Physicochemical Properties of Novel Copolymers of Quaternary Ammonium UDMA Analogues, Bis-GMA, and TEGDMA

This study aimed to elucidate the physicochemical properties of copolymers comprising 40 wt.% bisphenol A glycerolate dimethacrylate (Bis-GMA), 40 wt.% quaternary ammonium urethane-dimethacrylate analogues (QAUDMA-m, where m corresponds to the number of carbon atoms in the N-alkyl substituent), and 20 wt.% triethylene glycol dimethacrylate (TEGDMA) copolymers (BG:QAm:TEGs). The BG:QAm:TEG liquid monomer compositions and reference compositions (40 wt.% Bis-GMA, 40 wt.% urethane-dimethacrylate (UDMA), 20 wt.% TEGDMA (BG:UD:TEG) and 60 wt.% Bis-GMA, 40 wt.% TEGDMA (BG:TEG)) were characterized in terms of their refractive index (RI) and monomer glass transition temperature (Tgm) and then photocured. The resulting copolymers were characterized in terms of the polymer glass transition temperature (Tgp), experimental polymerization shrinkage (Se), water contact angle (WCA), water sorption (WS), and water solubility (SL). The prepared BG:QAm:TEG liquid monomer compositions had RI in the range 1.4997–1.5129, and Tgm in the range −52.22 to −42.12 °C. The BG:QAm:TEG copolymers had Tgp ranging from 42.21 to 50.81 °C, Se ranging from 5.08 to 6.40%, WCA ranging from 81.41 to 99.53°, WS ranging from 25.94 to 68.27 µg/mm3, and SL ranging from 5.15 to 5.58 µg/mm3. Almost all of the developed BG:QAm:TEGs fulfilled the requirements for dental materials (except BG:QA8:TEG and BG:QA10:TEG, whose WS values exceeded the 40 µg/mm3 limit).


Introduction
Over the past three decades, quaternary ammonium methacrylates (QAMs) have attracted the attention of scientists in the field of antibacterial dimethacrylate-based dental composites [1,2]. This is due to the fact that QAMs show high antibacterial activity caused by the presence of quaternary ammonium (QA) moieties in their structures. As the QA moiety contains a positively charged quaternary nitrogen atom, it can adsorb on the negatively charged bacteria cell surface. As a result, the long alkyl chain attached to the quaternary nitrogen can interact with the lipid chains in a cell wall and disturb the cell's electric balance. This causes an increase in the bacteria cell osmotic pressure which leads to its death [3]. What is more, QAMs can be permanently embedded into composite matrices via copolymerization of their double bonds with double bonds of other dimethacrylates constituting the composite matrix [4]. Thus, QAMs may offer long-term antibacterial activity [5,6]. Various QAMs have been described in the literature, including 2-(dimethylamino)ethyl methacrylate derivatives with chloride [7][8][9], bromide [10][11][12][13][14][15], and iodide [16,17] counter ions, quaternary ammonium derivative of bisphenol A glycerolate dimethacrylate [18], and fully aliphatic [19] and cycloaliphatic [20,21] urethanedimethacrylates. Such QAMs represent potential antibacterial components of dental composites because they offer high antibacterial activity against various strains, including glycerolate dimethacrylate [18], and fully aliphatic [19] and cycloaliphatic [20,21] urethane-dimethacrylates. Such QAMs represent potential antibacterial components of dental composites because they offer high antibacterial activity against various strains, including Streptococcus mutans [4,5,10,[13][14][15]18,20,21], Staphylococcus aureus [5,18], and Escherichia coli [18]. However, the well-known QAMs induce a loss of mechanical properties and an increase in the amount of water absorbed by the material and in residual fraction release [4,5,10,[13][14][15]18,19,21,22]. Nevertheless, the field of antibacterial dimethacrylates has continued to evolve, and there are still many possibilities to design new compounds with chemical structures that are suitable for obtaining dental composites with appropriate physicochemical, mechanical, and antibacterial characteristics.
Proper functioning and longevity of dental restorations equally depends on the physicochemical properties and mechanical performance. The most important physicochemical properties of dental materials' matrices include (i) DC-which should exceed 0.55 to provide clinical efficacy [26]; (ii) S e -which should be as low as possible, because the lower the S e, the narrower the marginal gaps between the reconstruction and adjacent tooth tissue, and therefore, the smaller the area privileged for bacteria growth [27]; (iii) Tg p -which should be higher than 40 • C, ensuring that the material is in a glassy state in a temperature range occurring in the oral cavity [28]; (iv) WS-which should not exceed 40 µg/mm 3 [29], because the excess of water absorbed by the material can cause its swelling and decrease its mechanical properties [30]; and (v) SL-which should not be higher than 7.5 µg/mm 3 [29], because usually, the higher the SL, the lower the mechanical stability of the restoration [31].
Despite the crucial role of the dental composites' physicochemical properties in governing their stability, longevity, and functionality, these aspects are rarely examined. In most cases, studies on dental composite matrices modified with QAMs are only limited to the examination of their antibacterial activity, DC, and mechanical properties.
The aim of this study was the preliminary assessment of the applicability of BG:QAm:TEGs as matrices for dental dimethacrylate-based composites from the perspective of their physicochemical properties. These evaluations were based on measurements of the RI and Tg m of liquid monomer compositions and the Tg p , S e , theoretical polymerization shrinkage (S t ), WCA, WS, and SL of corresponding copolymers. The DC [32] was also discussed.

