Structure⁻Properties Relations for Polyamide 6, Part 1: Influence of the Thermal History during Compression Moulding on Deformation and Failure Kinetics.

The deformation and failure kinetics of polyamide 6 samples prepared by several thermal histories were investigated by tests at different temperatures and relative humidities. PA6 samples were produced in quiescent condition and multiple cooling procedure. A characterization was performed to investigate the effect of the different thermal histories and the effect of hydration on both structures and glass transition temperature. The mechanical properties were investigated by tensile and creep tests at different temperatures and relative humidity. In order to describe the experimental results, the Ree⁻Eyring equation, modified with the "apparent temperature", was employed. In addition, the results of time-to-failure (creep tests) were described by the use of the "critical strain" concept. Eventually, a link between the Eyring theory and the structure evolution was made, i.e., a relation between the rate factors and the average lamellar thickness.

Structure-Properties Relations for Polyamide 6, Part 2: Influence of Processing Conditions during Injection Moulding on Deformation and Failure Kinetics.

The effect of processing conditions during injection on the structure formation and mechanical properties of injection molded polyamide 6 samples was investigated in detail. A large effect of the mold temperature on the crystallographic properties was observed. Also the the effect of pressure and shear flow was taken in to consideration and analysed. The yield and failure kinetics, including time-to-failure, were studied by performing tensile and creep tests at several test temperatures and relative humidities. As far as mechanical properties are concerned, a strong influence of temperature and relative humidity on the yield stress and time-to-failure was found. A semi-empirical model, able to describe yield and failure kinetics, was applied to the experimental results and related to the crystalline phase present in the sample. In agreement with findings in the literature it is observed that for high mold temperatures the sample morphology is more stable with respect to humidity and temperature than in case of low mold temperatures and this effects could be successfully captured by the model. The samples molded at low temperatures showed, during mechanical testing, a strong evolution of the crystallographic properties when exposed to high testing temperature and high relative humidity, i.e., an increase of crystallinity or a crystal phase transition. This makes a full description of the mechanical behavior rather complicated.

Deformation-Induced Phase Transitions in iPP Polymorphs.

This detailed study reveals the relation between structural evolution and the mechanical response of α -, ß - and γ -iPP. Uni-axial compression experiments, combined with in situ WAXD measurements, allowed for the identification of the evolution phenomena in terms of phase composition. Tensile experiments in combination with SAXS revealed orientation and voiding phenomena, as well as structural evolution in the thickness of the lamellae and amorphous layers. On the level of the crystallographic unit cell, the WAXD experiments provided insight into the early stages of deformation. Moreover, transitions in the crystal phases taking place in the larger deformation range and the orientation of crystal planes were monitored. At all stretching temperatures, the crystallinity decreases upon deformation, and depending on the temperature, different new structures are formed. Stretching at low temperatures leads to crystal destruction and the formation of the oriented mesophase, independent of the initial polymorph. At high temperatures, above T α c , all polymorphs transform into oriented α -iPP. Small quantities of the initial structures remain present in the material. The compression experiments, where localization phenomena are excluded, show that these transformations take place at similar strains for all polymorphs. For the post yield response, the strain hardening modulus is decisive for the mechanical behavior, as well as for the orientation of lamellae and the evolution of void fraction and dimensions. ß -iPP shows by far the most intense voiding in the entire experimental temperature range. The macroscopic localization behavior and strain at which the transition from disk-like void shapes, oriented with the normal in tensile direction, into fibrillar structures takes place is directly correlated with the strain hardening modulus.

Time-dependent failure of amorphous poly-D,L-lactide: influence of molecular weight.

