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2008, 112, 3586-3589 Published on Web 03/05/2008
Size Dependence of Transition Temperature in Polymer Nanowires Sana Nakanishi,*,† Hirofumi Yoshikawa,† Satoru Shoji,† Zouheir Sekkat,†,‡,| and Satoshi Kawata†,§ Department of Applied Physics, Osaka UniVersity, 2-1 Yamadaoka Suita, Osaka 565-0872, Japan, School of Science and Engineering, Al Akhawayn UniVersity in Ifrane, 53000 Irfane, Morroco, and Nanophotonics Laboratory, Riken, 2-1 Hirosawa, Wako, Saitama, Japan ReceiVed: January 17, 2008; In Final Form: February 12, 2008
We studied the effect of changing temperature on the mechanical properties of nanosized poly(methyl methacrylate) wires fabricated by two-photon fabrication. At around room temperature, the nanowires showed a transition temperature where the shear modulus suddenly changed. This transition temperature was observed to decrease more than 40 K by decreasing the radius of the nanowires from 450 to 150 nm. This size is several times larger in nanowires than reported values of polymer thin film thickness showing a depression of the glass transition temperature.
Polymer materials are characterized by their notable flexibility compared with metallic or ceramic counterparts. This notable flexibility is induced by the motion of the polymer chains. Therefore the thermal characteristics of a polymer are a key issue for a deep understanding of mechanical properties in a flexible polymer. Indeed, among the properties of polymers, the glass transition is one of the most important parameters to describe the mechanical properties of polymer materials. As the temperature of a polymer increases above its glass transition temperature “Tg”, molecular chains of the polymer start microBrownian motion,1 thereby making the polymer softer and more flexible, i.e., rubbery. Recently it was suggested that the glass transition temperature Tg is not an invariant parameter but a size-dependent variable, especially when the thickness of the polymer film becomes less than 100 nm.2-3 The invention of the two-photon polymerization (TPP) method enables one to fabricate an arbitrary three-dimensional polymeric structure4-6 with spatial resolution less than 100 nm.7,8 Several micro/ nanomechanical devices consisting of polymer three-dimensional micro/nanostructures have been proposed.9-12 In seeking the practical application of such polymer micro/nanodevices, one needs to consider not only the geometrical parameters such as spatial resolution and surface roughness but also the size effect on the mechanical properties (elasticity, viscosity, etc.). In this article we studied the relationship between the shear modulus of free-standing polymer nanowires fabricated by TPP and the temperature of the polymer, and found a remarkable sizedependent change of the transition temperature. For studying the shear modulus of polymeric nanowires, we used TPP to fabricate a nanowire of photopolymerizable resin into the shape of a coil spring, a geometry capable of magnifying mechanical deformations.9 Figure 1a,b shows an optical micro* Corresponding author. E-mail:
[email protected]. † Osaka University. ‡ Al akhawayn University. § Riken. | Z. C. is also with the Hassan II Academy of Sciences and Technology, Rabat, Morocco.
10.1021/jp800453p CCC: $40.75
Figure 1. Optical microscope image of experimentally produced coilshaped nanowire in (a) stretched state and (b) relaxation state. (c-e) Schematic image of method of driving coil-shaped nanowires by laser trapping force. (f) Temperature dependence of stretch length.
