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Phase Transitions of Hexadecane in Poly(alkyl methacrylate) Core-Shell Microcapsules Jeremy K. Black,† Lauren E. Tracy,† Conor P. Roche,† Paul J. Henry,† Joseph B. Pesavento,‡ and Thorsteinn Adalsteinsson*,†,§ Department of Chemistry and Biochemistry and Center for Nanostructures, Santa Clara UniVersity, Santa Clara, California 95053, and Life Science DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: August 19, 2009; ReVised Manuscript ReceiVed: February 9, 2010
Microcapsules containing subfemtoliter volumes of n-hexadecane (HD) within a 4-40 nm thick shell of poly(alkyl methacrylates) were prepared. The size of the HD drop was varied between 50 and 140 nm. The alkyl substituents on the methacrylate monomer were varied to alter the surface tension between the HD and the polymer shell in order to investigate the effects of surface tension on the freezing point of the HD. The size dependence of the supercooling as predicted by the G-T equation was not observed in our systems. An effect on the magnitude of supercooling with variation in the side chains was observed, where freezing the HD in capsules with bulkier side chains requires a greater magnitude of supercooling. This is in agreement with the increased hydrophobic character of the polymers and also correlates with the decrease in glass transition temperature of the polymer. We also observed aging of the capsules, which could be accelerated by heating. Introduction Liquids in bulk phases have physical properties that are different from those in small volumes. Those observed in small volumes are influenced by the nature of the interface between the liquid and the dispersing phase or the surface that confines the volume. The effect of decreasing volume becomes more prominent as the surface area to volume ratio (S/V) increases. As the S/V ratio increases, the fraction of molecules in the system that are at the interface also increases, and thus, the system becomes more governed by interfacial properties. The change in physical properties such as solubility and phase transitions is therefore an important concept to understand when studying fluids in nanostructures and has critical implications in biological and material science. Phase transitions of water, hydrocarbon oils, and mercury droplets were intensively studied by Turnbull and co-workers in the 1950s and 1960s.1-3 These studies led to a theoretical description for the kinetics during nucleation and crystallization of phases in microdroplets. In these systems, phase transitions occur at significantly lower temperatures than in the bulk phase, often tens of degrees below the bulk-phase transition. The difference between phase transition in the bulk vs a microdroplet, ∆T ) Tb - Tµ, is called supercooling. Using kinetic arguments, the supercooling is due to the low probability for heterogeneous nucleation events that are needed to initiate the phase transition when the volume is microscopically divided.1-4 In the bulk phase, nucleation can occur from a heterogeneous impurity anywhere in the volume, and only a few nucleation sites are needed to initiate the transition. When the volume is defined by small, isolated droplets, each droplet requires its own nucleation site, or impurity, for heterogeneous nucleation to occur. When no heterogeneous sites are present, the phase * To whom correspondence should be addressed. E-mail: tadalsteinsson@ scu.edu. † Department of Chemistry and Biochemistry, Santa Clara University. ‡ Lawrence Berkeley Laboratory. § Center for Nanostructures, Santa Clara University.
transition must start as a random thermal event, i.e., homogeneous nucleation, the rate of which is a kinetic event and is an exponential function of temperature. The resulting suppression of nucleation events in microscopic volumes causes a significant shift in the phase transition to lower temperatures. Lowering of the transition temperature by 10-20 °C is typically assumed to be indicative of homogeneous nucleation. Supercooling is also a well-established thermodynamic concept that can be rationalized by the existence of an interface. A system that includes interfaces between two phases incurs a Gibbs free energy change. This affects the thermal behavior of the confined liquid, and one can expect to see influences on liquid miscibility, phase transition temperatures, and transition enthalpies, to name a few. The effect on the phase transition temperature is described by the Gibbs-Thomson equation (G-T)
∆T )
γijViTb γij )K× ∆tH · r r
(1)
where r is the radius of curvature for the bounding surface, γij is the interfacial tension between the two phases, Vi is the molar volume of the liquid, Tb is the transition temperature for the bulk phase, and ∆tH is the enthalpy of the phase transition. The G-T equation can be expected to give a reasonable description of the supercooling behavior so long as the encapsulated molecules do not partition substantially into the confining phase, e.g., water and microporous glass.5 Studies of the phase transition in hydrocarbon microdroplets have mainly focused on the phase behavior of oils in surfactant micelles.3,6-14 This body of work has included investigation of the effects of droplet collisions on nucleation rate,6-8 of solid impurities in the droplet,9-12 and of the emulsifier type on the phase transition, which should be analogous to varying the water/oil surface tension.11,13 G-T type 1/r dependence of supercooling is generally seen in studies where size and its distribution are well controlled. The agreement is much less definitive than what is seen for water confined in microporous
