# Polyethylene plastic degradation: The dual pathways from macroplastics to nanoplastics

Aicha El Kharraf, Murielle Rabiller-Baudry, Ana Pradas del Real, Christophe Sandt, Mark Wiesner, Gergely Nemeth, Dominique Bavay, Silvia Peraza Ku, Nathan Bossa, Melanie Davranche

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Aicha El Kharraf, Murielle Rabiller-Baudry, Ana Pradas del Real, Christophe Sandt, Mark Wiesner, et al.. Polyethylene plastic degradation: The dual pathways from macroplastics to nanoplastics. Journal of Hazardous Materials, 2025, 499, pp.140200. 10.1016/j.jhazmat.2025.140200. insu-05336289

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# Polyethylene plastic degradation: The dual pathways from macroplastics to nanoplastics

Aicha El Kharraf<sup>a,b,*</sup>, Murielle Rabiller-Baudry<sup>a</sup>, Ana Pradas Del Real<sup>c,d</sup>, Christophe Sandt<sup>c</sup>, Mark Wiesner<sup>e,b</sup>, Gergely Nemeth<sup>c</sup>, Dominique Bavay<sup>b</sup>, Silvia Peraza Ku<sup>a</sup>, Nathan Bossa<sup>e</sup>, Melanie Davranche<sup>e</sup>

## HIGHLIGHTS

* Direct evidence shows nanoplastics can detach from macroplastics, bypassing microplastic formation.

* Combined UV irradiation and mechanical stress drive early nanoplastic release from polyethylene.

* SEM and advanced spectroscopy confirm nanoscale fragments originate from embrittled macroplastic surfaces.

* Nanoplastics display higher oxidation and reactivity, raising concerns for pollutant transport and toxicity.

* Findings challenge current plastic degradation models, urging regulatory updates to include nanoscale risks.

## GRAPHICAL ABSTRACT

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Plastic breakdown

Nanoplastic

Fragmentation

Microplastics

Surface embrittlement

Plastic pollution

Environmental fate

## ABSTRACT

Plastics degrade in the environment through complex physical and chemical processes. Here, we demonstrate that nanoplastics (NPs) can be directly released from macroplastics, and not solely generated via intermediate microplastic (MP) fragmentation, thereby challenging the conventional stepwise degradation paradigm. Using a multi-technique approach combining scanning electron microscopy (SEM), µ-Raman spectroscopy, Chemical mapping (sSNOM), and optical infrared spectroscopy (OPTIR), we show that chain scission and oxidation drive structural embrittlement, promoting direct NP detachment from macroplastic surfaces. These findings emphasize the urgent need to reconsider plastic pollution mitigation strategies, moving beyond a sole focus on MPs and NPs to include macroplastics as a primary and active source of nanoscale pollutants. Expanding regulatory

<footnote><sup>a</sup> Institut des Sciences Chimiques de Rennes (ISCR) UMR 6226 – Univ Rennes, CNRS, Rennes F-35000, France</footnote>

<footnote><sup>b</sup> Géosciences Rennes UMR 6118 – Univ Rennes, CNRS, Rennes F-35000, France</footnote>

<footnote><sup>c</sup> Synchrotron SOLEIL, l'Orme des Merisiers, Départementale 128, Saint-Aubin 91190, France</footnote>

<footnote><sup>d</sup> Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTERre, Grenoble 38000, France</footnote>

<footnote><sup>e</sup> Duke University, Durham, NC 27708, USA</footnote>

<footnote>* Corresponding author at: Institut des Sciences Chimiques de Rennes (ISCR) UMR 6226 – Univ Rennes, CNRS, Rennes F-35000, France.
E-mail address: aicha.el-kharraf@univ-rennes1.fr (A. El Kharraf).</footnote>
frameworks to address plastic degradation across all size scales, from macro to nano, is essential to limit long-term environmental and ecological risks.

# 1. Introduction

Plastics have revolutionized modern society, offering unparalleled durability and versatility. However, their widespread use has escalated into an environmental crisis [1]. Since the mid-20th century, global plastic production has increased from approximately 2 million metric tons in 1950–413.8 million metric tons in 2023 [2]. With only 9% of plastic waste recycled, 12% incinerated, and 60% accumulating in landfills or the natural environment [3] projections indicate that by 2050, approximately 12 000 MMt of plastic waste will persist in ecosystems.

Among synthetic polymers, polyethylene (PE) stands out as the most widely produced, representing nearly one-third of annual global output and consistently identified as the dominant component of marine litter and environmental debris [1]. Its ubiquity in production and waste streams, combined with its semi-crystalline structure and the widespread use of additives that enhance durability [4], explains both its environmental persistence and its relevance as a model system. These characteristics make PE an ideal polymer for investigating degradation pathways and byproducts formation (Supplementary Fig. S1).

Despite extensive research on PE-based plastics degradation, most studies isolate specific processes such as macroplastic breakdown, microplastic (MP) formation, or nanoplastics (NP) behavior, failing to capture their interconnectivity [5]. This fragmented approach limits our understanding of plastic transformation pathways in natural environments [6]. In real-world environments, PE-based plastics degradation is driven by multiple abiotic factors: photo-oxidation, mechanical stress, and chemical interactions, these processes operate simultaneously, compounding their effects [7]. Traditionally, macroplastics have been presented as fragment into MPs which subsequently degrade into secondary NPs [8]. However, emerging evidence challenge this stepwise model, suggesting that NPs can detach directly from macroplastics, adding complexity to degradation dynamic [9], [10].

Three key gaps persist in current knowledge: (i) cross-scale fragmentation dynamics: Do NPs originate solely from MP breakdown, or can they detach directly from macroplastics? (ii) mechanistic interaction between oxidation and mechanical stress: How does photo-oxidation influences embrittlement and facilitates fragmentation across scales? (iii) comparative characterization of parent materials and degradation products: How do the chemical and physical properties of MPs and NPs differ from their parent macroplastic material?

