Metal-free construction of primary sulfonamides through three diverse salts

Article abstract of DOI:10.1039/c8gc03014f

In this report, the first metal-free construction of primary sulfonamides through a direct three-component reaction of sodium metabisulfite, sodium azide and aryldiazonium has been established. Readily available inorganic Na₂S₂O₅ and NaN₃ were applied as the sulfur dioxide surrogate and nitrogen source respectively. The widely used sulfonamide drugs Celecoxib and Sulpiride, which possess multiple heteroatoms and active hydrogen containing functional groups, are efficiently installed with -SO₂NH₂ groups at a late stage. Control experiments and kinetic studies demonstrated that aryl radicals, sulfonyl radicals and conjugated phosphine imine radicals are involved in this transformation.

Full text of DOI:10.1039/c8gc03014f

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Preparation of fluorinated PCL porous
microspheres and a super-hydrophobic coating on
Cite this: Nanoscale, 2018, 10, 18857
fabrics via electrospraying†
Haipeng Wang,‡a Wulong Li ‡a and Zhanxiong Li*a,b
In this study, fluorinated polycaprolactone (PCL) block polymers with different fluorine contents were syn-
thesized via atom transfer radical polymerization (ATRP). An electrospraying technique was used to
prepare fluorinated PCL microspheres with different microstructures. In contrast to the golf ball shape of
unmodified PCL microspheres displaying porous pits on the surface, block polymer PCL-PTFOA(2 h) and
PCL-PTFOA(6 h) microsphere surfaces displayed regular honeycomb-like pore structures. Thermally
induced and evaporation-induced phase separations are proposed as the main mechanisms involved in
the formation of the porous microstructures. The micro-phase separation between the two blocks of the
fluorinated PCL copolymer is another factor that promoted the uniform collapse on the microsphere
surface and the formation of its rugged wall. The surface roughness of the porous microspheres signifi-
cantly improved their hydrophobicity, generating coating contact angles on aluminium foil substrates that
measured as high as 162.4
1.5°, which revealed that the surfaces were super-hydrophobic. Lastly,
Received 18th July 2018,
Accepted 10th September 2018
cotton fabric was directly coated with the fluorinated polymer microspheres via electrospraying, resulting
in super-hydrophobic surfaces and CAs reaching 160.0 1.3°. The results demonstrate that electrospray-
ing is a simple, innovative and cost-effective method for preparing polymer microspheres with controlla-
ble microstructures for fabric coating applications.
DOI: 10.1039/c8nr05793a
rsc.li/nanoscale
to its ease of fabrication, absorptive qualities, and tailorable
properties.16–20 Many methods, such as emulsion,21–23 freeze
1. Introduction
Polymer micro/nanoparticles have attracted much attention drying,24 nanoprecipitation and hydrolysis,25 have been
recently because of their potential applications in many fields adopted to fabricate PCL microspheres with different mor-
as controlled release agents, catalysts, scaffolds, selective phologies, including solid, porous and hollow structures. The
separation agents, and chemical sensors.1–6 It is desirable for use of an emulsion (e.g., oil in water (o/w) emulsion) and sub-
pores to be introduced into the surface of spheres because a sequent solvent evaporation is the most popular method to
porous structure possesses a higher specific surface area, a prepare PCL microspheres. However, these methods usually
lower density, and better permeability, all of which are have limitations, such as complicated processes, extreme con-
required in many demanding applications.7–9 Biocompatible ditions, poor universal applicability, or uncontrollable micro-
and/or biodegradable polymer microspheres and one- sphere morphology and size.
dimensional fibres are excellent candidates for biomedical,
Electrospraying is a dispersion method in which high
pharmaceutical, tissue engineering and degradable electronic voltage is applied to a needle tip containing a conductive
applications due to their unique properties.10–15 Trials have liquid to generate a spray. The electrostatic force overcomes
been conducted with poly-ε-caprolactone (PCL) in many novel the surface tension and produces near-monodisperse droplets
drug delivery systems and tissue engineering applications due with diameters varying from tens of nanometres to hundreds
of micrometres.26–29 In principle, this method is essentially
similar to that of electrospinning, with the most obvious
aCollege of Textile and Clothing Engineering, Soochow University, Suzhou 215021,
China. E-mail: lizhanxiong@suda.edu.cn; Fax: +86-512-67246786;
Tel: +86-512-67164993
bNational Engineering Laboratory for Modern Silk, Suzhou 215123, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nr05793a
difference between electrospraying and electrospinning being
the solution concentrations used in the process and the chain
entanglement density of the polymer solution.30 Higher solu-
tion concentrations tend to form continuous fibres in the
electrospinning process, while relatively lower concentrations
will generate micro/nanoparticles in the electrospraying
‡These authors contributed equally to this publication.
