Stimuli-Responsive Surface Localized Ionic Cluster (SLICs) Formation

Langmuir 2005, 21, 6753-6761

6753

Stimuli-Responsive Surface Localized Ionic Cluster (SLICs) Formation from Nonspherical Colloidal Particles David J. Lestage and Marek W. Urban* School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science Research Center, The University of Southern Mississippi, Hattiesburg, Mississippi 39406 Received January 11, 2005. In Final Form: April 8, 2005 Structural features of phospholipids provide a unique opportunity for utilizing these amphiphilic species to stabilize the synthesis of colloidal dispersion particles by controlling concentration levels relative to dispersion synthesis components. 1,2-Bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DCPC) phospholipid was utilized as cosurfactant in the synthesis of sodium dioctyl sulfosuccinate (SDOSS) stabilized methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal dispersions. Aqueous dispersions containing various concentration levels of DCPC result in the formation of cocklebur particle morphologies, and when prepared in the presence of Ca2+ and annealed at various temperatures, stimuli-responsive behaviors of coalesced films were elucidated. The formation of surface localized ionic clusters (SLICs) at the film-air (F-A) and film-substrate (F-S) interfaces is shown to be responsive to concentration levels of DCPC, Ca2+/DCPC ratios, and temperature. These studies show that it is possible to control stratification and mobility to the F-A and F-S interfaces during and after coalescence. Using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and internal reflection infrared imaging (IRIRI) spectroscopies, molecular entities responsible for SLIC formation were determined. These studies also show that stimuliresponsive behaviors during film formation can be controlled by colloidal solution morphologies and synergistic interactions of individual components.

Introduction Previous studies have shown that, when 1,2-Bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DCPC) was solubilized in an ethanol/H2O solution and heated above 38 °C followed by slow cooling, tubules with helical substructural ribbonlike features were formed1-3 which exhibit multilamellar structures.1,4-6 While their formation depends on solvent environments, when utilized as cosurfactant at low concentration levels with sodium dioctylsulfosuccinate (SDOSS) in colloidal dispersion synthesis,7 unique particle morphologies were formed as a result of SDOSS/DCPC ionic interactions at the particle interfaces. At similar concentration levels, hydrogenated soybean phosphocholine (HSPC) phospholipid combined with SDOSS surfactant provided methyl methacrylate/ n-butyl acrylate (MMA/nBA) copolymers with ionic stimuliresponsive behaviors allowing the formation of surface localized ionic clusters (SLICs)8,9 at the film-air (F-A) interface. In other studies10 utilizing 1-myristoyl-2-hydroxy-sn-glycero-3-phosphocholine (MHPC) at low concentration levels, p-MMA/nBA dispersions exhibited controllable responsiveness at the F-A interface stimulated by ionic strength changes of the aqueous phase. Interestingly enough, at higher concentrations, formation of SLICs resembling physiological lipid rafts11 was detected at the F-A interface. * To whom all correspondence should be addressed. (1) Yager, P. Biophys. J. 1985, 48, 899-906. (2) Yager, P.; Schoen, P. E. Mol. Cryst. Liq. Cryst. 1984, 106, 371-381. (3) Yager, P. Biophys. J. 1986, 49, 320. (4) Thomas, B. N.; Lindemann, C. M.; Clark, N. A. Phys. Rev. E 1999, 59, 3040-3047. (5) Thomas, B. N.; Corcoran, R. C.; Cotant, C. L.; Lindemann, C. M.; Kirsch, J. E.; Persichini, P. J. J. Am. Chem. Soc. 1998, 124, 12178. (6) Thomas, B. N.; Safinya, C. R.; Plano, R. J.; Clark, N. A. Science 1995, 267, 1635. (7) Lestage, D. J.; Urban, M. W. Langmuir 2005, submitted. (8) Dreher, W. R.; Urban, M. W.; Zhao, C. L.; Porzio, R. S. Langmuir 2003, 19, 10254-10259. (9) Lestage, D. J.; Urban, M. W. Langmuir 2004, 20, 7027-7035. (10) Lestage, D. J.; Urban, M. W. Langmuir 2005, 21, 2150-2157.

