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Localized Liquefaction Coupled with Rapid Solidification for Miniaturizing/Nanotexturizing Microfibrous Bioassemblies into Robust, Liquid-Resistant Sheet Dong Wu, Limei Li, Ying Wang, Zhengren Meng, Xueren Qian, Yongsheng Wang, and Jing Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04215 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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ACS Sustainable Chemistry & Engineering

Localized Liquefaction Coupled with Rapid Solidification for Miniaturizing/Nanotexturizing Microfibrous Bioassemblies into Robust, Liquid-Resistant Sheet Dong Wu, Limei Li, Ying Wang, Zhengren Meng, Xueren Qian, Yongsheng Wang, and Jing Shen*

Key Laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, 26 Hexing Road, Harbin 150040, China.

Corresponding author.

* Email addresses: [email protected]; [email protected] (J. S.)

ABSTRACT: Biopolymeric fibers with microscale diameters have long been commercially used to generate network-structured assemblies (paper-based products). Despite overwhelming productivity and widespread use, unconventional applications of these products are challenged by inherent imperfectness related to limited internal bonding, abundance of interfiber gaps, among others. Here, we demonstrate the use of a green, scalable concept involving localized liquefaction and rapid solidification to miniaturize microfibrous bioassemblies into nanotextured, delicately reorganized sheet. The time for bioassemblies-solvent interaction was identified as a critical factor. On the surface of reorganized bioassemblies, size-tunable outgrowths were formed

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due to nonsolvent-induced rapid phase transition. A bioassemblies-solvent contact time of 10 min resulted in surface nanostructuring. Densification of bioassemblies correlated well with the development of optical transparency.Reorganized bioassemblies exhibited pronounced mechanical robustness even after being soaked in water. Strong resistance to penetration by aqueous/nonaqueous liquids was identified. The liquid-contaminated surface of reorganized bioassemblies was easily cleanable. These features can be attributed to significant improvement of structural integrity. On the basis of the use of green, recyclable solvent/nonsolvent, the facile miniaturization of mass-producible microfibrous bioassemblies into delicately structured sheet with tunable functionalities would facilitate applications such as those related to advanced barrier packaging materials, sensors, and electronics.

KEYWORDS:

Natural

fibers,

Supramolecular

bioassemblies,

Structural

reorganization, Nanostructuring, Biopolymeric nanoproducts

INTRODUCTION As supramolecular bioassemblies predominately produced from naturally hollow fibers (with diameters and lengths of roughly 10 to 50 micrometers and 1 to 3 milimeters), paper is a ubiquitous, versatile, and low-cost material that can be tailored to various end-use applications. Biopolymeric fibers liberated from renewable feedstocks are key building blocks for commercial production of bioassemblies with tailorable properties. Structurally, these bioassemblies can simply be depicted as three-dimensional, porous, and microfibrous networks. Organization of fibers into 2


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such networksis commercially practiced using a highly scalable papermaking process involving colloidal interactions and hydrogen-bonding-directed assembly. It is interesting to note that, in the broad area of materials science and engineering, this process can be facilely redesigned to fabricate functional materials (e.g., graphene oxide paper/membrane) by assembling of liquid-dispersible components. 1,2 Tailoring structural characteristics and functionalities of fibers or their network-structured bioassemblies is a critical strategy toward designing bioproducts for highly efficient applications. Essentially, this strategy has been widely practiced in commercial pulp and paper manufacturing processes. Pulping, bleaching, and pulp refining result in tunable change of fiber characteristics. Wet-end and surface applications of additives, wet pressing, surface sizing/coating, finishing, and converting are used to tailor hydrophobicity, porosity, mechanical strength, smoothness, gloss, opacity, and ink-receptivity of bioassemblies to specific end-use requirements. This versatile strategy has huge potential in developing new bioproducts, opening the door for possible unconventional applications. Examples of reported functional bioassemblies-based products include superhydrophobic materials, 3

rewritable materials,

4-5

photocatalytic materials,6 shape-morphing materials,

antibacterial materials,

8-9

sensors,

Bioassemblies’

inherent

features,

10

energy-storage devices, including

lightweight,

11

7

among others.

cost-effectiveness,

renewability, and biodegradability, serve as a driving force for worldwide research and development activities. Green technologies have found widespread use in tailorableproduction of 3



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aforementioned fiber products. Sustainable bio-based additives, such as cationic starch and rosin dispersion, have been commercially used. Wet-end and surface applications of various bio-based additives, as substitutes for fossil-derived, non-sustainable ones, are believed to be an indispensable strategy toward a greener future.

12-14

In some cases, the use of bio-based or degradable additives can deliver

unique features, such as rapid water-dispersibility, good gas/liquid-barrier properties, and enhanced paper recycling. biotechnology

15-17

Producing safer/nontoxic commodities, use of

to enhance process efficiency, and closure of white water loops,

among others, are promising strategies in green technology portfolios. Despite numerous possibilities of using network-structured bioassemblies in diversified areas, highly efficient and advanced applications are somehow challenged by inherently limited mechanical strength.

