Anomalous Aging Response in
Two Phase Systems:
Creep and Stress Relaxation Differences in
Rubber-Toughened Epoxies

Dr. Andre Lee, Dept. of Materials Science and Mechanics Sponsored by: National Institute for Science and Technology / Research Excellence Funds

After quenching a glass-forming material from above the glass transition to below it, the system is out of equilibrium and the volume and enthalpy evolve spontaneously towards equilibrium in a process known as structural recovery. Associated with the structural recovery are changes in mechanical properties that have come to be known as physical aging. Physical aging is of considerable practical importance. In the case of engineering parts made of amorphous polymeric glasses (thermosetting epoxies, polycarbonate, etc.) for example, the initial properties of the material just after the thermal or thermo-mechanical forming process are not stable and will evolve towards a chronologically distant equilibrium during the service life of the part. More fundamentally, it is essential to characterize quantitatively the physical aging phenomena using well characterized materials in order to model the specific microscopic mechanisms which control the observed property evolution.

In our laboratory and others, the physical aging response of amorphous polymers has been extensively studied in both creep and stress relaxation experiments performed after a temperature jump from above the glass transition to below it. It is widely accepted that a time-aging time superposition principle adequately describes the changes in viscoelastic response that accompany the evolution of the non equilibrium thermodynamic properties of the polymers. However, most of these studies are restricted to homopolymer systems and relatively little experimation to study the physical aging response of multi-phase amorphous polymers and polymer blends.

Recently, we have completed a study of the physical aging response in stress relaxation and creep experiments for a series of rubber-toughened epoxies at temperatures between the glass transition temperature, Tg, of the rubber phase and that of the epoxy resin with rubber contents up to 10 wt%. At these aging temperatures the rubbery phase, which is in equilibrium, is mixed with the glassy phase, non equilibrium epoxy matrix, and one would not anticipate that the rubbery phase contributes to the structural recovery of the system. Hence, the aging should be determined by the glassy phase only. Surprisingly, while time-aging time superposition principle well describes the response of the neat resin and the rubber-toughened systems in creep experiments, it is found that in stress relaxation conditions, the time-aging time superposition principle cannot be applied for the rubber-toughened material. This observation is difficult to explain. It was expected that, because the rubber itself should not age, the response would be that of the neat resin. Yet, we observe that the influence of the structural recovery that occurs in the epoxy phase seems to be different depending upon whether it is probed by stressing the sample or straining it. Hence, one is lead to suggest that this apparently anomalous viscoelastic response is due to the two phase nature of the material. Unfortunately, there are few analyses of the expected viscoelastic response of two phase systems, which discuss the effects of aging on them.

A possible partial explanation of these results is found in the analysis of Brinson and Knauss. They performed a two dimensional finite element analysis, FEA, of the dynamic response of a system of an array of viscoelastic cylinders embedded in a viscoelastic matrix and compared the results of FEA to the bounds that would be expected from composite mixing rules that assumed either uniform stress or uniform strain in the system. Interestingly, depending on the relative magnitudes and relaxation times of the cylinder and matrix moduli, it was found that the response with frequency could change from one bound to another. Obviously such complicated behavior may explain why we see an apparent anomaly in the rubber-toughened epoxy. Does one have constant stress or strain in the system? Does the distribution change when one goes from creep to stress relaxation? In addition, it is possible that there are bulk modulus effects that might play a role in the two phase system.

The rubber-toughened epoxies probably need to be treated in a fully three dimensional analysis because they are systems of rubber spheres which are surrounded by the epoxy matrix. In beginning, we anticipated that the rubbery phase would have no impact on the aging and that the epoxy matrix phase would age. This, perhaps naively, assumes that working above the Tg of the rubber results in the rubber relaxation time being extremely short with a vanishingly small load supported by the rubber phase. This is true for the shear response of the rubber. However, in the three dimensional system, it is likely that bulk relaxation (the finite compressibility of the rubber) will be important. In this case, the bulk modulus of the rubber is only about 1/3 to 1/2 that of the glassy epoxy matrix and the response becomes a mixture of that of the rubbery and the glassy phases. Further, the densification of the glassy matrix during the aging experiment changes the system internal stress distribution. This would further complicate the behavior calculated by Brinson and Knauss and might provide an explanation as to why the creep and stress relaxation responses are so different in the aging experiment.

