acetylated starch blends: Effect of starch acetylation on the material properties

Journal Pre-proof Poly(lactic acid)/acetylated Starch Blends: Effect of Starch Acetylation on the Material Properties Rasool Nasseri, Robert Ngunjiri, Christine Moresoli, Aiping Yu, Zhongshun Yuan, Chunbao (Charles) Xu

PII:

S0144-8617(19)31120-8

DOI:

https://doi.org/10.1016/j.carbpol.2019.115453

Reference:

CARP 115453

To appear in:

Carbohydrate Polymers

Received Date:

4 June 2019

Revised Date:

3 October 2019

Accepted Date:

5 October 2019

Please cite this article as: Nasseri R, Ngunjiri R, Moresoli C, Yu A, Yuan Z, Xu C(Charles), Poly(lactic acid)/acetylated Starch Blends: Effect of Starch Acetylation on the Material Properties, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115453

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.

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Poly(lactic acid)/acetylated Starch Blends: Effect of Starch Acetylation on the Material

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Properties

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Rasool Nasseria, Robert Ngunjiria, Christine Moresolia,*, Aiping Yua,b,

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Zhongshun Yuanc and Chunbao (Charles) Xuc

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Department of Chemical Engineering, University of Waterloo, 200 University Avenue

b

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West, Waterloo, Ontario, Canada N2L 3G1

Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue

West, Waterloo, Ontario, Canada N2L 3G1 c

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Institute for Chemicals and Fuels from Alternative Resources, Department of Chemical

& Biochemical Engineering, Western University, London, Ontario, Canada N6A 5B9 *

To whom all correspondence should be addressed

Telephone: +1 (519) 888-4567, ext. 35254

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E-mail:[email protected]

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Graphical abstract

Highlights 

Higher thermal stability for PLA/acetylated starch blends compared to PLA



Different morphology was observed for PLA/acetylated starch blends

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Different mechanical properties were observed for PLA/acetylated starch blends

ABSTRACT This study investigated the acetylation of starch to improve its processability and compatibility with poly(lactic acid). The temperature at the maximum rate of degradation increased by 3.2%

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for poly(lactic acid) blends containing acetylated starch degree of substitution 2.5 compared to

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the blend containing neat starch. A biphasic morphology with distinct dispersed phase was predicted and observed experimentally for all blends except the blend containing acetylated

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starch degree of substitution 3. Acetylated starch induced plasticization and nucleation for all

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degree of substitution. The blend containing acetylated starch degree of substitution 2.5 had higher tensile strength (26%), and toughness (29%) compared to the blend containing neat

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starch. The superior mechanical properties of the blend containing acetylated starch degree of substitution 2.5 are attractive for medical implant applications. The continuous microstructure

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and transparency characteristics of the blend containing acetylated starch degree of substitution 3 are attractive for packaging applications.

KEYWORDS: acetylated starch, poly(lactic acid), phase behavior, thermal stability, thermal

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transition, mechanical properties, transport properties.

1. INTRODUCTION The increasing consumption of synthetic polymers has created significant environmental concerns and impact (Mose & Maranga, 2011; Nasseri & Mohammadi, 2014; Pimentel, Durães, Drummond, Schlemmer, Falcão & Sales, 2007; Schlemmer, de Oliveira & Araújo Sales, 2007).

4 One alternative to address these environmental concerns is the development of biodegradable polymers such as poly(lactic acid) (PLA), poly(hydroxyalkanoates) (PHAs), polycaprolactone (PCL) and polyesteramide (PEA). Among these biodegradable polymers, PLA is one of the most promising candidates due to the renewability of its feedstock, its compostability, and adequate mechanical properties and permeability (Ferri, Garcia-Garcia, Sánchez-Nacher, Fenollar &

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Balart, 2016; Zuo, Gu, Yang, Qiao, Tan & Zhang, 2014). The relatively high cost of PLA and its slow degradation rate, however, limit its application in daily usages. Blending PLA with low-

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cost biodegradable polymers such as starch can alleviate these shortcomings. Starch is a promising biodegradable material due to its low cost, abundance and renewability (Wang, Pan,

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Wu & Zhao, 2014). The high degree of crystallinity of native starch is the main obstacle in its

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thermal processing due to the lack of thermal transition before degradation (Orford, Parker, Ring & Smith, 1989) while its hydrophilic characteristics and poor compatibility limit its application

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in PLA blends.

Starch plasticization can improve its processability and compatibility with PLA. Finer

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thermoplastic starch (TPS) dispersion, higher tensile strength and modulus but lower crystallization rate were observed for PLA/TPS blends with high sorbitol/glycerol ratios (Li & Huneault, 2011). PLA/TPS blend containing 27 wt% sorbitol-plasticized TPS (36 wt% plasticizer) showed 28% increase in tensile strength and 6% increase in tensile modulus

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compared to glycerol-plasticized TPS. In spite of these improvements, plasticizers may induce and lead to undesired effects such as starch retrogradation and embrittling or phase separation ultimately creating biphasic blends and heterogeneous materials. Poor compatibility of starch with PLA remains an issue.

5 The use of compatibilizers has been examined as means to improve the compatibility between TPS and PLA. The compatibilization of 50:50 PLA/TPS (80 wt% starch and 20 wt% glycerol) with maleic anhydride, increased the tensile strength by 30% and led to smaller and more uniform TPS dispersed phase (Ren, Fu, Ren & Yuan, 2009). In another work, the compatibilization of PLA/TPS containing 30 wt% TPS (10 g of corn starch into 15 mL distilled

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water and 6.5 g of ethylene glycol) with 2 wt% maleic anhydride, increased the tensile strength by 10% when maleic anhydride was first mixed with PLA and subsequently added to TPS (Ali

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Nezamzadeh, Ahmadi & Afshari Taromi, 2017).