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process.31 Electrospraying has been experimentally demon- nonsolvent bath as the collector. Well-distributed coatings
strated as a viable process for generating micro- or nanometre with hydrophobic properties were successfully prepared to
droplets that have a surface charge. The highly charged dro- obtain super-hydrophobic surfaces. Cotton fabrics were pre-
plets consequently result in self-repelled particles without treated with a water-based polyacrylate emulsion adhesive and
coalescence.32 It is conceptually easy to control the size of elec- then coated with the microspheres via electrospraying. The
trosprayed particles from the nanometre to micrometre range electrosprayed fluorinated PCL microspheres with honeycomb-
by varying the solution flow rate, the applied voltage, the spray- like pore structures were randomly deposited on the surface of
ing distance, and the physical properties of the solution.33 In the cotton fabric, generating a microsphere deposition film
the micro- or nanometre range, the capability of producing with a micro/nanohierarchical structure. This structure endowed
polymer particles with controllable porous microstructures the surface of the cotton fabric with a higher surface rough-
using electrospraying is unmatched by other aerosol proces- ness, leading to super-hydrophobic performance. Because the
sing methods. In general, compared with other methods for coated superhydrophobic cotton fabrics reject many naturally
preparing porous polymer particles, there are several advan- occurring pollutants and are not easily soiled, they can be
tages of electrospraying, as follows:34–39 (1) relative ease of used to make, for example, contamination free canvases, tents,
setup, (2) open-atmosphere operation without the use of a table cloths and kitchen aprons. If the coating is thin enough
sophisticated chamber, (3) controllable particle sizes in a and does not affect the feel and breathability of the fabric,
narrow distribution via a cone-jet spraying mode, (4) high pro- superhydrophobic cotton can even be used to make garments
duction efficiency due to the direction of particles sprayed such as jackets and shirts that require minimal cleaning.
onto the collector under an electric field, and (5) well-
dispersed particles due to self-repellence resulting from the
electrical charges on the particles. Given these advantages, we
sought to use electrospraying as a means to fabricate porous
fluorinated PCL microspheres.
2. Experiment
2.1 Materials
To date, much work has been conducted to develop micro- Poly(ε-caprolactone) (PCL) (average Mn = 80 000 g mol−1) was
and nanomaterials to achieve super-hydrophobic surfaces40–44 purchased from Aldrich Chemistry (United Kingdom).
whose hydrophobicity is mainly determined by the chemical 6-Amino-1-hexanol (97%) and 4-dimethylaminopyridine
composition and the geometric architecture of the surface. (DMAP, 99%) were obtained from Aladdin (Shanghai, China).
Polymer coatings and inorganic colloid particulate coatings Succinic anhydride (SA, 99%) was obtained from Sinopharm
are popular approaches to obtain a rough surface by spherical Chemical Reagent Co., Ltd. 1H,1H,2H,2H-Perfluorooctyl acry-
protrusions,45–48 rendering an enhanced hydrophobic surface. late (TFOA) was supplied by Suzhou Zhongbo Chemical
Here, the particles are randomly stacked on a substrate, but Technology Co., Ltd. Copper(I) bromide (CuBr, 99%) and N,N,
the roughness obtained is not sufficient to acquire super- N′,N′,N″-pentamethyldiethylenetriamine (PMDETA, 99%) were
hydrophobicity. These particles are often coated with a thin purchased from Aladdin (Shanghai, China). CuBr was stirred
layer of hydrophobic fluoropolymer to decrease the surface for 5 h in acetic acid, filtered, washed three successive times
free energy.49–51 Moreover, raspberry-like particles have been with ethanol and dried in a vacuum. 2-Hydroxyethyl 2-bromoi-
developed for the fabrication of super-hydrophobic surfaces, sobutyrate (HEBiB, 95%) was obtained from Shanghai
whereby nano-sized guest particles are decorated onto much Bailingwei Chemical Technology Co., Ltd. Tetrahydrofuran
larger host particles.52–55 The hierarchical rough surface con- (THF) and 1,4-dioxane were supplied by Qiangsheng
tributes to a dramatic increase in hydrophobicity. However, Functional Chemicals Co., Ltd, and were purified by distilla-
preparation of hierarchical micro/nanoparticles with high tion. N,N′-Carbonyldiimidazole (CDI, 98%) was obtained from
surface roughness is usually a complicated and multi-step Energy Chemical (China). 2-Butanone was supplied by
process. Moreover, the particles are especially difficult to trans- Sinopharm Chemical Co., Ltd. Ethyl alcohol, methanol,
fer onto substrates. The electrospraying technique has been n-hexane and acetic acid were purchased from Qiangsheng
proven to be a convenient and effective method for obtaining Functional Chemicals Co., Ltd. 1,3-Bis(trifluoromethyl)-
microscale to nanoscale particles from a range of materials, benzene (98%) was obtained from Asic (Shanghai) Chemical
e.g., polymers56–59 and inorganic60 and hybrid com- Technology Co., Ltd. Aluminium oxide (200–300 mesh) was
pounds.61,62 During the process of electrospraying, vapour- supplied by Aladdin (Shanghai, China).
induced phase separation was responsible for the formation of
the porous structure and thus the rough surface, which con- Functional Chemical Co., Ltd. Adhesive (30% of polyacrylate
tributed to super-hydrophobicity. emulsion) was made by our laboratory and diluted to 3‰
Chloroform was purchased from Jiangsu Powerful
In this study, fluorine-containing polyacrylate chains were before use. Aluminium foil was purchased from Shanghai
introduced into PCL, and the fluorinated PCL microspheres Klinlai Plastic Co., Ltd. Cotton woven fabric was purchased
formed cratered structures with honeycomb-like pores that from Nantong Shengbaolu Textile Co., Ltd. Poly(tetrafluoro-
were fabricated by electrospraying the copolymer solutions ethylene) (PTFE) tubing (inner diameter, 1 mm) and 23 G
with a single solvent, instead of a blend containing a stainless steel needles (inner diameter, 0.5 mm; outer dia-
nonsolvent, directly onto substrates without using
a
meter, 0.6 mm) were acquired from Changsha Nanometer
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Instrument Technology Co., Ltd. Polypropylene (PP) syringes 1083.2 cm−1 (C–F), 1013.0 cm−1 (C–Br). 1H NMR (400 MHz,
of 10 mL volume were sourced from Jiangsu Kangyou Medical CDCl3), δ (ppm). For TFOA monomer, δ 6.45 (d, J = 17.3 Hz,
Equipment Co., Ltd.
