Enhancing mechanical and rheological properties of HDPE ﬁ lms through annealing for eco-friendly agricultural applications

: A composite of high-density polyethylene/ultra-high molecular weight polyethylene (HDPE/UHMWPE) was synthesized via a high-temperature melting process. We speci ﬁ cally examined the e ﬀ ects of annealing on the morphological and rheological properties of the composite. Using scanning electron microscopy and advanced rotational rheometry, we determined that annealing notably alters the composite ’ s microstructure, with a reduction in pronounced features post-treatment. Rheologically, UHMWPE content signi ﬁ - cantly enhanced mechanical properties, particularly in storage modulus ( G ′ ), loss modulus ( G ″ ), and complex viscosity (| η *|), which aligned with scaling law principles. Our ﬁ ndings highlight a distinct improvement in mechanical properties post-annealing, underscoring the potential of UHMWPE in enhancing polymer composite performance. The scaling laws relating G ′ , G ″ , and | η *| with UHMWPE were identi ﬁ ed as follows: (1) pre-c e : G ′ ∼ c 0.2 , g ″ ∼ c 0.07 , | η *| ～ c 0.07 and (2) post-c e : G ′ ∼ c 1.20 , g ″ ∼ c 0.6 , | η *| ∼ c 0.64 , with a forward shift in c e noted after annealing. The addition of UHMWPE signi ﬁ - cantly enhanced the system ’ s properties, particularly after annealing. This study o ﬀ ers crucial insights into optimizing HDPE plastic ﬁ lms for enhanced durability and sustainability, ideally suiting them for eco-friendly agricultural uses.


Introduction
Polyethylene (PE) plastic film, a crucial polymer material, has emerged as a cornerstone in modern agricultural production.The widespread adoption of plastic film mulching technology has significantly propelled agricultural productivity in China [1].In the year 2014 alone, the national consumption of plastic film astonishingly reached 1.425 million tons, covering nearly 300 million mu of agricultural land.This technique, primarily employed for its ability to elevate soil temperature and retain moisture, has proven to be instrumental in boosting crop yields by an impressive 20-50%.Such an increase is vital for bolstering Chinas food security.However, the extensive application of this technology has not been without its drawbacks, particularly leading to the adverse environmental impact known as "white pollution" due to film residues.
At the current stage, the primary strategy to combat this issue lies in the recycling and reusing of residual films.The challenge of recycling pollution predominantly stems from the quality and characteristics of the films used.Presently, the standards governing domestic film production are relatively low, coupled with insufficient management across the film's lifecyclefrom manufacturing and processing to usage and recycling.Consequently, the market is inundated with films that are ultra-thin, lack adequate strength, are prone to rapid aging, and have a short lifespan.This results in a proliferation of film fragments and residues in agricultural fields, presenting significant challenges in recovery and recycling.To address this, research and development focused on producing high-strength, high-quality, and cost-effective recyclable plastic films from the source is imperative.This approach is seen as a key solution for the prevention and control of residual film pollution [2].Furthermore, current strategies for film PE materials emphasize enhancing recycling rates and material performance improvement [3,4].Research exploring the recycling potential of waste high-density polyethylene (HDPE) films combined with natural fibers for creating decorative tiles revealed significant mechanical property enhancements [3].Another study demonstrated molecular weight adjustment of lowdensity polyethylene (LDPE) through ultra-high-speed extrusion to facilitate injection molding recycling, showing substantial energy savings and CO 2 emission reduction [5].Additionally, industrial-scale plastic pyrolysis research identified the impact of operational conditions on HDPE degradation mechanisms, proposing a simplified kinetic mechanism for primary product formation pathways.These studies indicate that technological innovations enable effective recycling of film-grade PE materials, offering sustainable alternatives to traditional materials with environmental and economic benefits.HDPE/UHMWPE films showcase significant advantages in agricultural film application and recycling, enhancing durability and reducing fragmentation.Their high recyclability allows for conversion into new plastic products, easing environmental impacts and cutting waste management costs.This promotes resource recycling, environmental stewardship in farming, and advances sustainable agriculture.
In the realm of engineering plastics, ultra-high molecular weight polyethylene (UHMWPE) stands out for its exceptional wear and impact resistance, self-lubrication, low water absorption, chemical corrosion resistance, and electrical insulation properties [6].However, UHMWPE's large molecular weight, extensive molecular chains, and numerous entanglement points [7][8][9] contribute to its significantly high melt viscosity [10,11].HDPE, another crucial material in mulching applications, is known for its good heat and cold resistance, chemical stability, and excellent rigidity and toughness.Yet, it falls short in aspects such as aging resistance and environmental stress cracking resistance.The incorporation of UHMWPE long-chain molecules into HDPE can effectively ameliorate these drawbacks [12,13], facilitating the development of agricultural film materials that are both high in strength and easily recoverable.
Annealing treatment plays a pivotal role in altering the structural properties of polymers.It serves to alleviate internal stresses incurred during processing, heighten crystallinity, and enhance crystallization, thereby improving the polymer's mechanical properties [14].Research by Medel has demonstrated that the annealing process can refine the morphology and structure of PE, thus bolstering its tensile strength and fracture toughness [15].Zhu's study on the effect of annealing temperature on polymer structure reveals that below 110°C, the crystalline morphology of PE remains largely unchanged, while above 138°C, the crystal thickness experiences a significant increase [16].Gaining insights into the impact of annealing mechanisms on the structure and properties of polymers is of considerable importance for polymer processing.In this study, HDPE and UHMWPE with varying concentrations were meltblended to examine the influence of annealing treatment on their rheological and morphological properties.The scaling relationship between these parameters and the concentration was explored using scaling theory.
2 Materials and methods

