Polymerization Kinetics of Poly(2-Hydroxyethyl Methacrylate) Hydrogels and Nanocomposite Materials

processes

Article Polymerization Kinetics of Poly(2-Hydroxyethyl Methacrylate) Hydrogels and Nanocomposite Materials

Dimitris S. Achilias * and Panoraia I. Siafaka

Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece; [email protected] * Correspondence: [email protected]; Tel.: +30-2310-99-7822

Academic Editor: Alexander Penlidis Received: 15 March 2017; Accepted: 19 April 2017; Published: 24 April 2017

Abstract: Hydrogels based on poly(2-hydroxyethyl methacrylate) (PHEMA) are a very important class of biomaterials with several applications mainly in tissue engineering and contacts lenses. Although the polymerization kinetics of HEMA have been investigated in the literature, the development of a model, accounting for both the chemical reaction mechanism and diffusion-controlled phenomena and valid over the whole conversion range, has not appeared so far. Moreover, research on the synthesis of nanocomposite materials based on a polymer matrix has grown rapidly recently because of the improved mechanical, thermal and physical properties provided by the polymer. In this framework, the objective of this research is two-fold: to provide a kinetic model for the polymerization of HEMA with accurate estimations of the kinetic and diffusional parameters employed and to investigate the effect of adding various types and amounts of nano-additives to the polymerization rate. In the ﬁrst part, experimental data are provided from Differential Scanning Calorimetry (DSC) measurements on the variation of the reaction rate with time at several polymerization temperatures. These data are used to accurately evaluate the kinetic rate constants and diffusion-controlled parameters. In the second part, nanocomposites of PHEMA are formed, and the in situ bulk radical polymerization kinetics is investigated with DSC. It was found that the inclusion of nano-montmorillonite results in a slight enhancement of the polymerization rate, while the inverse holds when adding nano-silica. These results are interpreted in terms of noncovalent interactions, such as hydrogen bonding between the monomer and polymer or the nano-additive. X-Ray Diffraction (XRD) and Fourier Transform Infra-Red (FTIR) measurements were carried out to verify the results.

Keywords: polymerization kinetics; HEMA; nanocomposites; in situ polymerization; nano-clays; nano-silica

1. Introduction Hydrogels are biomaterials with properties such as hydrophilicity, biocompatibility and high water absorption by swelling without dissolving, which have attracted great interest from several researchers worldwide [1]. They can be of natural origin (e.g., hyaluronic acid, chitosan, etc.) or synthetic (e.g., poly(ethylene glycol) (PEG), poly(lactic acid) (PLA), etc.). 2-hydroxyethyl methacrylate (HEMA) is a hydrophilic monomer used to prepare such synthetic polymeric (i.e., PHEMA) hydrogels. The polymer, PHEMA, due to its excellent biocompatibility and physicochemical properties, similar to those of living tissues, is widely used in many biomedical applications, such as drug-delivery systems, cell carriers, tissue engineering, contacts lenses, regenerative medicine, etc. [2–4]. Other applications of PHEMA include those presented by Kharismadewi et al. [5], as an adsorbent for the removal of the

Processes 2017, 5, 21; doi:10.3390/pr5020021 www.mdpi.com/journal/processes Processes 2017, 5, 21 2 of 19 methylene blue cationic dye from an aqueous solution and Moradi et al. [6] for the adsorption of Cu2+ and Pb2+ ions from aqueous single solutions [6]. The great interest in the polymerization of HEMA is based on the combination of methacrylate groups in its structure, resulting in relatively easy radical reactions, with the hydroxyl groups, that provide hydrophilicity. Thus, PHEMA allows obtaining tissue in-growth due to high permeability to small molecules and the soft consistency, which minimizes mechanical frictional irritation to surrounding tissues. The polymer (PHEMA) is a glassy amorphous material with low water absorption, high adhesion to glass and glass transition temperature ranging between 50 and 90 ◦C depending on the conditions of the polymerization process [7]. During the last decade, research on the synthesis of nanocomposite materials based on a polymer matrix has grown rapidly because of the improved mechanical, thermal and physical properties that are provided by the polymer. Several types of nano-fillers have been used, such as nano-clays, nano-silica, carbon nanotubes, fullerenes, graphene, etc. In this research, we investigated the formation of PHEMA-based nanocomposites with either a nano-organomodified montmorillonite (OMMT) or nano-silica. Incorporation of MMT is well known to improve the mechanical properties of the polymer matrix [8,9], whereas adding silica to PHEMA results in bioactive materials that keep the swelling properties of neat polymer; thus, they assure both morphological and bioactive characteristics [7]. Nanocomposites can be prepared by various methods, such as in situ polymerization, melt intercalation/exfoliation and solution casting. The in situ polymerization method was used here in order to obtain nanocomposites with a uniform dispersion of the filler in the polymer matrix. The kinetics of radical polymerization has been extensively studied for a long time. Differential scanning calorimetry (DSC) has long been used for monitoring polymerization reactions for several systems. It offers the advantage of continuous recording of the variation of the reaction rate with time through the measurements of the amount of heat released, since additional reactions are exothermic. This technique has been also used for recording the polymerization kinetics of HEMA and its copolymers with some dimethacrylate monomers (such as ethylene glycol dimethacrylate (EGDMA) or diethylene glycol dimethacrylate (DEGDMA)) [10–14]. Modeling of polymerization kinetics has been carried out using semi-empirical models, such as those developed for resins-curing [11] or based on the isoconversional principle [10]. A pioneering work on the polymerization kinetics of PHEMA crosslinked with EGDMA was presented thirty years ago by Mikos and Peppas [15]. Later on, the group of Bowman was the only one to develop kinetic models for the polymerization kinetics of HEMA based on the reaction mechanism [16,17]. A method for determining the kinetic parameters was also proposed [16]. In our previous investigation, we also used DSC measurements and compared the results obtained by a simple mechanistic model with those from isoconversional analysis [18]. Only the low degrees of conversion were investigated to avoid the diffusion-controlled phenomena. This research consists of two parts: In the first part, a detailed kinetic model is proposed for the polymerization kinetics of HEMA in bulk based on DSC measurements of the polymerization rate and the reaction mechanism, including also the effect of diffusion-controlled phenomena on the termination and propagation reactions. Polymerizations at different temperatures are carried out, and all kinetic parameters used are either based on accurate independent experimental measurements or estimated in this work. The aim of this part was to provide accurate kinetic data concerning the radical polymerization of HEMA over the whole conversion range. In the second part, the effect of adding different types and amounts of nano-particles on the polymerization kinetics is investigated. The in situ bulk radical polymerization is examined with two different types of nano-additives: an organo-modified montmorillonite, which is a 2D nanoclay, and a nano-silica, which can be considered a 3D nanoparticle. The polymerization kinetics is studied with DSC operating isothermally at different temperatures. Depending on specific interactions, a different effect on the polymerization kinetics was observed from those two nano-additives. Processes 2017, 5, 21 3 of 19

2. Materials and Methods

2.1. Materials The monomers used were composed of 2-hydroxyethyl methacrylate (HEMA), purchased from Aldrich with purity ≥99%. Before any use, it was passed at least twice through a disposable inhibitor-remover packed column, supplied from Aldrich, in order to remove the inhibitor included. The free radical initiator used, i.e., benzoyl peroxide (BPO) with a purity >97%, was provided by Fluka AG and purified by fractional recrystallization twice from methanol (purchased from Merck). For the preparation of the nanocomposites, two nanofillers were used: the first was a commercially-available organically-modified, with a quaternary ammonium salt (i.e., dimethyl hydrogenated tallow), montmorillonite clay, under the trade name Cloisite 15A, (provided by Southern Clay Products + Inc., Gonzales, TX, USA). The chemical structure of the ammonium salt was, N R2(CH3)2 where R is the hydrogenated tallow (∼65% C18, ∼30% C16, ∼5% C14), and its cationic exchange capacity (CEC) was 125 meq/100 g clay. Typical physical properties, according to the manufacturer, include: size, as measured by a transmission electron microscope for a PA6 nanocomposite, 75–150 nm × 1 nm; surface area 750 m2/g when exfoliated. The second nanoparticle used was hydrophilic fumed silica under the trade name Aerosil 200. 2 The average primary particle size was 12 nm, the specific surface area 200 m /g and the SiO2 content greater than 99.8% (from Evonik Resource Efficiency GmbH). All other chemicals used were of reagent grade.

2.2. Polymerization Kinetics Polymerization experiments were performed using the DSC Diamond (Perkin-Elmer, Waltham, MA, USA). For the temperature and enthalpy calibration of the instrument, indium was used. Polymerizations were run isothermally at temperatures of 52, 60, 72 and 82 ◦C. Although a signiﬁcant amount of heat is produced during the reaction, especially in the autoacceleration region, the equipment set up is so to maintain the reaction temperature constant (within ± 0.01 ◦C) during the whole conversion range. The liquid mixture of the monomer with the initiator at an initial concentration of 0.03 mol/L was placed into aluminum Perkin-Elmer pans, accurately weighted (approximately 10 mg), sealed and positioned into the appropriate holder of the instrument. In order to have an estimation of the overall reaction rate (dx/dt), the amount of heat released (d(∆H)/dt), the reaction exotherm in normalized values (W/g) was continuously recorded as a function of time. Then, the reaction rate is estimated from: dx 1 d(∆H) = (1) dt ∆HT dt where x denotes fractional conversion and ∆HT total reaction enthalpy. By integrating the area between the DSC thermogram and the baseline established after extrapolation from the trace produced when polymerization has been completed (no change in the heat produced), the degree of conversion can be calculated. In order to determine the total reaction enthalpy, a dynamic experiment followed, where samples were heated from the polymerization ◦ −1 temperature to 180 C at a rate of 10 K min . The sum of enthalpies of the isothermal (∆Hi) plus the dynamic (∆Hd) experiment was the total reaction enthalpy. The values thus estimated were always near 422 J/g, similar to the theoretical one obtained by dividing the standard heat of polymerization of a methacrylate double bond (i.e., 54.9 kJ/mol) over the monomer molecular weight (i.e., 130). The ultimate monomer conversion was estimated by the quotient ∆Hi/∆HT. In order to check for possible monomer evaporation during the reaction, the pans were weighed again after the end of the polymerization. A negligible monomer loss (less than 0.2 mg) was observed only in a few experiments. It should be noticed that at low polymerization temperatures (i.e., 50 or 60 ◦C), a small inhibition time was observed. This is due to the non-adequate removal of oxygen, dissolved in the monomer during the initial mixing, which was performed in air. Processes 2017, 5, 21 4 of 19 Processes 2017, 5, 21 4 of 19

