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Biodegradable polymer/graphene oxide
composite films
Ashutosh Kumar
Department of Mechanical Engineering
Indian Institute of Technology, PatnaIndia
Supervised by
Prof. Debes Bhattacharyya
Dr.Dongyan Liu
Centre of Advanced Composite Materials(CACM)
University of Auckland, New Zealand
2012
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Abstract
In the current age of growing environmental awareness and energy crisis
situation, biodegradable composites have gained wide acceptance in various
facets of engineering. Poly(lactic acid) (PLA) has several applications in various
areas such as in woven and non-woven fabrics, paper coatings, food and
medicine packaging, and biomedicine (sutures, scaffolds and implants).
This aliphatic polyester is prepared from lactic acid (therefore derived from
100% renewable sources, e.g. corn or sugarcane), and is biodegradable and
biocompatible.In order to make this material more attractive for someapplications, as a strong alternative to petrochemical plastics, some properties
should be improved, namely mechanical properties and gas barrier properties.
Graphene due its remarkable properties is centre of attraction for most of the
researchers these days. Our objective is to use graphene in composite as
reinforcement,a single atomic layer of carbon whose existence had beenknown for a long time but which was produced and identified only as recently
as 2004.Andre K. Geim and Konstantin S. Novoselov of the University ofManchester, UK, were awarded the 2010 Nobel Prize in Physics for their ability
to isolate this single sheet of carbon atoms.
The present work is to manufacture biodegradable polymer (PLA)/Grapheneoxide(GO) composites by twin extrusion and compression moulding methods
and characterize their mechanical and gas barrier properties. Five types of
PLA/GO films of different compositions were prepared and used in this study.
Theoretical modelling of gas barrier properties is done to compare the
experimental results with the prediction by various models proposed by
scientists. Effects of introducing the GO flakes into the matrix have been tried
to understand.
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Acknowledgement
I sincerely thank The Faculty of Engineering and CACM, The University of
Auckland, for providing me with this wonderful opportunity to work on aproject that was exciting and enthralling. I thank Prof. Anil K. Bhowmik, my
reverend Director, for giving me this opportunity to visit New Zealand and
work on a wonderful project.
This project has only been possible because of special contribution and great
assistance of many people.
Prof. Debes Bhattacharyya, your dedication, guidance and feedback, not to
forget your extensive manufacturing and composites knowledge, have beeninvaluable in assisting me throughout my project under your supervision.Its a
pleasure to thank Dr. Dongyan Liu for her constant inspirationand valuableinputs in manufacturing and experiments which helped to develop my
knowledge base in experimental research and made it possible to complete my
work well within time. Without her this never would have been possible.
I am grateful to all the technicians for their kind attention and help. Jos, forpatiently explaining the safety measures vital for working in the lab and
helping me out in doing the tensile test of films . Steve for providing essential
equipments required at all the stages of manufacturing. Jimmy, Shane and
Callum, for their practical inputs and assistance in various stages of
manufacturing and processing.
My sincere appreciations to my colleagues Kalyan and Vijay for all their
support to make this project possible.
Last but not the least, I thank my parents and my little sister for their love andsupport, and for standing by me at all times.
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Contents
Abstract.2
Chapter 1
Introduction.5
1.1 Composites.....5
1.2 Film Composites and its applications....5
1.3 Manufacturing techniques and instruments used6
1.3.1 Twin Extruder6
1.3.2 Compression moulding....71.3.3Scanning Electron Microscope..71.3.4 Differential Scanning Calorimetry..8
1.3.5 Optical Microscope91.4 Permeability10
1.4.1 Factors affecting permeability.13
1.4.2 Applications of permeability to industry..14
1.4.3 Theoretical Modelling of Gas Barrier Properties.14
Chapter 2
Materials19
2.1 PLA............19
2.2 Graphene Oxide..20
Chapter 3
Manufacturing Procedures and its Description...21
3.1 Preparation of Graphene Oxide Film..21
3.2 Preparation of Composite Films.24
Chapter 4
Testing Methods...28
4.1 SEM of Fracture Surface of Films..28
4.2 Films under Optical Microscope....31
4.3 Tensile test...32
4.4 DSC of the films....35
4.5 Permeability test and Theoretical Model....36
Chapter 5
5.1 Results and Discussions45
References..48
Appendices.......50
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Chapter 1: Introduction1.1 CompositesA composite is a material which is a mixture of two or more distinct materialswhere each material has different physical and chemical properties. Moreover,
a composite usually possesses properties superior to its constituents. It
consists of a matrix(dispersion phase) and a reinforcement(dispersed
phase).Matrix and reinforcement offer different properties.
Matrix
Transfers load to the reinforcement Holds the dispersed phase Provides chemical and temperature resistance
Reinforcement
Provides strength and stiffness Impact resistance Enhances gas barrier properties
1.2 Film Composites and its applicationsA thin film is a layer of material ranging from fractions of a nanometer(monolayer) to several micrometers in thickness. Films have been used in
industry for manifold purposes be it packaging industry,microelectronic
integrated circuits, magnetic information storage systems, optical coatings or
wear resistant coatings. However, the mechanical performance of these
materials tends to depend on fabrication and post-processing parameters.
With the purpose of improving the mechanical and gas barrier properties of
films a relatively novel idea of mixing GO in PLA matrix is used in this research
project.
PLA/GO composites can be prepared by melt-blending, solvent-casting or in
situ polymerization. In this work twin-extrusion and compression moulding are
used to obtain thin films and characterise them.
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1.3 Manufacturing techniques and instruments used1.3.1 Twin ExtruderIn the twin extruder, raw material in the form of small beads is gravity fed from
a top mounted hopper into the barrel of the extruder [11].The material enters
through the feed throat and comes into contact with the screw. The rotating
screw pushes the beads forward into the barrel which is heated to the desired
melting temperature of the polymer fed. A heating profile is set for the barrel
in which three or more independent PID controlled heater zones gradually
increase the temperature of the barrel from the rear.Extra heat is contributed
by the intense pressure and friction taking place inside the barrel.
