Oligomers as Triggers for Responsive Liquid Crystals

Article abstract of DOI:10.1021/acs.langmuir.8b01944

We report an investigation of the influence of aqueous solutions of amphiphilic oligomers on the ordering of micrometer-thick films of thermotropic liquid crystals (LCs), thus addressing the gap in knowledge arising from previous studies of the interactio

Full text of DOI:10.1021/acs.langmuir.8b01944

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Oligomers as Triggers for Responsive Liquid Crystals
Young-Ki Kim, Krishna R Raghupathi, Joel S Pendery, Piyachai Khomein, Uma
Sridhar, Juan J. de Pablo, Sankaran Thayumanavan, and Nicholas L. Abbott
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01944 • Publication Date (Web): 31 Jul 2018
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Oligomers as Triggers for Responsive Liquid Crystals
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YoungꢀKi Kim,† Krishna R. Raghupathi,‡ Joel S. Pendery,† Piyachai Khomein,‡ Uma Sridhar,‡
Juan de Pablo,§,#,* S. Thayumanavan,‡,* and Nicholas L. Abbott†,*
†Department of Chemical and Biological Engineering, University of WisconsinꢀMadison, 1415
Engineering Drive, Madison, WI 53706
‡Department of Chemistry, University of Massachusetts Amherst, 710 North Pleasant Street,
Amherst, MA 01003
§Institute for Molecular Engineering, University of Chicago, Chicago, IL 60637, USA
#Argonne National Laboratory, Argonne, IL 60439, USA
*corresponding authors
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ABSTRACT: We report an investigation of the influence of aqueous solutions of amphiphilic
oligomers on the ordering of micrometerꢀthick films of thermotropic liquid crystals (LCs), thus
addressing the gap in knowledge arising from previous studies of the interactions of monomeric
and polymeric amphiphiles with LCs. Specifically, we synthesized amphiphilic oligomers (with
decyl hydrophobic and pentaethylenegylcol hydrophilic domains) in monomer, dimer and trimer
forms, and incubated aqueous solutions of the oligomers against nematic films of 4’ꢀpentylꢀ4ꢀ
biphenylcarbonitrile (5CB). All amphiphilic oligomers caused sequential surfaceꢀdriven
orientational (planar to homeotropic) and then bulk phase transitions (nematic to isotropic) with
dynamics depending strongly on degree of oligomerization. The dynamics of the orientational
transitions accelerated from monomer to trimer, consistent with the effects of an increase in
adsorption free energy. The mechanism underlying the orientational transition, however,
involved a decrease in anchoring energy and not change in easy axis of the LC. In contrast, the
rate of the phase transition induced by absorption of oligomers into the LC decreased from
monomer to trimer, suggesting that constraints on configurational degrees of freedom influence
the absorption free energies of the oligomers. Interestingly, the oligomerꢀinduced transition from
the nematic to isotropic phase of 5CB was observed to nucleate at the aqueousꢀ5CB interface,
consistent with surfaceꢀinduced disorder underlying the aboveꢀreported decrease in anchoring
energy caused by the oligomers. Finally, we provided proofꢀofꢀconcept experiments of the
triggering of LCs using a trimeric amphiphile that is photocleaved by UV illumination into
monomeric fragments. Overall, our results provide insight into the rational design of oligomers
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that can be used as triggers to create responsive LCs
.
KEYWORDS: Interfaces, Ordering, Nematic, Phase transition, Adsorbates, Stimuliꢀresponsive
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INTRODUCTION
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Responsive materials undergo changes in structure and properties as a consequence of their
interactions with their environment, including via electrical and magnetic fields,1ꢀ2 mechanical
shear,3ꢀ4 changes in pH,5ꢀ8 UV irradiation,9ꢀ13 exchange of heat,14ꢀ16 and electrolyte
composition.17ꢀ18 This growing class of materials is enabling many potential applications, such
as the basis of sensors,3, 6ꢀ8, 12, 17 actuators,2, 8ꢀ11 and for achieving controlled release of active
chemical agents.4ꢀ5, 13ꢀ15, 18 Liquid crystals (LCs), in particular, offer substantial opportunities for
the design of responsive materials due to their ability to amplify molecularꢀlevel stimuli into the
optical scale.19ꢀ22 In this context, past studies23ꢀ30 have reported on the interactions of monomeric
and polymeric amphiphiles with aqueous interfaces of waterꢀimmiscible LC films and droplets.
Both monomers21ꢀ26 and polymers19ꢀ21, 26ꢀ30 have been reported to adsorb to LC interfaces and
trigger changes in ordering of the LCs that lead to easily quantified optical signals.
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LCs are anisotropic condensed phases that possess both the mobility of a fluid and the long
range order of crystalline solids.31ꢀ32 The long range orientational order present in a nematic LC
phase, where the constituent molecules align along a preferred axis called the director, gives rise
to elasticity and the tendency of the LC director to assume an unperturbed, homogeneously
aligned state. At the boundaries of a LC, the orientations assumed by the LC are determined by
intermolecular interactions with the confining medium, and deviations away from the preferred
orientation are energetically unfavorable. Consequently, in confined LC systems, the director
profile is determined by a competition between the anchoring energy (WL2) at the boundary of
the LC and the elastic energy (KL) in the bulk of the LC, where W is the surface anchoring
energy density, K is the Frank elastic constant of the LC, and L is a characteristic length of the
LC system.4, 31ꢀ34 As noted above, amphiphilic adsorbates can modify the anchoring of LCs at
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interfaces, and the elastic properties of the LCs can transmit and amplify these change in LC
anchoring throughout the volume of the LC that lies within ~100 µm of the interface.19ꢀ22 Since
LCs are birefringent, the optical appearance of the LCs, as observed using polarized optical
microscopy, change with the director profile of the LCs.35ꢀ36 Therefore, it is possible to infer the
presence and behavior of amphiphiles at LC/aqueous interfaces by observing the optical
responses of LC films.19ꢀ22
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Figure 1. a,b) Molecular structure of UVꢀcleavable oligomer amphiphile (UV3) in trimer form a) before and b) after
the cleavage reaction. Hydrophilic and hydrophobic groups are shown in green and red, respectively. cꢀe) Molecular
structures of oligomer amphiphiles with a hydrophilic polyethylene glycol functional group (shown in green) and
hydrophobic aliphatic tail (shown in red) in c) monomer (O1), d) dimer (O2), and e) trimer forms (O3).