All of the experiments were performed at least twice, and the best results are included in the ResultsAll section. of the experiments were performed at least twice, and the best results are included in the Results section. 2.3. Synthesis of Nanocomposites 2.3. Synthesis of Nanocomposites For the synthesis of the nanocomposite materials, initially, the appropriate amount (1, 3 or 5 wt%)For of the the synthesis nano-particles of the nanocomposite (i.e., OMMT or materials, silica) was initially, dispersed the appropriate in the monomer, amount using (1, 3 ormagnetic 5 wt %) andof the ultrasound nano-particles agitation. (i.e., In OMMT the final or homogeneous silica) was dispersed suspension, in the initiator, monomer, BPO using at a magneticconcentration and 0.03ultrasound mol/L, was agitation. added, In and the the ﬁnal mixture homogeneous was degass suspension,ed by passing the initiator,nitrogen BPOand immediately at a concentration used. The0.03 procedure mol/L, was followed added, for and the the measurement mixture was of degassed the reaction by passing rate with nitrogen time was and exactly immediately the same used. as thatThe reported procedure previously. followed for the measurement of the reaction rate with time was exactly the same as that reported previously. 2.4. Measurements 2.4. Measurements X-ray diffraction (XRD) patterns of the materials were obtained using an X-ray diffractometer (3003X-ray TT, Rich. diffraction Seifert, (XRD)Ahrenburg, patterns Germ ofany) the materialsequipped werewith a obtained CuKa generator using an (λ X-ray = 0.1540 diffractometer nm). Scans were(3003 taken TT, Rich. in the Seifert, diffraction Ahrenburg, angle range Germany) 2θ = 1°–10°. equipped with a CuKa generator (λ = 0.1540 nm). ◦ ◦ ScansDetails were takenof the in chemical the diffraction structure angle of rangeneat PHEM 2θ = 1 A–10 and. its nanocomposites were identified by recordingDetails their of the IR chemical spectra. structure The particular of neat PHEMA FTIR andinstrument its nanocomposites used was werethe identiﬁedSpectrum by1 spectrophotometerrecording their IR spectra. from Perkin-Elmer. The particular The FTIR spectra instrument were usedrecorded was theover Spectrum the range 1 spectrophotometer4000–500 cm−1 at a −1 −1 resolutionfrom Perkin-Elmer. of 1 cm−1, The and spectra 32 scans were were recorded averaged over to thereduce range noise. 4000–500 cm at a resolution of 1 cm , and 32 scans were averaged to reduce noise. 3. Results 3. Results 3.1. Polymerization Kinetics of HEMA 3.1. Polymerization Kinetics of HEMA Bulk free-radical polymerization kinetics of PHEMA has been investigated previously by our Bulk free-radical polymerization kinetics of PHEMA has been investigated previously by our group at reaction temperatures ranging from 50–80 °C [18,19]. Similar, though slightly different group at reaction temperatures ranging from 50–80 ◦C[18,19]. Similar, though slightly different reaction temperatures were also used here in order to check previous results and have appropriate reaction temperatures were also used here in order to check previous results and have appropriate reaction rate data. The variation of heat released with time at temperatures of 52, 60, 72 and 82 °C is reaction rate data. The variation of heat released with time at temperatures of 52, 60, 72 and 82 ◦C is illustrated in Figure 1a. From these data, the reaction rate was estimated according to Equation (1). illustrated in Figure1a. From these data, the reaction rate was estimated according to Equation (1). By integrating this equation, the monomer double bond conversion was calculated and is plotted in By integrating this equation, the monomer double bond conversion was calculated and is plotted in Figure 1b at all reaction temperatures studied. Figure1b at all reaction temperatures studied.

Figure 1. Variation of the amount of heat released ( a) and conversion ( (b)) with time during during the the bulk bulk radicalradical polymerization of HEMA at several constant temperatures.

The curves shown in Figure 11 exhibitexhibit thethe typicaltypical characteristicscharacteristics ofof aa radicalradical polymerizationpolymerization reactionreaction withwith diffusion-controlleddiffusion-controlled phenomena phenomena affecting affecting the the reaction reaction rate. rate. The featureThe feature points points are brieﬂy are brieflyanalyzed analyzed below. below. As is wellAs is known, well known, the radical the radical polymerization polymerization mechanism mechanism includes includes mainly mainly three three steps (i.e., decomposition of an initiator to primary radicals, reaction of these radicals with

Processes 2017, 5, 21 5 of 19 steps (i.e., decomposition of an initiator to primary radicals, reaction of these radicals with monomer molecules to form macro-radicals, propagation of the macro-radicals by reacting with several monomer molecules and, as the final point, termination of these macroradicals for the formation of the final polymer). At the early stages of polymerization (low conversions), ‘classical’ free-radical kinetics apply, with a purely chemical control of the reaction [20]. This is denoted by an almost constant polymerization rate and an almost linear dependence of monomer conversion with time. After a certain point in the region of 7%–12% conversion, an increase in the reaction rate takes place accompanied by an increase in the conversion values. This is the well-known gel-effect or autoacceleration phenomenon, which is attributed to the effect of diffusion-controlled phenomena mainly on the termination reaction. Accordingly, in reactions carried out without any solvent, in bulk, as the concentration of the macromolecules increases, the diffusion of the macro-radicals in space in order to find one another and react is significantly reduced. This results in a local increase in concentration, leading to increased reaction with monomer molecules and, hence, to an increased polymerization rate [20]. At higher degrees of conversion, the macro-radical chains are even more restricted in their movement, and their center-of-mass diffusion becomes very slow. However, termination of these macro-radicals is continued at a smaller rate, by means of their implicit movement caused by the addition of monomer molecules at the chain end. This diffusion mechanism is the so-called ‘residual termination’ or ‘reaction diffusion’. The higher the propagation reaction rate, the more likely is the reaction-diffusion to be rate determining. Afterwards, the polymerization rate falls significantly and tends asymptotically to zero. This stage corresponds to the well-known glass-effect. This is attributed to the effect of diffusion-controlled phenomena on the propagation reaction since at such high monomer conversions, even the small monomer molecules are hindered in their movement to find a macro-radical and react [20]. All of these phenomena are quantified next. As was reported previously, the free-radical polymerization mechanism constitutes mainly three steps: initiation (with an initiator decomposition kinetic rate constant, kd, and initiator efficiency, f ), propagation (with propagation rate constant kp) and termination (with rate constant kt). After a number of assumptions, including the steady-state approximation (rate of change of radical concentration with time equal to zero), the long chain hypothesis (consumption of monomer only in the propagation reactions) and negligible chain transfer to the monomer rate constant compared to propagation, the polymerization rate, Rp, is expressed as a function of conversion, X, by:

1/2 1/2 dX f kd 1/2 ∼ f kd 1/2 Rp = = kp [I] (1 − X) = ke f f (1 − X) with ke f f = kp [I] (2) dt kt kt

In order to estimate the overall kinetic rate constant, keff, Equation (2) can be integrated assuming that the initiator concentration, [I], the initiator efﬁciency, f, and all kinetic rate constants are constant.

− ln(1 − X) = ke f f t (3)

It should be noted that the assumptions used to result in Equation (3) are valid only at low degrees of monomer conversion. Then, from a plot of −ln(1 − X) vs. t, the slope of the initial linear part is directly equal to keff. Such plots at conversion values in the range of 1%–7% have been created and illustrated in Figure2a. The data followed very good straight lines at high temperatures (i.e., 72 and 82 ◦C), whereas a slight curvature appeared at the lower temperature of 52 ◦C (the correlation 2 coefﬁcient, R , ranged from 0.992–0.999). Then, from the keff values measured at different temperatures, the overall activation energy of the polymerization rate, Eeff, can be estimated from the slope of ln(keff) versus 1/T assuming an Arrhenius-type expression. The individual activation energies of the elementary reactions, i.e., propagation (Ep), initiation (Ei) and termination (Et), are correlated to Eeff according to the following equation, extracted from Equation (2):

Eeff = Ep + 1/2(Ei − Et) (4) Processes 2017, 5, 21 6 of 19

The Arrhenius-type plot is illustrated in Figure2b. As can be seen, all data follow a very good straight line, and the slope estimated provides an activation energy equal to 90.6 ± 1.1 kJ/mol (R2 = 0.999). This value is close to the literature value of PHEMA found in our previous publication, Processes 2017, 5, 21 6 of 19 i.e., 89 ± 3.1 kJ/mol (R2 = 0.997) [18], and also near to that of PMMA, i.e., 84 kJ/mol [21]. It should be notednoted that lowerthat lower values values have have been been reported reported in in the the literatureliterature (i.e., (i.e., 56.7 56.7 [10]; [10 ];63.6 63.6 [12]; [12 73.2]; 73.2 kJ/mol kJ/mol [11]), [11]), thoughthough they they have have been been estimated estimated by by integral integral data data onon non-isothermalnon-isothermal polymerization polymerization that thathave have been been provenproven to be to not be strictlynot strictly correct. correct. In In addition, addition, the the commercially-availablecommercially-available monomer, monomer, HEMA, HEMA, used usedby by differentdifferent groups groups of authorsof authors is notis not pure pure and and includes includes inhibitors or or other other products, products, which which affect affect the the polymerizationpolymerization kinetics. kinetics.

FigureFigure 2. Estimation 2. Estimation of the of effective the effective kinetic kinetic rate constant, rate constant,keff, using keff, experimentalusing experimental data indata the conversionin the regionconversion 1–7% at severalregion 1–7% temperatures at several (a temperatures) and the Arrhenius-type (a) and the Arrhenius-type plot to calculate plot the to overallcalculate (effective) the activationoverall energy (effective) of HEMA activation polymerization energy of HEMA (b). polymerization (b).

