d e n t a l m a t e r i a l s 3 6 ( 2 0 2 0 ) 698–709

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jo ur nal ho me pag e: www.int l .e lsev ierhea l th .com/ journa ls /dema

Surface dissolution and transesterification ofthermoset dimethacrylate polymer bydimethacrylate adhesive resin and organiccatalyst-alcohol solution�

Santhosh Basavarajappaa,∗, Leila Perea-Loweryb,Abdullah Maghram Alshehria, Abdul Aziz Abdullah Al-Kheraifa,Jukka P. Matinlinnac, Pekka K. Vallittud

a Dental Biomaterial Research Chair, Dental Health Department, College of Applied Medical Sciences, King SaudUniversity, Riyadh 11433, Saudi Arabiab Institute of Dentistry, Department of Biomaterials Science, University of Turku, FI-20520 Turku, Finlandc Faculty of Dentistry, The University of Hong Kong, Applied Oral Sciences & Community Dental Care, DentalMaterials Science, Hong Kong SAR, PR Chinad Professor and Chair of Biomaterials Science and Turku Clinical Biomaterials Centre – TCBC, Institute of Dentistry,University of Turku and City of Turku Welfare Division, Oral Health Care, FI-20520 Turku, Finland

a r t i c l e i n f o

Article history:

Received 26 December 2019

Received in revised form

18 February 2020

Accepted 12 March 2020

Keywords:

Dissolution

Transesterification

Dimethacrylate

a b s t r a c t

Objectives. To evaluate transesterification based dissolution of dimethacrylate and epoxy

polymers, the former containing ester groups. Polymer substrates were treated with an

adhesive resin (StickTM Resin) and an organic catalyst-alcohol solution (ethylene glycol

and triazabicyclodecene). The surface was chemically and nanomechanically analyzed with

Fourier Transform-Infrared (FTIR) spectroscopy, surface profile peak (Rp) and nanohardness

and modulus of elasticity.

Methods. A total of 100 specimens each of light-cured dimethacrylate polymer and heat-

cured diepoxy polymer were prepared. 20 specimens were randomly selected and used as

control group (0 s). The remaining specimens were randomly divided into 40 each for treat-

ment with an StickTM resin and ethylene glycol + triazabicyclodecene. Within each group

the 40 specimens were randomly subdivided into 20 each for treatment at 5 min and 24 h,

Epoxy resin with 10 specimens for FTIR and nanohardness and modulus of elasticity, and the other 10

R analyses.

for SEM and surface Ester groups

Alcoholysisp

Results. Dimethacrylate polymer showed a reduction in the nanohardness and modulus

of elasticity, Rp values and SEM also showed significant topographical changes after being

treated with either StickTM resin or ethylene glycol + triazabicyclodecene, whereas epoxy

resin substrate did not. FTIR analyses affirmed changes in the intensity of ester groups.

� Study conducted in: Dental Biomaterials Research Chair, Dental Health Department, College of Applied Medical Sciences, King SaudUniversity, Riyadh 11433, Kingdom of Saudi Arabia.

∗ Corresponding author at: Dental Health Department, College of Applied Medical Sciences, King Saud University, Riyadh 11433, SaudiArabia

E-mail address: sbasavarajappa@ksu.edu.sa (S. Basavarajappa).https://doi.org/10.1016/j.dental.2020.03.0050109-5641/© 2020 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

d e n t a l m a t e r i a l s 3 6 ( 2 0 2 0 ) 698–709 699

Significance. Ester group containing dimethacrylate polymer showed a reduction in NMP

within 5 min of exposure to the treatment agents with softening by solution ethylene gly-

col + triazabicyclodecene associated to the reduction of ester groups in the polymer structure

by transesterification. Epoxy polymer without ester groups was not affected by surface

softening with treatment agents. Adhesive resin caused surface swelling.

© 2020 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

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. Introduction

olymer dissolution has been extensively studied from var-ous perspectives of polymer and engineering sciences.issolution of polymers and the polymer matrix of resinomposites has a great impact to success of multimaterialental restorations. Dental resin composite encompasses ofve basic and essential components, namely a cross-linkedolymer matrix, inorganic fillers, X-ray opacifiers, initiatorystem and a silane coupling agent [1]. Composite resins inentistry usually consist of bisphenol A-glycidyl methacry-

ate (bis-GMA), triethylene glycol dimethacrylate (TEGDMA),rethane dimethacrylate (UDMA), ethoxylated bisphenol--dimethacrylate (bis-EMA), with bis-GMA being the mostommonly used [2]. Interfacial adhesion of resin compositeayers usually occurs via free radical polymerization of incre-

ental layers of composite by help of oxygen inhibited layern the substrate surface [3,4]. Interfacial adhesion of this kindannot be obtained when the substrate is aged or does not con-ain fresh oxygen inhibited surface layer. That said, adhesionetween aged or indirectly fabricated resin composite device

s based on other mechanisms such as dissolution of the sur-ace and polymer chain entanglement between the dissolvedubstrate and monomers of the adherent. Chemically, cross-inked polymers are known as thermosetting polymers andhey are difficult to dissolve without strong chemicals, highressure or temperature [5].

