Surface Phase Partitioning in Film Formation of Waterborne Polyurethanes. Monte Carlo Simulations and Internal-Reflectance IR Imaging

Abstract

Understanding the film formation mechanisms, in particular for thermosetting polymeric systems, has attracted considerable interests of experimentalists 1-3 as well as theoreticians 4-7 in recent years. While laboratory measurements provide the opportunity to examine the effect of external and internal stimuli (temperature, molecular weight, reactivity of individual components, solubility, and others) on the film formation, 8-10 computer simulations offer other advantages in understanding the macroscopic properties evolving from the microscopic details in response to such stimuli. 11 For example, the effects of molecular weight, driving field, and temperature on the film formation can be studied effectively. 12,13 This is particularly important when multicomponent systems are considered; even though there are difficulties associated with computational analysis because of the increased number of parameters and chemical reactivity of the system, computer simulations remain the useful theoretic tool to probe such a complex system. However, when theoretical and experimental studies are carried out concurrently, a wealth of complementary information can be obtained regarding the highly complex multicomponent systems. One example of such complexity is represented by the two-component waterborne polyurethane (2K WB PUR) polymers, 14 which appear to be quite sensitive to numerous external and internal stimuli, such as relative humidity, miscibility of individual components, reactivity, evaporation rates, temperature, as well as rheological characteristics. In essence, in addition to chemical reactions, hydrophobic and polar interactions, which are not easily measurable, may play an important role. Further complications arise when such systems are constrained by the surface of the substrate on which the film formation occurs. Thus, a number of physical and chemical processes come into play simultaneously, which may be manifested by stratification, miscibility, or phase separation during the film formation, to name just a few.In this paper, we combined computer simulations 13,15,16 and compared them with the experimental findings from the recently developed internal-reflection IR imaging (IRIRI) 17 measurements in an effort to correlate the morphological evolution of the film formation in 2K WB PUR. Before we focus on these results, let us first identify the macroscopic morphological changes resulting from the changes of the cross-linking conditions. As shown in Figure 1, the optical images of the same PUR films cross-linked at different temperatures indicate significant morphological changes. Thus, the main questions are as follows: (1) What is the origin of the morphological features in addition to the chemical reactions leading to the formation of polyurethane/polyurea (PUR/PUA)? (2) What is the observed surface heterogeneity/roughness, the interface/ surface phenomenon, the reflection of the bulk behavior at the film surface, or both? To address these issues, polyurethane films were prepared using polyester resin disp...

Biocompatibility of poly(ether)urethane‐gold nanocomposites

“…The similarity of waterborne PU to amphiphilic block copolymers may lead to possible nanometer-sized micelle formation. 22,23 These features were different from the long, thin lamellae (5-10 nm and 40-100 nm in length), commonly observed in the solvent-borne PU. 18 These relatively larger micelles (averaged 92.8 nm in diameter) on the surface of the waterborne PU may contribute to the inferior phase separation and mechanical properties relative to the solvent-borne PU.…”

Dynamic colloidal processes in waterborne two-component polyurethanes and their effects on solution and film morphology

“…While it is generally accepted that these systems are highly sensitive to a number of minute process and environmental variables, such as temperature, humidity, stoichiometric imbalance, pH, or shear history, to name a few, the precise roles of such variables prior to and/or during film formation have only recently begun to be addressed [18][19][20][21][22][23][24][25][26][27]. Since the dynamics of WB 2K-PURs in the aqueous state are likely to affect the final properties of resulting films, the objectives of these studies are to correlate solution morphological features with dynamics of colloidal processes in WB 2K-PUR reactive dispersions as well as to correlate solution morphological features with film morphology and film formation.…”

Film Growth and Surface Roughness with Effective Fluctuating Covalent Bonds in Evaporating Aqueous Solution of Reactive Hydrophobic and Polar Groups: A Computer Simulation Model

“…but it is a considerable improvement over the previous studies. [17,18] Our simulation involves thermodynamic equilibration via stochastic motion of each constituent and segmental moves and kinetic reaction while the aqueous component continues to evaporate. Depending on the rate of reaction, the thermal equilibration can be arrested by covalent bonding.…”

“…Due to complexity, it is also not feasible to incorporate the thermodynamic equilibration involving stochastic mobility of constituents and kinetic reactions with covalent bonding in a self-consistent analytical theory. [15] Therefore, computer simulations [16][17][18] remain the primary tool to complement and understand the laboratory observations. [1] Some attempts have been made recently to study the film growth in such a multicomponent (H, P, and A) system via computer simulations.…”

“…[1] Some attempts have been made recently to study the film growth in such a multicomponent (H, P, and A) system via computer simulations. [17,18] The constituents (H, P, and A) are represented by particles with appropriate characteristics, such as molecular weights, reaction functionality, and phenomenological interactions in such coarse grained model on a discrete lattice. The mixture equilibrates as the particles execute their stochastic motion via Metropolis algorithm and water component evaporates from the top of the sample.…”