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ArticlePhoto-oxidative Degradation and Biodegradation of Poly(etherblock-amide) Multiblock Copolymers
Yingjun An, Adchara Padermshoke, Thinh Van Nguyen, Hiroyasu Masunaga, Risa Yokoyama,
Maina Yonemura, Dai-ichiro Kato, Yutaka Kobayashi, Hiroshi Ito, Sono Sasaki, and Atsushi Takahara*

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ABSTRACT: This study investigated the photo-oxidative degradation and biodegradation behavior of poly(ether-block-amide)(PEBA) multiblock copolymers with varying amide segments andpolyethylene glycol (PEG) contents. The copolymer films were
exposed to ultraviolet (UV) light in an artificial weathering machine
to simulate the photo-oxidative degradation of these polymers in theenvironment. The biodegradation behavior of both the original andUV-exposed PEBA multiblock copolymer films was studied byimmersing them in extracted seawater for 28 d. The degree of
biodegradation was evaluated on the basis of the amount of carbon
dioxide generated. The changes in the morphology, chemical
structure, mechanical properties, molar mass, and microstructure of
the PEBA multiblock copolymer films after UV exposure andbiodegradation tests were investigated using laser microscopy, Fourier transform infrared spectroscopy, tensile testing,nanoindentation, size-exclusion chromatography, and wide-angle X-ray scattering. The PEBA multiblock copolymer with a higher
PEG content exhibited faster photo-oxidative degradation under UV exposure, whereas the nylon-6-segmented PEBA multiblock
copolymer demonstrated excellent biodegradability. Furthermore, the biodegradability of the PEBA multiblock copolymers
significantly improved after UV exposure. The biodegradability of the PEBA multiblock copolymers was confirmed through fieldtests conducted in Ainan, Ehime, Japan. It was observed that a nylon-6-segmented PEBA multiblock copolymer containing 56 wt %PEG underwent complete degradation in the marine environment after six months. The findings of this study provide valuable
insights into the design and optimization of biodegradable polymers to alleviate microplastic pollution in the environment.
KEYWORDS: poly(ether-block-amide) multiblock copolymer, photo-oxidation, biodegradation, field test, biodegradable polymer

1. INTRODUCTION
Plastic products are lightweight, durable, and inexpensive,
which makes them suitable for numerous daily applications.
However, the widespread use of plastic products has led tosignificant environmental problems such as the generation anddisposal of plastic waste, particularly mismanaged plastics.1−4
Mismanaged plastics often contaminate the ocean, where they
persist for long periods and have devastating impacts on
marine ecosystems.1 They pose a serious threat to marine life
as they can entangle and kill marine animals indiscriminately.5,6Moreover, several plastics break down into smaller particles
over time. Carpenter and Smith7 first reported plastic particles
on the surface of the Sargasso Sea in 1972, and the number ofplastic particles in the ocean increased exponentially from 3500
pieces per square kilometer in the Sargasso Sea area to 105−106 pieces per square kilometer in the East Asian seas in 2016.8
Plastic particles have numerous negative effects on marineorganisms and humans.9 To address plastic waste pollution, it
is imperative to understand the degradation behavior ofplastics in the natural environment.10,11 Most plastics are© 2024 American Chemical Society

exposed to sunlight and further fragment into particles throughabrasion by sand, rock, and wave actions.12,13 Researchers haveclassified plastic particles into four categories based on theirsize: macroplastics (>2.5 cm), mesoplastics (5 mm−2.5 cm),
microplastics (MPs; 1 μm−5 mm), and nanoplastics (<1μm).14 Previously, we studied the photo-oxidative degradationand fragmentation behavior of polyolefins.15,16 Our findings
indicate that polyolefin plastics become brittle after ultraviolet(UV) exposure, making them more susceptible to fragmentation into MPs. The development of biodegradable polymers
that can be degraded into carbon dioxide (CO2) and water by
microorganisms in sediments or soil is promising for mitigating
plastic waste pollution.5 Certain polyamides17−19 and polyReceived: March 25, 2024
Revised: April 26, 2024
Accepted: April 30, 2024
Published: May 15, 2024

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ArticleFigure 1. Schematic of the preparation of PEBA multiblock copolymer films and setup for the UV exposure and biodegradation tests of PEBA
multiblock copolymers.

