Development of photoresponsive coumarin-modified ethylene-co-vinyl alcohol copolymers with antifouling behavior

Article abstract of DOI:10.1016/j.reactfunctpolym.2020.104750

In this work, the modification of ethylene-co-vinyl alcohol (EVOH) copolymer with a coumarin derivative, 7-(hydroxyethoxy)-4-methyl coumarin (HEOMC), has been performed through the incorporation of different reactive groups, p-nitrophenyl chloroformate, succinic anhydride and phthalic anhydride as different linkers between coumarin and EVOH with the aim to obtain photo-responsive materials with improved protein absorption resistance. After purification, the modification degree was calculated by nuclear magnetic resonance and ultraviolet-visible spectroscopy. The thermal properties revealed the crystallinity loss and higher maximum degradation temperatures when EVOH is modified with coumarin derivative. Copolymers were successfully photo-crosslinked through the photo-responsive behavior of coumarin moieties. The micro-hardness test shown an increase of the mechanical performance due to the coumarin presence. The water contact angle measurements revealed that while coumarin-modified EVOH copolymers present a higher hydrophilic behavior than EVOH, the photo-dimerization process mainly leads to slightly deviations on the surface wettability of neat EVOH. Finally, the absorption of bovine serum albumin onto the surface of the coumarin-modified materials revealed enhanced antifouling properties, particularly after the photo-crosslinked process. Therefore, the introduction of coumarin through different chain extenders leads to functional materials with tunable photo-responsive performance and protein absorption resistance.

Full text of DOI:10.1016/j.reactfunctpolym.2020.104750

Reactive and Functional Polymers 157 (2020) 104750
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Development of photoresponsive coumarin-modified ethylene-co-vinyl
alcohol copolymers with antifouling behavior
⁎
Cástor Salgado , Marina P. Arrieta, Alberto Chiloeches, Alexandra Muñoz-Bonilla, Laura Peponi,
⁎
Daniel López, Marta Fernández-García
Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), C/Juan de la Cierva 3, 28006 Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, the modification of ethylene-co-vinyl alcohol (EVOH) copolymer with a coumarin derivative, 7-
(hydroxyethoxy)-4-methyl coumarin (HEOMC), has been performed through the incorporation of different re-
active groups, p-nitrophenyl chloroformate, succinic anhydride and phthalic anhydride as different linkers be-
tween coumarin and EVOH with the aim to obtain photo-responsive materials with improved protein absorption
resistance. After purification, the modification degree was calculated by nuclear magnetic resonance and ul-
traviolet-visible spectroscopy. The thermal properties revealed the crystallinity loss and higher maximum de-
gradation temperatures when EVOH is modified with coumarin derivative. Copolymers were successfully photo-
crosslinked through the photo-responsive behavior of coumarin moieties. The micro-hardness test shown an
increase of the mechanical performance due to the coumarin presence. The water contact angle measurements
revealed that while coumarin-modified EVOH copolymers present a higher hydrophilic behavior than EVOH, the
photo-dimerization process mainly leads to slightly deviations on the surface wettability of neat EVOH. Finally,
the absorption of bovine serum albumin onto the surface of the coumarin-modified materials revealed enhanced
antifouling properties, particularly after the photo-crosslinked process. Therefore, the introduction of coumarin
through different chain extenders leads to functional materials with tunable photo-responsive performance and
protein absorption resistance.
Ethylene-co-vinyl alcohol copolymers
Coumarin
Modification
Reactive groups
Photo-responsive
Coating
Antifouling
1
. Introduction
properties. For this reason, an optimal method to reduce their affinity
against water molecules is the partial modification of –OH groups.
The development of copolymers is an effective tool for modifying
Thus, EVOH copolymers can be easily modified to obtain materials with
tunable properties [5].
polymeric systems and therefore, to increase the number of applications
in the industry. Ethylene-co-vinyl alcohol (EVOH) copolymer is a
thermoplastic and semicrystalline copolymer synthesized from ethylene
and vinyl acetate and posterior hydrolysis. EVOH presents good me-
Coumarin is a natural compound habitually used in polymer
chemistry due to their photo-responsive properties [6,7]. Coumarin
derivatives are able to experiment reversible photo-dimerization and
pho-cleavage processes under UV irradiation. At wavelengths close to
365 nm, the double bond near to the lactone group undergoes a
[2π + 2π] cycloaddition to form a cyclobutane ring [8]. The dimers can
be photo-cleaved into monomers under irradiation of a UV light
(λ < 260 nm). These photochemical properties are used in the de-
velopment of different photo-responsive polymeric systems [9,10] like
micelles [11], nanogels [12], nanocomposites [13] as well as shape
memory polymers [14].
chanical and optical properties, it keeps low permeability to O and
2
CO , and consequently it is a very suitable material for the development
2
of inner layers in high barrier multilayer films for food packaging [1].
The hydroxyl groups of EVOH provide a high cohesive energy to the
system, by leading to a decrease in the available free volume for ex-
change of gas and high oxygen barrier properties [2], also providing
suitable hydroxyl sites to be easy used for further functionalization [3].
The ethylene molar content in EVOH is essential to determine char-
acteristics like the melting point (T
m
) or the glass transition tempera-
Besides, many coumarin derivatives have the capability to stabilize
free radicals, generally leading to good anti-oxidant and anti-microbial
activity [15]. These properties have also been used to enhance the
thermal stability of several polymers [7,16], or to provide antimicrobial
ture (T ), which are also important for the processability of EVOH [4].
g
However, the presence of –OH groups triggers the absorption of
water and provokes a decrease in the physical and mechanical
⁎
Corresponding authors.
E-mail addresses: castorsalgado@ictp.csic.es (C. Salgado), martafg@ictp.csic.es (M. Fernández-García).
https://doi.org/10.1016/j.reactfunctpolym.2020.104750
Received 12 September 2020; Received in revised form 2 October 2020; Accepted 11 October 2020
Available online 13 October 2020
1381-5148/ © 2020 Elsevier B.V. All rights reserved.

