Multiphase Polymers - American Chemical Society

2 Morphological Studies of PCP/MDI/BDO-Based Segmented Polyurethanes

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A. L . C H A N G

1

and E . L . T H O M A S

1

Department of Chemical Engineering, University of Minnesota, Minneapolis, M N 55455

Folymethane samples based on polycaprolactonediol/4,4'diphenylmethane diisocyanate/1,4-butanediol from 1/2/1 to 1/6/5 (PCP/MDI/BDO) mole ratio were phase separated by DSC, WAXS, electron diffraction, and TEM. Soft [ ( PCP/MDI ) i] and hard [ ( MDI/BDO ) j] segment sequences can crystallize. As-reacted samples contained a more ordered, hard-segment phase while solution-cast samples contain a more ordered, soft-segment phase. The observed increase in melting point and decrease in line- w i d t h of the hard segment, WAXS reflections suggest an increase of domain size with increased hard-segment content. The fractional degree of crystallinity of the hard-segment phase is approximately constant. All solvent (DMF) cast samples except sample 1/2/1 contained a spherulitic superstructure. Bright field, defocus micrographs of solution-cast films of samples 1/5/4 and 1/6/5 exhibit a 400 A scale granularity suggestive of hard-segment-rich domains.

T p h e interesting elastomerie properties of polyurethanes are currently attributed to the formation of microdomains (1,2,3,4). Thermoplastic polyurethane elastomers are multiblock copolymers consisting of short, immobile, polyurethane sequences ( "hard" segments ) connected via long and flexible chains ( "soft" segments ). A variety of aliphatic and aromatic diisocyanates with diol or diamine chain extenders have been used for the hard segment with typically 1,000-3,000 molecular weight polyether or polyester polyols for the soft segment. During polymerization Current address: Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01002. 1

0-8412-0457-8/79/33-176-031$05.50/0 © 1979 American Chemical Society

Cooper and Estes; Multiphase Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

32

MULTIPHASE

POLYMERS

and solidification, the hard and soft portions of the multiblock polyurethane chain undergo microphase separation into hard and soft domains. T h e strong polar bonding of the hard segments causes the hard domains to act as pseudocrosslinks for the flexible soft-segment phase. Mechanical properties can then be conveniently tailored by varying the ratio of hard-to-soft phase. The substantial work on polystyrene/polybutadiene and polystyrene/ Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0176.ch002

polyisoprene blends and diblock and triblock copolymer systems has lead to a general understanding of the nature of phase separation in regular block copolymer systems ( 5 , 6 ) .

T h e additional complexities of multi-

blocks with variable block length as well as possible hard- and/or softphase crystallinity makes the morphological characterization of polyurethane systems a challenge. In this chapter we investigate the morphology of a series of polyurethanes

based

on polycaprolactone

polyol

( P C P ),

diphenylmethane

diisocyanate (MDI), and butanediol ( B D O ) . Samples of as-batch-reacted and solution-cast polymers were examined by optical microscopy, transmission electron microscopy, electron and x-ray diffraction, and differential scanning calorimetry. Our interest is to provide a mapping of the size and shape of the domains ( and any superstructure such as spherulites ) and the degree of order as a function of the fraction of each phase present. Chain regularity and block length as well as thermal history during and after polymerization all play important roles in determining the degree of phase separation as well as the degree of order of the soft- and hard-segment domains. Better phase separation is favored for nonpolar soft-segment systems and with longer sequence lengths of the respective hard/soft

segments.

Both soft and hard domains in polyurethanes can be amorphous or partially crystalline, depending on the particular system. crystalline systems, the ordered hard-segment

F o r partially

phase is thought to be

composed of fringed lamellae domains of thickness (dimension parallel to chain axis)

equal to the hard-segment

length and lateral width

( dimensions normal to chain axis ) of less than a few hundred Angstroms (3).

Occasionally there is a spherulitic superstructure (3,6,7).

Although

the block length and volume fraction of each type of segment will influence the overall domain morphology, both bicontinuous (8) and discreteisolated (3,4,5)

domain morphologies have been proposed.

Providing the fact that a block is regular, block length is important in determining crystallinity. For polyester-based polyurethanes, Seefried et al. ( 9 ) have shown that a polycaprolactonediol with M

n

> 3000 was

necessary for soft-segment crystallinity. Hard-segment crystallization can occur for much shorter block lengths. Harrell (10) prepared a systematic series of polymers with monodisperse hard-segment sizes and showed that


2.

