Synthesis of Network Polymers by Means of Addition Reactions of Multifunctional-Amine and Poly(Ethylene Glycol) Diglycidyl Ether Or Diacrylate Compounds

polymers

Article Synthesis of Network Polymers by Means of Addition Reactions of Multifunctional-Amine and Poly(ethylene glycol) Diglycidyl Ether or Diacrylate Compounds

Naofumi Naga 1,2,* , Mitsusuke Sato 2, Kensuke Mori 1, Hassan Nageh 3 and Tamaki Nakano 3,4

1 Department of Applied Chemistry, College of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan; [email protected] 2 Graduate School of Science & Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan; [email protected] 3 Institute for Catalysis and Graduate School of Chemical Sciences and Engineering, Hokkaido University, N 21, W 10, Kita-ku Sapporo 001-0021, Japan; [email protected] (H.N.); [email protected] (T.N.) 4 Integrated Research Consortium on Chemical Sciences, Institute for Catalysis, Hokkaido University, N 21, W 10, Kita-ku Sapporo 001-0021, Japan * Correspondence: [email protected]

Received: 18 June 2020; Accepted: 3 September 2020; Published: 8 September 2020

Abstract: Addition reactions of multi-functional amine, polyethylene imine (PEI) or diethylenetriamine (DETA), and poly(ethylene glycol) diglycidyl ether (PEGDE) or poly(ethylene glycol) diacrylate (PEGDA), have been investigated to obtain network polymers in H2O, dimethyl sulfoxide (DMSO), and ethanol (EtOH). Ring opening addition reaction of the multi-functional amine and PEGDE in H2O at room temperature or in DMSO at 90 ◦C using triphenylphosphine as a catalyst yielded gels. Aza-Michael addition reaction of the multi-functional amine and PEGDA in DMSO or EtOH at room temperature also yielded corresponding gels. Compression test of the gels obtained with PEI showed higher Young’s modulus than those with DETA. The reactions of the multi-functional amine and low molecular weight PEGDA in EtOH under the speciﬁc conditions yielded porous polymers induced by phase separation during the network formation. The morphology of the porous polymers could be controlled by the reaction conditions, especially monomer concentration and feed ratio of the multi-functional amine to PEGDA of the reaction system. The porous structure was formed by connected spheres or a co-continuous monolithic structure. The porous polymers were unbreakable by compression, and their Young’s modulus increased with the increase in the monomer concentration of the reaction systems. The porous polymers absorbed various solvents derived from high aﬃnity between the polyethylene glycol units in the network structure and the solvents.

Keywords: gel; porous polymer; multi-functional amine; poly(ethylene glycol); network structure; mechanical property

1. Introduction Network structure, such as molecular and geometric structures, in chemically synthesized gels strongly aﬀects their properties. Polymerization (or copolymerization) of bi-functional (or multi-functional) monomers and crosslinking of liner pre-polymers using chemical reactions are widely used to synthesize polymer network in chemical gels. We have been developing several types of gels synthesized by addition reactions between multi-functional monomers as “joint unit” sources

Polymers 2020, 12, 2047; doi:10.3390/polym12092047 www.mdpi.com/journal/polymers Polymers 2020, 12, 2047 2 of 17 and α,ω-bifunctional monomers as “linker unit” sources in some solvents, based on the joint and linker concept. In our previous studies, addition reactions of multi-functional cyclic siloxane or cubic silsesquioxane, thiol, polyol, and acrylate as the multi-functional monomers and α,ω-diolefins, dithiol, diazide, divinyl ether, and diamine compounds as the bi-functional monomers successfully yielded the gels [1–3]. The gels formed a homogeneous network structure with extremely narrow mesh size distribution, and their mesh size could be controlled by the molecular length of the bi-functional monomers. However, those gels were too fragile to evaluate their mechanical properties. We have been trying to synthesize the gels with high mechanical properties using conventional addition reactions of generic chemicals and solvents. Various hydrogels with high mechanical properties have been developed by polymerization of functionalized poly(ethylene glycol) (PEG) [4–23]. The hydrogels with PEG moiety are expected to be applied as a biomaterial. The PEG linkage shows high affinity with various organic solvents, and the corresponding gels should be synthesized in organic solvents, so-called organogels. The PEG unit results in good ionic conductivity for cation carriers. The organogels with the PEG unit should be usable for structure material ionic conducting gels. Prior to the development of the ionic conducting gels, we investigated a basic study of the molecular design of organogels with the PEG unit based on the joint and linker concept. In addition, we expected to prepare the porous polymers by means of polymerization induced phase separation by control of affinity between the polymer network and the solvent in the reaction system. We selected bi-functional PEG as the linker monomer. For example, the gels were synthesized by addition reactions of a primary tri-amine, tris(2-aminoethyl)amine (TAEA), as the multi-functional monomer, and polyethylene glycol diglycidyl ether (PEGDE) or polyethylene glycol diacrylate (PEGDA) as the bi-functional monomer in some organic solvents [24]. The reactions yielded the gels with high mechanical properties. We also found the reaction of TAEA and low molecular weight PEGDA in ethanol (EtOH) yielded porous polymers induced by phase separation during the network formation. These polymers showed features derived from the PEG unit in the network structure. Although TAEA is available as a research reagent from some chemical companies, it is not suitable for applications in large scale from industrial perspectives of availability and cost. As the next step, we focus on commercial availability and variation of molecular structure of the multi-functional amine monomer, which can react with PEGDE or PEGDA. We select polyethyleneimine (PEI) as a multi-functional monomer. PEI has primary, secondary, and tertiary amines with branched structure. Diethylenetriamine (DETA) was also used as a corresponding small molecular amine monomer. These chemicals are industrially produced and widely used as washing (water-treatment) agents, adhesives, cosmetics, cultivation, treatment of fiber and paper, chelate, coatings, ion-exchange resins, epoxy resin curing agent, and so on. In this paper, we describe the synthesis of gels by addition reaction of PEI or DETA, as the multi-functional monomer, and PEGDE using the ring opening addition reaction [25] (Scheme1) or PEGDA using the aza-Michael addition reaction [26] (Scheme2). We study the e ffect of the network structure, feed ratio of amine monomer to PEGDE or PEGDA, and features of the solvents (H2O, dimethyl sulfoxide (DMSO), EtOH) on the mechanical properties of the resultant gels. The porous polymers were also obtained in the reactions of PEI or DETA with low molecular weight PEGDA in EtOH, and the structure and properties of the porous polymers were also investigated. Polymers 2020, 12, x 3 of 17

Polymers 2020, 12, 2047 3 of 17 Polymers 2020, 12, x 3 of 17

PEI PEGDE DMSO, PPh3, 90 °C PEI (n = 9: PEGDE400) (n = 23: PEGDE1000)PEGDE or DMSOH2O, room, PPh temp., 90 ° C (n = 9: PEGDE400) 3 DETA (n = 23: PEGDE1000) or H2O, room temp.

PEI – PEGDE gel DETA

PEI – PEGDE gel Scheme 1. Synthesis of gels by ring opening addition reaction of polyethyleneimine (PEI) or diethylenetriamineScheme 1. Synthesis (DETA of) gelsand polyethylene by ring opening glycol addition diglycidyl reaction ether ( ofPEGDE polyethyleneimine). DMSO, dimethyl (PEI)

sulfoxide.orScheme diethylenetriamine 1. Synthesis of (DETA) gels by and ring polyethylene opening addition glycol reaction diglycidyl of polyethyleneimine ether (PEGDE). ( DMSO,PEI) or dimethyldiethylenetriamine sulfoxide. (DETA) and polyethylene glycol diglycidyl ether (PEGDE). DMSO, dimethyl

sulfoxide.

