Conformation and structural changes of diblock copolymers with octopus-like micelle formation in the...

Conformation and structural changes of diblock

copolymers with octopus-like micelle formation in

the presence of external stimuli

K Dammertz, A M Saier, O Marti and M Amirkhani

Institute for Experimental Physics, Ulm University, Albert-Einstein-Allee 11, 89081

Ulm, Germany

E-mail: [email protected]

Abstract. External stimuli like vapors or electric fields can be used to manipulate

the formation of AB-diblock-copolymers on surfaces. We study the conformational

variation of PS-b-PMMA, PS and PMMA adsorbed on mica and their response to

saturated water or chloroform atmosphere. Using specimen with only partial polymer

coverage, new unanticipated effects were observed. Water vapor, a non-solvent for

all three polymers, was found to cause high surface mobility. In contrast, chloroform

vapor (a solvent for all three polymers) proved to be less efficient. Furthermore, the

influence of an additionally applied electric field was investigated. A DC field oriented

parallel to the sample surface induces the formation of polymer islands which assemble

into wormlike chains. Moreover, PS-b-PMMA forms octopus-like micelles (OLMs) on

mica. Under the external stimuli mentioned above, the wormlike formations of OLMs

are able to align in the direction of the external electric field. In absence of an electric

field, the OLMs disaggregate and exhibit phase separated structures under chloroform

vapor.

PACS numbers: 82.35.Jk, 82.35.Gh, 83.80.Uv, 68.47.Pe, 68.08.-p, 68.08.Bc, 68.35.bm

Submitted to: J. Phys. D: Appl. Phys.

marti

Schreibmaschinentext

Appeared in 2014 J. Phys. D: Appl. Phys. 47 175301 (http://iopscience.iop.org/0022-3727/47/17/175301)

Diblock copolymers with octopus-like micelle formation under external stimuli 2

1. Introduction

Block copolymers (BCPs) consist of two or more covalently bonded incompatible

blocks [1]. Due to the different chemical nature and conformational flexibility of

each block, BCPs can undergo distinct phase transitions and therefore spontaneously

form nanostructures via the molecular self-assembly process [2]. The morphology

of bulk BCPs is determined by the Flory Huggins segmental interaction parameter,

the total degree of polymerization, the volume fraction of the blocks and boundary

conditions [3, 4, 5]. Additionally, when a thin BCP film is confined to the surface of a

solid substrate and is exposed to gas environment, its nanoscale morphology strongly

depends on interfacial interactions (polymer-substrate and polymer-atmosphere) as well

as commensurability effects between the film thickness and the natural spacial period

of BCPs [5, 6, 7].

Studying ultra thin polymer films is of great interest, both for applications and for the

gain of scientific knowledge. When the characteristic length scales of a film approach

molecular dimensions, the long-range interactions are cut off at a finite thickness [8]. If

enough mobility can be provided to the molecules, lowering the thickness of a polymer

film destabilizes its surface, which in turn greatly affects the film stability.

To provide mobility or alter the conformation and morphology of BCPs, external stimuli

like thermal annealing [9], solvent vapor annealing (SVA) [10, 11], chemically patterned

substrates [12], electric fields (EFs) [13, 14, 15] or solvent evaporation [16] can be used.

Solvent annealing is one of the most widely applied technique. During SVA, polymeric

systems take up solvent molecules and thus affect the film morphology by changing the

polymer-polymer and polymer-surface interactions. In general, small solvent molecules

increase the mobility of polymers due to plasticizing effects and may also function

as a lubricant between the molecules and the substrate [17, 18]. Additionally, the

final structure of BCPs can be varied by choosing different solvents, dependent on

their particular selectivity [19, 20, 21]. Therefore, SVA can also be used to cause

destabilization and dewetting of polymeric systems [22]. In this case, dewetting occurs

due to short-range polar interaction forces between the polymer, the surface and the

solvent.

Furthermore, for BCPs, the interaction of each block with solvent molecules offers

the possibility to manipulate the dewetting process by using different solvents. In

addition to the dewetting process of polymers, which may occur for any type of

polymer with sufficient mobility, the morphology of thin film BCPs can be affected

by commensurability effects. Typically, a thin film with a thickness below 10 nm is

incommensurate with the natural period of the polymer film [23].

