Calorimetric Analysis of Thermal Phase Transitions in Functionalized

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J. Phys. Chem. B 2007, 111, 1334-1342

Calorimetric Analysis of Thermal Phase Transitions in Functionalized Microgels Todd Hoare* and Robert Pelton† Department of Chemical Engineering, McMaster UniVersity, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7 ReceiVed: October 22, 2006; In Final Form: December 14, 2006

Differential scanning calorimetry (DSC) is used to investigate the thermal phase transitions of a range of N-isopropylacrylamide (NIPAM)-based, carboxylic acid-functionalized microgels with well-defined radial and chain functional group distributions. The transition enthalpies of protonated microgels can be correlated with the hydrophobicity of the functional comonomer, while the transition enthalpies for ionized microgels can be correlated with the degree of microgel deswelling achieved across the thermal phase transition. The peak widths at half-height vary inversely with the average length of NIPAM blocks in each of the microgels, as calculated using a kinetic copolymerization model. Deconvolution of the asymmetric DSC thermograms is accomplished using a two-transition model, thought to relate to core-shell-type transitions induced by the significant local heterogeneities within the functionalized microgels. The ratio between the two transition temperatures of these deconvoluted peaks is a useful quantitative probe of the radial functional group distribution. An additional, low-temperature transition is also observed in the thermogram of the vinylacetic acid-functionalized microgel, indicative of the occurrence of local chain rearrangements prior to the macroscopic phase transition in this microgel. Complementary light scattering analysis suggests that microphase separation may account for this additional transition peak.

Introduction Microgels based on poly(N-isopropylacrylamide) (NIPAM), first prepared in our laboratory,1 are of significant scientific and technological interest due to their ability to undergo a volume phase transition at ∼32 °C.2 Incorporating functional comonomers into the microgels shifts the volume phase transition temperature (VPTT) to higher or lower temperatures (via the inclusion of more hydrophilic or hydrophobic comonomers) and changes the temperature range over which the phase transition is completed. The volume phase transitions of the resulting microgels are typically analyzed using a range of analytical techniques, including quasi-elastic light scattering, electrophoresis, potentiometric and conductometric titration, and turbidimetry. For the most part, these techniques probe bulk, macroscopic properties of the microgels such as hydrodynamic diameter, surface charge density, bulk composition, and refractive index. Differential scanning calorimetry (DSC) is a good complement to these methods given its ability to track the volume phase transition on the microscopic level. Heat effects across the phase transition are associated primarily with the aggregation of NIPAM-rich chains and the concurrent changes in polymerwater and water-water hydrogen bonding interactions. Comparisons between the DSC heat release profiles and the other macroscopic variables across the thermal phase transition may therefore yield insights into the mechanisms of microgel deswelling and the effects of microgel morphologies on the phase transition behavior. DSC is a well-established technique for investigating phase transitions and has been applied extensively in the characterization of PNIPAM-based polymers. Schild and Tirrell3 performed * To whom correspondence should be addressed. Phone: 1-905-5259140 ext. 27342. Fax: 1-905-528-5114. E-mail: [email protected]. † E-mail: [email protected].

DSC analyses on linear PNIPAM chains and reported a decreasing lower critical solution temperature (LCST) as the PNIPAM molecular weight increases over the same 32-35 °C temperature range observed in turbidimetric analysis. Several studies have also investigated bulk PNIPAM-based hydrogels using both low4- and high5-sensitivity DSC analysis. However, relatively few studies have applied DSC to microgel analysis. Snowden et al. reported the first DSC analysis of PNIPAMbased microgels,6 showing a reversible, cooperative, but asymmetrical deswelling transition. The asymmetry was even more apparent when the microgels were functionalized with 5 mol % acrylic acid;7 the thermograms, while still reversible, exhibited a distinct shoulder at higher temperatures which mandated a two-transition fit of the experimental DSC profiles. Woodward et al.8 performed DSC measurements on nonfunctionalized PNIPAM microgels with different cross-linker contents and again observed that two-transition fits were required to capture the shape of the thermogram. In this case, the microgels are purified by dialysis such that the linear polymers typically produced in microgel synthesis remained in the analyzed sample. The sharper, low-temperature transition observed was therefore attributed to oligomer aggregation, while the broader, highertemperature transition was linked to the conformational transition of the microgel. Woodward et al. observed distinct trends in terms of the overall transition enthalpies, the midpoint transition temperature, and the breadth/cooperativity of the phase transition as a function of cross-linker content. Gan and Lyon also applied peak width analysis techniques to probe the kinetics of microgel deswelling as a function of hydrophobic surface modification.9 Other applications of DSC in the context of studying microgels have primarily focused on low-sensitivity analyses for extracting the VPTT.10-14 In all these cases, however, a comprehensive analysis of the shapes and magnitudes of the phase transition thermograms was unnecessary to

