Synthesis of poly(n-alkyl acrylamides) and evaluation of nanophase separation effects by temperature-dependent infrared spectroscopy

Common linear polymers are known to undergo phase changes at the glass-transition temperature (Tg) and the melting point (Tm). In recent years, it has also been shown that molecules with long aliphatic side chains can give rise to a backbone-independent melting phenomenon, known as nanophase separation. This effect describes the self-assembly — independent of the polymer backbone — of alkyl side chains into semi-crystalline nanostructures. This work presents optimized, gram scale synthesis routes for dodecyl and octadecyl acrylamide and their respective homopolymers. Differential scanning calorimetry (DSC) experiments detected a broad endothermal signal for poly(n-dodecyl acrylamide) at − 29 °C and a narrower, more intense signal for poly(n-octadecyl acrylamide) at 34 °C. These signals indicate the nanophase separation TM of the alkyl side chains. We undertook the first temperature-controlled infrared spectroscopy investigations of these materials revealing a clear hypsochromic shift of the C–H stretching signals above TM and the amide I signal shifts that occurred only above and below Tg. These results provide further evidence, that the side chains act independently of the polymer backbone and show that infrared spectroscopy is a powerful tool for monitoring conformational changes in polymer side chains.

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New Pressure-Sensitive Acrylic Adhesives for Low Energy Substrates Prepared via UV-Induced Telomerization with a Fluorine-Based Telogen

Novel pressure-sensitive adhesives (PSA) for low energy substrates were prepared by a solvent-free UV-initiated telomerization process using n-butyl acrylate, butyl methacrylate, and lauryl methacrylate (LMA), with trifluoroethanol (TFEtOH) as a telogen, and acylphosphine oxide (APO) as a radical photoinitiator. A crosslinking monomer (an aliphatic urethane acrylate, L9033) and a radical UV-photoinitiator (α-hydroxyalkylphenone) were also tested as components of the adhesive compositions. The influence of LMA and TFEtOH on the UV-phototelomerization process kinetics and the physicochemical features of the obtained fluorotelomers, as well as the concentration of L9033 on the PSA adhesion to a polyethylene surface, were investigated. FT-IR results indicated that the fluorine groups were successfully introduced into the telomer structure. The highest adhesion relative to a polyethylene substrate (12.3 N/25 mm), and the highest hydrophobicity (with a contact angle of 95° for a water/PSA system) were observed for adhesives based on a telomer syrup containing 5 wt. parts of TFEtOH and 30 wt. parts of LMA (per 100 wt. parts of the monomer mixture). Additionally, it was revealed that a higher aliphatic urethane acrylate content and a higher UV dose increased the adhesion feature.

Enhanced Adhesion and Cohesion of Bioinspired Dry/Wet Pressure-Sensitive Adhesives

The byssus-mediated adhesion of marine mussels is a widely mimicked system for robust adhesion in both dry and wet conditions. Mussel holdfasts are fabricated from proteins that contain a significant amount of the unique catecholic amino acid dihydroxyphenylalanine, which plays a key role in enhancing interfacial adhesion to organic and inorganic marine surfaces and contributes to cohesive strength of the holdfast. In this work, pressure-sensitive adhesives (PSAs) were synthesized by copolymerization of dopamine methacrylamide (DMA) with common PSA monomers, butyl acrylate and acrylic acid, with careful attention paid to the effects of catechol on adhesive and cohesive properties. A combination of microscopic and macroscopic adhesion assays was used to study the effect of catechol on adhesion performance of acrylic PSAs. Addition of only 5% DMA to a conventional PSA copolymer containing butyl acrylate and acrylic acid resulted in 6-fold and 2.5-fold increases in work required to separate the PSA from silica and polystyrene, respectively, and a large increase in 180° peel adhesion against stainless steel after 24 h storage in both ambient and underwater conditions. Moreover, the holding power of the catechol PSAs on both steel and high-density polyethylene under shear load continuously increased as a function of catechol concentration, up to a maximum of 10% DMA. We also observed stark increases in shear and peel adhesion for the catecholic adhesives over PSAs with noncatecholic aromatic motifs, further underlining the benefits of catechols in PSAs. Overall, catechol PSAs perform extremely well on polar and metallic surfaces. The advantage of incorporating catechols in PSA formulations, however, is less straightforward for peel adhesion in nonpolar, organic substrates and tackiness of the PSAs.

