李培芳 閆壽科 任忠杰,*

(1內(nèi)蒙古民族大學(xué)物理與電子信息學(xué)院,內(nèi)蒙古通遼028043; 2北京化工大學(xué)材料科學(xué)與工程學(xué)院,化工資源有效利用國家重點實驗室,北京100029)

利用光學(xué)顯微鏡研究溶劑誘導(dǎo)的梯形苯基聚倍半硅氧烷結(jié)晶

李培芳1閆壽科2任忠杰2,*

(1內(nèi)蒙古民族大學(xué)物理與電子信息學(xué)院,內(nèi)蒙古通遼028043;2北京化工大學(xué)材料科學(xué)與工程學(xué)院,化工資源有效利用國家重點實驗室,北京100029)

通過溶劑誘導(dǎo)結(jié)晶的方法研究了梯形苯基聚倍半硅氧烷(PPSQ)的球晶結(jié)構(gòu).采用光學(xué)顯微鏡研究了溶液的濃度、溶劑的揮發(fā)時間以及溫度等影響因素對梯形苯基聚倍半硅氧烷的球晶結(jié)構(gòu)形貌的影響.實驗結(jié)果表明:稀釋溶液濃度與提高結(jié)晶溫度在改變球晶的形態(tài)方面具有相同的效果.球晶的尺寸會隨著溶液濃度的降低或者結(jié)晶溫度的升高而增加.這都歸因于梯形苯基聚倍半硅氧烷在二甲苯中的溶解度的增加.在不改變其它結(jié)晶條件的情況下，延長溶劑的揮發(fā)時間也會形成更大的球晶.當(dāng)在光學(xué)顯微鏡下旋轉(zhuǎn)樣品時球晶的結(jié)構(gòu)不會發(fā)生改變，這表明球晶具有均一的晶體學(xué)取向結(jié)構(gòu).負性球晶的特征也表明梯形苯基聚倍半硅氧烷的分子鏈是沿著球晶的切線方向排列.考慮到梯形苯基聚倍半硅氧烷具有剛性的分子鏈，還提出了梯形苯基聚倍半硅氧烷可能的球晶結(jié)構(gòu)模型.

梯形苯基聚倍半硅氧烷;結(jié)晶;溶劑誘導(dǎo);球晶;剛性分子鏈

1 Introduction

Polyphenylsilsesquioxane(PPSQ),a rigid double chain ladder polymer,exhibits many outstanding performances,such as excellent thermal and oxidative stability,good electric insulating property,and selective gas permeability.Even though the high rigidity of the molecular chains,it is soluble in many organic solvents,e.g.,toluene,benzene,xylene and tetrahydrofuran.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 1960 by Brown et al.,2some papers and patents focusing on the solution properties,3-5solid state structure6,7and its possible application8,9have been published.The crystallization behavior of PPSQ has been,however,less concerned.Only recently,Li et al.10,11have reported their observation on the single crystal structure of PPSQ obtained in different solvents at variable temperatures by the optical microscope.Spherulitic structure of PPSQ has,however,not been reported yet.This is caused by the difficulty in crystallization of PPSQ under normally used crystallization conditions.

PPSQ is very difficult to crystallize under thermal condition from the melt and normal solution casting or spin coating process owing to its high chain rigidity.Solvent-induced crystallization 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 effective glass transition temperature(Tg)of the polymer and encourage the occurrence of crystallization at temperatures well below the Tg.12-14Therefore,solvent induced crystallization of PPSQ by xylene is studied.The influences of the original solution concentration,the solvent evaporation time,and the crystallization 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.

2 Experimental

m-phenyldiamine,toluene,1,4-dioxane,xylene,thionyl chloride(AR,Sinopharm Chemical Reagent Beijing Co.,Ltd); phenyltrichlorosilane(AR,Acros);m-phthalic acid(AR,Sinopharm Chemical Reagent Beijing Co.,Ltd).

PPSQ used in this study was synthesized in our group.15Firstly,a perfect ladder m-phenylenediimino-bridged polyphenylsiloxane was prepared by stoichiometric hydrolysis/dehydrochlorination-condensation reaction.Then the bridge of m-phenyldiamine was wiped off by isophthalyl chloride stepwise following condensation to get perfect ladder PPSQ according to the literature.16,29Silicon nuclear magnetic resonance (29Si-NMR)spectrum and matrix-assisted laser desorption/ ionization time of flight mass spectrometry(MALDI-TOFMS)suggest the presence of good ladder regularity.The molecular structure of PPSQ is shown in Scheme 1.

Scheme 1 Molecular structure of PPSQ

The samples for optical microscopy observation were prepared by dropping the PPSQ xylene solution with concentrations ranging from 0.3%to 2%(w)onto clean glass slides inside 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 controlled.The experimental temperatures range from 65 to 85°C, while the solvent evaporation times are set for 3,8 d.

For optical microscopy(OM)observation,an Olympus BH-2 optical microscope(Japan)was used in this study.The pictures were taken under crossed polarizers at room temperature.To clarify the optical character of the spherulites,a primary red filter(λ-plate)was used.

