wu-et-al-visualization-of-macrophase-separation-and-transformation-in-immiscible-polymer-blends.pdf

wu-et-al-visualization-of-macrophase-separation-and-transformation-in

1. Visualization ofMacrophase
Separation and Transformation
in Immiscible Polymer Blends
Zhiyuan Wu
1
, Chunyu Zhang
2
*, Youliang Zhu
1
, Zhongyuan Lu
1
, Heng Liu
2
, Bin Xu
1
, Xuequan Zhang
2
& Wenjing Tian
1
*
1
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun,
Jilin 130012,
2
Key Laboratory of Rubber-Plastics, Ministry of Education, Shandong Provincial Key Laboratory of
Rubber-Plastics, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao,
Shandong 266042
*Corresponding authors:[email protected];[email protected]
Cite this:CCS Chem.2023, 5, 718–728
DOI:10.31635/ccschem.022.202101726
Identification and visualization of phase structures in-
side polymer blends are of critical importance in the
understanding of their intrinsic structure and dynamics.
However, the direct optical observation of the individ-
ual component phase in a dense bulk material poses
asignificant challenge. Herein, three-dimensional
fluorescence imaging of phase separation and real-
time visualization of phase transformation in immisci-
ble polymer blends of polypropylene and polystyrene
is realized through multiphoton laser scanning micros-
copy. Owing to the specificfluorescence behavior of
the cyanostyrene derivative 2-(4-bromophenyl)-3-
(4-(4-(diphenylamino)styryl)phenyl)fumaronitrile, the
high-contrast imaging of the macrophase of the com-
ponent polymer in two and three dimensions with a
maximum depth of 140μm and a high signal-to-noise
ratio of 300 can be achieved. Detailed spectroscopic
and structural studies reveal that the distinctivefluo-
rescence features of each phase domain should origi-
nate from the formation of a completely different
aggregate between probes and component polymer.
Furthermore, visualizations of the internal morphology
deformation and macrophase transformation were re-
alized by employing a stretched dumbbell sample un-
derconstanttension.
Keywords:fluorescent probes,fluorescent selectivity,
phase separation, multiphoton laser scanning micros-
copy, visualization
Introduction
Polymer blends can provide excellent performance that
can be achieved by combining various polymers with
distinctive properties into a single material in many ways.
1,2
These multi-component systems typically lead to a wide
range of phase behaviors that directly influence the asso-
ciated physical properties and play a crucial role in the
final application.
3
Thus, identifying the spatial structure of
their individual phases is critical to understanding the
relationships among the formation, structure, dynamics,
and functionality of blend materials. Most polymer blends
RESEARCH ARTICLE
Received: Dec. 17, 2021 | Accepted: Apr. 8, 2022 | Published: May 10, 2022
DOI:10.31635/ccschem.022.202101726
Citation:CCS Chem.2023, 5, 718–728
Link to VoR:https://doi.org/10.31635/ccschem.022.202101726
718

2. are immiscible, that is, each component phase usually
separates into distinct, macroscopic domains during the
mixing process, termed as macrophase-separated struc-
tures, which greatly affects the blend properties.
4
For
example, a co-continuous structure can lead to maximum
contributions of stiffness, hardness, and impact properties
of each component simultaneously, which can then
be used in a variety of applications, such as electrically
conductive blends and tissue scaffolds.
5
Asignificant
complication in blending is that the resulting structures
are three-dimensional (3D) and have an interpenetrating
anisotropic structure. Furthermore, the macrophase-
separated domains and structures are usually formed and
observed on the micrometer scale,
6–9
which makes it diffi-
cult to clearly identify the intrinsic structures of phase
behavior, especially for the structures inside the polymer
matrix. Some analytical techniques,
5,10,11
such as transmis-
sion electron microscopy,
12
scanning electron microscopy
(SEM),
13
and atomic force microscopy,
14
are used to char-
acterize the surface topography, size, and distribution of
the dispersed phase in polymer blends. Although these
methods offer excellent spatial resolution, they are usually
confined to the material surfaces or cross-sections that
require an invasive sample preparation process.
15,16
The
lack of the third dimension leads to a misinterpretation
of the true phase structure.
17–19
More complicated
morphologies will usually result in a less convincing inter-
pretation of the surface images. Moreover, they do not
directly provide intuitive insight into the morphologies
and are unable to provide a representation of these inter-
nal complex structures.
20
Therefore, the development of
microscopes that are capable of 3D images for internal
complex structures is anticipated.
Recent advances influorescence-based techniques
have emerged as a powerful tool to characterize
morphologies and facilitate a 3D visualization of the
exact morphology.
21–30
Lopez-Barron and Macosko
31
char-
acterized the interface between the two phases of an
immiscible polymer blend made offluorescently-labeled
polystyrene (PS) and styrene-ran-acrylonitrile copoly-
mer by laser scanning confocal microscopy. Tang et al.
32
observed the macrodispersion of montmorillonitefillers
labeled with tetraphenylethene in a polymer matrix.
However, visualization and precise localization of individ-
ual phase domains in a densely packed polymer mixture,
especially for the internal phase structures, are still rare.
Several factors limit the performance of currentfluores-
cence microscopy to image macrophase structures of
polymer blends, such as difficult or even impossible iden-
tification of different polymer species, an inability to dis-
tinguish the localization of phase-separated domains
inside the sample, or unsatisfactoryfluorescence imaging
efficiency and contrast.
33–38
These restrictions mainly in-
volve the emission behavior offluorescent probes in the
polymer mixture, which ultimately influences thefinal
imaging resolution, signal-to-noise ratio, and the visuali-
zation of an individual polymer within the bulk blends.
Therefore, a direct 3D imaging technique for polymer
blends without any invasive processes remains a signifi-
cant challenge but would provide valuable information
about both their intrinsic structures and dynamics.
