# Influence of thiocarbonylthio- RAFT agents on the homogeneity of polymer network and mechanical properties of 3D printed polymers

Emile Goldbach, Xavier Allonas, Celine Croutxé-Barghorn, Christian Ley, Lucile Halbardier, Gildas L'Hostis

► To cite this version:

Emile Goldbach, Xavier Allonas, Celine Croutxé-Barghorn, Christian Ley, Lucile Halbardier, et al. Influence of thiocarbonylthio-RAFT agents on the homogeneity of polymer network and mechanical properties of 3D printed polymers. European Polymer Journal, 2023, 188, pp.111947. 10.1016/j.eurpolymj.2023.111947. hal-05073936

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# Influence of thiocarbonylthio- RAFT agents on the homogeneity of polymer network and mechanical properties of 3D printed polymers

Emile Goldbachª, Xavier Allonasª, Celine Croutxé-Barghorn*ª, Christian Leyª, Lucile Halbardierª, Gildas L'Hostisᵇ

RAFT agents based on thiocarbonylthio- group were used in an acrylate formulation containing a phosphine oxide as photoinitiator. After optimization of the RAFT agent concentration, its effect on the polymer properties was studied. It turns out that the conversion at the gel point increases when RAFT agents are used. Similarly, a decrease in the relaxation temperature is noticed as well as an increase in the homogeneity of the polymer network. Due to the reduction of internal defects, this leads to better mechanical properties as exemplified by toughness measurement of 3D printed samples.

## Introduction

As radical polymerization usually presents a low control of the polymer structure, many efforts have been devoted since the 90's to circumvent this issue by introducing dormant species and transfer reactions.¹⁻² Among reversible deactivation radical polymerization (RDRP) processes, atom transfer radical polymerization (ATRP),³ nitroxide mediated polymerization (NMP)⁴ and reversible addition-fragmentation chain transfer (RAFT)⁵ are the most common approaches to control molar masses of the polymer chains and provide narrow molar mass distribution. RAFT polymerization is particularly attractive since RAFT agents are available for a wide range of monomer types (acrylates, methacrylates, styrene, acrylamides, methacrylamides, vinyl esters, vinyl amides).

Although that this technology was commonly used in thermal-initiated radical polymerization, the efficiency of RAFT-mediated photopolymerization was also evidenced.⁶⁻⁷ Photopolymerization presents several advantages in comparison to thermal-initiated polymerization, such as a huge increase of the reaction speed⁸ and its spatio-temporal control. These advantages have permitted to develop innovative 3D printing applications in stereolithography and digital light processing (DLP). To go further in this amazing field, several constraints must be overcome. As acrylate resins are mostly implemented in the 3D area, the resulting polymers are mostly brittle and/or exhibit insufficient toughness.⁹ As a consequence, addition of RAFT agents to acrylate formulations has been investigated¹⁰. RAFT agent was used as an iniferter⁶,¹¹,¹² under visible light irradiation. However, the low rate of photodissociation of the RAFT agent was found detrimental for DLP printing. To circumvent this problem, the potential of PET-RAFT systems has been investigated.¹³⁻¹⁸ The principle is to add a photosensitizer to the formulation. After being excited, it transfers the energy to the RAFT agent, thereby allowing the dissociation of the latter. More recently, RAFT agents were used in combination with photodissociative Type I photoinitiators in order to effectively reduce the retardation related to the RAFT agents.¹⁹⁻²¹ For example, the improvement of mechanical properties by addition of cyanomethyl dodecyl trithiocarbonate (CDT) as RAFT agent was reported.¹⁹ It was shown that the elongation at break increases and the Young's modulus and the stress at break decrease in presence of a limited amount of RAFT agent. However, the characterization of the resulting polymeric material was carried out on samples exhibiting different monomer conversions. Therefore, it is tricky to conclude that the increase of the elongation at break results from the network modification and

<footnote>a) Laboratoire de Photochimie et d'Ingénierie Macromoléculaires, Université de Haute Alsace, 3b rue Alfred Werner, 68093 Mulhouse Cedex, France. * E-mail: celine.croutxebarghorn@uha.fr</footnote>

<footnote>b) Laboratoire de Physique et Mécanique Textiles (LPMT), Université de Haute Alsace, 11 rue Alfred Werner, 68093 Mulhouse, France</footnote>

<footnote>† Electronic supplementary information (ESI) available: Materials and methods used for this study. See DOI: XXXXXX</footnote>
not from the lower material conversion. Methyl 2-(n-butylthiocarbono-thioylthio) propanoate (MBTP) was also employed as RAFT agent to increase the elongation at break and the break stress at iso-conversion.²¹ As only one RAFT agent has been studied, it may be difficult to understand the impact of the nature of the substituents of the thiocarbonylthio structure on the mechanical properties of the network. Concerning the toughness, which is a crucial characteristic of acrylate materials in 3D printing, the use of a resin based on a PET-RAFT mechanism has been proved to be beneficial¹³ even though the polymeric materials obtained exhibit different monomer conversions that also influence the final mechanical properties.

