Transcrystallization in syndiotactic polypropylene induced by high-modulus carbon fibers
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Polymer Bulletin 41, 239–245 (1998) * Corresponding author Polymer Bulletin Springer-Verlag 1998 Transcrystallization in syndiotactic polypropylene induced by high-modulus carbon fibers Chang-Mou Wu 1 , Ming Chen 1 , József Karger-Kocsis 2,* 1 Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan 80424, ROC 2 Institut für Verbundwerkstoffe GmbH, Universität Kaiserslautern, Postfach 3049, D-67653 Kaiserslautern, Germany, e-mail: [email protected] Received: 31 March 1998/Revised version: 29 May 1998/Accepted: 10 June 1998 SUMMARY It was first shown that transcrystallization can be induced in syndiotactic polypropylene (sPP) when a carbon fiber (CF) of high-modulus (HM) is embedded in the melt of sPP crystallizing under quiescent conditions. High-tenacity carbon fiber (HTCF), on the other hand, did not cause transcrystalline growth. Coating of HMCF by silicon carbide (SiC) stopped the transcrystallization of sPP. The difference in the morphology of the transcrystalline layer between isotactic PP (iPP) and sPP was revealed by phase contrast light (PCLM), scanning electron (SEM) and atomic force microscopy (AFM) taken from the etched surface of single fiber microcomposite specimens. 1. INTRODUCTION Transcrystallization caused by heterogeneous nucleating agents in semicrystalline polymers is a well-known phenomenon. Essential prerequisite of transcrystallization is the presence of active nuclei on the surface of the substrates (e.g., fillers, reinforcements) in high density. The closely spaced nuclei hinder the lateral extension of spherulites which are then forced to grow in one direction, namely perpendicular to the substrates’ surface. Since the density of the nuclei on the substrates’ surface is higher than in the bulk polymer a columnar morphology, termed to transcrystallinity, appears. Transcrystallization was first reported in 1952 [1] and became topic of numerous studies up to now. This phenomenon is reviewed for isotactic polypropyene (iPP) in refs. [2-5]. Nevertheless, it is not yet fully understood which are the controlling factors of transcrystallinity (why given fibers induce transcrystallization and other not). The still ongoing interest on this phenomenon is fueled by the belief that the resulting interphase morphology may promote the stress transfer from the matrix toward the reinforcement and thus polymeric composites of improved mechanical performance can be produced. It should be underlined here that transcrystallinity develops only in quiescent melt. In
Transcript of Transcrystallization in syndiotactic polypropylene induced by high-modulus carbon fibers
Polymer Bulletin 41, 239–245 (1998)
* Corresponding author
Polymer Bulletin Spr inger-V er lag 1998
Transcrystallization in syndiotactic polypropylene inducedby high-modulus carbon fibers
Chang-Mou Wu1, Ming Chen1, József Karger-Kocsis2,*
1 Institute of Materials Science and Engineering, National Sun Yat-Sen University,Kaohsiung, Taiwan 80424, ROC2 Institut für Verbundwerkstoffe GmbH, Universität Kaiserslautern, Postfach 3049,D-67653 Kaiserslautern, Germany, e-mail: [email protected]
Received: 31 March 1998/Revised version: 29 May 1998/Accepted: 10 June 1998
SUMMARYI t w a s f i r s t s ho w n tha t t r a ns c r ys ta l l i za t i o n c a n b e i nd uc e di n s ynd i o ta c t i c p o l yp r o p yl e ne ( s P P ) w he n a c a r b o n f i b e r ( CF )o f hi gh- mo d ul us ( HM) i s e mb e d d e d i n the me l t o f s P Pc r ys ta l l i z i ng und e r q ui e s c e nt c o nd i t i o ns . H i gh- te na c i tyc a r b o n f i b e r ( HT CF ) , o n the o the r ha nd , d i d no t c a us et r a ns c r ys ta l l i ne gr o w th. Co a t i ng o f HMCF b y s i l i c o n c a r b i d e( S i C ) s to p p e d the t r a ns c r ys tal l i za t i o n o f s P P . T hed i f fe r e nc e i n the mo r p ho l o gy o f the t r a ns c r ys ta l l i ne l a ye rb e tw e e n i s o ta c t i c P P ( i P P ) a nd s P P w a s r e v e a l e d b y p ha s ec o nt r a s t l i ght ( P CLM) , s c a nni ng e l e c t r o n ( S E M) a nd a to mi cfo r c e mi c r o s c o p y ( AF M) ta ke n f r o m the e tc he d s ur fa c e o fs i ngl e f i b e r mi c r o c o mp o s i te s p e c i me ns .
