Functionalized alginate with liquid-like behaviors and its application in wet-spinning

Accepted Manuscript Title: Functionalized alginate with liquid-like behaviors and its application in wet-spinning Authors: Zhen Sang, Wenqian Zhang, Zhiyuan Zhou, Huakang Fu, Yeqiang Tan, Kunyan Sui, Yanzhi Xia PII: DOI: Reference:

S0144-8617(17)30793-2 http://dx.doi.org/doi:10.1016/j.carbpol.2017.07.027 CARP 12544

To appear in: Received date: Revised date: Accepted date:

25-4-2017 14-6-2017 9-7-2017

Please cite this article as: Sang, Zhen., Zhang, Wenqian., Zhou, Zhiyuan., Fu, Huakang., Tan, Yeqiang., Sui, Kunyan., & Xia, Yanzhi., Functionalized alginate with liquid-like behaviors and its application in wet-spinning.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.07.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Functionalized alginate with liquid-like behaviors and its application in wet-spinning

Zhen Sanga, ‡, Wenqian Zhang a, ‡, Zhiyuan Zhou b, Huakang Fu c, Yeqiang Tan a, *, Kunyan Sui a, *, Yanzhi Xia a

a) Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of Shandong Province, Institute of Marine Biobased Materials, School of Materials Science and Engineering, Qingdao University, Qingdao 266071 China; b) Sandy Spring Friends School, Maryland 20860 USA; c) Zhejiang Juhua Research Institute of New Materials Co. Ltd., Hangzhou 310027 China.

Corresponding

authors.

E-mail address: [email protected] (T. TAN); [email protected] (K. SUI) Tel/Fax: +86-571-85950961

Highlights 

Novel

solvent-free

alginate

fluids

were

successfully

prepared

by

an

environmental-friendly method of one-step acid-base reaction. 

Excellent flowability of alginate fluids can be effectively adjusted by variation of molecular weight of alginate, mass ratio of alginate to PEG-substituted tertiary amines and PEG chain length.



The fibers prepared from alginate fluids with improved spinning efficiency exhibited double tensile strength and elongation at break in comparison with those of conventional spinning solutions. 1

Abstract: Alginate is a kind of marine-derived plant polysaccharide with useful properties including inherent flame-retardancy and biocompatibility, yet poor flowability and low processing efficiency induced by high viscosity impede its further industrial applications. In this study, PEG-substituted tertiary amines were adapted to functionalize alginate with different molecular weight via acid-base reaction to improve the flowability. Based on alginate with low molecular weight, alginate fluids exhibited excellent flowability at room temperature in the absence of solvent. For alginate with high molecular weight, gelatinous precipitated phase exhibited significant shear-thinning properties and higher solid content despite lack of solvent-free flowability, which was applied to wet-spinning. The alginate fibers exhibited increased tensile strength by 104% and elongation at break by 132% compared with conventional alginate fibers, and the spinning efficiency was significantly improved. The proposed strategy is expected to extend to highly efficient processing of other polysaccharides to obtain high-performance biomedical materials.

Keywords: Alginate; Rheology; Solvent-free fluids; Highly efficient spinning; Fibers; Mechanical properties.

1. Introduction Alginate, derived from marine brown algae, is a kind of linear polysaccharide copolymer consisting of (1→4)-linked β-D-mannuronic acid (M units) and its C-5 epimer α-L-guluronic acid (G units) (Pawar & Edgar, 2011). Owing to the abundance resources

and

biocompatibility

useful

properties

(Chiaoprakobkij,

including

inherent

Sanchavanakit,

flame-retardancy

Subbalekha,

Pavasant

and &

Phisalaphong, 2011; Kong, Quan, Jian & Xia, 2008; Sennerby, Röstiund, Albrektsson & Albrektsson, 1987; Srinivasan, Jayasree, Chennazhi, Nair & Jayakumar, 2012), and it exhibits broad application prospects in biological and medical fields (Lee, Ahn, 2

Bonassar & Kim, 2013; Sun & Tan, 2013), and food (Ack, Da & Kieckbusch, 2012; de Souza et al., 2009), cosmetics and textile industries (Darka et al., 2010; Qin, 2008; Qin, Hu & Luo, 2006). However, alginate solutions ordinarily exhibit high specific viscosity at rather low concentration and undergo sol-gel transition at 7.6-8.0 wt.% (Liu, Qian, Shu, Lu & Tong, 2003). Tong et al claims that physical gelation in aqueous solutions of sodium alginate is mainly induced by the interaction between repeat units of the macromolecules rather than molecular mass. And electrostatic interactions of intra- and inter-molecular hydrogen bonds play a key role between repeat units (Mazur, Buchner, Bonn & Hunger, 2013). Therefore, the high viscosity and low processing efficiency impede the industrial applications of alginate, and flowability improvement is the key to solve this issue. Giannellis and co-workers have pioneered an approach to prepare functionalized inorganic nanoparticles with liquid-like behaviors (Bourlinos et al., 2005b). Organic canopy layers were electrostatically grafted onto the surface of charged nanoparticles, such as silica (Bourlinos et al., 2005b), iron oxides (Zong, Wang, Zhao, Wang, Pan & He, 2013) and titania (Bourlinos et al., 2005a; Yao et al., 2014), providing a flowing medium for nanoparticle ionic fluids. In a different context, solvent-free fluids of DNA oligonucleotides were prepared by ion-exchange with a PEG-tailed quaternary ammonium cation (Bourlinos et al., 2005c), and liquid proteins based on electrostatic force between cationized ferritin and anionic polymer surfactant were reported by Mann (Perriman, Cölfen, Hughes, Barrie & Mann, 2009). Inspired by the works of solvent-free DNA liquids and protein ionic liquids combined with the presence of carboxyl group in every saccharide ring of alginate, the similar strategy of acid-base reaction was adapted to improve the flowability of alginate. Herein, alginates with different molecular weight were selected to react with PEG-substituted tertiary amines. A new kind of biomacromolecule solvent-free liquid was obtained for alginate with low molecular weight; a gel-like product with higher solid content and significant shear-thinning properties was obtained for alginate with high molecular weight to produce alginate fibers with excellent mechanical properties. 3

