Patent Grant
7632544
The present invention relates to a novel method for making nanopatterned templates which can be subsequently used for preparing nanoscale industrial objects. More specifically, the present invention relates to a novel method which utilizes oriented degradable diblock copolymer thin films to form nanopatterned templates. The nanopatterned templates so formed from the method of the present invention can also be referred to as “nanoreactors”, which are articles containing a series of periodic nanostructured porous channels that can be used as molds, masks, or other types of templates, to subsequently manufacture products in nanoscale dimensions. The nanopatterned templates of the present invention can be advantageously and cost-effectively manufactured which can be subsequently utilized in healthcare, semiconductor, as well as many other industrial applications.
In recent years, the science involving the manufacturing and applications of nano-dimensioned (“nanomaterials”) has become one of the most promising and creative research areas. One convenient way to prepare materials in nanoscale dimension is to provide nanopatterned templates, i.e., “nanopatterns” with periodic porous nanostructured articles, for the growth of nanomaterials. These nanopatterned templates can be considered as “nanoreactor” for producing nanomaterials. More recently, extensive studies to exploit the concept of nanoreactors have been carried out in different research areas, and wide varieties of nanomaterials and nanoarrays have thus been obtained. Different methods for nanopatterning such as photolithography, soft lithography, scanning probe lithography, electronlithography (i.e., top-down methods) and self-assembly of living cells, surfactants, dendrimers and block copolymers (i.e., bottom-up methods) have been proposed and examined. For a recent review, see C. Park, J. Yoon, E. L. Thomas, Polymer 2003, 44, 6725-6760.
Among these studies, the formation of nanopatterns from the self-assembly of block copolymers driven by the immiscibility between the constituted blocks can be efficiently, economically achieved due to the ease of processing. Examples of this study can be found in Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525-557. For such nanopatterns to prove useful in nanoapplications, it is necessary to generate thin-film samples with well-oriented periodic arrays over large area. Different approaches to control over the orientation of phase-separated microdomains (MD) have been achieved by using (1) solution casting, see, e.g., G. Kim, M. Libera, Macromolecules 1998, 31, 2569-2577; P. Mansky, C. K. Harrison, P. M. Chaikin, R. A. Register, N. Yao, Appl. Phys. Lett. 1996, 68, 2586-2588; and R. G. H. Lammertink, M. A. Hempenius, J. E. van der Enk, V. Z.-H. Chan, E. L. Thomas, G. J. Vansco, Adv. Mater. 2000, 12, 98-103; (2) shear fields, see, e.g., G. Kim, M. Libera, Macromolecules 1998, 31, 2569-2577; P. Mansky, C. K. Harrison, P. M. Chaikin, R. A. Register, N. Yao, Appl. Phys. Lett. 1996, 68, 2586-2588; and R. G. H. Lammertink, M. A. Hempenius, J. E. van der Enk, V. Z.-H. Chan, E. L. Thomas, G. J. Vansco, Adv. Mater. 2000, 12, 98-103; (3) electric fields, see, e.g., T. L. Morkved, M. Lu, A. M. Urbas, E. E. Ehrichs, H. M. Jaeger, P. Mansky, T. P. Russell, Science 1996, 273, 931-933; and T. Thurn-Albrecht, J. Schotter, G. A. Kästle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C. T. Black, M. T. Tuominen, T. P. Russell, Science 2000, 290, 2126-2129; (4) patterned substrates, see, e.g., P. Mansky, Y. Liu, E. Huang, T. P. Russell, C. J. Hawker, Science 1997, 275, 1458-1460; E. Huang, L. Rockford, T. P. Russell, C. J. Hawker, Nature 1998, 395, 757-758; L. Rockford, Y. Liu, P. Mansky, T. P. Russell, Phys. Rev. Lett. 1999, 82, 2602-2605; and J. Heier, J. Genzer, E. J. Kramer, F. S. Bates, G. Krausch, J. Chem. Phys. 1999, 111, 11101-11110; (5) temperature gradients, see, e.g., T. Hashimoto, J. Bodycomb, Y. Funaki, K. Kimishima, Macromolecules 1999, 32,952-954; and (6) epitaxial crystallization, see, e.g., J. C. Wittmann, B. Lotz, Prog. Polym. Sci. 1990, 15, 909-948; C. De Rosa, C. Park, E. L. Thomas, B. Lotz, Nature 2000, 405,433-437; and R.-M. Ho, P.-Y. Hsieh, W.-H. Tseng, C.-C. Lin, B.-H. Huang, B. Lotz Macromolecules 2003, 36, 9085-9092.
