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Synthesis and Characterization of the Block Copolyimides and Nanocomposite Photoluminescent Polyimide Films

Author: Shafiq urRehman
Tutor: Chen ChunHai
School: Jilin University
Course: Applied Chemistry
Keywords: Polyimide Block Fluorescence Segments Quantum Dots The polymer matrix Glass transition temperature polyimide polymer Polyamide acid WAXD molecular Composite films The aqueous phase carbonyl inorganic Main chain Water-soluble PAA constants
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Type: PhD thesis
Year: 2013
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Abstract


This dissertation describes the synthesis and characterization of the block copolyimide films and nanocomposite photoluminescent polyimide films. Aromatic polyimides are an important class of polymers used extensively in a variety of high performance materials and composites because of their outstanding physical properties considerably high glass transition temperatures, high resistance to chemicals and radiations, comparatively low dielectric constants, and superior mechanical properties. Some wide-ranging applications of aromatic polyimides include microelectronics, optoelectronics, automotive, aerospace engineering, advance textiles, and membranes technology. The recent trend is the development of multifunctional aromatic polyimides that will attain all desired application-dependent properties simultaneously.In the first part of this thesis, synthesis and characterization of a series of block and random copolyimide films synthesized from various molar ratios of two diamines, rigid2-(4-Aminophenyl)-5-aminobenzimidazole (APBI) and flexible4,4’-oxydianiline (ODA) by polycondensation with dianhydride3,3’,4,4’-biphenyltetracarboxylic dianhydride (sBPDA), is reported. Rigid rod-like heterocyclic diamine APBI diamines produced to rigid blocks while flexible diamine ODA gave rise to relatively flexible segments in block copolyimides. Six block copolyimide films were prepared with systematic variation of block length and block contents of both rigid (APBI-sBPDA) and semi-flexible (ODA-sBPDA) blocks. Thermal and mechanical strength properties of block copolyimides were optimized by controlling block length and contents. Dianhydride-terminated poly(amic acid) oligomer and diamine-terminated poly(amic acid) oligomer were prepared separately and then combined together. The contents of APBI ranged from10to60mol%in copolyimides, Then polymer films were obtained by conventional solution casting method after thermal imidization at elevated temperatures. For comparison purpose, random copolyimides and homopolyimide were also prepared with monomer compositions corresponding to block copolyimides. The copolyimide films obtained by thermal imidization of poly(amic acid)(PAA) solutions, were characterized by-TMA, DMA, TGA, DSC, WAXD. FTIR. tensile testing, water uptake, and dielectric constant measurements, to evaluate their thermal, mechanical strength and electrical properties in details. Rigid heterocyclic diamine APBI with interchain hydrogen bonding capability, led to low CTE, high Tg. high thermal stability and better mechanical properties. Increasing the APBI mol%caused a gradual decrease in the CTE and increase in Tg, thermal stability and tensile strength properties of the copolyimides films. Moreover, significantly enhanced thermal and mechanical properties of the block copolyimides were also found as compared to random copolyimides. The block copolyimide with APBI content of60mol%, achieved excellent properties, i.e., a low CTE (4.7ppm/K), a high Tg at377℃,5%weight loss at562℃and a tensile strength at198MPa. This can be interpreted because of comparatively higher degree of molecular orientation in block copolyimides. The remarkably low CTE values of block copolyimides can be explained in terms of locally ordered microdomains in block copolyimides. These microdomains are not related to crystalline structure but based on highly oriented and well-ordered regions formed by rigid APBI blocks. Microdomains order increases with block length and become more random as block length decreases. The highly oriented and well-ordered block copolyimide structure efficiently promoted molecular orientation in block copolyimides as compared to random copolyimides. The combined effect of heat resistant benzimidazole units and high rigidity led to better thermal stability of copolyimides. TGA analysis indicates that block copolyimides showed slightly higher Tg5than corresponding random copolyimides. This may be due to rigid APBI segments formation, leading to better packing and therefore to higher thermal stability. Tg increased with increasing the rigid APBI block length and molar ratio. Addition of APBI would increase the chain rigidity and therefore increased the potential barrier to rotation, which would result in higher Tg values. Mechanical properties results indicate that tensile strength and tensile modulus increased linearly with increase in APBI mol%. APBI based rigid polymer structure plays a major role in the improvement of mechanical properties. Extensive delocalization of π electrons in PBIs is also well known for their outstanding mechanical properties. Water uptake in copolyimides with higher APBI contents exhibited slightly higher water absorption. This can be attributed to higher concentration of hydrophilic imidazole N-H. Dielectric constants of copolyimides range from2.67to3.05and relatively lower than those of conventional polyimides, but somewhat higher as