10.1021/jp066916v CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007

Functionalized Microgel Thermal Phase Transitions

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1335

TABLE 1: Radial and Chain Functional Group Distributions in Functionalized Microgelsa

a In the radial density profiles, a higher pixel density (i.e., a darker region) corresponds to a higher local -COOH concentration. b Calculated via group contribution methods in the protonated state;17 log Kow(NIPAM) ) 0.66.

meet the objectives of the DSC experiments and/or was excessively difficult to perform given the undefined heterogeneity present in the microgels being studied. In our previous work,15,16 we prepared a series of microgels containing the same bulk carboxylic acid content (6.5 mol %) via the copolymerization of five different carboxylic acidfunctionalized comonomers: methacrylic acid (MAA), acrylic acid (AA), vinylacetic acid (VAA), fumaric acid (FA), and maleic acid (MA). Significantly different radial and chain functional group distributions were observed in the different microgels, distributions which were linked primarily to the different kinetic reactivities of the functional comonomer. Table 1 summarizes the different physical properties of the comonomers used and the corresponding differences in both the radial and chain functional group distributions in the microgels. The monomer hydrophobicity is represented by the octanol-water partition coefficient log Kow:17 the higher the log Kow value, the more the monomer partitions into the octanol phase of an octanol-water mixture and the higher the monomer hydrophobicity. The radial and chain distributions were calculated via copolymerization kinetics modeling16 and were confirmed experimentally via transmission electron microscopy (TEM) imaging,16 potentiometric titration,18 and dimensionless plot analysis.19 The radial distributions are presented graphically in terms of pixel density: the darker the pixel, the higher the local -COOH concentration. The chain distributions are presented in terms of the average number of consecutive functional monomer and N-isopropylacrylamide monomer units incorporated into the subchains of each of the microgels (i.e., the average block lengths of each monomer). The physical factors regulating the magnitudes of the calorimetric enthalpies, peak shapes, and transition temperatures of DSC scans are typically extremely difficult to identify in polyelectrolyte hydrogels due to the heterogeneity of the local chain morphologies (and thus the local phase transition temperatures) within the gels. In this paper, the comprehensive local morphological knowledge acquired for our series of -COOHfunctionalized microgels is applied to analyze the DSC responses of functionalized poly(N-isopropylacrylamide)-based microgels. Experimental Section Microgel Preparation. Microgels are prepared according to the recipes reported previously.15,16 Each functionalized microgel (unless otherwise noted) contains 6.5 mol % -COOH groups

on a bulk basis, allowing direct comparisons to be drawn between the different functionalized microgel properties without compensating for differences in the overall gel compositions. The microgels are purified via repeated (minimum five-cycle) ultracentrifugation, facilitating the removal of any water-soluble oligomers contained in the microgel. By eliminating these oligomers, it is assured that none of the deconvoluted DSC peaks can represent a linear polymer phase transition.8 Differential Scanning Calorimetry. DSC analysis was performed using a MicroCal VP-DSC microcalorimeter. Scans were performed at a slow rate of 10 °C/h, spanning a temperature range of 5-80 °C for the pH 3.5 scans and 5-100 °C for the pH 10 scans. Microgel suspensions were prepared at concentrations of 10 mg/mL in 1 mM KCl and adjusted to the desired pH via the addition of NaOH/HCl. Baseline scans using 1 mM KCl solutions (also used as the reference solution) were performed prior to sample injection and were subtracted from the sample data to isolate the heat associated with gel phase transition. Three replicate forward and backward scans were performed on each microgel suspension to ensure reversibility; all reported results are averages of these six scans, with the error bars corresponding to the standard deviation of the individual scans. No active temperature compensation was performed to maximize the sensitivity of the results. The reported curves were not smoothed or manipulated in any way outside of a baseline subtraction and a concentration normalization, both of which are performed using Microcal Origin 5.0. Light Scattering. Particle size and light scattering intensity measurements were performed on 15 ppm microgel suspensions in 1 mM KCl using a Lexel 95 ion laser operating at 514 nm and a BI-9000AT digital autocorrelator (Brookhaven Instruments Corp.). Measurements are performed at a 90° scattering angle using a 100 mW laser power. Error bars represent the standard deviation of five replicate particle size measurements. Results For each of the DSC runs reported in this study, very slow scan rates were used to achieve more reliable and accurate line shapes for the conformational transitions6 and thus enable effective deconvolutions of the thermograms into their constituent transition peaks. No hysteresis was observed between the forward and backward scans, further suggesting that thermodynamic equilibrium was achieved throughout the scans. Forward and reverse prescans were also performed prior to data

1336 J. Phys. Chem. B, Vol. 111, No. 6, 2007

Figure 1. Temperature dependence of the pH 3.5 hydrodynamic diameters of functionalized microgels compared with a nonfunctionalized PNIPAM microgel.