The proposed strategy is expected to extend to highly efficient processing of other polyelectrolyte polysaccharides to obtain high-performance biomedical materials.

2. Materials and methods 2.1 Materials Alginic acid with low molecular weight was supplied by Aladdin Industrial Corporation, and sodium alginate (SA) with high molecular weight was provided by Kangtong

Marine

Fiber

Corporation.

PEG-substituted

tertiary

amines

(C18H37)N[(CH2CH2O)mH][(CH2CH2O)nH], named as 1810, 1820, 1830 and 1860 (m+n=10, 20, 30, 60), were purchased from Haian Chemical Corporation. Anhydrous ethanol (AR), chlorine hydride (HCl), and anhydrous calcium chloride (CaCl2) were provided by Sinopharm chemical Reagent Corporation. Deionized water (18.25 MΩ cm) was prepared in a Sartorius Arium 611 system and used throughout the experiments. 2.2 Preparation of alginate fluids For low molecular weight alginate, alginic acid (1g) was dispersed in deionized water (50 mL) by stirring. PEG-substituted tertiary amine was subsequently dropped and stirred for 24h. The functionalized alginate spontaneously formed a separate layer at the bottom of the reaction beaker. The precipitation was collected and dried overnight at 70 °C to obtain the final products. For high molecular weight alginate, alginic acid was firstly produced according to the report of Babak (Babak, Skotnikova, Lukina, Pelletier, Hubert & Dellacherie, 2000). SA powders (14 g) were dispersed in the mixture of HCl solution (0.6 M, 200 mL) and ethanol (200 mL) by stirring overnight at room temperature, which was then filtered, washed by the ethanol and dried in 30 °C to obtain alginic acid with high molecular weight. The following procedures were in accordance with the low molecular weight system. The products originating from low molecular weight and high molecular weight alginate were named as Alg/PEG-substituted tertiary amines (Lx) and Alg/PEG-substituted tertiary amines (Hx), respectively, and x represented mass ratio of PEG-substituted tertiary amines to alginic acid. 4

2.3 Spinning of alginate fibers The prepared precipitation in high molecular weight system were wet-spun using 10 mL plastic syringes. A needle with an inner diameter of 1.2 mm was employed as the spinneret. The extrusion rate of the syringe pump was set at 600 μL·min-1 and 3 wt.% aqueous solutions of CaCl2 were used as coagulation mediums. These fibers were then collected from the coagulation bath after 2 h, washed in deionized water and thereafter completely dried in air overnight. Upon addition, conventional alginate spinning solution with upper concentration limit (5 wt.%) was also selected to wet-spinning by the same method for comparison. 2.4 Characterizations Molecular weight (Mw) and dispersity indexes (DI) were determined using gel permeation chromatography (GPC) measurements carried out on GPCmax TDA305 (Malvern Co. Ltd., Germany) equipped with Viscotek TriSEC Model 302 triple detector array (refractive index detector, viscometer detector, and laser light scattering detector) using TSK gel PWXL columns in series (A6000MXL/CGuard + C-MBLMWXL; Malvern Co. Ltd., Germany). 0.1 M sodium nitrate aqueous solution was used as the eluent at a flow rate of 0.7 mL·min-1. The eluent was monitored by a RI-8020 differential refractometer at 40 °C and polymer concentration was 2 mg·mL-1. Solid-state

13

C-nuclear magnetic resonance (Bruker AVANCE III HD, Switzerland)

was carried out using a 400 MHz apparatus to determine the M/G ratio of alginate with different molecular weight and linkage style between alginic acid and PEG-substituted tertiary amines. The M/G ratio was calculated by AM /AG ratio, which represented for the integral areas of curve-fitting analysis of

