Maleimide Polymer Networks with

Article pubs.acs.org/IECR

Recyclable Diels−Alder Furan/Maleimide Polymer Networks with Shape Memory Effect Mengjin Fan, Jialin Liu, Xiangyuan Li, Junying Zhang,* and Jue Cheng* Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *

ABSTRACT: A series of recyclable networks with shape memory effect was prepared by the Diels−Alder reaction between furan-functionalized poly(hydroxyaminoethers) (PHAEs) and 1,5-bis(maleimido)-2-methylpentane (MPDBMI). Compression molding was used to recycle the MPDBMI/PHAE films. The Diels−Alder and retro-Diels−Alder reactions between PHAEs and MPDBMI were confirmed by Fourier transform infrared, differential scanning calorimetry (DSC), and solubility test. Furthermore, the mechanical properties and shape memory properties of the original and recycled MPDBMI/PHAE networks were systematically investigated by a tensile test and a quantitative shape memory evaluation method, respectively. DSC measurement indicated that the retro-Diels−Alder reaction will occur upon heating, and the solubility test showed that the recycled MPDBMI/PHAE films may still be considered as recyclable. Moreover, the tensile test at room temperature disclosed that both the original and recycled MPDBMI/PHAE networks exhibited tough properties: all specimens exhibited either the ductile plastic fracture feature with the appearance of yielding phenomenon or the rubber property with large elongation at break. Quantitative shape memory evaluation revealed excellent shape memory performance of the original and recycled MPDBMI/ PHAE networks.

1. INTRODUCTION Dynamic covalent polymers represent the polymeric materials that possess dynamic covalent bonds and exhibit their dynamic properties under suitable conditions (e.g., heat and ultraviolet).1−4 The reversible nature of the dynamic covalent bonds whose association and dissociation can be controlled within a defined conditions range makes dynamic covalent polymers good candidates for recyclable,5−8 self-healing9−11 and drugrelease12 materials. To date, several reversible reactions, such as the radical exchange reaction,13,14 the Diels−Alder reaction15−17 and photoreversible olefin cycloadditions,18,19 have been used to prepare dynamic covalent polymers, and among these reactions, the Diels−Alder reaction may be the most studied.9,15 Thermally reversible Diels−Alder reactions between various furan and maleimide derivatives16,17 have attracted particular attention probably because of the fact that these reactions can take place under mild conditions with high chemoselectivity. By incorporatiion of the furan and maleimide groups into polymers, the resulting materials may exhibit their thermally healable or recyclable properties. Chen and co-workers9,20 prepared several thermally remendable and highly cross-linked polymeric materials and found that the Diels−Alder connections and disconnections are thermally reversible in a cyclic manner. Tian and co-workers21 showed a thermally remendable epoxy resin system containing labile furan/maleimide connections and stable epoxy/anhydride connections and found that the cured epoxy resin can be mended in a controlled manner. Zhang and co-workers22 reported a thermally healable and recyclable conetwork with triple-shape memory effect and found that the recycled material still exhibited good shape memory property. © XXXX American Chemical Society

Recently, shape memory epoxy resins (SMEPs) are of interest23,24 because of their easy tuning25,26 in thermal and thermomechanical properties by simply varying the formulations and the excellent shape memory properties including rapid shape memory response, high shape fixation, and high shape recovery. Previously, we27,28 prepared several SMEPs based on two bisphenol A type epoxy resins containing two and six oxyethylene units (DGEBAEO-2 and DGEBAEO-6; see Scheme 1) and found that these intrinsically toughened SMEPs exhibited relatively large deformation under suitable conditions and good shape memory effect with high shape fixation and high shape recovery, but, of course, these SMEPs are not recyclable. In the present work, furan-functionalized poly(hydroxyaminoethers) (PHAEs) were first synthesized from DGEBAEO-2, DGEBAEO-6, and furfurylamine (see Scheme 2). In these PHAEs, rigid and flexible units are connected together with covalent bonds; thus, when they are cross-linked with 1,5-bis(maleimido)-2-methylpentane (MPDBMI), ordered, and intrinsically toughened “epoxy−amine” networks containing reversible Diels−Alder connections can be formed (see Scheme 3). At low temperatures (e.g., 120 °C), retro-Diels−Alder adduct uncoupling becomes preponderant and thus the polymers can be recycled under this situation. Received: July 15, 2014 Revised: September 22, 2014 Accepted: September 26, 2014