In this paper, the impact of two different RAFT agents on the mechanical properties of a DLP photocurable acrylate formulation composed of acrylate monomers, photoinitiator and RAFT agent will be studied. First of all, the optimal RAFT agent concentration was determined by studying polymerization kinetics. Then, the network structure resulting from the radical controlled polymerization will be investigated. Finally, the mechanical changes of polymeric materials exhibiting the same monomer conversion will be characterized, with a focus on elongation at break and toughness.

## Results and discussion

The formulation studied contains 90 wt% ethoxylated (3) bisphenol A diacrylate (SR 349), 10 wt% 2-hydroxyethyl acrylate (HEA) and 0.9 wt% diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) (Scheme 1). The cyanomethyl dodecyl trithiocarbonate (CDT) and 4-cyano-4-(phenylcarbonothioylthio) pentanoic acid (CPADB) were chosen as RAFT agents, because of their commercial availability. The compositions of the formulations are reported in Table S1. The kinetics of the reaction were studied by following the decrease of the acrylate function at 1636 cm⁻¹ by real-time Fourier transform infrared spectroscopy (RT-FTIR). Irradiation was carried out with a LED (Ufiber, Uwave) at 385 nm and 9.8 mW/cm², which are similar to the exposure conditions of the DLP printer used in this study.

<figure><img src="image_2.png" /><figcaption>Ethoxylated (3) Bisphenol A Diacrylate (SR 349)</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>2-Hydroxyethyl acrylate (HEA)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Cyanomethyl dodecyl trithiocarbonate (CDT)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>4-Cyano-4-(phenylcarbonothioylthio) pentanoic acid (CPADB)</figcaption></figure>
**Scheme 1.** Structures of the molecules used in this study.

Several concentrations of CDT and CPADB were investigated in order to study the effect of the presence of RAFT agent on the photopolymerization process (Figure 1). The inhibition time, maximal polymerization rate and final conversion are reported in Table 1. A relevant parameter corresponding to the time to reach 80% of the final conversion (t<sub>80%</sub>) is also reported, which is approximately the time to reach the conversion plateau.

As expected, the formulation without any RAFT agents react pretty quickly and reach a final conversion (DBC<sub>max</sub>) of 74±2 % due to vitrification.
t<sub>80%</sub> value was found to be about 5 s for the reference system (with no RAFT agent). Adding CPADB or CDT in the formulation has several effects.
At low RAFT agent concentration (11.2 mM and 11.2, 40.3, 60.4 and 80.5 mM for CPADB and CDT, respectively), the final conversion still reaches
approximately 73 % after 120 s. However, polymerization rate is gradually reduced and inhibition period and t<sub>80%</sub> are gradually increased (see
Table 1) when the RAFT concentration increases. 16.2 s and 21.5 s are now required to reach 80% of conversion for formulations containing 11.2
mM of CPADB or 80.5 mM of CDT, respectively.

<figure><img src="image_7.png" /><figcaption>**Figure 1.** Kinetic profile of double bond conversion (DBC) for TPO based formulations with different concentrations of a) CPADB and b) CDT.</figcaption></figure>

This results can be explained by the mechanism of RAFT polymerization and in particular, the stability of the intermediate 2 by the mesomeric
donor group Z, and the chemical structure of the leaving R group (Figure S1).<sup>22</sup> The benzoic group of the CPADB leads to a better stabilization of
the intermediates 2 and 4 than the CDT does, due to its attractive mesomeric effect. Existence and stability of the radical intermediates account
for species of lower reactivity in the medium, slowing down the overall polymerization rate. In addition, the R* bulky group of CPADB, as a 4-
cyano-4-methyl pentanoic acid, also contributes to an increase of the induction time.<sup>23</sup> This explains the higher retardation induced by CPADB
at isoconcentration (t<sub>80%</sub> = 5.44 s and t<sub>80%</sub> = 16.2 for formulations containing 11.2 mM of CDT and CPADB, respectively). This conclusion is in line
with a recent paper evidencing the decrease of the polymerization rate with the decrease of the enthalpy of stabilization due to the Z group donor of several thiocarbonylthio compounds.²⁴

At high concentrations, a decrease of the final conversion is observed (see for example CPADB at 22.6 mM or 33.9 mM). This may be attributed to recombination reactions with the RAFT intermediated species (recombination of intermediate 4 either with a similar radical, see Figure S2, or with a growing chain, see Figure S3),²⁵ or trapping reactions of primary radicals from TPO with RAFT agents.

