Physicochemical and Mechanical Properties of Bis-GMA/TEGDMA Dental Composite Resins Enriched with Quaternary Ammonium Polyethylenimine Nanoparticles

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Article Physicochemical and Mechanical Properties of Bis-GMA/TEGDMA Dental Composite Resins Enriched with Quaternary Ammonium Polyethylenimine Nanoparticles

Izabela M. Barszczewska-Rybarek 1,* , Marta W. Chrószcz 1 and Grzegorz Chladek 2

1 Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, 44-100 Gliwice, Poland; [email protected] 2 Department of Engineering Materials and Biomaterials, Faculty of Mechanical Engineering, Silesian University of Technology, 44-100 Gliwice, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-32-237-1793

Abstract: Modification of dental monomer compositions with antimicrobial agents must not cause deterioration of the structure, physicochemical, or mechanical properties of the resulting polymers. In this study, 0.5, 1, and 2 wt.% quaternary ammonium polyethylenimine nanoparticles (QA-PEI-NPs) were obtained and admixed with a Bis-GMA/TEGDMA (60:40) composition. Formulations were then photocured and tested for their degree of conversion (DC), polymerization shrinkage (S), glass transition temperature (Tg), water sorption (WS), solubility (SL), water contact angle (WCA), flexural modulus (E), flexural strength (σ), hardness (HB), and impact resistance (an). We found that the DC, S, Tg, WS, E, and HB were not negatively affected by the addition of QA-PEI-NPs. Changes in these values rarely reached statistical significance. On the other hand, the SL increased upon increasing the QA-PEI-NPs concentration, whereas σ and a decreased. These results were usually statistically Citation: Barszczewska-Rybarek, n I.M.; Chrószcz, M.W.; Chladek, G. significant. The WCA values increased slightly, but they remained within the range corresponding to Physicochemical and Mechanical hydrophilic surfaces. To conclude, the addition of 1 wt.% QA-PEI-NPs is suitable for applications in Properties of Bis-GMA/TEGDMA dental materials, as it ensures sufficient physicochemical and mechanical properties. Dental Composite Resins Enriched with Quaternary Ammonium Keywords: quaternary ammonium polyethylenimine derivatives; nanoparticles; dental composite; Polyethylenimine Nanoparticles. mechanical properties Materials 2021, 14, 2037. https:// doi.org/10.3390/ma14082037

Academic Editor: Javier Gil 1. Introduction The development of dental materials with antibacterial activity represents a challenge Received: 26 March 2021 in modern biomaterial science engineering [1,2]. Currently used commercial dental restora- Accepted: 16 April 2021 Published: 18 April 2021 tive composites ensure satisfactory mechanical properties, biocompatibility, aesthetics, and economics [3,4]. One of the biggest remaining problems is the marginal gap formation

Publisher’s Note: MDPI stays neutral between the restoration and the adjacent tooth tissue due to polymerization shrinkage [5–7]. µ with regard to jurisdictional claims in The narrow gap width (clinically acceptable marginal gaps vary between 30 and 200 m [8]) published maps and institutional affil- creates a favorable place for dental plaque accumulation [7,9]. Bacterial metabolic processes iations. lead to the emergence of secondary caries, inflammation reactions, and in extreme cases, dental restoration failure [9–11]. The most frequent method for providing dental restorative composites with antibacterial properties is a physical modification by admixing a bioactive compound [1]. Several low molecular weight inorganic and organic substances have been successfully Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. added to commercial dental restorative composites to achieve materials with antimicrobial This article is an open access article activity. Zinc oxide [9,12,13], titanium dioxide [14,15], calcium phosphate [16,17], gold, and distributed under the terms and silver [18,19] are the most popular examples of the first group. The latter group is repre- conditions of the Creative Commons sented by antibiotics [20], chlorhexidine [9,21,22], furanone, ursolic acid, benzalkonium Attribution (CC BY) license (https:// chloride, triclosan, and methacryloyloxydodecyl pyridinium bromide [9]; however, toxic creativecommons.org/licenses/by/ responses of tissues adjacent to the reconstructions modified with these substances are 4.0/). often reported [12–22].

Materials 2021, 14, 2037. https://doi.org/10.3390/ma14082037 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 20

Materials 2021, 14, 2037 chloride, triclosan, and methacryloyloxydodecyl pyridinium bromide [9]; however,2 toxic of 20 responses of tissues adjacent to the reconstructions modified with these substances are often reported [12–22]. AA needneed forfor lessless cytotoxiccytotoxic antibacterialantibacterial agentsagents hashas ledled toto thethe developmentdevelopment ofof dentaldental materialsmaterials based based on on quaternary quaternary ammonium ammonium compounds compounds (QACs). (QACs). The The first first proposals proposals of of QACsQACs usageusage inin dentistrydentistry includedincluded bioactivebioactive compositionscompositions forfor externalexternal applicationsapplications ontoonto thethe surfacessurfaces ofof teeth,teeth, suchsuch asas thethe protectiveprotective toothtooth coatingcoating consistingconsisting ofof polyethyleniminepolyethylenimine (PEI)(PEI) quaternizedquaternized withwith fattyfatty acidacid [[23],23], andand cleaningcleaning compositioncomposition comprisingcomprising PEIPEI andand lowlow molecularmolecular weightweight quaternaryquaternary ammoniumammonium biocidebiocide (alkyltrimethylammonium(alkyltrimethylammonium halide) [[24].24]. TheThe short-lastingshort-lasting antibacterial antibacterial effect effect of of these these materials materials has has disqualified disqualified them them from from common com- usage.mon usage. The long-lasting The long-lasting antibacterial antibacterial effect effect can be can achieved be achieved by the by chemical the chemical modification modifi- ofcation dental of dimethacrylatedental dimethacrylate matrices matrices with mono- with mono and dimethacrylate- and dimethacrylate monomers monomers having hav- the quaternarying the quaternary ammonium ammonium group. Theygroup. are They covalently are covalently bonded bonded to a composite to a composite matrix duematrix to copolymerizationdue to copolymerization with dental with dimethacrylates.dental dimethacrylates. This solution This solution represents represents a dynamically a dynami- de- velopingcally developing research research area; however, area; however, materials materials of satisfactory of satisfac physicomechanicaltory physicomechanical performance per- haveformance not been have achieved not been so achieved far [25]. so Other far [25]. studies Other are studies conducted are conducted on the physical on the modifica- physical tionmodification of dental compositesof dental composites by admixing by particlesadmixing functionalized particles functionalized with quaternary with ammoniumquaternary groups.ammonium The initial groups. idea The comprised initial idea the application comprised of the a bioactive application filler, of achieved a bioactive by grind- filler, ingachieved of a methacrylate-silica by grinding of a methacrylate composite whose-silica matrixcomposite was whose modified matrix by copolymerization was modified by withcopolymerization methacryloyloxydodecylpyridinium with methacryloyloxydodecylpyridinium bromide (MDPB). Thebromide antibacterial (MDPB). activity The anti- of thatbacterial material activity was of weak that material due to the was screening weak due of to quaternary the screening ammonium of quaternary groups ammonium [26]. To overcomegroups [26]. the To disadvantages overcome the of disadvantages the MDPB-based of filler,the MDPB crosslinked-based quaternaryfiller, crosslinked ammonium qua- polyethylenimineternary ammonium nanoparticles polyethylenimine (QA-PEI-NPs) nanoparticles were (QA synthesized-PEI-NPs) [27 were]. Their synthesized polycationic [27]. characterTheir polycationic resulted incharacter a higher resulted antibacterial in a higher activity antibacteria comparedl with activity the MDPB-basedcompared with filler, the allowingMDPB-based QA-PEI-NPs filler, allowing to be used QA- inPEI smaller-NPs to amounts be used [in1]. smaller amounts [1]. QA-PEI-NPsQA-PEI-NPs areare obtainedobtained fromfrom linearlinear polyethyleniminepolyethylenimine (PEI)(PEI) inin aa three-stagethree-stage processprocess thatthat includes crosslinking, crosslinking, telomerization, telomerization, and and quaternization. quaternization. In Inthe the first first stage, stage, linear linear PEI PEIis crosslinked is crosslinked with with an alkyl an alkyl dihalide, dihalide, usually usually 1 to 1 20 to mol 20 mol.%.% 1,5- 1,5-dibromopentane.dibromopentane. This This re- reactionaction results results in in the the formation formation of of the the insoluble insoluble nanoparticle nanoparticle core from whichwhich shortshort PEIPEI chainschains protrude.protrude. TheyThey are are terminated terminated with with primary primary amino amin endo end groups groups that that are are subjected subjected to telomerization.to telomerization. The The two two methods methods that that may may be used be used alternatively alternatively include include N-alkylation N-alkylation and reductiveand reductive amination, amination, but N-alkylation but N-alkylation is primarily is primarily used whenused octylwhen bromide octyl bromide is used is as used the N-alkylationas the N-alkylation agent. Thisagent. step This determines step determines the hydrophobicity the hydrophobicity of the nanoparticles, of the nanoparticles, which iswhich responsible is responsible for the for interactions the interactions between between QA-PEI-NPs QA-PEI and-NPs lipids and lipids constituting constituting bacterial bac- membranes.terial membranes. Quaternization, Quaternization, also called also N-methylation, called N-methylation, is performed is performed in the final in stage. the final The quaternarystage. The quaternary ammonium ammonium groups are createdgroups duringare created the reaction during ofthe secondary reaction of and secondary tertiary aminoand tertiary groups amino with iodomethane.groups with iodomethane. Their presence Their provides presence QA-PEI-NPs provides withQA-PEI antibacterial-NPs with activityantibacterial (Figure activity1)[1,27 (Figure–29]. 1) [1,27–29].