10.1021/jp9080355 2010 American Chemical Society Published on Web 03/05/2010
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SCHEME 1: Schematic Structure of the Monomers Used in This Work
glasses, however.5,15-18 This is an expected outcome to some extent, because oil-emulsion droplets are generally much larger than the pore sizes in glasses. Nonetheless, there are numerous other challenges in applying the equation directly to hydrocarbon/ water emulsions, including the presence of trace impurities that may have a significant influence on the phase transition,9 and rapid micelle-to-micelle exchange during the phase transition. Earlier, we studied the phase transition of n-hexadecane (HD) droplets encapsulated within a thin polystyrene shell and observed a weak inverse dependence on radius for supercooling according to the G-T equation.19 One of the more intriguing observations was that with repeated freeze-thaw cycles we observed a reduction of supercooling from the range expected for homogeneous nucleation to a behavior typical of heterogeneous nucleation. We attributed this observation to a failure of the poly(styrene) capsule due to the contraction of the encapsulated HD upon freezing. The contraction was postulated to produce a point-failure in every capsule, which then becomes a heterogeneous nucleation site. The transition from homogeneous nucleation to heterogeneous nucleation was correlated to the cooling rate of the capsule sample and the thickness of the polymer shell. In this work, we study microcapsules containing HD, where we vary both the size of the oil droplet and the hydrophobicity of the encapsulating polymer. The hydrophobicity is varied by changing the alkyl ester group along the backbone of a family of poly(alkyl methacrylates). The variation in the side groups also changes the glass transition temperature of the polymer, which in turn should affect the flexibility of the polymer to respond to the change in volume upon freezing of the HD. Methods and Materials All chemicals used were purchased from Sigma-Aldrich. The alkyl methacrylate monomers (see Scheme 1), with the exception of n-hexyl methacrylate, were purified by double distillation at 21 Torr and stored under an inert atmosphere before use. Sodium dodecyl sulfate (SDS) was purified by double recrystallization from 2-propanol. 1-Dodecanol, n-hexadecane (HD), and 2,2′azobisisobutyronitrile (AIBN) were used as received. Deionized water was further purified by passing through a Millipore filter. Fresh-drawn water samples had a resistivity of 18 MΩ. Synthesis of Polymer Capsules. The capsules were prepared by the direct mini-emulsion polymerization method initially described by Tiarks et al.23 (The formation and stability mechanism underlying mini-emulsions has been discussed in numerous following publications from the groups of Landfester20,21 and Antonietti.22 For further discussion, see the cited references and references therein.) For a typical preparation, a stock surfactant solution was first prepared by dissolving 1.00 g of SDS and 0.010 g of 1-dodecanol in 100.0 g of water. The monomer/oil mixture (3.0 mL) was prepared by combining the monomer and HD in the desired ratio, and approximately 5 mg of AIBN initiator per gram of monomer was added to the mixture. After being stirred briefly, the solution was separated
from undissolved AIBN and added to 15.0 mL of surfactant solution contained in a 20 mL scintillation vial. The vial was sealed with a rubber septum, and the solution was stirred vigorously under argon flow for 0.5 h at room temperature. The vial was then placed in a water bath and the solution was sonicated using a tip sonicator (Misonix Microson Cell Disruptor XL) on high power for up to 3 min while the solution was gently stirred. The sonicated solution was stable for several hours with no change in particle size according to dynamic light scattering (DLS). Polymerization was initiated by heating the solution with gentle stirring in an aluminum bead bath at 88 °C. The reaction was complete in 90 min. The resulting microcapsules were purified by centrifugation/ decant/dispersion cycles. Mixtures were centrifuged for 0.5 h at 21 000 rpm to separate the capsules from the dispersant. After centrifugation, approximately 90% of the liquid could be removed. Three such cycles should therefore remove 99.9% of impurities and unreacted monomers from the microcapsules. More than five centrifugations led to irreversible coagulation of the particles, most likely due to the removal of surfactant from the capsule surface. However, five or more repetitions were possible if 0.01 mg/mL SDS solution was added when redispersing the particles. Particles with a volume fraction greater than 30% HD were easily purified by this method and were collected as a floating layer after centrifugation. It was more difficult to isolate capsules with approximately 30% HD fraction because of the similarity of the densities of the