Wetting behaviors of hyperbranched polymer composites within ordered porous template under vibration.
With the emergence of fabrication process based on micropolymer flow, e.g., micro/nano injection molding, the flow enhancement, and intermolecular entanglements reduction are attracting more attention from scientist around the world. Investigations of different process improvement methodologies, such as surface modification of micromolds, are more intensive due to their theoretical and practical significance (1-3).
Anodized alumina oxide (AAO) template is one of the most popular templates for micro/nano wire and tube fabrications. It contains pores with diameters tunable from a few tens of nanometers up to the micrometer range with high aspect ratios. Since it can be used as a mold for producing polymeric micro/nano products, studies of capillary behaviors of polymeric melts or solutions in AAO membranes are increasing (4-15). Most researches focused on the wetting mechanisms and morphologies of micro/nano rods or tubes. To our knowledge, there is no study on improving the polymer microflow by adding hyperbranched polymers into molding materials. Studies on improving the microflow with dynamic external forces such as high frequency vibration (16) are also limited.
Hyperbranched polymers are relatively new in polymer industry and they have many special characteristics compared with their linear homologue due to the unique three-dimensional dendritic structures. Their excellent thermal, optical, adsorption, and desorption properties have facilitated their applications in blending, coatings, and drug delivery systems (17), (18).
In this work, we will study the effects of introducing hyperbranched polyester (HBP) into cyclic olefin copolymer (COC) matrix on the microflow of COC melts in AAO micromold. The flow behaviors of the composite under vibration will also be investigated.
COC: The COC particles (5103LS) were purchased from Polyplastics, Japan. They were dewatered by being kept in a vacuum oven at 60 [degrees]C for more than 72 h before being used.
HBP: The hyperbranched polymer (PFH-64-OH) was purchased from Polymer Factory Company, Sweden. Each molecule of the polymer has on average 64 -OH terminated end groups. The theoretical molecular weight is 7300 g/mol and the polydispersity index is 1.27.
COC Samples. The COC particles were hot-pressed into a plate with thickness of about 1 mm under pressure of 0.3 MPa and temperature of 175[degrees]C.
COC-HBP Composite. The COC-HBP composite was prepared by solution blending method. 9.5 g COC and 0.5 g HBP were weighted and put together in 200 mL tetrahydrofuran (THF, AR). The blending solution was vibrated in an ultrasonic equipment for 60 min. Then it was poured into beaker containing large amount of deionized water. The composite blends were thus precipitated. The blends were put into vacuum oven and kept at 60CC for more than 48 h. Then the blends were also hot-pressed into a plate with thickness of about 1 mm under pressure of 0.3 MPa and temperature of 175[degrees]C. This composite was named as h-COC.
Anodized aluminum oxide (AAO) membranes (Whatman, U.K.), with pore size of 200 nm, were selected as ordered porous mold. Before usage, the molds were soaked in ethanol for 30 min.
In the first set of experiments, no vibration was applied. Five mold temperature, 150[degrees]C, 160CC, 170[degrees]C, 180[degrees]C, and 190[degrees]C, were used. The molding pressure was 0.6 MPa. The molding time was fixed at 15 min.
In the second set of experiments, temperature was fixed at 150[degrees]C. Four vibration frequencies, e.g., 1 kHz, 5 kHz, 10 kHz, and 100 kHz were applied. The molding time was also fixed at 15 min.
After the molding, the AAO membranes were dissolved using a 5 wt% sodium hydroxide solution in water/methanol (8:2) at room temperature for 12 h. Then the polymer samples were thoroughly washed by deionized water and dried under room temperature.
A home-made vibration assembly as shown in Fig. 1 was used in dynamic experiments. Sine signals with IV amplitude were generated by a signals generator and were amplified by a power amplifier (E-471, PI, Germany), which was connected to the pizeoelectronic transformer. The vibration amplitudes of the pizeoelectronic transformer measured with a laser displacement meter (LC-240, Japan) were around 0.10 [micro]m. A back pressure of 0.6 MPa was loaded on the samples throughout the experiments.