FigureFigure 1.1. TheThe chemicalchemical structurestructure ofof QA-PEI-NPs.QA-PEI-NPs.

The antimicrobial activity of QA-PEI-NPs has been widely examined [1]. It has been demonstrated in many studies that QA-PEI-NPs exert antibacterial activity against many

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bacteria strains. The Gram-positive bacteria, such as Staphylococcus aureus [29–31] and Streptococcus mutans [32] showed greater susceptibility to the antibacterial activity of QA- PEI-NPs than the Gram-negative bacteria, such as Escherichia coli [33] and Pseudomonas aeruginosa [29,31]. In those studies, the influence of the crosslink density, N-alkylation agent, N-alkylation degree, and quaternization degree on the antibacterial activity of QA- PEI-NPs was also demonstrated. It can be concluded that the highest efficiency could be achieved using 1,5-dibromopentane as the crosslinker in a ratio of 0.04 mol/mol PEI [32], 1- bromooctane N-alkylation agent in the ratio of 1 mol/mol PEI [29,30,32], and iodomethane quaternization agent in the ratio of 3 mol/mol PEI [30]. In other studies, the antibacterial activity of commercially available dental composite restorative materials enriched with QA-PEI-NPs was investigated. Dimethacrylate resin-based composites modified with 0.5 to 2 wt.% QA-PEI-NPs showed activity against the following bacterial strains: S. mutans—the bacteria identified the most with tooth decay [28,32,34,35], S. aureus [36], P. aeruginosa [36], E. coli [36], Staphylococcus epidermidis [36], Enterococcus faecalis [35–38], Actinomyces viscosus [34,35], and Lactobacillus casei [35]. It was found that 1 wt.% QA-PEI-NPs was sufficient to achieve a good antibacterial effect against the Gram-positive bacteria S. mutans, S. aureus, A. viscosus, E. faecalis, and L. casei. This concentration did not alter the cytotoxicity of the original material and did not exert harmful effects on living tissues. The Gram-positive bacteria, S. epidermis as well as Gram- negative bacteria, P. aeruginosa and E. coli, showed weaker responses to the presence of QA-PEI-NPs, and their higher content (2 wt.%) was required to achieve complete bacterial growth inhibition. The presence of 2 wt.% QA-PEI-NPs slightly increased the material’s cytotoxicity [36,39,40]. Based on the above, QA-PEI-NPs have been shown as highly efficient antibacterial agents for dental restorative materials based on dimethacrylate resins; however, there is still not enough knowledge of the influence of how QA-PEI-NPs affect the structural, physicochemical, and mechanical properties of this type of dental material matrix. It is well known that the deterioration of any property can negatively affect the composite performance and even reduce its lifetime [41]; therefore, this expertise is required for the development of new dental restorative materials modified with QA-PEI-NPs toward solutions aimed at achieving a set of optimum characteristics. The degree of conversion (DC) of double bonds is a particularly important structural property for the proper functioning of dental materials based on dimethacrylate resins. It determines their mechanical and physicochemical properties, and its reduction weakens the mechanical properties of a composite [41]. Shvero et al., by modifying the Filtek Supreme XT composite (3M ESPE Dental) with 2 wt.% QA-PEI-NPs did not observe a significant change in the DC, which increased from 51 to 53% after adding QA-PEI-NPs [35]. Beyth et al. studied the effect of incorporating 1 wt.% QA-PEI-NPs on the flexural properties of two commercial composites: Z250 (3M ESPE Dental) and Filtek Flow (3M ESPE Dental) [28]. The addition of QA-PEI-NPs to both materials resulted in a statistically insignificant reduction in the modulus. The flexural strength of the Z250 composite increased insignificantly, whereas that of the Filtek Flow composite decreased by approximately 40%. This article aims to provide the lacking knowledge about the structure-property relationships of dental dimethacrylate polymer networks modified with QA-PEI-NPs. The research was carried out on the composition of two commercial dimethacrylate dental resins—60 wt.% bisphenol A glycerolate dimethacrylate (Bis-GMA) (Scheme1) and 40 wt.% triethylene glycol dimethacrylate (TEGDMA) (Scheme1) enriched with 0.5, 1, and 2 wt.% QA-PEI-NPs. The cured materials were tested for their degree of conversion (DC), density (dp), and polymerization shrinkage (S), glass transition temperature (Tg), water sorption (WS), and solubility (SL), water contact angle (WCA), flexural modulus (E), flexural strength (σ), hardness (HB), and impact resistance (an). Materials 2021, 14, x FOR PEER REVIEW 4 of 20

Materials 2021, 14, x FOR PEER REVIEW 4 of 20 Materials 2021, 14, 2037 4 of 20

Scheme 1. The chemical structures of Bis-GMA and TEGDMA.

2. SchemeMaterials 1. The and chemical Methods structures of Bis-GMA and TEGDMA.

2.1. Materials Scheme2. Materials 1. The andchemical Methods structures of Bis-GMA and TEGDMA. 2.1.PEtOx Materials (poly(2-ethyl-2-oxazoline), 50 kDa, Sigma Aldrich, St. Louis, MO, USA), HClaq (hydrochloric acid 37 wt.% in H2O, Acros Organics, Geel, Belgium), NaOH (sodium hy- 2. MaterialsPEtOx (poly(2-ethyl-2-oxazoline),and Methods 50 kDa, Sigma Aldrich, St. Louis, MO, USA), HClaq droxide, Chempur, Piekary Śl., Poland), diethyl ether (Chempur, Piekary Śl., Poland), 1,5- 2.1.(hydrochloric Materials acid 37 wt.% in H2O, Acros Organics, Geel, Belgium), NaOH (sodium hy- dibromopentane (Acros Organics, Geel, Belgium), 1-bromooctane (Acros Organics, Geel, droxide, Chempur, Piekary Sl.,´ Poland), diethyl ether (Chempur, Piekary Sl.,´ Poland), PEtOx (poly(2-ethyl-2-oxazoline), 50 kDa, Sigma Aldrich, St. Louis, MO, USA), HClaq Belgium),1,5-dibromopentane methyliodide (Acros (Acros Organics, Organics, Geel, Geel, Belgium),Belgium), 1-bromooctaneNaHCO3 (sodium (Acros bicarbonate, Organics, (hydrochloric acid 37 wt.% in H2O, Acros Organics, Geel, Belgium), NaOH (sodium hy- Chempur,Geel, Belgium), Piekary methyliodide Śl., Poland), (Acros anhydrous Organics, ethanol Geel, (Stanlab, Belgium), Lublin, NaHCO Poland),(sodium hexane bi- droxide, Chempur, Piekary Śl., Poland), diethyl ether (Chempur, Piekary Śl.,3 Poland), 1,5- (Chempur,carbonate, Piekary Chempur, Śl., Piekary Poland),Sl.,´ Bis Poland),-GMA anhydrous (bisphenol ethanol A glycerolate (Stanlab, dimethacrylate, Lublin, Poland), dibromopentane (Acros Organics, Geel, Belgium), 1-bromooctane (Acros Organics, Geel, Sigmahexane-Aldrich, (Chempur, St. Louis, Piekary MO,Sl.,´ USA), Poland), TEGDMA Bis-GMA (triethylene (bisphenol glycol A glycerolate dimethacrylate, dimethacrylate, Sigma- Belgium), methyliodide (Acros Organics, Geel, Belgium), NaHCO3 (sodium bicarbonate, Aldrich,Sigma-Aldrich, St. Louis, St. MO, Louis, USA), MO, CQ USA), (camphorquinone, TEGDMA (triethylene Sigma-Aldrich, glycol dimethacrylate,St. Louis, MO, USA), Sigma- Chempur, Piekary Śl., Poland), anhydrous ethanol (Stanlab, Lublin, Poland), hexane DMAEMAAldrich, St. (dimethylaminoethyl Louis, MO, USA), CQ methacrylate, (camphorquinone, Sigma- Sigma-Aldrich,Aldrich, St. Louis, St. MO, Louis, USA) MO, were USA), (Chempur, Piekary Śl., Poland), Bis-GMA (bisphenol A glycerolate dimethacrylate, usedDMAEMA as received. (dimethylaminoethyl methacrylate, Sigma-Aldrich, St. Louis, MO, USA) were Sigma-Aldrich, St. Louis, MO, USA), TEGDMA (triethylene glycol dimethacrylate, Sigma- used as received. 2.2.Aldrich, Synthesis St. Louis, MO, USA), CQ (camphorquinone, Sigma-Aldrich, St. Louis, MO, USA), DMAEMA (dimethylaminoethyl methacrylate, Sigma-Aldrich, St. Louis, MO, USA) were 2.2.1.2.2. Synthesis of Linear Polyethylenimine (PEI) used as received. 2.2.1.Linear Synthesis polyethylenimine of Linear Polyethylenimine (PEI) was obtained (PEI) via the acidic hydrolysis of poly(2-ethyl- Linear polyethylenimine (PEI) was obtained via the acidic hydrolysis of poly(2-ethyl- 2-2.2.oxazoline) Synthesis (PEtOx) (Scheme 2) [42]. 2-oxazoline) (PEtOx) (Scheme2)[42]. 2.2.1. Synthesis of Linear Polyethylenimine (PEI) Linear polyethylenimine (PEI) was obtained via the acidic hydrolysis of poly(2-ethyl- 2-oxazoline) (PEtOx) (Scheme 2) [42].

