Chemistry

Extremely-strong long-chain polyamide elastomers with programmable supramolecular interactions and oriented crystalline microstructures

Synthesis of long-chain polyamides

Lately, a number of approaches have been developed to organize polyamides with practical facet teams30,31,32,33. Nonetheless, the synthesis of monomers was principally concerned with tedious work-up and comparatively low yields. On this work, we developed environment friendly synthesis of diene monomers that bear two secondary amide bonds in the primary chain hooked up with pendant facet teams. As proven in Fig. 1a, an α, ω-diene amide monomer with hydroxyl facet group (N,N’-(2-hydroxypropane-1,Three-diyl)bis(undec-10-enamide), UDA) was first ready by way of amidation of methyl 10-undecenoate with 1,Three-diamino-2-propanol34,35,36. The response conversion into UDA monomer throughout this step is quantitative (~100%). The hydroxyl group in UDA was then reacted with butyric anhydride to introduce a bigger pendant group that would later inhibit crystallization of its polymer. The resultant monomer 1,Three-di(undec-10-enamido)propan-2-yl butyrate is labeled as BUDA. The profitable synthesis of UDA and BUDA was confirmed by Fourier-transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (1H NMR), and carbon-13 nuclear magnetic resonance spectroscopy (Supplementary Fig. 1). Thiol-ene addition polymerization of each diene monomers was carried out to afford polyamides (Fig. 1b, labeled as PUDA and PBUDA, respectively). The existence of hydrogen bonded amide teams together with linear alkyl chains would end in extremely crystalline polymers for PUDA. Certainly, PUDA homopolymer (P7) is a extremely crystalline and brittle materials (Supplementary Fig. 2a). Then again, the presence of pendant teams might interrupt the formation of crystalline domains in PBUDA (P0, clear in Fig. 1c). Thus, we predict copolymer of each monomers (P(UDA-co-PBUDA) (opaque in Fig. 1c) might have the existence of a two-phase morphology with nanocrystalline domains dispersed in an amorphous matrix. The exact management of feed ratios of co-monomers would enable the facile tuning of crystallinity with the help of programmable supramolecular hydrogen bonding.

To know the impact of monomeric compositions on microstructures and thermomechanical properties of practical polyamides, we ready a collection of P(UDA-co-BUDA) copolymers with the fraction of UDA various from 10 to 80 mol% (polyamides P1–P6 in Supplementary Desk 1). The formation of those polyamide copolymers was confirmed by 1H NMR (Supplementary Fig. Three), gel permeation chromatography (Supplementary Fig. four), and thermogravimetric evaluation (Supplementary Fig. 5). All these polyamides have comparatively excessive molecular weight (>20,000 g mol−1) and good thermal stability.

Differential scanning calorimetry (DSC) was carried out to estimate the microstructure of polymers. Determine 2a reveals warmth stream curves of DSC for P0–P7. All copolymers exhibited glass transition temperature (Tg) far under room temperature (−29.four to −21.6 °C). Furthermore, DSC curves present distinct melting and crystallization processes for each PUDA homopolymer (P7) and copolymers (P1–P6). The melting temperature (Tm) of P1–P7 elevated with the rise of UDA content material. PUDA homopolymer (P7) has the very best Tm at ~122.Three °C and an enthalpy of fusion (ΔHm) of 595.5 J g−1, whereas PBUDA homopolymer (P0) was not noticed with a melting level. The DSC heating curves for polymers with excessive UDA content material (P5–P7) present two distinct melting peaks, which could be interpreted by a coexistence of the γ-phase and α′-phase crystals at elevated temperature37,38. These outcomes recommend the incorporation of UDA with the hydroxyl facet group facilitates the formation of crystalline domains: the upper the UDA content material, the upper melting temperature.

Fig. 2Fig. 2

Characterization of polyamides P0–P7. a Differential scanning calorimetry (DSC) heating curves; b one-dimensional (1D) wide-angle X-ray diffraction (WAXD) profiles; c Fourier-transform infrared spectroscopy (FT-IR) spectra within the 1500–1700 cm−1 area for P0–P7; d the change of FT-IR peak depth of free/disordered/ordered hydrogen bonded C=O for P0–P7 as a operate of UDA content material; e variable temperature FT-IR spectra of P6 within the 1500–1700 cm−1 area; f the change of FT-IR peak depth of free/disordered/ordered hydrogen bonded C=O for P6 as a operate of temperature; g proposed molecular fashions for crystalline area and amorphous area

