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  纖維素納米晶（CNC）具有可再生、生物相容好、無毒、可生物降解、機(jī)械性能優(yōu)異、比表面積大、水分散性好等特性，在石油開采、聚合物復(fù)合材料增強(qiáng)、水凝膠、氣凝膠、超級電容器、節(jié)能建筑、化妝品、造紙、涂料、液晶和廢水處理等領(lǐng)域顯示出廣闊的前景（圖1）。然而，CNC的親水表面限制了其進(jìn)一步應(yīng)用。對CNC表面進(jìn)行聚合物接枝不僅可以改變其表面的親疏水性能，而且可以賦予CNC多種其它功能（電導(dǎo)率、pH 或溫度響應(yīng)性、反應(yīng)性等）。其中，通過表面引發(fā)的原子轉(zhuǎn)移自由基聚合（SI-ATRP）可以很好地控制CNC表面聚合物的接枝密度、接枝鏈長和末端接枝等，進(jìn)一步拓展了纖維素納米晶的應(yīng)用領(lǐng)域。

圖1. 納米纖維素的廣泛應(yīng)用

  華南師范大學(xué)張振副研究員和周國富教授、法國波爾多大學(xué)Gilles Sèbe教授以及西南大學(xué)黃進(jìn)教授等人在Journal of Applied Polymer Science期刊發(fā)表了題為“Grafting polymers from cellulose nanocrystals via surface-initiated atom transfer radical polymerization”的綜述文章，系統(tǒng)地闡述了近年來通過SI-ATRP對CNC表面進(jìn)行聚合物接枝的調(diào)控和機(jī)制，包括接枝密度、接枝鏈長及接枝區(qū)域的調(diào)控；并對其在納米復(fù)合材料用填料、刺激響應(yīng)材料、載體劑和其它領(lǐng)域的應(yīng)用進(jìn)行了總結(jié)（圖2）。最后，針對目前通過SI-ATRP在CNC表面接枝聚合物存在的問題，該綜述進(jìn)一步指出CNC表面接枝聚合物的面臨的發(fā)展機(jī)遇和挑戰(zhàn)。

圖2. 通過SI-ATRP對CNC表面調(diào)控及其性能和應(yīng)用示意圖

  ATRP是一種活性聚合，可以通過調(diào)節(jié)單體和引發(fā)劑的比例以及單體轉(zhuǎn)化率，有效調(diào)節(jié)聚合物的分子量。SI-ATRP作為一種“grafting from”的接枝聚合物方法，和“grafting to”的聚合物接枝方法相比，具有空間位阻小、接枝密度高的優(yōu)點。由于SI-ATRP這種可控的接枝鏈長和接枝密度，被廣泛用于各種基體或納米材料的表面高分子接枝改性。目前已經(jīng)通過SI-ATRP在納米纖維素尤其是纖維素納米晶上接枝了各種不同結(jié)構(gòu)的高分子（圖3），近年來在這一領(lǐng)域也發(fā)表了相關(guān)綜述性文章，然而關(guān)于SI-ATRP對CNC表面接枝聚合物的接枝密度、接枝聚合物鏈長和接枝區(qū)域的精準(zhǔn)調(diào)控機(jī)制討論的綜述還很少，該綜述著重討論了通過SI-ATRP在CNC表面接枝高分子調(diào)控的機(jī)制及其對性能的影響和相關(guān)應(yīng)用。

圖3.通過 SI-ATRP 在納米纖維素表面上接枝的各種聚合物

  接枝聚合物的密度：通過SI-ATRP在CNC表面接枝聚合物首先需要在CNC表面引入SI-ATRP的引發(fā)位點Br，目前一般采用BIBB的酰溴和CNC表面的羥基的酯化反應(yīng)，CNC表面Br的含量很大程度上決定了接枝密度。然而這一過程仍然面臨著BIBB利用率低、Br含量相對較低、有機(jī)溶劑用量多、CNC的內(nèi)部結(jié)構(gòu)容易遭到破壞等缺點。在SI-ATRP的過程中，引發(fā)效率也會影響接枝密度；由于SI-ATRP過程中空間位阻較小，一般CNC-Br的引發(fā)效率比較高。

