Design of Self-Cross-Linkable Poly(n-butyl acrylate)-co-poly[N-(hydroxymethyl)acrylamide] Amphiphilic Copolymers toward Elastic and Self-Healing Properties

Abstract

A huge challenge for developing self-healing materials is achieving a good compromisation between mechanical properties and self-healing efficiency. For this purpose, a facile route by introducing N-(hydroxymethyl)­acrylamide (NMA) as a thermal-catalyzed self-cross-linker as well as a self-healing material into a soft polymer system for tuning the mechanical properties in an easy way, resulting in elastic and self-healing properties through a covalent and dynamic network simultaneously, represents an exciting avenue for the development of self-healing materials. Specifically, we propose a simple strategy to synthesize a self-cross-linkable poly­{(n-butyl acrylate)-co-[N-(hydroxymethyl)­acrylamide]} (PBA x -co-PNMA y ) amphiphilic copolymer prepared by radical polymerization method, where x and y are BA and NMA ratios, respectively, based on the monomer composition in the obtained copolymer. The mechanical properties and self-healing efficiency of the copolymer can be easily tuned by controlling the monomer ratios and varying the self-cross-linking reaction conditions. PBA0.8-co-PNMA0.2 in bulk solid state possesses a stretchability of up to 191%, maximum stress of 571 kPa, and a self-healing efficiency of 90% in ambient conditions without any interventions. Owing to the hydrophobic nature of PBA in the copolymer system, self-healing can be triggered even underwater. Furthermore, a microscale thin film bestowed with identical self-healing and mechanical properties can be fabricated and the behavior of the copolymer in thin-film form was inspected using a pseudofreestanding tensile tester machine. This work provides insight into the future design of materials with elastic, self-cross-linking, and self-healing properties, which are adjustable depending on the desired applications.

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“…After 6 h cross-linking, the −OH peaks of the copolymer and the 1.5 wt % nanocomposite are red-shifted to 3350 and 3355 cm –1 , respectively, together with a decrease in the characteristic peak intensities, indicating the loss of the OH group in the polymer chain. Such changes of signals confirm the breakage of the OH bond and the formation of a covalent bond during the cross-linking reaction . On the contrary, all of the amine (−NH and C–N), carbonyl (CO), and carboxyl (−COOH)) groups of the 6c-1.5 wt % nanocomposite film were shifted, implying that the introduction NGQDs into the polymeric network not only enhances the covalent bonding but also triggers the improvement of non-covalent interactions in the nanocomposite system (Figure S4b).…”

“…Adjustable Mechanical Properties of the Nanocomposite Films. Following our previous study, the reported PBA x -co-PNMA y synthesized using the ATRP method 25 ( 1 H NMR, GPC, and FTIR profiles are included in Figures S3 and S4) was combined with NGQDs via simple blending as illustrated in Figure 1 (see the Experimental Section/Methods for details). The aim of the experimental strategy is the construction of hydrogen bonds between the functional groups of NGQDs and the side chains of the NMA-segment in a copolymer, enacting as the main cross-linking site in the nanocomposite system.…”

“…The sample preparation and characterization procedures are described in detail in the characterization section. Under standard 1 H NMR acquisition conditions, the decrease in the identified NMA signals at a, b, and c along with the increase in the cross-linking degree were associated with the respective decrease in the number of protons at the −OH, −CH 2 , and −NH groups, influenced by the conversion of the covalent linkages in the intergroup of methylol acrylamide 25,27 The peaks of the −OH group (5.12−5.85 ppm) and −NH group (7.76−7.83 ppm) from the NMA unit are deshielding. Siskos et al discovered that the shifting of the proton signal to higher frequencies is associated with a shortening of the H-bond distance of donor−acceptor, meaning that the shorter the H-bond, the larger the proton deshielding.…”

“…Such changes of signals confirm the breakage of the OH bond and the formation of a covalent bond during the cross-linking reaction. 25 On the contrary, all of the amine (−NH and C−N), carbonyl (CO), and carboxyl (−COOH)) groups of the 6c-1.5 wt % nanocomposite film were shifted, implying that the introduction NGQDs into the polymeric network not only enhances the covalent bonding but also triggers the improvement of noncovalent interactions in the nanocomposite system (Figure S4b). The impact of NGQDs on the cross-linking degree has also been successfully monitored based on the changes of the integral area of the −OH stretching group in the range of 3235−3744 cm −1 in the FTIR spectra.…”

“…With this in mind, designing an elastic, self-healing, and swelling-resistant polymer matrix that simultaneously shows water permeability provides a distinct advantage for the application in wearable pH sensors on the human skin. In our previous report, we have successfully developed a skinlike copolymer (poly­{( n -butyl acrylate)- co -[ N -(hydroxymethyl)­acrylamide]}, marked as PBA x - co -PNMA y ), which provides a new insight into a copolymer with ease of tunability and controllability of both mechanical and self-healing properties through thermal self-cross-linking. The BA unit acts as a soft segment with hydrophobicity to prevent the swelling behavior in an aquatic environment, while the NMA unit promotes elastic and self-healing features through the controllable covalent cross-linking sites with dynamic hydrogen-bonding networks.…”