particle and dispersant. Addition of a small amount of methanol to the mixture allowed isolation in a shorter time, but this method was suspected to alter the polymer shell. Capsules with an HD fraction less than 30% settle to the bottom of the centrifuge tube. Purification of the heavier particles was done at 15 000 rpm to avoid clumping in the final step. The approximate concentration of the samples after purification was 0.15 g of solids (polymer and oil) per mL of capsule dispersion. Thermal Measurements of the Encapsulated HD. The thermal behavior of the encapsulated HD was investigated by differential scanning calorimetry (DSC), using a Mettler-Toledo DSC 823e instrument. Samples were analyzed within 3 days of their synthesis because of aging effects that otherwise occurred. For a typical run, a 10-20 µL sample of the capsule dispersion was placed in a 40 µL aluminum DSC crucible. The lid of the crucible was punctured using a 50 µm diameter pin before being crimped closed. This ensures that the pressure inside the capsule remains constant and that the measured heat flow corresponds to the change in enthalpy of the system. The reference pan was left empty, and an identical hole was punctured in the lid. In a typical DSC scan, the sample was first cooled at a -3 K/min rate from room temperature to -7 °C. It was then heated to room temperature at a rate of 3 K/min. This cycle was then immediately repeated three more times to test for possible failure of the capsules. A failure was indicated by a significant shift in the onset temperature in the system, and/or by the appearance of a secondary peak in the DSC scan. The onset temperature of
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the transition was determined as the intersect between the baseline (cubic spline) and a tangential line drawn through the point of maximum slope for each peak. The onset temperature was independent of the DSC heating rate between 0.5 and 10 K/min. Dynamic Light Scattering (DLS) Measurements. A 90Plus Particle Size Analyzer (Brookhaven Instruments) outfitted with an avalanche photodiode was used for particle sizing. The autocorrelation function for the scattering was measured at a 90° angle to the incidence beam. The DLS samples were prepared by prefiltering 4 mL of 10 µM potassium chloride through a 0.20 µm Whatman organic membrane filter into a clean poly(styrene) cuvette. The filtered solvent was first tested to ensure the absence of particles. Approximately 0.05 mL (one drop) of microcapsule suspension was added and gently mixed until the dispersion appeared homogeneous. After allowing 5 min for thermal equilibration, 10 1-min scans were collected and averaged. Single values outside 1.5σ of the measured average were discarded. The size was calculated assuming spherical geometry, using the refractive index of poly(methyl methacrylate) and the viscosity of water at 25.0 °C. Every sample reported had a continuous population distribution around the average. Samples with excess surfactant showed bimodal distributions. Transmission Electron Microscopy (TEM). NegatiWe Staining. A 3 µL portion of a 1:1000 dilution of the sample was pipetted onto a carbon coated 200 mesh copper EM grid (Ted Pella). After sitting for 1 min, the sample was blotted using Whatman filter paper. A 3 µL portion of 2% uranyl acetate (Electron Microscopy Sciences) was applied for 1 min and then blotted. Grids were dried for 30 min before use in the EM. Negative stain images were recorded using a JEOL 3100 FEG transmission electron microscope operating at an accelerating voltage of 300 keV. Images were recorded using a digital micrograph at varying magnifications onto a Gatan digital CCD and stored as Gatan image format files. Cryo-EM. A 3 µL portion of an undiluted sample was pipetted onto a 200 mesh copper lacey Formvar carbon coated EM grid (Ted Pella). After sitting at room temperature and humidity for 1 min, the sample was blotted using Whatman filter paper in an FEI Vitrobot. The grid was then plunged into liquid ethane and stored under liquid nitrogen. Cryo-EM images were recorded using a Phillips CM200 FEG transmission electron microscope operating at an accelerating voltage of 200 keV. In order to visualize structural details of the particles, the image was defocused from 2 to 20 µm. Images were recorded using a digital micrograph at varying magnifications with a Gatan digital CCD and stored as Gatan image format files. Final analysis of the images was done using ImageJ by NIH. Results and Discussion Capsule Synthesis. The mini-emulsion synthesis of the microcapsules results in particles with low polydispersity. A polydispersity of 1.08 down to 1.005 was commonly observed for capsules of poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), poly(n-butyl methacrylate) (PnBMA), poly(t-butyl methacrylate) (PtBMA), and poly(n-hexyl methacrylate) (PnHMA). This low polydispersity is attributed to the high shearing force during the sonication step.24 The exact mechanism is not fully understood,25 but Antonietti26 proposed that following sonication the Laplace pressure between the oil droplet and the water is higher than the surface tension. This difference makes the formation of larger droplets highly unfavorable, and the ripening of the microcapsules from smaller
Black et al.
