中圖分類號(hào):069 文獻(xiàn)標(biāo)志碼:A
Research Progress on High Performance, Sustainable and Functional Properties of Benzoxazine Resins
XIELin,LUYin,YAO Zhenhao,SHENGWeichen,ZHANGKan (Institute of Polymer Materials, School ofMaterials Science and Engineering,Jiangsu University, Zhenjiang212013, Jiangsu, China)
Abstract:Benzoxazineresins,asanovel class ofthermoseting polymer materials,have garnered significant atention from bothacademiaandindustryduetotheiruniquestructuraldesignability.Withtherapidadvancementsinhigh-end technological fieldsand thecontinuous enhancementof environmental awarenessamong humans,the future development trendofbenzoxazineresins isinevitably groundedin highperformanceandmultifunctionalitywithapredominantfocuson sustainability. Our research team insist on the novel synthetic approach to benzoxazine molecules,aiming to explore the relationship between the molecular structure,thecuring mechanisms of thermoseting resin monomersand the resulting material properties through the design and regulationof molecular structures and hydrogen bonding.We innovate highperformance benzoxazine resin systems,develop methods for optimizing the performanceof sustainable bio-based thermosetingresins,andfabricatenewbenzoxazine-basedfunctionalmaterials.Thispaperreviews therecent progresmade byourteam inthehigh-performanceenhancement,greening,and functionalizationof benzoxazineresins,whilealso projecting future research directions for this field.
Key words: benzoxazine resin; high performance; functional application; bio-based; molecular design
為了滿足電工電子和航空航天等先進(jìn)技術(shù)領(lǐng)域的特殊需求,研究者們對(duì)高性能材料的開發(fā)日趨重視。熱固性樹脂因其質(zhì)輕和效優(yōu)而被認(rèn)為是高性能聚合物材料基體必要的候選之一。聚苯并噁嗪(PBZ)樹脂作為一種較為新型的熱固性樹脂,由苯并噁嗪單體熱開環(huán)固化得到,具有較高的熱穩(wěn)定性[1-4]、可調(diào)控的力學(xué)性能[5.6]、突出的阻燃性[7.8]、較低的表面自由能[9,10]、低介電常數(shù)[11-13]、固化過程中近零收縮性[14,15]、低吸水性以及良好的黏附性[12,16,17]等諸多優(yōu)點(diǎn),可應(yīng)用于航空航天、電工電子、涂料和黏合劑等眾多技術(shù)領(lǐng)域[18](圖1),被視為傳統(tǒng)環(huán)氧類、雙馬來酰亞胺類和酚醛類樹脂的有力替代品[19]。