CONSULTANT’S CORNER

Plastics – It’s All About Molecular Structure

By Jeffrey A. Jansen Senior Managing Engineer & Partner, The Madison Group

The characteristic properties exhibited by plastics arethe direct result of the unique molecular structure ofthese materials. Taking that a step further, the variation

within the properties demonstrated by different plastics aris-es from diversity in their structure. Plastics are polymers ofvery high molecular mass. To enhance their properties, theyoften contain additives, such as fillers and reinforcements,anti-degradants and stabilizers, flame retardants and plas-ticizers. However, the underlying attributes of a plasticmaterial are determined by the polymer.

PolymerizationPolymers are macromolecules that are based on a structurebuilt up, chiefly or completely, from a large number of similarstructural units bonded together. Often called chains, thepolymer consists of repeating units, similar to links. Polymers

are formed through a process known as polymerization, inwhich monomer molecules are bonded together through achemical reaction that results in a three-dimensional networkof long individual polymer chains consisting of smaller repeat-ed units.

There are two basic types of polymerization reactions —addition and condensation. Addition polymerization is theformation of polymers from monomers containing a car-bon-carbon double bond through an exothermic additionreaction. Significantly, this reaction proceeds without theloss of any atoms or molecules from the reacting monomers.Common materials produced through addition polymeriza-tion include polyethylene, polypropylene, poly(vinyl chloride),and polystyrene as represented in Figure 1.

In contrast, condensation polymers are formed by a step-wise reaction of molecules with different functional groups.The reaction is endothermic and produces water, or other

Figure 1. Addition reaction mechanism showing styrene monomer polymerizing into polystyrene.

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small molecules such as methanol, as a byproduct. Commonpolymers produced through condensation reactions includethermoplastic polyesters, polyacetal, polycarbonate andpolyamides as represented in Figure 2.

Addition polymers form high-molecular-weight chains rap-idly, and tend to be higher in molecular weight than

condensation polymers. Comparing polymers produced viathe two different mechanisms, addition polymers are gen-erally chemically inert due to the relatively strongcarbon-carbon bonds that are formed. Condensation poly-mers tend to be susceptible to hydrolytic moleculardegradation through exposure to water at elevated temper-atures, through a mechanism that resembles the reversionof the initial liberalization reaction.

By using different starting materials and polymerizationprocesses and techniques, polymers having different molec-ular structures can be produced (see Fig. 3). 

The fundamental differences between the properties ofthese different types of polymers are attributable to thevarying functional groups within the molecular structure.These differences include mechanical, thermal and chemicalresistance properties. As such, it is important to select thecorrect type of plastic based upon the requirements of theapplication.

Intermolecular BondingAs indicated, polymerization results in the formation of mul-tiple individual polymer chains made up of repeating units.A key aspect of polymeric materials is that the chains areentangled within each other. The individual chains are notcovalently bonded to each other, but instead rely on inter-molecular forces, such as Van der Waals forces, hydrogenbonding, and dipole interactions, to keep the chains fromdisentangling. This results in a structure that is similar to abowl of spaghetti noodles (Fig. 4).

Figure 3. Polymers contain a wide variety of functionalgroups, responsible for the diversity in physical properties.

Figure 4. Polymer chains consist of a high number of repeatingunits, and are entangled to form a spaghetti-like structure.

Figure 2. Condensation reaction mechanism showing the polymerization of a polyamide from a diacid and a diamine.

Molecular WeightThrough the polymerization process, materials of relativelyhigh molecular weight, macromolecules, are produced. A keyparameter of a polymer is its molecular weight. Molecularweight is the sum of theatomic weights of the atomscomprising a molecule. Forexample, the molecularweight of polyethylene is cal-culated by multiplying themolecular weight of therepeating ethylene function-al group times the numberof units comprising thechain. Thus, for polyethyl-ene (Fig. 5), where therepeating unit contains twocarbon atoms and fourhydrogen atoms, the molec-ular weight is 28n, where nrepresents the number of repeating segments. Most com-mercial polymers have an average molecular weight between10,000 and 500,000.

Higher molecular weights are associated with longer molec-ular chains, and this results in a greater level of entanglement.This has important implications, as higher-molecular-weightgrades of plastics will have superior mechanical, thermaland chemical resistance properties compared with lower-molecular-weight grades of the same material.

It is important to remember that the polymerizationprocess is a chemical reaction, and while carefully controlled,there is some inherent variation. This results in polydispersity,or polymer chains of unequal length. Because of this, com-mercial plastics have polymers with a molecular weightdistribution. Simply put, molecular weight distribution rep-resents the relative amounts of polymers of differentmolecular weights to comprise a given specimen of thatmaterial. Unlike molecular weight, the relationship betweenmolecular weight distribution and end properties is not uni-form. For example, in comparing two similar materials withdifferent molecular weight distributions, in general the mate-rial with a wider distribution will exhibit better ductility andimpact resistance, but will demonstrate reduced strengthand stiffness.

Because of the structure of the molecules, polymeric mate-rials have different properties compared to other materials,like metals. Specifically, the relatively high molecular weight

and long polymer chain length result in entanglement, andthe lack of covalent intermolecular bonds facilitates polymerchain mobility. This combination of entangled mobile chainsresults in viscoelasticity.

Viscoelasticity is the property of materials that exhibitboth viscous and elastic characteristics when undergoing defor-mation. Viscous materials, like honey, resist shear flow andstrain linearly with time when a stress is applied. Elastic mate-rials, such as a steel rod, strain when stressed and quicklyreturn to their original state once the stress is removed. Vis-coelastic materials have elements of both of these propertiesand, as such, exhibit time-dependent strain. 