Applying an external electric field is another common method to influence the

conformation and dynamics of a polymer chain. The distinctive dielectric properties

of the two BCP blocks cause different torques for each component in the presence of

EFs, competing with interfacial energies due to BCP-substrate and BCP-atmosphere

interactions [24]. Furthermore, fluctuations at the interface of microdomains will be


amplified [25]. Hence, EFs, as well as SVA, can destabilize and dewet a polymeric

system [26, 27].

In recent years, the effect of external stimuli on thin diblock copolymer (DBCP) films

has been extensively investigated. However, the behavior of single chains or isolated

islands of DBCPs exposed to electric fields and/or long term solvent annealing was

not studied so far. SVA was applied to several different types of single polymer chains

[28, 29, 30, 31, 17], where it was found that a repetitive exposure of P2VP to water and

ethanol could cause a change in conformation from an elongated to a collapsed polymer

form [31]. Kumaki et al. transfered single polymer chains of PS-b-PMMA from a water

surface onto mica substrates and analyzed their response in humid atmosphere [32].

In our work, we study the conformation and configuration of DBCPs with micelle-

like structure under the influence of SVA and EFs. When a substrate is coated by a

DBCP solution of appropriate low concentration, an octopus-like micelle (OLM) may

form instead of an uniform thin polymer film. For this structure, each polymer chain

is in contact with a few other chains and can be distinguished from other molecules

by imaging techniques. Hence, the effect of external stimuli on DBCPs can be studied

on a microscopic level. The structure has two major differences compared to ultra-

thin films; first, islands are clearly separated from each other, cutting of the long-range

forces parallel to the substrate. Second, both DBCP blocks are in direct contact with

the substrate and upper surface.

2. Experimental Methods

2.1. Sample Preparation

Polystyrene (PS, Mw=101.000), Poly(methyl methacrylate) (PMMA, Mw=120.000) and

Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA, Mn=104.000, block ratio

1:1) were purchased from Sigma-Aldrich. We diluted the polymer samples in chloro-

form (SupraSolv grade, Merck), purchased from VWR international. Solutions with

concentrations from 0.002 to 2 g/l were spin-cast (Laurell Technologies Corporation,

WS-400B-6TFM/Lite) on freshly cleaved muscovite mica (Plano GmbH) with 3000 rpm

for 80 seconds.

Right after the preparation, we placed the samples either in a custom build high voltage

vapor chamber (Figure 1) or put them directly into a desiccator to dry them in vacuum

for at least 1 day. The samples prepared for the chamber were not dried in the beginning

to allow a higher surface mobility during the experiment.

The solvent and/or a horizontal electrical DC field (30 kV/cm) oriented parallel to the

surface was applied at room temperature. The exposure times which lead to a specific

conformation of PS, PMMA or PS-b-PMMA strongly depend on the quality of the used

mica sheet itself. Since muscovite-mica is a natural product, we left our samples for a

’sufficient time’ in the vapor chamber (in this work always for one week) to be sure that

no further changes are expected from longer times. However, if the surface quality is


poor (for instance mica sheets with higher roughness), all movements are suppressed,

resulting in a weak reaction.

2.2. Setup

Our high voltage chamber (Figure 1), completely built from PTFE, consists of carefully

isolated electrodes with a removable sample holder in the middle and teflon tubes to

apply vapors to the system. Different vapor molecules can pass into the chamber by

slowly purging nitrogen gas (N5, IndustrieGase AG, Germany) through a gas washing

bottle filled with the desired solvent.

Figure 1. Schematic top view of the high voltage chamber. Setup to apply

vapors and/or a parallel electric DC field.

2.3. AFM

We performed our measurements with a MultiMode SPM (Bruker MMAFM-2, 1997) in

tapping mode, using standard silicon cantilevers (Olympus OMCL-AC240TS, resonance

frequency 70 kHz, spring constant 2 N/m, tip radius 6-9 nm). Pictures were taken with

512 x 512 pixel at 1 Hz scanning frequency. Image analysis and editing was done with

NanoScope(TM) and Gwyddion [33].