collection to ensure that each sample has the same thermal history and no irreversible transition effects are reflected in the results. DSC scans were performed at two pH values, chosen according to our previous titrations of these microgels.23 At pH 10, all of the -COOH groups in each of the microgels are ionized, providing a clear picture of the impact of pH-sensitive functional groups on microgel phase transitions. At pH 3.5, >98% of the functional groups in AA-NIPAM, VAA-NIPAM, or MAA-NIPAM and 96% of the functional groups in FANIPAM are protonated. Thus, the pH 3.5 data represent the phase transition of the microgels in their predominantly protonated state. Microgel Transitions in Acidic Conditions (pH 3.5). The hydrodynamic diameter versus temperature profile for each of the functionalized microgels at pH 3.5 is shown in Figure 1. Overall, each of the microgels shows a very similar thermal deswelling profile. Only a ∼2-3 °C difference exists between the volume phase transition temperatures of the different functionalized microgels, a difference which can be attributed primarily to the relative hydrophilicities of the comonomers. Microgels prepared with functional monomers more hydrophilic than NIPAM in the protonated state (AA, MA, and FA) exhibit deswelling profiles which show broader volume phase transitions and slightly higher onset phase transition temperatures. The least hydrophilic of these monomers (AA) exhibits the lowest VPTT shift (∼0.5 °C), while the more hydrophilic FA and MA monomers produce VPTT shifts of 1-2 °C. Correspondingly, microgels prepared using more hydrophobic comonomers (VAA and MAA) exhibit phase transition temperatures 0.5-1 °C lower than the nonfunctionalized microgel. The overall magnitude of thermal deswelling achieved is also similar for each of the microgels regardless of the distribution of the functional monomers within the microgel matrix. The corresponding DSC thermograms for the microgels at pH 3.5 are shown in Figure 2, while Table 2 shows the overall enthalpy ∆H, the midpoint transition temperature Tm, and the peak width at half-height T1/2 measured from these thermograms. The enthalpy ∆H represents the area under the thermogram, while the midpoint transition temperature represents the temperature at which the highest Cp value is recorded. Several parallels are observed between the hydrodynamic diameter deswelling profiles in Figure 1 and the DSC thermograms in

Hoare and Pelton

Figure 2. Differential scanning calorimetry thermograms for each of the functionalized microgels as measured at pH 3.5.

TABLE 2: Thermal Transition Microgel Properties at pH 3.5 As Measured via DSC Analysis pH 3.5 microgel

∆H (kJ/(g of dry microgel))

Tm (°C)

T1/2 (°C)

MAA-NIPAM AA-NIPAM VAA-NIPAM FA-NIPAM MA-NIPAM NIPAM

34.7 ( 0.4 32.7 ( 0.2 41.7 ( 0.6 30.7 ( 0.2 32.5 ( 0.2 37.6 ( 0.2

34.5 ( 0.1 35.2 ( 0.2 33.7 ( 0.3 35.8 ( 0.4 35.5 ( 0.3 33.8 ( 0.1

5.7 ( 0.1 5.6 ( 0.1 5.2 ( 0.1 6.9 ( 0.7 6.0 ( 0.3 4.2 ( 0.1

Figure 2. Each DSC transition is centered at 35 ( 1 °C ,and the same ∼2 °C temperature range exists between the VPTT of the microgel with the lowest (VAA-NIPAM) and highest (FA-NIPAM) transition temperatures in the midpoint transition temperature data in Table 2. The microgels prepared via the copolymerization of comonomers more hydrophobic than NIPAM (VAA, MAA) exhibit lower VPTT values than those prepared with more hydrophilic comonomers (AA, FA, MA). Finally, none of the ∆H results for the different functionalized microgels deviates by more than (20% from the mean transition enthalpy for all the microgels at pH 3.5, consistent with the similar volumetric deswelling ratios observed for each of the microgels in Figure 1. However, a clear correlation is observed in the DSC data between ∆H and the comonomer hydrophilicity: the more hydrophilic the comonomer, the less heat is absorbed upon microgel deswelling. VAA is significantly more hydrophobic than FA in the protonated state; correspondingly, the overall transition enthalpy of VAA-NIPAM is more than 35% higher than that of FA-NIPAM despite the fact that the two microgels exhibit the same bulk carboxylic acid content, a similar radial functional group distribution, and a similar degree of volumetric deswelling across the thermal phase transition (Figure 1). The DSC transitions are also significantly broader than the particle size transitions, spanning a 20-25 °C overall temperature range (from baseline to baseline) compared to the 10-15 °C range observed in the particle size measurements. This significant broadening of the phase transition range upon ionization suggests that the functional groups are significantly more influential than the inter-cross-link subchain length distributions in controlling the shape of functionalized microgel DSC thermograms. Consistent with previously reported microgel DSC data,8 each DSC thermogram is asymmetric in that a distinct tail is observed at the higher temperature side of the distribution for each

Functionalized Microgel Thermal Phase Transitions

Figure 3. Temperature dependence of the pH 10 (ionized state) hydrodynamic diameter for each of the functionalized microgels tested compared with a nonfunctionalized PNIPAM microgel. Data are included for a VAA-NIPAM microgel containing half the total number of functional groups (3.3 mol %).