Due to difficulties in obtaining interfacial adhesionetween layers of thermosetting resin composites, IPN poly-er matrix has been developed for the fields of dental

aboratory technology, prosthodontics and recently in alsoestorative dentistry [6–9]. IPNs are preferred rather thanomopolymers or copolymers due to their higher flexural

oughness, improved handling properties and most impor-antly due to bonding of veneering resin composites, lutingements and adhesive resins adequately to the underlyingubstrate [10–13]. In the so-called semi-IPN matrix resin com-osites the bonding interface is called the secondary IPN [14].he dissolution behavior and kinetics of thermoplastic poly-ers by solvents has been the basis for the clinical use of

emi-IPN resins. However, although in the semi-IPN matrix ofome dental resin composites, the polymer matrix providesmproved adhesion properties over the cross-linked polymersf dimethacrylates and epoxies, nevertheless, cross-linkedolymers are still the most commonly used components inlling resin composites and root canal posts [15].

Dissolution and thickness of the adhesive interfaceetween the already cured polymer substrate and new resinepends upon factors such as contact wetting time on the

substrate surface, temperature and dissolving and swellingcapacity of the monomers of the adherent on the substratesurface. Theoretically, it is also possible to induce forma-tion of an adhesive interface by degrading the covalentlybound polymer structure of the substrate surface for allow-ing monomers of the adhesive or adherent to penetrate intothe polymer. The higher the cross-linking density is, the moredifficult the dissolving and surface swelling is to obtain. bis-GMA monomer-based filling resin composite and epoxy basedresin composite in root canal posts have high cross-linkingdensity and, therefore they are challenging to be adhered.Epoxy resins of several types are commonly used engineer-ing resin materials and they represent a step growth ofpolymerization when mixed with a fluid pre-polymer and areactive epoxide group as a hardener [16]. Beside of usingepoxy as dental material, epoxy and dimethacrylates, are alsoused in some surgical fiber-reinforced composite implants[17,18].

In polymer dissolution caused by the solvent involvesa two-step process which include diffusion of the solventand chain disentanglement. A glassy polymer, which isamorphous and un-crosslinked, when in contact with athermodynamically adaptable solvent, is penetrated by thesolvent. Two separate interfaces are formed in the polymer,one between the glassy polymer surface and the gel layer,and the another one between the gel layer and the solvent,eventually leading to plasticization. Gradually the polymeris dissolved once the induction time passes or cracks whenthere is an absence of the gel layer formation [19]. Variousparameters that govern the solubility of the polymer dissolu-tion and the dissolution process includes the solvent diffusionco-efficient, molecular weight of the polymer, cross-linkingdensity, temperature, and pressure [20] and thermodynamicbehavior of the solvent [21]. One potential alcohol solution,which could be used in dissolving the polymer substrate isethylene glycol. Solubility parameters of ethylene glycol (16.30�) [22] when compared to the solubility parameters of epoxyresin (18.2 �) [23] and dimethacrylate (18.8 �) indicate that itcould potentially dissolve the polymer [24].

The other mechanism to obtain monomer diffusion intothe surface layer of a cross-linked polymer substrate is basedon using a transesterification reaction (called also alcoholy-sis), where e.g. the hydroxyl group of ethylene glycol reactswith the ester group of the epoxy polymer [25]. Cross-linkedpolymer dissolution by transesterification has gained interestfrom the interfacial adhesion perspective and also based on

its applications in various areas such as medical and indus-trial field. Such examples of application are found for instancein membrane research, recycling of plastics, control of drug

l s 3 6 ( 2 0 2 0 ) 698–709

Fig. 1 – Chemical formula for bisphenol A-glycidyl

700 d e n t a l m a t e r i a

delivery with time release, and tissue regeneration research[26]. The transesterification reaction of epoxy polymer withester bonds in the chemical structure has been shown totaken place in a reasonable period of time but at the elevatedtemperature of 180◦ C with ethylene glycol and acceleratorof 1,5,7-triazabicyclo[4,4,0]dec-5-ene commonly known as tri-azabicyclodecene. Triazabicyclodecene is a guanidine solublebase and can act as a catalyst in transesterification. Degrada-tion and dissolution starts from the polymer surface and thusthe polymer softens. Surface hardness could therefore be usedas an indicator of dissolution or degradation. The most com-mon epoxy resin is bis-phenol-based and there are not esterbonds in its chemical structure. This is why transesterificationreaction cannot take place whereas some other epoxies can bedissolved and degraded by transesterification.

