HybrasurfsâA New Class of Hyperbranched Surfactants - Langmuir

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Hybrasurfs – A new class of hyperbranched surfactants Subramaniam Ramakrishnan, and N. S. Shree Varaprasad Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02022 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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Hybrasurfs – A new class of hyperbranched surfactants N. S. Shree Varaprasad and S. Ramakrishnan* Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore, 560012 INDIA E-mail: [email protected]

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ABSTRACT A hyperbranched polyester carrying peripheral allyl groups was prepared by meltcondensation of a suitably designed AB2 monomer bearing two allyl ester groups and one hydroxyl group. The periphery of the hyperbranched polymer was co-clicked with two different organic thiols, namely hexadecane thiol and 3-mercaptopropionic acid, using the thiol-ene reaction. Three different samples with varying mole-fractions of the hydrophilic carboxylic acid groups were prepared; the conformational adaptability of the hyperbranched polymer backbone permitted these amphiphilic systems to form Janus structures that exhibit surfactant-like properties and, therefore, we have termed them Hybrasurfs. These polymers behave like clusters of surfactants that have been stitched at the waist by the HB polymer backbone; the Langmuir isotherms revealed the formation of a monolayer, and in two of the samples having higher mole-fractions of hexadecyl segments a weak inflection in the isotherm is seen; this suggests a densification, typically implying the crystallization of the alkyl segment at the air-water interface. The monolayers were transferred onto a substrate and their heights were estimated using AFM; the values thus obtained were in reasonable agreement with the expected value. The water contact angles of the substrates bearing the transferred monolayers of the three different samples (transferred at two different points along the isotherm) were measured; it was seen that the sample carrying the highest mole-fraction of hexadecyl chains exhibited a significantly larger contact angle when compared to the other two samples. Interestingly, these hybrasurfs also formed vesicles in water and were shown to encapsulate water-soluble dyes, such as EiosinY. Thus, this class of readily accessible amphiphilic HB polymers that behave as a cluster of surfactants opens some interesting possibilities for further exploration.

Keywords: Hyperbranched polymers; amphiphilic; thiol-ene click reaction; Janus structure; Langmuir isotherms.


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INTRODUCTION Surfactants are a remarkably diverse class of molecules that covalently bring together two or more segments with distinct solubility characteristics; these form a variety of different aggregated structures in aqueous solution, ranging from simple micelles to elongated wormlike micelles, and a variety of bilayer structures, such as vesicles, bilayer discs, bilayer lamellae, etc.1,2 Chemists have examined a wide structural range of amphiphilic molecules, such as bola-amphiphiles,3 gemini-surfactants,4,5 dendritic surfactants, 6,7 etc.; each of these exhibit unique aggregation properties that have been often rationalized based on the relative volume-fractions of the hydrophobic and hydrophilic segments, and the relative flexibility of the entire molecule. Dendrimers are an interesting class of macromolecules that possess a highly symmetric branched topology;8 selective placement of hydrophobic and hydrophilic segments on diametrically opposite sides has been used to prepare Janus structures, and these have been shown to form a variety of different morphologies triggered by the immiscibility of the two peripheral segments.9 Like Dendrimers, hyperbranched polymers (HBPs) are also highly branched polymers but they carry numerous randomly located branching defects that arise because they are typically prepared in a single-step via a statistically random growth process.10-12 Despite the presence of these branching defects (often referred to as Linear (L) defects) several of the unique properties of Dendrimers are also exhibited by HBPs, especially in systems that are amphiphilic; the simple and straightforward preparation of HBPs, therefore, gives them an edge in terms of developing scalable applications. Recently, we have developed several simple methods to prepare HBPs that carry numerous clickable functional groups at the periphery; these could be either allyl or propargyl groups, both of which were shown to be quantitatively clickable using thiol-ene and azide-yne click reactions, respectively.13-17 Hetero-functional HBPs possessing more than one type of peripheral segment, such as alkyl and PEG, were shown by us to readily transform to Janus or Tripodal-type systems.18,19 In a series of papers, Tsukruk and co-workers20-23 examined the behaviour of amphiphilic hyperbranched polymers and demonstrated that the presence of significant linear defects and high polydispersity are not impediments to the formation of uniform supramolecular nano-fibrils at the air-water interface. Based on these reports, it is evident that the HB polymer backbone possesses adequate conformational freedom to reconfigure and 3|Page ACS Paragon Plus Environment

