Morphological characterization of the novel fine structure of the
PMMA/PVDF blend

著者

Akihiro Naruke, Xiaobin Liang, Ken Nakajima, Toshio Nishi

journal or publication title

Polymer Journal

volume54

page range

783–792

year

03 March 2022

出版社

Springer Nature

権利This version of the article has been accepted for
publication, after peer review (when applicable) and issubject to Springer Natureʼs AM terms of use, but is not
the Version of Record and does not reflect postacceptance improvements, or any corrections. The
Version of Record is available online at: http://dx.doi.org/10.1038/s41428-022-00625-z

URL

https://research.konicaminolta.com/jp/report/
doi: /10.1038/s41428-022-00625-z

Polymer Journal
https://doi.org/10.1038/s41428-022-00625-z

ORIGINAL ARTICLE

Morphological characterization of the novel ﬁne structure of the
PMMA/PVDF blend
Akihiro Naruke1,2 Xiaobin Liang1 Ken Nakajima
●

●

1

●

Toshio Nishi31234567890();,:

1234567890();,:

Received: 2 December 2021 / Revised: 7 January 2022 / Accepted: 15 January 2022
© The Society of Polymer Science, Japan 2022

AbstractPoly(methyl methacrylate) (PMMA)/Poly(vinylidene ﬂuoride) (PVDF) blends are well known as polymer pairs with uniqueand complex properties. Using low voltage scanning transmission electron microscopy (LV-STEM), we found that there
were previously unseen nanoscale structures inside them. The heterogeneous structures in the blends were formed on
the same scale that did not deviate from the size of a single molecular chain, regardless of the solution casting, melt mixing,
mixing ratios, and even amorphous or crystalline state of PVDF. The characteristics of these structures were discussed inaccordance with thermophysical properties and intermolecular interaction properties, which provides the following new
ﬁndings for the ﬁrst time. First, the mixing ratio-dependent melting point depression exhibited by PMMA/PVDF blends isdue to the size of their interface region. Second, the complex thermophysical behavior of the solution-casting blends iscaused by the presence of ultraﬁne PVDF crystals. Finally, LV-STEM visualized the molecular miscibility of PMMA and
PVDF in the melt-mixing samples.

Introduction
Polymer blends, in which crystalline polymers are added toamorphous polymer matrices, have been applied to various
products because of their ability to impart a sharp change inphysical properties due to the phase transition of the crystalline polymers while maintaining the high dimensional
stability of the amorphous polymer matrices. Here, physicalproperties mean, for example, viscosity change due to heat.
One example among industrial products is the toner used in
multifunction printers. The purpose of adding crystalline

Supplementary information The online version contains
supplementary material available at https://doi.org/10.1038/s41428022-00625-z.
* Ken Nakajima
nakajima.k.aa@m.titech.ac.jp
1

School of Materials and Chemical Technology, Tokyo Institute ofTechnology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552,
Japan

2

Advanced Technology Center, Corporate R&D Headquarters,
KONICA MINOLTA, INC., No.1 Sakura-machi, Hino-shi, Tokyo
191-8511, Japan

3

Tokyo Institute of Technology, Meguro-Ku, Tokyo 152-8552,
Japan

polymers is to melt the toner in a short period of time and to
ﬁx it to the paper, thereby achieving excellent energysaving effects [1]. To spread the effect of the added crystalline polymer throughout the blend, crystalline polymers
should be ﬁnely dispersed in the amorphous polymer matrix
on the order of nanometers. Therefore, for appropriatematerial design, it is preferable to use polymer pairs thatexhibit miscibility in the molten state. In other words, weshould aim to design materials in such a way that a compatible solid-state is derived from a miscible molten state.
On the other hand, when the crystalline polymer is aminor component in such miscible crystalline/amorphous
polymer blends, the behavior of the crystalline polymer isextremely unstable, which makes stable manufacture andstorage of products difﬁcult. The low stability of such
blends is mainly due to the following two factors. One is thechange in the state of crystalline polymers, including theirpresence in an amorphous state. Another is the melting
temperature of the crystalline polymer, which considerably
varies depending on the mixing ratio. In the past and currently, these compounds have been frequently studied using
poly(methyl methacrylate) (PMMA) and poly(vinylidene
ﬂuoride) (PVDF) blends [2–5]. For example, it is wellknown that PVDF exists in an amorphous state in PMMA/PVDF blends obtained by melt mixing when the ratio of
PVDF is less than 50 weight% [2]. It is also known that theA. Naruke et al.

