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Col lo id Polym er Sc ienc e Colloid PolymerSci. 260, 31- 36 (1982)

O p t i c a l p r o p e r t i e s a n d s t r u c t u r e o f d r a w n p o l y e t h y l e n e t e r e p h t a l a t e - p o l y -e thy lene f ilms : )W . W e n i g a n d R . H a m m e lLabo ra to r ium fCir Ang ewa ndte Phys ik , Universi t~i t - G H - Du isburg

Abstract Films made by coex trus ion o f po lye thy lene te reph ta la te and low dens i typo lye thy lene exh ib i t specu la r re f lec t ion o f l igh t when s tre tched. Un l ike the behav iour o ff i lms made of pure PET , wh ere specu la r re f lec tion is ra re and does occur a t ex tremely h ighs tre tch ing ra tes on ly , PET /LD PE f i lms show specu la r re f lec t ion independe n t o f thestretching rate .A f i lm, con ta in ing 70% PE T and 30% LD PE, p repared by coex trus ion o f the twocomp onents , has b een inves tigated by sca nn ing e lec tron mic roscopy , l igh t mic roscopy andby measuring the op t ica l p roper t ie s wi th a spec trophotomete r . The (uns tre tched) , , a sprep ared sample has been compare d with a sample s tretched to it = 4.The op t ica l measurements show as a resu l t o f the s t re tch ing a s t rong decrease o f thetrans mitt ance and an increase of the remittance. W hile there is no drastic change of thedispe rsion (which is onl y s light), the increase of the refractive in dex indicates som e strain-induced c rys ta l l iza t ion .The scann in g e lec tron mic rographs show long , need le l ike v o ids and ind ica te a f rac t iona t ionof the tw o co m pon ents as a result of the s tretching. This frac tiona tion has been investigatedby hea t ing the samples up und er the l igh t mic roscope : whi le the s t re tched sample show s asepara t ion o f the compo nents , the uns tre tched sample does n o t .Key words PE T/P E, Specular Reflection, O ptical Properties , Structure.

1 I n t r o d u c t i o nT h e o r i e n t a t i o n o f P o l y e t h y l e n e t e r e p h t a l a t e ( P E T )f i lm s h a s b e e n w id e ly i n v e s t i g a t e d [ 1 ] . I t i s k n o wn ,t h a t a m o r p h o u s P E T m a y c r y s t a l l i z e w h e n o r i e n t e da n d t h a t t h e r e s u l t i n g m e c h a n i c a l p r o p e r t i e s d e p e n do n th e s t r e t c h in g r a t e . Un ia x i a l s t r e t c h in g o f PE Tf i lm s h a s b e e n in v e s t i g a t e d b y Bia n g a r d i a n d Z a c h -m a n n [ 2 , 3 ] . T h e a u th o r s f i n d , t h a t t h e s t r u c tu r e o fa m o r p h o u s P E T s t r e t c h e d w i t h d i f f e r e n t r a t e s a td i f f e r e n t t e m p e r a tu r e s a n d c r y s t a l l i z e d a f t e r wa r d s d e -p e n d s m a in ly o n t h e b i r e f r i n g e n c e o f t h e sa m p le a f t e rs t r e t c h in g b e f o r e c r y s t a l l i z a t i o n . Wi th i n c r e a s in gd r a win g r a t e a l l s t r u c tu r e s o f s t r e t c h e d m a te r i a l f r o msp h e r u l i t e s t o l a m e l l a e sp l i t u p i n to m o sa i c b lo c s a r ef o u n d . A l s o t h e n u m b e r o f t i e - m o l e c u le s i nc r e as e w i t hthe s t re tch ing ra te .H i n r i c h s e n e t a l [ 4 ] i n v e s t i g a t e d t h e o r i e n t a t i o nm e c h a n i s m d u r i n g b i a x i a l d r a w i n g o f P E T f i l m s . A tl o w d r a w r a t i o s , t h e s t r u c t u r a l d e f o r m a t i o n s f o l l o wt h e k n o w n o r i e n t a t i o n m e c h a n i s m s [ 5 - 7 ] , w h i l e a th i g h e r d r a w r a ti o s t h e a u t h o r s c o n c l u d e f r o m a*) D edicated to Prof. D r. F. H. M tiller.