然而,苯并噁嗪樹脂依然存在高固化溫度、低交聯(lián)密度、質(zhì)脆低韌以及合成原料化石基來源(煤炭、石油、天然氣)占比大的弊端。幸運(yùn)的是,苯并噁嗪具有靈活的分子結(jié)構(gòu)設(shè)計(jì)性[20-22],可通過自然界中廣泛存在的生物基酚類(如丁香酚[23,24]、腰果酚[25]、愈創(chuàng)木酚[26,27])、生物基胺(如糠胺[28]、硬脂胺[29]、脫氫樅胺[30])和生物基醛(如糠醛和水楊醛[31)的獨(dú)特化學(xué)結(jié)構(gòu)來調(diào)控固化后樹脂的性能。然而,開發(fā)性能優(yōu)于普通化石基苯并噁嗪樹脂的生物基聚苯并噁嗪熱固性材料仍然是一個(gè)巨大的挑戰(zhàn)。本團(tuán)隊(duì)在苯并噁嗪領(lǐng)域深耕多年,通過設(shè)計(jì)新型化學(xué)結(jié)構(gòu)以及構(gòu)筑苯并噁嗪分子內(nèi)氫鍵,在引人不同功能基團(tuán)的基礎(chǔ)上深人探究了苯并噁嗪樹脂的構(gòu)效關(guān)系,進(jìn)一步采用可再生原料合成了新型生物基苯并噁嗪樹脂,開發(fā)了一系列潛伏催化固化與本征阻燃型生物基樹脂,還通過苯并噁嗪樹脂的功能化來拓展其在復(fù)合材料、潤滑油等領(lǐng)域的新應(yīng)用。
圖1聚苯并噁嗪樹脂的合成及其應(yīng)用Fig.1Synthesis and application of PBZ resin
本文綜述了團(tuán)隊(duì)近年來在苯并噁嗪樹脂高性能化、綠色化與功能化三方面的工作,具體包括:高性能鄰位酰胺/酰亞胺苯并噁嗪的開發(fā);生物質(zhì)黃酮和香豆素基苯并噁嗪樹脂的合成、固化機(jī)理與性能研究,尤其詳細(xì)介紹了分子內(nèi)氫鍵的調(diào)控機(jī)制和潛伏自催化固化機(jī)理;以苯并嚏嗪為基體制備石墨烯材料,并研究其在熱傳導(dǎo)、電磁屏蔽和摩擦等領(lǐng)域的應(yīng)用。
1高性能苯并噁嗪樹脂
苯并噁嗪樹脂作為一類極具價(jià)值的高分子材料,其獨(dú)特的分子骨架和可靈活調(diào)節(jié)的功能基團(tuán)配置,賦予了材料多變的性能水平。通過對(duì)合成原料的精心篩選和對(duì)苯并噁嗪結(jié)構(gòu)中取代基的相關(guān)改性,可以實(shí)現(xiàn)對(duì)該材料物理化學(xué)性能的精確調(diào)控,從而滿足多樣化的應(yīng)用場景需求。在苯并噁嗪樹脂的開發(fā)過程中,分子設(shè)計(jì)發(fā)揮著核心作用,這是因?yàn)榉肿釉O(shè)計(jì)不僅能夠塑造樹脂的網(wǎng)絡(luò)結(jié)構(gòu),還影響著樹脂的固化行為以及固化后材料的熱學(xué)行為和力學(xué)特性等。如何通過巧妙的分子設(shè)計(jì),彌補(bǔ)苯并噁嗪樹脂性能上的短板,合成兼具低固化溫度(易加工)和高熱穩(wěn)定性(高性能)的苯并噁嗪樹脂已然成為當(dāng)前研究的重點(diǎn)。針對(duì)這一挑戰(zhàn),本團(tuán)隊(duì)以引人耐熱基團(tuán)為切入點(diǎn),設(shè)計(jì)合成了以鄰位酰胺結(jié)構(gòu)、鄰位酰亞胺結(jié)構(gòu)為代表的高性能苯并噁嗪熱固性樹脂新體系。
1.1鄰位酰胺官能化苯并噁嗪樹脂
鄰位酰胺官能化苯并噁嗪是由引入了鄰位酰胺基團(tuán)的酚與胺及醛發(fā)生曼尼??s合反應(yīng)生成[32],典型結(jié)構(gòu)如圖2(a)所示。鄰位酰胺基團(tuán)與噁嗪環(huán)中的氧原子相鄰,致使酰胺基團(tuán)中的NH與噁嗪環(huán)中的O形成分子內(nèi)氫鍵(圖2(b)),而酰胺基團(tuán)若在嘎嗪環(huán)對(duì)位只能形成分子間氫鍵。這一獨(dú)特結(jié)構(gòu)促使鄰位酰胺苯并噁嗪在熱固化過程中能夠在較低溫度下實(shí)現(xiàn)開環(huán)聚合,且無需添加引發(fā)劑或催化劑[33]。密度泛函理論(DFT)的有關(guān)計(jì)算與實(shí)驗(yàn)闡釋了鄰位與對(duì)位酰胺官能化苯并噁嗪單體的分子結(jié)構(gòu)差異。