利用光学显微镜研究溶剂诱导的梯形苯基聚倍半硅氧烷结晶

利用光学显微镜研究溶剂诱导的梯形苯基聚倍半硅氧烷结晶李培芳1闫寿科2任忠杰2,*(1内蒙古民族大学物理与电子信息学院,内蒙古通辽028043;2北京化工大学材料科学与工程学院,化工资源有效利用国家重点实验室,北京100029)摘要:通过溶剂诱导结晶的方法研究了梯形苯基聚倍半硅氧烷(PPSQ)的球晶结构.采用光学显微镜研究了溶液的浓度、溶剂的挥发时间以及温度等影响因素对梯形苯基聚倍半硅氧烷的球晶结构形貌的影响.实验结果表明:稀释溶液浓度与提高结晶温度在改变球晶的形态方面具有相同的效果.球晶的尺寸会随着溶液浓度的降低或者结晶温度的升高而增加.这都归因于梯形苯基聚倍半硅氧烷在二甲苯中的溶解度的增加.在不改变其它结晶条件的情况下，延长溶剂的挥发时间也会形成更大的球晶.当在光学显微镜下旋转样品时球晶的结构不会发生改变，这表明球晶具有均一的晶体学取向结构.负性球晶的特征也表明梯形苯基聚倍半硅氧烷的分子链是沿着球晶的切线方向排列.考虑到梯形苯基聚倍半硅氧烷具有刚性的分子链，还提出了梯形苯基聚倍半硅氧烷可能的球晶结构模型.关键词:梯形苯基聚倍半硅氧烷;结晶;溶剂诱导;球晶;刚性分子链中图分类号:O641An Optical Microscopy Study on the Solvent-Induced CrystallineMorphology of Ladder PolyphenylsilsesquioxaneLI Pei-Fang 1YAN Shou-Ke 2REN Zhong-Jie 2,*(1College of Physics and Electronic Information,Inner Mongolia University for the Nationalities,Tongliao 028043,Inner Mongolia,P .R.China ;2State Key Laboratory of Chemical Resource Engineering,School of Material Science andEngineering,Beijing University of Chemical Technology,Beijing 100029,P .R.China )Abstract:Spherulites of rigid chain ladder polyphenylsilsesquioxane (PPSQ)were produced through solvent-induced crystallization.The influences of solution concentration,solvent evaporation time,and temperature on the spherulitic morphology of PPSQ were studied by optical microscopy.Diluting the solution was found to have the same effect as elevating the crystallization temperature.The size of the spherulites increases with either decreasing concentration or increasing temperature.This is caused by the enhanced solubility of PPSQ in xylene under such conditions.Extending the solvent evaporation time while leaving the other conditions unchanged also leads to the formation of larger spherulites.The invariance of spherulitic structure upon sample rotation suggests that the spherulites possess a uniform crystallographic orientation.The negative optical characteristics of spherulites indicates that the PPSQ molecular chains are oriented in the tangential direction of the spherulite.Considering the high rigidity of the molecules,an organization model of PPSQ spherulite is proposed.Key Words:Polyphenylsilsesquioxane;Crystallization;Solvent induction;Spherulite;Rigid chain[Article]物理化学学报(Wuli Huaxue Xuebao )Acta Phys.⁃Chim.Sin .2012,28(2),494-498FebruaryReceived:October 11,2011;Revised:November 9,2011;Published on Web:November 23,2011.∗Corresponding author.Email:renzj@;Tel:+86-10-64426375.The project was supported by the National Natural Science Foundation of China (21104002,11164020).国家自然科学基金(21104002,11164020)资助项目ⒸEditorial office of Acta Physico ⁃Chimica Sinicadoi:10.3866/PKU.WHXB201111233494LI Pei-Fang et al .:An Optical Microscopy Study on the Solvent-Induced Crystalline Morphology of Ladder PPSQNo.21IntroductionPolyphenylsilsesquioxane (PPSQ),a rigid double chain lad-der polymer,exhibits many outstanding performances,such as excellent thermal and oxidative stability,good electric insulat-ing property,and selective gas permeability.Even though the high rigidity of the molecular chains,it is soluble in many or-ganic solvents,e.g.,toluene,benzene,xylene and tetrahydrofu-ran.The intriguing properties and easy solubility made it to be an attractive research object both in scientific and practical view points.1Since the first report of synthesis of ladder-like PPSQ in 1960by Brown et al .,2some papers and patents focus-ing on the solution properties,3-5solid state structure 6,7and its possible application 8,9have been published.The crystallization behavior of PPSQ has been,however,less concerned.Only re-cently,Li et al .10,11have reported their observation on the single crystal structure of PPSQ obtained in different solvents at vari-able temperatures by the optical microscope.Spherulitic struc-ture of PPSQ has,however,not been reported yet.This is caused by the difficulty in crystallization of PPSQ under nor-mally used crystallization conditions.PPSQ is very difficult to crystallize under thermal condition from the melt and normal solution casting or spin coating pro-cess owing to its high chain rigidity.Solvent-induced crystalli-zation is an effective method to promote the crystallization of rigid polymers that cannot crystallize or crystallize very slowly under thermal conditions.It was reported that the interaction between the solvent and polymer chains can lower the effec-tive glass transition temperature (T g )of the polymer and en-courage the occurrence of crystallization at temperatures well below the T g .12-14Therefore,solvent induced crystallization of PPSQ by xylene is studied.The influences of the original solu-tion concentration,the solvent evaporation time,and the crys-tallization temperature were followed.According to the optical feature of the observed spherulites,a model reflecting the chain arrangement of PPSQ in spherulite is proposed in this study.2Experimentalm -phenyldiamine,toluene,1,4-dioxane,xylene,thionyl chlo-ride (AR,Sinopharm Chemical Reagent Beijing Co.,Ltd);phenyltrichlorosilane (AR,Acros);m -phthalic acid (AR,Sino-pharm Chemical Reagent Beijing Co.,Ltd).PPSQ used in this study was synthesized in our group.15Firstly,a perfect ladder m -phenylenediimino-bridged polyphe-nylsiloxane was prepared by stoichiometric hydrolysis/dehy-drochlorination-condensation reaction.Then the bridge of m -phenyldiamine was wiped off by isophthalyl chloride step-wise following condensation to get perfect ladder PPSQ ac-cording to the literature.16,29Silicon nuclear magnetic resonance (29Si-NMR)spectrum and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS)suggest the presence of good ladder regularity.The molec-ular structure of PPSQ is shown in Scheme 1.The samples for optical microscopy observation were pre-pared by dropping the PPSQ xylene solution with concentra-tions ranging from 0.3%to 2%(w )onto clean glass slides in-side a cylinder container at desired temperature.The container is covered with a lid and contains different volumes of extra solvent.The solvent can only escape through a small gap on the lid.In this way,the solvent evaporation rate was well con-trolled.The experimental temperatures range from 65to 85°C,while the solvent evaporation times are set for 3,8d.For optical microscopy (OM)observation,an Olympus BH-2optical microscope (Japan)was used in this study.The pictures were taken under crossed polarizers at room tempera-ture.To clarify the optical character of the spherulites,a prima-ry red filter (λ-plate)was used.3Results and discussionThe PPSQ molecules have a rigid backbone.Experimental results demonstrate that the PPSQ sample prepared by com-mon solution casting or spin coating methods is amorphous.This is nothing surprising since this kind polymer generally can hardly crystallize.Solvent induced crystallization of rigid chain polymers is,however,frequently reported.Therefore,crystallization of PPSQ induced by xylene was checked.Fig.1a shows an optical micrograph of PPSQ,which is solvent in-duced by xylene from 2%(w )solution at 65°C by controlling the complete solvent evaporation for 3d.It can be seen thattheFig.1Optical micrographs of solvent-induced crystallized PPSQ with different concentrations (w )at 65°C for different time(a)2%,3d;(b)0.3%,3d;(c)2%,8d;(d)0.3%,8dScheme 1Molecular structure of PPSQ495Acta Phys.⁃Chim.Sin .2012V ol.28PPSQ forms small spherulites.The randomly dispersed spheru-lites of different sizes exhibit a regular round shape and display clearly the characteristic “Maltese Cross ”.This indicates theoccurrence of PPSQ crystallization.17The glass transition tem-perature of PPSQ was estimated by differential scanning calo-rimetry to be ca 185°C.This is much higher than the crystalli-zation temperature used,i.e.,65°C.Normally,crystallization of a polymer at temperatures lower than its T g cannot happen owing to the low chain mobility.Therefore,the occurrence of PPSQ crystallization at 65°C indicates that the mobility of the amorphous PPSQ molecular chains is enhanced when they in-teract with the solvent molecules.18In other words,the solvent is speculated to reduce the relaxation time and therefore to de-press the glass transition temperature of PPSQ due to the addi-tional free volume introduced by the solvent.As a conse-quence,solvent induced crystallization of the PPSQ takes place.It should be pointed out that solvent-induced crys-tallization is a very complex phenomenon,in which the concen-tration of the solution,the solvent evaporation time and the temperature play very important roles.19Therefore,the influenc-se of the above mentioned factors on the crystallization behav-ior of the PPSQ were studied.We have first checked the influence of solution concentra-tion on the crystallization behavior of PPSQ.It was found that the size and number of the PPSQ spherulites change slightly with the decrease of the solution concentration.Fig.1b shows a representative optical micrograph of PPSQ,which was solvent induced by xylene from 0.3%(w )solution at 65°C for 3d.One can notice that with decreasing the solution concentration,the size of the spherulites increases somewhat,while the num-ber of the spherulites decreases evidently.This is caused by the decreased nucleation ability with decreased solution concentra-tion,leading to less spherulite formed and limited impinge-ment of the spherulites.It may also imply an increased crystal growth rate of PPSQ in the dilute solution.These changes re-sult from the fact that a better solubility of PPSQ in the dilute solution can be achieved with respect to the concentrated solu-tion.20Even though xylene is a good solvent for PPSQ,the sol-ubility of PPSQ in xylene is,however,expected to be reduced with increasing PPSQ amount.This will lead to the aggrega-tion of the PPSQ molecules on the one hand,which can en-hance the nucleation of the PPSQ,and reduce the diffusion rate as well as diffusion length of the PPSQ molecules in the solu-tion on the other hand.The drop in diffusion rate will reduce the crystal growth rate,while a reduction in diffusion