1999 рік

The thermoplastic elastomer (TPE) prepared by dynamic vulcanization is a two-phase material in which crosslinked rubber particles are densely dispersed in a ductile polymer matrix. The TPE shows an excellent strain recovery, even though the matrix consists of ductile plastics. This behavior was exemplified in a 50/50 poly(butylene terephthalate)(PBT)/ethylene rubber blend. Finite element method (FEM) analysis revealed that: (1) the low stress evolved in PBT matrix with bulk deformation, especially in the ligament matrix between rubber particles in stretching direction, is locally preserved within an elastic limit and it acts as an in situ formed adhesive for interconnecting the rubber particles, and (2) the volumeric strain of rubber particles with high Poisson’s ratio provides the contractile stress to heal the plastically deformed PBT phase outside the ligament matrix. Such strain mechanisms were supported by the polarized FT-Raman spectroscopy in terms of the peak shift caused by chain distortion, its anisotropy, and the gauche-to-trans transformation associated with plastic deformationm, in comparison with those in neat PBT.

Детальніше...  

A novel polymeric emulsifier, SMTAPE, was prepared by end-capping sulfopolyester polyol with TMA in this study. This polymeric emulsifier is applied for BMA emulsion polymerization. Also investigated herein is how SMTAPE’s chemical structures, e.g. SSIPM contents and molecular weights, influence the behavior of emulsion polymerization and subsequent PBMA lattices. Experimental results indicate that SMTAPE emulsifiers with a shorter average hydrophobic length stabilize the PBMA latex by the depletion mechanism. Although possessing good mechanical stability, using those emulsifiers has a higher coagulum during the synthesis stage. However, the SMTAPE emulsifiers with a longer average hydrophobic length anchor strongly on the PBMA latex particles and stabilize them by an electrostatic force with a negligible amount of steric stabilization. Although possessing better synthesis stability, using these emulsifiers has a poor mechanical stability. In addition, according to these experimental results, formation of the subsequent PBMA latex is attributed to a coagulation process of particle precursors, micellar particles as well as existing PBMA latex particles.

Детальніше...  

The thermodynamics of mixing of two miscible or immiscible polymers with a solid has been considered. It is shown theoretically that the introduction of a specific filler in binary polymer mixtures increases the thermodynamic stability of the ternary system. The compatibilizing effect of the filler depends on the change in free energy of mixing between the two polymers, the effect being more pronounced for immiscible systems.

Детальніше...  

The low-temperature notched Izod impact strength of poly(butylene terephthalate), PBT, blends with acrylonitrile–butadiene–styrene terpolymers, ABS, can be improved by incorporating methyl methacrylate, glycidyl methacrylate, ethyl acrylate terpolymers, MGE, as a reactive compatibilizer. However, fracture properties depend on the order of mixing of the blend components. When all components of compatibilized blends are melt mixed together in a single-pass extrusion, the low-temperature toughness is improved; however, the roomtemperature impact strength is reduced relative to uncompatibilized blends. Using a two-pass extrusion method where PBT and MGE are melt mixed together in a first extrusion prior to incorporating ABS in a second extrusion, the room-temperature impact strength is superior to that of blends prepared by the single-pass method. When a two-pass method is used where ABS and MGE are combined prior to extrusion with PBT, the impact properties at all temperatures are inferior to those of uncompatibilized blends. Evidence is presented which suggests that residual acids present in emulsion-made ABS material may cause a crosslinking reaction involving the epoxide functionality of MGE terpolymers resulting in a deleterious effect on the ABS mechanical properties and its blends with PBT. By changing the order of mixing, the sequence of chemical reactions which occur can be controlled to optimize blend properties.

Детальніше...  

The present investigation deals with the comb-like complexes made of poly(ethylene imine) (PEI) and stearic acid (SA) or oleic acid (OA). On cooling the mixture of dimethylformamide solutions of PEI and carboxylic acid to room temperature, the corresponding complexes were phase-separated and precipitated. Structures and liquid crystallinity of the PEI–OA and PEI–SA complexes were studied by means of various techniques. It was confirmed by infrared spectroscopy studies that the carboxylic acid molecules are linked by ionic bonds to form the comb-like complexes. The complexes give a narrow peak at 2u ˆ 28–38 in their wide-angle X-ray diffractograms, indicating that the side aliphatic chains are packed in layer structure with a spacing of 3.2–4.4 nm. In spite of the rather weak secondary valence force between an imine group of PEI and a carboxylic end group, the layer structures of the complexes are maintained even at the temperatures above the melting point of the side chains. Both of the comb-like complexes form optically positive spherulitic textures from the melt, indicating that the layer structures are oriented concentrically and consequently side aliphatic chains are oriented in a radial direction. Concentric arrangement of the layer structure is characteristic of the smectic-type liquid crystal. These results suggest the liquid crystallinity of the comb-like complexes are made of PEI and carboxylic acid.

Детальніше...  

A method for determining diffusion coefficients during shear by dynamic light scattering is presented. The direct determination of diffusion coefficient during flow is hampered by the domination of the convection terms over the diffusion terms except in a very narrow azimuthal scattering angle range that is impossible to reach experimentally. The method described here offers the advantage to this constraint. The technique is based on a double measurement of homodyne spectrum of light scattered from small scatterers immersed in a medium subjected to a simple shear flow. This can be applied to polymer solutions. Verification is provided using suspensions of polystyrene spheres in Newtonian media and a transparent rheometer.

