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Title: Flexible starch-polyurethane films: Effect of mixedmacrodiol polyurethane ionomers on physicochemicalcharacteristics and hydrophobicity
Authors: N.L. Tai, Raju Adhikari, Robert Shanks, PeterHalley, Benu Adhikari
PII: S0144-8617(18)30673-8DOI: https://doi.org/10.1016/j.carbpol.2018.06.019Reference: CARP 13693
To appear in:
Received date: 13-1-2018Revised date: 2-6-2018Accepted date: 4-6-2018
Please cite this article as: Tai, NL., Adhikari, Raju., Shanks, Robert.,Halley, Peter., & Adhikari, Benu., Flexible starch-polyurethane films:Effect of mixed macrodiol polyurethane ionomers on physicochemicalcharacteristics and hydrophobicity.Carbohydrate Polymers (2018),https://doi.org/10.1016/j.carbpol.2018.06.019
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Flexible starch-polyurethane films: Effect of mixed macrodiol polyurethane ionomers on physicochemical characteristics and hydrophobicity
N. L. Tai1,2, Raju Adhikari2, Robert Shanks1, Peter Halley3, Benu Adhikari1,2*
1School of Science, RMIT University, Melbourne, VIC 3083, Australia
2CSIRO Materials Science and Engineering, Clayton South, VIC 3169, Australia
3School of Chemical Engineering, University of Queensland, St Lucia, QLD 4072, Australia
*Corresponding author: phone: +61 3 99259940, fax: +61 3 99253747, email:
Highlights
Films were produced by blending anionic poly(ether-ester)urethane (AEEPU) and
starch
Molecular entanglement and hydrogen bonding occurred between starch & AEEPU
Miscibility and compatibility between starch & AEEPU were significantly high
Flexibility, hydrophobicity and transparency of these films were close to that of LDPE
Starch-AEEPU films can be used in packaging applications as an alternative of LDPE
Abstract
One of the most critical limitations in synthesizing starch-polyurethane (PU) hybrid materials
is their microphase separation caused by physical incompatibility. This paper reports that
the physical incompatibility and microphase separation between starch and PU can be
overcome by using specifically designed anionic poly(ether-ester) polyurethane (AEEPU).
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The AEEPU was synthesised by preparing isocyanate (NCO)-terminated prepolymer using
Isophorone diisocyanate (IPDI), 2,2-bis(hydroxymethyl)propionic acid (BMPA), poly
(ethylene glycol) (PEG) and polycaprolactone (PCL). This AEEPU was physically mixed with
glycerol plasticized high amylose starch (HAGS) at HAGS to AEEPU mass ratios of 90/10,
80/20, 70/30, 60/40, 50/50. Higher AEEPU content in HAGS-AEEPU increased surface
hydrophobicity and elasticity while the Young’s modulus remained unaffected. HAGS-AEEPU
film at 50:50 ratio was comparable to LDPE film in terms of elongation at break (187%),
Young’s modulus (383 MPa), and contact angle (112˚) and good transparency. These starch-
PU films are expected to find increased application as biodegradable packaging materials.
Keywords: High amylose starch, anionic polyurethane, packaging films, hydrogen bonding,
hydrophobicity, strength and flexibility.
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1. Introduction
There is an increasing demand for biodegradable packaging materials obtained primarily
from renewable sources. Starch is one of the most promising biopolymers to replace
synthetic polymers in packaging applications due to its availability, renewability and ease of
chemical modification (Lu, Xiao, & Xu, 2009; Sweedman, Tizzotti, Schäfer, & Gilbert, 2013;
Tharanathan, 2005). Starch-based materials are currently used in daily life in food and non-
food applications such as wrappings, mulch films, bags, paper laminations, blow-molded
bottles, boxes, cutlery and trays (Glenn, Orts, Imam, Chiou, & Wood, 2014; Tang, Kumar,
Alavi, & Sandeep, 2012). However, the application of starch as a major component of
primary or stand-alone packaging has some major limitations due to its inherent brittleness,
weak moisture resistance and sensitivity to environmental relative humidity (Averous, Moro,
Dole, & Fringant, 2000; Thunwall, Kuthanová, Boldizar, & Rigdahl, 2008). It does not
perform in the same manner as existing olefin-based films such as polyethylene. Hence, the
physical structure and chemical composition of starch have to be modified to improve the
physicochemical characteristics of starch-based packaging materials before they can be used
as major component of primary or ‘standalone’ packaging. Starch-based biodegradable
packaging materials can be developed using a range synthetic yet biodegradable polymers.
Among those polymers, polyurethane (PU) is highly versatile. The hard and/or soft segment
of polyurethanes (PUs) can be altered to tailor their properties for a wide range of
applications and as a result, there is an increasing interest in developing starch-PU hybrid
materials for packaging application.
PUs are usually hydrophobic in nature and are insoluble in water. Thus, water dispersible
polyurethane ionomers are synthesised by incorporating hydrophilic soft segments and ionic
species in their chain (Daemi, Barikani, & Barmar, 2014; Lee, Wu, & Jeng, 2006). Ionic PU
usually contains pendant carboxylic or sulphonic groups, or quaternary ammonium salts
(Mohaghegh, Barikani, & Entezami, 2005; Rengasamy, 2013). These ionic moieties act as
internal emulsifiers or self-emulsifiers and improve the dispersibility of PUs in water. In
aqueous medium, these ionomers act as electrically charged moieties and exert coulombic
force. The interaction between these charged moieties of polymeric matrix strengthens
intermolecular forces and results into physical crosslinking (Frisch & Xiao, 1995; Rengasamy,
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2013). These electrically charged moieties possess high surface energy and provide the
necessary driving force for film formation once the water is removed by evaporation.
Conventional PUs are incompatible with starch. Attempts have been made in the past to
improve the compatibility between PU and starch by primarily modifying the chemical
structure of PU. Starch–PU composite have been usually prepared via chemical grafting,
reactive extrusion or physical mixing methods. In our previous studies, we reported the
formulation and characteristics of thermoplastic starch-PU films, where chemical grafting of
polyurethane to starch was carried out (Tai, Adhikari, Shanks, & Adhikari, 2017a, 2017b). PEG
microdiol was used in the soft segment of those PUs which improved the ductility and water
resistance of the starch-PU films. However, the films were opaque and their opaqueness was
attributed to the degree of immiscibility and partial crosslink between starch and PU. In this
paper, instead of preparing starch-PU hybrid film by chemical grafting, we made use of many
advantages that come with physical mixing of PU ionomer with starch. It is possible to
physically mix starch with a suitably functionalised PU ionomer. The ionic nature of PU
ionomer makes it possible to blend it with starch without significant phase separation.
To date, PU has been used, rather than starch, as the major component in starch-PU hybrid
composite (Cao, Zhang, Huang, Yang, & Wang, 2003; Y. Lu, Tighzert, Dole, & Erre, 2005;
Travinskaya, Savelyev, & Mishchuk, 2014). Studies have shown that a well dispersed starch-
PU hybrid material contains up to 20 % w/w starch, above which phase separation occurs
and leads to poor mechanical properties. It is desired that the starch component in these
starch-PU films is significantly increased from current 20% (w/w). Although starch-PU
blends prepared by physical mixing have been studied in the past, the structure-properties
relationship of these material is not satisfactory understood. Because of the above reasons,
the formulation and physicochemical properties of starch-PU ionomer as a hybrid materials
warrants further research so that they can be used as primary packaging materials.
