Polymer Grafting of Unsaturated Polymers to Provide New Material Properties: A New Avenue for Polymer Upcycling

Abstract

Polymer materials are prevalent in society for a number of reasons such as a wide range of properties that have found uses in a many applications. Furthermore, polymer materials are much cheaper to synthesize and process into materials as compared to inorganic materials. However, the issues associated with plastic pollution are an ever-growing problem. To this end, to combat the current global waste disposal issues, new solutions are necessary to convert plastic waste into desirable and useful materials. Post-polymerization functionalization is an important set of chemistry techniques to modify as-synthesized polymers to find uses in different applications. The primary scope of this dissertation work is to investigate methods of post-polymerization functionalization of unsaturated polymers, utilizing their allylic carbons, with the goal of finding new and effective methods of valorization for commercially available polymers.

This dissertation starts with two studies on using in-situ polymerization to graft poly(styrene) (PS) from the midblock of poly(styrene-butadiene-styrene) (SBS). The highlighted chemistry uses benzoyl peroxide as an allylic hydrogen abstractor, forming an allylic radical on the backbone of SBS, which then polymerizes styrene monomer. For the samples in this study, the SBS was dissolved in styrene monomer at various volume fractions of SBS (φ_SBS), ranging from 50% SBS / 50% styrene monomer (φ_SBS = 50%) down to 10% SBS / 90% styrene monomer (φ_SBS = 10%). It was first shown that after the polymerization, a nanostructural transition was observed from disordered spheres of the neat SBS to lamellar and coexisting lamellar/disordered spheres after grafting. The samples were then subjected to tensile analysis where it was found that all grafted samples outperformed the neat SBS, with the sample with an initial volume fraction of SBS being 30% (φ_SBS = 30%) achieving the highest tensile strength and yield strength. It was hypothesized that these changes in mechanical properties were due to polymer grafting and not the increase of PS wt% within the samples.

The PS grafted SBS samples were then further analyzed via in situ small-angle X-ray scattering (SAXS) during uniaxial extension. For the neat SBS, there was minimal change in the SAXS pattern during deformation until ~140% strain, at which the sample began to disorder until failure. It is hypothesized that the SBS, with an initial disordered spheres morphology, experiences a simple chain pull-out mechanism during deformation, which leads it to failure. For φ_SBS = 30%, which initially had a lamellar morphology, the deformation was more complex. It was determined that during strain, the randomly oriented lamellae underwent a preferred deformation – the lamellae grains oriented parallel to the strain direction experienced a decrease in domain spacing while the grains oriented perpendicular experienced higher domain spacings. At a certain point, the perpendicularly oriented lamellar are torn apart, leading to formation of micro-fibrils, similar to poly(ethylene). Furthermore, it is believed that due to the polymer grafting, the PS grafts act as anchor points which help to strengthen the material from chain pull-out, despite the nanostructure being lamellar. 

Due to the limitations of the in-situ polymerization chemistry, such as being unable to control any aspect of the polymer grafting (graft density and graft M_n) as well as crosslinking of the final product due to the free radical chemistry, a new method of allylic bromination and atom-transfer radical polymerization (ATRP) was developed. This method offers a new way to control graft density and graft M_n. In this study, three polymers were investigated: poly(cyclooctadiene) (PCOD), poly(butadiene), and SBS. It was shown in this study that the graft density can effectively be tuned by changing the molar ratio of N-bromosuccinimide to the unsaturated polymer. Furthermore, it was also shown that the graft M_n was tuned by changing the reaction time, where longer times allowed for higher M_n values. This chemistry is being tested for uses such as changing mechanical properties of polymers as well as the surface chemistry properties of films. 

Overall, the methods shown in this dissertation provide insight into various methods of post-polymerization functionalization to repurpose commercially available unsaturated polymers for potential valorization.