Model networks of end-linked polydimethylsiloxane chains

Summary Elastomeric networks of high extensibility were prepared by end-linking mixtures of vinyl-terminated polydimethylsiloxane chains having molecular weights of approximately 600 and 11,000 g mol^–1, with silanes chosen to give junction functionalities ranging from 3 to 8. The resulting bimodal networks were studied in elongation, at 25 °C, to their rupture points, and in swelling equilibrium in benzene at room temperature. The elongation moduli [f ^*] were found to be in satisfactory agreement with previous results obtained by end-linking hydroxyl-terminated polydimethylsiloxane chains. Values of [f ^*] at low and moderate deformations gave relatively low values of the ratio of elasticity constants 2C ₂/2C ₁, which is a measure of the extent to which the elongation changes from approximately affine to nonaffine as the elongation increases. The low values obtained for this ratio are presumably due to diminished interpenetration of configurational domains in the case of very short chains. In spite of its small magnitude, 2C ₂/2C ₁ does show some decrease with increase in , as predicted by the recent molecular theory of rubberlike elasticity developed by Flory. The swelling equilibrium results were also found to be in satisfactory agreement with theory. The elongation moduli increased significantly at high elongations, and the values of the elongation at which the upturn was first discernible were very nearly independent of , This is consistent with the interpretation of this anomalous behaviour in terms of limited chain extensibility. The maximum extensibility generally decreased somewhat with increase in and this caused a decrease in both the ultimate strength and the toughness of the elastomer, as measured by the energy required for rupture.     

Thermomechanics of bimodal end-linked polydimethylsiloxane networks with one of the chains much short

Summary A novel equation of state for bimodal networks with extremely short chains as solid intrusions is developed proceeding from the van der Waals theory of molecular networks. Stress-strain and thermoelastic measurements on bimodal end-linked polydimethylsiloxane model networks can satisfactorily be described. The physical reasons behind that representation will be discussed.Dedicated to Professor Dr. K. Ueberreiter on his 70th birthday.     

Model networks of end-linked polydimethylsiloxane chains. XIV. Stress-strain,thermoelastic, and birefringence measurements on the bimodal networks at very low temperatures

Bimodal networks consisting of very short and relatively long poiydimethylsiloxane (PDMS) chains were studied from 30 to ?52°C in an attempt to elucidate the anomalous increases in modulus [f^*] exhibited by such elastomeric materials at high elongations. Temperature was found to have very little effect on (i) the elongation at which the upturn in [f^*] becomes discernible, (ii) the elongation at which rupture occurs, and (iii) the total increase in [f^*] up to the rupture point. The standard force-temperature (“thermoelastic”) plots were linear, but gave values of the energetic contribution to the total force which were significantly smaller than those universally obtained on unimodal, long-chain PDMS networks. Birefringence-temperature relations were also found to be linear, and yielded values of the optical-configuration parameter and its temperature coefficient which were in satisfactory agreement with the corresponding values reported for unimodal PDMS networks. These results indicate that even at very low temperatures the observed increases in modulus (and consequent improvements in ultimate strength) are due to non-Gaussian effects arising from limited chain extensibility, rather than from intermolecular reinforcing effects such as strain-induced crystallization.     

Neutron scattering and swelling properties of end-linked polydimethylsiloxane networks

Small angle neutron scattering measurements were performed on polydimethylsiloxane-toluene solutions and gels at different degrees of swelling. The scattering signal of the gel was decomposed into a solution-like part and a static part. The thermodynamic information obtained from the solution-like part of the signal is compared with macroscopic swelling pressure observations.     

Prediction of structure and properties of end-linked networks

Rate theory has been developed for RA₂+RB₄ polymerisations including intramolecular reaction and the general behaviour of RA₃, RA and stoichiometric RA₂+RB₃ and RA₂+RB₄ polymerisations compared regarding the moduli of networks formed at complete reaction. The predictions for RA₂+RB₃ and RA₂+RB₄ polymerisations are used to analyse existing experimental data on gel points and moduli in polyurethane polymerisations and networks. By fitting experimental gel points, improved predictions of moduli at complete reaction are achieved.     