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segregate incompatible segments. We describe here a new class of surfactants based on hetero-functionalized HBPs; here, long chain alkyl substituents and carboxylic acid groups have been incorporated at the periphery. We show that such systems indeed reconfigure to exhibit surfactant-like properties and therefore have termed them as hybrasurfs; the uniqueness of these amphiphilic systems comes from the fact that there are multiple polar head-groups and hydrophobic tails. When compared to simple linear polysoaps, the reorganization of the segments to adopt a Janus conformation, that presents all the polar heads to one side and hydrophobic tails to the other, should occur more readily; this, we believe is because the branched topology restricts the overall conformational degrees of the freedom, significantly. Experimental Section Solvents, purified by standard procedures,24 were used for synthesis of all the compounds. 5-Hydroxy isophthalic acid, magnesium turnings, allyl bromide, hexadecane thiol and 3mercaptopropionic acid were purchased from Aldrich Chemical Co. All 1H NMR spectra were recorded in a Bruker AV 400 MHz spectrometer using suitable deuterated solvents and tetramethylsilane (TMS) as an internal reference. GPC measurements were done using a Viscotek triple detector analyzer (TDA) model 300 system, with refractive index (RI), differential viscometer (DV) and light scattering (LS) detectors, which are connected in series with each other. The separation was achieved with the help of two PL gel mixed bed columns (300 x 7.5 mm) operated at 30° C using THF as an eluent. Universal calibration curve based on the data from the RI and DV detectors using narrow polystyrene standards were used to determine the molecular weights. Thermal analysis was performed using a Perkin Elmer differential scanning calorimetric (DSC 8000) instrument; measurements were carried out at a heating rate of 10 deg/min under a dry N2 atmosphere. Typically, 3 - 4 mg of sample was initially heated to a temperature above its melting point (to make sure that, the sample flows and makes complete contact with the surface of the pan) and then cooled; two subsequent heating and cooling runs were performed to ensure reproducibility. Since the melting transitions reflect the melting of the hexadecyl segments, the enthalpies associated with these transitions were normalized with respect to the weight percent of these segments in the sample; this, in turn, was estimated from the copolymer composition by NMR. NIMA 1232D1D2 Langmuir−BlodgeD trough was used for the LB studies of HEXPA4|Page ACS Paragon Plus Environment

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XY. A control water isotherm was recorded, and no significant surface pressure was noticed until the barrier is fully closed; such a flat baseline was ensured before beginning the experiment. Atomic Force Microscopy (AFM) measurements were done using Multimode SPM (Digital Instruments, Santa Barbara, CA) equipped with NanoScope IV A controller. FESP (Veeco) tips of force constant 1−5 N/m and resonance frequency of 75 kHz were used for imaging the sample, measurements were done in the tapping mode and the images were analyzed using the software provided with the instrument. Dynamic light scattering (DLS) measurements were done with Zetasizer Nano ZS90, molecular/particle size and zeta potential analyzer, (Malvern, UK). Confocal fluorescence analyses were done with Leica SP5 upright confocal microscope with 63x oil-immersion objective at a wavelength of 514nm. Contact Angle measurements were done on dry (air dried) silicon wafer with the monolayer of the polymer. Dimethyl, 5-hydroxyisophthalate (1). 5-Hydroxy isophthalic acid (15 g, 82.4 mmol) was taken in methanol (150 mL) along with a few drops of H2SO4; the contents were refluxed for 12 h. Then, water was added, and the product was extracted with ethyl acetate and the organic layer is washed with NaHCO3 till it gets neutralized and passed through Na2SO4 and dried. Yield was 98 %. MP:160°C 1

H NMR (CDCl3, δ ppm): 8.2 (s, 1H, ArCH); 7.76 (d, 2H, ArCH); 6.28 (s, 1H, ArOH); 3.94 (s, 6H,

