PTMA Polyurethane

14 The Structure of the Hard Segments in MDI/diol/PTMA Polyurethane Elastomers

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 1, 2015 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch014

J. BLACKWELL, M. R. NAGARAJAN, and Τ. Β. HOITINK Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44106

Polyurethane elastomers d e r i v e t h e i r e l a s t o m e r i c p r o p e r t i e s from phase s e p a r a t i o n o f the hard and s o f t copolymer segments, such t h a t the hard (urethane) segment domains serve as c r o s s l i n k s between the amorphous s o f t segment domains, which a r e u s u a l l y p o l y e s t e r s o r p o l y e t h e r s . We a r e i n t e r e s t e d i n the systems i n which the hard segments a r e prepared from d i p h e n y l methane 4 , 4 - d i i s o c y a n a t e (MDI) w i t h a l i n e a r d i o l as the c h a i n extender: f

- [ - 0 - CO - NH

C

H

2 ^ ^ - NH - CO - 0 - ( 0 Η ) - ] ~ 2

χ

n

The s o f t segments are p o l y ( t e t r a m e t h y l e n e adipate) (PTMA; ^ * 2089). I n these p r e p a r a t i o n s the hard domains a r e c r y s t a l l i n e and there has been c o n s i d e r a b l e i n t e r e s t i n the way the hydrogen bonding and c h a i n packing c o n t r i b u t e t o the s t r u c t u r a l s t a b i l i t y . The mechanical p r o p e r t i e s a r e dependent on the c h o i c e of c h a i n extender, which a f f e c t s the extent o f phase s e p a r a t i o n . Development o f c r y s t a l l i n i t y i s thought t o be an important f a c t o r c o n t r o l l i n g phase s e p a r a t i o n , and we a r e i n v e s t i g a t i n g the e f f e c t on the molecular conformation, packing, and the c r y s t a l l i t e per f e c t i o n , due t o v a r i a t i o n o f the c h a i n extender. In our i n i t i a l s t u d i e s we concentrated on the s t r u c t u r e o f poly(MDI-butandiol). I n i t i a l proposals f o r the s t r u c t u r e o f these hard segments were made by Bonart and co-workers (1-3) and by Wilkes and Yusek. (4) The x-ray p a t t e r n o f the o r i e n t e d an nealed elastomer f i l m showed a s i n g l e r e f l e c t i o n which c o u l d be assigned t o the hard segments, a t d ~ 7 . 9 Â , a z i m u t h a l l y i n c l i n e d at ^ 30° t o the m e r i d i a n . This r e f l e c t i o n must o r i g i n a t e from Bragg planes i n c l i n e d a t ^ 60° t o the c h a i n a x i s , and Bonart et a l proposed t h a t these planes a r i s e from a s t a g g e r i n g o f adj a c e n t chains so t h a t i n t e r m o l e c u l a r hydrogen bonds can be formed between the urethane groups. Formation o f C = 0 · · · Η - Ν hydrogen bonds which a r e approximately p e r p e n d i c u l a r t o the c h a i n neces s i t a t e s s t a g g e r i n g o f the c h a i n s , as occurs f o r example i n the s t r u c t u r e o f the α-form o f Nylon 66. ( 5 )

