Spatially resolved photoactivation of dynamic exchange reactions in 3D-printed thiol–ene vitrimers

Abstract

The covalently cross-linked network structure of vitrimeric polymers is able to be reorganised by a thermoactivated exchange of covalent bonds. Despite numerous options available for steering the exchange rate in dynamic polymer networks, a spatiotemporal onset, macroscopically discernible as a drop in viscosity, is most often difficult to realise. Reported here is the application of a photolatent transesterification catalyst, which releases a strong guanidine base upon irradiation with 405 nm LED light. Incorporated in a visible-light-cured thiol–ene polymer matrix providing ample hydroxy and ester moieties, spatially resolved catalyst activation enables a selective rearrangement of the network topology via dynamic transesterification. Owing to the use of an efficient oligoacylgermane as radical photoinitiator, which absorbs at 450 nm, a wavelength-orthogonality between the light-mediated curing reaction (step growth polymerisation of the thiol–ene resin) and the light-induced cleavage of the photolatent base catalyst is achieved. Moreover, the fast cure rate allows a fabrication of objects by digital light processing 3D printing. Stress relaxation studies confirm excellent creep properties prior to network activation, i.e. in the presence of the unactivated catalyst, and a fast dynamic exchange of covalent bonds, once the catalyst is released by irradiation with 405 nm light. In addition, a spatially controlled activation of transesterification reactions in selected areas of the 3D-printed polymer structure is demonstrated by a reshaping experiment. To reobtain the shape stability of the unactivated polymer network and to preserve the (reorganized) topology, the active catalyst can be removed by a simple heat treatment.

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Introduction

Conventional covalently cross-linked polymers, i.e. thermosets and elastomers, exhibit favourable strength to weight ratios, creep resistance even at high operating temperatures as well as distinct chemical resistance properties. On the other hand, thermomechanical welding, mending and recycling strategies as applied for thermoplastic polymers cannot be pursued.^1–3

So-called vitrimeric polymers aim to combine the beneficial aspects of both polymer classes. Their three-dimensional covalently cross-linked network topology adopted from thermosets and elastomers is able to be reorganised by an associative bond-forming, bond-breaking reaction sequence in response to an external stimulus (most often heat). Due to the resulting characteristic decrease in viscosity according to the Arrhenius relation, vitrimers are recyclable and repairable similar to thermoplastic materials.^1,3,4 Despite different possibilities to regulate the exchange rate in vitrimers, the implementation of a precisely defined temperature decoupled and spatially resolved onset of bond exchange reactions represents a challenging task.^5,6

In 2018, Bowman et al. introduced a convenient approach for the spatially resolved on-demand activation of base-catalysed thiol-thioester exchange reactions within a covalently cross-linked network structure. Triggered by the light-mediated cleavage of a photolatent base (PLB), i.e. release of a strong amine base, they were able to control the onset of dynamic bond exchange in a spatiotemporal manner.⁷ Our research group successfully transferred this concept to the – in various industrially relevant polymer structures implementable – transesterification mechanism, which relies on an exchange between ester links and hydroxy groups.⁸

Within subsequent studies, we combined the holistic idea of latent catalysis of dynamic transesterification in vitrimer systems with digital light processing (DLP) 3D printing.^9–11 Whereas thermally latent catalysts can be used regardless of the absorption characteristics of the photoinitiator, photolatent catalysts require an (at least) sequence-dependent wavelength-orthogonality.¹⁰ To achieve this fundamental prerequisite, the emission range of the light source used for DLP 3D printing must be adequately separated from the excitation window of the photolatent catalyst. Thus, the latency of the light-sensitive catalyst persists in the cured network, whilst a second range of wavelengths allows spatiotemporal activation.^9,11

We activated a Brønsted photoacid at 365 nm on-demand in a thiol-acrylate network polymerized at 405 nm via a dual-wavelength DLP 3D printing system.⁹ In a second study, an organophosphate with an at 370 nm photolabile o-nitrobenzyl protection group was synthesized and incorporated in a similar thiol-acrylate resin system designed for DLP 3D printing at 450 nm.¹¹ The established wavelength-orthogonality and spatiotemporal activation of dynamic transesterification was successfully demonstrated by stress relaxation and reshaping experiments.^9,11

Herein, we expand the family of catalysts for this concept to photolatent bases, releasing the active species at 405 nm. Advancing from thiol-acrylate photopolymers, which suffer from low creep resistance in the non-activated state, we used a significantly more stable thiol–ene resin system. To make it applicable for visible light DLP 3D printing (and avoid premature activation of the photolatent base), we employed a high-performance free-radical germanium-based photoinitiator absorbing at 450 nm. The finally achieved efficient spatiotemporal activation of dynamic transesterification is demonstrated through stress relaxation studies and locally induced shape changes in a 3D-printed object (Fig. 1).

