Microstructural Behaviors of Matrices Based on Polylactic Acid and

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Biotechnology and Biological Transformations

MICROSTRUCTURAL BEHAVIOR OF MATRICES BASED ON POLYLACTIC ACID AND POLYHYDROXYALKANOATES Juan Carlos Alzate Marin, Sandra Rivero, Adriana Pinotti, Alejandro Horacio Caravelli, and NE Zaritzky J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b01506 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Journal of Agricultural and Food Chemistry

MICROSTRUCTURAL BEHAVIOR OF MATRICES BASED ON POLYLACTIC ACID AND POLYHYDROXYALKANOATES Alzate Marin, Juan Carlos1; Rivero Sandra1,2*; Pinotti Adriana1,3; Caravelli Alejandro1; Zaritzky, Noemí Elisabet1,3 1

CIDCA (Centro de Investigación y Desarrollo en Criotecnología de Alimentos), 47 y 116

S/N, La Plata (B1900AJJ), Buenos Aires, Argentina – (CONICET, UNLP, Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CICPBA)). 2

Facultad de Ciencias Exactas, Universidad Nacional de La Plata (UNLP). 3

Facultad de Ingeniería, Universidad Nacional de La Plata (UNLP).

* Corresponding author: Rivero Sandra ([email protected])

Tel.:/Fax:

+54

0221-424-9287|

425-4853

|

489-0741

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Abstract

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Individual films of polyhydroxyalkanoates (PHA) or polylactic acid (PLA) and their

3

blends were developed by solvent casting. PHA was obtained from activated sludges

4

from a wastewater treatment system at laboratory-scale. This work was focused on

5

analyzing the microstructural properties and thermal behavior of either, PHA or PLA

6

individual films as well as their blends. The behavior against biodegradation process of

7

individual films and blends was examined from the microstructural point of view. ATR-

8

FTIR spectra indicated the existence of a weak molecular interaction between the

9

polymers. The formulation of blend films improved the PLA crystallinity; additionally,

10

it induced the polymer recrystallization phenomenon, since PHA crystallized acting as a

11

PLA nucleating agent. This phenomenon explains the improvements in water vapor

12

barrier film properties. The blends exposed to a biodegradation process showed an

13

intermediate behavior between PLA and PHA leading to a consistent basis for designing

14

systems tailored to a particular purpose.

15

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Keywords: Biopolymers, polymer blends, infrared spectroscopy, crystallinity,

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modulated DSC, morphology

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INTRODUCTION

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Polyhydroxylalkanoate (PHAs) are a family of bio-polyesters synthesized by

25

microorganisms from various carbon sources, and they are accumulated intracellularly

26

under nutrient stress and act as a carbon and energy reserve.1 PHAs can be produced by

27

both pure and mixed microbial cultures. Mixed cultures have the advantage that they do

28

not need sterile conditions and are better able than pure cultures to adapt to complex

29

substrates such as industrial wastes.2 For this, wastes from dairy industry, sugar

30

industry, forestry, and biodiesel production can be utilized.3 Cheese whey is a surplus

31

material from dairy industry, which is mainly discharged to both soils and water bodies

32

causing serious environmental problems. PHA production from cheese whey could

33

revalorize this dairy sub-product and, at the same time, solving the waste disposal

34

problems.

35

A relevant characteristic of PHA is its versatility, since there are more than a hundred

36

different monomers, hydroxyvalerate, butyrate, among others. The properties of the

37

synthesized polymer are modified depending on the position of its functional groups and

38

degrees of polymerization. This chemistry allows PHAs to be tailored to provide similar

39

properties to traditional thermoplastics such as polyethylene (PE) and polypropylene

40

(PP) while maintaining biodegradability.4

41

Polylactic acid (PLA), a thermoplastic aliphatic polyester derived from lactic acid (2-

42

hydroxy propionic acid), can be produced by condensation and polymerization directly

43

from its basic building block lactic acid, which is derived by fermentation of sugars

