J. Phys. Chem. B 2008, 112, 14379–14389
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Amphiphilic Siloxane Phosphonate Macromolecule Monolayers at the Air/Water Interface: Effects of Structure and Temperature† Israel Cabasso* and Elvira Stesikova‡ The Michael Szwarc Polymer Research Institute, Department of Chemistry State UniVersity of New York - esf, Syracuse, New York 13210 ReceiVed: February 21, 2008; ReVised Manuscript ReceiVed: April 28, 2008
A comprehensive study is reported of Langmuir-Blodgett (LB) films (spread at the air/water interface using the Langmuir balance technique) composed of surface active, nonionic, and OH-free amphiphilic siloxane phosphonate ester macromolecules. Analysis is made on three molecular structures in the form of linear polymer poly(diethylphosphono-benzyl-Rβ-ethyl methylsiloxane) (PPEMS), cyclic oligomer methylphosphonobenzyl-Rβ-ethyl cyclosiloxane (MPECS), and copolymer poly(PEMS-co-DMS). The surface pressure-surface area (π-A) isotherms of homopolymer at 3-40 °C show a clear temperature-induced phase transition (plateaus at πt ∼ 17-19 mN/m) below 10 °C. The magnitude of the transition substantially increases upon lowering the temperature (∂∆At/∂T ∼ -0.1 nm2 unit-1 deg-1 and ∂πt/∂T ∼ -0.25 mN m-1 deg-1). The positive entropy and enthalpy gain infers that strong coupling with the subphase and excess hydration attributed to hydrogen bonding between the PdO bond and the subphase prevails at low temperatures. The cyclic oligomer MPECS forms a condensed monolayer at the air/water interface that does not display a similar transition in the experimental temperature range. The temperature sensitivity of MPECS film is observed only in the collapsed region. The nature of the interaction with the subphase is similar for MPECS and PPEMS, indicating that the size and thermal mobility are the controlling factors in these processes. The elasticity plot reveals two distinct states (above and below transition). This observation is supported by BAM images that show irregular spiral structures below 10 °C. The transition occurring in the copolymer at 20 °C is due to relaxation of the PDMS component. The two maxima shown in the elasticity plot indicate additive fractions of PPEMS and PDMS. The surface areas of these macromolecules in the relaxed (1.48 nm2/unit) and packed (0.45 nm2/unit) forms obtained by PM3 modeling agree well with the experimental data and seem to indicate that the siloxane chain is being lifted off the subphase by the hydrophobic phenylic part of the molecule.1 Introduction The success in growing semiconducting nanoparticles in ultrathin Langmuir-Blodgett (LB) films (being cast and oriented at water-air interfaces) of polystyrene phosphonate esters and its blends with cellulose acetate and with vinylbenzyl phosphonate (VBP) monomers and polymers (PSP)1–3 lead us to the inspection of the field in which phosphonate moieties would be combined with polysiloxane matrices that could be deposited as LB films or monolayers.4 Polymers that contain a phosphonate ester functionality, -R′PO(OR)2 can chelate and dissolve large quantities of metal salt (e.g., uranyl nitrate) within their matrices.1–4 The phosphonyl group, PdO, is a strong nucleophile and interacts with water and other proton donor solvents through hydrogen bonding. Several phosphonate-containing polymers have been synthesized in our laboratory and were proven to produce effective LB films.1–3,5,6 However, these polymers have a relatively high glasstransition temperature, Tg, of ∼10-100 °C. Therefore, they have lower free-volume and polymer chain flexibility than polysiloxanes. The linear polysiloxane-based macromolecule, such as polydimethylsiloxane (PDMS), is known for its low Tg and hydrophobic properties; however, its Si-O-Si groups impart unique amphiphilic characters on PDMS that allow the formation of stable condensed type monolayers at the air/water † Part of the “Janos H. Fendler Memorial Issue”. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Current address: BASF Corp., Wyandotte, MI 48192.
