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NBS

PUBLICATIONS NISTIR 88-3873

Small Angle X-Ray Study

of the Deformation of

4,4'-Diphenylmethane

Diisocyanate/1 ,4'-Butanediol

(MDI/BDO) Based Polyurethanes

R. M. Briber, P. Sung and J. D. Barnes

U.S. DEPARTMENT OF COMMERCE

National Institute of Standards and Technology

(Formerly National Bureau of Standards)

Institute for Materials Science and Engineering

Polymers Division

Polymer Blends Group

Gaithersburg, MD 20899

October 1988

-QC

100

• U56

#88-3873

1988

c.2

NISTIR 88-3873

Small Angle X-Ray Study

of the Deformation of

4,4'-Diphenylmethane

Diisocyanate/1 ,4'-Butanediol

(MDI/BDO) Based Polyurethanes

R. M. Briber, P. Sung and J. D. Barnes

U.S. DEPARTMENT OF COMMERCE

National Institute of Standards and Technology

(Formerly National Bureau of Standards)

Institute for Materials Science and Engineering

Polymers Division

Polymer Blends Group

Gaithersburg, MD 20899

October 1988

National Bureau of Standards became the

National Institute of Standards and Technology

on August 23, 1988, when the Omnibus Trade and

Competitiveness Act was signed. NIST retains

all NBS functions. Its new programs will encourage

improved use of technology by U.S. industry.

U.S. DEPARTMENT OF COMMERCE

C. William Verity, Secretary

NATIONAL INSTITUTE OF STANDARDS

AND TECHNOLOGY

Ernest Ambler, Director

ABSTRACT

Small angle x-ray scattering (SAXS) has been used to study the effect of

deformation on the hard segment domain morphology of a series of segmented

polyurethanes based on 4 , 4' -diphenylme thane diisocyanate/1 , 4 ' -butanediol

(MDI/BDO) as the hard segment and polytetramethylene oxide (PTMO) as the soft

segment. At relatively low elongations (100-200%) the hard segment lamellae

orient and align with their long axis perpendicular to the deformation

direction. As the deformation increases (200-400%) these aligned domains begin

to break down and the SAXS intensity decreases correspondingly. At the largest

deformations (400-500%) a new component to the scattering appears as a streak

running perpendicular to the deformation direction and subsequent optical

microscopy reveals the presence of crazes. Crazes have been observed at

elongations close to failure in all three polyurethanes studied so far, yet the

presence of crazing has not been recognized in the scientific literature as

being important in understanding the deformation behavior of polyurethanes.

2

INTRODUCTION

Polyurethanes are segmented copolymers where the two different blocks on

the polymer chain are thermodynamically incompatible in the normal range of use

temperatures. This leads to microphase separation of the two blocks (hard and

soft segments) and at compositions of -50% hard segment and less, hard segment

microdomains on the order of 10 nm in size are formed which act as reinforcing

physical crosslinks for the soft segment matrix. These hard segment domains

give polyurethanes their desirable properties of high toughness and the ability

to tailor the properties to meet specific end uses by varying the ratio of hard

and soft segment [1].

Polyurethanes based on 4,4' diphenylme thane diisocyanate (MDI) and 1,4-

butanediol (BDO) as the hard segment (henceforth termed MDI/BDO) combined with

various soft segments are very important commercially [1]. In this work we

will study the system consisting of MDI/BDO as the hard segment and

polytetramethylene oxide (PTMO, prepolymer molecular weight Mn =1000) as the

soft segment. The focus of the work presented here will be the changes that

occur in the hard segment domain morphology with deformation as studied by

small angle x-ray scattering (SAXS)

The morphology of MDI/BDO based

polyurethanes in the isotropic state has been the subject of numerous articles

in the literature [2-8] but the behavior with deformation has not been studied

with the same depth.

