1996 Annual Report
Technical Activities: POLYMER CHARACTERIZATION
The Polymer Characterization program provides measurement methods, data, and standard reference materials needed by U.S. industry, research laboratories, and other federal agencies to characterize polymers for processibility, properties, and performance. Molecular weight and molecular weight distribution are the molecular characteristics of polymers that most affect their processing, properties and performance. Properties and performance, particularly mechanical attributes, may also vary widely depending on the solid state structure formed during processing. Furthermore, unlike other materials, polymers exhibit mechanical properties that are both time and rate dependent, even at sub-ambient temperatures. As a result, the focus of the program is on techniques that measure molecular weight and its distribution, the solid state structure of polymers, and time and rate dependent (viscoelastic) mechanical properties.
Primary methods employed for molecular weight characterization are dilute solution light scattering and osmometry. Chromatographic techniques, which require calibration by standards of known molecular mass, provide information on molecular weight distribution. Recent activities exploit advances in mass spectrometry using mass assisted laser desorption ionization (MALDI) to develop the method as a primary tool for the determination of the molecular weights of synthetic polymers. Solid state structure of polymers is elucidated using spectroscopic methods such as solid state nuclear magnetic resonance (NMR) and infrared spectroscopy. Other techniques such as small and wide angle x-ray scattering, and dilatometry are also used in this endeavor.
Adequate characterization of the time, rate and strain dependencies of mechanical properties of polymer requires a large number of tests. For this reason, methods are developed that reduce the number of tests necessary to describe the nonlinear thermo-viscoelastic properties of polymers. The approach applies the theoretical frameworks of continuum mechanics (solid and fluid rheology) and micro- to meso-scale physical models to interrelate the mechanical responses under different loading conditions. Model predictions are tested using the torsional dilatometer, conventional rotational rheometry tools and servo-hydraulic mechanical test devices. The combination of experimental and modeling provides the basis for both improved models and the development of new experimental methods for measuring material properties.
The polymer industry and standards organizations assist in the identification of current needs for standard reference materials. Based on these needs, research on characterization methods and measurements are conducted leading to the certification of standard reference materials. Molecular standards are used primarily for calibration of gel permeation chromatographs, the principal method employed by industry for assessing molecular weight and molecular weight distributions. Melt flow standards are used in the calibration of instruments used to determine processing conditions for thermoplastics.
Significant Accomplishments
Designed and constructed apparatuses for performing mechanical tests under conditions of sudden changes in concentrations of plasticizers.
Standard Reference Materials
Charles Guttman, William Blair, Bruno Fanconi, John Maurey
Objectives
Provide the U.S. polymer industry with standards for calibration of instruments used in the control of the synthesis and processing of polymers. Principal polymer standards are certified for molecular weight and melt flow rate; the former are used to calibrate gel permeation chromatographs and the latter to calibrate melt flow indexers.
Technical Description
New Polyethylene Molecular Weight Standards
Polyethylene is the dominant commercial polymer in the United States and worldwide. High temperature (150C) Size Exclusion Chromatography (SEC), although a relative method requiring calibration, is the most commonly used method to establish the molecular weight of these polymers. Yet, few SEC calibration standards are available on the market for the calibration of high temperature SEC. Molecular weight fractions with Mw/Mn of less then 1.2 are the most useful materials for calibrating SEC. The only commercial supplier of sharp fractions of polyethylene offers repackaged NIST polyethylene standard reference materials. These SRM's will soon be out of stock.
The lack of commercial standards arises out of the difficulties obtaining sharp molecular weight fractions of these materials and in measuring absolute molecular weights by light scattering or osmometry at temperatures as high as 150 C.
The original fractionation of a whole polyethylene that provided the fractions previously certified also yielded other fractions including ones with molecular weights around 5,000, 70,000 and 160,000 g/mole in sufficient quantity to produce three additional narrow fraction polyethylene SRM's.
These new materials along with current SRM 1482 and 1483 would provide a set of polyethylene fractions having molecular weights of
5,000
14,000 SRM 1482
32,000 SRM 1483
75,000
160,000
g/mole. This will provide the polymer industry with an adequate set of molecular weight calibrants for polyethylene. This work is supported by the Standard Reference Materials Program at NIST.
Recertification of Standard Reference Materials
In addition to producing new SRM's measurements are conducted leading to recertification of SRM's that are out of stock, exceeded normal shelve life, or repackaged for market reasons. If the SRM is out of stock then a complete certification must be carried out on a new batch of material, preferably one with molecular weight characteristics similar to those of the material being replaced. Light scattering and osmometry are the customary techniques used in complete recertifications. In cases where shelve life or repackaging are issues the approach is to use indirect measurements of molecular weight such as SEC or viscometry and compare results with those obtained during the original certification.
Accomplishments
New Polyethylene Molecular Weight Standards
All three polyethylene fractions have been purified by recrystallisation. Preliminary SEC analysis was conducted to check for homogeneity. Instrumentation required for the certification, refractometer and light scattering apparatus, were modified for high temperature operations. Sufficient amounts of the solvent , alpha - chlorornaphthalene have been purified.
Polystyrene SRM 706
When the packaged inventory of SRM 706 was depleted the Standard Reference Materials Program, SRMP, proceeded to bottle the remaining polystyrene pellets from the original material. Size exclusion chromatography was used to check bottle-to-bottle variability and a recertification by light scattering was carried out.
Polyethylene SRM 1482, a narrow molecular weight fraction
Due to a dwindling supply of 1482, SRMP repackaged the existing stock, reducing the amount of material from 1 g To 0.4 g. Size exclusion chromatography measurements were made to ascertain bottle-to-bottle variability of the newly packaged material and intrinsic viscosity measurements were made to insure that the material had not changed since the original certification
Planned Outcomes
The three polyethylene standards, together with existing standards will improve calibration of size exclusion chromatographs by providing the polymer industry with molecular mass standards covering the range 5000 to 160,000 g/mol.
Mass Spectrometry of Polymers
Charles Guttman
Objectives
Improve calibration standards for size exclusion chromatographs (SEC) to make these instruments, widely used by the polymer industry, more reliable for characterizing the molecular weight and distribution of polymers. Develop mass spectrometry as a primary tool for the determination of the molecular weights of synthetic polymers that will become the next generation calibrants for SEC.
Technical Description
Recent advances in Matrix Assisted Laser Desorption Ionization (MALDI) Time of Flight (TOF) Mass Spectrometry (MS) extend the range of detectability of whole undegraded polymer molecules with molecular masses up to 300,000 g/mol. An evaluation of MALDI TOF MS was made to determine its usefulness in determining the absolute molecular mass of synthetic polymers which would become molecular weight Standard Reference Materials (SRM) for calibration of size exclusion chromatographs. Initially, the repeatability of the spectra obtained on current molecular mass SRM's will be tested as a function of sample preparation methods, as well as instrument operation parameters and data analysis.