In the future, aging response of other multi-phase polymer systems and polymer blends will be examined, and finite element analysis will be performed to establish how the different aspects of the viscoelastic response of such two phase materials can affect their creep, relaxation and aging behavior.

Figure 1. (ABOVE) Small-strain stress relaxation modulus curves for DGEBA/D400 quenched from 62oC to 33oC and kept at 33oC for 3 days. The different curves were measured at various values of aging time. (0.5 hours, 1 hours, 2 hours, and 4 hours)

Figure 2. (ABOVE) Creep compliance curves for DGEBA/D4 with applied stress of 1 MPa quenched for 62oC to 33oC and kept at 33oC for 3 days. The different curves were measured at various values of aging time. (0.5 hours, 2 hours, 8 hours, 32 hours, and 64 hours)

Figure 3. (ABOVE) Creep compliance curves for 10 wt% CTBN rubber toughened DGEBA/D400 with applied stress of 2 MPa was quenched for 62oC to 25oC and kept at 25oC for 1 day. The different curves were measured at various values of aging time. (0.5 hours, 1 hour, 2 hours, 4 hours, 8 hours, 16 hours)

Figure 4. (ABOVE) Small-strain stress relaxation modulus curves for 10 wt% CTBN rubber toughened DGEBA/D400 with applied strain of 0.2% was quenched for 62oC to 25oC and kept at 25oC for 1 day. The different curves were measured at various values of aging time. (0.5 hours, 1 hour, 2 hours, 4 hours, 8 hours, 16 hours)

THE COMPOSITE CENTER JOINS
THE INFORMATION
SUPERHIGHWAY

The NSF Center’s Technology Transfer Office announces the development of a World Wide Web Server for both NSF Center and CMSC related activities. The World Wide Web, or WWW to the internet community, is rapidly becoming the information superhighway’s means of providing knowledge to network users. The WWW interface allows users to receive formatted text, graphics, pictures, animation, movie clips and sound over the electronic superhighway. The information can be read on a variety of platforms with WWW programs, including DOS, Windows, Macintosh, and Unix. The Web server at the NSF Center is loaded with information concerning research projects, principal investigators, and other composites-related activities at Michigan State University.

Interested persons can reach our server by typing the following Uniform Resource Locator (URL) into their WWW interface:
http://cmscsun.egr.msu.edu/
The World Wide Web server is an ongoing project here at the NSF Center and will always be expanding its knowledge database. If you are interested in including information concerning your CMSC project, please contact Michael Bogdan at (517) 353-9854.

FOURTH ANNUAL
TECHNICAL SYMPOSIUM

The NSF Center on Low-Cost High-Speed Polymer Composites Processing at Michigan State University and the Michigan Materials and Processing Institute (MMPI) are pleased to announce the Fourth Annual Technical Symposium on Polymer Composites Processing. The symposium will be held on Thursday, June 8 at the Marriot in East Lansing, Michigan.

The symposium is a one day event which will cover the composites materials and processing research accomplishments of the NSF Center and MMPI over the past year. Four Technical Sessions will review the research accomplishments of the NSF Center and MMPI. The morning’s two concurrent sessions will focus on the NSF Center’s Polymer Processing Projects and MMPI Technical Committee 1, Structural Applications. The afternoon’s two concurrent sessions will continue focusing on the NSF Center and MMPI Technical Committee 3, Recycling and Waste Management. An elaborate poster session (approximately 50 posters) will follow both the morning session and afternoon session. Presentations will be given by researchers from Michigan State University, the University of Michigan, Wayne State University, Michigan Technological University, the Advanced Materials Engineering Experiment Station in Midland, Michigan, and the University of Detroit Mercy. This is an excellent opportunity for individuals to view the composites research programs of these leading edge organizations and interact with faculty and student researchers.

There is a $25.00 symposium fee which includes registration, a continental breakfast, and lunch. For additional information about the symposium, please contact the NSF Center’s Technology Transfer Office at (517) 353-3969, or the Composite Materials and Structures Center at (517) 353-5466.

REQUEST FOR NEW AND RENEWAL PROPOSALS
and
JOINT CFMR/CMSC PROPOSALS

1995-1996 Research Excellence Fund

Emphasis Area: Materials Research
Thrust Area: COMPOSITE MATERIALS

The Research Excellence Fund (REF) of the State of Michigan will be will be available to support research in the area of composite materials for the period July 1, 1995 to June 30, 1996. The following information is provided to assist faculty in the preparation of NEW AND RENEWAL proposals as well as JOINT CFMR/CMSC proposals to be considered for support from these funds. The deadline for submitting proposals under this program is 5:00 PM on May 31, 1995.

