Chapter 6
Flow-Induced Structuring and Conformational Rearrangements in Flexible and Semiflexible Polymer Solutions Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 14, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0597.ch006
A. J. McHugh, A. Immaneni, and B. J. Edwards Department of Chemical Engineering, University of Illinois, Urbana,IL61801
This article presents a discussion of two examples of the rheo-optics of flow-induced structure formation in solutions of flexible and semi-rigid chain systems. Polystyrene solutions show shear-thickening, turbidity, and associated dichroism maximization during flow which can be related to the formation of micron-sized aggregates. Hydroxypropylcellulose in acetic acid represents a semi-rigid system which can undergo a conformational change under shear flow which is distinct from the overall molecular ordering. Such changes can be conveniently monitored in terms of the circular birefringence. The molecular ordering and supermolecular structuring that can be induced by flow in otherwise isotropic, polymer solutions and blends have been extensively studied using optical and rheo-optical techniques. Supermolecular structure formation is generally accompanied by significant increases in the solution turbidity and small angle light scattering (1,2,3,4,5,6), as well as measurable increases in the viscosity with shear rate (i.e., shear-thickening) (3,7,8). Our recent studies with solutions of an optically active, semi-rigid system (hydroxypropylcellulose (HPC) in acetic acid), demonstrate that flow can also induce intramolecular, conformational changes, which appear as measurable changes in the optical rotatory power (i.e., circular birefringence) (9,10). Since most of the above-noted studies have been extensively reviewed elsewhere (11,12) (or will be the subjects of further discussion in this symposium text), this chapter will simply focus on a discussion of two examples from our studies of flexible and semi-rigid systems which illustrate the above phenomena. The principal technique we use is modulated polarimetry in combination with in-situ measurement of the shear stress/viscosity behavior.
0097-6156/95/0597-0075$12.00/0 © 1995 American Chemical Society
Nakatani and Dadmun; Flow-Induced Structure in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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FLOW-INDUCED STRUCTURE IN POLYMERS
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 14, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0597.ch006
Rheo-optical Preliminaries The flow cell used in our studies is a small gap (0.5 mm) Couette device in which the outer cylinder is rotated while torque is monitored at the stationary inner cylinder. The optical properties of interest are simultaneously measured in theflow/shearplane by passing a laser beam (λ = 632.8 nm) down the fluid gap (d = 7.745 cm). The optical train consists of a laser beam which passes through a polarizer aligned with its transmission axis at 90° relative to the flow direction, followed by a photoelastic modulator (PEM) oriented at 45°, before entering the flow cell. Depending on the properties to be measured, additional optical elements are placed on either side of the flow cell. The PEM produces a sinusoidal dependence in the polarization of the light, 5pem= sin(cot). The configurations used in our studies are listed in Table 1. The leading letter indicates the property whose signal would normally be dominant in the particular configuration. The first two harmonics of the output intensity (7 ,/ ω) are registered by lock-in amplifiers and the dc component of the signal, 1^, is isolated using a low-pass filter. The digitized signals are transferred to a computer by the lock-ins, where ratios of the two harmonic intensities with respect to the dc intensity are formed as follows: A
ω
bu
,
1ω
Rm 2 ω
^ 2/ / (Α) Λ
.
2
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2
The Jj (i = 1,2) are Bessel functions of the first kind of order i and A is the adjustable amplitude of the photoelastic modulator which is set such that J (A) = 0. In the most general case (i.e. one in which the fluid exhibits circular as well as linear optical properties), these ratios will be functions of all the optical properties, i.e., linear dichroism (Δη"), linear birefringence (Δη'), their respective orientation angles (fa, fa), circular birefringence (An' ) and circular dichroism (An" ). For the wavelength used in our studies, the dichroism is due to scattering as opposed to absorption. Output ratios for each configuration listed in Table 1 can be determined using the Jones calculus. This results in a set of complex, non-linear, coupled equations which are functions of the optical properties (9,10,12). These can be simplified depending on the relative magnitudes of the optical properties one is measuring. However, care needs to be taken, particularly in the analysis of circular properties, since terms which might otherwise be thought negligible, can have important consequences for the interpretation of the dynamical behavior (9,10). 0
c
c
Flexible Chain Behavior Our rheo-optical studies offlexible-chainsystems have focused on dilute polystyrene solutions (c/c* ~ 0.1-1.1) in decalin, with fractionated polymer molecular weights ranging from 4.3 χ 10 to 17.9 χ 10 . The viscosity and linear optical properties have been monitored at 25 °C over the range of shear rates (500 s" to 8000 s") where both shear-thinning and shear-thickening occur. Figure 1 shows an example which illustrates the steady-state viscosity and dichroism behavior of the lowest molecular weight solutions that exhibit the phenomenon of shear-thickening (MW = 1.54 χ 10 , denoted PS2). At the lowest concentration, the scattering dichroism rises with shear 5
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Nakatani and Dadmun; Flow-Induced Structure in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 14, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0597.ch006
6.
Flexible and Semiflexible Polymer Solutions
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