Preparation and Properties of Polylactide Bio-composites with ...

Chiang Mai J. Sci. 2018; 45(5) 2059

Chiang Mai J. Sci. 2018; 45(5) : 2059-2068http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Preparation and Properties of PolylactideBio-composites with Surface-modified Silica ParticlesNarisara Jaikaew [a], Atitsa Petchsuk [b] and Pakorn Opaprakasit* [a]

[a] School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology

(SIIT), Thammasat University, Pathum Thani, 12121, Thailand.

[b] National Metals and Materials Technology Center (MTEC), Thailand Science Park, Pathum Thani,

12120, Thailand.

* Author for correspondence; e-mail: [email protected]

Received: 1 November 2017

Accepted: 30 April 2018

ABSTRACT

Biodegradable plastics have become interesting alternative materials in packagingapplications, because of their lower environmental impacts. Among these, polylactide (PLA)has played a key role, pertaining to many of its excellent properties. However, itscommercialization as packaging materials for products sensitive to light, gas, or temperaturechanges, such as fresh vegetables and fruits, is challenging, due to its moderate gas transmissionrates, and lower mechanical properties, compared to conventional plastic products.One effective method to solve this problem is the adding of inorganic fillers to the PLA resinto improve its properties. In this study, PLA bio-composite films have been developed byintroducing low-cost silica particles as a reinforcing agent. To effectively achieve propertyenhancements, compatibility between the 2 components needs to be improved. Surfacemodification of the inorganic silica particles is conducted by coating with poly (lacticacid-grafted-chitosan) copolymer (PCT). Commercial silica particles (c-silica), with average particlesize of 1-5 μm, were coated with PCT copolymer, employing a phase inversion emulsification(PIE) technique. After the coating process, the average size of the modified particles (m-silica)decreases to 17 nm, as the coated PCT layers on the particle surfaces prevent agglomerationof the silica particles by providing steric repulsion. The resulting m-silica and c-silica are thenused in the preparation of PLA/silica bio-composite films by varying silica types and theircontents from 0-5.0 wt%. Thermal and mechanical properties, light transmission, and gaspermeability of the 2 bio-composite systems, are compared. PLA/m-silica exhibitgreater tensile behaviors at all particle compositions. Variations in CO

2/O

2 permeability of

bio-composite films can be optimized by changing the silica compositions. At 5.0 wt% ofm-silica, the highest decrease in light transmission at 4, 8, and 16% for UV-A, UV-B, and visibleregions, respectively, are observed. The resulting PLA/m-silica bio-composite films have highpotential for use as smart packaging for fresh vegetables and fruits.

Keywords: packaging, polylactide, silica particles, bio-composite, permeability,light transmission

2060 Chiang Mai J. Sci. 2018; 45(5)

1. INTRODUCTION

Biodegradable polymers have attractedvast interest, due to environmental awarenessconcerning global warming, rapid decreaseof petroleum resources, plastic waste andpollution problems. Among these, polylactideor polylactic acid (PLA), a thermoplasticaliphatic polyester, is one of the most attractiveenvironmental-friendly materials, due to itsdegradability, ease of processability, relativelylow cost of production, renewability, andgood mechanical strength [1, 2]. PLA can besynthesized by either polycondensation oflactic acid, or ring-opening polymerization(ROP) of lactide dimer, in which the latter isthe most effective process to obtain highmolecular weight PLA. However, the polymerexhibits poor thermal stability and low meltstrength. This limits its use in several ways[3-6].

Addition of fillers to polymers,generating composite materials, is aneffective method to improve their properties,especially for PLA [7, 8]. The propertyimprovement is achieved by strong interactionsbetween the filler’s particles and the polymermatrix. Various composites of PLA withother biodegradable/biocompatible materialshave been prepared to enhance its thermaland mechanical properties, but still retain itsbiocompatibility and degradability [9-11].Many inorganic fillers have been used forpreparing polymeric composite materials[12, 13]. Among these, silica (SiO2

) is one ofthe most popular, due to its low cost andhighly abundance in nature [14]. There aremany sources of silica such as sand, clay, andash. PLA bio-composites have been widelystudied by many researchers due to theirunique properties, such as biodegradability,biocompatibility, good barrier property,low cost, and high modulus. However,modification of silica particles is required toimprove the compatibility between the

particles and the polymer matrix for achievingenhanced characteristics.