1H), 6.13 (d, J = 17.3, 10.5, 2.0 Hz, 1H), 5.89 (d, J = 10.5 Hz,
1H), 4.53–4.40 (m, 2H), 2.63–2.42 (m, 2H). For PTFOA polymer,
δ 4.36 (s, 2H), 3.80 (s, 2H), 2.47 (s, 2H), 2.11 (s, 1H), 1.27–2.01
2.2 Synthesis of fluorinated PCL block copolymer
2.2.1 Preparation of PCL-COOH.63 Activation of PCL by (s, protons of the structural unit on the PTFOA main chain),
aminolysis (PCL-OH). One gram of PCL was dissolved in 0.90 (s, 3H). (These results are shown in ESI S.1 and S.2.†)
20 mL of re-distilled 1,4-dioxane in a three-necked flask under
2.2.3 Preparation of PCL-b-PTFOA copolymer. One gram of
magnetic stirring at 37 °C. Subsequently, 1.1 g of 6-amino-1- PCL-COOH was dissolved in 30 mL of dry THF in a three-
hexanol was added to the mixture and the reaction took place necked flask under magnetic stirring at room temperature.
under a N2 atmosphere for 8 h. The resulting product was pre- Subsequently, 0.94 g of N,N′-carbonyldiimidazole was added to
cipitated twice in n-hexane. Then, the precipitated PCL-OH the mixture. After stirring for 2 h under a N2 atmosphere, all
was washed thoroughly in methanol, ethyl alcohol and doubly PTFOA was dissolved in dry THF/1,3-bis(trifluoromethyl)-
distilled water, yielding a pure product. The resulting PCL-OH benzene (3 : 1, total volume of solution was 24 mL) and was
was dried in an oven under vacuum at 37 °C for 24 h.
introduced into the mixture containing PCL-COOH. The reac-
In a three-necked flask, 1.0 g of PCL-OH and 1.14 g of SA tants were stirred for 3 h at 50 °C. After the completion of the
were dissolved in 20 mL of newly distilled 1,4-dioxane at room reaction, the product was precipitated twice in n-hexane. Then,
temperature. Subsequently, 0.4 g of K2CO3 and 0.35 g of DMAP the product was redissolved in THF and precipitated in de-
were added to the flask. The mixture was stirred for 120 min at ionized water to completely remove imidazole by-products
70 °C–80 °C. After the reaction, the insoluble carbonate by-pro- again. The resulting product was washed with a large amount
ducts were removed via filtration and the filtrate was washed of ethyl alcohol and deionized water to obtain the pure modi-
with an aqueous solution containing acetic acid (0.2 g mL−1). fied product, PCL-b-PTFOA. Finally, the resulting PCL-PTFOA
Subsequently, the product was precipitated twice in deionized was dried under reduced pressure at 37 °C for 24 h to constant
water to remove all of the unreacted SA and DMAP. Finally, the weight. According to the above procedure, PCL-PTFOA(2 h)
product was washed with a large amount of deionized water and PCL-PTFOA(6 h) were prepared by modifying PCL with
and ethyl alcohol and dried under reduced pressure at 37 °C PTFOA(2 h) or PCL-PTFOA(6 h), respectively. The fluorinated
for 24 h, yielding activated PCL via carboxylation (PCL-COOH).
