Multiphase Polymers: Blends and Ionomers - American Chemical

Chapter 9

Ternary Phase Diagrams for Interpenetrating Polymer Networks Determined During Polymerization of Monomer II

Downloaded by UNIV LAVAL on October 19, 2015 | http://pubs.acs.org Publication Date: July 21, 1989 | doi: 10.1021/bk-1989-0395.ch009

L. H. Sperling, C. S. Heck, and J. H. An Center for Polymer Science and Engineering, Department of Chemical Engineering, Materials Research Center, Whitaker Laboratory #5, Lehigh University, Bethlehem, PA 18015

Important aspects of polymer I/monomer II/polymer II ternary phase diagrams were determined for the system cross-poly butadiene-inter cross-polystyrene as monomer II, styrene, is polymerized. Information on the mechanisms of phase separation suggest first nucleation and growth, followed by a modified spinodal decomposition. Studies on the same system by small-angle neutron scattering and light-scattering both yield negative diffusion coefficients, but different numerical values of both the diffusion coefficients and the domain sizes. As a class, multicomponent polymer materials encompass polymer blends, grafts, blocks, AB-crosslinked copolymers, and interpenetrating polymer networks, IPN's. Each of these represents a distinct way of joining two or more polymers by a variety of methods, plain or fancy. Together, they constitute one of the fastest growing fields within polymer science, because with an increasing understanding of the interrelationships among synthesis, morphology, and mechanical behavior. Thus, we can fabricate materials which are tough impact-resist ant plastics, reinforced elastomers, semi-permeable membranes, biomedical materials, sound and vibration dampers, or a host of other materials (1-4). Interpenetrating polymer networks are defined as a combination of two polymers, each in network form. From a practical point of view, an IP Ν is comprised of two polymers which cannot be separated chemically, do not dissolve or flow, and are not bonded together. Like most other multicomponent polymer materials, IPN's usually phase separate due to their very small entropy of mixing. However, the presence of the crosslinks tends to reduce the resulting domain size, hence yielding a unique method of controlling the final morphology. D E V E L O P M E N T O F T H E IPN C O N C E P T There are two general methods of synthesizing IPN's, see Figure 1. For sequential IPN's, polymer network I is synthesized, and monomer II plus crosslinker and activator is swelled in and polymerized. For simultaneous interpenetrating polymer networks, SIN's, both monomers and their respective crosslinkers and activators are mixed together and polymerized, usually by separate and non-interfering kinetic methods such as stepwise and chain polymerizations. Of course, there are many 0097-6156/89/0395-0230$06.00A) ο 1989 American Chemical Society

In Multiphase Polymers: Blends and Ionomers; Utracki, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

SPERLING ET AL.

Ternary Phase Diagramsfor Polymer Networks


(3) S e q u e n t i a l IPN

O O O Δ Δ O O O Δ Δ O O O Ο monomer I + Acrosslinker I

network I

•· · •· · •

O O O Δ Δ O O O

•· · mixture o f Ο monomer I Δcrossiinker I • monomer I I Acrosslinker II

•

ft monomer I I + • Acrosslinker II

s w o l l e n network I

Figure 1.

(5)Simultaneous IPN

1

I

P N

Schematic of I P N synthetic methods.



232

MULTIPHASE POLYMERS: BLENDS AND IONOMERS

intermediate methods. Combined with choices of bulk, emulsion, suspension, and solution polymerization, very many possibilities exist. Since this paper will be restricted to sequential IPN's based on crosspolvbutadiene-inter-cross-polystyrene. PB/PS, it is valuable to examine the range of possible compositions, see Figure 2 (£). The PB/PS IPN polymer pair models highimpact polystyrene, and in fact, many of the combinations made are actually more impact resistant than the commercial materials. In general, with the addition of crosslinks, especially in network I, the phase domains become smaller. The impact resistance of high-impact polystyrene, upper left, is about 80 J/m. In the same experiment, the semi-I IPN, middle left is about 160 J/m, and the full IPN, lower left, is about 265 J/m (6). Since the commercial material had perhaps dozens of man-years of development, and the IPN composition was made simply for doctoral research with substantially no optimization, it was obvious that these materials warranted further study. Shortly thereafter, Yeo, et al. (7.8) showed that the domain size depended quantitatively on the crosslink density, the interfacial surface tension, and the temperature. For many compositions involving non-polar polymer pairs and moderate crosslinking levels, domain sizes of the order of 50-100 nm were predicted. While the theory was developed for spheres for simplicity, it was already known that both phases tended to be cocontinuous, especially for midrange compostions

