Grafted Interpenetrating Polymer Networks - Advances in Chemistry

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Grafted Interpenetrating Polymer Networks Barry J. Bauer, Robert M. Briber, and Brian Dickens Polymers Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899

A new class of interpenetrating polymer network (IPN) has been studied in which grafting reactions between the two components are varied. Small-angle neutron scattering of grafted and nongrafted IPNs shows that grafting greatly enhances the miscibility of the components. Five nonfunctionalized poly(methyl methacrylates) (PMMAs) with alkacrylate, methacrylate, acrylate, andα-methylstyreneend groups were dissolved in styrene-divinylbenzene and polymerized. Small-angle X-ray scattering was used to characterize the extent of phase separation. The uniformity of the IPNs is strongly dependent on the grafting efficiency. Grafted and nongrafted IPNs were also made from the PMMAs and polyethylene glycol diacrylates. Thermal studies showed one transition in the grafted samples and two distinct transitions in a nongrafted sample.

M

I S C I B L E P O L Y M E R B L E N D S A R E C O M B I N A T I O N S of two high molecular weight polymers that exist as a single phase; that is, the polymers dissolve in one another to form a polymer-polymer solution. Only a relatively few polymer pairs form miscible blends, largely because of the very low entropy of mixing of polymers. Although entropy of mixing always favors miscibility, it depends on the number of molecules per unit volume. Therefore, the larger the polymer molecules, the smaller the number of molecules per unit volume and the lower the entropy of mixing. Because the heat of mixing of polymer pairs is generally unfavorable, polymers generally do not form miscible blends. Interpenetrating polymer networks (IPNs) are similar to polymer blends in that they are mixtures of two polymeric components. Unlike conventional

This chapter not subject to U.S. copyright Published 1994 American Chemical Society

In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch007

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INTERPENETRATING POLYMER NETWORKS

polymer blends, however, IPNs are made from components that are mixed while at least one of the polymers is in the form of a monomer. Whereas polymeric components have very little entropy of mixing, typical monomers are small molecules and hence have appreciable entropy of mixing. There fore, many polymer-monomer combinations are miscible. A n important class of IPNs is sequential IPNs where there are two sequential, independent polymerizations. A polymer is formed through any conventional polymerization process and then swollen by or dissolved in a monomer of a different type. A second polymerization takes place and forms the second polymeric component in the presence of the first. During the second polymerization, entropy of mixing is lost as the molecules increase in size, and phase separation generally occurs, which results in the two-phase morphology common to IPNs. The phase separation occurs for the same reason that most polymer blends are immiscible: the unfavorable heat of mixing of most polymer pairs becomes larger than the effect of the entropy of mixing and causes the phase separation at some point during the second polymerization. One or both of the polymerizations form a cross-finked network I P N component. It is difficult to judge the effects of the cross-links on the phase boundaries of the system when the two polymers are extremely immiscible because phase separation always occurs during the polymerization. The presence of cross-finks may suggest that miscibility would be enhanced over that of a blend because the network acts as a mesh to restrict diffusion of the other polymer and thus to inhibit phase separation. O n the other hand, the formation of the network during the second polymerization may exclude the linear chains of the other polymer and thereby cause phase separation. To study the effects of cross-links on the phase separation in IPNs, it is therefore best to make the IPNs from polymers that are known to form miscible blends. Blends and IPNs of the polymers can then be studied by small-angle scattering techniques to measure thermodynamic parameters and to map out phase diagrams as a function of cross-link density (J). The first attempts to synthesize miscible IPNs were made by Millar (2) in 1960. The IPNs were synthesized by initial polymerization of a polystyrene-divinylbenzene network ( P S - D V B ) and then swelling the network with more styrene and D V B and polymerization of the second network. Although this work is assumed to have produced two cocontinuous networks as a single phase, the two components are identical, and any phase separation would be difficult to detect. Seigfried et al. (3) modified this type of I P N slightly by the inclusion of small quantities of diene monomer in the second polymerization. This procedure allowed selective staining of the second network for mi croscopy. Indications that phase separation may have taken place during the second polymerization were present. Frisch et al. (4) were first to successfully produce IPNs from polymers that form miscible blends. Polystyrene (PS) and polyphenylene oxide (PPO)


