Tap Tom Engels

Time-dependent failure in load-bearing polymers: a potential hazard in structural applications of polylactides

With their excellent biocompatibility and relatively high mechanical strength, polylactides are attractive candidates for application in load-bearing, resorbable implants. Pre-clinical studies provided a proof of principle for polylactide cages as temporary constructs to facilitate spinal fusion, and several cages already made it to the market. However, also failures have been reported: clinical studies reported considerable amounts of subsidence with lumbar spinal fusion cages, and in an in vivo goat study, polylactide spinal cages failed after only three months of implantation, although mechanical testing had predicted sufficient strength for at least eight months. The failures appear to be related to the long-term performance of polylactides under static loading conditions, a phenomenon which is common to all glassy polymers and finds its origin in stress-activated molecular mobility leading to plastic flow. This paper reviews the mechanical properties and deformation kinetics of amorphous polylactides. Compression tests were performed with various strain rates, and static stress experiments were done to determine time-to failure. Pure PLLA appeared to have a higher yield strength than its co-polymers with d-lactide, but the kinetic behaviour of the polymers was the same: an excellent short-term strength at higher loading rates, but lifetime under static stress is rather poor. As spinal implants need to maintain mechanical integrity for a period of at least six months, this has serious implications for the clinical application of amorphous polylactides in load bearing situations. It is recommended that standards for mechanical testing of implants made of polymers be revised in order to consider this typical time-dependent behaviour.

Processing-induced properties in glassy polymers : development of yield stress in polycarbonate

Abstract A new method is employed to directly predict the development of yield stress distributions in injection molded products of glassy polymers. The approach is based on the results of a study on the temperature dependence of the evolution of yield stress during annealing of polycarbonate below Tg. In combination with the process-related thermal history, derived from numerical simulations of the injection molding process, an estimate of the yield stress distribution throughout a product can be obtained. Computed yield stresses of injection molded plates prove to be in excellent agreement with the experimental values, including their dependence on mold temperature and plate thickness.

Numerical simulation of flat-tip micro-indentation of glassy polymers: influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments.

The effect of physical aging on the embrittlement of steam-sterilized polycarbonate

Polycarbonate is known to suffer from dramatic reductions in ductility upon exposure to hot, humid environments, such as during steam sterilization. Two phenomena have been proposed to be the main causes of this embrittlement: hydrolysis and microcavity formation. The present study focuses on a third phenomenon, whose contribution to the embrittlement has until now been considered insignificant: (physical) aging. By studying the influence of steam sterilization on the tensile deformation behavior of polycarbonate, it is shown that aging actually is one of the dominant factors in the embrittlement.

Predicting the Yield Stress of Polymer Glasses Directly from Processing Conditions: Application to Miscible Systems

Abstract A previously developed model which predicts the yield stress of a polymer glass directly from processing conditions is applied to a system of miscible polymers. The selected system consists of a blend of polycarbonate with polyester and three blend compositions of increasing weight percentages polyester are investigated with respect to their aging kinetics. Based on these kinetics, the yield stress as it results form the thermal history experienced during processing is predicted and found to be in good agreement with experimental results. The parameters governing the evolution of the yield stress are shown to follow the rule of mixtures, enabling the prediction of the yield stress of any blend composition.

Mechanical characterization of glassy polymers : quantitative prediction of their short- and long-term responses

In this chapter the relevant phenomena that dominate the mechanical performance of polymer glasses are discussed. Mechanical properties as measured in short-term experiments are directly linked to their long-term performance. The molecular processes that are of influence are investigated. The chapter is divided into two sections: The first deals with the phenomenology of the deformation, and the second addresses the constitutive modeling and precise characterization of polymers, needed to model their mechanical response.

Photonic shape memory polymer with stable multiple colors

A photonic shape memory polymer film that shows large color response (∼155 nm) in a wide temperature range has been fabricated from a semi-interpenetrating network of a cholesteric polymer and poly(benzyl acrylate). The large color response is achieved by mechanical embossing of the photonic film above its broad glass transition temperature. The embossed film, as it recovers to its original shape on heating through the broad thermal transition, exhibits multiple structural colors ranging from blue to orange. The relaxation behavior of the embossed film can be fully described using a Kelvin–Voigt model, which reveals that the influence of temperature on the generation of colors is much stronger than that of time, thereby producing stable multiple colors.