Polymer-2020_Plasticized poly(lactic acid) reinforced with antioxidant covalent organic frameworks

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Polymer 196 (2020) 122466
Available online 12 April 2020
0032-3861/© 2020 Elsevier Ltd. All rights reserved.
Plasticized poly(lactic acid) reinforced with antioxidant covalent organic 
frameworks (COFs) as novel nanofillers designed for non-migrating active 
packaging applications 
Paloma García-Arroyo a, Marina P. Arrieta a,*, Daniel Garcia-Garcia b,**, 
Rocío Cuervo-Rodríguez a, Vicent Fombuena b, María J. Manche~no a, Jos�e L. Segura a,*** 
a Departamento de Química Org�anica I, Facultad de CC. Químicas, Universidad Complutense de Madrid, Madrid, 28040, Spain 
b Instituto de Tecnología de Materiales, Universitat Polit�ecnica de Val�encia, Alcoy, 03801, Spain 
A R T I C L E I N F O 
Keywords: 
poly(lactic acid) 
Covalent organic frameworks 
Non-migration antioxidant activity 
A B S T R A C T 
A 2D Covalent Organic Framework (named [HC≡C]0.5-TPB-DMTP-COF) was synthesized and post synthetically 
functionalized with dopamine via Copper-catalyzed azide–alkyne cycloaddition (CuAAC) reaction (COFDOPA) to 
obtain 2D nanoparticles with antioxidant activity. COFDOPA nanoparticles were exfoliated into nanosheets 
(COFDOPA-e) and incorporated into plasticized poly(lactic acid) (PLA) matrix with 15 wt% of acetyl trybutyl 
citrate (ATBC) to develop non-migratory sustainable packaging. The effect of COFDOPA and exfoliated COFDOPA-e 
(in 0.5, 1 and 3 wt%) on the structural, thermal and mechanical properties of PLA-ATBC was studied. The 
bionanocomposites loaded with low amounts of COFDOPA-e (0.5 wt% and 1 wt%) resulted optically transparent 
and showed good interfacial adhesion, increased crystallinity, thermal and mechanical performance. Moreover, 
the overall migration level assayed in a fatty food simulant was below the migration limits required for food 
packaging materials and showed effective antioxidant activity. Thus, these bionanocomposites show great po-
tential as non-migration antioxidant materials with interest in the sustainable food packaging field. 
1. Introduction 
The use of biopolymers in the food packaging sector has raised in the 
last years due to their non dependence on non-renewable resources and/ 
or to the reduction of plastic waste after their useful life [1–3]. Among 
biopolymers, poly(lactic acid) (PLA) has received most attention for 
food packaging during the last two decades, due to the fact that it 
combines both sustainable characteristics, it is produced from the 
fermentation of starch obtained from renewable resources (i.e.: sugar 
cane and corn), and it is also compostable [2,4]. Nowadays PLA can be 
obtained through the revalorization of agri-food by-products as alter-
native feedstock for fermentative lactic acid (LA) production (i.e.: cob 
and corn stalks, bakery, coffee mucilage, brewer’s spent grains, etc.) to 
avoid food resources competition for its production [5,6] and it can also 
be recycled [7,8]. Moreover, considering the intended use, PLA’s 
properties such as high transparency, availability in the market at 
competitive cost and easy processability through the same processing 
technology already used for traditional petroleum-based thermoplastics, 
make it the most widely used biopolymer in the food packaging field [1, 
2,9]. However, it also presents some disadvantages such as the sensi-
tiveness to thermal degradation, as well as poor barrier and mechanical 
performance, which hinder its industrial exploitation [1,10,11]. In fact, 
after processing PLA still yields stiff and brittle material, and thus, other 
additives (i.e.: plasticizers, compatibilizers etc.) are frequently added, 
especially for those applications where flexibility is an essential issue 
such as the case of films [1,12]. In this sense, citrate based esters have 
been widely used as biopolyesters additives [13–15] and are accepted 
for food contact applications [16]. 
Another interesting approach that is growing up in the PLA based 
food-packaging sector is focused on the development of bio-
nanocomposites as a promising route to increase PLA crystallization rate 
with the further enhancement of the thermo-mechanical performance, 
by a simple blending approach [10,17,18]. Two-dimensional nano-
particles are able to accelerate PLA crystallinity as heterogeneous 
* Corresponding author. 
** Corresponding author. 
*** Corresponding author. 
E-mail addresses: marrie06@ucm.es (M.P. Arrieta), dagarga4@epsa.upv.es (D. Garcia-Garcia), segura@quim.ucm.es (J.L. Segura). 
Contents lists available at ScienceDirect 
Polymer 
journal homepage: http://www.elsevier.com/locate/polymer 
https://doi.org/10.1016/j.polymer.2020.122466 
Received 26 February 2020; Received in revised form 3 April 2020; Accepted 6 April 2020 
mailto:marrie06@ucm.es
mailto:dagarga4@epsa.upv.es
mailto:segura@quim.ucm.es
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Polymer 196 (2020) 122466
2
nucleation agents being an effective way to improve the final perfor-
mance of bionanocomposites. In this context, the chemistry of linking 
molecular building blocks by strong covalent bonds has gained consid-
erable interest for crystalline extended structures, since it allows 
yielding several new classes of porous materials such as covalent organic 
frameworks (COFs) [19]. COFs comprise an emerging class of materials 
based on the atomically precise organization of organic subunits into 
porous crystalline structures connected by strong covalent bonds with 
predictable control over composition, topology and porosity [19,20]. 
Layered COFs have been selected as a potential source for the production 
of 2D-polymers [20]. In fact, COFs posses special advantageous features 
such as low mass density, permanent porosity and high thermal and 
chemical stability [21,22]. The high thermal stability of COFs makes 
them interesting candidates for their use as biopolymer nanoreinforce-
ments that will be processed at high temperature (around 180–200 �C). 
In this context, COFs have already been prepared by extrusion process 
[23], but they have not been extruded as thermoplastic polymers ad-
ditives. Moreover, the COFs unique properties associated with reduced 
dimensionality, well-defined in-plane structure, and tuneable function-
alities [20], make them an excellent solution as polymeric nano-
reinforcements able to enhance crystallinity to improve biopolymers 
performance. It is known that, polymer reinforcement nanosheets may 
amend the structure-property features of biopolymers to a great extent 
when dispersed uniformly [24]. Layered structures of COFs have been 
successfully exfoliated into nanosheets by sonication-assisted exfoliation 
method as a simple and green route to obtain COF nanosheets [25,26]. 
