Porous epoxy phenolic novolac resin polymer microcapsules: Tunable release and bioactivity controlled by epoxy value

Porous epoxy phenolic novolac resin polymer microcapsules: Tunable release and bioactivity controlled by epoxy value

Accepted Manuscript Title: Porous Epoxy Phenolic Novolac Resin Polymer Microcapsules: Tunable Release and Bioactivity Controlled by Epoxy Value Author...

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Accepted Manuscript Title: Porous Epoxy Phenolic Novolac Resin Polymer Microcapsules: Tunable Release and Bioactivity Controlled by Epoxy Value Authors: Xian-peng Zhang, Jian Luo, Tong-fang Jing, Da-xia Zhang, Bei-xing Li, Feng Liu PII: DOI: Reference:

S0927-7765(18)30101-2 https://doi.org/10.1016/j.colsurfb.2018.02.026 COLSUB 9166

To appear in:

Colloids and Surfaces B: Biointerfaces

Received date: Revised date: Accepted date:

25-10-2017 6-1-2018 11-2-2018

Please cite this article as: Xian-peng Zhang, Jian Luo, Tong-fang Jing, Da-xia Zhang, Bei-xing Li, Feng Liu, Porous Epoxy Phenolic Novolac Resin Polymer Microcapsules: Tunable Release and Bioactivity Controlled by Epoxy Value, Colloids and Surfaces B: Biointerfaces https://doi.org/10.1016/j.colsurfb.2018.02.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Porous Epoxy Phenolic Novolac Resin Polymer Microcapsules: Tunable Release and Bioactivity Controlled by Epoxy Value Xian-peng Zhang,a,b,1 Jian Luo,a,b,1 Tong-fang Jing,a,b Da-xia Zhang,a,c Bei-xing Li,a,b

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Feng Liua,b * Key Laboratory of Pesticide Toxicology & Application Technique, Shandong

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Agricultural University, Tai’an, Shandong 271018, P. R. China

College of Plant Protection, Shandong Agricultural University, Tai’an, Shandong

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271018, P. R. China c

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University, Tai’an, Shandong 271018, China

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Research Center of Pesticide Environmental Toxicology, Shandong Agricultural

X.Z. and J.L. contributed equally to this work.

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Corresponding author.

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E-mail address: [email protected] (F. Liu)

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Graphical Abstract

Highlights 

Porous EPN-MCs were fabricated by interfacial polymerization.



Facile tunable release and bioactivity of the MCs were controlled by epoxy value.



The porosity on MC surface decreased with an increasing of epoxy value.



The cured shells had larger degrees of crosslinking with higher epoxy values.



EPN-MCs prepared with higher epoxy values exhibited better thermal stability.

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Abstract

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Microcapsules (MCs) prepared with diverse wall material structures may exhibit

different properties. In this study, MCs were fabricated with three kinds of epoxy phenolic novolac resins (EPNs), which possessed unique epoxy values as wall-forming

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materials by interfacial polymerization. The effects of the EPN types on the surface

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morphology, particle size distribution, encapsulation efficiency, thermal stability as

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well as release behavior and bioactivity of the MCs were investigated. In all three samples, the MCs had nearly spherical shapes with fine monodispersities and sizes in

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the range of 7-30 μm. Scanning electron microscopy (SEM) images showed that some small pores (ranging from 50 nm to 400 nm) appeared on the microcapsule surfaces

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and that the porosity decreased with an increasing of epoxy value. The X-ray

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diffractometer (XRD) analysis indicated that the cured EPN shells had larger degrees of crosslinking with higher epoxy values, leading to better thermal stabilities.

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Moreover, the release rate of the core material (pendimethalin) decreased with an increasing of epoxy value and thus resulted in a lower herbicidal control efficacy. The results of our research will enhance the potential application of EPNs as smart wallforming materials to prepare porous MCs for controlled release.

Keywords: Porous microcapsules; Epoxy phenolic novolac resin; Wall material structure; Tunable release behavior; Bioactivity

1. Introduction

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Due to typical core–shell structures, microcapsules (MCs) have attracted increasing attention for the practical applications in many fields, such as biotechnology,

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pharmaceuticals, food industries, agriculture, phase change materials, and chemical

materials [1-7]. A wide variety of materials have been used as capsule shells, including polymers, inorganic solids, and natural products [8-10]. Many physical or chemical

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strategies have also been utilized to fabricate various functionalized spherical MCs,

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among which interfacial polymerization is the most common used because of its

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simplicity, high encapsulation ability, fast reaction speed and easy industrialization

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[11,12].