BACKGROUND

MDI/BDO hard segments have been shown to form two distinct crystal

3
structures in the isotropic state (type I and type II) and a third crystal

structure is found on deformation (type III) [9]. Previous work has shown that

the type II crystal structure (the more highly ordered of the two structures

which appear in the isotropic state) forms lamellar hard segment domains about

12run in width by about 50-70nm in length for high hard segment content

materials [10], with smaller lamellae being formed for lower hard segment

content materials. A wide angle x-ray scattering (WAXS) deformation study on

the same type of materials as studied in this paper has shown that, at least at

the higher hard segment contents, (46 wt%) there is a continuous shift in the

hard segment chain axis WAXS reflection to smaller angles with increasing

deformation. This indicates that the hard segment chain conformation is

changing continuously with deformation [11]. The length of the hard segment

repeat unit increases with deformation, shifting towards the chain conformation

characteristic of the type III crystal structure. At large elongations (500%),

there remains extensive disorder between neighboring chains along the

deformation direction and the type III structure is not obtained without

subsequent annealing in the deformed state.

Small angle x-ray scattering of polyurethanes has been studied by

numerous authors [1,7,8,12-17] but only a relatively small number of these were

concerned with morphology changes that occurred with deformation [15-17]. In

general it has been observed by both infrared dichroism and WAXS at low

elongations (<100%) in polyurethanes the hard segment chains initially orient

perpendicular to the deformation and subsequently at larger deformations

(>100%), orient parallel to the deformation direction [18-20]. In this work

only the deformation behavior at strains greater than 100% will be considered

in detail.

4
EXPERIMENTAL

The polyurethanes were supplied in the form of melt pressed sheets 15 cm

square and 0.12 cm thick by the Upjohn Corporation under the trade name

Pellethane [21]. The polyurethanes are based on MDI/BDO as the hard segment

and PTMO (M^ =1000) as the soft segment. Three samples of different hard

segment content termed Pellethane 80A, 90A and 55D (increasing hard segment

content) were studied and the molar ratios, average hard segment length

(assuming a most probable sequence length distribution [22-24]) and weight

percent hard segment are given in table 1. All samples were annealed at 140°C

for 12 hours before any mechanical testing or SAXS experiments were performed

in order to provide an equivalent and reproducible thermal treatment and

morphology for the samples. Mechanical testing was performed using a TiniusOlsen testing machine [21]. The strain rate for all deformation experiments

was 10 cm/min. To prepare the samples for SAXS the specimens were stretched to

the desired elongation and then clamped using a pair of steel plates with a 2cm

by 1cm hole machined in the center. Gauge marks drawn with a pen on the

samples were used to measure the strain and to assure that the clamping

procedure did not result in significant change in the deformation. SAXS was

performed using the NBS 10 meter digital SAXS camera. This camera utilizes a

12 kW Rigaku-Denki rotating anode x-ray generator producing Cu Ka (A=1.5418)

radiation which was passed through a graphite monochromator [21]. The data was

collected using a two dimensional position sensitive gas filled detector

manufactured by TEC [21]. The data was corrected for parasitic scattering,

5
sample thickness and transmission, normalized to a constant monitor count and

then put on a absolute scale using a secondary standard which had been

calibrated against a Lupolen standard. The data was then circularly averaged

in the case of the isotropic samples and sector averaged (25° sector width)

parallel and perpendicular to the deformation direction for the deformed

samples

.

RESULTS AND DISCUSSION

Figure 1 shows the typical stress strain curves for the 80A, 90A and 55D

samples (circles, squares and triangles respectively). SAKS patterns were

taken at elongations of 0%, 100%, 200%, 300% (80A, 55D) , 350% (90A) , 400% (80A,

55D) , 450% (90A, 55D) and 500% (80A) . The figures 2a-f, 3a-e and 4a-f are the

two dimensional SAXS patterns from the three polyurethanes at the different

levels of deformation. The intensity scale is mapped onto a false color scheme

with the scale shown at the edge of each of the figures. With increasing

intensity the colors change in the order black-red-yellow-white-green-bluepurple. A clear peak in the scattering pattern is observed for all three

samples at 0% deformation (2a, 3a, and 4a)

.

The circularly averaged data for

the 0% deformation samples was Lorentz corrected by multiplying the intensity

data by q

2 (q is the scattering vector given by q=47rsin#/A) as is normally done

for lamellar systems [25]. This data for the three isotropic samples is shown

in figure 5. The positions of the peak and the equivalent lamellar spacings,

d, calculated using Bragg's law (d=A/2sin0), are given in table 3.

