高分子结晶理论的新概念与新进展.pdf

32007208231收稿,2007210216修稿;国家自然科学基金(基金号50773082,20574069,50621302,20490220)资助项目;33 通讯联系人,E2mail:scjiang 综述高分子结晶理论的新概念与新进展3温慧颖1,2,4 蒙延峰3 蒋世春133 安立佳1(1中国科学院长春应用化学研究所高分子物理与化学国家重点实验室 长春 130022)(2东北林业大学工程技术学院 哈尔滨 150040)(3鲁东大学化学与材料学院 烟台 264025)(4中国科学院研究生院 北京 100049)摘 要 回顾了传统的高分子结晶成核与生长模型,指出了该模型在应用中遇到的一些问题;同时总结了Strobl根据近年小角X射线散射结果提出的高分子结晶新机理2中介相机理.介绍了Strobl等构建的热动力学图解对熔体、中介相和片晶的转变过程,阐述了各相间的平衡转变温度、潜在的转变热以及表面自由能,说明了处于熔体和晶体之间的中介相的热动力学性质是理解高分子结晶过程的重要依据.关键词 高分子结晶,中介相,预有序,片晶1高 分 子 结 晶 传 统 理 论 Lauritzen2Hoffman(LH)理论 相对于小分子而言,高分子很难得到完美的结晶,只能得到部分结晶的结构,因此通常称之为半结晶高分子.高分子结晶一直是高分子物理领域内具有挑战性的基础问题之一.高度缠绕并互相贯通的高分子链段以及高分子的拓扑连通性质,变成一个有序结晶的过程似乎永远也不会完成.然而,这样的高分子不但完成了结晶过程,而且与那些非聚合物系统相比高分子晶体由于分子链的排列和堆积不同拥有众多的形态结构和与众不同的结晶过程.小分子结晶由成核和生长过程控制.第一步骤是成核过程,当新的胚核尺寸增大到临界值越过成核自由能位垒后,便可以稳定存在并能继续生长.人们把这种尺寸大于某一临界值的胚核称为新相的核心或晶核.下一步骤就是生长过程,在这个过程中不存在明显的位垒.很长时间以来,传统的高分子结晶主导理论由Lauritzen和Hoffman提出,即LH理论14.该理论是建立在小分子结晶的成核与生长理论基础上的,但是长期以来被用来解释高分子的结晶过程.LH理论描述高分子的结晶行为时有一个假设:就是认为作为结晶初始态的非晶态是由一种或者多种组分组成的均相体系.众所周知,一般情况下组成高分子的亚结构单元是折叠链片晶.Hoffman等将折叠链片晶的形成认为是高聚物分子以链序列的方式从各向同性的熔体中直接附在生长面上的过程,是一个一步过程,并且每个序列长度和片层厚度相当.首先,以上一次形成的光滑晶面为基底先结晶上去一段高分子链段,这一过程类似成核过程,为了区别成核过程称之为次级成核,速率为i,该过程是决定晶体生长速率的决定性步骤;然后,沿此所谓的“晶核”向两侧迅速地铺展,速率为g.在这个模型中,一个主要结论就是得到了稳定的最小片层厚度2 F,为折叠链表面能,F为自由能密度(与过冷度T成比例).另一个主要结论就是给出一个简单的生长速率方程G2exp(-KTT),在这里参数K和温度没有关系.经典的成核生长理论认为结晶必须经历先成核而后生长的过程.Hoffman进一步提出Regime Transition模型,如图1所示.在Regime,高温段为成核控制过程(gi),Regime,中温段为成核与生长同时控制的过程,Regime,低温段为生长控制过程,3个Regime区的生长速率存在固定的比例关系.2 传统理论面临的问题在高分子的结晶研究过程中,逐渐出现了一些经典的成核和生长理论难以解释的现象.作为占据高分子结晶领域的主导理论,LH理论被许多第2期2008年2月高 分 子 学 报ACTA POLY MERICA SINICANo.2Feb.,2008107 1994-2008 China Academic Journal Electronic Publishing House.All rights reserved.http:/Fig.1Schematic of Hoffmans regime transitiona)Regimecontrolled by nucleation process;b)Regimecontrolled by nucleation process and growthprocess simultaneously;c)Regimecontrolled bygrowth process工作者质疑并且不断改进57,但是无论怎么样争论,都是建立在同一个前提条件下,即认为在成核和结晶尚未开始前系统是均相的.然而越来越多的研究倾向于认为,在晶体形成之前,高分子链进行着有利于晶体形成的构象调整和取向变化,即存在着一个预有序相.这个与传统成核与生长的一步结晶过程的背离可以追溯到奥斯特瓦尔德的阶段定律(Ostwalds“rule of stages”)8.该定律说明了晶体总是首先形成晶核然后转变为介晶或者结晶的结构,这种转变不同于宏观稳定的晶型改变.该结构是亚稳定性的,具有纳米尺寸的大小.同时,愈来愈多的实验结果也表明了预有序相结构的存在920.Kanig早在1983年就通过TEM观察PE结晶,发现先于结晶片层有预有序相现象的出现9,10.后来Tashiro通过FTIR发现归于预有序相 六方相的谱带先于正交晶相结晶的谱带出现,证明了预有序相的存在.随着晶相谱带强度的增加,预有序相 六方相谱带强度同时发生降低11.最近Li等也通过FTIR发现剪切的iPP熔体中存在构象有序现象17.Hsiao等18,19用同步辐射SAXS和WAXS同时在线研究iPP和PBT的结晶过程,结果发现tSAXStWAXS,即在三维有序晶体形成之前,从熔体结晶的系统中已经出现了密度起伏,显示有序的层状结构.同时对于iPP还发现tLSTacTam.既然晶体的化学势始终低于中介相的化学势,那么中介相在宏观系统中只能是亚稳定的.然而,对于有着纳米尺寸的较小对象,稳定性就可以反转.较薄的中介层有着具有较低的表面自由能故比具有同样厚度的晶粒有着较低的Gibbs自由能.