At the front of the barrel, the molten plastic leaves the screw and travels
through a screen pack to remove any contaminants in the melt.The screens
are reinforced by a breaker plate (a thick metal puck with many holes drilledthrough it).After passing through the breaker plate molten plastic enters the
die. The die is what gives the final product its profile and must be designed so
that the molten plastic evenly flows from a cylindrical profile, to the product's
profile shape. Long continuous strands of polymer are obtained from the
extruder that was used.
Twin screw extruders are usually run starve fed. There is an independent
control of both the feed rate and the screw speed.
Fig.1.1- Twin extruder, CACM, University of Auckland.
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1.3.2 Compression mouldingIt is a manufacturing technique in which the desired object moulds are
compressed under high pressure and temperature. The moulds are inserted
between the two metallic plates which are connected to hydraulic pumps to
apply pressure. The advantage of compression moulding is its ability to mould
large, fairly intricate parts. Also, it is one of the lowest cost moulding methods
compared to other methods such as transfer moulding and injection moulding;
moreover it wastes relatively less material, giving it an advantage when
working with expensive compounds. However, compression moulding often
provides poor product consistency and difficulty in controlling flashing, and it is
not suitable for some types of parts. Fig.1.3 shows hydraulic press for
compression moulding.
Fig.1.2- Hydraulic Press (for compression moulding), CACM , University of Auckland
1.3.3 Scanning Electron MicroscopeSEM is a type of microscope which enables us to observe morphology of
materials at micro and nano-level. In this microscope an electron beam is
emitted over the desired region and the sample response is sensed that
reflects its topography. The response is due the conductivity of the sample
being observed. The signals produced by SEM contain data about its
composition, topography and other properties. Wide range of magnification is
possible i.e. from 10 to 500,000. Its magnification doesnt depend on the
power of the objective lenses.
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Glass Transitions Melting and Boiling Points Crystallization time and temperature Percent Crystallinity Heats of Fusion and Reactions Specific Heat Oxidative/Thermal Stability Rate and Degree of Cure Reaction Kinetics Purity
Fig.1.5- Differential scanning Calorimetry setup, courtesy: CACM University of
Auckland
1.3.5 Optical Microscope
Optical microscope is an instrument that uses visible light and a system of
lenses to magnify images of small samples.All modern optical microscopes
designed for viewing samples by transmitted light share the same basic
components of the light path, listed here in the order the light travels through
them:
In addition the vast majority of microscopes have the same 'structural'
components:
Ocular lens (eyepiece) Objective turret or Revolver or Revolving nose piece (to hold multiple
objective lenses)
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Objective Focus wheel to move the stage ( coarse adjustment, fine adjustment) Frame Light source Diaphragm and condenser lens Stage (to hold the sample)
Fig.1.6- Optical Microscope , Plastic Centre, University of Auckland
1.4 Perm eabi l i t y
To quantify and characterize the barrier properties of a polymer film or
membrane, the most frequently measured and reported quantity is the
permeability P.
Permeability P is a measure of the amount of gas that passes through a film of
thickness l and area A within a finite amount of time t.
= ()()()()() (1-1)
Permeability or transmission rate is dependent upon two factors: the solubility
of a gas or vapour and the rate of diffusion through the barrier. In order for
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permeation to occur, these two mechanisms one thermodynamic (solubility)
and the other kinetic (diffusion) must both occur.
= (1-2)where, D and S represent the diffusion and solubility coefficient respectively
and P is the permeability.
Diffusion through a polymer occurs by small molecules passing through voids
and other gaps between the polymer molecules (free volume) [48].If the
speed at which a molecule diffuses through a polymer obeys Ficks first and
second laws, as is the case for oxygen permeating through an MFC under
standard conditions, it is termed Fickian diffusion[7,8,9]:
Ficks first law:
= (1-3)
whereJ is the steady-state flux per unit area, D is the diffusion coefficient and
Cthe gas concentration.
It is well known that Ficks first law is also analogous to Darcys law, which can
be used to predict the permeability of a homogeneous system to gases or
liquids.
= (1-4)
Solubility is determined by the enthalpy change on dissolution of the molecule
in the polymer matrix and the volume available for occupation. The solubility is
in particular influenced by the state of the polymer; if it is in the rubbery state
then most common gases in polymers follow Henrys law behaviour.
However in glassy polymers, Henrys law is observed for the more non-
condensable gases (O2, helium, H2, N2, argon) while condensable gases such as
CO2 more accurately follow the dual-sorption model [7].
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Four stages are involved in the permeation of a gas through a film or polymer
matrix ,and they are [50]:
1. Absorption into the surface of the polymer
2. Solution of the gas or vapour into the polymer matrix
3. Diffusion through the wall along a concentration gradient
4. Desorption from the other surface
There are no universally accepted units for gas transmission through polymer
films or sheets; however a few common terminologies are defined in the ASTM
Standard D3985-05 [51] to report permeability.
Oxygen transmission rate (OTR) is the quantity of oxygen gas passing through
a unit area of the parallel surfaces of a plastic film per unit time under the
conditions of test. The SI unit for OTR is mol/m2 s, however it is usually
recorded as cm3. (STP)/m2 day[12, 15].
Oxygen permeance (PO2) takes into account the pressure difference between
the two sides of the film as shown in Equation (2-4) . The SI units of Permeance
are mol/(m2.s.Pa).
PO2 = (1-5)
Permeance does not take into account the thickness of the material and hence
is only useful when comparing specimens of similar thickness.
Oxygen permeability coefficient(P) is the product of permeance andthickness.