計(jì)算結(jié)果表明,分子內(nèi)氫鍵的存在顯著降低了 O-CH2 鍵的總電子密度,進(jìn)而降低了該鍵的鍵能,使鄰位酰胺苯并噁嗪更容易發(fā)生熱開環(huán)聚合。實(shí)驗(yàn)表征進(jìn)一步表明,在差示掃描量熱(DSC)測試中,相較于對(duì)位異構(gòu)體,鄰位酰胺苯并嘌嗪的熱開環(huán)固化峰值溫度 (Tp) 降低約 40% ,且基于Starink法計(jì)算得到的活化能也更低[34]。含有噻吩基團(tuán)的鄰位和對(duì)位酰胺苯并噁嗪單體中,噻吩中硫元素可以競爭調(diào)控NH與噁嗪環(huán)中的O形成分子內(nèi)氫鍵鍵能,對(duì)其分子內(nèi)可能形成的各種氫鍵(包括分子內(nèi)五元環(huán)、六元環(huán)氫鍵以及分子間氫鍵)強(qiáng)度的計(jì)算結(jié)果(圖2(c))進(jìn)一步證明,酰胺鍵中的NH與噁嗪環(huán)中的O形成的分子內(nèi)氫鍵越強(qiáng),相應(yīng)苯并嘌嗪的聚合溫度越低[35]。
圖2(a)典型的鄰位酰胺型苯并嘌嗪結(jié)構(gòu)式;(b)鄰位酰胺苯并嘌嗪(oHBA-a)形成分子內(nèi)氫鍵,對(duì)位酰胺苯并噁嗪(pHBA-a)形 成分子間氫鍵[33];(c)富含噻吩基團(tuán)的苯并嘌嗪中可能形成各種氫鍵的強(qiáng)度對(duì)比[35] Fig.2(a)Typicalstructuralfulaeoftoamidetypebenzoxaie;(b)ItraolelardrogenbdginBA-aditeolular hydrogen bonding in pHBA a[33] ; (c) Qualitative strength of hydrogen bonding in thiophene rich benzoxaiznes[35]
此外,鄰位酰胺型苯并嚏嗪在后固化階段,酚羥基和其鄰位酰胺基團(tuán)之間會(huì)發(fā)生分子內(nèi)熱環(huán)化反應(yīng),脫去水分子形成苯并噁唑環(huán)貫穿于樹脂網(wǎng)絡(luò)結(jié)構(gòu)中(圖3(a))[32],不但熱穩(wěn)定性進(jìn)一步提升,介電性能也得到優(yōu)化?;卩徫货0繁讲⑧燕旱谋讲f唑熱環(huán)化反應(yīng)并結(jié)合苯并噁嗪靈活的分子設(shè)計(jì)性,將多種功能基團(tuán)引入鄰位酰胺苯并噁嗪單體可以實(shí)現(xiàn)性能調(diào)控:(1)引人三氟甲基使樹脂的介電常數(shù) (εr) 降至 2.42~2.19 ,介電損耗(tan δ 降至 0.012~0.008 ,而玻璃化轉(zhuǎn)變溫度 (Tg) 高達(dá) 354°C ,且熱穩(wěn)定性顯著提升,質(zhì)量損失 5% 溫度(Td5) 和 10% 質(zhì)量損失溫度( (Td10) 分別達(dá)到 417°C 和 512°C[36] ;(2)引入炔基可在聚合過程中環(huán)化形成苯環(huán)從而改善綜合性能,樹脂的介電常數(shù)降至 2.55~2.31 ,介電損耗降至 0.008~0.004 ,且 Td10 為 445°C,800°C 下的殘?zhí)悸?(Yc) 為 66%137] ;(3)引入乙烯基可以與硅氧烷發(fā)生硅氫加成反應(yīng)進(jìn)而制備含硅氧烷鏈的主鏈型苯并噁嗪樹脂,結(jié)合苯并噁唑熱環(huán)化處理,樹脂也展現(xiàn)出優(yōu)異的介電性能(介電常數(shù)為 2.52~2.13 ,介電損耗為 0.056~ 0.008)和熱穩(wěn)定性( Tg 高達(dá) 400°C , Td10 為 4459C )(圖3(b))[38]。以上結(jié)果表明,鄰位酰胺苯并噁嗪在賦予樹脂單體低固化溫度的同時(shí),能進(jìn)一步通過后固化過程苯并噁唑化提升樹脂的綜合性能。該系列樹脂在電子封裝領(lǐng)域展現(xiàn)出良好的應(yīng)用潛力。
圖3(a)鄰位酰胺苯并噁嗪的熱固化機(jī)理[32l;(b)含三氟甲基、炔基和硅氧烷的鄰位酰胺苯并嘌嗪的結(jié)構(gòu)式
Fig.3(a)Tealcurigmechanisofrtamdebzoaie32;truturalfulaeofoAmdebzoxaziwithrfoeth alkynyl and siloxane functionalities
