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The effect of chemically coated nanofiber reinforcement on biopolymer based nanocomposites

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The effect of chemically coated nanofiber reinforcement on biopolymer based nanocomposites

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The aim of this work was to explore how various surface treatments would change the dispersion component of surface energy and acid-base character of hemp nanofibers, using inverse gas chromatography (IGC), and to investigate the effect of the incorporation of these modified nanofibers into a biopolymer matrix on the properties of their nano-composites. Bio-nanocomposite materials were prepared from poly (lactic acid) (PLA) and polyhydroxybutyrate (PHB) as the matrix, and the cellulose nanofibers extracted from hemp fiber by chemo-mechanical treatments. Cellulose fibrils have a high density of –OH groups on the surface, which have a tendency to form hydrogen bonds with adjacent fibrils, reducing interaction with the surrounding matrix. It is necessary to reduce the entanglement of the fibrils and improve their dispersion in the matrix by surface modification of fibers without deteriorating their reinforcing capability. The IGC results indicated that styrene maleic anhydride coated and ethylene-acrylic acid coated fibers improved their potential to interact with both acidic and basic resins. From transmission electron microscopy (TEM), it was shown that the nanofibers were partially dispersed in the polymer matrix. The mechanical properties of the nanocomposites were lower than those predicted by theoretical calculations for both nanofiber-reinforced biopolymers.
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THE EFFECT OF CHEMICALLY COATED NANOFIBER
REINFORCEMENT ON BIOPOLYMER BASED
NANOCOMPOSITES
Bei Wang, and Mohini Sain*
The aim of this work was to explore how various surface treatments
would change the dispersion component of surface energy and acid-
base character of hemp nanofibers, using inverse gas chromatography
(IGC), and to investigate the effect of the incorporation of these modified
nanofibers into a biopolymer matrix on the properties of their nano-
composites. Bio-nanocomposite materials were prepared from poly
(lactic acid) (PLA) and polyhydroxybutyrate (PHB) as the matrix, and the
cellulose nanofibers extracted from hemp fiber by chemo-mechanical
treatments. Cellulose fibrils have a high density of –OH groups on the
surface, which have a tendency to form hydrogen bonds with adjacent
fibrils, reducing interaction with the surrounding matrix. It is necessary to
reduce the entanglement of the fibrils and improve their dispersion in the
matrix by surface modification of fibers without deteriorating their
reinforcing capability. The IGC results indicated that styrene maleic
anhydride coated and ethylene-acrylic acid coated fibers improved their
potential to interact with both acidic and basic resins. From transmission
electron microscopy (TEM), it was shown that the nanofibers were
partially dispersed in the polymer matrix. The mechanical properties of
the nanocomposites were lower than those predicted by theoretical
calculations for both nanofiber-reinforced biopolymers.
Keywords: Cellulose nanofibers, Nanostructure, Microfibrils, Biopolymers, Inverse gas chromatography
Contact information: Centre for Biocomposite and Biomaterials Processing, Faculty of Forestry, 33
Willcocks Street, University of Toronto, Toronto, Ontario, Canada, M5S 3B3; *Corresponding author:
m.sain@utoronto.ca
INTRODUCTION
Miniturization is a continuing trend in the development of technology. The prefix
“nano” has become applied to new classes of materials intended for manufacturing, e.g.
nano-materials and nanocomposites. Unfortunately, not many of the most recent
developments of this nature are able to satisfy the core concept of sustainability. One way
to address issues related to sustainability is to incorporate renewable materials as
miniaturized elements of construction materials (Sain and Oksman 2006). The backbone
of a plant or tree is a polymeric carbohydrate with an abundance of tiny structural entities
known as “cellulose fibrils”. These fibrils are comprised of different hierarchical
microstructures commonly known as nano-sized microfibrils, having high structural
strength and stiffness (Wang and Sain 2007). Biopolymers from renewable resources
have attracted much attention lately. Renewable sources of polymeric materials offer an
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answer to maintaining sustainable development of economically and ecologically
attractive technology. In recent years, scientists and engineers have been working
together to use the inherent strength and performance of these nano-fibrils, combined
with natural green polymers, to produce a new class nano-materials.
Poly(lactic acid) (PLA) is a class of crystalline polymers with relatively high
melting point (Mohanty et al. 2000). Recently PLA has been highlighted because of its
availability from renewable resources such as corn and sugar beets. PLA is synthesized
by the condensation polymerization of D- or L-lactic acid or ring-opening polymerization
of the lactide (Lunt 1998). Advanced industrial technologies of polymerization have been
developed to obtain high molecular weight pure PLA, which leads to a potential for
structural materials with enough lifetime to maintain mechanical properties without rapid
hydrolysis. Poly(β-hydroxybutyrate) (PHB) is a biotechnologically produced polyester
that constitutes a carbon reserve in a wide variety of bacteria and has attracted much
attention as a biodegradable thermoplastic polyester (Holmes 1988). However, it suffers
from some disadvantages compared with conventional plastics, for example, brittleness
and a narrow processability window.
