N.m.r. studies on the effect of water on the glass transition of polystyrene

These investigations are concerned with water-polymer interactions in polymer latices. It is known that water can act as a plasticizer for many solid polymers and cause a reduction in the glass transition temperature, T_g, of the amorphous regions. Experiments were carried out to determine whether pulsed n.m.r. techniques could be used to study the T_g of a polymer suspension and hence the influence of water and electrolyte on it. From T₁ and T₂ proton relaxation measurements as a function of temperature on polystyrene latex systems it was shown that the presence of water lowers the T_g of the polymer particles (by about 10°C), the effect being slightly greater in the presence of concentrated electrolyte. The extent of electrolyte penetration into the particles was deduced by studying relaxation as a function of particle diameter in latices containing paramagnetic Mn²⁺ ions. Using simple theories of relaxation and spin diffusion it was concluded that for all but the smallest particles electrolyte penetration is restricted to a very thin shell of the order of 1 nm. These conclusions were supported by the results of similar measurements on PTFE particles.     

Thickness variation in the thermoforming of poly(methyl methacrylate) and high-impact polystyrene sheets

The influence of material flow properties on the variation of wall thickness in a thermoformed part was investigated by measuring the thickness reduction at the pole of free-formed axisymmetric domes of poly(methyl methacrylate) and high-impact polystyrene. It was found that at a given pole height, the thickness reduction in poly(methyl methacrylate) was less than in high-impact polystyrene, i.e., the wall thickness in a part formed from poly(methyl methacrylate) will be more uniform than in a part formed from high-impact polystyrene by the same technique. This difference in formability was ascribed to a difference in the dependence of the flow stress σ at the thermoforming temperatures on time. The flow stress of both materials was given by σ = Kt^m′?ⁿ, but whereas n was approximately 1 for both materials, m′ was ?0.052 and ?0.33 for poly(methyl methacrylate) and high-impact polystyrene, respectively. A physical argument and simple analysis led to the conclusion that a large (negative) value of the “stress relaxation index” in a material reduces the degree of uniformity of sheet thickness in a formed part.     

Relaxation processes in the surface layers of polymers at the interface

The proton spin-lattice relaxation and dielectric relaxation were studied in some polymers at the solid–polymer interface was constructed from several filled polymers. A useful model of surface layer which can be considered as consisting of a great number of small solid particles covered with a polymer layer. The following systems were studied: polystyrene, poly(methyl methacrylate), their copolymers and cellulose acetate in the presence of different content of fine particles of aerosil and Teflon. It was established that the decrease of surface layer thickness shifts the minimum of spin-lattice relaxation time T₁ of high temperature process to higher temperature and minimum T₁ of low temperature process to lower temperature. The same was found for dielectric losses reflecting the motion of side groups and of segments. From temperature dependence of T₁ and tan δ for both relaxation processes the apparent energies of activation were calculated. On the base of dielectric relaxation data the circular diagram of complex dielectric constant was constructed and by the Cole-Cole method the dispersion parameter α for polymers at the interface was calculated. These data also show the broadening of relaxation spectra in surface layers. The results are discussed in terms of the restriction of possible conformation of chains at the interface and their interaction with surface. It was established that character of molecular motion changes at the interface is dependent on the mode of molecular motion.     

Microemulsion polymerization of styrene using developed redox initiation system and study of rheological properties of obtained latices

Microemulsion polymerization of styrene was kinetically studied using a potassium persulfate (KPS)/P‐methyl benzaldehyde sodium bisulfite (MeBSBS) adduct as the developed redox pair initiation system. The rate of microemulsion polymerization of styrene was found to be dependent on the initiator, emulsifier, and monomer to the powers of 1.4, −0.77, and 0.83, respectively. The apparent Arrhenius activation energy (E_a) estimated for the microemulsion polymerization system was 6.5 × 10⁴ J/mol. Also, the morphological parameters were studied at different initiator concentrations. The rheological measurements for the prepared microemulsions were carried out to investigate the effect of the preparation parameters on the rheological behavior of the polystyrene microemulsions. The rheological flow curves of the polystyrene microemulsion latices prepared at different temperatures were carried out, and we found that the plastic viscosity and Bingham yield values of the flow curves increased with an increasing reaction temperature. That may be due to the cage effect of the prepared polymer particles, which trapped the medium molecules. The plastic viscosity increased with increasing emulsifier concentration while the Bingham yield value decreased. For the polystyrene microemulsion prepared in the presence of different initiator concentrations, the plastic viscosity and Bingham yield increased with increasing initiator concentration. This trend was found to be the same for the microemulsion latices prepared in the presence of different monomer concentrations. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1240–1249, 2000     

