Processing and Characterization of a Novel Bioabsorbable Polymer for Biomedical Applications

Author

Katherine Atchley, University of Tennessee - Knoxville

Date of Award

12-2006

Degree Type

Thesis

Degree Name

Master of Science

Major

Engineering Science

Major Professor

Joseph E. Spruiell

Committee Members

Roberto S. Benson, Kevin M. Kit

Abstract

A series of amino acid based bioanalogous polymers have been synthesized by Dr. C. C. Chu at Cornell University for future use in biomedical applications. These poly(ester amide)s have been designed to be bioabsorbable, with particular interest in internal fixation and drug delivery applications. The polymers PEA 4-Phe-4, 4-Phe-Das, and 8-Phe-Das were studied; however, due to better material properties, the focus mainly centered on 4-Phe-4. Since these are new polymers, processing conditions needed to be determined and the structure and properties characterized. The fibers were also processed post-extrusion in an effort to induce orientation and crystallinity within the polymer chains.

All three PEAs were extruded into monofilament fibers by way of a single-screw extruder. The 4-Phe-4 was processed at temperatures ranging from 150° – 195°C, and the 4-Phe-Das and 8-Phe-Das were processed at 135° and 140° C respectively. Post- extrusion drawing and annealing resulted in an increase in orientation, however crystallinity could not be induced by these methods. A nucleating agent was mixed with the 4-Phe-4 which seemed to result in an increase in the elastic modulus of the fiber. As with the drawing and annealing, the polymer remained amorphous following the addition of the nucleating agent.

Basic material properties were obtained for all three polymers. PEA 4-Phe-4 has a Tg of 59°C and a Tm of 109°C in its as received form. Once spun, the Tm disappears and the Tg lowers to 52±1.5°C. This loss of Tm corresponds to wide angle x-ray diffraction data collected and reinforces that the fibers are indeed amorphous. The 4-Phe-4 fibers have an elastic modulus ranging from 1606±188 MPa for the as spun fiber to 1919±203 MPa for the most highly drawn fiber. Similarly, the yield strength ranges from 11.4±0.75 – 113.1±22 MPa, and ultimate tensile strength ranges from 73±5 – 215±22 MPa. Birefringence ranges from 0.0026±0.00023 – 0.01812±0.00053. Dilute solution viscosity measurements provide an intrinsic viscosity of 0.55 dL/g for the as received polymer and 0.44 dL/g for the spun fibers.

Sodium benzoate was used as a nucleating agent during melt spinning. Once the nucleating agent was mixed with the 4-Phe-4, the Tg decreased further to 48°C. As before, there was no Tm present, which was confirmed by the WAXD data. The elastic modulus increased to 4246±430 MPa for the spun fiber and 5763±458 MPa for the fibers most highly drawn. The yield strength is within the same range as the original 4-Phe-4 fiber within reasonable error, ranging from 52±13 – 101.5±5 MPa. The ultimate tensile strength decreased, however, ranging from 63.5±2 – 122±17 MPa. Birefringence values were also slightly lower than the pure 4-Phe-4, ranging from 0.00168±0.00012 –0.01488±0.0017.

Finally, pure 4-Phe-4 was compression molded into film and drawn using a biaxial stretcher. Birefringence measurements for the drawn film are low compared to similar draw ratios in the fiber, ranging from 0.0011±0.00006 – 0.0042±0.00021. These drawn film samples were also subjected to creep testing using a DMA. Results show that 4-Phe-4 creeps at 37°C under a constant load.

The 4-Phe-Das and 8-Phe-Das behaved similarly throughout testing. Both polymers have low intrinsic viscosities (0.27 dL/g for both. The Tg for both is approximately 40°C in the as received form, and 58±0.5°C once spun into fiber. As spun 4-Phe-Das fibers have an elastic modulus of 2337±518 MPa, an ultimate tensile strength of 100±8 MPa, and a birefringence of 0.00341±0.0023. Corresponding data for 8-Phe-Das fibers reveal an elastic modulus of 1761±407 MPa, an ultimate tensile strength of 63±6 MPa, and a birefringence of 0.00103±0.0003.

Drawing appeared to provide some short range order and orientation within both the pure and nucleated 4-Phe-4 fibers. This can be seen to some extent through WAXD, but is most noticeable in the birefringence data. This chain orientation also leads to a significant increase in mechanical properties. Due to extreme brittleness, the 4-Phe-Das and 8-Phe-Das polymers were unable to be drawn, and their mechanical and optical properties suffered. Annealing did not have as great of an effect on the mechanical properties as drawing, but shrinkage tests conducted in heated water confirm that annealing helps to stabilize the fiber length and reduces shrinkage when heated above the Tg of the polymer.

The method of processing seemed to give the 4-Phe-4 fibers an advantage over the film when measuring their optical properties. Both fiber and film were drawn to similar draw ratios, yet the birefringence values of the drawn fiber are much higher than those of the drawn film. It seems that the initial orientation provided by the melt spinning processes lead to greater orientation during drawing. It is also likely that the higher draw temperatures for the film allowed the chains more freedom during drawing, therefore lowering their overall orientation.

Creep studies conducted at physiological temperature (37°C) show that the 4-Phe-4 film creeps significantly under constant load for prolonged periods of time. This may make the idea of application in a drug delivery system more feasible than internal fixation.

Recommended Citation

Atchley, Katherine, "Processing and Characterization of a Novel Bioabsorbable Polymer for Biomedical Applications. " Master's Thesis, University of Tennessee, 2006.
https://trace.tennessee.edu/utk_gradthes/1492