(324b) Fabrication of Mechanically Activatable Polymer Adhesive Via Kinetically Controlled Crosslinking

Dynamically responsive polymeric materials with on-demand crosslinking capabilities have gained significant attention by virtue of their applications in the field of adhesives, coatings, biomedical engineering, etc. Such systems often rely on external stimuli such as heat or light^1-2, limiting them to specific chemistries and not applicable in dark or protected environments. On the other hand, mechanical triggers (such as stress, shear, ultrasound, etc.) could be used in such environments, yet they are less explored in synthetic materials.³ In contrast, nature demonstrates excellent mechano-responsive properties. Actin cytoskeletal filaments that polymerize under cell strains⁴ and fibrin network that form during blood clotting⁵ are few examples. These systems often contain specific cryptic sites that are sterically shielded within their protein structures. Once the mechanical perturbations are applied, these cryptic sites get exposed and activate a series of biochemical reactions leading to the stiffening in these systems. Inspired by these, few synthetic systems demonstrating force-induced crosslinking have been reported.^6-7

Here, we describe a nature-inspired approach to create a polymer network system that demonstrates the mechano-induced crosslinking (Figure 1). We employed acetoacetoxyethyl methacrylate (AAEM) and aminoethyl methacrylate (AEM) as the cross-linkable monomers because this chemistry is widely used in the industry for a variety of applications due to the resulting networks’ strong mechanical properties, and excellent adhesion characteristics.

Methods.

Polymer Synthesis: All the polymers used in this study were synthesized through thermo-initiated free radical copolymerization using AIBN as the radical initiator in the DMSO solvent. The total monomer concentration used was 2 M. A series of polymers were synthesized by polymerizing 2-hydroxyethyl methacrylate (HEMA) monomer with a reactive crosslinker monomer (AAEM or AEM). The amount of reactive crosslinker monomer was varied between 5 -20 mol%. The crude polymer was purified by precipitating in diethyl ether. For shielded polymers, HEMA was systematically replaced with PEG containing monomer (PEGMA).

Polymer characterization: The chemical structures of synthesized polymers were confirmed through proton NMR spectroscopy, and the ratio of monomers in each polymer was determined by integrating the area under the respective peaks. The polymers’ molecular weight was determined by gel permeation chromatography (GPC) with 2,2,2-trifluoroethanol (TFE) as eluent and carried out on an Agilent 1200 system. The glass-transition temperatures T_g of the polymers were determined by using a differential scanning calorimeter (DSC, TA Instruments, Model Q1000).

Rheological Studies: The crosslinking kinetics and the rheological parameters of the polymer mixture solutions and gel networks were studied by a parallel plate rheometer (AR2000, TA Instruments) using small amplitude oscillatory time sweep test. In the case of shear-induced crosslinking, the large amplitude oscillations were used to induce the crosslinking under shear. For ultra-sound induced crosslinking, the polymer mixture was placed in an ultrasonic bath (Fisher Scientific, FS20D, 40 kHz frequency, 80 W) for 24 h. The polymer mixture was subjected to small amplitude oscillatory time sweep test at different time intervals to determine the kinetics and storage modulus. The gel-point for each crosslinking process was determined by using a time-resolved rheometry analysis.⁸

Results.

Two sets of polymers were synthesized by copolymerizing reactive monomer, AAEM, or AEM with 3-hydroxyethyl methacrylate (HEMA) and were well-characterized (Table 1). To install the mechano-responsive crosslinking, we have systematically replaced the HEMA with poly(ethylene glycol) (PEG)-methacrylate (PEGMA), which act as shielding groups to the reactive crosslinkers (Figure 1c). These side chains either slows down or inhibits interchain crosslinks depending upon their conformations (Figure 1c).

The solutions of the respective polymers (without shielding groups) were mixed to get the crosslinked network as an organogel, which confirms the feasibility of the crosslinking chemistry (Figure 1a, 1b). The storage modulus of the polymer system was studied as a function of crosslinking time on a parallel plate rheometer (Figure 2). When no strain was applied, no appreciable crosslinking was seen for both unshielded polymers and shielded polymers. However, under the shear strain, the unshielded polymers showed tremendous crosslinking with the storage modulus increased to about 5 orders higher than the initial value (black solid). The same for shielded polymer under shear strain only increased to 3 orders of magnitude compared to the initial value (red solid). It is interesting to note that the shielded polymer showed a slower crosslinking kinetics compared to unshielded polymers, which confirms that the crosslinking kinetics could be modulated in presence of PEG shielding groups. These preliminary observations indicate the applicability of this approach to synthesize mechanically activatable polymer adhesives with a long shelf-life.

Interestingly, the crosslinking kinetics could also be altered by ultrasound irradiation (Figure 3). The respective polymers mixture solution was kept in a bath sonicator, and it was periodically subjected to small amplitude time-sweep rheology to monitor the crosslinking. The polymer mixture that was subjected to ultrasound irradiation (green) showed faster-crosslinking kinetics, and its storage modulus (G’) was increased to about 4 orders of magnitude higher compared to the initial value within 3 h. Further, it was evident that the ultrasound irradiation is superior to shear forces for unshielding the PEG groups from the higher G’ of the ultrasound irradiated sample compared to the sheared sample (Figure 3, green and red). However, the G’ of this ultrasound applied polymer mixture slightly decreased after ~ 8 h, probably due to polymer chain scission from continuous ultrasound irradiation.

In summary, we demonstrated a simple and scalable approach to install mechano-sensitive, on-demand crosslinking in polymer systems using an industrially relevant crosslinking chemistry. The approach is based on bio-inspired shielding of crosslinkable reactive groups. The crosslinking could be initiated by both mechanical stress and ultrasound. This study paves the way for the commercialization of new types of coatings and adhesives.

Future work.

The preliminary results suggest that our approach of shielding of reactive crosslinkers with PEG groups is an effective strategy for installing force-responsive crosslinking. We plan to conduct further experiments to establish all the aspects of this approach. We will be quantifying the amount of unreacted crosslinkers as a function of crosslinking time to corroborate it with rheological results. The effects of PEG chain length on shear-induced crosslinking will be studied. The ultra-sound induced crosslinking will be further investigated to determine the optimum conditions to minimize the chain scission and to maximize the crosslinking.

References.

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8 Mours et al. Rheol. Acta 1994, 33 (5), 385-397,