PEI DMSO PEGDA EtOH (n = 4: PEGDA200) PEI DMSO (n = 9: PEGDAPEGDA4 00) EtOH (n = 14: PEGDA600) DETA (n = 4: PEGDA200) (n(n = =23 9:: PEGDAPEGDA41000)00) (n = 14: PEGDA600) DETA (n = 23: PEGDA1000) PEI – PEGDA gel, porous polymer

Scheme 2. Synthesis of gels and porous polymers by aza-Michael addition reaction of PEI or DETA Scheme 2. Synthesis of gels and porous polymers by aza-Michael additionPEI – reactionPEGDA ofgel PEI, porous or DETA polymer and polyethylene glycol diacrylate (PEGDA). and polyethylene glycol diacrylate (PEGDA). 2. MaterialsScheme and2. Synthesis Methods of gels and porous polymers by aza-Michael addition reaction of PEI or DETA 2. Materialsand polyethylene and Methods glycol diacrylate (PEGDA). 2.1. Materials 2.1.2. MaterialsMaterials and Methods PEI (Polyethylenimine300) was commercially obtained from Junsei Chemical Co., Ltd. (Tokyo, Japan),PEI and(Polyethylenimine300) used as received. was The commercially structure of PEI obtained is as follows:from Junsei molecular Chemical weight: Co., Ltd. 300, (Tokyo amine, 2.1. Materials Japanvalue:), 21and mmol used/ g,as[primary received.amine] The structure/[secondary of PEI amine] is as follow/[tertiarys: molecular amine] = weight:45%:35%:20%. 300, amine DETA value: was 21commercially mmol/g,PEI (Polyethylenimine300) [primary obtained amine] from Tokyo/[secondary was Chemical commercially amine] Industry/ [tertiaryobtained Co., Ltd. amine]from (Tokyo,Junsei = 45% Chemical Japan),:35%:20%. and Co., used DETALtd. without (Tokyo was , commerciallyfurtherJapan), purification.and used obtained as received. PEGDE from Tokyo samples,The s tructureChemical PEGDE400 of Industry PEI (epoxy:is as Co.,follow 263Ltd.s: g molecular/(eq.)Tokyo, and Japan PEGDE1000weight:), and 300, used amine (epoxy: without value: 574 furtherg21/eq.), mmol/g, were purification. kindly [primary donated PEGDE amine] from samples,/[secondary NOF CORPORATION PEGDE400 amine] (epoxy:/[tertiary (Tokyo, 263 amine] Japan), g/eq.) and and = 45% were PEGDE1000:35%:20%. used without (epoxy: DETA further 574 was g/eq.),purification.commercially were kindly PEGDA obtained donated samples, from from Tokyo PEGDA200, NOF Chemical CORPORATION PEGDA400, Industry PEGDA600,Co., (Tokyo, Ltd. (Tokyo, Japan and), PEGDA1000, Japan and were), and used used were without without kindly furtherdonatedfurther purification. purification.from Shin-Nakamura PEGDA PEGDE samples, samples, Chemical PEGDA200, PEGDE400 Co., Ltd. (Wakayama,PEGDA400, (epoxy: 263 Japan),P g/eq.)EGDA600, and and were PEGDE1000and purifiedPEGDA1000, (epoxy: by passing were 574 kindlythroughg/eq.), donated were Al2O 3 kindly columnfrom Shin donated before-Nakamura use from to NOFremove Chemical CORPORATION polymerization Co., Ltd. (Wakayama, inhibitors.(Tokyo, Japan Japan High), purity) and, and were were grade used purified DMSO without andby passingEtOHfurther were throughpurification. commercially Al2O PEGDA3 column obtained samples, before from use KantoPEGDA200, to remove Chemical PEGDA400, polymerization Co., Inc. (Tokyo,PEGDA600, inhibitors. Japan), and and High PEGDA1000, used purity as received. grade were DMSOTriphenylphosphinekindly and donated EtOH from were (PPh Shin commercially3)- (KantoNakamura Chemical obtaine Chemical Co.,d from Co., Inc., KantoLtd. Tokyo, (Wakayama, Chemical Japan) was Co., Japan commercially Inc.), and(Tokyo, were obtained, Japan purified), and and by waspassing dissolved through in theAl2 solventO3 column of the before reaction use systemto remove before polymerization use. inhibitors. High purity grade DMSO and EtOH were commercially obtained from Kanto Chemical Co., Inc. (Tokyo, Japan), and

Polymers 2020, 12, x 4 of 17

Polymers 2020, 12, 2047 4 of 17 used as received. Triphenylphosphine (PPh3) (Kanto Chemical Co., Inc., Tokyo, Japan) was commercially obtained, and was dissolved in the solvent of the reaction system before use. 2.2. Synthesis of Network Polymers 2.2. Synthesis of Network Polymers DETA has two primary amines, NH2 group, and one secondary amine, NH group. That means DETA has two primary amines, NH2 group, and one secondary amine, NH group. That means ﬁve active hydrogens or three amines in one DETA molecule. The reactions of DETA and PEGDE five active hydrogens or three amines in one DETA molecule. The reactions of DETA and PEGDE or or PEGDA were conducted under the feed ratio of Case 1: [N] (mole of amines in DETA) = [epoxy PEGDA were conducted under the feed ratio of Case 1: [N] (mole of amines in DETA) = [epoxy or or acrylate]) or Case 2: [H] (mole of active hydrogens in DETA) = [epoxy or acrylate]), as shown in acrylate]) or Case 2: [H] (mole of active hydrogens in DETA) = [epoxy or acrylate]), as shown in Scheme3. In the same way, the reactions of PEI-PEGDE and PEI-PEGDA (Case 1: [N] (mole of amines Scheme 3. In the same way, the reactions of PEI-PEGDE and PEI-PEGDA (Case 1: [N] (mole of amines in PEI) = [acrylate or acrylate], Case 2: [H] (mole of active hydrogens in PEI) = [acrylate or acrylate]) in PEI) = [acrylate or acrylate], Case 2: [H] (mole of active hydrogens in PEI) = [acrylate or acrylate]) were deﬁned as Case 1 or Case 2, respectively. were defined as Case 1 or Case 2, respectively.

SchemeScheme 3. 3. FeedFeed ratio ratio models models of of DETA DETA and PEGDE defi deﬁnedned in the present experiments.

Synthesis of G Gelsels A reaction of PEI with PEGDE400 (monomer concentration: 30 30 wt wt %, %, Case Case 1) 1) is described as a reference. PEI PEI (0.15 (0.15 g, g, [ [N]N] 2.5 2.5 mmol), mmol), PEGDE400 PEGDE400 (0.67 (0.67 g, g, epoxy epoxy 2.5 2.5 mmol), mmol), and and DMSO DMSO (0.92 (0.92 mL) mL) were were added to to a a 20 20 mL mL vial vial and and stirred stirred by vortex by vortex mixer mixer for several for several minutes minutes to make to a makehomogeneous a homogeneous solution. solution.DMSO solution DMSO ofsolution PPh3 (0.84 of PPh mL,3 (0.84 0.076 mL, mM, 0.076 2.5 mM, mol% 2.5 to mol% epoxy to group) epoxy was group) added was to added the reaction to the reactionsolution. solution. The reaction The solutionreaction wassolution introduced was introduced to an ampoule to an of ampoule 10 mL. Afterof 10 themL. ampoule After the was ampoule sealed, wasthe reactionsealed, the system reaction was keptsystem at 90 wa◦Cs kept for 24 at h. 90 The °C reactionfor 24 h. with The PEGDE1000reaction with or PEGDE1000 DETA was conducted or DETA wasunder conduc the sameted procedures. under the The same reactions procedures. in H2O The were reactions conducted in at H room2O were temperature conducted without at room PPh3. temperatureA reaction without of PEI PPh with3. PEGDA400 (monomer concentration: 30 wt %, Case 1) in DMSO is described as a reference.A reaction PEI of (0.15 PEI g,with [N] PEGDA400 2.5 mmol), PEGDA400 (monomer (0.635 concentration: g, epoxy 2.5 30 mmol),wt %, Case and DMSO 1) in DMSO (1.67 mL) is describedwere added as toa reference. a 20 mL vial, PEI and(0.15 stirred g, [N] by2.5 vortexmmol), mixer PEGDA400 for several (0.635 minutes g, epoxy to 2.5 make mmol), a homogeneous and DMSO (1.67solution. mL)The were reaction added solution to a 20 wasmL introducedvial, and stirred to an ampouleby vortex of mixer 10 mL. for After several the ampouleminutes wasto make sealed, a homogeneousthe reaction system solution. was kept The atreaction room temperature solution was for introduced 24 h. The reaction to an ampoule with PEGDA200, of 10 mL. PEGDA600, After the ampoulePEGDA1000, was DETA,sealed, orthe in reaction EtOH was system conducted was kept under at room the same temperature procedures. for 24 h. The reaction with PEGDA200, PEGDA600, PEGDA1000, DETA, or in EtOH was conducted under the same procedures. 2.3. Analytical Procedures 2.3. AnalyticaViscosityl Procedures of the reaction systems was traced by a tuning fork vibration viscometer, SV-1A (A&D Company Limited, Tokyo, Japan), equipped with a block heater. The measurements were conducted at room temperature or 90 ◦C.

