Crystalline polymers, relaxations

Optical and electro-optical behavior of side-chain liquid crystalline polymers are described 350-351>. The effect of flexible siloxane spacers on the phase properties and electric field effects were determined. Rheological properties of siloxane containing liquid crystalline side-chain polymers were studied as a function of shear rate and temperature 352). The effect of cooling rate on the alignment of a siloxane based side-chain liquid crystalline copolymer was investigated 353). It was shown that the dielectric relaxation behavior of the polymers varied in a systematic manner with the rate at which the material was cooled from its isotropic phase. 

For glassy and crystalline polymers there are few data on the variation of stress relaxation with amplitude of deformation. However, the data do verily what one would expect on the basis of the response of elastomers. Although the stress-relaxation modulus at a given time may be independent of strain at small strains, at higher initial fixed strains the stress or the stress-relaxation modulus decreases faster than expected, and the lloltz-nuinn superposition principle no longer holds. 

For elastomers, factorizability holds out to large strains (57,58). For glassy and crystalline polymers the data confirm what would be expected from stress relaxation—beyond the linear range the creep depends on the stress level. In some cases, factorizability holds over only limited ranges of stress or time scale. One way of describing this nonlinear behavior in uniaxial tensile creep, especially for high modulus/low creep polymers, is by a power... 

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

Crystallinity—about.i to 15% (213,232). The creep of plasticized poly(vinyl chloride) polymers as a function of temperature, concentration, and kind of plasticizer has been studied by many workers, including Aiken et ai. (232), Neilscn ct ai. (234), and Sabia and Eirich (243). These last workers also studied stress relaxation (244). In the case of crystalline polymers, plasticizers and Copolymerization reduce the melting point and the degree of Crystallinity. These factors tend to increase the creep and stress relaxation, especially at temperatures approaching the melting point. 

While no direct evidence of liquid crystallinity in PET-BB copolymers has been reported, the high-BB-content copolymers have been shown to possess morphologies similar to those of liquid crystalline polyesters [40], and show major changes in both melt relaxation times and fiber tensile moduli, suggestive of structural organization in a frustrated liquid crystalline polymer (LCP) (Table 6.3 and Figure 6.4) [41, 42],... 

The major results described could be partially anticipated from those previously reported for linear polyethylene (17) as well as those for cis polyisoprene. (] ) For the latter polymer, by taking advantage of its crystallization kinetic characteristics, it was possible to compare the relaxation parameters of the completely amorphous and partially crystalline polymer (31% crystallinity) at the same temperature, 0°C. This is a unique situation and allows for some unequivocal comparisons. It was definitively observed that for all the carbons of cis polyisoprene the T] s did not change with crystallization. 

FIGURE 14.10 Logarithm of the relaxation modulus as a function of temperature for three polymer samples. Sample (a) is (largely) crystalline vinyl pol5uner sample (b) is an amorphous vinyl polymer that contains light cross-linking and sample (c) is an amorphous vinyl pol5uner. The Tg for the amorphous polymer is about 100°C and the for the crystalline polymer is about 180°C. 

Gray and McCrum735 used the Hashin-Shtrikman theory to explain the origin of the y relaxation in PE and PTFE, Maeda et al.745 have given exact analyses of several two phase models for semi-crystalline polymers and Buckley755 represented a biaxially oriented sheet of linear polyethylene by a two phase composite model. 

Photoluminescence intensity of the amorphous polymers was generally much larger than that of the more crystalline polymers. The energy level of the lowest singlet excited state Es was evaluated to be 2.5-2.7 eV for the amorphous polymer pristine films, and 2.0 eV for the more crystalline polymers. The Stokes shifts were also observed to be much larger for the amorphous polymer films compared with those of the more crystalline polymer films. This indicates a larger structural relaxation of the amorphous polymers following photoexcitation. 

As revealed from Eqs. (1) and (2), or their candid forms (4) and (5), the longitudinal relaxation is determined by the spectral densities in the order of o>h toc, whereas the transverse relaxation involves the contribution from the zero frequency component Jo(0). In the case of solid matter, tc is generally very long. Hence, the transverse relaxation is predominantly determined by the zero frequency component Jo(0). In Eq. (5), for example, the zero frequency term (the first term) dominates the other terms that are reciprocally proportional to Tc for co2x2 1. Tic increases as xc increases (i.e. as the material under consideration becomes solider), whereas T2c decreases infinitely as xc increases. For example, Tic is generally in an order of several tens several hundreds of seconds for the crystalline component and in an order of a few tenths of a second for rubbery components of polymers. On the other hand, T2c is of an order of a few tens of microseconds for the crystalline or glassy component and a few milliseconds for the rubbery component of polymers. In this work, Tic and T2c are used for characterizing different components in crystalline polymers. 

Spin-Lattice and Spin-Spin Relaxations. In order to determine the content of these crystalline and noncrystalline resonances, the longitudinal and transverse relaxations were examined in detail. It was first confirmed that the noncrystalline resonance of all samples is associated with Tic in an order of 0.45-0.57 s. Hence, the noncrystalline component of all samples comprises a monophase, in as much as judged only by Tic. However, it was found that the noncrystalline component of drawn samples generally comprises two phases with different T2C values amorphous and crystalline-amorphous interphases. The dried gel sample does not include rubbery amorphous material it comprises the crystalline and rigid noncrystalline components. However, the rubbery amorphous phase with T2C of 5.5 ms appears by annealing at 145 °C for 4 minutes. For the orthorhombic crystalline component, three different Tic values, that suggest the distribution of crystallite size, were recognized for each sample, as normal for crystalline polymers [17,54, 55]. The Tic and T2C of all samples examined are summerized in Table 6. 

On the other hand, in the solid-state high resolution 13C NMR, elementary line shape of each phase could be plausibly determined using magnetic relaxation phenomenon generally for crystalline polymers. When the amorphous phase is in a glassy state, such as isotactic or syndiotactic polypropylene at room temperature, the determination of the elementary line shapes of the amorphous and crystalline-amorphous interphases was not so easy because of the very broad line width of both the elementary line shapes. However, the line-decomposition analysis could plausibly be carried out referring to that at higher temperatures where the amorphous phase is in the rubbery state. Thus, the component analysis of the spectrum could be performed and the information about each phase structure such as the mass fraction, molecular conformation and mobility could be obtained for various polymers, whose character differs widely. 

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