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Influence of microstructure on the dynamic mechanical behavior of polymeric composites containing structured latexes
Colloids and Surfaces A: Physiochemical and Engineering Aspects 153 Ž1999. 271]284
Influence of microstructure on the dynamic mechanical behavior of polymeric composites containing structured latexes J.H. Ana,U , J.M. Park b, J.H. Kimc a
Department of Polymer Science and Engineering, SungKyunKwan Uni¨ ersity, Jangan-gu, Suwon, Kyunggi-do, 440-749, South Korea b Korea Chemical Co. Ltd., Mabook-ri, Kuseong-myun, Yongin, Kyunggi-do, South Korea c Department of Chemical Engineering, Yonsei Uni¨ ersity, Shinchon-dong, Seodemoon-gu, Seoul, 120-749, South Korea
Abstract The dynamic mechanical behavior and mechanical behavior of polymeric composites containing three different kinds of dispersed phase are discussed. The first system has a structure of island-in-island type of morphology which is formed by phase separation inside of the crosslinked latex particles. The second system has a dispersed phase with a core-shell type of morphology which is prepared by heterocoagulation of large amphoteric latex and small anionic latex. The last one makes use of hollow particles containing a central void of 0.4 mm. The surface of the hollow particle has been modified to provide chemical bonds with a matrix phase composed of epoxy resins. The effect of morphology, the characteristics of the shell and the extent of interfacial bonding on the dynamic mechanical behavior or fracture behavior are discussed. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Dynamic mechanical behavior; Island-in-island morphology; Core-shell type; Hollow particles; Polymeric composites
1. Introduction Since a considerable fraction of polymeric materials are utilized as multi-phase forms such as
U
Corresponding author.
polymer blends and composites, many experimental and theoretical attempts have been made to provide suitable tools for predicting structure ]property relationships of multi-phase polymer systems. Most properties as well as mechanical behavior in multi-phase polymeric materials depend on two main factors: the first one is related to the properties of each constituent, while
0927-7757r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 Ž 9 8 . 0 0 7 2 2 - 5
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J.A. An et al. r Colloids and Surfaces A: Physiochem. Eng. Aspects 153 (1999) 271]284 Table 1 Properties of PS particles
Styrene Žgrl.a DVB Žgrl.a Average particle size Žmm. Glass transition temp. Ž8C.b a b
Fig. 1. Schematic description of sample preparation for composite sample with island-in-island dispersed phase.
the other is dependent on the physical arrangement of phases Žmorphology.. The dynamic mechanical properties of amorphous, homogeneous polymers are largely determined by molecular relaxation processes. Dynamic mechanical measurements have been also used in studies of multi-phase polymer systems such as evaluating miscibility w1]4x, interface w5]7x, damping characteristics w8,9x, and morphological variation w10]12x. Even though dynamic mechanical spectra may provide a conclusive basis for the distinction between compatible and incompatible blends w13,14x, the analysis of the heterogeneous system is more complicated since the behavior of blends or composites reflects molecular relaxation processes characteristic of each phase, but the behavior is significantly affected by morphology. Also, in many cases, these two factors are closely inter-related and cannot be separated. In some
PSŽ1.
PSŽ2.
PSŽ3.
10.40 0.11 0.5 105.2
10.29 0.21 0.5 107.9
9.98 0.53 0.5 115
Based on water. Determined by DSC.
cases, the effect of interfacial characteristics or modified relaxations which might arise from molecular mixing or chain constraints on the surface of fillers cannot be ignored w15x. In the efforts of developing mathematical modeling, a number of semi-empirical and theoretical formulae have been developed. However, there is no theory to account for all the factors mentioned above. On the other hand, even after assuming that we have a suitable model, there exists another problem in verifying the model experimentally since it is often hard to obtain a sample which has a dispersed phase with uniform size and distribution as most model equations assume. In many multi-phase polymeric materials, the morphology is developed by the phase separation during mixing or curing. Therefore, it offers very limited ability to control morphology, and hence systematic experiments to study structure ]property relationships are difficult. In this respect, latex particles are promising candidates as the dispersed phase. From literature review, a number of studies could be found which deal with blends containing latex particles as the dispersed phase or latex blends w16]22x. Table 2 Properties of PS network
N-PSŽ1. N-PSŽ2. N-PSŽ3. a b
Volumetric swelling ratio at 408Ca
Glass transition temperatureb Ž8C.
1.84 1.72 1.14
105.7 107.7 115.8
With n-butyl methacrylate. Determined by DSC.
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Fig. 2. Effect of PS particle contents on tan d of composite sample. The crosslink density of PS particles are fixed using particle ‘PSŽ1.’ in Table 1 w23x. Žv. C-PSŽ1.rPBMA-25%; ŽB. C-PSŽ1.rPBMA-15%; ŽI. C-PSŽ1.rPBMA-5%.
In this contribution, we will discuss the dynamic mechanical and mechanical behaviors of three different systems which contain latex particles as the dispersed phase, which includes islandin-island type of morphology, heterocoagulated core-shell and interface-modified hollow particles. 2. Polymer composite with island-in-island type of dispersed phase
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Fig. 3. Effect of PS particle contents on modulus of composite sample. The crosslink density of PS particles are fixed using particle ‘PSŽ1.’ in Table 1 w23x. Žv. C-PSŽ1.rPBMA-25%; ŽB. C-PSŽ1.rPBMA-15%; ŽI. C-PSŽ1.rPBMA-5%.
glass mold made of two glass plates using silicon gaskets and is clamped with small spring loaded clamps to follow the shrinkage during the polymerization. The mold is placed between two UV sources Ž12 W, 365 nm high pressure mercury lamp. and polymerized for 1 or 2 days in the temperature controlled box while the temperature is maintained the same as that of the swelling step. In this type of composite sample, the PS parti-
2.1. Experimental The composite samples containing dispersed phase with an ‘island-in-island’ type of morphology are prepared as summarized in Fig. 1. First crosslinked polystyrene ŽPS. particles are prepared from styrene ŽAldrich. and divinyl benzene ŽDVB; Aldrich. by emulsifier-free emulsion polymerization at 708C using potassium persulfate ŽPolymer Laboratory. as an initiator. The crosslink density of the particle is controlled by adjusting the amount of DVB with 1, 2, and 5 wt.%. The dried latex particles are mixed with a mixture of 98 parts of n-butyl methacrylate ŽBMA; Aldrich., 2 parts of tetraethylene glycol dimethacrylate ŽTEGDM; Polymer Laboratory. and benzoin ŽAldrich.. This mixture is kept in a sealed container at 408C until an equilibrium swelling is attained. After equilibrium, this mixture is poured into a
Fig. 4. Tan d behavior when the crosslink density of PS particle is varied while the particle loading is fixed at 15% w23x. Žv. C-PSŽ5.rPBMA-15%; ŽB. C-PSŽ2.rPBMA-15%; ŽI. CPSŽ1.rPBMA-15%.
