Thermal and mechanical properties of polypropylene in the thermoplastic elastomeric state

One result of the discovery of homogeneous metallocene stereospecific catalysts is the ability to prepare polypropylene in a stereoblock form in which the isotactic stretches give crystallites acting as temporary crosslinks in an elastomeric network structure. The fact that these elastomers are thermoplastic and thus reprocessible increases the importance of establishing their structure-property relationships. In this report, the dependence of their physical properties on isotactic pentad content, molecular weight, and possible strain-induced crystallization are described. Thermal evaluations and mechanical tests of these materials under oscillatory strain, continuous extension and near-equilibrium uniaxial and biaxial elongation showed that they were multiphase, tough elastomeric materials. Their moduli and tensile strengths increased with increase in % isotactic pentad content and with increase in molecular weight. Equilibrium stress-strain measurements showed the occurrence of strain-induced crystallization in uniaxial, but not in biaxial, deformations.     

Synthesis,structure, and properties of hybrid organic–inorganic composites based on polysiloxanes. II. Comparisons between poly(methylphenylsiloxane) and poly(dimethylsiloxane), and between titania and silica

The work reported in the preceding article in this series is extended by consideration of polysiloxane–ceramic composites based on atactic poly(methylphenylsiloxane) (PMPS) elastomers instead of poly(dimethylsiloxane). The former is noncrystallizable because of its stereochemically irregular structure, while the latter is crystallizable. In addition, some composites were prepared by the in situ precipitation of titania instead of silica. The resulting materials were characterized using differential scanning calorimetry, equilibrium stress–strain measurements in elongation, small-angle neutron scattering, and transmission electron microscopy. The moduli of the PMPS elastomers were found to increase significantly with increase in amount of either type of filler, with reinforcing upturns at high elongation in the case of the silica. Because the PMPS elastomers were amorphous, it is obvious that strain-induced crystallization is not required for these upturns in modulus. Titania did not give as good reinforcement as did silica, at least in the case of PMPS. Differences in interactions between the polymer and the two fillers are obviously important in this regard, but differences in particle morphology probably also contribute. Specifically, the titania “particles” were significantly larger than the silica particles when observed in TEM, and appeared to be much more porous. The actual domain size as measured by scattering, however, was only approximately 5% larger. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1191–1200, 1998     

Scratching behavior of elastomeric poly(dimethylsiloxane) coatings

Scratch testing has been performed on elastomeric poly(dimethylsiloxane) (PDMS) coatings on stainless steel with a spherical indenter. The friction coefficient (horizontal‐to‐normal force ratio) during scratching decreases with increasing normal load. This result can be explained by assuming that during scratching the contact area is determined by elastic deformation and the horizontal force is proportional to the contact area. With increasing driving speed, the friction coefficient increases, but the rate of increase decreases; this suggests that the scratching of the PDMS coating is a rate process and that the viscoelastic property of the coating influences its frictional behavior. Below a critical normal load, which increases with the coating thickness, the PDMS coating recovers elastically after being scratched so that there are no scratch marks left behind. Above the critical normal load, the coating is damaged by a combination of delamination at the coating/substrate interface and through‐thickness cracking. When the coating is damaged, there is an increase in the friction coefficient, and the friction force displays significant fluctuations. Furthermore, the critical normal load increases with the driving speed; this implies that time is needed to nucleate damage. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1530–1537, 2002     

Connecting the wetting and rheological behaviors of poly(dimethylsiloxane)-grafted silica spheres in poly(dimethylsiloxane) melts

Using dynamic light scattering, mechanical rheometry, and visual observation, the static wetting behavior of PDMS-grafted silica spheres (PDMS-g-silica) in PDMS melts is related to their rheology. A phase diagram is mapped out for a constant grafted chain length as a function of grafting density and free polymer chain length. The transition between stable and aggregated regions is determined optically and with dynamic light scattering. It is associated with a first-order wetting transition. In the stable region Newtonian behavior is observed for semidilute suspensions. The hydrodynamic brush thicknesses, deduced from viscosity measurements, correspond closely to values obtained from self-consistent field calculations for the various parameter values. At the transition, the brush collapses suddenly and shear-thinning and thixotropy appear. The rheology indicates a degree of aggregation that increases with increasing length of the free polymer, as suggested by the theory.     

