In the past ten years there has been a flurry of activity in the synthesis of new specialty polymers,largely as a result of the increased need for high technology materials. Interest is mainly shown in two distinctcategories of polymers: a) polymers which are used in very small quantities to fulfill critical needs as a part ofdevice systems, and b) high-performance engineering polymers which significantly extend their mechanicaland thermal properties for structural applications. Polyimides and their unparalleled versatility have captured the attention and imagination of scientists and engineers.This article describes some of the recent work done by the author's group on the rational design at the molecular level and the synthesis of polyimides that have unusual structures and novel properties.

The morphological changes of a side-chain liquid-crystalline polymethacrylate during isotropi-zation and liquid-crystallization transitions were studied by means of polarizing microscopy. These tran-sitions were found to be composed of the initiation of a new phase at local places of the old phase matrix andthe growth of the new phase domains. The kinetics of the liquid-crystallization of the polymer from anisotropic melt to a smectic mesophase was also investigated. The isothermal process of the transition can be described by the Avrami equation.The values of the Avrami exponent were found to be around 2.6,which is lower than the value usually obtained for crystallization transition of polymers,but larger than that reported for liquid-crystallization transition of main-chain polymers.These results may indicate the difference in growth geometry of new phase during transition between crystallization and liquid-crystallization in general and between liquid-crystallization of main-chain and side-chain polymers.It was found that the liquid-crystallization of the used side-chain polymethacrylate may occur at small undercoolings with high transformation rate similar to that of main-chain polymers and small-molecule liquid crystals,while the crystallization of polymers can only proceed at large undercoolings.These phenomena can be explained by the idea that the surface free energy of nucleus during liquid-crystallization transition is less than that for crystallization,and evidence was obtained from analysis of the temperature dependence of the transformation rate.

The synthesis of pseudo- and semi-interpenetrating polymer networks (IPNs) based on poly-dimethylsiloxane (PDMS) and polystyrene (PS) is described. IPNs were obtained by simultaneous and in situsequential synthesis procedure. The preliminary studies on IPNs properties such as transition temperature,microphase separation and mechanical behaviors have been carried out by using differential scanningcalorimetry (DSC) and scanning electron microscopy (SEM). The experimental evidence clearly showed thatsemi-IPNs obtained by sequential synthesis procedure have higher interpenetrating extent than pseudo-IPNs synthesized by simultaneous procedure.Over the full composition,the PDMS/PS IPNs are immiscible.The pseudo-IPNs microphase separation can be greatly subdued through are formation of grafting bonds between two networks as well as the kinetic rate-matching of the individual network crosslinking.

By this Monte Carlo simulation we studied the glass transition of polymethylene using themodified bond-fluctuation model combined with considering the rotational-isomeric state model. Theconfigurational properties in the polymethylene (PM) melts, such as the mean length, the mean energy perbond and the mean square radius of gyration were monitored. We found that the chains cannot be in theequilibrium states after a very long time when the temperature of the dense PM chains decreases to 120 K. As the melt vitrifies,these quantities gradually become independent of temperature in a narrow range.The glass transition temperature Tg depends upon the chain length of PM chains,and extrapolation to (CH)2 gives Tg=212K.The dynamics in the PM melts was also studied.It was found that the diffusion coefficients can be described by the Vogel-Fulcher law and the Vogel-Fulcher temperature Tg is 124K.The method may be used to investigate the glass transition of other real polymer chains.

The procedure for preparing a new type of uniform and porous chitosan gel from citric acidmedium is described. Its swelling behavior in different media was compared with those of the gels preparedby other methods. The ultrastructure of the xerogel prepared from citric acid was characterized using electronmicroscopy (SEM).

Erwinia (E) gum, an extracellular polysaccharide, is composed of fucose, galatose, glucose andglucuronic acid. Its viscosity behavior was investigated by a low-shear-rate multiball viscometer and arotational viscometer. Its weight-average molecular weight Mw and intrinsic viscosity [η] in 0.2 mol/L NaClaqueous solution were measured by light scattering method at 35℃ and viscometry at 25℃ and found to be1.06×106 g/mol and 1050 mL/g, respectively, and its aggregates in aqueous solution were proved by gel permeation chromatography (GPC).These results indicated that E gum in water has exceedingly high viscosity and exhibiys Binham fluid behavior,owing to its aggregation.The viscosity of E gum decreased with increasing temperature,and the turning point appeared at 38℃ for dilute solution and 80℃ for concentrated solution suggesting that the affregates of E gum in water started to disaggregate under these temperatures.In addition,the aggregates can be disrupted by adding either acid or base.The experimental results indicated that the E gum is a good thickening agent,and its fluid behavior is similar to xanthan.