melting point of PVDF is considerably lower than that ofpure PVDF in blends where PVDF is fully crystallized bycast blending and that the degree of melting point depression depends on the ratio of PVDF [3]. In addition, thecompatibility and/or miscibility of the two polymer pairs, aswell as the crystallization behavior of PVDF, depend on thethree crystal forms (α, β, γ) of PVDF and the difference inthe tacticity of PMMA [6–8].
These peculiar behaviors of PMMA/PVDF blends havebeen analyzed using various techniques, including the analysis of the crystalline state of PVDF using thermal analysis
and X-ray diffraction measurements, morphological considerations using optical microscopy, and interpretation of therheological properties of blends [9–15]. In addition to theseevaluation methods, analysis from a detailed perspective has
been actively performed. It is difﬁcult to interpret PMMA/
PVDF, which is believed to exhibit molecular miscibility,
only by macroscopic analysis, which inevitably requires
analysis and discussion at the molecular scale. Such evaluation methods are mainly related to the intermolecular interactions between PMMA and PVDF, and the following three
are examples. The ﬁrst is the evaluation of molecular mobility
by measuring the relaxation time using pulsed nuclear magnetic resonance [16, 17]. The second is the analysis of thehydrogen bonding between the carbonyl group of PMMA andpolar units of PVDF by Fourier transform infrared spectroscopy (FT-IR) [18–20]. The third is the evaluation of themobility of the PMMA side chain by dielectric relaxation[21, 22]. For morphological evaluation, which is important forinterpreting phenomena, only macro- to microscale observation methods have been used. This refers, for example, to theobservation of spherulites by polarized optical microscopy[10] or by scanning electron microscopy [23]. These
approaches are used because most of the previous studies
have focused on PVDF-rich blends, in which PVDF crystallizes relatively stably. Therefore, PVDF formed a large
spherulitic structure in the blends, and morphological analysis
was sufﬁciently conducted by the abovementioned microscopes. However, based on the abovementioned background,
focusing on PVDF-poor blends where PVDF is unstable andmacroscopic spherulites cannot be identiﬁed, morphologicalanalysis at the nanoscale becomes necessary. However, few
such studies have been conducted to date.
We believe that one of the factors governing the complex
behavior of PMMA/PVDF blends is the internal structure at
the nanoscale, which has not been observed thus far, and oneof the objectives of this study is to visualize it. Polymer blends
that exhibit miscibility, such as PMMA/PVDF, form solid
blends from molecularly mixed states. A variety of state-ofthe-art microscopes are candidates for observing nanoscale
microstructures. Among them, we chose electron microscopy,
especially low voltage scanning transmission electron microscopy (LV-STEM). LV-STEM, due to its low acceleration

voltage, can provide imaging by thermal diffuse scattering,which is an advantage for contrast generation between lightelements [24]. Additionally, it is not necessary to use pretreatments such as heavy metal staining or removing one
material by dry or wet etching, which sometimes destroy theoriginal structures of the samples. Moreover, knock-on damage
caused by a high acceleration voltage, which is often a concern
with ﬂuoropolymers such as PVDF, can be reduced.
In this study, LV-STEM was used to observe the internal
structures of PMMA/PVDF blends for the ﬁrst time. The
obtained images were discussed in relation to the physicalproperties of the blends. Speciﬁcally, the melting point
depression of PVDF crystals in PMMA/PVDF blends,
which has been extensively studied in the past, was discussed based on these internal structures. Moreover, we also
added new insights into the complex melting behaviors of
the blends, which cannot be expressed only by the melting
point depression. These were also explained as the intermolecular interactions between PMMA and PVDF.

Materials and methods/experimental
procedure
Materials and sample preparation
PVDF (average Mw ~120,000) and PMMA (average Mw~534,000) in pellet form were purchased from Merck
KGaA, Germany. Throughout this paper, the ratio of PVDF
in the blend is reported on a weight basis. The solutioncasting PMMA/PVDF blends in ﬁlm form were cast from
N,N-dimethylformamide (DMF) solution onto a glass surface at room temperature. The initial concentration of eachpolymer in DMF was ~3 g/100 mL. After the mixtures were
cast, DMF was allowed to evaporate very slowly under astream of air at reduced pressure (34 kPa) for at least
1 week. The resulting ﬁlms were subjected to further drying
under vacuum at room temperature for 1 week as previously
reported [3]. The melt-mixing PMMA/PVDF blends were
produced with a twin-screw kneader using DSM Xplore,
Netherlands Model MC 5 at 200 °C. Considering that thestate of the blend, especially the amorphous-crystalline
states of PVDF, can be changed depending on the coolingconditions, the melt-mixed strands were cooled by leaving
at room temperature or quenching with ice water.

Measurement of thermal properties
Thermal analysis by DSC was performed using Hitachi
High-Tech Science Corporation, Japan Model EXTAR
6000, at 10 °C/min temperature increase condition. A previous study [3] reported that the melting temperature of
PVDF crystals in PMMA/PVDF blends decreased as theMorphological characterization of the novel ﬁne structure of the PMMA/PVDF blend

weight fraction of PVDF decreased and that the melting
point depression that generally occurred in amorphouscrystalline polymer blends was expressed as Eq. (1),




1
1
RV2 lnϕ2
1
1
2

¼
þ

þχ
ϕ
ϕ
Tm0 ðblendÞ Tm0 ðpureÞ
m2 m1 1 12 1
ΔH 0 V1 m2

ð1Þ
where Tm0 ðblendÞ and Tm0 ðpureÞ are the equilibrium melting
points of the blend and of the pure crystallizable component,
respectively. ΔH0 is the heat of fusion of the perfectly
crystallizable polymer per mole of the repeat unit, V is themolar volume of the repeating units of the polymers, m is thedegree of polymerization, and ϕ is the volume fraction of thecomponent in the blend. Subscripts 1 and 2 refer to theamorphous and crystalline polymers, respectively. R is theuniversal gas constant, and χ12 is the polymer–polymer
interaction parameter. However, of note, these are considerations of the melting point depression, or melting
behavior, from a thermodynamic point of view and do notinclude considerations from a morphological point of view.

