Facile fabrication of color tunable film and fiber nanocomposites via thiol click chemistry

Article abstract of DOI:10.1021/ma401636e

A simple method for the fabrication of nanocomposite materials using thiol click chemistry is reported. The thiol click nanocomposite materials produced each displayed distinctive colors which were found to be dependent on both the ligand used to function

Full text of DOI:10.1021/ma401636e

Article
pubs.acs.org/Macromolecules
Facile Fabrication of Color Tunable Film and Fiber Nanocomposites
via Thiol Click Chemistry
Darryl A. Boyd,*,† Jawad Naciri,† Jake Fontana,† Dennis B. Pacardo,‡ Adam R. Shields,† Jasenka Verbarg,†
Christopher M. Spillmann,† and Frances S. Ligler‡
†Center for Bio/Molecular Science & Engineering, Naval Research Laboratory, 4555 Overlook Ave SW, Washington, D.C. 20375,
United States
‡Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, EB3, Mail
Stop 7115, Raleigh, North Carolina 27695-7115, United States
S
* Supporting Information
ABSTRACT: A simple method for the fabrication of nano-
composite materials using thiol click chemistry is reported. The
thiol click nanocomposite materials produced each displayed
distinctive colors which were found to be dependent on both the
ligand used to functionalize the nanoparticles and the concen-
tration of nanoparticles in the materials. Functionalized metallic
nanospheres were combined with thiol click solutions forming
viscous prepolymer solutions which were then polymerized upon
UV light exposure. Films were fabricated in a custom-built film
mold, and microfibers were fabricated using hydrodynamic
focusing in a microfluidic channel. For this study, three unique
thiolated ligandsincluding a newly synthesized ligandwere used to functionalize the nanospheres, thus assisting in the facile
incorporation and stability of the nanospheres within the polymers. In comparison to a previously reported method in which
thiol−ene nanocomposite films were fabricated, the method reported herein reduces the fabrication time from weeks to minutes.
Furthermore, the method in this report is expanded to also include fabrication of thiol−yne nanocomposites. Young’s moduli and
glass transition temperatures were determined for the materials, while UV−vis spectroscopy, transmission electron microscopy,
and optical analyses were also performed in order to characterize the nanocomposites.
concluded that the nanospheres were well dispersed within the
polymer matrix.14
INTRODUCTION
■
In recent years thiol click chemistrysometimes called “Hoyle
click chemistry” in honor of the substantial contributions
Professor Charles Hoyle made in establishing this research
areahas earned a prominent place in materials science
because of the adaptability of the thiol click reaction platform
and the ease with which these reactions can be performed.1−4
Because of the well-known strength of the gold−thiol bond
(∼45 kcal/mol),5 there are many reports of thiolated molecules
being used to form or modify self-assembled monolayers
(SAMs) deposited on various materials including gold
nanospheres (AuNS).5−8 Some of these reports specifically
utilize thiol click chemistry as a means of probing and
immobilizing species on surfaces, including gold.9−13
Despite the use of thiol click chemistry for modification of
gold surfaces, reports describing the inclusion of metallic
nanoparticles (NPs) into thiol click polymer materials are not
common. In one report, AuNS were incorporated into thiol−
ene films using a process that required up to 3 weeks of stirring
to incorporate the AuNS into the prepolymer.14 A gold
nanopowder was precomplexed with a trithiol during the
stirring process, followed by the addition of a trialkene to the
gold−thiol dispersion. This prepolymer material was photo-
polymerized on glass slides upon UV light exposure. The report
Recently, we reported the fabrication of thiol−yne and
thiol−ene microfibers using hydrodynamic focusing in micro-
fluidic channels.15−17 Those reports highlighted our ability to
fabricate thiol click fibers with nonround cross-sectional shapes
on the micrometer scale as well as the ability to perform
postpolymerization modifications on the surface of the fibers by
altering the prepolymer stoichiometry.15−17 The advantages of
the fiber fabrication method employed included the structural
uniformity of the fibers, the easily controlled production
conditions, and the ability to perform surface modifications on
the fibers. The possibility that inclusions, like metallic
nanoparticles, could be covalently coupled into these fibers,
thus altering the material properties, was intriguing.
Utilizing a process for the facile phase transfer and surface
functionalization of metallic NPs,18 in conjunction with the
microfluidic production of thiol click microfibers,15−17 we now
report a rapid, facile method for inclusion of AuNS into thiol
click fibers. We also use the same prepolymer preparation to
Received: August 4, 2013
Revised: November 21, 2013
Published: January 7, 2014
© 2014 American Chemical Society
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1
(77%) of 2. H NMR (CDCl3): δ 1.28−1.40 (m, 4H), 1.8−1.93 (m,
4H), 3.42 (t, 2H), 4.03 (t, 2H), 6.93 (2H, ArH), 8.05 (2H, ArH).
Synthesis of 4-(Hex-5-yn-1-yloxy)phenyl 4-((6-Bromohexyl)-
oxy)benzoate (3). To a mixture of 2 (2 g, 6.64 mmol), 4-(hex-5-yn-
1-yloxy)phenol (1.26 g, 6.66 mmol), and DMAP (68 mg, 0.54 mmol)
in 80 mL of dichloromethane was added 1-[3-(dimethylamino)-
propyl]-3-ethylcarbiimide methiodide (EDC·CH3I) (2.65 g, 8.94
mmol). The mixture was stirred for 24 h at room temperature. After
evaporation of solvent, the residue was subjected to column
chromatography on silica gel with hexane/ethyl acetate (9/1) as the
eluting solvent to yield 2.9 g (93%) of 3. 1H NMR (CDCl3): δ 1.28−
1.40 (m, 4H), 1.6 (m, 2H), 1.8−1.95 (m, 7H), 2.24 (m, 2H), 3.40 (t,
2H), 4.03 (t, 2H), 4.04 (t, 2H), 6.92 (4H, ArH), 7.1 (2H, ArH), 8.15
(2H, ArH).
fabricate thiol click nanocomposite films. In addition, the
control of the chemistry at the surface of the AuNS provided an
opportunity to evaluate the effect of functionalizing the AuNS
with different ligands on the mechanical properties of the
materials as well as assess the ability to cross-link NPs to a
polymer matrix via thiol click chemistry. For this purpose, the
synthesis of a new ligand was performed and is fully outlined in
this report. Finally, we analyzed the optical properties of the
materials via optical microscopy, transmission electron
microscopy (TEM), and UV−vis spectroscopy.
