The Influence of Processing Conditions on the Thermo-physical Properties and Morphology of Polycarbonate / Poly (butylene terephthalate) Blends

The objective of this work is to determine the effect of four process variables on the properties of blends composed of bisphenol-A polycarbonate (PC) and poly (butylene terephthalate) (PBT) polymers which are compounded using a large scale commercial extruder. The four variables studied are blend composition, specific energy consumption, residence time and shear rate. The last three factors were varied using the extruder screw speed and feed rate. The PC/PBT blends, commercially known as XENOY, were compounded using a WP ZSK 58 mm co-rotating twin screw extruder at the facility of SABIC Innovative Plastics in Cobourg Ontario. The extruder was instrumented to measure online the die pressure, specific energy consumption and blend temperature. The blends were characterized using differential scanning calorimetry, (DSC), scanning electron microscopy, (SEM), gel permeation chromatography, (GPC), and melt volume flow rate, (MVR). After processing, the blend properties determined were melting temperature, glass transition temperature, crystallinity, amorphous phase weight fraction, amorphous phase composition, phase morphology, PBT-rich-phase size, blend molecular weight distribution, and MVR. Using principles available in the literature, a linear regression model was developed to relate the process variables with the online measured properties and output blend properties. Fitting this model allowed the relative importance of each process variable to be estimated for each property. An attempt was also made to identify the general type of PC/PBT blend studied and how it compares with published PC/PBT blend data. It was found that the blends studied were well stabilized since there was no evidence of significant co-polymer formation during processing. Small decreases in molecular weight were attributed to mechanical degradation. Blending increased the crystallization and melting temperatures, as well as blend crystallinity. No practically significant difference in melting temperatures was observed between the different processing conditions. Analysis of glass transitions indicated that the blend components were partially miscible. The amorphous phase compositions were unaffected by blend composition or processing; however, the weight fraction PC-rich-phase present in the blend was strongly influenced by the screw speed. The phase structure of as-extruded blends could not be resolved using the SEM. Therefore, the blends were annealed to coarsen the phases. After annealing, a continuous PC-rich-phase and a discrete PBT-rich-phase were observed. The PBT phase size increased with increasing PBT content. No other statistically significant effects on phase size were observed but this is not conclusive due to the large scatter in the measurements. MVR was primarily influenced by blend composition and specific energy consumption, with the effects of composition being dominant. Further study using higher imaging resolution is required if the phase structures of as received blend pellets are to be characterized. Contrary to current practice, it is recommended that the Utracki-Jukes equation be used rather than the Fox equation for determining amorphous phase composition from glass transition data in PC/PBT blends

Development of an Injectable Hybrid-Hydrogel Using Oxidized-Alginate and N-Succinyl-Chitosan

