Top–down meets bottom–up: A comparison of the mechanical properties of melt electrospun and self-assembled 1,3,5-benzenetrisamide fibers
Graphical abstract
Introduction
The controlled fabrication of well-defined microscopic fibrillar structures has become one of the main topics in materials science [1], [2], [3]. Networks and nonwovens based on these structures possess exceptional properties, such as high surface area, possibility for easy functionalization and superior mechanical strength [4]. These are promising features for applications such as tissue engineering, drug delivery, sensors, micro-/nanoelectromechanical systems (MEMS/NEMS), and filtration [3], [4], [5], [6], [7]. Especially fibers of sub-micron or nanoscale diameters are of interest due to their surface to volume ratio and the possibility to form structures with small mesh-sizes. Two approaches are feasible to access these length scales: Bottom–up approaches rely on the self-assembly of smaller units (even single molecules) to hierarchical structures [8], [9]. Top–down approaches, such as electrospinning, shape the materials directly into the desired structure. Especially for fibers and nonwovens, a great variety of structures has been demonstrated [10].
Both techniques are complementary in various ways: Since self-assembly allows simultaneous formation and growth of many fibers in a given volume, it is preferable in terms of processing times, especially for upscaling. In addition, if the processing conditions are chosen well, smaller fiber diameters are accessible in a more simple fashion than in electrospinning [9], [11]. On the downside, self-assembled fibers have smaller length and random orientation since they grow from many nuclei.
The advantage of electrospinning is that fibers can easily be formed with macroscopic length and well-defined orientation on macroscopic length scales. This even allows the controlled formation of superstructures at the micrometer level and above. However, the processing times are longer since electrospinning is a sequential process, in which the time necessary to form fibers is proportional to the total fiber length.
To offer the highest flexibility, it would be desirable to switch from one to the other approach for the same class of materials – especially for the formation of hierarchically organized structures which span multiple length scales.
Lately, the self-assembly of 1,3,5-benzenetrisamides (BTAs) into fibrillar structures has attracted increasing research interest [12], [13]. The benzene core realizes a planar and symmetric moiety and three amide groups allow the formation of strong hydrogen bonds between adjacent molecules resulting in supramolecular architectures [14]. BTAs are well-known as nucleating agents for polyvinylidene fluoride and polypropylene [15], [16], [17], [18]. Moreover, they are applied as organo- and hydrogelators [19], [20], [21], [22], as additives to improve the charge storage capability of electret materials [23] and as supramolecular materials [24], [25].
In addition to their bottom–up properties, we recently reported on the melt electrospinning of various BTAs and 1,3,5-cyclohexanetrisamides into defined fine fibers with a narrow size distribution [26]. Although a high molecular mass polymer is not essential for obtaining uniform electrospun fibers [27], electrospinning of low molecular weight substances is still unusual. BTAs form macrodipoles along the main axis of the column during the supramolecular assembly process within external electric fields and consequently offer excellent pre-conditions for electrospinning [28], [29], [30]. The melt electrospinning of BTAs is an exciting new top–down approach for self-assembling systems. It offers the possibility to overcome the strict limitation of the self-assembly conditions and consequently opens up a wide field of new applications for BTAs.
For all applications, a reasonable mechanical stability is an essential prerequisite. However, regardless by which means the BTA fibers are prepared, the mechanical characterization on a micron- or sub-micron scale requires sophisticated methods. A powerful technique is nanomechanical bending experiments, which have been used for the mechanical investigation of polymer nanofibers [31], [32], [33], biological materials [34], [35], [36], [37], [38], CNTs [39], [40], and nanowires [41], [42], [43]. In previous studies, we performed bending experiments on BTA micro- and nanofibers obtained via controlled self-assembly from nonpolar solvents. The experiments demonstrated that their molecular architecture allows control over the fiber morphology without decreasing their mechanical stability [44], [45].
In this work, we address the question whether the properties of BTA fibers are affected by using a top–down approach instead of a bottom–up approach. For that purpose, we prepare fibers of the same 1,3,5-benzenetrisamide via self-assembly from solution and melt electrospinning. This allows us for the first time to compare crystal structure, morphology and nano-mechanical properties of BTA fibers prepared from the same material.
Section snippets
Results and discussion
For the comparison, we prepared fibers of N,N′,N″-tripropyl-1,3,5-benzenetricarboxamide 1 (Scheme 1) via bottom–up (in the following termed SA-fibers) as well as top–down techniques (in the following termed ES-fibers). The SA-fibers were produced via controlled self-assembly by cooling of a solution of 1 in 2,2,4,4,6,8,8-heptamethylnonane (HMN). As top–down approach, we used melt electrospinning.
In order to investigate structural features on the Ångström-scale, we performed XRD measurements on
Conclusions
In this study we demonstrated for the first time that mechanically robust BTA fibers can be accessed via bottom–up- and top–down-approaches. We prepared self-assembled (SA) and melt electrospun (ES) fibers from the same compound 1 and obtained fibers with an average diameter of 1.2 ± 0.7 μm and 0.8 ± 0.2 μm for SA- and ES-fibers, respectively. On the Ångström-scale, XRD measurements show the same crystal structure of the fibers, independently of the preparation method. On the microscopic scale,
Material
N,N′,N″-tripropyl-1,3,5-benzenetricarboxamide 1 was synthesized according to the literature [50]. The melting temperature of 1 is 289 °C and was determined in a Perkin Elmer Diamond DSC (heating rate: 10 K/min, nitrogen flow: 20 mL/min). The temperature at a 10% weight loss is 351 °C. The measurement was performed in a Mettler SDTA 851 TGA at 10 K/min (nitrogen flow: 60 mL/min). Isothermal TGA runs at the spinning temperature (290 °C) under nitrogen atmosphere were performed to verify the
Acknowledgments
This work received financial support from the German Research Foundation (Deutsche Forschungsgemeinschaft) within the SFB 840, project B8. We thank Doris Hanft for help with the synthesis of the 1,3,5-benzenetrisamide, Martina Heider and Dr. Beate Förster from Bayreuth Institute of Macromolecular Research for support with the SEM images and Andreas Timme and Marina Behr for the XRD measurements. DK, JCS, BRN and JWN acknowledge the support of the Elite Network of Bavaria.
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Julia C. Singer and Daniel Kluge contributed equally to this work.