P = PO2x t (1-6)
While the SI unit is mol/m2s Pa, authors in literature usually report
permeability as cm3 (STP).mm/m2 day.atm which is the same as saying a cubic
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centimeters of gas that passes through a square meter of film in a day when
the gas pressure differential on one side of the film, at a specified temperate,
is one atmosphere greater than that on the other side[13,14].
1.4.1Factors influencing permeability
There are a number of factors which influence diffusion and solubility and
hence permeability. These include:
1. Crystallinity
2. Filler particle3. Molecular orientation
4. Temperature
5. Pressure
6. Humidity
Crystallinity affects permeability as the chains are highly ordered in crystalline
regions compared to amorphous regions and hence there should be very little
free volume and the path should be extremely tortuous. The amount of freevolume depends on density and polymer characteristics. In crystalline regions
this provides less free volume while amorphous regions will depend upon the
direction of the penetrant [20].
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Filler material also affects the diffusion behaviour because if the particle is
impenetrable as it creates a more tortuous path for the diffusing molecule.
The reason for this is for every 5C increase in temperature, a 30 to 50% rise in
permeability occurs. From the mass transport equation, the flow of a gas andpartial pressure difference is affected by temperature changes. At the
molecular level, increasing temperature leads to a rise in the mobility of the
molecular chains and thermal expansion leads to a reduction in density. This
results in more free volume and thus higher solubility since free volume is
directly proportional to free volume [19].
1.4.2Applications of permeability to industry
Gas barrier properties are most important to the packaging industries. One of
the requirements of the packaging material is to prevent passage of gases like
oxygen to prevent degradation of stored material. There is also water and
other gases such as carbon dioxide and nitrogen which are important factors to
consider in packaging materials.
Preventing oxygen from entering a package is an important requirement for
most food products. If oxygen is allowed in the package, this will break down
organic materials initiating or accelerating the decay process which is themechanism for staleness and loss of nutritive value. On the other hand, to
maintain the bright red colour in meat, a high rate of oxygen transmission is
required while a low water transmission rate is required to prevent
drying the meat .Oxygen permeability plays an important role in maintaining
the quality of milk. High oxygen permeability of package will accelerate the
oxidation reaction of inside milk and in turn causes quality deterioration.
1.4.3 Theoretical modelling of Gas Barrier Properties
Factors under consideration during modelling would include the dispersed
phases aspect ratio, orientation, dispersion, shape and volume fraction, as
well as the density and crystallinity of the matrix and the affinity between the
constituent polymers and diffusing species [16, 17, 18] .
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The majority of models found in literature only take into account a couple of
the variables mentioned above.
Nielsens model for filled polymer systems
Nielsens model is based on the tortuosity factor where the filler particles are
impenetrable to a diffusing gas or liquid molecule,resulting in the diffusing
molecules following a tortuous path through the polymer. The relation is as
follows:
=
(1-7)
where, Pand Pm are the permeability of the composite and pure polymer,is the volume fraction of the matrix polymer and is the tortuosity factorwhich is the ratio of the distance a molecule must travel to get through the
film to the shortest route. If the filler particles are circular or rectangular the
tortuosity factor is represented by the following:
= 1 + ( 2) (1-8)
where, L the length of the filler, Wis the filler thickness and is the volumefraction of the filler or reinforcement. This model represents the ideal casewhere the particles are completely exfoliated and uniformly dispersed along
the preferred orientation in the polymer matrix.
Series and parallel model
The series and parallel models represent the upper and lower bounds for
permeability modelling. The upper bound is represented by the parallel model
where the reinforcing phase is orientated parallel to the direction of
permeation.The series model is the lower bound case where the reinforcing
phase is orientated across the direction of permeation [25].
Parallel model: = + (1-9)
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Series model:
= + (1-10)where, Pis the permeability, is the volume fraction and the subscripts mand f denote the matrix and reinforcing polymers.Geometric Mean Model
If one assumes a random distribution of phases, the film permeability can be
estimated using the weighted geometric mean of the polymer permeabilities
via a model known as the Geometric Mean Model[26]:
= + (1-11)
Generalised Maxwell-Rayleigh relationship
There also exists the Rayleigh relation for cylinders. This differs from Maxwells
equation[26] (also known as Maxwells relation for spheres) only by the value
of the shape factor defined in the following generalised equation as f, which is
equal to 1 for Rayleighs relation and 2 for Maxwells relation.
= 1 + (1+)( 1)( +) ( 1) (1-12)where,fis the shape factor in this equation. Whenfapproaches infinity
Equation (1-11) reduces down to the parallel model (1-9). Forf = 0 the
equation becomes the Series model (1-10). Whenf = 2, the equation
represents Maxwells equation for spheres. Lastly iff = 1, we get Rayleighs
relation [66] for long transverse cylinders.
Lewis and Nielsen equation for two-phase systems
A theoretical model developed by Nielsen [24, 26] to predict the elastic
modulus of two phase systems has also been applied by others to the
prediction of electrical and thermal conductivity. Likewise, permeability can be
predicted from component values using this model:
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= 1+1 (1-13)where
A = ke - 1, where ke is the Einstein coefficient and is equal to 1.5 for
fibres and 2.5 for spheres.
Also:
= 1
+(1-14)
= 1 + 1 (1-15)where represents the maximum fibre packing fraction. This value is 0.785for square packing, 0.82 for random packing and 0.907 for hexagonal packing.
Bttcher formula
Bttcher gave a formula that was originally applicable for random dispersion of
spherical particles which was later modified to a more general form that could
be applied to ellipsoidal shaped particles. This equation is [21]:
()+() +
()+() = 0 (1-16)
whereA is related to the shape of the reinforcement,A=1/3 for spheres and
A=0.5 for rods.
