Dynamic mechanical properties of poly-α-olefins containing ring structures in side chains

Dynamic loss modulus curves have been determined over a temperature range beginning at liquid nitrogen temperature for poly-α-olefin polymers containing various ring structures, i.e., phenyl, cyclohexyl, cyclopentyl, and naphthyl, in the side chain. Glass transition and appropriate secondary relaxation temperatures were observed for each polymer. Separation of each pendant ring structure from the main backbone chain by successive additions of methylene units results in lower glass-transition temperatures. Comparison of polymers with similar side chains and different ring structures shows that the respective glass-transition temperatures decrease in the order naphthyl > cyclohexyl > phenyl > cyclopentyl. Secondary relaxation peaks were obtained at about ?150°C for polymers containing the cyclohexyl and cyclopentyl rings. A similar peak was observed for the polymer possessing a phenyl ring separated from the main chain backbone by two methylene units. The comparable polymer containing the naphthyl ring structure exhibited a broad secondary relaxation peak centered at ?20°C. The polymers possessing cyclohexyl rings separated from the main chain backbone by one or two methylene units had an additional low temperature peak at ?80°C. The molecular mechanism associated with this relaxation may be related to intramolecular transformations of the cyclohexyl ring between its “chair–chair” conformations.     

Pockels-effect relaxation in poled side chain polymers: Decoupling of chromophor reorientation from α-relaxation

 The relaxation of the polar order of poled side chain polymers carrying NLO-active chromophores was monitored by Pockels-effect relaxation studies. Dielectric relaxation investigations were performed in order to analyze the coupling or decoupling of the chromophore reorientation to the relaxation modes of the side chain polymers. It was found that the chromophores perform their own reorientation relaxation mode both in the molten and the glassy state, which is not coupled to backbone relaxations. The chromophore reorientation process is characterized by a narrow distribution of the relaxation times and high activation energies. Studies on physical aging reveal that the chromophore reorientation is controlled by the free volume. The chromophore reorientation process can be influenced by the chemical linkage of the chromophore to the polymer backbone and by the nature of the backbone. Received: 1 August 1996 Accepted: 22 November 1996     

Organic thin film pyrosensors

Recent developments in sensor technology show the suitability of thin pyroelectric films for the construction of pyroelectric sensor devices made from polymer materials. These materials are used for its advantageous thermal properties and also for its suitable frequency behaviour. Pyroelectrical measurements have been provided on thin films based upon poly(vinyl alcohol)s and poly(siloxane)s with azobenzene side chains. Relaxation behaviour and stability of poled pyroelectric polymers are discussed and related to the results of dipole relaxation measurements provided at our laboratories.     

Poling and crosslinking processes in NLO polymers

A series of photocrosslinkable polymers bearing hyperpolarizable side chain chromophores was synthesized, poled and evaluated on the basis of the thermal stability of Second Harmonic Generation (SHG). Photoinitiation allowed for control of the onset of curing. Crosslinking was monitored by infrared spectroscopy and optimal conversion was achieved by applying a slow temperature ramp during exposure. The ultimate stability of the poled polymers was directly related to the number of crosslinking substituents that were attached to the chromophore pendant group. With two reactive groups per chromophore significant SHG was retained at temperatures above the initial polymer glass transition temperature. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2769–2775     

Nonlinear optical properties of some side chain copolymers based on benzoxazole containing chromophores

Acrylate‐methylmethacrylate copolymers have been synthesized for nonlinear optical applications. Acrylate monomer units are characterized by the presence in the side chain of phenylbenzoxazole groups containing electron donor‐electron acceptor substituents. The phase behavior of all polymers has been investigated by DSC, X‐ray diffraction and polarizing microscopy: two of them exhibit liquid crystalline behavior of smectic type. For four polymers, nonlinear optical properties have been examined by second harmonic generation measurements on thin films (∼ 1 μm thickness) electrically poled by corona discharge. Second order susceptibility coefficients d₃₃ and average relaxation times 〈τ〉, relative to the time stability of the chromophore poling, have been measured. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 603–608, 1999     

Large electro‐optic activity and enhanced temporal stability of methacrylate‐based crosslinking hyperbranched nonlinear optical polymer

