Photopolymerisation

A photopolymerisable formulation typically consists of a reactive diluent, a cross-linker and a photoinitiator. However, most formulations contain additional components (Figure 2) all influencing its reactivity, viscosity, reaction mechanism and the properties of the resulting polymer [2][3]:

 High molecular weight monomers, i.e. cross-linkers with more than one reactive group define the mechanical properties of the resulting polymer.

 Mono- and multifunctional reactive diluents affect the number of reactive groups, decrease the viscosity of the formulation and additionally tune the mechanical properties.

 A photoinitiator that meets the emission spectra of the used light source efficiently creates radicals upon its activation.

 A solvent swells the polymer network decreasing the stiffness and strength of the obtained structure.

 Filler materials influence the Young’s modulus² and/or other functional properties of the final polymer structure.

 If the monomers are very reactive, inhibitors can prevent premature polymerisation scavenging formed radicals.

 Bioactive stimuli can be added to the formulation. These substances are major determinants for cell behaviour and can be conjugated to the scaffold material [5].

Adding solvents and filler materials decrease the shrinkage during polymerisation to obtain better shape accuracy and reduce internal stresses.

2 The Young’s modulus is also known as tensile modulus or elastic modulus. It is a value describing the stiffness of an elastic material. It is calculated as the ratio between stress and strain along an axis in the linear range, where Hooke’s law can be applied [4].

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Figure 2 Basic building block of photopolymerisable formulations used for lithography based AMT [2]

In photopolymerisation, initiators dissolve into radicals that break the double bonds of the monomers (cross-linkers and reactive diluents) and start the solidification in the radical chain reaction polymerisation. In the following section, we will explain the mechanisms of the PIs’

decay into radicals and the process of radical chain growth polymerisation.

6.1.1 Photoinitiator

PIs are the key substance of photopolymerisable formulations. They are UV or VIS sensible and convert radiation energy into chemical energy dissolving into radicals [6] (Figure 6a), molecules that have unpaired electrons on an otherwise open shell configuration. As they are highly reactive, they can react with another molecule breaking its double bonds [7]. This initiation step starts the free radical polymerisation chain reaction.

In conventional photopolymerisation, one absorbed photon elevates the PI molecule from a lower (S0) to a higher and short-lived (S1) vibrational energy level, both of them being singlet states with spin zero.³ Rather than immediately decaying to the ground state simply emitting fluorescence or converting the energy into internal heat, the PI decays to a long-lived triplet state via inter system crossing. The spin of the molecule is now one.⁴ Depending on the molecule, the PI in the excited triplet state can create radicals via the monomolecular type 1 or the bimolecular type 2 mechanism of radical formation.

3 The spin is measured in reduced Planck’s constant

4 Triplet is referred to the three possibilities for the secondary spin quantum number (1,0,-1).

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Figure 3 a) type I initiator Irgacure 369 cleaves from the excited triplet state forming two radicals that can start polymerisation b) an amine (2) quickly transfers an electron to the type II initiator benzophenone (1);

proton transfer leads to a reactive amine that can start polymerisation. The PI itself recombines [8].

- or -cleavage leads to photo-fragmentation and radical formation of type I PIs. This usually takes place next to an aromatic carbonyl group and thus results in the formation of one or two benzoyl radicals, which are capable of starting the polymerisation. One typical example of a

-cleavage initiator is Irgacure 369 (Figure 3a).

Chromophores such as benzophenones and analogues as well as donors (co-initiators) such as alcohols, ethers and amines are the basis of bimolecular type II PIs. They create radicals via hydrogen abstraction or electron transfer. In hydrogen abstraction, a benzophenone in the triplet state, for example, abstracts hydrogen from an alcohol, ether or amine. Whereas the formed alcohol, ether or amine radicals start the polymerisation, benzophenone radicals recombine to form a non-reactive dimer. Figure 3b shows the latter type II reaction. An electron is abstracted from the amine (2) and transferred to the excited ketone (1). Subsequent proton transfer renders reactive amine radicals that can start the polymerisation. The benzophenone radicals recombine. Beside amines, ethers or alcohol, also monomers or the formed polymer chains can serve as donors [9][8][10].

For 1PP, the reaction mechanism of type I initiators is usually more efficient. It is much simpler and requires shorter excited state lifetimes not necessitating any interaction with another molecule. For 2PP, however, type I initiators are rare as shifting their absorption spectra is complicated. The design of 2PIs will be addresses in section 7.2.

6.1.2 Cross-linking of monomers

Radicals formed in the initiation process (Figure 6a) cross-linked viscous monomers to form a polymer. In the simplest case, a photopolymerisable formulation consists of only one type of monomer and the PI. Figure 4 shows the cross-linking of acrylates (AC), common monomers in photopolymerisation. In the propagation step (Figure 4b), the PI radicals break the double bonds of the carbonyl group and add onto the acrylate monomers rendering the nearest carbon a radical. The formed molecule can add another acrylate, which creates another anchor point on the molecule.

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Figure 4 The principle of Initiation (a) and Propagation (b), modified from [6] and [11]

This process continues until a chain termination reaction occurs. Recombination (Figure 5(1)) involves two unpaired electrons from two reactive molecules that bond together to form a non-reactive molecule. In disproportionation (Figure 5(2)), a radical attaches to a single C-H bond from another reactive molecule. Both reaction partners can form two non-reactive molecules, one with a single and one with a double bond at the end of the chain. Termination can also happen through random incidentally existent inhibitors such as aerial oxygen. This is a common challenge in stereolithography (see section 6.2). The radical chain propagation terminates integrating a peroxide group.

Figure 5 Termination through Recombination (1) and Disproportionation (2) or oxygen inhibition (3), modified from [6] and [11]

Branched polymers develop via chain transfer reactions (Figure 6). A reactive chain transfers its unpaired electron to a random unreactive C-H bond anywhere in the middle of the reaction partner’s chain. While this terminates the chain growth of the considered molecule, it creates a reactive anchor point in the chain of the reaction partner. As other molecules attach, a new chain starts to propagate leaving a branched polymeric structure [11].

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Figure 6 Chain transfer, R stands for the acrylate branch modified from [6] and [11]

Photopolymerisation based AMT restricts the curing of the formulation to the respective layered cross-sections of the CAD (see Figure 1c). It can be distinguished between different lithography based AMTs regarding their way of spatially controlling the exposure [1]. In the following section, we will present laser-scanning and photomask based AMT.