We have addressed the above challenge and devel-
oped a specific strategy of manipulating dye aggregates
to identify polymer domains with or without phenyl
moieties, where different aggregation pathways lead to
two distinct emission properties of a specific label-free
fluorescent probe, 2-(4-bromophenyl)-3-(4-(4-(diphe-
nylamino)styryl)phenyl)fumaronitrile (TB). By exploiting
TB in a way to selectively light up the target polymer, we
have demonstrated ultrafast and non-invasive imaging of
immiscible polymer blends composed of polypropylene
(PP) and PS using a multiphotonfluorescence imaging
technique (multiphoton laser scanning microscopy,
MLSM). The distinctfluorescence of polymer domains
with or without phenyl moieties allows us to visualize
3D structures of individual phases inside the polymer
mixtures. This excellent depth resolution can clearly ob-
serve a more accurate ellipsoid radius and the transfor-
mation from a co-continuous phase to a sea-island phase
under different annealing rates. Furthermore, we can also
track the phase distribution in a stretchable dumbbell
sample of PS/PP under constant tension in real time. The
“invisible”information relative to the internal morphology
deformation in the polymer specimens is transformed to
visiblefluorescent signals, which provides an essential
insight into the relationship between the microstructure
and mechanical properties of the polymer blends.
Experimental Methods
All-atom molecular dynamics simulation of
the TB-doped PSfilms and TB-doped PPfilms
An all-atom molecular dynamics (MD) simulation system
consists of four TB molecules and 20 PS or PP chains with
a length of 50 repeating units for each. Simulations were
carried out using the Forcite module of the Materials
Studio (Accelrys Inc., San Diego, CA, United States) with
COMPASSII forcefield. The composite structure was
equilibrated for 1 ns to fully mix the TB molecule with PS
or PP chains by the simulation under the NPT ensemble at
room temperature and 1 bar pressure.
General preparation procedure of TB-doped
polymerfilms
Polymer and TB stock solutions were prepared by dis-
solving 1 g of polymer sample in 100 mL toluene and
dissolving 0.001 g of TB in 2 mL toluene, respectively.
Polymer blend solution with a PP mass fraction of 80%
were prepared by mixing 0.4 mL of PP solution and 0.1 mL
RESEARCH ARTICLE
DOI:10.31635/ccschem.022.202101726
Citation:CCS Chem.2023, 5, 718–728
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3. of PS solution. Afterward, 0.1 mL TB solution was mixed
with 0.5 mL of as-prepared polymer solution under ultra-
sonication for about 2 h, generating a homogeneous
solution with a polymer concentration of 10 mg/mL and
1.0 wt % content of TB. Uniform thinfilms of TB-doped
polymers were fabricated by droplet coating the mixed
solutions of TB and polymer blends onto quartz plates.
The samples was kept at 180 °C for 10 min to volatilize
toluene, and then the temperature was reduced to room
temperature at rates of 1, 10, and 50 °C/min, respectively.
General preparation procedure of TB-doped
PS/PP bulk
First, we mixed the TB (1.0 wt %) with PS and PP in a
WLG10G twin-screw extruder (Shanghai Xinshuo Preci-
sion Machinery Co., Ltd., Shanghai, China) at 180 °C
extruder temperature and 60 r/min screw speed. Tensile
test specimens with dimensions of 75×10×2 mm made
of TB/PS/PP were produced by injection molding, con-
ducted by a WZS10D injection molding machine (Shang-
hai Xinshuo Precision Machinery Co., Ltd., Shanghai,
China) at 180 °C injection temperature, 10 MPa packing
pressure, and 5 s packing time.
Results and Discussion
Developing a specificfluorescence probe
for polymer blends
Our strategy to visualize and analyze the structures of
immiscible polymer blends depends on the aggregation of
thefluorescent probe, which differs from conventional
methods that often need to label the polymer with a
fluorescent probe through a complicated synthetic pro-
cedure. We synthesized a specificfluorescent probe TB
(Supporting Information Figures S1–S3)thatcanboostits
emission when doping in polymers containing a benzene
ring and can totally quench its emission in other polymers.
Consequently, an appropriate binary polymer blend, con-
taining benzene-based PS and its immiscible pair PP, is
expected to serve as a potential imaging target by simply
doping with thefluorescent probe TB (Figure1a). The
experimental samples can be prepared by using simple
solution-processed methods, such as spin coating or
droplet coating, in which 1 wt % TB is doped into the
polymer.
The emission behaviors of TB in both PS and PP matri-
ces were initially investigated by steady-state spectros-
copy. As expected, the TB-doped PSfilms exhibited bright
redfluorescence, whereas the TB-doped PPfilms and TB
films exhibited almost no emission. The PL spectrum of
TB-doped PSfilms showed a strong emission band peak
at 614 nm, where the photoluminescence quantum yield
(Φ
F) was as high as 34%. In contrast, TB-doped PPfilms or
pure TBfilms exhibited almost no emission and their PL
spectra were a straight line along the bottom (Figure1c).
Furthermore, we found that TBfilms show an obviously
red-shifted absorption band compared with that of TB in
tetrahydrofuran (THF) solution, indicating that the exis-
tence of strong intermolecular interactions leads to ag-
gregate formation in TBfilms, as shown in Figure1b.
Importantly, when doping PS and PP with TB, the absorp-
tion band of the TB-doped PSfilms showed a red-shift
compared with the TB-doped PP films. These observa-
tions suggested that TB probably exhibits different inter-
molecular interactions with a PS or PP polymer chain,
because the optical properties of the dye molecules
strongly depend on the aggregate structure or packing
arrangement.
To gain further insight into the interactions between TB
and polymer in a matrix, the compatibility of TB with the
polymer was evaluated by the Flory–Huggins interaction
parameter (χ), whose value can be derived from the
melting point of TB in the presence of a polymer (T
M
mix
).
