Accepted Manuscript Title: Inducing PLA/starch compatibility through butyl-etherification of waxy and high amylose starch Author: Obiro Cuthbert Wokadala Naushad Mohammad Emmambux Suprakas Sinha Ray PII: DOI: Reference:
S0144-8617(14)00575-X http://dx.doi.org/doi:10.1016/j.carbpol.2014.05.095 CARP 8964
To appear in: Received date: Revised date: Accepted date:
2-4-2014 19-5-2014 21-5-2014
Please cite this article as: Wokadala, O. C., Emmambux, N. M., and Ray, S. S.,Inducing PLA/starch compatibility through butyl-etherification of waxy and high amylose starch, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.05.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights Compatibility of both waxy and high amylose starches with PLA was improved.
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Butyl-etherified high amylose starch was more compatible with PLA
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PLA/butyl-etherified waxy starch gives better mechanical properties
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Starch butyl-etherification did not affect the stability of PLA/starch composites
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Inducing PLA/starch compatibility through butyl-etherification of waxy
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and high amylose starch
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Obiro Cuthbert Wokadalaab, Naushad Mohammad Emmambuxc, Suprakas Sinha
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Rayabc
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a
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Research, 1-Meiring Naude Road, Brummeria, Pretoria 0001, Republic of South Africa
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b
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South Africa
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c
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Abstract
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In this study, waxy and high amylose starches were modified through butyl-etherification to facilitate
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compatibility with polylactide (PLA). Fourier transform infrared spectroscopy, proton nuclear magnetic
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resonance spectroscopy and wettability tests showed that hydrophobic butyl-etherified waxy and high
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amylose starches were obtained with degree of substitution values of 2.0 and 2.1 respectively.
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Differential scanning calorimetry, tensile testing, and scanning electron microscopy (SEM)
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demonstrated improved PLA/starch compatibility for both waxy and high amylose starch after butyl-
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etherification. The PLA/butyl-etherified waxy and high amylose starch composite films had higher
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tensile strength and elongation at break compared to PLA/non-butyl-etherified composite films. The
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morphological study using SEM showed that PLA/butyl-etherified waxy starch composites had a more
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homogenous microstructure compared to PLA/butyl-etherified high amylose starch composites.
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Thermogravimetric analysis showed that PLA/starch composite thermal stability decreased with starch
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butyl-etherification for both waxy and high amylose starches. This study mainly demonstrates that
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PLA/starch compatibility can be improved through starch butyl-etherification.
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DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial
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Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg,
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Department of Food Science, University of Pretoria, Pretoria, South Africa
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Highlights
Compatibility of both waxy and high amylose starches with PLA was improved.
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Butyl-etherified high amylose starch was more compatible with PLA
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PLA/butyl-etherified waxy starch gives better mechanical properties
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Starch butyl-etherification did not affect the stability of PLA/starch composites
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Key words
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Polylactic acid (polylactide), modified starch, compatibility, biopolymer composites
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1 Introduction
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Renewable biodegradable materials have increasingly become important due to decreasing stocks
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and the negative environmental effects on of fossil fuel derived materials (Mohanty, Misra & Drzal,
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2002). Polylactide (PLA) is the most widely researched renewable biodegradable material for a vast
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range of industrial applications (Auras, Lim, Selke & Tsuji, 2011; Ray, 2012). PLA is linear
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hydrophobic polyester that is; biodegradable, synthesized controllably; inherently biocompatible; and
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has good mechanical properties. The most limiting short-coming to the wide application of PLA is its
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high price relative to other competing polymers. In order to reduce the relative expense of PLA,
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attempts have been made to blend PLA with starch (Avérous & Halley, 2009). Utilization of starch in
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polymer composites has become attractive due to its ready availability, biodegradability, renewability,
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ease of chemical modifications, relatively low cost compared to other commercial biodegradable
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materials.