In this context, COFs have been recently used as 2D-nanoreinformce-
ment for biopolymers matrices such as poly(vinyl alcohol) (PVA) [27] 
and chitosan [28]. Nonetheless, COFs can be specifically designed to be 
easily post-synthetically functionalized to introduce specific function-
alities according to the intended application [29] by a simpler approach 
than those required to functionalize other 2D-polymer such as graphene, 
which nanosheets have been widely used as nanofiller to enhance the 
performance of biopolymers [30], at the same time that maintain their 
crystalline and porous features [31]. Moreover, two-dimensional 
layered COFs can be successfully exfoliated into nanosheets just under 
ultrasonication [25,26]. Among porous materials, COFs posses partic-
ular advantageous characteristics for food packaging applications with 
respect to their closest congeners metal-organic frameworks (MOFs), 
since they are often composed of light atoms (i.e.: H, B, C, N and O) and 
have metal-free constitution avoiding the toxicity of metal ions[31,32]. 
Although more effort are needed to ensure COFs no toxicity, their 
biocompatibility and toxicity has been widely investigated as drug 
carriers in biomedical applications, as documented elsewhere [32–34]. 
On the other hand, lipid oxidation is a major reason that affects the 
sensory properties of food, and also represents a potentially harmful to 
human health. Thus, many smart packaging approaches have been 
focused on lipid oxidation prevention by incorporating antioxidants into 
the packaging materials instead of directly into the foodstuff. A broad 
range of natural compounds have been proposed as antioxidant agents 
for active packaging technology including synthetic and natural anti-
oxidants such as iron and ferrous oxide, ascorbic acid, sulphites, cate-
chols and enzymes such as glucose oxidase [35–37]. Among natural free 
radical scavenging antioxidants, compounds that contain catechols are 
reported to have high antioxidant capacity and have been widely stud-
ied as biomimetics of mussel adhesive proteins [38]. In this context, 
inspired by the natural mussel adhesion chemistry, dopamine has been 
extensively used to develop catechol-functionalized polymers and 
nanocomposites with antioxidant activity [39–41]. However, most 
active packaging technologies depend on the migration of the active 
agents from the packaging to the foodstuff in a time-released manner 
(Fig. 1-a) [42]. Nowadays, due to the fact that migration phenomenon 
represents a potential consumer concern and there are continues regu-
latory changes, the diffusion of the active agents suffer certain draw-
backs since sometimes results difficult to achieve a sustained release in a 
controlled way with the further maintenance of the active functionality 
as well as the structural and/or mechanical performance of the pack-
aging material [43,44]. Thus, minimizing migration of additives from 
packaging materials into foodstuff during storage represents the main 
challenge for the next generation of active food packaging materials 
[45]. Thus, non-migratory packaging systems are gaining interest in the 
active food packaging sector (Fig. 1-b). 
The main objective of this work was to develop high performance 
biodegradable non-migratory active packaging bionanocomposites 
based on PLA reinforced with dopamine functionalized COF obtained 
via the Copper-catalyzed azide–alkyne cycloaddition (CuAAC) process, 
the premier example of click chemistry reactions. For this purpose, the 
cavities of the selected starting COF (named as [HC≡C]0.5-TPB-DMTP- 
COF) [46] were decorated by covalently bonding a dopamine azide by 
means of the CuAAC reaction (Fig. 2). The starting COF was synthesized 
following an already reported procedure [46] and it was 
post-synthetically functionalized with a suitably functionalized dopa-
mine derivative via the CuAAC reaction (COFDOPA). The functionalized 
COFDOPA and exfoliated COFDOPA (COFDOPA-e) were further used for the 
development of novel PLA bionanocomposite films with active proper-
ties. The effect of functionalized COFDOPA as well as exfoliated 
Fig. 1. Schematic representation of: a) migratory and b) non-migratory active food packaging. 
P. García-Arroyo et al. 
Polymer 196 (2020) 122466
3
COFDOPA-e in PLA-ATBC blends was evaluated with the aim of obtain-
ing biodegradable films based on PLA with functional properties for 
food packaging applications. The influence of COFDOPA on the 
PLA-ATBC matrix was evaluated by studding the structural, thermal and 
mechanical performance of the bionanocomposites. Considering that the 
bionanocomposites should be thermally processed at high temperature 
for their scale up production from lab-level to the industrial plastic 
packaging sector, special attention was paid on the thermal degradation. 
Moreover, since these bionanocomposites are intended for 
non-migration antioxidant packaging applications, the antioxidant 
effectiveness was demonstrated in a fatty food simulant. 
2. Experimental section 
2.1. Chemicals and materials 
The following reagents were commercially available and were used 
as received: CuI, o-DCB, n-butanol, NaN3, Tf2O, dopamine hydrochlo-
ride salt, ZnCl2, Et3N, DIPEA. 2,5-dimethoxyterephtaldehyde (DMTA) 
[46], 2,5-bis(prop-2-in-1-yloxy)terephtaldehyde (BPTA) [47], 1,3,5-tri-
s-(4-aminophenyl)benzene (TAPB) [47], TPB-DMTP-COF [46], dopa-
mine azide [48] and TfN3 [49] were prepared according to reported 
procedures. Poly(lactic acid) (PLA ErcrosBio® LL703, density 1.25 g 
cm� 3, Mn ¼ 23,250 Da [13]) was gently supplied by Ercros S.A. (Bar-
celona, Spain). Acetyl tributyl citrate (ATBC) (98% purity, Mw ¼ 402 g 
mol� 1 and Tm ¼ � 80 �C) was purchased from Sigma-Aldrich. Chloro-
form (CHCl3 Sigma-Aldrich) was used as solvent for PLA based film 
preparation. 2,2-diphenyl-1-picrylhydrazyl (DPPH) 95% free radical 
were supplied by Sigma Aldrich (Madrid, Spain). 
2.2. Synthesis and dopamine functionalization of COF 
2.2.1. Synthesis of [HC�CH]0.5-TPB-DMTP-COF 
[HC≡CH]0.5-TPB-DMTP-COF was synthesized following an already 
reported recipe [46]. In brief, DMTA (11.9 mg, 0.06 mmol), BPTA (14.5 
mg, 0.06 mmol), TAPB (29.1 mg, 0.08 mmol) and o-DCB/n-Butanol (2 
mL/2 mL) and acetic acid (6 M, 0.4 mL) were reacted in a Pyrex vessel 
and 27.3 mg (53%) of a yellow solid were obtained, after Soxhlet 
extraction in THF (see Scheme 1 ESI). 
