Dynamic mechanical and melt rheological properties of sulfonated poly(butylene succinate) ionomers
Introduction
An ionomer is defined as an ion-containing polymer with a small amount (usually up to 10∼15 mol%) of ionic groups along the backbone chains or as pendant groups. Ionomers have been extensively studied because of the significant changes in their physical properties due to the formation of ionic aggregates, such as enhanced mechanical properties, high melt viscosity, and increased thermal properties 1., 2., 3., 4., 5.. Several models, including the core-shell model of MacKnight et al. the hard-sphere liquid-like interference model of Cooper et al. and the Eisenberg–Hird–Moore (EHM) model have been proposed to explain the significant changes in physical properties caused by ionic interactions 6., 7., 8.. They suggested that ion pairs aggregate to form multiplets, which restrict the mobility of the hydrocarbon polymer chain surrounding them. Based on EHM model, Nishida et al. investigated the dynamic mechanical properties with respect to sodium sulfonated poly(styrene–ethylene–butylene) random ionomer and the filler concept was applied to explain the increase in the storage modulus in the ionic plateau region [9]. Using dynamic mechanical analysis, Kim et al. confirmed that polystyrene (PS)- and poly(methyl acrylate) (PMA)-based ionomers have two glass transition temperatures, and found that both matrix Tg and cluster Tg are affected by the size of multiplets and the nature of ionic groups 10., 11.. Despite these studies, little is known about the dynamic mechanical or rheological properties of semi-crystalline polyester ionomers: i.e. how ionic aggregates affect dynamic mechanical and melt rheological properties.
In the present study, we prepared PBS ionomer by two-step polycondensation to extend the fields of application of biodegradable polymers. Copolymerization and blending are important methods for the improvement of the mechanical properties of polymers. However, there are several problems affecting the physicochemical properties of polymers prepared by these methods. For instance, aliphatic copolyesters contain 10–30 mol% of secondary monomers to reduce the crystallinity and thereby enhance their biodegradation rate and ductility. However, aliphatic copolyesters of these compositions show significant melting point depression, due to imperfect lateral packing and/or isomorphism, adversely affecting the temperature range over which they can be used 12., 13.. Previously, we analyzed poly(butylene adipate) ionomers that were synthesized to resolve these problems of parent poly(butylene adipate), and confirmed that the incorporation of ionic groups into the main chain led to a reduction of crystallinity without a striking depression of the melting point [14]. Therefore, we expected that introduction of ionic groups into the PBS main chains might not only lead to improved extensibility due to the effective reduction of crystallinity but also provide crucial information of melt rheological property due to the ionic interactions. Here, we report the syntheses, dynamic mechanical properties, and melt rheological properties of PBSi prepared in the presence of dimethyl 5-sulfoisophthalate sodium salt (DMSI) as a secondary monomer.
Section snippets
Materials and polymerization
Succinic acid, 1,4-butanediol, and DMSI (Aldrich) were used without further purification. Titanium tetrabutoxide (Aldrich) was also used as received. PBS and its ionomers were prepared by two-step polycondensation [15]. First, calculated amounts of succinic acid and 1,4-butane diol (1:1.2 by mole ratio) were introduced into a reactor equipped with an overhead stirrer and a temperature control device. The monomer mixture was melted and stirred at 20 °C for 2 h, evolving water condensate. Then,
Synthesis of PBSi
PBS and PBSi were synthesized by a two-step process consisted of direct esterification and polycondensation. DMSI was restrictively incorporated up to 5 mol% because DMSI has poor solubility in the reaction mixture. Fig. 1 shows a representative 1H NMR spectrum of PBSi-3. The complete disappearance of methyl hydrogens in DMSI at 3.95 ppm confirmed that ionic monomers were covalently incorporated into the PBSi chains. The integration ratio between the peak at 4.45 ppm arising from methylene
Conclusions
PBSis were successfully synthesized in the presence of DMSI by two-step melt polymerization. The DMA data and SEM images provide evidence for cluster formation. The rates of increase in matrix Tg and cluster Tg were slightly lower than those of amorphous polymer-based ionomers, and these PBSis showed marked differences in matrix Tg and cluster Tg, which was attributable to the lower level of clustering. The clusters of PBSi-3 in the bulk phase measured about 40 nm, demonstrating that
Acknowledgements
This work was supported by the Brain Korea 21 Project, the Basic Research program of the Korea Science and Engineering Foundation (grant No. R01-1999-000-00194-0) and the Korea–Japan Basic Scientific Promotion Program (grant No. F01-2001-000-20038-0).
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