Physicochemical Characteristics and Slow Release Performances of

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Physicochemical Characteristics and Slow Release Performances of Chlorpyrifos Encapsuled by Poly(butyl acrylate-costyrene) with the Cross-linker Ethyleneglycol Dimethacrylate Yu Wang, Zideng Gao, Yang Li, Sainan Zhang, Xueqin Ren, and Shuwen HU J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b01378 • Publication Date (Web): 06 May 2015 Downloaded from http://pubs.acs.org on May 12, 2015

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Journal of Agricultural and Food Chemistry

1

Physicochemical Characteristics and Slow release Performances of

2

Chlorpyrifos Encapsulated by Poly(butyl acrylate-co-styrene) with the

3

Cross-linker Ethyleneglycol Dimethacrylate

4

Yu Wang, Zideng Gao, Yang Li, Sainan Zhang, Xueqin Ren*, Shuwen Hu*

5

(College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China)

6 7

Abstract

8

Chlorpyrifos’ application and delivery to the target substrate needs to be

9

controlled to improve its use. Herein, poly(butyl acrylate-co-styrene) (poly(BA/St))

10

and poly(BA/St/ethylene glycol dimethacrylate (EGDMA)) microcapsules loaded

11

with chlorpyrifos as a slow release formulation were prepared by emulsion

12

polymerization. The effects of structural characteristics on the chlorpyrifos

13

microcapsules particle size, entrapment rate (ER), pesticide loading (PL), and release

14

behaviors in ethyl alcohol were investigated. FT-IR and TGA analysis confirmed the

15

successful entrapment of chlorpyrifos. The ER and PL varied with the BA/St monomer ratio,

16

chlorpyrifos/monomer core-to-shell ratio and EGDMA cross-linker content with consequence that

17

suitable PL was estimated to be smaller than 3.09% and the highest ER observed as 96.74%. The

18

microcapsules particle size (88.36–101.8 nm) remained mostly constant. The extent of

19

sustainable release decreased with increasing contents of BA, St, or chlorpyrifos in oil

20

phase. Specifically, adequate degree of cross-linking with EGMDA (0.5–2.5%)

21

increased the extent of sustainable release considerably. However, higher levels of

22

cross-linking with EGDMA (5–10%) reduced the extent of sustainable release. 1

ACS Paragon Plus Environment


23

Chlorpyrifos release from specific microcapsules (monomer ratio 1:2 with 0.5% EGDMA or 5 g

24

chlopyrifos ) tended to be a diffusion-controlled process, while others the kinetics probably

25

indicated the initial rupture release.

26

Keywords

27

Microcapsule, slow release, chlorpyrifos, poly(butyl acrylate-co-styrene),

28

ethyleneglycol dimethacrylate, emulsion polymerization.

29

2


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Introduction

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Chlorpyrifos (O,O-diethyl O-(3,5,6,-trichloro-2-pyridyl phosphorothioate)) is a

32

broad-spectrum, chlorinated organophosphate insecticide, acaricide and nematicide 1.

33

It is used worldwide as a moderately toxic organophosphate pesticide against diverse

34

chewing and sucking mouthparts pests on rice, wheat, cotton, fruit, vegetables, and

35

tea trees through the triple effects of stomach action, contact poison, and fumigation.

36

Because of its high volatilization and widespread use, chlorpyrifos represents one of

37

the most significant sources of organophosphate exposure during application,

38

resulting in adverse effects such as pollution of surface/underground water and

39

biological systems. Additionally, its high toxicity to humans can result in severe

40

illnesses, such as prostrate cancer 2, and respiratory problems 3. In China, the high

41

volatilization and easy spreading of chlorpyrifos in the soil and water lead to low

42

utilization efficiencies. As a result, chlorpyrifos needs to be repeatedly applied for

43

effective performance, and that consequently increases the level of adverse health

44

risks to both the environment and humans. Thus, control over the application of

45

chlorpyrifos in the agricultural sector is necessary.

46

Microcapsule-controlled release technology has been widely used for the

47

delivery of target molecules over the past decades. Microencapsulation offers

48

advantages such as target molecule protection, controlled and triggered release, and

49

the development of new product features 4. It has been recognized as a new

50

technology and promising solution in the fields of pharmaceuticals, biotechnology,

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pesticides, environmental engineering, cosmetics, coatings, and food chemistry 5. 3



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Additionally, microcapsule technology has bright prospects in the agriculture sector to

53

prepare controlled release formulation systems for pesticides. It is an effective

54

approach to efficiently increase the performance level of pesticides, and reduce

55

human labor and hazard risks to the environment and humans.

56

In recent years, many papers have reported the preparation of pesticide-based

57

microcapsules. Numerous methods were investigated toward the encapsulation of

58

pesticides namely, interfacial polymerization

59

condensation

60

successfully achieved the preparation of wall-type material membranes and

61

chlorpyrifos-encapsulating matrices. The materials included synthetic polymers (e.g.,

62

polyamide and polyurethane) prepared by interfacial polymerization and natural

63

polymeric membranes (e.g., alginate, chitosan, starch, and cellulose) prepared by

64

condensation methods. However, studies typically focused on the properties of the

65

encapsulating units only rather than on the release profiles of the encapsulated target

66

pesticide 16-18.

67

11

and solvent evaporation

12-15

6-9

, in situ polymerization

10

, and

methods. Some of these methods

It has been reported that some wall-type materials such as urea formaldehyde, 19

68

prepared by in situ polymerization

, afford short sustainable release profiles,

69

whereas other materials, such as polylactic acid 20, 21 and mesoporous silica 22, afford

70

longer sustainable release profiles. In contrast, emulsion polymerization affords the

71

synthesis of microcapsules with a relatively wide range of sustainable release profiles

72

for the delivery of active ingredients by varying the contents and ratios of typical

73

monomers including butadiene, styrene, acrylonitrile, acrylate ester, and methacrylate 4


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23-26

74

ester

. Furthermore, the preparation process of emulsion polymerization for

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chlorpyrifos won’t involve any organic solvent, such as toluene, xylene and

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cyclohexanone etc., which might be considered more environmentally friendly. And

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the chlorpyrifos-loaded microcapsules emulsion can be directly sprayed onto plants

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following synthesis, thereby not requiring the dispersion of the pesticide

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microparticles in water before application. The chlorpyrifos-loaded microcapsules

80

prepared by emulsion polymerization have been discussed in several patents, and in

81

several research papers

82

characteristics of wall materials on the sustainable release of chlorpyrifos

83

encapsulated in microcapsules are rare.