18

Superstrong resistance against various

liquids would also be needed in applications where frequent or long-term contact with liquids is a necessity. Further, regular bioassemblies have porous structures and rough surfaces, which is oftentimes a huge challenge for efficient use as substrates for functional materials such as electronics.

19

Basically, the structures of regular

bioassemblies are not as “fine” or “delicate” as most plastics, and intermolecular bonding interactions would need to be enhanced and finely tuned to meet the requirements of unconventional applications. In the context of global trend of a greener

economy,

tailorable

engineering

of

mass-producible,

sustainable

bioassemblies would bridge the gap between conventional pulp and paper industry and advanced applications. However, global research and development activities in 4


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this area are somehow limited. Therefore, there is an ongoing urgent need to unlock the possibilities of designing engineered bioassemblies for unconventional applications. In this current study, with the premise of using green processes in mind, we directed our efforts toward tailoring the characteristics of microfibrous bioassemblies by combining localized liquefaction with rapid phase transition to possible unconventional applications (e.g., advanced barrier packaging materials, bio-based vessels, and bio-based electronics). We envisaged that, on the basis of using green solvent and nonsolvent for sequential post-treatments of bioassemblies, localized liquefaction followed by rapid solidification would lead to structural reorganization, densification, miniaturization, and nanotexturization. These changes would facilitate the use of restructured bioassemblies in diversified/unconventional applications. EXPERIMENTAL SECTION Materials Network-structured bioassemblies (cotton-derived circular qualitative filter papers), with a diameter of 18 cm and an ash content of 0.01%, were purchased from Funshun Civil Administration Filter Paper Co. Ltd (China). Anhydrous zinc chloride, anhydrous ethanol, glycerol, and hydrochloric acid were of analytical reagent grade, purchased from Tianjin Basifu Chemicals Co. Ltd (China) and Tianjin Northern Tianyi Chemicals Co. Ltd (China), Tianjin Tianli Chemicals Co., Ltd (China), and Beijing Chemical Works (China), respectively. Anhydrous zinc chloride was dried at 105 oC for 24 h prior to use. Anhydrous ethanol was used as received. Glycerol and 5



hydrochloric acid were diluted with distilled water prior to use.

Structural Modification of Microfibrous Bioassemblies A facile route involving the use of a green solvent (zinc chloride trihydrate) and a green nonsolvent (anhydrous ethanol) was adopted to reorganize network-structured, microfibrous bioassemblies into “finer” structures. The solvent was prepared by sufficiently mixing anhydrous zinc chloride with distilled water (molar ratio of zinc chloride to water: 1/3). This solvent was preheated to 75 oC prior to its interaction with bioassemblies. Anhydrous ethanol was used as the green nonsolvent to induce rapid phase transition (solidification) and nanotexturization. In a typical set of experiments, bioassemblies (a sheet of untreated paper) were added into a transparent self-sealing plastic bag, followed by the addition of zinc chloride trihydrate (30 ml). The bag was then sealed, and bioassemblies were allowed to interact with solvent at 75 oC for 10 min, resulting in localized liquefaction. After this interaction, the solvent was removed. Solvent-treated bioassemblies were subjected to interaction with anhydrous ethanol (50 ml) for 20 min, and then sufficiently washed with distilled water. Blotting sheets (Hangzhou Fuyang Beimu Pulp and Paper Co., Ltd., China) were used to remove the residual water on the surface of bioassemblies. These bioassemblies were soaked in glycerin solution (5 g of glycerin dissolved in 95 g of water) for 30 min. Again, blotting sheets were used to remove the residual liquids. The resulting wet bioassemblies were finally subjected to pressing/drying (110 oC, 4MPa) with a R32022015 hot press machine (China).

6


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In another set of experiments, for comparison purposes, bioassemblies were soaked in zinc chloride trihydrate (150 ml) pre-added to an open plastic container, and the time for bioassemblies-solvent interaction was 5 s. This short-time treatment resulted in minor liquefaction of bioassemblies. Upon removal of the solvent, solvent-treated bioassemblies were allowed to interact with anhydrous ethanol (150 ml) for 90 s, followed by sufficient washing (using distilled water). The residual water on the surface of bioassemblies was subsequently removed with blotting sheets. These wet bioassemblies were pressed/dried (110 oC, 4MPa). For comparison purposes, at a constant temperature of 75 oC, bioassemblies were also soaked in zinc chloride trihydrate for 2 h, forming a clear solution. This biopolymeric solution was casted onto a polyfluorotetraethylene plate, and then immersed in anhydrous ethanol (150 ml). The as-formed biopolymeric film was sequentially washed with a dilute hydrogen chloride solution and distilled water, and soaked in a glycerin solution (5 g of glycerin dissolved in 95 g of water) for 30 min. Subsequently, the film was dried at 60 oC for 2 h, followed by pressing/drying (110 oC, 4MPa). In the case of bioassemblies not subjected to treatment with solvent (i.e., untreated bioassemblies), they were sequentially immersed in anhydrous ethanol and distilled water, and the residual water was removed with blotting sheets. These wet bioassemblies were pressed/dried (110 oC, 4MPa). It is noteworthy that, unless otherwise stated, all biopolymeric samples (i.e., untreated and restructured bioassemblies) mentioned throughout this research paper were conditioned in a desiccator for at least 24 h prior to analyses/tests. 7