A methacrylate‐based crosslinking hyperbranced polymers have been synthesized through initiator‐fragment incorporation radical polymerization and used for the temperature stable electro‐optic (EO) polymer application. This polymer consists of methyl methacrylate, 2‐metacryloxyethyl isocyanate, and ethylene glycol dimethacrylate (EGDMA) monomers. The use of EGDMA as a bifunctional unit resulted in the solvent‐soluble crosslinking hyperbranched chain, so that the EO polymer enhanced glass transition temperatures. A phenyl vinylene thiophene vinylene bridge nonlinear optical chromophore was attached to the polymer backbone as the side‐chain by a post‐functionalization reaction. The loading concentration of the chromophore was varied between 30 and 50 wt % by simply changing the mixing ratio of the precursor polymer to the chromophore. The synthesized EO polymers produced optical quality films with a light propagation loss of 0.61 dB/cm in a slab waveguide at 1.31 μm. The electrically poled film had an EO coefficient (r₃₃) of 139 pm/V at 1.31 μm. The EO crosslinking hyperbranced polymer had a high‐glass transition temperature of 170 °C, and exhibited excellent temporal stability of the EO activity at 85 °C for 500 h. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Characterization of the motion of spin probes and spin labels in amorphous polymers with two-dimensional field-step ELDOR

The motion of nitroxide spin probes and spin labels in amorphous polymers is studied below the glass transition temperature with a two-dimensional pulsed electron double-resonance experiment. Polystyrene and a liquid crystalline side group polymer are studied using both spin probes and spin labels covalently bound to specific sites along the polymer chain. Two methyl acrylic polymers differing only in their side group structure and polyvinylacetate are compared and large differences in the molecular dynamics deduced from both the nuclear and the electron spin relaxation rates are observed as the glass transition is approached. The results demonstrate the complexity of small amplitude motion in simple polymers below the glass transition temperature and show that it is very sensitive to the packing in the polymer. © 1996 John Wiley & Sons, Inc.     

Optical noise and dynamical properties of liquid crystal comb polymers with different mesogenic groups

The light scattered by a comb polymer with a polyacrylamide main chain and biphenyl-based mesogenic units containing the substituent R CN in the biphenyl core is characterized by a stationary noise which has been measured between 70 C and 140 C in the frequency interval 5 10 - 2 nu 10Hz. This optical noise is analysed by means of a specific procedure introduced to clarify the role of random movements of segments of the main chains on the side chain fluctuations. The RMS amplitude of the random molecular movements, the relaxation times of the side chain and main chain fluctuations, and the damping coefficients governing their motion are investigated as functions of temperature. The results are compared with those recently obtained on a similar polymer characterized by R H. Common features and differences are brought into evidence and discussed in terms of the different mesophase structures exhibited by the two polymers in the temperature interval considered.     

STATISTICS OF A NOVEL CLASS OF LIQUID CRYSTAL POLYMERS

A novel class of the liquid crystal polymers with mesogens laterally attached to flexible main chain backbones has been synthesized recently by us. While the flexible backbones tend to take configuration of maximum entropy the mesogenic side groups tend to orientate because of the anisotropie soft dispersion forces. A mean field theory is presented to describe the competition between these two trends which makes it possible for this class of polymers to show anisotropic (liquid crystal) phase within certain temperature range where the polymer main chains and the side chain mesogens will take approximately parallel arrangement, which is different from the normal side chain liquid crystal polymers. The temperature range of the liquid crystal phase, its ordering and phase transition all depend on the flexibility of the backbone, the strengthof the anisotropic forces of the side groups and the hinge elasticity. The results show that the liquid crystalisotropic phase transition is of the first order. The phase diagram, i. e. the dependence of the transition temperature on the structure of the polymers is also given.     

两种高Tg非线性光学聚合物母体及掺杂,极化的研究

利用具有非线性光学活性的对硝基苯胺掺杂高玻璃化转变温度的聚芳醚砜(PES-C)和聚芳醚酮(PEK-C),得到了两种掺杂含量较高的掺杂型非线性光学聚合物体系。电晕极化表明较高的取向和较慢的松驰。     

Liquid crystalline side chain polymer with a poly (Geraniol-co-MMA) backbone and phenylbenzoate mesogenic group: synthesis and characterization

A new side chain liquid crystalline polymers have been synthesized and characterized in which [geraniol-co-MMA] polymer are used as a backbone linked via polymethylene spacer to phenyl benzoate mesogenic group. The polymer exhibits enantiotropic liquid crystallinity with nematic phase and does not exhibit side chain crystallization .A clear difference between the nature of the mesophase is evidenced between [Geraniol-co-MMA] main chain and methacrylate polymers .The LC polymer exhibit glass transition at 40 °C. In a comparative analysis, we discuss the relevance of polymer backbone in the synthesis of side chain liquid crystalline polymers.     

