An official website of the United States government
This work reveals the influence of pendant hydrogen bonding strength and distribution on self-assembly and the resulting thermomechanical properties of A-AB-A triblock copolymers. Reversible addition-fragmentation chain transfer polymerization afforded a library of A-AB-A acrylic triblock copolymers, wherein the A unit contained cytosine acrylate (CyA) or post-functionalized ureido cytosine acrylate (UCyA) and the B unit consisted of n-butyl acrylate (nBA). Differential scanning calorimetry revealed two glass transition temperatures, suggesting microphase-separation in the A-AB-A triblock copolymers. Thermomechanical and morphological analysis revealed the effects of hydrogen bonding distribution and strength on the self-assembly and microphase-separated morphology. Dynamic mechanical analysis showed multiple tan delta (δ) transitions that correlated to chain relaxation and hydrogen bonding dissociation, further confirming the microphase-separated structure. In addition, UCyA triblock copolymers possessed an extended modulus plateau versus temperature compared to the CyA analogs due to the stronger association of quadruple hydrogen bonding. CyA triblock copolymers exhibited a cylindrical microphase-separated morphology according to small-angle X-ray scattering. In contrast, UCyA triblock copolymers lacked long-range ordering due to hydrogen bonding induced phase mixing. The incorporation of UCyA into the soft central block resulted in improved tensile strength, extensibility, and toughness compared to the AB random copolymer and A-B-A triblock copolymer comparisons. This study provides insight into the structure-property relationships of A-AB-A supramolecular triblock copolymers that result from tunable association strengths.
more »
« less
Unveiling the Role of Compositional Drifts on the Tack of Pressure‐Sensitive‐Adhesives
Abstract Pressure‐sensitive‐adhesives (PSAs) are pervasive in electronic, automobile, packaging, and biomedical applications due to their ability to stick to numerous surfaces without undergoing chemical reactions. These materials are typically synthesized by the free radical copolymerization of alkyl acrylates and acrylic acid, leading to an ensemble of polymer chains with varying composition and molecular weight. Here, reversible addition−fragmentation chain‐transfer (RAFT) copolymerizations in a semi‐batch reactor are used to tailor the molecular architecture and bulk mechanical properties of acrylic copolymers. In the absence of cross‐links, the localization of acrylic acid toward the chain ends leads to microphase separation, creep resistance, and enhanced tack. However, in the presence of Al(acac)3crosslinker, the creep resistance remains unchanged and mostly the large‐strain mechanical properties are affected. This behavior is attributed to microphase separation, but also to a change in the energy required to break physical associations, and untangle and elongate associative polymers to large deformations.
more »
« less
- Award ID(s):
- 2004167
- PAR ID:
- 10473461
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Macromolecular Chemistry and Physics
- Volume:
- 224
- Issue:
- 24
- ISSN:
- 1022-1352
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Biobased poly(γ-methyl-α-methylene-γ-butyrolactone) (PMMBL), an acrylic polymer bearing a cyclic lactone ring, has attracted increasing interest because it not only is biorenewable but also exhibits superior properties to petroleum-based linear analog poly(methyl methacrylate) (PMMA). However, such property enhancement has been limited to resistance to heat and solvent, and mechanically both types of polymers are equally brittle. Here we report the expeditious synthesis of well-defined PMMBL-based ABA tri-block copolymers (tri-BCPs)—enabled by dual-initiating and living frustrated Lewis pairs (FLPs)—which are thermoplastic elastomers showing much superior mechanical properties, especially at high working temperatures (80–130 °C), to those of PMMA-based tri-BCPs. The FLPs consist of a bulky organoaluminum Lewis acid and a series of newly designed bis(imino)phosphine superbases bridged by an alkyl linker, which promote living polymerization of MMBL. Uniquely, such bisphosphine superbases initiate the chain growth from both P-sites concurrently, enabling the accelerated synthesis of tri-BCPs in a one-pot, two-step procedure. The results from mechanistic studies, including the single crystal structure of the dually initiated active species, detailed polymerizations, and kinetic studies confirm the livingness of the polymerization and support the proposed polymerization mechanism featuring the dual initiation and subsequent chain growth from both P-sites of the superbase di-initiator.more » « less
-
Abstract The ability to modulate polyacrylamide hydrogel surface morphology, rheological properties, adhesion and frictional response is demonstrated by combining acrylic acid copolymerization and network confinement via grafting to a surface. Specifically, atomic force microscopy imaging reveals both micellar and lamellar microphase separations in grafted copolymer hydrogels. Bulk characterization is conducted to reveal the mechanisms underlying microstructural changes and ordering of the polymer network, supporting that they stem from the balance between hydrogen bonding in the substrate‐grafted hydrogels, electrostatic interactions, and a decrease in osmotically active charges. The morphological modulation has direct impacts on the spatial distribution of surface stiffness and adhesion. Furthermore, lateral force measurements show that the microphase separations lead to speed and load‐dependent lubrication regimes as well as spatial variation of friction. A proof of concept via salt screening demonstrates the dynamic control of surface morphology and adhesion. This work advances the knowledge necessary to design complex hydrogel interfaces that enable spatial and dynamic control of surface morphology and thereby of friction and adhesion through modulation of hydrogel composition and surface confinement, which is of significance for applications in biomedical devices, soft tissue design, soft robotics, and other engineered tribosystems.more » « less
-
If a binary liquid mixture, composed of two alternative species with equal amounts, is quenched from a high temperature to a low temperature, below the critical point of demixing, then the mixture will phase separate through a process known as spinodal decomposition. However, if the two alternative species are allowed to interconvert, either naturally (e.g., the equilibrium interconversion of enantiomers) or forcefully (e.g., via an external source of energy or matter), then the process of phase separation may drastically change. In this case, depending on the nature of interconversion, two phenomena could be observed: either phase amplification, the growth of one phase at the expense of another stable phase, or microphase separation, the formation of nongrowing (steady-state) microphase domains. In this work, we phenomenologically generalize the Cahn–Hilliard theory of spinodal decomposition to include the molecular interconversion of species and describe the physical properties of systems undergoing either phase amplification or microphase separation. We apply the developed phenomenology to accurately describe the simulation results of three atomistic models that demonstrate phase amplification and/or microphase separation. We also discuss the application of our approach to phase transitions in polyamorphic liquids. Finally, we describe the effects of fluctuations of the order parameter in the critical region on phase amplification and microphase separation.more » « less
-
null (Ed.)The increasing demand for bioderived plastics and rubbers and the large supply of glycerol makes it an excellent starting chemical for the production of biopolymers. Little success in commercially viable glycerol polymers has yet to be realized. In particular, high molecular weight thermoplastics have been especially elusive due to the multifunctional nature of glycerol. This work details the production of glycerol–acrylic biopolymers. By esterifying glycerol with acrylic acid, and subsequent RAFT polymerization to suppress the gelation, we were able to achieve glycerol thermoplastics with high molecular weights (1 MDa). After studying the thermal/mechanical properties of the polymer, it was found that these glycerol polymers had a high degree of tack. When added to wood as an adhesive, it was found that performance was comparable or exceeded standard wood adhesives such as Poly (Methylene diphenyl diisocyanate) (PMDI) and formaldehyde based adhesives. This yields wood adhesives that have less toxicity, lower environmental impact, and higher renewability.more » « less
