Diffusion-Weighted PFGNMR Study of Molecular Level Interactions of

Mar 12, 2013 - Antony K. Van Dyk,. §. John J. Rabasco,. § ... Company, Spring House, Pennsylvania 19477, United States. •S Supporting Information...
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Diffusion-Weighted PFGNMR Study of Molecular Level Interactions of Loops and Direct Bridges of HEURs on Latex Particles Kebede Beshah,*,† Aslin Izmitli,‡ Antony K. Van Dyk,§ John J. Rabasco,§ James Bohling,§ and Susan J. Fitzwater∥ †

Analytical Sciences, ‡Formulation Sciences, §Dow Coatings Materials, and ∥Materials Science and Engineering, The Dow Chemical Company, Spring House, Pennsylvania 19477, United States S Supporting Information *

ABSTRACT: Viscosity building in latex coatings to provide desirable shear thinning rheological properties is a key property commercially achieved with hydrophobically modified ethylene oxide urethane (HEUR) rheology modifiers (RMs). Prior studies focused on the aqueous solution properties of HEURs, resulting in the well-known transient network model that describes solution rheology reasonably well. Relatively fewer studies have probed the molecular level interactions between the hydrophobe groups of HEUR and latex surfaces under conditions of realistic latex volume fractions and HEUR concentration. The presence of ubiquitous surfactant and oligomer molecules in the latex aqueous phase makes it difficult to detect these interactions directly for any off-the-shelf (industrial) materials. In this work, we outline the use of pulsed field gradient (PFG) NMR spectroscopy as diffusion-weighted filter to remove the signals of low molecular weight species in order to detect hydrophobe end groups and urethane linkers. This in situ approach does not have any perturbation issues that are inherent in prior methods involving centrifugation and avoids the questions raised by the use of custom pyrene hydrophobes in fluorescence spectroscopy. From this study we conclude that there are no HEUR transient network structures present in HEUR−latex composites with less than about 2% HEUR and 30% latex relevant for coatings applications. Our results explain the shear thinning rheology of latex−HEUR composites based on molecular level interactions between hydrophobe end groups and urethane linkers of HEURs and latex particles to produce HEUR PEO loops on latex and direct bridges between pairs of latex particles.

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INTRODUCTION Hydrophobically modified ethylene (oxide) urethane (HEUR) thickeners have been widely used in the paint and coatings industry for over 25 years.1 Their rheology has been extensively studied, both in solution2 and in formulated paints and coatings.3 These nonionic associative thickeners are typically made of a water-soluble poly(ethylene oxide) (PEO) backbone with internal, terminal, and in some specific cases even with pendant hydrophobes.4 PEO and other components are linked together with urethane (carbamate) groups based on isocyanate chemistry,5,6 acetal−polyether or ketal−polyether groups with base-catalyzed gem dihalide chemistry,7 base-catalyzed epichlorhydrin or xylene dichloride chemistry,8 or aminoplast−ether (polyuril ether) chemistry.9 Pure HEUR polymers in aqueous solution above a threshold aggregation concentration (referred to as the critical aggregation concentration, cac) form flower micelles where hydrophobic groups assemble to form flower-like structures.10 At higher concentrations these flower micelles are bridged11 through PEO chains of the HEUR polymer backbone and above about 2% concentration form a 3-dimensional network.12 Micelle formation is a dynamic process; hence, the network is © 2013 American Chemical Society

transient in nature. Various experimental techniques have been used to characterize their solution oscillatory13 and steady shear14 rheology, including neutron scattering,15 light scattering,16 fluorescence spectroscopy,17,18 and NMR.19 Theoretical approaches, for example of Tanaka and Edwards,20 describe most of the salient features of steady shear behavior. However, while a Green−Tobolsky transient network model is capable of describing rheological behavior in terms of relaxation time corresponding to the hydrophobe pull-out time from flower micelles, unresolved issues remain. For example, Suzuki et al.18 and others report that hydrophobe pull-out does not occur at a significant rate and propose instead that shear thinning is attributed to flow induced disruption of the network connectivity through conversion of superbridges into superloops, both having the same core structure. While the rheology of HEUR polymers in solution is reasonably well understood, the rheology and conformation of HEUR molecules in formulated paints are more complicated. Received: January 17, 2013 Revised: February 19, 2013 Published: March 12, 2013 2216

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In particular, while it is recognized that HEUR polymers adsorb on the latex surface via their hydrophobic end groups forming a thin PEO shell,21,22 it remains unclear how the component contributions of core−shell hydrodynamic volume, particle aggregation, viscous drag on aggregated particles, and HEUR transient network in solution affect rheology. Early research in this field proposed that in the presence of latex particles HEUR polymers form a transient network of flower micelles which coexists with latex. In this transient network latex coexistence model, some of the network nodes were thought to be occupied by latex particles, with HEUR bridges to the transient network.23 Composite system viscosity increases by about 100fold (to viscosity greater than 2000 cP), from component 30% latex and 1% HEUR solution viscosities of around 20 cP, were proposed due to enhanced network connectivity introduced by the latex particles. On the other hand, Hemker et al.24 and Richey et al.25 reported that in the presence of latex particles, and below HEUR−surfactant saturation of the latex surface, a HEUR transient network was not detected based on their fluorescence study using pyrene as the hydrophobe group. Later, Macdonald and co-workers19,26 employed a pulsedgradient spin−echo nuclear magnetic resonance (PGSENMR) technique to quantitatively measure the fraction of HEUR bound with latex particles. For example, Uemura and Macdonald26 reported that at 1% (w/w) HEUR and 4% polystyrene latex loading the fractional population of bound HEUR is 0.53. They started with a dilute solution (8.1% w/w latex in D2O), and the aliquot (mixing of latex solution and HEUR solution) was centrifuged and the pellet fraction was resuspended to its original volume. We mention that dilution, centrifugation, and the associated procedure arguably introduces artifacts in adsorption quantification. In this work, we primarily focus on understanding the molecular level interactions of HEUR molecules with latex particles. The typical formulation used in this work was with latex 30% (w/w) and HEUR concentration 1% (w/w) which is representative of concentrations in commercial paints and coatings. For simplicity, pigment (TiO2) particles were not used in this study. Our experimental design circumvents some issues raised with previous experiments that advanced our understanding of molecular level interactions. For example, questions remain about the relevance of pyrene probe to alkyl hydrophobes of commercial interest. In addition, the fluorescence experiment is limited to only one size hydrophobe, pyrene, unlike the commercial HEUR polymers that have a wide range of hydrophobe end groups (C8−C20) and cannot account for interactions of some internal hydrophobes from the urethane linkers that our current experimental approach enables. Most prior studies are based on experimental procedures that involve centrifugation to separate the supernatant from the latex particles in order to study the amount of HEUR in the aqueous phase. We will show this approach perturbs the weakly bound hydrophobes leading to apparent detection of free HEUR in the aqueous phase. Our in situ approach to detecting free HEUR in the aqueous phase circumvents this apparent perturbation. Our approach has been a challenge in the past due to interference of ubiquitous signals from emulsion polymers that overwhelm the hydrophobe signals of HEUR in latex−HEUR composites. We have used PFGNMR experiments as diffusion filter technique to reduce or eliminate undesirable low molecular weight signals of emulsion polymers in the aqueous phase while keeping the high molecular weight signals of HEUR hydrophobes.

