Photo‐Crosslinked Elastomeric Bimodal Poly(trimethylene carbonate) Networks

Using methacrylated poly(trimethylene carbonate) oligomers unimodal (prepared from one macromer) and bimodal (prepared from two macromers with different molecular weights) photo‐crosslinked networks and structures are prepared by stereolithography. The obtained biodegradable networks are flexible and elastic. Compared to the corresponding unimodal networks, the tensile properties of bimodal poly(trimethylene carbonate) (PTMC) network films are significantly enhanced. Resilient materials with increased toughness and suture retention strengths are obtained. The mechanical properties of the bimodal networks compare favorably with those of unimodal networks prepared previously from PTMC macromers with much higher molecular weights. Tough porous PTMC structures with designed diamond pore network architectures can also be readily prepared by stereolithography. Upon swelling of these PTMC structures in a solvent, the pore sizes and pore size distribution increases while the porosity decreases.     

Experimental and theoretical investigations on petroleum-based elastomers with two cross-linking systems of different lengths viewed as bimodal networks

A review with 36 references discussing the chemistry and the structure-property relationship of elastomers cured with two cross-linking systems of different chain lengths such as sulfur and the polymerization products of p-benzoquinone and viewed as bimodal networks. These exceptional networks have shown remarkable improvements in the overall mechanical properties which are anticipated to be due to the non-Gaussian effects known for bimodal networks and evident by the anomalous upturn in the modulus values in Mooney-Rivlin stress-strain data representations. Proton and ¹³C NMR as well as energy minimization calculations were used to study the chemical structures and single chain contributions of polyquinones. Nuclei bending of these oligomers have shown to be greatly influenced by the restricted torsional behavior due to the presence of the hydrogen bonds between the benzenoid nuclei. Intrinsic atomic-level forces for the networks were evaluated using molecular dynamics techniques and showed that while the forces acting on the junction points of the cross-linking segments and the elastomeric chains had no apparent change as a consequence of the networks' bimodal formation, forces acting on the short chains of the bimodal networks are of much higher values as compared to those of unimodal networks. The presence of the relatively long polyquinone chains in the bimodal networks has caused the short sulfur chains to stretch to its maximum extensibility and no longer can increase its end-to-end distance separation by simple rotations about its skeletal bonds. Limited chain extensibility of the short chains resulting from the deformation of the bond angles and bond lengths has lead to higher potential energies. Studies on the swollen bimodal networks have validated the above conclusions since swelling of the networks will prevent the elastomeric chains from undergoing possible strain-induced crystallization during the stress-strain experiments and any abnormalities in the mechanical behavior of these networks must be therefore the result of the limited extensibility of the short chains of the networks.     

Some dynamic mechanical properties of unimodal and bimodal networks of poly(dimethylsiloxane)

Summary Elastomeric networks of polydimethylsiloxane prepared by end-linking chains having molecular weights in the range 18,500 to 220 g mol^-1 were studied from -128 to 50°C using a Rheovibron DDV III Viscoelastometer. In the case of the unimodal networks, the glass transition temperature T_g was generally insensitive to degree of cross-linking. The intensity of the tan δ relaxation, however, increased by over an order of magnitude over the range of cross-link densities investigated. Bimodal networks prepared from mixtures of relatively long and very short PDMS chains also had values of T_g which were insensitive to degree of cross-linking. Finally, as expected, the intensities of the tan δ peak for the bimodal networks could not be explained on the basis of simple additivity of contributions from the relatively long and the very short network chains.     

Polysiloxane elastomers with bimodal chain-length distributions

Most elastomers are prepared by an adventitious, random cross-linking process and thus have a broad, unimodal distribution of network chain lengths. It is possible, however, to obtain materials of controlled chain-length distribution by restricting the reactivity of the chains to their ends, and then end linking these chains with a multi-functional reactant. The networks of this type that have proved to be of greatest interest consist of short chains end linked with long chains to yield abimodal distribution of network chain lengths. These bimodal networks have unusually high extensibility for their values of the modulus and ultimate strength, and thus considerable toughness. Most such elastometers have been prepared from chains of poly(dimethylsiloxane), by carrying out either a condensation reaction between hydroxyl-terminated chains and tetraethoxysilane, or an addition reaction between vinyl-terminated chains and a poly(methylhydrogen siloxane) oligomer. The present review discusses the preparation of such materials, the characterization of some of their properties, and the interpretation of these properties in terms of the molecular theories of rubberlike elasticity.Presented at the XXVIth Silicon Symposium, Indiana University-Purdue University at Indianapolis, March 26–27, 1993.     