Scheme 2. The PEI formation via the acidic hydrolysis of PEtOx. Scheme 2. The PEI formation via the acidic hydrolysis of PEtOx. PEtOx (5.03 g) was dissolved in 20 mL distilled water and added to a round-bottom ﬂaskPEtOx equipped (5.03 withg) was a condenser,dissolved in thermometer, 20 mL distilled and magneticwater and stirrer. added Then, to a round 15 mL-bottom of HClaq

flask(37 wt.%equipped in H 2withO) was a condenser, added to the thermometer, flask, and the and reaction magnetic mixture stirrer. was Then, refluxed 15 mL for of 6 HCl h withaq (37Schemethe wt.% use 2. ofin The theH2 O) oilPEI bath.was formation added After via coolingto the the acidic flask, to room hydrolysis and temperature, the ofreaction PEtOx. unreacted mixture HClwas andrefluxed newly-formed for 6 h withpropionic the use acidof the were oil bath. removed After fromcooling the to reaction room temperature, mixture under unreacted reduced HCl pressure. and newly The- formedpowderPEtOx propionic residue (5.03 wasacidg) was dissolved were dissolved removed in distilled in from20 mL water,the distilled reaction and thewater mixture surplus and underadded acid was reduced to neutralizeda round pressure.-bottom with Theflask5 N powderNaOH equipped residue solution with was toa condenser, pH dissolved 9–10. The thermometer, in distilled resulting water, white and andmagnetic PEI the precipitate surplus stirrer. wasacid Then, then was 15 recrystallizedneutralized mL of HClaq with(37from wt.%5 N distilled NaOH in H2 water,O)solution was dissolved added to pH to9– in10.the methanol, Theflask, resulting and and the purifiedwhite reaction PEI by mixtureprecipitate precipitating was was refluxed into then ice-cooled recrys- for 6 h tallizedwithdiethyl the from ether.use distilled of After the oil filtrating,water, bath. Afterdissolved pure cooling PEI in was methanol, to room dried temperature, under and purified reduced unreacted by pressure precipitating HCl for 5and days.into newly ice- - cooledformed diethyl propionic ether. acid After were filtrating, removed pure from PEI the was reaction dried mixture under underreduced reduced pressure pressure. for 5 2.2.2. Synthesis of QA-PEI-NPs days.The powder residue was dissolved in distilled water, and the surplus acid was neutralized with 5QA-PEI-NPs N NaOH solution were synthesized to pH 9–10. via The an N-alkylation resulting white method PEI [ 30precipitate]. The solution was ofthen PEI recrys- (1.77 g, tallized0.04 mol) from in 20 distilled mL absolute water, ethanol dissolved was introducedin methanol, into and a round-bottom purified by precipitating flask equipped into with ice a- cooledcondenser, diethyl thermometer, ether. After and filtrating, magnetic pure stirrer. PEI Then, was 1,5-dibromopentane dried under reduced (0.04 pressure mol/mol for PEI 5 days.unit) was added, and the reaction was refluxed for 24 h with the use of the oil bath. In the second stage, the N-alkylation reaction was carried out. 1-bromooctane (1 mol/mol PEI unit) was added into the same flask, and the mixture was refluxed for 24 h. The resulting hydrobromic

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acid was neutralized with NaHCO3 (1.25 mol/mol 1-bromooctane) by heating under reflux for 24 h. The quaternization reaction was carried out in the same flask. The reaction mixture was refluxed for 48 h after adding iodomethane (3 mol/mol PEI unit). The resulting hydroiodic acid was neutralized with NaHCO3 (1 mol/mol PEI unit), by heating under reflux for 24 h. The QA-PEI-NPs were first purified by precipitation in distilled water. The filtered residue was then washed several times with hexane and distilled water, centrifuged, and lyophilized to dryness.

2.3. Curing Procedure The 60 wt.% Bis-GMA and 40 wt.% TEGDMA composition was prepared and 0.4 wt.% CQ was added as a photoinitiator. The composition was divided into four parts. Three were admixed with 0.5, 1, and 2 wt.% QA-PEI-NPs. One composition was left unmodified for comparison purposes. Mixing was performed in a dark room, initially with a spat- ula, and then, resins enriched with QA-PEI-NPs were homogenized with an ultrasonic stirrer. A reducing agent—1 wt.% DMAEMA was admixed to each composition before its polymerization. All compositions were subjected to photocuring with a UV–VIS lamp, having 127 mm in diameter, and emitting UV–VIS radiation in the range from 280 to 780 nm with the exitance of 2400 mW/cm2 (Ultra Vitalux 300, Osram, Munich, Germany) for 1 h, from a distance of 15 cm. Polymerizations were carried out in square-shaped glass molds measuring 90 mm × 90 mm × 4 mm (length × width × thickness), Petri dishes measuring 40 mm × 4 mm (diameter × thickness), and disc-like Teflon molds measuring 15 mm × 1.5 mm (diameter × thickness). The oxygen inhibition was reduced by covering the polymerizing system with a PET film.

2.4. Instrumental Analysis 2.4.1. Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR) The 1H NMR spectra were collected with a 300 MHz NMR spectrometer (UNITY/INOVA, Varian, Palo Alto, CA, USA). PEtOx, PEI, and QA-PEI-NPs were analyzed in CDCl3, CD3OH, and DMSO solutions, respectively. TMS was used as an internal reference each time.

2.4.2. Infrared Spectroscopy (FTIR) A Spectrum Two (Perkin-Elmer, Waltham, MA, USA) Fourier-transform infrared spectrometer (ATR-FTIR) was used in this study. The attenuated total reﬂectance sampling method was used to conﬁrm the chemical structure of PEtOx, PEI, and QA-PEI-NPs. The same apparatus was used to characterize the monomer and polymer compositions enriched with QA-PEI-NPs. Uncured samples were analyzed as thin layers closed between two KBr pellets. The cured samples were powdered and sieved to a grain size less than 25 µm. They were analyzed as KBr pellets, one week after curing. The FTIR spectra were used for the DC determination, using the following equation:  AC=C  AAr polymer DC(%) = 1 −  × 100 (1)  AC=C  AAr monomer

−1 where AC=C is the absorbance of the band at 1637 cm , corresponding to the C=C stretch- −1 ing vibrations in the methacrylate group, AAr is the absorbance of the band at 1608 cm , corresponding to the carbon-carbon stretching vibrations in the benzene ring.

2.4.3. Dynamic Light Scattering (DLS) The QA-PEI-NPs sizes were measured at 25 ◦C by a dynamic light scattering in- strument (DLS, Zetasizer Nano-S90, Malvern Technologies, Malvern, UK). Before measurements, QA-PEI-NPs were dispersed in distilled water and placed in a poly(methyl methacrylate) cuvette. Materials 2021, 14, 2037 6 of 20

2.4.4. Density and Polymerization Shrinkage

The liquid compositions were tested for density (dm) with a pyknometer according to ISO 1675 [43]. The cured materials were tested for density (dp) utilizing an analytical balance equipped with a density determination kit operating on Archimedes’ principle. The 0.01 mg analytical balance (XP Balance, Mettler Toledo, Greifensee, Switzerland) was used for measuring sample mass. The polymerization shrinkage (S) was calculated with the following equation:

d S(%) = 1 − m × 100 (2) dp

where dm is the monomer density, and dp is the polymer density.

2.4.5. Differential Scanning Calorimetry (DSC)

The glass transition temperature (Tg) was determined utilizing differential scanning calorimeter DSC 822e (Mettler Toledo, Greifensee, Switzerland), following the procedure described in the standard ISO 11357-2:2020 [44]. Samples of cured materials weighed about 2.5 mg. Experiments were carried out in the air, at the temperature range from −40 to 200 ◦C, with a heating rate of 10 K/min.

2.4.6. Water Sorption and Solubility The water sorption (WS) and solubility (SL) were determined following the procedure described in the standard ISO 4049 [45], utilizing a 0.01 mg analytical balance (XP Balance, Mettler Toledo, Greifensee, Switzerland). Disc-like specimens of cured materials measuring ◦ 15 mm × 1.5 mm (diameter × thickness) were dried at 100 C to constant mass (m0). The specimens were then introduced into glass containers with distilled water and kept for 7 days at room temperature. Just after removing from water, the specimens were thoroughly dried with absorbing paper and weighed (m1). The WS was calculated with the following equation:

µg m − m WS = 1 0 (3) mm3 V

where m1 is the mass of the swollen specimen, m0 is the initial mass of the dried specimen, and V is the initial volume of the dried specimen. The SL was determined by drying those specimens to a constant mass (m2), and calculating its value with the following equation:

µg m − m SL = 0 2 (4) mm3 V

where m2 is the mass of the dried specimen after water storage.