Results
In this study, eight compositions of dimethacrylate monomers were prepared. Six In the BG:QAm:TEG notation, the m still corresponds to the number of carbon atoms in the N-alkyl substituent. Later, Cm is used to denote the length of the N-alkyl substituent in QAUDMA-m and BG:QAm:TEG.   the reference copolymers (dmBG:UD:TEG = 1.115 g/cm 3 , dmBG:TEG = 1.102 g/cm 3 ) (p ≤ 0.05), whereas the remaining BG:QAm:TEGs had dm values similar to the reference copolymers (p > 0.05) Table 1. The properties of studied monomer compositions: the length of the N-alkyl substituent in QAUDMA-m (Cm), molecular weight (MW), double-bond concentration (xDB), refractive index (RI) and density (dm). Lowercase letters indicate statistically insignificant differences with a column (p > 0.05, non-parametric Wilcoxon test).    Table 2 summarizes the dp, St, and Se of the studied copolymers. The dp of BG:QAm:TEGs ranged from 1.174 to 1.216 g/cm 3 and decreased as the Cm increased. BG:QA8:TEG had a higher dp than BG:TEG (dp = 1.206 g/cm 3 ) (p ≤ 0.05), while  Table 2 summarizes the d p , S t , and S e of the studied copolymers. The d p of BG:QAm:TEGs ranged from 1.174 to 1.216 g/cm 3 and decreased as the Cm increased. BG:QA8:TEG had a higher d p than BG:TEG (d p = 1.206 g/cm 3 ) (p ≤ 0.05), while BG:QA10:TEG and BG:QA12:TEG had d p values similar to that of BG:TEG (p > 0.05). The remaining BG:QAm:TEGs had d p values lower than those of BG:TEG (p ≤ 0.05). The BG:QAm:TEGs generally had lower d p than BG:UD:TEG (d p = 1.215 g/cm 3 ) (p ≤ 0.05), except for BG:QA8:TEG, which had a similar d p (p > 0.05). The S t of BG:QAm:TEGs ranged from 8.90% to 9.81% and decreased as the Cm increased. All of the studied BG:QAm:TEGs had lower S t values compared with the reference samples (S tBG:UD:TEG = 12.90%, S tBG:TEG = 11.61%) (p ≤ 0.05). The opposite relationship was observed for S e . Specifically, the S e of BG:QAm:TEGs ranged from 5.08% to 6.40% and increased as the Cm increased. Compared with the reference samples (S eBG:UD:TEG = 8.35%, S eBG:TEG = 8.07%), all of the studied BG:QAm:TEGs had lower S e (p ≤ 0.05).

Properties of Copolymers
The Tg p of BG:QAm:TEGs ranged from 42.21 to 50.81 • C and increased as the Cm increased. All of the studied BG:QAm:TEGs had lower Tg p values compared with the reference copolymers (Tg pBG:UD:TEG = 55.90 • C, Tg pBG:TEG = 61.46 • C) (p ≤ 0.05). Representative DSC thermograms of the copolymers are shown in Figure 3.