interface. Various LB films of functionalized polysiloxane matrices have been studied, among them are derivatives of naphthalocyanate-polysiloxanes,7 polysiloxanes with mesogenic side chains,8 polysiloxane copolymers,9 and oligosiloxanes grafted to the acrylic chain.10 In this report, we wish to present a new type of amphiphilic macromolecule LB film, poly(diethylphosphonobenzyl-R,βethylmethylsiloxane) (PPEMS), which is based on phosphonate ester moieties appended to the polysiloxane main chain. This polymer is a viscous liquid at room temperature and has a Tg of -24 °C. Our preliminary attempt to spread PPEMS at the water-air interfaces seems to indicate that cohesive forces between the hydrophobic benzyl-R,β-ethylmethylsiloxane unit chains and hydrogen bonding interaction between the nucleophilic phosphonyl group (PdO) and the water subphase determine the molecular conformation and properties at the air/ water interfaces. Therefore, the strong influence of temperature on the properties of PPEMS films deposited at the air/water interface has been anticipated. This impelled us to launch an investigation on the relationship between the temperature influence on the monolayer properties and the size of the amphiphilic molecule. Only a few temperature studies are reported on LB film constructed by molecules having essentially the same chemical structure but varying in size.11–14 An analysis of these systems indicates that the mobility of the polymer backbone is hindered due to self-organization of the molecules and the slow relaxation processes, which are
10.1021/jp801557g CCC: $40.75 2008 American Chemical Society Published on Web 07/09/2008
14380 J. Phys. Chem. B, Vol. 112, No. 46, 2008 contrary to a fast reorganization of the small molecules (i.e., cyclic oligomer, for this discussion) that occurs in monolayers upon compression. The variation of collapse pressure πc,15–19 film expansions,12,13 and/or a phase transition from expanded to condensed state11,13,20,21 were reported to occur in polymer films with temperature change. The nature of temperatureinduced phase transitions in polymer films has been primarily (but not always) related to specific interactions of the amphiphiles with the subphase. Laschewsky et al.11 showed that lipids in the form of monomers and polymers exhibit a coexistence of solid and fluid analogue phases characterized by the elevation of transition pressure with temperature. Rettig et al.12 reported that polysiloxanes modified with mesogenic side chains show film contraction and collapse pressure, πc, and elevation with temperature increase, while the corresponding monomer exhibits the same change in πc and a phase transition between the liquid and condensed state. The study of nonionic, hydroxyl-free amphiphilic polysiloxane phosphonate LB film has never been reported according to our best knowledge. Identification of the properties and conditions that are required to obtain such LB films, at the air/water interface, play an important role in the ability to control growth and size of the nanoparticles on the monolayer and 3Dcomposite LB films. Following these considerations, this work presents a comparable study on LB film properties (elasticity, phase transition, collapse pressure, change in enthalpy and entropy) of linear polymer (PPEMS) and cyclic methyldiethylphosphonobenzyl-R,β-ethylmethylsiloxane (MPECS), and copolymer poly(PEMS-co-DMS) at the air/water interface, by the Langmuir balance technique and Brewster Angle Microscopy (BAM). Of special interest in this study has been the influence of the temperature on the properties of the siloxane phosphonate LB film. Combined with computational modeling, the hydration mechanism of hydrophilic phosphonate ester pendent groups with a water subphase, as related to the conformational change of PPEMS and the hydrophobic ethylbenzene link during compression, has been proposed. In addition, characterization of the temperature-induced phase transition with the thermodynamic parameters was obtained and compared with other polymer systems. This investigation proves that there is a significant difference of LB film properties among the three different molecular structures. Experimental Materials. Methylphosphonobenzyl-Rβ-ethyl cyclosiloxane (MPECS), poly(diethylphosphono-benzyl-Rβ-ethyl methylsiloxane) (PPEMS), and copolymer poly(PEMS-co-DMS) (1:1, molarratio)werepreviouslysynthesized,asdescribedelsewhere.22,23 Chloroform (Spectroscopy grade, Fisher Scientific) was used without further purification and was stored over molecular sieves. Water was purified by a Millipore ion exchange and filtration system with a resistivity of 18 MΩ · cm. The polymer (PPEMS) was characterized by 1H (300 MHz) and 13C (75 MHz) NMR spectroscopy (Bruker AMX 300) using CDCl3. The polymer molecular weight was determined with a Waters’ gel-permeation chromatography (GPC) system relative to polystyrene standard. PPEMS, having a number averaged molecular weight Mn of 22 000 (degree of polymerization 70, polydispersity 1.8), and MPECS cyclics containing 4-6 units (Scheme 1) and a copolymer with Mn ∼ 4500 (degree of polymerization 20) were used for this study. Note that PPEMS is a random copolymer of R- and β-ethyl methylsiloxane isomers.