13

C-NMR spectra in the

range of 60-90 ppm for M blocks composed of peak B and peak C and G blocks constituted by peak A, peak D and peak E (Fig. 1) (Salomonsen, Jensen, Larsen, Steuernagel & Engelsen, 2009). Rheological measurements were performed on a MCR 301 rheometer (Anton Paar) equipped with parallel plate grippers of 25 mm in diameter and the gap distance was set at 0.3 mm. A steady sweep was conducted at 25 °C controlled by a Peltier temperature controller at a shear rate ranging of 0.01-103 5

s-1. Dynamic frequency (ω) sweeps were performed from 0.1 to 500 rad·s-1 in the linearity region. Strain sweeps from 0.1% to 1000% were conducted at ω =10 rad·s-1 to determine the linear region. Temperature ramps were implemented from 25 to 80 °C at a rate of 2 °C·min-1 (ω = 10 rad·s-1). Fourier transfer infrared (FT-IR) spectra were measured with a Vector 22 spectrometer in transmission mode using the KBr disk method at the wavelength range of 500-4000 cm-1. Tensile strength and elongation at the break of the fibers were measured by a mechanical tester (WDW-5T) at strain rates of 1 mm·min-1, 5 mm·min-1 and 10 mm·min-1 with the gauge length of 35mm. The tensile fracture surfaces of fibers were observed under a scanning electron microscope (SEM, JEOL JSM-6309LV) at an acceleration voltage of 15 kV. The constituent proportions were evaluated by a thermo gravimetric analyzer (TGA, Q50, TA, USA) at a heating rate of 20 °C·min-1 under nitrogen. X-ray diffraction (XRD) was carried out using a Rigaku X-ray generator equipped with Cu K radiation ( = 0.154 nm) (Rigaku Co. Ltd., Japan). The elemental analysis of carbon, hydrogen, and nitrogen for Alg/PEG-substituted tertiary amines was undertaken with an Elemental Analyzer (Make – M/s Elementar, Germany; Model – Vario EL III). Water content of the alginate fluids was accurately estimated by a volumetric Karl Fischer titration (Mettler Toledo V30, Switzerland).

3. Results and discussion 3.1. Characterizations of alginate with low and high molecular weight Macromolecular characteristics including Mw and M/G content of alginates with low and high molecular weight were determined by GPC and solid-state

13

C-NMR

spectra. For alginate with low molecular weight, Mw and dispersity index (Mw/Mn) were determined to be 7.0 kDa and 1.8, respectively; for alginate with high molecular weight, Mw and Mw/Mn were determined to be 323 kDa and 2.6, respectively (Fig. S1). With regards to chemical composition, the M/G ratio for alginate with low and high molecular weight was calculated to be 1.43 and 1.25, respectively (Fig. 1 and Table S1). 3.2 Preparation and characterizations of solvent-free alginate liquids with low 6

molecular weight Solvent-free alginate liquids were prepared by acid-base reaction between carboxylate groups and ammonium cation, and titration curve was employed to discuss the relationship between mass or mole ratio and flowability behaviors (Liu, Yingbo, Zhang, Qiao & Liu, 2016). For 1810 as an example, the pH of the reaction during the acid-base titration process was monitored (Fig. 2A). The pH value of the solution increases gradually upon addition of 1810, indicating increasing degree of acid-base reaction between carboxylate groups and ammonium cation. The mole ratio (0.72) and mass ratio (3) of PEG-substituted tertiary amine to alginic acid corresponding to the dramatic change around pH = 5 is regarded as a critical ratio to obtain solvent-free alginate fluids by using the minimum amount of 1810, which is originating from the realization of fully dissolved state of alginic acid to effectively react with PEG-substituted tertiary amines. It is worth noticing that the true equivalent point for acid-base reaction occurs around pH = 6.3 with a 1:1 stoichiometric ratio, proved by the slowdown of pH growth beyond this point. The dissolving process is outlined in Fig. 2D. In light of lower initial pH (2.5) than the pKa of active-site carboxyl group (3.8), alginic acid is originally insoluble in water due to negligible dissociation of carboxyl groups and strong intra- and inter-chain hydrogen bonds. Minor ionization of carboxyl groups on alginic acid surface enables them to suspend in water, corresponding to the pH value of 2.5. After introducing 1810 with the measured pKa of 8.9 close to theoretical pKa of ammonia (9) (Rayer, Sumon, Jaffari & Henni, 2014), tertiary amines firstly get a proton from carboxyl group by acid-base reaction to generate pronated amine, and then the acid-base reaction of carboxyl group and pronated tertiary amines build up strong ionic pairs between carboxylate groups (-COO-) and ammonium cation (

), which further promotes the ionization

of carboxyl groups of alginic acid combined with weakened inter-molecular hydrogen bonds due to steric hindrance of long PEG chains, resulting in the complete dissolution of alginic acid with increasing degree of neutralization around pH = 5. In the final state, alginic acid chains exhibit a random coil conformation and the mixture 7