ArOCH3). 4-Bromobutylacetate (2).25 1,2-Dibromoethane (92.74 g, 493 mmol) and Mg turnings (12g, 493 mmol) were taken in 300 mL dry ether at N2 atmosphere was stirred for 3 h at RT. Once the evolution of ethylene gas stopped, indicating the formation of MgBr2, the ether was removed under reduced pressure. 200 mL of dry acetonitrile was added to the solid mass along with 40 mL of dry THF and 51 mL of acetic anhydride. The mixture was stirred for 16 h at RT, neutralized with aq. NaHCO3 solution, extracted with ethyl acetate, dried over anhydrous Na2SO4, concentrated and distilled using a Kugelrohr (at 120°C/2.0 mm of Hg) to get the desired product. Yield = 95%. 1

H NMR (CDCl3, δ ppm): 4.0 (t, 2H, BrCH2CH2CH2CH2OCOCH3); 3.42 (t, 2H,

BrCH2CH2CH2CH2OCOCH3); 2.0 (s, 3H, BrCH2CH2CH2CH2OCOCH3); 1.88-1.94 (m, 2H, BrCH2CH2CH2CH2OCOCH3); 1.75-1.79 (m, 1H, BrCH2CH2CH2CH2OCOCH3). 5|Page ACS Paragon Plus Environment

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5-(4-Hydroxybutoxy)benzene-1,3-dioic acid (3).15 A mixture of 1 (10 g, 47.62 mmol), 2 (13.9 g, 71.43 mmol), K2CO3 (39.5 g, 285.7 mmol) and catalytic amount of KI were taken in MEK (200 mL) and refluxed at 80 °C under N2 atmosphere for 12 h. The completion of the reaction was confirmed by TLC; once completed, excess solvent was removed, water was added, and the product was extracted with ethyl acetate, dried over anhydrous Na2SO4 and concentrated under reduced pressure. The excess of 2 may be removed by washing the residue with pet ether. Further the residue was refluxed at 60°C with NaOH (10.4 g, 262.24 mmol) and methanol as solvent for 12 h. Solvent was removed, and the residue was neutralized with the addition of dil. HCl. The white precipitate obtained was filtered and dried under reduced pressure. Overall yield was 90% and MP: 184°C. 1

H NMR (CD3OD, δ ppm): 8.2(t, 1H, ArCH); 7.7(d, 2H, ArCH); 4.1(t, 2H, ArCH2CH2CH2CH2OH);

3.64(t, 2H, ArCH2CH2CH2CH2OH); 1.86-1.93(m, 2H, ArCH2CH2CH2CH2OH); 1.7-1.77(m, 2H, ArCH2CH2CH2CH2OH). Diallyl, 5-(4-hydroxybutoxy) isophthalate (4).15 The compound 3 (5g, 19.6 mmol) was taken along with NaHCO3 (3.29g, 39.2 mmol), allyl bromide (4.7g, 39.2 mmol) in DMSO as solvent. The mixture was stirred at RT for 36 h. The completion of the reaction was confirmed by running TLC; once complete, water was added, and the product was extracted with ethyl acetate, dried over anhydrous sodium sulphate and dried under reduced pressure to give the product in 90% yield. 1

H NMR (CDCl3, δ ppm): 8.29 (d, 1H, ArCH); 7.74 (d, 2H, ArCH); 5.9-6.0 (m, 2H,

ArOCH2CHCH2); 5.2-5.4 (m, 4H, ArOCH2CHCH2); 4.8 (m, 4H, ArOCH2CHCH2); 4.08 (t, 2H, ArOCH2CH2CH2CH2OH);

3.7

(t,

2H,

ArOCH2CH2CH2CH2OH);

