Comparative studies on effects of silica and titania nanoparticles on crystallization and complex segmental dynamics in poly(dimethylsiloxane)

Effects of in situ synthesized silica and titania nanoparticles, 5 and 20-40 nm in diameter, respectively, on glass transition and segmental dynamics of poly(dimethylsiloxane) networks were studied by employing differential scanning calorimetry, thermally stimulated depolarization currents and broadband dielectric relaxation spectroscopy techniques. Strong interactions between the well dispersed fillers and the polymer suppress crystallinity and affect significantly the evolution of the glass transition in the nanocomposites. Next to the α relaxation associated with the glass transition of the bulk amorphous polymer fraction, two more segmental relaxations were recorded, originating from polymer chains restricted between condensed crystal regions (α_c-relaxation) and the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface (α′ relaxation), respectively. Interactions with the polymer were found to be stronger in the case of titania than silica, leading to an estimated interaction length of around 2 nm for silica and at least double for titania nanocomposites.     

Effect of side chain architecture on dielectric relaxation in polyhedral oligomeric silsesquioxane/polypropylene oxide nanocomposites

Segmental and normal mode dynamics in polyhedral oligomeric silsesquioxane (POSS)/poly(propylene oxide) (PPO) non-reactive and reactive nanocomposites were investigated using a broadband dielectric relaxation spectroscopy (DRS) over wide ranges of frequency and temperature. Three POSS reagents with varying side chain architecture were selected for the study: OctaGlycidyldimethylsilyl (OG), TrisGlycidylEthyl (TG) and MonoGlycidylEthyl (MG). Spectra of OG and TG show a segmental (α) process at lower frequency and a local (β) relaxation at higher frequency, while MG displays only a local relaxation. Neat PPO has both segmental and normal mode (α_N) process. In POSS/PPO non-reactive nanocomposites, the presence of OG and TG causes a decrease in the time scale of α_N and α relaxation, while MG has no impact on the dynamics of PPO. Chemical reactions in POSS/PPO reactive nanocomposites lead to the formation of nanonetworks. Prior to the onset of reaction, POSS nanoparticles promote the motions of PPO chains, decrease the time scale of relaxation and give rise to thermodielectrically simple spectra. During the reaction, however, the network formation leads to spectral broadening and a gradual increase in the time scale of both segmental (α) and normal mode (α_N) relaxation. A detailed account of the effects of structure, concentration and dispersion of POSS in the matrix, molecular weight of PPO, extent of reaction and temperature on the molecular origin, temperature dependence and spectral characteristics of relaxation processes in POSS/PPO nanocomposites is provided.     

Relaxation behavior of layered double hydroxides filled dangling chain-based polyurethane/polymethyl methacrylate nanocomposites

A series of dangling chain based-polyurethane/poly(methyl methacrylate) (DPU/PMMA) filled with exfoliated layered double hydroxides (LDH) were synthesized by methyl methacrylate in-situ intercalative polymerization. The dangling chains were introduced by using vegetable oils as chain extender. The effect of dangling chain and the contents of LDH on the molecular dynamics of DPU/PMMA was investigated by a combination of dynamic mechanical analysis and broadband dielectric relaxation spectroscopy. Compared with polyurethane/poly(methyl methacrylate) (PU/PMMA) without dangling chain, the glass transition temperature (T_g) of DPU/PMMA shifts to lower temperature and the segmental dynamics becomes faster. A plateau with a high loss factor value above T_g significantly broadens the damping temperature range due to the synergy effect between the dangling chains and LDH layers. In DPU/PMMA/LDH nanocomposites, the α-relaxation associated with the glass transition of the polymer matrix becomes slower with the increase of LDH content, which indicates a restricted molecular mobility in the interfacial regions between polymer and LDH. However, the local relaxations at relatively low temperature remain unaffected by dangling chain or the addition of LDH. When the LDH content increases, Maxwell–Wagner–Sillars (MWS) interfacial polarization process caused by charge accumulation at interfaces becomes faster because of the smaller mean distance d between the exfoliated LDH layers.     

