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Low-Molecular-Weight, High-Mechanical-Strength, and SolutionProcessable Telechelic Poly(ether imide) End-Capped with Ureidopyrimidinone Ke Cao† and Guoliang Liu*,†,‡ †

Macromolecules Innovation Institute (MII) and ‡Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *

ABSTRACT: Solution-processable poly(ether imide)s (PEIs) with ureidopyrimidinone (UPy) end groups were prepared by incorporating monoisocyanato-6-methylisocytosine into amineterminated PEI oligomers. After functionalization with UPy end groups, PEI with a molecular weight as low as 8 kDa (8k-PEIUPy) can be solution-cast to form films. Tensile tests revealed that 8k-PEI-UPy had an outstanding Young’s modulus higher than those of state-of-the-art high-molecular-weight commercial PEIs. The tensile strength, maximum elongation, and Young’s modulus of 8k-PEI-UPy were 87.2 ± 10.8 MPa, 3.10 ± 0.39%, and (3.20 ± 0.14) × 103 MPa, respectively. The discovery herein significantly advances the chemistry of high-temperature PEI resins. UPybased supramolecular chemistry is an effective and general strategy to achieve outstanding mechanical properties for PEI oligomers.

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INTRODUCTION Poly(ether imide) (PEI) is a high-temperature engineering thermoplastic with outstanding mechanical properties, thermal stability, and chemical resistance.1−4 Because of the excellent properties, PEI is widely used as matrix resins,5,6 adhesives,7,8 and coatings9,10 in fields such as aerospace and microelectronics. 11−17 High-molecular-weight 2,2-bis[4-(3,4dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) based PEI can be processed at ∼340 °C due to the flexible linkages in PEI backbones, i.e., ether (−O−) 18 and isopropylidene [−C(CH3)2−].19 However, the required processing temperature is high, and as a result, the process is energy-inefficient. Furthermore, it can lead to slow thermal degradation when the polymer is processed in air.2 To overcome these drawbacks possessed by high-molecular-weight PEIs, our strategy herein is to synthesize PEI oligomers that allow for lower processing temperatures and alternative methods such as solution-casting. To ensure that the PEI oligomers can be linked to form PEIs with high molecular weights and maintain the mechanical strength, we have introduced interactive end groups. Specifically, by end-capping PEI with ureidopyrimidinone (UPy) that enables interactive hydrogen bonding, PEI can form supramolecular polymers with excellent mechanical properties. Hydrogen-bonded supramolecular polymers, especially those based on UPy quadruple hydrogen bonding, have been studied extensively. The first report by Meijer and co-workers20 showed that UPy-based telechelic oligomers formed linear supramolecular polymers. The complementary quadruple hydrogen bonds between UPy groups have a dimerization constant of Kdim ∼ 107 M−1 in chloroform (CHCl3).21,22 Since then, many © XXXX American Chemical Society

have prepared UPy-based supramolecular polymers to improve mechanical properties and increase functionalities of materials. For example, Stang and co-workers have coupled UPy with other orthogonal interactions such as coordination-driven selfassembly and host−guest interactions to achieve supramolecular polymers with high structural ordering23 and complex functionalities.24 Guan and co-workers utilized UPy cross-linker to increase the stiffness of elastomers without sacrificing extensibility.25 Wang and co-workers combined UPy hydrogen bonding and host−guest interactions to prepare multiple hydrogen-bonding interlocked catenanes.26 The more widely used method is to modify the end groups of nucleophilic hydroxyl- or amine-terminated oligomers with electrophilic monoisocyanato-6-methylisocytosine (UPy-synthon) to create UPy-functionalized supramolecules with increased mechanical strength while inheriting the processability of oligomers.27−30 Meijer and co-workers attached UPy to oligomers of polydimethylsiloxanes, poly(ethylene oxide)s, polyethers, and polycarbonates.27,31,32 By incorporating UPy into low-molecular-weight polyesters, Long and co-workers successfully lowered the melt viscosity but maintained strong mechanical properties of polyesters.33 Similarly, they used UPy to increase the Young’s moduli of poly(ethylene-co-propylene)s.34 Nasseri and co-workers enhanced the storage moduli of poly(ethyleneco-vinyl alcohol)s by grafting UPy onto the side chains.35 Weder and co-workers showed that UPy-functionalized side chains increased mechanical properties of poly(methacrylReceived: January 26, 2017 Revised: February 20, 2017