For PHEMA, the propagation activation energy has been estimated by Buback et al. using the Forpulsed PHEMA, laser polymerization the propagation (PLP)/size-exclusion activation energy chromatography has been estimated (SEC) technique by Buback and found et al. equal using to the pulsed21.9 laser ± 1.5 polymerization kJ/mol [22]. Moreover, (PLP)/size-exclusion for the BPO initiato chromatographyr, the decomposition (SEC) activation technique energy and is foundequal to equal to 21.9143± kJ/mol1.5 kJ/mol [23]. Then, [22]. using Moreover, the value for estimated the BPO for initiator, the overall the (effective) decomposition activation activation energy (i.e., energy is equal90.6 tokJ/mol) 143 kJ/moland Equation [23]. (4), Then, the activation using the energy value of estimatedthe termination for thereaction overall can be (effective) estimated. activation This energywas (i.e., found 90.6 to kJ/mol) be 5.6 andkJ/mol, Equation very close (4), theto the activation value proposed energy offor the MMA termination polymerization reaction (i.e., can be 5.89 kJ/mol [24]). estimated. This was found to be 5.6 kJ/mol, very close to the value proposed for MMA polymerization Furthermore, in order to provide values for the individual kinetic rate constants of the HEMA (i.e., 5.89 kJ/mol [24]). polymerization, we used for the initiator BPO the values reported in [23], i.e., Furthermore, in order to provide values for the individual kinetic rate constants of the HEMA kd = 5 × 1016 exp(−143,000/RT) s−1 and f = 0.5. Then, from Equation (2) and assuming that [I] ≅ [I]0, the × 16 polymerization,values of (kp0/ we√kt0) usedcan be for estimated the initiator and included BPO thein Table values 1. The reported assumption in [23 of], a i.e.,nearlykd constant= 5 10 exp(−143,000/RT) s−1 and f = 0.5. Then, from Equation (2) and assuming that [I] =∼ [I] , the values initiator√ concentration is valid since in the time horizon where the values of keff were estimated,0 the of (kpratio0/ koft0 )[I]/[I] can be0 was estimated never lower and includedthan 0.998. in For Table the1. propagation The assumption rate constant, of a nearly that constant reported initiator by concentrationBuback [22], is i.e., valid ln since(kp (L/mol/s)) in the time= 16.0 horizon − 2634/T where(K), can the be used. values In ofsuchkeff a wereway, the estimated, termination the rate ratio of constant can be estimated, and values at the temperatures investigated are included in Table 1. [I]/[I]0 was never lower than 0.998. For the propagation rate constant, that reported by Buback [22], i.e., ln (kp (L/mol/s)) = 16.0 − 2634/T (K), can be used. In such a way, the termination rate constant Table 1. Values of kinetic and diffusion-controlled parameters obtained in this study during bulk can be estimated, and values at the temperatures investigated are included in Table1. polymerization of HEMA with the benzoyl peroxide (BPO) initiator.

Table 1. Values of kinetic andkp0/√ diffusion-controlledkt0 kp0 [22] parameterskt0 obtainedR in this study during bulk T (°C) keff (min−1) Ap fcp At fct polymerization of HEMA(L/mol/s) with the1/2 benzoyl (L/mol/s) peroxide (L/mol/s) (BPO) initiator.(L/mol)

52 0.00597 1.1278√ 2685 5.667E6 3.20 0.595 0.0500 1.40 0.083 kp0/ kt0 k [22] k R T (◦C)60 k (min0.01400−1) 1.4014 p0 3262 5.78E6t0 2.01 A0.64 0.0485f 1.35A 0.084f eff (L/mol/s)1/2 (L/mol/s) (L/mol/s) (L/mol) p cp t ct 72 0.04235 1.7257 4295 6.195E6 1.34 0.85 0.0481 1.28 0.0876 52 0.00597 1.1278 2685 5.667E6 3.20 0.595 0.0500 1.40 0.083 82 0.10222 2.0641 5326 6.657E6 0.91 1.14 0.0445 1.39 0.0858 60 0.01400 1.4014 3262 5.78E6 2.01 0.64 0.0485 1.35 0.084 72 0.04235 1.7257 4295 6.195E6 1.34 0.85 0.0481 1.28 0.0876 82In order 0.10222 to simulate the 2.0641 experimental 5326 data over 6.657E6 the whole 0.91 conversion 1.14 range, 0.0445 using Equation 1.39 0.0858 (2), the next step is to use appropriate equations accounting for the variation of the termination and propagation rate constants during polymerization. Though a number of sophisticated models have

Processes 2017, 5, 21 7 of 19

In order to simulate the experimental data over the whole conversion range, using Equation (2), the next step is to use appropriate equations accounting for the variation of the termination and propagation rate constants during polymerization. Though a number of sophisticated models have been developed [20], we followed here the one proposed by Bowman et al. [16], for the following reasons: it is rather simple and allows estimation of all parameters directly from the simulation of experimental data without integrating the reaction rate; it was originally developed for methacrylate polymerization, such as HEMA; and it is based on DSC data on the reaction rate, as those reported here. In brief, the effect of diffusion-controlled phenomena on the termination and propagation rate constants is taken into consideration using the following equations [16]:

1 1 1 = + h i (5) kp kp0 kp0 exp −Ap 1/Vf − 1/Vf ,cp

1 1 1 = + h i (6) kt kt0 kt,res + kt0 exp −At 1/Vf − 1/Vf ,ct where kt,res denotes the contribution of the reaction-diffusion term on the termination rate constant, estimated from: kt,res = Rkp[M] (7)

Vf is the fractional free volume of the system, related to the fractional free volumes of monomer (Vf,m) and polymer (Vf,p) from: Vf = Vf ,mφm + Vf ,p(1 − φm) (8) with: 1 − X φm = (9) 1 − X + X(ρm/ρp)

Vf ,m = 0.025 + αm(T − Tg,m) (10)

Vf ,p = 0.025 + αp(T − Tg,p) (11)

φm is the volume fraction of the monomer; α denotes the coefficient of expansion; Tg the glass transition temperature; and ρ density. Subscripts m and p are associated with the monomer and polymer, respectively. In the above set of equations, the thermal expansion coefficients, glass transition temperatures ◦ −1 and densities of monomer and polymer were taken from Goodner et al. [16], as αm = 0.0005 C , ◦ −1 ◦ ◦ 3 3 αp = 0.000075 C , Tg,m = −60 C, Tg,p = 55 C, ρm = 1.073 g/cm , ρp = 1.15 g/cm . The value of the polymer glass transition temperature has been also experimentally measured by Bolbukh et al. [7], and the monomer density has been calculated by Buback and Kurz [22]. The initial concentration of the monomer can then be estimated as [M]0 = ρm/MWm = 1.073 × 1000/130 = 8.25 mol/L. The parameters of the diffusion-controlled model that have to be evaluated are Ap, Vf,cp, At, Vf,ct and R. To determine these parameters, we followed a slightly modified procedure from that described by Goodner et al. [16]. Accordingly, every set of these parameters can be estimated from linear plots, if we focus only on the particular region of the diffusion-controlled phenomena on the termination and propagation rate constants. As was mentioned above, during polymerization, four regions can be identified. At the early stages (less than 10%), no diffusional limitations on either termination or propagation reactions occur. The second region is that of the gel-effect region, where strong diffusional limitations are present on the termination rate constant, which decreases by several orders of magnitude, while kp remains constant. The latter also holds in the third region, where the decrease in kt slows down due to the so-called reaction diffusion controlled termination. This means that although macroradicals cannot easily move in space, they can implicitly move by the addition of monomer molecules. In the fourth region, propagation becomes also diffusion controlled, and kp significantly reduces with conversion. Processes 2017, 5, 21 8 of 19

Initially, in order to estimate the parameter, R, introduced to account for the reaction-diffusion controlled termination, it is assumed that the particular term dominates kt in this region (near 40–50% conversion), and thus, kt can be set equal to kt,res, i.e., kt = kt,res = R kp [M]. Under this condition, the polymerization rate, Equation (2), is expressed as:

1/2 1/2 dX f kd[I] kp f kd[I](1 − X) Rp = = kp (1 − X) = (12) dt Rkp[M] R[M]0

The initiator concentration was assumed approximately equal to its initial value, [I]~[I]0, since during the whole polymerization time and for all temperatures investigated, the initiator consumption never exceeds 1% (calculated according to its decomposition rate constant). In the reaction-diffusion region, propagation is not diffusion-controlled and kp is approximated by kp0. Then, by rearranging Equation (12), the term, R, can be estimated from: ! k f k [I](1 − X) = p0 d R 2 (13) Rp[M]0

By plotting the right-hand side of Equation (13) as a function of the conversion, a straight horizontal line should be obtained. This has been done for all temperatures investigated, and at the particular region where this holds, the parameter R was evaluated and reported in Table1. Following, the diffusion-controlled parameters for the propagation reaction (i.e., Ap and Vf,cp) can be estimated from the results obtained in the fourth region, where the termination reaction is still reaction-diffusion controlled, but the propagation rate constant is also diffusion-controlled. Then, squaring Equation (12), rearranging and taking the natural logarithm leads to: " # kp f k [I](1 − X) 1 Ap 0 d − = − ln 2 1 Ap (14) RpR[M]0 Vf Vf ,cp

Thus, plotting the left-hand side of Equation (14) vs. 1/Vf will provide Ap from the slope and Vf,cp from the intercept. Finally, the gel-effect region can be analyzed similarly. In this region, kp can again be approximated by kp0. During autoacceleration, termination is governed by center-of-mass diffusional limitations, while the reaction-diffusion is still negligible. Under these conditions, the expression for kt is reduced to:

kt0 kt = h i (15) 1 + exp At 1/Vf − 1/Vf ,ct

Again, squaring Equation (2), rearranging and taking the natural logarithm leads to:   2 kt0Rp 1 At ln − 1 = At − (16) 2 2 V V kp0 f kd[I](1 − X) f f ,ct