CHANG

AND THOMAS

33

Segmented Polyurethanes

c r y s t a l l i z a t i o n o c c u r r e d e v e n f o r chains c o n t a i n i n g o n l y a single, h a r d segment repeat u n i t ( i t s h o u l d b e n o t e d that H a r r e l T s p o l y m e r has n o hydrogen b o n d i n g ) . Interestingly, he reported that increasing the h a r d segment b l o c k l e n g t h d i d n o t a p p r e c i a b l y affect t h e d e g r e e o f o r d e r w i t h i n t h e h a r d d o m a i n s , t h o u g h t h e m e l t i n g p o i n t of t h e phase w a s r a i s e d . W i l k e s (7,11,12)

hard-segment

and co-workers have characterized

t h e m o r p h o l o g y o f these s p e c i a l l y p r e p a r e d p o l y m e r s a n d h a v e f o u n d Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0176.ch002

both

s p h e r u l i t i c superstructure

a n d d o m a i n structures.

D o m a i n size

a p p a r e n t l y i n c r e a s e d w i t h increase i n h a r d - s e g m e n t c o n t e n t as e v i d e n c e d b y s h a r p e n i n g o f the w i d e - a n g l e x-ray ( W A X S ) reflections There

(12).

are relatively few direct transmission, electron

microscopy

( T E M ) studies o f d o m a i n structures i n p o l y u r e t h a n e s (3-7,13,14,15).

No

d i s t i n c t m i c e l l e - l a m e l l a e platelets h a v e b e e n o b s e r v e d i n t h e u r e t h a n e systems thus s t u d i e d . Instead, t h e d o m a i n structures w h i c h h a v e b e e n observed

generally appear

diameter

(4,13,14,15).

sions

as i s o l a t e d e q u i a x e d

R a n d o m l y oriented

of 300-600 Â have been

system (3,6).

observed

fibrils

grains 3 0 - 5 0 0 Â i n w i t h lateral d i m e n -

in a polyether/MDI/BDO

It r e m a i n s t o b e seen w h a t r e l a t i o n t h e e q u i a x e d grains

( d o m a i n s ) a n d t h e fibrils o b s e r v e d b y t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y have w i t h SAXS

the micelle-lamellae

structures

inferred from W A X S a n d

(12,16).

B o n a r t has p r o p o s e d t w o - d i m e n s i o n a l a n d t h r e e - d i m e n s i o n a l m o d e l s of M D I / B D O h a r d - s e g m e n t ( p a r a ) crystals (17,18,19).

Arrangements

w e r e c o n s t r u c t e d that p r o v i d e o p t i m u m h y d r o g e n b o n d i n g . T h e 7.9 Â p a r a c r y s t a l l i n e , h a r d - s e g m e n t reflection is t h o u g h t

to arise

from the

h y d r o g e n b o n d c o n t a i n i n g planes i n c l i n e d 3 0 ° t o t h e c h a i n axis.

Quite

h i g h temperatures a n d l o n g a n n e a l i n g times ( 1 9 0 ° C , 12 h r ) are r e p o r t e d l y r e q u i r e d f o r significant h a r d - s e g m e n t ( M D I / B D O ) c r y s t a l l i n i t y (2). S e v e r a l S A X S studies o n M D I / B D O p o l y u r e t h a n e s h a v e s h o w n a discrete s m a l l - a n g l e m a x i m u m i n t h e 200 Â r a n g e

(3,12,16,20).

This

m a x i m u m has b e e n a t t r i b u t e d to t h e average center-to-center s p a c i n g o f the h a r d - s e g m e n t d o m a i n s . O n e w o u l d expect a systematic v a r i a t i o n i n the p o s i t i o n of this m a x i m u m w i t h c o m p o s i t i o n , b u t this h a s not a l w a y s been observed

(3,16).

T h e i n f l u e n c e o f soft-segment

hydrogen bond

a b i l i t y a n d h a r d - s e g m e n t b l o c k size o n t h e phase s e p a r a t i o n have b e e n c l e a r l y s h o w n i n S A X S studies. (M„ =

F o r t h e same m o l e c u l a r w e i g h t p o l y o l

1000), a 1/2/1 p o l y e s t e r / M D I / B D O system w a s s i n g l e p h a s e d

(i.e., c o m p a t i b l e ) w h e r e a s t h e 1/2/1 p o l y e t h e r / M D I / B D O system w a s p h a s e separated

(20).

I n a d d i t i o n to m i c r o p h a s e structures, M D I / B D O - b a s e d p o l y u r e t h a n e systems

h a v e e x h i b i t e d s p h e r u l i t i c superstructure.

Characterization of

t h e b i r e f r i n g e n c e o f t h e spherulites w a s u s e d t o d e t e r m i n e t h e o r i e n t a t i o n o f the h a r d - s e g m e n t d o m a i n s ( 7 ) . H o w e v e r , b e c a u s e o f t h e sensi-


34

MULTIPHASE

POLYMERS

t i v i t y o f the o p t i c a l a n i s o t r o p y to t h e exact c o n f o r m a t i o n of t h e a r o m a t i c r i n g s as w e l l as to the c h a i n o r i e n t a t i o n , b i r e f r i n g e n c e

alone

cannot

determine hard-segment orientation w i t h i n the spherulite ( 3 ) . The

dynamic

mechanical

properties

(torsion

p e n d u l u m ) of the

present P C P / M D I / B D O p o l y u r e t h a n e system h a v e b e e n s t u d i e d b o t h as a f u n c t i o n of p o l y o l m o l e c u l a r w e i g h t a n d as a f u n c t i o n of h a r d segment concentration.