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Bruggeman formula
The differential effective medium (DEM) theory was introduced by Bruggeman
to estimate the effective thermal conductivity of composites at high volume
fractions [27]. Bruggeman formula for spherical particles is given by:
1 3
= (1 ) 1 (1-17)The permeability for random-oriented laminated flat particles is:
+2+2 = 1 (1-18)
Higuchi Model
Higuchi demonstrated when particle-particle interactions were neglected, the
model led to the well known Rayleigh-Clausius-Mosotti equation (labelled
Maxwell equation in this report). In another paper by Higuchi et al. [23] the
same principles from an earlier paper were used to derive a model to predict
the permeability of two-phase mixtures. His model is represented by:
= 1 + 3[1(1)] (1-19)The quantity Kinvolves the distribution function for random spheres and is a
function of the volume fraction of the reinforced polymer. Higuchi found
K = 0.78 provided a good fit between the experimental data and the
predicted model. When K = 0, this model reduces down to the Maxwell
equation for random spheres. is a measure of the permeability differencebetween the two phases and is given by:
= +2 (1-20)
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Chapt er 2 : Mater ia ls
2.1 PLA(Poly-lac t ic Ac id)
PLA [22] stands for poly-lactic acid and is a thermoplastic aliphatic polyester
derived from renewable resources, such as corn starch, tapioca products
(roots, chips or starch) or sugarcanes. It can biodegraded under certain
conditions, such as the presence of oxygen, and is difficult to recycle.Bacterial
fermentation is used to produce lactic acid from corn starch or cane sugr.
PLA polymer 2002D[28] , that we are using , is a clear sheet grade and
processeseasily on conventional extrusion and thermoforming equipment.Its
specific gravity is 1.24.Its glass transition temperature is in between 60-65 C
.Its melting temperature is 210 C. Its tensile yield strength at 60MPa.PLA is
PLA is used to make clear compostable containers and PLA lining is used in
cups and containers as an impermeable liner. PLA is biodegradable, and fully
compostable. It uses 65 percent less energy to produce than conventional oil-
based plastics and generates 68 percent fewer greenhouse gasses and contains
no toxins.
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Fig. 2.1- PLA Pellets, CACM, University of Auckland
2.2 Grap hene Oxi de (GO)
Graphene oxide is oxidised form of Graphene , the two dimensional sheet of
sp2
hybridised carbon atoms which has evolved as a material with remarkable
mechanical, electrical and thermal properties. Heaps of research has been
carried out and still being carried out to derive various applications of
graphene and its derivatives in the field of nano electronic devices, composite
materials and gas sensors,biomedical applications and energy storage devices.
Graphene oxide sheets have been used to prepare a strong paper-like material.
Graphene oxide is prepared by oxidation with strong oxidizers. It typically
preserves the layer structure of the parent graphite, but the layers are buckled
and the interlayer spacing is about two times larger (~0.7 nm) than that of
graphite. Besides oxygen, epoxide groups (bridging oxygen atoms), other
functional groups experimentally found are: carbonyl (=CO), hydroxyl (-OH),
phenol groups [2,3,4] attached to both sides.
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Fig.2.2 Graphene Oxide structure
Highly oxidized Graphene oxide is insulating where as graphene is
exceptionally conducting this is mainly due to the extensive presence sp3
carbon atoms of oxygen (highly electronegative) containing functional groups
which do not allow free movement of electrons.[29]It is almost asemiconductor, with differential conductivity between 1 and 510
-2S/cm at a
bias voltage of 10 V.[31]Its conductivity can be varied by varying the level of
oxidation, temperature and other environmental factors.[30]
Graphene oxide sheets have tensile modulus of 32 GPa [32] whereas of
graphene its 130GPa.[33]Its spring constant is also very high. Such chemically
and structurally tuned graphene sheets hold significant promise for novel
sensors, membrane based NEMS devices, transparent conductors for
optoelectronic applications, smart composite materials, and others.
Chapter 3 : Manufac t ur ing Procedures and i t s
Descr ip t ion
3.1 Preparation of Graphene Oxide film
Modified Hummers Method
Procedure:
1. First of all, 2g. of powdered Graphite and 1.5g. of sodiumnitrate are taken and poured into a flask containing
H2SO4(concentrated 66%).
2. Then, the mixture is stirred for 30min.at 600 rpm usingmagnetic stirrer.
3. Then the flask is transferred to an icebath which had beencooled down to 0o C.
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4. 6 g. of KMnO4 is slowly added to the flask at room temparatue.5. Then, the flask is transferred to a water bath to maintain a
temperature of 35 3o
C for 60 min.
6. Then the suspension is diluted with 100 ml. of water veryslowly causing violent effervescence and increase in
temperature.
7. The temperature is maintained at 80oC in hot waterbath for 15min.
8. The solution is again diluted to 300ml. with warm water9. Followed by, addition of 10 ml. of H2O2(30%) which results in
the colour change of the suspension to yellow.
10.Then, 200 ml. of HCl (20%) is added to the suspension.11.The suspension is left overnight.
Fig.3.1-GO suspension, Chemistry Lab. , CACM, University of Auckland.
12.Filtration and of the suspension is done till the pH of thesuspension turns neutral.
Fig.3.2 Filtration by suction
13.After filtration centrifugation of the GO is done.
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Fig.3.3-Centrifugation at Chemistry Lab., University of Auckland, Tamaki
Campus.
14.Then, ultrasonification of GO particles is done to break theminto finer particles.
Fig.3.4 Ultrasonification at Chemisty Lab. ,University of Auckland, Tamaki Campus.
Fig.3.5 Ultrasonifier
15.The Graphitic oxide is heated in the oven for drying.
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Fig. 3.6 Oven Dryer @ Chemistry Lab.,University of Auckland, Tamaki Campus.
16.The film that we get is as shown below.
Fig.3.7 GO film
3.2Preparation of Composite Films
Five different types of films have been prepared.