Addressing these gaps is crucial for accurately assessing plastic persistence, transport, and ecological impact. This study develops a novel experimental framework integrating photo-oxidation and mechanical stress, to model real-world plastic weathering. Whereas most studies have examined these processes separately, focusing either on embrittlement induced by photooxidation (UV) [11,12] or fragmentation driven by mechanical stress [13], this approach reproduces their combined action, thereby capturing their synergistic effects and providing a more realistic representation of plastic degradation. By employing a custom designed reactor, we bridge the gap between field scale and lab scale assessments. Coupled with advanced multiscale characterization techniques (e.g., Scanning Near-field Optical Microscopy (s-SNOM), Optical Photothermal Infrared (OPTIR), µ-Raman, AFM), this framework enables a comprehensive understanding of PE-based plastics degradation across scales. These insights refine current environmental fate models and inform regulatory strategies addressing NPs pollution.

# 2. Materials and methods

## 2.1. Material preparation and pre-treatment

### 2.1.1. Polyethylene plastic and characterization

Commercial formulated polyethylene (HDPE) plastic, in the form of spherical cover with a thickness of 1.2 mm and a diameter of 5 cm, served as the model material for this study. The choosing plastic contains serval inorganic additives (plenty of metal element were quantified using ICP-MS).

Pristine plastics were characterized for chemical and elemental composition using ATR-FTIR and ICP-MS. The detailed protocols and instrument settings are provided in Supplementary Information (supplementary Fig. S2).

## 2.2. Experimental setup for degradation

Polyethylene plastics were subjected to UVC irradiation and mechanical stress in a laboratory reactor. The setup consisted of a UVC LED system (2.18 W cm⁻¹ at 254 nm) integrated with a PTFE-coated magnetic stirring unit operating at 400 rpm (shear stress of 0.746 N m⁻²; detail about the estimation of shear force generated within the reactor is explained in the Supplementary Information S2). The temperature within the reactor was maintained between [25–42] °C, with a relative humidity of 50 %. Experiments were conducted in triplicate using PTFE beakers over a 1000-hour period. Each polyethylene sample (43.5 ± 0.1 g) was immersed in 506 ± 6 mL of Milli-Q water, adjusted to an ionic strength of 1 Mm by NaCl. Sequential sampling was performed at predefined intervals to monitor the progressive transformation of both the solid and aqueous phases. At each time point (7, 28, 360, 700, and 1000 h), aged polyethylene fragments (3 ± 1 g) were retrieved and desiccated, while the aqueous phase (21 ± 6 mL) was filtered through a 0.8 µm glass fiber membrane to separate microplastic fractions (5 mm – 1 µm) from nanoscale particles (1 µm – 1 nm). Further procedural details are provided in the Supplementary Information (Methods S2)

## 2.3. Quantification of end-products (MPs/NPs) generation

Mass loss associated with plastic degradation was monitored through gravimetric analysis. Samples (bulk plastic) were weighed at each exposure time point using a high-precision microbalance (Cubis SARTORIUS, accurately: 1 mg), allowing for the quantification of progressive material loss under experimental conditions. The percentage mass loss was determined by this formula:

$$ \% \text{Weight loss} = \frac{m_i - m_f}{m_i} \quad 100 \qquad (1) $$

With $m_i$ initial mass and $m_f$ final mass.

Total Organic Carbon (TOC) analysis was performed to quantify the release of dissolved organic carbon (DOC) into the aqueous phase as a result of plastic breakdown. Measurements were conducted using a Total Organic Carbon analyzer (Shimadzu TOC-5050A) on samples collected at predefined time intervals (0 h, 7 h, 28 h, 360 h, 700 h, and 1000 h). Prior to analysis, all samples were filtered through 0.8 µm glass fiber membranes. The accuracy of quantification was estimated at ±3 %, based on calibration with a standard potassium hydrogen phthalate solution.

Nanoparticle Tracking Analysis (NTA) was employed to assess the hydrodynamic size distribution and concentration of nanoplastics released during polyethylene degradation. Measurements were conducted at predefined time points (0 h, 7 h, 28 h, 360 h, 700 h, and
1000 h) to track the temporal evolution of nanoplastic formation. Prior to analysis, liquid samples (<0.8 µm) were subjected to ultrasonic treatment for 10 min to minimize particle aggregation. NTA measurements were performed at ambient temperature using a NanoSight LM20 system (Malvern Panalytical) at the Institute of Chemical science (ISCR), University of Rennes (France). Additional technical specifications are provided in the Supplementary Information (Methods S3).

## 2.4. Characterization of end-products physical properties

The structural evolution of polyethylene and its degradation byproducts (aged macroplastics, MPs, and NPs) was examined using field-emission scanning electron microscopy (FE-SEM, JEOL JSM 7100 F EDS EBSD Oxford, 10 kV). The measurements were performed at ScanMAT platform, university of Rennes, France. To enhance conductivity and imaging resolution, samples were sputter-coated with a thin Au layer and analyzed under high vacuum. Morphological transformations, including crack formation, surface roughness, and porosity evolution, were assessed across multiple magnifications to capture degradation dynamics from macro- to nanoscale.

The surface morphology and roughness of unaged and aged formulated PE-based plastics were assessed using a stylus profilometer (Bruker Dektak 150) at the Shared Materials Instrumentation Facility (SMIF), Duke University (USA). Surface profiling was performed by scanning the samples with a diamond-tipped stylus, capturing electromechanical responses and topographical variations.

To ensure measurement reliability, roughness analysis was conducted at three distinct positions per sample. The large evaluation surface area (0.28 mm × 0.21 µm) enabled the calculation of mean surface roughness using Gwyddion software. Measurement resolution was set to 0.5 µm to optimize the accuracy of surface profiling.

## 2.5. Characterization of end-products chemical properties

The physicochemical properties of micro- and nanoplastics were characterized using scattering-type scanning near-field optical microscopy (s-SNOM, IR-neaSCOPE, Attocube system AG) at the SMIS beamline of Synchrotron SOLEIL, Saint-Aubin, France. This technique integrates the molecular specificity of infrared (IR) spectroscopy with the nanoscale spatial resolution of atomic force microscopy (AFM), enabling detailed analysis of both structural and chemical transformations [14].

s-SNOM imaging was conducted at two specific vibrational bands: 1465 cm⁻¹ (CH₂ deformation, confirming polyethylene identity), and 1728 cm⁻¹ (ketone functional groups, oxidation marker) over 30 × 30 µm regions with a 7 ms integration time per point.

