Chain Conformation and Local Rigidity of Soluble Polyimides(II):Isomerized Polyimides in THF

Two soluble isomerized polyimides（PIs） synthesized from 2,2＇-bis（3,4-dicarboxyphenyl）hexafluoropropane dianhydride（6FDA） with either 2,2＇-dimethylbenzidine（2,2＇-DMB） or 3,3＇-DMB were investigated by means of size-exclusion chromatography coupled with multi-angle laser light scattering,a viscometer and a refractive index detector in tetrahydrofuran（THF） with tetrabutylammonium bromide（TBAB） at 35 ℃.The corresponding parameters related to conformations α and ν,evaluated from the scaling relationships [η]=K η M α and R g =K g M ν,respectively,were 0.66±0.01 and 0.55±0.02 for poly（6FDA/3,3＇-DMB）,and 0.67±0.01 and 0.56±0.01 for poly（6FDA/2,2＇-DMB）,indicating a random coil conformation for both the samples in this mobile system.The persistence length l p and shift factor M L（relative molecular weight per unit contour length） were estimated from the relationship between intrinsic viscosity and molecular weight for the wormlike cylinder model proposed by Bohdanecky.Both l p and M L showed that the two PIs in THF are flexible chains and exhibit some local rigidity to some extent.

Properties,Morphology and Structure of BPDA/PPD/TFMB Polyimide Fibers

The mechanical properties of fibers were notably improved by incorporating 2,2＇-bis（trifluoromethyl）benzidine（TFMB） into 3,3＇,4,4＇-biphenyltetracarboxylic dianhydride（s-BPDA） and p-phenylenediamine（PPD） backbone.The best strength and modulus of BPDA/PPD/TFMB polyimide（PI） fiber（diamine molar ratio of PPD/TFMB= 90/10） were 1.60 and 90 GPa,respectively,which was over two times that of BPDA/PPD PI fiber.SEM image showed that the cross-section of fibers at each stage was round and voids free.Besides,the ＂skin-core＂ and microfibrillar structure were not observed.The thermal properties of PI fibers were also investigated.The results showed that the fibers owned excellent thermal stability,moreover,the structural homogeneity of fibers were significantly improved by heat-drawn stage.The T g values were found to be around 300 °C by dynamic mechanical analysis（DMA）.Wide angle X-ray diffraction（WAXD） and small angle X-ray scattering（SAXS） experiments indicated that the order degree of longitudinal and lateral stacks,the molecular orientation and the structural homogeneity of fibers were improved in the preparation process of fibers.

The Hysteretic Behavior of Angular Dependence of Exchange Bias in NiFe/granular-FeMn-MgO Bilayers

We investigate the hysteretic behavior of the angular dependence of exchange bias for a series of polycrystalline NiFe/(FeMn)_(1-x)(MgO)_(x) bilayers with varying x.For x=0.025,the antiferromagnetic layer is of the largest degree of the fcc(111)preferred texture.At the same x,both the exchange field and blocking temperature acquire maximal values.In particular,the hysteretic behavior of the angular dependence between clockwise and counter-clockwise rotations shows minimal angular shift.These results can be explained in terms of the thermal activation model.

Photodynamic Therapy of Cancer Cells with Methanesulfonate Salts of 5-Aminolevulinic Acid and Its Derivatives

A series of 5-aminolevulinic acid and its alkylester methanesulfonates was exploited to photodynamic therapy（PDT） of human lymphocytic cells, U-937 in vitro. The PDT efficiency is influenced by the concentration and incubation time. Generally, for ALA and its alkylester methanesulfonates, the cell survival rate decreases and the accumulation ability of PplX increases with the concentration and incubation time. We found that the longer carbon chain methanesulfonates（C5-S, C6-S, C8-S） exhibit better PDT effect than ALA methanesulfonate. This possibly provides a promising route to the clinical application ofPplX-mediated PDT to cancer cell.

Flexible Synthesis of Enantiomeric and Racemic 3-Hydroxymethyl-1,2,3,4-tetrahydroisoquinolines via Bischler-Napieralski Reaction

With methyl ester of N-acylated L-3-（3,4-dihydroxyphenyl）alanine（L-DOPA） as starting material, racemic derivatives of 3-hydroxymethyl-1,2,3,4-tetrahydroisoquinoline were obtained with cyclization by Bischler-Napie- ralski（BN） reaction followed by reduction of methyl ester group to hydroxymethyl, while the optical pure enantiomers were prepared by the reduction of the methyl ester group to hydroxymethyl prior to cyclization with BN reaction. Racemerization took place in the BN stage in the presence of methyl ester, an electron-withdrawing group adjacent to chiral center. Therefore, the sequence of synthetic stages of cyclization and reduction, and subsequently whether there is electron-withdrawing group or not determines the chiral optical activity of the products.

Temperature Dependence of Chain Conformation and Local Rigidity of Isomerized Polyimides in Dimethyl Formamide

Temperature dependence of chain conformation and local rigidity of two soluble isomerized polyimides （PIs）, poly（hexafluorodianhydride/3,3＇-dimethylbenzidine）[poly（6FDA/3,3＇-DMB）] and poly（hexafluorodianhy- dride/2,2＇-dimethylbenzidine）[poly（6FDA/2,2＇-DMB）] were investigated by dilute solution viscosity, size exclusion chromatography（SEC） coupled with multi-angle laser light scattering, viscometer, and refractive index detector in dimethylformamide（DMF） with either 0.1 mol/L LiBr or 3.1 mmol/L tetrabutylammonium bromide（TBAB） in the temperature range of 30 to 50 ~C. The scaling relationships of [t/]=K^M~ and Rg=KgMv obtained are employed to in- vestigate the temperature dependence of chain conformation for the two polyimides. The values of a and v are in the range of 0.66--0.69 and 0.53--0.56, respectively, for poly（6FDA/3,3＇-DMB）, meanwhile they are in the range of 0.64--0.68 and 0.53--0.56, respectively, for poly（6FDAJ2,2＇-DMB）. These results reveal that random coil conforma- tions for both Pls are not affected visibly with increasing temperature from 30 ~C to 50 ~C. However, values of more exact intrinsic viscosity from dilute solution measurement indicate there is only tiny coil extension or shrinkage for both PIs with temperature rising. Parameters related to chain flexibility of polymer, including persistence length lp, shift factor ME（relative molecular weight per unit contour length） and backbone diameter d are estimated from the relationship between intrinsic viscosity and molecular weight for the continuous wormlike cylinder model, which indicates that two samples are flexible chains, only the chain of poly（6FDA/3,3＇-DMB） is stiffer than that of poly（6FDA/2,2＇-DMB） slightly.