In view of these studies, and considering the fact that lipid rafts may serve numerous biological applications, concentrations of phospholipids as well as particle morphology of colloidal dispersions appear to be the primary factors which allow the formation of unique films with stimuli-responsive characteristics. Particularly, DCPC provides the ability to generate nonspherical colloidal particles with cocklebur-shape morphologies which, at elevated concentration levels, may generate desirable properties of coalesced films. For example, DCPC phospholipid contains photopolymerizable moieties which may be incorporated in the synthesis of p-MMA/nBA colloidal dispersions stabilized with SDOSS cosurfactant. The primary objectives of the present efforts are to determine film formation processes resulting from the presence of DCPC at different concentration levels. These species are capable of forming tubules below 38 °C, which in the presence of colloidal particles are affixed to the particle surface, thus forming cocklebur particle morphologies. While the presence of amphiphilic DCPC will assist in colloidal particle surface stability, its ability to assemble tubule structures and photopolymerize1,3,12,13 opens up a number of possibilities for creating unique particle interfaces7 with inter- and intraparticle adhesion characteristics as well as stimuli-responsive behaviors in the presence of Ca2+ ions and at elevated temperatures. Experimental Section MMA and nBA monomers (Aldrich Chem. Co.) were copolymerized using a semicontinuous process outlined elsewhere14 and adapted for small-scale polymerization, as reported elsewhere.9 The reaction flask was placed in a water bath set at 72 (11) Pike, L. J. Biochem. J. 2004, 378, 281-292. (12) Svenson, S.; Messersmith, P. B. Langmuir 1999, 15, 4464-4471. (13) Thomas, B. N.; Lindemann, C. M.; Corcoran, R. C.; Cotant, C. L.; Kirsch, J. E.; Persichini, P. J. J. Am. Chem. Soc. 2002, 124, 12271233. (14) Davis, S. D.; Hadgraft, J.; Palin, K. J. Encylopedia of Emulsion Technology; Marcel Dekker: 1985; Vol. 2.

10.1021/la050084v CCC: $30.25 © 2005 American Chemical Society Published on Web 06/11/2005

6754

Langmuir, Vol. 21, No. 15, 2005

Lestage and Urban

Table 1. Particle Size and Composition of Random Copolymerized p-MMA/nBA Colloidal Dispersions with SDOSS/DCPC Mixture 0.08a

0.17a

0.3a

0.5a

1.0a

p-(MMA/nBA) SDOSS/DCPC




composition

p-(MMA/nBA) SDOSS


DDI (%) methyl methacrylate(%) n-butyl acrylate(%) SDOSS(%) K2S2O8 (%) DCPC(%)

67.50 15.60 15.60 0.780 0.520 0.000

67.50 15.60 15.60 0.767 0.507 0.026

solids (%) particle size (nm)

32.5 154

32.5 161

Components: 67.50 15.60 15.60 0.748 0.500 0.052 32.5 162

67.50 15.60 15.60 0.716 0.480 0.104

67.30 15.70 15.70 0.690 0.450 0.160

67.30 15.60 15.60 0.690 0.498 0.312

32.5 159

32.700 146

32.700 148

CaCl2/DCPC

concn (mM)

concn (mM)

concn (mM)

concn (mM)

concn (mM)

concn (mM)

1.0/1.0

0.0

0.7

1.4

2.7

4.1

8.1

a

w/w % (of total monomer) of DCPC incorporated in colloidal dispersion synthesis.

2 h and spectroscopic analysis was conducted at 25 °C. Microscopic attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy measurements were conducted on the filmair (F-A) and film-substrate (F-S) interfaces using a Bio-Rad FTS-7000 FT-IR single-beam spectrometer with 4 cm-1 resolution and a 2 mm Ge crystal with a 45° angle maintaining constant contact pressure between the crystal and the specimens. All spectra were corrected for spectral distortions using software for the Urban-Huang algorithm.16

Results The objective of these studies is to develop polymeric films with stimuli-responsive properties. For that reason, p-MMA/nBA colloidal particles were prepared using 0.08, 0.17, 0.3, 0.5, and 1.0 w/w % DCPC as cosurfactant with SDOSS. Their structures are shown below. Figure 1. TEM micrographs at 12,000x of p-MMA/nBA colloidal particles containing (A) 0.08; (B) 0.17; (C) 0.3; (D) 0.5; (E, F) 1.0 w/w % DCPC; and a scanning electron micrograph at 10,000x of neat DCPC tubules. °C and purged using N2 gas. The reaction flask was charged with 10 mL of DDI water, and while purging for 30 min, the content was stirred at 350 rpm. At this point, 20 w/w % potassium persulfate initiator (KPS: Aldrich) solution (DDI and KPS) and 10 w/w % of the pre-emulsion solution (DDI, SDOSS, monomers, and phospholipids, if applicable) were added. The SDOSS levels were in the range of 2.5 wt % of total monomer, thus generating 26.0 mM aqueous solutions that are above the CMC of SDOSS (0.10-0.14 wt % of water,15 or 2.0 mM). After 30 min, pre-emulsion and initiator solutions were fed at 0.394 and 0.095 g/min into the vessel over a period of 3 and 3.5 h, respectively. Upon completion, the reaction continued for 1 h after which time the temperature was raised to 85 °C. Upon cooling, the emulsion was filtered twice, and particle size analysis was performed using a Microtrac Nanotrac particle size analyzer model UPA250. In a typical experiment, the standard deviation was (10 nm. Colloidal dispersion particle morphologies were analyzed using a Zeiss EM 109-T transmission electron microscope (TEM) in which colloidal dispersions were diluted at a 20:1 vol. ratio (DDI H2O: dispersion) and deposited on Formvar coated copper TEM grids. CaCl2 (Aldrich) aqueous solutions were prepared at 1.0/1.0 CaCl2/ DCPC molar ratios by solubilizing the reagent in DDI water at 0.7, 1.4, 2.7, 4.1, and 8.1 mM. Table 1 provides details of colloid formulations and the resulting particle size. Such colloidal dispersions were cast onto a poly(tetrafluoroethylene) (PTFE) mold to achieve free-standing colloidal films with an approximate film thickness of 100 µm. The films were allowed to coalesce for 72 h in a controlled environment at 60% relative humidity (RH) and 23 °C. Annealing experiments were conducted in such a way that each specimen was exposed to the desired temperature for

