Initial investigations have been made into the use of high resolution nuclear magnetic resonance (NMR) for the characterization of radiation effects in graphite and Kevlar fibers, polymers, and the fiber/matrix interface in graphite/polyetherimide composites. Sample preparation techniques were refined. Essential equipment has been procured. A new NMR probe was constructed to increase the proton signal-to-noise ratio. Problem areas have been identified and plans developed to resolve them

Ferguson, Milton W.

English

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13C NMR Analysis of the Effects of Electron Radiation on 
Graphite/Polyetherimide Composites 
FINAL REPORT 
NASA Grant NCC1-115 
ABSTRACT 
Initial investigations have been made into the use of high 
resolution nuclear magnetic resonance (NMR) for the 
characterization of radiation effects in graphite and Kevlar 
fibers, polymers, and the fiber/matrix interface in 
graphite/polyetherimide composites. Sample preparation 
techniques were refined. Essential equipment has been procured. 
A new NMR probe was constructed to increase the proton signal-to- 
noise ratio. Problem areas have been identified and plans 
d e v e l o p e d  to r e s o l v e  t h e m .  
https://ntrs.nasa.gov/search.jsp?R=19890010834 2020-03-20T03:57:15+00:00Z
TABLE OF CONTENTS 
I . INTRODUCTION ................................ 1 
I 1  . SAMPLE PREPARATION .......................... 3 
I 1 1  . EQUIPMENT ................................... 4 
IV . CONCLUSION .................................. 6 
I. INTRODUCTION 
During recent years, there has been considerable interest in 
fiber-reinforced polymeric matrix composites. The use of 
composites for aircraft structural components has increased due 
to their outstanding combination of high strength, high modulus, 
and low density. In addition, fabrication techniques permit the 
tailoring of these materials to meet specific load and stiffness 
requirements. However, if these composites are to be utilized as 
efficient, high performance materials in aerospace applications, 
their long-range radiation durability must be established. 
Initial performance studies focused on the properties of the 
fibers and the polymeric matrix, but it was soon realized that 
the interactions a t  the fiber/matrix interface were highly 
significant. Since interfacial bonds transmit stress between the 
fiber and the matrix, the mechanical properties of the material 
may ultimately depend on the integrity of the interface. The 
interface also occupies a prominent role in the determination of 
the modes of failure and the behavior at failure of the material. 
(1,2) 
In order to understand the modifications to the composite 
caused by radiation, characterization of the interface in terms 
of its chemical behavior must be accomplished. High energy 
electrons interact with the composite by exciting the bonding 
electrons of the material to higher energy states or by ionizing 
the atoms. This produces radicals or ionic systems. The 
subsequent molecular changes caused by the radiation-induced 
1 
radicals may alter the mechanical properties of the material. 
The focus of this project was to investigate the use of high 
resolution nuclear magnetic resonance ( N M R )  measuring techniques 
to characterize and evaluate the fiber/matrix interface before 
and after irradiation with 1-MeV electrons. NMR affords the 
opportunity to identify and relate the electronic and geometric 
structure of the effected molecules to radiation-generated 
changes in the properties of the material. 
In order to accomplish these long-range goals, the first 
year research effort described in this report involved 
optimizing NMR techniques for the study of pulverized graphite 
and Kevlar 4 9  fibers, graphite/Ultem composites, and polymers 
(Ultem, Mylar, Kapton, and Lexan). Baseline characterization 
studies of these materials were,initiated. This work provided 
the necessary groundwork for further studies. 
2 
11. SAMPLE PREPARATION 
In determining the sample preparation techniques to be used, 
the requirements of both the NMR spectrometer and the electron 
accelerator must be considered. The target area of the 
accelerator is approximately 15 cm X 20 cm which restricts the 
amount of sample to be irradiated, yet sufficient quantities of 
sample must be generated in order to detect an N M R  signal. 
It has been established that pulverization of the fibers, 
polymers, and composites is necessary in order t o  maximize the 
volume of material packed into the N M R  tube and therefore to 