microgel. This tail can be attributed to a combination of two effects: variations in the local PNIPAM subchain lengths between cross-linking points within the microgel and shifts in the local VPTT values according to the distribution and hydrophilicity of the -COOH comonomers within the microgel. Schild and Tirrell showed that PNIPAM linear chains of lower molecular weight exhibited higher VPTT values than chains of higher molecular weight,3 while the copolymerization of more hydrophilic monomers shifts the VPTT to higher temperatures by shifting the hydrophilic-hydrophobic balance regulating microgel deswelling. Microgels prepared with more hydrophilic comonomers (FA and MA) exhibit broader overall transitions (i.e., higher T1/2 values) and broader tails compared to microgels prepared with more hydrophobic comonomers (VAA, MAA). As the functional monomer becomes more hydrophilic with respect to NIPAM and more radially localized within the microgel, the difference between the local volume phase transition temperatures within the microgel increases and the overall phase transition broadens. This explains why AANIPAM (in which functional groups appear to be relatively uniformly distributed throughout the microgel network) exhibits a similar T1/2 value to MAA-NIPAM (in which -COOH groups appear to be mostly core-localized) despite the significantly higher hydrophilicity of AA compared to MAA. Microgel Transitions in Basic Conditions (pH 10). The hydrodynamic diameter versus temperature profile for each of the functionalized microgels in the ionized state (pH 10) is shown in Figure 3. In general, each deswelling profile spans a larger temperature range (15-30 °C) and yields a lower overall degree of microgel deswelling compared to the corresponding microgel in acidic conditions. However, in contrast to the pH 3.5 protonated microgel data, significant differences exist between the ionized state deswelling profiles of the functionalized microgels. While only a 2-3 °C difference was observed in the onset VPTT values of the functionalized microgels at pH 3.5, onset VPTT values for the ionized microgels span a range of at least 30 °C: ∼38 °C for MAA-NIPAM, ∼48 °C for AA-NIPAM, ∼49 °C for MA-NIPAM, ∼53 °C for FANIPAM, and >70 °C for VAA-NIPAM. The shapes of the deswelling profiles are also significantly different according to the comonomer used to prepare the functionalized microgels. A distinct two-step deswelling transition is observed for MAA-

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1337

Figure 4. DSC thermograms (excess heat capacity versus temperature) for each of the functionalized microgels studied in the ionized state (pH 10).

TABLE 3: Thermal Transition Microgel Properties at pH 10 as Measured via DSC Analysis pH 10 microgel

∆H (kJ/(g of dry microgel))

Tm (°C)

T1/2 (°C)

MAA-NIPAM AA-NIPAM VAA-NIPAM FA-NIPAM MA-NIPAM NIPAM

23.4 ( 0.6 4.2 ( 0.2 5.0 ( 0.2 10.8 ( 0.2 11.9 ( 0.2 36.4 ( 0.2

37.2 ( 0.2 52.4 ( 0.2 53.4 ( 0.2 48.9 ( 0.4 46.2 ( 0.5 33.9 ( 0.2

11.2 ( 0.1 18.2 ( 0.4 16.4 ( 0.4 19.0 ( 0.9 17.3 ( 0.7 4.4 ( 0.1

NIPAM (thought to relate to the sequential collapse of the NIPAM-rich shell and MAA-rich core), the diacid-functionalized microgels exhibit continuous deswelling profiles over extremely broad (∼25-30 °C) temperature ranges, and VAA-NIPAM exhibits essentially no deswelling transition whatsoever up to 70 °C. The corresponding DSC thermograms for each of the ionized microgel thermal transitions are shown in Figure 4, while Table 3 shows ∆H, Tm, and T1/2 extracted from the thermograms. The transition temperatures and enthalpies vary much more widely between the different microgels under basic conditions compared to acidic conditions, in direct correlation with the particle size profiles. MAA-NIPAM exhibits an overall transition enthalpy double that of the diacid-functionalized monomers and approximately five times higher than AA-NIPAM and VAANIPAM in the ionized state. A strong correlation is also observed between the midpoint transition temperatures and the particle size data. The MAA-NIPAM transition occurs at temperatures 15-16 °C lower than that of AA-NIPAM and 6-8 °C lower than the diacid-functionalized microgels, in parallel to the deswelling data shown in Figure 3. The DSC results further suggest that a relationship exists between the radial functional group distribution and both the transition temperature and the transition enthalpy. For the monoacid-functionalized microgels, Tm increases and ∆H decreases systematically as the radial functional group distribution becomes more surface-localized. The diacid-functionalized microgels, meanwhile, yield lower Tm and ∆H values than would be expected on the basis of their apparently surface-localized functional group distributions. This observation directly corresponds with the particle size profiles in Figure 3, in which MANIPAM and FA-NIPAM show significantly lower VPTT values and larger overall degrees of deswelling compared to VAANIPAM despite containing a similar surface-localized functional

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Figure 5. Correlation between the measured calorimetric enthalpy of the microgels under both basic and acidic conditions and the overall deswelling ratio (normalized to the fully swollen microgel at 20 °C and pH 10).