The Research Excellence Fund (REF)

The Research Excellence Fund (REF) is a State of Michigan program designed to "focus resources on a limited number of specific basic and applied research proposals of outstanding quality which will contribute to economic development and job creation within the State of Michigan. Each project undertaken by an institution must be in a field where quality and institutional commitment are already strong."”...Dept. of Management and Budget REF Memorandum, 1986.

Michigan State University has designated the Biotechnology and Materials areas as their emphasis areas for REF. In the materials area, fundamental materials research, electronic and surface properties of materials and composite materials have been designated as the major thrust areas which are the recipients of REF monies according to the program guidelines given by the State. Any faculty member may apply for REF funding from any REF emphasis area.

In order to comply with the intent of this funding in the composite materials area, research thrusts have been identified which combine both the strengths of the faculty and a high potential for both economic development and job creation. This funding is not intended be a continuing long-term sole source for funding or to replace externally fundable research. It is intended to provide support to start new and novel research efforts, encourage new multi-investigator, interdisciplinary efforts and attract external funding in support of faculty research which will contribute to the long-term economic vitality of the State of Michigan as stated in the REF objectives.

In additions to individual faculty proposals, there are opportunities for:

The proposals will be peer-reviewed during the month of June. Announcements of new awards and renewals will be made shortly thereafter. Proposals that are not received in time for this peer-review will not be eligible for 1995-96 funding.

NEW, RENEWAL AND/OR JOINT CFMR/CMSC PROPOSALS ARE SOLICITED IN THE FOLLOWING THRUST AREAS:

RESEARCH THRUSTS FOR 1995/96

To be eligible for REF funds from this area, research proposals must be focused around some aspect of Composite Materials. For this purpose a composite is defined as any material composed of two or more distinct constituents fabricated into a final form suitable for engineering applications in which the constituents can be readily identified and exhibit an interface between them. In this context, engineering refers to not only structural applications but other applications such as thermal, electrical, etc. independent of the size of the part. Polymeric, metallic, ceramic, concrete and biomaterials are included in this definition as matrices. Particulates, films, and discontinuous and continuous fibers are included as reinforcements. Layered composites such as adhesively bonded joints would be included under this definition.

REF proposals are solicited under any of the following thrusts:

1. REDUCED COST COMPOSITE CONSTITUENTS (FIBER OR MATRIX)
2. NEW HIGH PERFORMANCE COMPOSITE MATERIALS
3. NEW AND/OR NOVEL COMPOSITE PROCESSING AND FABRICATION METHODS
4. PERFORMANCE AND DESIGN MODELLING
5. ADVANCED DETECTION AND CONTROL METHODS FOR COMPOSITE MANUFACTURING
6. ENVIRONMENT EFFECTS ON COMPOSITE PERFORMANCE
7. FUNDING FOR NSF IUCRC OR OTHER CENTER START-UP GRANTS

Highlights of the SAMPE
Michigan Regional Meet

A very eventful SAMPE Michigan Regional Meet was held on the 22nd March ’95 at the Kellogg Center with highlights being the Student Poster contest and the invited talk by Dr. Gilbert Chapman of Chrysler. Dr. Chapman gave a very enlightening talk on “Automotive Industry Requirements for the Development and Application of Polymer Composites” and highlighted the role of Automotive Composites Consortium.

Students from Michigan State University and Michigan Molecular Institute participated in the Poster contest, where the winner will present at the 40th International SAMPE Symposium and Exhibition, May 8-11, Anaheim, CA. The posters were evaluated by a panel of judges consisting of Donald Melotik (Ford), John Scanlon (AutoAir), Gilbert Chapman (Chrysler), and Murali Vedula (Michigan Chapter). Murty Vyakarnam was announced the winner of the Poster contest and won a cash prize of $500.

The event provided an excellent opportunity for interaction between students and the participants from the industry. The Regional Meet was co-sponsored by the Michigan and MSU Student Chapters.

Upcoming Conferences and Seminars ... May/June95

Composite Materials and Structures Center
College of Engineering
Michigan State University
East Lansing, MI 48824-1326

Please send any comment or request to

CompositeWEB page