Many studies have been done on extracting cellulose microfibrils from various
natural sources and on using them as reinforcement in composite manufacturing
(Bhatnagar and Sain 2005; Chakraborty et al. 2006; Nakagaito and Yano 2005; Sain and
Bhatnagar 2003). The use of cellulose nanofibers as nanoreinforcement is a new field in
nanotechnology, and as a result there are still some disadvantages. Firstly, the separation
of nanoreinforcement components from natural materials and the associated processing
techniques have been limited to the laboratory scale (Oksman et al. 2006). Secondly, the
fiber isolation process consumes a large amount of energy, water, and chemicals. The
production is time-consuming and is still associated with low yields. Thirdly, due to their
strong hydrogen bonding between cellulose chains, it is necessary to reduce the
entanglement of the fibrils and improve their dispersion in the solid phase polymer matrix
by surface modification of nanofibers without deteriorating their reinforcing capability. It
has been reported that the surface modifications of cellulose nanofibers to make them
compatible with non-polar solvents or non-polar polymers. Such an approach has been
attempted for polyolefins and other commodity polymers (Goussé et al. 2004). The
treatment of the fibers may be by bleaching, grafting of monomers, acetylation, and so
on. In this way, high performance composite materials can be processed with a good level
of dispersion. Interaction of cellulose with surfactants has been another way to stabilize
cellulose suspensions into non-polar systems (Heux et al. 2000).
Poor interfacial adhesion between nanofibers and the polymer matrix leads to a
decline in mechanical properties of nanocomposites. In recent years a deeper under-
standing has been achieved related to surface phenomena. This has led to an introduction
of more sophisticated approaches, which allow for a study of thermo-dynamic and kinetic
information. One technique, which has been shown to be very valuable, is inverse gas
chromatography (IGC). In IGC, a solid material under investigation is used as the
stationary phase. An empty column is filled with the (porous) material (adsorbent) and
the adsorbate molecules in the mobile phase probe the surface of the adsorbent
(Thielmann 2004). The surface energy of a material can be described by the sum of a
dispersion component and a specific interaction component (Gulati and Sain 2006). The
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dispersion component refers to London dispersion forces, and the specific component
refers to the polar, ionic, electrical, magnetic, metallic, and acid-base interactions.
Fowkes and Mostafa (1978) proposed that dispersion forces and acid-base interactions
are the primary forces operating across the interface. IGC is an alternative method for
measuring the changes in the thermodynamic properties of a nanofiber surface after
treatment and for estimating the London dispersion component of the surface free energy
of nanofibers (before and after treatment). Gulati and Sain (2006) reported that
alkalization and acetylation make the hemp fibers amphoteric, thereby improving their
potential to interact with both acidic and basic resins.
The goal of this work was to explore how various surface treatments would
change the dispersion component of surface energy and the acid-base character of hemp
nanofibers, using IGC. The cellulose nanofibers were extracted from hemp by chemo-
mechanical treatments. PLA- and PHB-based nanocomposites using cellulose nanofibers
were prepared by injection molding and hot compression. The cellulose nanofibers used
in this study were treated by five different chemicals. Uncoated cellulose was used as a
reference. Transmission and scanning electron microscopy were used to investigate the
nano-structure of the nanocomposites and the dispersion of fibers within the matrix. The
potential use of chemically coated nanofibers as reinforcing agents in biocomposites was
also explored. The mechanical properties of the nanocomposites were studied by means
of tensile testing.
EXPERIMENTAL
Materials
Matrix
Poly (lactic acid) (PLA), Nature WorksTM 4031D, was supplied by Cargill Dow
LLC, Minneapolis, USA. The material has a density of 1.25 g/cm3, a glass transition
temperature (Tg) of 58 ºC, and a melting point of 160 ºC. Polyhydroxybutyrate (PHB),
Biomer-P226 biodegradable polymer, was supplied by Biomer, Krailling, Germany. The
material has a density of 1.17 g/cm3 and melting point of 173 ºC.
Reinforcement
The raw material used in this study was hemp fibers (Cannabis sativa L.) from
southwestern Ontario, Canada (Hempline Inc., ON). These fibers have diameters of
approximately 22-25 µm and lengths of 15-25 mm. The cellulose nanofibers were
extracted from hemp fiber by chemo-mechanical treatments. Isolated nanofibers were
shown to have diameters between 50-100 nm and lengths in the micrometer scale, which
results in a very high aspect ratio (87.5).
Chemicals
Reagent grade chemicals were used for fiber isolation and bleaching, namely,
sodium hydroxide, hydrochloric acid, sodium chlorite, chlorine dioxide, peroxide, and
sulfuric acid. Michem® Prime EAA (ethylene acrylic acid) copolymer dispersions-4983R
(Michelman, Inc., Cincinnati, OH) was the dispersant, which exhibits excellent adhesion
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to cellulosic substrates. Styrene Maleic Anhydride resins (SMA®) from Sartomer
Company (Exton, PA) are low molecular weight styrene/maleic anhydride copolymers.
Hydrophobic SMA resins are used as surface sizing compounds for paper and cross-
linking agents for powder coatings. Kelcoloid HVF and LVF are stabilizers used for fiber
coating. Kelcoloids (International Specialty Products, Wayne, NJ) are made of propylene
glycol alginates (PGA), copolymers of mannuronic and guluronic acids. The key function
of PGAs is to help stabilize an emulsion or high-solids suspension. Guanidine
hydrochloride, 50940 BioChemika (Fluka Chemie AG, Buchs, Switzerland) was used for
the fiber coating. It was originally designed for refolding of proteins.