Compared to the previously mentioned technique,14 the
method reported herein drastically decreased the amount of
time needed to produce thiol click materials with well
distributed AuNS from weeks to minutes. Control of the
composition impacted both the strength and color of the
nanocomposites. The process for fabricating nanoparticle-
containing thiol click materials may find use in the production
of antibacterial coatings,19,20 development of optical materials/
devices,21,22 and applications in microcontact printing and soft
lithography.23−25
Synthesis of 4-(Hex-5-yn-1-yloxy)phenyl 4-((6-
Mercaptohexyl)oxy)benzoate (4). A stirred solution of 3 (2.9 g,
6.15 mmol) in 30 mL of THF was cooled to −10 °C under nitrogen.
Hexamethyldisilathiane (1.4 mL, 6.68 mmol) and tetra-n-butylammo-
nium fluoride (TBAF) (1 M solution in THF, 6.28 mL) were added.
The reaction mixture was stirred at this temperature for 10 min and
poured into a saturated solution of ammonium chloride. The solution
was extracted with ether, washed with brine, and dried over
magnesium sulfate. After evaporation of solvent, the residue was
subjected to column chromatography on silica gel with hexane/ethyl
acetate (9/1) as the eluting solvent to yield 2 g (77%) of 4 as white
EXPERIMENTAL SECTION
Materials and Techniques. Pentaerythritol tetrakis(3-mercapto-
propionate) (PETMP), 1,7-octadiyne (ODY), 1,4-butanediol divinyl
ether (BDDVE) (Chart 1), 2,2-dimethoxy-2-phenylacetophenone
■
1
crystals. H NMR (CDCl3): δ 1.25−1.32 (m, 3H), 1.35−1.40 (m,
2H), 1.5−1.63 (m, 4H), 1.75−1.95 (m, 5H), 2.24 (m, 2H), 2.5 (q,
2H), 4.03 (t, 2H), 4.04 (t, 2H), 6.92 (4H, ArH), 7.1 (2H, ArH), 8.15
(2H, ArH). Anal. Calcd for C25H30O4S: C, 70.39; H, 7.09. Found: C,
70.30; H, 7.0.
Chart 1. Reagents Used in Fabricating Thiol Click
Nanocomposites
Synthesis of 4-(5-Hexenyloxy)phenyl 4-((6-Mercaptohexyl)-
oxy)benzoate (5). Ligand 5 was synthesized as previously
described.18,26
Surface Functionalization of Nanospheres. Metallic nano-
sphere surfaces were functionalized as previously described.18 Briefly,
three different ligands were used to functionalize AuNS (Chart 2). To
Chart 2. Ligands Used To Functionalize 10 nm AuNS: 4
(Alkyne−Thiol Ligand); 5 (Vinyl−Thiol Ligand); 6 (1-
Dodecanethiol)
(DMPA), 1-dodecanethiol, tetrahydrofuran (THF), chloroform,
poly(ethylene glycol) (PEG), and all the starting materials used in
the ligand syntheses were purchased from Sigma-Aldrich and used as
received. 10 nm gold and 20 nm silver nanospheres were purchased
from SPI Supplies.
1H NMR spectra were recorded on a Varian VXR-400 spectrometer
using CDCl3 (internal reference 7.26) as the solvent. All column
chromatography was performed using Mallinckrodt Chemicals silica
grade 62 (60−200 mesh). Analytical thin chromatography (TLC) was
performed on Merck silica gel 60 F254 plates.
Synthesis of Ethyl 4-((6-Bromohexyl)oxy)benzoate (1). To
ethyl 4-hydroxybenzoate (7.30 g, 44 mmol), 6-bromohexan-1-ol (8 g,
44.2 mmol), and triphenylphosphine (11.52 g, 44 mmol) was added
100 mL of THF. To this mixture was added dropwise a solution of 10
mL of THF and diethyl azodicarboxylate (DEAD) (7.42 mL, 46.9
mmol). The reaction mixture was stirred overnight at room
temperature. After evaporation of solvent, the residue was subjected
to column chromatography on silica gel with dichloromethane as the
eluting solvent. The product was further crystallized from hexane to
an empty 20 mL borosilicate glass vial, 5 mg of ligand (4, 5, or 6) was
added, followed by 1 mL of THF; the contents were agitated and
mixed together (vial 1). In a separate 20 mL borosilicate glass vial, 1
mL of citrate-stabilized 10 nm gold nanospheres was added (vial 2).
The contents of vial 1 were added to vial 2, the vial was capped, and
the combined contents were shaken vigorously. The resulting solution
was allowed to sit in the vial (now uncapped and open to air), causing
the now functionalized (and phase transferred) nanospheres to form a
thin film layer along the wall of the vial. After about 30 min the vial
was decanted, leaving behind only the nanosphere thin film. To
resuspend the nanospheres, 1 mL of chloroform was added to the vial,
removing the nanosphere film from the vial wall. To prepare the
functionalized nanospheres for incorporation into polymer materials,
the chloroform nanosphere suspension was concentrated by slight
heating on a hot plate at 50 °C while being blown dry with N2(g) over
the top of the open vial until only a few microliters of the suspension
remained. In addition to AuNS, silver nanospheres (AgNS) were also
functionalized and incorporated into thiol−yne polymers (Supporting
Information).