1.2鄰位酰亞胺官能化苯并噁嗪樹脂
本團(tuán)隊(duì)首次觀察到鄰位酰亞胺型苯并噁嗪中的噁嗪環(huán)負(fù)電荷氧原子與酰亞胺功能團(tuán)之間存在分子內(nèi)排斥作用,從而導(dǎo)致C-N鍵旋轉(zhuǎn)受抑制而產(chǎn)生阻轉(zhuǎn)異構(gòu),該異構(gòu)體特征在核磁共振(NMR)測試中得到充分驗(yàn)證,在DFT計(jì)算中也有體現(xiàn)(圖4(a))[39]。本團(tuán)隊(duì)進(jìn)一步合成了多種具有阻轉(zhuǎn)異構(gòu)特征的鄰位酰亞胺型苯并噁嗪單體,并通過DFT模擬計(jì)算對(duì)其空間結(jié)構(gòu)進(jìn)行了深入研究(圖4(b))[40]。
鄰位酰亞胺型苯并噁嗪樹脂經(jīng)固化后同樣可進(jìn)一步后固化形成聚苯并噁唑樹脂。然而,鄰位酰亞胺進(jìn)行苯并噁唑熱環(huán)化的機(jī)理與鄰位酰胺苯并噁嗪有所差異[41:酰亞胺基團(tuán)會(huì)先與苯并嘌嗪開環(huán)生成的酚羥基反應(yīng),釋放出二氧化碳分子,進(jìn)而形成苯并噁唑基團(tuán)(圖4(c)),且該反應(yīng)溫度顯著高于鄰位酰胺苯并噁嗪樹脂的后固化溫度。
此外,引人包括馬來酰亞胺[42-45]、降冰片烯[4,43,46-51]、炔基[4,52]、氰基[47,3,54]、鄰苯二甲腈[51,55]、環(huán)丁烯[46]等具有交聯(lián)反應(yīng)活性的基團(tuán)到苯并嚏嗪單體結(jié)構(gòu)中,樹脂的熱學(xué)和力學(xué)性能進(jìn)一步提高。以降冰片烯和炔基同時(shí)引入鄰位酰亞胺苯并噁嗪單體為例,炔基有效降低了單體的固化溫度,降冰片烯則顯著提升了樹脂的交聯(lián)密度(圖4(d)),最終獲得的高性能樹脂具有 250qC 的 Tg 和 4589C 的 Td10[4] 。
綜上,鄰位酰胺和鄰位酰亞胺型苯并噁嗪樹脂以及相應(yīng)的后固化得到的聚苯并噁嗪的性能數(shù)據(jù)均匯總于表1中。
2生物基高性能苯并噁嗪熱固性樹脂
當(dāng)前,化學(xué)工業(yè)的原料大多源于化石基資源。化石基資源不僅面臨枯竭的困擾,還給生態(tài)環(huán)境造成破壞。因此,為推動(dòng)高性能苯并噁嗪熱固性樹脂朝著可持續(xù)方向發(fā)展,借助第四代化工路線的生物質(zhì)平臺(tái)化合物來制備苯并嚏嗪樹脂刻不容緩。在生物基苯并嚏嗪樹脂的開發(fā)過程中,原料的篩選與分子設(shè)計(jì)至關(guān)重要。通常,相較于脂肪族原料,采用酚和芳香族或類芳香族胺合成的苯并噁嗪樹脂在性能上更具優(yōu)勢。基于上述認(rèn)知,本團(tuán)隊(duì)系統(tǒng)開展了生物基高性能苯并噁嗪樹脂的研究工作。
2.1黃酮類生物基苯并噁嗪樹脂
黃酮類化合物是一類廣泛存在于藍(lán)莓、蔬菜、茶葉、香料以及豆制品中的有機(jī)酚化合物[56-59],按結(jié)構(gòu)可分為二氫黃酮(如橙皮苷和柚皮素(NAR))、黃酮醇(如槲皮素和山奈酚)、黃酮(如芹菜素(API)和木犀草素)、異黃酮(如大豆素和染料木素)以及花色素(如飛燕草素和芍藥素)[60]。該類化合物含有一種類似于脫氧安息香的獨(dú)特化學(xué)結(jié)構(gòu),能夠在燃燒中迅速碳化成致密的碳層,因此有助于提升苯并噁嗪樹脂的熱穩(wěn)定性和阻燃性[61]。此外,黃酮類化合物結(jié)構(gòu)中含有羥基、羰基、醚等基團(tuán),不僅具有構(gòu)建氫鍵的潛力,還賦予其抗菌、抗炎、抗病毒、抗氧化等多種生理活性,在醫(yī)學(xué)和健康領(lǐng)域有著廣闊的應(yīng)用前景[62-65]。
圖4(a)鄰位酰亞胺苯并噁嗪的阻轉(zhuǎn)異構(gòu)機(jī)理[39l;(b)多種含有阻轉(zhuǎn)異構(gòu)構(gòu)象的鄰位酰亞胺苯并嘌嗪單體[40];(c)鄰位酰亞胺苯 并嘌嗪的固化與后固化行為[41l;(d)含有降冰片烯和炔基的鄰位酰亞胺苯并噁嗪樹脂[4] Fig.4(aMechsmoftroomezatiothideeoe39l;(ierettysoftdebezaeatos; (c)Curingandpostcurigbhaviorofortoamidebenzoxazine41]; (dorth-midebenzoxaziecontainingnorboeneandaky4]
