Comparison of the Diffusion Coefficients Obtained for Latex Film Formation Studied by FRET and Pyrene Excimer Formation

Transcription

1 Comparison of the Diffusion Coefficients Obtained for Latex Film Formation Studied by FRT and Pyrene xcimer Formation Remi Casier, Jean Duhamel, Mario Gauthier Institute for Polymer Research, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Introduction Over the years, latex dispersions have been widely used for many industrial applications. One particular application is the formation of coherent coatings used for both decorative and protective properties. Starting with an aqueous latex dispersion, film formation is typically divided into three stages: water evaporation, particle deformation, and particle coalescence. 1 In the first stage, a latex dispersion is deposited on a substrate and the water is evaporated off, leaving a layer of packed particles. In order for the subsequent stages of film formation to occur, the film must be heated above a minimum temperature, namely the minimum film formation temperature (MFT). As the layer of packed particles is heated above the MFT, the particles will begin to deform, filling in the voids left behind from the first stage. With continued annealing of the film, the polymer chains constituting the latex particles will begin to diffuse across the particle boundaries, resulting in the formation of polymer-polymer entanglements between adjacent particles and ultimately the coalescence into one continuous film. It is during this final stage of film formation where a film develops both its mechanical integrity and the ability to protect a surface. As such, a quantitative measure of the extent of coalescence between the particles is of particular interest. Over the past 30 years, several techniques have been developed to quantitatively probe the polymer diffusion in latex films. Two such methods use small angle neutron scattering (SANS) 2 and fluorescence resonance energy transfer (FRT). 3 In FRT measurements, a film first prepared with equal amounts of latex particles labeled with two carefully chosen dyes. One of the dyes is a donor, typically phenanthrene, and the other an acceptor, typically anthracene. Using time-resolved fluorescence, the fluorescence decay of the donor is acquired. Before the particles coalesce, the donors will emit primarily with their natural lifetime. As IPD occurs, the donors and acceptors will begin to mix, resulting in an increase in the amount of FRT occurring between the donors and acceptors. The extent of FRT is then monitored over annealing time to give the fraction of mixing, or extent of coalescence, between latex particles over time. Although FRT provides a powerful tool to probe polymer diffusion, the same information can be found in a much simpler way using pyrene excimer formation. When a pyrene fluorophore is excited by light, it will emit as a monomer. However, if this excited pyrene directly encounters another ground-state pyrene, an excimer is formed. Using a steady-state spectrofluorometer, the fluorescence intensity of monomer and excimer is measured, and the ratio of the intensity of excimer emission over that of the monomer, namely the I/IM ratio, is calculated. The higher the local concentration of pyrene, the more likely an excited monomer is to form an excimer, resulting in a higher I/IM ratio. Using this principle, a latex consisting of polymers randomly labeled with pyrene will have a high local pyrene concentration, resulting in a high I/IM ratio. A film prepared from a small amount of this fluorescent latex in a matrix of native, or non-fluorescent, latex will initially exhibit this high I/IM ratio. As film formation occurs, and the polymer chains containing pyrene diffuse into the surrounding native particles, the local pyrene concentration decreases, resulting in a lower I/IM ratio. By monitoring the I/IM ratio over annealing time, the fraction of mixing between latex particles can be determined. 1