PCL block copolymers were synthesized according to
2.2.2 Synthesis of PTFOA polymer. The PTFOA was pre- Scheme 1. FT-IR ν (KBr): 2950.4 cm−1, 2866.8 cm−1 (–CH/–CH2),
pared via ATRP by using 1H,1H,2H,2H-perfluorooctyl acrylate 1728.0 cm−1 (CvO), 1242.0 cm−1, 1185.6 cm−1, 1142.0 cm−1
(TFOA) as the monomer and 2-hydroxyethyl 2-bromoisobuty- and 1083.2 cm−1 (C–F), 1241.5 cm−1 (C(O)–O), 1178.2 cm−1
rate (HEBiB) as the initiator. A typical procedure is described (O–C(C)). 1H NMR (400 MHz, CDCl3): for PCL, δ 4.05 (t, J =
below. In a three-necked flask, a solution of the initiator 6.7 Hz, 2H), 2.31 (t, J = 7.6 Hz, 2H), 1.65 (d, J = 11.3, 6.8 Hz,
HEBiB (42 μL, 0.29 mmol) and PMDETA (64 μL, 0.32 mmol) in 4H), 1.37 (d, J = 15.2, 6.8 Hz, 2H). For PCL-PTFOA copolymer,
6 mL dry 2-butanone was slowly stirred for 10 min. Then, CuBr 1H NMR (400 MHz, CDCl3): δ 4.30 (s, 2H), 4.06 (s, 2H), 2.42 (s,
(0.041 g, 0.29 mmol) was rapidly added to the flask and the 2H), 2.31 (s, 2H), 1.65 (s, 4H), 1.39 (d, J = 6.0 Hz, 2H). 19FNMR
solution was degassed with three freeze–pump–thaw cycles. δ (376 MHz, CDCl3): −79.84 (3F, CF3
After CuBr was thoroughly dissolved, the monomer TFOA −120.20 (2F, CF3CF2CF2), −121.29 (2F, CF3(CF2)2CF2
(6.1 g, 14.5 mmol) dissolved in 2 mL dry 2-butanone was (2F, CF3(CF2)3CF2), −124.80 (2F, CF3(CF2)4CF2). (These results
̲
), −111.99 (2F, CF3CF2
̲ ),
̲
̲
), −121.91
̲
̲
added to the flask. Then, the solution system was degassed are discussed in ESI S.3–S.5.†)
and heated at 78 °C under N2. After the completion of the reac-
tion, the reaction mixture was exposed to air to quench the
2.3 Electrospraying
reaction. The polymerization solution was then diluted with The electrospray apparatus consists of a digital syringe pump
tetrahydrofuran (THF)/1,3-bis(trifluoromethyl)-benzene (3 : 1, loaded with a syringe, a high-voltage generator and a grounded
total volume of solution was 40 mL) and filtered over alu- collector. PCL, PCL-PTFOA(2 h) and PCL-PTFOA(6 h) copoly-
minium oxide to remove the catalyst, and the resulting eluate mers were dissolved in chloroform for at least 5 h at room
solution was concentrated and then precipitated in methanol temperature, generating a homogeneous and stable 3 wt%
to obtain the light yellow gummy solid product. The product solution. In the electrospraying process, the solution was
was redissolved in 2 mL 1,3-bis(trifluoromethyl)-benzene and placed into a 10 mL syringe and continuously pushed by the
precipitated in n-hexane again. This cycle was repeated three syringe pump to a stainless steel nozzle bent at a right angle
times to obtain a pure product. Lastly, the product was dried with an internal diameter of 0.5 mm, which was connected to
under reduced pressure at 55 °C for 24 h to constant weight. a high-voltage power generator. The flow rates of the three
Polymerization was carried out for 2 h or 6 h to produce the copolymer solutions were 0.3 mL h−1, 1 mL h−1, and 1 mL h−1
,
fluoropolymer intermediates with different molecular weights: respectively. A direct-current high-voltage generator was used
FT-IR ν (KBr): 3450.7 cm−1 (–OH, from HEBiB initiator), to provide a voltage of 12.5 kV of potential difference between
2920.7 cm−1 and 2974.5 cm−1 (–CH/–CH2), 1636.4 cm−1 the nozzle and the grounded collector. The spraying distance
(CvC), 1242.0 cm−1
,
1185.6 cm−1
,
1142.0 cm−1 and between the nozzle and the collector was maintained at 15 cm.
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Scheme 1 Synthetic route of the fluorinated PCL block copolymer.
The working temperature was 20
2 °C, and the relative microspheres on the fibre. A schematic of the electrospraying
humidity was controlled in the range of 50–60%. A flat sheet process is shown in Scheme 2.
of aluminium foil (15 × 20 cm2) and a rotating cylinder collec-
tor covered by an immobilized piece of cotton woven fabric
2.4 Characterization
(10 × 23 cm2) were used as the collector apparatus. The cotton Fourier transform infrared spectroscopy (FT-IR) spectra were
woven fabric was immersed in adhesive for 1 h and finished obtained on a NICOLET 5700 spectrometer using KBr pellets.
by a laboratory padder with an uptake of 115–120%. The elec- 1H NMR spectra were recorded on an INOVA-400 spectrometer
trospray deposition method was used to form a polymer par- in CDCl3 or CDCl3/Freon-113 with tetramethylsilane (TMS) as
ticle deposit film. The thickness of the coating layer depended an internal standard. All the chemical shifts were expressed in
on the flow rate. Hence, the electrospraying time of the three ppm. The molecular weight and molecular weight distri-
copolymers was 180 min, 60 min, and 60 min. Furthermore, butions of PCL, PCL-PTFOA(2 h) and PCL-PTFOA(6 h) were
the fabrics coated by microspheres were immediately placed measured on gel permeation chromatography (GPC) columns
into an oven and baked at 50 °C for 24 h to crosslink the (Waters Microstyragel) (guard, 105, 103, and 102 Å),
Scheme 2 Schematic diagram of the electrospraying apparatus.
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THF eluent at 35 °C, flow rate = 1.00 mL min−1. The detectors
consisted of a differential refractometer (Waters 410, λ =
930 nm) and a multi-angle laser light scattering (MALLS)
3. Results and discussion
3.1 Fluorinated PCL block copolymer
detector (Wyatt Technology DAWN EOS, 30 mW, λ = 690 nm). Fluoropolymers, PTFOA, with terminal hydroxyl groups were
Absolute molecular weights were determined with the dn/dc synthesized via atom transfer radical polymerization (ATRP)
values of PCL, PCL-PTFOA(2 h), PCL-PTFOA(4 h), PCL-PTFOA using 1H,1H,2H,2H-perfluorooctyl acrylate (TFOA) as the
(6 h) and PCL-PTFOA(8 h) (5 mg mL−1) using Wyatt ASTRA monomer, CuBr/(N,N,N′,N′,N″-pentamethyldiethylenetriamine)
software.
