CORE Metadata, citation and similar papers at core.ac.uk Provided by Publications of the IAS Fellows Colloid Polym Sci (2002) 280: 886–891 DOI 10.1007/s00396-002-0697-9 ORIGINAL CONTRIBUTION G. Baskar Synthesis and solution properties of comblike S. Ramya A.B. Mandal polymers from octadecyl methacrylate and acrylic acid Abstract Amphiphilic polymers ing of 83 mol% ODMA and Received: 23 October 2001 Accepted: 8 March 2002 consisting of a statistical distribution 17 mol% acrylic acid behaves as an Published online: 22 May 2002 of octadecyl methacrylate (ODMA) ionomer, capable of forming col- Ó Springer-Verlag 2002 and acrylic acid in respective molar lapsed-coil structures at concentra- ratios of 83–22 and 17–78 mol% tions of 0.02 gml–1 and above as and in a molecular-weight range of shown by a very high I3/I1 of 1.20 4 –1 2.35–4.70·10 gmol have been (I3/I1 of pyrene in THF is 0.85). In synthesized. The series of polymers contrast, the poly(octadecyl meth- consisting of various mole fractions acrylate) homopolymer and the sets of ODMA and acrylic acid are ex- of copolymers consisting of a very pected to exhibit unique character- high proportion of acrylic acid to an istics resembling ionomer to extent of 73 mol% and above con- hydrophobically modified polyelec- tribute to almost negligible or very trolytes. The changes in the I3/I1 small changes in I3/I1 similar to the G. Baskar (&) emission intensity ratios of pyrene, homopolymer, poly(octadecyl Industrial Chemistry Division, Central Leather Research Institute, occurring in the presence of tetra- methacrylate), suggesting the for- Adyar, Chennai 600 020, India hydrofuran (THF) solutions of the mation of random-coil structures. E-mail: [email protected] polymers have been taken as the Tel.: +91-44-4911386/4911108 main basis for inferring solution Fax: +91-44-4911589 structures. The polymers are found Keywords Comblike polymers Æ S. Ramya Æ A.B. Mandal to form random-coil to collapsed- Copolymer of octadecyl methacry- Chemical Laboratory, coil/aggregated structures in THF late and acrylic acid Æ Solution Inorganic and Physical Chemistry Division, Central Leather Research Institute, solvent depending on the copolymer properties in tetrahydrofuran Æ Adyar, Chennai 600 020, India compositions. The polymer consist- Fluorescence Introduction formation of versatile mesophase structures. Different types of polymers, block copolymers [7, 8, 9, 10], stars, Amphiphilic polymers composed of hydrophilic and graft copolymers [11, 12, 13, 14, 15], segmented block hydrophobic segments a exhibit characteristic tendency copolymers [16, 17] and polysoaps [18, 19, 20, 21, 22, 23] to undergo self-organization processes in the presence of contributing to the versatile molecular architecture with solvents, forming superstructures on a mesomorphic distinct subgroups provide classic examples of polymeric scale [1, 2, 3, 4, 5, 6]. Such amphiphilic polymers prove amphiphiles. The kinds of aggregated structures exhib- more advantageous than simple surfactants in view of ited by a polymeric amphiphile, and arising from various the diversity in the arrangements of the hydrophobic kinds of forces, depend on the amphiphilic architecture and hydrophilic segments, the length and branching of and the polarity of the solvent. Polymeric amphiphiles, the hydrophobic tail, the molecular architecture afforded especially those consisting of ionizable comonomers, by varying the polymer geometry and, thus, enabling the exhibit interesting solution properties. The solution 887 structures of these polymeric amphiphiles are controlled derived from acrylic acid. It has been shown that poly- by various parameters, such as the ionic content of the meric gels consisting of a very low amount of the C18 polymers, the polarity of the solvent and the degree of side chain is capable of forming hydrophobic aggregates ionization. These structures provide a molecular basis over a wide pH range, in contrast to similar polymeric for understanding the kinds of forces controlling intra- systems consisting of side chains varying from C8 to C12 molecular and intermolecular association processes. [27]. The polymeric amphiphiles consisting of a C18 side Furthermore, they are also highly useful in under- chain thus become highly significant in view of the standing some significant phenomena, such as the or- promotion of hydrophobic forces, in addition to crys- dering of lipids and the regulation of protein talline properties. Recently, such kinds of polymers association/dissociation arising as a consequence of hy- consisting of acrylic/methacrylic acid have been suc- drophobic association inside biomembranes [24]. It is cessfully used in miniemulsion polymerization of styrene well known that polymers consisting of 15 mol% ionic [28]. In