Accomplishments
After an extensive survey and evaluation of available instrumentation a procurement requisition for a MALDI TOF MS was completed leading to the successful procurement of an instrument scheduled for delivery in February, 1997.
A collaboration with Rohm and Haas provided an opportunity to gain experience on a MALDI instrument and study SRM l487, a poly(methyl methacrylate), PMMA, SRM having a narrow molecular weight distribution centered near 6300 g/mol. The mass spectrum revealed features attributable to - methyl styrene, the initiator used in the anionic polymerization of this material. It was found that each PMMA polymer molecule contained zero to seven -methyl styrene units. The molecular weight distribution, MWD of polymer chains containing a fixed number of -methyl styrene units was obtained. The MWD, Mw and the average number of -methyl styrene units at a given molecular mass determined from MALDI TOF MS compared well with those obtained from more traditional methods such as ultracentrifugation and Size Exclusion Chromatography (SEC).
Recently, some researchers reported limited success in comparing the MWD of synthetic polymers obtained from SEC with the MWD obtained from time of flight mass spectrometry (TOF-MS). As part of the effort to apply MALDI-MS to the determination of the absolute molecular weights and molecular weight distributions of synthetic polymers, a general methodology was developed for comparison of the MWD obtained from MALDI TOF MS data of number of molecules of a particular mass with the MWD obtained from SEC data of concentration of molecules of a particular retention time.
Although the determination of the absolute molecular weights and molecular weight distributions of synthetic polymer SRM's by MALDI TOF MS appears promising, it must be verified that the detected molecular weight distribution is identical to that of the polymer.
External Collaborations
A collaboration with Dr.Paul Danis, a research chemist at Rohm and Hass, resulted on a joint publication on the MALDI TOF MS of NIST SRM 1487, a poly(methyl methacrylate).
Planned Outcomes
The methods developed in this project will simplify and improve the use of size exclusion chromatography to measure the molecular weight distribution (MWD) of polymers. Common industrial practice is to measure MWD by size exclusion chromatography (SEC) that requires calibration by a set of polymers of known molecular weight. Since standards are unavailable for most polymers crude approximations are used which introduce uncertainties in the measurement results. The methods developed will yield standards of known MWD adequate for SEC calibration. The availability of such standards for a wide range of commercial polymers will provide the polymer industry and research community with reliable calibrants for SEC to optimize the synthesis, processing and performance properties of polymers.
Outputs
Publications
Charles M.Guttman, The Relationship between the Signals from Size Exclusion Chromatography and Time of Flight Mass Spectrometry to a Polymer Molecular Weight Distribution, Polymer Preprints, vol. 37 #1, March 1996.
Charles M. Guttman, William R. Blair, Paul O. Danis, Mass Spectroscopy and SEC of SRM 1487, a Low Molecular Weight Poly(methyl methacrylate) Standard, Journal Polymer Science, Part B: Polymer Physics, submitted.
Presentations
Charles M.Guttman, The Relationship between the Signals from Size Exclusion Chromatography and Time of Flight Mass Spectrometry to a Polymer Molecular Weight Distribution, ACS Spring, 1996 Meeting, New Orleans, LA.
Charles M. Guttman, William R. Blair, Paul O. Danis, Mass Spectroscopy and SEC of SRM 1487, a Low Molecular Weight Poly(methyl methacrylate) Standard, Polymer Physics Gordon Conference, Kingsport, RI, July, 1996.
Characterization of Polymers by Spectroscopic Techniques
David VanderHart
Objectives
The objectives are to develop and use Nuclear Magnetic Resonance (NMR) techniques for characterization of molecular and microstructural level features that control properties of polymers.
Technical Description
Conceptually, the NMR work is aimed both at uncovering new insights into polymer structure and at the continuing development of NMR techniques for characterizing polymers. The following techniques have been drawn upon in our studies.
Proton spin diffusion is characteristic of an extended network of dipolar-coupled protons and is typical of organic solids; it is evidenced by the flow of proton polarization following the imposition of a polarization gradient. This flow is in a direction that restores spin equilibrium. Thus, properly designed proton spin diffusion experiments yield information about spatial relationships and domain size in organic polymers.
Multiple pulse proton techniques offer a way to quench spin diffusion and simultaneously, especially in the presence of magic angle sample spinning (MAS), achieve some chemical shift resolution for protons in solids.
Paramagnetically induced proton relaxation is the enhancement of proton relaxation rates which results from proximity to unpaired electrons. These electrons themselves relax, thereby causing large fluctuations in the local magnetic fields at the protons; and this, along with spin diffusion, is the mechanism for perturbing the proton relaxation.
13C spectra, obtained using MAS, offer the best chemical resolution for organic solids. Also, one can often distinguish signals arising from the same chemical species in different states of order, e.g. crystal or non-crystal. Efforts continue to develop strategies for improving resolution in order to provide greater ability to distinguish not only chemically different species but also chemically similar species in physically different sites.
Applications of these techniques to polymers include the following.
Develop a better understanding of the morphology which evolves when a semicrystalline polymer is mixed with an ionomer. The ionomer possesses a few 'decorations' which strongly interact with the semicrystalline polymer; in the absence of these few strong interactions, there would be no compatibility. The morphology and its stability reflect the interplay between the favorable energetics of crystallization and the tenacity of the strong intermolecular interactions. Furthermore, the work includes studies of the effects of the extent of chemical substitution and the influence of polar diluents, such as water, on the blend morphology.
Develop a better understanding of chemical changes and their uniformity in high-temperature chars when additives are used to promote char formation. The search for less toxic flame retardant polymers is currently intense worldwide. This research seeks to get at the mechanism of enhanced char formation.
External Collaborations
Yi Feng of AT&T Bell Laboratories, Murray Hill, NJ
Professor Robert Weiss of Univ. of Connecticut, Storrs, CT
R.H. Atalla of the USDA, Forest Products Laboratory, Madison WI
Professor Norritsu Terashima of Nagoya, Japan
Accomplishments
For the ionomer blend (metal-sulfonated poly(styrene)/methylated poly(amide)), experiments were carried out to determine the relative importance of temperature versus water in aging. Experiments showed that water was much more effective than a 20 C temperature rise. The morphological changes during aging are also interesting: for a blend consisting of 18% Zn++-containing ionomer (12% of the rings decorated), the overall dimension for compositional heterogeneity is 20 - 25 nm and this does not change with aging. However, with aging, the size of the polyamide crystals increases and the ionomer becomes increasingly demixed from the non-crystalline poly(amide). Thus, the smaller-scale morphology is demixing while the larger-scale morphology is stable.