0097-6156/81/0172-0179$05.00/0 © 1981 American Chemical Society

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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The above s t u d i e s provided i n s i g h t i n t o the s t r u c t u r e o f the hard segments, but more d e t a i l e d proposals c o u l d not be de veloped without a knowledge o f the stereochemistry o f the MDI u n i t . I n a d d i t i o n t o the standard bond lengths and angles f o r the phenyl, urethane, and b u t a n d i o l groups, we need t o determine ( i ) the bond angle a t the c e n t r a l C-CH -C b r i d g e ; ( i i ) the o r i e n t a t i o n o f the phenyl r i n g s w i t h respect to each other and to the c e n t r a l C-CH -C plane; ( i i i ) the degree o f p l a n a r i t y o f the urethane groups and t h e i r o r i e n t a t i o n s w i t h respect t o t h e i r adjacent phenyls; and ( i v ) the conformation o f the - ( C H ) ^ - c h a i n and i t s o r i e n t a t i o n w i t h respect t o the diphenylmethane diurethane. In order t o determine the above parameters we are i n v e s t i g a t i n g the s t r u c t u r e s o f model compounds prepared by capping MDI w i t h a l c o h o l s . The f i r s t o f these s t r u c t u r e s was methanol-capped MDI (6) [MeMMe, CH (C H NHC00CH ) ]. This compound c r y s t a l l i z e s i n two polymorphic s t r u c t u r e s , which w i l l be designated MeMMeI and I I . The s t r u c t u r e o f MeMMeI has been determined i n t h i s l a b o r a t o r y , (6) and r e v e a l s many f e a t u r e s which can be expected t o occur i n the polymer s t r u c t u r e . The conformation o f the molecule i s shown i n f i g . l a and the bc_ p r o j e c t i o n of the c r y s t a l s t r u c ture i s shown i n f i g . l b . The c e n t r a l C-CH -C b r i d g e angle i s 114.5° and the phenyl planes are mutually i n c l i n e d a t 90.0°. Despite the chemical symmetry, the molecule i s not p h y s i c a l l y symmetrical, and the two ends o f the molecule are d i f f e r e n t i a t e d by A and Β l a b e l s i n f i g . 1 (a and b ) ; t h i s asymmetry probably occurs i n order t o o p t i m i z e the hydrogen bonding and packing.* The phenyl groups are i n c l i n e d a t 74.5°(A) and 34.7°(B) to the b r i d g e C-CH -C plane. The urethane groups are p l a n a r and i n c l i n e d a t 39.4°(A) and 10.2°(B) to t h e i r adjacent phenyls; the t e r m i n a l methyl carbons l i e approximately i n the planes o f the urethane groups. The B-urethane groups form C = 0 ··· Η - Ν hydro gen bonds approximately i n the b£ plane, as seen i n f i g . l b . The molecules are stacked on top o f each other along the a. a x i s (per p e n d i c u l a r to the be p l a n e ) , and the A-urethane groups are hydro gen bonded to each other along t h i s s t a c k . In c o n s i d e r i n g the type o f packing p o s s i b l e i n the polymer s t r u c t u r e , i t i s c l e a r t h a t the hydrogen bonding seen f o r the Aurethanes i s l i k e l y to occur f o r the polymer, b u t not that f o r the B-urethanes. Another f e a t u r e o f the MeMMe s t r u c t u r e i s t h a t the molecules have c r y s t a l l i z e d end-to-end: see the shaded molecules i n f i g , l b . A model f o r the polymer c h a i n was d e r i v e d (9)


2

2

2

2

6

i+

3

2

2

2

* I t i s i n t e r e s t i n g that i n the s t r u c t u r e o f MeMMeII, as d e t e r mined by Born e t a l , (7) the molecules are symmetrical, w i t h the two ends o f the molecule r e l a t e d by a twofold a x i s through the CH group. This twofold molecular symmetry a l s o occurs f o r HO-BMB-OH (butandiol-capped MDI; CH [C H NHC00(CH )^OH], as de termined i n t h i s l a b o r a t o r y . (8) However, n e i t h e r "of these two s t r u c t u r e s has i n t e r m o l e c u l a r hydrogen bonding networks t h a t could be analogous to s i t u a t i o n i n the polymers. 2

2

6

i+

2



Figure 1. Structure of MeMMe (methanol-capped MDI) (6): (a) conformation molecule; (b) be projection of the structure showing the packing and intermole hydrogen bonding.



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by a d d i t i o n o f a -CH2-CH2- c h a i n t o form the b u t a n d i o l u n i t , which was assumed to have a p l a n a r z i g z a g conformation (as i n p o l y e t h y l e n e ) . The s t a c k i n g o f the molecules along the a. a x i s of the MeMMe s t r u c t u r e was r e t a i n e d f o r the polymer, such t h a t the ac p r o j e c t i o n o f the polymer s t r u c t u r e i s as shown i n f i g . 2, where the hydrogen bonding o f the A-urethane groups can be seen. The B-urethane groups are hydrogen bonded t o the next sheet o f c h a i n s , and thus the s t r u c t u r e i s s t a b i l i z e d by hydrogen bonding i n two (approximately p e r p e n d i c u l a r ) d i r e c t i o n s . This proposed s t r u c t u r e has a t r i c l i n i c u n i t c e l l w i t h dimensions a. = 5.2A, b = 4^81, c = 35.OA, α = 121°, 3 = 116°, and γ =85°. The space group i s P I s i n c e the center o f symmetry between molecules i n the MeMMe s t r u c t u r e r e l a t e s the two monomer u n i t s forming the polymer f i b e r repeat. C o n f i r m a t i o n f o r t h i s s t r u c t u r e was obtained i n l a t e r x-ray work, (10) i n which a more h i g h l y r e s o l v e d x-ray f i b e r diagram was obtained f o r the poly(MDI-butandiol) hard segments. Twelve unique r e f l e c t i o n s were observed and these were indexed by a t r i c l i n i c u n i t c e l l w i t h dimensions a. = 5.05Â, ID = 4.67Â, £ = 37.9Â, α = 116°, 3 = 116°, and γ =83.5°. Given the experimental uncer t a i n t y i n the measured d-spacings, and the assumptions i n the model b u i l d i n g , the agreement between the observed and p r e d i c t e d u n i t c e l l s i s s t r i k i n g . The most s i g n i f i c a n t d i f f e r e n c e i s probably i n the f i b e r repeat ( c ) , which i n d i c a t e s a more elonga ted conformation than t h a t p r e d i c t e d from the "monomer" s t r u c ture. This paper d e s c r i b e s a c o n t i n u a t i o n o f the above work, f i r s t l y t o apply c o n f o r m a t i o n a l a n a l y s i s t o p r e d i c t the c o n f o r mation o f the poly(MDI-butandiol) c h a i n , and secondly to extend these x-ray d i f f r a c t i o n and model b u i l d i n g techniques t o i n v e s t i g a t e the s t r u c t u r e s formed w i t h other c h a i n extenders, n o t a b l y p r o p a n d i o l and ethylene g l y c o l . More d e t a i l e d accounts o f t h i s work have been p u b l i s h e d elsewhere. (11,12) EXPERIMENTAL Specimens The polyurethane specimens used i n t h i s r e s e a r c h were generously provided by Dr. C.S. Schollenberger o f B.F. Goodrich Co., B r e c k s v i l l e , Ohio. The specimens were prepared from r e actant mixtures c o n t a i n i n g the r a t i o 6 moles MDI:5 moles d i o l : 1 mole PTMA ( M * 2089), corresponding t o approximately 50% hard segment content; the d i o l c h a i n extenders were ethylene g l y c o l (EDO), p r o p a n d i o l (PDO), and b u t a n d i o l (BDO). Oriented f i l m s o f the BDO and PDO polymers were prepared by s l o w l y s t r e t c h i n g 1mm t h i c k unoriented f i l m s t o 700% e x t e n s i o n on a s t a i n l e s s s t e e l rack a t 130°C over a p e r i o d o f 7 days. As w i l l be seen, these specimens showed no evidence f o r s o f t seg ment c r y s t a l l i n i t y . This procedure was u n s u c c e s s f u l f o r the EDO n