A careful design of chemical composition and physical properties of PU is the most
important step towards obtaining starch-PU packaging materials with desired
physiochemical and mechanical properties. The structure-function of PU indicates that
polyol soft segments play an important role in its properties (Hu & Mondal, 2005; Rahman
et al., 2013). It is shown that polyester polyol-based PU provides better mechanical
properties but it gets hydrolysed quite easily. On the other hand, polyether polyol-based PU,
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shows good resistance to hydrolysis, good water-vapor permeability and flexibility
(Fuensanta et al., 2017). Segmented PUs are prepared by blending polyester and polyether
polyols to produce PUs with targeted properties. Polyol blends are prepared using two or
more polyols with different properties to achieve better properties than that of either one
or to achive synergistic improvement of a targeted property (Cohn, Stern, González, &
Epstein, 2002; Shokrolahi & Yeganeh, 2014). To achieve targeted or specific physical
properties, a mixed polyol has to be used as a soft segment of PUs.
In the above context, the objective of this study was to design and synthesise anionic PU
using a suitable mixture of hydrophobic polyester (PCL) and hydrophilic polyether polyol
(PEG) as soft segment. We hypothesised that this approach can improve the structural
compatibility between starch and polyurethane and help develop functionally superior
starch-PU packaging films. To the best of our knowledge, no study has so far been
undertaken in developing starch-anionic PU films via physical mixing using mixed
polyester-polyether macrodiols. The particle size distribution, zeta potential, FTIR, SEM and
dynamic mechanical analysis (DMA) experiments were conducted to probe the interaction
between gelatinized starch and this newly synthesised anionic PU. The mechanical
properties and surface hydrophobicity of the starch-anionic PU films were measured and
explained. In short, this research shows that starch-anionic PU films can be developed and a
high degree of compatibility can be achieved between the anionic PU and gelatinized high
amylose starch. This research also shows that a simple physical blending is adequate to
develop highly compatible starch-PU films.
2. Materials and methods
2.1 Materials
High amylose (HA) corn starch (Gelose 80) with amylose-to- amylopectin ratio of 80:20 and
moisture content of 14.16 % was obtained from Ingredion ANZ Pty Ltd, New South Wales
(Australia). Glycerol (99.5%, G) and isophorone diisocyanate (98%, IPDI), 2,2-
bis(hydroxymethyl)propionic acid (98%, BMPA), sodium dodecyl sulfate (99%, SDS),
ethylenediamine (99.5%, EDA), and triethylamine (99.5%, TEA) were purchased from Sigma
Aldrich, New South Wales (Australia). Poly (ethylene glycol) (Mw = 1000 g/mol) (PEG 1000)
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and polycaprolactone (Mw = 1000 g/mol) (PCL 1000) were purchased from Thermo Fisher
Scientific, Victoria (Australia) and ERA polymer Pty Ltd, New South Wales (Australia)
respectively. LDPE snap seal bag was purchased from Coles Supermarket (Melbourne,
Australia). The polyols were dried under vacuum at 90 ˚C for at-least 12 h prior use. The HA
starch was used as received. The moisture content of starch was measured by gravimetric
method and was compensated for while preparing starch-water slurry before gelatinization.
2.2 Methods
2.2.1 Synthesis of anionic poly(ether-ester) urethane (AEEPU) dispersion
The PU was synthesized as reported by Adhikari et al. (Adhikari, Casey, Bristow, Freschmidt,
& Hornbuckle, 2017). The polymerization reaction was carried out under N2 atmosphere in a
three-necked round-bottom flask equipped with a mechanical stirrer. An NCO/OH ratio of
1.0 was used with 1.5 wt% of BMPA to create ionic species. The degassed polyol PEG 1000,
PCL 1000 and BMPA were added into the reactor and heated for 1 h at 90˚C until BMPA was
dissolved. Subsequently, IPDI was added to the mixture using a syringe. This mixture was
stirred continuously and was allowed to react for 4 h at 90˚C to obtain NCO terminated
prepolymer. Neither catalyst nor organic solvents were used in this process because the
resulting material is intended for food application. The prepolymer was then cooled to 50˚C
and TEA was added to the fully reacted mixture using a syringe to neutralize the reaction.
This neutralization reaction was continued for 30 min. After neutralization, deionized water
containing 2 wt% SDS was added into this neutralised prepolymer and then stirred
vigorously to yield the dispersion. Once the temperature of this dispersion was brought
down to 25˚C, EDA (diluted in deionised water) was added drop wise to trigger the chain
extension reaction. The stirring continued until NCO peak disappeared in FTIR spectra
indicating the completion of the reaction. The anionic polyurethane dispersion was
prepared as 20 % (w/w) solid content and was stored at ambient temperature.
2.2.2 Gelatinisation of starch
The HA starch and glycerol were added into distilled water to maintain a total solid
concentration of 5% (w/w). The HA starch: glycerol dry solid ratio was maintained at 80:20
(w/w). Gelatinisation was carried out using a pressurized BiotageTM microwave reactor
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(Biotage AB, Sweden) at 140 ˚C under constant stirring (Tai et al., 2017a). The headspace
pressure during gelatinization ranged from 700 to 800 kPa. The suspension was held at
140 ˚C for 15 min before cooling-down. The starch was fully gelatinized, i.e., starch granules
were fully ruptured and eliminated under this optimized gelatinized condition. The
microwave reactor was able to provide an effective volumetric heating to the starch
suspension.
2.2.3 Preparation of HAGS-AEEPU films
The HAGS-AEEPU blends were prepared by physically blending AEEPU dispersion and
gelatinised starch. The temperature of HAGS and AEEPU solutions and HAGS-AEEPU blends
was maintained at 50˚C using a magnetically stirred and temperature-controlled water bath.
The stirring was carried out for 30 min at 400 rpm. HAGS: AEEPU with the same solid
content 5 % (w/w) but different ratios (90:10, 80:20, 70:30, 60:40 and 50:50) were
thoroughly blended for 30 min at 50˚C. These blends were then cast onto a petri dish and
then dried at ambient temperature (20±1oC) for 72 h to produce films. These films were
conditioned in a desiccator containing magnesium nitrate (52.9% RH) for at-least 72 h prior
to analysis. The films are labelled as HAGS, HAGS10AEEPU, HAGS20AEEPU, HAGS30AEEPU,
HAGS40AEEPU and HAGS50AEEPU. The indicating numbers (10, 20, 30, 40 and 50) refer to
the mass ratio of AEEPU in the film. The residual moisture contents of the films ranged from
9.56±0.50 % to 11.59±0.66 % depending on the HAGS content in the film (Section 3.9).
2.3 Characterization of AEEPU and HAGS-AEEPU films
2.3.1 Measurement of particle size and zeta potential
The weight averaged particle size distribution and zeta potential (‐potential) of the AEEPU
dispersion were measure at 25 ± 1 °C. The particle size distribution of these dispersions was
measured using a Zetasizer (Zen3600, Malvern Instruments, Worcestershire, UK) using a
dynamic light scattering (DLS) technique. The zeta potential of dispersions was measured
using laser Doppler micro-electrophoresis. This method measures how fast a particle moves
in a liquid when an electrical field is applied.