13C CP/MAS-RMN, δ (ppm): 154.1, 148.7, 141.2, 128.7, 122.7, 
116.9, 110.7, 80.3, 73.3, 57.2, 54.6. ATR-FTIR, ν (cm� 1): 2357, 1590, 
1500, 1462, 1410, 1369, 1286, 1206, 1177, 1144, 1031, 974, 876, 826, 
725, 691, 671, 627, 598, 574. 
2.2.2. Synthesis of COFDOPA 
To obtain COFDOPA, a suspension of 27.69 mg of CuI and 100 mg of 
[HC≡CH]0.5-TPB-DMTP-COF was prepared in a mixture of THF/H2O 
(13 mL; 3/1) and it was further purged with Argon for 5 min. N,N-dii-
sopropylethylamine (DIPEA) was then added (87 μL, 0.50 mmol) and 
the mixture was purged with Argon to finally add 0.28 mL (0.28 mmol, 
1 M THF) of azide 1. The obtained suspension was stirred overnight 
under argon at room temperature. After the reaction time, the solid was 
filtered and then washed with H2O, MeCN and THF, to be further dried 
yielding a reddish solid (170 mg, COFDOPA) (Fig. 2). 
13C CP/MAS-RMN, δ (ppm): 154.5, 148.8, 140.9, 137.8, 128.4, 
122.8, 117.4, 110.0, 56.8, 54.6. FTIR (ATR), ν (cm-1): 1586, 1491, 1461, 
1410, 1287, 1208, 1176, 1144, 1031, 878, 825, 787, 691, 663, 574. 
ATR-FTIR, ν (cm� 1): 1587, 1491, 1410, 1144, 1031, 825. 
2.3. Characterization techniques 
13C cross-polarized magic angle spinning solid-state NMR (13C CP/ 
MAS NMR) were recorded on a 400 MHz spectrometer Wide Bore 
(probe: Hv/X BB of 4 mm), using a sample rotation frequency of 12 kHz 
and a ZrO2 rotor of 2.5 mm. 
Mass spectra were determined using a Matrix-Assisted Laser 
Desorption/Ionization-Time Of Flight (MALDI-TOF). 
UV–vis spectra were obtained with a UV–vis spectrometer for solid 
samples. The base line was register using Teflon. 
Fourier Transform infrared spectroscopy (FTIR) spectra are reported 
in wavenumbers (cm� 1). Solids were analyzed by attenuated total 
reflectance (ATR) on a diamond plate. Elemental Analysis (EA) were 
determined by means of a LECO CHNS-932 elemental analyser. 
Powder X-ray diffraction (PXRD) measurements were carried out 
with a PANalytical X’Pert PRO Powder system using Kα (λ ¼ 1.5406 Å) 
for values of 2θ from 1� to 10� range and/or X’PERT MPD with con-
ventional Bragg-Brentano geometry for values of 2θ from 1.8� to 35�. 
The COF pore volume and surface area were calculated from the 
nitrogen sorption isotherm at 77 K determined by means of a Brunauer- 
Emmett-Teller (BET). 
Thermogravimetric analysis was performed by means of a TGA-Q-50 
instrument on a platinum plate, samples were heatedunder nitrogen 
atmosphere at a heating rate of 10 �C/min. 
2.4. Exfoliation of COFDOPA 
A suspension of COFDOPA in CHCl3 (0.4 mg mL� 1) was submitted to a 
tip-sonication treatment in a Fisherbrand™ Q500 Sonicator (13 mm 
diameter 4220 probe, 50 Hz at 30% amplitude, Fisher Scientific) for 90 
min in an ice bath. In order to avoid the inherent shortcomings of son-
ication treatment for the exfoliation of 2D materials such as the gener-
ation of high local temperature due to sonication-induced cavitation 
Fig. 2. Schematic synthesis of COFDOPA. 
P. García-Arroyo et al. 
Polymer 196 (2020) 122466
4
[50], the sonication process was performed in cycles of 15 min in an ice 
bath. The resulting suspension was centrifuged at 1000 rpm for 5 min to 
eliminate non exfoliated COFDOPA. 
2.4.1. Characterization of exfoliated COFDOPA 
The particle size of sonicated assisted COFDOPA dispersed in CHCl3 
was determined by means of a dynamic light scattering analyser (DLS, 
Zetasizer Nano series ZS, Malvern Instrument Ltd., U.K.) at 20 �C. 
COFDOPA-e nanosheets were observed by Transmission Electron 
Microscopy (TEM, JEOL JEM-1010) operating at 100 kV. One droplet of 
COFDOPA-e suspension was deposited on carbon-coated copper grids and 
dried at room temperature during 20 min before observation. 
2.5. Bionanocomposites preparation 
Bionanocomposites films were prepared by solvent casting method. 
PLA pellets (0.8 g) were dissolved in 20 mL of CHCl3 and stirred at room 
temperature. Then, ATBC was added to obtain PLA-ATBC in 85:15 
proportion, following the preparation method of previous works [18, 
51]. PLA-ATBC films were loaded with the required amount of synthe-
sized COFDOPA or exfoliated COFDOPA (COFDOPA-e) to obtain bio-
nanocomposites reinforced with 0.5, 1 and 3 wt% of dopamine 
functionalized COF with respect to PLA matrix. Predetermined amounts 
of dopamine functionalized COFs (COFDOPA or COFDOPA-e) suspension 
in chloroform were mixed with the previously prepared plasticized PLA 
solution. It has been shown that sonication is an useful method to ho-
mogeneously disperse hydrophilic particles into PLA based polymeric 
solutions [9,15,52]. Therefore, an ultrasonic treatment was further 
conducted in a Transsonic 460 Elma ultrasonicator for 2 min in an ice 
bath, to obtain a good dispersion of both COFDOPA and COFDOPA-e into 
the plasticized PLA matrix. The resulting suspensions were cast onto a 
50 mm-diameter glass mould and then CHCl3 was eliminated at 60 �C for 
1 h. To complete the drying process, the films were put for about 24 h at 
40 �C and finally dried under vacuum to eliminate the traces of solvent. 
Table 1 summarizes the obtained formulations as well as the proportion 
of each component. 