基于黃酮類化合物的特點(diǎn),本團(tuán)隊(duì)首先以二氫黃酮類三酚——柚皮素為酚源合成了生物基苯并噁嗪單體(NAR-fa),研究表明柚皮素中的羰基可與其鄰位羥基形成穩(wěn)定的分子內(nèi)氫鍵,賦予該苯并嚏嗪潛伏自催化固化的特性,固化溫度低至 166°C ;將其作為改性劑添加到商業(yè)化雙酚A型苯并噁嗪及白藜蘆醇基苯并噁嗪樹脂中,可顯著降低樹脂固化溫度,同時(shí)有效提升樹脂的耐熱性與阻燃性[6]。鑒于芹菜素與柚皮素結(jié)構(gòu)相似且兼具碳碳雙鍵及分子內(nèi)氫鍵的特征,將芹菜素與糠胺(fa)合成了新型全生物基雙苯并嚏嗪(API-fa)[67]。苯并吡酮和API-fa中的碳碳雙鍵以及呋喃環(huán)能夠形成額外的交聯(lián)網(wǎng)絡(luò),可顯著提高生物基苯并噁嗪聚合物的熱性能和阻燃性能:其 Tg 高達(dá) 376°C ,在 800°C 下的殘?zhí)悸?(Yc) 為 66% ,微型量熱燃燒法(MCC)測試數(shù)據(jù)顯示該材料的放熱速率(HRR)僅為 20.2J/(g?K) ,總釋放熱(THR)為 9.4kJ/g 。研究表明,基于白楊素(CHR)制備的生物基聚苯并噁嗪中存在顯著的分子內(nèi)氫鍵作用[8],特別是 OH…N 鍵可以保護(hù) -OH 基團(tuán)不與水相互作用,使聚合物涂層展現(xiàn)出優(yōu)異的疏水性[6.70]。本團(tuán)隊(duì)另一項(xiàng)研究表明,白楊素與炔胺合成的生物基苯并噁嗪作為綠色添加劑具有顯著優(yōu)勢:可以有效降低環(huán)氧樹脂和雙馬來酰亞胺體系的聚合溫度,同時(shí)提升其熱穩(wěn)定和阻燃性能[71]。作為一種結(jié)構(gòu)獨(dú)特的黃酮類化合物,山奈酚(KAE)與芹菜素相比具有顯著差異:其4個(gè)羥基中有2個(gè)與酮基相鄰且其中一個(gè)為非酚羥基,與糠胺結(jié)合后會(huì)形成五元或六元?dú)滏I環(huán),進(jìn)而合成富含氫鍵的苯并嘌嗪樹脂。基于該樹脂基體制備的碳纖維增強(qiáng)復(fù)合材料展現(xiàn)出優(yōu)異的性能,與化石基苯并噁嗪復(fù)合材料相比,其 Tg 、抗拉強(qiáng)度和楊氏模量分別提提升 108% 1 28% 和 82.7% ,在交通運(yùn)輸和航空航天領(lǐng)域顯示出巨大的應(yīng)用潛力[72]。本團(tuán)隊(duì)最近以7-羥基黃酮(HYD)、白楊素、芹菜素、木犀草素(LUT)等合成單、雙以及三官能苯并噁嗪單體,探究了噁嗪環(huán)數(shù)量和氫鍵對(duì)聚合物固化機(jī)理和熱性能的影響,進(jìn)一步證明含有分子內(nèi)氫鍵的黃酮基苯并噁嗪具有潛在的自催化作用,從而能夠降低固化溫度,且其固化溫度與苯并噁嗪單體中噁嗪數(shù)量成反比[73]。典型的含分子內(nèi)氫鍵的黃酮基苯并噁嗪單體的結(jié)構(gòu)和相應(yīng)的潛伏催化固化機(jī)理如圖5所示,相關(guān)的熱性能與阻燃性能數(shù)據(jù)匯總于表2。
表1鄰位酰胺和鄰位酰亞胺聚苯并噁嗪和聚苯并嘌唑的熱性能與介電性能
Table1Thermalanddielectric propertiesofortho-amide andortho-imidepolybenzoxazine and polybenzoxazol
圖5(a)含分子內(nèi)氫鍵的黃酮基苯并嘌嗪單體結(jié)構(gòu)示意圖;(b)以山奈酚基苯并嘎嗪單體為例的分子內(nèi)氫鍵潛伏催化固化機(jī)理[72] Fig.5(a)Structuraldiagramofflavonoid-basedbenzoxazinemonomerscontainingintramolecularhydrogenbond;(b)ntramolecular hydrogen bond latent catalytic curing mechanism of kaempferol-based benzoxazine monomer[72]
表2黃酮基苯并噁嗪樹脂的熱性能與阻燃性能
Table 2Thermal properties and flame retardancy of flavonoid-based benzoxazine resins
2.2香豆素類苯并噁嗪樹脂
