Explanations for water whitening in secondary dispersion and emulsion polymer films

Abstract

The loss of optical transparency when polymer films are immersed in water, which is called “water whitening,” severely limits their use as clear barrier coatings. It is found that this problem is particularly acute in films deposited from polymers synthesized via emulsion polymerization using surfactants. Water whitening is less severe in secondary dispersion (SD) polymers, which are made by dispersing solution polymers in water without the use of surfactants. NMR relaxometry in combination with optical transmission analysis and electron microscopy reveal that some of the water sorbed in emulsion polymer films is contained within nanosized “pockets” or bubbles that scatter light. In contrast, the water in SD polymer films is mainly confined at particle interfaces, where it scatters light less strongly and its molecular mobility is reduced. The addition of surfactant to a SD creates a periodic structure that displays a stop band in the optical transmission. The total amount of sorbed water is not a good indicator of polymers prone to water whitening. Instead, the particular locations of the water within the film must be considered. Both the amount of water and the size of the local water regions (as are probed by NMR relaxometry) are found to determine water whitening. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1658–1674

Nuclear Magnetic Resonance Study of Water-Polymer Interactions and Self-Diffusion of Water in Polymer Films

“…The two exponential fit to the CPMG curves could be alternatively associated with pores of two sizes (small and large pores). A rough estimation for the pore sizes could be made from T 2 -CPMG-data [6] [35] [36].…”

“…This impacts the performance in polymer formulations [3] [5]- [7] [9]. The properties and applications of copolymer materials and polymer films in the presence of water have been investigated extensively using several techniques including NMR-methods [6] [8]- [12]. NMR has been used to estimate the amount of water as well as water self-diffusion in porous polymer materials [6] [9] [10] [12].…”

“…Water is known to interact strongly with a variety of polymeric materials having polar functional groups [1] [6]- [9] and both reversible and irreversible effects are known to occur [3] [5] [6] [11]. Short-term water exposure tends to give rise to reversible effects such as plasticization of the matrix, while longer-term exposures may lead to irreversible chemical changes in the system.…”

“…When polymers are processed from solvent, an initially homogeneous solution passes through a series of aggregated states prior to the formation of a homogenous film that is insoluble in water. Different types of solvent molecules could be distinguished in these partly dried films, e.g., in the polymer/solvent interface and also solvent inside the swollen polymer [1] [2] [6]. The formation of films from polymer solutions is still something of a mystery and there is little fundamental understanding about how molecular level interactions can lead to the specific properties of the films [9] [10].…”

“…A major factor is the copolymer composition (e.g., hydrophobic moiety and weak polyacid) [14] [15] which influences the removal of solvent at film formation. Another essential question is how the dried film interacts with water [6] [9]- [12].…”