Rheological Considerations for Dual-Extrusion Melt Processing of Dissimilar Polymers in Composite Structures
| dc.contributor.author | Mansfield, Craig Daniel | en |
| dc.contributor.committeechair | Bortner, Michael J. | en |
| dc.contributor.committeechair | Baird, Donald G. | en |
| dc.contributor.committeemember | Dillard, David A. | en |
| dc.contributor.committeemember | Davis, Richey M. | en |
| dc.contributor.committeemember | Martin, Stephen Michael | en |
| dc.contributor.department | Chemical Engineering | en |
| dc.date.accessioned | 2025-01-22T09:01:03Z | en |
| dc.date.available | 2025-01-22T09:01:03Z | en |
| dc.date.issued | 2025-01-21 | en |
| dc.description.abstract | Gel spinning is the current industrial method of choice for combining ultra-high molecular weight (UHMW) polymer resins with a substrate support polymer resin to produce composite filaments with a porous structure and high surface area per unit volume (specific area). Gel spinning is typically used to overcome a wide gap between the maximum processing temperature of the UHMW resin and the minimum processing temperature of the substrate resin and to avoid the high melt viscosity of the UHMW resin, but requires the costly recovery of toxic solvents. The UHMW resin is used because it forms a stable gel phase in the presence of water; a lower molecular weight resin (LMW) simply dissolves. A dual-extrusion process, which minimizes residence time with mismatched temperatures, was used to render a melt-based scheme practical. Dual-extrusion involves the separate plastication of materials prior to combination in a low residence time mixing head to form a desired composite. In this work, the UHMW and LMW resins were both poly(ethylene oxide) (PEO), and the substrate was polyarylsulfone (PAS). The initial focus of this dissertation is to investigate the rheology of PEO when subjected to temperatures beyond which it is known to degrade. Literature indicated PEO undergoes non-oxidative thermal degradation above 200°C and PAS is processed up to 350°C. Dynamic oscillatory shear rheometry was used to study 0, 25, 40, 50, 60, and 75wt% UHMW PEO in LMW PEO to take advantage of the sensitivity of viscosity to changes in molecular weight and material configuration, indicating degradation. Samples were exposed to 220, 230, 240, 250, 275, and 300°C temperatures for 5 minutes to explore conditions that could result in sample degradation. The viscosity decreased less with increasing UHMW PEO content for samples exposed to the same temperature and the viscosity decreased more with increasing exposure temperature for samples with the same UHMW PEO content. Parameters were regressed from observed data to predict the change in molecular weight via empiricisms relating the viscosity to molecular weight, shear rate, temperature, and time. This regression yielded a single master curve describing the behavior of PEO across all conditions, stable and degrading. The purpose of the second part of this work is to investigate the utility of the correlation developed with PEO in the first part with respect to characterizing an additional polymer resin, PAS, predicting the processing conditions for combining PEO and PAS in the dual-extrusion process, predicting the degradation of PEO in the dual-extrusion process, and characterizing the structure of the resulting composites with comparison to expectations from literature. The overall goal of eliminating the need for a toxic solvent in phase inversion gel spinning by changing to a melt process with dual-extrusion leaves theory and enters practice in this part. The correlation developed for PEO in the first part was used to regress parameters for PAS, extending the use case to an additional class of polymer resin. The regressions for both PEO and PAS were used to select processing conditions for operating the dual-extrusion process to yield composite filaments. Samples were produced with a range of compositions and prepared for microscopy as is, after etching with water, or after rinsing with water to remove extractables. Extractable content was characterized by the change in dry mass before and after rinsing samples using optical and scanning electron microscopy techniques. The observed excess extractables content of rinsed samples agreed with prediction from the regression for PEO and microscopy indicated qualitatively similar structure to similar gel spun materials in literature. | en |
| dc.description.abstractgeneral | Gel spinning is the current industrial method of choice for combining ultra-high molecular weight (UHMW) polymer resins with a substrate support polymer resin to produce composite filaments with a porous structure and high surface area per unit volume (specific area). Gel spinning is typically used to overcome a wide gap between the maximum processing temperature of the UHMW resin and the minimum processing temperature of the substrate resin and to avoid the high melt viscosity of the UHMW resin, but requires expensive toxic solvent recovery and recycling. The UHMW resin is used because it forms a stable gel phase in the presence of water; a lower molecular weight resin (LMW) simply dissolves. A dual-extrusion process, which minimizes residence time with mismatched temperatures, was used to render a melt-based scheme practical. Dual-extrusion involves the separate plastication of materials prior to combination in a low residence time mixing