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Native starch consists of granules (1-100 μm) which are made up of two glucose polymers; an
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essentially linear amylose and a highly branched amylopectin (Tester, Karkalas & Qi, 2004). Some
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starch referred to as waxy starch may contain high amylopectin (>85% w/w) while other types contain
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a high amount of amylose (40-80 %, high amylose starches) (Copeland, Blazek, Salman & Tang,
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2009; Tester, Karkalas & Qi, 2004). Amylose is suggested to behave as a linear polymer which
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facilitates intra- and extra-molecular interactions while amylopectin forms less aligned structures due
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to a highly branched structure (Liu, Xie, Yu, Chen & Li, 2009).
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Starch granules provide a major hindrance to application of starch based composite materials since
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they prevent intimate interaction between starch molecules and other polymers (Chivrac, Pollet &
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Avérous, 2009). Although the disruption of starch granules through production of thermoplastic starch
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can reduce the detrimental effect of granules, the hydrophilic nature of thermoplastic starch hinders
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compatibility with hydrophobic polymers such as PLA. Studies utilizing three-component PLA-
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compatibilizer-starch systems have demonstrated improvements in PLA/starch compatibility (Huneault
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& Li, 2007; Zhang & Sun, 2004). Some studies have focused on utilizing two-component PLA-starch
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systems through chemical modification of starch with maleate-group (Dubois & Narayan, 2003;
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Hwang, Shim, Selke, Soto-Valdez, Rubino & Auras, 2013; Raquez, Nabar, Narayan & Dubois, 2008;
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Wootthikanokkhan et al., 2012), and epoxy resins (Xiong et al., 2014).
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There have been no studies reported on butyl-etherification (C4) for inducing PLA/starch compatibility.
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In addition, no studies that have considered the potential effects of higher amylopectin and higher
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amylose proportions in starch on compatibility with PLA. The objective of the present study therefore,
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was to determine the effect of butyl-etherification of waxy and high amylose starch on PLA/starch
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compatibility.
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2. Experimental
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2.1. Materials
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The PLA utilized in the present research was a commercial grade PLA 2002D® (Natureworks LLC,
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USA). The PLA had an average molecular weight (Mw) 235 kg/mol, and a D-isomer percentage of
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about 4%. The density of the PLA was 1.24 g/cm while the glass transition and melting temperatures
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were approximately 60 °C and 153 °C respectively. Waxy maize starch (S9679; about 100%
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amylopectin) and high amylose maize starch (S4180; about 70% amylose) were obtained from Sigma
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Aldrich (USA). Butylene oxide (1,2-epoxybutane), sodium hydroxide, sodium sulphate and other
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chemicals were of analytical grade and were obtained from Merck (South Africa).
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2.2. Preparation of butyl-etherified starch and wettability test
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Butyl-etherification of high and low amylose starch was done according to Bien, Wiege and Warwel
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(2001) with some modifications using a Parr stirred reactor (Parr Instrument Company, USA). The
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molar ratio of both sodium hydroxide and sulphate to starch was 1:2 while the molar ratio of butylene
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oxide to starch (as glucose) was 1:7. The reaction material was heated at 140 ⁰C with a stirring at 250
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RPM for 100 min. Non-butyl-etherified waxy and high amylose starch controls were prepared without
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addition of the butylene oxide and precipitated with access acetone. All the samples were freeze dried
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to obtain the butyl-etherified and non-butyl-etherified starch material. The hydrophobic character of
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the prepared starch was assessed by testing the wettability in acetone/water solutions of 90% and
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45% v/v. The starches (2g) were dispersed in an acetone/water (90%) and stirred vigorously for 30
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minutes at room temperature. The appearance of the solutions was recorded. More water was added
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to the dispersions/solutions to a 30% acetone/water ratio and the appearance of the resultant
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solutions also recorded.
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2.3. Proton nuclear magnetic resonance spectroscopy (1H-NMR) and degree of
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substitution (D.S)
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The grafting of the hydroxybutyl-group onto the starch molecules was confirmed with H-NMR using a
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Varian Mercury 400 MHz NMR spectrometer (Varian Mercury, USA). The measurements were done
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with dimethyl sulfoxide-d6 (D-DMSO). The D-DMSO peak was used as the internal standard and the
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degree of molecular substitution was then determined from the ratio of the integrals of the alkyl
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protons and the glycosyl ring protons according to De Graaf, Lammers, Janssen and Beenackers
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(1995).