To set the stage, Figure 1 illustrates TEM micrographs of the resulting p-MMA/nBA particles ranging in size from 152 to 165 nm, and Table 1 provides details of the colloid formulations. As seen, at low concentration levels ranging from 0.08 to 0.17 w/w % DCPC in Figure 1, A and B, the particles are prepared with electron-rich entities detected at the particle surfaces. Although we realize that due to TEM limitations these data do not clearly identify the particle morphologies, chemically the same particles as large as 5 µm exhibit a cocklebur shape.7 Thus, it is assumed that the electron density changes shown in Figure 1 and the ability of DCPC to form tubules2 (Figure 1, G) result in cocklebur morphologies. These data show that increased concentrations up to 0.3 w/w % (Figure 1, C) result in gradually pronounced cocklebur morphologies manifested by a higher coverage of particle surfaces, but for 0.5 w/w % (D) and 1.0 w/w % (E) DCPC, the presence of densely packed surface entities is detected. It should be noted that a fraction of particles prepared at 1.0 w/w

(15) Industries, C.; Cytec Industries: Emulsion Polymers and Specialties: Stamford, CT, 2002.

(16) Urban, M. W. Attenuated Total Reflectance Spectroscopy of Polymers - Theory and Practice: Washington, D. C, 1989.

SLIC Formation from Nonspherical Colloidal Particles

Langmuir, Vol. 21, No. 15, 2005 6755

% DCPC (Figure 1, E) tend to adhere to form clusters with external tubule structures projecting away from the surface and one example of aggregate formation is illustrated in Figure 1, F. Such particles were allowed to coalesce to form uniform films. In an effort to identify processes at the F-A and F-S interfaces after film formation we utilized ATR-FTIR spectroscopy. As shown in the past, this approach allows us to effectively obtain molecular level information from these interfaces. Due to the inherent ability of DCPC to self-assemble into helically wound tubules (38 °C) we focused on the effect of temperature on the migration of DCPC in p-MMA/nBA colloidal films and its interactions with SDOSS and mobility of both components. The remaining portion of the Results is organized into the following sections: Responses at the F-A Interface, Responses at the F-S Interface, and Ionic Strength of Colloidal Solutions and Film Formation. Responses at the F-A Interface. Colloidal films containing 0.08, 0.17, 0.3, 0.5, and 1.0 w/w % DCPC were annealed at temperatures ranging from 25 to 150 °C and analyzed using ATR-FTIR in transverse magnetic (TM) and electric (TE) polarizations from which dichroic ratios (R)17 were determined and are listed in Table 2. Figure 2 illustrates ATR-FTIR spectra recorded from the F-A interface of coalesced films containing 0.08 w/w % DCPC and annealed from 25 to 150 °C. As seen, the bands at 1310 and 1300 cm-1 are detected, indicating the presence of phospholipid entities in the form of surface localized ionic clusters (SLICs)10 at the F-A interface. However, the detection of the bands at 1272 and 1255 cm-1, which in the previous studies10 indicated temperature-induced crystalline forms not present in individual synthesis components, reveals the presence of PO4- entities of DCPC hydrophilic groups.18 Furthermore, as shown in Trace A′ of Figure 2, the band at 1061 cm-1 due to P-O-C entities19 exhibits higher magnitude in TM polarization, thus lower R values (Table 2), which reveal a preferential perpendicular orientation of these species at the F-A interface.17 Upon annealing at 50 °C, the bands at 1272, 1255, and 1061 cm-1 shown in Traces B/B′ are enhanced, and higher R values indicate a preferential parallel orientation of the PdO moieties at 1272 and 1255 cm-1. The bands at 1310, 1300, 1272, 1255, and 1061 cm-1 are not detected above 50 °C, but at 125 °C the 1310, 1255, and 1061 cm-1 bands are present (Traces E/E′). It should be noted that analysis of polarized spectra allows us to identify preferential orientation of molecular segments of each species. For the purpose of these studies we focused on hydrophilic entities because these species are primarily responsible for DCPC and SDOSS interactions. ATR-FTIR spectra recorded from the F-A interface of p-MMA/nBA films containing 0.17 w/w % DCPC (not shown) revealed the presence of the 1310, 1300, 1272, 1255, and 1061 cm-1 bands at 25 °C, but suppressed band intensities are detected compared to the films containing 0.08 w/w % (Figure 2, Traces A/A′). Annealing at temperatures up to 75 °C reveals an increase in these bands, but at higher temperatures these bands are not detected. At the same time, the band at 1046 cm-1 is elucidated upon annealing at 150 °C, indicating the presence of

