Correlating In-Depth Mechanistic Understanding with Mechanical

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

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Correlating In-Depth Mechanistic Understanding with Mechanical Properties of High-Temperature Resistant Cyclic Imide Copolymers Kristina J. Jovic,§ Thomas Richter,∥ Christiane Lang,§ James P. Blinco,*,§ and Christopher Barner-Kowollik*,§ §

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School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia ∥ Evonik Resource Efficiency GmbH, Kirschenallee, 64293 Darmstadt, Germany S Supporting Information *

ABSTRACT: We break new ground in the in-depth mechanistic understanding of the molecular behavior of tertbutyl methacrylate (t-BMA) and N-isopropylacrylamide (NIPAM) copolymers upon thermal treatment and correlate this understanding with the mechanical properties of the resulting materials. We explorevia nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic measurementsthe formation of cyclic anhydride and imide structures and exploit high-resolution mass spectrometry (HRMS) coupled to size-exclusion chromatography (SEC) as well as tandem MS (MS/MS) to correlate the time and temperature dependent cyclization processes. Critically, we propose a reshuffling mechanism describing dynamic ring-opening events involving the anhydride and imide structures, which enables the in-depth understanding of the dynamic material properties. Drawing on information from low-molecular-weight model systems, we translate our findings to high-molar-mass polymers, showing that our mechanistic model enables the understanding of the material properties of the polymers.

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INTRODUCTION Polyimides have received increased interest for applications as advanced materials1−5 because of their outstanding thermal stability,6 enhanced chemical resistance,7 and excellent mechanical8 and electrical insulation properties.9,10 In addition to their utilization as high-temperature stable materials, they are also employed as fibers,11 foams,12 sealants, and membranes.13,14 One specific class of polyimides are poly(methacrylimides) (PMI). PMIs can exceed a mechanical stiffness having a Young’s modulus of greater than 6 GPa, a Tg close to 200 °C, and feature high-temperature resistance.15 Currently, the most common way to synthesize PMI is via freeradical copolymerization (FRP) of methacrylonitrile (MAN) and methacrylic acid (MAA) in bulk. Subsequent heating above 150 °C induces ring closure to provide the PMI units.16 However, MAN is toxic to the environment, as well as very expensive and difficult to access from an industrial point of view.17 Since the 1960s, several PMI manufacturers have been attempting to synthesize MAN-free PMI structures. One avenue is to cyclopolymerize various N-dimethacrylimides wheredepending on the N-residualglutarimides and succinimides are formed during the polymerization process.8,18−20 Alternatively, PMIs are accessible via the aminolysis of poly(methyl methacrylate) (PMMA) with a primary amine in a polymer-analogous reaction.21−26 The temperature resistance of the strictly alternating copolymer of styrene and © XXXX American Chemical Society

maleic anhydride can be increased by using primary amines for polymer-analogous imidization reactions.27−31 In situ imidization upon heating of bi- or ternary copolymers has been shown by the group of Ritter and others.32−34 Furthermore, Ritter and colleagues postulated a mechanism for the intramolecular imidization of their copolymer system constituted of tert-butyl methacrylate (t-BMA) and various N-methacrylamides. As a first step, isobutene is eliminated from the polymer side chain, followed by the loss of water due to the imidization reaction. Employing infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, the authors assigned characteristic imide vibrational modes and proton resonances. Based on 1H NMR spectroscopy, they were also able to calculate the degree of imidization by considering the integrated resonances of amide and imide protons. The calculation of the molar content of amide, imide, ester, and anhydride functionalities, however, was not possible. This is due to the fact that the characteristic resonance for the anhydride groups (two methyl groups at the backbone) overlaps with other resonances (methyl group of methacrylic acid, methyl group of imides), impeding a quantitative evaluation. Criticallyand surprisingly for such an important polymer classto date no molecular evidence that unambiguously links Received: June 25, 2018 Revised: September 28, 2018