Polymers 2020, 12, x 5 of 17

Viscosity of the reaction systems was traced by a tuning fork vibration viscometer, SV-1A (A&D Company Limited, Tokyo, Japan), equipped with a block heater. The measurements were conducted at room temperature or 90 °C. FT-IR spectra of reaction solutions and gels were recorded on a Jasco FT/IR-410 (JASCO Polymers 2020, 12, 2047 5 of 17 Corporation, Tokyo, Japan). The samples were put between KBr-Real Crystal IR-Card and Slip (International Crystal Laboratories, NJ, USA), and 30 scans were accumulated from 4000 to 500 cm−1. FT-IRThe m spectraechanical of reaction properties solutions of the and gels gels or wereporous recorded polymers on awere Jasco investigated FT/IR-410 (JASCO by the Corporation, compression Tokyo,test using Japan). Tensilon The samples RTE-1210 were (ORIENTEC put between Co. KBr-Real LTD., Crystal Tokyo, IR-Card Japan). andThe Slip test (International samples, wet Crystal gels as 1 Laboratories,prepared state NJ, or USA), dried and porous 30 scans polymers were accumulated, were cut to from0.7–1 4000 cm tocube 500s, cmand− .pressed at a rate of 0.5 mm/minThe mechanical at room temperature. properties of Compression the gels or porous test ofpolymers the gels were was investigated conducted atby 24 the h compression after the gel testformation using Tensilon. RTE-1210 (ORIENTEC Co. LTD., Tokyo, Japan). The test samples, wet gels as preparedScanning state or electron dried porous microscopy polymers, (SEM) were images cut to 0.7–1of the cm porous cubes, polymers and pressed were at a acquired rate of 0.5 by mm a /JEOLmin at(Tokyo, room temperature. Japan) JSM Compression-7610F microsco testpe of the with gels a waslower conducted secondary at 24 electron h after the image gel formation.detector at an accelerationScanning voltage electron of microscopy 3.0 kV. (SEM) images of the porous polymers were acquired by a JEOL (Tokyo,The Japan) surface JSM-7610F area of microscopethe porous polymers with a lower was secondary measured electron by nitrogen image sorption detector using at an accelerationan Autosorb voltage6AG (Quantachrome of 3.0 kV. , FL, USA), and determined by the Brunauer–Emmett–Teller (BET) equation. The surface area of the porous polymers was measured by nitrogen sorption using an Autosorb 6AG3. Results (Quantachrome, and Discussion FL, USA), and determined by the Brunauer–Emmett–Teller (BET) equation.

3.3.1. Results Sythessis and of Discussion Network Polymers by Means of Ring Opening Addition Reaction of PEI or DETA and PEGDE 3.1. Sythessis of Network Polymers by Means of Ring Opening Addition Reaction of PEI or DETA and PEGDE Ring-opening addition reaction of PEI or DETA and PEGDE was conducted in H2O, DMSO, or EtOH.Ring-opening The reactions addition in H2O reaction successfully of PEI produced or DETA andthe PEGDEgels at room was conductedtemperature in Hwithout2O, DMSO, catalyst. or EtOH.The reactions The reactions at relatively in H2O low successfully temperatures produced and/or the without gels at room catalyst temperature did not gel without in DMSO catalyst.. The Thecorresponding reactions at reactions relatively in lowthe presence temperatures of PPh and3 as/ orthe without catalyst catalystat 90 °C didsuccessfully not gel inyie DMSO.lded the Thegels. correspondingThe reaction systems reactions in inEtOH the presence did not gel of PPhdespite3 as the catalystpresence at of 90 the◦C catalyst. successfully The reactions yielded the must gels. be Theconducted reaction at systems the temperature in EtOH did less not than gel the despite boiling the point presence of EtOH of the (60 catalyst. °C). The The reaction reactionss under must 60 be °C conductedin EtOH would at the temperaturenot sufficient less to form than thethe boilinggels. point of EtOH (60 ◦C). The reactions under 60 ◦C in EtOHThe would reaction not su inﬃ cientDMSO to formwas traced the gels. by FT-IR spectroscopy. Figure 1 shows the FT-IR spectra of the The PEI reaction-PEGDE400 in DMSO reaction was system, traced by monomer FT-IR spectroscopy. concentration: Figure 301 shows wt % the in FT-IR DMSO. spectra Intensity of the of PEI-PEGDE400absorption peaks reaction at about system, 800 cm monomer−1 (Figure concentration: 1 (i)) derived 30 from wt %epoxy in DMSO. group Intensity and at about of absorption 1150 cm−1 1 1 peaks(Figure at about1 (ii)) 800and cm 1600− (Figure cm−1 (Figure1 (i)) derived 1 (iii)) fromderived epoxy from group amine and groups at about almost 1150 cmdisappeared− (Figure1 in (ii)) the 1 andspectrum 1600 cm of− the(Figure gel, 1indicating (iii)) derived the fromaddition amine reaction groups of almost epoxy disappeared group with inamine the spectrum would progress of the gel,successfully. indicating the addition reaction of epoxy group with amine would progress successfully.

(a)

(i) (ii) (iii)

(b)

Figure 1. FT-IR spectra of polyethyleneimine (PEI)-polyethylene glycol diglycidyl ether (PEGDE)400

reaction system of Case 1: (a) before reaction and (b) after reaction at 90 ◦C, solvent: dimethyl sulfoxide (DMSO), monomer concentration: 30 wt%.

The viscosity of the PEI or DETA-PEGDE reaction systems, monomer concentration: 30 wt% in H2O, was traced by the tuning fork vibration viscometer at room temperature (Supplementary Polymers 2020, 12, 2047 6 of 17

Material Figures S2 and S3). An inflection point of the profile was defined as the gel formation time (not theoretical gelation time), summarized in Table1. The gel formation time of the PEI-PEGDE400 reaction system (run 1) was shorter than that of the PEI-PEGDE1000 system (run 5) owing to the higher epoxy (amine) concentration in the reaction system with the same monomer concentration. The PEI-PEGDE400 reaction system of Case 2 (run 2) showed longer gel formation time than that of Case 1 (run 1). The FT-IR spectra of PEI-PEGDE400 gel obtained in the reaction system of Case 2 (Figure S1) showed a similar profile to that of the corresponding gel obtained in the reaction systems of Case 1 (Figure1). A comparison of these FT-IR spectra indicates that there was not much di fference in reaction conversions of these gels. In the case of the reaction of Case 2, addition reaction of primary amine with epoxy would proceed in two steps, as reported in the curing process of diamine and bisphenol A type diepoxy [27]. The first step is the reaction of a primary amine and an epoxy, which forms a secondary amine. After completion of the first step reaction, the secondary amine reacts with another epoxy. The longer gel formation time of Case 2 would be caused by the two steps’ reactions. Steric hindered secondary amine formed by a reaction of primary amine and one epoxy is also possible. The reaction system of PEI-PEGDE400 (run 1) caused the gel to form faster than that of DETA-PEGDE400 (run 3). The concentration of epoxy group (0.96 mol/L) of the PEI-PEGDE400 reaction system was almost same as that of the DETA-PEGDE400 reaction system (1.02 mol/L). One explanation of the result is that the inter-penetration of the polymer networks derived from high molecular weight and highly functionalized PEI, which should play a role of a physical crosslinking point, would occur during the reaction, and attained a short gel formation time. The viscosity of the PEI-PEGDE400 reaction system in DMSO at 90 ◦C was also traced. The gel formation times of the reaction systems in DMSO were longer than those in the corresponding reaction systems in H2O. Low basicity of amines of PEI in low polar solvent of DMSO may lower the reaction rate, which should cause longer gel formation times in DMSO.

Table 1. Gel formation time and mechanical properties of polyethyleneimine (PEI) or diethylenetriamine (DETA)-polyethylene glycol diglycidyl ether (PEGDE) gels, monomer concentration: 30 wt%. DMSO, dimethyl sulfoxide.

Amine/ Gel Young’s Stress at Strain at Epoxy a Run Amine PEGDE PEGDE Solvent Formation Modulus Break Break (mol/L) Feed Time (min) (kPa) (kPa) (%)

1 PEI 400 Case 1 H2O 0.96 35 426 400 26.5 2 PEI 400 Case 2 H2O 1.03 78 540 530 24.4 3 DETA 400 Case 1 H2O 1.02 128 241 228 44.4 4 DETA 400 Case 2 H2O 1.09 328 344 39.2 5 PEI 1000 Case 1 H2O 0.49 132 210 317 41.0 6 PEI 1000 Case 2 H2O 0.51 228 340 36.1 7 PEI 400 Case 1 DMSO 1.03 94 210 466 37.3 8 PEI 400 Case 2 DMSO 1.10 135 265 388 37.7 a Molar concentration of epoxy group of PEGDE.