undergoes a simultaneous phrase separation by forming gelatinous precipitated phase. The amphiphilic chains of PEG-substituted tertiary amines are “dressing” on them linked via ionic pairs. Strong electrostatic complexation weakens the solubility of ionized alginate in water, combined with inter-chain hydrogen bonds of PEG and hydrophobic association of alkane chain for PEG-substituted tertiary amines, which cause the occurrence of phase separation to obtain a transparent and homogeneous precipitated phase. As shown in Fig. 2B, upper-layer liquids generally become yellow with the increment of 1810 concentrations and the states of precipitation change from colloid to transparent and homogeneous solutions or gels. The obtained production after drying overnight at 70 °C in vacuum was placed on an individual glass slide at room temperature, and Alg/1810 (L1/3/5) exhibited a soften solid, gel-like, liquid-like state, respectively. To exhibit their flowability, we dropped them to 60 ° glass slide slopes at 60 °C. Flowing down to the bottom of slope driven by gravity can be observed immediately for Alg/1810 (L5), and Alg/1810 (L3) exhibited higher viscosity and flowed slowly (Fig. 2C). As a ‘solvent’ in solvent-free alginate liquids, PEG-substituted tertiary amines can help alginic acid move apart to a certain distance with less inter-molecular hydrogen bonds or recombine with other ions nearby, and this effect is able to cause the continual departing-recombining motion of PEG-substituted tertiary amines ions that favors smooth slipping between the adjacent blocks and subsequently the fluids of corresponding polymer (Perriman, Cölfen, Hughes, Barrie & Mann, 2009). The possibility of ester linkage between alginic acid and PEG-substituted tertiary amines except for neutralization were deeply discussed. As shown in solid-state

13

C-NMR

spectrum of Alg/PEG-substituted tertiary amines (Fig. S2), two strong peaks at δ = 31.2 ppm and δ = 71.6 ppm represent for the existence of methylene and polyethylene oxide of PEG, and the peak at δ = 176 ppm consists with the chemical shift of carboxyl group of raw alginate (Fig. 1). If ester linkages existed, corresponding chemical shift should move to high field, which did not appear in the

13

C-NMR

spectrum of Alg/ PEG-substituted tertiary amines, consisting with the fact that ester 8

linkage can hardly be formed under the condition without coupling agent and dehydrating agent (Yang & He, 2012). It can be concluded that neutralization rather than ester linkage between alginic acid and PEG-substituted tertiary amines occurred in the prepared alginate fluids. In addition, three kinds of characterizations are also employed to confirm the chemical composition of solvent-free fluids. Firstly, the water content of ionic liquids was accurately determined by a volumetric Karl Fischer titrator (Fig. S3). It is found that 0.3 wt.% water content for Alg/1810 (L3) and 0.6 wt% water content for Alg/1810 (L5), which are both negligible. Secondly, according to TGA analysis (Fig. S4), the weight loss of Alg/1810 (L3/5) before 100 °C is lower than 0.6 wt.%. Thirdly, based on weigh measurement, these products maintain constant weight in the last three measurements during dehydration. Therefore, it can be concluded that the influence of residual water on the flowability of alginate-based ionic liquid can be ignored. Based on the linkages form of ionic pairs and the absence of water, final alginate liquids can be regarded as a kind of novel ionic liquids. Dynamic frequency and temperature sweep curves were employed to quantitatively evaluate the flowing behaviors of alginate fluids. As shown in Fig. 3, storage modulus (G') almost equal loss modulus (G") in the measured frequency range for Alg/1810 (L3) displaying the characteristic of gel transition point, and a gel to liquid transformation presents as heating to 34 °C, which is similar with reported solvent-free DNA liquids (Bourlinos et al., 2005c). Moreover, Alg/1810 (L5) exhibits more significant liquid-like characters because G" always exceed G' by a large margin in the measured frequency and temperature range. Under the same mass ratio, PEG chain length obviously affects the formation. As shown in Fig. 4, the rheological behaviors of Alg/1820 (L5) with higher modulus are similar to these of Alg/1810 (L5), showing liquid-like behaviors in both dynamic frequency curves and temperature sweep curves; Alg/1830 (L5), however, exhibits gel-like behaviors; wax solids are even shown in Alg/1860 (L5) (Fig. S5). The increasing of PEG chain length under the same mass ratio implies that molar ratio of PEG-substituted tertiary amines to alginate decreases, leading to less neutralization 9

between amines and carboxylate groups, which can explain deteriorative flowabilities of the products with longer PEG chains, such as 1830 and 1860. According to the evidences above, we conclude that the formation of alginate fluids can be easily tuned by regulating the mass fraction of alginic acid, temperature and chain length of PEG in surfactants. Furthermore, the prepared alginate fluids possess the features of both ionic liquid and polysaccharide, leading to wide application in various fields, such as modification of super capacitors (Ma, Mu, Zhang, Sun, Peng & Lei, 2013; Yamagata, Soeda, Ikebe, Yamazaki & Ishikawa, 2013), enhancement of CO2 absorption (Andrewlin, Park, Petit & Park, 2014; Raman, Gurikov & Smirnova, 2015), illustration for models of polymer composite (Li et al., 2016; Praveena, Ravindrachary, Bhajantri & Ismayil, 2014; Wei et al., 2014). 3.3 Preparation of gel-like PEG-substituted tertiary amines/Alg with high molecular weight and application in highly efficient spinning Beyond the above mentioned, another highly anticipated application is taking advantage of alginate with excellent flowability as main source of spinning in textile industry. Alginate fibers are naturally green multifunctional fiber material, yet the poor mobility and low processing efficiency induced by high viscosity at low concentration impedes its industrial applications. The purpose of exploration solvent-free alginate fluids in this paper is to develop a new type of spinning solution that can realize a highly efficient processing strategy. On the other hand, considering the requirement of the mechanical properties used as fiber materials, sufficiently high molecular weight is essential. In despite of same preparation process and similarity phase

separation

phenomena

for

alginate

with

high

molecular

weight/

PEG-substituted tertiary amines during acid-base reaction, a soft and transparent membrane was formed after dehydration of centrifuged precipitation (Fig. S6), owing to severe intermolecular entanglements of polymer chains after solvent removal. The transparent membrane morphology of production implies a completely solvation of alginic acid and flowability of precipitated phase. Naturally, we turned our attention to transparent and homogeneous precipitation with high solid content. Alg/1820 10

