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Starch-based Blends,
Composites and
Nanocomposites
Edited by

Visakh P. M.
Tomsk Polytechnic University, Russia
Email:

Long Yu
CSIRO, Clayton, Australia
Email:


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Preface
This book summarizes many of the recent research accomplishments in the
area of starch-based polymer blends, composites and nanocomposites such
as starch: structure–property relationships, preparation and characterization
of starch nanocrystals, natural fibre-reinforced thermoplastic starch composites, applications of starch nanocrystal-based blends, composites and
nanocomposites, chemically modified thermoplastic starches, outstanding
features of starch-based hydrogel nanocomposites, starch-based blends,
fracture and failure of starch-based composites, application of starch
nanocomposites in the food industry and effect of additives on the properties of starch. As the title indicates, the book emphasizes the various aspects of starch-based blends, composites and nanocomposites, and it is
intended to serve as a ‘‘one-stop’’ reference resource for important research
accomplishments in this area. This book should be a very valuable reference
source for university and college faculties, professionals, postdoctoral research fellows, senior graduate students and researchers in R&D laboratories
working in the area of starch-based blends, composites and nanocomposites. The various chapters in this book have been contributed by prominent
researchers in industry, academia and government/private research laboratories across the globe, and provide an up-to-date record of the major
findings and observations in the this subject area.
The first chapter gives an overview of the state-of-the-art, new challenges
and opportunities in starch-based studies and research, preparation and
characterization and applications of starch-based blends, composites and
nanocomposites and also future trends in this area.
Chapter 2 provides up-to-date information about starch and its structure–
property relationships. The complexes of starch with other components,
starch hydrolysis, starch modification methods (chemical, physical, enzymatic and genetic/biotechnological modification), industrial production
RSC Green Chemistry No. 37
Starch-based Blends, Composites and Nanocomposites
Edited by Visakh P. M. and Long Yu

r The Royal Society of Chemistry 2016
Published by the Royal Society of Chemistry, www.rsc.org

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Preface

of starches and their uses are also discussed. Chapter 3, on the preparation
and characterization of starch nanocrystals, critically reviews recent advances in the preparation of starch nanocrystals and reports the critical steps
needed to modify them chemically for new applications. Novel and improved
techniques are outlined in order to optimize the preparation of starch
nanocrystals from a wide range of starch sources. Advanced methods used to
characterize starch nanocrystals and to understand their interactions with
polymer matrices are also discussed. In addition, the physicochemical
properties, including mechanical, barrier, morphological, thermal, and
swelling and emulsification, are presented with potentially new insights into
the development of starch nanocrystals for industrial applications. Chapter
4, on natural fibre-reinforced thermoplastic starch composites, covers aspects such as natural fibre, starch, thermoplastic starch and natural fibrereinforced thermoplastic starch composites. Of interest here is that the
authors prepared an environmentally friendly composite where the matrix
(sugar palm starch) and fibre (sugar palm fibre) were derived from one
source, i.e. the sugar palm tree. The resulting materials are termed ‘‘biocomposites’’ or ‘‘green’’ composites, which are considered to be totally
biodegradable.

Chapter 5 concentrates mainly on applications of starch nanocrystalbased blends, composites and nanocomposites and highlights recent research on the preparation, characterization and properties of polymeric
matrix–starch nanocrystal nanocomposites. The results discussed indicate
that starch nanocrystals were able to improve the properties of polymers
because strong interactions between the matrix and the nanofillers were
formed. The particular characteristics, such as the polymer used as the
matrix and the amount of nanofillers, that also affected the properties of
the nanocomposites are discussed. Chapter 6 considers the chemical
modification of thermoplastic starch. Topics covered include starch as an
industrial raw material, destructuring of starch by extrusion, chemical
modification of starch by reactive extrusion, depolymerization of starch by
reactive extrusion of thermoplastic starch and starch modification with citric
acid. The aim of this chapter is to review the chemical modification of starch
as thermoplastic starch by extrusion processing or other similar techniques
in which the material is in the melt state. Generally, this process is conducted in the presence of plasticizers such as glycerol, glycols and urea.
Chapter 7 discusses the outstanding features of starch-based hydrogel
nanocomposites and is focused on the formulation of functional materials
with enhanced properties for more suitable applications in different fields.
Relevant aspects of the current knowledge of starch-based hydrogel nanocomposites are discussed, mainly those based on structure–property relationships, and also specific functionalities for a given application. Further
comprehensive studies will broaden the understanding of structure–
property relationships. Chapter 8, on starch-based blends, includes topics
such as the preparation, modification and applications of starch-based
blends. Chapter 9 covers the fracture and failure of starch-based composites


