UV+EB Technology introduces “Professor’s Corner.” This quarterly column will provide science-based information to people employed in the UV/EB industry or in related academic research. Topics may range from the fundamentals of polymer chemistry to the detailed science of photopolymerization and advanced topics selected to enhance the technical skills of chemists, engineers and technicians employed in the field. A key objective is to assist technical personnel in achieving a deeper understanding of the chemistry and technology of UV/ EB polymerization. A second objective is to provide a framework for clarification of terminology used in the industry. Finally, a third objective is to create a forum for readers to submit technical questions that can be addressed by various experts in the field. This column is not intended to be a substitute for what the reader may learn from technical papers presented in UV+EB Technology. Rather it is to serve as an instructive supplement to enhance the reader’s scientific understanding of those articles.
The need for this column arises from the fact that many technical people in the UV/EB industry, including those with chemistry and chemical engineering degrees, have had little or no formal academic preparation in polymer chemistry. Thus, they find themselves employed in a highly specialized field of polymer science, but without a detailed understanding of the chemistry and physics thereof. The learning curve for the new employee is steep. Various laboratory observations may be inadequately interpreted, sometimes resulting in significant and long-lasting misunderstandings.
For many, the lack of academic polymer preparation was not a matter of choice. Rather, universities and colleges often have failed to provide courses in polymer chemistry, particularly at the undergraduate level. Many reasons might be advanced for this situation, but the fact remains that – although as many as 50% of all chemists in the US work with polymers at some point in their career (according to the Polymer Division of the American Chemical Society¹) – most enter the field with inadequate preparation in polymer science. The raison d’etre for this column, then, is to address this situation.
A brief introduction to polymer science
Polymer science is an interdisciplinary subject that relies on chemistry, physics and engineering. In an industrial setting, the successful polymer professional also should have a working knowledge of business-related subjects such as marketing and sales, and product management. Polymers are pervasive in the modern world and are present in one form or another in most materials we come in contact with every day. Therefore, it may be surprising to know that, until the 1920s and ’30s, most chemists did not understand the fact that polymers consisted of molecules of enormous size and molecular mass. This idea of “macromolecules” was first introduced by Hermann Staudinger in 1920²,³. Colligative properties, such as boiling point elevation and freezing point depression of polymer solutions, had indicated that molecular masses were very large. Even so, many prominent chemists flatly denied the fact, preferring to believe that polymers consisted of physical aggregates of smaller molecules amounting to colloids. (Colloidal dispersions are not true solutions and, therefore, do not exhibit colligative properties.) Along with Staudinger, Wallace H. Carothers accepted the “macromolecular hypothesis” and, based on that, was able to develop polyamides (nylons) and polyesters for the DuPont company4. Thus, Carothers’ work clearly demonstrated the validity of the macromolecular hypothesis, and thereafter it was no longer seriously debated.
So, what is a polymer? How do you define it for those asking what your work involves? Of course, it is a material consisting of truly gigantic molecules that are made from a very large number of smaller molecules known as monomers or repeat units. These small molecules are linked together chemically into exceptionally long chains known as “macromolecules,” and it is precisely this macromolecular structure that provides polymers with their unique properties when compared with properties of lower molecular mass materials, i.e., metals, ionic solids, molecular solids, etc. The properties we so clearly associate with polymers begin to emerge when the chain length reaches a point where the macromolecules become entangled with one another. This chain length is known as the “critical chain length,” Zc.
But, the structure/property differences with non-polymeric materials are much more complicated. Polymers can be categorized by their overall structure into at least three types. Some polymers have a predominantly “linear” structure, with individual long-chain macromolecules, while others are “branched” polymers, wherein shorter – but significant – branches emerge from the sides of the long chains, a.k.a., the “backbone” of the polymer. As examples, high-density polyethylene (HDPE) contains a high percentage of linear polymers, while low-density polyethylene (LDPE) is branched. Finally, many important polymers are “crosslinked.” The individual macromolecules are chemically bonded together, forming a three-dimensional network. Discounting imperfections at the molecular level, it may be useful to think of a crosslinked polymer sample as a “single molecule” of essentially infinite molecular mass. All commercially significant UV/EB or energy-cured polymers are crosslinked. Thus, understanding the structure/property relationships of crosslinked polymer systems is of paramount importance to the UV/EB professional.
In the next edition of “Professor’s Corner,” we will analyze further the differences among polymeric and non-polymeric materials and look for reasons for these many differences.
What are your technical questions about polymer science, photopolymerization or other topics concerning the chemistry and technology of UV/EB polymerization? Your questions will help guide future topic decisions for this column. Please submit your questions via email directly to Dianna Brodine, managing editor for UV+EB Technology, at [email protected].
References for further study
- ACS Polymer Chemistry Division, https://www.acs.org/content/acs/ en/careers/college-to-career/chemistry-careers/polymers.html
- https://www.acs.org/content/dam/acsorg/education/whatischemistry/ landmarks/staudingerpolymerscience/foundation-of-polymer-science-by-herman-staudinger-commemorative-booklet.pdf
Byron K. Christmas, Ph.D.
Professor of Chemistry, Emeritus
University of Houston-Downtown