2.3.2 Determination of molecular weight
The molecular weight distribution of AEEPU samples was determined using gel permeation
chromatography (GPC). Shimadzu chromatograph system equipped with RDI-10A refractive
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index detector was used for this purpose. This instrument contained four Waters Styragel
columns (HT2, HT3, HT4, and HT5), a CMB-20A controller system, a SIL-20A HT autosampler,
a LC-20AT tandem pump system, a DGU-20A degasser unit, and a CTO-20AC column oven.
N,N-Dimethylacetamide (DMAc) containing 4.34 g/L lithium bromide (LiBr) was used as an
eluent with a flow rate of 1 mL/min at 80 °C. Number (Mn) and weight average (Mw)
molecular weight of synthesised copolymers were evaluated using Shimadzu LC Solution
software. The GPC columns were calibrated with low dispersity polystyrene standards (Cohn
et al.) and the molar mass is expressed as PSt equivalent.
2.3.3 Determination of molecular interaction between HAGS and AEEPU
Specific spectral “signatures” of the HAGS and AEEPU were acquired using an attenuated
total reflectance (ATR) FTIR spectroscope (Nicolet 6700, Thermo Scientific, USA) with a
diamond coated zinc selenide crystal plate (reflection plate with pressure arm). The spectra
were collected in 650 to 4000 cm-1 range with automatic signal gain. A total of 16 scans
were performed and averaged for each sample at a resolution of 4 cm-1.
2.3.4 Acquiring microstructure images
Microstructure of film samples was captured using a field emission scanning electron
microscope (FESEM) (Zeiss Merlin, Germany). The images of top surface and internal
structure (cross section) of the films were acquired. To acquire the cross section, the film
samples were immersed in liquid nitrogen for 5 min and then fractured. The samples were
mounted on specimen stubs and sputtered with a thin layer of Iridium in order to make
them conductive. The images were acquired at an accelerating voltage of 3kV at 20,000×
magnification.
2.3.5 Measurement of mechanical properties
The mechanical properties (tensile strength, Young’s modulus and elongation at break) of
the films were measured using Instron universal testing machine (Instron 5565, USA). Tests
were carried out at ambient temperature in accordance with ASTM D1708 (ASTM, 2013a).
The width and grip distance of the test films were 5 ± 0.03 mm and 22 ± 0.05 mm,
respectively and the cross-head speed of 10 mm.min-1 was used. Five replicate runs were
carried out for each sample and the data points were averaged. The tensile strength
(σmax) was calculated by dividing the maximum force by cross-section area. Young's modulus
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(E) was calculated using the linear part of the stress versus strain curve. The percentage
elongation at break (εB) was calculated using equation (1).
ε𝐵 (%) = 𝐿−𝐿0
𝐿0 × 100 (1)
where, L and L0 are displacements (mm) at break and at the start of experiments,
respectively.
2.3.6 Measurement of light transparency of the films
Light transparency of the films was measured in wavelength range of 200 to 800 nm using a
UV-visible spectroscope (Lambda 1050, Perkin Elmer, Llantrisant, UK) according to the
method reported by Rao et al. (Rao, Kanatt, Chawla, & Sharma, 2010) and Shiku et al. (Shiku,
Hamaguchi, & Tanaka, 2003). Transparency of the films was calculated by using equation (2)
given below.
𝑇ransparency =𝑙𝑜𝑔10(𝑇600)
𝑥 (2)
where, T600 is the transmittance (%) of light at 600 nm and x is the film thickness (mm). The
wavelength of 600 nm is commonly used to determine the transparency of packaging films
(Monjazeb Marvdashti, Koocheki, & Yavarmanesh, 2017; Rao et al., 2010).
2.3.7 Measurement of contact angle
The contact angle (CA) of water on the film surface was measured using a tensiometer (CAM
200, KSV instruments LTD, Finland) connected to a high resolution digital camera (BASLER,
A602i, Germany). The angle formed at solid-liquid-air triple point was used to determine the
CA using static sessile drop method. The film was mounted on a glass plate with double
sided tape. Then, a drop of Mili-Q water was deposited on the film surface and the image of
this drop was recorded. The CA value was determined using image analysis software
(Attension Theta SFE). The plug-in program measured the drop profile using edge detection
algorithm and calculated the CA using ellipse approximation. Measurements were made at
five different spots of the film surface and the data was averaged.
The hydrophobicity of the film, which is the adhesion of water on the solid surface, was
measured making use of CA of water. When the solid surface holds water droplet less
strongly, its hydrophobicity is higher. The wettability or work of adhesion (WSL)(mN.m-1) of a
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solid surface is commonly calculated making use of contact angle using Young–Dupré
equation given by equation (3) (Fleming et al., 2011).
𝑊𝑆𝐿 = 𝛾𝐿𝑉 (1 + cos 𝜃) (3)
where, LV is the surface tension of the deionized water at test temperature (20°C)
72.8 mN.m-1.
2.3.8 Determination of crystalline/amorphous nature of the films
Wide angle X-ray diffraction (WAXD) was used to determine the crystalline/amorphous
nature of the HAGS-AEEPU films. Films were affixed to a Si zero background plate with
Kapton® tape. A diffractometer (Rigaku SmartLab, Japan) equipped with a rotating CuKα
anode source (45kV, 200mA) and a Hypix 3000 detector was used to obtain X-ray
diffractograms. The diffractometer was operated in glancing incidence mode with 1 mm
incidence and receiving slits and a beam limiting mask of 10 mm. Data was collected over
the 2θ range of 2° to 90° with a step size of 0.01° and a step rate of 1˚.min-1. The angle
between the X-ray source and specimen (Omega) (Ω) was fixed at 1.5°. Analysis of XRD data
was carried out using Bruker’s XRD program (EVA™v4.2).
2.3.9 Determining miscibility of the blends through dynamic mechanical property
The dynamic mechanical loss tangent of the HAGS-AEEPU films was determined using a
dynamic mechanical analyser (PYRIS DiamondTM, Perkin-Elmer, Japan) in tension mode at an
oscillating frequency of 1 Hz. Heating was carried out from -100 to 80 ˚C at a rate of
5oC·min-1. Each film sample (10 mm length x 10 mm width) was fixed on a twin grip clamp.
A thin layer of petroleum jelly grease was applied to each film prior testing to minimize
moisture loss during measurement. The loss tangent (tan 𝛿) was determined. The peak
temperature of the tan 𝛿 curve (Ttan 𝛿) was defined as the glass transition temperature (Tg)
of the samples (Menard, 2002).
2.3.10 Measurement of glass transition of AEEPU film
The glass transition temperature (Tg) of AEEPU film was also determined by a differential
scanning calorimeter (DSC) (DSC 3, Metler Toledo, New Castle, USA), equipped with a
quench cooling accessory. The system was calibrated using melting temperature (Tm) and
heat of fusion (ΔHm) of Indium (Tm = 156.6 ˚C, ΔHm= 28.51 J.g-1) and zinc (Tm = 419.5 ˚C,
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ΔHm = 107.03 J.g-1). The samples (about 3 mg) were weighed onto hermetically sealable
aluminium pans and an empty pan was used as a reference. The samples were scanned
at 10 oC.min-1 over a temperature range of -100 – 50 ˚C; to remove the thermal history of
the films. The samples were scanned twice at the specified temperature range.