Cabasso and Stesikova SCHEME 1: Chemical Structures of (A) Poly(diethylphosphonobenzyl-r,β-ethylmethylsiloxane) (PPEMS) and (B) Methyldiethylphosphonobenzyl-r,β-ethylmethyl siloxane (MPECS)
Surface Pressure-Area (π-A) Isotherm Measurements. A commercial Lauda Model P Film balance was used for π-A isotherm measurements. A Zenith Data System with a precision of 0.005 nm2/monomer and 0.2 mN/m controlled operation of the film balance. The spreading solutions were prepared by dissolving the PPEMS and MPECS in HPLC grade chloroform to give a total concentration of 1-5 × 10-3 mol/L. The surface of the aqueous solution contained in the trough was cleaned by water aspiration several times prior to spreading the solution. An appropriate amount of spreading solution (typically 30 µL aliquot) was spread onto the clean water surface. At least 15 min was allowed for evaporation of the spreading solvent. All the results presented in this study were obtained using a continuous compression method with a constant barrier speed of 5 cm2/min. The π-A isotherms recorded at different temperatures from 3 to 40 °C were readily reproducible. The subphase temperature was controlled to ( 0.5 °C by circulating thermostatted fluids through a heat exchanger in the trough. Brewster Angle Microscopy (BAM). A homemade Brewster angle microscope consisting of an He-Ne laser (Hughes, 3235 HP-PC, 20 mW) light with a wavelength of 632.8 nm was p-polarized through a polarizer (NRC RSA-2) and subsequently impinged upon the water surface. The angle of incidence was set at 53.1°, which is the Brewster angle for the air/water interface. The angle yielding the reflected light of minimum intensity was practically adopted. An optical filter (black Teflon) was placed at the bottom of the trough under the observed area to suppress the reflection of light transmitted through the interface. The beam, reflected in the presence of an insoluble monolayer, was imaged on a CCD camera (MTI CCD 72 camera, sensitivity ca. 0.002 lux), viewed on a monitor and videotaped. BAM images were recorded during compression and expansion of LB films. Differences in reflectivity probed by a p-polarized laser light beam are visible with a resolution limited by an optical diffraction of 30-40 µm. Results and Discussion Surface Area-Surface Pressure π-A Isotherms. The π-A isotherms of the macromolecules of interest to this study are shown in Figure 1A. The isotherm of PDMS is compared to its phosphonate ester derivatives: the cyclic oligomer MPECS, homopolymer PPEMS, and the copolymer poly(PEMS-coDMS). A clear dependence of film properties on the molecular composition is observed. The PDMS monolayer forms a single condensed phase at a relatively low surface pressure, which is
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J. Phys. Chem. B, Vol. 112, No. 46, 2008 14381 acceptor” with a relatively strong affinity to the water subphase and the large and bulky hydrophobic section. The isotherm is comparable, to some extent, with other amphiphilic polymers, such as poly(N-isopropyacrylamide),13 poly(styrene phosphonate),1 liquid-crystalline polysiloxane with mesogenic side groups,18 fluorinated maleimidecopolymers,26 PVP,27 and PEO,28 which show a relatively strong affinity to the water subphase. Therefore, it is certainly reasonable to conclude that grafting of benzyl phosphonate ester on the siloxane chain leads to a drastic change in the nature of the macromolecule films. In fact, after modification, these isotherms seem to be controlled by the amphiphilic properties of the phosphonate moiety. However, this requires a complete reassessment of configuration of the macromolecular siloxane chain that produces the surface active films. Any change in the π-A isotherm, as is related to molecular structure in the film, can be discussed in terms of elasticity, Es (i.e., the reciprocal of compressibility), which is defined as the film resistance to the change in the surface area, at a given temperature. Hence, elasticity can be obtained from the firstorder derivative of a π-A isotherm (eq 1)
∂π ( ∂A )
Es ) -A
Figure 1. (A) Surface pressure-surface area π-A isotherms of siloxane polymers at the air/water interface acquired at 20 °C and (B) surface elasticity vs surface area Es-A plots. Offset values: PDMS, 44 mN/m; PDMS-co-PPEMS, 70 mN/m; PPEMS (20 °C), 100 mN/m; PPEMS (3 °C), 154 mN/m; and MPEMS, 200 mN/m.