(mIn T E M studies by Fernandez, et al. (9) on thin-sliced materials, it was shown that early in the polymerization of the styrene in PB/PS IPN's the domains tended to be spherical, while later in the polymerization the domains tended to be ellipsoidal in nature. The latter were modeled as irregularly shaped cylinders, which resemble ellipsoidal structures on thin sectioning. In more recent experiments involving smallangle neutron scattering, SANS, it was concluded that the phase separation involved a mixture of nucleation and growth, and spinodal decomposition kinetics (10). THEORY From a thermodynamic point of view, phase diagrams may be constructed by changing the temperature (11). pressure (12), or composition of a material. The present experiments are concerned with changes in composition at constant temperature and pressure, leading to a ternary phase diagram with polymer network I at one corner, monomer II at the second corner, and polymer network II at the third corner. According to classical concepts, at first there should be a mutual solution of monomer II in network I, followed by the binodal (nucleation and growth kinetics) and finally the spinodal (spinodal decomposition kinetics). It must be emphasized that the two kinetic schemes are quite different, Figure 3 (10). Nucleation and growth kinetics occur frequently in the precipitation of salts from saturated solutions. This is the kinetic model usually taught in undergraduate classes. Spinodal decomposition, much less understood, was first studied by Cahn (13) and Cahn and Hilliard (14)· It involves the formation of compositional waves, the amplitudes of which grow with time. While the wave length may be initially constant, frequently a coarsening effect is noted with time. Often, dual phase continuity in the form of cylinders within a matrix is observed. The more polymer physicists study the kinetics of phase separation, the greater the importance of spinodal decomposition in modern polymer science. A powerful instrument for the study of polymers in the bulk state, including during polymerization, is small-angle neutron scattering, SANS. Studies involving SANS utilize differences in scattering length, rather than differences in refractive index, as for visible light. Usually, differences in scattering length are brought about by using deuterated molecules as a portion of the sample. For many irregularly


Ternary Phase Diagrams for Polymer Networks


9. SPERLING ET AL.

Figure 2. Transmission electron micrographs of six polybutadiene/polystyrene sequential IPN s and related materials, the polybutadiene portion stained with osmium tetroxide. Upper left: high-impact polystyrene, commercial. Upper right: a similar composition made quiescently. Middle left: semi-I IPN, PB (only) crosslinked. Middle right: semi-II IPN, PS (only) crosslinked. Lower left: full IPN, both polymers crosslinked. Lower right: full IPN, PB with higher crosslink level. (Reproduced from ref. 5. Copyright 1976 American Chemical Society.)


233

234



Nucleation & Growth positive diffusion coefficient

ί

Col

rui

lade:,

Lnri_ri_rL, Distence

Spinodal

Decomposition

'negative diffusion coefficient t2

ΙΛΛΛΛ,, Distance

Figure 3. Kinetics of phase separation in multicomponent polymer (Reproduced from ref. 10. Copyright 1988 American Chemical Society.)

materials.


9. SPERLING ET AL.

Ternary Phase Diagramsfor Polymer Network

shaped domain structures, the theory of Debye et al. (15) provides an excellent approximation. Debye defined a correlation length, a, as the average distance from a random position and in a random direction in the sample to the first domain interface. From the correlation length, several other quantities can be calculated. The correlation length, £, for phases 1 and 2, is defined as the average distance across a random domain by a randomly placed line: I, = a / ( l - ^ )

(la)

l = a/(W,)

(lb)

and 2

where the quantity φ represents the volume fraction of the phase in question. The specific surface area, S p, is defined as the surface area per unit volume between domains of the two phases:


S

Sp = 4M /a S

(2)

2

The wavelength, A, of a system, assuming spinodal decomposition kinetics (16). is equated to the maximum wave number, β , τη

A = 2*/j9

(3)

m

Assuming that a shoulder or maximum is found in a scattering pattern, the wavelength is determined directly. Otherwise, algebraic techniques may be employed (1Û). A n important quantity that can be calculated is the diffusion coefficient, D. A s illustrated in Figure 3, nucleation and growth results in a positive diffusion coefficient, because diffusion of the separating component is measured as movement from the original concentration to the depleted zone ahead of the growing new phase domain. B y contrast, spinodal decomposition diffusion is measured as the spontaneous movement from the original composition to a new, more concentrated phase. In terms of growth rate of the phase domain amplitude, R(/?) and the diffusion mobility M, Κ(β)

