Structure-Property Relations in Polymers - American Chemical Society

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8 Applications of Raman Spectroscopy to the Study of Polydiacetylenes and Related Materials S. H. W. Hankin and D . J. Sandman* G T E Laboratories I n c o r p o r a t e d , 4 0 Sylvan R o a d , Waltham, MA 02254

Raman spectroscopy is a useful and sensitive probe of the structure and properties of polydiacetylene (PDA) materials. Using 1064, 632.8, 514.5, 488.0, and 457.9 nm as wavelengths of excitation, Raman spectra were observed using light polarized in the direction of the chain axis for the PDA from 1,6-di-N-carbazolyl-2,4-hexadiyne (DCH) and 1,1,6,6-tetraphenylhexadiynediamine (THD) and for chemically modified versions of these materials. From the changes in the spectra with wavelength of excitation, it was deduced that these pristine PDA single crystals have disordered surface phases. For poly-DCH that has gained six Br atoms per repeat unit, Raman spectra provide a direct indication of the extensive conversion of the PDA backbone structure to that of a mixed polyacetylene. The normal modes associated with triple- and double-bond stretching on the PDA backbone provide a useful structure-property relationship for the characterization of the products of diacetylene polymerizations where the degree of definition does not approach that of fully polymerized single crystals.

POLYDIACETYLENES ARE A CLASS OF POLYMERS w i t h conjugated backbones available i n the f o r m o f macroscopic single crystals (1-3) i n certain cases. A s single crystals, polydiacetylenes ( P D A ; 1) are the best d e f i n e d class o f organic polymers, a n d it is expected that the properties o f these electrically insulating * Corresponding author. Department of Chemistry, University of Massachusetts, Lowell, M A 01854 0065-2393/93/0236-0243$6.00/0 © 1993 American Chemical Society

Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.


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materials might serve as models f o r the properties o f other conjugated polymers that are less w e l l defined. A t the present time, conjugated polymers i n their insulating forms are u n d e r investigation f o r applications such as photoconductivity, t h i r d - o r d e r nonlinear optical p h e n o m e n a , a n d sensors based o n c h r o m i c changes (4). A recent report o f electroluminescence ( 5 ) may presage another application o f these materials. A l l o f these potential uses involve a knowledge o f the electronic spectrum i n the solid state. Therefore, the s t r u c t u r e - p r o p e r t y relationships associated w i t h the solid-state spectra o f conjugated polymers are important to the understanding a n d use o f these materials. T h e solid-state electronic spectra o f P D A crystals are usually discussed i n the same framework as neutral molecular crystals ( 6 ) , that is, E(k)

=E

0

(1)

+ D + I(k)

E(k) is the observed solid-state transition energy, E is the gas phase transition energy for an isolated moiety, D summarizes the energetics o f the gas-to-crystal shift, a n d summarizes the exciton transfer interaction between translationally equivalent a n d nonequivalent moieties. F i g u r e 1 displays the solid-state electronic spectra o f two P D A crystals w i t h chemically related side chains: p o l y - D C H (l,6-di-iV-carbazolyl-2,4-hexadiyne; l a ) a n d p o l y - T H D (1,1,6,6-tetraphenylhexadiynediamine; l b ) ( 7 ) . Because b o t h crys0

RCH

Id

R=

le

R=

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r

2

4

E

T

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15

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Raman Spectroscopy and Polydiacetylenes

550 1

17

450

1

1

r

19

21

23

25

Energy (10 cm ) 3

1

Figure 1. Near-normal incidence h-axis reflection spectra of poly-DCH ( ) and poly-THD (—). Arrows indicate wavelengths of Raman excitation. (Reproduced with permission from reference 7. Copyright 1987 Elsevier Science Publishers B.V.)

tals are completely p o l y m e r i z e d , they have " i n f i n i t e conjugation lengths." Therefore, the longer wavelength absorption o f p o l y - D C H versus p o l y - T H D is d u e to the environment s u r r o u n d i n g the conjugated chains; that is, there is a larger c o n t r i b u t i o n to D a n d I(k) i n 1 f o r p o l y - D C H c o m p a r e d to p o l y - T H D . F o r the P D A f r o m the bis-p-toluenesulfonate o f 2,4-hexadiyn1,6-diol ( P T S ; lc), it is recognized that the environment contributes about 1 5 % to E(k) (8). I n recent years, extension o f the molecular crystal frame w o r k o f P D A to other conjugated polymers ( 9 , 10) has b e e n useful. R a m a n spectroscopy, especially resonance R a m a n ( R R ) spectroscopy, is recognized as a valuable t o o l for the study o f the structure a n d properties o f conjugated polymers i n general (11, 12) a n d P D A i n particular (11-13). A n important factor i n this utility is the lack o f interference d u e to strong fluorescence f r o m P D A crystals. T h e intensities o f R a m a n bands associated w i t h totally symmetric vibrational modes strongly c o u p l e d to the electroni cally excited state m a y b e enhanced b y a factor as large as 1 0 . R R is particularly attractive f o r the study o f conjugated polymers because the enhanced intensity permits isolation o f the R a m a n bands o f the c h r o m o p h o r e . R R spectroscopy c a n give i n f o r m a t i o n about the properties o f a species i n a n 6


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electronic excited state. T h e excitation profile can be obtained b y measuring the R a m a n cross section for a vibrational m o d e as a f u n c t i o n o f incident p h o t o n energy, a n d an estimation o f the strength o f the interaction between an electronic excited state a n d a vibrational m o d e may be obtained f r o m an excitation profile. E x c i t a t i o n profiles have b e e n obtained for several vibra tional modes o f p o l y - P T S (11, 12). T o measure an excitation profile, a vibrational m o d e cannot change its R a m a n shift as the incident excitation energy is changed; that is, there must be no dispersion (14). Strong dispersion o f b a n d shapes a n d peak positions is often f o u n d i n R R spectra o f conjugated polymers that are less o r d e r e d than P D A crystals, such as polyacetylene [ ( C H ) ; 2] (14, 15), polythiophenes (3) (16, 17), and disordered films o f soluble P D A such as the P D A f r o m the bis(butoxycarbonylmethyleneurethane) o f 5 , 7 - d o d e c a d i y n - l , 1 2 - d i o l ( 4 B C M U ; Id) (18). These materials are usually studied i n t h i n - f i l m f o r m . X