Characterization of Untreated and Structurally Reorganized Bioassemblies Scanning electron microscopy (SEM) images of untreated and structurally reorganized bioassemblies were collected with a JEOL JSM-7500F Scanning electron microscope (Japan) to identify surface and cross-sectional morphologies. A Microsoft PowerPoint 2013 software was used to produce colored SEM images, without changing the original morphological features. On the basis of SEM images, the dimensions of micro/nano-outgrowths were estimated with a Nano Measurer 1.2 software (Fudan University, China). Surface topographies of untreated and structurally reorganized bioassemblies were characterized with a Multimode 8 Atomic Force Microscope (AFM) (USA), and

images were collected. To further assess the

structure-related impact of combined post-treatments of bioassemblies, we also tested some samples with a Nicolet 6700 Fourier-transform infrared spectrometer (FTIR) (USA), a D/Max-2200 PC X-ray diffractometer (XRD) (Japan), an Escalab 250Xi X-ray photoelectron spectrometer (XPS) (USA), and a Bruker Avance III-HD 500 nuclear magnetic resonance spectrometer (NMR) (Germany) (Figure S1). It should be noted that the coloring of original SEM images is examplarily shown in Figure S2. Determination of Critical Properties of Untreated and Structurally Reorganized Bioassemblies Density The thickness (or caliper) of a given biopolymeric sample was measured with a ZUS-4 Paper Thickness Tester (China). The density (g/cm3) was then calculated by

8


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dividing the grammage of the sample by its thickness. Optical Transparency Biopolymeric samples were conditioned in a chamber containing a saturated solution of sodium chloride for at least 48 h, and placed on paper-strips with printed Chinese characters for “Northeast Forestry University”. Digital photographs were then taken to visually assess optical transparency. To test the light transmittances (%) of samples at different wavelengths, a T6 UV-Vis spectrophotometer (China) was used. Untreated and structurally reorganized bioassemblies were carefully attached to inside walls of sample cells prior to light transmittance tests. Mechanical Strength A ZL-300A tensile strength tester (China) was used to determine the tensile strength (MPa) of unwetted and wetted samples. Prior to wet tensile strength tests, samples were soaked in distilled water for 1 h, followed by the removal of water with blotting sheets. The ratio of wet tensile strength to dry tensile strength was used to evaluate the wet strength performance. Busting strength tests (kPa) were conducted with a BSM-1600 bursting strength tester. To visually assess the interaction of samples with water and disintegration behaviors (a reflection of wet strength characteristics), samples were were collected.

soaked in distilled water for varying times, and digital photographs In addition, the pronounced mechanical robustness of structurally

reorganized bioassemblies was evaluated by testing the possibility of using wet and dry strips to support a metal flattener (10 kg). To identify the impact of the number of 9



freeze-thaw cycles on strength properties, the selected sample was repeatedly subjected to freezing (about -20 oC, 15 min) and thawing (using distilled water to reduce the temperature, followed by water removal and drying at 60 oC). Bioassemblies-Liquid Interaction Untreated and structurally reorganized bioassemblies, equipped as filters, were combined with glass filtration sets. Various liquids, such as soy sauce, vinegar, beer, colored water, colored ethanol, and grease solution (mixture of castor oil, toluene, and n-heptane), were used to interact with biopolymeric samples. Digital photographs were taken to assess liquid-barrier properties. Water contact angle measurements (using 5 µL of water droplets) were performed with an OCA20 optical contact angle measuring system (Germany). To preliminarily assess the cleanabilities of untreated and structurally reorganized bioassemblies fouled by liquids, samples (strips) were attached onto transparent glass panes with double-sided adhesive taps, followed by pouring liquids (castor oil, ketchup, and milk) onto samples to allow bioassemblies-liquid interaction for 10 min. Subsequently, running tap water was used to clean liquid-contaminated surfaces.

After cleaning, the samples (together with

glass panes) were placed on colored paper strips, and digital photographs were taken. RESULTS AND DISCUSSION Proposed Process Concept and Its Impact on Morphological/Topographical Characteristics and Optical Transparency of Bioassemblies Lignocellulosic bioresources are generated in natural processes (e.g., photosynthesis

10


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and water cycle). Converting these renewable biopolymeric complexes into sustainable products that will ultimately biodegrade in the natural world after their life cycles is a guiding strategy toward sustainability.