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EXPERIMENTAL DETAILS

Samples. The basic structure of telechelic HEUR associative thickeners comprises poly(ethylene glycol) (Mw 8000−10 000) chains that are linked linearly with diisocyanates and end-capped by hydrophobe end groups.27 A series of HEUR samples were synthesized with nonylphenol and C4 to C12 hydrophobe end groups for this study.28 The total hydrophobe end group comprises the alkyl alcohol used to end-cap and the hydrophobe contribution from the urethane (carbamate) groups used to connect the alcohol to the poly(ethylene glycol) chain. The number-average molecular weights of the HEUR molecules range from 24K to 35K based on GPC measurement. The diisocyanates used for urethane linkers for this study are either hexamethylene diisocyanate (HDI) or H12MDI (also known as DesW (DESMODUR W)).

Table 1. HEUR Samples sample HEUR10 HEUR12 HEUR14 HEUR16 HEUR18 HEURNP HEURHDI

idealized structure C4H9-DesW-PEG8K-DesW-PEG8K-DesW-PEG8K-DesWPEG8K-DesW-C4H9 C6H13-DesW-PEG8K-DesW-PEG8K-DesW-PEG8K-DesWPEG8K-DesW-C6H13 C8H17-DesW-PEG8K-DesW-PEG8K-DesW-PEG8K-DesWPEG8K-DesW-C8H17 C10H21-DesW-PEG8K-DesW-PEG8K-DesW-PEG8K-DesWPEG8K-DesW-C10H21 C12H25-DesW-PEG8K-DesW-PEG8K-DesW-PEG8K-DesWPEG8K-DesW-C12H25 NP-EO4-HDU-PEG8K-HDU-PEG8K-HDU-PEG8K-HDU-EO4NP C10H21-HDU-PEG8K-HDU-PEG8K-HDU-PEG8K-HDUPEG8K-HDU-C10H21

Figure 1. Idealized chemical structure and a schematic representation of a typical telechelic HEUR thickener molecule where the lines represent the polyethylene chains of 8K−10K, the open ovals represent the urethane linkers, and the closed ones are the hydrophobe end groups.

The emulsion polymers in this study were synthesized using standard polymerization techniques,29 with butyl acrylate and methyl methacrylate copolymers as the major composition of the latex particles, particle size 130 nm diameter by BI90, and acrylic or methacrylic acid at low levels used as ionic stabilizers, along with surfactants as additional ionic and nonionic stabilizers.30 Rheology Measurements. Rheology modifier (RM) and binder mixtures were prepared with 1% rheology modifier concentration and 28% solids. The rheology modifier solids were synthesized as described above and dissolved in DI water. Rhoplex AC-261 latex emulsion was added to the RM−water mixture, mixed in a high-speed mixer, and kept on a roller for 1.5 h before measuring rheology. Rheology measurement was performed with an Anton Paar MCR 301 rheometer in the automated mode. In this mode, a six-axis robot aspirates and dispenses the sample with 2.5 mL Eppendorf syringes while another one cleans the measuring system. The measurements were carried out at room temperature using a cone and plate geometry with 50 mm diameter and 0.5° cone angle with a sample size of 350 μL. The sample was first presheared with a shear rate of 5 s−1 and then allowed to equilibrate for 1 min. Afterward, a steady state flow test was performed from 0.1 to 12 000 s−1 shear rates where 18 measurement points were collected with logarithmically varying measurement point 2217

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Figure 2. 1H NMR spectra of 1% HEUR in water. The left one has DesW (H12MDI) diisocyante linker while the right one has HDI linker. The signals designated by R are the alkyl end group signals while those denoted by u are urethane linker signals from H12MDI (DesW).

Figure 3. 1H spectra of 1% RM in water (top) and chloroform (bottom). The amphiphilic HEUR molecule forms transient network structure in water, but not in CDCl3. The line widths of the molecule indicate the restricted mobility of HEUR in water compared to CDCl3; nevertheless, we observe all signals quantitatively, especially the hydrophobe groups in the network junctions. Note the split signal at about 3.2 ppm in water due to the aromatic hydrophobe and HDU interaction in the network junction that is not observed in CDCl3 as we do not have such junctions. to filter out the signals of low molecular weight species including lowMw oligomers in the aqueous phase of emulsion polymers for the unique detection of associative thickener moieties with minimal interference from other signals of aqueous phase materials of emulsion polymers.33 The RF gradient pulse (small delta) was typically applied for 3−5 ms and a diffusion time (big delta) of less than 30 ms for the diffusion filter experiments. The latter was kept to smaller values whenever possible to reduce the degree of signal attenuation of the broader polymer signals due to spin−spin relaxation time (T2) effects. All samples were made in water with a capillary insert containing D2O

duration starting from 30 s at the lowest shear rate and going down to 5 s at the highest. NMR Experiments. NMR experiments were performed on Bruker AVANCE III 400 and 600 MHz spectrometers with maximum pulsed field gradient (PFG) strength of about 53 G/cm. For PFGNMR experiments, we used the longitudinal eddy-current delay experiment with bipolar (LEDbp) pulse pairs to reduce artifacts arising from eddy current and phase distortions.31 The PFGNMR experiments are primarily used to measure the self-diffusion coefficients of various species in solution.32 In this particular application, we use it primarily 2218

