Azobenzene-Based (Meth)acrylates: Controlled Radical

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Azobenzene-Based (Meth)acrylates: Controlled Radical Polymerization, Photoresponsive Solid−Liquid Phase Transition Behavior, and Application to Reworkable Adhesives Shotaro Ito,† Aishi Yamashita,‡ Haruhisa Akiyama,*,‡ Hideyuki Kihara,† and Masaru Yoshida*,† †

Research Institute of Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST Chugoku), 3-11-32, Kagamiyama, Higashihiroshima, Hiroshima 739-0046, Japan ‡ Research Institute of Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8568, Japan S Supporting Information *

ABSTRACT: Atom transfer radical polymerization was employed to induce the living polymerization of an azobenzene-containing monomer, 10-[4-(4-hexylphenylazo)phenoxy]decyl acrylate, with the resulting polyacrylate and the corresponding polymethacrylate undergoing a reversible solid−liquid phase transition under isothermal conditions caused by the photoinduced change of the azobenzene moiety shape. Irradiation-induced property changes were investigated by nuclear magnetic resonance, ultraviolet−visible (UV− vis) absorption spectroscopy, and dynamic viscoelasticity measurements, with focus on the effects of the main chain chemical structure and molecular weight. Azobenzene moiety photoisomerization and the concomitant phase transition were faster for the polyacrylate than for the polymethacrylate, which indicated the strong influence of the main chain structure. Finally, phototuning of the adhesion strength using azopolymer-bonded glass substrates was studied by single lap shear tests, with the maximum adhesion strength of >3 MPa being comparable to that of commercial hot-melt adhesives. Irradiation with UV light for only 15 s lowered the adhesion strength to 90% and relatively narrow molecular weight distributions (Table 2), with the [M]0/[I]0 ratio variation allowing polymers with three different Mns to be successfully synthesized (PM1−PM3). The thermal properties of the obtained polymers were probed by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD), with the phase transition temperatures listed in Table 2 determined by DSC on the second cooling scan at 10 °C min−1 (see Figure S1 for DSC thermograms and Figures S2−S7 for WAXD patterns). Thus, PA4 was shown to adopt liquid crystalline states below 107 °C by POM observations. In the temperature range of 107−76 °C, fan-shaped textures were observed with planar alignment, while a dark field was observed under homeotropic alignment, suggesting a smectic A phase (SmA). The smectic phase was also confirmed by WAXD measurements (Figure S2), with diffraction peaks at 2θ = 2.58° (layer distance d = 3.43 nm). The SmA phase transformed to the smectic B phase (SmB) below 76 °C, as demonstrated by the WAXD patterns in which the peak at d = ca. 0.44 nm (see Figure S2) is sharpened: this suggests strong lateral correlation among the azobenzene side groups. The diffraction patterns did not change below glass transition temperature (52 °C). On the other hand, while PM1 similarly showed SmA between 102 and 94 °C, it adopted a smectic C phase (SmC) below 94 °C, as demonstrated by the WAXD patterns in which the layer distance shortens from ca. 3.5 to ca. 3.2 nm (Figure S5−S7). A strong lateral correlation among the azobenzene side groups was observed below 59 °C, suggesting a higher-order smectic phase, such as smectic F or I. Although the glass transition of

Figure 2. 1H NMR spectrum of PA4 in CD2Cl2.

Figure 3. MALDI-TOF MS spectrum of PA4.

Mn ratio becoming smaller at high monomer conversions; e.g., a relatively small Mw/Mn of less than 1.20 was obtained at >30%

Figure 4. (a) First-order kinetic plots and (b) dependence of molecular weight (Mn,GPC, filled circles) and molecular weight distribution (Mw/Mn,GPC, open circles) on AzA monomer conversion for ATRP in the presence of EBiB, CuBr, and dnbpy in anisole solution at 80 °C. D

DOI: 10.1021/acs.macromol.8b00156 Macromolecules XXXX, XXX, XXX−XXX

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Table 2. Molecular and Thermal Characteristics of Polymers Used for the Observation of Photoresponsive Properties sample

Mn,GPC‑LSa (g mol−1)

Mw,GPC‑LSa (g mol−1)