Fig. 1 Schematic representation of the manufacturing process of a thiol–ene polymer through DLP 3D printing (including chemical structures of the resin components) followed by a light-triggered spatially resolved catalyst activation, i.e. release of an active transesterification catalyst, and a thermo-activated local rearrangement of the cross-linked network structure via a dynamic exchange between ester links and hydroxy groups.

Results and discussion

For efficient radical curing by visible light in an extended wavelength region (well above 400 nm), Norrish type I photoinitiators such as diacylgermanes, e.g. the commercially available Ivocerin®, or tetraacylgermanes are prominent candidates.¹² However, due to an improved performance at wavelengths around 450 nm in combination with its generically good solubility, a recently introduced oligoacylgermane derivative (Ge-PI) was selected, and synthesized according to the literature.¹³ The product's chemical structure was confirmed by ¹H and ¹³C NMR spectroscopy (Fig. S1, ESI†).

Applied in a thiol–ene-curable resin system, the air and moisture-tolerant click polymerisation mechanism enables quantitative yields even under mild reaction conditions.¹⁴ Ester links and hydroxy groups necessary for transesterification were implemented through a targeted selection of monomers. As a multifunctional thiol-based crosslinker, pentaerythritol tetrakis(3-mercaptopropionate) partially esterified with an average degree of 75% (PETMP 75) was used. Trimethylolpropane diallyl ether (TMPDE) served as the alkene component.

To be able to provide the spatially resolved activation of dynamic transesterification within the network structure, the photolatent base 1,1,3,3-tetramethylguanidine phenylglyoxylate (TMG-PLB) was synthesised in an adapted manner according to the literature, and incorporated into the resin system.¹⁵¹H and ¹³C NMR spectra (Fig. S2, ESI†) correspond to the proposed chemical structure shown in Fig. 1 along with the chemical structures of all other formulation components. Although according to the studies of Barner-Kowollik and co-workers the photochemical reactivity of a substance cannot always be directly deduced from its spectral absorption, UV-Vis measurements (Fig. 2a) indicate the requisite inactivity of TMG-PLB at 450 nm.¹⁶ Irradiation with 405 nm LED light, on the other hand, allows the selective release of the strong guanidine base 1,1,3,3-tetramethylguanidine (TMG, pK_a of the corresponding acid is 23.3 in acetonitrile) as an active transesterification catalyst.¹⁷ This behaviour is further confirmed by pH measurements in an aqueous solution containing 0.06 mol L⁻¹ TMG-PLB. Upon irradiation with 450 nm LED light, the pH remained constant in the neutral range even after an exposure dose of 3900 mJ cm⁻² (Fig. S3, ESI†). Thus, photochemical excitation, i.e. cleavage, is prevented during DLP 3D printing. Conversely, a significant shift to 8.4 was detected after an exposure to 820 J cm⁻² 405 nm LED light due to the release of TMG-PLB (Fig. 2b).

Fig. 2 (a) UV-Vis absorption spectra of TMG-PLB (1 × 10⁻³ and 1 × 10⁻⁴ mol L⁻¹ in acetonitrile) and Ge-PI (1 × 10⁻⁴ mol L⁻¹ in acetonitrile). (b) pH measurements in an aqueous solution of TMG-PLB (0.06 mol L⁻¹) upon irradiation with 405 nm LED light (967 mW cm⁻¹). The release of TMG is shown as an inset in the schematic representation.