esters20−22 exhibit excellent biodegradability in soil and marine
environments. However, balancing performance, cost, andbiodegradability remains a critical challenge in the development of biodegradable polymers.
Commercial poly(ether-block-amide) (PEBA) is a multiblock copolymer composed of a polyamide (nylon) hard
segment and a polyether (PE) soft segment. It is widely used invarious industries owing to its unique properties.23 The
physical properties of PEBA multiblock copolymers can be
tailored by modifying their molecular structure and segmentcomposition. These copolymers have been extensively studied
from academic and technological perspectives. PEBA is widelyused in sports equipment, medical devices, footwear,
automotive components, electronics, and various otherindustrial applications. In addition, PEBA copolymers havebeen extensively investigated for their potential as CO2/N2
separation membranes owing to their microphase-separated
structures and excellent gas-separation properties.24 PEBA has
recently gained recognition as a top sustainable elastomer.25
However, it is essential to consider the environmental
degradation behavior of the PEBA multiblock copolymers. In
general, polymers degrade in the environment through two
main routes: photo-oxidation induced by sunlight exposureand biodegradation by microorganisms in the soil orsediment.10 Bei et al.26 investigated the photodegradation
behavior of block copoly(caprolactone-ethylene glycol). They
found that while pure poly(ethylene glycol) (PEG) alone
cannot undergo photodegradation, the presence of carbonylgroups in the polycaprolactone segment catalyzes the breakageof PEO chains through a series of UV-initiated free radical
reactions. Kawai27 investigated the biodegradation behavior of
PEs and noted that PEG exhibits biodegradability.28 Negoro etal.29,30 isolated several bacterial strains capable of utilizing
nylon-6 oligomers as sole sources of carbon and nitrogen. Our
previous study showed that photo-oxidized nylon-6 isdegradable in seawater.5 Therefore, the biodegradability ofPEBA multiblock copolymers can be adjusted by selecting
biodegradable polyamide (nylon) hard and PE (PEG) soft
segments.
In this study, we investigated the environmental degradation
behavior of PEBA multiblock copolymers containing nylon-6
or nylon-12 hard segments with varying PEG contents (56, 48,
and 24 wt %) under laboratory conditions. We simulated thephoto-oxidative degradation and biodegradation of thesepolymers using a weathering machine and biochemical oxygen
demand (BOD) tests in extracted seawater. Changes in themorphology and microstructure of these copolymers wereanalyzed using polymer science. Additionally, we discuss theeffects of the nylon segments and PEG content on the photooxidation and biodegradation of PEBA multiblock copolymers.
Field tests were conducted in the ocean to verify thebiodegradability of PEBA multiblock copolymers under marine
environmental conditions.

2. EXPERIMENTAL SECTION
2.1. Materials. Three types of PEBA multiblock copolymer pellets(MH1657, MV1041, and MV1074) were obtained from Arkema,
Japan. 2,2,2-Trifluoroethanol (TFE) (Fluorochem Ltd.), sodium
trifluoroacetate (NaTFA) (>98.0%) (Tokyo Chemical Industry),
sodium dodecyl sulfate (SDS) (>98.0%) (FUJIFILM Wako Pure
Chemical Corporation), and a 5-cyano-2,3-ditolyl tetrazolium
chloride (CTC) rapid straining kit (DOJINDO Laboratories,
Kumamoto, Japan) were used as received.
2.2. Preparation of PEBA Multiblock Copolymer Films. The
raw material pellets were dried in a vacuum chamber at 70 °C for 4 h
before use. PEBA multiblock copolymer films were prepared using acompression molding machine (MP-WNH, Yamato Scientific Co.,
Ltd.). First, the pellets were heated and melted for 3 min at 240, 200,
and 220 °C for MH1657, MV1074, and MV1041, respectively. The
pellets were pressurized at 5 MPa for 1 min between polytetrafluoroethylene films. Subsequently, they were cooled at 20 °C with apressure of 5 MPa for 2 min using a pressing machine (MP-WC,
Yamato Scientific Co., Ltd.) equipped with a thermal controllingsystem (CF-A611, Yamato Scientific Co., Ltd.). The thickness of theresulting film was approximately 200 μm.
2.3. UV Exposure Test. The photo-oxidative degradation
behavior of the PEBA multiblock copolymer films was investigatedusing an artificial weathering test chamber (Xe-1, Q-SUN, Q-LAB
Co., Ltd.) equipped with a xenon lamp. PEBA multiblock copolymer
films were exposed to UV irradiation at 125 W m−2 (λ = 300−400nm) and a black panel temperature of 63 °C for up to 4 d. The total
UV irradiation in the accelerated UV exposure experiment was 43.2
MJ m−2, which is equivalent to 1.7 months of UV exposure in Choshi,
Japan.
2.4. Biodegradation Test in Extracted Seawater. The
biodegradation tests of the PEBA multiblock copolymer films in
extracted seawater were based on the method proposed by Taguchi etal.31 The extracted seawater was prepared by mixing 600 mL ofseawater with 100 g of sediment (particle size <2 mm). Seawater andsediment were collected from Imajuku, Nishi-ku, Fukuoka, Japan(33.6185°N, 130.2325°E) on 2023/09/21. After the mixture wassonicated at 31 kHz for 10 s, it was immediately filtered using a filter
bag with a 10 μm pore size. The viable cells of the normal andextracted seawater were stained by a CTC rapid staining kit andcounted in a Fuchs−Rosenthal counting chamber using a fluorescencemicroscope (Nikon Eclipse E400).32 A 300 mL glass bottle with 75
mL of extracted seawater and approximately 20 mg (1 × 1 cm2) offilms was placed in an incubator (25 °C) for 28 d to conduct thebiodegradation test. A BOD sensor (OxiTop-IDS; WTW, Weilheim,
Germany) was used to detect consumed O2 during the biodegradation test. The degree of biodegradation (%) was evaluated using6141

