10-5 which is an order of magnitude improvement over most designs. This will allow determination of the dielectric properties of low loss polymers with much greater speed than previously possible.

Work completed in FY 1979 includes a step function generator that can put out simultaneous positive and negative steps of 100 V matched to one part in 105 with a 10 μs rise time. This matching is necessary to accurately subtract out the constant capacitance term. A detector has been completed and is being trimmed. This detector will be able to resolve charge changes to 0.1 pc with a 10 μs response time. These and previously completed or obtained components complete the instrumentation.

One other needed piece of support work that has been completed is the development of an efficient numerical Laplace transform technique. The transform gives accurate results over six decades in log time and uses 36 preselected data samples. This performance is quite superior to methods in the literature.

The recording of data for the thermal pulse experiment and the transfer of these data to the Interdata Laboratory computer has been completed and work has started on writing the needed programs for data analysis. When completed this automation will provide a convenient method of determining polarization distributions in polymer films as an aid to studying their piezoelectric and charge transfer properties.

Ultrasonic Hydrophone

A. S. DeReggi, S. C. Roth, J. M. Kenney, and S. Edelman

This project is sponsored by the Bureau of Radiological Health which is also contributing the service of a guest worker. The early work was described in the annual report for last year.

The objective of the project is to develop a piezoelectric polymer hydrophone for characterizing, point-by-point, the acoustic field beamed by ultrasonic transducers such as those used in biomedical equipment. Accurate knowledge of the details of the acoustic field is not otherwise obtainable. The knowledge can be used to ensure both the safety and adequacy of ultrasonic dosage and for research in ultrasonic treatment.

During this year, a number of prototype hydrophones were built and tested at NBS and at the Bureau of Radiological Health. The hydrophone is a taut membrane of polyvinylidene fluoride (PVDF), 50 μm thick, with a piezoelectrically active central spot of about one millimeter diameter. The close impedance match of PVDF to water makes the hydrophone acoustically transparent in use. Thus, whether the hydrophone is held stationary or is scanned across the field, the acoustic impedance presented to the source is essentially the same as it would be if the probe were not present. The response of the probe is flat between 0.5 and 10 MHz. The thickness resonance is about 20 MHz.

The hydrophone was described in a paper presented by the authors listed above and G. Harris of the Bureau of Radiological Health at the joint meeting of the Acoustical Society of America and the Acoustical Society of Japan, November 27-December 1, 1978. J. Acoust. Soc. Am., 64, S. 55 (1978).

These ar

Plans are currently under way to extend the work to linear arrays, planar arrays to two-dimensional arrays conformed to special shapes such as parabolic dishes. rays are of interest in medical imaging systems.

Fabrication of Piezoelectric Polymer Film

A. J. Bur, J. M. Kenney, and M. G. Broadhurst

The Department of the Navy is supporting a project to fabricate piezoelectrically active PVF2 specimens with thicknesses of 0.81 mm (32 mil). A thick transducer is

desired because, relative to the thin film PVF2 transducers which we have routinely prepared, a thick transducer will have a lower capacitance and a correspondingly higher voltage response for a given strain.

The problem in preparing thick transducer specimens is that the dielectric strength of the specimen is usually lower than that of a thin film of the same material. A commercially available thick film was found to have such a low dielectric strength that the minimum poling field of 800 kV/cm could not be reached. Consequently, we began to prepare thick specimens from PVF2 powder starting material. The planned fabrication procedure will be: (1) vacuum molding of bubble-free disked shaped specimens; (2) converting the PVF2 crystal structure from a to ẞ phase by tensile extension or by rolling; (3) evaporating electrodes; (4) poling at 800 kV/cm or higher at 80 °C. At this time, bubble-free disk shaped specimens have been prepared but poling has not been carried out.

POLYMER STANDARDS FOR CONTROL AND EQUITY

L. E. Smith

Task Leader

Molecular weight and molecular weight distributions are the most important determinants of the useful properties of high polymers. They are of primary importance in determining both the processability of the raw materials and the characteristics of the fabricated products. Directly or indirectly, molecular weight is an important part of the specifications used in the commerce involving the tens of millions of tons of plastics produced every year. The ability to determine molecular weight reliably and reproducibly in different laboratories is thus important to both plastics producers and fabricators.

This task has four main technical activities, all aimed at this overall objective of providing a reliable base for the measurement system of molecular properties for use in commerce. First, the current needs of the measurement system used in industry are addressed by projects on the production of standards for molecular weight calibration and research on methods of improving the utilization of gel permeation or size exclusion chromatography which has become a widely used industrial measurement technique. Classical absolute molecular weight determinations are highly labor intensive, which limits the speed of standard reference material production and consumes large amounts of capital per standard. Our highest priority is therefore placed on the development of an absolute molecular weight detector for a gel permeation chromatograph which will be able to produce standards at a much faster rate without a sacrifice in quality. Elastomer SRM's are produced for quality control and specification by the rubber industry. The second technical activity provides the scientific basis for the molecular characterization methods of the future by exploring techniques and theories which meet needs we see growing in the plastics industry. Examples of this are our projects concerned with block copolymers and blends which are rapidly growing segments of total plastics use. The third and fourth activities involve the development and application of molecular characterization methods to particularly critical plastics applications where market forces are not sufficient to attract sufficient industrial effort. Our present work in these activities relates to the use of ultra high molecular weight polyethylene for orthopedic implant use and the development and application of gas permeation measurement methods and standards.