Samples were deposited on gold-coated substrates and dried for 24 h in a desiccator. Additional instrumental specifications are provided in the Supplementary Information (Methods S4).

Prior to analysis, unaged and aged formulated PE-based plastics were pre-treated by placing them in a desiccator for 24 h to eliminate residual moisture. For micro- and nanoplastics, the gold-coated substrate previously used in s-SNOM analysis was repurposed to ensure consistency across measurements.

Optical Photothermal Infrared (OPTIR) spectroscopy was conducted on unaged, aged, and degraded microplastics using a miRAge OPTIR microscope (Photothermal Spectroscopy Corp) at Synchrotron SOLEIL, Saint-Aubin, France. Chemical mapping was performed by raster scanning regions of 50 and 62 µm² dimension with steps of w nm in x and y in the 900–1800 cm⁻¹ spectral range, using a linear scan speed of 100 µm s⁻¹ and a step size of 500 nm, corresponding to the instrument's maximum spatial resolution. Two target wavelengths were selected: 1465 cm⁻¹ (CH₂ deformation) to confirm polyethylene identity, particularly for smaller particles, and 1728 cm⁻¹ (ketones) to assess oxidation. Increased intensity in chemical images indicated higher concentrations of the mapped functional groups.

Oxidative transformation of polyethylene plastic was evaluated by determining the relative absorbances of characteristic infrared absorption bands associated with carboxylic acid (1718 cm⁻¹), ketone (1728 cm⁻¹), ester (1746 cm⁻¹), and vinyl (1648 cm⁻¹) functional groups. These intensities were normalized to the CH₂ deformation band at 1465 cm⁻¹ to compute oxidation indices [15]. The indices were determined using the following expressions [16]:

$$ \text{Carbonyl index} = \frac{A_{1718}}{A_{1465}} \qquad (2) $$

$$ \text{Ketone index} = \frac{A_{1722}}{A_{1465}} \qquad (3) $$

$$ \text{Ester index} = \frac{A_{1746}}{A_{1465}} \qquad (4) $$

$$ \text{Vinyl index} = \frac{A_{1648}}{A_{1465}} \qquad (5) $$

With A is the bands area. Additional details on instrumental conditions and data processing are provided in the Supplementary Information (Methods S5).

Raman spectroscopy was performed to characterize the structural evolution of unaged, aged, and microplastic samples using a confocal Raman microscope (Horiba Jobin Yvon LabRam Aramis Raman/PL System) at the SMIF facility, Duke University (USA). The system was equipped with a 633 nm excitation laser, and spectra were acquired at 100 × magnification to optimize laser focus.

Crystalline (αc) and amorphous (αa) fractions were calculated using the following expressions [17]:

$$ \alpha_c = \frac{I_{1418}}{0.46(I_{1296} + I_{1302})} \qquad (6) $$

$$ \alpha_a = \frac{I_{1075}}{0.79(I_{1296} + I_{1302})} \qquad (7) $$

where I₁₄₁₈ corresponds to the orthorhombic crystalline phase, I₁₀₇₅ to the amorphous phase, and I₁₂₉₆ and I₁₃₀₂ to CH₂ twisting modes used as internal references. Additional acquisition parameters, data treatment, and crystallinity calibration procedures are provided in the Supplementary Information (Methods S6).

# 3. Results

## 3.1. Surface degradation and multiscale fragmentation of polyethylene-based plastic revealed by SEM

SEM analysis revealed the progressive structural breakdown of PE plastics under combined photo-oxidative and mechanical stress, from macroplastic fragmentation to the formation of MPs and NPs. Pristine plastic displayed a smooth and defects-free surface (Fig. 1a, b). In contrast, aged PE plastic showed significant surface degradation, with cracks, increased roughness, and microvoids (Fig. 1c, d). Notably, nanoscale fragments (red-circled, Fig. 1d) detach directly from the macroplastic surface, providing the first direct evidence of macro-to-nano fragmentation. This challenges the conventional view that secondary NPs originate exclusively from MPs breakdown [18], [19]. Further fragmentation produced MPs with irregular, highly textured and fissured surface (Fig. 1e, f), indicative of embrittlement, and progressive structural weakening. Continued degradation led to NPs release from MPs, as nanoscale fragments detached from their surfaces (Fig. 1h), reinforcing the role of photo-oxidative and mechanical stress in nanoscale fragmentation [19,20]. These findings reveal a multiscale degradation mechanism in which secondary NPs arise not only from MPs disintegration but also directly from macroplastics, uncovering a previously unrecognized pathway in environmental plastic breakdown.
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Multiscale degradation pathways of formulated polyethylene plastics: direct and indirect nanoplastic formation revealed by SEM. SEM panels (left): (a–b) Pristine PE surfaces prior to exposure, showing smooth and featureless morphologies. (c–d) Aged PE after 1000 h of UVC irradiation and mechanical stress; panel (e) highlights a micrometric fragment of ~3 µm resting on the embrittled surface, while panel (f) shows nanoscale fragments detaching at crack edges, with individual sizes of ~147 nm and 549 nm (circled/highlighted). (g) Microplastics (MPs) released into the aqueous phase; fracture lines and surface cracks are marked with arrows, while circled/highlighted regions show MPs bearing attached sub-micron particles, indicating their role as transitional intermediates between macroplastics and secondary nanoplastics. (h) Another example of MPs carrying attached nanoscale debris, supporting their role as fragmentation intermediates. (i) Higher-magnification view of (f), revealing nanoscale and microscale fragments detaching from the MP surface, with sizes of ~1.3 µm, 830 nm, and 322 nm. Scale bars as shown in panels. Scheme (right): The diagram summarizes the two fragmentation pathways illustrated by the SEM images. In the direct pathway, photo-oxidation and embrittlement of macroplastic surfaces lead to nanoscale detachment (panels c–d). In the indirect pathway, macroplastics fragment into MPs (panels e–f), which subsequently release secondary NPs from their surfaces (panels g–h). Together, these results demonstrate that nanoplastics arise both through direct release from macroplastics and stepwise fragmentation via MP intermediates.</figcaption></figure>