Mw/Mn,GPCb

Mw/Mn,GPC‑LSa

dn/dc

phase transitionc

PA4 PA5 PA6 PM1 PM2 PM3

8420 13900 22000 12800 21200 33100

9500 16300 28700 15600 26300 40100

1.10 1.11 1.20 1.20 1.15 1.13

1.13 1.17 1.31 1.22 1.24 1.21

0.179 0.169 0.167 0.156 0.162 0.162

G52SmB76SmA107I G55SmB79SmA112I G58SmB80SmA113I SmForI59SmC94SmA102I SmForI57SmC97SmA106I SmForI56SmC97SmA107I

a Determined by GPC equipped with a multiangle laser light scattering detector. bDetermined by GPC using polystyrene standards. cPhase transition temperatures were determined by the second cooling DSC scan at 10 °C min−1, and phase transitions were also observed by POM. “G52SmB76SmA107I” represents polymer phases: isotropic phase (I) above 107 °C, smectic A (SmA) phase between 76 and 107 °C, smectic B (SmB) phase between 52 and 76 °C, and glassy state (G) under 52 °C.

The absorption intensity of the photoirradiated PA4 thin film was much higher than that of the thermally annealed one (Figure 5a black and dashed lines) due to the difference of the corresponding azobenzene alignment directions.49 After thermal annealing, the azobenzene moieties tended to align perpendicularly to the substrate and thus decrease its surface energy. Subsequent photoirradiation randomized the above alignment or made it nearly parallel to the substrate, resulting in higher UV absorbance. Although such UV absorption change was also observed for PM1, the corresponding difference was relatively small, suggesting that the flexibility of the polymer main chain influences the dynamic behavior of azobenzene unit alignment. The temporal evolution of absorption spectra was observed for PA4 and PM1 films sequentially irradiated with UV and visible light. As shown in Figures 6a and 6c, the initially strong π−π* bands of both PA4 and PM1 lost intensity on irradiation with UV light, while n−π* bands concomitantly gained intensity. On irradiation with visible light, the π−π* band intensity increased, being accompanied by an intensity decrease

PM1−PM3 was not observed by DSC, dynamic viscoelasticity measurements showed the glass transition temperature to be around 50 °C, as will be discussed later. Photoisomerization Behavior. UV−vis absorption spectra of PA4 and PM1 in chloroform solution were recorded to check the presence of the azobenzene chromophore. As shown in Figures 5a and 5b (gray lines), both absorption spectra

Figure 5. UV−vis absorption spectra of (a) PA4 and (b) PM1 in CHCl3 solution (gray solid lines), as thermally annealed thin films (dashed lines), and thin films sequentially irradiated with UV and visible light (solid lines).

showed an intense π−π* band in the UV region and a weaker n−π* band in the visible region, clearly indicating the trans conformation of the azobenzene moiety.51 The absorption maxima of PA4 and PM1 were observed at 352 and 351 nm, respectively, with the respective monomer unit-based molar absorption coefficients (1.75 × 104 and 1.70 × 104 M−1 cm−1) being in good agreement with those of the corresponding azobenzene derivatives.50 To study photoswitching in the solid state, thin films prepared on quartz plates by spin-coating 4 wt % chloroform solutions of PA4 or PM1 were annealed at 140 °C for 30 min. The average film thickness measured by AFM was 325 and 194 nm for PA4 and PM1, respectively. In the initial state, the π−π* band of PA4 was observed at a shorter wavelength (316 nm) than that of the corresponding chloroform solution (Figure 5a dashed and gray lines), which indicated strong parallel stacking of azobenzene moieties (H-aggregation).52,53 Once the polymer film was irradiated with UV light, the π−π* band intensity decreased and increased upon subsequent irradiation with visible light and shifted to a longer wavelength (330 nm, solid line), which suggested more loose azobenzene packing. On the other hand, the π−π* band of thermally annealed PM1 was observed at 330 nm, which was indicative of decreased H-aggregation degree.