group distribution. This is consistent with the responses of the diacid-functionalized microgels observed over the pH phase transition in our previous work.19 Swelling “frustrations” are induced by the pairing of two -COOH groups within the diacid residues on the same polymer subchain, reducing the effective charge-charge repulsion driving swelling (or opposing deswelling) in the diacid-functionalized microgels. The asymmetry of the DSC transition is preserved in the ionized state, with distinctive tails visible at the high-temperature end of each of the functionalized monomer thermograms. However, compared to the pH 3.5 protonated microgel profiles, the ionized state transitions for the functionalized microgels are all broader (∼40 °C overall range), lower in intensity, and shifted toward higher temperatures, mirroring the particle size profiles. Repulsive charge-charge interactions between ionized -COOH groups effectively compete with the NIPAM-NIPAM interactions to reduce the cooperativity of the phase transition, broaden the transition temperature range, and reduce the overall degree of deswelling achieved. Discussion DSC Correlations. The DSC results can be correlated to both experimental, physical property data as well as model-based compositional data to better understand the mechanistic origins behind the observed trends in the transition enthalpy and peak width at half-height values. Figure 5 shows the relationship between the change in the microgel diameter (normalized to the fully swollen microgel diameter at pH 10 and 20 °C) and the overall transition enthalpy across the volume phase transition under both basic and acidic conditions. A linear correlation is observed between the degree of deswelling and the transition enthalpy for the ionized microgels (pH 10). This result is physically reasonable since more NIPAM chain aggregation processes occur and more hydrogen bonding interactions are disrupted as microgels continue to deswell, increasing the enthalpic impact of the phase transition. Thus, the same radial and chain distribution factors influencing the deswelling also regulate the magnitude of the heat release across the phase transition in the ionized state. Based on this result, the local charge density predominantly regulates the DSC response in the presence of charges by effectively

Hoare and Pelton competing with the NIPAM aggregation interactions to limit the degree of macroscopic chain collapse observed. DSC may therefore be useful as a probe of the local charge density in charged functionalized microgels. Although the spread in the ∆H values is significantly smaller under acidic conditions, no direct correlation is observed between the degree of deswelling and the transition enthalpy at pH 3.5. This is consistent with our hypothesis that the monomer hydrophobicity predominantly determines the DSC response in the absence of charges through the formation of looser or denser microscopic chain aggregates upon macroscopic chain collapse. The trend in the T1/2 values can also be described quantitatively according to the kinetic modeling approaches we have previously reported.16 Since T1/2 is fundamentally a representation of the cooperativity of the phase transition, the average length of thermosensitive poly(NIPAM) blocks in the microgel Vav4 is expected to influence the value of T1/2. The value of Vav4 was calculated for each microgel by dividing the gel into 20 equal-volume shells and applying a terminal copolymerization kinetics model to predict the mole fractions of NIPAM, MBA cross-linker, and functional monomer in each shell according to the relative reactivities of the comonomers.16 The average number of consecutive monomers of any one type in a shell can then be predicted as

Vav4 )

1 1 ) 1 - p44 p41 + p42 + p43

(1)

where Vav4 is the average number of consecutive monomer 4 units incorporated into the polymer (where monomer 4 ) NIPAM for this calculation) and pab is the probability of monomer b reacting with a polymer radical capped with monomer a, defined as

pab )

kabNaNb

∑b

b ) (1,2,3,4)

(2)

kabNaNb

Here, kab is the reaction rate constant of a radical of type a reacting with a monomer of type b and Na and Nb are the number of moles of components a and b, respectively, within the shell (as calculated using the kinetic model). For the calculations performed in this work, monomer 1 is the functional comonomer (AA, MAA, FA, MA, or VAA), monomer 2 is the unreacted, divinyl cross-linker, and monomer 3 is the singly reacted, pendent cross-linker with an estimated reactivity one-fourth that of the divinyl cross-linker. The results for each shell are then weight-averaged according to the total amount of NIPAM present per shell to yield an average NIPAM block length. The correlation between the kinetic model-predicted average NIPAM block lengths in each of the functionalized microgels and the peak widths at half-height measured via DSC is shown in Figure 6 for both the protonated and ionized microgels. The peak width at half-height increases as the average NIPAM block length decreases for microgels in both the protonated and ionized states. At pH 3.5, the hydrophobicity of the comonomers is just slightly higher (MAA, VAA) or slightly lower (AA, FA, MA) than that of NIPAM such that the presence of the functional monomer units play only a minimal role in “interrupting” the NIPAM-NIPAM hydrophobic interactions which drive microgel collapse. Thus, the dependence of T1/2 on the average NIPAM block length is very weak in the protonated gels. Conversely, when the functional groups are ionized, the charged groups are highly effective at interrupting the cooperative

Functionalized Microgel Thermal Phase Transitions

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1339

Figure 6. Correlation between the kinetic model-predicted average NIPAM chain length and the peak width at half-height from DSC analysis.