Microstructures and supramolecular hydrogen bonding

Extensive-angle X-ray diffraction (WAXD) was additional used to probe crystallization behaviors of homopolymers of UDA and BUDA, in addition to their copolymers (Fig. 2b). Besides PBUDA homopolymers, all different polymers have been noticed with a broad baseline and sharp peaks with different depth, indicating the coexistence of each crystalline and amorphous microstructures. The crystalline and amorphous peaks have been deconvoluted by way of peak becoming (Supplementary Fig. 6). The crystalline peak at 2θ round 21.2° corresponds to the γ-crystalline kind as reported for polyamides39. The diploma of crystallinity (Xc) was calculated primarily based on the fraction of areas underneath crystalline peaks over the full areas underneath each crystalline and amorphous areas. Supplementary Desk 2 summarizes the height positions and Xc for P0–P740. Xc will increase with the rise of UDA content material within the copolymers, in good settlement with the DSC knowledge. The scale of crystalline domains (t) could be estimated from the Scherrer’s system: t = λ/B cos θ, the place λ is the wavelength of X-ray, B is the complete width at half-maximum of diffraction peaks, and θ is the diffraction angle. In consequence, the dimensions of crystalline domains is simply ca. 6.7–eight.5 nm for the copolymers. Though well-aligned UDA segments induce the expansion of crystallites, the pendant butyrate teams hinder the alignment polyethylene-like chains on the spine and additional depress the general power of intermolecular van der Waals interactions. Supplementary Determine 7 reveals a polarized optical micrograph (POM) of a consultant copolymer P4, the practically darkish picture additionally indicated that the crystals are too small to be visualized by POM.

It was conceptualized that supramolecular hydrogen bonding ought to play a important function within the crystallization of polyamides. As hydrogen bonding is temperature-sensitive, these copolymers have been characterised by variable temperature FT-IR spectroscopy. When C=O teams of amides kind hydrogen bonds, their infrared absorption peak could shift. Determine 2c reveals FT-IR spectra of P0–P7 within the 1500–1700 cm−1 area. The peaks close to 1620–1680 cm−1 are assigned to the stretching of carbonyl teams. This area of carbonyl stretching comprises three distinct contributions: free carbonyl teams at 1678 cm−1, disordered hydrogen bonded carbonyl teams at 1646 cm−1, and ordered hydrogen bonded carbonyl teams at 1626 cm−1, the latter two of which point out the formation of hydrogen bonding in polyamides41. Determine 2nd reveals the adjustments of peak intensities of free/disordered hydrogen bonded/ordered hydrogen bonded carbonyl teams as a operate of UDA content material for polymers P0–P7 at room temperature. It may be seen that the height depth of ordered hydrogen bonded teams vastly elevated with the rise of UDA content material, demonstrating that the presence of extra UDA facilitated the formation of hydrogen bonding. Determine 2e reveals variable temperature FT-IR spectra of a consultant copolymer P6 within the 1500–1700 cm−1 area, which signifies affiliation/dissociation of hydrogen bonds underneath completely different temperature. With the rise of temperature, the absorption peaks at 1646 and 1626 cm−1 decreased, and a broad peak close to 1678 cm−1 regularly elevated, indicating the weakening and dissociation of hydrogen bonds. Determine 2f reveals the adjustments of peak depth of free/disordered hydrogen bonded/ordered hydrogen bonded C=O for P6 as a operate of temperature. There’s a transition occurred from 80 °C to 120 °C, which is in wonderful settlement with the melting outcomes by DSC. This steered that the ordered hydrogen bonds exist principally inside the crystalline area. Based mostly on these observations, molecular fashions for crystalline and amorphous phases have been proposed in Fig. 2g. The massive butyrate facet teams of BUDA disrupt chain regularity and like to remain within the amorphous area, whereas hydroxyl teams facilitate crystallization. General, it may be concluded that along with intermolecular van der Waals interactions from linear alkyl chains, the formation of semicrystalline microstructures was largely facilitated by inter/intra-molecular hydrogen bonding.