this study, we report the synthesis of four sets of comonomer or less are typically classified as ionomers. polymeric amphiphiles consisting of octadecyl methac- On the other hand, polyelectrolytes consisting of mainly rylate (C18 side chain) and acrylic acid in mole percent ionic monomer residues, when hydrophobically modi- ratio of 83:17, 45:55, 27:73 and 22:78 in a molecular- fied (2–5%) exhibit unique rheological properties and weight range 1–4·104 gmol–1. These polymers do not find immense potential as thickening agents. Typically, exhibit solubility in an aqueous medium, especially at an ionomer is capable of forming structures ranging low pH. We chose to investigate the solution behavior of from ionic clusters to hydrophobic aggregates similar to these polymers in a nonpolar solvent, namely tetra- micellar assemblies, depending on the solubility of the hydrofuran (THF) (dielectric constant, 7.5). THF is a backbone in the solvent under investigation and the very common solvent in molecular-weight estimations polarity of the solvent. This has been well demonstrated using gel permeation chromatography (GPC) and also in the solution behavior of many polymers, for example, different application systems. The solution structure of polyurethane-derived ionomers [25]. Whereas in the case these sets of copolymers were investigated in comparison of hydrophobically modified polyelectroytes, similar to that of poly(octadecyl methacrylate) homopolymer of aggregated structure formation occurs, depending on the similar molecular weight in order to investigate the in- nature of the solvent, as in case of ionomers, and the fluence of acrylic acid comonomer on the solution interplay between electrostatic interaction and the hy- structure. Various techniques, such as fluorescence drophobic aggregation process mainly controls the so- spectroscopy, viscosity, light scattering and neutron lution structures. In this context, it becomes highly scattering, are generally employed to investigate the significant to investigate the solution structure of a set of solution behavior of polymeric amphiphiles. The fluo- polymeric amphiphiles consisting of various composi- rescence spectroscopy technique proves highly useful in tions of hydrophobic and hydrophilic comonomers in investigations on the solution behavior of polymers. It is the chosen solvent. Such a set of polymers would cover a well known that the use of changes in the fluorescence range of polymeric materials varying from ionomers to behavior of an external fluoroprobe, like pyrene, in the hydrophobically modified polyelectrolytes. Such studies presence of polymer solutions can throw light on the are expected to throw useful light on the control of so- microenvironment of polymer solutions [29]. In this lution structures by the manipulation of ionic comono- study, the changes in the emission characteristics of a mer compositions. fluoroprobe, like pyrene, in THF solutions of the iono- Polymeric amphiphiles consisting of long side-chain mers investigated were taken as the main evidence for and ionic monomers are currently drawing significant probable changes in the microenvironment of polymer attention in view of their unique features. The long side- solutions arising as a consequence of specific organiza- chain group provides surfactant-like structures capable tion processes. of forming polysoap structures similar to micellar as- semblies and contributes significantly to surface-energy modifications. The ionic comonomer, of course, controls Experimental electrostatic interactions. Kuo et al. [26] reported the synthesis of a low-molecular-weight copolymer of Materials dodecyl methacrylate and acrylic acid. The polymer Octadecyl methacrylate, azobis(isobutyronitrile) (AIBN), both exhibits high polydispersity in its molecular weight and from Aldrich, USA, and ethyl methyl ketone (EMK) from Sisco has been demonstrated to undergo typical aggregation Laboratories, Mumbai, India, were used as received. The acrylic processes in water at high pH. The copolymer has been acid (Sisco Laboratories) used in the polymerization reactions was successfully used in emulsion polymerization of styrene. purified by distillation. The high-performance liquid chromatog- raphy (HPLC) grade THF used in the fluorescence experiments was The influence of the length of the side chain on the as- from S.D. Fine Chemicals, India. The solvents, petroleum ether sociation properties, especially in an aqueous medium, and methanol employed in the isolation of the polymers were of has been well demonstrated using polymeric amphiphiles Analar grades and were obtained from Glaxo, Mumbai, India. 888 Synthesis of the ionomers The set of polymers consisting of a statistical distribution of non- polar octadecyl methacrylate and polar