2.3.11 Statistical analysis
Statistical analysis was performed using IBM’s statistical software (SPSS®, version 24, IBM
Corp.). All experimental measurements were conducted at least in triplicate and data points
are expressed as the mean ± standard deviation where feasible. To detect any significant
effects of treatments, data for each experiment was separately tested using analysis of
variance (ANOVA, P < 0.05). The significant difference between any two mean values was
determined using post hoc comparison test (Duncan’s Multiple Range Test, DMRT) at 95%
confidence level (P = 0.05).
3. Results and discussion
3.1 Synthesis of anionic poly(ether-ester) urethane (AEEPU) and HAGS-AEEPU hybrid films
The schematic diagram for the synthesis of anionic poly(ether-ester) urethane (AEEPU) is
presented in figure 1 A. The dispersions of these anionic PUs in water were stable up to 20%
(w/w) solid content. This good dispensability of AEEPU in water can be attributed to the
incorporation of BMPA in the backbone of PU (Fuensanta et al., 2017). The BMPA
deprotonated carboxylate ion in the polymer serves as anionic core and acts as internal
emulsifier. BMPA carries carboxylate groups which are chemically bond to the surface of PU
and they form an electrical double layer with their counter-ions (TEA). The interaction
between BMPA and TEA generates anionic AEEPU. The negatively charged surface of AEEPU
causes repulsion among AEEPU particles and enables their better dispersion in water.
However, this highly hydrophilic nature of carboxylic groups imparts an increased affinity
with water in packaging films, which is a major disadvantage when greater surface
hydrophobicity or water repellency is desired (Barikani, Valipour Ebrahimi, & Seyed
Mohaghegh, 2007; Honarkar, Barmar, & Barikani, 2015). Because of this reason, the lowest
possible concentration of BMPA was used in this study. Furthermore, a small amount of
external emulsifier (SDS) was added to improve the dispersibility of AEEPU in water. Studies
have shown that SDS is able to penetrate starch granules and form amylose-SDS inclusion
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complexes, which ultimately affects the elastic properties (Debet & Gidley, 2006; Svensson,
Autio, & Eliasson, 1998). The presence of internal and external emulsifiers can also promote
interaction between HAGS and AEEPU.
The intermolecular interactions occurring between HAGS and AEEPU are postulated
schematically in Figure 1B. The AEEPU ionomers (-COO-NR3+) and SDS are electrically-
charged particles and aggregate via coulombic forces as double layer. When AEEPU and
HAGS are blended, these coulombic forces act between ionic centers and hydrogen bonds
are formed between ionic centers and HAGS; urethane linkages and HAGS; SDS and HAGS.
This interaction among the charged particles in polymeric matrix strengthened the
intermolecular forces and resulted into physical crosslinking. This entanglement of polymer
chains allowed more intercalation between HAGS and AEEPU, which ultimately improved
compatibility between HAGS and AEEPU.
Theoretically, the increased inter-chain interactions between polymers should result into
more cohesive structure. However, it is difficult for molecules with branched chains, large
structure, and big particles to come together and intercalate. This is due to the increase of
free volume between the bulky chains which usually lowers the interchain interactions
(Gündüz & Kısakürek, 2004) which ultimately results into poor dispersion. The effect of
particle size on the miscibility of starch and PU dispersions is discussed further in section 3.2.
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Figure 1. (A) Schematic diagram showing the synthesis of water dispersible anionic polyurethane (AEEPU); (B) proposed/postulated interaction between HAGS and AEEPU.
3.2 Particle size, molecular weight and zeta potential HAGS-AEEPU films
The average particle size and electrostatic charge density (‐potential) play important role
in stability of dispersion and wettability which, in turn, dictate the miscibility between starch
and polyurethane (Yuan, Wang, Cui, & Peng, 2016). It is commonly accepted that the
dispersions with particle size smaller than 200 nm are stable; while those with size larger
than 1000 nm are unstable (Saw, 2000; Yuan et al., 2016). Three different dispersions of PU:
poly(ether) urethane (EPU) (PEG polyol), poly(ether-ester) urethane (AEEPU) (PEG/PCL
polyol) and poly(ester) urethane (CPU) (PCL polyol) with different ratio of polyol soft
segment were prepared as shown in Table 1. The EPU, AEEPU and CPU dispersion had an
average particle size of about 23 nm, 45 nm and 958 nm, respectively (Table 1). All three
samples showed a unimodal particle size distribution with low polydispersity indicating that
that they produced homogenous dispersion. The molecular weight of all three types of PU
dispersions correlated with their Z-averaged particle size and that the higher the molecular
weight the bigger the particle size of the PU dispersion.
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The repulsive forces (higher negative ‐potential) between particles are responsible for the
overall stability of the dispersion (Saw, Brooks, Carpenter, & Keight, 2003). The ‐potential
values of EPU, AEEPU and CPU dispersion are – 7.3 mV, -17.9 mV and -47.5 mV, respectively.
Theoretically, the higher the absolute ‐potential value, the stronger the “repulsive force” is,
and more stable the emulsion is. The magnitude of ‐potential of AEEPU dispersions
increased with the increase in polyester polyol (PCL) content. Although EPU (PEG polyol) had
lower magnitude of ‐potential, yet, its aqueous dispersion was stable as indicated by the
almost similar particle size and ‐potential values of EPU (PEG polyol) measured at the
beginning and at five month of storage. This observation showed that the hydrophilic chain
(-CH2-CH2-O-) present in both BMPA and PEG increased the dispersibility and stability of
dispersions. This is because PEG is a strong hydrophile (also acts as non-ionic surfactant)
that migrates to the oil-water interface and decreases the interfacial tension (Hou, Ding,
Zhang, Sun, & Shan, 2015). CPU (PCL polyol) possessed high ‐potential value due to its
highly polarized ester carbonyl oxygen (Oester) which ultimately enhanced the interaction
between ionomers (Shokrolahi & Yeganeh, 2014).
In order to achieve good miscibility between starch and PU, it is essential to control the
particle size of the PU dispersions. The dispersions containing particles with < 20 nm in
diameter are stable and possess high surface energy both of which impart strong driving
force during film formation. Small particles are desirable because they are able to fill the
void spaces in the packaging structure (Saw, 2000; Serkis, Poręba, Hodan, Kredatusová, &
Špírková, 2015). The aqueous dispersion of starch showed bimodal particle size distribution
with the average particle size ranging from 131 nm to 867 nm. The particle size distribution
was affected by the amylose and amylopectin content of the starch and corroborated with
GPC bimodal distribution profile. We noticed that starch had difficulty in mixing with both
EPU (PEG) and CPU (PCL) and phase separation occurred in these mixtures. However, no
such phase separation was observed in starch-AEEPU (PEG/PCL) dispersions. The poor
miscibility of EPU, despite its much smaller size can be attributed to the weak ether bond of
PEG. This alkoxy oxygen exhibits weaker affinity to proton and it is less effective as hydrogen
bond acceptor (Scerba et al., 2012). Due to these reasons the EPU tends to coagulate itself
rather than forming hydrogen bond with HAGS. The large particles of CPU have greater
tendency to flocculate even though CPU has two strong ester carbonyl oxygen groups.