In the laboratory study we investigated the effect of a den-tal adhesive resin and an ethylene glycol + triazabicyclodeceneblend on a dimethacrylate polymer surface to reduce thesurface hardness by dissolution or by transesterification(alcoholysis) of the substrate surface. The control surfacewas a diepoxy polymer surface. The dissolution molecularmechanism was studied by analyzing and characterizing thechemical structure along with the surface profile and its effecton nanomechanical properties of the polymer at different timeintervals.

2. Materials and methods

2.1. Specimen preparation

Dimethacrylate polymer specimens were prepared usinga mixture of 70% bis-phenol-A-glycidyldimethacrylate (bis-GMA), 30% tri(ethyleneglycol) dimethacrylate (TEGDMA), 0.7%2-(dimethylamino) ethyl methacrylate (DMAEMA), 0.7% cam-phoroquinone. The unfilled resin was distributed into siliconemolds and pressed against two glass plates to obtain an evenand flat surface with 3 mm thickness. The mixture was firstlight-cured with a hand-held light polymerizing unit (EliparTM

S 10; 3 M ESPE, Minnesota, USA) for 40 s and then post-cured in an oven (TargisTM Power, Ivoclar, Vivadent, Schaan,Lichtenstein) at 95◦C for 25 min. The samples were groundusing 1200 grit silicon carbide grinding paper under runningwater to obtain a uniform flat surface. Next, the specimenswere cleansed using deionized water in an ultrasonic bath(Quantrex 90, L&R Ultrasonics, Kearny, New Jersey, USA) fora period of 10 min and allowed to dry at ambient laboratorytemperature (23 ◦C ± 1 ◦C) for 60 min. Control substrate spec-imens of bisphenol-based diepoxy resin were prepared bymixing 100 parts by weight of an elastomeric casting resin(Component A Biresin® U1419, Sika, Germany) and 16 partsby weight of an amine (Component B Biresin® U1458, Sika,Germany) which were mixed manually stirring in a baker. Theamine accelerator for curing of epoxy monomers results inpolymer without ester bonds and thus, the epoxy polymersserved as negative control for transesterification. A polyvinyl

siloxane putty (Affnis Putty, Coltene Whaledent, Altstatten,Switzerland) material was used to prepare a mold for prepar-ing the substrate specimens and to standardize their size andshape. The mixture was then placed in a vacuum to degas

methacrylate (bis-GMA), triethylene glycol dimethacrylate(TEGDMA), and diepoxy monomer. Ester groups are circled.

for 10 min and subsequently poured into polyvinyl siloxanemolds, and pressed against two glass plates to obtain an evenand flat surface with 3 mm thickness. The mixture was thencured at 80 ◦C for 8 h.

A total of 100 specimens (10 mm × 5 mm × 3 mm) each oflight-cured dimethacrylate polymer and heat-cured diepoxypolymer was prepared. Chemical formulaes of the monomersare shown in Fig. 1. The specimens were randomly assignedinto two groups of 40 each for treatment with an dimethacry-late adhesive resin (StickTM Resin,) and a mixture of 75%ethylene glycol and 25% 1,5,7 triazabicyclo[4,4,0]dec-5-ene and20 specimens were used as control, being not treated withthe alcohol blend. The specimens within each group were fur-ther randomly grouped into two groups with 20 specimens fortreatment at varying time intervals of 5 min, and 24 h respec-tively. Within each treatment time 10 specimens were usedfor chemical surface characterization and nanomechanicalproperties, and the other 10 for surface visual analyses andsurface profile peak (Rp) analyses. The specimens were keptin dark and in an incubator maintained at a temperature of37 ◦C ± 1 ◦C during the treatment with the StickTM resin andethylene glycol + triazabicyclodecene blend.

2.2. Experimental method

The specimens of both the groups were evaluated at thebaseline and after treatment with the two treatment agentsStickTM resin and ethylene glycol + triazabicyclodecene and atdifferent treatment times (5 min, and 24 h). The specimenswere subjected to the following analyses: nanomechanicalproperties, surface visual analyses, and chemical surface char-acterization.

2.2.1. Surface profile peakThe Rp of the samples was measured using a non-contactoptical profilometer (Bruker, Contour GT, Tucson, AZ, USA).

