(College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,China)
DOI: 10.6043/j.issn.0438-0479.201709014
备注
为了探测聚合物材料受力时的损伤和断裂机理,延长材料的使用寿命和耐久度,将一种基于罗丹明衍生物的力致变色力敏团共价连接到聚甲基丙烯酸甲酯(PMA)中,并通过溶液超声、固体压缩、紫外-可见吸收光谱和荧光光谱对其力致变色性能进行了表征.结果显示,含罗丹明力敏团的PMA受力后从无色转变为红色,其550 nm处的紫外-可见吸收峰和567 nm处的荧光发射峰强度均明显提升,这种转变可通过加热(70 ℃)恢复到初始状态,且能够多次重复.以上实验结果可通过外力诱导的罗丹明力敏团开环反应和加热导致的逆反应加以解释.结果表明合成的罗丹明力敏团是一类较为理想的损伤探测力敏团,可用于力响应材料的开发.
Stress-sensing in polymeric materials is important to prevent destructive failure,interpret failure mechanism,and improve the lifetime and durability of the product.In this study,a mechanochromic mechanophore based on a rhodamine derivative was synthesized,and the mechanophore was covalently linked to poly(methyl acrylate)(PMA).Then its mechanochromic properties were investigated by ultrasound sonication,compression,UV-vis and fluorescence spectroscopies.Under mechanical force,the rhodamine-containing PMA changed from colorless to pink(activation),accompanying the increase of absorption at 550 nm and fluorescence intensity at 567 nm,while the pink color faded upon heating at 70 ℃,and such an activation-fading of color were repeatable.These results could be interpreted as a consequence of force-induced ring-opening and heat-induced ring-closure reactions of rhodamine mechanophore.The results showed that the rhodamine mechanophore could be an ideal force-probe for stress-sensing applications and stress-responsive polymers.
引言
力敏团是一类受到机械力作用后能够选择性地发生特定化学反应和(或)物理转变的基团、复合体或聚集体,也是制备力响应智能材料的重要组件[1-2].高分子力化学是以高分子链为介质研究高分子骨架内力敏团受外力刺激响应的科学[3-5].近年来,高分子力化学得到前所未有的关注,力敏团以及具有特异力响应功能(包括机械力诱导的变色[6-17]、发光[18-19]、交联[11,20-22]、应力缓解[23-25]、小分子释放[26-29]等)的聚合物材料大量出现.在诸多力响应特性中,力致变色(包括颜色、吸收、荧光变化中的一种或多种)和力致发光可将力敏团所受的机械能转变为光学信号.这种特性可用于探测材料的受力情况并预报潜在的损伤,同时也是从分子尺度研究聚合物材料断裂机理的有力工具[30-31].此外,具有力致变色功能的材料还可用于制备各种力感应和显示的器件,应用于记忆存储、安全通信、人体运动检测等领域.
力致变色力敏团的力响应原理是利用力化学反应和(或)物理转变生成具有特殊光学性质的产物(或中间体),包括体系共轭结构的改变[6,11,14,16]、体系基态-激发态分布的改变[19]、特殊自由基结构的生成[17,22]以及超分子聚集体结构的改变[32-33].Davis等[6]首次将具有力致变色特性的螺吡喃应用到高分子力化学研究中.本课题组也在近期合成了具有不同连接位点的螺吡喃力敏团,并与超分子作用相结合,获得了同时具有共价和非共价力敏团的力响应高分子体系,进而深入研究了共价和非共价力敏团之间力响应的相关性[9-10,12-13].近期,本课题组合成了一个螺噻喃力敏团,同时具有力致变色和力致交联的特性[11].然而螺噻喃活化所需外力远低于高分子主链断裂所需外力,因此本课题组转向与螺吡喃具有相似结构的罗丹明衍生物的研究.
