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Polymerization Kinetics of Cyanate Ester Confined to Hydrophilic Nanopores of Silica Colloidal Crystals with Different Surface-G

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Polymerization Kinetics of Cyanate Ester Confined to Hydrophilic Nanopores of Silica Colloidal Crystals with Different Surface-G polymers Article Polymerization Kinetics of Cyanate Ester Confined to Hydrophilic Nanopores of Silica Colloidal Crystals with Different Surface-Grafted Groups Andrey Galukhin 1,* , Guzel Taimova 1, Roman Nosov 1, Tatsiana Liavitskaya 2 and Sergey Vyazovkin 1,2,* 1 Alexander Butlerov Institute of Chemistry, Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russia; [email protected] (G.T.); [email protected] (R.N.) 2 Department of Chemistry, University of Alabama at Birmingham, 901 S. 14th Street, Birmingham, AL 35294, USA; [email protected] * Correspondence: [email protected] (A.G.); [email protected] (S.V.) Received: 29 September 2020; Accepted: 11 October 2020; Published: 12 October 2020 Abstract: This study investigates the kinetics of confined polymerization of bisphenol E cyanate ester in the nanopores of the three types of silica colloidal crystals that differ in the concentration and acidity of the surface-grafted proton-donor groups. In all three types of pores, the polymerization has released less heat and demonstrated a very similar significant acceleration as compared to the bulk process. Isoconversional kinetic analysis of the differential scanning calorimetry measurements has revealed that the confinement causes not only a dramatic change in the Arrhenius parameters, but also in the reaction model of the polymerization process. The obtained results have been explained by the active role of the silica surface that can adsorb the residual phenols and immobilize intermediate iminocarbonate products by reaction of the monomer molecules with the surface silanols. The observed acceleration has been quantified by introducing a new isoconversional-isothermal acceleration factor Zα,T that affords comparing the process rates at respectively identical conversions and temperatures. In accord with this factor, the confined polymerization is 15–30 times faster than that in bulk. Keywords: cyanate esters; polymerization kinetics; confinement; colloidal crystals; thermal analysis; isoconversional kinetic analysis 1. Introduction Exploring chemical reactions under confinement, e.g., inside nanopores [1–3], nanobrushes [4], or self-assembled monolayers [5–8], has been of major research interest in the areas of catalysis, polymer, supramolecular, and biochemistry. Studying the effects of confinement on the reactivity is a key to understanding of biological systems, where many processes in living cells occur in a confined space [9], as well as chemical systems, where various novel materials are created under spaciously confined conditions that give rise to unusual properties and chemical mechanisms [10]. Confined polymerization attracts research attention as a means of controlling the properties of polymers, such as molecular weight distribution [11–13], crystallinity [14–16], tacticity [13,17,18], and glass transition temperature [19–21]. Currently, the most studied type of confined polymerization is radical polymerization. For this type of polymerization the changes in the reactivity of monomers have been explained as the effects of confinement on the rates of the initiation [11] and termination [17,22]. Polycyclotrimerization of cyanate esters is another type of reaction for which the effect of confinement on both reactivity of the reactant monomer and product polymer has been well documented [23–28]. Polymers 2020, 12, 2329; doi:10.3390/polym12102329 www.mdpi.com/journal/polymers Polymers 2020, 12, 2329 2 of 17 Polymers 2020, 12, x FOR PEER REVIEW 2 of 17 Foracceleration these reactions, in terms there of the have reaction been no mechanisms. specific studies The ofincreased the significant reactivity acceleration of monomers in terms has of been the reactionattributed mechanisms. mostly to increased The increased collision reactivity efficiency of monomersinduced by hasthe beennanopore attributed surface mostly in the to layer increased of the collisionadsorbed e ffimonomerciency induced [25,26,28]. by the Although nanopore surfacethe effect in the of layer the ofsurface the adsorbed silanols monomer groups [ 25on,26 ,28the]. Althoughpolymerization the eff ofect cyanate of the surfaceesters has silanols been invoked, groups on it has the polymerizationnever been studied of cyanate in detail esters [29,30]. has been invoked,This itcurrent has never study been focuses studied on in the detail thermal [29,30 polymerization]. of bisphenol E cyanate ester under both Thisbulk currentand confined study focusesconditions. on theSilica thermal colloidal polymerization crystals (SCCs) of are bisphenol used as E a cyanateconfining ester medium. under bothTo the bulk best