Backbone Stiffness Research Articles

Study of the mechanism of the polymerisation of α-leaving group-α′-polariser- p-xylene Indications for a free radical mechanism

Conjugated polymers show great promise for use in light emitting diodes (LED's). Unfortunately, the stiff backbone of these polymers results in insoluble and infusible materials which are thereby difficult to process. This problem can be overcome by using a precursor route. In our research group a novel route to stable non-aqueous processible precursor polymers has been developed. The general concept of this new route is the base-induced polymerisation of p-xylene-derivatives. We here report on the study of the mechanism of the polymerisation of these p-xylene-derivatives. Important indications for a free radical polymerisation mechanism are found. With an understanding of this mechanism, it is possible to control and modify this synthesis of PPV and its derivatives in a rational manner which results in control of yield and molecular weight of the polymer.

Polarized and depolarized dynamic light scattering of a rodlike polyelectrolyte in a strong acid

AbstractUnusual dynamic light scattering behavior of poly(benzoxazole), a stiff backbone rodlike polymer, has been found in methane sulfonic acid/methane sulfonic acid anhydride. A very rapid, angle‐independent decay mode characterized the short‐time autocorrelation functions at various levels of added salt, sodium methane sulfonate, in both polarized and depolarized dynamic light scattering experiments. In some ways, the behavior qualitatively resembles the long‐predicted “plasmon” mode, which has never been seen in more conventional polyelectrolyte systems. However, calculations based on a point charge model are inconsistent with the plasmon mode, even allowing for the high viscosity of the methane sulfonic acid solvent. The absence of angle dependence means that the rapid decay mode cannot be explained by the diffusion of oligomers. The meaning of the rapid, angle‐independent decay mode remains unclear.

XPS analysis of fluorinated polyimides

Five different fluorinated polyimides were characterized using X-ray photoelectron spectroscopy (XPS). Elemental compositions and polyimide stoichiometries as elucidated with XPS are reported. The fluorinated polyimides derived from corresponding poly(amic acid)s were completely imidized after thermal cure at 350°C, giving rise to a single N 1s peak at 400.8 eV. Interestingly, fluorine segregation on the cured polyimide surface was found to be a function of the polyimide backbone stiffness.

Rayleigh scattering of Mössbauer radiation in oriented fibres of hydrated biopolymers

The Rayleigh scattering of Mossbauer radiation (RSMR) has been measured on films of highly oriented hydrated polynucleotides (A-NaDNA) and polysaccharides (Na-hyaluronate). Both DNA and hyaluronate (HA) have helical secondary structures with a similar pitch (28.2 A for A-DNA, and 32.8 A for Na-HA), but they differ in the basic elements which make up the helices and in the extent of water-biopolymer interactions. These differences are responsible for the diverse stiffness of the polymer backbone, and also affect the dynamics of the first hydration layers. For both samples the elastic scattering intensity shows a sharp peak at about 2 A−1 only for samples oriented withQ parallel to the fibre direction. Its position is close to that of the first maximum in the structure factor of bulk water; it is, however, much narrower than in pure H2O and it is similar to a crystalline Bragg peak. It can be attributed to an ordered structure of water along the double helices. From the temperature dependence of the elastic intensity under the peak maximum, the mean square displacement of water oxygens in the direction parallel to the helices has been deduced. The thermal diffuse scattering intensity is also peaked at the sameQ values of the elastic intensity, indicating the presence of coherent vibrational excitations propagating along the ordered water filaments.

Statistical mechanical theory for liquid‐crystalline polymer solutions

AbstractA statistical mechanical theory is presented for liquid‐crystalline polymer solutions, where the liquid‐crystalline polymer is regarded as a wormlike spherocylinder interacting by the hard‐core repulsion and a soft (attractive or electrostatic) interaction with other spherocylinders. The theory is based on the following two assumptions: (1) the intramolecular excluded volume effect is not effective and (2) the interaction between pair of spherocylinders can be treated by the “single contact approximation”. These assumptions are relevant to a polymer with a stiff backbone and not very long contour length. The theory is used to predict thermodynamic and spatial properties of isotropic solutions and also isotropic‐liquid crystal phase equilibrium behavior. The predicted solution properties and phase behavior are favorably compared with experimental results for some selected stiff‐chain liquid‐crystalline polymers.