scope image of an experimentally produced coil-shaped nanowire with coil radius R of 4.5 µm and number of turns N of 5. © 2008 American Chemical Society
Letters For applying force onto the nanowire by means of laser trapping,a bead of 1.0 µm radius was attached to one end of the nanowires. The other end was connected to an anchor fixed to the glass substrate. The photopolymerizable resin we investigated here is a mixture of monomer (ethyl methacrylate, 50 wt %, Wako Pure Chemical Industries, Ltd.), cross-linker (dipentaerythritol hexaacrylate, 47 wt %, Kyoeisha Chemical Co., Ltd.), photoinitiator (benzil, 1.5 wt %, Wako Pure Chemical Industries, Ltd.), and sensitizer (2-benzyl-2-(dimethylamino)4′-morpholinobutyrophenone, 1.5 wt %, Aldrich Chemical Co., Inc.). The detailed procedures of the fabrication of the nanowire by TPP are explained in ref 6. For controlling the temperature of the nanowire, we used a heating and cooling device (LK600PH, Japan High Tec Corp.) mounted on a sample stage of an optical microscope. The sample cell with coil-shaped nanowires was placed in this device. In order to measure the stiffness of the nanowires, we focused the light from a 1064 nm cw Nd:YVO4 laser by an objective lens (40×, N.A. ) 0.6, LUCPlanFLN, Olympus) onto the bead in the sample cell and pulled on it by moving the focal position (Figure 1c-e). For evaluating the stiffness, we stretched the coil at 10 pN by means of laser trapping and measured the length of maximum extension of the coil. The movement up to the maximum extension where the bead escaped from the laser trap was sufficiently slow that the effect of viscous resistance was negligible. From the extension length, the spring constant was determined according to Hooke’s law, F) kx,9 where F is the trapping force of 10 pN, and x is the stretch length. For calibration of laser trapping force, we used 1 µm radius beads dispresed in the sample cell. The sample cell was quickly dragged to determine the stage velocity, at which the bead escapes. The calibration method of laser trapping force is shown in detail elsewhere.13 Finally, shear modulus G was calculated from the spring constant k and the geometrical parameters of the coil R, N, and the radius of the nanowire r, using the following equation: G ) 4NR3k/r4.14 First, we measured the maximum extension length while changing the temperature of the polymer nanowire every 1.5 °C from 15 °C to 44 °C. We performed the measurement five times at each temperature. From the five data we calculated an average value and standard deviation. The results are shown in Figure 1f with solid squares and error bars. The maximum length was initially about 4.5 µm and almost constant from 15 °C to 25 °C. Then, it started to increase gradually, and saturated to 6.0 µm at about 40 °C. This result indicates that the polymer nanowire underwent a sort of phase transition between 25 °C and 40 °C. After the first measurement, we cooled the nanowire from 44 °C to 6 °C and performed the same measurement again while heating from 15 °C to 44 °C. The results are shown as solid triangles in Figure 1f. As the transition behavior was seen to be almost the same in the first and second measurements, the transition we observed is a reversible and reproducible reaction, and the temperature change was slow enough to maintain thermal equilibrium. For comparison, the glass transition temperature of the polymer in the bulk state was measured as 152 °C by means of differential scanning calorimetry15 The transition temperature we observed with the nanowire is significantly smaller than that of the bulk. We fabricated eight coils with the same design as in Figure 1 but with different radii of the nanowires r by changing the laser power for TPP. In order to determine the r, we fabricated straight test wires as well, and we measured the radius of the wires using scanning electron microscopy (SEM). Typical SEM images of one nanowire are shown in Figure 2a,b. The radius of the nanowire is defined as r ) xrlrv, where rl and rv are the
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Figure 2. SEM images of nanowires in (a) top view and (b) side view. (c) Nanowire length measured from SEM image vs average radius. (d) Temperature dependences of stretch length of different sized nanowires.