Moreover, when MCs are applied in specific applications, controllable release behavior of the core material between the external environment and the MCs would be

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highly desired. More importantly, this transfer property is considered to be greatly

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affected by the microstructure of the MC wall materials (including the surface morphology, physical and chemical properties, porosity, and thickness). For instance,

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some previous studies have reported that MCs fabricated with different shell materials, crosslinking agents, or soft segments have obvious differences in morphology, encapsulation efficiency, thermal stability, release behavior, and other properties [1316]. Therefore, it is crucial to investigate the structures of wall materials in depth.

Particularly, the relationship between the capsule wall structure and the versatile function and application performance of MCs is urgently essential to explored. In recent years, epoxy resins have drawn increasing attention in many fields because of their superior chemical resistance, excellent mechanical and thermal properties, and

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low price [17,18]. Epoxy resins are very reactive, which allows for the use of a wide

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variety of curing agents, such as imidazoles, amines, anhydrides, polyamides and so on [19-22]. The polymerization reaction of epoxy groups in epoxy resins occurs and then becomes highly crosslinked network structure [23,24]. Therefore, epoxy resins are

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promising candidates as wall-forming materials. Furthermore, there is limited literature

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relevant to MCs prepared with epoxy resin polymers as shells [20,21,25], and the

Pendimethalin

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effects of the epoxy values on epoxy resin MCs have not been reported. (N-(l-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine),

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dinitroaniline preemergence herbicide, was chosen as a model pesticide in this study due to its hydrophobic characteristic, which is similar to the active ingredients used in

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the agrochemicals field. Nevertheless, it is highly toxic to aquatic organisms and easily

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lost through volatilization and photodegradation [26,27]. Therefore, microencapsulated pendimethalin can improve its stability and decrease its toxicity to non-target

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organisms.

In this paper, we prepared pendimethalin MCs using interfacial polymerization, in

which three kinds of epoxy phenolic novolac resins (EPNs) with unique epoxy values and 2,4,6-tris(dimethylaminomethyl)phenol (DMP) were used as wall-forming

materials and a crosslinking agent, respectively. The effects of the epoxy value on the surface morphology, particle size, encapsulation efficiency, and thermal properties as well as release properties and bioactivity of the MCs were investigated. The results of our research will provide an in-depth investigation of EPNs in the microencapsulation

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field for controllable release.

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2. Materials and methods 2.1. Materials

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Pendimethalin (purity 98%) was supplied from Shandong Huayang Technology Co.

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(Shandong, China). Fig. 1 A shows the chemical structures of the pendimethalin. Three

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EPNs with different epoxy values used as wall-forming materials were purchased from

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Shanghai Resin Plant, China. The actual epoxy values (0.418, 0.473 and 0.536 mol per

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100 g resin, respectively) of used EPNs are listed in Table S1, and the method of determination of epoxy values is provided in the Supporting Information. DMP, used as a crosslinking agent, was purchased from Aladdin Reagent Co. (Shanghai, China).

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Fig. S1 (Supporting Information) shows the chemical structures of the EPN and DMP.

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Polyoxyethylene sorbitan monooleate (Tween-80) used as an emulsifier was obtained from Tianjin Chemical Regents Factory (Tianjin, China). All other reagents were of

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analytical grade and used without further purification. Deionized water was used in all experiments.

2.2. Preparation of the EPN-MCs

Three kinds of MCs (samples MC1, MC2, and MC3) were prepared using an interfacial polymerization method, and the whole procedure was outlined in Fig. 1 B. First, the oil phase, including one of the three EPNs, pendimethalin, and xylene, was mixed with an aqueous solution containing Tween-80 in a three-neck round-bottomed

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flask equipped with a water thermostat bath and mechanism stirring. Then, the above mixture was carried out at 10000 rpm for 120 s with a homomixer at room temperature

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to form an oil-in-water emulsion. Subsequently, the DMP solution (5%, w/w) was

added into the above emulsion dropwise at a stirring rate of 400 rpm. Finally, the

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polymerization reaction proceeded at 70 °C for 5 h to form a cured shell. Fig. S2

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(Supporting Information) shows the polymerization mechanism of the EPN and DMP.