in agreement with other reports.17,24,25 The onset surface area, A0, of PDMS is 0.23 nm2/unit, and the isotherm levels off at π ∼ 9.5 mN/m. Granick et al.17 studied the cyclic form of PDMS and reported only minute differences between the cyclic and the linear form. All three phosphonated derivatives of polysiloxane films shown in Figure 1 differ substantially from PDMS. Their isotherms exhibit a highly compressed phase more than three times higher than collapse pressure and a large collapse area. In addition, the isotherm of copolymer poly(PEMS-coDMS) exhibits a large plateau region that cannot be found in π-A isotherms of PDMS nor in those of PPEMS at the same temperature (20 °C). The latter shows a clear temperatureinduced phase transition in the films at low temperature (3 °C). The isotherm of homopolymer PPEMS at 20 °C exhibits slow increases of surface pressure with packing density at the beginning, followed by a sharp increase as the film is further compressed, and finally reverts to a second slow increase stage. Furthermore, PPEMS film exhibits higher collapse pressure than PDMS (πc ∼ 33.2 mN/m) with a larger collapse surface area (Ac ∼ 0.44 nm2/unit). The isotherm does not exhibit any plateau region or observable phase transition before πc. These types of isotherms are characteristic for nonionic hydroxyl-free amphiphilic polymers, where their hydrophilic moiety is a “proton
T
(1)
The plot of Es vs surface area of the PPEMS film is shown in Figure 1B. The film is highly compressible in the surface area range from onset to the inflection point in the isotherm. As molecules pack, the elasticity increases to a maximum value of ∼53 mN/m where (∂2π/∂A2)r ) 0, at the corresponding surface area A ∼ 0.71 nm2/unit (Table 1). Beyond the inflection point up to the collapse point, the elasticity decreases. A collapse point Ac (0.44 nm2/unit) is obtained by linear extrapolation of the steepest part of the elasticity curves to the abscissa and represents the “collapse” of the monolayer indicating the possibility of multilayer formation. The isotherm of the cyclic analogue, MPECS, shows some similarities to that of PPEMS at 20 °C and has a collapse pressure of ∼32.0 mN/m, but a much smaller surface area (Ac ∼ 0.32 nm2/unit). In comparison, the onset surface area of PDMS is 0.23 nm2/ unit. This value agrees with the calculations of the caterpillarlike atom model,25 which assumes that every oxygen and silicon atom is adsorbed at the surface and that each monomer unit occupies an area of 0.227 nm2/unit. The primary elasticity peak of PDMS appears at ∼23 mN/m, corresponding to a surface area of ∼0.20 nm2/unit, indicating a PDMS conformation where the Si-O groups are adsorbed on water and the polymer backbone is extended parallel to the surface. As suggested by Noll et al.,24 the onset surface area configuration is a hydrated caterpillar-type structure, where the collapse configuration represents a dehydrated intermeshed form of this same extended configuration. Upon further compression, the siloxane chain is lifted out of water, whereby the macromolecule buckles and begins to coil into a series of helices. The nature of the pendant organic group has an important effect on these various configurations. For sake of this discussion, we adopted the model given by Noll et al.24 When the chains of the methyl-substituted siloxane are spread on the air/water interface, the substituent R and methyl groups could be arranged along the extended chain in three conformations, the Rs, βs, and γs forms. In the Rs form, the substituent R alternates regularly from side to side, while the methyl groups lie in a zigzag form above the extended chain. The arrangement in the βs form is just an opposite of the Rs form. Both Rs and βs forms are syndiotactic. In the γs form, all R substituents and methyl groups are aligned toward opposite sides of the extended chain, i.e., isotactic form.
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Figure 2. Geometry of PPEMS in packed form (A) and relaxed form (B) optimized with the PM3 method.
PPEMS is a methyl-R-siloxane, where R is a hydrophilic substituent (i.e., -C6H4CH2PO(OEt)2). It is impossible for PPMS to adopt the γs form because of the bulky nature of the substituents. The βs form is also unfavorable because this form prevents the hydrophilic substituents from interacting with water. Thus, it appears that the most favorable conformation of PPEMS is to adopt an Rs form, in which the substituents are directed laterally and methyl groups are arranged above the siloxane chain. This form allows PPEMS to have maximum interaction with water. Thus, considering the amphiphilic properties of PPEMS (and MPECS), it is reasonable to suggest a horizontal chain orientation of this macromolecule with the PdO groups (i.e., phosphonyl ester) being submerged in the subphase until a point of collapse. Molecular modeling (PM3 method) also supports this contention. The model assumes that the film is in the liquid phase and the phosphonate ester is totally submerged, while the siloxane and ethylbenzyl units are adsorbed on water. In the relaxed arrangement, the maximum surface area is allowed for the segmental motion of the adsorbed unit for the modeling purpose. This is an essential consideration for the calculation of Ao because it allows for no contacts between units just before the onset of surface pressure occurring, where an unlimited available area for segmental motion prevails. Figure 2 illustrates the conformation of the molecular model of PPEMS. The molecular units remain adsorbed on the water surface tightly packed in an “umbrella” conformation (Figure 2A) until a collapse point. This model yields a surface area of 0.45 nm2/unit for tightly
TABLE 1: Viscoelastic Properties of the Different Macromolecule Films at the Air/Water Interface and 20 °C condensed phase
expanded phase
A (nm2/ π Es Es π A macromolecule (mN/m) (nm2/unit) (mN/m) (mN/m) unit) (mN/m) PPEMS PDMS PDMSco-PPEMS MPECS PPEMSa a
53 23 33
0.71 0.20 0.20
17 3 30
30
0.71
8
50 41
0.47 0.64
24 23
42
1.06
13
At 3 °C.