1. INTRODUCTIONT r a ns c r ys ta l l i za t i o n c a us e d b y he te r o ge ne o us nuc l e a t i nga ge nts i n s e mi c r ys ta l l i ne p o l yme r s i s a w e l l - kno w np he no me no n. E s s e nt i a l p r e r e q ui s i te o f t r a ns c r ys ta l l i za t i o ni s the p r e s e nc e o f a c t i v e nuc l e i o n the s ur fa c e o f thes ub s t r a te s ( e . g. , f i l l e r s , r e i nfo r c e me nts ) i n hi gh d e ns i ty.T he c l o s e l y s p a c e d nuc l e i h i nd e r the l a te r a l e xte ns i o n o fs p he r ul i te s w hi c h a r e the n fo r c e d to gr o w i n o ne d i r e c t i o n,na me l y p e r p e nd i c ul a r to the s ub s t r a te s ’ s ur fa c e . S i nc e thed e ns i ty o f the nuc l e i o n the s ub s t r a te s ’ s ur fa c e i s hi ghe rtha n i n the b ul k p o l yme r a c o l umna r mo r p ho l o gy, te r me d tot r a ns c r ys ta l l i n i t y, a p p e a r s . T r a ns c r ys ta l l i za t i o n w a s f i r s tr e p o r te d i n 1 9 5 2 [ 1 ] a nd b e c a me to p i c o f nume r o us s tud i e s upto no w . T hi s p he no me no n i s r e v i e w e d fo r i s o ta c t i cp o l yp r o p ye ne ( i P P ) i n r e fs . [ 2 - 5 ] . Ne v e r the l e s s , i t i s no tye t fu l l y und e r s to o d w hi c h a r e the c o nt r ol l i ng fa c to r s o ft r a ns c r ys ta l l i n i t y ( w hy gi v e n f i b e r s i nd uc et r a ns c r ys ta l l i za t i o n a nd o the r no t ) . T he s t i l l o ngo i ngi nte r e s t o n thi s p he no me no n i s fue l e d b y the b e l i e f tha t ther e s ul t i ng i nte r p ha s e mo r p ho l o gy ma y p r o mo te the s t r e s st r a ns fe r f r o m the ma t r i x to w a r d the r e i nfo r c e me nt a nd thuspolymer ic composi tes of improved mechanical per formance c a nb e p r o d uc e d . I t s ho ul d b e und e r l i ne d he r e tha tt r a ns c r ys ta l l i n i t y d e v e l o p s o nl y i n q ui e s c e nt me l t . In
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s he a r e d me l t no t r a ns c r ys ta l l i za t i o n b ut c yl i nd r i t i c gr o w th( i nd uc e d b y α - p ha s e r o w nuc l e i [ 5 ] ) o c c ur s . Cyl i nd r i t i cc r ys ta l l i za t i o n i s a s p e c i a l v a r i a nt o f s e l f nuc l e a t i o n.Ac c o r d i ng to the a utho r s ’ kno w l e d ge no r e p o r t w a s p ub l i s he do n the t r a ns c r ys ta l l i za t i o n i n s P P . Wha t i s the r e a s o nb e hi nd ? T hi s i s l i ke l y a n e f fe c t o f the p r e s e nt c o mme r c i a ls P P gr a d e s ma nufa c tur e d b y me ta l l o c e ne c a ta l ys ts . T he yp o s s e s s mo d e r a te ta c t i c i t y, s te r e o - a nd r e gi o r e gul a r i t y a ndthus the us ua l s ub s t r a te s a r e una b l e to ge ne r a te the i rt r a ns c r ys ta l l i za t i o n. Ke e p i ng i n mi nd tha t the mo s ts t r a i ght fo r w a r d e xp l a na t i o n o f t r a ns c r ys ta l l i za t i o n i s thee p i ta xi a l o v e r gr o w th o n s ui ta b l e c r ys ta l l i ne s ub s t r a te s [ 6 -7 ] , a s l o ng a s s P P o f hi gh ta c t i c i t y i s no t a v a i l a b l e ther i ght p o l i c y i s to fo c us o n the s ub s t r a te s . T he hyp o the s i so f th i s w o r k w a s tha t c a r b o n f i b e r s ( CF ) w i th the hi ghe s tc r ys ta l l i n i t y a nd mo s t p e r fe c t c r ys ta l l i ne s t r uc tur e ma yt r i gge r t r a ns c r ys ta l l i za t i o n i n s P P .