c o m p a r i s o n o f p o l a r i z a t i o n m i c r o s c o p e e x p e r i m e n t sa n d X - r a y w i d e a n g l e a n a l y s i s t h a t t h e a l i g n m e n t o ft h e a m o r p h o u s r e g io n s i n t o t h e s e c o n d d r a w d i r e c t io no c c u r s e a s i e r t h a n t h a t o f t h e c r y s t a l l i t e s . T h e r e su l t ssu g g e s t , t h a t t h e s t r u c tu r a l t r a n s f o r m a t io n s o c c u r o nth e su p e r s t r u c tu r a l l e v e l r a th e r t h a n o n th e m o le c u l a rscale.Be s id e s t h e m e c h a n ic a l p r o p e r t i e s a l so t h e t h e r m a lb e h a v i o u r o f d r a w n P E T f il m s d e p e n d s o n t h e s t re t c h -in g c o n d i t i o n s [ 8] .I f lo w d e n s i t y p o l y e t h y l e n e ( L D P E ) i s b l e n d e d i n t ot h e P E T , b o t h m e c h a n i c a l a n d t h e r m a l p r o p e r t ie s a r ea l t e r e d , a s h a s b e e n o b se r v e d w i th o th e r i n c o m p a t ib l eb lends [9 ] .A f i l m , p r e p a r e d b y c o e x t r u s i o n o f P E T a n d L D P E( 7 0% P E T , 3 0 % L D P E ) e x hi b it s s p e c u l a r r e f l e c ti o na l r e a d y a t v e r y l o w d r a w i n g r a t e s . W i t h p u r e P E T ,sp e c u la r r e f l e c t i o n i s v e r y r a r e a n d c a n o n ly b eo b se r v e d a t e x t r e m e ly h ig h d r a win g r a t e s [ 1 0 ] .I n t h i s p a p e r , we in v e s t i g a t e t h e o p t i c a l p r o p e r t i e sa n d s t r u c t u r e o f a P E T / L D P E f i l m s h o w i n g s p e c u l a rr e f l e c t i o n . T h e o b se r v e d f a c t , t h a t sp e c u l a r r e f l e c t i o nc a n n o t b e a c h i e ve d w i t h f il m s p r e p a r e d f r o m r e g r a n u -] a t e d P E T / L D P E b l e n d s w i l l b e c o n s i d e r e d .

K 74

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32 Colloid and Polymer Science, VoL 260. No. 1 1982)S A M P L E P O S i T i O N

F O R T R A N S M I T T A N C EM E A S U R E M E N T\ ]

\ ]R E F L E C T A N C E

S T A N D A R D SFig. 1. In tegra t ing sphere o f specular ref lec t ion type

2 . E x p e r i m e n t a lA com mercia l fi lm of th ickness d = 0 .1 mm conta in ing 70% PE Ta nd 30% LDP E, p r e pa re d by c oe x t r u s ion o f bo t h c ompone n t s , wa sinves tigated bo th in the as prepa red (uns t re tched) s ta te andstretched to ~ = 4.F o r t h e o p ti c al m e a su r em e n ts a D I A N O / H A R D Y 1 r e c or d in gs pe c t r opho t ome t e r wa s u s e d i n c on j unc ti on w i t h a PDP 11 c omp u-ter . The w avelength range of the spect ro photo mete r was 380 to 700nanometers . An in tegra t ing sphere was used tha t a l lowed a l lmeasurem ents acco rding to AST M D 1003 [11] ( see fig . 1). Thesphere d iam eter of the ref lec t ion sphere was 8 inches .A H i tachi S 520 scanning e lec t ron micros cope was used for theSEM measurem ents af ter cut t ing the samples by use of a micro tom eand coat ing wi th gold (a luminum coated samples have a lso beeninvestigated). A large num ber of pictures have been shot and it wasfound , tha t the var ious s t ruc tures d isplayed in the figures are typica land found all over the sample.Pieces of f i lm have been heated up un der the opt ica l microsco pein a M ET TL ER ho t stage both in the as prep ared as wel l as in thestretched state. D iffere nt heatin g rates have been applied, a tem pera -ture cont ro l ler a l lowing the programming of the heat ing ra te .