A vinyl monomer having an electron-donating moiety, p-(N,N-dimethylamino)styrenc (DMAS),was synthesized. It was combined with benzoyl peroxide (BPO) to form a redox initiation system to initiatethe polymerization of methyl methacrylate (MMA). UV spectra measurements show that DMAS enters thePMMA chain as well. Both DMAS and its polymer P(DMAS) display strong fluorescence, and thefluorescence can be quenched by electron-deficient compounds such as methacrylonitrile, fumaronitrile and ethyl methacrylate etc.Moreover,DMAS can also form charge transfer complex (CTC) with strong electron acceptors such as tetracyanoethylene (TCNE).The difference between the photochemical properties of DMAS and P(DMAS) were explained in terms of molecular structure change and polymer conformation effect in solution.In addition,the CTC and exciplex formation of DMAS or P(DMAS) with C60 were also studied.

A series of new water-soluble bifunctional comonomers having both carboxyl and alkylpolyoxyethylene groups, such as sodium mono(ethyl polyoxyethylene) maleate (ZE series) with variousmolecular weights of polyoxyethylene ethyl ether, were synthesized and characterized. The effects of thestructural factor, the amount and feeding mode of the comonomers, the initiator concentration andpolymerization temperature on the stability of emulsifier-free emulsion copolymerization of methylmethacrylate (MMA) and butyl acrylate(BA) in the presence of a small amount of ZE with potassium persulfate as initiator were investigated.Stable almost monodispersed MMA/BA/ZE emulsifier-free latex particles were prepared.

A new series of chiral shish-kebab type liquid crystal block copolymers that form the smecticC(Sc*) phase was synthesized by solution polycondensation. The copolymers were characterized by GPC.DSC. TG, POM. X-ray diffraction and polarimeter. The copolymers 7 entered into liquid crystal phase whenthey were heated to their melting temperatures (Tm) and the copolymers 8 were in liquid crystal phase at roomtemperature with low viscosities. The smectic sanded texture or focal-conic texture were observed on POM.All the chiral block copolymers showed high optical activity .No racemization has happened.Temperature-variable X-ray diffraction study together with POM and polarimetric analysis realized that they are chiral smecticC(Sc*) phase.Thus we offer in this report the first example of shish-kebab type liquid crystal block copolymers that from a chiral smectic C(Sc*)phase.The variation of melting and isotropization temperatures with molecular structure was also discussed.

The photosensitive initiating system composed of 7-diethylamino-3-(2'-benzimidazolyl)coumarindye (DEDC) and diphenyliodonium hexafluorophosphate (DIHP) which act as the sensitizer and the initiatorrespectively, can be used to initiate the polymerization of methyl methacrylate (MMA). The results showedthat when exposed to visible light, coumarin dye/iodonium salt undergoes quick electron transfer from DEDCto DIHP and free radicals are produced. The visible light photoinduced reaction between DEDC and DIHP is mainly through the excited singlet state of DEDC and thus it is a little sensitive to O2.The influence of concentration of DEDC,DIHP and MMA on the rate of photopolymerization of MMA was also investigated.

The adsorption of linear poly(N-isopropylacrylamide) (PNIPAM) chains on surfactant-freepolystyrene (PS) nanoparticles was used as a model system to study the hydrophobic adsorption of polymeron the surface, because the hydrophobility of PNIPAM can be continuously varied by a small temperaturechange. The adsorption was investigated by a combination of static and dynamic laser light scattering (LLS)measurements, In static LLS, the absolute excess scattered light intensity led to the amount of PNIPAMa dsorbed on the surface.In dynamic LLS,the hydrodynamic thickness of the adsorbed PNIPAM layer was accurately measured.For a given particle concentration,the adsorption increases as the PNIPAM concentration and the incubation temperature increase.The average density of the adsorbed PNIPAM layer is reciprocally proportional to the number of the PNIPAM chains on the surface,revealing a simple scaling of the chain density distribution.The adsorption follows the Langmuir's isotherm.The enthalpy change estimated from the adsorption at 25℃ and 30℃ is slightly positive,indicating that the adsorption involves the coil-to-globule transition of the chains on the surface.

FT-Raman spectroscopy was used to monitor the polymerization of acrylic acid in aqueoussolution. A simple method to avoid the noise in the background during the signal processing via Fouriertransformation was used in this work. The effects of the amount of initiator used on the polymerization arestudied. When the amount of the initiator in the polymerization was increased, both the rate and extent ofpolymerization of acrylic acid will be increased.

Ethyl-cyanoethyl cellulose [(E-CE)C]/cross-linked polyacrylic acid [PAA] molecular compositeswith cholesteric order were prepared. It was found that the macromolecular cholesteric structure was changedwith the swelling of PAA in the composites. The selective reflection of the cholesteric phase shifted to thelonger wavelength and the X-ray diffraction angle shifted to the high angle direction during swelling, whichsuggested that the cholesteric pitch and the number of the layers of ordered (E-CE)C chains in the cholesteric phase were increased.