Discussion
In this study, novel dimethacrylate liquid monomer compositions and their corresponding copolymers consisting of 40 wt.% Bis-GMA, 40 wt.% QAUDMA-m, and 20 wt.% TEGDMA (BG:QAm:TEGs) were prepared, and their physicochemical properties were discussed. It is essential to understand these fundamental properties in order to assess the potential applicability of such copolymers in newly designed dental composite matrices; however, they are rarely studied.
The organic matrix in a dental composite is responsible for the proper functioning and longevity of dental restoration materials. This matrix sticks the filler particles together, transfers stress to those particles, and gives shape to the restoration. Therefore, the matrix should have adequate physicochemical properties and should be stable over the entire desired service time and intraoral temperature range.

Properties of Liquid Monomer Compositions
The RI defines the optical properties of a composite matrix, which should provide similar transparency to enamel [33]. All of the prepared BG:QAm:TEGs met the requirements for dental materials, according to which the RI should be within the range from 1.46 to 1.55 [34]. These results suggest that all tested BG:QAm:TEGs have suitable aesthetic properties.
The Tg m of BG:QAm:TEGs ranging from −52.22 to −42.12 • C indicates that all the monomeric systems exist in a liquid state at working temperatures. The Tg m values were analyzed to assess the molecular mobility of liquid monomers; the higher the Tg m , the lower the molecular mobility [35]. The lowest Tg m was recorded for BG:QA18:TEG, which had the highest Cm; its Tg m value was 4.23 • C higher than that of BG:TEG and 7.33 • C lower than that of BG:UD:TEG. In contrast, BG:QA8:TEG, which had the shortest Cm, was characterized by the highest Tg m ; its Tg m value was 14.33 • C and 2.77 • C higher than those of BG:TEG and BG:UD:TEG, respectively. In general, all of the studied BG:QAm:TEGs were characterized by higher Tg m values than BG:TEG ( Figure 2). Relative to the BG:UD:TEG reference, BG:QA8:TEG had a higher Tg m , BG:QA10:TEG had a similar Tg m , and the remaining BG:QAm:TEGs had lower Tg m . These results suggest that the prepared BG:QAm:TEG monomeric systems had lower molecular mobility than BG:TEG. Accordingly, relative to the BG:UD:TEG reference, BG:QA8:TEG showed lower molecular mobility, BG:QA10:TEG showed similar molecular mobility, and the remaining BG:QAm:TEGs showed higher molecular mobility. These distinctions can be explained by differences in the character and the resulting strength of intermolecular interactions between the monomer molecules. Among the two references, BG:UD:TEG had a lower molecular mobility than BG:TEG (i.e., Tg m BG:UD:TEG > Tg m BG:TEG ) because of the presence of urethane groups in the UDMA monomer. These moieties are willing to form strong hydrogen bonds with the hydroxyl groups of Bis-GMA, which are also stronger than any other hydrogen bonds occurring in systems comprising Bis-GMA, UDMA, and TEGDMA [36]. Owing to the fact that QAUDMA-m molecules can form strong hydrogen bonds with the hydroxyl groups of Bis-GMA as well as UDMA, all of the BG:QAm:TEG liquid monomer compositions were characterized by lower molecular mobility than BG:TEG. Analysis of the Tg m values further revealed that the Cm affects the Tg m . Specifically, Tg m decreased (i.e., molecular mobility increased) as Cm increased. This relationship can be attributed to the increasing distance between monomer molecules caused by the increasing Cm, which weakens the intermolecular interactions.