The synthesis and characterization of polystyrene/magnetic polyhedral oligomeric silsesquioxane (POSS) nanocomposites

Fc-CHCH-C₆H₆-(C₅H₉)₇Si₈O₁₂ (POSS1, Fc: ferrocene) which contain both metal and CC double bond was firstly synthesized by Wittig reaction. The chemical structure of POSS1 was characterized by FTIR, ¹H, ¹³C and ²⁹Si NMR, mass spectrometry and elemental analysis, and the magnetic property of POSS1 have also been studied. Polystyrene composites containing inorganic-organic hybrid polyhedral oligomeric silsesquioxane (POSS1) were prepared by bulk free radical polymerization. XRD and TEM studies indicate that POSS1 is completely dispersed at molecular level in PS matrix when 1 wt% POSS1 is introduced, while some POSS1-rich nanoparticals are present when content of POSS1 is beyond 3 wt%. GPC results show that molecular weight of the PS/POSS1 nanocomposites are increased with addition of POSS1. TGA and TMA data show the thermal stabilities of PS/POSS1 nanocomposites have been improved compared to neat PS. The PS/POSS1 nanocomposites also display higher glass transition temperatures (T_g) in comparison with neat PS. Viscoelastic properties of PS/POSS1 nanocomposites were investigated by DMTA. The results show the storage modulus (E′) values (temperature>T_g) and the loss factor peak values of the PS/POSS1 nanocomposites are higher than that of neat PS. Mechanical properties of the PS/POSS1 nanocomposites are improved compared to the neat PS.     

Polymer dynamics in rubbery epoxy networks/polyhedral oligomeric silsesquioxanes nanocomposites

Dielectric techniques, including thermally stimulated depolarization currents (TSDC, ?150 to 30°C) and, mainly, broadband dielectric relaxation spectroscopy (DRS, 10^?2 – 10⁶ Hz, ?150 to 150°C) were employed, next to differential scanning calorimetry (DSC), to investigate molecular dynamics in rubbery epoxy networks prepared from diglycidyl ether of Bisphenol A (DGEBA) and poly(oxypropylene)diamine (Jeffamine D2000, molecular mass 2000) and modified with polyhedral oligomeric silsesquioxanes (POSS) units covalently bound to the chains as dangling blocks. Four relaxations were detected and analyzed: in the order of increasing temperature at constant frequency, two local, secondary γ and β relaxations in the glassy state, the segmental α relaxation associated with the glass transition and the normal mode relaxation, related with the presence of a dipole moment component along the Jeffamine chain contour. Measurements on pure Jeffamine D2000 helped to clarify the molecular origin of the relaxations observed. A significant reduction of the magnitude and a slight acceleration of the α and of the normal mode relaxations were observed in the modified networks. These results suggest that a fraction of polymer is immobilized, probably at interfaces with POSS, due to constraints imposed by the covalently bound rigid nanoparticles, whereas the rest exhibits a slightly faster dynamics due to increaseof free volume resulting from loosened molecular packing of the chains (plasticization by the bulky POSS units).The increase of free volume is rationalized by density measurements. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dielectric properties of nylon 6/clay nanocomposites from on-line process monitoring and off-line measurements

Nylon 6/clay nanocomposites were studied by dielectric relaxation spectroscopy (DRS) to correlate morphology and microstructure with relaxation behavior of the polymer matrix at the molecular level. Partially exfoliated clay microstructure was achieved by extruding nylon 6 with surfactant-treated montmorillonite clays. A new on-line dielectric slit die sensor was used to examine the melt state properties during extrusion compounding. Solid state properties were probed by off-line DRS over a temperature range from −50 to 180 °C in a frequency range from 10⁻³ to 10⁶ Hz. Using non-linear regression methods in conjunction with the temperature-frequency positions of relaxations observed in the dielectric loss data, the experimental data were fit with the Havriliak-Negami and Cole-Cole dielectric relaxation functions corrected for electrode polarization and DC conductivity. Characteristic frequency, relaxation strength, and DC conductivities were extracted from curves with overlapping relaxation modes. Two dielectric relaxations were observed in the composite melt: the α relaxation associated with molecular segmental motion, and a Maxwell-Wagner relaxation (MW) resulting from interfacial polarization at the resin/clay interface. Analysis of the solid-state data yielded a comprehensive master plot of dielectric relaxations attributed to segmental and local molecular dynamics and other relaxations resulting from water and Maxwell-Wagner interfacial polarization. The impact of clay fillers is seen in nearly all relaxation processes changing both characteristic frequency and strength of the relaxation.     