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2.4. Fourier-transform infrared (FTIR) measurements
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The FTIR of the modified starches and latter, the PLA/starch composite films, was assessed using a
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Perkin-Elmer spectrum 100 FTIR spectrometer (Perkin-Elmer, UK) with a diamond/ZnSe crystal under
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universal attenuated total reflectance. The scanning was done on the films at a resolution of 4 cm
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with 32 scans in the range 800-4000 cm . The individual curves (for each sample) were normalized to
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enable comparison of peaks intensities.
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2.5. Preparation of PLA/butyl-etherified starch composite
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PLA pellets were dried overnight at 80 ⁰C. The starches (butyl-etherified and non-butyl-etherified
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waxy or high amylose) and PLA at different levels of addition (10, 20, 30, 40% w/w of PLA) were
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accurately weighed and manually mixed. The mixtures were melting blended in a HAAKE PolyLab OS
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Rheomix (Thermo Electron Co., USA) at 165 ºC for 10 min with a rotor speed of 80 rpm. The samples
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were then compression moulded into thin films (about 0.5-0.8 mm thick) with a total residence time of
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10 minutes at 165 ⁰C using a Carver compression mould (Carver, USA).
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2.6. Thermal analysis
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Differential scanning calorimetry (DSC) was done using a DSC-Q2000 instrument (TA Instruments,
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USA). The samples (about 7.5 mg) were heated at a rate of 10 ⁰C/min from -20 ⁰C to 200 ⁰C. They
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were then held at 200 ⁰C for 5 min in order to remove the thermal history. Cooling was then done at a
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rate of 10 ⁰C/min to -20 ⁰C and heated again 10 ⁰C/min to 200 ⁰C. The glass transition temperature
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(Tg), crystallization temperature (Tc), melting temperature (peak) (Tm) were determined from the
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second heating scan of thermogram. The degree of crystallinity ( ) was determined according to
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Battegazzore, Alongi and Frache (2014) using Equation 1.
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d
..................................................................Equation 1
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where:
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starch weight percentage in the composites and
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93J/g (Iannace & Nicolais, 1997). The experiment was performed in triplicate.
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2.7. Thermogravimetric analysis
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Thermogravimetric analysis of the PLA-Starch blends was done using a Q500 (TA Instruments, USA)
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thermogravimetric analyser (TGA) instrument. The samples of similar shape and size and about 13.5
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± 0.5 mg were used. The samples were heated from 20 ⁰C at a rate of 20 ⁰C /min up to 600 ⁰C under
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nitrogen. The resultant data was analysed using TA Universal analysis software. The temperature
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(Tmax) at which maximum rate of degradation occurs, the thermal degradation onset temperature (5%
is the melting enthalpy of a given sample,
is the
is the standard PLA crystal melting enthalpy of
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weight loss, T5%) and the char value (% residue at 600 ºC) were determined. The samples were
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analysed in triplicate.
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2.8. Mechanical/Tensile Properties
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The tensile strength, elongation at break, and modulus of the PLA/butyl-etherified starch composite
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films and their respective controls were determined according to ASTM D882 using an Instron 5966
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tester (Instron Engineering Corporation, USA). The prepared films were cut into test stripes of 100mm
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by 10 mm. The test stripes were conditioned in a moisture equilibration chamber with saturated
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magnesium nitrate solution (RH 50%, 25 ⁰C) for 1 week. Tests were done immediately after
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equilibration and at least 10 runs were done for each treatment and the average values reported with
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a coefficient of variation of 10% at most.
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2.9. Morphological characterization
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The cross-sectional morphology of the PLA/starch composite films at 10% w/w and 30% w/w starch
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addition levels was determined using a scanning electron microscopy (SEM) (JEOL model JSM-
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7500F) equipment with an accelerating voltage of 5 kV. The film samples were placed in liquid
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nitrogen for 30 minutes and then freeze fractured. They were sputter coated with carbon in order to
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enhance conductivity. The size of distinct areas observed was determined using ImageJ software
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(National Institutes of Health, USA). The Ferret’s diameter (longest distance along a given particle)
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was measured for each distinct particle and reported as an average of at least 50 particles.