Figure 1. Hydrodynamic radius (DLS) for the capsules. The capsule sizes shown here represent syntheses in which the amount of surfactant and sonication time and temperature were according to the method described. The inset figure shows raw DSC data from two samples of PMMA capsules.
to larger is slow in comparison with the rate of breakdown and reformation during sonication. The low polydispersity of the mini-emulsion droplets is a key aspect for our studies, since we are analyzing the behavior of the encapsulated oil phase as a function of microcapsule size. The idea that no intermicelle exchange occurs between the emulsion droplets is probably not strictly true,27 but our observations are in agreement with an exchange rate that is slow in comparison with the lifetime of the polymerization. The overall conversion of monomer to polymer in the miniemulsion synthesis was never better than 50%, yet GPC analysis of PMMA and PEMA samples showed that the molecular weight of the polymer was greater than 600 kg/mol, and several samples showed molecular weights in excess of 2000 kg/mol. This behavior in the polymerization of the acrylamide is in agreement with the behavior of mini-emulsion synthesis of poly(styrene) colloids.28 Figure 1 shows the average hydrodynamic radius of the microcapsules for the various polymers as a function of the volume fraction of HD used in the synthesis. The variation in capsule size was greatest for PtBMA and least for PMMA. Size was primarily controlled through the sonication time prior to polymerization, and it was found that sonicating for approximately 40 s provided microcapsules of consistent size. This is in agreement with previously published work from our group and elsewhere.19,29 Mini-emulsion droplets are known to have extraordinarily low polydispersity in comparison with emulsions obtained by stirring or agitation. This is a commonly observed phenomenonforsuccessfullyestablishedmini-emulsionsystems.24,30 Here, we obtained polydispersity between 1.008 and 1.080 (DLS) on a regular basis. This low dispersion in diameter is improved by longer sonication times, as long as the dispersion is kept at room temperature. Low polydispersity is typical for mini-emulsions when an appropriate sonication time and power is used. Figure 2 shows a TEM image of the capsule samples. Figures from various points in the sample showed the population of large particles with a diameter in the 150 nm range, which matches the measured diameter from the DLS. The TEM images also showed a large number of particles measuring 40-60 nm. This size was also seen in the DLS scans, but the peak was removed by the centrifugation/decant purification. The samples used in the TEM were not purified in the same way as the DLS samples and therefore include micelles and residual polymer molecules. The total number of capsules imaged was insufficient for full statistical analysis to compare with the statistical distribution seen in the DLS.
Phase Transitions of Hexadecane
Figure 2. Transmission electron micrographs of a capsule sample. The sample in question consisted of 80% hexadecane with PMMA shell coating. The main figure shows a negative stained image with several large particles measuring approximately 150 nm across. The image also has several small particles measuring in the 40 nm range. These particles are likely residual surfactant micelles or excess polymer molecules. The scale bar is 500 nm. The inset figure shows a Cryo-TEM image. The scale bar is 100 nm. The figure indicates that the capsules may have surface structures, or wrinkles.
Figure 3. Raw DSC scans for f(HD) ) 0.5 PMMA capsules. The scan order of the curves is green, red, blue.
The inset figure shows a cryo-TEM image of two capsules of the larger variety. This image shows that the capsules appear to have either surface structures, or wrinkles on the surface. This indicates that the polymer layer is not a uniform or continuous polymer film surrounding the core. The conditions in the cryo-TEM environment are unlikely to lead to drying out of the capsule during imaging, but such drying (pruning) is evident when we have attempted to image these capsules in scanning electron microscopy (SEM). Small variations in the overall particle diameter were seen as a function of the concentration of surfactant and the ratio of oil phase to water used in the synthesis. In cases where too little surfactant was used, a polymer layer was formed on top of the reaction vial. The variation in microcapsule size at a constant oil fraction should be a function of the interfacial tension between the monomer and the adjacent water phase in the dispersion. The greater variation at higher oil volume fractions may also be due to the slightly greater variation in the microcapsule size, as the capsules were not as easily formed. Freezing Behavior of the HD in PMMA Capsules. Figure 3 shows a typical DSC scan for a PMMA capsule with f(HD) ) 0.5. The inset boxes are magnifications of the phase transitions observed in the scans. The left inset shows the freezing
