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A new approach for mechanisms of ferroelectric crystalline phase formation in PVDF nanocomposites

15 May 2014-Physical Chemistry Chemical Physics (The Royal Society of Chemistry)-Vol. 16, Iss: 22, pp 10679-10687

TL;DR: Using time-resolved Fourier transform infrared spectroscopy, a new mechanism for ferroelectric polymorph formation of poly(vinylidene fluoride) (PVDF) nanocomposites is proposed and different crystalline polymorph formation should be inferred as a transition in the reeling-in rate dependence of the friction coefficient on nanocomPOSites rather than as a change in the relative rates of secondary nucleation and substrate completion.

AbstractThis paper proposes a new mechanism for ferroelectric polymorph formation of poly(vinylidene fluoride) (PVDF) nanocomposites. Utilizing time-resolved Fourier transform infrared spectroscopy (FTIR), the real-time investigation of the conformational changes of the PVDF chain segment during crystallization of neat PVDF and the corresponding nanocomposite was performed. Whilst PVDF–clay nanocomposites exhibited mainly the β crystal phase coexisting with the γ phase at low Tc (Tc 155 °C). Experimental results were compared with predictions of the Lauritzen and Hoffman (LH) model and discrepancies were observed between model predictions and experiments. We then recalled the Brochard-de Gennes (BD) model and proposed that different crystalline polymorph formation should be inferred as a transition in the reeling-in rate dependence of the friction coefficient on nanocomposites rather than as a change in the relative rates of secondary nucleation and substrate completion. Combining LH and BD models we proposed a new mechanism to answer the contradictory questions associated with nanocomposite polymorphism. The coexistence of different polymorphs in nanocomposites was proposed to be associated with the coexistence of fast and slow moving chains, which were recognized as the free and adsorbed chains by nanofillers.

Topics: Nucleation (51%), Crystallization (50%)

Summary (2 min read)

LH model

  • Real-time FTIR studies reveal the presence of both b and g phases in nanocomposites, where the b/g ratio increased with decreasing temperature.
  • To obtain Teqm , neat PVDF and nanocomposite melting points (Tm0) measured by DSC were plotted vs. crystallization temperature (Tc) and extrapolated to the line where Tm = Tc (Fig. 10).
  • Comparing Fig. 11a with Fig. 2 one can readily understand that the change in the slope of the LH plot is not related to regime I/II transition but it is more likely due to the different equilibrium melting points of b and g polymorphs.
  • Here the authors propose that different crystalline polymorph formation should be inferred as a transition in the reeling-in rate dependence of the friction coefficient rather than a change in the relative rates of secondary nucleation and substrate completion.

Brochard-de Gennes (BD) model

  • A fundamental understanding of the crystallization requires insight into how a chain deposits from the melt onto the crystal growth front.
  • Therefore, in this slow velocity regime, the friction coefficient is independent of velocity, but is strongly dependent on molecular weight.
  • Furthermore, in this regime, the tethered chain starts to disentangle which results the lower friction coefficient.
  • The friction in the marginal and stick regimes is mostly owing to the presence of entanglements.

Characterization

  • X-ray diffraction measurements were performed on a Panalytical XRD instrument.
  • Sections were imaged using a Gatan Orius SC1000 digital camera on a JEOL 2100 transmission electron microscope (TEM) operating at an accelerating voltage of 200 kV.
  • The mixtures were then pressed into disks with a thickness of B0.5 mm.
  • Each sample was kept at 210 1C for 10 min to erase any thermal history, before instantly cooling down to Tc after which a time-resolved FTIR measurement was conducted.

Morphology of nanocomposites

  • Fig. 5a presents the WAXD patterns of nanoclays and PVDF nanocomposites.
  • The appearance of this peak could be attributed to a partially collapsed structure resulting from quaternary ammonium degradation.
  • SEM and TEM images of nanocomposites are shown in Fig. 5b and c SEM images of nanocomposites demonstrate that clay tactoids are aligned in the flow direction.
  • From the TEM images one can easily understand that PVDF did not form exfoliated structure.