2.5.1. Bionanocomposites characterization 
The absorption spectra of nanocomposites, obtained in the 700-250 
nm region, were assayed by means of a Perkin-Elmer (Lambda 35, USA) 
UV-VIS spectrophotometer. 
Nanocomposite film colour properties were evaluated in the CIELAB 
colour space by using a KONICA CM-3600d COLORFLEX-DIFF2, Hunt-
erLab, Hunter Associates Laboratory, Inc, (Reston, Virginia, USA). The 
instrument was calibrated with a white standard tile. Yellowness index 
(YI) and colour coordinates, L (lightness), a* (red-green) and b* (yellow- 
blue) were measured at random positions over the film surface. Average 
values of five measurements were calculated. Total colour difference 
(ΔE) of each bionanocomposite was calculated with respect to the 
control PLA-ATBC film using Equation (1). 
ΔE¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Δa�2 þ Δb�2 þ ΔL�2
p
(1) 
FESEM micrographs of the cryo-fractured cross-sections of the films 
were acquired with a field emission scanning electron microscope 
(FESEM) ZEISS model ULTRA 55 (Eindhoven, The Netherlands). Image 
acquisitions were conducted at an accelerating voltage of 5 kV. Prior to 
be observed fractured surfaces of samples were coated with a thin layer 
of platinum in a high vacuum sputter coater EM MED20 from Leica 
Microsystem (Milton Keynes, United Kingdom). 
Thermogravimetric analysis (TGA) was conducted in Linseis TGA 
PT1000 (Selb, Germany) under isothermal mode at 180 �C for 20 min as 
well as under dynamic mode with a heating rate of 10 �C/min, from 30 
to 700 �C, in a nitrogen atmosphere (flow 20 cm3/min). The onset 
degradation temperature (T0) was determined at 1% of mass loss, while 
temperatures of the maximum decomposition rate (Tmax) were calcu-
lated from the first derivative of the TGA curves (DTG). 
DSC experiments were conducted in a TA Instrument Q200 calo-
rimeter (New Castle, DE, USA). Samples were subjected to a thermal 
cycle consisting in a first heating scan from room temperature to 190 �C 
at a heating rate of 10 �C/min, followed by a cooling process down to 0 
�C and subsequent heating up to 250 �C, at 10 �C/min. The glass tran-
sition temperature (Tg) was taken at the mid-point of heat capacity 
changes. The melting temperature (Tm) and cold crystallization tem-
perature (Tcc) were obtained from the first heating while the degree of 
crystallinity (χc) was calculated through Equation (2): 
χc¼ 100% ​ � ​
�
ΔHm � ΔHc
ΔHcm
�
​ � ​
1
WPLA
(2) 
where ΔHm is the melting enthalpy, ΔHcc is the cold crystallization 
enthalpy, ΔHmc is the melting heat associated to pure crystalline PLA 
(93 J/g) [53] and WPLA the weight fraction of PLA in the sample. 
The nanomechanical properties of films were measured by a nano-
indenter machine G-200 (Agilent Technologies, Santa Clara, CA, USA). 
All samples were indented in the same experiment. Indentations were 
performed at maximum 1000 nm constant depth using a Berkovich 
diamond tip. An array of 8 indentations was performed for each sample. 
The function area used to estimate the contact area at low depths was 
previously calibrated in fused silica. The stiffness required to calculate 
the contact area beyond indenter and elastic modulus was obtained by 
means of the continuous stiffness measurement (CSM) technique [43, 
54]. The CSM technique provides in-depth profiles of the hardness (H) 
and elastic modulus (E). 
Overall migration studies were conducted in a faty food simulant 
(Simulant D1 ¼ ethanol 50% v/v), by the total immersion of the bio-
nanocomposite films in food simulant D1 (area-to-volume ratio ¼ 6 
dm2/kg food simulant) at 40 �C for 10 days [55]. After 10 days films 
were removed and the food simulant was almost evaporated on a 
heating plate and then dried in an oven at 110 �C for 30 min to get the 
residue, which was weighed in an analytical balance Sartorius BP211D 
(Goettingen, Germany) to � 0.1 mg to determine the overall migration 
expressed as mg/kg of food simulant. 
In order to corroborate that dopamine functionalized COFs were not 
migrating from the packaging to the food simulant, the obtained residue 
after the overall migration tests was redissolved in 2.25 mL of 50% v/v 
ethanol and the antioxidant activity was determined according to the 
DPPH-method, by determining the reduction of the absorbance at 517 
nm by means of a UV–Vis Varian Cary spectrophotometer. The radical 
scavenging activity (RSA) was determined according to Equation (3). 
RSA ð%Þ¼
AControl � Asample
AControl
� 100% (3) 
where Acontrol is the absorbance of 2,2-difenil-1-picrylhydrazyl (DPPH) 
in ethanolic solution and Asample the absorbance of DPPH after 15 min in 
Table 1 
Bionanocomposite film formulations. 
Materials PLA (wt 
%) 
ATBC (wt 
%) 
COFDOPA (wt 
%) 
COFDOPA-e (wt 
%) 
PLA 100 
PLA-ATBC 85 15 
PLA-ATBC- 
COFDOPA0.5 
84.58 14.93 0.5 
PLA-ATBC- 
COFDOPA1 
84.15 14.85 1 
PLA-ATBC- 
COFDOPA3 
82.45 14.55 3 
PLA-ATBC- 
COFDOPA0.5e 
84.58 14.930.5 
PLA-ATBC- 
COFDOPA1e 
84.15 14.85 1 
PLA-ATBC- 
COFDOPA3e 
82.45 14.55 3 
P. García-Arroyo et al. 
Polymer 196 (2020) 122466
5
contact with each sample. The results were expressed as radical scav-
enging activity (RSA), expressed as the equivalent of gallic acid (GA) 
concentration (mg/kg) by using a calibrated curve of gallic acid con-
centration versus RSA (%). 
To evaluate the non migration radical scavenging capacity of bio-
nanocomposites the DPPH-method was also used. Double sided, total 
immersion migration tests were performed by total immersion of films in 
a glass vial containing a DPPH solution in methanol (area-to-volume 
ratio ¼ 6 dm2/L DPPH). Bionanocomposites were stored at room tem-
perature in the dark for 10 days to assure full contact between the 
bionanocomposite films and the DPPH solution. The antioxidant ability 
was measured by the scavenging of DPPH radical through the action of 
the antioxidant bionanocomposites that decolourizes the DPPH solution 
using Equation (3). Where Acontrol is the absorbance of the DPPH solu-
tion in contact with plasticized PLA film (PLA-ATBC). 