A series of p o l y u r e t h a n e s w i t h P C P / M D I / B D O


m o l e r a t i o of 1/2/1 w i t h v a r i a b l e M =

n

of t h e soft-segment

polyol ( M

n

340, 530, 830, 1250, 2100, a n d 3130) e x h i b i t e d a single c o m p o s i t i o n a l

dependent T

g

polyols ο Γ Μ the 3 1 3 0 - M

n

consistent w i t h a c o m p a t i b l e

(noncrystalline)

system f o r

of 340-2100 (e.g., 3 7 - 1 3 w t % h a r d s e g m e n t ) ( 9 ) .

η

p o l y o l , soft-segment

two-phase morphology.

For

crystallization occurred indicating a

T h e dynamic mechanical

properties

for t w o

different m o l e c u l a r w e i g h t p o l y o l s (830 a n d 2100 M ) w e r e also s t u d i e d n

as a f u n c t i o n of h a r d - s e g m e n t c o n c e n t r a t i o n (21). 830-M

n

T h e lower T

g

p o l y o l system w a s v e r y sensitive to the h a r d - s e g m e n t

whereas the lower T

g

content

of t h e 2100 M - p o l y o l system d i d n o t s i g n i f i c a n t l y n

Table Mol Ratio PCP/MDI/BDO

of t h e

Wt %

MDI

0 •11 19 26 31 35 38 74

1/0/0 1/1/0 1/2/1 1/3/2 1/4/3 1/5/4 1/6/5 0/1/1

As-Reacted WAXS b

XL-S XL-S \. 44 4.6, A — 4 . 4 4.6,4.1,3.75, A — 4 . 4 4.6,4.1,3.75, A — 4 . 4 4.6,4.1,3.75, A — 4 . 4 A—8.4, 4.9,4.6,4.1,3.75

Wt Hard

I. 52°C DM F Spherulite Size (μηι)

Fraction Segment (%) 0 0 13 23 31 38 43 100

—64 ° C

—

-40 -40 -32 -30 -30 + 125°C 52°C Electron

50 30 None < -5 5 10-30 10-30 50

DMF Diffraction"

XL-S XL-S XL—S* X L - S * , A—4.4 ( w e a k ) X L - S * , A—4.4 X L - S * , A—4.4 X L - S * , A—4.4 4.6, 3.9

* From Ref. 21 for a PCP diol of M . = 2100. _ The random copolymer samples are based on a PCP diol of M„ = 2.000 ; X L - S : crystalline P C L reflections; X L - S * : crystalline soft segment in limited regions of sample; A - X . X : diffuse halo centered at X X A. 6


2.

Segmented Voleur ethanes

CHANG AND THOMAS

35

increase over the same range of hard-segment concentration (see Table I ). This would indicate that the relative degree of phase separation was much greater for the higher molecular weight polyol system.


Experimental T h e polyurethane samples were kindly supplied by F . E . Critchfield of Union Carbide Corporation. T h e samples were made by a one-step batch process with curing at 145 ° C for 16 hr. Details of the polyurethane polymerization are described elsewhere (9). T h e hard segment consists of 4,4'-diphenylmethane diisocyanate ( M D I ) and 1,4-butanediol ( Β D O ) ; ( M D I / B D O ) i , and the soft segment consists of M D I and polycaprolactone diol ( P C P ) with M = 2000; ( P C P / M D I ) , . Transmission electron micrographs and diffraction patterns were obtained using a J E O L 100CX electron microscope operated at 100 K e V . Wide-angle x-ray patterns were taken with a flat film camera using nickel-filtered copper radiation with a Phillips-Norelco generator operated at 30 K V and 20 m A. T h e D S C experiments were carried out using a Perkin-Elmer D S C II at a heating rate of 2 0 ° C / m i n . Sample size was approximately 10 mg. Calibration was done with an In standard . Eight different polymers were studied: polycaprolactone homopolymer ( P C L ) ; the two regular pure segment copolymers, P C P / M D I and M D I / B D O , and five random copolymers with P C P / M D I / B D O mole ratios varying systematically from 1/2/1 to 1/6/5 (see Table I). Sample opacity increases strongly with increased hard-segment content for the five random copolymer samples. Both regular pure segment copolymers are opaque. T w o types of samples were prepared for detailed morphological examination. Sections of the as-polymerized material were examined directly by W A X S and D S C . Samples also were prepared by casting films from a solution of the polymer in dimethyl formamide ( D M F ) at 5 2 ° C . Solvent was allowed to slowly evaporate, and the films were dried by annealing for 670 hr at 5 2 ° C , followed by slow cooling to room temperature. T h i n films suitable for electron microscopy were cast from 0.5 wt % polymer in D M F onto clean glass slides, and after 20 hr of annealing at 5 2 ° C , floated off on distilled water and mounted on 300-mesh copper grids. n

Experimental

Results

Differential Scanning Calorimetry. Figures 1 and 2 show 320-520 Κ D S C scans of the as-reacted polymer and the D M F solution cast samples at heating rate of 2 0 ° C / m i n .