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When flocculation occurs, even the higher charge density (higher ‐potential values) of
CPU does not improve the miscibility between CPU and starch. Because of its large particle
size, CPU is not effective in penetrating the HAGS chain. Thus, to achieve a good mixing
between HAGS and AEEPU it is essential to work out a suitable PCL to PEG ratio.
Table 1. Particle size, zeta potential, molecular weight and stability of anionic polyurethane
and HAGS dispersions.
Formulation
PCL in
the
mixture
PEG in
the
mixture
Molecular
weight
(Mn)
PDI
(Mw/Mn)
Particle size (Z-
average size)
(d.nm)
Zeta potential
(mV)
Stability
(over 6
months)
Appearance
EPU 0 1 50911 4.39 23.38 ± 6.99a -7.16 ± 1.12a Stable Translucent
AEEPU 0.5 0.5 49275 2.63 45.52 ± 2.48a -17.9 ± 1.71b Stable Translucent
CPU 1 0 130460 3.21 958.32 ± 135.06b -47.5 ± 3.97c Unstable White
HAGS - - 1667072* 1.13 867.7 ± 10.08c
-4.37 ± 1.21d
Turning gel
when cool
down
Milky white
gel 75765** 2.05 131.7 ± 8.05d
The values with different lower-case letters in superscript at the same column are significantly different
(p<0.05). HAGS= high amylose glycerol plasticized starch film; EPU= Anionic polyurethane with PCL: PEG ratio
at 0:1; AEEPU is 0.5:0.5; CPU is 1:0.
*21.7% of molecular weight (Mn) falls at 1667072.
** 78.2% of Molecular weight (Mn) falls at 75765.
3.3 IR absorption characteristics of HAGS and HAGS-AEEPU films
FT-IR spectroscopy is used to characterize the interaction and compatibility between HAGS
and Anionic PU. The FT-IR spectra of Anionic PU, HAGS and HAGS-PEG-PU films are
presented in Figure 2.
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Figure 2. FTIR spectra of HAGS, AEEPU and HAGS/AEEPU hybrid films; (A) N-H stretching
region; (B) C=O stretching region; (C) C-O stretching region. HAGS= high amylose glycerol
plasticized starch film, HS= short form of HAGS; AEEPU= Anionic polyurethane. Thus
numbers (20 and 40) indicate the ratio of AEEPU in the HAGS films.
In the case of HAGS a broad band of hydroxyl (-OH) peak, unsymmetrical and symmetrical
stretching vibrations of C-H were observed at 3289 cm-1, 2927 cm-1 and 2890 cm-1,
respectively. The bands at 997 and 1076 cm-1 are due to the characteristic anhydro-glucose
ring (C-O) of starch (Worzakowska, 2016; Zhang & Han, 2006). The anionic PUs (AEEPU)
showed stretching vibrations of N-H at 3459 cm-1 and 3348 cm-1 indicating to the presence
of free and hydrogen-bonded amine groups. They also showed stretching vibrations of C=O
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at 1726 cm-1 and 1662 cm-1 due to the presence of free urethane carbonyl and hydrogen-
bonded urea carbonyl respectively (Liao et al., 2014; Travinskaya et al., 2014). The C-O-C
stretching vibrations of soft polyol segments of polyether and polyester polyol were
observed at 1100 cm-1 and 1220 cm-1, respectively (Daemi, Barikani, & Barmar, 2013a; Liu et
al., 2011).
When different chemical groups interact at the molecular level (i.e, hydrogen bonding or
other interactions occur) shifting of peak position/width and intensity of the spectral bands
of participant groups are observed in FTIR spectra (Monjazeb Marvdashti et al., 2017; Xu, Li,
Kennedy, Xie, & Huang, 2007). These changes in spectral properties can be used as
indicators of miscibility of polymers. Hydrogen bonding interaction usually shifts the
stretching frequency of participating groups (e.g. N-H, O-H, C=O and C-O) either towards
lower or higher wavenumbers accompanied by increased intensity and broadening of the
peak. The shifting of FTIR spectra wavenumber indicates whether the components of the
composite film are facilitating or restricting via the formation of hydrogen bonds (Wanchoo
& Sharma, 2003).
Figure 2 (A) shows that N-H or OH stretching region of AEEPU, HAGS and HAGS-AEEPU
samples. By adding small amount of AEEPU into the starch, the NH groups of AEEPU
(absorption band at 3348 cm-1) were superseded by the intensity of OH groups of HAGS.
However, the intensity of OH groups of HAGS decreased with the increase of HAGS/AEEPU
ratio from 20 to 40 %. This observation indicated that a considerable amount of OH groups
of HAGS has participated in the intermolecular hydrogen bonding with AEEPU.
When AEEPU was added to HAGS the doublet bands of the urethane carbonyl groups
(Figure 2 (B)) at 1727 cm-1 and 1646cm-1 were shifted to 1730 cm-1 and 1649 cm-1,
respectively. The shifting of these peaks can be attributed to the hydrogen bonding either
between O-H group of HAGS and ether/ester carbonyl linkage of polyol, or urethane
carboxylate groups of polyurethane, or N-H bonds of urethane (Daemi et al., 2014; Daemi,
Barikani, & Barmar, 2013b; M. Zhang, Song, Wang, & Wang, 2012). The increased intensity
of the peaks at higher AEEPU ratio provided further proof that stable hydrogen bonds were
formed between the AEEPU and HAGS molecules. This observation also indicated that the
number of reactive carbonyl group was increased as they were contributed by AEEPU. These
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observations suggested that higher physical entanglements and, hence, better miscibility
was achieved between HAGS and AEEPU.
The HAGS glycosidic bond stretching (C-O in C-OH peak) of HAGS shifted from 997 cm-1 to in
between 1000 cm-1 and 1021 cm-1 after mixing with AEEPU (Figure 7(B)). The shifting of the
peak can be related to the formation of hydrogen bond between OH and ether oxygen
group (C-O-C vibration) of PEG or ester (COO vibration) groups of PCL soft segment in AEEPU
(Barikani & Mohammadi, 2007; Travinskaya et al., 2014). The increase of AEEPU content in
HAGS/AEEPU ratio shows the shifting of 997 cm-1 peak to higher wavenumber. These
observations indicate that stronger bonding or interaction had occurred among the HAGS
and AEEPU polymer chains. The observed characteristics of FTIR spectra revealed that the
hydrogen bonding between HAGS and AEEPU was the main mode of interaction which
imparted increased degree of compatibility of between HAGS and AEEPU.