Synthesis of homopolymers and copolymers of bulky fumarates, and their spreading behaviour in insoluble monolayers

A series of fumaric esters and amides having bulky substituents were prepared, and their homo- and copolymerisations with electron-rich comonomers were investigated. As the resulting polymers are characterised by rather stiff polymer backbones, they represent weakly amphiphilic polymers of the “hairy rod” type which are accessible by radical polymerisation. The spreading behaviour at the air-water interface was studied, with respect to the influence of the substituents and of the comonomers used on monolayer properties.

A photoluminescence study of poly(phenylene vinylene) derivatives: The effect of intrinsic persistence length

We report the results of light scattering, absorption, excitation, and emission spectroscopy of three polyphenylene vinylene (PPV) derivatives; poly[2-methoxy, 5-(2′-ethyl-hexyloxy-p-phenylene- vinylene] (MEH-PPV), poly[2-butoxy, 5-(2′-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV), and poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV) in solution with p-xylene. We find that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the polyphenylene vinylene backbone and that this change in stiffness has dramatic effects on the photoluminescence of polyphenylene vinylene. We have determined the luminescence quantum efficiencies of the polyphenylene vinylene derivatives relative to a known standard, Rhodamine 6G, and find that the photoluminescence can be greatly enhanced by increasing the intrinsic stiffness of the polymer backbone. The stiffest polymer, poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV), has a quantum efficiency of 0.66±0.05. The quantum efficiency decreases to 0.22±0.05 for poly[2-butoxy, 5-(2′-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV) and 0.20±0.05 for poly[2-methoxy, 5-(2′-ethyl-hexyloxy-p-phenylene-vinylene] (MEH-PPV), the most coiled derivative. Excitation profiles of the three derivatives also show an increase in nonradiative decay at high energies when the polymer assumes a more coiled comformation. Thus, the quantum yields are dependent on pump energy.

Structural Effects on the Transport of Water in Polyimides

The influence of molecular primary and secondary structure on the transport of water has been investigated for a series of chemically similar polyimides using an IR ATR method. The diffusivity of water generally decreased with increased chain backbone stiffness (as determined from the mechanical β transition), and the activation energies for diffusion generally increased with backbone stiffness. For a given backbone composition, the diffusivity was also shown to markedly increase as the density of the amorphous phase decreased. The vibrational spectra show that the absorbed water partitions into a bimodal distribution of sites, presumably representing single and clustered water molecules

Analytic PRISM theory of structurally asymmetric polymer blends and copolymers

Analytic PRISM theory with the new molecular closures is applied to determine the effective chi-parameters and spinodal instability curves for structurally asymmetric polymer alloys. Compressibility effects are found to be very important, and the use of a literal incompressible RPA-like approximation is shown to incur qualitative errors in most cases. A rich and nonadditive dependence of phase transition temperatures and apparent SANS chi-parameters on backbone stiffness asymmetry, attractive interaction potential asymmetry, and thermodynamic variables is found for binary homopolymer blends. A novel strategy for designing miscible mixtures based on a cancellation, or compensation, of the relevant asymmetries is identified. The influence of chain stiffness asymmetry in blends characterized by specific interactions is also studied. Generalization of the analytic PRISM theory to mixtures of random copolymers and periodic block copolymer melts is presented. All the rich behavior predicted for phase-separating homopolymer mixtures is again found for these systems, plus additional non mean field effects associated with random copolymer composition and block architecture. The theory is applied semiquantitatively to interpret recent experiments on polyolefin blends, diblock copolymers, and random copolymer alloys. Theoretical predictions are made which qualitatively account for recent experimental observations of a strong influence of stiffness asymmetry on phase separationmore » temperatures, and the breakdown of the mean field random copolymer approach. Anomalous behavior is also predicted for deuterated mixtures due to an interference between the consequences of stiffness asymmetry and enthalipic interactions. The physical mechanism for the many non-Flory-Huggins effects predicted by the compressible PRISM theory is local, scalar density correlations, which appears to be different than the nematic fluctuation mechanism suggested by recent field theoretic work.« less