Visualization of morphology
The internal structure was visualized with STEM using
Hitachi High-Tech Corporation, Japan Model S-4800 at anaccelerating voltage of 30 kV. Samples for observation
were obtained using a Leica Microsystems GmbH, Germany Model UC-6 ultramicrotome to prepare thin sectionsof ~100 nm thickness at room temperature.

Evaluation of interactivity
The miscibility of PMMA and PVDF in the melt stateaccounts for the hydrogen bonding between the C=O carbonyl group of PMMA and the CH2 unit of PVDF [18, 19].
The presence or absence and the degree of hydrogenbonding can be evaluated by observing the C=O peak of
PMMA obtained by FT-IR measurements. If hydrogenbonding is formed, this peak is shifted to a lower wavenumber due to the vibration limitation caused by the bonding. This was measured by attenuated total reﬂection (ATR)/
FT-IR (Thermo Fisher Scientiﬁc, USA Model AVATAR370) with Ge prism at a wavenumber resolution of 1 cm−1.

Results
Solution-casting samples
Figure 1 shows thermograms of the solution-casting blendsof PMMA/PVDF with various mixing ratios and pure PVDF
measured at a heating rate of 10 °C/min. Endothermic peaks
from the melting of crystalline PVDF were observed in all

Fig. 1 Thermograms of solution-casting PVDF and PMMA/PVDF
blends with various mixing ratios obtained at a heating rate of 10 °C/
min. The arrows indicate the low-temperature melting peaks that weare focusing on

blends, including blends with low PVDF ratios. The melting
curves were shifted toward lower temperatures withdecreasing PVDF ratio while maintaining their shapes.
However, of note, in the blends with a low PVDF ratio, thepresence of small peaks at lower temperatures than the main
melting peak was evident. Speciﬁcally, those peaks exist at
temperature that is slightly higher than 100 °C for the blend
with PVDF 20% and at temperature that is slightly higher
than 110 °C for the blend with PVDF 50%, as indicated by
the arrows. These small endothermic peaks were also
observed in a previous study [3], while their origin was notdiscussed. Thus, we added a detailed consideration to thisinformation in “Discussion” section. Figures 2 and 3 show
how the melting temperature Tm and total endothermic area
due to the melting of crystalline PVDF depend on the weightfraction of PVDF in the blends. The data from the previous
study [3] were also superimposed in the ﬁgures. The melting
point is determined by the well-established method [25],which identiﬁes the temperature at which the last detectabletraces of crystalline nature disappear upon heating. The
dashed line in Fig. 4 represents a marginal case where there
is no interaction between the components and PVDF is
assumed to be fully crystallized from the blend. The proximity of the plot of measurements to this dashed line indicates that nearly all crystallizable PVDF in the blend is
crystallized. The plots of the melting point of pure PVDF
and the melting point depression of the blends and theendothermic area close to the dashed line shown in Figs. 2
and 3 replicate the results of the previous study [3]. The
melting enthalpy of pure PVDF in this experiment was 58.1
J/g, which agrees well with the previous study [26]. The
melting point depression of PVDF crystals in blends is the
direct consequence of the negative value of the interaction
parameter χ12 shown in Eq. (1) [3]. Equation (1) is often
used to determine the miscibility of polymer pairs, and there
are many studies on this subject [27–30].
Figure 4 shows cross-sectional STEM bright-ﬁeld images of the blends with various mixing ratios. The dark

A. Naruke et al.

contrast between PVDF crystals and amorphous PMMA
domains was successfully obtained using LV-STEM withan acceleration voltage of 30 kV, without the use of pretreatment methods such as heavy metal staining or etching.

Melt-mixing samples

Fig. 2 Dependence of melting temperature (Tm) of the solution-casting
blends on the weight fraction of PVDF. The black dots are the datafrom this study, and the dots made of white dotted lines are the data by
Nishi et al. [3]Figure 5 shows the internal structures of melt-mixing blendswith PVDF 50 and 20% produced under several different
cooling conditions. Cooling at −20 °C/min means thatsamples of melt mixing were held at 200 °C for 30 min in
the DSC pan and then cooled at −20 °C/min using the DSC
program. Here, no endothermic peaks were observed in the
DSC measurements of any blends (see Fig. S1 in Supplementary Information). Thus, all PVDF of any blends existed
in an amorphous state. Figure 5 shows that ﬁne sea–island
structures are formed in all blends. The size of the islands isapproximately 300 nm in diameter, and there is no change
in size between blends with PVDF ratios of 50 and 20%,nor is there any change in the structure with cooling conditions. We will mention in more detail in the discussionsection the nature of the islands with dark contrast, but they
are judged to be PMMA/PVDF mixtures with a higher
PVDF ratio than the sea region.

FT-IR measurement of solution-casting and meltmixing samples

Fig. 3 Dependence of the endotherm area due to the melting of PVDF
crystals in the solution-casting blends on the weight fraction of
PVDF. The black dots are the data from this study, and the dots madeof white dotted lines are the data by Nishi et al [3]contrasts in the image are crystalline PVDF domains,
indicating the presence of nanoscale ﬁne structures inside
the blends that are considerably smaller than the 0.5 µm
scale bar. Speciﬁcally, when the PVDF ratio was 20%,
crystalline PVDF formed islands ~200 nm in diameter, andwhen the ratio was 50%, the PVDF crystalline islands wereinterconnected. In the case of the ratio of 80%, the PVDF
crystals dominate the ﬁeld of view. In this study, theFigure 6 shows the absorbance of the C=O band of the
carbonyl group of PMMA in blends prepared by solution
casting and melt mixing with a PVDF ratio of 50%,
obtained by FT-IR measurements. As mentioned above,
when PMMA and PVDF interact to form hydrogen bonds,
this peak shifts to a lower wavenumber, and it is known thathydrogen bonding is generated only when PVDF is in theamorphous state [18]. Figure 6 shows that, compared to thepure PMMA peak shown in the dotted line, there was no
shift in the solution-casting blend shown in the solid line,
whereas the melt-mixing blend shown in the dashed line
showed a shift to a lower wavenumber, indicating the formation of hydrogen bonding. This is consistent with theresult of the DSC measurement in this study, i.e., only
crystalline PVDF exists in the solution-casting blends, andonly amorphous PVDF exists in the melt-mixing blends.