本文中合成了一种基于罗丹明衍生物的力致变色力敏团,并以其合成活性自由基引发剂,进而制备聚合物链中部具有单个力敏团的高分子链,并对合成的聚合物开展溶液和固体力化学研究.
1 实验部分
2 结果与讨论
2.1 罗丹明力敏团溶液的力化学反应采用GPC测得所合成聚合物的分子质量分别是64,45和20 ku,其GPC折光指数(RI)流出曲线如图2所示.不同分子质量的聚合物将用于研究该类聚合物力化学反应的分子质量依赖性和热效应对其结构的影响.
(a)不同超声时间的聚合物溶液;(b)紫外-可见吸收光谱;(c)荧光光谱(激发波长550 nm);(d)力作用下的开环反应.
图3 超声前后PMA-Rho-PMA(64 ku)溶液的颜色和光谱变化及力作用下的开环反应
Fig.3 Colour and spectra of PMA-Rho-PMA(64 ku)solutions before and after sonication,and force-induced ring-opening reaction
图2 不同分子质量PMA-Rho-PMA的GPC的RI流出曲线
Fig.2 GPC RI reflex trace of PMA-Rho-PMA with different molecule masses将PMA-Rho-PMA(64 ku)的四氢呋喃溶液进行超声处理,检测罗丹明力敏团在稀溶液中的力化学性能.由图3可以看出,随着超声的进行溶液颜色逐渐从无色转变为粉红色并不断加深(图3(a)),相应的紫外-可见吸收光谱在516和550 nm处出现了特征吸收峰(图3(b)),而荧光光谱在567 nm处出现了特征峰(图3(c)).超声过程中溶液内部产生空泡,当气泡破裂时会在溶液中产生瞬间的强拉伸流场,造成聚合物分子链的拉伸[2].以上结果表明在外力作用下,罗丹明力敏团发生了活化,导致体系颜色改变.由于酸可以使罗丹明发生开环反应并且开环产物结构已知[37],所以在初始聚合物溶液中加入盐酸并检测其荧光光谱.实验结果显示酸化产物的荧光特征峰与超声产物的完全一致(图3(c)),表明罗丹明力敏团在外力作用下发生了开环反应(图3(d)),从而引起体系颜色变化.
同样地,将PMA-Rho-PMA(45 ku)和PMA-Rho-PMA(20 ku)溶液进行超声实验.如图4所示,超声后在PMA-Rho-PMA(45 ku)溶液中可以观测到罗丹明力敏团的活化,而PMA-Rho-PMA(20 ku)溶液的紫外-可见吸收光谱则没有显著变化.这表明罗丹明力敏团的力化学反应与聚合物链长呈明显的正相关性[38],含有罗丹明力敏团的聚合物其主链必须大于特定长度,才能够有效地将溶液流场产生的拉伸应力传递给分子内的罗丹明力敏团,并进一步引发活化[39].此外,为了排除超声过程中伴随的热效应对罗丹明力敏团开环反应的影响,将聚合物溶液加热到70 ℃并保持3 h,此时其紫外-可见吸收光谱没有发生变化,表明热效应无法使得罗丹明力敏团发生开环反应.
(a)超声-室温循环;(b)超声-加热循环;(c)聚合物溶液超声后在70 ℃下加热1 h.
图5 PMA-Rho-PMA(64 ku)溶液超声-室温循环、超声-加热循环的紫外-可见吸收以及溶液颜色变化
Fig.5 UV-vis absorbance of PMA-Rho-PMA(64 ku)solution during sonication-room temperature cycles and sonication-heating cycles and the color changes
图4 PMA-Rho-PMA(20 ku)和PMA-Rho-PMA(45 ku)超声前后的紫外-可见吸收光谱
Fig.4 UV-vis absorbant spectra of PMA-Rho-PMA(20 ku)and PMA-Rho-PMA(45 ku)before and after sonication进一步研究此力化学反应的动力学.通过检测PMA-Rho-PMA(64 ku)溶液在550 nm处的吸光度(A550)可以得到反应速率常数.假设在超声时间范围内逆反应可以忽略(闭环反应,理由见下节)并且开环反应遵循一级反应动力学,即
At=A0(1-exp(-kt)),(1)
其中,A0和At分别是体系在550 nm的初始吸光度和超声t min后的吸光度,k是高分子链中罗丹明力敏团力化学开环反应的表观反应速率常数.拟合结果得到k=8.0×10-3 min-1.k的数值与聚合物分子质量相关,分子质量越大,反应速率常数越大[40].