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The work reported in this paper is motivated by the longꢀterm goal of designing responsive LC
systems based on stimuliꢀtriggered changes in the degree of oligomerization of amphiphiles. For
example, Figs. 1a and 1b show the design of a trimeric amphiphile that incorporates a functional
group that is cleaved upon illumination with UV light, thus converting the trimeric amphiphile
(Fig. 1a, UV3) to three monomeric fragments and a “core” (Fig. 1b). While this strategy of UV
(or chemicallyꢀtriggered) disassembly of oligomers appears a promising one for the design of
responsive LC systems, a key knowledge barrier to the rationale design of such systems is the
gap in understanding of how oligomeric amphiphiles interact with LCs. Whereas, as noted
above, the behaviors of amphiphilic polymers and monomers have been widely studied at LC
interfaces,19ꢀ22 oligomers have not been investigated. Past studies have established, however, that
the interactions of adsorbates/absorbates with nematic LCs are strongly dependent on the
configurational degrees of freedom of the adsorbates/absorbates and the extent to which
configurational degrees of freedom are modified by the orientationally ordered environment of
the LCs.7, 29, 37ꢀ40 For example, it is well known that flexible polymers segregate from nematic
LCs because the orientational order of the LC reduces the configurational degrees of freedom of
the polymer chain (entropic penalty).41ꢀ42 Accordingly, we hypothesized that the degree of
oligomerization may impact the miscibility of amphiphiles with LCs and thus the response of
LCs to exposure to oligomers.
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Motivated by this gap in knowledge, in this paper, we report a study of the interactions of
oligomeric amphiphiles with nematic LC films, focusing on both dynamic and equilibrium
responses of the LCs, as a function of the degree of oligomerization. The molecular structures of
the oligomers used in our study are shown in Figs. 1cꢀe. Each amphiphilic unit is composed of a
hydrophobic decyl sideꢀchain (shown using red in Figs. 1cꢀe) and a hydrophilic pentaethylene
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glycol (PEG, shown using green in Figs. 1cꢀe) sideꢀchain linked via an aromatic core. The
monomer (O1), dimer (O2), and trimer (O3) are composed of the amphiphilic units, linked
through a methylated amide. This amphiphile design was chosen for its facile synthesis, which
allows introduction of sideꢀchain functionalities selected to provide a response to the targeted
external stimulus. In the second part of the paper, we use structureꢀproperty relationships
developed in the first part of the paper to guide the design and interpretation of experiments
using the photoresponsive trimer shown in Fig. 1a.
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METHODS
O1 (86 %yield)
Jones Reagent
2.7 mL / mmol
DCM/SOCl2
TEA
O2 (72 %yield)
O3 (66 %yield)
Acetone, 0 oC
76 %
1)
Compound A
Compound B
Compound D
2) MeI/NaH, THF
DCC/TEA
K2CO3
DCM, reflux
55 %
DMF/140 oC
6 hrs 58 %
Compound E
Compound C
1. SOCl2/CF 2. TEA
H
N
H
N
N
UV3
H
25 %
CuSO4. 5H2O
NaAscobate
THF/H2O, 29 %
Trimer Precursor
Scheme 1. Synthesis of oligomer amphiphiles.
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Materials and Characterizations. All chemicals and reagents were purchased from commercial
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sources and were used as received, unless otherwise mentioned. HꢀNMR spectra were recorded
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on 400 MHz Bruker NMR spectrometer using the residual proton resonance of the solvent as the
internal standard. Chemical shifts are reported in parts per million (ppm). When peak
multiplicities are given, the following abbreviations are used: s, singlet; d, doublet; t, triplet; m,
multiplet. 13CꢀNMR spectra were proton decoupled and measured on a 500 MHz Bruker
spectrometer with 125 MHz frequency by using carbon signal of the deuterated solvent as the
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internal standard. H NMR of the methylated amphiphiles showed incorrect integrations and
were rounded to the expected values. To clearly confirm the formation and purity of those
products mass spectrometry was performed and reported.
Preparation of Jones Reagent. Chromium trioxide (7 g, 70 mmol) was dissolved in 10 mL of
water in a 50 mL Erlenmeyer flask. The solution was then cooled using an ice bath followed by
slow addition of 6.1 mL of concentrated sulfuric acid (18 M). The resultant solution was then
diluted with 20 mL of water with constant stirring.
Synthesis of Acid Precursor. Compound A (0.77 g, 1.49 mmol) was dissolved in acetone and
cooled using an ice bath. Compound A was synthesized following the previously reported
procedure.43 To this solution was added a cooled solution of Jones reagent (4 mL, 2.7
mL/mmol) drop wise, and stirred for 4 hours. Upon complete conversion of the reaction
(checked by thin layer chromatography), the reaction mixture was filtered to remove a green
precipitate formed. To this filtrate was added excess of isopropyl alcohol and filtered again to
remove any new precipitate formed. The reaction mixture was then concentrated in vacuo and
the residue was dissolved in water and the pH of this aqueous phase was adjusted to pH<3,
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followed by extraction with ethyl acetate (2x), the combined extracts were then dried over
anhydrous Na2SO4. Upon evaporation of the solvent, the crude product was purified by silica gel
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column chromatography (using combiflash) to afford 0.6 g (76%) of compound B. HꢀNMR
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(400 MHz; Acetoneꢀd6): δ 7.17ꢀ7.16 (m, 2H), 6.75 (t, J = 2.3Hz, 1H), 4.18 (t, J = 4.7 Hz, 2H),
4.04 (t, J = 6.5 Hz, 2H), 3.84 (t, J = 4.7 Hz, 2H), 3.68ꢀ3.54 (m, 14H), 3.47ꢀ3.45 (m, 2H), 3.28 (s,
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3H), 1.82ꢀ1.75 (m, 2H), 1.53ꢀ1.45 (m, 2H), 1.41ꢀ1.28 (m, 14H), 0.88 (t, J = 6.9 Hz, 3H). Cꢀ
NMR (125 MHz; Acetoneꢀd6): δ 170.9, 167.3, 161.27, 161.09, 133.3, 108.8, 108.6, 106.8, 72.7,
71.44, 71.29, 71.25, 71.06, 70.3, 68.92, 68.75, 60.5, 58.8, 32.6, 26.7, 25.7, 23.3, 20.8, 14.50,
14.36.
