Low frequency excitation in amorphous acrylic polymers

Low frequency Raman depolarized spectra from poly(methyl methacrylate) have been determined at temperatures between 293 K and 85 K in the frequency region $\text{5}\text{cm}{}^{\mathrm{?1}}\text{<}\text{v}\text{?}\text{<2000}\text{cm}{}^{\mathrm{?1}}$. All the reduced intensity spectra resolve into two bands: a broad and intense band at 91 cm^?1 and a narrower and weaker band at 22 cm^?1. The Raman intensities do not depend on temperature which implies first order Raman scattering. Room temperature low frequency Raman spectra were also recorded from poly(ethyl methacrylate) and poly(-n-butyl methacrylate). In the two PMMA substitutes the high frequency band is shifted to lower values as the side group mass is increased and show a Debye-like character. The lower frequency band cannot be resolved in the spectrum of PEMA and PnBMA. The two bands at 91 cm^?1 and 22 cm^?1 are attributed to side group and backbone motions respectively. By studying PMMA samples of different tacticity no dependence of the band frequency on chain configuration has been detected. The density of states g(Ω) obtained through the low frequency Raman spectrum has been used to calculate the specific heat C_v. The values of C_v obtained are lower than those measured calorimetrically but varied with temperature in a similar manner in the region 0.8<T<4K.     

Monitoring the chemical crosslinking of propylene polymers through rheology

The chemical crosslinking of propylene polymers is described; the reaction has been performed under dynamic conditions, using a peroxide and a furan or bis-maleimide-based coagent as crosslinking promoter; the reaction mechanisms have been investigated through FT-IR spectrometry. Crosslinking experiments have been carried out while keeping constant the amount and the nature of peroxide and increasing the coagent concentration; four different coagents have been used. The samples have been studied, in the molten state, by measuring the melt flow rates (MFR) and the values of G′ (storage modulus), G″ (loss modulus), Tg(δ) (ratio G″/G′), and η^∗ (complex viscosity), in the frequency range 10⁻²-10² rad s⁻¹. Another set of measurements has been performed on the same materials, collecting for each sample the values of η⁰ (zero shear viscosity) and J_e⁰ (steady state creep compliance) by creep-creep recovery experiments. The effects of crosslinking degree estimated by gel content and through rheological measurements allows one to evaluate the efficiency of the coagent both in terms of prevention of β-scission and promotion of the crosslinking reactions. These results are discussed with reference to the selection of proper conditions of the crosslinking process in order to achieve a material having desired MFR, rheological behavior and melt elasticity for selected application.     

Study of rotational mobility of stable nitroxide radicals in solid polymers

The rotational mobility of stable nitroxide radicals in PS, PMMA, PVC, PP and PE has been studied over a wide temperature range by the e.s.r. method. At temperatures T lower than T_g, spin probes act as kinetically independent particles, the rotational frequency of which is deter mined mainly by the micropore dimensions of the polymer and depends indirectly on the mobility of segments or side groups.     

Optically active polymers containing sidechain trans-stilbene chromophores

Optically active copolymers of (?) menthyl acrylate (MtA) with trans-4-hydroxystilbene acrylate (SA), with a wide composition range, have been prepared using AIBN as the radical initiator. Reactivity ratio values of the comonomers, evaluated by the Kelen-Tüdös method, have allowed us to show that SA exhibits a higher reactivity compared with MtA. Mean sequence lengths of both co-units and their distribution in the macromolecules have been evaluated by statistical calculations. Chiroptical properties of side-chain stilbene chromophores have been investigated by circular dichroism measurements and related to the composition and distribution of monomeric units.     

Photoresponsive polymers: 2. The monolayer behaviour of photochromic polymers containing aromatic azobenzene residues

The monolayer characteristics of photochromic azo containing addition and condensationpolymers have been investigated under different conditions of illumination. Possible orientations of the polymers in the monolayers have been proposed. It was found that trans → cis photoisomerization of these materials was not always possible in the Langmuir films and photoinduced changes in monolayer area with time were followed for those polymers which showed uninhibited photoisomerization.     

