THERMODYNAMICS OF POLY(ETHYLENE OXIDE)-POLY(VINYL ACETATE) BLENDS

Heat-of-mixing data, obtained on blends of poly(ethylene oxide) (PEO) with whole and fractionated poly(vinyl acetate) (PVAc), were used to feed Patterson's theory of polymer-polymer miscibility. Negative values of mixing enthalpy, contact-energy term, interaction'' parameter and excess volume were obtained only for blends with the lowest molecular weight PVAc fraction. These results show that miscibility of PVAc with PEO strongly depends on its molecular weight. The calculated unfavourable excess volume term of the Patterson equation is small in comparison with the absolute value of the interaction term. Therefore, miscibility of PEO and low-molecular-weight PVAc is dictated by the weak specific interactions between different repeat units and by the entropic gain in the mixing process.

STATISTICAL THERMODYNAMICS OF POLYDISPERSE POLYMER BLENDS WITH STRONG INTERACTION

A statistical thermodynamics theory of polydisperse polymer mixtures with strong interaction between dissimilar components based on a lattice fluid model is formulated. Expressions for the free energy, equation of state, phase stability and spinodal for a polydisperse, binary polymer mixture with strong interaction are derived.

MISCIBILITY AND PHASE-BEHAVIOR IN BLENDS OF POLY(HYDROXYETHER OF BISPHENOL-A) WITH POLY(ETHYLENE OXIDE-CO-PROPYLENE OXIDE)

The miscibility of poly(hydroxyether of bisphenol A) (phenoxy) with a series of poly(ethylene oxide-co-propylene oxide) (EPO) has been studied. It was found that the critical copolymer composition for achieving miscibility with phenoxy around 60-degrees-C is about 22 mol % ethylene oxide (EO). Some blends undergo phase separation at elevated temperatures, but there is no maximum in the miscibility window. The mean-field approach has been used to describe this homopolymer/copolymer system. From the miscibility maps and the melting-point depression of the crystallizable component in the blends, the binary interaction energy densities, B(ij), have been calculated for all three pairs. The miscibility of phenoxy with EPO is considered to be caused mainly by the intermolecular hydrogen-bonding interactions between the hydroxyl groups of phenoxy and the ether oxygens of the EO units in the copolymers, while the intramolecular repulsion between EO and propylene oxide units in the copolymers contributes relatively little to the miscibility.

THE THERMAL-STABILITY OF POLYETHYLENE BLENDS DURING HEAT-PROCESSING AND SHORT-TERM PHOTOOXIDATION

The melt flow behaviour of LDPE/HDPE blends with various compositions have been determined by melt flow index (MFI) measurement. The effects of stabilizers, photo-sensitizers, multiple extrusions and short-term photooxidation have been studied. The results show that there is no marked thermal stability difference between homopolymers and blends without multiple extrusions, no matter whether stabilizers or photo-sensitizers are added. Multiple extrusions or photo-sensitizers reduce their thermal stability, shown by the decrease in MFI. The decrease in MFI of photooxidized samples does not imply serious structural change and shows that the active species formed during photooxidation induce a crosslinking reaction in the melt indexer. Multiple extrusions increase the number of active species formed in LDPE or blends and lead to an obvious decrease in MFI. It is suggested that LDPE and LDPE-rich blends after short-term photooxidation can be characterized by MFI measurement. In contrast, HDPE cannot be characterized by this method due to its linear structure.

A HIGH-RESOLUTION SOLID-STATE NMR-STUDY OF THE MISCIBILITY, MORPHOLOGY, AND TOUGHENING MECHANISM OF POLYSTYRENE WITH POLY(2,6-DIMETHYL-1,4-PHENYLENE OXIDE) BLENDS

An extended Goldman-Shen pulse sequence was used to observe indirectly the proton spin diffusion in the blends of polystyrene (PS) with poly(2,6-dimethyl-1,4-phenylene oxides) (PPO). The results indicate that the average distance between PS and PPO is less than 5 angstrom in the intimately mixed phase, but there are heterogeneous domains on a 100-angstrom scale. The data of spin relaxation of carbons, T1(C), for homopolymers and their blends suggest that there is a strong pi-pi electron conjugation interaction between the aromatic rings of PS and those of PPO, while the aromatic rings of PPO drive the aromatic rings of PS to move cooperatively. It is the cooperative motion that markedly improves the impact strength of PS.