Nanoconfined segmental dynamics in miscible polymer blend nanocomposites: the influence of the geometry of nanoparticles

The influence of nanoconfinement on segmental relaxation behavior of poly(methyl methacrylate) and poly(styrene-ran-acrylonitrile) miscible blend and its nanocomposites with spherical and layered nanoparticles have been investigated. Dynamic mechanical analysis was employed to examine the effect of geometry of nanoparticles on the temperature dependence and relaxation function breadth of segmental dynamics (α-relaxation) in the glass transition region. The maxima of the loss modulus curves were used to fit to the Vogel–Fulcher–Tamman equation to describe the temperature dependence of the characteristic relaxation times. Furthermore, the T _g-normalized semi-logarithmic Arrhenius plots (fragility plots) were exploited to indicate the changes in cooperative segmental motions across the glass transition. The master curves for relaxation modulus were also constructed for each sample as a function of time using the time–temperature superposition principle. The investigated nanocomposites showed a narrower segmental dispersion in the glass transition region compared to the neat systems. The relaxation modulus master curves were fitted by the Kohlrausch–Williams–Watts (KWW) function. It was observed that the distribution parameter of segmental relaxation time increased with addition of nanoparticles which was correlated with a decrease in fragility index. In addition, the increase of the KWW distribution parameter (β _KWW) for spherical silica nanocomposites was less than that for nanocomposites prepared with layered silicates (organoclay).     

Type-A dipole moment and segmental dynamics of poly(styrene oxide)

We report the dielectric properties of poly(styrene oxide)s (PSO) in bulk and concentrated solution states. Since the structure of PSO is asymmetric along the backbone, the repeat unit of PSO is expected to possess the non-zero component of the dipole moment p_A parallel to the chain contour as well as the perpendicular component p_B. The former and latter cause the dielectric normal mode and segmental mode relaxations, respectively. Contrary to the above mentioned expectation the temperature dependence of ε″ exhibits only the primary (α) relaxation and weak secondary relaxation (β) in the glassy state. No loss peak due to the normal mode relaxation was observed in the frequency region expected from the viscoelastic terminal relaxation in bulk and toluene solutions. The dielectric behaviours of the α relaxation in the bulk state were analyzed in detail and the parameters of the Vogel-Fulcher and the Havriliak-Negami equations were determined. The Kirkwood correlation factor was determined to be 0.36. The ¹³C NMR spectra indicate that the present PSO samples contain about 2% head-to-head linkages. This cannot be the origin of the disappearance of the normal mode. We conclude that p_A of PSO is too small to be detected. The p_A calculated with molecular orbital methods supports this conclusion.     

Physical properties and dielectric behavior of the poly(urethane-urea) based on o-dianisidine and renewable cross-linkers

The novelty of the poly(urethane-urea) series consists in inclusion of o-dianisidine units in the main chain and cross-linking with renewable biomaterials, unused compounds so far in the synthesis of the poly(urethane-urea) (Tween 20, Span 20, Phloroglucinol). The effects of these components on the structure, surface, thermo-mechanical properties and dielectric behavior of the obtained poly(urethane-urea) were investigated by Fourier transform infrared (FTIR) spectroscopy, thermo-gravimetric analysis, static contact angles, broadband dielectric spectroscopy, and mechanical testing. The FTIR spectra showed that the urethane hydrogen bonds decreased with the increase of o-dianisidine content. Such that, at the increase of the o-dianisidine content, decreased the thermo-mechanical properties, and increased strongly the water contact angle from 83 to 108°. By dielectric relaxation spectroscopy was studied the molecular dynamics within the polymeric matrices with identical soft segments but different structure of the hard domains. These poly(urethane-urea) materials exhibit two secondary relaxations (β and γ) and a relaxation process α, corresponding to the segmental movements in the soft phase, which occurs around the temperature of −50°C independent of the measurement frequency. o-Dianisidine prevents the formation of all the urethane hydrogen bonds and so increases the chains mobility and dipoles polarization of polymer matrix, thus increasing the dielectric constants.     

