Melt viscosity glass transition temperature

Plasticizers are reducing the modulus, tensile strength, hardness, density, melt viscosity, glass transition temperature, electrostatic chargeability and volume resistivity of a polymer. In contrast, they are increasing the flexibility, elongation at break, toughness, dielectric constant and power factor (2). 

Because of their novel topologies, polyrotaxanes have properties different from those of conventional polymers. Solubility, intrinsic viscosity, melt viscosity, glass transition, melting temperature and phase behavior can be altered by the formation of polyrotaxanes. The detailed changes are related both to the properties of the threaded cyclics and to the backbone and the threading efficiency. 

The second method is quite harsh but similar to RESS process as they both involve use of SFCO as a solvent rather than an anti-solvent. This process involves dissolving the SF in molten solute and the resulting supercritical solution fed via an orifice into a chamber to allow rapid expansion under ambient conditions [17], The dissolved gas decreases the viscosity of the molten compound and so the gas saturated liquid phase is expanded to generate particles from materials that are not necessarily soluble in SF. The presence of the CO allows the material to melt at temperature significantly lower than the normal melting or glass transition temperature. 

Plasticizers are organic substances of low volatility that are added to plastics compounds to improve then-flexibility, extensibility, and processability. They increase flow and thermoplasticity of plastic materials by decreasing the viscosity of polymer melts, the glass transition temperature (Tg) the melting temperature (Tn), and the elasticity modulus of finished products [53]. 

Combs, R. L. and Nation, R. G., Relationships among melt flow, glass transition temperature and inherent viscosity of tfamnoplastic polyestms, J. Pofym, ScL, 30, 407-414 (1970). 

Figures 16 [p.237] and 17 illustrate respectively the crystal growth rates and the crystallization half-time as a function of undercooling for the four polyolefins. Analyzing the figures, it is not possible to draw any general conclusion on the dependences of the crystallization rate on the bulkiness of the polyolefin side group. Other variables, like molecular mass, crystallization condition, melting, and glass transition temperatures, as well as viscosity of the melt, are involved and should be taken into account for defining the crystallization process of these important polymeric materials.

Since successful commercialization of Kapton by Du Pont Company in the 1960s (10), numerous compositions of polyimide and various new methods of syntheses have been described in the Hterature (1—5). A successful result for each method depends on the nature of the chemical components involved in the system, including monomers, intermediates, solvents, and the polyimide products, as well as on physical conditions during the synthesis. Properties such as monomer reactivity and solubiHty, and the glass-transition temperature,T, crystallinity, T, and melt viscosity of the polyimide products ultimately determine the effectiveness of each process. Accordingly, proper selection of synthetic method is often critical for preparation of polyimides of a given chemical composition. 

The Arrhenius equation holds for many solutions and for polymer melts well above their glass-transition temperatures. For polymers closer to their T and for concentrated polymer and oligomer solutions, the WiUiams-Landel-Ferry (WLF) equation (24) works better (25,26). With a proper choice of reference temperature T, the ratio of the viscosity to the viscosity at the reference temperature can be expressed as a single universal equation (eq. 8) ... 

As the temperature is decreased, free-volume is lost. If the molecular shape or cross-linking prevent crystallisation, then the liquid structure is retained, and free-volume is not all lost immediately (Fig. 22.8c). As with the melt, flow can still occur, though naturally it is more difficult, so the viscosity increases. As the polymer is cooled further, more free volume is lost. There comes a point at which the volume, though sufficient to contain the molecules, is too small to allow them to move and rearrange. All the free volume is gone, and the curve of specific volume flattens out (Fig. 22.8c). This is the glass transition temperature, T . Below this temperature the polymer is a glass. 

The glass transition temperatures of the nylons appear to be below room temperature so that the materials have a measure of flexibility in spite of their high crystallinity under general conditions of service. The polymers have fairly sharply defined melting points and above this temperature the homopolymers have low melt viscosities. Some thermal properties of the nylons are given in Table 18.4. 

Material added to a plastic to increase its workability and flexibility. Plasticizers tend to lower the melt viscosity, the glass transition temperature and/or the elastic modulus. 

Temperature has a complex effect on crystallization rate. Initially, as the temperature falls below the equilibrium melting temperature, the crystallization rate increases because nucleation is favored. However, as the temperature continues to fall, the polymer s viscosity increases, which hampers crystallization. As a rule of thumb, a polymer crystallizes fastest at a temperature approximately mid-way between its glass transition temperature and its equilibrium melting temperature. 

At room temperature, atactic polystyrene is well below its glass transition temperature of approximately 100 °C. In this state, it is an amorphous glassy material that is brittle, stiff, and transparent. Due to its relatively low glass transition temperature, low heat capacity, and lack of crystallites we can readily raise its temperature until it softens. In its molten state, it is quite thermally stable so we can mold it into useful items by most of the standard conversion processes. It is particularly well suited to thermoforming due to its high melt viscosity. As it has no significant polarity, it is a good electrical insulator. 

Solid-state polycondensation (SSP) is thus a technique applied to thermoplastic polyesters to raise their molecular weight or IV. During solid-state polycondensation, the polymer is heated above the glass transition temperature and below the melt temperature of the polymer either under an inert gas or under vacuum. Increasing the intrinsic viscosity requires a residence time of up to 12 h under vacuum or under inert gas, at temperatures from 180 to 240 °C. 

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