General procedure for synthesis of Oligomers O1 to O3. To a solution of compound B in
methylene chloride was added excess of thionyl chloride (SOCl2) and the mixture was refluxed
for 4 hours. The reaction mixture was then concentrated in vacuo to remove unreacted thionyl
chloride. The crude acid chloride product was then dried for an additional 2 hours under vacuum
and redispersed in methylene chloride (DCM). To this solution of acid chloride was then added
appropriate equivalents of the corresponding amine (1 equivalent dimethylamine for O1, 0.5
equivalent N,N’ꢀdimethyl ethylene diamine for O2) and triethylamine (TEA) and stirred at room
temperature for 24 hours. The reaction mixture was then concentrated in vacuo and the residue
was dissolved in water and extracted twice with ethyl acetate, the combined extracts were then
dried over anhydrous Na2SO4. Upon evaporation of the solvent, the crude product was purified
by silica gel column chromatography (using combiflash) to obtain oligomers O1 and O2
respectively. O3 was synthesized in accordance with a previously reported procedure.43ꢀ44
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Monomeric Oligomer (O1): HꢀNMR (400 MHz; CDCl3): δ 6.51 (m, 2H), 6.49 (m, 1H), 4.10
(t, J = 4.8 Hz, 2H), 3.92 (t, J = 6.6 Hz, 2H), 3.84 (t, J = 4.8 Hz, 2H), 3.73ꢀ3.63 (m, 14H), 3.54
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(dd, J = 5.7, 3.6 Hz, 2H), 3.37 (s, 3H), 3.08 (s, 3H), 2.96 (s, 3H), 1.75 (dt, J = 14.4, 7.0 Hz, 2H),
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1.44ꢀ1.33 (m, 14H), 0.88 (t, J = 6.8 Hz, 3H). Cꢀ NMR (125 MHz; CDCl3): δ 171.4, 160.4,
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159.9, 138.2, 105.8, 105.4, 102.7, 72.0, 70.92, 70.72, 70.69, 70.67, 70.62, 69.7, 68.4, 67.7, 59.1,
39.6, 35.4, 32.0, 29.68, 29.67, 29.48, 29.43, 29.28, 26.1, 22.8, 14.2. MALDIꢀToF m/z 578.40
(C30H53NO8+Na+ requires 578.3698).
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Dimeric Oligomer (O2): 1HꢀNMR (400 MHz; Acetoneꢀd6): δ 6.53ꢀ6.38 (m, 6H), 4.15ꢀ3.55
(m, 44H), 3.46 (dd, J = 5.8, 3.9 Hz, 4H), 3.28 (s, 6H), 3.05 (s, 6H), 1.77ꢀ1.67 (m, 4H), 1.48ꢀ1.30
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(m, 28H), 0.88 (t, J = 6.6 Hz, 6H). Cꢀ NMR (125 MHz; Acetoneꢀd6): δ 171.4, 161.0, 160.8,
140.0, 106.1, 105.9, 103.1, 72.7, 71.4, 71.1, 70.2, 68.7, 68.5, 58.8, 44.8, 37.8, 23.3, 14.4. ESIꢀ
MS m/z 1131.7344 (C60H104N2O16+Na+ requires 1131.7298).
Trimeric Oligomer (O3): 1HꢀNMR (400 MHz; Acetoneꢀd6): δ 6.57ꢀ6.41 (m, 9H), 4.16ꢀ3.55
(m, 60H), 3.46 (t, J = 4.8 Hz, 6H), 3.28 (s, 9H), 3.06 (s, 3H), 2.66 (s, 3H), 1.79ꢀ1.65 (m, 6H),
1.33 (d, J = 75.8 Hz, 42H), 0.88ꢀ0.86 (m, 9H). 13Cꢀ NMR (125 MHz; Acetoneꢀd6): δ 172.0,
171.6, 161.21, 161.06, 161.00, 160.94, 160.74, 139.7, 106.32, 106.24, 106.11, 105.91, 105.54,
105.43, 103.70, 103.52, 103.1, 102.9, 72.6, 71.37, 71.32, 70.28, 70.13, 68.82, 68.72, 68.58,
68.43, 58.8, 23.3, 14.4. MALDIꢀToF m/z 1685.493 (C90H155N3O24+Na+ requires 1685.0898).
Synthesis of compound C and D. 5ꢀhydroxyꢀ2ꢀnitrobenzyl alcohol (500 mg, 2.96 mmol) was
dissolved in DMF (10 mL). Potassium carbonate (820 mg, 5.94 mmol) and nꢀdecyl bromide (720
o
mg, 3.26 mmol) were added into the solution. The reaction was stirred at 140 C for 6 hours
under argon atmosphere. The reaction was then cooled down to room temperature and 90 mL of
water was added to the reaction. The product was extracted into ethyl acetate and washed with
water and brine and dried over sodium sulfate. The product was further purified by flash column
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chromatography (hexane/ethyl acetate) to give yellow liquid compound C (530 mg, 58%). Hꢀ
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NMR (CDCl3): δ 8.17 (d, 1H), 7.19 (d, 1H), 6.88 (dd, 1H), 4.98 (s, 2H), 4.07 (t, 3H), 1.82 (m,
2H), 1.46 (m, 2H), 1.40ꢀ1.20 (b, 12H), 0.88 (t, 3H).
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Compound C (530 mg, 1.71 mmol) and 2ꢀazido acetic acid (190 mg, 1.88 mmol) were
dissolved in dichloromethane (10 mL). N,N’ꢀdicyclohexyl carbodiimide (DCC, 387 mg, 1.88
mmol) was then added to the reaction followed by triethylamine (TEA, 0.26 mL, 1.88 mmol).