2.2 罗丹明力敏团力化学反应的可逆性分析采用PMA-Rho-PMA(64 ku)进行超声-加热循环实验,以考察其受力开环反应的可逆性.
将PMA-Rho-PMA(64 ku)溶液超声10 min后室温放置10 min,其紫外-可见吸收光谱在室温放置后并没有明显改变.第一次超声后其A550为0.037; 室温放置10 min后略微上升到0.040; 重复以上步骤3次,发现其A550在室温放置后均变化不明显,如图5(a)所示.PMA-Rho-PMA(64 ku)溶液超声10 min后,其A550从0升至0.037; 停止超声,并将溶液在70 ℃下加热10 min后,A550降至0.029; 重复以上步骤3次,可看到PMA-Rho-PMA(64 ku)溶液的A550在超声后上升而在加热后下降,如图5(b)所示.以上结果表明罗丹明力敏团的开环产物在室温下比加热条件下稳定,在室温下需要相当长的时间(大于12 h)才可恢复,而在加热条件下可以快速恢复到初始的闭环状态,并且这一过程可以至少循环3次.此外,活化开环后的罗丹明力敏团于70 ℃加热1 h后可完全恢复到闭环状态,溶液颜色发生明显变化,如图5(c)所示.已有文献[41-43]的研究结果表明罗丹明衍生物的闭环和开环状态可以重复多次.由此表明,PMA-Rho-PMA(64 ku)的力致变色特性可以重复利用.
2.3 罗丹明力敏团的固体力化学反应(a)压缩前聚合物膜;(b)压缩后聚合物膜;(c)加热(70 ℃,1 h)后颜色恢复;
(d)压缩后薄膜在550 nm激发光下的荧光光谱;(e)567 nm处荧光强度随压强的变化.
图6 PMA-Rho-PMA(64 ku)膜压缩前后的状态以及荧光光谱
Fig.6 The state and fluorescence spectra of PMA-Rho-PMA(64 ku)film before and after compression由于力响应材料在实际应用中多处于固态,所以进一步研究了罗丹明力敏团在固体中的活化.将PMA-Rho-PMA(64 ku)进行溶液铸膜,完全干燥后的PMA-Rho-PMA(64 ku)膜为无色透明状态,如图6(a)所示.将聚合物膜用油压机进行压缩以考察罗丹明力敏团在固体当中是否能够被活化.如图6(b)所示,经过200 MPa压缩后的薄膜在接触位置呈现粉红色,经过紫外光照射后有荧光,这表明罗丹明力敏团在固体状态下可以活化开环,出现力致变色现象.此外,对压缩活化后的固体进行加热处理,罗丹明力敏团活化产生的红色褪去(图6(c)),表明固态下开环的罗丹明力敏团通过加热也能够恢复到原来的闭环状态[14].
进一步的定量研究表明,罗丹明力敏团在固体中的活化存在一个压强临界值.如图6(d)所示,薄膜在567 nm处的荧光发射峰强度随着施加压强的增大而增强,并且这种趋势只有在较大的压强下才明显:当压强小于100 MPa时,荧光强度几乎没有变化,表明罗丹明力敏团几乎未活化; 当压强大于100 MPa后,荧光强度显著增强.图6(e)是薄膜在567 nm处的荧光强度随施加压强变化的曲线,可以看出罗丹明力敏团活化的临界压强在200 MPa附近.因此,本研究合成的罗丹明力敏团可以作为一种理想的损伤探测基团,在实际应用中当共价融入聚合物链的罗丹明力敏团发生变色时,就表明体系进入危险区域,需要进行干预以避免灾难性破坏发生.