Na+SO3-- - -H2O interactions20 of SDOSS hydrophilic ends at the F-A interface. In p-MMA/nBA colloidal films containing 0.3 w/w % DCPC spectral intensities (not shown) due to PO4- entities at 1272, 1255, and 1061 cm-1 are not detected at the F-A interface for p-MMA/nBA films annealed at the temperature range from 25 to 150 °C. However, upon annealing at 75 °C, the band at 1123 cm-1 is detected in the TE and TM polarizations, indicating the presence of C-O-C21 linkages of DCPC, and upon annealing at 150 °C, Na+SO3-- - -H2O interactions of SDOSS manifested by the 1046 cm-1 band are again detected at the F-A interface. As was indicated in the Experimental Section, all spectroscopic analysis was conducted at 25 °C. ATR-FTIR spectra of p-MMA/nBA films shown in Figure 3, Traces A-D′, illustrate that at 0.5 w/w % DCPC concentration levels, crystalline species represented by the bands at 1310 and 1300 cm-1 are detected at the F-A interface after annealing at 50 °C. Similarly to Figure 2, Traces B/B′ recorded from the F-A interface of films containing 0.08 w/w % DCPC and annealed at 50 °C, the PdO entities are detected at 1272 and 1255 cm-1 with the R > 1.0 (Table 2), thus indicating a preferential parallel orientation of amphiphilic entities, whereas the P-O-C segments represented by the band at 1061 cm-1 are normal to the F-A interface with R ) 0.28. However, at 75 °C, these bands are not observed, but reappear at 100 °C (Traces D/D′) with reduced intensities, whereas in films containing 0.08 w/w % DCPC, they were present up to 75 °C annealing temperature and were not detected again until 125 °C. Figure 4, Traces A/A′, represent ATR-FTIR spectra of films containing an excess of DCPC at 1.0 w/w %. As seen, such films exhibit SLIC entities at ambient conditions, as demonstrated by the presence of the bands at 1310 and 1300 cm-1, PdO vibrations at 1272 and 1255 cm-1, and P-O-C entities at 1061 cm-1. At higher temperatures, however, as seen in Traces B-C′, these bands are no longer present at the F-A interface, but the PdO segmental vibration bands at 1272 and 1255 cm-1 have consolidated into one band at 1264 cm-1. Responses at the F-S Interface. As we recall the analysis of the F-A interface, when p-MMA/nBA films containing 0.08 w/w % DCPC were annealed at 100 °C, SLICs were not observed at the F-A interface (Figure 2, Traces D/D′), but are detected at the F-S interface. These results indicate that mobility to the F-A and F-S interfaces depends on DCPC concentration levels and temperature. Figure 5, Traces A/A′, show the presence of the band at 1300 cm-1 with the R ) 2.0 (Table 2), which indicates a preferential parallel orientation of SLIC molecular segments, but the band at 1310 cm-1 is not detected. Furthermore, the bands at 1272, 1255, and 1061 cm-1 are enhanced, revealing the presence of PO4- species with preferential perpendicular orientation to the F-S interface as R < 1.0, but upon annealing above 100 °C, these entities are not detected. When annealed above the melting temperature of SDOSS (150 °C), bands at 1210 and 1046 cm-1 are detected, which indicate the presence of SO3-Na+ entities of SDOSS.22 ATR-FTIR spectra recorded from the F-S interface of p-MMA/nBA films containing 0.17 w/w % DCPC (not

(17) Hobbs, J. P.; Sung, C. S. P.; Krishnan, K.; Hill, S. Macromolecules 1983, 16, 193-199. (18) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6 ed.; John Wiley and Sons: New York, 1998. (19) Lambert, J. B.; Shurvell, H. F.; Lightner, D.; Cooks, R. G. Introduction to Organic Spectroscopy; Macmillan Publishing Company: New York, 1987.

(20) Evanson, W.; Urban, M. W. J. Appl. Polym. Sci. 1991, 42, 22872296. (21) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: 1991. (22) Evanson, W.; Thorstenson, T. A.; Urban, M. W. J. Appl. Polym. Sci. 1991, 42, 2297-2307.