Polymorph formation

  • To better understand the mechanism and kinetics involved in skeletal and chain conformational changes, which are responsible for the formation of different polymorphs of PVDF, timeresolved FTIR as a powerful tool was used.
  • Real-time FTIR studies were conducted in the temperature range of 140–160 1C.
  • Fig. 6 displays typical time-dependent spectral variations of neat PVDF and the corresponding nanocomposite in the region of 1500–550 cm 1 during isothermal crystallization at 150 1C.
  • By subtracting the initial spectrum of the melt state (at 210 1C) from the consecutive spectra, a difference spectrum Pu bl is he d on 1 0 A pr il 20 14 .

Conclusion

  • Although nucleation effects of nanofillers are undeniable hypotheses proposed till now are unable to provide an explanation for the coexistence of different polymorphs as well as changes in the polymorph ratio with temperature.
  • Combining LH and BD models the authors proposed a new mechanism to answer the contradictory questions associated with nanocomposite polymorphism.
  • If g varies with undercooling by several orders of magnitude, V could fall into different velocity regimes for which the reeling-in rate dependence of the friction coefficient would be different which in turn causes transition from the marginal velocity regime to the Rouse velocity regime.
  • This change in the velocity regime is responsible for the change in the polymorph ratio.
  • Nevertheless, the coexistence of different polymorphs in nanocomposites is associated with the coexistence of fast and slow moving chains, which were recognized as the free and adsorbed chains by nanofillers.

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Thisisthepublishedversion:
Abolhasani,MohammadMahdi,Naebe,MinooandGuo,Qipeng2014,Anewapproachfor
mechanismsofferroelectriccrystallinephaseformationinPVDFnanocomposites,Physical
chemistrychemicalphysics,vol.16,no.22,pp.10679‐10678.
AvailablefromDeakinResearchOnline:
http://hdl.handle.net/10536/DRO/DU:30063732
Reproducedwiththekindpermissionofthecopyrightowner
Copyright:2014,RSCPublications