Bimodal grain structure effect on the static and dynamic mechanical properties of transparent polycrystalline magnesium aluminate (spinel)

Transparent polycrystalline magnesium aluminate (spinel) with bimodal and unimodal grain structures were prepared. The influence of grain size distribution on static and dynamic mechanical properties were systematically investigated. The results showed that bimodal grain structure spinel has larger flexural strength (236.31 MPa) compared to unimodal grain structure spinel (221.38 MPa). Whereas, their values of hardness are very similar (15.1 vs 14.7 GPa) and fracture toughness remains unchanged (1.1 MPa∙m^1/2 for both spinel). Although static compression strength of bimodal grain structure spinel (1236 MPa) is higher than that of unimodal one (1078 MPa) due to a smaller average grain size in the former, the negative effect of bimodal grain structure reduced the spinel strength compared to theoretically predicted value. Bimodal grain structure spinel shows slightly lower increment (49%) in compression strength from static to dynamic loading compared to that of unimodal one (57%) due to a decreased strain-rate sensitivity ascribed to bimodal grain structure. A brittle mode in inelastic deformation at Hugoniot elastic limit was demonstrated in both bimodal and unimodal grain structures. Bimodal grain structure has an influence on the Hall-Petch-like relation of yield strength under planar impact loading.     

Application of statistical mechanics to the analysis of various physical properties of elastomeric networks — a review

This paper examines the application of the statistical mechanics to the analysis of various physical properties of the elastomeric networks. The equilibrium properties of rubber-like networks are discussed, and also some dynamic properties, such as the relaxation spectrum of Gaussian networks. The paper covers a large spectrum of properties of polymer networks such as: fluctuations and chain dimensions in unimodal and bimodal network, effects of entanglements and constraints on the elastic properties of the network, segmental orientation, liquid-crystalline networks, small angle neutron scattering from networks, strain birefringence, elastic properties of filled networks, strain induced crystallization etc. The paper shows that the statistical mechanics can be successfully used to the analysis of almost all physical properties of rubber-like networks.     

Some evidence on pore sizes in poly(dimethylsiloxane) elastomers having unimodal,bimodal, or trimodal distributions of network chain lengths

Summary Unimodal, bimodal, and trimodal networks were prepared by end linking functionally-terminated chains of poly(dimethylsiloxane). The resulting materials were characterized using a thermoporometric technique in which freezing points or melting points are determined for solvent absorbed into the network stuctures. The extent to which the normal melting point is suppressed depends on how much the solvent is constrained within the network pores. Several well-defined melting points were observed for some of the multimodal networks, which is consistent with their unusual distributions of network chain lengths.     

Large-Scale Structures in Bimodal Poly(dimethylsiloxane) Elastomers

Small-angle X-ray and neutron scattering are used in conjunction to study structures in a poly(dimethylsiloxane) network having a bimodal distribution of network chain lengths, a type of elastomer of considerable interest because of its unusually good mechanical properties. The scaling regimes for the scattering are compared and contrasted for varying degrees of network swelling. The excess scattering at small angles in the swollen state is associated with large-scale structures, likely high cross-link density clusters with reduced swelling, topologically trapped within the network mesh. These results indicate the presence of supramolecular structures with apparently smooth surfaces even in the non-swollen state for this type of elastomer. Optical microscopy of unimodal and bimodal elastomers having comparable crosslink staining does in fact suggest the presence of smooth surfaced, micron-scale islands of higher cross-link density. Finally, the effects of the molecular weight distribution of the network mesh demonstrate that bimodal networks can have supramolecular structures that are smaller but more random than their unimodal counterparts.     