The synthesis and characterization of new thermoplastic poly(carbonate-urethane) elastomers derived from HDI and aliphatic–aromatic chain extenders

New thermoplastic poly(carbonate-urethane) elastomers (TPCUs) were prepared by a one-step melt polymerization from 20–80 mol% poly(hexane-1,6-diyl carbonate) diol of as a soft segment, hexane-1,6-diyl diisocyanate and 2,2′-[methylenebis(1,4-phenylenemethylenethio)]diethanol, 3,3′-[methylenebis(1,4-phenylenemethylenethio)]-dipropan-1-ol or 6,6′-[methylenebis(1,4-phenylenemethylenethio)]dihexan-1-ol (H) as new chain extenders at the NCO/OH molar ratio of 1 in the presence of dibutyltin dilaurate as a catalyst. The structures of the TPCUs were examined by FTIR spectroscopy, X-ray diffraction analysis, scanning electron microscopy and atomic force microscopy (AFM). The TPCUs were also characterized by physicochemical, thermal (by differential scanning calorimetry (DSC) and thermogravimetry) and tensile properties as well as Shore A/D hardness. The resulting TPCUs were colorless polymers, showing ordered structures including semicrystalline, with the highest ability to crystallize exhibited by the polymers derived from diol H. The polymers with the soft-segment content of 40–80 mol% (56.4–25.7 wt% of hard segments) exhibited a microphase separation shown by DSC and AFM. The TPCUs showed tensile strength in the range of 8.8–23.3 MPa and elongation at break in the range of 240–670%.     

Preparation and characterization of some unusually transparent poly(dimethylsiloxane) nanocomposites

A technique was developed for preparing poly(dimethylsiloxane) nanocomposites having unusually high transparencies as quantitatively judged by ultraviolet–visible spectroscopy. The method was the in situ generation of silica particles by a two‐step sol–gel procedure in which the required water of hydrolysis was simply absorbed from the air, and the catalyst was generated in situ from a tin salt. Electron microscopy showed that the phase‐separated silica domains were very small (30–50 nm in diameter) and well dispersed, as expected from the transparency of the composites. Stress‐strain measurements in tension indicated that the particles provide very good reinforcement. Ultra‐small‐angle X‐ray scattering data showed that the domain morphology depends strongly on catalyst, but weakly on loading level. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1897–1901, 2003     

Preparation and properties of poly(vinyl alcohol)/silica nanocomposites derived from copolymerization of vinyl silica nanoparticles and vinyl acetate

Nanocomposites of poly(vinyl alcohol)/silica nanoparticles (PVA-SNs) were prepared by in-situ radical copolymerization of vinyl silica nanoparticles functionalized by vinyltriethoxysilane (VTEOS) and vinyl acetate with benzoyl peroxide (BPO, i.e., initiator), subsequently saponified via direct hydrolysis with NaOH solution. The resulting vinyl silica nanoparticles, PVA-SNs were characterized by means of fourier transformation spectroscopy (FTIR), transmission electron microscopy (TEM) and the elemental analysis method. Effects of silica nanoparticles on viscosity and alcoholysis of PVA-SNs were studied by a ubbelohode capillary viscometer and the back titration method. The morphological structure of PVA-SN films was investigated by scanning electron microscopy (SEM). Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and tensile test were used to determine the thermal and mechanical properties of PVA-SN films. The results indicated that the content of vinyl groups on the surface of the vinyl silica nanoparticles was up to 3.02 mmol/g and vinyl silica nanoparticles had been successfully copolymerized with vinyl acetate. Furthermore, compared to pure PVA, silica nanoparticles bonded with polymer matrix in a low concentration affected the viscosity and alcoholysis of the PVA-SNs materials. At the same time, it resulted in the improvement of the thermal and mechanical properties of the PVA-SN materials due to a strong interaction between silica nanoparticles and the polymer matrix via a covalent bond. It could be found that the optical clarity of the membrane was changed through UV-Vis absorption spectrum due to the introduction of silica nanoparticles.     

