Molecular Simulation of Physical Properties of Hindered-Amine Light Stabilizers in Polyethylene Shyamal K. Nath, Juan J. de Pablo, and Anthony D. DeBellis Contribution from the Department of Chemical Engineering, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706, and AdditiVes DiVision, Ciba Specialty Chemicals Corporation, Tarrytown, New York 10591 ReceiVed NoVember 30, 1998. ReVised Manuscript ReceiVed March 2, 1999 Abstract: Novel Monte Carlo methods are used to simulate the excess chemical potential of commercial hindered-amine light stabilizers (HALS) in polyethylene. The solubility change incurred by increasing the size of the pendant tail of HALS molecules by a single methylene segment is estimated using simulations. Our results indicate that HALS molecules undergo a conformational transition at intermediate pendant-tail chain lengths. Our results also indicate that, above the boiling temperature of a particular stabilizer HALS, a segmental chain length increase raises its solubility in polyethylene. Similar phenomena are observed for small alkanes in polyethylene both theoretically and experimentally. The relative solubility of HALS of different lengths appears to increase with decreasing temperature. Results of molecular dynamics simulations yield diffusion coefficients that are consistent with available experimental data. Introduction Polymers can be highly reactive toward their environment; chain scissions and crosslinking can alter the molecular weight of a material considerably, thereby leading to deterioration of mechanical properties. In general, condensation polymers containing functional groups in their backbone, most notably polyesters, polyamides, and polyurethanes, are more prone to hydrolytic and biodegradation than polymers containing a carbon-carbon backbone (polyolefins). Polyolefins are not too susceptible to biodegradation unless they are first oxidized. Exposure to light, however, accelerates oxidation. The more oxidizable polyolefins (e.g., polyethylene or polypropylene) are highly sensitive to photooxidation, and the rate of physical deterioration of these polymers can be accelerated by almost an order of magnitude in the presence of light. The use of polymeric materials for increasingly demanding applications has led to numerous studies of polymer stability; 1-4 a number of effective stabilizers have now been developed. Hindered-amine light stabilizers (HALS) represent some of the most widely used photostabilizers for polyolefins, and several experimental studies of their properties have appeared in the literature. 5-7 For a stabilizer to be effective in preventing oxidation, it has to be chemically reactive and it has to meet certain physical requirements when dispersed in the host polymer. The stabilizer must be present at sufficiently high concentrations for effective protection of the polymer. One of the major problems encoun- tered in polymer stabilization is the physical loss of stabilizers from the polymer, which is directly related to their affinity for the polymer and their diffusion in the polymeric matrix. In some cases the loss of a stabilizer from the polymer can be accelerated if the stabilizer has a strong affinity for the surrounding medium (compared to that of the polymer); such effects, however, are not considered in this work. In the polymer stabilization community, the efficiency of a stabilizer is loosely defined as its ability to protect a polymer from degradation (under weathering conditions, accelerated or otherwise) over time. Empirically, the efficiency of a stabilizer for protection of a polyolefin (the so-called “protection time”) has been found to be correlated to the logarithm of the ratio S 2 /D, where S is the solubility (in wt %) of the stabilizer in the polymer and D is its diffusion coefficient (in cm 2 /s). 8,9 Protection time is determined experimentally by exposing polymer samples, stabilized with a known amount of the relevent additive, to accelerated weathering, followed by mechanical testing against failure at various times. Experimental studies have shown that this relationship is approximately valid for HALS-stabilized polyolefins. 9-11 Additives with a higher S 2 /D are indeed found to have a higher efficiency. In order to design better, more effective stabilizers, it would be useful to understand the factors which govern the solubility and diffusion behavior of such molecules in polyolefins. This will permit screening of promising additives before they are actually synthesized in the laboratory. In this work we demon- strate that molecular simulations can provide valuable informa- tion toward that goal. We do so by examining the solubility of a particular class of hindered-amine light stabilizers in poly- ethylene. Novel Monte Carlo methods are employed to calculate the relative solubility of a homologous series of HALS (1) Hawkins, W. L. In OxidatiVe and CombustiVe ReViews; Tipper, C. F. H., Ed.; Elsevier Publishing: Amsterdam, 1965; Vol. 1, p 170. (2) Allara, D. L.; Edelson, D. Rubber Chem. Technol. 1972, 45, 437. (3) Mill, T.; Mayo, F., Richardson, H.; Irwin, K.; Allara, D. L. J. Am. Chem. Soc. 1972, 94, 6802. (4) Gugumus, F. In Oxidation Inhibition in Organic Materials; Pospisil, J., Klemchuk, P. P., Eds.; CRC Press: Boca Raton, 1990: Vol. 1, p 61. (5) Allen, N. S. Chem. Soc. ReV. 1986, 15, 373. (6) Klemchuk, P. P.; Gande, M. E. Polym. Degrad. Stab. 1988, 22, 241. (7) Denisov, E. T. Polym. Degrad. Stab. 1989, 25, 209. (8) Moisan, J. Y. In Polymer Permeability; Comyn, J. Ed.; Elsevier: London, 1985; p 119. (9) Malik, J.; Hrivik, A.; Tomova, E. Polym. Degrad. Stab. 1992, 35, 61. (10) Moisan, J. Y.; Lever, R. Eur. Polym. J. 1982, 18, 407. (11) Malik J.; Tuan D.Q.; Spirk E. Polym. Degrad. and Stab. 1995, 47, 1. 4252 J. Am. Chem. Soc. 1999, 121, 4252-4261 10.1021/ja984107m CCC: $18.00 © 1999 American Chemical Society Published on Web 04/15/1999