The physicochemical properties of interpenetrating polymer networks based on a polyurethane and a polyurethane-acrylate

Polymer Science U.S.S.R. Vol. 20, pp. 51--61. ~ ) Pergamon Pre~ Ltd. 1978. Printed in Poland

0052-$950/78/0101-0051507.50/0

THE PHYSICOCHE~MICAL PROPERTIES OF INTERPENETRATING POLYMER NETWORKS BASED ON A POLYURETHANE AND A POLYURETHANEACRYLATE* Yu. S. LIPATOV,L. V. KAmrBX~OVA, T. S. KmnxMovx and L. M. SEROEYEVA Institute of the Chemistry of Macromolecular Compounds, Ukr.S.S.R. Academy of Sciences

(Received 7 February 1977) Interpenetrating polymer networks (IPNs) based on a polyurethane and a polyurethane-acrylate, have been obtained. The nature of the interaction between the components of the I P N was investigated by infrared and NMR spectroscopy, the molecular mobility was studied over a wide range of temperatures and the energies of activation for the thermal transitions found to occur in the system are calculated. The sorption of dioxan vapour by the IPNs was studied. The free energy of rrlixing of the networks in the IPNs has been calculated for the first time, and also the free energy of swelling and the partial specific enthalpies and entropies of the IPNs. I t is shown that despite the facts that the components of the IPNs are not compatible and that they do not interact chemically, a loose transitional region is formed between the two component network phases, and this determines the combination of properties of the system as a whole.

THE synthesis of i n t e r p e n e t r a t i n g p o l y m e r n e t w o r k s (IPNs) is a p r o m i s i n g m e t h o d for p r o d u c t i o n of c o m p o s i t e p o l y m e r i c m a t e r i a l s b a s e d on crosslinked p o l y m e r s . I n r e c e n t t i m e s m u c h a t t e n t i o n has b e e n p a i d to p r e p a r a t i o n a n d s t u d y o f t h e p r o p e r t i e s of I P N s [1-3]. W e h a v e p r e v i o u s l y e x a m i n e d the p h y s i c o c h e mical p r o p e r t i e s of I P N s b a s e d on a p o l y u r e t h a n e a n d a c o p o l y m e r of s t y r e n e a n d divinylbenzene, a n d established certain characteristics associated w i t h t h e f o r m a t i o n in this t y p e of s y s t e m , o f a t r a n s i t i o n a l region w i t h p r o p e r t i e s differing f r o m those of t h e c o m p o n e n t networks. I n t h e p r e s e n t p a p e r we p r e s e n t t h e results o f a n investigation o f t h e s t r u c t u r e a n d t h e r m o d y n a m i c p r o p e r t i e s o f I P N s o b t a i n e d f r o m a p o l y u r e t h a n e (PU). a n d a p o l y u r e t h a n e - a c r y l a t e (PUA). For preparation of the IPNs an oligourethane-acrylate was synthesized from ethylene glycol methacrylate, toluylene di-isocyanate and poly(propylene glycol) with M = 700, by t h e method described in reference [4]. The adduct of trimethylolpropane with toluylene di-isocyanate and poly(propylene glycol) with M = 2000 were used for preparation of the *Vysokomol. soyed. A20: No. 1, 46-54, 1978. 51

YD'. S. LIPATOV 85 a/.