(PMDETA) as a catalyst system and 2-hydroxyethyl 2-bromoiso-
The static contact angles (CAs) and sliding angles (SAs) of butyrate (HEBiB) as the initiator.
the PCL, PCL-PTFOA(2 h), and PCL-PTFOA(6 h) polymeric
The active hydroxyl groups were bonded at the end of PCL
membranes were evaluated at room temperature and via aminolysis with 6-amino-1-hexanol. A terminal carboxyl
ambient humidity on a Krüss DSA100 instrument equipped was introduced by reacting with the ending hydroxyl of succi-
with a video camera. For each test, a 6 μL water and nic anhydride (SA) to fabricate the reactive polymer
hexadecane droplet was used. The CA and SA values were the PCL-COOH.
averages of three separate measurements made on different
The PTFOA molecule was immobilized on PCL via esterifi-
areas, which were estimated by the instrument software cation by using CDI as an activator under mild conditions to
according to the fitting method employing the Young–Laplace obtain hydrophobic copolymers (as shown in Scheme 1).
equation.
As shown in Table 1, the polydispersity of PCL, PCL-PTFOA
The surface free energies of the PCL and PCL-PTFOA copo- (2 h) and PCL-PTFOA(6 h) was determined as 1.60, 1.53 and
lymer films were estimated according to the method proposed 1.55, respectively, indicating that the polydispersity of PCL
by Owens and Wendt.64 Water and hexadecane were respect- only slightly changed after chemical modification. Our pre-
ively adopted as the hydrophilic and oleophobic liquids for the vious study reported that the fission of the PCL macromolecule
analysis. The contents of the chemical elements on the film chain led to a molecular weight decrease (the average Mn of
and microsphere surface were examined using energy-disper- PCL is 80 000 g mol−1, and the Mn of PCL-OH is 77 414
sive X-ray spectrometry (EDS).
g mol−1 after modification) during the aminolysis process.
Surface tension was examined with an OCAT21 surface While it can be observed that the molecular weight of
tension detector via the du Nouy ring method in chloroform PCL-PTFOA(2 h) was similar to that of PCL, the molecular
for 3 wt% polymer solutions. Viscosity was measured with a weight of the PCL-PTFOA(6 h) copolymer was even higher than
SNB-2 digital rotational viscometer at room temperature.
that of PCL because of the introduction of additional fluoro-
The surface morphology and size of electrospraying polymer block chains. The fluorine contents (atomic%) of
particles were studied under a Hitachi S4800 scanning electron PCL-PTFOA(2 h) and PCL-PTFOA(6 h) copolymer films were
microscope (SEM) with an accelerating voltage of 3–5 kV. The 1.31% and 4.02%, respectively. The fluorinated PCL block
particle size and distribution were determined via SEM image copolymer with longer fluoropolymer chains, and thus a
analysis using image processing software (ImageJ). To investi- higher molecular weight and higher fluorine content, pos-
gate the internal structures, the microspheres were immersed sesses better hydrophobicity.
in liquid nitrogen for 24 h and then slowly ground in liquid
nitrogen. The powder was dried at 37 °C, and the cross-section
morphology of the microspheres was observed using SEM.
3.2 Electrosprayed fluorinated PCL microspheres with a
honeycomb structure
The washing test was performed using a rotor washing
machine (Wenzhou Fangyuan Instrument Co., Ltd, China)
under the following conditions: 40 °C, 30 min, 30 cycles per
min with 4 g L−1 of standard detergent. The mechanical pro-
perties of the coated fabrics were measured using a universal
testing machine (WDW-200, Shenzhen SANS Test Machine Co.,
Ltd, Shenzhen, China).
Typical electrospraying equipment essentially consists of a
syringe connected to a metallic needle, a high-voltage power
supply, and a grounded collector. A digital pump is used for
high-precision control of the flow rate of the polymer solution
stored in the syringe. The setup is the same as that employed
for electrospinning. Whether fibres or particles are produced
depends on electrospray parameters, such as applied voltage,
Table 1 Molecular characteristics and surface properties of the copolymers
Fluorine
γSV
Surface energyd
c
a
a
a
Polymers
PCL
Mn
89 477 143 814 1.60
Mw
Mw/Mn
contentb (%) CAwater (°)
CAhexadecane (°) (mN m−1
)
(mN m−1
)
Viscositye (mPa s)
—
1.31
4.02
96.8 1.0
108.0 0.8 102.2 0.6
128.4 1.3 113.7 0.4
0
28.72
8.99
3.29
29.9
21.7
19.2
27.5
20.6
17.5
PCL-PTFOA(2 h) 86 917 133 107 1.53
PCL-PTFOA(6 h) 97 060 150 226 1.55
a Determined by GPC measurement. b Determined by EDS measurement. c Surface energy obtained indirectly from the contact angle (two fluid
method). d Surface tension in chloroform for 3 wt% solutions via the du Nouy ring method. e Viscosity tested with a digital rotational viscometer.