By plotting the left-hand side of Equation (16) vs. 1/Vf, the last two parameters, At and Vf,ct, can be determined. Using the above procedure, all of the parameters were estimated, and results at different polymerization temperatures are included in Table1. It is seen that, as the polymerization temperature increases, the reaction-diffusion parameter, R, decreases, whereas Ap increases. In contrast, parameters, Vf,cp, Vf,ct and At are slightly affected by temperature. Thus, it seems that increased reaction temperatures lead to a higher mobility of the monomer molecules, and diffusion controlled phenomena affect the propagation reaction at higher conversions. In contrast, higher reaction temperature results in lower termination rate constants in ProcessesProcesses 20172017, 5, 521, 21 9 of9 19of 19 to the macroradicals to move in space before freezing and terminate only by the implicit addition of monomerthe reaction-diffusion molecules. It should region. be This note meansd here that that the the increased estimated temperatures values of R provide lie in between greater mobilitythe values of to0.703 the macroradicalsand 9.353 calculated to move for in spacerigid beforeor totally freezing flexible and chains terminate [25]. only Moreover, by the implicit Goodner addition et al. of[16] monomer molecules. It should be noted here that the estimated values of R lie in between the values provided a value for R equal to four, which follows our estimations quite well if we extrapolate our of 0.703 and 9.353 calculated for rigid or totally ﬂexible chains [25]. Moreover, Goodner et al. [16] vales to a temperature near 35 °C, where these authors carried out their experiments. provided a value for R equal to four, which follows our estimations quite well if we extrapolate our Using the parameters reported in Table 1, the termination and propagation rate constants can be vales to a temperature near 35 ◦C, where these authors carried out their experiments. estimated at different temperatures, and results appear in Figure 3. It can be observed that kp Using the parameters reported in Table1, the termination and propagation rate constants can be maintains a constant value up to 55–75% depending on the temperature. Afterwards, it drops estimated at different temperatures, and results appear in Figure3. It can be observed that kp maintains approximatelya constant value 1.5 uporders to 55–75% of magnitude depending due on to the the temperature. effect of diffusion-cont Afterwards,rolled it drops phenomena. approximately The kt curve shows a more complex evolution. It drops quickly from its initial value, as growing 1.5 orders of magnitude due to the effect of diffusion-controlled phenomena. The kt curve shows macro-radicalsa more complex experience evolution. diffusio It dropsn quicklylimitations from and its initial autoacceleration value, as growing occurs. macro-radicals After a reduction experience by two ordersdiffusion of magnitude limitations and and a autoacceleration conversion approximat occurs. Afterely equal a reduction to 50%, by reaction-diffusion two orders of magnitude termination and startsa conversion to dominate, approximately and the equaldecrease to 50%, in k reaction-diffusiont is much more terminationgradual. This starts slow todominate, decrease andover the the conversiondecrease inrangekt is muchfrom 45–65% more gradual. is due Thisto the slow fact decrease that the over reaction-diffusion the conversion rangetermination from 45–65% rate constant, is due kt,resto, is the proportional fact that the reaction-diffusionto the monomer terminationconcentration, rate [ constant,M], denotedkt,res ,by is proportionalthe unreacted to thedouble-bonds’ monomer concentration.concentration, Since [M], polymerization denoted by the unreactedproceeds, double-bonds’the latter is still concentration. decreasing. SinceWhen polymerization the glass-effect appearsproceeds, and the kp latterstarts is to still drop, decreasing. kt again When decreases the glass-effect through appearsthe reaction and -diffusionkp starts to proportionality. drop, kt again Moreover,decreases from through Figure the 3, reaction-diffusion it can be observed proportionality. that at lower temperatures, Moreover, from wher Figuree both3, it the can small be observed molecule andthat the at lowermacro-radical temperatures, mobility where is bothlower, the the small effe moleculect of diffusion-contro and the macro-radicallled phenomena mobility is lower,is more pronouncedthe effect of in diffusion-controlled both kt and kp. phenomena is more pronounced in both kt and kp.

107 82 oC 72 oC 6 10 k o t 60 C 52 oC

105

4 k 10 p

103

102 kineticrate constants (L/mol/s) 0 20 40 60 80 100 Conversion (%)

Figure 3. Variation of the termination and propagation rate constants estimated from Equations (5) Figure 3. Variation of the termination and propagation rate constants estimated from Equations (5) and (6) with conversion at several reaction temperatures, using the parameters reported in Table1. and (6) with conversion at several reaction temperatures, using the parameters reported in Table 1.

Finally,Finally, as as can can be be seen seen in in Figure Figure 4,4 ,using using this this simplified simpliﬁed model, model, an an extremely extremely good simulation of theof experimental the experimental data at data all atdifferent all different temperatures temperatures was obtained. was obtained. Both the Both initial the initialrate and rate the and location the andlocation value of and the value maximum of the maximumrate of polymerization rate of polymerization are reproduced are reproduced to within a to few within percent a few in percent all cases. in all cases. Furthermore, the simulation captures the decrease in the rate almost exactly. What is Furthermore, the simulation captures the decrease in the rate almost exactly. What is important to important to note from the results shown in Figure4 is that all parameters used were determined from note from the results shown in Figure 4 is that all parameters used were determined from the the previous polymerization study, and the ﬁtting of the reaction rate did not employ any additional previous polymerization study, and the fitting of the reaction rate did not employ any additional adjustable parameter. adjustable parameter.

Processes 2017, 5, 21 10 of 19

ProcessesProcesses2017, 52017, 21, 5, 21 10 of 1910 of 19

Figure 4. Comparison of the experimental and simulated rate curves for HEMA polymerization at FiguredifferentFigure 4. Comparison temperatures. 4. Comparison of theof the experimental experimental and and simulatedsimulated rate rate curves curves for for HE HEMAMA polymerization polymerization at at differentdifferent temperatures. temperatures. 3.2. Polymerization Kinetics during the Formation of PΗΕΜΑ/OMMT Nanocomposites 3.2. Polymerization Kinetics during the Formation of PΗΕΜΑ/OMMT Nanocomposites 3.2. Polymerization Kinetics during the Formation of PHEMA/OMMT Nanocomposites Subsequently,Subsequently, the the effect effect of of adding adding aa commercialcommercially-availablely-available nano-OMMT, nano-OMMT, namely namely Cloisite Cloisite 15A, 15A, on theSubsequently,on thepolymerization polymerization the effect kinetics kinetics of adding of of HEMAHEMA a commercially-available atat several several temperatures temperatures was nano-OMMT, was investigated. investigated. namely Nanocomposites Nanocomposites Cloisite 15A, onof the HEMAof polymerization HEMA with with different different kinetics amounts amounts of HEMA ofof nano-montmorillonite at several temperatures were were prepared. was prepared. investigated. Nanocomposites of HEMATheThe withmorphology morphology different amountsof of thethe nanocomposites ofnanocomposites nano-montmorillonite waswas examined examined were prepared.using using X-ray X-ray analysis. analysis. The XRDThe XRD diffractogramsThediffractograms morphology of ofthe the nanocomposites of nanocomposites the nanocomposites with 1, 1, 3 3 wasand and 5 examined5wt% wt% OMMT OMMT using (i.e., (i.e., Cloisite X-ray Cloisite 15A), analysis. 15A), as well as Theas well of XRD as of diffractogramsthe thepristine pristine nano-clay ofnano-clay the nanocomposites appear appear in in FigureFigure with 5.5. ForFor 1, the the 3 andcommercial commercial 5 wt % OMMT, OMMT OMMT, a large (i.e., a large peak Cloisite peak was 15A), measuredwas measured as well at as at of2.99° the2.99° pristine denoting denoting nano-clay a da-spacing d-spacing appear of of 2.97 in2.97 Figure nm,nm, closeclose5. For to to thethe the commercialvalue value re portedreported OMMT, by theby the amanufacturer, large manufacturer, peak was i.e., measured3.15 i.e., nm. 3.15 atnm. 2.99All◦ nanocompositesAlldenoting nanocomposites a d-spacing presented presented of 2.97 clear, clear, nm, but close lowerlower to thein in intention valueintention reported peaks peaks at by2 atθ angles the2θ angles manufacturer, equal equal to 2.07, to 2.21 i.e.,2.07, and 3.15 2.21 nm. and 2.34° for the materials with 1, 3 and 5 wt% OMMT, respectively. These correspond to d001-spacing of All2.34° nanocomposites for the materials presented with 1, clear,3 and but 5 wt% lower OMMT, in intention respectively. peaks atThese 2θ angles correspond equal toto 2.07,d001-spacing 2.21 and of 4.29, 4.02 and 3.79 nm, respectively (Figure 5). All of these values were higher compared to the 2.34◦ for the materials with 1, 3 and 5 wt % OMMT, respectively. These correspond to d -spacing of 4.29,pristine 4.02 and nano-clay. 3.79 nm, This respectively shift suggests (Figurean increase 5). inAll the of basal these spacing values of thewere silicate higher platelets, compared001 which to the 4.29,pristine 4.02is attributed andnano-clay. 3.79 to nm, the This respectively penetration shift suggests (Figureof the an macromolec5 ).increase All of these inular the valueschains basal wereintospacing the higher clayof the comparedplatelets. silicate From platelets, to the these pristine which nano-clay.is attributedobservations This to shift andthe suggests consideringpenetration an increasethat of the the measured in macromolec the basal peaks spacingular are ratherchains of the weak silicateinto and the platelets, broad, clay itplatelets. can which be said is From attributed that these toobservations thethe penetration morphology and of considering of the the macromolecular nanocomposites that the measured produced chains into peakswas the mainly are clay rather intercalated platelets. weak From andand partially thesebroad, observations itexfoliated. can be said and that consideringthe morphologyThe that effect the of of measuredthe the nanocompositesamount peaks of nanofiller are rather produced on weak the variationwa ands mainly broad, of the intercalated it canpolymerization be said and that partiallyrate the and morphology doubleexfoliated. of the nanocompositesbondThe conversioneffect of the producedwith amount time wasat of two nanofiller mainly constant intercalated temperon theatures variation and appears partially of thein exfoliated.Figurespolymerization 6 and 7, respectively. rate and double bondTheThe conversion effectpresence of theof with the amount timenano-clay at of two nanoﬁller was constant found on to temper theenha variationnceatures polymerization appears of the polymerization in kineticsFigures leading 6 and rate 7,to and respectively.higher double bondTheconversion conversionpresence valuesof with the at timenano-clay a specific at two reaction was constant found time. temperatures to enhance polymerization appears in Figures kinetics6 and leading7, respectively. to higher The presence of the nano-clay was found to enhance polymerization kinetics leading to higher conversion values at a specific reaction time. conversion values at a speciﬁc reaction time. PHEMA + 1wt% OMMT 160 PHEMA + 3wt% OMMT PHEMA + 5wt% OMMT PHEMA + 1wt% OMMT 160120 PHEMA + 3wt% OMMT Cloisite 15APHEMA + 5wt% OMMT

12080 Cloisite 15A Intensity (a.u) Intensity

8040 Intensity (a.u) Intensity 0 40 0246810 2è (degress)

0 0246810 2è (degress)

Figure 5. XRD diffraction patterns of the organomodiﬁed montmorillonite (OMMT) (Cloisite 15A) used and the PHEMA/OMMT nanocomposites.

