丙烯酰胺在聚乙二醇水溶液中聚合初期的液滴形成與成長(zhǎng)過程

1、丙烯酰胺在聚乙二醇水溶液中聚合初期的液滴形成與成長(zhǎng)過程     呂挺單國(guó)榮(化學(xué)工程國(guó)家重點(diǎn)實(shí)驗(yàn)室(浙江大學(xué))浙江大學(xué)化工系杭州310027)摘要用動(dòng)態(tài)激光光散射(DLS)在線觀測(cè)了丙烯酰胺(AM)在聚乙二醇(PEG)水溶液中聚合初期液滴的出現(xiàn)、生長(zhǎng)及聚并過程,考察了PEG分子量和濃度對(duì)聚合初期液滴尺寸的影響;用透射電鏡(TEM)對(duì)聚合初期液滴形態(tài)的演變進(jìn)行了觀察,發(fā)現(xiàn)與DLS結(jié)果能很好吻合.用分光光度計(jì)對(duì)聚合體系分相點(diǎn)進(jìn)行確定,采用溴化法測(cè)定了聚合體系臨界分相時(shí)的轉(zhuǎn)化率.隨PEG分子量或濃度的升高,臨界分相轉(zhuǎn)化率逐漸減小;隨溫度的升高,臨界分相轉(zhuǎn)化率先

2、減小后增大,在50左右出現(xiàn)最小值.用凝膠滲透色譜(GPC)對(duì)聚合體系臨界分相時(shí)聚丙烯酰胺(PAM)的分子量進(jìn)行了研究,變化趨勢(shì)與臨界分相轉(zhuǎn)化率的變化一致.在上述基礎(chǔ)上,提出了AM在PEG水溶液中聚合初期的液滴形成、成長(zhǎng)機(jī)理.關(guān)鍵詞丙烯酰胺,雙水相聚合,臨界分相轉(zhuǎn)化率,臨界分子量聚丙烯酰胺(PAM)是一種十分重要的水溶性高分子,廣泛應(yīng)用于污水處理、石油開采、造紙等行業(yè).其制備方法主要是水溶液聚合,但仍存在聚合過程中的高黏度導(dǎo)致攪拌與傳熱等問題.反相(微)乳液聚合、在醇/水體系中的分散聚合,盡管克服了上述困難,并得到較高分子量的PAM,但引入的有機(jī)溶劑會(huì)造成污染,產(chǎn)品需要再分離.近年來,出現(xiàn)了水溶

3、性單體的雙水相聚合, Song等1, 2研究了丙烯酰胺(AM)在硫酸銨水溶液中的雙水相聚合,單國(guó)榮等35研究了AM在聚乙二醇(PEG)水溶液中的聚合動(dòng)力學(xué)、產(chǎn)品穩(wěn)定性.雙水相聚合反應(yīng)是一種將水溶性單體、引發(fā)劑及分散介質(zhì)溶解在水中,形成均相水溶液,一定條件下進(jìn)行聚合反應(yīng),體系發(fā)生相分離,形成互不相容的水溶性聚合物分散液的聚合反應(yīng).關(guān)于傳統(tǒng)分散聚合的成核機(jī)理,許多學(xué)者進(jìn)行了深入研究612,認(rèn)為隨著引發(fā)劑的分解,聚合物自由基在連續(xù)相不斷增長(zhǎng),達(dá)到臨界鏈長(zhǎng)時(shí)從連續(xù)相析出,并迅速聚并形成初級(jí)核,初級(jí)核不穩(wěn)定,繼續(xù)聚并直至被足夠的穩(wěn)定劑穩(wěn)定,成核過程結(jié)束.成核過程時(shí)間很短,大約在轉(zhuǎn)化率為0·1%

4、時(shí)結(jié)束,之后不再形成新核,依靠聚合物粒子內(nèi)部的聚合以及不斷吸附連續(xù)相的聚合物自由基與鏈終止聚合物進(jìn)行粒子的增長(zhǎng).目前,雙水相聚合成滴機(jī)理的研究很少,Wang等13研究AM/甲基丙烯酰氧乙基三甲基氯化銨在硫酸銨水溶液聚合體系,認(rèn)為反應(yīng)一開始聚合物自由基與鏈終止聚合物聚集在一起形成聚合物相,隨著界面張力的增強(qiáng),小液滴在攪拌下逐漸形成.對(duì)雙水相聚合初期液滴的形成、成長(zhǎng)尚鮮有文獻(xiàn)報(bào)道.1實(shí)驗(yàn)部分1·1實(shí)驗(yàn)原料    聚乙二醇(PEG,分子量為4000、6000、10000、20000,分別記為PEG4000、PEG6000、PEG10000、PEG20000 )