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dx.doi.org/10.1021/ie5028183 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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C). 1H NMR data of MPDBMI (δ, ppm): 6.71 (d, 4H,  CH−), 3.49 (t, 2H, −NCH2CH2−), 3.36 (m, 2H, −NCH2CH(CH3)−), 1.87 (m, H, −CH(CH3)−), 1.49−1.80 (m, 2H, −NCH2CH2−), 1.04−1.43 (m, 2H, −CH(CH3)CH2−), 0.86 (d, 3H, −CH(CH3)−). 13C NMR data of MPDBMI (δ, ppm): 171.0 and 170.8 (−CO), 134.0 (CH−), 43.7 (−NCH2CH(CH3)−), 37.8 (−NCH2CH2−), 32.2 (−CH(CH3)CH2−), 31.2 (−CH(CH3)−), 25.8 (−NCH2CH2−), 17.2 (−CH(CH3)−). 2.2. Synthesis of Poly(hydroxyaminoethers) (PHAEs) Derived from DGEBAEO-2, DGEBAEO-6, and Furfurylamine. PHAEs can be obtained by reacting the epoxy resin with a stoichiometrically equivalent amount of furfurylamine at 55 °C for 3 h, 100 °C for 3 h, and 130 °C for 8 h in the absence of solvent. The detailed formulations in the present work are presented in Table S1 in the Supporting Information (SI). Note here that this series of PHAEs was marked as PHAE06(x), where x represents the weight fraction of DGEBAEO-6 in the epoxy component (DGEBAEO-2 and DGEBAEO-6). 2.3. Preparation of Cross-Linked MPDBMI/PHAE Networks. PHAE was dissolved in an appropriate amount of dichloromethane, and a stoichiometric amount of MPDBMI was added and dissolved in the resulting solution with constant stirring. Then the reactant solution was poured into a mold precoated with mold release agent. Dichloromethane was allowed to volatilize gradually from the solution for about 12 h, and after that time, the precured films were carefully peeled away from the mold and postcured at 70 °C for 2 h, 60 °C for 2 h, and 50 °C for 30 h. The detailed formulations for preparation of the cross-linked films are shown in Table S2 in the SI. 2.4. Preparation of Recycled MPDBMI/PHAE Networks. To investigate the recyclability of the cross-linked MPDBMI/PHAE networks, original MPDBMI/PHAE films were cut into small pieces, and then these small pieces were compression-molded into new films in a square stainless mold with 50 × 50 × 0.6 mm3 inner size using a 70911-24B powder press machine (Tianjin New Technical Instrument Co. Ltd., China). The following thermal program was used in our study: the samples were heated to 135 °C and maintained at this temperature for 5 min, subsequently compression-molded under 10 MPa for 5 min, and then naturally cooled to room temperature with the pressure loaded. After compression molding, the resulting films were also postcured at 70 °C for

Scheme 1. Chemical Structures of Epoxy Resins, Furfurylamine, and Bis(maleimide) Used in This Work

As shown in Scheme 1, DGEBAEO-6 is more flexible than DGEBAEO-2 and the molecular weight of DGEBAEO-6 is larger than that of DGEBAEO-2; thus, the flexibility and crosslink density of the shape memory MPDBMI/PHAE networks can be tuned by varying the relative content of DGEBAEO-6. In our study, Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), and solubility testing were used to study the thermal reversibility of the original and recycled MPDBMI/PHAE networks. Furthermore, the mechanical and shape memory properties were respectively investigated by a tensile test and a qualitative shape memory evaluation method.