In our previous work, we havesuccessfully prepared silica particles fromrice husk ash (RHA) and modified theirsurfaces with poly(lactic acid-grafted-chitosan)copolymer (PCT) by employing a one-stepcoating method. The modified particles wereused as fillers in the preparation of PLA/silicabio-composite films to enhance theirmechanical properties and gas permeabilityfor use as packaging materials [15].

In this study, surface modification ofcommercial silica particles is conducted bycoating with PCT copolymer employing, aphase inversion emulsification (PIE). Thetechnique provides micro- or nano-particleswith a narrow particle size distribution, andlow process cost [16]. The resulting modifiedsilica particles (m-silica) are then used asreinforcing material for commercial PLAfilms. Thermal, mechanical, and lighttransmission properties, and gas permeabilityof the bio-composite films containinguntreated commercial silica (c-silica) andm-silica, are compared. The materials arehighly suitable for use in active packagingapplications.

2. MATERIALS AND METHODS

2.1 MaterialsPoly(lactic acid-grafted-chitosan) copolymer

(PCT) was synthesized in this laboratory,according to the procedure previouslyreported [15]. Commercial silica (SigmaAldrich, USA) with an average particle sizeof 1-5 μm, was used. In the phase inversionemulsification (PIE) process, poly(vinylalcohol) (PVA) (Sigma Aldrich, USA; degreeof hydrolyzed, 87-90%; molecular weight3×104 g/mol) and sodium dodecyl sulfate(SDS; Sigma Aldrich, analytical reagent), wereused as surfactants. Chloroform (CHCl

3),

Chiang Mai J. Sci. 2018; 45(5) 2061

(Sigma Aldrich, analytical grade) was usedas a solvent. Polylactide (PLA) 4043D pelletswere purchased from Nature Works (USA)and used in the preparation of compositematerials.

2.2 Experimental Methods2.2.1 Preparation of modified silicaparticles (m-silica)

Modified silica particles (m-silica), codedas m-silica, were prepared by coating c-silicawith PCT copolymer, employing a PhaseInversion Emulsification (PIE) method, inwhich aqueous and oil phases are prepared.The aqueous phase consists of 10 g c-silicaparticles, dispersed in 0.1 M NaOH solutioncontaining SDS (12.5%wt of water). The oilphase contains 2.5 g of PCT copolymer and2 g of PVA dissolved in CHCl3

solvent.Both SDS and PVA act as surfactants, andmigrate from the bulk to the interface,leading to a decrease in the interfacial tension.The water phase was added dropwise intothe oil phase at a 1 ml/min rate, withcontinuous stirring at 500 rpm. When thewater phase was completely added, a milkymixture was obtained, and the mixture wasfurther stirred for 1 h. The mixture was thenplaced in a fume hood for one night tocompletely evaporate the CHCl

3 solvent.

The sample was then centrifuged, wherethe precipitants were separated and dried ina vacuum oven at 60 °C for 24 h.

2.2.2 Fabrication of PLA/silicabio-composite films

The modified silica particles (m-silica) andcommercial silica (c-silica) were blendedwith PLA by an internal mixer (Chareon Tut,Thailand) at 170 °C for 15 min. The contentsof the particles were varied from 0.5 to5.0 wt%. Films of neat PLA, PLA/c-silica,and PLA/m-silica were prepared by acompression molding machine (Chareon Tut,

Thailand) at 170 °C for 15 min. The specimenswere prepared in a rectangular sheet form,with a 15 mm width and 100 mm of gaugelength for the tensile test according toASTM D882. For the gas permeability test,films with an average thickness of 40-50 μmwere prepared by the same method.