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A number of chemical modification of starch have been considered as means to improve its compatibility with PLA and ultimately its processability. The butyl-etherification of waxy and

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high amylose starch produced PLA/butyl-etherified starch blends with increased homogeneity of the microstructure, about 90% increase in tensile strength and 2.5-fold increase in elongation at

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break at the expense of lowering the temperature of the maximum degradation rate for PLA/butyl-etherified waxy starch blend (80/20 wt/wt) compared to PLA/native waxy starch

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blends (Wokadala, Emmambux & Ray, 2014). In a different study, chemical modification of cassava starch with poly(2-Ethylhexyl Acrylate) was carried out to increase the compatibility between starch and PLA. PLA/starch-g-poly(2-Ethylhexyl Acrylate) blend containing 10 wt% modified starch showed about 33-fold increase in elongation at break and 21-fold increase in

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toughness (Bunkerd, Molloy, Somsunan, Punyodom, Topham & Tighe, 2018). In another work, relative efficiency of three different compatibilization strategies for PLA/TPS blend, namely in situ formation of urethane linkages, coupling with peroxide between starch and PLA, and the addition of PLA-grafted amylose were evaluated. The best compatibilization efficiency was

6 observed when PLA-grafted amylose was used where tensile strength of PLA/TPS blend containing 60wt% TPS increased by 60% (Schwach, Six & Avérous, 2008). The acetylation of starch, a relatively simple chemical modification which consists of the replacement of the hydroxyl groups in the glucose units with acetyl groups, has been overlooked and has received limited attention for the development of PLA blends. Most attention has been

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given to PLA/ Acetylated starch (AS) foam materials. AS is characterized by its degree of substitution, which refers to the substitution of the hydroxyl groups of the glucose units with

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acetyl groups. The benefits of acetylation include the alteration of the hydrophobicity of starch

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(Ashogbon & Akintayo, 2014), the weakening of the intermolecular hydrogen bonding and the decreased in crystallinity, thus facilitating the mobility of the molecules providing an opportunity

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to participate in glass transition (Fringant, Desbrières & Rinaudo, 1996). Improved thermal stability has also been reported after acetylation (Garg & Jana, 2011; Zhang, Zuo, Wu, Wang &

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Gao, 2012). PLA blends containing glycerol plasticized AS (GPAS), with unknown DS, have shown improvements in their mechanical properties (Zhang, Ran, Zhuang, Yao & Dong, 2009).

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A 2.5-fold increase in elongation at break of PLA was observed with addition of 20 wt% of GPAS at the expense of 34% decrease in tensile strength and about 10°𝐶 decrease in the temperature of the maximum degradation rate. The effect of DS on the properties of PLA/GPAS

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was not discussed in their study.

The main objective of our study was to address the lack of knowledge on the role of the degree of substitution (DS) of acetylated starch (AS) in PLA/AS blends. AS with defined DS, covering the full DS spectrum, were used to prepare PLA/AS blends. The first step was to investigate the thermal stability by thermogravimetric analysis (TGA), the morphology of the PLA/AS blends by comparing phase behavior predictions based on thermodynamic principles to visual

7 observations obtained by scanning electron microscopy and the thermal transitions and crystallization behavior obtained by differential scanning calorimetry (DSC). Finally, the mechanical properties, water uptake and water vapor permeability of the PLA/AS blends were used to assess two potential applications, medical implants and packaging materials.

2. MATERIALS AND METHODS

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2.1. Materials

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PLA 4032D (Mn=58000 g/mol, D-content 1.8 %) was provided by NatureWorks Inc (USA). Corn starch was purchased from local Ontario market (BulkBarn®, product code: 000260). The

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amylose content of the starch was estimated to be 25.4±1.6 wt% (Mahmood et al’s method

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(Mahmood, Turner & Stoddard, 2007), details in supplementary information). Acetic anhydride (≥ 97.0%) and acetic acid (≥ 99.7%) were obtained from CALEDON laboratory chemicals

(Canada).

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2.2. Methods

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(Canada). Dimethyl sulfoxide (HPLC grade, ≥ 99.9%) was purchased from Sigma-Aldrich

2.2.1. Acetylation of starch

Corn starch was vacuum-dried at 55 ℃ for 24 hours prior to acetylation. Native starch was coded

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as DS0. AS with DS, 0.5, 1.5, 2.5 and 3, were produced and coded as DS0.5, DS1.5, DS2.5, and DS3, respectively. A typical acetylation reaction would be initiated by placing 15 g vacuumdried corn starch, acetic anhydride (amount defined by the molar ratio between starch and acetic anhydride and desired DS) in 250 mL three neck glass reactor. A mass of 0.375 g of toluene sulfuric acid as the catalyst would be mixed with acetic acid and subsequently adding this

8 mixture to the 250 mL three neck glass reactor equipped with a nitrogen stream. The reactor was heated at 135 ℃ in an oil bath with magnetic stirring. After 180 min, the reaction was stopped by placing the reactor in water. The cooled mixture was transferred into a 500 mL beaker and was precipitated with methanol as the non-solvent. The residual viscous product was rinsed with methanol and recovered into a 500 mL beaker. The precipitated AS was filtered and rinsed with

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an additional 200 mL of methanol. The mixture was filtered again and the final AS was vacuumdried at 45℃.