The melting point of the TB-doped PS or TB-doped PP
mixture with different mass fractions was performed by
differential scanning calorimetry. As shown inSupporting
Information Figure S4, TB showed varying degrees of
melting point depression in the PS and PP mixture. Thus,
as shown in Figure1d, we can obtain the values of the
interaction parameter of TB with different polymers
through Flory–Huggins lattice theory.
39,40
In the mixture
of TB and PS, the interaction parameter is−0.11, which
indicates that they are miscible. The negative interaction
parameter reflects a great thermodynamic driving force
for mixing TB with PS, which suggests TB has good
dispersion in the PS matrix and tends to form an isolated
state. In contrast, the mixture of TB and PP results in a
largely altered interaction parameter of 0.65. The highly
positive value of the interaction parameter indicates that
TB and PP are immiscible, and the large difference be-
tween the interaction parameters suggests that TB under-
goes a different assembly process when mixed with PS
and PP. It is worth noting that TB molecules possess a
large dipole moment because of the asymmetric geomet-
ric and strong intramolecular charge transfer state, which
usually leads to the formation of compact packing and
strong intermolecular interactions. Indeed, strongπ–π
interactions and a closed packed structure are observed
for the TB crystal (Supporting Information Figure S5),
which also exhibits no emission similar to its films
(Supporting Information Figure S6a). Thesefindings indi-
cate that TB would present a different aggregate structure
in a PS and PP polymer matrix, resulting in distinct emis-
sion behavior. Additionally, to explore thefluorescence
properties of TB-doped polymers with or without phenyl
moieties, a variety of polymers with and without phenyl
moieties were chosen, such as PS, poly-a-methyl styrene
(PAMS), styrene butadiene styrene block copolymer
(SBS), polyethersulfone (PES), PP, polyvinyl pyrrolidone
RESEARCH ARTICLE
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4. (PVP), polyetherpolyol (PMPO/POP), and polycaprolac-
tone (PCL). As shown inSupporting Information Figure
S6, TB exhibited strong emission in PS, PAMS, SBS, and
PES, but showed weak emission in PP, PVP, PMPO, and
PCL. This suggests that TB can serve as a unique probe for
the visualization of phase separation of biphasic blend
polymers with and without phenyl moieties.
We further investigated the excited-state dynamics of
TB in a PS and PP polymer matrix. Time-resolvedfluores-
cence spectra of TB shown in Supporting Information
Figure S6breveal a significantly increasedfluorescence
lifetime (τFL) of TB-doped PSfilms (5.01 ns) compared
with that of TB-doped PPfilms (1.38 ns), as well as TBfilms
(2.65 ns). Furthermore, a long fluorescence lifetime
was also observed in TB-doped rigid polymers, as
listedSupporting Information Table S1. Because the PL
quantum yieldΦF equals the product ofτFL and the
radiative deactivation rate (kr), kr and the non-radiative
deactivation rate (knr) can be approximately estimated
(Supporting Information Table S1). Compared with TB-
doped PPfilms, there is one order of magnitude increase
of kr in TB-doped PSfilms, while knr decreases to one-
sixth. This shows that thefluorescence transition is favor-
able in TB-doped PSfilms but inhibited in TB-doped PP
films. It is worth noting that TB has no emission in diluted
THF solution but exhibits boosted emission at 77 K
(Supporting Information Figure S7). The frozen solution
provides a rigid environment tofix the isolated TB mole-
cules and restrict the intramolecular motions, which
results in enhanced emissions compared with that in so-
lution at room temperature. This is in good agreement
with the strong emission observed when TB is doped into
the PS matrix, where PS is analogous to a solid solution.
However, TBfilms also show a low kr similar to the TB-
doped PPfilm, which suggests that TB probably forms
aggregates to cause emission quenching.
Figure 1| Investigating photophysical properties of TB. (a) Luminous mechanism of the TB (1.0 wt %) doped in PS and
PP and molecular structures of the polymers (PS and PP) andfluorescent probe (TB). (b) UV–vis absorption and (c) PL
emission spectra (excitation wavelength: 365 nm) of the TB in THF solution, TBfilms, 1.0 wt % TB-doped PSfilms and
1.0 wt % TB-doped PPfilms. (d) Free energy of mixing TB with PS, PP using melting point depression measurements.
RESEARCH ARTICLE
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5. Excited state dynamics of TB/PS and TB/PP
polymer mixtures
To explore the origin of the distinctivefluorescence be-
havior of TB in various aggregate states, we carried out
ultrafast transient absorption (TA) spectroscopy to study
the internal excited-state dynamics. Femtosecond-
resolved TA spectra of TB are shown in Figures2a,2b,
2d,and2e. After excitation, the characteristic excited-
state absorption (ESA) bands at 500–750 nm were ob-
served in THF solution within a few ps. Fast intramolecular
non-radiative processes, including structural evolution
accompanied by vibration and rotation, can dissipate the
excited-state energy of TB on the sub-ps and ps scales.
The significant changes in ESA within 2 ps is likely due to
the configurational evolution after photoexcitation (Supp-
orting Information Figure S8). In sharp contrast, the char-
acteristic stimulated emission (SE) bands at 580–610 nm
overlapping with the ESA bands at 610–780 nm were
observed in TB-doped PSfilms. Notably, a constant de-
crease at the SE band was observed, which is in good
agreement with its PL spectrum at 625 nm (Figure2a)and
should result from the radiative S
1–S
0decay. This spectral
evidence indicates that a large number of excitons are
deactivated through the radiative pathway, which is con-
sistent with a high radiative deactivation rate. The all-atom
MD simulations predict the dispersed state of TB in differ-
ent polymer matrixes. As indicated in Figure2c,TBwas
mono-dispersed in the PS matrix and tended to an isolat-
ed state owing to the good compatibility with PS. Conse-
quently, the radiative transition of excited TB can be
predominant when TB is mono-dispersed in a rigid PS
matrix and intramolecular vibration and rotation are
suppressed.