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for the lock signal. In some instances additional water presaturation pulse is employed in the PFGNMR pulse sequence to help with suppression of the strong water signal to reduce dynamic range and radiation dumping effects.

while the other two urethane linkers are in the middle of the HEUR chain that are completely soluble in water, hence the 50:50 ratio between these signals in the inset. The line widths for these two soluble urethane linkers in water is essentially the same as those in CDCl3. Note that there are only four EO groups between the nonylphenol end group and the urethane group closest to it. We did not observe similar effects for urethane linkers closest to the NP when we used (EO)23 spacers instead of (EO)4. This observation gives us a unique tool in the latex−HEUR composite adsorption isotherm. Molecular Level Detection of HEUR−Latex Composites. Our intent is to detect molecular level interaction between HEUR molecules and latex particles, which has been reported before from fluorescence spectroscopy study using pyrenes as end groups.24,25 We develop a method that allows us to study these interactions between HEURs and latexes that are used for coatings applications at volume solids and concentrations representative of commercial applications. In other words, the emulsion polymers and HEUR molecules are not designed as specific model systems but off-the-shelf industrial grade materials. In coatings and other industrial application we realize that surfactants and other oligomeric species in emulsion polymers may compete for adsorption sites on the latex surfaces; hence, we would like to observe these interactions in situ. First, we demonstrate the complex nature of NMR signals of emulsion polymers that hindered in situ detection of these interactions by NMR, or any other analytical method, up to now. The 1H NMR signal of an emulsion polymer comprises of two distinct components arising from the two phases as shown in Figure 4. The latex particles with immobilized polymers

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RESULTS AND DISCUSSION HEUR Molecules in Water. We will first study HEUR only solution in water to lay the baseline of the NMR signals in order to better interpret the data from the latex−HEUR composites. The details of the resulting transitional networks of HEURs in water have been studied and reported earlier, and we will not go in any detail in this report.3,21,26,34,35 The NMR signals of such transient networks have the characteristic poly(ethylene glycol), hydrophobe end group, and urethane linker signals readily detected in aqueous solutions. Typical spectra of 1% HEUR solution in water are shown in Figure 2 for HEURs with the two types of urethane linkers used in this study. In this work, our focus is to monitor the hydrophobe group signals in order to identify their location either in the continuous water phase or on the surface of the latex particles. To this end we have used a HEURNP molecule of idealized structure: NP-EO4−HDU-PEG8K-HDU-PEG8K-HDU-PEG8KHDU-EO4−NP, where NP is nonylphenol, EO4 is a tetramer of ethylene oxide, and HDU is the urethane (carbamate) linker moiety of hexamethylene diisocyanate. The presence of an aromatic species as part of the hydrophobe end group enables the unique molecular level detection of network junctions that is enhanced by the aromatic ring current shift on the end group alkyl chain and near neighbor urethane linker that is shown to be part of the network junction. We note that while we form a transient network structure of HEURNP in a polar solvent, water, we do not expect any network formation in chloroform. Figure 3 shows that even though we observe that the line widths of the HEURNP signals in water are slightly broader than the ones in CDCl3 due to the reduced mobility resulting from the transient network structure,19 we detect all signals of the HEURNP molecule in H2O quantitatively as shown from the integration of the data: aromatics at 7 ppm, urethane NH (proton) signals at 4.2 ppm, HDU at 3.2, 1.5, and 1.4 ppm, and the branched alkyl signals of nonyl end group at 1.5−0.5 ppm. In other words, we have the sensitivity and resolution to detect the hydrophobe signals as long as they are in the aqueous phase. A closer look at the spectra in Figure 3 shows that the urethane signals are split in two (see inset) in the aqueous solution where one of the pair is shifted upfield by about 0.1 ppm and is slightly broader. This is in contrast to the spectrum of HEURNP solution in CDCl3, where we observe a single peak for each methylene moiety of the urethane linker; we are ignoring the splitting due to JH−H coupling. We attribute this shifted and broader signal to the proximity of some of the HDU to aromatic end groups of NP, hence the ring current shift from the aromatic group. In other words, nonylphenol and HDU close to the end group are part of the “transiently” immobilized network junction. The line width of the shifted signal is broader due to the restricted motion of the urethane group in the hydrophobe−hydrophobe interacting network junctions compared to other HDU signals that are soluble in the water phase and more mobile (see the structure on top of Figure 3). The molecular weight of HEURNP is about 25K, and we have on average two urethane linkers close to the end group that are part of the hydrophobic group forming the network junction,

Figure 4. 1H NMR signals of polymers in latex particles and any hydrophobe groups of surfactants and rheology modifiers that are adsorbed to the particle surface are broad and indistinguishable with each other while the signals from aqueous phase materials of emulsion polymers give sharp peaks over a narrow chemical shift range similar to any polymer solution spectrum.

forming the hard phase have broad signals owing to the restricted motion of the polymers in the latex particles; these line widths could be as high as 50 ppm. On the other hand, the continuous aqueous phase contains all soluble components including some oligomers from emulsion polymerization and other partially solubilized components such as surfactants with significant mobility. Hence, the 1H NMR signal of the aqueous phase has a range of about 10 ppm, much narrower than the polymer signal in the latex particles. The aqueous phase signal is highlighted by a box and expanded in the inset of Figure 4. This distinct delineation of spectral features of the two phases enables us to identify whether a molecule or part of a molecule is associated with the latex particle or solubilized in the aqueous 2219