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BOD and the theoretical oxygen demand (ThOD) according to thefollowing equations

Each set of the spectra was normalized to the respective intensity of
the amide I band at approximately 1640 cm−1.
The thermal properties of the PEBA multiblock copolymers werecharacterized using differential scanning calorimetry (DSC) (DSCvesta, Rigaku, Japan) under nitrogen atmosphere at a heating rate of 10°C min−1.
Synchrotron X-ray scattering experiments were performed at theBL38B1 beamline station of SPring-8 (RIKEN). The X-ray wavelength (λ) was 0.1 nm for the WAXS measurements and the sampleto-detector distance was approximately 0.28 m. Measurements were
performed in transmission geometry at room temperature with an
exposure time of 10 s for each sample. The WAXS data were collected
online using a two-dimensional detector (PILATUS3X 2M). Cerium
oxide was used as the standard sample for the calibration of WAXS.
The degree of crystallinity (χc) of hard segment nylon phase wascalculated from the scattering intensity of the crystal and amorphous
components Ac and Aa in the WAXS profiles using the following
equation

BODsample = (O2 uptake sample (mg L 1)

O2 uptake1

blank (mg L ))i BOD yzzz × 100%
biodegradation degree (%) = jjj
k ThOD {After the biodegradation experiment, the films were collected fromthe extracted seawater and immersed in an SDS solution to extract thebiofilm from the surface of the PEBA multiblock copolymer films.33
The biodegraded films were immersed in 30 mL of SDS solution (0.2
wt %) and shaken for 48 h using a rotating shaker. The films werethen rinsed with deionized water five times to remove the residual
SDS and dried at 40 °C under vacuum for 48 h.
Figure 1 illustrates the sample preparation and setup for UV
exposure and biodegradation tests of the PEBA multiblock copolymer
films. Following these tests, the changes in surface morphologies,
mechanical properties, chemical structure, and microstructure of theNy6/PEG56, Ny12/PEG48, and Ny12/PEG24 films were characterized using laser microscopy, tensile tests, nanoindentation,
attenuated total reflectance (ATR) Fourier transform infrared (FTIR), and wide-angle X-ray scattering (WAXS).
2.5. Characterization. The mechanical properties of the PEBA
multiblock copolymer films were measured using a tensile tester(RTI-1225, TENSILON) at a strain rate of 100% min−1. The Young’s
modulus (E), strength at break (σb), and elongation at break (εb)
values of the films were the average of at least four measurements.
The surface mechanical properties of the PEBA multiblock copolymer
films were investigated using a nanoindentation tester (OPX-UNHT,
Anton Paar) equipped with a typical Berkovich three-sided pyramidal
diamond indenter tip. Measurements were performed in load-control
mode with a load and unload rate of 2 mN min−1. The maximum load
was 1 mN. A linear load-hold-unload profile for each indentation wasmaintained for 30−10−30 s. The plasticity index (ψp) of the PEBA
multiblock copolymer films was evaluated using the following
equation34
P =AL

ArticleWAXS(%) = Ac /(Ac + Aa ) × 100%cwhere Ac and Aa were evaluated by peak decomposition from the onedimensional (1D) WAXS profiles using PeakFit software; Gaussianfunctions were used to fit the amorphous halo, and Gaussian/Lorentz
profiles were used for the scattering peaks.

3. RESULTS AND DISCUSSION
3.1. Properties of PEBA Multiblock Copolymer Films.
The thermal properties, mechanical properties, and chemicalcompositions of the three PEBA multiblock copolymers are
summarized in Table 1. The chemical structure andTable 1. Physical Properties of the PEBA Multiblock
Copolymer Films