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flow rate, collection distance, environmental conditions, and cosity (as shown Table 1). Nevertheless, the fluorinated PCL
solution properties (solution viscosity, solution conductivity, microspheres have a lower average particle size than that of
surface tension, etc.). When a high voltage is applied to the PCL. This size difference is due to the fluoropolymer solution
metallic needle, the pendant polymer solution at the nozzle droplets more easily breaking up into secondary droplets (or
will be subjected to the electrical field and the induced satellites) and producing smaller microspheres under electro-
charges are evenly distributed over the surface. Under the static repulsion and the coulombic force exerted by the electri-
electrostatic repulsion and the coulombic force caused by the cal field. Furthermore, the Wigner distribution maxima
electrical field, the polymer solution droplet is stretched into a (WDM) of the three electrosprayed particles is 6.7 μm, 5.25 μm,
conical jet and further breaks into small droplets, provided and 5.75 μm, respectively, and it can be clearly seen that their
that the applied electric field is sufficiently high. Polymer par- microsphere size distribution is uniform and narrow in Fig. 1.
ticles are finally obtained when the solvent is completely evap-
orated during the process.
The surface morphologies of the PCL (a), PCL-PTFOA(2 h)
(b) and PCL-PTFOA(6 h) (c) microspheres are shown in Fig. 2.
Fig. 1 shows the SEM images and size distributions of PCL Electrospraying is a very convenient but complicated process
(a), PCL-PTFOA(2 h) (b) and PCL-PTFOA(6 h) (c) microspheres. during which solvent evaporation and polymer diffusion play
The images clearly show that the three polymer microspheres important roles in determining the morphology of the final
were fabricated with uniform, dispersive and perfect spherical product. It can be observed that PCL formed a golf ball-shaped
features via electrospraying technology using only a single microsphere with many porous pits on the surface, while
solvent. The average particle size of the PCL, PCL-PTFOA(2 h) PCL-PTFOA(2 h) and PCL-PTFOA(6 h) displayed regular honey-
and PCL-PTFOA(6 h) microspheres is 6.69 μm, 5.50 μm, and comb-like pore structures on the microsphere surface. The
5.64 μm, respectively. Compared to PCL, a faster flow rate was rapid evaporation of chloroform due to less PCL and chloro-
observed in producing PCL-PTFOA(2 h) and PCL-PTFOA(6 h) form interaction early in the solution’s ejection from the
microspheres, as there was lower surface tension and lower vis- nozzle and passage to the collector account for these pore
Fig. 1 SEM images and size distributions of PCL (a), PCL-PTFOA(2 h) (b) and PCL-PTFOA(6 h) (c) microspheres.
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Fig. 2 Magnified SEM images of PCL (a), PCL-PTFOA(2 h) (b) and PCL-PTFOA(6 h) microspheres (c).
structures. Chloroform molecules were expulsed rapidly from to overcome the resistance of the solution and migrated to the
the electrosprayed droplets; thus, PCL polymer chains precipi- interior of the droplet because of the concentration difference.
tated and linked in the final stage of the process, resulting in This process favours the shrinking and surface collapse of the
golf ball-shaped particles with closed pores on the surface. copolymer microsphere. However, the PTFOA component in
With respect to solvent evaporation, thermally induced and the copolymer is poorly compatible with PCL due to the lower
evaporation-induced phase separations are the pertinent surface energy. After the PCL segment entangled and solidified
phase separation processes for pore formation in electro- to form the spherical shell, the incompatible PTFOA chain
sprayed PCL porous particles.65,66 Solvent evaporation and eva- moved away from the PCL shell and migrated to the chloro-
porative cooling result in a solution that is thermodynamically form phase in the interior of the droplet. Consequently, micro-
unstable, which is the driving force for phase separation. phase separation between the two blocks of the copolymer
During the solvent evaporation and the resulting phase separ- occurred, promoting the uniform collapse of the microspheres
ation, the polymer solution develops into a polymer-rich and a and the formation of a rugged wall surrounding the surface
polymer-poor phase.66,67 The polymer-rich phase begins to pores. As a result, the microspheres with honeycomb-like pores
entangle and shrink, gradually forming the solidified shell. can be obtained after the solvent has completely evaporated.
The polymer-poor phase, which mainly consists of chloroform,
escapes the droplet surface and forms surface pores after com- wrinkles of PCL-PTFOA(2 h) (1) and PCL-PTFOA(6 h) (2) micro-
plete evaporation. sphere surfaces as captured by testing via energy-dispersive
Fig. 3 shows the fluorine content in the holes and on the
Compared to PCL microspheres, the surface morphologies X-ray spectrometry (EDS). The SEM images reveal that the two
of fluorinated PCL microspheres have a notable difference of copolymer microspheres have a porous and honeycomb-like
distinct honeycomb-like pore structures on their surface. The morphology. Furthermore, it can be observed that all pores are
introduction of a fluoropolymer chain (PTFOA) into PCL surrounded by a furrowed wall and that the PCL-PTFOA(2 h)
lowered its surface tension and viscosity greatly, which facili- microsphere surface displays a smaller pore size and thicker
tated the phase separation. During the electrospraying process, wall than that of the PCL-PTFOA(6 h) microsphere. This differ-
after the polymer solution formed the polymer-rich phase and ence can be attributed to the PCL-PTFOA(6 h) copolymer being
the polymer-poor phase, the concentrated polymer phase soli- equipped with a longer fluorinated chain segment and the
dified rapidly and formed the spherical matrix, whereas the copolymer solution having a lower surface tension and lower
polymer chain at the interface between the two phases tended viscosity resulting from a higher fluorine content. The longer
Fig. 3 High-magnification SEM images of PCL-PTFOA(2 h) (1) and PCL-PTFOA(6 h) (2) microspheres obtained by electrospraying a solution directly
onto the substrate and F element analysis of holes and wrinkles on the surface of the two copolymer microspheres.
