It has been several months since our last blog entry and it got us thinking about the passage of time.  In addition, this is the season we look back on all that has happened in the past year and peer forward to the next.  In the spirit of this blog and the New Year’s tradition we celebrate the changes that materials have undergone in the past 365 days and how they will realize their potential in 2016!

Understanding how materials have changed in this past 365 days is the key to predicting the way they will look on January 1, 2017. What causes this change?  What characteristics of a material are impacted by these changes? And in keeping with the topic of this blog, do voids, defects and The Space Between solids contribute to aging in materials?

Do Materials Age?

Yes, of course, all materials, both organic (human skin, paint, polymers) and inorganic (bones, rocks, metal, ceramics), are older and have changed to some degree over the past year.  The change in materials with time is simply referred to as “aging”; just as we humans refer to our own passage through time.  Whether we are looking in the mirror or out from a window, aging happens in front of our eyes every moment, it is a continuous process. New Year just happens to be a mark of time. The process of aging is everywhere: collagen and elastin break down, paint fades, a pothole appears, iconic structures like the Colosseum crumble, polymers become brittle and crack, and the foam in our running shoes degrades (whether we are running in them or not) providing a less cushioning effect.  With time all things age!

Aging in materials science and engineering is, in its simplest sense, a time-dependent change in a material and material property.  There are two distinct types of aging processes, physical and chemical aging.  The chemical aging process is what we are most familiar with, and includes changes like the oxidation of steel leading to rust and ultraviolet light can break polymer chains causing depolymerization leading to brittleness.  Physical aging is also a common polymer aging mechanism though it seems to be less appreciated.  During physical aging there is no change in chemical composition.  Voids tend to play an important role in this type of aging. Various environmental factors that a material is exposed to drive these changes: temperature, wind, radiation, humidity, water, cyclic mechanical loads, freeze/thaw cycles, friction or combinations of these.

For example, weathering can cause large chunks of the Colosseum in Rome to break away, see Figure 1a.  A one rainstorm may only dislodge a single flake of volcanic ash in the ancient, porous tuff walls and pillars, see Figure 1b (Calcaterra, Cappelletti et al. 2000) Tuff, known as piperno in Italian, is a porous rock made by the compression of hot ash that was ejected from an erupting volcano. Despite the porous nature, piperno was widely used as a material of construction in ancient Rome.  Considering weathering and earthquakes, all in all the Colosseum is still structurally sound after 2000 years (Cerone, Croci et al. 2000)!

This is in contrast to the chair that is near a hiking trail in the state of Massachusetts (USA).  GMG has witnessed its deterioration since it mysteriously appeared in 2011.  Obviously, this chair was not designed to withstand the high heat, freezing cold, snow, rain, and small animals.  It was designed for the more temperate conditions that prevail in the interior of a house.  Though it was not in the greatest shape when it first appeared near the hiking trail, deterioration happened swiftly.  The chair would have eventually aged and looked this way (without the leaves and pine needles) even if used inside of a house but having been exposed to the harsh New England weather accelerated the aging process.

Let’s focus on this chair and the physical aging that is occurring in the polymer materials; foam cushions (polyurethane) and fabric (polyester). What role did voids play in this aging and what are the accompanying changes in material properties?

Polymer Aging and Voids

Before discussing aging in polymers, let’s go over some background in polymer structure on the molecular level.  The total volume of a polymer is the sum of both “solid” polymeric chains and of void space.  The solid portion consists of the carbon “backbone” chain including any of its side groups, i.e. the volume taken up by the polymer molecules.  Everything not part of the polymeric chain is intrinsic void.  Intrinsic voids appear because of the nature of the material, natural processes, processing limitations, and/or aging during service.  These intrinsic voids between the polymeric chains constitute what is called free volume.

Free volume is a very important concept in polymers and dates back to Eyring’s work in 1936 (Eyring 1936). In the simplest sense, free volume is The Space Between or void between polymeric chains and allows the polymer chains to vibrate, rotate, and undergo molecular motion. In order for these modes to occur a void of critical size and volume must be present. Paraphrasing Sears and Darby (Sears, Darby 1982), an increase in free volume permits increased motion of polymer molecules. Thus, a study of the thermomechanical properties of polymers is nothing but a study of changes in free volume in the polymer. It is easy to see that the hotter a polymer, the more free volume, and consequently the softer it becomes. At higher temperatures, the polymeric chains can move more freely because there is more (and larger) free volume space surrounding the polymeric chains.

When a thermoplastic polymer begins to flow, the free volume has reached a critical size and volume allowing the polymer chains to move freely.  Thermoplastic polymers are often “melt processed” in this flowing state and formed into components or parts.  These components are typically cooled quickly into a solid form.  Because of the fast cooling there is excess free volume.  As this component ages this excess free volume collapses.  The volume difference caused by the reduction in free volume is responsible for physical aging; more specifically the free volume will diffuse to the surface of the polymer and vanish.  This physical aging is accompanied by many mechanical and physical changes such as surface roughening, increase in density, higher strength and modulus, lower failure strain, higher softening temperature.