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cles which contain acrylic component as sub-domains form dispersed phase in the matrix phase composed of PBMA. The morphology inside of the dispersed phase will be determined by the balance between the polymerization of the acrylic components and phase separation similar to an interpenetrating polymer network ŽIPN. w24x. The dispersed phase of these composite samples could be considered as a small-scale sequential type of IPN. The controlling factor in determining the amount of included phase inside the dispersed phase and the morphology will be the crosslink density of the PS particles. In order to determine the amount of acrylic component inside of the domains, the bulk size of the PS network is prepared from styrene and DVB, of which the glass transition is matched to that of PS particles based on DSC measurement. This PS network is allowed to swell with butyl acrylate until equilibrium and the volumetric swelling ratio is measured to estimate the amount of butyl acrylate inside of the PS particles in the composite samples. The properties of PS particles and PS networks are summarized in Tables 1 and 2, respectively. Since separate characterization of the dispersed phase in the composites is impossible, macroscopic samples of sequential IPN composed
Fig. 5. Modulus behavior when the crosslink density of PS particle is varied while the particle loading is fixed at 15% w23x. Žv. C-PSŽ5.rPBMA-15%; ŽB. C-PSŽ2.rPBMA-15%; Ž'. CPSŽ1.rPBMA-15%.
of the same species are prepared separately. Starting with a PS network of which the glass transition temperature is matched with corresponding PS particles, an equilibrium amount of BA and TEGDM mixture was allowed to swell into the PS network. After equilibrium, the excess monomer was removed and photopolymerized in the same manner as the preparation of composite samples. The first letter in the sample specification stands for the kind of sample; C Žcomposite., N ŽPS networks., IPN Žinterpenetrating networks.. The number in the parentheses following PS stands for the crosslinking level of PS particles and the last % is the weight % of PS particles mixed with matrix forming monomers. 2.2. Results and discussion Figs. 2 and 3 present the tan d, and modulus behavior of the composite sample with different loading of PS particles. Since the same kind of PS particles Žsame crosslink density. are used, the character and size of the dispersed phase could be considered identical. Two transition peaks are observed; the one around 608C corresponds to polyŽbutyl methacrylate . matrix while the other one at higher temperature represents the character of PS-rich dispersed phase. As the loading of
Fig. 6. Comparison of tan d behavior among composite sample, PS network ŽN-PS., and PSrPBMA IPN w23x. Žv., ŽB. N-PS; Ž^. C-PSŽ2.rPBMA-25%; Žv. IPN-PSŽ2.rPBMA.
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PS particles is decreased, the peak representing the dispersed phase becomes less distinct while the peak position seems to be invariant to PS particle content. On the other hand, if the volume of the dispersed phase is controlled by changing the crosslink density of PS particles, quite different behavior could be found as shown in Figs. 4 and 5. In this series of samples, if the crosslink density of PS particles is increased, the occluded amount of acrylic component inside of the PS domain will be decreased and the effective volume fraction of the dispersed phase will be decreased. In Fig. 4, as the crosslink density of PS particles is increased, the peak height representing the PS-rich domain becomes less developed and the sample, C-PSŽ5.rPBMA-15%, which is expected to have the smallest effective disperse-phase volume fraction including acrylic occlusions inside the PS domains, shows the smallest height of the PS transition peaks. On the other hand, the peak height of PBMA transitions seems to be decreasing relatively as the crosslink density decreases. This feature suggests that the peak height depends on the effective volume fraction of dispersed phase rather than true volume of PS component. Compared to the previous case where the particle loading is changed while the property of dispersed phase is fixed, the peak position representing the dispersed phase seems to move to-
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ward higher temperature as the crosslink density is increased. Another difference between the above two cases is found in modulus behavior. The modulus in the plateau region above 1008C is increased as the crosslink density of PS particle is increased ŽFig. 5.. On the other hand, when the loading of the particles with fixed crosslink density is varied, not much variation could be observed as shown in Fig. 5. In Fig. 6, the tan d behavior of composite samples are compared with PBMA network, PS network, and PSrPBMA IPN which is expected to have similar morphology to that of the dispersed phase in the composite sample. The PS tan d maximum of the composite sample appears 5]108C lower than that of IPN or PS networks. The possible explanation for this feature could be found in the morphological difference between composite and IPN. Above all, the PS phase shows macroscopic continuity throughout the sample in IPN w25x. On the other hand, the PS rich domains in the composite samples form a discrete phase even though there is a continuity of PS components in the individual PS rich domains. Also, in contrast to composites with inert filler, the PBMA component occluded in the PS rich domains might have some degree of connectivity with the matrix phase, which may affect dynamic mechanical response of the PS rich domains.
Table 3 The reaction makeup for large particle ŽLP., small particle ŽSP., and matrix particle ŽMP. Component
Methyl methacrylate ŽMMA. Styrene ŽST. Butyl acrylate ŽBA. Acrylic acid ŽAA. 2-Ethyl aminoethyl methacrylate ŽDEAEM. Sodium dodecyl sulfate ŽSDS. Potassium persulfate ŽKPS. 2,29-AzobisŽ2-amidino propane. hydrochloride ŽAIBH. Azobisisobutyronitrile ŽAIBN. 1,4-Butanediol dimethacrylate Acetone Distilled water
Reaction makeup Žg. LP
SP-S
SP-M
X-SP-M
MP
180 ] ] 0.9 1.8 ] ] 1.8 ] ] ] 1800
] 197.1 72.9 1.35 ] 52 ] ] 5.5 ] 17.5 1530
197.1 ] 72.9 1.35 ] 52 ] ] 5.5 ] 17.5 1530
197.1 ] 72.9 1.35 ] 3.9 2.7 ] ] 1.35 ] 1522
500 ] ] ] ] 7.2 3.4 ] ] ] ] 1272
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Table 4 Particle sizes and particle size distributions for LP, SP and MP
LP SP-M SP-S X-SP-M MP
Number average diameter Ž Dn ; nm.
Weight average diameter Ž Dw ; nm.
Dw rDn
295 48 41 74 103
301 51 46 78 110
1.02 1.06 1.12 1.05 1.07
3. Polymer composites with heterocoagulated core-shell structures 3.1. Experimental Samples are prepared in the following steps. First, large particles ŽLP. and small particles ŽSP. for heterocoagulation and matrix particle ŽMP. are prepared by emulsifier-free emulsion polymerization at 708C for 24 h. In the synthesis of
LP composed of methyl methacrylate ŽMMA; Junsei Chemical Co.. , 2-ethyl aminoethyl methacrylate ŽDEAEM; Junsei Chemical Co.. and acrylic acid ŽAA; Junsei Chemical Co.., the isoelectric point was controlled by adjusting the relative amount of amine and carboxyl functional comonomers in the medium of optimized pH range. Two kinds of small anionic particles, SP-M composed of MMA, AA, and n-butyl acrylate ŽBA. and SP-S composed of styrene ŽST., AA, and BA are prepared by micro-emulsion polymerization. Another kind of SP-M, X-SP-M which is the crosslinked version of SP-M is prepared by adding 1,4-butanediol dimethacrylate as crosslinker. The reaction makeup and particle characterizations are summarized in Tables 3 and 4. Heterocoagulation is carried out with the prepared LP and SP. The diluted LP and SP latexes are diluted and its pH values were adjusted to 10. Then two latexes are blended and the pH of blended latex is changed from 10 to 3 with 0.5 N
Fig. 7. Electron micrographs of particles at each heterocoagulation step. Ža. LP before heterocoagulation; Žb. SP before heterocoagulation; Žc. just after heterocoagulation; Žd. after annealed.