Processes 2017, 5, 21 11 of 19

Processes 2017, 5, 21 11 of 19 ProcessesFigure 2017 5. , XRD5, 21 diffraction patterns of the organomodified montmorillonite (OMMT) (Cloisite 15A)11 of 19 used and the PHEMA/OMMT nanocomposites. Figure 5. XRD diffraction patterns of the organomodified montmorillonite (OMMT) (Cloisite 15A) ProcessesFigure2017, 5 ,5. 21 XRD diffraction patterns of the organomodified montmorillonite (OMMT) (Cloisite 15A) 11 of 19 used and the PHEMA/OMMT nanocomposites. used and the PHEMA/OMMT nanocomposites. 100

100 10080

80 8060

60 60 40

40 40 PHEMA

Conversion (%) 20 PHEMA+1 wt% OMMT PHEMA PHEMA+3 wt% OMMT

Conversion (%) 20 PHEMAPHEMA+1 PHEMA+5 wt% wt% OMMT OMMT

Conversion (%) 20 PHEMA+1 wt% OMMT 0 PHEMA+3 wt% OMMT PHEMA+3PHEMA+5 wt% OMMT 0 5 10 15 20 PHEMA+5 25 wt% 30 OMMT 35 40 0 0 0 5 10 15Time 20 (min) 25 30 35 40 0 5 10 15 20 25 30 35 40 Time (min) Figure 6. Effect of the amount of nano-OMMT on the variation of polymerizationTime (min) rate (a) and Figure 6. Effect of the amount of nano-OMMT on the variation of polymerization rate (a) and conversionFigureFigure 6. 6. Effect( bEffect) with ofof time thethe duringamountamount polymerization ofof nano-OMMTnano-OMMT of onHEMA on the the variationat variation 60 °C. of of polymerization polymerization rate rate (a) (aand) and conversion (b) with time during polymerization of HEMA at 60 °C. conversionconversion (b ()b with) with time time during during polymerization polymerization of HEMA at at 60 60 °C.◦C. 100 100 100 80 80 80 60 60 60 40 40 40 PHEMA Conversion (%) PHEMA Conversion (%) 20 PHEMA PHEMA+1 wt% OMMT Conversion (%) 20 PHEMA+1 PHEMA+3 wt% wt% OMMT OMMT 20 PHEMA+1PHEMA+3 wt% OMMT PHEMA+5 wt% OMMT PHEMA+3PHEMA+5 wt% OMMT 0 PHEMA+5 wt% OMMT 0 0 02468100246810 0246810 TimeTime (min) (min) Time (min) FigureFigure 7. 7.Effect Effect of of the the amount amount of of nano-OMMTnano-OMMT onon the variationvariation ofof polymerizationpolymerization rate rate (a )( aand) and FigureFigure 7. 7.Effect Effect ofof thethe amountamount ofof nano-OMMTnano-OMMT on on the the variation variation of of polymerization polymerization rate rate (a) (aand) and conversionconversion (b )(b with) with time time during during polymerization polymerization ofof HEMAHEMA atat 8282 °C.°C. conversionconversion (b ()b with) with time time during during polymerization polymerization of HEMA at at 82 82 °C.◦C. TheThe effective effective rate rate constants constants were were estimatedestimated at both 60 andand 8282 °C°C according according to to the the procedure procedure The effective rate constants were estimated at both 60 and 82 °C◦ according to the procedure describedThe effective in the rateprevious constants section. were The estimated relative amounts at both of 60 k andeffeff compared 82 C according to that of toneat the PHEMA, procedure describeddescribed in inthe the previous previous section. section. The The relativerelative amountsamounts of kkeff comparedcompared to to that that of of neat neat PHEMA, PHEMA, appear in Figure 8. It was observed that, at relatively low temperatures (i.e., 60 °C), the relative appeardescribedappear in in Figure theFigure previous 8. 8.It Itwas was section. observed observed The relative that,that, atat amounts relativelyrelatively of lowkeff compared temperaturestemperatures to that (i.e., (i.e., of 60 neat60 °C), °C), PHEMA, the the relative relative appear effective rate constant significantly increases with the amount of the nano-OMMT,◦ whereas at higher effectiveineffective Figure rate8. rate It constant was constant observed significantly significantly that, at increases relativelyincreases withwith low temperaturesthethe amount ofof (i.e., thethe nano-OMMT, nano-OMMT, 60 C), the relative whereas whereas effective at athigher higher rate temperatures, a much lower increase is clear. temperatures,constanttemperatures, signiﬁcantly a mucha much increaseslower lower increase increase withthe is is clear. amountclear. of the nano-OMMT, whereas at higher temperatures, a much lower increase is clear.

Figure 8. Relative effective kinetic rate constants obtained during polymerization of PHEMA/n-OMMT

nanocomposites at two temperatures as a function of the amount of n-OMMT. Processes 2017,, 5,, 2121 12 of 19

Figure 8. Relative effective kinetic rate constants obtained during polymerization of Processes 2017, 5, 21 12 of 19 PHEMA/n-OMMT nanocomposites at two temperatures as a function of the amount of n-OMMT.

Furthermore, the overall activation energy of thethe polymerization was calculated according to −1 the procedure procedure described described in in Section Section 3.1, 3.1 and, and a value a value equal equal to 76.5 to 76.5 kJ mol kJ mol−1 for−1 thefor nanocomposite the nanocomposite with with5 wt% 5 wtnano-clay % nano-clay was obtained. was obtained. This is This lower is lowercompared compared to the to corresponding the corresponding neat PHEMA neat PHEMA (i.e., −1 (i.e.,90.6 kJ 90.6 mol kJ−1 mol). In− general,1). In general, one would one would assume assume higher higheractivation activation energy energy of the nanocomposites of the nanocomposites due to thedue extra to the barriers extra barriers that the thatnano-filler the nano-ﬁller adds into adds the polymerizing into the polymerizing mixture. The mixture. reason The for reason the enhanced for the polymerizationenhanced polymerization rate can be rate found can if be one found examines if one polymerization examines polymerization at a microscopic at a microscopic level, presented level, presentedin the Discussion in the Discussion section. section.

3.3. Polymerization Kinetics during the Formation of PHEMA/SilicaPΗΕΜΑ/Silica NanocompositesNanocomposites In this section,section, the effect ofof addingadding differentdifferent amountsamounts ofof nano-silicanano-silica in thethe polymerizationpolymerization of HEMA isis investigated.investigated. The The results results of of the the variation variation of theof the polymerization polymerization rate rate and and conversion conversion with with time timeat 82 at◦C 82 are °C included are included in Figure in Figure9. In contrast 9. In cont torast the nano-OMMT,to the nano-OMMT, adding adding nano-silica nano-silica seems toseems retard to retardthe polymerization the polymerization rate and rate shift and the shift conversion the conversi vs.on time vs. curves time curves to higher to higher reaction reaction times. times. In order In orderto have to safehave results, safe results, the experiments the experiments were were repeated repeated also atalso lower at lower reaction reaction temperatures. temperatures. Results Results are areillustrated illustrated in Figure in Figure 10. It is10. seen It thatis seen at all that different at all reaction different temperatures, reaction temperatures, the effect of nano-silicathe effect onof nano-silicathe polymerization on the polymerization rate is the same. rate The is the reaction same. is Th retarded,e reaction and is retarded, the conversion and the curves conversion are shifted curves to arehigher shifted times to withhigher increasing times with amounts increasing of the amounts nano-silica of the added. nano-silica added.

100

PHEMA Conversion (%) PHEMA Conversion (%) 20 PHEMA/1PHEMA/1 wt%wt% silicasilica PHEMA/3PHEMA/3 wt%wt% silicasilica PHEMA/5PHEMA/5 wt%wt% silicasilica 0 01234567 Time (min) Time (min)

Figure 9.9. EffectEffect of of the the amount amount of nano-silica of nano-silica on the on variation the variation of polymerization of polymerization rate (a) and rate conversion (a) and ◦ (conversionb) with time (b during) with time polymerization during polymerization of HEMA at of 82 HEMAC. at 82 °C.

100 100 100

80 80 80

60 60 60

40 40 40

PHEMA PHEMA Conversion (%) PHEMA Conversion (%) PHEMA Conversion (%) 20 20 Conversion (%) 20 PHEMA/1wt%PHEMA/1wt% silicasilica PHEMA/1PHEMA/1 wt%wt% silicasilica PHEMA/3wt%PHEMA/3wt% silicasilica PHEMA/3PHEMA/3 wt%wt% silicasilica PHEMA/5wt%PHEMA/5wt% silicasilica PHEMA/5PHEMA/5 wt%wt% silicasilica 0 0 0 1020304050 0 20 40 60 80 100 120 Time (min) Time (min) Time (min) Time (min) Figure 10. EffectEffect of of the the amount amount of of nano-silica on on the variation of conversion with time during polymerization of HEMA atat 6060 ((aa)) andand 5252 ◦°CC( (b).