Improved mechanical properties of ATBN-toughened epoxy networks by controlling the phase separation scale

Amine-terminated butadiene acrylonitrile (ATBN) rubber-toughened epoxy networks with different phase separation scale were formulated by adjusting acrylonitrile content in ATBN, and the effect of phase separation on stress–strain behavior, impact strength, dynamic mechanical thermal properties and thermo-gravimetric performance of the toughened epoxy networks were studied in detail. Scanning electron microscope analysis demonstrated that phase separation occurred in the toughened networks, forming a two-phase morphology, and the size of rubber particles was highly dependent on the acrylonitrile content in ATBN. Toughened epoxy network with larger phase separation scale exhibited longer ultimate elongation, and smaller phase separation scale was proved to be more effective in improving impact strength. Furthermore, bimodal rubber-particle distributed epoxy networks were obtained by addition of two types of ATBN simultaneously. The impact strength of the bimodal rubber-particle distributed epoxy network showed a great increase of 47% without sacrificing other mechanical properties, as compared to those of unimodal rubber-particle distributed epoxy networks.     

Effect of non-Gaussian chains on fluctuations of junctions in bimodal networks

A simple tetrahedron model is used to study the effect of non-Gaussian chains on fluctuations of junctions in bimodal networks. The four chains are assumed to meet at a junction with their other ends being fixed at the vertices of the tetrahedron. It is assumed that the angles between mean end-to-end vectors of all four chains connected at the junction are tetrahedral, but the lengths of edges of the tetrahedron may differ due to the difference in the lengths of the chains. The central junction is free to fluctuate, subject to the constraints imposed by the pendant chains. The long chains are chosen to be Gaussian. The short chains are assumed to be non-Gaussian. Calculations show that the non-Gaussian nature of the short chains imposes severe restrictions on the fluctuations of the central junction. The strength of these restrictions directs attention to the importance of anharmonic modes in networks.     

Effect of chain length distribution on thermal characteristics of model polytetrahydrofuran (PTHF) networks

A series of model polytetrahydrofuran (PTHF) networks were synthesized via end-linking reactions of α, ω-allyl PTHF oligomers with a stoichiometric tetrafunctional crosslinker. The telechelic PTHF oligomers were synthesized by living cationic ring-opening polymerization of tetrahydrofuran followed by a termination reaction with allyl alcohol. Networks thus prepared have well-controlled architecture in terms of the inter-crosslink chain length (M_c) and chain length distribution: resulting in unimodal, bimodal and clustered structures. Unimodal network was prepared by using polymer chains of same molecular weight, bimodal networks were synthesized by using two groups of polymer chains with different average molecular weights, and the clusters are prepared by incorporating clusters of networks with small molecular weight chains in a network matrix made of longer chains. Thermal characteristics of these model networks were investigated as a function of crosslink density, as well as inhomogeneities of crosslink distribution using DSC. We demonstrate that glass transition temperature (T_g) and crystallization behavior (melting temperature and crystallinity) of the networks are both strongly influenced by crosslink density (M_c). By comparing the unimodal, bimodal and clustered networks with similar average M_c, the effects of inhomogeneities in the crosslink distribution on the thermal properties were also investigated. Results show that inhomogeneities have trivial influence on T_g, but strongly affects the crystallization behavior. Moreover, the effects of the content ratio and length ratio between long and short chains, and the effects of cluster size and size distribution on the thermal characteristics were also studied.     

Coagent-induced transformations of polypropylene microstructure: Evolution of bimodal architectures and cross-linked nano-particles

Polypropylene is transformed by simultaneous, radical-mediated chain scission and cross-linking to generate branched architectures. While macroradical fragmentation reduces the molar mass of the dominant chain population, cross-linking by triallyl trimesate (TAM) activation yields a minority population of hyper-branched chains that is less susceptible to molecular weight loss. This disparity in chain reactivity leads to bimodal molecular weight and branching distributions. Furthermore, a precipitation polymerization of TAM can proceed concurrently with PP branching to produce a low yield of cross-linked, TAM-rich nano-particles. The mechanisms through which unimodal composition and molecular weight distributions evolve toward a bimodal condition are discussed, along with the factors that lead to particle formation.     