Structure and properties of crystalline polymers

Starting with a structural study of the crystallization behaviour of poly(vinyl alcohol), the author has been analysing the crystal and molecular structures of crystalline polymers for the past 35 years. One of the characteristic points of the methods used is the co-operative use of X-ray diffraction and infrared and Raman spectroscopy (normal coordinate treatment). There are many examples of the application of this technique to a series of polyethers, polythioethers, polyesters, polymer complexes etc. Furthermore, the intra- and intermolecular energy calculations have succeeded in accelerating the structural analyses of important but complicated polymer materials and in revealing the factors governing the stable crystal structure and molecular conformation of a polymer. Poly(ethylene oxygenzoate) x form and double-stranded helices of isotactic poly(methyl methacrylate), the first double helix ever found for synthetic polymers, are used as typical for the application of this method. The structural interpretation of the mechanism of optical compensation in racemic polymers is also a good example. The energy calculations have developed to the stage where the stability of two crystal forms of polyethylene can be discussed in terms of free energy. Utilizing the structural data thus accumulated and the spectroscopically obtained interaction parameters, the structure-property relationship has been clarified quantitatively by lattice dynamical theory. The calculated crystallite moduli of polymer chains agree well with the observed ones for many polymers such as poly-p-phenylene terephthalamide, etc. The study has been advanced by a new method of calculation of the three-dimensional elastic constant tensor and its application to polyethylene, poly(vinyl alcohol), nylon 6 etc. As an extension the general method of calculating the piezoelectric constant tensor has also been derived and successfully applied to poly(vinylidene fluoride) form I, resulting in the interpretation of the origin of macroscopic piezoelectricity of this polymer.     

Hierarchical nanophase-separated structures created by precisely-designed polymers with complexity

This review paper summarizes recent advances in self-assembly of complex polymers, focusing on three characteristic polymeric systems. The first is star-branched polymers of the ABC type, the second one consists of multiblock terpolymers with different chain lengths, while the third comprises supramacromolecular assembly systems with hydrogen and ionic bonding interactions between different polymer species. A quasicrystalline tiling structure with mesoscopic length scale has been found for the first star polymer system as well as the periodic Archimedean tiling structures, and moreover three-dimensional Zincblende network structure has been discovered. Furthermore the hierarchical structures having two length scales have been also found for the ABC star molecules whose chain length ratios, that is, A/B and/or A/C are larger than approximately five. Hierarchical structures with double periodicity have been observed for the hexablock and undecablock terpolymers and it has been revealed that their morphology changes systematically depending on composition of polymeric species. Poly(4-hydroxystyrene) (H) homopolymer was found to be dissolved into microdomain of poly(2-vinylpyridine) formed by poly(styrene-b-2-vinylpyridine) due to hydrogen bonding interaction, resulting in the origin of morphological transitions depending on the composition of H homopolymer added. Hierarchical structures possessing double periodicity have been found for poly(isoprene-b-2-vinylpyridine)/poly(styrene-b-4-hydroxystyrene) blends depending on both volume fractions of component polymers and blend ratio. Blends of different homopolymers with several complementary nucleotides or acid/base moieties on chain ends have been confirmed to show nanophase-separated structures as a result of successful formation of “supramacromolecules”.     

Proton spin-lattice relaxation in vinyl polymers

Proton spin—lattice relaxation times and Overhauser enhancements in vinyl polymers have been calculated for three correlation time distributions, and compared with experimental measurements on solutions of poly(vinyl acetate) in toluene-d₈. The results show that of the models considered, only the conformational jump model offers a consistently successful interpretation of the polymer motion.     

Molecular motions in poly(dimethyl siloxane) oligomers and polymers

The glass transition temperatures of fifteen samples of poly(dimethyl siloxane) polymers and oligomers have been measured by differential scanning calorimetry. Polymers with M_n>2400 exhibit an asymptotic value of T_g(∞)=148K, but for shorter chains the T_g is lower. The thermomechanical behaviour has also been examined using a torsional braid analyser and this has served to confirm the chain length dependence of T_g. A sub-glass transition, located in the temperature range 80–120K for short chain samples, has been attributed to methyl group rotation.     

Composite nanofibers of conducting polymers and hydrophobic insulating polymers: Preparation and sensing applications

Various conducting polymer/hydrophobic insulating polymer (CP/HIP) composite nanofibers have been prepared by electrospinning and vapor deposition polymerization (VDP) with benzoyl peroxide (BPO) as oxidant. BPO is soluble in N,N-dimethylformamide (DMF) and can form homogenous solutions with hydrophobic polymers such as poly(methyl methacrylate) (PMMA) and polystyrene (PS). High-quality nanofibers of PMMA or PS containing a certain amount of BPO were produced by electrospinning and used as the templates for VDP of pyrrole, 3,4-ethylenedioxythiophene (EDOT), and aniline. The non-woven mats of the resulting CP/HIP composite fibers can be used as the high-sensitive sensing elements of gas sensors. A gas senor based on polypyrrole (PPy)/PMMA composite fibers was fabricated for sensing ammonia or chloroform vapor, and exhibited greatly improved performances comparing with those of the device based on a PPy flat film.     