The stoichiometry of all photopolymer samples prepared was adjusted to a 1 : 1 equivalence between allyl moieties and thiol groups and contained the visible light photoinitiator Ge-PI (readily soluble in TMPDE) in a concentration of 0.15 mol% relative to the thiol functions. TMG-PLB as the photolatent transesterification catalyst was incorporated in a concentration of 5.0 mol% relative to the ester links present. For following the curing kinetics, FTIR measurements were performed under irradiation with a 450 nm LED lamp which matches the emission spectrum of the DLP 3D printer used. The initially present thiol (2569 cm⁻¹) and alkene (1645 cm⁻¹) absorptions bands decreased in intensity throughout the curing process whilst hydroxy groups (3485 cm⁻¹) and ester links (1740 cm⁻¹), essential for dynamic transesterification, remained unaffected (Fig. S4a, ESI†). With an irradiation dose of 6 J cm⁻², full conversion of the thiol and alkene moieties was observed at comparable rates (Fig. S4b, ESI†).^10,18

On the obtained covalently cross-linked 1 mm thick polymer sample, stress relaxation measurements were performed in the linear viscoelastic range of the material at a deformation of 3%. Amplitude sweep experiments are shown in Fig. S5 (see ESI†). To investigate the light-mediated activation of TMG-PLB, measurements prior to and after different irradiation doses with 405 nm LED light were conducted at 70 °C (Fig. 3a). The unexposed, i.e. unactivated, sample showed only a negligible relaxation to 90% of the normalised relaxation module (G_(t)/G₀) after 10 h. Compared to the thiol-acrylate systems available in the literature, this is a distinct improvement.^9,11 At temperatures applicable to trigger stress relaxation in a pronounced manner after releasing the transesterification catalyst, they tend to relax already well below 80% even in their unactivated state. Thiol–ene networks are less prone to polymerization shrinkage and related internal stresses leading to undesired stress relaxation, i.e. creep. Thus, they retain their dimensional stability, important for numerous practical applications.^5,19

Fig. 3 (a) Normalised stress relaxation curves acquired at 70 °C prior to and after different irradiation doses of 405 nm LED light. (b) Comparison of the stress relaxation behaviour at 70 °C of an unactivated, a photoactivated (580 J cm⁻² 405 nm LED light) and a thermally deactivated (210 °C for 10 min under nitrogen atmosphere) sample. (c) Stress relaxation data obtained from samples activated with an irradiation dose of 580 J cm⁻² (405 nm LED light) in a temperature range of 70 to 100 °C. (d) Arrhenius plot of the characteristic relaxation times (τ) obtained with a decrease of the respective normalized relaxation modulus to 1/e. Insets show the corresponding equation and fitting parameters. E_a is the activation energy, R the universal gas constant and R² the coefficient of determination.

In contrast, for the sample activated with an irradiation dose of 116 J cm⁻², a distinctive drop in (G_(t)/G₀) to 64% was observed after the same measurement time of 10 h (Fig. 3a). Due to the enhanced release of TMG, this trend intensified up to an irradiation dose of 580 J cm⁻² by which an (almost) complete relaxation was reached. The pronounced change in behaviour achieved can be attributed to the light-triggered release of TMG catalysing topological rearrangements against the applied force via dynamic transesterification.

Apart from the ability of an efficient light-mediated release of TMG as transesterification catalyst, the thermal stability of the whole (latent) vitrimer system is of major importance. Thermogravimetric analysis (TGA) of pure TMG-PLB under nitrogen atmosphere showed a total mass loss of 1% at 164 °C (Fig. S6, ESI†). Incorporated in the thiol–ene polymer network used, this temperature is shifted to 178 °C. Furthermore, a significant change in the rate of mass loss was observed at a temperature of about 220 °C. The corresponding total mass loss of about 5 wt% at this point can be attributed to the theoretical mass fraction of TMG-PLB in the polymer system. From about 290 °C onwards, the thiol–ene network itself started to degrade, which was further verified by analysis of a sample without incorporated TMG-PLB. Altogether, the introduced photolatent vitrimer system exhibits a thermal stability that considerably exceeds the temperature range necessary for an efficient bond exchange via dynamic transesterification.

Since the TMG released by exposure to 405 nm LED light for the catalysis of dynamic transesterification remains in the polymer system, the obtained modified network structure exhibits a limited shape stability (over an extended period of time). In a stress relaxation measurement conducted at room temperature (G_(t)/G₀) decreased to 84% after 10 h (Fig. S7, ESI†).