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DOI: 10.1021/acs.macromol.8b01347 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the thermal and mechanical properties of the generated polymers with the molecular changes along the lateral polymer chain has been reported. Herein, we close this mechanistic gap, while providing a novel access route to high-performance polyimides. We initially investigate low-molecular-weight copolymer systems entailing tert-butyl methacrylate (t-BMA) and N-isopropylacrylamide (NIPAM), and we map the molecular changes via hyphenated size exclusion chromatography (SEC) and high-resolution Orbitrap electrospray ionization mass spectrometry (SEC-ESI MS) in addition to 1 H, 13C NMR, and IR spectroscopy as well as TGA. On the basis of our in-depth molecular assessment, we provide a complete mechanism for the formation of intramolecular generated imides driven by a thermodynamically initiated reshuffling process. We underpin our mechanistic proposal using tandem mass spectroscopy (MS/MS) and by synthesizing a model copolymer based on tert-butyl acrylate and Nisopropylacrylamide. Through this all-encompassing approach, we are able to provide the unambiguous identification of the reshuffling process and its correlation to the material’s properties. Subsequent to establishing the mechanism of intramolecular imide formation, we focus on the correlation between the molecular structure and the properties of high-molecularweight copolymers with varying molar ratios of t-BMA and NIPAM. Specifically, we demonstrate how the glass transition temperature (Tg) of the copolymers after heat treatment correlates with the polymer composition andmost importantlythe number of formed imide species. The formation of imide structures in the high-molecular-weight copolymers is confirmed via solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The mechanical properties and their relationship to the molecular structure of the heat treated polymers are established via nanoindentation, allowing the definition of the optimum ratio of t-BMA to NIPAM for the imidization process to gain the highest E-Modulus and Tg.

Table 1. Molecular Characteristics of the Synthesized Prepolymers P1−P8 P1 P1* P2 P3 P4 P5 P6 P7 P8

mol %a NIPAM

Mnb / g·mol−1

D̵ b

Tgc/°C

35 35 0 14.5 21.3 33.3 45.5 75.2 100

2100 1600 280000 293000 330000 377000 480000 302100 370000

1.7 2.0 2.0 1.9 1.8 2.4 2.7 1.2 2.8

72 95.4 123.8 123.6 124.6 124.9 120.2 139.8

a Molar ratio of the comonomers in the copolymer, calculated via 1H NMR from the ratio of the resonances of the single proton of the isopropyl-group and the nine protons of the tert-butyl-group. b Determined via SEC in THF as the eluent (35 °C, 1 mL·min−1), calibrated with PMMA standards. cDetermined via differential scanning calorimetry.

closure, four new compounds can be formed, that is, methacrylic acid (species 3) by reacting with an acrylic acid moiety (species 5), leading to the formation of the mixed anhydride (species 7), whereas the reaction of two acrylic acids (species 5) leads to the formation of the symmetrical anhydride (species 8). In addition, new cyclic imide structures are generated from the reaction between N-isopropylmethacrylamide (species 4) and a methacrylic acid (species 3) forming an imide (species 11) with two methyl groups along the backbone, as well as an imide (species 10) without methyl groups from the reaction between N-isopropylacrylamide (species 2) and acrylic acid (species 5). In total, three different anhydride, as well as three different imide species, are anticipated to form from the set of carboxylic acid and Nisopropylamine species. As mentioned above, ring-closing reactions are induced via thermal curing, therefore it is of high relevance to determine the optimal curing conditions, leading therefore to a high Tg. Temperature and curing time are key determinants to achieve optimal curing conditions. Thus, systematic experiments were conducted to investigate different curing temperatures (190, 210 and 230 °C). According to differential scanning calorimetry (DSC), the highest Tg was achieved with a curing temperature of 210 °C (see Supporting Information (SI), Figure S1). In concomitant TGA measurements, the initial decrease in mass commences at around 210 °C, which can be attributed to the release of isobutene (see SI, Figure S2).35 Compared with the ring-closing imidization reaction, the loss of isobutene is the energetically determining step (Nphenylmalimide: ΔH+R = 12.6 kcal , tert-butylacetate: ΔH+R =