2

= -Όβ

Α

- 2Μ β Ί

(4)

and 7 is gradient energy coefficient divided by the second derivative of the Gibbs free energy with respect to composition. The scattered intensity (either light or neutrons) at an arbitrary angle over time yields the quantity R, and hence D. SANS P A T T E R N S D U R I N G A P O L Y M E R I Z A T I O N Figure 4 (10) shows the SANS scattering patterns at various stages of a PB/PS IPN polymerization. A t 3 % polystyrene, the low scattering intensity and small angular dependence suggest that phase separation has not yet taken place. The next three data curves show a shoulder, which can be interpreted according to equation (3) as domains of the order of 60 nm. Then the shoulder disappears, suggesting greater disorder. The latter is bourn out by electron microscope studies, see Figure 5 (17). A t low conversions, spherical domains are formed, followed by what appear to be ellipsoidal structures. These can be modeled as truncated irregularly shaped cylinders. A t latter conversions, where the shoulders in the scattering patterns vanish, the structures appear more blurred. It is thought that this blurring may


235



236

Figure 4. SANS intensities of a PB/PS IPN, on samples made by adding limited amounts of styrene monomer, and polymerizing to completion. Number following the S represents the weight-fraction of polystyrene. (Reproduced from ref. 10. Copyright 1988 American Chemical Society.)


Ternary Phase Diagrams for Polymer Netwofa


9. SPERLING ET A L

Figure 5. Collage of PB/PS photomicrographs produced by evaporating the un reacted monomer before T E M .


237

238

MULTIPHASE POLYMERS: BLENDS AND

IONOMERS

arise from the increased internal viscosity of the material on high conversion, and subsequent decrease in a diffusional control of phase separation by any mechanism. Following equation (1), the polystyrene transverse lengths were found to increase with weight fraction of polystyrene, Figure 6 (U£). This suggests an increase in the average polystyrene domain diameter with increase in conversion. On the other hand, the specific surface area, Figure 7 (18). is seen to go through a broad maximum near midrange compositions. The apparent decrease in S p during the latter stages of conversion might be indicative of the blurred order observed via T E M . Thus, the two experiments, highly complementary, are in agreement. S


L I G H T - S C A T T E R I N G S T U D I E S AS A F U N C T I O N OF

TIME

A series of PB/PS IPN's were photopolymerized by ultraviolet light in a special glass sandwich cell. The sample could be removed periodically from the photopolymerization box, and the scattering intensity, I, recorded while still in the sandwich cell. Then the sample was returned to the photopolymerization box. Time of polymerization, t, was recorded as the time the sample spent in front of the UV light. The light-scattering instrument was a Brice-Phoenix 2000 photometer, using the mercury blue line, 435 nm. The angle used below, after correction for refraction, was approximately 20°. For nucleation and growth, assuming that the polymerization of monomer II forms spheres of constantly increasing volume yields (19) I = kt

2

(5)

The basis for equation (5) is that, for spheres small in comparison to the wavelength of the radiation, the scattering intensity increases as the square of the volume. A plot of scattered intensity vs. time squared should be linear if nucleation and growth kinetics are followed. As shown in Figure 8, three portions of the scattering pattern are distinguishable. A t very short times, the scattering intensity is low, and increasing rapidly. This is taken as the onset of phase separation. A t times longer than about 4xl0 minutes squared, the curve drifts upward, suggesting that nucleation and growth kinetics may not be strictly followed. For spinodal decomposition, Lipatov, et al. (1£) assumed that I depends on the square of the difference in refractive index of the original phase and the new phase, but that the phase dimensions are constant. Then, 5

0nl(/?,t) = ίηΙ(β,Ο)

+ 2R(/?)t

(6)

This suggests a plot of inl vs. time. As shown in Figure 9, a straight line is obtained for times above 600 minutes, corresponding to the region of upward drift in Figure 8. However, the data are also compatible with spinodal decomposition kinetics throughout the polymerization, see dashed line. Thus, the experiment appears indecisive. The most likely answer is that both kinetic mechanisms are active through the larger portion of the polymerization, and/or the theory illustrated above in equations (5) and (6) does not accurately portray the physical situation. For example, even though the data in Figure 8 and 9 represent conversions up to about 4 0 % polystyrene, the transverse lengths increase as shown in Figure 6. The data in Figures 8 and 9 can be converted to weight percent conversion making use of earlier experiments by Fernandez at al. (20). The beginnings of a phase diagram can be determined making use of all of the data now available, see Figure 10. The reaction follows the line of arrows. The dashed line is the binodal, where nucleation and growth begins in the range of approximately 3-6% polystyrene.


