NERSC Initiative for Scientific Exploration (NISE) 2011 Awards

Self-Healing of Polymer Films

Gary Grest, Sandia National Laboratories

Associated NERSC Project: Drying and Self-Assembly of Nanoparticle Suspensions (m1151)

Polymers play an important role in a wide range of applications from coatings and packaging to lithography and structural materials. After time due to either mechanical stress or aging, cracks can develop in the polymer leading to mechanical failure of the structure. The goal of the proposed project is to examine how cracks in a polymer self-heal when heat is applied to raise the temperature about the glass temperature. The proposed simulations will correlate the evolution of the polymer interface to the mechanical strength of the crack. Correlations between interface structure, entanglements, and mechanical properties will then be determined. These will provide fundamental understanding of the factors that control the fracture and shear strength of polymer interfaces and develop models for predicting performance.

The strength of polymeric materials has a direct effect on their performance in a wide range of polymer components. After time due to either mechanical stress or aging, cracks can develop in the polymer leading to mechanical failure of the structure. The goal of the proposed project is to examine how cracks in a polymer self-heal when heat is applied to raise the temperature about the glass temperature. The proposed simulations will provide direct insight into the connection between the microscopic structure of polymer interfaces and their macroscopic mechanical properties. Complete information about the state of the polymer interface during self-healing, including the entanglements, density profile and distribution of short chains and chain ends will be available. Then mechanical properties including failure stress, fracture energy, and friction will be calculated for a range of interdiffusion times and compared to the mechanical properties of the bulk polymer. A parametric study will allow the trends with interdiffusion time and polymer stiffness to be determined. The impact of these factors on failure stress under tensile and shear loading will be evaluated. The results will test existing models, guide the development of new models, and aid in the interpretation of experiments and the improvement of the self-healing of polymeric materials.

The goal of this project is to examine how the shear and tensile strength of self-healing polymer interface depends on the degree of interdiffusion across the interface. The mechanism of this increase will be determined by combining microscopic analysis of structure with the evaluation of macroscopic mechanical properties. The macroscopic properties of bulk polymers are strongly influenced by the topological entanglements between polymer chains. The role of entanglements near interfaces in determining the increase of the interfacial strength with interdiffusion time will be determined. One question will be how entanglements between polymers from opposite sides of the crack affect strength, particularly in the presence of short chains which occur due to chain scission at the interface. Is entanglement density alone sufficient to confer strength, or do the entanglements need to be between polymers from the two sides of the joint? The first entanglements are likely to form near chain ends. Do they strengthen the interface or are they easy to pull out? There is a substantial literature on thermal welding and the related process of crack-healing. Experiments exhibit a power law dependence of fracture energy on contact or welding time that can be explained by models based on the rate of interdiffusion. However there are several microscopic models for how interdiffusion strengthens the interface based on the number of chains crossing the interface, the interpenetration depth, “effective crossings” by some minimum depth, and an effective entanglement density. The mechanism must involve entanglements in some manner since they are critical to the bulk fracture energy, but the details remain unknown.

In this initial study of self-healing of a crack, we will follow the interdiffusion of polymers across a interface produced by cutting all bonds along a polymer chain which cross the mid-plane of the polymer. Due to the slow time scales we will model the polymer using a well established bead-spring model. By varying the bending stiffness between monomers along the polymer chain, we can vary the entanglement length N_e. In the proposed study we will model long polymer chains of length N=500 beads so that the polymer is well entangled. We will study a full flexible chain which has an entanglement length N_e ~ 85 and a more rigid polymer which has an entanglement length N_e~28 so that we can compare the effect of self-healing on polymer stiffness. Since the average end to end distance of these two systems are approximately 28 and 36, respectively, the system size has to be quite large so that the simulation cell is much larger than the end to end distance in all three spatial directions. Thus the two simulations proposed here are for 9600 chains of length 500 or 4.8 million monomers. Based on the known diffusion times for these two systems in the bulk, the simulations will be need to run for between 300 to 500 million time steps for the crack to heal. At different stages of the self-healing, we will quench the temperature to below the glass transition temperature and determine the mechanical strength of the interface and compare to that of the bulk. We will also do a primitive path analysis to determine the degree of entanglement between the two sides of the crack.