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2.2.2. Preparation of PLA/AS blends

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PLA and AS were dried overnight at 80°𝐶 in a vacuum oven. The PLA/AS blends were produced in a SJSZ-07A lab-scale twin screw extruder (Ruiming Plastics Machinery, Wuhan

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city, China) where the blend components were mixed at 180°𝐶 with a screw speed of 30 rpm for

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7 min. Based on previous work (Zhang, Ran, Zhuang, Yao & Dong, 2009) the amount of AS was chosen to be 15 wt% in order to achieve a droplet-matrix morphology and to be able to

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understand the effect of an interface. The mixing temperature, speed and time were identified from preliminary experiments such as to minimize changes in the color of PLA as an indication of its degradation. The extruded PLA/AS blends were then pelletized and compression molded into tensile and impact specimens using a hot press (PHI, Pasadena Hydraulic Inc., USA) at

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200°C with 70 MPa pressure for 4 minutes. Thin films (~100μm) of PLA/AS were prepared for water absorption and water vapor permeability analysis using the above compression molding procedure. The unprocessed and processed PLA and the PLA/AS blends are presented in Table 1. Table 1. Composition and processing time of the PLA and PLA/AS blends

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Material

AS (wt%) 0 0 15 15 15 15 15

Processing time (min) 0 7 7 7 7 7 7

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UPLA (unprocessed) PPLA (processed) PLA/DS0 PLA/DS0.5 PLA/DS1.5 PLA/DS2.5 PLA/DS3

PLA (wt%) 100 100 85 85 85 85 85

2.2.3. Determination of the degree of substitution (DS) for acetylated starch (AS)

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The DS of AS was estimated by proton nuclear magnetic resonance (1H NMR) spectra as the

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ratio between the area of acetyl CH3 at 2.15 ppm to the area of starch CH at 3.5-5.5 ppm (Shogren & Biswas, 2010). All spectra were obtained with a Varian Inova 600 NMR

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spectrometer equipped with a Varian 5 mm triple-resonance indirect-detection HCX probe at

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25°𝐶 using DMSO as the solvent. A sample of 1H NMR spectra of DS3 is presented in supplementary information (Figure S1).

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2.2.4. Wide Angle X-ray Diffraction (WAXRD)

The degree of crystallinity of AS was measured by WAXRD (D8 focus, Bruker) with CuKα1 radiation and 1.5406 Å wavelength operated at 40 kV and 30 mA. The X-ray diffraction patterns

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were recorded in a 2𝜃 angle range of 3-40 with step of 0.02° and speed of 1 sec/step. The degree of crystallinity was estimated with the method of (Nara & Komiya, 1983). The area under the diffractogram was assigned to the crystalline and the amorphous domains by creating a smooth curve that connected the baseline of the peaks using OriginPro 8.5 software (Supplementary Information Figure S2). The area above the smooth curve was considered as the crystalline domain and the area between the smooth curve and the linear baseline was considered

10 as the amorphous domain. The degree of crystallinity was calculated from the area of the peak for the crystalline domain and the sum of the area of the crystalline domain and the amorphous domain: 𝜒𝑐𝐴𝑆 (%) =

𝐴𝑐 𝐴𝑐 +𝐴𝑎

× 100

(1)

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where 𝜒𝑐𝐴𝑆 is degree of crystallinity, 𝐴𝑐 area of the peak assigned to the crystalline domain and

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𝐴𝑎 area of the peak assigned to the amorphous domain. 2.2.5. Scanning electron microscopy (SEM)

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The cross-sectional morphology of the samples was investigated by SEM (Quanta FEG 250,

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USA) with an electron voltage of 10 kV. The specimens were frozen under liquid nitrogen, then fractured, mounted, coated with gold and observed. ImageJ software was used to estimate the

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volume equivalent average diameter of the dispersed phase. At least 10 distinct elements of the dispersed phase were selected for the size analysis.

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2.2.6. Thermogravimetric analysis (TGA)

Thermogravimetric analysis was performed in a Q500 TA instrument (USA). Each sample (about 20 mg) was heated from 25°𝐶 to 600°𝐶 at 10°𝐶/min heating rate under nitrogen

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atmosphere (40.0 ml/min) to prevent thermo-oxidative degradation. The mass change over time data was analyzed with the TA Universal Analysis version v5.5.20 software to estimate the temperature for the onset of degradation (𝑇5% ) and the temperature at the maximum rate of degradation (𝑇𝑚𝑎𝑥 ) of the PLA/AS blends. 2.2.7. Differential Scanning Calorimetry (DSC)

11 The thermal transitions of the PLA/AS blends were investigated with a Q2000 TA instrument (USA). A sample (15-20 mg) was placed in an aluminum pan. The sample was initially heated from 25°𝐶 to 190°𝐶 at 10°𝐶/min heating rate to eliminate all thermal history. Next, the sample was cooled to 25°𝐶 and then heated again to 190°𝐶 (10°𝐶/min heating rate). An empty pan was used as a reference. The glass transition temperature (𝑇𝑔 ), cold crystallization temperature (𝑇𝑐𝑐 )

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and melting temperature peak (𝑇𝑚 ) were determined from the second heating scan of the thermogram using TA Universal Analysis version v5.5.20 software. When multiple endothermic

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peaks were present, the peak temperature of the largest endotherm was taken as 𝑇𝑚 . The degree of crystallinity (𝜒𝑐 ) was defined as follows (Battegazzore, Alongi & Frache, 2014; Wokadala,

Δ𝐻𝑚

0 ×𝛿 𝛥𝐻𝑚 𝑃𝐿𝐴

) × 100

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𝜒𝑐 (%) = (

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Emmambux & Ray, 2014):

(2)

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0 where Δ𝐻𝑚 is the enthalpy of melting for the second heating scan, 𝛥𝐻𝑚 the melting enthalpy for

100 % crystalline PLA (93.6 J/g (Iannace & Nicolais, 1997)), and 𝛿𝑃𝐿𝐴 is the mass fraction of

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PLA in the PLA/AS blends. 2.2.8. Tensile properties

The tensile properties were measured at 25°𝐶 using an Instron 3369 tensile tester (USA),

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according to ASTM D1708 with crosshead speed of 10mm/min. Ten specimens of each material were analyzed. All samples were conditioned 24 h at ambient conditions prior to testing. 2.2.9. Izod impact strength