BLACKWELL ET AL.

MDI/diol/PTM A Polyurethane Elastomers

1

Figure 2. The ac projection of the structure of poly (MDI-butandiol) hard segments proposed by Blackwell and Gardner (9).


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polymer, i n that the f i l m s always broke during s t r e t c h i n g above 60°C. The l a t t e r specimens were o r i e n t e d by s t r e t c h i n g 400% a t room temperature. The x-ray p a t t e r n s obtained showed both hard and s o f t segment c r y s t a l l i n i t y .


X-ray D i f f r a c t i o n X-ray d i f f r a c t i o n p a t t e r n s were recorded u s i n g a S e a r l e t o r o i d a l f o c u s i n g camera w i t h n i c k e l f i l t e r e d CuKa r a d i a t i o n generated by a Rigaku-Denki r o t a t i n g anode source. Conformational A n a l y s i s The c h a i n conformations o f poly(MDI-BDO), poly(MDI-PDO), and poly(MDI-EDO) were i n v e s t i g a t e d (11) using both s e m i e m p i r i c a l p o t e n t i a l energy f u n c t i o n s and CNDO/2 quantum mechanical methods. (13) Standard i n t e r a t o m i c d i s t a n c e s and angles were used f o r the diphenylmethane and - ( C H ) - u n i t s . S i m i l a r para meters f o r the urethane group were obtained from a survey o f 24 c r y s t a l s t r u c t u r e s of low molecular weight urethane compounds. The s e m i e m p i r i c a l c a l c u a l t i o n s determined the t o t a l conforma t i o n a l energy by summing non-bonded (van der Waals), t o r s i o n a l , angular deformations, and e l e c t r o s t a t i c e n e r g i e s . P o i n t charges f o r the c a l c u l a t i o n o f the e l e c t r o s t a t i c c o n t r i b u t i o n s were c a l c u l a t e d u s i n g the CNDO/2 method. The c o e f f i c i e n t s used f o r the p o t e n t i a l f u n c t i o n s were due to Sheraga and co-workers. (14) CNDO/2 c a l c u l a t i o n s were a l s o used to estimate the e f f e c t s of i n t e r a c t i o n o f the π-orbitals of the phenyl and urethane groups. 2

X

RESULTS AND DISCUSSION Conformational A n a l y s i s o f Poly(MDI-BDO) The conformation o f the poly(MDI-BDO) c h a i n i s d e f i n e d by s i x t o r s i o n angles as shown below. (CH ) ^ — ^ C 0 N H H K \ 2

/V-àr€H —Λ-^-NHCO.O—^(CH^ 2

The phenyl and urethane groups a r e p l a n a r and a r e assumed t o be r i g i d ; the conformation o f the - ( C H ) - c h a i n must a l s o be de f i n e d . The d e f i n i t i o n s o f the o r i g i n s f o r the s i x t o r s i o n angles are given i n reference 11. This polymer has a long chemical repeat and conformational a n a l y s i s would be a formidable task were i t not t h a t the x-ray data i n d i c a t e a h i g h l y extended conformation: two monomer u n i t s repeat i n 37.9Â, i . e . a r i s e o f 1 8 . 9 5 Â per repeat. Consequently, the conformation w i l l be determined by i n t e r a c t i o n s between contiguous groups. Conformational energy c a l c u l a t i o n s have been used to i n v e s t i g a t e the phenyl-phenyl, the pheny1-urethane, and, a f t e r c o n s i d e r a t i o n o f the p o s s i b l e - ( C H ) - conformations, the 2

if

2

i+


14.

