Nanocellular polymer foams have shown significant potential for industrial applications because of their superior thermal, mechanical, and optical properties. Some of these properties may be further improved by enhancing the ordering of cell structures. However, it is challenging for conventional foaming methods to control both the cell size and ordering at the nanoscale. Here, we show an innovative method to produce highly ordered nanocellular polymer foams by incorporating the self-assembly of an asymmetric diblock copolymer with the UV-induced chemical foaming technique. The minor domains are designed to generate a gaseous compound from the partial cleavage of the functional group. It is demonstrated that the gas-producing reaction can be accelerated at a temperature low enough to prevent melting of the whole self-assembled template, by mixing a small amount of photoacid generator into the copolymer, followed by UV irradiation. The result is the production of polymer foams with the nanoscale cells highly aligned to the self-assembled domains.

In this study, we have investigated a UV-induced chemical foaming method to create a nanocellular polymer thin film. A small amount of photoacid generator (PAG) was mixed with poly(methyl methacrylate-co-tert-butyl acrylate) (PMMA-co-PtBA). After UV irradiation followed by heating, tert-butyl ester group in the PtBA was deprotected with the acid generated from the PAG, and isobutene gas was produced in the polymer matrix. The resulting polymer foamed with the gas had cell size and cell density ranging from 100 to 200 nm and from 1 × 10 13 to 9 × 10 13 cells/cm 3 , respectively. Several key parameters were found that would affect largely on the formation of the nanocellular structure, i.e., foaming time and temperature, UV dose, molecular weight of polymer.

Random or block copolymers that include the tert-butyl ester may be used to generate nanocelluar foams. In this paper, we demonstrated the synthesis of asymmetric PS-b-PtBMA using the organotellurium mediated radical polymerization (TERP). The TERP is suitable for block copolymer synthesis with precise control of the molecular weight (M n ) while having low dispersity (Ð). Our results show that for M n = 23.75 kg/mol, relatively low dispersity Ð = 1.18 has been achieved with 87% degree of livingness after the PS polymerization. However, at M n > ~50 kg/mol, Ð was largely increased due to the high viscosity of the polymer solution at high conversions.

A method to fabricate highly ordered nanocellular foam by incorporating the UV-induced chemical foaming technique with self-assembly of a block copolymer, poly(methyl methacrylate-block-tert-butyl acrylate) (PMMA-b-PtBA), was reported in our previous communication [Rattanakawin, P. ACS Macro Lett. 2020, 9 (10), 1433−1438. Cells with a size of a few tens of nanometers were successfully generated within the cylindrical PtBA-rich domains by heating the UV-irradiated selfassembled PMMA-b-PtBA. Here, we show how other selfassembled morphologies of PMMA-b-PtBA, such as lamellae and PMMA-rich cylinders, affect the formation of nanocellular foams. It is demonstrated that cells generated from the lamella templates are largely expandable compared to those from the cylindrical templates, mainly due to the increase in gas amount produced within the PtBA-rich domains. Meanwhile, the lamellar framework is no longer maintained and transformed into a microcellular structure when the foaming temperature is increased near the glass transition temperature of PMMA. We show that such a drastic change in cell structure may be mitigated by increasing the molecular weight of PMMA.