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Macromolecules amide).36 All these studies focused on incorporating UPy into polymers whose glass transition temperatures (Tg) are lower than 100 °C due to the concerns of thermal stability of UPy. With Tg above 200 °C, PEIs were never functionalized by UPy but simple hydrogen bonding end groups such as uracil37 and benzimidazole,38,39 yielding mild improvement in mechanical properties of PEI fibers.40,41 In model product studies, however, Armstrong and Buggy reported that UPy did not degrade until ∼240 °C,42,43 suggesting that UPy is applicable to polymers with high Tgs such as PEI. In this work, we describe the synthesis of thermally stable and solution-processable UPy-terminated PEI oligomers by functionalizing amine-terminated PEIs (PEI-NH2, Scheme 1)

stable by itself, the UPy end group exhibited excellent thermal stability after being incorporated into PEI oligomers.

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EXPERIMENTAL SECTION

Materials. 2,2-Bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) was supplied by SABIC and subjected to a heating−cooling cycle to remove any residual moisture before use. Phthalic anhydride (PA) was provided by SABIC and used as received. m-Phenylenediamine (mPD, 99%) was purchased from Sigma-Aldrich and purified by sublimation before use. 6-Methylisocytosine (MIC, 98%), hexamethylene diisocyanate (HMDI, 99%), o-dichlorobenzene (oDCB), dibutyltin dilaurate (95%), tetrahydrofuran (THF, 99.9%), and silica gel were purchased from Sigma-Aldrich and used as received. Hexyl isocyanate (98%) was purchased from TCI chemicals and used as received. Hexanes was purchased from Fisher Chemical. CHCl3 was obtained from Spectrum Chemical. Methanol (MeOH) was obtained from Pharmco-AAPER. All deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. All solvents were used as received. Analytical Methods. Proton nuclear magnetic resonance (1H NMR) spectroscopy characterization was performed on a Varian Unity 400 at 399.98 MHz in deuterated chloroform. Thermogravimetric analysis (TGA) was performed by heating the samples to 600 °C under a nitrogen flush of 60 mL/min on a TA Instruments Q500 TGA. Differential scanning calorimetry (DSC) was performed under a nitrogen flush of 50 mL/min at a heating rate of 10 °C/min on a TA Instruments Q1000 DSC, which was calibrated using indium (melting point (mp) = 156.60 °C) and zinc (mp = 419.47 °C) standards. Tg was measured as the midpoint of the transition in the second heating ramp. Polymers were dissolved in CHCl3 and cast into a Teflon Petri dish, followed by slow evaporation of the solvent and drying the film at 180 °C in vacuo. Chloroform size exclusion chromatography (SEC) provided absolute molecular weights using a Waters 1515 isocratic HPLC pump and a Waters 717plus autosampler with Waters 2414 refractive index and Wyatt MiniDAWN MALLS detectors (flow rate 1.0 mL/min). The column set consisted of two Shodex KF-801 columns and a guard column with the same stationary phase of crosslinked polystyrene. The columns and detectors were maintained at 35 °C. Tensile testing was performed on a 5500R Instron universal testing at a cross-head speed of 5 mm/min; the tensile strength, maximum elongation, and Young’s moduli are reported based on an average of five specimens. The viscosity measurements were carried out using a Brookfield viscometer (LVDV-E model) at room temperature. Synthesis of UPy-Synthon. UPy-synthon was synthesized following a method described in a previous report (Scheme 2).44 A suspension of MIC (12 g, 95.92 mmol) and 6.2-fold excess of HMDI (100 g, 595 mmol) was stirred at 100 °C for 24 h. The white product was precipitated in hexanes, washed with hexanes three times, filtered, and then dried at 40 °C in vacuo. 1H NMR (400 MHz, CDCl3, δ): 13.1 ppm (s, 1H, −NH−C(CH3)= (a)), 11.9 ppm (s, 1H, −NH−CO− NH−CH2− (b)), 10.2 ppm (s, 1H, −NH−CO−NH−CH2− (c)), 5.8 ppm (s, 1H, −CHC(CH3) (d)), 3.3 ppm (m, 4H, −NH−CO− NH−CH2−, −CH2NCO− (e)), 2.2 ppm (s, 3H, −NHC(CH3) CH−CO− (f)), 1.3−1.7 ppm (m, 8H, −(CH2)4− (g)). ESI-MS calcd: M = 293.1 g/mol; found: m/z 294.2. [M + H]+, 316.1 [M + Na]+, 587.3 [2M + H]+, 609.3 [2M + Na]+. Synthesis of PEI-NH2. Scheme 1 illustrates the general synthetic procedure and reaction conditions for an amine-terminated poly(ether imide) from BPADA and mPD with a stoichiometric imbalance. An exemplary synthesis of a 2500 g/mol PEI is described. A three-neck 500 mL round-bottomed flask, equipped with an overhead stirring rod, a Dean−Stark trap, and a nitrogen inlet, was charged with BPADA (16.800 g, 32.28 mmol), mPD (4.363 g, 40.35 mmol), and 60 mL of oDCB and then purged with N2. Subsequently, the slurry was heated to 180 °C and stirred for 12 h and then heated to 380 °C in a metal bath for 30 min. The entire reaction was conducted in a constant N2 stream. The oligomer was recovered by dissolution in CHCl3 and

