pubs.acs.org/Macromolecules Published on Web 12/22/2009 r 2009 American Chemical Society Macromolecules 2010, 43, 975–985 975 DOI: 10.1021/ma9020483 Effect of Hydroplasticization on Polymer Diffusion in Poly(butyl acrylate-co-methyl methacrylate) and Poly(2-ethylhexyl acrylate-co-tert-butyl methacrylate) Latex Films Mohsen Soleimani, †,‡ Jeffrey C. Haley, ‡ Willie Lau, § and Mitchell A. Winnik* ,†,‡ † Department of Chemical Engineering, University of Toronto, Toronto, Ontario, Canada, M5S 3E5, ‡ Department of Chemistry, University of Toronto, Toronto, Ontario, Canada, M5S 3H6, and § Dow Advanced Materials, The Dow Chemical Company, 727 Norristown Road, Spring House, Pennsylvania 19477 Received September 14, 2009; Revised Manuscript Received November 23, 2009 ABSTRACT: We compare the influence of humidity on the polymer diffusion rate in films formed from two different polymer latex samples whose polymers have the same glass transition temperature (T g ≈ 12 °C) but different hydrophilicity: poly(butyl acrylate-co-methyl methacrylate), P(BA-MMA), and the more hydro- phobic poly(2-ethylhexyl acrylate-co-tert-butyl methacrylate), P(EHA-tBMA). The diffusion process was monitored by fluorescence resonance energy transfer (FRET) at 25 °C and at 0, 23, 54, 85 and 98% relative humidities. The results show that the polymers diffused more rapidly in films aged at higher humidities, and thus were characterized by higher apparent diffusion coefficients (D app ). By performing a master curve analysis, we obtained humidity related shift factors (a H ). Not all the water taken up by these latex films contributes to enhance diffusion rates. Some of the water absorbed at high humidities is present in the form of water pools and microcavities (free water) and does not actively contribute to plasticization. We used FTIR spectra to obtain information about how water resides in the copolymer films. Although water is poorly miscible with most polymers, our results show that water molecules dispersed molecularly among the chains are highly efficient as a plasticizer and a promoter of polymer diffusion in latex films. Introduction Plasticizing polymers by incorporating small molecules as additives is a common strategy for modifying the mechanical properties of polymers. 1 At the molecular level, plasticizers affect intermolecular interactions and therefore chain relaxation dy- namics by decreasing the internal friction coefficient among polymer chains. In this context, a variety of different organic compounds are intentionally introduced to polymer systems as plasticizers. For instance, in the coatings industry, volatile organic compounds (VOC) are often added to aqueous polymer latex formulations to decrease the modulus of polymer particles. In the presence of these volatile plasticizers, the particles become soft enough to deform and pack to yield transparent and void- free continuous films upon water evaporation. Over a longer time scale, as the VOCs escape to the atmosphere, the glass transition temperature (T g ) of the polymer film increases, and the film hardness increases. Hydroplasticization, plasticization by moisture, often occurs unintentionally since water has limited but non-negligible solu- bility in many polymers. Despite its poor miscibility, moisture can lead to dramatic changes in the mechanical properties of a polymer. Hydroplasticization is more prominent for applications in which polymers are exposed to water rich media such as body fluids (e.g., tablet coatings, drug delivery systems, and implants) 2-5 or ambient moisture (e.g., polymer adhesives, automotive parts, latex paints, and coatings). 6-8 This problem is accentuated in water-based paints since they often contain salts and other hydrophilic additives. 8 Moisture absorption into a polymer matrix is driven by os- motic pressure and proceeds until the activity of water absorbed into the polymer is equal to that in the vapor phase. For glassy polymers such as epoxy resins, the swelling stress may cause irreversible physical damage such as microcracking and crazing. 9 Moreover, the absorbed water can change the glass transition temperature of the polymer through the specific interaction of water molecules, disrupting interchain bonding networks 10 or by the composition dependence of the glass transition in miscible polymer-diluent systems. 11-13 A large number of relationships between T g and the phase composition of amorphous polymers have been developed. Aside from empirical equations, most of the theoretically driven rela- tions such as the Fox, Gordon-Taylor or Kelly-Bueche equa- tions are based on polymer free volume theory. For predicting the T g of a moisture-softened polymer, the Couchman-Karasz equation is widely used. This expression modifies the Fox equation by taking into account changes in the specific heat capa- city of water and that of the polymer (4C p ) at T g . 14 However, applying any predictive model requires a detailed understanding of how water resides in the polymer. The absorbed water may phase separate, as reported for many amorphous and crystalline polymers, such as epoxy resins, 19,23 cellulose esters, 15 poly(methyl methacrylate), 16 and poly(vinyl alcohol). 17,18 The nature of absorbed water into epoxy resin has been thoroughly investigated. 12-23 Results based on quadrupole-echo NMR spectroscopy have indicated that water forms a single phase in epoxy polymers in which the water molecules present are more mobile than hydrogen-bonded water but less mobile compa- red to free water. 12,13 However, measurements employing other experimental techniques (such as dielectric measurements, FTIR, ATR-FTIR, UV-reflection, and positron annihilation lifetime spectroscopy) have been interpreted as indicating a dual nature of water in epoxy resin: a combination of molecularly bound water plus the presence of a more mobile water fraction localized in holes and cavities. 19-23 *To whom correspondence should be addressed.