Glycerol Acrylate-Based Photopolymers with Antimicrobial and Shape-Memory Properties

In this paper, for the first time, photopolymers were synthesized from glycerol acrylates with different numbers of functional groups, 2-hydroxy-3-phenoxypropyl acrylate, glycerol dimethacrylate or glycerol trimethacrylate, without and with the addition of vanillin styrene. The photocuring kinetics were monitored by real-time photorheometry. The mechanical, rheological, thermal, antimicrobial and shape-memory properties of the photopolymers were investigated. All polymers synthesized demonstrated antibacterial activity against Escherichia coli and Staphylococcus aureus, as well as antifungal activity against Aspergillus flavus and Aspergillus niger. 2-Hydroxy-3-phenoxypropyl acrylate-based polymers showed thermoresponsive shape-memory behavior. They were able to maintain their temporary shape below the glass transition temperature and return to their permanent shape above the glass transition temperature. Synthesized photopolymers have potential to be used as sustainable polymers in a wide range of applications such as biomedicine, photonics, electronics, robotics, etc.


Introduction
Biobased materials are a viable alternative to petroleum-based materials, as they are environmentally friendly and come from biorenewable sources, such as plants and living systems [1,2].These materials can be chemically modified by the addition of functional groups to produce monomers suitable for polymerization [3].A biobased material with great potential for polymer synthesis is glycerol, which is obtained as the main by-product of the biodiesel production process [4].Glycerol is widely used in the polymer industry as a feedstock for polymer synthesis [5].Another example of promising materials is vanillin [6].It is a biobased aromatic compound that can be extracted directly from plants or obtained through the chemical modification of lignin [7].Vanillin-based materials have been successfully used in polymer synthesis and have shown high mechanical strength and good antimicrobial properties [8].Both vanillin and glycerol derivatives were used successfully in the photopolymerization of biobased polymers [9,10].
Photopolymerization is an environmentally friendly polymerization process that follows the principles of Green Chemistry [11].It is a fast process that starts rapidly after irradiation, and polymers can be produced in a few minutes, which is a huge advantage compared to thermal polymerization [12].Moreover, photopolymerization is an energyefficient process and can be performed at room temperature [13].It is an easily controlled process, since it only starts in irradiated areas, which makes photopolymerization suitable for optical 3D printing technology [14].In addition, photopolymerization can be used as a simple way to obtain polymers with unique properties such as antimicrobial activity or shape-memory [15].Shape-memory materials are smart materials that can change their shape when triggered by a stimulus [16].The wide range of stimuli, such as light, magnetic, thermal, chemical or electrical [17], allows for the use of shape-memory polymers in the production of smart materials [18].Thermoresponsive shape-memory polymers obtain temporary shapes when they are deformed at a temperature higher than their glass transition temperature, and then, cooled below it again [19].They can be used in various application areas such as biomedicine, photonics, electronics, robotics, etc. [20].
Antimicrobial materials are very important nowadays as they can prevent the spread of diseases [21].There are two main types of antimicrobial materials: natural and synthetic [22].Synthetic antimicrobial polymers are mostly obtained through the incorporation of biocidic organic compounds such as essential oils or biocidic inorganic particles such as metal ions into the polymeric material.The main disadvantage of these approaches is that their antimicrobial activity decreases over time [23].Due to this, the most advantageous approach is the chemical modification of polymers, i.e., the covalent incorporation of antimicrobial fragments [24].The main application areas for antimicrobial polymers are the medicine, textile and food industries [25].For example, the use of antimicrobial polymers in the manufacture of medical devices could stop most hospital-acquired infections, which occur because medical devices provide the right conditions for microbes to grow [26].Antimicrobial food packaging could stop disease transmission through food and even prevent food from spoiling during storage [27].
In this study, three glycerol derivatives, 2-hydroxy-3-phenoxypropyl acrylate, glycerol dimethacrylate and glycerol trimethacrylate, were used in free-radical photopolymerization without and with the addition of vanillin styrene, which was used to improve the mechanical properties of the resulting polymers.A total of 3 mol.% of photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide was used in the compositions.This photoinitiator was selected due to its photobleaching effect and its ability to cure deep layers of resin [28].The aim of this study was to develop antimicrobial thermoresponsive shape-memory photopolymers.For the first time, vanillin styrene was used in photopolymerization.It was determined that the addition of vanillin styrene increased the rigidity and thermal stability of the polymer prepared with 2-hydroxy-3-phenoxypropyl acrylate, but also, the bulk structure of this substituent increased the free volume in the polymer network, which could have contributed to the slightly inferior antimicrobial properties of the polymer films.

