Mobility Of Chain Segments Research Articles

Thermal and dynamic mechanical behavior of poly(vinyl chloride)/wood flour composites

AbstractThermal and dynamic mechanical behaviors of wood plastic composites made of poly vinyl chloride (PVC) and surface treated, untreated wood flour were characterized by using differential scanning calorimetry and dynamic mechanical analysis. Glass transition temperature (Tg) of PVC was slightly increased by the addition of wood flour and by wood flour surface treatments. Heat capacity differences (ΔCp) of composites before and after glass transition were markedly reduced. PVC/wood composites exhibited smaller tan δ peaks than PVC alone, suggesting that less energy was dissipated for coordinated movements and disentanglements of PVC polymer chains in the composites. The rubbery plateaus of storage modulus (E′) curves almost disappeared for PVC/wood composites in contrast to a well defined plateau range for pure PVC. It is proposed that wood flour particles act as “physical crosslinking points” or “pinning centers” inside the PVC matrix, resulting in the absence of the rubbery plateau and high E′ above Tg. The mobility of PVC chain segments were further retarded by the presence of surface modified wood flour. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Dynamic Mechanical and Dielectric Properties of Nanocomposites of Natural Rubber (NR), Carboxylated Styrene Butadiene Rubber (XSBR) Latices and their Blends

Abstract The viscoelastic and dielectric properties of nano structured layered silicates reinforced natural rubber (NR), carboxylated styrene butadiene rubber (XSBR) and their blends have been analyzed. The viscoelastic properties such as storage modulus, loss modulus, damping behavior and glass transition temperature of nano filled latices have been investigated. Upon the addition of filler, the storage modulus of nanocomposites was found to increase due to the enhancement in stiffness of the material. Due to the restricted mobility of polymer chain segments, the damping values decreased as a function of filler loading. An investigation of the viscoelastic properties revealed that there was a strong interaction between the polymer and the filler. Latex nanocomposites was characterized by X-ray diffraction technique. The enhanced d values indicated the intercalation of polymer chain into the layers of silicates. The dielectric properties of nanocomposites have been investigated as a function of frequency in the range of 50Hz–100KHz. The effect of frequency on dielectric permittivity (Ε′), dielectric loss (Ε″), dissipation factor (tan δ) and volume resistivity (ρv) of latex nanocomposite have been measured under alternating current. The dielectric permittivity of the samples was found to be higher upon the incorporation of nano fillers. The volume resistivity decreased due to the enhanced conductivity of filled samples.

Non-isothermal crystallization kinetics and melting behaviors of nylon 11/tetrapod-shaped ZnO whisker (T-ZnOw) composites

Differential scanning calorimetry (DSC) was used to investigate the non-isothermal crystallization kinetics and melting behaviors of nylon 11/T-ZnOw composites. For non-isothermal studies, the Avrami equation modified by Jeziorny, the Ozawa model and the equation combining the Avrami and Ozawa theories were employed. It was found that the Ozawa model failed to describe the non-isothermal crystallization process for both nylon 11 and nylon 11/ T-ZnOw composites, while the Avrami equation modified by Jeziorny and the equation combining the Avrami and Ozawa theories exhibited great advantages in treating the non-isothermal crystallization kinetics and gave good results. The results showed that the addition of T-ZnOw influenced the mechanism of non-isothermal crystallization and promoted the crystallization rate of PA11 matrix, which indicated that the T-ZnOw might act as a nucleating agent. Further analysis based on the Hoffman–Lauritzen theory was presented. The results revealed that the surface fold free energy σ e and the absolute values of effective activation energy E α for PA11/T-ZnOw composites were higher than those for neat PA11, which indicated that the addition of T-ZnOw hindered the mobility of chain segments. Both nylon 11 and nylon 11/ T-ZnOw composites exhibited two melting endotherms, while the equilibrium melting point obtained for PA11/T-ZnOw composites was higher than that for neat PA11.