Although the intramolecular vibration and rotation are
greatly suppressed in the solid state, excimers are likely to
form in TBfilms upon excitation, owing to the molecular
stacking and intrinsic D–A structure of TB. A“dark”excited
state can be formed within 1 ps, as evidenced by the
gradually increasing ESA, which then undergoes a non-
radiative pathway (Supporting Information Figure S9).
More importantly, an obvious change of the ESA in
TB-doped PPfilms was also observed within 25 ps (Supp-
orting Information Figure S10), which lagged compared
with that of TB in solution. This may originate from a slow
configurational evolution after photoexcitation, where the
intramolecular motions are not fully restricted by the PP
matrix. Similar to TBfilms, the gradually increased and
blue-shifted ESA indicated that the radiative transition of
excited TB was inhibited, because a molecular aggregate
was formed following the initial structural evolution. These
findings imply that the“dark”excited state should origi-
nate from the formation of a unique aggregate of TB in the
PP matrix, which agrees with the closed stacking of TB
molecules from MD simulations (Figure2f).
3D visualization of the phase separation
of PS/PPfilms by MLSM
Owing to the unique photophysical properties of TB
doped in different polymer matrixes, we explored the
potential application in directly imaging the morphologies
Figure 2| Excited state dynamics of TB in different states. TA spectra of (a) TB-doped PSfilms, (b) TB-doped PPfilms,
TB in (c) THF and (d) TBfilms. Excitation: 400 nm, probe light: 450–780 nm. Snapshot of the all-atom MD simulations
of (e) TB/PSfilms and (f) TB/PPfilms.
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6. of polymer blends byfluorescence microscopy. For this
purpose, PS and PP without anyfluorescent probe labels
were chosen as binary immiscible polymer pairs, and TB
served as thefluorescence probe with a low doping con-
tent of 1%. The blendfilms underwent a simple mixing
process with different annealing treatments to obtain
various phase-separated structures. As shown inSupp-
orting Information Figure S11, a surface image of a phase-
separated structure between the PS and PP phase
domains was clearly observed byfluorescence microsco-
py, where the PS domains showed strong emission and the
PP domains exhibited nofluorescence. This indicates that
TB exhibits distinctfluorescence properties in different
phase domains when doping in a PP/PS blend mixture.
The results imply that the assembly processes of TB with
PS and PP in the blend mixture are independent of each
other, and the emission behavior is identical to that from
doping with the single polymer. As shown inSupporting
Information Figure S12,TB/PSfilms with different TB
ratios had strongfluorescence emission while thefluores-
cence of TB/PPfilms was always weak. It means that
TB can keep the samefluorescent characteristics when
different mass fractions of TB are doped in polymers.
Considering the significant difference of emission intensity
in blends, we next sought to realize 3D visualization of the
phase-separated structure inside blends by MLSM. It is
worth noting that TB not only exhibits a unique emission
behavior but also possesses highly nonlinear optical prop-
erties. Our previous study demonstrated that TB shows a
large two-photon absorption cross-section in its aggre-
gate state, which is beneficial for two-photonfluorescence
(2PF) under two-photon excitation (Supporting Infor-
mation Figure S13). More importantly, the assembly
process with PS enhances the radiative transition of TB,
which results in a high photoluminescence quantum yield
of a TB-doped PS mixture. These advantages of the
unique features of TB are critical for the high imaging
resolution and quality of TB-doped polymer blends.
Figure3presents MLSM images of PS/PP binary poly-
mer blends, where the blendfilm was prepared by simply
mixing in toluene followed by droplet coating the mixed
solutions of TB and polymers onto quartz plates. The
samples were kept at 180 °C for 10 min to volatilize the
toluene solvent, and then the temperature was cooled to
room temperature at a rate of 1 °C/min. Femtosecond
lasers with wavelengths of 800 nm were focused on
the surface of thefilms through an objective lens (20×).
The imaging time for each frame was approximately 4 s
with 10-μm steps in thezdirection. Interestingly, the image
shows a distinguishable phase-separated structure of
polymer blends, in which the red region represents the
PS phase domain and the black region corresponds to the
PP phase domain. The boundary between the two regions
is well de
fined and was easy to identify one from the other.
The strong red signal from the PS domain is attributed to
the two-photonfluorescence of TB, but, in sharp contrast,
no signal from the PP domain was detected owing to
the aggregate-caused quenching. In addition to these
two-dimensional images, we successfully obtained 3D
reconstructed images via the z-scanning technique
(Figures3aand3b). The images clearly show two distinct
red and black regions in which the PS and PP domains are
separated from each other, even in thez-axis direction.
Eventually, the highest signal-noise ratio (SNR) of 220 and
maximum imaging depth of 110μmwereachievedinthe
blends. Compared withfluorescence microscopy, there
was a 100-times enhancement of the SNR by MLSM
(Supporting Information Figure S14), which helped to
eliminate the interference of backgroundfluorescence
and achieve a high resolution. Owing to high-quality
imagingoftheblend,theisolatedislandstructureswith
different radii of the PS domain were obtained from the
scanning images of each section at every 10μm. Notably,
we were able to directly observe the true ellipsoidal
shape of PS from the side view of the panoramic recon-
structedfluorescence micrograph, where most of the PS
ellipsoids with a maximum diameter of approximately
25μm are arranged spatially separated from one anoth-
er. Remarkably, we found that some of adjacent spheri-
cal PS domains have already merged into one observed
in the depth of 40μm, whereas they are completely
separated in upper section of 10 and 20μm, respectively
(Figures3c–3e). These observations are consistent with
Ostwald ripening theory that states that components of
the discontinuous phase can diffuse to form larger dro-
plets through the continuous phase. In other words, the
diffusion and merging process of the isolated PS phase
probably occur through the continuous PP phase.