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After cooled, the MC suspensions were obtained and used for the measurement of

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encapsulation efficiency and particle size. Additionally, the MC suspensions were also

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washed three times with deionized water and then dried in a vacuum oven at 40 °C for 72 h for further characterization.

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2.3. Characterization of the EPN-MCs

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The surface morphology of the obtained MCs was investigated using optical microscopy (OM, OLYMPUS CX41-32RFL; Tokyo, Japan) and scanning electron

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microscopy (SEM, S-4800, Hitachi, Tokyo, Japan). The particle size and size distribution of the prepared MCs were measured with a

laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., Malvern, UK).

The chemical structures of the MCs and their components were analyzed using an FTIR spectrometer (Nicolet Instrument, Madison, Wisconsin). The crystallinities of the cured capsule shells were analyzed using X-ray diffractometer (XRD, Bruker, Discover 8 advance) operating at 40 kV and 40 mA with

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Cu Kα radiation.

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The thermal properties of the three MC samples were analyzed by thermogravimetric

analysis (TGA, SDT Q600, TA, USA). The samples of approximately 5 mg each were heated at a heating rate of 10 °C/min under a nitrogen atmosphere (50 mL/min) from

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25 °C to 700 °C.

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2.4. Determination of the encapsulation efficiency

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The encapsulation efficiency of the MCs was tested following a method modified

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from MT 189, CIPAC [28]. Accurately weighed 0.5 g of the MC suspensions, which contained uniform pendimethalin (M0), was placed into a glass bottle (100 mL). Then, deionized water (10 mL) and hexane (50 mL) were added to the bottle. The bottle was

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placed on a roller immediately and rolled horizontally at 60 ± 10 rpm. After 15 min

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(±10 s), the bottle was taken out from the roller, and the upper hexane layer was immediately extracted for measurement, which contained all the unencapsulated

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pendimethalin (Mt). The amount of pendimethalin was monitored using a highperformance liquid chromatography (HPLC) system (Agilent 1200; Agilent Technologies; Santa Clara, CA) equipped with an ultraviolet detector. The chromatographic separation was conducted by an Agilent Diamonsil C18 column (250

mm × 4.6 mm; i.d., 5 µm). The mobile phase was composed of methanol and water (90:10, v/v) at a flow rate of 1 mL/min, and the UV detector was set at a wavelength of 238 nm. The injection volume was 20 µL, and the column temperature was room temperature. The amount of M0 was obtained from the standard curve of pendimethalin

Encapsulation efficiency (%) = (M0 − Mt) / M0 × 100

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encapsulation efficiency was calculated using the following equation:

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and the detailed procedure is provided in the Supporting Information. The

where M0 is the total amount of pendimethalin in the 0.5 g MC suspensions, and Mt is

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2.5. Release properties of the EPN-MCs

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the amount of pendimethalin in the upper hexane layer.

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The release properties of the three obtained MCs were examined by adapting a previously reported method [29]. The dried MC samples were accurately weighed (0.5

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g) and then transferred to a 250 mL three-neck round-bottomed flask equipped with a stirrer. Next, a hexane/methanol mixture (170:10, v/v) was added and used as a release

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medium and stirred at a rate of 400 rpm at 30 °C. Subsequently, 0.5 mL of liquid was

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removed at various time intervals and added to the same volume of release medium immediately. The amount of pendimethalin (Ct) was measured, and the concentration

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of pendimethalin in the 0.5 g dried MC sample (C0) was also calculated using a HPLC system as described above in section 2.4. The cumulative release proportion was calculated using the following equation:

Cumulative release proportion (%) = (Ct / C0) × 100 where Ct and C0 are the pendimethalin concentration of the release medium at time of t and in the 0.5 g dried MC sample, respectively.

