Molecular Design Guidelines for the Fabrication of Polymer Hybrid Materials with ‘Self-Heal Ability’

 

The concept of self-healing polymers has generated considerable interest in recent years as it provides the opportunity to design materials with significantly extended lifespan by allowing them to recover from structural damage and resume their functionality. However, current approaches to self-healable polymers, can be complicated and limit economics and scalability. A novel concept based on Van der Waals-driven self-healing in copolymers with ‘lock-and-key’ architecture has emerged to endow engineering-type polymers with the capacity to recover from structural damage. Complicating the realization of ‘lock-and-key’-enabled self-healing is the tendency of copolymers to form nonuniform molecular architectures during polymerization reactions. This limits favorable site interactions and renders the evaluation of van der Waals-driven healing difficult.

Firstly, in Chapter 4 to 6, methods for the synthesis of lock-and-key copolymers with prescribed architecture were developed to overcome this limitation and enable the deliberate synthesis of ‘lock- and-key’ architectures most conducive to self-healing. The effect of chain architecture on the material’s recovery behavior was also evaluated for three poly(n-butyl acrylate/methyl methacrylate) [P(BA/MMA)] copolymers with different architecture: alternating (alt), statistical (stat), and gradient (grad). Copolymers with alt and stat architecture displayed a ten-fold increase of recovery rate compared to the grad copolymer variant despite of a similar overall glass transition temperature. Alternating copolymers provide the most rapid recovery, while at the cost of weaker mechanical properties and a more expensive synthesis. Considering the competing parameters, our study suggests that statistical copolymers might be considered as viable option to facilitate van der Waals-driven self-healing.

However, the mechanical properties of statistical copolymers are not yet qualified for engineering materials use, and the performance at extreme conditions is also limited. Tethering of polymer chains to the surface of nanoparticles to form ‘brush particles’ (aka ‘hairy particles’) could offer a solution. Brush particles has been shown to provide one-component hybrid materials with improved properties. Recent advances in the surface-initiated controlled radical polymerization (SI-CRP) significantly expanded the control over structural parameters of surface-bound polymer ligands, such as degree of polymerization, dispersity, or grafting density. This enabled the synthesis of brush particle systems with structure and dynamical properties that are tunable across the range of soft (polymer-like) to hard (particle-like) interactions, providing opportunities for the design of hybrid materials with novel functionalities.

Based on these advancements, Chapter 7 to 8 provided a strategy to extend the lock-and-key approach to silica nanoparticle systems using surface-initiated atom transfer radical polymerization (SI-ATRP). The effect of surface polymerization on the mechanical and recovery behavior will be discussed. The topological constraints in brush particle solids will be shown to result in unexpected property combinations, such as ‘heal and recall’ behavior, i.e. the combination of self-healing and shape memory functionality that indicates new opportunities for BA/MMA based brush particle solids.

Despite the recent progress made in the field of SH polymers, a major challenge remains the realization of SH capability in polymers with sufficiently high modulus (> 1 GPa) to be suitable for engineering applications. This is because the mechanism of SH relies on sufficient chain mobility to enable structure reformation. Chapter 9 to 10 explored a new templating approach to realize high modulus polymer hybrid materials with SH ability. The approach comprises the dissolution of linear butyl acrylate/methyl methacrylate (BA/MMA) copolymer filler with SH ability within a BA/MMA copolymer brush particle template structure. The selective increase of the MMA content of the brush P(BA-s-MMA) enables the formation of a rigid (high Tg) matrix while maintaining the solubility of the linear P(BA-s-MMA) (low Tg) filler within the interstitial regions of the brush template. The mobility of the linear SH copolymer across the interstitial regions gives rise to the self-heal ability of the rigid hybrid. This unique combination of stiffness and self-healing capability provides opportunities for applications such as self-healing coatings and photonic materials.

Additionally, a recently published project focusing on synthesis of high molecular weight polyisoprene grafted silica brush particles (Chapter 11) is included in appendices.