3. Results and Discussion

3.1. Conformational behavior of PS-b-PMMA on mica

The AB-diblock-copolymer PS-b-PMMA contains one polar (PMMA) and one non-polar

(PS) block and is therefore able to form OLMs on mica (Figure 2). While depositing


a PS-b-PMMA solution on surface, the PMMA blocks will be randomly trapped and

adsorbed in a predominant elongated conformation due to their strong affinity to mica.

On the contrary, the PS chains remain in the solvent till all chloroform is evaporated.

Therefore, the PS chains are forced to collapse into flat globular islands at the end of

the evaporation process. At a polymer concentration high enough that the distance be-

tween two BCP chains is comparable to one polymer length, the PS blocks of different

polymer chains assemble into bigger globules. The size of resulting OLMs is concen-

tration dependent and reaches from single polymers (Figure 2a) up to big agglomerates

covering large areas (Figure 2c-d). The height of single PS cores ranges from 1-3 nm,

the strongly adsorbed star-like PMMA shell around is between 0.3 nm and 0.8 nm thick.

Hence, single PMMA blocks can be easily identified.

As long as the polymer concentration is low (0.02-0.2 mg/ml), islands with a diameter

between 0.5 nm and 2 nm are well distributed and cover approximately 2-25 % of the

sample surface (Figure 2a,b). For higher concentrations, the diameter of the PS cores

increases up to 30 nm (Figure 2b) until they loose their round appearance (Figure 2c)

and start to accumulate into a dense film (Figure 2d).

For this work, a polymer concentration that provides well separated, not too small

OLMs is most suited to analyze the molecular response to external stimuli. Hence,

we used polymer concentrations of approximately 0.02-0.2 mg/ml for all measurements

presented here. Due to strong interactions of PS-b-PMMA with preparation tools like

vessels or pipes, the concentration is slightly changing for different samples.

The formation of OLMs was also theoretically studied a few years ago [34]. Williams

describes the influence of good and bad solvent conditions to irreversibly grafted poly-

mer chains. A collapse of these chains and the formation of OLMs can be observed by

varying the grafting density under bad solvent conditions, which resembles our evapo-

ration process.

To confirm that the surface pattern formation of PS-b-PMMA depends only on the in-

teraction energy between all molecular components of the system and not on the surface

structure of mica, the experiment was repeated on silicon. We found that the overall

structural formation of PS-b-PMMA is the same. Furthermore, dip-coated samples are

indistinguishable from spin-coated samples (see Supporting Information). Hence, the

conformation and position of the micelles is just driven by the competing interaction

energies of the present materials. Using dynamic light scattering (DLS), we could also

show that the micelles are not present in solution. Accordingly, the patterns are formed

during the coating process. In addition to our experimental work, we performed sim-

ulations which likewise support our hypothesis on the origin of OLMs (manuscript in

preparation). Our results agree therefore quite well with existing studies about the for-

mation of OLMs as a random adsorption process of free PMMA blocks from solution and

a subsequent collapse of the PS chains as a result of the evaporating solvent [34, 35, 36].


Figure 2. PS-b-PMMA spin-cast on mica. Polymer concentration 0.002 (a), 0.02

(b), 0.2 (c) and 2 (d) mg/ml. OLMs with appropriate density are formed between 0.02

and 0.2 mg/ml.

3.2. Influence of water, ethanol and chloroform vapor

A very useful and common way to move polymers on surfaces is to apply a suitable vapor

to the system. In our study, we tested the response of PS, PMMA and PS-b-PMMA to

water vapor, a non-solvent, and chloroform vapor, a solvent for all three polymers. The

experiments were also done in combination with an applied electric field, described in

section 3.3.

Figure 3. PS 0.02 mg/ml on mica. Initial conformation (a), under water (b) and

under chloroform atmosphere (c).

Figure 4. PMMA 0.02 mg/ml on mica. Initial conformation (a), under water (b)

and under chloroform atmosphere (c).


Considering the two blocks of PS-b-PMMA separately, one can observe that PS ho-

mopolymers form homogeneously distributed globules with a concentration dependent

size on mica (Figure 3a). The height of spin-cast PS globules for a polymer concentration

of 0.02 mg/ml varies between 6-8 nm. Contrary to the formation process of an OLM, no

strongly adsorbing PMMA block is present. Therefore, the PS chains are slightly more

mobile during the evaporation process and form bigger and rounder globular islands.