Preparation of Polymer Specimens
The initial mixtures containing 1 mol of the glycerol derivative (HPPA, GDMA or GTMA), 0.1 mol of VS (in B1-B3 cases) and 3 mol.% of TPO were stirred with a magnetic stirrer at 40 • C for 10 min until a homogenous mixture was obtained.The mixtures were then poured into Teflon molds (45 mm × 10 mm × 3 mm) and cured for 1-3 min (until the samples had hardened) under a UV/Vis lamp (Helios Italquartz, model GR.E 500 W, Milan, Italy) with an intensity of 310 mW/cm 2 .The compositions of the resins are presented in Table 1.

Preparation of Polymer Specimens
The initial mixtures containing 1 mol of the glycerol derivative (HPPA, GDMA or GTMA), 0.1 mol of VS (in B1-B3 cases) and 3 mol.% of TPO were stirred with a magnetic stirrer at 40 °C for 10 min until a homogenous mixture was obtained.The mixtures were then poured into Teflon molds (45 mm × 10 mm × 3 mm) and cured for 1-3 min (until the samples had hardened) under a UV/Vis lamp (Helios Italquartz, model GR.E 500 W, Milan, Italy) with an intensity of 310 mW/cm 2 .The compositions of the resins are presented in Table 1.

Characterization Techniques
A Spectrum BX II FT-IR spectrometer (Perkin Elmer, Llantrisant, UK) was used to record Fourier transform infrared spectroscopy (FT-IR) spectra.The reflection was measured during the test.The range of wavenumbers was (4000-650) cm −1 .

Characterization Techniques
A Spectrum BX II FT-IR spectrometer (Perkin Elmer, Llantrisant, UK) was used to record Fourier transform infrared spectroscopy (FT-IR) spectra.The reflection was measured during the test.The range of wavenumbers was (4000-650) cm −1 .
Soxhlet extraction was used to determine the yield of insoluble fractions.Polymer samples of 0.4 g were extracted with acetone for 24 h.A swelling test was also performed using two different solvents, acetone and toluene.The methodology of these experiments was described in a previous publication [29].
Thermal characteristics of the polymers were investigated using thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA).During DMTA, the shear mode was used with a frequency of 1 Hz, a shear strain of 0.1% and a normal force of 5 N.The mechanical characteristics of the synthesized polymers were determined by a tensile test.Young's modulus, tensile strength and elongation at break were determined.The methodology of these experiments was described in a previous publication [29].
The collected data were statistically analyzed using ANOVA for the Microsoft Excel program.The results are presented as the average values ± standard deviation.The estimated p-value was below 0.05 within the groups.

Real-Time Photorheometry
UV/Vis curing tests were performed with glycerol-based resins containing 0.1 mol of VS (only in B1-B3 cases) and 3 mol.% of TPO on a MCR302 rheometer (Anton Paar, Graz, Austria) equipped with a plate/plate measuring system using the methodology described in a previous publication [29].In all cases, shear mode with a frequency of 10 Hz and a shear strain of 0.1% was used.Five measurements were made for each formulation, and results with a standard deviation of less than 5% were obtained.