T w o broad endothermic transition regions

are observed—a very broad and weak transition near 3 5 0 ° C

and a

narrower, stronger transition below 5 0 0 ° C . F o r the as-reacted polymer, the high temperature peak (circa 490 K ) shifts upwards by about 1 0 ° C as the hard-segment concentration in creases from 13-43%.

Scans through the P C L melting range indicate


36


MULTIPHASE POLYMERS

Figure 1.

DSC scans at heating rate of 20°C/min of the as-reacted PCP/ MDI/BDO samples over the 320-520 Κ range



2.

CHANG AND THOMAS

350

Segmented

40Ô

37

Polyurethanes

^ ^

"

450

^

5ÔÔ

Temperature, Κ

Figure

2.

DSC scam at heating rate of 20°C/min of DMF solution PCP/MDI/BDO samples over the 320-520 Κ range

cast

no significant soft-segment crystallinity is present. For the solvent-cast samples, the high temperature peak (circa 455 K) also shifts upwards with increasing hard-segment content by about 10°C. The solvent cast 1/2/1 sample has no high temperature peak but does show a small softsegment melting endotherm at about 40°C for a scanning rate of 5°C/min. The heat of fusion of the high temperature transition increases with hard-segment content for both types of samples, with the respective D M F cast samples having much lower values than the as-reacted samples (see Table I I ) .

Table I I . Mol Ratio PCP/MDI/ BDO

Wt t tion Hard Segment

1/2/1 1/3/2 1/4/3 1/5/4 1/6/5

13 23 31 38 43

rac

As-Reacted Polymer :L_ &h( cal/'g T . (Κ) 0.2

1.7

2.8 4.2 5.2

479484 494 493 490

Solution Cast Polymer . _ T m (K) àh cal/g t

0 0.4 1.1 1.2 2.6


— 445 456 458 458

38

MULTIPHASE POLYMERS

Diffraction.

W i d e - a n g l e x - r a y s c a t t e r i n g patterns o f t h e as-reacted

p o l y m e r i n d i c a t e m i c r o p h a s e s e p a r a t i o n is o c c u r r i n g b y t h e a p p e a r a n c e o f c r y s t a l l i n e h a r d - s e g m e n t reflections (see T a b l e I a n d F i g u r e 3 ) ( 2 , 3 ) . T h e s e reflections b e c o m e

sharper a n d stronger w i t h increasing h a r d -

s e g m e n t content. S a m p l e 1/2/1 w i t h o n l y 1 3 % h a r d segment shows o n l y o n e d i f f u s e h a l o c e n t e r e d a t 4.4 Â . T h e r e is n o x - r a y e v i d e n c e o f softs e g m e n t c r y s t a l l i n i t y i n a n y o f t h e as-reacted r a n d o m c o p o l y m e r samples.


S a m p l e 1/1/0 ( p u r e soft s e g m e n t ) e x h i b i t s s h a r p c r y s t a l l i n e reflections w h i c h i n d e x w e l l w i t h t h e k n o w n c r y s t a l s t r u c t u r e o f P C L (22). E l e c t r o n d i f f r a c t i o n o f s o l u t i o n cast films a n n e a l e d a t 5 2 ° C

(which

is just b e l o w t h e m e l t i n g p o i n t o f P C L h o m o p o l y m e r ) shows soft-segment

Figure 3. WAXS patterns of the as-reacted PCP/MDI/BDO polymers: (upper left) 11211, (upper right) 11312, (lower left) 11413, and (lower right) 1/6/5. All samples except upper left exhibit hard-segment crystalline reflections.


2.

CHANG AND THOMAS

39

Segmented Folyurethanes

c r y s t a l l i n i t y (see F i g u r e 4 ) . T h e a m o u n t o f soft-segment

crystallinity

increases w i t h t h e w e i g h t f r a c t i o n of soft segment, b u t these c r y s t a l l i n e soft-segment film.

regions a p p e a r d i s c r e t e l y ( a n d r a n d o m l y ) t h r o u g h o u t t h e

It has n o t b e e n p o s s i b l e t o p i n d o w n t h e i r l o c a t i o n w i t h i n t h e

spherulites. No

distinct hard-segment

reflections

a r e v i s i b l e i n s olution-c as t

samples of t h e 1/2/1-1/6/5 r a n d o m c o p o l y m e r s , i n s t e a d a s t r o n g h a l o


c e n t e r e d at 4.4 Â is o b s e r v e d i n s i d e t h e 4 . 1 - a n d 3 . 7 - Â P C L reflections. T h e pure, hard-segment

s a m p l e has t w o b r o a d r i n g s c e n t e r e d

at 4.6

a n d 3.9 A . Microscopy. T h e m o r p h o l o g i e s o f t h e p u r e , soft-segment ( PCP/MDI )

a n d pure, hard-segment copolymer

copolymer

( M D I / B D O ) are

Figure 4. Electron diffraction patterns of DMF solution cast samples: (upper left) 1/3/2, (upper right) J/5/4, (lower left) 11615, and (lower right) 0/1/1 (pure hard segment)


40

MULTIPHASE POLYMERS

s h o w n i n F i g u r e 5.