J. Phys. Chem. B, Vol. 114, No. 12, 2010 4133 exotherms for three scan cycles. The DSC curves change slightly with each cooling cycle, and a broad side-peak is observed to form with repeated cooling. The side-peak is more significant for samples where the fraction of HD is low. In particular, repeated thermal cycling of samples with 20% HD showed nearly complete disappearance of the main peak and the formation of a broad peak at 15° higher temperature. We have previously characterized this behavior as failure of the capsules.19 We believe this failure may involve the formation of a point-failure within the capsule that becomes a site for heterogeneous nucleation in the freezing transition. Samples with higher HD fractions did not show a capsule-failure signature, and the evolution of the side-peak was significantly less than what is shown in Figure 3. It is interesting to note that pure HD mini-emulsions do not show point-failure behavior in the DSC scan, but the onset of the phase transition is at a higher temperature than for the PMMA/HD capsules. A sample without the surfactant, i.e., pure HD in water, had nonrepeating DSC behavior between samples. The melting peaks, illustrated in the magnified region, show an almost negligible change with repeated thawing of the capsules. Both the freezing and melting behavior of HD as seen here is markedly different from the behavior we have previously reported for poly(styrene)/HD capsules.19 Influence of the Droplet Size on Supercooling. For this aspect of our work, we chose to focus on PMMA capsules. This choice was made due to the well-documented physical behavior of the PMMA polymer. Increasing the HD fraction, f(HD), in the formulation of the microcapsules, results in larger oil droplets confined within the polymer shell. The increase in f(HD) goes hand in hand with the capsule wall becoming thinner. The overall size of the microcapsule also increases with increasing f(HD). This increase is least pronounced for PMMA (see open squares in Figure 1). Our DLS measurements can be used to estimate both the thickness of the capsule wall and the actual size of the encapsulated HD. First, the capsules are large on a molecular scale, so the hydrodynamic radius is identical to the hard-sphere radius of the microcapsules. If we assume that the monomer volume is close to being identical to the volume of the resulting polymer, the radius of the oil droplet is simply
Roil ≈ RH · f(HD)1/3
(2)
where f(HD) ) V(HD)/V(total). (The second approximation is not strictly correct; we would expect the density of the polymer to be slightly higher than the density of the monomer. Corrections for this difference are largely speculative, however, since the polymer density may also be different with film thickness and the radius of the capsule.) A plot of the hydrodynamic radius of the microcapsules versus f(HD) and the calculated oil radius versus f(HD) is shown in Figure 4. The oil radius is a linear function of f(HD). The origin of this linearity is unknown to us at this point. The variation in supercooling, ∆T, as a function of f(HD) is shown in Figure 5. ∆T here is the difference between the onset temperature of the freezing during the first cooling run and the onset of the freezing transition for a bulk sample of HD. This is the definition for supercooling used in the G-T equation. The small variations in ∆T of 0.2 K are very close to matching the reproducibility of the DSC measurements that we achieved. In contrast, the supercooling for the pure mini-emulsion is significantly less than when PMMA is present. Supercooling
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Figure 4. Hydrodynamic radius and calculated oil droplet size as a function of the volume fraction of HD used in the formulation for synthesizing the capsule. The equation for the linear regression is Roil(calc) (nm) ) 13.4 + 120 × f(HD).
Figure 5. Extent of supercooling as a function of HD fraction. Three separate measurements are shown for f(HD) ) 0.50 and for f(HD) ) 1.0 to demonstrate reproducibility of ∆T.
Figure 6. Extent of supercooling as a function of inverse oil radius. The data only include PMMA/HD capsules. The size of the oil droplets was calculated using the linear regression equation in Figure 4.
in a nonpolymerized MMA/HD sample showed a nonrepeating DSC behavior, so we are confident that the HD is confined within the PMMA shell. According to the G-T equation, we expect the supercooling to be linear with inverse size of the droplet. This type of graph is shown in Figure 6. The oil droplet size, Roil, is calculated using the equation given in Figure 4. Due to the small variation in ∆T and the scattering in the DSC measurements between different capsule preparations, we have a harder time drawing meaningful conclusions between ∆T and Roil, although Figure 6 appears to be nonlinear, with the general trend that the supercooling increases as the droplet size decreases. Supercooling as a Function of Polymer Type. In addition to the 1/r size dependence of ∆T, the G-T equation explains
Black et al.
Figure 7. Extent of supercooling for the liquid-solid phase transition of HD encapsulated within different polymer capsules. The capsule diameter was adjusted to 160 ( 10 nm with f(HD) ) 0.5. The two data sets in the figure, open squares and open circles, are done independently.