1. Pure PLA2. PLA + 0.5 % GO (Master Batch)3. PLA + 1% GO (Master Batch)4. PLA + 0.5 % GO (Non-master Batch)5. PLA + 1% GO (Non-master Batch)
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Preparation of Master Batch
Master Batch is prepared by mixing 0.5 g. and 1 g. of GO and 19.5 g. and 19 g.
of PLA respectively in solvents DMF+THF in the ratio 1:1 . GO and PLA are
dispersed in the solvent and they are dried in an oven under vacuumconditions. We get solid stuff as show in the figure.
Fig.3.8 Master Batch
Procedure :
1. First PLA pellets are ground into smaller granules (
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Fig.3.10-Grinder
3. GO is mixed with PLA in 1 and 0.5 weight percent for the non masterbatch.
4. For master batch we mix 80 gm of PLA granules to each concentration(0.5 and 1 wt. per cent).
5. The mixtures are again grinded in a grinder for proper mixing.6. Pure PLA and the four mixtures are first extruded into long strands using
twin extruder (see figure).
Fig.3.11- Long strands of PLA/GO being enrolled.
7. Then, the strands are cut into smaller size pieces using palletizer.
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Fig.3.13- Palletized PLA, PLA+GO(0.5%)& PLA+GO(1%)
8. After pelletisation keep the pelletised pieces are kept in vaccum oven for3 hours .
Fig.3.14-Vacuum Drier, CACM, University of Auckland.
9. 2.5 gm of each concentration (including pure PLA pallets) are weighedand hot compression moulded into thin sheets using the hydraulic press.
Fig.3.15 -Films after compression moulding.
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Chapter 4: Testing Methods
4.1 SEM of Fractured surface of Films
SEM samples were prepared by cutting films into thin strips. These strips weredipped into Liquid nitrogen and were fractured inside the liquid nitrogen.
These fractured surfaces were observed under Scanning Electron
Microscope(SEM) and following images were obtained. GO samples were also
observed under SEM.
PurePLA
PLA + 0.5 %
GO(MB)
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PLA + 1 %
GO(MB)
PLA + 0.5 %
GO(NMB)
PLA + 1 %
GO(NMB)
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GO film
GO
suspension
Reduced
GO
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4.2Films under Optical Microscope
Films were reviewed under optical microscope, since after SEM test the two
phases (matrix and reinforcement) we not separately visible. So, under 100x
and 400x magnification films were observed and picture of the morphology
were capture which as shown below.
PLA + 1 %
GO
PLA + 0.5 %
GO
Pure PLA
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4.3Tensile Test
The films were cut into thin strips of 13mm width using the click press
machine(fig.4.1) , which were then again cut into dumbbell shaped strips using
the dumbbell shaped blades and a press machine.These strips are then tested
in the INSTRON 5660 where the gauge length fixed is 25 mm , full scale load is
1000N.
Fig.4.1Click press
Fig.4.2 Stress v/s Strain Graph
obtained for PLA
Fig.4.3 Stress v/s Strain Graph
obtained for PLA/GO(0.5%)
Fig.4.4 Stress v/s Strain Graph
obtained for PLA/GO(1%)
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To summarize the results obtained in the tensile test, we got the best set of
properties for PLA+0.5%GO .The mechanical properties are almost identical for
both master batch and non master batch films. Not much improvement is
observed by introducing GO in the matrix by twin extrusion and hot
compression moulding method, this is owed to the fact that above 150o
C
there is some change in the properties of GO .The table shows the results
obtained from the test.
Table4.1 Tensile Test Results
Material Modulus(GPa)
YieldStrength
(MPa)
UltimateTensile
Strength
(MPa)
MaximumLoad
(N)
Pure PLA 3.16 52.92 53.03 30.60
PLA+0.5%GO
(MB)
3.83 57.83 57.83 32.43
PLA+ 1%GO
(MB)
3.50 53.30 53.30 31.96
PLA + 0.5%
GO (NMB)
3.52 56.70 56.70 32.10
PLA + 1% GO
(NMB)
3.39 54.04 54.07 31.95
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4.4 DSC Test for the Films
Differential scanning calorimetry (DSC) was done on the twin-extruder and
compression moulded films of PLA and GO in order to determine the thermal
behaviour of the films.52 mg. of each of the three films(PLA,PLA+0.5% GO
MB,PLa+1% GO MB ) was cut and sealed to be tested and loaded in the
machine. The results are presented in Figure below.
Fig.4.7.DSC Test graphs obtained after analysis
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Table4.2 DSC Test Results Summary
S. No. Material
Glass
Transition
Temperature
Onset of
Crystallization
Temperature
Degree of
Crystallinity
(%)
1. Pure PLA 56.40oC 108.32oC 0.27
2. PLA+0.5%GO 56.77oC 105.74o C 0.86
3. PLA + 1% GO 55.48o C 97.80o C 4.12
DSC tests reveal that the glass transition temperature of all the films remain
very close to 56oC with not much variation. From this result it is inferred that
there is not much of a strong interaction or bonding between the interface of
the materials. Though the increase in concentration of GO in the matrix has led
to a decrease in the onset of crystallisation temperature i.e. presence of GO
induces nucleation in the matrix. Also, crystallinity of the films has increased
with increase in the concentration of GO in the films.
4.3 Permeability Modelling and oxygen gas barrier results:
Permeability testing was successfully done using the MOCON OX-TRAN 2/10
machine.The films were cut from the edges to fit in the fixture for testing and
the process took around 10 hours for completion. The films under exposure to
the oxygen were 50 cm2
and thickness varied with each film. Ambienttemperature fixed was 23
oC and the standard procedure of testing was
followed which involved 10 cycles of 30 minutes each and a conditioning
period of 3 hours. The results obtained are shown in Table
In section 1.4.3 a brief description of the various models found in literature
was provided. Most of the models are conductivity or elasticity models for two
phase materials modified because of their close analogy to permeability. To
check the applicability of the models, the models were plotted againstexperimental data found in table 4.3. All these models use the permeability of
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matrix. The reinforcement is highly impermeable as found in literature so the
permeability value for GO is taken to be zero.