Dielectric investigation of molecular dynamics of blends: III. Effect of molecular weight in TMPC/PS blends

Dielectric and calorimetric measurements have been carried out for tetramethyl polycarbonate/polystyrene (TMPC/PS) blends with different compositions. The effect of varying the molecular weight of the weakly polar component (PS) on the molecular dynamics of the polar segments of TMPC has been thoroughly studied over wide ranges of frequency (10⁻²−10⁵ Hz), temperature (50–220°C) and number average molecular weight, M̄_n, (6500–560 000 g mol⁻¹). All blends were found to be compatible regardless of the composition ratio and the molecular weight of PS. Some new and interesting experimental findings have been observed concerning the effect of molecular weight on the glass temperature and on the broadness of the glass transition and relaxation. Neither the kinetics nor the distribution of relaxation times of the local process observed in pure TMPC was affected by blending with PS, regardless of the composition ratio or the molecular weight of PS. It has been concluded that the mixing of the polymeric components to form a homogeneous single phase (compatible blend) does not take place on a segmental level but on a structural one. The size of this structural level has been suggested to have the same volume as the cooperative dipoles, which is assumed to be the minimum volume responsible for a uniform glass transition (10–15 nm). The molecular weight dependence of the relaxation characteristics of the glass process and temperature could be attributed to the variation in the size and packing of the structural units.     

NMR and sedimentation studies of a polymeric steric stabilizer for alumina

The sedimentation behavior of alumina powder has been studied in the presence of poly-vinylpyrrolidone (PVP) and poly(vinylpyrrolidone-co-vinyl acetate) (PVP/VA) in both thermodynamically “good” and “poor” solvents for the PVP homopolymer. PVP/VA provides higher sediment densities than does its PVP homopolymer counterpart. Solutionstate ¹³C-T₁ spin-lattice relaxation measurements were made on analogous mixtures both with and without alumina powder. The NMR results suggest that the PVP/VA copolymer is anchored to the alumina powder surface by means of VA moieties, whereas the PVP moieties extend into the continuous phase of the slurry medium. Thus, the higher settling densities that are observed in the presence of PVP/VA can be attributed to a steric stabilization mechanism. © 1993 John Wiley & Sons, Inc.     

Dielectric Investigation of Molecular Dynamics of Blends: IV. Effect of Blending on the Normal Mode Process of Polyisoprene/Polystyrene Blends

Polyisoprene/polystyrene (PI/PS) blends have been prepared and investigated for compatibility using dielectric and calorimetric measurements. Various blends were prepared from polystyrene (number average molecular weight, — _n=160 000 g mol^-1) and polyisoprene with — _n values of 13 800, 40 500 and 130000gmol^-1. Dielectric measurements have been carried out over a wide frequency range (10^-2–10⁶Hz) and in the temperature range of the glass and normal mode processes (-70 to +70°C). The glass transition, as well as the corresponding relaxation process, of polyisoprenes were shifted to higher temperatures in the different blends, indicating compatibility. The blends showed a lower critical solution temperature (LCST) at temperatures above 105°C. It was surprising to find that blending of polyisoprene with polystyrene led to a great shift to higher values in the relaxation frequency of the normal mode process for the isoprene segments. The measurements showed that the relaxation time of the normal mode process in the blends was longer than that of the glass process by a constant factor (3·2 decades), regardless of the molecular weight of the polyisoprenes used in the blends. This finding implied that the domain length responsible for the compatibility of the two polymers was consistent regardless of the molecular weight used (where — _n> — _c, the critical molecular weight). In view of the results obtained, and by using a molecular model, it was possible to determine the size of the structural domains responsible for the compatibility. The value obtained (16·7nm) is very similar to that suggested to be responsible for the glass transition in pure polymers. © 1997 SCI.     