2.2.3 Surface studies of silica particlesSurface morphology of c-silica and

m-silica particles was examined by SEM(JEOL-JSM-7800F). The samples wereprepared by a goal coating technique.Surface chemical compositions of thematerials were characterized by SEM, withenergy dispersive X-ray analysis (EDX).

2.2.4 Characterization of bio-compositefilms

Thermal behavior of bio-compositefilms was studied by Differential ScanningCalorimetry (DSC822e Mettler Toledo).The evaluation was by a heat-cool-heatstandard method from -20 to 200 °C, at aheating and cooling rate of 20 °C/min.The first heating step is employed to erasethe sample’s thermal history.

The degree of crystallization is calculatedby the following equation:

Xc = × 100%

Where, ΔHm

is the enthalpy of meltingand ΔH

m° denotes the enthalpy of fusion

for fully crystalline PLA, which has a valueof 93.1 J/g [17].

Tensile behaviors of the bio-compositefilms at various particle compositions werestudied by a Universal Testing Machine(T-series Materials testing machine, modelH5TK, Tinius Olsen LTD., UK) equippedwith a 100 N load cell, with a crossheadspeed of 50 mm/min at ambient temperature.At least six specimens were examined for

ΔHm

ΔHm°

2062 Chiang Mai J. Sci. 2018; 45(5)

each test, and an averaged value was reported.Break energy of the samples was calculatedfrom the area under the stress-strain curves.

Permeability tests of the bio-compositefilms were conducted. Oxygen permeabilitywas recorded on a Mocon Instrument(Ox-Tran Model 2/21), following ASTMD3985, at 23 °C, 0% relative humidity.Carbon dioxide permeability was measuredon a Mocon Instrument (Permatran-C Model4/41), at 23 °C and 0% relative humidity.Water vapor permeability was measured onan Illinois Instrument (Model 7200), usingASTM F1249-01, at 38 °C and 90% relativehumidity and 1 atm pressure. UV-Visible-NearIR transmittances of the composite filmswere determined by a spectrophotometer(Agilent Technologies, CARY 7000).The measurements were conducted from250-2500 nm in the transmission mode.

3. RESULTS AND DISCUSSION

3.1 Surface Morphology of Silica ParticlesProperties of silica particles before

and after surface modification, in terms ofsize, shape, and surface morphology, areexamined. In general, inorganic particleswith hydrophilic hydroxyl groups can easilyadhere to each other through a hydrogenbonding network, leading to the formationof aggregate. SEM images of c-silica andm-silica particles, at magnifications of 25000and 50000X, are compared in Figure 1.Both c-silica and m-silica particles show anirregular shape. However, c-silica has a highercontent of aggregate, compared to m-silica.This is likely because the PCT copolymerchains can act as polymeric surfactants byadsorbing on the surface of silica particles,interrupting physical interactions betweensilica particles. The average particle size ofnon-aggregated particles is determinedfrom SEM images by using J-image software.The average sizes of c-silica and m-silicaparticles are 58 and 17 nm, respectively.The m-silica also shows more uniform sizedistribution than c-silica. These result fromeffective coating by the PIE method.

Figure 1. SEM images of (a), (b) c-silica and (c), (d) m-silica, at 25000 and 50000X magnification.

(a) (b)

(c) (d)

25000X 50000X

Chiang Mai J. Sci. 2018; 45(5) 2063

SEM-EDX results on surface compositionsare summarized in Table 1, where 23.4 and7.3 wt% Si contents are observed for c-silicaand m-silica, respectively. The relatively lowerweight percentages of Si and O atoms,and higher C content of m-silica reflect the

presence of PCT copolymers on particlesurfaces. It is expected that the remainingpolymeric surfactant, present as a coatinglayer on particle surfaces, also plays a role inenhancing compatibility with the PLAmatrix.