Table4.3 Permeability Test Results
Sample Transmission rate
cc/[m2-day]
Permeability
cc-mm/[m2-day]
Permeability
cc-mil/[m2-day]
Pure PLA 74.95 35.41 548.96
PLA+0.5%GO 69.14 13.27 522.64
PLA+1 % GO 70.10 12.75 502.30
These models provide a good starting point for predicting the permeability in
MFC. Each of them follows the same downward trend seen in the experimental
data and with some modification to the gradient; these models should provide
a more accurate representation of the data.
As earlier mentioned in section 4.1.3 the parallel and series model give the
upper and lower bound of the gas permeability value of composites. Fig.4.8
shows the plot of oxygen permeability v/s GO volume fraction. It can be seen
from the figure that the experimental data falls well within the upper and
lower bounds predicted by the models.
Fig.4.8 Permeability v/s GO volume fraction
-100
0
100
200
300
400
500
600
-0.002 0 0.002 0.004 0.006 0.008 0.01
OxygenPermea
bility(cc-mil/[m2-day)
Reinforcement(GO) concentartion(vol%)
Experimental
Parallel
Series
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Fig.4.9 shows the comparison of experimental and predicted oxygen gas
permeability values by the Maxwell-Rayleigh model and geometric mean
model for the composite. Though all the models overestimate the permeabilityvalues for the composite, the geometric mean model is the closest of the
existing models.
Fig.4.9 Permeability v/s GO volume fraction
Four other models were used for prediction of oxygen gas permeability of thecomposite films (Lewis-Nielson, Bottcher, Bruggeman and Higuchi) shown in
fig. 4.10. All these models over predict the permeability for low contents of
PET. The permeability values predicted by these models are within the same
vicinity of each other. The predicted values of oxygen gas permeability by
Bottcher and Bruggeman are almost coinciding and the hence the graphs of
the two models are overlapping.
490
500
510
520
530
540
550
560
0 0.002 0.004 0.006 0.008 0.01
OxygenPermeability(cc-mil/[m2-da
y])
Reinforcement(GO) concentration (vol %)
Experimental
Geometric mean
Maxwell
Rayleigh
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Fig.4.10Permeability v/s GO volume fraction
Modification of models
In this section each of the models listed in section 4.1.3 have been modified to
make them more applicable to the composite film. This is done by fitting the
models to the experimental results by changing the various factors present in
the models. The equations or formula listed in table 4.4 have been modified to
fit the experimental results.
Table 4.4
S.No. Model Name Equation /Formula Involved
1. Maxwell-Rayleigh
equation = 1 + (1 +)( 1)
( +) ( 1)
2. Lewis and Nielsenequation for two-phase
systems
= 1 +1
490
500
510
520
530
540
550
560
0 0.002 0.004 0.006 0.008 0.01
OxygenPermeability(ccmil/[m2-day])
Reinforcement(GO) concentration (vol %)
Experimental
Lewis-Nielsen
Bottcher
Bruggeman
Higuchi
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3. Bottcher formula ( ) + ( ) +( ) + ( ) = 0
4. Bruggeman Formula
1 3= (1 ) 1
5. Higuchi model = 1 + 3[1 (1 )2]
Maxwell-Rayleigh equation
The generalised Maxwell-Rayleigh equation takes into account the models
derived by Maxwell and Rayleigh for spheres and long transverse cylinders
respectively, through a shape factor that also incorporates the parallel and
series models. A recap of which model corresponds to the relevant shapefactor is given in Table 4-4.
Table 4.4
Model Shape factor (f)
Maxwell 2Rayleigh 1
Parallel Series 0
Using the shape factors predicted by Maxwell and Rayleigh the value of
permeability found out exceeds that of the experimental results.
The shape factor is determined to fit the experimental results and is found out
to be f = 0.094.
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Fig.4.11 Permeability v/s GO volume fraction
Lewis and Nielsen equation for two-phase systems
The Lewis and Nielsen equation is based on the model derived by Halpin and
Tsai. They extended the Halpin-Tsai equation to include maximum packing
fraction of the filler which is considered to be important for viscosity of
suspensions and they pointed out the relation between the shape factor
constant (A) and the generalised Einstein coefficient.
The shape factor constant A = Ke1 and the value of Ke is found out to be1.095 that best fits with the experimental data.
490
500
510
520
530
540
550
560
0 0.002 0.004 0.006 0.008 0.01
OxygenPermeability(ccmil/[m2-day])
Reinforcement(GO) concentration (vol %)
Experimental
f = 0.0944
Maxwell(f = 2)
Rayleigh( f = 1)
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Fig.4.12 Permeability v/s GO volume fraction
Bottcher formula
Bttcher derived a formula to correlate the dielectric behaviour of powder
with bulk based on the earlier derivation from Bruggeman. Figure 4.13 shows
the plot of permeability v/s GO volume fraction in which the shape factor A
assumed was 1/3 and which is modified to 0.905 to match the experimental
results so that the best fit is obtained.
Table 4.5
Shape of dispersedphase
Shape factorA
Sphere 1/3
Rods 1/2
Experimental
fitting 0.905
490
500
510
520
530
540
550
560
0 0.002 0.004 0.006 0.008 0.01
O
xygenPermeability(cc-mil/[m2-day])
Reinforcement(GO) concentration (vol %)
Experimental
Lewis-Nielson
Modified Lewis-Nielson
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Fig.4.13 Permeability v/s GO volume fraction
Bruggeman Formula
Bruggemans formula was originally developed to predict the conductivity in
spherical particles. The formula proposed is shown below.