Recently, a very rapid route to generate oriented Microdomains for use as a water-permeable membrane by spin coating for poly(styrene)-b-poly(ethyleneoxide) (PS-PEO) has been reported; within seconds, arrays of nanoscopic cylindrical domains of PEO were produced in a glassy PS matrix to open a novel route towards water permeable membranes with well-defined channel size. Their results are reported in Z. Q. Lin, D. H. Kim, X. D. Wu, L. Boosahda, D. Stone, L. LaRose, T. P. Russell, Adv. Mater. 2002, 14, 1373-1376. It has also been reported that block copolymers containing polyesters become the novel families for the preparation of nanoporous materials where polyester blocks might be selectively degraded, particularly by hydrolysis treatment, see, e.g., H. Tsuji, Y. Ikada, J. Polym. Sci., Part A: Polym. Chem., 1998, 36,59-66; and A. S. Zalusky, R. Olayo-Valles, C. J. Taylor, M. A. Hillmyer, J. Am. Chem. Soc. 2001, 123, 1519-1520. In the later article, it was also reported that ordered nanoporous polymers from poly(styrene)-b-poly(D,L-lactide) (PS-PLA) block copolymers has been successfully achieved by simply chemical etching of the PLA in the bulk.
The primary object of the present invention is to develop an improved method for making industrial objects with nanoscale dimensions, or nanomaterials. More specifically, the primary object of the present invention is to develop an improved method for preparing nanopatterned templates, which can be subsequently utilized for making nanomaterials.
In the present invention, it was discovered that large-scale, well-oriented nanochannel arrays in the form of thin films can be efficiently and cost-effectively produced by using degradable block copolymers. In some of the preferred embodiments the details of which will be discussed below, a series of degradable block copolymers, poly(styrene)-b-poly (L-lactide) (PS-PLLA), with PLLA hexagonal cylinder (HC) morphology has been synthesized. By selecting appropriate solvent for spin coating, the formation of large-size, oriented microdomains of PS-PLLA thin films where the axis of hexagonal cylinder morphology is perpendicular to the substrate (i.e., perpendicular morphology) was successfully achieved. Subsequently, nanopatterned templates were prepared after hydrolysis treatment.
Bulk samples of block copolymers were prepared by solution casting from dichloromethane (CH2Cl2) solution (10 wt % of PS-PLLA) at room temperature. Hexagonal cylinder nanostructures of amorphous PS-PLLA were identified by Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Similar results for various PS-PLLA samples having different molecular weights were also obtained. Thin films of the block copolymer were formed on different substrates by simply spin coating from dilute chlorobenzene (C6H5Cl) solution (1.5 wt % of PS-PLLA) at room temperature without further treatment. Well-oriented, perpendicular Microdomains was obtained as evidenced by scanning probe microscopy (SPM). The effect of alignment was further confirmed by the TEM images where the projected images reflected perpendicular cylinders on the substrate. As evidenced by selected area electron diffraction experiments, no crystalline diffraction was identified; suggesting that amorphous or low crystallinity samples were obtained after spin coating. With the process of the present invention, the oriented microdomains can be as large as several cm2 in area.
