# RAFT-mediated thermoplastic material by photopolymerization: Controlling liquid-solid transition for vat photopolymerization in 3D DLP printing

Emile Goldbach, Xavier Allonas, Lucile Halbardier, Christian Ley, Celine

Croutxé-Barghorn

► To cite this version:

Emile Goldbach, Xavier Allonas, Lucile Halbardier, Christian Ley, Celine Croutxé-Barghorn. RAFT-mediated thermoplastic material by photopolymerization: Controlling liquid-solid transition for vat photopolymerization in 3D DLP printing. European Polymer Journal, 2023, 197, pp.112335. 10.1016/j.eurpolymj.2023.112335. hal-05073945

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# RAFT-mediated thermoplastic material by photopolymerization: controlling liquid-solid transition for vat photopolymerization in 3D DLP printing †

Emile Goldbachª, Xavier Allonas*ª, Lucile Halbardierª, Christian Leyª, Celine Croutxé-Barghornª

In this paper, the impact of the solidification process on a 3D DLP printed thermoplastic material based on a monofunctional acrylate resin containing a RAFT agent is investigated. First, the conversion and the molar mass at the point of solidification were determined using photorheology and SEC experiments. These results were compared to those obtained through the photopolymerization occurring in the vat, i.e. for solidification of the material immersed in the formulation. Interestingly, the conversion increases and the mass molar decreases at the point of solidification with the addition of the RAFT agent. Moreover, the material photopolymerized without RAFT agent evolves through an intermediate semi-solid state during the solidification process. This state does not appear for the RAFT-mediated material, resulting in a better liquid-to-solid separation and, ultimately, in a better repeatability for the printed material.

**Keywords:** RAFT photopolymerization, photopolymerized thermoplastic, 3D printing, vat photopolymerization, controlled polymerization

## Introduction

3D printing is one of the most promising manufacturing technology developed over the past years for the fabrication of complex objects used in a wide variety of applications like aerospace, automotive industries, (bio)medical, architecture and more.<sup>1,2</sup> Especially, digital light processing (DLP) 3D printing holds central place among these technologies, providing high resolution and printing efficiency.<sup>3,4</sup> In that case, materials are usually based on multifunctional monomers, allowing good liquid-to-solid transition thanks to the formation of a covalent polymer networks<sup>5</sup>. However, unlike thermoplastics materials, these thermosets material cannot be reprocessed or recycled (due to their intrinsic infusibility and insolubility).<sup>6,7</sup> From that perspective, some researches has been made recently in order to print a thermoplastic material with sufficient mechanical properties by DLP.<sup>8-14</sup> The main issue is the diffusion and the dissolution of the polymer chains in the resin during the printing process. In order to avoid it, the liquid-solid transition needs to be fast (which is usually ensured by a high polymerization rate) and the resulting polymer should have a low solubility in the resin.<sup>8</sup> For example, using 4-acryloylmorpholine<sup>8</sup> or isobornyl acrylate<sup>9</sup>, thermoplastic materials were successfully printed by DLP. However, both materials present poor mechanical properties, with stress at break reaching 10 MPa at a strain of 10 % for the 4-acryloylmorpholine based material and 8 MPa at a strain of 7 % for the isobornyl acrylate based material. Nevertheless, their fusibility capacity was successfully demonstrated. In order to increase the mechanical properties of the material, few other approaches have

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

<footnote>† Electronic supplementary information (ESI) available: Materials and methods used for this study. See DOI: XXXXXX</footnote>
been investigated. A stoichiometric combination of 1,6-hexanedithiol and diallyl terephthalate allowed to print high mechanical properties material¹⁰. The printed material crystalizes in few minutes after the reaction, a fact that leads to enhanced mechanical properties: the material reaches a stress at break of 25 MPa at a strain of 800 %. An alternative approach was based on covalent adaptive networks using a monomer or a combination of substituted monomers with functions able to give (like -OH or -NH functions) or accept (like -C=O functions) hydrogen bonds. Furthermore, these monomers were also mixed with zinc salt in order to create ionic bonds¹¹⁻¹³. As the weak interactions ensure a solidification of the polymerized material, the glass transition temperature of the material doesn't need to be higher than the printing temperature. Therefore, elastomers with good mechanical properties (strain at break up to 800 % for a stress at break of 4 MPa) have been printed by DLP. The mechanical properties can be tailored according to the glass temperature of the final material (for a material with a Tg of 80 °C, the material break at a stress of 21 MPa and a strain of 10 %, and for a material with a Tg of 30 °C, the material break at a stress of 0.4 MPa and a strain of 550 %)¹³. Thanks to the weakness of the hydrogen bonds, these materials are still fusible like classic thermoplastic polymers. However, the previous approaches are limited by the choice of monomer, and there is no general solution which could be applied to a wide variety of acrylates. Furthermore, there is a real lack of data about the solidification process and the impact of the liquid-solid transition on the printed material, which seems to be crucial for DLP printing of thermoplastics¹⁰⁻¹³, was not investigated.