热力学动力学关联了3个转变温度Tam,Tac,Tmc分别对应着热转变hca=ha-hc和hma=ha-hm.由于gam和gac可以通过热熵变化sma=sa-sm和sca=sa-sc表示.可以如下表示:(Tmc-Tac)sca=(Tmc-Tam)sma(4)Fig.7Temperature dependencies of the bulk chemicalpotentials ofthemesomorphic(hexagonal)andthecrystalline(orthorhombic)phaseThe potentials are referred to the chemical potential of themelt and denotedgamandgacrespectively43.即:hmahcasmasca=Tmc-TacTmc-Tam(5)图8是热力学动力学示意图,其中有两条没有被共聚物单元或立构整规性影响的结晶线和重结晶线,还有一条熔融线.包含了4个不同的相,熔体、中介层(m)、原始晶(cn)和稳定晶体(cs).该示意图描绘了4个相的稳定范围和转变线.变量依旧是温度和片层厚度倒数.相转变条件分别为Tmcn、Tacn、Tmcs、Tacs和Tam.需要注意的是图中的交点Xs和Xn,在Xn处中介层、本地晶体和熔体都有着相同的Gibbs自有能,在Xs处对于稳定晶同样适用.Xs和Xn控制熔融之后等温结晶的实施.为了和实验相一致,可以设计两个步骤.在图中分别表示路线A和路线B.路线B通过高温结晶来实现,在开始点,标记为1,这时,分子链开始从熔体中以较小的厚度粘附在中介层的横向面上.这个层开始自发的增厚直到达到一个转变线Tmcn,由此达到了另一个点2,同时本地晶体开始快速增长.随后的稳定过程使它们达到了一个较低的自由能状态.加热时晶粒保持稳定直至上升到与晶体融化相联系的Tacs.路线A即所谓的低温结晶,开始是一样的,链序列粘附到自发增厚的中介层上,然后,一旦到达Tmcn,本地晶开始形成随后稳定.当开始加热稳定晶体的时候,开始时刻他们保持住他们的结构.继续加热,发现Tmcs转变线先于熔融线达到,也就是意味着与路线B直接熔融不同而先会经历中介相的结构.进一步的加1112期温慧颖等:高分子结晶理论的新概念与新进展 1994-2008 China Academic Journal Electronic Publishing House.All rights reserved.http:/热会使中介相持续的重结晶(图83a至83b).最后到达相交点Xs,这时晶体开始熔解.Fig.8(Tn-1)2phase diagram for polymer layers in a melt(“a”)dealing with 3 phases:mesomorphic“m”,native crystalline“cn”and stabilized crystalline“cs”.Two pathwaysfor an isothermal crystallizationfollowed by heating,A(low crystallization temperatures)and B(high crystallizationtemperatures);The experimental“crystallization line”is identicalwithTmcn,the“melting line”is identical withTacs,the“recrystallization line”is to be identified withTmcs43.根据Scherrer方程,片晶的亚结构2小晶块的直径可以由WAXS的衍射线宽来得到,如图9所示.可以看到不同的sPP及其共聚物的所有D-1200(T)处在一条共同线上,这如同sPP结晶线一样延伸出去终止于Tc=195.同时,对结晶过程进一步研究发现,温度控制的结晶过程的特征时间符合定律lg T,表示特征时间,例如到达最终结晶度一半所需要的时间.随着温度变化的关系可以理解成为晶体的生长也是如同成核步骤一样与一个活化位垒有关.由实验研究得到球晶生长速率对温度的依赖性的关系可以利用方程(6)表示.u=u0exp-TAT-TVexp-TGTzero-T(6)在实验的温度范围内起到决定作用的是第二个指数因子.Strobl利用中介相模型对方程(6)中第二个指数因子由温度决定的活化位垒的本质加以研究,并且回答了零生长速率Tzero的表示意义.图2的多步模型包含了如下几个活化位垒,首先,链序列往中介层生长面上粘附过程应该是占主导的,有的实验结果支持这个假设.在序列生成前的熔体是线团状态,为了粘附就需要一个活化过程来使之转变成全面伸直的形态.中介层和晶体不同之处是容许构象的变化.这个伸直长度最少要达到中介层的初始厚度值.序列中单体的数量n3,由下面的公式来决定.n3=2amTamhma1Tam-T(7)因为链的伸直导致了与链序列长度有关的熵的降低,由此需要引入了一个熵活化位垒:-Skn3(8)位垒转变发生的概率如下:expSk=exp-ConstTam-T(9)假如Tzero与Tam一致,这和方程(6)的实验结果一致.因此,到达Tam的距离就是控制高温区高分子结晶生长速率的因素.Fig.9Different samples ofsPP(sPP,sP(P2co2O)x,andthe commercial FinasPP)crystallized at various temperaturesTc:crystallization lined-1cversusTcdetermined by SAXS(from Ref 32)and lateral coherence lengthsD200derivedfrom the line width of the 200 reflection(filled symbols)444 结语近年来关于高分子结晶机理的争论成为热点,逐渐出现了传统的成核和生长模型不能解释的一些现象.Strobl借助于一些新的仪器和手段,得到了有关高分子结晶的一些新结果,并且提出了高分子结晶的新模型.