T h e observed morphology depends on the sample

preparation technique, BDO )

m e l t east films of

yielding very large (0°-90°)

spherulites.

( P C P / M D I ) and ( M D I /

diameter)

nonbanded, positively

S o l u t i o n - c a s t films sometimes


biréfringent

(50-μ,τη


exhibit

CHANG AND THOMAS

Segmented

Ρolyurethanes

41


2.

Figure 5. Optical micrographs of pure soft- and pure hard-segment copoly mers: (upper left) compression-molded soft segment, flower left) DMF solutioncast soft segment, (upper right) compression-molded hard segment, (lower right) DMF solution-cast hard segment. All micrographs are for crossed pohrizers except lower right.



MULTIPHASE POLYMERS



2.

CHANG AND THOMAS

Segmented

F olyur ethanes

43

Figure 6. Transmission electron micrographs of solution-cast films of: (upper left) pure soft-segment copolymer and (lower left, above) pure hard-segment copolymer



MULTIPHASE POLYMERS



2.

CHANG AND THOMAS

Segmented Polyurethanes

45

Figure 7. Transmission electron micrographs of solution-castfilmsof: (upper left) 1/3/2, (lower left) 1/4/3, and (above) 1/6/5. Spherulite size decreases with decreasing hard-segment content.


MULTIPHASE POLYMERS


46

Figure 8. Bright field, defocus micrographs of solution castfilmsof: (a) sample 1/5/4 and (b) sample 1/6/5 (underfocus for both micrographs = 2 μπι)


2.

CHANG AND THOMAS

Segmented

47

Polyurethanes

banded spherulites, but the optical anisotropy of the random copolymer and pure hard-segment spherulites is greatly diminished. Transmissionelectron micrographs show that the detailed morphology of the pure soft and pure hard spherulites are quite different (see Figure 6). T h e pure soft-segment

spherulites

consist

of the familiar branching, radiating

lamellae, whereas the pure hard-segment spherulites exhibit a radiating, rough fibrous texture.


Figure 7 shows that spherulite size decreases markedly with decreasing hard-segment content.

Interestingly, the spherulitic structure of the

intermediate composition random copolymers

(23%, 31%, 38%, and

43% ) still resembles that of the pure hard segment even though the hard segment is the minor component. 1/4/3,

and 1/3/2

Samples 0/1/1,

1/6/5,

1/5/4,

all contain coarse, fibrous spherulites with rough

boundaries. Sample 1/2/1, which contains the smallest amount of hard segment, exhibits no detectable superstructure. Visualization of the microphase domain morphology proved difficult. Figure 8 shows a dark granularity on the 4 0 0 - Â scale in the defocused bright-field image of samples 1/6/5

and 1/5/4.

T h e size and amount

(as well as the visibility) of this structure decreases with decreasing hard-segment content.

Films of less than 38% hard segment

appear

rather uniform in contrast at high resolution.

Discussion As-Reacted Samples. T h e thermal transition behavior of the random copolymer samples is similar to that of other previously studied M D I / B D O - b a s e d segmented polyurethanes (23,24).

Depending on the ther-

mal history of the sample, up to three endothermic transitions, all believed to be associated

with disordering of ( para ) crystalline

hard-segment

regions, have been observed. Sample annealing has been shown to cause the two lower endotherm peaks to shift to higher temperatures and become merged.

Further annealing then causes an additional upwards

shift of this combined lower peak until only a single high-temperature peak remains.

T h e annealing behavior is interpreted in terms of the

rearrangement

of small, disordered hard-segment

crystalline domains

regions

into

more

(24).

T h e observed weak, single low-temperature transition with a single, strong high-temperature transition suggests that ordered

hard-segment

domains can be directly formed in the as-reacted ( and cured ) polymers. T h e observed increase in the melting point of the random copolymers with increase in the average hard-segment length suggests an increase of domain size with increased hard-segment content.

T h e intensity and

American Chemical Society Library 1155 13th St. N. W. Cooper and Estes; Multiphase Polymers Washington, C. Washington, 20C38 Advances in Chemistry; American ChemicalD. Society: DC, 1979.

