No abstract

Ionomers are polymers containing a low mole fraction of ionic groups bound to the polymer backbone. These ionic groups produce major changes in their structure and mechanical properties. Recently, we introduced a new family of crosslinked poly(Bd)/poly(Bd-co-MAA) core shell nanoparticles (1,3butadiene and methacrylic acid) that could be ionically crosslinked and cast as nanostructured ionomer films from aqueous dispersions [Pinprayoon et al., Soft Matter, 2011, 7, 247]. The MAA units in the core-shell particles were neutralised by Zn 2+ . Here, we explore the structure-property relationships for these new architecturally controlled nanocomposites by investigating 6 new poly(Bd)/poly (Bd-co-MAA) dispersions and films. In this study we varied the extent of covalent crosslinking in the core and the shell at constant ionic crosslinking for the first time. We used dynamic mechanical thermal analysis to establish a general phase map for the new nanostructured ionomers. Stress-strain data show that our nanostructured films have well controlled, and adjustable, modulus and strain at break values. The data show that the core-shell nanoparticle geometry allows the often observed trade-off between elasticity and ductility to be tuned in a manner that is not possible for conventional ionomers. We show that the chain transfer agent (CTA) concentrations used during the preparation of the nanoparticle cores and shells can be used to independently tune the mechanical properties of the films. This is due to variation of the extents of covalent crosslinking. The results of this study should apply to other covalently crosslinked core-shell nanoparticles containing RCOOH groups in the particle shells.

Ionomers are polymers which contain ionic groups that are covalently bound to the main chain. The presence of a small percentage of ionic groups strongly affects the polymer's mechanical properties. Here, we examine a new family of nanostructured ionomer films prepared from core-shell polymer nanoparticles containing acrylonitrile (AN), 1,3-butadiene (Bd) and methacrylic acid (MAA). Three new AN-containing dispersions were investigated in this study. The core-shell nanoparticles contained a PBd core. The shells contained copolymerised Bd, AN and MAA, i.e., PBd-AN-MAA. Three types of crosslinking were present in these films: covalent crosslinks (from Bd); strong physical crosslinks (involving ionic bonding of RCOO(-) and Zn(2+)) and weaker physical crosslinks (from AN). We examined and compared the roles of AN and ionic crosslinking (from added Zn(2+)) on the structure and mechanical properties of the films. The FTIR spectroscopy data showed evidence for RCOOH-nitrile hydrogen bonding with tetrahedral geometry. DMTA studies showed that AN copolymerised within the PBd-AN-MAA phase uniformly. Tensile stress-strain data showed that inclusion of AN increased elasticity and toughness. Analysis showed that about 33 AN groups were required to provide an elastically-effective chain. However, only 1.5 to 2 ionically bonded RCOO(-) groups were required to generate an elastically-effective chain. By contrast to ionic bonding, AN inclusion increased the modulus without compromising ductility. Our results show that AN is an attractive, versatile, monomer for increasing the toughness of nanostructured ionomers and this should also be the case for other nanostructured polymer elastomers.

Vinyl-functionalised pH-responsive hollow particles have been prepared for the first time and used to construct physical and doubly crosslinked (DX) hydrogels. The pH-responsive DX hydrogels do not redisperse, have a high storage modulus with interconnected porosity and should be useful for regenerative medicine applications. Hollow particles continue to intrigue researchers 1-7 and have a number of potential applications including intracellular delivery, 3 drug delivery 4,8,9 and feedback active coatings. 10 Recently, hollow particles have shown good promise for cartilage regeneration. 11 Although the hollow particles provided a beneficial morphology, 11 the dispersions did not form gels and would have limited potential to support load immediately after injection. Injectable gels that support load and also possess high internal porosity have excellent potential for regenerative medicine applications. Hydrogels are widely studied in the context of tissue engineering. 12,13 Unfortunately, it is difficult to control their porosity and its interconnectivity at the micrometre length scale. Recently, we showed that physical gels can be prepared using pH-responsive hollow particles. 14,15 The particle shells were crosslinked using reversible disulfides bonds. The physical gels were not permanent (no covalent interlinkages between particles) and redispersed if placed in water. This severely limits their potential applications. One possible method to achieve durable hydrogels is to covalently interlink swollen microgel particles. 16 However, those systems lacked interconnected porosity, which is essential for many regenerative medicine applications. In the present study we show for the first time that vinyl-functionalised pH-responsive hollow particles can be covalently interlinked to give injectable pH-responsive hydrogels with interconnected micrometre-scale porosity. The starting point for our new hydrogels is collapsed hollow particle dispersions of non-crosslinked poly(MMA-co-MAA). MMA and MAA are methylmethacrylate and methacrylic acid. The dispersion was prepared using a method established earlier 15-see Scheme S1 (ESI †). The number-average molecular weight and polydispersity for poly(MMA-co-MAA) were 27 300 g mol À1 and 1.9. The hollow particles form because the amphiphilic copolymer phase separates at the CH 2 Cl 2 /water interface. 14,15 The poly(MMAco-MAA) particles dissolve when the pH is increased to the pK a unless the shells are crosslinked. 15 Previously, we used cystamine for that purpose. Here, we functionalised the particles with aminoethyl methacrylate (AEM) and prepared the first examples of pHresponsive hollow particles crosslinked with vinyl groups, i.e., poly(MMA-co-MAA)/AEM particles (Step 1, Scheme 1). Vinyl functional groups provide a more robust source of crosslinking (cf. cystamine 15). (The methods used here are described in the ESI †.) The crosslinking first occurred within the shells (intra-shell crosslinking) and these particles are termed singly crosslinked (SX) particles. The SX partic...

Heteroaggregation of dispersions has attracted much interest in the literature, especially when one or more components are stimulus responsive. Here, we study binary mixtures of microgels (MG) and starlike copolymers for the first time. The study investigated the use of complementary hydrogen bonding between carboxylic acid and amide groups to construct heteroaggregates and gels that contained temperature-and pH-responsive components. The pH-responsive MG contained methacrylic-acid and had an apparent pK a of 8.2. Two new star-like copolymers were introduced which comprised a cationic backbone with poly(N-isopropylacrylamide) side-chains. They are abbreviated as M1-PNP. A combination of complementary hydrogen bonding and hydrophobic interactions was shown to cause formation of heteroaggregates for mixed MG/M1-PNP dispersions at room temperature and at pH values less than the MG pK a . MG/M1-PNP heteroaggregate formation occurred over a wide pH-range and also in the presence of 0.2 M NaNO 3 . The heteroaggregates exhibited temperature-dependent hydrodynamic diameters and zeta potentials. Concentrated MG/M1-PNP dispersions formed selfsupporting hybrid gels at 45 C and gel formation also occurred over a wide pH range. The gels contained 80% MG with respect to total polymer content and were remarkably ductile. They had yield strains greater than or equal to 290%. There was evidence that the elasticity and ductility of the hybrid gels were controlled by the MG and M1-PNP components, respectively. The new M1-PNP star-like copolymers introduced here had superior temperature-triggered gel-formation properties compared to related copolymers and should be a versatile system for conferring temperature-responsive gelation properties to polymer colloids containing carboxylic acid groups.