The effect of swelling on the stress-strain isotherms and ultimate properties of polydimethylsiloxane networks in elongation

Summary Gamma radiation was used to prepare three cross-linked, unfilled samples of highly elastomeric polydimethylsiloxane. Portions of each sample were studied in elongation to their rupture points, at 30 °C, in both the unswollen state and swollen with low molecular weight dimethylsiloxane fluid. Values of the volume fraction ν₂ of polymer in the networks ranged from 1.00 to 0.40. None of the stress-strain isotherms obtained showed any upturn in the reduced force or modulus [f ^*] at high elongation, an observation in agreement with the conclusion that such increases in [f ^*], when observed, are due to strain-induced crystallization. All of the isotherms were well represented by the semi-empirical equation [f ^*]=2C ₁+2C ₂ α ⁻¹, whereα is the elongation, and 2C ₁ and 2C ₂ are the Mooney-Rivlin constants. Values of 2C ₂, which serves as a measure of the extent to which [f ^*] varies with elongation, showed a decrease with decrease in ν₂, and generally also with decrease in degree of cross-linking. The ultimate properties reported were the valuesλ _r and [f ^*], of the total elongation and the reduced force, respectively, at the rupture point. Decrease in degree of cross-linking causes a significant increase inλ _r and a significant decrease in [f ^*] _r ; decrease in ν₂, however, has only a relatively small effect onλ _r and [f ^*] _r . With 3 figures and 1 table     

Effect of pendent chains on the interfacial properties of thin polydimethylsiloxane (PDMS) networks

The interfacial properties of end-linked polydimethylsiloxane (PDMS) films on silicon are examined. Thin cross-linked PDMS films (～10 μm thick) were synthesized over a self-assembled monolayer supported on a silicon wafer. By systematically varying the concentration of monofunctional PDMS in a mixture with telechelic precursor molecules, structures ranging from near-ideal elastic networks to poorly cross-linked networks composed of a preponderance of dangling/pendent chains were synthesized. Lateral force microscopy (LFM) employing bead probes was used to quantify the effect of network structure on the interfacial friction coefficient and residual force. Indentation measurements employing an AFM in force mode were used to characterize the elastic modulus and the pull-off force for the films as a function of pendent chain content. These measurements were complemented with conventional mechanical rheometry measurements on similar thick network films to determine their bulk rheological properties. All networks studied manifested interfacial friction coefficients substantially lower than that of bare silicon. PDMS networks with the lowest pendent chain content displayed friction coefficients close to 1 order of magnitude lower than that of bare silicon, whereas networks with the highest pendent chain content manifested friction coefficients about 3 times lower than that of bare silicon. At intermediate sliding velocities, a crossover in the interfacial friction coefficient was observed, wherein cross-linked PDMS films with the least amount of pendent chains exhibit the highest friction coefficient. These observations are discussed in terms of the structure of the films and relaxation dynamics of elastic strands and dangling chains in tethered network films.     

Synthesis and properties of monodisperse polydimethylsiloxane networks

α,ω-Divinyl-terminated polydimethylsiloxane polymers with low polydispersivity were synthesized via anionic polymerization of hexamethylcyclotrisiloxane (D₃). Five “monodisperse” polymers with different molecular weights were hydrosilylated with tetrakisdimethylsiloxysilane in the presence of platinum catalysts and inhibitors to form crosslinked networks. Control studies to determine the effect of platinum, silicon hydride, and heat were performed to verify that no redistribution occurred during hydrosilylation. The properties of these cured samples were characterized chemically by degradation with trifluoromethanesulfonic acid in the presence of hexamethyldisiloxane, infrared spectroscopy, and nuclear magnetic spectroscopy. Mechanical properties were studied by measurements of Shore A hardness, dynamic shear modulus, and tensile modulus. The relationship between the molecular weight of the vinyl polymer and the final properties of the cured networks was measured. © 1993 John Wiley & Sons, Inc.     

A molecular dynamics simulation study on polymer networks of end-linked flexible or rigid chains

The differences in formation and structural properties of polymer networks consisting of end-linked flexible or rigid chains were studied by molecular dynamics simulation. Networks were formed from monodisperse, linear, short, flexible or rigid chains with functional end groups and a stoichiometric ratio of trifunctional cross-linkers. The rigid chains had a rodlike shape defined by an angle potential, while the flexible chains had no angle potential. In order to understand the influence of chain rigidity, all parameters of precursor chains (length, reactivity, bond potential, nonbonding potential) were the same, with the exception of the angle potential. The system density rho, corresponding to the concentration of monomer in solvent, was varied from 0.01 to 0.11. Different network structures resulting from the different processes of network formation were observed. Simulations showed that the flexible chains created an inhomogeneous network on a large scale via microgel cluster formation, in agreement with experimental observations, whereas the rigid chains rapidly created a homogeneous network in the entire system volume without first generating microgel clusters, with the additional difference that they gave rise to mutually interpenetrating networks at the local scale.     