The mechanical properties of the PEI or DETA-PEGDE gels were investigated by compression test. Figure2 shows the stress–strain curves of PEI or DETA-PEGDE400 gels, monomer concentration: 30 wt % in H2O, and the results are summarized in Table2. The gels obtained in the reaction of Case 2 showed higher Young’s modulus than the corresponding gels obtained in the reaction of Case 1. One explanation of the result is that the higher epoxy concentration in the reaction systems of Case 2 should yield gels with high crosslinking density. The gels with PEI showed hard and brittle features in comparison with those with DETA. The result can be explained by the inter-penetration of the polymer networks derived from the speciﬁc structure of PEI, as described above. The PEI-PEGDE1000 gels showed soft and ﬂexible features in comparison with the PEI-PEGDE400 gels owing to the lower crosslinking density, derived from the low epoxy concentration in the reaction systems. The PEI-PEGDE400 gels prepared in DMSO showed lower Young’s modulus and higher strain at break Polymers 2020, 12, 2047 7 of 17

than the corresponding gels prepared in H2O. The result can be explained by the high aﬃnity between Polymersthe polymer 2020, 12, network x and DMSO, which should make the gels soft and ﬂexible. 7 of 17

600

(b)

500

400 (a) (d)

300 (e) Stress [kPa] Stress

200 (c)

100

0 0 10 20 30 40 50 Strain [%]

FigureFigure 2. 2. StressStress–strain–strain curves curves of of PEI PEI-PEGDE400-PEGDE400 g gels:els: (a ()a Case) Case 1 1and and (b ()b Case) Case 2, 2,DETA DETA-PEGDE400-PEGDE400 gels: gels: (c) Case 1 and (d) Case 2, and PEI-PEGDE1000 gel: (e) Case 1; solvent: H O, monomer concentration: (c) Case 1 and (d) Case 2, and PEI-PEGDE1000 gel: (e) Case 1; solvent: H2O,2 monomer concentration: 3030 wt wt %. %. Table 2. Gel formation time and mechanical properties of PEI or DETA-polyethylene glycol diacrylate Table(PEGDA) 1. gels,Gel solvent: formation DMSO, time monomer and m concentration:echanical properties 30 wt%. of polyethyleneimine (PEI) or diethylenetriamine (DETA)-polyethylene glycol diglycidyl ether (PEGDE) gels, monomer Amine/ Gel Young’s Stress at Strain at concentration: 30 wt%. DMSO, dimethyl sulfoxideAcrylate. Run Amine PEGDA PEGDA a Formation Modulus Break Break (mol/L) Feed Time (min)Gel (kPa) (kPa) Strain(%) Amine/ Young’s Stress 9 PEI 200 Case 1Solvent 1.53 Epoxy a Formation 3 540 871 24.4at Run Amine PEGDE PEGDE Modulus at 10 PEI 400 Case 1 1.05(mol/L) 5Time 850 811Break 29.6 feed (kPa) Break 11 PEI 400 Case 2 1.12 135(min) 740 777( 31.5%) 12 PEI 600 Case 1 0.80 11 265 784(kPa) 32.6 113 PEI PEI 400 1000 Case Case 1 1H2O 0.530.96 123 35 552426 751400 26.5 37.1 214 PEI PEI 400 1000 Case Case 2 2H2O 0.551.03 78 471540 664530 24.4 50.0 315 DETA DETA 400 400 Case Case 1 1H2O 1.141.02 16128 552241 751228 44.4 37.2 416 DETA DETA 400 400 Case Case 2 2H2O 1.091.09 52 471328 664344 39.2 50.4 5 PEI 1000 Casea Molar 1 concentrationH2O of acrylate0.49 group132 of PEGDA. 210 317 41.0 6 PEI 1000 Case 2 H2O 0.51 228 340 36.1 3.2.7 Synthesis PEI of Network400 Polymers Case by 1 MeansDMSO of Aza-Michael 1.03 Addition 94 Reaction of210 DETA or466 PEI and37.3 PEGDA 8 PEI 400 Case 2 DMSO 1.10 135 265 388 37.7 Aza-Michael additiona reactionMolar concentration of PEI or DETAof epoxy and group PEGDA of PEGDE was. conducted in H2O, DMSO, or EtOH at room temperature. The reaction systems in H2O once formed gels, and then turned 3.2.them Synthesis into solution of Network owing Polymers to the by hydrolytic Means of Aza degradation-Michael Addition of ester Reaction group of of PEGDA. DETA or The PEI reactionsand in PEGDADMSO successfully yielded the gels. The molecular weight of PEGDA aﬀected the production state of the reaction in EtOH. The reactions with relatively high molecular weight, PEGDA, PEGDA400, Aza-Michael addition reaction of PEI or DETA and PEGDA was conducted in H2O, DMSO, or PEGDA600, and PEGDA1000, in EtOH yielded the gels. By contrast, the reactions with low molecular EtOH at room temperature. The reaction systems in H2O once formed gels, and then turned them weight, PEGDA and PEGDA200, in EtOH formed some states, gel, precipitated polymer, or porous into solution owing to the hydrolytic degradation of ester group of PEGDA. The reactions in DMSO polymer, depended on the reaction conditions. We shall return to this point later. successfully yielded the gels. The molecular weight of PEGDA affected the production state of the Figure3 shows FT-IR spectra of the PEI-PEDA400 reaction system, monomer concentration: 30 reaction in EtOH. The reactions with relatively high molecular weight, PEGDA, PEGDA400, wt% in DMSO. The intensity of the absorption peaks at about 700 cm 1 (Figure3 (i)) derived from PEGDA600, and PEGDA1000, in EtOH yielded the gels. By contrast, the reactions− with low molecular acrylate and 1150 cm 1 (Figure3 (ii)) and 1600 cm 1 (Figure3 (iii)) derived from amine groups was weight, PEGDA and PEGDA200− , in EtOH formed some− states, gel, precipitated polymer, or porous polymer, depended on the reaction conditions. We shall return to this point later. Figure 3 shows FT-IR spectra of the PEI-PEDA400 reaction system, monomer concentration: 30 wt% in DMSO. The intensity of the absorption peaks at about 700 cm−1 (Figure 3 (i)) derived from acrylate and 1150 cm−1 (Figure 3 (ii)) and 1600 cm−1 (Figure 3 (iii)) derived from amine groups was

Polymers 2020, 12, 2047 8 of 17

Polymers 2020, 12, x 8 of 17 decreased (almost disappeared) in the spectrum of the gel owing to the addition reaction of the acrylate groupdecreased with amine.(almost disappeared) in the spectrum of the gel owing to the addition reaction of the acrylate group with amine.

(a)

(ii) (i) (iii) (b)

FigureFigure 3. 3.FT-IR FT-IR spectra spectra of of PEI-PEGDA400 PEI-PEGDA400 reaction reaction system system of of Case Case 1 1 (a ()a before) before reaction, reaction, and and (b ()b after) after reaction,reaction, solvent: solvent: DMSO, DMSO, monomer monomer concentration: concentration: 30 30 wt wt %. %.