1
yield 10 g (69%) of white crystals 1. H NMR (CDCl3): δ 1.28−1.38
(m, 7H), 1.8−1.94 (m, 4H), 3.42 (t, 2H), 4.03 (t, 2H), 4.38 (q, 2H),
6.92 (2H, ArH), 7.99 (2H, ArH).
Synthesis of 4-((6-Bromohexyl)oxy)benzoic Acid (2). To a
solution of 1 (10 g, 30.4 mmol) in 150 mL of methanol and 40 mL of
water was added LiOH·H2O (3.75 g, 90 mmol). The reaction mixture
was heated to 50 °C for 4 h. After evaporation of the solvent, the
residue was neutralized by a mixture of HCl/H2O. The suspension was
filtered and washed with cold water. The crude product was
crystallized from a mixture of hexane/ethanol (9/1) to yield 7 g
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Prepolymer Solution Preparation. Thiol−yne solutions were
made by combining 1 equiv (0.010 mol) of ODY with 1 equiv (0.010
mol) of PETMP. Thiol−ene solutions were made by combining 2
equiv (0.010 mol) of BDDVE with 1 equiv (0.005 mol) of PETMP.
Either a thiol−yne or a thiol−ene solution was added to a vial
containing concentrated, functionalized nanospheres. Vials were then
sonicated until the nanospheres were dispersed in the thiol click
solution (<5 min). DMPA photoinitiator (2 mol %) was added to the
vial containing the suspension, and the vial was sonicated for an
additional 2 min. All suspensions were immediately transferred to a
syringe for introduction into either the film holder or the microfluidic
channel under limited light conditions to prevent premature
polymerization.
RESULTS AND DISCUSSION
Bifunctional Ligand Synthesis. The synthesis steps
leading to the preparation of the di-end-functional (i.e.,
bifunctional) alkyne−thiol ligand 4 are shown in Schemes 1
■
Scheme 1. Synthesis of Alkynyl−Thiol Ligand Precursor (3)
Off-stoichiometry prepolymer solutions were prepared as described
above; however, their component ratios were adjusted as follows. For
materials with excess thiols, thiol−yne solutions were made by
combining 0.005 mol of ODY with 0.010 mol of PETMP. Thiol−ene
solutions were made by combining 0.010 mol of BDDVE with 0.010
mol of PETMP. For thiol−yne materials with excess alkynes, solutions
were made by combining 0.010 mol of ODY with 0.005 mol of
PETMP.
Material Fabrication. Thiol click films were fabricated by
introducing the prepolymer into a holder designed and fabricated in-
house using 3D printer technology (Figure S1). The film thickness was
∼0.40 mm. Film polymerization was initiated by UV irradiation at 365
nm and ∼200 mW/cm2. Thiol click fibers were fabricated as previously
described.15 Briefly, fibers were produced in a microfluidic channel that
had eight diagonal shaping stripes. The prepolymer, which served as
the core solution, was introduced into the channel simultaneously with
a nonpolymerizable sheath solution of matching phase and viscosity.
The grooves directed the sheath solution entirely around the
prepolymer solution to create the round shape and to move the
prepolymer stream away from the channel walls prior to polymer-
ization. Fiber polymerization was also initiated by UV irradiation at
365 nm and ∼200 mW/cm2. All fibers were fabricated with
sheath:core flow-rate ratios of 25:1. Thiol click fibers were fabricated
with AuNS concentrations ranging from 3.0 × 10−6% to as much as 1.8
× 10−4%. All AuNS concentrations in this report are given in vol %.
Mechanical Properties. The Young’s moduli (E) values were
determined using dynamic mechanical analysis (DMA) as previously
described.15 The E-moduli were determined at 30 °C for all thiol−ene
fibers because 30 °C is above the glass transition temperature (Tg) for
native thiol−ene. For all of the thiol−yne fibers, E-moduli values were
determined at 80 °C, which is above the Tg for native thiol−yne. The
force was ramped from 0.004 N to an upper limit of 3.300 N at a rate
of 0.005 N/min. A slope was then calculated from a plot of the stress
versus strain curve at low strain.
Scheme 2. Synthesis of Alkynyl−Thiol Ligand (4)
The Tg values for the nanocomposites were determined by
differential scanning calorimetry (DSC) using a 2920 Modulated
DSC (TA Instruments). Materials were cooled to −40 °C and heated
to 100 °C, and then the cycle was repeated. The Tg values were taken
from the second heating cycle.
Transmission Electron Microscopy (TEM) Analysis. Prior to
TEM sample preparation, glass knives were first prepared from
ultramicrotome glass strips (Ted Pella, Inc.) employing a Leica EM
KMR3 glass knifemaker. The glass knives were then used to prepare
ultrathin samples of thiol-polymer encapsulated AuNS using a Leica
EM UC7 microtome generating approximately 130 nm sections. The
sample cuts were then applied to copper grids (Ted Pella, Inc.) and
air-dried at room temperature. The TEM imaging was performed
using JEOL 2000FX S/TEM with LaB6 electron source, operated at
200 kV with magnification of 25K and 60K.
UV−vis Spectroscopy Analysis. UV−vis spectroscopy was
performed on thiol click nanocomposites using a Varian Cary 5000
UV−vis−NIR spectrophotometer (Agilent Technologies).
