Article pubs.acs.org/Langmuir
Latex Barrier Thin Film Formation on Porous Substrates Afsaneh Khosravi, Julia A. King, Heather L. Jamieson, and Mary Laura Lind* School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, United States S Supporting Information *
ABSTRACT: Here we present the formation of thin layers of barrier polymers onto mesoporous and macroporous substrates via dip coating of latex solutions. We investigated four commercially available latex solutions: polytetrafluoroethylene (PTFE), perfluoroalkoxy fluorothermoplastic (PFA), polyvinylidene chloride (PVDC), and polyolefin-based latex (Hypod). We examined the latex film formation on porous polymeric and ceramic substrates with a broad range of pore sizes from 10 to 200 nm. Our results show that both characteristics of the latex solution [glass transition temperature (Tg), particle size, and crystallinity] and the characteristics of the porous substrate (pore size and hydrophobicity) impact the film formation. We confirmed the defect-free, barrier nature of our latex thin films through scanning electron microscopy (SEM), atomic force microscopy (AFM), and hydraulically driven water permeation tests. Additionally, we found that latex concentration (not dipping time) is the most important parameter determining ultimate latex film thickness. We obtained defect-free films from PVDC and Hypod, which are “soft” polymers (Tg < room temperature), on mesoporous substrates under the conditions of slow evaporation rate of the solvent from these latex solutions. PTFE and PFA, which are “hard” polymers (Tg > room temperature), did not form continuous films on porous substrates. coalesce and fuse together.22 Coalescence of polymer beads into a film is thermodynamically favorable because the thin film has less surface area than the individual particles; thus, the thin film has a lower state of free energy than the same mass of individual particles.17 The micromechanical processes during particle deformation have been disputed.21,17,23,20,8,24 Researchers have proposed four main theories to account for the origin of the deforming forces.25 First, in the dry sintering theory, Dillon postulated that the interfacial tension between the particles and surrounding air (γPA) drives the particle deformation, and deformation occurs after all the solvent has evaporated.23 Second, Brown17 hypothesized that the capillary force resulting from the presence of liquid menisci (with negative curvature) between closely packed particles (γ WA ) is responsible for the deformation of the latex particles and film formation. Also, Brown hypothesized that the rate of water removal, particle size, surface tension of the emulsion, and the rheological properties of the polymer determine the degree of coalescence of the polymer particles.17 Brown proposed a balance between promoting forces, which encourage particle deformation, and resisting ones, which impede particle deformation. He indicated that deformation will occur as long as the capillary pressure is greater than the elastic response of the particles (determined by the polymer’s shear modulus).1,26,19
1. INTRODUCTION Latex solutions consist of polymer particles dispersed in an aqueous medium.1 Using water as a solvent has many advantages over organic solvents, including no volatile organic compounds (VOC), reduced odor, inflammability, and nontoxicity.2,3 Polymer latexes are frequently coated onto solid substrates to make thin films for applications ranging from paint to encapsulated vitamins.4−6 The formation of a continuous polymer film from a latex solution occurs in three stages: evaporation, deformation, and coalescence. In the first stage, water evaporates at a constant rate from the deposited latex solution;7−9 this concentrates the particles into a dense pack of spheres.10−12 In the second stage, the spherical particles begin to deform into rhombic dodecahedra. The rate of water evaporation decreases and the polymer particles contact each other; this forms liquid menisci between the close-packed particles and forms capillary pressure between the particles. This capillary pressure deforms the polymer particles to fill the void space between them.13,11,12,14 In the third stage, the individual particles coalesce into a film. Polymer chains interdiffuse across particle−particle boundaries, and the individual particles become indistinguishable. Factors influencing polymeric interdiffusion on nonporous substrates are the molecular weight of the polymer,15 the film formation temperature, the extent of cross-linking,16 the spatial distribution of polymer chain ends at the interface, and the electrostatic and steric stabilization of the latex.17,4 After coalescence is complete, the film has its final toughness and strength.13,18−21,14 The physical and mechanical properties of the film are dependent on the extent to which the latex particles © 2014 American Chemical Society
Received: July 16, 2014 Revised: October 16, 2014 Published: October 27, 2014 13994
dx.doi.org/10.1021/la502812d | Langmuir 2014, 30, 13994−14003
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Table 1. Properties of the Latex Particles and Porous Substrates polymer
substrates
G