Figure 6. UV−vis absorption changes of (a, b) PA4 and (c, d) PM1 thin films upon irradiation with (a, c) UV light at 20 mW cm−2 and (b, d) visible light at 40 mW cm−2. The irregularity at 330 nm was caused by spectrometer light source switching. E

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Macromolecules Table 3. cis-Azobenzene Moiety Fractions Observed during Thermal and Photochemical Treatment fraction of cis-azobenzene moieties (%) thermal treatmenta UV irradiationb subsequent visible light irradiationc

PA4

PA5

PA6

PM1

PM2

PM3

0 76 ± 2 6

0 82 ± 2 4

0 81 ± 11 8

0 59 ± 6 6

0 45 ± 9 5

0 59 ± 14 5

a Rapid cooling to room temperature from 140 °C. bIrradiation with 365 nm UV light at 50 mW cm−2 for 5 min. The reported values are averages of three measurements with standard deviations. cSequential 5 min irradiations with UV and 520 nm visible light (40 mW cm−2).

Figure 7. Temperature dependence of storage (G′, solid line) and loss (G″, dotted line) moduli as well as that of the loss tangent tan δ (dashed line) of (a) PA4 and (b) PM1 (cooling process).

to facilitate the photoisomerization of side-chain azobenzene moieties. Although molecular weight may also affect chain flexibility, its effect on the photoisomerization rate remained unclear, probably due to the relatively high molecular weights of polymers (>9.5 kg mol−1) used for these measurements. Polymers irradiated with UV light were subsequently irradiated with visible light for 5 min (520 nm, 40 mW cm−2), which induced the cis−trans azobenzene moiety isomerization (Table 3) and resulted in polymer resolidification, with the azobenzene moiety being in an almost photostationary state. Dynamic Viscoelastic Behavior. The temperature-dependent phase transition behavior of PAs and PMs was investigated by dynamic viscoelasticity measurements, with typical examples depicted in Figure 7. Specifically, samples of 8 mm diameter and 0.15 mm thickness were cooled from 120 to 30 °C, and their storage (G′) and loss (G″) moduli were recorded as a function of temperature. During cooling, G″ exceeded G′ by ca. 60 and 50 °C for PA4 and PM1, respectively, indicating fluidic behavior at higher temperature (Figure 7a,b). G′ and G″ sharply increased at two different temperatures for each sample, i.e., to ca. 80 and 60 °C for PA4 and 90 and 50 °C for PM1. The transition at higher temperatures was ascribed by DSC to the SmA−SmB transition for PA4 and SmA−SmC for PM1, while transition at lower temperatures was expected to be glass transition, although that of PM1 was not observed by DSC. No viscoelastic property changes corresponding to isotropic−SmA transitions (observed at ∼105 °C by DSC) were detected for PA4 and PM1 by dynamic viscoelasticity measurements, which agreed with the viscosity of LC polymers being higher in the isotropic phase than in the nematic phase.54 Below their glass transition temperatures (Tgs), the samples behaved elastically, since G′ exceeded G″. Both PAs and PMs exhibited similar G′ values of ∼100 MPa at 30 °C, which lay between those of typical epoxy resins55 and pressure-sensitive adhesives.56

of the n−π* band (Figures 6b and 6d). During irradiation with visible light, the absorption band at ∼360 nm corresponding to nonaggregated trans-azobenzene moieties initially gained intensity, with the subsequent gradual aggregation of these moieties resulting in an intensity increase of the band at ∼330 nm. Both polymers were photoequilibrated within an irradiation time of 30 s. 1H NMR spectra of the azopolymer films acquired after the photoreaction showed that cisazobenzene moieties were mainly (>95%) observed in the photostationary state upon 365 nm UV irradiation, being almost fully isomerized (>95%) to the trans-form upon 520 nm visible light irradiation. Thus, reversible cis−trans photoisomerization was observed even in solid-state polymers. To further investigate the photoisomerization rate in the depth direction of solid-state polymers, PA and PM were placed between two glass slides and spread by thermal treatment above an isotropic temperature. The polymer thicknesses equaled ∼20 μm, as measured by digital vernier calipers. On irradiation with UV light (365 nm, 50 mW cm−2), azobenzene moiety isomerization occurred, starting from the irradiation surface, and the polymer near this surface was liquefied. During irradiation, the isomerization-caused liquefaction proceeded to deeper polymer parts, while the polymer at the opposite side remained solid. After 5 min UV irradiation, the glass slides could be easily separated by application of a small stress, with the solid polymer remaining on the bottom glass and the liquefied polymer remaining on the top one. Subsequently, the polymer was completely dissolved in CDCl3 in the dark and characterized by 1H NMR to determine the trans-/cis-azobenzene moiety ratio. The obtained results (Table 3) clearly showed that the isomerization rates of PAs exceeded those of PMs. Considering the fact that PAs and PMs exhibit almost identical azobenzene moiety concentrations, the above behavior was attributed to structural differences between polymethacrylate and polyacrylate main chains. Specifically, the higher flexibility of the polyacrylate main chain was thought F