To ensure the reliability of experimental results, three identical experiments were repeated for each test, and five measurements were taken to get the average wetting displacement.
Dynamic mechanical properties were measured with a diamond analyzer (PE Instruments). Samples were cut into 25 x 5 x 1 [mm.sup.3] sized rectangles. All experiments were performed in tensile mode from 25[degrees]C to 180[degrees]C at a heating rate of 3[degrees]C/min. The frequency of the applied force was 5 Hz.
Scanning Electronic Microscopic Observation
The polymer samples were dipped into liquid nitrogen for 5 min. They were then taken out and broken into pieces for further cross-sectional morphology observation. Ag slurry was used to bond the broken samples onto testing plates. A thin layer of gold was sputtered on the samples before scanning electronic microscopy (SEM) observation (Leica Stereoscan 440).
RESULTS AND DISCUSSION
Effects of HBP on the Mechanical Properties of COC
To evaluate the effects of addition of HBP on the dynamic mechanical properties of COC matrix, DMA measurements were performed and the results were shown in Fig. 2. From the curves of storage modulus ([G.sup.']) and loss modulus ([G.sup."]) with temperature, it is obvious that the mechanical loss of h-COC sample is lower than that of pure COC sample when temperature is below 120[degrees]C. It suggested a much better molecular segmental mobility in h-COC sample than pure COC in this temperature region. This may be attributed to the spherical-shape and lubricating effect of HBP additives, which reduced the physical entanglements between COC macromolecules (19).
Effects of HBP on the Wetting Displacements of COC Without Vibration
Figure 3 shows the SEM images of the top view and the side view of the AAO template that has through holes of average diameter of ~200 nm. Figure 4 presented the SEM images of side views of microfibers of COC and h-COC materials wetted in AAO template under 160[degrees]C and 190[degrees]C, respectively.
Figure 5 presented the wetting displacements of the COC and h-COC samples in AAo templates in 15 min under different temperatures. It is clear that when temperature increased from 150[degrees]C to 180[degrees]C, the wetting displacements increased from 0.39 [micro]m to 5.72 [micro]m and from 0.52 [micro]m to 6.53 [micro]mi for COC and h-COC samples, respectively. This suggested that the h-COC melts can run faster than pure COC melts within microchannels of AAO template when temperature is below 180[degrees]C.
Generally speaking, the surface tension and viscosity of the liquid, the size of the capillary, and the length of the channel determine the rate of liquid flow in capillary (20), (21):
Dz/dt = R[gamma] cos [[thate].sub.c]/ (4[eta]z) (1)
where z is the length of melt column, t is time, [eta] is the viscosity of polymer melt, R is the hydraulic radius, [gamma] is surface tension, and [[thate].sub.c] 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. The viscosity of the polymer melts usually decreased with the increase of temperature:
In [eta] = In A + [DELTA] [E.sub.[eta]]/RT (2)
where In in the equation represents the logarithm based on 'e' A is a constant related to molecular structures, R is gas constant, [DELTA][E.sub.[eta]] is the active energy of polymer flowing.
In the temperature region of 150-180 [degrees]C, the growing wetting displacements with the increased temperature is attributed to the reduced viscosity of the two materials. However, as the temperature increased beyond 180 [degrees]C, the wetting displacement of h-COC sample decreased.
In the first temperature region, it is because the addition of spherical-shaped HBP molecules caused the reduction of intermolecular entanglements of COC molecules. The reduced intermolecular entanglements induced the increase of wetting displacements in microchannels. As temperature increased beyond 180[degrees]C, phase separation might have occurred in h-COC composite (22), (23). The phase separation might cause local aggregation and entanglement of HBP molecules. The pore walls of AAO are amorphous and contain water, electrolyte anions, and positively charged defects. Therefore, it can interact well with polar molecules, especially HBP molecules with theoretical 64--OH groups. The aggregated HBP may stick to the walls and partly block the microchannels, causing a slow down of microflow (24-26).
