Glassy blends

Through the understanding of the nonequilibrium changes in the glassy state of miscible blends, the excess volume of mixture is analyzed, and is related to the nonequilibrium enthalpy of mixing. In contrast to the multi-phase systems, the presence of a maximum yield stress in a miscible glassy blend at a critical concentration is predicted as a function of the nonequilibrium interaction. In accordance with Eq. (59), the total volume of a compatible blend is written as... 

The Metravib Micromecanalyser is an inverted torsional pendulum, but unlike the torsional pendulums described eadier, it can be operated as a forced-vibration instmment. It is fully computerized and automatically determines G, and tan 5 as a function of temperature at low frequencies (10 1 Hz). Stress relaxation and creep measurements are also possible. The temperature range is —170 to 400°C. The Micromecanalyser probably has been used more for the characterization of glasses and metals than for polymers, but has proved useful for determining glassy-state relaxations and microstmctures of polymer blends (285) and latex films (286). 

Whilst the volume production of completely new polymers which have achieved commercial viability in recent years has been small, the development of polymer blends has been highly significant. Of these the most important involve a glassy... 

Following the success in blending rubbery materials into polystyrene, styrene-acrylonitrile and PVC materials to produce tough thermoplastics the concept has been used to produce high-impact PMMA-type moulding compounds. These are two-phase materials in which the glassy phase consists of poly(methyl methacrylate) and the rubbery phase an acrylate polymer, usually poly(butyl acrylate Commercial materials of the type include Diakon MX (ICI), Oroglas... 

The term ABS was originally used as a general term to describe various blends and copolymers containing acrylonitrile, butadiene and styrene. Prominent among the earliest materials were physical blends of acrylonitrile-styrene copolymers (SAN) (which are glassy) and acrylonitrile-butadiene copolymers (which are rubbery). Such materials are now obsolete but are referred to briefly below, as Type 1 materials, since they do illustrate some basic principles. Today the term ABS usually refers to a product consisting of discrete cross-linked polybutadiene rubber particles that are grafted with SAN and embedded in a SAN matrix. 

Blending the glassy phase polymer with another polymer of higher Tg such as a polycarbonate. 

The process of blending with another glassy polymer to raise the heat distortion temperature is not restricted to polycarbonate, and the polysulphones are obvious candidates because of their higher Tg. One blend has been offered (Arylon T by USS Chemicals) which has a higher softening point than the ABS-polycarbonates. 

Blending of ABS with an acrylic material such as poly(methyl methacrylate) can in some cases allow a matching of the refractive indices of the rubbery and glassy phases and providing that there is a low level of contaminating material such as soap and an absence of insoluble additives a reasonable transparent ABS-type polymer may be obtained. More sophisticated are the complex terpolymers and blends of the MBS type considered below. Seldom used on their own, they are primarily of use as impact modifiers for unplasticised PVC. 

The commercial success of ABS polymers has led to the investigation of many other polyblend materials. In some cases properties are exhibited which are superior to those of ABS and some of the materials are commercially available. For example, the opacity of ABS has led to the development of blends in which the glassy phase is modified to give transparent polymers whilst the limited light aging has been countered by the use of rubbers other than polybutadiene. 

Blends have also been produced containing neither acrylonitrile and styrene in the glassy phase nor polybutadiene in the rubbery phase. 

Note In polymers, because of the low mobility of polymer chains, particularly in a glassy state, metastable mixtures may exist for indefinite periods of time without phase separation. This has frequently led to confusion when metastable miscible polymer blends are erroneously claimed to be miscible. 

Melt-processable polymer blend or copolymer in which a continuous elastomeric phase domain is reinforced by dispersed hard (glassy or crystalline) phase domains that act as junction points over a limited range of temperature, or... 

Apparently all four curves intersect at the same point, namely at the temperature at which A and B have the same E-modulus. Each blend has the same modulus at this temperature. This situation may occur when B is a glassy amorphous polymer and A... 

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