Furthermore, the effective rate constants were estimated at all four temperatures, according to the procedure described in the previous section. The relative amounts of keff compared to that of neat Processes 2017, 5, 21 13 of 19 Processes 2017, 5, 21 13 of 19 Furthermore, the effective rate constants were estimated at all four temperatures, according to theProcesses procedureFurthermore,2017, 5, 21described the effective in the previous rate constants section. were The esrelativetimated amounts at all four of ktemperatures,eff compared to according that of13 neat of to 19 PHEMAthe procedure appear described in Figure in 11. the It previouscan be noticed section. that, The at relativeall reaction amounts temperatures, of keff compared the relative to that effective of neat ratePHEMA constant appear significantly in Figure 11.decreases It can be with noticed the amou that, ntat allof thereaction nano-silica, temperatures, reaching the almost relative 70% effective of the PHEMA appear in Figure 11. It can be noticed that, at all reaction temperatures, the relative effective initialrate constant value when significantly adding 5% decreases of the nano-additive. with the amou nt of the nano-silica, reaching almost 70% of the initialrate constant value when signiﬁcantly adding 5% decreases of the nano-additive. with the amount of the nano-silica, reaching almost 70% of the initial value when adding 5% of the nano-additive.

Figure 11. Relative effective kinetic rate constants obtained during polymerization of Figure 11. Relative effective kinetic rate constants obtained during polymerization of PHEMA/nano-silicaFigure 11. Relative nanocomposites effective kinetic at temperatures rate constants of 52, 60, 72obtained and 82 °Cduring as a function polymerization of the amount of PHEMA/nano-silica nanocomposites at temperatures of 52, 60, 72 and 82 ◦C as a function of the ofPHEMA/nano-silica the additive. nanocomposites at temperatures of 52, 60, 72 and 82 °C as a function of the amount amount of the additive. of the additive. Finally, in order to estimate the effect of the nano-silica on the overall activation energy of the polymerization,Finally, in in order orderArrhenius-type to to estimate estimate plotsthe the effect effectwere of constr of the the nano-silicaucted nano-silica at all on amounts onthe the overall overall of activationthe activation nano-additive energy energy of and the of illustratedpolymerization,the polymerization, in Figure Arrhenius-type Arrhenius-type12. Initially, plots it can plots were be were constrseen constructed uctedthat the at alleffective at amounts all amounts kinetic of the rate of nano-additive the constant nano-additive of andthe nanocompositesillustratedand illustrated in Figure inis always Figure 12. slightly12Initially,. Initially, lower it can itcompared can be beseen seen to that neat that thePHEMA the effective effective at all kinetictemperatures kinetic rate rate constant constantand decreased of of the withnanocomposites the amount is of always the nano-silica. slightly lower Moreover, compared the to activation neat PHEMA energies at all all temperatures temperaturesestimated were and 90.6 decreased decreased ± 2.4, 90.8with ± the3.9 and amount 91.2 of± 3.2 thethe kJ/mol nano-silica.nano-silica. for the Moreover,nanocomposites the activation with 1, 3 and energies 5 wt% estimated nano-silica, were respectively. 90.6 ±± 2.4,2.4, These90.8 ± values3.93.9 andand are 91.2 91.2 similar ±± 3.23.2 kJ/mol to kJ/mol neat for PHEMA for the the nanocomposites nanocomposites (i.e., 90.6 kJ/mol) with with showing 1, 1, 3 3and and a5 5 slightlywt% wt % nano-silica, nano-silica, increasing respectively. respectively.trend with theThese amount values of arethe similarnano-silica to neatneat added. PHEMAPHEMA (i.e.,(i.e., 90.690.6 kJ/mol)kJ/mol) showing showing a a slightly increasing trend with the amount of the nano-silica added.

FigureFigure 12. 12. Arrhenius-typeArrhenius-type plots plots toto calculate calculate the the ov overallerall (effective) (effective) activation activation energies energies of of PHEMA/nano-silicaFigurePHEMA/nano-silica 12. Arrhenius-type nanocomposites. nanocomposites. plots to calculate the overall (effective) activation energies of PHEMA/nano-silica nanocomposites.

Processes 2017, 5, 21 14 of 19

4. DiscussionProcesses 2017, 5, 21 14 of 19 4. DiscussionAlthough the polymerization kinetics of HEMA have been investigated in the literature [12–17], the developmentAlthough the of a polymerization model, including kinetics both kineticof HEMA and ha diffusion-controlledve been investigated parametersin the literature that have[12–17], been estimatedthe development from experimental of a model, measurements, including both valid kinetic over and the diffusion-controlled whole conversion range,parameters has not that appeared have sobeen far. In estimated this framework, from experimental in the first partmeasurements, of this research, valid anover accurate, the whole rather conversion simple, kineticrange, modelhas not for theappeared polymerization so far. In of this HEMA framework, was provided, in the first based parton of this experimental research, an DSC accurate, measurements. rather simple, The variationkinetic ofmodel the polymerization for the polymerization rate with of timeHEMA at was several prov isothermalided, based reactionon experimental temperatures DSC measurements. was presented, andThe kinetic variation rate constantsof the polymerization and diffusion-controlled rate with time parameters at several wereisothermal evaluated. reaction It was temperatures found that was using thesepresented, parameters and thekinetic evolution rate constants of the experimental and diffusio polymerizationn-controlled parameters rate with time were can evaluated. be simulated It was very well.found All that parameters using these estimated parameters at different the evolution reaction of temperatures the experimental are includedpolymerization in Table rate1. Thesewith time could becan used be in simulated the development very well. of All more parameters complex estimate modelsd for at different the simulation reaction of temperatures the polymerization are included kinetics of HEMAin Table and1. These its copolymers. could be used in the development of more complex models for the simulation of the polymerizationIn the second kinetics part, nanocomposites of HEMA and its of copolymers. PHEMA with several relative amounts of nano-clay and nano-silicaIn the were second prepared part, nanocomposites in order to investigate of PHEMA theeffect with several of the nano-additive relative amounts on theof nano-clay polymerization and kineticsnano-silica of HEMA. were prepared The in situ in order bulk to radical investigate polymerization the effect of the technique nano-additive was investigated. on the polymerization Isothermal kinetics of HEMA. The in situ bulk radical polymerization technique was investigated. Isothermal experimental data were obtained from DSC measurements. It was found that the inclusion of the experimental data were obtained from DSC measurements. It was found that the inclusion of the nano-montmorillonite results in a slight enhancement of the polymerization rate, while the inverse nano-montmorillonite results in a slight enhancement of the polymerization rate, while the inverse holds when adding nano-silica. holds when adding nano-silica. The interpretation of these results can be carried out in terms of specific interactions and The interpretation of these results can be carried out in terms of specific interactions and particularlyparticularly the the formation formation ofof intra- and and inter-chain inter-chain hydrogen hydrogen bonds bonds between between the monomer the monomer and the and thepolymer polymer molecules. molecules. The The monomer, monomer, 2-hydroxyethyl 2-hydroxyethyl methacrylate, methacrylate, contains contains one hydroxyl one hydroxyl (–OH) (–OH)and andone one carbonyl carbonyl (C=O) (C=O) group group on its on molecule. its molecule. The C=O The group C=O acts group only as acts the only proton as acceptor, the proton while acceptor, the whileOH thegroup OH acts group as both acts the as proton both thedonor proton and acceptor donor and [26]. acceptor Hydrogen [26 bonding]. Hydrogen between bonding the monomer between thehydroxyl monomer group hydroxyl and carbonyl group oxygen and carbonyl atom strengthen oxygen atoms the positive strengthens partial the charges positive at the partial carbonyl charges C atatom the carbonyl and at the C double atom and bond, at theas shown double schematicall bond, as showny in Scheme schematically 1, leading into Schemea significant1, leading charge to a significanttransfer in chargethe transition transfer state in of the propagation transition state[27]. In of the propagation polymer, PHEMA, [27]. In theboth polymer, OHOH PHEMA, and bothC =OHO···HOOH typesand Cof =hydrogen-bondsO ··· HO types can of hydrogen-bondsoccur (Scheme 1). can Not occur only (Scheme the dimer1). Notstructure only the( dimerOH structure OH ), but (OH also··· theOH aggregate··· ), but alsostructure the aggregate( OHOH structureOH (···) haveOH ···beenOH found··· OH in··· many) have beensystems, found including in many liquid systems, alcohols including and solid liquid polymers alcohols [26]. and It solidhas been polymers found [that26]. 53.7% It has of been the foundOH thatgroup 53.7% on of the the PHEMA OH group side on chain the PHEMA terminal side contributes chain terminal to the OH contributesOH type to the of OHhydrogen-bond,··· OH type of = hydrogen-bond,while the remaining while the47.3% remaining are engaged 47.3% in are the engaged OHO in theC OHtype··· of Ohydrogen= C type bond, of hydrogen at ambient bond, at ambienttemperature temperature [26]. [26].

O C CH 3 O O C H2 H2C C O C O C O CH O C C CH 2 2 CH2 O H2 HO CH2 H O H2C O C H C OH O 2 O H OH H2C H2 O H C C C O CH2 2 C O C HO H C C O C 2 O O H2 HO CH2 H2C CH3 CH2 HO

SchemeScheme 1. 1.Schematic Schematicillustration illustration ofof thethe polymerizationpolymerization of of HEMA HEMA to to PHEMA PHEMA and and the the intra- intra- and and inter-chaininter-chain hydrogen hydrogen bonds bonds formed. formed.

