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Chapter 2
Materials and Methods
Abstract
The characteristics of all the materials used in the study and the details of
the experimental techniques employed to get the final conclusion are presented
in this chapter. The methods of preparation of both TPEs and TPVs are
explained. The particulars of the analysis of phase morphology by scanning
electron microscopy and atomic force microscopy are incorporated. The
details of experimental setup and temperature programme for the spherulite
growth of PP in presence of NR and NBR using polarising microscope
coupled with hot-stage; the procedure for the study of bulk, fractionated and
self seeding crystallization; and measuring the Wide–angle X-ray scattering
(WAXS) pattern are included. Experimental setup for the isothermal and non-
isothermal crystallization kinetics has been discussed in detail. Information
about mechanical and dynamic mechanical tests, together with IR-strain
measurements for NBR/PP TPVs and nano-indentation for the NR/PP TPEs
are integrated.
88 Chapter 2
2. Experimental
2.1. Materials
Isotactic polypropylene, PP, (Koylene M3060) having melt flow index
(MFI) of 3g/10 min and density 0.905gm/cm3 at 23°C was kindly supplied by
Indian Petro Chemical Ltd, Vadodara, India. Natural rubber (NR, ISNR-5) and
epoxydised natural rubber (ENR25) were supplied by RRII, Kottayam, India.
Acrylonitrile co-butadiene rubber (Chemaprene N 3309) with acrylonitrile
content 34% was supplied by Synthetic and Chemicals, Bareli UP, India.
Hydroxy terminated natural rubber was synthesized in SCS, MGU. Exxon,
Germany provided the commercially available maleic anhydride
functionalized polypropylene, ExxelorTM PO 1020 with 0.5-1 wt% MA
content. Carboxylated polypropylene and NBR-MAPP graft copolymer
prepared from MA-PP and amine terminated NBR, (Hycar, Bayer, Germany)
were used for the compatibilization.
2.2. Methods: Melt Blending
NR/PP blends were prepared by melt mixing PP with NR in a Rheocord
at 180oC and 60 rpm. For the dynamic vulcanization, sulphur based curatives
were added to the blend in the mixing chamber where the vulcanization is
completed in 10 minutes.
NR/PP blends for the fractionated crystallization analysis, were prepared
by melt mixing PP with masticated NR in a Rheocord at 190°C and 60 rev/min
and to avoid the coalescence phenomena of the dispersed iPP phase, sulphur
based cure system was loaded in the blend using a two-roll mill and
vulcanized at 150°C (static vulcanization).
Materials and Methods 89
For the preparation of NBR/PP blends initially the components were
melt mixed in a Brabender Plasticoder PL 2000 at 65rev. min-1at 190°C for 4
min.
NBR/PP blends for the fractionated crystallization analysis, were
prepared by melt mixing PP with NBR in a Brabender Plasticoder at
65rev/min at 190°C for a mixing time of 6min and NBR phase was vulcanized
using sulphur based cure system. The compatibilizer were prepared at 180°C,
in the same brabender plasticoder, initially the MA-PP was melted and then low
molecular weight liquid NBR (Hycar ATBN X 16) is added and mixing
completed within 6min.
The compatibilizer were prepared at 180°C, in the same Brabender
Plasticoder. Initially the MA-PP had been melted and then low molecular
weight liquid NBR (Hycar ATBN X 16) is added and the mixing completed
within 6min.
For the preparation of dynamic vulcanizates initially the components
were melt mixed in a Brabender Plasticoder PL 2000 at 65rev. min-1 at 190°C.
Compatibilizer, curing agent and other additives were added following the
time-torque curve given in the fig 2.1. with the help of a computer attached to
the brabender. In Fig. 2.1, I, II, III and IV indicate order addition of each
additive and the corresponding time. (I-iPP, II-NBR, III-Compatibilizer, IV-
ZnO and Stearic Acid, V-Curing Agent, VI-Atioxidant) Mixing time was fixed
for 15min.
90 Chapter 2
0 2 4 6 8 10 12 14 160
2
4
6
8
10
12
14
16
18
VI
VIII
II
I
IV
Torq
ue (N
m)
Time (min)
Figure 2.1. Torque development during the dynamic vulcanization as a function of time.
Samples were hot pressed at 190°C under 70-bar pressure in a hydraulic
press. It is clear from the graph that at first the torque is high as we introduce
the iPP, when the temperature of the sample increases torque comes down.