Properties of Copolymers
The polymerization shrinkage (S) was determined to be a factor influencing the restoration shrinking [37]. This phenomenon results from double-bond polymerization, whereby the van der Waals forces between the monomer molecules are replaced by much stronger covalent bonds. As a result, the entire system shrinks, and marginal gaps are formed between the tooth filling and adjacent tissue [38]. The higher the S value, the greater the marginal gap; therefore, the S should be as low as possible to enable optimal restoration implantation. In addition, the narrow space between the restoration and the tooth tissue creates an environment that is extremely conducive to bacteria colonization, which can lead to secondary caries or gum inflammation [27].
The S t can be calculated considering that (i) shrinkage results from the polymerization of the methacrylate group and (ii) the reduced molar volume of methacrylate groups is equal to 22.5 cm 3 /mol [39]. The S t values of all studied BG:QAm:TEGs were lower than those of the reference copolymers and decreased as Cm increased ( Table 2). The highest S t was recorded for BG:QA8:TEG, which was 15.50% and 23.95% lower than those of BG:TEG and BG:UD:TEG, respectively. In contrast, BG:QA18:TEG had the lowest S t , which was 23.34% and 31.01% lower than those of BG:TEG and BG:UD:TEG, respectively. Thus, all tested BG:QAm:TEG liquid monomer compositions theoretically shrink less than the reference compositions. In addition, increasing the Cm reduced the degree of shrinking because increasing the MW of the QAUDMA-m decreased the x DB (Table 1).
In practice, S t is always higher than S e because polymerization is never complete. The S e was calculated according to the d m (Table 1) and d p ( Table 2). Similar to S t , the S e values of BG:QAm:TEGs were lower than those of the reference copolymers owing to the higher MW of BG:QAm:TEG monomer systems. In contrast to S t , the S e values of BG:QAm:TEGs increased as C m increased ( Table 2). The highest S e was recorded for BG:QA18:TEG, which was 20.69% and 23.35% lower than those of BG:TEG and BG:UD:TEG, respectively. The BG:QA8:TEG had the lowest S e , which was 37.05% and 39.16% lower than those of BG:TEG and BG:UD:TEG, respectively. These relationships can be explained based on the DC. Table 2 indicates that the DC values of BG:QAm:TEGs increased as Cm increased; therefore, S e also increased.
The Tg p is related to the stiffness of dimethacrylate polymer networks that create the dental composite matrix; the higher the Tg p , the stiffer the polymer network [40]. The Tg p values of BG:QAm:TEGs were lower than those of the reference copolymers and increased as Cm increased (Figure 3). The reduction in BG:QAm:TEG stiffness in comparison to the reference copolymers can be attributed to the presence of long N-alkyl chains that act as pendant groups and loosen the polymer network structure. The lowest Tg p was recorded for BG:QA8:TEG, which was 19.25 • C and 13.69 • C lower than that of BG:TEG and BG:UD:TEG, respectively. In contrast, BG:QA18:TEG had the highest Tg p , which was only 10.65 • C and 5.09 • C lower than that of BG:TEG and BG:UD:TEG, respectively. Considering the target applications of these materials (dental restorations), the Tg p should be higher than the maximum intraoral temperature to ensure that the material will be in a glassy state, with stable mechanical properties over the entire range of working temperatures [28]. For this reason, the Tg p should not be lower than 40 • C because the temperature of the oral cavity during a fever usually does not exceed 39 • C [41]. Therefore, it can be concluded that the Cm did not negatively affect the Tg p to a significant extent because the Tg p values of all studied BG:QAm:TEGs were higher than 40 • C.