Imaging Studies. Images of the films and fiber coils showing the
color of the materials were taken using a digital camera (Canon
Powershot G9). Optical microscope images were obtained using an
Olympus BX51 microscope (Olympus NDT Inc.) that was outfitted
with an Abrio IM imaging system (CRi).
and 2. Specifically, ethyl 4-((6-bromohexyl)oxy)benzoate (1)
was synthesized by coupling ethyl 4-hydroxybenzoate with 6-
bromohexan-1-ol under standard Mitsunobu conditions (di-
ethyl azodicarboxylate (DEAD)/triphenylphosphine).27 The
acid derivative 2 was obtained through basic hydrolysis of 1
using LiOH in a methanol/water mixture. Esterification of the
acid 2 with 4-(hex-5-yn-1-yloxy)phenol in the presence of 1-[3-
(dimethylamino)propyl]-3-ethylcarbiimide methiodide and
(dimethylamino)pyridine (DMAP) led to 3.28 The final
product 4 was prepared using the trimethylsilylthioxy
dehalogenation procedure.29 The bifunctional vinyl−thiol
ligand 5 was synthesized as previously described (Chart
2).18,26 The monofunctional ligand 6 was chosen as a
comparative functionalization compound to ligands 4 and 5
because it was made up of a long hydrophobic carbon chain
with only a thiol end group to bind gold and no other click
reactive end group (Chart 2).
Fabrication of Materials. Though the incorporation of
AuNS into thiol−ene materials had previously been reported by
Hoyle and co-workers,14 we were able to improve upon this
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method by decreasing the fabrication time scale from weeks to
minutes. We employed a unique procedure for the surface
functionalization of metallic NPs recently reported.18 The
surfaces of the AuNS were modified with a monolayer of thiol-
terminated ligand; the AuNS were then concentrated and
combined with either a thiol−yne (ODY + PETMP) or a
thiol−ene (BDDVE + PETMP) solution. Once the solutions
were prepared, they were utilized for the production of either
films or fibers (Scheme 3). We should note that though the use
during the initial 5 h, followed by consistent stirring for 1−3
weeks depending on the concentration of AuNS. Only after the
AuNS were visibly well dispersed in the thiol material was the
alkene added, thus completing the AuNS thiol−ene prepolymer
preparation.14
In the method used in this report, AuNS were functionalized
with ligands prior to their complexation into the thiol−yne or
thiol−ene prepolymers. Because the thiol−yne and thiol−ene
prepolymers were largely composed of hydrophobic compo-
nents, it is plausible that the surface functionalization of the
AuNS with a monolayer of hydrophobic ligand made them
more compatible with the prepolymers. This ligand−prepol-
ymer compatibility would thus lend itself to much faster
complexation of the AuNS within the prepolymer matrix.
To ensure the facile complexation of the AuNS into the
prepolymers, nanospheres functionalized using 4 (which had an
alkyne terminus) were incorporated into thiol−yne materials
because the thiol−yne solution also contained an alkyne
(ODY). Similarly, AuNS-functionalized with 5 were incorpo-
rated into thiol−ene materials that contained the alkene
BDDVE (Chart 2).
Scheme 3. Ligand-Functionalized AuNS Were Incorporated
into a Thiol Click Prepolymer Solution, and Then
Transferred via Syringe to Either a Holder for Film
Fabrication or Introduced into a Microfluidic Channel for
a
Fiber Fabrication
Mechanical Properties. Having previously reported the
strengths of both thiol−yne and thiol−ene microfibers using
the same monomer components as the ones used in this
current report,15 the effect on the mechanical properties
generated by the incorporation of AuNS into those materials
was measured and compared to the native materials. The
previous report documented greater Young’s modulus (E-
modulus) values for native thiol−yne fibers compared to native
thiol−ene fibers,15 and thus we expected thiol−yne nano-
composites would have greater E-modulus values than thiol−
ene nanocomposites when equivalent AuNS concentrations
were present. Because we were introducing nanoscale metallic
particles into thiol click matrices, and based on some
precedence in the literature,33−35 we also expected to see an
increase in E-modulus for both thiol−yne and thiol−ene
following AuNS incorporation. Finally, the monofunctional
ligand 6 (Chart 2) should not form bonds with thiol−yne or
thiol−ene; therefore, nanocomposite fibers that contained
AuNS functionalized with ligand 6 were made to compare
the effect of using bifunctional versus monofunctional ligands
on the materials properties.
As expected, thiol−yne nanocomposite fibers indeed had
greater E-modulus values than thiol−ene nanocomposite fibers
with equivalent concentrations of AuNS (Table 1). This result
was likely due to the additional π bond available for cross-
linking in thiol−yne polymers, which creates a greater cross-link
density, thus conferring greater E-modulus values.15,36,37
Comparing rows (a) through (e) in Table 1, divergent E-
modulus behavior was observed between thiol−yne and thiol−
ene fibers. As AuNS concentration increased, thiol−yne fibers
returned an increase in E-modulus. In contrast, none of the
thiol−ene nanocomposite fibers had an E-modulus greater than
that of native thiol−ene fibers, and there was no easily
discernible E-modulus trend observed for the thiol−ene
nanocomposite fibers as the AuNS concentration increased
(Table 1). Both outcomes are reasonable given that in some
reports nanocomposites have been shown to have enhanced
mechanical strength when compared to their neat (i.e., native)
material counterparts,33−35 while others show decreased
mechanical strength.30,38
a
Polymerization of materials occurred via UV light irradiation.
of 1,7-octadiene might have been a better thiol−ene alkene
choice for comparison with the thiol−yne components we
used, we chose to use BDDVE in the thiol−ene polymer to
better compare the results with previous characterization of this
material.15
Fiber fabrication proceeded as previously described.15 For
the hydrodynamic focusing procedure to work properly, it was
imperative to minimize boundary effects between the core
(prepolymer) and sheath solutions. Increasing the concen-
tration of AuNS increased the viscosity of the prepolymer
solution. The consequent viscosity mismatch between the core
and sheath solutions can be mitigated by either adjusting the
sheath and core flow rates or by using a more viscous sheath
material as the AuNS concentration increases. Fabrication of
the thiol click nanocomposite films only mandated that the
prepolymer solution be exposed to UV light, thus requiring
even less time to produce films than fibers. In order to fabricate
uniform films, a small mold for shaping the prepolymer solution
was made in-house (Figure S1). From start to finish, the
nanocomposite films were fabricated in less than 30 min, while
fibers could be fabricated in an hourwhich includes the time
required to set up the microfluidic channel apparatus.