20-24

The state-of-the-art pulping

(fiber liberation) and papermaking (fiber reassembling) processes essentially involve such sustainable conversions. In lignocellulosic matrices, fibers (thread-like plant cells with high aspect ratios) are delicately inter-bonded, packed, and confined in their native manners. Liberation of fibers from their native matrices allows for reassembling them into sheet-shaped structures (microfibrous bioassemblies) for various applications. On the basis of well-established industrial practices of fiber liberation and reassembling, our proposed process concept concerning delicately reorganizing regular microfibrous bioassemblies toward diversified/unconventional applications is schematically illustrated in Figure 1. The novelty of the concept is mainly indicated by the right part of the figure. Specifically, this concept features in the combination of partial liquefaction and rapid solidification to miniaturize microfibrous bioassemblies into densified/miniaturized sheet with nanotextured surface. Basically, key elements of this process concept are: (1) existing state-of-the-art industrial processes associated with fiber liberation and reassembling are the foundation of the concept; (2) microfibrous bioassemblies are subjected to combined post-treatments of localized liquefaction and rapid solidification; (3) combined post-treatments involve the integrated use of a green solvent and a green nonsolvent that are easily recyclable; (4) combined post-treatments would result in densified, mechanically robust, and liquid-barrier structures with enhanced 11



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intermolecular bonding interactions and realignment of structural units; and (5) in-situ formation of nanotextured surface would be induced by rapid solidification. A pronounced feature of the facile concept is that it would be readily integrated into existing industrial processes of fiber liberation and reassembling. Also, the use of green, recyclable solvent/nonsolvent would fit well into sustainability. Essentially, the proposed process concept is related to the realignment and redistribution of biopolymeric structural units to generate new structures with enhanced compactness. In this green process, no covalent bonds and no toxic byproducts would be formed, and the enhancement of hydrogen bonding interactions would be a key factor governing the characteristics of structurally reorganized bioassemblies (Figure S1). In accordance with widely practiced surface treatment processes of commercial microfibrous bioassemblies, the green solvent and the green nonsolvent could be recycled

in

envisaged

continuous

industrial

operations,

without

causing

pollution-related problems. Due to the fact that structural reorganization could result in

densified,

delicately

bonded,

mechanically

robust,

and

liquid-resistant

bioassemblies, unconventional applications such as those related to their use as substrates for electronic circuits, actuators, and many others would be facilitated. The use of green processes to dissolve/gelatinize cellulose or cellulose-rich materials is a promising strategy toward sustainable, diversified, and value-added applications.

25-27

Once efficiently dissolved/gelatinized cellulosic materials can be

converted into new structures.

28

In line with the proposed process concept, we used

an inorganic molten salt hydrate, i.e., zinc chloride trihydrate (with a zinc chloride to 12


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water molar ratio of 1/3), as the green solvent, to induce localized liquefaction of network-structured, microfibrous bioassemblies. The interaction of cellulose-rich bioassmeblies with solvent can lead to in-situ localized formation of “fluidizable” macromolecules, which would serve as building blocks to form structurally reorganized, consolidated structures. This non-derivatizing solvent has an ionic liquid nature, and its good recyclability and ease of preparation are pronounced advantages. 29

It is worth noting that, zinc chloride is non-toxic and inexpensive,

30

and it has

found commercial use in pharmaceuticals, cosmetics, and personal care products. The use of zinc chloride solution for treatment of microfibrous bioassemblies to produce vulcanized sheets has been a commercial practice, although the process efficiency may still remain to be further improved. The combination of zinc chloride with sodium hydroxide to post-treat microfirbrous bioassemblies was recently found to result in the fabrication of high-strength antibacterial sheets.

31

Once mixed with

water to form a trihydrate (instead of other forms of hydrates), zinc chloride can exhibit the strongest capability as regards cellulose dissolution/gelatinization, possibly due to the unique features of the solvent system (e.g., strong hydrogen-bond-donating capacity and nonpolar nature).

29, 32

Evidently, the use of zinc chloride trihydrate as

the green solvent for cellulose or cellulose-rich materials is a facile, proven approach. On the other hand, we used anhydrous ethanol as the green nonsolvent to induce rapid solidification of dissolved/gelatinized fractions of microfibrous bioassemblies. This nonsolvent can be facilely recycled in integrated processes, facilitating mass production of structurally reorganized high-performance bioassemblies. 13



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Figure 1. Sustainable, scalable conversion of biopolymeric feedstocks into delicately structured bioassemblies with nanotextured surface for diversified/unconventional applications.

We studied the surface morphological characteristics of bioassemblies before and after structural reorganization (Figure 2). Untreated bioassemblies were distinctly porous, with well-discernible fibers (ribbon-like structures) and interfiber gaps (Figure 2a,b). Under a high magnification of 10, 000 times, filaments/fibrils associated with untreated bioassemblies were observable (Figure 2c). It is noted that, for such bioassemblies constructed in typical commercial processes with a huge production capacity, microfibers are merely bonded at contact points. These bioassemblies are basically made from randomly deposited microfibers with initial tube-like

structures.