Methods
Nanofiber isolation
The isolation of hemp nanofibers is a multi-step process. Chemical and
mechanical treatments were applied to the fiber to make nanofibers. The chemical
treatment included pre-treatment, acid hydrolysis, and alkaline treatment. The mechanical
treatment was comprised of two parts: cryocrushing and high pressure defibrillation.
Details of the nanofiber isolation process are outlined in the author’s previous publication
(Wang et al. 2007).
Nanofiber chemical coating
Cellulose nanofibers were stored in water suspension after the chemo-mechanical
isolation. Different types of chemicals were added to the suspension containing
nanofibers in the proportion 1:2 (w/w), using an estimated weight of the cellulose
nanofibers. In order to improve the dispersion of the coated nanofibers, the suspensions
were prepared with continuous stirring by magnetic stirrer for 24 h at a room temperature.
The suspensions containing nanofibers were freeze-dried in a Multi-Drier freeze-drying
machine (Frozen in Time, Ltd.).
Processing of nanocomposites
This project was focused on synthesizing nano-biocomposites, using PLA and
PHB in the solid phase, by injection molding or hot compression. A solid-phase
compounding method was used to mix the freeze-dried nanofibers with PHB in a high-
intensity kinetic mixer (Werner and Pfleiderer Gelimat) at 3200 rpm with tip speed of 23
m/s. Product was discharged at a pre-set temperature of 150 °C. Test samples were
compression-molded with a WABASH Hot Press into sheet form. The mold temperature
was 180 °C, and the pressure was 50 MPa. PLA composites containing 5 wt.% SMA-
coated nanofibers were prepared by melt blending the polymer with the fiber, using a
Brabender mixer (C.W. Brabender Instruments Inc., NJ). The compounding temperature
was 170 °C, and the rotating screw speed was 60 rpm for 5 min. Then the compound was
granulated, using a C.W. Brabender Granulator (C.W. Brabender Instruments Inc., NJ).
The granulates were then pre-heated to 100 °C for 1 h and injection molded using an
Engel Injection molder (Model ES-28, ON, Canada) equipped with a standard ASTM
mold for tensile, flexural, and impact test specimens. The typical injection molding
conditions were: injection temperature 180 °C, injection time: 8 s, cooling time 25 s, and
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mold opening time 2 s. All composites contained 5 wt.% loading of nanofibers with
respect to total weight of the composite.
Column preparation and IGC procedure
IGC measurements were done with a Perkin-Elmer Autosystem XL Gas Chro-
matograph (GC) fitted with a flame ionization detector. To ensure flash vaporization, the
injection port was kept at 423 K. All stationary phases, including 2-4g uncoated hemp
nanofibers (HPN) or coated-HPN, were dried in an oven at 70 °C for 24h and packed
under vacuum with a vibrator into a copper column (length 33 cm and internal diameter
of 4 mm) of which the end was plugged with glass wool. The columns were maintained
overnight at 105 °C in a nitrogen stream to remove moisture and other volatiles from the
cellulose fibers before each experiment. The columns were first cleaned with acetone
before use to get rid of greases used in copper processing.
The IGC probes used in the present study were chromatography grade solvents
(Sigma-Aldrich). The probes were used without further treatment. Their physicochemical
properties are listed in Table 1. Helium was used as the carrier gas. The corrected flow
rate of helium was 10 mL/min. Small quantities of probes were injected into the column
using Hamilton syringes. Peaks were found to be symmetrical and the area under each
peak was directly related the amount adsorbed/desorbed. In the present study, the
temperature dependence was determined within the temperature range 40 to 100 °C.
Averages of three measurements were taken to calculate retention volumes, with air as
the marker.
Table 1. Physicochemical Properties of the IGC Probes used In the Present
Study (Schultz et al. 1987; Guttmann 1983)
Probe Area (Aº 2) γld (mJ/m2) DN AN Character
Hexane 51.5 18.4 0 0 Neutral
Heptane 57 20.3 0 0 Neutral
Octane 62.8 21.3 0 0 Neutral
Nonane 68.9 22.7 0 0 Neutral
Chloroform 44 25.9 0 23.1 Acidic
Ethyl Acetate 48 16.5 17.1 9.3 Amphoteric
Ethyl Ether 47 15 19.2 3.9 basic
Tetrahydrofuran (THF) 45 22.5 20.1 8 basic
Acetone 42.5 16.5 17 12.5 Amphoteric
Microscopy characterization
The nanostructure of the composites was examined in a transmission electron
microscope (TEM), Hitachi H-7000 TEM at an acceleration voltage of 100 kV. To
examine the nanocomposites, the samples were cut and polished to rectangular sheets,
embedded in epoxy, and allowed to cure overnight. The final ultra-microtoming was
performed with a diamond knife at room temperature, generating foils approximately 90
nm in thickness. These foils were gathered onto Cu grids.
A scanning electron microscope (JEOL JSM-840, Tokyo, Japan) (SEM) was used
as a routine for microstructural analysis of the nanofibers with and without surface
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coatings. All images were taken at an accelerating voltage of 15 kV. The sample surfaces
were coated with a thin layer of gold on the surface, using an Edwards S150B sputter
coater (BOC Edwards, Wilmington, MA) to provide electrical conductivity.