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RESULTS AND DISCUSSION To examine the imidization mechanism, initially lowmolecular-weight polymers, P1 and P1* with Mn from 2100 to 1600 g·mol−1, were synthesized while high-molecular-weight polymers (P2−P8 with Mn from 280 to 480 kg·mol−1) with variable NIPAM (14−75 mol %) content were generated to assess the structure/mechanical property relationships. The molecular characteristics of each polymer (P1−P8) are collated in Table 1. The polymer-analogous imidization reaction was induced by thermal treatment of the synthesized copolymers (190−230 °C). As proposed by Ritter and colleagues,32 the reaction commences with a syn-elimination of isobutene, leading to the formation of free acid units (Scheme 1, species 3) along the polymer chain. The mechanistic cascade commences with the cleavage of isobutene and the formation of the resulting methacrylic acid groups (Scheme 1, species 3), cyclic anhydrides (Scheme 1, species 6) or cyclic imides (Scheme 1, species 9). Furthermore, and in a critical extension to the basic mechanismwe propose that a reopening of already formed imides and anhydrides occurs (a and b, Scheme 1), leading to a reshuffling of the N-isopropylamine and acid residuals. Because of this reshuffling process, new species are formed where the N-isopropylamine moiety features a methyl group in the 2-position (species 4) and the acrylic acid moiety features a proton in the 2-position (species 5). Upon ring

mol

40.5 kcal ).36,37 Subsequently, the duration of the heat exposure mol was varied from 30 min to 4 h keeping the temperature constant at 210 °C. The low-molecular-weight polymer P1 was initially analyzed via ATR-IR spectroscopy in its original state as well as after 4 h of exposure to 210 °C under a nitrogen atmosphere (Figure 1). The characteristic IR-bands were assigned using values previously reported in literature.38 The bands at 1803 cm−1 (CO symmetric vibration, Figure 1, ⧫) and 1765 cm−1 (CO asymmetric vibration, Figure 1,⬢) can be assigned to the carbonyl vibration of the formed anhydrides (species 6, 7, and 8). Over time, the number of anhydrides slightly increases. B


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Scheme 1. Exemplary Mechanistic Reaction Sequence of t-BMA/NIPAM Copolymers under Thermal Stress (Species 1 and 2) and the Resulting Reaction Intermediates of the Imidization Processa

a

Elimination of isobutene leads to the formation of methacrylic acid (species 3), followed by an initial anhydride (species 6) or imide ring closure (species 9). Further thermal treatment causes the cyclic structures to ring-open, yielding the new species 4 and 5 from species 9. Subsequent ringformation produces new imides (species 11 and 10), as well as new anhydride species (species 7 and 8).

to the CH-deformation vibration of the N-isopropyl group and undergo a significant change in shape. In summary, IR spectroscopy allows identification of the reaction products after heat treatment (i.e., acrylic acids, anhydrides, and imides). However, it was not possible to distinguish between the variant species (e.g., anhydrides species 6, 7, and 8 or imides species 9, 10, and 11), as proposed in our mechanism (Scheme 1). To further elucidate the molecular transformation of P1 under thermal stress, the products were assessed via 1H NMR spectroscopy (Figure 2, refer to the schematic drawings within the figure for resonance assignments). The release of isobutene in the initial step is evidenced by the decrease of the resonance at δ = 1.46 ppm ((CH3)3, c), which disappears completely within the first 2 h of thermal treatment (refer to 1H NMR spectra Figure S3 in the SI).32 The resonances at δ = 7.33 ppm (h), δ = 7.25 ppm (g) and δ = 3.75 ppm (e) are associated with the aromatic and the benzylic methyl protons of benzyl mercaptan, respectively. The resonance of residual water in acetone-d6 changes its shape because of interaction with the methacrylic acid protons in the backbone, which is formed during the thermal treatment (species 3). The formation of imide structures is correlated with a resonance at δ = 4.83 ppm