These first results allow us to determine the highest concentration of each RAFT agent which does not lead to any detrimental decrease of the final conversion (about 73%). The corresponding values are 11.2 mM and 80.5 mM for CPADB and CDT, respectively. In the following work, formulations containing 11.2 mM CPADB and 11.2, 40.3, 60.4 80.5 mM CDT will be investigated with a view to compare the impact of the two RAFT agents at same concentration (11.2 mM) and at maximal concentration before the decrease of the final conversion. The inhibition time, the polymerization rate, the final conversion and t₈₀% are reported in Table 1. It should be noted that the relative amount of light absorbed by the RAFT agents is negligible with respect to that of TPO (16.6% for 201.3 mM CDT and 3.4% for 11.2 mM CPADB at 385 nm). This slight absorption could contribute to an additional polymerization delay; however, this effect could not explain the significant decrease in final conversion (Table S2 and Table S3).

<table><caption>**Table 1.** Inhibition time (t<sub>inhib</sub>), maximal polymerization rate (R<sub>pmax</sub>), time to reach 80% of the final conversion (t<sub>80%</sub>) and final conversion (DBC<sub>max</sub>) for formulations without RAFT, with 11.2, 40.3, 60.4, 80.5 mM CDT and with 11.2 mM CPADB.</caption><thead><tr><th rowspan="2">RAFT agent</th><th rowspan="2">0 mM</th><th colspan="4">CDT</th><th rowspan="2">CPADB<br>11.2 mM</th></tr><tr><th>11.2 mM</th><th>40.3 mM</th><th>60.4 mM</th><th>80.5 mM</th></tr></thead><tbody><tr><td>Inhibition time t<sub>inhib</sub> (s)</td><td>0.48</td><td>0.49</td><td>0.85</td><td>1.57</td><td>2.23</td><td>3.27</td></tr><tr><td>Maximal polymerization rate R<sub>pmax</sub> (s<sup>-1</sup>)</td><td>0.71</td><td>0.42</td><td>0.15</td><td>0.12</td><td>0.08</td><td>0.05</td></tr><tr><td>Time to reach 80% of the final conversion t<sub>80%</sub> (s)</td><td>5.0</td><td>5.4</td><td>10.6</td><td>13.3</td><td>21.5</td><td>16.2</td></tr><tr><td>Final conversion (DBC<sub>max</sub>%)</td><td>73.5</td><td>70.5</td><td>74.1</td><td>72.5</td><td>73.5</td><td>73.4</td></tr></tbody></table>

As the RAFT agent induces a large inhibition time, it is important to verify that the formulation is still printable. Therefore, the Jacobs working curves were determined by measuring the thickness (Cd) of cured with the light dose (E) applied. These two parameters are linked together by the depth of penetration Dp of the light and the critical energy Ec, which is the minimum energy required to reach the gel point (equation 1).²⁶

27

$$C_d = D_p \times \ln \frac{E}{E_c} \quad (\text{eq. 1})$$

These photonic parameters are crucial in order to calculate the light dose necessary to print a given thickness of resin. These working curves (385 nm, 9.8 mW/cm²) with 11.2, 40.3, 60.4, 80.5 mM CDT, with 11.2 mM CPADB or without RAFT agents are shown in Figure 2.