AUL
AL

a

where AL and AUL are the areas under the loading and unloading
curves, respectively (Figure S4). The plasticity index (ψp) of each
PEBA multiblock copolymer film was the average of at least fivemeasurements. Both the tensile and nanoindentation tests of the
PEBA multiblock copolymer films were conducted at room
temperature (25 °C).
The surface morphologies of the initial, UV-exposed, andbiodegraded PEBA multiblock copolymer films were observed usinga laser microscope (VK-X260, Keyence, Japan).
The weight-average molar mass (Mw), number-average molar mass
(Mn), and polydispersity (D) of the initial, photo-oxidative degraded,
and biodegraded Ny6/PEG56 films were measured by size-exclusion
chromatography (SEC) using an instrument (Prominence-i LC2030C, Shimadzu) equipped with a guard column (Tosoh TSKgel
Guard Column SuperH-L), two columns (Tosoh TSKgel column
SuperH 2500 and 4000), and a refractive index detector (RID-20A,
Shimadzu) at a flow rate of 0.4 mL min−1 at 40 °C. TFE with 10 mM
NaTFA was used as the eluent. The sample solution was prepared bydissolving 3 mg of the film in 3 mL of a TFE solution with 10 mM
NaTFA. Poly(methyl methacrylate) with different molar mass (Mn =675−589,000; Mw/Mn = 1.02−1.26) was used as the standard to
calibrate the SEC system.
FT-IR spectra of the PEBA multiblock copolymer films werecollected using an FT-IR spectrometer (Nicolet iN10, Thermo Fisher
Scientific) equipped with a mercury cadmium telluride detector and asingle-reflection ATR accessory with a germanium crystal (incident
angle: 34.5°). All spectra were collected from 4000 to 675 cm−1 with aresolution of 2 cm−1. Signals were obtained by averaging 64 scans.

PEBA

PA
segment

PEG content(wt %)aTm(°C)b

χc-Nylon(%)c

E (MPa)d

MH1657
MV1074
MV1041

Nylon-6Nylon-12
Nylon-12

56
48
24

202.8
156.3
170.3

64.8
48.9
49.6

52.3 ± 4.0
62.7 ± 2.5
158.9 ± 7.9

NMR. bDSC. cWAXS, and. dTensile test.

compositions of PEBA multiblock copolymers were determined by 1H nuclear magnetic resonance (1H NMR). Both
PEBA MV1074 and MV1041 are nylon-12-segmented multiblock copolymers, whereas PEBA MH1657 is a nylon-6segmented multiblock copolymer. PEG served as the soft
segment in all three copolymers, with PEG contents in
MH1657, MV1074, and MV1041 of 56, 48, and 24 wt %,respectively. Hereafter, these copolymers will be referred to as
Ny6/PEG56, Ny12/PEG48, and Ny12/PEG24. The melting
temperatures (Tm) of Ny6/PEG56, Ny12/PEG48, and Ny12/
PEG24 were 202.8, 156.3, and 170.3 °C, respectively. The
nylon-6-segmented copolymer exhibited a higher Tm comparedto the nylon-12-segmented copolymers. Additionally, for thenylon-12-segmented copolymers, Tm decreased with increasing
PEG content. The crystallinities of hard segment nylon phasein Ny6/PEG56, Ny12/PEG48, and Ny12/PEG24 were 64.8,
48.9, and 49.6%, respectively. In the PEBA multiblock
copolymer films, the nylon segments formed a crystalline
structure, while the PEG segments existed in an amorphous
phase, as revealed by the DSC thermograms and WAXS
profiles of the Ny6/PEG56, Ny12/PEG48, and Ny12/PEG24
films. Specifically, the DSC thermograms of the PEBA
multiblock copolymers (Figure S2a) indicate that the Tm of
the PEG segments is −1.5 °C, significantly lower than room6142

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ArticleFigure 2. (a) Strain−stress curves of the initial and UV-exposed (16 h) PEBA multiblock copolymer films. (b) Young’s modulus (E), elongation atbreak (εb), and strength at break (σb) of the initial and 16 h UV-exposed PEBA multiblock copolymer films.

Figure 3. (a) Load−displacement curves of the initial and 2 d UV-exposed PEBA multiblock copolymer films. (b) Plasticity index of the PEBA
multiblock copolymer films.

