Complex Sequence-Defined Heteropolymers Enable Controlled Film Growth in Layer-By-Layer Assembly

Abstract

Digitally-encoded poly(phosphodiesters) (d-PPDE) with highly complex primary structures are evaluated for layer-by-layer (LbL) assembly. To be easily decoded by mass spectrometry (MS), these digital polymers contain many different monomers: 2 coding units allowing binary encryption, 1 cleavable spacer allowing controlled MS fragmentation, and 3 mass tags allowing fragment identification. These complex heteropolymers are therefore composed of 6 different motifs. Despite this strong sequence heterogeneity, it is found that they enable a highly controlled LbL film formation. For instance, a regular growth is observed when alternating the deposition of negatively-charged d-PPDE and positively-charged poly(allyl amine hydrochloride) (PAH). Yet, in this approach, the interdistance between consecutive coded d-PPDE layers remains relatively small, which may be an issue for data storage applications, especially for the selective decoding of the stored information. Using poly(sodium 4-styrene sulfonate) (PSS) as an intermediate non-coded polyanion, it is shown that a controlled interdistance between d-PPDE layers can be easily achieved, while still maintaining a regular LbL growth. Last but not least, it is found in this work that d-PPDE of relatively small molecular weight (i.e., significantly smaller than those of PAH and PSS) still enables a controlled LbL assembly.

Keywords: data storage; digital polymers; layer‐by‐layer assembly; sequence‐controlled polymers; thin films.

Figure 1

a) General molecular structure of the digital heteropolymer studied in this work. b) Molecular structures of non‐coded polyanion and polycations that were used in the construction of layer‐by‐layer (LbL). The digital multilayer film was prepared following two routes. Route 1: alternating deposition cycle of coded d‐PPDE (red) and non‐coded PAH (blue). Route 2: four‐step PSS/PAH/d‐PPDE/PAH deposition cycle.

Figure 2

a) Monitoring of film thickness by ellipsometry after deposition of each polyelectrolyte layer in Route 2. In this approach, a 4‐step PSS/PAH/d‐PPDE/PAH deposition cycle was followed to deposit 40 consecutive layers. The grey diamond, purple triangles, red circles, and blue squares represent PEI, PSS, d‐PPDE, and PAH layers, respectively. b) Monitoring of film thickness by ellipsometry after deposition of each coded d‐PPDE layer. The dashed line represents a linear fit showing that the growth of the initial layers is influenced by the substrate and is not representative of that of the bulk film.^{[ 38 ]}