As can be seen from the results collected in Table 2, the RAFT agents increase the Ec values from 5.8 mJ/cm² for the reference formulation to 83.0 mJ/cm² for CDT and to 11.2 mJ/cm² for CPADB. Similarly, CPADB and CDT present a different influence on the Dp value. The increase of Dp with increasing CDT concentration is not fully understood and would deserve a dedicated study. Nevertheless, both formulations are still operative for 3D printing.
<figure><img src="image_8.png" /><figcaption>**Figure 2.** Working curves of formulation with without RAFT, with 11.2, 40.3, 60.4, 80.5 mM CDT and with 11.2 mM CPADB.</figcaption></figure>

<table><caption>**Table 2.** E<sub>c</sub> and D<sub>p</sub> values for formulations without RAFT, with 11.2, 40.3, 60.4, 80.5 mM CDT and with 11.2 mM CPADB.</caption><thead><tr><td>RAFT agent</td><td>Critical energy E<sub>c</sub> (mJ/cm<sup>2</sup>)</td><td>Depth penetration D<sub>p</sub> (μm)</td></tr></thead><tbody><tr><td>None</td><td>5.6</td><td>328</td></tr><tr><td>CDT 11.2 mM</td><td>5.9</td><td>345</td></tr><tr><td>CDT 40.3 mM</td><td>27.7</td><td>537</td></tr><tr><td>CDT 60.4 mM</td><td>47.7</td><td>618</td></tr><tr><td>CDT 80.5 mM</td><td>83.0</td><td>1346</td></tr><tr><td>CPADB 11.2 mM</td><td>17.9</td><td>232</td></tr></tbody></table>

To investigate more deeply the impact of the CDT and the CPADB on the photopolymerization process, the formation of the polymer network was studied by photorheometry. Gel point, storage and loss modulus were determined using an irradiation of 9.8 mW/cm² at 385 nm (Figure 3).

<figure><img src="image_9.png" /><figcaption>**Figure 3.** Storage (dotted line) and loss modulus (solid line) measured during irradiation for formulations with CDT, CPADB or without RAFT agent.</figcaption></figure>
It is worth comparing the time needed to reach the gel point (GP) as determined from crossing point between storage and loss moduli, and the corresponding double bond conversion (DBC) obtained from RT-FTIR at the same irradiation time (Table 3).

As expected, due to the retardation caused by the RAFT mechanism, the time at gel point increases from 0.74 s to 8.76 s and 9.76 s for 11.2 mM CPADB and 80.5 mM CDT, respectively. Interestingly, the RAFT agent increases the conversion at the gel point (from 1.1% to 6.1 % and 14.2 % for 11.2 mM CPADB and 80.5 mM CDT, respectively).

<table><caption>**Table 3.** Time and DBC at gel point for formulations without RAFT, with 11.2, 40.3, 60.4, 80.5 mM CDT and with 11.2 mM CPADB.</caption><thead><tr><td>RAFT agent</td><td>Time at GP (s)</td><td>DBC at GP (%)</td></tr></thead><tbody><tr><td>None</td><td>0.50 ± 0.01</td><td>1.1 ± 0.3</td></tr><tr><td>CDT 11.2 mM</td><td>0.87 ± 0.01</td><td>6.7 ± 0.4</td></tr><tr><td>CDT 40.3 mM</td><td>3.62 ± 0.05</td><td>10.1 ± 0.4</td></tr><tr><td>CDT 60.4 mM</td><td>5.7 ± 0.1</td><td>12.2 ± 0.4</td></tr><tr><td>CDT 80.5 mM</td><td>10.3 ± 0.1</td><td>14.4 ± 0.1</td></tr><tr><td>CPADB 11.2 mM</td><td>5.1 ± 0.2</td><td>6.1 ± 0.2</td></tr></tbody></table>

These photorheological experiments afford interesting information close on the polymer network to the gel point. In order to complete the whole picture about the network formation, DMA tests were performed on the different formulations (Figure 4). Films were prepared in laminar mode between two internal sheet of polypropylene of 100 µm and two external plate of glass separated with a spacer of 100 µm made of tape. The formulation was irradiated for 5 min at 385 nm with an irradiance of 10 mW/cm².

<figure><img src="image_10.png" /><figcaption>**Figure 4:** DMA tests on films based on formulations a) without any RAFT agent, with 11.2 mM CPADB and 11.2 mM CDT and b) without any RAFT agent and with 11.2, 40.3, 60.4, 80.5 mM CDT.</figcaption></figure>
Figure 4 shows the tan(δ) curves obtained for the different formulations, which traduce the relaxation of the polymers chains during the heating of the material. From these curves, the relaxation temperature (T<sub>α</sub>) and the full width at half maximum (FWHM) were extracted (Table 4).