Additionally, all CO vibrations of the other functions can be found at 1660 cm−1; however, they cannot be assigned to a specific structure. The formation of imides over time can be observed via the increase of the band at 1669 cm−1 (CO, imide, Figure 1, ★). The decrease of the band at 1541 cm−1 is associated with the NH stretching vibration of the N-isopropyl group from N-isopropylacrylamide (species 2) and underpins the transformation into the cyclic imide structure (species 9, 10, and 11) (Figure 1, ◁). The −CNC− axial stretching mode of the imide is identified at 1223 cm−1 (Figure 1,▼), whereas the vibrational band at 1022 cm−1 (Figure 1, ▲) is assigned to the −CNC− transverse stretching mode. The band at 1250 cm−1 refers to the C−O stretch of t-BMA (Figure 1, □), whereas the band at 1136 cm−1 is associated with the CHdeformation vibration of t-BMA (Figure 1, ▷) and the band at 845 cm−1 is assigned to the CH3 rocking frequencies of t-BMA (Figure 1, ☆), both signals are decreasing because of the loss of isobutene upon heating resulting in methacrylic acid species. The C−O stretching mode of the formed methacrylic acid is observed at 1320 cm−1 (Figure 1, ■). The sharp bands at 1394 and 1373 cm−1 (Figure 1, ▽ and △), visible in the initial IR spectrum of the untreated polymer P1, are assigned C


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Orbitrap electrospray ionization mass spectrometry (SEC-ESI MS).39−41 The spectra for the copolymer P1 (Figure 3) were detected in positive-ion mode and were assigned in the range of 1000− 4000 m/z (refer to the Supporting Information section, Table S1, S2, S3 and S4). The representative region of 1110−1320 m/z illustrates the single charged spectra of P1 before thermal treatment, after thermal treatment for 30 min, as well as after 1 and 2 h of heat exposure (Figure 3, from top to bottom). Starting with the parent polymer P1 before thermal treatment, each signal a1−a9 can be identified as combinations of the two comonomers t-BMA (species 1) and NIPAM (species 2) with benzyl mercaptan as an end group (e.g., a5, m/z = 1225.7867, Δm/z = 0.0023). Whereas the signals α−η are combinations of the two comonomers of t-BMA and NIPAM featuring a hydroxyl end group, potentially due to small residuals of water in the system (Figure 3, P1). After 30 min at 210 °C, new signals are observed, which can be assigned to chains featuring the new species 3 in addition to the initial species 1 and 2 (e.g., b3, m/z = 1311.8253, Δm/z = 0.0002). Species 3 is formed due to a syn-elimination of isobutene from the t-BMA units. Thermal treatment at 210 °C for 1 h leads to an increase of the signals for chains entailing species 3, while signals containing only species 1 decrease. In addition, several new signals are observed, which can be attributed to chains featuring newly formed species. Specifically, the ring-closing reaction between species 3 and species 2 results in cyclic imides (species 9), while the condensation of two species 3 leads to the formation of a symmetrical cyclic anhydride (species 6) (e.g., c25, m/z = 1284.7877, Δm/z = 0.0099). As proposed in Scheme 1, a reopening followed by a reshuffling is observed, leading to the formation of species 4 and 5 (Figure 3, 1h). After 2 h of thermal treatment, only small traces of the signals assigned to the starting material remain. Instead, the formation of additional imides (species 10 and 11) as well as anhydrides (species 7 and 8) can be observed (Figure 3, 2h). These newly formed imides and anhydrides are the product of the cyclization of the previously formed species 4 and 5 in combination with the initially observed species 2 and 3 (e.g., 31, m/z = 1270.6962, Δm/z = 0.0055). For longer curing

Figure 1. Attenuated total reflection infrared (ATR-IR) spectra of P1 before (black line) and after 4 h (red line) of heat treatment at 210 °C under a nitrogen atmosphere. The structures depicted within the figure highlight the respective functional groups that can be assigned to the observed vibrations (for details, refer to the main text).