In addition, the use of Cu++ as a paramagnetic metal ion in the ionomer caused the proton relaxation to be dominated by the fluctuating electrons. As a result of analyzing the heterogeneity of proton relaxation, two conclusions were reached: a) In the pure ionomer where 12% of the aromatic rings are randomly sulfonated, the range of variation of local Cu ion concentrations, averaged over a distance scale of 14 nm, was less than a factor of 1.35 for 95% of such domains. b) In a blend of 75% ionomer (with 2.3% of the rings decorated) and 25% poly(amide), phase separation takes place where all or most of the poly(amide) mixes with 12-16% of the ionomer in one phase; the remainder of the ionomer goes to the other phase. It was expected that the portion of the ionomer in the mixed phase would possess above-average decoration levels. Surprisingly, the data suggested that the ionomer in the mixed phase had a near-average level of decoration.
The work on polymer chars is at a preliminary stage and is being carried out with Dr. Jeff Gilman of the Fire Science Division at NIST. The 13C spectra, elemental analysis and electron spin resonance (ESR) spectra were obtained on chars of poly(vinylacetate) containing silica gel and potassium carbonate additives that promote the level of charring. The ESR results show that there is a substantial number of free radicals in the chars which have the potential of reducing 13C signals. Although 13C spectra are obtained on the chars it is likely the less than one-half of the total 13C are measured. The question is whether the spectra, while not quantitative, are representative of the whole. To address this, proton 'spin counting' experiments are performed to see if the proton signal intensity is consistent with elemental analysis. The results to date indicate that a larger fraction of the protons are observed than the 13C's. Furthermore, the relaxation of the protons is inhomogeneous on a T1H timescale of a few hundred ms. Hence, there is considerable inhomogeneity of composition in the chars and that raises more serious questions about the representative character of the 13C spectra. Another consideration which can be invoked to explain inhomogeneously reacted region of chars is the difference between physically encapsulated and unencapsulated regions. This aspect is also under investigation.
With respect to the issue of spectral resolution in 13C spectra with magic angle spinning, linewidth and transverse-relaxation (T2C) data have been accumulated on crystalline polyethylene, a polymer whose carbons have strong dipolar couplings with protons. These data were taken at different spinning speeds, proton rf-decoupling field strengths, static fields and different choices of the proton rf frequency in the vicinity of the proton resonance. The resolution achieved for the 13C's of polyethylene is a function of all of these parameters. Of particular interest is the observed dependence on the static field which points to the important role that the proton chemical shift anisotropy plays at higher static fields. The second important finding is the rather long lifetimes of the proton spin states for those protons attached to the 13C nuclei. A recognition of this longer lifetime will allow certain modulation schemes to be effective in enhancing resolution.
Outputs
Publications
N. Terashima, R.H. Atalla and D.L. VanderHart, Solid State NMR of Specifically 13C Enriched Lignin in Wheat Straw, submitted to Phytochemistry.
D. L. VanderHart and G.B. McFadden, Some Perspectives on The Interpretation of Nmr Spin Diffusion Data in Terms of Polymer Morphologies, Solid State NMR (in press)
Presentations
D. L. VanderHart, Y. Feng, C.C. Han and R. Weiss, Morphological Investigation of Sulfonated Poly(styrene) Ionomers and their Blends with a Poly(amide) Using 13C NMR and Proton Spin
Diffusion Methods, American Physical Society Meeting, St. Louis, March 1996.
D. L. VanderHart, NMR Investigations of Polymer Morphology Using Proton Spin
Diffusion, Chemistry and Applied Science Department Seminar, William and Mary University, April 1996.
D. L. VanderHart, Morphological Study of a Sulfonated Poly(styrene)/Poly(amide) Blend
Using Solid State NMR, 38th Rocky Mountain Conference on Analytical Chemistry, Denver, Colorado, July 1996.
Characterization of Polymer Morphology by Microscopy Techniques
C. L. Jackson
Objectives
The objectives are to utilize optical and electron microscopy to study the morphology of polymeric materials and to investigate the potential of new microscopy techniques, such as scanning force microscopy, cryo-TEM, and TEM with image processing, to solve problems in polymer morphology.
Technical Description
The characterization polymer morphology is important to better understand the relationship between structure and properties in many materials. Techniques used include transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical and atomic force microscopy. The combination of microscopy techniques with other methods such as scattering, diffraction or spectroscopic methods gives a more complete description of the structure of complex blends, composites or other multiphase materials. Many types of materials have been studied to date, including polymer blends, block copolymers, liquid crystalline polymers, and organic-inorganic hybrid materials.
External Collaborations
A collaboration with Dr. Henri Chanzy of CNRS in Grenoble, France has been initiated to add measurement capability in the area of cryo-TEM. This method allows solutions or suspensions of polymer aggregates, micelles or dendrimers, for example, to be imaged directly in vitrified solvent. This method has not been widely used on synthetic polymers. Preliminary work is being done at NIH in collaboration with Drs. F. Booy and A. Stevens.
Accomplishments
From TEM studies of graft copolymers of polystyrene (PS) and poly(methacrylic acid) (PMMA), the dried micelles were found to be about one-third the size of the solubilized micelles as measured by dynamic light scattering and small angle neutron scattering. This implies that the PS cores are swollen and the PMMA chains in the corona are almost fully extended in solution. >From the TEM studies, the polydispersity of the micelles as well as the number of aggregates per micelle were determined.
Other recent studies have focused on phase separated polymer blends containing block copolymer as a compatibilizer where the kinetics of phase separation at constant and shallow quench depths was studied by light scattering techniques. As block copolymer is added, the kinetics of phase separation slows dramatically. TEM specimens were prepared under similar conditions to compare the real-space morphology on these complex ternary systems.
Outputs
Publications
C. L. Jackson, L. Sung, and C. C. Han, Evolution of Phase Morphology in Compatibilized Polymer Blends at Constant Quench Depths: Complimentary Studies by Light Scattering and Transmission Electron Microscopy, Polym. Eng. Sci., submitted.
C. L. Jackson, B. J. Bauer, A. I. Nakatani and J. D. Barnes, Synthesis of Organic-Inorganic Materials from Interpenetrating Polymer Network Chemistry, Chem. Mater., 1996, 8, 727-733.
C. L. Jackson, L. Sung and C. C. Han, Morphology and Phase Separation Kinetics of a Compatibilized Blend, SPE Antec Proc., 1996, 1599.
Presentations
C. L. Jackson, Vitrification and Crystallization of Organic Liquids Confined to Nanoscale Pores, American Chemical Society, Orlando, FL, August, 1996.
C. L. Jackson, Morphology Evolution and Phase Separation Kinetics in a Polymer Blend with Diblock Copolymer Additive, Gordon Conference on Polymer Physics, Newport, RI, July 1996.
C. L. Jackson, Morphology and Phase Separation Kinetics of a Compatibilized Blend, Society of Plastics Engineers, Indianapolis, IN, May 1996.
C. L. Jackson, Micellar Structure of a Model Ionic Graft Copolymer, American Physical Society, St. Louis, MO, March 1996.