Table 1. Surface chemical compositions of c-silica and m-silica particles.Elements

COSi

c-silicawt %19.457.223.4

Atomic %26.859.413.8

m-silicawt %56.836.07.3

Atomic %65.331.13.6

3.2 Bio-composite Film PropertiesThermal properties of PLA, PLA/

c-silica, and PLA/m-silica bio-compositefilms at various particles compositions werestudied by DSC, as shown in Figures 2 and 3.All samples have a semi-crystalline structure,with glass transition temperature (T

g) at

60-62 °C, and crystalline melting temperature(T

m) at 149-151 °C. Cold crystallization (T

c)

is observed at 119-124 °C. In addition,physical aging behavior is observed in allsamples from their corresponding 1st heatingscan thermograms, as a sharp peak locatedimmediately following the glass transition.This is because the samples are stored in adesiccator at ambient temperature, withcontrolled humidity (%RH) of 25-50%,for 2 weeks before DSC testing. Physical agingis a natural phenomenon that significantlyaffects the physical properties of amorphousor glassy polymers (or polymers with Tg

).During aging, the material becomes moreand more glass-like and less rubber-like, i.e.,stiffer and more brittle. The effects of agingtakes place at temperatures below T

g, as the

molecular mobility is not quite zero, with aslow and gradually approach to equilibrium.The change in physical properties with agingcan have significant economic implications,and in some cases, it is necessary to predictlong-term behavior from an accelerated

short-time test. Permeability of packagingmaterials is also affected by physical aging.

Figure 2. DSC thermograms (1st heating scan)of (a) PLA, and PLA/m-silica bio-compositefilms at various m-silica compositions: (b) 1.0,(c) 1.5, (d) 2.0, (e) 2.5, and (f) 5.0 wt%.

Figure 3. DSC thermograms (2nd heatingscan) of (a) PLA, and PLA/m-silica bio-composite films at various m-silica compositions:(b) 1.0, (c) 1.5, (d) 2.0, (e) 2.5, and (f) 5.0 wt%.

2064 Chiang Mai J. Sci. 2018; 45(5)

Results from the 2nd heating scan alsoshow a similar trend to that from the 1st heatingscan. All samples have a semi-crystallinestructure with T

g from 59-61 °C, and T

m from

150-152 °C. Tc is observed at approximately

119-121 °C for films containing m-silicawith a composition from 0 to 2.0 wt%.This slightly increases to 123 and 124 °Cfor those containing 2.5 and 5.0 wt%m-silica compositions, respectively.

Results on the thermal properties ofPLA, PLA/c-silica, and PLA/m-silicabio-composite films, derived from DSCthermograms, are summarized in Table 2.From 1.0 to 5.0 wt% of m-silica contents,no significant difference in crystallinity (Xc

)of bio-composite films (19-21%) is observedwhen the silica contents are varied. However,the films with m-silica particles exhibit lower

Tc, compared to the corresponding films

containing unmodified silica. At both2.5 and 5.0 wt% of particle compositions,PLA/c-silica bio-composite films showlower X

c values than neat PLA. This is due

to the larger particle size and the presenceof aggregates in the unmodified silica, as seenfrom SEM images. Aggregates are observedin c-silica and have a tendency to agglomeratewithin the PLA matrix. Fillers with smallerparticle size provide a larger specific surface,and hence, higher efficiency as nucleatingagents for polymer chains. In contrast, thepresence of fillers with a large particlesize can retard the crystalline formation ofpolymer chains. c-silica tends to agglomerateand has a larger particle size in compositesystems, leading to an increase in T

c and a

decrease in Xc of the composite films.

Table 2. Thermal properties of PLA, PLA/c-silica and PLA/m-silica bio-composite films,derived from DSC thermograms.