3. Results and discussion 
3.1. Synthesis, functionalization and characterization of COFs and COFs 
nanosheets 
The dopamine functionalized COF (COFDOPA) was prepared by a two- 
step reaction sequence starting from a three-component condensation of 
DMTA; TAPB and BPTA to obtain [HC�C]0.5-TPB-DMTP-COF [46], 
followed by CuAAC reaction with dopamine azide as shown in Fig. 2. 
The successful grafting of dopamine into the COF structure was 
corroborated by 13C RMN by checking the disappearance of the signals 
corresponding to the characteristic Csp signals of the alkyne moieties at 
80 and 73 ppm (Fig. S1) as well as by ATR-FTIR by checking the 
disappearance of alkyne groups (2359 cm� 1) in the 
[HC�CH]0.5-TPB-DMTP-COF structure and the disappearance of azide 
band of dopamine azide (2098 cm� 1) (Fig. S2). The powder X-ray 
diffraction (PXRD) confirmed that the crystalline structure is maintained 
in COFDOPA (Fig. 3-a). 
The thermogravimetric analysis (TGA) reveals that the COFDOPA is 
stable up to 300 �C (Fig. S3). The nitrogen sorption isotherms of 
[HC�C]0.5-TPB-DMTP-COF reveals the sorption curve of a typical type 
IV isotherm (Fig. 3-b) characteristic of mesoporous materials. While the 
surface area, pore volume and pore size derived from (Fig. 3-b, Fig. S4) 
of [HC�C]0.5-TPB-DMTP-COF are in good accordance with already 
published works [31,46,56]. As expected, the incorporation of dopa-
mine into the COF structure leads to a reduction of the BET surface area 
and pore volume (Fig. 3-b). 
Correspondingly, the pore size distribution, determined by means of 
the non-local density functional theory (NLDFT) method, showed a pore 
size reduction (from 2.3 nm to 1.4 nm, Fig. S4). Sonication has been 
proposed as simple, cost effective and scalable top-down method for the 
exfoliation of 2D materials into nanosheets [50,57]. Thus, a suspension 
of COFDOPA in CHCl3 was submitted to an ultrasonic treatment as it is 
schematically represented in Fig. 4. The colloidal character of the 
resulting suspension was corroborated by the Tyndall effect upon irra-
diation with a laser beam [26]. The hydrodynamic sizes of exfoliated 
COFDOPA nanosheets were measured by DLS. The exfoliated COFDOPA 
reaches a monomodal size distribution in 90 min of sonication (6 cycles 
of 15 min), with an average value of 255 nm, ranging from 190 nm to 
300 nm (see DLS graph in the scheme, Fig. 4). TEM micrograph (Fig. 4) 
shows the structural aspect and the actual dimension of the COFDOPA-e 
nanosheets. Transparent COFDOPA-e nanosheets with lateral dimensions 
around 100 nm are observed. 
Transparency is one of the most important properties in food pack-
aging for consumer acceptance. The visual appearances of the films are 
shown in Fig. 5-a, while the absorption spectra of PLA, PLA-ATBC as well 
as COFDOPA and exfoliated COFDOPA-e bionanocomposites are displayed 
in Fig. 5-b. Neat PLA film proved to be very transparent, while the 
incorporation of ATBC leads to the most transparent film showing the 
highest transmission in the visible region of the spectra (400–700 nm). 
The PLA-COFDOPA and COFDOPA-e bionanocomposites resulted trans-
parent, but a reddening effect appeared which was more evident with 
increasing COF content. It has been observed that catechols produced 
red and amber tonality of PLA matrix based formulations [43,58]. 
The colour parameters of bionanocomposites were also studied in the 
CIELab space (see values for each film in Fig. 5-a). PLA and plasticized 
PLA (PLA-ATBC) showed the highest L value confirming the character-
istic high brightness of PLA. L was significantly affected by COFDOPA and 
COFDOPA-e presence. From the visual appearance of films (Fig. 5-a), it is 
evident that the addition of COFDOPA to the mainly transparent and 
colorless PLA-ATBC resulted in colour changes getting some reddish 
tone in all bionanocomposites. Therefore, dopamine decorated COF 
produced a significant deviation towards red, revealed by the positives 
values of the a* coordinate which resulted higher with increasing 
amount of COFDOPA and COFDOPA-e. The bionanocomposite films also 
presented increasing positive values of b* coordinate, indicative of an 
increment in yellowness. It should be highlighted that bio-
nanocomposites loaded with low amounts of exfoliated COFDOPA-e (0.5 
wt% and 1 wt%) showed higher lightness, transparency and somewhat 
less reddish effect due to the better dispersion of COF nanosheets in the 
polymeric matrix. 
In Fig. 6 are shown the FESEM micrographs of the cross-fractured 
sections of PLA, plasticized PLA (PLA-ATBC) and both COFDOPA and 
COFDOPA-e based bionanocomposite films. As expected PLA exhibits a 
regular and smooth surface of amorphous polymers (Fig. 6-a), whereas 
Fig. 3. a) Comparative PXRD patterns of [HC�C]0.5-TPB-DMTP-COF (red) and 
COFDOPA (blue) and b) Comparative N2 (77 K) sorption isotherms of 
[HC�C]0.5-TPB-DMTP-COF (red) and COFDOPA (blue). (For interpretation of the 
references to colour in this figure legend, the reader is referred to the Web 
version of this article.) 
P. García-Arroyo et al. 
Polymer 196 (2020) 122466
6
plasticized PLA films showed a clear more plastic deformation and no 
apparent phase separation (Fig. 6-b) as a result of the homogeneous 
dispersion of ATBC in PLA matrix confirming the efficiency of ATBC as 
plasticizer for PLA matrix [18,59]. The fracture surface of PLA-ATBC--
COFDOPA0.5 bionanocomposite exhibits a rougher surface, but with the 
presence of small voids (Fig. 6-c). 