Structural Characterization of Polymers by Small Angle X-Ray Scattering
John D. Barnes
Objectives
A Small Angle X-Ray Scattering Facility is maintained both to serve as a user facility for outside industrial clients and to support research within the Polymers Division. The project objectives are focused on maintaining an up to date, state of the art SAXS Facility that develops and incorporates improvements in analysis methods and instrumentation that expand the use of small angle x-ray scattering (SAXS) as a technique for characterizing materials and processes.
Technical Description
By maintaining a state of the art SAXS facility, Polymers Division researchers are able to perform in house research relevant to ongoing programs as well as collaborative research with both industrial and academic scientists. Of particular interest is the two-dimensional capability of the NIST area detector and development of special techniques such as oblique incidence scattering, elevated temperature capabilities and deformation stages for studies. In addition, analytical capabilities are constantly upgraded and implemented as the need or opportunity arises.
External Collaborations
The SAXS facility was used during the year for projects with outside users that fall into three categories. First, there were several extensive collaborations with industrial research laboratories in which the results were of importance to the company and also resulted in technical publications. W.R. Grace uses the oblique incidence technique to describe the through thickness orientation of developmental films, the Dow Chemical Company examines the morphology of injection molded syndiotactic polystyrene, and Air Products Company studies the microstructure of high performance polyurethanes. The SAXS facility is also used for proprietary measurements when the industrial client, i.e. W.R. Grace, is interested in protecting sensitive data. Finally, the facility attracts collaborators from the academic community and here work was undertaken with Michigan Technological Institute in the study of the microstructure of block copolymers under shearing deformations.
Planned Outcomes
NIST researchers and external uses from industry, government and academic laboratories will have available a state-of-the-art small angle x-ray facility operated by experts who can assist in collection and analysis of data.
Accomplishments
Outputs
Publications
C.L. Jackson, B.J. Bauer, A.I. Nakatani, and J.D. Barnes, Synthesis of Hybrid Organic-Inorganic Materials from Interpenetrating Polymer Network Chemistry, Chem. Mater., 1996, 8, 727-733.
J.D. Barnes, G.B. McKenna, B.G. Landes, R.A. Bubeck, and D.A. Banks, Morphology of Syndiotactic Polystyrene as Examined by Small Angle X-Ray Scattering, Proc. SPE Annual Technical Conference, 1996, 1532-1535.
R. Hu, W.S. Lambert, and J.D. Barnes, Structure-Property Correlatiion in Stretched LLDPE Film Using Oblique Incidence SAXS, Proc. SPE Annual Technical Conference, 1996, 1839-1843.
Nonlinear Viscoelasticity of Solid Polymers
G.B. McKenna, C.R. Schultheisz, D.M. Colucci, and P.A. O'Connell
Objectives
The objectives are to develop measurement methods and analytical approaches for the analysis of the nonlinear viscoelastic response of polymers. In addition, evaluations are made of constitutive relationships, with the emphasis on material clock or reduced time models, that allow the description of the mechanical response to complex mechanical and thermal histories.
Technical Description
Improvements in measurement methods are sought through modifications of conventional testing techniques such as extension and compression measurements in stress relaxation, creep or torsional measurements, the latter including torsional dilatometry. The experimental design is based on concepts from finite elasticity theory of incompressible materials for evaluation in compressible, viscoelastic materials in order to use simple measurements in a single geometry to predict properties under other test geometries. Thermal histories and multiple step strain and stress histories are applied to the materials to evaluate viscoelastic constitutive models based on material clock or reduced time concepts. Furthermore, a complete database for a single, well defined, glassy polymer is being developed for publication as a reference for evaluation of material models. The material itself can be distributed as a research material to interested and qualified laboratories.
External Collaborations
The major interaction is with the GM/GE ATP project on "Thermoplastic Engineering Design" in which viscoelasticity data on the project's polycarbonate thermoplastic material are provided to the GM and GE researchers for use in Numerical Modeling being developed at GM and GE. In addition, collaborative work with the University of Leeds in Leeds, UK, demonstrated the ability to extend finite elasticity concepts to finite viscoelasticity as a means of determining material model parameters from simple tests (uniaxial extension) to predict the response in shear and equi-biaxial extension.
Planned Outcomes
Accomplishments
Outputs
Publications
C. R. Schultheisz, D. M. Colucci and G. B. McKenna, Experimental Evidence and Modeling of the Differing Time Scales of Structural Recovery and Mechanical Relaxation Observed During Aging of Glassy Polymers, Proceedings of the Society for Experimental Mechanics, VIII International Congress on Experimental Mechanics, June 10-13, 1996, Nashville, Tennessee.
A.S. Wineman and G.B. McKenna, Determination of the Strain Energy Density Function for Compressible Isotropic Nonlinear Elastic Solids by Torsion-Normal Force Experiments, In Nonlinear Effects in Fluids and Solids, ed. By M.M. Carroll and M. Hayes, Plenum Press, New York, 1996.
J.J. Pesce and G.B. McKenna, A Comparison of Torque and Normal Force Responses in Polycarbonate in One and Two Step Torsional Histories with a Material Clock Model, in Mechanics of Plastics and Plastic Composites, 1995,ed by M.C. Boyce, American Society of Mechanical Engineers, 1995,MD-68 and AMD-215, 309-336.
C.R. Schultheisz, G. B. McKenna, Y. Leterrier and E. Stefanis, A Comparison of a and ate from Simultaneous Volume and Mechanical Measurements, Proceedings of the Society of Experimental Mechanics, Spring Conference, June 12-14, 1995, Grand Rapids, MI.
C.R. Schultheisz, D.M. Colucci, G. B. McKenna, and J.M. Caruthers, Modeling the Differing Time Scales of Structural Recovery and Mechanical Relaxation in Aging Experiments, in Mechanics of Plastics and Plastic Composites, 1995,ed by M.C. Boyce, American Society of Mechanical Engineers, 1995, MD-68 and AMD-215, 251-282.
D.M. Colucci, P. A. O'Connell and G.B. McKenna, Volume Changes in Thermoplastics in Creep and Stress Relaxation Experiments in Tension and Compression, (1996) SPE ANTEC, 1543.
P. A. O'Connell and G.B. McKenna, Large Deformation Response of Polycarbonate: Time-Temperature and Time-Aging Time Superposition. (1996) SPE ANTEC, 2262.
J.-J. Pesce and G. B. McKenna, Prediction of the Sub-Yield Extension and Compression Responses of Glassy Polycarbonate from Torsional Measurements, (J. Rheology, Submitted).
P.A. O'Connell and G.B. McKenna, Large Deformation Response of Polycarbonate: Time-temperature and Time-Aging Time Superposition, in Polycarbonate Science and Technology, ed. by D.G. LeGrand and J.T. Bendler, Marcel Dekker, New York, (Submitted).