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2.6. Herbicidal activity assays of the EPN-MCs The herbicidal activities of pendimethalin EPN-MCs against Echinochloacruss-galli

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L. (BYG) and Amaranthus retroflexus L. (RPW) were conducted in a greenhouse pot

culture experiment. Forty weed seeds were planted in a plastic pot (15 cm diameter, 15 cm height) filled with a 1:5 (v/v) mixture of sand and loam soil with all necessary

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nutrients (pH = 7.4). The seeds were covered with 0.5 cm of sand/soil mixture, and each

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pot was watered through the bottom by infiltrating irrigation to ensure optimal soil

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moisture. In the weed control experiments, four test concentrations (300, 450, 675 and 1012 g a.i./ha) of the three MC samples and pendimethalin emulsifiable concentrate

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(EC) were applied to the surface of the mixed soil soon after sowing, and the same amount of deionized water was sprayed as a blank control. The application was made

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with compressed air by moving the nozzle cabinet sprayer equipped with one Teejet

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9503EVS flat fan nozzle and adjusted to deliver 450 L/ha. Then, the pots were placed inside a greenhouse and kept under the conditions of 25 ± 2 °C and 70 ± 2% humidity.

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The stem control efficacy and fresh weight reduction were studied after 21 days of herbicide treatment. The treatments were arranged in a complete randomized design and were repeated three times. The stem control efficacy was calculated using the following equation:

Stem control efficacy (%) = (N0 - N1) / N0 × 100 where N1 and N0 are the number of plants in the treatment and blank control, respectively.

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The fresh weight reduction was calculated using the following equation: Fresh weight reduction (%) = (R0 - R1) / R0 × 100

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where R1 and R0 are the total fresh weights of the plants in the treatment and blank control, respectively.

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3.1. Chemical structure of the EPN-MCs

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3. Results and Discussion

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The chemical structure characterization of the EPN-MCs was carried out using FTIR spectroscopy. The FTIR spectra of the three samples were similar. Fig. 2 shows the

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infrared spectrum of MC3 as a representative, and the other FTIR spectra of the EPNMCs are shown in Fig. S3 (Supporting Information). The specific absorption of the

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epoxy ring at 910 cm-1 disappeared after polymerization (Fig. 2A) [30], indicating that the epoxy groups in EPN reacted completely. The peaks at 1536 cm-1 and 1324 cm-1 are

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assigned to the stretching vibrations of the nitro groups, and the peaks at 1248 cm-1 and

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1187 cm-1 are attributed to the C─N stretch of pendimethalin (Fig. 2C) [31]. In addition, the characteristic peaks of pendimethalin are clearly seen in the MCs with core material, and no new bands are observed (Fig. 2B), indicating that pendimethalin was successfully encapsulated by the EPN-based shell and no reaction between pendimethalin and EPN occurred.

3.2. XRD analyses of the three cured EPN shells XRD patterns of the three cured EPN shells are shown in Fig. 3. The broad diffraction peaks are displayed in the 2θ range of 11-28°, indicating that all cured systems were amorphous. However, the difference in the center of the broad peaks indicates that the

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degree of crystallinity was different in the three cured polymers; the peak of the MC3

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sample was sharper compared to those of MC2 and MC1. Calculated from the location

of the peaks, the d-space in the MC1, MC2 and MC3 cured systems were 6.71, 5.73 and 4.67 Å, respectively, suggesting that the MC3 cured system was more densely

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compacted.

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The actual epoxy values of used EPNs are listed in Table S1 (Supporting

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Information). MC1 has the lowest epoxy value (approximately 0.418 mol per 100 g of resin), while those of MC2 and MC3 are higher (approximately 0.473 and 0.536 mol

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per 100 g of resin, respectively). Therefore, the relative content of epoxy groups varied, while the EPN amount was fixed in this study. When the polymerization reaction

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between the epoxy groups and amine groups takes place at the interface [25], a larger

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concentration of epoxy groups may exhibit a higher degree of crosslinking. The change of crystallinity can explain the degree of crosslinking of the samples, which is regulated

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by the epoxy groups in the EPNs. Thus, the choice of an EPN with a larger epoxy value can form more compact cured EPN-polymers, which possess higher degrees of crosslinking.

