58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Nature elegantly utilizes the interaction between disparate building blocks to generate biological systems with novel material properties. It has been suggested that these observations may offer clues in the design of synthetic composites, which link units of dissimilar mechanical properties. The goal of this two-year New Directions research program was to fabricate biologically-inspired, interpenetrating polymer networks (IPNs) to create biologically-inspired polymeric systems that incorporate synergistic interactions between mechanically-distinct network phases. We propose to assay the role of supramolecular interactions in IPNs utilizing polybutadiene and poly(dimethylsiloxane) (PDMS) oligomeric cores and end-functionalized with 2-ureido-4-[1H]-pyrimidone (UPy) as the supramolecular linkage. These model supramolecular systems will inform our ultimate goal of the design of robust, biomimetic functional IPNs via strategic control of phase interactions.  

(1) Mechanistic understanding of the interplay of covalent and non-covalent interactions in photocurable supramolecular elastomers

The structural interplay between the covalent network afforded through photocrosslinking and the supramolecular network formed via UPy aggregation was the focus of this investigation. Hydroxyl-terminated PB (1.2, 3.0 kg/mol; 20% 1,2-vinyl content) was functionalized with supramolecular UPy end groups (SPB1200; SPB3000) via well-established, high yielding protocols (Scheme 1). For control studies, hydroxyl-terminated poly(ethylene-co-butylene) (PEB3000) was also functionalized with UPy linkages. These supramolecular material systems (SPB1200, SPB3000, SPEB3000) and PB were irradiated at 320-390 nm for 5, 10, 20, 30, 40, 50, 60 and 80 minutes. This chosen range of UV irradiation wavelengths avoids the maximum absorption wavelength of isocytosine group in the UPy aggregates, which is below 300 nm[1]. It is acknowledged that curing may be hindered via photocrosslinking in air, but this represents the most industrially-relevant fabrication approach. To compensate for scavenging of the produced free radicals by environmental oxygen, an excess of photoinitiator was included in the film.

It was determined via spectroscopic studies that curing of the pendant 1,2 vinyl groups (994 cm^-1) within the PB core dominated up to ~50 min curing time beyond which degradation of the main chain 1,4 site (967 cm^-1) (Figure 1). The inset pictures of Figure 1b (SPB1200) and Figure 1d (SPB300) as a function of UV exposure time highlight the degradation that occurs with longer cure times. UV-irradiation was accompanied by an expected increase in the glass transition temperature (T_g) as the crosslink density was increased until ~50 min exposure time; above this cure time, T_gwas observed to decrease due to the onset of dominant degradation. This degradation effect resulted in an increase in hydrophilicity due to the abundance of oxygen-rich surface groups. Acceleration of the cure rate was observed due to the presence of UPy aggregates (confirmed by X-ray studies) in SPB1200 and SPB300 compared to PB1200 and PB3000.

(2) Mechanical response of photocurable supramolecular elastomers 

An understanding of the balance between curing and degradation in this supramolecular material platform is critical to the role of the supramolecular phase interactions in IPNs and has led to the development of mechanically-robust, elastomers. Stress-strain experiments were conducted for SPB1200 and SPB3000 and compared to PB1200 and SPB3000. Interestingly, a range of mechanical behavior was obtained as a function of irradiation time up to the onset of degradation (Figure 2). The UPy functionality enhanced the tensile strength, modulus, and toughness irrespective of cure level of SPB1200 when compared to PB1200. Cured SPB3000 samples displayed lower or comparable extensibility, comparable modulus, and enhanced tensile strength and modulus compared to PB3000. Increasing the cure time promoted the development of the crosslinked network, which hindered the reorganization of the UPy aggregates upon deformation at higher crosslink densities. It is important to note that a higher concentration of UPy groups is incorporated into SPB1200 compared to SPB3000, which led to superior mechanical properties. Our initial probe of the mechanics of these overlaid covalent and supramolecular networks suggests that a 5 min cure time yields the optimal microstructural organization that allows rearrangement of the supramolecular network in response to deformation. Future studies are underway to quantify the contributions to the network morphology.

Figure 2 - Tensile behavior of cured SPB1200 and SPB 3000 compared to PB1200 and PB3000  

(3) Mechanical response of semi-IPN networks

To enhance mechanical properties, such as toughness, modulus, strength, and elongation, a model semi-IPN system was developed. The semi-IPN sample contains 70 wt% of a UV crosslinkable PB3000 (hyrdroxyl terminated polybutadiene; M_n = 3000 g/mol) and 30 wt% of a linear SPEB3000 (supramolecular poly(ethylene-co-butylene); M_n = 3000 g/mol). As expected, the mechanical properties of semin-IPN were increased compared to the UV cured PB3000. In particular, the toughness of the semi-IPN was dramatically increased by two orders of magnitude for 30 min-cured samples in Figure 3a. The modulus, strength, and elongation-at-break of 30 min-cured semi-IPN samples were also increased Figure 3b-d. This significant enhancement (~2x) of mechanical toughness is attributed to the interaction of the chemically-crosslinked and physically-crosslinked supramolecular; a molecular level understanding of this synergistic effect is currently being investigated.

Figure 3 - Comparison of mechanical properties (toughness (A), modulus (B), tensile strength (C), and elongation-at-break (D)) for PB3000 and semi-IPN (PB3000/SPEB3000 70/30 w/w)

(4) Synthetic pathway toward UPy-functionalized PDMS-co-PVMS Synthesis of UPy end-capped PDMS containing pendant vinyl groups was accomplished via ring opening polymerization of cyclic siloxanes (Scheme 2). We are currently working on mechanical characterization of these photocurable supramolecular materials.