The vast time scale difference between Hoyle and co-workers
method of AuNS inclusion and the method reported herein is
likely due to the interphase between the AuNS surface and the
prepolymer matrix (i.e., AuNS local environment).30−32 Hoyle
and co-workers employed a method of precomplexing the
AuNS directly to the prepolymer matrix by dissolving Au
nanopowder in the thiol material. It was this early step in the
process that required alternating sonication and stirring steps
Comparing rows (c) and (f) in Table 1 provided the
opportunity to determine if there was any difference in E-
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a
Table 1. Thiol Click Fiber Mechanical Properties
average E-modulus (MPa)
Tg (°C)
vol % AuNS
ligand on AuNS
AuNS Ene (30 °C)
AuNS Yne (80 °C)
AuNS Ene
AuNS Yne
(a)
(b)
(c)
(d)
(e)
(f)
0.0
N/A
5.3 0.8
3.2 0.4
4.5 0.2
3.2 0.3
3.5 0.3
3.9 0.4
15.0 2.0
14.7 1.1
21.6 0.5
22.0 1.8
32.6 4.6
21.7 4.2
−35
−22
−21
−17
−10
−9
47
6
3.0 × 10−6
4.5 × 10−5
9.0 × 10−5
1.8 × 10−4
4.5 × 10−5
4 or 5
4 or 5
4 or 5
4 or 5
6
10
13
22
17
Average E-moduli and Tg values for thiol click nanocomposite fibers. (a) Native thiol−ene and thiol−yne fibers with no AuNS,15 (b)−(e) fibers
fabricated using bifunctional ligands 4 (for AuNS Yne) or 5 (for AuNS Ene), (f) fibers fabricated using ligand 6. Each data set was performed in
a
triplicate; error given as standard deviation in megapascals (MPa).
modulus for fibers fabricated with ligand 6 monofunctionalized
AuNS versus fibers fabricated with ligands 4 or 5 bifunction-
alized AuNS. The data showed that there was no appreciable
difference in the E-modulus between fibers that utilized the
bifunctional ligands (4 and 5) versus those that used the
monofunctional ligand 6 for both AuNS thiol−yne and AuNS
thiol−ene fibers at the 4.5 × 10−5% concentration level. These
results suggests that for thiol−yne and thiol−ene the presence
of AuNS may impact the strength of the polymer either by
increasing it (as seen in thiol−yne) or by decreasing it (as seen
in thiol−ene), but the impact is not due to cross-linking of the
nanospheres with the polymer.
prepolymer solutions became increasingly more pronounced.
For example, thiol−yne prepolymer solutions with a low
concentration of AuNS appeared faint pink in color. As the
AuNS concentration increased, the thiol−yne prepolymer
solutions became increasingly magenta and ultimately became
a dark purple for the highest concentrations. The materials
made from these solutions maintained colors similar to their
prepolymer solutions following photopolymerization (Figure
1).
Hoyle and co-workers reported that as the AuNS
concentration increased, the glass transition temperatures
(Tg) of their thiol−ene material also increased.14 Rows (a)
through (e) of Table 1 show agreement with this trend for the
thiol−ene materials reported. However, unlike thiol−ene, there
was a dramatic drop in the Tg from native thiol−yne to the
thiol−yne nanocomposite with the least concentration of
AuNS. Following this initial decrease in Tg, the thiol−yne
nanocomposites did show increasing Tg values as AuNS
concentration increased (Table 1).
Figure 1. Photograph images of thiol click fiber coils. The top row
gives the concentration of AuNS within the fibers in vol %. The first
column shows native fibers. Thiol−ene fibers were fabricated with
AuNS functionalized with ligand 5, and thiol−yne fibers were
fabricated with AuNS functionalized with ligand 4. Fibers in the
column designated “4.5 e-5 % Ligand 6” contained AuNS function-
alized with the monofunctional ligand 6.
The difference in both E-modulus and Tg behavior between
thiol−yne and thiol−ene may be attributed to the fact that,
though similar, there are differences in thiol−yne and thiol−ene
chemistries. Specifically, the kinetics that occur during the rapid
photopolymerization process differs because of the additional π
bond present in thiol−yne reactions.36,37 Perhaps this affects
the way the functionalized AuNS are incorporated into thiol−
yne. It is also possible that the difference in the ligands and the
thiol click components used have an effect on the region
between the AuNS and the bulk polymer (i.e., the interphase).
Even with the similarities in the ligands (4 and 5) and the
prepolymer components used for thiol−yne and thiol−ene
nanocomposites, it is probable that the interphase is affected by
even the minor differences, and this would impact the
mechanical properties of the materials.30 In either case, no
broad trends or analogies about thiol−yne versus thiol−ene
nanocomposites should be drawn from these data because these
results are likely unique to the thiol click components used.37
Visible and Optical Properties of Thiol Click Nano-
composites. A notable observation related to the dispersion of
AuNS in the thiol click materials was the uniformity of the
color of each material type. AuNS thiol−yne and AuNS thiol−
ene prepolymer solutions, as well as their subsequent polymers,
each maintained a color similar to the characteristic color of the
AuNS used in fabricating them when the bifunctional ligands 4
and 5 were used to modify the AuNS surfaces (Figure 1). As
the AuNS concentration increased, the color within the
Interestingly, when the AuNS were functionalized with the
monofunctional ligand 6, the resulting polymer materials did
not retain the purple-gray color of the prepolymer solution
(Figure S3). Instead, the polymers were more comparable to
the colorless native thiol click materials (Figure 1) than to their
counterparts using the bifunctional ligands. Because it is known
that nanoparticles which are surface modified with ligands
having functional termini can participate in cross-linking and in
click reactions,19,39−43 it is plausible that the bifunctional
ligands (4 and 5) participated in cross-linking the AuNS to
available thiols in the prepolymer solutions upon photo-
polymerization, thus influencing the interparticle spacing after
polymerization. This could not have been the case when using
ligand 6 due to the lack of a clickable functional group at the
end of the ligand opposite the gold-bound thiol. Therefore, our
initial conjecture is all the prepolymer solutions expressed
color, regardless of the surface modifying ligand used, because
of the proximity and distribution of the AuNS within the
prepolymer matrix. Upon photopolymerization, the prepol-
ymers containing AuNS functionalized with ligands 4 or 5
formed an aligned polymer backbone that allowed the AuNS,
which were now cross-linked to the polymer backbone, to
maintain close proximity with one another. This close
proximity of the AuNS in the polymers allowed the
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nanocomposites to maintain a color similar to the one
expressed in the prepolymer. When the AuNS were function-
alized with ligand 6, the purple gray color of the prepolymer
was not maintained upon polymerization because the AuNS
were not covalently attached to the polymer backbone due to
the lack of end-terminal bifunctionality of ligand 6.
