1999 рік

An experimental study was carried out to examine the effects of added salt on the rheology of four model hydrophobic alkali-soluble associative polymer (HASE) solutions. With increasing salt concentrations, the solution viscosities of all the model polymers decreased continuously and the viscoelastic properties of the hydrophobe-containing polymer solutions changed from predominantly elastic to viscous behavior. Model polymers with stronger hydrophobic associations were more susceptible to the presence of added salt, however such effects were weakened at high shear stresses. At a certain level of added salt, the model polymer with hydrophobes consisting of C20 alkyl chain demonstrated a unique shear-thickening behavior at high shear stresses. The critical shear stress at which the shear-thickened viscosity reached the maximum value was independent of the salt concentration, but was proportional to the polymer concentration. The shear superimposed oscillation technique was employed to probe the shear-induced structural changes of the model polymer that contributed to the shear-thickening behavior. With increasing applied shear stresses, a corresponding increase in the storage modulus (G 0 ) was observed at the shear-thickening region where the viscosity increased with applied stresses. This observation indicated that the shear-thickening behavior is caused by the formation of a larger number of network junctions. The increase in the network junction densities was attributed to the conversion of intra- to inter-molecular associations.

Детальніше...  

The second-order nonlinear optical (NLO) itaconite copolymer was synthesized to study the effect of temperature and time on the electrooptic effect. The aminonitrostilbene chromophore was synthesized to be bound to itaconate polymer backbone. Particularly, two chromophoric groups were present to enhance the dipole moment in one repeating unit of polyitaconate. The resistance and electric potential between the two electrodes were characterized with temperature. The electro-optic signal and coefficient were investigated using the modified reflection technique. The time-resolved study gave much information on how the electro-optic property can be brought about. Temporal stability was also observed at various temperatures, which is around and far below glass transition temperature.

Детальніше...  

A series of new soluble poly(amide-imide)s were prepared from the diimide-dicarboxylic acid, 3,3 0 ,5,5 0 -tetramethyl-2,2-bis[4-(4-trimellitimidophenoxy)phenyl]propane (TBTPP), with various diamines by the direct polycondensation in N-methyl-2-pyrrolidinone using triphenyl phosphite and pyridine as condensing agents. The new diimide-dicarboxylic acid TBTPP bearing isopropylidene and methyl-substituted arylene ether units was synthesized by the condensation reaction of 3,3 0 ,5,5 0 -tetramethyl-2,2-bis[4-(4-aminophenoxy)phenyl]propane with trimellitic anhydride. All the polymers were obtained in quantitative yields with inherent viscosities of 0.58-1.03 dl g ⫺1. The polymers were amorphous and most of them were readily soluble in aprotic polar solvents such as N-methyl-2-pyrrolidinone, N,N-dimethylacetamide, and N,N-dimethylformamide, as well as in less polar solvents such as dimethyl sulfoxide, m-cresol, pyridine and g-butyrolactone, and also even in tetrahydrofuran. The glass transition temperatures of the polymers were determined by the differential scanning calorimetry (DSC) method and were in the range 240–278°C. These polymers were stable up to 450°C and lose 10% weight in the range of 454–484°C and 448–482°C in nitrogen and oxygen, respectively. The polymer films had a tensile strength range of 98–122 MPa and a initial modulus range of 2.7– 3.8 GPa.

Детальніше...  

The reactive interface formed between an immiscible polymer pair, amorphous polyamide (aPA) and functionalized polysulfone (PSU), was studied by ellipsometry. Diblock copolymers were formed in situ by an interfacial reaction between the amine-terminated aPA and the end-functionalized PSU, whereas graft copolymers were formed between the amino-chain end groups of aPA and the PSU functionalized in the middle of the chains. It was found that the interfacial reaction significantly increased the interfacial thickness, even reaching the size larger than the coil size of the copolymers formed at the interface. The interface structure and the properties were affected by the position and type of functional groups incorporated to the PSU. The reactive interface resulting in the graft copolymers tended to be thicker than that forming the block copolymers. However, the thinner interface demonstrated a superior adhesion to the thicker interface.

Детальніше...  

The melting behavior of blends of polypropylene (PP) and maleic anhydride grafted PP (mPP), crystallized both non-isothermally and isothermally, have been investigated by differential scanning calorimetry (DSC). Also the relationship between co-crystallization, phase separation and the corresponding crystallization conditions in the blends is presented. It has been found that either co-crystallization or phase separation in the blends can be obtained, depending on the crystallization conditions. If the cooling rate is very slow in the non-isothermal case, or the crystallization temperature is relatively high in the isothermal case, phase separation between PP and mPP molecules will be observed in the blends. Otherwise, co-crystallization between different molecules will dominate in the blend system. An effort was made to elucidate these phase phenomena with the aid of kinetic parameters by isothermal crystallization. Fractional values of the Avrami exponent are obtained, which range from 1.93 to 3.21, indicating an average contribution of simultaneous occurrence of various types of nucleation and growth of crystallization. The half-life t 1/2 of crystallization shows that incorporation of mPP in PP can either have little effect or greatly increase the crystallization rate of PP, depending on the crystallization temperatures. At higher crystallization temperatures, the crystallization rate of the blends is higher than both the pure homopolymers, resulting in phase separation morphology. The results by small angle X-ray scattering (SAXS) are consistent with those of the DSC measurements. It has been concluded that the crystallization kinetics, not the thermodynamics, mainly determine the final phase morphologies in this particular system.