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Preface


ix

with topics such as the fracture and failure of starch reinforced with natural
fibers and the fracture and failure of starch-based nanocomposites (starch
reinforced with carbon nanotubes and with nanoclays), and summarizes
future trends. Chapter 10 surveys the applications and potential of starch
nanocomposites in the food industry. The major topics include nanotechnology in foods, starch as the matrix with different nanofillers and
starch nanoparticles. The final chapter, on the effects of additives on the
properties of starch, provides an overview of recent progress in the interaction between starch and its additives. It includes the effect of additives on
the properties of starch, the mechanism of the interactions between starch
and additives and recent applications of additives in starch-based products.
The editors would like to express their sincere gratitude to all the contributors to this book, whose excellent support ensured the successful
completion of this venture. We are grateful to them for the commitment and
the sincerity they have shown towards their contributions. Without their
enthusiasm and support, the compilation of this book would have not been
possible. We also thank all the reviewers who devoted their valuable time to
make critical comments on each chapter. Finally, we thank the publisher,
the Royal Society of Chemistry, for recognizing the demand for such a book,
for realizing the increasing importance of the area of starch-based blends,
composites and nanocomposites and for initiating this new project.
Visakh P. M.


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Contents
Chapter 1

Starch: State-of-the-Art, New Challenges and Opportunities
Visakh P. M.
Starch: Introduction and Structure–Property
Relationships
1.2 Preparation and Characterization of Starch
Nanocrystals
1.3 Natural Fibre-reinforced Thermoplastic Starch
Composites
1.4 Applications of Starch Nanocrystal-based Blends,
Composites and Nanocomposites
1.5 Chemically Modified Thermoplastic Starches
1.6 Outstanding Features of Starch-based Hydrogel
Nanocomposites
1.7 Starch-based Blends
1.8 Fracture and Failure of Starch-based Composites
1.9 Application of Starch Nanocomposites in the Food
Industry
1.10 Effect of Additives on the Properties of Starch
References

1

1.1


Chapter 2 Starch: Introduction and Structure–Property
Relationships
Khongsak Srikaeo
2.1

Introduction

1
2
4
4
5
6
7
8
9
9
10

17

17

RSC Green Chemistry No. 37
Starch-based Blends, Composites and Nanocomposites
Edited by Visakh P. M. and Long Yu
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2.2

Starch Structure
2.2.1 Starch Molecular Structure
2.2.2 Minor Components in Starch Granules
2.3 Starch Functionalities
2.3.1 Phase Transition of Starch
2.3.2 Starch in Limited Water Systems
2.3.3 Glass Transition in Starch
2.4 Starch–Lipid Complexes
2.5 Starch Hydrolysis
2.5.1 Acid Hydrolysis
2.5.2 Starch Digestive Enzymes
2.5.3 Human Digestive System
2.5.4 Starch Digestibility
2.6 Modification of Starch
2.6.1 Chemical Modification
2.6.2 Physical Modification
2.6.3 Enzymatic Modification

2.6.4 Genetic/Biotechnological Modification
2.7 Industrial Production of Starch and its Uses
2.7.1 Industrial Production of Starch
2.7.2 Utilization of Starches
References
Chapter 3 Preparation and Characterization of Starch Nanocrystals
Mehran Ghasemlou, Seyed Mohammad Taghi Gharibzahedi
and Marlene J. Cran
3.1
3.2
3.3

Introduction
Starch
Synthesis of Starch Nanocrystals
3.3.1 Preparation Protocols
3.3.2 Acid Hydrolysis
3.3.3 Chemical Modification
3.4 Characterization of Starch Nanocrystals
3.4.1 Mechanical Characteristics
3.4.2 Barrier Properties
3.4.3 Swelling Characteristics
3.4.4 Thermal Properties
3.4.5 Morphological Characteristics
3.4.6 Emulsification Characteristics
3.4.7 Crystallinity Characteristics
3.5 Conclusion
References