(m) (species 9, 10 and 11), whereas the signal assigned to the free isopropyl group of NIPAM at 3.96 ppm diminishes (f) (species 2). Employing the ratio of the integrals off m and f, the relative content of formed imides can be calculated. After 1 h at 210 °C, 27% of NIPAM has been transformed into cyclic imide structures, and after 8 h, the cyclic imide content increased to 78%. In addition, an overall broadening and shift of the backbone proton resonances is visible, impeding further conclusions on the mechanism. In-line with conclusions from ATR-IR, the 1H NMR spectrum underpins the formation of anhydrides and imides; however, a quantitative and dynamic picture as a function of time only emerges when the fate of the functional groups within an individual chain is followed. Such a detailed molecular transformation mapping is accessible via hyphenated size exclusion chromatography (SEC) and high-resolution

Figure 2. 1H NMR spectra of P1 before heat exposure (top), after 1 h at 210 °C (middle) and after 4 h (bottom) at 210 °C in acetone-d6. D


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Figure 3. Representative section between m/z = 1100 and 1320 of the SEC-ESI MS spectrum of the copolymer P1 before thermal treatment, after 30 min, 1 and 2 h at 210 °C. Spectra were recorded in THF/methanol in positive-ion mode. The mass spectrum was derived from the spectra recorded at 19.26−19.39 min of the SEC experiment. Refer to the SI (Table S1−S4) for a complete assignment of the peaks.

(MS/MS), the other approach compares our findings with a model copolymer (P1*) constituted of tert-butyl acrylate (tBA) and N-isopropylacrylamide (NIPAM), which has been thermally treated to induce imidization. For the ESI (+)-CID MS study, we selected a characteristic peak in the spectrum of P1 after 2 h of thermal treatment at 210 °C, which was obtained in positive mode in THF/ methanol (3/2) containing 50 μM sodium trifluoroacetate (Figure 3, bottom). As the precursor ion, d37 (m/z = 1297.6596) was selected (Figure 4, red dotted line), which is assigned to the sodium adduct of a polymer chain entailing

times, intensity changes of the previously identified signals as well as an alteration of the different species due to the dynamic behavior of the reshuffling process within a polymer chain are visible. All t-BMA units have been transformed to species 3, which participates in the cyclization reactions. The exemplary spectral assignments and isotopic pattern simulations in comparison with experimental patterns of the spectra discussed above can be found in the SI (Figure S8 and Figure S9). It must be noted that the same chemical composition of a polymer chain can originate from different combinations of the species discussed above leading to isobaric structures. For example, a combination of species 2 and 3 has the same chemical composition and thus the same m/z value as the combination of species 4 and 5. An alternative isobaric structure is species 9 and X (Scheme 2). Mechanistically, it is Scheme 2. Isobaric Structure of Species 9 and Potential Species Xa

a

Species 9 contains a methyl group along the polymer backbone and an isopropyl group as an N-residual, whereas species X shows no methyl group in the polymer backbone, yet features an additional methyl moiety at the N-residual leading to a tert-butyl residual.

Figure 4. ESI (+)-CID MS spectrum of a selected ion of P1 after 2 h of thermal treatment at 210 °C, d37 (m/z = 1297.6596, Figure 3 bottom spectrum), detected as the sodium adduct in THF/methanol (3/2) containing 50 μM sodium trifluoroacetate, utilizing a CID fragmentation energy of 50 eV. The fragmentation pattern can be assigned to different copolymer building blocks. The selected structures depicted within the figure can only be obtained if a reshuffling mechanism is operational (see SI, Table S5 for a complete peak assignment).

very unlikely that species X is generated, yet nevertheless it has to be unambiguously established that only species 9 occurs within the polymer structure. To resolve this mechanistic dichotomy, two approaches were employed to verify our proposed mechanism (Scheme 1). One powerful approach is to exploit tandem mass spectrometry E