RAFT polymerization is already used for the 3D DLP printing of thermosets materials. The living characteristic of the RAFT polymerisation allows a new wide range of applications, such as the printing of formulations containing reactive prepolymers¹⁵⁻¹⁹, or the insertion of monomers in an already 3D printed material²⁰⁻²². In this paper, attempt was made to print a RAFT-mediated acrylate thermoplastic material by DLP. As the RAFT polymerization allows a good control of the molar mass and the polydispersity²³⁻²⁸, the state of the material during the solidification process will be investigated and compared under different conditions. To this end, a model will be developed in order to predict the molar mass and conversion into the material in function of the printing parameters. Finally, the impact of the solidification process on the material quality will be explored.

## Materials and Methods

The formulation used in this paper (Scheme 1) is based on isobornyl acrylate (Sartomer) as monofunctional monomer, and contains 30 mM of diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO, IGM Resins) as photoinitiator and 1-((2-Methyl-4-((2-methylphenyl)-diazenyl)phenyl)-diazenyl)naphthalene-2-ol (Sudan IV, Sigma-Aldrich) as a UV blocker. Cyanomethyl-dodecyl-trithiocarbonate (CDT, TCI Chemicals) was chosen as the RAFT agent, as it presents a good control of the acrylate radical polymerization¹⁴, ²⁹. The detailed composition of all formulations investigated during this work is reported in Table S1.

The acrylate double bond conversion was followed by Real Time Fourier Transform Infrared Spectroscopy (RT-FTIR). The measurement was achieved on Bruker Vertex 70 spectrometer, with a spectral resolution of 4 cm⁻¹ and averaged over 30 scans. 25 µm thick samples were analysed in laminar mode between two 10 µm films of polypropylene and two BaF₂ pellets. Samples were irradiated with a 385 nm LED (UWave UFIBER) at an irradiance of 9.8 mW/cm². The conversion was determined by following the decrease of the acrylate double bond at 1404 cm⁻¹.

The rheological measurements were achieved by an Anton Paar MCR302 Rheometer with parallel plates (PP), at 25°C with an air gap of 50 µm.
The frequency used during the test was 50 rad/s with a deformation of 0.2 %. Samples were irradiated with a 385 nm LED (UWave UFIBER) at an irradiance of 9.8 mW/cm².

For size exclusion chromatography experiments, the samples were prepared by putting 40 mg of the formulation between two glass plates separated with a spacer of 25 µm. The formulation was irradiated with a 385 nm LED (UWave USTRAIGHT) at an irradiance of 10 mW/cm². The resulting material was solubilized in 2 mL of tetrahydrofuran (THF) and then analysed by a Shimadzu LC-20AD liquid chromatograph mounted with size exclusion chromatography column VARIAN PLgel 5 µm. The solvent used (THF), kept at 30°C, was injected in the column with a flow of 1 mL/min. The chromatograph was coupled with a Shimadzu RID-10A differential refractometric detector.

3D printing working curves were performed on a glass plate by irradiation with a 385 nm LED (M385L3 from Thorlabs) at an irradiance of 9.8 mW/cm². The thickness of the resulting photopolymer was measured with an optical profilometer (Alti Surf 500 from Altimeter) mounted with a 2 mm chromatic detector. 3D printed objects were fabricated using an DLP printer (Asiga UV Max) by additive manufacturing of 90 layers of 100 µm cured with an irradiance of 9.8 mW/cm² at a wavelength of 385 nm.