Tailoring the surface properties of poly(dimethylsiloxane) microfluidic devices

Poly(dimethylsiloxane) (PDMS) is an attractive material for microelectrophoretic applications because of its ease of fabrication, low cost, and optical transparency. However, its use remains limited compared to that of glass. A major reason is the difficulty of tailoring the surface properties of PDMS. We demonstrate UV grafting of co-mixed monomers to customize the surface properties of PDMS microfluidic channels in a simple one-step process. By co-mixing a neutral monomer with a charged monomer in different ratios, properties between those of the neutral monomer and those of the charged monomer could be selected. Mixtures of four different neutral monomers and two different charged monomers were grafted onto PDMS surfaces. Functional microchannels were fabricated from PDMS halves grafted with each of the different mixtures. By varying the concentration of the charged monomer, microchannels with electrophoretic mobilities between +4 x 10(-4) cm2/(V s) and -2 x 10(-4) cm2/(V s) were attainable. In addition, both the contact angle of the coated surfaces and the electrophoretic mobility of the coated microchannels were stable over time and upon exposure to air. By carefully selecting mixtures ofmonomers with the appropriate properties, it may be possible to tailor the surface of PDMS for a large number of different applications.     

Synthesis,structure, and properties of hybrid organic–inorganic composites based on polysiloxanes. I. Poly(dimethylsiloxane) elastomers containing silica

Various synthetic protocols were used to prepare several classes of polysiloxane–silica filler systems. The structures of these fillers and their interactions with the polysiloxane matrices were studied using small-angle X-ray and neutron scattering. In addition, the mechanical properties of the composites were characterized using equilibrium stress–strain isotherms in elongation. The results indicated that manipulation of the chemical reactions used to generate the filler can lead to a wide range of complex structures and unusual properties. Some of the observed mechanical properties were correlated with information on the composite structures and on elastomer–filler interactions. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1167–1189, 1998     

Rheological properties of poly(dimethylsiloxane)-poly(diethylsiloxane) blends

The rheological behavior of poly(dimethylsiloxane)-poly(diethylsiloxane) blends in the range 20 to 100°C, including the region of existence of poly(diethylsiloxane) in mesomorphic and amorphous states has been studied by capillary viscometry. The flow of these blends within the studied shear stress interval between 10³ and 10⁵ Pa obeys a power law. If poly(diethylsiloxane) is introduced in the mesomorphic state and serves as a matrix, the blends behave as viscoplastic bodies and feature the yield stress. The flow of these blends is accompanied by an appreciable orientation of the poly(diethylsiloxane) phase. Blends in which the mesomorphic poly(diethylsiloxane) is a disperse phase flow as abnormally viscous fluids in which poly(diethylsiloxane) plays the role of a structuring filler. The isotropization of poly(diethylsiloxane) leads to a reduction in its viscosity and, accordingly, in the viscosity of the blend. The logarithm of the effective viscosity of such blends both at the constant shear rate and constant shear stress is the linear function of their composition. The addition of poly(diethylsiloxane) to poly(dimethylsiloxane) strongly affects the degree of swelling of an extrudate at the exit of a capillary, and this parameter depends on the phase state of poly(diethylsiloxane) and its content in the blend. Upon incorporation of a small amount of a poly(dimethysiloxanel)-poly(diethylsiloxane) block copolymer (compatibilizer) into the blend, the viscosity of the blend approaches that of the predominant component. This phenomenon is apparently related to the fact that the block copolymer facilitates development of a more uniform morphology of the blend, in particular, the continuous dispersion phase. This factor, along with the specifics of the deformational behavior of poly(diethylsiloxane), also manifests itself during drawing and subsequent shrinkage of crosslinked resins prepared from the blends under study.     

