This work details an integrated investigation
of liquid crystal
(LC) oligomers that combines experiments and molecular dynamics simulations
to obtain a detailed understanding of the molecular structure of LC
oligomers and the mechanism underlying their phase transition temperatures.
We synthesized and characterized a series of LC oligomers prepared
from different lengths of methylene spacers in the reactive LC monomers
and n-alkylamine chain extenders via the aza-Michael addition reaction. In parallel,
we performed isothermal–isobaric (NPT) ensemble
coarse-grained molecular dynamics (CG-MD) simulation of analogue mesogens
that are connected to flexible spacers and extenders at varying temperatures,
spacer lengths, and extender lengths. This approach allowed the effect
of length in the flexible spacer as well as in the chain extender
on the nematic–isotropic transition temperature (T
ni) to be determined. The results showed that increasing
the length of the extender decreases T
ni for LC oligomers and amplifies the decrease of T
ni in LC oligomers when the spacer length is short. We
infer that the combination of spacer and extender changes the shape
anisotropy of LC oligomers, changing the packing behavior of constituent
mesogens, thus affecting their ability to transition from the isotropic
to the nematic phase. The detailed molecular structure–property
relationships formulated enable prescribing design rules for LC oligomers
geared toward molecularly engineered shape changing materials.
Linear and nonlinear rheological properties of model comb polystyrenes (PS) with loosely to densely grafted architectures were measured under small and medium amplitude oscillatory shear (SAOS and MAOS) flow. This comb PS set had the same length of backbone and branches but varied in the number of branches from 3 to 120 branches. Linear viscoelastic properties of the comb PS were compared with the hierarchical model predictions. The model underpredicted zero-shear viscosity and backbone plateau modulus of densely branched comb with 60 or 120 branches because the model does not include the effect of side chain crowding. First- and third-harmonic nonlinearities reflected the hierarchy in the relaxation motion of comb structures. Notably, the low-frequency plateau values of first-harmonic MAOS moduli scaled with M w − 2 (total molecular weight), reflecting dynamic tube dilution (DTD) by relaxed branches. Relative intrinsic nonlinearity Q0 exhibited the difference between comb and bottlebrush via no low-frequency Q0 peak of bottlebrush corresponding to backbone relaxation, which is probably related to the stretched backbone conformation in bottlebrush.
Graphite was functionalized electrochemically in a potassium fluoride solution and used to prepare polyimide (PI)/graphene nanohybrid films. The as-made electrochemically fluorinated graphene (EFG) was used to prepare nanohybrid films with colorless PI, which was synthesized from 4,4 ′ -(hexafluoroisopropylidene) diphthalic anhydride and bis(trifluoromethyl) benzidine by in situ polymerization. The surface functionalization of graphite was characterized by powder XRD, TEM with energy dispersive X-ray spectroscopy elemental mapping, X-ray photoelectron spectroscopy, Raman spectroscopy, and TGA. The microstructure of the films was characterized by Fourier transform IR spectroscopy, XRD and SEM. The film properties were measured using a universal testing machine, TGA, dynamic mechanical analysis, four-point probe, UV-visible spectroscopy and water contact angle analysis. EFG improved the tensile strength and modulus of the nanohybrid films by 20% and 50%, respectively. The glass transition temperature and electrical conductivity of the nanohybrid films were 12 ∘ C and nine orders of magnitude higher than those of the neat PI film, respectively. The nanohybrid film maintained 80% optical transmittance even after the addition of 0.1 wt% EFG.
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