5、、丙烯酰胺,均購于ACROSORGANICS.過硫酸銨(APS)、偶氮二異丙基咪唑啉鹽酸鹽(VA044)、硫代硫酸鈉、碘化鉀、溴酸鉀、溴化鉀、淀粉指示劑均為分析純,市售.聚合用水為去離子水.1·2丙烯酰胺濃度及反應(yīng)轉(zhuǎn)化率的測(cè)定按國(guó)標(biāo)GB12005·3-89溴化法測(cè)定AM濃度,計(jì)算聚合過程中AM的轉(zhuǎn)化率.1·3聚合初期液滴尺寸和形態(tài)的表征將一定量的PEG、AM與水混合,溶解并降溫后加入引發(fā)劑,接著取其4 mL并用0·2m濾孔膜過濾后置于5mL的比色皿中,并在保護(hù)氣體下密封.然后將比色皿置于動(dòng)態(tài)光散射(DLS)(Zetasizer3000HSA,Malver

6、n)檢測(cè)槽中,迅速升溫至60,開始計(jì)時(shí),在線檢測(cè)聚合初期液滴的尺寸,每檢測(cè)次耗時(shí)約4 min.取少量反應(yīng)液分散在甲醇中,取少許滴于銅網(wǎng)上,干燥后用透射電鏡(TEM) ( JEOL JSM-1230EXT20)觀察形態(tài).1·4臨界分相轉(zhuǎn)化率及臨界分子量的測(cè)定稱取一定量的AM、PEG、去離子水,加到500mL的夾套反應(yīng)釜中,邊攪拌(75 r/min)邊通保護(hù)氣體,升至指定溫度,加入引發(fā)劑,開始計(jì)時(shí).定時(shí)取樣,用分光光度計(jì)(Shimadzu UV-3150)測(cè)其透光度.同時(shí)用溴化法測(cè)定轉(zhuǎn)化率,將透光度開始下降時(shí)的轉(zhuǎn)化率作為臨界分相轉(zhuǎn)化率.待反應(yīng)體系分相后加入終止劑對(duì)苯二酚停止反應(yīng),高速離心

7、取其上層清液,去除水分,用甲醇洗滌數(shù)次去除PEG.得到的產(chǎn)品干燥后用GPC(Waters 1525泵、2414示差檢測(cè)器、717自動(dòng)進(jìn)樣器)測(cè)定分子量,作為分相時(shí)的臨界分子量.色譜柱由PL aquagel-OH 50、30、10型3柱串聯(lián).測(cè)試條件,流動(dòng)相0·1 mol/L NaNO3水溶液、流速0·8mL/min、試樣濃度0·3%、進(jìn)樣量50L、柱溫30.2結(jié)果與討論2·1初期液滴尺寸與形態(tài)演變采用DLS在線觀測(cè)聚合反應(yīng)初期PAM液滴的形成過程.反應(yīng)開始后的前幾次掃描中均未能檢測(cè)到析出的PAM液滴,說明此時(shí)體系并未分相.隨著反應(yīng)的繼續(xù)進(jìn)行, DLS檢測(cè)

8、到約為3050 nm的微液滴,然后迅速增大,PAM液滴滴徑分布系數(shù)(DDI)先增大后減小,當(dāng)轉(zhuǎn)化率達(dá)到6%左右時(shí),PAM液滴滴徑達(dá)到了約200 nm.隨著分散介質(zhì)PEG分子量或濃度的減小,聚合物液滴穩(wěn)定性下降,聚并加劇,液滴尺寸增大更加迅速,滴徑分布系數(shù)變小,如圖1和2所示.采用透射電鏡(TEM)對(duì)聚合初期的聚合物液滴形態(tài)進(jìn)行了觀察.由于在樣品制備及TEM觀察過程中會(huì)失水,不能確切地反映液滴的實(shí)際尺寸,但由于其液滴里富集了高含量的PAM,其高黏度足夠使其在樣品制備或觀察過程中維持原來的形態(tài),因此液滴形態(tài)仍是準(zhǔn)確的.如圖3(a)所示,在轉(zhuǎn)化率較低時(shí),其液滴形態(tài)為大小不均的球狀;隨后出現(xiàn)了橢球狀形

9、態(tài)的液滴(圖3b所示),這是由于微液滴間發(fā)生聚并,但其液滴內(nèi)部的高黏度阻礙了液滴間的擴(kuò)散所致;隨著反應(yīng)的繼續(xù)進(jìn)行,從連續(xù)相析出聚合物液滴的速率增加,聚并Fig. 1EffectofPEG molecularweighton the droplets size anddistribution in the initial stage ofpolymerizationPEG=12 g, AM=4 g, H2O=84 g, APS=0·01 g,T=60Fig. 2Effect of PEG concentration on the droplets size anddistributio