Alkoxy‐substituted liquid‐crystalline aromatic copolyesters, 2

AbstractNumber‐average molecular weights, Mn, up to 25 000, and inherent viscosities, ηinh, up to 2,88 dL/g, were found for the soluble members of an alkoxy‐substituted copolyester series (HTO5‐X series). A liquid‐crystalline (high‐temperature) phase was distinguished by IR and wide‐angle X‐ray scattering on molten and quenched samples in form of a nematic sanidic phase. In this phase no relationship exists between the orientation of the aromatic polyester backbone and that of the laterally attached alkoxy groups. Further, it was proven by IR that only the polyester backbone can be aligned by shear stress. If the applied shear stress is very strong, the orientation of the stiff backbone becomes sensitive to disturbances caused by mechanical forces.

Langmuir-Blodgett films of photochromic polyglutamates 3. Spectroscopic studies on LB films of photochromic polyglutamates with alkylspacers of different length

Abstract Poly (5-(2-(4-(4-hexylphenylazo) phenoxy) ethyl)- l -glutamate) (P2) and poly(5-(6-(4-(4-hexylphenylazo) phenoxy) hexyl)- l -glutamate (P6) are “hairy rod” polymers (i.e stiff polymer backbone with flexible side chains) bearing photochromic azobenzene moieties in the side chain coupled to the main chain via spacers of different length. Laagmuir-Blodgett films of these polymers were investigated using X-ray diffraction, FTIR and UVVIS spectroscopy. The structure of freshly prepared LB films is very similar for both polymers and is built up by deformed “hairy rods” (i.e with a non-symmetrical distribution of side chains around the polymer backbone) arranged in layers with main chains preferentially oriented in the dipping direction. The side chains are preferentially oriented normal to the surface of the substrate, additionally the projection of the long axis of the azobenzene moiety into the film plane lies preferentially parallel to the main chains. On isomerization of the azobenzene moieties by irradation, the molecular arrangement is changed dramatically. The layered structure is lost, due to rearrangement of the deformed “hairy rods” to a more symmetrical distribution of the side chains around the main chain, whereas the preferred orientation of the main chains is retained. In addition, for P2 projection of the azobenzene long axis changes from being parallel to the main chain to a more perpendicular orientation. Due to group interaction, as in the combined main chain and side chain liquid crystals, the preferred orientation of the chromophores parallel to the main chain is preserved in the case of P6.

The Effect of Chain Stiffness in Lightly Crosslinked Epoxies

Abstract Three monomer pairs were used to examine the effect of increasing backbone stiffness on material properties. Bisphenol F, resorcinol, and 2,7-dihydroxynaphthalene and their diglycidyl ether analogs of increasing stiffness were chosen for ease of processing and to avoid the complicating influences of steric hindrance, polar interactions between chains, and liquid crystallinity. Oligomeric precursors were solution polymerized to insure stoichiometric imbalance-determined crosslink density. The Tg s of the Bisphenol F, resorcinol, and 2,7-dihydroxynaphthalene LXTs are 56, 67, and 105°C, respectively, by DSC. The modulus of the Bisphenol F and resorcinol LXTs are 3.04 and 3.47 GPa, respectively.

Novel Template Guided Synthesis of Poly Aniline

In acidic medium, polyaniline has positive charges on its molecular backbone. These charges come from either protonation at the nitrogen atom sites or polarons in its β-electronic system [1]. Although polyaniline, due to its positive charges, is a polycation, yet its physical properties are quite different from the classical polyelectrolytes. For example, the polycation vinylbenzyltrimethylammonium chloride is soluble in water but polyaniline molecules (and other conducting polymers too) tends to aggregate into intractable solid. Conducting polymers aggregate into intractable solid because of their stiff molecular backbone and the strong short range attractive forces between βclouds of adjacent polyaniline chains. The insolubility of polyaniline poses problem for solution processing of polymers. There have been a heavy effort towards the resolution of this problem [2].