2. EXPERIMENTAL SECTION 2.1. Materials. DGEBAEO-2 (epoxy equivalent 224.2 g/ mol), DGEBAEO-6 (epoxy equivalent 309.6 g/mol), and 1,5bis(maleimido)-2-methylpentane (MPDBMI) were synthesized in our laboratory; 2-furfurylamine was obtained from Tianjin Heowns Biochemical Technology Co., Ltd. (Tianjin, China); dichloromethane was available from Sinopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China). FTIR data of MPDBMI (νmax, cm−1): 3452 (CO), 3099 (C−H), 2939 (C−H of alkyl groups), 1700 (CO), 1410 and 1118 (C−N−

Scheme 2. Synthesis of PHAEs Derived from Epoxy Resins and Furfurylamine

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Scheme 3. Preparation of MPDBMI/PHAE Networks

2 h, 60 °C for 2 h, and 50 °C for 30 h. The recycle process of the cross-linked MPDBMI/PHAE films is shown in Figure 1.

(HT6E−HT5−HT3) and a Waters 2410 refractive index detector. Tetrahydrofuran with a flow rate of 1 mL/min was used as the eluent, and polystyrene standards were used to construct a calibration curve. Thermal analyses of the original and recycled MPDBMI/ PHAE networks were performed on a TA Instruments Q20 differential scanning calorimeter equipped with an RCS 90 cooling system with the following thermal program (dry N2 was used as the protective atmosphere): heating from −30 to +205 °C at a heating rate of 10 °C/min, then cooling to −30 °C at the maximum cooling rate, and finally heating to 215 °C at a heating rate of 10 °C/min. The tensile properties at 25 °C were investigated using an Instron 1185 universal tester. The original (about 0.3 mm in thickness) and recycled (about 0.7 mm in thickness) films were cut into dumbbell-shaped bars (50 mm × 4 mm × thickness). The crosshead speed was set at 10 mm/min. Dynamic mechanical analysis (DMA) was carried out with a TA Instruments Q800 dynamic mechanical analyzer from −10 to +75 °C at a heating rate of 3 °C/min. The film tension clamps were used in the DMA experiments, and the constant frequency was set at 1 Hz. The width of the specimen for DMA experiments was about 6.5 mm, and the thickness of the specimen coincided with that of the specimen for tensile experiments. The storage modulus, loss modulus, onset temperature of the glass transition zone defined by the intersection temperature of the two tangents to the E′temperature curve at the glass transition drop (TgE′),26,29 and glass transition temperature defined by the tan δ peaks (TgDMA) are available from the DMA spectrum.

Figure 1. Recycle process of the cross-linked MPDBMI/PHAE films.

2.5. Measurements. The FTIR spectra of epoxy resin, MPDBMI, and PHAE were recorded on a Nicole Nexus 670 FTIR spectrometer in the 4000−400 cm−1 range using KBr pellets, and the FTIR spectra of the MPDBMI/PHAE films were recorded on a Bruker Alpha-T FTIR spectrometer equipped with a single-reflection ATR sampling module in the 4000−600 cm−1 range. 1H and 13C NMR spectra were recorded on a Bruker Advance 400 spectrometer at ambient temperature using CDCl3 as the solvent. Gel permeation chromatography (GPC) was carried out at 30 °C using a Waters GPC system equipped with three Styragel columns C



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The shape memory effect properties of the original and recycled MPDBMI/PHAE specimens were quantitatively evaluated on the Q800 dynamic mechanical analyzer under the controlled force mode. First, a specimen (about 4 mm in width and 0.3 mm in thickness for the original specimens and about 3 mm in width and 0.7 mm in thickness for the recycled ones) was equilibrated at its TgE′ for 5 min, and then a force ramp (0.3 N/min for the original specimens and 0.6 N/min for the recycled ones) was applied until a relatively large strain was reached [about 50% for MPDBMI−PHAE06(100), 45% for MPDBMI−PHAE 0 6 (75), and 40% for MPDBMI− PHAE06(50)]. Second, the specimen was cooled to TgE′ − 30 °C at a cooling rate of 3 °C/min with the force unchanged. Third, the force was subsequently released, and the specimen was maintained at TgE′ − 30 °C for 5 min. Finally, the specimen was reheated to TgE′ + 20 °C [about TgE′ + 15 °C for MPDBMI−PHAE06(50) to avoid possible retro-Diels−Alder reaction] at a heating rate of 3 °C/min, followed by equilibration at TgE′ for 5 min to start the next shape memory cycle. Figure 2 shows one typical shape memory cycle.