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
behavior [17], respectively. Coumarin can also be effective as anti-
fouling agent due to its lipophilic character and planar molecular
structure [18,19]. In opposition to metallic biocides, coumarin is an
environmental-friendly system to be incorporated in polymer coatings,
affecting the biofilm formation and the progress of fouling sequence, as
is the case when coumarin is incorporated into the matrix paints [20].
In this study, commercial EVOH copolymer with 56% of vinyl al-
cohol content is modified with coumarin to give enhanced and new
properties such as photo-responsive behavior and antifouling proper-
ties. The EVOH is previously activated following three different stra-
tegies by incorporation of p-nitrophenyl chloroformate, succinic anhy-
dride or phthalic anhydride. The substitution degree in each case is
2.3. Synthesis of HEOMC-succinic acid (HEOMCO)
The synthesis of the coumarin derivative was achieved by following
the procedure described in a previous work [13]. In a rounded flask,
HEOMC (2.10 g, 9.52 mmol) was dissolved in 25 mL of DCM and TEA
(5.32 mL, 38.1 mmol) was added. In another flask, succinic anhydride
(1.52 g, 14.26 mmol) and DMAP (57.5 mg, 0.47 mmol) were dissolved
in 30 mL of DCM. The solution of HEOMC was poured into the second
flask and the mixture was stirred at room temperature for 72 h. The
solution was filtered, washed with a 5% HCl solution and purified by
recrystallization in ethanol to obtain 2.44 g (yield ca. 80%) of
HEOMCO.
1
1
calculated by nuclear magnetic resonance ( H NMR) and ultraviolet-
H NMR (400 MHz, DMSO, ppm): δ 7.70 (d, J = 8.7 Hz, 1H),
visible (UV–vis) spectroscopy. The resulting materials with coumarin
are characterized by Fourier Transform Infrared Spectroscopy (FTIR),
7.05–6.96 (m, 2H), 6.23 (d, J = 1.5 Hz, 1H), 4.42–4.27 (m, 4H), 2.41
1
3
(d, J = 1.3 Hz, 3H). C NMR (101 MHz, DMSO, ppm): δ 173.8, 172.6,
1
13
H and C NMR. The photo-responsive properties are studied to eval-
161.7, 160.6, 155.1, 153.8, 127.0, 113.8, 112.9, 111.8, 101.9, 67.0,
uate the photo-dimerization and photo-cleavage of grafted coumarin
moieties. In order to use these materials as potential polymer coatings,
the thermal behavior, microhardness, surface wettability and anti-
fouling properties are also analyzed.
62.8, 29.1, 18.6. Electron-ionization mass spectroscopy (EI-MS): m/z
+
calculated for C16
H
16
O
7
[M + H] : 320.0896, found: 320.0902. FTIR:
−1
ν (cm ) = 3098, 3025, 2993, 2955, 2931, 1726, 1656, 1606, 1556,
1391, 1369, 1339, 1301, 1208, 1148, 1080, 1029, 983, 905, 875, 851,
8
33, 795, 747, 705, 667.
2
. Experimental section
2
.4. Synthesis of HEOMC-phthalic acid (HEOMC-PH)
In a rounded flask, 5 g of HEOMC (22.70 mmol) and 4.7 mL of TEA
2.1. Materials
(
34.05 mmol, 1.5 eq.) were dissolved in 100 mL of anhydrous THF
EVOH copolymer with 56% of vinyl alcohol (den-
under Argon atmosphere. In another two-necked rounded flask pro-
vided with a condenser, 5 g of phthalic anhydride (34.05 mmol, 1.5 eq.)
and 138 mg of DMAP (1.13 mmol, 0.05 eq.) were dissolved in 30 mL of
anhydrous THF. Afterward, the solution of HEOMC was added with a
syringe over the phthalic solution and the mixture was stirred at reflux
for 72 h. The mixture was cooled down to room temperature and the
salt of TEA was filtered. Then, the solution was concentrated, washed
with aqueous 5% HCl and extracted with DCM. The product was dried,
concentrated and purified by precipitation in cold DCM to obtain 6.2 g
−
1
−1
sity = 1.15 g cm , melt flow index = g 10 min ) was supplied by
Dupont,
1,4-dioxane,
2,2-dimethoxypropane,
N,N′-dicyclohex-
ylcarbodiimide (DCC), dichloromethane (DCM), ethyl acetoacetate,
resorcinol, 2-bromoethanol, ethyl acetate, 4-(dimethylamino)pyridine
(
DMAP), triethylamine (TEA), potassium carbonate, N,N-di-
methylformamide (DMF), phosphorous pentoxide, p-nitrophenyl
chloroformate, succinic anhydride, tetrahydrofuran (THF), phthalic
anhydride, bovine serum albumin-fluorescein isothiocyanate conjugate
(
BSA-FITC) and Trizma® hydrochloride buffer solution (pH = 7.4) were
(
yield ca. 78%) of a white solid (HEOMC-PH).
supplied by Sigma-Aldrich. Acetone, DMF, n-hexane, chloroform,
ethanol and methanol, were supplied by Scharlau. Sulfuric acid (98%),
dimethyl sulfoxide (DMSO) and pyridine were supplied by Panreac. N-
methyl-2-pyrrolidone (NMP) and deuterated DMSO were supplied by
Merck. TEA was distillated before used; the other products were used as
received.
1
H NMR (500 MHz, DMSO‑d , ppm): δ 13.28 (s, 1H), 7.82–7.73 (m,
6
1
1
H), 7.66 (d, J = 8.8 Hz, 1H), 7.65–7.61 (m, 3H), 7.02 (d, J = 2.4 Hz,
H), 6.99 (dd, J = 8.8, 2.5 Hz, 1H), 6.20 (d, J = 1.3 Hz, 1H), 4.64–4.55
1
3
(
m, 2H), 4.42–4.38 (m, 2H), 2.38 (s, 3H). C NMR (101 MHz,
DMSO‑d
6
, ppm): δ 167.8, 167.5, 161.2, 160.1, 154.7, 153.3, 132.2,
1
6
31.9, 131.4, 131.1, 129.0, 128.2, 126.5, 113.3, 112.4, 111.3, 101.4,
6.3, 18.1. Electron-ionization mass spectroscopy (EI-MS): m/z calcu-
+
2.2. Synthesis of coumarin derivative
lated for C20
H
16
O
7
[M + H] : 368.0896, found: 368.0889. FTIR: ν
−1
(
cm ) = 1719, 1675, 1614, 1394, 1285, 1269, 1213, 1134, 1075,
The synthesis of the coumarin derivative was carried out by fol-
880, 675.
