Model Of Polymer Degradation Research Articles

A model of polymer degradation and erosion for finite element analysis of bioresorbable implants

Finite element analysis is a powerful tool for the design of bioresorbable medical implants made of aliphatic polyesters such as bioresorbable vascular scaffolds. However polymer erosion has been traditionally modelled using empirical rules rather than differential equations. The rule-based models are difficult to implement in a finite element analysis. Consequently, these models have been limited to simple geometries such as plates or spheres. This paper presents a set of differential equations that govern the hydrolytic chain scission and bulk erosion of bioresorbable implants where polymer erosion is modelled using a differential equation instead of empirical rules. These differential equations can be conveniently solved using a commercial finite element package to calculate the molecular weight and mass loss as functions of time for bioresorbable implant made of aliphatic polyesters. A case study of Absorb bioresorbable vascular scaffolds (BVSs) is presented using data obtained from the literature, where 98 Absorb BVSs were implanted in 40 porcine coronary arteries. It is demonstrated that the finite element model can fit the data of both molecular weight and mass loss as functions of time to an accuracy of approximately 5%. The finite element model and the back-calculated model parameters can be used to design future implants that degrade in a controlled pattern with required mechanical performance.

Influence of the polymer degradation on enhanced oil recovery processes

• A novel 2-D chemical reservoir simulator is proposed using flux limiting functions. • A new polymer degradation model is used to calculate the aqueous phase properties . • The oil desaturation curve is modified to consider the viscoelastic effects. Polymer flooding is one of the most common and technically developed chemical Enhanced Oil Recovery (EOR) processes. Its main function is to increase the carrying phase’s (i.e., water or brine) viscosity in order to mobilize the remaining trapped oil. Many numerical simulators have been developed during the last 30 years considering the influence of the polymer molecules on the viscosity as well as on other physical parameters (e.g., diffusion, adsorption). Nevertheless, there are certain phenomena which were not previously considered, for instance, the interfacial effects of hydrophobically modified polymers. Furthermore, the degradation of the polymer molecules in a harsh environment such as the one found in porous media is well known. This causes a deterioration on the viscosifying properties, diminishing the efficiency of the method. It is important also to consider the effect of the polymer viscoelasticity on the microscopic sweeping efficiency, lowering the residual oil saturation , which has not been properly addressed. A new compositional 2D numerical simulator is presented for polymer flooding in a two-phase, three-component configuration, considering all these physical effects present in porous media and using a fully second-order accurate scheme coupled with total variation diminishing (TVD) functions. Results demonstrated that degradation cannot be considered negligible in any polymer EOR process, since it affected the viscoelastic and viscosifying properties, decreasing the sweeping efficiency at both micro- and macroscopic scales. This simulator will allow setting the desired designing properties for future polymers in relationship with the characteristics of the oil field to be exploited.

Biosynthesis of Oligodextrans Through Synergistic Catalysis of Dextransucrase and Dextranase in a Recycle Membrane Bioreactor

A continuous synthesis process for the production of oligodextrans from sucrose by using dextransucrase and dextranase in a recycle ultrafiltration membrane bioreactor was developed. During continuous oligodextrans synthesis, changes in weight-average molecular weight (Mw) and polydispersity index (D value) in the second feed tank (i) were monitored by high-performance gel permeation chromatography. Mw decreased with increasing reaction time, resulting in an increasingly homologous solution of low Mw oligodextrans (approximately 1 × 104 Da). The D value of the oligodextrans produced was nearly 1.0. In addition, the polymer degradation model well described the kinetics of Mw changes in oligodextrans. Results showed that recycle membrane bioreactor coupled with dextransucrase and dextranase is a simple and effective method that can be used to produce and improve the quality of oligodextrans.

