Date of Award

5-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Hirt, Douglas E

Committee Member

Guiseppi-Elie , Anthony

Committee Member

Bruce , David

Committee Member

Webb , Ken

Abstract

The major drawbacks of PLA are its poor toughness and lack of readily reactable groups. Unfortunately, typical methods of PLA toughening are associated with significant modulus and/or ultimate tensile strength (UTS) loss. The main objective of this research was to toughen PLA, with minimal modulus and/or UTS loss, and introduce reactive groups into the PLA matrix in one step. Initially, this objective was divided into two separate parts: PLA surface modification followed by toughening.
PLA film was solvent cast from chloroform solution and was surface modified using a sequential two-step photografting approach. Benzophenone was photografted onto the film surface in Step 1 followed by photopolymerization of hydrophilic monomers, acrylic acid and acrylamide, from the film surface. The resultant films were characterized using ATR-FTIR spectroscopy, water contact angle goniometry, transmission FTIR microspectroscopy, and tensile testing. The effect of the reaction solvent (ethanol and water) in Step 2 on PLA film surface and bulk properties was also studied. There was significant penetration of monomers into the films when ethanol was used as the reaction solvent, resulting in significant toughness loss. This monomer penetration into the films was successfully reduced by using water instead of ethanol as the reaction solvent in Step 2 and resultant films showed higher toughness than films surface-modified using ethanol as the reaction solvent in Step 2. It was also observed that solvent cast PLA film retained approximately 13 wt% chloroform, as characterized using thermogravimetric analyses (TGA). The presence of residual chloroform in the film specimens is undesirable from a biocompatibility standpoint. Therefore, further work was conducted on melt-processed films where residual solvent from the film-formation method would not be an issue.
Addition of a small amount of poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] (PHBHHx) to PLA improved the toughness of the resultant melt-processed blend from 4 ± 2 MPa for neat PLA to 175 ± 35 MPa for PLA-PHBHHx blend (90 wt% PLA). PLA-PHBHHx blend films were melt-processed using a single screw extruder. These polyblend films appeared to be non-compatible as characterized using dynamic mechanical analyses (DMA). PLA-PHBHHx blend films underwent rapid physical aging losing their toughness from 175 ± 35 MPa (right after extrusion) to 68 ± 34 MPa (day 3). The blend films were surface modified using the sequential two-step photografting protocol using water as the reaction solvent in Step 2. PLA-PHBHHx blend films lost approximately 95% of their toughness on surface modification due to UV-assisted solvent induced crystallization as characterized using wide angle X-ray diffraction (WAXD) analyses.
A novel reactive blending approach was developed to toughen PLA with minimal modulus and UTS loss and introduce reactive groups into the PLA matrix. PLA was reactive blended with a stiffening polymer, poly(acrylic acid) (PAA), followed by physical blending with a toughening polymer, poly(ethylene glycol) (PEG), in solution. The modified PLA was extruded into films using a co-rotating twin-screw extruder and characterized using tensile testing, differential scanning calorimetry (DSC), DMA, and toluidine-blue-surface-staining. This material exhibited, for the first time, approximately 10 fold increase in PLA's toughness without significant modulus and/or UTS loss and also introduced a controlled concentration of surface modifiable reactive acid groups into the PLA matrix in one step.

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