S.A. Ketcham, J.M. Niemiec, and G.B. McKenna, Extension and Compression of Elastomeric Butt Joint Seals, J. Engineering Mechanics, 1996, 122, 669-677.
Presentations
P.A. O'Connell and G.B. McKenna, Sub-Tg,Time-Temperature Superposition in Polymers at Large Deformations, Society of Rheology Annual Meeting, Sacramento, CA, October, 1995.
J.J. Pesce and G.B. McKenna, Prediction of the Extension and Compression Response of Glassy Polycarbonate in the Nonlinear Range from Torsional Measurements, Society of Rheology Annual Meeting, Sacramento, CA, October, 1995.
G.B. McKenna, Structural Recovery and Nonlinear Viscoelasticity in Glassy Polymers, IBM Almaden Research Center, San Jose, CA, October, 1995.
G.B. McKenna, The Impact of Physical Aging and Nonlinear Viscoelasticity on the Performance of Polymers and Composites, School of Chemical Engineering, Purdue University, Lafayette, IN, October, 1995.
D.M. Colucci, A Thermoviscoelastic Approach in Modeling the Glass Transition Region, University of Maryland, Baltimore Campus, Department of Mechanical Engineering, Baltimore, MD, November 1995.
J.J. Pesce and G.B. McKenna, Comparison of Torque and Normal Force Responses of Polycarbonate in One and Two Step Torsional Histories with a Material Clock Model, ASME Winter Annual Meeting, San Francisco, CA, November, 1995.
G.B. McKenna, Measurement of Material Properties in Nonlinear Viscoelasticity of Solid Polymers, Eindhoven University, Eindhoven, The Netherlands, February, 1996.
G.B. McKenna, Time-Temperature, Time-Aging Time and Time-Strain Superposition in Polymers Below the Glass Transition, Institut Charles Sadron, Strasbourg, France, February, 1996.
P.A. O'Connell and G.B. McKenna, Sub-Tg, Time-Temperature, Time-Aging Time and Time-Strain Superposition in Polymers at Large Deformations, Division of High Polymer Physics, American Physical Society March Meeting, St. Louis, MO, March, 1996.
W.H. Han, F. Horkay and G.B. McKenna, Evaluation of Some Rubber Elasticity Models from Mechanical and Swelling Measurements, Division of High Polymer Physics, American Physical Society March Meeting, St. Louis, MO, March, 1996.
G.B. McKenna, The Need for Efficient Methods for the Determination of Material Parameters in Non-linear Materials, Institute for Mechanics and Materials Think Tank on The Role of Theory and Experiment in the Physics and Mechanics of Solids, Houston, TX, March, 1996.
D.M. Colucci, C.R. Schultheisz, and G.B. McKenna, A Thermoviscoelastic Analysis of the Differences in Time Scales Evidenced by Structural Recovery and Physical Aging, American
Physical Society Annual Meeting, March 1996, St. Louis, MI.
G.B. McKenna, Efficient Experimental Methods for the Measurement of Nonlinear Material Parameters, University of Nebraska-Lincoln, Lincoln, NB, April, 1996.
P.A. O'Connell and G.B. McKenna, Large Deformation Response of Polycarbonate: Time-Temperature and Time-Aging Time Superposition. SPE ANTEC, Indianapolis, IN, May, 1996.
D.M. Colucci, P.A. O'Connell, and G.B. McKenna, Volume Changes in Thermoplastics in Stress Relaxation Experiments in Tension and Compression, 1996 Society of Plastics Engineers Annual Meeting, May 1996, Indianapolis, IN
D.M. Colucci, A Comparison of the Predictive Capabilities of Several Nonlinear Viscoelastic Constitutive Models, Materials Lab Directorate, Wright Patterson Air Force Base, Dayton, OH, May 1996.
G.B. McKenna, D.M. Colucci and P.A. O'Connell, Relaxation Behavior of Polycarbonate below the Glass Transition, Thermoplastic Engineering Design Meeting with GM and GE, General Electric CRD, Schenectady, NY, May, 1996.
W.H. Han, Glenn Brown Award Lecture entitled Computational Modelling of Orientation and Flow Instabilities, Textures, Rheology, and Optics of Shearing Nematic Liquid Crystalline Materials, 16th Biennial International Liquid Crystal Conference, Liquid Crystal Institute, Kent State University, Kent, OH, June, 1996.
D.M. Colucci, P.A. O'Connell, and G.B. McKenna, Experimental Investigation of the Nonlinear Viscoelastic Response and Subsequent Volume Changes of Two Engineering Polymers in Tension and Compression, VII International Congress on Experimental and Applied Mechanics, Special Symposium: Time Dependent Materials, Nashville, TN, June 1996.
D.M. Colucci, P.A. O'Connell, and G.B. McKenna, Single Step Stress Relaxation Responses of Polycarbonate: Biaxial Measurements, 1996 American Society of Mechanical Engineers,
Mechanics and Materials Conference, June 1996, Baltimore, MD
J.J. Pesce and G.B. McKenna, Prediction of the Extension and Compression Responses of Glassy Polycarbonate in the Nonlinear Range from Torsional Measurements Using a Compressible Valanis-landel Analysis, ASME Spring Meeting, Johns Hopkins University, Baltimore, MD June, 1996.
P.A. O'Connell and G.B. McKenna, Sub-Tg, Time-Temperature and Time-Strain Superposition in Polymers at Large Deformations, ASME Spring Meeting, Johns Hopkins University, Baltimore, MD June, 1996.
Physical Aging of Polymers
G.B. McKenna, C.R. Schultheisz, D.M. Colucci, P.A. O'Connell, M. L. Cerrada
Objectives
The objectives are to develop new methods and incorporate known methods of measurement to determine the interrelationships between the thermodynamics and kinetics of glass formation and to relate these to the underlying microstructure of the glass through appropriate physical models.
Technical Description
Engineering polymers are generally used below their glass transition temperature with the result that their properties are constantly evolving due to the non-equilibrium state of the glassy phase. In addition, the actual path of pressure and temperature by which the polymer reaches the glassy state impacts the resulting mechanical properties. Therefore it is important to have a constitutive framework in which to cast the evolution of the material properties with history in order to predict the resulting material behavior both during and subsequent to processing. In this project, measurements of the volume recovery and mechanical response are made on both amorphous and semi-crystalline polymers. In the case of the amorphous polymers the procedures chosen are classical and torsional dilatometry and stress relaxation measurements after temperature jumps in order to establish the range of validity of the current models of structural recovery and physical aging. In addition, the physical aging data generated on poly(carbonate) are part of a larger data base of viscoelastic data for an amorphous, glassy polymer. Because the ideas currently used as the basis of understanding the structural recovery and physical aging are known to be inadequate, the experiments are designed to provide insight into where improvements in the models can be made. An example is the observation that the mechanical response of the polymer does not evolve at the same rate as does the volume, giving rise to questions about the validity of simple free volume descriptions of glassy kinetics.