Microisotropic structure in solutions of polymers differing in molar mass and backbone stiffness

Correlation distance of polarizability fluctuations, a c , was measured by elastic light scattering in moderately concentrated solutions of polymers differing in molar mass M w . In very dilute solutions the relationship a c→o=( R ̄ g 2) 1 2 /√6 holds true, where ( R ̄ g 2) 1 2 ) is the mean square radius of gyration of the isolated macromolecule. For flexible-chain polymers in the absence of strong intermolecular interactions the distance a c decreases with polymer concentration c because of shrinkage of swollen macromolecular coils. The dependences of 1n ( a c /√ M w ) on 1n c follow a single master curve for all fractions of different molar mass. On the other hand, in the presence of specific intermolecular interactions (such as hydrogen bonds), a c increases with concentration as a result of association phenomena. At a given concentration the size of associates is proportional to ( R g 2) 1 2 . The character of the concentration dependence of a c is governed by the type of intermolecular interactions operative in the system.

Molecular and mechanical aspects of helicoid development in plant cell walls

AbstractThe view is presented that extracellular architecture in plant cell walls results from an interplay between molecular self‐assembly and mechanical reorientation due to growth forces. A key initial self‐assembly step may involve hemicelluloses. It is suggested that hemicelluloses may self‐assemble into a helicoid via a cholesteric liquid crystalline phase; the detailed molecular structure of hemicelluloses (stiff backbone, bulky side chains, and the presence of asymmetric carbon atoms) is shown to be consistent with cholesteric requirements for such self‐assembly. Since hemicelluloses are hydrogen‐bonded to the periphery of cellulose microfibrils, the cellulose could then itself become helicoidally arranged. Such ‘universal plywood’ structure is found in the walls of a wide variety of plants, and in several types of cell (including wood). The permanent effects of growth stresses on patterns seen in sections of helicoids are displayed by computer graphics plots, and the expected changes in stiffness are calculated.

High‐modulus polydiacetylene single‐crystal fibers

AbstractThe relationship between structure and mechanical properties of polydiactetylene single‐crystal fibers has been studied in detail. It is shown by transmission electron microscopy that the fibers have a high degree of internal perfection, with the polymer molecules aligned parallel to the fiber axes. The fibers of the dicarbazolyl derivative investigated were found to have a Young's modulus of 45 GPa and fracture strengths of up to 1.5 GPa, the strengths being controlled by defects such as surface steps. It is shown that the stiffness of the polymer backbone is similar to that of polyethylene, and the theoretical strength of the polydiacetylene singlecrystal fibers is determined to be about 3 GPa, corresponding to a fracture strain of between 6 and 8% and a force required to break molecules to the order of 3 nN. The derivative studied is also found to have good thermal stability, not degrading below 300°C, and excellent creep resistance up to at least 100°C. The possibility of using the single‐crystal fibers in composites is also discussed.

Viscosity-molecular weight relationships, intrinsic chain flexibility, and dynamic solution properties of guar galactomannan

Molecular theories of the viscosity of dilute and more concentrated solutions of random-coil polymers have been applied to five molecular-weight fractions of guar galactomannan. The variation in intrinsic viscosity ([η]; dL.g −1) with molecular weight (M r) followed the relationship [η] = 3.8 × 10 −4) M r 0.723, consistent with random-coil behaviour. The intrinsic stiffness of the galactomannan backbone was estimated by evaluating the “characteristic ratio” C ∝, which is ∼ 12.6 and agrees well with results for carboxymethylcellulose, which has a closely similar, β-(1→4)-linked polymer backbone. At intermediate concentrations (c), up to specific viscosities of ∼ 10, η sp was proportional to c 1.3, while at higher concentrations, η sp was ∼c 5.1. This exponent is higher than is usually observed for polymers interacting purely by physical entanglement, and is evidence for more specific polymer-polymer interactions (“hyperentanglements”). The concentration dependence and the time-scale of entanglement coupling have been monitored by both steady-shear and oscillatory measurements, and a generality of response has been demonstrated.