TheThe viscosity viscosity of of the the PEI-PEGDA PEI-PEGDA reaction reaction systems, systems monomer, monomer concentration: concentration: 30 30 wt% wt% in in DMSO, DMSO, waswas traced traced at at room room temperature temperature to to estimate estimate the the gel gel formation formation time time (Supplementary (Supplementary Material Material Figures Figures S5 andS5 and S6), S6) summarized, summarized in Table in Table2. The 2. The gel formationgel formation time time of theof the reaction reaction systems systems of of Case Case 1 increased1 increased withwith the the increase increase in in the the molecular molecular weight weight of of PEGDA. PEGDA The. The concentration concentration of of acrylate acrylate group group of of PEGDA PEGDA inin the the reaction reaction systems systems increased increased with with the the decrease decrease in in the the molecular molecular weight weight of of PEGDA PEGDA under under the the samesame monomer monomer concentration, concentration, as as summarized summarized in in Table Table2. 2 The. The higher higher molar molar concentration concentration of of the the acrylateacrylate (and (and amine) amine) group group in in the the reaction reaction system system with with low low molecular molecular weight weight PEGDA200 PEGDA200 yielded yielded the the polymerpolymer network network with with high high crosslinkingcrosslinking density,density, whichwhich should cause the the short short gel gel formation formation time time in inthe the homogeneous homogeneous phase phase.. The The gel gel formation formation time time of of the the PEI PEI-PEGDA400-PEGDA400 reaction reaction system system of of Case Case 1 1(5 (5min) min) was was much much shorter shorter than than that that of of Case Case 2 2(135 (135 min). min). The The FT FT-IR-IR spectrum spectrum of ofthe the PEI PEI-PEGDA400-PEGDA400 gel gelobtained obtained from from the the reaction reaction system system of ofCase Case 2 (Figure 2 (Figure S4) S4) showed showed a similar a similar profile proﬁle to tothat that of ofthe the gel gelobtained obtained from from the thereaction reaction system system of Case of Case 1 (Figure 1 (Figure 3). The3). conversions The conversions of acrylate of acrylate group group in the ingels theof gelsCase of1 and Case Case 1 and 2 evaluated Case 2 evaluated from peak from intensity peak intensityof (i) in Figures of (i) in3 and Figure S4 3based and Figureon the intensity S4 based of 1 onunchanged the intensity peak of (at unchanged 965 cm−1) peak were (at about 965 cm62%− )and were 56 about%, respectively. 62% and 56%, Although respectively. the conversions Although of theacrylate conversions groups of in acrylatethe gel of groups Case 2 in were the a gel little of Casesmaller 2 were than athat little of smallerCase 1, this than would that of not Case necessarily 1, this wouldmean nota difference necessarily in the mean reaction a diﬀ rateerences of inCase the 1 reaction and Case rates 2. The of longer Case 1 gel and formation Case 2. The time longer of the gelPEI- formationPEGDA400 time reaction of the PEI-PEGDA400 system of Case reaction 2 can be system explained of Case by 2st caneric be hindered explained proton by steric transfer hindered of the protonsecondary transfer amine of the formed secondary by a amine reaction formed of a byprimary a reaction amine of a primary and an amineacrylate, and as an reported acrylate, in as a reportedcomputational in a computational study of an studyaza-Michael of an aza-Michael addition reaction addition [28 reaction]. The gel [28 formation]. The gel formationtimes of the times DETA of - thePEGDA400 DETA-PEGDA400 reaction systems reaction were systems longer were than longer the thancorresponding the corresponding PEI-PEGDA400 PEI-PEGDA400 reaction reaction systems systemsdespite despite the similar the similar acrylate acrylate concentration, concentration, as observed as observed in in the the reaction reactionss of of PEI oror DETA-PEGDEDETA-PEGDE systems.systems. The The results results can can be be explained explained by by the the inter-penetration inter-penetration of of the the polymer polymer networks networks derived derived from from PEI,PEI, which which would would be be formed formed during during the the reaction. reaction. TheThe gel gel formation formation times times of of the the reaction reaction systems systems in in EtOH EtOH were were also also determined determined (Supplementary (Supplementary MaterialMaterial Figures Figures S7 S7 and and S8), S8), and and the the results results are are summarized summarized in Tablein Table2. The 2. T correspondinghe corresponding reactions reaction in s DMSOin DMSO and and EtOH EtOH showed showed similar similar gel formationgel formation times. times. TheThe mechanical mechanical properties properties of of the PEI or or DETA DETA-PEGDA-PEGDA gels gels in in DMSO DMSO were were investigated investigated by bycompression compression test. test. Figure Figure 4 shows4 shows the stress the stress–strain–strain curves curves of PEI ofor DETA PEI or-PEGDA400 DETA-PEGDA400 gels, monomer gels, monomerconcentration: concentration: 30 wt % 30in DMSO, wt % in andDMSO, the and results the are results summarized are summarized in Table in 2 Table. The2 PEI. The or PEI DETA or - DETA-PEGDE400PEGDE400 gels obtained gels obtained in the in reactions the reactions of Case of Case 1 showed 1 showed a higher a higher Young’s Young’s modulus modulus than than the corresponding gels of Case 2. These results are opposite to those of the PEI or DETA-PEGDE400 gels described above. One explanation of the results is the lower reaction conversions in the reaction systems of Case 2 with PEGDA400, as observed in FT-IR spectroscopy (Figure S4). The reaction

Polymers 2020, 12, 2047 9 of 17 the corresponding gels of Case 2. These results are opposite to those of the PEI or DETA-PEGDE400 Polymersgels described 2020, 12, xabove. One explanation of the results is the lower reaction conversions in the reaction9 of 17 systems of Case 2 with PEGDA400, as observed in FT-IR spectroscopy (Figure S4). The reaction systems systems with low reaction conversion should yield gels with low crosslinking density and soft with low reaction conversion should yield gels with low crosslinking density and soft features. The features. The gels with PEI showed hard and brittle features in comparison with those with DETA, gels with PEI showed hard and brittle features in comparison with those with DETA, as observed in as observed in the PEI or DETA-PEGDE400 gels. The result can be explained by the inter-penetration the PEI or DETA-PEGDE400 gels. The result can be explained by the inter-penetration of the polymer of the polymer networks derived from the specific structure of PEI, as described above. The PEI- networks derived from the speciﬁc structure of PEI, as described above. The PEI-PEGDA600 and PEGDA600 and PEGDA1000 gels showed soft and flexible features in comparison with the PEI- PEGDA1000 gels showed soft and ﬂexible features in comparison with the PEI-PEGDA400 gel owing PEGDA400 gel owing to the lower crosslinking density derived from lower acrylate concentration in to the lower crosslinking density derived from lower acrylate concentration in the reaction system. the reaction system.

1000

(a) (b) 800 (e) (c)

(d) 600

Stress [kPa] Stress 400

200

0 0 10 20 30 40 50 60 Strain [%] Figure 4. Stress–strain curves of PEI-PEGDA400 gels: (a) Case 1 and (b) Case 2, DETA-PEGDA400 gels: Figure 4. Stress–strain curves of PEI-PEGDA400 gels: (a) Case 1 and (b) Case 2, DETA-PEGDA400 (c) Case 1 and (d) Case 2, and PEI-PEGDA400 gels: (e) Case 1; solvent: DMSO, monomer concentration: gels: (c) Case 1 and (d) Case 2, and PEI-PEGDA400 gels: (e) Case 1; solvent: DMSO, monomer 30 wt%. concentration: 30 wt%. The mechanical properties of the PEI or DETA-PEGDA gels in EtOH were also investigated by the Table 2. Gel formation time and mechanical properties of PEI or DETA-polyethylene glycol diacrylate compression test, and the results are summarized in Table3. The gels in EtOH showed a much lower (PEGDA) gels, solvent: DMSO, monomer concentration: 30 wt%. Young’s modulus than the corresponding gels in DMSO. One explanation of the results is the strict diﬀerence in the phase of the gels. The reactions in DMSOGel occurred in the homogeneous phase, and yielded transparent gels. By contrast,Amine/ the gels preparedFormatio in EtOH slightlyYoung’s turned white.Stress ThisStrain would be Run PEGD Acrylate a an intermediateAmine state betweenPEGDA homogeneous gel and porousn polymerModulus induced at by Break phase separation.at Break A (mol/L) The phase separation occurredFeed in the reactions with lowTime molecular ( weightkPa) PEGDA200(kPa) in EtOH,(%) as described below. Slight phase separation in PEI or DETA-PEGDA(400,(min) 600, 1000) reaction systems in EtOH9 wouldPEI make the200 gels soft.Case 1 1.53 3 540 871 24.4 10 The aza-MichaelPEI 400 addition Case reaction 1 of PEI1.05 or DETA and5 PEGDA200850 in EtOH yielded811 the gels29.6 and porous11 polymersPEI depended400 onCase the 2reaction 1.12 conditions. 135 Figure 5 shows740 the production777 diagram31.5 of PEI-PEGDA20012 PEI reaction600 systemsCase in 1 EtOH at0.80 20 ◦ C. The porous11 polymers265 were obtained784 under32.6 the feed molar13 ratiosPEI of PEI/DEGDA2001000 Case in the 1 range0.53 of 2 /12–2/14123 and monomer552 concentrations 751 in the37.1 range of 20–2514 wt %.PEI The feed1000 molar ratioCase of 2 PEI/DEGDA2000.55 of Case 2 corresponds471 to 2/7.9.664 That means50.0 the porous15 polymersDETA were400 obtained Case under 1 conditions1.14 in which16 the molar concentrations552 751 of acrylate37.2 groups were16 higherDETA than that400 of activeCase hydrogens. 2 The1.09 unreacted52 acrylate groups471 of PEGDA200664 would50.4 form dangling chains in the reactiona Molar system, concentration which wouldof acrylate be usablegroup of to PEGDA yield porous. polymers. The mechanical properties of the PEI or DETA-PEGDA gels in EtOH were also investigated by the compression test, and the results are summarized in Table 3. The gels in EtOH showed a much lower Young’s modulus than the corresponding gels in DMSO. One explanation of the results is the strict difference in the phase of the gels. The reactions in DMSO occurred in the homogeneous phase, and yielded transparent gels. By contrast, the gels prepared in EtOH slightly turned white. This would be an intermediate state between homogeneous gel and porous polymer induced by phase separation. The phase separation occurred in the reactions with low molecular weight PEGDA200 in

Polymers 2020, 12, x 10 of 17

EtOH, as described below. Slight phase separation in PEI or DETA-PEGDA(400, 600, 1000) reaction systems in EtOH would make the gels soft.

Table 3. Gel formation time and mechanical properties of PEI or DETA-PEGDA gels, solvent: EtOH, monomer concentration: 30 wt %.