T h e R R studies o f p o l y - P T S , - D C H , a n d - T H D have b e e n p e r f o r m e d w i t h relatively thick single crystals. F o r such samples w h e r e the absorption coefficient exceeds 1 0 throughout the wavelengths o f interest, it is important to recognize that R R spectroscopy is p r i m a r i l y a probe o f the surface regions o f the crystal (11). 5

O u r initial motivation i n applying R R spectroscopy to the study o f P D A was to learn greater detail about the structural changes that occur i n the course o f the c h e m i c a l modification o f p o l y - D C H (19). T h e R R spectra o f pristine p o l y - D C H a n d - T H D have b e e n s t u d i e d to reveal changes that o c c u r f r o m c h e m i c a l modification. D u r i n g these studies, we f o u n d it useful to study R a m a n spectra o f b o t h pristine a n d chemically m o d i f i e d P D A w i t h m u l t i p l e wavelengths of excitation. W e also i n c l u d e d F o u r i e r transform ( F T ) R a m a n excitation at 1064 n m , w h i c h is a wavelength w h e r e the materials u n d e r investigation are transparent (20). H e r e i n w e report o u r investigations o f R a m a n spectra o f single crystals o f p o l y - D C H and - T H D w i t h fight p o l a r i z e d



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along the p o l y m e r chain d i r e c t i o n as a function o f wavelength o f excitation, focusing p r i m a r i l y o n the n o r m a l modes associated w i t h t r i p l e - a n d d o u b l e b o n d stretching. F r o m these spectra, w e deduce evidence that the crystals have distinct surface phases, w h i c h is a previously unappreciated point. W e discuss the changes to the R a m a n spectra o f these materials that occur o n c h e m i c a l modification a n d h o w R a m a n spectroscopy has b e e n used to further understanding o f s t r u c t u r e - p r o p e r t y relationships i n these materials. T h e crystal structures o f p o l y - D C H a n d - T H D are k n o w n (21) and, thus, spectro scopic studies o f these materials a d d detail to substances o f k n o w n crystal a n d molecular structure. Because all results o f diacetylene polymerizations d o n o t lead to single crystal products, i t is possible, w i t h certain assumptions (given i n the following text), to use R a m a n spectroscopy to deduce i f these less w e l l - d e f i n e d polymers have the usual e n - y n e backbone structure established (21) f o r the best defined P D A .

Experimental Details General. M e l t i n g points are uncorrected. E l e m e n t a l analyses were p e r f o r m e d b y Schwarzkopf M i c r o a n a l y t i c a l Laboratory, W o o d s i d e , N e w Y o r k . T h e * H Ν M R spectrum was r e c o r d e d o n a 9 0 - M H z spectrometer (Varian) i n C D C 1 solution w i t h tetramethylsilane as reference. Samples o f p o l y - D C H (30), p o l y - T H D ( 7 ) , b r o m i n a t e d p o l y - D C H (19), a n d l,6-bis(3',6'-dibromoN-carbazolyl)-2,4-hexadiyne ( D C H B r ; 4 a ) (22) were either synthesized as previously described or were available f r o m earlier studies. T h e G a m m a c e l l at Brandeis U n i v e r s i t y was t h e source o f t h e C o radiation used i n these studies. 3

4

6 0

R—C H 2~-C=G—C=0—CH gr-R

4b

American Chemical Society Library 1155 16th St., N.W.Relations in Polymers Urban and Craver; Structure-Property Washington. 20036 Advances in Chemistry; American D.C. Chemical Society: Washington, DC, 1993.


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I n v i e w o f o u r d e d u c t i o n that P D A crystals have distinct surface phases, it was d e e m e d appropriate to analyze o u r samples o f P D A - D C H a n d - T H D for trace metals b y X - r a y fluorescence ( X R F ) a n d for surface contamination b y electron spectroscopy for c h e m i c a l analysis ( E S C A ) . A recently synthe sized sample o f P D A - T H D contained F e at a less than 10 p p m level a n d C u a n d B i at 3 0 - 4 0 - p p m levels b y X R F analysis whereas less than 10 p p m o f each o f these elements was f o u n d i n an o l d e r sample; o u r samples o f P D A - D C H contained less than 10 p p m o f each o f these elements. E S C A studies o f P D A - D C H a n d - T H D reveal atomic ratios for C a n d Ν compara b l e to those expected for b u l k material. T h e atomic percentages o f oxygen observed b y E S C A w e r e 7.4 a n d 5 . 0 % , respectively, for P D A - T H D a n d - D C H . N o evidence for specific C - O single- or d o u b l e - b o n d species was found.

Raman Spectroscopy. T h e light sources for these experiments w e r e an argon i o n laser ( C o h e r e n t Innova 70-4) w i t h 2 - 5 m W o f excitation at the samples i n the 514.5-, 488.0-, a n d 457.9-nm fines a n d a 1 0 - m W h e l i u m - n e o n laser (Spectra Physics) for 632.8-nm light w i t h < 1.5 m W at the samples. N o sample decomposition was observed w i t h laser excitation at these powers. T h e dispersive device is a spectrometer (Spex 1877 Triplemate) w i t h the entrance slit at 1 c m , the slit i n the center o f the bandpass m o n o c h r o m a t o r at 8 m m , a n d the slit at the entrance to the dispersive m o n o c h r o m a t o r at 50 μηι, T h e front bandpass m o n o c h r o m a t o r consists o f two 600 g r o o v e / m m gratings, a n d spectra w e r e obtained w i t h the 6 0 0 - g r o o v e / m m grating (band pass ± 5 c m at 1 9 , 4 3 6 c m ) i n the t h i r d monochromator. L i g h t was detected at 90° to the exciting laser b e a m b y an optical m u l t i c h a n n e l analyzer ( E G & G P A R C O M A III). Samples w e r e m o u n t e d o n an XYZ stage a n d aligned parallel to the i n c o m i n g light. T h e collection t i m e for the m u l t i c h a n n e l analyzer was set to maximize the signal o n the detector. D a m a g e to samples was observed at excitation powers near 50 m W ( n o r m a l survey scan power) d u r i n g the initial experiments o n these samples. T h i s damage ap p e a r e d as w h i t e n i n g o f the sample; that is, a " b l e a c h i n g o u t " o f the sample color. Subsequent experiments using p o l y - T H D a n d 488.0-nm excitation (23) demonstrated the deterioration o f the R a m a n signal w i t h laser powers above 17 m W at the sample that was manifested b y a decrease i n the ratio o f the heights o f the double- and t r i p l e - b o n d shirts w i t h increased exposure to the laser. - 1