Dynamic mechanical properties of polyurethane block polymers

The dynamic mechanical properties of polyester and polyether urethane block polymers have been investigated at four frequencies (3.5, 11, 35 and 110 Hz) in the temperature range of — 150 to 200°C. The existence of a two phase structure was demonstrated in these systems by the observation of two major transition regions corresponding to (1) the glass transition temperature (T_g) of the ester or ether soft segments, and to (2) the softening temperature of the aromatic-urethane hard segments. Several secondary relaxations were observed in addition to the two major relaxations. It was possible to assign molecular mechanisms to each of these relaxations. All relaxation phenomena were greatly influenced by the molecular weight of the prepolymer, weight percent of hard segments, and thermal history. An increase in the molecular weight of the prepolymer above 1,000 at constant hard segment content resulted in a semi-crystalline material, which possessed a lower T_g for the macroglycol segments. Annealing to enhance crystallinity increased the T_g of the soft segments, consistent with the usual observation in semicrystalline homopolymers. These findings suggest that the relaxation mechanisms of polyurethane block polymers are not only influenced by the degree of crystallinity, but also by the nature of the domain structure.     

Modeling of the viscoelastic behavior of amorphous polymers by the differential and integration fractional method: the relaxation spectrum H(τ)

The dynamic mechanical behavior of poly(methyl methacrylate) (PMMA) from deep in the glass to the glass transition region has been studied by DMTA and analyzed by using a phenomenological fractional model in which the dynamic stress appears as a non-integer-order derivative of the strain. In order for the model to accurately represent the experimental data, three non-integer values for the derivative order are required. These values are related to two relaxation mechanisms. In the low temperature region (i.e. the β relaxation of PMMA), the derivative order is smaller and near 0.2, which indicates behavior close to the ideal elastic solid (glassy). For higher temperatures (between the β and the α relaxations), the derivative order is higher, indicating more viscoelastic behavior. In this work, modeling of the viscoelastic behavior of polymers using the fractional calculus approach is presented and the extended fractional solid (EFS) model is used to fit the experimental data of PMMA. In addition, the continuous relaxation spectrum H(τ) of PMMA is calculated from the model using the inverse Stieljes integral transformation. Finally, the effect of thermal treatment on the non-integer model parameters and on the distribution of relaxation times is obtained.     

Evaluation of local polarity of polymer solids by a rigid fluorescent probe of carbazole-terephthalate cyclophane

A novel rigid fluorescent probe, carbazole-terephthalate cyclophane (Cz-TP) was applied to evaluate local dielectric constants (ε) of various polymer solids in a wide range of temperatures. For poly(vinylidene fluoride), the ε increased above the glass transition temperature (T_g), due to relaxations of the polar segment -(CH₂CF₂)- of the main chain. For poly(alkyl methacrylate)s, the ε increased above the T_g or the melting temperature of the side chain, where motions of the polar ester groups are activated. For cyanoethylated polymers, the ε increased owing to motions of the polar cyano groups at the end of the side chain and the ε corresponded to the dielectric constant evaluated by dielectric relaxation measurement at a high frequency, because the Cz-TP exciplex has a lifetime of tens of nanoseconds. For a cyanoethylated polymer with a high content of cyano groups, the ε was larger at low temperatures than the dielectric constant obtained by the macroscopic dielectric relaxation measurement. These results show that the Cz-TP molecule is a useful probe for evaluation of the local polarity in polymer solids over a wide temperature range and can detect even a small change in ε at transition temperatures such as glass transition, side-chain melting, and side-chain relaxation.     