Discussion
Solution-casting samples
First, we consider the melting point depression derived by
the Nishi-Wang equation from a morphological point ofview. In Eq. (1), the degree of melting point depression onMorphological characterization of the novel ﬁne structure of the PMMA/PVDF blend

Fig. 4 STEM bright-ﬁeld images of solution-casting PVDF/PMMA blends in various mixing ratio. a 20/80, b 50/50, c 80/20. Scale bar is 0.5 μm

Fig. 5 STEM bright-ﬁeld imagesof melt-mixing PMMA/PVDF
blends by various coolingprocesses from melting at 200 °C. a PVDF 20% cooled at RT, b
PVDF 50% cooled at RT, c
PVDF 20% quenched in ice
water, and d PVDF 20% cooledin DSC at −20 °C/min. Scalebar is 0.5 μm

Fig. 6 Carbonyl stretching absorption of pure PMMA and PMMA 50/
PVDF 50 blends. The dotted line shows Pure PMMA, the solid line
shows Solution-casting blend, and the dotted dashed line shows Meltmixing blend

the left-hand side is a function of ϕ1. This is because m isextremely large and χ12 is inherent for each polymer pairunder certain conditions. This is supported by the DSC

measurements with various mixing ratios, i.e., varying ϕ1.
However, it is not clear why the degree of melting point
depression varies with ϕ1. What we focused on was the sizeof the interface regions between PMMA and PVDF where
they contact and can interact with each other. The size of theinterface regions of the sea-island structure in twodimensional images, such as in this study, corresponds tothe perimeter of the island. The length of the perimeterchanges with the area of the islands; thus, it needs to benormalized. The value of the perimeter divided by the area
is always constant, regardless of the number of islands
inside the ﬁeld of view, as long as the size of the islands isconstant, and they are not in contact with each other. Fig. 7
shows this ratio, the perimeter divided by the area of thedark contrasts, which corresponds to PVDF domains calculated by image analysis of the STEM images of eachblend. The analysis was performed on four images acquiredin the size range of 100–150 µm2, and for the sake of analysis, the contrasts between dark and bright are reversed
from the original images (see Figs. S2–S4 in SupportingA. Naruke et al.

Fig. 7 Dependence of the perimeter divided by the area of PVDF
domains on the weight fraction of PVDF. The perimeter and area arecalculated by image analysis

Information). As a result, the perimeters varied with themixing ratio. The original STEM images in Fig. 4 show thatthe periodicity of the structures is the same regardless of the
PVDF ratio. However, as the mixing ratio of PVDF
increased, PVDF in the form of connected islands became
more prominent. This connectivity is the cause of thedecrease in the perimeters. Next, to clarify the relationshipin-between, the melting point of each blend is plottedagainst the perimeter/area and shown in Fig. 8. The curve of
the plots is almost the same as that shown in Fig. 2, where
the horizontal axis is the weight fraction of PVDF. This isevidence that the size of the interface regions is closely
related to the melting point depression.
Next, we discuss a complex behavior that cannot be
described in the single word “melting point depression”,
especially in blends with low PVDF ratios of 20 and 50%.
The presence of an endothermic peak appears at a temperature lower than the main endothermic peak of eachsample, as mentioned above. In Fig. 9, the melting peaks of
PVDF for each blend are shown after normalization by thepeak top values as well as the baseline correction. Of note,the endothermic peak at a temperature lower than the main
melting peak is indicated by the arrows. Furthermore, theintensity of this peak increases as the ratio of PVDF
decreases. The existence of this peak and its origin have notbeen mentioned in previous studies, and the origin of thispeak is discussed in correlation with the newly visualized
nanostructure for the ﬁrst time.
First, the most convincing phenomenon suggested by the
low-temperature endothermic peak is the melting of “ﬁnercrystalline PVDF. However, we would like to touch on theother possibilities as well, unraveling past research

Fig. 8 Dependence of melting temperature (Tm) of the solution-casting
blends on the perimeter divided by the area of PVDF domains. The
plots are similar to Fig. 2 with PVDF weight fraction on thehorizontal axis