1.78

(m,

4H,

ArOCH2CH2CH2CH2OH); 1.97 (s, 1H, ArOCH2CH2CH2CH2OH). Polymerization of 4. The monomer 4 (1.5g, 4.48 mmol) along with 2 mol % of dibutyl tin dilaurate (DBTDL) was taken in a test tube shaped polymerization vessel. The mixture was degassed for about 10 min and maintained at 110°C under continuous N2 purge to attain a homogeneous mixture of the monomer and the catalyst. The polymerization was carried out at 150°C under continuous N2 purge for about 45 min with constant stirring. Subsequently, the polymerization was continued in Kugelröhr at 150°C for a period of 45 mins under reduced pressure (2 Torr) with continuous rotation. The polymer obtained was dissolved in CHCl3 and precipitated twice in methanol; isolated yield was 80%. 6|Page ACS Paragon Plus Environment

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1

H NMR (CDCl3, δ ppm): 8.26(s, 1H, ArCH); 7.72(s, 2H, ArCH); 6.0(t, 1H, ArOCH2CHCH2); 5.2-

5.4(m, 2H, ArOCH2CHCH2); 4.8(s, 2H, ArOCH2CHCH2); 4.41(s, 2H, ArCH2CH2CH2CH2-); 4.1(s, 2H, ArCH2CH2CH2CH2-); 1.98(s, 4H, ArCH2CH2CH2CH2-). Clicking with hexadecane thiol and 3-mercapto-propionic acid. A mixture of the polymer, HBP–allyl (1 g, 3.62 mmol), hexadecane thiol (1.037 g, 3.62 mmol) and 3-mercaptopropionic acid (384.2 mg, 3.62 mmol) (for the feed of 50% of each of hexadecane thiol and mercapto succinic acid) were taken in a quartz test tube along with the photo-initiator 2,2-di-methoxy 2-phenyl acetophenone (92.78 mg, 0.18 mmol). The reaction mixture was dissolved in 10 mL CHCl3 and was irradiated with UV light (250 W Hg vapor lamp) for 8 h. The polymer solution was concentrated and precipitated in methanol twice. Similarly, the other clicked products viz, HEXPA3070 and HEXPA7030 were prepared with use of different compositions of hexadecane thiol and 3-mercaptopropionic acid. In the case of HEXPA7030, the product was precipitated in petroleum ether to give overall yield of 80 %. 1

H NMR (CDCl3, δ ppm): 8.22(s, 2H, ArCH); 7.72(s, 4H, ArCH); 4.42(s, 8H, -

OCH2CH2CH2CH2OArOCH2);

4.1

(s,

4H,

-OCH2CH2CH2CH2OAr-);

2.79(s,

2H,

ArOCH2CH2CH2SCH2CH2COOH); 2.63-2.66(t, 4H, ArOCH2CH2CH2SCH2CH2-); 2.52(s, 2H, ArOCH2CH2CH2SCH2CH2(CH2)13CH3); 2.1(s, 4H, ArOCH2CH2CH2SCH2CH2-);

1.99 (s, 8H, -

OCH2CH2CH2CH2OArOCH2); 1.234(m, 26H, ArOCH2CH2CH2SCH2CH2(CH2)13CH3); 0.864(t, 3H, ArOCH2CH2CH2SCH2CH2(CH2)13CH3). AFM studies. A solution of the polymer at a concentration 0.25 mg/mL in THF was spin coated on a pretreated mica sheet and AFM images were recorded before and after annealing at a temperature above the Tg of the HBP. Langmuir isotherms. NIMA 1232D1D2 Langmuir−BlodgeD trough was used for the LB studies of HEXPA-XY. A control water-isotherm was recorded (also with 0.1 N NaHCO3 solution) and no significant surface pressure was noticed until the barrier is fully closed; such a flat baseline was ensured before beginning the experiment. HEXPA-XY solutions (0.5 mg/mL) were prepared in CHCl3 (HPLC grade). Polymer solution in requisite amount was spread on the trough. The layer was compressed at a rate of 50 cm2/min and iso-cycles were recorded at a rate of 300 cm2/min. The plots were normalized with respect to the weight of the polymer sample delivered on to the air-water interface. 7|Page ACS Paragon Plus Environment