<table><caption>**Table 4.** Relaxation temperature (T<sub>α</sub>) and full width at half maximum (FWHM) of the tan(δ) peak for the formulations without any RAFT agent, or with 11.2, 40.3, 60.4, 80.5 mM CDT or 11.2 mM CPADB.</caption><thead><tr><td>RAFT agent</td><td>Relaxation temperature t<sub>α</sub> (°C)</td><td>Full Width at Half Maximum FWHM (°C)</td></tr></thead><tbody><tr><td>None</td><td>63.4 ± 0.7</td><td>27.1 ± 0.7</td></tr><tr><td>CDT 11.2 mM</td><td>59.8 ± 1.0</td><td>26.9 ± 0.8</td></tr><tr><td>CDT 40.3 mM</td><td>57.3 ± 0.7</td><td>26.1 ± 0.4</td></tr><tr><td>CDT 60.4 mM</td><td>57.4 ± 0.9</td><td>25.4 ± 0.6</td></tr><tr><td>CDT 80.5 mM</td><td>53.2 ± 0.8</td><td>23.6 ± 0.6</td></tr><tr><td>CPADB 11.2 mM</td><td>56.6 ± 1.2</td><td>24.6 ± 0.4</td></tr></tbody></table>

Keeping in mind that the conversion is similar for all the samples, the FWHM value is a good parameter to estimate the homogeneity of the network. The results obtained with CDT show a decrease of the full width at half maximum of 13 % at 80.5 mM CDT with respect to the sample without RAFT agent. Concerning CPADB, a decrease of 9.4 % was noticed at 11.2 mM.

In radical photopolymerization, the formation of 3D network starts with the formation of initial short 3D clusters during the early time of irradiation (Scheme 2). Then, the clusters are growing and link together at the gel point. Further irradiation will contribute to increase the cross-link density, although that some inhomogeneity is created within the material due to the lack of mobility. Adding a RAFT agent in the formulation will lead to two major changes in the network formation. As the RAFT agent inhibit the growing clusters and create new ones during the pre-equilibrium step, it will globally increase the number of clusters before the gel point. When the gelation takes place with a decreased inter-cluster volume, a fact which explains the increase of conversion at gel point. As the conversion at gel point is nearly the same at RAFT isoconcentration (DBC at GP = 6.7 ± 0.4 % and DBC at GP = 6.1 ± 0.2 % for formulations containing respectively 11.2 mM CDT and 11.2 mM CPADB), it can be assumed that the number of initial clusters depends mostly of the concentration of RAFT agent, and not of the Z group structure.

Furthermore, keeping in mind that the RAFT agent continuously transfers radicals from growing clusters to dormant clusters during the main equilibrium, it allows a simultaneous growth of the clusters, which results in a better homogeneity of the network. DMA results shows that the homogeneity between the clusters is mainly governed by the Z group structure. Indeed, at isoconcentration (11.2 mM), CPADB leads to a significant decrease of the full width at half maximum, which is not observed at the same extent with CDT. This is certainly due to the benzoyl group of the CPADB, which results in a better stabilisation of the intermediate 4 (Figure S1). The relaxation temperature T<sub>α</sub> also decreases from 63.4°C to 53.2°C and 56.6°C for materials containing 80.5 mM CDT and 11.2 mM CPADB respectively. This is probably due to the homogeneization of the network, which diminishes the amount of entangled chains, and so reduces the amount of energy necessary to relax the polymer chains.
<figure><img src="image_11.png" /><figcaption>**Scheme 2.** Formation of the network for A) conventional free radical polymerization and B) RAFT-mediated controlled radical polymerization

Finally, the mechanical properties of these materials were investigated through tensile tests curves (Figure 5 and Table 5). Tensile tests were repeated 5 times for each formulation.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 5:** Mechanical tests on films based on formulations a) without any RAFT agent, with 11.2 mM CPADB and 11.2 mM CDT and b) without any RAFT agent and with 11.2, 40.3, 60.4, 80.5 mM CDT.</figcaption></figure>
The samples made with CDT or CPADB exhibit a lower Young's modulus than the sample without RAFT agent (Figure 5). In addition, the stress at break decreases ($\sigma_b$ (80.5 mM CDT) = 24.6 ± 1.6 MPa and $\sigma_b$ (11.2 mM CPADB) = 22.2 ± 2.4 MPa) in comparison to the reference ($\sigma_b$ (TPO) = 30.2 ± 4.0 MPa). This is certainly due to the decrease of the $T_a$, which allows the material to be deformed with less energy. Furthermore, the elongation at break increases for the formulations containing RAFT agents in comparison to the reference sample.