圖4. 通過SI-ATRP在CNC表面接枝聚合物的示意圖

  ATRP過程中需要無氧操作，而且需要大量的CuBr作為催化劑。ARGET ATRP是指在ATRP過程中加入過量的還原劑，和ATRP相比具有反應(yīng)條件溫和、不需要嚴(yán)格除氧和催化劑用量少等優(yōu)點。通過對比SI-ATRP和SI-ARGET ATRP發(fā)現(xiàn)，在常規(guī)的SI-ATRP中，聚合過程中鏈增長速率相對較慢，CNC-Br上的Br引發(fā)位點幾乎都能引發(fā)SI-ATRP，因此接枝密度相對較高（圖5）。而在SI-ARGET ATRP中， CNC-Br上只有少部分Br被引發(fā)，聚合過程中鏈增長速率相對較快，較長的聚合物鏈屏蔽了周圍Br的引發(fā)，因此CNC-Br的引發(fā)效率較低，接枝密度相對較低。

圖5. Br-CNC的SI-ATRP和SI-ARGET ATRP接枝機(jī)理示意圖

  接枝聚合物的鏈長：SI-ATRP 最重要的優(yōu)勢之一是對接枝聚合物鏈長的準(zhǔn)確控制，由于均相和非均相體系中的聚合機(jī)理可能不同，人們已經(jīng)做了許多努力來證明SI-ATRP的反應(yīng)活性。表征接枝聚合物的直接方法是通過 GPC 測量裂解的接枝聚合物的分子量和分散性。接枝聚合物通常需要在高溫下用強(qiáng)酸或強(qiáng)堿進(jìn)行裂解，為了克服這個問題，學(xué)者們通常會在體系中引入犧牲引發(fā)劑。如通過采用紫外光可裂解聚合物進(jìn)行 SI-ATRP 接枝，證實了接枝聚合物和游離聚合物之間的等效性。紫外光可裂解帶有 Br 引發(fā)位點的光連接劑，在犧牲引發(fā)劑的存在下進(jìn)一步通過SI-ATRP接枝聚苯乙烯。結(jié)果發(fā)現(xiàn)接枝聚苯乙烯的分子量與游離 PS 的分子量非常吻合。由此說明，聚合物-g-CNC 中接枝聚合物的鏈長可以通過單體與犧牲引發(fā)劑的摩爾比或單體轉(zhuǎn)化率進(jìn)行調(diào)節(jié)。

  CNC的末端聚合物接枝改性：通過控制聚合物在納米粒子表面接枝的區(qū)域可以實現(xiàn)CNC表面的不對稱接枝。例如通過硫酸水解等方法可獲得兩端帶有不同的官能團(tuán)的CNC，即非還原端帶有羥基側(cè)基，還原端帶有醛基。通過對CNC的還原端進(jìn)行區(qū)域選擇性改性可實現(xiàn)CNC表面的不對稱接枝。如圖6所示，首先采用亞次氯酸鈉（NaClO₂）將CNC表面的醛基氧化為羧基，然后分別與乙二胺和2-溴異丁酸N-羥基琥珀酰亞胺酯反應(yīng)，進(jìn)而通過SI-ATRP，可以在CNC的還原端接枝不同的聚合物單體，使CNC的一端接枝上斑片狀或多毛聚合物。通過TEM照片，可以觀察到金納米顆粒標(biāo)記的斑塊狀的CNC，表明其表面聚合物的成功接枝。

圖6.（a）通過SI-ATRP對CNC的還原端改性；（b）通過SI-ATRP對CNC進(jìn)行表面均勻改性；（c）一端接枝聚合物并用金納米粒子標(biāo)記的斑片狀CNC的TEM圖像

  性能與應(yīng)用：該綜述還進(jìn)一步探索了利用SI-ATRP制備的聚合物接枝纖維素納米晶Polymer-g-CNC的性能以及在納米復(fù)合材料增強(qiáng)劑、刺激響應(yīng)性復(fù)合材料等方面的應(yīng)用。

  總結(jié)與展望：由于CNC優(yōu)異的性能，開發(fā)低成本、高性能的纖維素基可生物降解材料是替代化石基塑料的有前途的替代品。近年來，CNC生產(chǎn)已朝商業(yè)化方向發(fā)展。然而，CNC 的親水表面和表面官能團(tuán)缺乏（除了羥基）阻礙了它們更廣泛的應(yīng)用和商業(yè)化。在此背景下，通過聚合物接枝對 CNC 進(jìn)行表面改性可用于調(diào)節(jié)其表面性質(zhì)，并賦予 CNC 多種功能，可以進(jìn)一步促進(jìn)其在許多未開發(fā)領(lǐng)域的應(yīng)用。總之，通過 SI-ATRP 從 CNC 接枝聚合物是一種非常有前途的調(diào)整納米纖維素表面性質(zhì)的方法。