27-30

. However, reports on the effects of the structural

84

In the present study, butyl acrylate (BA), styrene (St), and cross-linker ethylene

85

glycol dimethacrylate (EGDMA) were used to synthesize microcapsules by emulsion

86

polymerization. Two series of chlorpyrifos-loaded polyacrylate microencapsulating

87

systems were prepared i.e., microcapsules of chlorpyrifos-loaded poly(BA/St) and

88

chlorpyrifos-loaded poly(BA/St/EGDMA) (poly(BA/St) cross-linked with EGDMA).

89

The glass transition temperature (Tg) of poly(butyl acrylate) is low, and hence

90

features an elastomeric state (soft and sticky) at room temperature, whereas

91

polystyrene features a glassy state (hard and brittle) because of its higher Tg. Hence, it

92

is expected that copolymerization of BA and St whose Tg can be easily designed by

93

Fox equation may generate microcapsules with the required hardness and viscosity.

94

Furthermore, the addition of cross-linker EGDMA is expected to afford the synthesis

95

of a denser microcapsule wall material or better connections among the microcapsule 5



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96

particles. Thus, copolymerization between BA and St or between BA/St and EGDMA

97

would afford microcapsules with fine characteristics (hardness, viscosity, and dense)

98

to achieve long sustainable release properties as desired. More particularly, the

99

preparation conditions of the chlorpyrifos microcapsules i.e., the effects of monomer

100

ratio (BA/St), core-to-shell ratio, and content of cross-linker EGDMA in oil phase on

101

the characteristics of the microcapsules (particle size, entrapment rate, and pesticide

102

loading), especially, the sustained release performance were investigated.

103

Materials and methods

104

Materials

105

BA (chemical pure, National Medicine Group Chemical Reagent Co., Ltd.), St

106

(chemical pure, National Medicine Group Chemical Reagent Co., Ltd.), nonylphenol

107

polyoxyethylene (OP-10), sodium dodecyl sulfate (SDS), ammonium persulfate (APS;

108

analytical pure, National Medicine Group Chemical Reagent Co., Ltd.), sodium

109

bisulfite (SBS; analytical pure, Beijing Chemical Reagent Co.), chlorpyrifos (Jiangsu

110

YiJin Agrochemical Co., Ltd.), and EGDMA (99%, J&K Beijing Science and

111

Technology Co., Ltd.) were used in this study.

112

Preparation of chlorpyrifos microcapsules

113

The

chlorpyrifos-loaded

poly(BA/St)

and

chlorpyrifos-loaded

114

poly(BA/St/EGDMA) microcapsules were synthesized by emulsion polymerization in

115

a 250-mL four-neck flask equipped with a mechanical stirrer, a heating mantle with a

116

digital thermometer, and a condenser. In a typical synthesis, exact amounts of

117

chlorpyrifos were dissolved in a mixture of BA, St, and EGDMA to form the oil 6


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118

phase. Then, the oil phase was poured into distilled water (30 g) containing OP-10

119

(0.15 g) and SDS (0.4 g) and emulsified using a homomixer at 1500 rpm for 10 min to

120

form the pre-emulsion. Another solution that was prepared by dissolving OP-10 (0.1 g)

121

and SDS (0.2 g) into distilled water (25 g) was added to the four-neck flask with

122

mechanical stirring at 300 rpm. The initiator solution was prepared by dissolving APS

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(0.25 g) and SBS (0.25 g) in distilled water (15 g). One-third of the prepared

124

pre-emulsion was added to a four-neck flask under nitrogen gas, and maintained at

125

80 °C. Then, one-third of the initiator was introduced to initiate polymerization. The

126

remainder of the pre-emulsion and initiator solutions was slowly dropped into the

127

reaction mixture over a period of 1 h. The reaction was continued further for 3 h

128

under nitrogen gas. The resulting emulsion was collected after cooling to room

129

temperature for further characterization. The formulations employed for the synthesis

130

of the chlorpyrifos microcapsules are listed in Table 1.

131

Effect of different formulations on the characteristics of microcapsules

132

The individual effects of different factors on particle size, entrapment rate,

133

pesticide loading, and slow release behaviors were examined. More specifically, the

134

effects of the monomer ratio, core-to-shell ratio, and EGDMA cross-linker content in

135

the oil phase on the characteristics of the microcapsules were studied. The BA/St

136

monomer ratios (w/w) in the oil phase were 1:5, 1:2, 1:1, 2:1, and 5:1 in the oil phase.

137

The chlorpyrifos/monomer core-to-shell ratios (w/w) in the oil phase were 1:30, 5:26,

138

10:21, and 15:16. The contents of cross-linker EGDMA in the oil phase (relative to

139

EGDMA/monomer ratio (mol %/mol %)) were 0, 0.5, 1.0, 2.5, 5, and 10%. 7



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Characterization

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Entrapment rate and pesticide loading

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The two parameters, entrapment rate (ER, %) and pesticide loading (PL, %),

143

were used to characterize the efficiency of the encapsulation process. ER is defined as

144

the ratio between the weight of encapsulated chlorpyrifos and the theoretical weight

145

of chlorpyrifos during the preparation process,

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ER % =

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PL is defined as the weight of encapsulated chlorpyrifos divided by the weight of

148

× 100 %.

the corresponding chlorpyrifos-containing microcapsules,

149

PL % =

150

Prior to measuring ER and PL of the pesticide microcapsules, the emulsion was

151

demulsified. Typically, the emulsion (2 mL) was added to a 1:1 (w/w) mixture of

152

ethanol and 2% CaCl2 aqueous solution. The demulsified pesticide microcapsule

153

solution

154

chlorpyrifos-containing microcapsules. Then, the microcapsules were washed with 50

155

wt.% ethanol/water solution once and distilled water twice to remove any

156

non-encapsulated chlorpyrifos, followed by drying at 45 °C for 24 h.

was

filtered

using

a

mutche

× 100 %.

filter

to

obtain

aggregates

of

157

The microcapsule aggregates (~0.3 g) were introduced into 50-mL centrifuge

158

tubes containing n-hexane (20 mL). The encapsulated chlorpyrifos was extracted into

159

n-hexane upon stirring at 150 rpm for 4 h at 25 °C. The concentration of chlorpyrifos

160

in

161

spectrophotometer (UNICO Shanghai Instrument Co., Ltd., China) at 292 nm and the

n-hexane

was

analyzed

on

a

WTF

UV-2102PC

8


ultraviolet–visible

Page 9 of 34


162

weight of encapsulated chlorpyrifos was calculated.

