Molecular Chain Flexibility and Dielectric Loss at High-Frequency:Impact of Ester Bond Arrangement in Poly(ester imide)s

The evolution of high-frequency communication has accentuated the significance of controlling dielectric properties in polymer media.Traditionally,it has been theorized that rigid molecular chains lead to lower dielectric loss.However,the validity of this proposition at high frequencies remains uncertain.To scrutinize the correlation between chain flexibility and dielectric properties,we synthesized six poly(ester imide)s(PEIs)with systematically varied molecular chain flexibilities by modifying the ester's substitution on the aromatic ring.The introduction of ester bonds bestowed all PEI films with a low dielectric dissipation factor(D_(f)),ranging from 0.0021 to 0.0038 at 10 GHz in dry conditions.The dry D_(f)displayed a pattern consistent with volume polarizability(P/V).Unexpectedly,PI-mmm-T,featu ring the most flexible molecular chain,exhibited the lowest dielectric loss under both dry(0.0021@10 GHz)and hygroscopic(0.0029@10 GHz)conditions.Furthermore,the observed increase in D_(f)after humidity absorption suggests that the high dielectric loss of PEI in applications may be attributed to its hygroscopic nature.Molecular simulations and characterization of the aggregation structure revealed that the smaller cavities within flexible molecular chains,after close stacking,impede the entry of water molecules.Despite sacrificing high-temperature resistance,the precursor exhibited enhanced solubility properties and could be processed into high-quality films.Our research unveils new insights into the relationship between flexibility and highfrequency dielectric loss,offering innovative perspectives on synthesizing aromatic polymers with exceptional dielectric properties.

Chain Flexibility and Connectivity:The Uniqueness of Polymer Crystallization

Theory of polymer crystallization is listed as one of the top ten current challenges in polymer community.Clarifying the uniqueness of polymer crystallization should be the first step to search for the roadmap for targeting this challenge.Chain flexibility and connectivity are two peculiar characteristics of polymers,which differentiate their dynamics and structures from that of small molecules.The uniqueness of polymer crystallization must also stem from these two peculiar characteristics,which,however,has not been attracted significant attention.A local structure order may be essential in assisting the transformation of flexible chain to rigid segment with conformational ordering.The entanglement pressure and entanglement free energy may help to understand how entanglement affect nucleation in polymer network and why the interlamellar amorphous layer always accompany the formation of crystal layer.Some recommendations will be given for future study.

SYNTHESES AND SPECTROSCOPIC PROPERTIES OF PORPHYRIN-PHTHALOCYANINE MITH FLEXIBLE POLYATONIC CHAINS

The syntheses of porphyrin-phthalocyanine heterodimers with flexible polyatomic chains especially designed for the studies of intramolecular photoinduced processes are presented along with the effect of mutual orientation of the chromophore subunits on intramolecular energy transfer efficiency.

Selective Modification of β-Cyclodextrin on the Secondary Side 4:Preparation of Arylated β-Cyclodextrins with a Flexible Chain

Mono 3 -deoxy-(N' -benzoyl-ethylenediamino)-β -CD, mono 3 -deoxy-(N' -benzimidoyl -ethylamino)-β-CD, and mono 3 -deoxy-(N' - salicylidene-ethylenediamino)- β -CD, each of which has a flexible chain that bonds the aryl moiety on the secondary side of β-CD, were prepared inthis research. The reaction processes might involve the formation of mono(2, 3 -manno-epoxide )- β -CD as an intermediate in our reaction conditions.

Density functional theory for molecular orientation of hard rod fluids in hard slits

A density functional theory （DFT） is used to investigate molecular orientation of hard rod fluids in a hard slit. The DFT approach combines a modified fundamental measure theory （MFMT） for excluded-volume effect with the first order thermodynamics perturbation theory for chain connectivity. In the DFT approach, the intra-molecular bonding orientation function is introduced. We consider the effects of molecular length （i.e. aspect ratio of rod） and packing fraction on the orientations of hard rod fluids and flexible chains. For the flexible chains, the chain length has no significant effect while the packing fraction shows slight effect on the molecular orientation distribution. In contrast, for the hard rod fluids, the chain length determines the molecular orientation distribution, while the packing fraction has no significant effect on the molecular orientation distribution. By making a comparison between molecular orientations of the flexible chain and the hard rod fluid, we find that the molecular stiffness distinctly affects the molecular orientation. In addition, partitioning coefficient indicates that the longer rodlike molecule is more difficult to enter the confined phase, especially at low bulk packing fractions.