## 3.2. Quantifying polyethylene-based plastic degradation: linking mass loss to end-products release and nanoplastic early-stage formation

Degradation of PE initiated with the rapid appearance of nanoscale fragments. NTA revealed that particles below 100 nm were already detected after 7 h of exposure, followed by a gradual release of larger NPs, reaching an average size of 170 ± 30 nm at 1000 h (Fig. 2a). Besides the increase of the size, the variance of size increases as well refereeing to the rise of heterogeneity of NPs population with time exposure. Simultaneously, NPs concentration increased sharply within the first 7 h (22 ± 6) × 10⁶ particles mL⁻¹, peaked at (16 ± 4) × 10⁷ particles mL⁻¹ at 700 h, and then stabilized (Fig. 2b). This early-stage NPs release aligns with previous findings showing direct detachment of nanoscale fragments from degrading plastic matrices [9,21]. Quantification of the organic fraction < 0.8 µm (corresponding mostly to nanoplastics, and potentially to oligomers) using a TOC analyzer further confirms this trend, confirming the onset of NPs release at the initial stages of degradation (Fig. 2d).

Gravimetric analysis revealed a progressive mass loss of 33.9 ± 0.6 % at 1000 h (Fig. 2c), with the most pronounced decline between 200 h and 700 h, coinciding with peak NPs concentrations (Fig. 2b). This trend indicates an accelerated degradation phase, during which

plastic fragmentation intensifies. While mass loss confirms material breakdown, it does not differentiate whether NPs or MPs dominate the release. However, MPs are not stable end-products where continuous degradation weakens them, leading to secondary fragmentation and further fragmentation due to sustained environmental stress. [19], [22]. SEM imaging (Fig. 1e, f) supports this process, showing MPs with fractured, porous surfaces, indicative of oxidative degradation and mechanical disintegration. Further SEM analysis confirms NPs release, reinforcing the micro to nano fragmentation pathway (Fig. 1h), which occurs alongside direct NPs release from macroplastics (Fig. 1d).

These findings provide quantitative evidence that PE-based plastic degradation leads to the simultaneous MPs and NPs release, rather than a strictly hierarchical fragmentation process. After 1000 h of aging, PE plastic released secondary NPs and MPs at concentrations of 435.3 ± 0.3 mg g⁻¹ (size distribution from <100 nm to 170 ± 30 nm after 7 h and 1000 h of aging respectively) and 213.49 ± 0.06 µg g⁻¹ (100 ± 200 µm, see supplementary information), respectively. The strong correlation between plastic mass loss and particle formation underscores the importance of accounting for both size fractions in environmental assessments of plastic pollution.
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Temporal evolution of PE-based plastic degradation under combined UVC irradiation and mechanical stress. Data points represent mean values and shaded areas indicate standard deviation (n = 3). (a) Hydrodynamic size distribution of nanoplastics determined by NTA (nm). (b) Nanoplastic concentration measured by NTA (particles mL⁻¹). (c) Mass loss of PE fragments normalized to initial mass (mg g⁻¹), determined by gravimetric analysis. (d) Nanoplastics concentration (<$0.8 µm$), released into the aqueous phase and normalized to polymer mass (mg g⁻¹), measured by TOC analysis.</figcaption></figure>

### 3.3. Breaking down polyethylene-based plastic: surface transformations driving micro- and nanoplastic formation

Atomic force microscopy (AFM) and profilometry analyses revealed key surface deterioration patterns in PE-based plastics during the degradation characterized by increasing roughness and formation of deep pits. Unaged plastic (Fig. 3a, b) displayed a smooth surface with low roughness (40 ± 6 nm, supplementary Tab. S1) and shallow pits (500 ± 400 nm), indicating a stable polymer matrix. In contrast, aged PE (Fig. 3c, d) exhibited a tenfold increase in roughness (400 ± 200 nm) and significantly deeper pits (8000 ± 2000 nm, Fig. 3d), confirming severe embrittlement and mechanical weakening. These structural changes correlate with polymer mass loss (33 %, Fig. 2c), indicating that surface degradation is a primary driver of fragmentation. As roughness increases, the polymer matrix weakens, promoting fragments detachment and the formation of MPs and NPs. The emergence of deep surface pits (Fig. 3d) accelerates this process, as embrittlement facilitates crack propagation and material loss [23].

MPs (Fig. 3e, f) exhibited the highest roughness (500 ± 200 nm, supplementary Tab. S1) and pronounced surface irregularities, surpassing those of aged PE-based plastic, suggesting that MPS are particularly vulnerable to further breakdown [24]. AFM profiling (Fig. 3f)

revealed deep pits that act as initiation points for continued degradation, supporting the progressive detachment of smaller particles. In contrast, NPs (Fig. 3j) had lower roughness (300 ± 200 nm, supplementary Tab. S1) but retained nanoscale surface heterogeneities, confirming that degradation persisted at the nanoscale. The depth of pits in MPs (Fig. 3f) further demonstrates how mechanical and oxidative stress progressively destabilize the surface, driving continuous fragmentation [24,25].

Profilometry depth analysis (Fig. 3d, f) confirms that degradation primarily affects the outer layers with roughness increasing as material loss progresses. The formation of deep cracks in aged plastic facilitates MP release (Fig. 3e), which then degrades further into NPs [21,26]. These findings provide quantitative evidence of plastic progressive degradation, highlighting how surface deterioration governs the transition from macroplastics to MPs and ultimately to NPs.