2.4.7. Water Contact Angle The cured materials were tested for the water contact angle (WCA) utilizing a go- niometer (OCA 15EC, Data Physics, Filderstadt, Germany). Deionized water (4 µL) was dropped onto the tested surface via the sessile drop method.

2.4.8. Mechanical Properties Flexural Properties The ﬂexural modulus (E) and ﬂexural strength (σ) were tested using a universal testing machine Zwick Z020 (Ulm, Germany), following the procedure described in the standard ISO 178 [46]. Specimens of cured materials, measuring 80 mm × 10 mm × 4 mm Materials 2021, 14, 2037 7 of 20

(length × width × thickness) were cut out from the pre-formulated molds and sanded clean. E and σ were calculated with the following equations:

P l3 E (MPa) = 1 (5) 4bd3δ and 3Pl σ(MPa) = (6) 2bd2

where P1 is the load at the selected point of the elastic region of the stress-strain plot, P is the maximum load, l is the distance between supports, b is the sample width, d is the sample thickness, and δ is the deﬂection of the sample at P1.

Hardness The hardness (HB) was determined utilizing a VEB Werkstoffpr˝ufmaschinenmachine (Leipzig, Germany), following the procedure described in the standard ISO 2039-1 [47]. The disc-like specimens of cured materials, measuring 40 mm × 4 mm (diameter × thickness) were sanded clean before testing. HB was calculated with the equation:

0.21 Fm (h−h )+0.21 HB(MPa) = r (7) πdhr

where Fm is the test load, d is the diameter of the ball intender (d = 5 mm), h is the immersion depth, and hr is the reduced depth of immersion (hr = 0.25 mm).

Impact Strength

The impact strength (an) was determined utilizing the Charpy impact tester (Zwick RKP 450, Ulm, Germany), according to the standard ISO 179-1 [48]. Tests were performed on specimens of cured materials measuring 80 mm × 10 mm × 4 mm (length × width × thickness), which were cut out from the pre-formulated molds and sanded clean. an was calculated with the equation:

kJ A a = n (8) n m2 bh

where An is the force causing fracture of a specimen, b is the specimen width, and h is the specimen thickness.

2.5. Statistical Analysis A set of results achieved for ﬁve specimens was analyzed using the Statistica 13.1 software (TIBCO Software Inc., Palo Alto, CA, USA). The one-way ANOVA with Tukey’s HSD post hoc tests were performed (α = 0.05). The results were expressed as mean values, and their associated standard deviations (SD).

3. Results In this work, the composition of two common dental resins 60 wt.% Bis-GMA and 40 wt.% TEGDMA was modiﬁed by the physical admixing of QA-PEI-NPs. The latter was synthesized according to procedures described in the literature. First, linear PEI was obtained via the acidic hydrolysis of PEtOx [42]. The formation of PEI was conﬁrmed by 1H NMR and ATR-FTIR measurements. The 1H NMR spectrum of PEI revealed the disappearance of signals from protons present in the -CH2CH3 PEtOx substituent, located at 1.13, 2.31, and 2.42 ppm (Figure2). It also indicates that PEtOx hydrolysis was complete. Since the local chemical environment surrounding the hydrogen nuclei of –CH2CH2– groups was changed, they gave a single peak located at 2.73 ppm. The ATR-FTIR spectrum of PEtOx exhibited an absorption peak of the >C=O group in the amide group located at MaterialsMaterials 2021 2021, ,14 14, ,x x FOR FOR PEER PEER REVIEW REVIEW 88 of of 20 20

synthesizedsynthesized according according to to procedures procedures d describedescribed in in the the literature. literature. First, First, linear linear PEI PEI was was ob- ob- tainedtained via via the the acidic acidic hydrolysis hydrolysis of of PEtOx PEtOx [42]. [42]. The The formation formation of of PEI PEI was was confirmed confirmed by by 1H 1H NMRNMR and and ATR ATR-FTIR-FTIR measurements. measurements. The The 1H 1H NMR NMR spectrum spectrum of of PEI PEI revealed revealed the the disap- disap- pearance of signals from protons present in the -CH2CH3 PEtOx substituent, located at pearance of signals from protons present in the -CH2CH3 PEtOx substituent, located at Materials 2021, 14, 2037 1.13,1.13, 2.31, 2.31, and and 2.42 2.42 ppm ppm (Figure (Figure 2). 2). It It also also indicates indicates that that PEtOx PEtOx hydrolysis hydrolysis was was complete. complete.8 of 20 Since the local chemical environment surrounding the hydrogen nuclei of –CH2CH2– Since the local chemical environment surrounding the hydrogen nuclei of –CH2CH2– groupsgroups was was changed, changed, they they gave gave a a singl singlee peak peak located located at at 2.73 2.73 ppm. ppm. The The ATR ATR-FTIR-FTIR spec- spec- trumtrum of of PEtOx −PEtOx1 exhibited exhibited an an absorption absorption peak peak of of the the >C=O >C=O group group in in the the amide amide group group located located 1646 cm ,−1 which disappeared as PEI was formed. A peak of the –NH– group located at atat 1646 1646 cm cm−1−1, ,which which disappeared disappeared as as PEI PEI was was formed. formed. A A peak peak of of the the – –NHNH–– group group located located at at 3357 cm −1 was observed instead (Figure3). 33573357 cm cm−1 was was observed observed instead instead (Figure (Figure 3). 3).

11 FigureFigureFigure 2. 2.2. The TheThe 1 HH NMR NMR spectra spectra of of PEtOx PEtOx and and PEI. PEI.

FigureFigureFigure 3. 3.3. The TheThe ATR ATR-FTIRATR-FTIR-FTIR spectra spectra of of PEtOx PEtOx and and PEI. PEI.

InInIn the thethe second second stage, stage, stage, PEI PEI PEI was was was crosslinked crosslinked crosslinked with with with 1,5 1,5 1,5-dibromopentane-dibromopentane-dibromopentane (0.04 (0.04 (0.04 mol/mol mol/mol mol/mol PEI PEI unit),PEIunit), unit), N N-alkylated-alkylated N-alkylated with with with1 1-bromooctane-bromooctane 1-bromooctane ( 1(1 mol/mol mol/mol (1 mol/mol PEI PEI unit), unit), PEI unit),and and quaternized quaternized and quaternized with with iodo- withiodo- methaneiodomethanemethane (3 (3 mol/mol mol/mol (3 mol/mol PEI PEI unit). unit). PEI unit).The The formation formation The formation of of QA QA of-PEI-PEI QA-PEI-NPs-NPs-NPs was was confirmed confirmed was conﬁrmed by by 1H 1H byNMR NMR 1H NMR and ATR-FTIR measurements. The signals at 0.87, 1.20–1.50, and 1.72 ppm from protons of the bromooctane substituent (–CH3 and –(CH2)6– groups) were present in the 1H NMR spectrum of QA-PEI-NPs (Figure4), but they were not observed in the 1H NMR

spectrum of PEI (Figure2). The signal coming from the -CH 2- groups of PEI changed its position and moved from 2.74 ppm to 3.32 ppm due to changes in the chemical environment surrounding the hydrogen nuclei. It overlapped with the signals of the remaining –CH2– group of the octyl chain and -CH3 groups neighboring the quaternary nitrogen (Figure4). The ATR-FTIR spectrum of QA-PEI-NPs exhibited characteristic absorption peaks of the MaterialsMaterials 2021 2021, ,14 14, ,x x FOR FOR PEER PEER REVIEW REVIEW 99 of of 20 20

andand ATR ATR-FTIR-FTIR measurements. measurements. The The signals signals at at 0.87, 0.87, 1.20 1.20––1.50,1.50, and and 1.72 1.72 ppm ppm from from protons protons ofof the the bromooctane bromooctane substituent substituent ( –(–CHCH3 3and and – –(CH(CH2)26)–6– groups) groups) were were present present in in the the 1H 1H NMR NMR spectrumspectrum of of QA QA-PEI-PEI-NPs-NPs (Figure (Figure 4), 4), but but they they were were not not observed observed in in the the 1H 1H NMR NMR spectrum spectrum ofof PEI PEI (Figure (Figure 2). 2). The The signal signal coming coming from from the the -CH-CH2-2 -groups groups of of PEI PEI changed changed its its position position andand moved moved from from 2.74 2.74 ppm ppm to to 3.32 3.32 ppm ppm due due to to changes changes in in the the chemical chemical environment environment sur- sur- Materials 2021, 14, 2037 9 of 20 roundingrounding the the hydrogen hydrogen nuclei. nuclei. It It overlapped overlapped with with the the signals signals of of the the remaining remaining ––CHCH2–2– groupgroup of of the the octyl octyl chain chain and and - CH-CH3 3groups groups neighboring neighboring the the quaternary quaternary nitrogen nitrogen (Figure (Figure 4). 4). TheThe ATR ATR-FTIR-FTIR spectrum spectrum of of QA QA-PEI-PEI-NPs-NPs exhibit exhibiteded characteristic characteristic absorption absorption peaks peaks of of the the −1 amineamine group group (3446 (3446 and and 1620 1620 cm cm−1),), −– –CH1CH3 3and and – –CHCH2–2– groups groups (2954, (2954, 2925, 2925, 2852, 2852, and and 1465 1465 amine group (3446 and 1620 cm ), –CH3 and –CH2– groups (2954, 2925, 2852, and cm−1−1), and quaternary nitrogen (956 cm−1−1) (Figure 5). cm1465), and cm −quaternary1), and quaternary nitrogen nitrogen (956 cm (956) (Figure cm−1 )5). (Figure 5).