Dependence of the elastomeric properties of bimodal networks on the lengths and amounts of the short chains

Bimodal elastomeric networks were synthesized by tetrafunctionally end-linking very short and relatively long hydroxyl-terminated chains of poly(dimethylsiloxane). Decrease in the molecular weight of the short chains (from 880 to 660 to 460 g mol^?1) generally results in significant increases in ultimate strength and energy for rupture. Decrease in the number of short chains, however, causes these two quantities to go through a maximum. Too many short chains gives essentially a brittle thermoset, and this precludes any upturn in the modulus from the limited extensibility of the short chains. Having too few short chains makes their limited extensibility irrelevant since the entire macroscopic deformation can then be taken up by the long chains present in the network.     

The influence of the molecular weight distribution of network chains on the mechanical properties of polymer networks

End-linked poly(dimethylsiloxane) (PDMS) networks with different molecular weight distributions (MWD) of the primary chains were prepared and investigated by isothermal stress-strain measurements. We found a lowering of the elastic modulus with increasing broadness of the MWD. The observed range of the moduli seems not to be restricted to the region limited by the classical models of rubber elasticity. This result is based on our own experimental investigations and on a reanalysis of data taken from the literature. In the case of nearly monodisperse distributions (M _n /M _w 1) the effect of configurational restrictions of the network strands probably dominates. In the opposite case (M _n /M _w 1), we discuss that spatial clustering of the crosslinks may reduce the effective number of elastically active network junctions.     

Contribution of entanglements to the modulus of end-linked poly(ethylene oxide) networks

Poly(ethylene oxide) networks were made at 75 °C by end-linking of short linear chains with an excess of Tolonate HDT, a tri-functional aliphatic isocyanate. Molecular weights of poly(ethylene oxide)were 4000, 6000 and 12000. The observed elastic moduli were much higher than predicted from affine theory, indicating a large contribution from entanglements in contrast to the findings of previous studies. With increasing excess of cross-linker, the contribution from entanglements levels off at a value of approximately 5 MPa which is in good agreement with the rubber plateau modulus of pure poly(ethylene oxide).     

Surface properties of a series of amphiphilic dendrimers with short hydrophobic chains

A series of tree-shaped, amphiphilic dendrimers was synthesized. The products belong to the family of one-directional arborols of the form ([9]-n), where the notation signifies that each molecule has nine hydroxyl groups ([9]-) as the hydrophilic head and an alkyl chain as the hydrophobic moiety (n = 6, 8, or 10 carbon atoms). The surfactant character changes dramatically as the number of methylene groups increases. The critical micelle concentration of [9]-6 was determined, and pressure-area isotherms of the less soluble [9]-8 and [9]-10 were obtained. Large structures existed atop the spread layers. Large structures were also found in solutions of [9]-6.     

Elastomeric properties of end-linked networks of high cross-link functionality. Accounting for possible changes in effective functionality with extent of reaction and chain-length distribution

This study reanalyzes some elastomeric properties in elongation reported for poly(dimethylsiloxane) (PDMS) networks of high cross-link functionality which had been prepared by using multifunctional siloxane oligomers to end link vinyl-terminated PDMS chains. The extent of reaction of the vinyl end groupsP _vi spanned the range of 0.40 to 0.95. These networks had elongation moduli that significantly exceeded the values predicted by the Flory-Erman theory, except at very low values ofP _vi. Trends in their stress-strain isotherms, as characterized by the Mooney-Rivlin constants 2C ₂ and the ratio 2C ₂/C₁, also appeared to be different from those predicted by theory. Neglected in such standard analyses, however, was the fact that the segments between cross-links along the junction precursor molecules can themselves act as short network chains, contributing to the modulus and giving a strongly bimodal distribution of both network chain lengths and cross-link functionalities. Of particular interest is the apparent change in functionality with extent of reaction and chain length distribution. The results thus obtained do suggest strong dependence of the observed values of the phantom modulus on the network chain-length distribution, particularly at very small values of the ratio of the length of the short chains to the long ones. Calculations based on recognition of these complications can be used to characterize more realistically the deformation of such networks. The results give much better agreement with experiment. Such behavior could be an important characteristic of elastomeric networks in general.Also, a preliminary attempt was made to bridge theory with experiment based on Kloczkowski, Mark, and Erman's recent theory of fluctuations of junctions in regular bimodal networks. The agreement between theory and experiment thus obtained is rather satisfactory and lends further support to assumptions that take into account the possibly bimodal nature of these high-functionality networks.     

Oligomer-to-polymer transition in short ethylene glycol chains connected to mobile hydrophobic anchors.

