46400-G7
Novel Degradable Polymer Synthesis to Investigate Network Formation and Structure
Radical polymerizations are used in a wide variety of fields ranging from the fabrication of coatings and adhesives, and more recently for medical use as biomaterials (e.g., bone cements, dental implants) and it is important to better understand their networks for better design. In recent years, for use in the fields of drug delivery and tissue engineering, degradable polymers that form via a radical polymerization are being developed. In addition to expanding the overall potential of radically polymerized materials, these new polymers open up avenues to further investigate fundamental structure-property relationships between precursor molecules and final network properties. Specifically, we proposed to utilize the controlled synthesis of a novel macromer that forms a crosslinked polymer via a radical polymerization to better understand how the macromer structure influences the reaction behavior, degradation, and mechanical properties of formed networks while using isolation and analysis of network kinetic chains to gain more insight into this behavior.
During the first year, macromers with a variety of molecular weights, branching, and functionalization (i.e., number of reactive groups) were synthesized and polymerized into networks and the reaction behavior and final network properties (i.e., mechanics in tension and compression, glass transition temperature, degradation behavior). To accomplish the aims of this work, it was necessary to synthesize a novel polymer that had tunable structural features. Specifically, we modified precursors to a tough biodegradable elastomer, poly(glycerol sebacate) (PGS) with acrylates to impart control over the crosslinking process and allow for more processing options. The work involved the condensation polymerization of sebacic acid (diacid) with glycerol (tri-ol) under high temperature and vacuum. The precursor was then acrylated and the acrylation efficiency was changed through the molar ratio of reagents.
The acrylated-PGS (Acr-PGS) macromers are capable of crosslinking through free radical initiation mechanisms (e.g., redox and photo-initiated polymerizations) and the molecular weight (time of condensation reaction) and branching (introduction of di-ol over tri-ol) of the precursors are readily altered during synthesis. The molecular weights of the prepolymer as defined by GPC increased with reaction time and ranged from ~4.06 kDa to 23.46 kDa, illustrating the tunability of molecular weight. As with most condensation reactions, prepolymers were polydisperse (2.01 to 4.60) and generally increased with reaction time. The % acrylation ranged from ~9.6 to ~88.0%. Six Acr-PGS macromers were chosen to represent a range of molecular weights and acrylations, and thus, provide insight into the relationships between macromer structure and network properties.
The photopolymerization reaction was investigated by introducing 0.5 wt% photoinitiator (DMPA) into the Acr-PGS macromer and monitoring the consumption of the acrylate group peak in real time with exposure to 365 nm ultraviolet light. The maximum conversion occurred after ~8 minutes. The large difference in acrylate absorption between initial and final time points and the near baseline level at the final time point indicate a high level of conversion of the acrylate group to crosslinks. The gelation time was defined as the point when a slowly stirring stirbar was stopped after injection of the macromer into a vial. For this system, gelation occurred at ~5 minutes and a maximum temperature of ~30°C (starting from room temperature) was observed.
Polymer slabs for mechanical and degradation analysis were prepared using photopolymerization. The Young’s modulus was determined from the slope of the linear portion of the plot (<20% strain) and varied (~0.15 to 30 MPa) depending on the Acr-PGS macromer. The % strain at break also varied (~5 to 200%) depending on the Acr-PGS macromer used for network formation. In general, the Young’s modulus increased as the degree of acrylation increased for a given molecular weight. As expected, the % strain at break increased as the Young’s modulus and % acrylation decreased for a given molecular weight. Furthermore, the Young’s modulus and % strain at break increased with increasing molecular weight for similar degrees of acrylation.
Relationships between macromer structure and network properties can easily be drawn from this data. For instance, increases in the % acrylation lead to increases in the number of crosslinks formed, which is associated with an increase in the modulus of a resulting material and a decrease in the ability to elongate a material before failure. Additionally, more elastomeric-like features are obtained as the molecular weight of the prepolymer is increased. This is a clear demonstration that small modifications during synthesis can lead to drastic differences in the properties of the resulting material.
Over the upcoming year, more work will be performed to assess kinetic chain lengths in networks through degrading them and assessing the molecular weight of degradation products which consist of small molecules and kinetic chains. Additionally, this work will be accompanied by the development of a kinetic gelation model to compare with obtained experimental results. This information can then be translated into the design of better polymeric materials for numerous fields and applications.
