Polymerization shrinkage, photoinitiated

The nature and type of initiation scheme plays an important role in the performance of the adhesive [194,202-204]. Stresses due to polymerization shrinkage lead to the creation of a gap between the adhesive and tooth material. In the case of bulk chemical initiation, shrinkage stresses tend to create gaps at all interfaces, drawing material inward isotropically. With a photoinitiation scheme, polymerization begins at the free surface and pulls the material away from the dentin towards the free surface [194]. Thus the gap is created at the... 

The introduction of so-called hybrid resins represents the beginning of the next phase of materials development. Hybrid resins comprise a minor fraction of acrylic-based component that guarantees rapid aoss-linking to reach form stability within short processing times. The majority of the resin is constituted by epoxy-ftmctional compounds exhibiting comparatively low polymerization shrinkage during slow cationic cure. Development of hybrid resin formulations requires careful choice and adjustments of the included compounds, especially of cationic and radical photoinitiators. 

A liquid preparation with solid polystyrene (0.6 g) dissolved in liquid styrene monomer (1.5 mL) was cast against a mold. Polymerization was accomplished with UV irradiation (21°C, 18 h). Solid PS was included to reduce the degree of shrinkage that occurred when monomeric styrene was photopolymerized [85]. In a similar manner, PMMA dissolved in MMA was cast against a Si master. Upon UV polymerization (with BME as the photoinitiator), a PMMA chip is formed. Nearly 100 PMMA chips can be replicated using a single Si master [223]. 

Cationic cure mechanisms are an alternative approach to uv curing. This involves the photogeneration of ions, which initiate ionic polymerization. This process is not subject to oxygen inhibition, as are some of the free radical mechanisms. Cationic cure mechanisms generally also provide less shrinkage and improved adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and other basic materials. The acidic species can also promote corrosion. As a result, the vast majority of uv formulations are acrylate-based and cure by a free radical mechanism. 

These results point to two processes, premature radical chain termination and film shrinkage, which compete in determining the ultimate polymerization conversion efficient of multifunctional acrylates. It is obvious that critical attention must be paid to the pulse repetition rate, photoinitiator concentration, and acrylate functionality in developing any photopolymerizable system for laser-initiated polymerization. Future publications on laser-initiated polymerization of multifunctional acrylates will deal with monomer extraction of partially polymerized films, mechanical properties of laser polymerized films, and the Idnetics of single-pulsed systems. 

The laser irradiation induced the formation of polymer cylinders in the bulk of the film. Acrylic monomers underwent a volume decrease of 10 to 15% when polymerized. The underlying reason was that the gradient of chemical composition resulting from the spatially controlled conversion of monomer into polymer immediately induced the flowing of monomer and photoinitiator molecules to the illuminated areas. Hollows created by volume shrinkage were almost immediately filled by reactive species. 

The most simple cycle is the 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (1 in Scheme 3.3). This and equivalent monomers (structures 2,3 and 4 in Scheme 3.3) have been polymerized using different catalytic means. The team of Loy and Rahimian [187, 188] carried out the polymerization of all these monomers using a basic catalyst (i.e. tetrabutylammonium hydroxide TBAH) and Bronsted adds, namely formic and triflic adds or cationic photoinitiators. Monomer 3, a soUd, was either polymerized in THF or copolymerized with 1. The polymerization was very fast (<30 min), and resulted in a material that could resist temperatures of up to 500 °C. The authors were confident that these dicydic monomers, produced via crossUnked materials, would in time replace the conventional sol-gel technique, as the polymerization is straightforward, solvent-free, has less than 5wt% shrinkage, and generates no porosity in the final material. 

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