However, the outstanding temperature stability of the thiol–ene polymer matrix can be exploited for the thermal removal of TMG that had been released for the catalysis of dynamic transesterification. In this way, the reorganised network structure can be preserved and an undesired network mobility prevented as demonstrated by stress relaxation measurements (Fig. 3b). The photoactivated sample (580 J cm⁻² 405 nm LED light) used was heat-treated at 210 °C under nitrogen atmosphere for 10 min prior to the measurement. As a consequence of the thermal removal of TMG, only a relaxation to 95% of (G_(t)/G₀) was observed after 10 h and thus the dimensional stability of the unactivated sample was successfully recovered.

An Arrhenius-type temperature dependency of the characteristic relaxation time (τ) is one of the most important features of vitrimers.^20,21 In respect thereof, stress relaxation measurements were conducted on activated samples (580 J cm⁻² 405 nm LED light) between 70 and 100 °C (Fig. 3c). Due to the accelerated dynamic exchange between ester links and hydroxy groups, stress relaxation proceeded more rapidly toward higher temperatures. The respective values for τ can be obtained at the point (G_(t)/G₀) equals (1/e) as a generic exponential decrease, i.e. (G_(t)/G₀) = exp(−t/τ), is followed. Plotted against (1/T), a linear trend according to the Arrhenius relation is observable (Fig. 3d). For this reason, a globally constant temperature independent network connectivity or rather a vitrimeric associative bond exchange mechanism is confirmed.^1,20,21

The associative nature of the dynamic exchange between ester links and hydroxy groups was further verified by swelling experiments. For the unactivated network, a mass swelling ratio of 2.21 ± 0.01 and a gel fraction of 88.4 ± 2.07% were determined. Activation with 405 nm LED light (580 J cm⁻²) followed by temperature-induced dynamic transesterification at 100 °C for 90 min resulted in a slightly decreased mass swelling ratio (2.05 ± 0.11) and a slightly increased gel fraction (95.9 ± 0.78%). These changes are most likely caused by the formation of additional cross-links during light-induced activation of the catalyst. Since Ge-PI absorbs significantly better at 405 than at 450 nm (Fig. 2a), remaining traces of the photoinitiator may induce the conversion of unreacted monomer residues. Compared to other photocured thiol–ene polymers, the obtained gel fractions are in the upper range, and moreover no indications of a dissociative bond exchange mechanism, i.e. a degradation of the network structure, are observable.²²

To analyse the influence of stress relaxation on the mechanical properties of the polymer network, tensile testing was performed (Fig. S8, ESI†). Unactivated dumbbell test specimens exhibited a breaking stress of 0.84 ± 0.04 MPa at a strain of 16.4 ± 0.94%. After the release of TMG under irradiation with 405 nm LED light (580 J cm⁻²) and subsequent storage at 100 °C for 90 min, a reduced breaking stress of 0.74 ± 0.01 MPa was determined at a strain of 13.0 ± 0.06%. This minor increase in brittleness is in accordance with the higher gel fraction obtained for the activated sample with a slightly increased cross-link density. The relaxation process itself was not effective in improving the tensile testing properties, albeit the potential for the relief of internal stresses was very limited, based on the stress relaxation measurement of the unactivated sample (Fig. 3a).

Differential scanning calorimetry (DSC) measurements confirm that the activation process, i.e. the photocleavage of TMG-PLB, which is accompanied by the formation of additional cross-links has only a marginal influence on the fundamental network properties (Fig. S9, ESI†). The shift in the glass transition temperature from −23 to −21 °C between the unactivated and the activated sample is negligibly small.

From an application perspective, the resin formulation's viscosity of 165 mPa s (Fig. S10, ESI†) is beneficially low for DLP 3D printing.^9,11 Owing to the efficient photoinitiator Ge-PI, present in the low concentration of 0.15 mol% (relative to thiol functions), layers with a constant thickness of 50 μm could be printed with an exposure time of only 3.5 s at 450 nm. The achievable spatial printing resolution was determined to be at least 1.00 mm by fabricating a comb-like structure including holes and bars in varying sizes (Fig. S11, ESI†). However, for in z-direction larger objects, which require printing of about more than 50 layers, the spatial resolution is considerably limited due to the enhanced influence of scattered light.