44

from carbohydrates sources, such as corn starch, cassava roots, chips or starch, or

45

sugarcane.5 Polylactic acid is produced at the largest industrial scale of all

46

biodegradable polymers and it is considered as the most promising candidate for 2 ACS Paragon Plus Environment

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replacing conventional plastics.6 It is being used in biomedical applications, for bottle

48

production, filament production for 3D printing, and in compostable food packaging. It

49

is also being evaluated as a material for tissue engineering. Mass production has

50

reduced the cost of PLA production, making it an economically viable choice for

51

fabrication of containers, plastic bags and fibers. Commercial-scale plants today

52

produce hundreds of thousand tons of PLA per year.7

53

By controlling the molecular architecture suppliers are able to tailor the polymer to

54

specific applications. Consequently, new and improved grades of bioplastic resins are

55

constantly introduced in the market. NatureWorks LLC, focuses on a multi-step

56

procedure involving the condensation reaction of aqueous lactic acid to produce low

57

molecular weight PLA pre-polymer. It is converted into a blend of lactide stereoisomers

58

in order to produce a large spectrum of PLA grades.8 The films based on PLA have

59

excellent optics, good machinability, barrier and mechanical properties.5

60

PLA and PHA polymers are polyesters and are used in consumer products by a widely

61

industrial sector due to their biocompatibility and sustainability.9 They have comparable

62

thermal behavior to some conventional polymers and this has generated much interest in

63

exploring their physical and structural properties to identify potential applications.10

64

The combination of two polymers allows the design of new materials with tailorable

65

properties differing meaningfully from those of each component adjusting the

66

advantages of each polymer in order to obtain a material for different applications. In

67

this sense, the formulation of blend systems is easier and faster than copolymerization

68

methods.11 There are many studies focused on PLA modification such as the addition of

69

modifiers, copolymerization.12 The addition of a highly ordered stereochemical

70

structure such as PHA could improve the film properties based on PLA.13,14 In this



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sense, blending PLA with another bio-based and biodegradable material as PHA in

72

order to obtain tailor-made materials, constitutes a promising alternative.

73

This work was focused on analyzing the microstructural properties and thermal

74

behavior of either, PHA or PLA individual films as well as their blends. Furthermore,

75

the behavior against biodegradation process of individual films and PLA/PHA blends

76

was examined from the microstructural point of view.

77

78

MATERIALS AND METHODS

79

Poly lactic acid (PLA) grade 4043D in pellet form (98% L-lactide with a D-isomer

80

content of approximately 2%) designed for the production of films, was purchased from

81

Natureworks® under the trademark IngeoTM. As well known, the production of PLA

82

involves the conversion of starch in dextrose via a hydrolysis process and the

83

fermentation by microorganisms to synthesize the chain of polylactide polymer.

84

Polyhydroxylalkanoate (PHA) was obtained from activated sludges from a wastewater

85

treatment system at laboratory-scale (CIDCA). A two-stage biological system was

86

proposed to produce PHA by aerobic mixed culture from hydrolyzed cheese whey. The

87

first step involves the selection of PHA-accumulating organisms using an aerobic

88

sequencing batch reactor (SBR) operated under a feast-famine regime. Then, a stage of

89

accumulation of PHA in an aerobic batch reactor was used, which was inoculated with

90

biomass taken from SBR enriched in PHA-accumulating bacteria.

91 92

Extraction and quantification of PHA

93

Extraction and characterization of PHA from batch reactor biomass were performed

94

following the method proposed by Venkateswar Reddy et al.15 A copolymer P (HB-HV) 4 ACS Paragon Plus Environment

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with a purity of 96 % and a molar relation 95:5 of hydroxybutyrate (HB) and

96

hydroxyvalerate (HV) respectively was obtained.