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(small delta) and the diffusion time (big delta) of the PFGNMR experimental parameters in Figure 5 for diffusion filter applications; the intensity of the signal is attenuated at a much faster rate for higher diffusion coefficient (D) species (low Mw) compared to higher Mw species with orders of magnitude lower diffusion coefficients. Hence, we can selectively detect the signals of HEURs by eliminating the signals of aqueous phase emulsion species that would overwhelm the end group and urethane linker signals of HEUR molecules. In Figure 6, we show the expanded region of the spectrum from 2.2 to 0.6 ppm for clarity as the signals of interest (alkyl end group and urethane linkers) of HEUR molecule are mostly in this region of the spectrum (see Figure 2). The top two spectra (A and B) are obtained for 30% solid BA/MMA latex. With our experimental condition of 5 ms small delta, 20 ms big delta, and 50 G/cm gradient amplitude of the PFGNMR pulse sequence, we are able to eliminate all signals of aqueous phase species of the emulsion in this range owing to their low Mw and high diffusion coefficient (Figure 6B). Figure 6C shows the conventional 1H NMR spectrum of 1% HEUR and 30% latex composite. As expected, the spectrum in this region is essentially similar to Figure 6A (latex only) since the major components of the aqueous phase materials are from the emulsion polymer. The peaks in Figure 6C are much broader than the latex only signals (Figure 6A) due to increased viscosity of the HEUR−latex composite. Upon application of the PFGNMR sequence for diffusion weighted filter, we obtain the spectrum in Figure 6D for HEUR−latex composite; this spectrum is essentially similar to the HEUR only signal shown in Figure 6E without the signals labeled as R to be discussed later. This experimental approach permits the detection of low level components of HEUR in latex composites in situ. Figure 6D is expanded vertically for better observation and not to scale with respect to Figure 6C. We have included the intensity profile of the signals as a function of PFG strength in the Supporting Information to illustrate the parameters window used to remove the signals of aqueous phase emulsions without perturbing the HEUR signals. We have extended the diffusion filtered method to twodimensional NMR experiments by incorporating the longitudinal eddy-current delay experiment with bipolar pulses prior to any desired 2D NMR pulse sequence. We have incorporated the filter experiment on most of the 2D NMR pulse sequences that are generally essential for structural elucidation of polymers such as COSY, HSQC, HSQC-TOCSY, and HMBC. An example of the pulse sequence and 2D NMR spectrum obtained with the diffusion filter is shown below where we use the PFGNMR diffusion-weighted filter for a twodimensional heteronuclear single quantum coherence (HSQC) experiment. The diffusion filter part of the pulse sequence is essentially the longitudinal eddy-current delay experiment with bipolar (LED-bp) pulse pairs,31 and the second part is the 2D HSQC pulse sequence using trim pulses for INEPT (insensitive nuclear enhanced by polarization transfer) sequence.36 In order to minimize signal attenuation of the diffusion filtered signal after LED-bp, we used milder gradient conditions than we used in the one-dimensional version of the experiment. These parameters are optimized by monitoring the first scan of the 2D experiment for the desired level of attenuation of undesirable low-Mw signals without significantly attenuating the desired ones. In most cases, a small delta (gradient pulse width) of 2−3 ms and big delta (diffusion time) of about 20 ms are sufficient

phase. For instance, amphiphilic surfactants have two components: a hydrophobic alkyl group as well as a hydrophilic group such as poly(ethylene glycol) (PEG). As depicted in Figure 4, when the hydrophobic alkyl group (solid line on the particle surface in Figure 4) is adsorbed onto the surface of the latex particle, it has a restricted mobility (short T2, broad signals), while the hydrophilic PEG segment (dashed line in Figure 4) is expected to be in the water phase with substantial dynamics (long T2, narrower signals). Thus, the 1H NMR signal of the hydrophobic alkyl segment of the surfactant is part of the broad signal shown below, while the hydrophilic PEG signal is part of the aqueous phase signals in the expanded inset labeled as PEG. Associative HEUR thickeners have essentially similar amphiphilic architecture as the surfactant described above, except both ends of HEUR molecules are hydrophobic groups. So, if the hydrophobe groups are adsorbed to the latex surface, their signals will be broad and indistinguishable from the broad signals of the polymers in the latex particles. If they are in the aqueous phase, we will detect them similarly to what we saw in Figure 3. In other words, the signals of the hydrophobe groups in the aqueous phase would be detected in the box of Figure 4. As alluded to above, we face a challenge that previously prohibited the detection of these aqueous phase signals of HEUR end groups and urethane linkers in industrial grade latex due to the presence of ubiquitous surfactant and synthesis byproduct molecules such as oligomers in the aqueous phase of emulsion polymers. The signals from these aqueous phase species of emulsion polymers dominate the spectrum making it harder, if not impossible, to detect the very low intensity signals from the hydrophobe end groups as well as urethane linkers of HEURs. One approach to circumvent this problem is to use ion exchange resins to remove any undesirable species from the aqueous phase.26 We have observed that the ion exchange approach also physically removes some surfactants and other surface adsorbed components of the emulsion polymers that could modify the hydrophobicity of the latex particle surfaces, thus altering the interactions with HEUR molecules. As stated above, we develop a method that would study the systems with minimal, if any, perturbation to the system that is used for coatings applications. PFGNMR experiment as diffusion weighted filter permits the separation of signals of lower molecular weight species such as surfactants and any other oligomers from the emulsion polymers since these low-Mw species have much higher diffusion coefficients than the HEUR molecules of interest. As we have illustrated earlier33 and can be observed from the equation in Figure 5, one can optimize the gradient pulse width

Figure 5. Longitudinal eddy-current delay experiment with bipolar (LEDbp) pulse pairs to reduce artifacts arising from eddy current and phase distortions.31 In the current application, small and big delta are kept constant and the experiment is performed as a one-dimensional experiment to eliminate signals of fast diffusing species in order to selectively detect high-Mw signals in polymer mixtures. 2220

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Figure 6. An expanded region of 1H NMR signal showing spectral region of interest. (A) Conventional 1H NMR signal of 30% (solid) latex. (B) Diffusion-weighted (filtered) spectrum of (A); since the signals in (A) are due to low-Mw species in the aqueous phase, they are eliminated by the PFGNMR diffusion weighted filter. (C) Conventional 1H NMR spectrum of 1% HEUR and 30% latex. (D) Diffusion filtered spectrum. (E) 1% HEUR solution in water for reference. The diffusion filtered spectra are obtained with 4 ms small delta, 30 ms big delta, and 53 G/cm.

