Thiol-functionalized nanogels as reactive plasticizers for crosslinked polymer networks

Abstract

Significant efforts have been expended to mitigate plasticizer migration from crosslinked methacrylic and poly(vinyl chloride) polymer networks by synthesizing reactive plasticizers that can blend homogenously within the networks to reduce polymer property change, acute toxicity and downstream environmental effects of plasticizer migration with limited and varying amount of success. We hypothesized that appropriate thiol-functionalized nanogels synthesized using the same monomers as the parent network to generate highly compact, crosslinked structures will form thermally stable, homogenous networks and perform as optimal reactive plasticizers. Nanogels were synthesized via a thiol-Michael addition solution polymerization and incorporated at different mass ratios within a polyethylene glycol 400 urethane dimethacrylic monomer to form photo-crosslinked networks. While maintaining the inherent hydrolytic stability, thermal stability and biocompatibility of the parent matrix at ~99% acrylic group conversion, the PEG400 urethane dimethacrylic -nanogel networks retained optical clarity with >90% visible light transmission at 20wt% nanogel concentration within the matrix. The addition of the nanogels also enhanced the elongation of the parent matrix by up to 320%, while a 37°C reduction in glass transition temperature (∆T_g) and ≥50% reduction in modulus was observed. A 52% reduction in the shrinkage stress of the material was also noted. The results indicate that the application of thiol-functionalized nanogels as plasticizers to alter the bulk properties of the parent matrix while mitigating plasticizer migration by covalently crosslinking the nanogels within the polymer matrix provides a simple yet efficient technique to generate network-specific plasticizers with the ability to alter targeted properties within polymers.

Keywords: Crosslinked networks; Nanogels; Phthalate mimics; Plasticizers; Thiols.

Fig 1

The glass transition temperature (T_g) of the of E9NG films at different nanogel mass fraction estimated from the peak of the tan delta curve shows a decrease in the T_g as a function of nanogel loading when compared to the E9 parent matrix (A). As the nanogel content within the parent matrix increases, an increase in the elongation of the material is also observed (B).

Fig 2

The decrease in glass transition temperature (T_g) of the E9NG formulations as a function of increasing viscosity can be used as a formulation-aid to estimate the T_g vs Viscosity measurements in designing polymer networks.

Fig 3

(A) The ability of nanogels to alter hydrophilicity of the parent polymer network is shown as a measure of the equilibrium water content (EWC) measured as a function of nanogel mass fractions(B). The contact angle of a water drop measured on the surface of the polymer films with different nanogel mass fractions indicate an increased surface hydrophilicity at 20 % NG loading.

Fig 4

Refractive Index (RI) of polymer networks remain unchanged at different nanogel mass fractions. No significant changes in RI as a function of nanogel content is observed in the wet and dry RI of E9NG matrix (A). Light transmittance of the E9NG polymer network at different nanogel mass fractions follow a similar trend indicating that the bulk network maintains its translucency (B).

Fig 5

Thermogravimetric analysis of the E9NG networks indicate that the thermal stability of the parent E9 networks was largely unaltered up to 200 °C.

Fig 6

The hydrolytic stability tests of the E9NG networks in DI water at 70 °C for 28 days captures the enhanced stability of plasticized E9NG networks.

Fig 7

Cell morphology after 48 h incubation with (A) media only (control), (B) E9 film (C) E9NG-10 film and (D) E9NG-20 film. The spindle morphology of L929 mouse fibroblast cells are retained after incubating with E9NG at different nanogel mass fraction, indicating the cytocompatibility of these materials.

Scheme 1

Thiol-functionalized nanogels are synthesized and dispersed within the PEG400 UDMA matrix and subsequently photopolymerized. Polymer networks generated from different mass fractions of nanogels (0, 5, 10, 20and 30 %) within the PEG400 UDMA matrix are characterized to study the plasticizing effect of the nanogels.

Scheme 2

Schematic representing the monomers used in this study and the synthesis of the thiol-functionalized nanogels via thiol-methacrylate Michael-addition reaction using three methacrylate monomers, PEG400 UDMA, GMA, and UDMA in the molar ratio of 30:30:40. The step-growth reaction with a thiol to methacrylate functional group ratio of 1.5:1 was catalyzed by a mixture of trimethylamine and triphenylphosphene in a solution polymerization reaction with 4× excess of solvent to yield thiol-functionalized nanogels.