3 结 论
本研究合成了一种基于罗丹明衍生物的力致变色力敏团,并通过SET-LRP的方法将其引入到高分子链中,随后考察其在溶液和固体中的力化学性质.实验结果表明,含有罗丹明力敏团的PMA分子链必须超过特定长度才能够有效地将溶液流场产生的拉伸应力传递给分子内的罗丹明力敏团,即该力敏团的活化存在分子质量依赖性.该力敏团在受到外力作用后发生开环反应,生成红色产物.该产物在516和550 nm处具有特征吸收峰并且具有荧光(发射峰567 nm).开环反应生成的红色产物在室温下较稳定,需经过较长(大于12 h)时间才可从开环状态缓慢转变为无色的闭环状态,而加热可加快这一过程,同时该修复过程具有可逆性,并可重复多次.另外,在无共价键或超分子交联的PMA固体中也可实现罗丹明力敏团的力致活化,说明该罗丹明力敏团对力十分灵敏,是一种较为理想的损伤探测分子.
1.1 试 剂3-羟基-N,N-二乙基苯胺(纯度98%)、3-氨基苯酚(纯度99%)、2-溴-2-二甲基丙酰溴(纯度98%)、三(2-二甲氨基乙基)胺(纯度99%),购自安耐吉公司; 邻苯二甲酸酐(纯度99%)、乙二胺(纯度99%)、丙烯酸甲酯(纯度98%)、二甲基亚砜(DMSO,无水溶剂级),购自阿拉丁公司; 铜丝(纯度100%)购自阿法埃莎公司; 氮气(纯度99.99%)购自林德气体有限公司; 甲苯、氢氧化钠、盐酸、甲醇、浓硫酸、二氯甲烷、无水硫酸镁、乙醇、四氢呋喃、三乙胺和氯化钠均为分析纯,氘代三氯甲烷(CDCl3,纯度99.8%)、氘代甲醇(MeOD,纯度99.8%)、硅胶(300～400目),购自厦门绿茵试剂玻仪有限公司.
1.2 仪 器Bruker AV500核磁共振(NMR)谱仪(氘代试剂为CDCl3和MeOD,内标物为四甲基硅烷(TMS))、Sonics VC505超声波细胞破碎仪、岛津 UV2550 紫外-可见分光光度计、F7000 荧光光谱仪、Esquire 3000 Plus-ESI 电喷雾离子阱质谱(MS)仪、Waters 凝胶渗透色谱(GPC)仪、上海宝灿油压千斤顶、IKARCT加热磁力搅拌器、IKARV10controlV-C控制型旋转蒸发仪.
图1 罗丹明力敏团和PMA-Rho-PMA的制备
Fig.1 Synthesis of the rhodamine mechanophore and PMA-Rho-PMA1.3 制备合成1.3.1 合成路线设计罗丹明力敏团及含有该力敏团的聚丙烯酸甲酯(poly(methyl acrylate),PMA)聚合物PMA-Rho-PMA的合成路线如图1所示:化合物1、2和3的合成方法
参考文献[34-36]; 化合物3与2-溴-2-二甲基丙酰溴进一步反应得到罗丹明力敏团(化合物4),并以其作为聚合反应的引发剂,通过单电子转移活性自由基聚合(SET-LRP)方法聚合得到PMA-Rho-PMA.