Sample

PLA

m-silica

c-silica

Silica(%)

01.01.52.02.55.02.55.0

1st ScanT

g

(°C)6060616262626261

Tc

(°C)121120119121123124126125

Δ(Hc)

(J/g)-19-20-21-20-21-19-13-15

Tm

(°C)150149150150150151151151

Δ(Hm)

(J/g)1920212021201515

Xc(%)2122222123211616

2nd ScanT

g

(°C)5959606159606060

Tc

(°C)124123122123125125127125

Δ(Hc)

(J/g)-18-19-20-20-20-19-13-19

Tm

(°C)150150150150151152152151

Δ(Hm)

(J/g)1819201920191420

Xc(%)1921222122211521

Tensile behavior of bio-composite filmsis examined by a Universal Testing Machine(UTM), as summarized in Figure 4. Tensilestrength, elongation at break, modulus, andbreak energy of bio-composites containingm-silica and c-silica are slightly lower thanthat of neat PLA film. In fillers/polymermixtures, filler particles function as a stressconcentrator for the matrix. However, this

may adversely promote crack initiation ifthe interfacial adhesion between fillers andthe polymer matrix are poor, due mainly tothe incompatibility between the 2 components.It is clearly observed that composite filmsconsisting of m-silica show greater tensileproperties than the corresponding filmswith c-silica. This is due to smaller size andmore uniform dispersion of m-silica particles

Chiang Mai J. Sci. 2018; 45(5) 2065

in the PLA matrix. The presence of a PCTcopolymer coating layer on the silica surfaceplays a key role in promoting compatibility

between m-silica and the PLA matrix, andalso enhancing dispersion in the compositesystem.

Figure 4. Tensile strength (a), elongation at break (b), modulus (c), and break energy (d) ofPLA, PLA/c-silica, and PLA/m-silica bio-composite films at various silica compositions.

Gas permeability is a crucial factor forpackaging design because gases such asoxygen, carbon dioxide, and water vapor havestrong effects on shelf-life and quality of foodsand fresh products [18]. The requirementsfor gas permeability of polymer films for aspecific product may vary, depending ontheir activities during storage, such asrespiration and oxidation reactions offresh fruits and vegetables. Those activitiesproduce specific gases such as carbon dioxide(CO2

), oxygen (O2), and water vapor (WV),

which change the atmosphere within thepackaging. It is important to study the gaspermeability of polymer films in order toensure effective preservation of products.Gas permeability of the resulting compositesfilms are examined, in terms of oxygen,carbon dioxide, and water vapor permeability,as summarized in Table 3. Water vapor

permeability of the composite containingc-silica remains significantly unchanged, andthat of m-silica increases around 2.5 times,compared to neat PLA. This is mainly dueto the incorporation of more polar PCTcopolymer chains into the hydrophobic PLAmatrix. Given that gas diffusion throughpolymeric films is strongly dependent onsize, shape, polarity, and solubility of the gasmolecules, the presence of more polar groupsin a composite is favorable for polar watermolecules [19]. The selectivity value, whichrepresents the permeability ratio of differentgases, is also calculated in terms of CO

2/O

2,

CO2/WV, and O

2/WV. Variations in the CO

2/

O2 permeability ratios of bio-composite films

are observed with changes in types of silicaparticles (c-silica and m-silica) and theircompositions. The ratio for PLA/m-silicabio-composite films containing 2.5 and

70

60

50

40

30

20

10

0

Ten

sile

Str

engt

h (M

pa)

0 0.5 1 1.5 2 2.5 5Silica Composition (wt%)

m-silica

c-silica

(a) (b)

m-silica

c-silica

0 0.5 1 1.5 2 2.5 5Silica Composition (wt%)

3.5

3

2.5

2

1.5

1

0.5

0

Elo

ngat

ion

at b

reak

(%)

(d)

m-silica

c-silica

0 0.5 1 1.5 2 2.5 5Silica Composition (wt%)

1.5E+06

Bre

akE

nerg

y (M

J/m

3 )

1.0E+06

5.0E+05

0.0E+00

(c)

m-silica

c-silica

0 0.5 1 1.5 2 2.5 5Silica Composition (wt%)

2400

2300

2200

2100

2000

1900

1800

Mod

ulus

(M

Pa)

2066 Chiang Mai J. Sci. 2018; 45(5)

5.0 wt% of m-silica are 0.05 and 0.87,respectively, while those of PLA/c-silicafilms are much higher at 9.33 and 2.33,

for 2.5 and 5.0 wt% of c-silica compositions,respectively.