In this context, Elangovan et al. developed biodegradable composites 
based on PLA reinforced with 1, 3 and 5 wt% of MOFs (Basolite™ C-300) 
and observed the formation of voids which has been ascribed to a poor 
interfacial adhesion between PLA and MOF [60]. In this work, these 
voids disappeared in the case of exfoliated COFDOPA-e in PLA-ATBC--
COFDOPA0.5e (Fig. 6-d). Similar behaviour is observed with the addition 
of 1 wt% of COFDOPA and COFDOPA-e with somewhat rougher fracture 
surfaces (Fig. 6-e and Fig. 6-f) due to the reinforcing effect of COF. In the 
case of bionancomposites loaded with 3 wt% of COFDOPA and COFDOPA-e 
different regions were clearly visible with ductile and brittle behaviour 
as well as the presence of holes, suggesting that COFs are not well 
dispersed with a resultant poor interfacial adhesion, probably due to the 
high amount of COFDOPA and COFDOPA-e in this formulations. Mu et al. 
synthesizedan imine-based COF which was further exfoliated into mi-
croparticles (mCOF) by ultrasound treatment in the presence of the 
surfactant sodium ligninsulfonate to increase the dispersion of the COF 
into chitosan matrix. They observed that mCOFs at 3.2 wt% lead to 
agglomeration in chitosan nanocomposite, even in the presence of the 
surfactant [28]. 
These materials are intended for food packaging applications, and 
the most used processing technologies in the plastic food packaging field 
are those based on melt-processing approaches (i.e.: extrusion, injection 
moulding, film forming, etc.). Thus the thermal stability of each 
formulation was studied by isothermal TGA at the typical extrusion 
processing temperature of PLA polymer of 180 �C (Fig. 7). In fact, 
although COFs has been prepared by extrusion process [23], polymeric 
nanocomposites reinforced with COF have been only prepared by sol-
vent casting approaches [27,28]. Under isothermal conditions all COF-
DOPA and COFDOPA-e loaded plasticized PLA matrix showed higher 
thermal stability than neat PLA and plasticized PLA-ATBC, particularly 
exfoliated ones. Considering that there are not research works related 
with the use of COFs as polymer reinforcements for melt extruded 
polymeric nanocomposites for industrial purposes, the isothermal TGA 
results showed that the formulation developed here ensure enough 
thermal stability for melt-processing in the plastic food packaging 
sector. The effect of COFDOPA and COFDOPA-e on the thermal properties 
of plasticized PLA films was also investigated by dynamic TGA mea-
surements (Fig. 7-b and c, Table S1). All plasticized formulations 
showed lesser thermal stability than that of neat PLA film since the 
plasticizer decreased the onset degradation temperature (T0) of PLA, in 
accordance with previous reported plasticized PLA with citrate esters 
[13,15]. The incorporation of COFDOPA and COFDOPA-e shifted the onset 
degradation temperature of PLA-ATBC to higher values (between 9 �C 
and 22 �C) and mainly maintain the maximum degradation temperature 
(in the range of 353–356 �C). The delay of the beginning of the thermal 
decomposition process indicates an effective stabilizing effect of COF-
DOPA on the plasticized PLA matrix leading to an improvement of the 
thermal stability of the bionanocomposites. In fact, COFDOPA has shown 
effectiveness to increase the thermal stability of PLA better than other 
reinforcements such as MOFs, which were able to increase the onset 
thermal degradation as a maximum of 5 �C [60,61]; or expanded 
graphite which leads to composites characterized by polymeric matrix 
thermal stability slight improved with respect to neat PLA [62]. Mean-
while, Mu et al. were able to increase only 2 �C of chitosan thermal 
stability by adding 1.6 wt% of microsized COFs, while higher amounts 
did not increase the onset thermal degradation [28]. The same authors 
further obtained COFs nanosheets and developed PVA nanocomposites, 
increasing the onset degradation temperature between 1 and 4 �C with 
0.3 and 1.2 wt% of COFs nanosheets [27]. 
DSC analysis was used to investigate the glass transition (Tg), cold 
crystallization (Tcc), melting temperatures (Tm) and crystallinity (χc) of 
PLA, PLA-ATBC and PLA-ATBC bionanocomposites. The DSC thermal 
parameters (Table S1) were obtained from the second heating scan 
(Fig. 8) to remove the effect of the thermal history. Moreover, it was 
Fig. 4. Schematic exfoliation process of COFDOPA. 
Fig. 5. a) Visual appearance of films with their respective CIELab colour co-
ordinate values and b) UV–vis measurements of COFDOPA based bio-
nanocomposite films. (For interpretation of the references to colour in this 
figure legend, the reader is referred to the Web version of this article.) 
P. García-Arroyo et al. 
Polymer 196 (2020) 122466
7
performed immediately after the melt-quenching to simulate the in-
dustrial production of bionanocomposites by melt-blending approaches. 
As expected, the Tg of plasticized formulations was significantly lower 
than that of pure PLA film thanks to the increased polymer chains 
mobility, confirming the success of ATBC to act as PLA plasticizer [43, 
63]. The addition of ATBC also reduced the cold crystallization tem-
perature of PLA, due to the fact that ATBC plasticizer interacts well with 
PLA polymeric matrix enhancing its slow crystallization rate [13,64]. 
This effect was enhanced by the combination of ATBC with COF nano-
sheets, showed by somewhat additional decrease of the cold crystalli-
zation temperature, suggesting that COFs also favour the nucleation 
effect and the crystal growth of PLA. In fact, the PLA-ATBC-COF bio-
nanocomposites showed enlarged degree of crystallinity with respect to 
PLA-ATBC (χc PLA-ATBC ¼ 12.0%), particularly in bionanocomposites 
containing exfoliated COF (COFDOPA-e) (χc PLA-ATBC-COFDOPA0.5e ¼
18.8%; χc PLA-ATBC-COFDOPA1e ¼ 20.0% and χc PLA-ATBC-COFDOPA3e 
¼ 19.0%). The higher crystallinity achieved by exfoliated samples 
(PLA-ATBC-COFDOPA-e bionanocomposites χc values are between 
18.8% and 20.0%) than for non exfoliated samples (PLA-ATBC-COF-
DOPA bionanocomposites χc values are between 15.3% and 18.7%) is 
because of the better dispersion of exfoliated COF into plasticized PLA 
matrix, and thus promoting a higher nucleation effect (see Table S1). It is 
known that from the melt PLA crystallizes in the α form at temperatures 
higher than 120 �C, while the α0 form appears when PLA crystallizes 
below 110 �C [65]. In neat PLA sample, a small exothermic peak prior to 
the melting endotherm is observed (Fig. 8). This small exothermic peak 
has been related to the transition of the disordered (α0) to ordered (α) 
crystals, suggesting that a great fraction of the polymer is in an 
Fig. 6. FESEM image of the cryofracture surface of bionanocomposite films. 