Chain packing versus chain mobility in semialiphatic BTDA-based copolyimides

A series of partially aliphatic copolyimides derived from 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), 4,4′-diaminodiphenylmethane (DDM) and 1,6-diaminohexane (DAH) was synthesized. Solubility tests and structural characteristics, evaluated by wide-angle X-ray diffraction measurements (WAXD) did not vary steadily with the content of the flexible segment. On the dynamic mechanical analysis (DMA) the evolution of the secondary relaxations (γ and β) suggested an increased chain tightness as more aliphatic component is included. The onset temperature of α-relaxation decreased regularly with the content of aliphatic sequence. During the α-relaxation the trend of E′, E″ and tan δ with temperature, and at multiple frequencies, evidenced the overlapping of phenomena. The increased mobility determines the formation of oriented structures that behave as constraints. Simultaneously, as the flexible content (DAH segment) is more important, orientation relaxations take place with increasing temperature. The equilibrium between stiffening and flowing makes the polyimides with high aliphatic content behave like elastomers.     

Rheological study of the gelation of cross-linking polyhedral oligomeric silsesquioxanes (POSS)/PU composites

The gelation process of N-Phenylaminomethyl-POSS/PU (polyurethane) nanocomposites during curing and at the stable state after curing was investigated by rheology. An increase in dynamic shear moduli, G′ and G″, was observed during the dynamic temperature ramps of the sample. In time-resolved mechanical spectroscopy (TRMS), G′ and G″ increased with curing time at constant curing temperatures over a wide of frequencies. The gelation time of the composites decreased with the rise of curing temperature or with the increase of POSS concentration. The relaxation exponent n at the critical gel was around 0.73. The gel stiffness S decreased as curing temperature increased or as POSS concentration increased. After the completion of the curing reaction, the critical concentration of POSS beyond which the gelation of POSS/PU composites would happen was found around 2.5 wt%. The viscoelastic properties of crosslinking POSS/PU fitted time–temperature-superposition well which implied the incorporation of multifunctional POSS into PU increased the homogeneity of crosslinking POSS/PU composites. Surprisingly, the modulus of the fully cured materials between 2.7 wt% and 6 wt% could also be supposed onto a master curve at high temperature, which implied self-similarity of network near the critical gel. The similar microstructure of POSS/PU at stable state was also confirmed by TEM. The network formation mechanism and the fine structure of the crosslinking POSS/PU were firstly investigated which would provide technical and theoretical basis for other hybrid crosslinking systems.     

Dielectric relaxation in montmorillonite/polymer nanocomposites

We report dielectric relaxation behavior in blends of sodium montmorillonite particles (MM) with a series of polymers (i.e., polyisoprene (PI), poly(propylene glycol) (PPG), and poly(butylene oxide) (PBO)). These polymers are known to exhibit the dielectric normal mode due to the fluctuation of the end-to-end vector as well as the segmental mode due to local, segmental fluctuations. The data indicate that all blend systems exhibit an additional relaxation process at a temperature region below the glass transition temperature, T_g, of the pure polymer component. The intensity of the new relaxation process increases with the content of MM and hence the relaxation process can be assigned to the segmental motion of the chains intercalated in the interlayers of MM. On the other hand, the relaxation time of the normal mode reflecting the fluctuation of the end-to-end vector is the same as the neat polymers but the intensity of the relaxation process increases due to enhancement of the internal electric field by MM.     

Mechanical relaxations in polybutene-1 and poly-4-methylpentene-1

Dynamic mechanical measurements between — 180°C and 180°C were made on both isotropic and drawn samples of polybutene-1 (PB-1) and poly-4-methylpentene-1 (P4MB1) over a wide frequency range by the use of a torsional pendulum (0.3–3 Hz), a viscoelastic spectrometer (5–90 Hz) and ultrasonic technique (3 MHz). The relaxation peaks were identified and the associated activation energies determined from Arrhenius plots. For PB-1 it was observed that orientation reduces the height and shifts up the temperature of the α_a-peak associated with large scale main-chain motion in the amorphous regions, but has little effect on the β-peak associated with side-group motion. In addition to the α_a and β relaxations a high-temperature crystalline relaxation (α_c) is also observed in P4MP1. For both the α_c and β relaxations the mechanical loss at 45° to the draw direction is much larger than that at 90°, which indicates that shear processes are involved in these relaxations.     