Self-Calibrating Gel Permeation Chromatography

P. H. Verdier

Gel permeation chromatography (GPC) is a widely accepted technique for estimating the molecular weight distribution (MWD) of high polymers. However, the usefulness of the conventional GPC apparatus is limited by the need to provide calibrants for each polymer measured of known molecular weight over the entire molecular-weight range in which the MWD is significantly different from zero. The calibration depends, among other things, upon the chemical nature, degree of branching, etc., of the polymeric material, so that each new material requires a fresh calibration. The so-called "universal calibration" hypothesis, while useful, is limited to comparisons of polymers of similar shape, and in any event is inadequate for quantitative determinations. Some instruments, an example of which is commercially available, attempt to circumvent the need for calibrants by adding a single-angle light scattering detector to the usual concentration-sensitive detector. However, this does not allow the extrapolation to zero scattering angle which is required in principle to relate scattering intensity to molecular weight. In addition, qualitative information on the variation of scattering with angle, normally required to give assurance that meaningful results are being obtained, is not available.

We are designing and constructing a light-scattering detector for the GPC which measures, in real time, the scattering intensity as a function of scattering angle, and which is controlled by a dedicated minicomputer in a way that allows scattering to be measured as a function of scattering angle and concentration and the results extrapolated to zero scattering angle and concentration. The instrument will allow continuous monitoring of the variation of scattering with angle. This will allow immediate identification of difficulties such

as association, microgel formation, etc., which could affect the validity of the molecular weight obtained, an important consideration for work on new and unstudied materials. In addition to molecular weight, the mean-square radius (radius of gyration) will be obtained as a function of molecular weight, at least in the higher ranges of molecular weight, providing useful information for the characterization of branched polymers.

During the current fiscal year, we have completed construction of a prototype of the scattering cell and its mounting, the two most critical components of the instrument. In the coming fiscal year we expect to complete design and construction of the remainder of the instrument and to begin testing it.

Recycle Gel Permeation Chromatography

F. L. McCrackin and H. L. Wagner

Recycle gel permeation chromatography (GPC, also called size exclusion chromatography) was studied theoretically and experimentally as a method to measure the column spreading parameter and the the polydispersity of nearly monodisperse polymer standards. The width of the recycled chromatograms could be accounted for by four causes: (1) the volume of solution injected into the column, (2) the column spreading, (3) the molecular weight distribution of the polymer, and (4) spreading due to the pump. A method was developed to separate the spreading into contributions due to each of the individual causes. Previous analysis of recycled GPC considered the width of the chromatograms as due to only the column spreading and molecular weight distribution of the polymer. By taking the other contributions of the spreading of the chromatograms into account, much more accurate values of the column spreading parameter and polydispersity of the polymer are obtained.

The recycle experiments were carried out with a set of crosslinked polystyrene columns of nominal pore size of 103, 104, 105 and 106 A in a GPC system with a differential refractometer as a detector. The columns were calibrated in tetrahydrofuran and toluene with 5 narrow distribution anionic polystyrenes ranging in molecular weight from 9,000 to 498,000, including SRM 1478 at 36,000 and SRM 705 at 175,000.

To reduce the extent of interference by spurious peaks, which normally occur at high retention volumes after the polymer peak has eluted with distortion of the subsequent recycle peaks, most recent runs were made in toluene. The use of this solvent, together with additional changes in the injection procedure, reduced this interference but did not eliminate it entirely.

Previously, all the various components of spreading were obtained by a least squares fitting of a function of the total spreading to a polynomial in the number of cycles. To improve the precision of the results, we made use of the observation that the spreading due to the injection and the pump was the same for all the samples. These two compo

nents of the spreading were then determined separately by measuring the spreading with the columns removed from the chromatograph. Knowing these, it was possible to determine the remaining components, the column spreading and polydispersity, from a least squares fitting of a function of the total spreading to a simple linear relationship in the number of cycles.

W n

Results from this analysis are shown in Table 1. The polydispersities, M/M of the anionic polystyrenes are quite low, ranging from 1.0027 to 1.09. The spreading due to the columns increases with molecular weight from 0.2 ml at a molecular weight of 104 to 0.6 ml at a molecular weight of 2 x 105.

Such measurements now allow the spreading to be measured for any set of columns without using recycle by measuring SRM 750 or 1478.

A chromatogram is obtained for one of these polystyrenes run through the columns, and the spreading due to the columns is then found by subtracting the spreading due to the known polydispersity from the total spreading.

Many of the procedures used by industry for quality control and commercial specifications of rubber depend on standard reference materials (SRM) which are maintained by the Polymer Science and Standards Division. This project renews SRM's as stocks are depleted and produces new SRM's needed by industry for control of processes and commercial specifications which are outside the range of properties available with current SRM's.

The limited resources of the SRM program have prompted us to try to elicit support for an industrial research associate program that would assist in the maintenance of the current program and in the development of new SRM's. Although ASTM Committee D-11 on Rubber and Rubberlike Materials has set up a task group to try to get support for the rubber SRM program from the rubber industry, nothing has been accomplished in this regard since December 1977 and the industry appears content to let the rubber SRM program remain static. In response to this, the Polymer Science and Standards Division has been making efforts to maintain the important SRM's and to eliminate those which are selling at very low levels. The net result will be a streamlined rubber SRM program which serves industry in an efficient manner with materials which are certified by NBS to have certain physical and/or chemical properties rather than having NBS serve as a warehousing distributor for industrial grade materials.

As part of the program to maintain the current important SRM's, we have contracted for the production of a new Butyl Rubber SRM, which will be certified for Mooney Viscosity. In addition, one material, N-t-butyl-2-benzothiozole sulfenamide, a rubber accelerator, will be renewed as an SRM. This material is certified only for homogeneity and fits into the category of warehousing an industrial grade material, but will be renewed to avoid a major disruption in the ASTM standards which rely on this material.