Figure 7. A 2D HSQC experiment with diffusion filter sequence in order to selectively eliminate signals of low molecular weight species in a polymer matrix.

to obtain the diffusion-filtered 2D NMR spectrum using about 50 G/cm gradient amplitudes. Figure 8 shows 2D HSQC spectra without (left) and with (right) diffusion filter for a HEUR−latex composite in water. The one-dimensional proton spectrum of the same composite before and after PFG filter is shown on the top of the corresponding 2D plot. As shown on the right of Figure 8, we are able to eliminate undesirable low molecular weight signals arising from the emulsion polymerization of the latex from interfering with the assignment of the signals. The predominant signal we detect without interference in the spectrum is that of HEUR, as it is the high molecular weight signal in the composite. We also detect some methyl group signals at about 0.9 ppm from the aqueous phase of the emulsion polymer as we did not completely eliminate the oligomer signal with our milder PFG conditions discussed above. This capability allows us to study very complex composites without any perturbation of the system, which could have some effect on the degree and type of interactions between HEUR molecules and the emulsion polymer system as applied to coatings. Needless to say, we can apply this method for detecting polymer signals

Figure 8. Left: 2D HSQC spectrum of 1% HEUR and 30% (solid) latex composite. The right 2D spectrum is obtained using the diffusion filtered experiment shown in Figure 7. The 1H one-dimensional spectra are also the respective before and after diffusion filter experiment as illustrated in Figure 5. 2221

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observe any off-the-shelf HEUR and latex composite interaction on a molecular level, we have also observed the interactions of internal urethane linkers with latex particles surfaces. For a hexamethylene diisocyanate linker, we observed that about 40% of the urethane linkers are adsorbed to more hydrophilic latex surface and more than 60% for more hydrophobic latexes. We will discuss this further in subsequent pages. The observation that hydrophobe end groups are not detected in the aqueous phase of 1% HEUR−30% latex composites confirms a HEUR−latex composite structure consisting of loops and direct bridges22,24,25 as shown in Figure 10. Some researchers proposed that viscosity depends

selectively in any solution and in the presence of undesirable low molecular weight species from unreacted species and/or reaction byproducts. In Figure 9, we illustrate the diffusion filtered NMR signals of 1% HEURHDI molecules ((hexamethylene diisocyanate

Figure 9. (A) 1% HEUR and 30% latex (hydrophilic surface) composite in water. (B) 1% HEUR in water. Note the arrows indicate where the C10 end group signals should be in the composites but are not detected.

Figure 10. A cartoon representation of the old (left) and current (right) model based on the PFGNMR result for 1% HEUR and 30% latex composite. The small black dots represent the end groups of HEUR molecules. In the old model, these hydrophobes are believed to form the transient network structures in the aqueous phase.

(HDI) linkers and C10 alkyl end group) with 30% (solid) latex in water. Figure 9B is the reference spectrum of 1% HEURHDI in water where we observe the signals of every component of the HEUR molecule quantitatively. Figure 9A shows the spectrum of 1% HEUR−30% latex (solid) composite in water. Both spectra are obtained by PFGNMR diffusion filter experiment; we did not need the filter experiment in Figure 9B, but we did it for comparison under similar conditions. We readily observe that the composite signal in Figure 9A is much broader due to the increased viscosity as a result of the network structure of HEUR with the latex particles. The other striking difference we observe in the composite spectrum of Figure 9A is the absence of any signal of the alkyl hydrophobe end group, labeled a, b, c, and d in Figure 9B. We expected to have a significant reduction of the intensity of the hydrophobe group due to its interaction to the latex surface; we recall from Figure 4 that if the hydrophobe group is adsorbed to the latex surface, its mobility is restricted significantly so that the signals get too broad, similar to the copolymers in the latex particle, to be detected uniquely in this spectral range. We acquired a significant number of scans on a 600 MHz spectrometer with cryoprobe to ascertain our limit of detection, and we conservatively estimate that we would have been able to detect the hydrophobe signal if there were more than 0.1% of the hydrophobe group in the aqueous phase. So, for all practical purposes, all alkyl hydrophobe end groups are adsorbed onto the latex surfaces at 1% HEUR and 30% latex solid composites in water. This observation is the same from C10 to C16 hydrophobes and with varying hydrophilicity of latex surfaces of BA/MMA emulsion polymers used in coatings applications. Our data are consistent with Richey et al.’s results where they observed no pyrene end groups in the aqueous phase by fluorescence spectroscopy.24,25 In addition to being able to

upon a quantity of HEUR hydrophobe forms flower micelles and a HEUR transient network coexisting in solution.23 Note that this coexistence model20,23,27,37 is a natural extension of the transient network structure of associative thickeners in water where some of the hydrophobe nodes were thought to be occupied by latex particles while the rest of the threedimensional transient network structure remained intact38 as shown on the left in Figure 10. In other words, the latex particles were believed to provide additional junction points to the three-dimensional transient network structure of HEURs in water and thus provide an increase in transient network viscosity. The component viscosities of 30% latex and 1% HEUR (C12 equiv alkyl hydrophobe) are of the order of 20 cP, while the viscosity of the composite is of the order 2000 cP, even at concentrations where no HEUR transient network exists. While some flower micelles and transient network may arise for higher HEUR concentrations (including those beyond commercial interest in paints and coatings), we note that viscosity increases monotonically as HEUR concentration increases through levels where no transient network is present. In particular, viscosity does not exhibit a step change increase at higher HEUR concentrations where HEUR flower micelles and transient network may arise. Furthermore, at high HEUR concentrations above about 5%, we observe that the rate of viscosity increase plateaus with increasing HEUR concentration. Consequently, we identify HEUR direct bridges and loop conformations as the primary molecular basis for viscosity generation in latex paints and coatings. As with any weak van der Waals type hydrophobe− hydrophobe interaction, adsorption−desorption exchange of the hydrophobe groups of HEUR molecules with the latex particles occurs. The stronger the interactions between the end groups hydrophobes and the latex particle surfaces, with larger 2222