The reaction was stirred at room temperature for 12 hours. All solid was then removed my
filtration and the product was purified by flash column chromatography (hexane/ethyl acetate) to
give yellow liquid compound D (370 mg, 55%). 1HꢀNMR (CDCl3): δ 8.21 (d, 1H), 7.03 (d, 1H),
6.91 (dd, 1H), 5.66 (s, 2H), 4.05 (t, 2H), 4.01 (s, 2H), 1.82 (m, 2H), 1.46 (m, 2H), 1.40ꢀ1.20 (b,
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12H), 0.88 (t, 3H). CꢀNMR (CDCl3) : δ 167.8, 163.8, 140.1, 134.3, 128.4, 114.9, 113.5, 69.2,
64.7, 50.6, 32.0, 29.7, 29.4, 29.1, 26.0, 22.8, 14.2. ESI/Mass: Calc. (C19H28N4O5+Na+) 415.1958
found 415.2733.
Synthesis of trimer precursor. Compound E (400 mg, 0.94 mmol), which was synthesized
following the previously reported procedure,44 was dissolved in chloroform (1 mL). Thionyl
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chloride (1 mL, 13.8 mmol) was added to the reaction and it was stirred at 60 C for 6 hours.
Solvent and the excess of SOCl2 were removed under vacuum to give the acid chloride
derivative. Then, it was dissolved again in chloroform (1 mL). 1,7ꢀdimethyl ethylene triamine
(26 mg, 0.20 mmol) was added followed by TEA (0.4 mL, 2.8 mmol). 1,7ꢀdimethyl ethylene
triamine was synthesized following the previously reported procedure.45 The reaction was
stirred at room temperature for 14 hours. The product was purified by flash column
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chromatography (dichloromethane/methanol) to yield a viscous yellow liquid (95 mg, 25%). Hꢀ
NMR (CDCl3): δ 6.68ꢀ6.57 (m, 6H), 6.46ꢀ6.34 (m, 3H), 4.57 (s, 6H), 3.99 (m, 6H), 3.78 (b, 6H),
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3.72ꢀ3.62 (b, 42H), 3.54 (t, 6H), 3.37 (s, 9H), 2.66 (s, 3H), 2.22 (s, 6H). ESI/Mass: Calc.
(C69H101N3O24+Na+) 1378.6673 found 1378.6738.
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Synthesis of UV-cleavable trimer (UV3). Trimer precursor (95 mg, 0.07 mmol) and compound
D (117 mg, 0.30 mmol) were dissolved in tetrahydrofuran (THF). Copper sulfate pentahydrate
(75 mg, 0.30 mmol) and sodium Lꢀascobate (60 mg, 0.30 mmol) were added to the solution.
Couple drops of water were added into the mixture. The reaction was stirred at 60 oC for 6 hours.
The solid was removed by filtration and the filtrate was purified by flash column
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chromatography to yield a viscous yellow liquid product (51 mg, 29%). HꢀNMR (CDCl3): δ
8.20 (d, 3H), 7.78 (s, 3H), 6.95ꢀ6.90 (m, 6H), 6.61ꢀ6.60 (m, 3H), 6.59 (t, 3H), 6.57ꢀ6.56 (m, 3H),
5.64 (s, 6H), 5.30 (s, 6H), 5.22 (s, 6H), 4.10 (t, 6H), 4.06 (t, 6H), 3.83 (t, 6H), 3.70ꢀ3.60 (b,
50H), 3.54 (m, 6H), 3.37 (s, 9H), 3.08 (s, 6H), 1.83 (m, 6H), 1.48 (m, 6H), 1.40ꢀ1.30 (b, 36H),
0.88 (t, 9H). ESI/Mass: Calc. (C126H185N15O39+Na+) 2555.2852 found 2555.1102.
RESULTS and DISCUSSION
Our initial experiments were performed to determine if amphiphilic oligomers O1-O3 would
partition to LC interfaces and influence the ordering of LCs. To this end, a LC film (10 µm in
thickness) was prepared by depositing nematic 4’ꢀpentylꢀ4ꢀbiphenylcarbonitrile (5CB) into a
gold TEM grid placed on a glass substrate functionalized with dimethyloctadecyl[3ꢀ
(trimethoxysilyl)propyl]ammonium chloride (DMOAP). The DMOAP functionalization was
performed to impose a homeotropic orientation on 5CB at the treated substrate.46ꢀ47
Subsequently, the LC film was immersed into an aqueous bath containing 10 mM phosphate
buffered saline (PBS) at pH 7.4 and the relevant amphiphilic oligomer (Fig. 2a).
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Figure 2. a, b) Schematic illustration (side view) and optical micrographs (top view, crossed polars) of 5CB films
hosted in a TEM grid (10 µm in thickness) supported on a DMOAPꢀtreated glass substrate and immersed under a
PBS aqueous solution. Dashed lines in a) indicate the orientation of LC. cꢀh) Sequential micrographs of the LC film
following addition of O2 (C2=0.08 mM) into the aqueous phase at c) 4, d) 5, e) 11, f) 12, g) 20, and h) 31 hours.
o
Insets in d) and h) are the corresponding conoscopic textures. The experiments were performed at 25 C. Scale bar,
200 µm.
We first investigated the influence of the dimeric oligomer (O2) dispersed in the aqueous
solution on the orientation of 5CB at the LC/aqueous interface by characterizing the optical
response of LC films using polarizing optical microscopy (POM).19, 21 Each LC film, when
immersed under aqueous PBS (no oligomer), exhibited a Schlieren texture with dark brushes
(Fig. 2b) that correspond to regions of the LC in which the director is aligned either parallel or
perpendicular to one of the crossed polarizers.31ꢀ32, 35 Because 5CB anchors with a perpendicular
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orientation (homeotropic) at the DMOAPꢀcoated glass substrate, the bright optical appearance of
the LC film arises from planar anchoring of nematic 5CB at interfaces to the bulk aqueous
phases with neutral pH and low ionic strength.19, 21 These soꢀcalled hybrid boundary conditions
induce a continuous bend of the director within the 5CB film (Fig. 2a). We note that the
Schlieren texture also indicates that the azimuthal anchoring of the LC at the aqueous interface is
degenerate (Fig. 2b). Following the introduction of 0.08 mM O2 into the bulk aqueous phase, we
made two key observations. First, we observed the brightness of the LC film to diminish (Fig.
2c), forming a dark monodomain after 5 hours (Fig. 2d, 1st transition). When observing the dark
state of the LC under conoscopic observation, we found a “Maltese cross” isogyre (inset in Fig.