packed and 1.48 nm2/unit for completely relaxed arrangements (Figure 2B) of the macromolecule segments on the surface. These calculations are in good agreement with the observed values of surface area at the onset and collapse of PPEMS monolayer (Table 2). Similar “flat” conformations on the water surface have been established for monolayers of PMMA and poly(n-BuMA) with carbonyl groups submerged in water.29 It is noteworthy to indicate that the affinity of the phosphonyl group to water is much stronger than that of the carbonyl ester. The former binds more water molecules because its dipole moment (3.6 D) is 3-fold higher than the latter (1.2 D).30 This value suggests a horizontal chain orientation of the macromolecule with the PdO groups submerged in the water subphase due to their high
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Figure 3. Surface pressure-surface area π-A isotherms of PPEMS films spread at the air/water interface acquired at various subphase temperatures (3-40 °C).
affinity to water, whereas molecular units remain adsorbed on the water surface. The π-A isotherm of poly(PEMS-co-DMS) (1:1, mol:mol) copolymer (Figure 1A) exhibits a clear transition between liquidexpanded (LE) and liquid-condensed (LC) phases. The transition pressure, πt, defined as a midpoint of the transition zone, was found to be 16 mN/m. The collapse pressure, πc, increased to 39.0 mN/m compared to the PPEMS, implying that the presence of the dimethylsiloxane stabilizes the copolymer and prevents it from early collapse. The onset area, Ao (1.29 nm2/unit), is found within the range of PPEMS. The LE phase is observed at low π on the copolymer film adopting an expanded conformation on the surface, with the phosphonate groups submerged in water. The transition between two phases in the copolymer is clearly shown on the elasticity curve that displays a separate peak for each individual phase (Figure 1B). Such a transition is not preserved in the PPEMS films at high temperatures. The maximum elasticity, ∼30 mN/m, of the LE phase occurred at a surface area of ∼0.71 nm2/unit, the same as that of PPEMS, but the latter has much higher elasticity, ∼53 mN/m (Table 1). The maximum elasticity, ∼33 mN/m, of the LC phase occurred at A ∼ 0.2 nm2/unit which is similar to that of PDMS, but the latter has a lower elasticity (∼23 mN/ m). The elasticity reveals two maxima that are additive fractions of PPEMS and PDMS. These results may indicate that some dimethylsiloxane segments in the copolymers are lifted-off from the interface, yielding a less than expanded surface area at the point of maximum elasticity, or may form loops. In either case, the deviation from the monolayer structure avoids collapse. The π-A isotherms of PPEMS acquired at different subphase temperatures (3 to 40 °C) are shown in Figure 3. The isotherms reveal clear temperature dependence and a temperature induced transition. Above 10 °C, the isotherms exhibit only marginal changes with temperature (Figure 3 and Table 2). The onset surface area, Ao ∼ 1.40 nm2/unit, is invariant. The collapse pressures, πc, are found in the range of 31-36 mN/m, and the corresponding collapse surface areas, Ac, are within the range of 0.33-0.44 nm2/unit. A significant change in the shape of the π-A isotherms is evident below 10 °C, where a transition plateau between LE and LC phases appears. The plateau occurs in a narrow range of surface pressure at the corresponding surface pressure range of 15-19 mN/m and disappears gradually as temperature increases from 3 to 10 °C. The appearance of a
TABLE 2: Characteristics of PPEMS Monolayer Films at the Air/Water Interface with Different Temperatures temperature (°C)
Ao (nm2/unit)
πc (mN/m)
Ac (nm2/unit)
π (mN/m)
3 5 7 10 20 30 40
1.80 1.76 1.46 1.40 1.40 [1.48]a 1.40 1.40
35.9 35.0 34.3 35.8 33.2 33.0 31.0
0.34 0.35 0.35 0.34 0.44 [0.45]a 0.40 0.33
19.0 17.5 17.0 -
a
By PM3 molecular modeling.
plateau in the π-A isotherms is accompanied by a significant increase in the onset surface area, Ao, up to 1.76 nm2/unit inferring the formation of a more expanded phase at low temperature. Generally, according to the two-dimensional phase rule established by Crisp,31 the appearance of a flat portion of a π-Α isotherm results from a phase transition. The monolayer film behavior, depicted in Figure 3, does not completely satisfy this criterion, since the pressure in the “transition” zone is not constant but exhibits some elevation with compression. This can be attributed to the relatively fast compression speed and/ or may indicate that changes in the film are less defined and occur under a range of conditions, due to the structural and compositional distribution. Consequently, the observed plateau is attributed to the transition between more or less expanded films due to specific properties of the phosphonated polysiloxanes that are elaborated below. The temperature-induced phase transition in PPEMS film, signified by the plateau, indicates that beyond a certain temperature (in this case 14) exhibited a transition from expanded to condensed states with positive ∂π/∂T. The corresponding change of heat, calculated in the same manner as in the present study, was in good
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Figure 7. BAM images of PPEMS LB films observed during measurement of π-A isotherms at 20 °C: (a) initial state, A0 ∼ 2.0 nm2/unit; (b) π ∼ 20 mN/m; (c) π ∼ 30 mN/m; (d) collapse state, π ∼ 33 mN/m; (e) π ∼ 0 mN/m, immediately after decompression; and (f) π ∼ 0 mN/m, 5 min after decompression. (The scale bar represents 500 µm.)