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The compressed monolayer was transferred on to a piranha–treated silicon wafer at two different surface pressures (where change in slope and at solid phase) and is used for AFM studies and contact angle measurements. Vesicle formation (Hydrophilic dye). A HEXPA-XY solution of 0.25mg/mL in CHCl3 was taken in a vortex bottle and the solvent was removed with slow rotation in the presence of N2 gas, to get a very thin layer of polymer on the glass wall. After thorough drying under reduced pressure, 1 mL of distilled water, along with the dye (Eosin y), was added to the flask, and the contents were kept for hydration in refrigerator for 12 h. It is then sonicated at around 60°C (5 mins), cooled (2 mins) and finally vortexed (5 mins). This sequence of steps was repeated 4-5 times. RESULTS AND DISCUSSIONS As described earlier,13-17 the parent hyperbranched polyester bearing numerous clickable allyl end-groups was prepared by the self-condensation of an AB2 type monomer, namely diallyl, 5-(4-hydroxybutyloxy) isophthalate (Scheme S1). The single-step melt condensation route to this parent polyester makes this an attractive molecular scaffold for generating a variety of interesting derivatives. Previously, we had shown that the peripheral allyl groups of this polyester were readily clicked with a variety of different hydrophobic and hydrophilic thiols in a quantitative manner; this yielded both organic and water-soluble HB polymeric derivatives.13-17 Here, we clicked the peripheral allyl groups simultaneously using two thiols, namely hexadecane (C16) thiol and 3-mercaptopropionic acid (scheme 1); by varying the ratio of the two thiols the hydrophilic-lyophilic balance (HLB) of the resulting HB polyester can be readily fine-tuned. Based on our earlier studies,18,19 it was anticipated that the distinct solubility characteristics of the two peripheral segments would lead to a reconfiguration of the HB polymer backbone to generate a system where the long alkyl chains and the carboxylic acid groups self-segregate leading to the formation of a “clustertype surfactant”, as schematically shown in Scheme 1. The thiol-ene click reaction was carried out in a single-step using the required mole-ratio of the two thiols under standard photo-initiated reaction conditions.26 The molecular weights (Mw) of the parent polyester and those after clicking ranged from 12,000 to 40,000, as determined by GPC, using a universal calibration method.


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Scheme 1. Preparation of Hybrasurfs and schematic depiction of the structural reorganization to generate Janus structures.

The proton NMR spectra of the parent HB polyester and those of the peripherally clicked polymers are shown in Figure 1; polymers with three different compositions of the two peripheral segments were prepared, namely HEXPA-7030, HEXPA-5050 and HEXPA-3070; here the first two digits of the suffix represents the mole-percent of hexadecane thiol in the feed, whereas the second two digits is the mole-percent of 3-mercaptopropionic acid.


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1

Figure 1. H NMR spectra of the parent HB polyester along with those of the hybrasurfs, namely HEXPA7030, HEXPA5050 and HEXPA3070. The expanded insets show the weak residual peaks from the unreacted allyl groups.

The spectra of the parent HB polyester clearly show the presence of the remaining oneequivalent of allyl group per repeat unit, as expected; the peaks at ~ 6.0 ppm and in the region 5.3 to 5.5 ppm represent the vinyl protons, whereas that at 4.8 ppm is due to the methylene proton of the allyl group. Upon clicking, these peaks almost completely disappear; the intensity of the residual unreacted allyl peaks (as shown in the expansion) amounts to less than 5 %. Further, the peak due to the allylic methylene protons moves upfield and merges with methylene protons of the polymer backbone at ~4.4 ppm; the relative intensity of this peak is clearly seen to double in all three clicked derivatives (see Figure S1). By comparing the spectra of the hybrasurfs with those of the model HBPs carrying only hexadecyl or only propionic acid segments (see Figure S2), it was possible to assign the different peaks associated with each of these segments; thus, the peak at 2.4 ppm was assigned to hexadecyl segment while that at 2.6 ppm to the propionic acid unit. From the relative intensities of these two peaks the composition of the hybrasurfs was readily

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estimated; these are listed in Table 1. The estimated composition using NMR deviated only slightly from the feed composition, as evident from the Table. HBP

% Feed

From NMR

HEXPA3070

30:70

40:60

HEXPA5050

50:50

54:46

HEXPA7030

70:30

66:34

Table 1: Table showing the % in the feed and mole-percentages of hexadecane thiol and 3-mercaptopropionic acid estimated from their proton NMR spectra.