Materials and instruments
Main raw materials: HDPE, grade 8008; melt flow rate: 15 g per 10 min, sourced from China Petroleum and Chemical Corporation.UHMWPE, molecular weight (M w ) = 3 million, obtained from Beijing No. 2 Auxiliary Plant.The virgin resins without pretreatment were used.Antioxidant 1010 [17] procured from Guangdong Dongguan Huichen Chemical Co., Ltd, China.
Tensile testing: The UTM6502 Electronic Universal Testing Machine, provided by Sansi Zongheng Technology (referred to as the testing machine henceforth), is employed to assess the mechanical properties of all mulching films, focusing on longitudinal tensile strength and transverse tensile strength.The tensile test for films conforms to the National Standard GB/ T1040., "Determination of Tensile Properties of Plastics -Part 3: Test Conditions for Films and Sheets," with a tensile rate set at 50 mm•min −1 .

Experimental process 2.2.1 Preparation of blends
HDPE/UHMWPE blends were prepared by melt blending method.Weigh 15 g of evenly stirred HDPE/UHMWPE mixture, add antioxidant 1010 to prevent material oxidation [17], then add it to the torque rheometer with a volume of 20 mL, adjust the temperature of zones I, II, and III to 160°C, rotate at 60 rpm, and take out the material after internal mixing for 5 min.Refer Table 1 for the specific proportion of blends.

Rheological sample preparation
The prepared blend sample was put into a disc mold with a diameter of 200 mm and a thickness of 1.5 mm, and was formed by using a flat curing press, controlling the temperature of the upper and lower templates at 160℃, pressure at 20 MPa, and melting molding for 5 min.

Annealing treatment
The blend samples were put into an oven with nitrogen, and the oven was controlled at 160℃ for 5 h for thermal annealing.After annealing, they were cooled in a rheological mold for shaping.

Testing and characterization 2.3.1 Characterization of morphology and structure
The samples were brittle broken in liquid nitrogen, and the cross-section samples were dried and sprayed with gold in vacuum.The cross-section morphology was observed by scanning electron microscope.

Characterization of rheological properties
Advanced rotary rheometer, parallel plate mode, parallel plate diameter 20 mm, test temperature 160℃, dynamic frequency scanning, fixed strain value 1.0%, scanning frequency range 10 2 -10 −2 rad•s −1 .The whole test process is protected by nitrogen to prevent sample aging.

