Shrinkage Of Photopolymerization

Photopolymerization has been widely accepted and is still growingdue to enormous advantages including high reaction rate and mildpolymerization condition, and temporal and spatial control of initiation.However, polymerization shrinkage is a major drawback for currentfree-radical photopolymerizable systems. Polymerization shrinkageinduces many problems and limitates the application of photocurablematerials. Herein, the new real-time method is developed to monitor theshrinkage process. The shrinkage process associated withphotopolymerization has been systematically investigated. In addition, forthe aim of decreasing the shrinkage, the novel solid-statephotopolymerization and hybrid photopolymerization has been relized,and we have proved these novel photopolymerization with low shrinkage.The main work is showed as the following.1. The laser reflection method is developed to measure the real-timeshrinkage. The accuracy, repeatability of this method has been wellproved. This method could be widely used for different photo-curablesystem.The photo-DMA method is developed to simultaneously measure thereal-time shrinkage and modulus development. This method could beused for the photo-curable dental composites.2. Various acrylate monomers are used to investigate thephotopolymerization shrinkage process. The concentration of doublebonds and degree of double bond conversion are key factors to infleuncethe shrinkage process. The shrinkage factor of acrylate, which iscalculated from the curve of maximum shrinkage versus theconcentration of double bonds, is23.98ml/mol, which means15.72mL of shrinkage produced by every mole of double bond polymerized. Theinfluence of chemical structure on the shrinkage process has beenevaluated. With increase of the molecular length by rising the degree ofethoxylation, the shrinkage rate and percentage shrinkage has beendecreased. Decrease of the functionality accompanied with the reducingof the molecular weight, the concentration of double bonds may be notdecreased, so percentage shrinkage may be not reduced by decreasing thefunctionality. Methacrylate could decrease the shrinkage rate andpercentage shrinkage, but unfortunately reduced the degree of conversion.The compositional effect on shrinkage process of phtocurableformulation has been evaluated. The final shrinkage of fomulation can becalculated according to the contribution of monomers and oligomers.Because the oligomer has the higher molecular weight than monomer,shrinkage rate and final shrinkage could be easily decreased by increasingthe weight percentage of oligomers. Using ethoxylated monomers candecrease the shrinkage rate and percentage shrinkage. Photo-initiator candecrease the percentage shrinkage and shrinkage rate when lowconcentration of photo-initiator is used. It is due to the decrease ofconversion, and concentration of photo-initiator has no significantinfluence on the maximum shrinkage.3. The free-radical solid-state photopolymerization of octadecylacrylate has been achieved without polymerization kinetics beendeteriorative influenced by mobility restriction. The XRD, DSC, andreal-time FTIR proved that similarity of hexagonal packing between themonomers and polymers facilitates the photopolymerization in the solidstate. Unlike the normal liquid-state photopolymerization, the solid-statephotopolymerization is insensitive to the oxygen due to the fact that thediffusion of oxygen into the sample has been slow down. Moreimportantly, due to the more compact packing of monomers, theshrinkage of solid-state photopolymerization is obviously low (1%).The rapid free-radical solid-state photopolymerization of octadecylmethacrylate has been achieved with the increasing the polymerizationreactivity of methacrylate. The polymerization rate and double bondconversion in the solid state (5oC and15oC) is higher than those in the liquid state (25oC and35oC). The polymerization rate of crystallineoctadecyl methacrylate significantly exceeds that of highly functionaldimethacrylates (EGDAB and DEGDMA). Based on DSC, XRD andreal-time FTIR results, octadecyl methacrylate polymerized in the solidstate without destroying the hexagonal packing of crystalline long alkylchains, and the the hexagonal packing of long alkyl chains are preservedas the hexagonal packing of long alkyl side-chains of polymers. GPCshows that the molecular weight of polymers is still high, and themolecular weight distribution is significantly decreased by solid-statephotopolymerization. We inferred that the crystalline structure, in whichmethacrylic double bonds are concentrated, facilitated rapid minimallyactivated propagation and suppressed bimolecular termination. Moreimportantly, because obvious shrinkage occurs before polymerization dueto crystallization, the shrinkage of solid-state photopolymerizationwithout transition of liquid to solid is obviously low(1%).4. The rapid photopolymerization of hybrid thiol-epoxy/thiol-acylatesystem has been achieved. ITX/TBD HBPh4photoinitiating system thatgenerated both free radical and strong base (TBD) upon UV exposure(320-500nm), could induced the relatively fast and essentiallyquantitative thiol-epoxy click reaction, radical-mediated thiol-acrylatereaction and thiol-acrylate Micheal addition reaction. The results showthat hybrid systems have much lower polymerization shrinkage comparedwith single acrylate system, and polymerization shrinkage decreases withincreasing the percentage weight of thiol-epoxy. Besides, the mechanicalproperty and hardness of thiol-acylate system has been improved bythiol-epoxy reaction. With rising the percentage weight of thiol-epoxy,the store modulus, the glass transition temperature, and hardness hassignificantly increased.