and of confinedour knowledge, conditions. this Silicais the colloidal first study crystals that (SCCs)makes areuse usedof SCC as ato confining explore medium.confined Tochemical the best reactions. of our knowledge, Chemical this modification is the first studyof SCC that is makesused to use reveal of SCC the to influence explore confined of the nature chemical of reactions.surface-grafted Chemical proton modification-donor groups of SCC on is usedthe reactivity to reveal theof influencethe confined of the monomer. nature of surface-graftedThe resulting proton-donorreactions have groups been studied on the by reactivity differential of the scanning confined calorimetry monomer. (DSC) The resulting, and their reactions kinetics have has been studiedanalyzed by by di meansfferential of scanningthe isoconversional calorimetry methodology (DSC), and their [31]. kinetics The analysis has been reveals analyzed that confinement by means of thecauses isoconversional a change of the methodology reaction kinetics [31]. The and analysis mechanisms reveals of thatpolymerization confinement that causes has abeen change linked of the to reactionthe surface kinetics silanol and groups mechanisms participating of polymerization in the polymerization that has been process. linked to the surface silanol groups participating in the polymerization process. 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials Ammonium hydroxide solution (25% of NH3, TatChemProduct, Kazan, Russia), tetraethylorthosilicateAmmonium hydroxide (TEOS, solution >99.9%, (25% of NHALDRICH3, TatChemProduct, Chemistry Kazan,, Saint Russia), Louis, tetraethylorthosilicate MI, USA), (TEOS,tetrabutylammonium>99.9%, ALDRICH hydroxide Chemistry, solution Saint (40% Louis, in water, MI, USA), Fluka tetrabutylammonium Analytical, Saint Louis, hydroxide MI, USA solution), 1,3- (40%propanesultone in water, Fluka (98%, Analytical, ALDRICH Saint Chemistry Louis, MI,, USA),Saint 1,3-propanesultoneLouis, MI, USA), (98%,bisphenol ALDRICH E (>98%, Chemistry, TCI, SaintZwijndrecht, Louis, MI, Belgium USA), bisphenol), phosphorus E (>98%, pentoxide TCI, Zwijndrecht, (99%, Vect Belgium),on, Saint phosphorus Petersburg, pentoxide Russia (99%,), n-hexane Vecton, Saint(>98%, Petersburg, EKOS-1, Moscow, Russia), n -hexaneRussia), (>toluene98%, EKOS-1, (99.5%, Moscow, EKOS-1, Russia), Moscow, toluene Russia (99.5%,), and EKOS-1,tetrahydrofuran Moscow, Russia),(THF, >98%, andtetrahydrofuran ChemMed, Kazan, (THF, Russia>98%,) ChemMed,were purchased Kazan, and Russia) used were without purchased additional and usedpurification. without additionalBisphenol purification.E cyanate ester Bisphenol of purity E cyanate99.5% was ester synthesized of purity 99.5% according was synthesizedto the previously according described to the previouslysynthetic procedure described synthetic(Figure 1) procedure [32,33]. The (Figure purity1) [ 32of, 33synthesized]. The purity monomer of synthesized was no monomer less than was 99.5% no lessaccording than 99.5% to high according-performance to high-performance liquid chromatography liquid chromatography (HPLC). Absolute (HPLC). ethanol Absolute was obtained ethanol was by obtainedconsecutive by consecutivedistillations distillations of 96% ethanol of 96% over ethanol CaO and over CaH CaO2. and Deionized CaH2. Deionized water (18.2 water MΩ) (18.2 was MobtainedW) was obtainedby Arium by mini Arium instrument mini instrument (Sartorius (Sartorius,, Goettingen, Goettingen, Germany Germany).). Figure 1. Scheme of synthesis of target cyanate ester. 2.2. Methods Scanning electronelectron microscopy microscopy (SEM) (SEM) measurements measurements were were carried carried out using out ausing Merlin a field-emissionMerlin field- high-resolutionemission high-resolution scanning electronscanning microscope electron microscope (Carl Zeiss, (Carl Oberkochen, Zeiss, Oberkochen, Germany). Germany) An UP200Ht. An ultrasonicUP200Ht ultrasonic homogenizer homogenizer (Hielscher ( Ultrasonics,Hielscher Ultrasonics, Teltow, Germany) Teltow, wasGermany used) forwas all used sonications. for all Asonications. MF48 centrifuge A MF48 (AWEL, centrifuge Blain, (AWEL France), Blain, was France used) forwas all used centrifugations. for all centrifugations. A LF-7/11-G1 A LF- furnace7/11-G1 (furnaceLOIP, Saint (LOIP, Petersburg, Saint Petersburg, Russia) was Russia used) was for calcinationused for calcination and sintering. and sintering. Contact angles Contact were angles measured were measured using a drop shape analyzer DSA100 (KRUSS GmbH, Hamburg, Germany); the water drop Polymers 2020, 12, 2329 3 of 17 using a drop shape analyzer DSA100 (KRUSS GmbH, Hamburg, Germany); the water drop volume was 2 µL. High-performance liquid chromatography (HPLC) analyses were carried out with a Dionex Ultimate 3000 chromatograph (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an UV detector and a Dionex Acclaim 120 chromatographic
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