Modeling of Biodegradable Polyesters With Applications to Coronary Stents

The interest in biodegradable polymers for clinical and biomedical engineering applications has seen a dramatic increase in the last 10 years. Recent innovations include bioresorbable polymeric stents (BPS), which are temporary vascular scaffolds designed to restore patency and provide short-term support to a blocked blood vessel, before becoming naturally resorbed over time. BPS offer possibilities to overcome the long-term complications often observed with the permanent metallic stents, well established in the treatment of coronary and peripheral artery disease. From the perspective of designing next generation BPS, the bulk degradation behavior of the polymer material adds considerable complications. Computational modeling offers an efficient framework to predict and provide understanding into the behavior of medical devices and implants. Current computational modeling techniques for the degradation of BPS are either phenomenologically or physically based. In this work, a physically based polymer degradation model is implemented into a number of different computational frameworks to investigate the degradation of a number of polymeric structures. A thermal analogy is presented to implement the degradation model into the commercially available finite-element code, abaqus/standard. This approach is then applied to the degradation of BPS, and the effects of material, boundary condition, and design on the degradation rates of the stents are examined. The results indicate that there is a notable difference in the molecular weight trends predicted for the different materials and boundary condition assumptions investigated, with autocatalysis emerging as a dominant mechanism controlling the degradation behavior. Insights into the scaffolding ability of the various BPS examined are then obtained using a suggested general relationship between Young's modulus and molecular weight.

Aging of polyamide 11. Part 3: Multiscale model predicting the mechanical properties after hydrolytic degradation

ABSTRACTA holistic general multiscale model of polymer degradation has been applied to predict the mechanical properties of polyamide 11 after the hydrolytic ageing. Results for elastic modulus, tensile strength, and embrittlement threshold have been compared with experimental aging in deoxygenated water at 120°C. For all studied properties the modeled trend is close to the experimental test results confirming hydrolysis induced chain scission and chemicrystalization as the two main mechanisms of property change. This suggests that the multiscale modeling methodology can provide a valuable alternative to accelerated aging tests. The model also indicated that the crystalline phase does play a role in the plastic deformation. Moreover, the mechanical equilibrium between effects of macromolecule degradation and an increased degree of crystallinity has been described. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42792.

Mathematical Solvability of a Caputo Fractional Polymer Degradation Model Using Further Generalized Functions

The continuous fission equation with derivative of fractional orderα, describing the polymer chain degradation, is solved explicitly. We prove that, whether the breakup rate depends on the size of the chain breaking up or not, the evolution of the polymer sizes distribution is governed by a combination of higher transcendental functions, namely, Mittag-Leffler function, the further generalizedG-function, and the Pochhammer polynomial. In particular, this shows the existence of an eigenproperty; that is, the system describing fractional polymer chain degradation contains replicated and partially replicated fractional poles, whose effects are given by these functions.

Employment of polymer degradation models in population balance equations describing flocculation with sonicated polymers

Flocculation process based on water soluble polymers as flocculating agents is still intensively researched nowadays, despite the introduction of these materials about fifty years ago. Dual polymer flocculation or application of new, more sophisticated macromolecules is proposed as a new approach. Simultaneously, more detailed mathematical models of flocculation are developed. The employment of polymer degradation model in population balance equations (PBE) is proposed in the paper. The suspension of chalk in distilled water was flocculated using three types of ultrasonically conditioned commercial flocculants. The experiments were performed using an apparatus for on-line determination of particle size distribution (PSD). The obtained results were compared with simulations using the Class Method. It was proved that the kinetic constant in the aggregation kernel can be described by a function similar to the one showing a change of mean molecular weight of polymer with sonication time.

Application of PSO Algorithm in Parameter Optimization of Biodegradable Medical Polymer Degradation Model

The particle swarm optimization (PSO) algorithm is a stochastic global optimization technique based on swarm intelligence. It possesses advantages such as being a simple principle, few parameters and easy to be realized. In this paper, an optimization model is established to solve the difficulty in selecting parameters and improve the simulation accuracy of the biodegradable medical polymer degradation model. When modeling, the particle swarm optimization (PSO) algorithm is proposed to solve the model and calculate undetermined parameters of the biodegradable medical polymer degradation equations. A comparative analysis of the calculation results is progressed. It shows that parameters determined by optimization model make the simulation results of degradation model more close to the experiment results. Using this method to solve the model is more accurate and efficiency than determining parameters artificially. It also shows that the particle swarm optimization algorithm used to optimize parameters have practical significance and application value.