PVT measurements of a glass forming polymer (poly(carbonate)) are also used to understand how the glass transition and the glassy state depend on path. In particular, first time measurements that demonstrate the existence of an isochoric (constant volume) glass transition have been made.
To assess microstructural changes in amorphous polymers a new capability has been added to the Small Angle X-ray Scattering (SAXS) facility to permit studies of the wavelength dependence of the structural evolution of amorphous polymers after temperature jump experiments. Such experiments will determine the existence of differences in kinetics of structural recovery depending on microstructural size scale in a way that is consistent with free volume distribution models of the glassy state.
Models of the aging response of semi-crystalline polymers are even less successful than those for the amorphous polymers. To better describe such processes physical aging data are obtained on semi-crystalline films and injection molded syndiotactic polystyrene. New non-contact techniques based on laser extensometry are being developed to study anisotropic structural recovery in semi-crystalline films.
External Collaborations
The classical dilatometry studies on poly(carbonate) are part of collaborative work with the GM/GE ATP project on Thermoplastic Engineering Design. Similarly, the linear viscoelastic response of poly(carbonate) to physical aging is part of this project. The data produced here are given to GM and GE for incorporation into numerical models used in the program. The physical aging work on anisotropic films is performed in collaboration with researchers at Kodak. The work on the PVT response of poly(carbonate) and on isochoric glass transition was performed in collaboration with researchers from Wright Laboratories and Michigan State University.
Planned Outcomes
Accomplishments
Outputs
Publications
J. M. Niemiec, C.L. Schutte, C.R. Schultheisz and G. B. McKenna, Time-Temperature and Time Aging Time Superposition in Polycarbonate Below the Glass Transition, Proceedings ANTEC, SPE, 2402-2408 (1995).
J.-J. Pesce, J.M. Niemiec, M. Y. Chiang, C. L. Schutte, C.R. Schultheisz, and G.B. McKenna Characterization of Polymers in the Glass Transition Range: Time-Temperature and Time-Aging Time Superposition in Polycarbonate, in Current Research in the Thermo-Mechanics of Polymers in the Rubbery-Glassy Range, ed. by M. Negahban, American Society of Mechanical Engineers, New York, 1995, AMD-Vol. 203, pp 77-88.
I. Spinu and G.B. McKenna, Physical Aging of Thin Films, Proceedings ANTEC, SPE, 2684-2697 (1995).
G.B. McKenna, Dynamics and Mechanics Below the Glass Transition: The Non-Equilibrium State, J. Comp. Matls. Science, 1995, 4, 349-360.
J. Beckmann, G.B. McKenna, D.H. Banks, B.G. Landes and R.A. Bubeck, The Physical Aging Response of Syndiotactic Poly(styrene), (1996) SPE ANTEC, 2272.
G.B. McKenna, Comments on the paper 'Isobaric Volume and Enthalpy Recovery of Glassses II. A Transparent Multiparameter Model, J. Polymer Science, Polymer Physics Edition, (In Press).
D.M. Colucci, C.R. Schultheisz and G.B. McKenna, Dynamics of Structural Recovery and Mechanical Response of Polymeric Liquids Near to the Glass Transition, Proceedings of the Materials Research Society 1995 Fall Meeting: Disordered Materials Symposium, Boston, MA.
D.M. Colucci, G.B. McKenna, A. Lee, D.B. Curliss, K.B. Bowman and J.D. Russell, Isochoric and Isobaric Glass Formation: Similarities and Differences, J. Polymer Science, Physics Edition, (submitted).
Presentations
C.R. Schultheisz, D.M. Colucci and G.B. McKenna, "Structural Recovery and Physical Aging: A Thermoviscoelastic Analysis of the Differences in Rates of Evolution," 67th Annual Meeting of the Society of Rheology, Sacramento, California, October, 1995.
G.B. McKenna, "Physical Aging and its implications for the long term performance of polymers and composites," Golden Gate Polymer Forum, Mountain View, CA, October, 1995.
G.B. McKenna, "Structural Recovery and Nonlinear Viscoelasticity in Glassy Polymers," IBM Almaden Research Center, San Jose, CA, October, 1995.
G.B. McKenna, "Structural Recovery and Its Implications for the Long Term Performance of Polymers and Composites," Department of Chemical Engineering, University of Pittsburgh, November, 1995.
C.R. Schultheisz, D.M. Colucci, G.B. McKenna and J. M. Caruthers, "Modeling of Different Time Scales of Structural Recovery and Mechanical Relaxation Observed in Aging Experiments," ASME Winter Annual Meeting, San Francisco, CA, November, 1995.
D.M. Colucci, C.R. Schultheisz, and G.B. McKenna, "Prediction of Torsional Dilatometry Responses Using a Thermoviscoelastic Constitutive Model", American Institute of Chemical Engineers National Meeting, November 1995, Miami, FL.
D.M. Colucci, G.B. McKenna, and A. Lee, "Observation of an Isochoric Glass Transition", Materials Research Society Fall Meeting, November 1995, Boston, MA.
D.M. Colucci, C.R. Schultheisz, and G.B. McKenna, "Dynamics of Structural Recovery and Mechanical Response of Polymeric Liquids Near to the Glass Transition", Materials Research Society Fall Meeting, November 1995, Boston, MA.
P.A. O'Connell and G.B. McKenna, "Time-Temperature and Time-Aging Time Superposition in Polymers Below The Glass Transition," Materials Research Society, Winter Meeting, December, 1995.
G.B. McKenna, "Time-Temperature, Time-Aging Time and Time-Strain Superposition in Polymers Below the Glass Transition," Institut Charles Sadron, Strasbourg, France, February, 1996.
D.M. Colucci, "Volume Change Measurements in Isotropic and Anisotropic Deformations of a Polymer Glass: A Comparison of Experiment with Statistical Mechanical and Phenomenological Models", National Institute of Standards and Technology, Gaithersburg, MD, February 1996.
G.B. McKenna, "Physical Aging in Polymers and Its Implications for Long Term Performance of Polymers and Composites," Lehigh University Department of Chemical Engineering, Bethlehem, PA, March, 1996.
D.M. Colucci and G.B. McKenna, "Prediction of Pressure-Volume-Temperature Behavior Including the Glass Transition Temperature Using the Simha-Somcynsky and Gibbs-DiMarzio
Equations of State", American Physical Society Annual Meeting, March 1996, St. Louis, MO.
D.M. Colucci, C.R. Schultheisz, and G.B. McKenna,"A Thermoviscoelastic Analysis of the Differences in Time Scales Evidenced by Structural Recovery and Physical Aging", American
Physical Society Annual Meeting, March 1996, St. Louis, MO.