Processes 2017, 5, 21 15 of 19 Processes 2017, 5, 21 15 of 19 When the nano-clay, OMMT, is added to the system, the clay platelets are partially exfoliated due toWhen the ultrasound the nano-clay, agitation OMMT, and is polymerization, added to the system, by the insertion the clay plateletsof the macromolecular are partially exfoliated chains in betweendue to the the ultrasound clay galleries, agitation as was and observed polymerization, from XRD by the measurements. insertion of the Then, macromolecular the HEMA-HEMA chains interactionsin between thewith clay the galleries, hydrogen as bonding was observed are disrupted, from XRD by measurements.the existence of the Then, clay the platelets HEMA-HEMA inserted ininteractions between withthe themonomer hydrogen molecules bonding areand disrupted, macromolecular by the existence chains, ofas the shown clay platelets schematically inserted in Schemebetween 2. the This monomer could result molecules in more and reactive macromolecular monomer molecules chains, as shownduring schematicallypolymerization in that Scheme could2. Thisfacilitate could the result reaction in more rate, reactive resulting monomer in higher molecules kinetic rate during constants polymerization and less overall that couldactivation facilitate energy. the Thereaction disruption rate, resulting of hydrogen in higher bonding kinetic between rate constants HEMA and molecules less overall is thus activation consistent energy. with The the disruption increased rateof hydrogen of monomer bonding addition between to the HEMA macroradicals molecules comp is thusared consistent to that within neat the polymerization. increased rate of Moreover, monomer ataddition high temperatures, to the macroradicals monomer compared molecules to that and in neat macroradicals polymerization. have Moreover, enough atmobility, high temperatures, and their movementmonomer moleculesis not affected and macroradicalsmuch by the havepresence enough of the mobility, side hydrogen and their bonds. movement However, is not at affected lower temperatures,much by the presence hydrogen of the bonding side hydrogen significantly bonds. decr However,eases the at lower reactivity temperatures, of radicals hydrogen and monomer bonding molecules,signiﬁcantly and decreases as a result, the reactivitythe presence of radicals of nano-c andlays, monomer which disrupt molecules, those and bonds, as a result, contributes the presence much toof a nano-clays, higher polymerization which disrupt rate those (Figure bonds, 7). contributes much to a higher polymerization rate (Figure7).

Scheme 2. Schematic illustration of the polymerization of HEMA to PHEMA in the presence of Scheme 2. Schematic illustration of the polymerization of HEMA to PHEMA in the presence of nano-OMMT. nano-OMMT.

In contrast, polymerization of of HEMA HEMA in in the presence of nano-silica resulted in retarded polymerization rates and lower effective kinetic ra ratete constants compared to neat PHEMA. It seems that the presence of hydroxyl groups in the surfacesurface of the nano-silica results in the formation of hydrogen bonds between the polymer and the nano-additive, resulting in lower reactivity of the monomer. Intra-molecular HEMA-HEMA hydrogen bonding interactions are disturbed and replaced by inter-molecular hydrogen bonds between the hydroxyls in the surface of silica with carbonyls and/or hydroxyls present in the PHEMA macromolecular chain, as shown schematically in

Processes 2017, 5, 21 16 of 19

Processes 2017, 5, 21 16 of 19 and/or hydroxyls present in the PHEMA macromolecular chain, as shown schematically in Scheme3. ThisScheme gives 3. riseThis to gives a reduction rise to ina reduction the polymerization in the polymerization rate compared rate to neatcompared HEMA, to more neat pronouncedHEMA, more as pronouncedthe amount of as nano-silica the amount is increased.of nano-silica It should is increased. be noted It here should that similarbe noted results here havethat similar been observed results haveduring been polymerization observed during of HEMA polymerization in polar solvents, of HEMA such in aspolar DMF solvents, [28]. such as DMF [28].

HEMA + Silica OH CH3 HO OH H2 C O C HO OH

O C C CH2 H2 HO OH H O OH OH O H HO OH H2 H C C C O 2 HO OH C O C H2 HO OH CH3 OH

O C

O O C H2C O C O CH2 CH2 O HO CH OH H C 2 2 O C HO OH H2C OH O OH H2C OH HO CH2 HO HO OH OH OH HO OH O C O C HO OH H C 2 O O HO HO OH CH2 H2C OH CH2 HO

Scheme 3. 3. SchematicSchematic illustration illustration of the of polymerization the polymerization of HEMA of HEMAto PHEMA to PHEMAin the presence in the of presence nano-silica. of nano-silica. The above assumptions, concerning hydrogen bonding of PHEMA during polymerization, were partiallyThe verified above assumptions,by FTIR measurements. concerning The hydrogen FTIR spectra bonding of neat of PHEMA and during the nanocomposites polymerization, were partiallyrecorded, verified and details by FTIR in measurements.the regions of intere The FTIRst are spectra shown of in neat Figure PHEMA 13. Particularly, and the nanocomposites Figure 13a wereshows recorded, the IR spectra and details in the inO–H the stretching regions of region interest, whereas are shown Figure in Figure 13b shows 13. Particularly, the corresponding Figure 13 IRa spectrashows thein the IR spectraC=O stretching in the O–H region. stretching region, whereas Figure 13b shows the corresponding IR spectraSeveral in the contributions C=O stretching from region. 3100–3700 cm−1 are identified in the O–H stretching region. According to MoritaSeveral et al. contributions [26], the band from at 3536 3100–3700 cm−1 is cm attributed−1 are identiﬁed to hydrogen-bonded in the O–H stretching hydroxyl region. groups, According whereas atto 3624–3660 Morita et cm al.−1 [ 26to ],free the OH. band According at 3536 to cm Figure−1 is 13a, attributed signals toat the hydrogen-bonded latter region were hydroxyl not identified groups, in whereasany material at 3624–3660 investigated. cm− 1Therefore,to free OH. it Accordingseems that to there Figure are 13 nota, signalsmany free at the hydroxyls, latter region i.e., were the OH not groupsidentiﬁed are innot any donating material hydrogen investigated. bonds. Therefore, Moreover, it a seems dominant that therepeak at are 3536 not cm many−1 was free recorded hydroxyls, for −1 neati.e., thePHEMA, OH groups meaning are not donatingthe existence hydrogen of hydr bonds.ogen-bonded Moreover, ahydroxyls. dominant peakIn the at 3536PHEMA/SiO cm was2 nanocomposites, this peak was shifted to lower wavenumber at 3430 cm−1, providing evidence of the gradual association of the OHOH type of hydrogen bonds with the addition of the nanosilica.

Processes 2017, 5, 21 17 of 19

recorded for neat PHEMA, meaning the existence of hydrogen-bonded hydroxyls. In the PHEMA/SiO2 Processes 2017, 5, 21 17 of 19 nanocomposites, this peak was shifted to lower wavenumber at 3430 cm−1, providing evidence of the gradual association of the OH ··· OH type of hydrogen bonds with the addition of the nanosilica. In the C=O stretching region, two contributions around 1730 and 1637 cm−1 were identified. In the C=O stretching region, two contributions around 1730 and 1637 cm−1 were identified. Again, these bands are assigned to free C=O groups and hydrogen-bonded carbonyl groups, Again, these bands are assigned to free C=O groups and hydrogen-bonded carbonyl groups, respectively [26]. The peak position and width of the band around 1730 cm−1 slightly changed with respectively [26]. The peak position and width of the band around 1730 cm−1 slightly changed with the the addition of the nanofiller, whereas for the band around 1637 cm−1, the position again slightly addition of the nanofiller, whereas for the band around 1637 cm−1, the position again slightly changed, changed, although its width varied significantly. Particularly, the peak at 1637 cm−1 almost although its width varied significantly. Particularly, the peak at 1637 cm−1 almost disappeared at the disappeared at the PHEMA/OMMT nanocomposite, whereas it became very broad and intensive in PHEMA/OMMT nanocomposite, whereas it became very broad and intensive in the PHEMA/silica the PHEMA/silica materials. Therefore, it seems that only a small number of hydrogen bonded C=O materials. Therefore, it seems that only a small number of hydrogen bonded C=O appears in neat appears in neat PHEMA, which is negligible in PHEMA/OMMT nanocomposites. In contrast, in the PHEMA, which is negligible in PHEMA/OMMT nanocomposites. In contrast, in the C=O stretching C=O stretching region, it was found that the association of the C = OHO -type of hydrogen bond region, it was found that the association of the C = O ··· HO-type of hydrogen bond occurs to a large occurs to a large extent when silica is used. extent when silica is used.

100 100 PHEMA PHEMA+ 3wt% OMMT PHEMA+3 wt% SiO2 80 80

60 60

40 3536 40 1637 1730 Transmittance (%)

20 Transmittance (%) 20 PHEMA PHEMA+ 3wt% OMMT 3430 PHEMA+3 wt% SiO2 0 0 3600 3300 3000 2700 2100 1800 1500 -1 -1 Wavenumber (cm ) Wavenumber (cm )

FigureFigure 13. 13.Details Details of of the the FTIR FTIR spectra spectra of of neat neat PHEMA PHEMA and and PHEMA PHEMA nanocomposites nanocomposites with with 3 wt3 wt% % −1 −1 OMMTOMMT or or 3 3 wt wt% % silica silica in in the the region region 3750–2700 3750–2700 cm cm−1 ((a)) andand 2100–15002100–1500 cmcm−1 (b(b).).