BLACKWELL

MDI/diol/PTMA

ET AL.

Polyurethane Elastomers

urethane - ( C H ^ ^ i n t e r a c t i o n s . These r e s u l t s have then been combined t o p r e d i c t the conformation o f the polymer c h a i n . For the phenyl-phenyl i n t e r a c t i o n s , the energy o f the d i phenylmethane u n i t was minimized as a f u n c t i o n o f φ} and φ f o r d i f f e r e n t values o f the C-CH -C b r i d g e angle, τ. Minimum energy occurs f o r τ = 110°, the t e t r a h e d r a l angle. However there i s a s u b s i d i a r y minimum a t τ = 118.3°, w i t h energy 1.9 kcal/mole highei than the τ = 110° minimum. The s u b s i d i a r y minimum i s a t Φχ,Φ = (-60°,-60°) f o r which the phenyl r i n g s are mutually p e r p e n d i c u l a i and e x a c t l y gauche t o the c e n t r a l CH group ( c . f . τ=114.5° i n the s t r u c t u r e o f MeMMeI. (6) This s u b s i d i a r y minimum a l s o leads to a more extended conformation and f o r these reasons was s e l e c ted f o r the diphenylmethane s e c t i o n o f the polymer c h a i n . For the phenyl-urethane i n t e r a c t i o n s , minimum energy occurs a t X = ±90°, f o r which the phenyl and urethane planes are perpendicu l a r . The most extended conformation i s Χ ,X =-90°,+90°; a monomer repeat o f c_ =18.95Â cannot be achieved f o r the other combinations o f Χχ,Χ f o r any conformation o f the r e s t o f the chain. The above conformation: Χ = -90°, φ = -60°, φ = -60°, X = +90° has a l e n g t h o f 14.0Â measured from the t e r m i n a l oxygens of the urethane groups. Minimum energy f o r the 0-(CH ) -0 sect i o n i s f o r an a l l - t r a n s conformation which i s coplanar w i t h the urethanes, ω , ω = 180°,180°, i . e . the s e c t i o n -NH-C0-0-(CH ) -O-CO-NHi s a p l a n a r z i g z a g . Examination o f s u b s i d i a r y m i n i m a c o n t a i n i n g gauche bonds show t h a t the monomer repeat o f _c = 18.95A cannot be a t t a i n e d w i t h any but the a l l - t r a n s conformation. Thus the p r e d i c t e d polymer conformation i s d e f i n e d by the f o l l o w i n g t o r s i o n angles ω! = 180°, Χ = -90° , φ! = -60°, φ = -60°, χ = +90°, ω = 180°, w i t h an a l l - t r a n s - ( C H ) - u n i t . The monomei repeat f o r our model was 18.95Â which matches the observed value, ( V a r i a t i o n o f the bond lengths and angles i s p o s s i b l e w i t h i n the standard ranges, r e s u l t i n g i n a v a r i a t i o n o f approximately ±0. 5Â i n the monomer repeat ; the exact agreement i s c o n t r i v e d f o r convenience i n l a t e r packing a n a l y s e s . ) A p r o j e c t i o n o f the pred i c t e d conformation i s shown i n f i g . 3. We can be reasonable c o n f i d e n t t h a t the a c t u a l t o r s i o n angles i n the s o l i d s t a t e s t r u c t u r e w i l l be c l o s e t o the p r e d i c ted v a l u e s , except f o r X j and X , which d e f i n e the phenylurethane o r i e n t a t i o n s . The model d i p h e n y l - d i u r e t h a n e s t r u c tures (6-8) have X angles i n the range 10-37°. The d e v i a t i o n s from X = 90° c o u l d be due t o e l e c t r o n d e r e a l i z a t i o n between the phenyl and urethane groups, which would f a v o r the s t e r i c a l l y d i s a l l o w e d X = 0° conformation. Such e l e c t r o n d e r e a l i z a t i o n has been considered elsewhere f o r aromatic polyamides and p o l y e s t e r s . However, a s i m p l e r e x p l a n a t i o n may be t h a t s i g n i f i c a n t space occurs between the molecules i f they are hydrogen bonded i n the X = ±90° conformations, and t h a t t h i s i s e l i m i n a t e d when X i s v a r i e d i . e . the urethanes are t w i s t e d away from a p o s i t i o n 2

2


18

2

2

χ

2

2

ι

χ

2

2

2

χ

1+

2

2

lf

Q

χ

2

2

2

2

I+

2



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APPLICATIONS

18.95 Â Figure 3. Projection of the minimum energy conformation of poly(MDI-BDO) (11). The fiber repeat is 37.9 Â and contains two monomers, that is, the monomer repeat along the fiber axis is 18.95 Â.