- 1

Preparation of P o l y - D C H B r 9 . P o l y - D C H (204 m g , 0.50 m m o l ) a n d b r o m i n e (10 m L ) w e r e heated at reflux w i t h magnetic stirring for 24 h . T h e mixture was c o o l e d a n d d i l u t e d w i t h carbon tetrachloride (10 m L ) , a n d the solid p r o d u c t was isolated b y suction filtration o n a sintered glass f u n n e l . T h e straw-colored solid was w a s h e d w i t h C C 1 u n t i l the washings w e r e colorless and then v a c u u m - d r i e d to give 5 6 0 - m g product. X - r a y p o w d e r 4


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diffraction revealed t h e solid t o b e amorphous. T h e F o u r i e r transform infrared ( F T I R ) spectrum was indistinguishable f r o m that o f p o l y - D C H B r (19). Analysis—Calculated for C H N Br : C, 32.07; H, 1.44; N, 2.49; Br, 64.00. Found: C, 32.24; H, 1.42; N, 2.23; Br, 64.28. The observed analysis corresponds to C H Ν Br . 8

30

16

2

9

30

1 5 8

1φ8

8 9 9

Synthesis and Polymerization of the Bis-p-chlorocinnamate of 10 12-Docosadiyn-l,22-diol. T o a solution o f s u b l i m e d p - c h l o r o e i n Downloaded by EAST CAROLINA UNIV on April 8, 2018 | https://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch008

9

namoyl chloride (24) (14.9 g, 74 m m o l ) a n d 10,12-docosadiyn-l,22-diol (4b; 10.8 g, 32.3 m m o l ) i n tetrahydrofuran ( T H F , 100 m l ) , a solution o f p y r i d i n e (5 m L ) i n T H F (20 m L ) was a d d e d dropwise. T h i s mixture was refluxed f o r 5 h. T h e c o o l e d reaction mixture was p o u r e d into water (500 m L ) a n d the solid product, w h i c h t u r n e d light b l u e i n r o o m light, was collected o n a B u c h n e r f u n n e l . T h e solid was dissolved i n c h l o r o f o r m , a n d this solution was extracted twice w i t h aqueous N a H C O . T h e C H C 1 solution was d r i e d over M g S 0 , filtered, a n d evaporated to give a solid (20.2 g, 9 4 % yield) that was recrystall i z e d f r o m acetone to give 4b, 16.5 g ( 7 7 % yield), m p 8 4 - 8 5 °C. T h e melt was stable a n d n o evidence o f reaction was n o t e d o n heating 4b at 7 0 - 7 5 ° C for 2 days. T h e I R spectrum o f 4b [mineral o i l (Nujol)] exhibited t h e following significant absorptions (reciprocal centimeters): 1703, 1630, 1485, 1180, 1085, 1006, 980, 815, 720. T h e U N M R spectrum o f 4b exhibited the following: δ 7 . 2 - 7 . 8 ( m , 8 H , aromatic), δ 6 . 3 - 6 . 5 ( 4 H , - C H = C H - ) , δ 4 . 2 (t, J = 6 H z , 4 H , - O C H ) , δ 2.2 (t, J = 6 H z , - C H = C - ) , δ 1.1-1.8 ( m , 2 8 H ) . T h e X - r a y p o w d e r pattern o f 4b exhibited the f o l l o w i n g reflections ( d ; angstroms): 33.1, 18.1, 11.3, 8.50, 5.60, 5.36, 5.15, 5.07, 4.89, 4.69, 4.54, 4.32, 4.26, 4.18, 3.98. Analysis—Calculated for C H Cl O : C, 72.37; H, 7.30; CI, 10.68. Found: C, 72.09; H, 7.37; Cl, 10.44. Exposure o f 4b to C o g a m m a radiation (at least 50 M r a d over 3 0 days) converted i t to a coppery b r o w n crystalline solid. E x t r a c t i o n o f this solid w i t h hot hexane ( i n w h i c h 4b is soluble) f o r 6 h d i d not result i n a weight loss. T h e F T I R spectrum o f this p o l y m e r was similar to that o f 4b a n d exhibited the following: 2920, 2851, 1714, 1638, 1592, 1492, 1466, 1407, 1321, 1274, 1253, 1233, 1224, 1203, 1184, 1171, 1108, 1091, 1013, 9 8 1 , 821, 720. T h e X - r a y p o w d e r diffraction o f this p o l y m e r revealed t h e f o l l o w i n g reflections ( d , angstroms): 6.67, 5.96, 5.56, 5.41, 4.91, 4.76, 4.56, 4.39, 4.18, 4.05, 3.84. s

3

4

L

2

2

40

48

2

4

6 0

Raman Spectra of Single Crystals of and PDA-THD

PDA-DCH

F i g u r e 2 a displays a n F T - R a m a n spectrum o f a p o l y - D C H single crystal obtained w i t h 1 0 6 4 - n m excitation (20). F o r the study o f P D A , 1064 n m is a n extremely useful wavelength because P D A are transparent at this wavelength and there is n o fluorescence to compete w i t h R a m a n processes. Previously



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(b) Figure 2. FT-Raman spectra (1064-nm excitation) of poly-DCH (a) and polyDCHBr (b). (Reproduced with permission from reference 20. Copyright 1990 Gordon and Breach.) 6


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r e p o r t e d R a m a n spectra o f p o l y - D C H used o n l y 632.8-nm excitation (12, 26);

25,

the spectrum i n F i g u r e 2a is i n good agreement w i t h these reports. I n

this spectrum the n o r m a l m o d e , w h i c h p r i m a r i l y involves t r i p l e - b o n d stretch ing (12), cm