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©
the Owner Societies 2014
Phys. Chem. Chem. Phys.
Cite this: DOI: 10.1039/c4cp00031e
A new approach for mechanisms of ferroelectric
crystalline phase formation in
PVDF nanocomposites
Mohammad Mahdi Abolhasani,*
a
Minoo Naebe
b
and Qipeng Guo
b
This paper proposes a new mechanism for ferroelectric polymorph formation of poly(vinylidene fluoride)
(PVDF) nanocomposites. Utilizing time-resolved Fourier transform infrared spectroscopy (FTIR), the real-
time investigation of the conformational changes of the PVDF chain segment during crystallization of
neat PVDF and the corresponding nanocomposite was performed. Whilst PVDF–clay nanocomposites
exhibited mainly the b crystal phase coexisting with the g phase at low T
c
(T
c
o 155 1C), the coexistence
of g and b crystalline phases was found at a high T
c
temperature range (T
c
4 155 1C). Experimental
results were compared with predictions of the Lauritzen and Hoffman (LH) model and discrepancies
were observed between model predictions and experiments. We then recalled the Brochard-de Gennes
(BD) model and proposed that different crystalline polymorph formation should be inferred as a transi-
tion in the reeling-in rate dependence of the friction coefficient on nanocomposites rather than as a
change in the relative rates of secondary nucleation and substrate completion. Combining LH and BD
models we proposed a new mechanism to answer the contradictory questions associated with nano-
composite polymorphism. The coexistence of different polymorphs in nanocomposites was proposed to
be associated with the coexistence of fast and slow moving chains, which were recognized as the free
and adsorbed chains by nanofillers.
1. Introduction
In the last decade, ferroelectric crystalline polymorphs of
poly(vinylidene fluoride) (PVDF); b and g, have been widely explored
in PVDF based nanocomposites.
1–11
Priya and Jog
12–14
for the first
time showed the effectiveness of organically modified mont-
morillonite on induction of the b polymorph in PVDF film.
However, after Priya and Jog’s investigation, further aspects of this
phenomenon were studied by other research groups.
1–11,15–20
It has
been shown that addition of different types of nanofillers into
PVDF can lead to the coexistence of a, b and g crystalline phases.
Studies conducted by Giannelis et al.
19
and Ramasundaram et al.
20
have been largely used to explain the origin of ferroelectric phase
formation. According to Giannelis and Ramasundaram similar
crystal lattices between clay and the b polymorph; and the presence
of an ion–dipole interaction between exfoliated nanoclay layers and
PVDF chains in the molten state are likely to be responsible for the
formation of ferroelectric crystalline polymorphs in PVDF.
However, there are still many debates with regard to hypo-
theses mentioned above. Asai et al.
17,18
used a modified layered
titanate having different charge density and different crystal
lattice parameters in their study and found that these fillers
greatly contributed to the enhancement of the formation of
both g and b phase crystals. There are also some other reports
regarding the formation of ferroelectric crystalline polymorphs
of PVDF in the presence of nanotubes,
21
graphene
22
and ferrite
nanofillers.
23
All these fillers have different crystal lattice para-
meters and different interaction with PVDF; though, the for-
mation of g and b phase crystals in these nanocomposites
cannot be explained by Giannelis and Ramasundaram hypo-
theses. Furthermore, in this work as well as some other studies
it has been observed that crystallization at different tempera-
tures results in formations of different polymorphs for example
PVDF/HTO exhibited mainly a phase crystals coexisting with g
and b phases at a low T
c
range (110–135 1C) while a major g
phase crystal coexisting with b and a phases appeared at high
T
c
(140–150 1C).
18
None of the above-mentioned phenomena can completely
be explained by Giannelis and Ramasundaram hypotheses.
This has motivated us to reinvestigate the formation mecha-
nism of different polymorphs of PVDF by a new more mecha-
nistic approach. In this paper our aim is to answer two
questions i.e. why different polymorphs coexist in nanocompo-
sites? And why the ratio of these polymorphs changes with
a
Chemical Engineering Department, University of Kashan, Kashan, Iran.
E-mail: abolhas ani@kashanu.ac.ir
b
Institute for Frontier Materials, Deakin University, Australia
Received 3rd January 2014,
Accepted 9th April 2014
DOI: 10.1039/c4cp00031e
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temperature? To find answers for these two questions we
have used the well-known Lauritzen–Hoffmann (LH)
24–27
and
Brochard-de Gennes (BD) models.
28–34
2. Theory
LH model
Since a comprehensive explanation of the LH model is beyond
the scope of the present paper, we refer the interested readers
to the appropriate literature for a more complete overview.
27,35
Three regimes in the LH model are anticipated based on the
relative rates of surface nucleation (i) and substrate completion
(g). In regime I, (i) is much slower than (g). Thus, nucleation is
the rate-controlling step that results in the subsequent expres-
sion for the growth rate:
G
I
= b
0
iL (1)
where, b
0
is the thickness of a monomolecular layer and L is the
substrate length. In regime II, the nucleation rate (i) is compar-
able to the substrate completion rate (g) therefore multiple
nuclei compete to complete a new crystal layer. The growth rate
in regime II is determined by the following expression:
G
II
= b
0
(2ig)
1/2
(2)
when the temperature is further lowered, (i) becomes much
faster than (g). This means that there is inadequate space for
significant substrate completion. In this case, the growth rate is
again controlled by the nucleation rate, (i). This temperature
range is defined as regime III. Growth in regime III is char-
acterized by a rate given by:
G
III
= b
0
iL
0
(3)
where L
0
is the distance between niches and is only 1.5 to 2.5 a
0
,
where a
0
is the width of the stem. When discussing the LH
model it is important to note how Hoffman and Miller
27
applied the theory of forced reptation into the LH model. They
named the reeling-in rate of chain segments, r, as the ratio of
the undercooling-dependent crystallization force, f
c
, and the
friction coefficient, x,(r = f
c
/x). The substrate completion rate, g,
is relative to the reeling-in rate, g = r (a
0
/lg*), where lg * is the
initial lamellar thickness. They assumed that the friction
coefficient is independent of the reeling-in rate. Fig. 1 shows
the schematic representation of three different crystal growth
regimes proposed by Hoffmann et al.
35
Hoffman et al.
27,35
derived expressions for i and g, thus gave
the expression for the growth rate as follows:
G ¼ G
0
exp
DG
Z
RT
c

