Fully sustainable poly­[HPMC-g-(PMVL-b-PLLA)] graft block copolymer thermoplastics were prepared from hydroxypropyl methylcellulose (HPMC), β-methyl-δ-valerolactone (MVL), and l-lactide (LLA) using a facile two-step sequential addition approach. In these materials, rubbery PMVL functions as a bridge between the semirigid HPMC backbone and the hard PLLA end blocks. This specific arrangement facilitates PLLA crystallization, which induces microphase separation and physical cross-linking. By changing the backbone molar mass or side chain composition, these thermoplastic materials can be easily tailored to access either plastic or elastomeric behavior. Moreover, the graft block architecture can be utilized to overcome the processing limitations inherent to linear block polymers. Good control over molar mass and composition enables the deliberate design of HPMC-g-(PMVL-b-PLLA) samples that are incapable of microphase separation in the melt state. These materials are characterized by relatively low zero shear viscosities in the melt state, an indication of easy processability. The simple and scalable synthetic procedure, use of inexpensive and renewable precursors, and exceptional rheological and mechanical properties make HPMC-g-(PMVL-b-PLLA) polymers attractive for a broad range of applications.

Poly­(lactide) (PLA), a compostable bioderived polyester, can be produced at a cost and scale that makes it an attractive replacement for nondegradable petroleum-derived thermoplastics. However, pristine PLA is brittle and unsuitable for use in applications where high impact strength and ductility are required. In this work we demonstrate that poly­(l-lactide) (PLLA) in the glassy state can be toughened significantly via addition of an amphiphilic diblock polymer. Notably, a PLLA blend containing only 5 wt% poly­(ethylene oxide)-b-poly­(butylene oxide) (PEO–PBO) exhibited tensile toughness and notched Izod impact strength over an order of magnitude higher than neat amorphous PLLA without a significant reduction in transparency or elastic modulus. For a series of PLLA blends containing PEO–PBO of fixed composition (∼70% volume fraction PBO), the toughness was inversely related to the molar mass of the added modifier with the highest toughness observed for the blend containing the smallest diblock (∼7 kg/mol). Interestingly, at fixed composition and molar mass poly­(l-lactide)-b-poly­(butylene oxide) (PLLA–PBO) exhibited a substantial but reduced toughening efficiency compared to PEO–PBO. We attribute this difference to a change in the solubility parameter of the amphiphilc block. Using TEM, we show that the greatest toughening is observed when the diblock modifier forms small cylindrical micelles that are well dispersed in the PLLA matrix. This morphology is facilitated by a negative Flory–Huggins interaction parameter (χ) between PEO and PLLA. These insights suggest a new and versatile strategy for the facile and efficient toughening of brittle thermoplastics.

Graft block polymers are defined by several architectural parameters, including backbone flexibility, graft density, backbone length, side-chain composition, and sidechain length. In this work we probe the impacts of each of these parameters on the phase behavior, rheological properties, and mechanical performance of these materials. Specifically, we examine two sets of materials prepared from backbones of different inherent flexibility. One set was prepared from poly[(n-butyl acrylate)-co-(2-hydroxyethyl acrylate)] (B x E y ) copolymers; the other was prepared from hydroxypropyl methyl cellulose (HPMC) samples. Sequential ring-opening transesterification polymerization from these hydroxyl-functionalized macroinitiatiors yielded a diblock graft architecture containing a rubbery interior block and semicrystalline exterior blocks tethered to a flexible (B x E y ) or rigid (HPMC) backbone. Good control over side-chain molar mass and composition and judicious choice of the graft block segments enabled the preparation of materials that were either ordered or disordered in the melt state. In the former case, crystallization destroys existing order in the material; in the latter case crystallization induces new microphase separation in the bulk. Many of the structure−mechanical property relationships observed for graft block copolymers with rigid backbones are maintained for graft block polymers with semiflexible backbones, including the tendency for samples to remain transparent when stretched. However, interestingly, the effects of graft density and backbone length are quite different depending on the rigidity of the backbone.