- 1

is f o u n d at 2081 c m

- 1

, a n d shifts at 1491, 1466, 1450, a n d 1420

are associated w i t h a F e r m i resonance (12)

that involves the d o u b l e -

b o n d , carbazole, a n d methylene groups. B e a r i n g i n m i n d that n o two single crystals are identical, the f o l l o w i n g observations concerning spectra o f polyDCH

obtained w i t h the other wavelengths o f excitation are relevant. A s

expected for good crystals, the spectra obtained w i t h 632.8-, 514.5-, a n d Downloaded by EAST CAROLINA UNIV on April 8, 2018 | https://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch008

4 8 8 . 0 - n m excitation show no dispersion o f the R a m a n signal. W i t h excitation at 514.5, 488.0, a n d 457.9 n m , some crystals show the R a m a n spectrum o n a luminescent b a c k g r o u n d . F i g u r e 3 displays a representative spectrum o f a p o l y - D C H crystal obtained w i t h 457.9-nm excitation. I n addition to

the

luminescence, the spectrum i n F i g u r e 3 reveals considerable b r o a d e n i n g a n d loss o f resolution o f the Raman-shifted lines, especially i n the d o u b l e - b o n d region, w h e n c o m p a r e d to the spectrum i n F i g u r e 2a a n d to spectra reported w i t h 632.8-nm excitation. Initially, w e were quite p u z z l e d b y the appearance o f the spectrum i n F i g u r e 3; w e w i l l r e t u r n to this issue later.

2200

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Raman Shift (cm ) -1

Figure 3. Raman spectrum (457.9-nm excitation) of

poly-DCH.


600

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F i g u r e 4 shows the F T - R a m a n spectrum o f a single crystal o f p o l y - T H D . T r i p l e - b o n d shifts o f 2111 a n d 2099 c m " a n d a d o u b l e - b o n d stretch at 1485 cm are revealed. W i t h 632.8-nm excitation ( F i g u r e 5a), a t r i p l e - b o n d shift is observed at 2115 c m a n d a d o u b l e - b o n d shift is n o t e d at 1485 c m , i n accord w i t h an earlier observation (7). 1

- 1

- 1

- 1

T h e spectrum obtained w i t h 514.5-nm excitation ( 2 7 ) is markedly dif ferent than those o f p o l y - T H D observed w i t h 1064- a n d 632.8-nm light. A b r o a d laser-induced emission p e a k i n g at a Stokes shift o f about 2300 c m completely obscures any hint o f a R a m a n spectrum i n F i g u r e 5 b . T h i s emission is noteworthy because its m a x i m u m is f o u n d at a wavelength comparable to t h e m a x i m u m associated w i t h t h e excitonic absorption o f p o l y - T H D (7). Additionally, fluorescence f r o m high-quality P D A crystals is negligible (28) a n d observed emissions i n solid P D A are d u e t o material disordered b y mechanical damage o r photooxidation. I f the usual " m i r r o r i m a g e " relationship between absorption a n d fluorescence spectra is assumed, it is estimated that the absorption m a x i m u m leading to the emission i n F i g u r e 5b w o u l d b e at 4 5 0 - 4 6 0 n m . A n absorption m a x i m u m i n this wavelength region is k n o w n to be associated w i t h disordered P D A materials (29).


- 1

T h e spectrum observed w i t h 457.9-nm excitation ( F i g u r e 5c) is clearly different f r o m that i n F i g u r e 5a because the shifts associated w i t h triple- a n d

c

Φ

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Raman Shift (cm- ) 1

Figure 4. FT-Raman (1064-nm excitation) spectrum of

poly-THD.


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(a)


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(c)

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l

1

i

l

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2400 2200 2000 1800 1600 1400

Raman Shift (cm- ) 1

Figure 5. Raman spectra of poly-THD at 632.8 (a), 514.5 (b), and 457.9 nm (c) wavelengths of excitation. (Reproduced from reference 27. Copyright 1991 American Chemical Society.)

d o u b l e - b o n d stretching are observed at 2105 a n d 1504 c m " , respectively. Spectra observed w i t h 488.0-nm excitation reveal a R a m a n spectrum super i m p o s e d o n an emission. T h e data i n F i g u r e 5 are f o u n d i n crystals freshly p o l y m e r i z e d i n inert atmosphere, i n crystals exposed t o ambient conditions for extended t i m e periods, a n d i n crystals w i t h freshly cleaved surfaces. F i g u r e 5 displays spectra r e c o r d e d at 20 °C, b u t spectra obtained at —100 °C do not differ significantly. T h e data i n F i g u r e 5 are not the result o f t h e r m a l decomposition i n d u c e d b y heating o f the crystal b y the incident laser beams. D e c o m p o s i t i o n , however, m a y b e i n d u c e d b y irradiating the crystals w i t h 1


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higher laser p o w e r (23). N o t e further that the data i n F i g u r e 5 do not differ significantly for p o l y - T H D crystals p o l y m e r i z e d thermally or w i t h C o g a m m a radiation.