(H2/2.5/3) was obtained by mixing the high molecular weight alginic acid with different multiple surfactant. The homogeneity of precipitation is shown in Fig. 5B, combined with titration curves (Fig. S8). The results shows that precipitation turns to be homogeneous and transparent gel at the mass ratio of 1820/Alg = 2.5. As shown in Fig. 5A, G' are always higher than G'' in the measured temperature and frequency range suggesting a weak gel characteristic for all selected samples. Given the reduction of high solid content with increasing 1820, Alg/1820 (H2.5) precipitation were determined to apply to spinning, which has the alginate content of 10 wt.% calculated by volatile water content and feed ratio, and conventional spinning solution with upper concentration limit (5 wt.%) was selected as comparison due to unfeasible dissolution for alginate spinning solution with 6 wt.% or higher concentration which show high viscosity and gel-like behaviors (Fig. S7). According to Fig. 6, steady shear sweep curve of Alg/1820 (H2.5) precipitation exhibit more significant shear-thinning properties than that of 5% SA solution, the viscosity of Alg/1820 (H2.5) becomes lower at shear rate above 6.3 s-1 serving as a reference of shear rate during spinning to roughly evaluate the flowability of spinning solutions, which contributes to energy conservation and easier regulation during spinning. The alginate fibers were prepared using a routine wet-spinning method by the schematic diagram of the self-built device as indicated in Fig. 7A, and the obtained fibers with good appearance is shown in Fig. 7B. Based on SEM observations, the surface of Alg/1820 (H2.5) fiber exhibits smoother than that of 5% SA fiber, but both show obvious interlaced mesh of smaller sub-fibrils on their surface (Fig. 7D1 and D3). Both the roughness along the inner perimeter of the spinneret holes and the shrinkage upon drying of the fibers are postulated as the main reasons for the formation of these streaks (Watthanaphanit, Supaphol, Tamura, Tokura & Rujiravanit, 2010). By comparison with Alg/1820 (H2.5) fiber, 5% SA fibers exhibit loose interlaced mesh with more defects related to poor flowability and low solid content of spinning solution, leading to relatively low tensile strength and elongation at break. 11

Moreover, the tensile cross-section of Alg/1820 (H2.5) fiber looks compact, uniform and remains no gaps (Fig. 7D2 and D4). The polarizing microscope of Alg/1820 (H2.5) fiber demonstrates clear anisotropy and brighter polarizing which are responsible for higher orientation than that of 5% SA (inserts in Fig. 7C). XRD is deemed as an effective tool to characterize the crystallinity. Compared with conventional fiber, Alg/1820 (H2.5) fiber exhibits two higher characteristic peaks 2 of ∼ 15° and ∼ 21.5° corresponding to (110) and (002) reflections, respectively, which indicates higher crystallinity and better orientation of alginate chains for Alg/1820 (H2.5) fiber (Fig. S9). The scattering peak (2) at ∼15° corresponds to the lateral packing among molecular chains, and the peak at ∼21.5° is from the layer spacing along the molecular chain direction, in agreement with the characteristic crystallization of alginate fibers (Pawel Sikorski, Frode Mo, Gudmund Skjåkbræk & Stokke, 2007). The reason why higher crystallinity and better orientation of alginate chains for Alg/1820 (H2.5) fiber is that the migration of relative large PEG-substituted amine molecule from fiber to coagulation medium by ion-exchange with Ca2+ during cross-linking induces the orientation of alginate chains. Therefore, the role of PEG-substituted tertiary amines in this system is to form high solid content of spinning solution and induce orientation and crystallinity of alginate chains during cross-linking. Based on traditional view, some scholars deem that the diffusion of Ca2+ causes the combination of G blocks of alginate and Ca2+ to form ‘egg-box’ structures (Han et al., 2012). Nevertheless, a study reported by Fang (Pawel Sikorski, Frode Mo, Gudmund Skjåkbræk & Stokke, 2007) that the popular ‘egg-box’ model may not be the only possible structure. Ca2+ may combined not only with short G blocks but with MG blocks to destroy original electrostatic interaction between carboxyl groups of alginic acid and tertiary amines of PEG-substituted tertiary amines, and then surfactant in Alg/1820 (H2.5) fibers can be exchanged. The illustrating model is shown in Fig. 8. To confirm the chemical composition of Alg/1820 (H2.5) fibers, elemental analysis was employed. As shown in Table S2, the nitrogen content in Alg/1820 (H2.5) fibers was calculated to be 0.02%, almost the same with that of 5% 12

SA fibers (0.01%), indicating that PEG-substituted tertiary amines were fully removed by diffusion of Ca2+ during cross-linking. The minor difference of nitrogen content is attributed to system error. In addition, almost identical FT-IR spectra (Fig. S10), TGA curves (Fig. S11), and