PU. The IPNs were prepared in the following way. The oligourethane-acrylate, to which the initiator (AZBN) had been added, was mixed with the PU in the common solvent, methylene chloride. Films of the resulting solution were poured on to Teflon, and were then kept under vacuum at room temperature to remove the solvent. The films so obtained were cured in vacuo at 75° for 48 hr. Samples of IPNs with different proportions of the components--from 0 to 100~o by weight of PUA, were investigated. Infrared and NMR spectroscopy were used for study of the IPNs, and the sorption of solvent vapours by the latter was also studied. Broad band NMR spectra were recorded in a TsLA spectrometer [5],* at a working frequency of 20 MHz in the temperature range of 140-360°K. The temperature was controlled within -t- 1°. The second moments of the spectra, AH], were obtained by numerical integration. The energies of activation for the processes occurring in the polymers, were calculated according to the Gutowsky-Pake formula [6]. The infrared spectra of the IPNs and of the individual networks, were recorded in a UR-20 spectrometer at room temperature. Specimens for the infrared measurements were in the form of thin films (about 3 ~), cut on a microtome. For the sorption experiments I P N films of thickness 0.5 ~ were used. The change in weight of the specimen during sorption was determined by means of lYlcBain balance with molybdenum springs of sensitivity 3-4 rag/ram. Temperature control during the sorption experiments was withint ±0.1% Sorption was studied at 300° a n d \ 313°K.

]'2~

20

'

,% 60PUAw I.f00

0"6 1"0 (Ah ~i.~ FIG. I. Dependence of (zIHl)lnt on I P N composition.

Investigation of the I P N s by wide-line N M R and infrared spectroscopy. Analysis of the infrared spectra showed t h a t t h e y consist of the spectra of t h e indiv i d u a l P U a n d P U A n e t w o r k s superimposed on one another. T h e y c o n t a i n t h e b a n d s characteristic of t h e c o m p o n e n t s of the I P N a n d no new b a n d s are present. T h e position a n d w i d t h of the b a n d s are n o t altered. These results indicate t h a t c h e m i c a l reaction does n o t occur in the I P N s , in spite o f the similar chemical n a t u r e o f the components. Despite t h e absence of chemical reaction, t h e f o r m a t i o n of intermolecular bonds o f a different k i n d in the s y s t e m c a n n o t h o w e v e r b e neglected. T h e n a t u r e of the interaction b e t w e e n the c o m p o n e n t s o f the I P I ~ s can be j u d g e d f r o m the N M R spectra. The N M R spectra of the I P N s c o n t a i n two comp o n e n t s , which is typical of two phase systems. I t is well k n o w n t h a t the a d d i t i v i t y * We are deeply grateful to I. Ya. Slonim for provision of the NMR spectrometer used in these exper~nents.

Interpenetrating polymer networks

58

rule is observed in the recording of the absorption lines of two component systems. Additivity must also be observed in the case of the second moments of the lines [7]. Then AHI can be represented b y the formula

(ztH])l,== 2

2

~Ol (AH22)1-~ - ~2 (z~g22)2~ - (z~H~)int,

(1)

2

where (AH~)I' 3, (AH2)I and (AH~)2 are the second moments of the absorption lines of the two component mixture and of the first and second components respect~ively, (AH~)int is a term taking account of interaction, and (01 and @, the fractions of the first and second components in the mixture. The quantity (AH2)lnt can have positive or negative values when there is interaction between the components of the system, even though there is no chemical reaction. W h e n there is no interaction in a two phase system (AHl)lnt~0. 2

4Hz, gauss

E~,10-s,J/mole 3O

+ \~

//4

f

I

I

I

L

I

I

I

I

~

20 O0 6O 80 I00 'qo

~ZO FIO.

300

PUA , wf.. ~

T,°K FIG. 3

2

Fzo. 2. Temperature dependence of AHI: 1--PU; 4--PUA; 2, 3--INPs with PU: PU• ratios by weight of 0.50 : 0.50 (2) and 0.05 : 0.95 (3). FIG. 3. Dependence of the energy of activation of transition II on IPN composition. 2

Figure 1 shows the dependence of (AH2)lnt on the composition of the I P N . It is seen the (AH~)Int is negative for all the I P N compositions. The negative values of (AH2)lnt indicate increase in molecular mobility in the system, because of loosening of the structure of the IPN, probably as a result of formation of a. transitional layer in systems of this type. The temperature dependence of AH~ was used for study of molecular mobility in the IPNs. Figure 2 shows the temperature dependence of AH~ for P U , P U A and some IPNs of different composition. Similar curves were obtained for the other IPNs. The decrease in AH~ as the temperature is increased occurs in stages, each change in AH~ corresponding to a distinct change in the nature o f molecular motion in the polymers. The P U shows two transitions, a low tempera-