6 0

Because R R spectroscopy o f P D A b u l k single crystals is p r i m a r i l y a p r o b e o f the surface region o f these crystals (12), w e associate the emission i n F i g u r e 5b w i t h a disordered p o l y - T H D surface structure. A g a i n , w e l l - o r d e r e d P D A crystals do not emit fight (28), a n d the additional features i n the 457.9-nm p o l y - T H D spectrum correspond w e l l to features f o u n d w i t h 632.8n m excitation (7). Therefore, the spectrum i n F i g u r e 5c is that o f a disor d e r e d surface phase o f p o l y - T H D , possibly oligomerie. A disordered surface phase o f p o l y - T H D c o u l d arise i f the surface o f the material fails to c o n t r o l the topochemical a n d topotactic solid-state p o l y m e r ization o f T H D m o n o m e r to the same extent as the b u l k o f the crystal (21). Therefore, a surface structure for p o l y - T H D w o u l d be distinct i n o r i g i n f r o m the photooxidized surface o f p o l y - P T S (8). A corollary to the p r e c e d i n g statement o n the o r i g i n o f a surface phase i n p o l y - T H D is that other P D A crystals might be expected to have disordered surface structures. I n v i e w o f our deductions c o n c e r n i n g the spectra i n F i g u r e 5, w e associate the b r o a d e n e d spectrum i n F i g u r e 3 w i t h the presence o f a disordered surface phase o f p o l y - D C H . T h e spectrum o f p o l y - P T S obtained w i t h 406.8-nm excitation (30) reveals b r o a d e n e d satellites associated w i t h b o t h the double- a n d t r i p l e - b o n d stretching modes, a n d may indicate a surface phase o n this crystal. I n v i e w o f these observations, it is conceivable that R R spectra o f P D A materials obtained w i t h wavelengths o f excitation significantly shorter than m a x i m u m absorption wavelengths may b e d o m i n a t e d b y d i s o r d e r e d surface phases. F r o m the foregoing s u m m a r y a n d earlier w o r k o n p o l y - P T S (12) a n d the P D A f r o m the bis-ethylurethane o f 5 , 7 - d o d e c a d i y n - l , 2 - d i o l ( E T C D ; le) ( 3 1 , 32) a useful s t r u c t u r e - p r o p e r t y relationship emerges. F o r P D A w i t h absorp t i o n maxima i n the 6 2 0 - 6 6 0 - n m range (blue spectra), the n o r m a l m o d e associated w i t h t r i p l e - b o n d stretching is f o u n d at a shift o f 2 0 8 0 - 2 0 9 0 c m , whereas P D A w i t h absorption m a x i m a i n the 5 4 0 - 5 7 0 - n m range ( r e d spectra) exhibit the t r i p l e - b o n d stretch at a shift o f 2 1 1 0 - 2 1 2 0 c m " . - 1

1

Raman Spectra of Chemically Modified Poly-DCH P D A crystals have van der Waals tight-packed crystal structures a n d are not expected to have any particular c h e m i c a l reactivity because d i f f u s i o n o f reagents is not facile i n crystalline regions o f polymers. T h i s was the state o f affairs before o u r observation o f the reaction o f p o l y - D C H w i t h b r o m i n e (19). I n addition to b r o m i n e , chlorine a n d concentrated n i t r i c acid can also react w i t h p o l y - D C H to give compositionally w e l l - d e f i n e d materials that are homogeneous w h e n examined b y electron microscopy. T h e reactions w i t h


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b r o m i n e y i e l d m u c h detailed i n f o r m a t i o n a n d are o f great interest. B r o m i n e reacts w i t h p o l y - D C H crystals to f o r m n e w homogeneous materials that gain three to nine B r atoms p e r repeat unit, d e p e n d i n g o n the experimental conditions. T h e reactions are anisotropic, a n d crystallographic o r d e r is re tained, especially i n materials that have gained u p to six B r atoms p e r repeat unit. A l t h o u g h the formation o f the material w i t h six B r atoms p e r repeat unit ( p o l y - D C H B r ) involves a single crystal-single crystal transformation, the p r o d u c t crystal is disordered. T h e nature o f the reaction products was d e d u c e d f r o m C cross-polarization-magic angle s p i n n i n g nuclear magnetic resonance ( C P - M A S N M R ) spectroscopy (22) rather than b y single crystal X - r a y crystallography. These N M R studies reveal that, f o r the case o f p o l y - D C H B r , b r o m i n e selectively substitutes the carbazole rings i n the 3 and 6 positions, a n d the d i m i n u t i o n o f signal i n the t r i p l e - b o n d c h e m i c a l shift region indicates the conversion o f the triple b o n d to a b r o m i n a t e d double b o n d ; that is, the P D A structure o f the starting p o l y m e r is converted to a m i x e d polyacetylene structure, as shown i n Scheme I. 6


1 3

6

T h e conclusion that p o l y - D C H is converted to a m i x e d polyacetylene structure is a n indirect conclusion i n that it is d e d u c e d f r o m a loss o f a signal. O u r interest i n a direct i n d i c a t i o n o f such a structure motivated us to study R a m a n spectroscopy o f the b r o m i n a t e d materials ( 2 0 , 26, 33) because ( C H ) had b e e n studied extensively b y R a m a n techniques (11, 14, 15). X

O u r R a m a n studies o f b r o m i n a t e d p o l y - D C H have focused o n samples that have gained five to six o r eight to nine B r atoms p e r repeat unit. These studies illustrate the advantage o f using m u l t i p l e wavelengths o f excitation, i n c l u d i n g those i n a transparent region. T h e solid-state electronic spectrum o f the material that gained five to six B r atoms p e r repeat unit revealed a shoulder near 630 n m a n d additional absorption throughout the visible region (26, 33). W e initially felt that 632.8-nm light w o u l d excite the well-crystal l i z e d regions o f the sample a n d that 488.0-nm fight w o u l d probe disordered

Br

Scheme 1. Conversion of poly-DCH

to

Br

poly-DCHBr . 6


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conjugated chains w i t h 514.5 n m as a possible intermediate case. T h e spectra observed w i t h 488.0- a n d 5 1 4 . 5 - n m excitation are largely d o m i n a t e d b y b a c k g r o u n d emission (33). W i t h 632.8-nm excitation (26, 33), R a m a n shifts o f 2107 c m " w i t h a shoulder near 2130 c m " were revealed. T h i s h i n t o f m o r e than one acetylenic species is consistent w i t h N M R w o r k (22) that revealed more than one residual acetylenic resonance. I n the region o f d o u b l e - b o n d stretching, shifts o f 1427, 1454, 1469, 1486, a n d 1522 c m " , similar to those o f the pristine p o l y m e r , w e r e noted. F o r the b r o m i n a t e d material, these vibrations are likely to be d u e to a composite o f different species that contain b r o m i n a t e d double bonds i n the extended conjugated backbone and, possibly, F e r m i resonance that involves a backbone d o u b l e b o n d methylene group a n d a 3,6-dibromocarbazolyl group. T h e most i n formative R a m a n spectrum o f this material was obtained w i t h 1064-nm excitation (20) a n d is shown i n F i g u r e 2b. C o m p a r i s o n o f this spectrum w i t h that o f the pristine p o l y m e r ( F i g u r e 2a) reveals that the shifted lines i n the b r o m i n a t e d p o l y m e r are somewhat broader. T h e shift at 1518 c m " is the strongest feature i n F i g u r e 2b, a n d w e recall that a R a m a n shift near 1520 c m " is characteristic o f an extended polyene chain (34). W e conclude, for p o l y - D C H B r , that the R a m a n spectra obtained w i t h 632.8- and 1064-nm excitation strongly reinforce the conclusion (22) that this material has b e e n extensively converted to a m i x e d polyacetylene structure. 1