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187

p e r p e n d i c u l a r to the phenyl r i n g s . The energy i n c r e a s e f o r the molecular conformation would be more than compensated by t h a t due to the packing e f f e c t s . V a r i a t i o n of X has l i t t l e e f f e c t on the l e n g t h of the monomer repeat, owing to the f a c t t h a t the t e r m i n a l oxygen of the urethane group i s almost c o l i n e a r w i t h the N-C (phenyl) bond. Table 1 shows the monomer repeats f o r centrosymmetric chains w i t h a range of values of Xj and X , where i t i s seen t h a t e/2 does not vary by more than 0.2Â. We are now c o n s i d e r i n g the packing of the poly(MDI-BDO) chains i n the proposed t r i c l i n i c u n i t c e l l , and w i l l r e p o r t on t h i s i n due course. 2

Poly(MDI-PDO) and Poly(MDI-EDO) X-ray d i f f r a c t i o n p a t t e r n s of o r i e n t e d f i l m s of the BDO, PDO, and EDO p r e p a r a t i o n s (12) are shown i n f i g . 4. The p o l y (MDI-BDO) p a t t e r n has been analyzed p r e v i o u s l y and i n d i c a t e s a t r i c l i n i c u n i t c e l l i n which the base plane i s t i l t e d , i . e . i t i s not p e r p e n d i c u l a r to the f i b e r a x i s , such t h a t the chains are i n a staggered hydrogen bonded a r r a y . The conformational a n a l y s i s above shows t h a t the polymer c h a i n i s f u l l y extended, w i t h the a l l - t r a n s - ( C H ^ ^ - u n i t coplanar w i t h the urethane groups. The x-ray p a t t e r n of poly(MDI-PDO) and the EDO p r e p a r a t i o n can be seen to be q u i t e d i f f e r e n t from t h a t of poly(MDI-BDO). Poly(MDI-PDO) shows m e r i d i o n a l r e f l e c t i o n s a t d=16.2Â, 8.1Â, and ^4.OA, and an e q u a t o r i a l a t d - 4.7Â, w i t h a row l i n e at t h a t p o s i t i o n . This p a t t e r n i s c h a r a c t e r i s t i c of a m o n o c l i n i c u n i t c e l l w i t h the base plane p e r p e n d i c u l a r to the f i b e r a x i s , i . e . the chains are not staggered but are arranged i n r e g i s t e r . The poly(MDI-BDO) and poly(MDI-PDO) specimens had been prepared i n the same manner, and the lower q u a l i t y of the x-ray p a t t e r n f o r the l a t t e r i n d i c a t e s a lower degree of c r y s t a l l i n e order i n the poly(MDI-PDO) hard segments. I n the p a t t e r n of the EDO preparat i o n , m e r i d i o n a l r e f l e c t i o n s are seen a t 15. OA and 7.5Â, and there i s a s t r o n g e q u a t o r i a l a t d=4.05Â. This specimen had been prepared at room temperature, and consequently c r y s t a l l i n e r e f l e c t i o n s f o r the s o f t segments are to be expected. The e q u a t o r i a l r e f l e c t i o n matches t h a t a t d=4.05A f o r the α-form of PTMA, (15) the more probable polymorph g i v e n the method of prepa r a t i o n . However the m e r i d i o n a l r e f l e c t i o n s are not observed f o r e i t h e r a- or $-PTMA and these can be assigned to the poly(MDIEDO) hard segments. Analogy w i t h the poly(MDI-PDO) p a t t e r n suggests t h a t the PDO and EDO hard segments have s i m i l a r c h a i n conformations, w i t h monomer repeats of 16.2 and 15.OA respec t i v e l y , and are packed i n an unstaggered a r r a y . The 1.2Â d i f f e r e n c e i n the monomer repeats f o r poly(MDI-PDO) and poly(MDI-EDO) i s probably due to d e l e t i o n of a trans-CH?group: i n p o l y e t h y l e n e the repeat i s 1.27Â per CH group. However, the d i f f e r e n c e between the poly(MDI-BDO) repeat of 18.95A and t h a t f o r poly(MDI-PDO) of 16.2Â cannot be e x p l a i n e d so simply. 2



APPLICATIONS


188

Figure 4. X-ray diffraction patterns of oriented polyurethanefilms:(a) MDI/ BDO/PTMA stretched 700% at 130°C (10); (b) MDI/PDO/PTMA stretched 700% at 130°C (12); (c) MDI/EDO/ΡΊΜΑ stretched 400% at room tempera ture (12). In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