48

MULTIPHASE POLYMERS

l i n e w i d t h o f t h e h a r d - s e g m e n t W A X S reflections also i n d i c a t e t h a t c r y s t a l size is i n c r e a s i n g w i t h i n c r e a s e d h a r d - s e g m e n t Figure

9 s h o w s that

content.

t h e heat o f f u s i o n increases

approximately

l i n e a r l y w i t h h a r d - s e g m e n t c o n t e n t f o r samples c o n t a i n i n g greater t h a n 23 w t % h a r d segment.

T h e i n c r e m e n t a l heat o f f u s i o n w h e n r e l a t e d t o

the incremental weight fraction of h a r d segment f o r r a n d o m copolymers of greater t h a n 2 3 w t % h a r d segment shows that t h e f r a c t i o n a l d e g r e e


of c r y s t a l l i n i t y o f t h e h a r d segment p h a s e is a p p r o x i m a t e l y constant ( a t a b o u t 4 7 % b a s e d o n a n estimate o f 35.5 c a l / g f o r t h e p u r e , h a r d - s e g m e n t h e a t o f f u s i o n ( 2 9 ) ) (see T a b l e I I ) . T h e d e v i a t i o n f r o m l i n e a r i t y o f t h e h e a t o f f u s i o n d a t a at l o w w e i g h t f r a c t i o n h a r d s e g m e n t m a y b e a t t r i b u t a b l e t o a m i n i m u m c r i t i c a l h a r d s e g m e n t l e n g t h necessary f o r c r y s t a l l i z a tion.

T h e w e i g h t f r a c t i o n o f h a r d segment o f b l o c k l e n g t h e q u a l t o o r

greater than k consecutive hard-segment units m a y b e estimated u s i n g P e e b l e s (30)

t h e o r e t i c a l d a t a ( w h i c h assumes c o m p l e t e

reaction w i t h equal diisocyanate reactivity). a theoretical hard-segment

stoichiometric

F r o m F i g u r e 9 w e see f o r

distribution a block length of k >

6 is t h e

m i n i m u m effective b l o c k l e n g t h f o r c r y s t a l l i n e h a r d segment, w h i c h i s a m u c h h i g h e r v a l u e t h a n f o r H a r r e l F s (10) p o l y m e r (k > 1 ) .

7b

wt % Hard

Segment

Figure 9. Plot of heat of fusion vs. weight fraction of hard segment for each random copolymer, assuming k > I or k > 6, where k is the number of diisocyanate (hard-segment) units betwen two consecutive macrodiol (soft-segment) units. (Peebles (30) calculation of hard, block-length distribution in segmented polyurethane block copolymer is applied.)


2.

CHANG AND THOMAS

49

Segmented Poly methanes

T h e present results are, however, in general agreement with those of Harrell (10),

that is, longer hard-segment block length increases

size of hard-segment domains ( hence higher T

m

the

) but does not affect the

degree of order within the hard-segment domains (hence constant, hardsegment degree of crystallinity ). T h e absence of any soft-segment endotherm and crystalline W A X S reflections implies a noncrystalline softsegment phase in the as-reacted samples.


Solvent-Cast Samples.

T h e D S C scans of the D M F - s o l v e n t cast

samples at heating rate of 2 0 ° C / m i n also show two endothermic transi tions, but both peaks are shifted down in temperature with the as-reacted samples.

in comparison

As well, the heat of fusion of the high

temperature peak is less than in the respective as-reacted sample Table II).

(see

This would indicate a lower ordering of the hard-segment

phase and perhaps a poorer degree of phase separation in solution-cast films.

Interestingly, sample 1/2/1

shows a small melting endotherm at

approximately 4 0 ° C at heating rate of 5 ° C / m i n , indicating the presence of crystalline soft-segment regions. T h e electron diffraction results support the occurrence of soft-seg ment crystallinity and a more disordered hard-segment solvent-cast

samples.

phase in the

Moreover, electron diffraction indicates isolated

crystalline soft-segment regions persist even in samples of up to

43%

hard segment. Electron diffraction is very sensitive to small local fluctua tions in the overall structure because the diffraction patterns can be obtained from regions less than 1 μχη in diameter and less than .l-/i.m thick, whereas D S C and W A X S , of course, measure bulk polymer which yields averages over the whole sample. The spherulite morphology of the solvent-cast

samples

depends

strongly on the hard-segment content. T h e rough, fibrous-texture spherulites of the pure hard-segment copolymer are retained in the

1/6/5-1/3/2

random copolymer samples, but the average spherulite size decreases markedly with decreasing hard-segment content.

T h e spherulite mor

phology is therefore controlled by the first solidifying component and T

g

for pure hard segment are 2 3 2 ° and 1 2 5 ° C (25)

soft segment 6 0 ° and — 64 ° C (26))

m

even at quite low proportion of that

component (e.g., as low as 23 wt % hard segment). 1/2/1

(T

and for pure

Films of sample

are not spherulitic and appear quite uniform at high resolution

even though regions of crystalline soft segment are present. T h e low degree of hard-segment crystallinity in the

solvent-cast

samples with the added problem of loss of crystallinity from electronbeam damage prevented visualization of either the hard- or soft-segment domains by dark-field microscopy. Therefore bright-field defocus electron microscopy was used to enhance contrast between the microphases 28).