temperature, suggesting the presence of PEG segments withlower molar mass in the amorphous phase. Furthermore,
Figure S2b displays the 1D WAXS profiles of the Ny6/PEG56,
Ny12/PEG48, and Ny12/PEG24 films, revealing the formation of the α-crystal form of nylon-635 in the Ny6/PEG56
film and the γ-crystal form of nylon-1236 in the Ny12/PEG48
and Ny12/PEG24 films. Finally, the Young’s modulus of the
PEBA multiblock copolymers decreased with increasing PEG
content, measuring 52.3, 62.7, and 158.9 MPa for Ny6/
PEG56, Ny12/PEG48, and Ny12/PEG24, respectively.
3.2. Mechanical Properties after UV Exposure. Thechanges in the mechanical properties of the PEBA multiblock
copolymer films after UV exposure were investigated using
tensile tests. Because of the rapid degradation of PEG in the
Ny6/PEG56, Ny12/PEG48, and Ny12/PEG24 films after 2 dof UV exposure, the UV-exposed films became too brittle to be
fixed between the chucks. Therefore, the PEBA multiblock
copolymer films after 16 h of UV exposure were used fortensile tests. Figure 2a displays the strain−stress curves of theinitial and UV-exposed Ny6/PEG56, Ny12/PEG48, andNy12/PEG24 films. The mechanical properties of the PEBA
multiblock copolymer films significantly deteriorated after 16 hof UV exposure. Particularly, for the Ny6/PEG56 and Ny12/
PEG48 films with higher PEG contents, the fracture stress andstrain almost decreased to 0 after 16 h of UV exposure. Figure
2b shows the Young’s modulus (E), elongation at break (εb),
and strength at break (σb) of the initial and UV-exposed Ny6/
PEG56, Ny12/PEG48, and Ny12/PEG24 films. The values of
E for Ny6/PEG56, Ny12/PEG48, and Ny12/PEG24 were 52,
63, and 159 MPa, respectively, and remained almost constantafter 16 h of UV exposure. However, εb and σb of all three
PEBA multiblock copolymer films were significantly reduced
after UV exposure. After 16 h of UV exposure, the εb of Ny6/
PEG56, Ny12/PEG48, and Ny12/PEG24 decreased from1341, 1703, and 946% to 6, 10, and 543%, respectively, whileσb decreased from 39, 50, and 58 MPa to 2, 5, and 32 MPa,
respectively. After the UV exposure test, the Ny12/PEG24
films with a relatively lower PEG content exhibited larger εband σb values than those of the Ny6/PEG56 and Ny12/PEG48
films. The PEBA multiblock copolymer with a higher PEG
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content was more susceptible to UV aging. These changes inmechanical properties suggest that photo-oxidative degradationof PEBA multiblock copolymers likely occurred in the PEG
segment, a hypothesis supported by the FT-IR spectra andsurface mechanical property analysis of UV-exposed PEBA
multiblock copolymer films.
The surface mechanical properties of the PEBA multiblock
copolymer films were investigated by nanoindentation. Figure
3a shows the load−displacement curves of the initial and 2 d
UV-exposed Ny6/PEG56, Ny12/PEG48, and Ny12/PEG24
films. The plasticity index (ψp) of these films was evaluatedfrom the load−displacement curves, as depicted in Figure 3b.
The ψp values of the Ny6/PEG56 and Ny12/PEG48 films
were 0.58 and 0.41, respectively. After UV exposure in theweathering machine for 2 d, these values increased to 0.71 and
0.68, respectively. The ψp values of the initial and 2 d UVexposed Ny12/PEG24 films were 0.56 and 0.53, respectively,
remaining almost constant with increasing UV irradiation.
Notably, for the PEBA multiblock copolymers with higher
PEG contents, ψp increased after UV exposure, indicating
degradation of the PEG soft segments during UV exposure.
The changes in ψp across films with various PEG contentssuggest that degradation is more likely to occur in PEBA
multiblock copolymers with higher PEG contents.
3.3. IR Spectral Changes Induced by UV Exposure.
Both Raman and FT-IR spectroscopy are frequently used to
study the structure of polymers. However, FT-IR spectroscopy
is much more sensitive to the bonding states of oxygen andcarbon in degraded polymers.37 FT-IR spectroscopy is avaluable tool for identifying polymer degradation products,15,38
and in this study, it was employed to analyze the photooxidation behavior of PEBA multiblock copolymer films.
Figure 4 shows the normalized FT-IR spectra of the Ny6/
PEG56, Ny12/PEG48, and Ny12/PEG24 films before andafter UV exposure. The initial films exhibited characteristic
absorption bands originating from the PEG segments39

