Stress optical measurements

Dynamic viscoelastic and stress-optical measurements are reported for blends of crosslinked random copolymers of butadiene and styrene prepared by anionic polymerization. Binary blends in which the components differ in composition by at least 20 percentage units give 2 resolvable loss maxima, indicative of a two-phase domain structure. Multiple transitions are also observed in multicomponent blends. AU blends display an elevation of the stress-optical coefficient relative to simple copolymers of equivalent over-all composition. This elevation is shown to be consistent with a multiphase structure in which the domains have different elastic moduli. The different moduli arise from increased reactivity of the peroxide crosslinking agent used toward components of higher butadiene content. 

Stress Birefringence. Confirmation of the multiphase nature of the polyblends described may be obtained by stress-optical measurements. Since this technique apparently has not been used before to demonstrate incompatibility in polyblends, a brief description is given of the rationale behind the method. 

In the solid phase, support for a similar contention has come from the drawing of polyethylene terephthalate, combined with stress optical measurements, and also for the behaviour of craze fibrils in a range of glas polymers. 

Flow into either sharp-edged or tapered die was studied using stress-optical measurements. Distribution of shear stress and normal stress difference was obtained for PS melt — neither stresses nor velocities showed secondary motion. 

Dynamic mechanical and dynamic stress-optical measurements have been performed on side-chain LCEs with different crosslinking densities [8]. It was found that the relaxation strengths depend strongly on the crosslinking density demonstrating that the vicinity of the crosslinking points perturbs the liquid crystalline order. In this experiment the samples were exposed to a time-dependent stress <7(t), and the time-dependent responses, that is the time-dependent strain e(t) and the time-dependent birefringence An (t), were measured. The real part... 

The results and the analysis of detailed stress-optical measurements in the immediate vicinity of the phase transition have been reported [3,4]. In Fig. 8 the reciprocal... 

Detailed stress-optical measurements have been analyzed to yield further information [4]. In Fig. 10 the birefringence (order parameter) was plotted as a function of reduced temperature for several nominal stresses <7 . These results were combined with the predictions of the Landau model and static stress-strain curves and led to a number of interesting consequences. In Fig. 11 the shift in the phase transition temperature is plotted as a function of nominal stress and shifts of up to 7.5 K were found compared to maximum displacements by electric and magnetic fields of about 5 mK in low molecular weight materials. In Fig. 12 the birefringence An is shown as a function of strain X=L/Lq at constant nominal stress f7n = 2.11xlO Nmm. A strictly... 

This result is not contrary to the conclusions following from the stress optical measurements at temperatures above T, where the LC-elastomer is optically negative. In this case the optical axis of the network corresponds to the axis of deformation. In the LC-state the characterization "optically positive" is refered to the optical axis of the LC-phase, which must not necessarily coincide with the axis of deformation. 

Regarding the LC-elastomer (below T ) as a "two phase-system" one can assign one optical axis to the anisotropic LC-state of the mesogenic side chains and one to the anisotropic orientation of the network chains. The second one corresponds to the optical axis as usually determined by stress optical measurements of conventional... 

Values for the extension ratios, estimated by Kramer and colleagues [36, 42] and also by Ward and co-workers [18, 22] from analysis of optical interference patterns, compare reasonably well with estimates of the network extensibility from small-angle neutron scattering data [36] or stress-optical measurements [43]. It was therefore proposed that the criterion for craze failure, and hence crack propagation via a craze, is to assume that the entangled strands crossing the section of the craze at the crack tip break due to the development of the critical stress at the crack tip cTfaii,... 

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