1 Figure 4. The1 H NMR spectrum of QA-PEI-NPs. FigureFigure 4. 4. The The 1 HH NMR NMR spectrum spectrum of of QA QA-PEI-PEI-NPs.-NPs.

FigureFigureFigure 5. 5. The 5.TheThe ATR ATR ATR-FTIR-FTIR-FTIR spectra spectra spectra of of ofQA QA QA-PEI-NPs.-PEI-PEI-NPs.-NPs.

The dimensions of the QA-PEI-NPs were analyzed utilizing the DLS technique (Figure6). Three fractions of particles were detected with an average size of (i) 151 nm (±28 nm) that constituted 18.9% of all particles, (ii) 732 nm (±222 nm) that constituted 73.9% of all particles, and (iii) 5212 nm (±467 nm) that constituted 7.2% of all nanoparticles. The Z-average size was

509.1 nm. Materials 2021, 14, x FOR PEER REVIEW 10 of 20

The dimensions of the QA-PEI-NPs were analyzed utilizing the DLS technique (Fig- ure 6). Three fractions of particles were detected with an average size of (i) 151 nm (±28 nm) that constituted 18.9% of all particles, (ii) 732 nm (±222 nm) that constituted 73.9% of Materials 2021, 14, 2037 all particles, and (iii) 5212 nm (±467 nm) that constituted 7.2% of all nanoparticles. The10 of Z 20- average size was 509.1 nm.

Figure 6. TThehe DLS size distribution profile proﬁle of QA-PEI-NPs.QA-PEI-NPs.

The obtained QA-PEI-NPsQA-PEI-NPs were were introduced introduced into into the the Bis-GMA/TEGDMA Bis-GMA/TEGDMA composition composi- tionin the in amountsthe amounts of 0.5, of 1,0.5, and 1, and 2 wt.%. 2 wt.%. The The modified modified compositions, compositions, as wellas well as oneas one free free of ofQA-PEI-NPs, QA-PEI-NPs, were were hardened hardened by by photocuring photocuring with with a a commoncommon dentaldental initiating system. Prepared materials were tested for selected physicochemicalphysicochemical andand mechanicalmechanical properties.properties. In Table 1 1,, thethe results results of of density density and and polymerization polymerization shrinkage shrinkage areare summarized. summarized. AsAs can be seen,seen, thethe densitydensity of of uncured uncured compositions compositions (d m(d)m did) did not not change change after after adding adding QA-PEI- QA- 3 3 NPs (1.13 g/cm ). Curing3 increased the density (d ), which ranged from 1.22 to 1.24 g/cm PEI-NPs (1.13 g/cm ). Curing increased the densityp (dp), which ranged from 1.22 to 1.24 g/cmand slightly3 and slightly increased increased upon upon increasing increasing the concentration the concentration of QA-PEI-NPs; of QA-PEI-NPs; however, however, the theneat neat polymer polymer and and the materialthe material containing containing 0.5 wt.%0.5 wt.% QA-PEI-NPs QA-PEI-NPs had had the samethe same density. density. The Theaddition addition of 1 wt.%of 1 wt.% QA-PEI-NPs QA-PEI resulted-NPs res inulted a statistically in a statistically insignificant insignificant (p > 0.05) (p increase> 0.05) inin the dp. The addition of 2 wt.% QA-PEI-NPs caused a 2% increase in the dp compared with crease in the dp. The addition of 2 wt.% QA-PEI-NPs caused a 2% increase in the dp com- p ≤ paredthe neat with polymer. the neat The polymer. latter difference The latter was difference statistically was statistically significant ( significant0.05). (p ≤ 0.05).

Table 1. The monomer density (dm), polymer density (dp), and polymerization shrinkage (S) of studied materials. Statistically Table 1. The monomer density (dm), polymer density (dp), and polymerization shrinkage (S) of studied materials. Statisti- ≤ signiﬁcantcally significan differencest differences (p (0.05)p ≤ 0.05) are are marked marked with with the the lower lower case case letters, letters whereas, whereas statistically statistically insigniﬁcantinsignificant differences ((p > 0.05) are marked with the upper case letters.

3 3 dm (g/cmdm (g/cm) 3) ddpp (g/cm(g/cm)3) SS (%) (%) SampleSample Name Name (p = 0.9999)(p = 0.9999) ((pp= = 0.013)0.013) (p(p == 0.007) 0.007) AverageAverage SDSD AverageAverage SD SD Average Average SD SD 0%0% QA-PEI-NP QA-PEI-NP 1.131.13 0.06 0.06 1.221.22A,B,c A,B,c 0.010.01 7.377.37 A,B,cA,B,c 0.700.70 0.5%0.5% QA-PEI-NP QA-PEI-NP 1.131.13 0.03 0.03 1.221.22A,D,e A,D,e 0.010.01 7.377.37 A,D,eA,D,e 0.580.58 B,D,F B,D,F 1%1% QA-PEI-NP QA-PEI-NP 1.131.13 0.05 0.05 1.231.23 B,D,F 0.010.01 8.138.13 B,D,F 0.720.72 2% QA-PEI-NP 1.13 0.05 1.24 c,e,F 0.01 8.87 c,e,F 0.63 2% QA-PEI-NP 1.13 0.05 1.24 c,e,F 0.01 8.87 c,e,F 0.63

The results in Table 1 1 also also indicateindicate thatthat therethere isis aa relationshiprelationship betweenbetween thethe amountamount ofof QAQA-PEI-NPs-PEI-NPs and polymerization shrinkage shrinkage (S). (S). The The S S values ranged from 7.37 to 8.87% and increased as the concentrationconcentration of QA-PEI-NPsQA-PEI-NPs increased.increased. Compared with the neat ppolymer,olymer, this parameter firstfirst increased when the concentration of QA-PEI-NPsQA-PEI-NPs exceeded 0.5 wt.%. The The addition addition of 1 and and 2 2 wt.% wt.% of of QA QA-PEI-NPs-PEI-NPs increased the S values, which were respectively, statisticallystatistically insignificantinsignificant ((pp> > 0.05)0.05) andand statisticallystatistically significantsignificant (p(p≤ ≤ 0.05). The latter inc increaserease was 20%. The DC in cured materials was also determined using FTIR spectroscopy. Figure 77 shows thethe representativerepresentative FTIR FTIR spectra spectra of of the the composition composition containing containing 0.5 wt.%0.5 wt.% QA-PEI-NPs QA-PEI- NPsin its in liquid its liquid and curedand cured forms. forms. The resultsThe results for the for DC the are DC summarized are summarized in Figure in Figure8. The 8. neat The neatpolymer polymer was characterizedwas characterized by a by DC a ofDC 68.08%. of 68.08%. Generally, Generally, the additionthe addition of QA-PEI-NPs of QA-PEI- NPsincreased increased the DC the values,DC values, which which ranged ranged from from 70.44 70.44 to 70.77%; to 70.77%; however, however, all the all differences the differ- p encesin the in DC the values DC values were notwere statistically not statistically significant significant ( > 0.05). (p > 0.05).

Materials 2021, 14, x FOR PEER REVIEW 11 of 20

MaterialsMaterials 2021 2021, 14, ,14 x ,FOR x 2037 FOR PEER PEER REVIEW REVIEW 11 11of 20of 20

(a) (b)

Figure 7. The representative FTIR spectra of the Bis-GMA/TEGDMA composition containing 0.5 wt.% QA-PEI-NPs in its (a)( a) (b()b ) uncured (a) and cured (b) forms. FigureFigure 7. 7.The The representative representative FTIR FTIR spectra spectra of ofthe the Bis Bis-GMA/TEGDMABis-GMA/TEGDMA-GMA/TEGDMA composition composition composition containing containing containing 0.5 0.5 wt.% wt.% QA QA QA-PEI-NPs-PEI-PEI-NPs-NPs in in its its uncured (a) and cured (b) forms. uncureduncured (a) ( aand) and cured cured (b) ( bforms.) forms.