97 98

Film preparation

99

The films were prepared by dissolving PHA and PLA in chloroform 1 and 2 % (w/v),

100

respectively, under stirring for 3 h. Mixtures were prepared by PLA/PHA following

101

different proportions: 20/80, 40/60, 60/40, 80/20 (w/w) which were stirred for 30 min at

102

60 °C.

103

Filmogenic solution of single PLA, PHA and their blends were cast into Petri dishes of

104

glass (9 cm diameter) which were left under a fume hood. Later, the samples were dried

105

in a vacuum oven at 60 °C to assure the solvent removal from the matrices. The

106

solutions were cast maintaining a constant mass / molding area ratio in order to assure a

107

uniform thickness in the different samples, due to the control solutions (PLA, PHA) had

108

different concentrations.

109

Film thickness was measured by means of a coating thickness gauge Check Line DCN-

110

900 (New York, USA) for non-conductive materials on non-ferrous substrates. For each

111

specimen, at least fourteen measurements at different positions were taken.

112 113

Water vapor permeability

114

Water vapor permeability (WVP) tests were carried out according to the technique

115

described by Rivero et al.16 based on a modified ASTM16 method E96. A permeation

116

cell maintained at 20 °C was used for the assays. Eight measurements were performed

117

once steady-state conditions were reached. All measurements were performed at least

118

four times for each specimen.



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Thermal analysis by MDSC

121

Thermal analysis of the samples was conducted by the modulated differential scanning

122

calorimetry (MDSC) technique. Q100 (TA Instruments, USA) controlled by a TA 5000

123

module (TA Instruments, New Castle, USA) and equipped with a cryogenic quench

124

cooling accessory, under a N2 atmosphere was used. This technique allows measuring

125

simultaneously the calorific capacity of the sample and differentiating between

126

reversible events (dependent on the calorific capacity) and irreversible (dependent on

127

the time and the temperature) that the sample experienced. Likewise, it allows a major

128

resolution of complex transitions.

129

A standard heating ramp of 10ºC min-1 with a modulation temperature amplitude of 0.5

130

ºC and a modulation period of 60 s were used. The first scan (heating) was performed in

131

order to delete the thermal story from -50ºC up to 200ºC. Once the first scan was

132

completed, the sample was cooled to -80 °C and then a second heating scan was

133

recorded. Informed results were the average of two replicates to ensure reproducibility

134

of the results.

135

The results were analyzed by using the Universal Analysis V1.7F software (TA

136

Instruments). Different parameters were determined from the obtained thermograms:

137

melting temperature (Tm) and enthalpy corresponding to the area of the endothermic

138

peak (∆H ), crystallization temperature during the cooling stage (T ) and crystallization

139

enthalpy (∆H ) and glass transition temperature (Tg) determined from the reversible

140

signal.

m

c

c

141 142

ATR-FTIR spectroscopy

143

The interactions between the system components was analyzed using the ATR-FTIR

144

technique. Spectra of the samples were registered by using the Thermo Scientific 6 ACS Paragon Plus Environment

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Nicolet iS10 FT-IR Spectrometer (Madison, USA) in the wavenumber region 4000-400

146

cm-1 performing 64 scans at a 4 cm-1 resolution. Specimens were placed onto the

147

diamond ATR crystal (Smart iTX accessory) and the software Omnic 8 (Thermo

148

Scientific, Madison, USA) was used for the data analysis process. To ensure

149

reproducibility of the results, the tests were performed in triplicate.

150 151

X-ray diffraction

152

X-ray diffractograms of individual PVA and PLA films and their blends were evaluated

153

by using a X'Pert Pro P Analytical Model PW 3040/60 (Almelo, The Netherlands)

154

operated at room temperature. The CuKα radiation (1.542 Å) was generated at 30 mA

155

and 40 kV. Scattered radiation was detected in an angular range of 3–60° (2θ) at a step

156

size of 0.02°. The crystallinity degree (CD) was calculated following the procedure

157

described in previous works.18,19

158 159

Structural studies by SEM

160

Microstructure of the matrices was examined by using a FEI model Quanta 200

161

scanning electron microscope (The Netherlands). Individual films and blend matrices

162

were immersed in liquid nitrogen and fractured cryogenically. Samples were mounted

163

onto metal stubs by using a double-sided tape and observed at low pressure and an

164

acceleration voltage of 12.5 kV, without any metal or carbon coating.