G.B. McKenna, "Viscoelasticity and Aging in Polymer Glasses: Questions of Measurement and Analysis," Virginia Polytechnic and State University, Blacksburg, VA, April, 1996.
P.A. O'Connell and G.B. McKenna, "Large Deformation Response of Polycarbonate: Time-Temperature and Time-Aging Time Superposition." SPE ANTEC, Indianapolis, IN, May, 1996.
J. Beckmann, G.B. McKenna, D. H. Banks, B.G. Landes and R.A. Bubeck, "The Physical Aging Response of Syndiotactic Poly(styrene)," SPE ANTEC, Indianapolis, IN, May, 1996.
C. R. Schultheisz, D. M. Colucci and G.B. McKenna, "Experimental Evidence and Modeling of the Differing Time Scales of Structural Recovery and Mechanical Relaxation Observed During Aging of Glassy Polymers," Society for Experimental Mechanics, VIII International
Congress on Experimental Mechanics, June, 1996, Nashville, Tennessee.
C. R. Schultheisz and G. B. McKenna, "Volume Changes in Polycarbonate Following Temperature Jumps," American Society of Mechanical Engineers, 1996 Mechanics and Materials Conference, June, 1996, Baltimore, Maryland.
G.B. McKenna, "The Dynamics of Polymers Below the Glass Transition," Gordon Research Conference on Polymer Physics, Newport, RI, July, 1996.
C. R. Schultheisz and G.B. McKenna, "Volume Evolution of Lexan LS-2 Polycarbonate," Semiannual Meeting GE/GM Advanced Technology Project on Thermoplastic Engineering Design, October, 1996, Buffalo, New York.
Organizing Activities
G.B. McKenna and S.C. Glotzer co-organized a Symposium on "40 Years of Entropy and the Glass Transition," at the March Meeting of the American Physical Society, Division of High Polymer Physics, St. Louis, MO, March, 1996.
Finite Element Analysis in Polymer Mechanics.I: Dimensional Stability
M.Y.M. Chiang
Technical Objectives
The objective is to determine the impact of specific parameters, e.g., deviations from material linearity, inhomogeneity of temperature, etc., on the residual stresses and resulting dimensional stability or warpage of polymer-based components.
Technical Description
The project implements nonlinear viscoelasticity into finite element codes that are used to analyze the impact of thermal history on the residual stresses in printed wiring board type composites and injection molded parts. Initially, a simplified micro-mechanical model is used in a linear thermo-viscoelastic finite element analysis to examine the role of material viscoelasticity (stress relaxation or creep) on dimensional changes. The analysis incorporates material properties determined in our laboratory for the thermo-viscoelastic response of an epoxy. The model, based on a unit representative cell and classical laminate theory, serves to demonstrate the time-dependency of the inner layer dimensions during the cooling process after the part is removed from the mold. Long term behavior is explored in a numerical simulation which includes stress relaxation below the glass transition temperature (Tg). In this model, the relaxation response at a given temperature, T, is described by substituting a temperature-dependent reduced time factor into a formerly temperature-independent relaxation function (time-temperature superposition, TTS).
In addition to considerations of time-temperature superposition behavior, it has been observed that polymeric materials undergo a continuous evolution of properties below the glass transition temperature and that the changes in viscoelastic properties can be well represented by a time-aging time superposition principle. Hence, the material response at different aging times may be treated rather than the response at different temperatures T. A circular plate geometry is chosen to predict the warpage caused by residual stresses, which are induced by the quenching from the temperature near Tg to room temperature. The plate is initially assumed to be stress free and viscoelastic . The analysis includes a heat transfer and subsequent stress analysis.
External Collaborations
This work has been performed in collaboration with the GM/GE ATP project on Thermoplastic Engineering Design.
Planned Outcomes
Accomplishments
Outputs
Presentations
M.Y.M. Chiang and G.B. McKenna, The Effect of Age Dependent Behavior on Shape Changes in a Polymeric Part Subjected to Non-uniform Cooling, Materials Research Society, Winter Meeting, December, 1995.
M.Y.M. Chiang, Effect of Time-Temperature Superposition on Dimensional Stability in Micro-mechanics Model, Polymer Composite Laboratory, Korea Institute of Science and Technology, Seoul, Korea, September, 1996.
Finite Element Analysis in Polymer Mechanics. II: Fracture
M.Y.M. Chiang
Technical Objectives
The objective is to combine numerical solutions with experimental observations to determine the impact of material elastic-plastic and visco-plastic constitutive properties on strain localization in fracture of thin adhesive bonds. The effects of viscoelasticity and aging on the localization event are investigated.
Technical Description
This investigation combines experiments and finite element analysis with emphasis on the local deformation at the crack tip to establish fracture criteria which are independent of specimen geometry. In the experiments, the bond thickness is the prime variable, ranging from 25 m to 420 m. End-notched flexure and the Butterfly adhesive bonding test specimens are used to study constrained interfacial crack propagation. Experiments were performed using an epoxy adhesive; the crack tip region was observed with the aid of a high-magnification video system.
Large strain incremental plasticity finite element analysis was used to study numerically the same specimen geometry and test conditions as measured experimentally. The deformation and mechanism of fracture during the entire event of crack propagation, which includes the onset of crack propagation, was modeled.
External Collaborations
This work was carried out in collaboration with the Tel Aviv University, Israel.
Planned Outcomes
Accomplishments
The local shear strains estimated from the finite element analysis and those measured experimentally are in reasonable agreement. The analysis provides quantitative insights into the mechanics of other failure modes observed in the experiments. The onset of a detrimental microdebond ahead of the crack tip is controlled by the bond-normal tensile stress. Hydrostatic tension seems responsible for the development of a large void at the crack tip which temporarily arrests the crack while the principal tensile stress at the crack tip seems to govern crack kinking. All of these failure modes are activated under large strains, which highlights the rule of plasticity in the fracture of polymer joints.
Outputs
Publications
M.Y.M. Chiang, Modified Mohr-Coulomb Plasticity Model for Nonlinear Fracture Analysis of Polymeric Interlayer, ANTEC '95, Society of Plastic Engineers, Boston.
H. Chai and M.Y.M. Chiang, A Crack Propagation Criterion Based on Local Shear Strain in Adhesive Bonds Subjected to Shear, J. of Mech. Phys. of Solids (submitted ).
M.Y.M. Chiang and H. Chai, A Large Strain Analysis of Constrained Interfacial Crack Propagation Based on Local Shear Deformation - Part I, Int. J. Solids and Structures (submitted).
H. Chai and M.Y.M. Chiang, A Large Strain Analysis of Constrained Interfacial Crack Propagation Based on Local Shear Deformation - Part II, Int. J. Solids and Structures (submitted).
Presentations
M.Y.M. Chiang, Finite Element Analysis of Strain Localization and Finite Strain Fracture in Adhesive Bonds, North Carolina State University, MARS Mission research Center, Raleigh, NC, March, 1996.