### 3.4. Chemical transformation of polyethylene during plastic degradation: functional group evolution (OPTIR, s-SNOM)

Environmental degradation of PE-based plastic induces progressive oxidation, leading to structural and chemical changes that affect fragmentation pathways and environmental interactions. OPTIR and s-SNOM analyses were used to track oxidation progression by mapping
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Surface roughness evolution and fragmentation pathway of polyethylene-based plastic during aging (AFM, Profilometry) (a, b) Unaged plastic with low roughness and shallow pits. (c, d) Aged plastic showing increased roughness and deep pit formation due to oxidative embrittlement. (e, f) MPs exhibiting the highest roughness and pronounced surface irregularities, facilitating fragmentation. (j) NPs with lower roughness but retaining nanoscale heterogeneities, confirming ongoing degradation. Profilometry and AFM analyses illustrate the progressive transition from macroplastics to MPs and NPs.</figcaption></figure>
ketone functional groups (1728 cm⁻¹) and quantifying additional
oxygenated species using carbonyl indexes.

Oxidation during PE plastic degradation was primarily observed
through changes in the carbonyl spectral region (1600–1800 cm⁻¹).
Infrared spectral analysis revealed a progressive increase in carbonyl
absorption with aging (Fig. 4). Aged PE macroplastic exhibited pro-
nounced ketone and ester bands, while MPs showed an additional ab-
sorption band at 1718 cm⁻¹, corresponding to carboxylic acids,
indicating further oxidative progression. The most intense carbonyl
signals were detected in NPs, evidencing that oxidation is more homo-
geneous, reflecting a more advanced oxidation state [27,28], suggesting
that advanced oxidation state could favored particles detachment. To
quantify these trends, we calculated the ketone index as a marker of
oxidative intensity, and used chemical imaging of the 1728 cm⁻¹ band to
map the spatial distribution of oxidation. This combined approach
captures both the chemical evolution and heterogeneity of oxidation
across degradation steps.

Spectra deconvolution (supplementary Fig. S5) further revealed a
sequential oxidation hierarchy, where ketones forming first, follow by
esters and ultimately carboxylic acids (ketones < esters < acids) [29,
30]. This oxidation pathway aligns with established polyolefins degra-
dation mechanisms, where initial carbonyl formation leads to more
reactive oxidative species [31,32], corresponding to ketones
(1728 cm⁻¹), esters (1746 cm⁻¹), and carboxylic acids (1718 cm⁻¹)
(Fig. 4). These change increase the hydrophilicity and surface reactivity
of MPs and NPs, affecting their interaction with pollutants and potential
bioavailability [18,33](supplementary Tab. S2).

The ketone index increases significantly from 0.006 ± 0.001 in unaged macroplastic to 0.2 ± 0.2 in aged macroplastic, 0.4 ± 0.1 in MPs, and 0.7 ± 0.3 in NPs (supplementary Tab. S3), indicating that oxidation intensifies as fragmentation progresses [29,34].

Simultaneously, the vinyl index (marker of chain scission) increased
from 0.010 ± 0.008 in unaged macroplastic to 0.073 ± 0.002 in MPs

(supplementary Tab. S3), confirming that oxidative degradation is
accompanied by polymer backbone cleavage [35]. The formation of
vinyl groups (Norrish reactions) during chain scission was correlated
with major changes in PE-based plastic mechanical properties, particu-
larly a rapid transition from ductile to brittle failure, accelerating frag-
mentation [36,37]. This shift is linked to secondary crystallization at the
polymer surface, which generates localized stress points and structural
defects, promoting crack initiation and propagation [38,39]. These
findings confirm that oxidation not only alter chemical composition but
also weakens the mechanical integrity of PE macroplastic, ultimately
influencing MPs and NPs release.

Chemical mapping of ketone functional groups at 1728 cm⁻¹ (Fig. 4) revealed heterogeneous oxidation patterns in aged PE macroplastic and MPs, with oxidation hotspots concentrated around cracks and mechanical stress points [40], where degradation is most advanced. The oxidation-rich zones suggests that oxidation spreads along structurally weakened aera (specifically the amorphous regions, where chain mobility and radical propagation are favored), accelerating embrittlement and subsequent fragmentation. Chemical map (Fig. 4c, red panel), shows elevated carbonyl absorption near a surface crack, supporting this spatial correlation between structural defects and oxidative stress. Similar behavior has been reported by Bracco et al. [41] and Ter Halle et al. [8], who observed preferential oxidation in amorphous domains during UV aging of LDPE, HDPE, and MPs. In contrast, NPs exhibited a more homogeneous oxidation pattern, like due to their higher surface area to volume ratio, which enhance oxygen diffusion and oxidative aging [30].

3.5. *Polyethylene-based plastic degradation unveiled: crystallinity and molecular changes via micro-Raman spectroscopy*

μ-Raman spectroscopy provided critical insights into the structural
evolution of PE plastics during degradation, revealing changes in

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Mapping oxidation in polyethylene-based plastic: infrared spectroscopy and chemical imaging (OPTIR, s-SNOM). (a) Average IR spectra (10 spectra for each type of sample) of unaged PE-based plastic (black), aged PE-based plastic (red), MPs (blue), and NPs (green), showing progressive oxidation through the emergence and intensification of carbonyl-related peaks. (b) Optical microscopy images of degraded PE-based plastic at various fragmentation stages. (c) chemical images at 1728 cm⁻¹ illustrating heterogeneous oxidation patterns in aged PE and MPs, becoming more uniform in NPs.</figcaption></figure>
crystallinity, molecular conformation, and polymer chain integrity. The average Raman spectra (Fig. 4) of unaged plastic, aged plastic, and MPs showed progressive modifications in characteristic vibrational bands (supplementary Tab. S3). Unaged plastic exhibited well defined peaks associated with its semi-crystalline nature, including a prominent crystalline band at 1418 cm⁻¹ (orthorhombic phase) and amorphous bands at 1440 cm⁻¹ and 1460 cm⁻¹ [31,35]. Upon aging, the 1418 cm⁻¹ peak intensity increased significantly while the amorphous bands decreased indicating crystallization during degradation. MPs displayed an even higher degree of crystallinity, suggesting that embrittlement caused by increased crystallinity promotes fragmentation [32,33].