165 166

Biodegradation test

167

A series of plastic pots (400 cm3) were used as soil containers, and their microflora

168

present was used as a degrading medium. Square specimens of 3x3 cm with a thickness

169

of approximately 25 µm on average were put into plastic mesh to allow the access of 7 ACS Paragon Plus Environment


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moisture and microorganisms and to retrieve the degraded specimens. In order to

171

guarantee an aerobic degradation the film samples were buried. The samples were

172

conditioned at relative humidity of 50 % and a temperature of 20ºC. Containers were

173

sprayed regularly with a defined and constant amount of water. Similar conditions were

174

employed by Rivero et al.20 At specific time intervals, specimens were recovered from

175

the soil and cleaned with a brush to avoid any possible damage of the structure.

176

In accordance with the described methodology in ASTM D5988-0321, a ratio of 1 g of

177

compost per 25 g of soil was used. Studies of the morphology of the degraded films

178

were performed by means of SEM, MDSC, and ATR-FTIR analysis.

179 180

Statistical analysis

181

For statistical analysis, version 10.0 of Systat-software (SYSTAT, Inc., Evanston, IL,

182

USA) was used. Analysis of variance (ANOVA), linear regressions, and Fisher's Least

183

Significant Difference for mean comparison test were performed. The significance level

184

used was 0.05.

185 186

RESULTS AND DISCUSSION

187

Thermal analysis

188

The thermograms obtained by means of MDSC from the PLA, PHA, and their blends

189

can be observed in Fig. 1, where the stages of heating and cooling are shown. The

190

temperatures of the thermal transitions and the enthalpy values (J g-1) of the

191

characteristic events during heating and cooling stages obtained from MDSC curves, are

192

summarized in Table 1.

193

As it is possible to see in Fig. 1a, the thermal analysis allowed observing a peak for each

194

individual film, attributed to the melting of the crystalline domain corresponding to 8 ACS Paragon Plus Environment

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PLA and PHA at 147.7 ºC and 172.7 °C, with associated enthalpies of 20.1 and 86.6 J g-

196

1

197

crystalline populations (crystalline fraction) which exist in the material when the

198

melting occurs. Thellen et al.4 using MDSC reported that the crystal melting is produced

199

at higher temperatures in the samples of PHA containing the lower percentage of

200

valerate, because as valerate content increases, the onset of crystal melting occurs at

201

lower temperatures. In the present study, PHA thermograms evidenced the presence of a

202

peak with a shoulder, which could represent the melting of crystal lamellae as was

203

reported by Thellen et al.4 for extruded PHA.

204

On the other hand, PHA films exhibited a Tg at -4.7 °C, whereas PLA showed a marked

205

transition at 58.1 °C (Fig. 1c). da Silva et al.22 reported values of glass transition

206

temperatures ranging from 2.7 to -4.4 ºC for PHA matrix obtained by casting.

207

The thermograms of all the mixtures exhibited peaks of recrystallization of PLA at 107

208

°C, this transition being more evident with higher PLA proportions in the mixture (Fig.

209

1a). These results would indicate that the incorporation of PHA contributes to the

210

process of recrystallization of PLA in all the blend formulations. This effect was more

211

marked in the blend 80/20, behavior that could be explained bearing in mind that a low

212

proportion of crystalline domains of PHA dispersed in a continuous matrix of PLA

213

induced the phenomenon of nucleation of this latter polymer. The presence of double

214

melting temperatures in the PLA/PHA blends corresponds to both, the melting of “as

215

formed” and recrystallized polymer.10

216

According to Abdelwahab et al.10 and Ikehara et al.23, the recrystallization strongly

217

depends on the melting temperature difference among the components of the mixture.