6756


Lestage and Urban

Table 2. Dichroic Ratios (Ra) of Selected IR Bands dichroic ratios (R) of specific molecular segments annealing T (°C)

SLIC 1310 cm-1

SLIC 1300 cm-1

PdO 1272 cm-1

PdO 1255 cm-1

P-O-C 1061 cm-1

SO31046 cm-1

25 50 75 100 125 150

2.000 0.667 1.000 n/a 1.000 n/a

2.100 0.654 1.000 1.000 n/a n/a

0.08 w/w % DCPC F-A 1.000 4.600 1.011 n/a 0.833 n/a

1.000 4.212 1.030 n/a 1.000 n/a

0.331 0.161 n/a n/a 0.500 1.000

n/a n/a 1.000 1.000 1.000 0.895

25 50 75 100 125 150

n/a 1.000 1.000 1.000 1.000 n/a

n/a 1.300 1.299 2.000 1.000 n/a

0.08 w/w % DCPC F-S n/a 1.110 1.000 0.625 1.000 n/a

n/a 1.121 1.000 0.588 1.000 n/a

1.000 1.351 0.909 0.869 1.000 1.000

n/a 1.000 1.000 1.000 n/a 1.000

25 50 75 100 125 150

1.000 1.000 1.000 1.000 1.000 n/a

1.000 1.000 1.000 1.000 1.000 n/a

0.17 w/w % DCPC F-A 1.112 0.833 2.100 1.000 n/a n/a

1.180 0.900 2.150 1.000 n/a n/a

0.594 1.310 0.500 1.000 1.000 n/a

n/a n/a 1.000 1.000 1.000 1.200

25 50 75 100 125 150

1.100 1.000 1.000 1.000 n/a n/a

1.000 1.000 1.000 1.000 n/a n/a

0.17 w/w % DCPC F-S 1.000 1.000 1.000 n/a n/a n/a

1.000 1.000 1.000 n/a n/a n/a

0.990 1.000 1.000 1.000 1.000 1.000

n/a n/a 1.000 1.000 1.000 1.150

25 50 75 100 125 150

n/a n/a n/a n/a n/a n/a


0.30 w/w % DCPC F-A n/a n/a n/a n/a n/a n/a


n/a n/a n/a n/a n/a 1.000

1.000 1.000 1.000 1.000 n/a 1.111

25 50 75 100 125 150



0.3 w/w % DCPC F-S n/a n/a n/a n/a n/a n/a


n/a n/a n/a n/a n/a 1.000

1.000 n/a n/a n/a n/a 1.012

25 50 75 100 125 150

n/a 0.823 n/a 1.000 n/a n/a

1.000 0.833 n/a 1.000 n/a n/a

0.50 w/w % DCPC F-A n/a 1.900 n/a 1.210 n/a n/a

n/a 1.833 n/a 1.150 n/a n/a

1.000 0.278 n/a 0.357 n/a 1.000

n/a n/a n/a 1.000 1.000 1.210

25 50 75 100 125 150

n/a 1.000 1.000 1.000 1.100 n/a

n/a 1.000 2.000 1.000 1.430 n/a

0.50 w/w % DCPC F-S n/a 0.833 0.833 0.741 1.000 n/a

n/a 0.834 0.833 0.833 1.000 n/a

n/a 2.200 1.000 1.000 1.400 1.000


25 50 75 100 125 150

0.909 n/a n/a 0.833 n/a n/a

0.830 n/a n/a 0.909 n/a n/a

1.0 w/w % DCPC F-A 0.833 n/a n/a 1.250 1.000 n/a

0.870 n/a n/a 1.311 1.000 n/a

1.000 n/a n/a 1.000 n/a 1.000

n/a n/a n/a n/a 1.000 1.000

25 50 75 100 125 150

3.000 1.600 2.000 1.800 1.000 n/a

3.200 1.850 2.230 1.630 1.100 n/a

1.0 w/w % DCPC F-S 1.000 1.150 1.000 1.000 1.090 n/a

1.000 1.222 1.000 1.000 1.079 n/a

0.909 0.333 0.495 0.833 0.769 1.000

n/a n/a n/a n/a 1.000 1.300


Langmuir, Vol. 21, No. 15, 2005 6757

Table 2 (Continued) 1.0/1.0 CaCl2/DCPC

1310 cm-1

dichroic ratios (R) of specific molecular segments 1300 cm-1 1272 cm-1 1255 cm-1 1061 cm-1

F-A F-S

n/a 1.510

n/a 1.750

F-A F-S

n/a n/a

n/a 1.000

F-A F-S

n/a n/a

F-A F-S F-A F-S

0.08 w/w % DCPC n/a 0.500

1046 cm-1

n/a 0.550

n/a 1.430

1.000 n/a

0.17 w/w % DCPC n/a 1.000

n/a 1.000

1.000 1.000

1.000 n/a

n/a n/a

0.30 w/w % DCPC n/a n/a

1.020 1.130

1.450 1.020

1.550 2.000

n/a 1.030

n/a 1.040

0.50 w/w % DCPC n/a 0.971

n/a 1.000

n/a 1.200

1.000 n/a

n/a n/a

n/a n/a

1.0 w/w % DCPC 1.000 1.000

1.000 1.000

n/a n/a

n/a 1.000

a R ) A /A , where A and A are band areas for parallel (TE) and perpendicular (TM) polarizations. R values were obtained by ratioing | ⊥ | ⊥ the baseline-corrected IR bands of interest. For each band, the same wavenumber range was used to determine the band area for TE and TM polarizations.