33

The

somehow

not-so-densely-packed

14


structures

of

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bioassemblies with limited bonding sites would contribute negatively to structural integrity and compactness. Under our experimental conditions, the interaction of bioassemblies with zinc chloride trihydrate for 5 s followed by anhydrous-ethanol-induced rapid solidification resulted in noticeable surface morphological changes: (1) porous bioassemblies became denser and more closely packed; and (2) flake-like micro-outgrowths were generated in-situ on the surface of bioassemblies or discernible fibers (Figure 2d-f,m). In a recent study conducted by Hideaki et al.,

34

cellulosic fiber networks

(microfibrous bioassemblies) were soaked in a melted ionic liquid for 5 to 60 s, followed by being immersed in ethanol (98%) and subsequent washing/drying, the surface of networks was then partially covered with film-like outgrowths, and interfiber pores were still readily observable. The difference between our findings and those reported by Hideaki et al. may be closely related to the inherent characteristics of solvents and non-solvents used in the treatments of cellulosic networks. Particularly, anhydrous ethanol may induce a more rapid/drastic phase transition in comparison to 98% ethanol. Surprisingly, very drastic surface morphological changes were observed when bioassemblies interacted with zinc chloride trihydrate for 10 min (localized liquefaction) followed by rapid solidification (Figure 2g-i,n). As a result of macromolecular reassembly, the surface of bioassemblies was substantially reorganized into a finer, much more compact structure. Microfibers with characteristic native architectures were no longer observable, and scattered nano-outgrowths were 15



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formed. These changes clearly demonstrate that surface morphological characteristics can be finely tuned by changing process conditions, particularly in the case of the time for the contact of bioassemblies with solvent. Patterned surfaces with size-controllable outgrowths would be designed to suit specific end-use applications. The interaction of bioassemblies with zinc chloride trihydrate for 2 h led to compete liquefaction (dissolution/gelatinization), and the resulting biopolymeric film was nanostructured; The nano-outgrowths of biopolymeric film were essentially much smaller

and

more

uniformly

distributed

than

those

pertaining

to

a

bioassemblies-solvent contact time of 10 min (Figure 2j-l,o). Therefore, the proposed process concept could lead to tunable surface nanostructuring of bioassemblies, and bioassemblies-solvent contact time would be a decisive factor. It is interesting to note that, surface nanostructuring is a promising strategy to endow materials with improved performances or new functionalities.

35-37

Due to the formation of

nano-structures on the surface of bioassemblies, a large surface area would be constructed, potentially leading to enhanced surface-related interactions in various applications such as their use as substrates for functional materials (e.g., sensors and electronics).

16


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Figure 2. Surface morphologies of bioassemblies and the estimated dimensions of 17



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outgrowths.

a-c. SEM images of untreated bioassemblies. d-i. SEM images showing the formation of micro/nano-textured surfaces as a result of treatment of bioassemblies by combining localized liquefaction with rapid solidification. j-l.

SEM

images

showing

nano-morphologies

of

structurally

reorganized

bioassemblies formed by combing complete liquefaction with rapid solidification. m-o. Particle size distributions pertaining to f, i, and l, respectively, estimated with a Nano Measurer Software. Note that microfibrous bioassemblies interacted with

solvent for 5 s (d-f), 10 min

(g-i), and 2 h (j-l).

We also evaluated the topographical characteristics of bioassemblies before and after structural reorganization by AFM imaging (Figure 3). Untreated bioassemblies used in this work were found to have a somehow “coarse” surface (Figure 3a). Treating bioassemblies by soaking in solvent for 5s followed by rapid solidification resulted in very pronounced topographical change, due to the formation of microsized outgrowths (Figure 3b). In this case, flake-like projections and “hill and valley” structures can easily be observed. This distinct feature of micro-roughness of reorganized bioassemblies can clearly be related to rapid phase transition induced by anhydrous ethanol. In a recent study conducted by Yu et al.,38 the use of anhydrous ethanol facilitated the generation of hierarchical surface roughness for the development of superhydrophobicity, thanks to rapid phase change (solidification). 18


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Quite interestingly, very flat/delicate bioassemblies with nano-outgrowths were formed when bioassemblies-solvent interaction time was 10 min or 2 h (Figure 3c,d), demonstrating very good agreement with the SEM images shown in Figure 2. Again, these morphological/topographical results indicate that the proposed concept can be employed to design bioassemblies with tunable structural characteristics.

Figure 3. Topographical images of bioassemblies. 19



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a. AFM image of untreated bioassemblies. b-d.

AFM

images

of

structurally

reorganized

bioassemblies

with

bioassemblies-solvent contact times of 5 s, 10 min, and 2 h, respectively. Microfibrous bioassemblies constructed by a conventional papermaking process are oftentimes not quite optically transparent. A high opacity of bioassemblies is a prerequisite for printing and writing, so that “show-through” can be reduced. The limited optical transparency of regular bioassemblies is due to fact that the abundance of microcavities within the porous bioassemblies would cause light scattering.

