Letter pubs.acs.org/macroletters
Effect of Methoxy Substituent Position on Thermal Properties and Solvent Resistance of Lignin-Inspired Poly(dimethoxyphenyl methacrylate)s Shu Wang,† Alexander W. Bassett,§ George V. Wieber,† Joseph F. Stanzione, III,§ and Thomas H. Epps, III*,†,‡ †
Department of Chemical & Biomolecular Engineering and ‡Department of Materials Science & Engineering, University of Delaware, Newark, Delaware 19716, United States § Department of Chemical Engineering, Rowan University, Glassboro, New Jersey 08028, United States S Supporting Information *
ABSTRACT: The macromolecular properties, including the glass transition temperature (Tg) and solvent resistance, of lignin-inspired poly(dimethoxyphenyl methacrylate)s were controlled by varying the position of the dimethoxy substituents in the constituent monomers. For the four dimethoxyphenyl methacrylate isomers investigated, with substituents at different locations on the phenyl ring (i.e., 3,5-; 2,3-; 2,4-; and 2,6-), the Tg’s of the resulting polymers spanned a wide range from less than 100 °C to greater than 200 °C. Rotational freedom and segmental interactions were responsible for the varying Tg’s. The polymers were thermally stable in air up to ∼300 °C, providing a suitable thermal processing window. The poly(dimethoxyphenol methacrylate) homopolymers also exhibited remarkably different solvent resistances to organic solvents, including tetrahydrofuran and chloroform. Furthermore, by copolymerizing various dimethoxyphenyl methacrylate isomers, the Tg and solvent resistance of the resulting macromolecules could be tuned independently to gain enhanced control over materials design. The ability to manipulate properties through isomer composition in this lignin-inspired system may facilitate usage in applications such as components in coatings, thermoplastics, thermoplastic elastomers, and other materials.
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interactions between ester and methyl groups, and the sp2hybridized interactions between phenyl rings.16−18 Tg’s from 300 °C) in comparison to alkyl- and aldehyde-substituted poly(phenyl methacrylate)s.10,21 This enhancement in thermal stability 804
DOI: 10.1021/acsmacrolett.7b00381 ACS Macro Lett. 2017, 6, 802−807
Letter
ACS Macro Letters
P(2,4D) and P(2,3D) was intermediate to the above two polymers. Although the Tg of P(2,3D) was ∼3 °C lower than the Tg of P(2,4D), P(2,3D) was more resistant to both THF and CHCl3 than P(2,4D), with the difference in THF being the more significant of the two. It is understood that the solvent swelling of polymers is dependent on a number of factors including polarity, dielectric permeability, basicity, and molar volume of the solvent; electrophilicity of the polymer; and the solubility parameter difference between the polymer and solvent.38−40 For swelling in halogenated hydrocarbons and ethers, the electrophilicity of the polymer is known to play an important role.38 m-Methoxy moieties are electron-withdrawing groups, whereas p-methoxy moieties are electron-donating groups.41,42 This factor likely influenced the relative affinities of P(2,4D) and P(2,3D) to THF and CHCl3, with P(2,4D) demonstrating a greater degree of swelling. The solvent resistance of the P(2,6D-co-2,4D) and P(2,6D-co-3,5D) copolymers also was measured and compared with that of the homopolymers (Figure 3). The solvent resistance of the copolymers was lower than P(2,6D) but higher than P(2,4D) and P(3,5D), in accordance with the trend of Tg. Moreover, it is worth noting that the statistical copolymers P(2,6D-co-2,4D) and P(2,6D-co-3,5D) had nearly identical solvent resistance to P(2,3D) in THF despite the copolymers having almost 50 °C higher Tg’s, whereas P(2,3D) had much greater solvent resistance than P(2,4D) in THF even though the two polymers had very similar Tg’s. These comparisons indicate that the Tg and the solvent resistance of poly(dimethoxyphenyl methacrylate)s can be tuned independently through variations in dimethoxy isomer content. In summary, the position (ortho, para, and meta) of the dimethoxy moieties on the phenyl rings of lignin-inspired dimethoxyphenyl methacrylate isomers strongly impacted the Tg and solvent resistance of the resulting polymers. The rotational freedom as well as the segmental interactions were the major contributing factors for wide-ranging Tg’s. Tg’s from less than 100 °C to greater than 200 °C were accessible from the poly(dimethoxyphenyl methacrylate) homopolymers and statistical copolymers. Additionally, all poly(dimethoxyphenyl methacrylate)s exhibited excellent thermal stability up to ∼300 °C in air [more stable than methyl-, ethyl-, and aldehyde-substituted poly(phenyl methacrylate)s], and the effect of the substituent location on thermal degradation of the dimethoxy-based polymers was minimal. Both the Tg and electrophilicity of the monomers appeared to play significant roles in the solvent resistance behavior, affecting the diffusion of solvent molecules in the polymer films and affinity of the polymers to the solvents. Furthermore, the Tg and solvent resistance of this family of poly(dimethoxyphenyl methacrylate)s can be tuned independently. Thus, this study provides a robust foundation toward the design and development of lignin-inspired poly(dimethoxyphenyl methacrylate)s as potential components in coatings, thermoplastics, thermoplastic elastomers, and other applications.