Only for the 1/5/4

and 1/6/5

(27,

samples was a distinct domain


50

MULTIPHASE POLYMERS

morphology evident.

Regions w h i c h appear d a r k i n the underfocused

m i c r o g r a p h are regions c a u s i n g a greater p h a s e c h a n g e of t h e t r a n s m i t t e d electrons a n d h e n c e are h a r d - s e g m e n t - r i c h r e g i o n s . I s o l a t e d d o m a i n s of 2 0 0 - 5 0 0 Â d i a m e t e r as w e l l as o c c a s i o n a l short, m e a n d e r i n g e l o n g a t e d d o m a i n s 1000-4000 Â l o n g a n d 300

Â w i d e appear rather

uniformly

d i s t r i b u t e d w i t h i n the s p h e r u l i t e s a n d i n t e r s p h e r u l i t i c r e g i o n s . T h e r o u g h radiating

fibrous

texture

a n d t h e b a n d i n g of t h e p u r e

hard-segment


s p h e r u l i t e s are a t t r i b u t e d to mass t h i c k n e s s contrast c a u s e d b y v a r i a t i o n s in

film

thickness

d u r i n g g r o w t h of the

solvent-cast

spherulitic

S o l v e n t c a s t i n g tends to decrease the c r y s t a l l i n i t y of t h e

films.

hard-segment

p h a s e as reflected i n the o b s e r v e d l o w e r h a r d - s e g m e n t m e l t i n g p o i n t s a n d heats of f u s i o n a n d the d e c r e a s e d b i r e f r i n g e n c e of the s p h e r u l i t e s . T h i s l o w e r degree of o r d e r i n the h a r d - s e g m e n t p h a s e is a t t r i b u t e d to t h e r a p i d u p w a r d s passage of t h e h a r d - s e g m e n t T

m

and T

g

t h r o u g h the sol-

v e n t c a s t i n g t e m p e r a t u r e d u r i n g t h e last stages o f solvent e v a p o r a t i o n , r e s t r i c t i n g c r y s t a l l i z a t i o n of t h e h a r d - s e g m e n t

sequences.

Overview D S C , W A X S , electron diffraction, a n d electron microscopy support a phase-separated

m o r p h o l o g y f o r a l l five of t h e

random copolymer

P C P / M D I / B D O - s e g m e n t e d polyurethanes studied. T h e slight increase o b s e r v e d b y S e e f r i e d et a l . ( 9 )

i n the l o w e r t e m p e r a t u r e T

g

with hard-

segment c o n c e n t r a t i o n is t h e r e f o r e l i k e l y a t t r i b u t a b l e t o i n c r e a s e d r e i n f o r c e m e n t of the soft-segment m a t r i x b y a n i n c r e a s e d n u m b e r of d i s p e r s e d hard-segment domains. E s t a b l i s h i n g the o c c u r r e n c e of p h a s e s e p a r a t i o n b y e x a m i n i n g t h e c r y s t a l l i n i t y of e i t h e r t h e h a r d - o r soft-segment p h a s e c a n b e m i s l e a d i n g s i n c e a s m a l l v o l u m e f r a c t i o n of a p a r t i a l l y c r y s t a l l i n e p h a s e m a y not b e e a s i l y d e t e c t e d (e.g., s a m p l e 1/2/1 appears a m o r p h o u s b y W A X S ) , a n d / or, of course, p h a s e s e p a r a t i o n m a y o c c u r w i t h o u t either p h a s e b e i n g o r d e r e d . A l s o t h e d e t e c t a b i l i t y of d o m a i n structures b y T E M is sensitive t o t h e d o m a i n size a n d the e l e c t r o n d e n s i t y d i f f e r e n c e b e t w e e n t h e phases. E v e n i f the e l e c t r o n d e n s i t y of the phases r e m a i n s constant f o r the v a r i o u s compositions,

the

projected

electron

density difference for r a n d o m l y

o r i e n t e d d o m a i n s w i l l t e n d t o a v e r a g e o u t unless the d o m a i n size is o n t h e o r d e r of t h e s a m p l e t h i c k n e s s .

Thus, bright

field

electron m i c r o -

g r a p h s of ( t y p i c a l l y ) 5 0 0 - Â t h i c k films enhances t h e v i s i b i l i t y of l a r g e r domains w h i l e obscuring smaller domains. Although

short-segment

c o m p a t i b l e , t h e 1/2/1

sequences are

expected

to be

the

most

r a n d o m c o p o l y m e r w i t h a n average hard-block-

s e q u e n c e l e n g t h of o n l y t w o u n i t s does e x h i b i t a p h a s e - s e p a r a t e d

mor-

p h o l o g y — a s reflected for the as-reacted sample i n hard-segment crystal-


2.