Detection of sub-glass transition events in amorphous polymers using differential scanning calorimetry: 3. Further observations on polymers containing pendant cycloalkyl rings

Heat capacity (C_p) contributions have been calculated from the partition function which describes the conformational energies of cycloalkyl rings containing from 9 to 12 carbons. The magnitude of these contributions depends on the number of conformations available to the ring system, and is a maximum for cyclodecane. The heat capacity has been measured as a function of temperature for two dicycloalkyl itaconate polymers, one containing pendant cyclodecyl rings, and the other pendant cyclododecyl rings. In the case of the former polymer an inflection in the C_pT curve is observed below the glass transition temperature. The experimental values associated with this inflection correlate with the calculated contribution from the conformational energies of the cyclodecyl rings, and this supports our assignment of the molecular origin of glassy state relaxations in such polymers. Any similar contributions to the glassy state heat capacities of the other polymers in the series are below the levels of detection of our present equipment.     

Detection of sub-glass transition events in amorphous polymers using differential scanning calorimetry

The variation of the heat capacity (C_p) as a function of temperature has been measured for a sample of poly(di-cyclo-octyl itaconate) using a differential scanning calorimeter. Two inflexions in the C_pT curve have been detected. One, observed at 390K has been assigned to the glass-rubber transition, the other, located at 140K, is a consequence of intramolecular motion in the pendant cyclo-octyl rings. The heat capacity contribution calculated from the partition function describing a series of cyclo-octane conformers agreed closely with the experimental values associated with this lower inflexion, and supported our interpretation of the molecular origins of this transition.     

Tensile strength elevation of brittle polymers by entanglements

Plots of tensile strength (T) versus reciprocal number average molecular weight (M^?1) have been made using previously reported data for linear polymers tested in the glassy state. Over a wide range of molecular weights there is conformity to Flory's empirical equation T = A - BM^?1, in which A and B are constants. Values of M obtained by extrapolation to T = 0 correlate with critical values of molecular weight which are diagnostic of incipient formation of an entangled network. The entanglement thesis is developed further by reference to a model in which brittle strength is attributed to the breaking of covalent backbone bonds. Theoretical values are calculated which exceed experimental values by a factor of only three. Such close agreement is attributed to the insensitivity of glassy linear polymers to flaws.     

Low temperature dielectric relaxation in polymers containing an aromatic group in the main chain

Dielectric measurements of samples of polymers containing phenylene rings in the back-bone chain have been obtained in the frequency range 62Hz to 100kHz and at temperatures between 4·2K and 330K. Most of the polymers with a phenylene ring in the main chain exhibit a dielectric relaxation in the temperature range 120 to 185K (62Hz). In poly(2,6-dimethyl-p-phenylene oxide) a single relaxation region occurs, ranging from 125K (62Hz) to 170K (100kHz). In poly(phenylene sulphide) a very broad relaxation region occurs over the temperature range ～70K to 190K. Poly(p-xylylene) exhibits a well defined relaxation centred at 185K (62Hz) moving to 250K (100kHz). In poly(dichloro-p-xylylene) the relaxation region is in the same temperature range as for poly(p-xylylene), although less sharp; in poly(monochloro-p-xylylene) the beginnings of relaxation region are visible at 330K (62Hz). In all these polymers the low temperature relaxation region is believed to be caused by a local re-orientation of the phenylene ring, or substituted phenylene ring. The substitution of a single chlorine atom on the phenylene ring causes the relaxation to appear at higher temperatures, whereas the disubstitution causes the relaxation region to re-appear at a similar temperature to that for the unsubstituted polymer. The activation energies of these processes lie in the range 30–45MJ/kg mol (7–11 kcal/mol). The temperatures and activation energies of this process appear to depend on the units adjacent to the phenylene ring, and on steric and polar effects caused by substituents on the ring.     