An interesting point is that the material based on formulation containing 60.4 mM CDT presents a higher strain break ($\epsilon_b$ = 14.1 ± 3.3) than the material based on formulation containing 11.2 mM CPADB ($\epsilon_b$ = 8.8 ± 3.8). Keeping in mind that the CDT sample present a higher conversion at gel point and a lower full width at half maximum, it seems that the increase of the elongation break is mainly due to the increase of the initial clusters than the homogeneity between the clusters.

<table><caption>**Table 5.** Stress and elongation at break, toughness values for formulations without any RAFT agent, with 11.2, 40.3, 60.4, 80.5 mM CDT and 11.2 mM CPADB.</caption><thead><tr><th>RAFT agent</th><th>Stress at break σ<sub>b</sub> (MPa)</th><th>Strain at break ε<sub>b</sub> (%)</th><th>Toughness K<sub>ic</sub> (MPa/cm<sup>1/2</sup>)</th></tr></thead><tbody><tr><td>None</td><td>29.3 ± 4.0</td><td>5.1 ± 1.8</td><td>0.82 ± 0.03</td></tr><tr><td>CPADB 11.2 mM</td><td>22.2 ± 2.4</td><td>8.8 ± 3.8</td><td>0.94 ± 0.01</td></tr><tr><td>CDT 11.2 mM</td><td>22.8 ± 3.1</td><td>8.0 ± 1.2</td><td>0.81 ± 0.02</td></tr><tr><td>CDT 40.3 mM</td><td>23.5 ± 1.9</td><td>9.5 ± 1.0</td><td>0.96 ± 0.06</td></tr><tr><td>CDT 60.4 mM</td><td>23.0 ± 1.5</td><td>14.1 ± 3.3</td><td>1.03 ± 0.07</td></tr><tr><td>CDT 80.5 mM</td><td>24.6 ± 1.6</td><td>23.2 ± 3.2</td><td>1.05 ± 0.06</td></tr></tbody></table>

Acrylates are known to exhibit poor toughness. Applications of acrylate based systems would be more interesting in 3D printing if this property was increased. Therefore, the impact of the RAFT agent on the final toughness was evaluated on formulations with and without RAFT using the ISO 13586:2018 standard: cracked tensile specimens (Figure 6) were manufactured by 3D DLP Printing and then studied in three-point bending contraction test.

The toughness $K_{ic}$ can be calculated from charge at break $F_Q$ with equation 2.

$$K_{ic} = \frac{f(a/W) \times F_Q}{\sqrt{W \times h}} \quad (\text{eq. 2})$$

where a is the length of the crack, W is the width of the tensile specimen and h is the height of the tensile specimen. $f(a/W)$ is calculated by the following equation 3.

$$f(a/W) = 6\alpha^{1/2} \frac{1.99 - \alpha(2.15 - 3.93\alpha + 2.7\alpha^2)}{(1 + 2\alpha)(1 - \alpha)^{3/2}} \quad (\text{eq. 3})$$

where $\alpha = \frac{a}{W}$. The results are collected in Table 5.

A significant increase of the toughness was observed for materials based on formulations with 40.3, 60.4 and 80.5 mM CDT (+17.1%, +25.6% and +28.0% in comparison of the reference, respectively) and with 11.2 mM CPADB (+14.6% in comparison of the reference). The toughness seems to increase with the cluster homogeneity in the material. Furthermore, as the material based on 60.4 mM CDT formulation reaches a
lower homogeneity than the material based on 11.2 mM CPADB formulation, the higher toughness seems to be due to the higher RAFT concentration, and so to the higher number of initial clusters in the material.

<figure><img src="image_13.png" /><figcaption>**Figure 6.** a) Picture of a cracked tensile specimen (100x20x7.5 mm³) based on a formulation without any RAFT agents. b) Three-point bending contraction test on cracked tensile specimens based on formulations without RAFT, with 11.2 mM CDT and with 11.2 mM CPADB. c) Three-point bending contraction test on cracked tensile specimens based on formulations without RAFT and with 11.2, 40.3, 60.4, 80.5 mM CDT.</figcaption></figure>

## Conclusion

RAFT agents have an impact on the photopolymerization of acrylates, through the formation of more homogeneous 3D polymer network. Consequently, a significant improvement of the mechanical properties is noticed, with increased elongation at break values for samples exhibiting the same conversion. The toughness is also improved by about 20-25%. This clearly evidences the potential of RAFT agents in 3D printing of multifunctional acrylate formulations. Therefore, it is important to keep in mind that the mechanical properties, as the network structure, depends strongly of the monomer structure. Further works are in progress in order to highlight the effect of RAFT agents on monomers with different structures and functionalities.

## Notes and references

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