3.4 Morphology and structure of HAGS and HAGS/AEEPU films
Figure 3 shows the SEM micrographs of cryogenically-fractured surface (cross-section) and
top surface of the HAGS and HAGS-AEEPU films. These micrographs provide insights on the
characteristics of the HAGS-AEEPU film structure. The microstructure of cross-section and
top surface of HAGS film (Figure 3(A)) shows a coarse surface morphology (topography),
most probably due to the radially orientated crystalline and amorphous layers of HAGS
(Appelqvist & Debet, 1997). The presence of AEEPU resulted into smoother surface
topography and more cohesive network structure in HAGS-AEEPU films. This may be due to
the fact that the AEEPU occupied the vacant space surrounding the HAGS molecules through
the formation of hydrogen bonds. The smaller particle size of AEEPU and also electrostatic
interactions of the carboxylate group of PU facilitated this occupation. The ionic nature of
the PU chains and the presence of ionic emulsifiers in its formulation increased the
molecular level interaction and altered morphology and physical properties of HAGS-AEEPU
blends (Daemi et al., 2014). The carboxylate groups of AEEPU helped make its dispersion in
water more stable and improved its miscibility with starch. The small particle size of AEEPU
also allowed it to be readily and homogeneously dispersed into HAGS.
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A HAGS-to-AEEPU ratio of 60/40 or higher is required to achieve a good interaction between
starch and AEEPU molecules. As can be seen from Figure 3 (B) & (C), the (surface and cross-
sectional) roughness of the films with HAGS-to-AEEPU ratios of 80/20 and 70/30 became
less apparent and the network of the films became denser with the increase of AEEPU ratio
to 60/40 or 50/50. This 40-50% percentage of AEEPU, on solid basis, is required to generate
sufficient number of hydrogen bonds with OH groups of HAGS. The cross-section of the
HAGS-AEEPU films of 60/40 and 50/50 showed smooth and homogenous surface (Figure 3
(D) & (E)). This observation suggested that there was a good interfacial adhesion between
the HAGS and AEEPU. The hydrogen bonding and the intense electrostatic interaction
between anionic carboxylate groups of PU and OH groups of HAGS helped promote the
interaction between these two polymers. In addition, the surface micrograph of the films
also showed that the HAGS were covered with AEEPU-rich phase (60/40 or 50/50 ratio in Fig
3(D) & (E)). This increased compatibility between AEEPU and HAGS enables development of
HAGS-AEEPU films with better physicomechanical properties.
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Figure 3. Scanning electron microscope micrographs of the cross-section (left) and surface
(Wright, Li, & Guo) of (A) HAGS; (B) HS20AEEPU; (C) HS30AEEPU; (D) HS40AEEPU; (E)
HS50AEEPU. HAGS= high amylose glycerol plasticized starch film; HS= short form of HAGS;
AEEPU= Anionic polyurethane; 20, 30, 40 and 50 indicate the ratio of AEEPU in the HAGS
film.
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3.5 Crystalline and amorphous characteristics of HAGS and HAGS-AEEPU films
The molecular interaction between HAGS and AEEPU was also confirmed from the data
obtained from WAXD. Figure 4 shows the WAXD diffractograms of HAGS, AEEPU and
HAGS-AEEPU films. The scattering intensity of these diffractograms was normalized with
respect to the film thickness. HAGS showed a type B crystalline structure (Wang, Wang, Yu,
& Wang, 2014) with diffraction peaks at 5.5 ˚, 14.3, 17˚, 19.6 ˚, and 21.4˚ (2θ). Both HAGS
and AEEPU shared diffraction peaks around 19.5˚ (2θ). The HAGS peak was assigned to the
crystalline V-amylose–lipid complexes (Cheetham & Tao, 1998; Waduge, Hoover, Vasanthan,
Gao, & Li, 2006) whereas the AEEPU film had amorphous structure with a single broad peak.
The diffraction peaks at 2.2°, 4.4° and 6.6° (2θ) indicated the presence of SDS in the
formulation. However, since SDS had very low concentration in all the formulations
concentration, it is not expected to affect the crystallinity of the films. Interestingly, the
diffraction peaks of polyols that typically appear at 19.2˚ and 23.3˚ (2θ) in PEG (Tai et al.,
2017a) and at 21˚, 22˚ and 23.7˚ in PCL (2θ) (Fuensanta et al., 2017) were not observed.
These results suggested that the soft segment of PU (comprised of polyol) either did not
crystalize or it was overshadowed by the hard segment. This could also possibly be due to
strong interaction of urethane linkage with oxygen molecules of polyols in soft segment
(Doseva, Shenkov, Vasilev, & Baranovsky, 2004; Skarja & Woodhouse, 1998).
The incorporation of AEEPU into HAGS disrupted the crystalline structure of HAGS film. The
intensity of crystalline HAGS peaks appearing at 5.5 and 17 ° (2θ) decreased substantially
with the increase of PU content while the intensity of the peak appearing at 21.4 ° (2θ)
decreased to a lesser degree. Shifting of some peak was also observed. Most notably the
minor HAGS peak previously observed at 14.3° (2θ) shifted towards lower angles with the
increase of AEEPU content. This indicated to an increase in d-spacing due to AEEPU
intercalation. Films with HAGS/AEEPU ratio of 60/40 (HS40AEEPU) and 50/50 (HS50AEEPU)
had a characteristic and prominent broad amorphous peak at 19.5˚ (2θ) as expected from
the diffractograms of AEEPU. This notable decrease in the crystallinity of starch in
HAGS-AEEPU accompanied with the shifting of peak positions indicated that the AEEPU
molecules were able to intercalate into the interior of HAGS. This intercalation is augmented
by the electrostatic and intermolecular interactions between the urethane groups of AEEPU
and the OH groups of HAGS. These interactions led to the formation of intermolecular
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hydrogen bonds and prevented the recrystallization of HAGS (Xu et al., 2007). The
diffraction peaks of HAGS-AEEPU films also became broader indicating the increased
compatibility between starch and AEEPU (Abugoch, Tapia, Villamán, Yazdani-Pedram, &
Díaz-Dosque, 2011; Martins et al., 2012). A new, albeit less prominent peak appeared in the
HAGS-AEEPU films at around 12.8° (2θ). The intensity of this new peak increased with the
increase in AEEPU content. It has to be noted that the peak of starch-only film appeared at
14.3° (2θ). The original diffraction peak of starch in HAGS-AEEPU films consistently shifted to
lower angles when the AEEPU content in the films increased indicating that HAGS and
AEEPU are compatible (Xu et al., 2007).
Figure 4. X-Ray diffraction patterns of HAGS, AEEPU and HAGS-AEEPU films. HAGS= high
amylose glycerol plasticized starch film; HS= short form of HAGS; AEEPU= Anionic
polyurethane; 20, 30, 40 and 50 indicate the ratio of AEEPU in the HAGS film.
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3.6 Optical transparency of HAGS-AEEPU films
Light transmission property of packaging materials is important for their application. We
had reported earlier that the crosslinking of starch and PU in starch-PU films resulted into
less transparent films (Tai et al., 2017a). Figures 5 shows the transparency of HAGS-AEEPU
films determined using equation (2) based on absorption band at 600 nm (visible region).
The transparency of HAGS-AEEPU film is then benchmarked against commonly used
synthetic film. LDPE film showed the highest and the HAGS film showed the lowest
transparency. The film transparency of HAGS-AEEPU films increased with the increase of
AEEPU content. This may be due to the fact that the interaction between the HAGS and
AEEPU could alter the refractive index of the HAGS-AEEPU film, thereby increasing the
transparency (Rao et al., 2010). The increased transparency of the HAGS-AEEPU films also
indicates that these two polymers have good miscibility (Monjazeb Marvdashti et al., 2017;
Xu et al., 2007). Results presented in Figure 5 showed that the transparency of
HAGS40AEEPU and HAGS50AEEPU films were comparable to that of the LDPE film, which
means that the HAGS-AEEPU film could be used in transparent packaging with confidence.