存在于種子、根和葉的次生代謝物中的香豆素及其衍生物是用于制備熒光化學(xué)傳感器[74]和防污涂料[55]等高分子材料的家族型天然化合物,4-甲基-7-羥基香豆素[75,76]和7-羥基香豆素[77]合成單官的苯并噁嗪樹脂的相關(guān)研究工作此前也有報(bào)道。Sini等[78研究表明,相比于雙官或者多官的苯并噁嗪樹脂,單官的苯并噁嗪樹脂往往交聯(lián)密度低,存在大量的結(jié)構(gòu)缺陷,故而在熱穩(wěn)定性方面稍遜一籌。本團(tuán)隊(duì)以秦皮乙素(6,7-二氫香豆素)制備了雙官生物基苯并噁嗪單體,經(jīng)高溫固化后形成的樹脂體系交聯(lián)密度增大, Tg 可以達(dá)到 261‰ ,熱穩(wěn)定性得到顯著提升( Td10 和 Yc 分別為 407‰ 和 57%[79] ),而且新獲得的苯并噁嗪樹脂在無鹵、無磷的條件下也表現(xiàn)出UL-94V-0 級(jí)別的優(yōu)異阻燃性,尤其最高HRR僅為 2.51J/(g?K) (圖6(a))。此外,二氫香豆素憑借異常靈活的分子設(shè)計(jì)能力與酪胺制得了生物基雙苯并噁嗪[80]。該方法不僅簡單環(huán)保,而且得到的富含氫鍵的生物基聚苯并噁嗪具有優(yōu)異的熱穩(wěn)定性,其黏附強(qiáng)度(圖6(b))和低表面能特性也極為出色。此外,含香豆素的苯并噁嗪在 365nm 波長紫外光照下能誘導(dǎo)產(chǎn)生 [2π+2π] 環(huán)加成反應(yīng),產(chǎn)生新的光聚體(圖6(c)),進(jìn)而增強(qiáng)材料的耐熱性能[81]。
圖6香豆素類苯并噁嗪樹脂優(yōu)異的(a)阻燃性[79],(b)黏附強(qiáng)度[80]以及(c)光、熱固化機(jī)理[81]Fig6Excelnt(a)eretadntl,siogthndlgt,talrigmhnisofouriois
3苯并噁嗪樹脂的功能化應(yīng)用
近年來,研究者們已成功開發(fā)了一系列適用于不同領(lǐng)域的功能性苯并噁嗪材料,涵蓋阻燃材料[3]、抗菌材料[82]、高效碳吸附材料[83]以及高性能摩擦材料[84]等。
3.1苯并噁嗪/石墨烯復(fù)合材料
聚苯并噁嗪本身存在脆性高、電絕緣性強(qiáng)以及熱傳導(dǎo)性能差等問題[85-87],因而僅依靠調(diào)控苯并噁嗪單體結(jié)構(gòu)和聚合方法難以滿足其在導(dǎo)電、導(dǎo)熱等應(yīng)用中的性能需求。目前,通過化學(xué)或物理手段將相應(yīng)功能性納米材料引入苯并噁嗪基體以構(gòu)筑復(fù)合材料,是一種提升其傳導(dǎo)性能的有效途徑。
受到磚泥結(jié)構(gòu)的啟發(fā),本團(tuán)隊(duì)在一種含有鄰位酰胺結(jié)構(gòu)單體的基礎(chǔ)上引入呋喃基團(tuán)增強(qiáng)基體與石墨烯之間的結(jié)合力,即利用呋喃官能團(tuán)中的共軛二烯與石墨烯上的碳碳雙鍵發(fā)生Diels-Alder反應(yīng)并通過真空過濾技術(shù)實(shí)現(xiàn)石墨烯與苯并嘎嗪復(fù)合材料的有效組裝,成功制備出如圖7(a)所示的層狀結(jié)復(fù)合薄膜,幾乎每一層石墨烯均平行于復(fù)合薄膜片材的表面,且相鄰石墨烯層沿面內(nèi)方向呈現(xiàn)高度取向性[83]。這種排列方式顯著增大了接觸面積,同時(shí)固化后聚合物支撐的石墨烯結(jié)構(gòu)促進(jìn)了致密網(wǎng)絡(luò)的形成,為聲子傳播提供了低熱阻路徑,極大地提升了熱傳導(dǎo)效率。含有鄰位酰胺結(jié)構(gòu)的苯并噁嗪化合物在熱活化固化過程中能夠發(fā)生獨(dú)特的結(jié)構(gòu)重排,生成具有較低介電常數(shù)和損耗因子的聚苯并噁唑[44],顯著增強(qiáng)聚合物基質(zhì)的剛性(圖7(b))。當(dāng)石墨烯質(zhì)量分?jǐn)?shù)僅為 46.8% 時(shí),復(fù)合薄膜的面內(nèi)熱導(dǎo)率可高達(dá) 39.2W/(m?K) ,顯著優(yōu)于純苯并噁嗪樹脂的熱導(dǎo)率 (0.31W/(m?K)) 。此外,復(fù)合薄膜還具有優(yōu)異的電磁屏蔽性能,層層堆疊的石墨烯片顯著增強(qiáng)了電磁波的層間反射,而致密的磚/砂狀微觀結(jié)構(gòu)則大幅提升了材料的有效表面積,從而構(gòu)建了一道高效的電磁波屏障(圖7(c)),因此,聚苯并噁嗪/石墨烯薄膜展現(xiàn)出高達(dá) 1.04dB/μm 的電磁干擾(EMI)屏蔽效率。