12 The Izod impact strength of the notched specimens was measured in an Instron CEAST 9050 impact tester (USA), according to ASTM D256. At least six specimen of each material were tested. All samples were conditioned 24 h at ambient condition prior to testing. 2.2.10. Water absorption Each specimen (30mm×13mm×3mm) was dried overnight at 50°𝐶 in a convection oven. Each

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specimen was then immersed in water maintained at 30°𝐶. The weight change was recorded at

the precision of 10-4 g. The water uptake was calculated as: 𝑤𝑡 −𝑤0 𝑤0

× 100

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𝑀𝑡 (%) =

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different time intervals, after removing the surface water from the sample, using a balance with

(3)

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where 𝑤0 and 𝑤𝑡 are the initial weight of the specimen before immersion and the weight of the

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specimen after water immersion, respectively. 2.2.11. Water vapor permeability (WVP)

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The WVP test was conducted according to ASTM E96/E96M. Circular thin films (~100μm thickness) were mounted and sealed on the open mouth of cylindrical containers containing about 100g of calcium chloride. The mass of each container and film was measured to determine its initial mass before its exposure to constant relative humidity of 50 % and temperature of 23°𝐶

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conditions in an environmental chamber (Sanyo versatile environmental test chamber, Japan). The mass of the container and film was measured at regular intervals until reaching the linear mass increase over time. After that, the increase in the mass of the container and film over time were measured and WVP (g.h-1.m-1.Pa-1) was calculated as: 𝑊𝑉𝑃 =

𝑚×𝑑 𝐴×𝑡×𝑃

(4)

13 where 𝑚 (g) is the mass increase of the container and film, 𝑑 (m) the film thickness (in the range of 0.08×10-3 to 0.13×10-3 m), 𝐴 (m2) the exposed area of the film (29.22×10-4 m2), 𝑡 (h) is the duration of permeation, and 𝑃 (Pa) is the water vapor partial pressure difference across the film (1396 Pa). Three films of each material were analyzed and reported as average results. 2.2.12. Phase Diagram Prediction

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In order to predict the phase diagram of PLA/AS blends, the compressible regular (CRS) model

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was employed. In the CRS model, the total change in free energy upon mixing per unit volume is given by (Ruzette & Mayes, 2001):

NA νA

Ln ϕA +

ϕB ρ̃B NB νB

Ln ϕB ) + ϕA ϕB (ρ̃A δA,0 -ρ̃B δB,0 )

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ϕA ρ̃A

∆g mixing =k B T (

(5)

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Where 𝑘𝐵 is Boltzmann constant, 𝜙𝑖 the volume fraction, νi the hard core segment volume, ρ̃ i the

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reduced density at temperature 𝑇, 𝑁𝑖 the number of segments (molecular weight/repeating unit molecular weight (𝑀𝑖 ) ) and δi,0 the solubility parameter at zero Kelvin for component i. 𝐴 and 𝐵

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denote PLA and AS, respectively.

The hard core segment volume is defined as:

νi = 𝑀𝑖 /𝑁𝐴𝑣 𝜌𝑖∗

(6)

𝑀𝑖 is the molecular weight of the repeating unit and 𝑁𝐴𝑣 is Avogadro’s number. The parameter

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𝜌𝑖∗ is the hard core density and was estimated from the density at temperature 𝑇 (𝜌𝑖 (𝑇)): 𝜌𝑖∗ = 𝜌𝑖 (𝑇)𝑒𝑥𝑝(𝛼𝑖 𝑇)

(7)

where 𝛼𝑖 is the volumetric thermal expansion coefficient. The hard core density and the density at temperature 𝑇 were used to calculate the reduced density at temperature 𝑇:

14 𝜌̃𝑖 (𝑇) = 𝜌𝑖 /𝜌𝑖∗

(8)

The parameter δi,0 was obtained from the reduced density and the solubility parameter at 298𝐾: 2 𝛿𝑖,0 = 𝛿𝑖2 (298)⁄𝜌̃𝑖 (298)

(9)

The solubility parameter at 298𝐾 was estimated by Van Krevelen and Hoftyzer (VKH) (Van

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Krevelen & Te Nijenhuis, 2009) group contribution method. A compressible blend should be stable with respect to the composition and the volume fluctuations for phase stability. Since the

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simplified free energy expression given by the CRS model is based on the properties of the pure components, the effect of the volume fluctuation was neglected. Therefore, the criterion for local

∂ϕ2A

= kBT (

ρ̃A ϕA NA νA

+

2

ρ̃B ϕB NB νB

) − 2(ρ̃A δA,0 -ρ̃B δB,0 ) > 0

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∂2 Δgmixing

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stability in a binary polymer mixture can be presented by (Rubinstein & Colby, 2003):

(10)

2.3. Statistical analysis

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At the spinodal condition, which is the limit of local stability, this value equals zero.

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Multiple-comparison Bonferroni t-test at a significance level of 𝛼=0.1 was employed to evaluate the significance of the difference between means of properties that have replicate measurements. All reported error bars represent standard error.

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3. RESULTS AND DISCUSSION 3.1. Thermal stability The effect of acetylation in altering the thermal stability of AS and PLA/AS blends was obtained by TGA and the associated DTGA characteristics (steps of mass loss and the temperature of the DTGA peaks (𝑇𝑚𝑎𝑥 )). High thermal stability is desired when producing PLA blends.