2 xperimental Latex Preparation: A semi-continuous emulsion polymerization technique was utilized for the preparation of the fluorescent latex particles. A reactor was initially charged with deionized water and the dioctyl sodium sulfosuccinate (AOT) surfactant. As the reactor heated to 80 C, a monomer feed was prepared containing n-butyl methacrylate (BMA), 1-pyrenemethoxydiethoxyethoxy methacrylate (PyG 3 MA), AOT, and water. To the heated reactor, ammonium persulfate was added followed by the addition of the pre-emulsified monomer feed over a three hour period. The resulting latex was filtered, producing a pyrene-labelled poly(n-butyl methacrylate) latex (Py-PBMA-latex). The non-fluorescent latex were prepared in a similar manner, using a monomer feed containing only BMA, AOT and water. In total, four latexes were prepared: two fluorescent and two non-fluorescent. Latex characterization: Particle size and particle size dispersity (PSD) were determined using dynamic light scattering (DLS). Gel permeation chromatography (GPC) was used to characterize the polymer chains. Differential refractive index (DRI), UV absorbance, light scattering, and viscosity detectors were used in conjunction to provide the absolute molecular weight and dispersity (Ð) of the polymers. UV absorption was used to determine the incorporation of PyG 3 MA into the copolymer backbone. Steady-State Fluorescence: Steady-state fluorescence spectra were acquired using a front-face geometry setup. The films were excited at 344 nm, and the emission was scanned from 350 to 600 nm in one nm increments. The I/IM ratio was calculated by dividing the area underneath the excimer from 500 to 530 nm (I) by the area underneath the fourth monomer peak from 492 to 498 nm (IM). Film Formation: Two films were studied, one containing polymers with relatively high molecular weight, and the other prepared with polymers having lower molecular weights. Overviews of the properties of the two films are given in Table 1. ach film contained 5 wt% fluorescent latex mixed with 95 wt% nonfluorescent latex. Among the two latexes in a film, the particle size, PSD, M w, and Ð were kept as similar as possible. The films were prepared by depositing the latex mixture on a quartz plate and allowing the solution to dry under nitrogen overnight. After drying, the films were annealed at a constant temperature. After annealing for a given period of time, the fluorescence of the film was acquired by quickly cooling the film to room temperature, to halt polymer diffusion, and acquiring the steady-state fluorescence spectrum. The film was then re-heated for further annealing. This process was repeated to obtain the steady-state fluorescence of the film for increasing annealing times. Table 1: Overview of the film compositions. Film Latex Latex Pyrene Content (mol%) Particle Size (nm) PSD M w (kg mol -1 ) Đ Weight Fraction 1 Py-PBMA-Latex PBMA-Latex , Py-PBMA-Latex PBMA-Latex

3 Flu. Int. (a.u.) Flu. Int. (a.u.) Results and Discussion For each film, the steady-state fluorescence spectrum was monitored over time for a total of nine annealing temperatures ranging from 75 to 119 C. Figure 1 displays the change in the steady-state fluorescence spectrum for Film 1 at an annealing temperature of 102 C. The film exhibited the highest amount of excimer before annealing, corresponding to an I/IM ratio of As film formation occurred, and the polymer chains containing pyrene diffused into the surrounding particles, the fluorescence intensity of the excimer decreased, reaching a value of 0.04 for a fully annealed film. Similar changes in the fluorescence spectrum were observed for both Films 1 and 2 over all temperature ranges. A) Increasing Annealing Time Wavelength (nm) Wavelength (nm) Figure 1: Steady-state fluorescence spectra obtained for A) Film 1 containing Py-PBMA-Latex-1 annealed at 102 C and expanded area between 500 and 530 nm corresponding to the excimer fluorescence. Top to bottom: t an = 0, 25, 110, 560 min., and (for a homogeneous film). λ ex =344 nm. Using the I/IM ratios, the fraction of mixing between adjacent latex particles (f m ) within a film at annealing time t was calculated via quation 1. I I I M I t M t0 f m ( t) (1) I I I M t I M t0 In Figure 2, the fraction of mixing as a function of annealing time is given for both Films 1 and 2 at an annealing temperature of 102 C. Both plots exhibit a rapid increase in f m at early times, followed by the slow, continuous increase at longer annealing times. These characteristic profiles were found to be in very good agreement to the profiles found by previous FRT measurements. 3,4,6 As expected from previous studies, the film containing the lower molecular weight polymer coalesced more quickly than the film containing the high molecular weight polymer. 3

4 D (nm 2 /s) D (nm 2 /s) f m (a.u.) f m (a.u.) A) Annealing Time (min.) Annealing Time (min.) Figure 2: Fraction of mixing of A) Film 1 (Py-PBMA-Latex-1: M w = 820 kg/mol) and Film 2 (Py-PBMA- Latex-2: M w = 360 kg/mol) annealed at 102 C. Following the methodology used for FRT studies, 3,4 the apparent diffusion coefficient (D) of the fluorescently-labeled polymer was calculated using a Fickian diffusion model. Figure 3 displays the diffusion coefficients found for both films at the nine chosen annealing temperatures ranging from 75 to 119 C. A) Time (min.) Time (min.) Figure 3: Plot of the apparent diffusion coefficients as a function of annealing time for A) Film 1, containing high molecular weight polymer chains (Py-PBMA-Latex: M w = 820 kg/mol, Ð = 1.9) and Film 2, containing a lower molecular weight polymer (Py-PBMA-Latex: M w = 360 kg/mol, Ð = 1.8). From top to bottom T an = 119, 112, 111, 102, 98, 94, 88, 84, and 75 C. By observing the changes in D in Figure 3, several trends become apparent. Firstly, an increase in annealing temperature corresponds to an increase in the measured diffusion coefficients. Higher annealing temperatures lead to more thermal energy, an increase in Brownian motion of the polymer 4