BLENDS OF POLY[3,3-BIS(CHLOROMETHYL)OXETANE] WITH POLY(VINYL ACETATE) .1. THERMAL AND MECHANICAL-PROPERTIES

Blends of poly[3,3-bis(chloromethyl)oxetane] (Penton) with poly(vinyl acetate) were prepared. Compatibility, morphology, thermal behavior, and mechanical properties of blends with various compositions were studied using differential scanning calorimetry (DSC), dynamic mechanical measurements (DMA), tensile tests, and scanning electron microscopy (SEM). DMA study showed that the blends have two glass transition temperatures (T(g)). The T(g) of the PVAc rich phase shifts significantly to lower temperatures with increasing Penton content, suggesting that a considerable amount of Penton dissolves in the PVAc rich phase, but that the Penton rich phase contains little PVAc. The Penton/PVAc blends are partially compatible. DSC results suggest that PVAc can act as a beta-nucleator for Penton in the blend. Marked negative deviations from simple additivity were observed for the tensile strength at break over the entire composition range. The Young's modulus curve appeared to be S-shaped, implying that the blends are heterogeneous and have a two-phase structure. This was confirmed by SEM observations.

TENSILE PROPERTIES OF BLENDS OF POLY(ETHER SULFONE) WITH A POLY(ETHER IMIDE)

Blends of poly(ether sulphone) (PES) with a poly(ether imide) (PEI) in various proportions were prepared by the coprecipitation method. Mechanical properties and morphology of the blends were studied using tensile tests and scanning electron microscopy (SEM). The tensile moduli exhibit positive deviations from simple additivity. Marked positive deviations were also observed for ultimate strength. These results suggest that the PEI/PES blends are mechanically compatible. SEM study revealed that the blends are not homogeneous and the polymers are immiscible on the segmental level. However, the dispersions of the blends are rather fine. The interfaces between the two phases are excellently bonded; PEI and PES appear to interact well.

TENSILE PROPERTIES OF BLENDS OF PHENOLPHTHALEIN POLY(ETHER ETHER KETONE) WITH A POLY(ETHER IMIDE)

Blends of phenolphthalein poly(ether ether ketone) (PEK-C) with a poly(ether imide) (PEI) in various proportions were prepared by the coprecipitation method. Mechanical properties and morphology of the blends were studied using tensile tests and scanning electron microscopy (SEM). It was found that the tensile moduli exhibit positive deviations from simple additivity. Marked positive deviations were also observed for ultimate strength. These results suggest that the PEI/PEK-C blends are mechanically compatible. SEM study shows no evidence of phase separation, supporting the idea that the blends are compatible.

EPITAXIAL CRYSTALLIZATION OF HDPE ON IPP IN HDPE/IPP BLENDS

Unique crystalline morphologies of solution-cast films of HDPE/iPP blends were investigated by means of transmission electron microscopy (TEM), electron diffraction, metal shadowing and specimen-tilt techniques. The unique morphologies come from an epitaxial crystallization of HDPE on iPP. The contact planes of the two kinds of crystals are (100) of HDPE and (010) of iPP, while the intercrossing angle between their chain axes is about 50-degrees. The HDPE existed with different crystalline morphologies in the two kinds of crystalline regions of iPP spherulites, i.e. cross-hatched and single-crystal-type structures. Based on structural analysis, two models of epitaxial growth of HDPE on iPP are proposed.