Broadband dielectric spectroscopic characterization of the hydrolytic degradation of carboxylic acid-terminated poly(d,l-lactide) materials

Broadband dielectric spectroscopy was used to examine carboxylic acid-terminated poly(d,l-lactide) samples that were hydrolytically degraded in 7.4 pH phosphate buffer solutions at 37°C. The dielectric spectral signatures of degraded samples were considerably more distinct than those of undegraded samples and a Tg-related relaxation associated with long range chain segmental mobility was seen. For both degraded and undegraded samples, a relaxation peak just beneath a DSC-based Tg was observed, which shifts to higher frequency with increasing temperature. Thus, this feature is assigned as the glass transition as viewed from the dielectric relaxation perspective. Linear segments on log–log plots of loss permittivity vs. frequency, in the low frequency regime, are attributed to d.c. conductivity. An upward shift in relaxation peak maximum, fmax, observed especially after 145d of immersion in buffer, implies a decrease in the time scale of long range segmental motions with increased degradation time.Permittivity data for degraded and undegraded materials were fitted to the Havriliak–Negami equation with subtraction of the d.c. conductivity contribution to uncover pure relaxation peaks. Parameters extracted from these fits were used to construct Vogel–Fulcher–Tammann–Hesse (VFTH) curves and distribution of relaxation time, G(τ), curves for all samples. It was seen that the relaxation times for the α-transition in both degraded and undegraded samples showed VFTH temperature behavior. G(τ) curves showed a general broadening and shift to lower τ with degradation, which can be explained in terms of a broadening of molecular weight within degraded samples and faster chain motions.

Viscoelastic properties of in situ composite based on ethylene acrylic elastomer (AEM) and liquid crystalline polymer (LCP) blend

Viscoelastic properties of in situ ethylene acrylic elastomer composites containing 10 and 20 wt.% of liquid crystalline polymer (LCP) were measured in dynamic mechanical experiments. The results show the significant improvement in the storage modulus of the composites with increasing LCP content. All the composites exhibits a single relaxation, which is glass to rubber transition, observed as a peak in the loss modulus at 1 Hz. The time–temperature superposition principle was applied for the pure ethylene acrylic elastomer (AEM) and its composites, in order to evaluate the changes in the viscoelastic properties of AEM by the addition of LCP. Temperature dependence of the shift factors were described using both the Arrhenius and William–Landel–Ferry (WLF) model at the glass transition region. The activation energies of the shift factors are increases with increasing temperature due to complexicity in the motions of the polymer chains, which is further confirmed by the large activation entropies. The principle of time–temperature superposition is used to study changes in the relaxation spectra of in situ AEM system. It is suggested that the change in the viscoelastic behavior, in the case of LCP reinforced AEM, is due to altered molecular mobility of the polymer chain segments in the vicinity of the LCP phase.

On promoting dispersion and intercalation of bentonite in high density polyethylene by grafting vinyl triethoxysilane

Vinyl triethoxysilane grafted high density polyethylene (HDPE-g-YDH151)/Bentonite (BT) composites were prepared via melt compounding and compared with HDPE/BT composites. FTIR proved that HDPE-g-YDH151 is chemically bonded to BT sheets. XRD, SEM and TEM results indicated that the introduction of YDH151 promoted the dispersion and intercalation of BT into HDPE. Consequently, HDPE-g-YDH151/BT composites show satisfied mechanical properties, e.g., the composite with 2phr BT has an elongation at break 29% higher and Young’s modulus 32% higher than that of HDPE-g-YDH151.Comparatively, BT aggregated in HDPE/BT composites and all the mechanical properties decreased. Because of high interfacial adhesion between HDPE-g-YDH151 matrix and exfoliated BT, which reduces the mobility of crystallizable PE chain segments, and subsequently reduces the crystallization ability. In comparison, the addition of BT to HDPE did not affect the crystallization behavior of the later.