Synthesis and characterization of azo–bisbenzylidene‐based polymers for second order nonlinear optics

Main chain polymers containing azo and bisbenzylidene moiety were synthesized by polycondensation method. The successful polymerization reaction was characterized by UV–vis absorption, FT‐IR and NMR spectroscopy. The resulting polymers were soluble in polar solvents like dimethyl formamide (DMF) and showed good thermal stability up to 250 °C. These polymers were blended with poly methyl methacrylate (PMMA) and corona poled for characterizing second harmonic generation (SHG) property. Temperature stability study of SHG intensity of poled polymer showed that it is stable up to 80 °C. To improve temperature stability further the crosslinking of polymer under UV light is proposed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4317–4324     

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer-solvent mixtures

The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K₂₂ and viscosity coefficient γ₁. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K₂₂ coupled with large increases in γ₁. The decrements in K₂₂ are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.     

Effect of pressure and temperature on the dynamics of bulk polyethylacrylate and polyethylmethacrylate polymers studied by photon correlation spectroscopy

Photon correlation spectroscopy has been used to study relaxation processes in bulk polymers: PEA and PEMA, above the glass transition temperature under high pressure. In the case of PEA one relaxation process was observed over the whole pressure range of 1–2000 bar. This process is assigned to the localized backbone motion of the polymer chain. In PEMA two overlaping relaxation modes have been found. Quantitative characterization of these two modes was possible after their separation in the time domain, which was achieved by applying high pressure to the sample. It was found that an additionalα-methyl group in PEMA (in comparison with PEA) causes an appearance of an additional relaxation mode which can be assigned to the side group motion. Moreover, the localized backbone motion in PEMA is more cooperative because of a hindrance effect due to the presence of the additionalα-methyl side group. The obtained results are in a good agreement with the previous dielectric relaxations studies.     

Low-temperature mechanical relaxations in polymers containing aromatic groups

Studies have been made of the secondary relaxation processes in the solid state of a number of polymers containing aromatic groups in the polymer chain. The polymers investigated include one, polystyrene, with the aromatic group in side-chain positions, and six high polymers in which phenylene rings lie in the main backbone chain. In polystyrene, wagging and torsional motions of the side chain phenyl rings give rise to a low-temperature δ-relaxation which is centered at 33°K (1.7 Hz) and which has an activation energy of about 2.3 kcal/mol. Most of the polymers with phenylene rings in the main chain exhibit a low-temperature relaxation in the temperature region from 100°–200°K. This relaxation process is centered at 159°K (0.54 Hz) in poly-p-xylylene, at 162°K (0.67 Hz) in polysulfone, and at 165°K (1.24 Hz) in poly(diancarbonate). In poly(2,6-dimethyl-p-phenylene oxide), two overlapping low-temperature relaxations are found, one in the range 125–140°K and the other near 277°K (ca. 1 Hz). The low-temperature secondary relaxation process in all of these polymers is believed to be associated with local reorientational motion of the phenylene, or substituted phenylene, rings or with combined motion of the phenylene rings and nearby chain units. For these low temperature relaxation processes, the activation energy is about 10 kcal/-mole. The temperature location of the relaxation appears to depend on the specific units to which the phenylene rings are attached and on steric and polar effects caused by substituents on the ring. In the poly-p-xylylenes the relaxation is shifted to much higher temperatures by a single Cl substitution on the ring but remains at essentially the same temperature position when dichlorosubstitution is made. The effects of water on the magnitude and temperature location of the observed low temperature relaxations, as well as the implications of the study for other polymers containing aromatic groups in their backbone chains, are discussed.     

