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Enhanced Polymer Melts Flow though Nanoscale Channels under Vibration Jie Kong,†,‡,* Yan Xu,† Kai-Leung Yung,† Yunchuan Xie,† and Lan He† Department of Industrial and Systems Engineering, The Hong Kong Polytechnic UniVersity, Hung Hom, Kowloon, Hong Kong, P. R. China and Department of Applied Chemistry, School of Science, Northwestern Polytechnical UniVersity, Xi’an, 710072, P. R. China ReceiVed: October 16, 2008; ReVised Manuscript ReceiVed: NoVember 21, 2008
The enhanced poly(-caprolactone) melts flow behaviors though nanoscale channels under vibration were observed. The effect of vibration on the nanoflow is dependent on the vibration frequency of piezoelectric transducer that generates the vibration fields. The flow rate of poly(-caprolactone) melts in nanochannels increases with the increase of vibration frequency within the range from 2.0 to 14.0 kHz. The observed enhanced flow through nanochannels under vibration is a new nanoscale phenomenon, which is potential in vibration-assisted nanofluidic, nanoimprint lithography, and micro-/nanoinjection molding etc. Introduction The behavior of liquids or melts flowing through nanoscale channels is an active focus of current research especially with the development of micro-/nanotechnologies, which include capillary lithography, nanoimprint lithography, electro-spinning, and micro-/nanoinjection molding.1-9 These micro-/nanotechnologies play an important role on the fabrication of plastic micro/nano biosensors, bioreactors and biochips.10-15 With respect to these micro/nano technologies mentioned above, the favorable flow behavior of polymer melts through nanometerscale channels, pores or layers is strategic. Compared with the fluidics hydrodynamic of simple liquid through micro- or macrochannels, however, the flow behavior of polymer melts through nanochannels is difficult to be obtained16-21 and is strongly determined by the so-called nanoscale effects and surface interactions, such as capillary effect, surface energy, wettibility property, and the roughness of inner walls.22-25 In order to achieve the favorable flow of polymer melts through nanoscale channels, the wetting phenomenon of liquid on a high-energy solid surface should be adequately noticed. If the nanoscale channels are prepared using solid materials with high surface energy, the polymer melts with low surface energy can favorably wet and permeate into them due to wetting actions. So in an adequate time scale the flow of polymer melts in nanoscale channels can be conveniently observed.26 However the nanoflow rate is low to some degree if only depending on wetting actions. The low flow rate of polymer melts may cause an obstruction for developing nanoimprint lithography or micro/ nano injection molding technology, where the favorable flow rate of polymer melts through nanochannels is a key factor. So enhancing the flow rate of polymer melts through nanoscale channels is crucial for either fundamental or application research of micro/nano technology. As we know, thermo-pumping technology27,28 and electric field29 have been applied to boost the capillary flow of fluids in nanochannels. Experimental studies of the nanomeniscus between a vibrating nanoneedle of an AFM machine and liquids were also performed.30 It was found that the damping coefficient between the nanoneedle and liquids increased with the increase * Corresponding author. E-mail:
[email protected]. † The Hong Kong Polytechnic University. ‡ Northwestern Polytechnical University.