18

19
26
27
27
31
31
32
33
33
34
35
36
38
38
40
41
43
45
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Chapter 4 Natural Fibre-reinforced Thermoplastic
Starch Composites
J. Sahari, S. M. Sapuan, Y. A. El-Shekeil,
M. R. Ishak and R. Akhtar
4.1
4.2

Introduction
Natural Fibres
4.2.1 Introduction

4.2.2 Natural Fibre Composites
4.2.3 Preparation of Natural Fibres
4.2.4 Characterization of Natural Fibres
4.2.5 Treatment of Natural Fibres
4.2.6 Advances in Natural Fibres:
Nanotechnology
4.2.7 Sources of Nanocellulose
4.2.8 Preparation of Nanocellulose
4.3 Starch
4.3.1 Introduction
4.3.2 Preparation of Starch
4.3.3 Characterization of Starch
4.4 Thermoplastic Starch (TPS)
4.4.1 Introduction
4.4.2 Characterization of TPS
4.5 Natural Fibre-reinforced Thermoplastic Starch
Composites (NFTPSs)
4.5.1 Introduction
4.5.2 Characterization of NFTPSs
4.6 Conclusion
References

Chapter 5 Applications of Starch Nanocrystal-based Blends,
Composites and Nanocomposites
Veroˆnica M. A. Calado and Andresa Ramos
5.1
5.2
5.3

Introduction

Preparation of Starch Nanocrystals
Composites
5.3.1 Preparation of Composites
5.3.2 Techniques for Characterizing
Composites
5.4 Conclusion
References

109

109
110
110
111
112
113
116
117
118
118
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118
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124
124
125
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Chapter 6 Chemical Modification of Thermoplastic Starch
Antonio Jose´ Felix Carvalho
6.1
6.2
6.3

Introduction

Starch as an Industrial Raw Material
Destructuration of Starch by Extrusion:
Thermoplastic Starch
6.4 Chemical Modification of Starch by Reactive
Extrusion
6.4.1 Grafting with Maleic Anhydride (Starch-g-MA)
6.4.2 Diisocyanates
6.4.3 Transesterification
6.4.4 Esterification
6.4.5 Epoxidation
6.4.6 Reaction with Vinyl Monomers
6.4.7 Hydrolysis and Glycolysis
6.5 Depolymerization of Starch by Reactive
Extrusion of TPS
6.6 Starch Modification with Citric Acid in the Melt
State: Reactive Extrusion of TPS
6.7 New Challenges and Opportunities
Acknowledgements
References
Chapter 7 Outstanding Features of Starch-based Hydrogel
Nanocomposites
Antonio G. B. Pereira, Andre´ R. Fajardo, Artur J. M. Valente,
Adley F. Rubira and Edvani C. Muniz
7.1
7.2

Introduction: Starch Properties
Hydrogels
7.2.1 Starch-based Hydrogels
7.2.2 Characterization of Starch-based Hydrogels

7.3 Starch Hydrogel Composites
7.4 Applications of Starch-based Hydrogels
7.5 Future Trends in Starch-based Hydrogels
7.6 Conclusion
References
Chapter 8 Starch-based Blends
Jiwei Li and Xuegang Luo
8.1

Introduction

217

217
219
221
223
224
224
226
226
226
226
226
227
227
229
230
230


236

236
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241
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8.2

Starch-based Blends
8.2.1 Partially Biodegradable Starch-based
Blends

8.2.2 Completely Biodegradable Starch-based
Blends
8.3 Modification of Starch-based Blends
8.3.1 Gelatinization
8.3.2 Amylose/Amylopectin Ratio
8.3.3 Additives
8.3.4 Radiation
8.3.5 Filling Modification
8.3.6 Ternary Blending
8.4 Preparation of Starch-based Blends
8.4.1 Casting
8.4.2 Extrusion
8.4.3 Film Blowing
8.4.4 Injection Molding
8.4.5 Compression Molding
8.4.6 Foaming
8.4.7 Spinning
8.5 Applications of Starch-based Blends
8.5.1 Agricultural Applications
8.5.2 Packaging
8.5.3 Pharmacy and Biomedicine
8.5.4 Absorbent Materials
8.6 Conclusion
References