13

C-NMR spectra (Fig. S12) of Alg/1820 (H2.5) and 5% SA

fibers were obtained, which further proved that residual amount of PEG-substituted tertiary amines in fibers was negligible. Three different strain rates are discussed, namely 1 mm·min-1, 5 mm·min-1 and 10 mm·min-1. As shown in Table. S3, elongation at break decreases with the increasing of stretching rate, while tensile modulus change in opposite direction, showing a trend from tough to brittle behaviors for Alg/1820 (H2.5) and 5% SA fibers, attributing to strong intermolecular interaction induced by high orientation and regulation of chains, which results in decreased elasticity and easy broken. In addition, at all the three rates, the tensile strengths of Alg/1820 (H2.5) fibers are twice higher than those of 5% SA fibers and generally keep at the same level. High crystallinity, high orientation and dense chain aggregation owing to high spinning concentration attribute to the enhancement of mechanical properties of Alg/1820 (H2.5) fibers. Based on the standard of national fiber testing, fibers’ length under 35mm should be used the strain rate of 5 mm·min-1, therefore its conclusion was summarized as following. The Alg/1820 (H2.5) fibers exhibited a tensile strength of 104.0±5.2 MPa that increased by 104%, and an elongation at break of 7.2±0.8 % that increased by 132% in comparison with the fibers obtained by 5 wt% conventional spinning solution, implying that the spinning efficiency was significantly improved due to higher alginate content of Alg/1820 (H2.5) precipitation (10 wt%). To accord with the requirement of the textile industrial, conventional units for Alg/1820 (H2.5) fibers and 5 wt% fibers were offered as well, which are calculated to be 1.65 g/denier and 0.71 g/denier, respectively (Table. S3).

4. Conclusions We have developed a facile method for preparation of alginate fluids in absent of solvent by one-step acid-base reaction between alginic acid and PEG-substituted tertiary amines. The flowability of alginate fluids can be tuned by PEG chain length, 13

temperature, mass ratio of PEG-substituted tertiary amines and molecular weight of alginate. Functionalized alginic acid with low molecular weight showed excellent solvent-free fluids at room temperature, while for high molecular weight, gel-like precipitation in the phase separation structure of acid-base reaction exhibited significant shear-thinning properties and higher solid content in despite of lacking solvent-free flowability, which was directly applied in the routine spinning. Alginate fibers were prepared using Alg/1820 (H2.5) precipitation with high polymer content (10 wt.%) through wet spinning routine, exhibiting increased tensile strength by 104% and increased elongation at break by 132% in comparison with fibers obtained by conventional spinning solution (5 wt.%), and the spinning efficiency was significantly improved. The surfactant could be completely removed by Ca2+ during cross-linking and easily recycled. The proposed strategy is expected to extend to highly efficient processing of other polyelectrolyte polysaccharides to obtain high-performance biomedical materials.

Acknowledgments This work was partially supported by the National Nature Science Foundation of China (No. 51403113 and 51573080), Natural Science Foundation for Distinguished Young Scientists of Shandong Province (BS2014CL007), Postdoctoral Science Foundation of China and Shandong Province (2016T90610, 2015M571994 and 201501007), Project of Shandong Province Higher Educational Science and Technology Program (J14LA19), and Program for Taishan Scholar of Shandong Province.

14

References Ack, B., Da, S. M., & Kieckbusch, T. G. (2012). Natamycin release from alginate/pectin films for food packaging applications. Journal of Food Engineering, 110(1), 18–25. Andrewlin, K. Y., Park, Y., Petit, C., & Park, A. H. (2014). Thermal stability, swelling behavior and CO2 absorption properties of Nanoscale Ionic Materials (NIMs). Rsc Advances, 4(110), 65195-65204. Babak, V. G., Skotnikova, E. A., Lukina, I. G., Pelletier, S., Hubert, P., & Dellacherie, E. (2000). Hydrophobically Associating Alginate Derivatives: Surface Tension Properties of Their Mixed Aqueous Solutions with Oppositely Charged Surfactants. Journal of Colloid & Interface Science, 225(2), 505-510. Bourlinos, A. B., Chowdhury, S. R., Jiang, D. D., An, Y. U., Zhang, Q., Archer, L. A., & Giannelis, E. P. (2005a). Layered organosilicate nanoparticles with liquidlike behavior. Small, 1(1), 80-82. Bourlinos, A. B., Herrera, R., Chalkias, N., Jiang, D. D., Zhang, Q., Archer, L. A., & Giannelis, E. P. (2005b). Surface-Functionalized Nanoparticles with Liquid-Like Behavior. Advanced Materials, 17(2), 234-237. Bourlinos, A. B., Ray Chowdhury, S., Herrera, R., Jiang, D. D., Zhang, Q., Archer, L. A., & Giannelis, E. P. (2005c). Functionalized Nanostructures with Liquid-Like Behavior: Expanding the Gallery of Available Nanostructures. Advanced Functional Materials, 15(8), 1285-1290. Chiaoprakobkij, N., Sanchavanakit, N., Subbalekha, K., Pavasant, P., & Phisalaphong, M. (2011). Characterization and biocompatibility of bacterial cellulose/alginate composite sponges with human keratinocytes and gingival fibroblasts. Carbohydrate Polymers, 85(3), 548-553. Darka, M., Zoran, S., Marija, R., Tamara, R., Petar, J., Jovan, N., & Maja, R. (2010). Functionalization of polyester fabrics with alginates and TiO2 nanoparticles. Carbohydrate Polymers, 79(3), 526-532. 15