Figure 2. Polarized ATR-FTIR spectra at the F-A interface depicting 1350-950 cm-1 region of MMA/nBA copolymer films containing 0.08 w/w % DCPC annealed at (A) 25, TE; (A′) 25, TM; (B) 50, TE; (B′) 50, TM; (C) 75, TE; (C′) 75, TM; (D) 100, TE; (D′) 100, TM; (E) 125, TE; (E′) 125, TM; (F) 150, TE; (F′) 150 °C, TM for 2 h.

shown) revealed no significant spectral features until annealed above 38 °C where a shoulder at 1046 cm-1 due to Na+SO3-- - -H2O vibrations is detected at 75 °C. Elevated temperatures up to 150 °C elicit an increase in magnitude of the 1046 cm-1 band with R > 1.0 which indicates a preferential parallel orientation of S-O entities with respect to the F-S interface. Similarly, the F-S interface spectra (not shown) of p-MMA/nBA films containing 0.3 w/w % DCPC and annealed at elevated temperatures reveal no spectral changes except at 150 °C, where the bands at 1046 and 1025 cm-1 are detected indicating the presence of the S-O and O-C-C22 segmental motions of SDOSS. Figure 6 illustrates ATR-FTIR spectra recorded at the F-S interface of p-MMA/nBA films containing 0.5 w/w % DCPC, and the 1310, 1300, 1272, 1255, and 1061 cm-1 bands are detected which exhibit enhanced magnitude at 100 °C, as shown in Traces A/A′. Recalling Figure 3, Traces

Figure 3. Polarized ATR-FTIR spectra at the F-A interface depicting 1350-950 cm-1 region of MMA/nBA copolymer films containing 0.5 w/w % DCPC annealed at (A) 25, TE; (A′) 25, TM; (B) 50, TE; (B′) 50, TM; (C) 75, TE; (C′) 75, TM; (D) 100, TE; (D′) 100 °C, TM for 2 h.

D/D′ (100 °C) of the F-A interface containing 0.5 w/w % DCPC, these bands are no longer detected. As seen, when annealing from 100 to 150 °C, the bands at 1310 and 1300 cm-1 due to SLICs, and at 1272, 1255, and 1061 cm-1, corresponding to PO4- species, decrease at the F-S interface. At the same time, SDOSS is observed at the F-S interface, which is manifested by the presence of the S-O and O-C-C stretching vibrations at 1046 and 1025 cm-1. Similarly, for higher concentration levels of DCPC at 1.0 w/w %, coalesced films exhibit SLIC and PO4entities at the F-S interface (not shown) as the bands at 1310, 1300, 1272, 1255, and 1061 cm-1 no longer detected at the F-A interface at 50 °C are now present at the F-S interface spectra. Furthermore, as annealing temperature increases, a decrease in the magnitude of these species is observed, and the band at 1025 cm-1 appears upon annealing at 125 °C which is enhanced after annealing at 150 °C, revealing the presence of SDOSS O-C-C species at the F-S interface. With these data in mind, let us focus on the chemical composition of SLICs. For that reason we utilized internal reflection infrared imaging (IRIRI) to obtain chemical information from the F-S interface areas where SLICs

6758


Figure 4. Polarized ATR-FTIR spectra at the F-A interface depicting 1350-950 cm-1 region of MMA/nBA copolymer films containing 1.0 w/w % DCPC annealed at (A) 25, TE; (A′) 25, TM; (B) 50, TE; (B′) 50, TM; (C) 75, TE; (C′) 75 °C, TM for 2 h.

Figure 5. Polarized ATR-FTIR spectra at the F-S interface depicting 1350-950 cm-1 region of MMA/nBA copolymer films containing 0.08 w/w % DCPC annealed at (A) 100, TE; (A′) 100, TM; (B) 125, TE; (B′) 125, TM; (C) 150, TE; (C′) 150 °C, TM for 2 h.

were detected. In particular, cross-polarized optical microscopy revealed bi-refringent entities at the F-S interface of ambiently coalesced 1.0 w/w % DCPCcontaining films with irregular geometries shown in Figure 7A, which were chemically analyzed using IRIRI, and the results are shown in Figure 7B. Spectral maps were collected from the same areas A, B, and C are shown in Figure 7C. As seen, IRIR images in Figure 7B for 1300 (SLIC), 1255 (PdO), and 1061 cm-1 (P-O-C) exhibit similar features manifested by colors which are generated by detection of high and low concentration levels of the species to which they are tuned. For example, the spectral image of the 1046 cm-1 band due to SO3-- - -H2O interactions illustrates low concentration levels of SDOSS where high levels of SLIC and DCPC components are present, and the image of the band at 1148 cm-1 is due to O-C-C