2. EXPERIMENTALSThe sPP used was provided by Fina Research (Feluy, Belgium)a nd s ho w e d the fo l l o w i ng c ha r a c te r i s t i c s : ta c t i c i t y ( r r r r b ynuc l e a r ma gne t i c r e s o na nc e , NMR) =0 . 6 8 , Mw =1 2 8 kg/ mo l ,M w / M n =1 . 6 5 , me l t i ng a nd c r ys tal l i za t i o n te mp e r a tur e s ( b yd i f fe r e nt i a l s c a nni ng c a l o r i me t r y, DS C) 1 2 4 ° C a nd 6 3 ° C,r e s p e c t i v e l y. F o r c o mp a r i s o n p ur p o s e a n i s o ta c t i c P P ( i P P )homopolymer (Novolen® 100N, BASF, Ludw igshafen, Germany)w a s us e d i n f i l m fo r m ( thi c kne s s : 5 0 µ m) .P o l ya c r yl o ni t r i l e - b a s e d hi gh- te na c i ty ( HT CF , Id e mi ts u Ko s a nCo . , Chi b a , J a p a n) a nd me s o p ha s e p i tc h- b a s e d hi gh- mo d ul usc a r b o n f i b e r s ( HMCF , p r o d uc e d a t the C l e ms o n Uni v e r s i t y,Clemson, SC, USA) were used as heterogeneous nucleants . Thel a t te r w a s a v a i l a b l e i n r i b b o n a nd c - s ha p e fo r ms ,r e s p e c t i v e l y ( F i gur e 1 ) . No ne o f the CF s c o nta i ne d a nys i z i ng o r s ur fa c e f i n i s h. S o me HMCF s , o n the o the r ha nd ,w e r e c o a te d b y s i l i c o n c a r b i d e ( S i C ) v i a a c he mi c a l v a c uumd e p o s i t i o n ( CVD) p r o c e s s .
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Single fiber microcomposi tes were produced by “sandwiching”the CF b e tw e e n tw o f i l ms o f i P P a nd s P P ( p r o d uc e dp r e v i o us l y) w hi c h w a s he a te d to e r a s e the me l t me mo r y ( Tf )p r i o r to c o o l i ng to the i s o the r ma l c r ys ta l l i za t i o nte mp e r a tur e ( Tc ) . T he fo l l o w i ng c o nd i t i o ns w e r e s e t fo r s P Pa nd i P P , r e s p e c t i v e l y:T f =1 8 0 [ 8 - 9 ] a nd 2 0 0 ° C [ 5 ] , ho l d i ng t i me : 5 mi nT c =1 1 0 a nd 1 3 3 ° C, ho l d i ng t i me : c a . 1 h.T he a b o v e Tf v a l ue s a l o ng w i th the ho l d i ng t i me w e r ed e f i n i te l y s uf f i c i e nt to d e s t r o y a l l nuc l e i a nd p r o d uc e ame l t o f “ b l a nk me mo r y” .T he c r ys ta l l i za t i o n o f the q ui e s c e nt me l t w a s o b s e r v e d i n ahot s tage (THM S600 of Linkam, Water fie ld, UK) under crossedp o l a r s . S i ngl e f i b e r mi c r o c o mp o s i te s w e r e a l s o p r o d uc e d b yp l a c i ng o ne f i b e r o n the P P me l t s ur fa c e p r i o r to c o o l i ng toT c . T he s e s p e c i me ns w e r e l a te r e tc he d c he mi c a l l y a c c o r d i ngto the p r o c e d ur e o f O l l e y a nd Ba s s e t t [ 1 0 ] i n o r d e r to s tud ythe i nte r p ha s e mo r p ho l o gy b y p ha s e c o nt r a s t l i ght mi c r o s c o p y( P CLM; Le i tz , We tz l a r , Ge r ma ny) , s c a nni ng e l e c t r o nmi c r o s c o p y ( S E M, ; J e o l 5 4 0 0 , T o kyo , J a p a n) a nd a to mi c fo r c emi c r o s c o p y ( AF M; Na no S c o p e , D i gi ta l Ins t r ume nts , S a ntaBa r b a r a , CA, US A) . T he to p o gr a p hy o f the e tc he d s a mp l e s w a ss c a nne d ( c a . 1 Hz) i n ta p p i ng mo d e o f the AF M.