3.3. Morphologies of the EPN-MCs

All three prepared MCs had nearly spherical shapes with good monodispersities (Fig. 4A-C). The SEM images (Fig. 4D-F) showed that the MCs had big dimples but smooth and compact surfaces, indicating that the EPN-based capsule shell was soft. Interestingly, some small pores were observed on the MC surfaces with diameters

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ranging from 50 to 400 nm, and the porosity decreased with an increasing of epoxy value. In the literature, Xie et al. fabricated porous MCs templated by nonionic

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surfactant micelles above the cloud point and proved the pore forming mechanism of temperature-sensitive templates [32]. Similarly, DMP, a temperature-responsive

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reversible material, which is easily dissolved in cold water and insoluble in water above

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its so-called cloud point, was employed as a crosslinking agent in this study. Therefore,

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the formation of small pores on the MC surface may be due to the reversible behavior

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of DMP. The detailed pore forming mechanism needs further examination.

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3.4. Particle size distributions and encapsulation efficiencies of the

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The particle size distributions and encapsulation efficiencies of the MCs prepared

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with the aforementioned three EPNs are shown in Fig. 5. The MC1 sample exhibits a relatively wider size distribution (MC1: 4.2-49.8 μm, MC2: 3.5-44.4 μm and MC3: 2.9-

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41.8 μm), and the particle size distributions become narrower with an increasing of epoxy value (Fig. 5A). Similarly, the entrapment rate increases slightly with an increase in the number of epoxy groups (Fig. 5B). Moreover, the encapsulation efficiencies of all three obtained MC samples were above 91%, indicating that the EPN-MCs contained pendimethalin in the core and that the wall-forming material (EPN) used in

this study is feasible and practical for encapsulating oily core materials. The mean diameters of three MCs are slightly decreased (ranging from 19.7 to 17.6 μm), suggesting that the number of epoxy groups has no considerable influence on the MC size.

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3.5. Thermal stabilities of the EPN-MCs

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The thermal stabilities of the obtained MCs were evaluated using thermogravimetric

analysis. As shown in Fig. 6, the weight loss below 120 °C was mainly attributed to the evaporation of adsorbed water. Furthermore, the MCs experienced two-step main

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decomposition profiles. The first-step weight loss of the MCs occurred between 130

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and 160 °C, corresponding to the escape of encapsulated xylene (boiling point: 137–

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142 °C) from the MCs. The second decomposition step at 180-350 °C is attributed to the degradation of pendimethalin and part of the epoxy polymer shell. Comparably, the

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rapid weight loss onset temperatures of the three MC samples (in the order of MC1, MC2, and MC3) were 200, 220, and 225 °C, respectively, indicating that the thermal

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stability of MC3 was better than those of MC2 and MC1. Along with the XRD results,

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it is concluded that the EPN shell with a larger number of epoxy groups has better thermal properties due to its highly crosslinked molecular structure, and it can protect

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the core quite well. In addition, the three MCs decomposed substantially above 350 °C with approximately 12% of the original mass remaining at 700 °C, indicating that the EPN-based polymeric shell has excellent thermal stability.

3.6. Controlled release behaviors of the EPN-MCs

Fig. 7 shows the release characteristics of the three MC samples in hexane-methanol mixed release medium. The three samples have similar release models, and the release curves show three release stages. First, the release rate of the core material was slow in the first hour, suggesting that the MC wall had superior chemical resistance. Second,

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the cumulative release of the core material was higher than 80% during the fast release stage. Subsequently, the release rate increases slowly in the slow-release stage. Among

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the three MC samples, the release rate of MC1 was the fastest, followed by MC2 and

then MC3. As previously described, MCs have integrated and compact shell structures

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(Fig. 4), and the transfer of the core material mostly occurs through permeation into the

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outer environment. Therefore, the structure of the capsule wall is the critical factor that

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influences the release of the core material. In this study, the crosslinking degree and

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porosity of the capsule shell were the main factors that determined the transfer rate of

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the core material for similar MC sizes (Fig. 5B). Based on the above analysis, the capsule shell of MC3 was much more compact and had lower porosity compared to

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MC2 and MC1, leading to more difficult release of the core by osmosis. A similar result was obtained by Liang and co-workers, where the degradation profile of polymeric

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capsules was controlled by varying the degree of crosslinking in the capsules, which further influenced the release properties [33]. Liu et al. also reported that

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chlorantraniliprole-loaded MCs with a high porosity could achieve faster release rate [34]. Thus, it can be concluded that the release rate of the loaded cargo from the EPN wall can be controlled by varying the degree of crosslinking or the porosity of the EPN.