Particle‐size‐dependent properties of sulfonated polystyrene nanoparticles

The influence of sulfonation reaction time, temperature and the parent polystyrene (PS) particle size on the degree of sulfonation (DS), ion exchange capacity (IEC), morphology and glass transition temperature (T_g) of sulfonated polystyrene (SPS) particles was investigated. A longer reaction time (ca 2 h) at 40 °C and a smaller particle size resulted in SPS particles with a high DS. It was found that a larger PS particle size did not readily yield SPS particles with a high DS even with a longer reaction time. Contrary to the popular belief in the literature that a higher DS ensures a high IEC, we observed that the proportionality of IEC to DS is primarily controlled by the SPS particle size. Larger IEC values were obtained for larger particles rather than smaller ones despite their similar DS, owing to the presence of strong interactions between ? SO₃H groups within the particles in the latter case which restricts the availability of free H⁺ for ion exchange. The SPS particles displayed a core‐shell morphology in which the outer shell appeared because of sulfonation on the PS. The DS value and the SPS particle size significantly influenced the shell thickness and thereby the morphology of the SPS particles. Copyright © 2012 Society of Chemical Industry     

Effect of molecular weight of an anionic dispersant on the properties of BaTi4O9 slurries

The colloidal stability of BaTi₄O₉ (BT₄) aqueous suspensions with poly(acrylamide‐co‐4‐carboxylamino‐4‐oxo‐2‐butenate) (PAC) of different molecular weights at pH 9 has been investigated by means of zeta potential, adsorption, sedimentation, and particle size measurements. The results indicate that PAC could improve the dispersion of the particles from agglomeration. The resulting suspensions became more stabilized, and contained powder with smaller particle size. Consequently, the compacts with PAC exhibited better properties in terms of density and dielectric constant than those without any polymer present. The performance of PAC increased with decreasing polymer molecular weight. Clearly, PAC1 (M_w = 1.8 × 10⁴) was most effective in dispersing the BT₄ particles, and stabilizing the ceramic suspensions. This is attributed to the highest adsorption of this polymer onto BT₄ powder, and causes strongest electrostatic repulsions among solid particles. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Dielectric investigation of the molecular dynamics of blends (II): polymers with dissimilar molecular architecture (PS/TMPC blend)

The molecular dynamics of polystyrene/tetramethyl polycarbonate (PS/TMPC) blends were investigated using dielectric spectroscopy in the frequency and temperature ranges 10^?2?10⁷ Hz and 50–220°C, respectively. Blends with different compositions, namely 12.5, 25, 50, 75 and 87.5 wt% PS/TMPC were found to be compatible over the entire composition range and showed one glass relaxation process corresponding with the transition observed by calorimetric measurements. It was found that neither the kinetics or the distribution of relaxation times of the local process observed in pure TMPC were affected by blending. This fact implies that the local environment of the segments is not changed upon blending. Furthermore, these blends showed that the blending of TMPC with weakly polar polystyrene does not cause any change in the dipolar interaction of the TMPC segments, so that the relaxation strength varies linearly with composition. It is concluded that the polymeric chains of the TMPC and PS are not mixed at a segmental level, but at a higher structural level lying somewhere between segmental and molecular. This conclusion is in good agreement with that obtained in the case of blends composed of polymers with similar molecular architecture.     

Effects of extrusion on the structure and properties of high-impact polystyrene

A commercial heat-resistant polystyrene (M?_n = 7 × 10⁴, M?_w = 3 × 10⁵), containing 9 percent cis-1, 4-polybutadiene, was extruded either repeatedly (2 to 8 times) at 220°C, or else only once at a higher temperature (up to 290°C). Neither treatment significantly altered the melt rheology at 220°C (pseudoplastic, with n = 0.39), or the tensile modulus (1.5 GPa) and yield stress (20 MPa), or the material's rubber content, determined by both infrared spectrophotonietry and Wijs iodometry. Other properties, only slightly affected by recycling at 220°C, were changed after one extrusion at 290°C: elongation at tensile failure was reduced by 57 percent; in impact testing the strength was 29 percent less, and the mode of fracture (revealed by scanning electron microscopy of the surfaces) became brittle instead of ductile; the rubber particles seen in the transmission electron microscope had agglomerated and lost sphericity; and the ratio of weight-to-number-average molecular weight of the polystyrene component, calculated from gel permeation chromatograms, increased by 93 percent. Mechanical spectra (Rheovibron), from ?120 to + 120°C at 110 Hz, changed gradually with increasingly harsh treatment of the material, a peak emerging at ～50°C due to a beta relaxation of the polystyrene. Thus, good properties were retained after normal processing, but were lost after shearing at too high a temperature, probably because of destruction of entanglements and of the bonds between polystyrene and rubber.     

