Nanofluidics Approach to Separate between Static and Kinetic

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Nanofluidics approach to separate between static and kinetic nano-confinement effects on the crystallization of polymers Afef Houachtia, Pierre Alcouffe, Gisele Boiteux, Gerard Seytre, Jean-Francois Gerard, and Anatoli Serghei Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b00185 • Publication Date (Web): 26 May 2015 Downloaded from http://pubs.acs.org on May 29, 2015

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Nano Letters

Nanofluidics approach to separate between static and kinetic nano-confinement effects on the crystallization of polymers

Afef Houachtia1, Pierre Alcouffe1, Gisèle Boiteux1, Gérard Seytre1, Jean‐François Gérard2, Anatoli Serghei1*

1

Université Claude Bernard Lyon1, Ingénierie des Matériaux Polymères, CNRS-UMR 5223, 69622 Villeurbanne, France 2

INSA de Lyon, Ingénierie des Matériaux Polymères, CNRS-UMR 5223, 69622 Villeurbanne, France

Here we report a nanofluidics approach which allows one to discriminate, for the first time, between static and kinetic effects on the crystallization of polymers in 2-dimensional nanoconfinement. Nanofluidics cells designed to monitor in real time – via permittivity measurements – the flow process of polymers into cylindrical nanopores were employed to investigate the crystallization of poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) under static and under kinetic confinement conditions. A significant separation between static confinement effects and flow effects in confinement is reported. A characteristic time is deduced, to quantify the impact of flow on the crystallization process of polymers taking place under conditions of 2D geometrical nanoconfinement.

Keywords: nanofluidics, crystallization of polymers, nanopores, confinement, ferroelectric polymers, AAO membranes.

* corresponding author: [email protected]

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The crystallization process of polymers under conditions of geometrical nanoconfinement1-18 represents a topic of large scientific interest in the recent years. Nanoporous media of controlled pore size distribution and porosity have been often employed as a confining geometry.12-20 Changes in the crystallization process due to finite size effects as well as orientation effects imposed by the presence of interfaces have been reported.1,15,19,20 In addition to static factors related to the dimensionality and the morphology of the geometrical nano-confinement and to the influence of the interfacial interactions, kinetic aspects might come into play when nano-confined polymers are investigated under flow. However, until now, the investigations on the crystallization of polymers in nano-confinement have been only carried out under static conditions: usually, the nanoporous media are filled by the polymer under study and the crystallization process is subsequently characterized by various experimental techniques. No in-situ kinetic studies on the crystallization of nanoconfined polymers measured during the flow process have been reported so far. Here, we demonstrate an experimental approach that allows one to investigate the crystallization process of nano-confined polymers under kinetic conditions, using nanofluidics cells designed to monitor in real-time the flow process of polymers into cylindrical nanopores. Based on dielectric measurements on poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE), we report the ability to discriminate, for the first time, between static and kinetic effects on the crystallization of polymers in 2D nano-confinement. A significant separation between static confinement effects and flow effects in confinement becomes thus possible by this approach. The importance of this development consists in the fact that it gives a quantitative estimation of the impact of the flow process on the properties of confined polymers. To demonstrate our novel nanofluidics approach, the polymer PVDF-TrFE was chosen because of its ferroelectric and piezoelectric properties, which give rise to numerous applications in different industrial domains such as acoustic components, electrical equipments, medicalinstrumentation, robotics, security devices, optical devices. Our study gives evidence for the ability of inducing – by the flow process in nanoconfinement – orientation effects in the conformation of the polymer chains, which could represent a powerful tool for enhancing the performance of polymer nano-materials. To carry-out the nanofluidics investigations, highly ordered nanoporous membranes (AAO membranes) with a well-defined distribution of nanopores were prepared in a two-step electrochemical anodization process of pure aluminum.21,22 Basically, the anodization process consists in applying a voltage between an aluminum plate and a Platinum electrode, both immersed in an acid solution of controlled temperature. The process is carried-out in a two2 ACS Paragon Plus Environment