5 chains, and ultimately larger diffusion coefficients. Secondly, there is an observed decrease in D with increasing annealing time. This decrease has been observed previously in FRT measurements, and is attributed to the dispersity of the polymer chains. At early times, the diffusion of the small chains is primarily observed. These small chains quickly reach equilibrium in the film, at which point the diffusion coefficient begins to be dominated by the larger, and diffusively slower, polymer chains. In agreement with this analogy, D is larger for Film 2, which contains the lower molecular weight polymers. Based on the method developed by the Winnik group, 3,4 the apparent activation energy of diffusion ( a ) of the polymer was calculated next. By plotting log(d) as a function of 1/T for a fixed f m, an Arrhenius equation can be used to calculate a. In Figure 4, the plots of Films 1 and 2 found by pyrene excimer formation are compared to a plot found using FRT measurements. In both Films 1 and 2, linear trends are observed for all f m s corresponding to an a of 170 ± 22 kj/mol. This value is not only very similar to 160 kj/mol found by FRT measurements 4 for PBMA, but is also close to the value of 151 kj/mol found for PBMA in the bulk by dynamic mechanical analysis. 5 A) C) T (K -1 ) T (K -1 ) T (K -1 ) Figure 4: Arrhenius plots used to find the activation energy of the diffusion coefficient for A) Film 1 (M w = 817 kg/mol, Ð = 1.9, a = 180 ± 8 kj/mol), Film 2 (M w = 356 kg/mol, Ð = 1.8, a = 160 ± 21 kj/mol), and C) a PBMA film using FRT measurements (M w = 420 kg/mol, Ð = 5.0, a = 159 kj/mol). 4 The fraction of mixing was held constant for each series, as indicated in the figures. To further confirm the validity of the a values, master curves for the diffusion coefficients were prepared next. Using an Arrhenius equation, the diffusion coefficients were normalized to a chosen reference using both temperature and a. Figure 5 compares the results of the master curves, as a function of f m, prepared from Films 1 and 2 to that of a PBMA film probed by FRT. Although the film probed by FRT contains very short polymer chains (M w = 38 kg/mol), the only major difference should lie with the order of magnitude of D. For both Films 1 and 2, the calculated a values enable one to build a continuous master curve of log(d) versus f m in good agreement with previous studies. 3,4 5

6 A) C 112 C 119 C 75 C 111 C 102 C 98 C 88 C 84 C C 112 C 94 C 84 C 75 C 111 C 102 C 98 C 88 C f m C) C 107 C f m 74 C 56 C Figure 5: Diffusion coefficients and their master curves obtained for A) Film 1 and Film 2 using a reference temperature T 0 = 75 C, and C) a PBMA film probed by FRT (M n = 38 kg/mol, T 0 = 56 C). 4 References: f m 1. Gauthier, C.; Guyot, A.; Perez, J.; Sindt, O. Film Formation and Mechanical Behavior of Polymer Laticies. Film Formation in Waterborne Coatings, Washington, DC: American Chemical Society. Chapter 10, 1996, pp Hahn, K.; Ley, G.; Schuller, H.; Obethür, R. On Particle Coalescence in Latex Films. Colloid Polym. Sci. 1986, 264, Zhao, C., Wang, Y., Hruska, Z., Winnik, M. Molecular Aspects of Latex Film Formation: An nergy-transfer Study, Macromolecules 1990, 23, Ye, X.; Farinha, J. P. S.; Oh, J. K.; Winnik, M. A.; and Wu, C. Polymer Diffusion in PBMA Latex Films Using Polymerizable Benzophenone Derivative as and nergy Transfer Acceptor. Macromolecules 2003, 36, Child, W.; Ferry, J. Dynamic Mechanical Properties of Poly-n-Butyl Methacrylate. Journal of Colloid Science 1957, 12,