The effect of curing of unsaturated polyester resin on the dynamic relaxation time

The effect of hot curing of unsaturated polyester resin on the dynamic relaxation time was studied using dielectric measurements along with two dynamic mechanical measurement methods. It was found that the dynamic response during cure was a material frequency dependent property and did not depend on the measurement method. All relaxation times, measured during cure, by all three measurement methods used, converged to a single equation: τ(t)_av=at^b where t= curing time, a, b=constants. The increase of the relaxation time during cure followed the same trend as a friction factor, which was found to increase with conversion. The crosslinking density was found to increase slowly with conversion, while the relaxation time increased exponentially. These two different modes of behavior during cure explain the high resolution of dynamic measurements as a cure monitoring tool, which can easily detect small curing changes. This behavior of the relaxation time was explained by the sharp rise of activation energy due to a parallel decrease of free volume at high conversion.     

The gel-phase modulus of high-impact polystyrene from dynamic mechanical spectroscopy

High-impact polystyrene (HIPS) constitutes a mechanically attractive composite, consisting of a glassy matrix and a rubberlike particle phase (gel phase). Dynamic mechanical spectroscopy was performed for the polystyrene matrix for three different types of HIPS as well as for the concentrated gel-phase material, at the vicinity of the respective glass-transition temperatures (T_g). An approximate estimation of the gel-phase modulus was attempted by using known mechanical models. A comparison with experiments was also made. The modulus of the composite was found to be lower than the theoretical lower bound for particulate composites. This was attributed to a separate phase between gel particles and the matrix. A diffusion-type variation of the modulus of this mesophase layer was estimated, and a correlation between calculated fitting parametric exponents and impact behavior of HIPS was found. Moreover, the T_gs of the materials under investigation were also measured with two independent methods. It was found that all types of HIPS presented higher T_gs than the pure matrix by 5 to 10°C with the highest T_g found being that of the gel-enriched material. The shift of T_gs to higher temperatures was attributed to an eventual increase of the effective cross-link density of the matrix because of grafting.     

The extensional flow of poly(methyl methacrylate) and high-impact polystyrene at thermoforming temperatures

The work described in the present paper was performed to establish stress–strain–time relationships at plastic sheet thermoforming temperatures. The relationships are correlated with sheet-forming “formability”. Specimens of poly(methyl methacrylate) at 165°C and high-impact polystyrene at 122°C were extended to large strains at constant cross-head velocities. Initial strain rates were between 4.2 × 10^?3/sec and 1.6 × 10^?1/sec. It was found that the flow stress σ was related to the true strain ε and the elapsed time t by a relation σ = Kt^m′εⁿ, where K is a constant and n and m′ are indices. The value of n for both materials was approximately one. The value of m′ was ?0.052 and ?0.33 for poly(methyl methacrylate) and high-impact polystyrene, respectively. Tests were also performed in which the cross-head velocity was increased in steps. It was found that the flow stress in these tests followed the same relationship as in the constant cross-head velocity tests.     

Surface-Enhanced Vibrational Spectroscopy: SERS and SEIRA

With respect to the origin of single-molecule sensitivity in surface-enhanced Raman scattering, elastic scattering and emission spectra were investigated for Ag particles adsorbed with dye. The scattering peak observed at 600–650 nm was extinguished during the inactivation process of an enormous SERS signal, whereas localized surface plasmon (LSP) peaks located at 520 nm and 730 nm did not change significantly. The scattering peak at 600–650 nm arises from increased electromagnetic coupling between the LSP of adjacent Ag particles through dye molecules. In addition, distinct emission peaks were observed at 550–600 nm and 600–750 nm for hot Ag particles with adsorbates. These bands were attributed to emissive relaxation of metal electrons and fluorescence of molecules, respectively. Furthermore, the shorter wavelength peak showed invariant Stokes shift irrespective of excitation wavelengths, most probably arising from inelastic scattering of excited electrons by adsorbed molecules. The adsorbed state of CO and related species on the Pt film electrode was investigated using attenuated total reflection—surface-enhanced infrared absorption spectroscopy. Intermediate species were found on the bare Pt surface in 1 mM CH₃OH + HClO₄ solutions at +0.2 V ≤ E ≤ +0.6 V that give absorption peaks at 1405 cm⁻¹ and 1300 cm⁻¹. These bands can be attributed to carbonate species or -COH. Water molecules located at the hydrophobic interfaces between CO and electrolyte solutions were evidenced by a quite high OH stretch absorption at 3664–3646 cm⁻¹, as well as a lower broad peak at ca. 3480 cm⁻¹.     