1


1

1

1

6

I f p o l y - D C H crystals are refluxed w i t h excess b r o m i n e without stirring, materials that have gained approximately eight B r atoms p e r repeat u n i t are isolated (19). T h e s e materials have some crystallinity a n d show a shoulder near 450 n m i n their solid-state electronic spectra. I n contrast to the c o p p e r - b r o n z e color o f p o l y - D C H B r , p o l y - D C H B r is straw-colored, w h i c h suggests extensive disruption o f the conjugated backbone structure. Solid-state N M R studies (22) indicate that the carbazole rings are also b r o m i n a t e d i n the 3 a n d 6 positions a n d that r e m a i n i n g b r o m i n e atoms are involved i n convert i n g the conjugated backbone to a partially saturated backbone. E l e m e n t a l analysis o f a recently isolated material indicates that nine B r atoms p e r repeat unit are gained b y refluxing p o l y - D C H a n d b r o m i n e with magnetic stirring. A l t h o u g h R a m a n a n d F T I R spectra o f p o l y - D C H B r a n d — B r are indistin guishable, the B r material is amorphous b y X - r a y diffraction. T o date, an informative R a m a n spectrum o f these materials has b e e n obtained only w i t h 632.8-nm excitation; the spectra obtained w i t h shorter wavelengths are d o m i nated b y emission. F i g u r e 6, for example, reveals the spectrum obtained at 488.0 n m ; features near shifts o f 2100 a n d 1500 c m " are barely noticeable. F i g u r e 7 displays the spectrum obtained w i t h 632.8-nm excitation, w h e r e b r o a d e n e d shifts near 2140, 1467, 1453, a n d 1420 c m " are noted. T h e s e shifts are sufficiently similar to the shifts o f the pristine p o l y - D C H that w e assume they are due to some l o w concentration o f conjugated segments that r e m a i n at the e n d o f the reaction; such segments w o u l d dominate the interaction o f this solid w i t h 6 3 2 . 8 - n m light. 6

8

8

9

9

1

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1800

1600

1400

1200

1000


Figure 6. Raman spectrum (488.0-nm excitation) of poly-DC

HBr . 8

Just as the m o n o m e r D C H B r (4a) was u s e d as a m o d e l c o m p o u n d f o r the solid-state N M R studies ( 2 2 ) , w e h o p e d to b e able to use it as a m o d e l for R a m a n studies o f a b r o m i n a t e d carbazole. T h e spectra, however, w e r e d o m i n a t e d b y emission for wavelengths o f excitation b e t w e e n 457.9 a n d 632.8 n m . O n l y the 632.8-nm spectrum gave hints o f R a m a n lines near 1467, 1325, and 1222 c m " . T h e d o m i n a t i o n b y emission i n attempts t o r e c o r d R a m a n spectra o f 4a are not surprising i n v i e w o f the reported ( 3 5 ) phosphorescence o f 3 , 6 - d i b r o m i n a t e d carbazoles. 4

1

R a m a n spectra o f chemically m o d i f i e d versions o f p o l y - D C H are most informatively studied using m u l t i p l e wavelengths o f excitation. T h e R a m a n spectra o f p o l y - D C H B r a n d ~~ B r c o n f i r m the conclusions o f the solid-state N M R studies ( 2 2 ) that although these materials are homogeneous w h e n examined b y electron microscopy, they are not homogeneous at the m o l e c u lar repeat level. 6

8

Raman Spectra of PDA Materials Less Well Defined Than Single Crystal N o t a l l P D A materials are as w e l l d e f i n e d as the foregoing examples o f fully p o l y m e r i z e d single crystals. F o r example, there are m a n y instances w h e r e



258


\L

2200

ι 2100

ι 2000

ι 1900

ι 1800

ι

1700

ι 1600

ι 1500

ι 1400


Figure 7. Raman spectrum (632.8-nm excitation) of

poly-DCHBr . 8

P D A p o l y m e r crystals contain significant amounts o f m o n o m e r that d o not polymerize due to a m i s m a t c h b e t w e e n m o n o m e r a n d p o l y m e r lattices (21). Additionally, i t does not follow that i f a diacetylene m o n o m e r reacts to give a c o l o r e d product, this c o l o r e d p r o d u c t has the e n - y n e (21) backbone struc ture. O t h e r reaction pathways are conceivable. Therefore, i n the absence o f a complete crystal structure, other means o f p r o o f o f p o l y m e r backbone structure are highly desirable, especially f o r insoluble materials. U s e f u l experimental techniques for this purpose i n c l u d e X - r a y p o w d e r diffraction [observation o f a 4.9-Â repeat distance (21)], solid-state C N M R , and R a m a n spectroscopy. A p p l i c a t i o n o f R a m a n techniques f o r this purpose assumes that the observed spectra are representative o f the b u l k o f a solid sample, not just the surface. 1 3

I f a strongly c o l o r e d solid material is t h e p r o d u c t o f a diacetylene polymerization a n d i t exhibits a R a m a n spectrum w i t h shifts near 2100 a n d 1500 c m , i t is reasonable t o conclude that the material has the e n - y n e backbone structure because w e l l - d e f i n e d P D A crystals have R a m a n spectra that exhibit t h e n o r m a l modes associated w i t h triple- a n d d o u b l e - b o n d stretching near 2100 and 1500 c m . T h i s approach is i m p l i c i t i n the study o f P D A L a n g m u i r - B l o d g e t t films ( I I ) , where excitation profiles have b e e n obtained i n certain cases. - 1