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were made into IPNs by selectively cross-linking the components, and miscibility was maintained as judged by glass-transition temperature (T ) measurements and microscopy. Because blends of these two polymers have a large, negative interaction parameter (5) and remain miscible under all conditions, there was no way to conclude whether the IPNs were more or less miscible than the blends. Coleman et al. (6) used Fourier transform infrared (FTIR) spectroscopy to study blends of ethylene-vinyl acetate copolymers and various oligomers that could form a cross-linked phase. Although the blends were highly miscible due to hydrogen bonding interactions, phase separation occurred each time a network was formed through the condensation reaction. Work at the National Institute of Standards and Technology (NIST) (1) on blends of deuterated polystyrene (PSD) and polyvinyl methyl ether) ( P V M E ) has shown that there is a very favorable interaction in this system at temperatures below 100 °C. However, semi-IPNs made by dissolving linear P V M E in styrene-DVB mixtures and polymerizing at 70 °C phase separated upon polymerization (7). Felisberti et al. (8) also showed that polymerization of styrene-DVB that contains dissolved P V M E causes phase separation. Single-phase IPNs were produced by Felisberti et al. by lightly cross-linking functionalized PS in the presence of P V M E with the polymers dissolved in solvent, but phase separa tion occurred as the cross-link density increased. Fay et al. (9) also made P S - P V M E IPNs that were hazy, which suggested that phase separation had occurred. Control of the polymerization conditions enabled control of the resulting morphology, to yield large amounts of partial mixing. This result caused a very wide T range and good mechanical damping properties of the material. IPNs made from standard polystyrene (PSH) and P S D were studied recently (10). These polymers form nearly athermal mixtures that allow easy study of the effects of linear chain molecular weight and the cross-link density. The interaction parameter between P S H and P S D is negligible compared to the effects of cross-links. A n increase in the linear chain molecular weight or an increase in the cross-link density destabilizes the I P N and eventually causes phase separation. In a related study (11), miscible P S D - P V M E blends were cross-linked by gamma irradiation to produce many types of cross-links that include grafts between the two different polymers. These materials showed greatly in creased miscibility compared to blends. Frisch and Zhou (12) studied simultaneous full IPNs made from compo nents that do not form miscible blends. In many of the IPNs, Frisch and Zhou found a single T , which they interpreted as the result of miscibility of the components of the I P N . Note, however, a single T would be found in systems where weak phase separation had occurred.


g

g

g

g


182



Radical polymerizations can result in chain transfer to polymer-forming grafted polymer structures (13). Some researchers have suggested that graft ing one polymer type onto another in an I P N may effect the miscibility of the components (8). To examine the effect of grafting on I P N miscibility, IPNs with various amounts of grafting between components have been made and have been studied by small-angle scattering and thermal measurements (14, 15).

Theoretical Details The thermodynamics of IPNs were first described by Binder and Frisch (16), who used classical rubber elasticity to describe the elastic contributions to the free energy by the polymer network. The equation has the form ^

+ λ% + λ | ) - Β ν 1 η ( λ λ λ )

= ^Αν(λ*

χ

χ

γ

(1)

ζ

where Af is the free energy, k is Boltzmann's constant, Τ is the tempera ture, is the extension ratio in the i t h direction and is the ratio of the deformed length to the undeformed length, ν is the average number of elastically effective cross-links per unit volume, and A and Β are characteris tic constants. The scattering from the IPNs measures concentration fluctua tions, so eq 2 is used to express the extension ratio in terms of φ, the volume fraction of the network, and φ , the volume fraction of the network where the chains are relaxed: 5

λ = (φ /φ) 5

(2)

1 / 3

These equations are an extension of the work of Flory and Rehner (17) on the swelling of networks. The scattering from IPNs can be calculated from the foregoing thermo dynamic relationships via a procedure described by Onuki ( 18). The resultant scattering for zero angle scattering is given by d\Af/kT)

Β

Αφ /

θφ

ϋ,Ν.φ

ϋ,Ν.φ /

2

2

5

1

3

v N (l

3

h

h

- φ)

χ

Κ

v

S(0)

0

V

;

S(0) is the zero angle scattering intensity, k is a contrast factor calculated from the polymeric repeat units, v is a reference volume, N is the degree of polymerization between cross-links, JV is the degree of polymerization of the linear chains, and χ is the Flory-Huggins interaction parameter. Equation 3 was derived for a semi-IPN; that is, a two-component I P N with one cross-fin ked polymer and one linear polymer. n