Physical nature of complex structural relaxation in polysiloxane - PMpTS: α and α′ relaxations

Structural relaxation processes in poly(methyl-para-tolyl-siloxane) (PMpTS) polymers of three molecular weights were studied using dynamic light scattering. Two relaxation processes: the usual α and an additional slow one α′ were observed and studied as function of temperature and molecular weight. Contrary to the structural relaxation, we find that in a plot T-T_g the relaxation times for the α′ process for all molecular weights do not collapse to a single curve. For one of the samples the light scattering correlation functions were compared with the corresponding functions obtained by means of mechanical relaxation, dielectric spectroscopy and computer simulations. The simulations show that the bimodal distribution, i.e. the α relaxation and the slow (α′) process are contained in the correlation functions of most of the probes (optical anisotropy, dipole moment, chain bond, density) in agreement with experimental observations.     

Segmental dynamics in hybrid polymer/POSS nanomaterials

Of the many nanomaterials available today, polyhedral oligomeric silsesquioxanes (POSS) are in a class of their own as they hold the capability to combine unique reactive inorganic–organic hybrid chemical compositions with nano-sized dimensionally stable cage structures. Depending on the structure and reactivity of their vertex groups, POSS may be blended in a polymer matrix, grafted as side chains, lie on the main macromolecular contour or even act as large, multifunctional chemical crosslinks. POSS is known to influence polymer segmental dynamics with several accelerating or decelerating mechanisms, that often lead to a significant decrease or increase of the glass transition temperature (T_g), respectively. This review explores these mechanisms with respect to the chemical nature of the organic substituents and the resulting particle–polymer interactions; the synthesis route, the chain topology, and the degree of dispersion. T_g vs content data are compiled from the primary literature in a series of comparative graphs. It will be shown that the dependence of T_g on the composition of the POSS nanomaterials can be often discussed and considered in terms similar to those used for polymer blends and copolymers.     

Glass transition temperatures of poly(hydroxystyrene-co-vinylpyrrolidone-co-isobutylstyryl polyhedral oligosilsesquioxanes)

A series of poly(hydroxystyrene-co-vinylpyrrolidone-co-isobutylstyryl polyhedral oligosilsesquioxanes) (PHS-PVP-POSS) hybrid polymers with various POSS contents was prepared by free radical copolymerization of acetoxystyrene, vinylpyrrolidone with styrylisobutylpolyhedral oligosilsesquioxanes (POSS), followed by selective removal of the acetyl protective group. The POSS content of a hybrid polymer can be effectively controlled by varying the feed ratios of reactants. The T_g of the POSS hybrid increases with the POSS content of PHS-PVP-POSS hybrids. The mechanism of T_g enhancement in these PHS-PVP-POSS hybrids was investigated using DSC, FTIR and GPC. The formation of the physically cross-linked POSS in these hybrid polymers trends to restrict polymer chain motion and results in significant T_g increase.     

Epoxy nanocomposites with octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane

The POSS-containing nanocomposites of epoxy resin were prepared via the co-curing reaction between octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane (OpePOSS) and the precursors of epoxy resin. The curing reactions were started from the initially homogeneous ternary solution of diglycidyl ether of bisphenol A (DGEBA), 4,4′-Diaminodiphenylmethane (DDM) and OpePOSS. The nanocomposites containing up to 40 wt% of POSS were obtained. The homogeneous dispersion of POSS cages in the epoxy matrices was evidenced by scanning electronic microscopy (SEM), transmission electronic microscopy (TEM) and atomic force microscopy (AFM). Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) showed that at the lower POSS concentrations (<30 wt%) the glass transition temperatures (T_gs) of the nanocomposites almost remained invariant whereas the nanocomposites containing POSS more than 40 wt% displayed the lower T_gs than the control epoxy. The DMA results show that the moduli of the nanocomposites in glass and rubbery states are significantly higher than those of the control epoxy, indicating the nanoreinforcement effect of POSS cages. Thermogravimetric analysis (TGA) indicates that the thermal stability of the polymer matrix was not sacrificed by introducing a small amount of POSS, whereas the properties of oxidation resistance of the materials were significantly enhanced. The improved thermal stability could be ascribed to the nanoscaled dispersion of POSS cages and the formation of tether structure of POSS cages with epoxy matrix.