2d) that indicates uniform homeotropic ordering of the LC across the LC film.16 Second, after 11
hours of incubation, bright domains emerged (Fig. 2e) and grew to cover the entire LC film (Fig.
2f, 2nd transition). Subsequently, dark domains appeared at the edges of the birefringent LC
films, forming isolated bright domains that decreased in size (Fig. 2g) until the film assumed
again a uniform dark appearance (Fig. 2h, 3rd transition). The final state exhibited a dark texture
(no Maltese cross) under conoscopy (inset in Fig. 2h).
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The 1st transition (Figs. 2bꢀd) reported above (dark state with Maltese cross) reveals that O2
within the bulk aqueous phase initially partitioned onto the LC/aqueous interface to cause an
orientational transition from an initial tangential orientation to a homeotropic orientation. This
type of transition in orientation can occur through at least two mechanisms19ꢀ21: (i) a change in
the easy axis, i.e. the preferred orientation of the director is changed by interactions of the alkyl
chains of O2 with mesogens at the interface, or (ii) a decrease in the surface anchoring energy
(WL2) that allows the orientation of LC at the aqueous interface to relax to that imposed by the
DMOAPꢀtreated glass substrates (to minimize the elastic energy (KL)). To reveal the origin of
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the ordering transition, we prepared two 5CB films supported on glass substrates that were
coated with either DMOAP or polyimide (PI) that was rubbed to achieve unidirectional planar
alignment. The thickness of each LC film was 10 µm. This film thickness was selected to
generate easily quantified changes in optical retardance of LC films with an effective
birefringence of 0≤∆n≤0.2, where ∆n=0.2 is the birefringence of 5CB1.
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Figure 3. Optical micrographs (top view, crossed polars) and director profiles (side view) of 5CB films supported
on glass substrates coated with aꢀc) DMOAP and dꢀf) a rubbed polyimide following contact with a,d) air and
aqueous solutions b,e) without and c,f) with O2 (C2=0.15 mM). Micrographs in c) and f) were obtained after
incubation for 5 hours. R, P and A indicate the rubbing direction and orientation of polarizer and analyzer,
respectively. Dashed lines in the second and fourth columns indicate the LC orientation. The experiments were
performed at 25 oC. Scale bar, 200 µm.
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In air, the 5CB film supported on the DMOAPꢀtreated glass surface was uniformly
homeotropic (Fig. 3a), consistent with past reports of homeotropic anchoring of 5CB at
interfaces to air,19 whereas the 5CB film supported on the rubbed PIꢀcoated surface exhibited a
birefringent texture corresponding to a hybrid configuration arising from planar anchoring at the
PI interface and homeotropic anchoring at the air interface (Fig. 3d). In aqueous PBS, because
the LC/aqueous interface anchors the LC in a degenerate tangential orientation, the DMOAPꢀ
supported film showed the Schlieren texture (Fig. 3b), as reported above. In contrast, the LC film
supported on the PIꢀcoated surface exhibited a uniform planar alignment, leading to a bright
appearance without dark brushes and a coloration corresponding to higher order optical
retardance (Fig. 3e) as compared to that exhibited in air. Finally, we added O2 into the overlying
aqueous phase (C2=0.15 mM) and equilibrated the 5CB films for 4 hours. As O2 molecules
partitioned to the LC/aqueous interface, the DMOAPꢀsupported film restored the original dark
appearance (Fig. 3c). In contrast, the PIꢀsupported film retained the bright optical texture and
coloration (Fig. 3f) observed before the introduction of O2 (Fig. 3e, uniform planar anchoring).
If the adsorption of O2 onto the LC/aqueous interface had modified the easy axis of the LC, the
PIꢀsupported film would have assumed an optical appearance similar to the PIꢀsupported film in
air (Fig. 3d, hybrid anchoring). Our observations, therefore, suggest that the mechanism by
which O2 causes the orientational transition reported in Figs. 2bꢀd (1st transition) involves a
decrease in the anchoring strength of the 5CB at the aqueous interface. This mechanism contrasts
to past studies of conventional surfactants and phospholipids which have established that the
amphiphiles cause a change in orientation of the easy axis.19ꢀ21 Additional observations
described below support the proposal that O2 lowers the anchoring energy of the LC by inducing
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local disorder (melting) at the interface of the LC, a proposal that is also supported by molecular
simulations to be published elsewhere.
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As noted above, following the 1st transition to the dark homeotropic state (Fig. 2d), the LC
films immersed in aqueous O2 exhibited a subsequent transition to a transient bright state (Figs.
2eꢀg) and then a second and final dark state (Fig. 2h). The final dark state (Fig. 2h) differed from
the homeotropic state discussed above (Fig. 2d) in two important ways. First, whereas the edge
of the LC within each grid square was bright in the first dark state (Fig. 2d; due to interaction of
the LC with the vertical surfaces of the grid), no bright edges were present in the final dark state
(Fig. 2h). Second, in contrast to the first dark state, as noted above, conoscopic observation of
the film in the final dark state did not generate a Maltese cross (insets in Figs. 2d and 2h). These
two observations suggest that the final dark state arises from either (i) the absence of the 5CB in
the grid (due to dewetting or solubilization of the LC) or (ii) the formation of an isotropic phase
(I) of 5CB. Reflection mode imaging of the grid confirmed that 5CB remained in the grid (Fig.
S1), and thus we concluded that the final dark state corresponded to an isotropic phase of 5CB.
That is, the orientational transition that was driven by the interaction of O2 with the aqueous
interface of nematic 5CB was followed by a phase transition which transformed the nematic 5CB
to an isotropic phase.
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Figure 4. a) Absorbance of aqueous solutions with O2 (initial C2=0.3 mM) before (Apre, solid line) and after 5 hours
of incubation against the 5CB film (Apost, dotted line) along with its difference spectrum (∆A= Apost –Apre, dash dot
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line). b) ∆A with respect to initial C2. The experiments in a) and b) were performed at 25 C. c) Phase diagram of
5CB as a function of the concentration of O2 doped in 5CB ( C25CB ).
