Amorphous polymers density

Thickness of polymer membrane across which diffusion is occurring Weight fraction of molecules that he within the size range i Density of a totally amorphous polymer Density of a completely crystalline polymer Density of polymer specimen for which f>ercent crystahinity is to be determined... 

Acetate and triacetate polymers are white amorphous soHds produced in granular, flake, powder, or fibrous form. They are used as raw materials in the preparation of fibers, films, and plastics. Polymer density varies and ranges from 100 kg/m for the fibrous form to 500 kg/m for granules. Acetate polymer is shipped by trailer tmck, railroad freight car, or multiwaH bags. 

Barrier Properties. VinyUdene chloride polymers are more impermeable to a wider variety of gases and Hquids than other polymers. This is a consequence of the combination of high density and high crystallinity in the polymer. An increase in either tends to reduce permeabiUty. A more subtle factor may be the symmetry of the polymer stmcture. It has been shown that both polyisobutylene and PVDC have unusually low permeabiUties to water compared to their monosubstituted counterparts, polypropylene and PVC (88). The values Hsted in Table 8 include estimates for the completely amorphous polymers. The estimated value for highly crystalline PVDC was obtained by extrapolating data for copolymers. 

Crystallization kinetics have been studied by differential thermal analysis (92,94,95). The heat of fusion of the crystalline phase is approximately 96 kj/kg (23 kcal/mol), and the activation energy for crystallization is 104 kj/mol (25 kcal/mol). The extent of crystallinity may be calculated from the density of amorphous polymer (d = 1.23), and the crystalline density (d = 1.35). Using this method, polymer prepared at —40° C melts at 73°C and is 38% crystalline. Polymer made at +40° C melts at 45°C and is about 12% crystalline. 

In principle the heat required to bring the material up to its processing temperature may be calculated in the case of amorphous polymers by multiplying the mass of the material (IP) by the specific heat s) and the difference between the required melt temperature and ambient temperature (AT). In the case of crystalline polymers it is also necessary to add the product of mass times latent heat of melting of crystalline structures (L). Thus if the density of the material is D then the enthalpy or heat required ( ) to raise volume V to its processing temperature will be given by ... 

In the crystalline region isotactic polystyrene molecules take a helical form with three monomer residues per turn and an identity period of 6.65 A. One hundred percent crystalline polymer has a density of 1.12 compared with 1.05 for amorphous polymer and is also translucent. The melting point of the polymer is as high as 230°C. Below the glass transition temperature of 97°C the polymer is rather brittle. 

In most cases crystal densities differ from the densities of amorphous polymers. This leads to differences in refractive index, which in turn cause scatter of light at boundaries between amorphous and crystalline zones. Such materials are opaque except in certain instances where the crystal structure can be carefully oriented to prevent such scatter of light. 

A suitable approach to the equilibration of an amorphous polymer system at bulk density becomes much more likely when the fully atomistic model in continuous space is replaced by an equivalent coarse-grained model on a lattice with sufficient conformational flexibility. Different strategies, which seek results at different levels of detail, can be employed to create an appropriate coarse-grained model. Section 4 (Doruker, Mattice) describes an approach which attempts to retain a connection with the covalent bonds in the polymer. The rotational isomeric state (RIS) [35,36] model for the chain is mapped into... 

In addition to temperature and concentration, diffusion in polymers can be influenced by the penetrant size, polymer molecular weight, and polymer morphology factors such as crystallinity and cross-linking density. These factors render the prediction of the penetrant diffusion coefficient a rather complex task. However, in simpler systems such as non-cross-linked amorphous polymers, theories have been developed to predict the mutual diffusion coefficient with various degrees of success [12-19], Among these, the most notable are the free volume theories [12,17], In the following subsection, these free volume based theories are introduced to illustrate the principles involved. 

Why is the density of an amorphous polymer greater in its glassy state than in its rubbery state ... 

Vc crystalline Va, amorphous). The densities of the pure crystalline (pc) and pure amorphous (pa) polymer must be known at the temperature and pressure used to measure p. The value of pc can be obtained from the unit cell dimensions when the crystal structure is known. The value of pa can be obtained directly for polymers that can be quenched without crystallization, polyfethylene terephtha-late) is one example. However, for most semi-crystalline polymers the value of pa is extrapolated from the variation of the specific volume of the melt with temperature [16,63]. 

Little is known about the variation of the critical stress ", with structure and temperature. For the polyethylene discussed abovedecreased from 620 psi at 22X to 39general trend with all polymers. Turner (84) found that the value of (r(. for polyethylenes increased by a factor of about 5 in going from a polymer with a density of 0.920 to a highly crystalline one with a density of 0.980. Reid (80,81) has suggested that for rigid amorphous polymers. ", should be proportional t° (Tt - T) For brittle polymers, the value of ", may be related to the onset of crazing. 

Based on the results obtained to date, which have been summarized above for several different semicrystalline polymers— linear and low density (branched) polyethylene, polytrimethylene oxide, polyethylene oxide and cis polyisoprene—it is concluded that the relatively fast segmental motions, as manifested in Tq, are independent of all aspects of the crystallinity and are the same as the completely amorphous polymer at the same temperature. Furthermore, it has previously been shown that for polyethylene, the motions in the non-crystalline regions are essentially the same as those in the melts of low molecular weight ii-alkanes. (17)... 

Crystalline polymers tend to have greater mechanical strength, higher melting points, and higher densities than amorphous polymers. On the other hand, they are usually much less transparent (more opaque) because light is reflected or scattered at the hour dary between the crystalline ana amorphous... 

Examples of crystalline polymers are nylons, cellulose, linear polyesters, and high-density polyethylene. Amorphous polymers are exemplified by poly(methyl methacrylate), polycarbonates, and low-density polyethylene. The student should think about why these structures promote more or less crystallinity in these examples. 

The value of TMWV is dependent on the cohesive energy density (CED) of amorphous polymers, the extent of crystallinity in crystalline polymers, and the effect of reinforcements in polymeric composites. Thus, while a low molecular weight amorphous polymer may be satisfactory for use as a coating or adhesive, a chain length generally above 100 is often required if the polymer is to be used as an elastomer or plastic. 

Free-radical polyolefin reactions form polymers with many mistakes in addition to the ideal long-chain alkanes because of chain-branching and chain-termination steps, as discussed. This produces a fairly heterogeneous set of polymer molecules with a broad molecular-weight distribution, and these molecules do not crystallize when cooled but rather form amorphous polymers, which are called low-density polyethylene. 

The density and n value of a polymer crystal are greater than those of an amorphous polymer. Many polymers are opaque because of the presence of ordered clusters of crystals called spherulites which have different n values. ptfe, which is highly crystalline, is opaque but amorphous polycarbonate (PC), PMMA, and PS are noncrystalline and clear. 

Crystalline polymers such as high-density polyethylene (hope), PP, PTFE, and polyoxymethylene (POM) exhibit somewhat higher X values than amorphous polymers such as low-density polyethylene (ldpe), atactic PS,... 

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