Dynamic response of transiently trapped entanglements in polymer networks

The structure and viscoelastic response of polymer networks are highly sensitive to the presence of pendant chains. These imperfections, that are unavoidable produced during a cross-linking reaction, reduce the cross-linking density and affect the damping response of elastomers. In this work the dynamics of pendant chains present in a cross-linked network is investigated using end-linked poly(dimethyl-siloxane) networks with well defined structure. For this purpose, model networks containing 10 and 20 wt% of two different monodisperse pendant chains with molecular weights well above the critical entanglement molecular weight and some of their blends were prepared. It was found that, within this range of concentration of pendant chains, the long-time dynamic response of the networks was nearly insensitive to the content of pendant material but deeply influenced by the average molar mass of these defects. While the equilibrium behavior of the networks can be well described by a mean field theory for rubber elasticity, the long time relaxational dynamics can be rationalized in terms of the Pearson-Helfand picture for the arm retraction process. Within this theoretical picture, the dynamics can be explained in terms of the molecular architecture of the network, the Rouse time and the weight average molar mass of the pendant material.     

Bimodal networks and networks reinforced by the in situ precipitation of silica

The goal of primary interest in these investigations was the development of novel methods for preparing elastomeric networks having unusually good ultimate properties. The first technique employed involves endlinking mixtures of very short and relatively long functionally-terminated chains to give bimodal networks. Such (unfilled) elastomers show very large increases in reduced stress or modulus at high elongations because of the very limited extensibility of the short chains present in the networks. The second technique employs the in situ precipitation of reinforcing silica either after, during, or before network formation. The reaction involves hydrolysis of tetraethylorthosilicate, using a variety of catalysts and precipitation conditions, and the effectiveness of the technique is gauged by stress-strain measurements carried out to yield values of the maximum extensibility, ultimate strength, and energy of rupture of the filled networks. Information on the filler particles thus introduced is obtained from density determinations, light scattering measurements, and electron microscopy.     

Unimodal and bimodal networks of physically crosslinked polyborodimethylsiloxane: viscoelastic and equibiaxial extension behaviors

Unimodal and bimodal networks of physically crosslinked polyborodimethylsiloxane (PBDMS) were prepared by end-linking hydroxy-terminated polydimethylsiloxane (PDMS) with boric acid. Their viscoelastic and equibiaxial extension behaviors were investigated. Three PDMS precursors with different number-average molecular weight ( M ¯ n $ {\overline{M}}_n $ ) were employed, of which the shortest chain had M ¯ n $ {\overline{M}}_n $ lower than the entanglement molecular weight. Bimodal networks were prepared from the mixture of the shortest and the longer PDMS chains. Linear viscoelastic behavior of unimodal network of the shortest chain gave the best fit to the Maxwell model with single relaxation time of 1.59 s, and equilibrium elastic modulus (G _e ) of the network was well-explained by phantom network model. The unimodal networks from the other two long chain precursors, however, showed multi-relaxation behavior with the longest relaxation times of 1.00–1.26 s. Moreover, their G _e was close to affine model and deviated from the phantom model with trapped entanglement factors of ~ 0.13. The bimodal networks with high mole percentage of short chains gave G _e values approximate to the predicted values of phantom model. Such bimodal networks showed an extremely large increase in modulus at high biaxial extension, attributed by the limited extensibilities of short chains and un-relaxed crosslinked junctions.     

Einfluß des Quellmittels auf die Bestimmung der Netzkettenmolmasse durch Kompressionsmessungen

Uniaxial compression of poly(dimethylsiloxane) networks swollen in various solvents or solvent mixtures was investigated. Assuming a deformation model, the molar mass of the network chains can be calculated from the modulus G* measured at small values of deformation (0.9 < λ < 1.0). Depending on the swelling degree, the calculated molar mass of the network chains indicates characteristic features. A strong dependence in this regard was found for unimodal networks in the area of low swelling degrees, i. e. up to 3–4. This behaviour can be described in terms of a transition from affine to free-fluctuating phantom networks. In the area of higher swelling degrees, a further increase of the calculated molar mass of the network chains was observed. Networks synthesized in the presence of a solvent were found to behave similarly. Networks synthesized from different polymers were found to behave in a distinctly different manner.     