39

However, dense networks/bioassemblies constructed using nanocellulosic fibers (other than conventional microsized cellulosic fibers) as basic structural units or building blocks can be optically transparent. 40-41 We examined the impact of structural reorganization on optical transparency (Figure 4). As shown in Figure 4a, untreated bioassemblies were very opaque, and structural reorganization had a noticeable transparency-enhancing effect. Very slight structural reorganization pertaining to a bioassemblies-solvent contact time of 5 s only had a negligible effect on transparency, and the Chinese characters for “Northeast Forestry University” were not discernible. However, the transparency of bioassemblies was greatly enhanced under experimental conditions as regards a bioassemblies-solvent contact time of 10 min. Evidently, on the basis of well-established papermaking process, combined post-treatments of the resulting bioassemblies involving localized liquefaction and rapid solidification can result in fine tuning of transparency, and the time for liquefaction (dissolution/gelatinization) 20


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can be a critical factor. The change in transparency/opacity can be closely related to the delicate, tunable alteration of structural characteristics of bioassemblies. The observed transparency of completely reorganized bioassemblies pertaining to complete liquefaction was close to that of partially reorganized bioassemblies with a bioassemblies-solvent contact time of 10 min. Light transmittance as a function of wavelength (200 to 800 nm) shown in Figure 4b was in good agreement with the above-mentioned photographic information. It is noteworthy that the UV spectrophotometric method is quite effective in evaluating light transmittances of biopolymeric materials across various wavelengths.

42

It is reasonable that, if

biopolymeric structural units are densely packed without generating large spaces to efficiently scatter light, the resulting bioassemblies would have a high optical transmittance.43 Thus, the combination of localized liquefaction with rapid solidification would result in tunable packing density (hugely dependent upon bioassemblies-solvent

contact

time)

and

consequently

transmittance.

21


controllable

optical


Figure 4. Impact of structural reorganization on optical transparency. a. Digital photographs of bioassemblies placed on colored strips with printed Chinese characters for “Northeast Forestry University”. From top to bottom: untreated bioassemblies, reorganized bioassemblies with bioassemblies-solvent contact times of 5 s, 10 min, and 2 h, respectively. b. Light transmittance as a function of wavelength. U pertains to untreated bioassemblies. R-a, R-b, and R-c pertain to reorganized bioassemblies with bioassemblies-solvent contact times of 5 s, 10 min, and 2 h, respectively. Mechanical Robustness of Structurally Reorganized Bioassemblies Biopolymeric paper-based products (bioassemblies) are promising substrates for the production of “green” advanced materials such as sensors. 44 Mechanical robustness is a prerequisite for most of these applications.

18

In terms of regular products for

conventional applications, strength enhancement is commercially achieved by using hydrogen-boding polymers to enhance intermolecular bonding interactions in fiber networks. Starch-based polymeric materials are typical additives for paper 22


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strengthening applications.

45

Nanocellulosic film or nanopaper is known to have a

much higher strength in comparison to regular cellulosic paper, and indeed it has gained widespread interest in developing mechanically robust materials for tailored applications. However, currently available strategies for the production of this innovative

material are hugely challenged by limited scalability. If the

mass-producible regular paper-based products are facilely engineered/post-treated toward tunable mechanical robustness, huge opportunities of highly efficient advanced applications would be envisageable. We studied the strength properties of bioassemblies before and after structural reorganization

(Figure

5).

Structural

reorganization

pertaining

to

a

bioassemblies-solvent contact time of 5 s led to a more than 5-fold increase in dry tensile strength, an about 29-fold increase in wet tensile strength, an about 2-fold increase in bursting strength, and an about 4-fold increase in wet-tensile-strength-ratio (Figure 5a,b,c,d). Thus, the mechanical strength of dry/wet bioassemblies was markedly improved as a result of mild structural reorganization. It is worth noting that both dry and wet mechanical strength properties of biopolymeric products are critical in many applications where contact with water or aqueous liquids is necessary, even in the case of common paper-based products such as banknotes and maps. For nanocellulosic assemblies, a pronounced enhancement of wet strength by entangling with algal polysaccharides was recently reported by Benselfelt et al.

46

When

bioassemblies interacted with solvent for 10 min followed by post-processing, the enhancements of dry tensile strength, wet tensile strength, bursting strength, and 23



wet-tensile-strength ratio were approximately 10, 40. 6, and 3 folds, respectively (Figure 5a,b,c,d). The impact of structural reorganization on enhancement of wet strength was also successfully demonstrated by evaluating the change of disintegration behavior in water (Figure 5e). Furthermore, distinguished mechanical robustness of reorganized bioassemblies pertaining to a bioassemblies-solvent contact time of 10 min was clearly identified, since both unwetted and wetted strips were capable of supporting a metal flattener (10 kg) (Figure 5f). Videos showing the mechanical robustness of bioassemblies as regards the metal flattener can be found in the “ASSOCAITED CONTENT” section. As a result of structural reorganization, the formation of a more closed/compact structure with drastically increased number of intermolecular bonds would explain the mechanical robustness of dry/wet bioassemblies.