3.7. Biological activities of the EPN-MCs

Fig. 8 shows the herbicidal activities of pendimethalin MC and EC samples against BYG and RPG in concentrations ranging from 300 to 1012 g a.i./ha. The control efficacies of the MCs were relatively low compared to that of pendimethalin EC sample under the same concentration, which is due to the barrier of the capsule shells. The fresh

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weight reduction of BYG and RPG reached 90% in four samples with 1012 g/ha. Among three MC samples, the stem control efficacy and fresh weight reduction of MC1

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sample were the highest with the same dosage, and which decreased with an increasing

of epoxy value in the EPN. It is obvious that the herbicidal activity results corresponded

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to the TGA analysis and release properties of the MCs mentioned above. Briefly, a

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faster release rate of the core from the MCs can lead to higher bioactivity.

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Currently, regulating different responses in the release of MCs to meet various applications is a research hotspot. Some attempts have been made to achieve this goal.

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For instance, modifying the structure of wall-forming materials has been effective to trigger the core release in specific environments, such as pH-, UV- or temperature-

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induced stimuli [35-37]. Simultaneously, some studies have reported controlled release properties by regulating the size and porosity of the MCs [34,38]. Epoxy resins are

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potential wall-forming materials used to fabricate MCs with porous walls, although few

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have been reported. In this study, we found that the epoxy value can influence the structure of the EPN shell (including the morphology, porosity and crosslinking degree) and further regulate the application performance (release behavior and bioactivity) of the encapsulated cargo, which can act as a useful reference. Nevertheless, the influences of the DMP content on the porosity and the MC size on release as well as the feasibility

of the loaded EPN-MCs in the field of agriculture or other potential applications in similar fields, such as drug/vaccine delivery, need to be further examined.

4. Conclusions

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In summary, we prepared three EPN-MCs for encapsulating pendimethalin via an interfacial polymerization method. All three MCs had spherical profiles in the sizes of

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7-30 μm with good monodispersities, and encapsulation efficiencies higher than 91%. The porous MCs with tunable surface porosities were obtained by varying the epoxy

value in the EPN. In addition, the chemical structure and thermal property analyses of

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the three MC samples suggested that an increase in the number of epoxy groups can

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lead to a higher crosslinking density of the capsule wall and further influence the release

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characteristics and bioactivities of the encapsulated core. In conclusion, the EPN-MCs with porous surfaces can achieve various release responses via regulating the epoxy

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value, and thus can fit different applications. The results of this research provide a valuable reference, and we envision that the EPNs will be promising candidates as wall-

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forming materials to fabricate porous MCs in specific applications for controllable

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release and selective permeation.

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Acknowledgements This work was supported by grants from the National Key Research Development

Program of China (2017YFD0200300) and National Natural Science Foundation of China (31772203).

Appendix A. Supplementary data

Figs. S1 and S2 display the chemical structures of EPN and DMP and their Polymerization mechanism, respectively. Fig. S3 shows additional FTIR spectra of MC1 and MC2. The determination methods of epoxy value and M0, and the additional data of the actual epoxy value were also included.

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Fig. 1. (A) Chemical structure of pendimethalin; (B) Flowchart of the preparation

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method of the EPN-MCs.

Fig. 2. FTIR spectra of (A) the MCs without core material; (B) the MCs with core material; (C) pendimethalin.

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Fig. 3. XRD patterns of the cured EPN-polymers: (A) MC1, (B) MC2 and (C) MC3.

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Fig. 4. Optical microscopy and SEM images of the MCs prepared with different EPNs: (A, D) MC1, (B, E) MC2 and (C, F) MC3.

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Fig. 5. (A) Particle size distributions and (B) encapsulation efficiencies and mean

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diameter of the MCs prepared with different EPNs.

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Fig. 6. TGA curves of the MCs prepared with different EPNs.

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Fig. 7. Release properties of the MCs prepared with different EPNs.

Fig. 8. Herbicidal activities of the EPN-MCs against (A, B) BYG and (C, D) RPG.