- 1


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W e recently reported the synthesis a n d study o f the solid-state reactivity o f a series o f bis-p-chlorocinnamates o f diacetylene diols (24). T h e ester o f 10,12-docosadiyn-l,22-diol (4b) is thermally stable, b u t is p o l y m e r i z e d b y 370-nm light a n d C o g a m m a radiation t o a coppery b r o w n solid whose X - r a y p o w d e r diffraction reveals a reflection at d = 4.91 Â. T h e R a m a n spectrum (632.8-nm excitation) o f this material is shown i n F i g u r e 8, a n d the observation o f shifts at 2 1 3 3 a n d 1448 c m suggests t h e usual e n - y n e structure for this material. 6 0

- 1

In the course o f o u r studies o f the crystal a n d molecular structure a n d solid-state reactivity o f D C H B r , (4a) (36), w e also studied the properties o f the dark violet largely amorphous solid f o r m e d b y extended heating above 200 °C. T h e R a m a n spectrum (632.8-nm excitation) revealed shifts at 2128, 1481, a n d 1464 c m , a n d this i n f o r m a t i o n is sufficient t o conclude that the p r o d u c t o f t h e r m a l solid-state p o l y m e r i z a t i o n o f D C H B r has the usual e n - y n e structure.


4

- 1

4

Conclusions R a m a n spectroscopy,

using excitation i n b o t h transparent

a n d absorbing

regions, w i l l continue to b e a valuable t o o l i n the study o f the structure a n d

^ C l - ^ ~ ^ - CH= CH— C — Ο—(CH )g — C = C 2

Monomer; Co Gamma Polymerization

2200

2100

2000

1900

1800

1700

1600

1500

Raman Shift (cm ) -1

Figure 8. Raman spectrum (632.8-nm excitation) of the PDA from monomer 4b.


1400

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properties o f P D A materials a n d other conjugated polymers. T h e n o r m a l modes associated w i t h t r i p l e - a n d d o u b l e - b o n d stretching, f o u n d near shifts o f 2100 a n d 1500 c m " , respectively, constitute a very useful s t r u c t u r e - p r o p erty relationship f o r the characterization o f m o r e complex P D A materials than those emphasized i n earlier studies a n d h e r e i n . A s an example o f a m o r e complex P D A system, a recent study o f phase separation i n cross-polymerized diacetylenes may b e c i t e d ( 3 7 ) .


1

T h e initial comprehensive studies o f R R spectroscopy b y Batchelder a n d co-workers ( 8 , I I , 12), p r i m a r i l y w i t h P D A - P T S single crystals, are o f major value because the measured excitation profiles allow estimation o f the inter action between an electronic excited state a n d a vibrational mode. A d d i t i o n ally, these studies point the way t o w a r d the study o f more complex systems. T h e studies w i t h single crystals o f P D A - D C H a n d - T H D are an important extension o f this w o r k because they reveal that dispersion i n R R spectra o f P D A crystals may b e a relatively widespread p h e n o m e n o n . T h e d e d u c t i o n that d i s o r d e r e d P D A surface phases, likely f o r m e d b y failure o f the crystal surface to control the topochemical a n d topotactic polymerization, are the structural source o f the dispersion clearly raises an important issue i n the study o f conjugated polymers for electronic a n d optical applications. D i s p e r s i o n is a c o m m o n feature o f the R R spectra o f conjugated macro molecules less structurally o r d e r e d than P D A single crystals. T h e three models (namely, amplitude mode, conjugation length, a n d effective conjuga tion coordinate) used to explain the features o f R R spectra o f less-ordered conjugated polymers w e r e c o m p a r e d recently ( 3 8 ) . W i t h reference to e q 1, a given sample o f a partially crystalline conjugated p o l y m e r m a y b e expected to have a variety o f conjugation lengths (hence d i f f e r i n g E ) , a variety o f local environments o f varying crystallinity (hence variation i n D ) , and, i n the spirit of our discussions, the possibility o f distinct surface structures. 0

T h e r e are further indications that the surface topographies o f P D A crystals are distinct f r o m the b u l k a n d may b e quite complex. A n atomic force microscopy study ( 3 9 ) c o n c l u d e d that the substituent positions o f a P D A crystal surface layer w e r e different f r o m those o f the b u l k . Indications that the surface structure o f the P D A o f the bis-isopropylurethane o f 5,7-dodec a d i y n - l , 1 2 - d i o l varies f r o m crystal to crystal were f o u n d i n o u r R R spectral studies using 514.5-nm excitation at ambient temperature (40). T h i s w o r k indicated that the n u m b e r o f shifted fines associated w i t h a given n o r m a l mode, the relative intensity o f the lines o f a given mode, a n d the amount o f b a c k g r o u n d luminescence change f r o m crystal to crystal. W e conclude that the foregoing studies are a useful b e g i n n i n g to understanding the potential complexity o f the surfaces o f P D A crystals.

Acknowledgments T h e authors thank D r . M . J . S m i t h ( N i c o l e t Instruments) f o r r e c o r d i n g the F T - R a m a n spectra displayed i n this w o r k . C o m p o u n d 4 b was synthesized b y


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D r . R . A . H a a k s m a . T h e authors thank E . Yost f o r the synthesis o f the most recent samples o f T H D m o n o m e r a n d polymer. X - r a y p o w d e r patterns were furnished b y D r . G . P . H a m i l l a n d M . J. D o w n e y . D . R . H a m m o n d s u p p l i e d F T I R spectra, F . X . P i n k p e r f o r m e d the X R F a n d E S C A analyses, a n d Β . M . F o x m a n (Brandeis University) facilitated access to the G a m m a c e l l .