Fig. 9 Melting peaks of solution-casting PVDF and PMMA/PVDF
blends. Baseline corrected and standardized by peak top value. The
arrows indicate the low-temperature melting peaks that we arefocusing on

examples. In other words, it is necessary to conﬁrm whether
the low-temperature endothermic peaks could be due tophenomena other than the melting of crystalline PVDF. If
this peak is not caused by melting, there are two possibilities. The ﬁrst is the glass transition temperature, Tg, of theblend. The Tg values of PMMA and PVDF used in thisexperiment were 97 °C and −40 °C, respectively [31]. The
Tg of the blend varies between the Tg of each depending on
the mixing ratio and miscibility. Thus, the Tg of the blend is
expected to be clearly lower than the Tg of PMMA, 97 °C.
In fact, as shown in Fig. 10, thermograms of the lowtemperature range, the glass transition occurs at even lower
temperatures than the low-temperature endothermic peaks
shown by the arrows. In these blends, the glass transition
occurs very slowly, making it difﬁcult to distinguish theMorphological characterization of the novel ﬁne structure of the PMMA/PVDF blend
Table 1 Area ratio of PVDF domains calculated by image analysis of
STEM images for various solution-casting blends
PVDF weightfraction (%)
20

50

Fig. 10 Thermograms in the low temperature range of solution-casting
PVDF and PMMA/PVDF blends. The arrows indicate the lowtemperature melting peaks that we are focusing on

exact positions. The exception is the blend with PVDF
20%, and it is judged that Tg exists at ~30–40 °C. The
second possibility is the endothermic peak at ~60 °C, which
is known by several previous studies to be peculiarly
exhibited in PVDF. The origin is not clear, while the following possibilities have been proposed [32]: upper glasstransition [31], reorganization within conformationally disordered crystals [33], molecular motions corresponding torelaxation in the crystalline/amorphous interface [34], andmelting of paracrystalline domains [35]. However, this peakis observed in this experiment at 60 °C, which is completely
different from the low-temperature endothermic peak.
Although the assignment of the peak at 60 °C is out of ourscope, we now conclude that these two possibilities are
excluded.
From the viewpoint of the low-temperature melting peakobserved in DSC measurements, there have been similar
studies in the past. Poly(butylene succinate) possesses a
low-temperature endothermic peak similar to that in thisexperiment, and its origin has been discussed [36, 37].
These studies lead to the conclusion that in nonisothermal
crystallization from melts, especially when the cooling rateis large, components that melt at low temperatures areproduced. This occurs due to the low thermal stability of thecrystallites. The ﬁndings in these studies do not directly
apply to our study because the mixing and crystallization
processes are different. However, the fact that crystals in
different states can be produced from a single polymer
provides some support for the low-temperature melting
peak observed in this study. It is difﬁcult to further verify its
origin from the thermogram alone. However, further discussion can be made by correlating the results of LV-STEM
observations as follows.
The area of the domains was calculated from the binary
images used to measure the perimeter length earlier and areshown in Table 1. In the blend with 80% PVDF, the area
ratio of dark contrast was almost the same as the PVDF
ratio. However, in the blend with PVDF 50%, only ~70% of80

Analysis number

Area ratio of PVDF
domains (%)

n1

10

n2

10

n3

11

n4

11

n1

36

n2

32

n3

39

n4

38

n1

78

n2

78

n3

75

n4

77

the PVDF ratio appears as the area ratio of dark contrasts,
and in the blend with PVDF 20%, only approximately halfof the PVDF ratio appears as the area ratio of dark contrasts.
Two aspects are necessary for the discussion based on theseresults. First, it is worth repeating that almost all crystallizable PVDF in the blend was crystallized regardless of the
PVDF ratio, based on Fig. 3. This indicates that the area
ratio must be equal to the PVDF ratio regardless of the
PVDF ratio. Second, PVDF is a semicrystalline polymer. In
our measurement, the melting enthalpy of pure PVDF is58.1 J/g, and the corresponding crystallinity becomes 55%
based on 104.1 J/g of perfect crystal [34]. This means thateven if all crystallizable PVDF is crystallized, it contains a
45% amorphous component. If this crystallinity is directly
multiplied by the PVDF ratio of the blend with PVDF 80%,
the dark contrast ratio of 77 would be too high, assumingthat only the crystallized area was observed. However, thisstatement is negated in terms of the spatial resolution of
STEM and the image analysis, as explained below.
Figure 4a shows that there are even more light and dark
contrasts in the dark domains corresponding to PVDF (seeFig. S5 in the supplementary information, more highresolution images). We speculate that these light and dark
contrasts are generated in accordance with the amorphousand crystalline regions of PVDF. The amorphous component of semicrystalline polymers is typically located atinterlamellar or interﬁbrillar areas. Their scales varydepending on the composition and the process of crystallization. For example, a value of 2.5 nm for the interlamellar thickness was reported for pure PVDF [38]. The
size of the interﬁbrillar region is measured to be several toseveral tens of nm, based on the image in Fig. 4a. These
scales are very small compared to the spatial resolution ofboth STEM and image analysis. As a result, both amorphous regions together with the crystalline region of