lowing the procedure of Ling et al. [21] with some variations. Equi-
molar amounts of resorcinol (35.6 g, 0.32 mol) and ethyl acetoacetate
2.5. Synthesis of EVOH-HEOMC (ENH) copolymer
(
42.15 g, 0.32 mol) were dissolved in 130 mL of 1,4-dioxane. Then,
9
.7 mL of concentrated sulfuric acid (182 mmol) were added dropwise
2.5.1. Modification of EVOH with p-nitrophenyl cloroformate: EN
EVOH was activated by following the procedure described by
Sánchez-Chaves et al. [22]. Firstly, 5 g (135 mmol) of EVOH were
dissolved in 170 mL of NMP at 80 °C in a double-walled reactor and
then cooled to 0 °C. Afterwards, 9.3 mL (114 mmol) of pyridine and
23 g (114 mmol) of p-nitrophenyl chloroformate were added and stirred
for 24 h. The modified EVOH was isolated by precipitation with ethanol
and purified by reprecipitation from DMF in ethanol. Then, the product
(EN) was dried at vacuum in the presence of phosphorous pentoxide to
obtain a white solid with a substitution degree (SD) of 99%.
and stirred for 3 h at 65 °C. The reaction was cooled down to room
temperature and the white precipitate was filtered and washed with
hexane to obtain pure 7-hydroxy-4-methylcoumarin (HMC). Later,
HMC (4 g, 22.7 mmol) was poured in a two-neck round-bottom flask
and dissolved in 20 mL of DMF. Then, 2-bromoethanol (4.3 g,
3
4.4 mmol and potassium carbonate (6.3 g, 45.6 mmol) was added
under stirring. The reaction mixture was stirred for 24 h at 88 °C, then
cooled down and poured into ice water; the crude product was filtered
and recrystallized twice in ethyl acetate to obtain 35.2 g of HEOMC
(
(
yield ca. 50%).
1
2.5.1.1. EN. 1H NMR (DMSO‑d6, 500 MHz): δ 8.07 (s, 2H), 7.41–7.26
H NMR (400 MHz, CDCl , ppm): 7.51 (d, 1H; J = 8.8 Hz), 6.88
3
1
3
dd, 1H; J1 = 8.8 Hz, J2 = 2.8 Hz), 6.83 (d, 1H, J = 2.8 Hz), 6.15 (d,
(m, 2H), 5.16–4.76 (s, 1H), 2.19–1.26 (m, 6H). C NMR (DMSO‑d ,
6
1
2
H, J = 1.2 Hz), 4.15 (t, 2H, J = 4.4 Hz), 4.02 (q, 2H, J = 4.9 Hz),
.40 (d, 3H, J = 1.2 Hz), 2.09 (t, 1H, J = 6.0 Hz, OH). FTIR: ν
126 MHz): δ 155.1, 152.2, 144.9, 126.1, 122.3, 80.1, 78.1, 76.1, 74.4,
−1
72.7, 37.8, 37.3, 33.6, 32.8, 28.6, 24.4, 20.1. FTIR: ν (cm ) = 3419,
2931, 1755, 1669, 1616, 1594, 1522, 1493, 1347, 1260, 1206, 1164,
1090, 1050, 1011, 858, 760, 722, 681.
−
1
(
cm ) = 3427, 2954, 1704, 1621, 1554, 1388, 1370, 1295, 1272,
1
268, 1153, 1074, 1042, 892, 893, 867, 846, 817, 752, 573, 526, 441.
2

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
2
.5.2. Incorporation of HEOMC to EN
126 MHz): δ 167.7, 166.7, 132.4, 131.6, 130.7, 130.3, 128.5, 127.7,
A mixture of HEOMC (10.6 g, 48.2 mmol) and TEA (10 mL,
2.3 mmol) was incorporated to a solution of EN (5.6 g, 24.1 mmol of
74.7, 71.4, 68.3, 37.0, 33.0, 28.9, 24.5, 24.2, 20.1. FTIR: ν
−1
7
(cm ) = 2928, 1701, 1599, 1579, 1283, 1261, 1127, 1070, 1039,
791, 742, 705, 676.
reactive groups) in 280 mL of DMF. Catalyst DMAP (147 mg, 1.2 mmol)
was added to the mixture and stirred 72 h at 70 °C. The solution was
cooled at r.t. and precipitated over distillated water. The product was
purified by precipitation from DCM in methanol to obtain 4.4 g of a
light-yellow solid with a 62% of SD.
2.7.2. Incorporation of HEOMC to EP
In a two necked rounded flask, 5.5 g (45 mmol) of EP and 6.7 g
(31 mmol) of HEOMC were dissolved in 125 mL of anhydrous DMF. In
another flask, 6.3 g (31 mmol) of DCC and catalyst DMAP were dis-
solved in 20 mL of anhydrous DMF. The solution was incorporated over
the first flask with a syringe and was stirred at 60 °C for 24 h. The
solution was precipitated over distillated water; the solid obtained was
dried with phosphorous pentoxide and purified by dissolution in DCM
and precipitation over methanol twice. The white solid obtained was
dried at vacuum (8.9 g with a substitution degree of 40%).
2
2
3
1
3
1
.5.2.1. ENH. 1H NMR (DMSO‑d6, 500 MHz): δ 7.59 (s, 1H), 6.89 (s,
H), 6.13 (s, 1H), 4.89–4.62 (m, 1H), 4.44 (s, 2H), 4.31 (s, 2H), 2.35 (s,
1
3
H), 2.07–0.99 (m, 6H). C NMR (CDCl , 126 MHz): δ 160.8, 159.5,
3
54.3, 153.8, 152.5, 125.8, 113.1, 111.8, 111.0, 101.1, 66.1, 65.2,
−
1
4.5, 33.1, 28.4, 20.6, 17.5. FTIR: ν (cm ) = 2928, 1713, 1611, 1510,
389, 1250, 1200, 1147, 1070, 1013, 989, 846, 784, 731, 704.