To demonstrate the spatiotemporal activation of dynamic exchange reactions in a macroscopic manner, reshaping was performed on a 3D-printed object (Fig. 4). In a first step, only the right ear of the 1 mm thick rabbit was irradiated with 405 nm LED light (580 J cm⁻²) to trigger the spatially resolved release of TMG as active base catalyst. Subsequently, both ears were fastened in a bent position, and the whole setup was stored at 100 °C for 90 min. Throughout this period, the network topology of the light-exposed, i.e. activated, right ear was reorganised against the applied force or rather according to the predefined bent shape by a base-catalysed dynamic exchange between ester links and hydroxy groups. Thus, a new shape was obtained which remained in position even after the fixation unit was removed. The left ear, however, immediately returned to the entropically more favourable straight position initially defined via photopolymerization during DLP 3D printing. To confirm the preserved photolatency of TMG-PLB in this part of the object, the left ear was also exposed to 405 nm LED light (580 J cm⁻²), and both ears were fastened again in the bent position. After 90 min at 100 °C and removal of the fixation unit, the left ear retained its bent position as well caused by a rearrangement of the network structure via dynamic transesterification.

Fig. 4 Reshaping experiment with an object manufactured by DLP 3D printing at 450 nm. Spatially resolved shape changes are induced through a locally controlled activation of the photolatent base catalyst TMG-PLB by irradiation with 405 nm LED light and a reorganisation of the network structure of the respective area via base-catalysed dynamic transesterification.

Conclusions

The spatiotemporal activation of vitrimeric properties has been successfully demonstrated within a covalently cross-linked thiol–ene photopolymer. Upon irradiation with 405 nm LED light, a photolatent base catalyst was activated under the release of a strong guanidine base acting as an efficient transesterification catalyst. Owing to a present sequence-dependent wavelength-orthogonality, i.e. inactivity of the photolatent base at higher wavelengths, light-mediated curing at 450 nm was applicable. The high-performance oligoacylgermane photoinitiator used enabled an efficient sample preparation process via DLP 3D printing including exposure times of only 3.5 s. Stress relaxation measurements, prior to and after irradiation with 405 nm LED light, highlighted an outstanding change in the rate of dynamic transesterification within the polymer structure. As a consequence of the light penetration depth, limitations arise in terms of object geometry, transparency and thickness. The possibility of a spatially resolved onset of exchange reaction was confirmed by local shape changes in the structure of a 3D-printed object. Although activation by light provides a very precise spatial resolution, diffusion of the activated catalyst cannot be excluded over an extended time period. With a simple heat treatment, however, the active transesterification catalyst can be removed from the polymer which leads to a preservation of the (reorganized) network topology. Finally, the introduced photolatent thiol–ene vitrimer provides an important alternative to the existing acrylate-containing systems. First, comparable relaxation times can be achieved at significantly lower temperatures and second, the creep resistance in the unactivated state is noticeably improved.

Experimental

Materials

All chemicals were used as received without further purification. Bruno Bock (Germany) kindly provided pentaerythritol tetrakis(3-mercaptopropionate) partially esterified with an average degree of 75% (PETMP-75). In its synthesis, 75% of the total amount of hydroxy groups of pentaerythritol were esterified with 3-mercaptopropionic acid. Trimethylolpropane diallyl ether 90 (TMPDE) was supplied by Perstorp Holding AB (Sweden) free of charge. Phenylglyoxylic acid was purchased from Alfa Aesar (USA), while 1,1,3,3-tetramethylguanidine was obtained from Sigma Aldrich (USA). Water for pH measurements (HPLC Gradient Grade) and all other chemicals used were received from Carl Roth (Germany). 1,3,5-(Carbonyl-tris(2,4,6trimethylbenzoyl)germyl)-benzene (Ge-PI) was obtained from the materials reported in the literature used for synthesis.¹³

Synthesis of TMG-PLB

1,1,3,3-Tetramethylguanidine phenylglyoxylate (TMG-PLB) was synthesised in an adapted manner according to the literature.¹⁵ In a 100 mL round-bottom flask, phenylglyoxylic acid (5.00 mmol, 750 mg) was dissolved in 30 mL chloroform. 1,1,3,3-Tetramethylguanidine (5.00 mmol, 580 mg) dissolved in 20 mL chloroform was added dropwise under vigorous stirring over about 5 min. After at least 15 h of stirring at room temperature, about 40 mL of diethyl ether were added, and stirring was continued until the product started to precipitate. To complete the precipitation process, the mixture was left at room temperature for about 2 h. The product obtained as a colourless solid was repeatedly washed with diethyl ether and dried under vacuum (4.65 mmol, 1.233 g, 93%). ¹H NMR (300 MHz, CDCl₃): δ 9.13–8.66 (bs, 1H), 8.10–7.95 (d, 2H), 7.55–7.46 (m, 1H), 7.46–7.35 (m, 2H), 2.99–2.85 (bs, 12H) ppm. ¹³C NMR (300 MHz CDCl₃): δ 196.2, 171.5, 162.4, 134.5, 133.0, 129.8, 128.5, 39.8 ppm.