Tensile testing
The mechanical behavior of nanofiber-blend-PHB film or nanofiber-blend-PLA
nanocomposite was tested by an Instron 5860 (Grove City, PA) in tensile mode with a
load cell of 2 kN or 30 kN in accordance with ASTM D 638. The specimens were cut in a
dumbbell shape with a die ASTM D 638 (type V). Tensile tests were performed at a
crosshead speed of 2.5 mm/min. The values reported in this work result from the average
of at least 5 measurements.
BACKGROUND
Determination of the Acid-Base Characteristics of Lignocellulosic Surfaces
by IGC
The surface energy of a material can be described by the sum of the London
dispersion component and specific interactions. Thus, the work of adhesion can be
written as,
Wa = Wad + WaAB (1)
where Wa, Wad, and WaAB are the total work of adhesion, the work of adhesion due to
dispersion forces and acid-base interactions, respectively. Acid-base interactions are
useful for surface modification (Dwight et al. 1990). Hence, in order to design new
modification methods for improving fiber-matrix adhesion and meaningful interpretation
of the existing methods, quantitative determination of surface acid-base characteristics of
natural fibers is important. Data generated in this study explored surface modification for
lignocellulosic fibers and their compatibilization with biopolymers.
Background of IGC
IGC has become a widely used technique to characterize the surface properties of
organic and inorganic materials. Acid-base probes are used to measure the acid-base
characteristics of the solid surface, and saturated n-alkane probes are used to measure the
dispersion component of the surface energy of interaction. In the present study, retention
times of saturated n-alkane and acid-base probes injected at infinite dilution were used to
calculate the dispersion component (γsd) of the surface energy, the free energy of
adsorption (GAB), and the enthalpy of adsorption (HAB) corresponding to acid-base
surface interactions. Papirer’s approach, as described by Schultz et al. (1987; 1991), was
used to estimate the acceptor (KA) and donor (KD) parameters of the test substrates.
The fundamental parameter in the IGC measurements is the specific retention
volume, Vn, defined as the volume of carrier gas required to elute a probe from a column.
Vn is related to experimental variables by the following equation,
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V
n = F*(Tr-To) (2)
where Tr and To are the retention times of the probe and the air marker, respectively; F* is
the corrected flow rate of the carrier gas, defined as,
F
* = FJ (3)
where F is the corrected gas flow rate in mL/min; J is the correction factor for the gas
compressibility,
J = 1.5 [(Pi/Po)2-1]/[(Pi-Po)3-1] (4)
where Po is the carrier gas pressure at the column outlet, and Pi is the carrier gas pressure
at the column inlet.
The interaction of neutral probes, such as saturated n-alkanes, with the substrate
material is dominated by the van der Waals dispersion forces of interaction. Molar free
energy of adsorption is related to net retention volume by the following relation,
G = RT ln(Vn) + C (5)
where R is the gas constant, T is the column absolute temperature, and the value of C
depends on the reference state. The free energy of adsorption is related to work of
adhesion by the following relation (Mukhopadhyay and Schreiber 1995),
G = NaWa = 2Na(γsd)1/2(γld)1/2 + C (6)
where N is Avogadro’s number; a is the surface area of a single probe; Wa is the work of
adhesion; γld is the dispersion component of the surface energy of the probe; and γsd is the
dispersion component of the total surface energy of the interacting solid. Combining
equation (5) and (6), we get:
RT ln(Vn) = 2Na(γsd)1/2(γld)1/2 (7)
A plot of RT ln(Vn) versus 2Na(γld)1/2 should give a straight line with slope (γsd)1/2
in the case of probes interacting only due to dispersion component of surface energy.
From the slope of the straight line γsd can be calculated.
The free energy of adsorption (GAB) corresponding to the specific acid-base
interactions is related to the enthalpy of adsorption (HAB) by,
GAB = HAB - TSAB (8)
where SAB is the entropy of adsorption corresponding to the specific acid-base
interactions. A plot of GAB versus T (temperature) should yield a straight line with
intercept equal to HAB. The enthalpy of adsorption corresponding to the specific acid-
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base interaction is related to the acceptor and donor parameters, KA and KD of the fibers.
According to Saint-Flour and Papirer (1982),
HAB = KADN + KDAN (9)
where DN and AN are the donor and acceptor numbers, respectively, of the acid-base
probe as defined by Guttmann (1983). A plot of HAB/AN versus DN/AN should yield a
straight line with slope KA and intercept KD. According to Schultz (1987) the specific
interaction parameter, I, for acid-base interactions can be defined as,
I = KAf KDm + KAm KDf (10)
where the superscripts f and m refer to fiber and matrix, respectively.
RESULTS AND DISCUSSION
Dispersion Component of The Surface Energy
Preliminary experiments were performed on the coated and uncoated cellulose
powders to determine the optimum chromatographic conditions for reproducible
measurements of the retention times of the probes. The chromatographic peak shape of
each probe had to be as symmetrical as possible. The dispersion component of uncoated
and chemically coated hemp fibers was calculated from a plot of RT ln(Vn) versus
2Na(γld)1/2. The values for the dispersion component, γsd, of the surface energy at different
temperatures are summarized in Table 2.