The evolution of the crystalline, amorphous, and interface (semi-ordered zones where crystalline and amorphous phases meet) fractions (supplementary Tab. S4) further support this trend, revealing a progressive transformation of PE plastic microstructure. The crystalline fraction increased from 0.44 ± 0.08 in unaged plastic to 0.65 ± 0.07 in aged plastic and remained high in MPs (0.6 ± 0.2). This increase confirms that degradation-driven molecular rearrangement leads to a more ordered structure [42]. This transformation is governed by two competing degradation mechanisms: chain scission, which enhances molecular mobility and promotes recrystallization, and crosslinking, which stiffens the polymer matrix by forming intermolecular bonds [43, 44]. While chain scission facilitates molecular reorganization, crosslinking increases polymer stiffness, both contributing to embrittlement and fragmentation [45].

A direct correlation between crystallinity and mechanical brittleness was observed when comparing Raman data with surface roughness (Fig. 3c, e). As crystallinity increased, roughness also rose in aged plastic and MPs, showing that oxidation-induced embrittlement destabilizes the surface during degradation, the amorphous phase, which provides flexibility, increased from 0.077 ± 0.002 in unaged plastic to 0.12 ± 0.05 in aged plastic but decreased to 0.09 ± 0.02 in MPs. Meanwhile, the crystalline regions remained dominant making MPs brittle and prone to further fragmentation into NPs [46].

IR spectroscopy tracked the formation of vinyl groups, providing further evidence of degradation pathways. The vinyl index increased from 0.010 ± 0.008 in unaged PE-based plastic to 0.024 ± 0.008 in aged PE-based plastic and 0.073 ± 0.002 in MPs (supplementary Tab. S3, Fig. S8). This trend aligns with activation energy values where vinyl formation (20–46 kJ mol⁻¹) [47–49] requires less energy than carbonyl formation (46–95 kJ mol⁻¹), confirming that chain scission occurs before oxidation [35]. The simultaneous increase in crystallinity and vinyl content suggests that degradation start with backbone cleavage, creating unsaturated vinyl sites before oxidation progresses.

The relationship between crystallinity and vinyl formation (supplementary Fig. S8) underscores the instability of MPs. With high crystallinity and rough surfaces, these particles are brittle and prone to further breakdown. This structural weakening accelerates the formation of smaller fragments, reinforcing the role of crystalline phase transformation in PE-based plastic degradation and NPs generation.

# 4. Discussion

## 4.1. Rethinking the degradation paradigm

Conventional models of plastic weathering describe a strictly sequential hierarchy, where macroplastics degrade into microplastics (MPs) before further breaking down into nanoplastics (NPs) [5,50]. However, the findings challenge this assumption by demonstrating that NPs can also form directly from macroplastics, bypassing the MP stage. This discovery is critical, as it suggests that NP production may occur much faster in the environment than current models predict, leading to potential underestimations of NPs concentrations in natural ecosystems.

SEM imaging provides direct evidence of nanoscale fragments detaching from macroplastic surfaces, confirming the existence of parallel degradation pathways. These findings expand upon previous work

by Ekvall et al. (2019) [20] and Lambert et Wagner. (2016) [9], which reported early-stage NP release under mechanical stress. The results reinforce the need to reassess degradation models to incorporate both direct and sequential NPs formation.

## 4.2. Temporal dynamics and synergistic effects

The pronounced increase in NPs release and mass loss between 200–700 h highlights a synergistic interplay between photo-oxidation and mechanical stress. At the earliest stage (≤200 h), oxidative aging generated carbonyl-rich fissures that rendered macroplastic surfaces mechanically fragile. These weakened zones fragmented rapidly under shear, releasing nanoscale fragments directly from macroplastics and demonstrating the direct fragmentation pathway. This progression reveals a critical mechanistic coupling: photo-oxidation primes the polymer matrix for failure, while mechanical stress accelerates disintegration through these vulnerable sites, thereby amplifying NPs release beyond what either process could achieve alone.

During the intermediate phase (200–700 h), the sharp rise in mass loss (Fig. 2c) and in the NPs concentration (Fig. 2b) coincide with the appearance of MPs bearing cracked and oxidized surfaces (Fig. 1e–h). These MPs act as transient intermediates that, under continued oxidation and shear, readily disintegrated and released additional nanoscale fragments. Hence, while direct macro-to-NP detachment dominates early degradation, the MP-to-NP pathway becomes the primary amplifier of nanoscale output during extended weathering.

At later stages (>700 h), MPs persist only as unstable residues and continue to fragment, sustaining NPs release even as bulk mass decline slowed. Collectively, these results provide compelling evidence for a dual-pathway model of degradation, in which direct and indirect routes operate on different timescales. Explicit recognition of this temporal interplay is essential for accurate environmental fate modeling, which to date has relied largely on strictly sequential assumptions.

## 4.3. Mechanistic underpinnings of fragmentation

By applying both µ-Raman spectroscopy with IR spectroscopy (OPTIR, s-SNOM) to the same samples (pristine PE-based plastics, aged PE-based plastics, and MPs), we provide molecular level insights into the degradation process. Raman data show a gradual increase in bulk crystallinity over time, accompanied by an early rise in the vinyl index revealing (supplementary Fig. S8) that chain scission occurs at the initial stages of degradation. This correlation between vinyl group formation and localized crystallinity changes is in agreement with induces embrittlement [35], generating mechanically weak zones where cracks propagate, facilitating oxygen diffusion and oxidation [51].

Chemical mapping further highlights the heterogeneous nature of oxidation, with carbonyl-rich zones concentrated around crack sites. OPTIR analysis shows that oxidation progress sequentially, from ketones to esters and ultimately to carboxylic acids with the highest oxidation observed in NPs. AFM and profilometry confirm that MPs exhibit increased surface roughness, particularly in crack-prone regions, confirming their mechanical instability. These findings underscore that MPs are not stable degradation end-products but transient intermediates that continue to fragment into NPs. This instability has major environmental implications. If MPs are prone to continuous breakdown, it suggests that nanoplastic generation occurs much more rapidly in the environment than previously assumed.