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step procedure, to ensure nanoporous membranes with an ordered distribution of nanopores (Fig. 1a,b,c). The pore diameter, the center-to-center distance between the pores and the thickness of the porous membrane can be controlled by adjusting the anodization conditions: type of electrolyte, concentration, temperature, anodization time and the applied voltage. After the second anodization, the AAO membranes were detached from the aluminum substrate by applying a voltage in a solution of 2,3-butane-diol.23 This leads to AAO membranes with nanopores having their both ends open, which represents an essential condition to be fulfilled in order to carry-out the flow experiments. For the current study, AAO membranes with a pore diameter of 40 nm and with a typical length of 100 microns were used. In order to discriminate between static confinement effects and flow effects in confinement, the experimental procedure was designed to allow investigations upon varying the flow conditions (i.e. the flow time) while keeping constant the size of the geometrical confinement (i.e. the pore diameter). The kinetics of the capillary flow of polymers into nanopores can be followed by using on optical microscope. This is exemplified in Fig 1d, for nanofluidics investigations on PVDF-TrFE (Mw=170.000 g/mol, purchased from Piezotech). A bulk layer of PVDF-TrFE was placed on top of the AAO membrane and the sample was heated slightly above the melting point of the polymer in order to initiate the flow process (Tm=156 oC for PVDFTrFE). After carrying-out the nanofluidics experiment a certain time t at a constant temperature T, the flow process was stopped by quenching the sample to room temperature. Upon immersing the sample into liquid nitrogen, the AAO membranes were broken in order to take optical images of the cross-section (Fig. 1d). The kinetics of the capillary flow is clearly reveled in these images due to the optical contrast between the nanopores filled with the polymer and the empty nanopores. Carrying-out the nanofluidics experiments for different flow times allows one to quantitatively analyze the kinetics of the capillary flow of PVDFTrFE into nanopores (Fig. 1e). A linear relationship between the length of the capillary flow and the square root of the flow time is observed. This is in agreement with the Washburntheory of the capillary flow,24 which gives the equation: γ (θ) / √

η

() =

(eq. 1)

L(t) being the flow distance, R the pore radius, γ the interfacial tension, cos(θ) the wetting angle, η the polymer viscosity and t the flow time. One can thus conclude that no 3 ACS Paragon Plus Environment

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deviations from the Washburn-theory of capillary flow is observed for PVDF-TrFE flowing into nanopores of 40 nm diameter. The nanofluidics investigations on PVDF-TrFE were carried out based on dielectric measurements by Broadband Dielectric Spectroscopy. The dielectric properties of PVDFTrFE in the bulk are discussed in Fig. 2, which shows the real part of the complex permittivity ε’ (Fig. 2a,b) and its first derivative in respect to temperature

(ε) ()

(Fig. 2c,d) measured at

different frequencies of the applied electric field, upon cooling (Fig. 2a,c) and heating (Fig. 2 b,d). The first derivative of the permittivity is directly proportional to the first derivative of density, as a result of the Clausius-Mosoti relation.25 Since phase transitions are accompanied by changes in density, the first derivative of permittivity allows one to detect the phasetransitions of polymeric materials by means of a dielectric measurement.26 For PVDF-TrFE in the bulk, a crystallization process and a ferroelectric Curie transition is observed on cooling (Fig. 2c) and a Curie transition followed by the melting process is detected on heating (Fig. 2d). The dielectric data were taken at different frequencies, but no frequency dependence was observed in the first derivative of permittivity. This is a typical behavior for a dielectric response dominated by density fluctuations characteristic for phase transitions.25 It is also observed that the crystallization/melting process and the Curie transition have opposite signs in the first derivative of permittivity: while the Curie transition is manifested by a positive peak, the crystallization and melting are characterized by negative peaks. This is due to opposite variations in density (decreasing, respectively, increasing) which take place when the polymer undergoes the observed phase transitions. In order to monitor in real time the flow process of polymers into the nanopores, nanofluidics cells have been fabricated using the AAO membranes27 (Fig. 3a). Thin layers of gold were deposited by sputtering on both surfaces of the empty AAO membranes. The sputtering conditions were optimized in order to ensure that the nanopores remain open after the metallization process. The gold layers, used as electrodes, were connected to the dielectric spectrometer in order to allow the monitoring of the flow process by means of a permittivity measurement (Fig. 3a). Since the electrodes are deposited on the empty cell (that is, before the polymer is brought in contact with the nanofluidics cell) no contributions from the bulk layer of PVDF-TrFE are measured by this approach. The bulk layer is only serving as a reservoir, to sustain the flow process of PVDF-TrFE into the nanopores. The strength of this approach relies thus in its unique ability of directly measuring – during the flow process – the contribution of the nanoconfined polymer separated from that of the bulk. 4 ACS Paragon Plus Environment

