FlexiTac: Dual-Network Adhesive

• FlexiTac is a dual-network, solvent-free adhesive that harnesses an ABA triblock copolymer with in situ–polymerized poly(n-butyl acrylate) for tunable mechanical properties.
• It utilizes self-assembled PMMA microdomains as reversible crosslinks, enabling robust adhesion and high stretchability for applications such as soft robotics and wearable electronics.
• Innovative design, scalable UV-curing, and eco-friendly processing make FlexiTac a promising solution for creating mechanically transparent, durable seams in flexible substrates.

FlexiTac is a solvent-free, architected, dual-network adhesive system designed for applications demanding a combination of high strength, stretchability, and efficient, environmentally responsible processing. The formulation draws direct inspiration from natural systems—such as tendons and spider silk—that couple stiff, structural domains with soft, extensible matrices (Lana et al., 2 Jan 2025). FlexiTac uses a self-assembling ABA triblock copolymer scaffold (poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate), PMMA-b-PnBA-b-PMMA) and integrates an in situ–polymerized secondary network of poly(n-butyl acrylate), conferring tailorable mechanical properties and robust adhesion profiles suitable for flexible substrates including stretchable fabrics, soft robotics, flexible electronics, and sports apparel.

1. Chemical Design and Synthesis

FlexiTac’s primary network is based on an ABA triblock copolymer, wherein the A segments are glassy PMMA and the B segment is rubbery PnBA. The typical commercial batch utilizes f_PnBA ≈ 0.70 and f_PMMA (end-blocks) ≈ 0.30, with Mn,total ≈ 66,000 g/mol. Synthesis commonly employs ATRP or RAFT protocols:

• Step 1: PMMA macro–chain transfer agent (CTA) is initiated (AIBN or CuBr/ligand), yielding PMMA–Br (Mn ≈ 20,000 g/mol).
• Step 2: Chain extension with nBA forms PMMA–PnBA–Br.
• Step 3: Final extension with MMA produces the ABA triblock.

The process’s mechanistic shorthand (e.g., RAFT) can be summarized:

$$\text{Initiation:} \quad I \rightarrow 2\,R\cdot \ \text{Propagation:} \quad PMMA\text{–CTA} + R\cdot \rightarrow PMMA\text{–R}\cdot \rightarrow PMMA\text{–PnBA–CTA} \rightarrow \ldots$$

Self-assembly of PMMA end-blocks in the absence of solvent leverages the Flory–Huggins parameter ($\chi_{PMMA-PnBA} \approx 0.04$–$0.06$). PMMA microdomains organize into a spherical/cylindrical morphology, with theoretical domain spacing:

$d \approx aN^{2/3}\chi^{1/6}$

where $a \approx 0.5$ nm and $N$ is the total degree of polymerization. Empirical measurement via SAXS yields $d \approx 19.6$ nm.

2. Physical Crosslinking and Morphology

PMMA microdomains act as physical crosslinks, creating a network that provides mechanical integrity while allowing the PnBA matrix to accommodate strain.

Key Morphological Data:

Crosslinker (EGDMA)	Mean D (nm)	Spacing d (nm)	Morphology
0 mol %	7.0 ± 1.5	14.6 ± 2.1	Uniform spheres
5 mol %	6.4 ± 2.2	17.7 ± 2.8	Broader size distribution
10 mol %	18 ± 3	–	Cylinders + spheres

SAXS profiles show a principal peak at $Q^* = 0.032\,\text{\AA}^{-1}$ (yielding $d = 19.6$ nm), and TEM confirms domain diameters $D \approx 6$–$7$ nm (center-to-center spacing $\approx$ 14–18 nm). According to self-consistent field theory (Matsen-Thompson), $f_{PMMA} = 0.30$ gives a spherical phase with a bridging chain fraction $\phi_{bridge} \approx 75$–$80$\%.

3. Secondary Network Formation

A secondary, covalently crosslinked PnBA network is established in situ by free-radical photopolymerization:

• Monomer: n-butyl acrylate (BA), selective “solvent” at curing.
• Photoinitiator: Irgacure 819 at 1–2 mol % (w.r.t. C=C).
• Crosslinker: ethylene glycol dimethacrylate (EGDMA), 0–10 mol % (w.r.t. BA).
• UV irradiation: 7.5–9 mW/cm² for 5–30 min.