3 Results and discussion

Fig.1 Optical micrographs of solvent-induced crystallized PPSQ with different concentrations(w)at 65°C for different time(a)2%,3 d;(b)0.3%,3 d;(c)2%,8 d;(d)0.3%,8 d

The PPSQ molecules have a rigid backbone.Experimental results demonstrate that the PPSQ sample prepared by common 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 induced by xylene from 2%(w)solution at 65°C by controlling the complete solvent evaporation for 3 d.It can be seen that the PPSQ forms small spherulites.The randomly dispersed spherulites of different sizes exhibit a regular round shape and display clearly the characteristic“Maltese Cross”.This indicates the occurrence of PPSQ crystallization.17The glass transition temperature of PPSQ was estimated by differential scanning calorimetry to be ca 185°C.This is much higher than the crystallization temperature used,i.e.,65°C.Normally,crystallization of a polymer at temperatures lower than its Tgcannot 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 interact with the solvent molecules.18In other words,the solvent is speculated to reduce the relaxation time and therefore to depress the glass transition temperature of PPSQ due to the additional free volume introduced by the solvent.As a consequence,solvent induced crystallization of the PPSQ takes place.It should be pointed out that solvent-induced crystallization is a very complex phenomenon,in which the concentration of the solution,the solvent evaporation time and the temperature play very important roles.19Therefore,the influencse of the above mentioned factors on the crystallization behavior of the PPSQ were studied.

Fig.2 Optical micrographs of solvent-induced crystallized PPSQ with different concentrations at 85°C for 3 dw/%:(a)2,(b)1,(c)0.5,(d)0.3

We have first checked the influence of solution concentration 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 3 d. One can notice that with decreasing the solution concentration, the size of the spherulites increases somewhat,while the number of the spherulites decreases evidently.This is caused by the decreased nucleation ability with decreased solution concentration,leading to less spherulite formed and limited impingement of the spherulites.It may also imply an increased crystal growth rate of PPSQ in the dilute solution.These changes result from the fact that a better solubility of PPSQ in the dilute solution can be achieved with respect to the concentrated solution.20Even though xylene is a good solvent for PPSQ,the solubility of PPSQ in xylene is,however,expected to be reduced with increasing PPSQ amount.This will lead to the aggregation of the PPSQ molecules on the one hand,which can enhance the nucleation of the PPSQ,and reduce the diffusion rate as well as diffusion length of the PPSQ molecules in the solution 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 spherulites,which can restrain the further propagation of the crystals. All these result in the formation of relatively smaller spherulites.

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 evaporation rate under otherwise unchanged condition,the spherulite size increases evidently.Now the spherulites can reach 100 micrometers in diameter for the low concentration sample.This is easily understood since extending solvent evaporation time means an extension of crystallization time,which results generally 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 crystallized at 85°C for 3 d.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 300 micrometers. Considering that high solubility can also be reached by elevating 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 good agreement with the results observed for the samples with different concentrations at lower temperature.

Fig.3 Optical micrograph of PPSQ spherulite obtained from 0.5%(w)solution at 85°C for 3 d under a primary red filter(λ-plate)

Fig.4 Sketch of the possible spherulitic structure for PPSQ

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 present stage.To disclose the chain organization of PPSQ in the spherulites,detailed optical microscopy observations were performed.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 polarizers.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 crystallographic orientation is equivalent within the resolution employed and the major refractive index(generally in molecular chain direction)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.3 shows an optical micrograph of a PPSQ sample obtained from 0.5%(w)solution at 85°C for 3 d under a primary red filter. The appearance of the yellow first and third while the blue second and forth quadrants indicate that the PPSQ spherulites are optically negative.Considering that the radial arranged molecular chains always exhibit a positive character,23the negative optical characters of the PPSQ spherulites indicate that the PPSQ molecular chains in spherulite are oriented perpendicular to radial direction.This kind organization has been most frequently observed for flexible chain polymers with folded chain lamellae 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 direction,the spherulitic structure of the PPSQ may be as that sketched in Fig.4.The PPSQ spherulites are composed of crystalline lamellae,while the crystalline lamellae are produced through parallel alignment of the PPSQ molecules.

4 Conclusions

The spherulites of PPSQ were obtained by the method of solvent-induced crystallization.OM results indicated that the radius 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 time.Comparing with 65°C,the more perfect spherulites were obtained under 85°C.The lamella may arrange along the radial 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 proposed.

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October 11,2011;Revised:November 9,2011;Published on Web:November 23,2011.

An Optical Microscopy Study on the Solvent-Induced Crystalline Morphology of Ladder Polyphenylsilsesquioxane

LI Pei-Fang1YAN Shou-Ke2REN Zhong-Jie2,*
(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 and Engineering,Beijing University of Chemical Technology,Beijing 100029,P.R.China)

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.

Polyphenylsilsesquioxane;Crystallization;Solvent induction;Spherulite;Rigid chain

10.3866/PKU.WHXB201111233www.whxb.pku.edu.cn

*Corresponding author.Email:renzj@mail.buct.edu.cn;Tel:+86-10-64426375.

The project was supported by the National Natural Science Foundation of China(21104002,11164020).

國家自然科學(xué)基金(21104002,11164020)資助項目

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