Monitoring of Photocuring Kinetics by Real-Time Photorheometry
Real-time photorheometry was used to investigate the photocuring kinetics of the resins.The results are summarized in Table 2.The storage modulus represents the rigidity of the polymers.The dependences of the storage modulus of resins B1-B6 on irradiation time are shown in Figure 2.More rigid polymers were obtained from compositions with VS (B1 and B2) compared to compositions without VS (B4 and B5) due to the two rigid benzene rings in the VS structure.The only exceptions were the polymers prepared with GTMA, as the value of G' of polymer B3 prepared with VS was slightly lower than that of polymer B6.The reason for this might be the high number of functional groups of GTMA, in which molecules tend to react with each other instead of VS.
Photocuring (determined by gel point) was faster when VS was not used in the compositions, because acryl groups are more reactive than vinyl groups [30].However, the photocuring of GTMA-based polymer B3 with VS was slightly faster (t gel = 2.4 s) than that of GTMA-based polymer B6 (t gel = 2.6 s).This can be explained by the higher number of functional groups present in compositions with GTMA and faster formation of spatial hindrances that slow down photopolymerization, because the remaining groups cannot reach each other [31].
due to the free volume in the polymer network.Also, an increased number of functional groups increases the shrinkage of resultant polymers due to the increased formation of short covalent bonds [32].A high number of functional groups results in a more rigid polymer network, which lowers the volume of the resulting polymer.Compositions B1 and B4 formed a linear polymer, which shrank less than the cross-linked polymers B2, B3, B5 and B6 [33].The rate of photocuring also influenced the shrinkage of polymers, and the gel point value of polymer B1 was higher than that of polymer B4, indicating slower photocuring and less shrinkage [34].

Characterization of Polymer Structure
The chemical structure of the polymers was confirmed by FT-IR spectroscopy.The FT-IR spectra of the polymers were compared with those of the monomers in order to observe the decrease in the signals of the functional C=C groups and the appearance of Polymer shrinkage was in the range of 2-14%.Higher shrinkage values were observed for compositions with VS (B2 and B3); the only exceptions were compositions B1 and B4 prepared with HPPA.Polymers B2 and B3 shrunk more than polymers B5 and B6 due to the free volume in the polymer network.Also, an increased number of functional groups increases the shrinkage of resultant polymers due to the increased formation of short covalent bonds [32].A high number of functional groups results in a more rigid polymer network, which lowers the volume of the resulting polymer.Compositions B1 and B4 formed a linear polymer, which shrank less than the cross-linked polymers B2, B3, B5 and B6 [33].The rate of photocuring also influenced the shrinkage of polymers, and the gel point value of polymer B1 was higher than that of polymer B4, indicating slower photocuring and less shrinkage [34].