The method of the present invention can be tailored for great varieties of end use applications. For example, different substrates including glass slide, carbon-coated glass slide, indium tin oxide (ITO) glass, silicon wafer, silicon oxide, inorganic light emitted diode and alumina have been used for nanopatterning. Large-size, oriented perpendicular Hexagonal cylinder morphology was obtained. However, the bottom morphology of the nanopatterns appeared well-defined nanostructures under SPM examination only if the film was treated by hydrolysis (i.e., degradation of PLLA); suggesting that there is always a thin layer of PLLA formed on the substrate after spin coating (for instance, ca. 5 nm as estimated by volume fraction for cast film on glass slide having 50 nm thickness). Similar to the recent studies of electric field alignment of block copolymers; the surface effect from coated substrate on morphology is always existent. The effects of substrate affinity and interfacial energy have been examined in this study. The surface tension of PLLA (˜38.27 mN/m) is lower than that of PS (˜40 mN/m) besides the affinity of PLLA with hydrophilic substrate is slightly higher than that of PS. As a result, PLLA favors to preferentially segregate on the substrate so as to form PLLA thin layer. The formation of PLLA thin layer can be avoided by spin-coating the samples at temperature above Tg,PLLA but below Tg,PS. The behavior has also been observed in different cases; the glass transition temperature of at least one of the blocks should be below processing temperature in order to ease the substrate influence. As a result, the nanopatterned textures were thus schematically illustrated in
The method of the present invention can be utilized to extend the applicability of nanopatterns. It is possible to have tunable film thickness and dimension for nanopatterning. Reasonably wide range of film thickness from 20 nm to 160 nm can be obtained by simply controlling the spin rate of coating as expected. Oriented PS-PLLA samples can be achieved regardless of thin-film thickness. Also, the surface topography of formed nanopatterns is very smooth; the averaged roughness is in the range of 0.4 nm by SPM roughness evaluation. Different domain sizes as determined by TEM, SAXS and SPM were obtained by controlling molecular weight of PS-PLLA. Following the successful procedure for hydrolysis of PLA, well-oriented, perpendicular hexagonal cylinder nanochannel arrays were simply obtained within hour by using a sodium hydroxide solution of methanol/water (0.5M solution was prepared by dissolving 2 g of sodium hydroxide in an 40/60 (by volume) solution of methanol/water) at 60° C. for the degradation of amorphous PLLA. Consequently, nanopatterned templates over large area in addition to uniform surface with controlled thickness and domain size in the form of thin films were successfully prepared on different substrates.
In summary, in the present invention, we have presented an excellent and quick way to prepare large-scale microdomains for PS-PLLA diblock copolymers. Owing to the hydrolysis character of the polyester components, the formation of the ordered nanohole arrays provides a simple path to prepare nanopatterned templates for nanoapplications.
The present invention will be described in detail with reference to the drawing showing the preferred embodiment of the present invention, wherein:
The present invention will now be described more specifically with reference to the following examples. It is to be noted that the following descriptions of examples, including the preferred embodiment of this invention, are presented herein for purposes of illustration and description, and are not intended to be exhaustive or to limit the invention to the precise form disclosed.
A mixture of styrene (46 mL, 400 mmol), BPO (0.39 g, 1.6 mmol) and 4-OH-TEMPO (0.33 g, 1.92 mmol) (molar ratio of 4-OH-TEMPO/BPO =1.2) was preheated in a round-bottom flask (250 mL) in nitrogen atmosphere at 95° C. for 3 h to allow BPO to decompose completely. The system was then heated at 130° C. for another 4 h to yield PS-TEMPO-4-OH. The resulting polystyrene was precipitated with methanol (300 mL) from a THF (50 mL) solution.
The product was then recrystallized twice from CH2Cl2 (40 mL)/MeOH (200 mL) mixed solution, and collected by vacuum filtration to give white solids. The final solid was washed by 100 mL MeOH and dried in vacuo overnight to form PS-2 [yield: 32.6 g (78%). Mn=20900 and PDI=1.17. 1H NMR (CDCl3): 6.46-7.09 (br, 5H, ArH), 1.84 (br, 1H, CH), 1.42 (br, 2H, CH2)]. All manipulations were carried out under a dry nitrogen atmosphere. Solvents, benzoyl peroxide, styrene, L-lactide, and deuterated solvents were purified before uses.
A typical ring-opening polymerization procedure was exemplified by the synthesis of CP-4. [(η3-EDBP)Li2]2[(η3-nBu)Li(0.5Et2O)]2 (0.11 g, 0.1 mmol) was added to 4-hydroxy-TEMPO-polystyrene (PS-2, 4.18 g, 0.2 mmol) in 20 mL of toluene at 0° C. The mixture was stirred at room temperature for 2 h, and then dried under vacuum. The resulting product (lithium alkoxide macroinitiator) was dissolved in CH2Cl2 (20 mL) and L-lactide (2.16 g, 15 mmol) in CH2Cl2 (10 mL) was added. While the mixture was stirred for 4 h, conversion yield (74%) of poly(L-lactide) was analyzed by 1H NMR spectroscopic studies.