Different from other methods,
41–44
we demonstrate the
successful imaging of binary immiscible polymer blends
via non-covalentfluorescent probes by MLSM. We directly
observed an integral and sharp 3D image of the compli-
cated morphologies inside blends owing to excellent SNR
and depth resolution, which is notoriously difficult to
realize by conventional electronic and optical microscopy
techniques.
Visualization of the effect of annealing rate on
the phase separation of PS/PPfilms by MLSM
MLSM imaging allows us to directly evaluate the dynamic
properties of phase separation in immiscible polymer
blends. According to Oswald ripening theory, the phase
separation of polymer blends is a thermodynamically-
driven process, which suggests that the thermal annealing
process will have a significant impact on the phase-
separated structure. Figure4shows the images of TB-
doped PS/PP blends with different annealing rates. The
images were obtained through MLSM with 10-μmstepsin
thezdirection (Supporting Information Figures S15–S17).
The well-defined co-continuous structures in blends
were observed when the temperature dropped at a rate
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7. of 50 °C/min (Figure4aandSupporting Information
Video S1), where the PS and PP components showed
3D spatial interpenetrating and intertwining structures.
With cooling rates down to 10 °C/min, we found that the
discrete, large, nucleated islands or elongated islands are
not fully separated, and there are some interconnected PS
domains inside the blend matrix as shown in Figure4band
Supporting Information Video S2. By further decreasing
the cooling rate to 1 °C/min, the ideal droplet or isolated
island particles of PS were well dispersed in the blend
matrix, and each droplet particle was completely separat-
ed (Figure4candSupporting Information Video S3).
These imaging data present the formation of various
morphology types by controlling the annealing condition,
from a co-continuous structure to an unstable intermedi-
ate hybrid structure, and eventually transforming into a
dispersed morphology, which can be clearly revealed
by 3D imaging. Additionally, as shown in Figures4dand
4e, the imaging data reveal high SNR (as high as 300) and
a depth resolution up to 140μm within PS/PP polymer
blends under various annealing treatments, which indicate
that TB has excellent thermostability and photostability. It
should be noted that 3D data of the morphology can
provide useful and additional structural information. For
instance, there are some small PP particles with a radius of
approximately 3μm that are only observed in the section
at a depth of 40μm, while these are not observed in the
upper or lower sections (Figure4a). In contrast to the
isolated adjacent PS particles in the upper section, we
found that they tend to approach and partly connect with
each other in the deeper matrix of blends (Figure4b),
which agrees with Ostwald maturation that claims that the
diffusion of particles occurs with increasing coalescence
time.
Visualization of the effect of constant force on
the PS/PP bulk’s phase distribution by MSLM
Taking these advantages of MLSM imaging to visualize
the internal structure, we attempted to directly visualize
the phase transformation dynamics of PS/PP polymer
blends by using a practical bulk sample. A dumbbell
Figure 3| 3D visualization of the phase separation of TB/PS/PPfilms. (a) Top view of 3D reconstructed two-photo
fluorescence imaging of TB/PS/PPfilms from a depth of 0–110μm. (b) Side view of stacked imaging of TB/PS/PPfilms.
The MSLM scans TB/PS/PPfilms every 10μm in the range of 0–120μm. PS region imaging at a depth of 10μm(c),20μm
(d), and 40μm (e) Insert: partial enlargement of PS phase. Scale bar: 40μm.
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8. sample is often used in most mechanical tests for poly-
mers to show the stress concentration in the transition
area. Especially for immiscible polymer blends that pos-
sess a phase separated structure, stretching the sample is
often accompanied by a phase transformation between
binary phase domains inside the blend matrix. Wefirst
performed 3D imaging of a PS/PP dumbbell sample that
was prepared as shown inSupporting Information Figure
S18, in which the distribution of the PS phase was clearly
observed from the red region, and the black regions
represented the PP phase. The images showed that PS
forms a sea-island phase-separated structure. After
annealing the sample at 180 °C (experimental details are
included in theSupporting Information), it was found
that the average particle size of PS obviously increased,
and several small PS particles merged into a large one, as
shown inSupporting Information Figure S18. This was
ascribed to the static coalescence behavior that occurs
during the annealing process, providing direct evidence
of thermodynamically-driven phase transition of polymer
blends. The use of this strategy to image the phase
separated structure and understand the pathway of the
assembly process provides a novel alternative to more-
common electronic and optical methods.
We then sought to evaluate the phase transformation
dynamics for the PS/PP dumbbell sample under constant
force stretching by means of real-time imaging. Although
the limitation of collecting signal and image processing
depends on the Z-scan technique, directly imaging in the
dumbbell sample without any invasive and damage pro-
cess remains an opportunity to reveal the changes of
phase structure, including the size, shape, and distribution
of the phase. To realize observation of the internal struc-
ture in a stretched sample, we introduced a lab-made
stretcher integrated with the specimen stage of MSLM,
as shown inSupporting Information Figure S19, which was
well matched with the working distance of the objective
lens. Tensile tests were carried out with a crosshead speed
of 4 mm/min at room temperature. Figure5apresents a
picture of the tensile sample after stretching for 30 s that
clearly shows the“red”undeformed region and the
“white”deformed region under visible light. 3D images
of both undeformed and deformed regions clearly show
the phase structure of PS and PP domains inside blends,
respectively. Remarkably, PS particles with a red signal
from the undeformed region were uniformly dispersed in
the dark PP matrix, whereas a huge dark groove corre-
sponding to the PP domain was found in the deformed
Figure 4| Visualization of the phase separation of TB/PS/PPfilms under different annealing rates. (a–c) Fluorescent
3D reconstructed imaging and section imaging (20, 40, and 60μm) of 1.0 wt % TB-dopedfilms of TB/PS/PP (20/80,
w/w) with different annealing rates. Temperature drops at rates of 50, 10, and 1 °C/min, respectively. (d–f) The 2PF
intensity and SNR plots of the TB/PS/PP (20/80, w/w)films processed with different annealing rates at an imaging
depth range from 0 to 120μm. The temperature drops at rates of 50, 10, and 1 °C/min, respectively. Scale bar: 40μm.
