The star of the study is polyolefin elastomer, or POE, a flexible ethylene-octene copolymer that has become a favorite candidate for cushioning applications because it is soft, tough, and relatively inexpensive. POE foams, however, have a stubborn weakness: their melt strength is limited. When a molten sheet of POE is foamed, the growing gas bubbles, or cells, can merge with one another in a process called coalescence, and the resulting cellular structure can collapse or shrink after foaming. The result is an unstable foam with poor dimensional stability and disappointing mechanical performance. To combat this, the researchers turned to a two-pronged strategy: chemical crosslinking to stiffen the polymer network, and blending in a second elastomer, olefin block copolymer, or OBC, to reinforce the melt and refine the cell structure.

The chemistry behind the crosslinking step is straightforward but consequential. The team added 0.9 parts per hundred resin of bis(1-(tert-butylperoxy)-1-methylethyl)benzene, known in the trade as BIPB, a peroxide that decomposes under heat and generates free radicals. These radicals abstract hydrogen atoms from the polymer chains, creating chain radicals that couple with one another to form covalent crosslinks. The crosslinked network behaves like a molecular scaffolding: during foaming it holds the expanding melt together, preventing neighboring cells from rupturing and merging. The researchers confirmed the extent of crosslinking by measuring the xylene-insoluble fraction, the portion of the material that will not dissolve in hot solvent because it is tied into a network. As OBC content rose from zero to 50 parts per hundred resin, this insoluble fraction climbed from 82.2 percent to 90.9 percent, a clear sign that the blend was forming a progressively denser crosslinked architecture.

The OBC itself is a fascinating material. Produced by chain-shuttling polymerization, it consists of alternating blocks of crystallizable ethylene-octene sequences and amorphous octene-rich segments. The hard crystalline blocks act as physical crosslinks that melt at elevated temperature, while the soft amorphous blocks provide elasticity. In the POE/OBC blends, the OBC increased both the storage modulus and the complex viscosity of the melt, exactly the rheological changes that stabilize growing bubbles. At the same time, the overall apparent crystallinity of the composites dropped dramatically, from 18.1 percent down to 7.3 percent as OBC loading increased. This reduction in crystallinity matters for foaming because crystals can act as nucleation sites and also stiffen the matrix; less crystallinity means a softer, more deformable cell wall that can stretch further before failing.

With the materials in hand, the team employed supercritical carbon dioxide foaming, a technique that has become the gold standard for environmentally friendly polymer foam production. In this process, the polymer is saturated with carbon dioxide pressurized above its critical point, where the gas takes on liquid-like density and gas-like diffusivity, allowing it to dissolve into the polymer in large quantities. When the pressure is released or the temperature is raised, the dissolved gas comes out of solution and nucleates billions of tiny bubbles. Because carbon dioxide is nontoxic, nonflammable, and leaves no residue, the method avoids the ozone-depleting and greenhouse-active blowing agents that older foam processes relied on. The researchers selected foaming conditions of 100 degrees Celsius and 15 megapascals, a window where the crosslinked blends could expand uniformly without collapsing.

The foaming results revealed a composition-dependent balancing act. The formulation containing 20 parts per hundred resin of OBC, designated F-OBC-20, achieved the highest volume expansion ratio of the series, reaching 8.90 plus or minus 0.13, meaning the foam swelled to nearly nine times the volume of the original solid. At the other end of the spectrum, the formulation with 50 parts of OBC, F-OBC-50, produced the finest cellular architecture: an average cell size of just 54.3 plus or minus 5.1 micrometers and an extraordinary cell density of 1.66 times ten to the seventh cells per cubic centimeter. These microcellular structures are prized because smaller, more numerous cells distribute stress more evenly and trap gas more effectively, improving both mechanical performance and insulation.

The functional payoffs were substantial. Ball rebound resilience, a standard measure of how much energy a foam returns after an impact, increased from approximately 31 percent for the base formulation to over 43 percent for the OBC-rich composites. That improvement translates directly into cushioning materials that feel springier and recover their shape more completely after repeated compression, a property that matters enormously in footwear midsoles and sports protective equipment. Simultaneously, the thermal conductivity of the foams fell from 0.0819 to 0.0607 watts per meter-kelvin, a roughly 26 percent reduction. Lower thermal conductivity means better insulation, and the finer cells of the high-OBC foams contribute by suppressing convection and radiation within the cellular structure while the polymer itself conducts less heat.

Not every property moved in a favorable direction, and the authors are candid about the trade-offs. The specific compressive stress at 50 percent strain, which measures the load-bearing capability of the foam normalized by its density, decreased as OBC content rose. In other words, the bouncier, better-insulating foams were somewhat softer under sustained compression. This is the classic dilemma of foam design: expansion, cellular refinement, resilience, insulation, and compressive strength do not all improve together at a fixed crosslinker loading. The value of the study lies precisely in quantifying these trade-offs, giving manufacturers a data-driven basis for choosing a formulation that matches a specific application, whether that is a shoe midsole that prioritizes energy return or an insulation panel that prioritizes low thermal conductivity.

The broader significance of the work extends beyond the specific POE/OBC system. Supercritical carbon dioxide foaming is rapidly becoming the preferred route for thermoplastic and elastomeric foams across the industry, and studies like this one show how molecular design, crosslinking chemistry, and process conditions can be orchestrated together to overcome the intrinsic limitations of a given polymer. The Beijing team’s approach of using a block copolymer to simultaneously boost melt strength, refine cells, and enhance elasticity could inspire similar strategies in other elastomer families, from thermoplastic polyurethanes to poly(ether-block-amide)s, where shrinkage and cell instability have long frustrated foam producers.

For consumers, the research points toward a future in which the foam inside a sneaker, a yoga mat, or a building panel is lighter, springier, and more insulating than what is available today, all manufactured with a benign gas instead of legacy chemical blowing agents. For the polymer science community, the study provides a rigorous, quantitative demonstration that composition is a powerful lever: by simply adjusting the ratio of two compatible olefin elastomers while holding the peroxide crosslinker constant, the researchers swept across a wide property space, from high-expansion foams to microcellular insulators. As demand grows for sustainable, high-performance cellular materials, that kind of precise compositional control may prove to be the difference between a foam that fails in service and one that endures.

Subject of Research: Supercritical CO2 foaming of crosslinked POE/OBC elastomer composites for enhanced foam resilience and thermal insulation

Article Title: Supercritical CO2 foaming of POE/OBC composite foams with enhanced resilience and thermal insulation

Article References: Supercritical CO2 foaming of POE/OBC composite foams with enhanced resilience and thermal insulation. (n.d.). https://doi.org/10.1007/s00289-026-06700-2

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06700-2

Keywords: polyolefin elastomer, olefin block copolymer, supercritical CO2 foaming, polymer foams, crosslinking, BIPB peroxide, resilience, thermal insulation, cell morphology, melt strength, cushioning materials, Polymer Bulletin