Figure 3. FTIR spectra of DGEBAEO-6 and PHAE06(100).

amine. The appearance of broad absorption around 3450 cm−1 (corresponding to the stretching vibration of the hydroxyl groups) in the FTIR spectrum of PHAE06(100) is attributed to the generation of secondary hydroxyl groups by the copolymerization between epoxy and amine groups. The absorptions at 3142 and 3114 cm−1 correspond to the stretching vibration of C−H in furan rings. Additionally, the GPC values of number-average molar mass (Mn) and polydispersity index (PDI) for the PHAEs are summarized in Table S3 in the SI. 3.2. Diels−Alder and Retro-Diels−Alder Reactions between PHAEs and MPDBMI. FTIR spectra of PHAE 06 (100), MPDBMI, and cross-linked MPDBMI− PHAE06(100) are presented in Figure 4. As shown in the

Figure 2. Typical stress−strain−temperature diagram for quantitative shape memory evaluation.

Furthermore, the shape fixation (Rf) and shape recovery (Rr) can be calculated as follows: shape fixation: shape recovery:

R f (N ) = R r(N ) =

ε u (N ) × 100% εm(N )

(1)

εm(N ) − εf (N ) × 100% εm(N ) − εf (N − 1) (2)

where εm represents the strain before the force is released, εu represents the fixed strain of the unloaded sample after stabilization at TgE′ − 30 °C for 5 min, and εf represents the final strain of the sample in the shape memory cycle.30

Figure 4. FTIR spectra of PHAE06(100), MPDBMI, and cross-linked MPDBMI−PHAE06(100).

3. RESULTS AND DISCUSSION 3.1. Synthesis of PHAEs. As an example, the FTIR spectra of DGEBAEO-6 and PHAE06(100) are shown in Figure 3. It can be observed that the absorption at 914 cm−1 (corresponding to the asymmetrical stretching of the epoxy group) disappears after the reaction of DGEBAEO-6 and furfuryl-

FTIR spectrum of MPDBMI−PHAE06(100), the absorption at 696 cm−1 assigned to maleimide ring deformation31 disappears almost completely and an absorption at 1769 cm−1 specific to the furan/maleimide adduct32−34 obviously appears, indicating that the Diels−Alder reaction between the furan and maleimide groups took place successfully. To further investigate the retroDiels−Alder reaction, a cross-linked MPDBMI−PHAE06(100) D



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sample was heated to 200 °C at 10 °C/min in the DSC cell (protected by dry N2 atmosphere), and then it was immediately cooled to room temperature with maximum cooling rate. The FTIR spectrum of MPDBMI−PHAE06(100) after heat treatment is shown in part II of Figure S1 in the SI. A decrease in the intensity of the absorption at 1769 cm−1 and a significant increase in the intensity of the absorption at 696 cm−1 can be observed, indicating that the retro-Diels−Alder reaction took place upon heating to 200 °C. Finally, MPDBMI− PHAE06(100) after heat treatment was maintained at 60 °C for 1 h, and the resulting FTIR spectrum is shown in part III of Figure S1 in the SI. With the recovery in the intensity of the absorption at 1769 cm−1 and the decrease in the intensity of the absorption at 696 cm−1, we can conclude that the Diels−Alder bonds regenerated again at 60 °C. Moreover, the retro-Diels−Alder reaction of the cross-linked MPDBMI/PHAE networks was investigated by DSC, and the DSC thermograms of the original and recycled MPDBMI− PHAE06(100) samples are shown in Figure 5. During the first