Anion exchange membranes with eight flexible side-chain cations for improved conductivity and alkaline stability

Increasing the local charge density of flexible side-chain cations in the hydrophilic segments of anion exchange membranes(AEMs)is helpful for improving their properties.However,due to limitations of structural design strategies and available synthetic methods,very few AEMs with more than four flexible side-chain cationic groups in hydrophilic segments have been reported.In order to further improve the hydroxide conductivity,alkaline stability and dimensional stability,herein we report a series of AEMs containing eight flexible side-chain cations in hydrophilic segments,based on poly(aryl ether sulfone)s(PAES).The synthesis,ion exchange capacity(IEC),water absorption,dimensional swelling,alkaline stability and hydroxide conductivity of the obtained membranes(PAES-8TMA-x)were examined and the relationships between structures and properties of different types of AEMs were also systematically compared.The resulting AEMs with IEC values of1.76–2.76 mmol g^-1 displayed comprehensively desirable properties,with hydroxide conductivities of 62.7–92.8 m S cm^-1 and dimensional swelling in the range of 8.3%to15.8%at 60℃.The IEC and hydroxide conductivity for a representative sample,PAES-8TMA-0.35,maintained 82.2%and 79.6%of the initial values after being immersed in2 mol L^-1 Na OH at 90℃ for 480 h,respectively.This study expands the design and preparation of AEMs containing high local densities of flexible side chain cations,and provides a new strategy for new AEM materials.

Thermal properties of polyurethane elastomer with different flexible molecular chain based on para-phenylene diisocyanate

This work focuses on the relationship between flexibility of molecular chains and thermal properties of polyurethane elastomer（PUE）, which laid the foundation of further research about how to improve thermal properties of PUE. A series of PUE samples with different flexibility of molecular chains was prepared by using 1,4-butanediol（1,4-BDO）/bisphenol-a（BPA） blends with different mole ratios including9/1, 8/2, 7/3, 6/4 and 5/5. As comparison, PUE extended with pure 1,4-BDO and BPA was also synthesized.These samples were characterized by differential scanning calorimetry（DSC）, thermogravimetric analysis（TGA）, dynamic mechanical analysis（DMA）, etc. The results showed that with the decrease in flexibility of molecular chains the glass transition temperature（Tg） increased and low-temperature properties became worse. Besides, all samples had a certain degree of microphase separation, and soft segments in some samples were crystallized, i.e. the decreasing flexibility of molecular chains led to the impossibility of chains tightly packing and crystalline domains forming so that the degree of microphase separation decreased and the thermal properties became worse.

Research on Improving the Thermal Conductivity of Epoxy Resin with Flexible Assisted Rigid Groups

Epoxy resins are cross-linked polymeric materials with typically low thermal conductivity.Currently,the introduction of rigid groups into epoxy resins is the main method to improve their intrinsic thermal conductivity.The researchers explored the relationship between the flexible chains of epoxy monomers and the thermal conductivity of the modified epoxy resins(MEP).The effect of flexible chain length on the introduction of rigid groups into the cross-linked structure of epoxy is worth investigating,which is of great significance for the improvement of thermal conductivity of polymers and related theories.We prepared a small molecule liquid crystal(SMLC)containing a long flexible chain via a simple synthesis reaction,and introduced rigid mesocrystalline units into the epoxy resin via a curing reaction.During high-temperature curing,the introduced mesocrystalline units underwent orientational stacking and were immobilized within the polymer.XRD and TGA tests showed that the ordering within the modified epoxy resin was increased,which improved the thermal conductivity of the epoxy resin.Crucially,during the above process,the flexible chains of SMLC provide space for the biphenyl groups to align and therefore affect the thermal conductivity of the MEP.Specifically,the MEP-Ⅵcured with SMLC-Ⅵcontaining six carbon atoms in the flexible chain has the highest thermal conductivity of 0.40W·m^(-1)·K^(-1),which is 125%of the thermal conductivity of SMLC-IV of 0.32 W·m^(-1)·K^(-1),111%of the thermal conductivity of SMLC-Ⅷof 0.36W·m^(-1)·K^(-1),and 182%of the thermal conductivity of pure epoxy of 0.22 W·m^(-1)·K^(-1).The introduction of appropriate length flexible chains for SMLC promotes the stacking of rigid groups within the resin while reducing the occurrence of chain folding.This study will provide new ideas for the enhancement of thermal conductivity of cross-linked polymeric materials.

Enhanced energy storage characteristics of the epoxy film with rigid phenyl-flexible etherified methylene chains

The introduction of highly polarized flexible segments into polymer molecular chains is an effective means to improve the dielectric constant and mechanical flexibility of polymers,which is important for improving the energy storage characteristics and applicability for the roll-to-roll process of metallic film capacitors.However,the introduction of flexible segments may adversely affect the heat resistance of polymer materials,which limits their application at high working temperatures.In this work,we have prepared a novel epoxy film with excellent comprehensive properties by introducing rigid phenyl and flexible etherified methylene side chains.For instance,the optimal epoxy film showed an energy storage density of 7.06 J cm^(-3)and 85%charge-discharge efficiency at room temperature and 420 kV mm^(-1).At 200 kV mm^(-1)and 110°C,a working condition for the application of the electric vehicle,the prepared film still showed an energy storage density of 1.5 J cm^(-3)and charge-discharge efficiency of 86%,which is 3 times that of BOPP film.This work provides an idea for material designs of high-performance polymer film capacitors.