Rheology of Blends of a Rodlike Polymer (PBO) and its Flexible Chain Analog
- PDF / 272,748 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 9 Downloads / 149 Views
RHEOLOGY OF BLENDS OF A RODLIKE POLYMER (PBO) AND ITS FLEXIBLE CHAIN ANALOG V. J. SULLIVAN AND G. C. BERRY Carnegie Mellon University, Dept. of Chemistry, 4400 Fifth Avenue, Pittsburgh, PA 15213
ABSTRACT
Rheological properties of isotropic solutions of rodlike poly(p-phenylene benzbisoxazole), PBO, flexible chain poly(2,5-benzoxazole), ABPBO, and their Measurements include steady state miscible blends in solution are described. properties (the viscosity and recoverable compliance as functions of shear dynamic compliance), and (the recoverable transient properties rate), mechanical properties (the loss and storage compliances as functions of frequency). The relaxation spectrum of the blends is broader than that for the rodlike chain, and tends to occur at longer times, reflecting a viscosity enhancement that occurs with the blends. The measured zero shear viscosities for rod and blend solutions are compared with predictions based on the model of Doi and Edwards.
INTRODUCTION
Rodlike polymers containing heterocyclic aromatic groups in the main agents in rod-flexible chain chain have shown promise as reinforcing Above a critical composition for formation of a nematic phase, composites [1]. rod-coil-solvent mixtures typically exhibit a broad biphasic regime, in which the flexible chain is predicted to be essentially excluded from the nematic phase (2]. In order to maintain an intimate dispersion of the rodlike chain in the flexible chain matrix, fibers and films are processed from the isotropic phase. It is therefore important to understand the dynamics of isotropic mixtures of rods and flexible chains. The model of Doi and Edwards for isotropic solutions of caged rods treats the rod constraint release mechanism, for movement of a rod into a new cage, as the translation of a rod along its length by a distance proportional to its contour length [3]. This treatment gives a result for the rotational diffusion of a caged rod which leads to an expression for the zero shear viscosity 71o given by
no/,s = KM[7] (cL/ML)
3
f
(1)
f = (1- B(c/c*)]-2 where 71 is the solvent viscosity, c, M, L and (71] are the rod concentration, molecular weight, length and intrinsic viscosity, respectively, ML = M/L and K and B are parameters determined by fitting the equation to experimental data. Here, f is a crowding factor accounting for the severe restriction of the rod rotation which occurs at concentrations approaching (but less than) the In critical rod concentration cR* for the formation of an ordered phase [4]. In the following, eq. 1 will be practice, B is found to be close to unity (4]. used to predict the viscosities of both rod and blend solutions.
Mat. Res. Soc. Symp. Proc. Vol. 171. '1990 Materials Research Society
142
EXPERIMENTAL
Static and Dry polymers were received from the Dow Chemical company. dynamic light scattering and intrinsic viscosity measurements give a weight average chain length Lw = 170 nm for PBO, and Lw = 320 nm for ABPBO. Rheological experiments included steady state viscosity and recovery, transie
Data Loading...