BLACKWELL


189

From the c o n f o r m a t i o n a l a n a l y s i s poly(MDI-BDO) i s seen t o be f u l l y extended, w i t h an a l l - t r a n s b u t a n d i o l u n i t . Thus i t must be concluded t h a t the poly(MDI-PDO) and poly(MDI-EDO) conforma t i o n s a r e c o n t r a c t e d and that there a r e some gauche conformations i n the 0-(CH ) -0 s e c t i o n s o f the c h a i n s . Our procedure i n p r e d i c t i n g poly(MDI-PDO and poly(MDI-EDO) conformations has been t o take the extended minimum energy con formation f o r the diphenylmethane diurethane s e c t i o n (Χχ = -90°, Φΐ = -60°, Φ = -60°, Χ = +90°) and to l o o k a t p o s s i b l e conforma t i o n s o f the d i o l s e c t i o n s , comparing t h e i r r e l a t i v e energies and f i b e r r e p e a t s . (12) These conformations a r e d e f i n e d by the t o r s i o n angles ω , ω , ω and [ f o r poly(MDI-PDO) o n l y ] ω , as shown below. 2


MDI/diol/PTMA

ET AL.

x

2

2

1

2

3

4

-O^H ^H -^H --^C0NI^ 2

2

2

ΛΗ:Η -Λ

/)^C00-^H —^Η ^Η ·^

2

2

2

2

The chemical repeats a r e symmetrical and i t i s l i k e l y t h a t t h i s w i l l l e a d t o symmetry i n the conformations on e i t h e r s i d e o f the center o f the d i o l u n i t i . e . ω = ω and ω = ω f o r poly(MDI-PDO) and ω = - ω f o r poly(MDI-EDO). These symmetries l e a d t o monomer repeats. The x-ray data appear to be compatible w i t h m u l t i p l e repeats e.g. where s u c c e s s i v e monomer u n i t s a r e r e l a t e d by a screw a x i s , and we a r e i n the process o f c o n s i d e r i n g these op t i o n s . However, there i s no p o s i t i v e x-ray evidence f o r anything more complex than a monomer r e p e a t , and the m u l t i p l e repeat pos s i b i l i t i e s w i l l not be d i s c u s s e d f u r t h e r . An energy map was obtained f o r the ω ω conformations f o r poly(MDI-PDO) and the p o s i t i o n s and r e l a t i v e energies o f the minima a r e l i s t e d i n t a b l e 2. Not s u r p r i s i n g l y , the lowest energy i s f o r ω ,ω =180°,180°, corresponding to the f u l l y ex tended a l l - t r a n s conformation. This has a monomer repeat o f 17.6A which i s s i g n i f i c a n t l y longer than the observed v a l u e o f 16.2Â. I n a d d i t i o n i t can be seen t h a t such a c h a i n cannot form s t r a i g h t i n t e r m o l e c u l a r hydrogen bonds s i n c e the N-H groups o f urethanes adjacent t o the c e n t r a l - ( C H ) - a r e p o i n t i n g i n the same d i r e c t i o n . The next lowest minimum, 0.9 kcal/mole above the a l l - t r a n s conformation i s f o r ω , ω = 180°,±60° (+60° and -60° correspond to the gauche " and gauche"" conformations f o r the CH groups) . This trans-gauche^-gauche"*"-trans (and trans-gauche"-gauche""-trans) o p t i o n has a f i b e r repeat of 16.20Â. F o l l o w i n g t h i s , the t h i r d minimum i s 1.4 kcal/mole h i g h e r than a l l - t r a n s a t ω , ω = ±80°,180° ( d e v i a t i o n from ±60° i s due to the CH - c a r b o n y l i n t e r a c t i o n ) . This corresponds t o the gauche"*"-trans-trans-gauche"*" (and gauche"trans-trans-gauche ) conformation, and has a f i b e r repeat o f 16.24Â. The second and t h i r d conformations a r e both compatible w i t h the observed monomer repeat. The remaining minima a r e f o r a l l - g a u c h e o p t i o n s , of which the most extended conformation has a repeat of 15.31Â, s i g n i f i c a n t l y s h o r t e r than the observed value. 1

1

4

2

3

3

1 ?

