Abstract
We describe the influence of competing self-organizing phenomena on the formation of cholesteric mesophase in liquid crystalline brush block terpolymers (LCBBTs) and liquid crystalline random brush terpolymers (LCRBTs) containing chromonic molecules. A library of LCBBTs and LCRBTs are synthesized using ring-opening metathesis polymerization (ROMP) of norbornene side-chain functionalized monomers comprising cholesteryl mesogen (NBCh9), chromonic xanthenone (NBXan), and poly(ethylene glycol) (NBMPEG). Compression molded films of LCRBTs containing chromonic molecules display multilevel hierarchical structure in which cholesteric mesophase co-exists with π–π stacking of the chromonic mesophase along with PEG microphase segregated domains. This is unexpected as conventional LCBCPs and LCBBCs that lack chromonic molecules do not form cholesteric mesophases. The presence of π–π interactions modifies the interface at the IMDS so that both chromonic and cholesteric mesophases coexist leading to the manifestation of cholesteric phase for the first time within block architecture and is very reminiscent of previously published LCRBCs without chromonic molecules. The key to the observed hierarchical assembly in these LCBBTs containing chromonic molecules lies in the interplay of LC order, chromonic π–π stacking, PEG side chain microphase segregation, and their supramolecular cooperative motion. This unique “single component” polymer scaffold transforms our capacity to attain nanoscale hierarchies and optical properties from block architecture similar to nanoscale mesophases resulting in random architecture.
Highlights
Polymer based 1D photonic nanostructures with domain sizes comparable to the wavelength of light are formed by using (i) small molecular additives to swell domains of linear block copolymers (BCPs),[1,2,3] (ii) self-assembling ultra-high molecular weight brush block copolymers (Brush BCPs),[4,5,6] or (iii) 1D photonic domains from cholesteric (N*) liquid crystalline random copolymers (LCPs).[7,8,9,10] cholesteric mesophases and their consequent stimuli-responsive liquid crystalline properties have never been observed using block copolymers (BCPs), brush block copolymers (Brush BCPs), liquid crystalline block copolymers (LCBCPs) or liquid crystalline brush block copolymers (LCBBCs).[11]
Existence of a cholesteric mesophase above $81 C is supported by UV-vis studies under re ectance mode. In both block terpolymers and random terpolymer lms, we observe a peak at 371 nm which is a further indication that p–p* transition that occurs in this system from stacked chromonic molecules, Fig. 6.43,44 in all these samples p–p chromonic samples co-exist with cholesteric mesophase and in three samples, p–p stacking, PEG microphase segregation and cholesteric mesophase co-exist
We report the synthesis and characterization of random and block terpolymers by ring-opening metathesis polymerization (ROMP) of NBCh9, NBXan, and NBMPEG
Summary
Polymer based 1D photonic nanostructures with domain sizes comparable to the wavelength of light are formed by using (i) small molecular additives to swell domains of linear block copolymers (BCPs),[1,2,3] (ii) self-assembling ultra-high molecular weight brush block copolymers (Brush BCPs),[4,5,6] or (iii) 1D photonic domains from cholesteric (N*) liquid crystalline random copolymers (LCPs).[7,8,9,10] cholesteric mesophases and their consequent stimuli-responsive liquid crystalline properties have never been observed using block copolymers (BCPs), brush block copolymers (Brush BCPs), liquid crystalline block copolymers (LCBCPs) or liquid crystalline brush block copolymers (LCBBCs).[11]. Chromonic mesogens possess liquid-like mobility and retain long-range order at certain concentrations in forming either chromonic N or C phase, which are retained in thermotropic dried lms.[30,31] The intermolecular distance between stacked aggregates in chromonic molecules is known to be 0.34 nm, an indication of p–p stacking arising from aromatic cores which can be observed even at lower concentrations of chromonic systems it is difficult to ascertain if N or C chromonic phase is produced.[23] In this effort, we determine the co-existence of p–p stacking of xanthenone, conventional LC interactions and microphase segregation of PEG domains in block and random terpolymers using X-ray scattering techniques and UV analysis of compression molded samples and spin cast lms for SEM.
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