3. RESULTS AND DISCUSSIONSTranscrystalliz at ionF i gur e 2 d e p i c ts ho w e f f i c i e nt α - p ha s e nuc l e a nt the HMCF i ni P P i s . T hi s i s c o nfo r m w i th o ur p r e v i o us r e s ul ts [ 5 , 1 1 ] .
HMCF d o e s i nd uc e t r a ns c r ys ta l l i za t i o n i n s P P ( F i gur e 3 a )w he r e a s HT CF no t ( F i gur e 3 b ) .T he thi c kne s s o f the t r a ns c r ys ta l l i ne l a ye r i n b o th s P P( F i gur e 3 a ) a nd i P P ( F i gur e 2 ) i s ma tc he d w i th the s i ze o fthe s p he r ul i te s , a s e xp e c te d .T he nuc l e a t i o n e f f i c i e nc y o f HMCF d i mi ni s he s a f te r S i Cc o a t i ng ( F i gur e 4 a ) . In c a s e o f p o o r S i C - c o a t i ng the no n-c o a te d ( “ b a r e ” ) s i te s s t i l l p r e s e r v e the i r nuc l e a t i o na b i l i t y ( F i gur e 4 b ) . Co ns i d e r i ng the fa c t tha t HMCF s e xhi b i tthe hi ghe s t c r ys ta l l i n i t y a nd mo s t p e r fe c t c r ys ta l l i nel a t t i c e s t r uc tur e , o ne c a n c o nc l ud e tha t t r a ns c r ys ta l l i n i t y
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i n s P P d e p e nd s a l s o o n c r ys ta l l i n i t y p a r a me te r s o f thes ub s t r a te , v i z . HMCF . T hi s a s s ump t i o n s ho ul d b e c he c ke d ,ho w e v e r , b y a s s e s s i ng the c r ys ta l l i n i t y p a r a me te r s o f HMCF .F ur the r i nv e s t i ga t i o ns a r e a l s o ne e d e d to c l a r i fy theme c ha ni s ms o f t r a ns c r ys ta l l i ne o v e r gr o w th ( w he the r o r no t o fe p i ta xi a l o r i g i n [ 6 - 7 ] ) . Re me mb e r tha t S i C i s c r ys ta l l i ne ,a s w e l l . T he l a c k o f t r a ns c r ys ta l l i n i t y w i th S i C - c o a te d HMCFhi nts tha t the r e q ui r e me nts o f e p i ta xy ( l a t t i c e ma tc h i n thec r ys ta l l i ne s t r uc tur e b e tw e e n the s ub s t r a te a nd p o l yme r ,p o l yme r d e p o s i t i o n o nto the s ub s t r a te e tc . ) a r e no mo r e me t .T he r e fo r e , p r o v i d i ng the HMCF w i th v a r i o us c r ys ta l l i nec o a t i ngs w o ul d b e a v e r y us e ful s t r a te gy to c he c k thec o nt r o l p a r a me te r s a nd me c ha ni s ms o f e p i ta xi a l gr o w th.
I t i s w o r th e mp ha s i z i ng the d i f fe r e nc e i n the s p he r ul i t i cmo r p ho l o gy b e tw e e n i P P ( F i gur e 2 ) a nd s P P ( F i gur e s 3 a nd 4 ) .In i P P w e l l - d e v e l o p e d s p he r ul i te s a r e p r e s e nt w he r e a s i n s P Pa d i s o r d e r e d f i ne s p he r ul i t i c te xtur e c a n b e r e v e a l e d .