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Fig. 9. Schematic description of sample preparation for composite containing heterocoagulated core-shell particles. Fig. 8. Particle size distribution before and after pH adjustment for heterocoagulation w26x. Ža. Before pH adjustment; Žb. after pH adjustment.
HCl solution. The pH adjusted mixtures are annealed at 25 or 708C for 24 h to form shell structures in emulsion state. Fig. 7 shows the electron micrographs of particles at each step of heterocoagulation. Also, in Fig. 8, the typical variation of particle size Žmeasured with capillary hydrodynamic fractionation . before and after the heterocoagulation is shown. Comparison of particle size variation suggests that the surface of LP is not completely covered with SP. The percent coverage based on UV absorbance characterization is around 80%. Solid specimens are made by mixing dried heterocoagulated particles and MP, and then comTable 5 Calculated content of SP in heterocoagulated particles and in the composite sample Content of SP Ž%.
SP-M
SP-S
X-SP-M
In heterocoagulated particle In composite samples
38.93 11.68
34.25 10.27
53.18 15.96
pression molding in the heated press. The content of heterocoagulated particles are adjusted to 30%, then the SP content in the final molded sample would be around 10% as summarized in Table 5. Also, for comparison, simple blend is made by mixing SP and MP. A schematic description of overall sample preparation is given in Fig. 9. 3.2. Results and discussion In Fig. 10, dynamic mechanical behavior of four different particles, LP, SP-S, SP-M and MP, are shown. The glass transition temperature of SP-S and SP-M are 75 and 658C, respectively, while LP and MP have rather similar transition temperatures, 154 and 1478C. In Fig. 11, tan d and modulus behavior of two composite samples, the one containing heterocoagulated particle with SP-S and the other with SP-M, are compared. In both modulus and tan d plot, two transitions representing polystyrene phase could be observed. On the other hand, in the case of SP-M, only one transition corresponds to matrix or core phase appears. Even if we used a different set of heterocoagulated particles annealed at 258C, exactly the
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Fig. 10. Dynamic mechanical behavior of latex particles w26x. Žv. Matrix particle ŽMP.; Ž'. large particle ŽLP.; Ž%. small particle, SP-S; Ž. Small particle, SP-M.
same result is obtained; one transition with SP-M and two transitions with SP-S. One possible explanation for single Tg of the composite sample with SP-M would be that SP-M has molecular level miscibility with matrix or core material, PMMA. In order to check this possibility, DSC thermograms were taken ŽFig. 12.. All of the samples show two separate glass transition regions, confirming the presence of SP phase even though the characteristic tan d peak repre-
Fig. 11. Dynamic mechanical behavior of the composite sample containing 30% of heterocoagulated particles annealed at 708C w26x. Ža. Storage modulus wŽ`. CŽ70.-SP-S; Ž'. CŽ70.-SPMx; Žb. tan d wŽ`. CŽ70.-SP-S; Ž'. CŽ70.-SP-Mx.
senting SP-M phase cannot be observed. Considering that dynamic mechanical behavior depends not only on the properties of each phase but also on the morphology, these features suggest that the difference in dynamic mechanical behavior could be attributed to the structural difference of heterocoagulated particles between SP-M and SP-S. The small particles SP-M and SP-S are composed of polyŽMMA-BA-AA. and
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Fig. 13. Schematic description of structural development during the annealing of heterocoagulated particles.
series of experiments. In this case, a crosslinked version of SP-M, X-SP-M is used instead of SP-M in the heterocoagulation step. Fig. 14 shows the dynamic mechanical behavior of the composites sample containing X-SP-M. Regardless of the annealing temperature, the transition around 708C representing X-SP-M could be observed. 4. Polymeric composites containing hollow particles Fig. 12. DSC thermogram of composite samples containing 30% of heterocoagulated particles annealed at 708C or 258C w26x. Ža. CŽ70.-SP-M; Žb. CŽ25.-SP-M; Žc. CŽ70.-SP-S; Žd. CŽ25.-SP-S.
polyŽST-BA-AA., respectively. Therefore, SP-M could be considered to be more compatible with LP than SP-S in a relative sense. Furthermore, the amine moiety in the LP phase may further facilitate this difference. Therefore, as illustrated in Fig. 13, SP-M may diffuse into core LP during the annealing period while the less compatible SP-S forms separate shell structures. This result may be substantiated by another
4.1. Experimental Hollow polymer particles were prepared by polymerizing a hydrophobic polymer shell around a hydrophilic polymer core, then recovering and drying the particles w27x. Typical cores were soft carboxyl-containing copolymers which have been neutralized; typical shells were hard polystyrene or polyŽmethyl methacrylate .. The volume of core must be great enough so as to shrink and leave a void upon drying; the shell must be strong enough to maintain its integrity during drying. These core-shell particles were prepared by multi-staged emulsion polymerization process which com-
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methacrylate ŽMMA., acrylonitrile ŽAN. and methacrylic acid ŽMAA.. By changing the composition of the monomer mixture, the glass transition temperature or the content of carboxyl group of the newly formed shell was controlled. The reaction makeup is summarized in Table 6, in which the amount of newly formed shell is about 20 vol.% based on the total volume of the final particles and the content of MAA is varied 1, 3, 5, 7 wt.%. In Fig. 15, the particle size distribution before and after the modification is shown. After the modification, the emulsion particles are freeze-dried and mixed with epoxy resin Ždiglycidyl ether of bisphenol A; DGEBA., and then cured using aminoethyl piperazine ŽAEP. as a curing agent. The content of hollow particle is fixed at 10 wt.% and 2 h of post-curing at 1008C is employed after the 1 h of room-temperature curing. Fig. 16 shows the sample preparation schematically. 4.2. Results and discussion
Fig. 14. Dynamic mechanical behavior of the composite samples containing 30% of heterocoagulated particles with X-SPM w26x. Ža. Storage modulus wŽ`. CŽ70.-X-SP-M, Ž'. CŽ25.-XSP-Mx; Žb. tan d wŽ`. CŽ70.-X-SP-M, Ž'. CŽ25.-X-SP-Mx.
prised: Ži. preparing the core by polymerizing carboxylic acid monomers with other monoethylenically unsaturated monomers; Žii. encapsulating the core with a rigid polymer shell by polymerizing shell-forming monomers, in the presence of the core particles; and Žiii. neutralizing with ammonia so as to swell the core and form particles which, when dried, contain a single closed cell of void. An additional emulsion polymerization step was employed in the presence of the core-shell structural latexes in order to change interfacial characteristics when introduced into the matrix composed of epoxy. This modification was done by delayed adding of monomer mixture composed of butyl acrylate ŽBA ., styrene ŽSt., methyl
In Fig. 17, the electron micrographs of the fractured surface of epoxy prepared with the hollow particles with different contents of MAA are compared. As the content of MAA is increased in the surface modification steps, better interfacial bonding occurs between the particles and the matrix, as expected, through the reaction between MAA and epoxy matrix. This expectation could be confirmed in the electron micrographs showing more deformed spheres with increasing MAA content. If the toughening mechanism in this system is based on the deformation of the hollow particle itself, better toughness is expected for the samples with higher content of MAA. However, this is not the case as shown in Fig. 18. The composite containing the unmodified hollow particles show only marginal increase in the K IC value compared with the control sample. If the surface of the hollow particle is modified, an increase in the fracture toughness could be observed. However, contrary to the expectation based on electron micrographs, the fracture toughness shows maximum for the sample with 1 wt.% of MAA and shows a decrease with further increase of MAA in the modification steps. The
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Table 6 Reaction makeup for surface modification of the hollow particles Part
Components
Modification of hollow particles Žg. HŽ60-1.
