Bulk Compliance of Polymers
Experiments and Molecular Dynamics Simulations
Sandeep Sane, Wolfgang Knauss*, Tahir Cagin** and William Goddard III**
*Graduate Aeronautical Laboratories, ** Materials and Process Simulation Center
California Institute of Technology
Pasadena, CA 91126
One of the most often overlooked properties in the mechanical characterization of viscoelastic polymers is the time-or frequency-dependent bulk response. For isotropic material descriptions in the linear theory of viscoelasticity, two material characteristics are needed. One describes the time-dependence of the shear behavior, the other the bulk (volumetric) response. Thus usual engineering analyses are performed only with the shear description, constituting only half of the material information. The reason for this shortcoming is the great difficulty associated with the experimental methods available for this purpose. To accomplish this goal, one needs to assess exceedingly small volume variations with a high degree of accuracy. Experimental set up was developed for carrying out such precise measurements by confining a polymer sample in a small cavity and applying cyclic pressure using piezoelectric transducers. Tests were conducted on Poly (Methyl Methacrylate) or PMMA to measure the bulk compliance over a wide range of temperatures and frequencies.
One potential means of eventually circumventing laboratory measurements is to perform molecular dynamics simulations. This approach becomes viable only after its shortcomings and pitfalls have been explored and removed. On the other hand, it appears that bulk property determinations should be one of the more readily successful computations in the physical property determinations of polymers by computational means. Consequently, we are attempting to compare molecularly based computations with laboratory measurements. Models of amorphous PMMA were built and constant pressure-temperature dynamics were performed to compute the bulk compliance as a function of temperature. The results were then compared with the experimental measurements. Though simulations were able to capture the essential features of the bulk behavior of PMMA, further refinements are required to model the complete description.
Experimental Results on Poly (Methyl Methacrylate) or PMMA
Molecular Dynamics Simulations of PMMA
Figure shows an example of the PMMA sample. Five such samples were built with different densities and random dihedral distribution. Each sample contains 3 polymer chains with 50 monomers (2256 atoms in total). Structures were optimized through repeated cycles of energy minimization and temperature annealing.
Samples were subjected to Constant Pressure and Temperature Dynamics (NPT dynamics) to determine the isothermal bulk compressibility of PMMA. Multiple samples were used.
Comparison between Experimental Results and MD Simulations