Exposure test
The mulching film exposure test is an experimental method designed to assess the durability and performance of mulching films under sunlight exposure.Samples of mulching films listed in Tables 1 and Table 2 are measured for their weight-average molecular weight (M w ) and number-average molecular weight (M n ) both before exposure and after 60 days of exposure.The method for calculating the polydispersity index (PDI) is as follows: Here, M w refers to the weight-average molecular weight and M n denotes the number-average molecular weight.
All data points depicted in the figures represent the average values derived from systematic experimental observations.Double-logarithmic plots can be used to identify how properties such as G′, G″, and |η*| change with UHMWPE concentration in HDPE/UHMWPE composites.These figures have been created using the ggplot2 package in R.

Morphological characterization of HDPE/
UHMWPE blends pre-and post-annealing HDPE and UHMWPE, sharing identical basic units, exhibit excellent compatibility, resulting in a homogeneous blend system.Figure 1(a)-(d) depicts the cross-sectional morphology of UHMWPE/HDPE blends with varying UHMWPE mass fractions of 0%, 0.5%, 1%, and 5%, respectively.Observation from the micrographs reveals that the incorporation of UHMWPE into the matrix induces a distinct filamentous structure within the cross-section of the blend.In contrast, pure HDPE samples devoid of UHMWPE exhibit only a folded block distribution, lacking any filamentary features.
As the concentration of UHMWPE increases, the filamentous structure becomes increasingly pronounced.The fundamental particle morphology of commercially synthesized UHMWPE is often described as a highly entangled spherical configuration [18].The emergence of the filamentous structure aligns with the formation of a "shish-kebab" microstructure [19][20][21], attributed to the elongation of UHMWPE molecular coils, resonating with the filamentous appearance depicted in the figures.Studies by Hsiao et al. have also demonstrated that blending UHMWPE with low molecular weight PE facilitates the orientation of long-chain, highly entangled molecules to form a "shish-kebab" microstructure [13,[22][23][24].
The annealing process facilitates the mobility of molecular segments, promoting a more thorough fusion within the blend.Figure 2(a)-(d) illustrates the cross-sectional morphology of the blend system, pre-and post-annealing, with UHMWPE concentrations of 1%, 1% (post-annealing), 5%, and 5% (post-annealing), respectively.It is distinctly evident from the images that the filamentous structure diminishes following annealing.During this process, the osmotic pressure exerted by the low molecular weight HDPE enables the long chains of UHMWPE molecules to diffuse within the HDPE melt.This diffusion mitigates the high degree of entanglement typically observed in UHMWPE primary particles.Consequently, UHMWPE molecular coils and HDPE molecular chains achieve a more homogeneous state, impeding the tensile transformation of UHMWPE molecules that is necessary for the formation of the filamentous entanglement structure.

Rheological characterization of HDPE/
UHMWPE blends pre-and post-annealing there is a slight increase in the modulus and viscosity of each component.The pure HDPE sample also demonstrates a modest enhancement in mechanical properties following annealing, owing to the more orderly arrangement of HDPE molecular segments and improved crystallinity, resulting in enhanced mechanical performance [25].
When correlated with the morphological characterization results, the annealing process appears to facilitate a more regular arrangement of HDPE molecules, a more comprehensive intermingling of UHMWPE molecular chains with HDPE molecular chains, and a tendency toward a more uniform and complete network structure within the system, thereby improving its mechanical properties.
Figure 6 illustrates the loss tangent (tan δ = G″/G′) of HDPE/UHMWPE blends with different UHMWPE concentrations, both pre-and post-annealing, against scanning  frequency.At the rheological working temperature of 160°C, the pure HDPE sample is in a viscous flow state, with G″ > G′, leading to a high tan δ value.Annealing of the pure samples results in increased crystallinity of HDPE post-cooling and more regular molecular arrangements, thus increasing G′ and slightly decreasing the tan δ value.UHMWPE primary particles, characterized by their high entanglement, behave as rigid entities with G′ significantly exceeding G″, resulting in a very small tan δ value.As the blend is increasingly supplemented with UHMWPE particles, the system's tan δ value progressively decreases.However, post-annealing, the tan δ value conversely increases due to the annealing process causing UHMWPE molecular chains to extend and their degree of entanglement to decrease, leading to a reduction in UHMWPE particles' G′ and a more pronounced increase in tan δ with rising UHMWPE content.The state change of UHMWPE particles plays a crucial role in the mechanical property alterations observed in the blends.