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synthesized six copolymers with a higher molar mass (Mn ∼ 350 000 g·mol−1) and varied the NIPAM content between 14.5 mol % (P3) and 75.2 mol % (P8). Additionally, we synthesized the two homopolymers p(t-BMA) (P2) and p(NIPAM) (P9). The change in the glass transition temperature (Tg) of the copolymers P2−P8 was examined as a function of the curing temperature and heating time, respectively. Initially, different curing temperatures were examined (190 °C, 210 and 230 °C). With 210 °C the highest Tg was achieved; therefore, this temperature has been chosen as an ideal curing temperature. At a lower temperatures, imides cannot be generated effectively as evidenced by IR spectroscopy (SI, Figure S13). At 230 °C, a discoloration of the previously white polymer was observed, which indicates potential onset of degradation of the polymeric material. Subsequently, we investigated the effect of different heating times at 210 °C on the glass transition temperature of the polymers (Figure 5). The Tg of the starting materials P3−

anhydride (species 6) and imide (structures 9, 10, and 11) as well as noncyclic carboxylic acid (species 3) and Nisopropylamine species (species 4). Complete fragmentation was achieved with 50 eV as CID fragmentation energy, resulting in fragment ions below m/z = 800. All fragments were assigned to building blocks of the original polymer chain (refer to SI, Table S5). The most intense ion (signal 3, m/z = 150.0901) represents species 4, featuring a methyl group and the N-isopropyl group. Further evidence for our mechanism can be found in signal 5 at m/z = 191.0928, which can be assigned to the newly formed species 10, a symmetrical cyclic imide without methyl groups. We further observed combinations of several characteristic species underpinning our proposed mechanism. For example, the signal at m/z = 276.1219 (signal 12) is assigned to a combination of species 4 and species 8, and the signal at m/z = 345.2162 (signal 17) to a combination of species 4 and species 11. On the basis of the fragmentation pattern presented in Figure 4, we were able to evidence the existence of the newly formed species due to the occurrence of the suggested reshuffling mechanism. Further, we verified our proposed mechanism by utilizing a model copolymer (P1*) composed of tert-butyl acrylate (tBA) and N-isopropylacrylamide (NIPAM) after thermal treatment at 210 °C for 1 h. P1* is a simplified version of P1, due to the lack of methyl groups along the polymer backbone only the N-residual group has to be investigated closely. It has to be proven that only a N-isopropyl and no Ntert-butyl group exists to underpin our reshuffling mechanism. According to ESI MS (+) of the heat-treated copolymer consisting of t-BA and NIPAM in THF/methanol containing 50 μM sodium trifluoroacetate, the cyclic imide Nisopropylimide with a repeating unit of m/z = 167.0946 is clearly visible. Next to N-isopropylimide, only the starting materials NIPAM and acrylic acid are present, no N-tertbutylimide (species X, Scheme 2) is observed (e.g., Peak b, m/ z = 373.1926, Δm/z = 0.00, see SI, Figure S11, Table S6). The absence of the N-tert-butyl group (species X, Scheme 2) unambiguously confirms that the additional species formed in our original copolymer are a product of the proposed reshuffling process. In summary, we were able to verify our proposed molecular reshuffling mechanism depicted in Scheme 1 using low-molarmass copolymers for in-depth analysis via various techniques. Based on ATR IR and 1H NMR spectroscopy, we were able to show that cyclic imide and anhydride structures are formed. However, it could not be distinguished between individual species. By utilizing high-resolution SEC-ESI MS, an in-depth analysis of the formed products was possible, identifying the hypothized cyclic anhydride and imide structures (Species 6 and 9) and, additionally, new species originating from a dynamic reshuffling process. On the basis of SEC-ESI MS data, we were able to show a stepwise change of the molecular structure of the copolymers during thermal treatment as a function of the curing time. Critically, we observed the formation of new species which helped to understand the mechanism. We further verified our suggested reshuffling mechanism by analysis via ESI (+)-CID MS and the comparison to a model copolymer with less variety in the possible thermal products (t-BA/NIPAM). After we established the molecular formation mechanism of intramolecular imide formation, we moved to investigate the influence of various amide contents in the copolymer on the resulting mechanical properties after thermal treatment. We

Figure 5. Effect of curing time on the Tg of the polymers P3 (+), P4 (●), P5 (■), and P6 (▲) at 210 °C. The inset depicts a representative DSC curve of P5 after 1 h.