Детальніше...  

Yield behaviour and plastic resistance of the (100)[001] chain slip system in a-crystals of isotactic polypropylene (iPP) were studied in samples of biaxially oriented films of plain iPP and blends of iPP with hydrogenated oligo(cyclopentadiene) (HOCP). Due to the orientation process the films investigated exhibit sharp and nearly one-component texture with the (010) plane parallel to the film surface and the chain direction parallel to the direction of final drawing (transverse direction, TD). The films were studied in tension at various angles with respect to the orientation direction. It was found that the yield stress obeys the Coulomb yield criterion, provided that the angle between the chain orientation axis and tensile axis is within the range of 30–50°. The results suggest that the (100)[001] chain slip is active as a single deformation mechanism in this range of sample orientation. The critical resolved shear stress necessary to activate this slip, t c, was determined for iPP and iPP/HOCP blend samples. It was also found that the slip process is sensitive to the stress normal to the slip plane, similarly to the slip processes observed in linear polyethylene crystals. The value of t c determined for plain iPP was 22.6 MPa, while for the 8:2 blend its value increased to 35.5 MPa. The increase is caused most probably by the presence of a small amount of HOCP molecules incorporated within iPP crystals, as well as by the layers of higher concentration of HOCP located at crystal–amorphous interfaces, which both cause immobilization of a part of dislocations and consequently an increase of the yield stress observed in the blend samples. The third probable cause of the increase of yield stress in blends may be an increase of the glass transition temperature of the amorphous phase of iPP in the blend, as compared with plain iPP samples.

Детальніше...  

Polyimides with unique thermomechanical characteristics and film forming capabilities have emerged as choice material for diversified applications in advanced technologies. Wide ranging properties could be achieved for polyimides by incorporating them with many metal salts, complexes or organic materials. The inadvertent crosslinking was introduced due to metal salt incorporation and unique polyimide alloys are prepared by blending the crosslinked and uncrosslinked polyamic acid components followed by thermal imidization. The blend components were synthesized by the reaction of polyamic acid with varying concentrations of a crosslinker (here antimony pentachloride or APC) as high as 1.67 × 10 22 mole/dm 3 (i.e. hypothetically calculated Critical Crosslinker Concentration) up to lowest of 1.67 × 10 26 mole/ dm 3. This communication delves into the synthesis and characterization of polyimide blends and alloys prepared by varying degrees of crosslinking introduced via APC–Amic Acid reactions. The polyimide alloys were prepared by thermal imidization of polyamic acid blends at different curing temperature starting from 508 to 3508C. The degree of imidization, mechanical properties and the residual solvent content for blends having varying mole fractions of crosslinked or uncrosslinked components have been studied. The resultant APC-PI blends have exhibited synergism on mechanical properties. The feasibility of preparing polyimide alloys with synergistic combinations of crosslinked and uncrosslinked polyimide components was concluded.

Детальніше...  

A series of alkyl pyridinium bromide maleate and succinate diester surfactants has been used as stabilizers in the free radical emulsion polymerization of styrene. High conversions of styrene were achieved readily with the maleate surfactants, but the succinates gave rise to varying levels of retardation. In two of the latter cases, use of the surfactant at a concentration below its crucial micelle concentration appears to be a key factor, but the other retardations are difficult to account for. Little difference in reactivity and latex properties was found with the maleate surfactants despite the position of the maleate group being very different in the series of molecules examined. Chemical incorporation of the maleate surfmers into the polymer particles appears to occur early on in the polymerizations, but a sufficiently high proportion of groups is on the surface to yield stable latex emulsions. The surface tensions of the final latices produced with both the maleate and succinate surfactants are high, suggesting a large proportion of succinate species are also surface adsorbed. However, the surface tensions of these latices are a little lower than those of latices produced with the maleates, suggesting the proportion of solubilized succinate surfactant is a little higher. This seems to be important in terms of the robustness of the stability of the latices to the addition of electrolytes, the succinatestabilized species being consistently more tolerant. This suggests that while chemically bound surfactants can improve some of the properties of latices, e.g. the hydrophobicity of films, this might be at the expense of latex stability.

Детальніше...  

Conventional expandable polystyrene (PS) is prepared via the suspension polymerisation of styrene containing pentane isomers as a blowing agent. The polystyrene beads are expanded by heating above the glass transition temperature, Tg. During the expansion process, pentane, which contributes to the formation of ozone in the lower atmosphere, is released into the environment. The objective of this study was to replace pentane isomers with a harmless blowing agent, namely water. The procedure adopted to use water as a blowing agent was as follows. Water was emulsified by means of a suitable emulsifier in a prepolymerised styrene/polystyrene mixture possessing a high viscosity in order to fixate the emulsified water droplets. Subsequently, this inverse emulsion was suspended in water containing a suitable suspending agent and polymerisation was completed. This procedure resulted in spherical PS beads containing small water droplets which were capable of expanding the PS matrix upon heating above Tg.

Детальніше...