(83d) Effect of Backbone and End-Group Regioisomerism on Thermomechanical Properties of Vanillin-Based Polyurethane Networks

Thermosetting networks are widely used due to their excellent thermomechanical properties and stability that results from the presence of permanent covalent cross-links. Studies have shown that the molecular architecture of the monomers and cross-linkers, in addition to their functionality, can strongly affect the mechanical properties of the resulting network. One of the modes by which monomer and cross-linker architecture affect material properties is by influencing crosslink density. Cross-link density and distribution can vary depending on the functionality and geometry of the monomer and cross-linker and are one of the primary determinants of material property. Unlike small molecules, modulation of the spacing, regioselectivity, and sterics of a multifunctional unimolecular oligomer (i.e., a macromer) is non-trivial and often requires extensive synthetic manipulations. Due to these synthetic challenges, studies into how macromer structure affects network connectivity and material properties are lacking. Addressing this question requires studying network formation with well-defined and easily accessible functional macromers. Sequence-defined macromers fill a void that exists between precise small molecule monomers and large homo- and copolymers. When assembled from functional monomers, these well-defined unimolecular macromers can serve as versatile building blocks for a wide range of polymeric materials and networks.

Recently, our research group developed a robust platform for the synthesis of sequence-defined oligocarbamates (SeDOCs) using a sustainable feedstock, vanillin, and high-yielding reactions. The SeDOCs have modifiable pendant groups and a rigid aromatic vanillin-based backbone. Previous work done in the research group shows the importance of sequence on the properties of cross-linked networks. This work inspired further investigations, carried out in this study, where we investigate the effect of regioisomeric backbone and end-group substitutions on network topology and material properties. In this study, we show that the SeDOC platform can be used to create and tune network properties by varying backbone structure and end-group composition. To study how changes in backbone and end-group structure affect network topology and properties, four macromers (two pairs of regioisomers) were synthesized and cross-linked using multivalent thiols. Each oligomer is named according to its monomeric units, where “A” represents an allyl pendant group, “m” represents an isovanillin backbone unit (1,3 connectivity) and “p” represents a vanillin backbone unit (1,4 connectivity). The end-groups used to cap the macromers were guaiacol (G), pentanol (P), and neopentanol (N). The four macromers that were studied were AmAG, ApAG, ApAN, and ApAP. The AmAG and ApAG macromers are regioisomers in their backbone, while ApAN and ApAP are regioisomers at their end-group.

The 1H NMR spectra of the AmAG and ApAG regioisomers shared similar spectral features with notable differences in the splitting patterns of the benzylic protons and the allylic protons. This hinted at differences in conformation or electronic environment. Using 1H–13C HSQC technique, we confirmed the presence of eight methylene protons (four benzylic and four allylic) for both macromers. The four benzylic protons in ApAG were separated both in the 13C and 1 H spectra, indicating that the two benzyl groups in the macromer were in different environments. However, the benzylic protons in AmAG were separated only in the 13C spectra suggesting that the two benzyl positions in the macromer were in identical chemical environments. This observation speaks to how a single positional change in the macromer backbone can alter the molecular electronic environment. Variable temperature NMR (VT-NMR) was also used to confirm the structural dynamics of the macromers and the results concurred with the conclusions from the HSQC spectra.

To study the effect of regioisomerism on the physical properties of the macromers, viscosity measurements were performed using a parallel plate rheometer. The measured viscosities were compared to motor oil and honey in order to provide a physical frame of reference using common household items. The viscosities of the SeDOCs with rigid aromatic guaiacol end groups are comparable to honey and the viscosities of SeDOCs with aliphatic end groups are comparable to motor oil. The viscosity clearly decreases with increasing end-group rotational flexibility, i.e., going from a rigid guaiacol cap in AmAG and ApAG to aliphatic end caps in ApAN and ApAP. Flexible chains in the molecular structure increase the available free volume in the fluid, which results in enhanced mobility and diminished resistance to flow. Interestingly, the subtle change in the backbone structure from a 1,3 substitution in AmAG to a 1,4 substitution in ApAG led to a ∼40% decrease in the viscosity.

