Dianjun Yao - Academia.edu (original) (raw)
Papers by Dianjun Yao
Macromolecules, 2005
Self-assembly of the perylene segment leads to vesicle formation of perylene end-capped poly(dime... more Self-assembly of the perylene segment leads to vesicle formation of perylene end-capped poly(dimethylsiloxane) (PDMS) in nonaqueous media. The perylene self-assembles into crystalline aggregate, with a well-defined melting temperature and reversible melting and crystallization. The T m of the perylene aggregates increases with an increase in the length of the PDMS chain. The absorption spectra depend on the solvent used. TEM images show that a trilayer vesicle structure is formed with hexane mixtures. Thus, the packing of perylene depends on the choice of the solvent system. This initial attempt to prepare such vesicles in nonaqueous media may be applicable to similar systems.
This manuscript has been reproduced from the microfilm master. UMI films the text directly from t... more This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand corner and continuing from left to right in equal sections with small overlaps.
Polymer, 2005
The crystalline conformations of perylene polyimides (PPIs), with alkyl spacers varying in length... more The crystalline conformations of perylene polyimides (PPIs), with alkyl spacers varying in length from C 3 to C 12 are discussed. Although modeling of single chains would suggest the possibility that those with odd number of CH 2 groups adopt a flat helical shape, X-ray diffraction suggests that the chains with odd and even spacers have the same conformation, due to the necessity to pack the perylene units. Upon annealing, the UV-vis absorption maximum red-shifts by about 10 nm, indicating enhanced p-stacking between the perylenes. Several changes in the spectra are seen with an increase in annealing time, at a given temperature. With an increase in annealing time, X-ray diffraction patterns show changes in the number of reflections, indicative of a crystal to mesogenic transition. The time required for this transition increases with a decrease in the spacer length, and annealing temperature, suggesting that the dynamics of this transition is very slow. q Polymer 46 4390-4396 www.elsevier.com/locate/polymer 0032-3861/$ -see front matter q
Polymer, 2005
A unique crystallization and melting behavior of a novel semicrystalline polyimide derived from 3... more A unique crystallization and melting behavior of a novel semicrystalline polyimide derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 1,4-bis(3-aminopropyl)piperazine were studied by differential scanning calorimetry (DSC) with and without temperature modulation and wide-angle X-ray diffraction (WAXD). Polymer samples isolated from a chloroform solution showed melting transitions in the DSC. However, WAXD traces showed crystallinity only after annealing above the glass transition, for about 2h. For samples crystallized from the melt, crystallization could be achieved only in a narrow crystallization range of 200–220°C, after 10h. A maximum crystallinity of this polyimide was found to be 30%. Two distinct melting transitions were observed by DSC, which could be explained using a partial disordering—reorganization—final melting model.
European Polymer Journal, 2006
A series of perylene-containing random copolyimides with different ratios of diaminododecyl peryl... more A series of perylene-containing random copolyimides with different ratios of diaminododecyl perylene tetracarboxylic dianhydride (PPI-12) and 4,4 0 -(hexafluoro-isopropylidene) diphthalic anhydride (6FDA) have been studied. Copolymerization improves the solubility and the copolyimide with a low percentage (less than 20% of total dianhydrides) of perylene becomes highly soluble in conventional solvents, such as, CHCl 3 , CH 2 Cl 2 , or THF etc. The positions of the absorption maxima in the UV-visible spectra do not change with 6FDA concentration, but the relative intensities do. Although the homopolymer PPI-12 does not show a distinct glass transition, its T g was derived by extrapolation of the DSC results with the copolymers. The copolymers are semi-crystalline up to a concentration of 60% (mol) 6FDA. It is of interest that the spacings of X-ray reflections also do not change with the addition of 6FDA. This is rationalized on the basis that the conformation of the 6FDA segment does not cause any significant difference in the preferred conformational shapes of the homopolymer of PPI-12. Due to this ''conformational isomorphism'' 6FDA is not a significant chain ''kinker'' in this case.
Macromolecules, 2005
... The diffraction data were recorded from 2θ = 2° to at least 2θ = 40° with a step size of 1.2°... more ... The diffraction data were recorded from 2θ = 2° to at least 2θ = 40° with a step size of 1.2°/min. The data were analyzed using the Jade 5 XRD Pattern Processing software by MDI Materials Data Inc. ... Y.-Y.; Brannan, AK; Davis, HT; Bates, FS J. Phys. Chem. B 2002, 106, 3354. ...