1.3.2 化合物1的合成与表征将3-羟基-N,N-二乙基苯胺(5 g,30.26 mmol)和邻苯二甲酸酐(4.7 g,31.77 mmol)装入具玻璃节门的反应瓶中,用30 mL甲苯溶解.将体系密封,于140 ℃、氮气气氛下回流3 h.室温冷却至50～60 ℃,加入约30 mL 35%(质量分数)的氢氧化钠溶液,于90 ℃下反应6 h.边搅拌边将产物倒入300 mL水中,用10 mol/L的盐酸调节pH至7左右,室温放置2 h,过滤得固体产物.将产物用水洗涤、抽滤、真空干燥,得浅粉色固体(产率67%).1H-NMR(500 MHz,V(CDCl3):V(MeOD)=1:5): δ 1.18(d,J=7.11 Hz,1H),3.41(q,J=6.94 Hz,4H),6.10(d,J=3.87 Hz,1H),6.13(s,1H),6.87(d,J=9.08 Hz,1H),7.34(d,J=7.34 Hz,1H),7.56(t,J=7.68 Hz,1H),7.64(t,J=7.68 Hz,1H),8.06(d,J=7.57 Hz,1H).
1.3.3 化合物2的合成与表征将化合物1(1 g,3.19 mmol)和3-氨基苯酚(0.35 g,3.19 mmol)混合均匀,冰水浴下逐滴加入96%(质量分数)浓硫酸使其溶解,将体系置于100 ℃下反应3 h.反应完成后冷却至室温,将产物倒入100 mL冰水中,强烈搅拌,用饱和氢氧化钠水溶液缓慢调节pH至7左右.用二氯甲烷萃取3次,有机层用无水硫酸镁干燥,过滤,减压旋转蒸发除去溶剂,真空干燥.再用硅胶柱层析分离,洗脱剂为V(二氯甲烷):V(甲醇)=5:1,得到红色固体(产率30%).1H-NMR(500 MHz,MeOD):δ 1.29(t,J=7.11 Hz,6H),3.65(q,J=7.09 Hz,1H),6.75(s,1H),6.77(s,1H),6.90(s,1H),6.99(d,J=9.62 Hz,1H),7.19(d,J=8.84 Hz,1H),7.24(d,J=6.71 Hz,1H),7.27(d,J=9.49 Hz,1H),7.60(t,J=6.55 Hz,1H),7.64(t,J=6.35 Hz,1H),8.08(d,J=8.47 Hz,1H).
1.3.4 化合物3的合成与表征将化合物2(180 mg,0.46 mmol)溶于20 mL乙醇中,将乙二胺(0.2 mL,3.45 mmol)溶于少量乙醇后逐滴加入,体系回流15 h至红色褪去,减压旋转蒸发除去溶剂.粗产物用二氯甲烷溶解,用水萃取3次,有机层用无水硫酸镁干燥,过滤,减压旋转蒸发除去溶剂,真空干燥,得红色固体(产率94%).1H-NMR(500 MHz,CDCl3):δ 1.18(t,J=7.06 Hz,6H),2.42(t,J=6.58 Hz,2H),3.15～3.25(m,2H),3.35(q,J=7.11 Hz,4H),6.28～6.31(m,2H),6.38(d,J=2.50 Hz,1H),6.43(s,1H),6.45(s,1H),6.47(d,J=2.11 Hz,1H),7.07～7.10(m,1H),7.45～7.49(m,2H),7.91～7.93(m,1H).13C-NMR(500 MHz,CDCl3):δ 12.56,40.67,43.71,44.33,64.84,97.76,101.59,105.32,108.38,109.19,111.51,122.82,123.78,128.18,128.65,128.99,131.15,132.51,147.89,148.91,153.03,153.06,153.31.MS(电喷雾电离(ESI)正离子模式),[M]+:m/z=429.2.