163

Particle size measurement

164

The mean diameter of the microcapsule particles was calculated using a

165

HORIBA LA-950 laser particle analyzer (HORIBA Co., Ltd., Japan) at the Beijing

166

Centre for Physical & Chemical Analysis (Beijing, China)

167

Fourier transform infrared (FTIR) spectroscopy

168

For sample analysis, the microcapsules obtained from the demulsified solution

169

described above were dried at 40 °C overnight. All the FTIR measurements were

170

conducted on a Nicolet NEXUS-470 spectrometer (Madison, WI, USA).

171

Thermo-gravimetric analysis (TGA)

172

Thermo-gravimetric analysis for the chlorpyrifos microcapsules was conducted

173

by using a TGA/DSC simultaneous thermal analyzer (Mettler Toledo, Switzerland) at

174

a heating rate of 10 °C/min under a nitrogen atmosphere from 25 °C to 500 °C.

175

Transmission electron microscopy (TEM)

176

Prior to TEM analysis, the emulsion was diluted 600-fold, followed by negative

177

staining. TEM photographs were taken on a JEM-1230 transmission electron

178

microscope (JEOL Japan Electronics Co., Ltd., Japan) operating at an accelerating

179

voltage of 80 kV at the required magnification (80,000–200,000×) at room

180

temperature.

181

Slow release behaviors of chlorpyrifos microcapsules

182

For the chlorpyrifos release experiments, a chlorpyrifos microcapsule emulsion

183

(5.0 g) was injected into a dialysis bag with molecular weights of 8,000–12,000. The 9



184

dialysis bag was placed in a conical flask (250 mL) containing absolute ethyl alcohol

185

(100 mL) as the release medium under static conditions at 25 °C. The amounts of

186

chlorpyrifos released at different time intervals were measured by recording the

187

absorbance of the release medium at 291 nm on an ultraviolet–visible

188

spectrophotometer (UNICO Shanghai Instrument Co., Ltd., China).

189

Results and Discussion

190

FTIR spectroscopy analysis of chlorpyrifos microcapsules

191

To assess the successful encapsulation of chlorpyrifos, the FTIR spectra of the

192

chlorpyrifos, poly(BA/St) microcapsules, and chlorpyrifos-loaded poly(BA/St)

193

microcapsules were recorded (Figure 1). The spectrum of chlorpyrifos (Figure 1a)

194

featured stretching vibration peaks corresponding to pyridine ring C=N, aromatic

195

P–O–C, aliphatic P–O–C, and P=S at 1549.64, 1169.59, 852.84, and 1016.97 cm−1,

196

respectively. These characteristic peaks are typical of chlorpyrifos. The poly(BA/St)

197

microcapsules (Figure 1b) were characterized by the absorption peak at 1727.69 cm−1

198

(corresponding to C=O), which proved the existence of BA/St copolymer. Moreover,

199

the characteristic absorption peak of C=C belonging to monomer BA and St at 1637 cm−1 was

200

absent, which proved the successful preparation of the poly(BA/St) wall material. The

201

characteristic peaks observed in both Figure 1a and Figure 1b were present in the

202

spectrum of the chlorpyrifos-loaded poly(BA/St) microcapsules (Figure 1c), thereby

203

confirming the successful entrapment of chlorpyrifos by poly(BA/St) wall material.

204

TG analysis of chlorpyrifos microcapsules

205

Figure 2 demonstrated TGA thermogram of prepared poly(BA/St) chlorpyrifos 10


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206

microcapsules (Figure 2. c) comparing with pure chlorpyrifos (Figure 2. a), blank

207

poly(BA/St) microcapsule (Figure 2. b) and physical mixture of chlorpyrifos and

208

blank poly(BA/St) microcapsule (Figure 2. d). TG curves presented the weight loss of

209

pure chlorpyrifos occurred at 143 °C while that of blank poly(BA/St) microcapsule

210

occurred at 360 °C. TG curves of poly(BA/St) chlorpyrifos microcapsules showed

211

two stage degradation with an initial weight loss from 234 °C to 301 °C and a second

212

from 360 °C to 425 °C. Comparing with TG curves of pure chlorpyrifos and blank

213

poly(BA/St) microcapsule, the first stage might result from the gasification of

214

chlorpyrifos and the second one was attributed to the decomposition of poly(BA/St).

215

It also proved that the chlorpyrifos was encapsulated by poly(BA/St) microcapsule

216

successfully. Additionally, TG curves of poly(BA/St) chlorpyrifos microcapsules had

217

the same degradation feature as the TG curves of physical mixture of chlorpyrifos and

218

blank poly(BA/St) microcapsule, might imply the physical combination between

219

chlorpyrifos and poly(BA/St) walls material.

220

Particle size of chlorpyrifos microcapsules

221

In order to investigate the diameter feature of microcapsule pesticide sample

222

prepared in this study, particle size distribution analysis was conducted. The particle

223

size distribution of M3 was demonstrated in Figure 3. The diameter presented an

224

approximately normally distribution and the particle size mostly distributed at about

225

90 nm with the mean diameter of 90.21 nm. For other samples, similar distribution

226

was observed. It revealed a narrow and concentrated distribution of the particle size of

227

each sample. As to the mean diameter, presented on Table 1, mean diameters of 11



228

sample P1-P3 were slightly bigger than others, which probably indicated the influence

229

of chlorpyrifos content on particle mean diameter. When only considering the

230

monomer ratio (M1-M5), M2 is slightly bigger than others’ formulation (M1, M3, M4

231

and M5) where no significant difference was observed. Similarly, the mean diameters

232

of E1-E5 were almost the same (about 90nm), which indicated that the change of

233

EGDMA content had little effect on particle size. These results demonstrated that the

234

content of the monomer, pesticide, and EGDMA minimally influenced the particle

235

size. Consequently, it could be inferred that the release performance of the

236

microcapsules would not be influenced by the diameter of the microcapsules that

237

remained mostly unchanged across the different synthesis conditions studied herein.