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Figure 8. Temperature dependence of G′ (solid line) and tan δ (dashed line) observed for (a) PA4 (green), PA5 (blue), and PA6 (red) and (b) PM1 (green), PM2 (blue), and PM3 (red) during cooling. Real Tg values are as follows: PA4, 53 °C; PA5, 56 °C; PA6, 58 °C; PM1, 50 °C; PM2, 51 °C; PM3, 51 °C.

Figure 9. Time-dependent changes of G′ (solid line), G″ (dotted line), and tan δ (dashed line) of (a) PA4 and (b) PM1 upon irradiation with 365 nm UV light (100 mW cm−2) at 30 °C.

Figure 8a shows the temperature dependence of G′ for PA4−PA6, revealing that the modulus change temperature increased with increasing molecular weight. Moreover, tan δ peaks at ∼50 °C corresponded to glass transition temperatures, which also increased with increasing molecular weight, as confirmed by the results of DSC measurements. On the other hand, the G′ profiles of PM1−PM3 showed that the glass transition temperature of these samples was almost constant regardless of the molecular weight (Figure 5b, Tg ≈ 50 °C). At a temperature slightly higher than Tg (55 °C), PMs with lower molecular weights showed lower G′ values. Importantly, low G′ facilitated azobenzene moiety alignment during the glass transition and therefore increased polymer elasticity, accounting for the sharp increase of G′ observed for PM1. On the other hand, the slow increase of G′ observed during the glass transition of PM3 was ascribed to the azobenzene moieties of this higher-molecular-weight polymer being more difficult to align during the glass transition due to high polymer viscosity. In the DSC thermograms (Figure S1d−f), exothermic peaks corresponding to the phase transition of SmC to SmF (I) at around 60 °C also support the difficulty in the alignment of azobenzene moieties around this temperature. The heat of the phase transition for PM1 is much larger than those for PM2 and PM3, suggesting a high degree of azobenzene alignment in the low-molecular-weight PM polymer. The degree of G′ increase (Figure 8a) and the phase transition heat of PAs (Figure S1a−c), however, did not depend on the molecular

weight, probably because even high-molecular-weight polymers of this class were flexible enough for such alignment. Thus, the effect of molecular weight on the G′-increasing behavior in the glass transition of azobenzene-containing poly(meth)acrylates was shown to depend on the chemical structure of the main chain. The photochemically induced modulus changes of PAs and PMs under isothermal conditions were also investigated by dynamic viscoelasticity measurements at a controlled temperature of 30 °C, with typical results shown in Figure 9. Notably, within 1 min after starting UV light irradiation (100 mW cm−2), the G′ values of PA4 and PM1 immediately decreased from 100 to 0.1 MPa (irradiation started at 2 min in Figure 9), with additional irradiation (15 min) further decreasing G′ and G″ and making G″ eventually exceed G′, which indicated a transition to the liquid phase. The rates of UV irradiationinduced G′ and G″ decrease appeared to be similar for PA4 and PM1, whereas the corresponding tan δ increase rates were different. Since tan δ is calculated as G″/G′, larger tan δ values imply more fluidlike behavior. Upon 1 min UV irradiation, the tan δ of PA4 exceeded a value of two, while that of PM1 approximately equaled unity. During 15 min irradiation, tan δ of PA4 always exceeded that of PM1, implying that PA4 was more fluid than PM1 at a given irradiation time. The higher rate of modulus change observed for PA4 was consistent with the higher photoisomerization rate measured for this polymer by 1H NMR. Accordingly, the higher chain flexibility of PAs G