of vibration frequency. Meanwhile, the enhancement of solid particle movements in micro/nano channels using vibration has been observed recently.31,32 To the best our knowledge, an enhanced nanoflow by vibration for polymer melts has never been reported up to date although the enhancement of polymer melt nanoflow is critical and significant for many applications. So in this paper, we report for the first time an enhanced flow of polymer melts through nanoscale channels under vibration. Possible applications of our finding include, but are not limited to, vibration-assisted nanofluidics, nanoimprint and micro/nano injection molding etc. Experimental Section Materials. The biodegradable poly(-caprolactone) with number-averaged molecular weight (Mn) of 80 000, melting temperature (Tm) of 60 °C and melt flow index (MFI) of 1.0 g/10min (125 °C/44 psi) was purchased from Sigma-Aldrich. Nanoporous alumina templates with the diameters of 120-220 nm (Whatman Co Ltd., U.K.) are through-hole, freestanding disks with the channels length of about 54 µm. The chemical reagents including NaOH, ethanol and deionized water employed were purchased from Alfa Aesar China Co. Ltd. Achievement of Nanoflow of Poly(E-caprolactone) Melts. The flow of poly(-caprolactone) melts through nanoscale channels were conducted on a self-made apparatus illustrated in Figure 1. Parts 1-6 of the apparatus denote the pressure meter, temperature controller, alternating current generator, piezoelectric amplifier, piezoelectric transducer, and oscilloscope, respectively. The nanoporous alumina template was put below the poly(-caprolactone) melts, whose channels were perpendicular to polymer film and steel substrate. The polymer was heated by thermal cycle springs and surrounded with the hot gas of 0.6 MPa pressure. The experimental procedure is described as follows. The pressure of hot gas was controlled using pressure meter (1), and the mold enclosed with poly(-caprolactone) was heated by thermal cycle springs (Hotset, Forsteppe Asia Limited, Hong Kong) with the temperature controller (2) (KTM4, Panasonic, Japan). The poly(-caprolactone) and nanoporous alumina template enclosed in mold were heated to 90 °C and kept for 5 min. Then the alternating current with a set frequency, such as 2.0 kHz, was supplied from a generator (3) (MSO6054A,
10.1021/jp809164k CCC: $40.75 2009 American Chemical Society Published on Web 12/16/2008
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Figure 2. Illustration of polymer melt flow in nanochannels: (a) schematic of vibration and nanoflow; (b) l is the displacement and θ is the dynamic contact angle.
caprolactone) nanofibers/nanotubes arrays were released from the alumina nanopores after removing the template in a NaOH solution. At last, the poly(-caprolactone) nanofibers/nanotubes were characterized using Scanning Electron Microscopy (SEM, Leica Stereoscan 440) and Field-emission Scanning Electron Microscopy (FE-SEM, JEOL JSM-6335F). Before the measurements, all the samples were coated with Au film with thickness of 5.0 nm. Results and Discussion
Figure 1. Schematics of apparatus for achieving flow of polymer melts through nanochannels under vibration: (1) pressure meter, (2) temperature controller, (3) alternating current generator, (4) piezoelectric amplifier, (5) piezoelectric transducer, (6) oscilloscope. (7) Photo of vibration mold.
Agilent Technologies). Through a piezoelectric amplifier (4) (HVPZT, Physik Instrument, Germany), the current was transmitted to the piezoelectric transducer (5) (P-244.1X, Physik Instrument, Germany). Due to the piezoelectric effects, the vibration was generated from piezoelectric transducer and transmitted to the polymer melts and nanochannels. Depending on the fixed pressure (0.6 MPa) of hot gas surrounding the poly(-caprolactone) melts, the polymer melts were in the sustained vibration fields during their infiltration into nanoscale channels. Meanwhile, the alternating current can be introduced into an oscilloscope (6) (TDS2014, Tektronix) to monitor its frequency, amplitude, and waveforms. Characterization of Flow Behaviors. The displacement of polymer melts into nanochannels were deduced by statistical length of the poly(-caprolactone) nanofibers/nanotubes formed in alumina nanopores. The experimental procedure is as following. First, poly(-caprolactone) film with a thickness of about 300 µm and nanoporous alumina template with the diameters of 120-220 nm were assembled in the mold as presented in Figure 1. The nanochannels of alumina nanoporous template were perpendicular to the substrate of mold. Subsequently, under the vibration with a frequency, the poly(-caprolactone) melts infiltrated into the nanochannels at 90 °C and 0.6 MPa. The experimental temperature is far above the melting temperature (Tm, 60 °C). The poly(-caprolactone)/ nanoporous alumina template were taken out of the mold after they were cooled to the ambient temperature. Then the poly(-