The development of the molecular configuration resembles a dewetting process, driven

by the repulsive interaction between PS and the polar surface of mica.

In the presence of water vapor, we observed a slight movement and deformation of the

PS globules (Figure 3b). This reaction is a consequence of the strong attractive interfa-

cial energies between water and mica. For this reason, water tends to interact strongly

with the surface and is therefore applying pressure to the PS islands. As PS is a non-

polar polymer, the separated globular islands can not be moved over long distances. The

minimum and maximum height of the globules, ranging from 5-8 nm, is not significantly

changing compared to the initial state (Figure 3a).

As chloroform is a good solvent for PS, it likewise elongates on the surface when exposed

to chloroform vapor. The sample areas are mainly covered with extended chains, but

agglomerates with peak heights up to 100 nm can also be found (Figure 3c). However,

most of the structures are between 2 nm and 8 nm in height. Since applying chloroform

vapor to our system results in a very thin solvent film parallel to the mica surface, the

PS chains favor a flat, elongated conformation. Due to the very short timescale of the

subsequent evaporation process and the molecular configuration parallel to the surface,

the PS chains are not able to recollapse into a globular conformation after drying the

sample. Compared to water vapor, the conformational change and movement of PS is

much stronger under chloroform vapor.

Spin-cast PMMA homopolymers show an adsorbed extended conformation on mica,

covering as much surface area as possible (Figure 4a).

Contrary to the case of PS, we observed a quite strong movement of PMMA chains un-

der the influence of water vapor. For long exposure times (7 days), a distinct translation

of the chains accumulating into big, flat polymer domains (height less than 1 nm) or

globular structures (height 2-3 nm) could be found (Figure 4b). By carefully scanning

many areas on the samples, we observed elongated as well as collapsed conformations,

mainly concentrated in one region. Additionally, wide fields of the samples are clean.

Since PMMA contains hydrophilic ester groups, it can weakly interact with water, al-

though PMMA is indissoluble in water. Furthermore, PMMA chains are able to ’swim’

on water surfaces [32, 37]. As already stated above, water has a very strong affinity to

mica and is therefore able to displace molecules with lower interaction energies from the

surface. PMMA can therefore be desorbed and moved by water molecules ’crawling’

below the chains. The added mobility of such a system is very high, which explains

the observed strong horizontal translation of the polymers. As PMMA still has the

tendency to interact with mica, the polymer chains are more or less dragged into wide,

flat areas and do not accumulate so frequently into higher structures.


Under chloroform atmosphere, we observed an agglomeration process of PMMA chains

with a subsequent formation of partitioned areas on the surface (Figure 4c). The height

of these pancake-like PMMA structures is mostly below 1 nm, as expected for single

chains deposited side by side. However, at the center of these pancake-like structures, we

often found big PMMA agglomerates up to 60 nm in height. In contrast to the results

under water vapor, many parts of the sample are still covered with elongated PMMA

chains. We could not find any totally polymer free area which means that the overall

translation distance is smaller under the influence of chloroform vapor. Additionally,

many regions with weak or no reaction to the solvent stimulus could be observed. The

main reason for this differing behavior is the low attractive interaction of chloroform

with mica. Hence, PMMA is able to interact freely with the surface as well as with

the solvent molecules. Because PMMA does not really favor any of them (chloroform is

not a good solvent for PMMA), the resulting mobility is smaller compared to a system

exposed to water vapor. However, because the self-interaction of PMMA chains sur-

rounded by chloroform molecules may dominate, thicker structures can also be formed.

The same behavior was found for all concentrations tested (0.001-2.5 mg/ml), ranging

from single, well separated molecules up to thin continuous films.

The reaction of PS-b-PMMA to water vapor is shown in Figure 5. After long expo-

sure times (7 days), we found a collapse of PMMA strands forming a corona around PS

islands. In addition, the OLMs start to displace slowly and agglomerate into long worm-

like chains. The mean height of the OLMs is the same as for the initial state, 1-3 nm.

