- Properties quantifying transport of small molecules; such as solubilities of small molecules in a polymer, extent of swelling and/or dissolution by small molecules, diffusivities of small molecules through the polymer, and overall permeabilities of a polymer to different small molecules.
- Heat resistance properties; such as thermooxidative stability (measured in an environment containing oxygen, most commonly air), thermal stability (measured in a non-oxidative environment, such as under a nitrogen blanket or in vacuum), and (c) fire resistance (which can be assessed in several different ways).