Figure 8. BAM images of PPEMS LB films observed during m π-A isotherm measurements at 5 °C: (a) π ∼ 15 mN/m; (b) π ∼ 17 mN/m; (c) π ∼ 30 mN/m; (d) collapse state, π ∼ 33 mN/m; (e) π ∼ 16 mN/ m, after decompression; (f) π ∼ 0 mN/m. (The scale bar represents 500 µm.)
agreement with the heat of fusion of alkyl chains due to the side chain crystallization. However, PPEMS is an awkward molecule that cannot be crystallized. In fact, dehydration of the water shells does not lead to reorganization into a neat structure, but rather, dehydrated phosphonic heads of the bulky rigid benzene ring segments apparently repel each other. Thus, “crystallization” that otherwise decreases the entropy in other systems such as PEO does not occur. The plateau surface area, ∆At, decreases from 0.26 to 0.08 nm2/unit as temperature increases from 3 to 7 °C. This negative ∂∆At/∂T value, -0.09 nm2unit-1deg-1, may comply with the view that partial disruption of hydration shells upon compression of the amphiphilic polymer also entails disruption of the excess water molecules of the hyper-expanded polymer at low temperatures. Brewster Angle Microscope (BAM) Images. The BAM images of polymer films taken during acquiring π-A isotherms are presented in Figure 7. Noncompressed PPEMS films (π ) 0 mN/m, A ∼ 2.0 nm2/unit, Figure 7a) are entirely uniform, and the intensity of the reflected light is hardly higher than that of the bare water surface. The film appears as a “wood pattern” with variable thicknesses at π ∼ 20 mN/m (Figure 7b), where elasticity revealed the presence of a “critical” point at 17 mN/
m, beyond which the elasticity progressively decreases. Increasing surface pressure up to 30 mN/m (Figure 7c) prompted the appearance of a series of contrasting stripes further suggesting formation of a multilayer. Progressive development of a thick phase is evident in the collapsed state (π ∼ 33 mN/m, Figure 7d). The decompression of the film followed the reverse order where the thick scattered domains were observed at π ∼15 mN/ m. The decompression results in the decay of these domains into much smaller aggregates, ∼40 µm in diameter (Figure 7e), completely disappear after several minutes (Figure 7f). The π-A isotherm is supported by the BAM observation, whereas upon compression, the pressure dissipates through the polymer/water subphase interaction while maintaining its lateral structure. It seems that above 17 mN/m, due to the reduction of surface area, polymer segments aggregate into multilayer structures of higher density. In fact, dehydration of phosphonate groups reduces the distance between the phosphonyl groups, causing film collapse at high pressure. The BAM images of PPEMS films observed at 5 °C (Figure 8) show that the first aggregates (or condensed form) in the form of round shaped domains, ca. 200-500 µm in diameter, were observed at the onset of the plateau (π ∼15 mN/m) (Figure
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Figure 9. Compression and expansion π-A isotherm of PPEMS at the air/water interface (5 °C).