DOI: 10.1021/acs.macromol.8b00156 Macromolecules XXXX, XXX, XXX−XXX

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of failure modes is the residual stress in specimens. Since the thermal expansion coefficient of polymers is more than 10 times greater than that of glass,57,58 residual stress accumulated at the polymer/glass interface during the rapid cooling of thermally bonded specimens (from 150 to 25 °C within 5 min). To verify this, the thermally bonded specimen using PA4 was cooled from 150 to 25 °C at a cooling rate of 1 °C min−1. Since the cooling rate did not affect the adhesion strength (Figure S8), the residual stress in the specimens was not the main reason for the different adhesion strengths of the thermally and photochemically bonded specimens. The content of trans-/cis-azobenzene moieties is another possible reason for the observed failure mode differences. Polymers sequentially irradiated with UV and visible light contained ∼5% of the cis-form, whereas the thermal process produced polymers containing 100% trans-azobenzene (Table 3). The residual cis-isomer possesses a large dipole moment due to its bent structure, allowing stronger polymer−glass substrate interactions. We found that the thermally bonded specimens irradiated with 405 nm light (25 mW cm−2 for 1 h at 25 °C) contain ∼5% of cis-azobenzene, and those irradiated with 520 nm light (40 mW cm−2 for 1 h at 25 °C) contain ∼2% of cisazobenzene. Because the polymer remained in solid form during this process, the residual stress did not significantly change from that in the original thermally bonded specimen. Thus, we can verify the effect of cis-azobenzene content and that the adhesion strengths improved with the increase of cisazobenzene fraction from that in the original thermally bonded ones (Figure S8). Therefore, the improved adhesion strength of photochemically bonded specimens was possibly due to the stronger polymer−glass substrate interactions of the cisazobenzene moiety than that of the trans one. For both PA and PM series, the adhesion strength increased with increasing molecular weight. Comparison of PAs and PMs with similar molecular weights (PA5 and PM1 or PA6 and PM2) showed that the adhesion strengths of the latter were slightly higher than those of the former. Adhesion strength is generally governed by two main properties of the adhesive, namely its cohesive and substrate adhesion forces, and further studies are thus required to clarify which force plays the dominant role in enhancing adhesive strength upon increasing the molecular weight. In our previous report on polyacrylates with side-chain azobenzene moieties, the smaller-molecularweight polymer exhibited higher adhesion strength,31 in stark contrast to the results presented herein. At present, we think that the above difference is caused by the different terminal groups of these polymers. Since the polymers used in our previous work were prepared by free-radical polymerization using 2,2′-azobis(isobutyronitrile) and tert-dodecylmercaptan, the structure of their terminal groups is not clear, in contrast to the present case. Notably, the incorporation of certain functional groups into the chain end may affect adhesive properties, with the contribution of these terminal groups to adhesion strength expected to be higher for low-molecularweight polymers. Since the ATRP method was herein used to polymerize acrylate monomers, various functional groups can be incorporated into the azobenzene-containing poly(meth)acrylates using functional initiators or postpolymerization modification methods,59 which requires the effect of chain end groups on adhesion strength to be investigated in more detail. Adhesion strength phototuning was carried out by irradiation with UV light. Specifically, photochemically bonded specimens

should lead to faster photoisomerization and concomitant solid−liquid phase transition. Note that the absolute values of G′, G″, and tan δ were not valid, since they were determined for heterogeneous samples, i.e., for those where only the parts near the irradiation surface were liquefied during UV irradiation, whereas the rest remained solid. Phototuning of Adhesion. For single lap shear tests and stress−strain curve measurements, adhesion specimens prepared by bonding two glass plates with polymers were used. Two kinds of specimens were prepared, namely thermally and photochemically bonded ones. The former ones were prepared by spreading thermally melted polymer between glass plates (followed by cooling), whereas the latter ones were fabricated by sequentially irradiating thermally bonded specimens with UV and visible light. Notably, photochemically bonded specimens were not prepared by spreading photoliquefied polymers due to the long time (>30 min) required to completely liquefy the powdered polymers. The bonding area and thickness were set to ∼75 mm2 and ∼10 μm, respectively. For each sample, critical stresses of five specimens were measured, and the results were reported as averages with standard deviations (Figure 10). The representative stress− strain curves of PA4 and PM1 are shown in Figure 11.