Mechanical and Morphological Properties of Waterborne ABA Hard-Soft-Hard Block Copolymers Synthesized by Means of RAFT Miniemulsion Polymerization

High molecular weight waterborne ABA block copolymers of styrene (St) and 2-ethylhexyl acrylate (2EHA) containing hard and soft domains were synthesized by means of RAFT (mini)emulsion polymerization using a bifunctional symmetric S,S-dibenzyl trithiocarbonate (DBTTC) RAFT agent. Miniemulsion polymerization was initially used for the synthesis of the A-block, which forms hard domains, followed by 2EHA pre-emulsion feeding to build the B-block soft domains. Polymerization kinetics and the evolution of the Molecular Weight Distribution (MWD) were followed during the synthesis of different ABA block copolymers. The thermal properties of the final symmetric block copolymers were studied on dried films by means of DSC. It was found that the block copolymers have two glass transitions, which indicates the presence of a two-phase system. Phase separation was investigated by means of microscopic techniques (AFM and TEM) and SAXS, both of the particles in the latex form, as well as after film formation at room temperature and after different post-treatments. Films were annealed at temperatures well above the glass transition temperature (T_g) of the hard phase to study the bulk morphology of the films after complete particle coalescence. Moreover, for comparison purposes, the films were re-dissolved in THF, and films were again cast directly from the homogeneous THF solutions. As THF is a good solvent for both blocks, such films serve as a reference for the equilibrium morphology. Finally, DMTA studies of the films annealed at different temperatures were performed to correlate the morphology changes with the mechanical properties of the block copolymers.

Pressure-Sensitive Adhesives under the Influence of Relative Humidity: Inner Structure and Failure Mechanisms

Model pressure-sensitive adhesive (PSA) films of the statistical copolymer P(EHA-stat-20MMA), which comprises 80% ethylhexyl acrylate (EHA) and 20% methyl methacrylate (MMA), are studied. The PSA films are stored under different relative humidities from <2% to 96% for 24 h and subsequently investigated concerning the near-surface composition profile by measuring X-ray reflectivity (XRR) and tack performance. For both types of measurements, special custom-made sample environments are used, which ensure constant temperature and relative humidity during the XRR and tack measurements. Different failure mechanisms of the adhesive bond are found by adjusting the relative humidity. XRR measurements evidence enrichment layers in vicinity to and at the surface depending on the provided relative humidity during the postproduction treatment, which also influence the tack performance. This finding is supported by tack measurements using punches with different roughness.

Thermodynamic Interactions between Polystyrene and Long-Chain Poly(n-Alkyl Acrylates) Derived from Plant Oils

Vegetable oils and their fatty acids are promising sources for the derivation of polymers. Long-chain poly(n-alkyl acrylates) and poly(n-alkyl methacrylates) are readily derived from fatty acids through conversion of the carboxylic acid end-group to an acrylate or methacrylate group. The resulting polymers contain long alkyl side-chains with around 10-22 carbon atoms. Regardless of the monomer source, the presence of alkyl side-chains in poly(n-alkyl acrylates) and poly(n-alkyl methacrylates) provides a convenient mechanism for tuning their physical properties. The development of structured multicomponent materials, including block copolymers and blends, containing poly(n-alkyl acrylates) and poly(n-alkyl methacrylates) requires knowledge of the thermodynamic interactions governing their self-assembly, typically described by the Flory-Huggins interaction parameter χ. We have investigated the χ parameter between polystyrene and long-chain poly(n-alkyl acrylate) homopolymers and copolymers: specifically we have included poly(stearyl acrylate), poly(lauryl acrylate), and their random copolymers. Lauryl and stearyl acrylate were chosen as model alkyl acrylates derived from vegetable oils and have alkyl side-chain lengths of 12 and 18 carbon atoms, respectively. Polystyrene is included in this study as a model petroleum-sourced polymer, which has wide applicability in commercially relevant multicomponent polymeric materials. Two independent methods were employed to measure the χ parameter: cloud point measurements on binary blends and characterization of the order-disorder transition of triblock copolymers, which were in relatively good agreement with one another. The χ parameter was found to be independent of the alkyl side-chain length (n) for large values of n (i.e., n > 10). This behavior is in stark contrast to the n-dependence of the χ parameter predicted from solubility parameter theory. Our study complements prior work investigating the interactions between polystyrene and short-chain polyacrylates (n ≤ 10). To our knowledge, this is the first study to explore the thermodynamic interactions between polystyrene and long-chain poly(n-alkyl acrylates) with n > 10. This work lays the groundwork for the development of multicomponent structured systems (i.e., blends and copolymers) in this class of sustainable materials.

Adhesion on Nanoorganized Multilayers

Nanostructured multilayers composed of alternate organic (alkyldithiol) and metallic (gold) layers are grafted onto glass plates and prepared in order to modify the mechanical and local dissipative properties of a thin surface layer of the substrate. The adhesion phenomenon between a polyisoprene elastomer and these layers is presented and verified by two theories, namely, Johnson, Kendall, Roberts (JKR) and linear elastic fracture mechanics. The increase in adhesion with contact time following a power law has been clearly noted.