1
2
.6. Synthesis of EVOH-SUCCINIC-HEOMC (ESH)
2.7.2.1. EPH. H NMR (500 MHz): δ 7.97–7.11 (m, 5H), 6.84 (s, 2H),
6
.09 (s, 1H), 5.47–4.75 (m, 1H), 4.71–4.03 (m, 4H), 2.25 (s, 3H),
13
2.6.1. Modification of EVOH with succinic anhydride (ES)
2.05–0.25 (m, 6H). C NMR (DMSO, 126 MHz): δ 167.2, 166.6, 161.5,
160.4, 155.0, 153.1, 139.2, 131.8, 129.0, 126.7, 113.7, 112.7, 111.7,
101.6, 75.7, 72.9, 70.0, 66.7, 63.9, 34.0, 31.6, 30.5, 29.5, 26.0, 25.3,
In a two-necked rounded flask, 5 g (135 mmol) of EVOH were
dissolved in 100 mL of DMSO at 80 °C. The mixture was cooled at room
temperature and 14 g (151 mmol) of succinic anhydride, 10.6 mL
−1
24.4, 18.4. FTIR: ν (cm ) = 2929, 2855, 1713, 1650, 1613, 1512,
1448, 1389, 1367, 1345, 1259, 1135, 1068, 848, 770, 740, 706.
Finally, all materials were processed into films by compression
molding in a hot press (Dr. COLLIN 200 × 200) at 100 °C by using a
(
135 mmol) of TEA and 200 mg (1.6 mmol) of DMAP were added. The
mixture was stirred for 5 days until it changed to dark and then was
precipitated over distillated water and purified by precipitation from
THF in hexane to obtain 12.4 g of a dark solid with SD of 89%. The
modified copolymer was dissolved in 200 mL of THF to avoid the
crosslinking.
2
1
13
film mold (50 × 50 mm ). H NMR and C NMR spectra are depicted
in the Supporting Information (Fig. S1 to S17).
2
.8. Characterization techniques
2
.6.1.1. ES. 1H NMR (DMSO‑d
, 500 MHz): δ 12.15 (s, 1H), 4.77 (s,
13
6
1
1
H), 2.50–2.38 (m, 4H), 1.73 (s, 2H), 1.42 (s, 2H), 1.18 (s, 2H).
C
All molded compressed EVOH copolymers were elucidated by H
1
3
NMR (DMSO‑d
6
, 126 MHz): δ 173.5, 172.3, 171.9, 67.1, 38.3, 37.9,
NMR and C NMR spectra at 500 MHz in a Varian System 500 spec-
trometer by using deuterated dimethyl sulfoxide (ca. 8% w/v). Fourier
transform infrared (FTIR) spectra were acquired with a Spectrum One
−
1
2
8.9, 25.2. FTIR: ν (cm ) = 2929, 1707, 1408, 1379, 1159, 992, 953,
8
28.
−1
FTIR spectrometer with a resolution of 4 cm
and 16 accumulation
−1
2.6.2. Incorporation of HEOMC to ES
scans (Perkin-Elmer Co., Norwalk, CT, USA) in the 4000–650 cm
7
g (80 mmol) of ES were previously dissolved in 70 mL of THF and
region at room temperature. The modification degree of EVOH copo-
lymers was also confirmed by UV spectra recorded in a NanoDrop one
(Thermo Fisher Scientific). Coumarin modified EVOH copolymers were
irradiated at 365 nm with 5 × 8 W 365 nm - UV lamps for 210 min to
produce the coumarin [2 + 2] cycloaddition reaction, while they were
irradiated at 254 nm with 5 × 8 W 254 nm - UV lamps during 180 s for
the photo-cleavage reaction. Thin copolymer films were casted over the
external surface of a quartz cell and the UV spectra were acquired with
a Perkin Elmer Lambda 35 UV/VIS spectrometer (Norwalk, CT, USA)
from 400 to 200 nm. The yield of photo-dimerization reaction was
obtained from the absorbance at 320 nm (A 320) using the Eq. (1) [23]:
were added to 250 mL of DMF. After that, 11.8 g (54 mmol) of HEOMC,
1
3.8 g (67 mmol) of DCC and catalyst DMAP were added to the solution
under stirring at 60 °C for 48 h. Then, the mixture was cooled to room
temperature, precipitated over distillated water and purified by pre-
cipitation from DCM in methanol. The modified copolymer was dried at
vacuum to obtain 10.1 g of a white solid with a SD of 55%.
2
1
2
1
1
2
1
7
.6.2.1. ESH. 1H NMR (DMSO‑d
, 500 MHz): δ 8.16 (s, 1H), 7.60 (s,
6
H), 6.89 (s, 2H), 6.14 (s, 1H), 4.76–4.69 (m, 1H), 4.35 (s, 2H), 4.25 (s,
1
3
H), 2.56–2.29 (m, 7H), 1.76–1.05 (m, 6H). C NMR (DMSO‑d ,
6
26 MHz): δ 171.8, 169.4, 161.1, 159.9, 154.6, 153.1, 126.3, 113.2,
A 3200 A 320f
12.2, 111.2, 101.2, 66.4, 62.4, 32.4, 31.6, 30.1, 29.4, 29.1, 28.7, 28.5,
Photo dimerization (%) =
× 100
A 3200
(1)
−
1
5.4, 25.1, 24.5, 18.0. FTIR: ν (cm ) = 2928, 2854, 1724, 1658,
612, 1525, 1388, 1369, 1345, 1263, 1146, 1070, 1013, 986, 847, 808,
32, 704.
A320
0
and A320
f
are the absorbance values measured at 320 nm
before and after the irradiation at t time, respectively.