238

Effect of monomer ratio on characteristics of chlorpyrifos microcapsules

239

As observed in Table 2 and Figure 4, when the BA/St monomer ratio changed

240

from 1:5 to 1:1, ER and PL increased from 81.28 to 89.31% and 2.62 to 2.88%,

241

respectively. In contrast, when the BA/St monomer ratio changed from 1:1 to 5:1, ER

242

and PL decreased from 89.31 to 73.62% and 2.88 to 2.37%, respectively. As deduced,

243

excess amounts of BA and St were detrimental to the encapsulation of chlorpyrifos as

244

indicated by the corresponding lower measured ER and PL values when compared

245

with those of other formulations comprising lower amounts of BA and St. At

246

excessive amounts of St (M1), the resulting poly(BA/St) became hard and brittle

247

because of its higher Tg over 55 °C. Hence, the polymer was prone to rupture during

248

the encapsulation of chlorpyrifos. Similarly, when the content of BA in the

249

formulation was high (M4, M5), poly(BA/St) featured soft and sticky characteristics 12


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250

with the lower Tg of about -21 and -39 °C, and hence was not adequately

251

mechanically stable for the effective entrapment of the pesticide. Therefore, the ER

252

and PL values of the microcapsules were satisfactory at moderate monomer ratios of

253

1:2 and 1:1 owing to the desirable soft and hard characteristics of poly(BA/St). The

254

monomer ratio of 1:2 was studied further in the subsequent experiments.

255

Effect of core-to-shell ratio on characteristics of chlorpyrifos microcapsules

256

The influence of different chlorpyrifos/monomer BA and St core-to-shell ratios

257

on ER and PL was investigated (Table 3, Figure 5). The weight of the oil phase was

258

kept constant and the total weight of chlorpyrifos and monomer BA and St was 31 g.

259

Only the weight of chlorpyrifos was varied from 1 to 15 g to achieve

260

chlorpyrifos/monomer BA and St weight ratios of 1:30, 5:26, 10:21, and 15:16. As

261

observed in Figure 5, PL increased from 2.74 to 29.11% when the loading content of

262

pesticide increased owing to the increased content of pesticides in the oil phase. In

263

contrast, ER decreased from 85.14 to 60.15% when the loading content of

264

chlorpyrifos increased from 1 to 15 g. These results showed that moderate dosages of

265

monomer in the chlorpyrifos emulsion preparation process afforded desirable ER and

266

PL values, whereas similar weight levels of chlorpyrifos and monomer did not result

267

in desirable ER values. However, higher PL values were obtained. This phenomenon

268

was attributed to ineffective chlorpyrifos entrapment owing to the inadequately dense

269

polymer obtained in the presence of relatively low monomer dosages. Optimum

270

weight ratios of the pesticide and monomer were determined at 1:30–5:26 for the

271

effective encapsulation of chlorpyrifos into poly(BA/St) microcapsules by emulsion 13



272 273 274

polymerization. Effect of content of cross-linker EGDMA on characteristics of chlorpyrifos microcapsules

275

EGDMA, a widely employed cross-linker for emulsion polymerization, will

276

form a dense network within the microcapsule wall material or generate links among

277

the microcapsule particles upon copolymerization with BA and St. In the present

278

study, the effect of EGDMA on the characteristics of the microcapsules was

279

investigated by varying its contents at 0, 0.5, 1.0, 2.5, 5, and 10% (relative to the

280

EGDMA/monomer ratio, mol/mol). The results are shown in Table 4. As observed in

281

Figure 6, ER and PL both reached a maximum when an EGDMA concentration of 0.5%

282

was used, and then decreased with increasing concentrations of cross-linker EGDMA.

283

The observed increase in ER and PL may be attributed to the denser structure of

284

poly(BA/St/EGDMA) relative to that of poly(BA/St) owing to the higher degree of

285

cross-linking. In contrast, the considerable decrease in ER and PL with further

286

increases in the amount of EGDMA may be attributed to the excessively high degrees

287

of cross-linking owing to the higher content of cross-linker. Higher degrees of

288

cross-linking would result in tighter connections among the polymer chains, thereby

289

inhibiting effective encapsulation of the pesticide.

290

Pesticide release study

291

An important aim of the present study was to study the release performance of

292

chlorpyrifos from the poly(BA/St) and poly(BA/St/EGDMA) microcapsules. First, the

293

effect of the monomer ratio (BA/St) used in emulsion polymerization on the release 14


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294

profile is discussed. The pesticide microcapsule emulsion was introduced into a

295

dialysis bag and the release of the pesticide from the dialysis bag was monitored over

296

~170 h. The results are shown in Figure 7. As observed, the release of chlorpyrifos

297

from the poly(BA/St) microcapsules M1 and M5 prepared with monomer ratios of 1:5

298

and 5:1 was fast. A sustainable pesticide release over ~72 h was observed. A slower

299

release profile was obtained for the chlorpyrifos-loaded poly(BA/St) microcapsules

300

M4 prepared with a monomer ratio of 2:1. The release profile of the microcapsules

301

M2 prepared with a monomer ratio of 1:2 was satisfactory: a lower chlorpyrifos

302

release was observed at the early stages of the release studies that lasted for 168 h.