Characterization of Polymer Structure
The chemical structure of the polymers was confirmed by FT-IR spectroscopy.The FT-IR spectra of the polymers were compared with those of the monomers in order to observe the decrease in the signals of the functional C=C groups and the appearance of new signals as a result of the formation of C-C bonds.As an example, the FT-IR spectra of GDMA, VS and polymers B2 and B5 are presented in Figure 3.The signals of the acryl groups at 1637 cm −1 and vinyl groups at 1581 cm −1 are reduced in the polymer spectra.In addition, a newly formed C-C bond signal is visible at 1156 cm −1 in the spectra of the polymers.These changes indicate that polymers were formed during photocuring.
To confirm the formed structure of the polymers, the yield of the insoluble fraction was calculated after Soxhlet extraction and swelling tests were performed.The results are presented in Table 3.The yield of the insoluble fraction of the polymers was in the range of 86-98%.The highest value was shown by GTMA-and VS-based polymer B3 (98%) due to the densely cross-linked network.The lowest value was shown by an HPPA-based polymer without VS (B4).The reason for this is the physical network of the linear macromolecules, which is formed through the steric entanglement of large aromatic substituents, and the resulting steric constraints prevent the linear macromolecules from entangling while dissolving in the solvent.In all cases, the higher amount of yield of the insoluble fraction was obtained for polymers prepared with VS compared to the corresponding polymers without VS.
new signals as a result of the formation of C-C bonds.As an example, the FT-IR spectra of GDMA, VS and polymers B2 and B5 are presented in Figure 3.The signals of the acryl groups at 1637 cm −1 and vinyl groups at 1581 cm −1 are reduced in the polymer spectra.In addition, a newly formed C-C bond signal is visible at 1156 cm −1 in the spectra of the polymers.These changes indicate that polymers were formed during photocuring.To confirm the formed structure of the polymers, the yield of the insoluble fraction was calculated after Soxhlet extraction and swelling tests were performed.The results are presented in Table 3.The yield of the insoluble fraction of the polymers was in the range of 86-98%.The highest value was shown by GTMA-and VS-based polymer B3 (98%) due to the densely cross-linked network.The lowest value was shown by an HPPA-based polymer without VS (B4).The reason for this is the physical network of the linear macromolecules, which is formed through the steric entanglement of large aromatic substituents, and the resulting steric constraints prevent the linear macromolecules from entangling while dissolving in the solvent.In all cases, the higher amount of yield of the insoluble fraction was obtained for polymers prepared with VS compared to the corresponding polymers without VS.Two solvents, the polar solvent acetone and the non-polar solvent toluene, were selected for the swelling test (Table 3, Figure 4).The highest swelling values were shown by HPPA-based polymers B1 and B4 in acetone.The reason for this is the low dense physical network of these polymers, caused by linear macromolecules and bulk aromatic VS substituents, and their solubility in acetone.However, their swelling values in toluene were  Two solvents, the polar solvent acetone and the non-polar solvent toluene, were selected for the swelling test (Table 3, Figure 4).The highest swelling values were shown by HPPA-based polymers B1 and B4 in acetone.The reason for this is the low dense physical network of these polymers, caused by linear macromolecules and bulk aromatic VS substituents, and their solubility in acetone.However, their swelling values in toluene were lower (1.48-9.14%)indicating that these polymers are not easily soluble in toluene.GDMA-and GTMA-based polymers (B2, B3, B5 and B6) showed higher swelling values in toluene than in acetone due to their structure being more similar to the non-polar solvent toluene than to the polar solvent acetone [35].
Polymers 2024, 16, x FOR PEER REVIEW 7 of 12 lower (1.48-9.14%)indicating that these polymers are not easily soluble in toluene.GDMA-and GTMA-based polymers (B2, B3, B5 and B6) showed higher swelling values in toluene than in acetone due to their structure being more similar to the non-polar solvent toluene than to the polar solvent acetone [35].

Thermal Properties
Thermogravimetric analysis and dynamic mechanical thermal analysis were selected to determine the thermal characteristics of the synthesized polymers.The results are presented in Table 4.The thermal decomposition of polymers occurred in one or two steps,

Thermal Properties
Thermogravimetric analysis and dynamic mechanical thermal analysis were selected to determine the thermal characteristics of the synthesized polymers.The results are presented in Table 4.The thermal decomposition of polymers occurred in one or two steps, depending on the polymer (Figure 5).The thermal decomposition of polymers B1-B3 (prepared with VS) occurred in one step, while the thermal decomposition of polymers B4-B6 (prepared without VS) occurred in two steps.The reason for this was the presence of a higher amount of low-molecular-weight compounds and/or oligomers that were not incorporated into the polymer network of polymers B4-B6, which thermally decompose easier than the cross-linked parts of these polymers.The highest thermal stability was demonstrated by polymer B3 (T dec-10% = 372 • C).This polymer had the highest yield-ofinsoluble-fraction value, which makes it thermally stable at higher temperatures.Polymers B1 and B4 composed of linear macromolecules had a temperature of 10% weight loss in the range of 312-314 • C. All polymers demonstrated higher thermal stability compared to the GDMA-and vanillin acrylate-based polymers used for additive manufacturing (T dec-10% = 250-280 • C) [36].The glass transition temperature of the synthesized polymers was in the range of 19-55 °C.The DMTA curves are presented in Figure 6.Polymers B1-B3 prepared with VS were determined to have a higher glass transition temperature than the corresponding polymers prepared without VS (B4-B6).This was because of the aromatic fragments of VS in their structure.Polymers B1 and B4 showed the lowest Tg values due to their linear macromolecules.The highest values of Tg were demonstrated by polymers B2 and B5 prepared with GDMK.These polymers have a cross-linked structure and the highest values of insoluble fraction, which results in higher values of Tg.The glass transition temperature of the synthesized polymers was in the range of 19-55 • C. The DMTA curves are presented in Figure 6.Polymers B1-B3 prepared with VS were determined to have a higher glass transition temperature than the corresponding polymers prepared without VS (B4-B6).This was because of the aromatic fragments of VS in their structure.Polymers B1 and B4 showed the lowest T g values due to their linear macromolecules.The highest values of T g were demonstrated by polymers B2 and B5 prepared with GDMK.These polymers have a cross-linked structure and the highest values of insoluble fraction, which results in higher values of T g .
were determined to have a higher glass transition temperature than the corresponding polymers prepared without VS (B4-B6).This was because of the aromatic fragments of VS in their structure.Polymers B1 and B4 showed the lowest Tg values due to their linear macromolecules.The highest values of Tg were demonstrated by polymers B2 and B5 prepared with GDMK.These polymers have a cross-linked structure and the highest values of insoluble fraction, which results in higher values of Tg.

