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Effects of Filler Particle/Elastomer Distribution and Interaction on Composite Mechanical Properties Liliane Bokobza,* Gilles Garnaud, and James E. Mark† Laboratoire Physico-Chimie Structurale et Macromole´ culaire, Ecole Supe´ rieure de Physique et de Chimie Industrielles de la Ville de Paris, 10, Rue Vauquelin, 75231 Paris Cedex 05, France
Jagdish M. Jethmalani, Edward E. Seabolt, and Warren T. Ford Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received May 9, 2001. Revised Manuscript Received October 2, 2001
Some new characterization results are reported for composites prepared from methyl acrylate monomer and from reinforcing silica particles at various degrees of dispersion. In some cases, 3-(trimethoxysilyl)propyl methacrylate groups were grafted onto the silica (PMA) through participation in the methyl acrylate polymerization used to form an elastomeric PMA matrix. In some cases, the usual random dispersion of the silica particles was “aged” or converted into regular arrays within the monomer prior to its polymerization. As an alternative, placing chloropropyltrimethoxysilane groups on the particle surfaces was used to obtain random arrangements in which the strong bonding between the particles and elastomer was suppressed. In another approach, the particle dispersion was first dried and then blended into the monomer before its polymerization, thereby giving an aggregated arrangement. These various composites were characterized with regard to their mechanical properties in elongation (using techniques allowing a close appproach to elastic equilibrium), and with regard to chain orientation (using birefringence measurements and infrared spectroscopy). The elastomers having randomly dispersed and regularly dispersed silica dispersions were very similar in mechanical properties and chain orientation, but extensibility was significantly improved by decreasing the strength of the particle-elastomer bonding. Additional improvements in extensibility, and associated increases in toughness, were obtained when these same particles were aggregated.
Introduction The materials that are the focus of this study are some novel polymer-silica composites that were first studied because of their unusual optical properties. The first series of such composites consists of poly(methyl acrylate) (PMA) composites reinforced with 3-(trimethoxysilyl)propyl methacrylate coated particles of silica (TPM-silica).1-3 This methacrylate acted as a bonding agent, in that its methacrylate functionalities were used for bonding with poly(methyl acrylate) through participation in the methyl acrylate polymerization used to form an elastomeric matrix. In addition, the trimethoxysilyl groups were hydrolyzed in a sol-gel reaction that attaches them to the surfaces of the amorphous silica particles, which are nearly monodisperse in size (approximately 150 nm diameter). If the system was “aged”, as described below, then the resulting composite * Corresponding author. Telephone: +33 1 40 79 45 15. Fax: +33 1 40 79 46 86. E-mail:
[email protected]. † Permanent address: Department of Chemistry and the Polymer Research Center, The University of Cincinnati, Cincinnati, OH 452210172. E-mail adresses:
[email protected], jemcom.crs.uc.edu. (1) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1994, 6, 362. (2) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. Hybrid OrganicInorganic Composites; Mark, J. E., Lee, C. Y-C., Bianconi, P. A., Eds; American Chemical Society, Washington, DC, 1995. (3) Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1996, 8, 2138.
Scheme 1. Sketch of the Structure of the PMA/TPM-Si Composites6
materials exhibited iridescence and other novel optical properties, due to Bragg diffraction of visible light by the colloidal “crystalline” arrays. The regular particle arrangements obtained were stable, since the methacrylate groups of the TPM were chemically bonded to the polymeric matrix, as shown in Scheme 1.6 Most of the characterization work on these materials have involved their optical properties.3 Mechanical properties thus far have been limited to nonequilibrium measurements in elongation, on an Instron tester, and some biaxial measurements by the inflation of thin sheets of the materials.4-6 Other properties remain largely unknown, although there has been some char(4) Pu, Z.; Mark, J. E.; Jethmalani, J. M.; Ford, W. T. Polym. Bull. 1996, 37, 545.