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fluorinated polymer chain segment enhances the micro-phase golf ball shape. Furthermore, comparing the two fluorinated
separation between the two blocks in PCL-PTFOA(6 h). Lower PCL copolymer microsphere coatings reveals that the CA of
surface tension and lower viscosity further facilitate the PCL-PTFOA(2 h) and PCL-PTFOA(6 h) microsphere coatings
migration of the fluorinated segments and their shrinkage with honeycomb-like pore structures greatly increases to 162.4
toward the interior of the droplet and generate the porous
1.5° and 158.2
1.0° respectively. This change occurs
morphology with a larger pore size and thinner walls. The EDS because the microspheres possess a porous surface morphology
analysis revealed that the fluorine content in the hole was that forms a secondary roughness on the microsphere coatings.
higher than that on the surface wrinkle for both fluorinated These micro/nanohierarchical structures create a larger surface
copolymer microspheres. This finding confirmed that the roughness, allowing more air to be trapped in the pores on the
fluorinated segment tended to move to the interior during the coating surface, leading to a higher CA. Meanwhile, the fluori-
microsphere formation.
nated chains present play a role in enhancing the hydrophobi-
The internal structures of the microspheres were investi- city of the coated substrate because of its low surface free
gated by viewing cross sections, as shown in Fig. 4. The micro- energy. Although the fluorine content is lower for PCL-PTFOA
sphere formation process could be considered as consisting of (2 h), it is interesting that the CA of its microsphere coating is
two steps. The first step is electrospraying, which is higher than that of PCL-PTFOA(6 h). This higher CA occurs
accompanied by rapid evaporation of the solvent, and the because during the coating of PCL-PTFOA(2 h), denser and
second step is the formation of microspheres. Macrovoids smaller pores formed on the microsphere surface, and these
throughout the cross section of the microsphere that display a pores can capture more air and increase hydrophobicity.
uniform cellular structure can be seen in Fig. 4a (PCL micro- Additionally, the oil can wet the surface and spread over the
sphere), while in Fig. 4b, only slight pits are observed on the coating surface more easily for PCL-PTFOA(2 h) microsphere
cross section of the PCL-PTFOA(2 h) microspheres. coatings because of the capillary effect (Fig. 5).68
Furthermore, the cross section of the PCL-PTFOA(6 h) micro-
spheres becomes smooth and solid. The PCL polymer solution
possesses a higher surface tension and viscosity, causing PCL
polymer chains to precipitate rapidly and form links in the
processing of solvent evaporation. Hence, the solvent cannot
quickly escape from the electrosprayed droplets, thus forming
inner cavities and producing a porous final structure. The
intertwisting action of the fluorinated PCL copolymer chains is
weaker because of lower intermolecular forces; therefore, the
solvent is easily expulsed and evaporates from the electro-
sprayed droplets. In contrast, the fluorinated PCL copolymer
molecules migrate more easily to the interior of the droplet,
thus filling in the cavity and forming solid microspheres.
3.4 Direct coating of cotton fabrics
Cotton fabrics were coated with microspheres via electrospray-
ing. To provide adhesion between the cotton fabric and the
electrosprayed microspheres, the cotton fabrics were pretreated
with a water-based polyacrylate emulsion adhesive and coated
with one of the three polymers via electrospraying. The
polymer microspheres firmly adhered to the cotton fabrics
after crosslinking by drying. Fig. 6a–c show the morphology of
the cotton fabrics coated with the electrosprayed microspheres
of PCL, PCL-PTFOA(2 h) and PCL-PTFOA(6 h), respectively. It is
clear that the polymer microspheres are evenly distributed on
the surface of the cotton fabrics. Additionally, the thin mem-
branes formed by the adhesive, which immobilized the
spheres on the cotton fabrics and provided strong adhesion,
3.3 Microsphere coating and super-hydrophobicity
Surface wettability is a very important aspect of material pro- are visible on the cotton fibre surface. Fig. 6a′–c′ show that
perties and can affect the practical application of materials in after a standard washing process with soap, the number of
many fields. In this work, the porous microstructure of the microspheres on the cotton fabric decreased slightly, but
electrosprayed microsphere could significantly improve the abundant microspheres still adhered to the fabric. This result
surface roughness of coatings, increasing the surface hydro- indicates that electrosprayed microspheres adhere well to
phobicity. The flat PCL film possesses a CA of only 96.8 1.0° cotton fibres due to the adhesive force provided by polyacry-
(Table 1). However, the CA significantly increases to 150.7
late. The water contact angle test shows that the cotton fabrics
1.6° for the electrosprayed PCL microspheres with a porous coated with PCL-PTFOA(2 h) or PCL-PTFOA(6 h) electrosprayed
Fig. 4 Cross-section SEM images of the copolymer microspheres: PCL (a), PCL-PTFOA(2 h) (b), and PCL-PTFOA(6 h) (c).