Lestage and Urban

Figure 6. Polarized ATR-FTIR spectra at the F-S interface depicting 1350-950 cm-1 region of MMA/nBA copolymer films containing 0.5 w/w % DCPC annealed at (A) 100, TE; (A′) 100, TM; (B) 125, TE; (B′) 125, TM; (C) 150, TE; (C′) 150 °C, TM for 2 h.

segments of the polymer matrix.21 The spectra were collected from the regions labeled A-D, and areas A and C, corresponding to Traces A and C, contain the highest concentration levels of SLIC (1300 cm-1) and DCPC (1255 and 1061 cm-1) entities. In contrast, areas B and D consist of the polymer matrix represented by the O-C-C vibrations at 1148 cm-1 and SDOSS-H2O interactions (1046 cm-1). Ionic Strength of Colloidal Solutions and Film Formation. Ionic strength of aqueous environments in which colloidal particles are synthesized also plays a significant role.9,10,23,24 Previous studies7 revealed that synthesis of SDOSS/DCPC stabilized p-MMA/nBA dispersions in the presence of CaCl2 limited cocklebur morphology formation as the SO3-- - -+N(CH3)3 ionic interactions between SDOSS and DCPC were disrupted by Ca2+ ions. Furthermore, even post addition of Ca2+ may have very drastic effects on stimuli-responsive behaviors in phospholipid-containing colloidal systems.9,10 With this in mind, SDOSS/DCPC stabilized p-MMA/nBA dispersions containing 0.08, 0.17, 0.3, 0.5, and 1.0 w/w % DCPC were prepared in the presence of a 1.0/1.0 CaCl2/ DCPC molar ratio (Table 1), and the resulting films were analyzed. Figure 8, A, Traces A-B′ illustrate ATR-FTIR spectra recorded from the F-A and F-S interfaces of films of coalesced p-MMA/nBA colloidal particles containing 0.08 w/w % DCPC after preparation in 1.0/1.0 CaCl2/DCPC ionic solution. As seen, the spectra recorded from the F-S interface reveal bands at 1310 and 1300 cm-1, thus indicating the presence of SLICs and the R values in Table 2 reveal a preferential parallel orientation of these species at the F-S interface. At the same time, R values of 0.5 and 0.6 for the bands at 1272 and 1255 cm-1, respectively, indicate that the PdO entities are preferentially aligned normal to the F-S interface, and the enhanced band intensities suggest the formation of an ordered crystalline phase. The intense band detected at 1061 cm-1 with R ) 1.4 is indicative of ordered P-O-C segments with preferential parallel orientation near the F-S interface. (23) Yacoub, A.; Urban, M. W. Biomacromolecules 2003, 4, 52-56. (24) Dreher, W. R.; Urban, M. W. Macromolecules 2003, 36, 1228.


Langmuir, Vol. 21, No. 15, 2005 6759

Figure 7. Optical micrograph (A) depicting SLICs at the F-S interface of 1.0 w/w % DCPC-containing p-MMA/nBA films with IRIR images (B) tuned to 1300, 1255, 1148, 1061, and 1046 cm-1 with regions labeled A-D corresponding to Traces A-D (C).

Figure 8. A - Polarized ATR-FTIR spectra depicting the 1350-950 cm-1 region of MMA/nBA colloidal dispersions containing 0.08 w/w % DCPC and 1.0/1.0 CaCl2/DCPC ionic solution recorded from the (A) F-A, TM; (A′) F-A, TE; (B) F-S, TM; (B′) F-S, TE interface. B - Polarized ATR-FTIR spectra depicting the 1350-950 cm-1 region of MMA/nBA colloidal dispersions containing 0.3 w/w % DCPC and 1.0/1.0 CaCl2/DCPC ionic solution recorded from the (A) F-A, TM; (A′) F-A, TE; (B) F-S, TM; (B′) F-S, TE interface. C - Polarized ATR-FTIR spectra depicting the 1350-950 cm-1 region of MMA/nBA colloidal dispersions containing 0.5 w/w % DCPC and 1.0/1.0 CaCl2/DCPC ionic solution recorded from the (A) F-A, TM; (A′) F-A, TE; (B) F-S, TM; (B′) F-S, TE interface.