χ

2

2

2

χ

3

2

4

2

1

2

2


190

U R E T H A N E CHEMISTRY AND

The two p o s s i b l e conformations of poly(MDI-PDO) are shown i n f i g . 5. We are c u r r e n t l y c o n s i d e r i n g packing models f o r these conformations, which may a l l o w f o r s e l e c t i o n of one or the other as the more l i k e l y . However, evidence i n favor of the lower energy form (trans-gauche -gauche -trans) comes from the s t r u c ture of another model compound determined i n t h i s l a b o r a t o r y : butandiol-capped MDI[CH {C H NHC00(CH ) 0H} ; HO-BMB-OH], which i s shown i n f i g . 6. The t e r m i n a l -C^-0 chains are approximately p l a n a r , but are gauche to the urethane: ω ,ω = 170.5,69.5°. As i n d i c a t e d i n f i g . 6, the C ··· C d i s t a n c e corresponding to the poly(MDI-PDO) repeat i s 16.34Â. I t should be noted t h a t i n the HO-BMB-OH s t r u c t u r e χ = 10.4°, i . e . the phenyl and urethane groups are almost coplanar. However, symmetrical v a r i a t i o n of Xj and X changes the f i b e r repeat by no more than ±0.2Â, i . e . c_ i s r e l a t i v e l y i n s e n s i t i v e to the phenyl-urethane o r i e n t a t i o n s , as was the case f o r poly(MDI-BDO). The same energy c a l c u l a t i o n s can be used to i n v e s t i g a t e the p o s s i b l e poly(MDI-EDO) conformations. The energy minima and monomer repeats are l i s t e d i n t a b l e 3. The a l l - t r a n s and t r a n s gauche"*"- trans conformations have monomer repeats t h a t are s i g n i f i c a n t l y longer than the observed value of 15.0Â, which c o i n c i d e s w i t h t h a t f o r the t h i r d minimum: gauche"*"-trans-gauche". This conformation i s proposed f o r the poly(MDI-EDO) s t r u c t u r e , and i s shown i n f i g . 7. As was the case f o r the other polymers, symmetr i c a l v a r i a t i o n of Xj and X have l i t t l e e f f e c t on the f i b e r repeat. We are c u r r e n t l y s t u d y i n g packing models f o r a l l three polymers, which should throw l i g h t on the more l i k e l y X angles. At t h i s p o i n t the f o l l o w i n g c o n c l u s i o n s can be drawn from these i n v e s t i g a t i o n s . The BDO hard segments have b e t t e r c r y s t a l l i n e order than the PDO hard segments, suggesting more e f f e c t i v e phase s e p a r a t i o n i n the BDO p r e p a r a t i o n . This may have the simple e x p l a n a t i o n t h a t the poly(MDI-BDO) chains can c r y s t a l l i z e more e a s i l y , s i n c e i n the c r y s t a l s t r u c t u r e they have the lowest energy f u l l y extended conformation, which forms s t r a i g h t i n t e r molecular hydrogen bonds i n both dimensions p e r p e n d i c u l a r to the chain a x i s . Poly(MDI-PDO) however cannot form an e q u i v a l e n t hydrogen bonding network i f i t adopts the lowest energy f u l l y extended conformation, and thus w i l l need to rearrange to a h i g h e r energy c o n t r a c t e d conformation. This s t r u c t u r e w i l l then be s t a b i l i z e d by i n t e r m o l e c u l a r hydrogen bonds, but w i l l be of h i g h e r energy than the poly(MDI-BDO) s t r u c t u r e , and thus there w i l l be l e s s of a d r i v i n g f o r c e f o r phase s e p a r a t i o n due to c r y s t a l l i z a t i o n of the hard segments. The r e s u l t s f o r poly(MDIEDO) are more s u r p r i s i n g i n t h a t one might expect t h a t t h i s polymer would adopt a s t r u c t u r e s i m i l a r to poly(MDI-BDO), s i n c e EDO has an even number of CH groups. I t may be however t h a t the ethylene g l y c o l c h a i n i s too s h o r t to a l l o w adequate packing of the diphenylmethane diurethane u n i t s i n the extended conformation, r e s u l t i n g i n adoption of a c o n t r a c t e d conformation more analogous to poly(MDI-PDO). +

2


APPLICATIONS

+

6

lt

2

lt

2

+

1

2

2

2

2


14.

BLACKWELL

MDI/diol/PTMA

ET AL.


Table 1 Minimum Energy Conformations of the MDI-butandiol Fragment


I n i t i a l Conformation (degrees) φ

Relative Energy (kcal/mole)

Length o f the MDI-butandiol Fragment c_/ 2

No.

ωχ

1

-180

Xl - 90

Φι -60

-60

90

180

0.00

18.95

2

-180

-100

-60

-60

80

180

0.32

19.02

3

-180

-110

-60

-60

70

180

0.88

19.06

4

-180

-120

-60

-60

60

180

1.62

19.09

5

-180

-130

-60

-60

50

180

2.58

19.06

6

-180

-140

-60

-60

40

180

3.92

19.02

7

-180

-150

-60

-60

30

180

5.35

18.95

8

-180

-160

-60

-60

20

180

7.03

18.86

9

-180

-170

-60

-60

10

180

9.12

18.77

Χ

2

2

ω

2

Table 2 Minimum Energy Conformations of MDI-PDO Fragment Conformational Angles (degrees) ωχ

ω

180

φ

Fiber Repeat c(A)

X2

ω

-60

90

180

180

0.00

17.60

-60

-60

90

60

180

0.90

16.20

-90

-60

-60

90

180

80

1.39

16.24

-90

-60

-60

90

80

60

1.64

15.31

180

Xl -90

Φι -60

180

60

-90

80

180

60

80

2


2

3

Table 3 Minimum Energy Conformations of MDI-EDO Fragment Conformational Angles (degrees)