218

When the Tm difference is very large, the component with the higher melting

219

temperature crystallizes first, and its spherulites contribute to fill all the volume.

, respectively. The intensity of the melting peak is related to the number of crystals or



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The blend 20/80 exhibited a peak at 171.9 °C because of the melting of the PHA

221

crystalline fraction and a shoulder at 148.4 °C corresponding to the melting of PLA.

222

The blends 80/20, 60/40 and 40/60 exhibited two endothermic transitions; the enthalpy

223

associated with the first event diminished with the incorporation of a higher proportion

224

of PHA to the blend, while the enthalpy of the endothermic peak, characteristic of the

225

PHA-crystalline phase, became more prominent (Fig. 1a and Table 1). These findings

226

were in accordance with a higher crystalline degree.

227

From the obtained DSC curves, two Tg values were observed irrespective of the blend

228

composition. These results suggest that the analyzed blends PLA/PHA turned out to be

229

immiscible in the amorphous state (Fig. 1c).

230

According to Lipatov and Alekseeva24, the appearance of two transitions associated to

231

the glass transition temperatures supports the existence of a partially miscible system,

232

that is, materials with heterogeneous biphasic structure.

233

During the cooling step, the PHA exhibited a peak of crystallization at 58.7 °C with an

234

associated enthalpy of 48.4 J g-1, whereas the thermogram of the PLA did not show this

235

transition (Fig. 1b and Table 1). Taking these results into account, it is possible to infer

236

that PHA crystallinity is greater than that of PLA.

237

The thermal analysis of PLA/PHA mixtures showed that the peak of crystallization

238

attributed to the PHA was observed only in the mixtures 20/80 and 40/60, with a minor

239

enthalpy associated regarding a single PHA matrix (Fig. 1b). On the other hand, the

240

thermograms of individual PLA film and the mixtures 60/40 and 80/20 did not exhibit

241

this thermal transition (Fig. 1b). Nevertheless, an event was observed at 57.7°C

242

attributed to the glass transition temperature of the PLA-enriched phase. Similar results

243

were informed by Arrieta et al.12, Furukawa et al.25 and Zhang and Thomas11 indicating


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that these results would be a signal of the fact that the PHA in the mixtures does not

245

crystallize during the cooling stage when it is in minor proportion.

246 247

Microstructural analysis

248

FTIR analysis

249

From ATR-FTIR spectra, the functional characteristic groups of the PLA and PHA were

250

identified (Fig. 2a and b). They are consistent with those reported in the literature.5,26,27

251

In PLA spectrum, the band located at 1746 cm-1 is a strong peak that corresponds to the

252

stretching of carbonyls’ amorphous phase.5 The bands located at 1180 and 1080 cm-1

253

belong to the asymmetry and the symmetry stretching vibration of C–O–C. As known,

254

PLA is a hydrophobic polymer due to the presence of –CH side groups. The peaks at

255

about 2997 cm-1 and 2946 cm-1 correspond to the –CH asymmetric and symmetric

256

stretching vibrations of –CH groups in the side chains, respectively (data not shown).

257

On the other hand, in PHA spectrum the peak at 1719 cm-1, is attributed to the stretching

258

of the crystalline carbonyl group, and the stretching of the carbonyl group of the

259

amorphous phase resulted virtually imperceptible. Samples 40/60 and 20/80 exhibited

260

peaks at 1748 cm-1 and 1755 cm-1 (Fig. 2b), which are assigned to the amorphous and

261

crystalline bands of PLA respectively.24

262

Furthermore, Fig. 2a and b depicts the ATR-FTIR spectra of PLA/PHA blend matrices.

263

Blend spectra of composition 80/20 and 60/40 followed a similar pattern to that of the

264

pristine PLA, while the mixtures 40/60 and 20/80 were identified mainly with the PHA

265

spectrum. All of them unfolded both bands corresponding to the stretching of carbonyls

266

located at 1746 and 1719 cm-1 due to PLA and PHA phase, respectively (Fig. 2a and b).