The displacement lt and rate dlt/dt used to describe the flow behaviors of poly(-caprolactone) melts through nanoscale channels under vibration are presented in Figure 2. The driven force p is mainly generated from partial or complete wetting of poly(-caprolactone) melts with low surface energy on the alumina surface of nanochannels with high surface energy. It is mainly dependent on the surface tension, hydraulic radius of nanochannel, and contact angle.33 Depending on the wetting driven force, the poly(-caprolactone) melts infiltrated into nanochannels.34 The displacement lt at time t, and the rate dlt/ dt were deduced by statistical length of the poly(-caprolactone) nanofibers/nanotubes formed in alumina nanopores. Additionally the gravitational influence was negligible.35 In a vibration field, after wetting poly(-caprolactone) melts into alumina nanochannels with the diameters of 120-220 nm and the length of 54 µm (Figure 3), the poly(-caprolactone) nanofibers/nanotubes with an diameter ranged from 120 to 240 nm are aligned in a direction perpendicular to the remaining bulk poly(-caprolactone) substrate as presented in Figure 4. The length or height of the generated poly(-caprolactone) nanofibers/nanotubes can represent the trace of polymer melts infiltrated into nanochannels before they become condensed matter, which can be conveniently determined from their crosssection (Figure 4a). It is interesting that different one-dimensional nanostructures, i.e. nanofibers and nanotubes, are generated after the wetting process because of the nonuniform distribution of nanopores sizes. As we know, the wetting temperature defined as “wetting transition temperature” is considered as one of key factors to the formation of nanotubes or nanorods of polymers.35 Hereby, a “critical transition size” is suggested. For the nanopores with the diameter higher than the “critical transition size”, polymer melts in center can not be synchronizing with the spreading of melts near to inner wall in a suitable time-scale, resulting in the generation of nanotubes. According to the results of SEM observation, the “critical transition size” for this poly-
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Figure 3. SEM images of nanochannels of nanoporous alumina template employed: (a) cross-section of nanochannels in alumina template; (b) top view of nanochannels.
(-caprolactone) wetting system should be a value within 140 and 160 nm. The effects of frequency of vibration field generated by piezoelectric transducer on the nanoflow behaviors of poly(-caprolactone) melts were studied. The cross-section images of nanofibers/nanotubes formed under vibration fields with different frequency and wetting time are presented in Figure 5. The vibration frequency ranged from 2.0 kHz to 14.0 kHz was employed because of the frequency limitation of piezoelectric transducer (Figure 1(5)). Figure 6 shows the relationship between the displacement lt and the vibration frequency, in which the wetting time is fixed at 15 min. First, after wetting 15 min under static condition, i.e. without vibration, the displacement of poly(-caprolactone) melts in nanochannels is about 2.6 µm and the according aspect ratio of nanofibers/nanotubes infiltrated into nanochannels is about 10, which is much lower than the length of nanochannels. When the wetting system of poly(-caprolactone) melts and nanochannels is in the vibration fields with frequency ranged from 2.0 kHz to 14.0 kHz, the displacement (4.2-9.0 µm) within the same time (15 min) and on the same pressure (0.6 MPa) are enhanced obviously compared with a reference baseline of static condition. With the increase of vibration frequency, the displacement is increasing to the value of 9.0 µm under vibration with the frequency of 14.0 kHz. The according aspect ratio of nanofibers/nanotubes infiltrated into nanochannels is about 30. In addition, the relationship between the displacement lt and the time t in vibration fields with a frequency of 5.0 kHz in Figure 7 also shows that the speed of polymer melts through nanoscale channels under vibration is faster than that obtained without vibration.
Figure 4. SEM images of poly(-caprolactone) nanofibers/nanotubes formed after being released form nanochannels: (a) cross-section of nanofibers/nanotubes arrays; (b) magnified local zone of nanofibers/ nanotubes arrays; (c) top view of nanofibers/nanotubes arrays.
The observed nanoflow of polymer melts through nanoscale channels is similar to the wetting process. Although the fundamentals of equilibrium wetting phenomenon have been well explored, the dynamic process (or called three-phase contact line motion problem), which is particularly important for many potential practical applications, remains poorly understood.36-41 Different models have been established to try to explain the phenomena.36-39 The mechanisms of dynamic wetting are not yet sufficiently understood. It is acknowledged that the pressure drop across the liquid/vapor interface is one of the sources that drive the infiltration. Besides static contact angle, there is a dynamic contact angle that depends not only on the wetting properties of solid walls but also on the moving speed of the
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Figure 5. SEM images of cross-section of poly(-caprolactone) nanofibers/nanotubes arrays formed under different infiltration conditions: (a) infiltrating 15 min without vibration, (b) infiltrating 15 min under vibration with frequency of 2.0 kHz, (c) infiltrating 15min under vibration with frequency of 5.0 kHz, (d) infiltrating 15min under vibration with frequency of 10.0 kHz, (e) infiltrating 40 min without vibration, and (f) infiltrating 70 min under vibration with frequency of 5.0 kHz.
