Polystyrene elastomeric networks

Characterization of elastomeric networks by swelling equilibrium measurements may take advantage of the applicability of the constrained junction theory already demonstrated for mechanical testing. Use of the interpenetration concept (equation 124 or 125), and of topological expressions for (equations 47 or 60) cause equation (156) to depend only on and x for a perfect network, or on M, M , and x for a randomly cross-linked network. An example of such an application was given by Erman and Baysal. Two cross-linked polystyrene networks were... 

IPNs are also attractive for development of materials with enhanced mechanical properties. As PDMS acts as an elastomer, it is of interest to have a thermoplastic second network such as PMMA or polystyrene. Crosslinked PDMS have poor mechanical properties and need to be reinforced with silica. In the IPNs field, they can advantageously be replaced by a second thermoplastic network. On the other hand, if the thermoplastic network is the major component, the PDMS network can confer a partially elastomeric character to the resulting material. Huang et al. [92] studied some sequential IPNs of PDMS and polymethacrylate and varied the ester functionalities the polysiloxane network was swollen with MMA (methyl methacrylate), EMA (ethyl methacrylate) or BuMA (butyl methacrylate). Using DMA the authors determined that the more sterically hindered the substituent, the broader the damping zone of the IPN (Table 2). This damping zone broadness was also found to be dependant on the PDMS content, and atomic force microscopy (AFM) was used to observe the co-continuity of the IPN. 

In commercial thermoplastic S-B-S rubber, the end-block phase is present in a smaller proportion with a styrene-to-butadiene (end-block-to-midblock) ratio in the range 15 85 to 40 60 on weight basis. The useful temperature range of S-B-S copolymer lies between the Tg of polybutadiene and polystyrene. Below the Tg of polybutadiene, the elastomeric midblocks become hard and brittle. Above the Tg of polystyrene, the domains soften and cease to act as cross-links for the soft midblocks. Between the Tg of both homopolymers, however, the hard styrene domains prevent the flow of the soft elastomeric butadiene midsegments through a network similar to vulcanized rubber. Therefore, within normal use temperature, S-B-S block copolymer retains the thermoplasticity of styrene blocks and the toughness and resilience of the elastomer units. 

Styrene block copolymers are the most widely used TPEs, accoimtingfor close to 45% of total TPE consumption worldwide at the close of the twentieth century. 1 They are characterized by their molecular architecture which has a hard thermoplastic segment (block) and a soft elastomeric segment (block) (see Fig. 3.2). Styrenic TPEs are usually styrene butadiene styrene (SBS), styrene ethylene/butylene styrene (SEBS), and styrene isoprene styrene (SIS). Styrenic TPEs usually have about 30 to 40% (wt) bound styrene certain grades have a higher boimd styrene content. The polystyrene endblocks create a network of reversible physical... 

More recent SANS experiments on stretched networks were performed by Hinkley et al. (99) and by Clough et al. (100,101). Hinkley et al. (99) prepared blends of polybutadiene and polybutadiene-de. Both polymers were made by the living polymer technique, end-capped with ethylene oxide, and water-washed to yield the dihydroxy liquid prepolymer. Uniform networks were prepared by reacting the prepolymers with stoichiometric amounts of triphenyl methane triisocyanate. The value of using polybutadiene over polystyrene, of course, is that the networks are elastomeric at ambient temperatures. 

At room temperature, these polystyrene domains are hard and act as physical cross-hnks, tying the elastomeric midsegments together in a 3-dimensional network. In some ways, this is similar to the network formed by vulcanizing conventional rubbers using sulfiir cross-links. The difference is that in thermoplastic elastomers, the domains lose their strength when the material is heated or dissolved in solvents. This allows the polymer or its solution to flow. 

Styrenic block copolymer (SBS) TPEs are multiphase compositions in which the phases are chemically bonded by block copolymerization (see chapter Introduction to Plastics and Polymers). At least one of the phases is a hard styrenic polymer. This styrenic phase may become fluid when the TPE composition is heated. Another phase is a softer elastomeric material that is rubber-like at room temperature. The polystyrene blocks act as cross-links, tying the elastomeric chains together in a three-dimensional network. SBS TPEs have no commercial applications when the product is just a pure polymer. They must be compounded with other polymers, oils, fillers, and additives to have any commercial value. 

Polymer networks are conveniently characterized in the elastomeric state, which is exhibited at temperatures above the glass-to-rubber transition temperature T. In this state, the large ensemble of configurations accessible to flexible chain molecules by Brownian motion is very amenable to statistical mechanical analysis. Polymers with relatively high values of such as polystyrene or elastin are generally studied in the swollen state to lower their values of to below the temperature of investigation. It is also advantageous to study network behavior in the swollen state since this facilitates the approach to elastic equilibrium, which is required for application of rubber elasticity theories based on statistical thermodynamics. ... 

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