how interfacial tension between the polymer and oil, γp/o, should affect the supercooling. In this work, we chose to vary γ by varying the ester side chains on the polymer backbone rather than by varying the encapsulated oil, which would change both Tm and ∆fH in the transition. For this purpose, PMMA, PEMA, PnBMA, PtBMA, and PnHMA capsules measuring 160 ( 10 nm with f(HD) ) 0.5 were produced. Organization and orientation of the side chains of several of the polymers (PMMA, PEMA, PnBMA, and PnHMA) upon wetting was studied by M. Clarke et al.31 using sum frequency generation spectroscopy. In their work, the interfacial polymer reorganized its side chains when the continuous phase was altered. In particular, methyl groups on the side chains tended to become buried when the interface changed from being air/ polymer to water/polymer interface. For polymers with low glass transition temperature, Tg, the backbone also reorganized in response to the change in the continuous phase. In addition to the change in Tg and side chain mobility, variation in the polymer backbone also alters the solubility of the polymer in the oil phase. In practice, these factors greatly affected the yield and stability of the capsules, with Tg appearing to be the most important. PnHMA capsules were difficult to purify without incurring a significant loss in mass. Furthermore, the capsules showed a significant aging effect in DSC scans taken of the same samples over consecutive days (samples were stored at room temperature). Here, we include data from PnHMA samples along with data from other capsules in this work for the purpose of completeness, but these data have little or no analytical value. Figure 7 shows the supercooling we measured for polymers with different side chains. The ordering of the polymer type in the figure is in the order of a perceived complexity, or size, of the side chain, viz., methyl, ethyl, n-butyl, t-butyl, and n-hexyl. The scattering of ∆T in the PnHMA capsule samples is most notable. This demonstrates that these capsules were not stable, most likely due to the low glass-transition temperature of the polymer. The apparent trend in Figure 7 is that, with greater volume of the side chain, the extent of supercooling increases. With ∆T in the excess of 15 °C, we are most likely observing freezing of the HD through homogeneous nucleation, rather than the transition being nucleated from the oil/polymer interface. This means that the presence of the different side chains increases the kinetic stability of the HD (increases ∆Gq) and the specific mobility of the side chains may not be of greater importance. In order to correlate the supercooling with the surface tension, we need the γ for the polymer/oil interface (γp/o). The poly(alkyl
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TABLE 1: Collection of Literature Values Pertaining to the Polymers Studied Here polymer
Tg (°C)33
γp/a (mN/m)
source
PMMA PEMA PnBMA PtBMA PnHMA
108 65 20 110 -5
38.3, 43.2 33.6, 35.9 28.8, 34.6 18.1, 30.5 31.1
Kwok 2000,34 Lee 199135 Kwok 2000,34 Wu 197136 Kwok 2000,34 Wu 197136 Grundke 2005,37 Wu 197136 Clarke 200631
TABLE 2: Supercooling of Encapsulated HD, the Polymer/ Air/Water Contact Angle, and the Calculated Surface Tension between the HD and the Polymera polymer
∆T (K)
θp/a/w
γp/HD (mN/m) (eq 3)
γmid (mN/m)
PMMA PEMA PnBMA PtBMA PnHMA
16.0 ( 0.2 16.4 ( 0.2 16.5 ( 0.1 16.9 ( 0.1 15.9 ( 0.7
66 73 84 80 78
11-16 6.6-9.9 1.8-7.5 -8.9-3.5 4.1-5.0
13.8 7.8 4.7 -2.7 4.6
a Our best estimate for the contact angle for all of the polymers is between 10 and 12°. A choice of constant contact angle is analogous to shifting γp/o by a constant value from γp/a.
methacrylate)/oil and poly(alkyl methacrylate)/water interface has been studied in great detail in the literature. From this body of work, it is clear that the surface tension between polymers and adjacent liquids is a function of the method of preparation and rearrangement of the side chains of the polymer in response to the nature of the liquid that wets the surface. Such work requires experimental procedures that were not available to us, but we will attempt to give a reasonable estimate of this value as follows. In its simplest form, the surface tension between a solid (p) and liquid that wets it (l), γp/l, can be calculated from the tangential angle between the solid, air, and liquid, θp/a/l, that is stabilized when a small liquid drop rests on a flat polymer film. A well-known connection between these values is Young’s equation:
γp/l ) γp/a - γl/a × cos(θp/a/l)
(3)
where γp/a is the surface tension between the polymer and air and γl/a is the surface tension between the liquid and air. The surface tension between HD and air is 28.12 mN/m.32 The polymer/air surface tensions reported in the literature vary slightly with the measurement method; values for γp/a derived from contact angle measurements are collected in Table 1. Table 2 summarizes the results from our DSC measurements as well as values we measured for the polymer/air/water contact angle, θp/a/w. (The contact angle was measured using a spincoated polymer film on a silica surface. The polymer was obtained by mini-emulsion synthesis using f(HD) ) 0.01, followed by purification via repeated dissolution/precipitation in THF/water/methanol.) Our measurements for the contact angle for PMMA, PEMA, and PnBMA are in agreement with values reported by Clarke et al.,31 although our value is otherwise somewhat low in comparison with what is commonly observed in the literature (70-76°). Our attempts at directly measuring the HD/polymer contact angle were inconclusive using the instrumentation available to us. In general, the HD wetted the polymer surface with a final contact angle between 10 and 12°, but this angle tended to decrease with time. If we assume that θp/HD/a is the same (11°) for all of the polymers, we can estimate the interfacial tension between the HD and the oil. This value
Figure 8. Average supercooling as a function of the midpoint of the estimated surface tension.