Electrorheology of suspensions of mesostructured and mesoporous silica in poly(dimethylsiloxane)

The effect of electric fields on the viscosity and shear stress of dispersions of mesostructured silicon dioxide powders and the products of their thermal treatment, that is, mesoporous silicas, in silicone oil is investigated. Mesostructured silicon dioxide powders are prepared using different structuring reagents, namely, octylamine, dodecylamine, and polyethylenimine. It is shown that the shear stresses developing in systems containing mesostructured powders as dispersed phases under the action of electric fields are several times higher than those in dispersions of mesoporous materials. The shear stresses developing in the systems containing powders of mesostructured materials as dispersed phases under the effect of electric fields depend on the nature of organic substrates present in pores and the conductivity of the systems. Electrorheological effect in dispersions of mesoporous silicas slightly depends on the structure of the materials. The observed synergism may be related to the interactions between the components of a mesostructured material, which manifest themselves as a rise in interfacial polarization. The stress-strain curves were measured upon tensile and compression in electric fields of different strengths.     

NMR measurement of the surface dynamics of poly(dimethylsiloxane) adsorbed on silica gel

CPMAS-DD ¹³C NMR spectroscopy was used to examine the mobility of poly(dimethylsiloxane) adsorbed on silica gel (PDMS/SiO₂) at submonolayer coverages. The spin-lattice relaxation time in the rotating frame (T_1ρH) decreased linearly with increasing loading. This is consistent with a decrease in the mobility of the polymer segments as the loading is increased. The decrease in mobility results from interpolymer interference. We propose a model that explains these results in terms of a surface intrinsic viscosity that incorporates the polymer-polymer interactions on the surface.     

Effect of thermal degradation on rheological properties for poly(3-hydroxybutyrate)

Thermal degradation at processing temperature and the effect on the rheological properties for poly(3-hydroxybutyrate) have been studied by means of oscillatory shear modulus and capillary extrusion properties, with the aid of molecular weight measurements. Thermal history at processing temperature depresses the viscosity because of random chain scission. As a result, gross melt fracture hardly takes place with increasing the residence time in a capillary rheometer. Moreover, it was also found that the molecular weight distribution is independent of the residence time, whereas the inverse of the average molecular weight is proportional to the residence time. Prediction of average molecular weight with a constant molecular weight distribution makes it possible to calculate the flow curve following generalized Newtonian fluid equation proposed by Carreau as a function of temperature as well as the residence time.     

Synthesis and thermal properties of poly(methyl methacrylate)-graft-(cellobiosylamine-C15)

Model experiments for synthesis of a comb-shaped copolymer with cellulose side-chains were performed with cellobiose derivatives. A novel cellobiose monomer, N-(15-methacryloyloxypentadecanoyl)-2,3,6-tri-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-β-d-glucopyranosylamine (2) was prepared from heptaacetylcellobiosyl- amine. Homopolymerization of cellobiose monomer 2 and copolymerization of monomer 2 with methyl methacrylate (MMA) were performed using 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator to obtain homopolymers 3-i (i = 1–4) and copolymers 3-i (i = 5–7), poly(methyl methacrylate)-graft-(heptaacetylcellobiosylamine-C15). The size exclusion chromatography—multi-angle laser light scattering (SEC-MALS) measurements revealed that comb-shaped homopolymers 3-i (i = 1–4) had more compact structures compared to copolymers 3-i (i = 5–7) at the same elution volume. Selective deacetylation of polymers 3-i (i = 1–7) gave novel cellobiose polymers 4-i (i = 1–7), poly(methyl methacrylate)-graft-(cellobiosylamine-C15). The amide linkages between cellobiose moiety and long-chain alkyl group, and the ester linkages between PMMA main-chain and long-chain alkyl group remained after deprotection. The differential scanning calorimetry (DSC) measurements revealed that the T _gs of the polymers 4-i (i = 1, 5, 6, 7) increased with increasing cellobiose composition in the polymers. It was indicated that cellobiose moieties of polymers 4-i (i = 1, 5, 6, 7) reduced the mobility of PMMA main-chain.     