303

The maximum sustainable release content of the pesticide was 63.9%. These results

304

may be explained by the different chlorpyrifos microcapsule characteristics as a result

305

of the different monomer ratios used in the formulations as observed in Table 2. The

306

ER and PL values of the chlorpyrifos-loaded microcapsules M1 and M5 were

307

relatively lower, which not only led to poor chlorpyrifos encapsulation, but also to a

308

high content of chlorpyrifos retained on the microcapsule surface or in the water

309

phase. Thus, the fast release profile was caused by the fast dissolution of chlorpyrifos

310

in the external phase at the early stages of the release process and prompt release of

311

the pesticide from the mechanically weak chlorpyrifos-loaded poly(BA/St)

312

microcapsules. Furthermore, the Tg of M1 was much higher than the release

313

temperature of 25 °C while the Tg of M5 was much lower than 25 °C. A relatively

314

large difference between Tg and environmental temperature might lead to a relatively

315

complete polymer state transitions to elastomeric state for M5 and glassy state for M1, 15



316

which seemed to be conducive to the chlorpyrifos release in the present study. The

317

rapid early release observed for the microcapsules M4 was due to the same

318

phenomena occurring in microcapsules M1 and M5 as discussed previously. However,

319

the slower release profile was due to the stronger mechanical property of the

320

microcapsules. Poly(BA/St) microcapsules M2 and M3 had mechanical properties

321

suited for entrapping the pesticide as well as obtaining high ER and PL values. Tg of

322

M2 and M3 were close to the release temperature, which might let the microcapsule

323

had characteristics of both elastomeric state and glassy state. It seemed to be very

324

suitable for slow release of chlorpyrifos.Therefore, both the early release and

325

sustainable release (>168 h) profiles of these particular pesticide microcapsules were

326

satisfactory. More importantly, our results indicated that the poly(BA/St)

327

microcapsules prepared at a monomer ratio of 1:2 (M2) were the best candidate for

328

achieving sustainable release of chlorpyrifos.

329

Secondly, the effect of varying the EGDMA cross-linker content in the

330

microcapsule formulation on the release profile of chlorpyrifos was investigated. The

331

content was varied from 0 to 10%, while the BA/St monomer ratio was fixed at 1:2.

332

The results are depicted in Figure 8. As observed, the sustainable release of

333

chlorpyrifos loaded in the microcapsules M2-E3 prepared with increasing EGDMA

334

contents from 0 to 2.5% increased. Further increases in the EGDMA content above

335

2.5% led to shorter sustainable releases (E4-E5). The reduction in the sustainable

336

release behavior may be due to the dense poly(BA/St/EGDMA) network that prevents

337

effective release of chlorpyrifos from the microcapsule. Moreover, more EGDMA 16


Page 16 of 34

Page 17 of 34


338

reacted with BA and St, resulting in tighter and denser polymer chains, consequently

339

leading to the decrease in sustainable release. In contrast, increasing the content of

340

EGDMA from 2.5 to 10% resulted in considerably fast release profiles of chlorpyrifos.

341

The fast release profiles may be due to the low mechanical stability of the

342

microcapsules in ethyl alcohol release medium owing to the greater extent of

343

cross-linking by EGDMA. Besides, the relatively lower ER and PL values resulted in

344

a higher level of pesticide retainment on the microcapsule surface or in the water

345

phase that may account for the rapid initial release rate.

346

To

further

understand

the

release

profiles

of

the

pesticide-loaded

347

EGDMA-cross-linked microcapsules, TEM analysis was conducted on the

348

pesticide-loaded microcapsules prepared with different cross-linker EGDMA contents.

349

The morphology and dispersion of the microcapsules can be observed in Figure 9.

350

The dark circles of microcapsule from TEM might also represent the relative thin

351

shell polymer forming by the slowly dropped process of monomer, which indicated a

352

core-shell structure of these microcapsules. Figure 9a shows the homogeneous

353

dispersion of the chlorpyrifos-loaded poly(BA/St) microcapsules prepared in the

354

absence of cross-linker EGDMA. In contrast, connections among the pesticide-loaded

355

microcapsules prepared in the presence of 5% EGDMA were obvious (Figure 9b).

356

The external connections among the microcapsules that resulted in denser structures

357

led to slower release of chlorpyrifos when compared with that observed for

358

microcapsules prepared without EGDMA. Unfortunately, excessive connections led to

359

the aggregation of the microcapsules that were prepared in the presence of 10% 17



360

EGDMA (Figure 9c). Furthermore, the inhomogeneous microcapsule structure may

361

be prone to rupture during the microcapsule synthesis and pesticide release processes.

362

Therefore, poly(BA/St/EGDMA) microcapsules prepared in the presence of 5–10%

363

EGDMA featured fast release profiles, whereas those prepared in the presence of

364

0.5–2.5% featured slower release characteristics.

365

Figure 10 shows that the length of sustainable release of chlorpyrifos decreased

366

significantly with increasing loading contents of chlorpyrifos (1–10 g). Accordingly,

367

changes in the loading content of chlorpyrifos altered the chlorpyrifos/monomer BA

368

and St core-to-shell ratio. More specifically, increasing the dosage of pesticide led to

369

decreasing dosages of the monomer at a given combined pesticide and monomer

370

content. The polymer matrix prepared with a lower amount of monomer was not

371

dense enough for chlorpyrifos entrapment. Based on the results discussed previously,

372

the lower ER of the microcapsules P1 and P2 loaded with 5 and 10 g pesticide is

373

another reason for the rapid release profile observed. These results indicated that a

374

core-to-shell ratio of 1:30 was optimum to achieving good release profiles, whereas

375

microcapsules loaded with higher contents of chlorpyrifos (5–10 g) featured faster

376

release profiles.

377

Chlorpyrifos Release Kinetics

378

All of the slow release profile curves were composed of the gradual release curve

379

followed by constant release curve that implied the fully release of chlorpyrifos. In

380

order to investigated the release mechanism of chlorpyrifos-loaded microcapsule in

18


Page 18 of 34

Page 19 of 34


381

the present study, data of the gradual release curve from chlorpyrifos release

382

experiments were fitted to the following equation 31: !

= #$ %

"

383 384 385

!

is the amount of chlorpyrifos released at time t,

"

is the total amount of

chlorprifos in microcapsule, k is a release constant and n is a diffusional exponent. As descripted in previous study

20, 32, 33

, the diffusion exponent n was equal to

386

0.43 for a Fickian diffusion system, when the diffusion of chlorpyrifos was controlled

387

by its concentration difference, and diffusional exponent n was close to 0.85 for a

388

degradation release system, when the release of chlorpyrifos was controlled by the

389

degradation of microcapsules.

390 391

Slow release data were fitted with nonlinear regression. The release constant, diffusional exponent and the correlation coefficient R were presented in Table 5.