15 3.1.1. Acetylated starch The TGA and DTGA of acetylated starch according to DS are presented in Figure 1A and B and Table 2. Two different mass losses could be observed reflecting either the condensation of hydroxyl groups (200°𝐶-300°𝐶) or the decomposition of acetyl groups (315°𝐶-415°𝐶). The magnitude and location of these two mass losses changed according to the acetylation level (ie

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DS). Native starch (DS0) showed one major mass loss in the 245-600°𝐶 temperature range representing the condensation of hydroxyl groups. Two mass losses were present in AS with DS

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ranging from 0.5 to 2.5. The TGA of AS with DS0.5 showed the acetyl group decomposition

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peak as a shoulder to the right of the hydroxyl group condensation peak. The shift to lower temperatures of the hydroxyl group condensation peak could reflect degradation of starch and

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destruction of crystalline structure occurring during the acetylation reaction (Tupa, Ávila Ramírez, Vázquez & Foresti, 2015). In contrast, the TGA of DS3 showed one major mass loss

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associated with the decomposition of acetyl groups. The absence of the mass loss associated with the hydroxyl condensation supports the removal of hydroxyl groups (Figure 1A). The

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temperature at the maximum rate of degradation (𝑇𝑚𝑎𝑥 ) of DS3 was observed at 367°𝐶, about 50°𝐶 higher than the highest rate of mass loss of native starch (DS0), indicating that acetylation of starch increased its thermal stability as reported previously (Tupa, Maldonado, Vázquez &

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Foresti, 2013; Tupa, Ávila Ramírez, Vázquez & Foresti, 2015; Wang et al., 2008).

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Figure 1. TGA (A and C) and DTGA (B and D) curves of PLA and PLA/AS blends containing AS with different DS

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(10°C /min heating rate, nitrogen conditions).

3.1.2. PLA/AS blends

The thermal characteristics of acetylated starch when incorporated in PLA blends influenced

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somewhat the thermal characteristics of the PLA/AS blends (TGA (Figure 1C and D) and the onset of degradation (𝑇5% ) and 𝑇𝑚𝑎𝑥 presented in Table 2). Note that 𝑇5% of the processed PLA (PPLA) was lower compared to the unprocessed PLA (UPLA), reflecting the thermal and shear induced degradation taking place during the processing, while 𝑇𝑚𝑎𝑥 remained unchanged. The addition of native starch (DS0) to PLA did not affect 𝑇5% but increased 𝑇𝑚𝑎𝑥 by 7°𝐶 for the PLA/DS0 blend. In contrast, the PLA/DS0.5 blend, its 𝑇5% decreased by 26°𝐶 and decreased by

17 6°𝐶 for the PLA/DS1.5 blends compared to PLA/DS0 blends while 𝑇𝑚𝑎𝑥 decreased by 47°𝐶 for the PLA/DS0.5 blend and decreased by 5°𝐶 for the PLA/DS1.5 blend. The PLA/DS2.5 and PLA/DS3 blends displayed higher 𝑇5% and 𝑇𝑚𝑎𝑥 compared to PPLA and other PLA/AS blends. The higher thermal stability of these two PLA/AS blends reflect the positive effect of acetylation in improving the thermal stability of PLA materials. A slight increase of thermal stability has

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been reported in the literature for PLA/TPS blends (Li & Huneault, 2011; Wang, Yu, Chang & Ma, 2008).

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Table 2. Temperature of the onset of degradation (𝑇5% ) and temperature at the maximum rate of degradation (𝑇𝑚𝑎𝑥 ) of AS, PLA and PLA/AS blends.

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𝑻𝒎𝒂𝒙 (°𝑪) 313 291 274/368 279/370 367 341 342 349 302 344 360 350

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𝑻𝟓% (°𝑪) 80 126 130 148 324 308 293 294 268 288 316 306

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Sample DS0 DS0.5 DS1.5 DS2.5 DS3 UPLA PPLA PLA/DS0 PLA/DS0.5 PLA/DS1.5 PLA/DS2.5 PLA/DS3

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3.2. Phase behavior and morphology of PLA/AS blends The phase behavior of the PLA/AS blends according to DS was first predicted and then compared to experimental observations obtained by SEM. The predictions of the phase behavior of PLA/AS blends were obtained with the compressible regular solution (CRS) model and the properties of the pure components (Ruzette & Mayes, 2001).

18 The properties of AS were measured experimentally (details are provided in Supporting Information) and obtained from the literature for PLA and are summarized in Table S3 of Supporting Information. These properties were used to estimate ∆g mixing and the corresponding phase behavior of the PLA/AS blends according to PLA volume fraction. The phase diagram of the PLA/AS blends is shown in Figure 2. The volume fraction of PLA in

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the blends investigated in this study, estimated from the density of the pure components, was in the range of 0.85-0.87. The phase behavior prediction indicates immiscibility for all blends

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except PLA/DS3 at processing (453K) and room (298K) temperature conditions which was

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confirmed by SEM imaging of cryofractured cross-sections of PLA/AS blends (Figure 3). Two

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distinct phases could be clearly observed for all PLA/AS blends except for DS3.

Figure 2. Phase diagram predictions of PLA/AS blends according to PLA volume fraction for different DS of AS.

The interfacial region of the dispersed and the continuous phase indicates a visible gap in the PLA/DS0 blend (arrow, figure 3B) while no visible gap was observed in the other biphasic blends (figure 3D, F, and H). The presence of a visible gap in PLA/DS0 could reflect the high degree of crystallinity of native starch (DS0), 36.2% (Table 3) preventing interdiffusion of the

19 PLA and the starch phases at the interface. The absence of visible gap in the PLA/DS0.5, PLA/DS1.5 and PLA/DS2.5 may be associated with some interdiffusion of the phases due to the higher thermodynamic affinity between phases. The effect of starch chemical modifications in reducing the visible gap between phases was reported previously for PLA/silane-modified cassava starch blend (Jariyasakoolroj & Chirachanchai, 2014) and PLA/maleic anhydride grafted

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wheat starch blends (Zhang & Sun, 2004). The covalent bonding occurring between starch and PLA in PLA/silane-modified cassava starch and PLA/maleic anhydride grafted wheat starch

