Effect of Molecular Weight and Layer Thickness on the Dielectric Breakdown Strength of Neat and Homopolymer Swollen Lamellar Block Copolymer Films

Abstract

Designing next-generation lightweight pulsed power devices hinges on understanding the factors influencing the energy storage performance of dielectric materials. Polymer dielectric films have a quadratic dependence of energy storage on the voltage breakdown strength, and strategies to enhance the breakdown strength are expected to yield a path toward high energy storage densities. Highly stratified lamellar block copolymer (L-BCP) films of model polystyrene-<i>b</i>-polymethylmethacrylate (PS-b-PMMA) exhibited as much as ~50% enhancement in breakdown voltage ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) (225% increase in stored energy density, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>U</mml:mi> <mml:mo>∼</mml:mo> <mml:msup> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> <mml:mn>2</mml:mn></mml:msup> </mml:mrow> </mml:math> ) compared to unordered as-cast L-BCP films. Such an energy density using amorphous polymer is on par with industry-standard semicrystalline biaxially oriented polypropylene (BOPP) and as such a notable development in the field. This work develops a deeper understanding of the molecular mechanisms of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> </mml:math> enhancement in L-BCP films, relating <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> </mml:math> directly to molecular weight ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>M</mml:mi> <mml:mtext>n</mml:mtext></mml:msub> </mml:mrow> </mml:math> ), with interpretation to effects of chain-end density and distribution, interface formation, layer thickness, and their relative contributions. As-cast disordered L-BCP films show decreasing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> </mml:math> with decreasing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>M</mml:mi> <mml:mtext>n</mml:mtext></mml:msub> </mml:mrow> </mml:math> similar to homopolymer studies because of the increase of homogeneously distributed chain ends in the film. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> </mml:math> increases significantly in parallel ordered L-BCP films because of the combination of interface formation and spatial isolation of the chain ends into segregated zones. We further confirm the role of chain ends in the breakdown process blending a low <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>M</mml:mi> <mml:mtext>n</mml:mtext></mml:msub> </mml:mrow> </mml:math> L-BCP with matched <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>M</mml:mi> <mml:mtext>n</mml:mtext></mml:msub> </mml:mrow> </mml:math> homopolymers to attain the same layer spacing as neat L-BCP of higher <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>M</mml:mi> <mml:mtext>n</mml:mtext></mml:msub> </mml:mrow> </mml:math> . <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> </mml:math> shows a significant decrease at low homopolymer fractions because of increased net chain-end density within swollen ordered L-BCP domains in wet-brush regime, followed by increased <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> </mml:math> because of layer thickness increase via segregated "interphase layer" formation by excess homopolymers. Notably, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>E</mml:mi> <mml:mrow><mml:mtext>BD</mml:mtext></mml:mrow> </mml:msub> </mml:mrow> </mml:math> of homopolymer swollen L-BCPs is always lower than that of neat L-BCPs of the same domain spacing because of overall adverse chain-end contribution from homopolymers. These findings provide important selection rules for L-BCPs for designing next-generation flexible electronics with high energy density solid-state BCP film capacitors.