Restricted chain segment mobility in poly(amide) 6/clay nanocomposites evidenced by quasi-isothermal crystallization

Modulated temperature differential scanning calorimetry is used to explore the interactions between a poly(amide) 6 matrix and various types of clay reinforcement. During quasi-isothermal crystallization of the polymer/clay nanocomposites, an excess contribution is observed in the recorded heat capacity signal, due to reversible melting and crystallization. It is proposed that the magnitude of this excess contribution can be used to qualify the polymer/clay interfacial interaction, as it is directly linked to the segmental mobility of the polymer chains in the interphase region, where both the crystalline and amorphous polymer fractions are affected. It is shown that the interfacial interaction strongly depends on the type of clay filler used. These interactions play a key role in the development of specific material properties for the different types of nanocomposites. A simple interphase model for the poly(amide) 6/clay nanocomposites is proposed.

Glass transition and the origin of poly(p-phenylene sulfide) secondary crystallization

The effect of low temperature cold-crystallization on quenched poly(p-phenylene sulfide) (PPS) amorphous phase behaviour was systematically investigated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) over the entire range of the process, from its early stage to the end. For the first time a well resolved double glass transition of partially cold-crystallized samples was evidenced. A T(g) steady increase was observed during the primary crystallization process, due to the reduction of amorphous chain segmental mobility imposed by the growing rigid phase. A shift of the relaxation temperature of about 10 degrees C was recorded at the end of the primary crystallization process. As the secondary crystallization takes place a new glass transition appears at higher temperature. For longer annealing time the lower T(g) disappears while the intensity of the upper one increases. The upper temperature glass transition of semi-crystalline PPS is explained as a consequence of the PPS secondary cold-crystallization process. In the light of the thermal and dynamic mechanical results, an interpretation is given of step-wise double crystallization, evidenced in non-isothermal cold-crystallization experiments carried out at low heating rate and followed by means of FT-IR techniques.

Thermoelastic Behavior of Polyurethanes Reinforced with the In situ‐Generated Sodium Silica‐Polyphosphate Nanophase

Segmented polyurethane‐based nanocomposites prepared by reaction between the organic precursor (macrodiisocyanate, MDIC) and the inorganic precursor (sodium silica‐polyphosphate, SSP) were characterized by differential calorimetry, wide‐angle and small‐angle X‐ray diffraction, and stretching calorimetry techniques. Steric restrictions on the mobility of soft MDIC chain segments by a spatial network of self‐associated stiff segments were assumed responsible for the overshoot of experimental values of the glass transition temperature above that expected for completely miscible components. The initial spatial network of self‐associated stiff segments of the MDIC was destroyed by the randomly distributed SSP particles, giving rise to their own fractal‐like structure. Low reinforcement efficiency of the SSP nanophase was tentatively attributed to the relatively large size of its spatial aggregates. The scale of particle displacements within an infinite cluster during stretching is likely to be negligibly small.

Gamma radiation induced effects on silica and on silica–polymer interfacial interactions in filled polysiloxane rubber

We report in our studies to assess the impact of gamma radiation on silica and on the silica–polymer interface in filled polysiloxane rubber. Electron spin resonance (ESR) and solid-state nuclear magnetic resonance (NMR) studies have been performed on samples exposed to gamma radiation. In an effort to probe directly the effect of gamma radiation on the silica surface, we employed 1H and 29Si NMR. Our ESR studies show trapped paramagnetic species (positive holes and/or trapped electrons) within the host silica matrix for all samples exposed to gamma radiation. A sample of pure cab-o-sil irradiated to a dose of 50 kGy also shows an ESR signal. Our studies on real-time aged samples (derived from field trials) also show ESR signatures indicative of silica based trapped paramagnetic species. The growth of trapped paramagnetic species as a function of gamma dose was investigated. This shows that the build up of trapped species occurs rapidly at low gamma dose before reaching saturation at about 20–30 kGy. Radiation induced band gap excitation is the likely process leading to the creation of these paramagnetic species which may be trapped in regions of local charge deficit within the silica matrix. Species that are not trapped may take part in silica surface reactions leading to changes in filler–polymer interfacial interactions. NMR studies combined with ammonia modified swell studies have shown increased polymer segmental chain mobility (softening) at low gamma dose indicative of a possible reduction in filler–polymer interfacial interactions. For those samples exposed to high gamma dose, our ammonia modified swell studies suggest increased polymer–filler interactions presumably through silica–polymer crosslinking effects. Our 1H and 29Si NMR studies on irradiated silica suggest that the silica surface is sensitive to gamma radiation. Our observations are important as they highlight the need to better control the quality (size, purity, etc.) of the silica constituent in filled polymer components used in gamma radiation environments.