Chapter 9 Fracture and Failure of Starch-based Composites
Celina R. Bernal
9.1
9.2


Introduction
Fracture and Failure of Starch Reinforced with
Natural Fibers
9.3 Fracture and Failure of Starch-based
Nanocomposites
9.3.1 Starch Reinforced with Carbon
Nanotubes
9.3.2 Starch Reinforced with Nanoclays
9.4 Summary and Future Trends
References

264
264
268
274
274
275
278
283
285
289
291
291
292
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295
296
296
297
298

299
300
301
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305
305

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Chapter 10 Application of Starch Nanocomposites in the Food
Industry
Khongsak Srikaeo

10.1
10.2

Introduction
Nanotechnology in Foods
10.2.1 Application of Nanotechnology in Food
Production and Nutrition
10.2.2 Application of Nanotechnology in Food
Packaging
10.2.3 Other Applications
10.2.4 Safety Assessment of Nanotechnology
in Foods
10.3 Starch Nanocomposites in the Food Industry
10.3.1 Starch Matrix with Different Nanofillers
10.3.2 Starch Nanoparticles (SNPs)
10.4 Conclusion
References

Chapter 11 Effects of Additives on the Properties of Starch
Wei Wang, Hong Yang and Min Cui
11.1
11.2

11.3

11.4

Introduction
Starch–Hydrocolloid Blends
11.2.1 Introduction

11.2.2 Characteristics of Starch–Hydrocolloid
Blends
11.2.3 Mechanisms of Starch–Hydrocolloid
Interactions
11.2.4 Applications of Starch–Hydrocolloid
Blends
Starch–Protein Blends
11.3.1 Introduction
11.3.2 Characteristics of Starch–Protein Blends
11.3.3 Mechanisms of Starch–Protein
Interactions
11.3.4 Applications of Starch–Protein Blends
Starch–Lipid Blends
11.4.1 Introduction
11.4.2 Characteristics of Starch–Lipid Blends
11.4.3 Mechanisms of Starch–Lipid Interactions
11.4.4 Applications of Starch–Lipid Blends

352

352
353
353
359
362
363
367
368
374
393

394

403

403
404
404
404
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412
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11.5

Blends of Starch and Salts, Sugars and Other
Additives
11.5.1 Effects of Acids on the Properties of Starch
11.5.2 Effects of Alkalis on the Properties of
Starch
11.5.3 Effects of Salts on the Properties of Starch
11.5.4 Effects of Sugars on the Properties of
Starch
11.5.5 Effects of Amino Acids on the Properties of
Starch
11.6 Conclusions and Future of Starch Blends
Acknowledgements
References
Subject Index

422
422
423
423
424
425
425
426
426
433



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07:56:41.
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CHAPTER 1

Starch: State-of-the-Art, New
Challenges and Opportunities
VISAKH P. M.
Tomsk Polytechnic University, Lenin Av. 30, 634050 Tomsk, Russia
Email:

1.1 Starch: Introduction and Structure–Property
Relationships
Starch is a polysaccharide consisting of D-glucose units, referred to as
homoglucan or glucopyranose, and has two major biomacromolecules –
amylose and amylopectin. Amylopectin is a much larger molecule than
amylose, with a molecular weight of 1Â107–1Â109 and a heavily branched
structure built from about 95% a-(1-4) and 5% a-(1-6) linkages. Amylopectin unit chains are relatively short compared with amylose molecules,
with a broad distribution profile. Starch varieties contain primarily two
different types of anhydroglucose polymers, amylase and amylopectin.
Both amylose chains and the exterior chains of amylopectin can form
double helices, which in turn may associate to form crystalline domains. In
most starches these are confined to the amylopectin component. Double
helices form more or less ordered arrays where the ordered structures are

crystalline entities. The starch granule is a very complex structure, the
complexity being built around variations in the composition (a-glucan,
moisture, lipid, protein and phosphorylated residues) and structure of the
components. In wheat, the starch surface protein friabilin has attracted
much attention because of its proposed association with grain hardness.1–4
RSC Green Chemistry No. 37
Starch-based Blends, Composites and Nanocomposites
Edited by Visakh P. M. and Long Yu
r The Royal Society of Chemistry 2016
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2