Molecular weight and degree of acetylation of high-intensity ultrasonicated chitosan

The influence of high-intensity ultrasonication on the molecular weight and degree of acetylation of chitosan was investigated. High-molecular weight shrimp chitosan was purified by alkaline precipitation and dialysis from aqueous solution. A 1% (w/v) chitosan in 1% (v/v) aqueous acetic acid was sonicated for 0, 0.5, 1, 5, 15, and 30 min at 25 °C at power levels of 16.5, 28.0 and 35.2 W cm −2. Degree of acetylation was determined by high-pressure liquid chromatography with photodiode array at 210 nm, monitoring acetyl groups released after complete hydrolysis and deacetylation of samples. Average molecular weight of ultrasonicated chitosan was determined by measurements of intrinsic viscosity of samples. The degree of acetylation of purified chitosan was 21.5%. Results indicated that neither power level nor sonication time altered the degree of deacetylation of chitosan molecules. Intrinsic viscosity of samples decreased exponentially with increasing sonication time. Rates of intrinsic viscosity decrease increased linearly with ultrasonic intensity. Results were analyzed in terms of molecular weight decreases. The Schmid polymer degradation model was used to analyze the molecular weight decay as a function of sonication time. A first order relationship for the dependence of the reaction rate on the ultrasonic intensity was suggested. Results of this study indicate that high-intensity ultrasonication can be utilized to reduce molecular weight of chitosan while maintaining the degree of acetylation.

Modelling thermal degradation of polymers using single-step first-order kinetics

A theoretical framework for characterising single-step Arrhenius degradation kinetics in terms of a characteristic temperature and temperature range is developed. It is demonstrated that for the purposes of practical calculation, the reaction order may be assumed to be unity and also that a first-order approximation to an nth-order TG curve remains a good approximation over an order-of-magnitude variation in heating rate. This fact implies that when the pyrolysis of much larger samples of material is modelled, the error involved in using first-order kinetics is small. The equivalent first-order approximation is then applied to a global in-depth model of polymer degradation in order to predict mass loss rates in bench-scale experiments such as the cone calorimeter test. The mass loss rate curves obtained from the equivalent first-order approximation are found to compare well with the full nth-order model. Finally, an estimate of the average or steady mass loss rate is developed which fully accounts for the interaction between the degradation kinetics, the external heat flux, the heat losses and the latent heat of vaporisation.

A thermal model of polymer degradation during selective laser sintering of polymer coated ceramic powders

Reports that measurable amounts of polymer degradation occur during the fabrication of objects from polymer coated ceramic powders by selective laser sintering (SLS). Argues that because the binder is important in achieving strong green parts that can be handled with minimal breakage during post‐processing operations, it is essential to minimize the extent of binder losses. As the first step towards understanding the mechanisms of binder degradation, this paper presents a thermal model of the physical system, noting that the agreement between theory and experiment are good. The model is used to help determine the most influential parameters affecting binder losses during fabrication from polymer coated powders. Predicts that adjustments to laser beam diameter, laser scanning distance and gaseous environment will strongly affect polymer binder degradation during processing. Further predicts correctly that polymer degradation during SLS processing is not sensitive to the inherent degradation kinetics of the polymer.

A kinetic model of polymer degradation during extrusion

Investigations and numerical representation of chemical transformation mechanisms occurring during polymer processing have recently become a very important problem. In this paper a model put forward to describe the interrelation of mechanical degradation and thermal oxidation phenomena during PE extrusion. This model permits determination of changes in molecular weight, the amounts of reacted oxygen and the quantities of accumulated peroxide together with the amount of the inhibitor used during extrusion. These parameters have a significant affect on the physical and mechanical properties of the polymer and make it possible to assess its lifetime in finished products under operating conditions.

An explicit solution to a discrete fragmentation model

The discrete binary fragmentation equation is solved explicitly far a model where tnenetrateoibreakupoiaparlicie oishek, o,,equals(k - lj/(k+l),andthedaughter-ri~e distribution b,lx equals 2/(k-1). This system is closely related to a model of polymer degradation considered by Simha, in which ax = I and b,,* is as above. In the continuum limit, both of these models go over to a continuous fragmentation model in which all particles break with an equal rate, a(x) = I, and the daughter-size distribution is uniform, b(yl x) = Z/x, which is at the borderline of the shattering transition.