Mechanical and swelling properties of PDMS interpenetrating polymer networks

Poly(dimethylsiloxane) (PDMS) interpenetrating networks (IPNs) of two different molecular weight PDMS were prepared. Six series of IPNs were obtained by first tetra-functionally end-linking long vinyl-terminated PDMS (molar mass 23 × 10³ or 21 × 10³ g mol⁻¹) neat or in a 50% solution with unreactive PDMS chains. These networks were then dried and swollen with short reactive telechelic PDMSs (molar mass 800, 2.3 × 10³ or 5.7 × 10³ g mol⁻¹) that were subsequently end-linked. The mechanical, toughness and swelling properties of these IPNs were investigated. We found that the correlation between modulus (E) and equilibrium swelling (Q) in toluene of the PDMS IPNs obeys a scaling relation identical to that of a normal unimodal PDMS network. This result strongly suggests effective load transfer between the networks. The results of the elastic modulus and of the toughness of the networks represented by the energy required to rupture them were analyzed in terms of a recent model by Okumura [Europhys Lett 2004;67:470.]. Although the modulus results are in reasonable agreement with the equal-stress model of Okumura, the toughness results are not. In addition, our measured toughness decreases instead of increases with composition in an opposite trend to that predicted by the equal-strain model. An empirical model based on fracture mechanics gives a good representation of the toughness data.     

A study on blends of different molecular weights of polypropylene

Blends of small percentage of plastic grade polypropylene with fiber grade polypropylene are studied in unoriented and oriented states. A 3% blend sample has a higher spherulitic growth rate, and improved mechanical behavior in drawn fiber state as compared to the parent sample. These changes are related to the presence of bimodal and trimodal crystal texture of polypropylene in the blend, respectively. At a higher blend percent, specifically at 10 and 15%, the mechanical properties of the drawn fiber are inferior and these are related to partial phase segregation of the components.     

Monte Carlo simulation of two interpenetrating polymer networks: Structure, swelling, and mechanical properties

The swelling and mechanical properties of various interpenetrating polymer networks (IPNs) were studied. Six networks made from permutations of a moderately crosslinked polyelectrolyte network (ref), a moderately crosslinked neutral polymer network (net1), and a highly crosslinked polyelectrolyte network (net2) were first swollen in water and structural properties such as end-to-end chain lengths and radial distribution functions were compared with the component networks' equilibrium properties. The swelling of composite IPNs was discussed in terms of a balance between the osmotic pressure due to mobile counterions and the restoring force of the network chains, which act in parallel to counteract the osmotic swelling. For the ref-net2 system, the strong stretching of net2 chains increases the network restoring force and the further swelling due to the counterions is suppressed. The swollen networks were then uniaxially stretched, and equilibrium stress-strain plots were obtained up to high extension ratios. The equilibrium volume decreased upon uniaxial extension, and the elastic moduli of IPNs of the A-A type were slightly greater than that of their respective single networks.     

Dynamic Mechanical Thermal Analysis and Rheological Properties of Synthesized Polypropylene Reactor Blends Using Homogeneous Binary Metallocene Catalyst

A homogenous binary metallocene catalytic system comprising of isospecific rac-Me₂Si(2-Me-4-Ph In)₂ZrCl₂(I) producing high molecular weight isotactic polypropylene and oscillating (2-Ph In)₂ZrCl₂(II) precursor producing low isotactic elastomer polypropylene at three varying molar ratios of two types of catalysts was used to synthesize polypropylene reactor blends. Dynamic mechanical thermal analysis and rheological properties along with molecular weight of synthesized polypropylene reactor blends were studied and correlations among these properties were established. It was found that molar ratio of catalysts is a significant factor in determination of molecular weight and its distribution. The produced polymers with unimodal molecular weight distribution showed intermediate modulus during dynamic mechanical thermal experiment, while the ones with bimodal molecular weight distribution exhibited a kind of phase separation at low temperature. Depending on strength of the developed structures, determined by the presence of interfacial connectors, the modulus could be adjusted. Origins of other types of relaxations and their differences for each type of the developed products were discussed in detail. From rheological results and particularly the relaxation curves, the characteristics of the chain structure of the synthesized reactor blends could be resolved. It was revealed that one of the synthesized polymers had long chain branches unlike the rest of the samples having linear chain structure.