Table 3. Gas permeabilities: CO2, O

2, and water vapor (WV)

, of PLA, PLA/c-silica, and

PLA/m-silica bio-composite films containing different particle compositions.

Sample

PLA2.5 m-silica5.0 m-silica2.5 c-silica5.0 c-silica

O2

(cm3⋅mm/m2⋅day⋅atm)3118332315

CO2

(cm3⋅mm/m2⋅day⋅atm)259

282834

WV(g/m2⋅day)

1856531822

CO2/O

2

0.810.050.879.332.26

CO2/

WV1.380.160.521.551.54

O2/

WV1.723.270.610.170.68

It is important to investigate the lighttransmission behavior of composite films,especially in UV, visible, and near infraredregions, in order to design packagingmaterials for each food products. Plasticmaterials tend to undergo photo-chemicaldegradation when exposed to UV radiation(200-400 nm). Also, food products whichexhibit high sensitivity towards visible lighttend to spoil in lighting conditions rangingfrom 400 to 700 nm. Light transmissioncurves of the composite films, measuredby UV-Vis-NIR spectrometry, are shownin Figure 5. Detailed results on %T forPLA/m-silica composite films in UV andvisible regions are provided in Table 4.The transparency percentage of light (%T)decreases for PLA/m-silica composite films,compared to neat PLA, with respect to

an increase in the m-silica composition.This indicates that m-silica particles act as alight absorbing agent for UV (300-400 nm)and Visible (400-765 nm) regions, andprevent the transmission of light throughthe films. Neat PLA film allows the passageof 72, 83, and 88% of radiation in the UV-A,UV-B, and visible regions, respectively.The %T values tend to decrease in UVregions with increasing m-silica content in thebio-composite films. At 5.0 wt% of m-silicacomposition, the film shows a maximumdecrease in %T of 4, 8, and 16%, of radiationin UV-A, UV-B, and Visible regions,respectively. The reduction in %T values isdue to light absorptivity of chitosan in thePCT copolymer structure, coated on them-silica’s surface, which agrees with thatreported by Rapa et al [20].

Figure 5. UV-Visible-NIR transmittance spectra of PLA, PLA/m-silica bio-composite filmsat 2.5, and 5.0 wt% compositions.

Chiang Mai J. Sci. 2018; 45(5) 2067

Table 4. Percentage of light transmission (%T) values for PLA/m-silica composite films.

4. CONCLUSIONS

The surfaces of commercial silica particles(c-silica) are successfully modified by coatingwith PCT copolymer, employing a phaseinversion emulsification (PIE) technique.This effective coating step leads to a decreasein average size of the resulting modifiedsilica particles (m-silica) to around 17 nm,as a result of steric repulsion from PCTcopolymer coated layers, which preventsagglomeration of the silica particles.The resulting modified particles are thensuccessfully used in the preparation ofPLA/silica bio-composite films by varyingthe silica contents (from 0-5.0wt%), and typesof silica particles. PLA/m-silica films exhibitgreater tensile behaviors at all compositions,compared to the correspondingbio-composite films with unmodified c-silicaparticles. The CO2

/O2

permeability ratioof bio-composite films can be tuned byvarying the types of silica particles (c-silicaand m-silica) and their compositions.Light transmission reduction in both the UVand visible regions is achieved in PLA/m-silicabio-composite films. The composite filmshave high potential for use as smart andactive packaging.

ACKNOWLEDGEMENTS

The authors acknowledge financialsupport from the National ResearchUniversity (NRU) grant, provided fromThe Office of Higher Education Commission

(OHEC) and the Center of Excellence inMaterials and Plasma Technology (CoE M@PTech), Thammasat University. N.J. is gratefulfor scholarship support from The RoyalGolden Jubilee (RGJ) Ph.D. program of theThailand Research Fund (PHD/0026/2558).The authors gratefully acknowledge theinstrument support from Center of ScientificEquipment for Advanced Research, Officeof Advanced Science and Technology,Thammasat University.

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