Consequently, the AuNS were not as uniformly distributed in
close proximity to one another; thus, little to no color was
expressed in those polymer materials.
bifunctional ligand was used to functionalize the AuNS and
much farther apart in the composites using AuNS function-
alized with the monofunctional ligand. Within the pockets of
concentrated AuNS, there were clear differences based on the
type of ligand employed.
When monofunctional ligand 6 was used, the AuNS were in
intimate contact with each other, forming dense aggregates. In
contrast, AuNS modified with ligand 4 maintained an
interparticle spacing, even within the pockets of concentrated
nanoparticles (Figure 3). This interparticle spacing likely led to
the preservation of the plasmon resonance, which was similar to
that expected of an individual AuNS (Figure 4). In general,
To further investigate the distribution of the AuNS within
the thiol click nanocomposites, optical microscope images of
thiol−yne (Figure 2) and thiol−ene (data not shown) fibers for
Figure 4. Spectral analysis of AuNS incorporated into thiol−yne films.
Solid gray (3.0 ×10−6%) and solid black (4.5 × 10−5%) lines represent
films fabricated with ligand 4-functionalized AuNS. Dashed gray (3.0
×10−6%) and dashed black (4.5 × 10−5%) lines represent films
fabricated with ligand 6-functionalized AuNS. Inset shows enlarged
view of low absorbing films.
Figure 2. Representative optical microscope images of thiol−yne
fibers. Except for the fiber designated “Ligand 6”, thiol−yne fibers were
fabricated with ligand 4 functionalized AuNS. Scale bars represent 100
μm.
these data support the conclusion that the covalent attachment
of the AuNS to the polymer is related to the ligand type used to
functionalize the AuNS, which affects the interparticle spacing.
Thus, the color of the nanocomposites can be tuned by
functionalizing the AuNS with different ligands.
Spectral Analysis. Analysis in the UV−vis spectrum was
performed on thiol−yne nanocomposite films to further
determine the AuNS distribution within the polymers. The
characteristic wavelength for 10 nm Au nanospheres in water is
between 510 and 525 nm;45 thus, materials containing well-
dispersed 10 nm Au nanospheres should have a λmax in or near
this range.45−47 Two sets of films (one using ligand 4-
functionalized AuNS and the other using ligand 6-function-
alized AuNS) were analyzed at two different AuNS
concentrations (3.0 ×10−6% and 4.5 × 10−5%).
each discrete concentration were taken. If the AuNS were
flocculating within the nanocomposites, then those flocculates
might have been visible by optical microscopy.44 In each case,
however, little to no flocculation was apparent (Figure 2).
Because visualizing nanoparticle aggregation is difficult using
optical microscopy due to the dimensions and concentrations
of the AuNS used, we also examined the materials via TEM
(Figure 3). The TEM images showed numerous pockets of
AuNS that were in close proximity to each other when the
Although none of the films had a λmax within the ideal 510−
525 nm range, the films made using ligand 4-functionalized
AuNS had λmax values ∼535 nm (Figure 4), while increasing the
AuNS content from 3.0 ×10−6% to 4.5 × 10−5% significantly
increased the absorbance of the material. For the same AuNS
concentrations, films fabricated using ligand 6-functionalized
AuNS returned low absorbance values within the visible range
of the spectrum, with a λmax value of ∼550 for the 4.5 × 10−5%
AuNS sample. The red-shift observed for the AuNS within the
nanocomposites was likely due to the high refractive index of
thiol click polymers.48,49
Overall, these data agreed with both the optical microscopy
and TEM data and suggested that the AuNS were individually
incorporated within the thiol−yne polymers when the bifunc-
Figure 3. Representative TEM image of 10 nm AuNS within a thiol−
yne nanocomposite (4.5 ×10−5 AuNS vol %). AuNS functionalized
with ligand 4. Scale bar represents 50 nm.
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tional ligands were used to functionalize the AuNS. The lack of
color in the nanocomposites that used ligand 6-functionalized
AuNS might have been the result of the aggregated AuNS being
in pockets too far from each other to contribute considerably to
the color of the composite (Figure 1).
Off-Stoichiometry Thiol Click Nanocomposites. Mak-
ing stoichiometrically imbalanced thiol−ene materials can alter
the materials properties;50 thus, it seemed reasonable that the
same would be true if AuNS were included in the material
matrix. In an effort to determine if and how the properties of
thiol click materials with AuNS inclusions were altered, off-
stoichiometry thiol−yne and thiol−ene nanocomposite fibers
were fabricated (Figure 5) and their E-moduli determined
In the case of off-stoichiometry thiol−ene nanocomposite
fibers with excess thiols, the color of the material was a brighter
shade of purple than their stoichiometric counterparts (Figure
5, row a). The off-stoichiometry thiol−yne nanocomposite
fibers with excess thiols appeared to be a darker magenta than
their stoichiometric complements. The off-stoichiometry thiol−
yne nanocomposite fibers with excess alkynes appeared darker
than, but comparable in color to, those with excess thiols
(Figure 5, row a).