Additionally, a closer examination of the DSC thermograms reveals that the Tg1 value of the recycled samples is slightly higher than that of the original ones and the enthalpy value during the first scan (ΔH1) of the recycled samples is slightly lower than that of the original ones; the increase in the Tg1 value and the decrease in the ΔH1 value for the recycled samples may be respectively attributed to the removal of little residual solvent and the destruction of some maleimide groups during the recycle process. The DSC values of Tg1, Tg2, and ΔH1 together with the exothermic peak temperature during the first heating scan (Tp1) are collected in Table 1. As the relative content of DGEBAEO-6 in the epoxy component decreases from 100 to 50 wt %, Tg1 increases from 23.3 to 40.8 °C for the original MPDBMI/PHAE networks and from 24.9 to 42 °C for the recycled ones. The increase in Tg1 is mainly due to the increase in the cross-link density and rigidity of the cross-linked network because DGEBAEO-2 has lower molecular weight and more rigid units than DGEBAEO-6. The reversibility of the recycled MPDBMI/PHAE networks was further verified by the solubility test. As shown in Figure 6, after 30 min at 120 °C, all of the recycled MPDBMI/PHAE films were almost completely dissolved in dimethyl sulfoxide (DMSO). This result suggests that the recycled MPDBMI/ PHAE films may still be considered as recyclable. 3.3. Mechanical Properties of the Original and Recycled MPDBMI/PHAE Films. Typical stress−strain curves of the original and recycled MPDBMI/PHAE specimens at room temperature are presented in Figure 7, and the tensile data are given in Table 2. For both the original and recycled MPDBMI/PHAE specimens, it can be clearly observed that the tensile stress decreases and the elongation at break increases with increasing relative content of DGEBAEO-6. This result may be attributed to the fact that, as the relative content of DGEBAEO-6 increases, the cross-link density of the network decreases and the flexibility of the network increases. Both MPDBMI−PHAE06(50) and MPDBMI−PHAE06(75) exhibit a ductile plastic fracture feature with the appearance of yielding phenomenon, and MPDBMI−PHAE06(50) shows the rubber property with large elongation at break due to its relatively low glass transition temperature. The tough properties suggest that these cross-linked MPDBMI/PHAE networks are suitable for shape memory applications even after being recycled. 3.4. Shape Memory Properties of the Original and Recycled MPDBMI/PHAE Networks. It was reported by Feldkamp and Rousseau26,29 that the ultimate strain of their SMEPs could be improved significantly by simply decreasing the deformation temperature to coincide with the onset temperature of the glass transition (TgE′). In our present work, TgE′ was also chosen as the deformation temperature for the original and recycled MPDBMI/PHAE specimens, and the values of TgE′ and TgDMA are listed in Table 3. The three-dimensional stress−strain−temperature diagrams for quantitative shape memory evaluation of the original and recycled MPDBMI−PHAE06(100) networks are shown in

Figure 5. DSC thermograms of the original and recycled MPDBMI/ PHAE samples.

heating, a broad and significant endothermic peak can be observed, indicating that depolymerization of the MPDBMI/ PHAE networks occurred by the retro-Diels−Alder reaction. In the following second heating scan, a slight exothermic peak in the range of about 50−100 °C was observed; this exothermic peak is ascribed to the Diels−Alder reaction between the free furan and maleimide groups produced by the retro-Diels−Alder reaction during the first heating scan. The value of Tg obtained during the first heating scan (Tg1) is significantly higher than that obtained during the second heating scan (Tg2) because of the fact that, during the first heating scan, retro-Diels−Alder reaction occurred and thus the cross-link density of the networks decreased.

Table 1. DSC Values for the Original and Recycled MPDBMI/PHAE Samples original

recycled

sample

Tg1 (°C)

Tg2 (°C)

ΔH1 (J/g)

Tp1 (°C)

Tg1 (°C)

Tg2 (°C)

ΔH 1 (J/g)

Tp1 (°C)

MPDBMI−PHAE06(100) MPDBMI−PHAE06(75) MPDBMI−PHAE06(50)

23.3 33.6 40.8

2.6 11.8 21.3

72.2 76.0 80.8

123.6 121.8 122.4

24.9 35.5 42.0

2.8 12.3 20.5

69.4 75.0 77.4

124.0 126.1 126.2

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Figure 6. Solubility test for the recycled MPDBMI/PHAE networks.