Dilute solution behaviour of polymers near the phase separation temperature

Light scattering and viscosity measurements performed in the temperature range between the θ-point and the temperature of phase separation with dilute solutions of polystyrene in cyclohexane and poly(acrylic acid) in dioxane have indicated a different mechanism of precipitation in the two systems. While polystyrene molecules give rise to progressively growing clusters the poly(acrylic acid) coils assume compact forms before precipitating. Phase separation is achieved in the former case by extensive clustering, in the latter by particle coalescence. The dimensions that the compact form of the poly(acrylic acid) molecule can reach before precipitation sets in correspond to a solid-like density inside the molecular domain. From the experimental data the temperatures characteristic for cluster formation (T_c) and for coil to globule transition (T_g) have been obtained.     

Deformation zones and entanglements in glassy polymers

Deformation zones are regions of drawn but unfibrillated material which grow from crack tips in thin films of glassy polymers which have a low value of I_e′, the chain contour length between entanglements. The growth of these zones is observed optically and their final structure characterized by transmission electron microscopy. By microdensitometry of the electron image plate the average value of the extension ratio within the deformation zone, λ_DZ, is measured. Such deformation zones have been grown in thin films of four homopolymers and a series of polymer blends. It is found that λ_DZ is approximately 0.6 λ_max, where λ_max is a predicted maximum extension ratio derived from a simple model in which the entanglement points are assumed to act as permanent crosslinks with no chain slippage or scission occurring. This value of λ_DZ is lower than the extension ratios previously measured for crazes grown in the same polymers; typical λ_craze values lie much closer to λ_max. This result can be rationalized by realizing that at least a limited degree of chain scission/slippage must occur during crazing to permit the generation of the void-fibril network. For those polymers where both crazing and deformation zones may form, the latter grow rapidly whereas the formation of crazes requires longer times. This observation also indicates the importance of the kinetic process of chain scission/chain slippage for crazing. Annealing of the polymer films below the glass transition temperature leads to an increased tendency for crazing relative to the growth of deformation zones.     

Effect of temperature and frequency in fatigue of polymers

The effects of frequency and temperature on fatigue crack propagation rate in poly(methyl methacrylate) and polycarbonate have been studied using centrally notched plate specimens cycled in tension between constant stress intensity limits. Crack growth was monitored at frequencies between 0.1 Hz and 100 Hz and at temperatures between ?60°C and 40°C. A linear relationship between the cyclic crack growth rate $\text{d}\text{(2a)}\text{d}\text{N}$ and appropriate levels of toughness, K, has been proposed: $\text{d}\text{(2a)}\text{d}\text{N}\text{= A?}{}^{\mathrm{\alpha }}$, where $\text{? =}\text{(λ ? λ}{}_{\mathrm{<mtext>th</mtext>}}\text{)}\text{(K}{}^2{}_{\mathrm{<mtext>1</mtext><mtext>C</mtext>}}\text{? K}{}^2{}_{\mathrm{<mtext>max</mtext>}}\text{)}$, λ = K²_max ? K²_min, λ_th is the threshold limit and A and α are constants. Also, the influence of mean stress intensity was briefly discussed.     

Fluorescence polarization changes in electrically oriented solutions of dye-tagged polymers

Dilute aqueous solutions of poly(methacrylic acid) (M = 7.8 × 10⁵) and sodium polystyrene sulphonate (M = 1.3 × 10⁵) when tagged with the dye acridine orange, have been subjected to pulsed electric fields. Changes have been recorded for the polarized components of the fluorescence with each polymer system, during the application of the electric fields. From these fluorescent changes, which were recorded for wavelengths above 515 nm when radiation of 488 nm was incident, the nature of binding of the dye to these polymers has been estimated. For each system, the planar dye molecules appear to bind parallel to the polymer backbone.     

Influence of molecular weight on dynamic crossover temperature in linear polymers

Dielectric and light scattering spectra of two linear polymers, polyisoprene (PIP) and polystyrene (PS), were analyzed in broad temperature and frequency range above the glass transition temperature, T_g. The crossover temperature, T_C, was estimated using two approaches: (i) derivative analysis of relaxation times proposed by Stickel and (ii) Mode-Coupling Theory approach. Both estimates provide consistent values. T_C varies with molecular weight (MW) in both polymers, while the ratio T_C/T_g changes significantly with MW in PS only. It appears that the segmental relaxation time at T_C has value τ(T_C) ∼ 10⁻⁷ s for both polymers independent of MW and similar to the value reported for many non-polymeric glass-forming systems. No sign of the dynamic crossover has been observed in the chain relaxation around T_C of the segmental dynamics.