24


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Figure 5. Strength properties of bioassemblies. a-d. Dry tensile strength (DTS), wet tensile strength (WTS), bursting strength (BS), 25



Page 26 of 43

and wet-tensile-strength ratios (WTSR) of different samples. U pertains to untreated bioassemblies.

R-a

and

R-b

pertain

to

reorganized

bioassemblies

with

bioassemblies-solvent contact times of 5 s and 10 min, respectively. e. Digital photographs showing disintegration behaviors of untreated bioassemblies soaked in distilled water for 1 d (left) and structurally reorganized bioassemblies (with a bioassemblies-solvent contact time of 5 s) soaked in distilled water for 45 d (right). f-g. Digital photographs showing mechanical robustness of structurally reorganized bioassemblies with a bioassemblies-solvent contact time of 10 min (unwetted and wetted, from left to right) in supporting a metal flattener (10 kg). Unwetted strip had a width of 15 mm. For wetted strip, water-strip contact time during wetting was 5 min.

Reorganization-Induced Densification and Miniaturization Densification of materials such as those derived from lignocellulosic resources is a universally efficient strategy to enhance mechanical strength largely due to increased number of intermolecular bonds and improved alignment of structural units.

47-48

Other structure-dependent properties may also be impacted by densification. We found that the structural reorganization concept demonstrated in our study led to pronounced densification (Figure 6a,b,c). As a result of combined post-treatments of bioassemblies involving localized liquefaction and rapid solidification, the initial interfiber spaces/pores were substantially collapsed, resulting in a denser, more compact structure (Figure 6a,b). Under our experimental conditions pertaining to a

26


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bioassemblies-solvent contact time of 10 min, a more than 70% increase in density was achieved (Figure 6c). The densification impact was in good consistency with the results pertaining to mechanical strength enhancement shown in Figure 5, since internally reassembled dense structure would have more internal bonds. Similarly, structural densification of bioassemblies occurs naturally in wet pressing of the commercial paper production process, although the internal pores of bioassemblies would not be efficiently eliminated partly due to the non-pliable nature of fibers. We also found that the densification of bioassemblies was accompanied by remarkable miniaturization, as shown in Figure 6d. Strong et al. recently demonstrated an interesting process of miniaturizing paper via periodate oxidation, and potential applications would include its use in microfabrication.

49

Similarly, our demonstrated

process concept of structural reorganization of bioassemblies using green solvent/nonsolvent would also find possible use in similar applications.

27



Page 28 of 43

Figure 6. Densification and miniaturization of bioassemblies as a result of structural reorganization. Note that bioassemblies interacted with solvent for 10 min. a-b. SEM images of cross-sections of untreated and structurally reorganized bioassemblies (U and R-b) showing pronounced densification. c. Reorganization-induced increase of density. d. Reorganization-induced miniaturization (samples shown in the photograph were conditioned in in a chamber containing a saturated solution of sodium chloride).

Impact of Freeze-Thaw Cycling on Mechanical Robustness Freeze-thaw cycling has long been used to assess the mechanical durability and aging characteristics of materials under environmental conditions.

50

We evaluated the

impact of freeze-thaw cycling on strength properties of structurally reorganized 28


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bioassemblies pertaining to a bioassemblies-solvent contact time of 10 min (Figure 7). Encouragingly, freeze-thaw cycling was found to have no significant impact on dry tensile strength, wet tensile strength, bursting strength, and wet-tensile ratio. Thus, the mechanically robust bioassemblies would be applicable to certain extreme environmental conditions involving frequent freezing and/or thawing.

Figure 7. Strength properties of structurally reorganized bioassemblies (with a bioassemblies-solvent contact time of 10 min) as a function of number of freeze-thaw cycles. DTS, WTS, BS, and WTSR pertain to dry tensile strength, wet tensile strength, bursting strength, and wet-tensile-strength ratio, respectively.

Interaction

of

Aqueous

and

Nonaqueous

Liquids

Bioassemblies 29


with

Reorganized


The use of materials in aqueous systems is a common practice.

Page 30 of 43

51

In this regard,

liquid-resistance of materials would be a necessity in certain cases. Essentially, superior liquid-resistant properties of cellulosic paper/film are highly desirable for applications such as packaging.

52

However, compared to synthetic plastics,

applications of bioplymeric microfibrous products are commercially strongly hampered by limited resistance to penetration by liquids. The surface anchorage of barrier coatings is oftentimes used to endow cellulosic products with liquid-barrier properties.

53

A recently published interesting work by Zhang et al.

54

pertains to

anchoring of a regenerated cellulosic coating (via in-situ formation) to paper, resulting in pronounced liquid-resistance as well as mechanical robustness. As an alternative to the use of barrier coatings, structural reorganization of microfibrous bioassemblies involving the combination of localized liquefaction and rapid solidification was found to be very effective in the development of liquid-barrier properties (Figures 8-9). As shown in Figure 8, reorganized bioassemblies with a bioassemblies-solvent contact time of 10 min had a strong resistance to penetration by soy sauce, vinegar, beer, and colored water. As a result of structural reorganization, the water-contact angle for reorganized bioassemblies increased from 0o to 71.6o, still less than 90o. This finding may possibly be explained by the polysaccharide-related nature of the surface, where hydroxyl groups are still available. Reorganized bioassemblies were also found to resist the penetration by colored ethanol, grease solution, as well as acidified and alkalinized

water

(Figure

9).

