Amorphous phase density

The amorphous phase differs from the mesophase and the crystalline phase by a clearly lower value of density. The amorphous phase density depends on the internal orientation of the fiber. Us value is in the range 1.335-1.357 g/cm. In the case of a very high orientation, it can even reach the value 1.363 g/cm-. ... 

Using the mentioned values, an amorphous phase density of 1.241 g/cm3 was calculated, which seems to be an acceptable value. (The x(c) values of the virgin powder sample and the sample after compression moulding at 280°C (2.5 minutes) can be calculated using the Hf(max.)-value of 242 J/g (see 9.2.2) because both the DSC and the XRD measurement were performed on PK copolymer with only an amorphous and a S-crystalline phase. This XRD value does not hold, however, for PK copolymer with both an a-crystalline and a S-crystalline phase). 

The dimensional stability of molded components benefits from several unique aspects of SPS. First, the amorphous and crystalline phases have equivalent density, and this tends to minimize the crystalline gradients that are typical of semicrystalline, thermoplastic materials. A comparison of the amorphous phase density with the crystalline phase density of SPS and other semicrystalline thermoplastics is shown in Table 15.3. 

In addition, to account for the density change during crystallization, Nakamura multiplied the right side of Equation (7.30) by an empirical factor, pjpi, equal to the ratio of semicrystalline phase density to amorphous phase density. 

Density. Density of LLDPE is measured by flotation in density gradient columns according to ASTM D1505-85. The most often used Hquid system is 2-propanol—water, which provides a density range of 0.79—1.00 g/cm. This technique is simple but requires over 50 hours for a precise measurement. The correlation between density (d) and crystallinity (CR) is given hy Ijd = CRj + (1 — Ci ) / d, where the density of the crystalline phase, ify, is 1.00 g/cm and the density of the amorphous phase, is 0.852—0.862 g/cm. Ultrasonic methods (Tecrad Company) and soHd-state nmr methods (Auburn International, Rheometrics) have been developed for crystallinity and density measurements of LLDPE resins both in pelletized and granular forms. 

Density. Although the polymer unit cell dimensions imply a calculated density of 1.33 g/cm at 20°C, and extrapolation of melt density data indicates a density of 1.13 g/cm at 20°C for the amorphous phase, the density actually measured is 1.15—1.26 g/cm, which indicates the presence of numerous voids in the stmcture. 

A more polar comonomer, eg, an AN comonomer, increases the water-vapor transmission more than VC when other factors are constant. For the same reason, AN copolymers are more resistant to penetrants of low cohesive energy density. AH VDC copolymers, however, are very impermeable to ahphatic hydrocarbons. Comonomers that lower T and increase the free volume in the amorphous phase increase permeability more than the polar comonomers higher acrylates are an example. Plasticizers increase permeabiUty for similar reasons. 

In the studies carried out by one of the authors [52], the values of Ea and E were determined for PET fibers of the microfibrillar and of the lamellar substructure. The results have been presented in Tables 8 and 9. The results obtained show that for both types of substructure the resistance to deformation, that is, the value of E, depends on the degree of molecular orientation of the amorphous material of the fiber fa) and the density of this amorphous phase of the fiber da)- However, this dependence assumes a different form for the microfibrillar and for the lamellar substructure. In the first case, it has the form ... 

The rate of 7-radiation-induced cross-linking in the crystalline and amorphous regions of a crystallizable polychloroprene has been measured by Makhlis et al. [75] who have found a considerably lower cross-link density and less degradation in the crystalline portion of the rubber. The cross-links have been posmlated to be mainly intramolecular in crystalline regions and intermolecular in the amorphous phases. 

The dynamic mechanical behavior indicates that the glass transition of the rubbery block is basically independent of the butadiene content. Moreover, the melting temperature of the semicrystalline HB block does not show any dependence on composition or architecture of the block copolymer. The above findings combined with the observation of the linear additivity of density and heat of fusion of the block copolymers as a function of composition support the fact that there is a good phase separation of the HI and HB amorphous phases in the solid state of these block copolymers. Future investigations will focus attention on characterizing the melt state of these systems to note if homogeneity exists above Tm. 

However, at some specific pressure the high-density polymorph becomes mechanically unstable. This low-pressure limit is seldom observed, since it often corresponds to negative pressures. When the mechanical stability limit is reached the phase becomes unstable with regard to density fluctuations, and it will either crystallize to the low-pressure polymorph or transform to an amorphous phase with lower density. 

As discussed above, many polymers contain some crystalline structures when they are solidified. These polymers are referred to as semicrystalline resins. These crystalline structures can be observed using microscopy as shown in Fig. 2.12 for PP and sPS resins. As shown schematically in Fig. 2.13 and discussed above, not all portions of the polymer chains are incorporated into the crystalline structure. Instead, the portions of the chains that are not crystallized make up the amorphous phase. Solid density is the most commonly used method for measuring the... 

Silica has 22 polymorphs, although only some of them are of geochemical interest—namely, the crystalline polymorphs quartz, tridymite, cristobahte, coesite, and stishovite (in their structural modifications of low and high T, usually designated, respectively, as a and jS forms) and the amorphous phases chalcedony and opal (hydrated amorphous silica). The crystalline polymorphs of silica are tectosilicates (dimensionality = 3). Table 5.68 reports their structural properties, after the synthesis of Smyth and Bish (1988). Note that the number of formula units per unit cell varies conspicuously from phase to phase. Also noteworthy is the high density of the stishovite polymorph. 

Note 4 The degree of crystallinity can be determined by several experimental techniques among the most commonly used are (i) X-ray diffraction, (ii) calorimetry, (iii) density measurements, and (iv) infrared spectroscopy (IR). Imperfections in crystals are not easily distinguished from the amorphous phase. Also, the various techniques may be affected to different extents by imperfections and interfacial effects. Flence, some disagreement among the results of quantitative measurements of crystallinity by different methods is frequently encountered. 

The search for a substance harder than diamond has recently focussed on buckminsterfullerene, Ceo. Above 8 GPa and 700 K, buckminsterfullerene forms two amorphous phases which are harder than diamond—the samples scratched diamond anvils when squeezed between them Although harder than diamond, these phases have a lower density and are semiconducting. 

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