10、n in the initial stage ofpolymerizationPEG20000+H2O=96 g, AM=4 g, APS=0·01 g,T=60加劇,較小液滴優(yōu)先聚并,使得液滴尺寸變大,且較為均一(圖3c所示).這些現(xiàn)象均與動(dòng)態(tài)光散射結(jié)果相一致,但與Wang等13研究所得結(jié)果有很大的差異.這是由于文獻(xiàn)13是在硫酸銨水溶液體系中聚合,聚合初期的聚合物液滴含水率較高,內(nèi)部黏度較低,且硫酸銨體系中液滴穩(wěn)定性較差,其形態(tài)隨著攪拌速率變化而改變.2·2臨界分相轉(zhuǎn)化率與臨界分子量為了進(jìn)一步理解聚合初期液滴的形成機(jī)理,對(duì)AM在PEG水溶液中聚合分相時(shí)的臨界分相轉(zhuǎn)化率及臨

11、界分子量進(jìn)行了測(cè)定.將反應(yīng)液透光度下降時(shí)作為體系分相點(diǎn),此時(shí)的轉(zhuǎn)化率作為臨界分相轉(zhuǎn)化率(Xc).Xc隨PEG分子量(圖4)、PEG濃度(圖5)的增加而下降,因此DLS能較早檢測(cè)到微液滴(圖1、圖2所示);隨溫度的上升(3075)先減小后升高, 50時(shí)的Xc達(dá)到了最小值(約為1% )(圖6).這與PEG-PAM-H2O雙水相體系相圖研究結(jié)果一致5.Fig. 3Dropletmorphologies in the initial stageConversion: a) 2·27%; b) 3·43%; c) 5·78%; PEG20000=12 g, AM=4 g,

12、H2O=84 g, APS=0·01 g,T=60Fig. 4Effect of PEG molecular weight on the criticalconversionPEG=20 g, AM=6 g, H2O=74 g, APS=0·0025 g,T=60Fig. 5Effect of PEG concentration on the criticalconversionPEG20000+H2O=94 g, AM=6 g, APS=0·0025 g,T=60在PEG濃度為20%的反應(yīng)體系中,當(dāng)PEG分子量為400時(shí),隨著聚合反應(yīng)的進(jìn)行,體系始終清澈,未見相

13、分離,最終產(chǎn)品PAM分子量較低(數(shù)均58800、重均433600).這是由于PEG分子量下降,端羥基濃度增大,自由基向其鏈轉(zhuǎn)移增多所致.而在PEG20000作為分散介質(zhì)體系中,用量減少至4%時(shí),轉(zhuǎn)化率達(dá)到12·5%左右才發(fā)生分Fig. 6Effect ofpolymerization temperature on the criticalconversionPEG20000=20 g, AM=6 g, H2O=74 g, from 30to50withVA044=0·0035 g, from 55to75withAPS=0·0025 gFig. 7Effect o

14、f PEG molecular weight on the criticalmolecularweightPEG=20 g, AM=6 g, H2O=74 g, VA044=0·0035 g,T=50相,但體系不穩(wěn)定,產(chǎn)品結(jié)塊嚴(yán)重.Yasuda等12曾對(duì)苯乙烯在乙醇中分散聚合的成粒機(jī)理進(jìn)行了研究,并測(cè)定了其臨界分子量,發(fā)現(xiàn)當(dāng)攪拌轉(zhuǎn)速為30 r/min時(shí)所測(cè)得的臨界分子量最大,但隨聚合溫度及單體轉(zhuǎn)化率的變化并不顯著.圖7、圖8、圖9是不同反應(yīng)條件下AM在PEG水溶液中聚合臨界分相時(shí)的PAM臨界分子量(Mc),其變化趨勢(shì)與Xc相一致.隨聚合溫度上升,Mc先減小后增大,在50時(shí),出現(xiàn)了最小

15、值,約為2227.隨PEG濃度或分子量的減小,Mc逐漸增大,從連續(xù)相析出的PAM鏈聚并形成的初始微液滴尺寸也隨之增大(圖1、圖2所示).Fig. 8Effect of PEG concentration on the criticalmolecularweightPEG20000+H2O=94 g, AM=6 g, VA044=0·0035 g,T=50Fig. 9Effect ofpolymerization temperature on the criticalmolecularweightPEG20000=20 g, AM=6 g, H2O=74 g, from 30to50w

16、ithVA044=0·0035 g, from 55to75withAPS=0·0025 g2·3聚合初期液滴形成、成長(zhǎng)機(jī)理反應(yīng)開始后,由于引發(fā)劑分解,聚合物自由基不斷生成并增長(zhǎng),見圖10中的和.當(dāng)聚合物自由基達(dá)到臨界鏈長(zhǎng)時(shí),從連續(xù)相中析出,并迅速聚并,形成初級(jí)核,見圖10中的.初級(jí)核不穩(wěn)定,繼續(xù)聚并,形成微液滴,見圖10中的,到此為止,將其定義為液滴形成期.Fig. 10Schematic representation of the initial droplets formation and growthmechanism ofAM aqueous two-p