Again it increases with the addition of NBR, and then the torque again comes
down. The compatibilizer is added and mixing continues till the torque levels
off. Then stearic acid and ZnO are added. After sometime torque again levels
off. Addition of curing agent leads to a substantial increase in the torque due
to the cross-linking of the rubber phase and thereby exerting grater resistance
to rotation. Thus the progress of the vulcanization can be conveniently
followed by monitoring torque during the mixing.
Materials and Methods 91
2.3. Analytical methods
2.3.1. Morphological characterization
The morphology of the NR/PP and NBR/PP blend was analyzed by
electron microscopy, using a SEM Philips microscope on cryogenically
fractured surfaces of the samples. Before the electron microscopy observation,
the surfaces were coated with Au–Pd alloy with a SEM coating device (SEM
Coating Unit). The SEM samples were prepared as follows: the strips cut from
compression-moulded samples were fractured in liquid nitrogen. NR and NBR
phases were removed by benzene and chloroform respectively. After etching
the phases, samples were dried in a vacuum oven at 70°C for 12 h and were
coated with gold using sputter coater. Then their SEM micrographs were
taken.
For the uncompatibilized and compatibilized blends micrographs were
obtained with average numbers and volume diameters (dn, dw and dv) of the
dispersed domains, using Image analysis software. The dispersity (D) of the
sizes obtained and the average number of particles/cm3 were also calculated
using the following equations.
dn = ∑ ∑ iii ndn / (2.1)
dw = iiii dndn∑ ∑/2 (2.2)
dv = iiii dndn 34 /∑ ∑ (2.3)
where ni is the number of droplets of ‘i’ with diameter di.
The polydispersity was evaluated with the equation D = dv/ dn
92 Chapter 2
The volume fraction of the dispersed phase was calculated from the following
relationship
Xv = (Xp/ρd)/[Xp/ρd+(1-Xp)/ρm] (2.4)
where Xp is the weight fraction of the minor phase and ρd is the dispersed
phase density.
The average particle number per cm3 was determined from
Nn = Xv 1cm3/[π/6(dn)3] (2.5)
The values calculated from the above equations could be used for
explaining the action of compatibilizer.
Atomic force microscopic examinations of the NBR/PP TPVs were
done using cryo-cut specimen with liquid nitrogen cooled diamond cutter.
Measurements were carried out in air at ambient conditions (25°C) with a
Nanoscope III Atomic Force Microscope, made by Digital Instruments Inc.,
USA. The experiments were carried out in tapping mode with constant
amplitude, using micro fabricated cantilevers. The scanning was done using
Olympus – Tapping Mode Etched Silicon Probe with square pyramid in shape.
The characteristics of the probe are: Force constant (K), 42 N/m; Nominal tip
radius of curvature less than 10 nm; cantilever length having a length of 160
µm and a tip height of 10 µm. Cantilever configuration: Rectangular Substrate
fits standard cantilever holder; Reflective coating with Aluminium; Tip half
angle of 17° on each side, 0° on front and 35° back. Images were analysed
using a Nanoscope imaging processing software.
Materials and Methods 93
2.3.2. Thermal analysis: fractionated crystallization
A TA DSC-2920 Instrument was used to determine the fractionated
crystallization behavior of PP and NR/PP blends. The following conditions
were applied for the study of the confirmed crystallization behavior of PP/NR
blends.
Stage 1. Heating of the sample in the calorimeter at a rate of 10°C/min from –
20 to 220°C.
Stage 2. Isothermal annealing at 220°C for 5 mins; this was done to remove
any crystalline nuclei if present.
Stage 3. Cooling of the sample in the calorimeter to -20°C at a rate of
10°C/min
Stage 4. Heating the sample in the calorimeter again to 220°C at a rate of
10°C/min.
From the heating and cooling curves the melting and crystallization
parameters were estimated.
Mettlor Toledo DSC 820 system was used to analyze the thermal
behavior of the following NBR/PP blends having compositions 100/00, 98/02,
95/05, 90/10, 85/15 and 80/20 in non-isothermal mode. Procedure adopted
was similar to the case of PP/NR TPEs.