The parameter Rp was selected instead of Ra for the analysesbecause it was aimed to demonstrate possible dissolution ofthe highest topographical peaks on the substrate surface. Pro-filometer is equipped with a fully automated turret Nanolens

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Table 1 – Statistical results of surface profile peak (Rp)(�m) of epoxy resin and dimethacrylate polymer treatedwith resin SR and EG + TBD. SR = StickTM resin,EG + TBD = Ethylene glycol + Triazabicyclodecene.

Epoxy resin Control 5 min 24 h

SRMean 56.94a,A 63.17b,A 48.84c,A

S.D 0.85 0.93 0.88

EG + TBDMean 56.94a,A 55.11b,B 55.87b,B

S.D 0.85 0.83 0.68

Dimethacrylate polymerSRMean 41.47aA 47.52b,A 36.33c,A

S.D 0.74 0.88 0.80

EG + TBDMean 41.47a,A 54.61b,B 32.65c,B

S.D 0.74 0.91 0.94

Different superscript small letters in the same row indicate statis-tically significant at p < 0.05.Different superscript capital letters in the same column indicatestatistically significant for each group at p < 0.05.

Table 2 – Statistical results of nanohardness (GPa) andthe Young’s modulus of elasticity (GPa) of epoxy resintreated with resin (SR) and EG + TBD. SR = StickTM resin,EG + TBD = Ethylene glycol + Triazabicyclodecene.

Nanohardness Control 5 min 24 h

SRMean 0.202a,A 0.171a,A 0.312c,A

S.D 0.030 0.050 0.040

EG + TBDMean 0.202a,A 0.324b,B 0.327b,A

S.D 0.030 0.080 0.090

Young’s modulus of elasticitySRMean 3.433a,A 3.878a,A 6.427b,A

S.D 0.870 0.590 0.620

EG + TBDMean 3.433a,A 6.742b,B 6.434b,A

S.D 0.870 0.900 0.890

Different superscript small letters in the same row indicate statis-tically significant at p < 0.05.

d e n t a l m a t e r i a l s

FM module and a programmable X, Y and Z axis that providesigh-resolution data. The high-resolution data are converted

nto accurate 3D object code (simple vision 64). Five readingsere taken on each surface at randomly opted locations with

mm apart, and their mean value was taken.

.2.2. Nanomechanical properties nanoindenter (Bruker, Tucson, AZ, USA) with a three-sidederkovich diamond indenter was used to test the nanohard-ess and modulus of elasticity. With preset loading andnloading values of 0.2 mN/s and 0.2 mN/s respectively with

maximum load of 50 mN and a resting time of 10 s, thendenter was pressed onto the surface of the specimen. Thexperiment was carried out at 23 ◦C in a closed chamber underow noise conditions.

.2.3. Fourier Transform-Infrared (FTIR) spectroscopicnalyses

surface chemical characterization of the specimens waserformed using FTIR (Thermo Scientific, iD7 ATR, Canada).ttenuated Total Reflection (ATR), the most popular sampling

echnique for FTIR analyses was employed in the wave-umber range 750–4000 cm−1. The specimens were subjectedo ATR element made of diamond, wherein the infrared beamnters the ATR crystal at an angle of 45◦ which is totallyeflected at the crystal to specimen interface. The IR spectrumas plotted as the wavenumber versus reflectance.

.2.4. Scanning electron microscopy (SEM)pecimens were subjected to surface visual analyses of SEMicrographs (SEM, BOC EDWARDS, PV25MK, West Sussex,

ngland, UK). The SEM device was at the acceleration voltagef 1 kV and a 700× magnification.

.3. Statistical analysis

tatistical analysis of the numerical data was performed usingPSS v. 20. One-way ANOVA and the Tukey’s post hoc tests werepplied. Linear and non-linear regression analyses were per-ormed for NMP for the two solvents at different time intervals.he significance level was determined to be p ≤ 0.05.

. Results

he Rp analysis showed that the epoxy substrate had higher Rp

alues than the dimethacrylate polymer substrate at the base-ine, but no clear trend of the surface profile peak change ofhe substrates at different time points was seen (Table 1). How-ver, statistically significant differences were found betweenime points and certain substrate-treatment agent combi-ations (Table 1). Dimethacrylate polymer substrate showed

significant reduction in the Rp when it was treated withtickTM resin and ethylene glycol + triazabicyclodecene athe time point of 24 h. SEM images showed topographi-

al changes of the dimethacrylate substrate surface aftereing treated either with StickTM resin, or with ethylene gly-ol + triazabicyclodecene for 24 h, whereas no changes wereeen of the epoxy substrate (Figs. 2 and 3).