likely was a result of the relatively high dissociation energy of the CH3−OC6H3 bond.34 In addition to thermal properties, solvent resistance is an important parameter in the design of polymeric materials. The solvent resistance of the lignin-inspired polymers was probed by solvent vapor annealing (SVA). Complete details on the SVA procedure can be found in the literature,35 and additional information is located in the SI. Briefly, polymer thin films were cast on ultraviolet ozone-treated silicon wafers, and the films then were exposed to solvent vapor in a flow chamber. The solvent vapor swelled the film, leading to an increase in film thickness.36 Herein, the least amount of swelling correlated to the best solvent resistance. Two commonly employed solvents, THF and CHCl3, were probed. In SVA experiments with both solvents, two solvent vapor concentrations (partial pressures) were used; solvent partial pressures were controlled by adjusting the relative flow rate of a solvent-rich vapor stream to an N2 diluent stream prior to introduction into the SVA chamber (details are located in the SI). To avoid start-up effects associated with glassy polymers, the SVA experiments were initiated at a high solvent concentration to ensure maximum plasticization,35 and all experiments were run until film thickness equilibrated. For each solvent, the initial vapor concentration was chosen so as not to dewet the films. The swelling data in the form of change in polymer volume fraction (ϕ, defined as the ratio of the initial film thickness to the solvent-swollen film thickness) are presented in Figure 3.
Figure 3. Change in polymer volume fraction during the SVA of polymer films (a) in THF at an average laboratory temperature of 25.5 °C and (b) in CHCl3 at an average laboratory temperature of 26.5 °C. p/psat is the ratio of the partial pressure of solvent in the chamber to its saturated partial pressure (using Antoine equation37). Note: all samples at a given solvent partial pressure were run simultaneously (i.e., in the same SVA chamber at the same time) to reduce experimental variability.
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ASSOCIATED CONTENT
S Supporting Information *
P(2,6D) had the best resistance, while P(3,5D) had the worst resistance with respect to the two solvents studied. P(2,6D) had the highest Tg and the largest diffusion limitations of the solvent molecules in the films. On the contrary, P(3,5D) had the lowest Tg and the smallest barrier to solvent diffusion and ultimate swelling. The solvent resistance to THF and CHCl3 of
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00381. Materials and experimental methods, polymerization information and polymer characteristics, Tg of all 805
DOI: 10.1021/acsmacrolett.7b00381 ACS Macro Lett. 2017, 6, 802−807
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ACS Macro Letters
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polymers and Tg,∞ for all homopolymers, and DSC thermograms for all homopolymers and statistical copolymers (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shu Wang: 0000-0002-1917-1796 Thomas H. Epps III: 0000-0002-2513-0966 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NSF grant CHE-1507010 to T.H.E. The University of Delaware (UD) NMR facility was supported by the Delaware COBRE program with a grant from NIH NIGMS (1 P30 GM110758-01). The authors thank the UD Advanced Materials Characterization Laboratory for use of DSC and TGA instruments. The authors also acknowledge Dr. Jillian A. Emerson for assistance in the film casting and SVA experiments and Dr. Shuang Liu for assistance with size exclusion chromatography experiments.
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