P8 were in a range close to 124 °C and a general increase of Tg 40 to 60 °C was observed in all copolymers upon heating. For the two homopolymers a starting Tg of 95 °C (Lit. 107 °C)42 for P2 and 139 °C (Lit. 143−148 °C)43 for P8 was determined which was increased after thermal treatment 169 °C (P2) and 144 °C (P8). The great increase of ∼70 °C for P2 could be explained by anhydride formation, whereas for P8 there is nearly no increase of the Tg, because p(NIPAM) does not undergo the formation of cyclic structures. The most significant increase in Tg was observed for P5 (t-BMA/ NIPAM (33.3 mol % NIPAM), Figure 5, ■). After thermal treatment for 1 h at 210 °C, the Tg was approaching 190 °C. The increase in Tg is smaller for polymers with a higher content of NIPAM (P7, ▲), whereas polymers with a lower NIPAM content (P3, + and P4, ●) show a similar increase in Tg as P5. Generally, we observed the highest values for the Tg in all materials after 1 h of curing time, and afterward the Tg decreases for longer heating times, reaching a plateau for treatments exceeding 4 h, which is still significantly higher than the Tg of the starting material. As depicted in Figure 5, a sharp increase of the Tg from the starting material to the heat treated polymers is visible due to the formation of imides and anhydrides. The formation of F


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the formation of hydrogen bonds between species 6/9 and species 2 and/or 3. For a copolymer with a higher or equal amount of NIPAM (P7), the probability of two neighboring NIPAM groups is very high. As a result, no cyclic structures can be formed because NIPAM units do not react with each other. On the other hand, if the amount of t-BMA (P4) is too high, mainly anhydrides will be formed which also leads to a decrease of the Tg and E-modulus.

cyclic structures leads to a higher rigidity and stability and, consequently, to an increase in the glass transition temperature. After 1 h, the highest Tg is observed. According to the previously performed SEC-ESI MS spectra of P1, mainly species 2, 3, and 6 are present at this stage, while only a small number of imides (species 9) is formed. Anhydrides are more readily formed during the beginning of the reaction (e.g., anhydride formation of poly(acrylic acid) EA = 38 kcal·mol−1); however, imides are thermodynamically preferred (e.g., cyclization of N-phenylnaphthalimide EA = 17 kcal·mol−1) and are formed in a significant amount within 2 h of heating. The decrease in Tg with longer heating times and thus an increase in the number of cyclic imides could potentially be due to the loss of intramolecular hydrogen bonds, which can only form from the noncyclic N-isopropylamine structures (species 2) or the free carboxylic acid groups (species 3). An additional effect for the decrease in Tg may also be found within the reshuffling mechanism itself. Due to the reopening of cyclic structures and the formation of species 4 and 5, Tg decreases slightly; however, at one point, an equilibrium between opening and closing of the cyclic structures is established, leading to a stabilization of the Tg value. In general, an excess of the t-BMA units seems favorable to achieve high Tg values in the cured material, potentially due to lower statistical probability of two or more neighboring NIPAM units which will impede imide and anhydride formation. For the determination of the E-Modulus, nanoindentation was performed on the polymers P3−P7. To avoid foam formation induced by the cleavage of isobutene, the heat treatment of the polymers was performed in a hot melt press from RANDAL employing a pressure of 5 kN at 210 °C for 1 h. The thin, transparent foils were subsequently analyzed by nanoindentation. Commencing with an initial simultaneous increase of the Tg and E-modulus, both material properties start to decrease again after they reached a maximum at 33.3 mol % NIPAM. Importantly, a closer inspection of Figure 6 indicates that the mechanical properties of P3−P7 are in good correlation with their measured Tg. The E-modulus shows a similar behavior as the Tg, implying that a slight excess of t-BMA in the copolymer leads to the highest Tg as well as the highest E-modulus (P5, 33.3 mol % NIPAM). This behavior is once more explained by