9. SPERLING ET AL.

Ternary Phase Diagramsfor Polymer Networks

500

400

• ISeries Ρ • iSeries S • ISeries Ο •ISeries M

300

I PS (A)


200

100

10 20 30 40 50 60 70 80 90 100 PS content C Vol. %) Figure 6. Increase in the polystyrene transverse lengths with polystyrene content. (Reproduced from ref. 18. Copyright 1988 American Chemical Society.)

300

^

• ISeries • '.Series • ISeries •ISeries

Ρ S Ο M

200 Ssp (m /gm) 2

100

10

2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 100 P S content ( V o l . %)

Figure 7. The specific interfacial surface area goes through a broad maximum as polystyrene content increases. (Reproduced from ref. 18. Copyright 1988 American Chemical Society.)


239



240

Figure 9. Spinodal decomposition kinetics for PB/PS IPN polymerization. Same data as in Figure 8. Dashed line is the best fit assuming spinodal decomposition throughout.


Ternary Phase Diagrams for Polymer Network


SPERLING ET AL.



242


The solid line is on the spinodal, where spinodal decomposition begins at approximately 20-25% polystyrene. When the line joining Polymer I and Polymer II is reached, polymerization is completed, producing a sample of 80% polystyrene. While the phase diagram is only for one composition, perhaps it will be helpful in suggesting future experiments. Existing theory of phase separation for IPN's (21) predict domain wavelengths which are both larger and smaller than the typical distances between network crosslinks. In the latter case, they anticipate a coarsening of the structure until the domain size becomes comparable to the distances between crosslinks. This latter corresponds to the 60 nm domain size determined from the shoulder of the SANS patterns. Lastly, it is interesting to look at the diffusion coefficients, see Table I. The values of D are all negative, supporting the idea that spinodal decompositon is important. However, the values found for light-scattering do not agree with those found by SANS, and are not in proportion to their respective wavelengths. The question arises, are the two experiments actually measuring different quantities? Other data in the literature, also shown in Table I, show similar differences.

Table I. Summary of Diffusion Coefficient and Wavelength Results From Several Studies

Investigators Sperling, L. H., et al. An, J. H., et al. Hill, R. G., et al. Nojima, S., et al. Hashimoto, T . , et al. Lipatov, Y. S., et al. (a) (b)

(c) (d) (e) (f)

Method Light Scattering SANS SANS Light Scattering Light Scattering Light Scattering

Diffusion Coefficient D(m /s) 2

8

-1.8xl0" -l.lxlO" -l.lxlO" -9.6x1ο" -9.5xl0" -4.1xl0"

2

1 1

7

8

Wavelength (nm) 1,600 54 30

1,970 1,500

Ref. (a) (b) (c) (d) (e)

ω

L. H. Sperling, C. S. Traubert, and J. H. An., Polym. Mat. Sci. Eng., 58, 889 (1988). J. H. An and L. H. Sperling, in "Cross-Linked Polymers: Chemistry, Properties, and Applications," R. A. Dickie, S. S. Labana, and R. S. Bauer, Eds., ACS Symp. Series No. 367, American Chemical Society, Washington, D.C., 1988. R. G. Hill, P. E. Tomlins, and J. S. Higgins, Polymer, 2g, 1708 (1985). S. Nojima, Y. Ohyama, M . Yamaguchi, and T. Nose, Polym. J . , 14, 907 (1982). T . Hashimoto,, J. Kumahi, and H. Kawai, Macromolecules, 16, 641 (1983). Y. S. Lipatov, O. P. Grigor'yeva, G. P. Kovernik, V . V . Shilov, and L. M. Sergeyeva, Makromol. Chem., 186. 1401 (1985).

T H E PAST. PRESENT. AND F U T U R E OF IPN RESEARCH The first modern scientific paper on IPN's was written by Millar in 1960 (22). In 1979, there were a total of about 125 papers and about 75 patents in the field of IPN's, and three products. By 1985, there were several hundred papers and patents, and fifteen products. Today, the production of papers and patents is well over 100 per year. Also, the fraction of papers and patents that utilize IPN notation is