Glass transition (T
g
) and melting (T ) temperatures were de-
m
2.7. Synthesis of EVOH-PHTHALIC-HEOMC (EPH)
termined by Differential Scanning Calorimetry (DSC). Measurements
were performed with a TA Instruments DSC Q2000 (New Castle, DE,
USA) under nitrogen atmosphere (50 mL/min). Samples were sealed in
aluminum pans and heated from −80 to 100 °C at 10 °C/min. The
2.7.1. Modification of EVOH with phthalic anhydride (EP)
5
g (135 mmol) of EVOH were dissolved in 200 mL of DMF at 80 °C,
the temperature was cooled down to 60 °C. Then, 16.8 g (113 mmol) of
phthalic anhydride, 16 mL (113 mmol) of TEA and catalyst DMAP were
added. After 2 h, the product was precipitated over distillated water
and further purified by re-precipitation from a mixture of acetone/
methanol 1/1 over distillated water. A white solid was obtained and
dried at vacuum (10.9 g with a substitution degree of 98%).
degree of crystallinity (X ) was calculated from the enthalpy of a 100%
c
crystalline poly(vinyl alcohol) (PVA) (157.8 J/g) [24]. TA Instruments
TGA Q500 thermal analyzer was used to perform the thermogravi-
metric measurements under dynamic mode using nitrogen atmosphere
(flow rate of 60 mL/min). Samples were heated from room temperature
to 700 °C at a heating rate of 10 °C/min. The initial degradation tem-
peratures (T ) were obtained at 5% of mass loss and temperatures at the
0
2
.7.1.1. EP. 1H NMR (DMSO‑d
6
, 500 MHz): δ 13.09 (s, 1H), 7.72–7.32
maximum degradation rate (Tmax) were obtained from the first deri-
vative of the TGA curves (DTG). Microindentation measurements were
1
3
(
m, 4H), 5.27–4.95 (m, 1H), 2.00–1.18 (m, 6H). C NMR (DMSO‑d ,
6
3

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
obtained with a Vickers indentator attached to Leitz microhardness
group (56%), and four protons of the ethylene (44%) that are con-
(
MH) tester. Experiments were undertaken at 22 °C with contact load of
sidered in the equation.
9
80 N and a contact time of 25 s. MH (in MPa) were calculated ac-
SD(without coumarin) (%)
cording to the relationship: MH = 2 sin 68 P/d [2], where P (in N) is
the contact load and d (in mm) is the diagonal length of the projected
indentation area.
Integral of methine at 4.5 ppm × (2 × 0.56 + 4 × 0.44)
=
Integral of methylene at 2.1 0.6 ppm × 0.56
(2)
Surface wettability of the films was obtained with a KSV Theta
goniometer by static water contact angle (WCA) measurements in
contact mode. The volume of the droplets was 7.0 μL and the contact
angle was determined as the average value of 4 drops of distilled water
randomly deposited with a syringe the film surface. The surface of the
films was observed by scanning electron microscopy (SEM) using a
Philips XL30 microscope with an acceleration voltage of 25 kV. Prior to
SEM observation, the films were coated with gold. Antifouling prop-
erties of EVOH derivatives were obtained by immersing the films
The incorporation of coumarin to the EVOH copolymer is confirmed
1
13
13
by H NMR, C NMR and FTIR. In the Fig. 2 the most important
C
NMR and FTIR signals are shown. The presence of the carbonyl group in
the copolymer ENH, belonging to the p-nitrophenyl chloroformate is
1
3
confirmed by the signal at 154 ppm in the C NMR spectrum, as can be
observed in the Fig. 2A and marked with a discontinuous blue rec-
tangle. Carbonyl group can also be distinguished by FTIR with a
−1
shoulder at 1735 cm corresponding to the C]O stretching vibration
(
Fig. 2B). Also, the end of reaction can be followed by FTIR (Fig. 2B),
−1
(
1 × 1 cm) in solutions 0.01 M of bovine serum albumin-fluorescein
where aromatic bands at 1520 and 1350 cm
disappear (marked as
isothiocyanate conjugate (BSA-FITC) in Trizma hydrochloride buffer
solution for 3 h at 25 °C. Then, the film was removed from the solution
and the unbound protein concentration was analyzed in the resultant
solution by fluorescence spectroscopy in a Synergy HTX Multi-Mode
Reader spectrophotometer (Bio-Tek) at an excitation wavelength of
blue discontinuous circles) and the coumarin lactone band at
−
1
1
615 cm
Meanwhile, in coumarin modified copolymers (ENH, ESH and EPH)
the SD was calculated by comparison of the HEOMC aromatic signal at
appears (marked as red discontinuous circle).
6
.1 ppm and the methine signal at 4.5 ppm of modified EVOH (Eq. 3).
4
80 nm and an emission wavelength of 525 nm. Briefly, 100 μL of each
Therefore, the SD is referred to the amount of coumarin that has reacted
with the modified EVOH.
solution was placed in a 96-well round-bottom microplate. All films
were analyzed in triplicate, and blank experiments were also performed
with the Trizma buffer solutions. The fluorescence intensity of each
sample was measured and the data were converted to concentration of
unbound protein using the corresponding standard curve. From this
data, the amount of BSA-FITC adsorbed onto the surface of the film was
calculated by subtracting the unbound concentration from the initial
Integral of HEOMC at 6.1 ppm
Integral of methine at 4.5 ppm
SD(with coumarin) (%) =
(3)
3.2. Characterization of the copolymer ESH obtained by route B
concentration and the represented in ng cm−1
.
The modification of EVOH with succinic anhydride (ES), and sub-
1
sequently modification with HEOMC (ESH) were also confirmed by H
NMR and FTIR spectroscopy (see Fig. 3). Methine signals in EVOH (3.3
to 3.9 ppm) are shifted to 4.8 ppm when EVOH is modified with suc-
cinic anhydride (pictured with blue discontinuous circle in Fig. 3A).
The incorporation of coumarin to ES (Fig. 3A) shifts the methylene
bands of HEOMC at 3.7 and 4.1 ppm to 4.2 and 4.3 ppm in ESH (red
discontinuous circle), respectively. In addition, the methine group at
3
. Results and discussion
The different synthetic routes followed to obtain EVOH containing
coumarin groups are represented in Scheme 1. Notice that although all
the obtained coumarin derivatives presented similar coumarin group
the chemical structure of the resulting copolymers significantly differs.