de Souza, W. C., de Rezende, S. T., Viana, P. A., Falkoski, D. L., Reis, A. P., Machado, S. G., de Barros, E. G., & Guimarães, V. M. (2009). Treatment of soy milk with Debaryomyces hansenii cells immobilised in alginate. Food Chemistry, 114(2), 589-593. Han, X. J., Dong, Z. Q., Fan, M. M., Liu, Y., Li, J. H., Wang, Y. F., Yuan, Q. J., Li, B. J., & Zhang, S. (2012). pH-induced shape-memory polymers. Macromolecular Rapid Communications, 33(12), 1055. He, Y., Zhang, N., Gong, Q., Qiu, H., Wang, W., Liu, Y., & Gao, J. (2012). Alginate/graphene oxide fibers with enhanced mechanical strength prepared by wet spinning. Carbohydrate Polymers, 88(3), 1100-1108. Kong, Q., Quan, J. I., Jian, Y. U., & Xia, Y. (2008). Preparation and Properties of Alginate Salt Fibers: An Inherent Flame-Retardant, Biodegradable Fiber. (pp. 48-51). Lee, H. J., Ahn, S. H., Bonassar, L. J., & Kim, G. H. (2013). Cell(MC3T3-E1)-Printed Poly( ϵ -caprolactone)/Alginate Hybrid Scaffolds for Tissue Regeneration. Macromolecular Rapid Communications, 34(2), 142–149. Li, P., Zheng, Y., Shi, T., Wang, Y., Li, M., Chen, C., & Zhang, J. (2016). A solvent-free graphene oxide nanoribbon colloid as filler phase for epoxy-matrix composites with enhanced mechanical, thermal and tribological performance. Carbon, 96, 40-48. Liu, X., Qian, L., Shu, T., Lu, L., & Tong, Z. (2003). Sol-gel transition and gel point determination for aqueous solutions of sodium alginate. Acta Polymerica Sinica, 288(4), 484-488. Liu, Y., Yingbo, R., Zhang, B., Qiao, X., & Liu, C. (2016). Tuning of Ionic Interaction and Rheological Properties of Nanoscale Ionic Materials. Chemical Journal of Chinese Universities, 37(4), 767-774. Ma, G. F., Mu, J. J., Zhang, Z. G., Sun, K. J., Peng, H., & Lei, Z. Q. (2013). Preparation of Polypyrrole/Sodium Alginate Nanospheres and Their Application for High-Performance Supercapacitors. Acta Physico-Chimica Sinica, 29(11), 2385-2391. Mazur, K., Buchner, R., Bonn, M., & Hunger, J. (2013). Hydration of Sodium 16

Alginate in Aqueous Solution. Macromolecules, 47(2), 771-776. Pawar, S. N., & Edgar, K. J. (2011). Chemical modification of alginates in organic solvent systems. Biomacromolecules, 12(11), 4095-4103. Pawel, S., Frode, M., Gudmund Skjåkbræk, A., & Stokke, B. T. (2007). Evidence for Egg-Box-Compatible Interactions in Calcium−Alginate Gels from Fiber X-ray Diffraction. Biomacromolecules, 8(7), 2098-2103. Perriman, A. W., Cölfen, H., Hughes, R. W., Barrie, C. L., & Mann, S. (2009). Solvent-free protein liquids and liquid crystals. Angewandte Chemie, 48(34), 6242-6246. Praveena, S. D., Ravindrachary, V., Bhajantri, R. F., & Ismayil. (2014). Free volume-related microstructural properties of lithium perchlorate/sodium alginate polymer composites. Polymer Composites, 35(7), 1267–1274. Qin, Y. (2008). Alginate fibres: an overview of the production processes and applications in wound management. Polymer International, 57(2), 171-180. Qin, Y., Hu, H., & Luo, A. (2006). The conversion of calcium alginate fibers into alginic acid fibers and sodium alginate fibers. Journal of Applied Polymer Science, 101(6), 4216-4221. Raman, S. P., Gurikov, P., & Smirnova, I. (2015). Hybrid alginate based aerogels by carbon dioxide induced gelation: Novel technique for multiple applications. The Journal of Supercritical Fluids, 106, 23-33. Rayer, A. V., Sumon, K. Z., Jaffari, L., & Henni, A. (2014). Dissociation Constants (pKa) of Tertiary and Cyclic Amines: Structural and Temperature Dependences. Journal of Chemical & Engineering Data, 59(11), 3805-3813 Sa, V., & Kornev, K. G. (2011). A method for wet spinning of alginate fibers with a high concentration of single-walled carbon nanotubes. Carbon, 49(6), 1859-1868. Salomonsen, T., Jensen, H. M., Larsen, F. H., Steuernagel, S., & Engelsen, S. B. (2009). Direct quantification of M/G ratio from

13

C CP-MAS NMR spectra of

alginate powders by multivariate curve resolution. Carbohydrate Research, 344(15), 2014-2022. 17