The two dimensional patterns for the samples stretched to 100% and 200%

elongation are given in figures 2b , c 3b , c and 4b ,

c

respectively. The

6
deformation direction is shown by the arrow. The patterns show an clear

evidence of the deformation with maximum due to the lamellar periodicity

orienting along the meridian. The scattered intensity is also diminished in

the equatorial region of the pattern. This indicates an alignment of the hard

lamellae with their long axis perpendicular to the deformation direction and

consequently the hard segment chain axis is oriented along the deformation

direction.

Figures 2d, 3d and 4d show the two dimensional SAXS data obtained at 300%

elongation for samples 80A and 55D and 350% for the 90A sample. Figures 2e,

3e and 4e show the data for the 80A, 90A and 55D samples at 400%, 450% and 400%

deformation respectively. Figures 2f and 4f show the data at 500% and 450% for

the 80A and 55D samples.

The first obvious trend observable in the data is that as the deformation

starts (100%) all the samples exhibit orientation of the hard segment lamellae

along the direction perpendicular to the deformation. At slightly higher

elongations (200%) the orientation of the domains is increased, but at the same

time it becomes apparent, especially in the 55D sample that the scattered

intensity is decreasing. Figures 6a, b 7a, b and 8a, b show I(q) versus q plots

along the equator and meridian for the data presented in the two dimensional

scattering patterns for the 80A, 90A and 55D samples respectively. The data

presented in figures 6-8 was averaged over 25° wide sectors perpendicular

(equatorial) and parallel (meridian) to the deformation direction. The

decreasing intensity with deformation is clearly visible in the I(q) versus q

plots where the scattering maximum decreases continuously with deformation both

along the equator and the meridian.

In the two dimensional patterns there is an additional effect that is

7
observable at the larger deformations. There is a pronounced streaking in the

equatorial regions of the scattering pattern clearly visible in figures 2d, 3d

and 4d, though it is beginning to show up at 200% deformation (figures 2c, 3c,

and 4c) . This streaking appears to start earlier for the higher hard segment

content 55D sample but becomes the most pronounced for the 80A sample at 500%

deformation. The 80A sample deformed to 500% exhibits a large increase in the

equatorial scattering at the smallest angles which is clearly visible as a

region of purple in figure 2f. The increased scattering is also present in the

equatorial sector average in figure 6a where the scattering at 500% deformation

is much larger in the q region between 0.01-0.04 A" 1

. One possible explanation

for the increased scattering is void formation at the largest deformations and

indeed the 90A and 55D samples begin to scatter light, becoming slightly hazy,

shortly before failure (-350%). The 80A sample begins whitening at about 450%,

eventually becoming opaque at failure. Closer examination of an 80A sample at

large deformations by optical microscopy reveals that the source of the stress

whitening is crazing. Figures 9a and b are optical micrographs taken using

dark field and phase contrast optics of an 80A sample stretched to 500%. and

700% deformation respectively. An arrow on each micrograph indicates the

deformation direction. Figure 9a is at lower magnification and the crazing is

seen as lines running perpendicular to the deformation direction. In figure 9b

at higher magnification the larger deformation has caused the crazes to open up

and the internal structure can be seen to consist of fibrils running

perpendicular to the craze boundaries (parallel to the deformation direction)

.

Optical microscopy on the 90A and 55D samples also reveals the presence of

crazes at large deformations, with the number density of crazes appearing to

decrease with increasing hard segment content at constant elongation. Crazing

8
has been reported in polyurethanes only once before [26] and the presence of

crazes prior to failure is not widely appreciated. The streaking the SAXS

patterns observed along the equator is probably due to elliptical voids (with

the major axis oriented along the deformation direction) occurring between the

fibrils inside the crazes rather than due to scattering from periodicity

between the separate crazes (which is on the order of tens of microns and would

be at too small an angle to resolve by SAXS) . At the large deformations

attained shortly before failure the hard segment domain structure is

essentially completely destroyed and the scattering from the crazes dominates

the SAXS patterns

.