Gel Amine/ Young’s Stress Strain Run PEGD Acrylate a Formatio Amine PEGDA Modulus at Break at Break A (mol/L) n Time Feed (kPa) (kPa) (%) (min) 17 PEI 400 Case 1 0.79 13 92.1 223 27.1 18 PEI 400 Case 2 0.64 41 67.2 226 28.1 Polymers19 2020, 12PEI, 2047 600 Case 1 0.60 17 62.7 231 1030.5 of 17 20 PEI 1000 Case 1 0.39 1000 37.4 233 37.1 21Table 3. GelPEI formation 1000 time andCase mechanical 2 properties0.28 of PEI or DETA-PEGDA32.4 gels, solvent:195 EtOH,42.6 22monomer DETA concentration: 400 30 wtCase %. 1 0.86 63 81.3 86 37.0 23 DETA 400 Case 2 0.66 134 73.5 191 50.6 a Amine/ Gel Young’s Stress at Strain at Molar concentrationAcrylate of acrylate group of PEGDA. Run Amine PEGDA PEGDA Formation Modulus Break Break a (mol/L) The aza-Michael addition reactionFeed of PEI or DETATime and (min) PEGDA200(kPa) in EtOH(kPa) yielded the(%) gels and porous17 polymers PEI depended 400 on Case the 1 reaction 0.79 conditions. 13Figure 5 shows 92.1 the production 223 diagram 27.1 of PEI18-PEGDA200 PEI reaction 400 systems Case in 2 EtOH 0.64at 20 °C. The 41 porous polymers 67.2 were 226 obtained under 28.1 the feed19 molar PEI ratios of 600PEI/DEGDA200 Case 1 in the 0.60 range of 2/12 17–2/14 and 62.7monomer concentrations 231 30.5 in the range20 of 20 PEI–25 wt %. 1000 The feed Case molar 1 ratio 0.39 of PEI/DEGDA200 1000 of Case 37.4 2 corresponds 233 to 2/7.9. 37.1 That 21 PEI 1000 Case 2 0.28 32.4 195 42.6 means22 the DETA porous polymers 400 were Case obtained 1 0.86 under conditions 63 in which 81.3 the molar 86 concentrations 37.0 of acrylate23 DETAgroups were 400 higher Case than 2 that of 0.66 active hydrogens. 134 The 73.5 unreacted 191 acrylate gr 50.6oups of PEGDA200 would form danglinga Molar concentration chains in ofthe acrylate reaction group system, of PEGDA. which would be usable to yield porous polymers.

Figure 5. Production diagram of PEI-PEGDA200 reaction system in EtOH, reaction temperature: 20 ◦C, ∆ gel, porous polymer, precipitate. Figure• 5. Production diagram of PEI-PEGDA200 reaction system in EtOH, reaction temperature: 20 °C , Δ gel, ● porous polymer, □ precipitate. Figure6 shows the production diagram of DETA-PEGDA200 systems in EtOH, monomer concentration: 20 wt %. The porous polymers were obtained under the wide range of reaction conditions. Figure 6 shows the production diagram of DETA-PEGDA200 systems in EtOH, monomer The equivalent molar feed of active hydrogen to acrylate, feed molar ratio of DETA/PEGDA200: 2/5 concentration: 20 wt %. The porous polymers were obtained under the wide range of reaction corresponding to Case 2, must be suitable in this reaction system to obtain the porous polymers conditions. The equivalent molar feed of active hydrogen to acrylate, feed molar ratio of at a wide range of polymerization temperatures. The eﬀect of the monomer concentration and DETA/PEGDA200: 2/5 corresponding to Case 2, must be suitable in this reaction system to obtain the polymerization temperature on the production state of the DETA-PEGDA200 system was investigated porous polymers at a wide range of polymerization temperatures. The effect of the monomer in the reactions of Case 2, as shown in Figure7. The reactions with low monomer concentration, concentration and polymerization temperature on the production state of the DETA-PEGDA200 15 wt%, yielded the porous polymer at a wide range of reaction temperatures, from 20 to 50 C. By system was investigated in the reactions of Case 2, as shown in Figure 7. The reactions with◦ low contrast, the reactions with high monomer concentrations, 25–30 wt%, yielded the porous polymers at low reaction temperatures, 10–15 ◦C. One explanation of the results is that the polymerization (gel formation) rate tends to be much higher than the phase separation rate in the reactions with high monomer concentrations owing to the high crosslinking density in the reaction system, and low reaction temperatures should be necessary to induce the phase separation by reducing the solubility of the polymer network. Polymers 2020, 12, x 11 of 17 Polymers 2020, 12, x 11 of 17 monomer concentration, 15 wt%, yielded the porous polymer at a wide range of reaction monomertemperatures, concentration, from 20 to 50 15 ° C. wt%, By contrast, yielded the the reactions porous with polymer high monomer at a wide concentrations, range of reaction 25–30 temperatures,wt%, yielded fromthe porous 20 to 50 polymers °C. By contrast, at low reaction the reactions temperatures, with high 10 monomer–15 °C. One concentrations, explanation 25of– the30 wt%,results yield is ed that the the porous polymerization polymers at (gel low fo reactionrmation )temperatures, rate tends to 10 be– 15 much °C. One higher explanation than the of phase the resultsseparation is that rat e the in the pol ymerizationreactions with (gel high for monomermation) rat concentrationse tends to be owing much to higher the high than crosslinking the phase separationdensity in rat thee reactionin the reactions system , withand highlow reactionmonomer temperatures concentrations should owing be to necessary the high tocrosslinking induce the Polymers 2020, 12, 2047 11 of 17 densityphase separation in the reaction by reducing system the, and solubility low reaction of the polymer temperatures network should. be necessary to induce the phase separation by reducing the solubility of the polymer network.

Figure 6. Production diagram of DETA-PEGDA200 reaction system in EtOH, monomer concentration Figure 6. Production diagram of DETA-PEGDA200 reaction system in EtOH, monomer concentration Figurein reaction 6. Production solution: diagram20 wt%, of△ DETAgel, ●- PEGDA200porous polymer, reaction □ systemprecipitate. in EtOH, monomer concentration in reaction solution: 20 wt%, gel, porous polymer, precipitate. in reaction solution: 20 wt%, △4 gel,• ● porous polymer, □ precipitate.

Figure 7. Production diagram of DETA-PEGDA200 reaction system in EtOH, Case 2, gel, porous Figure 7. Production diagram of DETA-PEGDA200 reaction system in EtOH, Case 2, △4 gel, •● porous polymer, precipitate. Figurepolymer, 7. Production □ precipitate. diagram of DETA-PEGDA200 reaction system in EtOH, Case 2, △ gel, ● porous polymer,The surface □ precipitate. structure of the PEI or DETA-PEGDA200 porous polymers prepared in EtOH was observedThe bysurface SEM. structure Figure8 shows of the the PEI SEM or DETA images-PEGDA200 of the PEI-PEGDA200 porous polymers (a, b, c) prepared and DETA-PEGDA200 in EtOH was (d,observed e,The f) drys urface by porous SEM. structure Figure polymers of 8 the shows obtained PEI or the DETA from SEM- thePEGDA200 images reaction of theporous systems PEI -polymersPE withGDA200 20, prepared 25, (a, and b, c)30 in and wt%EtOH DETA of was the- observedmonomerPEGDA200 by concentrations. (d, SEM. e, f) dry Figure porous The 8 shows morphologypolymers the SEMobtained of the images polymersfrom of the the reaction obtained PEI-PE systemGDA200 from thes with reaction (a, 20, b, c)25, systemsand and DETA30 withwt%- PEGDA20020of the wt% monomer monomer (d, e, concentrations.f) dry concentration porous polymers The was morphology formed obtained by offrom connected the thepolymers reaction spheres obtained system about sfrom with 10 µ the m20, inreaction 25, both and reaction30systems wt% ofsystems,with the 20monomer wt% as shown monomer concentrations. in Figure concentr8a,d. Theation The m w increaseorphologyas formed of theof by the monomerconnected polymers concentration spheres obtained about from in 10 thethe μm reactionreaction in both systems, systemsreaction with25systems, wt%, 20 wt% as decreased shown monomer in the Figure concentr size 8a of,dation the. The spheres,w increaseas formed as ofshown theby connectedmonomer in Figure concentrationspheres8b,e. about Co-continuous in10 the μm reaction in both monolithic reactionsystems, systems,structure25 wt%, as decreased was shown observed in theFigure insize the8a of,d polymers . theThe spheresincrease obtained, of as the shown frommonomer inthe Figure reaction concentration 8b, systemse. Co -incontinuous the with reaction 30 wt monolithic systems, % of the 25monomerst ructure wt%, decreased wasconcentration, observed the size asin shown the of polymers the in spheres Figure obtained8, c,f. as These shown from structures in the Figure reaction should 8b ,e systems. be Co induced-continuous with by 30 the wt monolithic spinodal% of the stdecompositionmonomerructure was concentration, observed during the in as network theshown polymers in formation Figure obtained 8c in,f. theThese from reaction structures the reactionsystems should [ systems29– be32 ].induced In with the 30 caseby wtthe of % spinodal the of high the monomermonomerdecomposition concentration, concentration during the as reaction networkshown (30in formation Figure wt%), 8c the ,inf. phase Thesethe reaction separationstructures system should wass [ ﬁxed29 be–32 induced at]. In the the early bycase the stage of spinodal the of high the decompositionspinodal decomposition during the of network the co-continuous formation monolithicin the reaction structure system owings [29– to32]. the In highthe case polymerization of the high rate with the high crosslinking density in the reaction system. The structure of connected spheres in Figure8a,b,d,e would be formed at the late stage of the spinodal decomposition owing to the low polymerization rate in the reactions with a lower monomer concentration (15 and 20 wt %). Polymers 2020, 12, x 12 of 17 monomer concentration reaction (30 wt%), the phase separation was fixed at the early stage of the spinodal decomposition of the co-continuous monolithic structure owing to the high polymerization rate with the high crosslinking density in the reaction system. The structure of connected spheres in Polymers 2020, 12, 2047 12 of 17 Figure 8a,b,d,e would be formed at the late stage of the spinodal decomposition owing to the low polymerization rate in the reactions with a lower monomer concentration (15 and 20 wt %).