Figure 8. DC values of the materials. All the results for DC were statistically insignificant (p > 0.05). FigureFigureFigure 8. DC8. DCDC values values values of ofthe of the the materials. materials. materials. All All All the the the results results results for for DC DC were were statistical statistical statisticallyly lyin significan insigniﬁcantinsignificant (pt (>p > 0.05). 0.05).0.05).In Figure 9, the Tg results are summarized. Generally, the Tg slightly increased due to In Figure9, the T g results are summarized. Generally, the Tg slightly increased due the introduction of QA-PEI-NPs into the neat polymer (Tg = 54.19 °C) and ◦ranged from to theIn introductionFigure 9, the T ofg results QA-PEI-NPs are summarized. into the neat Generally, polymer the (T Tgg slightly= 54.19 increasedC) and ranged due to 55.47 toIn 58.96Figure °C; 9, however,the Tg◦ results the T areg decreas summarized.ed upon Generally, increasing the the T concentrationg slightly increased of QA due-PEI to- thefrom introduction 55.47 to 58.96 of QAC;- however,PEI-NPs into the Ttheg decreased neat polymer upon (T increasingg = 54.19 °C) the and concentration ranged from of NPs.the introductionAll the differences of QA in-PEI the-NPs Tg values into the did neat not polymerhave statistical (Tg = 54.19 meaning °C) and(p > 0.05).ranged from QA-PEI-NPs. All the differences in the Tg values did not have statistical meaning (p > 0.05). 55.4755.47 to to58.96 58.96 °C; °C; however, however, the the Tg T decreasg decreaseded upon upon increasing increasing the the concentration concentration of ofQA QA-PEI-PEI- - NPs.NPs. All All the the differences differences in inthe the Tg T valuesg values did did not not have have statistical statistical meaning meaning (p (>p 0.05).> 0.05).

Figure 9. Tg values of the materials. All the results for Tg were statistically insigniﬁcant (p > 0.05). Figure 9. Tg values of the materials. All the results for Tg were statistically insignificant (p > 0.05).

In Figure 10 the results for water sorption (WS) are summarized. Its values slightly Figure 9. Tg values of the materials. All the results for Tg were statistically insignificant (p > 0.05). Figureincreased 9. Tg v afteralues theof the introduction materials. All of the QA-PEI-NPs results for T intog were the statistical neat polymerly insignifican (WS =t 31.5 (p > µ0.05).g/mm 3)

Materials 2021, 14, x FOR PEER REVIEW 12 of 20 Materials 2021, 14, x FOR PEER REVIEW 12 of 20

Materials 2021, 14, 2037 12 of 20 In Figure 10 the results for water sorption (WS) are summarized. Its values slightly increasedIn Figure after 10 the the introduction results for ofwater QA -sorptionPEI-NPs (WS)into the are neat summarized. polymer (WS Its values= 31.5 µ slightlyg/mm3) increasedand ranged after from the 34.63 introduction to 36.59 µofg/mm QA-3PEI; however,-NPs into WS the decreased neat polymer upon (WS increasing = 31.5 µ theg/mm con-3) 3 3 andcentrationand ranged ranged offrom QA from 34.63-PEI 34.63- NPs.to 36.59 to All 36.59 µ g/mm theµ differencesg/mm; however,; however, in WS the decreased T WSg values decreased upon did notincreasing upon have increasing statistical the con- the centrationmeaningconcentration ( p of > 0.05). QA of-PEI QA-PEI-NPs.-NPs. All the All differences the differences in the in Ttheg values Tg values did did not not have have statistical statistical meaningmeaning (p > (p 0.05).> 0.05).

Figure 10. WS values of the materials. All the results for WS were statistically insignificant (p > 0.05). FigureFigure 10. 10.WS WSvalues values of the of materials. the materials. All the All results the results for WS for were WS were statistical statisticallyly insignifican insignificantt (p > 0.05). (p > 0.05). FigureFigure 11 11summarizes summarizes the thewater water solub solubilityility (SL) results. (SL) results. Its values Its values increased increased after QA after- Figure 11 summarizes the water solubility (SL) results. Its values3 increased after QA- PEIQA-PEI-NPs-NPs were introduced were introduced into the into neat the polymer neat polymer (SL = 2.17 (SL =µg/mm 2.17 µg/mm) and ranged3) and rangedfrom 2.54 from PEI-NPs were introduced3 into the neat polymer (SL = 2.17 µg/mm3) and ranged from 2.54 to 2.5410.48 to µg/mm 10.48 µ. g/mmIn addition3. In, addition,SL increased SL increasedupon increasing upon increasingthe concentration the concentration of QA-PEI- of to 10.48 µg/mm3. In addition, SL increased upon increasing the concentration of QA-PEI- NPs.QA-PEI-NPs. These increases These were increases usually were statistically usually significant statistically (p significant ≤ 0.05). The (p only≤ 0.05). statistically The only NPs. These increases were usually statistically significant (p ≤ 0.05). The only statistically insignificantstatistically difference insignificant (p > difference0.05) in the (p SL> 0.05)values in was the found SL values for the was neat found polymer for the and neat insignificant difference (p > 0.05) in the SL values was found for the neat polymer and materialpolymer containing and material 0.5 wt.% containing QA-PEI 0.5-NPs. wt.% This QA-PEI-NPs. corresponded This to corresponded 17% growth. to The 17% SL growth. val- material containing 0.5 wt.% QA-PEI-NPs. This corresponded to 17% growth. The SL val- uesThe of SLmaterials values conta of materialsining 1 containingand 2 wt.% 1 QA and-PEI 2 wt.%-NPs QA-PEI-NPswere respectively were 87% respectively and 383% 87% ues of materials containing 1 and 2 wt.% QA-PEI-NPs were respectively 87% and 383% higherand 383%than the higher neat than polymer. the neat polymer. higher than the neat polymer.

≤ FigureFigure 11. 11.SL SLvalues values of the of the materials. materials. Statistical Statisticallyly significan significantt differences differences (p ≤ (p 0.05)0.05) are aremarked marked with with Figurethethe lower lower11. caseSL casevalues letters letters, of, whereas the whereas materials. statistically statistically Statistical insignificant insignificantly significan differencest differences differences (p > ( 0.05)p(p> ≤ 0.05)0.05) are marked areare markedmarked with withwith the the theupperupper lower case casecase letters. letters.letters , whereas statistically insignificant differences (p > 0.05) are marked with the upper case letters. TheThe results results for for the the water water contact contact angle angle (WCA) (WCA) are are summarized summarized in Figures in Figures 12 12and and 13. 13 . TheThe values results increased for the water due contact to the angle introduction (WCA) ofare QA-PEI-NPssummarized in into Figures the neat 12 and polymer 13. The values increased◦ due to the introduction of QA◦ -PEI-NPs into the neat polymer (WCA The= 81.08(WCA values°) and = increased 81.08 ranged) and frdueom ranged to 84.26 the from introductionto 89.01 84.26°. They to 89.01of also QA. -increasedPEI They-NPs also into upon increased the increasing neat upon polymer increasingthe concen- (WCA the =tration 81.08concentration° )of and QA ranged-PEI of-NPs. QA-PEI-NPs. from These 84.26 increases to These 89.01°. increases usuallyThey also usuallydid increased not didshow notupon statistical show increasing statistical significance the significance concen- (p > tration(p > 0.05),of QA except-PEI-NPs. for theThese material increases containing usually 2 did wt.% not QA-PEI-NPs. show statistical The significance WCA of the (p latter > material was statistically significantly higher (p ≤ 0.05), compared the neat polymer with as well as all the materials modified with 0.5 and 1 wt.% QA-PEI-NPs.

Materials 2021, 14, x FOR PEER REVIEW 13 of 20

0.05), except for the material containing 2 wt.% QA-PEI-NPs. The WCA of the latter material was statistically significantly higher (p ≤ 0.05), compared the neat polymer with as well as all the materials modified with 0.5 and 1 wt.% QA-PEI-NPs. Materials 2021, 14, 2037 13 of 20

(a) (b) (a) (b)

FigureFigure 12. 12.TheThe images images of deionized of deionized water water droplets droplets on onthe the studied studied surfaces surfaces obtained obtained from from the the goniometry goniometry camera camera for forthe the (a) (neata) neat polymer, polymer, (b ()b material) material containing containing 0.5 0.5 wt.% wt.% QA QA-PEI-NPs,-PEI-NPs, ( (cc)) material material containing 11 wt.%wt.% QA-PEI-NPs,QA-PEI-NPs, and and (d ()d material) ma- terialcontaining containing 2 wt.% 2 wt.% QA-PEI-NPs. QA-PEI-NPs.

≤ FigureFigure 13. 13.WCAWCA values values of the of the surfaces. surfaces. Statistical Statisticallyly significan signiﬁcantt differences differences (p ( p≤ 0.05)0.05) are are marked marked with withthe the lower lower case case letters, letters whereas, whereas statistically statistically insigniﬁcant insignificant differences differences ( p(p> > 0.05) 0.05) are are marked marked with with the theupper uppercase caseletters. letters.