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hydrophobes (greater than C14) and/or more hydrophobic latex surfaces, the lower the probability for these exchanges and much shorter (≪1 ms) lifetime of the hydrophobe group in the aqueous phase. As a result, these exchanges are mainly between loop to loop, loop to direct bridge, and possibly direct bridge to direct bridge. Formation of bridges, loops, and chain transport between surfaces has been studied by Gao, Dewalt, and OuYang39 and Meng and Russell40 and is dependent on HEUR hydrophobes, concentrations, molecular weight, relaxation times, latex concentration, hydrophobicity, particle size, etc. Thus, the frequency of exchange and lifetime of the hydrophobe end groups in the aqueous phase is dependent on many factors, primarily on the hydrophobe size and hydrophobicity of the latex surface. Based on NMR accessible time scales, the hydrophobe’s lifetime in the aqueous phase during the exchange is expected to be in the submillisecond time frame with negligible probability of forming hydrophobe− hydrophobe interactions in the aqueous phase for composites with less than about 2% HEUR and 30% latex solid. In our model for HEUR concentrations below 2%, without added surfactant, and latex solid concentration of 15−30%, there are negligible “free” HEUR molecules in the aqueous phase, but if we increase the amount of HEUR, or add surfactant, with fixed latex solids, we will saturate the surfaces and some of the HEUR end groups will be in the aqueous phase forming the transient networks similar to the cartoon on the left of Figure 10.20,23,27,37 The HEUR structure with nonylphenol end group (HEURNP) is ideal to illustrate the adsorption isotherm, especially the formation of aqueous phase hydrophobe−hydrophobe interactions at higher HEUR levels. As shown in Figure 3, transient network formation of this unique HEUR end group in the aqueous phase is characterized by an upfield shift of some urethane signals due to their proximity to aromatic species in the hydrophobe network junctions in the aqueous phase. We can monitor the intensities of those signals, together with the methyl groups of the branched nonyl end groups, as an indication that there are hydrophobe end groups in the aqueous phase that are not adsorbed to latex particles. Figure 11 shows the diffusion filtered 1H NMR signals of these composites with varying concentrations of HEURNP and fixed latex solids (15%). We have plotted the signals with normalized intensities of the urethane signals for better observation and comparison of changes in the spectrum as we increase the HEUR levels. We notice the growing methyl end group signal at about 0.8 ppm starting with HEURNP amount between 2 and 3%, indicating the presence of end groups in the aqueous phase. In addition, we also note that the urethane signals at 1.25, 1.4, and 3.05 ppm, highlighted by dashed arrows, increase in their intensities simultaneously with methyl group signals. As we have illustrated in Figure 3, these are the result of hydrophobe− hydrophobe associations in the aqueous phase from the transient network nodes of HEUR molecules. These unique signals serve as a vivid marker for the formation of transient networks in the aqueous phase as soon as the hydrophobe groups appear in the aqueous phase at higher concentrations of HEUR. A quantitative plot of adsorbed and free methyl group (from nonylphenol) is shown in Figure 12 as percent of the total end group of HEUR used. We have set the integration of the methyl group signals for 5% HEUR only in water at 5% for a reference. The circles denote the total methyl end group for each level of HEUR used. Below 2% HEUR level, we do not observe any

Figure 11. PFGNMR signals of 1−5% HEUR with 15% (solid) latex composite. As we increase the amount of HEUR with fixed latex level, we detect hydrophobe end group signals from methyl groups when HEUR level added is more than 2%, which indicates that some of the end groups of the RM are now in the aqueous phase of the RM−latex complex. The signals highlighted by dashed arrows 3.05, 1.4, and 1.25 ppm are the HDU signals that form transient networks in the aqueous phase. The signal intensity is normalized to HDU signals for better visualization of the spectral line shapes. Quantitative measure of this data set is shown in Figure 12.

end group nonylphenol signal in the aqueous phase; hence, all hydrophobe end groups are attached to the latex surface with a structure of loops and direct bridges as shown by the cartoon on the bottom left of Figure 12. At higher RM levels (>2%), we observe nonylphenol end groups that associate in the aqueous phase as highlighted by the cartoon on the right side of Figure 12 where the dots outside of the circles denote the hydrophobe groups associating in the aqueous phase. The presence of these associations is ascertained from the shift of the urethane signals highlighted by dashed arrows at 3.05, 1.4, and 1.25 ppm as discussed in Figure 11. Saturation levels (of HEUR and surfactant) on the surface of the latex particle are approached when the RM level exceeds about 4%. Note that our latex solids level for this experiment was set at 15% and this saturation RM concentration level could be higher for higher latex solids and smaller particle size which have higher total surface area. The adsorption isotherm plot is dependent on many factors including the HEUR end group as well as the emulsion polymer and the surfactant concentration and molar mass. In other words, the point at which hydrophobe groups in the aqueous phase are detected and the saturation point is approached varies based upon factors such as hydrophilicity and particle size of the latex surface, alkyl hydrophobe end group length and type, surfactant levels, etc. Overall, our result is consistent with a Langmuir type L4 adsorption isotherm model of the fluorescence data reported for pyrene end groups.22,24,25 2223

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Figure 12. Adsorbed and free methyl (Me) end group as a function of HEUR levels in HEUR−latex composites. Me adsorbed is determined from a difference of the total added and detected in the aqueous phase. The cartoons show the core−shell structures of latex and HEUR with loops and possibly bridges at low HEUR levels and with transient network formation in the aqueous phase at high levels of HEUR.