exp
DG
KT
c

(4)
where G
0
is a constant, R the gas constant, K Boltzmann
constant (1.38 10
16
erg K
1
), T
c
crystallization temperature
in K, DG
Z
the activation energy for the diffusion of the crystal-
lizing segment across the phase boundary, and DG* the free
energy of crystallization of the initial lamella. Considering
G = CK
n
1/n
for the overall crystallization rate, where C is a
constant and assuming s = 0.1 (DH
f
)(a
0
b
0
)
1/2
here DH
f
is the
heat of fusion per unit volume and s the free energy of the side
surface of the nucleus, the below equation is obtainable:
fK
n
ðÞ¼
1
n
ln K
n
ln f
2
þ
C
1
RC
2
þ T
c
T
g

0:2 T
eq
m
ln f
2
DT
¼ ln A
0
K
g
fT
c
DT
(5)
where C
1
and C
2
universal constants are 4120 cal mol
1
and
51.6 K, respectively, T
g
is the glass transition temperature in K,
k
g
is the nucleation factor, f the correction factor for heat of
fusion, T
eq
m
is the equilibrium melting temperature (in K) of the
blend and DT = T
m
T
c
. The schematic plot of f (K
n
) vs. 1/fT
c
DT
for three different regimes is shown in Fig. 2.
Brochard-de Gennes (BD) model
A fundamental understanding of the crystallization requires insight
into how a chain deposits from the melt onto the crystal growth
front. More specifically, one should consider the dependence of
Fig. 1 Schematic representation of three different crystal growth regimes.
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Phys. Chem. Chem. Phys.
the chain friction coefficient, x, of a tethered chain on the
reeling-in rate. De Gennes
28–32
discussed the related process of
pulling a chain by its head at a velocity V with a force F in a melt
of similar chains. A chain attached to a substrate can not
reptate freely and the motion of the tethered chain is restricted
by the wall. Three velocity regimes have been predicted theore-
tically and confirmed experimentally. Fig. 3 shows the sche-
matic representation of three different velocity regimes
proposed by de Gennes.
At low velocity, the polymer chain is weakly deformed. On
the time scale allowed for a melt chain to move an entangle-
ment length, the melt chain must travel along its tube to be
disentangled from the tethered chain.
29
They obtained the
following expression for the friction coefficient in this regime:
x =(N
4
/N
e
3
) x
r
(6)
where N is the number of repeat units for the melt chain, N
e
is
the number of repeat units between two entanglements and x
r
is the monomeric friction coefficient. Therefore, in this slow
velocity regime, the friction coefficient is independent of velo-
city, but is strongly dependent on molecular weight. This
regime is also called the stick regime due to the very large
friction coefficient.
With an increase in velocity, the tethered chain begins to
deform; this regime is called the marginal regime. The tethered
chain can be imagined as a trumpet made by a series of blobs
as shown in Fig. 3. One can visualize that the blob size D would
decrease with increasing velocity until it reaches D* that is the
distance between two entanglements. At this point the thres-
hold velocity V* is obtained. When V 4 V*, a constant force is
expected. The following equation for the marginal regime has
been proposed:
x = kT/(aN
e
1/2
V) (7)
Fig. 2 Schematic representation of three regimes in LH theory.
Fig. 3 Schematic of conformational change of a chain with increasing
velocity.
Fig. 4 Evolution of (a) friction force (F) and (b) the friction coefficient (x)
with velocity in different regimes. Please note that V p r p g. Therefore
this figure demonstrates the dependence of the reeling-in rate and the
substrate completion rate on the friction coefficient either.
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where a is the size of the repeat unit. Clearly, the friction
coefficient x declines with increasing velocity in this regime,
which shows the development of slippage. Furthermore, in this
regime, the tethered chain starts to disentangle which results
the lower friction coefficient.
The friction in the marginal and stick regimes is mostly
owing to the presence of entanglements. However, at high
enough velocity the Rouse friction, which always acts on every