hydrophobic associations and a steep linear correlation is observed between the T1/2 values and the average NIPAM block lengths. Thus, a link can be drawn between the cooperativity of the phase transition, the peak width at half-height from the DSC analysis, and the average length of NIPAM-only segments in a microgel. Together with the ∆H versus degree of deswelling correlation given in Figure 5, these data show how an improved understanding of the underlying gel morphology can be applied to understand the mechanistic reasons behind the calorimetric responses of microgels across their thermal phase transition. Peak Shape Analysis. The distinct differences in the shapes of the high-sensitivity DSC thermograms also yield insights into the underlying mechanisms of microgel deswelling. Each of the microgels exhibits an asymmetric transition with a distinct tail at the upper temperature end of the distribution. Extensive DSC scans were performed to confirm that these tails are thermodynamic features and not kinetic effects. Based on the average collective diffusion coefficient for PNIPAM gel systems (D ∼ 10-7 cm2/s) and the maximum size of the microgels used in this study (d ) 450 nm), the collective diffusion approach of Tanaka and co-workers20 can be used to estimate the time τ required to achieve swelling equilibrium according to

τ)

(

)

〈d2〉 (450 × 10-7 cm)2 ) 0.0033 s ) 6D 6(10-7 cm2/s)

(3)

Comparing this time to the 10 °C/h scan rate used for the DSC analysis, deswelling kinetics should not impact the DSC result and true thermodynamic swelling features should be captured throughout the phase transition. This conclusion is supported by a range of other observations. Good reversibility is observed for each functionalized microgel, slower scans (5 °C/h) produce the same DSC curve shape ((1% error at each temperature),

and an increase in the postscan thermostat time from 20 to 200 min induces no significant change in the overall transition enthalpy or the shape of the thermogram. To fit the thermograms to independent thermodynamic phase transitions, two transition peaks are required: a large transition typically centered near the center of the overall thermogram and a lower-intensity transition centered at higher temperatures. Table 4 shows the midpoint transition temperatures calculated for each of the two transitions isolated. Deconvolution was performed by fitting the experimental data to a non-two-state model using the Levenberg-Marquardt nonlinear least-squares fitting method in the Microcal VP-DSC software. The key parameter of interest from this analysis is Tm2/Tm1, the ratio between the midpoint transition temperatures of the two deconvoluted transition peaks. Under both basic and acidic conditions, the Tm2/Tm1 ratio increases systematically as functional groups become more radially localized at the microgel surface. This general trend can be understood by interpreting the two deconvoluted transitions as representing a core-shelltype collapse induced by the local compositional inhomogeneities within the microgel. As functional monomers are localized toward the more lightly cross-linked microgel surface, the difference between the volume phase transition temperatures of the NIPAM-rich core and the functional monomer-rich shell increases, increasing the Tm2/Tm1 ratio. At pH 3.5, the VPTT difference is related primarily to the different monomer hydrophilicities, while, at pH 10, the presence of ionized monomers accounts for the VPTT difference. Thus, the Tm2/Tm1 ratios can generally be used as a quantitative parameter defining the radial functional group distribution. It must be noted, however, that the ionized VAA-NIPAM microgel does not explicitly follow this trend. The slightly lower than expected Tm2/Tm1 ratio observed in VAA-NIPAM is likely a function of the significantly lower local cross-link density present in the VAA-NIPAM microgel due to the chain-transfer mechanism of monomer incorporation.15 This reduces the elastic resistance to the collapse of the ionized network and lowers the effective transition temperature of the functional group-rich shell. The VAA-NIPAM microgel also exhibits unique behavior in terms of the shape of its DSC thermogram at pH 10. A second macroscopic DSC transition is observed prior to the major phase transition in the thermogram of the ionized VAA-NIPAM microgel. Figure 7 shows the deconvoluted DSC thermograms of VAA-NIPAM microgels containing three different VAA contents, while Table 5 shows the corresponding thermodynamic properties of the deconvoluted transitions. The smaller, lowtemperature transition is labeled as transition 3. Assuming the energy per NIPAM monomer involved in the aggregation is equal for each functionalized microgel, the van’t Hoff enthalpy associated with each of the transitions (∆Hv) is proportional to the cooperativity of the associated phase transition. Several comments can be made regarding these data in relation to the additional transition 3 signal observed in the VAA-NIPAM thermograms. The enthalpy associated with the additional transition is low, and no statistically significant

TABLE 4: Midpoint Transition Temperatures (Tm) for Each of the Two Major Deconvoluted Transitions in Functionalized Microgels in Both the Protonated and Ionized State pH 3.5

pH 10

microgel

Tm1 (°C)

Tm2(°C)

Tm2/Tm1

Tm1 (°C)

Tm2 (°C)

