Polymer Stability & Degradation
Our research interests have included the investigation into mechanisms of polymer degradation both reversible and irreversible. Reversible processes are often referred to as physical aging or creep and stress relaxation. These processes are thermodynamically reversible given the appropriate processing conditions. The non-reversible processes are those that involve chain cleavage that ultimately can result in both chain molar mass reduction as well as molar mass increase, i.e. chain cross-linking. The particular process and the resultant material characteristics are a function of many factors that include polymer chemistry, synthetic method, thermal history, environment, time, temperature, and pressure. Below are a few examples of some our work in this area.
Polymer stability as a function of processing
Physical aging is a process that affects the amorphous phase of the polymer only. It is dependent upon time and temperature below the Tg for a given polymer. Our work focused on a thermoplastic poly(aryelene ether bisphenol A imide). We examined the changes in the specific volume of the polymer after aging at Tg-10 C for times upto 104seconds. The most significant change is in the overall elongation to failure, reduction from ca. 160% to ca. 16%. These changes are completely reversible. The diffusion rate of the polymer chain dictates the rate and magnitude of the aging effects and reversibility.
Another processing based issue is associated with fiber processing. As an example, ultra high modulus polyethylene fibers are manufactured by a gel spinning method in which the ultra high molar mass polyethylene are drawn to extreme elongations. The fibers exhibit interesting morphologies that are attributed to polymer microfibrils which are bundles of the polyethylene chains. It is also common to observe bundle bunching at regular intervals on some of these fibers. Sometimes there are cracks that form perpendicular to the long axis of the fibers. The cracks are actual microfibril fractures. The density of these cracks control the overall mechanical strength and stability of the fibers.
Polymer stability as a function of chain architecture
The objective of this work was to study fracture behavior of an amorphous thermoplastic polymer. Theory indicates that after the ultimate mechanical stress is achieved at the critical molar mass for entanglements, end groups have a neglible effect on the chain diffusion kinetics. We selected a series of polycarbonates in which the end group chemistry and thus molar mass were varied systematically to alter the chain reputation behavior. We examined the influence on the fracture mechanics and correlated the results to the molar mass of the end groups. The optical images below represent the mesoscale changes that arise during fracture of paracumylphenol endcapped poly(bisphenol A carbonate) structure at a molar mass of 26 and 43 kg/mol. It is also evident in the stress-strain response that the endgroups govern the overall mechanical behavior of these polymers.
Figure 1: Room temperature stress-strain behavior of PEI after aging at 195 C for 10,000 min, and then equilibrated at Tg + 25 C. Crosshead speed = 0.10 in/min.
Figure 2: FIber degradation – Ultra high modulus polyethylene: (top) stress cracks; (bottom) xylene stripped
Figure 3: Fracture surface of polycarbonate with paracumylphenol end groups (top) Mw=26.4 kg/mole and (bottom) Mw=26.4 kg/mole in the quenched states. (Optical micrograph at 50x)
Figure 4: Room temperature stress-elongation plot of polycarbonate with paracumylphenol end groups in the quenched state. Crosshead speed is 0.5 inch/min.
Figure 5: Bioactive fibers: (left) individual fiber, mag bar = 100 microns0; (right) higher mag = 5 micron