Ordered Nanoarrays from Pore-Filling Nanoporous Templates and Corresponding Nanoscale Spatial Effect

利用奈米孔洞模板製備奈米有序陣列及其空間侷限效應

羅冠昕(Kuan-Hsin Lo)

指導教授 ： 何榮銘

國立清華大學/工學院/化學工程學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00010

摘要

A series of degradable BCPs, polystyrene-b-poly(L-lactide) (PS-PLLA), with PLLA hexagonal cylinder (HC) morphology has been synthesized in this study. Well-oriented, perpendicular PLLA cylinders of PS-PLLA thin films were efficiently achieved by spin coating using appropriate solvents regardless of the use of substrates. After hydrolysis of PLLA, well-oriented nanoporous templates over large area in addition to uniform surface with controlled thickness and domain size were obtained; providing a simple and efficient path to prepare topographic nanopatterns for optical and biological applications. Nanoporous templates with well-oriented periodic arrays have numerous potential applications through pore-filling process. It is important to examine the features of pore-filling process for the nanoporous templates so as to create various templated nanomaterials for practical applications.
To achieve efficient pore-filling process, specific treatments on the polymeric templates were carried out. The pore-filling process involves the trust of capillary force driven from the tunable wetting property of solution for the templates. By taking advantage of the pore-filling process, in-situ formations of CdS nanostructures can be achieved by introducing the solution of Cd ions followed by the treatment of H2S vapor as reduction agent. Various shapes of the cluster of CdS nanocrystals including tubular-like and cylinder-like CdS nanostructures in the templates can thus be obtained through different pore-filling processes including air-block releasing and directed capillary force methods. Interesting spectroscopic results were found in ultraviolet (UV) and potoluminescence (PL) spectrum, indicating that the emission intensity of the CdS nanoarray can be modulated by pore-filling process.
Furthermore, the same pore-filling process for the nanoporous PS template was developed for hybridization by exploiting the directed capillary force with the tunable wetting property of pore-filling solution. For medical application, we elucidated the feasibility of using the nanoporous PS templates for the controlled release of drugs through pore-filling sirolimus (a potent immunosuppressive drug) into the templates. Consequently, after the pore-filling process, sirolimus-loaded cylindrical and lamellar nanoarrays can be obtained. A comparison with those of macroscale templates indicated that the developed nanoporous templates can successfully entrap the loaded drug in nanoscale pores, markedly increasing the duration of drug delivery. As a result, the size, geometry, and depth of the nanoscale pores of the nanoporous templates can be readily controlled to regulate the drug release profiles.
For optical application, we aim to examine the feasibility of forming nanostructured thin films with cylindrical nanoarray by pore-filling chromophore/dispersant mixture or hydrophilic conjugated polymers into the nanoporous PS template. Through a specific pore-filling process, i.e., a solvent-annealing process, the optical materials can be introduced into the template to form well-defined nanoarrays with specific optical characters. It is important to realize the corresponding specific properties for the control of hybridized nanostructures. First, a simple method to generate ordered 1-pyrenebutanol/isochromanone (PY/CM) nanoarrays through a pore-filling process for nanoporous polymer templates was developed so as to enhance the chromophore luminescence of the PY. Fluorescence results combining with the morphological evolution examined by scanning probe microscopy revealed that the enhanced luminescence intensity reaches the maximum intensity as the nanopores of template are completely filled by the mixture of PY/CM. The variation is attributed to nanoscale spatial effect on the enhanced mixing efficiency of PY and CM, i.e., the alleviation of self-quenching problem, as evidenced by the results of attenuated total reflection Fourier transform infrared spectroscopy combining with grazing incident wide-angle X-ray (GI-WAXD) diffraction. The nanostructured thin film gives better dispersion for the pyrene in the pyrene/dispersan mixture due to nanoscale spatial effect so as to inhibit the self-quenching problem, resulting in the enhanced luminescence.
Moreover, the introduction of dispersants not only results in the disassociation of non-emission pyrene aggregates but also drives the anisotropic orientation of pyrene molecules due to nanoscale spatial effect. Consequently, the behavior of chromophore luminescence such as monomer and excimer emissions as well as self-quenching is strongly dependent upon the mixing ratio of PY and CM. With the increase of CM loading, the self-quenching problem can be significantly alleviated. The alleviation of self-quenching problem is attributed to the disassociation of non-emission PY aggregates, as evidenced by the results of 2D grazing incidence wide-angle X-ray diffraction (GI-WAXD). The variation is attributed to nanoscale spatial effect on the enhanced mixing efficiency of PY and CM to molecular level. Comparatively, the degree of enhanced luminescence reaches a maximum at specific mixing ratio and then decreases with the further increase of dispersant loading, suggesting that the perfect mixing ratio can be found so as to significantly alleviate the self-quenching problem. Polarized photoluminescence (PL) spectroscopy and grazing incidence Fourier transform infrared spectroscopy (GI-FTIR) were used to characterize the molecular orientation of PY molecules; the spectroscopic results indicate an anisotropic orientation of PY molecules along the cylindrical direction of nanopores. The anisotropic orientation of PY molecules is attributed to nanoscale spatial effect, suggesting that the induced anisotropy gives rise to the increase of PY emission and its lifetime of the PY molecules. The controlling molecular orientation of PY into nanostructure is important for enhanced luminescence of the pyrene molecules, in particular for the fabrication of device performance.
In contrast to chromophore molecules, the same pore-filling method (that is the solvent-annealing process) was used to create the ordered conjugated polymer nanoarrays so as to enhance the efficiency of PL. PL results combining with the morphological evolution examined by scanning probe microscopy revealed that the enhanced PL reaches the maximum intensity as the template pores are completely filled by conjugated polymers, similar to the result of the PY molecules. Polarized PL spectroscopy and GI-FTIR were used to examine the chain orientation of templated conjugated polymer; the spectroscopic results indicate a parallel chain orientation along the direction of nanopores. The induced alignment of the conjugated polymer chains is attributed to the nanoscale spatial effect so as to increase the PL intensity and the lifetime of the conjugated polymer. The enhanced luminescence of chromophore or conjugated polymer nanostructure is highly promising for use in designing luminescent nanodevices.

關鍵字

團聯共聚合物 ； 奈米孔洞模板 ； 發光材料

並列摘要

無資料

並列關鍵字

Block copolymers ； Nanostructures ； Nanoporous materials ； Photoluminescence ； template

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