The mixture was then quenched by the addition of an aqueous acetic acid solution (0.35 N, 20 mL) and the polymer was precipitated on pouring into n-hexane (300 mL) to give white solids. The product was purified by precipitation from CH2Cl2 (30 mL)/Hexane (150 mL) mixture solution. The final crystalline solid was precipitated from CH2Cl2 (30 mL)/MeOH (150 mL) and dried under vacuum at 50-60° C. overnight to yield 3.02 g of PS-b-PLA(CP-4) (yield: 48%). Mn=46700 and PDI=1.17. 1H NMR (CDCl3): 6.46-7.09 (br, 5H, ArH), 5.16 (q, 1H, CH(CH3), J=7.2 Hz), 1.84 (br, 1H, CH), 1.58 (d, 3H, CH(CH3), J=7.2 Hz), 1.42 (br, 2H, CH2). 1H and 13C NMR spectra were recorded on a Varian VXR-300 (300 MHz for 1H and 75 MHz for 13C) or a Varian Gemini-200 (200 MHz for 1H and 50 MHz for 13C) spectrometer with chemical shifts given in ppm from the internal TMS or the central line of CHCl3. The GPC measurements were performed on a Hitachi L-7100 system equipped with a differential Bischoff 8120 RI detector using THF (HPLC grade) as an eluent. Molecular weight and molecular weight distributions were calculated using polystyrene as standard.
A number of poly(styrene)-poly(L-lactide) (PS-PLLA) chiral block copolymers were prepared. On the basis of molecular weight and volume ratio, these PS-PLLAs are designated as PSxx-PLLAyy (ƒPLLAν=z), wherein xx and yy represent the molecular weight of PS and PLLA divided by one thousand measured by NMR, respectively, and z indicates the volume fraction of PLLA. In these calculations, the densities of PS and PLLA are assumed to be 1.02 and 1.18 g/cm3, respectively.
Another series of PS-PLLA copolymers with different volume ratios were prepared by the same two-step “living” polymerization sequence. On the basis of molecular weight and volume ratio, these PS-PLLAs were designated as PSx-PLLAy (ƒPLLAν=z), with x and y representing the numbers of the repeating unit for PS and PLLA blocks and z representing the volume fraction of PLLA (calculated by assuming that densities of PS and PLLA were 1.02 and 1.18 g/cm3, respectively). Bulk samples of the block copolymers were prepared by solution casting from dichloromethane (CH2Cl2) solution (10 wt % of PS-PLLA) at room temperature.
Crystallization of PLLA in PS-PLLA gave rise to significant changes for microphase-separated morphology of PS-PLLA as observed in our laboratory. It is possible to destroy the formed microstructures so as to form crystalline morphology. DSC experiments were carried out in a Perkin Elmer DSC 7. For instance, PLLA blocks of PS29-PLLA22 (ƒPLLAν=0.37) melt at around 165° C. The maximum crystallization rate of PLLA blocks is at ca. 95° C. in accordance with exothermic response (i.e., the occurrence of crystallization) at different isothermal crystallizations. However, no significant exothermic response was observed under fast cooling. The glass transition temperatures of PLLA and PS are approximately 51.4° C. and 99.2° C., respectively.
SAXS experiments were conducted at the synchrotron X-raybeam-line X3A2 at the National Synchrotron Light Source in Brookhaven National Laboratory. The wavelength of the X-ray beam is 0.154 nm. The zero pixel of the SAXS patter was calibrated using silver behenate, with the first-order scattering vector q* (q*=4λ−1 sin θ, where 2θ is the scattering angle) being 1.076 nm−1. Time-resolved SAXS experiments were carried out in a heating chamber with step temperature increasing. Degradation temperature was identified by the disappearance of scattering peaks.