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In the cases of the 80/20, 60/40, and 40/60 blends, bands in the C-O-C and C-C

268

stretching and the CH deformation mode region (1300-1000 cm-1), are visualized at

269

about the same wavenumbers as those of pure PHA (Fig. 2a and b).

270

It is noteworthy that the intensities of these bands were interdependent and mutually

271

dependent, one being increased at the expense of the other, meaning that the ratio of

272

intensities of these two bands changed in agreement with the ratio of the individual

273

components. These results allowed inferring the immiscibility of these components and

274

that there were no strong molecular interactions between both polymers.

275 276

X-Ray diffraction

277

The typical diffraction patterns of individual PHA, PLA, and composite films are

278

depicted in Fig. 3. X-ray diffractogram of PLA films exhibited a pattern corresponding

279

to an amorphous material with a peak located at 2θ = 17 º associated with the reflection

280

of the crystals of the polymer;13,28 the estimated crystallinity degree was 14.3 % (Table

281

insert in Fig. 3). On the contrary, PHA is a highly ordered polymer and it is known to

282

crystallize in an orthorhombic cell.10

283

X-ray diffraction analysis was used to determine the crystalline structure and the

284

crystallinity degree of the blends. The PHA spectrum showed diffraction peaks placed

285

at 2θ = 13.8° and 17°, corresponding to (020) and (110) planes, respectively.11

286

Additionally, reflections at 2θ = 20.3, 22.4, 25.4 and 30.8 were detected, which are

287

characteristics of the PHA polymer, its CD being of 18.3 %.14

288

The inclusion of PHA in the blends increased the crystallinity of the materials. The

289

highest crystallinity degrees of 60/40 and 40/60 blends were confirmed by means of X-

290

rays (Table inserted in Fig. 3). The diffractograms showed peaks of higher intensity in

291

relation to individual matrices. 12 ACS Paragon Plus Environment

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This behavior would allow inferring that the PHA induces the recrystallization of the

293

PLA due to its capacity to act as an agent of nucleation. According to Furukawa et al.24

294

and Zhang and Thomas,11 it generates a better packing density of the polymeric

295

segments and promotes a better adhesion and interaction at the interface level. However,

296

the microstructure, porosity and permeability of the matrix seem to depend on the

297

proportions PLA/PHA as presented below.

298 299

SEM observations

300

From the macroscopic point of view, the blend of the two polymers appeared to be well

301

mixed in the matrices with no apparent phase separation, although, FTIR showed

302

immiscibility of both components. The average thickness of the films was 25 µm, which

303

was corroborated by microscopic analysis. As it can be seen in Fig. 4, all matrices

304

presented homogeneous appearance irrespective of the PLA/PHA ratio.

305

Films showed important differences as it was analyzed in the cross section of single or

306

blend matrices. PLA, individual films showed homogeneous appearance, without pores

307

and with a good structural integrity. PHA polymer led to films whose cross section was

308

rough giving a structure with a fibrous appearance (Fig. 4).

309

The cross-section and the surface of the mixtures 20/80 and 40/60 revealed the presence

310

of an orderly structure attributed to the growth and crystallization of the spherulites of

311

PHA, proving to be a network formed by co-domains corresponding to the different

312

polymers with a granular interspersed appearance and a repetitive pattern. It is important

313

to remark that the highest proportion of PHA in the blends (PLA/PHA= 20/80),

314

evidenced a porous microstructure due to the structural arrangement between the

315

polymers. According to Abdelwahab et al.,10 the interface interactions between PHA



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316

and PLA phases could influence the nucleation phenomenon as supported by the XRD

317

results.

318

These micrographs reinforced the results of immiscibility obtained by other techniques.