HŽ60-3.
HŽ60-5.
HŽ60-7.
ŽH10-1.
HŽ110-1.
200 21 0.4
200 21 0.4
200 21 0.4
200 21 0.4
200 21 0.4
200 21 0.4
10.4 0.1
10.4 0.1
10.4 0.1
10.4 0.1
10.4 0.1
I
H Žhollow particle. DDI water APS
II
DDI water CO-436
10.4 0.1
III
Butyl acrylate Styrene MAA MMA Acrylonitrile
3.74 3.93 0.19 10.28 0.56
3.74 3.93 0.56 9.91 0.56
3.74 3.93 0.94 9.54 0.56
3.74 3.93 1.31 9.16 0.56
8.72 3.93 0.19 5.3 0.56
0 3.93 0.19 14.02 0.56
Fig. 16. Schematic presentation of sample preparation steps for polymeric composites containing surface modified hollow particles.
Fig. 15. Particle size distribution before and after the surface modification of hollow particles. Ža. Before modification; Žb. after modification.
detailed mechanism of toughening in this system is a subject of further study. The effect of MMA content can be viewed more obviously in the behavior of tensile strain. All samples containing hollow particles exhibit much lower tensile strain compared with the neat epoxy ŽFig. 19.. Such a decrease in the tensile strain becomes more severe as the MMA content is increased, confirming the chemical reaction between the MAA functional of the hollow particle and epoxy matrix. Also, it is worthwhile to note that the modulus of the
composite samples are similar to that of the neat epoxy, reflecting the rigidity of hollow particles composed of polystyrene and poly Žmethyl methacrylate .. By changing the composition of monomer mixture in the surface modification step, the glass transition temperature of the shell could be varied. In Fig. 20, the fracture toughness values are compared among samples with different shell Tg . The best one is found to be the sample with a medium Tg value of 608C while the other two specimens with 108C or 1108C shell are slightly worse. However the difference is only marginal suggesting that the thickness Žaround 15 nm based on particle size measurement. is not thick enough to provide an effect similar to that of rubbery particles. Also in tensile modulus, the specimen with the shell of 608C seems to be the best while the one with the lowest Tg is the worst. This is probably due to the poor mechanical properties
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Fig. 17. Electron micrograph of the fractured surface of the composite samples containing hollow particles. The charge of hollow particles are fixed at 10 wt.% and all of the samples are post-cured at 1008C for 2 h. Ža. Composite using unmodified hollow particle; Žb. composite using hollow particle with 1 wt.% MAAwHŽ60-1.x; Žc. composite using hollow particle with 3 wt.% MAAwHŽ60-3.x; Žd. composite using hollow particle with 5 wt.% MAAwHŽ60-5.x; Že. composite using hollow particle with 7 wt.% MAAwHŽ60-7.x.
of a shell with low Tg or the possible deformation and destruction of the shell during the sample preparation steps. 5. Conclusions It has been shown that dynamic mechanical measurement is a useful tool to characterize the polymeric composites containing various types of dispersed phase formed from structured latexes. In the composites having disperse phases with
island-in-island type of morphology, the height of the tan d peak of the dispersed phase is found to be generally dependent on the effective volume fraction rather than the overall composition. Therefore, the occluded portion inside of the dispersed phase seems to behave as a part of the dispersed domains. When the dispersed domains are composed of core-shell particles via heterocoagulation, the dynamic mechanical response representing the shell phase is observable only if either the shell phase has enough incompatibility
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Fig. 18. Comparison of fracture toughness among epoxy, epoxy with unmodified hollow particle and epoxy with modified hollow particles.
Fig. 19. Comparison of stress-strain behaviour among epoxy, epoxy with unmodified hollow particle and epoxy with modified hollow particles.
toward the core phase material or the shell phase is crosslinked, suggesting that the shell phase cannot maintain its shape during the annealing period if the shell phase possesses a certain degree of compatibility with the core phase. Hollow particles are also examined as toughening agents for the epoxy. The addition of unmodified hollow particle can improve the toughness of the epoxy resin as studied by Bagheri and Pearson w28x. On the other hand, if the carboxyl moiety
Fig. 20. Comparison of fracture toughness among samples containing a modified layer with different Tg .
which can react with epoxy matrix, is incorporated into the surface of the hollow particle, the toughness is further improved. However, such improvement decreases with the increased carboxyl content, suggesting there exists an optimum level of the interfacial bonding. The steep decrease in tensile strain value for epoxy-containing hollow particles suggests that the hollow particles behave as a typical hard filler.
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Acknowledgements The authors thank the Korea Science and Engineering Foundation and Center for Advanced Functional Polymers ŽGrant No. 97K3-1005-0307-3. for the financial support. References w1x J.D. Keitz, J.W. Barlow, D.R. Paul, J. Appl. Polym. Sci. 29 Ž1984. 3131. w2x K. Liang, G. Banhegyi, F.E. Karsz, W.J. MacKnight, J. Polym. Sci. Polym. Phys. Ed. 29 Ž1991. 649. w3x J.K. Yeo, L.H. Sperling, D.A. Thomas, Polym. Eng. Sci. 21 Ž1981. 696. w4x J.A. Manson, L.H. Sperling, Polymer Blends and Composites, Plenum Press, New York, 1976. w5x J.L. Gomez Ribelles, J. Mano Sebstia, R. Marti Soler, M. Monleon Pradas, A. Ribes Greus, J.J. Suay Anton, J. Appl. Polym. Sci. 42 Ž1991. 1647. w6x J. Ding, C. Chen, G. Xue, J. Appl. Polym. Sci. 42 Ž1991. 1459. w7x M. Kodama, I. Karino, J. Appl. Polym. Sci. 32 Ž1986. 5057. w8x R.D. Corsaro, L.H. Sperling ŽEds.., Sound and Vibration Daming with Polymers, ACS Symposium Series 424, ACS, Washington, DC, 1990. w9x D. Klempner, C.L. Wang, M. Ashitiani, K.C. Frisch, J. Appl. Polym. Sci. 32 Ž1986. 4197. w10x J.K. Yeo, L.H. Sperling, D.A. Thomas, Polym. Eng. Sci. 22 Ž1982. 190. w11x D.J. Hourston, R. Satgurunathan, H.C. Varma, J. Appl. Polym. Sci. 34 Ž1987. 901. w12x K.W. McLaughlin, Polym. Eng. Sci. 299 Ž1989. 1580. w13x S. Krause, in: D.R. Paul, S. Newman ŽEds.., Polymer Blends, Academic Press, New York, 1978, Chap. 3, p. 15.