To understand the effect of composition and regioisomerism on the thermal and mechanical properties of thermosetting networks, the allyl groups in the SeDOCs were cross-linked with a tetravalent and trivalent thiol via thiol–ene reactions. The size of the crosslinkers were kept small, such that their chain flexibility would not mask the effect of SeDOC structure. The four SeDOCs and two cross-linkers were used to prepare eight unique thioether linked networks. The gel fraction results indicate that the extent of cross-linking is similar (>90%) for all eight networks. We also examined the initial rate of cross-linking across all networks using a model kinetics experiment with a monothiol such that –ene conversion could be measured via 1H NMR. The kinetics data shows similar reaction kinetics across all SeDOCs. This indicates that the rates and extent of cross-linking are not affected by the SeDOC structure. It follows then that any differences in the network properties can be attributed to monomer structure and composition.

The fractional free volume (FFV_h), an empirical parameter that characterizes static hole volume in polymers in the glassy state, was measured for all eight cross-linked networks at −10 °C. In the glassy state, higher static hole volumes are associated with polymer networks that are unable to pack efficiently due to rigid or bulky groups with constrained centers or shapes. The data showed that the AmAG SeDOC networks had the highest FFV_h in both the tetrathiol and trithiol cases. As indicated earlier, high FFV_h values are associated with polymer networks that are unable to pack efficiently due to rigid or bulky groups with constrained centers or shapes. This suggests that materials with high FFV_h values have a high barrier to bond rotation, which is required for polymer unwinding and segmental motion of chains. Thus, our results suggest that the AmAG network is geometrically constrained in the glassy state relative to the other SeDOC networks, and this should translate to higher glass transition temperature (T_g).

The T_g of the cross-linked networks were evaluated by differential scanning calorimetry (DSC). Overall, the data showed higher T_g’s for networks made with the tetrathiol cross-linker relative to those made with the trithiol cross-linker. We also see a general trend of decreasing T_g with increasing end-group flexibility as we go from AmAG and ApAG with rigid aromatic end-groups to ApAN and ApAP with flexible aliphatic end-groups. The data showed that the AmAG networks, made with both tetrathiol and trithiol, had the highest T_g values. This observation was consistent with the conclusions from the FFV_h data. In addition to DSC measurements, T_g was also determined from DMA studies via tan δ plots. Similar trends seen in the DSC data were also recorded in the tan δ plots. T_g decreases were observed with increasing end-group flexibility. The difference in the T_g of the networks made with AmAG relative to ApAG (~10^oC) could be due to the higher molecular rigidity or constrained shape of the AmAG macromer within the network. This effect is more pronounced in the tetrathiol cross-linked networks than in the trithiol networks. We posit that the additional connectivity in the tetrathiol amplifies the geometric constraints of the macromers in the networks. Above T_g, all networks had similar storage moduli values in the rubbery regime. At room temperature, the storage modulus largely follows similar trends to the T_g and viscosity measurements. The FFV_h values of the networks are correlated with the measured T_g. Networks with higher T_g values comprise chains that are composed of structurally “constrained” moieties, as observed in the viscosity data. This molecular constraint results in the accumulation of larger static hole volumes in the glassy state, hence high FFV_h. Polymeric networks that are unable to pack efficiently due to these molecular constraints, for the same reason, have a high barrier to bond rotation, which is required for polymer unwinding and segmental motion of chains. Thus, networks with poor packing efficiencies due to geometric constraints have a high FFV_h and high T_g relative to others. Overall, our results, when paired with the gel fraction and kinetics data, support the conclusion that the observed differences in thermomechanical properties are related to the impact of SeDOC structure (i.e. regioisomerism). This response in material properties to the subtle change in backbone connectivity could be used to alter the properties of commercial resins, e.g., tuning the mechanical, self-healing, and adhesive properties of phenylene diamine-based epoxy resins. Armed with the versatility of the SeDOC platform, the next step beyond backbone conformation and terminal group composition is to combine these elements along with sequence to generate multifunctional macromers with multiple handles that can be used to further tune material properties.