A new oxidised-alginate / N-succinyl-chitosan hydrogel system with potential biomedical applications has been explored for precursor solution viscosities within the injectable range (<~0.2 Pa·s). When fully developed, its advantages may include: 1) good mechanical properties due to covalent crosslinking; 2) increased degrees-of-freedom to tailor properties and unique stress strain response due to the hybrid structure; 3) suitability for cell encapsulation / use as an injectable gel due to biocompatibility / mild formation conditions; and 4) excellent adoptability due to readily available and low cost raw ingredients. The current work is focused on two initial studies of the system: 1) development of hybrid hydrogel compositions to meet target viscosity and stiffness ranges that are appropriate for common hydrogel applications, and 2) development and validation of models to predict hydrogel swelling and stiffness behaviour. As gel systems become more complex, predictive tools are crucial for more focused development work so that end users can tailor gel systems to their precise requirements. In this work, we have undertaken the development and/or advancement of three predictive models: 1) advancing current swelling models to apply to hybrid systems containing polyampholytes; 2) refining membrane osmometry models to handle polyampholytes and to incorporate modern corrections for non-ideal solutions; and 3) proposing a new stiffness model for hybrid gel systems. These models were verified using a comprehensive experimental program for which new experimental equipment was custom-designed and built. To form the new gels, limit-oxidized alginate (~45% repeat units modified) and six N-succinyl-chitosans (22 to 52% primary amine substitution) were synthesized from commercially available polymer. Solutions were prepared using phosphate buffered saline. A viscosity guideline of (<~0.2 Pa·s) was obtained from the injectability literature. This was easily met by oxidized-alginate solutions. N-succinyl-chitosan was limited to five substitutions (22 to 48%), with respective concentrations between 4.8 and 2.0 w/w%. Gels were formed by blending alginate and chitosan solutions at five different ratios (2:1 to 1:9). Gel stiffness was characterized in the 'as cast' state by compression testing. Swelling was characterized in phosphate buffered saline. Our results showed that the stiffness model provided a good fit (largest p-value <0.03). From the model it was found that limit-oxidized-alginate is 16 times less stiff than N-succinyl-chitosan. This combined with the restricted injectable concentration range resulted in a maximum stiffness of 7 kPA, which is below the target window for muscle tissue (i.e. between 10 to 30 kPa). To simultaneously achieve both targets further investigation is recommended using higher concentrations of lower molecular weight chitosan or stiffer oxidized polysaccharides. For N-succinyl-chitosan, the membrane osmometry model gave an excellent fit (largest p-value ~10⁻⁷). A key finding was that the Manning-Oosawa model for ion condensation does not accurately predict behaviour. The relatively fast rate of oxidized-alginate degradation prevented osmometry from being used to collect empirical ion condensation data. Without this data, classical swelling theory could not adequately predict experimental response. In future work, conductivity experiments are recommended to characterize ion condensation. Recent models by Trizac and O'Shaughnessy are recommended as improvements to the swelling theory. The primary scientific contributions from this work are: 1) the first systematic characterization of the stiffness and swelling of injectable oxidised-alginate / N-succinyl-chitosan hydrogels; 2) the first analysis of polyampholyte ion condensation by membrane osmometry since the advent of modern ion activity models in the early 1990's; 3) the aforementioned advancement of osmometry and swelling theory; and 4) the proposal and successful experimental verification of a new model for hybrid gel stiffness. In addition, a number of other contributions were also made, mainly: 1) a membrane osmometer was designed and built to characterize small samples of polyelectrolytes at elevated temperature and in concentrated chloride solutions, 2) N-succinylation was found to fit a log-linear empirical correlation with respect to input reagent concentrations; 3) a small increase in chitosan moisture affinity and thermal stability was observed with increasing N-succinyl substitution; 4) anomalous phase separation and rheological behaviour was observed in N-succinyl-chitosan solutions near the solubility limit; and 5) polyampholyte solution rheology was found to scale differently than that of polyelectrolytes. Although the stiffness is below what was anticipated, the oxidised-alginate / N-succinyl-chitosan hydrogel system shows good potential. Provided the stiffness limitation can be overcome, the existence of high strain 'secondary stiffness' behaviour provides the potential for a much closer match to the non-linear elastic response of muscle tissue than is currently possible with single component systems. In addition, interesting phase behaviour near the N-succinyl-chitosan solubility limit may allow independent control of pore size and stiffness via a heterogeneous structure

On the impact of powder cohesion on the bulk properties of a powder bed in Additive Manufacturing using Discrete Element Method (DEM) simulations

In powder based Additive Manufacturing (AM) a uniform and compact spread of particles is required which can then be accurately fused layer by layer to form final products. As powders are spread, several parameters control the quality of the final powder bed layer; namely, spreader type, powder grain shape, powder characteristics and ambient manufacturing conditions. Utilising discrete element method (DEM) simulations this paper investigates the effect of cohesion on the quality of the powder bed post spreading. However, only cohesion due to the formation of liquid bridges as a result of moisture content of the powder is considered in this work. Simulations are run with a realistic spreader (geometry of which was created from data points from manufacturing equipment used within industry), alongside realistic particle shapes created via Multi-Sphere Approximations (MSA) of models derived from powder X-ray microtomography images, see Figure 1. A random selection of powder particles is chosen and used within simulations, with the resolution of these particles being controlled via a surface smoothing factor [1] to ensure an acceptable balance of accuracy and computational cost. Simulations are run with an appropriate subset of the total number of particles to yield a statistically accurate representation of the grain population to identify the effects of cohesion on the final quality of the powder bed layer. In this paper for the first time, the relationship between the moisture content and powder bed quality is investigated and the simulation results indicate that the cohesion has a strong effect on the powder bed quality which is quantified via a surface roughness parameter and powder's bulk density

Application of Digital Image Correlation Method to Biogel

This study adopts the digital image correlation (DIC) method to measure the mechanical properties under tension in agarose gels. A second polynomial stress– strain equation based on a pore model is proposed in this work. It shows excellent agreement with experimental data and was verified by finite element simulation. Evaluation of the planer strain field by DIC allows measurement of strain localization and Poisson’s ratio. At high stresses, Poisson’s ratio is found to exceed the standard assumption of 0.5 which is shown to be a result of pore water leakage. Local failure strains are found to be approximately twice those determined by crosshead displacements. Viscous properties of agarose gels are investigated by performing the tensile tests at various loading rates. Increases in loading rate do not cause much difference in the shape of stress– strain curves, but result in increases in ultimate stress and strain