maximize the N M R  signal. Due to the requirements of the 
accelerator facility, the pulverization must be done after 
irradiation. The pulverization,must also be performed as quickly 
as possible in order to minimize the time the samples are at room 
temperature between cessation of irradiation and the N M R  
measurements. It was found that standard grinding techniques 
could be used provided that the samples were cut into small 
pieces and ground with frequent additions of dry ice. 
3 
111. EQUIPMENT 
Part of this project included the procurement of a new pulse 
programmer which was essential to the planned crosspolarization- 
decoupling experiment to observe the 13C spectra. 
was delivered seven months late. The company, NOVEX Inc., went 
out of business soon after delivery. To this date, the programmer 
fails to operate properly. (NOVEX Inc. had been selected as the 
supplier because they had manufactured most of the other 
components of the spectrometer and we were concerned about 
compatibility.) 
The programmer 
The magic angle spinning apparatus which is necessary to 
accomplish high resolution NMR is operational. However, it is 
also necessary to attain adequa,te field stabilization. At this 
time, the field stabilization equipment still allows a field 
drift of approximately 3 gauss/l0 kilogauss per hour. This 
degree of drift does not permit the magic angle spinning 
technique to achieve the needed line narrowing. 
Baseline measurements of the proton (hydrogen) spectrum in 
various polymers (Ultem, Mylar, Kapton, and Lexan) and in Kevlar 
4 9  fibers were attempted using the previous pulse programmer 
while efforts were being made to resolve difficulties with the 
new pulse programmer. During these investigations, the existence 
of a small but discernible proton signal in the probe itself, due 
to the presence of plastics and epoxy, was discovered. The broad 
proton lines and relatively low signal-to-noise in the materials 
t o  be studied required the use of high powers and long-term 
averaging. While greatly improving the signal-to-noise level, 
4 
this also magnified the normally insignificant background signal 
from the probe materials. In many cases, the background signal 
was the same order of magnitude as the desired signal. This 
problem was solved by the construction of an all-new probe. 
Each step in the probe construction was carefully monitored 
to eliminate all materials containing the hydrogen atom (protons). 
An etching technique (hydrofluoric acid and coil-shaped wax resist 
patterns) was used to make glass coil forms. 
also designed to maximize signal-to-noise by improving impedance 
matching, with an emphasis on reducing the coil's impedance and 
any stray capacitance which could reduce efficiency. 
A new circuit was 
Thenew probe was tested using Ultem and found to have an 
acceptable signal-to-noise ratio. The new probe design also 
yielded a nearly ideal resonance frequency response curve and 
good tuning character is t ics . 
5 
IV. CONCLUSION 
Despite the development of the new probe, the field drift 
problem mentioned previously ( 3  gauss/l0 kilogauss per hour) 
smeared out any detail in the proton spectrum. (The entire proton 
spectrum is only 10 ppm wide o r  about 0.1 gauss in a 10 kilogauss 
field.) We were unable to achieve the desired high resolution 
conditions. With such featureless proton spectra it was not 
possible to discern any differences between the various samples 
of the study. 
This project was plagued with a series of problems including 
the late delivery of the pulse programmer which does not yet 
function, field stabilization difficulties, and the necessity of 
developing a new probe, which made it impossible to obtain any 
usable results within the time constraints allowed. With more 
time and resources the field stabilization problem and pulse 
programmer problem can be solved and high resolution conditions 
can be reached. 
Our plan is to continue this project without N A S A  funding 
and to ultimately achieve the goals outlined in the proposal. 
6 
REFERENCES:  
1. L.J. Broutman, Interfaces in C o m p o s i t e s ,  ASTM STP 4 5 2 ,  
American Society for Testing and Materials, 27 (1969) 
452, American Society for Testing and Materials, 90 (1969) 
- 
2. G.A. Cooper and A. Kelly, Interfaces in Composites, ASTM STP 
.. 
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A (13)C NMR analysis of the effects of electron radiation on graphite/polyetherimide composites

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