Most of the theoretical and computational descriptions of the phase behavior of block copolymers describe the chain ensembles of perfect and uniform polymers. In contrast, experimental studies on block copolymers always employ materials with disperse molecular makeup. Although most polymers are so-called monodisperse, they still have a molecular weight dispersity. Here, we describe the synthesis and properties of a series of discrete length diblock co-oligomers, based on oligo-dimethylsiloxane (oDMS) and oligo-lactic acid (oLA), diblock co-oligomers with highly noncompatible blocks. By utilizing an iterative synthetic protocol, co-oligomers with molar masses up to 6901 Da, ultralow molar mass dispersities (Đ ≤ 1.00002), and unique control over the co-oligomer composition are synthesized and characterized. This specific block co-oligomer required the development of a new divergent strategy for the oDMS structures by which both bis- and monosubstituted oDMS derivatives up to 59 Si-atoms became available. The incompatibility of the two blocks makes the final coupling more demanding the longer the blocks become. These optimized synthetic procedures granted access to multigram quantities of most of the block co-oligomers, useful to study the lower limits of block copolymer phase segregation in detail. Cylindrical, gyroid, and lamellar nanostructures, as revealed by DSC, SAXS, and AFM, were generated. The small oligomeric size of the block co-oligomers resulted in exceptionally small feature sizes (down to 3.4 nm) and long-range organization.
An experimental study is presented
in which we compare the bulk
phase behavior of discrete and (partially) disperse diblock co-oligomers
(BCOs) with high χ–low N. To this end,
oligomers of dimethylsiloxane (oDMS) and lactic acid
(oLA) were synthesized, each having either a discrete
number of repeat units or a variable block length. Ligation of the
blocks resulted in oDMS–oLA BCOs with dispersities ranging from <1.00001 to 1.09, as revealed
by mass spectroscopy and size exclusion chromatography. The phase
behavior of all BCOs was investigated by differential scanning calorimetry
and small-angle X-ray scattering. Compared to the well-organized lamellae
formed by discrete oDMS–oLA, we observe that an increase in the dispersity of these BCOs results
in (1) an increase of the stability of the microphase-segregated
state, (2) a decrease of the overall degree of ordering, and (3) an increase of the domain spacing.
Precise oligomeric materials constitute a growing area of research with implications for various applications as well as fundamental studies. Notably, this field of science which can be termed macro‐organic chemistry, draws inspiration from both traditional polymer chemistry and organic synthesis, combining the molecular precision of organic chemistry with the materials properties of macromolecules. Discrete oligomers enable access to unprecedented materials properties, for example, in self‐assembled structures, crystallization, or optical properties. The degree of control over oligomer structures resembles many biological systems and enables the design of materials with tailored properties and the development of fundamental structure–property relationships. This Review highlights recent developments in macro‐organic chemistry from synthetic concepts to materials properties, with a focus on self‐assembly and molecular recognition. Finally, an outlook for future research directions is provided.
Crystallinity is seldomly utilized
as part of the microphase segregation
process in ultralow-molecular-weight block copolymers. Here, we show
the preparation of two types of discrete, semicrystalline block co-oligomers,
comprising an amorphous oligodimethylsiloxane block and
a crystalline oligo-l-lactic acid or oligomethylene
block. The self-assembly of these discrete materials results in lamellar
structures with unforeseen uniformity in the domain spacing. A systematic
introduction of dispersity reveals the extreme sensitivity of the
microphase segregation process toward chain length dispersity in the
crystalline block.
Intriguingly, little
is known about the impact of dispersity on
the crystallization driven self-assembly (CDSA) of amphiphilic block
copolymers in aqueous media. Here, we investigate the influence of
dispersity on the CDSA of ABA-type amphiphilic block co-oligomers
(ABCOs). Two pairs of ABCOs are synthesized comprising discrete (Đ = 1.00) or disperse (Đ =
1.20) isotactic l-lactic acid 16-mers as the semicrystalline
hydrophobic block and either oligo(ethylene glycol) methyl ether (MeOoEG)
or oligo(tetraethylene glycol succinate) (oTEGSuc) as the discrete
hydrophilic block. Self-assembly studies in water with 10% THF reveal
uniform nanofibers/2D sheets for the discrete oligomers, but such
structural regularity is largely compromised in the disperse oligomers.
The results are corroborated by sharp melting transitions in both
solution and bulk for the discrete ABCOs, unlike their disperse analogues
that show a lack of crystallization. Interestingly, the discrete MeOoEG-LLA
oligomer reveals crystallization driven gelation, illustrating the
contrasting differences between the discrete oligomers and their disperse
counterparts.
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