length will cause larger material depletion zones around the spheru-lites,which can restrain the further propagation of the crystals.All these result in the formation of relatively smaller spheru-lites.The solvent evaporation rate,which is controlled by the time need for evaporating the solvent completely,shows also great influence on the crystalline morphology of the PPSQ.As shown in Fig.1(c,d),with the decrease of the solvent evapora-tion rate under otherwise unchanged condition,the spherulite size increases evidently.Now the spherulites can reach 100mi-crometers in diameter for the low concentration sample.This is easily understood since extending solvent evaporation time means an extension of crystallization time,which results gener-ally in bigger spherulites.The influence of temperature on the spherulitic morphology of PPSQ is well illustrated in Fig.2.The optical micrographs were taken from samples with different concentrations crystal-lized at 85°C for 3d.From Fig.2,we see well-developed spherulites for all samples with any concentrations.At high concentration,e.g.,2%(w ),spherulites of tens of micrometers in diameter have been created.At low concentration,e.g.,0.3%(w ),the diameter of the spherulites exceeds 300micrometers.Considering that high solubility can also be reached by elevat-ing the dissolved temperature,these results clearly indicate that improved solubility reduces the nucleation ability but enhances the diffusion rate and length of the polymer chains,which lead to formation of larger spherulites.This is in goodagreementFig.2Optical micrographs of solvent-induced crystallized PPSQ with different concentrations at 85°C for 3dw /%:(a)2,(b)1,(c)0.5,(d)0.3Fig.3Optical micrograph of PPSQ spherulite obtained from 0.5%(w )solution at 85°C for 3d under a primaryred filter (λ-plate)496LI Pei-Fang et al .:An Optical Microscopy Study on the Solvent-Induced Crystalline Morphology of Ladder PPSQNo.2with the results observed for the samples with different concen-trations at lower temperature.The above results demonstrate that the PPSQ can form spherulites under the existence of solvent.The arrangement of molecular chains in the spherulite is,however,not clear at pres-ent stage.To disclose the chain organization of PPSQ in the spherulites,detailed optical microscopy observations were per-formed.We first chose the same sample as shown in Fig.2.The observed spherulites exhibit black Maltese cross,a peculiar characteristic of polymeric spherulites under crossed polariz-ers.By rotating the sample about the incident beam axis,the characteristic Maltese cross remained unchanged.This implies that in all radial directions of the spherulite the crystallograph-ic orientation is equivalent within the resolution employed and the major refractive index (generally in molecular chain direc-tion)of the spherulites is either parallel or perpendicular to the radial direction.21,22Moreover,a primary red filter was used to clarify the PPSQ chain orientation in the spherulites.Fig.3shows an optical micrograph of a PPSQ sample obtained from 0.5%(w )solution at 85°C for 3d under a primary red filter.The appearance of the yellow first and third while the blue sec-ond and forth quadrants indicate that the PPSQ spherulites are optically negative.Considering that the radial arranged molecu-lar chains always exhibit a positive character,23the negative op-tical characters of the PPSQ spherulites indicate that the PPSQ molecular chains in spherulite are oriented perpendicular to ra-dial direction.This kind organization has been most frequently observed for flexible chain polymers with folded chain lamel-lae grown in radial direction.In the present case,chain folding of the PPSQ is highly impossible.Therefore,the whole rigid PPSQ molecules in the spherulite should arrange in tangential direction.Therefore,we can only conjecture on the underlying physics of the crystallization process and crystalline structure of the PPSQ.Considering that the spherulites generally grow from the center parts,i.e.,the nuclei,outward in the radial di-rection,the spherulitic structure of the PPSQ may be as that sketched in Fig.4.The PPSQ spherulites are composed of crys-talline lamellae,while the crystalline lamellae are produced through parallel alignment of the PPSQ molecules.4ConclusionsThe spherulites of PPSQ were obtained by the method of sol-vent-induced crystallization.OM results indicated that the radi-us of solvent-induced spherulites increased with decreasing the concentration of PPSQ under the same temperature.The size of spherulites became bigger following the longer induced paring with 65°C,the more perfect spherulites were obtained under 85°C.The lamella may arrange along the radi-al direction of the spherulite and the molecular chain vertically arrange into the lamella confirmed by OM.Considering the high rigidity of the molecules,an organization model of the PPSQ spherulite is 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