Glassy polymers homopolymers

The other important influence of the bulk properties of the homopolymers is on the fibril stress, i.e. the craze stability. Even if, as pointed out above, the crazing stress for a glassy polymer does not vary much, it is well known that the molecular weight of the polymer has a profound effect on its fracture toughness [58]. 

In Table 6.1 some of the most often-used monomers are shown with their reactivity, expressed in terms of the Alfrey-Price Q and e values [4] (see Section 1.6.4 of Chapter 1), their water solubilities, boiling points and the homopolymer Tg values. The table is arranged in order of decreasing polymer Tg values, splitting the list into two, with the upper half representing glassy polymers at room temperature and rubbery polymers taking the bottom half. 

A very large body of data on the gas permeability of many rubbery and glassy polymers has been published in the literature. These data were obtained with homopolymers as well as with copolymers and polymer blends in the form of nonporous dense (homogeneous) membranes and, to a much lesser extent, with asymmetric or composite membranes. The results of gas permeability measurements are commonly reported for dense membranes as permeability coefficients, and for asymmetric or composite membranes as permeances (permeability coefficients not normalized for the effective membrane thickness). Most permeability data have been obtained with pure gases, but information on the permeability of polymer membranes to a variety of gas mixtures has also become available in recent years. Many of the earlier gas permeability measurements were made at ambient temperature and at atmospheric pressure. In recent years, however, permeability coefficients as well as solubility and diffusion coefficients for many gas/polymer systems have been determined also at different temperatures and at elevated pressures. Values of permeability coefficients for selected gases and polymers, usually at a single temperature and pressure, have been published in a number of compilations and review articles [27—35]. 

Tables 61.2-61.17 list references to many recent and some earlier permeability measurements made with various pure gases and membranes cast from different classes of rubbery and glassy polymers, but mainly homopolymers.

The idea described above for glassy amorphous homopolymers can be extended to include miscible amorphous polymer blends, such as PS/PPO. Furthermore, a low degree of covalent cross-links can be considered as equivalent to entanglements for controlling the deformation mode. The strand density of cross-linked polymers is defined as the sum of the entanglement density and the covalent cross-link density [18] as... 

A copolymer in region 5 is atypical amorphous, glassy polymer hard, rigid, and usually brittle. Again, if the polymer is pure, it will be perfectly transparent. PMMA (Lucite, Plexiglas) and PS are familiar examples of homopolymers with these properties. 

In this chapter, we have presented the rheological behavior of homopolymers, placing emphasis on the relationships between the molecular parameters and rheological behavior. We have presented a temperature-independent correlation for steady-state shear viscosity, namely, plots of log ri T, Y) r](jiT) versus log or log j.y, where Tq is a temperature-dependent empirical constant appearing in the Cross equation and a-Y is a shift factor that can be determined from the Arrhenius relation for crystalline polymers in the molten state or from the WLF relation for glassy polymers at temperatures between and + 100 °C. 

In general, as the ion content is raised, the modulus or stiffness of the ionomer is increased, as shown by the data in Fig. 2. While the increase is much greater in the elevated temperature range, where the polymer is acting more like a crosslinked rubber, there is still a significant increase in the glassy modulus below Tg. For example, for the PMMA-based ionomer of Fig. 2, the modulus at 30°C is almost 20% above that of the homopolymer for an ionomer having an ion content of 12.4 mol%. For the... 

Triblock copolymers, as shown in Fig. 5.8 d), comprise a central homopolymer block of one type, the ends of which are attached to homopolymer chains of another type. As with other block copolymers, the components of triblocks may be compatible or incompatible, which will strongly influence their properties. Of particular interest are triblocks with incompatible sequences, the middle block of which is rubbery, and the end blocks of which are glassy and form the minor phase. When such polymers phase-segregate, it is possible for the end blocks of a single molecule to be incorporated into separate domains. Thus, a number of rubbery mid-block chains connect the glassy phases to one another. These materials display rubber-like properties, with the glassy domains acting as physical crosslinks. Examples of such materials are polystyrene/isoprene/polystyrene and polystyrene/polybutadiene/polystyrene triblock copolymers. 

NMR has not been widely employed to study dynamics in block copolymer melts, although field gradient NMR can provide a wealth of information on the diffusion of block copolymer chains (Fleischer et al. 1993). The orientation of a deuterated homopolymer in a lamellar diblock copolymer (in a glassy state) was determined using 2H NMR by Valic et al. (1994,1995). Other applications of NMR to probe polymer chain dynamics and details of experimental protocols are described by Bovey and Jelinksi (1989). 

The mechanical admixture of low molecular weight monomers into polymers normally in the glassy state at room temperature in order to increase the flexibility and softness of the polymer has great technical importance. Thus, such plasticizers as di-2 ethyl n-hexyl phthalate are frequently incorporated into polyvinyl chloride, homopolymer or copolymer, to increase the flexibility and commercial value of this resin. Cast (1953) as well as Hellwege, Knappe and Semjonow (1959) have... 

The homopolymers poly(methyl methacrylate) and poly-(ethyl methacrylate) are compatible with poly(vinylidene fluoride) when blended in the melt. True molecular com-patibility is indicated by their transparency and a single, intermediate glass transition temperature for the blends. The Tg results indicate plasticization of the glassy methacrylate polymers by amorphous poly(vinylidene fluoride). The Tg of PVdF is consistent with the variation of Tg with composition in both the PMMA-PVdF and PEMA-PVdF blends when Tg is plotted vs. volume fraction of each component. PEMA/PVdF blends are stable, amorphous systems up to at least 1 PVdF/I PEMA on a weight basis. PMMA/ blends are subject to crystallization of the PVdF component with more than 0.5 PVdF/1 PMMA by weight. This is an unexpected result. 

Polymers of type shown on in Figure 4.28(b) were also investigated. For materials with five PV units, well-defined nanostructures were observed which are spaced 8 nm from center to center and have a length of 80 nm. Electron diffraction measurements show that the rods are packed into the same structure as the poly(p-phenylene vinylene) homopolymer. The rods are again perpendicular to the surface. When only two PV units are present, no nano-scale organization is observed and glassy solids were obtained instead. This latter observation shows that in order to obtain ordered nanostructures, a rod containing two PV units is not sufficient. 

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