Compared to free PMMA chains under water vapor, we did not observe an extended

state for the PMMA block when bound to PS. As described above, water molecules try

to squeeze in between the PMMA chains and the substrate. Since one OLM can not

be displaced so easily due to its relatively inert PS core, the PMMA chains are forced

to collapse. Nevertheless, the presence of the polar polymer seems to provide enough

mobility to enable larger displacements of OLMs compared to pure PS which lead to

partial aggregation. Another interesting effect can be seen in Figure 5b. The formation

of a hole in a dense layer of OLMs shows that the accumulation process needs some kind

of nucleation core which results from local density fluctuations or contamination. Hence,

the development of the found structures starts at one specific point and proceeds from

the inner area to the outer side. Therefore, starting from a nucleus, small aggregations

can move as well as single OLMs and build up bigger clusters.

Under chloroform atmosphere, the initially formed surface micelles disaggregate due to

the elongation of the PS part (Figure 6). As already observed for PMMA, the poly-

mers agglomerate into large non-continuous pancake-like islands. For PS-b-PMMA, the

height of the accumulated polymers mostly ranges from 2-4 nm, but also structures up

to 140 nm high can be found. The main hypothesis is the same as already described for

the single components of PS-b-PMMA. The additional mobility of the block polymer is

high compared to PMMA homopolymers, but lower as for free PS. Additionally, due to

the repulsive interaction between the two components of the block-polymer and their

different affinity to the solvent as well as to the surface, phase separation (spinodal


decomposition) occurs. Therefore, structures with PMMA chains in the inner region of

one polymer domain surrounded by dense elongated PS chains are formed. Furthermore,

some single chains are not yet incorporated and driven towards an already agglomerated

area.

Figure 5. PS-b-PMMA in water vapor. Polymer concentration 0.02 mg/ml,

exposure to water vapor for 7 days. (a)-(c) show different sample regions.

Figure 6. PS-b-PMMA in chloroform vapor. Polymer concentration 0.02 mg/ml,

exposure to chloroform vapor for 7 days. (a)-(c) show different sample regions.

3.3. Influence of an external electric field

To effectively displace polar polymer chains by DC fields oriented parallel to the sample

surface, one has to provide mobility to the polymer chains. If an external EF is applied

to a dry sample, the adsorption force dominates and therefore no effect can be observed.

As described above, single PMMA chains react under saturated water atmosphere with

strong translation and agglomeration into flat polymer areas (height below 1 nm). In

combination with EFs, PMMA chains form ’droplets’ of approximately 2-6 nm height,

which attach to each other and produce randomly oriented wormlike structures (Figure

7a,b). Due to the polar nature of the PMMA side chains, such polymer droplets with

well defined, circular boundaries can be created. The resulting structures act more as

particles than single chains. After one polymer droplet is formed, it exhibits a macro-

scopic dipole which leads to the observed development of wormlike chains in the same


way as it is well known for the behavior of polar particles.

Figure 7. PMMA and PS in 30 kV with water. PMMA (a,b) and PS (c,d)

concentration 0.02 mg/ml, exposure to water vapor and a parallel DC field of 30kV

for 7 days.

Compared to the initial state, PS globules show an increased undulation of the

boundary and a strong tendency to fuse into bigger globules (height between 5-25 nm,

Figure 7c,d). Additionally, we observed a development of short wormlike chains due to

small induced dipoles. Similar to the behavior of PMMA, the described structures do

not orient in the direction of the external EF.

PS-b-PMMA OLMs also produce wormlike chains in EFs under water vapor (Figure

8). As described above, the PMMA corona collapses in humid atmosphere, and the

height of OLMs does not change compared to the initial state. Since the parallel DC

field affects the polar parts in the collapsed PMMA corona and induces a small dipole

in PS cores, the anticipated mobility of OLMs is much stronger under EFs due to

interfacial fluctuations, resulting in a translation of all OLMs attaching to wormlike

structures (Figure 8a and b, 9a and b). Although they contain a relatively weak mobile

PS core, the OLMs can move over very long distances. Furthermore, we could observe

a clear alignment in the direction of the EF (Figure 8c, 9c) for some samples. The

final structure attained after one week strongly depends on the quality of the surface

(e.g. surface roughness or contamination). If the exposure time to external stimuli is

longer than 7 days or the surface has a lower roughness, the OLMS can also fuse at

the contact points (Figure 9). In contrast, if a sufficient polymer density fluctuation or

surface contamination is missing, the essential nucleation centers may be more difficult

to form. The adjacent formation of wormlike structures can therefore be appreciable

slowed down (even after 5 days, no significant reaction can be observed).