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Structure of the molecules used in this study.</figcaption></figure>

# Results and discussion

## Photopolymerization process

The polymerization kinetics of formulations containing isobornyl acrylate as monomer, TPO as photoinitiator and Sudan IV as photoabsorber were studied by real-time Fourier transform infrared spectroscopy (RT-FTIR). CDT was used as RAFT agent at different concentrations (Table S1).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Double bond conversion measured during irradiation (385 nm, 9.8 mW/cm²) for formulations with different CDT concentrations.</figcaption></figure>

As can be seen in Figure 1 and Table 1, when the concentration of CDT increases, the maximum polymerization rate decreases and the inhibition time increases, as reported elsewhere¹⁴. This could be attributed to the decrease in active species, as a consequence of the presence of dormant chains (Scheme S1).³⁰ Furthermore, the final conversion, which is almost stable up to 80 mM of CDT, dramatically decreases for the formulation containing 100 mM of CDT (from 87.1 % to 50.8 %). This can be explained by the increase in termination reactions due to the presence of a stable radical intermediate (2 or 4 in Scheme S1) formed during the main equilibrium of the RAFT mechanism.³¹ Consequently, only formulations containing a CDT concentration ranging from 0 to 80 mM will be investigated.

<table><caption>**Table 1.** Inhibition time t<sub>inhib</sub>, maximum polymerization rate R<sub>p</sub><sup>Max</sup> and final conversion of formulations at different CDT concentrations.</caption><thead><tr><td>[CDT] (mM)</td><td>t<sub>inhib</sub> (s)</td><td>R<sub>p</sub><sup>Max</sup> (mol.L<sup>-1</sup>.s<sup>-1</sup>)</td><td>Final conversion (%)</td></tr></thead><tbody><tr><td>0</td><td>1.0</td><td>0.95</td><td>91.5</td></tr><tr><td>30</td><td>4.6</td><td>0.57</td><td>92.1</td></tr><tr><td>45</td><td>8.2</td><td>0.48</td><td>90.1</td></tr><tr><td>64.8</td><td>10.3</td><td>0.32</td><td>89.5</td></tr><tr><td>80</td><td>18.3</td><td>0.22</td><td>87.1</td></tr><tr><td>100</td><td>38.2</td><td>0.09</td><td>50.8</td></tr></tbody></table>

## Photopolymer characterization by photorheology and SEC

As the RAFT mechanism is well known to reduce the chain length of the material, photorheological studies are essential to characterize the solidification process. Therefore, storage and loss moduli of the different formulations were determined using an irradiation of 9.8 mW/cm² at 385 nm (Figure 2).
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Storage and loss moduli measured during irradiation (385 nm, 9.8 mW/cm²) for formulations with different CDT concentrations.</figcaption></figure>

As the final material consists of linear polymer chains, the crossing point between the storage and loss moduli curves corresponds to the solidification point³² (Table 2). Photorheology (Figure 2) shows that the solidification point is gradually retarded when CDT concentration increases (from 4.3 s without CDT to 55.6 s for 80 mM of CDT). As all the experiments were performed at the same irradiance, it is possible to determine at which time t the solidification point is obtained (Figure 2) and to use the results obtained by RT-FTIR to determine the conversion at which this solidification point is obtained (from Figure 1).

It could be observed that a high conversion is required to solidify the thermoplastic material (approximately 60 %) in comparison to a classic thermoset material (typically less than 30 %)¹⁴,³³. Furthermore, the conversion at solidification increases with the CDT concentration (from 58.4 % for the reference to 79.2 % for 80 mM CDT).

<table><caption>**Table 2.** Time at solidification point $t_{solidRheo}$ (obtained from photorheology experiments), conversion ($C_{solidRheo}$) and molar mass at solidification ($M_{solidRheo}$) for formulations with different CDT concentrations.</caption><thead><tr><td>[CDT] (mM)</td><td>t<sub>solidRheo</sub> (s)</td><td>C<sub>solidRheo</sub> (%)</td><td>M<sub>solidRheo</sub> (g/mol)</td></tr></thead><tbody><tr><td>0</td><td>4.3</td><td>58.4</td><td>169,000</td></tr><tr><td>30</td><td>13.1</td><td>59.7</td><td>22,100</td></tr><tr><td>45</td><td>20.4</td><td>63.9</td><td>18,600</td></tr><tr><td>64.8</td><td>31.4</td><td>65.3</td><td>15,000</td></tr><tr><td>80</td><td>55.6</td><td>79.2</td><td>14,700</td></tr></tbody></table>