The free-radical reaction scheme:

$$\mathrm{Irg}^\cdot \xrightarrow{h\nu} 2\,\mathrm{R}\cdot \ \mathrm{R}\cdot + \mathrm{CH_2=CHCOOnBu} \longrightarrow \mathrm{R'-CH_2-CH^\cdot-COOnBu} \ \text{EGDMA crosslinking}: \mathrm{CH_2=CHCOOCH_2CH_2OCOCH=CH_2} + \text{growing radical} \rightarrow \text{network node}$$

Kinetics is monitored by FTIR (vinyl peaks at 1619 & 1636 cm⁻¹ disappear after 15 min UV), gravimetry/TGA shows mass loss $\leq 0.5$\% (conversion $>99.5$\%), and GPC confirms $>75$\% triblock retention.

4. Mechanical Characterization

Mechanical properties are tunable by crosslinking density and cure time.

• DMA (Tension mode):
  • Neat triblock: $E' \approx 2.1$ MPa, $E'' \approx 0.48$ MPa @ 1 Hz.
  • Dual-network, no crosslinker: $E' \approx 0.2$–$0.6$ MPa.
  • With 10 mol % EGDMA: $E' \rightarrow 2.8$ MPa, $E'' \rightarrow 0.84$ MPa.
• Uniaxial Tensile Tests:
  • Young’s modulus increases from $0.17$ MPa (0% XL) to $1.18$ MPa (10% XL) in the initial linear regime.
  • Final regime (pre-failure): $0.34 \rightarrow 2.35$ MPa.
  • Affine network fit: $\sigma(\lambda) = G(\lambda - \lambda^{-2})$, with $G = E/3$, $G \approx 0.06$–$0.40$ MPa.

5. Adhesion Performance

Adhesive performance is evaluated on neoprene lap joints via tensile testing.

Joint Type	Failure Stress (MPa)	Strain at Failure (%)
Neoprene Ctrl	3.25	235
Glue only	0.57	80
Glue + Seam	0.94	106
Sewn	0.84	114

“Glue + Seam” matches/semi-exceeds sewn joint strength, with stress–strain curves nearly overlaying pristine neoprene up to $\sim$100\% strain, yielding a “mechanically transparent” seam. Adhesive seams fail via cohesive cracking through the bulk; sewn seams initiate failure at needle punctures, introducing barrier integrity issues.

6. Application Examples

In neoprene composites (70% foam, 30% nylon), FlexiTac penetrates porous substrates pre-cure, locks onto filaments via in situ BA swelling, and cures under mild UV (7.5 mW/cm$^2$, $<30$ min) to produce water-tight, stretchable seams. Notable implementation domains include:

• Soft-robot joints requiring $>200$\% strain durability.
• Wearable e-textiles needing robust sweat/wash barriers.
• Sportswear with hand-feel–matched, mechanically transparent seams.

7. Advantages, Scalability, and Generalization

FlexiTac is solvent-free, eliminating drying time, emissions, and exposure risks. BA monomer serves as a swelling medium and is entirely consumed, supporting eco-compatibility. Tunability is achieved by adjusting EGDMA crosslinker density (0–10 mol\%), enabling $E_{initial} \approx 0.17$–$1.18$ MPa and allowing for graded-stiffness or transparent seams. Primerless application permits delivery via syringe, roller, or A/B mixing gun for roll-to-roll integration, and UV-cure modules can be embedded in industrial lines.

The dual-network paradigm illustrates a reproducible strategy: (1) a block copolymer supplies reversible, physical crosslinks; (2) an in situ–polymerized homopolymer locks the morphology; (3) mechanical properties are governed by crosslink density and block fractions. For FlexiTac, block chemistry and monomers are selected for glass transition, surface energy, and polymerization compatibility, while crosslink density is tuned for modulus, toughness, and adhesion energy.

In summary, FlexiTac embodies the PMMA-b-PnBA-b-PMMA/poly-nBA dual network concept, providing independently tunable physical and chemical crosslinks for scalable, high-strength, and high-stretch applications, with mechanical matching optimized for flexible substrates (Lana et al., 2 Jan 2025).