1.3.5 化合物4的合成与表征将化合物3(186 mg,0.434 mmol)置于圆底烧瓶中,橡胶塞塞住瓶口,用注射器注入10 mL无水四氢呋喃使其溶解.随后加入三乙胺(0.38 mL,2.778 mmol),将体系置于冰浴中,搅拌下用注射器缓慢滴入2-溴-2-甲基丙酰溴(0.32 mL,2.604 mmol),滴加完毕后继续搅拌反应24 h.减压旋转蒸发除去溶剂,用氯化钠水溶液洗涤2次.再用硅胶柱层析分离,洗脱剂为二氯甲烷,得浅红色固体(产率65%).1H-NMR(500 MHz,CDCl3):δ 1.20(t,J=7.07 Hz,6H),1.27(b,2H),1.95(d,J=2.56 Hz,6H),2.07(d,J=2.89 Hz,6H),2.97～3.01(m,2H),3.36(q,J=7.04 Hz,4H),6.33(dd,J=8.92,2.53 Hz,1H),6.42(d,J=2.51 Hz,1H),6.47(d,J=8.89 Hz,1H),6.67(d,J=8.53 Hz,1H),6.94(dd,J=8.5,2.2 Hz,1H),7.04～7.06(m,1H),7.48～7.50(m,2H),7.71(d,J=2.15 Hz,1H),7.78～7.96(m,1H).13C-NMR(500 MHz,CDCl3):δ 8.71,12.65,32.08,32.48,39.44,41.05,44.47,46.10,60.85,62.66,65.18,98.00,103.95,108.31,108.97,115.32,115.45,123.27,123.88,128.38,128.47,128.66,130.15,133.14,138.80,149.29,152.49,152.95,153.56,169.94,170.22,172.09.MS(ESI正离子模式),[M]+:m/z=749.3.
1.3.6 含有单个罗丹明力敏团的PMA的合成将化合物4(15 mg,0.02 mmol)置于50 mL具玻璃节门的反应瓶中,加入计量的丙烯酸甲酯和等体积的DMSO,搅拌均匀,氮气氛下加入计量长度的铜丝(0.23 cm/mL)与三(2-二甲氨基乙基)胺(Me6-TREN)(1 μL,0.004 mmol).随后密封体系,经过冷冻—抽真空—解冻循环操作3次后,保持真空条件,于室温下反应3 h.待溶液变得黏稠,打开玻璃节门通大气终止聚合反应.反应完成后,加入二氯甲烷稀释溶液,在甲醇中沉淀出聚合物; 再用二氯甲烷溶解,于甲醇中沉淀,反复操作3次后得到半透明沉淀; 收集沉淀,真空干燥,得白色透明聚合物.通过调整化合物4和丙烯酸甲酯的比例可获得不同分子质量的聚合物,以PMA-Rho-PMA(m ku)表示,其中m ku表示分子质量.
1.4 溶液超声实验和表征将10 mL 5 mg/mL的聚合物四氢呋喃溶液置于超声波破碎仪中,氮气鼓泡30 min除去氧气.超声条件如下:温度-5～0 ℃,功率500 W,脉冲间隔 1 s.超声过程中在固定时间点(0,10,30,60,120和180 min)取样,样品一部分进行GPC测试,另一部分稀释后测定其紫外-可见吸收光谱和荧光光谱.
1.5 溶液超声-加热循环实验和表征取10 mL 5 mg/mL的聚合物乙腈溶液进行超声,超声10 min后室温放置10 min并测定其紫外-可见吸收光谱,以上步骤重复3次.另取10 mL 5 mg/mL的聚合物乙腈溶液进行超声,超声10 min后在70 ℃下加热10 min,冷却至室温后测定其紫外-可见吸收光谱,以上步骤重复3次.
1.6 固体压缩实验和表征将聚合物溶于二氯甲烷中,待溶液均匀,将其移入聚四氟乙烯模具中进行溶液铸膜,放于通风处自然干燥3 d,进一步抽真空除去溶剂.聚合物薄膜在油压千斤顶下以不同压强(5,10,50,100,220,300,500,625,700 MPa)压缩10 min,然后测定压缩样品的荧光光谱.
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