= (1 ) 1 Fig. shows that the above formula over predicts the value of oxygen gas
permeability. Modification made in the above formula so that it can be
applicable for flaky GO particles in the PLA matrix is that the index of = 13is changed to 0.54 to match with the experimental results obtained.
490
500
510
520
530
540
550
560
0 0.002 0.004 0.006 0.008 0.01
OxygenPermeability(cc-mil/[m2-da
y])
Reinforcement(GO) concentration (vol %)
Experimental
Bottcher
Modified Bottcher
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Fig.4.14 Permeability v/s GO volume fraction
Higuchi model
Higuchi derived his model from the theory used to develop his new
relationship for dielectric properties of two-phase mixtures. The quantity K is
considered to be the shape factor in this model and the value used in 0.78 wasbased on dielectric constant data for powders and suspensions. Using the
results from experiment, the constant K was modified to fit the experimental
data and was found to be 3.49.
490
500
510
520
530
540
550
560
0 0.002 0.004 0.006 0.008 0.01
Ox
ygenPermeability(cc-mil/[m2-
day]
)
Reinforcement(GO) concentration (vol%)
Experimental
Bruggeman
Modified Bruggeman
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Fig.4.14 Permeability v/s GO volume fraction
5.1 Results and Discussion:
Films of PLA(Poly-Lactic Acid) and Graphene oxide were successfully
manufactured and their mechanical, thermal and oxygen gas barrier properties
were characterised. The results can be summarised as follows:
1. Tensile Test results show that the modulus of elasticity , yield strengthand ultimate tensile strength improved with the addition of 0.5(wt%)
GO in the PLA matrix. Though the improvement was not significant in
the light of the fact that the tensile strength of Graphene is
remarkable ~1GPa, nevertheless, the properties did not deteriorate
on addition of GO.
It was also observed that the increase in the concentration of GO in
the PLA matrix led to a decline in the mechanical strength. 1% GO inPLA showed lower values of modulus of elasticity, yield strength,
490
500
510
520
530
540
550
560
0 0.002 0.004 0.006 0.008 0.01
Oxyge
nPermeabilty(cc-mil/[m2-day])
Reinforcement(GO) concentration (vol%)
Experimental
Higuchi
Modifiied Higuchi
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ultimate tensile strength and maximum load. This behaviour of the
PLA/GO film is owed to the fact that the increase in GO content led to
non-uniform dispersion, agglomeration and formation of lumps
increasing the non-homogeneity of the samples.
2. To gain more insight of the trends in mechanical strength of thesamples, the SEM test for the fractured surface of the films was done.
The pure PLA fracture surface showed a more regular pattern of scaly
surface. The roughness of surface increases with increase in GO
content. This, in a way, testifies for the formation of lumps and non-
homogeneity in the films with the increase in GO content. But, the
main objective of this test was to observe the GO particles inside the
PLA matrix, which could not be done as the two phases (matrix andreinforcement) were not distinguishable, may be due to less
resolution. Difference in the surfaces of Master Batch and non-
master batch is due to the process of manufacture, one being more
dispersed due to use of solvent and the other being directly
incorporated.
3. Due to our inability to observe the GO particles dispersed in PLAdistinctly, we took resort to optical microscope to explore more.
The two phases of films were, now, clearly distinguishable and the GO
matter dispersed in the PLA matrix could be easily viewed. The GO
was randomly distributed in the PLA matrix and looked like dark
patchy flat paper or flaky structured. The large GO particles were
easily visible while there were many smaller particles dispersed but
could not be seen properly.
4. After the morphology of the films became clear, the thermalproperties of the films were tested by DSC technique. The resultsobtained were quite satisfactory and in congruence with the trends
observed in the mechanical strength of the films. The glass transition
temperature of all the three films was around 56oC, which shows that
the interaction at the interface of the two materials is not strong
enough to bring about much improvement in mechanical properties.
Another interesting feature observed was the decrease in the onset
of crystallisation temperature of the films with the increase in the GO
content in the matrix. This shows that the presence of GO induces the
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nucleation process to occur faster. The %crystallisation of films was
calculated using the data obtained in the test and it was found that
with increase in GO concentration it increased.
5. The oxygen gas barrier properties testing was the last test conductedon the films, as the material under consideration was perceived to be
a potential material to be used in packaging industry. The
permeability of pure PLA film was found to be 548.96 cc-mil/[m2-day]
which is quite high very close to the value in the other reference
texts. The addition of 0.5% GO (MB) showed around 10% decrease in
permeability which as a matter of fact is astounding looking the
amount of GO used. The value of permeability found was 489.08 cc-
mil/[m2-day]. But, further increase in GO content led to a decrease in
barrier properties and hence, rise in the permeability value for 1% GO(MB) in PLA matrix. The actual value that was observed for 1% GO film
was 500.3271 cc-mil/[m2-day]. This was not expected in light of the
permeability theories given by researchers (Lewis-Neilson, Maxwell-
Rayleigh, Higuchi). The oxygen gas barrier test for the non- master
batch samples was also done. The permeability value for GO(0.5%) in
PLA was found out to be 522.64 cc-mil/[m2-day] and that of GO(1%) in
PLA was found to be 502.30 cc-mil/[m2-day], which was in accordance
of the theoretical gas barrier models which predict that with increase
in the reinforcement percent the gas barrier properties increases.6. In Section 6.2, some of the models found in literature were used to
predict the oxygen gas permeability value of the composite films and
was matched with the experimental data of non-master batch
PLA/GO films. Each of the models were modified to make them more
applicable to the experimental data for PLA/GO(NMB) composite
films. None of the modified shape factors provided an ideal fit to the
data in particular for models like Lewis-Nielsen, Maxwell-Rayleigh,
Bottcher and Bruggeman. Shape factors or indices were changed to fit
the formulae to match with the experimental data.