The WCA was measured to assess the hydrophilicity of the copolymer surfaces. Common dental composite fillers [42,43] and dental tissues [44] are both highly hydrophilic. Therefore, the higher the matrix hydrophilicity, the greater its ability to physically bond with the filler and tissue adjacent to the restoration. The hydrophilicity can be expressed according to the WCA, i.e., if WCA < 90 • , the surface is hydrophilic, and if WCA > 90 • , the surface is hydrophobic [45]. The WCA values of the studied BG:QAm:TEGs increased as Cm increased. As shown in Figure 4, increasing the Cm from C8 to C14 caused only a slight increase in WCA, which remained lower than 90 • . Therefore, the surfaces of these polymers could be classified as hydrophilic. Further lengthening the Cm to C16 and C18 significantly increased the WCA, thereby changing the nature of the surface from hydrophilic to hydrophobic. These results suggested that the main factor influencing the WCA was the length of the N-alkyl substituents. The trend observed for WCA is consistent with published reports, which indicate that the hydrophobic character of the alkyl chains increases with their length [46]. The slower increase in WCA among BG:QAm:TEGs with C8 to C14 relative to those with C16 and C18 was attributed to the dominant influence of the hydrophilic quaternary ammonium group [47] in the BG:QAm:TEGs with shorter Cm. In the remaining BG:QAm:TEGs, the hydrophobic nature of the N-alkyl chains played a key role. The lowest WCA was recorded for BG:QA8:TEG, which was 5.96% lower than that of BG:TEG and 0.80% higher than that of BG:UD:TEG. The BG:QA18:TEG was characterized by the highest WCA, which was 14.97% and 23.24% higher than that of BG:TEG and BG:UD:TEG, respectively. These results confirm that the surfaces of all studied BG:QAm:TEGs were less hydrophilic than that of BG:UD:TEG. Compared with BG:TEG, only the surfaces of BG:QA16:TEG and BG:QA18:TEG were more hydrophobic, whereas the remaining BG:QAm:TEGs were more hydrophilic.
Dental materials are constantly exposed to moisture, which results in their swelling due to water absorption [30]. This can compensate for the decrease in material volume caused by polymerization [48]. However, excessive WS leads to excess swelling, which may deteriorate the restoration's mechanical properties, and in extreme cases, cause mechanical damage [49]. Therefore, according to the standard, ISO 4049, the WS of dental materials should not exceed 40 µg/mm 3 [29]. The WS values of the studied BG:QAm:TEGs were higher than those of the reference copolymers and decreased as Cm increased. The lowest WS was recorded for BG:QA18:TEG, which was 41.67% and 121.52% higher than that of BG:TEG and BG:UD:TEG, respectively. The BG:QA8:TEG was characterized by the highest WS, which was 272.86% and 483.01% higher than that of BG:TEG and BG:UD:TEG, respectively. The BG:QAm:TEG copolymers with Cm from C12 to C18 met the requirements for dental materials because their WS values were lower than 40 µg/mm 3 . The high WS of studied BG:QAm:TEGs can be explained based on several factors. The most influential factor is likely the presence of two quaternary ammonium groups in the QAUDMA-m, which are primed to absorb water [20]. Additionally, as the MW of QAUDMA-m increased (i.e., with increasing Cm), the molar ratio of QAUDMA-m in the BG:QAm:TEG decreased; therefore, the proportion of hydrophilic quaternary ammonium groups in the BG:QAm:TEGs decreased, thereby reducing the WS. The reduction in WS as the Cm increased can be explained by the increasing hydrophobicity of the longer N-alkyl chains [46]. It is also possible that the longer N-alkyl chains can shield the positively charged quaternary nitrogen ion, thus limiting its ability to absorb water [50].
Leachability of the residual monomers to water (SL) is another parameter that should be controlled when considering materials for dental applications. The SL should be as low as possible because excessive leaking decreases the restoration's lifetime and promotes the deterioration of its mechanical properties [31]. Therefore, according to the standard, ISO 4049, the SL of dental materials should not exceed 7.5 µg/mm 3 [29]. The SL values of the studied BG:QAm:TEGs were higher than those of the reference copolymers and were unaffected by the Cm. The lowest SL was recorded for BG:QA8:TEG, which was 230.13% and 359.82% higher than that of BG:TEG and BG:UD:TEG, respectively. In contrast, BG:QA14:TEG had the highest SL, which was 257.69% and 398.21% higher than that of BG:TEG and BG:UD:TEG, respectively. However, all of the studied BG:QAm:TEGs met the requirements for dental materials because their SL values were lower than 7.5 µg/mm 3 . The significantly higher SL of BG:QAm:TEGs can be explained by the presence of quaternary ammonium groups in QAUDMA-m. As previously stated, these groups have a high affinity to water; therefore, they can easily migrate through the polymer network with water.