w14x L. Nielsen, R.F. Landel, Mechanical Properties of Polymers and Polymeric Compounds, Chap. 4, Marcel Dekker, New York, 1994. w15x M. Kodama, I. Karino, J. Appl. Polym. Sci. 32 Ž1986. 5345. w16x J.Y. Cavaille, R. Vassoille, G. Thollet, L. Rrios, C. Pichot, Colloid Polym. Sci. 269 Ž1991. 248. w17x B. Schlund, J. Guillot, C. Pichot, A. Cruz, Polymer 60 Ž1989. 1853. w18x J.Y. Cavaille, C. Jourdan, X.Z. Kong, J. Perez, C. Pichot, J. Guillot, Polymer 27 Ž1986. 693. w19x C. Jourdan, J.Y. Cavaille, J. Perez, Polym. Eng. Sci. 28 Ž1988. 1318. w20x J.-E.L. Jonsson, H. Hassander, L.H. Jansson, B. Tornell, Macromolecules 24 Ž1991. 126. w21x K.M. O’Connor, S.L. Tsaur, J. Appl. Polym. Sci. 33 Ž1987. 2007. w22x P. Cousin, P. Smith, J. Polym. Sci. Polym. Phys. Ed. 33 Ž1995. 1681. w23x K.R. Kim, J.H. An, K.W. Cho, C.E. Park, J. Appl. Polym. Sci. 47 Ž1993. 305. w24x L.H. Sperling, IPN and Related Materials, Plenum Press, New York, 1981. w25x J.H. An, L.H. Sperling, in: R.A. Dickie, S.S. Labana, R.S. Bauer ŽEds.., Cross-Linked Polymers; Chemistry, Properties, and Applications, ACS Symposium Series 367, Chap. 19, ACS, Washington, DC, 1988. w26x H.-J. Ha, Y.-J. Park, J.-H. An, J.-H. Kim, J. Appl. Polym. Sci. Žin press.. w27x J.W. Vanderhoff, J.M. Park, M.S. El-Aasser, in: E.S. Daniel, E.D. Sudol, M.S. El-Aasser ŽEds.., Chap. 17, ACS Symposium Series 492, ACS, Washington, DC, 1992. w28x R. Bagheri, R.A. Pearson, Polymer 37 Ž1996. 4529.
Influence of microstructure on the dynamic mechanical behavior of polymeric composites containing structured latexes J.H. Ana,U , J.M. Park b, J.H. Kimc a
Department of Polymer Science and Engineering, SungKyunKwan Uni¨ ersity, Jangan-gu, Suwon, Kyunggi-do, 440-749, South Korea b Korea Chemical Co. Ltd., Mabook-ri, Kuseong-myun, Yongin, Kyunggi-do, South Korea c Department of Chemical Engineering, Yonsei Uni¨ ersity, Shinchon-dong, Seodemoon-gu, Seoul, 120-749, South Korea
Abstract The dynamic mechanical behavior and mechanical behavior of polymeric composites containing three different kinds of dispersed phase are discussed. The first system has a structure of island-in-island type of morphology which is formed by phase separation inside of the crosslinked latex particles. The second system has a dispersed phase with a core-shell type of morphology which is prepared by heterocoagulation of large amphoteric latex and small anionic latex. The last one makes use of hollow particles containing a central void of 0.4 mm. The surface of the hollow particle has been modified to provide chemical bonds with a matrix phase composed of epoxy resins. The effect of morphology, the characteristics of the shell and the extent of interfacial bonding on the dynamic mechanical behavior or fracture behavior are discussed. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Dynamic mechanical behavior; Island-in-island morphology; Core-shell type; Hollow particles; Polymeric composites
1. Introduction Since a considerable fraction of polymeric materials are utilized as multi-phase forms such as
U
Corresponding author.
polymer blends and composites, many experimental and theoretical attempts have been made to provide suitable tools for predicting structure ]property relationships of multi-phase polymer systems. Most properties as well as mechanical behavior in multi-phase polymeric materials depend on two main factors: the first one is related to the properties of each constituent, while
0927-7757r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 Ž 9 8 . 0 0 7 2 2 - 5
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Styrene Žgrl.a DVB Žgrl.a Average particle size Žmm. Glass transition temp. Ž8C.b a b
Fig. 1. Schematic description of sample preparation for composite sample with island-in-island dispersed phase.
the other is dependent on the physical arrangement of phases Žmorphology.. The dynamic mechanical properties of amorphous, homogeneous polymers are largely determined by molecular relaxation processes. Dynamic mechanical measurements have been also used in studies of multi-phase polymer systems such as evaluating miscibility w1]4x, interface w5]7x, damping characteristics w8,9x, and morphological variation w10]12x. Even though dynamic mechanical spectra may provide a conclusive basis for the distinction between compatible and incompatible blends w13,14x, the analysis of the heterogeneous system is more complicated since the behavior of blends or composites reflects molecular relaxation processes characteristic of each phase, but the behavior is significantly affected by morphology. Also, in many cases, these two factors are closely inter-related and cannot be separated. In some
PSŽ1.
PSŽ2.
PSŽ3.
10.40 0.11 0.5 105.2
10.29 0.21 0.5 107.9
9.98 0.53 0.5 115
Based on water. Determined by DSC.
cases, the effect of interfacial characteristics or modified relaxations which might arise from molecular mixing or chain constraints on the surface of fillers cannot be ignored w15x. In the efforts of developing mathematical modeling, a number of semi-empirical and theoretical formulae have been developed. However, there is no theory to account for all the factors mentioned above. On the other hand, even after assuming that we have a suitable model, there exists another problem in verifying the model experimentally since it is often hard to obtain a sample which has a dispersed phase with uniform size and distribution as most model equations assume. In many multi-phase polymeric materials, the morphology is developed by the phase separation during mixing or curing. Therefore, it offers very limited ability to control morphology, and hence systematic experiments to study structure ]property relationships are difficult. In this respect, latex particles are promising candidates as the dispersed phase. From literature review, a number of studies could be found which deal with blends containing latex particles as the dispersed phase or latex blends w16]22x. Table 2 Properties of PS network
N-PSŽ1. N-PSŽ2. N-PSŽ3. a b
Volumetric swelling ratio at 408Ca
Glass transition temperatureb Ž8C.
1.84 1.72 1.14
105.7 107.7 115.8
With n-butyl methacrylate. Determined by DSC.
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Fig. 2. Effect of PS particle contents on tan d of composite sample. The crosslink density of PS particles are fixed using particle ‘PSŽ1.’ in Table 1 w23x. Žv. C-PSŽ1.rPBMA-25%; ŽB. C-PSŽ1.rPBMA-15%; ŽI. C-PSŽ1.rPBMA-5%.