There are three main factors that will affect the viscoelas-ticity of a plastic part — temperature, strain rate, and time.Because of this, plastics are temperature, strain rate andtime sensitive. Temperature is the most obvious of thesefactors. Polymers exhibit a comparatively high level of changein physical properties over a relatively small temperaturerange. As the temperature is increased, the polymer chainsare positioned further apart. This results in greater free vol-ume and kinetic energy, and the chains can slide past oneanother and disentangle more easily.

As strain rate — the speed at which load is applied — isincreased, the polymer chains do not have enough time toundergo ductile yielding, and they will disentangle throughan increasingly brittle mechanism. This is why plastics aremuch more susceptible to impact failures than they are over-load failures, which occur at more moderate strain rates.

The inherent viscoelastic nature of polymeric materialsproduces movement within the polymer chains under con-ditions of applied stress. This results in time dependencywithin polymeric materials. Because of this molecular mobil-ity, plastic materials will exhibit differences in their long-termand short-term properties due to the application of stressover time. This means that the properties of a plastic material,such as strength and ductility, are not static, but will decreaseover time. This often leads to creep and stress relaxationwithin plastic materials.

Crystalline/Amorphous StructureAnother fundamental characteristic of polymeric materialsis the organization of their molecular structure. Broadly,plastics can be categorized as being semicrystalline or amor-phous. Understanding the implications of the structure, andspecifically, the crystallinity, is important as it affects materialselection, part design, processing and the ultimate anticipatedservice properties.

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CONSULTANT’S CORNER

Plastics – It’s All About Molecular Structure ________________________

Figure 5. The repeating unit ofpolyethylene consists of twocarbon atoms with pendanthydrogen atoms.

Most non-polymeric materials form crystals when theyare cooled from elevated temperatures to the point of solid-ification. This is well demonstrated with water. As water iscooled, crystals begin to form at 0°C as it transitions fromliquid to solid. Crystals represent the regular, orderedarrangements of molecules, and produce a distinctive geo-metric pattern within the material. With small molecules,such as water, this order repeats itself and consumes a rel-atively large area relative to the size of the molecules, andthe crystals organize over a relatively short time period.

However, because of the rather large size of polymer mol-ecules and the corresponding elevated viscosity,crystallization is inherently limited, and in some cases, notpossible. Polymers in which crystallization does occur stillcontain a relatively high proportion of non-crystallized struc-ture. For this reason, those polymers are commonly referredto as semicrystalline. Polymers, which because of their struc-ture, cannot crystallize substantially are designated asamorphous (Fig. 6).

Amorphous polymers have an unorganized, loose struc-ture. Semicrystalline polymers have locations of regularpatterned structure bounded by unorganized amorphousregions. While some modification can be made through theuse of additives, the extent to which polymers are semicrys-talline or amorphous is determined by their chemicalstructure, including polymer chain length and functionalgroups.

The ordered arrangement of the molecular structure asso-ciated with crystallinity results in melting when a sufficienttemperature is reached. Because of this, semicrystallinepolymers such as polyethylene, polyacetal and nylon willundergo a distinct melting transition, and have a meltingpoint (Tm). Amorphous polymers, including polystyrene,polycarbonate and poly(phenyl sulfone), will not truly melt,

but will soften as they are heated above their glass transitiontemperature (Tg). This is represented by the differential scan-ning calorimetry (DSC) thermograms (Fig. 7).

The difference between semicrystalline and amorphousmolecular arrangement also has an implication on themechanical properties of the material, particularly as theyrelate to temperature dependency. In general, amorphousplastics exhibit a relatively consistent modulus over a tem-perature range. However, as the temperature approachesthe glass transition temperature of the material, a sharpdecline occurs. In contrast, semicrystalline plastics exhibitmodulus stability below the glass transition temperature,

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Figure 6. Structural representation of semi-crystalline and amorphous polymers.

Figure 7. DSC thermogram six showing a melting endothermfor a semicrystalline polymer and a glass transition for anamorphous material.

Figure 8. Graphical representation of the changes in moduluscharacteristic of semicrystalline and amorphous polymers.

which is often subambient, but show a steady declinebetween the glass transition temperature and the meltingpoint (Fig. 8).

Due to their viscoelastic nature, time and temperatureact in the same way on polymeric materials. Because ofthis, the changes within the material as a function of timecan be inferred from the stability of the material versustemperature.

Aside from the time and temperature dependence, otherkey properties of polymeric materials are determined bytheir semicrystalline/amorphous structure. Some general-

izations of characteristic properties are listed in Table 1. Plastics continue to be used in increasingly diverse and

demanding applications. Given the cost of product failure,it is very important that the right material be chosen specif-ically for each situation. Because the base polymerdetermines many of the critical performance characteristicsof the plastic resin, it is essential that the correlation betweenmolecular structure and performance be understood. Thedifference between success and failure can hinge on theimplications of molecular weight, molecular weight distri-bution, and crystalline/amorphous structure.

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Plastics – It’s All About Molecular Structure ________________________CONSULTANT’S CORNER

ABOUT THE AUTHORJeffrey A. Jansen is seniormanaging engineer and apartner with The MadisonGroup, a Madison, Wis.-based provider of consultingservices to the plastics indus-try. He is an expert in failureanalysis; material analysis,identification and selection;and aging studies for plasticand rubber components. Asenior member of SPE,Jansen also is a past chair-man of SPE’s Failure Analysis & Prevention SpecialInterest Group.

Table 1

Semicrystalline• Distinct and sharp melting point• Opaque or translucent• Better organic chemical resistance• Higher tensile strength and modulus• Better creep and fatigue resistance• Higher density• Higher mold shrinkage

Amorphous• Soften over a wider range of temperature• Transparent• Lower organic chemical resistance• Higher ductility• Better toughness• Lower density