2.3.3. Self-nucleation experiments
Successive self-annealing experiments were performed to initiate self-
nucleation in the NR/PP blend system. These experiments were also
performed using TA DSC (2920) Instrument. A procedure similar to the one
reported elsewhere [1] was adopted in theses experiments. In the beginning of
94 Chapter 2
the self-seeding techniques, the following steps were taken for erasing of
thermal history and creating an initial standard state of the material. Samples
were first heated to 220ºC at a heating rate of 10ºC /min and kept there for 10
minutes isothermally. Subsequently the samples were cooled to -20ºC at a
cooling rate of 10ºC/min. In the next step, the samples were heated to 165ºC
(just above the melting temperature of iPP). Then the samples were annealed
for five minutes to generate few crystals which are temperature resistant at that
temperature. Then the samples were cooled to -20ºC, at a cooling rate of
10ºC/min and kept it isothermally for 5 min. In the final step the samples were
heated to 160ºC and annealed at this temperature for 5 min for creating more
crystals and then cooled to -20ºC and kept for 5 min.
Mettlor Toledo DSC 820 system was used to analyze the thermal
behavior of the PP/NBR TPEs. Procedure adopted was similar to the case of
PP/NR TPEs.
2.3.4. Isothermal crystallization kinetics
A TA Instrument DSC 2920 apparatus was used to follow the
crystallization kinetics of PP and PP/NR blends. Baseline calibration,
calibration for the temperature scale and cell constant were carried out using a
pure indium standard (TºM= 156.6ºC) according to the procedure suggested
by TA Instrument in the instrument manual. To minimise the thermal lag
between polymer sample and DSC furnace, each sample pan was loaded with
a microtome cut piece from compression-moulded sheets of the blend having
the same thickness. Sample mass was kept around 10mg. Also for each
measurement new specimens were used and the specimen compartment was
flushed with dry N2 50ml/min to avoid any kind of oxidative degradation. The
Materials and Methods 95
following conditions were applied for the study of the melting and
crystallization behavior of NR/PP blends.
Stage 1. Heating of the sample in the calorimeter at a rate of 20°C/min from
–20 to 220°C
Stage 2. Isothermal holding segment at 220°C for 5 mins; this was done to
remove any crystalline nuclei if present.
Stage 3. Cooling of the sample in the calorimeter to Tc at a rate of
20°C/min and kept there for 60 min.
Stage 4. Heating the sample in the calorimeter again to 220°C at a rate of
20°C/min.
From the exothermic curves, parameters of the crystallization kinetics
were estimated.
2.3.4.1. Kinetic model- Avrami
The model proposed by Avrami is based on the concept that
crystallization process is usually treated as a series of two-stage process: the
primary and the secondary crystallization stage. The crystallization process is
labeled by temperature dependence. The heat evolved during isothermal
crystallization (∆Hc) and the crystalline conversion (Xt) at constant
temperature is recorded as a function of time. Crystalline conversion can be
calculated using the eq. 2.6 [3-6]
0
0
( / )
( / )
t
tt
dH dtQXQ
dH dt dt∞
∞
= =∫
∫ (2.6)
In this equation, dH/dt is the rate of heat evolution during crystallization,
96 Chapter 2
Q∞ is the total heat evolution at the end of the crystallization and tQ is
the total quantity of heat evolved up to a time ‘t’. The volume fraction of the
crystalline material could be computed based on the following eq.
( ) 1 exp( )ntX Kt= − − (2.7)
X(t) is the volume fraction of the crystalline material at time ‘t’ and
isothermal crystallization temperature T. ‘n’ is the Avrami exponent related to
the nucleation type and crystal growth geometry. The crystallization rate
coefficient, K is a parameter of crystallization growth rate and is related to the
nucleation type, crystal growth geometry and crystallization temperature.
Based on these parameters, the crystallization half time, t1/2 which is a measure
of crystallization rate can be calculated from the following equation:
1
1/ 2 (ln 2 / ) nt k= (2.8)
2.3.4.2. Tobin’s model
While Avrami model is appropriate for the initial stages of
crystallization, good prediction can be obtained only up to 30% relative
crystallinity, since it does not consider the impingement of spherulites. Tobin
[7-9] proposed a theory of phase transformation kinetics with growth site
impingement to describe the isothermal crystallization process of polymers.
According to this approach, the equation of phase transition is explained on
the basis of Tobin exponent [10].
According to Tobin,
1
t
t
nt
t nt
k tXk t
=+
(2.9)
Materials and Methods 97
Strictly speaking, Xt is the relative crystallinity as a function of time, kt
is the Tobin crystallization rate constant, and nt is the Tobin exponent. Based
on this proposition, the Tobin exponent nt does not need to be integral, since it
is controlled directly by different types of nucleation and growth mechanism.