In this contribution, we will discuss the dynamic mechanical and mechanical behaviors of three different systems which contain latex particles as the dispersed phase, which includes islandin-island type of morphology, heterocoagulated core-shell and interface-modified hollow particles. 2. Polymer composite with island-in-island type of dispersed phase
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Fig. 3. Effect of PS particle contents on modulus of composite sample. The crosslink density of PS particles are fixed using particle ‘PSŽ1.’ in Table 1 w23x. Žv. C-PSŽ1.rPBMA-25%; ŽB. C-PSŽ1.rPBMA-15%; ŽI. C-PSŽ1.rPBMA-5%.
glass mold made of two glass plates using silicon gaskets and is clamped with small spring loaded clamps to follow the shrinkage during the polymerization. The mold is placed between two UV sources Ž12 W, 365 nm high pressure mercury lamp. and polymerized for 1 or 2 days in the temperature controlled box while the temperature is maintained the same as that of the swelling step. In this type of composite sample, the PS parti-
2.1. Experimental The composite samples containing dispersed phase with an ‘island-in-island’ type of morphology are prepared as summarized in Fig. 1. First crosslinked polystyrene ŽPS. particles are prepared from styrene ŽAldrich. and divinyl benzene ŽDVB; Aldrich. by emulsifier-free emulsion polymerization at 708C using potassium persulfate ŽPolymer Laboratory. as an initiator. The crosslink density of the particle is controlled by adjusting the amount of DVB with 1, 2, and 5 wt.%. The dried latex particles are mixed with a mixture of 98 parts of n-butyl methacrylate ŽBMA; Aldrich., 2 parts of tetraethylene glycol dimethacrylate ŽTEGDM; Polymer Laboratory. and benzoin ŽAldrich.. This mixture is kept in a sealed container at 408C until an equilibrium swelling is attained. After equilibrium, this mixture is poured into a
Fig. 4. Tan d behavior when the crosslink density of PS particle is varied while the particle loading is fixed at 15% w23x. Žv. C-PSŽ5.rPBMA-15%; ŽB. C-PSŽ2.rPBMA-15%; ŽI. CPSŽ1.rPBMA-15%.
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cles which contain acrylic component as sub-domains form dispersed phase in the matrix phase composed of PBMA. The morphology inside of the dispersed phase will be determined by the balance between the polymerization of the acrylic components and phase separation similar to an interpenetrating polymer network ŽIPN. w24x. The dispersed phase of these composite samples could be considered as a small-scale sequential type of IPN. The controlling factor in determining the amount of included phase inside the dispersed phase and the morphology will be the crosslink density of the PS particles. In order to determine the amount of acrylic component inside of the domains, the bulk size of the PS network is prepared from styrene and DVB, of which the glass transition is matched to that of PS particles based on DSC measurement. This PS network is allowed to swell with butyl acrylate until equilibrium and the volumetric swelling ratio is measured to estimate the amount of butyl acrylate inside of the PS particles in the composite samples. The properties of PS particles and PS networks are summarized in Tables 1 and 2, respectively. Since separate characterization of the dispersed phase in the composites is impossible, macroscopic samples of sequential IPN composed
Fig. 5. Modulus behavior when the crosslink density of PS particle is varied while the particle loading is fixed at 15% w23x. Žv. C-PSŽ5.rPBMA-15%; ŽB. C-PSŽ2.rPBMA-15%; Ž'. CPSŽ1.rPBMA-15%.
of the same species are prepared separately. Starting with a PS network of which the glass transition temperature is matched with corresponding PS particles, an equilibrium amount of BA and TEGDM mixture was allowed to swell into the PS network. After equilibrium, the excess monomer was removed and photopolymerized in the same manner as the preparation of composite samples. The first letter in the sample specification stands for the kind of sample; C Žcomposite., N ŽPS networks., IPN Žinterpenetrating networks.. The number in the parentheses following PS stands for the crosslinking level of PS particles and the last % is the weight % of PS particles mixed with matrix forming monomers. 2.2. Results and discussion Figs. 2 and 3 present the tan d, and modulus behavior of the composite sample with different loading of PS particles. Since the same kind of PS particles Žsame crosslink density. are used, the character and size of the dispersed phase could be considered identical. Two transition peaks are observed; the one around 608C corresponds to polyŽbutyl methacrylate . matrix while the other one at higher temperature represents the character of PS-rich dispersed phase. As the loading of
Fig. 6. Comparison of tan d behavior among composite sample, PS network ŽN-PS., and PSrPBMA IPN w23x. Žv., ŽB. N-PS; Ž^. C-PSŽ2.rPBMA-25%; Žv. IPN-PSŽ2.rPBMA.
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PS particles is decreased, the peak representing the dispersed phase becomes less distinct while the peak position seems to be invariant to PS particle content. On the other hand, if the volume of the dispersed phase is controlled by changing the crosslink density of PS particles, quite different behavior could be found as shown in Figs. 4 and 5. In this series of samples, if the crosslink density of PS particles is increased, the occluded amount of acrylic component inside of the PS domain will be decreased and the effective volume fraction of the dispersed phase will be decreased. In Fig. 4, as the crosslink density of PS particles is increased, the peak height representing the PS-rich domain becomes less developed and the sample, C-PSŽ5.rPBMA-15%, which is expected to have the smallest effective disperse-phase volume fraction including acrylic occlusions inside the PS domains, shows the smallest height of the PS transition peaks. On the other hand, the peak height of PBMA transitions seems to be decreasing relatively as the crosslink density decreases. This feature suggests that the peak height depends on the effective volume fraction of dispersed phase rather than true volume of PS component. Compared to the previous case where the particle loading is changed while the property of dispersed phase is fixed, the peak position representing the dispersed phase seems to move to-
275
ward higher temperature as the crosslink density is increased. Another difference between the above two cases is found in modulus behavior. The modulus in the plateau region above 1008C is increased as the crosslink density of PS particle is increased ŽFig. 5.. On the other hand, when the loading of the particles with fixed crosslink density is varied, not much variation could be observed as shown in Fig. 5. In Fig. 6, the tan d behavior of composite samples are compared with PBMA network, PS network, and PSrPBMA IPN which is expected to have similar morphology to that of the dispersed phase in the composite sample. The PS tan d maximum of the composite sample appears 5]108C lower than that of IPN or PS networks. The possible explanation for this feature could be found in the morphological difference between composite and IPN. Above all, the PS phase shows macroscopic continuity throughout the sample in IPN w25x. On the other hand, the PS rich domains in the composite samples form a discrete phase even though there is a continuity of PS components in the individual PS rich domains. Also, in contrast to composites with inert filler, the PBMA component occluded in the PS rich domains might have some degree of connectivity with the matrix phase, which may affect dynamic mechanical response of the PS rich domains.
Table 3 The reaction makeup for large particle ŽLP., small particle ŽSP., and matrix particle ŽMP. Component
Methyl methacrylate ŽMMA. Styrene ŽST. Butyl acrylate ŽBA. Acrylic acid ŽAA. 2-Ethyl aminoethyl methacrylate ŽDEAEM. Sodium dodecyl sulfate ŽSDS. Potassium persulfate ŽKPS. 2,29-AzobisŽ2-amidino propane. hydrochloride ŽAIBH. Azobisisobutyronitrile ŽAIBN. 1,4-Butanediol dimethacrylate Acetone Distilled water
Reaction makeup Žg. LP
SP-S
SP-M
X-SP-M
MP
180 ] ] 0.9 1.8 ] ] 1.8 ] ] ] 1800
] 197.1 72.9 1.35 ] 52 ] ] 5.5 ] 17.5 1530
197.1 ] 72.9 1.35 ] 52 ] ] 5.5 ] 17.5 1530
197.1 ] 72.9 1.35 ] 3.9 2.7 ] ] 1.35 ] 1522
500 ] ] ] ] 7.2 3.4 ] ] ] ] 1272
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Table 4 Particle sizes and particle size distributions for LP, SP and MP
LP SP-M SP-S X-SP-M MP
Number average diameter Ž Dn ; nm.