Детальніше...  

N-[4-(Azidocarbonyl)phenyl]maleimide (3) and N-[4-(N 0 -phenoxycarbonylamino)phenyl]maleimide (5) were polymerized and copolymerized with methyl methacrylate (MMA) or acrylic acid (AA) by initiation with AIBN. The resultant polymers were to have aromatic isocyanato groups when heated. In the differential scanning calorimetry measurement, the polymers from monomer 3 showed a strong and broad exotherm around 130°C. Thermogravimetric analysis also showed a weight decrease corresponding to the elimination of nitrogen at that temperature. The formation of isocyanato groups was confirmed by infra-red measurement with varying the cell temperature. The coupling reaction of the polymer with N-4-(nitrophenyl)-L-prolinol, resulting in the formation of urethane bonds, was carried out in a quantitative yield. The crosslinked films from copolymers of 5 and MMA or AA were obtained when heated above 200°C. The cured films were insoluble in common organic solvents. With the copolymers with AA it was expected that carboxylic groups would react with isocyanato groups generated from urethane units when heated. The copolymer was degraded above 150°C. After curing, its thermal stability improved dramatically, no weight loss being observed up to 300°C.

Детальніше...  

Calculations of normal modes of isotactic 1,4-trans-poly(1,3-pentadiene) and their dispersion within the first Brillouin zone (reduced zone scheme) are reported here. Calculated vibrational frequencies are compared with the reported IR spectroscopic data and a close agreement is found between the two. Characteristic features of dispersion curves, like bunching and repulsion, are observed and discussed. Frequency distribution function derived from dispersion curves is used to calculate the heat capacity in the temperature range 50–450 K.

Детальніше...  

A novel method for the preparation of solvent-free poly(ε-caprolactone) (PCL) microspheres from PCL/poly(ethylene glycol) (PEG) blends was developed and demonstrated. The particle size of the PCL spheres are in the range of 1 to 20 mm (mostly in the range 5–10 mm). The influences of the molecular weight of PCL, the molecular weight of PEG, the type of emulsifier, the concentration of plasticizer, the homogenization time and temperature on the characteristics of the PCL microparticles were also investigated. No changes in PCL molecular weight distribution and peak shape were observed, which indicates that the PCL matrix is stable during the fabrication process. However, the microspheres made from low-molecular-weight PCL coalesced into a continuous mass on drying and lost their properties as individual spheres. The surface of the PCL microspheres prepared from PEG 300 as the external phase at 70°C is much smoother than that of the other formulations. The major advantage of this polymer-blend melt technique compared with traditional methods for microsphere preparation is the elimination of methylene chloride or other organic solvents for polymer solubilization. Therefore, the toxicity associated with organic solvent residues resulting from conventional solvent preparation of microspheres is not present with this polymer-blend melt technique.

Детальніше...  

Deprotection of poly(2-trimethylsilyl-2-propyl methacrylate) was studied for various photogenerated acids. Deprotection of 2-trimethylsilyl-2-propyl (TMSP) ester did not occur in a catalytic manner when the photogenerated acid possessing a fluoride ion source such as SbF⫺ 6 or AsF⫺ 6 was used. Because the cleaved product, a -methyl-a -(trimethylsilyl)ethyl cation, undergoes rearrangement through methyl migration from silicon to carbon with nucleophilic attack on the silicon by the fluoride ion, the acid was consumed in the reaction. Fortunately, when the counterion of the acid does not provide a fluoride ion, e.g. sulfonate, the carbonium ion undergoes elimination to produce 2,2,3trimethyl-2-silabut-3-ene, and regenerates another acid. The thermal stability of TMSP ester was very similar to t-butyl ester, and acid catalyzed cleavage of the TMSP begins at 80°C.

Детальніше...  

The relaxation behavior of high density polyethylene functionalized with diethylmaleate has been determined by dynamic mechanical method and thermally stimulated depolarization technique. The transitions observed have been related to the structural parameters as well as the a -olefin comonomer content and degree of grafting, and compared with previous TSDC results on LLPE functionalized with the same polar group. The lowest relaxation is composed of a sub-g and a g mode assigned to localized motions of the polar groups and to the glass transition (at 137 K), respectively. The b -relaxation intensity grows with the grafting degree in agreement with its assignment to motions taking place in the interfacial regions. The behavior of the a -mode with different content of diethylmaleate has confirmed its dependence with the crystallite size.

Детальніше...