compared to fluorinated and nanoporous polyimides having ultra-low dielectric constants. Overall, block copolyimides demonstrated better thermal, mechanical and electric properties as compared to simple random copolyimides. APBI based block copolyimide systems can be promising candidates for base film materials in microelectronics applications.In the second part of dissertation, photoluminescent CdS quantum dots/polyimide nanocomposite films are reported. High quality luminescent CdS quantum dots were synthesized in aqueous medium and then incorporated in polymer matrix. Generally, quantum dots are prepared in organic solvents. However, the organic solvent method is complicated and unsafe to the environment due to the pyrolysis of lethal organometallic reagents. Therefore, in our studies, we prepared water based CdS quantum dots which are simpler to prepare and safer as compared to quantum dots prepared in organic solvents. This procedure includes direct aqueous synthesis route that is environment friendly and produced high quality luminescent water-soluble quantum dots. Accordingly, polyimides were also prepared from water soluble precursor i.e., poly(amic acid) salt rather than conventional poly(amic acid). Conventional preparation technique of polyimides via poly(amic acid) precursor possess some drawbacks, such as, serious damage to the environment because of the toxic volatile polar solvents, elevated thermal imidization temperature and weak hydrolytic stability of poly(amic acid) precursors. To deal with these shortcomings, we selected the water soluble precursor i.e., poly(amic acid) salt. Another major reason was the better incorporation of the water soluble CdS quantum dots in polymer matrix. Water soluble CdS quantum dots were very well dispersed in the poly(amic acid) salt. CdS quantum dots were characterized by XRD, photoluminescence and ultraviolet-visible spectroscopic techniques. Characterization results confirmed the successful synthesis of stable and high quality photoluminescent CdS quantum dots of3-5nm. Next step was the dispersion of water soluble CdS in the polymer matrix. As stated earlier, poly (amic acid) salt precursor was selected for this purpose. Poly (amic acid) was prepared from PMDA/ODA and converted into poly(amic acid) salt by addition of triethylamine. Poly(amic acid) salt solution was precipitated into acetone and vacuum dried. Dried powder was thoroughly dissolved in water. CdS quantum dots solution were mixed with poly(amic acid) salt solution. Nanocomposites films were prepared by solution casting and thermally imidized at300℃. Nanocomposites films were characterized by photoluminescence (PL) measurements, XRD. TEM, Thermomechanical analysis. PL spectra of films showed that nanocomposite films are photoluminescent. PL spectra indicate that there is minor hypsochromic shift and PL peaks are broad as compared to that of pure CdS quantum dots solution. This might be due to the effect of the polymer matrix and presence of organic solvent traces which shift the PL spectra into blue region. TEM micrographs clearly show the presence very well dispersed CdS nanocrystals in films with size of3-5nm. WAXD studies also evidenced the amorphous nature of polyimide films and characteristics CdS nanocrystals peaks. Nanocomposite films were also characterized to evaluate their thermal and mechanical properties. Incorporation of CdS did disturb the mechanical strength and thermal properties. A small rise in glass transition temperature was observed with higher CdS contents. Thermal stability was also enhanced. This phenomenon is common when inorganics were well dispersed in polymer matrix. Results showed that high quality photoluminescent CdS/polyimide nanocomposite films were successfully synthesized. CdS/polyimide nanocomposite films maintained the excellent thermal property of the host polyimide films. This unique approach of preparation of nanocomposite films via water soluble polymer precursor and CdS quantum dots, can find novel applications in electrical and optical devices.Chapter3reports the synthesis and characterization of six-membered naphthalene dianhydrides based polyimide films and their significantly enhanced thermal and hydrolytic stabilities.4,4’-Binaphthyl-1,1’,8,8’-tetracarboxylic dianhydride (BNTDA) and4,4’-Ketone binaphthyl-11’,8,8’-tetracarboxylic dianhydride (KBNTDA) were prepared by the dehalogenation-coupling of4-bromo-1,8-naphthalic anhydride and insertion reactions respectively. The structures of BNTDA and KBNTDA were characterized by FTIR,1H-NMR and13C-NMR. A series of polyimides were successfully synthesized from BNTDA, KBNTDA, and various diamines such as4,4’-diaminodiphenyl ether (ODA),4.4’-diaminodiphenylmethane (MDA), and2,5-bis (4-aminophenoxy)-biphenyl (p-TPEQ). The higher molecular weight polyimides exhibited better solubility in common aprotic solvents. The thermal characterization of polyimides by DMA, DSC and TGA techniques, demonstrated super thermal stability of polyimides containing naphthalimide. Glass transition temperature (Tg) of the all polyimides were above326℃and the5%weight loss temperature was above525℃in air and545℃in N2. Hydrolytic stability of the polyimide films was evaluated by immersing the films into deionized water,10%NaOH and10%H2SO4aqueous solutions. Mechanical properties remained stable before and after treatment. Six-membered polyimides derived from KBNTDA and BNTDA, exhibited better hydrolytic stability than those with five-membered phthalic anhydrides.

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