Evaluation of Polymer Relaxation Dynamics

Abstract

On the road towards developing truly self-healing polymer coatings, autonomous gap closure behavior is an important step. Stored entropic energy in a polymer could act as driving force for this closure. To make proper use of this concept it is important to develop characterization protocols able to relate entropy release to gap closure dynamics and ultimately to polymer architecture. Dynamic mechanical thermal analysis (DMA) and Laser Speckle Imaging (LSI) have previously been used in separate studies to quantify entropic energy change and measure local polymer dynamics, respectively. This work aims to develop an LSI-based protocol able to relate entropic induced length changes measured in DMA to polymer relaxation dynamics. To this purpose a set of non-healing epoxies with different molecular weight between crosslinks was tested both in DMA and LSI. A recently developed analysis protocol was used to extract, from DMA data, the entropy change at the glass transition region related to displacement (i.e. shape memory effect). With the help of a micro-tensile tester and a temperature control system, LSI allowed quantifying the local dynamics of the studied polymers as function of temperature and strain. The overall relaxation dynamics in stress relaxation tests measured with LSI were then compared to the DMA results. A relation between the strain relaxation dynamics in LSI, crosslinking density, entropy release and tan δ variation with temperature was found. This was done by comparing the frequency and intensity of local displacements measured with LSI to delta length and tan δ obtained with DMA. Furthermore, this research revealed an interesting discrepancy between local strain relaxation and stress relaxation times, as the local strain relaxation time extracted with LSI was shown to be longer than the overall stress relaxation time measured by the micro-tensile tester in the same test. The characterization and data analysis protocol explored here paves the way towards experimental methods that allow a more direct correlation between coating design parameters, such as crosslinking density and chain flexibility, and local entropy driven damage closure.