## 4.4. Environmental relevance and methodological considerations

The mechanistic validity of the approach reinforces the broader relevance of the conclusions. The shear stress applied in the experiments (0.746 N·m⁻²) is equivalent to shallow ocean waves at ~0.2 m depth (0.7–0.8 N·m⁻², Carniello et al., 2018 [52]), underscoring the ecological realism of the mechanical forcing. In parallel, UVC irradiation was not
intended to replicate the solar spectrum but to accelerate degradation processes that, under natural UVA/UVB exposure, would unfold over decades. Mechanistic studies have established that UVC triggers the same fundamental photochemical pathways as natural sunlight, chain scission and photo-oxidation through Norrish I and II reactions, ulti- mately producing equivalent degradation byproducts, though on com- pressed timescales [28,53,54].

Over 1000 h of irradiation, the system delivered a cumulative UV dose of ~7.85 × 10⁶ J cm⁻², compared to ~1.8 × 10⁴ J cm⁻² from nat- ural UVA+UVB under the same duration, equivalent to an acceleration factor of ~430 × . Therefore, 1000 h correspond to ~49 years of cu- mulative natural UV exposure. We emphasize that this represents a controlled dose acceleration rather than a replication of natural kinetics. However, when combined with environmentally realistic shear stress (0.746 N m⁻²), the setup effectively reproduces the coupled effects of sunlight and hydrodynamic forcing within a compressed timeframe.

Considering that the rupture force of a C–C bond is estimated to be around 5–7 nN [55,56], the applied mechanical stress of 0.746 N m⁻² in the reactor corresponds to approximately 7.46 nN over a 100 µm × 100 µm surface area. This stress level is therefore sufficient to induce homolytic C–C bond cleavage, leading to the generation of free radicals. This level of mechanical energy can thus promote the fragmentation of macro- into microplastics. Consequently, the applied shear stress likely amplifies each degradation step by generating mechano-radicals, thereby reproducing the synergy between photochemical and mechan- ical processes observed in natural environments.

4.5. Statistical correlations between chemical and physical parameters

To quantitatively assess the interplay between chemical and physical
parameter, we applied Q–Q analysis and correlation matrices. Q–Q plots
evaluate whether datasets follow a normal distribution and enable
comparison of shifts in mean (μ), variance (σ), and distribution shape
across degradation products, while correlation matrices quantify the
strength and direction of associations between chemical oxidation

(carbonyl index) and physical embrittlement (surface roughness, Ra).

The Q–Q analysis revealed clear statistical transitions (Fig. 6a). Unaged plastics displayed stable, narrowly distributed values (supplementary Tab. S5) for both carbonyl content (μ = 0.01, σ = 0, P = 0.79) and roughness (μ = 40.1 nm, σ = 5.9, P = 0.06), consistent with structural integrity. In contrast, aged plastics showed increased variability, with oxidation rising sharply (μ = 0.18, σ = 0.13, P < 0.0001) and roughness spreading widely (μ = 489.9 nm, σ = 141.9, P = 0.17), reflecting heterogeneous embrittlement. MPs inherited high surface roughness (μ = 521.5 nm, σ = 131.8, P = 0.22) but moderate oxidation (μ = 0.37, σ = 0.05, P = 0.23), whereas NPs exhibited the strongest oxidation signal (μ = 0.68, σ = 0.29, P = 0.04) alongside highly variable roughness (μ = 346.9 nm, σ = 186.4, P = 0.06). Together, these distributions confirm that NPs are chemically distinct and more oxidized than their precursors.

The correlation matrix further reinforced this coupling (Fig. 6b). Aged plastic roughness correlated strongly with NPs oxidation (r = 0.94), indicating that fissured, carbonyl-rich surfaces of aged plastics act as hotspots for nanoscale detachment. MPs, by contrast, showed negative correlations between roughness and oxidation (r = -1), suggesting that while they inherit physical fragility, they remain less oxidized than NPs. This strong negative correlation underscores that MPs, while mechanically weakened, do not accumulate the same level of oxidation as NPs, confirming their role as unstable, short-lived intermediates rather than chemically advanced degradation products. This duality reinforces the mechanistic interpretation: oxidation initiates embrittlement, embrittlement propagates fragmentation, and together these processes accelerate NPs release, with MPs serving as transient intermediates.

Overall, these statistical results provide robust quantitative support for the dual-pathway degradation model, demonstrating that chemical oxidation and physical roughening are not independent but co-evolving drivers of accelerated NPs release.

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Average Raman spectra (three spectra for each type of samples) of unaged PE-based plastic (black), aged PE-based plastic (red), and MPs (blue), highlighting structural modifications and crystallinity evolution. Corresponding optical images depict sample morphology at the analyzed regions.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Statistical coupling between chemical oxidation and physical roughening during PE-based plastic degradation. (a) Quantile-quantile (Q-Q) plots comparing the distributions of carbonyl index (chemical oxidation marker) and surface roughness (Ra, surface topography) across degradation stages: unaged macroplastic, aged macroplastic, microplastics (MPs), and nanoplastics (NPs). Shifts in mean (μ), variance (σ), and distribution shape indicate progressive embrittlement and oxidation, with NPs showing the highest oxidation (μ = 0.68, σ = 0.29, P = 0.0399) and strong variability in roughness (μ = 346.9 nm, σ = 186.4). (b) Correlation matrix between topography and carbonyl groups reveals significant positive correlations between aged plastic roughness and NP oxidation (r = 0.94), while MPs display negative correlations between roughness and oxidation (r = -1.0). Together, these analyses demonstrate that chemical oxidation and physical embrittlement are statistically coupled, jointly driving accelerated fragmentation and NP release.</figcaption></figure>

## 4.6. *Ecological and regulatory implications*

The environmental implications of this degradation process extend beyond simple particle size reduction. MPs, although transient, serve as critical vectors for pollutant transport, adsorbing and transferring contaminants across ecosystems [57] before fragmenting into NPs. Unlike MPs, NPs have the ability to penetrate biological membranes, leading to increased bioaccumulation risks. Their high surface-area-to-volume ratio and chemical reactivity further amplify their capacity to bind with pollutants [58], potentially altering contaminant bioavailability and toxicity [59]. These transformations significantly influence their interactions with contaminants, biological systems [59], and environmental compartments [59]. This heightened pollutant adsorption capacity may alter contaminant bioavailability, potentially increasing toxicity in exposed organisms [59] and facilitating long-range contaminant transport across ecosystems. Beyond their role as passive pollutant carriers, NPs themselves can exhibit direct toxic effects due to their small size, high surface area-to-volume ratio, and altered surface properties [60].