Figure 5. The effect of the concentration of AEEPU on the transparency of HAGS-AEEPU films
and LDPE film. HAGS= high amylose glycerol plasticized starch film; HS= short form of HAGS;
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AEEPU= Anionic polyurethane. The numbers (20, 30, 40 and 50) indicate the ratio of AEEPU
in the HAGS film.
3.7 Compatibility and miscibility between starch and AEEPU
The compatibility between two polymers determines their miscibility in their blend. The
compatibility greatly affects the mechanical properties of the resulting hybrid materials
(Zeng, Zhang, & Kennedy, 2005). Dynamic mechanical analysis was carried out to assess the
miscibility of the starch-AEEPU hybrid materials in molecular level. The temperature at
which tan δ attained its peak value was taken as the Tg (Averous et al., 2000; Menard, 2002).
Figure 7 presents the tan δ versus temperature plots of HAGS and HAGS-AEEPU hybrids
films. HAGS showed a distinct two tan δ transitions peaks at -64.8 ˚C and -8.4˚C which are
associated with - and α-relaxations of the HAGS, respectively (Averous et al., 2000; Lourdin,
Bizot, & Colonna, 1997; Mikus et al., 2014). The - and α-relaxations correspond to
glycerol-rich and starch-rich phases, respectively, and indicate to a partial miscibility of
these two components and resulting in heterogeneity in the HAGS system.
The Tg of AEEPU was observed at -31.7˚C. The incorporation of AEEPU led to gradual merge
of two Tg peaks into a single Tg peak, as observed in the case of HS50AEEPU. The starch rich
phase Tg at -8˚C, shifted to -15.7oC, -14.8 ˚C, -24.3˚C, -25.1˚C with the increase of AEEPU
content (HAGS-to-AEEPU ratios of 80/20, 70/30, 40/60 and 50/50 respectively). This
observation further indicates that better miscibility had occurred between the HAGS and
AEEPU. If HAGS and AEEPU had distinct phase separation, then, it was expected for these
two components to show two distinct Tg values (Zeng et al., 2005; K. Zhang, Nagarajan,
Misra, & Mohanty, 2014). The HS10AEEPU sample displayed three distinct Tg values
of -63.8˚C, -17.9˚C, and 3.0˚C respectively, which are associated to glycerol-rich, starch-rich
and AEEPU-rich phase regions. Furthermore, the peak height and width of peak associated
with Tg is a useful feature to analyse the molecular motion of polymers. It is observed that,
the - relaxations peak of HAGS appearing at -65˚C showed a decrease in its intensity and
increase in its width when the AEEPU content increased. This is especially prominent in the
case of HAGS-to-AEEPU ratios of 90/10, 80/20 and 70/30. This depression in peak height can
be attributed to the decrease in the mobility of the polymer chains in the vicinity of glass
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transition region and increased adhesion among the polymer chains (Maji, Guchhait, &
Bhowmick, 2009; Zhang et al., 2014). It is important to note here that only a single
endothermic peak (due to glass transition) was observed at higher AEEPU concentration
(HS40AEEPU and HS50AEEPU); thus, only a single Tg peak was visible. This observation
indicated that the increase of AEEPU urethane groups in HAGS-AEEPU formulations resulted
into increased physical anchoring between starch chains and restricted the segmental
movement of the polymer chains. This increased interaction between starch and AEEPU had
pronounced effect on the mechanical properties of starch-AEEPU hybrid films as discussed
in the ensuing section.
Figure 6. The variation of loss tangent (tan δ) of HAGS and HAGS-AEEPU films as a function
of temperature. HAGS= high amylose glycerol plasticized starch film; HS= short form of
HAGS; AEEPU= Anionic polyurethane; 20, 30, 40 and 50 indicate the ratio of AEEPU in the
HAGS-AEEPU films.
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3.8 Mechanical Properties of HAGS and HAGS-AEEPU films
The stress-strain curves of HAGS and HAGS-AEEPU films are presented in Figure 7. All the
films showed a distinct linear elastic region and the nature of deformation in these films
depended on the concentration of AEEPU. HAGS film showed brittle fracture with very little
ductility and very short necking due to their less densely packed intra-granular structure
(Figure 3 (A)). The ultimate fracture of HAGS films occurred immediately after the yield
point.
The stress-strain curves of HAGS-AEEPU show that certain amount of AEEPU is required to
improve the mechanical properties. Films with HAGS/AEEPU ratio at 90/10 and 80/20
showed no significant difference in mechanical properties when compared with HAGS films.
However, when the AEEPU content was increased to achieve HAGS-to-AEEPU ratio of 70/30
and or higher, the tensile strength and Young’s modulus decreased half of its value as
compared to neat HAGS, and the flexibility of the HAGS-AEEPU films increased significantly
(P<0.05). The stress-strain curves of HAGS-AEEPU films showed that the tensile strength
started to trend downward immediately after the ultimate tensile strength was reached.
This can be attributed to the necking or localized reduction of the local diameter of the
specimen (Budynas & Nisbett, 2010). The specimen begins to “neck” at location of weakness
where the area reduces dramatically. These observations indicate that the HAGS-AEEPU
films are not as tough as LDPE, yet they have good flexibility.
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Figure 7. Stress-strain curves for HAGS, LDPE and HAGS-AEEPU films. HAGS= high amylose
glycerol plasticized starch film; HS= short form of HAGS; AEEPU= Anionic polyurethane; 20,
30, 40 and 50 indicate the ratio of AEEPU in the HAGS film.
As can be seen from Figure 7, the HAGS film showed brittle characteristics with high tensile
strength (19.7 MPa), Young’s modulus (825 MPa) and lowest elongation at break (19.5 %). In
comparison, the LDPE plastic bag showed the highest tensile strength (27.5 MPa), lowest
Young’s modulus (262.5 MPa) and highest elongation at break at 149 %. One of the most
important parameters required for food packaging is the mechanical resilience of films
during handling, storage and shipping. The fragile nature of HAGS makes it less desirable
compared to the polyolefin based flexible packaging as can be seen from LDPE data.
Data presented in Figure 7 shows that the incorporation of AEEPU in HAGS improved the
ductility of the HAGS films. At lower AEEPU content (starch-to-AEEPU ratio: 90/10 and 80/20)
the HAGS-AEEPU films showed similar mechanical properties compared to those of HAGS.
This is because at these concentrations AEEPU contains fewer urethane groups to form
hydrogen bonds with available OH groups of HAGS. This results into poorer interaction
between HAGS and AEEPU molecules. On the contrary, film with higher AEEPU ratio
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(HS40AEEPU and HS50AEEPU) showed good elongation at break (113 % and 187 %) and
good Young’s modulus (439 MPa and 384 MPa), both of which are comparable to that of
LDPE. Compared to HAGS film, HS50AEEPU film showed 10-fold improvement in terms of
elongation at break without compromising its modulus. These improvements in film
properties were possible because the higher AEEPU content enabled better interaction
between AEEPU and HAGS molecules. This increased interaction increased the number of
intermolecular hydrogen bonds and decreased the intermolecular distance of starch
molecules and PU. Thus, an increase of AEEPU content in HAGS-AEEPU formulation leads to
high intermolecular cohesive force and improves the stress transferring or distributing
efficiency.