Table 2. Dispersion Component, γsd, of the Surface Energy of Lignocellulosic
Particles at Different Temperatures
γs
d (mJ/m2)
Material 313K 333K 353K 373K
Cellulose (Dorris and Gray 1979) 48 44 40 36
Uncoated HPN 42 40 34 28
SMA-Coated HPN 44 41 39 36
HVF-Coated HPN 46 44 43 38
LVF-Coated HPN 44 42 40 33
EAA-Coated HPN 46 43 41 37
Guanidium Hydrochloride Coated HPN 50 47 44 41
PLA 32 29 28 27
PHB 51 47 43 40
HPN: hemp nanofibers
The dispersion component of resin was also calculated similarly. The linear
relationship vs. n-alkane chain length illustrates that this technique works well in case of
natural fibers. Chemical treatments had the effect of increasing their respective γsd values
toward that of the cellulose powder. This is likely due to the dissolution of low energy
surface impurities and surface exposure of relatively higher energy cellulose. In the
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present study, the temperature dependence was determined in the temperature range 40 to
100 °C. Chemically coated fibers showed a negative temperature coefficient over this
entire range due to chemical rearrangements. The London dispersion component was
affected by the type of polymers and the treatment of fibers.
Acid-base Interactions of the Surface Energy
Values of the free energy of adsorption, GAB, corresponding to surface acid-base
interactions are summarized in Table 3. The corresponding values of the enthalpy of
adsorption, HAB, determined from the plots of GAB as a function of temperature (T) for
all the probes in all cases, are given in Table 4. Some probes showed a negative acid-base
free energy and enthalpy of adsorption on the cellulose. For example, the acid-base
interaction between HVF-coated HPN (hemp nanofibers) and the ethyl ether probe was
not favorable for adsorption. Considering that ethyl ether is basic (DN = 19.2), and the
HVF-coated HPN used in this study was found to have a basic characteristic (KD = 0.22),
this result is not surprising. A comparison of the enthalpy of adsorption between uncoated
and coated fibers indicated that the interactions between the probes and SMA- and EAA-
coated HPN were greater than those observed between the probes and uncoated HPN.
The uncoated HPN had relatively low donor (KD = 0.31) and acceptor (KA = 0.19)
parameters compared to the donor (KD = 0.77) and acceptor (KA = 0.34) parameters of
SMA-coated HPN. The significant increase in the acceptor parameter KA suggests that
coated fiber may interact more strongly with a matrix (Marcovich et al. 1996).
The KA and KD values for the respective fibers were estimated from the slope and
intercept of the respective linear regression lines of HAB/AN as a function of DN/AN.
These values are summarized in Table 5. Qualitatively, SMA-coated HPN showed
relatively higher acid-base characteristics than uncoated HPN. A similar trend was
observed in the case of EAA-coated HPN compared with uncoated HPN. The uncoated,
SMA-coated, HVF-coated, and LVF-coated HPN showed a basic surface characteristic.
KA and KD values appear to be consistent with the molecular structure of cellulose, where
the hydrogen atoms in the hydroxyl groups act as electron acceptors and the oxygen
atoms in the glycosidic linkages and hydroxyl groups act as electron donors. The EAA-
coated HPN showed an amphoteric surface characteristic, and guanidium hydrochloride
coated HPN showed a predominantly acidic characteristic. The relatively high KA value
indicates a surface that is rich in hydroxyl groups.
The surfaces of uncoated and chemically coated fibers were enriched by different
classes of chemicals and extractives. Hemp fibers were found to be basic, which is
probably due to presence of triglycerides, which exhibit a pronounced basic character
(Tshabalala 1997). The removal of extractives and hemicellulose by chemical treatments
had the effect of increasing the dispersion component of the surface energy of the HPN.
The polymer matrix PLA used in this study was found to have an acidic character. By
contrast, PHB showed a predominantly basic character, according to the KA and KD
values.
Values of the specific interaction parameter, as defined by Schultz et al. (1987),
were calculated for each type of fiber and resin combination. These values are shown in
Table 6. Acid-base interactions with PLA increased by SMA- and EAA-coated HPN, and
a very similar trend was observed for PHB matrix. EAA copolymers were used as a
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dispersant in this study, bringing together in one product the benefits of both ethylene and
acrylic acid. The crystalline structure of ethylene provides the barrier properties,
flexibility, and resistance to water and chemicals. The acrylic acid comonomer imparts
improved adhesion, hot-tack strength, and optical clarity. The nanofiber suspension
containing EAA dispersant remains homogeneous indefinitely. It has excellent adhesion
to cellulose and other polar substrates due to the high content of acrylic acid in the base
copolymer. The EAA dispersant also exhibits outstanding adhesion to polyethylene and
other plastics. Styrene maleic anhydride (SMA) resins are low molecular weight
styrene/maleic anhydride copolymers. Altering the styrene to maleic anhydride ratio
changes the hydrophilic/hydrophobic balance of the polymer. At their most hydrophilic,
SMA resins form high solids solutions and can be used to produce fiber dispersions.
These results are of special practical importance because surface acid-base interactions
may be implicated in the adhesion of coatings and finishes to polymer and other
lignocellulosic fibers. Adsorption occurred only when there was an exothermic interracial
acid-base interaction. The present paper is only focused on the material structure and
mechanical properties of SMA-coated HPN nanocomposites. The properties of EAA-
coated HPN nanocomposites were discussed in the author’s previous publication (Wang
and Sain 2007).