Weight average diameter Ž Dw ; nm.
Dw rDn
295 48 41 74 103
301 51 46 78 110
1.02 1.06 1.12 1.05 1.07
3. Polymer composites with heterocoagulated core-shell structures 3.1. Experimental Samples are prepared in the following steps. First, large particles ŽLP. and small particles ŽSP. for heterocoagulation and matrix particle ŽMP. are prepared by emulsifier-free emulsion polymerization at 708C for 24 h. In the synthesis of
LP composed of methyl methacrylate ŽMMA; Junsei Chemical Co.. , 2-ethyl aminoethyl methacrylate ŽDEAEM; Junsei Chemical Co.. and acrylic acid ŽAA; Junsei Chemical Co.., the isoelectric point was controlled by adjusting the relative amount of amine and carboxyl functional comonomers in the medium of optimized pH range. Two kinds of small anionic particles, SP-M composed of MMA, AA, and n-butyl acrylate ŽBA. and SP-S composed of styrene ŽST., AA, and BA are prepared by micro-emulsion polymerization. Another kind of SP-M, X-SP-M which is the crosslinked version of SP-M is prepared by adding 1,4-butanediol dimethacrylate as crosslinker. The reaction makeup and particle characterizations are summarized in Tables 3 and 4. Heterocoagulation is carried out with the prepared LP and SP. The diluted LP and SP latexes are diluted and its pH values were adjusted to 10. Then two latexes are blended and the pH of blended latex is changed from 10 to 3 with 0.5 N
Fig. 7. Electron micrographs of particles at each heterocoagulation step. Ža. LP before heterocoagulation; Žb. SP before heterocoagulation; Žc. just after heterocoagulation; Žd. after annealed.
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Fig. 9. Schematic description of sample preparation for composite containing heterocoagulated core-shell particles. Fig. 8. Particle size distribution before and after pH adjustment for heterocoagulation w26x. Ža. Before pH adjustment; Žb. after pH adjustment.
HCl solution. The pH adjusted mixtures are annealed at 25 or 708C for 24 h to form shell structures in emulsion state. Fig. 7 shows the electron micrographs of particles at each step of heterocoagulation. Also, in Fig. 8, the typical variation of particle size Žmeasured with capillary hydrodynamic fractionation . before and after the heterocoagulation is shown. Comparison of particle size variation suggests that the surface of LP is not completely covered with SP. The percent coverage based on UV absorbance characterization is around 80%. Solid specimens are made by mixing dried heterocoagulated particles and MP, and then comTable 5 Calculated content of SP in heterocoagulated particles and in the composite sample Content of SP Ž%.
SP-M
SP-S
X-SP-M
In heterocoagulated particle In composite samples
38.93 11.68
34.25 10.27
53.18 15.96
pression molding in the heated press. The content of heterocoagulated particles are adjusted to 30%, then the SP content in the final molded sample would be around 10% as summarized in Table 5. Also, for comparison, simple blend is made by mixing SP and MP. A schematic description of overall sample preparation is given in Fig. 9. 3.2. Results and discussion In Fig. 10, dynamic mechanical behavior of four different particles, LP, SP-S, SP-M and MP, are shown. The glass transition temperature of SP-S and SP-M are 75 and 658C, respectively, while LP and MP have rather similar transition temperatures, 154 and 1478C. In Fig. 11, tan d and modulus behavior of two composite samples, the one containing heterocoagulated particle with SP-S and the other with SP-M, are compared. In both modulus and tan d plot, two transitions representing polystyrene phase could be observed. On the other hand, in the case of SP-M, only one transition corresponds to matrix or core phase appears. Even if we used a different set of heterocoagulated particles annealed at 258C, exactly the
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Fig. 10. Dynamic mechanical behavior of latex particles w26x. Žv. Matrix particle ŽMP.; Ž'. large particle ŽLP.; Ž%. small particle, SP-S; Ž. Small particle, SP-M.
same result is obtained; one transition with SP-M and two transitions with SP-S. One possible explanation for single Tg of the composite sample with SP-M would be that SP-M has molecular level miscibility with matrix or core material, PMMA. In order to check this possibility, DSC thermograms were taken ŽFig. 12.. All of the samples show two separate glass transition regions, confirming the presence of SP phase even though the characteristic tan d peak repre-
Fig. 11. Dynamic mechanical behavior of the composite sample containing 30% of heterocoagulated particles annealed at 708C w26x. Ža. Storage modulus wŽ`. CŽ70.-SP-S; Ž'. CŽ70.-SPMx; Žb. tan d wŽ`. CŽ70.-SP-S; Ž'. CŽ70.-SP-Mx.
senting SP-M phase cannot be observed. Considering that dynamic mechanical behavior depends not only on the properties of each phase but also on the morphology, these features suggest that the difference in dynamic mechanical behavior could be attributed to the structural difference of heterocoagulated particles between SP-M and SP-S. The small particles SP-M and SP-S are composed of polyŽMMA-BA-AA. and
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279
Fig. 13. Schematic description of structural development during the annealing of heterocoagulated particles.
series of experiments. In this case, a crosslinked version of SP-M, X-SP-M is used instead of SP-M in the heterocoagulation step. Fig. 14 shows the dynamic mechanical behavior of the composites sample containing X-SP-M. Regardless of the annealing temperature, the transition around 708C representing X-SP-M could be observed. 4. Polymeric composites containing hollow particles Fig. 12. DSC thermogram of composite samples containing 30% of heterocoagulated particles annealed at 708C or 258C w26x. Ža. CŽ70.-SP-M; Žb. CŽ25.-SP-M; Žc. CŽ70.-SP-S; Žd. CŽ25.-SP-S.