Different superscript capital letters in the same column indicatestatistically significant for each group at p < 0.05.

Nanohardness and modulus of elasticity of the epoxyresin substrate according to the treatment with StickTM resinor ethylene glycol + triazabicyclodecene blend showed minorincrease by prolonging the treatment time up to 24 h (Table 2)which was also demonstrated by regression analyses withboth of the treatment agents (Figs. 4 and 5). Linear regres-sion analysies for nanohardness and modulus of elasticitywith treatment time for the epoxy resin with SR was (R2 = 0.96,p < 0.05) and (R2 = 0.98, p < 0.05) as shown in Fig. 4 and non

linear regression analyses for epoxy resin with ethylene gly-col + triazabicyclodecene (R2 = −0.28, p < 0.05) for hardness and(R2 = −0.49, p < 0.05) for modulus of elasticity Fig. 5.

702 d e n t a l m a t e r i a l s 3 6 ( 2 0 2 0 ) 698–709

Fig. 2 – SEM images for specimens of dimethacrylate polymer treated with resin SR and EG + TBD at baseline (control), 5 min,and 24 h. Original magnification ×700. SR = StickTM resin, EG + TBD = Ethylene glycol + Triazabicyclodecene.

th rethyle

Fig. 3 – SEM images for specimens of epoxy resin treated wiOriginal magnification ×700. SR = StickTM resin, EG + TBD = E

Nanohardness and modulus of elasticity of dimethar-cylate polymer substrate were reduced with both of thetreatment agents (Figs. 6 and 7, Table 3). Regression analy-ses for the dimethacrylate substrate with StickTM resin fornanohardness (R2 = −0.89, p < 0.05) and (R2 = 0.20, p < 0.05) formodulus of elasticity as shown in Fig. 6 and with ethyleneglycol + triazabicyclodecene (R2 = 0.03, p < 0.05) and (R2 = −0.79,p < 0.05) for nanohardness and modulus of elasticity respec-tively are shown in Fig. 7. The representative nano-indentation

curves and NMP plots during nano-indentation are presentedin Figs. 8 and 9 respectively.

sin SR and EG + TBD at baseline (control), 5 min, and 24 h.ne glycol + Triazabicyclodecene.

The FTIR analyses dimethacrylate substrate showed low-ered peak heights of C = O (1650–1750 cm−1) when thesurface was treated with ethylene glycol + triazabicyclodecenewhereas not much changes in the peak height were seenby the treatment with StickTM resin. Similarily, ethylene gly-col + triazabicyclodecene lowered the height of the peaks ofC-O (1141–1271 cm−1) which was not seen after treatmentwith StickTM resin (Fig. 10). With the epoxy substrate noeffect of ethylene glycol + triazabicyclodecene blend or StickTM

resin treatment was seen for the peak heights of C = O(1650–1750 cm−1) or C-C (1575–1625 cm−1). A significant peak

d e n t a l m a t e r i a l s 3 6 ( 2 0 2 0 ) 698–709 703

Fig. 4 – Linear regression analyses correlation between treatment time for nanohardness and the Young’s modulus ofelasticity for epoxy resin treated with StickTM resin at baseline (0 s), 5 min, and 24 h.

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ig. 5 – Regression analyses correlation between treatment tpoxy resin treated ethylene glycol + triazabicyclodecene ble

t wavenumber of 1720 cm−1 occurred on the epoxy surfacefter it was treated with StickTM resin for 24 h (Fig. 11)

. Discussion

he current laboratory study was undertaken to validate aew method which could be used to improve interfacial adhe-

ion between cross-linked thermoset resin composites andther resins, such as resin luting cements, core-built-up com-osites and veneering composites. The clinical benefits of aew bonding systems would therefore found in almost all of

for nanohardness and the Young’s modulus of elasticity fort baseline (0 s), 5 min, and 24 h.

fields of operative dentistry and dental technology. This studyalso focused on determining whether the fundamental basisof interfacial adhesion by dissolving the substrate could beimproved by transesterification, which has been tested widelywith the purpose of recycling plastics [26]. The requirement forthe transesterification reaction is the presence of ester groupsin the chemical structure of the polymer. The most commonlyused monomer systems of bis-GMA-TEGDMA contains estergroups and could therefore be used for the transesterifica-

tion to break down the surface layer and enable diffusion ofmonomers and to entangle to the substrate surface. Anotherpolymer, which is commonly used e.g. in the polymer matrix

704 d e n t a l m a t e r i a l s 3 6 ( 2 0 2 0 ) 698–709

Fig. 6 – Non-linear regression analyses correlation between treatment time for nanohardness and the Young’s modulus ofelasticity for dimethacrylate polymer treated with StickTM resin at baseline (0 s), 5 min, and 24 h.

