Pseudo-Component Modeling of Stress Relaxation Behavior of Self-Assembled Polycaprolactone Matrices

The objective of this work was to study the effect of strain rate to the stress relaxation behavior of a novel procedure for self-assembled polycaprolactone (PCL) films and propose a pseudo-component model to describe the stress-strain behavior of the material in multiple strain stages. The ramp-and-hold tests were carried out in wet conditions (phosphate-buffered saline, PBS) at 37°C using three different strain rates (0.6%/s, 1.0%/s, and 3.0%/s). The results showed that the effect of strain rate, at least in the range used in the experiment (0%/s -3%/s) was insignificant on stress relaxation behavior.

Composite models with three pseudo-components are chosen with six combinations from four pseudo-components – hyperelastic spring, spring-and-dashpot, retain, and reform. The pseudo-component called “reform” describes an internal structure that reorganizes to its original structure when held in an elongated state for a period of time, and “retain” describes an internal structure that holds its oriented structure while the internal stress is relieved. Both reform and retain account for necking in cross-section reduction due to strain, but the classic spring-and-dashpot model does not. These models were developed and tested for the stress response to five sequential periods of strain and hold. The differential equations representing stress- time relation of the pseudo-components were solved numerically by a semi-implicit finite-difference method using Visual basic programming.

Most of pseudo-components combinations are able to model the multi-strain testing with the value of sum of squares (SSD) lower than 15, but the combination of one hyperelastic and two spring-and-dashpot pseudo-components still gave the best fit. The combination of reform and retain gave a good fit with the SSD of 0.8 for self-assembled PCL film but this pseudo-components combination did not fit as well as the combination of one hyperelastic and two retain pseudo-components or the ones that has the spring-and-dashpot pseudo-component for the traditional chloroform-casted PCL film (SSD = 16.6). In summary, the proposed pseudo-component model approach shows potential for describing and understanding stress relaxation behavior of material.