Polymer Degradation
Cleavage of covalent bonds within the primary macromolecular backbone of a polymer reduces overall molecular mass. This destructive chemical process, designated as chain scission, undermines mechanical integrity by lowering tensile strength, fracture toughness and elasticity in sensor packaging resins and structural polymers. The phenomenon occurs when thermal energy, ultraviolet radiation, mechanical shear or oxidising chemical species dissociate main-chain carbon-carbon or carbon-oxygen bonds.
Degradation models track the reaction rate via molecular weight distribution analysis, and the mechanism terminates when radical intermediates stabilise through termination reactions, disproportionation or crosslink formation.
Molecular Pathway
Homolytic or heterolytic cleavage of backbone bonds generates reactive radical fragments along the polymer skeleton. When oxygen infiltrates the polymer matrix, these carbon-centered radicals combine with dissolved molecular oxygen, generating peroxy radicals. Hydrogen abstraction from adjacent polymer chains follows, forming unstable hydroperoxides that dissociate into further radical species and amplify fragmentation.
In fluoroelastomers and epoxy encapsulants deployed in industrial sensing heads, scission decreases network crosslink density, shifting the glass transition temperature downward. The resulting loss of molecular entanglements transforms ductile plastics into brittle matrices susceptible to microcracking under modest cyclic strains.
Analytical Characterisation
Metrology laboratories measure bond breakage through gel permeation chromatography, tracking changes in number-average and weight-average molecular weights. Decreases in zero-shear viscosity during rheological sweeps provide direct confirmation of shortening chain lengths. Gel content analysis separates soluble fragments from insoluble network fractions, isolating scission events from competing crosslinking reactions.
Fourier transform infrared spectroscopy complements molecular weight metrics by identifying terminal carbonyl, hydroxyl or carboxylic acid functional groups formed at cleaved chain ends.
Operational Penalty
Protective potting compounds experience functional breakdown when continuous bond cleavage alters sealing compliance. Sensor diaphragms encapsulated with degraded elastomers suffer zero-drift excursions as seal stress relaxation alters internal mechanical preloads. In optical sensing elements, low-molecular-weight degradation fragments migrate toward surfaces, causing optical hazing and transmission loss.
Premature seal rupture in downhole or chemical immersion probes traces directly to accumulated chain degradation within elastomeric O-rings.
