Understanding the Effect of the Dianhydride Structure on the Properties of Semiaromatic Polyimides Containing a Biobased Fatty Diamine

21 Oct.,2022

 

odpa anhydride

Effect of the Dianhydride Structure on the PIs Properties

The imidization of PAA into PI can be confirmed by IR spectroscopy using the disappearance of the amic acid peaks typically visible at 1716, 1640, and 1550 cm–1 in PAA spectra and the appearance of the characteristic peaks of imide bonds at 1770, 1710, 1360, and 745 cm–1 in PI spectra. The imidization reaction was confirmed for all of the polymers ( ) and supported by 1H NMR analysis (Figure S1, Supporting Information). The percentage yields were calculated by the standard approach shown in the SI (Page S-2). Yields of 83% for BPADA-D, 89% for ODPA-D, 74% for 6FDA-D, and 73% for BPDA-D were obtained.

As can be seen from the TGA curves (Figure S2, Supporting Information), all polymers show a high thermal stability independent of the dianhydride architecture with values for the onset degradation temperature (2% weight loss) at 330–380 °C (Table 1) similar to those of traditional commercial polyimides such as LaRC-IA. All samples showed only a small weight loss up to 0.6% until 200 °C suggesting that almost no solvent (toluene or DMAc) was entrapped during the imidization and that the monomers were fully reacted. The resulting polymers were fully soluble in common organic solvents, such as toluene, THF, and chloroform, which facilitates their processing (for example, in coatings, adhesives and thin films applications), as opposed to commercial polyimides.19 The GPC results are presented in Table 1. The small differences in the molecular weights might be due to various reasons, such as impurities or side-reactions but also differences in the electron affinities of the aromatic dianhydrides. As the susceptibility of the nucleophilic attack increases with the electrophilicity of the dianhydride group, the reactivity of the dianhydride monomer is related to its electron affinity: higher values indicate higher reactivity of the dianhydride. According to the literature,18,26 the electron affinities of the dianhydrides used in this work increase in the order BPADA-D < ODPA-D < BPDA-D < 6FDA-D.

Table 1

polymerMw (g/mol)Mn (g/mol)PDIDSC-Tg (°C)rheology-Tg (°C)BDS-Tgd (°C)TGA T (2% weight loss) (°C)density (g/cm3)BPADA-D29k18k1.62436203601.05ODPA-D32k16k2.01325113801.056FDA-D41k20k2.02540213301.12BPDA-D37k20k1.92233e, 46e183501.05Open in a separate window

The flexibility of the dianhydride moieties is reflected in the value of the Tg, where three polymers (BPADA-D, 6FDA-D, and BPDA-D) exhibit a similar Tg, ranging from 22 to 25 °C and ODPA-D showing a lower value of 13 °C. The Tg values of these PIs are incomparable to those of fully aromatic commercial PIs (200 < Tg < 400 °C),27 which naturally excludes the possibility for their use in high-temperature applications. However, their Tg’s are higher than the ones of other reported polymers that contain fatty acid dimer as the building block that remained significantly below room temperature. The ionic supramolecular networks prepared from fatty acid dimer by Aboudzadeh et al. exhibit their Tg’s in the range −29 <Tg < 10 °C.10 The fatty acid dimer based polyamides from Hablot at al. showed −17 < Tg < −5 °C,12 while the values in the range −10 < Tg < −0.9 °C were obtained in the fatty acid dimer based polyamides synthesized by van Velthoven et al.13

The ODPA and BPADA dianhydrides have a very flexible oxygen linker between the two phtalic anhydride parts of the molecule (Scheme 1). Comprising two oxygen linkers, BPADA may appear the most flexible out of the four dianhydrides used here, yet it does not exhibit the lowest Tg. This is likely to be caused by the increase in aromatic content compared to ODPA-D, where the extra flexibility of the ether linkage is counterbalanced by the increased number of aromatic rings. Despite the structural differences, BPDA-D and 6FDA-D display a similar Tg. The expected increase in backbone flexibility of the 6FDA-D polymer may be compensated by the extra bulkiness of the CF3 groups. A similar effect of dianhydride structure on the Tg of nonbranched fully20 and partially aromatic28 polyimides was reported in the literature, with the exception of BPADA. A detailed DSC analysis (Figure S3, Supporting Information) shows the absence of melting or crystallization peaks thereby reflecting the amorphous nature of all of the polyimides in their state just after synthesis.

Rheological temperature sweeps were performed at a very slow cooling rate (1 °C/min), and the results are shown in . a shows the values of the storage modulus (G′) and loss modulus (G″) while damping factors (tan δ) versus temperature at 1 Hz for all of the samples studied are shown in b.

The differences between the Tg values determined from the maximum of the tan δ peak (25 °C < Tg < 40 °C) arise from the effect of different aromatic dianhydride structures on the glass transition processes. The width of the curves indicate the breadth of the temperature range over which the glass transition occurs but also the polymer structural heterogeneity.29 The values of tan δ of these polymers (especially BPADA-D and 6FDA-D with values close to 2) are remarkably high over a broad range of near-room temperatures, which makes them great candidates in applications where high damping properties are required at ambiental conditions (noise or vibration insulating materials, shock absorbers, and sealants). In general, damping materials with tan δ>0.5 are considered for outdoor or machinery applications.30 (As seen in b, BPADA-D and 6FDA-D are displaying the narrowest tan δ peaks. A somewhat broader curve was obtained with ODPA-D and the broadest with BPDA-D. Moreover, the BPDA-D system exhibits stepwise glass transition (shown in b as (I) and (II)), as opposed to the other three (BPADA-D, ODPA-D, and 6FDA-D), which show single Tg.