Sennerby, L., Röstiund, T., Albrektsson, B., & Albrektsson, T. (1987). Acute tissue reactions to potassium alginate with and without colour/flavour additives. Biomaterials, 8(1), 49-52. Srinivasan, S., Jayasree, R., Chennazhi, K. P., Nair, S. V., & Jayakumar, R. (2012). Biocompatible alginate/nano bioactive glass ceramic composite scaffolds for periodontal tissue regeneration. Carbohydrate Polymers, 87(1), 274-283. Sun, J., & Tan, H. (2013). Alginate-Based Biomaterials for Regenerative Medicine Applications. Materials, 6(4), 1285-1309. Watthanaphanit, A., Supaphol, P., Tamura, H., Tokura, S., & Rujiravanit, R. (2010). Fabrication, structure, and properties of chitin whisker-reinforced alginate nanocomposite fibers. Journal of Applied Polymer Science, 110(2), 890-899. Wei, S., Zhang, X., Zhao, K., Fu, Y., Li, Z., Lin, B., & Wei, J. (2014). Preparation, characterization, and photocatalytic degradation properties of polyacrylamide/calcium alginate/TiO2 composite film. Polymer Composites, 08(3), 297-299. Yamagata, M., Soeda, K., Ikebe, S., Yamazaki, S., & Ishikawa, M. (2013). Chitosan-based gel electrolyte containing an ionic liquid for high-performance nonaqueous supercapacitors. Electrochimica Acta, 100(7), 275-280. Yang, J., & He, W. (2012). Synthesis of lauryl grafted sodium alginate and optimization of the reaction conditions. International Journal of Biological Macromolecules, 50(2), 428-431. Yao, C., Zhao, J., Ge, H., Ren, J., Yin, T., Zhu, Y., & Ge, L. (2014). Fabrication of dual sensitive titania (TiO2)/graphene oxide (GO) one-dimensional photonic crystals (1DPCs). Colloids & Surfaces A Physicochemical & Engineering Aspects, 452(1), 89-94. Zong, P., Wang, S., Zhao, Y., Wang, H., Pan, H., & He, C. (2013). Synthesis and application of magnetic graphene/iron oxides composite for the removal of U(VI) from aqueous solutions. Chemical Engineering Journal, 220(11), 45-52.

18

Fig. 1. Solid-state 13C-NMR spectra for alginate with low and high molecular weight (A); curve-fitting analysis of solid-state 13C-NMR spectra for ring carbons of alginate with low (B) and high (C) molecular weight in the range of 60-90 ppm.

19

Fig. 2. Titration curve for the acid–base reaction between 1810 and Alg (A); upper liquids (B1) and precipitations (B2) of Alg/1810; photographs of Alg/1810 placed on glass slides (C1) and flowing down 60° glass slopes (C2); mechanism between the alginic acid and the surfactant (D). The compositions of L1, L3 and L5 systems represent that the mass ratios of 1810 to low molecular weight alginic acid are 1, 3 and 5 times, respectively.

Fig. 3. Dynamic frequency (A) ranging from 0.1 to 500 rad·s-1 and temperature sweep curves (B) ranging from 25°C to 80 °C with ω= 10 rad·s-1 for Alg/ PEG-substituted tertiary amines fluids with different feed ratio. Alg/1810 (L3) and Alg/1810 (L5) stand for triple and five times of 1810 to alginic acid with low molecular weight, respectively. 20

Fig. 4. Dynamic frequency curves (A) ranging from 0.1 to 500 rad·s-1 and temperature sweep curves (B) ranging from 25°C to 80 °C with ω= 10 rad·s-1 for Alg/ PEG-substituted tertiary amines fluids with different PEG chain length. Alg/1810 (L5), Alg/1820 (L5) and Alg/1830 (L5) stand for five times of 1810, 1820 and 1830 to alginic acid with low molecular weight. The chain lengths of 1810, 1820 and 1830 are m+n = 10, 20 and 30, respectively, and m and n represent the addition of ethylene glycol units for two branches of PEG-substituted tertiary amines.

Fig. 5. Dynamic frequency sweep curves (A) ranging from 0.1 to 500 rad·s-1 at room temperature and photographs (B) of precipitation for Alg/1820 with different feed ratio. Hx represents x mass ratio of 1820 to alginic acid with high molecular weight.

21

Fig. 6. Steady viscosity as a function of shear rate for 5% SA and Alg/1820 (H2.5) spinning solutions at 25 °C.

Fig. 7. Wet-spinning equipment (A); photograph of Alg/1820 (H2.5) fibers (B); stress–strain curves and polarizing microscopes for and Alg/1820 (H2.5) fibers and 5% SA fibers (C); SEM images of tensile-broken Alg/1820 (H2.5) fibers (D1,D2) and 5% SA fibers (D3, D4). For both Alg/1820 (H2.5) fibers and 5% SA fibers, 10 mL plastic syringes and a metal needle with an inner diameter of 1.2 mm. The extrusion rate of the syringe pump at 600 μL·min-1 and the draw ratio = 1.15 were employed through wet-spinning and fibers are immersed at 3 wt.% CaCl2 coagulation bath for 2h.

22

Fig. 8. Mechanism between Alg/1820 (H2.5) and calcium ions during cross-linking

23