Elongational viscosity for miscible and immiscible polymer blends. I. PMMA and AS with similar elongational viscosity

The effects of miscibility and blend ratio on uniaxial elongational viscosity of polymer blends were studied by preparing miscible and immiscible samples at the same composition by using poly(methyl methacrylate) (PMMA) and poly(acrylonitrile-co-styrene) (AS). Miscible polymer blend samples for the elongational viscosity measurement were prepared by using three steps: solvent blends, cast film, and hot press. A phase diagram of blend samples was made by visual observation of cloudiness. Immiscible blend samples were prepared by maintaining the prepared miscible samples at 200°C, which is higher than cloud points using a LCST (lower critical solution temperature) phase diagram. The phase structure of immiscible blends was observed by an optical microscope. The elongational viscosity of all samples was measured at 145°C, which is lower than the cloud-point temperature at all blend ratios. The elongational viscosity of PMMA and AS was similar to each other. The strain-hardening property of miscible blends in the elongational viscosity was only slightly influenced by the blend ratio, and this was also the case with immiscible blends. The strain-hardening property was only slightly influenced, whether it was miscible or immiscible at each blend ratio. Polydispersity in molecular weight for blend samples was not changed by GPC (gel permeation chromatography) analysis. Almost no change in the polydispersity of the molecular weight for blends and the similarity of elongational viscosity between PMMA and AS resulted in little influence of the blend ratio and miscibility on the strain-hardening property. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 757–766, 1999     

Enthalpic effects on interfacial adhesion of immiscible polymers compatibilized with block copolymers

The influence of enthalpic interactions on interfacial adhesion between immiscible polymer matrices and reinforcing block copolymer segments has been studied using the transmission electron microscopic (TEM) methodology of Creton et al. We examined the behaviour of four statistical styrene-acrylonitrile (SAN) copolymers, each having different acrylonitrile (AN) content, blended with polystyrene (PS) as the minor component, and reinforced by three poly(methyl methacrylate-b-styrene) (PMMA-b-PS) block copolymers of differing molar masses, viz. 20000, 65000 and 680000 g mol⁻¹. These observations were compared with similar experiments on poly(methyl methacrylate) (PMMA) blended with PS and reinforced by PMMA-b-PS. Emulsification was observed with all three PMMA-b-PS copolymers. Crazes were formed in the SAN matrices and a statistical evaluation of interfacial failures was performed on the discrete PS domains that lay within the crazes. For the two block copolymers of higher molar mass, optimal reinforcement of the interfaces was observed independent of the SAN composition. With the 20000 block copolymer, however, the pattern of the interfacial failure depended strongly on the SAN composition. Specifically, it was observed that the fraction of the discrete particles that suffered interfacial failure, and led to the creation of large voids in the crazes in these blends, increased with increased AN content of the SAN matrix. Thus, we found that the fraction of discrete PS particles that produce large voids in crazes of blends containing SAN33 is always higher than in blends containing SAN15, when reinforced with the 20000 PMMA-b-PS. We infer that the critical molar mass required of a mechanically reinforcing copolymer depends on the short-range (attractive and repulsive) interactions between the blend components in the interfacial region. The TEM method could not, however, distinguish between reinforced and neat PMMA/PS blends, all of which showed strong adhesion. This is attributed to the comparatively diffuse interface in the PMMA/PS system, a consequence of the relatively weak repulsion between these two polymers.     

Free volume and dipole mobility in an epoxide oligomer before crosslinking

The free volume of a bisphenol‐A‐type epoxide oligomer (DGEBA) was studied using Williams–Landel–Ferry parameters and thermal expansion coefficients above and below the glass transition temperature (T_g). The values of the free‐volume fraction at the T_g are around 0.02 for the DGEBA oligomers having weight‐average molecular weights (M _w's) from 1396 to 2640. The dipole mobility, which was obtained from the analysis of the temperature dependence of the dielectric relaxation time, was compared with the segment mobility in terms of the critical volume for the transport of each moving unit. The critical volume for the segment transport increases with increase of the M _w of the oligomer. The critical volume for the dipole movement, on the other hand, is not different between the oligomers studied (1396 ≤ M _w ≤ 2640), which leads to that the dipole mobility in the epoxide oligomer is smaller than is the segment mobility. The low mobility of the dipole is considered to result from the molecular interaction restricting the dipole movement, especially in a smaller M _w oligomer. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 207–214, 1999     

Transport parameters and solubility coefficients of polymers at their glass transition temperatures

Many parameters of polymers exhibit breaks when temperature passes through glass transition. It is also often assumed that fractional free volume (FFV) at the glass transition temperature (T_g) has a standard value (the isofree volume concept). As gas diffusion (D) and permeability (P) coefficients depend on FFV, and mechanism of sorption and permeation is different above and below T_g, a question can be asked if D and P parameters of various gases in polymers have standard values at corresponding T_g, and, if not, how the values of D(T_g) and P(T_g) vary with T_g in different polymers. To examine this problem, two approaches were used: (1) extrapolation to T_g of numerous P and D values measured at ambient temperatures; (2) an analysis of direct data obtained in different polymers at their T_g. In both cases, qualitatively similar results were obtained: the D(T_g) and P(T_g) values increase with growing T_g independently of the nature of gas. Permselectivity P_i(T_g)/P_j(T_g) and selectivity of diffusion D_i(T_g)/D_j(T_g) are reduced when T_g increases. The dependence of the solubility coefficients S(T_g) = D(T_g)/P(T_g) is much weaker than those of D(T_g) and P(T_g). This conclusion was confirmed by the results of direct measurements of S in a wide range of temperature including T_g for several gas/polymer systems. An analysis of the results of positron annihilation studies of free volume in polymers led to the conclusion that the observed increases in the D(T_g) and P(T_g) values with T_g are caused mainly by thermal activation of diffusion processes at T_g. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1691–1705, 2000     

Attempt to correlate the yield processes above and below the glass transition in glassy polymers

This paper is concerned with a model which attempts to describe quantitatively, by the same elementary process, the yield behaviour above and below T_g, as well as the effect of annealing on the yield stress. This model links together theories we have previously proposed and relies on the following main assumptions: the deformation processes imply the cooperation of n activated segments and that the free energy increase of an activated segment depends on the structural state of the polymer. A satisfactory agreement is found with yield stress data on polycarbonate (PC), over a very large range of temperatures and strain rates. The correlation between the yield stress and the annealing treatment is also reasonable.