M.Y.M. Chiang and H. Chai, An Intrinsic Fracture Property for Highly Constrained Interlayer Subjected to Shear Loading, 19th Technical Conference, The Adhesion Society, Myrtle Beach, SC, March, 1996.
M.Y.M. Chiang, Study of Strain Localization of Thin Adhesive Bonds, Martin Chiang
Inst. of Applied Mechanics, National Taiwan University, Taiwan, September, 1996.
Viscoelastic Measurements in Plasticizing Environments
G.B. McKenna, F. Horkay, and W.H. Han
Objectives
The objectives are to develop measurement methodologies for determination of the creep response of polymers, including rubber and glasses, in plasticizing environments such as H2O and CO2. The test methods need to have the capability of measuring the material response to both constant activity (concentration) and transient conditions. In addition, the methods must include moderate pressure and temperature excursion capabilities to 10 M Pa and 150 C.
Technical Description
Small molecules are known to plasticize polymers and reduce the glass transition temperature Tg . Associated with such a decrease in Tg it is generally observed that the modulus decreases and the viscoelastic relaxation response shifts to shorter times. While such behavior anticipates that the concepts of reduced time, similar to time temperature superposition, should describe the response of polymer glasses to changes in small molecule concentration, it has not been fully demonstrated. A test methodology is under development in which the thermodynamic activity of a small molecule vapor or gas, specifically moisture and CO2, can be controlled much as temperature can be. Then, the equivalence of changing the activity to changing the temperature can be investigated. Furthermore, because the change in Tg with concentration reduces the temperature difference from Tg in isothermal experiments, activity jumps should be treatable in the same manner as are temperature jumps. If this is true, then it becomes possible to use the reduced time concepts, developed in nonlinear viscoelasticity and time-temperature and time-aging time superposition, to describe the impact of moisture or CO2 on polymer behavior.
The experimental procedure requires the ability to perform isothermal activity (relative humidity or pressure)-jump experiments on thin polymer samples and to determine the volume, mass and viscoelastic property changes after such a jump. The questions to be asked are: does the property evolution that results obey reduced time principles? do the observed reduced time variables (shift factors) obey the same kinetics as would result from temperature jump experiments? and can the behavior be modeled using the reduced time framework developed for volume and aging responses of polymers?
Accomplishments
Outputs
Publications
F.Horkay, W.K.Waldron, Jr., and G.B. McKenna, The Effect of Molar Volume on the Swelling of Poly(Dimethyl Siloxane) in Fluorocarbon Fluids, Polymer, 1996, 36, 4525. (In Press).
F. Horkay and G.B. McKenna, Gels, In Physical Properties of Polymers Handbood, ed. by J. E. Mark, AIP Press, American Institute of Physics, Woodbury, New York, 1996, 379-400.
G.B. McKenna and J.M. Crissman, Temperature and Crosslinking Effects on the Swelling of Poly(isoprene) in Isopiestic Conditions, J. Polymer Science, Physics Ed. (In Press).
F. Horkay, W.H. Han, G. B. McKenna, A Comparison of Some Rubber Network Models with Stress-Strain Data on Dry and Swollen Polymer Rubbers, J. Mathematics and Mechanics of Materials (submitted).
Chiang, M.Y.M. and McKenna, G.B., Report on the NIST Workshop of Hygrothermal Effect on the Performance of Polymers and Polymeric Composites, NISTIR 5826, April, 1996.
Presentations
W.H. Han and G.B. McKenna, The Physical Aging Response of Epoxy in Relative Humidity Jump Experiments, 6th International Workshop on Moisture in Electronic Packaging, U.S. Department of Commerce, NIST, Gaithersburg, MD, October, 1996.
Organizing Activities
M.Y.M. Chiang and G.B. McKenna co-organized a working group meeting with researchers from industry that followed up on issues identified in the Workshop on "Hygrothermal Effects on the Performance of Polymers and Polymeric Composites," held in FY 1995.
Measurement of Craze Initiation and Growth in Polymers
Principal Investigators: G.B. McKenna, J.D. Barnes, P.A. O'Connell, G. Gusler and M. Delin
Objectives
The objectives are to develop methods to measure the structural changes in polymers prior to crazing using small angle x-ray scattering techniques and to investigate the time evolution of such changes in poly(carbonate) and styrene-acrylonitrile copolymer. Instrumentation and methods are devised to investigate the craze initiation and growth responses during physical aging.
Technical Description
It is well known that physical aging affects the viscoelastic response of polymers. However, there have been few studies of the impact of aging on failure and fewer on a common precursor to failure referred to as crazing. Methods are devised to study the impact of the aging response on crazing and to understand the accumulation of material changes that lead to the craze initiation event. An optical approach is being taken to study the initiation of crazes and the impact of aging on the craze initiation and growth. Although crazing is a macroscopic phenomenon in the sense that crazes are readily visible to the naked eye, the events that lead to crazing are not clear. Therefore, SAXS methods are developed to examine the ways in which microstructural damage accumulates in the polymer prior to the craze initiation. In particular, a stress relaxation method was devised to initiate crazing that is controlled in such a way as to be able to perform SAXS measurements of structural change as a function of deformation and time after application of the deformation.
External Collaborations
This work is being performed in collaboration with researchers from Chalmers Institute of Technology in Goteburg, Sweden.
Accomplishments
Outputs
Publications
G. M. Gusler and G.B. McKenna, The Effect of Physical Aging on the Crazing Behavior of Polystyrene, ACS Polymer Preprints,1995, 36(2), 63-64.
G.M. Gusler and G.B. McKenna, The Effect of Physical Aging on the Craze Response of Polystyrene and a Styrene-Acrylonitrile Copolymer, (1996) SPE ANTEC, 1537.
G.M. Gusler and G.B. McKenna, The Craze Initiation Response of A Polystyrene and a Styrene-Acrylonitrile Copolymer During Physical Aging, Polymer Engineering and Science (submitted).
Presentations
G.M. Gusler, D.M. Colucci, and G.B. McKenna, Impact of Physical Aging on Crazing in Polystyrene, American Institute of Chemical Engineers National Meeting, November 1995, Miami, FL.
G.M. Gusler and G.B. McKenna, Crazing in a Polystyrene and a Styrene-Acrylonitrile Copolymer: Does Physical Aging Have an Effect? Materials Research Society, Winter Meeting, December, 1995.
G.M. Gusler and G.B. McKenna, The Impact of Physical Aging on Craze initiation in Polystyrene and a Styrene-Acrylonitrile Copolymer, Division of High Polymer Physics, American Physical Society March Meeting, St. Louis, MO, March, 1996.
G.M. Gusler and G.B. McKenna, The Effect of Physical Aging on the Craze Response of Polystyrene and a Styrene-Acrylonitrile Copolymer, SPE ANTEC, Indianapolis, IN, May, 1996.