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3.0. Results and Discussions
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3.1. Verification of starch butyl-etherification and hydrophobic character
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The equation for the butyl-etherification that was intended in this research is shown in Figure 1a while
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the H-NMR spectra for the butyl-etherified and non-butyl-etherified waxy and high amylose starches
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obtained are shown in Figure 1b. The assignment of peaks as done based on Teramoto, Motoyama,
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Yosomiya and Shibata (2003). The peaks at 4.0-5.6 ppm could be assigned to the hydroxyl groups of
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the glucose residues. These peaks decreased when both the non-modified waxy and high amylose
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starch were butyl-etherified (see Figure 1b). This indicated reduction in the glycosyl residue hydroxyl
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protons. Peaks corresponding to methyl/methylene protons were observed at 0.082 ppm (position 1 of
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Figure 1a), 1.24 ppm (positions 2 and 4 on Figure 1a), and 1.42 ppm PPM (position 3 on Figure 1a)
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were observed for the butyl-etherified waxy and high amylose starches. These indicated the grafting
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of the hydroxybutyl-group to the starch residues.
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Figure 1:
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amylopectin) Maize Starch (a) and the associated butyl-etherification equation (b).
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The FTIR spectra of non-butyl-etherified waxy starch, butyl-etherified waxy starch, non-butyl-etherified
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high amylose starch and butyl-etherified high amylose starch are shown in Figure 2. There was a
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strong broad peak for the non-butyl-etherified starches in the range 3200-3400 cm (Figure 2). This
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peak can be attributed to stretching of glycosyl –OH groups. This peak was diminished in the both the
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butyl-etherified waxy and high amylose starches (Figure 2). This indicated a decrease in the glycosyl
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–OH groups due to substitution by the hydroxybutyl-group. The FTIR spectra of the butyl-etherified
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waxy and high amylose starches also showed peaks at 2,800-2,995 cm
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vibration in the methyl/methylene component of hydroxybutyl group added (Figure 2). The
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methyl/methylene components of the hydroxybutyl group in the butyl-etherified waxy and high
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amylose starches also led to distinct –CH asymmetric and symmetric stretching vibrations peaks at
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1460 and 1360 cm . There was also an increase in the intensity of the peak at 1110 cm due C-O
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bond stretching which indicated of attachment of the hydroxybutyl group to the glucose residues in the
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starch. Similar results showing increases in decreases stretching of glycosyl –OH, incidence of new
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symmetric C-H vibrations and –CH asymmetric/symmetric vibrations have been reported for
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carboxymethylated (Yuen, Choi, Phillips & Ma, 2009) and propyl-etherified (Teramoto, Motoyama,
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Yosomiya & Shibata, 2003) starches.
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From the NMR and FTIR results (Figure 1b and 2 respectively), there was apparently no clear
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difference in substituted starch between the waxy and high amylose starch. The calculated degrees of
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substitution of the butyl-etherified high amylose starch (HAS) and waxy starch (D.S) were 2.1 and 2.2
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respectively. From the wettability test (Figure 3), it could be noted that the butyl-etherified waxy starch
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and high amylose starches were soluble in the relatively non-polar system (90% acetone) (Figure 3a).
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On changing of the system to a more polar one (30% acetone, 70% water), the modified starches
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precipitated (Figure 3b). This indicated that the starch modification by butyl-etherification gave a more
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hydrophobic starch for both waxy and high amylose starch when compared to the respective control
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samples that were not modified.
H-NMR spectra of butyl-etherified and non-modified high amylose and waxy (high
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Figure 2: FTIR Spectra of non-modified and butyl-etherified high amylose and waxy (high
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amylopectin) Starch.
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WS = Non-butyl-etherified waxy starch, BWS = butyl-etherified waxy starch, HAS = Non- butyl-
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etherified high amylose starch, BHAS = butyl-etherified high amylose starch.
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The curves presented were normalized individually and then plotted with vertical offsetting for clarity.