In order to get insights into this phenomenon formulations containing various CDT concentrations were characterized by SEC.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Polydispersity in function of the conversion of formulations with different CDT concentrations.</figcaption></figure>

As can be seen in Figure 3, the polydispersity is clearly reduced by the RAFT mechanism for all CDT concentrations (from 4-5 for the reference to less than 1.5 for all formulations with CDT at early stage of polymerization). The increase of polydispersity after 50% of conversion is due to the lack of mobility enhanced by the solidification of the material. To ensure that the number of active species is constant during the reaction, the time evolution of ln([M]₀/[M]) was investigated (Figure 4). Three regimes can be observed. First, there is an induction time due to the pre-equilibrium of the RAFT mechanism and the consumption of dissolved oxygen. Then, a constant evolution of ln([M]₀/[M]) can be observed, which accounts for a constant number of active species. Finally, termination reactions appear, as the material solidify. Therefore, the linear evolution of ln([M]₀/[M]) during the second regime and the decrease of the dispersity for formulations containing CDT indicates a good control of the reaction.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Time evolution of ln([M]₀/[M]) for formulations with different CDT concentrations.</figcaption></figure>

Also, the molar mass of the polymer is gradually reduced with the CDT concentration (Figure S1). This is due to the reinitiating step of the RAFT mechanism, which increases the quantity of growing chains in the material³⁴. Furthermore, unlike the reference, the change in molar mass with
the conversion seems to follows a linear relationship. This is in agreement with equation 1 which predicts the theoretical molar mass of the polymer $M_{n,th}$ (when the amount of radical species formed from the photoinitiator is neglected with respect to the quantity of RAFT agent) ³⁵.

$$M_{n,th} = \frac{C_v \times [M]_0 \times M_M}{[RAFT]} + M_{RAFT} \quad (eq. 1)$$

where $C_v$ is the acrylate double bond conversion during the reaction and $M_M$ and $M_{RAFT}$ are respectively the monomer and the RAFT agent molar mass. First, the measured molar mass seems to be overestimated in comparison to the theoretical molar mass (Figure 5). This may be explained by a not optimal calibration of SEC. Actually, the RAFT polymerization seems optimal (i.e. when there is no termination reaction) for formulations containing 64.8 and 80 mM CDT, as the evolution of the molar mass in function of the conversion is linear. However, the molar mass for formulations with 30 and 45 mM CDT slowly decreases with the conversion in comparison to the ideal case.

The prediction of the molar mass at given conversion values is a key point, as it would permit to know precisely the molar mass at the point of solidification. As the theoretical molar mass is slightly different than the measured molar mass, an empirical fitting of the plots shown in Figure 5 for 0 mM, 30 mM CDT, 45 mM CDT, and one for CDT both concentrations of 64.8 and 80 mM CDT were performed (see Figure S2, Figure S3 and Table S2). From these data, the molar mass at the point of solidification can be determined and reported in Table 2.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Molar mass changes with the conversion of formulations at different CDT concentrations in comparison to the theoretical molar mass.</figcaption></figure>

For a given conversion, when the CDT concentration increases, the number of growing chains increases and the molar mass of the polymer decreases. As a consequence, the solubility of the chains is favoured and therefore, whereas the molar mass at solidification decreases, the conversion at solidification increases with the CDT concentration.

## Behaviour during the vat photopolymerization process

In order to characterize the solidification process during a 3D printing experiment, Jacobs working curves were determined (see Figure 5).³³,³⁶ In these experiments, a large volume of photopolymerizable resin is irradiated from the bottom through a 1x1 cm² transparent window. Therefore,
various pillars of solid sample are created within the liquid formulation by delivering different exposure doses E (@385 nm, 9.8 mW/cm²). The height of each pillar C<sub>d</sub> is given by:

$$C_d = D_p \times \ln \frac{E}{E_c} \quad (\text{eq. 2})$$

with D<sub>p</sub> the depth of penetration of the light and E<sub>c</sub> the critical energy, which is the minimum energy required to reach the point of solidification. As can be seen in Figure 6 and Table 3, when the polymerization rate decreases with the CDT concentration, the critical energy E<sub>c</sub> gradually increases from 28 mJ/cm² for the formulation without CDT to 729 mJ/cm² for formulation containing 80 mM CDT. In the meantime, the depth penetration D<sub>p</sub> increases with the CDT quantity as a consequence of the photobleaching of the photoinitiator. It is worth noting that in this case, the solidification of the pillars occurs within the liquid resin, which could act as solvent, i.e. in immersed conditions. From these experiments, a conversion C<sub>solid</sub><sup>imm</sup> and molar mass M<sub>solid</sub><sup>imm</sup> at solidification can also be obtained.