Fig. 7 – Non-linear regression analyses correlation between treatment time for nanohardness and the Young’s modulus oflyco

elasticity for dimethacrylate polymer treated with ethylene g

24 h.

of root canal posts is epoxy. There are several types of epoxyresins and based on the polymerization system the polymerhas or has not ester groups in the chemical structure. It is notknown the exact composition of the epoxy resins which areused in fiber reinforced composite, root canal posts. For thisstudy we selected polymer substrates, which are known to dif-fer in this respect, namely bis-GMA-TEGDMA based polymer

with ester groups and amine accelerator resulting in epoxypolymer without ester groups in the structure. In fact, thiswas confirmed by the FTIR analysis of the epoxy, which didnot show the characteristic peak for the ester group. Alter-

l + triazabicyclodecene blend at baseline (0 s), 5 min, and

natively, the dimethacrylate polymer demonstrated carbonylgroup which is characteristics for the ester group. Some inter-esting physical and chemical observations of the behavior ofthe surfaces of dimethacrylate and epoxy polymer were made.These observations relate to the phenomena of polymer dis-solution and swelling.

The process of polymer dissolution by a solvent begins with

the formation of a surface layer, wherein the solvent invadesand drives the swollen polymer layer into the solvent. Theprocess of dissolution is gated by the polymer chain disen-tanglement and the polymer-chain diffusion adjacent to the

d e n t a l m a t e r i a l s 3 6 ( 2 0 2 0 ) 698–709 705

Fig. 8 – Graphs illustrating the loading and unloading of the force and plotted values for nanohardness and the Young’smodulus of the dimethacrylate polymer treated with ethylene glycol + triazabicyclodecene blend for 24 h.

Fig. 9 – Graphs illustrating the plotted values of contact depth (�m) (x-axis) plotted against surface nanohardness (GPa)(y-axis on the left) and elastic modulus (GPa) (y-axis on the right) of the epoxy polymer treated with ethyleneg

sebTsTtr(tilotp

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lycol + triazabicyclodecene blend for 24 h.

olvent-polymer interface. The organization of different lay-rs from the pure solvent to the pure polymer involves a liquided, gel bed, solid swollen layer and the infiltration layer [27].he infiltration of the solvent into the polymer increases thewollen polymer layer, until it reaches a quasistationary state.he dissolution of any thermoset polymer is generally of two

ypes: reaction-rate controlled (i.e. polymer-solvent diffusionate higher than the reaction rate), and diffusion controlledi.e. diffusion rate slower than the reaction rate). The dissolu-ion process is also influenced by the temperature. A decreasen temperature as shown a reduced formation of the gel layereading to building up of stresses, resulting in the formationf cracks [26]. Of the several factors governing the dissolu-ion rate of polymer, molecular weight and the polydispersitylayed a major role.

Polymers with high molecular weight and monodispersen nature have a relatively slower dissolution rate and viceersa [28]. Solvents with high diffusion rates and swelling

ower have a thorough effect on the polymer dissolution. Ifhe rate of relaxation of stresses within the glassy polymer

atrix is overtaken by the pressure buildup by the solvent,

it eventually leads in the fracture of the polymer [29]. A non-solvent molecule when added to the solvents has shown to bea major contributory factors for increasing the dissolution ofsolvents due to the increased plasticization nature of it [30].Given that, the solubility parameters of the solvent, wettabil-ity and surface tension have a role in polymer dissolution.On the other hand, polymers which have solubility param-eters with no greater difference to solvents, dissolve easily[31]. The results of the current study suggested a decrease ofthe surface nanohardness of the dimethacrylate surface bythe application of StickTM resin. Softening of the surface waslikely due to swelling of the surface of the substrate by dif-fusion of monomers of bis-GMA and TEGDMA. It should benoted that the softening was demonstrated by nanomechani-cal means. Surface softening occurred during the first minutesof time exposure and regardless of prolonging the treatmenttime up to 24 h, no further softening was observed. Thiscould be due to evaporation of the lower viscosity TEGDMA

monomer at RTP [32], which may have been more effectivedissolving component of the system. It is also possible thatsome degree of polymerization during the prolonged treat-

706 d e n t a l m a t e r i a l s 3 6 ( 2 0 2 0 ) 698–709

Fig. 10 – FTIR spectrum for specimens of dimethacrylate polymer substrate treated with resin SR and EG + TBD at baseline(control), 5 min, and 24 h. The arrows are depicting the peaks of carbonyl groups(C = O) and aromatic groups (C–O).SR = StickTM resin, EG + TBD = Ethylene glycol + Triazabicyclodecene.