Macromolecules, 2005
Self-assembly of the perylene segment leads to vesicle formation of perylene end-capped poly(dime... more Self-assembly of the perylene segment leads to vesicle formation of perylene end-capped poly(dimethylsiloxane) (PDMS) in nonaqueous media. The perylene self-assembles into crystalline aggregate, with a well-defined melting temperature and reversible melting and crystallization. The T m of the perylene aggregates increases with an increase in the length of the PDMS chain. The absorption spectra depend on the solvent used. TEM images show that a trilayer vesicle structure is formed with hexane mixtures. Thus, the packing of perylene depends on the choice of the solvent system. This initial attempt to prepare such vesicles in nonaqueous media may be applicable to similar systems.
This manuscript has been reproduced from the microfilm master. UMI films the text directly from t... more This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand corner and continuing from left to right in equal sections with small overlaps.
Polymer, 2005
The crystalline conformations of perylene polyimides (PPIs), with alkyl spacers varying in length... more The crystalline conformations of perylene polyimides (PPIs), with alkyl spacers varying in length from C 3 to C 12 are discussed. Although modeling of single chains would suggest the possibility that those with odd number of CH 2 groups adopt a flat helical shape, X-ray diffraction suggests that the chains with odd and even spacers have the same conformation, due to the necessity to pack the perylene units. Upon annealing, the UV-vis absorption maximum red-shifts by about 10 nm, indicating enhanced p-stacking between the perylenes. Several changes in the spectra are seen with an increase in annealing time, at a given temperature. With an increase in annealing time, X-ray diffraction patterns show changes in the number of reflections, indicative of a crystal to mesogenic transition. The time required for this transition increases with a decrease in the spacer length, and annealing temperature, suggesting that the dynamics of this transition is very slow. q Polymer 46 4390-4396 www.elsevier.com/locate/polymer 0032-3861/$ -see front matter q
Polymer, 2005
A unique crystallization and melting behavior of a novel semicrystalline polyimide derived from 3... more A unique crystallization and melting behavior of a novel semicrystalline polyimide derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 1,4-bis(3-aminopropyl)piperazine were studied by differential scanning calorimetry (DSC) with and without temperature modulation and wide-angle X-ray diffraction (WAXD). Polymer samples isolated from a chloroform solution showed melting transitions in the DSC. However, WAXD traces showed crystallinity only after annealing above the glass transition, for about 2h. For samples crystallized from the melt, crystallization could be achieved only in a narrow crystallization range of 200–220°C, after 10h. A maximum crystallinity of this polyimide was found to be 30%. Two distinct melting transitions were observed by DSC, which could be explained using a partial disordering—reorganization—final melting model.
European Polymer Journal, 2006
A series of perylene-containing random copolyimides with different ratios of diaminododecyl peryl... more A series of perylene-containing random copolyimides with different ratios of diaminododecyl perylene tetracarboxylic dianhydride (PPI-12) and 4,4 0 -(hexafluoro-isopropylidene) diphthalic anhydride (6FDA) have been studied. Copolymerization improves the solubility and the copolyimide with a low percentage (less than 20% of total dianhydrides) of perylene becomes highly soluble in conventional solvents, such as, CHCl 3 , CH 2 Cl 2 , or THF etc. The positions of the absorption maxima in the UV-visible spectra do not change with 6FDA concentration, but the relative intensities do. Although the homopolymer PPI-12 does not show a distinct glass transition, its T g was derived by extrapolation of the DSC results with the copolymers. The copolymers are semi-crystalline up to a concentration of 60% (mol) 6FDA. It is of interest that the spacings of X-ray reflections also do not change with the addition of 6FDA. This is rationalized on the basis that the conformation of the 6FDA segment does not cause any significant difference in the preferred conformational shapes of the homopolymer of PPI-12. Due to this ''conformational isomorphism'' 6FDA is not a significant chain ''kinker'' in this case.
Macromolecules, 2005
... The diffraction data were recorded from 2θ = 2° to at least 2θ = 40° with a step size of 1.2°... more ... The diffraction data were recorded from 2θ = 2° to at least 2θ = 40° with a step size of 1.2°/min. The data were analyzed using the Jade 5 XRD Pattern Processing software by MDI Materials Data Inc. ... Y.-Y.; Brannan, AK; Davis, HT; Bates, FS J. Phys. Chem. B 2002, 106, 3354. ...