P. García-Arroyo et al. 
Polymer 196 (2020) 122466
8
amorphous state as a result of the fast cooling applied [66]. The addition 
of ATBC shifted the melting temperature to lower values, while reduced 
the crystallization phenomenon just before heating. The reduction of 
disordered α crystals (α0) has been attributed to the higher chain 
mobility in plasticized systems [3,13]. Meanwhile COF did not produced 
significant changes on Tm values. 
The mechanical properties of all samples was assayed by means of a 
nanoindenter test. The mean values of elastic modulus (E) and hardness 
(H) for each sample was calculated in the range from 500 to 1000 nm 
and are shown in Fig. 9. As expected, plasticized showed lower E and H 
values than neat PLA, showing once again the effectiveness of ATBC to 
plasticize PLA. Meanwhile, COF added samples showed increased E and 
H values, particularly those reinforced with 0.5 wt% and 1 wt% of 
exfoliated COF (COFDOPA-e), confirming its reinforcing effect in accor-
dance with DSC experiments. Mu et al. were able to increase around of 
50% the chitosan Young modulus by adding between 0.4 wt% and 0.8 
wt% of mCOF. Mansour et al. recently measured the nanomechanical 
porperties of PLA/graphene nanocomposites based on commercial PLA 
with 10 wt% of graphene (BlackMagic3D) achieving an increase of 21% 
in the elastic modulus [67]. In our work, the bionanocomposites loaded 
with 0.5 wt% and 1 wt% of COFDOPA-e were able to increase the elastic 
modulus of plasticized PLA in 55.9% and 68.6%, respectively. As ex-
pected, plasticized showed lower E and H values than neat PLA, showing 
Fig. 7. a) Isothermal TGA, b) dynamic TGA and c) DTG of bio-
nanocomposite films. 
Fig.8. DSC second heating scan of bionanocomposite films. 
Fig. 9. Nanomechanical results: a) elastic modulus (E); b) nanohardness (H). 
P. García-Arroyo et al. 
Polymer 196 (2020) 122466
9
once again the effectiveness of ATBC to plasticize PLA. Meanwhile, COF 
added samples showed increased E and H values, particularly those 
reinforced with 0.5 wt% and 1 wt% of exfoliated COF (COFDOPA-e), 
confirming its reinforcing effect in accordance with DSC experiments. 
Mu et al. were able to increase around of 50% the chitosan Young 
modulus by adding between 0.4 wt% and 0.8 wt% of mCOF. Mansour 
et al. recently measured the nanomechanical porperties of PLA/-
graphene nanocomposites based on commercial PLA with 10 wt% of 
graphene (BlackMagic3D) achieving an increase of 21% in the elastic 
modulus [67]. In our work, the bionanocomposites loaded with 0.5 wt% 
and 1 wt% of COFDOPA-e were able to increase the elastic modulus of 
plasticized PLA in 55.9% and 68.6%, respectively. 
Overall migration results of PLA, plasticized PLA (PLA-ATBC) and 
bionanocomposites into the food simulant D1 after 10 contact days are 
showed in Fig. 10-a. The results showed that bionanocomposites loaded 
with high amount of non exfoliated COFDOPA, that is 1 wt% and 3 wt%, 
and those loaded with the higher amount of exfoliated COFDOPA-e of 3 
wt% exceed the legal overall migration limit (60 mg/kg of food simu-
lant). Meanwhile, PLA, PLA-ATBC and bionanocomposites loaded with 
the lowest amount of non exfoliated COFDOPA (PLA-ATBC-COFDOPA0.5) 
and those loaded with low amounts of exfoliated COFDOPA-e, that is with 
0.5 wt% and 1 wt% (PLA-ATBC-COFDOPA0.5e and PLA-ATBC-COFDO-
PA1e) resulted in migration levels well below the legal overall migration 
limit of 60 mg/kg of food simulant, thus demonstrating a non-migrating 
behaviour. The migrated compounds were redissolved in simulant D1 
and the radical scavenging activity was measured by DPPH method. 
There was not a decrease in the DPPH absorbance in the materials that 
did not exceeded the legal overall migration limit (PLA, PLA-ATBC, 
PLA-ATBC-COFDOPA0.5, PLA-ATBC-COFDOPA0.5e and PLA-ATBC- 
COFDOPA1e), demonstrating the non-migration behaviour. Meanwhile, 
the migrated compounds of materials that exceeded the overall migra-
tion limits presented antioxidant activity (PLA- ATBC-COFDOPA1 ¼
0.14 mg/dm2, PLA-ATBC-COFDOPA3 ¼ 0.16 mg/dm2and PLA-ATBC- 
COFDOPA3e ¼ 0.15 mg/dm2), suggesting that COFDOPA nanosheets were 
migrated from the packaging material to the food stuff. These results are 
in good agreement with FESEM results in which these materials showed 
poor interfacial adhesion. 
The antioxidant efficacy of the COFDOPA bionanocomposites as non- 
migration packaging materials was determined by measuring the free 
radical scavenging activity of the DPPH solution in direct contact with 
films (Fig. 10-b). Non containing COFDOPA bionanocomposite films (PLA 
and PLA-ATBC) were studied as control samples. As expected, control 
samples did not show any DPPH radical scavenging activity (not shown). 
Those materials that exceed the overall migration limit were those that 
revealed higher antioxidant ability, which improved with increasing 
amount of COFDOPA added. 
On the other side, those bionanocomposites that complied the 
legislation [55] in terms of overall migration levels (PLA-ATBC-COF-
DOPA0.5, PLA-ATBC-COFDOPA0.5e and PLA-ATBC-COFDOPA1e) pre-
sented lesser antioxidant activity, but still presenting antioxidant effect, 
demonstrating their efficacy as non-migration antioxidant materials. 
The antioxidant activity of COFDOPA is associated to dopamine nature as 
a polyphenol having the second hydroxyl group in ortho position [40]. 
The scavenging effect reported here for bionanocomposites loaded with 
low amounts of COFDOPA (between 1 wt% and 3 wt%) are in the range of 
polymers bearing catechol moieties [40,68]. 