References 1. Crystallographically Ordered Polymers; Sandman, D. J., Ed.; ACS Symposium Series 337; American Chemical Society: Washington, DC, 1987. 2. Polydiacetylenes; Cantow, H . J., Ed.; Advances in Polymer Science 63; Springer: Berlin, Germany, 1984. 3. Polydiacetylenes; Bloor, D.; Chance, R. R. Eds.; NATO ASI Series; Martinus Nijhoff: Dordrecht, Netherlands, 1985. 4. Sandman, D. J. 1991 McGraw-Hill Yearbook of Science and Technology; Mc Graw-Hill: New York, 1991; pp 71-75. 5. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature (London) 1990, 347, 539. 6. Wright, J. D. Molecular Crystals; Cambridge University Press: New York, 1987; Chapter 6, p 96. 7. Morrow, M. E.; White, K. M.; Eckhardt, C. J.; Sandman, D. J. Chem. Phys. Lett. 1987, 140, 263. 8. Batchelder, D. N . In Polydiacetylenes; Bloor, D.; Chance, R. R., Eds.; NATO ASI Series; Martinus Nijhoff: Dordrecht, Netherlands, 1985; pp 187-212. 9. Sandman, D. J.; Chen, Y. J., Synth. Metals 1989, 28, D613. 10. Bassler, H . In Optical Techniques to Characterize Polymer Systems; Bassler, H., Ed.; Elsevier: New York, 1989; pp 181-225. 11. Batchelder, D. N . In Optical Techniques To Characterize Polymer Systems; Bassler, H., Ed.; Elsevier, New York, 1989; pp 393-427. 12. Batchelder, D. N.; Bloor, D. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H.; Hester, R. E., Eds.; Wiley: New York, 1984; Vol. 11, pp 133-209. 13. Chance, R. R. In Encyclopedia of Polymer Science and Technology; Kroschwitz, J. I., Ed.; Wiley: New York, 1986; Vol. 4, pp 767-779. 14. Lichtman, L. S.; Sarhangi, Α.; Fitchen, D. B. Solid State Commun. 1980, 36, 869. 15. Schen, Μ. Α.; Chien, J. C. W.; Perrin, E.; Lefrant, S.; Mulazzi, E. J. Chem. Phys. 1988, 89, 7615. 16. Vardeny, Z.; Ehrenfreund, E.; Brafman, O.; Heeger, A. J.; Wudl, F. Synth. Metals 1987, 18, 183. 17. Botta, C.; Luzzati, S.; Bolognesi, Α.; Catellani, M . ; Destri, S.; Tubino, R. In Materials Research Society Symposium Proceedings; Chiang, L.-Y.; Chaikin, P. M.; Cowan, D . O., Eds.; Materials Research Society: Strasbourg, France, 1990; Vol. 173, pp 397-402. 18. Zheng, L. X.; Benner, R. E., Vardeny, Ζ. V.; Baker, G. L. Phys. Rev. Β 1990, 42, 3235. 19. Sandman, D. J.; Elman, B. S. Hamill, G. P.; Hefter, J.; Velazquez, C. S. In Crystallographically Ordered Polymers; Sandman, D. J., Ed.; ACS Symposium Series 337; American Chemical Society: Washington, DC, 1987; Chapter 9, pp 118-127. 20. Hankin, S.; Sandman, D. J. Mol. Cryst. Liq. Cryst. 1990, 186, 197. 21. Enkelmann, V. In Polydiacetylenes; Cantow, H . J., Ed.; Advances in Polymer Science 63; Springer: Berlin, Germany, 1984; pp 91-136. ;



262


22. Eckert, H.; Yesinowski, J. P.; Sandman, D . J.; Velazquez, C. S. J. Am. Chem. Soc. 1987, 109, 761. 23. Hankin, S.; Sandman, D. J.; Yost, E. A.; Stark, T. J. Synth. Metals 1992, 49, 281. 24. Sandman, D. J.; Haaksma, R. Α.; Foxman, Β. M. Chem. Mater. 1991, 3, 471. 25. Elman, B. S.; Thakur, M . K.; Sandman, D. J.; Newkirk, Μ. Α.; Kennedy, E. F. J. Appl. Phys. 1985, 57, 4996. 26. Sandman, D. J.; Chen, Y. J.; Elman, B. S.; Velazquez, C. S. Macromolecules 1988, 21, 3112. 27. Hankin, S.; Sandman, D. J. Macromolecules 1991, 24, 4983. 28. Bloor, D.; Rughooputh, S. D. D . V.; Phillips, D.; Hayes, W.; Wong, K. S. Electronic Properties of Polymers and Related Materials; Kuzmany, H.; Mehring, M.; Roth, S., Eds.; Springer: Berlin, Germany, 1985; pp 253-255. 29. Bloor, D. Photon, Electron, and Ion Probes of Polymer Structure and Properties; Dwight, D. W.; Fabish, T. J.; Thomas, H . R., Eds.; ACS Symposium Series 162; American Chemical Society: Washington, DC, 1981; pp 81-104. 30. Kuzmany, H.; Kurti, J. Synth. Metals 1987, 21, 95. 31. Sandman, D. J.; Chen, Y. J. Synth. Metals 1989, 28, D613. 32. Sandman, D. J.; Chen, Y. J. Polymer 1989, 30, 1027. 33. Hankin, S. H . W.; Sandman, D. J. Polymer Commun. 1990, 31, 22. 34. Shirakawa, H.; Ito, T.; Ikeda, S. Polymer J. 1973, 4, 460. 35. Yokoyama, M.; Funaki, M.; Mikawa, H . J. Chem. Soc., Chem. Commun. 1974, 372. 36. Sandman, D . J.; Velazquez, C. S.; Hamill, G. P.; Foxman, Β. M . Polym. Prepr. Am. Chem. Soc., Div. Polym. Chem. 1992, 33, 284. 37. Nitzsche, S. Α.; Hsu, S. L.; Hammond, P. T.; Rubner, M . F. Macromolecules 1992, 25, 2391. 38. Kurti, J.; Kuzmany, H . Phys. Rev. Β 1991, 44, 597. 39. Magonov, S. N.; Bar, G.; Cantow, H.-J.; Bauer, H.-D.; Müller, I.; Schwoerer, M . Polym. Bull. 1991, 26, 223. 40. Hankin, S. H . W.; Sandman, D . J. In Materials Research Society Symposium Proceedings; Chiang, L. Y.; Garito, A. F.; Sandman, D. J., Eds.; Materials Research Society: Strasbourg, France, 1992; Vol. 247, pp 661-667. RECEIVED for review May 14, 1991. ACCEPTED revised manuscript July 15, 1992.


Structure-Property Relations in Polymers - American Chemical Society

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