8a). The size of the aggregates progressively increases to ∼1 mm at π ∼ 17 mN/m (Figure 8b). At π ∼ 30 mN/m, very high contrast fractal-shape formations with smoothly curved boundaries are observed (Figure 8c). This pattern has not been observed at 20 °C and represents the state where the film is compressed over the critical point (π ∼ 19 mN/m at 7 °C) and below collapse. High brightness of the reflected light from a condensed layer (which can be attributed to a multilayer film structure) also occurs at π < πc at 20 °C. The same pattern is preserved in collapse, where thicker and larger spots are developed with similar fractal-shaped boundaries due to chain entanglement (Figure 8d). Upon decompression, the process is reversed; however, the resulting domains are much larger when compared with those obtained during compression. Thus, large aggregates are observed at π ∼ 16 mN/m (plateau region, Figure 8e), but they vanish completely when the initial state, π ) 0 mN/m and A ∼ 2.0 nm2/unit, is reached (Figure 8f). This observation is consistent with the elasticity plots (Figure 6) that reveal two peaks at low temperatures, regular-shaped dispersed structures at low surface pressure, and irregular pictorial domains of bright intensity at high surface pressure. The hysteresis shown in the compression and expansion π-Α isotherm (Figure 9) could be due to PPEMS chains that do not have enough time to recover from chain entanglements that were formed during the compression. Irregular spiral structures of surface domains at low temperatures have also been observed by others.43–46At low temperatures, PPEMS film resembles the interfacial behavior of oxyethylenated nonionic amphiphiles suggesting that hydration shells formed around phosphonate ester pendant groups play an important role in the relaxation mechanism upon compression. Note, however, that irregular pictorial structures appear at rather high surface pressure (π ∼ 30 mN/m) where the existence of hydration shells is unlikely based on the available surface area per polymer unit. BAM images of low molecular weight analogue MPECS are shown in Figure 10. No substantial temperature dependence is observed, and the sequences of images obtained at 20 and 5 °C are almost identical. The images seem to be very uniform and homogeneous, even at high surface pressure, ∼20 - 25 mN/ m, with no detectable change in the pattern corresponding to π ∼ 0 mN/m. At 30 mN/m, monolayers show a slight increase in thickness (Figures 10b and 10f). It should be emphasized that low molecular weight molecules tend to form more organized and uniform monolayers, in contrast to polymers that exhibit a wide spectrum of intermediate conformations between the “ideal” 2D monolayer and 3D coils (Figure 7c). When deviations from the “monolayer” state are induced by compression, they are more pronounced for PPEMS than for MPECS. The latter
Figure 10. BAM images of the MPECS monolayer observed during π-A isotherm measurements at 5 and 20 °C. At 20 °C: (a) initial state, π ∼ 0 mN/m; (b) π ∼ 30 mN/m; (c) collapse state, π ∼ 33 mN/m; (d) π ∼ 0 mN/m, after decompression. At 5 °C: (e) initial state, π ∼ 0 mN/m; (f) π ∼ 30 mN/m; (g) collapse state, π ∼ 33 mN/m; (h) π ∼ 0 mN/m, after decompression. (The scale bar represents 500 µm.)
does not exhibit bright reflections, even in a collapsed state due to relatively thin collapsed films. The compression of the MPECS monolayer is a reversible process, and no remnant aggregates are observed upon decompressing back to its initial state at both temperatures (Figures 10d and 10h). These results demonstrate that the films show remarkable reversibility and homogeneity at different stages of compression. BAM images of PPEMS films obtained at 5 °C (Figure 8) show striking differences from those at 20 °C, indicating
14388 J. Phys. Chem. B, Vol. 112, No. 46, 2008 important morphological changes. At 5 °C, PPEMS monolayers behave like a 2D-melt or very viscous liquid on the water surface. This fluid character, which is not observed at 20 °C, may arise from the enhanced hydrogen bonding interactions of phosphoryl groups with water molecules at lower temperature. The restricted mobility of polymer segments, strongly coupled to the subphase at 5 °C, manifests itself in the intriguing fractal patterns observed by BAM. Conclusion The results of this research show that amphiphilic polysiloxane macromolecules that consist of a siloxane backbone grafted by phosphonate ester groups can form stable Langmuir-Blodgett films spread at the air/water interface. The interaction of macromolecules with the subphase is primarily due to hydrogen bonding, typifying the behaviors of macromolecules in a LCST environment. While clear phase transition is observed at the polymer film below 10 °C, their cyclic oligomer seems to be transition-free in the range of the testing temperatures (3-40 °C). Since the nature of the interactions of these molecules with the subphase is similar (H-bonding with PdO), the differences between the polymer PPEMS and the cyclic oligomers MPECS are apparently due to size and mobility. The thermal mobility of the smaller molecules is substantially higher and is the dominating factor in such comparisons. The temperature-induced transition of PPEMS is reversible, and so are π-A isotherms below 10 °C at a surface pressure of ∼14-17 mN/m. One common explanation for this transition is the cleavage of interacting water shells upon compression. This water shell reconstitutes upon decompression. The magnitude of the transition is dependent