monomer and has contribution to overall friction force, would
become dominant.
28,33
The regime at V 4 V
Rouse
is named the
Rouse regime wherein the corresponding friction coefficient is:
x
Rouse
= Nx
r
(8)
Fig. 4 shows the variation of the friction coefficient in three
different velocity regimes proposed by de Gennes.
3. Experimental
Materials and sample preparation
PVDF (Kynar 710) MFR of 25 g/10 min (2328C/12.5 kg load)
from Arkema was used in this work. Cloisite 30B is organically
modified clay with a cation exchange capacity of 90 meq/100 g,
supplied by Southern Clay. All components were dried in a
vacuum oven at 80 1C for at least 12 h before processing. The
nanocomposite with 5 wt% nanoclay was prepared using a
Brabender type plastic mixer with a two rotors at a rotation
speed of 100 rpm at 190 1 C for 15 min. Samples were then hot
pressed at 200 1C to a 200 mm thick film and allowed to slowly
cool down to room temperature.
Characterization
X-ray diffraction measurements were performed on a Panaly-
tical XRD instrument. The data were recorded in the range
of 2y1 = 2–101. Samples were scanned continuously with a
0.51 scan step and 1 s scan time.
The composite samples were sectioned using a Leica UC6
ultramicrotome with a FC6 cryochamber at 120 1C, at a nominal
thickness of 70 to 80 nm. Sections were imaged using a Gatan
Orius SC1000 digital camera on a JEOL 2100 transmission
electron microscope (TEM) operating at an accelerating voltage
of 200 kV.
Scanning electron microscopy (SEM) was performed using a
Leica S440 instrument. Samples were cryogenically fractured in
liquid nitrogen and sputter-coated with a thin layer of gold
before imaging.
Differential scanning calorimetry (DSC) was conducted
using a TA Instrument Q200. For measuring the equilibrium
melting point, neat PVDF and nanocomposites were melted at
210 1C for 10 min then each sample cooled down to desired
isothermal temperature and maintained at that temperature
until the degree of crystallinity was not increased any more.
After completion of isothermal crystallization the sample is
subsequently reheated to 210 1C at a rate of 20 1C min
1
to
obtain the melting endotherm curve.
FTIR spectra were collected at 2 cm
1
nominal resolution
using a Bruker 70 spectrometer in transmission mode. The
spectra were obtained by averaging 32 scans with a mean
collection length of 1 s per spectrum. The background spectra
at the same crystallization temperature (T
c
) as the sample were
collected and used for reduction. The homogenous mixtures of
KBr powder and PVDF or nanocomposites (powder) in the
weight ratio of 95 : 5 were prepared. The mixtures were then
pressed into disks with a thickness of B0.5 mm. The disks were
placed in a custom made heating chamber, which allowed
reaching the desired T
c
in a short time. Each sample was kept
at 210 1C for 10 min to erase any thermal history, before
instantly cooling down to T
c
after which a time-resolved FTIR
measurement was conducted.
4. Results
Morphology of nanocomposites
Fig. 5a presents the WAXD patterns of nanoclays and PVDF
nanocomposites. The cloisite 30B has a d-spacing of 1.8 nm,
evidenced by the XRD peak at 2y–4.81. In the nanocomposites
containing 5 wt% clay, this peak is shifted towards the left
(lower angles), resulting in a diffused peak at 2y–2.51, corre-
sponding to a d-spacing of 3.4 nm. This suggests that the clay
forms an intercalated nanocomposite structure. This type of
structure is formed due to the interaction between the modified
clay and PVDF or because of shear induced intercalation. The
peak at 2y–5.81 corresponding to the d-spacing 1.4 nm could be
due to the second order diffraction d(002).
36
The appearance of
this peak could be attributed to a partially collapsed structure
resulting from quaternary ammonium degradation.
SEM and TEM images of nanocomposites are shown in
Fig. 5b and c SEM images of nanocomposites demonstrate that
clay tactoids are aligned in the flow direction. It can be seen
that the clay tactoids are dispersed uniformly into the PVDF
matrix. The thickness and length of the clay tactoids are found
to be in the range of 50–150 nm, respectively. From the TEM