Chapter 1

Integral proteins have a higher molecular weight than surface proteins
(B50–150 and B15–30 kDa, respectively) and include residues of enzymes
involved in starch synthesis, especially starch synthase. Starches also contain relatively small quantities (o0.4%) of minerals (calcium, magnesium,
phosphorus, potassium and sodium), which are, with the exception of
phosphorus, of little functional significance. As the starch paste cools, the
viscosity increases due to the formation of a gel held together by intermolecular interactions involving amylose and amylopectin molecules. The
retrogradation of amylose in processed foods is considered to be important

for properties relating to stickiness, ability to absorb water and digestibility,
whereas retrogradation of amylopectin is probably a more important
determinant in the staling of bread and cakes.
Most starches contain a portion that digests rapidly [rapidly digesting
starch (RDS)], a portion that digests slowly [slowly digesting starch (SDS)]
and a portion that is resistant to digestion [resistant starch (RS)].5 Starch
modification not only decreases retrogradation, gelling tendencies of pastes
and gel syneresis but also improves paste clarity and sheen, paste and gel
texture, film formation and adhesion.6 These highly functional derivatives
have been tailored to create competitive advantages in new products,
improve product aesthetics, lower recipe/production costs, eliminate batch
rejects, ensure product consistency and extend shelf-life while clearly making starch relevant in all stages of a food product’s life cycle.7 Modification of
starch is an ongoing process as there are numerous possibilities. There is a
huge market for the many new functional and added-value properties resulting from these modifications.

1.2 Preparation and Characterization of Starch
Nanocrystals
Acid hydrolysis is possibly the most common and optimized method to
produce starch nanocrystals.8 Acid treatment dissolves the regions of low
lateral order to reveal the concentric lamellar structure of starch granules. By
this approach, water-insoluble and highly crystalline residues may be converted into stable suspensions by a subsequent vigorous mechanical
shearing action.9 During acid hydrolysis, regions of low lateral order and
also amorphous phases in the starch granules start to dissolve, while the
highly crystalline water-insoluble lamellae remain undissolved.10 Le Corre
et al.11 conducted an experiment to determine whether starch from many
different sources could be used to prepare starch nanocrystals and if the
amylose content and/or botanic origin of the starch influenced their final
properties. Starch nanocrystals are reported to be derived from starch
granule crystallites12 and result from the disruption of the semicrystalline
structure of native starch granules at temperatures below the gelatinization

temperature. Under these conditions, the amorphous regions in starch
granules are hydrolysed, which allows the separation of nanoscale crystalline


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3

residues. Starch nanocrystals of different sizes and shapes can be obtained
depending on the origin of the starch and the isolation process. Xu et al.14
prepared starch nanocrystals from corn, barley, potato, tapioca, chickpea
and mung bean starches using an acid hydrolysis method.
´le
´ et al.15 studied the processing of nanocomposite materials conMe
sisting of natural rubber filled with waxy maize starch nanocrystals.
Angellier et al.16 employed starch nanocrystals in natural rubber composites
and found a remarkable enhancing effect, but when the starch nanocrystal
content exceeded 20%, the enhancement decreased. Bouthegourd et al.17
reported the extraction and characterization of potato starch nanocrystals
and their nanocomposites with a natural rubber latex matrix with the
preparation performed using sulfuric acid at 40 1C. Kim et al.18 claimed that
obtaining individual nanoparticles from starch was almost impossible
regardless of the origin of the starch. In another study by the same group,19 a