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Taking into account the hexagonal packing of the nanopores, the global dielectric permittivity of the nanofluidics cell can be expressed as: ∗ ∗ ∗ = (1 − ) ! + !!#$#%&

(eq. 2)

with '

∗ ()*)+,+-./

=

∗ (,+012.-

+−3

(eq. 3)

and

=

4 √5 6

7

7 8

(eq. 4)

99

where represents the porosity of the AAO membrane, :; the nanopores diameter, :?@AB? = TC −

DE FDGHIHJK

(eq. 5)

HLMNO Q HP

As evident from Fig. 5, this mathematical expression represents a good fitting function of the experimental data measured for Tkinetic. The fit gives Tstatic=109 oC, in excellent agreement with the direct determination of the crystallization temperature measured under 8 ACS Paragon Plus Environment

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static conditions (Fig. 4b). For the asymptotic behavior in the limit of long flow times, a value of T∞=124 oC is obtained. This indicates that, for PVDF-TrFE flowing into 40 nm nanopores, a shift by 15 oC in the crystallization process can be induced by the flow process. A value of tC=42 min is deduced for the characteristic flow time. This gives an estimate of the time scale governing kinetic effects on the crystallization process taking place under flow condition. Hence, the empirical function expressed by eq. (5) turns out to be useful in quantifying the relation observed between the flow time and the crystallization temperature. Investigations on PVDF-TrFE nanowires by complementary experimental techniques require one to detach, after performing the nanofluidics experiments, the bulk layer from the AAO membrane used as a nanofluics cell. Due to strong adhesion forces, the detachment turned out not to be possible for the polymer used in the current study. This is emphasizing once again the strength of the nanofluidics approach presented here, which is allowing measurements on the nanoconfined polymer while the bulk layer is still in contact with the measurement cell. To our best knowledge, no other development allowing in-situ investigations on the crystallization of nano-confined polymers under flow conditions has been reported in the literature so far. The polymer nanowires formed by the flow process can be released from the nanofluidics cell by completely or partially dissolving the AAO membrane in an aqueous solution of phosphoric acid. Typical images of PVDF-TrFE nanowires obtained by the flow process into nanopores are shown in Fig.6, after a partial dissolution of the AAO membrane carried-out for 3 minutes in an ultra-sound bath. Standing nanonowires with a high aspect ratio and with a well defined diameter (equal to the diameter of the nanopores) are clearly observable in the Fig. 6. In conclusion, the nanofluidics approach presented here allows one to clearly differentiate, for the first time, between static confinement effects and flow effects in confinement. It is demonstrated that kinetic experiments bring essential elements in understanding the crystallization of polymers under conditions of geometrical confinement. The experimental results brought clear experimental evidence that the flow process into nanopores can be used to manipulate, through orientation effects, the crystallization process of polymers. It was furthermore shown that the balance between kinetic and static effects can be adjusted by controlling the flow time. The ability to monitor and to control the flow process of polymers into cylindrical nanopores could represent thus a powerful tool for fabricating nano-materials with adjustable physical properties.


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Corresponding Author

Anatoli Serghei *E-mail [email protected].

Notes

The authors declare no competing financial interest.

Acknowledgements

We gratefully acknowledge the technical help of Laurent Cavetier in optimizing the dielectric cells for the nanofluidics investigations.