CONCLUSIONS

From the work currently being completed it has been shown that the domain

morphology first orients with the hard segment lamellae perpendicular to the

draw direction and then, with increasing deformation, the domains begin to

break down. As the sample approaches the elongation at fracture, e f , the hard

segment domains are completely destroyed and the material begins to exhibit

crazing. At slightly larger deformations the polyurethane fails. This

behavior is present in all the polyurethanes studied so far including

Pellethane 80A, 90A and 55D. It is not known whether the morphology of the

hard segment domains will return to the same (or similar) state upon releasing

of the strain (at strain levels less then € f ) after it has been held in the

stretched state for a period of time.

EXTENSIONS AND FUTURE WORK

Experiments are planned to extend the current work to study the recovery

9
of the polyurethanes after uniaxial stretching. Small angle x-ray scattering

will again be used to study the domain morphology both before and after the

releasing of the strain. This work should give valuable insight into the

deformation processes that occur in polyurethanes under cyclic deformation.

Once the morphology changes that occur during these "single cycle" experiments

are understood then experiments will be performed to examine the morphological

changes that occur during multicycle loading histories.

The multicycle experiments that are planned will involve changing the

variables which may influence the behavior of the polyurethanes under cyclic

deformation. These include varying the number of cycles, the initial and final

deformation of a given cycle and the frequency of the deformation. It may be

that the permanent deformation suffered by the hard segment domains in the

polyurethane is more sensitive to some of these variables than others. These

experiments should provide much needed insight into the morphological and

mechanical behavior of polyurethanes subject to cyclic loading histories.

10
LIST OF FIGURES

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Stress - strain curves for the samples 80A, 90A and 55D given by the

squares, triangles and circles respectively.

Two dimensional scattering patterns for the 80A sample as a function

of deformation: a 0% , b 100%, c 200%, d 300%, e 400%, f 500%.

Two dimensional scattering patterns for the 90A sample as a function

of deformation: a 0%, b 100%, c 200%, d 350%, e 450%.

Two dimensional scattering patterns for the 55D sample as a function

of deformation: a 0% , b 100%, c 200%, d 300%, e 400%, f 450%.

I(q)q2 versus q obtained for at 0% deformation for the 80A, 90A and

55D samples obtained by circularly averaging the data in figure 2.

I(q) versus q for the equatorial and meridional regions of the 80A

sample are presented in a and b respectively.

I(q) versus q for the equatorial and meridional regions of the 90A

sample are presented in a and b respectively.

I(q) versus q for the equatorial and meridional regions of the 55D

sample are presented in a and b respectively.

Optical micrographs of the 80A sample at 500% and 700% deformation

are shown in a and b respectively.

11
TABLE 1

sample molar ratio

PTMO/MDI/BDO

mole fraction

hard segment

weight %

hard segment

average hard

segment length

80A 1/2. 3/1.

3

0.56 26 2.3

90A 1/3. 3/2.

3

0.69 38 3.3

55D 1/4. 3/3.

3

0.77 46 4.3

1 assuming random polymerization the number average hard segment length can be

calculated from: N= 1/(1-X) where X is the hard segment mole fraction.

TABLE 2

e f (%) cr f (N/m2

) x 10“ 7

80A 600 6.34

90A 500 5,65

55D 480 6.41

TABLE 3

qm ax ( A_1 ) d spacing (A)