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725

9. region. This indicated that the transition from homoge-
neous system to inhomogeneous occurred in the blends.
Moreover, the distribution of PS particles became slightly
closer on both sides of the dark groove, and the size and
shape of PS particles exhibited no obvious changes, which
demonstrated that the PS phases had almost no strain
under the stress. In contrast, the dark PP phase exhibited a
large deformation region which correlated with the strain
of the blends (Supporting Information Figure S20). As
shown in Figure5b, we propose that the strain of PP in the
matrix was the main reason for the change in the PS phase
distribution. Meanwhile, the PS phase with almost no
strain moved slowly to both sides under tension. During
tensile testing, unlike stretchable organohydrogels with
binary cooperative phase,
45,46
thestressdoesnottrans-
fer from PP phase to PS phase directly when the blends
were stretched at a constant force due to the poor
compatibility between PS and PP. Therefore, the PS
phases did not exhibit cooperative deformation, which
led to the decreased tensile strength of the PP/PS blend
(Figure5c). Thesefindings were further confirmed by
SEM imaging of different areas of the stretched sample,
as shown inSupporting Information Figure S21,inwhich
PS particles were peeled from the PP matrix because
they were unable to produce strain together with the PP
phase.
Conclusions
In summary, we realize 3D visualization of internal macro-
phase-separated structures of immiscible blends of poly-
mer with and without phenyl moieties by manipulating
dye aggregates that exhibit the distinct luminescence
features in each polymer phase domain. No destructive,
complicated, and material-dependent chemical reaction
or modification is involved. It provides an emerging ap-
proach to the intuitive real-space representation of mor-
phological information and offers essential insight into
Figure 5| Visualization of phase distribution under constant tension. (a) Photographs of dumbbell-shaped samples
used for mechanical performance testing after stretching and 3D reconstructed image showing the phase distribution
of the polymer in deformed and undeformed regions. (b) Illustration of the deformation of PP (cyan-blue area) and PS
(red area) region under different stretching degrees during the stretching process. (c) The strain-stress curve of PP
(black line) and PP/PS blends (red line).
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10. complicated structures in polymer blends. More impor-
tantly, super high SNR (300) and deep imaging depth
(140μm) achieved by MLSM allowed for the non-invasive
and real time imaging of phase transfer dynamic process
in the practical polymer sample, which cannot be realized
by conventional electronic and optical microscopies. In
addition, we should emphasize that 3D visualization of
internal structures in polymer blends can be performed by
digital image analysis, which extracts useful and new
structural information from the obtained volume data.
This will help in unveiling the structure and properties of
complex structures and dynamics in both academic and
industrial areas.
Supporting Information
Supporting Information is availableand includes the
details of materials and characterization methods, the
experimental procedures of molecules,Figures S1–S21,
andTables S1.
Conflict of Interest
All authors declare no competingfinancial interest.
Funding Information
This work wasfinancially supported by the National
Natural Science Foundation of China (grant nos.
21835001, 52073116, and 51773080) and the JLU Science
and Technology Innovative Research Team (grant no.
2021TD-03).
References
1. White, J. L.; Bumm, S. H. Polymer Blend Compounding
and Processing. InEncyclopedia of Polymer Blends; Isayev,
A. I., Palsule, S., Eds.; Wiley-VCH: Weinheim,2011;pp1–26.
2. Bussink, J.; van de Grampel, H. T. Polymer Blends. In
Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH:
Weinheim,2000.
3. Paul, D. R. Polymer Blends: Phase Behavior and Prop-
erty Relationships. InMulticomponent Polymer Materials;
Paul, D. R., Sperling, L. H., Eds.; American Chemical Society:
Washington, DC,1985; Vol. 211, pp 3–19.
4. Robeson, L. Historical Perspective of Advances in the
Science and Technology of Polymer Blends.Polymers2014,
6, 1251–1265.
5. Pötschke, P.; Paul, D. R. Formation of Co-Continuous
Structures in Melt-Mixed Immiscible Polymer Blends. J.
Macromol. Sci. C2003,43,87–141.
6. Fortelný, I.;Živný, A.; JŮza, J. Coarsening of the Phase
Structure in Immiscible Polymer Blends. Coalescence or
Ostwald Ripening?J. Polym. Sci. Pol. Phys.1999,37, 181–187.
7. Yu, W.; Zhou, C.; Inoue, T. A Coalescence Mechanism for
the Coarsening Behavior of Polymer Blends During a
Quiescent Annealing Process. I. Monodispersed Particle Sys-
tem.J. Polym. Sci. Pol. Phys.2000,38, 2378–2389.
8. Toolan, D. T. W.; Hodgkinson, R.; Howse, J. R. Strobo-
scopic Microscopy-Direct Imaging of Structure Development
and Phase Separation During Spin-Coating.J. Polym. Sci. Pol.
Phys.2014,52,17–25.
9. Lorén, N.; Altskär, A.; Hermansson, A.-M. Structure
Evolution during Gelation at Later Stages of Spinodal Decom-
position in Gelatin/Maltodextrin Mixtures.Macromolecules
2001,34,8117–8128.
10. Mullin, N.; Hobbs, J. K. Direct Imaging of Polyethylene
Films at Single-Chain Resolution with Torsional Tapping
Atomic Force Microscopy.Phys. Rev. Lett.2011,107, 197801.