Positron annihilation study of density fluctuations in amorphous poly(ethylene terephthalate) films in terms of quasispinodal decomposition

Positron annihilation lifetime spectroscopy was applied to study poly(ethylene terephthalate). Changes in the positronium yield as a function of the temperature and as a function of the elapsed experiment time at predetermined constant temperatures were measured. The long-lived component was due to ortho-positronium ($\mathrm{o}\text{\ensuremath{-}}\mathrm{Ps}$) pick-off annihilation, and the temperature dependences of the $\mathrm{o}\text{\ensuremath{-}}\mathrm{Ps}$ lifetime $({\ensuremath{\tau}}_{3})$ and the corresponding intensity $({I}_{3})$ were in good agreement with the molecular mobility of the polymer chain segments. The peak positions of the $\ensuremath{\beta}$ $(74\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and $\ensuremath{\gamma}$ $(\ensuremath{-}66\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ dispersions obtained by dynamic mechanical measurements were in good correlation with the first and second transitions of ${\ensuremath{\tau}}_{3}$. The mechanical dispersion and the first transition appeared at around $70\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, while the dispersion and the second transition appeared around $\ensuremath{-}66\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The curves of ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ as a function of the elapsed experiment time can be divided into three stages: stage I, where ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ were maintained constant; stage II, where ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ tended to decrease; and stage III, where ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ tended to keep constant again. The higher was the annealing temperature, the shorter became the periods of stages I and II. This result was similar to that obtained by using time-resolved light scattering. Furthermore, time-resolved x-ray diffraction indicated no existence of crystallites in stage II. Thus, it turned out that the decreases of ${\ensuremath{\tau}}_{3}$ and ${I}_{3}$ with elapsed time in stage II corresponded to the molecular ordering arising due to the density fluctuations of the amorphous phase associated with quasispinodal decomposition.

Experimental evidence for reduced chain segment mobility in poly(amide)-6/clay nanocomposites

Poly(amide)-6/clay nanocomposites are investigated by means of modulated temperature differential scanning calorimetry. The importance of polymer–filler interaction is explored by comparing nanocomposites based on untreated and organically modified clay. During quasi-isothermal crystallization experiments, an excess contribution is observed in the recorded heat capacity signal due to reversible melting and crystallization. The magnitude of this excess contribution depends on the nanocomposite investigated. We suggest that it is directly related to the segmental mobility of the polymer chains in the interphase region. As such, the magnitude of this excess contribution can be used to quantify the efficiency of the polymer–clay interaction. Depending on the clay type used, differences in interfacial interaction can be achieved, which is of great importance with respect to the improvement of material properties. Based on thermal analysis results, a simple interphase model is proposed that is able to account for both the thermal and mechanical properties of poly(amide)-6/clay nanocomposites.

Mass uptake study of the diffusion of water and SBF into poly(2-hydroxyethyl methacrylate-co-tetrahydrofurfuryl methacrylate) containing aspirin or vitamin B12