10.1021/cm010462r CCC: $22.00 © 2002 American Chemical Society Published on Web 11/28/2001
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acterization work on properties such as scattering intensities on related systems.7-9 The present study provides some additional results on these systems, and extends the coverage to some related composites containing silica (CP-silica), in which chloropropyltrimethoxysilane replaces the 3-(trimethoxysilyl)propyl methacrylate in order to suppress the strong bonding between the silica and the polymer host matrix.10,11 The particle-bonded type of composite should give some additional insight into the long-standing question in the rubber industry on the repercussions of carrying out compounding to the extreme of dispersing reinforcing filler as completely as possible.12-16 One complication in the usual compounding situation is the fact that the shearing stresses sufficient to break up particle aggregation are also sufficient to cleave some of the elastomeric chains.12-16 Also of interest in this regard, of course, is the effect of filler-particle interactions on mechanical properties, in particular maximum extensibility. The extents of orientation of the chains during deformation is of considerable basic interest as well and, in the case of crystallizable elastomers, is of considerable practical importance since it leads to strain-induced crystallization and the tremendous reversible reinforcement it provides in an elastomer. One novelty of the present study is to go beyond the usual stress-strain measurements17,18 to gain new insights into these PMA composites, through characterization of chain orientation by state-of-the-art birefringence and infrared dichroism measurements. The newness of some of these techniques requires some of the background information presented below. The orientation of polymer chains under strain may be conveniently described by the second Legendre polynomial:19-22
1 〈P2(cos θ)〉 ) (3〈cos2θ〉 - 1) 2
(1)
where θ is the angle between the macroscopic reference axis (usually taken as the direction of strain) and the local chain axis of the polymer. The orientational be(5) Pu, Z. Ph.D. Thesis in Chemistry, The University of Cincinnati 1997. (6) Pu, Z.; Mark, J. E.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1997, 9, 2442. (7) Coltrain, B. K.; Landry, C. J. T.; O’Reilly, J. M.; Chamberlain, A. M.; Rakes, G. A.; Sedita; J. S., Kelts, L. W.; Landry, M. R.; Long, V. K. Chem. Mater. 1993, 5, 1445. (8) Jethmalani, J. M.; Sunkara, H. B.; Ford, W. T.; Willoughby, S. L.; Ackerson, B. J. Langmuir 1997, 13, 2633. (9) Jethmalani, J. M.; Ford, W. T.; Beaucage, G. Langmuir 1997, 13, 3338. (10) Rajan, G. Ph.D. Thesis in Chemistry, The University of Cincinnati, 2001. (11) Rajan, G.; Mark, J. E.; Seabolt, E. E.; Ford, W. T. J. Macromol. Sci., Pure Appl. Chem, in press. (12) Boonstra, B. E. In Rubber Technology; Morton, M., Ed.; Van Nostrand Reinhold: New York, 1973; p 51. (13) Boonstra, B. B. Polymer 1979, 20, 691. (14) Warrick, E. L.; Pierce, O. R.; Polmanteer, K. E.; Saam, J. C. Rubber Chem. Technol. 1979, 52, 437. (15) Rigbi, Z. Adv. Polym. Sci. 1980, 36, 21. (16) Medalia, A. I.; Kraus, G. In Science and Technology of Rubber, 2nd ed.; Mark, J. E., Erman, B., Eirich, F. R., Eds.; Academic: New York, 1994; p 387. (17) Mark, J. E.; Erman, B. Rubberlike Elasticity. A Molecular Primer; John Wiley & Sons: New York, 1988. (18) Erman, B.; Mark, J. E. Structures and Properties of Rubberlike Networks; Oxford University Press: New York, 1997.