Effects of Vibration on the Wetting Displacements of Pure COC and h-COC
Figure 6 illustrated the relation between wetting displacements and vibration frequency for COC and h-COC samples at 150 [degrees]C. It is apparent that the effects of frequency on the two types of samples are dramatically different. For the pure COC samples, the increased vibration frequency boosted the wetting displacements in microchannels all the way to 100 kHz. However, for the h-COC, the wetting displacement increased with frequency when frequency was below 5 kHz, but it started to decrease with frequency when frequency was over 5 kHz. When vibration frequency was below 10 kHz, the wetting displacements of h-COC were more than COC samples. However, when frequency was beyond 10 kHz, the wetting displacements of h-COC became less than pure COC; therefore, the benefit of adding HBP additives was lost. Figure 6 showed, as frequency increased from 1 kHz to 5 kHz, the wetting displacements increased from 0.59 [micro] to 0.78 [micro]m and from 1.06 [micro] to 1.21 [micro] for the pure COC and h-COC, respectively. When frequency increased from 5 kHz to 10 kHz, the wetting displacements of COC increased from 0.78 [micro]m to 1.02 [micro]m, while the wetting displacements of h-COC decreased from 1.21 [micro]m to 1.02 [micro].
The vibration enhancements on the wetting of polymer melt within the microchannels may be explained by the fact that vibration can reduce the intermolecular entanglements that caused the reduction in both dynamic contact angle [[theta].sub.c] and the viscosity of polymer melt. While the difference between pure COC and h-COC when frequency was beyond 5 kHz may be due to the phase separation of h-COC once again. Because the increase of vibration frequency will increase the diffusion mobility of molecules in h-COC melts. As the diffusion mobility exceeds certain level phase separation may occur, causing a preferential aggregation of HBP molecules. The pore wall of AAO can interact well with polar molecules, especially HBP molecules with theoretical 64--OH groups. The aggregated HBP may stick to the walls of microchannels, causing the slow down of microflow. In fact, the effects of frequency are similar to the effects of temperature.
Figure 7 presented SEM images of top morphology of different microfibers molded under 1 kHz and 10 kHz vibrating frequencies with mold temperature of 150[degrees]C. Figure 8 plotted the possible wetting model of the composites within the microchannels.
In summary, the effects of adding HBP into COC matrix on the micromolding of COC polymer were investigated. Experimental results show the introduction of HBP molecules additives boosted the microflow of COC melt due to the reduction of intermolecular caused by HBP molecules. However, if the temperature is too high, phase separation will occur, causing the reduction of the speed of microflow. Applying high frequency vibration can also boost micropolymer flow. However, it also boosts the phase separation in HBP-COC composites. Therefore, appropriate selection of temperature and vibration frequency is critical for the enhancement of polymer flow in microchannels. Results of this paper are very useful for micromolding-based polymer processing.
NOMENCLATURE AAO Anodized alumina oxide COC Cyclic olefin copolymer HBP Hyperbranched polyester SEM Scanning electronic microscopy THF Tetrahydrofuran
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Yun-Chuan Xie, (1), (2) Yan Xu, (1) Kai-Leung Yung (1)
(1) Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Horn, Kowloon, Hong Kong, People's Republic of China
(2) Department of Materials Chemistry, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China
Correspondence to: Y.-C. Xie; e-mail: firstname.lastname@example.org or Y. Xu; e-mail: email@example.com
Contract grant sponsor: Research Grants Council of Hong Kong Special Administrative Region Government; contract grant number: RGC, PolyU-5327/07E.
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|Author:||Xie, Yun-Chuan; Xu, Yan; Yung, Kai-Leung|
|Publication:||Polymer Engineering and Science|
|Date:||Jan 1, 2012|
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