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Built -Up of the Transcrystalline Laye rT he d i f fe r e nc e i n the t r a ns c r ys ta l l i ne mo r p ho l o gy b e tw e e ns P P a nd i P P b e c o me s p e r t i ne nt w he n v i e w e d b y P CLM a f te re tc hi ng. F i gur e 5 c o mp a r e s the P CLM p i c tur e s i n i P P ( F i gur e5 a ) a nd s P P ( F i gur e 5 b ) , r e s p e c t i v e l y. F ur the r mo r e , the f i nes t r uc tur e , v i z . l a me l l a r a r r a nge me nt , w i th i n thet r a ns c r ys ta l l i ne l a ye r i s a l s o d i f fe r e nt b e tw e e n i P P a nd s P Pw hi c h i s the o utc o me o f d i f fe r e nc e s i n the c r ys ta l l i nes t r uc tur e a nd c r ys ta l l i za t i o n b e ha v i o ur b e tw e e n i P P a nd s P P .In i P P b o th nuc l e a t i o n s i te s a nd i mp i nge me nt l i ne s b e tw e e nthe “ uni d i r e c t i o na l l y” gr o w i ng s p he r ul i te s c a n w e l l b eo b s e r v e d . F i gur e 5 a s ho w s a l s o the l a me l l a r l a y- up a nd i t so r i e nta t i o n. By c o nt r a s t , the f i ne s t r uc tur e o f thet r a ns c r ys ta l l i ne l a ye r i n s P P i s muc h l e s s r e s o l v e d . T hel a me l l a r l a y- up o f the s P P ( d i s o r d e r e d b und l e s o r f i b r i l s )i n the v i c i n i t y o f HMCF s e e ms to b e v e r y s i mi l a r w ha t w a so b s e r v e d i n s o me s i te s o f the b ul k ( s e e F i gur e 7 a ) w hi c h a r el i ke l y a ggr e ga te s o f l a r ge s i ngl e c r ys ta l s [ 1 2 ] .
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T he S E M p i c tur e ta ke n f r o m the i nte r fa c i a l r e gi o n o f thes P P / HMCF mi c r o c o mp o s i te s ho w s tha t the th i c kne s s o f thet r a ns c r ys ta l l i ne l a ye r i s c a . 1 0 µ m ( F i gur e 6 ) . F i gur e 6s ugge s ts tha t the o r i e nta t i o n o f the l a me l l a e i n thet r a ns c r ys ta l l i ne l a ye r d i f fe r s f r o m tha t o f the b ul k. T hi sb e c o me s o b v i o us i n the AF M p i c tur e s ( c f . F i gur e s 7 a nd 8 ) .T he AF M s c a ns ta ke n b y a mp l i tud e o s c i l l a t i o n r e v e a l the ma i nd i f fe r e nc e b e tw e e n the t r a ns c r ys ta l l i ne r e gi o n a nd the b ul k.In the i nte r fa c i a l t r a ns c r ys ta l l i ne r e gi o n the s P P l a me l l a es e e m to b e l a r ge r a nd / o r a l i gne d d i f fe r e nt l y f r o m tha t o fthe b ul k. A f l a t - o n typ e o v e r gr o w th [ 1 3 ] o f the s P P o n theHMCF s ub s t r a te c a n b e s up p o s e d b a s e d o n F i gur e 8 . T hemi c r o b e a m X- r a y s c a t te r i ng te c hni q ue i s t r us te d to s he dl i ght o n the l a me l l a r a r r a nge me nt i n the i nte r fa c i a l r e gi o n.
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4. CONCLUSIONSBa s e d o n thi s s tud y p e r fo r me d o n c a r b o n f i b e r ( CF ) - i nd uc e dt r a ns c r ys ta l l i za t i o n i n s ynd i o ta c t i c p o l yp r o p yl e ne ( s P P ) thefo l l o w i ng c o nc l us i o ns ma y b e d r a w n:i - t r a ns c r ys ta l l i za t i o n o f s P P c a n b e t r i gge r e d b y hi gh-
mo d ul us CF ( HMCF ) s ho w i ng the hi ghe s t c r ys ta l l i n i t y a ndmo s t p e r fe c t c r ys ta l l i ne s t r uc tur e . S he a thi ng thes ur fa c e o f HMCF b y S i C s to p s the gr o w th o ft r a ns c r ys ta l l i za t i o n. T hi s fa c t ma y b e t r e a te d a s a ni nd i r e c t e v i d e nc e fo r the e p i ta xi a l o r i g i n o ft r a ns c r ys ta l l i za t i o n.
i i - a c c o r d i ng to o p t i c a l , s c a nni ng e l e c t r o n a nd a to mi cfo r c e mi c r o s c o p i c i ns p e c t i o ns the l a me l l a r a r r a nge me nti n the t r a ns c r ys ta l l i ne l a ye r d i f fe r s f r o m tha t o f b ul ki n s P P . F ur the r i nv e s t i ga t i o n b y us i ng mi c r o fo c us X- r a ys c a t te r i ng a r e no w i n p r o gr e s s i n o r d e r to c l a r i fy thec ha r a c te r i s t i c s o f the l a me l l a r gr o w th.
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