Similar observations could be made from the broadband dielectric spectroscopy (BDS) data. The spectra corresponding to all four polymers over a wide range of frequencies at different temperatures are shown in Figure S4, Supporting Information. Tg was calculated from the temperature dependence of the segmental relaxation times (τmax). When this dependence follows a Vogel–Fulcher–Tammann (VFT) behavior, Tg is obtained by extrapolating the VFT fit to the temperature at which τmax is equal to 100 s (see the SI).31 The Tg’s calculated following this approach (Table 1) are similar to the values obtained by DSC and follow the same trend of increase among the four polymers investigated (ODPA-D < BPDA-D < BPADA-D < 6FDA-D), as seen by DSC and rheology. The temperature dependence of the relaxation times in the whole region of the segmental relaxation further supports the fact that the ODPA-D polymer has the least restricted dynamics (see the SI).

In addition to a third estimate of Tg, BDS gave some further insights on the structure and polymer architecture of the studied PIs.

By plotting the normalized dielectric loss vs normalized frequency, a clearer view on how the shape (symmetry and broadness) of the relaxation spectra varies is obtained. shows the normalized plots at a selected temperature (T = 50 °C) at which the dielectric relaxation is clearly observable as a well-resolved peak in the frequency window for all polymers. Depending on the nature of the dianhydride incorporated into the polymer, clear differences in the shape of the spectra can be noticed. Schönhals and Schlosser32 phenomenological model proposes that the shape of the normalized dielectric loss peak is related to the behavior of the polymer at low and high frequencies, controlled by inter- and intramolecular interactions, respectively. The application of such a model to the studied polyimides suggests that the differences on the low frequency side may be related to the changes in the dynamics of the main chain segments due to the contribution/restriction imposed by the different dianhydrides, while the variations on the high frequency side can be attributed to the influence of the dangling side chains.

In order to quantify these differences, the dielectric strength (Δε) and the shape parameters b and c derived from the Havriliak–Negami (HN) fitting function (see the SI) were calculated and shown in . The b and c parameters characterize the symmetric and asymmetric broadening of the relaxation time distribution, respectively. The term b*c is also shown in the same figure. In a log(ε″) vs log(f) plot, just as in , the shape parameters b and b*c correspond to the low- and high-frequency slopes of the relaxation function respectively, with regards to the position of the maximal loss.33

A detailed analysis of the plots in gives better insight into the effect of different dianhydrides on the resulting polymer architectures. An increase in dielectric strength (Δε) can be understood in terms of an increasing number of mobile dipoles (increasing fraction of polar molecules) involved in the relaxation, indirectly reflecting stronger molecular interactions. We can therefore infer that the BPADA-D system has higher dipole interactions than the rest of the PI systems with other dianhydrides. Recently, research on the relaxation behavior of thermo-reversible elastomer networks based on 2-ureido-4-pyrimidinone (UPy) dimers has been published by Luo et al.34 In their work they report on the appearance of a new relaxation, seen as a shoulder close to the segmental relaxation, ascribed to the dissociation dynamics of dimer complexes within the network. They conclude that this new relaxation is related to the local UPy dimers dynamics resulting from the dissociation and reformation of the complexes. They also report an increase of Δε due to the increased number of dissociated UPy units. In our work, there is no evidence of a new relaxation probably because they take place within the temperature range within the glass transition; therefore, any weaker relaxation is hidden by the strong segmental relaxation.

Second, the fact that the b parameter is higher for the BPADA-D system reflects a narrower and more symmetrical loss peak. In terms of the polymer architecture, this would suggest a more homogeneous structure formed by polymer chains with similar large scale motions. On the contrary, the lowest b value for the BPDA-D system suggests chain segments with different dynamics and thus a higher degree of structural heterogeneity, implying regions of distinct mobility. This hypothesis is confirmed by the presence of a stepwise glass transition, as reported by rheological measurements ( b). Moreover, the lowest b*c parameter for BPDA-D is further confirming heterogeneous dynamics of this sample with a more visible temperature dependence. As the temperature increases, the b*c parameter tends to increase, presuming the distinct heterogeneous mobility regions become more homogeneous, especially above 50 °C. As will be discussed later on, the increase on b*c values above 50 °C could be related to local ordering present in the BPDA-D sample appearing at around 50 °C.

Aromatic polyimides are well-known for forming charge transfer complexes (CTCs), which are widely claimed to be the reason behind their great mechanical and thermal properties, as well as their characteristic colors,18 due to their absorption characteristics tailings in the visible region caused by the intra and/or intermolecular charge transfer (CT) interactions of the PI backbones. Moreover, CTCs have a major effect on the chain packing.27,35 In order to investigate the origin of the different coloration, the polymer optical properties were tested by fluorescence spectroscopy as typically used in polyimide research to identify the formation of CTCs.18,35−39 The CTCs existence has been previously confirmed for not only fully aromatic PIs but also in the semiaromatic ones.40 Our results showed that CTCs in the branched polyimides are identified by a long-wavelength absorption at λ > 330 nm, similar to those reported in literature.37,38 As can be seen in , the four polyimides developed in this work are fluorescent in the CTC-region (400 nm < λem < 450 nm).

Guest Posts
*
*
* CAPTCHA
Submit