To provide insight into the mechanism through which O2 caused the 5CB film to undergo a Nꢀ
toꢀI phase transition, we measured the UVꢀvisible absorbance of each aqueous O2 solution
before (Apre) and after 5 hours of incubation (Apost) against the 5CB film. After incubation, the
5CB films were either isotropic (C2 = 0.3 mM) or nematic (C2 = 0.02, 0.04, 0.08, 0.15 mM (see
o
below)) phases at T = 25 C. For an aqueous solution with initial C2=0.3 mM (Fig. 4a), we
observed a significant decrease in absorbance following incubation against the 5CB (at λ=284
nm, absorbance of the aromatic groups of the dimer). The decrease in absorbance (∆A=ApostꢀApre)
indicates partitioning of O2 into the 5CB film. Consistent with this interpretation, the differential
absorbance (∆A<0) grew in magnitude with increase in the initial value of C2 (Fig. 4b). From the
measured ∆A of an aqueous solution of O2 (initial C2=0.3 mM) upon incubation with a 5CB film
(Fig. 4a), we calculated that the concentration of O2 dissolved within 5CB (C25CB ) increased to
14.5 wt% (Fig. S2 and SI text). To determine the influence of O2 on the phase behavior of 5CB,
we measured a phase diagram of 5CB doped with O2. As shown in Fig. 4c, with increase in
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C25CB , we observed a broadening of the temperature range over which coexisting N and I phases
(N+I) were observed along with a decrease in the clearing temperature (N+IꢀtoꢀI, TC). We
compared TC of a 5CB film incubated against 0.3 mM of aqueous O2 for 5 hours (16 °C, Fig.
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S3) to a calibration plot of TC measured as a function of C25CB dissolved into 5CB (Fig. 4c and
S4) to establish that the weight percentage of O2 absorbed into the LC film from the aqueous
solution was 13.9 wt%. This estimate is in good agreement with our independent measurement
based on the decrease in A (C25CB = 14.5 wt%, see SI text).
Figure 5. a) Schematic illustration (sideꢀview) of the LC film with a director profile (dotted lines) at the transient
bright state. Yellow bars and dashed lines indicate an ordering of LCs at interfaces and within the film, respectively.
b) Optical micrographs (topꢀview, crossed polars) and c) director profiles of bright domains in the LC film generated
o
by isotropic domains of 5CB at the LC/aqueous interface. The experiments were performed at 25 C. Scale bar, 200
µm.
Having established that the final dark state of the 5CB film contacting O2 corresponds to an
isotropic phase, we return to the observation of transient bright domains that precede this final
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state (Figs. 2dꢀf). We hypothesized that these domains arise from processes that take place
during the NꢀtoꢀI phase transition of the 5CB. As noted above, our experimental observations
indicate that the orientational transition caused by adsorption of O2 at the aqueous interface
(Figs. 2bꢀd) is due to a decrease in anchoring energy, an observation that is consistent with O2
disordering the nematic phase within a thin film located a few nanometers from the aqueous
interface. Guided by these observations, we hypothesized that nucleation of the isotropic phase
caused by uptake of O2 would occur at the aqueous interface of the LC film (Fig. 5a). To test
this prediction, we incubated a 40 µmꢀthick LC film against aqueous O2 (C2=0.3 mM) until
birefringent domains appeared in a homeotropic monodomain (Fig. 5b). By adjusting the vertical
location of the focal plane of the microscope, we established that the bright domains were
located near the aqueous interface (Fig. 5a). The bright domains arise from the orientational
anchoring of nematic 5CB at its isotropic phase, which was measured to be θ=63.5o from the
interface normal (Figs. 5a,c).48ꢀ49 This tilted orientation of the nematic phase is sufficient to
cause the bright birefringent states.16 We also interpret our observations to indicate that the
shapes of the domains of isotropic 5CB corresponded to spherical caps (Fig. 5). The interfacial
tension of the nematic/isotropic interface (σNI=1.5 ± 0.6 x 10ꢀ5 J/m2) is approximately two orders
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of magnitude lower than that of the Nꢀwater (σNW) or Iꢀwater (σIW) interfacial tensions (σNW
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7x10ꢀ3 J/m2, σIW = 6x10ꢀ3 J/m2 at T=35oC)50ꢀ51 and thus σNW>σIW>>σNI. To minimize interfacial
energies,52 therefore, the isotropic domain of 5CB forms a spherical cap protruding into the
nematic phase with the aqueous interface remaining largely flat (Fig. 5a). The diameter of each
isotropic domain is much larger than the depth of penetration of the cap into the nematic phase,
consistent with estimates of interfacial free energies (see SI). We also observed the isotropic
domains of 5CB at the aqueous interface to be mobile and to merge with isotropic regions of
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5CB that accumulated at the grid edges, a behavior that is also consistent with localization of the
isotropic domains at the aqueous interface (not solid/LC interface). Importantly, the observation
that the isotropic domains nucleated at the O2ꢀdecorated interface provides support for our
conclusion that O2 alters the orientation of the LC through surfaceꢀinduced melting of the
nematic phase.
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Figure 6. aꢀd) Schematic illustrations of LC film contacting an aqueous solution containing oligomeric amphiphiles,
yielding an anchoring transition from a) a planar to b) homeotropic orientation at LC/aqueous interfaces, and then a
phase transition from a nematic to c) nematicꢀisotropic coexistence and ultimately to d) an isotropic phase. eꢀg)
Timeꢀdependent anchoring and phase transitions of the LC films in aqueous solutions with e) O1, f) O2, and g) O3
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as a function of their concentrations. The experiments were performed at 25 C. P, H, N+I, and I indicate nematic
films with planar (yellow bar) and homeotropic anchoring (purple bar) at the LC/aqueous interface, formation of
isotropic droplets (green bar, nematicꢀisotropic coexistence phase), and complete transition to isotropic phase (blue
bar), respectively.