392

The release exponent n close to 0.43 of samples M2, M3, E3 and P1 with a high

393

R values that indicated good fitting of exponential equations, revealed the release

394

mechanism of these microcapsule to be controlled by diffusion. When the release

395

exponent n values were much lower than 0.43, there might be some other factors that

396

influenced the chlorpyrifos release system and the rapid initial release profile of these

397

sample probably implied the rupture of microcapsule in the early release stage. All n

398

values were below 0.43, proving that degradation of microcapusle was not the

399

dominant factor of the release.

400

As to the effects of monomer ratio on the microcapsule, release profiles of M2

401

and M3 with BA/St ratios of 1:2 and 1:1 fitted well with the equation while profiles of 19



402

microcapsules of other BA/St monomer ratio did not, which indicated the same

403

conclusion that the suitable monomer ratio for chlorpyrifos slow release were 1:2 and

404

1:1. The n value of E3 with 2.5% EGMDA was 0.428, which represent a good

405

diffusion system of chlorpyrifos. As a result, E3 with 2.5% EGMDA was the finest

406

slow release formulation for controlled chlorpyrifos. For the effects of different

407

core-to-shell ratios, results of slow release kinetics showed that M2 and P1 with

408

core-to-shell ratios of 1:30 and 5:26 basically matched the diffusion system while

409

ratio of 10:21 might lead to an initial rupture of microcapsule. It might be related with

410

the lower ER as a consequence of the higher PL.

411

A series of poly(BA/St) and poly(BA/St/EGDMA) microcapsules loaded with

412

various amounts of chlorpyrifos were prepared by emulsion polymerization. All

413

prepared microcapsules featured particles sizes of ~90 nm with a little difference

414

among each samples. The microcapsules show promise as a carrier for the slow

415

release of pesticide chlorpyrifos. FT-IR and TG analysis of the microcapsules

416

confirmed the successful entrapment of chlorpyrifos in the poly(BA/St) wall material;.

417

The microcapsule ER varied from 60.15 to 96.74%. The highest ER (96.74%) was

418

observed for microcapsules prepared with a BA/St monomer ratio of 1:2 in the

419

presence of moderate EGDMA cross-linker content of 0.5%. In contrast, lower ERs of

420

60.15–67.97% were observed for microcapsules prepared with higher contents of

421

chlorpyrifos (5-10 g). Variations in ER were consistent with variations in PL except

422

for the series of microcapsules prepared with different core-to-shell ratios. In this

423

series, maximum PL (29.11%) resulted in the lowest ER (60.15%). 20


Page 20 of 34

Page 21 of 34


424

The release performance of chlorpyrifos from the microcapsules was

425

characterized by the medium dialysis method. Microcapsules prepared with a

426

monomer ratio of 1:2 whose Tg was close to the release temperature displayed

427

optimum sustainable release of chlorpyrifos. In contrast, microcapsules prepared with

428

monomer ratios of 1:5 and 5:1 displayed significantly high pesticide release at the

429

early stages of the release process. Microcapsules featuring adequate degrees of

430

cross-linking by EGDMA (0.5–2.5% EGDMA) displayed prolonged sustainable

431

release profiles, whereas those prepared with higher EGDMA contents of 5–10%

432

displayed enhanced chlorpyrifos release. Additionally, microcapsules prepared with a

433

chlorpyrifos/monomer BA and St core-to-shell ratio of 1:30 displayed optimum

434

release profiles, whereas those prepared at other ratios displayed faster release profiles.

435

Chlorpyrifos release was characterized by nonlinear regression analysis. The diffusion

436

release from microcapsules was found due to diffusional exponent n closing to 0.43,

437

while for samples that had n values much lower than 0.43, their faster release might

438

be caused by the initial rupture of microcapsule.

439

The current study investigated the influences of monomer ratio, core-to-shell

440

ratio, and degree of cross-linking on the characteristics and sustainable release

441

behaviors of chlorpyrifos entrapped in poly(BA/St) and poly(BA/St/EGDMA)

442

microcapsules via emulsion polymerization. Moreover, the findings provide a

443

reference for the release profile studies of polyacrylate chlorpyrifos microcapsules.

444

References:

445

(1) Bending, G. D.; Lincoln, S. D.; Sorensen, S. R.; Morgan, J. A. W.; Aamand, J.; Walker, A. In-field 21



446

spatial variability in the degradation of the phenyl-urea herbicide isoproturon is the result of

447

interactions between degradative Sphingomonas spp. and soil pH. Appl. Environ. Microb. 2003, 69,

448

827-834.

449

(2) Fleming, L. E.; Bean, J. A.; Rudolph, M.; Hamilton, K. Cancer incidence in a cohort of licensed

450

pesticide applicators in Florida. J. Occup. Environ. Med. 1999, 41, 279-288.

451

(3) Salameh, P. R.; Baldi, I.; Brochard, P.; Raherison, C.; Abi Saleh, B.; Salamon, R. Respiratory

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symptoms in children and exposure to pesticides. Eur. Respir. J. 2003, 22, 507-512.

453

(4) Hack, B.; Egger, H.; Uhlemann, J.; Henriet, M.; Wirth, W.; Vermeer, A. W. P.; Duff, D. G.

454

Advanced agrochemical formulations through encapsulation strategies? Chem. Ing. Tech. 2012, 84,

455

223-234.

456

(5) Shim, T. S.; Kim, S. H.; Yang, S. M. Elaborate design strategies toward novel microcarriers for

457

controlled encapsulation and release. Part. Part. Syst. Char. 2013, 30, 9-45.

458

(6) Hong K.; Park S. Preparation of polyurethane microcapsules with different soft segments and their

459

characteristics. React. Funct. Polym. 1999, 42, 193-200.

460

(7) Hashemi S. A.; Zandi M. Encapsulation process in synthesizing polyurea microcapsules containing

461

pesticide. Iran. Polym. J. 2001, 10, 265-270.

462

(8) Hirech, K.; Payan, S.; Carnelle, G.; Brujes, L.; Legrand, J. Microencapsulation of an insecticide by

463

interfacial polymerisation. Powder Technol. 2003, 130, 324-330.