Getting back to the chair on the hiking trail, the aging that occurred is definitely a combination of physical and chemical aging.  But many of the macro-scale properties are attributed to the molecular-level changes in the void spaces of the polyester fibers and polyurethane foam used to make the chair.  When GMG sat in the chair, the fabric easily ripped and the foam lost much of its ability to “bounce back” once he sat on it.  In scientific parlance we say the strain to failure in the fabric was decreased when describing the fabric breaking and the compression set of the foam (permanent deformation) increased, i.e., the foam loses its  ability to bounce back to its original thickness.  These are classic signs of the loss of free volume and physical aging.

One interesting fact is that polymers are “thermal history-dependent.” Meaning, if two components were both melt processed but one component was cooled faster than another, they would start life with different amounts of free volume and thus age differently.  Also, unlike humans, the clock can be reset on thermoplastic polymers and “life” can start again, simply by heating the polymers back to the softening temperature where the polymer chains can move freely and reformed into a new part.  It is because of free volume (voids) we can reset the clock and recycle thermoplastic polymers.

In our book, Voids in Materials: From Unavoidable Defects to Designed Cellular Materials, we discuss the role that void space plays during physical and chemical aging in crystalline (metals and ceramics), amorphous (polymers and glasses) and composite materials.  We discuss polymer aging in more detail and examine the two different types of free volume, free volume hole and interstitial free volume.  The concept of diffusion as it pertains to free volume and vacancies is introduced and governing equations are presented.

Read more about ‘The Space Between’ and materials voids by Gary and Krishan:

• Biomaterials and Much Ado About Nothing(ness)
• Reverse-Engineering Bread
• The Space Between: How Voids in Materials Contribute to 21st Century Society
• To the Bottom of the Sea Using Buoyancy Voids
• A Cause and Solution to Global Warming

Gladysz and Chawla’s upcoming book Voids in Materials: From Unavoidable Defects to Designed Cellular Materials is now available on the Elsevier Store. Use discount code “STC215” and save up to 30% on your very own copy!

About the Authors

Dr. Gary Gladysz (Twitter: @GMGladysz) is an Associate at Empyreus Solutions, LLC, Seattle WA, USA, where he consults and leads university and government technical interactions. He received his PhD from the New Mexico Institute of Mining & Technology where he participated in the NATO Collaborative Program with the German Aerospace Institute (DLR).   Since receiving his PhD, he has led research efforts in university, government and industrial settings. He has extensive research experience designing and characterizing fibrous composite materials, ceramic composites, polymers, composite foams, and thin films.

As a Technical Staff Member at Los Alamos National Laboratory (LANL), he was technical lead for Rigid Composites and Thermoset Materials. In 2005 he was awarded the LANL Distinguished Performance Group Award for his work leading materials development on the Reliable Replacement Warhead Feasibility Project. He has served on funding review boards for LANL, National Science Foundation, ACS, and the Lindbergh Foundation. He has been guest editor on four issues of leading materials science journals, including Journal of Materials Science and Materials Science & Engineering: A. Dr. Gladysz has organized five international conferences/symposia on syntactic foams and composite materials. He started and currently chairs the ECI international conference series on Syntactic & Composites Foams. Dr. Gladysz currently lives near Boston, Massachusetts, USA.

Krishan Chawla is Professor Emeritus of Materials Science and Engineering at the University of Alabama at Birmingham, USA. He received his Ph.D. from the University of Illinois at Urbana-Champaign. His research interests encompass processing, microstructure, and mechanical behavior of materials. He has taught and/or done research at several universities around the world. Professor Chawla has served as a Program Director for metals and ceramics in the Division of Materials Research, National Science Foundation.

He is a fellow of ASM International. Among his other awards are: Distinguished Researcher Award at New Mexico Tech, Distinguished Alumnus Award from Banaras Hindu University, Eshbach Society Distinguished Visiting Scholar award at Northwestern University, Faculty Fellow award at Oak Ridge National Laboratory. Professor Chawla is the author or coauthor of various textbooks in the area of materials and serves on the editorial boards of a number of journals. He is editor of the journal International Materials Reviews.

References

CALCATERRA, D., CAPPELLETTI, P., LANGELLA, A., MORRA, V., COLELLA, A. and DE GENNARO, R., 2000. The building stones of the ancient centre of Naples (Italy): Piperno from Campi Flegrei. A contribution to the knowledge of a long-time-used stone. Journal of Cultural Heritage, 1(4), pp. 415-427.

CERONE, M., CROCI, G. and VISKOVIC, A., 2000. The structural behaviour of Colosseum over the centuries, International congress: More than two thousand years in the history of architecture, 2000.

EYRING, H., 1936. Viscosity, plasticity, and diffusion as examples of absolute reaction rates. The Journal of Chemical Physics, 4(4), pp. 283-291.

SEARS, J.K. and DARBY, J.R., 1982. Technology of Plasticizers. New York: John Wiley & Sons.