Scaling relationships in HDPE/UHMWPE blends pre-and post-annealing
Scaling theory, a framework for studying the critical transition phenomena in matter, establishes the relationship between critical exponents and scaling.This theory, with its universal applicability, is significantly important for understanding the rheological behavior of polymers [26].Figures 7-9 illustrate the low-frequency (ω = 0.92 rad•s −1 ) logarithmic relationships of log G″, log G′, and log |η*| against log (C UHMWPE %) in a double-logarithmic plot to derive the scaling relationships of G′, G″, and |η*| with UHMWPE concentration.Notably, the mechanical properties of the blends exhibited only marginal improvement postannealing, thus the scaling relationships showed no substantial alteration before and after the annealing process.Double-logarithmic plots can be utilized to identify how properties such as G′, G″, and |η * | vary with UHMWPE concentration in HDPE/UHMWPE composites.Significant shifts in data point trends and abrupt changes in the slope of trend lines signal a transformation in material properties.As depicted in Figures 7-9, all three graphs demonstrate that the relationships of G′, G″, and |η*| undergo noticeable changes in slope both before and after the critical concentration, pre-and post-annealing.From these double- logarithmic plots, the corresponding scaling relationships can be deduced as follows: (1) before the critical point: G′ ∼ c 0.2 , G″ ∼ c 0.07 , |η*| ∼ c 0.07 and (2) after the critical point: G′ ∼ c 1.20 , G″ ∼ c 0.6 , |η*| ∼ c 0.64 .This scaling relationship indicates that the scaling values post-critical point are substantially higher than those pre-critical point.The variation in scaling relationships around the critical point is associated with the critical entanglement phenomenon of polymer chains [27,28].As the concentration of UHMWPE increases, the proportion of long UHMWPE molecular chains in the system rises, reaching the critical entanglement concentration (c e ) of UHMWPE.This leads to mutual entanglement among long chains, significantly increasing the modulus and viscosity of the blend.
The c e of UHMWPE experienced a shift pre-and postannealing, with c e being 2.3% before annealing and 2.1% after, advancing the transition point post-annealing.This is because the UHMWPE molecular chains, originally highly entangled, become more extended post-annealing.These elongated chains then form mutual entanglements between UHMWPE particles, enabling the establishment of an entanglement network at a lower concentration post-annealing and enhancing the mechanical properties of the blends.The scaling relationships of G′, G″, and |η*| with UHMWPE concentration and the alteration in the critical entanglement concentration are profoundly significant for further exploring the motion of UHMWPE molecular chains during the annealing process.

Exposure testing for molecular weight alterations in HDPE/UHMWPE blends
Initially, the HDPE/UHMWPE blends showed uniform molecular weights (M w , M n ) and PDI.Post-exposure, significant changes were observed: both weight-average (M w ) and number-average (M n ) molecular weights increased with higher UHMWPE content, while PDI decreased.This indicates enhanced molecular stability and more uniform weight distribution in blends with increased UHMWPE, suggesting improved environmental resilience and consistent performance.These results highlight the significant impact of real-world conditions on the molecular integrity of polymer composites.