images one can easily understand that PVDF did not form
exfoliated structure.
Polymorph formation
Herein we intend to mainly focus on mechanisms of ferro-
electric polymorph formation in PVDF–clay nanocomposites.
To better understand the mechanism and kinetics involved in
skeletal and chain conformational changes, which are respon-
sible for the formation of different polymorphs of PVDF, time-
resolved FTIR as a powerful tool was used. Real-time FTIR
studies were conducted in the temperature range of 140–160 1C.
Fig. 6 displays typical time-dependent spectral variations of
neat PVDF and the corresponding nanocomposite in the region
of 1500–550 cm
1
during isothermal crystallization at 150 1C.
The frequencies and the vibrational assignments for a, g and b
phases are 763, 811 and 1273 cm
1
, respectively.
20
By subtracting the initial spectrum of the melt state (at
210 1C) from the consecutive spectra, a difference spectrum
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Journal ArticleDOI
01 Jan 1997-Polymer
Abstract: The rate of growth of chain-folded lamellar crystals from the subcooled melt of polyethylene fractions is treated in terms of surface nucleation theory with the objective of illuminating the origin of the chain folding phenomenon and associated kinetic effects in molecular terms. An updated version of flux-based nucleation theory in readily usable form is outlined that deals with the nature of polymer chains in more detail than previous treatments. The subjects covered include: (i) the origin of regimes I, II, III, and III-A and the associated crystal growth rates, including the effect of forced steady-state reptation and reptation of ‘slack’ in the subcooled melt; (ii) the variation of the initial lamellar thickness with undercooling; (iii) the origin of the fold surface free energy σ e and the lateral surface free energy σ; (iv) the generation and effect of nonadjacent events (such as tie chains) on the crystallinity and growth rates; and (v) ‘quantized’ chain folding at low molecular weight. The topological limitation on nonadjacent re-entry and the value of the apportionment factor ψ are discussed. Key experimental data are analysed in terms of the theory and essential parameters determined, including the size of the substrate length L involved in regime I growth. The degree of adjacent and/or ‘tight’ folding that obtains in the kinetically-induced lamellar structures is treated as being a function of molecular weight and undercooling. New evidence based on the quantization effect indicates a high degree of adjacent re-entry in regime I for the lower molecular weight fractions. The quality of the chain folding at higher molecular weights in the various regimes is discussed in terms of kinetic, neutron scattering, i.r., and other evidence. Application of the theory to other polymers is discussed briefly.

895 citations


Journal ArticleDOI
Abstract: the kinetics of polystyrene melt intercalation in organically modified mica-type silicates were studied using X-ray diffraction and transmission electron microscopy. By monitoring the change in the integrated intensity of the basal reflection of the silicate host, the rate of conversion from unintercalated to intercalated silicate was determined at various temperatures and for various molecular weights of polystyrene. Hybrid formation is limited by mass transport into the primary particles of the host silicate and not specifically by diffusion of the polymer chains within the silicate galleries. The activation energy of hybrid formation is similar to that previously measured for polystyrene self-diffusion in the melt, implying that the mobility of the polymer chains within the host galleries is at least comparable to that in the melt.

629 citations


Journal ArticleDOI
01 Dec 1992-Langmuir
Abstract: We discuss shear flows of a polymer melt near a solid surface onto which a few chains (chemically identical to the melt) have been grafted. At low shear rates a a*. This transition may be important in the processing of polymers, where a few chains from the melt can be bound on an extruder wall and play the role of the grafted chains.

454 citations