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Figure 3: Wettability of waxy (WS), butyl-etherified waxy (BWS), high amylose (HAS), and butyl-
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etherified high amylose (BHAS) starch in acetone/water solutions at 90% v/v (a) and after adding
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extra water to 30 % v/v (acetone/water) (b).
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BWS = butyl-etherified waxy starch, WS = Non-butyl-etherified waxy starch, BHAS = butyl-etherified
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high amylose starch, HAS = Non- butyl-etherified high amylose starch.
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3.2. Thermal properties
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Figure 4 shows the DSC thermograms of the PLA/hydrobutylated starch composites and the control
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samples (neat PLA and PLA/starch composites with non-butyl-etherified starches) for both waxy and
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high amylose starches. The PLA glass transition temperature (Tg) (60⁰C) decreased with increased
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amounts of butyl-etherified starches while addition of the non-butyl-etherified did not affect the glass
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transition. The Tg of PLA/starch blends containing butyl-etherified waxy starch were 58, to 54, 50 and
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47 ⁰C while that for the blends containing butyl-etherified high-amylose maize starch were 57, 50, 45
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and 38 ⁰C at 10, 20, 30 and 40% addition respectively. Addition of the control starches (non-butyl-
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etherified), did not affect the Tg of PLA. At the different starch addition levels, the blends containing
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butyl-etherified waxy starch lead to a lower reduction in Tg compared to those with butyl-etherified
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high-amylose starch (see Figure 4 a and b). A reduction in PLA Tg with addition of the butyl-etherified
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starches indicated that there was improved compatibility between PLA and the starches due to butyl-
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etherification. This was probably due to the increased hydrophobic character of the starch (as
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indicated by wettability test) which allowed for better distribution of the starch in the PLA matrix hence
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leading to a plasticizing effect. The increased hydrophobicity of the starches could have facilitated
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plasticizing effect of the starches PLA hence a decrease in Tg. Such plasticizing effects by modified
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starch were suggested to result from the presence of bulky added hydrophobic side chains on the
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starch by Aburto et al. (1997). The more bulky nature of the amylopectin molecules compared to
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amylose probably contributed to the greater plasticizing observed for butyl-etherified waxy starch
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compared to the butyl-etherified high amylose starch.
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The cold crystallization peak temperature decreased with increased amounts of added butyl-etherified
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waxy and high-amylose starches (see Figures 4a and b respectively). While those containing non-
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butyl-etherified starches did not show any clear crystallization activity (see Figures 4c and 4d). It
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should be noted that cold crystallization of PLA in a DSC experiment is an experimental condition
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generated phenomenon which depends on the cooling and heating conditions used. However, cold
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crystallization can be used for comparison of the crystallization between samples under the same
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experimental conditions. The PLA cold crystallization temperatures of the blends containing butyl-
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etherified waxy maize starch were 131, 126, 124 and 113 ⁰C; those for the blends with butyl-etherified
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high-amylose maize starch were 128, 109, 98 and 93 ⁰C at 10, 20, 30 and 40% starch respectively. It
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could be noted that, the cold crystallization temperatures were lower with addition of butyl-etherified
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high-amylose starch compared to butyl-etherified waxy starch at corresponding levels of addition.
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Butyl-etherification of starch apparently causes nucleating effects which are greater for high amylose
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starch compared to high amylopectin starch. Nucleating effects of modified starches on PLA were
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also recently reported by Hwang, Shim, Selke, Soto-Valdez, Rubino and Auras (2013) and Ouyang,
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Wang, Zhao, Liu, Li and Zhang (2012).
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Melting endotherm consisting of two co-operative melting peaks was observed at higher addition of
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both butyl-etherified waxy (see Figure 4a) and high amylose (see Figure 4b) starch. This effect was
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observed at only 40% w/w addition level for PLA/butyl-etherified-waxy-starch composites while it
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started at 20% addition PLA/butyl-etherified-high-amylose-starch composites. Co-operative-melting
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peaks indicate a meta-stable crystalline structure. Therefore, at a particular corresponding starch
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addition level, the PLA/butyl-etherified-waxy-starch composites crystallites were probably different
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from those in PLA/butyl-etherified-high-amylose-starch composites. This implies that the starch
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amylose/amylopectin ratio possible modulates the structure of PLA crystallites in PLA/butyl-etherified
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starch composites.