In InFigure Figure 14, 14the, theresults results for the for flexural the flexural modulus modulus (E) are (E) summarized. are summarized. Its values Its values de- creaseddecreased due to due the to introduction the introduction of QA of-PEI QA-PEI-NPs-NPs into the into neat the polymer neat polymer (E = 3731 (E =MPa) 3731 and MPa) rangedand rangedfrom 3557 from to 3712 3557 MPa to 3712. They MPa. decreased They decreased upon increasing upon increasing the QA-PEI the-NPs QA-PEI-NPs concentration,concentration, but these changes but these were changes not statistically were not statistically significant significant(p > 0.05). Compared (p > 0.05). with Compared the neatwith polymer, the neat the polymer, maximum the reduction maximum was reduction only 5%, was which only was 5%, whichfound for was the found material for the containingmaterial 2 containing wt.% QA- 2PEI wt.%-NPs. QA-PEI-NPs. In Figure 15, the results for the flexural strength (σ) are summarized. Its values decreased due to the introduction of QA-PEI-NPs into the neat polymer (σ = 85.18 MPa) and ranged from 36.76 to 63.09 MPa. They decreased upon increasing the concentration of QA-PEI-NPs, and all decreases were statistically significant (p ≤ 0.05). Compared with the neat polymer, the σ values of materials containing 0.5, 1, and 2 wt.% QA-PEI-NPs were 26, 38, and 57% lower, respectively. Materials 2021, 14, x FOR PEER REVIEW 14 of 20

Materials 2021, 14, x FOR PEER REVIEW 14 of 20 Materials 2021, 14, 2037 14 of 20

Figure 14. E values of the materials. All the results for E were statistically insignificant (p > 0.05).

In Figure 15, the results for the flexural strength (σ) are summarized. Its values decreased due to the introduction of QA-PEI-NPs into the neat polymer (σ = 85.18 MPa) and ranged from 36.76 to 63.09 MPa. They decreased upon increasing the concentration of QA- PEI-NPs, and all decreases were statistically significant (p ≤ 0.05). Compared with the neat polymer, the σ values of materials containing 0.5, 1, and 2 wt.% QA-PEI-NPs were 26, 38,

and 57% lower, respectively. FigureFigure 14. 14.E valuesE values of the of thematerials. materials. All Allthe theresults results for forE were E were statistical statisticallyly insignifican insigniﬁcantt (p > ( p0.05).> 0.05).

Figure 15. σ values of the materials. All the results for σ were statistically signiﬁcant (p ≤ 0.05) and Figure 15. σ values of the materials. All the results for σ were statistically significant (p ≤ 0.05) and theythey are aremarked marked with with the thelower lower case case letters letters.. In Figure 16, the hardness (HB) values are summarized, which slightly increased due to theIn introductionFigure 16, the of hardness QA-PEI-NPs (HB) into values the are neat summarized, polymer (HB which = 106.19 slightly MPa) increased and ranged due from Materials 2021, 14, x FOR PEER REVIEW 15 of 20 to the109.1 introduction to 111.81 MPa; of QA however,-PEI-NPs the into HB the decreased neat polymer upon increasing (HB = 106.19 the MPa) concentration and ranged of QA- fromPEI-NPs. 109.1 to All 111.81 the differencesMPa; however, between the HB HB decreased values were upon statistically increasing insigniﬁcant the concentration (p > 0.05). of QA-PEI-NPs. All the differences between HB values were statistically insignificant (p > 0.05). Figure 15. σ values of the materials. All the results for σ were statistically significant (p ≤ 0.05) and they are marked with the lower case letters.

In Figure 16, the hardness (HB) values are summarized, which slightly increased due to the introduction of QA-PEI-NPs into the neat polymer (HB = 106.19 MPa) and ranged from 109.1 to 111.81 MPa; however, the HB decreased upon increasing the concentration of QA-PEI-NPs. All the differences between HB values were statistically insignificant (p > 0.05).

FigureFigure 16. HB 16. valuesHB values of the of thematerials. materials. All Allthe theresults results for forHB HBwere were statistical statisticallyly ins insigniﬁcantignificant (p > (p > 0.05). 0.05).

In Figure 17, the impact strength (an) values are summarized, which decreased due to the introduction of QA-PEI-NPs into the neat polymer (an = 7.73 J/cm2) and ranged from 2.84 to 6.58 J/cm2. They decreased upon increasing the concentration of QA-PEI-NPs, and all these decreases were statistically significant (p ≤ 0.05). Compared with the neat polymer, the an values of the materials containing 0.5, 1, and 2 wt.% QA-PEI-NPs were 15, 39, and 63% lower, respectively.

Figure 17. an values of the materials. All the results for an were statistically significant (p ≤ 0.05) and they are marked with the lower case letters.

4. Discussion The recent trends in dental restorative composites have shown huge interest in the development of materials possessing antibacterial activity. The physical admixing of bioactive compounds is one of the simplest ways to achieve this goal. As many of the most popular biocides show toxic effects, the application of QA-PEI-NPs seems to be a promis- ing alternative. QA-PEI-NPs have already shown high antibacterial activity against many bacteria strains, including those responsible for tooth decay. When added to dental restorative composites, they successfully inhibit bacterial growth on their surfaces. Only 1 wt.% QA-PEI-NPs is sufficient to kill Gram-positive bacteria, whereas 2 wt.% is required to kill Gram-negative bacteria [1].

Materials 2021, 14, x FOR PEER REVIEW 15 of 20

Materials 2021, 14, 2037 Figure 16. HB values of the materials. All the results for HB were statistically insignificant (p > 15 of 20 0.05).

In Figure 17, the impact strength (an) values are summarized, which decreased due In Figure 17, the impact strength (an) values are summarized, which decreased due to the introduction of QA-PEI-NPs into the neat polymer (an = 7.73 J/cm2) and ranged from to the introduction of QA-PEI-NPs into the neat polymer (a = 7.73 J/cm2) and ranged 2.84 to 6.58 J/cm2. They decreased upon increasing the concentrationn of QA-PEI-NPs, and from 2.84 to 6.58 J/cm2. They decreased upon increasing the concentration of QA-PEI-NPs, all these decreases were statistically significant (p ≤ 0.05). Compared with the neat poly- and all these decreases were statistically signiﬁcant (p ≤ 0.05). Compared with the neat mer, the an values of the materials containing 0.5, 1, and 2 wt.% QA-PEI-NPs were 15, 39, polymer, the an values of the materials containing 0.5, 1, and 2 wt.% QA-PEI-NPs were 15, and 63% lower, respectively. 39, and 63% lower, respectively.

Figure 17. an values of the materials. All the results for an were statistically significant (p ≤ 0.05) and Figure 17. an values of the materials. All the results for an were statistically significant (p ≤ 0.05) andthey they are are marked marked with with the the lower lower case case letters. letters. 4. Discussion 4. Discussion The recent trends in dental restorative composites have shown huge interest in the developmentThe recent trends of materials in dental possessing restorative antibacterial composites activity.have shown The huge physical interest admixing in the of developmentbioactive compounds of materials ispossessing one of the antibacterial simplest ways activity. to achieve The physical this goal. admixing As many of bio- of the activemost compounds popular biocides is one of show the simplest toxic effects, ways the to achieve application this ofgoal. QA-PEI-NPs As many of seems the mos to bet a popularpromising biocides alternative. show toxic QA-PEI-NPs effects, the have application already of shown QA-PEI high-NPs antibacterial seems to be activity a promis- against ingmany alternative. bacteria QA strains,-PEI-NPs including have already those responsibleshown high for antibacterial tooth decay. activity When against added tomany dental bacteriarestorative strains, composites, including those they successfullyresponsible for inhibit tooth bacterial decay. When growth added on their to de surfaces.ntal restor- Only ative1 wt.% composites, QA-PEI-NPs they successfully is sufficient toinhibit kill Gram-positive bacterial growth bacteria, on their whereas surfaces. 2 wt.% Only is 1 requiredwt.% QAto-PEI kill-NPs Gram-negative is sufficient bacteriato kill Gram [1]. -positive bacteria, whereas 2 wt.% is required to kill Gram-negativeOn the bacteria other hand, [1]. each physical modification of a material based on a polymer network can negatively affect its molecular structure, which can deteriorate the material’s physicochemical and mechanical properties; therefore, in this study, we investigated how the physical modification of the Bis-GMA/TEGDMA polymer network with QA-PEI-NPs affected its structural, physicochemical, and mechanical properties. The composition of Bis-GMA 60 wt.% and TEGDMA 40 wt.% was chosen as the most representative example of a dental composite restorative material matrix [41]. In this study, QA-PEI-NPs were synthesized starting from PEtOx hydrolysis, which resulted in the formation of linear PEI (Figures2 and3), which was then subjected to crosslinking, N-alkylation, and quaternization. The latter reactions led to the formation of QA-PEI-NPs (Figures4 and5). The DLS measurements revealed that achieved QA-PEI-NPs were 151 nm in average size. Bigger particles, with average diameters of 732 nm, and 5212 nm (Figure6) can be recognized as agglomerates of QA-PEI-NPs [ 37]. This result is in agreement with the patent requirements, which states that QA-PEI-NPs should be from 10 to 10,000 nm in size. Preferred are particles less than 1000 nm in size, and most preferred are particles of up to 150 nm in size [27]. It is worth noting, that only 7.2% of all particle agglomerates were of about 5200 nm average size. The photocured materials were tested for the degree of conversion. The FTIR analysis did not reveal any significant impact of the QA-PEI-NPs presence on the DC. If one looks at the DC results in more detail, a slight increase in its values can be observed. It might be attributed to the increase in loop and cycle formation [41]. However, it can be concluded Materials 2021, 14, 2037 16 of 20