Resin preparation

Ge-PI (0.15 mol% relative to thiol functions, 1.16 × 10⁻² mmol, 20 mg) was dissolved in TMPDE (4.05 mmol, 869 mg) by sonication at 45 °C for about 15 min. Subsequently, PETMP-75 (2.57 mmol, 1000 mg) and TMG-PLB (5.00 mol% relative to thiol functions, 3.86 × 10⁻¹ mmol, 102 mg) were added and dissolved by means of a magnetic stirrer and sonication at 35 °C for about 20 min.

Specimen preparation

With exception of the 3D-printed objects used for spatially resolved reshaping and determining the printing resolution, all samples prepared from the resin formulation were cured with a 450 nm LED lamp (Zgood® wireless LED curing lamp). For that purpose, the resin formulation was poured into a circular silicone mould with a diameter of 10 mm, and placed in a distance of 3 cm under the lamp. The emission spectrum corresponding to these settings, including intensity information, is shown in Fig. S12a (see ESI†). After 90 s, the already solid specimen with a thickness of about 1 mm was turned and irradiated for another 90 s (3 J cm⁻² in total). Where smaller quantities were required for analysis, the specimens were manually cut into smaller pieces.

Digital light processing (DLP) 3D printing was carried out using a Lithoz (Austria) CeraFab 7500 printer operating at 450 nm. All structures were printed with a layer height of 50 μm and an exposure time of 3.5 s calculated from the intensity of 100 mW cm⁻² and the total dose of 350 mJ cm⁻² set on the printer. Fig. S12b (see ESI†) provides the measured emission spectrum, including intensity information, obtained at these settings. Due to the low quantity of resin (15 mL) used in relation to the size of the vat, the automatic squeegee system of the printer was applied with a rotation speed of 75° s⁻¹ and a rotation angle of 360° to redistribute the resin after printing an individual layer. For detaching the object from the vat, a tilt speed of 10° s⁻¹ and for attaching a tilt speed of 36° s⁻¹ was used. All printed objects were post-cured under the 450 nm LED lamp from both sides at a distance of 3 cm for 90 s (3 J cm⁻² in total).

Independent of the manufacturing method, all specimens prepared were placed in a convection oven for thermal annealing at 90 °C for 30 min. Activation of the specimens, i.e. cleavage of TMG-PLB, was carried out by means of a 405 nm LED Control 5S spot curing system from Opsytec Dr Gröbl (Germany). The lamp intensity was set to 100%, and the end of the light guide was placed 4 cm away from the surface of the specimen. The emission spectrum corresponding to these settings, including intensity information, is shown in Fig. S12c (see ESI†). Unless otherwise stated, specimen activation was performed with a total irradiation dose of 580 J cm⁻², i.e. a total exposure time of 10 min, applied to both sides of the respective specimen in equal amounts. Fig. S12d (see ESI†) provides an overview of the emission spectra of the light sources used in normalized representation.

Rheological investigations

Stress relaxation and amplitude sweep measurements were performed using a parallel-plate geometry setup with a diameter of 10 mm on a Physica MCR 501 rheometer from Anton Paar (Austria). Prior to any analysis, the respective about 1 mm thick circular specimen was equilibrated to the measurement temperature under an over the entire measurement procedure constant normal force of 15 N (automatically regulated by the gap size) for 10 min. For stress relaxation analysis, thereafter, a 3% step strain was applied and the relaxation modulus was monitored over time. Amplitude sweep experiments were carried out with an oscillation frequency of 1 Hz within the logarithmically ramped deformation range of 0.1 to 100%.