hydrolysis process combined with a physical treatment such as ultrasonication for the formation of a uniform dispersion of starch nanocrystal
was investigated.
Li et al. reported three stages corresponding to the stepwise hydrolysis of
the amorphous, semicrystalline and crystalline layers of the starch structure.20 Some authors have suggested that high-amylose starches are more
susceptible to acid hydrolysis than those with lower amylose contents,
which are more easily hydrolysed.21–23 This can be explained by either the
greater extent of starch inter-chain associations in the amorphous regions,
which are more compactly organized,24 or by the slower penetration of
hydrogen ions into the granules due to the limited swelling of high-amylose
starch.
Large-scale starch nanocrystals (10–50 nm) obtained from the acid
hydrolysis of amylopectin-rich waxy maize starch have been employed to
prepare nanobiocomposites with natural rubber using a mastication technique.25 Habibi and Dufresne26 found that the mechanical characteristics of
nanocomposite materials were improved by using chemically modified
starch nanocrystals, which resulted in better dispersion of the filler within
the matrix. Chen et al.27 reported a reduction in the moisture uptake of a
poly(vinyl alcohol) (PVA) matrix from 78 to 62% for the unfilled matrix and
40% w/w starch nanocrystal-reinforced composites, respectively. Angellier
et al.28 modified starch nanocrystals with alkenyl succinic anhydride or
isocyanates and observed that the toluene uptake of the composite was
higher than that of unmodified starch nanocrystals.
Thielemans et al.29 observed that the thermal behaviour of starch
nanocrystals was improved by grafting to alkyl polymer chains, which they
suggested may be due to the protective crystalline layer formed by the
oxygen-poor stearate surface. Namazi and Dadkhah30 found similar results
in relation to hydrophobically modified starch nanocrystals using octanoyl,
nonanoyl and decanoyl chloride in an aqueous medium under mild conditions. They evaluated the thermal properties of the starch nanocrystals
using thermogravimetric analysis (TGA) and observed that the



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decomposition onset temperature increased for modified starch nanocrystals, revealing their higher thermal stability than the unmodified form. The
synthesis of starch nanocrystals with various sizes and shapes has been
widely reported,31 with the industrially important platelet starch nanocrystals obtained from hydrolysis of native starch granules being relatively
easy to obtain with thicknesses of 6–8 nm, lengths of 40–60 nm and widths of
15–30 nm.32

1.3 Natural Fibre-reinforced Thermoplastic Starch
Composites
Starch can be processed into a mouldable thermoplastic known as thermoplastic starch (TPS). TPS is plasticized starch that has been processed
(typically using heat and pressure) to destroy completely the crystalline
structure of the starch to form an amorphous thermoplastic material. Water
contained in the starch and the added plasticizers play an indispensable role
because the plasticizers can form hydrogen bonds with the starch, replacing
the strong interactions between the hydroxyl groups of the starch molecules,
thus making the starch thermoplastic. McHugh et al. suggested that, owing
to its small size, glycerol was more effective than sorbitol in plasticizing the
starch.33 Many studies have been carried out on the preparation of TPS using
glycerol as plasticizer. Park et al. developed biodegradable thermoplastic
potato starch by using 30% glycerol as plasticizer.34 Averous and Boquillon
studied the thermal and mechanical behaviour of composites made from
TPS reinforced with agro-materials (cellulose and lignocellulose fibres).35

The TPS composite modulus displays a regular behaviour where the
reinforcement effect increases with increase in the fibre length from shortlength fibre (SF) to medium-length fibre (MF) and fibre content whereas the
elongation at break decreases with increase in fibre content and length.

1.4 Applications of Starch Nanocrystal-based Blends,
Composites and Nanocomposites
The main advantages of starch nanocrystals (SNCs) are their renewable nature, low cost, high barrier properties, availability, compatibilization with
biopolymers, high specific strength, non-abrasive and non-toxic nature that
allows easier processing even at high filling levels, biodegradability and a
relatively reactive surface. They are edible, versatile and light weight and
have a high aspect ratio, high specific strength and high modulus. Starch
nanoparticles and nanocrystals have many potential applications, such as
plastic fillers, food additives, drug carriers, implant materials, vehicles for
carrying bioactive substances and nutraceuticals, fillers in biodegradable
composites, coating binders, adhesives and a source of energy at the end of
their life cycle.36 The starch nanocrystals can also be used in biomedical,
biochemical and technological applications and as vehicles for carrying


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bioactive substances and nutraceuticals. However, they tend to aggregate
and settle down in aqueous solutions, which is a limitation to their

application in most biological and food systems.37 Wei et al.38 mentioned
that SNCs are crystalline platelets originating from the breakdown of the
semicrystalline structure of starch granules by acid hydrolysis of amorphous
parts. Because of their unique properties, SNCs have been widely used as
particle emulsifiers to prepare Pickering emulsions39 and as a reinforcement
to prepare nanocomposites, such as biodegradable films and natural
polymers.40–42