80A 0.056 110

90A 0.069 90

55D 0.069 90

12
REFERENCES

1. I. Goodman, "Recent Developments in Block Copolymers", No. 2, Elsevier

Applied Science Publishers, England, 1985

2. N.S. Schneider, C.R. Desper, J.L. Illinger, A.O. King and D Barr, J.

Macromol. Sci.-Phys., 1975, Bll, 527

3. A.L. Chang, R.M. Briber, P.E. Gibson, E.L. Thomas, R.J. Zdrahala, F.E.

Critchfield, Polymer, 1982, 23,1060

4. D.S. Huh and S.L. Cooper, Polym. Eng. Sci., 1971, 11, 369

5. J. Blackwell and C.D. Lee, J. Polym. Sci., Polym. Phys . Ed., 1984, 22, 759

6. T.R. Hesketh, J.W.C. Van Bogart, S.L. Cooper, Polym. Sci. and Eng., 1980,

20, 190

7. G.L. Wilkes, S.L. Samuels, R. Crystal, J. Macromol. Sci.-Phys., 1974, B10,

203

8. R.W. Seymour, J.R. Overton, L.S. Corley, Macromolecules, 1975, 8, 1975

9. R.M. Briber and E.L. Thomas, J. Polym. Sci., Polym. Phys. Ed., 1985, 23,

1915

10. R.M. Briber and E.L. Thomas, J. Macromol. Sci.-Phys., 1984, B22, 509

11. R.M. Briber and P. Sung, Polym. Comm., 1987, 28(6), 162

12. Z. Ophir, G. Wilkes, J. Polym. Sci., Polym. Phys. Ed., 1980, 18, 1469

13. J.T. Koberstein and R.S. Stein, J. Polym. Sci., Polym. Phys. Ed., 1983,

21, 1483

14. J.W.C. Van Bogart, P.E. Gibson, S.L. Cooper, J. Polym. Sci., Polym. Phys.

Ed., 1983, 21, 65

15. C.R. Desper, N.S. Schneider, J.P. Jasinski, J.S. Lin, Macromolecules, 1985,

18, 2755

16. M. Shibayama, Y. Ohki , T. Kotani, S. Nomura, Polym. Journ. , 1987, 19(9),

1067

17. E.S. Kong, G.L. Wilkes, J. Polym. Sci., Polym. Lett. Ed., 1980, 18, 369

18. R.W. Seymour, A.E. Allegreza, S.L. Cooper, Macromolecules, 1973, 6, 896

19. A.E. Allegreza, R.W. Seymour, H.N. Ng, S. L. Cooper, Polymer, 1974, 15, 433

20. R. Bonart, J. Macrmol. Sci.-Phys., 1968, B2(l), 115

13
21. Certain equipment, instruments or materials are identified in this paper

in order to adequately specify the experimental details. Such

identification does not imply recommendation by the National Institute of

Standards and Technology or the Food and Drug Administration nor does it

imply the materials or equipment identified are necessarily the best

available for the purpose.

22. L.H. Peebles, Macromolecules, 1974, 7, 872

23. L.H. Peebles, Macromolecules, 1976, 9, 58

24. H.K. Frensdorff, Macromolecules, 1971, 4, 369

25. R. Hosemann and S.N. Bagchi, "Direct Analysis of Diffraction by Matter",

North Holland Publishing Company, Amsterdam, Interscience Publishers Inc.,

New York, 1962

26. R. Caspary, H. Schnecko, Makromol. Chem. 1981, 182, 2109

14
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NBS-1 14A (rev. 2«ac) U.S. DEPT. OP COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NISTIR 88-3873

2. Performing Organ. Report No. 3. Publication Date

OCTOBER 1988

4. TITLE AND SUBTITLE

Small Angle X-Ray Scattering Study of the Deformation of

MDI/BDO Based Polyurethanes

5. AUTHOR(S)

R. M. Briber; P. Sung, J. D. Barnes

6. PERFORMING ORGANIZATION (If joint or other than N BS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Small angle x-ray scattering (SAXS) has been used to study the effect of

deformation on the hard segment domain morphology of a series of segmented

polyurethanes based on MDI/BDO as the hard segment and PTMO as the soft segment.

At relatively low elongations (100-200%) the hard segment lamellae orient and

align with their long axis perpendicular to the deformation direction. As the

deformation increases (200-400%) these aligned domains begin to break down and

the SAX intensity decreases correspondingly. At the largest deformations (400-500%)

a new component to the scattering appears as a streak running perpendicular to

the deformation direction and subsequent optical microscopy revealed the presence of

crazes. Crazes have been observed at elongations close to failure in all three

polyurethanes studied so far, yet the presence of crazing has not been recognized

in the scientific literature as being important in understanding the deformation

behavior of polyurethanes.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Polyurethanes, morphology. Small angle x-ray scattering, deformation, crazing

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