54

YU. S. LIPATOV e~ ~ . \

ture one at 140°K (I), brought about by rotation of methyl groups [8], and one at 200°K (II), relating to motion of the methylene and methine groups of the poly(propylene glycol). In the ease of the PUA there are three transitions, the low temperature transition (I) (140°K), a transition in the region of 230°K (II), due to motion of methylene and methine groups, and a transition above 320°K (III), brought about by the glass transition of the methacrylic part of the PUA [8].

° =

50 ~3 4,0

30

2 #0

20

I0_ 20 •

OV

0.2

o.~,

o.s

0"3

O'E

o~p/po

FIG. 4

0"7

0"8

-

PUA,~ FIG. 5

FIG. 4. Isotherms of sorption of dioxan at 300°K: 1 - - P U , 2 - - P U A , 3 - 5 - - I P l ~ s with ratios of P U : P U A b y weight of: 0.98 : 0.2 (3), 0.92 : 0.08 (4) and 0.80 : 0.20 (5). FIG. 5. Variation in sorption of dio~an at 300°K with change in IPI~ composition a t P/Po 0.75

(1) ~nd 0.60 (2).

All the IPNs also undergo three transitions. Retention of the thermal transitions peculiar to the individual networks, indicates the existence of two independent phases in the IPNs. In all these IPNs, however, transition II, due to motion of methylene and methine groups, covers a large temperature range, being more diffuse than in the PU and PUA (Fig. 2). This serves as an indirect indication that the I P N contains a transitional layer. We calculated Ea for the transitions observed in the IPNs and in the individual networks. The results of the calculations for transition II are presented in Fig. 3. Analysis of these results shows that Ea for the transitions in the IPNs is lower, at all ratios of the components, than the energies of activation for these

Interpenetrating polymer networks

55

transitions in the individual networks. Moreover it is seen from Fig. 3 t h a t the activation energy does not vary uniformly with change in composition. One of its lowest values occurs at a low concentration of one of the components of the IPN. This could be explained by formation of a transitional region with a looser structure, as a result of considerable association of the components, as we found in an I P N of another type [9]. As the concentration of the second network increases the amount of transitional region and hence the molecular mobility of the system as a whole increases. In the middle of the range of compositions some increase in Ea occurs, but nevertheless its value remains less than for the transition in the individual components. The relative increase in Ea in this region can be explained in the following way. In formation of an I P N where one of the component networks is present in an amount smaller than t h a t of the other network, the one that is present in the larger quantity forms a continuous matrix. The second network is dispersed, as it were, in the matrix of the first, i.e. it forms inclusions of another phase, of small dimensions, in the first. As a result of this the area of contact between the phases, and hence also the amount of boundary region, are very large, and they increase as the concentration of the second component increases. In the middle range of compositions neither network predominates in formation of a continuous matrix, and therefore the structure of the I P N is made up of large formations of each of the components, forming two phases. As a result of this the area of contact between the phases decreases and hence the amount of transitional region decreases. The energy of activation, Ea, of the transitions in the IPNs increases uniformly. All this is supported by morphological investigations, and by study of the sorption of dioxan vapour by these IPNs over a wide range of compositions.