DSC thermogram appeared no melting endotherm during heating. WAXD (Widel-Angle X-ray Diffraction) diffraction exhibited amorphous diffraction profile. A Siemens D5000 1.2 kW tube X-ray generator (Cu Kα radiation) with a diffractometer was used for WAXD powder experiments. The scanning 2θ angle ranged between 5° and 40° with a step scanning of 0.05° for 3 sec. The diffraction peak positions and widths observed from WAXD experiments were carefully calibrated with silicon crystals with known crystal size.
Transmission electron microscopy in bright field was performed with JEOL TEM-1200× transmission electron microscopy. Staining was accomplished by exposing the samples to the vapor of a 4% aqueous RuO4 solution for 3 hours.
The surface of the solution-casting PS-PLLA samples after hydrolysis was observed using AFM (Atomic Force Microscopy). A Seiko SPA-400 AFM with a SEIKO SPI-3800N probe station was employed at room temperature in this work. A rectangle-shaped silicon tip was applied in dynamic force mode (DFM) experiments using a type of SI-DF20 with a spring force contact of 19 Nm−1 and scan rate of 1.0 Hz.
Field emission scanning electron microscopy was used to observe the PS-PLLA samples from different views. Field emission scanning electron microscopy (FESEM) was performed on a Hitachi S-900 FE-SEM using accelerating voltages of 2-5 keV. Samples were examined either on the solution-cast surface or fractured cross sections of PS-PLLA thin films after hydrolysis. The samples were mounted to brass shims using carbon adhesive, and then sputter-coated with 2-3 nm of gold (the gold coating thickness is estimated from a calculated deposition rate and experimental deposition time).
A variety of PS-PLLA bulk samples were prepared by solution casting from dichloromethane (CH2Cl2) solution (10 wt % of PS-PLLA) at room temperature. Table 1 shows the number-average molecular weights (Mn), polydiversity (PDI), volume fraction of polystyrene, ƒPSν, d-spacing, and diameter of the samples so prepared. The number-average molecular weight of each component of the copolymers was measured from integration of 1H NMR measurement. The polydipersity was obtained from GPC analysis. The values listed under column [c] were obtained from calculation of TEM micrographs. The values listed under column [d] determined from first scattering peak of SAXS. And the values listed under column [e] were obtained from surface analysis of scanning probe microscopy (SPM).
The method of the present invention can be tailored for great varieties of end use applications. For example, different substrates including glass slide, carbon-coated glass slide, indium tin oxide (ITO) glass, silicon wafer, silicon oxide, inorganic light emitted diode and alumina have been used for nanopatterning. Large-size, oriented perpendicular hexagonal cylinder morphology was obtained. However, the bottom morphology of the nanopatterns appeared well-defined nanostructures under SPM examination only if the film was treated by hydrolysis (i.e., degradation of PLLA); suggesting that there is always a thin layer of PLLA formed on the substrate after spin coating (for instance, ca. 5 nm as estimated by volume fraction for cast film on glass slide having 50 nm thickness). Similar to the recent studies of electric field alignment of block copolymers; the surface effect from coated substrate on morphology is always existent. The effects of substrate affinity and interfacial energy have been examined in this study. The surface tension of PLLA (˜38.27 mN/m) is lower than that of PS (˜40 mN/m) besides the affinity of PLLA with hydrophilic substrate is slightly higher than that of PS. As a result, PLLA favors to preferentially segregate on the substrate so as to form PLLA thin layer. The formation of PLLA thin layer can be avoided by spin-coating the samples at temperature above Tg,PLLA but below Tg,PS. The behavior has also been observed in different cases; the glass transition temperature of at least one of the blocks should be below processing temperature in order to ease the substrate influence. As a result, the nanopatterned textures were thus schematically illustrated in
Finally,
As discussed above, the present invention discloses an efficient and cost-effective way to prepare large-scale microdomains from PS-PLLA diblock copolymers. Owing to the hydrolysis character of the polyester components, the formation of the ordered nanohole arrays provides a simple path to prepare nanopatterned templates for nanoapplications.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6054111 | Antonietti et al. | Apr 2000 | A |
| 6271308 | de Boer et al. | Aug 2001 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040265548 A1 | Dec 2004 | US |
