The glass transition temperature of a series of ethylene−vinyl alcohol copolymers which have been exposed to methanol is determined by dynamic mechanical measurements as a function of methanol concentration. The results of a number of sorption and desorption measurements taken in the temperature range 21−60 °C for the same polymer−solvent system are also reported. An interpretation of these data, which relates these two sets of results, is presented. This interpretation differs considerably from those currently available in the literature for sorption data similar to those reported here. The need to deconvolute from the sorption data the effect of macroscopic elastic constraints arising during the swelling process and in particular to distinguish features of the sorption curves which reflect true material properties of the system as opposed to simple geometrical effects is pointed out. Once this is done, the main qualitative features of our results, namely, initial sorption ∼ presence of sharp concentration fronts during sorption, and existence of two different desorption regimes, can be accounted for on the basis of a simple description of solvent transport based on Fick's law with a diffusion coefficient which changes suddenly from a low value characteristic of the glassy state to a high value typical of rubbery polymers at the concentration at which plasticization takes place. The geometry dependent features of the sorption curves can also be understood within this framework.

Fracture of a network under threshold conditions, Le., fracture at infinitely long time, is governed by the length of some characteristic elastically active strand. The threshold fracture energy, r0, is related to this strand length through T~ a {'Iz, where { is the number of units in the strand. Measurements of T, for a poly(dimethylsi1oxane) (PDMS) network gave { = 143 monomer units. This strand length compares well with the entanglement spacing for PDMS (Me = 135 monomer units). Characterization of the network with statistical theory indicated that the average length of a strand between active chemical junction points was 1900 monomer units. Therefore, approximately 14 entanglement couplings were trapped between active chemical junctions in the network. Our results show that entanglement couplings dominate the process of fracture under threshold conditions for a network with a relatively large number of trapped entanglement interactions.Entanglements have profound effects on the viscoelastic response of polymer systems.' They are found in systems of long linear chains, where neighboring molecules permeate regions occupied by other chains in the system. Overlapping chains physically entangle with each other and restrict the relative motion of large segments of neighboring chains. Chain reconfiguration for lengths greater than a critical scale, defined as the entanglement molecular weight (Me), is retarded to longer times due to the physical entanglements between neighboring chains.Relaxations on scales smaller than Me are apparently unaffected by the presence of the entanglements.Several accounts of the nature of entanglements have appeared in the recent l i t e r a t~r e .~-~ The most satisfying idea treats the restrictions imposed by entanglements in a distributed fashion. That is, the entanglements do not act at specific locations along the chains in the system. Rather, neighboring chains are thought to form a mesh, or tube, that restricts lateral movement and allows one-dimensional diffusion in the tube as a mechanism for relaxation. This theory, and several of its modification^,^ appears to capture the essential features of experimental observations. Recent theoretical and experimental work has also shown that entanglement couplings can dominate the smallstrain equilibrium modulus of a cross-linked rubber The network is formed by covalently linking together the chains of an entangled, linear system. In the process, a fraction of the entanglements present in the linear system before cross-linking are trapped between the chemically bound junction points in the network. Statistical theories have been developed to calculate the fraction of entanglements that are trapped in this process and to help quantify the contribution of trapped entanglements to the equilibrium modulus.'O~"Other ideas on network elasticity have treated the effects of junction fluctuations on the elastic m o d~l u s . '~~'~ Topological constraints only help suppress junction fluctuation in these theories while the reduced configurations av...

No abstract

SynopsisMean field theories developed by Kerner and van der Poel were applied to several binary elastomer blends. Calculations were compared with experimental small strain dynamic mechanical properties of the blends. Although the blends exhibit a wide range of detailed structures, the theories were able to model blends with well-defined included particles embedded in a matrix phase. Blends with a continuous morphology were not properly modeled with the Kerner or van der Poel theories.

Dynamic mechanical thermal analysis and transmission electron microscopy have been used to elucidate the structure of binary and ternary blends of NR, BIIR, and IM. Dynamic measurements at 10 Hz were able to resolve loss-tangent peaks into a major peak due to NR and a broad shoulder associated with BIIR and IM. Interpretation of these data in conjunction with electron micrographs indicate that the butyl polymers (BIIR and IM) form a second phase in a matrix of NR for compositions containing at least 67% NR. Dynamic mechanical properties and TEM micrographs of binary blends of NR with BIIR or IM show that the structure of these binary blends differ; IM forms larger, more distinct domains in the NR matrix. This difference in structure may result from the different molecular weights of the butyl polymers and the ability of BIIR to crosslink with NR. TEM micrographs of both binary blends indicate that carbon black is dispersed in the matrix material and is excluded from the isobutylene-rich domains, The two-phase structure of these blends and the partitioning of carbon black between the phases may enhance the fatigue lives of these composites. Cure temperatures in the range from 130°C to 170°C affected the properties and structure of only one blend studied in this work. This blend, an 80:20:20 mixture of NR, BIIR, and IM, respectively, was able to alter its morphology when the cure temperature was elevated. Material cured at 130°C contained domains with a wide variety of shapes and sizes; material cured at 170°C contained uniform, well-defined inclusions. This ternary blend was the only material that also exhibited a higher fatigue life when the cure temperature was raised. Achieving a well-defined dispersion in a two-phase elastomer blend apparently maximizes the fatigue life of the composite material.