The dispersion of crystalline microstructures in an amorphous matrix is essential to own excellent mechanical properties for a lot of of polyamide techniques42,43,44. For the copolymers of UDA and BUDA, an amorphous matrix envelopes nanocrystalline domains, the place there are wealthy inter/intra-molecular hydrogen bonds. The mechanical properties of copolymers with varied UDA contents have been measured by way of monotonic tensile deformation (Fig. 3a, Supplementary Fig. eight, and Supplementary Desk Three). In comparison with the non-crystalline homopolymer PBUDA, copolymer P4 has tensile power and Younger’s modulus at 18.four ± 2.1 and 149.6 ± Three.1 MPa, a rise of 429.6% and 521.four%, respectively. Furthermore, with the rise of UDA content material, the toughness elevated from 5.2 (P0) to 65.zero MJ m−Three (P4), a rise over 12.5-folds, demonstrating that the existence of nanocrystalline domains vastly enhances toughness. Copolymers P1–P3 have been additionally noticed with related traits at completely different ranges of enhance in mechanical properties (Fig. 3a). Copolymers P5–P6 with greater contents of UDA (60% and 80% respectively) weren’t topic to such comparisons, as they’re fairly brittle.

Fig. ThreeFig. 3

Step-cycle tensile deformation as a processing methodology to enhance mechanical properties. a Monotonic stress–pressure curves of P0–P4; b first and c second step-cycle tensile deformation of P4; d monotonic stress–pressure curves of ultra-strong elastomer four (uE4) throughout step-cycle tensile deformation; e monotonic stress-monotonic pressure curves of uE1–uE4; f monotonic stress–pressure curve of uE4-Cu (14.four mol%)

Preparation of uEs

It’s anticipated that the as-prepared movie samples of copolymers by answer casting don’t possess extremely oriented crystalline microstructures. Consecutive cyclic tensile deformation was utilized to copolymers P1–P4 to induce the alignment of microstructures45,46. After deformation, these samples have been labeled as uE1–uE4 (Fig. Three and Supplementary Figs. 9–11). The stress–pressure curves through the first and second step-cycle tensile deformation of P4 are proven in Fig. 3b and c. The massive hysteresis throughout every cycle is in settlement with the Mullin impact47. After the step-cycle tensile deformation, these copolymers have been remodeled into ultra-strong elastomers (uE1–uE4). For instance, the stress at break of uE4 is 126.Three MPa, greater than seven occasions of as-prepared P4 (Fig. 3e). Although the elongation at break of uE4 diminished to 30%, its elastic restoration is strikingly excessive at 94.5% (Fig. 3d). Equally, the excessive elastic restoration was additionally noticed for uE1, uE2, and uE3 at 96.9%, 96.7%, and 95% respectively (Supplementary Fig. 12). Collectively, these elastomers mixed ultrahigh tensile power and wonderful elasticity. The mechanical properties of uE1–uE4 are summarized in Supplementary Desk four. The mechanical properties of uEs could be exactly tuned by controlling the content material of UDA. Supplementary Determine 13 reveals uE4 fiber with a diameter of ~135 μm can simply maintain a weight of 200 g, implying the distinctive toughness.

To reveal that there’s a lot room for additional bettering mechanical power of those polyamides, we carried out a preliminary examine by introducing steel–ligand coordination. Cuprous–thioether coordination has been extensively noticed in organic techniques and lately utilized in artificial polymers48,49,50,51. Our polyamides comprise an affordable fraction of thioether teams alongside the spine. Thus, CuBr was used to induce cuprous–thioether coordination to strengthen the mechanical properties (Supplementary Fig. 22). After related tensile deformation processing, as proven in Fig. 3f and Supplementary Figs. 14–15, polyamides with 14.four mol% cuprous ions (to the sulfur atoms, labeled as uE4-Cu) exhibit stress at break at 211.2 MPa, greater than 65% enhance over uE4, whereas sustaining related elongation at break (27.9%). This substantial enhancement is a robust indication of the robustness of present buildings of polyamide techniques.

To put our work in context, we’ve got collected quite a lot of long-chain polyamides and polyesters reported in literature and in contrast their tensile power (Supplementary Desk 5). It’s fairly evident that the polyamides we ready possess the very best tensile power, even a lot greater than long-chain nylons (Nylon 12). Whereas it might not be truthful to match, our polymers even have higher tensile power than short-chain nylons (e.g., polyamide 6.6).