9. SPERLING ET AL.

Ternary Phase Diagrams for Polymer Network


increasing, showing that more scientists and engineers are cognizant of the growing body of literature surrounding the new field. (Many of the earlier papers spoke ol graft copolymers that were crosslinked, etc.) Interpenetrating polymer networks are important because their crosslinks offer a novel method of controlling domain size and shape; many mechanical properties such as impact strength depend on the size of the rubber domain. Thus, small, nearly uniform domains can be generated. Dual phase continuity offers many advantages, because rubber/plastic compositions yield tough, leathery materials. Many of the compositions described above, for example, contain two continuous phases, with cylinders of polystyrene meandering within the poly butadiene matrix. Since all IPN's are crosslinked, it may be that their greatest advantage will lie in products which are leathery or rubbery, but can not be permitted to flow. ACKNOWLEDGMENTS The authors wish to thank the National Science Foundation for support through Grant No. DMR-8405053, Polymers Program. The SANS experiments were performed at NCSASR, funded by NSF Grant No. 7724458 through interagency agreement No. 40-367-77 with D O E under contract DE-AC05-84R-21400 with Martin Marietta Energy System, Inc. The authors also wish to thank G. D. Wignall for his time and excellent suggestions thoughout this project.

LITERATURE CITED 1. 2.

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

11. 12. 13. 14. 15. 16. 17.

Sperling, L. H. Interpenetrating Polymer Networks and Related Materials; Plenum: New York, 1981. Xiao, H. X.; Frisch, K. C.; Al-Khatib, S. In Cross-Linked Polymers: Chemistry, Properties, and Applications; Dickie, R. Α.; Labana, S. S.; Bauer, R. S., Eds.; ACS Symposium Series No. 367; American Chemical Society: Washington, D.C., 1988. Lipatov, Yu. S.; Karabanova, L.; Sergeeva, L.; Gorbach, L.; Skiba, S. Vysokomol. Soedin, B, Krat. Soobshch 1986, 29, 274. Hourston, D. J.; Satgurunthan, R.; Varma, H. C. J. Appl. Polym. Sci. 1987, 33, 215. Donatelli, Α. Α.; Sperling, L. H.; Thomas, D. A. Macromolecules 1976, 9, 671. Donatelli, Α. Α.; Sperling, L. H.; Thomas, D. A. Macromolecules 1976, 9, 676. Yeo, J. K.; Sperling, L. H.; Thomas, D. A. Polymer 1983, 24, 307. Yeo, J. K.; Sperling, L. H.; Thomas, D. A. Polym. Eng. Sci. 1982, 22, 190. Fernandez, A. M.; Wignall, G. D.; Sperling, L. H. In Multicomponent Polymer Materials; Paul, D. R.; Sperling, L. H., Eds.; ACS Adv. in Chem. No. 211; American Chemical Society: Washington, D.C., Ch. 10. An, J. H.; Sperling, L. H. In Cross-Linked Polymers: Chemistry, Properties, and Applications; Dickie, R. Α.; Labana, S. S.; Bauer, R. S., Eds.; ACS Symposium Series No. 367; American Chemical Society: Washington, D.C., 1988; Ch. 19. Bauer, B. J.; Briber, R. M.; Han, C. C. Polym. Prepr. 1987, 28(2), 169. Lee, D. S.; Kim, S. C. Macromolecules 1984, 17, 268. Cahn, J. W. J. Chem. Phys. 1965, 42, 93. Cahn, J. W.; Hilliard, J. E. J. Chem. Phys. 1958, 28, 258. Debye, P.; Anderson, H. R.; Brumberger, H. J. Appl. Phys. 1957, 28, 679. Olabisi, O.; Robeson, L. M.; Shaw, M. T. Polymer-Polymer Miscibility; Academic Press: New York, 1979. Fernandez, A. M. Ph.D. Thesis, Lehigh University, Pennsylvania, 1984.


243

244 18.

19. 20. 21. 22.


An, J. H.; Sperling, L. H. In Cross-Linked Polymers: Chemistry, Properties and Applications; Dickie, R. Α.; Labana, S. S.; Bauer, R. S., Eds.; ACS Symposium Series No. 367; American Chemical Society: Washington, D.C., 1988. Lipatov, Y. S.; Grigor'yeva, O. P.; Kovernik, G. P.; Shilov, V. V.; Sergeyeva, L. M. Makromol. Chem. 1985, 186, 1401. Fernandez, A. M.; Widmaier, J. M.; Sperling, L. H. Polymer 1984, 25, 1718. Binder, K.; Frisch, H. L. J. Chem. Phys. 1984, 81, 2126. Millar, J. R. J. Chem. Soc. 1960, 1311.


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Multiphase Polymers: Blends and Ionomers - American Chemical

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