圖7基于真空過濾的聚苯并噁嗪/石墨烯導(dǎo)熱復(fù)合材料:(a)復(fù)合材料薄板的導(dǎo)熱系數(shù);(b)后固化過程中聚苯并嘌嗪結(jié)構(gòu)到聚 苯并嘎唑結(jié)構(gòu)的轉(zhuǎn)換;(c)EMI屏蔽機(jī)理[83] Fig.7Polybezoxazierapnetealconductivityompositebasdonvacmfiltratio:(a)ealcoductivityofcompoitet; (b)Conversion ofpolybenzoxazine structure to polybenzoxazole structure during postcuring;(c)EMI shielding mechanism[83]
聚苯并噁嗪/石墨烯薄膜經(jīng)進(jìn)一步熱處理后,聚苯并噁嗪成功完成了結(jié)構(gòu)重排并轉(zhuǎn)變?yōu)楦叨裙曹椀木郾讲f唑[83]。這一轉(zhuǎn)變顯著增強(qiáng)了聚合物與石墨烯之間的 π-π 相互作用,從而使復(fù)合薄膜的面內(nèi)熱導(dǎo)率從39.2W/(m?K) 進(jìn)一步提升至 47.8W/(m?K) 。此外,該轉(zhuǎn)化過程中形成了更密集交聯(lián)的網(wǎng)絡(luò)結(jié)構(gòu)和更小的石墨烯層間距,這種結(jié)構(gòu)變化不僅強(qiáng)化了電磁波的反射與散射效應(yīng),而且顯著提高了能量損耗,實(shí)現(xiàn)了更高的電磁屏蔽性能,從而使得聚苯并噁唑/石墨烯薄膜的電磁干擾屏蔽效率進(jìn)一步提升至 1.19dB/μm 。
3.2苯并噁嗪基激光誘導(dǎo)石墨烯
隨著機(jī)械系統(tǒng)對(duì)能效和耐用性的要求不斷提高,越來越多的潤滑劑被應(yīng)用于減少摩擦和磨損。潤滑劑的應(yīng)用不僅可以顯著提高機(jī)械系統(tǒng)的能源利用效率,還能有效延長部件的使用壽命。石墨烯材料因其獨(dú)特的低摩擦特性和優(yōu)異的抗磨損性能而備受關(guān)注,其高導(dǎo)熱性能夠快速散失滑動(dòng)產(chǎn)生的熱量,從而進(jìn)一步提升系統(tǒng)的穩(wěn)定性。然而,對(duì)于油基潤滑劑而言,如何在無需復(fù)雜化學(xué)改性的前提下實(shí)現(xiàn)超細(xì)碳質(zhì)顆粒的良好分散仍然是一個(gè)亟待解決的問題。
在此背景下,本團(tuán)隊(duì)通過“激光直寫技術(shù)\"直接在聚合物表面制備得到三維多孔石墨烯碳材料激光誘導(dǎo)石墨烯(LIG),基本實(shí)現(xiàn)從生物質(zhì)碳化合物到生物基苯并噁嗪,再到功能性碳材料轉(zhuǎn)變的路徑探索(圖 8(a,b))[84] 目前,已有大量關(guān)于利用苯并噁嗪樹脂制備LIG的研究報(bào)道,這些研究主要集中在光熱材料、超疏水材料和電磁屏蔽材料等領(lǐng)域的應(yīng)用[889]。本團(tuán)隊(duì)首次探討了此類LIG在摩擦材料中的潛在應(yīng)用[90],所得的LIG具有獨(dú)特的三維多孔結(jié)構(gòu)和疏水表面(圖8(c)),與未經(jīng)改性的其他碳基潤滑劑添加劑(如氧化石墨烯、單壁碳納米管、炭黑和還原氧化石墨烯)相比展現(xiàn)出更優(yōu)的分散性能,將 w=0.1% 的LIG添加到聚 a 烯烴基礎(chǔ)油(PAO4)中時(shí),PAO4的摩擦系數(shù)降低了約 22% ,磨損率系數(shù)降低了約 95% ,同時(shí)實(shí)現(xiàn)了優(yōu)異的減摩和抗磨效果。然而,在聚合物基底前驅(qū)體上形成LIG,將會(huì)極大地限制其在實(shí)際應(yīng)用中幾何形狀和尺寸的可控性。因此,本團(tuán)隊(duì)還基于液體生物基苯并噁嗪單體通過高能激光的光熱效應(yīng)有效促進(jìn)苯并噁嗪表面C一O和 c-N 鍵的斷裂,實(shí)現(xiàn)碳原子重新排列進(jìn)而生成一種3D多孔LIG潤滑添加劑[90]。與聚合物基底類似,由液體單體制備的LIG表面展現(xiàn)出豐富的多孔結(jié)構(gòu)和褶皺特征(圖8(d))。LIG參與了邊界潤滑膜的形成,顯著縮短了磨痕的寬度和深度。當(dāng) w(LIG)=0.5% 時(shí),PAO4潤滑劑的平均摩擦系數(shù)和磨損率比未添加LIG時(shí)分別降低了 52% 和 97% 。即使摩擦?xí)r間延長一倍,PAO4/LIG潤滑劑仍表現(xiàn)出穩(wěn)定的減摩抗磨性能,充分體現(xiàn)了三維多孔LIG作為潤滑添加劑的長壽命特性。