1.5 Chemically Modified Thermoplastic Starches
Grafting with maleic anhydride (starch-g-MA) is one of the most traditional
and relevant techniques used to compatibilize non-polar polymers, such as
polyolefins [e.g. low- and high-density polyethylene (LDPE and HDPE) and
polypropylene (PP)] and polar materials, such as starch. Diisocyanates, such
as 4,4 0 -diphenylmethane diisocyanate (MDI) and hexamethylene diisocyanate (HDI), have been employed for the reactive compatibilization of starch
with polyesters such as poly(lactic acid) (PLA).43–47
Starch can be esterified by processing TPS with acid anhydrides
and catalysts. The main anhydrides used are acetic, propionic, maleic and
succinic anhydride.48 Other reagents such as formic acid,49,50 acid halides and
vinyl acetate have also been reported.51 Other reactions may occur during the
esterification process, such as hydrolysis and glycosylation with glycerol.52
Propylene oxide is the most commonly used reagents in the preparation
of hydroxypropyl starch.53,54 Reaction with 3-chloro-2-hydroxypropyltrimethylamine chloride with reactive extrusion (REX), in the presence of 15
wt% glycerol, has been used to prepare cationic starch.55 Reactions with
vinyl acetate,56 styrene57 and acrylamide58,59 have been described in processes for the grafting of starch in the molten state. Reduction of the molar
mass of starch by glycolysis, catalysed by inorganic or organic acids, has
been used to prepare modified TPS with lower melt viscosity.60 Citric acid
was used to improve the compatibility of TPS and other polymers, including
LDPE.61,62 Blends with linear LDPE were described by Ning et al.63 In their
studies, it was shown that citric acid can indeed improve the compatibility of
the system. A recent study by our group also investigated blends of starch

modified with citric acid and polyethylene in a two-stage process. Citric acid
has also been used in nanocomposites of TPS with clays such as montmorillonite (MMT), for the purpose of modifying MMT to increase its rate of
exfoliation64–66 and to alter the properties of TPS so as to increase the
wettability of the clay. The future of the reactive extrusion of TPS lies in its
use in an intensive form so that starch can be radically modified, generating
new materials that can be tuned to a wide assortment of uses. Reactive
extrusion of TPS has proved to be a green process in that native starch is
used without previous modification and the reactions take place at the same
time as TPS is produced.


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1.6 Outstanding Features of Starch-based Hydrogel
Nanocomposites
The highly branched structure renders high molecular weight amylopectin
that is in general 1000 times higher than that of amylose. Indeed, amylopectin is a ‘‘titan’’ natural molecule: one of the largest with a molecular
weight of up to 400 Â106.1 The ratio of amylose to amylopectin in any native
starch is dependent on the source. In addition to these two main components, starch granules could also present other minor components in
their composition such as some particulate material (e.g. cell-wall fragments) or surface and internal components (e.g. proteins, enzymes, lipids,
amino and nucleic acids).67,68
Another important feature of starch is the large number of hydroxyl
groups in their backbone. These groups have great affinity for other hydroxyl