0

c

b


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183

The effect of cross-link density on the scattering intensity is shown by Figure 1 (10). Equation 3 indicates that a plot of inverse zero angle scattering versus inverse degree of polymerization between cross-links should be linear with a positive slope. The dashed line in Figure 1 is such a plot where the value of x/v is the value of the polymer blend; the symbols ( Δ ) represent experimental data and the solid line is a linear least squares fit of the data. Because k /S(0) decreases in Figure 1 as 1/N increases, the scattered intensity increases with increased cross-link density. The point at which the intensity goes to infinity is the spinodal point for the system. Therefore, increased cross-link density pushes the system toward phase separation. Samples made with higher cross-link densities were phase separated. Onuki has modified the original theory to include heterogeneities in the network (Onuki, Α., personal communication). This modification changes the effect of cross-links from stabilization of the system to destabilization of the I P N . Bastide et al. (19) formulated a similar theory to explain scattering results from siloxane networks. Heterogeneities describe natural cross-link concentration fluctuations present even in model networks. 0


n

C

18

N

15

Φ β ϋ

=

L

=

420 0.481

/

/

/

12

/ /

/

Χ

/

* /

ο β

Χ

/

/

6

V

l/v N a

c

Χ 10

5

(mole c m

—3

molecule)

Figure 1. Small-angle neutron scattering plots for k /S(0) versus 1/($N V ) (solid line) for PSH-PSD semi-II IPNs. The values calculated from equation 3 (dashed line) are given for the same system. n


C

C


184


Grafted polymers are more miscible than blends of ungrafted polymers. Although no theory for the scattering from grafted polymer networks exists, scattering from simple graft polymers has been studied extensively. Benoit and Hadziioannou (20) give equations for scattering from graft copolymers and from mixtures of block copolymers and homopolymers; their techniques can be used to predict the scattering from a graft copolymer-ungrafted polymer mixture. Figure 2 is a plot of the spinodal value of χ Ν versus polydispersity index, M / M , for polymers that are 100% grafted or 100% ungrafted. M and M are the weight average and number average molecular weights, respectively. A volume fraction of 0.5 of each component is present, Ν is the degree of polymerization of the graft (or the unattached chain), and χ is the interaction parameter. Because the scattering does not change significantly with the backbone length and number of grafts (20), the limiting case of the number of grafts going to infinity is taken. The lower line in Figure 2 is the spinodal value of a polymer blend: one component has degree of polymerization Ν and the other component has w

n

w

n

12

10

100% Grafted

Macro phase

Micro phase Ζ

5

X

0% Grafted

1.0

1.4

1.2

1.

1.8

2.0

Mw/Mn

Figure 2. Spinodal values of xN for grafted and nongrafted polymers versus polydispersity index. The volume fraction of each component is 0.5. w


7.

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185

Grafted IPNs

infinite degree of polymerization. The upper line is the spinodal value for a 100% grafted copolymer. The graft copolymer has a much higher value of XN than an ungrafted sample of the same material at any polydispersity index. The graft copolymer is stabilized by a factor of 11.4 for the monodisperse case and 8.0 for a polydispersity of 2. Therefore, grafted IPNs are probably much more likely than ungrafted IPNs to be stable as a single phase. The calculated scattering intensities can go to infinity in two ways (20). In polymer blends, the maximum scattering is at zero angle and any phase separation is macrophase separation, with characteristic sizes (commonly about 1000 nm) that are limited by kinetics. In grafted polymers, the maximum scattering occurs at a nonzero angle. Such samples separate into microphases if they are pushed into the two-phase region. The size scale is related to the inverse of the scattering angle at the peak maximum and is typically 10 nm. Figure 2 shows that the phase separation, dependent on the polydisper sity, can be either microphase or macrophase. A peak i n the scattering of a grafted sample indicates efficient grafting and suggests that microphase separation would occur for the appropriate xN .


W

w

Experimental Details Table I is a list of the characteristics of the poly(methyl methacrylate) (PMMA) and P S H macromonomers used to make grafted IPNs. A l l of the P M M A macromonomers have similar molecular weights and more than 90% functionalization with one of the chain ends being one of four copolymerizable groups; a fifth sample contains no end groups capable of radical copolymerization. The acrylate and methacrylate chain ends were attached through the ester linkages and the alkacrylate chain end was attached through the α-methylene group. Seven samples were made for the small-angle neutron scattering (SANS) experiments and their compositions are listed in Table II. A l l samples were approximately 50 w t % of one of the two polymeric components. The first polymer was either PS or P M M A . All of the P M M A samples were macromono-