These results corroborated well with the information obtained through SEM micrographs,
FTIR data, DMA and suggested that the good physical entanglements and mixing had
occurred between HAGS and AEEPU and they had good compatibility and miscibility.
3.9 Hydrophobicity of HAGS-AEEPU films
The surface hydrophobicity of HAGS-AEEPU films was determined in terms of contact angle
(CA) (Table 2). The surface is considered hydrophobic when its contact angle, measured
using pure water, is higher than 90° (ASTM, 2013b). The CA of LDPE film at 10 s (θ10s) was
102˚, showing a hydrophobic nature. HAGS film showed the lowest CA value (θ10s= 45˚) and
absorbed water rapidly due to the hydrophilic nature of starch and the presence of glycerol.
The hydroxyl groups of starch and glycerol in HAGS film preferably formed hydrogen bonds
with water (Tan, Su, Zhang, & Huang, 2015; Wei et al., 2016). The HAGS-AEEPU film showed
a significant increase (p<0.05) of CA to 115˚ (θ10s) in the case of HAGS/AEEPU at 50:50 ratio.
Even at higher HAGS/AEEPU ratio of 90/10, the CA value increased to 85˚ (θ10s). As shown in
SEM micrographs (Figure 3 (D) & (E)), the good miscibility between AEEPU in HAGS helped
create a dense film matrix by filling up the gap in HAGS’s porous structure. As explained
earlier, the strong electrostatic interaction of the carboxylate groups in AEEPU and its small
particle size allowed it to intercalate into the HAGS structure. Furthermore, the combined
effect of hydrogen bonding between urethane linkage (of AEEPU) and hydroxyl group of
HAGS led to a strong intra-chain interaction among starch molecules. This more compact
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structure of HAGS-AEEPU films provided greater resistance to the penetration of water into
the film (Tai et al., 2017a).
The high sensitivity of HAGS films to water was reflected in the change of CA in a 300 s time
frame (θ300s) which was of the order of 32% decrease (45˚ to 30˚) which again indicates to its
porous structure and hydrophilic nature. The change in CA in the case of HS10AEEPU and
HS20AEEPU was 23% and 16%, respectively. This extent of change of CA within 300 s time
frame is also significant and it can be attributed to the limited availability of urethane
groups in AEEPU to form hydrogen bonding with OH group of HAGS. In these formulations,
there are high number of uncovered or exposed OH groups and together with porous
structure of starch matrix, these film structures allow easier percolation of water. Change of
CA of HS40AEEPU and HS50AEEPU films was remarkably low, 5% and 2.4%, respectively. As
can be seen from SEM micrographs (Figure 3 (D) & (E)), the AEEPU was uniformly dispersed
with starch and anionic PU rich layer was preferentially formed on the film surface. This
polyurethane-rich layer was able to cover most of the OH groups of starch. As articulated
earlier, higher numbers of hydrogen bonds were formed between urethane groups of
AEEPU and OH groups of HAGS. These two factors were responsible for making the
HAGS-AEEPU film surface more resistance to water (stable CA), at the same time reducing
the moisture content of the film (Table 2). CA of these HAGS-AEEPU films was comparable
to that of LDPE film which showed 7.6 % decrease over 300 s.
The work of adhesion (WSL) is a fundamental parameter that depends on surface
characteristics and provides estimation of affinity of water to a given surface. WSL is
calculated using CA and surface tension of water using certain realistic assumptions
(Chaudhury, 1996). As can be seen from Table 2, HAGS and HS10AEEPU had rougher surface
and thus had higher value of WSL. This meant that these two films are expected to have
higher wettability. When the AEEPU content in HAGS-AEEPU films increased, the WSL was
decreased (Table 2) which indicated to increased hydrophobicity of these film surfaces.
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Table 2. Contact angle data of HAG and HAG-AEEPU films. The CA of LDPE film is provided
for comparison.
Sample
Moisture
content of the
films
(%)
Contact angle
CA at 10 s(˚) CA at 300 s(˚)
Change of CA
from 1 to 300 s
(%)
Work of
adhesion
(mN/m)
LDPE - 102.37 ± 2.37a 94.52 ± 1.16a 7.67 72.03 ± 1.58a
HAGS 11.59 ± 0.66a 44.57 ± 6.03b 30.02 ± 5.08b 32.50 145.91 ± 3.24b
HS10AEEPU 10.99 ± 0.27ab 84.88 ± 3.79c 65.02 ± 6.27c 23.40 110.24 ± 7.16c
HS20AEEPU 10.86 ± 0.43ab 104.40 ± 3.94a 87.82 ± 3.39d 15.88 81.16 ± 4.07d
HS30 AEEPU 10.57 ± 0.67ab 106.68 ± 3.48ad 93.66 ± 3.92a 12.20 73.45 ± 4.73a
HS40 AEEPU 10.17 ± 0.89bc 110.28 ± 2.71d 104.96 ± 2.32e 4.83 57.27 ± 2.74e
HS50 AEEPU 9.56 ± 0.5d 115.75 ± 5.54e 112.96 ± 4.63f 2.42 48.60 ± 4.30f
Mean values with different lower-case letters in superscript at the same column are
significantly different (p<0.05). HAGS= high amylose glycerol plasticized starch film; HS=
short form of HAGS; AEEPU= Anionic polyurethane. The numbers (20, 30, 40 and 50)
indicate the ratio of AEEPU in the HAGS film.
4. Conclusions
This study provides insights onto the structural properties of poly (ether-ester) urethane
(AEEPU), the miscibility/compatibility of the AEEPU-HAG blends and the network formation
and physicochemical properties and hydrophobicity of HAGS-AEEPU films. The water
dispersible AEEPU, was synthesised by a two-step method reacting diisocyanate (IPDI) with
polyester (PCL) and polyether (PEG) polyols. The BMPA was used as the source of anions in
the formulation. The prepolymer carboxyl groups were neutralized with TEA, dispersed in
water and the chain was extended using EDA.
FTIR, SEM, DMA and WAXD data of HAGS-AEEPU films showed that the AEEPU and HAGS
were compatible. This compatibility was found to be dependent on physical entanglement
and hydrogen bonding of ionic groups of the AEEPU and starch. Increasing AEEPU content
resulted in increased intermolecular hydrogen bonding between HAGS and AEEPU, which
improved the miscibility between HAGS and AEEPU and produced highly flexible and
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hydrophobic HAGS-AEEPU films. The blending of AEEPU into HAGS increased the
hydrophobicity and elongation in HAGS-AEEPU films without compromising their modulus
as comparing to that of LDPE. However, the mechanical strength of the film still needs
improving. The HAGS-AEEPU films with HAGS/AEEPU ratios of 60:40 and 50:50 produced
films with mechanical properties, water repellency and transparency almost similar to that
of LDPE.
ACKNOWLEDGEMENTS
The first author gratefully acknowledges the scholarship support from RMIT University and
CSIRO Manufacturing, Australia. The authors wish to thank Dr. Aaron Seeber and Dr Mark
Greaves for providing technical support and input during X-ray diffraction and SEM
experiments.
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