Curve fitting method could be followed to calculate the parameters of Tobin
crystallization kinetics.
2.3.5. Non-isothermal crystallization kinetics
The following conditions were applied for the study of the melting and
crystallization behavior of PP/NR blends.
Stage 1. Heating of the sample in the calorimeter at a rate of 20 K/min from
–20 °C to 220 °C.
Stage 2. Isothermal annealing at 220°C for 5 min; this was done to remove
any crystalline nuclei if present.
Stage 3. Cooling of the sample in the calorimeter to 25ºC at various cooling
rates.
Stage 4. Heating the sample in the calorimeter again to 220°C at a rate of
20 K/min.
Each sample was measured at least two times.
From the exothermic curves, parameters of the crystallization kinetics
were estimated.
Mettlor Toledo DSC 820 system was used to analyze the dynamic
crystallization behavior of the PP and PP/NBR TPVs in non-isothermal mode.
To minimize the thermal lag between polymer sample and DSC furnace, each
sample pan was loaded with a cut piece from compression-molded sheets of
98 Chapter 2
the blend having the same thickness. Samples of about 10mg were taken in
aluminium sample pan and the measurements were done in nitrogen
atmosphere. Each sample was measured at least two times. The samples were
initially scanned in the temperature interval from 25 to 220oC, at a heating rate
of 10 K/min then kept for 10min at 220oC to destroy the thermal prehistory of
the samples. The samples were then cooled down at a cooling rate of 12 K/min
to 25ºC. The samples were kept at 25ºC for five minutes and then heated to
220ºC at a heating rate of 10K/min. In the coming cooling stages, various
cooling rates were (10, 8, 6 and 4 K/min) applied. The temperature for melting
(220ºC) was chosen to be much higher than the melting temperature (Tm) of
the neat PP under study. The melting at this maximum temperature leaves only
temperature resistant heterogeneous nuclei of unknown nature. From the
exothermic curves, parameters of the crystallization kinetics were estimated.
From the heating and cooling curves the melting and crystallization
parameters were estimated. These include,
a) Onset of melting (Tmonset)
b) Melting point (Tm)
c) Onset of crystallization (Tconset)
d) Crystallization temparature (Tc)
e) Endset of crystallization (Tcednset)
f) Normalized value of heat of fusion (∆Hf)
g) Normalized value of heat of crystallization (∆Hc)
h) Percentage crystallinity Xc
The percentage crystallinity was calculated using the expression,
Materials and Methods 99
% Crystallinity = ∆Hf x 100/∆Hof (2.10)
Here ∆Hf was the heat of fusion of the sample and ∆Hs° is that of 100%
pure crystalline PP, which is taken as 209J/g. [11,12]
2.3.5.1. Ozawa method-Theoretical background
The most common approach used to describe the overall isothermal
crystallization kinetics is the Avrami model [3,13,24] in which the relative
crystallinity as a function of time θ(t) can be expressed as
( ) 1 exp ( ) anat K tθ = − − (2.11)
where Ka and na are the Avrami crystallization rate constant and the
Avrami exponent, respectively. The Avrami rate constant Ka is written in the
form of the composite Avrami rate constant Ka (i.e. Ka = Kan). It should be
noted that both Ka and na are constants, specific to a given crystalline
morphology and type of nucleation for a particular crystallization condition
[15]. Based on the original assumptions of the theory, the value of the Avrami
exponent na should be an integer ranging from 1 to 4.
In the study of non-isothernal crystallization using DSC, the energy
released during the crystallization process appears to be a function of
temperature rather than time as in the case of isothermal crystallization. As a
result, the relative crystallinity as a function of temperature θ(T) can be
formulated as
0
( / )( )
T
cT
c
dH dT dTT
Hθ =
∆
∫ (2.12)
where Tο and T represent the onset and an arbitrary temperatures respectively,
100 Chapter 2
dHC is the enthalpy of crystallization released during an infinitesimal
temperature range dT, and ∆HC is the overall enthalpy of crystallization for a
specific cooling condition.
To use Eq.(12) for the analysis of non-isothermal crystallization data
obtained by DSC, It must be assumed that the sample experience the same
thermal history as designated by the DSC furnace. This may be realized only
when the thermal lag between the sample and the furnace is kept minimal. If
this assumption is valid, the relation between the crystallization time t and the
sample temperature T can be formulated as
0T Ttφ−
= (2.13)
where φ is the cooling rate. According to the equation (13), the horizontal
temperature axis observed in a DSC thermogram for a non-isothermal
crystallization data can readily be transformed into the time scale. Based on
the mathematical derivation of Evans [6], Ozawa [16] extended the Avrami
theory [3-5] to describe the non-isothermal crystallization data.