CHANG AND THOMAS

Segmented

51

Polyurethanes

linity and i n soft-segment crystallinity i n the solution-cast sample. In general, the as-reacted samples contain a more ordered,

hard-segment

phase than solution-cast samples, while the solution-cast samples contain a more ordered, soft-segment phase than the as-reacted samples. This strong dependence of the order of both the soft- and hard-seg ment phases on the sample preparation technique may account for some of the previous disagreement on the extent of phase separation in a given


polyurethane. Acknowledgment T h e authors are pleased to acknowledge suport of a grant-in-aid from the Union Carbide Corporation. Literature

Cited

1. Cooper, S. L . , Tobolsky, Α. V . , J. Appl. Phys. (1966) 10, 1837. 2. Huh, D . S., Cooper, S. L., Polym. Eng.Sci.,(1971) 11, 369. 3. Schneider, N . S., Desper, C . R., Illinger, J. L . , King, A . O., Barr, D . , J. Macromol. Sci., Phys. (1975) B11, 527. 4. Koutsky, J. Α., Hein, Ν. V., Cooper, S. L . , J. Polym. Sci., Polym. Lett. Ed. (1970) 8, 353. 5. Beecher, J . F., Marker, L . , Bradford, R. D . , Aggarwal, S. L . , J. Polym. Sci., Part C (1969) 26, 117. 6. Aggarwal, S. L . , Polymer (1976) 17, 938. 7. Wilkes, G . L . , Samuels, S. L . , Crystal, R., J. Macromol. Sci., Phys. (1974) B10, 203. 8. Cooper, S. L . , Seymour, R. W . , West, J. C., "Encyclopedia of Polymer Science & Technology, Plastics, Resins, Rubbers, Fibers: Supplement Vol. 1—Acrylonitrile Polymers, Degradation to Vinyl Chloride," Herman F. Mark, Norbert M . Bikales, Eds., p. 521, Wiley-Interscience, New York, 1976. 9. Seefried, C . G . , Jr., Koleske, J. V . , Critchfield, F . E . , J. Appl. Polym. Sci. (1975) 19, 2493. 10. Harrell, L . L . , Jr., Macromolecules (1969) 2, 607. 11. Samuels, S. L . , Wilkes, G . L . , J. Polym. Sci., Polym. Lett. Ed. (1971) 9, 761. 12. Samuels, S. L . , Wilkes, G . L . , J. Polym. Sci., Polym. Symp. (1973) 43, 149. 13. Lagasse, R. R., J. Appl. Polym. Sci. (1977) 21, 2489. 14. Kimura, I., Ishihara, H . , Ono, H . , Yoshihara, N . , Kawai, H . , IUPAC —III, Macromol. Prepr., Vol. 1, 525, Boston, 1971. 15. Lagasse, R. R., Wischmann, Κ. B., Am. Chem. Soc., Div. Org. Coat. Plast. Chem., Prepr. (1977) 37, 501. 16. Wilkes, C . E . , Yusek, C . S., J. Macro. Sci. Phys. (1973) B7, 157. 17. Bonart, R., Morbitzer, L . , Henze, G . , J. Macromol. Sci. Phys. (1969) B3, 337. 18. Bonart, R., Morbitzer, L . , Muller, Ε . H . , J. Macromol. Sci., Phys. (1969) B3, 337. 19. Bonart, R., Angew. Makromol. Chemie (1977) 58/59, 259. 20. Clough, S. B., Schneider, N . S., King, A . O., J. Macromol. Sci., Phys. (1968) B2, 641. 21. Seefried, Jr., C . G . , Koleske, J. V . , Critchfield, F. E . , J. Appl. Polym. Sci. (1975) 19, 2503.


52

MULTIPHASE POLYMERS


22. Chatani, Y., Okita, Y., Tadokoro, H., Yamashita, Y., Polym. J. (1970) 1, 555. 23. Miller, G . W . , Saunders,J.H.,J.Appl.Polym. Sci. (1969) 13, 1277. 24. Seymour, R. W . , Cooper, S. L . , Macromolecules (1973) 6, 48. 25. MacKnight, W . J., Yang, M . , Kajiyama, T . , Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. (1968) 9, 860. 26. Heijboer, J., J. Polym. Sci., Polym. Symp. (1968) 16C, 3755. 27. Petermann, J., Gleiter, H . , Philos. Mag. (1975) 31, 929. 28. Christner, G . L., Thomas, E . L . , J. Appl. Phys. (1977) 48, 4063. 29. Kajiyama, T . , MacKnight, W . J., Polym. J. (1970) 1, 548. 30. Peebles, L . H . , Jr., Macromolecules (1976) 9, 58. RECEIVED June 5, 1978.


Multiphase Polymers - American Chemical Society

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