SynopsisThe types of molecular architecture commonly present in many commercial elastomers include long branching, which in extreme cases results in crosslinked gel network. This architecture was modeled by preparing a series of ethylene-propylene copolymer samples in which the degree of branching was systematically varied. The frequency-dependent viscoelastic properties of these model systems were measured over a temperature range of 80-23OoC with a Rheometrica Mechanical Spectrometer. Time-temperature superposition was employed to obtain master curves of the storage G' and loss G" moduli and complex viscosity. The viscoelastic properties of the model samples change systematically with the variations in molecular architecture. Specifically, the low frequency Newtonian viscosity behavior is progressively replaced by non-Newtonian power-law behavior, and the G' response relative to that of the G" is significantly enhanced as long-branching increases. The practical use of a modified Cole-Cole plot, in which the axes are expressed as the logarithms of G' and G", for analysis of molecular architecture is demonstrated. Changes in the long-branch architecture of the model samples were readily detected as systematic variations in shape and displacement of the modified Cole-Cole plot. On the other hand, the data of molecularly linear elastomer samples of different M, but similar MWDs were reduced to a single master Cole-Cole plot.

SYNOPSISThree samples of styrene-isoprene-styrene ( S-I-S ) block copolymers were chosen; copolymer A had 25% styrene, and copolymers B and C had 14% styrene. Copolymers A and B contained 20% diblock polymer and copolymer C contained 40% diblock polymer. All copolymers were mixed with a terpene type tackifier to make 56% and 48% weight tackifier concentration. These represent our model samples of pressure sensitive adhesives. The determination of tack, room-temperature peel-strength, and failure temperature under static shear were performed. The above results have been interpreted with the basic rheological data. The dynamic viscoelastic measurements and tensile stress-strain measurements were used. The effects of tackifier on the rubbery plateau moduli were treated with the GuthGold-type equation. The implications of the deviation from the equation are discussed in terms of the connectivity between polystyrene domains and the stability of the hard domains affected by inclusion of rubber segments.

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ABSTRACT:The viscosity of freshly prepared poly(vinyl chloride) (PVC) plastisol increases with time, and this phenomenon is called viscosity aging. The increase is rapid in the beginning and slows down to a quasistable value, but a very slow increase continues. The phenomenon may be a result of either the deagglomeration of agglomerated particles or the dissolution of low-molecular-weight PVC into the plasticizer. In this work, two typical commercial resins were used, one containing friable agglomerates and the other containing nonfriable agglomerates. With the friable-agglomerate resin, about 40% of the initially present agglomerates deagglomerated, whereas the viscosity increased in a week to twice the initial value. With the nonfriable-agglomerate resin, very fine and very low molecular weight particles, about 3% of all the particles, dissolved into the plasticizer in 2 days. The effect of the plasticizer type on the viscosity aging through deagglomeration was investigated with four plasticizers and three plasticizer blends. The emulsifiers used for polymerization, and retained through drying, affected the aging in the beginning. On the other hand, the viscosity after 1 week was free from the effect of the emulsifier and was affected only by the plasticizer type. With the exception of two blends, the 1-week viscosity was quantitatively related to the dielectric constant divided by the molecular weight of the plasticizer. For the plasticizer blends, one of the plasticizers could have a dominant effect on the promotion of deagglomeration.