Thermodynamics of sorption of dioxan by interpenetrating polymer networks. Analysis of the isotherms of sorption of dioxan vapour by the IPNs and by the individual components at 300°K (Fig. 4), showed that the isotherms of the IPNs lie above the curves for the separate networks. Figure 5 shows the variation in the amount of dioxan adsorbed by IPNs of different composition. The curves were obtained by intersecting the sorption isotherms at relative vapour pressures of the solvent of p/po~0.75 (curve 1) and 0.60 (curve 2). I t is seen that the amount adsorbed by the IPNs is more than the amount adsorbed by the individual components over almost the entire range of compositions. IPNs containing a smM1 amount of P U form the exception (curve 2). The increase in sorption does not follow the change in composition uniformly, however. I n the middle of the concentration range (40-60% of P U A in the IPN) sorption decreases appreciably. A similar pattern is obtained for sorption at 313°K. The greater amount of sorption in the IPNs in comparison with the individual networks can be explained in the first place by formation in these systems of the loose structured transitional region, mentioned above. I n addition to looseness of structure, defectiveness of the IPNs in comparison with the individual networks can also affect the sorption of a solvent vapour. The rates

56

Y u . 8. LIPATOV ~ o~.

of erosslinking of the component networks of the IPNs are different. Whereas the oligourethane-acrylate is completely cured in 5 hr, formation of the P U network requires 48 hr. Consequently the polyurethane network is formed in the presence of a crosslinked PUA network. In other words the polyurethane network is formed in the presence of a solid surface and, for reasons that we have discussed previously [10], defects can occur in this network because of decrease in the number of chemical branch points and the appearance of "free" ends. We have made some thermodynamic calculations on the basis of the above isotherms. The quantities calculated were free-energy of swelling, the partial specific enthalpy and entropy, and, for the first time, the free energy of mixing of the networks in the IPN. For calculation of the free energy of swelling, the change in the partial free energy of the solvent, d#l, was calculated from the relative vapour pressure of the solvent by means of the equation 1

A~,I= -~ 1¢T In P/Po

(2)

where M is the molecular weight of the solvent. The partial free energies of the IPl~s were calculated from the Gibbs-Duhem equation, which for the specific quantities is written as

Odg~

w~~

+ w2 0~_0wl=o,

(3)

where W1 and W2 are the weight fractions of the solvent and polymer respectively. In the explicit form the dependence of d/~2 on d/q is given by the equation

~

00

Since exact solution of equation (4) is impossible we obtain an approximate value from Simpsons formula [11], of the integral

,

~=-

zl/~x

fw,

w~ (A~I),

(5),

AUL'

where d/d is the lowest calculated value of the partial free energy, corresponding to the minimal sorption measured experimentally. Then a correction A for the, region unaccounted for is sought graphically [12]

Interpenetrating polymer networks

57

The free energy of mixing for the solutions was calculated from the equation

Ag~= W1At~I+ W2A~

(7>,

The calculated values are given in the Table. 0"6 ]

O"7 '

I

0"8 ~

I

0"9 '

I

I

!

~

--0.8 --l.g 0.2

PUA, w'f.% O'g 0"8

0"#

--244

i

- -3.2 a

-

/)

--0.o

-0.8 •-5.G 2

l+.

4g FIG. 6. Dependence of the free energy of mixing, Ag m, at 300°K on the weight fraction o f IPN, Wz, in an I P N - s o l v e n t system; 1 - - P U , 2 - - P U A , 3 - 5 - - I P N s with P U : P U A ratios b y weight of: 0.98 : 0.02 (3), 0.92 : 0.08 (4) and 0.60 : 0.40 (5); 6--variation in the m i n i m a l free energy of mixing, Agmtn, of the I P N - s o l v c n t system, with variation in I P N composition.

Figure 6a shows typical curves of the concentration dependence of the free energy of mixing of IPNs with the solvent. It is seen that the form of the c u r v e s is characteristic of systems with limited swelling. They are all downwardly convex, which indicates thermodynamic stability of the IPN-solvent system [14]. I t follows from reference [15] that thermodynamic stability of a polymer-solvent system requires a negative value of A9 m, and the lower the Agm~-f(W~). curve lies, the more stable is the system.

~8

Yu. S. LIeATOV e t a / .