groups with some difference of significant implication to viscosity build. The weaker binding energy between small hydrophobe end groups and latex surfaces leads to faster exchange rates and longer lifetime of the hydrophobe group in the aqueous phase compared to the more hydrophobic end groups. These relatively faster exchange rates and longer life times in the aqueous phase result in faster reorganization of loop and bridge conformations and smaller aggregates of latex particles that are relatively short-lived and rapidly deformed; hence, lower viscosity at zero to low shear rates. In addition, shorter adsorbed-state lifetimes and longer free-state lifetimes lead to faster reorganization of aggregates and more inter particle bridging so that higher shear rates will have less effect on viscosity compared to HEURs with larger hydrophobe end groups that have bigger and relatively longer lived aggregates at low shear rates. Another way to look at the interaction of latex particles with HEURs with small and large alkyl groups at high shear rates is the instantaneous availability of these end groups for direct bridge formation to a nearby latex particle. As we have pointed out above, large alkyl chains have much slower exchange rates with extremely short lifetimes in an aqueous media, hence a significantly reduced probability of bridge formation between particles that are moving fast relative to each other at high shear rates. In contrast, shorter alkyl end groups exchange faster and have longer lifetime in the aqueous phase and hence are more available for bridge formation albeit individually forming shorter-lived bridges. As a result, at high shear rates, composites with small alkyl end groups can form relatively more bridges compared to larger alkyl end groups. Structure−Property Correlation: Viscosity Profile. In Figure 14, we show the viscosity profile for composites of 1% HEUR with varying size of alkyl end groups (consisting of H12MDI linker and alkyl group to form an equivalent alkyl hydrophobe) and 28% latex particles. We used the same latex and same HEUR architecture and molecular weight in all these rheology experiments, except the size of the alkyl end groups of HEUR. The low shear rate viscosity of the latex−HEUR

Even though the preceding NMR study, and the molecular level model describing the interaction of HEUR molecules with latex particles, was done at zero shear, it gives us some insight to extend the model to higher shear rates consistent with the rheofluorescence spectroscopy results of Richey et al.25 and Suzuki et al.18,35 Our model proposes a consistent interaction between HEUR and latex surfaces in that there is no HEUR end group species in the aqueous phase below HEUR− surfactant−latex saturation except for a submillisecond lifetime, based on NMR time scales, even at higher shear rates. This model contrasts with previous assumptions by some researchers, based on a transient network model, where HEUR molecules detach from the particles and stay in the aqueous phase. HEUR end groups are expected to form more direct bridges at zero shear, and as the shear rate is increased, some direct bridges are reorganize to form loops, and since the lifetime of a hydrophobe in the aqueous phase is extremely short (≪1 ms), the detached hydrophobe end groups adsorb at the nearest hydrophobe surface available, mostly forming loops with both ends on the same latex particle. The relatively longer lifetime of adsorbed hydrophobes (milliseconds to seconds) results in a low probability of bridge reformation at shear rates greater than 1−1000 s−1. In other words, less aggregation of latex particles occurs as the shear rate is increased while HEUR molecules form more loop conformations on latex particles. This is particularly true for hydrophobe groups greater than C12. As we saw above, these larger hydrophobe end groups have much slower exchange rates due to their strong binding energy to latex particles that lead to larger latex aggregates at zero to low shear rates, resulting in high low-shear viscosity. At higher shear rates, reduced interparticle bridge formation reduces the equilibrium size of particle aggregates, resulting in reduced viscosity and the characteristic shear thinning property of HEUR−latex composites. The interactions with weaker hydrophobes (less than C12 alkyl end groups) during high shear rates is essentially the same as described in the previous paragraph for large alkyl end 2224

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We have extended these measurements to high shear rates of about 10 000 s−1 (ICI viscosity42), where we observed the reverse correlation between high shear viscosity and alkyl end group length. As shown in Figure 13, we propose more intraparticle loops with less bridging between particles. The longer time scale for dissociation of larger alkyl end groups tends to maintain the loops for longer periods with diminished probability to form bridges and aggregates of latex particles the necessary condition for high viscosity. On the other hand, the smaller alkyl end groups have much shorter time scales for dissociation from the latex surface, giving them a relatively higher probability to form bridges. These shorter lived but more numerous bridges are responsible for modestly larger aggregates and higher viscosity of composites with smaller alkyl end groups that we measured at high shear rates. Influence of Latex Surface Hydrophilicity. The other major factor in the HEUR−latex interaction is the hydrophobicity of the latex particle surface. We controlled the hydrophobicity of the latex particle surfaces by the amount of acid monomer stabilizer (acrylic acid or methacrylic acid) used in emulsion synthesis; one with lower acid content gives us the more hydrophobic latex particle surface that we confirmed by titration. For this study we used the same type and amount (1%) of HEUR12 to 30% latex solid with hydrophilic or relatively more hydrophobic particle surface. We also kept the particle sizes at about 130 nm. The PFGNMR parameters were set to shorter gradient pulses to remove only the signals of the very low molecular weight species of the aqueous phase material that would interfere with our detection of HEUR signals, but kept relatively higher Mw oligomers as we found them to be good internal standards to facilitate comparison of HEUR signal amplitude in the hydrophilic and hydrophobic latexes. Figure 15A is the spectrum for 1% HEUR12 in water as a reference for HEUR signals. As before, we show only the spectral region of interest for better visualization; the only signal not shown in the spectra is that of PEG at 3.6 ppm which dominates the spectra as shown in Figure 1. Figure 15B has two traces: the “latex” trace is the aqueous phase oligomers signal for the hydrophilic latex before HEUR addition, and the “composite” trace is for the composite of 1% HEUR12 and 30% solids of hydrophilic latex. We observe the urethane linker signals (u) of HEUR superposed on the latex oligomer signals. We also note that two peaks marked R (alkyl hydrophobe end

Figure 13. A cartoon representation of the molecular level interaction of HEUR and latex particles where direct bridges that are responsible to high viscosity are converted to mostly loops at high shear leading to shear thinning of HEUR−latex composites.