Fig. 11 – FTIR spectrum for specimens of epoxy resin treated with resin SR and EG + TBD at baseline (control), 5 min, and24 h. The arrows are depicting the peaks of carbonyl groups(C = O) and (C–C). SR = StickTM resin, EG + TBD = Ethyleneglycol + Triazabicyclodecene.

d e n t a l m a t e r i a l s 3 6

Table 3 – Statistical results of nanohardness (GPa) andthe Young’s modulus of elasticity (GPa) of dimethacrylatepolymer treated with resin SR and EG + TBD. SR = StickTM

resin, EG + TBD = Ethylene glycol + Triazabicyclodecene.

Nanohardness Control 5 min 24 h

SRMean 0.389a,A 0.308b,A 0.342b,A

S.D 0.060 0.070 0.070

EG + TBDMean 0.389a,A 0.328b,A 0.310b,A

S.D 0.060 0.06 0 0.050

Young’s modulus of elasticitySRMean 6.714a,A 6.033b,A 5.963b,A

S.D 0.610 0.820 0.720

EG + TBDMean 6.714a,A 5.797b,A 6.121b,A

S.D 0.610 0.61 0 0.540

Different superscript small letters in the same row indicate statis-tically significant at p < 0.05.

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Different superscript capital letters in the same column indicatestatistically significant for each group at p < 0.05.

ent time have hindered the dissolution and swelling of theurface. SEMs showed smoothening of the surface topographyy StickTM resin treatment which supports surface dissolu-ion and swelling to have occurred. The surface topographyhanges are similar which were earlier demonstrated withhermoplastic polymethyl methacrylate surface and solventf methylmethacrylate [33]. It is important to note that thehermoplastic polymer surface showed dissolution patternslready in one-minute treatment time, whereas, in this studyith thermoset polymer, a 24 h treatment time was required

or microscopically visible surface topography changes. To puthis to the context of interfacial bonding strength by sur-ace dissolution, these findings supports previous findingshat thermoplastic polymers and semi-IPN polymers do haveetter bonding characteristics than cross-linked thermosets

34–36].The FTIR results of the present study showed that the appli-

ation of StickTM resin on dimethacrylate polymer surface didot cause any significant changes in the FTIR spectrum ofhe substrate. Thus, the softening was likely related to sur-ace swelling and not to actual dissolution due to covalentlyound cross-linked polymer structure. Differently, applicationf solution ethylene glycol + triazabicyclodecene to the surfaceaused reduction of the surface hardness with simultaneoushange in the chemical structure as found by the FTIR anal-sis. The analysis showed that characteristic peaks at theavenumbers of 1650–1750 cm−1 reduced their height, which

uggests transesterification has occurred.Epoxy resins are widely used in the manufacturing of

ber reinforced composite constructs. Epoxy resins withhe ester bond undergo depolymerization when a mixturef alcohol and catalyst, by transesterification resulting in

he formation of di-ester and a tetra alcohol. Ethylene gly-ol + triazabicyclodecene mixture was used for dissolving theolymer. Triazabicyclodecene has shown to dissolve thor-ughly in ethylene glycol when compared and contrasted to

( 2 0 2 0 ) 698–709 707

other catalyst such as zinc Acetate and sodium hydroxide [37].Hence, triazabicyclodecene was considered as a highly effec-tive catalyst to be used as a catalyst along with ethylene glycolfor the dissolution of epoxy resin. A solvent without any cata-lyst, when ethylene glycol used alone was shown to increasethe mass of the epoxy resin due to diffusion of it in the layerof the polymer [38].

The process of dissolution of epoxy resin occurs by theprocess of alcoholysis/transesterification rate, and this isdirectly related to the concentration of the catalyst [39].Nuclear magnetic resonance spectroscopic (NMR) studieshave shown free ethylene glycol and end terminated ethy-lene glycol molecules upon treating epoxy resin in ethyleneglycol + triazabicyclodecene solution. This indicated that theester bond was cleaved by alcohol and resulted in the forma-tion of oligomers by depolymerized resin [40]. In the currentstudy we found that the absence of ester group in the epoxyresin, dissolution of the resin was not possible by ethylene gly-col by transesterification and the StickTM resin caused swellingof the polymer layer and softened it. The aforementionedmechanism is valid for epoxies containing ester groups andthe mechanism could be considered also to take place in otherester groups containing polymers, such as bis-GMA-TEGDMA.