Table I. Characteristics of Macromonomers Polymer PS PS PMMA PMMA PMMA PMMA PMMA

Functionality Methacrylate None Alkacrylate Acrylate Methacrylate α-M ethylstyrene None

M 13,000 13,000 7,200 7,100 4,600 6,400 7,900

n

4,200 4,600 3,800 4,300 3,600

%End Group

> 90 95 96 > 90 —


186


Table II. SANS Samples


Sample 1 2 3 4 5 6 7

Polymer I PS PS PS PS PS PMMA PMMA

13,000 13,000 13,000 13,000 13,000 7,200 7,200

DVB in II (wt%)

Graft

0.2 0.5 0.2 0.5 0.9 0.5 0.9

No No Yes Yes Yes Yes Yes

mers of the alkacrylate type, whereas the PS samples were either macromonomers of the methacrylate type of unfunctionalized. Both PS types had the same molecular weight and narrow molecular weight distributions. The cross-linked component in each sample of the semi-II IPNs was deuterated styrene, which provided the necessary contrast for the SANS experiment. The deuterated styrene was cross-linked with various amounts of divinylbenzene (DVB). The weight fraction of DVB listed in the table is based only on the second component. Small-angle X-ray scattering (SAXS) samples were synthesized by a semi-II synthesis; 50-wt% P M M A macromonomer was dissolved in 50-wt% styrene-divinylbenzene. All samples contained l - w t % divinylbenzene and 0.2-wt% azobis(isobutyronitrile) (AIBN) dissolved in the styrene. The samples were polymer ized at 60 °C for 18.5 h and at 110 °C for 3 h. Extractions were done twice with toluene. Samples for neutron scattering were synthesized as SAXS except that 0.1-wt% A I B N free radical initiator was used. The samples were polymerized in place in neutron scattering cells in an oven at 130 °C for 16 h. Gel permeation chromatography of the toluene extracts was carried out using ultraviolet and mass evaporative detectors to identify the amounts of P M M A macromonomer and polystyrene in the extract. Samples for the thermomeehanical analysis (TMA) studies were made from 0-50-wt% P M M A macromonomer; the remainder of the sample was polyethylene glycol diacrylate that contained l-wt% benzoyl peroxide. The T M A samples were polymerized into 1-mm-thick sheets at 60 °C for 24 h. The SANS measurements were performed at the NIST small-angle neutron scattering facility (21). The wavelength of the incoming neutron beam was set to 0.6 nm with Δ λ/λ = 25% by the use of a rotating velocity selector. The data were collected using a two-dimensional detector and corrected for empty cell scattering, incoherent scattering, and background. The scattering was put on an absolute intensity scale that used a secondary standard of silica gel. The SAXS measurements were carried out at the NIST SAXS facility. Two-dimensional scattering data were corrected for empty cell scattering and dark current. All samples were isotropic and the results were circularly averaged. The T M A was done with a thermomechanical analyzer (Perkin Elmer TMS-2). Sample thickness was monitored while the sample was being heated from —100 to 150 °C with a 10 °C/min heating rate. The rate of change in thickness with temperature is a measure of the thermal expansion coefficient, which has different values below and above TL.


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187

Grafted IPNs


Results Figure 3 is a plot of SANS scattering intensity versus the scattering vector q = (4ττ/λ) sin(0/2), where λ is the wavelength of the neutron beam and θ is the angle of the scattered beam. The top two curves are for the standard semi-II IPNs made from polystyrene that did not have macromonomer functionality and contained 0.5- and 0.2-wt% D V B , respectively. These results are in agreement with previous findings for such IPNs (10). If the D V B concentration were increased to 0.9 wt%, these samples would be phase separated. The lower three curves in Figure 3 are for samples made with macromonomers that are capable of grafting to the P S D component. The presence of very low scattered intensity indicates that these macromonomers are much more homogeneous and further from a phase transition than equivalent samples made from unfunctionalized polymers. This status indi cates that use of the macromonomer functionality, which allows grafting of the first polymer onto the network formed in the second step, greatly stabilizes the single-phase region. The solid triangle on the ordinate is the calculated value for zero angle scattering from an equivalent blend; that is, a

48 PSH/PSD Semi-II IPN 40

Ungrafted and Grafted

ο ο

• 0.2 % Ungrafted ο 0.5 % Ungrafted

32

Δ 0.2 % Grafted ο 0.5 % Grafted

24

• 0.9 % Grafted 16

0.00

0.02

0.06

0.04

0.08

q (A" ) 1

Figure 3. SANS scattering intensities for PSH-PSD grafted and ungrafted semi-II IPNs.