17、hase polymerization in aqueousPEG solution聚合反應(yīng)在連續(xù)相及聚合物相同時(shí)進(jìn)行,微液滴通過內(nèi)部聚合不斷長(zhǎng)大;同時(shí)連續(xù)相又不斷析出形成新的微液滴,液滴滴徑分布開始變寬,見圖10中的.隨反應(yīng)進(jìn)行,液滴數(shù)目不斷增多,尺寸較小的液滴優(yōu)先聚并,使得聚合物液滴滴徑增大且分布重新變窄,見圖10中的.實(shí)驗(yàn)中發(fā)現(xiàn),由于聚合初期液滴間的聚并較為激烈,當(dāng)聚合速率過快時(shí),在聚合初期即形成細(xì)凝膠狀PAM,產(chǎn)品靜置后極易分層.因此要制備較為穩(wěn)定的乳狀分散液產(chǎn)品,控制初期的聚合速率十分關(guān)鍵.3結(jié)論    AM在PEG水溶液中聚合開始后,由于引發(fā)劑分解,聚

18、合物自由基不斷生成并增長(zhǎng),當(dāng)聚合物自由基達(dá)到臨界鏈長(zhǎng)時(shí),開始從連續(xù)相中析出.Xc與Mc隨PEG分子量或濃度的增加而減少,隨著溫度的上升先減小后增大,在50時(shí)達(dá)到了最小值.隨后析出的聚合物鏈迅速聚并形成初級(jí)核,初級(jí)核不穩(wěn)定,繼續(xù)聚并形成微液滴,這個(gè)階段定義為液滴形成期.此后,聚合反應(yīng)在連續(xù)相及聚合物相同時(shí)進(jìn)行,微液滴通過內(nèi)部聚合不斷長(zhǎng)大,同時(shí)又從連續(xù)相析出新生成的微液滴,因此其液滴滴徑分布開始變寬.隨著反應(yīng)的繼續(xù)進(jìn)行,液滴數(shù)目不斷增多,尺寸較小的液滴優(yōu)先聚并,使得液滴尺寸增大,并變得較為均一.REFERENCES1SongB K,ChoM S,YoonK J,LeeD C. JApplPolym

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22、angW D,WangPX.EurPolym J,2006,42:12841297INITIAL DROPLETS FORMATION AND GROWTH PROCESS OFACRYLAM IDE DURING AQUEOUS TWO-PHASE POLYMERIZATION INAQUEOUS POLY(ETHYLENE GLYCOL) SOLUTIONL Ting, SHAN Guorong(StateKeyLaboratory ofChemicalEngineering(Zhejiang University),Department ofChemicalEngineering, Zh

23、ejiang University, Hangzhou310027)AbstractIn order to explore the droplet formation process of the aqueous two-phase polymerization ofacrylamide (AM) in aqueous poly(ethylene glycol) (PEG) solution, dynamic light scattering (DLS) wasemployed to investigate the appearance, the size and size distribut

24、ion of droplets in the initial stage ofpolymerization.The effects of PEG molecularweight and concentration on the droplets size and distributionwere studied. Itwas found that the droplets stability in aqueous PEG solution decreased, and the dropletsaggregatedwith each other to form larger andmore un

25、iform dropletswith decreasing PEG molecularweightorPEG concentration. The morphology of the droplets rich in polyacrylamide (PAM) was observed withtransmission electronmicroscopy (TEM).The droplets shape in the initial stage ofpolymerization is sphericalbutpolydisperse.However, some oval droplets fo

26、rmed in the produc,t while the conversion was still very low,which indicated some coalescence of droplets in this stage.As the polymerization proceeds, the droplet sizeincreased rapidly and the size distribution of droplets become uniform. These resultswere well in agreementwith the DLS results.W it

27、h using the spectrophotometer, the phase separation pointwas determined where thetransparence of reaction solution declined suddenly.The effectofvarious parameters on the critical conversionofphase separationwasmeasured bybromatingmethod. Itdecreasedwith the increase ofPEGmolecularweightorPEG concen

28、tration.W ith raising temperature from 30to 70, the critical conversion decreased at firs,tand then turned to increase when the temperature was at about50.The criticalmolecularweightwas alsosystemically investigated by gelpermeation chromatogram (GPC). Itcould be seen that the evolution ofcriticalmolecularweight correspondedwellwith thatof critical conversion atvarious reaction conditions.On the basisof above mentioned results, an initial droplet formation and growth mechanism of AM aqueous two-phasepolymerization in aqueous PEG solutionswas proposed.As the reaction starts, primary radi