Although the promising features of COFs, the concerns regarding 
their toxicity still need to be documented in detail. It should be taken 
into account that the design of skeletons without toxicity is a minimum 
requirement for COFs application in food contact materials. Neverthe-
less, since the nanosized pores of COFs are useful for drug carriers and it 
delivery, the biocompatibility and toxicity of some imine-linked 2D 
COFs skeleton have been already investigated. In fact, there are some 
COFs that are promising candidates for drug carriers which have shown 
an effective metabolic pathway [69,70]. For instance, drug-loaded COFs 
were prepared by immersing an imine-based COF into 5-fluorouracil 
(5-FU), an anticancer drug. By using MCF-7 cells (Michigan Cancer 
Foundation-7, a breast cancer cell line) as a typical example, the 
biocompatibility of these COFs has been investigated in vitro showing 
that the cell viability remains >80% upon a 24 h incubation in the 
presence of the bare COFs at a high concentration of 200 μg mL� 1, 
indicating the good biocompatibility of the COFs [69]. Antioxidant 
compounds, such as quercetin, have been also adsorbed into COFs for 
drug release applications. In this context, Vijay et al. synthesized an 
imine-based covalent organic framework which was loaded with quer-
cetin. They further demonstrated that the quercetin loaded COF suc-
cessfully killed most human breast carcinoma cells (MDA-MB-231 cells) 
by in vitro experiments, while COF without quercetin was non-toxic to 
the cells studied [71]. By offering the first report of the application of 
covalently functionalized active compound into COF structure for the 
development of non-migrating bionanocomposites, we expect that more 
functionalized COFs will be used for the development of novel smart 
packaging systems for a variety of functions and applications. For 
instance, by appending charged species to the COF skeletons, a guani-
dinium based ionic COF has shown antibacterial activity against both 
Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia 
coli) bacteria [72]. Doxorubicin (DOX), a widely used effectively 
anti-cancer drug, was successfully in situ encapsulated into 
[HC≡C]0.5-TPB-DMTP-COF by a one pot method showing an excellent Fig. 10. a) Overall migration and b) RSA of bionanocomposite films. 
P. García-Arroyo et al. 
Polymer 196 (2020) 122466
10
pH-responsive release property. Thus, we anticipate that such active (i. 
e.: antioxidant, antimicrobial, pH-responsive, etc.) COFs will facilitate 
innovations in the development of smart nanocomposite materials for 
food packaging applications. 
4. Conclusion 
An imine-based covalent organic framework was synthesized and 
functionalized with dopamine via click chemistry (COFDOPA) as anti-
oxidant agent. COFDOPA was successfully exfoliated into COFDOPA-e 
nanosheets and further incorporated into plasticized PLA matrix with 
ATBC. ATBC increased the flexibility of the films due to its ability to 
enhance ductile properties and to depress the Tg. Although transparent 
bionanocomposites were obtained, COFDOPA produced a reddish tone. 
COFDOPA and COFDOPA-e increased the crystallinity of PLA-ATBC blend 
since their well dispersion into the plasticized polymeric matrix facili-
tates PLA crystallization. Moreover, bionanocomposites containing 
COFDOPA and particularly those loaded with exfoliated COFDOPA-e 
showed significant improved thermal stability, shifting the onset ther-
mal degradation between 17 �C and 22 �C to higher values. The bio-
nanocomposites also showed improved mechanical performance. In 
fact, compared to plasticized PLA-ATBC, a 56% improvement in theelastic modulus was obtained for PLA-ATBC-COFDOPA0.5e and 69% for 
PLA-ATBC-COFDOPA1e. Non exfoliated COFDOPA incorporated in 1 wt% 
and 3 wt% were released from the packaging material to a fatty food 
simulant. Meanwhile, low amounts of exfoliated COFDOPA-e (0.5 wt% 
and 1 wt%) interacted better with the polymeric matrix and overall 
migration values were well below the legislation limits. The DPPH 
method revealed that dopamine inherent antioxidant activity was able 
to scavenge the DPPH radical when it was covalently bonded into COF 
nanosheets and well dispersed into PLA-ATBC matrix. Thus, PLA-ATBC 
based bionanocomposites incorporated with low amounts of COFDOPA-e 
(PLA-ATBC-COFDOPA0.5e and PLA-ATBC-COFDOPA1e) have shown 
their potential as non-migrating antioxidant materials for sustainable 
food packaging applications. 
Although there is still a long way before COFs applications in food 
contact materials, the present research may open a door for the use of 
covalent bonded active substance into COFs to design active nanofillers 
with specific functionalities as a route to prepare non-migrating active 
nanocomposites. 
Declaration of competing interest 
The authors declare that they have no known competing financial 
interests or personal relationships that could have appeared to influence 
the work reported in this paper. 
CRediT authorship contribution statement 
Paloma García-Arroyo: Investigation, Data curation, Visualization, 
Writing - original draft, Writing - review & editing. Marina P. Arrieta: 
Conceptualization, Investigation, Data curation, Visualization, Writing - 
original draft, Writing - review & editing, Funding acquisition. Daniel 
Garcia-Garcia: Investigation, Data curation, Visualization, Writing - 
original draft, Writing - review & editing. Rocío Cuervo-Rodríguez: 
Visualization, Writing - review & editing. Vicent Fombuena: Supervi-
sion, Visualization, Writing - review & editing, Funding acquisition. 
María J. Manche~no: Conceptualization, Investigation, Visualization, 
Writing - review & editing. Jos�e L. Segura: Conceptualization, Inves-
tigation, Visualization, Supervision, Writing - review & editing, Funding 
acquisition, Project administration. 
Acknowledgment 
This work was supported by the Spanish Government, Spain (pro-
jects: MAT2016-77608-C3-2-P, MAT2017-84909-C2-2-R and M.P. 
Arrieta postdoctoral contract: FJCI-2017-33536), the UCM, Spain and 
Santander Group, Spain (projects: INV.GR.00.1819.10759 and 
Santander-UCM PR87/19–22628) as well as Generalitat Valenciana, 
Spain (D. Garcia-Garcia postdoctoral contract: APOSTD/2019/201). P. 
García-Arroyo is gratefully acknowledged to the Comunidad de Madrid, 
Spain for a predoctoral contract. Dr. Emilio Ray�on is greatly acknowl-
edged for his assistance with nanomechanical measurements. Authors 
thank to Ercros S.A. for providing PLA pellets. 
Appendix A. Supplementary data 
Supplementary data to this article can be found online at https://doi. 
org/10.1016/j.polymer.2020.122466. 
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