VARIATION

IN

TJ~ BASED

Polymer

TI:t.~RMODYNAMIC FUNCTIONS ON A POLYURETHANE

Wi

DURING

SORPTION

OF

DIOXAN

BY

IPNs

AND A POLYURETHANE-ACRYLATE

J ~ t x l 0 -3

d g m x l 0 - a I ,~ H sx l 0 - 8 I T J S s x l 0 -s J/°K.kg

PU

0.9742 0-9465 0.9175 0.8908 0.8521 0.8040 0.7682

--0.7954 --1.5484 --2.3161 --3.1153 --3.9653 --4.9793 --6.1118

--2.4584 --3.9062 --4.9404 --5.6111 --6"2848 --6.8408 --7.0388

10.4976 18.3306 26.7607 36.1492 45.3468 57.1531 71.2229

11-2524 19.8789 29.0769 39.2646 49.3121 62-1325 77.3348

PUA

0.9776 0-9534 0.9260 0.9003 0.8602 0.8166 0.7989

--0.6698 -- 1.3071 --1.9954 --2.7277 --3.5190 --4.4380 --5.4080

--2.1101 --3.3674 --4-3731 --5-0450 --5.7735 --6.2789 --6-3534

14.1974 22.2536 30"3429 32.7361 38.7073 48.2444 58.7797

14.8673 26.3463 32.3384 35.4638 42.2263 52.6825 64.1878

I P N 8% P U A and 92% P U

0.9674 0.9368 0.9047 0.8696 0.8106 0.7664 0.7224

--0.9510 --1.8392 --2.7745 --3.7446 --4.8445 --6.1227 --7.5356

--3.0186 --4.5843 --5-7347 --6.6110 --7.6119 --8.0742 --8"2773

10.5829 21.6654 32.4945 43-5226 56.5933 73.1660 93.1563

11.5040 23.4046 35.2691 47.2672 61.4379 79.2887 100.7281

I P N 40 % P U A and ~o% P U

0.9655 0-9333 0.9005 0.8662 0.8334 0.8003 0.7433

-- 1.0048 --1.9686 --2.9153 --3.9150 --4.9496 --6.0608 --7-2084

--3.2217 --4.8801 --6.0181 --6-8659 --7"3984 --7.7418 --7.9532

22.2607 39.4082 54.9894 70.3013 84.3409 96.6016 108-1839

23.2656 41.3768 57.9017 74.2164 89.2906 102.6624 115.3923

I P N 50% P U A and 50% P U

0.9713 0.9399 0.9054 0.8722 0.8374 0.8135 0.7498

--0.7954 --1.6420 --2.5263 --3.4750 --4.4317 --5.4273 --6.4916

--2.6456 --4-2851 --5.5156 --6.3501 --6.9057 --7.1150 --7.3968

18.1766 34.1831 50.2168 66.4700 80.3543 91-5046 102.1725

18.9821 35.8251 52.7434 69.9450 84.7860 96.9319 108.6642

I P N 80~o P U A and 20% P U

0-9622 0-9317 0.9031 0.8715 0.8405 0.8066 0.7499

--1.1723 --2.1687 --3.1208 --4-1059 --5.0936 --6.1298 --7.2696

--3.5952 --5.1363 --6.1252 --6.9157 --7.4148 --7.7443 --7.9758

25.8099 45.5406 61.8164 77.3331 89-7507 100.6803 111.7176

26.9822 47.7094 64.9372 81.4391 94-8444 106.8102 118.8913

Interpenetrating polymer networks

s9

~ m

Figure 6b shows the variation of g , the minimal value for each sample, with change in composition. The graph shows that introduction of the second network increases the thermodynamic stability of t h e I P N - s o l v e n t system, i.e. mixtures of polymers have greater affinity with a solvent than each of the components separately. In the middle of the composition range the thermodynamic stability of the I P N - s o l v e n t systems decreases a little. This can be explained qualitatively b y the probability of some increase in polymer-polymer interaction, and consequently decrease in I P N - s o l v e n t interaction.