The microstructure and mechanical properties of a block copolymer modified commercial thermoset plastic formed from a bisphenol‐A based epoxy and a bio‐derived amine hardener (Cardolite® NC‐541LV) were investigated. A series of poly(ethylene oxide)‐b‐poly(butylene oxide) (PEO‐PBO) diblock copolymers was synthesized at fixed composition (31 ± 1% by volume PEO) and varying molecular weight expanding on a commercially available PEO‐PBO compound marketed by the Dow Chemical Company under the trade name FORTEGRA™ 100; direct application of any of these block copolymers resulted in little improvement of the poor fracture toughness of the cured material. Modification of the resin formulation and curing protocol led to the development of well‐defined spherical and branched worm‐like micelles containing a PBO core and PEO corona in the cross‐linked products as evidenced by transmission electron microscopy (TEM) and small angle X‐ray scattering (SAXS) measurements. Maximum fracture toughness (K1c) and a ninefold increase in the critical strain energy release rate (G1c) over the unmodified neat epoxy was achieved at 5 wt % loading of intermediate molecular weight PEO‐PBO, without measureable reductions in modulus, glass transition temperature or transparency. This study provides new strategies for engineering improved performance in thermoset materials using block copolymer additives that exhibit challenging mixing thermodynamic characteristics with the component monomers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 189–204

Binary composites formed by individually mixing exfoliated graphene oxide modified with amineterminated poly(butadiene−acrylonitrile) (GA) and a spherical micelle forming poly(ethylene oxide)-b-poly(ethylene-altpropylene) (OP) diblock copolymer with a thermoset epoxy, and the associated GA/OP/epoxy ternary composites, were prepared and studied as a function of the molecular weight M c between cross-links. The rigid GA filler dispersed well in the cured epoxies as established by transmission electron microscopy (TEM). The toughening efficacy of GA alone was found to depend strongly on the modifier concentration and the matrix cross-link density with an optimal 1.7-fold increase in the critical strain energy release rate (G Ic ) over the neat epoxy obtained with a 0.04 wt % loading in the most lightly cross-linked (M c = 6100 g/mol) material. Addition of 5 wt % OP to this epoxy resin enhanced G Ic by a factor of 12. Combining the hard GA and soft OP modifiers at the same loading levels (0.04 and 5 wt %, respectively) resulted in 18 times the G Ic of the unmodified material, a 31% improvement over the effect anticipated by simple addition of the fracture properties of the binary composites. Decreasing M c to 700 g/mol eliminated this synergistic effect while reducing the overall improvement in G Ic to just 3 times that of the neat epoxy. Topological features on the fracture surfaces, imaged using a scanning electron microscope (SEM), suggest that the synergistic toughening of the GA/OP/epoxy ternary composite involves concurrent mechanisms operating on different length scales, including micelle cavitation and graphene debonding, resulting in simultaneous shear yielding, crack pinning, and crack deflection.

Growing interest has been received in metallic foams for their combined features of metals and porous structures. Coating metals on polymers have been the most prevalent method to fabricate hybrid metallic foams to inherit both the merits of metals and the mechanical flexibility of polymers. However, direct coating metals on foams is challenging and requires tedious synthesis, such as electrolysis and chemical reduction. This work reported a facile strategy to build hybrid metallic foams via in situ foaming of liquid metals (LM) and polyurethane. The fluidity and incompatibility of LM with porous polyurethane allow the coating of LM on polymers. LM-Foams exhibit high electrical conductivity (3.9 × 104 S/m), low density (ρ < 1 g/cm3), phenomenal elasticity (recover at 95% strain), and excellent mechanical stability (stable with 1000 compressive cycles). Interestingly, the ease of deformation for fluidic fillers in elastic polyurethane generates additional resistive change under pressure, showing unique sensory behaviors which were not observed in conventional conductive foams, such as high response sensitivity (gauge factor >25), short response time (202 ms), and outstanding electrical stability. The nonuniform size distribution of pores leads LM-Foams to show unusual position-dependent sensitivity, enabling advanced applications as password pads and electrical protection foams.

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