polyŽST-BA-AA., respectively. Therefore, SP-M could be considered to be more compatible with LP than SP-S in a relative sense. Furthermore, the amine moiety in the LP phase may further facilitate this difference. Therefore, as illustrated in Fig. 13, SP-M may diffuse into core LP during the annealing period while the less compatible SP-S forms separate shell structures. This result may be substantiated by another
4.1. Experimental Hollow polymer particles were prepared by polymerizing a hydrophobic polymer shell around a hydrophilic polymer core, then recovering and drying the particles w27x. Typical cores were soft carboxyl-containing copolymers which have been neutralized; typical shells were hard polystyrene or polyŽmethyl methacrylate .. The volume of core must be great enough so as to shrink and leave a void upon drying; the shell must be strong enough to maintain its integrity during drying. These core-shell particles were prepared by multi-staged emulsion polymerization process which com-
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methacrylate ŽMMA., acrylonitrile ŽAN. and methacrylic acid ŽMAA.. By changing the composition of the monomer mixture, the glass transition temperature or the content of carboxyl group of the newly formed shell was controlled. The reaction makeup is summarized in Table 6, in which the amount of newly formed shell is about 20 vol.% based on the total volume of the final particles and the content of MAA is varied 1, 3, 5, 7 wt.%. In Fig. 15, the particle size distribution before and after the modification is shown. After the modification, the emulsion particles are freeze-dried and mixed with epoxy resin Ždiglycidyl ether of bisphenol A; DGEBA., and then cured using aminoethyl piperazine ŽAEP. as a curing agent. The content of hollow particle is fixed at 10 wt.% and 2 h of post-curing at 1008C is employed after the 1 h of room-temperature curing. Fig. 16 shows the sample preparation schematically. 4.2. Results and discussion
Fig. 14. Dynamic mechanical behavior of the composite samples containing 30% of heterocoagulated particles with X-SPM w26x. Ža. Storage modulus wŽ`. CŽ70.-X-SP-M, Ž'. CŽ25.-XSP-Mx; Žb. tan d wŽ`. CŽ70.-X-SP-M, Ž'. CŽ25.-X-SP-Mx.
prised: Ži. preparing the core by polymerizing carboxylic acid monomers with other monoethylenically unsaturated monomers; Žii. encapsulating the core with a rigid polymer shell by polymerizing shell-forming monomers, in the presence of the core particles; and Žiii. neutralizing with ammonia so as to swell the core and form particles which, when dried, contain a single closed cell of void. An additional emulsion polymerization step was employed in the presence of the core-shell structural latexes in order to change interfacial characteristics when introduced into the matrix composed of epoxy. This modification was done by delayed adding of monomer mixture composed of butyl acrylate ŽBA ., styrene ŽSt., methyl
In Fig. 17, the electron micrographs of the fractured surface of epoxy prepared with the hollow particles with different contents of MAA are compared. As the content of MAA is increased in the surface modification steps, better interfacial bonding occurs between the particles and the matrix, as expected, through the reaction between MAA and epoxy matrix. This expectation could be confirmed in the electron micrographs showing more deformed spheres with increasing MAA content. If the toughening mechanism in this system is based on the deformation of the hollow particle itself, better toughness is expected for the samples with higher content of MAA. However, this is not the case as shown in Fig. 18. The composite containing the unmodified hollow particles show only marginal increase in the K IC value compared with the control sample. If the surface of the hollow particle is modified, an increase in the fracture toughness could be observed. However, contrary to the expectation based on electron micrographs, the fracture toughness shows maximum for the sample with 1 wt.% of MAA and shows a decrease with further increase of MAA in the modification steps. The
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281
Table 6 Reaction makeup for surface modification of the hollow particles Part
Components
Modification of hollow particles Žg. HŽ60-1.
HŽ60-3.
HŽ60-5.
HŽ60-7.
ŽH10-1.
HŽ110-1.
200 21 0.4
200 21 0.4
200 21 0.4
200 21 0.4
200 21 0.4
200 21 0.4
10.4 0.1
10.4 0.1
10.4 0.1
10.4 0.1
10.4 0.1
I
H Žhollow particle. DDI water APS
II
DDI water CO-436
10.4 0.1
III
Butyl acrylate Styrene MAA MMA Acrylonitrile
3.74 3.93 0.19 10.28 0.56
3.74 3.93 0.56 9.91 0.56
3.74 3.93 0.94 9.54 0.56
3.74 3.93 1.31 9.16 0.56
8.72 3.93 0.19 5.3 0.56
0 3.93 0.19 14.02 0.56
Fig. 16. Schematic presentation of sample preparation steps for polymeric composites containing surface modified hollow particles.
Fig. 15. Particle size distribution before and after the surface modification of hollow particles. Ža. Before modification; Žb. after modification.
detailed mechanism of toughening in this system is a subject of further study. The effect of MMA content can be viewed more obviously in the behavior of tensile strain. All samples containing hollow particles exhibit much lower tensile strain compared with the neat epoxy ŽFig. 19.. Such a decrease in the tensile strain becomes more severe as the MMA content is increased, confirming the chemical reaction between the MAA functional of the hollow particle and epoxy matrix. Also, it is worthwhile to note that the modulus of the
composite samples are similar to that of the neat epoxy, reflecting the rigidity of hollow particles composed of polystyrene and poly Žmethyl methacrylate .. By changing the composition of monomer mixture in the surface modification step, the glass transition temperature of the shell could be varied. In Fig. 20, the fracture toughness values are compared among samples with different shell Tg . The best one is found to be the sample with a medium Tg value of 608C while the other two specimens with 108C or 1108C shell are slightly worse. However the difference is only marginal suggesting that the thickness Žaround 15 nm based on particle size measurement. is not thick enough to provide an effect similar to that of rubbery particles. Also in tensile modulus, the specimen with the shell of 608C seems to be the best while the one with the lowest Tg is the worst. This is probably due to the poor mechanical properties
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Fig. 17. Electron micrograph of the fractured surface of the composite samples containing hollow particles. The charge of hollow particles are fixed at 10 wt.% and all of the samples are post-cured at 1008C for 2 h. Ža. Composite using unmodified hollow particle; Žb. composite using hollow particle with 1 wt.% MAAwHŽ60-1.x; Žc. composite using hollow particle with 3 wt.% MAAwHŽ60-3.x; Žd. composite using hollow particle with 5 wt.% MAAwHŽ60-5.x; Že. composite using hollow particle with 7 wt.% MAAwHŽ60-7.x.
of a shell with low Tg or the possible deformation and destruction of the shell during the sample preparation steps. 5. Conclusions It has been shown that dynamic mechanical measurement is a useful tool to characterize the polymeric composites containing various types of dispersed phase formed from structured latexes. In the composites having disperse phases with
island-in-island type of morphology, the height of the tan d peak of the dispersed phase is found to be generally dependent on the effective volume fraction rather than the overall composition. Therefore, the occluded portion inside of the dispersed phase seems to behave as a part of the dispersed domains. When the dispersed domains are composed of core-shell particles via heterocoagulation, the dynamic mechanical response representing the shell phase is observable only if either the shell phase has enough incompatibility
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Fig. 18. Comparison of fracture toughness among epoxy, epoxy with unmodified hollow particle and epoxy with modified hollow particles.
Fig. 19. Comparison of stress-strain behaviour among epoxy, epoxy with unmodified hollow particle and epoxy with modified hollow particles.
toward the core phase material or the shell phase is crosslinked, suggesting that the shell phase cannot maintain its shape during the annealing period if the shell phase possesses a certain degree of compatibility with the core phase. Hollow particles are also examined as toughening agents for the epoxy. The addition of unmodified hollow particle can improve the toughness of the epoxy resin as studied by Bagheri and Pearson w28x. On the other hand, if the carboxyl moiety
Fig. 20. Comparison of fracture toughness among samples containing a modified layer with different Tg .
which can react with epoxy matrix, is incorporated into the surface of the hollow particle, the toughness is further improved. However, such improvement decreases with the increased carboxyl content, suggesting there exists an optimum level of the interfacial bonding. The steep decrease in tensile strain value for epoxy-containing hollow particles suggests that the hollow particles behave as a typical hard filler.
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