Refractive Index
The RI of the liquid monomer compositions was determined according to the standard, ISO 489:1999 [51], using a DR 6100T (Krüss Optronic, Germany) refractometer. Briefly, 2 mL of each liquid monomer composition was placed on the refractometer plate, and the measurement was performed at 20 • C.

Density and Polymerization Shrinkage
The d m was determined according to the standard, ISO 1675 [52], using a 1 mL pyknometer. The d p was determined using an analytical balance (XP Balance, Mettler Toledo, Greifensee, Switzerland) equipped with a density determination kit, which operated on the basis of Archimedes' principle.
The S e was calculated using Equation (1), where d m is the liquid monomer composition density, and d p is the copolymer density. The S t was calculated using Equation (2), where f is the methacrylate functionality (f = 2), ∆V is the decrease in molar volume attributable to the polymerization of one mole of methacrylate (∆V = 22.5 cm 3 /mol [39]), d m is the liquid monomer composition density, and MW is the liquid monomer composition molecular weight.

Glass Transition Temperature
The Tg m and Tg p were determined using a differential scanning calorimeter (DSC 3, Mettler Toledo, Greifensee, Switzerland) according to the standard, ISO 11357-2:2020 [53]. Briefly, 2 mg of sample were placed in a standard aluminum crucible and heated in air within the temperature range from −90 to 200 • C, with a heating rate of 10 K/min.
The Tg was taken as the midpoint of the transition region.

Water Contact Angle
The WCA was determined using an OCA15EC goniometer (Data Physics, Filderstadt, Germany) via the sessile drop method. Briefly, 4 µL of deionized water were dropped on the surface of disc-shaped copolymer samples (diameter × thickness = 15 mm × 1.5 mm).

Water Sorption and Solubility
The WS and SL were determined according to the standard, ISO 4049 [29], using disc-shaped copolymer samples (diameter × thickness = 15 mm × 5 mm).
Initially, the samples were dried at 100 • C in a conditioning oven until they reached a constant weight (m 0 ), which was usually achieved after 72 h. Then, samples were placed in glass vials containing distilled water and stored for seven days at room temperature. Next, samples were removed from the water, blotted dry, and weighted (m 1 ). Finally, the samples were once again dried to a constant mass (m 2 ) at 100 • C in a conditioning oven. Sample weights were measured using an analytical balance (XP Balance, Mettler Toledo, Greifensee, Switzerland) with 0.01 mg accuracy.
The WS and SL were calculated using Equations (3) and (4), respectively, where m 0 is the initial mass of the dried sample, m 1 is the mass of the swollen sample, m 2 is the mass of the dried sample after storage in water, and V is the initial volume of the dried sample.

Statistical Analysis
The experimental results reported herein were expressed as an average of five measurements and presented with the associated standard deviations (SD). The data were analyzed and compared using the non-parametric Wilcoxon test with a significance level (p) of 0.05 using Statistica 13.1 software (TIBCO Software Inc., Palo Alto, CA, USA).

Conclusions
The physicochemical properties of six dimethacrylate-based compositions comprising 40 wt.% Bis-GMA, 40 wt.% QAUDMA-m, 20 wt.% TEGDMA, and their corresponding copolymers mainly depended on the length of the N-alkyl substituent. It was observed that: (i) Tg m decreased with increasing Cm, whereas the opposite trend was observed for Tg p , (ii) S e increased with increasing Cm, (iii) hydrophobicity (determined from WCA) increased with increasing Cm, and (iv) WS decreased with increasing Cm. Only RI and SL were unaffected by the Cm.
In relation to the properties of the reference copolymers, the following conclusions were drawn. All liquid BG:QAm:TEG monomer compositions had suitable RI and Tg m . Moreover, all studied BG:QAm:TEG copolymers were characterized by low S values and Tg p values higher than 40 • C, suggesting that they would have stable mechanical properties over the working temperature range of dental restorations. The surfaces of two of the BG:QAm:TEGs adopted a hydrophobic character (BG:QA16:TEG and BG:QA18:TEG), whereas the remaining BG:QAm:TEGs had hydrophilic surfaces. All of the studied BG:QAm:TEGs had higher SL than the reference copolymers; however, the obtained SL values did not exclude their use as matrices of dimethacrylate-based dental composites. Only BG:QA8:TEG and BG:QA10:TEG did not fulfill the WS requirements because their WS values were higher than 40 µg/mm 3 ; this feature would prevent their use as matrices for dental composites.