STUDY ON STRUCTURE-PROPERTY RELATIONSHIP OF POLYIMIDE BLENDS .1. A DYNAMIC MECHANICAL STUDY OF THE MISCIBILITY OF POLYIMIDE POLYIMIDE BLENDS

Three pairs of polyimide/polyimide blends (50/50 wt%) with different molecular structures were prepared by two ways, i.e. mixing of the polyamic acid precursors with subsequent imidization, and direct solution mixing of the polyimides. The blends were studied with DMA technique. The results obtained show that all the blends prepared with these two different ways are miscible, as there existed only one glass transition temperature(Tg) for all the blends. It is suggested that the miscibility of these polyimide/polyimide blends is a result of the strong inter-molecular charge-transfer interaction between the chains of their components.

CRYSTALLIZATION OF POLY(ARYL-ETHER-ETHER-KETONE)S BLENDS

The crystallization and melting behaviour of poly(aryl-ether-ether-ketone) (PEEK) in blends with another polymer of the same family containing a bulky pendant phenolphthalein group (PEK-C) have been investigated by thermal methods. The small interaction energy density of the polymer pair (B = -8.99 J/cm3), evaluated from equilibrium melting point depression, is consistent with the T(g) data that indicate partial miscibility in the melt. Two conjugated phases are in equilibrium at 430-degrees-C: one is crystallizable and contains about 35 wt% of PEK-C; the other, containing only 15 wt% of PEEK, does not form crystals upon cooling and it interferes with the development of spherulites in the sample. The analysis of kinetic data according to nucleation theories shows that crystallization of PEEK in the explored temperature range takes place in Regime III and that a transition to Regime II might be a consequence of an increase in the amount of non-crystallizable molecules in the PEEK-rich phase. A composition independent value of the end surface free energy of PEEK lamellae has been derived from kinetic data (sigma-e = 40 +/- 4 erg/cm2) in excellent agreement with previous thermodynamic estimates. A new value for the equilibrium melting temperature of PEEK (T(m)-degrees = 639 K) has been obtained; it is about 30-degrees-C lower than the commonly accepted value and it explains better the "memory effect" in the crystallization from the melt of this high performance polymer.

SEM STUDY ON FRACTURE-BEHAVIOR OF ETHYLENE/PROPYLENE BLOCK COPOLYMERS AND THEIR BLENDS

The toughening effect of the separate phases of ethylene/propylene block copolymers and their blends was studied by scanning electron microscopy (SEM). The results obtained show that the interfacial adhesion between separate phases and the isotactic polypropene (iPP) matrix in ethylene/propylene block copolymers is strong at room temperature, but poor at low temperature; specimens exhibit tearing of separate phases during fracture at room temperature, but interfacial fracture between separate phases and the iPP matrix at low temperature. From the characteristics of fractographs of ethylene/propylene block copolymers and their blends, it could be concluded that the separate phases improve the toughness of specimens in several ways: they promote the plastic deformation of the iPP, and they can be deformed and fractured themselves during the fracture process. However, it was shown that the plastic deformation processes, such as multiple-crazing, shear yielding, etc. of the matrix are the dominant mechanisms of energy absorption in highly toughened ethylene/propylene block copolymers and their blends. The deformation and fracture of separate phases are only of secondary importance.

CHARACTERIZATION OF BLENDS OF POLYPROPYLENE WITH A SEMIRIGID LIQUID-CRYSTAL COPOLYESTER

Blends with a liquid-crystal polymers (LCP) as one component show, in general, very interesting properties. Reduction of shear visocity and improvement of mechanical properties are very remarkable. High melting temperatures and high costs of the LCP limit the use of these blends. A new class of thermotropic LCPs with flexible spaces, with relatively low melting temperatures, can overcome the first problem. In this work, rheological and mechanical properties of blends of polypropylene with low contents of this LCP are presented. Torque during extrusion and viscosity decrease with LCP content. Elastic modulus is remarkably increased when the LCP phase is oriented.