Dental News - New method allows production of customised photopolymers for dentistry

Dental Tribune International

Mon. 30. July 2018

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VIENNA, Austria: An increasing number of coating materials, including some for use in dentistry, are cured with light. Yet, homogeneous, custom polymer networks cannot be produced, and the materials tend to be brittle, which limits the commercial application of photopolymers. Researchers from TU Wien recently presented a method by which homogeneously cross-linked, tailored, tough photopolymers can be fabricated—even at high resolution for 3-D printing.

Light curing is usually a radical chain polymerisation. An initiator is split by light energy into radicals, which then attack the monomer, forming a new radical that becomes the starting point of a growing polymer network by attacking and binding additional monomers.

However, newer methods to better control radical photopolymerisation, and thus the material properties of the products, tend to slow down the curing process, which is not suitable for 3-D printing. A short irradiation phase is crucial for high spatial resolution and economical production times.

The research team has developed a new approach that allows custom methacrylate-based photopolymers to be produced without delaying the curing process. The success lies in adding an ester-activated vinyl sulphonate ester (EVS), which acts as a chain transfer agent. It is activated in that it easily splits off a portion of itself.

If the growing polymer network attacks the EVS instead of the next monomer, an intermediate is formed that rapidly splits into a terminated polymer chain in the network and a highly reactive radical, which starts a new chain reaction. The more EVS that is added, the shorter the average chain length in the polymer network. Since shorter polymer chains remain mobile for longer, the risk of shrinkage cracks during curing is significantly reduced. Unlike with conventional chain transfer agents, polymerisation is not inhibited because neither stable intermediates nor reversible reaction steps are involved, but the splitting off of the radical occurs preferentially.

As an example, the researchers printed a scaffold-like structure from a methacrylate copolymer. Individual layers of 50 μm in thickness were spatially well resolved. The material was very homogeneous, solid, but elastic and impact-resistant with high tensile strength, properties that can be adjusted by the amount of EVS added. Without EVS addition, the material was very brittle. This new approach paves the way for tough photopolymers for biomedical applications such as dental fillings.

The study, titled “Vinyl sulfonate esters: Efficient chain transfer agents for the 3D printing of tough photopolymers without retardation”, was published on 16 July 2018 in Angewandte Chemie International Edition.

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