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Fig. 5 SEM images of electrosprayed products with different copolymers: PCL (a), PCL-PTFOA(2 h) (b), and PCL-PTFOA(6 h) (c). Photo insets depict
the contact angles of the aluminium foil substrate coated with the corresponding electrosprayed products.
Fig. 6 SEM images of electrosprayed microsphere coated different materials on cotton fabric: (a) PCL, (b) PCL-PTFOA(2 h), and (c) PCL-PTFOA(6 h);
coated cotton fabrics after a soap wash: (a’) PCL, (b’) PCL-PTFOA(2 h), and (c’) PCL-PTFOA(6 h).
Fig. 7 Photograph of the wettability and SEM image of the cotton fabric coated with the porous PCL-PTFOA(6 h) microspheres.
microspheres possess a CA of 159.5 1.8° and 160.0 1.3°,
We conduct linear abrasion tests to fully study the mechani-
respectively, and are thus super-hydrophobic surfaces (inset in cal durability of the superhydrophobic fabric, and the method
Fig. 6b and c) as the micro/nanohierarchical structure endows is illustrated in Fig. 8a. The cotton fabric weighing 200 g is
the surface of the cotton fabric with a high surface roughness placed facedown to 600 mesh sandpaper and moved for
as well as very low surface free energy. To macroscopically 100 mm along the ruler, and then pulled back when reaching
display the super-hydrophobicity of the coated cotton fabric, the edge; this process was defined as 1 cycle. The water CAs
Fig. 7a shows a photograph of the wettability of the cotton fabric and SAs after the abrasion test are shown in Fig. 8b. It is
coated with the porous PCL-PTFOA(6 h) microspheres. Clearly, observed that CA values for water droplets placed on the fabric
the yellow potassium dichromate water solution droplets main- surface are varying from 159° in the initial state to 153° after
tain their spherical shape and stand very well on the cotton 15 cycles of abrasion, whereas the corresponding SA values are
fabric surface, indicating a hydrophobic coating performance.
ranging from 9° to 18°. Similarly, the PCL-PTFOA(2 h) micro-
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Fig. 8 The abrasion and mechanical durability of PCL-PTFOA(2 h) microsphere coated cotton fabric. (a) Sandpaper abrasion tests. (b) CA and SA
values of coated cotton fabrics.
Fig. 9 Magnified SEM images of the cotton fabric coated with (a) PCL, (b) PCL-PTFOA(2 h), and (c) PCL-PTFOA(6 h) microspheres.
sphere coated cotton fabric retains its water repellent property PCL-PTFOA(2 h) and PCL-PTFOA(6 h) copolymer microspheres
after other tests including washing and tape pressing, as possessing different microstructures were successfully pro-
shown in Fig. 8b. The above results indicate that the superhy- duced by electrospraying with a single solvent. The average
drophobic property will not be destroyed by mechanical particle sizes of the PCL, PCL-PTFOA(2 h) and PCL-PTFOA(6 h)
abrasion, tape adhesion and soaping.
microspheres are 6.69 μm, 5.50 μm and 5.64 μm, respectively.
Fig. 9 shows the magnified SEM images of the cotton fabric Their microsphere size distribution is narrow, and the Wigner
coated with microspheres. The microspheres are immobilized distribution maximum (WDM) is 6.70 μm, 5.25 μm and
on the cotton fabric by the adhesive. The three copolymer 5.75 μm, respectively. The SEM images show that the PCL
microsphere preparations show poorer uniformity when com- microsphere surfaces appear to have many porous pits similar
paring the deposition of microspheres using cotton fabric vs. to the shape and texture of a golf ball, whereas the PCL-PTFOA
aluminium foil as a collector. However, the microspheres still (2 h) and PCL-PTFOA(6 h) microsphere surfaces display regular
show good dispersal and maintain their spherical shape. In honeycomb-like pore structures. Thermally induced and evap-
comparison with the coatings on the aluminium foil substrate, oration-induced phase separations are considered to be the
the PCL microspheres display a golf ball shape, and the two pertinent phase separation processes that promote pore for-
fluorinated PCL microspheres produce surface roughening mation in electrospraying microspheres with porous surfaces
and display a cage-like structure. This arrangement may be and/or even inner porosity. For the fluorinated PCL copolymer
due to the kinetics that arise from the continually rotating microsphere, the micro-phase separation between the two
cylinder collector and nozzle giving rise to an unstable electric block chains is an additional factor promoting the uniform
field, resulting in a stronger whip effect.15 This phenomenon collapse of the copolymer microsphere surface and the for-
can create a greater roughness for the cotton fabric and thus mation of a rugged wall that surrounds the surface pores.
produce a super-hydrophobic microsphere coating on the Because of the high surface roughness, the CAs of coatings
surface of cotton fabric.
produced by the three electrosprayed microspheres with hier-
archically porous surfaces are 150.7 1.6°, 162.4 1.5° and
158.2 1.0°, respectively. Furthermore, fluorinated segments
enhance the hydrophobicity of the fluorinated PCL micro-
sphere coating due to its low surface free energy and provide a
4. Conclusions
In summary, fluorinated PCL copolymers with different fluo- super-hydrophobic coating surface. The cotton fabrics coated
rine contents were synthesized and characterized. PCL, directly with the microspheres of the fluorinated polymers via
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the financial support by the
National Natural Science Foundation of China (No. 51673137)
and the Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
ˇ
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