Discussion
The exploration of HDPE/UHMWPE blends offers compelling insights into the impact of UHMWPE concentration and annealing on the material's characteristics.Notably, the observed morphological transformations, particularly the development and eventual diminishment of filamentous structures upon annealing, resonate with existing literature findings.This study, delving into the synthesized HDPE/ UHMWPE composite, emphasizes the effects of annealing, alongside the morphological and rheological properties, and evaluates the environmental stress cracking resistance.Yin et al. [29] underscores the morphological evolution of UHMWPE from primary to swollen particles, which significantly influences HDPE's crystallization.This aligns with our observations of microstructural changes postannealing, where an increased UHMWPE concentration led to notable morphological alterations, as verified by SEM imaging.This transformation suggests that annealing  [30] examination of the influence of HDPE's melting flow rate (MFR) on the UHMWPE/HDPE blend's attributes complements our rheological analysis.The finding that neither excessively high nor low MFR of HDPE is beneficial for the blend's properties aligns with our results.Enhancements in mechanical properties were observed upon optimizing UHMWPE concentration and conducting post-annealing treatment.The significance of chain entanglement and its effect on blend properties is pertinent.Our study's scaling laws, particularly for storage and loss modulus post-annealing, suggest that annealing alters UHMWPE's molecular interactions within HDPE, leading to modifications in mechanical properties.
In comparison with Chen et al.'s findings [31], the superior stress cracking resistance of the LDPE/UHMWPE blend relative to the HDPE/UHMWPE blend offers insights into the varying impacts of polymer matrices on composite properties.The established correlation between morphological characteristics and blend composition aligns with our study.Our post-annealing morphological analysis and the consequent improvements in mechanical properties demonstrate a similar correlation, where the annealing-induced altered microstructure plays a pivotal role in enhancing the composite's mechanical performance.
Annealing emerges as a crucial factor in modulating these properties, heralding potential advancements in the development of high-strength, sustainable HDPE films for diverse applications, including eco-friendly agricultural practices.This integrated approach provides a comprehensive perspective on the material's behavior, paving the way for further research and application in the realm of sustainable material development.Additionally, they highlighted that blends with better-distributed molecular weights exhibit enhanced resistance to thermal and UV degradation, a critical aspect for applications in challenging environmental conditions.

Conclusions
The HDPE/UHMWPE blend, prepared via melt blending, exhibited a filamentous structure in its cross-section, becoming more pronounced with increased UHMWPE content.Post-annealing, this structure dissipated, resulting in a more uniform blend, thereby enhancing the system's mechanical properties.Rheological analysis revealed that UHMWPE addition improved the blend's mechanical characteristics.Annealing led to a more cohesive network structure within the system, aligning mechanical properties with morphological changes.The scaling relationship analysis of rheological data revealed that before the critical entanglement concentration: G′ ∼ c 0.2 , g″ ∼ c 0.07 , |η*|∼ c 0.07 and post-critical concentration: G′ ∼ c 1.20 , g″ ∼ c 0.6 |η*|∼ c 0.64 .Post-annealing, the system's critical entanglement concentration decreased, suggesting that UHMWPE forms a filamentous entanglement structures at a lower concentration, thus enhancing the blends' mechanical properties.This scaling relation and the shift in critical concentration are crucial for understanding UHMWPE chain mobility in the melt.The samples displayed consistent M w , M n , and PDI values.Following exposure testing, changes in these parameters were observed, with both weight-average and number-average molecular weights increasing and PDI decreasing with higher UHMWPE content.This trend highlights the blend's enhanced molecular stability and uniformity, crucial for creating plastic films that are both strong and sustainable, suitable for eco-friendly agricultural applications.

Figures 3 -Figure 1 :
Figures 3-5 display the storage modulus, loss modulus, and complex viscosity of HDPE/UHMWPE blends with varying UHMWPE concentrations, both before and after annealing, across different scanning frequencies.As depicted in these figures, the storage modulus (G′), loss modulus (G″), and complex viscosity (|η*|) exhibit a marked growth trend with the increase in UHMWPE concentration.This is attributable to the long molecular chains of UHMWPE, which impart a reinforcing effect on the system.Post-annealing,

Table 1 :
Composition ratios of the HDPE/UHMWPE blend system

Table 2 :
Changes in molecular weight of HDPE/UHMWPE blends after field exposure