The ingress of water and Kokubo simulated body fluid (SBF) into poly(2-hydroxyethyl methacrylate) (PHEMA), and its co-polymers with tetrahydrofurduryl methacrylate (THFMA), loaded with either one of two model drugs, vitamin B12 or aspirin, was studied by mass uptake over the temperature range 298–318 K. The polymers were studied as cylinders and were loaded with either 5 wt% or 10 wt% of the drugs. From DSC studies it was observed that vitamin B12 behaved as a physical cross-linker restricting chain segmental mobility, and so had a small anti-plasticisation effect on PHEMA and the co-polymers rich in HEMA, but almost no effect on the T g of co-polymers rich in THFMA. On the other hand, aspirin exhibited a plasticising effect on PHEMA and the co-polymers. All of the polymers were found to absorb water and SBF according to a Fickian diffusion mechanism. The polymers were all found to swell to a greater extent in SBF than in water, which was attributed to the presence of Tris buffer in the SBF. The sorptions of the two penetrants were found to follow Fickian kinetics in all cases and the diffusion coefficients at 310 K for SBF were found to be smaller than those for water, except for the polymers containing aspirin where the diffusion coefficients were higher than for the other systems. For example, for sorption into PHEMA the diffusion coefficient for water was 1.41 × 10−11 m2/s and for SBF was 0.79 × 10−11 m2/s, but in the presence of 5 wt% aspirin the corresponding values were 1.27 × 10−11 m2/s and 1.25 × 10−11 m2/s, respectively. The corresponding values for PHEMA loaded with 5 wt% B12 were 1.25 × 10−11 m2/s and 0.74 × 10−11 m2/s, respectively.

Influence of ionic aggregation on the surface energies of crystallites in poly(butylene terephthalate) ionomers

The influence of ionic comonomer units and ionic aggregation on the crystallization behavior and crystallite surface energies of poly(butylene terephthalate-co-5-sodiosulfoisophthalate) (PBTi) ionomers containing 3 and 5mol% dimethyl-5-sodiosulfoisophthalate monomer units is compared to that of pure poly(butylene terephthalate). The rate of crystallization of the ionomers, as determined from an Avrami analysis, was shown to decrease with an increase in the ionic comonomer unit content. This behavior was attributed to the presence of non-crystallizable comonomer units and the reduced mobility of the crystallizable chain segments due to ionic aggregation in the PBT ionomers as evidenced by the broad peak centered about −9.5ppm in the 23Na NMR spectrum. The surface free energies of the polymer crystallites were determined using the Avrami rate constants in a modified Laurizten–Hoffman method. The PBT ionomers showed a 35–70% increase in the product of the surface energies (σσe) over that observed for pure PBT. By assuming a constant lateral surface energy, this behavior was attributed to a 30–70% increase in the work associated with chain folding. Relative to crystallites formed in pure PBT, these data suggest that there is a more disordered crystalline fold surface in the ionomers due the perturbed development of crystallites in the presence of a strong electrostatic network.

Study of the nanomorphology of OC1C10-PPV/precursor-PPV blends by solid state NMR relaxometry

Films of conjugated polymer blends were studied by NMR relaxometry in order to improve the understanding of the nanomorphology and segmental chain mobility. Whereas optical microscopy was applied to obtain a rough impression about the surface morphology, the T1H and T2H NMR relaxation decay times were determined in the solid state (13C-CP/MAS and 1H-wideline NMR) to judge the homogeneity of the phase morphology, to estimate the size of phase separated molecular domains and to compare the local segmental chain mobility. Besides blend composition, important parameters investigated are the film processing technique (dropcasting vs. spincoating), the casting solvent and the casting substrate.

Study of the nanomorphology of OC 1C 10-PPV/precursor-PPV blends by solid state NMR relaxometry

Films of conjugated polymer blends were studied by NMR relaxometry in order to improve the understanding of the nanomorphology and segmental chain mobility. Whereas optical microscopy was applied to obtain a rough impression about the surface morphology, the T 1H and T 2H NMR relaxation decay times were determined in the solid state ( 13C-CP/MAS and 1H-wideline NMR) to judge the homogeneity of the phase morphology, to estimate the size of phase separated molecular domains and to compare the local segmental chain mobility. Besides blend composition, important parameters investigated are the film processing technique (dropcasting vs. spincoating), the casting solvent and the casting substrate.