that their influence on the DC can be neglected, when QA-PEI-NPs are added into the Bis-GMA/TEGDMA matrix at a maximum of 2 wt.%. The DC is closely related to the polymerization shrinkage (S), which is an inherent effect of the polymerization process. During the curing reaction, the monomer molecules transform into repeating units, the van der Waals forces are converted into covalent bonds, and the polymerizing system undergoes volumetric contraction. As a result, the marginal gap formation takes place. It creates favorable conditions for biofilm development, resulting in the development of secondary caries and periodontal diseases [49]. In this study, the polymerization shrinkage was determined by measuring the density before and after curing. The density of uncured compositions (dm) did not alter with the addition of QA- PEI-NPs. In the case of cured materials, the addition of 0.5 wt.% QA-PEI-NPs also did not influence the density (dp). An increase in the concentration of QA-PEI-NPs resulted in a maximum 2% increase in this parameter (Table1). As expected, the same relationship was observed for S. However, the scale of changes was greater. The neat polymer and material containing 0.5 wt.% QA-PEI-NPs were characterized by the same S value. An increase in the concentration of QA-PEI-NPs resulted in 10 and 20% increases in this parameter, respectively for samples containing 1 and 2 wt.% QA-PEI-NPs (Table1). The Tg of studied materials was determined as a crucial physicochemical factor of the polymer network constituting the dental composite matrix. A dental material must be able to withstand mechanical stress created by biting forces. It is only possible when the Tg exceeds the temperature in the mouth environment. The results showed that the modification of the Bis-GMA/TEGDMA composition with QA-PEI-NPs did not cause statistically meaningful changes in the Tg values. Since all tested materials had a Tg greater than 55 ◦C, it can be assumed that they would be in the glassy state once they were applied as a matrix in a dental composite. Water sorption (WS) is another crucial physicochemical property of dental materials because it is responsible for their dimensional stability. Excess water swelling causes the dental material to expand, which, in extreme cases, can cause teeth fracture; therefore, the WS value of a dental material should be investigated, and its value should not exceed 40 µg/mm3 [45]. As can be seen from Figure 10, the WS values usually did not change significantly due to the addition of QA-PEI-NPs. The largest increase of 16% was observed with the addition of 0.5 wt.% QA-PEI-NPs; however, the highest WS value (36.59 µg/mm3) falls within the scope of the standard ISO 4049. A dental material’s behavior in water is also usually characterized by its water solubility (SL). The maximum allowable soluble fraction is 7.5 µg/mm3 [45]. As can be seen from Figure 11, the SL value increased upon increasing the concentration of QA-PEI-NPs. The introduction of 0.5 wt.% QA-PEI-NPs negligibly increased the SL. Further increases in SL were significant and corresponded to 87 and 383% for materials containing, respectively 1 and 2 wt.% QA-PEI-NPs. The SL values of the materials containing 0.5 and 1 wt.% QA-PEI- NPs (2.54 and 4.05 µg/mm3, respectively) were lower than the limit defined in the standard ISO 4049. From this perspective, it can be concluded that the introduction of 0.5 and 1 wt.% QA-PEI-NPs into the Bis-GMA/TEGDMA matrix did not disqualify it from potential dental applications. The modification with 2 wt.% QA-PEI-NPs requires serious consideration because the SL value of this material was 40% higher than that specified in the standard ISO 4049. The results from the DC analysis suggest that such a significant increase in the SL values may be caused by the QA-PEI-NPs leaching. The general approach to the relationship between the DC and SL assumes that the lower the DC, the higher the SL [37]. As shown in Figure8, the DC was not significantly influenced by the presence of QA-PEI-NPs; therefore, the evidence suggests that QA-PEI-NPs were not firmly anchored within the Bis-GMA/TEGDMA matrix. The interactions between QA-PEI-NPs and water molecules were probably stronger than those between QA-PEI-NPs and the Bis-GMA/TEGDMA polymer network. It also indicates that, in those interactions, the hydrophilicity of QA-PEI- NPs, resulting from the presence of the quaternary ammonium groups, dominated their hydrophobicity, resulting from the presence of the N-octyl substituents. Materials 2021, 14, 2037 17 of 20

On the other hand, the water contact angle (WCA) results showed that the hydrophilicity of the tested surfaces decreased upon increasing the concentration of QA-PEI-NPs (Figure 13). These changes were usually insignificant, except for the material containing 2 wt.% QA-PEI-NPs, whose WCA increased by about 10% compared with that of the neat polymer; however, the WCA values of all tested surfaces were less than 90◦, which classifies them as hydrophilic [50]. The Bis-GMA/TEGDMA materials enriched with QA-PEI-NPs were also tested for selected mechanical properties. They showed different responses to various mechanical factors. The flexural modulus (E) of the studied materials changed insignificantly due to the addition of QA-PEI-NPs (Figure 14). Its values slightly decreased as the concentration of QA-PEI-NPs increased, but the maximum difference was only about 5% compared with the neat polymer. It might be attributed to the subtilities of the molecular structure of the Bis-GMA/TEGDMA polymer network. For example, an increase in the loop number could cause a decrease in E values (loops do not decrease the DC, but increase a network elasticity [41]). Conversely, the flexural strength (σ) decreased significantly upon increasing the concentration of QA-PEI-NPs (Figure 15). Compared with the neat polymer, the σ decreased by 26, 37, and 57% for materials containing 0.5, 1, and 2 wt.% QA-PEI-NPs, respectively. A similar result was observed by Beyth et al. in a study on a commercial flowable composite composed of 53 wt.% Bis-GMA/TEGDMA matrix and 47 wt.% zirconia/silica filler [28]. They found that the addition of 1 wt.% QA-PEI-NPs decreased the flexural strength by about 40%. Interestingly, the flexural strength of another commercial material, composed of 40 wt.% Bis-GMA/UDMA/Bis-EMA matrix and 60 wt.% zirconia/silica filler, insignificantly increased. This leads to the conclusion that the influence of the modification of dental dimethacrylates with QA-PEI-NPs cannot be generalized to all kinds of materials, and it probably depends on their chemical composition. TEGDMA is a known factor causing mechanical strength deterioration. Due to the lack of hydrogen donors to hydrogen bonds its molecule cannot form strong physical crosslinks. On the contrary, UDMA can participate in strong hydrogen bonding and therefore increases mechanical strength. It explains why the Bis-GMA/UDMA/Bis-EMA/QA-PEI-NP composite was less sensitive to flexural stress than that comprising Bis-GMA/TEGDMA/QA-PEI-NP [28]. It also explains the result of our study, and allows us to assume that the introduction of QA-PEI-NPs into materials containing lower amounts of TEGDMA or those without TEGDMA will have a less significant impact on the flexural strength [41]. A certain improvement in flexural strength might be also achieved by the application of silanized fillers. They generally increase the flexural strength, in contrast to unsilanized fillers, which reduce it [51]. The hardness (HB) of the studied materials was not negatively affected by the addition of QA-PEI-NPs (Figure 16). The HB slightly increased (Figure 16). This result is in agreement with the literature data, which indicates that the hardness of dimethacrylate polymers is less sensitive to the DC and physical crosslinking [41]. Finally, the impact resistance (an) of the studied materials was examined. This is a rarely investigated and problematic property because its values are usually lower than expected [52]. The an of studied materials decreased significantly upon increasing the concentration of QA-PEI-NPs (Figure 17). Compared with the neat polymer, the an value decreased by 15, 39, and 63% for materials containing 0.5, 1, and 2 wt.% QA-PEI-NPs, respectively. This behavior may be explained by the combination of several factors, including a low degree of physical crosslinking, incomplete conversion, formation of microgel agglomerates, weak intermolecular interactions between the Bis-GMA/TEGDMA matrix and QA-PEI-NPs [41,52,53], and even a presence of air bubbles trapped in a material [54]. Research is continuing in this area.

5. Conclusions This article can be recognized as an initial study on the physicochemical and mechanical properties of dental polymers modiﬁed with QA-PEI-NPs. It provided a new portion of Materials 2021, 14, 2037 18 of 20

information about properties of the most basic dental composition, Bis-GMA/TEGDMA, modified with QA-PEI-NPs. The results of this study showed that the addition of QA- PEI-NPs into the Bis-GMA/TEGDMA composition up to a maximum of 2 wt.% did not negatively affect many of its properties, including the degree of conversion, polymerization shrinkage, glass transition temperature, water sorption, flexural modulus, and hardness. On the other hand, QA-PEI-NPs increased the water solubility, whereas the bending strength and impact resistance decreased. Therefore, it is worth extending the research on other dimethacrylate systems, comprising UDMA and/or Bis-EMA, as well as a filler, to achieve the optimum physicomechanical performance.

Author Contributions: Conceptualization, I.M.B.-R.; methodology, I.M.B.-R. and M.W.C.; investi- gation, M.W.C. and G.C.; statistical analysis, G.C.; resources, I.M.B.-R. and M.W.C.; data curation, M.W.C. and G.C.; writing—original draft preparation, I.M.B.-R. and M.W.C.; visualization, M.W.C.; writing—review, supervision, project administration, funding acquisition, I.M.B.-R. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Rector’s grant for the scientific research and development activities in the Silesian University of Technology, grant number: 04/040/RGJ20/0111. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data supporting reported results are available from the authors. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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