319

As PLA proportion increased, the morphology of the matrices became more

320

homogeneous, less rough until the mixture 80/20 presented an appearance where the

321

PLA-enriched phase formed a continuous domain in which the PHA-enriched phase

322

was dispersed, homogeneously distributed as can be seen in Fig.4.

323 324

Water vapor permeability

325

Table 2 shows WVP values of individual films and PLA/PHA blends. The formulation

326

of blend matrices improved the barrier properties of the materials obtaining a reduction

327

of 40% in WVP values for the blends 40/60, 60/40 and 80/20 compared to the values of

328

the individual films (Table 2). Meanwhile, 20/80 did not show significant differences

329

(p>0.05).

330

According to Arrieta et al.,12 the crystallization of PLA reduces the water vapor

331

permeability since the crystals decrease the volume of amorphous phase generating a

332

path of greater tortuosity, reducing mass matter transfer.

333

The improvement of the barrier properties is related to the higher crystallinity of the

334

blend matrices PLA/PHA as observed by X Ray Diffraction technique and MDSC

335

Thermal analysis.

336 337

Soil biodegradation studies

338

The influence of the microstructure developed during the formulation of the blends on

339

the biodegradation process was examined. Morphological study by SEM during soil

340

biodegradation experiments was complementary to visual examination as it helped to 14 ACS Paragon Plus Environment

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confirm the existence of structural modifications in the films, allowing a detailed

342

evaluation of the degradation process.

343

The results showed that degradation behavior of bio-based polymers followed different

344

patterns regarding morphological characteristic of each matrix. As can be seen in Fig.

345

5a, the biodegradation occurred in both PHA and PLA phases.

346

The biodegradation assay revealed that PHA films exhibited remarkable biodegradation

347

at 30 days, while blend matrices (PLA/PHA) showed changes around 50 days, which

348

would indicate a delay in the process of biodegradation (Fig. 5a). According to Weng et

349

al.,27 PHA polymer could be biodegraded at a high rate, under composting as well as

350

soil environment conditions; in contrast even PLA was biodegradable under composting

351

conditions, the degradation rate in soil was slow.

352

SEM micrographs show the fracture surface of the PLA/PHA matrices after a

353

biodegradation process of 50 days (Fig. 5a). It can be seen that the biodegradation was

354

intensified with the higher proportions of PHA in blend system. As the proportion of

355

PHA increased, particularly from 40/60 to 20/80, the matrices showed greater interstices

356

or cavities (Fig. 4) which could facilitate the biodegradation process. Zhang and

357

Thomas11 explained that the inclusion of PHA improved the degradation degree of PLA

358

at room temperature. The authors reported that PHA and PLA exhibit different

359

degradation pattern. PHA is mainly degraded by the attack of various enzymes at the

360

surface. According to Weng et al.27 the degradation of PHA can be attributed to the

361

erosion catalyzed by bacteria from the surface to the interior.

362

In contrast, PLA degradation is produced throughout the whole sample, starting with a

363

non-enzymatically hydrolysis that leads to a reduction in the molecular weight.11 Then,

364

low molecular weight PLA diffuses out of the bulk polymer and can be metabolized by

365

microorganisms, producing water, carbon dioxide and humus.29 15 ACS Paragon Plus Environment


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On the other hand, the thermograms of blend matrices, 20/80, 40/60 and 60/40, exposed

367

to the biodegradation process for 50 days, experienced a decrease of the transition

368

enthalpy corresponding to the melting of the PHA crystalline fraction being more

369

marked in the blends with the greatest proportion of PHA (Fig. 5b). In this sense,

370

Dharmalingam30 found that the changes of melting enthalpy (∆Hm) for composite

371

material based on blends of PLA/PHA represent depolymerization. Consequently, this

372

phenomenon could indicate a higher sensitivity to the degradation process in the

373

presence of a higher proportion of PHA, independently of the rearrangement formed

374

between both polymers.

375

The chemical structure changes of the matrices exposed to the biodegradation process

376

were investigated by ATR-FTIR (Fig. 5c).