Scheme 1. Synthesis of Amine-Terminated Poly(ether imide) Oligomer (PEI-NH2)

Scheme 2. Synthesis of Monoisocyanato-6-methylisocytosine (UPy-Synthon)

Scheme 3. Synthesis of UPy-Terminated Poly(ether imide) (PEI-UPy)

with UPy-synthon (Schemes 2 and 3). Stoichiometric imbalance of BPADA and m-phenylenediamine (mPD) ensured the synthesis of amine-terminated PEI, which could react with UPy-synthon for PEI functionalization. Tuning the molecular weight of PEI oligomers allowed for synthesis of UPyterminated poly(ether imide) (PEI-UPy) with various molecular weights and enabled the determination of the lowest molecular weight for PEI film formation. By comparing the mechanical properties of PEI-UPy with benchmark commercial PEIs, we showed that UPy incorporated PEI oligomers with a molecular weight as low as 8 kDa possessed outstanding mechanical properties comparable to state-of-the-art highmolecular-weight PEIs. Surprisingly, although thermally unB

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Macromolecules precipitation into MeOH. The precipitate was filtered and washed with MeOH three times and dried in vacuo at 180 °C for 8 h. Synthesis of PEI-UPy. PEI-UPy was prepared by reacting PEINH2 oligomers with an excess of the UPy-synthon in CHCl3. A typical synthesis is described as follows. A flame-dried, 100 mL roundbottomed flask was charged with PEI-NH2 (molecular weight (Mn) = 3.1 kg/mol, 3.100 g, 1 mmol), UPy-synthon (1.173 g, 4 mmol), and CHCl3 (60 mL) and then purged with N2. The reaction mixture was stirred at 60 °C for 24 h. Subsequently, 2.346 g of silica gel and 0.5 mL of 3 wt % dibutyltin dilaurate in THF were added to the mixture and allowed to react at 60 °C for 1 h. The suspension was diluted by 120 mL of CHCl3 and filtered through Celite. The filtrate was precipitated in MeOH. The precipitate was filtered and washed with MeOH three times and dried in vacuo at 100 °C for 24 h.

PEIs were end-capped with PA (Scheme S1 and Figure S2) to allow for SEC characterization. Thermal Analysis of PEI-NH2. TGA and DSC were used to analyze the thermal properties of PEI-NH2. TGA revealed that PEI-NH2 oligomers had outstanding thermal stability, which is attributed to the strong imide rings in the PEI backbones. All PEI-NH2 oligomers degraded through a single degradation step in the range of 400−600 °C (Figure 2). The decomposition temperatures at 5% weight loss (Td,5%) were about 500−520 °C (Table 1).