Future research should focus on how these oxidation-induced chemical changes influence NPs toxicity compared to MPs, particularly in long-term exposure scenarios. Understanding these mechanisms is essential for predicting the long-term ecological effects of NPs, ensuring that regulatory responses reflect real-world environmental risks. Given their unique physicochemical properties, persistence, and bioavailability, NPs should be treated as a distinct class of pollutants, requiring dedicated risk assessment frameworks and environmental monitoring strategies. Without such efforts, their full ecological impact remains difficult to quantify, leaving significant gaps in our understanding of risks associated to plastic across ecosystems and food webs. Alongside, the impact of plastic additives, whether organic, inorganic, or organo-metallic, on degradation mechanisms, kinetic pathways, and the release and environmental fate of micro and nanoplastics remains significantly under-investigated. Notably, Benítez et al. (2013) [61] reported that the presence of cobalt stearate significantly enhances oxidative degradation of polymers under environmental conditions. This highlights the critical need to incorporate additive effects into predictive models of plastic aging and pollutant release.

Current plastic pollution regulations remain piece-meal, primarily

focusing on MPs [62] while overlooking both NPs and their macroplastic origins. While regulations remain often unclear on nanoparticles, the European Commission's recent REACH regulation restricts intentionally added manufactured MPs but does not address the environmental degradation of macroplastics, nor does it account for the formation of NPs [63]. Our findings demonstrate that macroplastics do not solely degrade into MPs but also directly generate NPs, highlighting a crucial gap in current regulatory approaches. Addressing plastic pollution requires moving beyond a microplastic-centered framework and adopting a comprehensive, multiscale strategy that considers all degradation pathways. Limiting macroplastic persistence in the environment would not only reduce MP formation but also significantly curb the release of NPs, which are more difficult to detect and regulate due to their small size and high surface reactivity. Intervening at the macroplastic level is therefore a proactive approach, preventing long-term pollution rather than merely managing its consequences.

To address the full life cycle of plastic degradation, an integrated regulatory strategy is urgently needed. This strategy should: (i) prioritize macroplastic source reduction; (ii) support the development of advanced detection technologies for monitoring micro and nanoplastics (MPs and NPs); (iii) strengthen waste management policies through improved collection systems and incentives for recycling and reuse; and (iv) promote the redesign of additives and polymer compositions in new plastics to enhance their recyclability and reduce environmental persistence. Notably, current evidence does not confirm that bio-sourced plastics are inherently safer than petroleum-based plastics in terms of their fragmentation into MPs and NPs.

## 5. Conclusion

This study reveals that polyethylene plastics degrade through dual fragmentation pathways, producing NPs both via conventional MPs disintegration and directly from embrittled macroplastic surfaces. By replicating environmentally relevant UV exposure and mechanical stress, we demonstrate that oxidation-induced chain scission and surface roughening drive early-stage NP formation, challenging the prevailing stepwise degradation paradigm. Our findings are underpinned by a robust, multiscale methodology combining high-resolution imaging (SEM, AFM), spectroscopic mapping (OPTIR, s-SNOM), and molecular
characterization (μ-Raman), enabling precise tracking of physical and chemical transformations across size scales.

The emergence of NPs directly from macroplastics redefines their role as an active source of nanoscale pollutants, with implications for environmental exposure models, risk assessment, and plastic pollution regulation. MPs, often considered stable degradation intermediates, are shown to be mechanically unstable and chemically evolving entities that further fragment under environmental stressors. This dynamic behavior accelerates NP release, increasing their potential for long-range transport, bioavailability, and pollutant vectorization.

By uncovering the chemical and structural drivers of nanoscale plastic formation, this work advances our understanding of plastic persistence in natural systems. It highlights the need for regulatory frameworks that address degradation pathways across the full plastic size continuum. Targeting macroplastic sources may offer the most effective strategy to mitigate the elusive and potentially hazardous footprint of nanoplastics, an urgent priority for environmental sustainability.

## Environmental Implications

Our study reveals that polyethylene macroplastics directly generate nanoplastics under environmental stress, challenging the traditional stepwise degradation paradigm. This overlooked nanoscale pathway accelerates pollutant release and increases the risk of nanoplastic bioavailability and toxicity due to their small size and surface reactivity. Current environmental models and regulations focusing solely on microplastics may underestimate nanoplastic pollution. Recognizing macroplastics as a primary source of nanoplastics is crucial to mitigate ecological and human health hazards. Integrating nanoscale fragmentation into risk assessments and developing targeted monitoring strategies are urgent priorities to address this emerging environmental threat.

## CRediT authorship contribution statement

**Melanie DAVRANCHE:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition, Formal analysis, Conceptualization.
**Nathan BOSSA:** Software. **ANA PRADAS DEL REAL:** Writing – review & editing, Validation, Software.
**Murielle RABILLER-BAUDRY:** Writing – review & editing, Resources, Project administration, Funding acquisition, Conceptualization.
**EL Kharraf Aicha:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Methodology, Investigation, Formal analysis, Data curation.
**Dominique BAVAY:** Conceptualization.
**Gergely NEMETH:** Software.
**Mark WIESNER:** Writing – review & editing, Validation, Supervision, Funding acquisition.
**Christophe SANDT:** Writing – review & editing, Validation, Software.
**Silvia Peraza KU:** Software.

## 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.

## Acknowledgements

The authors acknowledge financial support from CNRS 80PRIME (UltimatePlast project) and the French National Research Agency (ANR) [ref. NanoCigo ANR23-CE01-0002-01]. Through the support of the GeOHeLiS analytical platform at Rennes University, this publication is also supported by the European Union through the European Regional Development Fund (FEDER), the French Ministry of Higher Education and Research, the French Region of Brittany, and Rennes-Metropole.

# Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jhazmat.2025.140200.

## Data availability

Data will be made available on request.

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