We studied the structure of short ethylene glycol (EG) chains with N repeating units (EGN, N = 3, 6, 9, 12, and 15) connected to hydrophobic dihexadecyl chains by means of a combination of differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS/WAXS). These synthetic amphiphiles dispersed in water form planar lamellar stacks and hexagonal cylinders confining the EG chains to restricted geometries. Owing to the self-assembly of the anchoring points, the lateral density of EG chains in planar lamella can be quantitatively controlled. Furthermore, the chain-melting phase transition of the anchors enables us to "switch" the intermolecular distance reversibly. SAXS/WAXS results suggest that the shorter EG chains (N = 3, 6, and 9) assume a helical conformation in stacks of planar lamella. When the EG chains are further elongated (N = 12 and 15), the lamellar periodicities cannot be explained by a linear extrapolation of shorter oligomers, but can be interpreted well as polymer brushes following the scaling theorem. Such rich phase behaviors of EGN molecules can be used as a simple model of oligo/poly-saccharide chains on cell surfaces, which act not only as flexible repellers between neighboring cells but also as stable spacers for functional ligands.     

Stress relaxation and dynamic viscoelastic properties of end-linked poly(dimethyl siloxane) networks containing unattached poly(dimethyl siloxane)

Stress relaxation in uniaxial extension and dynamic shear moduli G′ and G″ have been studied in networks of vinyl-terminated poly(dimethyl siloxane) (PDMS) of five different molecular weights (M _n from 1800 to 29,200) crosslinked with cis-dichlorobis (diethyl sulfide) platinum (II) and containing 10 and 15 wt % of two samples of high-molecular-weight unattached linear hydroxyl-terminated PDMS (M _w 700,000 and 950,000). The M _w/M _n ratio of both the network prepolymers and the unattached linear species was approximately 2. In stress relaxation the stretch ratio was 1.25 or less and the shear relaxation modulus was calculated from the neo-Hookean stress-strain relation. In the dynamic measurements, the strain amplitude was 15% or less; after conversion to the timedependent shear relaxation modulus G(t) the two sets of measurements were combined and the contribution of the unattached species G₁(t) was calculated by difference. After multiplication by (1 − v)⁻¹G/G_e, where v₂ is the volume fraction of network, G is the plateau modulus of the uncrosslinked polymer, and G_e is the equilibrium modulus of the network containing unattached molecules, G₁(t) was compared with G₁₁(t), the relaxation modulus was essentially the same in both environments. The relaxation was slower in the networks than in the uncrosslinked polymer by 1 to 2 orders of magnitude, and it increased gradually with increasing G_e, which is a measure of total to pological obstacles represented by crosslinks plus trapped entanglements. A similar but less striking difference between relaxation in a network and in the homologous environment of a linear polymer was previously observed in end-linked polybutadiene networks and the butadiene phase of a styrene-butadiene-styrene block copolymer. It appears that, in these systems where the topology of the obstacles is fixed, the reptation is severely restricted or else alternative modes of configurational rearrangement which contribute to relaxation in the uncrosslinked polymer are suppressed.     

Crystallization of short aliphatic polymer chains. I. General chain-folding behavior

Short aliphatic polymer chains of different lengths were prepared by degrading polyethylene samples of appropriately chosen initial fold lengths to the chain lengths which correspond to a single chain traverse through the lamella. The resulting dicarboxylic acids were either used as such for further crystallization experiments or were first converted into diiodides to remove polar endgroups. The resulting short polymers all crystallized by chain folding even if the chains (peak of distribution) were only 1.5–4 times the length of a traverse through the lamella. In the diiodides the fold length varied continuously with crystallization temperature, as is usual in high molecular weight material, but with the dicarboxylic acids such variation, while observable, was only small. The effect of the molecular weight on the fold length due to its influence on supercooling at a given crystallization temperature has become apparent. Renewed degradation with nitric acid and subsequent GPC analysis of the degradation products confirmed the folded nature of the chains in the above crystals. This analysis combined with experiments on the reactivity of chain ends has led to the picture that each chain folds completely, once, twice etc. so that both folds and ends are in the surface zone but are located at varying heights, as appropriate to the overall layer thickness for the molecular weight distribution in question. This picture is consistent with other concurrent work.     

Cross-link network of polydimethylsiloxane via addition and condensation (RTV) mechanisms. Part I: Synthesis and thermal properties

A series of highly cross-linked polysiloxane was synthesised via hydrosilylation and condensation reaction. Structural identification using Fourier Transform Infrared (FTIR) and ¹H-NMR confirmed their chemical structures. Their thermal and, mechanical properties, and crystallinity, were analysed and related to the level of cross-link density. These systems displayed elevated thermal and hardness properties at an increased cross-link density. Furthermore, the level of crystallinity was reduced as displayed by XRD analysis. Along with this observation, the calculated fractional free volume (FFV) showed a decreasing trend leading to the ‘densification’ effect. It was envisaged that the linear polysiloxane chain segments aligned parallel to each other in a triclinic crystal system to generate a crystalline domain. The spacing between these stacking chains was found to be about 7.2 Å as measured from simulated XRD pattern.