圖8基于苯并嘎嗪激光誘導(dǎo)的石墨烯:(a)從生物碳資源向功能碳材料的轉(zhuǎn)化[84];(b)LIG的制備;(c)基于聚苯并嚏嗪的LIG;(d)基于液體生物基苯并嘌嗪單體的LIG[90]
Fig.8Laser-ducedgraeebasedonbezoxaie:(a)ransfoationfrobiologicalcarbonsoucs tofunctioalcarbomteal; (b)Preparation of LIG; (c)LIG based on polybenzoxazine; (d)LIG based on liquid bio-based benzoxazine monomer[90]
總結(jié)與展望
本文全面綜述了團(tuán)隊(duì)近十年來在高性能苯并噁嗪樹脂領(lǐng)域的探索與部分研究成果。通過靈活設(shè)計(jì)的分子結(jié)構(gòu)構(gòu)筑分子內(nèi)氫鍵,成功制備出兼具低溫固化、高熱穩(wěn)定性、阻燃性及介電性能優(yōu)良的苯并噁嗪熱固性樹脂。基于黃酮類和香豆素類生物質(zhì)合成生物基苯并噁嗪樹脂新體系,更是推動(dòng)苯并噁嗪樹脂朝著高性能、可持續(xù)發(fā)展方向邁進(jìn)了關(guān)鍵一步。此外,苯并噁嗪與石墨烯形成的復(fù)合材料在熱傳導(dǎo)、電子屏蔽和潤滑劑等多功能領(lǐng)域也取得顯著進(jìn)展,而采用激光燒蝕的方法,成功實(shí)現(xiàn)了從樹脂到功能性碳材料的轉(zhuǎn)變。
盡管高性能苯并噁嗪樹脂已取得一些重要突破,但其大規(guī)模商業(yè)化仍面臨諸多挑戰(zhàn),需在分子設(shè)計(jì)、原料來源、功能化應(yīng)用、工業(yè)化生產(chǎn)及樹脂回收再利用等方面進(jìn)行持續(xù)探索與優(yōu)化。具體可優(yōu)化方面如下:(1)分子設(shè)計(jì)優(yōu)化,深人研究樹脂構(gòu)效關(guān)系,精準(zhǔn)調(diào)控分子結(jié)構(gòu)與氫鍵,進(jìn)一步優(yōu)化固化溫度、熱穩(wěn)定性等關(guān)鍵特性,滿足航空航天、電工電子等高端領(lǐng)域的嚴(yán)苛要求;(2)綠色可持續(xù)發(fā)展,拓展生物質(zhì)原料的來源,提升苯并嚏嗪樹脂中生物基含量,突破性能瓶頸,實(shí)現(xiàn)對(duì)石油基產(chǎn)品的有效替代,推動(dòng)行業(yè)可持續(xù)發(fā)展;(3)功能化應(yīng)用,持續(xù)優(yōu)化與功能性納米材料的復(fù)合技術(shù),探索功能材料制備新工藝,拓展其在諸如新能源、生物醫(yī)學(xué)等前沿領(lǐng)域的應(yīng)用;(4)工業(yè)化推進(jìn),著力攻克量產(chǎn)技術(shù)難題,提高生產(chǎn)效率,降低成本,加速科研成果轉(zhuǎn)化,讓高性能苯并嚏嗪樹脂在各領(lǐng)域發(fā)揮更大價(jià)值,為材料科學(xué)發(fā)展注入新活力;(5)回收再利用,在苯并噁嗪結(jié)構(gòu)中引入特定可逆鍵實(shí)現(xiàn)可控降解,在保持高性能的同時(shí),實(shí)現(xiàn)特定條件下的降解和再生利用。
綠色與可持續(xù)發(fā)展是苯并噁嗪樹脂未來發(fā)展的主旋律,而高性能和多功能是苯并噁嗪發(fā)展的基石。不斷總結(jié)苯并噁嗪化合物的結(jié)構(gòu)特征并與性能關(guān)聯(lián),針對(duì)應(yīng)用目標(biāo)設(shè)計(jì)合成功能性的高性能苯并噁嗪樹脂,并實(shí)現(xiàn)特定條件下的降解與再利用,將助力推動(dòng)苯并嘌嗪樹脂的快速發(fā)展。
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(責(zé)任編輯:王吉晶)