3 . Resultsa O p t i c a l p r o p e r t i e s

Figure shows the reflectance as well as thet ra ns m it ta nce a s a f unc t i o n o f t he w a v e l en g t h f o r bo t hs a m ples . A s co u l d be ex pec t ed f ro m t he a ppea ra nce o ft he sa m pl es i n po l y ch ro m a t i c l i g h t t he e f fec t o fs tretching resul t s in an increase in ref lectance and adecrea s e i n tra nsm i tt ance t he w a v e l eng t h depen dences ho w i n g o n l y a s m a ll cha ng e.

H a ze v a l ues w h i ch ca n be o bt a ined f ro m t het ra ns m it ta nce m ea s u rem en t s s h o w t ha t t he un-s tretched sample can be cons idered to be t ransparentwhi le i t becomes t rans lucent a f t er s t retching [11] . Thisi s a lso apparent f ro m th e va lues o f the a lbedo

100

80

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100 I I I

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I - -I - -~ELOZ10 ) . a = rem ittance, b = transm ittance

b S t r u c t u r eThe scanning electron micrographs showing theunstretch ed sample fig. 4) display no indication of anoriented structure which confirms, that the extrusionof the blend does not impose a texture on the finalsample.Up on stretc hing the sample specular ref lect ionstarts to occu r at ~ ~ 2) undergo es a drastic change in

although it contains both the scattering and theabsorption coefficients.Remittance as well as transmittance values can becalculated by theor etical mo dels [12-14]. It turne s out,that the calculations have to be carried out using thethree-flux model proposed by Pauli and Eitle [15]rather than by the two-flux model of Kubelka andMunk [16], where infinite thickness of the sample isassumed. The results yielded are much the same thanthose obtained for pigmented media and indicate thepresence of large compa red to the wavelength of theincident light beam) particles in the stretched sample.Th e inte gral scattering appears to inc rease as a result ofthe stretching while the scattering shifts to smallerangles. Figure 3 shows the calculated deviations be-tween the theoretical and measured curves of bothremittance and transmittance.Refractive indices can be determined from themeasured remittance by use of the Fresnel and Beerequations or from the calculated intensities as well.The increase of the refractive index n = 1.554 for theunstretc hed and n = 1.584 for the stretched sample)indicates partial strain induced crystallization of thesample.Both samples do show some dispersion, thewavelength dependence of the refractive index is,however, rather weak. Fig. 4. Scanning electron micrograph of the unstretched sample.a: endview, b: sideview

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34 Colloid and Polymer Science, VoL 260. No. 1 1982)

Fig. 6. Splatview of the stretched sample

Fig. 5. M icrograph of the stretched sample a: endview b : sideview Fig. 7. Sideview of the stretched sample in a higher magnification

structure. Large voids of about 25 m in length and upto 1 /am wide occur (fig. 5) that are arranged almostregularly parallel to the film surface (see also fig. 6).The micrographs of the end view show, that theselong, needlelike voids fill the whole sample in analmost layerlike fashion. Moreover, as figure 7 sug-gests, the voids appear coated with a thin layer ofpolyeth ylene thus enhancing the specular reflection by

lowering the angle of total reflection thro ugh its lowerrefractive index. Possibly upon putting stress on thesample the material breaks inside the PE domainsbeing dispersed within the PET matrix. This wouldmean a further phase separation of the two compo-nents and should effect the melting properties, as canbe observed with PP/PE blends.

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Optical properties and structure of drawn polyethyleneterephtalate polyethylene films 35

Fig. 8. O ptical microg raphs of the stretched sample. The sample has been heated up to 270 ~ and kept at that temperature fo r 2 min. Themicrograp hs show the edge of the sample while cooling dow n to 7 -- 108 ~ a: T ~ 250 ~ b: T = 108 ~ 2 rain., c: T = 108 ~ 5 min.(Reproduced from colour prints)c Meltin g properties

In order to investigate the melting properties andprove the concept of phase separation as a conse-quence of stretching we heated up the samples andobserved the segregation of the two components byoptical microscopy. It has to be expected that thesegregation can be m ore r eadily complete d in a samplewhere the phases are already separated.