An experimental investigation of fracture by cavitation of model elastomeric networks

A new methodology to investigate the failure of elastomers in a confined geometry has been developed and applied to model end-linked polyurethane elastomers. The experimental in situ observations show that the elastomers fail by the growth of a single cavity nucleated in the region of maximum hydrostatic stress. Tests carried out at different temperatures for the same elastomer show that the critical stress at which this crack grows is not proportional to the Young's modulus E but depends mainly on the ratio between the mode I fracture energy G_IC and E. A reasonable fit of the data can be obtained with a model of cavity expansion by irreversible fracture calculating the energy release rate by finite elements with a strain hardening constitutive equation. Comparison between different elastomers shows that the material containing both entanglements and crosslinks is both tougher in mode I and more resistant to cavitation relative to its elastic modulus. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48:1409–1422, 2010     

Enhanced thermal and mechanical properties of poly (vinylidene fluoride) nanocomposites reinforced by liquid-exfoliated graphene

We report the preparation and enhanced thermal and mechanical properties of poly (vinylidine diflouride) (PVDF) nanocomposites reinforced by few-layer graphene flakes which are produced by the direct liquid-phase exfoliation of pristine graphite. Graphene flakes are found to homogeneously disperse in PVDF, reduce the bubble defects and thus the porosity of PVDF, and change PVDF’s crystallinity. Thermogravimetric analysis indicates that graphene can accelerate the fracture of hydrogen bond connecting PVDF and N-Methyl pyrrolidone molecules. 1.5?wt% graphene loading leads to around 20?°C enhancement in the melting temperature of PVDF. The mechanical properties like Young’s modulus (EIT), yield stress (σy), and hardness (H) of the nanocomposites are investigated by nanoindentation technique. A 1.0?wt% loading of graphene is found to increase EIT, σy, and H of PVDF by ～337%, ～102%, and ～228%, respectively.

• Highlights

• Few-layer graphene was produced by liquid-phase exfoliation.

• Graphene were added to PVDF to enhance thermal and mechanical properties of polymer.

• Mechanical properties of PVDF/graphene composite films were investigated by nanoindentation.

     

Effects of bond-angle inversion on the statistical properties of poly(dimethylsiloxane)

Evidence of only a low barrier to inversion in Si? O? Si sequences could be important with regard to the interpretation of the statistical properties of silicone polymers. The effects are estimated for the temperature coefficients of the unperturbed dimensions, dipole moments, and optical-configuration parameter for poly(dimethylsiloxane).     

Simulations on the reinforcement of poly(dimethylsiloxane) elastomers by randomly distributed filler particles

Monte Carlo computer simulations were carried out on filled networks of poly(dimethylsiloxane) (PDMS), which were modeled as composites of crosslinked chains and randomly arranged spherical filler particles. The primary concern of the investigation was the effect of the excluded volume of these particles on the elastomeric properties of the polymers. Calculations were carried out for PDMS chains with different molecular masses between crosslinks, and for filler particles with different sizes and at various volume percentages. Distributions of end-to-end vectors for both unfilled and filled networks were obtained using Monte Carlo simulations based on rotational isomeric state (RIS) theory. More extended configurations, with a higher end-to-end distance, were observed for networks filled with smaller particles. The nominal stress f* and the modulus or reduced nominal stress [f*] were calculated from the distributions of end-to-end vectors using the Mark-Curro approach. Relatively small filler particles were found to increase the non-Gaussian behavior and to increase the normalized moduli above the reference value of unity. Temperature effects on the stress were also investigated. © 1996 John Wiley & Sons, Inc.