Perhaps of even greater significance was the presentation of
color in all the off-stoichiometry fibers that contained AuNS
functionalized with the monofunctional ligand 6 (Figure 5, row
b). Both off-stoichiometry nanocomposite thiol−yne and
thiol−ene fibers with thiols in excess appeared gray in color,
while off-stoichiometry nanocomposite thiol−yne fibers with
alkynes in excess appeared purple in color. This was in contrast
to the stoichiometrically balanced fibers in which AuNS
functionalized with ligand 6 appeared relatively colorless
(Figure 1). In the cases where thiols were in excess, it is
conceivable that a percentage of the thiols from the monomer
material (PETMP) were able to bind to the gold, bringing the
AuNS into closer proximity to each other and thus generating
the slight coloration. A similar scenario is plausible when
alkynes were in excess. This is because it is known that the
surfaces of nanospheres tend to carry a positive charge and can
attract the negatively charged electrons of π bonds.51 That said,
the exact explanation for the slight coloration of the off-
stoichiometry materials with alkynes in excess is unclear. In all
cases, however, it is obvious that the imbalance of prepolymer
components had an impact on the organization of the AuNS
and consequently on the visible properties of the material.
The same polymers were characterized using DMA to
determine if the interactions that altered the colors also altered
the E-moduli (Table 2). Rows (a) through (c) of Table 2
indicated that for each AuNS concentration the thiol−ene
nanocomposite fibers fabricated with excess thiols in the
prepolymer showed greater E-modulus when compared to
native thiol−ene fibers that had thiols in excess, but no AuNS.
However, none of the AuNS thiol−ene fibers with excess thiols
had E-modulus values as great as the stoichiometrically
balanced fibers (Table 1). This is consistent with previous
research done with off-stoichiometry thiol−ene chemistry in
which it was determined that leaving thiols in excess caused the
materials fabricated to be softer than their stoichiometrically
balanced counterparts.50
Unlike off-stoichiometry thiol−ene fibers with excess thiols
in the prepolymer matrix, which increased in E-modulus with
the incorporation of AuNS, the E-modulus for off-stoichiom-
etry thiol−yne fibers with excess thiols in the prepolymer
trended downward as AuNS concentration increased. These
fibers had overall decreased E-moduli in comparison to their
stoichiometric counterparts at all AuNS concentration levels
(Table 2, rows (a)−(c)). For the related AuNS thiol−yne fibers
with excess alkynes, as AuNS concentration increased the E-
moduli values appeared to follow no discernible trend for the
data points analyzed (Table 2).
Comparing rows (c) and (d) of Table 2, the use of ligand 6
instead of the bifunctional ligands 4 and 5 had no statistical
impact on the E-moduli of the off-stoichiometry thiol click
nanocomposites with thiols in excess. This was not the case for
AuNS thiol−yne fibers that had alkynes in excess, which had
twice the E-moduli when ligand 6 was used to functionalize the
AuNS versus ligand 4.
Figure 5. Photograph images of off-stoichiometry thiol click fiber coils:
(a) fibers fabricated using bifunctional ligands 4 (for AuNS Yne) or 5
(for AuNS Ene); (b) fibers fabricated using monofunctional ligand 6.
All fibers depicted contained 4.5 × 10−5% AuNS.
a
Table 2. E-Moduli of Off-Stoichiometry Thiol Click Fibers
average E-modulus (MPa)
AuNS Ene
excess thiols
(30 °C)
AuNS Yne
excess thiols
(80 °C)
AuNS Yne
excess alkynes
(80 °C)
vol %
AuNS
(a)
(b)
(c)
(d)
0.0
0.3 0.1
1.2 0.5
1.3 0.1
1.3 0.4
11.1 1.4
6.9 0.6
5.0 0.9
4.6 0.4
3.8 0.7
14.5 1.6
9.3 1.1
20.9 1.7
3.0 ×10−6
4.5 × 10−5
4.5 × 10−5
a
Average E-moduli of off-stoichiometry thiol click nanocomposite
fibers. (a) Off-stoichiometry thiol−ene and thiol−yne fibers with no
AuNS, (b) and (c) fibers fabricated using bifunctional ligands 4 (for
AuNS Yne) or 5 (for AuNS Ene), (d) fibers fabricated using
monofunctional ligand 6. Each data set was performed in triplicate;
error given as standard deviation in MPa.
(Table 2). Using the specific components for thiol−yne
(PETMP + ODY) and thiol−ene (PETMP + BDDVE)
previously outlined, thiols were combined with an alkyne or
an alkene in a 2:1 ratio forming the prepolymer prior to
addition of AuNS. AuNS were then suspended in the
prepolymer solution following functionalization with one of
the ligands. Also, to determine the effect of leaving alkynes in
excess, thiol−yne nanocomposite fibers were fabricated in
which the thiols were combined with the alkyne in a 1:2 ratio.
Thiol−yne nanocomposite fibers were fabricated with AuNS
functionalized with either ligand 4 or ligand 6. Similarly, AuNS
thiol−ene fibers were fabricated with AuNS functionalized with
either ligand 5 or ligand 6. This afforded the opportunity to
again contrast the effect of functionalizing AuNS with
bifunctional (4 or 5) versus monofunctional (6) ligands on
the properties of the off-stoichiometry polymer materials.
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In general, off-stoichiometry thiol click nanocomposite fibers
tended to have decreased E-moduli when compared to
stoichiometrically balanced thiol click nanocomposite fibers.
Further investigation of off-stoichiometry thiol click nano-
composites with greater AuNS concentrations is required,
however, to extrapolate a well-substantiated rationale for the
data obtained.