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blends induced by the modification agent, can play a similar role as the interdiffusion of phases

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ultimately leading to the disappearance of a visible gap at the interface. Table 3. Properties of AS and PLA/AS blends

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PLA/AS Blend AS Volume Averaged Equivalent Diameter 𝑇𝑔 (μm) (°𝐶) 62 a 13.95±0.70 61 b 9.98±0.68 60 c 23.14±1.39 60 5.91±1.86d 61 61

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PPLA PLA/DS0 PLA/DS0.5 PLA/DS1.5 PLA/DS2.5 PLA/DS3

AS 𝜒𝑐𝐴𝑆 (%) 36.2 22.9 7.1 4.8 -

lP

Sample

𝑇𝑐𝑐 (°𝐶) 112 106 103 108 108 109

𝑇𝑚 (°𝐶) 170 169 169 169 169 170

𝜒𝑐 (%) 40.5 45.3 47.4 43.6 45.1 42.6

Different letters denote statistically significant different samples (significance level of α=0.1)

The volume average equivalent diameter of AS (assuming spherical geometry), presented in

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Table 3, decreased with increasing DS except for PLA/DS 1.5. The PLA/DS2.5 blend contains small and large size AS components which is reflected by the relatively high standard deviation compared to the average equivalent diameter. As in the case of immiscible synthetic polymer blends, the reduced interfacial tension between PLA and DS2.5 may have caused an effective tension transfer from the PLA matrix to the large AS droplets and their subsequent breaking in smaller droplets during their processing (Delaby, Ernst, Germain & Muller, 1994).

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Figure 3. SEM images of cryofractured cross-section of PLA/DS0 (A, B), PLA/DS0.5 (C, D), PLA/DS1.5 (E, F), PLA/DS2.5 (G, H), and PLA/DS3 (I, J) at two magnifications. The circled region in the lower magnification image (left hand side) is shown in the right hand side image at higher magnification. Arrows point at the interface. All samples contain 15 wt% of AS.

21

3.3. Thermal transition The thermal transitions of PLA/AS blends are shown in Figure 4A and summarized in Table 3. The temperature of the glass transition (𝑇𝑔 ), cold crystallization (𝑇𝑐𝑐 ), and melting (𝑇𝑚 ) of PPLA have been identified with vertical dashed lines to assist with the visualization of those thermal transitions in PLA/AS blends. The 𝑇𝑔 of all PLA/AS blends decreased by 1 to 2 °𝐶 compared to

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PPLA due to the plasticization effect of AS (Table 3). The 𝑇𝑐𝑐 decreased for all PLA/AS blends

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compared to PPLA. One can infer that AS may act as a nucleating agent for PLA by facilitating its crystallization. The role of native starch and AS as nucleating agent in PLA crystallization

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was explored by considering the degree of crystallinity of the PLA blends (𝜒𝑐 ) and the size and smoothness of the dispersed phase. It is commonly agreed that the nucleation effect is promoted

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when the nucleating agent is small, has high surface roughness, similar crystalline lattice

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structure to the matrix and high affinity with the matrix (Ning et al., 2012; Pukánszky & Fekete, 1999; Sarikhani, Nasseri, Lotocki, Thompson, Park & Chen, 2016; Wittmann & Lotz, 1990). Considering these factors, the role of AS as nucleating agent in PLA/AS blends was analyzed.

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The degree of crystallinity of AS in the blend might be different from 𝜒𝑐𝐴𝑆 before blending but still can be employed for qualitative comparison of the nucleants. The affinity of AS with PLA increased with increasing DS (Figure 2). The PLA/DS0.5 and PLA/DS3 blends showed the

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highest and lowest 𝜒𝑐 , differing by 5%. In the PLA/DS0.5 blend, the AS dispersed phase has a small average equivalent diameter, rough surface (Figure 2D) in addition to high degree of crystallinity, which should translate in the most efficient PLA nucleation. In the PLA/DS3 blend, there was no visible dispersed AS phase (Figure 2J) which may explain the lowest 𝜒𝑐 . The PLA/DS2.5 blend and the PLA/DS0 blend have similar 𝜒𝑐 but significantly different AS average equivalent diameter which may be induced by their different hydrophobicity and thermodynamic

22 affinity with the PLA matrix. The relationship between 𝑇𝑐𝑐 and 𝜒𝑐 is illustrated by the inversed linear relationship between these two parameters where high 𝑇𝑐𝑐 is associated with low 𝜒𝑐 (Figure 4B). The melting transition indicates the presence of an exothermic peak prior to the endothermic peak of melting for PLA/DS0 and PLA/DS0.5. Such an exothermic peak has been previously attributed to the PLA crystal transformation from the disordered 𝛼′ form to the more ordered 𝛼 form (Kawai et al., 2007). The small peak or shoulder at 162°𝐶 prior to the main

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melting peak for all blends reflects the characteristic melt-recrystallization behavior of the 𝛼

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form (Zhang, Tashiro, Tsuji & Domb, 2008).

Figure 4. (A) DSC thermograms of PLA and PLA/AS blends with vertical lines used to assist with the visualization of the key features and (B) linear correlation between the lowering of the temperature of cold crystallization compared to PLA and degree of crystallinity of PLA and PLA/AS blends. Dashed line represents the linear fit

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(R2=0.96).