Tm2/Tm1

MAA-NIPAM AA-NIPAM VAA-NIPAM FA-NIPAM MA-NIPAM

34.3 ( 0.1 34.9 ( 0.1 33.9 ( 0.1 35.9 ( 0.1 35.7 ( 0.1

34.4 ( 0.1 37.4 ( 0.1 37.4 ( 0.1 40.0 ( 0.1 39.5 ( 0.1

1.00 ( 0.01 1.07 ( 0.01 1.10 ( 0.01 1.11 ( 0.01 1.11 ( 0.01

42.5 ( 0.1 52.1 ( 0.1 51.4 ( 0.1 48.4 ( 0.1 46.0 ( 0.1

46.2 ( 0.1 62.1 ( 0.1 59.7 ( 0.3 59.5 ( 0.4 58.9 ( 0.1

1.09 ( 0.01 1.19 ( 0.01 1.16 ( 0.01 1.23 ( 0.01 1.28 ( 0.01

1340 J. Phys. Chem. B, Vol. 111, No. 6, 2007

Hoare and Pelton

Figure 8. Ratio between the change in the experimentally measured light scattering intensity and the change in the theoretical Mie light scattering intensity for each of the functionalized microgels as a function of temperature (pH 10).

Figure 7. DSC thermograms for VAA-NIPAM microgels containing different VAA contents (at pH 10): (a) 2.7 mol % VAA; (b) 3.3 mol % VAA; (c) 6.5 mol % VAA

particle size change is observed over the temperature range of the transition. This observation suggests that the process associated with this transition must be occurring primarily on the microscopic scale and is likely primarily driven by entropic considerations. The high (∆Hv)3 value indicates that the transition is highly cooperative, suggesting that large-scale polymer chain and/or solvent restructuring may be occurring. The transition temperature for transition 3 is also in range of the LCST of unmodified poly(NIPAM), suggesting that the transition may be associated with the local collapse of a NIPAM-

rich chains in the VAA-NIPAM microgel. Finally, the overall enthalpy related with the additional transition systematically decreases as the functional group loading decreases; no additional transition whatsoever is resolved for the 2.7 mol % VAA microgel. Thus, the transition appears to arise in response to an increase in the local charge density within the gel. The dimensionless plotting strategies developed in our previous work19 can be applied to explore the possible mechanistic reasons for this additional transition peak. Figure 8 compares the measured change in light scattering intensity at 90° and the theoretical change in the Mie light scattering intensity at the same angle for each of the microgels at pH 10. The theoretical Mie calculations are performed assuming that each microgel is homogeneous, allowing the overall refractive index of the microgel to be calculated as an additive fraction of the water and polymer fractions inside the microgel. Since the light scattering intensity increases as a near-exponential function of the refractive index according to Mie theory, an increase in microgel heterogeneity leads to an increase in the experimental light scattering intensity relative to the Mie prediction. Comparing Figure 8 with the hydrodynamic diameter profiles in Figure 3, each microgel shows the same general trend: a gradual decrease in the experimental to theoretical light scattering ratio at temperatures well below the VPTT, a sharp increase just below the VPTT, and (for those microgels whose transitions are largely complete by the 70 °C maximum temperature on our dynamic light scattering system) a further decrease above the VPTT toward a steady state. The NIPAM microgel profile, also representative of each of the pH 3.5 microgel profiles (not shown), most clearly shows each of these stages. This general profile suggests that the chain reorientation process observed in the pH 10 VAA-NIPAM DSC thermograms may be occurring via the entropically driven process of microphase separation. Microphase separation has previously been reported in the context of bulk functionalized PNIPAM systems21 and is illustrated schematically in Scheme 1. As the temperature is increased, the phase transition temperature of the NIPAM-rich chains in the microgel is reached, creating a driving force for polymer chain aggregation. However, gel collapse reduces the volume available for counterion diffusion

Functionalized Microgel Thermal Phase Transitions

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1341

TABLE 5: Thermodynamic Properties Calculated on the Basis of the Three-State Model for the DSC Thermograms of VAA-NIPAM Microgels with Differing Carboxylic Acid Contents (CU ) Cooperative Unit) no.

property

6.5 mol % -COOH

3.3 mol % -COOH

2.7 mol % -COOH

3

Tm (°C) ∆H (kJ/(g of microgel)) ∆Hv (kJ/(mol of CU)) Tm (°C) ∆H (kJ/(g of microgel)) ∆Hv (kJ/(mol of CU)) Tm (°C) ∆H (cal/(g of microgel)) ∆Hv (×104 kcal/(mol of CU))

35.6 ( 0.1 0.38 ( 0.01 900 ( 20 51.4 ( 0.1 2.2 ( 0.1 280 ( 20 59.7 ( 0.2 2.8 ( 0.1 180 ( 10

33.9 ( 0.1 0.15 ( 0.01 830 ( 10 49.1 ( 0.1 5.0 ( 0.1 280 ( 20 57.8 ( 0.2 6.1 ( 0.2 160 ( 10

no transition obsd no transition obsd 42.5 ( 0.1 10.6 ( 0.2 390 ( 20 48.0 ( 0.1 10.8 ( 0.2 200 ( 10

