Many key components of implantable medical devices are made from polymeric materials. The functions of these materials include structural support, electrical insulation, protection of other materials from the environment of the body, and biocompatibility, as well as other things such as delivery of a therapeutic drug. In such roles, the stability and integrity of the polymer, over what can be a very long period of time, is very important. For most of these functions, stability over time is desired, but in other cases, the opposite–the degradation and disappearance of the polymer over time is required. In either case, it is important to understand both the chemistry that can lead to the degradation of polymers as well as the kinetics that controls these reactions. Hydrolysis and oxidation are the two classes of reactions that lead to the breaking down of polymers. Both are discussed in detail in the context of the environmental factors that impact the utility of various polymers for medical device applications. Understanding the chemistry and kinetics allows prediction of stability as well as explanations for observations such as porosity and the unexpected behavior of polymeric composite materials in some situations. In the last part, physical degradation such interfacial delamination in composites is discussed.

We studied the hydrolysis kinetics of amorphous polylactide. It was found the hydrolysis rate had a slow-to-fast transition at a certain molecular weight (Mn). This transition was not correlated with the mass loss and water uptake of samples, nor the pH values of testing media. We speculated that this transition was due to the slow diffusion of polymer chain ends. The chain ends did not significantly promote the hydrolysis of samples until their concentrations (approximately 1/Mn) reached a critical value. The degradation tests were also conducted over a temperature range from 37 to 90 degrees C. A time-temperature equivalent relationship of degradation processes was established and a master curve spanning a time range equivalent to 3-5 years at 37 degrees C was constructed. This master curve can be used to predict polymer degradation processes based on accelerated tests. The functional time and disappearance time of degradable polymers were also discussed.

Block copolymer reduces particle size in immiscible polymer-polymer blends by suppressing droplet coalescence and by aiding droplet breakup through reduced interfacial tension. In this paper we separated coalescence from breakup and studied suppression of coalescence by block copolymers in a model blend composed of polystyrene (PS), high-density polyethylene (HDPE), and polystyrenepolyethylene (PS-PE) block copolymer. Coalescence was examined by monitoring particle size change vs shear strain while varying the shear rate, block copolymer concentration, molecular weight, and symmetry (coil size ratio of blocks). Even 0.5% PS-PE block copolymer significantly suppressed coalescence of HDPE droplets in a PS matrix. Assuming all of the PS-PE molecules were at the interface, the minimum concentration of block copolymers required to prevent coalescence under shear was found to be about 0.2 chain/nm 2 at a coalescence shear rate of 0.1 s -1 for PS-PE with a molecular weight of 20-20 kg/mol. We found that this minimum concentration decreased with shear rate and with increasing molecular weight of PS-PE. We also found that, with the total molecular weight being the same, a PS-PE with a larger PS block suppressed HDPE particle coalescence more efficiently than one with a smaller block. These results indicate that steric repulsive interactions due to the presence of block copolymer at the interfaces are more important than those due to interfacial tension gradients.

The location of poly(styrene-b-poly(methyl methacrylate)) (PS-b-PMMA) block copolymers in PMMA/poly(cyclohexyl methacrylate) (PCHMA) melt mixed blends was determined. Among all these components, only the PS portion of the block copolymer can be stained by ruthenium tetroxide and appears dark in transmission electron micrographs. Then the locations of block copolymer in the blends, i.e., interfaces and micelles, can be easily identified. Morphology and the aggregation of the PS-b-PMMA were studied as a function of molecular weight and molecular weight fraction. At low molecular weight PS-b-PMMA, a PMMA/PS-b-PMMA macrophase formed. At higher M n, small (<1 μm) minor phase (PMMA) drops coated with copolymer formed, but these drops also contained micelles even at low PS-b-PMMA concentration. Increasing the molecular weight of the PMMA first caused the drop size to increase, and then the copolymer micelles to relocated from the PMMA to the PCHMA phase. Increasing block copolymer concentration caused PMMA drop size to decrease roughly in proportion to interfacial coverage. The results are discussed qualitatively in terms of Leibler's wet−dry brush theory.

The process by which polymeric materials hydrolyze and disappear into their environments is often called erosion. Two types of erosion have been defined according to how the hydrolysis takes place. If hydrolysis occurs throughout the entire specimen at the same time, it is called bulk erosion. If the hydrolysis is mainly confined to a region near the surface of the specimen and the surface continuously degrades by moving inward, it is termed surface erosion. In this article, a kinetic relationship for bulk erosion is developed. This relationship provides a method for estimating the hydrolysis kinetic constants for bulk‐eroding polymers. This same relationship is also applicable to surface erosion at a microscopic level. Through its combination with a diffusion–reaction equation and the provision of moving boundary conditions, an analytical solution to the steady‐state surface‐erosion problem is obtained. The erosion rate, erosion front width, and induction time can all be expressed as simple functions of the rate of polymer bond hydrolysis, water diffusivity, and solubility, plus other parameters that can be experimentally determined. The erosion front width is the product of the induction time and the erosion rate. The ratio of the erosion front width to the polymer specimen thickness is a parameter that determines whether the specimen undergoes surface or bulk erosion. Theoretical results are compared with experimental observations from the literature, and agreement is found. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 383–397, 2005