4
.8 ppm in ES is slightly affected by the incorporation of HEOMC and is
3.1. Characterization of the copolymer ENH obtained by route a
shifted to 4.7 ppm in ESH. Similarly, the FTIR spectra confirmed the
successfully modification of the EVOH copolymer, obtained by this
route. In the FTIR spectra (Fig. 3B marked in blue characters), the ap-
pearance of the carbonyl stretching band belonging to the dimer C]O
EN modified copolymer was characterized by FTIR, 1H and 13
C
NMR. The most characteristic IR bands are shown in Fig. 1A. Coumarin
−
1
presents typical absorption bands at 1720 and 1615 cm , belonging to
−1
in carboxylic acids at 1710 cm and a shoulder of ester C]O stretch at
C]O and C]C lactone stretching vibration, respectively. The CeO
−1
−
1
1
725 cm
confirm the process [9]. In the FTIR spectra (Fig. 3B), the
ester stretching vibration band appears at 1278 and 1070 cm
and
−
−
1
1
modification of ES with HEOMC induces the decrease of the C]O acid
CeH aromatic out of plane bending vibration appears at 840 cm
21]. The most characteristic signals of EVOH appear at 1440 cm
−
1
dimer band. Furthermore, the presence of C]O (1655 cm ) and C]C
[
−1
−
1
−1
lactone (1615 cm ) bands of coumarin are marked in red; also a
(
methylene asymmetric stretch), 1330 cm
(methine asymmetric
−
1
−
1
displacement in the C]O stretching from 1705 to 1655 cm
is ob-
stretch), 1087 cm (CeO stretch) and 840 cm (deformation of CeH
−
1
served, triggered by hydrogen bond interaction with the carboxyl
groups of unmodified ES.
out of plane). The hydroxyl stretch (3320 cm ) and the methylene
−
1
anti-symmetric stretch (2900 cm ) bands are not shown in the Fig. 1A.
1
Fig. 1B shows H NMR signals of EN as well as those of HEOMC and
EVOH. Aromatic signals of HEOMC appear at 7.7, 6.9, and 6.2 ppm,
proton of hydroxyl group at 4.9 ppm, the methylene groups at 4.1 and
3.3. Characterization of the copolymer EPH obtained by route C
3
.7 ppm and the methyl group at 2.4 ppm [13]. EVOH signals appear in
Likewise, the introduction of phthalic anhydride and afterwards
1
three regions: from 4.1 to 4.7 ppm (hydroxyl groups), from 3.3 to
.9 ppm (methine groups) and from 1.2 to 1.4 ppm (methylene groups)
HEOMC into the EVOH copolymer was elucidated by H NMR and FTIR
3
spectroscopy. In the Fig. 4A the influence of the phthalic anhydride into
the EVOH chain (EP) is shown by the presence of the methine band at
5.1 ppm and also the aromatic protons of phthalic ring between 7.3 and
7.7 ppm (marked a with discontinuous blue circle). Moreover, in the
FTIR spectrum (Fig. 4B) the presence of carbonyl vibrational strength at
[
22]. These three groups of alcohol and methylene signals correspond
to the isotactic, heterotactic and syndiotactic arrangement in PVA [25].
The substitution degree (SD) of modified EVOH copolymers was
1
calculated by using H NMR. EVOH methylene signals (between 2.1 and
−
1
0
.6 ppm) were compared with methine signal (around 4.5 ppm) of
1705 cm
is also marked.
modified groups to determine the SD (Eq. 2) in the copolymer without
coumarin (EN). As EVOH is a copolymer with 56% of vinyl alcohol, the
methine signal is referred to 56 per each 100 repeating units. Besides,
the methylene signal contains two protons, which correspond to vinyl
The incorporation of HEOMC into the EP is also established by the
1
presence of new aromatic signals of HEOMC in the H NMR spectrum
(marked with a red discontinuous circle in Fig. 4A). Moreover, the
−
1
carbonyl vibrational strength of lactone coumarin bands at 1655 cm
4

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
Scheme 1. Synthetic routes of modified EVOH copolymers.
(
C=O) and 1615 cm−1 (C=C) are observed (Fig. 4B).
the same band. The photo-conversion degree of coumarin-modified
EVOH copolymers is shown in Fig. 5. The higher photo-dimerization
yields were obtained for the sample ESH, which is caused by a better
mobility of coumarin moieties linked to the succinic chain extender.
The results obtained for ENH and EPH are similar, with more rigid
spacer groups. The photo-cleavage process is a faster route and is in
equilibrium with other parallel reactions, i.e. asymmetric fragmenta-
tion [6]. The photo-cleavage process is highly influenced by the mo-
bility and rigidity of coumarin moieties. Also, the photo-cleavage pro-
cess in EPH is influenced by the absorption of phthalic ring at around
290 nm (see Fig. S18 in Supporting Information). In this case, phthalic
ring can absorb energy at wavelengths close to those of coumarin
The substitution degree of the copolymers EP and EPH were de-
termined thought the (Eqs. 24), and results are collected in Table 1.
3
.4. Photo-dimerization properties
Coumarin-modified EVOH copolymers were irradiated with a UV
lamp to study the photo-dimerization reaction of coumarin in the dif-
ferent systems. The photo-dimerization process was estimated with
UV–vis spectrophotometry by following the decreasing of the absorp-
tion band of coumarin double bond at 320 nm [21]. Likewise, the
photo-cleavage process of the coumarin dimer triggers the increase of
Fig. 1. FTIR (A) and 1H NMR (B) spectra of: EVOH, EN and HEOMC.
5

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
Fig. 2. 13C NMR (A) and FTIR (B) spectra of: EVOH, EN, ENH and HEOMC.
dimer, avoiding a more efficient photo-cleavage [26].
thermal transitions in EVOH (see Fig. 6A); the glass transition of EVOH
The SD of coumarin modified copolymers was also calculated by UV
spectroscopy in a Nano-drop and the Lambert-Beer's law (Eq. 4). The
modified copolymers were dissolved in DMF and the absorbance of
each sample was measured 4 times.