Fiber Repeat c(A)

2

ω

180

180

180

0.00

16.45

90

180

60

180

0.94

15.80

-60

90

60

180

-60

1.28

15.00

-60

90

60

60

-60

1.98

14.16

Xl -90

Φΐ -60

Φ2

X

-60

90

-90

-60

-60

-90

-60

-90

-60

ω 2

1

ω

3


191


192

URETHANE CHEMISTRY AND APPLICATIONS

Figure 5. Projection of predicted conformations of poly (MDI-PDO) (12): (a) trans-gauche -gauche -trans; fiber repeat 16.20 Â; (b) gauche -trans-transgauche ; fiber repeat 16.24 Â. +

+

+

+


ET

AL,

MDI/ diol/PTMA Polyurethane Elastomers


BLACKWELL


193


194


15.0

Figure 7. Predicted conformation for poly(MDI-EDO) (12). The ethylene glycol unit has the g a u c h e - t r a n s - g a u c h e conformation; thefiberrepeat is 15.0 A. +

+


I

14.

BLACKWELL E T AL.

MDI/diol/PTMA


195

We have r e c e n t l y obtained x-ray d i f f r a c t i o n diagrams o f e q u i v a l e n t p r e p a r a t i o n s using p e n t a n d i o l and h e x a n d i o l and a r e extending the above c o n s i d e r a t i o n s t o these and systems prepared w i t h other c h a i n extenders. Our research i s supported by A.R.O. grant number DAAG29-79G-0070.


Abstract We are using x-ray methods to determine the structure of the poly(MDI-diol) hard domains in polyurethane elastomers, in order to understand the effect of chemical structure on the crys talline order and extent of phase separation. Oriented films containing approximately 50% hard segments have been obtained for preparations using butandiol (BDO), propandiol (PDO), and eth ylene glycol (EDO) as the chain extender; the soft segment is polytetramethylene adipate (M =2089). Poly (MDI-BDO) is the most crystalline, and has a triclinic unit cell with a tilted base plane such that there is a staggering of adjacent chains in the crystal structure. In contrast, poly(MDI-PDO) and poly(MDI-EDO) have monoclinic unit cells with adjacent chains in register. In order to develop molecular models for these polymers we have used single crystal x-ray methods to determine the structures of methanol- and butandiol-capped MDI. Based on the data from these structures, conformational analysis has been used to predict the likely chain conformations for the polymers. A model has been developed for poly(MDI-BDO) in which fully extended chains are staggered to give a hydrogen bonding network in two dimensions perpendicular to the chain axis. In contrast poly(MDI-PDO) and poly(MDI-EDO) crystallize in higher energy contracted conforma tions, which are necessary for the non-staggered packing of the chains. This difference may explain the higher crystalline per fection of the hard segments in the butandiol preparation. n

Literature Cited 1. Bonart, R.J. J. Macromol. Sci. (Β), 1968, 2, 115. 2. Bonart, R.J.; Morbitzer, L . ; Hentze, G. J. Macromol. Sci. (Β), 1969, 3, 337. 3. Bonart, R.J.; Morbitzer, L . ; Muhler, E.H. J. Macromol. Sci. (B), 1974, 9, 447. 4. Wilkes, C.W.; Yusek, C. J. Macromol. Sci. (Β), 1973, 7, 157. 5. Bunn, C.W. and Garner, E.V. Proc. Roy. Soc. (London), 1947, A189, 39. 6. Gardner, K.H. and Blackwell, J. Acta Crystallogr., 1980, B36, 1972. 7. Born, L.; Hocker, J.; Paulus, H.; Wölfel, E. Krist. Struct. Commun., (in press); Hocker, J. and Born, L. J. Polymer Sci.Polymer Letters Edns., 1979, 17, 723. 8. Forcier, P.G. and Blackwell, J. Acta Crystallogr., 1980,B37, 286.




196

9. Blackwell, J. and Gardner, K.H. Polymer, 1979, 20, 13. 10. Blackwell, J. and Ross, M. J. Polymer Sci.-Polymer Letters Edns., 1979, 17, 447. 11. Blackwell, J. and Nagarajan, M.R. Polymer, 1981,22,202. 12. Blackwell, J.; Nagarajan, M.R.; Hoitink, T., submitted to Polymer. 13. Pople, J.A. and Severidge, D.L. "Approximate Molecular Or bital Theory"; McGraw-Hill, New York, 1970. 14. Ooi, T.; Scott, R.A.; Vanderkooi, G.; Scheraga, H.A. J. Phys. Chem., 1967, 46, 4410. 15. Minke, R. and Blackwell, J. J. Macromol. Sci.-Phys. (Β), 1979, 16(3), 407. RECEIVED May 11, 1981.


PTMA Polyurethane

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