-112

-8o o

7"

}:-

-16

0.75

0"85

0.95

~ 2.~

* ~ 0"8 O'Z

W2

Fie. 7

0"¢

0"6 0"8 ~.0 PUA , wf.

Fie. 8

l~o. 7. Concentration dependence of T.dS=for: 1--PU, 2--PUA, 3-6--IPNs with PU : PUA ratios by weight of 0.98 : 0.02 (3), 0.60 : 0.40 (4), 0.50 : 0.50 (5) and 0.20 : 0.80 (6). Fie. 8. Variation in the free energy of mixing, Ag*, of the networks in the IPNs, with change in composition. Knowing Apl and Ap2 at two temperatures the change in the partial specific enthalpy and entropy of the I P N can be found. The change in partial specific enthMpy was calculated from the formula [16]

AH~=

T1A#~ (T2)--T,Ag~ (T~)

TI--T,

(8)

and the change in partial specific entropy b y making use of the expression

TAS== AH2-=A~ .

(9)

60

Y u . S. LIPATOV e~ al.

I t is seen from the Table (the values of z/H~) that sorption of dioxan by the IPNs and by the separate components, occurs with absorption of heat, which is characteristic of polymers in the high elastic state. Figure 7 shows the concentration dependence of the partial specific entropy of some IPNs and of the components. The change in mobility of the polymer chains can be judged from these restilts [17], i.e. the greater the value of TztS~ the greater is their mobility or flexibility. It follows from Fig. 7 and the Table that the mobility of the chain segments between the branch points of the network in an I P N is greater than in the individual components. It increases as the proportion of the second network in the I P N is increased, on the PU or the PUA side. The mobility falls a littlo only in the middle of the composition range. Since the entropy is positive and fairly high, it is obvious that the main contribution to it is made by the positive, combinatorial entropy, relating to rearrangement of the positions of the mobile elements of the IPN-solvent system, and not by the negative, non-combinatorial entropy of mixing, due to the energy of interaction of the polymer with the dioxan vapour. I t is obvious that increase in mobility in an I P N with respect to the components, can be explained only by the emergence of additional freedom in achievem e n t of these rearrangements. As was pointed out above, this could happen only if there is a loose structured, transitional layer in the IPN, containing extra free volume. The expected decrease in the amount of transitional region in the middle of the concentration range, causes the mobility of the elements of I P N systems in this range of compositions to decrease. By the method proposed by Tager and others for mixtures of polymers [15], using the free energies of mixing of the IPNs, and of the components, with tho solvent, we calculated the free energy of mixing of the networks in the IPNs, of the Gibbs free energy z/g*, over a wide range of compositions (Fig. 8). I t is seen that the free energy of mixing is positive over the entire range of compositions. Therefore the constituent networks of the IPNs are thermodynamically incompatible. The phase diagram is bimodal. In the middle of the range of I P N compositions there is some relative increase in the thermodynamic stability of the system. This investigation of IPNs based on a polyurethane and a polyurethane-acrylate has thus shown that the components of the system are thermodynamically incompatible, and also that no chemical reaction takes place between them. I n contrast to other work on IPNs, in our work it is shown that despite thermodynamic incompatibility of the components, in systems of this type a transitional layer is formed between the two component phases. The entire combination of properties of the IP~Ns is determined by the existence and characteristics of this region. Particular emphasis must be placed on results reported in the present paper, indicating that' the packing density of the macromolecules

Interpenetrating polymer networks

61

in the YPNs is lower than the packing density in the individual networks and that the mobility of the chain segments between branch points in the networks is greater. Both these facts can be explained by formation of a loose, transitional region in systems of this type. The degree of structural looseness in the transitional layer is dependent on the ratio of the components. Translated by E. O. PHILLIPS REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13.

14. 15. 16. 17.

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