Figure 8. 8. ScanningScanning electron electron microscopy microscopy (SEM (SEM)) images images of (a–c of) PEI (a–-cPEGDA200) PEI-PEGDA200 and (d– andf) DETA (d–f-) PEGDA200DETA-PEGDA200 dry porous dry polymers porous polymers;; monomer monomer concentration concentration in the reaction in the system: reaction (a system:,d) 20 wt%, (a, d(b), 20e) 25wt%, wt%, (b, eand) 25 (c wt%,,f) 30 and wt% (c; ,feedf) 30 molar wt%; feedratio molar of PEI/PEGDA200: ratio of PEI/PEGDA200: 2/14, DETA/PEGDA200: 2/14, DETA/PEGDA200: 2/5. 2/5.

The porosity of the dry porous polymer was determined by the following equation: The porosity of the dry porous polymer was determined by the following equation:

Porosity푃표푟표푠푖푡푦[ [−]] = =1 1−W푊//ρ휌V푉 (1(1)) − − where W, ρ, and V are weight (g), true density (g(g/cm/cm3),), and and volume volume (cm (cm33)) of of the the porous porous polymer, polymer, respectively. The p porosityorosity of the dry porous polymers ranged from about 65 to 70%, and decreased with the increas increasee in the monomer concentration in in the the reaction reaction system, system, as as summarized summarized in in Table Table 4.. Porous polymers with PEI and DETA sho showedwed similar values of porosity. The s specificpecific surface area of the porous polymerspolymers waswas determined determined by by the the BET BET method method (Supplementary (Supplementary Material Material Figure Figure S9), S9), and and the theresults results are are summarized summarized in Table in Table4. The 4. The values values were were relatively relatively low low owing owing to the to notthe micro-porous,not micro-porous but, butmacro-porous macro-porous structure structure of the of the porous porous polymers. polymers. The The DETA-PEGDA200 DETA-PEGDA200 porous porous polymers polymers showed showed a alarger larger surface surface area area than than the the PEI-PEGDA200 PEI-PEGDA200 porous porous polymers. polymers. One One explanation explanation of theof the result result is that is that the theuneven uneven structure structure on theon the skeleton skeleton of theof the DETA-PEGDA200 DETA-PEGDA200 porous porous polymer, polymer, as observedas observed in Figurein Figure8f, 8fwould, would increase increase the the specific specific surface surface area. area. Mechanical properties of the PEI and DETA-PEGDA200 dry porous polymers, which were obtainedTable from 4. theStructure reactions and mechanicalwith the feed properties molar ofratio PEI of or DETA-PEGDA200PEI/PEGDA200: dry2/14 porous or DETA/PEGDA200: polymers. 2/5 (Case 2) in EtOH, were investigated by the compression test. Stress–strain curves of these porous Amine/ Monomer Specific Young’s polymersRun are shown Amine in FigurePEGDA200 9, and theConcentration results are summarizeda Porosity (%)in TableSurface 4. All the porousModulus polymers were soft and flexible, and molunbreakable/mol under(wt%) the pressure of 50 N. The compressedArea (m2/g) porous(kPa) polymers quickly24 returned PEI to the original 2/14 shape when 20 the pressure 71.4 was released ( 2.2Supplementary 4.5 Material Figure 25S10). Young’s PEI modulus 2/14 of the porous 25polymer increased 65.7 with the increas 1.8e in the 19.3 monomer concentration26 in PEI the reaction 2/ 14 system owing 30 to the increase 64.0 of the true 1.2density (decrease 22.2 of the porosity)27. The Young’s DETA modulus 2/5 Case values 2 of the 20 porous polymers 72.9 with PEI were 4.7 lower than 37.4 those with 28 DETA 2/5 Case 2 25 67.0 3.8 62.7 DETA. The branched PEI structure should induce soft and flexible features of the porous polymers. 29 DETA 2/5 Case 2 30 65.3 2.6 67.2

a Molar concentration of the acrylate group of PEGDA200 in the reaction system.

Mechanical properties of the PEI and DETA-PEGDA200 dry porous polymers, which were obtained from the reactions with the feed molar ratio of PEI/PEGDA200: 2/14 or DETA/PEGDA200: 2/5 (Case 2) in EtOH, were investigated by the compression test. Stress–strain curves of these porous polymers are shown in Figure9, and the results are summarized in Table4. All the porous polymers Polymers 2020, 12, x 13 of 17

Table 4. Structure and mechanical properties of PEI or DETA-PEGDA200 dry porous polymers.

Specific Amine/ Monomer Young’s Run Porosity Surface Amine PEGDA200 Concentration a Modulus (%) Area mol/mol (wt%) (kPa) (m2/g) 24 PEI 2/14 20 71.4 2.2 4.5 25 PEI 2/14 25 65.7 1.8 19.3 26 PEI 2/14 30 64.0 1.2 22.2 27 DETA 2/5 Case 2 20 72.9 4.7 37.4 28 DETA 2/5 Case 2 25 67.0 3.8 62.7 29 DETA 2/5 Case 2 30 65.3 2.6 67.2 a Molar concentration of the acrylate group of PEGDA200 in the reaction system. The dried porous polymer absorbs various solvents owing to the high affinity of the PEG linkage to the solvents. Figure 10 shows the absorption capacity of PEI or DETA-PEGDA200 porous polymers obtained from the reaction systems in EtOH with 20 wt % monomers. The polymers absorbed the solvents listed in Figure 10 and gained 100–350% in volume based on the original size. The solvents were absorbed by soaking to vacant space and swelling of the spheres in the porous polymers. The volume gain of the porous polymers by the absorption is induced by swelling of the spheres. The swelling ratio should be affected by the affinity between the polymer network and the solvents. The Polymers 2020, 12, 2047 13 of 17 affinity between PEG and the solvent was quantitatively evaluated by Hansen solubility parameter (Hansen distance: Ra = {4*(dD1 - dD2)2 + (dP1 - dP2)2 + (dH1 – dH2)2}1/2, where 1: PEG; 2: solvent; and weredD, soft dP, and and ﬂexible, dH are andthe energy unbreakable from distribution under the pressure forces, dipolar of 50 N. intermolecular The compressed force porouss, and polymers hydrogen quicklybond returned between to molecules, the original respectively) shape when [33] the. pressureHansen was distance released values (Supplementary of PEG-DMSO, Material PEG- Figuredichloromethane, S10). Young’s and modulus PEG-chloroform of the porous are polymer relatively increased smaller withthan theother increase solvents, in the indicating monomer high concentrationaffinity between in the reaction PEG and system these owing solvents to the (Supplementary increase of the true Materi densityal Table (decrease S1). The of the high porosity). affinity Thebetween Young’s the modulus polymer values network of the and porous DMSO, polymers dichloromethane, with PEI were or lower chloroform than those should with induce DETA. a The high branchedswelling PEI ratio structure of the porous should polymers. induce soft and ﬂexible features of the porous polymers.

