enee of olutions W. P. CONKER
AND
P. I. DONNELLY
Hercules Experiment S t a t i o n , W i l m i n g t o n , Del.
T h i s work was begun with the object of finding what information regarding the structures within viscose solutions can be uncovered from a study of the flow birefringence of these concentrated solutions. The high flow birefringence of viscose is unusual for homogeneous high polymer solutions of comparable molecular w-eight. A simple apparatus is described with which the magnitude and the relaxation time of flow birefringence can be measured. These two variables are interpreted in terms of
the size and the number of molecular aggregates in viscose solutions. Specifically i t has been found that the number, but not the size, of these aggregates is dependent upon viscose process variables, such as carbon disulfide content, dilution, aging, ripening, and source of cellulose. Thus flow birefringence is shown to be a fruitful tool to be added to the all-too-short list of methods for studying the complex character of viscose solutions at concentrations actually used for spinning.
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Low velocity gradients were obtained by allowing the viscose to drop under gravity through a cylindrical glass tube which vias held fixed between crossed Polaroids. The transmission of the system during flor was taken as a measure of the magnitude of the flow birefringence. All such measurements were referred t o a standard birefringent mica plate which could be inserted between the crossed ~Polaroidsso that the birefringence readings obtained over a long period of time were accurately comparable. -411 birefringence data were expressed in terms of the ratio of the increaFe in transmission during flow to the increase in transmission when the standard plate vias inserted between the crossed Polaroids. Readings were taken in such a manner as to eliminate the effect of differences in the color of solutions on the measurement of the birefringence. For the relaxation time determinations, the rate of disappearance of the birefringence was followed photoelectrically after the flow was suddenly stopped by a rapid-acting knife valve. It n-as found that the decay with time \vas largely exponential, as would be expected for a single relaxation process. For precise nieasurement the rate of decay of birefringence was compared with the rate of charging of a condenser of known capacity through a resistor of known resistance. Such charging is known to be exponential, having a time constant equal t o the product of the capacity times the resistance. The comparison was made on an oscilloscope screen.
ISCOSE solutions at a concentration level suitable for spinning show an easily discernible flow birefringence. For example, pint bottles of viscose inverted between L'crossed'' sheets of Polaroid become luminous as the liquid readjusts its position under the force of gravity. Quantitative measurements of the flow birefringence of solutions containing high polymer molecules have proved to be a powerful tool for determining the average molecular size and shape of many polymeric materials. However, only limited study of viscose solutions has yet been reported. Signer and Meyer (IO)published an admirable investigation of the molecular changes occurring during the ripening of viscose as revealed by the flow birefringence technique. Their work was concerned solely with the behavior of very dilute cellulose xanthate solutions which are not commercially useful and for which a relatively high shearing stress is necessary to bring about a detectable birefringence. When the present work \vas undertaken, it was suspected that additional colloidal structure may be present in commercial viscose which does not occur in dilute solutions, since the appearance of such a marked flow birefringence a t low shearing stresses is not normally found in solutions of other cellulose derivatives at comparable concentrations. The evidence confirming such a belief is reported here. Since this work was completed, Gonsalves ( 4 ) has published an investigation of the relaxation times in cellulose xanthate solutions with high cellulose content. His study also stresses the value of measurements obtained with the more concentrated solutions, but his data and interpretation differ somewhat from that reported hcie 8 s will be pointed out. The present study is concerned with both physical and chemical approaches toward the elucidation of the character and the cause of the flow birefringence in commercial viscose, especially at low velocity gradients. -
OPTICAL SYSTEV
The optical system, diagrammatically illustrated in Figure 1, was set up on a Cenco optical bench.
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APPARATUS
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During the course of the investigation three types of flow birefringence measurements were carried out: relaxation time, relative magnitude of birefringence at low velocity gradients, and dependence of birefringence upon velocity gradient.
The light source, 1, \vas a 50-candlepower Maxda bulb
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Figure 1. Optical System for Measuring Flow Birefringence and Relaxation Time 1136
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powered (automobile by a 6-volt type storage 1183) batt,ery through a rheostat for controlling the intensity. -4 cylindrical metal shield was placed around the bulb to minimize the stray light. The lens system produced an approximately parallel beam of light which could be almost completely polarized. The lens combination consisted of two condensing lenses, 2. (focal lengt,h 5 em., diameter
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
h .
40 mm.), and an f-3 photographic lens, 3, of 10.5 cm. focal length. Polaroids, 4, 10, were placed on opposite sides of the sample. The Polaroids were crossed with their axes 45' to the axis of the flow tube. A green filter combination, 5, composed of Corning No. 348 and No. 430, was used to obtain roughly monochromatic light. The rectangular aperture, 7 , restricted the light to the central part of the sample cell, 9. After passing through the cell, the light progessed through the second Polaroid, 10, onto the photomultiplier tube, 12 (RCA lP21). A 2 X 8 mm. slit, 11, restricted the view of the phototube to the illuminated center of the cell. The long axis of the slit was perpendicular to the cell tube axis. Sample cell 9 was a 150-cm. length of round glass tubing, I-cm. inside diameter. At the point of passage of the light, the tube was surrounded by a 1.5-inch (3.75-cm.) cubical glass cell, 8, having two flat glass windows. This cube was filled with water to eliminate the lens effect caused by the cylindrical sample cell. The birefringence standard, 6, was made b y splitting a sheet of clear Muscovite mica to obtain a strip having a uniform thickness of about 60 microns. The uniformity was checked by inserting it between two crossed Polaroids. The mica strip was cemented (Canada balsam) between a clear glass microscope slide and a dark green filter, Willson weld shade 6, having a transmitr tance of 0.25%. The transmission of the standard under crossed Polaroids was the same order of magnitude as that of the average viscose solutions. ELECTRICAL CIRCUIT
CHARQINI) TIME RELAXATION TIME