5.5. Conclusions Conclusions InIn this this study, study, the the polymerization polymerization kinetics kinetics of of PHEMA PHEMA was was studied studied both both experimentally experimentally using using DSCDSC measurements measurements and and theoretically. theoretically. A A simplified simplified kinetic kinetic model model was was developed developed taking taking into into account account bothboth the the chemical chemical reactions reactions and diffusion-controlledand diffusion-controlled phenomena phenomena and valid and over valid the whole over conversionthe whole range.conversion Kinetic range. and diffusion-controlled Kinetic and diffusion-controlled parameters were parameters evaluated, were and theevaluated, model was and able the to model simulate was successfullyable to simulate the variation successfully of the the polymerizationvariation of the ratepolymerization with time. rate The with main time. conclusion The main drawn conclusion from thedrawn study from on the the formation study on ofthe nanocomposite formation of nanoco materialsmposite based materials on PHEMA based is that on thePHEMA polymerization is that the ratepolymerization can be either rate enhanced can be oreither decreased enhanced depending or decreased on the depending type and on the the amount type and of thethe nanofilleramount of used.the nanofiller Thus, the used. effective Thus, overall the effective kinetic rateoverall constant kineti wasc rate found constant to increase was found with to the increase amount with of the the nano-filleramount of when the anano-filler nano-clay when was used, a nano-clay whereas thewas inverse used, whereas was observed the inverse when nano-silica was observed was used.when Itnano-silica seems that nano-compoundswas used. It seems having that surface nano-compounds functional groups, having such surface as hydroxyls functional in thegroups, case of such silica, as mayhydroxyls form hydrogen in the case bonds of silica, with themay carbonyls form hydrogen or hydroxyl bonds groups with the of thecarbonyls polymer, orresulting hydroxyl somehow groups of inthe the polymer, retardation resulting of the reaction. somehow In contrast,in the retardation the addition of ofthe clay reaction. platelets In maycontrast, result the in theaddition dissociation of clay ofplatelets hydrogen may bonds result existing in the in thedissociation polymer molecules,of hydrogen resulting bonds thusexisting in slightly in the higher polymer reaction molecules, rates. resulting thus in slightly higher reaction rates. Author Contributions: D.S.A. and P.I.S. conceived of and designed the experiments. P.I.S. performed the experiments. D.S.A. wrote the paper and performed the simulation analysis. Author Contributions: D.S.A. and P.I.S. conceived of and designed the experiments. P.I.S. performed the Conflictsexperiments. of Interest: D.S.A. Thewrote authors the paper declare and no performed conflict of the interest. simulation analysis.

ReferencesConflicts of Interest: The authors declare no conflict of interest.

1.ReferencesNogueira, N.; Conde, O.; Minones, M.; Trillo, J.M.; Minones, J.R. Characterization of poly(2-hydroxyethyl methacrylate) contact lens using the Langmuir monolayer technique. J. Colloid Interface Sci. 2012, 385, 1. 202–210.Nogueira, [CrossRef N.; Conde,][PubMed O.; Minones,] M.; Trillo, J.M.; Minones, J.R. Characterization of poly(2-hydroxyethyl methacrylate) contact lens using the Langmuir monolayer technique. J. Colloid Interface Sci. 2012, 385, 202–210. 2. Montheard, J.P.; Chatzopoulos, M.; Chappard, D. 2-Hydroxyethyl methacrylate (HEMA): Chemical properties and applications in biomedical fields. J. Macromol. Sci. C Polym. Rev. 1992, 32, 1–34.

Processes 2017, 5, 21 18 of 19

2. Montheard, J.P.; Chatzopoulos, M.; Chappard, D. 2-Hydroxyethyl methacrylate (HEMA): Chemical properties and applications in biomedical ﬁelds. J. Macromol. Sci. C Polym. Rev. 1992, 32, 1–34. [CrossRef] 3. Gupta, M.K.; Bajpai, J.; Bajpai, A.K. Optimizing the release process and modelling of in vitro release data of cis-dichlorodiamminoplatinum (II) encapsulated into poly(2-hydroxyethyl methacrylate) nanocarriers. Mater. Sci. Eng. C 2016, 58, 852–862. [CrossRef][PubMed] 4. Costantini, A.; Luciani, G.; Annunziata, G.; Silvestri, B.; Branda, F. Swelling properties and bioactivity of silica gel/pHEMA Nanocomposites. J. Mater. Sci. Mater. Med. 2006, 17, 319–325. [CrossRef][PubMed] 5. Kharismadewi, D.; Haldorai, Y.; Nguyen, V.H.; Tuma, D.; Shim, J.-J. Synthesis of graphene

oxide-poly(2-hydroxyethyl methacrylate) composite by dispersion polymerization in supercritical CO2: Adsorption behavior for the removal of organic dye. Compos. Interfaces 2016, 23, 7. [CrossRef] 6. Moradi, O.; Aghaie, M.; Zare, K.; Monajjemi, M.; Aghaie, H. The study of adsorption characteristics Cu2+ and Pb2+ ions onto PHEMA and P(MMA-HEMA) surfaces from aqueous single solution. J. Hazard. Mater. 2009, 170, 673–679. [CrossRef][PubMed] 7. Bolbukh, Y.; Klonos, P.; Roumpos, K.; Chatzidogiannaki, V.; Tertykh, V.; Pissis, P. Glass transition and hydration properties of polyhydroxyethylmethacrylate filled with modified silica nanoparticles. J. Therm. Anal. Calorim. 2016, 125, 1387–1398. [CrossRef] 8. Nikolaidis, A.K.; Achilias, D.S.; Karayannidis, G.P. Synthesis and characterization of PMMA/organomodified montmorillonite nanocomposites prepared by in situ bulk polymerization. Ind. Eng. Chem. Res. 2011, 50, 571–579. [CrossRef] 9. Achilias, D.S.; Siafaka, P.I.; Nikolaidis, A.K. Polymerization kinetics and thermal properties of poly(alkyl methacrylate)/organomodifiend montmorillonite nanocomposites. Polym. Int. 2012, 61, 1510–1518. [CrossRef] 10. Passos, M.F.; Dias, D.R.C.; Bastos, G.N.T.; Jardini, A.L.; Benatti, A.C.B.; Dias, C.G.B.T.; Maciel Filho, R. PHEMA Hydrogels. J. Therm. Anal. Calorim. 2016, 125, 361–368. [CrossRef] 11. Ning, L.; Xu, N.; Xiao, C.; Wang, R.; Liu, Y. Analysis for the reaction of hydroxyethyl methacrylate/benzoyl peroxide/polyethacrylate through DSC and viscosity changing and their resultants as oil absorbent. J. Macromol. Sci. Part A Pure Appl. Chem. 2015, 52, 1017–1027. [CrossRef] 12. Huang, C.-W.; Sun, Y.-M.; Huang, W.-F. Curing kinetics of the synthesis of poly(2-hydroxyethyl methacrylate) (PHEMA) with ethylene glycol dimethacrylate (EGDMA) as a crosslinking agent. J. Polym. Sci. A Polym. Chem. 1997, 35, 1873–1889. [CrossRef]

13. Kaddami, H.; Gerard, J.F.; Hajji, P.; Pascault, J.P. Silica-ﬁlled poly(hema) from hema/grafted SiO2 nanoparticles: Polymerization kinetics and rheological changes. J. Appl. Polym. Sci. 1999, 73, 2701–2713. [CrossRef] 14. Li, L.; Lee, L.J. Photopolymerization of HEMA/DEGDMA hydrogels in solution. Polymer 2005, 46, 11540–11547. [CrossRef] 15. Mikos, A.G.; Peppas, N.A. A model for prediction of the structural characteristics of EGDMA crosslinked PHEMA microparticles produced by suspension copolymerization/crosslinking. J. Controll. Release 1987, 5, 53–62. [CrossRef] 16. Goodner, M.D.; Lee, H.R.; Bowman, C.N. Method for determining the inetic parameters in diffusion-controlled free-radical homopolymerizations. Ind. Eng. Chem. Res. 1997, 36, 1247–1252. [CrossRef] 17. Hacioglu, B.; Berchtold, K.A.; Lovell, L.G.; Nie, J.; Bowman, C.N. Polymerization kinetics of HEMA/DEGDMA: using changes in initiation and chain transfer rates to explore the effects of chain-length-dependent termination. Biomaterials 2002, 23, 4057–4064. [CrossRef] 18. Achilias, D.S. Investigation of the radical polymerization kinetics using DSC and mechanistic or isoconversional methods. J. Therm. Anal. Calorim. 2014, 116, 1379–1386. [CrossRef] 19. Siafaka, P.; Achilias, D.S. Polymerization kinetics and thermal degradation of poly(2-hydroxyethyl methacylate)/organomodiﬁed montmorillonite nanocomposites prepared by in situ bulk polymerization. Macromol. Symp. 2013, 331–332, 166–172. [CrossRef] 20. Achilias, D.S. A review of modeling of diffusion controlled polymerization reactions. Macromol. Theory Simul. 2007, 16, 319–347. [CrossRef] 21. Achilias, D.S.; Verros, G.D. Modeling of Diffusion-Controlled Reactions in Free Radical Solution and Bulk Polymerization: Model Validation by DSC Experiments. J. Appl. Polym. Sci. 2010, 116, 1842–1856. [CrossRef] Processes 2017, 5, 21 19 of 19

22. Buback, M.; Kurz, C.H. Free-radical propagation coefﬁcients for cyclohexyl methacrylate, glycidyl methacrylate and 2-hydroxyethyl methacrylate homopolymerizations. Macromol. Chem. Phys. 1998, 199, 2301–2310. [CrossRef] 23. Siddiqui, M.N.; Redhwi, H.H.; Vakalopoulou, E.; Tsagkalias, I.; Ioannidou, M.D.; Achilias, D.S. Synthesis, characterization and reaction kinetics of PMMA/silver nanocomposites prepared via in situ radical polymerization. Eur. Polym. J. 2015, 72, 256–269. [CrossRef] 24. Zoller, A.; Gigmes, D.; Guillaneuf, Y. Simulation of radical polymerization of methyl methacrylate at room temperature using a tertiary amine/BPO initiating system. Polym. Chem. 2015, 6, 5719–5727. [CrossRef] 25. Achilias, D.S.; Kiparissides, C. Development of a general mathematical framework for modeling of diffusion-controlled free-radical polymerization reactions. Macromolecules 1992, 25, 3739–3750. [CrossRef] 26. Morita, S. Hydrogen-bonds structure in poly(2-hydroxyethyl methacrylate) studied by temperature- dependent infrared spectroscopy. Front. Chem. 2014, 2, 1–5. [CrossRef][PubMed] 27. Liang, K.; Hutchinson, R.A. Solvent Effects on Free-Radical Copolymerization Propagation Kinetics of Styrene and Methacrylates. Macromolecules 2010, 43, 6311–6320. [CrossRef] 28. Furuncuoglu Ozaltin, T.; Dereli, B.; Karahan, O.; Salman, S.; Aviyente, V. Solvent effects on free-radical copolymerization of styrene and 2-hydroxyethyl methacrylate: A DFT study. New J. Chem. 2014, 38, 170–178. [CrossRef]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).