Figure 10. Adhesion strengths of thermally bonded specimens (light gray bars), photochemically bonded specimens (dark gray bars), and photochemically bonded specimens irradiated with 365 nm UV light for 15 s (black bar).

Figure 11. Stress and strain curves of (a) PA4 and (b) PM1 for thermally bonded specimens (dashed line), photochemically bonded specimens (solid line), and photochemically bonded specimens irradiated with UV light for 15 s (gray solid line).

As shown in Figure 10, the adhesion strength of photochemically bonded specimens was always higher than that of thermally bonded ones. Moreover, the failure loci of thermally bonded specimens were always interfacial, while those of the photochemically bonded ones were of a mixed interfacial/ cohesive nature. One of the possible reasons for this difference H

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Macromolecules were irradiated with 365 nm UV light for 15 s (50 mW cm−2) just before the single lap shear test. During the above test (performed in the dark), the irradiated specimens were deformed (Figure 11), with the critical stresses being less than 0.2 MPa for all polymers. The drastic decrease of adhesion strength was caused by the liquefaction of the adhesives. Considering that the irradiation time was quite short, the formation of a thin layer of liquefied polymer near the irradiation surface effectively lowered the adhesion strength. The residual adhesion strength after UV irradiation increased with increasing molecular weight for both PAs and PMs, as shown in Figure 10. Based on 1H NMR measurements, which showed that the photoisomerization rate is not dependent on the molecular weight of PAs or PMs, the difference in the residual adhesion strength was ascribed to the fluidity of the photoisomerized polymers. Specifically, lower-molecular-weight polymers became more fluid upon UV irradiation, which led to decreased residual adhesion strength. Prolonged irradiation led to a further decrease of adhesion strength, with critical stresses for all specimens irradiated by UV light for 5 min being less than 0.05 MPa, which enabled their easy manual separation.

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AUTHOR INFORMATION

Corresponding Authors

*(H.A.) E-mail: [email protected]. *(M.Y.) E-mail: [email protected]. ORCID

Shotaro Ito: 0000-0001-5212-7466 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Hiroyuki Minamikawa for his assistance of the WAXD measurement. We also appreciate the measurement of spin-coated polymer film thickness by Dr. Hirohmi Watanabe and Ms. Mariko Takahashi. S.I. thanks the Sumitomo Electric Group CSR foundation for partial support of this research. The authors appreciate the support by the fundamental research fund of AIST.

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CONCLUSIONS We have successfully synthesized well-defined polyacrylates possessing a functional azobenzene moiety in each repeating unit by ATRP. Controlled polymerization was achieved at high monomer concentration, and MALDI-TOF MS analysis revealed the precisely controlled structures of the resulting polymers. UV−vis absorption measurements for thin PA and PM polymer films showed that the azobenzene moieties in solid-state polymers could be reversibly transformed into trans-/cis-isomers by irradiation with 365 nm UV light or 520 nm visible light under isothermal conditions. The rate of PA polymer isomerization was faster than that of PM polymers, while no effect of molecular weight on isomerization speed was observed. Thus, the main chain structure strongly influenced the photoisomerization rate in polymer films. Similarly, PA and PM polymers exhibited different changes of their viscoelastic properties upon irradiation with UV light. The phase transition of PA4 occurred more rapidly than that of PM1, and the resulting liquefied PA4 polymers showed more fluid behavior than PM1. The photochemical phase transitions of the fabricated polymers were used to prepare a reworkable adhesive, the adhesion strength of which could be reduced upon UV irradiation. The photochemically bonded specimen of PM3 showed the maximum adhesion strength of ∼3.7 MPa, which was almost equal to the typical adhesion strength of commercially available hot-melt adhesives.60 The adhesion strengths of all specimens drastically decreased to less than 0.2 MPa upon 15 s irradiation with UV light, which indicated that the above polymers can be used as reworkable adhesives to enable on-demand bonding and debonding by irradiation with visible and UV light.

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thermograms, temperature-dependent WAXD patterns, and additional adhesion strength tests (PDF)

REFERENCES

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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.8b00156. Details about the synthesis and characterization of AzA and AzM monomers and the PM polymer, DSC I

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Macromolecules

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DOI: 10.1021/acs.macromol.8b00156 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00156 Macromolecules XXXX, XXX, XXX−XXX