Material Structure
Figure 1 presents typical pictures of freeze-dried HPN. Figure 1(a) is a SEM
image of uncoated HPN. Each particle of HPN is an aggregation of cellulose fibers due to
the strong hydrogen bonds of adjacent molecules. The size of the fiber bundle is at the
µm level. Figure 1(b) shows a picture of SMA-coated HPN with a well-organized web-
like structure. The morphology of coated HPN appears distinguishable compared to
uncoated HPN. The SMA-coated fibers formed loose networks during freeze drying. It is
proved that SMA could reduce the entanglement of the nanofibers.
Fig. 1. Scanning electron micrographs of freeze-dried HPN samples: (a) uncoated and (b) SMA
coated.
20µm
20µm
a b
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Table 3. Free Energy of Adsorption, GAB, of the Acid-Base Probes at Different
Temperatures
GAB (KJ.mol-1)
Substrate/Probe 313K 333K 353K 373K
Uncoated HPN
Chloroform 3.76 1.50 -0.86 -2.37
Ethyl Acetate 5.36 4.61 3.25 1.66
Ethyl Ether 3.15 -0.20 -0.52 -3.06
Tetrahydrofuran (THF) 4.40 2.88 1.52 -1.32
Acetone 4.61 2.23 0.07 -2.37
SMA-Coated HPN
Chloroform 12.86 12.68 8.89 5.55
Ethyl Acetate 12.10 10.79 8.76 6.58
Ethyl Ether 8.20 7.45 6.39 5.42
Tetrahydrofuran (THF) 13.07 12.55 12.21 11.43
Acetone 12.86 11.91 11.28 9.21
HVF-Coated HPN
Chloroform 2.30 1.10 -1.63 -4.38
Ethyl Acetate 2.07 0.90 -0.80 -1.64
Ethyl Ether -1.05 -3.43 -7.34 -9.93
Tetrahydrofuran (THF) 2.31 0.41 -0.05 -0.97
Acetone 1.64 -0.14 -1.63 -4.63
LVF-Coated HPN
Chloroform 3.59 2.64 -0.17 -1.49
Ethyl Acetate 1.79 0.86 0.36 -0.73
Ethyl Ether -0.07 -1.20 -3.40 -4.53
Tetrahydrofuran (THF) 1.86 0.95 0.33 -0.26
Acetone 2.73 1.76 1.35 0.27
EAA-Coated HPN
Chloroform 12.70 11.33 10.66 9.83
Ethyl Acetate 11.71 10.99 9.99 9.08
Ethyl Ether 10.29 8.13 7.00 6.37
Tetrahydrofuran (THF) 13.56 12.47 11.54 10.78
Acetone 9.96 9.35 8.79 8.71
Guanidium Hydrochloride Coated HPN
Chloroform -1.87 -3.41 -4.81 -5.09
Ethyl Acetate -3.05 -3.57 -3.89 -4.04
Ethyl Ether -4.26 -4.54 -4.81 -5.09
Tetrahydrofuran (THF) -3.47 -3.48 -3.49 -3.50
Acetone 2.96 2.87 2.71 2.62
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Table 4. Enthalpy of Adsorption, HAB
HAB (KJ.mol-1)
Probe Uncoated
HPN
SMA-
Coated
HPN
HVF-
Coated
HPN
LVF-
Coated
HPN
EAA-
Coated
HPN
Guanidium
Hydrochloride
Coated HPN
Chloroform 5.70 16.43 5.04 5.65 13.45 1.04
Ethyl Acetate 6.83 14.20 3.34 2.58 12.67 2.81
Ethyl Ether 4.58 9.21 2.20 1.59 11.17 3.98
Tetrahydrofuran
(THF) 6.50 13.63 3.00 2.47 14.41 3.46
Acetone 6.91 14.21 3.88 3.47 10.28 3.09
Table 5. Surface Acid-Base Characteristics, KA and KD
Material KA (a.u.) KD(a.u.)
Uncoated HPN 0.19 0.31
SMA-Coated HPN 0.34 0.77
HVF-Coated HPN 0.07 0.22
LVF-Coated HPN 0.04 0.23
EAA-Coated HPN 0.49 0.45
Guanidium Hydrochloride Coated HPN 0.20 0.02
PLA 0.18 0.12
PHB 0.22 0.69
Table 6. Values of Specific Interaction Parameter
I = KAfKDm + KAmKDf PLA PHB
Uncoated HPN 0.08 0.20
SMA-Coated HPN 0.18 0.41
HVF-Coated HPN 0.05 0.10
LVF-Coated HPN 0.05 0.08
EAA-Coated HPN 0.14 0.44
Guanidium Hydrochloride Coated HPN 0.03 0.14
The processing of cellulose nanocomposites renders several challenges. The
major difficulty is to achieve uniformly dispersed nanofibers in the polymer matrix. The
nanofibers have a very large surface-to-volume ratio and have a tendency to aggregate
when dried. The injected composites were examined using a transmission electron
microscope (TEM) to study the composite morphology at nanoscale. Figure 2(a) shows
an overview picture of the PLA/SMA-coated HPN composite. It was difficult to see any
cellulose nanofibers in this sample. There are some dark spots, indicating that the
nanofibers were not uniformly dispersed in the PLA matrix, and it is possible that the
cellulose was degraded during processing. In Fig. 2(b) a more detailed view of the
composite with PLA is shown. It can be seen that the nanofibers were partly dispersed in
PLA. Agglomerates were present in the PLA/SMA-coated HPN nanocomposite. The
structure can therefore not be described as fully networked. The dispersion and
distribution of nanofibers can be affected and improved by optimizing the chemical
surface treatments and the compounding process. Figure 2(b) shows the presence of a
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non-homogeneous structure of nanofibers in the PLA based nanocomposites. This fact
will be reflected in the mechanical properties, since there is a strong link between the
morphology of nanocomposites and the improvement in properties of the polymer matrix.