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RESULTS AND DISCUSSION PEI-NH2 Synthesis. A stoichiometric imbalance of the monomers in step-growth polymerization can yield telechelic polymers with specific end groups.45−47 By adding excess mPD into the reaction mixture, we synthesized PEIs with amine end groups (Scheme 1). 1H NMR spectroscopy confirmed the amine end groups (Figure 1) and allowed for quantification of Figure 2. TGA thermograms display high thermal stability of PEI-NH2 oligomers. The inlet shows the zoomed-in curves around 5% weight loss.

DSC was used to determine the glass transition temperatures of the PEI-NH2 oligomers (Figure 3a). As shown by the DSC traces, Tg increased with Mn following the Flory−Fox equation:48 K Tg = Tg, ∞ − Mn (1)

Figure 1. 1H NMR spectra of (a) 8k-PEI-NH2, (b) 6k-PEI-NH2, (c) 4k-PEI-NH2, and (d) 2k-PEI-NH2 in CDCl3. Peak intensities are normalized to peak i. Peaks f, g, and h are signals of the repeating units, and peaks f′, g′, and h′ are signals of the end group.

the degree of polymerization and molecular weight of PEI-NH2 (Table 1 and Figure S1). The molecular weights of PEI-NH2 calculated via 1H NMR were further confirmed by determining the molecular weights of phthalic anhydride (PA)-terminated PEIs using SEC (Table 1). Since PEI-NH2 cannot be directly characterized by SEC due to the strong interactions between amine end groups and our SEC columns, the amine-terminated Table 1. Summary of Mn, Td,5%, and Tg of PEI-NH2 Oligomers sample

theor Mn (kDa)

NMR Mn (kDa)

SEC Mna (kDa)

8k-PEI-NH2 6k-PEI-NH2 4k-PEI-NH2 2k-PEI-NH2

7.8 6.0 4.3 2.5

8.0 5.6 4.5 3.1

8.4 7.1 5.8 3.1

Td,5%b (°C) 520 511 513 500

± ± ± ±

12 20 17 24

Tg (°C) 205 199 195 186

± ± ± ±

0 0 0 1

a The Mn of PA-terminated PEI (PEI-PA) was first determined by SEC. The Mn of PEI-NH2 was then estimated by taking consideration of the molecular difference between PEI-NH2 and PEI-PA. See the Supporting Information for more details. bThe TGA analyses were based on five repeating measurements.

Figure 3. (a) DSC traces of PEI-NH2 oligomers. The Tg increased with Mn. (b) Fitting of Tg to the Flory−Fox equation. The solid line is a linear fit of Tg with respect to 1/Mn (eq 1). K = 100 °C kg mol−1; Tg,∞ = 217 °C. C

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Macromolecules The linear fitting (Figure 3b) had an R2 value of 0.9985 and suggested an intrinsic glass transition temperature (Tg,∞) of 217 °C, which is in excellent agreement with the Tg of state-of-theart high-molecular-weight PEIs (Tg = 217 °C).49 PEI-UPy Synthesis. Reacting PEI-NH2 with excess UPysynthon at a ratio of [NH2]:[NCO] = 1:2 transformed PEINH2 into PEI-UPy. Noticeably, the viscosity of the products increased significantly compared to the precursors. The 1H NMR spectra of PEI-UPy had characteristic downfield signals of UPy groups (9.9, 11.7, and 13.2 ppm; Figure 4 inset),

at an acceptable level of functionalization,33−35 particularly for the purpose of this work. The incompleteness of the PEI-NH2 to PEI-UPy conversion is probably due to the low solubility of UPy-synthon in chloroform and the high viscosity of the reaction mixtures. The viscosity can be potentially reduced by reacting PEI-NH2 with a mixture of UPy-synthon and ethylene glycol-UPy. As reported previously by Scherman et al.,50 UPysynthon can react with PEI-NH2 while ethylene glycol-UPy can lower the virtual molecular weight and hence the viscosity. Thermal Analysis of PEI-UPy. PEI-UPy exhibited two stages of weight loss, as shown by TGA (Figure 5). The first

Figure 5. TGA thermograms exhibit thermal stability of PEI-UPy oligomers, UPy-synthon, and MIC.