groups (hydrogen bonding), which can act as a driving force to hold the
starch chains together in a regular pattern. Where such ordering occurs,
crystalline regions are deposited in the starch granules. Many research
groups have explored in depth the potential of starch-based hydrogels and
their derivatives (such as starch-based hydrogel composites) in a wide variety
of technological and biological fields. As a result, outstanding advances in
starch-based hydrogels have published in recent decades. The application of
hydrogels is no longer focused only on liquid absorption/retaining and the
advantages of this very promising class of materials are now being exploited in
the most varied industrial, technological and biotechnological sectors.69–72
Hydrogels prepared from biopolymers (mainly from polysaccharides) have
found great applicability as biomaterials.73–75 The interesting properties of
polysaccharides derive from their structure, which, in general, contains a
large number of functional groups (–COOH, –OH, –NH2, –NHOCCH3 and
–OSO3H) that can be crosslinked by reaction with a coupling agent or that
allow the insertion of crosslinkable groups or polymeric chains on the
polysaccharide backbone.76,77 Most of the hydrogels prepared by this
methodology show semi-interpenetrating network (semi-IPN) characteristics. IUPAC defines a semi-IPN as a polymeric material comprising at least
one network and at least one linear or branched polymer characterized by
the penetration of both on a molecular scale.78 Starch-based hydrogels
prepared from hydrophilic polymers/monomers, in a general way, have the
capacity to absorb and retain large amounts of liquid, which classifies these
hydrogels as superabsorbent. Raw starch is not so hydrophilic owing to its
granular structure, and for this reason the association of starch with more
hydrophilic polymers is required in order to prepare materials with a high
liquid uptake capacity. In polymer science, hydrogels have evolved into
materials with outstanding features and many potential applications, from
soil conditioners and hygienic products to tissue engineering, drug delivery
systems and imprinted polymers.79,80 Al et al.81 prepared superabsorbent
hydrogel composites by grafting acrylic acid onto a starch backbone at a

monomer to starch weight ratio of 1.5 (ca. 40 wt% maize starch) using


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N,N 0 -methylenebisacrylamide as crosslinker, cerium ammonium nitrate as
initiator and Na/MMT as the reinforcing phase. Eid82 reported the preparation of starch polyelectrolyte hydrogels [starch/polyvinylpyrrolidone,
starch/polyacrylamide and starch/poly(acrylic acid)] polymerized via gamma
irradiation loaded with silver nanoparticles prepared by in situ reduction of
silver nitrate with sodium borohydride, at room temperature.
The addition of active fillers can confer new properties on hydrogels,
broadening their range of applications. In this respect, silver nanoparticles
have been incorporated into hydrogels owing to their antibacterial features.
Magnetic nanoparticles have also been added to starch hydrogels, allowing
the modulation of release and absorption features by the use of an external
magnetic field and also magnetic separation from aqueous media. Hydrogels are also potential candidates for the transport and storage of acidresponsive drugs. Mauricio et al. also developed a novel synthetic strategy for
the synthesis of magnetite-nanostructured amylose microspheres.83

1.7 Starch-based Blends
Sabetzadeh et al.84 prepared LDPE–thermoplastic corn starch (TPCS) blends
containing different amounts of TPCS (0–40 wt%) and a constant amount of
LDPE-g-MA (3 wt%) by using a single-screw extruder. Tanrattanakul and
Panwiriyarat85 enhanced the compatibility of LDPE–cassava starch blends by

the addition of potassium persulfate (PPS) and benzoyl peroxide (BPO) to
increase interfacial adhesion between the LDPE matrix and starch granules.
Wang et al.86 prepared compatible TPS–PE blends by reactive extrusion and
reported that in the presence of dicumyl peroxide (DCP), the thermal plasticization of starch and its compatibilizing modification with polyethylene
(PE) were accomplished by one-step reactive extrusion in a single-screw
extruder. Thipmanee and Sane87 reported the compatibilizing effect of
zeolite 5A on linear low-density polyethylene (LLDPE)–TPS blends. The
mechanical properties of the starch–poly(e-caprolactone) (PCL) blends
become poorer with increasing starch content in the blend88 This may be
ascribed to the incompatibility between the hydrophobic PCL and the
hydrophilic starch.89,90 Tan et al.91 used synthesized starrch-modified polyurethane (St-PCL) as compatibilizer to compatilibilize the starch-PCL blend
and found that a smaller amount of St-PCL can effectively improve the
compatibility of the blends. Starch-g-PCL has been used as a compatibilizer
to enhance the interfacial adhesion between PCL and starch phases, and the
properties of the blend were obviously improved.92,93
Lai et al.94 used three types of TPS, namely potato starch, corn starch and
soluble potato starch, to blend with polyhydroxybutyrate (PHB), and the TPS
(soluble starch)–PHB blend gave the highest level of water absorption and
weight loss. PVA was also one of the first synthetic polymers to be blended
with starch and shows excellent mechanical properties and compatibility
with starch.95–97 Moreover, the PVA–starch blend is assumed to be biodegradable since both components are biodegradable in various microbial