Fig. 9. Optic al microgr aph of the edge of the unstretched sample atroo m temperature after keeping it at 280 ~ for 2 rain. (Repro ducedfrom colour print)

The results of these investigations are shown infigures 8 and 9. Upon heating up the stretched sampleto the melting temperature of polyethylene, the sam-ple changes its pattern and small elongated dots darkspots between crossed polarizers) become visible.Heating further up, a coalescence of these spotsbecome evident which occurs, however less pro-nounced, in the unstretched sample too.The figures 8 and 9 display the edge of the stretchedand the unstretched samples respectively. From thestretched sample molten material is expelled from thesample w hen kept at a temperature slightly below themaximum m elting temperature of PET the sample hasbeen kept at 270 ~ for 2 min.). Upon cooling dow n,no crystallization is observed above 110~ At108 ~ howev er, crystallization occurs as is typicalfor low density polyethylene. A wide angle X-rayexperiment confirms, that the expelled material isindeed LD PE. No spherulitic crystallization has beenobserved probably due to a small amount of PETleading to a partially solodification of the expelledLD PE at 250 ~ as displayed in figure 8.The unstr etched sample fig. 9) does not show sucha separation of the two components when treated thesame way. Figure 9 shows the unstretched sampleafter melting up 2 min. at 280 ~ followe d bycooling down to room temperature. Although somephase sepa ration inside the sample can be observed, noLDPE has been expelled from the blend.

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36 Colloid and Polymer Science, Vol. 260. No. 1 1982)

T h i s e x p la ins r e a d i ly t h a t a f i lm p r e p a r e d b ye x t r u s io n o f t h e r e g r a n u la t e d b l e n d h a s a l o we r t e n s i les t r e n g th a n d d o e s n o t sh o w a n y sp e c u la r r e f l e c t i o n .eferences

1 . S a m u e l s , R . J ., S t r u c t u r e d P o l y m e r P r o p e r t i e s , J o h n W i l e y a .S o n s N e w Y o r k , 19 7 4 ).2 . B i a n g a r d i , H . J . , H . G . Z a c h m a n n , J . P o l y m . S c i . , P o l y m .Sym p. 58 , 169 1977) .3 . B i a n g a r d i , H . J . , M a k r o m o L C h e m . 1 7 9 , 2 0 5 1 1 9 7 8) .4 . H i n r i c h s e n , G . , A . E b e r h a r d , U . L i p p e , H . S p r i n g e r , C o l l . a n dPo lym . Sci. 259, 73 1981).5 . A rgon , A . S ., Ph i l . Mag . 28 , 839 1973) .6 . D u c k e t t , R . A . , B . C . G o s w a m i , I . M . W a r d , A . M . Z i h l i f , J .Po ly m . Sci ., Po ly m . Phy s . Ed . 15 , 333 1977) .7 . F r i ed r ic h , K. , J . Mat . Sc i . 15 , 258 1980) ,8 . S p r i n g e r , H . , U . B r i n k m a n n , G . H i n r i c h s e n , C o l l . a n d P o l y m .Sci. 259, 38 1981).

9 . H a r r i s o n , I . , J . R u n t , J . P o l y m . S c i ., P o l y m . P h y s . E d . 1 8 , 2 2 5 71980).1 0 . B i a n g a r d i , H . J . , p r i v a t e c o m m u n i c a t i o n .1 1. A N S I / A S T M D 1 0 0 3 - 6 1 A m e r i c a n N a t i o n a l S ta n d a r d .12 . Orchard , S . E . , J . Op t . Soc . Am. 59 , 1584 1969) .13 . Mudge t , P . S . , L . W. R ichards , App l . Op t . 10 , 1485 1971) .14 . Giov ane l l i , R . G. , Op t . Ac ta , 2 , 153 1955) .1 5 . P a u l i , H . , D . E i t l e , C o l o u r 7 3 , p . 4 2 3 , A d a m H i l g e r , L o n d o n1973).16 . Kub e lka , P . , F . M unk , Z . techn . P hys . 12 , 593 1931) .R e c e i v e d S e p t e m b e r 2 8 , 1 9 8 1

A u t h o r s a d d r e s s :W e r n e r W e n i gU n i v e r s i t ~ i t - G H - D u i s b u r gL a b o r a t o r i u m f Ci r A n g e w a n d t e P h y s i kLotharstraf~e 654 1 00 D u i s b u r g 1 / W e s t G e r m a n y