3.4. Mechanical properties The mechanical properties of PLA and PLA/AS blends are summarized in Figure 5 and supplementary information, Table S4. It was not possible to obtain materials of sufficient quality

23 for the PLA/DS0.5 and PLA/DS1.5 blends. The materials had visible air bubbles, most probably induced by thermal degradation occurring during compression molding and supported by their low thermal stability (lower 𝑇5% ). The manipulation of the temperature, pressure and time conditions during compression molding could not alleviate these problems. Note that the extrusion of PLA for 7 minutes affected negatively all mechanical properties of

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PLA except its modulus. The mechanical properties of the PLA/DS0 blend were significantly lower than those of PPLA. The tensile strength, elongation at break and toughness dropped by

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30%, 15% and 32%, respectively which could reflect the visible gap observed by SEM (Figure

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3B) that could act as stress concentration points. The PLA/DS2.5 blend had significantly higher tensile strength, and toughness, 26%, and 29% compared to the PLA/DS0. The higher

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mechanical properties of the PLA/DS2.5 may reflect the absence of visible gap observed by SEM (Figure 3H) due to the effective tension transfer from the PLA matrix to the AS dispersed

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phase.

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Figure 5. Mechanical properties of PLA and PLA/AS blends; (A) stress-strain curves, (B) tensile strength, (C) toughness and (D) impact stress. Different letters denote statistically significant differences (Bonferroni t-test, α=0.1)

The biphasic PLA/DS2.5 blend had also statistically superior tensile strength and toughness

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properties compared to the single phase PLA/DS3 blend. The mechanical properties of the PLA/DS3 blend may reflect its morphology with no visible biphasic structure (Figure 2J) where the chains of the AS dispersed phase within the PLA matrix and may have interfered with the continuity of the PLA matrix and reduce the mechanical performance of the blend. Similar observations have been reported previously for poly (propylene carbonate)/AS blends where

25 changes of the microstructure from biphasic to single phase due to the increase of DS reduced the tensile and impact strength of the blend (Zeng, Wang, Xiao, Han & Meng, 2011). The mechanical and thermal properties of PLA/DS2.5 blend obtained in this study were compared to published results for PLA/modified-starch blends (Table 4). The comparison was based on the relative difference of the mechanical property for PLA/modified starch blends

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compared to PLA/neat starch blends and the absolute difference in the 𝑇𝑚𝑎𝑥 for PLA/modified starch blends compared to PLA/neat starch blends. The PLA/DS2.5 blend showed that

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acetylation was beneficial in increasing the mechanical and thermal properties, 26% increase in

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tensile strength, 10% increase in elongation at break and 11°C increase in 𝑇𝑚𝑎𝑥 . These improvements were lower than those obtained with the maleic anhydride and butyl-etherification

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treatments. For instance, addition of maleic anhydride to a PLA/TPS blend containing 30 wt% TPS increased the tensile strength by 5% and elongation at break by 1150% (Huneault & Li,

lP

2007). Butyl-etherification of starch, on the other hand, increased the tensile strength by 90% and elongation at break by 150%, while decreased the 𝑇𝑚𝑎𝑥 by 27°C for a PLA/Butyl-etherified

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starch blend comprising 20 wt% starch derivative (Wokadala, Emmambux & Ray, 2014).

26 Table 4. Comparison of mechanical and thermal properties of PLA/TPS blends and PLA/AS blends

(Wang, Yu, Chang & Ma, 2008)

-

-

+2

(Li Huneault, 2011)

3

20

+5

3.2

Plasticization with sorbitol

83% PLA 27% TPS

46

+28

addition of an amphiphilic molecule (Tween 60)

70% PLA 30% TPS

38

-8

addition of maleic anhydride

70% PLA 30% TPS

40

Waxy maize starch (S9679), Sigma Aldrich

Butyletherification

80% PLA 20% Butyletherified starch

Corn starch Ju-NengJing Corn

addition of maleic anhydride

2002D Natureworks

Wheat starch Supergel 1203, ADMOgilvy

50% PLA 50% TPS

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Cassava starch Tongchan Co.

Corn starch local Market

Acetylation

85% PLA 15% Acetylated Starch, DS2.5

&

+400

-

(Yokesahachart & Yoksan, 2011)

20

+1150

-

(Huneault Li, 2007)

+90

1.5

+150

-27

(Wokadala, Emmambux & Ray, 2014)

17.5

+60

2

+10

-

(Ren, Fu, Ren & Yuan, 2009)

36.2

+26

3.8

+10

+11

This study

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Plasticization with formamide and water

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50% PLA 50% TPS

Corn starch (Langfang Starch)

4042D Natureworks

4032D Natureworks (58kDa)

+4

Rel. Diff. (%)

4032D Natureworks

Tong-Jieiang Biomaterial (180kDa)

+106

Rel. Diff. (%)

PLA/TPS or native starch (%)

Composition

Wheat starch Supergel 1203, ADMOgilvy

2002D Natureworks

Reference

Elongation at break

PLA/TPS or native starch (MPa)

Modification method

+5

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Natureworks (220kDa)

𝑻𝒎𝒂𝒙 Abs. Diff. (°𝑪)

Tensile strength Starch type

22

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PLA type

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Mechanical properties

&

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3.5. Water uptake and vapor permeability properties The water uptake of PLA and PLA/AS blends thin films (100 m) with sufficient thermal stability for compression molding are presented in supplementary information (Figure S5) and the equilibrium water uptake reported in Table 5. The equilibrium water uptake of the PLA/AS blends was statistically higher than for PPLA and decreased with increasing DS. The water uptake profile of the PLA/DS0 and PLA/DS2.5 blends consisted in an initial steep increase

27 followed by a more gradual increase. The water uptake profile of the PLA/DS3 blend was similar to the PPLA profile with a moderate initial increase followed by negligible increase. The two different profiles may reflect the distinct biphasic microstructure of the PLA/DS0 and PLA/DS2.5 blends compared to the single phase microstructure of the PLA/DS3 and PPLA materials (Figure 2).