Table 3. Values of TgE′ and TgDMA Determined from DMA original

recycled

sample

TgE′ (°C)

TgDMA (°C)

TgE′ (°C)

TgDMA (°C)

MPDBMI−PHAE06(100) MPDBMI−PHAE06(75) MPDBMI−PHAE06(50)

25.1 35.5 44.6

34.9 45.9 56.0

27.9 39.6 46.4

36.7 49.5 57.8

4. CONCLUSION A series of cross-linked MPDBMI/PHAE networks with shape memory effect was prepared. The Diels−Alder and retroDiels−Alder reactions between PHAEs and MPDBMI were investigated by FTIR, DSC, and solubility testing. Additionally, the mechanical and shape memory properties of the crosslinked MPDBMI/PHAE networks were systematically studied by a tensile test and a quantitative shape memory evaluation method, respectively. The FTIR study indicated that the crosslinked MPDBMI/PHAE networks were successfully prepared in this study, the DSC measurement on the cross-linked networks showed that retro-Diels−Alder reaction will occur upon heating, and the solubility test revealed that the recycled MPDBMI/PHAE films may still be considered as recyclable. Moreover, the tensile test at room temperature showed that, for both the original and recycled MPDBMI/PHAE specimens, the tensile stress decreased and the elongation at break increased with increasing relative content of DGEBAEO-6; MPDBMI− PHAE06(50) and MPDBMI−PHAE06(75) exhibited a ductile plastic fracture feature with the appearance of yielding phenomenon, and MPDBMI−PHAE06(50) shows the rubber property with large elongation at break. Quantitative shape memory evaluation disclosed that both the original and

Figure 7. Typical stress−strain curves of the original and recycled MPDBMI/PHAE films.

Figure 7 [the three-dimensional stress−strain−temperature diagrams for MPDBMI−PHAE 06 (75) and MPDBMI− PHAE06(50) are shown in Figure S2 in the SI], and the values of shape fixation and shape recovery ratios are given in Table 4. From this table, one can observe that the shape fixation and shape recovery ratios of the original MPDBMI/PHAE are higher than 98%; moreover, the recycled MPDBMI/PHAE networks also exhibit excellent shape memory performance with high shape fixation (>99%) and high shape recovery (>97%).

Table 2. Tensile Properties of the Original and Recycled MPDBMI/PHAE Specimens at Room Temperature sample original

recycled

MPDBMI-LEP06(100) MPDBMI−LEP06(75) MPDBMI−LEP06(50) MPDBMI−LEP06(100) MPDBMI−LEP06(75) MPDBMI−LEP06(50)

strength at yield (MPa)

strain at yield (%)

18.5 ± 1.5 44.4 ± 2.3

4.5 ± 0.1 5.3 ± 0.3

16.0 ± 2.1 48.8 ± 3.1

5.9 ± 0.8 7.6 ± 0.4 F

strength at break (MPa) 15.6 30.2 27.4 9.3 30.2 36.4

± ± ± ± ± ±

2.0 1.1 1.5 1.0 2.2 6.2

strain at break (%) 173.0 142.7 21.8 176.9 159.0 9.8

± ± ± ± ± ±

7.2 4.5 4.9 10.1 15.6 1.5



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Table 4. Values of Shape Fixation and Shape Recovery for the Original and Recycled MPDBMI/PHAE Networks original

recycled

sample

shape fixation ratio (%)

shape recovery ratio (%)

shape fixation ratio (%)

shape recovery ratio (%)

MPDBMI−LEP06(100) MPDBMI−LEP06(75) MPDBMI−LEP06(50)

99.32 ± 0.02 99.08 ± 0.04 98.52 ± 0.10

99.66 ± 0.22 99.80 ± 0.25 99.44 ± 0.41

99.40 ± 0.05 99.33 ± 0.05 99.22 ± 0.02

99.74 ± 0.17 98.67 ± 1.47 97.74 ± 2.46

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 10 64425439. Fax: +86 10 64425439. *E-mail: [email protected]. Phone: +86 10 64425439. Fax: +86 10 64425439. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National High Technology Research and Development Program of China (Grant 2012AA03A205).

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Figure 8. Consecutive shape memory cycles for the original (a) and recycled (b) MPDBMI−PHAE06(100) networks.

recycled MPDBMI/PHAE networks exhibited excellent shape memory performance with high shape memory fixation and high shape memory recovery.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Text describing formulations of PHAEs (Table S1), formulations of cross-linked MPDBMI/PHAE films (Table S2), GPC values for PHAEs (Table S3), FTIR spectra of MPDBMIPHAE06(100) before and after heat treatment (Figure S1), and three-dimensional stress−strain−temperature diagrams for MPDBMI-PHAE06(75) and MPDBMI-PHAE06(50) (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. G



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