<figure><img src="image_8.png" /><figcaption>Figure 6. Jacobs working curves (JWC) for the formulations with different CDT concentrations.</figcaption></figure>

<table><caption>**Table 3.** Depth of penetration D<sub>p</sub> and critical energy E<sub>c</sub> of formulations with different CDT concentrations and conversion C<sub>solid</sub><sup>imm</sup> and molar mass M<sub>solid</sub><sup>imm</sup> at solidification as obtained in immersed conditions.</caption><thead><tr><th>[CDT] (mM)</th><th>D<sub>p</sub> (μm)</th><th>E<sub>c</sub> (mJ/cm²)</th><th>C<sub>solid</sub><sup>imm</sup> (%)</th><th>M<sub>solid</sub><sup>imm</sup> (g.mol<sup>-1</sup>)</th></tr></thead><tbody><tr><td>0</td><td>318</td><td>28</td><td>33.1</td><td>147,000</td></tr><tr><td>30</td><td>305</td><td>133</td><td>65.1</td><td>23,200</td></tr><tr><td>45</td><td>310</td><td>210</td><td>71.1</td><td>19,600</td></tr><tr><td>64.8</td><td>518</td><td>406</td><td>83.7</td><td>19,200</td></tr><tr><td>80</td><td>857</td><td>729</td><td>87.7</td><td>16,300</td></tr></tbody></table>

During the exposure process, a solid polymer is created as soon as the solidification point is reached. Increasing the irradiation time allows the solid to grow while the upper part of the solid always corresponds to the transition between the liquid and the solid.
Thanks to our previous works, it is possible to predict the conversion in the depth of a printed layer with the following model.33, 36 The light dose $E_z$ received by the formulation at a depth z is given by:

$$E_z = I_0 \times t_{\text{layer}} \times 10^{-\varepsilon Cz} \quad (\text{eq. 3})$$

Where $I_0$ is the irradiance, $t_{\text{layer}}$ is the irradiation time, $\varepsilon$ is the molar absorption coefficient and $C$ is the concentration of the absorbing species. As the depth penetration $D_p$ is defined by the depth where 63%33 of the initial light is absorbed by the formulation, the experimental absorptivity of each formulation $\varepsilon C$ can be calculated from:

$$\varepsilon C = -\frac{1}{D_p} \log(0.37) \quad (\text{eq. 4})$$

Using the experimental conversion curves obtained from RT-FTIR experiments allows the conversion $C_v$ to be predicted by an empirical model:

$$C_v = \frac{a \times (I_0 \times t_{\text{layer}} \times 10^{-\varepsilon Cz})^c}{b^c + (I_0 \times t_{\text{layer}} \times 10^{-\varepsilon Cz})^c} \quad (\text{eq. 5})$$

where the coefficients $a, b, c$ and the absorptivity $\varepsilon C$ values are reported in Table S3 for each formulations studied. Therefore, the conversion at solidification can be determined as well as the molar mass at solidification when the material is immersed in the formulation. As can be seen in Table 3, the results of conversion and molar mass at solidification for an immersed material presents a similar trend with respect to the results obtained by photorheology (Table 2).

## Impact of the liquid-solid transition during vat photopolymerization

However, the two methods used to investigate the conversion and the molar mass at solidification (photorheology and immersed polymerized sample) presents significant differences (Figure 7) which deserve further investigation.

<figure><img src="image_9.png" /><figcaption>Figure 7. Conversion at solidification obtained by photorheology and by Jacobs working curves for an immersed material.</figcaption></figure>

For the material based on formulations containing CDT, immersing the material shifts the solidification process to higher conversions. This can simply be explained by the dissolving process of the slightly polymerized surface. More interestingly, for the material containing no CDT, the
solidification appears at a lower conversion when the material is immersed. It turns out that the RAFT agent clearly affects the liquid-solid transition. This can be studied by observing the change in the storage modulus $G'$ with the conversion (Figure 8).