semi-minor axis (in the focal plane for TPP) and the semimajor axis (along the propagation direction of the laser for TPP), considering that the fabricated wire has an elliptical sectional shape according to the intensity profile of the laser focus for TPP.9 The relationship between r and the laser power for TPP is shown in Figure 2c. The typical temperature dependence of the maximum extension length of different nanowires is shown in Figure 2d. It is clearly seen that thinner nanowires show a lower transition temperature, and stretch more easily. In our previous work, we showed that the laser power for fabrication affects not only the size of the nanowire but also the progression of the polymerization reaction simultaneously.9 The molecular structure of the polymer in a nanowire depends on the reaction of photopolymerization. In the early stage of the polymerization process, produced polymer molecules do not have a three-dimensional network structure and are small enough to be soluble. With the progress of the polymerization process, polymer molecules grow and cross-link each other to form a three-dimensional network, which results in an insoluble polymeric solid.16 Additional polymerization increases the crosslinking density, which induces a large shear modulus. These factors, i.e., cross-linking density and polymerization degree, possibly affect the phase transition temperature.17-19 In order to compensate for the progression of the polymerization reaction,
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Letters
Figure 4. (a) Evaluating transition temperature from the relation between temperature and shear modulus. (b) Relationship between the average radius of nanowire and transition temperature. Figure 3. (a) Hardening of a nanowire by additional UV irradiation, evidenced by the decrement of stretch length. (b) Comparison of transition temperature of a nanowire before and after UV irradiation.
we exposed ultraviolet (UV) light (UV lamp; center wavelength 370 nm) onto the nanowires in order to complete the photopolymerization reaction in the nanowire. From the decrease of maximum stretching length, we observed that the nanowires became hardened by additional UV irradiation, which indicates that the additional UV exposure induces further polymerization with remnant monomer molecules, cross-linkers, and radical species in the nanowires. We exposed nanowires to the UV light until the maximum stretching length ceased to decrease (Figure 3a). We compared the transition temperature of nanowires before and after additional UV irradiation, as shown in Figure 3b. We found that the transition temperature was not affected by additional UV irradiation, although the shear modulus was increased by additional UV irradiation for each temperature. Accordingly, the progression of the polymerization reaction does not affect the shift in transition temperature in nanowires. For determination of the transition temperature, Tc, we used the following equation:20
G(T) ) Gb +
Ga - Gb T - Tc 1 + exp a
(
)
(1)
where G(T) is the shear modulus at temperature T, Tc is the transition temperature, Ga is the shear modulus well above the transition temperature, Gb is the modulus at well below the transition temperature, and a is the transition rate. When the temperature increases from below Tc to above Tc, microBrownian motion is induced in molecular chains of the polymer, and the shear modulus decreases. This is because the polymer chains are released from secondary bonds such as dipole-dipole
interaction, van der Waals force, and hydrogen bonds, which restrict the motion of the polymer chain below Tc. If we assume that the probability of the secondary bond breakage follows a Gaussian distribution, we can describe the shear modulus around Tc with eq 1. Typical relationships between shear modulus and temperature are shown in Figure 4a. Fitted curves using eq 1 are shown as solid lines. From this curve fitting, the transition temperature Tc of each of the nanowires was calculated (shown as dotted lines in Figure 4a). Figure 4b shows the relationship between the average radius of the nanowires and Tc. Tc shows an almost linear