Mismatched Thiol Click Nanocomposites. “Mis-
matched” thiol click nanocomposites, in which AuNS function-
alized using ligand 4 and AuNS functionalized using ligand 5
were incorporated into thiol−ene and thiol−yne polymers,
respectively, were also fabricated (Figure 6) and their E-moduli
determined (Table 3). This was done to determine the effect of
the bifunctional ligands on the properties of the materials.
ene fibers (Table 3). This suggested that though the color of
the material was affected by the ligand used to functionalize the
AuNS, the overall strength of the polymer was dominated by
the thiol−yne or thiol−ene monomer components used to
fabricate the material.
CONCLUSION
■
Functionalized Au nanospheres were incorporated into
polymer nanocomposites via photoinitiated thiol click chem-
istry, and the mechanical and optical properties of the materials
were reported. The ligands used to functionalize the AuNS
included a newly synthesized ligand which aided in determining
the effect of various ligands on the properties of the materials
fabricated. The nanocomposite films were made in a custom-
built mold, while fibers were fabricated using a microfluidic
sheath flow system.15,17 The process reported improved upon a
previously published method for thiol−ene nanocomposite
fabrication by drastically reducing the fabrication time from
weeks to minutes.14 Building on other prior work done using
anisotropic AuNPs in which nanocomposite colors were
dictated by the thickness of the material and the concentration
of the AuNPs,52,53 the work we presented outlined an
additional method for the tuning of colors and optical
properties in nanocomposites by using ligand functionalized
nanoparticles in the nanocomposite material matrix.
Figure 6. Photograph images of mismatched thiol click nanocomposite
fiber coils. Thiol−ene fibers contained AuNS functionalized with
ligand 4. Thiol−yne fibers contained AuNS functionalized with ligand
5.
Despite using less than a fraction of a vol % of AuNS in all
cases presented, the visible and tensile properties of the
materials fabricated were significantly altered when compared
to the native thiol click materials. The microscopic and
spectroscopic data indicated that the AuNS were well
distributed within all the materials fabricated using bifunctional
ligands. Thiol click nanocomposites fabricated with AuNS
modified with bifunctional ligands 4 or 5 displayed very
pronounced colors consistent with the concentration of the
AuNS introduced into the prepolymer, whereas materials
fabricated using AuNS surface-modified with monofunctional
ligand 6 did not. Although there may be other factors in play,
this difference is likely due to the availability of terminal π
bonds in ligands 4 and 5, which could provide for the covalent
attachment, and thus cross-linking,19 of the AuNS via those
ligands to thiols within the nanocomposite polymer ma-
trix.39−42
Further studies showed that the inclusion of AuNS in off-
stoichiometry thiol−yne and thiol−ene fibers tended to
decrease the E-modulus for the materials. However, the
imbalance in prepolymer functional components generated a
colored material when the monofunctional ligand 6 was used,
which was in contrast to the nearly colorless polymers
produced when using the same ligand−AuNS combination in
stoichiometrically balanced fibers. Finally, when the ligands
used to functionalize the AuNS were swapped between thiol−
yne and thiol−ene materials, it was determined that the ligands
played a key role in determining the expression of color while
the thiol click monomer component combinations dominated
the overall strength of the E-moduli.
a
Table 3. Young’s Moduli of Mismatched Thiol Click Fibers
average E-modulus (MPa)
mismatched AuNS Ene
mismatched AuNS Yne
vol % AuNS
(30 °C)
(80 °C)
3.0 × 10−6
4.5 × 10−5
2.7 0.1
3.1 0.3
7.6 0.9
5.9 0.6
a
Average E-moduli of mismatched thiol−ene and thiol−yne fibers
with AuNS inclusions. Each data set was performed in triplicate; error
given as standard deviation in MPa.
When the ligands used to functionalize the AuNS (4 and 5)
were swapped between thiol−yne and thiol−ene fibers to form
mismatched materials, the thiol−ene fibers incorporating AuNS
then appeared magenta while the thiol−yne fibers incorporat-
ing AuNS appeared dark purple. This was opposite of the
expression of color when AuNS with ligand 4 were used with
the thiol−yne polymers and AuNS with ligand 5 were used in
thiol−ene polymers (Figure 1). This suggested that ligands 4
and 5, which only differ in the presence of an alkynyl versus a
vinyl terminus (Chart 2), played a key role in the expression of
color in the materials. This is significant because it had been
previously shown that the color of nanocomposite materials
containing anisotropic AuNPs could be tuned by adjusting the
thickness of the material and by adjusting the volume fraction
of the AuNPs.52,53 In addition to further establishing those
points in materials using isotropic AuNS, we have now also
shown that the color of polymer nanocomposites can be
adjusted by functionalizing AuNPs with different ligand types,
even when the ligands are nearly identical.
Functionalizing the AuNS with specific ligand types and
tuning the prepolymer to have specified amounts of AuNS can
be used to impart targeted optical and tensile characteristics.
Polymers with AuNS inclusions, fabricated as outlined in this
work, may find use in a variety of materials science applications
including lithography, sensing, and optics.
Although the expression of color was swapped when the
ligands were swapped, mismatched AuNS thiol−yne fibers were
still greater in E-modulus than the mismatched AuNS thiol−
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail darryl.boyd.ctr@nrl.navy.mil; Tel 202-404-6140
(D.A.B.).
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Notes
The authors declare no competing financial interest.
D.A.B.: NRL/NRC Postdoctoral Fellow.
A.R.S. and J.V.: Former NRL/NRC Postdoctoral Fellow.
F.S.L.: Former NRL Employee.
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ACKNOWLEDGMENTS
■
Darryl A. Boyd, Jake Fontana, Adam R. Shields, and Jasenka
Verbarg performed this work while National Research Council
Postdoctoral Fellows. The work was supported by ONR/NRL
Work Unit 9899. The views are those of the authors and do not
represent the opinion or policy of the US Navy or Department
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Instrumentation Facility (AIF) at the North Carolina State
University, which is supported by the State of North Carolina
and the National Science Foundation.
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dx.doi.org/10.1021/ma401636e | Macromolecules 2014, 47, 695−704