including C−O and C−C stretching and CH2 rocking at 842
cm−1, CH2 rocking and CH2 twisting at 963 cm−1, C−O andC−C stretching and CH2 rocking at 1061 cm−1, C−O and C−
C stretching at 1105 cm−1, C−O stretching and CH2 rocking
at 1143 cm−1, CH2 twisting at 1241 and 1280 cm−1, and CH2
wagging at 1343 cm−1. After UV exposure, a drastic decrease in
the intensity of the bands at 1343, 1105, 963, and 842 cm−1
was observed, indicating the degradation of the PEG segmentsin the PEBA multiblock copolymers. Conversely, the intensityof the IR bands associated with carbonyl groups in the range
1780−1640 cm−1 increased after UV exposure, indicating thegeneration of carbonyl-containing products via photo-oxidation.
3.4. Biodegradability of PEBA Multiblock Copolymers. Biodegradation, another significant degradation pathway for polymers in the natural environment, was assessed forthe initial and 2 d UV-exposed PEBA multiblock copolymer
films by immersing them in extracted seawater. The extracted
seawater was used to expedite the biodegradation process forall samples under investigation. The concentration of viablemicroorganisms in the extracted seawater was 7.5 × 106 cells
mL−1, nearly 60 times higher than that in fresh seawater (1.2 ×
105 cells mL−1). To validate the reliability of the test, acellulose paper (filter paper 5C, ADVANTEC), known for its
excellent biodegradability, was employed as a reference.5
Figure 5a illustrates the degree of biodegradation of cellulose
and both the initial and UV-exposed Ny6/PEG56, Ny12/
PEG48, and Ny12/PEG24 films after 28 d of accelerated
biodegradation tests. The degree of biodegradation of cellulose
in the extracted seawater increased linearly with exposure timeafter a short induction time (∼2 d), reaching 13% after 28 d,suggesting that the test was reliable. The degree of
biodegradation of the initial Ny12/PEG48 and Ny12/PEG24
films was zero, indicating that they are not biodegradable in themarine environment. In contrast, the degree of biodegradationof the Ny6/PEG56 film increased with time, reaching 12%
after 28 d of biodegradation testing in the extracted seawater.
Figure 5b shows the photographs of the Ny6/PEG56, Ny12/
PEG48, and Ny12/PEG24 films in the extracted seawater.
Biofilm formation was observed on the Ny6/PEG56 film,
whereas no evidence of biofilm formation was observed on the
Ny12/PEG48 and Ny12/PEG24 films, confirming theirbiodegradability. This difference may be attributed to thehigher hydrophilicity of the nylon-6 segments compared to thenylon-12 segments, facilitating the formation of biofilms on thesurface of the Ny6/PEG56 film. Figure S6 shows the watercontact angle measurements of the Ny6/PEG56, Ny12/
PEG48, and Ny12/PEG24 films, which were 38.9, 49.7, and64.2°, respectively. Once microorganisms attached to thesurface of the Ny6/PEG56 films, biodegradation occurred inboth the PEG and nylon-6 segments. Bernhard et al.28 revealed
that PEGs with lower molar mass are entirely biodegradable bymarine microorganisms. Negoro et al.29 reported that nylon-6
oligomers could be enzymatically degraded by NylA, NylB, and
NylC hydrolases.40,41 The biodegradability of the nylon-6
oligomers was also tested in extracted seawater, as shown in
Figure S7. It was found that nylon-6 oligomers exhibited
excellent biodegradability in extracted seawater, suggesting thatnylon-6 segments in the Ny6/PEG56 copolymers can be
enzymatically degraded by similar hydrolases present in theextracted seawater. The enzymatic degradation of the PEBA
multiblock copolymers by NylB and NylC hydrolases was also
investigated. As shown in Figure S8, the Ny6/PEG56

Figure 4. Normalized ATR FT-IR spectra of initial and 2 d UVexposed PEBA multiblock copolymer films: (a) Ny6/PEG56, (b)Ny12/PEG48, and (c) Ny12/PEG24.
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ArticleFigure 5. (a) Changes in the biodegradation degree (%) of cellulose, initial, and UV-exposed PEBA multiblock copolymer films over time inextracted seawater. (b) Photographs of PEBA multiblock copolymer films submerged in extracted seawater for 28 d.

Figure 6. Laser microscopy images of initial, 2 d UV-exposed, and biodegraded PEBA multiblock copolymer films.

copolymer was degraded by NylB and NylC hydrolases. Field
tests conducted in the ocean further verified the biodegradability of the PEBA multiblock copolymers under marine
environmental conditions (Figures S12 and S13). The Ny6/
PEG56 films disappeared completely in the marine environ-ment, whereas no change was observed in the Ny12/PEG48
and Ny12/PEG24 films after a 6 month test in Ainan, Japan.
Notably, the Ny12/PEG48 and Ny12/PEG24 films became
biodegradable after UV exposure. As shown in Figure 5b, a
biofilm formed on the surface of the UV-exposed Ny12/6145