The specific time-dependent deformation response of amorphous poly(lactic acid) (PLA) is known to lead to rapid failure of these materials in load-bearing situations. We have investigated this phenomenon in uniaxial compression on P(L)DLLA samples with various molecular weights. The experiments revealed a strong dependence of the yield stress on the applied strain rate. Lower molecular weights showed identical deformation kinetics as higher molecular weights, albeit at lower stress values. This dependence on molecular weight was incorporated into an Eyring-equation by introducing mobility through a virtual temperature that is shifted by the deviation of the T(g) from T(g,∞). Stress-dependent lifetime of polymer constructs was described by the use of this modified Eyring-equation, combined with a critical plastic strain. This model proves useful in predicting the molecular weight dependence of the time to failure, although it slightly overestimates life time at low stress levels for a material with very low molecular weight. The versatility of the model is demonstrated on e-beam sterilized PLDLLA, where the resulting reduction in molecular weight induces a substantial decrease in lifetime. A single T(g) measurement provides sufficient information to predict the decrease in lifetime.

Time-dependent failure of amorphous polylactides in static loading conditions.

Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this study we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

Time-dependent mechanical strength of 70/30 Poly(L, DL-lactide): shedding light on the premature failure of degradable spinal cages.

STUDY DESIGN: In vitro studies on the mechanical strength of 70/30 poly(l,dl-lactic acid) (70/30 PLDLLA) cages. OBJECTIVE: To evaluate the effect of loading rate, humidity, temperature, and continuous static loading on the strength of 70/30 PLDLLA, to elucidate the mechanism of premature failure of degradable spinal cages observed in earlier studies. SUMMARY OF BACKGROUND DATA: Degradable 70/30 PLDLLA cages have been designed to withstand mechanical loads in a goat lumbar spine for at least 6 months. Yet mechanical failure was observed after only 3 months in vivo. We hypothesize that this observation can be related to the time-dependent nature of the polymer. METHODS: Degradable 70/30 PLDLLA cages were loaded to failure at loading rates between 10 and 10 mm/s under standard loading conditions (in air at room temperature: +/-23 degrees C). The experiments were also done at body temperature (37 degrees C) and under wet conditions. Furthermore, we determined the time-to-failure for 70/30 PLDLLA cages subjected to loads well below their instantaneous mechanical strength. RESULTS: The mechanical strength of 70/30 PLDLLA cages was lower for lower loading rates, higher temperature, and higher humidity. The cages already failed within less than 5 minutes when statically loaded at 75% of their strength, and within 1 day when loaded at about 50% of their strength. Extrapolation predicts cage failure at 3 months when loaded at 25% of their strength. CONCLUSION: Premature failure of 70/30 PLDLLA cages, as observed in vivo in earlier studies, is owing to mechanical loading and the time-dependent mechanical properties of the material. The standards for mechanical testing of implants made of strongly time-dependent materials like polylactide should be reconsidered.

Mechanical behaviour of a new acrylic radiopaque iodine-containing bone cement.

In total hip replacement, fixation of a prosthesis is in most cases obtained by the application of methacrylic bone cements. Most of the commercially available bone cements contain barium sulphate or zirconium dioxide as radiopacifier. As is shown in the literature, the presence of these inorganic particles can be unfavourable in terms of mechanical and biological properties. Here, we describe a new type of bone cement, where X-ray contrast is obtained via the introduction of an iodine-containing methacrylate copolymer; a copolymer of methylmethacrylate and 2-[4-iodobenzoyl]-oxo-ethylmethacrylate (4-IEMA) is added to the powder component of the cement. The properties of the new I-containing bone cement (I-cement) are compared to those of a commercially available bone cement, with barium sulphate as radiopacifier (B-cement). The composition of the I-cement is adjusted such that similar handling properties and radiopacity as for the commercial cement are obtained. In view of the mechanical properties, it can be stated that the intrinsic mechanical behaviour of the I-cement, as revealed from compression tests, is superior to that of B-cement. Concerning the fatigue behaviour it can be concluded that, though B-cement has a slightly higher fatigue crack propagation resistance than I-cement, the fatigue life of vacuum-mixed I-cement is significantly better than that of B-cement. This is explained by the presence of BaSO4 clumps in the commercial cement; these act as crack initiation sites. The mechanical properties (especially fatigue resistance) of the new I-cement warrant its further development toward clinical application.