The $G'$ change with the conversion is quite different for formulations with and without RAFT agent. For formulations containing CDT, $G'$ starts to significantly rise at a conversion of approximately 50 % (60% for 80 mM), and reaches rapidly almost 1000 MPa. In comparison, for the formulation containing no CDT, $G'$ starts to rise at a conversion of approximately 20-30%, and reaches only about 10 MPa. As $G'$ curve crosses $G''$ one at approximately 10000 MPa (Figure 2), there exists a zone between 10 MPa and 10000 MPa in which the material is more viscous than elastic ($G' < G''$), and is in a semi-solid state. The low conversion at the top of the polymerized sample without CDT can be explained by the formation of this intermediate state which is not considered as a solid by the rheological analysis. This was confirmed by the texture of the top of the sample which is very ductile in comparison to the samples made using CDT.

This effect can be explained by the differences in polydispersity and molar mass. When the formulation without CDT polymerizes, the growing chains reach a high molar mass at early conversion and have a poor solubility. As the polydispersity is high, there is a state when some growing chains precipitate, and other are still soluble in the formulation. This results in a phase where the material is solid due to the chains that precipitated and swelled with a huge quantity of monomer and lower molar mass chains. In comparison, as the CDT decreases the molar mass, when it will precipitate, the material will be swelled with less monomers, resulting in a more rigid material. Moreover, as the polydispersity is reduced, almost all growing chains will precipitate at the same time. As a consequence, the transition between liquid and solid state is more pronounced.

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Change in the storage modulus with the conversion.</figcaption></figure>

In order to study the impact of this liquid-solid transition on the 3D printing process, the repeatability was investigated by printing each formulation 4 times in order to determine the standard deviation $\Delta C_d$ of the thickness of each plots. Figure 9 shows the result for formulations containing 0 and 45 mM CDT (similar curves for formulations containing 30, 64.8 and 80 mM are shown in Figure S4).
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Values of C<sub>d</sub> and corresponding standard deviations showing the better repeatability for samples containing RAFT agent.</figcaption></figure>

It is clear from Figure 8 that the range of possible values for C<sub>d</sub> at the same light dose is much wide for the formulation without CDT compared to formulation containing CDT. This underlines the better repeatability (2.7 µm for 45 mM of CDT) achieved for formulations containing CDT with respect to the one without CDT (9.8µm). Therefore, the presence of CDT ensures a rapid liquid-solid transition (in term of conversion), which is key in DLP 3D printing in order to have a good resolution.⁹ Finally, the printing resolution were evaluated for a formulation containing 30 mM of CDT. For that, a slab mounted with ten slits and stems with decreasing thickness (from 500 µm to 50 µm, with a step of 50 µm) were printed by DLP. Concerning printing conditions, a layer thickness of 100 µm were chosen in order to minimise the printing duration. The time of irradiance of each layers (39,4 s, which correspond to a C<sub>d</sub> of 325 µm) were adjusted in order to maximise the number of slits and stems printed.
The printed material (Figure 10) presents a slit resolution of 200 µm and a stem resolution of 150 µm.

<figure><img src="image_12.png" /><figcaption>**Figure 10.** 3D DLP printing of a resolution piece based on a formulation containing 30 mM of CDT.</figcaption></figure>

This resolution is slightly better to resolution obtained in the literature for printed thermoplastic material based on common acrylate monomers (peak of 200 µm).⁸

## Conclusion

The solidification process of a photopolymerized thermoplastic material was investigated within the framework of 3D printing process. Adding a RAFT agent in the formulation leads at the solidification point to an increase of the molar mass and a decrease of the conversion. In addition, during the solidification process, the formulation without RAFT agent go through a state where the material is semi-solid. This is explained by the high dispersity of the polymer chains. As the formulations containing RAFT agent start to solidify at a higher conversion and with a lower
polydispersity, the liquid-solid transition is more marked. This allows a clear separation between the liquid and the solid state, leading to a better repeatability of the final material. Finally, a RAFT-mediated thermoplastic material were printed by DLP, which reached a resolution of 150 µm.

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Thermoplastic

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Reference

Better resolution

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Better repeatability

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RAFT

RAFT-mediated thermoplastic material printed by DLP