Simultaneous time-of-flight secondary ion MS quantitative analysis of drug surface concentration and polymer degradation kinetics in biodegradable poly(L-lactic acid) blends

This paper reports a new quantitative method of analyzing both the earliest stage of degradation of a polymer and the surface concentration of an additive using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The static SIMS spectra of triphenylamine (Ph₃N)/poly(L-lactic acid) (PLLA) (20:80 wt %) blend matrixes hydrolyzed in buffered conditions within a short-term (<48 h) period are simultaneously analyzed in the low-mass range for the surface accumulation profile of Ph₃N and in the high-mass range to determine the hydrolytic degradation kinetics of PLLA, respectively. The results provide new insight in evaluating the surface concentration of Ph₃N (pK_b ≈ 0) from the blends to see how it relates to the reactions (hydrolytic PLLA degradation) occurring in the surface region in the initial induction period over which negligible loss of polymer weight is observed. The relative PLLA surface degradation at pH 10.0 is ∼2 times faster than that at pH 7.4. The relative extent of increase in Ph₃N surface concentration assayed in pH 10.0 buffer system is 9 times greater than that at pH 7.4. The initial rapid increase in surface concentration of Ph₃N is related to but not singularly dependent on the rate of PLLA degradation at the surface of blend matrixes.