is obviously somewhat dependent on the choice made for γp/a, but this may give an insight into the connection between the supercooling and γp/o nevertheless. The calculated values are summarized in Table 2. According to the G-T theory, ∆T should increase with increasing γo/p. Judging from Table 2, we have the correlation that the polymer that shows the highest γ between the HD and polymer shows the supercooling that ranks lowest (PMMA with ∆T ) 16.0 K and γ ≈ 12 mN/m). The polymer that has the lowest surface tension shows the largest supercooling (PtBMA with ∆T ) 16.9 K and γ < 0 mN/m). Both polymers have very similar glass transition temperatures Tg. Figure 8 further demonstrates this trend. It should be noted here that the values of x for the points in the figure are arbitrarily chosen from the midpoints in the range for γp/HD. The error bars on the horizontal axis represent the lowest and largest value found in the literature. The error in the vertical axis represents the standard deviation in the measured values of supercooling from the values in Figure 7. The linear correlation in the figure is somewhat unexpected, given the numerous approximations and variation in surface tension values collected for the figure. The linear correlation between ∆T and γ is in agreement with the G-T equation, but the negative slope in the figure is not, since every coefficient in eq 1 should be positive. Effect of Extended Heating of the Capsules on HD Phase Transition. Our observation of the instability of the PnHMA capsules brought our attention to another observation made during this study. Capsules having a poly(alkyl methacrylate) backbone were observed to show decreasing ∆T results depending on the duration between the DSC measurements and the synthesis. This type of effect was not seen in poly(styrene) capsules, with DSC measurements on capsules synthesized 2 years ago in our laboratory still being consistent with the results obtained by Fette and Pham at that time.19 In contrast, PMMA capsules showed a drop in ∆T by several degrees over periods of several weeks. In order to accelerate aging of the capsules, samples were heated to 60 °C and held there for a specified period of time. This experiment was performed in a sealed DSC pan to prevent evaporation of water. Following this heating period, ∆T was measured as before. Figure 9 shows ∆T after varying heating times. The meaning of the symbols is explained in the figure caption. ∆T changed from 16 to 14 K after approximately 2 hours of heating. Supercooling of approximately 14 K is also seen in samples that are stabilized only by a surfactant (see f(HD) ) 1 in Figure 5). Incidentally, we see a similar shift in ∆T when the capsules are stored longer than 2 weeks at room temperature. This shift
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Figure 9. Supercooling for the HD in PMMA capsules as a function of heating time at 60 °C. The dashed line in the figure is merely to guide the eye. Squares and circles represent different experiments done independently three months apart. The dark symbols represent the onset temperature for the main transition peak, whereas the gray symbols represent the onset temperature for any secondary peak or shoulder in the transition.
in the phase transition may indicate dewetting of the PMMA from the HD oil droplet. As the polymer rearranges, the exposed oil droplet is stabilized by the surfactant. Figure 9 also shows a further change in ∆T that we did not observe at room temperature over the time span of approximately 6 months. Once ∆T leveled off, the DSC scan showed single freezing transitions, even for multiple cool-heat cycles. It is interesting to note here that the sample with 5 µL drops of HD and 20 µL of water does not show a clear transition for HD in the DSC scan. The DLS scans of heated DSC samples showed that the samples were either extremely aggregated or that the samples had picked up particular contamination. It is possible that the capsules form larger aggregates upon heating, especially if the surfactant coating undergoes rearrangement that exposes the polymer to the surrounding water phase. Conclusions In this work, we synthesized a series of alkyl-methacrylate capsules containing encapsulated n-hexadecane. The microcapsule sizes were in the range 100-280 nm with low polydispersity. The size varied with the type of polymer used in the synthesis, with PMMA capsules being the smallest and PnHMA the largest. This variance is in agreement with the expected differences in hydrophobicity of the polymer backbones, which should increase from the methyl group to the n-hexyl group. The PMMA-encapsulated HD could withstand multiple freeze-thaw cycles without affecting the integrity of the polymer shell to a significant extent. The capsules were more resilient to freezing when the