General reflections on the symposium on physical structure of the amorphous state

Abstract Following a discussion of the historical perspective of the problem of order in amorphous polymers and a definition of terms, this paper reviews various topics in the literature. This problem and the related one of order in liquid alkanes (as a model for polyethylene) and amorphous polymers were omitted from or inadequately treated at this symposium. These topics relate both to short-range order (>50 Å) and long-range order (>100 Å) including supramolecular amorphous structure. Among the former are depolarized Rayleigh scattering and heats of mixing of normal alkanes; positron annihilation in polyisobutylene, x-ray scattering, dielectric loss data, and the constancy of the Tg/Tm ratio. Among the latter are the Mooney-Rivlin C2 constant and the general implications of short-range order for a supramolecular structure in long-chain polymer. Since local order exists in simple liquids, it must be present in amorphous polymers. A term, correlation of molecular orientations, CMO, introduced by Bothorel for low molecular weight liquids, is employed to express a feeling for this short-range order. The connectedness of an ensemble of polymer chains implies at once some supramolecular structure (even though being X-ray amorphous and, therefore, not having supramolecular order), since any given polymer chain can experience several CMO's not only with itself but with one or more neighboring chains. This supramolecular structure is dynamic above but frozen below Tg. This postulated supramolecular structure may result in the nodules seen by electron microscopy. The key unresolved problem of this symposium concerns the reality of, and the kinetics for, the assumed reversible transformation at Tm: random interpenetrating coils in melt chain-folded crystals in solid This dilemma is posed by neutron scattering on polyethylene showing (in some cases) random coils in the melt and by various types of physical evidence (not cited at the Symposium) for chain-folded crystals in the solid. A semicollapsed coil model with local order, proposed independently by Lindenmeyer and by Arzhakov et al., is consistent with a) ¯r0 2 1/2 values observed in the amorphous state, b) the existence of local order, and c) the reversible melting-recrystallization kinetics. Factors which favor a high CMO (local order) are 1) close-packing possibilities (no or small side chains), 2) polarity, 3) a stiff backbone, 4) regularity of backbone structure, and 5) a small temperature interval between the measuring temperature and Tg, i.e., small free volume. Elastomers as a class, especially when observed at T ≥ Tg + 100 °C, are least likely to exhibit local ordering but still are not completely free from it. It is observed that the ratio C2/C1 of the Mooney-Rivlin constants decreases with average area per chain molecule, suggestive of local order induced by orientation. Finally, suggestions are made for topics at a future symposium on amorphous order.

Potentiometric titration of polyelectrolytes having stiff backbones

AbstractThe potentiometric titration curves of carboxymethylcellulose, which has a relatively stiff backbone, and also of poly(D‐glutamic acid) in the helical region are compared with the theoretical curves calculated assuming that the polyions are rods and have smeared charges on their surfaces. For carboxymethylcellulose good agreement is observed when its charge density is high, whereas as the charge density decreases the calculated curves deviate from the observed ones. The main reason for the disagreement at low charge densities may be attributed to the flexibility of the polymer backbone. For helical poly(D‐glutamic acid) satisfactory agreement between calculated and observed curves is found if a radius thicker than the realistic radius is employed. The reason for the excessively large radius may be attributed to the inapplicability of the smeared charge model.

Structure–physical property relationships in peroxide‐cured butadiene–styrene copolymers

AbstractStructure–physical property relationships in high‐vinyl butadiene–styrene copolymers have been determined for samples cured with dicumyl peroxide under the same conditions. Three different structures, butadiene–styrene–butadiene (B–S–B) triblocks, butadiene–styrene (B–S) diblocks, and random butadiene–styrene copolymers, have been examined. Flexural modulus increases with increasing styrene content owing to the inherent stiffness of a polystyrene backbone. Swelling increases whereas hardness and heat distortion temperature decrease with increasing styrene content. This behavior is explained by the decrease in crosslink density with increasing styrene content in all structures. Heat distortion temperatures of the B–S–B and B–S networks are superior to the heat distortion properties of the random structures. The B–S–B structure is the most solvent resistant, followed by the random copolymers, with the B–S structures swelling to the greatest extent. Swelling differences between the B–S–B and random networks decrease with increasing styrene content, while swelling differences between the B–S–B and B–S networks increase with increasing styrene content. These results are explained by the nature of the crosslinking reaction and the number of loose ends present in each network.