relationship with the radius of the nanowire, and decreased more then 40 K when the average radius decreased from 450 to 150 nm. Now the question remains as to the origin of the reduction of transition temperature in TPP-fabricated nanosized polymers. Several studies of polymer thin films show a size-dependent shift of the glass transition temperature, which notably appears when the thickness of the polymer becomes less than 100 nm.7,8,21-24 Although the phenomena are not completely clarified, the prevailing interpretation is based on the concept of a layer model.25-28 The surface layer facing free space is characterized by enhanced mobility and simultaneous packing density reduction of molecular chains as compared to the bulk polymer. The polymer chains near the surface being exposed to ethanol solution have a larger degree of freedom for molecular dynamics compared with the polymer chains being bound deep inside the bulk, which in turn gives rise to significant depression of the transition temperature. When the size of the polymer becomes nanoscale, the specific surface area becomes inversely proportional to the size reduction, and the surface-originated effect becomes dominant in the mechanical properties of the polymer. Actually, the size of the nanowires we studied is several times larger than the reported thickness of polymer films that showed this effect; however, we clearly observed a similar
Letters depression of transition temperature. Our result suggests that the thickness of the surface layer in nanowires is larger than in the case of polymer thin films. One possible factor of the increment of the surface layer thickness is the existence of an ethanol solution surrounding polymer nanowires. When a polymer is immersed in a solution, solution molecules penetrate into the polymer network and induce the polymer to swell. It is also known that penetration and sorption of small molecules into the polymer induces the mobility enhancement of polymer chains.29 Manoli et al.30 reported that the penetration of water or methanol molecules into a poly(methyl methacrylate) (PMMA) film is obvious from the surface to a depth of about 100 nm. For comparison, we measured the Tg of the bulk polymer after it was immersed in ethanol for more than 5 months as well. The ethanol-immersed polymer showed a glass transition temperature of ∼150 °C, which is almost the same as that of dried polymer, and we did not see any obvious change in the glass transition temperature before/after immersion in the case of bulk polymer. Another possible factor is the reduction of the structural dimension from a two-dimensional (2D) thin film to a one-dimensional (1D) thin wire. The reduction of structural dimension leads to an increase in specific surface area of the polymer and further reduces the transition temperature. Mundra et al.31 reported a similar result showing significant depression of the glass transition temperature from 2D to 1D form in PMMA fixed on a silica substrate. As mentioned above, the transition temperature Tc observed in this study is significantly small compared with that for the bulk state. It is known that PMMA shows another transition, called β transition, which appears below the glass transition temperature at around ∼30 °C.3,32 The β transition originates in the hindered rotation of the polymer side groups of PMMA. The phase transition we observed in nanowires might be the β transition. Although we do not yet have enough evidence for conclusive support, another possibility is that the glass transition temperature dropped to around 20 °C when the size of the polymer is reduced to ∼100 nm. We need a wider temperature range to identify the origin of this transition; however, at higher temperatures, convection and evaporation of ethanol surrounding the nanowires disturbed our measurements. In conclusion, we observed a size-dependent change of transition temperature in nanosized polymer wire fabricated by TPP. Tc decreased more than 40 K with a decrease of the radius of the nanowire from 450 to 150 nm. To our knowledge, this is the first report showing a size-dependent transition temperature of free-standing 1D polymer