Structural Characterization of Glassy and Rubbery Model Anionic

Chapter 21

Downloaded by UNIV OF MISSOURI COLUMBIA on November 26, 2013 | http://pubs.acs.org Publication Date: September 19, 2008 | doi: 10.1021/bk-2008-0996.ch021

Structural Characterization of Glassy and Rubbery Model Anionic Amphiphilic Polymer Conetworks 1

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Gergely Kali1,2, Theoni K. Georgiou , Béla Iván , Costas S. Patrickios , Elena Loizou , Yi Thomann , and Joerg C. Tiller 1,*

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Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus Department of Polymer Chemistry and Material Science, Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Pusztaszeri út 59-67, Hungary Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899 Freiburg Materials Research Center and Institute for Macromolecular Chemistry, Department of Chemistry, University of Freiburg, D-79104 Freiburg, Stefan-Meier-Strasse 21, Germany 2

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The structure of two series of model amphiphilic polymer conetworks based on the hydrophilic anionic methacrylic acid was characterized by atomic force microscopy (AFM) and small-angle neutron scattering (SANS). In the first series, the hydrophobic component was the glassy poly(methyl meth acrylate), while the rubbery poly(2-butyl-1-octyl methacrylate) constituted the hydrophobic segments in the second series. Each series comprised conetworks in which the hydrophile / hydrophobe ratio was systematically varied as well as conetworks with different architecture of the linear chain: ABA and BAB triblock and statistical. The A F M and SANS measurements indicated nanophase separation in the triblock copolymer-based conetworks and provided the spacing, size and shape of the formed nanodomains.

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© 2008 American Chemical Society

In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Introduction Amphiphilic polymer conetworks (APCNs) possess unique properties that are useful for different and important applications (1,2). These properties include the ability to interact and swell in polar and nonpolar solvents, good mechanical strength (compared to hydrogels), and the ability for nanophase separation (1-15). Several characterization methods have been used to investigate the nanophase separation in these materials: small-angle x-ray scattering (SAXS) (5-8), small-angle neutron scattering (SANS) (9,10), transmission electron microscopy (TEM) (77), solid state NMR (8,12), and atomic force microscopy (AFM) (5-7,11,13,14). However, there are not many literature reports where more than one of these methods were used to investigate nanophase separation and to give combined structural information. In this report, we study two series of model anionic APCNs and we extensively investigate their swelling and structural properties. Methacrylic acid (MAA), a hydrophilic, negatively ionizable monomer, was the common component in the two series. In the first series, the simple, commercially available and inexpensive methyl methacrylate (MMA) constituted the hydrophobic component. However, PMMA is glassy (tough), and likely to result in fragile materials with non-equilibrium frozen structures. In contrast, the hydrophobic component in the second series was the non-commercially available 2-butyl-l-octyl methacrylate (BOMA), whose polymers are rubbery (soft) and likely to lead to APCNs that are mechanically more robust and present easily accessible equilibrium morphologies.

Experimental Copolymer Synthesis All the conetworks in this study were synthesized by group transfer polymerization (GTP) (16). The hydrophilic monomer MAA was introduced via the polymerization of tetrahydropyranyl methacrylate (THPMA, in-house synthesized from dihydropyran and MAA), followed by the removal of the protecting tetrahydropyranyl group by acidic hydrolysis after conetwork formation. From the two hydrophobic monomers used in this study, M M A was commercially available, while BOMA was in-house synthesized from the corresponding alcohol and methacryloyl chloride. l,4-Bis(methoxytrimethylsiloxymethylene)cyclohexane (MTSMC, in-house synthesized) and ethylene glycol dimethacrylate (EGDMA) were used as the initiator and the cross-linker for the conetwork formation, respectively. Figure 1 shows the chemical structures and names of the repeating units of all monomers and the EGDMA cross-linker.



0

|

3

J

n

CH | 3

u

c=o o

poly(tetrahydropyranyl methacrylate) (PTHPMA)

poly(methacrylic acid) (PMAA)

2

CH |

3

2

2

H,

poly(ethylene glycol dimethaciylate) (PEGDMA)

II

o = -cC — C +-

1

0

CH

I

1 CH

0

c - c —

H

Figure 1. Chemical structures and names of the monomer and cross-linker repeating units.

poly (2-buty 1-1 -octy 1 methacrylate) (PBOMA)

1 CH poly (methyl methacrylate) (PMMA)

"-

2

•C-j—

H


00 00

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The conetworks were synthesized by sequential monomer / cross-linker additions (17-20). APCNs of different compositions were obtained by varying the comonomer feed ratio, while APCNs of different architectures were obtained by varying the order of reagents addition (monomers, cross-linker or initiator (10)). Before cross-linking, the linear copolymer precursors were sampled and were characterized in terms of their molecular weight (MW) and composition using gel permeation chromatography (GPC) and *H NMR spectroscopy, respectively. Figure 2 represents a typical structure of a model APCN.

Figure 2. A probable structure of a typical model conetwork. The grey and black colors represent hydrophilic and hydrophobic chains, while the EGDMA cross-linkers are shown as black circles. The number of arms at the cross-links is around 30.

Dynamic Mechanical Analysis (DMA) Investigations on the mechanical properties of hydrolyzed and uncharged conetworks were carried out using a Tritec2000 Triton Technologies dynamic mechanical analyzer. The measurements were performed in the compression mode at a single frequency of 1 Hz. The experiments were carried out at 25 °C and, during the measurements, the samples were immersed in water (pH ~ 8).

Measurements of the Degree of Swelling (DS) The hydrolyzed conetworks were cut into small pieces and dried under vacuum for three days at room temperature. The dry conetwork mass was determined gravimetrically, followed by the transfer of the conetworks in water. Twelve samples were allowed to equilibrate for three weeks in milli-Q



290 (deionized) water, covering a pH range between 2 and 13 and the whole range of the degree of ionization (DI) of the MAA units. The DSs were calculated as the ratio of the swollen conetwork mass divided by the dry conetwork mass. After the measurements of the DSs in water as a function.of pH, the water-swollen samples were dried in vacuum at room temperature for three days. A volume of 5 mL THF was transferred into the glass vials containing the dried conetwork samples, which were allowed to equilibrate again for three weeks. The THFswollen mass of each conetwork was determined gravimetrically, from which the DS in THF was calculated.

Small-Angle Neutron Scattering (SANS) All the (hydrolyzed) conetworks of this study were characterized using SANS in D 0 . The samples were in the uncharged state (pH ~ 8). SANS measurements were performed on the 30 m NG7 instrument at the Center for Neutron Research of the National Institute of Standards and Technology (NIST). The incident wavelength was X = 6 A. Three sample-to-detector distances, 1.00, 4.00 and 15.30 m, were employed, covering a grange [q is the scattering vector, with q = AnIX sin(0/2), 6 is the scattering angle] from 0.003 A" to 0.60 A" . The samples were loaded in 1 mm gap thickness quartz cells. The scattering patterns were isotropic, and, therefore, the measured counts were circularly averaged. The averaged data were corrected for empty cell and background. The distance between the scattering centers was estimated from the position of the intensity maximum,

Structural Characterization of Glassy and Rubbery Model Anionic

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