A. Naruke et al.

PVDF are incorporated into the dark contrast area. Thus,
the dark contrast ratio of 77% reproduced well the PVDF
composition of 80%. In addition, FT-IR measurements
show that there is no hydrogen bonding between PMMA
and PVDF in the solution-casting blend, meaning that
amorphous PVDF does not interact with PMMA in theblends. This suggests that amorphous PVDF is only present in the interlamellar and interﬁbrillar regions of thecrystalline PVDF domains.
We can conclude that the dark contrast area contains all
PVDF components for the blend of PVDF 80%. However,the data for both 20 and 50% show the deviation. Thisdeviation indicates the presence of crystalline PVDF that is
not visible in LV-STEM, which is a reasonable interpretation in terms of the resultant spatial resolution due to ourexperimental conditions. The thickness of the ultrathin
specimens used in this observation was approximately 100
nm, which is a typical thickness for polymeric materials.
Then, it is common sense that any objects consisting of lightelements with sizes extremely smaller than the specimen
thickness cannot be visualized by TEM.
The abovementioned result suggests the presence of ﬁne
crystalline PVDF that cannot be visualized by LV-STEM in
blends with low PVDF ratios and that these crystals
increase in correlation with a decrease in PVDF ratio. Thiscan be related to the presence of crystalline PVDF with alow-temperature melting peak observed by DSC. First, thislow-melting crystalline PVDF tends to increase withdecreasing PVDF ratio, as mentioned above, which issimilar to the increasing tendency of ﬁne crystalline PVDF
by LV-STEM. The low melting point indicates that the
crystals are ﬁne crystals with thin lamellar thicknesses,
implying that both the low-melting crystalline PVDF evidenced by thermograms and the invisible ﬁne crystals are
identical to each other. The increase in this invisible component with decreasing PVDF ratio in the blend is estimatedto be due to the Tg of the blend. Tg increases with a decreasein the PVDF ratio; at high Tg there is a decrease in molecular mobility. This means that as the PVDF ratio decreases,
the mobility of the PVDF molecules decreases, and as aresult, they do not gain the necessary mobility to form
domains.

Melt-mixing samples
The amorphous state shown by the blends with PVDF 50
and 20% in DSC measurements is consistent with a previous study [2]. The new ﬁnding in this study is the ﬁne seaisland structure visualized by LV-STEM. First, this structure was also observed in the ice-water quenched sample, as
shown in Fig. 5c, and therefore can be judged to reﬂect themelt miscibility of PMMA and PVDF. As mentioned above,
PMMA and PVDF are completely mixed in the molten

state, which can be described as molecular miscibility, butfrom the nanoscale point of view, it is newly clariﬁed thatthey form a ﬁne heterogeneous structure.
Next, in discussing the composition of each component
and the origin of structure formation, we consider the presence of an endothermic peak at ~60 °C, as previously
mentioned. In other words, it is necessary to check whether
the islands seen in LV-STEM observations are relevant toparacrystalline PVDF or other heterogeneous structures. For
this purpose, after the DSC measurement, the sample wascooled from 200 °C to room temperature at −20 °C/min in
DSC and measured again at 10 °C/min. In this 2nd run
measurement, the endothermic peak at ~60 °C disappeared(see Fig. S6 in Supplementary Information), which was also
conﬁrmed in a previous study [32]. The image in Fig. 5a
was obtained for the specimen cooled at RT, which corresponds to the 1st scan, i.e., the blend with an endothermic
peak near 60 °C. In contrast, the image in Fig. 5d was
obtained for the specimen with cooling at −20 °C/min in
DSC, which means a 2nd scan, where similar sea island
structures were visible. This fact excludes any possibility
that paracrystalline PVDF or other heterogeneous structures
are the cause of the ﬁne sea-island structure observed inthis study.
We are interested in the compositional attribution ofislands and seas, as well as their formation factors. For thisreason, the area ratio of islands with dark contrast wasdetermined by image analysis as in the solution casting
blend. Because the microstructure here is highly reproducible, the analysis was performed only on three images
with a smaller size of 5 µm2 (see Fig. S7 in Supplementary
Information). The obtained results show that the averagedark contrast area of the blend with 20% PVDF is 28%,
which is larger than the ratio of PVDF in the blend. This, atleast, indicates that the composition of the island is not pure
PVDF but a mixture with PMMA. Next, to determine thepresence or absence of PVDF in the sea region, we measured the number of islands in blends containing 20 and
50% PVDF (see Figs. S8 and S9 in Supplementary Information). Here, the contrast difference between the islands
and the sea in the PVDF 50% blend was small and could not
be binarized. Therefore, the number of islands was usedinstead. The number of islands in the same 14 µm2 size
images were ~78 and 129, respectively. These numbers
cannot simply be explained by the value of the PVDF ratio:
20 to 50% of the PVDF ratio is a 2.5-fold increase, whereas
78–129 islands are only a 1.6-fold increase. This means that
PVDF is present in the sea region of at least the PVDF 50%
blend. Furthermore, based on the similarity of the structure,
we believe that PVDF is also present in the sea region of the
20% PVDF blend.
Next, we discuss the state of PVDF molecules in this ﬁne
sea island structure and the factors responsible for its

Morphological characterization of the novel ﬁne structure of the PMMA/PVDF blend

formation. The PVDF used in this study has an Mw of~500,000, and its single molecular chain length is considered to be ~2 µm. This is derived from a value of ~3 nm/
12.5 monomer, which was obtained by computational
molecular modeling in a previous study [39]. The size of theisland identiﬁed in this study was ~300 nm and did notdeviate from the expected size of the random coil formed
from this molecular chain. This hypothesis is supported by
the fact that the size of the islands does not change between
the blends with PVDF ratios of 50 and 20%. Furthermore,this idea is strengthened by the similar island size of crystalline PVDF seen in solution-casting blends, which is~200 nm. As shown earlier, in solution-casting blends,
PMMA and PVDF exist in an incompatible manner, i.e.,there is no PMMA in the islands. Therefore, the slightly
smaller size for solution-casting blends is consistent with
our hypothesis. To further support this hypothesis, validation using PVDF with different molecular weights isneeded.