188


blend in which no cross-hnking is present. Therefore, the semi-II I P N structure seems to push the polymers toward phase separation, whereas the grafted I P N structure made from the macromonomers favors the single phase. Figure 4 gives the results for the grafted P M M A - P S D samples made with P M M A macromonomers. These scattering curves have prominent peaks. Previous work on blends cross-linked by exposure to gamma irradiation also gave scattering curves with similar peaks. These peaks were interpreted to be incipient microphase separation (II). There is no symbol on the ordinate to show where an equivalent blend would scatter because such a blend would be beyond the point of phase separation. The SANS results from these model systems demonstrate that there are great differences in the microuniformity of samples made with and without macromonomers. The polymer pairs used in the SANS studies represent the two limits of grafting efficiency and it is, therefore, likely that mixtures of macromonomers and unfunctionalized polymers would produce morphologies between the two limiting cases. Macromonomers can be made with a variety of end groups that are capable of copolymerization. The reactivity ratios of such a copolymerization will determine the uniformity of the resultant copolymers. Macromonomers generally have reactivity ratios similar to the ratios of small monomers of the same chemical nature as the macromonomer chain end (22). 12

ι

ι

ι

ι

I

10

S ω

6

ι

ι

V _

-

PMMA/PSD .

Grafted IPN

-

.

Ο 0.5 % DVB

_

0 0.9 % DVB t

0.00

0.01

i

0.02

l

l

0.03

0.04

f

ι

ι

0.05

0.06

0.07

0.08

q(Â" ) 1

Figure 4. SANS scattering intensities for PMMA-PSD grafted semi-II IPNs.



7. BAUER ET AL.

189

Grafted IPNs

SAXS is similar to SANS in that the range of scattering vector, q, is similar. The major difference between the two techniques is the factors that cause contrast to exist between the different components. Contrast in SANS can be conveniently obtained by replacing the hydrogen atoms in one component with deuterium atoms. Contrast in SAXS comes from differences in electron density, which are closely correlated with the mass density of the samples. PS and P M M A have mass densities of approximately 1.05 (23) and 1.19 g/cm (24). Although the contrast factor in SANS is very large compared with the SAXS contrast, there is still a large enough density difference for SAXS experiments to give useful results (for much of the time covered by this work, construction at the reactor site temporarily stopped SANS experiments, so the SAXS technique was developed as a replacement). In the five samples made, the only significant variable was the type of polymerizable end group on the macromonomer. Table III lists the composi tion of the SAXS samples. Samples made with alkacrylate, methacrylate, and acrylate groups were optically clear and appeared uniform. The α-methyl styrene sample was cloudy and the unfunctionalized sample was opaque; these conditions indicate ever stronger phase separation. Figure 5 plots the scattering curves on a logarithmic scattering intensity scale shifted in the y axis for clarity. The α-methylstyrene sample has a very rapid dropoff in scattering intensity with angle, which indicates phase separa tion. The alkacrylate sample has a peak in the scattering intensity that is similar to the peak seen in the SANS data. As with the SANS results, these samples seem to be very uniform. The four P M M A macromonomers differed only in the polymerizable end group, low molecular weight analogues of which are all known to copolymerize with styrene. The morphology of these samples was extremely varied, which indicates that slight differences in the polymerization kinetics may have produced profound differences in the morphology and hence properties. Because the macromonomers have molecular weights many times larger than the styrene comonomer, a 50-wt% mixture means that styrene is present in a large molar excess. Therefore, only one reactivity ratio, r , is important: the relative probability of a growing chain that contains the comonomer as 3

2

Table III. SAXS and Extraction Results of Grafted PS-PMMA IPNs End Group Alkacrylate Methacrylate Acrylate a-Methylstyrene None

r (monomer)