377

ATR-FTIR spectra corroborated the changes caused in the structure of the polymers.

378

After 50 days of exposure to the biodegradation process, ATR-FTIR spectra of PHA

379

and blend films showed that the bands lost definition.

380

For all samples assayed, the peaks in the 4000-3000 cm-1 region became broadened after

381

the biodegradation process (data not shown). According to Weng et al.,27 this fact was

382

attributed to the formation of hydroxyl and carboxylic groups. This was also confirmed

383

by the shift of the absorption peak of C=O stretching vibration after biodegradation.

384

Ludueña et al.31 pointed that the chemical and enzymatic hydrolysis are the main

385

mechanisms of chain rupture during the degradation process. Then biodegradation

386

process depends on water availability, which produces hydrolysis, and microbial attack

387

of the matrix. Water diffusion in the soil led to a hydrolytic degradation of the material

388

modifying the surface and visualizing a porous structure with evident signs of

389

degradation. Similar explanation was proposed by Rocca-Smith et al.32


Page 19 of 34


390

The biodegradability of blends was more marked in films formulated with increasing

391

proportions of PHA; similar results were reported by Zhang and Thomas.11

392 393

CONCLUSIONS

394

Thermal and microstructural studies revealed that blends of PLA/PHA based systems

395

were partially miscible. The formulation of blend matrices improved the crystallinity of

396

the PLA; additionally, it induced the polymer recrystallization process, since PHA

397

crystallized acting as a PLA nucleating agent. This phenomenon explains the

398

improvements in water vapor barrier film properties. Moreover, the blends exposed to a

399

biodegradation process showed an intermediate behavior between PLA and PHA

400

leading to a consistent basis for designing systems tailored to a particular purpose or

401

use.

402

This work contributes therefore to the knowledge of microstructure allow developing

403

and testing materials with specific technological applications.

404 405

ACKNOWLEDGMENTS

406

Authors acknowledge Ing. Javier Lecot, Lic. Diana Velasco, and Sr. Daniel Russo for

407

technical assistance.

408 409

FUNDING SOURCES

410

This work was supported by the Argentinean Agency for the Scientific and

411

Technological

412

Argentinean National Research Council (CONICET) (PIP 2013-0109) and University of

413

La Plata (Argentina).

Promotion

(ANPCyT)

(Project

PICT

2012-0415,

2014-1620),



Page 20 of 34

415

REFERENCES

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Bioresour. Technol. 2016, 218, 692-699.

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3. Mothes, G., Schnorpfeil, C., Ackermann, J.-U. Production of PHB from crude

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polyhydroxyalkanoate (PHA) films. J. Polym. Environ. 2008, 16(1), 1-11.

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acid multilayers. Adv. Mater. Lett. 2017, 8(6), 741-751.

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12. Arrieta, M.P., López, J., Hernández, A., Rayón, E. Ternary PLA–PHB–Limonene

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14. Arrieta, M.P., Fortunati, E., Dominici, F., López, J., Kenny, J.M. Bionanocomposite

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508

FIGURE CAPTIONS

509

Fig. 1 MDSC curves of individual (PLA, PHA) and blend films with different

510

proportions of PLA/ PHA showing: a) heating stage, b) cooling stage, and c) reversing

511

heat flow (W g-1) evidencing the glass transition temperatures (Tg) of each component.

512

Fig. 2 ATR-FTIR spectra of individual (PLA, PHA) and blend matrices with different

513

proportions of PLA/ PHA.

514

Fig. 3 X-ray diffractograms of individual (PLA, PHA) and blend films with different

515

proportions of PLA/ PHA. Table insert shows the crystallinity degree (%) of the

516

samples.

517

Fig. 4 Cross-section SEM micrographs of individual (PLA, PHA) and blend films with

518

different proportions of PLA/ PHA. Different letters indicate significant differences (p

519

Microstructural Behaviors of Matrices Based on Polylactic Acid and

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