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The fitting of the water uptake data for the entire time period with 𝑀𝑡 /𝑀∞ = 𝑘𝑡 𝑛 provided estimates of 𝑛 smaller than 1/2 for all PLA/AS blends and PPLA which is characteristics of

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pseudo-Fickian diffusion (Comyn, 2012) and reflects primarily the PLA behavior (Chow, Leu &

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Mohd Ishak, 2014), supplementary information (Table S5). Assuming that the water uptake follows the one-dimensional Fickian diffusion process, the water uptake at time 𝑡 was expressed

𝑀∞

8

= 1 − ∑∞ 𝑛=0 (2𝑛+1)2

𝜋

2 exp[

−𝐷(2𝑛+1)2 𝜋2 𝑡 𝑑2

]

(11)

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𝑀𝑡

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as (Rogers & Comyn, 1985):

where 𝑀∞ is the equilibrium water uptake of the specimen, 𝑑 the thickness of the specimen, and

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𝐷 the diffusion coefficient. For the initial linear portion of the Fickian diffusion curve, 𝐷 was estimated as: 𝐷=

𝜋𝑑 2 𝜃2 2 16𝑀∞

(12)

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where 𝜃 is the slope of the linear portion of 𝑀𝑡 versus 𝑡 1/2 curve (Chow, Leu & Mohd Ishak, 2014; Jiang, Kolstein, Bijlaard & Qiang, 2014). The water diffusion coefficients were estimated from equation 12 and presented in Table 5. The water diffusion coefficient of the PLA/DS0 and PLA DS2.5 blends, displaying biphasic microstructure, was about two-fold lower than for the PLA/DS3 single phase blend. It is well established that water absorption for blend materials is

28 influenced by the microstructure of the dispersed phase and the matrix and the degree of crystallinity. In this study, the lower water diffusion coefficient of the PLA/DS0 blend may be related to the high crystallinity of the native starch which could act as an impermeable barrier preventing diffusion of water in the starch dispersed phase and forcing water to follow longer and more tortuous paths for diffusion (Bhatia, Gupta, Bhattacharya & Choi, 2012). In contrast,

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the presence of crystalline regions in the matrix can immobilize adjacent amorphous chains and make a rigid amorphous phase which is less permeable (Choudalakis & Gotsis, 2009). The

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diffused interphase may also be less permeable forcing the water molecules to follow a tortuous path (Comyn, 2012). The presence of a biphasic microstructure in the PLA/DS2.5 blend together

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with the smaller size and larger number of AS dispersed phase may increase the tortuosity and

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explain the lower water diffusion coefficient compared to PPLA and PLA/DS0 blend. The single phase morphology of PPLA and PLA/DS3 suggest that the degree of crystallinity of the matrix

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will be the predominant factor influencing the water transport properties in these materials. In the PLA/DS3 blend, PLA has the higher 𝜒𝑐 (Table 3) reflecting its smaller water diffusion

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coefficient compared to PPLA.

Table 5. Equilibrium water uptake, water diffusion coefficient and WVP of PPLA and PLA/AS blends.

Material

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PPLA PLA/DS0 PLA/DS2.5 PLA/DS3

Equilibrium water uptake (%) 0.92±0.01a 3.38±0.03d 2.93±0.08c 1.71±0.02b

Diffusion coefficient (×10-12 m2.s-1) 7.23±0.55c 2.91±0.25a 2.02±0.13a 5.32±0.33b

WVP (×10-7 g.h-1.m-1.Pa-1) 0.67±0.02a 1.16±0.12b 1.52±0.08b 2.48±0.09c

Different letters denote statistically significant different samples (significance level of α=0.1)

The water vapor permeability (WVP) of films prepared with PLA and PLA/AS blends is reported in Table 5. The WVP for all PLA/AS blends is higher compared to PLA and increases

29 with DS. The presence of acetylated starch nor the increase DS have been able to lower the WVP. Similar increase in WVP for PLA blends have been reported previously for PLA/polyethylene glycol (PEG) blend comprising 9 wt% polyethylene glycol with around 42% increase in water vapor permeability compared to pure PLA (Byun, Kim & Whiteside, 2010). The authors suggest that the increase is due to the enhancement of free volume as a result of the

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plasticization effect of PEG on PLA which leads to faster diffusion of water vapor molecules.

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4. Conclusion

This study demonstrates the potential of starch acetylation as means to improve the compatibility

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of starch with PLA by alleviating the limitations in the processing of starch and by producing materials with good mechanical properties. The thermal stability of acetylated starch (AS) and

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PLA/AS blends was enhanced when hydroxyl groups in the starch molecule were replaced by

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acetyl groups. The predicted phase behavior was confirmed by experimental SEM observations of the morphology of PLA/AS blends. A biphasic microstructure was predicted and observed experimentally for all PLA/AS blends except PLA/DS3. The glass transition temperature of all

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PLA/AS blends decreased due to the plasticization of AS. The presence of AS in the PLA blends altered the crystallization behavior of PLA. The PLA/DS2.5 blend had the highest mechanical performance.

The PLA/DS3 displayed lower equilibrium water uptake but higher WVP

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compared to PLA/DS2.5. Future work will be focusing on the interfacial properties and crystallization behavior according to starch acetylation level. Acknowledgements The authors would like to gratefully acknowledge OMAFRA New Directions Program (20132016) for funding this study. Furthermore Dr. Duhamel and Dr. Gauthier (University of

30 Waterloo) are also thankfully acknowledged for providing some chemicals. Dr. Ghavami and Ms. Jenn Coggan (University of Waterloo) are acknowledged for the access to the compression

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molding and mechanical testing facilities.

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For Table of Contents use only

Poly(lactic acid)/acetylated Starch Blends: Effect of

of

Starch Acetylation on the Material Properties

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Rasool Nasseria, Robert Ngunjiria, Christine Moresolia,*, Aiping Yua,b, Zhongshun Yuanc and

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Chunbao (Charles) Xuc