Structure and Mechanical Properties of Polymers

Using molecular mechanics and molecular dynamics methods structure, energetic and thermal behavior and mechanical properties of various polymers were studied.

isolated polymer chains: Polyethylene,polyfuran, polyparaphenylene, polythiophene, DNA strands, Polypeptides
dendrimers: PAMAM, Ph-O, dendritic box.
crystalline polymers: Polyethylene, oriented polyimides, polyesthers, PTFE, Nylons.
amorphous bulk polymers: Polyethylene, polycarbonate.

In molecular mechanics studies: the objective was to study the structure, energetics, and mechanical properties of bulk polymers. Volumetric and structural properties of polymers were obtained by optimizing internal energy(enthalpy) and structure at zero pressure and temperature.

In mechanical property determinations three different methods are used.

Constant stress minimization to obtain induced strain components
Constant strain minimization to obtain internal stress components
One point calculation: using second derivatives for Born terms.

In second derivative calculations, using harmonic theory, thermodynamic state functions were also computed.

Molecular dynamics studies, thermal behavior of the polymers were explored. Isolated polymers were investigated for conformational behavior. In molecular dynamics studies of mechanical property determination c-Polyethylene is used as a test example. This was the first time that the fluctuation expressions [3] for elastic constants are used to evaluate elastic constants of a polymer at elevated temperature.

References

T. Cagin, N. Karasawa, S. Dasgupta and W. A. Goddard, III, "Thermodynamic and Elastic Properties of Polyethylene at Elevated Temperatures," Computational Methods in Materials Science, pp 61-66, 1992.
T. Cagin, W. A. Goddard, and M. L. Ary, "Canonical Dynamics Simulation of Single Chain Polyethylene," Comp. Polym. Sci. 1, 241 (1991).
J. R. Ray, Phys. Rep. 8, 1, (1988) and references therein.
P.Q. Qian, T. Cagin and Z.M. Chen, "Characterization of Polyesther properties by Molecular Simulation Technique", Proceedings of American Chemical Society, pp 92-3; 1993.
C.F. Fan, T. Cagin, Z.M. Chen and K.A. Smith, "Molecular Modeling of Polycarbonate: Force Fields, Structure and Mechanical Properties," Macromol. 27, 2383 (1994);
P.Q. Qian, T. Cagin and Z.M. Chen, "Characterization of highly oriented polyimides using Molecular Simulations," Proceedings of American Chemical Society, pp 96-7; 1993.
C.F. Fan, T. Cagin, W. Shi, and K.A. Smith, "Local Chain Dynamics of a Model Polycarbonate Near Glass-Transition Temperature - A Molecular Dynamics Study," Macromol. Theo. and Simul. , 6, 83-102 (1997).
P. Miklis, T. Cagin, W. A. Goddard, III, Dynamics of Bengal Rose Encapsulated in the Meijer Dendrimer Box, J. Amer. Chem. Soc., in press.

Collaborators

Drs. C. F. Fan, P. Qian, Z. M. Chen, N. Karasawa, S. Dasgupta, M.L. Ary ;
Professors J.R. Ray and W. A. Goddard, III.

High performance polymers

Stiff Chain Aromatic Heterocyclic Polymers

Light weight, high strength fibers and films produced from stiff chain aromatic heterocyclic polymers are good candidates for use as structural materials in advanced technological applications. Some examples of this kind of polymers we studied are

ABPBO poly-2,5-benzoxazole

ABPBT poly-2,6-benzothiazole

ABPBI poly-2,5-benzobimidazole

PBT poly-paraphenylene-benzothiazole

PBO poly-paraphenylene-benzoxazole

PBI poly-paraphenylene-benzobimidazole

Molecular mechanics and molecular dynamics methods used in studying the structure, dynamics, thermodynamics and mechanical properties of these high performance polymers. Semi-empirical solid state quantum mechanics programs are used in studying the elctronic structures of the same polymers.

Structure of PBZT Crystal.

Mechanical Properties

We subjected these polymers moderate incremental tension and compression to compute the finite difference elastic properties. [1]
Second, Extreme tensions and compressions are applied to investigate the structural transformations. [2]
Hessians for single crystals were computed using Dreiding Force field to obtain elastic constants at a single point. [1]
Harmonic theory is used to study the structural and thermodynamic stability as a function of temperature. [1, 2]
Molecular dynamics simulations on the isolated chains and in the bulk state are performed to investigate the rigidity, thermal behavior and mechanical properties[3].

Electronic Properties

Electronic structures and band gaps of PBO, PBI, and PBT are also studied using semi-empirical quantum mechanical methods. [4]

References

T. Cagin, "Mechanical Response of High Performance Polymers," in Materials Theory and Modeling, pp 321-4, 1993.
T. Cagin and W. A. Goddard, III, unpublished.
B. L. Farmer, T. Cagin, D. S. Dudis, W. W. Adams, "Molecular Modeling of Rigid Rod Polymers," in Deformation, Yield and Fracture of Polymers, 1991.
T. Cagin and D. S. Dudis, unpublished.

Collaborators

Drs. W. W. Adams, D. S. Dudis, and Profs. B. L. Farmer and W. A. Goddard, III.

tahir@wag.caltech.edu