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Next, we sought to determine the extent to which the degree of oligomerization impacted the
responses of the LC films to the amphiphiles (Figs. 6aꢀd). Specifically, we compared the
dynamics of the transitions triggered by O1 (monomer), O2 (dimer), and O3 (trimer), as a
function of their initial concentration in the bulk aqueous phase (Figs. 6eꢀg). We make three key
observations from the data in Figs. 6eꢀg. First, all three oligomers exhibited the same progression
of orientational and phase states. The concentrations at which they were observed and their
lifetimes, however, were significantly different for each oligomer. Second, with increase in
initial concentration of each oligomer in the aqueous solution, we observed the dynamics of the
anchoring transition to the homeotropic state and then phase transition to N+I or I state to
accelerate, indicating that the rates of uptake of the oligomers onto the LC/aqueous interfaces
and then into the bulk of the LC films were concentrationꢀdependent. Third, for a fixed
concentration, with increase in degree of oligomerization, we measured the dynamics of the
anchoring transition to the homeotropic orientation to accelerate. In contrast, the phase
transitions from N (homeotropic anchoring)ꢀtoꢀN+I or N+IꢀtoꢀI slowed with increase in degree
of oligomerization. For example, at C1=C2=C3=0.05 mM, we measured the transition to the
homeotropic orientation to be complete at 10.2 hours for O1, 7.1 hours for O2, 3.8 hours for O3,
whereas the N (homeotropic state)ꢀtoꢀN+I phase transition occurred at 12.3 hours for O1, 19.7
hours for O2, and >60 hours for O3. If O1ꢀO3 are compared at the same concentrations of
amphiphilic units in aqueous solutions (e.g., at C1=0.15, C2=0.1, C3=0.05), however, the LC
films assumed homeotropic orientations with comparable dynamics, suggesting that the ordering
of LC is strongly coupled to the interactions of the amphiphilic units of the oligomers;
homeotropic states were observed at 3.9 hours for C1=0.15, 3.8 hours for C2=0.1, and 3.8 hours
for C3=0.05. The distinct dynamics exhibited by the oligomers (at the same oligomer
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concentration) leads to the important conclusion that an increase in degree of oligomerization
will i) accelerate the interactions of oligomers with LCs at the LC/aqueous interface that cause
the homeotropic orientation and ii) suppress the absorption of oligomers into the bulk of the LC
films (thus slowing the transition to the isotropic phase).
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The behaviors of the monomeric (O1), dimeric (O2), and trimeric (O3) amphiphiles, as
described above, led us to hypothesize that UVꢀtriggered cleavage of a trimeric amphiphile
(UV3, Fig. 1a) should generate distinct changes in both equilibrium and dynamic orientational
states and phase states of films of 5CB. To test this prediction, we incubated an aqueous solution
of UV3 against 5CB (CUV=0.1 mM). The 5CB, which was supported on a DMOAPꢀcoated glass
substrate, initially exhibited a typical Schlieren texture (Fig. 7a). The film was observed to
undergo an anchoring transition to a homeotropic orientation over 5 hours (Fig. 7b). The
resultant homeotropic anchoring was maintained for several days without showing evidence of
birefringent domains, similar to our observations with trimeric amphiphile O3 (Fig. 6g). In
addition, we explored the reversibility of adsorption of UV3 at the LC/aqueous interface and
compared it to O3. When the overlying aqueous solution of UV3 (CUV=0.1 mM) was replaced by
water (CUV=0 mM) following 5 hours of incubation against the LC film (Figs. 7b,c), desorption
of UV3 occurred within 2 hours as evidenced by a transition in the optical appearance of the LC
film from a dark monodomain (Fig. 7b) to a birefringent Schlieren texture (Fig. 7c). To replace
the overlying aqueous solution by water, we first removed the aqueous UV3 and the LC sample
from a bath (a thin film of UV3 solution remained on and covered the LC surface). The empty
bath was rinsed to remove residual UV3 and then was refilled with water. Finally, the LC film
was submerged into the bath filled with water. We estimated the concentration of UV3 in the
final aqueous solution to be less than 5·10ꢀ4 mM. Similarly, when aqueous O3 (C3=0.1 mM) was
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replaced with water (C3=0 mM) following incubations of DMOAPꢀsupported LC films for 5
hours, we observed the homeotropic dark state to become birefringent, consistent with reversible
adsorption, as observed with UV3. When combined, these results reveal that O3 and UV3 trigger
similar changes in the LC.
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Figure 7. aꢀf) Optical micrographs (crossed polars) of 5CB films immersed in aqueous solutions with aꢀc) UV3
(CUV=0.1 mM) or dꢀf) cleaved UV3 (CCUV=0.1 mM) at a,d) 0 and b,e) 5 hours, and c, f) after incubation against
water for 2 hours. gꢀi) Optical micrographs of LC films g) equilibrated in aqueous UV3 (CUV=0.1 mM) for 4 hours
followed by h) 1 hour of UV irradiation and i) 2 hours incubation after dilution of UV3 concentration to 0 mM. The
experiments were performed at 25 oC. Scale bar, 200 µm.
Next, we explored how UVꢀtriggered cleavage products of UV3 influenced a film of 5CB. As
shown in Figs. 1a,b, UV cleavage generates three monomeric fragments and a core unit. We
irradiated an aqueous solution of UV3 (CUV=0.1 mM) with UV light (wavelength of 365 nm,
power 8 W) for an hour to cleave UV3 into fragments. Completion of the cleavage reaction was
verified by measurements of the UVꢀvisible absorbance of the aqueous UV3 solution (Fig. S5).
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Following incubation of the cleaved UV3 solution against a DMOAPꢀsupported LC film for 5
hrs, in contrast to the behavior of UV3 prior to cleavage (Figs. 7a,b), the LC did not exhibit a
dark appearance (Figs. 7d,e). This result is consistent with cleavage of UV3 and indicates that
the cleavage products of UV3 (monomeric fragments, Fig. 1b) caused neither an anchoring
transition nor phase transition in the 5CB. We note that the same concentration of the monomeric
amphiphile O1 (C1=0.3 mM) in an aqueous solution induces both anchoring and phase
transitions in 5CB films over 5 hours of incubation (Fig. 6e). The structures of O1 and the
fragments of cleaved UV3, however, differ in important ways (Figs. 1b,c): although both possess
decyl chains, the fragments of UV3 do not possess the hydrophilic pentaethyleneglycol chains of
O1 and thus are unlikely to be as water soluble (or as interfacially active) as O1. Because we
would expect there to be a strong thermodynamic driving force for partitioning of the
hydrophobic fragments of UV3 into the LC, the absence of both the anchoring and phase
transitions in the 5CB film leads us to conclude that the sparing water solubility of the UV3
fragments likely results in their aggregation in bulk aqueous solution and thus kinetically slow
partitioning into the LC on the experimental timeꢀscale. Additional results presented below
support this interpretation.