stage at ∼240 °C was due to the decomposition of 6methylisocytosine (MIC) moieties in PEI-UPy, which is suggested by the same onset degradation temperatures of PEI-UPy and pure MIC. The decomposition temperature of MIC moieties in PEI-UPy agreed well with the previous reports, whereas the urea bonds between alkyl chains and isocytosine rings were cleaved at ∼240 °C.42,43 It is noteworthy that the first degradation temperature of PEI-UPy (∼240 °C) was much higher than that of UPy-synthon (∼160 °C), which is due to the configurational change of −NCO after being incorporated into PEIs. In pure UPy-synthon, −CH2NCO degraded first at ∼160 °C, corresponding to a weight loss of 19%, followed by the cleavage of urea bonds at ∼240 °C, as shown by TGA (Figure 5). Once the −NCO group in UPysynthon was attached to PEIs, it changed to a urea bond that linked the PEI backbone with the hexyl chain. The TGA analyses showed that at ∼240 °C PEI-UPy had weight losses matching with the weight percentage of MIC in PEI-UPy oligomers of all molecular weights (Table 2). After cleaving the urea bond closest to the chain ends, the remaining polymer chains may react with one another to form PEIs of larger molecular weights, i.e., PEI-urea-hexyl-urea-PEI, similar to the formation of N,N-di-n-butylurea during the pyrolysis of N[(butylamino)carbonyl]-6-methylisocytosine in a previous report by Armstrong and Buggy.42 The weight loss at the second stage was mainly caused by the degradation of PEI backbones, similar to that of PEI-NH2. DSC measurements revealed that Tg of PEI-UPy increased with molecular weight, and there was a significant change in Tg after incorporating UPy into PEI-NH2 (Figure 6 and Table 2). In comparison with PEINH2, Tg of PEI-UPy increased by 8, 10, 13, and 19 °C for 8k-, 6k-, 4k-, and 2k-PEIs, respectively. The changes in Tg are more significant for the oligomers with lower molecular weights. Mechanical Properties. To test the mechanical properties of PEI-NH2 and PEI-UPy oligomers, we have solution-cast films using CHCl3. Among all the oligomers, only 8k-PEI-UPy

Figure 4. 1H NMR spectra of (a) 8k-PEI-UPy, (b) 6k-PEI-UPy, (c) 4k-PEI-UPy, and (d) 2k-PEI-UPy in CDCl3. Peak intensities are normalized to peak a for comparison.

confirming the successful incorporation of UPy moieties into the PEI oligomers. However, the three peaks shifted slightly in comparison with the original positions in the 1H NMR spectrum of UPy-synthon (Figure S3). Since there was excessive UPy-synthon in the reaction, adding silica gel removed the residual UPy-synthon in the reactor,27 as confirmed by 1H NMR spectroscopy (Figure S3, panels d and e). By determining the amount of PEI-NH2 in the product using 1H NMR, the conversion of PEI-NH2 to PEI-UPy was calculated (see Supporting Information). Although we have optimized the reaction conditions including solvent, temperature, and amount of UPy-synthon, the conversion of PEI-NH2 to PEI-UPy was not more than 90% (Figure S4 and Table 2). The degrees of UPy-functionalization of 2k-PEI-NH2 were 88%, 82%, 50%, and 78% in chloroform, DMF, DMSO, and NMP, respectively. The over 85% conversion in chloroform is Table 2. Degree of UPy Functionalization, MIC Weight Percent in PEI-UPy, Weight Loss of PEI-UPy at the First Degradation Stage, and Tg of PEI-UPy sample

degree of UPy functionalization (%)

MIC (wt %)

weight loss at 400 °C (%)

8k-PEI-UPy 6k-PEI-UPy 4k-PEI-UPy 2k-PEI-UPy

82 83 87 88

2.9 4.0 4.9 6.8

2.9 4.5 6.8 6.7

Tg (°C) 213 209 208 204

± ± ± ±

1 1 1 1 D

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Figure 6. DSC traces of PEI-UPy series.

and 8k-PEI-NH2 formed intact films (Figure 7). The filmforming ability of PEI seemed to mainly depend on the

Figure 8. Comparison of solution-cast (a) 8k-PEI-NH2 and (b, c) 8kPEI-UPy films. The 8k-PEI-UPy film was flexible and could be cut into dumbbell-shaped specimens, while the 8k-PEI-NH2 film cracked.