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The Tm decreased with the addition of the butyl-etherified waxy and high-amylose starches (see
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Figures 5a and 5b respectively). The addition of non-butyl-etherified starches at the different levels of
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addition did not affect the PLA melting temperature (see Figures 5a and 5b). The PLA melting
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temperature decreased with the addition of the butyl-etherified waxy and high-amylose starches (see
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Figures 5a and 5b respectively). Neat PLA had a melting temperature of 153⁰C. The melting
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temperatures of the blends containing butyl-etherified waxy maize starch were 152, 149, 147, and
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146⁰C; while those for the blends with butyl-etherified high-amylose maize starch were 151, 150, 149
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and 146 ⁰C at 10, 20, 30 and 40% starch respectively. A depression of the Tm is an indication of
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compatibility for polymer blends (Aburto et al., 1997). The butyl-etherification hence improved the
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PLA/starch miscibility for both waxy and high amylose starch but high amylose starch was apparently
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more compatible with PLA.
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The ∆H and
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than those of the composites with non-butyl-etherified starches at the different respective starch
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addition levels (see Figure 4). The ∆H and
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starches were higher than those of the composites with butyl-etherified waxy starch at the different
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levels of starch addition. The ∆H and
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amylose starch were 21 J/g (
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10, 20, 30 and 40% starch respectively. The ∆H and
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etherified waxy maize starch were 9J/g (
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37%) at 10, 20, 30 and 40% starch respectively. The ∆H and
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non-butyl-etherified high amylose maize starch were 1J/g (
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4J/g (
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of the composites with butyl-etherified waxy and high amylose starches were higher
of composites with butyl-etherified high amylose
of the PLA-starch composites containing butyl-etherified high
= 26%), 24J/g (
= 32%), 23J/g (
= 35%), and 22J/g (
= 40%) at
of the PLA-starch composites containing butyl-
= 11%), 12J/g (
= 16%), 12J/g (
= 19%), and 21/g (
=
of the PLA/starch composites with
= 1%), 3J/g (
= 4%), 4J/g (
= 7%)and
= 7%) while those for the composites with non-butyl-etherified waxy starch were 1J/g (
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=1%), 1 J/g (
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The higher ∆H and
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PLA/butyl-etherified waxy starch composites indicated that butyl-etherified high amylose starch
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probably had a higher PLA nucleation effect than butyl-etherified waxy starch. The stronger nucleating
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effect of the butyl-etherified high amylose starch on PLA is probably due close PLA/starch interactions
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in PLA/butyl-etherified high amylose starch composites as a result of the essentially linear structure of
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amylose.
= 1%), 2J/g (
= 2%) and 4J/g(
= 7%) at 10, 20, 30 and 40% starch respectively.
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values for PLA/butyl-etherified high amylose starch composites compared to
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Figure 4: DSC thermograms of PLA/starch composites with butyl-etherified waxy starch (a), butyl-
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etherified high amylose starch (b), non- modified waxy starch (c) and non-modified high amylose
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starch (d).
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BWS = butyl-etherified waxy starch, WS = Non-butyl-etherified waxy starch, BHAS = butyl-etherified
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high amylose starch, HAS = Non- butyl-etherified high amylose starch
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The numbers 10, 20, 30, 40 represent the percentage (% w/w) starch addition levels.
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Curves presented are representative second heating scan thermograms
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3.3 Thermal stability
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Thermogravimetric analysis measurements depend on the extent of heat induced bond cleavage in a
293
given system due to thermal energy being greater than the bond energies. Interpretation of TGA
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results however should be done with caution since TGA only measures the changes in weight due to
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released vaporized material after the bond breakages.
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The TGA and the derived TGA curves for the PLA/starch composites in the present study are shown
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in Figure 5 while the associated TGA parameters are shown in Table 1. The degradation onset (T5%)
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and maximum degradation rate temperatures (Tmax) of PLA were significantly (p