464

(9) Mihou, A. P.; Michaelakis, A.; Krokos, F. D.; Mazomenos, B. E.; Couladouros, E. A. Prolonged

465

slow release of (Z)-11-hexadecenyl acetate employing polyurea microcapsules. J. Appl. Entomol. 2007,

466

131, 128-133.

467

(10) Rochmadi, A. P.; Hasokowati W. Mechanism of microencapsulation with urea-formaldehyde 22


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polymer. Amer. J. Appl. Sci. 2010, 7, 39-45.

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(11) Mayya K. S.; Bhattacharyya A.; Argillier, J. F. Micro-encapsulation by complex coacervation:

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influence of surfactant. Polym. Int. 2003, 52, 644-647.

471

(12) Dailey O. D.; Dowler C. C. Polymeric microcapsules of cyanazine: preparation and evaluation of

472

efficacy. J. Agric. Food Chem. 1998, 46, 3823-3827.

473

(13) P Rez-Mart Nez, J. I.; Morillo, E.; Maqueda, C.; Gin S, J. M. Ethyl cellulose polymer

474

microspheres for controlled release of norfluazon. Pest Manag. Sci. 2001,57, 688-694.

475

(14) El Bahri Z.; Taverdet J. L. Elaboration and characterisation of microparticles loaded by pesticide

476

model. Powder Technol. 2007, 172, 30-40.

477

(15) Adak, T.; Kumar, J.; Shakil, N. A.; Walia, S. Development of controlled release formulations of

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imidacloprid employing novel nano-ranged amphiphilic polymers. J. Environ. Sci. Heal. B. 2012, 47,

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217-225.

480

(16) Zhang P.; Zhang Q.; Jiao, Q. Synthesis and characterization of microcapsules with chlorpyrifos

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cores and polyurea walls. Chem. Res. Chinese U. 2006, 22, 379-382.

482

(17) Zhu, L.; Wang, Z.; Zhang, S.; Long, X. Fast microencapsulation of chlorpyrifos and bioassay. J.

483

Pestic. Sci. 2010, 35, 339-343.

484

(18) Zhang, J.; Li, M.; Fan, T.; Xu, Q.; Wu, Y.; Chen, C.; Huang, Q. Construction of novel amphiphilic

485

chitosan copolymer nanoparticles for chlorpyrifos delivery. J. Polym. Res. 2013, 20, 107.

486

(19) Kumbar, S. G.; Kulkarni, A. R.; Dave, A. M.; Aminabhavi, T. M. Encapsulation efficiency and

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release kinetics of solid and liquid pesticides through urea formaldehyde crosslinked starch, guar gum,

488

and starch +guar gum matrices. J. Appl. Polym. Sci. 2001, 82, 2863-2866.

489

(20) Stloukal, P.; Kucharczyk, P.; Sedlarik, V.; Bazant, P.; Koutny, M. Low molecular weight 23



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poly(lactic acid) microparticles for controlled release of the herbicide metazachlor: preparation,

491

morphology, and release kinetics. J. Agric. Food Chem. 2012, 60, 4111-4119.

492

(21) Zhang, S. F.; Chen, P. H.; Zhang, F.; Yang, Y. F.; Liu, D. K.; Wu, G. Preparation and

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physicochemical characteristics of polylactide microspheres of emamectin benzoate by modified

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solvent evaporation/extraction method. J. Agric. Food Chem. 2013, 61, 12219-12225.

495

(22) Zhang, W.; He, S.; Liu, Y.; Geng, Q.; Ding, G.; Guo, M.; Deng, Y.; Zhu, J.; Li, J.; Cao, Y.

496

Preparation

497

silica-alginate-elements as controlled release carrier materials. ACS Appl. Mater. Inter. 2014, 6,

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11783-11790.

499

(23) Liu, D.; Ichikawa, H.; Cui, F.; Fukumori, Y. Short-term delayed-release microcapsules

500

spraycoated with acrylic terpolymers. Int. J. Pharm. 2006, 307, 300-307.

501

(24) Chen, C.; Chen, Z.; Zeng, X.; Fang, X.; Zhang, Z. Fabrication and characterization of

502

nanocapsules containing n-dodecanol

503

redox

504

(25) Swamy, B. Y.; Prasad, C. V.; Rao, K. C.; Subha, M. C. S. Preparation and characterization of poly

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(hydroxyl ethyl methyl acrylate-co-acrylic acid) microspheres for drug delivery application. Int. J.

506

Polym. Mater. 2013, 62, 700-705.

507

(26) Wu, H.; Xu, Y.; Liu, G.; Ling, J.; Dash, B.; Ruan, J.; Zhang, C. Emulsion cross-linked

508

chitosan/nanohydroxyapatite microspheres for controlled release of alendronate. J. Mater. Sci.-Mater.

509

M. 2014, 25, 2649-2658.

510

(27) Casana, G. V.; Gimeno, S. M.; Gimeno, S. B. Agrochemical formulations containing

511

microcapsules. U.S. Patent 08263530, 2012.

and

characterization

of

novel

by

functionalized

miniemulsion

prochloraz

polymerization

microcapsules

using

using

interfacial

initiation. Colloid Polym. Sci. 2012, 290, 307-314.

24


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512

(28) Casana, G. V.; Gimeno, S. M.; Gimeno, S. B. Composition for delivering e.g. pyrethroids

513

comprises microcapsules that each encloses microencapsulated material, where wall of microcapsule is

514

formed by polymerization of aliphatic/aromatic isocyanate, and acetylene carbamide derivative. U.S.

515

Patent 2012245027-A1, 2012.

516

(29) Murakami M.; Ogawa M.; Fujinoto, I.; Ohtsubo, T. New pesticidal compositions comprise

517

microcapsules microencapsulating organophosphorus compound e.g. chlorpyrifos, used to control

518

wood-injuring insects, termites and nuisance injurious insects and to produce insect-proof wood. U.S.

519

Patent 5929053-A, 1999.

520

(30) Yang C.; Pan I. Controlled release pesticidal microcapsule prodn.|by mixing vegetable oil,

521

pesticide and aq.urea-formaldehyde prepolymer, acidifying and cross-linking, giving high

522

encapsulation rate. U.S. Patent 5576008-A, 1996.