Figure 2: DSC thermograms of the hybrasurfs, along with that of the HB polyester clicked with hexadecane thiol alone; on this scale HEXPA-3070 does not reveal a melting point.

Having established the composition of the peripherally clicked HB polyesters, first we examine if the self-segregation of the two segments occurs in the solid state. For this, the DSC studies of the parent polyester and those of the hybrasurfs were carried out; the parent polyester exhibited only a glass transition temperature at ~14°C (see Table 2), whereas two of the hybrasurfs, carrying larger mole-fractions of the hexadecyl segments exhibited a clear melting transition, as seen in Figure 2. For comparison, the HB polyester carrying only hexadecyl units (HBPC16) was also synthesized and its DSC thermogram is included in Figure 11 | P a g e ACS Paragon Plus Environment

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2; the melting point of this model polyester was seen at ~29°C, whereas the melting points of HEXPA7030 and HEXPA5050 were only slightly lower at ~28° and 27°C, respectively. On the other hand, HEXPA3070 exhibited a very weak and significantly lower Tm at ~15° C (Figure S3). As mentioned earlier, the self-segregation of dissimilar peripheral segments could lead to the formation of Janus-type structures, wherein the alkyl segments and the carboxylic acid groups are segregated; the independent crystallization of the hexadecyl segments is evidently responsible for the melting peak in these samples. It is well-known that long alkyl chains, present even as pendant groups in linear polymers, tend to independently crystallize due to their strong propensity to form a paraffinic crystalline lattice;27 the closeness of melting temperatures of the model HB polyester carrying only hexadecyl chains (HBPC16) and those of the two hybrasurfs supports this hypothesis. These observations broadly concur with those reported by others in the case of similar peripherally functionalized dendrimers28 and hyperbranched polymers.29 Thus, the two dissimilar segments appear to self-segregate and generate the anticipated polysurfactanttype structures, as depicted in Scheme 1. This was further confirmed by SAXS measurements (Figure S4), which revealed the formation of a lamellar morphology in HEXAPA7030; however, the other two samples did not reveal any clear SAXS pattern. The efficacy of the self-segregation and further organization in bulk does appear to depend on the mole-fraction of the crystallizable cetyl segments. One can liken these systems to gemini surfactants, wherein two surfactants are linked together by a spacer segment; in this case, on the other hand, the reconfigured HB polymer backbone forms the linking segment between several surfactant molecules and thus could be viewed as oligo-surfactants. Both the slight lowering of the melting temperature and the significant broadening of the peak in the case of the hybrasurfs, suggests that the domain separation does not occur perfectly; further, when the hexadecyl mole-fraction is small (HEXPA3070) the melting peak is very weak, suggesting that a critical volume-fraction is needed to drive this self-segregation.


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Compound

Mn

Mw

PDI

Tm /Tg °C

HBP

3500

12700

3.6

14.3 (Tg)

HEXPA3070

8700

36800

4.2

15

21

HEXPA5050

10200

25100

2.5

27.38

72

HEXPA7030

9700

21000

2.1

27.98

114

29.59

98

C16

Enthalpy (J/g)

Table 2: Table showing the properties of the hybrasurfs with varying hydrophobic content; the enthalpies are normalized with respect to the mole-percent of hexadecane thiol.

Studies at the air-water interface As mentioned earlier, Tsukurk and coworkers20-23 have demonstrated that core-shell type amphiphilic hyperbranched polymers (with polar core) exhibit typical surfactant-type spreading behavior at the air-water interface; they also showed the formation of remarkably uniform nano-fibrils due to the segregation and crystallization of the long peripheral alkyl

segments. Figure 3: Langmuir isotherms for HEXPA7030 (black), HexPA5050 (red) and HEXPA3070 (blue). The arrows show the surface pressure at which the possible crystallization of the long carbon chains taken place.