The polymer structure of dimethacrylate includes predom-inantly the covalent bonds (phase of copolymer bis-GMA-TEGDMA) and weaker hydrogen and even more weaker thevan der Waals bonds along the polymer chains. Nevertheless,this reorganized polymer chain at the surface layer would havealso formed crystallized regions [41] resulting in the increaseof the surface hardness and modulus of elasticity [42]. Theactual dissolving stage in the polymer would have taken placeif the exposure time had been longer for more than 30 daysby alcoholysis [43]. Surface hardening of the polymer at thenanometer scale has also been demonstrated in our previousstudies by alcoholysis [44]. FTIR analysies of dimethacrylatepolymer suggested that after the treatment with ethylene gly-col + triazabicyclodecene softening of the polymer was likelydue to transesterification.

The various peaks in the IR spectra are related to thevarying intensity caused by the vibrations of different func-tional groups in the molecules. The band-width is related tothe chemical environment determined by the intermolecu-lar interactions, and the intensity of the peak is governedby distinct factors like absorbance, pathlength, absorptivityand concentration of the functional groups. Chemical char-acterizations of the components in the polymer matrix arediscrete to various carbon to oxygen (C-O or C = O or C-O-C), carbon to hydrogen (C-H), carbon to carbon (C = C) doublebonds. The wavenumbers in the group of 850–920 cm−1 areof C-H bonds, C-O and C-O-C group are shown in the regionof 1141–1271 cm−1 and C = O in the wavenumber range of1650–1750 cm−1 along with the aromatic bands [45].

Interestingly, we found that the peaks for carbonyl group(C = O) at wavenumber range of 1650–1750 cm−1 and C-Oat wavenumber range of 1141–1271 cm−1 was foreshortenedif the surface had been exposed to triazabicyclodecene on

dimethacrylate polymer. This may relate to the possible break-down of the polymer by dissolution into oligomers [25]. FTIRanalysis of the chemical structure of the dimethacrylate poly-mer dissolved in the ethylene glycol + triazabicyclodecene

l s 3

r

708 d e n t a l m a t e r i a

blend showed a characteristic change in the peak at 1730 cm−1

of ester moieties which was disappeared when compared tothe resin. This disappearance of the peak was attributed tothe alcoholysis reaction wherein the ester functional groupsare released upon the cleavage of the bond. The catalyticsystem of depolymerizing the epoxy resin is explained byaction of ethylene glycol + triazabicyclodecene first on theester by the attack on the di-substituted nitrogen at thecarbonyl group and forming betaine like intermediate. Fol-lowing this the protonated nitrogen drives the transitionalcompound of triazabicyclodecene amide and frees the alco-hol. This process of release of alcohol and diffusion of theethylene glycol + triazabicyclodecene solution into the poly-mer network, leads to rapid depolymerization. Nevertheless,the rate of dissolution of the dimethacrylate polymer is inter-related to the capability of the treatment agent to diffuse intothe polymer layer and alcoholysis (transesterification). Epoxyresin showed a significant peak at wavelength of 1720 cm−1

when treated with StickTM resin for 24 h, attributed to thealdehyde group. Previous studies [46,47] have demonstratedformaldehyde formation from the monomers of methacry-lates. Thus, it was expected that in the time period of 24 hsome of the monomers of StickTM resin are forming aldehy-des by oxidation reaction at the RTP conditions and this wasdetected by the FTIR at 1720 cm−1.

We could find that surface profile peak, Rp clearly was influ-enced by the treatment agents which was likely associated tothe swelling and resolidification of the polymer surface. Also,some residual monomers would have leached out from thepolymer matrix causing a minor effect in the dimensions ofthe polymer matrix [48]. From the clinical and dental technol-ogy point of view, the current results may have a significantimpact for better understanding of the effects of commonlyused dental adhesives on the interfacial adhesion of resincomposites of different kinds. However, further research isneeded to demonstrate the impact of transesterification to theadhesion or resin composites with different kinds of polymermatrix.

5. Conclusions

The bisphenol-based epoxy substrate was not affected bysurface softening with treatment agents of StickTM resinand ethylene glycol + triazabicyclodecene blend. Dimethacry-late polymer substrate reduced its surface nanohardnesswithin 5 min of time by application of StickTM resinor ethylene glycol + triazabicyclodecene blend on the sur-face. Softening of the dimethacrylate surface by ethyleneglycol + triazabicyclodecene blend was associated to thereduction of ester groups in the polymer structure, i.e. trans-esterification took place, whereas application of StickTM resincaused surface changes by surface swelling only.

Acknowledgments

The authors are grateful to the Deanship of Research Chairs,King Saud University, for funding through the Vice Deanshipof Research Chairs, Kingdom of Saudi Arabia, Riyadh.

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