Figure 14. Shear thinning viscosity profile of latex−HEUR composites as a function of shear rates for HEURs with varying end groups (hydrophobicity) and same latex.

composites is well correlated to the length of the alkyl end group: HEUR with C18 > C16 > C14 > C12 > C10. The dependence of low shear viscosity on hydrophobe end group size has been studied by other researchers.39,41 Annable et al.41 observed an order of magnitude increase in the relaxation time for a two-carbon increase in the hydrophobe size which influences the time scale of end group dissociation. Since we kept other parameters such as HEUR architecture, molecular weight, and the latex the same, the lifetime of an individual HEUR bridge between particles is, for the most part, proportional to the length of the alkyl end group.

Figure 15. An expanded 1H PFGNMR spectra of (A) 1% HEUR12 in water (references spectrum). (B) Spectra of 30% latex (hydrophilic) only and composite with 1% HEUR12. (C) Spectra of 30% latex (hydrophobic) only and composite with 1% HEUR12. The cartoons depict the adsorption of almost all hydrophobic species including the urethane linkers on hydrophobic latex surfaces. 2225

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CONCLUSION We have shown a new approach to study HEUR−latex interaction on a molecular level based on diffusion filter by PFGNMR. The method permits the detection of end group hydrophobe and urethane linker signals in the presence of ubiquitous aqueous phase species of emulsion polymers by eliminating the signals of the latter based on their diffusion coefficients. We have extended the diffusion filter technique to standard 2D NMR experiments relevant to polymer characterization. Our goal is to perform such studies on off-the-shelf industrial materials. Our results show that for composites of 30% solids latex and less than 2% HEUR, all end group hydrophobes are adsorbed to the latex particle surfaces, forming loops and direct bridges between particles. Direct bridges between particles form the basis for viscosity build at low shear rates and are progressively rearranged into fewer bridges and more intraparticle loops as the shear rate increases, which results in the characteristic shear thinning property of these polymer composites. We have also shown on a molecular level the interaction of urethane linker hydrophobes to latex surfaces and illustrated their consequences on the viscosity of the composite. Our results and molecular interaction model are consistent with rheology measurement over a wide range of shear rates. We demonstrate the absence of transient networks in the aqueous phase up to HEUR concentrations of commercial interest. Widely accepted prior models proposed that latex particles occupy some of the hydrophobe junction nodes of the three-dimensional transient network to produce enhanced viscosity. We note inconsistencies of these models with experiment and rheology and describe a molecular interaction model of direct bridges between latex particles and intraparticle loops.

group) are missing in the composite spectrum since all the alkyl end groups are adsorbed onto the latex surfaces. Even though this latex is relatively more hydrophilic, it has enough hydrophobic surface to adsorb all the alkyl end groups of HEUR. Figure 15C is for a composite with latex particles that have relatively fewer acid groups on the surface, which makes the latex surface relatively more hydrophobic. We observe a stark difference in that the composite signal is essentially the same as that of the latex only. We do not observe any of the alkyl end group (R) signals. A careful comparison of the latex and composite signals in Figure 15C shows that we observe only a trace of urethane linker signals, which means all of the R groups as well as almost all of urethane linkers (u) are adsorbed onto the more hydrophobic latex surface. The cartoons on the right side of Figure 15 show our depiction of the composite structure based on the NMR data. The open and closed circles denote urethane linkers and alkyl groups, respectively. For hydrophilic latex surface where most of the urethane linkers are detected in the aqueous phase, the HEUR molecules are expected to behave as telechelic structures with hydrophobic interactions mediated solely by their end groups which exchange and rearrange between latex particle surfaces during mixing and Brownian motion collisions.39,40 In contrast, the latex with more hydrophobic surface adsorbs most of the urethane linkers of the HEUR molecules. We expect these urethane linkers to form additional concerted hydrophobic associations, albeit weaker, that contribute to the HEUR association energy in concert with the larger terminal hydrophobes. Viscosity measurements of the two composites provide an interesting contrast. At low shear rates (less than 1 s−1), the composite with the more hydrophobic latex surface had higher viscosity. We attribute this to the increased binding energy provided by the more hydrophobic latex surface and the additional concerted interactions with the internal urethane linkers. We note that the viscosity of the HEUR−latex composite is dependent on the strength of their interaction energy.34c The more hydrophobic surface provides for stronger interaction, hence increasing the lifetime of the compound hydrophobe on the latex surface, which in turn increases the size of latex particle aggregates bridged by HEUR molecules. At high shear rates (10 000 s−1), we observe the reverse, where the viscosity for the more hydrophilic latex−HEUR composite is higher. As depicted in Figure 15B, the weaker interaction between the urethane linkers and the more hydrophilic latex surface leads to faster exchange rates, longer end-group hydrophobe lifetime in the aqueous phase, faster reorganization, and more interparticle bridging. As we discussed above, faster exchange rates and longer hydrophobe lifetimes in the aqueous phase lead to a higher probability of bridge formation, albeit with individually shorter-lived bridges. As a result, the more hydrophilic latex−HEUR composite exhibits more Newtonian rheology, with lower low-shear viscosity and higher high-shear viscosity. This observation is essentially similar to what we observed in the flow curve of Figure 14, except in that case the difference in hydrophobicity was from the HEUR end groups, while in this case the difference is in the apparent concerted contribution of the urethane junctions as additional hydrophobic intraloop contact points of HEUR molecules, with the hydrophobic end groups, on hydrophobic particle surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Discrimination window for diffusion-weighted PFGNMR filter; (2) adsorption isotherm by centrifugation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 215-619-5261. Notes

The authors declare no competing financial interest. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for in-depth discussions with our colleagues Drs. Barrett Bobsein, Dan Saucy, Alan Nakatani, Tirtha Chatterjee, Melissa Johnson, Valeriy Ginzburg, Ken Laughlin, Stewart Williams, and Jim Alexander.

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REFERENCES

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