Figure 8. PS-b-PMMA in 30 kV with water. Polymer concentration 0.02 mg/ml,

exposure to water vapor and a parallel DC field for 7 days. The inner image in (c)

shows the PSD of (c).

Figure 9. PS-b-PMMA under 30 kV in water vapor. Polymer concentration

0.02 mg/ml, exposure to water vapor and a parallel DC field for 8 days.

The power spectral density of Figure 8c (shown in the upper right corner of the

picture) indicates the observed directionality due to the influence of EFs. Based on the

average radial distribution of the autocorrelation function, we determined a character-

istic length scale d of 75 nm (see red line in Figure 8c). This length scale corresponds

to the average size of the polymer free areas between each aligned region, formed by

the attached chains shown in Figure 8c. Although we found numerous places where an

alignment can be clearly observed, many scan areas (maximum 12 µm2) show just a

wormlike formation.


Figure 10. PS-b-PMMA under 30 kV in chloroform vapor. Polymer

concentration 0.02 mg/ml, exposure to chloroform vapor and a parallel DC field for 7

days.

Under chloroform vapor in combination with an external EF, PS-b-PMMA form

pancake-like structures with a mean height between 2 nm and 6 nm (Figure 10). In

contrast to the effects found in chloroform atmosphere without EFs, no phase separa-

tion could be observed. A possible explanation for this characteristic response is the

dipolar nature of the used polymers, which enables high conformational fluctuations

under the influence of EFs. Hence, no equilibrium state can be achieved, essential to

initiate spinodal decomposition. Alignment of the pancake-like polymer structures has

not been observed in this experiment due to the lower mobility provided by chloroform

compared to water vapor, but is most likely after longer exposure times or under higher

field strength.

4. Conclusion

In this work, we presented the response of PS-b-PMMA OLMs as well as their single

components on mica to external stimuli (water vapor, chloroform vapor and parallel

DC field). In humid atmosphere, PMMA and PS-b-PMMA gain high mobility due to

the energetic competition between water molecules and PMMA chains regarding their

surface occupancy. Since PS is a non-polar polymer and the initially formed globules are

relatively big, the observed effects are smaller for this polymer. Subjected to an external

electric DC field of 30 kV/cm, all three polymers can form (partially aligned) wormlike

chains in their respective molecular conformation (see above). Under chloroform vapor,

the mobility of PS-b-PMMA and PMMA is lower compared to water vapor. The

resulting pancake-like polymer structures exhibit phase separated domains if no electric

field is applied. PS on the other hand clearly favors an interaction with solvent molecules

over one with the surface and therefore undergoes quite strong changes in conformation

and position. The predominant globules elongate completely in chloroform atmosphere

and are trapped in this conformation after drying the sample.

The methods presented to stimulate polymeric systems and the observed effects are

promising tools to manipulate polymeric materials in ultra thin films, applicable to


nanotechnology, condensed matter and life-science. With the proper choice of polymers,

vapors and surface characteristics, the desired systems can be altered on single molecule

level.

Acknowledgments

The authors thank Martin Muller (Institute of Experimental Physics, Ulm Univeristy)

for technical support, Jens-Uwe Sommer (Leibniz-Institute of Polymer Research Dresden

and Institute of Theoretical Physics, TU-Dresden) and Christoph Jentzsch (Leibniz-

Institute of Polymer Research Dresden) for helpful discussions.


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Supplementary data for

Conformation and structural changes of diblock

copolymers with octopus-like micelle formation in

the presence of external stimuli

K Dammertz, A M Saier, M Amirkhani and O Marti

Institute for Experimental Physics, Ulm University, Albert-Einstein-Allee 11, 89081

Ulm, Germany

E-mail: [email protected]

PACS numbers: 82.35.Jk, 82.35.Gh, 83.80.Uv, 68.47.Pe, 68.08.-p, 68.08.Bc, 68.35.bm

Submitted to: J. Phys. D: Appl. Phys.

2

Figure S1. PS-b-PMMA spin-cast on silicon. Polymer concentration 0.002 (a),

0.02 (b), 0.2 (c) and 2 (d) mg/ml.

Figure S2. PS-b-PMMA drop-coated on mica. Polymer concentration

0.02 mg/ml.

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