Rangaramanujam M. Kannan

Dynamics of each component in miscible blends of polyisoprene and polyvinylethylene

The dynamics of the individual components in 1,4-polyisoprene/polyvinylethylene (PIP/PVE) miscible blends are studied using dynamic stress-optical measurements. While the homopolymers are thermorheologically simple and obey the stress-optic rule, the blends show failure of time-temperature superposition and complex stress-optic behavior. The way in which the stress-optic rule fails reveals the relaxation dynamics of each species. The dynamic modulus and complex birefringence coefficient are analyzed to infer the relaxation of each component. The entanglement molecular weight, Me, and monomeric friction coefficient, ζ0, of each species as a function of blend composition and temperature are determined from the contribution of each species to the dynamic modulus. The effect of blending on Me of each component is small; however, its effect on ζ0 of each species is dramatic. Blending strongly speeds the rate of relaxation of the high Tg component (PVE), while more modestly slowing the relaxation of the low Tg component (PIP). The dynamics of each species have different temperature dependencies in the blend, which leads to the failure of the superposition principle. Furthermore, both the difference between the friction coefficients of the two species and the difference in their temperature dependencies is greater in blends rich in the high Tg material (PVE).

Stress‐optical manifestations of molecular and microstructural dynamics in complex polymer melts

Experimental methods are presented for investigating molecular and microstructural aspects of the rheology of polymer melts. Measurements of stress, birefringence, and dichroism are made simultaneously so that the connection between macroscopic viscoelasticity and the dynamics of the fluid microstructure can be established as directly and unambiguously as possible. We show how quantitative characterization of departure from the stress‐optic rule can expose distinct molecular and microstructural dynamics in complex polymer melts. This general approach is applied (1) to resolve the contributions of distinct species to the macroscopic stress, illustrated using a miscible polymer blend, (2) to discriminate between dynamical regimes that are controlled by conformational or microstructural relaxation, illustrated using an ordered block copolymer, and (3) to distinguish the dynamics of topologically distinct parts of a given molecule, illustrated using a side‐group liquid‐crystalline polymer.

The third-normal stress difference in entangled melts: Quantitative stress-optical measurements in oscillatory shear

AbstractStress-optical measurements are used to quantitatively determine the third-normal stress difference (N3 = N1 + N2) in three entangled polymer melts during small amplitude (<15%) oscillatory shear over a wide dynamic range. The results are presented in terms of the three material functions that describe N3 in oscillatory shear: the real and imaginary parts of its complex amplitude ψ3*= ψ3′- iψ3″, and its displacement ψ3d. The results confirm that these functions are related to the dynamic modulus by ω2ψ3*(ω)=(1-β)[G*(ω))−

Evolution of microstructure and viscoelasticity during flow alignment of a lamellar diblock copolymer

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Effect of mesophase order and molecular weight on the dynamics of nematic and smectic side-group liquid-crystalline polymers

G*(2ω)] and ω2ψ3d(ω)=(1- β)G′(ω) as predicted by many constitutive equations, where β = −N2/N1. The value of (1-β) is found to be 0.69±0.07 for poly(ethylene-propylene) and 0.76±0.07 for polyisoprene. This corresponds to −N2/N1 = 0.31 and 0.24±0.07, close to the prediction of the reptation model when the independent alignment approximation is used, i.e., −N2/N1 = 2/7 − 0.28.

Dynamics of disordered diblocks of polyisoprene and polyvinylethylene

The dynamics of oscillatory shear‐induced alignment in side‐group liquid‐crystalline polymers (SGLCPs) are investigated by measuring transmittance and viscoelasticity during and after the alignment process. We compare the dynamic moduli and transmittance of shear‐aligned samples with those of magnetically aligned samples. Relative to an unaligned nematic, magnetic alignment does not appreciably alter the dynamic moduli. This suggests that, at small strains, distortion of the polymer backbone dominates the linear viscoelastic response so that the moduli are insensitive to macroscopic director alignment. With increasing strain, deviations from linear behavior are manifested by an increase in the dynamic modulus. This strain hardening may be due to the tendency of the mesogens to orient perpendicular to the backbone in the present SGLCP, since the mesogens attached to a given strand may be forced to adopt an intermediate orientation between that dictated by the backbone segment and that dictated by the local...