Shape-Memory Properties of Polymers
Polymers B1 and B4 demonstrated thermo-responsive shape-memory properties, which were determined by Tg [37].These polymers were composed of macromolecules with hard and soft fragments.Soft aliphatic chains act as switchable units that allow the polymer to change its shape [38].These polymer samples were able to obtain a temporary shape and maintain it when the temperature was lower than their Tg.To demonstrate these properties, the polymer samples were heated above their glass transition temperature, and then, deformed to the desired temporary shape, in this case, a roll.When the temporary shapes were obtained, they were fixed by cooling the polymer samples below their Tg.The temporary shapes did not change over time while the samples were kept at

Shape-Memory Properties of Polymers
Polymers B1 and B4 demonstrated thermo-responsive shape-memory properties, which were determined by T g [37].These polymers were composed of macromolecules with hard and soft fragments.Soft aliphatic chains act as switchable units that allow the polymer to change its shape [38].These polymer samples were able to obtain a temporary shape and maintain it when the temperature was lower than their T g .To demonstrate these properties, the polymer samples were heated above their glass transition temperature, and then, deformed to the desired temporary shape, in this case, a roll.When the temporary shapes were obtained, they were fixed by cooling the polymer samples below their T g .The temporary shapes did not change over time while the samples were kept at a temperature below their T g .The ability of the samples to return to their permanent shape was verified by heating them above their T g .Polymers B1 and B4 were able to return to their permanent shapes in a short period of time, 5 s.The heating-cooling-heating cycle scheme of the polymer B1 sample is presented in Figure 7.
Polymers 2024, 16, x FOR PEER REVIEW 9 of 12 a temperature below their Tg.The ability of the samples to return to their permanent shape was verified by heating them above their Tg.Polymers B1 and B4 were able to return to their permanent shapes in a short period of time, 5 s.The heating-cooling-heating cycle scheme of the polymer B1 sample is presented in Figure 7.