523

(31) Korsmeyer R. W.; Gurny R., Doelker, E.; Buri, P.; Peppas, N. A. Mechanisms of solute release

524

from porous hydrophilic polymers. Int. J. Pharm. 1983, 15, 25-35.

525

(32) Zuleger S.; Lippold B. C. Polymer particle erosion controlling drug release. I. Factors influencing

526

drug release and characterization of the release mechanism. Int. J. Pharm. 2001, 217, 139-152.

527

(33) Asrar, J.; Ding, Y.; La Monica, R. E.; Ness, L. C. Controlled release of tebuconazole from a

528

polymer matrix microparticle: release kinetics and length of efficacy. J. Agric. Food Chem. 2004, 52,

529

4814-4820.

530

25



531 532

Figure 1. FTIR spectra of (a) chlorpyrifos, (b) poly(BA/St) (1:1, w./w.) microcapsules, and (c)

533

chlorpyrifos-loaded poly(BA/St) (1:1, w./w.) microcapsules.

534 535

Figure 2. TGA thermogram: (a) chlorpyrifos; (b) blank poly(BA/St) microcapsule; (c) poly(BA/St)

536

microcapsule of chlorpyrifos; (d) physical mixture of chlorpyrifos and blank poly(BA/St)

537

microspheres.

26


Page 26 of 34

Page 27 of 34


538 539

Figure 3. Particle size distribution of chlorpyrifos poly(BA/St) microcapsule with the monomer ratio

540

1:1 (BA/St w./w.)

541 542

Figure 4. Effect of BA/St monomer ratio in the oil phase on the ER and PL of the microcapsules.

27



543 544

Figure 5. Effect of additive amount of pesticide in the oil phase on the ER and PL of the microcapsules.

545 546

Figure 6. Effect of additive amount of EGDMA in the oil phase on the ER and PL of the microcapsules.

28


Page 28 of 34

Page 29 of 34


547 548

Figure 7. Profiles of chlorpyrifos release from formulations prepared using different ratios of BA/St

549

monomer.

550 551

Figure 8. Profiles of chlorpyrifos release from formulations prepared using different contents of

552

EGDMA (The BA/St monomer ratio was 1:2).

29



Page 30 of 34

553 554

Figure 9. TEM images of the different pesticide microcapsules prepared with (a) 0, (b) 2.5, and (c) 10%

555

EGDMA.

556 557

Figure 10. Profiles of chlorpyrifos release from microcapsules prepared with different loading contents

558

of chlorpyrifos (1–10 g).

559

Table 1. Particle size of the chlorpyrifos microcapsules prepared under different conditions.

Oil phase Mean Diameter Sample

Monomer

Chlorpyrifos

EGDMA/ monomer (nm)

(BA/St w./w.)

(g)

(mol./mol.)

M1

1:5

1.0

0

88.80±1.47 ef

M2

1:2

1.0

0

95.45±1.96 c

30


Page 31 of 34


M3

1:1

1.0

0

90.21±1.93 e

M4

2:1

1.0

0

87.40±1.75 ef

M5

5:1

1.0

0

89.21±0.42 ef

E1

1:2

1.0

0.5%

92.82±1.68 d

E2

1:2

1.0

1.0%

88.59±1.71 ef

E3

1:2

1.0

2.5%

89.82±1.66 ef

E4

1:2

1.0

5.0%

88.36±0.89 ef

E5

1:2

1.0

10%

90.66±0.57 de

P1

1:2

5.0

0

97.96±1.10 b

P2

1:2

10.0

0

101.8±1.21 a

P3

1:2

15.0

0

97.17±1.71 bc

560

Table 2. Characteristics of the pesticide microcapsules prepared under different BA/St monomer ratio

561

conditions.

Monomer

Designed

(BA/St

Tg

Entrapment Rate

Pesticide Loading

w./w.)

(°C)

(%)

(%)

M1

1:5

55.13

81.28±2.02 b

2.622±0.07 b

M2

1:2

25.02

85.14±2.80 b

2.747±0.09 b

M3

1:1

-0.023

89.31±2.21 a

2.881±0.07 a

M4

2:1

-21.18

76.37±1.37 c

2.464±0.04 c

Sample

Characteristic Parameter

31



M5

5:1

-39.31

73.62±2.34 c

Page 32 of 34

2.375±0.08 c

562

The designed Tg (glass transition temperature) was calculated by the Fox equation.

563

Table 3. Characteristics of the pesticide microcapsules prepared under different content of chlorpyrifos.


Chlorpyrifos Core-to-shell Sample (g)

Ratio (w./w.) Entrapment Rate (%) Pesticide Loading (%)

M2

1.0

1/30

85.14±2.02 a

2.747±0.09 d

P1

5.0

5/26

78.18±1.51 b

12.61±0.24 c

P2

10.0

10/21

67.97±2.87 c

21.93±0.93 b

P3

15.0

15/16

60.15±2.73 d

29.11±1.32 a

564

Table 4. Characteristics of the pesticide microcapsules prepared under different content of crosslinker

565

EGDMA.


EGDMA/monomer Sample

566

(mol./mol.)

Entrapment Rate (%)

Pesticide Loading (%)

M2

0

85.14±2.02 c

2.747±0.09 c

E1

0.5%

96.74±0.47 a

3.094±0.01 a

E2

1.0%

93.05±1.69 ab

2.951±0.05 b

E3

2.5%

90.02±1.89 b

2.783±0.06 c

E4

5.0%

86.11±3.24 c

2.557±0.10 d

E5

10%

85.78±1.45 c

2.359±0.04 e

Table 5. Parameters characterizing fitting of the model equation on chlorpyrifos gradual release data

Sample

k

n

R

M1

0.433

0.180

0.980

32


Page 33 of 34


M2

0.132

0.317

0.998

M3

0.120

0.398

0.993

M4

0.161

0.383

0.976

M5

0.234

0.316

0.985

E1

0.167

0.274

0.992

E2

0.126

0.288

0.998

E3

0.067

0.428

0.996

E4

0.187

0.327

0.964

E5

0.337

0.232

0.991

P1

0.086

0.431

0.997

P2

0.230

0.237

0.990

567

33



568

TOC Graphic

569

34


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Physicochemical Characteristics and Slow Release Performances of

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