Spin-label electron spin resonance studies of micellar dispersions of PEGs–PEs polymer-lipids

Conventional electron spin resonance (ESR) spectroscopy of different positional isomers of phosphatidylcholine spin labels ( n-PCSL; n=5, 7, 10, 12, 14, and 16) has been used to study micellar dispersions made of poly(ethylene glycol)s–phosphatidylethanolamines (PEGs–PEs) polymer-lipids. Such aggregates are currently used as long circulating drug delivery systems “in vivo.” We varied both the hydrocarbon chain length and the polymer size of the polymer-lipids. The dependence of the lipid-chain packing density on temperature and on label position as well as the flexibility and polarity profiles with position of chain labeling have been established for the PEGs–PEs micellar dispersions. The results show both similarity and differences either with common micellar dispersions of single chained lyso-palmitoylphosphatidylcholine (C 16Lyso-PC) or with lamellar dispersions of double chained dipalmitoylphosphatidylcholine (DPPC). Well defined chain flexibility gradients of the same overall shape are obtained in the considered dispersions. However, the mobility of the first acyl chain segments is appreciable higher in micelles of polymer-lipids than in bilayers of DPPC and it becomes indistinguishable at the chain termini. A trend of decreasing polarity on moving toward the bilayer interior is seen in DPPC bilayers, whereas biphasic polarity profiles are obtained in micelles of polymer-lipids and C 16Lyso-PC. Moreover, the properties of the PEGs–PEs micelles do not depend on the length of the hydrocarbon chain of the polymer-lipids but are slightly influenced by the size of the polymer.

Prediction of brittle-to-ductile transitions in polystyrene

In this study it is attempted to predict brittle-to-ductile transitions (BDTs) in polystyrene blends, induced either by an increase in temperature or by a decrease in inter-particle distance. A representative, two-dimensional volume element (RVE) of a polystyrene matrix with 20% circular voids, is deformed in tension. During deformation a hydrostatic-stress based craze-nucleation criterion [1] is evaluated. The simulations demonstrate that crazes initiate at low temperatures while a transition from crazing to shear yielding (BDT) is found around 75 °C. The numerical results correlate well with tensile tests on similar heterogeneous polystyrene. The presence of an absolute length, as experimentally found, is more difficult to explain. Near a free surface a T g-depression is measured for polystyrene and also the resistance to indentation in polystyrene is lower than expected from bulk properties. Both observations are rationalised by an enhanced segmental mobility of chains near a free surface. As a consequence of these findings, an absolute length-scale could be incorporated in the numerical simulations. For simplicity, the length-scale is modelled by taking a temperature gradient over a thin layer near the internal free surfaces of the RVE. Deformation of the RVE with different absolute length-scales shows that indeed also the experimentally found brittle-to-ductile transition can be predicted if the ligament thickness between the inclusions (‘voids’) in polystyrene is below a critical value of ca. 15 nm.

Study of the amorphous phase in semicrystalline poly(ethy1ene terephthalate) via dynamic mechanical thermal analysis

Characters of amorphous phase in semicrystalline poly(ethy1ene terephthalate) (PET) were investigated systematically via dynamic mechanical thermal analysis (DMTA) in this paper. It was found that the storage modulus (E') and the glass transition temperature (Tg) of semicrystalline PET changed with the degree of crystallinity (Xc). Tg showed good linearity with Xc. However, neither reduction of E' in the Tg region (ΔlgE') nor loss tangent (tanδ) at the Tg presented linearity with Xc, which suggests that the two-phase model was not suitable for semicrystalline PET. It was also confirmed that the physical aging reduced the chain segmental mobility, producing higher Tg.

Effect of the segmental mobility restriction on the thermoset chemical kinetics

AbstractThe cure kinetics of an epoxy–amine commercial thermoset system have been investigated with the isothermal differential scanning calorimetry technique. In particular, a kinetic study has been performed in the glass–transition zone, in which diffusion phenomena compete with the chemical transformations and the overall reaction rate is partially slowed by the reduced segmental chain mobility. A generalized form of the Vogel equation has been formulated to account for the effect of the increasing glass–transition temperature on the chain mobility and, therefore, on the overall reaction rate. The kinetic model has been expressed with two factors representing the chemical reaction rate and the segmental mobility reduction. As the main result, the activation energy relative to the diffusion phenomena has been found to be very low, having a value of 42.5 K ≈ 0.356 kJ/mol, which is compatible only with the small‐angle rotation of the reactive unit. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3757–3770, 2002