(T
g
) that appears at 54 °C, and the melting of crystalline domains (T ),
m
at 166 °C [27]. The enthalpy of this transition is 79.3 J/g, that corre-
sponds to X of 50.3%, which is in the range of this copolymer at the
c
performed conditions. Nevertheless, a total loss of crystallinity can be
observed when EVOH is modified, provoked by the disruption of the
crystalline order produced by the substituents [28]. The different series
are also displayed in Fig. 6. When EVOH is activated with p-nitrophenyl
chloroformate (Fig. 6B) presents higher stiffness due to the bulky sub-
|
Abs|
SD(UV ) (%) =
x100
Ci
(4)
Where |Abs| is the average absorbance and C
i
is the initial concentra-
stituent. Therefore, EN shows a higher T than EVOH, 67 °C. The in-
g
tion of each sample. The absorption coefficient of HEOMC
corporation of HEOMC leads to the elimination of nitrophenyl group
−
1
−1
(
(
(
ε
ε
ε
320
320
=
14,226
11,184
±
211 mol
L cm ), HEOMCO
−1
and increase the flexibility; then, ENH displays a T at 44 °C. The in-
g
−
1
=
±
517 mol
−1
L
cm
)
and HEOMC-PH
troduction of coumarin leads to intramolecular interactions between
themselves or with the free hydroxyl groups in EVOH [22]. In contrast,
the chemical modification of EVOH with succinic anhydride (Fig. 6C)
−
1
320 = 11,581 ± 562 mol
L cm ) were previously calculated in
DMF. These results are in agreement with the NMR measurements. The
compilation of all modified copolymers with molar ratio of each co-
monomer and the SD of coumarin are shown in Table 1.
provokes a diminishment on the T from 54 °C to 11 °C. Succinic
g
moieties provide higher mobility to the polymer matrix leading to a
It should be observed that SD values obtained by NMR and UV
spectroscopy are very similar and, therefore, both techniques could be
easily used to study coumarin-based systems.
reduction of the T value and as happen with EN, its insertion inhibit
g
the crystallization of EVOH [29]. The posterior incorporation of
HEOMC to obtain ESH, gives as result a movement restriction due to
coumarin moieties and then, the T rises up to 42 °C. Meanwhile, the
g
modification with phthalic anhydride introduces more rigidity to
3.5. Thermal properties
EVOH, resulting in the highest value of T (81 °C) (Fig. 6D). Again,
g
when HEOMC is incorporated to obtain EPH, this gives a slight flex-
The thermal properties of modified EVOH copolymers were studied
ibility even though its T
modification degree.
g
is still high, 72 °C in part due to its smaller
by DSC and TGA and their main results are summarized in Table 2.
The EVOH copolymer is composed of ethylene and vinyl alcohol
units, with inherent capacity to form three-dimensional networks and
with high crystallinity. Its corresponding transitions should appear
between these two homopolymers. The DSC heating curve shows two
In order to analyze the thermal stability of EVOH copolymers, TGA
measurements were performed. Weight loss and DTG results are shown
in Fig. 7. It is well-known that polyethylene acts as a thermal stabilizer
Fig. 3. 1H NMR (A) and FTIR (B) spectra of: EVOH, ES, ESH and HEOMC.
6

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
Fig. 4. 1H NMR (A) and FTIR (B) spectra of: EVOH, EP, EPH and HEOMC.
in EVOH copolymers, therefore two peaks of maximum degradation
temperature are shown in the DTG curves. The first one appears at
3.7. Surface wettability
3
96 °C, corresponding to vinyl alcohol units, which matches with the
The surface wettability of the coumarin-modified EVOH copolymers
before and after UV irradiation process was studied by water contact
angle. The results are also collected in Table 3 and can be considered as
hydrophilic surfaces with contact angles lower than 90°. As expected,
the irradiation of neat EVOH does not modify the surface, since it does
not contain coumarin groups; however, a film annealing can be per-
formed with an increase of crystallinity and higher rigidity, which is not
significant to modify its water contact angle. The value of neat EVOH
was in the range of already reported works of EVOH with 44% of
ethylene content [33]. Generally, the hydrophilic behavior of EVOH is
slightly increased when EVOH is modified with coumarin derivatives
because the modification intercepts the inter- and intramolecular bonds
between hydroxyl groups present in the vinyl alcohol fraction of EVOH
[33], leaving free hydroxyl groups able to interact with water at the
surface of the film and, thus, enhancing the surface wettability of the
materials. This is the case of ENH with the higher modification degree
with coumarin and free vinyl alcohol units. In the case of ESH and EPH,
with lower modification degrees, the rest of vinyl alcohol units are
capped with succinic and phthalic acid moieties, respectively. These
groups could interact between themselves or be exposed at the surface,
enhancing their wettability. The differences between ENH, ESH and
EPH are not significant.
PVA degradation [30]. The second peak at 444 °C corresponds to the
ethylene degradation [31]. In general, the degradation starts at lower
temperatures in modified EVOH copolymers. Nevertheless, the mod-
ification of the EVOH copolymer with coumarin induces higher max-
imum degradation temperatures corresponding to the ethylene de-
gradation. This behavior could be ascribed to the antioxidant ability of
coumarin [32], as it was already observed in other polymeric matrix
such as poly(ε-caprolactone) (PCL) [7]. This effect is especially marked
in the sample ENH, which presents the higher value of Tmax (462 °C)
due to the higher amount of coumarin in this formulation (see Table 1
with the highest degree of substitution). It should be noted that the
samples ESH and EPH show three degradation steps in the thermogram.
The lower substitution degree in these copolymers when coumarin is
incorporated leads to different interactions between structure of acti-
vator, unreacted hydroxyl groups and coumarin moieties.
3.6. Microhardness
The mechanical properties of coumarin-modified EVOH copolymers
were studied by microhardness test to evaluate the elastic recovery or
the resistance to deformation before and after photo-dimerization
process. The polymer modification usually leads to an arrangement
between the moieties, which can interact with themselves or show
entanglements. This behavior produces a variation in the permanent
plastic deformation at the surface of the polymeric film.
Additionally, SEM images are taken to analyze the surface since it is
well-known that the roughness of surface can influence the contact
angle [34,35]. It is observed in Fig. 8 that there are smooth and flat
surfaces, therefore, the differences between them have to be due to the
chemical structure.