havior can be described by birefringence, directly related to the second Legendre polynomial by the expression19,20
∆n ) [∆n]0〈P2(cos θ)〉
(2)
and by the closely related technique of infrared dichroism as well. According to the theory, the birefringence is related to the strain function by the expression23,24
∆n )
νkTC P(R2 - R-1) ) D1P(R2 - R-1) V
(3)
Here, ν/V represents the number of chains per unit volume, P is a factor equal to 1 for an affine network but is (1 - 2/φ) for a phantom network (φ being the junction functionality) and C is the stress-optical coefficient.18 In infrared spectroscopy, measurements of chain orientation can be performed by measuring the dichroic ratio R (R ) A|/A⊥) where A| and A⊥ are the absorbances of the investigated infrared band measured with radiation polarized parallel and perpendicular to the stretching axis, respectively. The orientation function is related to the dichroic ratio R by the expression
〈P2(cos θ)〉 )
(R - 1) 2 × ) (R + 2) (3 cos β - 1) D0P(R2 - R-1) (4) 2
where β is the angle between the transition moment vector of the vibrational mode considered and the local chain axis of the polymer and D0 is the configurational factor, which is shown to be inversely proportional to the number n of bonds in the chain between two junctions.25 One can express the orientation of the transition moment vector, 〈P2(cos γ)〉, with respect to the direction of stretch, by the expression
〈P2(cos γ)〉 )
(R - 1) (R + 2)
(5)
The quantity (R - 1)/(R + 2) is called the dichroic function. Thus, the present investigation was carried out to provide clarification of these issues, by preparing the PMA colloidal silica systems in such a way as to obtain particle dispersions that were either random, regular, or aggregated, with filler-elastomer interactions that were either very strong or quite weak. Of particular interest were the mechanical properties of these various types of filled elastomers and the degrees of chain orientation under deformation as gauged by birefringence measurements and infrared spectroscopy. Experimental Section Synthesis of the PMA/TPM)Silica and Its Dispersions in the PMA. Details of the preparation of TPM-coated silica particles and their use in ordered composite PMA films are (19) Bokobza, L.; Lapra, A. J. Polym. Sci., Polym. Phys. 2000, 38, 2449. (20) Bokobza, L. J. Polym. Int. 2000, 49, 743. (21) Amram, B.; Bokobza, L.; Queslel, J. P.; Monnerie, L. Polymer 1986, 27, 877. (22) Amram, B.; Bokobza, L.; Monnerie, L.; Queslel, J. P. Polymer 1988, 29, 1155. (23) Erman, B.; Flory, P. J. Macromolecules 1983, 16, 1601. (24) Erman, B.; Flory, P. J. Macromolecules 1983, 16, 1607. (25) Besbes, S.; Cermelli, I.; Bokobza, L.; Monnerie, L.: Bahar, I.; Erman, B.; Herz, J. Macromolecules 1992, 25, 1949.
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reported elsewhere.1-3 In the present investigation, the particle concentrations investigated were 35 and 40 wt %, and were obtained by diluting a master batch of 48 wt % dispersion with more methyl acrylate (MA) monomer. Each of the MA-silica dispersions3 was mixed with 0.25 wt % of the photoinitiator 2,2-dimethoxy-2-phenylacetophenone and was then poured into a sandwiched glass cell. Two of these cells had dimensions of 2.5 cm × 7.5 cm × 264 µm and two others 5.0 cm × 7.5 cm × 396 µm. Two of the mixtures were immediately photopolymerized, but two of the other cells were placed horizontally in an oven maintained at 25 °C for a period of 6-8 h.3 The two mixtures which had been immediately polymerized gave composites with randomly distributed particles, while the two placed in the oven had sufficient time for the particles to form highly regular colloidal crystals.3 In both cases, the photopolymerization of the monomer and the methacrylate groups on the TPM-silica gave permanence to the arrangements obtained. To form the aggregated TPM-silica, concentrated methanolic dispersions of 151 nm TPM-silica particles were dried at room temperature, followed by overnight vacuum-drying at 25 °C. The resulting aggregated particles were then redispersed in MA to form a concentrated dispersion of 53 wt % particles. As before, the desired particle concentrations of 35 and 40 wt % were obtained by diluting the 53 wt % dispersion with more MA. These mixtures were immediately photopolymerized, as described above. The densities required, for example for calculating volume fractions, were 1.22 g cm- 3 for the PMA, 2.05 g cm- 3 for the uncoated silica particles, and 1.80 g cm-3 for the TPM-silica. Electron microscope pictures demonstrating the structural features of these composites are shown elsewhere.6 Synthesis of the PMA/CP)Silica and Its Dispersions in the PMA. The coatings on these silica particles were obtained from chloropropyltrimethoxysilane,10,11 and these coated particles thus lack the methacrylate groups used in the TPM to bond the particles to the PMA.3,6 The desired samples of chloropropylsilica were prepared from colloidal silica particles which had an average diameter of 155 nm (according to dynamic light scattering measurements) and 3-chloropropyltrimethoxysilane (which had been distilled under vacuum). The methods employed were those reported for the preparation of TPM-silica from trimethoxypropylsilyl methacrylate.3 Chemical analysis of the dried particles gave the following results. Anal. Found: C, 9.91; H, 2.70; N,