difference in film flexibility shows that the incorporation of UPy significantly improved the mechanical properties of the 8k-PEI oligomer. We further compared the tensile properties of the solutioncast 8k-PEI-UPy film with standard PEI films including PEI(1) and PEI(2) (Figure 9 and Table 3). Surprisingly, the 8k-PEIUPy film, with a molecular weight of only 8.7 kDa (calculated by assuming full conversion from 8k-PEI-NH2 to 8k-PEI-UPy), showed higher Young’s modulus than PEI(1) (Mn,SEC = 16.9 kDa) and PEI(2) (Mn,SEC = 24.5 kDa), while its tensile strength and maximum elongation were comparable to those of PEI(1) and PEI(2) films. These results showed that the incorporation of UPy end groups is an effective approach to improve the mechanical properties of low-molecular-weight PEIs. Origin of Changes in Tg and Improved Mechanical Strength of PEI-UPy. To verify whether the increase of Tg results from the urea linkage or the end UPy moiety, PEIs terminated with urea bonds and flexible hexyl groups (PEIUHex, Scheme 4) were synthesized as a set of reference polymers for comparison (Scheme S2). PEI-UHex differs from PEI-NH2 by having a hexyl group at each end of the polymer chain linked by urea; PEI-UPy differs from PEI-UHex by having a UPy end group at each end of the polymer chain linked by another urea. Surprisingly, after incorporation of UHex, Tg decreased or did not change significantly compared with PEINH2 (Table 4 and Figure S5), which is attributed to the two competing factors: hydrogen bonding and flexible alkyl chains. Being locked in hydrogen bonds, polymer chain ends tend to be less free and an increase of Tg is expected. On the other hand, short flexible alkyl chains are known to generate free volume at polymer chain ends, and thus a decrease of Tg is anticipated.52 Combining these two factors, it seems that the effect of flexible alkyl group outweighs that of the hydrogen bonding, leading to a decrease of Tg in PEI-UHex. In contrast, the combining hydrogen-bonding effects from urea linkage and UPy in PEI-UPy prevail over the flexible alkyl chain effect, resulting in increased Tg compared to PEI-NH2. It is interesting that although UPy multiple hydrogen bonding dissociates at above 80 °C,53 UPy still played a role in the increase of the relatively high Tg of PEI. In addition, all PEI-UHex and PEIUPy oligomers started to degrade at ∼240 °C by cleaving the polymer chains at the outmost urea bond (Figure 5 and Figure S6).

Figure 7. Solution-cast (a) 8k-PEI-NH2, (b) 6k-PEI-NH2, (c) 4k-PEINH2, (d) 2k-PEI-NH2, (e) 8k-PEI-UPy, (f) 6k-PEI-UPy, (g) 4k-PEIUPy, and (h) 2k-PEI-UPy. Only 8k-PEI-NH2 and 8k-PEI-UPy formed films.

molecular weight, and the end group had a minor effect, in agreement with the common understanding that molecular weight is the dominant factor for controlling polymer physical properties.51 The inability to form films prohibited comparing mechanical properties of PEI-NH2 and PEI-UPy with Mn lower than 6 kDa. Nevertheless, the comparison between 8k-PEI-UPy and 8kPEI-NH2 highlighted the improvement in mechanical properties after functionalization of PEI-NH2 with UPy. Although both 8k-PEI-NH2 and 8k-PEI-UPy showed film-forming ability, the solution-cast 8k-PEI-NH2 film was fragile and could not be cut into dumbbell structures (Figure 8a). In contrast, 8k-PEIUPy could be cut and the resulting dumbbell structures showed great bending flexibility (Figure 8, panels b and c). The sharp E

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Macromolecules Scheme 4. Structure of PEI-UHexa

a

PEI-UHex differs from PEI-UPy by having no UPy end groups.