Investigation of thermal degradation of polystyrene by differential scanning calorimetry

Thermal degradation of polystyrene was performed in a differential scanning calorimeter under nitrogen atmosphere, and the glass transition temperature was measured continuously. From random chain scission kinetics and T_g-molecular weight relationships, the rate constant for random scission was obtained. The rate constant was found to undergo a drastic change at a critical molecular weight (∽45,000), which corresponds to a similar observation in relaxation studies. A viscosity-dependent mechanism for radical chain end termination is thus suggested.     

Rheoiogical studies of compaction for polystyrene powder

Polystyrene powder was compacted in a cylindrical die at constant compaction speed. Compaction was found to be influenced by test temperature, compaction speed and void fraction of the powder bed. The stress reduction behavior of an axial stress σ₁ and a radial stress σ₂ was observed at constant void fraction after compaction. The stress reduction thus obtained was interpretable by adopting an octahedral normal stress σ_oct and an octahedral shear stress τ_oct which were converted from σ₁ and σ₂. This behavior was attributable to the deformation of polystyrene around contact points and to a sliding mechanism between particles. It was ascertained that the relation between a shear stress τ and a normal stress σ obtained on compaction differed from that obtained in shear by a mechanism involving movement between polystyrene particles.     

Orientation and relaxation in uniaxially stretched atactic polystyrene

Infra-red measurements of the dichroic ratio of atactic polystyrene absorption bands provide a valuable method of determination of orientation as well as relaxation of chains during stretching. Different strain rates and temperatures of stretching were used. Orientation relaxation was analysed using Lodge's constitutive equation and a master curve was obtained at a reference temperature T₀ = 115°C. Orientation relaxation behaves similarly to mechanical relaxation and a scaling factor of 3.6 × 10^?8 m² N^?1 holds between the two sets of results.     

A New Method for Determination of the Adsorption Isotherm of SDS on Polystyrene Latex Particles Using Conductometric Titrations

Monodisperse polystyrene (PS) latex particles were prepared through a conventional batch emulsion polymerization procedure. After cleaning, the latex was subjected to a surfactant titration by sodium dodecyl sulfate (SDS) and the conductivity was monitored. Using the break point in the conductivity‐concentration curve, the adsorption area of the surfactant molecules at saturation, A_s, was determined as 46 Ų per SDS molecule. In order to determine the contribution of the different ionic species to the conductivity, a simple model based on experimental conductivity measurements was developed. The degrees of counterion binding to the micelles and to the surfactant anions adsorbed onto polymer particles were calculated as 0.770 and 0.849, respectively. The Langmuir adsorption isotherm was determined for the adsorption of SDS onto PS latex particles with Γ_∞ = 7.257 · 10^–10 mol/cm² and K = 1.208 · 10⁵ cm³/mol.     

Dielectric Relaxation Spectroscopy of Poly (Vinyl Chloride-co-Vinyl Acetate-co-2-Hydroxypropyl Acrylate)/Poly (Acrylonitrile-Butadiene-Styrene) Polymer Blend

The dielectric techniques used to investigate the relaxation behavior of poly (vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) (PVVH), poly (acrylonitrile-butadiene-styrene) (ABS), and its polyblends include broadband AC dielectric relaxation spectroscopy (DRS) in the frequency range from 10^?2 to 10⁵ Hz, and thermally stimulated depolarization current (TSDC) technique in the temperature range from 300 K to 413 K. It was observed that PVVH is characterized by a dipolar relaxation peak around 347 K and a space charge peak in the temperature range 353–383 K, whereas pure ABS is characterized by a dipolar relaxation peak at 389 K. On the other hand, polyblend samples are found to be characterized by two different relaxation peaks at 359 K and 387 K, respectively. The dielectric properties of pure materials and their polyblend are investigated. All samples are characterized by high dielectric constant (ε′) at very low and high temperature. The dielectric loss shows a single peak for pure materials, whereas polyblends show a broad peak at low frequency which could be attributed to the Maxwell-interfacial polarization (MWS) and another peak at high frequency which could be attributed to the dipolar relaxation. The temperature dependence of AC conductivity was investigated for all samples. The values of the exponent n suggest that the hopping mechanism dominates at lower temperatures.