Utilizing Polymerization-induced Phase Separation to Improve Dental Restorative Materials
Light is ideal to initiate the polymerization reaction during dental curing not only for the fact that it solves the biological incompatibility of thermal reactions but also because it allows for temporal and spatial control over the polymerization process. However, photo-polymerization is not without issues. Materials resulting from photo-initiation shrink in volume and experience a great deal of stress which leads to crack formation and other forms of damage.
To solve the volumetric shrinkage and stress increase experienced during photo-initiated curing, another polymerization method is under study. This process is called Polymerization Induced Phase Separation (PIPS). The process uses light as initiator but differs from conventional photo-polymerization in the fact that it results in the formation of a heterogeneous network which phase separates. This is accomplished in Caroline’s lab by the addition of non-reactive prepolymers of varying side chain length such as poly-(methyl, ethyl and butyl) methacrylates (PMMA, PEMA, PBMA). This results in a material which resists volumetric shrinkage and experiences reduced stress. However, there are a few limitations, such as understanding the curing mechanism, characterizing the monomer formulations as well as physical mechanism of shrinkage reduction, and optimizing the reaction time.
By modifying the bulk homo-polymer matrix TEGDMA, three advantages present themselves. The material is biologically compatible with other monomers, it is purely photo-initiated, and the reaction time is fast. The polymers and monomers need to be stable, so their thermal property was tested to see if they are stable at room temperature. The clouding point is measured by reducing the temperature. If the material becomes hazy, it is thermodynamically unstable. All materials tested were unstable at around 0 oC but that was okay because the materials would be used at room temperature.
Adding prepolymer increases the rate of conversion, but testing showed that adding too much actually decreased the rate of conversion as the viscosity increased, making it difficult for the termination step to occur. So while prepolymers help the kinetics, too many prepolymers will hinder the reaction and the right balance has to be found to bring about the phase separation.
The PIPS reaction occurs very early in the process and gelation always occurs afterwards, which is what is wanted. No matter what loading level, two phases of the same composition always formed, one purely in TEGDMA DNA and one rich in a mixture of TEGDMA DNA. This validated the techniques they had previously developed and proved they can make biologically compatible materials that undergo phase separation.
The proposed physical mechanisms show that the material will polymerize at different rates, one that will shrink as expected and one that will compensate for the volume shrinkage. To measure the polymerization stress the material was cured between two glass rods. If the material shrunk the beam was deflected. This experiment revealed a reduction of 40% in volume shrinkage, which is double what was expected. This indicated that PIPS was doing something beyond just adding volumetrically. The different mechanisms are 1) Nucleation and Growth and 2) Spinodal Decomposition. They depend on how the material is developed. Increasing the light intensity increases the rate of reaction. Only at high prepolymer loadings does the molecular weight of the prepolymer impact stress because the viscosity is so high the reaction is actually slowed down.
Caroline’s work has demonstrated PIPS is a purely photo-initiated, ambient cure polymerization and found that PIPS leads to additional polymerization stress reduction beyond what is expected from volumetric displacement. Future work will focus in understanding how the physical properties of the prepolymers also impact the PIPS process and extend to systems of co-polymers appropriate for dental resin formulations.
Caroline presented at Lafayette College on March 1, 2013.
Her PowerPoint is here: LafSeminar130301_3
Some reports contributed to by Caroline or related to her research are below:
OralBiologyReview_BowmanStansburyCramer_2011
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