1 2

SCHEME 1: Schematic Diagram of Gel Morphologies across the Volume Phase Transition. (a) Swollen Equilibrium State (T < VPTT); (b) Microphase Separated State, Just below the Macroscopic Onset VPTT; (c) Collapsed Equilibrium State (T > VPTT)

within the polyelectrolyte microgel matrix, an entropic cost of gel deswelling in ionized gels. In response, local swelling occurs in regions surrounding charged residues, while local collapse occurs in the NIPAM-rich regions, resulting in little or no net change in the gel dimensions despite the occurrence of significant internal gel restructuring. Since the Mie calculations are based on the measured particle size (which is constant just prior to the main phase transition region), the formation of locally collapsed, heterogeneous regions within the microgel with a higher local refractive index yield an increase in the experimental to theoretical scattering ratio, as observed in Figure 8 prior to the onset VPTT. The ∼20-30 nm correlation length (i.e., blob size) of the heterogeneities typically observed via neutron scattering analysis of PNIPAM-based microgels would enable ready observation of these locally collapsed regions via light scattering analysis.22 The concept of microphase separation is consistent with the appearance of the additional lower-temperature transition in the DSC thermograms of ionized VAA-functionalized microgels. VAA is incorporated into microgels predominantly via chain transfer to create a “hairy” microgel with functional groups primarily localized on polymer chain ends (i.e., functional monomers are incorporated primarily as end groups instead of within the polymer chains).15 Thus, the polymer chains supporting the “hairs” would primarily be composed of NIPAM residues, facilitating local chain aggregation at a significantly lower temperature than that required for bulk microgel deswelling. This is consistent with the low ∼35 °C transition temperature associated with the additional transition in Figure 7b,c. The elastic resistance to the aggregation of these NIPAM-rich subchains is minimized by the relatively low degree of crosslinking in the vicinity of the VAA residues, a result of chain transfer effectively “undoing” cross-linking points in the microgel by creating additional polymer chain ends. Thus, microphase separation could occur to a much greater extent in VAA-NIPAM compared to the other functionalized microgels in which functional groups are incorporated randomly within a more highly cross-linked polymer network. This explains why the additional DSC transition is observed only in the VAANIPAM thermogram. As the functional monomer loading

decreases, the driving force for microphase separation (i.e., the charge density of the microgel) would decrease; furthermore, less “de-cross-linking” would occur in the gel network due to the reduced degree of chain transfer, reducing the mobility of the subchains to microphase separate. Correspondingly, the additional DSC thermogram peak disappears as the VAA content of the microgel is reduced. Finally, the evidently large number of chain rearrangements which must occur on a local level to form microphase-separated domains would account for the high cooperativity of the additional DSC transition as inferred by van’t Hoff analysis. Thus, both turbidimetric and calorimetric analysis suggests that microphase separation occurs in functionalized microgel systems. Future reports will address the use of small-angle neutron scattering to confirm the presence of microphase-separated domains in these functionalized microgel systems. Conclusions (1) Transition enthalpies of functionalized PNIPAM microgels are directly correlated to the hydrophilicity of functional comonomers in the protonated state and the degree of gel deswelling across the thermal phase transition in the ionized state. (2) The peak width at half-height of the DSC thermograms decreases systematically as the average length of NIPAM-only monomer segments in the microgel (as predicted by copolymerization kinetics modeling) increases. (3) Under both basic and acidic conditions, functionalized microgel DSC thermograms must be deconvoluted into at least two theoretical thermodynamic transitions due primarily to the influence of the heterogeneous radial functional group distributions on the local phase transition temperatures within the gel matrix. (4) The ratio of the midpoint transition temperatures associated with the two deconvoluted thermodynamic phase transitions from the DSC thermograms is a useful quantitative diagnostic of the radial functional group distribution in microgels; the higher the temperature ratio, the more surface-localized the radial functional group distribution.

1342 J. Phys. Chem. B, Vol. 111, No. 6, 2007 (5) The presence of an additional, low-temperature macroscopic transition in the VAA-NIPAM microgel appears to be related to microphase separation prior to the macroscopic phase transition. Acknowledgment. Drs. Richard and Raquel Epand are gratefully acknowledged for the use of their DSC instrument. The Natural Sciences and Engineering Research Council of Canada (NSERC) is acknowledged for funding. Supporting Information Available: MathCad worksheet used to perform microgel Mie scattering calculations and full deconvolutions of each of the functionalized microgel thermograms at both pH 3.5 and pH 10. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247-256. (2) Pelton, R. H. AdV. Colloid Interface Sci. 2000, 85, 1-33. (3) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352-4356. (4) Inomata, H.; Goto, S.; Saito, S. Macromolecules 1990, 23, 48874888. Shibayama, M.; Morimoto, M.; Nomura, S. Macromolecules 1994, 27, 5060-5066. (5) Grinberg, N. V.; Dubovik, A. S.; Grinberg, V. Y.; Kuznetsov, D. V.; Makhaeva, E. E.; Grosberg, A. Y.; Tanaka, T. Macromolecules 1999, 32, 1471-1475. (6) Murray, M.; Rana, F.; Haq, I.; Cook, J.; Chowdhry, B. Z.; Snowden, M. J. J. Chem. Soc., Chem. Commun. 1994, 1803-1804.

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