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To investigate the in situ optical response of the LC film to UVꢀtriggered cleavage of UV3, we
equilibrated a 5CB film against an aqueous solution of UV3 (CUV=0.1 mM) to achieve a
homeotropic LC orientation and then irradiated the system (aqueous solution of UV3 and UV3
adsorbed on interface of LC) with UV light for 1 hour. Interestingly, we measured no detectable
departure of the LC film from the dark homeotropic state during or after UV irradiation (Figs.
7g,h). This result is significant in light of the observation, as reported above, that cleavage
products of UV3 present in aqueous solution do not impose a homeotropic orientation on the LC
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(Fig. 7e). We interpret this result to indicate that hydrophobic fragments generated by cleaving
UV3 adsorbed on the LC interface remain adsorbed to the interface of the LC (control
experiments performed without UV3 confirmed that UV exposure of the LC did not cause
homeotropic alignment). To provide additional insight into the dynamics of desorption of the
UV3 cleavage products from the aqueous interface of the LC, we exposed a LC sample (in
contact with aqueous UV3) to UV light and then equilibrated the sample against an aqueous
solution free of UV3 for 2 hours (Figs. 7h, i). We observed the sample to remain in the
homeotropic state. In contrast, we observed UV3 to desorb from the LCꢀaqueous interface in the
absence of UV irradiation within 2 hours (Figs. 7b,c). This result provides support for our
conclusion that the absence of reversible desorption of UV3 cleavage products is due to the
hydrophobic nature of the fragments (see above). However, additional experiments that
characterize the kinetics of desorption of UV3 fragments from the interface are needed to fully
understand the molecular origins of our observation.
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Overall, the results above indicate that the dynamics of the LC response to UV3 is altered by
UV cleavage of the trimeric amphiphile. Specifically, UV3 triggers a transition to homeotropic
anchoring (Fig. 7b) whereas the cleavage products of UV3 do not (Fig. 7e). In addition, UVꢀ
cleavage of the trimer adsorbed on the interface of the LC leads to distinct differences in the
dynamic response of the LC to contact with water: UV3 generates a transition from homeotropic
to planar anchoring (Fig. 7c) whereas the cleavage products of UV3 maintain the homeotropic
orientation of the 5CB (Fig. 7i). In addition to revealing that oligomers form the basis of a
promising class of triggers for the design of responsive LC systems, these results hint that an
understanding of the kinetics of these systems will be an important challenge to address in future
studies aimed at development of rational design principles.
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CONCLUSION
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In summary, we have characterized the interactions of nematic LC films with aqueous
solutions of amphiphilic monomers (O1), dimers (O2) and trimers (O3). Specifically, we found
that oligomeric amphiphiles O1-O3 caused nematic 5CB to undergo sequential anchoring
(planar to homeotropic) and phase transitions (nematic to isotropic) with dynamics that depended
strongly on degree of oligomerization. The dynamics reflect the relative rates of adsorption onto
the LC/aqueous interface and subsequent absorption into the LC film. Significantly, the two
transitions appear fundamentally connected, as the orientational and phase transition appear to
arise from amphiphileꢀinduced melting of the nematic phase of 5CB at aqueous interface and
within the bulk phase, respectively.
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The influence of degree of oligomerization on extent of partitioning of the oligomers into the
LC is consistent with the effects of constraints on configurational degrees of freedom of the
oligomers in the LC, which increases with molecular weight of the oligomers. We expect this
trend to be general to broad classes of oligomers and not be dependent on the details of the
molecular structure of the oligomers. However, other trends reported in our paper, such as the
decrease in anchoring energy of the LC at the aqueousꢀLC interface, are likely strongly
dependent on the details of the molecular structure of the oligomers (rigidity of molecules,
hydrophilic/hydrophobic balance). Indeed, past studies have demonstrated that small changes in
molecular structure of adsorbates (e.g., head group, length and/or branching of aliphatic tails)
can significantly influence LC ordering at LC/aqueous interfaces.53ꢀ56
We also explored the extent to which changes in degree of oligomerization can trigger LCs by
exploring the properties of a UVꢀcleavable trimeric oligomer (UV3) that was transformed into
three monomeric fragments and a core unit (λ=365 nm). We found that UV cleavage of the
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trimer led to striking changes in the dynamics of the response of the LCs, supporting our
conclusion that molecular triggering of LCs via changes in degree of oligomerization appears a
fruitful strategy for further exploration. For example, we envision future designs of ligandꢀ
functionalized oligomers that are targeted by specific antibodies. Because immunoglobulin G
(IgG) has two binding regions, we predict that binding of oligomers to the IgG will lead to a
change in the degree of oligomerization and thus dynamics of triggering of LCs. Alternatively,
enzymatically catalyzed crossꢀlinking of oligomers should lead to large changes in the dynamic
and equilibrium responses of LCs. These directions of inquiry and others will rely heavily on the
advances reported in this paper.
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ASSOCIATED CONTENT
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Supporting Information. The supporting information is available free of charge on the ACS
Publications website at DOI:
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Materials; Preparation of LC film; Calculation of radius and penetration depth of isotropic
domain; Calculation on the amount of O2 absorbed into a 5CB film; Optical micrograph of a
film of isotropic 5CB under reflection mode (Figure S1); Absorption of aqueous solutions with
respect to C2 (Figure S2); Phase transition of a film of isotropic 5CB incubated in an aqueous O2
upon cooling (Figure S3), Clearing temperatures of 5CB films measured as a function of the
concentration of O2 dissolved into 5CB (Figure S4), UVꢀvisible absorbance spectra of the
aqueous UV3 upon UV irradiation (Figure S5) (PDF)
AUTHOR INFORMATION
Corresponding Author
*Email: nlabbott@wisc.edu
*Email: depablo@uchicago.edu
*Email: thai@chem.umass.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We acknowledge support of this research from the Army Research Office through W911NFꢀ
15ꢀ1ꢀ0568 and W911NFꢀ17ꢀ1ꢀ0575. Additional support was provided by the National Science
Foundation (CBET 1508987 and DMRꢀ1435195). Partial support and facilities support from the
Wisconsin MRSEC is also acknowledged (DMRꢀ1121288 and DMRꢀ1720415).
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