Conclusion
By applying LV-STEM to PMMA/PVDF blends, theirnanoscale internal structures, never seen before, were visualized. By performing image analysis on these structures andcorrelating them with DSC and FT-IR measurements, thefollowing three new insights were obtained. First, the size ofinterface regions is closely related to the melting point
depression. Second, the low-temperature melting peaks of
DSC observed in solution-casting blends, especially with low
PVDF ratios, originate from ﬁne PVDF crystals that are difﬁcult to observe even by electron microscopy. The last insightis for melt-mixing blends where PVDF is in an amorphousstate, meaning that PMMA and PVDF are miscible. Even inthis state, a ﬁne heterogeneous structure was observed inblends, which is the visualization of the molecular miscibilityof PMMA and PVDF for the ﬁrst time.

Compliance with ethical standards
Conﬂict of interest The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard tojurisdictional claims in published maps and institutional afﬁliations.

References
1. Daimon K, Ninomiya M. EA-Eco toner. Fuji Xerox Technical
Reports. 2011.
2. Noland JS, Hsu NN-C, Saxon R, Schmitt JM. Compatible highpolymers; poly (vinylidene ﬂuoride) blends with homopolymersof methyl and ethyl methacrylate. Multicompon Polym Syst.
1971. https://doi.org/10.1021/ba-1971-0099.ch002

3. Nishi T, Wang TT. Melting point depression and kinetic effects ofcooling on crystallization in poly (viny1idene ﬂuoride)-poly(methyl methacrylate) mixtures. Macromolecules. 1975. https://doi.org/10.1021/ma60048a040
4. Aid S, Eddhahak A, Khelladi S, Ortega Z, Chaabani S,
Tcharkhtchi A. On the miscibility of PVDF/PMMA polymer
blends: thermodynamics, experimental and numerical investigations. Polym Test. 2019. https://doi.org/10.1016/j.polymertesting.
2018.11.036
5. Shi Z, Ma X, Zhao G, Wang G, Zhang L, Li B. Fabrication ofhigh porosity nanocellular polymer foams based on PMMA/PVDF blends. Mater Des. 2020. https://doi.org/10.1016/j.matdes.
2020.109002
6. Sasaki H, Bala PK, Yoshida H, Ito E. Miscibility of PVDF/PMMA blends examined by crystallization dynamics. Polymer.
1995. https://doi.org/10.1016/0032-3861(95)99296-7
7. Yoshida H. Structure formation of PVDF/PMMA blends studied.
J Therm Anal. 1997. https://doi.org/10.1007/BF01987426
8. Horibe H, Hosokawa Y, Oshiro H, Sasaki Y, Takahashi S, Kono
A, et al. Effect of heat-treatment temperature after polymer melt
and blending ratio on the crystalline structure of PVDF in a
PVDF/PMMA blend. Polym J. 2013. https://doi.org/10.1038/pj.
2013.53
9. Kwei TK, Patterson GD, Wang TT. Compatibility in mixtures ofpoly (vinylidene ﬂuoride) and poly (ethyl methacrylate). Macromolecules. 1976. https://doi.org/10.1021/ma60053a018
10. Wang TT, Nishi T. Spherulitic Crystallization in Compatible
Blends of Poly (vinylidene ﬂuoride) and Poly (methyl methacrylate). Macromolecules. 1977. https://doi.org/10.1021/ma60056a
034
11. Roerdink E, Challa G. Inﬂuence of tacticity of poly (methyl
methacrylate) on the compatibility with poly (vinylidene ﬂuoride).
Polymer. 1978. https://doi.org/10.1016/0032-3861(78)90034-4
12. Morra BS, Stein RS. Morphological studies of poly(vinylidene
ﬂuoride) and its blends with poly(methyl methacrylate). J Polym
Sci: Polym Phys Edition. 1982. https://doi.org/10.1002/pol.1982.
180201208
13. Okabe Y, Murakami H, Osaka N, Saito H, Inoue T. Morphology
development and exclusion of noncrystalline polymer during
crystallization in PVDF/PMMA blends. Polymer. 2010. https://doi.org/10.1016/j.polymer.2010.01.055
14. Hirata Y, Kotaka T. Phase separation and viscoelastic behavior ofsemicompatible polymer blends: poly (vinylidene ﬂuoride)/poly(methyl methacrylate) system. Polym J. 1981. https://doi.org/10.
1295/polymj.13.273
15. Zhang Y, Zuo M, Song Y, Yan X, Zheng Q. Dynamic rheology
and dielectric relaxation of poly (vinylidene ﬂuoride)/poly (methyl
methacrylate) blends. Compos Sci Technol. 2015. https://doi.org/
10.1016/j.compscitech.2014.10.024
16. Douglass DC, McBrierty VJ. Compatibility in PVF2/PMMA andPVF2/PEMA blends as studies by pulsed NMR. Macromolecules.
1978. https://doi.org/10.1021/ma60064a030
17. Papavoine CHM, Maas WEJR, Veeman WS, Buning GHW,
Vankan JMJ. Triple-resonance proton-ﬂuorine-19-carbon-13
CPMAS NMR study on the crystallization behavior of the
PMMA/poly(vinylidene ﬂuoride) 60/40 blend. Macromolecules.
1993. https://doi.org/10.1021/ma00076a0o46
18. Léonard C, Halary JL, Monnerie L. Hydrogen bonding in PMMAﬂuorinated polymer blends: FTi.r. investigations using ester model
molecules. Polymer. 1985. https://doi.org/10.1016/0032-3861(85)90084-9
19. Coleman MM, Painter PC. Hydrogen bonded polymer blends, Prog
Polym Sci. 1995.https://doi.org/10.1016/0079-6700(94)00038-4
20. Gregorio R Jr, Borges DS. Effect of crystallization rate on theformation of the polymorphs of solution cast poly (vinylidene