300

250

200

150 (f) (e)

Stress [kPa] Stress (d) 100 (c) (b) (a)

0 0 20 40 60 80 100 Strain [%]

Figure 9. Stress–strain curves of PEI-PEGDA200 (bold lines: a–c) and DETA-PEGDA200 (thin lines: d–fFigure) dry porous 9. Stress polymers;–strain curves monomer of PEI concentration-PEGDA200 in (bold the reaction lines: a system:–c) and (DETAa,d) 20-PEGDA200 wt %, (b,e) 25(thin wt lines: %, andd– (cf,)f dry) 30 wtporous %; feed polymers molar; ratio monomer of PEI concentration/PEGDA200: 2 in/7 orthe DETA reaction/PEGDA200: system: (a 2,/d5) Case20 wt 2). %, (b,e) 25 wt %, and (c,f) 30 wt %; feed molar ratio of PEI/PEGDA200: 2/7 or DETA/PEGDA200: 2/5 Case 2). The dried porous polymer absorbs various solvents owing to the high affinity of the PEG linkage to the solvents. Figure 10 shows the absorption capacity of PEI or DETA-PEGDA200 porous polymers obtained from the reaction systems in EtOH with 20 wt % monomers. The polymers absorbed the solvents listed in Figure 10 and gained 100–350% in volume based on the original size. The solvents were absorbed by soaking to vacant space and swelling of the spheres in the porous polymers. The volume gain of the porous polymers by the absorption is induced by swelling of the spheres. The swelling ratio should be affected by the affinity between the polymer network and the solvents. The affinity between PEG and the solvent was quantitatively evaluated by Hansen solubility parameter (Hansen distance: 2 2 2 1/2 Ra = {4*(dD dD ) + (dP dP ) + (dH dH ) } , where 1: PEG; 2: solvent; and dD, dP, and dH 1 − 2 1 − 2 1 − 2 are the energy from distribution forces, dipolar intermolecular forces, and hydrogen bond between molecules, respectively) [33]. Hansen distance values of PEG-DMSO, PEG-dichloromethane, and PEG-chloroform are relatively smaller than other solvents, indicating high affinity between PEG and these solvents (Supplementary Material Table S1). The high affinity between the polymer network and DMSO, dichloromethane, or chloroform should induce a high swelling ratio of the porous polymers. Polymers 2020, 12, 2047 14 of 17 Polymers 2020, 12, x 14 of 17

400

350

300

250

200

150 Volume gain[%] Volume

100

Absorption solvent

Figure 10. Absorption capacity of PEI-PEGDA200 (white) and DETA-PEGDA200 (gray) porous polymers,Figure 10.monomer Absorption concentration capacity in of reaction PEI-PEGDA200 system: 20 (white)wt %, feed and molar DETA ratio-PEGDA200 of PEI/PEGDA200: (gray) porous 2/7, polymers, monomer concentration in reaction system: 20 wt %, feed molar ratio of PEI/PEGDA200: DETA/PEGDA200: 2/5 (Case 2), preparation temperature: 20 ◦C. 2/7, DETA/PEGDA200: 2/5 (Case 2), preparation temperature: 20 °C. 4. Conclusions 4. ConclusionsThe network polymers with a PEG unit were successfully synthesized by ring opening addition reactionsThe of network conventional polymers multi-functional with a PEG unit amines, were successfully PEI or DETA, synthesized with PEGDE by ring in opening H2O at addition room temperaturereactions of without conventional a catalyst, multi or in-functional DMSO at 90 amines,◦C using PEI PPh or3 asDETA, the catalyst. with PEGDE The network in H2 polymersO at room weretemperature also synthesized without bya catalyst the aza-Michael, or in DMSO addition at 90 °C reaction using PPh of PEI3 as orthe DETA catalyst. with The PEGDA network in polymers DMSO orwere EtOH also at roomsynthesized temperature. by the aza The-Michael feed ratio addition of amines reaction to epoxyof PEI oror acrylateDETA with group, PEGDA structure in DMSO of amineor EtOH monomers, at room moleculartemperature. weight The offeed PEGDE ratio of or amines PEGDA, to andepoxy feature or acrylate of the group, solvent structure affected theof amine gel formationmonomers, time molecular and mechanical weight properties of PEGDE of the or resultantPEGDA, gels.and feature The increase of the of solvent the PEGDE affected or PEGDA the gel feedformation ratio (Case time 2)and induced mechan aical long properties gel formation of the resultant time owing gels. to The the increas steric hindrancee of the PEGDE of secondary or PEGDA aminesfeed ratio formed (Case by 2) the induced addition a reactionlong gel offormation a primary time amine owing and to one the epoxy steric or hindrance acrylate group.of secondary The gelsamines formed formed by higher by the feed addition ratio of reaction PEGDE of (Case a primary 2) showed amine a higher and one Young’s epoxy modulus or acrylate owing group. to the The highgels crosslinking formed by higher density. feed By ratio contrast, of PEGDE the gels (Case with 2) PEGDA showed showeda higher oppositeYoung’s resultsmodulus owing owing to to the the lowhigh reaction crosslinking conversions density. in theBy reactionscontrast, the of Case gels 2.with The PEGD increaseA showed of the molecular opposite results weight owing in PEGDE to the orlow PEGDA reaction increased conversions the gel in formation the reactions time of and Case decreased 2. The increase the Young’s of the modulus molecular of weight the gels in owing PEGDE toor the PEGDA low crosslinking increased the density gel formation under the time same and monomer decreased concentration. the Young’s modulus The reaction of the systems gels owing with to PEIthe showed low crosslinking a longer gel density formation under time the same and yielded monomer gels concentration. with a higher The Young’s reaction modulus systems than with the PEI correspondingshowed a longer reaction gel systems formation and time gels and with yielded DETA. gelsThese with results a higher indicate Young’s inter-penetration modulus than of the the polymercorresponding networks reaction derived systems from the and specific gels with structure DETA of. PEI,These which results would indicat playe inter a role-penetrat of the physicalion of the crosslinkingpolymer networks points. derived from the specific structure of PEI, which would play a role of the physical crosslinkingThe reaction points. of PEI or DETA with low molecular weight PEGDA200 in EtOH under specific conditionsThe yieldedreaction porousof PEI or polymers DETA with induced low bymolecular phase separation weight PEGDA200 during the in networkEtOH under formation. specific Theconditions morphology yielded of the porous porous polymers polymers induced showed by phase connected separation spheres during or a co-continuous the network formation. monolithic The structuremorphology depending of the on porous the reaction polymers conditions. showed The connected porous polymers spheres or showed a co-continuous flexible features monolithic and werestructure unbreakable depending by the on compression. the reaction Theconditions. porous The polymers porous absorbed polymers various showed solvents flexible owing features to the and highwere affi unbreakablenity with a PEG by the unit compression. in the polymer The network. porous polymers absorbed various solvents owing to the highAs affinity mentioned with above,a PEG unit the additionin the polymer reactions network. of the multifunctional-amines, PEI or DETA, and PEGDEAs or mentioned PEGDA based above, on the addition joint and reactions linker concept of the mustmultifunctional be usable methods-amines, toPEI synthesize or DETA the, and networkPEGDE polymers or PEGDA containing based on a the PEG joint unit. and We linker are studying concept the must characteristic be usable methods network structureto synthesize in the the network polymers containing a PEG unit. We are studying the characteristic network structure in the

Polymers 2020, 12, 2047 15 of 17 porous polymers, especially the formation process of the co-continuous structure, and applications such as column, support of conductive material, separator of battery, and scaﬀold of cell cultivation, among others, and the results will be reported elsewhere.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/9/2047/s1, Figure S1: FT-IR spectra of PEI-PEGDE400 reaction system Case 2; Figure S2: Time evolution of viscosity of PEI-PEGDE400 and PEI-PEGDE1000 reaction systems in H2O; Figure S3: Time evolution of viscosity of PEI-PEGDE400 reaction systems in DMSO; Figure S4: FT-IR spectra of PEI-PEGDA400 reaction system of Case 2; Figure S5: Time evolution of viscosity of PEI-PEGDA200, PEI-PEGDA400, and PEI-PEGDA600 reaction systems in DMSO; Figure S6: Time evolution of viscosity of PEI-PEGDA400 and DETA-PEGDA400 reaction systems in DMSO; Figure S7: Time-evolution of PEI-PEGDA400, PEI-PEGDA600, and PEI-PEGDA1000 reaction systems in EtOH; Figure S8: Time-evolution of viscosity of DETA-PEGDA400, DETA-PEGDA600, and DETA-PEGDA1000 systems in EtOH; Figure S9: Adsorption isotherm of PEI-PEGDA200 and DETA-PEGDA200 porous polymers; Figure S10: Cycle test (compression and release) of DETA-PEGDA200 porous polymer; Table S1: Affinity of PEG and solvents calculated by Hansen solubility parameters. Author Contributions: Conceptualization, N.N. and T.N.; analysis, H.N.; investigation, M.S. and K.M.; writing—original draft preparation, N.N.; writing—review and editing, T.N.; supervision, N.N. and T.N.; project administration, N.N.; funding acquisition, N.N. All authors have read and agreed to the published version of the manuscript. Funding: This work was partially supported by JSPS KAKENHI Grant Number 24550261 and in part by KAKENHI Grant Number JP19H02759. Conflicts of Interest: The authors declare no conflict of interest.

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