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CONCLUSIONS In summary, we have shown a novel access route to gain mechanistic insights of high-performance polyimides formed via heat treatment of a copolymer consisting of t-BMA and NIPAM. By using hyphenated size exclusion chromatography (SEC) coupled to a high-resolution Orbitrap electrospray ionization mass spectrometry (SEC-ESI MS), as well as 1H, 13 C NMR, and IR spectroscopy, we were able to map timedependent molecular changes along the polymer chain of the low-molar-mass model copolymer P1 (Mn = 2100 g·mol−1). Subsequent to the release of isobutene from t-BMA units, we observe the formation of cyclic anhydrides and imides over time. Critically, we identify the presence of newly formed species. To account for the formation of these species, we propose a dynamic ring-opening mechanism (Scheme 1). An in-depth characterization of the synthesized copolymer P1, as well as a second model copolymer P1* (Mn = 1600 g·mol−1) via MS/MS was conducted to further evidence the appearance of the occurring species. For P1, we were able to unambiguously identify polymer fragments containing the reshuffled species 4, 5, and 11. P1*, a copolymer featuring tertbutyl acrylate and N-isopropylacrylamide, was used to unambiguously evidence that the N-isopropyl group is stable under the employed conditions and is not affected by the free isobutene to form species X. In the second part of the current study, we established the correlation between structural and mechanical properties. On the basis of six different high-molecular-weight copolymers (Mn = 300−480 kg·mol−1) comprising different t-BMA/ NIPAM compositions, we submit that the highest Tg and Emodulus is achieved with a slight excess of t-BMA P5 (33.3 mol % NIPAM). A shift toward a higher content of either NIPAM (P7) or t-BMA (P4) leads to a decrease in Tg and Emodulus in both cases, which is due to the decrease in probability for the formation of cyclic imides. Variable temperatures (190, 210, and 230 °C) and curing times (from 30 min to 8 h) have been investigated. The highest Tg and E-modulus is obtained with a curing temperature of 210 °C and a curing time of 1 h. At temperatures lower than 210 °C, no isobutene is cleaved, and thus, no cyclization can occur. Temperatures higher than 210 °C lead to degradation of the polymer. After 1 h of curing time, cyclic formations (anhydrides and imides) start to form, and hydrogen bonds are established with neighboring NIPAM and methacrylic acid units, which leads to a high Tg and E-modulus. After curing times of longer than 1 h, mainly cyclic structures are present, impending hydrogen bonds formation.

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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.8b01347.

Figure 6. Relationship of Tg (black dots) vs E-Modulus (red dots) of P3−P7. All polymers have been heat treated for 1 h at 210 °C. G


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Additional figures, experimental procedures, instrumentation, synthetic procedures, and analysis of the compounds used (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

James P. Blinco: 0000-0003-0092-2040 Christopher Barner-Kowollik: 0000-0002-6745-0570 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.B.-K. and J. B. are grateful to Evonik Resource Efficiency GmbH (Darmstadt, Germany) for funding the current project and for the excellent collaboration. C.B.-K. additionally acknowledges continued support from the Queensland University of Technology (QUT) as well as the Australian Research Council (ARC) in the form of a Laureate Fellowship. The authors are grateful to the Australian Institute for Bioscience and Nanotechnology (AIBN, Dr. Lewis Chambers) for DSC access as well as to the Centre for Advanced Imaging at the University of Queensland (UQ, Dr. Ekaterina Strounina) for access to solid-state NMR and the UQ Faculty of Engineering (Dr. Michael Murphy) for access to a hot melt press. The data for the nanoindentation reported in this paper were obtained at the Central Analytical Research Facility operated by the Institute for Future Environments (QUT). Access to CARF is supported by generous funding from the Science and Engineering Faculty (QUT).

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ABBREVIATIONS t-BMA, tert-butyl methacrylate NIPAM, N-isopropylacrylamide NMR, nuclear magnetic resonance IR, infrared HRMS, high-resolution mass spectrometry SEC, size-exclusion chromatography PMI, poly(methacrylimides) Tg,, glass transition temperature FRP, free radical polymerization MAN, methacrylonitrile MAA, methacrylic acid PMMA, poly(methyl methacrylate) THF, tetrahydrofuran DSC, differential scanning calorimetry ATR-IR, attenuated total reflection infrared ESI, electrospray ionization CID, collision-induced dissociation.

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