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References

(1) Duran, H.; Steinhart, M.; Butt, H.-J.; Floudas G. Nano Lett. 2011, 11, 1671-1675. (2) García-Gutiérrez, M.-C. ; Linares, A.; Hernández, J. J.; Rueda, D. R.; Ezquerra, T. A.; Poza, P.; Davies, R. J. Nano Letters 2010, 10, 1472-1476. (3) Fischer, F. S. U.; Tremel, K.; Sommer, M.; Crossland E. J. C.; Ludwigs, S. Nanoscale 2012, 4, 2138-2144. (4) Taden, A.; Landfester, K. Macromolecules 2003, 36, 4037-4041. (5) Barroso-Bujans, F.; Palomino, P.; Fernandez-Alonso, F.; Rudic, S.; Alegría, A.; Colmenero, J.; Enciso, E. Macromolecules 2014, 47, 8729–8737. (6) Ho, R.-M.; Chiang, Y.-W.; Lin, C.-C.; Huang, B.-H. Macromolecules 2005, 38, 47694779. (7) Li, M.-C.; Chang, G.-W.; Lin, T.; Ho, R.-M.; Chuang, W.-T.; Kooi, S. Langmuir 2010, 26, 17640-17648. (8) Jiang, Q.; Ward, M. D. Chem. Soc. Rev. 2014, 43, 2066-2079. (9) Sun, Y.-S.; Chung, T.-M.; Li, Y.-J.; Ho, R.-M.; Ko, B.-T.; Jeng, U. S.; Lotz, B. Macromolecules 2006, 39, 5782-5788. (10) Chung, T.-M.; Wang, T.-C.; Ho, R.-M.; Sun Y.-S.; Ko, B.-T. Macromolecules 2010, 43, 6237-6240. (11) Michell, R. M.; Blaszczyk-Lezak, I.; Mijangos, C.; Müller, A. J. Polymer 2013, 54, 4059-4077. (12) Meng, L. ; Wu, H.; Huang, Y.; Su, Z. Macromolecules 2012, 45, 5196–5200. (13) Michell, R. M.; Lorenzo, A. T.; Müller, A. J.; Lin, M.-C.; Chen, H.-L.; Blaszczyk-Lezak, I.; Martín, J.; Mijangos C. Macromolecules 2012, 45, 1517-1528. (14) Suzuki, Y.; Duran, H.; Akram, W.; Steinhart, M.; Floudas G.; Butt, H.-J. Soft Matter 2013, 9, 9189-9198. (15) Lutkenhaus, J.L.; McEnnis, K.; Serghei, A.; Russell, T.P. Macromolecules 2010, 43, 3844-3850. (16) Reid, D. K.; Ehlinger, B. A.; Shao, L.; Lutkenhaus, J. L. J. Polym. Sci. B 2014, 52, 14121419. (17) Duran, H.; Gitsas, A.; Floudas, G.; Mondeshki, M.; Steinhart, M.; Knoll, W. Macromolecules 2009, 42, 2881-2885. (18) Martín-Fabiani, I.; García-Gutiérrez, M.-C.; Rueda, D. R.; Linares, A.; Hernández, J. J., Ezquerra, T. A.; Reynolds, M. ACS Appl. Mat. Inter. 2013, 5, 5324-5329. 11 ACS Paragon Plus Environment

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(19) Steinhart, M. ; Senz, S.; Wehrspohn, R.B.; Gosele, U.; Wendorff, J.H. Macromolecules 2003, 36, 3646-3651. (20) Steinhart, M. ; Göring, P.; Dernaika, H.; Prabhukaran, M.; Gösele, U.; Hempel, E.; Thurn-Albrecht, T. Phys. Rev. Lett. 2006, 97, 027801. (21) Masuda, H.; Fukuda, K. Science 1995, 268, 1466-1468. (22) Masuda, H. ; Hasegwa, F.; Ono, S.; J. Electrochem. Soc. 1997, 144, 127-130. (23) Zhao, S.; Roberge, H.; Yelon, A.; Veres, T. J. Am. Chem. Soc. 2006, 128, 12352-12353. (24) Washburn, E.W. Phys. Rev. 1921, 17, 273-283. (25) Kremer, F. in Broadband Dielectric Spectroscopy; Kremer, F., Schönhals, A., Eds.; Springer: Berlin, 2003. (26) Serghei, A.; Lutkenhaus, J.L..; Miranda, D.F.; McEnnis, K.; Kremer, F.; Russell, T.P. Small 2010, 6, 1822-1826. (27) Serghei, A.; Chen, D.; Lee, D.H.; Russell, T.P. Soft Matter 2010, 6, 1111.