The viscosity of the resin formulation was measured at 25 °C on an Anton Paar Physica MCR 102 rheometer. Using a CP50-2 cone with a diameter of 49.973 mm and an angle of 1.982°, the shear rate was increased via a linear ramp from 3 to 300 s⁻¹.

pH measurements

For monitoring the light-mediated catalyst activation, i.e. base release in solution at room temperature, TMG-PLB (1.2 mmol, 318.4 mg) was dissolved in 20.0 mL of water. 15.0 mL of the prepared solution (0.06 molar) were irradiated with 405 nm LED light (Control 5S spot curing system from Opsytec Dr Gröbl) under moderate stirring (400 rpm). At a set lamp intensity of 100% the end of the light guide was placed 4 cm above the surface of the liquid. Light exposure was interrupted for measuring the pH by means of Seven Direct SD20 pH meter equipped with an InLab® Max Pro-ISM electrode from Mettler Toledo (USA).

FTIR spectroscopy

Light-induced curing of the resin formulation was followed via FTIR spectroscopy in transmission mode on a Bruker (USA) Vertex 70 spectrometer. One drop of resin was spread between two CaF₂ discs and irradiated with 450 nm LED light (Zgood® wireless LED curing lamp) at a distance of 5 cm. The corresponding emission spectrum, including intensity information, is shown in Fig. S12a (see ESI†). All spectra were generated out of 16 scans obtained in the wavenumber range of 800 to 4000 cm⁻¹ with a resolution of 2 cm⁻¹.

Swelling experiments

For swelling, semi-circular, 1 mm thick specimens with a weight between 50 and 55 mg were separately immersed in tetrahydrofuran (15 mL) at room temperature. Mass swelling ratios, defined as the fractional increase in weight caused by solvent absorption, were calculated after 48 h. Subsequently, the specimens were dried at 65 °C for 80 h to determine the gel contents, defined as the percentage residual mass in relation to the dry initial mass. Mean values and standard deviations for the unactivated as well as the activated (580 J cm⁻² 405 nm LED light) and afterwards heat treated (100 °C for 90 min) samples were calculated from six specimens each.

DSC measurements

DSC measurements were conducted with about 10 mg of sample on a Mettler Toledo DSC 821e in standard 40 μL aluminium crucibles. A temperature program comprising six individual steps was followed with a standard heating/cooling rate of 10 °C min⁻¹ under nitrogen atmosphere: (1) isotherm at −50 °C for 10 min, (2) heating to 50 °C, (3) isotherm at 50 °C for 1 min, (4) cooling to −50 °C, (5) isotherm at −50 °C for 10 min, (6) heating to 50 °C. Glass transition temperatures were determined from step 6 (second heating cycle) as the midpoint in heat capacity.

Tensile testing

Position controlled tensile testing was performed with a crosshead speed of 5 mm min⁻¹ on a Z1.0 static materials testing machine from ZwickRoell (Germany). Flat gripping chucks with a 0.3 mm scale pattern were used for clamping 1.5 mm thick 3D-printed dumbbell test specimens (Fig. S8b, ESI†) in a distance of 22.0 mm. Reported mean values and standard deviations were calculated from three individual measurements each.

Other characterisation techniques

NMR spectroscopy was performed on a Bruker Avance III 300 MHz spectrometer in CDCl₃.

UV-Vis absorption spectra were recorded on a Cary 50 UV-Visible spectrophotometer from Varian (Australia) between 200 and 600 nm in acetonitrile. The used quartz cuvette had an optical path length of 10 mm.

Emission spectra of the light sources were obtained via an Ocean Insight (USA) STS-UV miniature spectrometer.

TGA was carried out on a Mettler Toledo TGA/DSC1 thermogravimetric analyser using 70 μL Al₂O₃ crucibles. Under a nitrogen purge of 50 mL min⁻¹, about 10 mg of sample were heated from 30 to 600 °C at a heating rate of 10 °C min⁻¹.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to thank Bruno Bock (Marschacht, Germany) for the custom synthesis of the thiol crosslinker (PETMP-75) and gratefully acknowledge financial support from FWF (Vienna, Austria; Project Number P 32606-N). The research work was performed within the COMET-Module project “Chemitecture” (Project-No.: 21647048) at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology and the Federal Ministry for Digital and Economic Affairs with contributions by Montanuniversitaet Leoben. The PCCL is funded by the Austrian Government and the State Governments of Styria, Lower Austria and Upper Austria.

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra, FTIR spectra with conversion plot, amplitude sweep experiments, TGA and DSC curves, viscosity measurement, photograph of an object illustrating the printing resolution and emission spectra of the light sources used. See DOI: https://doi.org/10.1039/d3py00377a

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