nanowires. According to our experimental result, the transition temperature is sensitive to the size of the polymer in nanoscale, and therefore, the result reported here is a very important clue toward a practical application of polymer-based micro/nanomechanical systems. The experimental data also suggest a novel functionality in polymer micro/nanodevices, which is reversible temperatureswitchable elasticity/viscoelasticity. This property itself is actually intrinsic in any polymer materials, and so this is not surprising; however, by designing the size of polymer structures at the nanoscale, it becomes possible to control the switching temperature.
J. Phys. Chem. B, Vol. 112, No. 12, 2008 3589 Acknowledgment. The authors acknowledge Prof. Masahiro Irie, Prof. Sinzaburo Ito, and Prof. Toshikazu Takigawa for their valuable comments and discussions on this work. This work was supported by Grant-in-Aid (A) 17201033, MEXT, Japan. References and Notes (1) Mark, J. E. Physical Properties of Polymer; American Chemical Society: Washington, D.C., 1984. (2) Roth, C. B.; Dutcher, J. R. Eur. Phys. J. E 2003, 12, s103. (3) Fukao, K.; Uno, S.; Miyamoto, Y.; Hoshino, A.; Miyaji, H. Phys. ReV. E 2002, 64, 051807. (4) LaFratta, C. N.; Baldacchini, T.; Farrer, R. A.; Fourkas, J. T.; Saleh, B. E. A; Naughton, M. J. J. Phys. Chem. B 2004, 108, 11256. (5) Pitts, J. D.; Campagnola, P. J.; Epling, G. A.; Goodman, S. L. Macromolecules 2000, 33, 1514. (6) Wu, D.; Fang, N.; Sun, C.; Zhang, X. Appl. Phys. Lett. 2002, 81, 3963. (7) Takada, K.; Sun, H. B.; Kawata, S. Appl. Phys. Lett. 2005, 86, 071122. (8) Xing, J.-F.; Dong, X.-Z.; Chen, W.-Q.; Duan, X.-M.; Takeyasu, N.; Tanaka, T.; Kawata, S. Appl. Phys. Lett. 2007, 90, 131106. (9) Nakanishi, S.; Shoji, S.; Kawata, S.; Sun, H.-B.; Appl. Phys. Lett. 2007, 91, 063112. (10) Kawata, S.; Sun, H. B.; Tanaka, T.; Takada, K. Nature, 2001, 412, 697. (11) Knoll, A.; Duerig, U.; Zueger, O.; Guentherodt, H.-J. Microelectron. Eng. 2006, 83, 1261. (12) Galajda, P.; Ormos, P. Appl. Phys. Lett. 2001, 78, 249. (13) Svaboda, K.; Block, S. M. Annu. ReV. Biophys. Biomol. Struct. 1994, 23, 247. (14) Ancker, C. J.; Goodir, J. N. J. Appl. Mech. 1958, 25, 466. (15) We measured the glass transition temperature of the polymer in the bulk state by means of differential scanning calorimetry in accordance with JIS K 7121 testing methods for the transition temperature of plastics standardized by the Japanese Standards Association. (16) Park, S. H.; Lim, T. W.; Yang, D.-Y.; Cho, N. C.; Lee, K.-S. Appl. Phys. Lett. 2006, 89, 173133. (17) Hu, L.; Frech, R.; Glatzhofer, D. T. Polymer 2006, 47, 2099. (18) Keymeulen, H. R.; Diaz, A.; Solak, H. H.; David, C.; Pfeiffer, F.; Patterson, B. D.; Friso van der Veen, J.; Stoykovich, M. P.; Nealey, P. F. J. Appl. Phys. 2007, 102, 013528. (19) Dalnokki-Veress, K.; Forrest, J. A.; Murray, C.; Gigualt, C.; Dutcher, J. R. Phys. ReV. E 2001, 63, 031801. (20) Peleg, M. Biotechnol. Prog. 1994, 10, 652. (21) Forrest, J. A.; Dalnoki-Veress, K. AdV. Colloid Interface Sci. 2001, 94, 167. (22) Alcoutlabi, M.; McKenna, G. B. J. Phys.: Condens. Matter 2003, 17, R461. (23) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Eur. Phys. Lett. 1994, 27, 59. (24) Prucke, O.; Christian, S.; Bock, H.; Ruhe, J.; Frank, C. W.; Knoll, W. Macromol. Chem. Phys. 1998, 199, 1435. (25) Fryer, D. S.; Peter, R. D.; Kim, E. J.; Tomaszewski, J. E.; de Pablo, J. J.; Nealey, P. F.; White, C. C.; Wu, W.-L. Macromolecules 2001, 34, 5627. (26) Mattson, J.; Forrest, J. A.; Boerjesson, L. Phys. ReV. E 2000, 62, 5187. (27) Ellison, C. J.; Torkelson, J. M J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2745. (28) Ellison, C. J.; Torkelson, J. M Nat. Mater. 2003, 2, 695. (29) McCormic, M.; Smith, R. N.; Graf, R.; Barrett, C. J.; Reven, L.; Spiess, H. W. Macromolecules 2003, 36, 3616. (30) Manoli, K.; Goustouridis, D.; Chatzadroulis, S.; Raptis, I.; Valamontes, E. S.; Sanopoulou, M. Polymer 2006, 47, 6117. (31) Mundra, M. K.; Donthu, S. K.; Dravid, V. P.; Torkelson, J. M. Nano Lett. 2007, 7, 713. (32) McCrum, N. G.; Read, B. E.; Willam, G. Anelastic and Dielectric Effects in Polymer Solids; Wiley: London, 1967.