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Figure captions

Figure 1: (a,b,c) images by Scanning Electron Microscopy of highly ordered nanoporous membranes prepared by electrochemical anodization of aluminum. (d) optical image in cross section of an AAO membrane partially filled with PVDF-TrFE, as a result of a nanofluidics experiment carried-out at 160 oC. (e) kinetics of the capillary flow of PVDF-TrFE into nanopores of 40 nm diameter, measured at 160 oC.

Figure 2: real part of the complex permittivity ε’ (a, b) and its first derivative in respect to temperature

(ε) ()

(c, d), measured at different frequencies of the applied electric field, upon

cooling (a, c) and heating (b, d).

Figure 3: (a) nanofluidics cells designed to monitor in real-time, by means of a permittivity measurement, the flow process of polymers into cylindrical nanopores. (b) the temperature program of a nanofluidics experiment. (c) the flow process of PVDF-TrFE into nanopores of 40 nm diameter monitored by means of a dielectric measurement. The time t0 indicates the begin of the flow process, which happens when the melting point of the polymer is reached during the heating cycle. The red points t1, t2, t3, t4, t5 and t6 schematically indicate the moment when, after a certain flow time, the PVDF-TrFE is crystallized during the flow process by cooling the sample to room temperature.

Figure 4: phase transitions of PVDF-TrFE measured on cooling, in the bulk (a) and in 40 nm nanopores under static conditions (b) and under kinetic conditions in dependence on the flow time (c).

Figure 5: the crystallization temperature of PVDF-TrFE measured under kinetic conditions, in dependence on the flow time.

Figure 6: image by SEM of PVDF-TrFE nanowires fabricated by the nanofluidics experiment.


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Figures

(a)

(b)

(c)

40 flow length [µm]

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(d)

35

30

25

(e)

0,5

1,0 time t

Figure 1


1,5 1/2

2,0 1/2

[hour ]

2,5

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PVDF-TrFE bulk, heating

PVDF-TrFE bulk, cooling

60

80 60

40

40

20

20

Curie transition

crystallization

Curie transition

(d)

(c) -80

melting

-40

0 40 80 120 Temperature [°C]

160

-80

-40

0 40 80 120 Temperature [°C]

Figure 2


160

first deriv. of ε' [a.u.]

first deriv. of ε' [a.u.]

ε'

80

(b)

(a)

frequency 1e5 Hz 1e4 Hz 1e3 Hz

ε'

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a)

flow of PVDF-TrFE into 40 nm nanopores

(b)

o

Temperature [ C]

160 140 120 100

melting of PVDF-TrFE

80 60 40 20

(c)

0,6

ε'(flow) - ε'(empty)

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0,4

t1

t2

t3

t t4 t5 6

empty 0,2 cell 0,0

t0=begin of the flow process 0

1

2

3

4

Time [hours]

Figure 3


5

6

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bulk crystallization

Curie transition

(a) (b) crystallization under static confinement

40 nm: no flow (complete filling)

crystallization under confinement 40 nm: flow 10 min during flow (c) 20 min kinetic 30 min vs. 60 min static 90 min 120 min 180 min flow

first derivative of ε' [a.u.]

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-40

0

40 80 120 o Temperature [ C]

Figure 4


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122 120 o

Tc(kinetic) [ C]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

118 116 114 112 110 0

40

80

120 160 200 240

flow time [min] Figure 5


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2 µm

Figure 6


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TOC Figure

flow of PVDF-TrFE into 40 nm nanopores

40 nm: no flow

120 80 40 0,6 t1

0,4

t2

t3

t t t4 t5 6 7

0,2 0,0

empty cell t0 begin of the flow process

0

1

2

3

4

5

6

Time [hours]

crystallization

Curie transition

flow time t1 kinetic t2 vs. t3 static t4 t5 t6 t7 80 120

40 nm: under flow

flow

melting of PVDF-TrFE

160

first derivative of ε' [a.u.]

o

ε'(flow) - ε'(empty) Temperature [ C]

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-40

0

40

o

Temperature [ C]


Nanofluidics Approach to Separate between Static and Kinetic

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