Quite

interestingly,

structurally

reorganized

bioassemblies exhibited a noticeable cleanability when fouled by castor oil, ketchup, 30


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and milk (Figure 10). The identified liquid-resistance and cleanablity of structurally reorganized bioassemblies may stem from structural densification,

realignment of

structural units, and nanotexturization. It is noted that surface nanostructuring can be correlated with liquid contact angle.

55

Arrays of nano-outgrowths on the surface of

reorganized bioassemblies would greatly affect liquid-assemblies interaction.

Figure 8. Interaction between typical aqueous liquids and bioassemblies. Reorganized bioassemblies with a bioassemblies-solvent contact time of 10 min showed strong resistance to penetration by soy sauce, vinegar, beer, and colored water. No penetration was observed after 30 days of contact. As compared with untreated bioassemblies, reorganized bioassemblieshad a higher water contact angle, but both had a hydrophilic nature (with contact angles of less than 90o).

31



Figure 9. Photographs showing the interaction of bioassemblies with typical organic liquids as well as acidic, neutral, and alkaline water. Reorganized bioassemblies with a bioassemblies-solvent contact time of 10 min showed a strong resistance to penetration by colored ethanol and grease solution (mixture of castor oil, toluene, and n-heptane, with volume percentages of 30%, 35%, and 35%, respectively). No penetration was observed within 12 hours. Reorganized bioassemblies were somehow transparent, and impermeable to acidified (left), original (middle), and alkalinized (right) water.

32


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Figure 10. Photographs showing the cleanabilities of untreated (top) and structurally reorganized (bottom) bioassemblies fouled by castor oil (left), ketchup (middle), and milk (right). The arrows indicate “before and after simple cleaning with running tap water”. Structurally reorganized bioassemblies (with a bioassemblies-solvent contact time of 10 min) (somehow transparent) were easily cleanable. Untreated bioassemblies (somehow opaque) were impermeable to three types of liquids and not cleanable. CONCLUSIONS A green, facile strategy of structural reorganization of network-structured, microfibrous bioassemblies involving loacalized liquefaction and rapid solidification was

demonstrated.

As

a

result

of

combined

post-treatments,

morphological/topographical and other structural characteristics of bioassemblies were noticeably altered. Structural reorganization led to realignment/redistribution of 33



structural units and formation of nanotextured surface. The time for the interaction of cellulosic networks with solvent was identified as a critical factor. Under conditions pertaining to a contact time of 10 min, combined post-treatments led to structural densification/miniaturization, development of optical transparency and mechanical robustness (in both dry and wet states), strong liquid-resistance, and noticeable cleanablity. The proposed process concept led to

the enhancement of structural

integrity/compactness of bioassemblies, which would be explained by tunable realignment of structural units. Altogether, these findings would facilitate the use of mass-producible bioassemblies in diversified, advanced applications, e.g., as high-performance substrates for functional materials including biosensors, electronics, and microfluidics. The combined processes of localized liquefaction and rapid solidification would also shed light on the development of miniaturized, robust nanoproducts from materials with varying characteristics. ASSOCIATED CONTENT

Videos 1 and 2 demonstrating the mechanical robustness of unwetted and wetted strips (structurally reorganized bioassemblies) in easily supporting a metallic flattener (10 kg) (AVI); Videos 3 and 4 demonstrating the mechanical weakness of untreated strips (unwetted and wetted bioassemblies without post-treatments) (AVI).

Supporting Information (FTIR/XRD/XPS/NMR spectra and the Coloring of SEM images) (PDF).

34


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AUTHOR INFORMATION Corresponding author. * Email addresses: [email protected]; [email protected] (J. S.) 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. ACKNOWLEDGEMENT This work was financially supported by the Fundamental Research Funds for the Central Universities of China (2572018CG04), the Program for New Century Excellent Talents in University (NCET-12-0811), and the Natural Science Foundation of China (218708046). The helps and valuable comments/suggestions from the Anonymous Reviewers and the Handling Editor (Prof. Lina Zhang) are greatly appreciated. REFERENCES (1) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and characterization of graphene oxide paper. Nature 2007, 448, 457, DOI 10.1038/nature06016. (2) Rashidi, F.; Kevlich, N. S.; Sinquefield, S. A.; Shofner, M. L.; Nair, S. Graphene oxide membranes in 35



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Synopsis A green concept for miniaturizing mass-producible, microfibrous bioassemblies into nanotextured sheet with interesting functionalities was demonstrated.

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Localized Liquefaction Coupled with Rapid ... - ACS Publications

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