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References
[1]Wikipedia.
[2]Lipatov YS. Interfacial effects in polymer blends. Review. Polymer ScienceUSSR (English Translation of Vysokomolekulyarnye Soyedineniya Series A)1978;20(1):1-18.
[3]Jeong H-K, Lee YP, Lahaye RJWE, Park M-H, An KH, Kim IJ, et al.Evidence of graphitic AB stacking order of graphite oxides. J Am Chem Soc2008;130:13626.
[4]Szab T, Berkesi O, Forg P, Josepovits K, Sanakis Y, Petridis D, et al.
Evolution of surface func-tional groups in a series of progressively oxidizedgraphite oxides. Chem Mater 2006;18:27409.
[5] Lerf A, He H, Forster M, Klinowski J. Structure of graphite oxide revisited.J Phys Chem B 1998;102:447782.[6]Massey, L K, "Permeability Properties of Plastics and Elastomers", 2003,Andrew Publishing.[7]W.F. Smith, Foundations of Materials Science and Engineering 3rd ed.,McGraw-Hill (2004)[8]H.C. Berg,Random Walks in Biology, Princeton (1977)
[9] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, JohnWiley & sons, (1976)[10] Skoog, Douglas A., F. James Holler and Timothy Nieman (1998).Principles of Instrumental Analysis (5 ed.). New York. pp. 805808.[11]Rauwendaal, Chris (2001), Polymer Extrusion, 4th ed, Hanser.[12]Hanne Larsen, Achim Kohlr and Ellen Merethe Magnus, "Ambient oxygeningress rate method", John Wilew & Sons, Packaging Technology and Science,Volume 13 Issue 6, Pages 233 241.
[13]F2622 Standard Test Method for Oxygen Gas Transmission Rate ThroughPlastic Film and Sheeting Using Various Sensors.[14]ASTM. Standard Test Method for Oxygen Gas Transmission RateThrough Plastic Film and Sheeting Using a Coulometric Sensor. D3985-02. p.458-463.[15]Yam, K. L., "Encyclopedia of Packaging Technology", John Wiley & Sons,2009.[16]Shields,R.J, Bhattacharyya, D., Fakirov, S., Oxygen permeabilityanalysis of micro-fibril reinforced composites from PE/PET blends, Composites
Part A: Applied Science and Manufacturing, Accepted, 2008;39:940-949.[17]Gas barrier properties of PP/EPDM blend nanocomposites
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Masoud Frounchi, Susan Dadbin , Zahra Salehpour , Mohsen Noferesti.[18]Gas transport properties of polyacrylate/clay nanocomposites preparedvia emulsion polymerization Jose M. Herrera-Alonso, Zdenka Sedlakova, EvaMaranda.
[19]Gas Permeability and Free Volume of Highly Branched SubstitutedAcetylene Polymers byYu. P. Yampolskii, A. P. Korikov, V. P. Shantarovich,K. Nagai, B. D. Freeman, T. Masuda, M. Teraguchi and G. Kwak.[20]Moisture Permeability of Polymers. I. Role ofCrystallinity and Orientation by S. W. LASOSKI, JR., and W. H. COBBS, JR.,
E.I.duPont de Nemours and Company, Film Department,Buffalo, New York.[21]Effective Medium Theories for Artificial MaterialsComposed of MultipleSizes of Spherical Inclusions in a Host Continuum William M. Merrill, Student
Member, IEEE, Rodolfo E. Diaz, Michael M. LoRe, Mark C. Squires, andNicolaos G. Alexopoulos, Fellow, IEEE[22]Sdergrd, Anders; Mikael Stolt (February 2002). "Properties of lactic acidbased polymers and their correlation with composition". Progress in PolymerScience27[23]Physical models of diffusion for polymer solutions, gels and solids byL. Masaro, X.X. Zhu.[24]Models for the Permeability of Filled Polymer Systems by Lawrence E.Nielsen at CENTRAL RESEARCH DEPARTMENT, MONSANTOCOMPANY ST., LOUIS, MISSOURI.[25] Polymer blends, Lloyd M. Robinson , Hanser.
[26]Characterisation of the Mechanical and Oxygen Barrier Properties ofMicrofibril Reinforced Composites by Ryan John Shields.[27]Generalized Bruggeman Formula for the Effective Thermal Conductivityof Particulate Composites with an Interface Layer by J. Ordez-Miranda J. J.Alvarado-Gil , R. Medina-Ezquivel.[28]Techical Data shee_2002D, by NatureWorks.[29] Boukhvalov, D. W.; Katsnelson, M. I. J. Am. Chem. Soc. 2008, 130,10697.[30] Tunable Electrical Conductivity of Individual Graphene Oxide Sheets
Reduced at Low Temperatures Inhwa Jung, Dmitriy A. Dikin,, Richard D.Piner, and Rodney S. Ruoff.[31] C. Gomez-Navarro et al. (2007). Nano Letters, volume 7, issue 11, page3499 doi: 10.1021/ nl072090c[32] "Graphene Oxide Paper". Northwestern University. Retrieved 2011-02-28.[33] Lee, C. et al. (2008). "Measurement of the Elastic Properties and IntrinsicStrength of Monolayer Graphene".Science 321 (5887): 385. Bibcode2008Sci...321..385L
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Appendix
Calculation of volume fraction:
Volume fraction of GO in the matrix is calculated from mass fraction of GO
used and density of PLA and GO.
= + (1 )Where, = volume fraction of GO
w = mass fraction of GO
= density of PLA matrix = 1.24 g/cc = density of GO = 1.48 g/cc.
Calculation of percent crystallinity:
= 100Where, = per cent crystallinity
= enthalpy of melting
=enthalpy of cold crystallization
= enthalpy of 100% crystalline sample of the polymer = 93 J/g. = mass fraction of polymer in the matrix.
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