Fig. 2. Transmission electron micrograph of the PLA/SMA-coated HPN composites: (a) an
overview and (b) detailed view.
Mechanical Properties of Nanocomposites
The chemical surface modifications of cellulose fibers were studied with the aim
of improving their interfacial compatibility with PLA and PHB, that is, to enhance the
mechanical properties of the ensuing composite. The mechanical properties of the
prepared nanocomposites are presented in Table 7. There were some improvements in the
properties of the nanocomposite materials, compared to pure PLA and PHB. Table 7 also
shows that the improvements were similar for both nanoreinforcements. The PHB/SMA-
coated HPN nanocomposite showed a 17% increase in the yield strength and a 24.5%
increase in modulus in comparison to PHB/uncoated HPN nanocomposite. There was a
35% increase in the yield strength and a 37% increase in modulus relative to pure PHB.
The PLA/SMA-coated HPN nanocomposite showed only a 3% increase in tensile
strength and a 7% increase in modulus compared to PLA/uncoated HPN. There was a
8.6% increase in tensile strength and a 10% increase in modulus, compared to pure PLA.
These results were lower than expected. Theoretical calculations were therefore
performed in order to better understand the results and to see the potential effect of both
nanoreinforcements.
1 µm 200 nm
b
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Table 7. Tensile Properties of the Nanocomposites.
Materials Max. Stress
(MPa) S.D. E-Modulus
(GPa) S.D.
PHB 15.32 1.00 1.41 0.16
PHB/Uncoated
HPN 17.68 1.68 1.55 0.11
PHB/SMA-coated
HPN 20.68 6.66 1.93 1.25
PLA 65.49 0.21 2.72 0.09
PLA/Uncoated
HPN 68.97 0.40 2.80 0.06
PLA/SMA-coated
HPN 71.14 0.64 2.99 0.01
The Halpin-Tsai equation was used to calculate the theoretical tensile modulus for
the two nanocomposite materials, see Eq. (11) – Eq. (14) (Agarwal and Broutman 1990),
E = Em (1 + ζηΦ)/(1-ηΦ) (11)
where Em is the Young’s modulus of the matrix, Ef represents Young’s modulus of the
filler, ζ is a shape parameter dependent upon filler geometry, orientation, and loading
direction, and η is given by,
η = (Ef/Em-1) / (Ef/Em+1) (12)
ζ = 2 × Length/Diameter (13)
Φ = volume fraction (14)
The Halpin-Tsai equation is normally used to predict the modulus for aligned
fiber composites, but it has been used before to predict the modulus of nanocomposites
(Wu et al. 2004; Fornes and Paul 2003). It was chosen because it demanded the least
amount of assumptions to be made about the materials. The Halpin-Tsai equation can
only be applied to predict the modulus of fiber/matrix nanocomposites in the range of low
fiber volume fractions. At high filler concentration, the predicted value is lower than the
experimental data. It is assumed that the filler apparent volume is related to the dispersion
of filler, and that the larger apparent volume may originate in better dispersion, which
results in a higher modulus of the composite. When the predicted values at filler volume
concentrations of less than 6%, it is well fitted to the experimental data (Wu et al. 2004).
By comparing model predictions with the two-dimensional finite element calculations for
discontinuous oriented square fiber-reinforced composites, Ashton et al. (1969) deter-
mined that ζ = 2 (length/diameter of the fiber) = 2 × aspect ratio provided good
agreement for longitudinal modulus.
The volume fraction of each nanoreinforcement was calculated using Eq. (15)
(Luo and Daniel 2003),
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Φf = (wf/ρf)/( (wf/ρf) + (1 – wf)/ρm) (15)
where, wf = 5%, ρcellulose = 1.58 g/cm3 (Ganster et al. 1999), ρSMA-coated HPN =1.70 g/cm3,
ρPLA = 1.25 g/cm3, and ρPHB = 1.17 g/cm3. The volume fractions for the PLA/uncoated
HPN and the PLA/SMA-coated HPN were determined to be 4% and 3.7%, respectively.
The volume fraction for the PHB/uncoated HPN and the PHB/SMA-coated HPN were
3.75% and 3.5%, respectively. The following data were used in the calculation: EPLA =
1.7 GPa, EPHB = 1.0 GPa, Ecellulose = 167.5 GPa (Petersson and Oksman 2006), aspect
ratio of uncoated HPN is 88, and aspect ratio of SMA-coated HPN is 82 (Wang et al.
2007). A comparison between the theoretical and experimental results can be seen in Fig.