Films containing 0.17 w/w % DCPC and prepared in the presence of 1.0/1.0 CaCl2/DCPC ionic strength solution elicit no spectral changes at the F-A or F-S interfaces (not shown). In contrast, at concentration levels of 0.3 w/w % DCPC, coalesced films demonstrate CaCl2 ionicresponsive behaviors in the F-A and F-S interface spectra, as shown in Figure 8B, Traces A-B′. As seen for 1.0/1.0 CaCl2/DCPC ratios, the bands at 1135, 1123, and

1089 cm-1 are detected at the F-A and F-S interfaces, thus indicating the presence of SDOSS SO3- entities along with C-O-C and P-O- segments of DCPC.9,10,21,23 Figure 8C, Traces A-C′, illustrate the spectra of p-MMA/ nBA films containing 0.5 w/w % DCPC recorded from the F-A and F-S interfaces. As shown in Traces B/B′, 1.0/1.0 CaCl2/DCPC ionic strength resulted in the formation of SLICs and facilitates the mobility of PO4- species to the

6760


Figure 9. Schematic depicting stimuli-responsive behaviors of coalesced p-MMA/nBA films containing concentrations of DCPC and annealed at elevated temperatures at the (I) F-A and (II) F-S interface.

F-S interface. Similarly to annealed films containing 0.5 w/w % DCPC, but not CaCl2 (Figure 2, Traces B/B′), the bands at 1310, 1300, and 1061 cm-1 are present. Discussion On the basis of the results described above, it is seen that there is a significant effect of temperature on stimuliresponsive characteristics during film formation. While it is apparent that the concentration of phospholipids provides a unique environment for solution morphologies, the effect of temperature is also very pronounced. As shown in Figures 2-4 after films are formed, the presence of SLICs at the F-A interface is detected, but different temperature regimes result in different film surface morphologies. To schematically illustrate these processes, Figure 9I, A-C, was constructed and illustrates that, for DCPC concentration ranges from 0.08 to 0.17 w/w % (Figure 9, I, A), DCPC and SLICs stratify at the F-A interface at ambient conditions. At higher temperatures, however, they are no longer detected, but SDOSS is elucidated above its melting temperature of 150 °C. In contrast, at 0.3 w/w %, SLICs are not detected regardless of annealing conditions, but DCPC is present when films are annealed above its transition temperature (38 °C) at 75 °C and the presence of SDOSS is detected at 150 °C. This is schematically shown in Figure 9I, B. At 0.5-1.0 w/w % concentration levels, as shown in Figure 9I, C, higher temperatures are required to mobilize SLICs and DCPC, but upon annealing at 150 °C, only SDOSS is present. Again, this is attributed to the fact that SDOSS melts, whereas DCPC does not. Postponing temporarily the discussion concerning this behavior, let us focus on the F-S interface.

Lestage and Urban

In contrast to the F-A interface, the responses of DCPC and SDOSS to temperature are substantially different at the F-S interface. As demonstrated in Figures 5-7, ATRFTIR spectra of p-MMA/nBA films containing 0.08 w/w % DCPC (Figure 5) revealed the presence of a crystalline phase of SLICs detected at the F-S interface, and annealing above 100 °C disrupts these entities, forcing them into the bulk of the film as they are no longer detected. However, when films were annealed at 150 °C, SDOSS is detected at the F-S interface. Similarly, films containing 0.17 w/w % DCPC and annealed at 75 °C reveal the presence of Na+SO3-- - -H2O interactions which are enhanced at higher annealing temperatures. These data indicate that SDOSS migration to the F-S interface is enhanced with elevated temperatures, but only after DCPC reorients in a liquid-crystalline state above 38 °C. In contrast for 0.3 w/w % DCPC, p-MMA/nBA films exhibit no responsiveness at the F-S interface, but upon annealing at 150 °C, SDOSS is detected. Thus, it is apparent that there is an optimal DCPC concentration, which occurs at 0.3 w/w % at which colloidal particles are stabilized by a uniform coverage of tubules (Figure 1C). Consequently, when these particles coalesce, films exhibit diminished free volume and mobility of low-molecular weight species. However, upon annealing at 150 °C, localized SDOSS ionic interactions with DCPC are disrupted as a result of melting, thus increasing its mobility to film interfaces. Figure 9II, A-C summarize the results obtained from the F-S interface of p-MMA/nBA films after annealing at elevated temperatures. For 0.08-0.17 w/w % DCPC concentration levels (A) stratification is observed above 50 °C. However, for 0.08 w/w % DCPC, SLIC and DCPC entities are detected, whereas only SDOSS is detected at 0.17 w/w % DCPC. For 0.3 w/w % DCPC (Figure 9II, B), annealing at 150 °C is required for SDOSS to be mobilized to the F-S interface of the films. However, as depicted in Figure 9II, C, at higher concentration levels of DCPC SLIC and DCPC molecular segments appear when films are annealed above ambient conditions, and again at 150 °C SDOSS migrates to the F-S interface. Considering that the tubule formation temperature is thermally induced (

Stimuli-Responsive Surface Localized Ionic Cluster (SLICs) Formation

Recommend Documents