虽然Strobl所提出的中介相的高分子结晶模型还没有被人们广泛地接受,但是其对于传统高分子结晶理论的挑战有目共睹.目前,Strobl提出的中介相模型引起了新的热点和讨论,对高分子结晶理论和现代高分子物理的发展产生了很大影响.在Strobl提出的中介相新模型的基础上,Sirota讨论了片层的粒状结构就是瞬时中介层的“手印”,其不断增厚逐渐达到高密度状态,而中介相的低界面能就是由于低密211高 分 子 学 报2008年 1994-2008 China Academic Journal Electronic Publishing House.All rights reserved.http:/度产生45.最近又有人讨论了中介相形成高分子的两种模型2低温结晶和高温结晶来讨论中介相在高分子结晶过程中的作用46.他们提出,只有低温结晶才符合奥斯特瓦尔德的阶段定律定律(Ostwalds rule).Cho等认为存在一个中间层介于结晶和熔融体之间,并且导致了最后的结晶有序.通过重结晶进一步证明了中介相在结晶过程中的重要作用47.我们工作组在对sPP剪切结晶的SAXS的研究过程中发现,在微晶尺度上和片晶尺度上的取向程度有着很大的差别,我们认为这是中介相的作用导致,中介层在剪切的作用下取向使得片晶尺度上的取向相对明显48.在目前高分子结晶的研究阶段,认为高分子结晶是一个存在中介亚稳定性相的多步骤过程是一个全新的尝试,它从全新的角度阐释了高分子结晶过程.REFERENCES1HoffmanJ D,Lauritzen J IJr.J Res Natl Bur 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A,Prez E.Macromolecules,2007,40:1775177847Cho T,Heck B,Strobl G.Chinese J Polym Sci,2007,25(1):839448Wen H Y,Jiang S C,Men Y F,Okuda H,An L J.Chinse J Polym Sci,2008,(2):153160THE NEW CONCEPTS AND NEW DEVELOPMENTS IN HTE THEORY OFPOLYMER CRYSTALLIZATIONWEN Huiying1,2,4,MENG Yanfeng3,JIANG Shichun1,AN Lijia1(1State Key Laboratory of Polymer Physics and Chemistry,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun130022)(2College of Engineering and Technology,Northeast Forestry University,Harbin150040)(3School of Chemistry&Material Science,Ludong University,Yantai264025)(4Graduate School of the Chinese Academy of Sciences,Beijing100049)AbstractRecently a debate about the initial crystallization process which has not been the hotspot for a long timesince the theory proposed by Hoffman2Lauritzen(LH)dominated the field arose again.For a long time the Hoffman2Lauritzen model was always confronted by criticism,and some of the points were taken up and led to modifications,but the foundation remained unchanged which deemed that before the nucleation and crystallization the system wasuniform.In this article the classical nucleation and growth theory of polymer crystallization was reviewed,and theconfusion of the explanations to the polymer crystallization phenomenon was pointed out.LH theory assumes that thegrowth of lamellae is by the direct attachment of chain sequences from the melt onto smooth lateral sides.Whereas,more and more experimental results show that before the crystallite appears polymer chains go through conformationadjustment and orientation transformation which favor the formation of crystallites,i.e.a preordering structure existsbefore or at the early stage of polymer crystallization.This is a deviation from the simple process