A. Naruke et al.
ﬂuoride). Polymer. 2008. https://doi.org/10.1016/j.polymer.2008.
07.01021. Hahn B, Wendorff J, Yoon DY. Dielectric relaxation of thecrystal-amorphous interphase in poly (vinylidene ﬂuoride) and its
blends with poly (methyl methacrylate). Macromolecules. 1985.
https://doi.org/10.1021/ma00146a024
22. Hahn BR, Herrmann-Schönherr O, Wendorff JH. Evidence for acrystal-amorphous interphase in PVDF and PVDF/PMMA blends.
Polymer. 1987. https://doi.org/10.1016/0032-3861(87)90404-6
23. Ma W, Zhang J, Wang X, Wang S. Effect of PMMA on crystallization behavior and hydrophilicity of poly(vinylidene ﬂuoride)/poly(methyl methacrylate) blend prepared in semi-dilute
solutions. Appl Surf Sci. 2007. https://doi.org/10.1016/j.apsusc.
2007.04.001
24. Libera MR, Egerton RF. Advances in the transmission electron
microscopy of polymers. Polym Microsc. 2010. https://doi.org/10.
1080/15583724.2010.493256
25. Hoffman JD, Weeks JJ. Melting process and the equilibrium
melting temperature of polychlorotriﬂuoroethylene. J Res Nat Bur
Stand Sect A. 1962:66A;13–28.
26. Neidhöfer M, Beaume F, Ibos L, Bernès A, Lacabanne C.
Structural evolution of PVDF during storage or annealing, Polymer. 2004. https://doi.org/10.1016/j.polymer.2003.12.066
27. Jo WH, Lee MR, Min BG, Lee MS. Miscibility of poly(ether
imide)/poly(ethylene terephthalate). Polym Bull. 1994. https://doi.
org/10.1007/BF00313482
28. Qiu Z, Yan C, Lu J, Yang W, Ikehara T, Nishi T. Various crystalline morphology of poly(butylene succinate-co-butylene adipate) in its miscible blends with poly(vinylidene ﬂuoride). J Phys
Chem. 2007. https://doi.org/10.1021/jp067606f
29. Wiranidchapong C, Rades T, Kulvanich P, Tucker IG. Method of
preparation does not affect the miscibility between steroid hormone and polymethacrylate. Thermochim Acta. 2009. https://doi.
org/10.1016/j.tca.2008.12.013
30. Chenyang X, Mengmeng Z, Liping Z, Jichun Y, Xiaojun C,
Yongjin L. Ionic liquid modiﬁed poly(vinylidene ﬂuoride):

crystalline structures, miscibility, and physical properties. Polym
Chem. 2013. https://doi.org/10.1039/C3PY00466J
31. Leonard C, Halary JL, Monnerie L, Micheron F. DSC studies on
the transitions in poly (vinylidene ﬂuoride) and some related
copolymers. Polym Bull. 1984. https://doi.org/10.1007/
BF00258030
32. Merlini C, Barra GMO, Araujo TM, Pegoretti A. Electricallypressure sensitive poly(vinylidene ﬂuoride)/polypyrrole electrospun mats. RSC Adv. 2014. https://doi.org/10.1039/
C4RA01058B
33. Loufakis K, Wunderlich B. Thermal analysis of the conformational disorder in semicrystalline poly (vinylidene ﬂuoride) andpoly(triﬂuoroethylene). Macromolecules. 1987. https://doi.org/10.
1021/ma00176a026
34. Teyssedre G, Bernes A, Lacabanne C. Inﬂuence of thecrystalline phase on the molecular mobility of PVDF. J Polym Sci:
Part B Polym Phys. 1993. https://doi.org/10.1002/polb.1993.
090311316
35. Nabata Y. Molecular motion in form II poly (vinylidene ﬂuoride).
Jpn J Appl Phys. 1990. https://doi.org/10.1143/JJAP.29.2782
36. Yasuniwa M, Satou T. Multiple melting behavior of poly(butylene
succinate). I. Thermal analysis of melt-crystallized samples. J
Polym Sci: Part B Polym Phys. 2002. https://doi.org/10.1002/
polb.10298
37. Qiu Z, Komura M, Ikehara T, Nishi T. DSC and TMDSC study ofmelting behaviour of poly(butylene succinate) and poly(ethylene
succinate). Polymer. 2003. https://doi.org/10.1016/j.polymer.
2003.10.045
38. Nakagawa K, Ishida Y. Dielectric relaxation and molecularmotions in polyvinylidene ﬂuoride with crystal form II. J Polym
Sci: Part B Polym Phys Ed. 1973. https://doi.org/10.1002/pol.
1973.180110804
39. Bystrov VS, Paramonova EV, Bdikin IK, Bystrova AV, Pullar
RC, Kholkin AL. Molecular modeling of the piezoelectric effectin the ferroelectric polymer poly (vinylidene ﬂuoride) (PVDF). J
Mol Model. 2013. https://doi.org/10.1007/s00894-013-1891-z