SAXS

% Macromonomer Reacted

Optical Note

0.5 (MMA) (26) 0.5 (MMA) (26) 0.7 (MA) (27) 1.3 (aMS) (28) 00

4 3 2 1 —

90 72 82 36 0

Clear Clear Clear Cloudy Opaque

2



190


4

gI

Ό.00

1

1

0.02

1

1

0.04

1

1

1

0.06

q (A" ) 1

Figure 5. SAXS log($cattering intensity) versus scattering vector qfor PMMA-PS IPNs. the terminal group to add a comonomer compared with addition of a macromonomer. A value less than 1 means that the macromonomer is preferentially incorporated, whereas a value greater than 1 means that the comonomer is preferentially added. Table III gives published values of reactivity ratios of styrene with small comonomers similar to the chain ends of the macromonomer used ( M M A is methyl methacrylate, M A is methyl acrylate, and α M S is a-methylstyrene). Extraction studies show that virtually all of the nonfunctionalized P M M A is not bonded to the PS network. The nonfunctionalized P M M A , therefore, did not become attached to the polystyrene network through grafting reac tions such as chain transfer to the polymer. Only 36-wt% of the a M S terminated P M M A was bonded to the network, whereas 72-90 w t % of the other polymers were bonded. This result is consistent with the expected reactivity ratios of the macromonomers. The sample that had the fewest grafted chains was very opaque and seemed to be strongly phase separated. The aMS-terminated P M M A , which had only 36% attached chains, was cloudy and translucent. This condition indicates phase separation to a lesser extent. The samples that had higher percentages of grafted chains seemed optically clear with no obvious phase separation.


7.

BAUER ET AL.

Miscible blends and copolymers (25) that form a single phase produce materials that have T s between the T s of the individual components. To test the effect of macromonomers on the thermal properties, two samples were prepared from 35-wt% P M M A and 65-wt% polyethylene glycol) (PEG) 400 diacrylate: one sample was prepared with P M M A macromonomer and the other sample was unfunctionalized. The P M M A s were very similar in molecu lar weight (see Table I). The major difference was that one P M M A was a macromonomer with more than 96% alkacrylate functionality at one end, whereas the other P M M A had a hydroxy group at one end that made it incapable of copolymerization with the P E G 400 diacrylate. The samples were cured as previously described and a T M A was performed on each. Figure 6 shows the T M A of the 35-wt% macromonomer sample that formed a grafted I P N . As described earlier, this sample displays a single transition. Figure 7 shows the results for a similar sample without the macromonomer functionality that forms an ungrafted I P N . Quite clearly, two thermal events take place. The lower transition is near the transition of the T of a pure P E G 400 diacrylate network, whereas the upper transition is near the transition of the P M M A macromonomer itself. The transitions could conceivably involve crystalline melting points, but because pure P E G net works and P M M A each only show T s, it seems more likely that the two transitions are T s. This sample seems to be strongly phase separated into domains of nearly pure components. g


191

Grafted IPNs

g

g

g

g

Summary Small-angle scattering is a powerful tool for study of the effects of I P N structure on the miscibility of the two components. Previous work (7, 10) showed that conventional semi-II IPNs are destabilized compared to blends of the same two components. Grafted IPNs are considerably more miscible than blends of the same polymers. Variation of the grafting efficiency greatly changes the resultant morphology of an I P N from very strong phase separation to high miscibility. The presence of a peak in the scattering data in some cases suggests that microphase separation may occur under the proper conditions. These results suggest that controling the chemistry of the I P N polymer ization (i.e., cross-link density and grafting) can give a wide variety of morphologies and, hence, properties. Miscible polymers can be made to phase separate easily by increasing the cross-link density and avoiding graft ing whereas immiscible polymers can be made miscible by providing func tional groups that will result in grafting between the two polymers. Control of the extent of phase separation from strong separation into nearly pure phases or weak separation with broad distributions of composi-



Figure 6. TMA scan of grafted semi-II IPN of PEO-PMMA.


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Grafted IPNs


7.

193

1

OH

I

Ο

"É CO

I

1 go



194


tions may be possible. Control of the characteristic size of phase separation from small microphase separated domains to much larger macrophase separa tion may also be possible. These results raise questions concerning earlier studies. In the P S - P S IPNs made by Millar (2), if there is no grafting between the two polymeric components, it is likely that the IPNs are phase separated to some extent; if grafting did occur, the samples may have been single phase. However, making strongly grafted IPNs was not the intent of the study. On the other hand, if IPNs are more miscible than blends of the same two polymers, unexpected grafting may have taken place. It is common to assume that the networks are independent in IPNs and that any grafting is inconsequential, but, as shown here, the resultant morphology will be strongly affected by grafting.

Acknowledgments Samples and characterization were supplied by DuPont. Certain equipment, instruments, and materials are identified in this paper to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology, nor does it imply the materials are necessarily the best available for the purpose.

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;

;

;

;

;


;


7. BAUER ET AL. Grafted IPNs

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RECEIVED for review November 26, 1991. ACCEPTED revised manuscript June 9, 1992.


Grafted Interpenetrating Polymer Networks - Advances in Chemistry

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