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PEG48 and Ny12/PEG24 films, which was attributed to theincreased hydrophilicity resulting from the generation ofcarbonyl-containing products during photo-oxidation. Oncethe biofilm was attached to the surface, biodegradation of the
PEG segments progressed over time. The degree of
biodegradation of the UV-exposed Ny12/PEG48 and Ny12/
PEG24 films in extracted seawater increased with exposure
time, reaching 10 and 5%, respectively, after 28 d of accelerated
biodegradation testing. The degree of biodegradation of theUV-exposed Ny6/PEG56 increased to 21.1%, which was
almost twice that of initial Ny6/PEG56 film. This observationsuggests that the biodegradability of Ny6/PEG56, Ny12/
PEG48, and Ny12/PEG24 copolymers was enhanced byphoto-oxidation.42
3.5. Surface Morphology after UV Exposure andBiodegradation. The surface morphologies of the initial, UVexposed, and biodegraded PEBA multiblock copolymer films
were examined by laser microscopy. Upon exposure to UV
irradiation in a weathering test chamber, the surfaces of the
PEBA multiblock copolymer films rapidly deteriorated. As
shown in Figure 6, the initial films exhibited a smooth surface
morphology. However, after 2 d of UV exposure in theweathering machine (equivalent to 26 d of UV exposure in
Choshi, Japan), cracks formed on the surfaces of all three
PEBA multiblock copolymer films. The number of cracks inthe films significantly increased with increasing UV irradiation(Figure S9). In particular, denser cracks were observed on thesurfaces of the UV-exposed PEBA multiblock copolymer films
with higher PEG contents. Surface deterioration was more
pronounced in the Ny6/PEG56 and Ny12/PEG48 films than
Ny12/PEG24 films. These changes in the surface morphologyof the UV-exposed films suggest the occurrence of photooxidation in the PEBA multiblock copolymers, with thosecontaining higher PEG contents exhibiting increased susceptibility to photo-oxidation. It is important to note that thephoto-oxidative degradation of PEBA multiblock copolymers
occurs at a much faster rate than that of nylon homopolymers.
The surface deterioration observed in the PEBA multiblock
copolymer films may be attributed to chain scission in the PEG
soft segments during photo-oxidation, as evidenced by FT-IR
and SEC analyses.
After the biodegradation tests, clear holes were observed on
the surface of the biodegraded Ny6/PEG56 films, confirming
their biodegradability in seawater. No surface changes wereobserved in biodegraded Ny12/PEG48 or Ny12/PEG24 films.
However, clear cracks appeared on the surface of the UVexposed PEBA multiblock copolymer films after the biodegradation test, indicating degradation of these films. Interestingly, the UV-exposed Ny6/PEG56 film degraded intofragments in the extracted seawater (Figure S10). For thenylon-12-segmented PEBA multiblock copolymer films,
significantly more cracks were observed on the surface of theUV-exposed Ny12/PEG48 film with a higher PEG content
than the UV-exposed Ny12/PEG24 films. This observationsuggests that the biodegradability of the UV-exposed PEBA
multiblock copolymers increased with increasing PEG content.
3.6. Molar Mass of UV-Exposed and Biodegraded
PEBA Multiblock Copolymers. After UV exposure andbiodegradation testing, the surfaces of the PEBA multiblock
copolymer films were visibly damaged, with obvious cracks
formed on their surfaces. Chain scission of the Ny6/PEG56
copolymer due to photo-oxidation and enzymatic degradation
was investigated using SEC. As shown in Figure 7, the molar

ArticleFigure 7. Distribution of molar mass of Ny6/PEG56 films before andafter UV exposure and biodegradation evaluated by SEC.

mass of the Ny6/PEG56 film decreased after the UV exposureand biodegradation tests. The Mw of the Ny6/PEG56
copolymers decreased from 1.3 × 105 to 7.2 × 103 and 5.5
× 104 g mol−1 after 2 d of accelerated UV exposure (21.6 MJm−2) and 28 d of biodegradation, respectively, confirming thatchain scission was induced by photo-oxidation and enzymatic
degradation. Particularly, for the UV-exposed Ny6/PEG56
copolymer, the photo-oxidative degradation products, PEG
and nylon-6 oligomers, further enhanced the biodegradabilityof the Ny6/PEG56 copolymer. It is noteworthy that the Mw of
UV-exposed Ny6/PEG56 increased slightly to 1.1 × 104 g
mol−1 after the biodegradation test in extracted seawater owing
to the removal of soluble photo-oxidation products with lower
molar mass.
3.7. Microstructure of UV-Exposed and Biodegraded
PEBA Multiblock Copolymer Films. Microstructuralchanges in the PEBA multiblock copolymers after UV exposureand biodegradation were investigated using WAXS. Only thenylon segments formed a crystalline structure in the PEBA
multiblock copolymers (Figure S11), indicating that the PEG
segment exists in the amorphous phase. Table 2 lists the degree
Table 2. Degree of Crystallinity of Hard Segment Nylon
Phase in the PEBA Multiblock Copolymers before and after
UV Exposure and Biodegradation
degree of crystallinity (%)
PEBA

initial

UV 2d

Bio

UV 2d + Bio

Ny6/PEG56
Ny12/PEG48
Ny12/PEG24

64.8
48.9
49.6

78.8
58.3
56.1

72.4
52.2
49.1

88.0
70.3
61.5

of crystallinity (χc) of hard segment nylon phase in the PEBA
multiblock copolymers before and after the environmental
degradation test. The χc of Ny6/PEG56, Ny12/PEG48, andNy12/PEG24 increased from 64.8, 48.9, and 49.6% to 78.8,
58.3, and 56.1%, respectively, after 2 d of UV exposure. The
degradation of the amorphous PEG segments with higher gas
solubility during the UV exposure test contributed to theincrease in χc. After the biodegradation test, the χc of Ny6/
PEG56 increased to 72.4% due to enzymatic degradation of
the PEG segments. The χc of the Ny12/PEG48 and Ny12/
PEG24 films remained almost constant after the biodegradation test, suggesting that biodegradation did not occur in these6146

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films. For the UV-exposed Ny6/PEG56, Ny12/PEG48, andNy12/PEG24 films, χc further increased to 88.0, 70.3, and61.5%, respectively, after biodegradation, corroborating thatthe photo-oxidative degraded products of the PEG segments
exhibit excellent biodegradability in seawater.