11. Keller, A. Morphology of Crystalline Synthetic Polymers.
Nature1953,171, 170
–171.
12. de Campo, C.; Dick, M.; Pereira dos Santos, P.; Haas
Costa, T. M.; Paese, K.; Stanisçuaski Guterres, S.; de Oliveira
Rios, A.; Hickmann Flôres, S. Zeaxanthin Nanoencapsulation
with Opuntia monacantha Mucilage as Structuring Material:
Characterization and Stability Evaluation Under Different
Temperatures.Colloids Surf.2018,558, 410–421.
13. Gu, B.; Sun, X.; Papadimitrakopoulos, F.; Burgess, D. J.
Seeing is Believing, PLGA Microsphere Degradation
Revealed in PLGA Microsphere/PVA Hydrogel Composites.
J. Control Release2016,228, 170–178.
14. Jabir, M. S.; Taha, A. A.; Sahib, U. I.; Taqi, Z. J.; Al-
Shammari, A. M.; Salman, A. S. Novel of Nano Delivery
System for Linalool Loaded on Gold Nanoparticles Conju-
gated with CALNN Peptide for Application in Drug Uptake
and Induction of Cell Death on Breast Cancer Cell Line.
Mater. Sci. Eng. C2019,94, 949–964.
15. Gramlich, M. W.; Bae, J.; Hayward, R. C.; Ross, J. L.
Fluorescence Imaging of Nanoscale Domains in Polymer
Blends Using Stochastic Optical Reconstruction Microscopy
(STORM).Opt. Express2014,22, 8438–8450.
16. Falsafi,S.R.;Rostamabadi,H.;Assadpour,E.;Jafari,S.
M. Morphology and Microstructural Analysis of Bioactive-
Loaded Micro/Nanocarriers via Microscopy Techniques;
CLSM/SEM/TEM/AFM. Adv. Colloid Interface Sci.2020,
280,102166.
17. Xue, L.; Li, W.; Hoffmann, G. G.; Goossens, J. G. P.; Loos,
J.; de With, G. High-Resolution Chemical Identification of
Polymer Blend Thin Films Using Tip-Enhanced Raman Map-
ping.Macromolecules2011,44, 2852–2858.
18. Momose, A.; Fujii, A.; Kadowaki, H.; Jinnai, H. Three-
Dimensional Observation of Polymer Blend by X-Ray Phase
Tomography.Macromolecules2005,38, 7197–7200.
19. Pyun, A.; Bell, J. R.; Won, K. H.; Weon, B. M.; Seol, S. K.;
Je, J. H.; Macosko, C. W. Synchrotron X-Ray Microtomogra-
phy for 3D Imaging of Polymer Blends. Macromolecules
2007,40, 2029–2035.
20. Koizumi,S.;Yamane,Y.;Kuroki,S.;Ando,I.;Nishikawa,
Y.; Jinnai, H. Three-Dimensional Observation of Phase-
Separated Poly(methyl methacrylate)/Poly(styrene-
ran-4-bromostylene) Blends by 3D NMR Microscopy with
X-Ray Microscopy.J. Appl. Polym. Sci.2007,103,470–475.
21. Yang, S.-G.; Wei, Z.-Z.; Cseh, L.; Kazemi, P.; Zeng, X.-b.;
Xie, H.-J.; Saba, H.; Ungar, G. Bowls, Vases and Goblets—The
RESEARCH ARTICLE
DOI:10.31635/ccschem.022.202101726
Citation:CCS Chem.2023, 5, 718–728
Link to VoR:https://doi.org/10.31635/ccschem.022.202101726
727

11. Microcrockery of Polymer and Nanocomposite Morphology
Revealed by Two-Photon Optical Tomography. Nat. Com-
mun.2021,12, 5054.
22. Onogi, S.; Shigemitsu, H.; Yoshii, T.; Tanida, T.; Ikeda, M.;
Kubota, R.; Hamachi, I. In Situ Real-Time Imaging of
Self-Sorted Supramolecular Nanofibres.Nat. Chem.2016,
8, 743–752.
23. Wang, S.; Liu, J.; Goh, C. C.; Ng, L. G.; Liu, B. NIR-
II-Excited Intravital Two-Photon Microscopy Distinguishes
Deep Cerebral and Tumor Vasculatures with an Ultrabright
NIR-I AIE Luminogen.Adv. Mater.2019,31, e1904447.
24. Qiu, Z.; Zhao, W.; Cao, M.; Wang, Y.; Lam, J. W. Y.; Zhang,
Z.; Chen, X.; Tang, B. Z. Dynamic Visualization of Stress/
Strain Distribution and Fatigue Crack Propagation by an
Organic Mechanoresponsive AIE Luminogen. Adv. Mater.
2018,30, e1803924.
25. Zhang, H.; Sun, J. Z.; Liu, J.; Kwok, R. T. K.; Lam, J. W. Y.;
Tang, B. Z. Visualizing and Monitoring Interface Structures
and Dynamics by Luminogens with Aggregation-Induced
Emission.J. Appl. Phys.2019,126, 050901.
26. Pujals, S.; Feiner-Gracia, N.; Delcanale, P.; Voets, I.;
Albertazzi, L. Super-Resolution Microscopy as a Powerful
Tool to Study Complex Synthetic Materials.Nat. Rev. Chem.
2019,3,68–84.
27. Wang, S.; Liu, J.; Feng, G.; Ng, L. G.; Liu, B. NIR-II Excit-
able Conjugated Polymer Dots with Bright NIR-I Emission for
Deep In Vivo Two-Photon Brain Imaging Through Intact
Skull.Adv. Funct. Mater.2019,29, 1808365.