wetting line. The infiltration speed is approximately an exponential function of time. It decreases faster at the beginning and approaches zero slowly. Since the wetting speed depends on the dynamic contact angle, the motion of wetting line stops when the dynamic contact angle approaches the equilibrium contact angle. Zhang and Russell et al.35 modeled the rate of the flow of polymer melt in nanopore with the Lucas-Washburn equation:42
dz/dt ) Rγ cos θc/(4ηz)
(1)
Here z is the length of melt column, t is time, η is the viscosity of melt, R is the hydraulic radius, γ is surface tension, and θc is contact angle. The equation shows the wetting velocity depends on the surface tension, liquid viscosity, diameter of micro/nano channels and the contact angle. On the other hand, the dynamic contact angle is dependent on the wetting velocity. This makes the analytical
modeling of wetting velocity (even without applying vibration) a difficult task. The observation of this paper that vibration enhances the nanoflow of polymer melts is the first time. It will be more challenging to explain its mechanism. Here, we only give a preliminary analysis. The first reason is the variation of the dynamic contact angle induced by vibration. As being assumed by other researchers,36 there is often no slippage at the liquid-solid boundary, while slippage occurs a few molecules away from the boundary. The slippage may be enhanced by vibration, which will alter the shape of the nanoflow curvature such that the dynamic contact angle is further away from the equilibrium angle, causing the increase in the infiltration driving force. Figure 8 shows a simple schematic of a dramatic variation of curvature and dynamic contact angle that may happen due to vibration. Moreover, when the frequency of vibration increases, slippage inside the melt may
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Kong et al. in melts. The frictions from inter or intra macromolecules chains or segments hinder the flow and motion driven by the wetting effects. Under the vibration fields with appropriate frequency, the proposed overlap pulsation shear effects reduce their viscosity. The similar results about the reduced dynamical apparent viscosity can be found in the polyethylene melts in oscillation molding apparatus43 or polypropylene melts with pulsatile pressure flow in a dynamic capillary rheometer.44 The reduced resistance of polymer melts along the inner walls of nanochannels will enhance flow rate of poly(-caprolactone) melts through nanochannels under vibration. Conclusions
Figure 6. Dependence of displacements of poly(-caprolactone) melts in nanochannels under vibration when wetting temperature is 9 0 °C, pressure is 0.6 MPa and wetting time is 15 min. The displacement of melts under static condition without vibration is 2.6 µm.
In conclusion, the enhanced flow behaviors of poly(-caprolactone) melts through nanoscale channels under vibration can be obtained and observed. It is found that when the vibration frequency is ranged from 2.0 kHz to 14.0 kHz, the flow sped of poly(-caprolactone) melts in nanochannels increases with the increase of vibration frequency. Under the vibration field, the reduced resistance of polymer melts along the inner walls of nanochannels and the variation of the dynamic contact angle induced by vibration are proposed to explain its mechanism preliminarily. Acknowledgment. The authors thank the finically supports from the Research Grants Council of Hong Kong Special Administrative Region Government (RGC, PolyU5314/05E) and Postdoctoral Fellow Grant Project of The Hong Kong Polytechnic University. References and Notes
Figure 7. Dependence of displacements of poly(-caprolactone) melts in nanochannels on the wetting time under static condition without vibration and under vibration with the frequency of 5.0 kHz.
Figure 8. Comparison of curvatures of nanoflows in static condition and vibration field: (a) static condition; (b) vibration field.
be further enhanced. The dynamic contact angle further deviated from its equilibrium and the infiltration speed further increased. In addition, when vibration is applied, shear rate in the polymer melt promotes the disentanglement of macromolecules and thus reduces the viscosity. According to the molecular dynamics, the nanoflow of polymer melts in nanochannels can be accomplished by the vermiculation of polymer chain coils
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