Mathematically, the relative crystallinity function of temperature θ(T) can be
represented as a function of cooling rate as
0
0( ) 1 exp n
Ktθφ
− = −
(2.14)
where KO is the Ozawa crystallization rate function, and nO is the Ozawa
exponent. It should be noted that the Ozawa kinetic parameters (ie., KO and nO)
hold similar physical meaning that of the Avrami.
Materials and Methods 101
2.3.6. WAXD analysis
Wide–angle X-ray diffraction (WAXD) measurements were performed
in the transmission mode with a Krystalloflex K70 (Siemens) instrument. The
CuKα radiation was filtered using Ni filter. The measurements were done in
the 2θ range of 5 to 50o with an interval of 0.05o. Cryotomed samples having
dimensions of 4x2x0.1 mm were used for this study. Thin samples were
isothermally crystallized at 124ºC using the same temperature programme
adopted in DSC measurements. Following blends of PP/NR having
compositions 100/00, 95/10, 90/10, 80/20, 70/30, 50/50 and 50/50 D were
used for the X-ray measurements. The background scattering corresponding to
an empty sample holder was subtracted from the experimental curves.
For the NBR/PP TPEs (minor amount of PP) samples having dimensions
of 40x20x1 mm were used for X-ray diffraction study.
The degree of crystallinity Xc.WAXD = Area of the six distinct crystalline
peaks at 2θ angles 14.05o, 16.85o, 18.45o, 21.6o and 28.65o/ total area under the
normalized WAXD curve including that under the amorphous halo. [17,18]
2.3.7. Hot stage-polarizing optical microscopy
The morphology and the isothermal spherulite growth rate were studied
by using a Zeiss Axioscope polarizing optical microscope, fitted with a
Linkam hot stage. The radial growth rate, G. dr/dt (where r is the radius of the
spherulites and t the time), was calculated by measuring the size of iPP
spherulites during the isothermal crystallization process. The standard
procedure adopted was given below. Thin films having a thickness of 30µm
were trimmed using a Mikrom microtome. Then the thin film was sandwiched
102 Chapter 2
between two microscope slides, heated at 220°C for 10 min, then cooled to Tc
and allowed to crystallize, entire procedure were done in dry nitrogen
atmosphere. The radial growth rate of a spherulite was monitored during
crystallization by taking photomicrographs automatically at appropriate
intervals of time with cross-polarized light. From the plots of r against the
time, G was calculated as the slope of the resulting straight lines.
In the case of NBR/PP TPEs, the optical hot-stage experiments were
conducted with a Leica hot-stage system with programmed temperature
controller and cooling system. The hot stage was mounted on a Leica DMLP
microscopy system equipped with a 20x (magnification 200 times) objective
and Carton CCD camera (http://www.carton-opt.co.jp.). Other experimental
conditions were similar to the case of NR/PP blend.
2.3.8. Tensile testing
Tensile measurements were performed in accordance with ISO R527
[19] at room temperature on an Zwick tensile tester with a crosshead speed of
20 mm/min. All samples were tested to stress at break. The Young’s modulus
(Ey) is determined from the initial slope of the stress–strain curve while the
tensile strength and elongation at break were calculated from the curve.
2.3.9. Dynamic mechanical analysis
The glass transition temperatures of the TPEs and the TPVs from NR/PP
blend were determined by dynamic mechanical analysis (DMA). Experiments
were performed in a TA 820 DMA, in tensile mode, at a frequency of 1 Hz.
Samples were heated from -80 to 80°C at a rate of 2°C/min. The peak
temperatures in the loss modulus were taken to define operational mechanical
glass transition temperatures. Thin sheets of NR/PP blend were prepared using
hot press, they heated up to 180°C at a heating rate of 10°C and compression
Materials and Methods 103
moulded at 180°C and cooled to 25°C. Rectangular thin film specimens having
3mm×30mm were cut from the moulded sheet and analyzed under tensile mode
in the DMA at 10Hz. Thermal procedures adopted were given below.
Step 1. Cooling to -80°C using liquid nitrogen.
Step 2. Isothermal for 10min at -80°C.
Step 3. Heating to 600C at a heating rate of 2°C/min.