AUTHOR INFORMATION

Corresponding Author

Atsushi Takahara − Research Center for Negative Emission
Technologies, Kyushu University, Fukuoka 819-0395, Japan;
orcid.org/0000-0002-0584-1525;
Email: takahara.atsushi.150@m.kyushu-u.ac.jp

4. CONCLUSIONS

Authors

Yingjun An − Research Center for Negative Emission
Technologies, Kyushu University, Fukuoka 819-0395, Japan
Adchara Padermshoke − Research Center for Negative
Emission Technologies, Kyushu University, Fukuoka 8190395, Japan
Thinh Van Nguyen − Research Center for Negative Emission
Technologies, Kyushu University, Fukuoka 819-0395, Japan;
orcid.org/0000-0002-2789-6797
Hiroyasu Masunaga − Japan Synchrotron Radiation ResearchInstitute, Sayo-gun, Hyogo 679-5198, Japan
Risa Yokoyama − Graduate School of Science and Engineering,Kagoshima University, Kagoshima 890-0065, Japan
Maina Yonemura − Graduate School of Science andEngineering, Kagoshima University, Kagoshima 890-0065,
Japan
Dai-ichiro Kato − Graduate School of Science andEngineering, Kagoshima University, Kagoshima 890-0065,
Japan
Yutaka Kobayashi − Research Center for GREEN Materials
and Advanced Processing, Yamagata University, Yonezawa,
Yamagata 992-8510, Japan
Hiroshi Ito − Research Center for GREEN Materials andAdvanced Processing, Yamagata University, Yonezawa,
Yamagata 992-8510, Japan; Graduate School of Organic
Materials Science, Yamagata University, Yonezawa,
Yamagata 992-8510, Japan; orcid.org/0000-0001-84328457
Sono Sasaki − Department of Biobased Materials Science,
Graduate School of Science and Technology and Faculty ofFiber Science and Engineering, Kyoto Institute of Technology,
Kyoto 606-8585, Japan; RIKEN SPring-8 Center, Sayo-gun,
Hyogo 679-5148, Japan; orcid.org/0000-0001-73749854
Complete contact information is available at:https://pubs.acs.org/10.1021/acsapm.4c00920

The photo-oxidative degradation and biodegradation behaviorof PEBA multiblock copolymers with nylon-6 or nylon-12 hard
segments and PEG soft segments were investigated based onpolymer science. Surface morphology analysis revealed that
surface deterioration induced by UV aging occurred readily in
PEBA multiblock copolymers, with more severe deterioration
observed in copolymers with higher PEG contents. The
mechanical properties of the PEBA multiblock copolymers
with higher PEG contents drastically deteriorated after short
UV exposure tests, indicating chain scission upon exposure.
Nanoindentation and WAXS analyses suggested that the PEG
soft segments degraded during UV exposure tests, leading toan increase in the plasticity index and degree of crystallinity.
ATR FT-IR spectroscopy further supported the idea thatphoto-oxidative degradation primarily occurs in PEG soft
segments with higher O2 solubility, as evidenced by the notabledecrease in the intensity of the characteristic absorption bandsof the PEG segments (C−O and C−C stretching) after short
UV exposure tests. Biodegradation tests demonstrated thatnylon-6-segmented PEBA multiblock copolymers with higher
PEG contents exhibited excellent biodegradability, whereas
nylon-12-segmented PEBA multiblock copolymers did notdegrade in the extracted seawater owing to poor hydrophilicity.
Biofilms were notably present on the surfaces of the Ny6/
PEG56 films, whereas no biofilms were observed on thesurfaces of the Ny12/PEG48 and Ny12/PEG24 films. The
biodegradability of the PEBA multiblock copolymers was
confirmed through field tests. The nylon-6-segmented PEBA
multiblock copolymer containing 56 wt % PEG underwent
complete degradation in the marine environment. Furthermore, the biodegradability of the PEBA multiblock copolymers
improved with UV aging, as the films became more hydrophilic
after UV exposure. The WAXS analysis revealed that
biodegradation occurred preferentially in the PEG soft
segments.

■

ArticleNotes

The authors declare no competing financial interest.

■

ASSOCIATED CONTENTACKNOWLEDGMENTS
This work is based on the results obtained from a project,
PNP18016, commissioned by the New Energy and Industrial
Technology Development Organization (NEDO). WAXS data
were obtained at the BL38B1 beamline station of the SPring-8
synchrotron facility in Japan with the approval of RIKEN. The
authors also thank Dr. Sinan Feng for the NMR measurement.

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsapm.4c00920.
Additional experimental details, including DSC thermograms, 1H NMR spectra, 1D WAXS profiles, load−
displacement curve of nanoindentation test, watercontact angle measurement of PEBA multiblock
copolymer films, biodegradation test of nylon-6
oligomers, enzymatic degradation test of initial andUV-exposed PEBA multiblock copolymers, laser microscopy images of PEBA copolymer films after 2 day and 4
day UV exposure test, photograph of Ny6/PEG56
fragments, molar mass of Ny6/PEG56 copolymers, andfield test of PEBA multiblock copolymer films (PDF)■

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