Accepted Manuscript

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:

benu.adhikari@rmit.edu.au

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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