The dynamic mechanical behavior of NBR/PP blends and dynamic
vulcanizates was studied using a rheometer of model ARES N2 Rheometric
Scientific under torsional mode. The testing temperature ranged from -100°C
to 150°C and the experiment was carried out at 10Hz at 2°C/min.
2.3.10. Nano-indentation
For NR/PP TPEs Instrumented microhardness tests were done using a
Nanoindenter XP, MTS Systems Inc., equipped with a diamond Berkovich
pyramide. The specimens, produced using a microscopic hot stage at a cooling
rate of 10K/min, had a thickness of 100 µm and were glued on metallic sample
holders. The indentation depth was 3 µm, the indentation rate 0.15µm/s. After
a holding time of 30s at maximum load to reduce creep effects, the specimens
were unloaded. The indentation hardness HIT was calculated at the maximum
load Fmax with the aid of the projected contact area Ap.. Accordingly [20,21]:
p
maxIT A
FH = (2.15)
If the deformation of the diamond indenter is neglected, the indentation
modulus EIT can be calculated from the unloading segment according to
[20,21]: the following equation.
104 Chapter 2
)1(A2
SE 2
p
IT ν−⋅π
= (2.16)
where ‘S’ is the contact stiffness dF/dh at maximum load. Because the
exact Poisson's number ν of the materials were unknown, the values of ν were
calculated using a simple rule of mixture. 0.38 and 0.5 are the values of PP
and the NR components respectively. Note, that value of ν is squared and so a
possible difference between the calculated and the real values has a relatively
small influence on the determined indentation modulus.
2.3.10. Infrared Strain Measurements
Films having thickness of 50±3µm were compression moulded at 190oC.
Samples having dimensions 20×10mm were cut for the spectroscopic analysis.
Figure 2.2. Instrumental setup for the infrared strain measurements.
FTIR measurements were carried out on a Perkin-Elmer FTIR
spectrometer S2000 and the experimental setup is shown in fig 2.2. The
Materials and Methods 105
spectrum examined covers the range between 585 to 3500cm-1 with a
resolution of 2 cm-1 and 16 spectra were taken during each measurement. The
duration of such a measurement was about 30s. The incident radiation area has
a diameter of 5 mm. The measurements were carried out with and without the
polarizer adjusted alternately parallel and perpendicular to the draw direction.
A stretching machine which allows for uniaxial stretching was placed into the
sample compartment of the spectrometer. In the tensile test the extension was
carried out incrementally in steps of 50% at room temperature. At the applied
strain, the specimen was held for 2 min to relax before IR measurements
started.
106 Chapter 2
2.4. References
1. Fillon B, Wittman JC, Lotz B, Thierry, A, J. Polym. Sci. B, 1993: 31; 1383.
2. Young RJ, Lovell PA, Introduction to Polymers, London: Chapman and
Hall; 1991.
3. Avrami M, J. Chem. Phys. 1939: 7; 1103.
4. Avrami M, J. Chem. Phys. 1940: 8; 212.
5. Avrami M, J. Chem. Phys. 1941: 9; 177.
6. Evans UR, Trans Faraday Soc. 1945: 41; 365.
7. Tobin MC, J. Polym. Sci., Polym. Phys. 1974 : 12; 399.
8. Tobin MC, J. Polym. Sci, Polym. Phys. 1976 : 14; 2253.
9. Tobin MC, J. Polym. Sci., Polym. Phys. 1977 : 15 ; 2269.
10. Young RJ and Lovel PA, Introduction to Polymers, Chapman and Hall,
London, 1991.
11. Patric S. Dai, Peggy Cebe, Malcolm Capel, Polym. 2002: 40; 1644.
12. Ana LN, Da Silva, Marisa CG. Rocha, Lea Lopes, Beatriz S. Chagas,
Fernanda MB. Coutinho. 2001: 81; 3530.
13. Avrami M, J. Chem. Phys. 1940 : 8; 212.
14. Avrami M, J. Chem. Phys. 1941: 9; 177.
15. Wunderlich B, Macromolecular Physics, Academic Press, New York, 1976:
2,5; 139.
16. Ozawa T, Polym. 1971: 12; 150.
17. Rabiej S, Eur. Polym. J. 1991: 27; 947.
Materials and Methods 107
18. Young RJ, Lovell PA, Introduction to Polymers, London: Chapman and
Hall; 1991.
19. ISO 2000: 14577.
20. Oliver WC. and Pharr GM, J. Mater. Res. 1992: 7; 1564.
21. ISO 2002: 14577.