Table 4. Comparison of Tg among PEI-NH2, PEI-UHex, and PEI-UPy mol wt (kDa)

Tg of PEI-NH2 (°C)

8 6 4 2

205 199 195 185

Tg of PEI-UHexa (°C) 200 189 195 185

(−5) (−10) (+0) (+0)

Tg of PEI-UPyb (°C) 213 209 208 204

(+8) (+10) (+13) (+19)

a Values in parentheses were the difference of Tg between PEI-UHex and PEI-NH2. bValues in parentheses were the difference of Tg between PEI-UPy and PEI-NH2.

Table 5. Solution Viscosities of 8k-PEI-UPy, 8k-PEI-UHex, and 8k-PEI-NH2 at a Concentration of 200 mg/mL sample

η (cP)

8k-PEI-UPy 8k-PEI-UHex 8k-PEI-NH2

2058 41.99 23.68

confirmed by 1H NMR spectroscopy. Despite the low molecular weights, TGA revealed excellent thermal stability of PEI-NH2 oligomers. DSC illustrated that Tg of PEI-NH2 increased following the classic Flory−Fox equation. UPyterminated telechelic PEIs were synthesized by functionalizing amine-terminated telechelic PEIs with UPy-synthon. After incorporating UPy, PEI had two-stage degradation behavior and the Tg increased. Effects of molecular weights and UPy incorporation on mechanical properties of PEI films were systematically studied. Only PEIs with Mn higher than 6 kDa showed film-forming capability regardless of the end groups. Incorporation of UPy end group significantly enhanced the flexibility of PEI. Importantly, PEI-UPy oligomers with Mn of only 8 kDa was solution-processable and exhibited outstanding mechanical strength comparable to high-molecular-weight state-of-the-art commercial PEIs. The findings herein provide insight into the design of low-molecular-weight PEIs with solution processability and outstanding mechanical properties, and thus, UPy-based supramolecular chemistry represents an effective and general approach to enhancing mechanical properties of high-temperature PEI oligomers.

Figure 9. Stress−strain curves obtained from tensile tests of (a) 8kPEI-UPy, (b) PEI(1), and (c) PEI(2) films.

Similarly, to determine whether the improved mechanical strength of 8k-PEI-UPy stemmed from the UPy moieties or the urea linkage, we utilized the reference polymers PEI-UHex to compare with PEI-UPy. The solution viscosities (η) of the 8kPEI-UPy, 8k-PEI-UHex, and 8k-PEI-NH2 were measured. Given the same polymer concentration (200 mg/mL, in CHCl3), the solution viscosity of 8k-PEI-UHex was only 1.8 times that of 8k-PEI-NH2, while the solution viscosity of 8kPEI-UPy was 87 times larger than that of the 8k-PEI-NH2. The significant increase in viscosity shows strong chain interactions resulting from the UPy moieties (Table 5), which ultimately improve the Young’s modulus of PEI-UPy.

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CONCLUSIONS Step-growth polymerization of BPADA and mPD at stoichiometric imbalance afforded the synthesis of amine-terminated telechelic PEIs. The molecular structures and Mn were

Table 3. Mechanical Properties of Solution-Cast 8k-PEI-UPy and State-of-the-Art Commercial PEIs: PEI(1) and PEI(2) sample

tensile strength (MPa)

maximum elongation (%)

Young’s modulus (MPa)

8k-PEI-UPy PEI(1): 16.9k-PEI PEI(2): 24.5k-PEI

87.2 ± 10.8 98.8 ± 1.1 99.0 ± 1.3

3.10 ± 0.39 4.98 ± 0.54 5.02 ± 0.40

(3.20 ± 0.14) × 103 (2.79 ± 0.09) × 103 (2.94 ± 0.12) × 103

F

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00156. 1 H NMR analysis of 2k-PEI-NH2, calculation of degree of polymerization from PA end-capped 2k-PEI-NH2, 1H NMR analysis of evolution from PEI-NH2 to PEI-UPy, calculation of degree of UPy functionalization, synthesis of PEI-UHex, DSC traces and TGA thermograms of PEIUHex (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (540) 231-8241 (G.L.). ORCID

Guoliang Liu: 0000-0002-6778-0625 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The material presented here is based upon work supported by SABIC (458369). The authors thank Prof. Timothy E. Long for insightful discussions and Mr. Joseph M. Dennis for assistance in SEC characterizations.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b00156 Macromolecules XXXX, XXX, XXX−XXX