on temperature, i.e., increasing substantially on lowering the temperature. Above 10 °C, there is no apparent transition, and surface area contraction is accompanied with the rapid rise of surface pressure (slight difference between isotherms can be related to the difference in thermal motion). The conjecture that MPECS film would have developed a transition at a much lower temperature than that shown by PPEMS is probably correct but cannot be directly proven for obvious reasons. The negative temperature coefficients of the transition (∂πt/ ∂T) indicate that the transition is an endothermic process and that the condensed state of the LB monolayer is characterized by higher entropy and enthalpy compared to the expanded state. The negative value of ∂∆At/T (plateau surface area) may comply with the view that partial disruption of hydration shells upon compression of the amphiphilic polymer could also entail disruption of the excess water molecules of the hyperexpanded polymer at low temperatures. In fact, BAM results seem to support the contention that PPEMS has stronger interactions with the subphase and exhibits a more viscous character of the film at low temperatures. Irregularly shaped elongated domains observed by BAM at lower substrate temperature suggest an increased repulsion of the polar groups of the polymer at higher compression. Compression of the monolayer is followed by partial disruption and breakage of the hydrogen-bonded network yielding film contraction. The condensed state of PPEMS monolayers can be characterized by a more compact arrangement of polymer segments and water molecules liberated from the monolayer. Such a mechanism is anticipated to result in the entropy and enthalpy gain for the system, which is indeed observed for the PPEMS film. The oligomer MPECS does not show a similar temperatureinduced transition. Quantitative analysis of the number of water molecules involved in the rupture of water shells is not available
Cabasso and Stesikova TABLE 4: Characteristics of the Phase Transition in the PPEMS Monomolecular Films at the Air/Water Interface ∆H ∆S ∆E temperature πt ∆At (°C) (mN/m) (nm2/unit) (kJ/mol) (kJ/mol · K) (kJ/mol) 3 5 7
19.0 17.5 17.0
0.26 0.14 0.08
10.8 5.8 3.4
39.1 21.1 12.0
13.8 7.3 4.2
to our knowledge. Many reports claim, for example, that in PEO copolymers the hydrophilic segment of macromolecules sinks (upon compression) into the subphase and self-organizes into ordered dehydrated structures that lead to a substantial decrease of entropy in the process. However, PPEMS and MPECS are awkward, irregular molecular structures, with quite large threedimensional hydrophobic units and would not organize into ordered structures upon contraction. This is being reflected by the positive entropy change that was calculated from the phase transition. Modeling of this macromolecule in the relaxed and packed form (the “umbrella” shape) seems to indicate that silicon chains are being lifted off the subphase by the ethylbenzene part of the unit that is linked to the hydrophilic phosphonate interacting with the subphase. However, the molecular structure is complex and does not resemble common surfactants, phospholipids, or polymers such as PPMA that do not contain hydroxyl or ionic groups. The arrangement of water molecules around the phosphonate ester is yet to be determined. Theoretical and experimental studies concerning this macromolecule cast as LB films are currently underway in our laboratory. PPEMS has a significant absorption capacity of transition metal salts (up to 30 mol %). In preliminary experiments, based on the results obtained in the present study, we have shown that nanostructured aggregates of chelated salts were converted into semiconductive domains, and multilayer films have been formed by transferring PPEMS monolayers onto solid plates. Acknowledgment. We acknowledge the invaluable contribution of Dr. Xinwei Wang, with the PM3 modeling and computations of the data, and Dr. Nicholas Kotov for helping with the BAM experiments. The Financial Support of an NIH Grant R01-DE06179 and the Membrane Center of Syracuse University Materials Science and Engineering is acknowledged. References and Notes (1) Yuan, Y.; Cabasso, I.; Fendler, J. H. Chem. Mater. 1990, 2, 226. (2) Yuan, Y.; Cabasso, I.; Fendler, J. H. Macromolecules 1990, 23, 3198. (3) Yuan, y.; Fendler, J. H.; Cabasso, I. Chem. Mater. 1992, 4, 312. (4) Lin, S.; Cabasso, I. J. Polym. Sci., Polym.Chem. 1999, 37, 4043. (5) Sun, J. P. D.; Cabasso, I. J. Polym. Sci., Polym. Chem. 1989, 27, 3985. (6) Yu, Z.; Zhu, W.; Cabasso, I. J. Polym. Sci., Polym. Chem. 1990, 28, 227. (7) Schwiegk, S.; Vahlenkamp, T.; Xu, Y.; Wegner, G. Macromolecules 1992, 25, 2513. (8) Chen, X.; Xue, Q.-B.; Yang, Q.-Z. Macromolecules 1996, 29, 5658. (9) Richardson, T.; Majid, W. H. A.; Cochrane, E. C. A. Thin Solid Films 1994, 242, 61. (10) Kunitake, M.; Nishi, T.; Yamamoto, H.; Nasu, K.; Manabe, O.; Nakashima, N. Langmuir 1994, 10, 3207. (11) Laschewsky, A.; Ringsdorf, H.; Schmidt, G.; Schneider, J. J. Am. Chem. Soc. 1987, 109, 788. (12) Retting, W.; Naciri, J.; Shashidhar, R.; Duran, R. S. Thin Solid Films 1992, 210, 114. (13) Liu, G.; Yang, S.; Zhang, G. J. Phys. Chem. B 2007, 111, 3633. (14) Hossain, M. M.; Iimura, K.; Kato, T. J. Colloid Interface Sci. 2006, 298, 348. (15) Wang, Y. F.; Chen, T. M.; Li, Y. J.; Kitamura, M.; Nakaya, T.; Sakurai, I. Macromolecules 1996, 29, 5810. (16) Kawaguchi, M.; Saito, W.; Kato, T. Macromolecules 1994, 27, 5882.
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