Title: Compatibilization of Immiscible Polymer Blends Using Polymer-Grafted Nanoparticles
Description: Recycling is one of the most important strategies for combating plastic pollution. However, before plastic waste can be converted into other items, the different types of plastic present in it must be sorted, a time-consuming and expensive process. Indeed, it is often more cost-effective to manufacture new plastic materials than to recycle existing plastic waste. Researchers are therefore attempting to eliminate the sorting process altogether and directly recycle the mixed plastic waste. While this would lead to phase-separated mixtures with temporally evolving domains and poor fracture toughness properties, these problems could be mitigated to some extent by incorporating surfactant-like macromolecular compatibilizers, such as block copolymers or random copolymers (RCPs). These compatibilizers preferentially localize at polymer/polymer interfaces, lowering droplet coalescence and interfacial tension in the process. Moreover, the macromolecular structure of these compatibilizers enables them to form entanglement networks across the interface, thus enhancing stress transfer and fracture toughness.

Nanoparticle (NP)-based compatibilizers have recently attracted attention due to their significantly stronger suppression of droplet coalescence under certain conditions. Unfortunately, while these compatibilizers work relatively well in oil/water emulsions, they perform poorly in immiscible polymer blends. This is because most polymer blends consist of hydrophobic components, making the NPs gravitate toward one of the bulk phases rather than the interface. Moreover, their rigid cores function as stress concentrators in polymer matrices, causing further deterioration to the fracture toughness properties of the blend. In this dissertation, we construct hybrid compatibilizers consisting of NP cores and outer grafted polymer layers. In this manner, the desired features of both macromolecules and NPs are combined into a single compatibilizer: the NP cores suppress droplet coalescence, while the polymer grafts direct the NPs to the interface and form entanglements. We investigate the effectiveness of these hybrid compatibilizers in three critical areas: NP localization control, droplet coalescence suppression, and fracture toughness enhancement.

In each area, we perform systematic studies using an immiscible polymer blend composed of poly(methyl methacrylate) (PMMA) and polystyrene (PS) in order to find the optimal compatibilizing effect as a function of graft chemistry, graft molecular weight, and grafting density. We demonstrate that the most efficient hybrid compatibilizers are those with a surfactant-like architecture. For example, silica NPs sparsely grafted with PS chains can form a dense monolayer packing at the immiscible PMMA/PS interface. In this example, surfactancy is derived from a balance of enthalpic interactions: the silica core strongly interacts with the PMMA phase, while the PS grafted layer mixes intimately with the PS phase. The hydrophilic–lipophilic balance is readily controlled by varying the contact area of each interaction through the grafting density or the graft molecular weight. Similarly, we show that silica NPs grafted with surfactant-like polymer chains, such as styrene–methyl methacrylate RCPs, can also localize at the PMMA/PS interface.

Here, surfactancy is derived mainly from the RCP grafts. There are two advantages to using RCP grafts. First, it allows for interfacial localization even if the grafted layer completely encapsulates the silica core (i.e., at high grafting densities). Second, RCP grafts can entangle on both sides of the interface and thus transmit stress more efficiently than PS grafts, which only entangle on the PS side of the interface. There are two advantages to using this latter approach. First, RCP grafts can entangle on both sides of the interface and thus transmit stress more efficiently than PS grafts, which only entangle on the PS side of the interface. Second, it allows for interfacial localization even if the grafted layer completely encapsulates the silica core (i.e., at high grafting densities). Our research shows that both forms of hybrid compatibilizers significantly outperform conventional ungrafted macromolecular compatibilizers in droplet coalescence suppression. Interestingly, coalescence can be suppressed even when the hybrid compatibilizers only partially cover the dispersed droplets.

We believe that this is due to the grafted layers forming strong entanglement networks around the droplets that function as barriers to coalescence. Linear rheology experiments corroborate this reasoning: the low-frequency storage moduli of the compatibilized blends approach a plateau when the NP grafting density is increased, suggesting the presence of a network structure at the interface. For fracture toughness experiments, we employ RCP-grafted NPs to exploit their entanglement on both sides of the interface. We show that when a moderate grafting density is used, the fracture toughness of the PMMA/PS interfaces exceeds that of the interfaces compatibilized with ungrafted RCP analogs. This again results from the brush entanglement network at the interface. Specifically, in the moderate grafting density zone, RCP brushes form a more connected entanglement network than ungrafted RCPs and are thus more efficient at transmitting stress across the interface. In summary, we have developed a method for accurately controlling the localization of NP-based compatibilizers in immiscible polymer blends.

We have also identified the grafting conditions under which these hybrid compatibilizers outperform conventional macromolecular compatibilizers in both droplet coalescence suppression and fracture toughness enhancement.