polymer shell was thin, corresponding to the majority of the capsular volume being HD. This indicates that flexibility of the shell is important for responding to shrinkage when the HD freezes. The overall size of the capsules was also affected by the total fraction of HD in the capsules, those having a greater fraction being larger. The cause of this is unknown, but an initial guess for this effect is that change in the oil viscosity may play a pivotal role in the final particle size during the sonication. Encapsulated HD showed significant supercooling for the freezing transition. The supercooling becomes larger as the oil droplet radius decreases. The relationship is not linear when ∆T is plotted against 1/roil, as would be expected from the G-T equation. The supercooling for the HD freezing was also affected by the type of polymer used to form the capsule. We saw a
Black et al. slight increase in ∆T with increased size of the ester side chain on the polymer. Plotting the supercooling against the estimated value for the interfacial tension between the polymer and oil γp/o results in a linear relationship but with a negative slope. Although the G-T equation predicts a linear relation between ∆T and γ, we would expect the trend to have a positive slope. We observed that the alkyl methacrylate/HD capsules changed when stored at room temperature for extended periods of time. This shelf life was investigated by heating PMMA capsules to a temperature close to the glass transition temperature Tg. Our model is currently that the polymer shell relaxes its conformation in a manner such that some part of the encapsulated oil becomes exposed to the surfactant layer that stabilizes the capsule suspension. This exposure alters the supercooling in the system to a value typical for the surfactant-stabilized oil droplet. With further heating, the capsules aggregate, and the encapsulated drop takes on a bulk-like phase transition behavior. Acknowledgment. The authors thank Santa Clara University for financial support for this work. J.K.B. was supported by a grant from Santa Clara University Faculty Development Office “EIBM#0057”. Lauren Tracy was supported by The Claire Luce Booth fellowship “DEAN1121”. GPC analysis of samples was done by Polymer Laboratories Inc. Trials in using SEM to image the capsules were done at the Center for Nanostructures at Santa Clara University by the group members in the Adalsteinsson group. The authors also thank Professor J. Gilbert at Santa Clara University for helpful comments on the manuscript. References and Notes (1) Turnbull, D. J. Chem. Phys. 1950, 18, 198–203. (2) Turnbull, D. J. Chem. Phys. 1952, 20, 411–424. (3) Turnbull, D.; Cormia, R. L. J. Chem. Phys. 1961, 34, 820–831. (4) Becker, R.; Doring, W. Ann. Phys. 1935, 416, 719–752. (5) Schreiber, A.; Ketelsen, I.; Findenegg, G. H. Phys. Chem. Chem. Phys. 2001, 3, 1185–1195. (6) Hindle, S.; Povey, M. J. W.; Smith, K. J. Colloid Interface Sci. 2000, 232, 370–380. (7) McClements, D. J.; Dungan, S. R. J. Colloid Interface Sci. 1997, 186, 17–28. (8) Clausse, D.; Gomez, F.; Pezron, I.; Komunjer, L.; Dalmazzone, C. AdV. Colloid Interface Sci. 2005, 117, 59–74. (9) Herhold, A. B.; Ertas, D.; Levine, A.; King, H. Phys. ReV. E 1999, 59 (6), 6946–6955. (10) Montenegro, R.; Landfester, K. Langmuir 2003, 19, 5996–6003. (11) Montenegro, R.; Antonietti, M.; Mastai, Y.; Landfester, K. J. Phys. Chem. B 2003, 107, 5088–5094. (12) Dickinson, E.; Ma, J. G.; Povey, M. J. W. J. Chem. Soc., Faraday Trans. 1996, 92, 1213–1215. (13) McClements, D. J.; Dickinson, E.; Dungan, S. R.; Kinsella, J. E.; Ma, J. G.; Povey, M. J. W. J. Colloid Interface Sci. 1993, 160, 293–297. (14) Herhold, A. B.; King, H. E.; Sirota, E. B. J. Chem. Phys. 2002, 116, 9036–9050. (15) Hansen, E. W.; Gran, H. C.; Sellevold, E. J. J. Phys. Chem. B 1997, 101, 7027–7032. (16) Schmidt, R.; Hansen, E. W.; Stocker, M.; Akporiaye, D.; Ellestad, O. H. J. Am. Chem. Soc. 1995, 117, 4049–4056. (17) Takamuku, T.; Yamagami, M.; Wakita, H.; Masuda, Y.; Yamaguchi, T. J. Phys. Chem. B 1997, 101, 5730–5739. (18) Bellissentfunel, M. C.; Lal, J.; Bosio, L. J. Chem. Phys. 1993, 98, 4246–4252. (19) Fette, E. V.; Pham, A.; Adalsteinsson, T. J. Phys. Chem. B 2008, 112, 5403–5411. (20) Landfester, K.; Willert, M.; Antonietti, M. Macromolecules 2000, 33, 2370–2376. (21) Landfester, K. Annu. ReV. Mater. Res. 2006, 36, 231–279. (22) Landfester, K.; Bechthold, N.; Tiarks, F.; Antonietti, M. Macromolecules 1999, 33, 5222–5228. (23) Tiarks, F.; Landfester, K. M.; Antonietti, M. Langmuir 2001, 17, 908–918. (24) Landfester, K.; Schork, F. J.; Kusuma, V. A. C. R. Chim. 2003, 6, 1337–1342. (25) Sudol, E. D.; El-Aasser, M. S. Emulsion Polymerization and Emulsion Polymers; John Wiley and Sons Ltd: Chichester, England, 1997.
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