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We carried out similar studies of the three different hetero-functionalized hyperbranched polymers. As seen from the Langmuir isotherms (Figure 3), the onset surface area (normalized w.r.t weight), at which the surface pressure begins to rise, is considerably different for the three polymers. Whereas HEXPA5050, bearing roughly equal mole-fractions of cetyl chains and carboxylic acid groups at the periphery, exhibits the lowest onset surface area, HEXPA3070 exhibits the largest value; furthermore, the later sample also shows no inflection point. The inflection in the isotherm is a reflection of densification at the air-water interface during the compression, and this could be attributed to the crystallization of the cetyl chains that get organized above the water surface;

30-32

in HEXAPA3070, the mole-

fraction of the cetyl chains are small and therefore even during compression they are unable to come close enough to crystallize. However, in the other two samples an inflection point is seen during the compression; this inflection occurs at a lower surface pressure for the sample having a larger mole-fraction of cetyl chains, namely HEXAPA7030. This reflects the relative areas of cross-section of the expanded hyperbranched backbone bearing the carboxylic acid units and those of all the cetyl chains; when the latter is larger the onset of crystallization occurs at lower surface pressures. The compressed monolayers were transferred onto a hydrophilic substrate, namely piranhatreated silicon wafer, and examined using AFM; the transfer was done at different surface pressures. Figure 4 shows the AFM images of the transferred monolayers from the air-water interface at different surface pressures for the three clicked HBPs; a, b and c are the images of HEXPA7030, HEXPA5050 and HEXPA3070, respectively, transferred at maximum compression; g, h and i were transferred just prior to compression, while d, e and f are images of monolayer transferred at intermediate surface pressures, near the point of inflection. Further, the water contact angle measurements were also done; these studies clearly indicate two trends – i) generally, higher the cetyl segment content higher is the contact angle, and ii) higher the surface pressure at which the monolayer is transferred, higher is the contact angle.


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Figure 4: AFM images of the monolayer transferred onto hydrophilic silicon wafer a, b, and c are images of monolayer of HEXPA7030, HEXPA5050 and HEXPA3070 transferred at a surface pressures of 55, 40 and 38 mN -1 -1 m respectively. d, e and f are transferred at a surface pressure of 27, 24 and 24 mN m and g, h and i are -1 transferred at 10 mN m (before any compression). Inset shows the contact angles for the corresponding monolayer and also the line profile of the image.

Both these observations are generally consistent with our expectations; however, the difference between HEXAPA7030 and HEXAPA5050 is small, and at some transfer pressures the later even exhibits a higher contact angle. Thus, it appears that once the mole-fraction of the hydrocarbon segment exceeds a critical value, further increase does not appear to


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change the packing at the air-water interface very significantly. In fact, it could also be that the hybrasurf that has similar mole-fractions of the two segments permits the most effective packing; thus, increase in the mole-fraction of either segments leads to poorer packing. It may be expected that the cetyl segments pack more effectively as the surface pressure is increased, and the higher values of contact angles is a reflective of the density of the packed hydrocarbon at the contact surface. Thermal annealing of spin-coated films To examine the morphology thin-film coatings of the hybrasurf prepared by simple spin coating, a THF solution of the polymer was spin-coated on freshly cleaved mica surface; it was noticed that fairly uniform films were formed, which improved substantially upon thermal annealing at a temperature just below the melting of the cetyl chains (Figure 5). The surface roughness of the region covered by the film clearly decreased upon annealing. Furthermore, it was noticed that, the most uniform coverage was observed in the case of HEXPA5050, whereas in the other two samples having unequal mole-fractions of the two segments, a typical island-and-hole morphology was observed. Thermally annealed films of HEXPA5050 evidently produced the smoothest films. Importantly, typical island-and-hole morphology was observed with the heights of the islands of annealed samples falling in the range of 1.5 – 2 nm.


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a

b

c

d

e

f

Figure 5: AFM images of spin-coated HEXPA-XY on freshly cleaved mica sheet. Images a, b and c represent films of HEXPA7030, HEXPA5050 and HEXPA3070, respectively, before annealing, while d, e, f represent those after thermal annealing. Inset shows the height profile of the films.