Mechanical Characteristics
A tensile test was performed to determine the mechanical characteristics of polymers B1-B6.The results are presented in Table 5. Polymers B1 and B4 prepared with HPPA were soft and flexible and showed high values of elongation at break and low Young's modulus values due to the linear macromolecule structure.The most rigid polymers and the highest Young's modulus values were obtained for polymers B2 and B5 prepared with GDMA.These polymers have the highest yield-of-insoluble-fraction values.Because of this, these polymers are mechanically stronger than the linear polymers B1 and B4, but their elongation-at-break values are lower.Polymers prepared with GDMA showed similar Young's modulus values to antimicrobial GDMA-based polymers suitable for optical 3D printing (Young's modulus = 76.6-190.7 MPa) [39].Higher values of Young's modulus and lower values of tensile strength  GDMA.These polymers have the highest yield-of-insoluble-fraction values.Because of this, these polymers are mechanically stronger than the linear polymers B1 and B4, but their elongation-at-break values are lower.Polymers prepared with GDMA showed similar Young's modulus values to antimicrobial GDMA-based polymers suitable for optical 3D printing (Young's modulus = 76.6-190.7 MPa) [39].Higher values of Young's modulus and lower values of tensile strength were shown by polymers B4-B6 prepared without VS compared to the corresponding polymers B1-B3 prepared with VS.The only exceptions are polymers B1 and B4 prepared with HPPA.In this case, higher values of Young's modulus were shown by polymer B1 prepared with VS.The reason for this is the presence of aromatic rings in the structure of VS, which increase the rigidity of the linear polymer B1 prepared with HPPA.However, the addition of VS to GDMA or GTMA had the opposite effect due to the increased the free volume in the polymer network, as it created a slightly more flexible polymer network, resulting in a lower Young's modulus and a slightly higher value of elongation at break.219.9 ± 3.5 1.9 ± 0.1 1.7 ± 0.0 B6 96.9 ± 1.1 0.3 ± 0.0 2.9 ± 0.1

Antimicrobial Activity of Polymers
The antibacterial and antifungal activity of the polymer films was investigated during direct contact of the bacterial or fungal suspensions with the polymer samples and by calculating the percent reduction of microbial spores after 1 h to see their dependence on the polymer structure.The results are presented in Table 6.All polymers were more active against the Gram-negative bacteria E. coli compared to the Gram-positive bacteria St. aureus.This is because St. aureus is more resistant to antibiotics and antimicrobial agents than E. coli [40].For example, the highest antibacterial activity was demonstrated by polymer B6, with a 99.12% reduction against E. coli and an 88.38% reduction against St. aureus.All polymers prepared with VS were less active against both types of bacteria compared to the corresponding polymers prepared without VS.The structure of VS increased the free volume and facilitated molecular motions, which could impact the decreased antibacterial properties of the polymer films.For example, the VS-based polymer B2 reduced the viability of St. aureus by 72.95%, while the corresponding polymer without VS (B5) reduced it by 78.09%.The reason for this is the high antimicrobial activity of glycerol derivatives [41].The lowest antibacterial activity was shown by polymers B1 and B4 prepared with HPPA, and the highest by polymers B3 and B6 prepared with GTMA.For example, HPPA-based polymer B4 reduced the viability of

Figure 2 .
Figure 2. Dependence of storage modulus of resins B1-B6 on irradiation time.

Figure 2 .
Figure 2. Dependence of storage modulus of resins B1-B6 on irradiation time.

Figure 4 .
Figure 4. Dependence of swelling value on the duration of swelling of polymers B1-B6 in acetone (a) and toluene (b).

Figure 4 .
Figure 4. Dependence of swelling value on the duration of swelling of polymers B1-B6 in acetone (a) and toluene (b).

3. 5 .
Mechanical Characteristics A tensile test was performed to determine the mechanical characteristics of polymers B1-B6.The results are presented in Table 5. Polymers B1 and B4 prepared with HPPA were soft and flexible and showed high values of elongation at break and low Young's modulus values due to the linear macromolecule structure.The most rigid polymers and the highest Young's modulus values were obtained for polymers B2 and B5 prepared with Polymers 2024, 16, 862 9 of 12

Table 1 . Compositions of resins B1-B6. Resin Glycerol Derivative Molar Ratio of Glycerol Derivative and Vanillin Styrene Amount of Photoinitiator TPO, mol.% *
* Calculated from the total mol amount of monomers.

Table 2 .
Mechanical and rheological characteristics of the resins.
* After 450 s from the start of irradiation.

Table 3 .
Characteristics of polymers.

Table 3 .
Characteristics of polymers.

Table 4 .
Thermal characteristics of polymers.

Table 5 .
Mechanical characteristics of polymers obtained by tensile test.

Table 6 .
Antimicrobial characteristics of polymers.