head to form a desired composite. In this work, the UHMW and LMW resins were both poly(ethylene oxide) (PEO), and the substrate was polyarylsulfone (PAS). The initial focus of this dissertation is to investigate the rheology of PEO when subjected to temperatures beyond which it is known to degrade. Rheology is the study of deformation of materials and rheometric tests, which involve the controlled deformation of materials, allow observation and calculation of important material properties, such as viscosity, modulus, yield strength, and strength at tensile or compressive failure. Literature indicated PEO undergoes non-oxidative thermal degradation above 200 °C and PAS is processed up to 350 °C. This implied that any melt based process would need to overcome a 150 °C temperature gap. Dynamic mode small amplitude oscillatory shear rheometry (SAOS) was used to study 0, 25, 40, 50, 60, and 75 wt% UHMW PEO in LMW PEO to take advantage of the sensitivity of viscosity to changes in molecular weight and material configuration, indicating degradation. SAOS is an exceptionally useful method for studying materials by imposing a very small, oscillating deformation on one side of a sample and measuring the torque required to keep the other side of the sample stationary. From this data, viscosity is trivially calculated. Samples were exposed to 220, 230, 240, 250, 275, and 300 °C temperatures for 5 minutes to explore conditions that could result in sample degradation. Viscosity is strongly dependent on molecular weight and temperature, typically increasing with increased molecular weight and decreasing with increased temperature. The viscosity decreased less with increasing UHMW PEO content for samples exposed to the same temperature and the viscosity decreased more with increasing exposure temperature for samples with the same UHMW PEO content. Parameters were regressed from observed data to predict the change in molecular weight via using a novel correlation that relates the viscosity to molecular weight, shear rate, temperature, and time. This regression yielded a single master curve describing the behavior of PEO across all conditions, stable and degrading. A master curve is made by transforming data collected at a wide range of conditions to data representative of a single set of reference conditions or in dimensionless form to represent any conditions. This is how data collected by a single instrument with a limited range of testable conditions can be used to predict material properties and behavior over significantly larger ranges. The purpose of the second part of this work is to investigate the utility of the correlation developed with PEO in the first part with respect to characterizing an additional polymer resin, PAS, predicting the processing conditions for combining PEO and PAS in the dual-extrusion process, predicting the degradation of PEO in the dual-extrusion process, and characterizing the structure of the resulting composites with comparison to expectations from literature. The overall goal of eliminating the need for a toxic solvent in gel spinning by changing to dual-extrusion, a melt process, leaves theory and enters practice in this part. The correlation developed for PEO in the first part was used to regress parameters for PAS, extending the use case to an additional class of polymer resin. The regressions for both PEO and PAS were used to select processing conditions for operating the dual-extrusion process to yield composite filaments. Sample composite filaments were produced with a range of compositions and prepared for microscopy as is, after etching with water, or after rinsing with water to remove extractables. Extractables include any materials that can be removed using a solvent, in this case water. The observed excess extractables content of rinsed samples agreed with prediction from the regression for PEO and confirmed that the predicted form and extent of degradation happened during melt processing. Extractable content was characterized by the change in dry mass before and after rinsing samples using optical and scanning electron microscopy techniques. Microscopy permitted identification of each material by comparison of images taken before and after etching with water. Microscopy observations indicated qualitatively similar structure to similar gel spun materials in literature, confirming that dual-extrusion is a potentially viable replacement for gel spinning for similar composites. | en |
| dc.description.degree | Doctor of Philosophy | en |
| dc.format.medium | ETD | en |
| dc.identifier.other | vt_gsexam:42201 | en |
| dc.identifier.uri | https://hdl.handle.net/10919/124293 | en |
| dc.language.iso | en | en |
| dc.publisher | Virginia Tech | en |
| dc.rights | In Copyright | en |
| dc.rights.uri | http://rightsstatements.org/vocab/InC/1.0/ | en |
| dc.subject | rheology | en |
| dc.subject | dual-extrusion | en |
| dc.subject | composites | en |
| dc.subject | structure | en |
| dc.subject | polymers | en |
| dc.title | Rheological Considerations for Dual-Extrusion Melt Processing of Dissimilar Polymers in Composite Structures | en |
| dc.type | Dissertation | en |
| thesis.degree.discipline | Chemical Engineering | en |
| thesis.degree.grantor | Virginia Polytechnic Institute and State University | en |
| thesis.degree.level | doctoral | en |
| thesis.degree.name | Doctor of Philosophy | en |
Files
Original bundle
1 - 1 of 1