Dielectric relaxation studies in poly(hexamethylenesebacate) (HMS), poly(2-methyl-2-ethyl propylenesebacate) (MEPS), poly(2,5-dimethylbutylene sebacate) (DBS), and block copolymers of HMS and MEPS

The α and β dielectric relaxations of poly(hexamethylenesebacate) (HMS), poly(2-methyl-2-ethyl propylenesebacate) (MEPS), poly(1,4- dimethylbutylene sebacate) (DBS) and block copolymers of HMS and MEPS have been studied. The α relaxation is amenable to a W.L.F. analysis and is associated with the glass transition of the polymers. This relaxation moves to higher temperatures with increasing HMS content in HMS/MEPS block copolymers. All the polymers studied exhibit psuedo-activation energies of ～32 kcal/mole at the glass transition. It is concluded that because the superposition principle is operative in the block copolymers, the glass transition must be very similar in both polymers and morphology and degree of crystallinity do not greatly affect this transition. The β relaxation which has been associated with segmental relaxation of polymethylene segments in polymers is also shown to be a function of HMS/MEPS block copolymer composition and chemical structure. This relaxation takes place at lower temperatures with increased HMS content in the blocks and also shifts to lower temperatures with side chain substitution adjacent to the carbonyl group in the polymer. It is concluded that the β relaxation takes place in the amorphous and crystalline regions of the polymer.     

Composite spectral density functions for the interpretation of the 13C NMR relaxation behavior of polymers containing hydrocarbon side chains. Free and restricted side chain motion

¹³C NMR NT₁ and NOE have been calculated by using composite spectral density functions describing polymer chain segmental motion and internal rotation of a hydrocarbon side chain attached to the polymer backbone. Numerical results at two magnetic fields are presented as a function of the various motional parameters characterizing the various models. NT₁ and NOE relaxation parameters are well behaved and appear to have practical value for describing the dynamics of these systems. The models have been applied to the relaxation data of poly(n-butyl methacrylate) and poly(n-hexyl methacrylate) in toluene solutions. The dynamics of the two polymers are characterized by a very localized backbone motion and restricted internal rotation about successive C? C bonds of the side chains. © 1995 John Wiley & Sons, Inc.     

Dipolar relaxation mechanisms in the vitreous state, in the glass transition region and in the mesophase, of a side chain polysiloxane liquid crystal

The dipolar relaxation mechanisms in a side chain liquid crystalline polysiloxane have been studied by Thermally Stimulated Discharge Currents (t.s.d.c.) and by Dielectric Relaxation Spectroscopy (d.r.s.). The study was carried out in a wide temperature range covering the vitreous phase, the glass transition region and the liquid crystalline phase. Different discharges were observed in the t.s.d.c. spectrum of this polymer which were attributed, in the order of increasing temperature, to local non-cooperative motions probably involving internal rotations in the spacer and in the alkyl group of the mesogenic moiety, to the Brownian motions of the main chain associated with the glass transition and to motions involving reorientations of the components of the dipole moment of the mesogenic side group in the liquid crystalline phase. The dielectric relaxation spectrum, on the other hand, is dominated by two relaxation processes both of which are above the measured glass transition temperature and shows also a much broader and less intense relaxation below the glass transition temperature which is attributable to local motions along the side groups. It is emphasized that the comparison between the d.r.s. and the t.s.d.c. results is not straightforward and that more research work is needed in order to enable a clear attribution of the relaxation processes at the molecular level, and an unambiguous interpretation of the results obtained by the two techniques.     

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer–solvent mixtures

Abstract

The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K ₂₂ and viscosity coefficient γ₁. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K ₂₂ coupled with large increases in γ₁. The decrements in K ₂₂ are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.     

Side chain crosslinking of aromatic polyethers for high temperature polymer electrolyte membrane fuel cell applications

Novel aromatic polymers bearing polar pyridine units in the main chain and side chain crosslinkable hydroxyl and propargyl groups have been successfully synthesized. The polymers have been investigated in terms of their critical properties related to their application in high temperature polymer electrolyte membrane fuel cells, such as doping ability, mechanical properties, and thermal stability. Crosslinked membranes were prepared by direct crosslinking of hydroxyl side chain groups with decafluorobiphenyl used for the first time as a crosslinking agent. However, further functionalization of hydroxyl groups to the propargyl derivative has also led to crosslinked polymers after thermal curing. Both types of crosslinked membranes exhibited higher glass transition temperatures as well as lower doping levels when doped in phosphoric acid compared with the non crosslinked analogs, confirming the formation of a successfully crosslinked network. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012