To know the substantial distinction of mechanical properties between as-prepared P1–P4 and tensile-deformed uE1–uE4, WAXD measurement was used to disclose how the microstructures might be rearranged by the cyclic tensile deformation. As proven in Fig. 4a, 2D WAXD sample of P4 is sort of isotropic. After cyclic tensile processing, an anisotropic 2D WAXD sample is clearly fashioned for uE4, the arrow represents the stretching course (Fig. 4b). The scattering depth was discovered to converge on the meridian, which signifies that crystalline domains are oriented alongside the tensile course. The orientation of crystal section can also be verified by the azimuthal angle at 2θ = 21° (Fig. 4c). In accordance with the WAXD evaluation (Supplementary Figs. 16–18), we proposed a mannequin on microstructure rearrangement of P(UDA-co-BUDA) copolymers throughout cyclic tensile deformation (Fig. 4d).

Fig. fourFig. 4

Microstructure evaluation of as-prepared and tensile-deformed copolymers. a, b Two-dimensional (2D) wide-angle X-ray diffraction (WAXD) patterns of P4 and ultra-strong elastomer four (uE4); c WAXD azimuthal depth profiles for P4 and uE4 at 2θ = 21°; d a proposed mannequin as an example the microstructure rearrangement of P(UDA-co-BUDA) copolymers throughout cyclic tensile deformation

AIE of polyamides

Surprisingly, each as-prepared and uni-directionally stretched polyamides exhibited sturdy luminescence. As proven in Fig. 5a and b, as-prepared P4 emits sturdy blue photoluminescence. The corresponding fluorescent spectrum reveals an emission peak at ~418 nm, whereas the UV–vis absorption peak is at 210 nm (Fig. 5c and Supplementary Fig. 19). Determine 5d–f present fluorescence pictures of stretched uE4 fibers noticed underneath microscope with a diode laser, which was excited with 340–380, 460–495, and 530–550 nm, respectively. Furthermore, Fig. 5g signifies that the emission coloration of P4 movie didn’t present apparent distinction throughout stretching, which was additional corroborated by fluorescent spectra of P1–P4 and uE1–uE4 (Supplementary Fig. 20). Conventional chromophore-containing fluorescent polymers don’t emit sturdy fluorescence in concentrated options or stable states as a result of aggregation-caused quenching24,52. Within the present case, the as-prepared polyamides with none standard chromophores exhibit sturdy emission on the stable state. We imagine that these polyamides have the properties of AIE (Supplementary Fig. 21), which was first found with chromophore-containing molecules by Tang and coworkers in 200125. Lately, artificial and pure chromophore-free polymers have been reported with AIE traits, on account of the formation of polar group clusters as a consequence of restriction of molecular motions27,53. Just a few analysis teams have reported polyamides with photoluminescence29,54,55,56. It’s usually believed that the sturdy hydrogen bonding induces the formation of native clusters of amides that facilitate fluorescence. We hypothesized that our polymers with wealthy amide and hydroxyl teams allow through-space conjugation by way of n–π* or π–π* transitions52. Thus, the as-prepared polyamides and elastomers might generate sturdy fluorescence within the stable state (Fig. 5h), which might be useful for the extension of those elastomers from bioplastics to different areas equivalent to biomedical purposes which can be worthy for exploration sooner or later.

Fig. 5Fig. 5

Aggregation-induced emission (AIE) properties of as-prepared polyamides and ultra-strong elastomers (uEs). a Picture of P4 underneath seen mild; b picture of P4 underneath 365 nm mild, emitting sturdy blue fluorescent; c ultraviolet–seen (UV–vis) absorption (crimson line) and fluorescent emission (black line) spectra of P4; d–f fluorescence pictures of uE4 fibers noticed underneath a microscope with a diode laser underneath different excited wavelengths (340–390, 460–495, and 530–550nm); g pictures of P4 throughout stretching excited by 365 nm UV lamp; h schematic illustration on the luminescent mechanism of polyamides

In abstract, biobased α, ω-diene amide monomers have been designed with pendant polar hydroxyl or non-polar butyrate teams. In contrast with conventional polyamides, these long-chain polyamides ready by thiol-ene addition polymerization have managed programmable supramolecular hydrogen bonding and tunable crystallinity with mechanical properties facilely adjusted by altering co-monomer compositions. By unidirectional step-cycle tensile deformation, ultra-strong elastomers have been obtained with out the addition of any fillers. It’s the formation of extremely aligned crystalline microstructures chargeable for the distinctive mechanical properties of elastomers. Furthermore, the clustered amide teams with molecular motions restricted within the mixture state allow these polyamides with luminescence, a phenomenon of AIE. This examine gives an strategy to pursuing biobased polymers derived from renewable pure merchandise, which mix ultrahigh mechanical power, wonderful elasticity, and powerful photoluminescence.


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