The MH results are shown in Table 3. In general, the increase in the
MH is related with the entanglement of the modified chains, as well as
the interactions between coumarin moieties. In the case of EVOH, the
irradiation could produce a film annealing with an increase of crys-
tallinity and higher rigidity. This fact is not possible in modified co-
polymers due to the absence of crystallinity. For that reason, it is ex-
pected that neat EVOH presents lower values of MH than modified
copolymers. Additionally, ENH shows high MH values, which is similar
to that of ESH and higher than that of EPH. These values are in con-
cordance with the already commented restriction on the mobility of
Meanwhile, the photo-crosslinking process in EVOH copolymers
bearing a coumarin moiety decreases the distance between hydro-
phobic groups in coumarin by forming the cyclobutane units [13] and it
could induce to a less hydrophilic behavior in the coumarin-modified
copolymers.
3.8. Antifouling properties
The protein absorption experiment was performed to study the
antifouling properties of the resulting coumarin-modified EVOH copo-
lymers before and after UV irradiation process. The samples with ap-
polymer chains, i.e. T (see Table 2) and the degree of coumarin in-
g
corporation. There are not significant variations in the MH with the
irradiation process with the exception of the sample ESH, which shows
the highest photo-crosslinking degree (75%) of coumarin. This behavior
can be ascribed to the higher flexibility of succinic extender that favors
the photo-dimerization process.
2
proximately 1 cm of surface area were incubated into the bovine serum
albumin solution. The results of BSA-FITC adsorbed mass per unit area,
m
ads, are shown in the Table 3. The absorption of BSA onto the surface
of the film is related with their hydrophobic performance [36]. Gen-
erally, the absorption of BSA onto the surface of the copolymers is re-
duced when EVOH is modified with coumarin [18]. In effect, from the
7

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
Table 1
Molar ratio and substitution degree of coumarin in EVOH copolymers.
Structure
Name
EN
x
y
z
–
SDNMR (%)
SDUV (%)
–
0.01
0.55
99 ±
5
ENH
0.21
0.35
–
62 ±
5
56 ±
2
ES
0.06
0.05
0.50
0.23
–
89 ±
5
5
ESH
0.28
55 ±
61 ±
7
EP
0.01
0.01
0.55
0.33
–
99 ±
5
5
EPH
0.22
40 ±
39 ±
7
data collected in Table 3, it is clearly observed that the modification
with coumarin enhances the protein-adsorption-resistance properties in
EVOH modified copolymers. The amount of BSA adsorbed on the sur-
faces is much smaller in coumarin derivatives films that the protein
adsorbed on EVOH. Moreover, the photo-crosslinking of coumarin
seems to reduce even more the adsorption of BSA. This behavior may be
due to different factors, including variation of the surface wettability
due to the changes in the chemical structure after photo-dimerization.
4
. Conclusions
Coumarin moieties as photo-responsive segments have been suc-
cessfully grafted to EVOH copolymers by activation with different chain
1
13
extenders. Copolymers were characterized by FTIR, H and C NMR.
8

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
Fig. 5. Photo-dimerization and cleavage process of coumarin and conversion degree of coumarin-modified EVOH copolymers at different times of photo-dimerization
left side) and photo-cleavage (right side) processes.
(
Table 2
The materials exhibited reversible photo-dimerization and photo-clea-
vage processes, whose conversion degree was conditioned by the nature
of the chain extender. Thermal properties reveal that the modification
induces to a complete loss of crystallinity and a significant alteration of
the glass transition temperature. Likewise, coumarin triggered a higher
thermal stability corresponding to the ethylene degradation of the final
material. The photo-crosslinking produces a slightly increase of the
hydrophilic behavior and also to render materials with enhanced an-
tifouling properties. Therefore, the coumarin moieties grafted onto
EVOH copolymers allowed to obtain interesting materials that could be
capable to act as photo-responsive polymeric coatings with tunable
hydrophilicity and antifouling performance by varying the chain
DSC and TGA thermal properties of coumarin-modified EVOH copolymers.
Sample
T
g
(°C)
T
m
(°C)
T
onset (°C)
T
max I (°C)
T
max II (°C)
Tmax III (°C)
EVOH
EN
54
67
44
11
42
81
72
166
–
329
225
239
220
216
195
195
396
271
303
283
196
237
198
444
461
462
322
340
295
308
–
–
–
ENH
ES
–
–
460
457
458
462
ESH
EP
–
–
EPH
–
Standard error: Temperature ± 1 °C.
Fig. 6. DSC thermograms of all series of coumarin-modified EVOH copolymers. EVOH (A), ENH family (B); ESH family (C) and EPH family (D).
9

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
Fig. 7. Thermogravimetric analysis of all series: TGA (A) and DTG (B) of ENH family; ESH family (B, C) and EPH family (E, F).
Table 3
extender used. These results suggest that both ENH and ESH copoly-
mers achieved the greatest photo-responsive, thermal, mechanical and
antifouling properties.
Surface properties of coumarin-modified EVOH copolymers. The irradiated
materials are depicted with “i-” before the name of the sample.
Sample
MH (MPa)
WCA (°)
m
ads (ng/cm2)
EVOH
i-EVOH
ENH
166 ± 10
188 ± 14
87 ±
86 ±
78 ±
85 ±
76 ±
80 ±
76 ±
83 ±
5
4
2
1
1
2
5
2
75 ±
75 ±
19 ±
n.d.
8 ±
1
2
1
Data availability
276 ±
266 ±
260 ±
232 ±
239 ±
2
5
5
1
2
The data that support the findings of this study are openly available
in Digital.CSIC at https://digital.csic.es/.
i-ENH
ESH
0
i-ESH
EPH
n.d.
18 ±
48 ±
0
4
i-EPH
235 ± 14
Declaration of Competing Interest
n.d. non detected adsorption.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
10

C. Salgado, et al.
Reactive and Functional Polymers 157 (2020) 104750
Fig. 8. SEM images of all materials before and after irradiation.
Acknowledgements
A. Marcos-Fernandez, Synthesis and characterization of a photo-crosslinkable
polyurethane based on a coumarin-containing polycaprolactone diol, Eur. Polym. J.
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Authors thank Spanish Ministry of Science and Innovation (Projects
MAT2016-78437-R MAT2017-88123-P), the Agencia Estatal de
Investigación (AEI, Spain) and Fondo Europeo de Desarrollo Regional
[
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