of nucleation andgrowth at the early stages of melt crystallization which can be gone back to Ostwalds“rule of stages”.Stroblsmulti2step crystallization mechanism based on recent SAXS results was summarized in the paper.The theory assumesfor the formation of polymer crystallites,a passage through a transient mesomorphic phase.It is proposed that theinitial step is always the creation of a mesomorphic layer which starts with an attachment of chain sequencesfrom themelt onto a growth face of a mesomorphic layer with minimum thickness and spontaneously thickens,up to a criticalvalue,where it solidifies through a cooperative structural transition.The transition produces a granular crystallinelayer,which transforms in the last step into homogeneous lamellar crystallites and this granular substructure isobserved for many semicrystalline polymers.Crystallization of polymers is controlled by many factors which compriseof the variation of the main structural parameters such as lamellar thickness,long spacing,crystallinity with thecrystallization temperature,the structural changes during recrystallization processes and the temperature dependenceof growth ratesetc.T o expatiate upon the transforming process among melting,mesophase and lamellae athermodynamics scheme was introduced.Many experiments indicate the existence ofmesophase,and thethermodynamic framework by Strobl is generally feasible for many situations.Indeed,it is the interference ofmesophase which generally dominants polymer crystallization by the selection of the lamellar thickness.Sirota arguedthat the granular structure of the lamellae was itself a fingerprint of the transient mesophase into which a lamella411高 分 子 学 报2008年 1994-2008 China Academic Journal Electronic Publishing House.All rights reserved.http:/originally grew,and it was then converted to the higher2density stable form;and the lower interfacial energy of themesophases is connected with their lower density.Based on Strobls suggestion two crystallization modes ofmesophase forming polymers have been offered which were partitioned by crystallization rates and only the secondcase by high rates was deemed to apply to the Ostwalds rule of stages.In conclusion,it is very important to newlyunderstand the polymer crystallization process for the thermodynamical properties of the mesophase between meltingand crystallite.KeywordsPolymer crystallization,Mesophase,Precursor Structure,Lamellae5112期温慧颖等:高分子结晶理论的新概念与新进展 1994-2008 China Academic Journal Electronic Publishing House.All rights reserved.http:/