Preparation of Vesicles Hollow spheres and vesicles are of great interest, as they could be used for encapsulating dyes/drugs; they find applications as carriers for drugs, artificial cells and as bio-reactors. The self-assembly of amphiphiles is the simplest way to encapsulate variety of guest molecules;33-36 this subject has been discussed extensively in a recent review by Zhou and Yan.37 In the present study, we examine the preparation of vesicles using all the three amphiphilic HBPs, and attempt to encapsulate a water-soluble organic dye, like Eiosin-Y; as these derivatized HBPs form Janus structures, it was expected that they could also form bilayers and consequently vesicles. In order to form vesicles, a thin layer of the HBP sample was coated on the inside surface of a glass vial by slow evaporation of a chloroform solution of the polymer under a N2 gas purge; water, along with Eosin-Y, was added to the vial, the film was hydrated and then sonicated at 80° C. The sonicated solution was cooled in an icewater bath, vortexed and heated again under sonication at 80° C; this cycle was repeated 4 times and then the solution was dialyzed to remove excess of un-encapsulated dye. For confocal microscopic studies, the dialyzed solution was drop-cast on to a microscope slide and examined using an excitation wavelength of 514 nm; Figure 6 shows confocal images 17 | P a g e ACS Paragon Plus Environment

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revealing the encapsulated dye. The images clearly reveal the formation micron-sized spherical vesicles; Eosin-Y dye appears to have been selectively taken up within the bilayer of the vesicles. Interestingly, the most uniform vesicles are formed in the case of HEXPA3070, which has a higher mole-fraction of the carboxylic acid functionality; this is unlike earlier observations in the solid state, where the other two samples with higher molefraction of hexadecyl chains appeared to self-segregate more effectively.

Figure 6: Confocal microscope images of Eosin-Y encapsulated vesicles formed by HEXPA-XY. (Inset is the image from different place of the same sample).

Conclusions In conclusion, we have demonstrated the utilization of a peripherally clickable HB polyester for the preparation of amphiphilic derivatives that carry different mole-fractions of hydrophobic cetyl segments and hydrophilic carboxylic acid groups at their molecular periphery. Even though these two types of segments are installed simultaneously in a statistically random fashion, the conformational adaptability of the HB polymer backbone permits the formation of a Janus-type structure, that resembles a cluster of surfactants stitched together as the waist; such self-segregation is reflected in the DSC thermograms that exhibit a melting peak associated with the cetyl chains and the Langmuir isotherms that reflect the formation compact monolayers, which when transferred on to a substrate leads to the formation of hydrophobic monolayer with contact angles as high as 100 degrees. Interestingly, spin coating hydrophilic mica substrates also generates similar hydrophobic surfaces, which improves significantly upon thermal annealing; this could be a useful process for the preparation of specialized surface coatings. Although, in this study we have utilized fairly long alkyl chains (C-16), in order to assist (and examine) crystallization-induced


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stabilization of the Janus structures, it results in poor water-solubility of the hybramers. However, this also presumably enables them to form vesicles in solution, as evident from the confocal fluorescence images of dye-encapsulated systems. In summary, it is evident that amphiphilic HBPs can be readily prepared via a single-step thiol-ene co-clicking reaction; it is evident that installation smaller alkyl chains and/or using more highly watersoluble segments will help further tune the hydrophilic-hydrophobic balance to render other interested surfactant systems. ACKNOWLEDGEMENTS We would like to thank the Department of Science and Technology, New Delhi, for the research grant (SR/S1/OC-84/2012) and for the award of J C Bose fellowship (2016-2021) to SR. Supporting Information Available. Monomer synthesis scheme, detailed NMR spectra, DSC thermograms and SAXS data. This material is available free of charge via the Internet at http://pubs.acs.org.


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FOR TABLE OF CONTENT USE ONLY Hybrasurfs – A new class of hyperbranched surfactants

N. S. Shree Varaprasad and S. Ramakrishnan*


HybrasurfsâA New Class of Hyperbranched Surfactants - Langmuir

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HybrasurfsâA New Class of Hyperbranched Surfactants - Langmuir