  論文下載鏈接：http://doi.org/10.1002/app.51458

作者信息：

Dr. Zhen Zhang is an Associated Researcher at South China Normal University (Guangzhou, China) since 2019. He received his Bachelor of Engineer degree in Polymers from East China University of Science and Technology (Shanghai, China) in 2009, Master of Science degrees in Materials from ECUST and Chemical Engineering from Chalmers University of Technology (Goteborg, Sweden) in 2012, and PhD degrees in Polymers from Bordeaux University (Bordeaux, France) and in Chemistry from University of Waterloo (Waterloo, Canada) in 2017, respectively. His research interests mainly focus on the preparation, surface modification and applications of nanocellulose.

張振副研究員，本科畢業(yè)于華東理工大學(xué)，碩士畢業(yè)于華東理工和瑞典查爾姆斯理工大學(xué)，博士畢業(yè)于加拿大滑鐵盧大學(xué)和法國波爾多大學(xué)，2019年加入華南師范大學(xué)華南先進(jìn)光電子研究院周國富教授團(tuán)隊，主要研究方向為納米纖維素（纖維素納米晶）的制備、改性和應(yīng)用，Pickering乳液，相變材料等，目前已經(jīng)在Chemical Engineering Journal, ACS Sustainable Chemistry & Engineering, Carbohydrate Polymers, Cellulose, Journal of Colloid and Interface Science, Advanced Sustainable Systems, ACS Applied Nano Materials和《化學(xué)進(jìn)展》等期刊發(fā)表一作或通訊作者論文20余篇。

Gilles Sèbe is an Associate Professor at the University of Bordeaux in France, within the Laboratoire de Chimie des Polymères Organiques (LCPO). After receiving his Ph.D. degree in wood chemistry in 1996, he did a one-year post-doc at McMaster University in Hamilton (Canada), and then joined the BioComposites Center in Bangor (UK) for a two years period. In 2000, he was appointed at the University of Bordeaux, doing research and teaching on wood and biopolymers chemistry. His current research at the LCPO aims at developing novel nanocellulose-based functional materials, through the chemical tailoring of the nanocellulose surface. 

Yelin Hou (候曄琳) recieved her bachelor''s degree of Light Chemical Engineering from Shaanxi University of Science and Technology. She will receive her master''s degree of light industry technology and engineering from Shaanxi University of Science and Technology, and she is now a PhD candidate at the doctoral school of Chemical Sciences in the University of Bordeaux under the supervison of Dr Gilles Sèbe. Her research interests mainly focus on cellulose nanocrystals and organic-inorganic nanocomposites.

Juan Wang (王娟) received her Bachelor degree in Material Chemistry from Qiuzhen College of Huzhou Normal College in 2020 (which is renamed to Huzhou College in 2021). She is currently an engineer in ScienceK Ltd. ScienceK (閃思科技 www.sciencek.com) is one of the leading pioneers in the commercial production, functionalization, and application of nanocellulose (especially cellulose nanocrystals, CNC) in China. 閃思科技ScienceK是中國納米纖維素生產(chǎn)、改性和應(yīng)用研發(fā)的先行者。

Prof. Jin Huang (黃進(jìn)教授) obtained Ph.D degree in Wuhan University, and serves as the full-professor in Southwest University (China) and the director of “Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing”. He carried out scientific research at Institute of Chemistry (Chinese Academy of Sciences), Wuhan University of Technology, Institut National Polytechnique de Grenoble, etc. His research focuses on "Polymer-Centered Soft-Matter Materials", "Sustainable Chemistry and Materials", "Flexible Materials and Devices", "Advanced Materials Manufacturing", etc.  The research on "cellulose nanocrystals-based materials" has achieved great progress. He has published over 200 peer-reviewed papers, and edited 3 monographs and wrote 8 book chapters.

Prof. Guofu Zhou (周國富教授) received his Ph.D in Chongqing University, 1986. He served Philips Research Laboratories in the Netherlands as a senior scientist and principal scientist from 1995 to 2011. He is currently a Professor at South China Normal University (SCNU) in the field of micro/nano optoelectronic materials, devices and flat panel displays. He is also a Co-founder & Director of Electronic Paper Display Institute of SCNU, National Center for International Research on Green Optoelectronics and joint research labs of SCNU-TUE. He is leading several national research projects and Sino-Dutch joint projects.