3. When comparing the results, one has to keep in mind that theoretical calculations are
based on PLA/cellulose and PHB/cellulose systems, where the nanoreinforcement is
aligned in the longitudinal direction and has perfect interfacial adhesion to the matrix.
From Fig. 3, we can draw the conclusion that both systems have large potentials for
strength development, which this experiment was unable to reach. One can also see that
the PLA system has the largest potential and that the PLA/nanofiber system, due to its
agglomerated structure, was farthest away from its theoretical value. When it comes to
the PLA/uncoated HPN and the PLA/SMA-coated HPN system, the uncoated nanofiber
PLA system should have higher theoretical tensile strength value, compared to the SMA
coated nanofiber PLA system, due to its lower volume fraction. In contrast, the chemical
treatments on the fiber surface increased the interfacial adhesion between fiber and
matrix; the experimental results showed the PLA/SMA-coated HPN system as having
higher tensile properties.
0.00
2.00
4.00
6.00
8.00
0 0.01 0.02 0.03 0.04 0.05
Volume Fraction
E [GPa]
Halpin-Tsai-PLA/Cellulose Halpin-Tsai-PHB/Cellulose PLA/Uncoated-HPN
PLA/Coated-HPN PHB/Uncoated-HPN PHB/Coated-HPN
Fig. 3. Experimentally measured tensile modulus data compared to theoretical predictions by
Halpin-Tsai.
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Wang and Sain (2007). “Coated nanofibers in composites,” BioResources 2(3), 371-388. 386
CONCLUSIONS
1. Inverse gas chromatography (IGC) at infinite dilution has proven to be a convenient
tool for measurement of surface energy and acid-base characteristics of natural fibers
and polymer matrix. Changes in final properties of the composites due to the effect of
various chemical treatments on the fiber surface can also be explained using this
technique. Acid-base interactions with PLA were increased by SMA- and EAA-
coated HPN, and the same trend was observed for the PHB matrix.
2. SEM pictures showed SMA-coated HPN having a well-organized web-like structure
and proved that the size of nanofibers is indeed in the nano-level. Current TEM
pictures showed the presence of a non-homogeneous structure of nanofibers in the
PLA based nanocomposites. The properties shown here will most probably be
improved if it is possible to disperse the nanofibers more evenly within the polymer
matrix. A uniform nanofiber dispersion in a matrix, coupled with a high aspect ratio
of the nanofibers will indicate a strong potential for the use of these biocom-posite
films.
3. In both PLA and PHB systems, the SMA-coated HPN as the reinforcement enhanced
the mechanical properties over the systems containing uncoated HPN or pure polymer.
The theoretical calculations made in this article showed that the PLA has the largest
potential to improve the mechanical properties, compared to the PHB system. This
experiment was a step in the direction of creating fully renewable biopolymer based
nanocomposites.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support of this study given by NSERC
(Natural Sciences and Engineering Research Council of Canada).
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Revised version received: June 19, 2007; Article accepted June 19, 2007; Published June
21, 2007
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Adsorption isotherms of some hydrocarbons on cotton cellulose paper and on thermomechanical pulp fibers are determined by a gas chromatographic technique. The isotherms are well described by the BET equation, and monolayer coverages are readily determined despite the absence of sharp knees in the isotherms. The molecular areas for the adsorbates on the two adsorbents are estimated by comparison with nitrogen adsorption data. The surface pressures, calculated from the isotherms, are analyzed in terms of Fowkes' theory. The London dispersion force contributions to the surface free energies of crystalline cellulose and of lignified wood fiber are believed to be 48 and 37 mN m−1, respectively.
Article
The role of Lewis acid-base interactions at the fiber-matrix interface in composites is studied with both glass and Teflon fibers. In the glass fiber case, surface chemistry is modified with amino-, methacryloxy- and glycidoxy-silane coupling agents (A-1100, A-174 and A-187, respectively). Silane adsorption mechanisms as well as the properties of filament-wound, unidirectional epoxy and polyester composites are explained by a combination of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and flow microcalorimetry. The heats of adsorption of pyridine and phenol prove that the coupling agents add acidic sites to the glass fiber surface as well as stronger basic sites. The subsequent adhesion of the matrix polymers and the short beam shear strengths of composites are explained on this basis. The Teflon fibers are first etched with sodium naphthalene solutions, and then sequentially hydroborated and acetylated, producing approximately monofunctional hydroxyl (acidic) and ester (basic) groups on the surfaces, as determined by XPS, FTIR, and electrophoretic mobility analyses. Composites prepared with the acetylated fibers and a chlorinated polyvinyl chloride (acidic) matrix are superior in tensile properties, and SEM fractography shows PTFE fibrillation, indicative of good fiber-matrix adhesion and stress transfer, in this case only.
Article
The final performance of a composite material depends strongly on the quality of the fibre-matrix interface. The interactions developed at the interface were studied using the acid-base or acceptor-donor concept.The surface characteristics of the carbon fibres and the epoxy matrix were studied using a tensiometric method and the inverse gas chromatography technique. Acid-base surface characters could be determined allowing the interactions at the interface to be described by a specific interaction parameter.It was shown that the shear strength of the interface, as measured by a fragmentation test, is strongly correlated to this specific interaction parameter, demonstrating the importance of acid-base interactions in the fibre-matrix adhesion.