28. Zhao, W.; Liu, Z.; Yu, J.; Lu, X.; Lam, J. W. Y.; Sun, J.; He, Z.;
Ma, H.; Tang, B. Z. Turning on Solid-State Luminescence by
Phototriggered Subtle Molecular Conformation Variations.
Adv. Mater.2021,33, 2006844.
29. Chen, Y.; Liu, Y.; Lu, S.; Ye, S.; Gu, H.; Qiang, J.; Li, Y.; Chen,
X. Photostimulated Spiropyran for Instantaneous Visualiza-
tion of Thermal Field Distribution and Flow Pattern.J. Am.
Chem. Soc.2020,142, 20066–20070.
30. Albertazzi, L.; Zwaag, D. v. d.; Leenders, C. M. A.; Fitzner,
R.; Hofstad, R. W. v. d.; Meijer, E. W. Probing Exchange Path-
ways in One-Dimensional Aggregates with Super-Resolution
Microscopy.Science2014,344,491–495.
31. Lopez-Barron, C. R.; Macosko, C. W. Characterizing
Interface Shape Evolution in Immiscible Polymer Blends via
3D Image Analysis.Langmuir2009,25, 9392–9404.
32. Guan, W.; Wang, S.; Lu, C.; Tang, B. Z. Fluorescence
Microscopy as an Alternative to Electron Microscopy for
Microscale Dispersion Evaluation of Organic-Inorganic Com-
posites.
Nat. Commun.2016,7, 11811.
33. Han, T.; Gui, C.; Lam, J. W. Y.; Jiang, M.; Xie, N.; Kwok,
R. T. K.; Tang, B. Z. High-Contrast Visualization and
Differentiation of Microphase Separation in Polymer Blends
by Fluorescent AIE Probes. Macromolecules2017,50,
5807–5815.
34. Nishikawa, Y.; Koga, T.; Hashimoto, T.; Jinnai, H. Mea-
surements of Interfacial Curvatures of Bicontinuous
Structure from Three-Dimensional Digital Images. 2. A Sec-
tioning and Fitting Method.Langmuir2001,17, 3254–3265.
35. Yang, R.; Jiao, Y.; Wang, B.; Xu, B.; Tian, W. Solid-State
Reversible Dual Fluorescent Switches for Multimodality Opti-
cal Memory.J. Phys. Chem. Lett.2021,12,1290–1294.
36. Qian, H.; Purwanto, N. S.; Ivanoff, D. G.; Halmes, A. J.;
Sottos, N. R.; Moore, J. S. Fast, Reversible Mechanochromism
of Regioisomeric Oxazine Mechanophores: Developing in
Situ Responsive Force Probes for Polymeric Materials.Chem
2021,7, 1080–1091.
37. Oneil, C. E.; Jackson, J. M.; Shim, S.-H.; Soper, S. A.
Interrogating Surface Functional Group Heterogeneity of
Activated Thermoplastics Using Super-Resolution Fluores-
cence Microscopy.Anal. Chem.2016,88, 3686–3696.
38. Doroshenko,M.;Gonzales,M.;Best,A.;Butt,H.-J.;
Koynov, K.; Floudas, G. Monitoring the Dynamics of Phase
Separation in a Polymer Blend by Confocal Imaging and
Fluorescence Correlation Spectroscopy.Macromol. Rapid.
Comm.2012,33,1568–1573.
39. Ye, L.; Hu, H.; Ghasemi, M.; Wang, T.; Collins, B. A.; Kim, J.
H.; Jiang, K.; Carpenter, J. H.; Li, H.; Li, Z.; McAfee, T.; Zhao, J.;
Chen, X.; Lai, J. L. Y.; Ma, T.; Bredas, J. L.; Yan, H.; Ade, H.
Quantitative Relations between Interaction Parameter, Mis-
cibility and Function in Organic Solar Cells.Nat. Mater.2018,
17, 253–260.
40.Marsac,P.J.;Li,T.;Taylor,L.S.Estimationof
Drug-Polymer Miscibility and Solubility in Amorphous Solid
Dispersions Using Experimentally Determined Interaction
Parameters.Pharm. Res.2009,26,139–151.
41. Qiang, Z.; Shebek, K. M.; Irie, M.; Wang, M. A Polymeriz-
able Photoswitchable Fluorophore for Super-Resolution Im-
aging of Polymer Self-Assembly and Dynamics.ACS Macro
Lett.2018,7, 1432–1437.
42. Qiang, Z.; Wang, M. 100th Anniversary of Macromolecular
Science Viewpoint: Enabling Advances in Fluorescence Mi-
croscopy Techniques.ACS Macro Lett.
2020,9,1342–1356.
43. Ito, S.; Funaoka, M.; Hanasaki, I.; Takei, S.; Morimoto, M.;
Irie, M.; Miyasaka, H. Visualization of the Microstructure and
the Position-Dependent Diffusion Coefficient in a Blended
Polymer Solid Using Photo-Activation Localization Micros-
copy Combined with Single-Molecule Tracking Based on
One-Color Fluorescence-Switching of Diarylethene.Polym.
Chem.2022,13, 736–740.
44. Chapman, D. V.; Du, H.; Lee, W. Y.; Wiesner, U. B. Optical
Super-Resolution Microscopy in Polymer Science. Prog.
Polym. Sci.2020,111, 101312.
45. Zhao, Z.; Zhang, K.; Liu, Y.; Zhou, J.; Liu, M. Highly
Stretchable, Shape Memory Organohydrogels Using
Phase-Transition Microinclusions.Adv. Mater.2017,29,
1701695.
46. Zhao, Z.; Liu, Y.; Zhang, K.; Zhuo, S.; Fang, R.; Zhang, J.;
Jiang, L.; Liu, M. Biphasic Synergistic Gel Materials with
Switchable Mechanics and Self-Healing Capacity. Angew.
Chem. Int. Ed.2017,56, 13464–13469.
RESEARCH ARTICLE
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