Introduction

Ethylene-methacrylic acid (E/MAA) copolymers were first developed over four decades ago. It was quickly realized that the MAA groups could be partially or fully neutralized with a metal cation – such as sodium, magnesium, zinc, etc. - to form a class of extraordinarily tough semicrystalline “ionomers” [1]. The improved mechanical properties of these ionomers has led to their widespread use as packaging films, scratch resistant coatings and as an interlayer in shatter-resistant glass windows. Despite decades of commercial utilization, the structure-processing-property relationships for these materials are only now being explored [2-4]. The aim of this investigation is to reveal the microstructural origins of the yield stress (σ_y) for E/MAA copolymers and ionomers.

Experimental

E/MAA copolymers and ionomers were provided by DuPont Packaging and Industrial Polymers. The copolymer base resins contained 6, 11.5, 15 and 19 wt% MAA (2.0, 4.1, 5.5 and 7.1 mol % MAA, respectively). The degree of neutralization varies between 0 and 83%, or 0 to 2.6 wt% Na. As-received pellets were melt-pressed into 0.2-0.5 mm thick sheets at 140-160 ºC in a PHI hot press and quenched to room temperature. The sheets were then stored in a desiccator at room temperature for three weeks. ASTM D1708 dogbones were stamped from these sheets for tensile testing. DMTA samples measuring approximately 5 x 22 mm² were cut from the same sheets to preserve the thermal history. Uniaxial tensile stress-strain curves were obtained with an Instron Model 5865, equipped with an Environmental Chamber 3111 retrofitted to control the testing temperature to within 0.3 ºC with a cycle time of 1 second. Samples were tested at temperatures between 0 and 35 ºC. Crosshead speeds ranged from 8.5 × 10^-3 to 8.5 mm/s, producing initial strain rates of 3.8 × 10^-4 to 3.8 × 10^-1 s^-1. Subambient temperatures were achieved by introducing a small quantity of dry ice into the chamber. DMTA measurements were performed at 1 Hz on a TA Instruments RSA 3, using the film fixture. Data were collected every 5 ºC for an effective heating rate of about 10 ºC/min.

Results and Discussion

Tensile tests of E/MAA copolymers and ionomers were performed over a range of temperatures and strain rates. The yield stress was taken as the stress exerted on the sample at a strain that corresponds to the intersection of the Young's modulus line and a line fit to the pseudo-linear region immediately following the yield point. This construction allows for the determination of σ_y even when a local maximum in stress – corresponding to the initiation of necking – is not observed.

As expected, σ_y increases as the temperature is lowered and as the strain rate is increased. For the unneutralized E/MAA copolymers, two regimes of behavior are observed. At low strain rates, σ_y is a very weak function of strain rate and is well-described by the polyethylene crystal plasticity expression of Shadrake and Guiu [5,6],

(1)

where K(T) is the shear modulus of the slip plane, B is the magnitude of the Burgers vector, r₀ is the dislocation core radius, l_c is the crystal thickness and ΔG* is the strain rate-dependent energy barrier for the nucleation of a [001] screw dislocation from one of the lateral faces of a polyethylene crystal. At higher strain rates, however, σ_y starts to increase very quickly with strain rate due to incomplete relaxation of the amorphous phase. DMTA tests reveal that, for the E/MAA copolymers, the incomplete relaxation corresponds to the glass transition temperature, T_g. The tensile stress contribution, σ_re, from a single incomplete relaxation process is given by the Ree-Eyring model [7],

(2)

where R is the gas constant, v is the activation volume, is the applied strain rate, is a constant pre-exponential factor and DH is the activation energy. Thus, when σ_re/T is plotted against , this regime appears as a straight line. Overall, we can describe the temperature and strain rate dependence of the E/MAA copolymer σ_y by the simple superposition of the two contributions so that,

. (3)

Eq. (3) also works well to describe the σ_y of E/MAA ionomers. However, over the range of conditions studied, the amorphous phase was not completely relaxed when the degree of neutralization was greater than 0.4 wt% Na. At this critical degree of neutralization, the stiff “regions of restricted mobility” [8] surrounding each ionic aggregate are large (or numerous) enough to impinge upon each other and nearby polyethylene crystals. The percolation of the stiff material through the amorphous phase is responsible for the elevated yield stress. Additional neutralization above the critical degree results in minimal gains in the measured yield stress; there is little advantage in neutralizing to stoichiometric equivalence.

References

[1] R.W. Rees, D.J. Vaughn, Polym. Prepr. Am. Chem. Soc., Div. Polym. Chem., 6, 296 (1965).

[2] K. Wakabayashi, R.A. Register, Polymer, 46, 8838 (2005).

[3] K. Wakabayashi, R.A. Register, Macromolecules, 39, 1079 (2006).

[4] R.C. Scogna, R.A. Register, Polymer, 49, 992 (2008).

[5] L.G. Shadrake, F. Guiu, Philos. Mag., 34, 565 (1976).

[6] L.G. Shadrake, F. Guiu, Philos. Mag. A., 39, 785 (1979).

[7] T. Ree, H. Eyring, J. Appl. Phys., 26, 793 (1955).

[8] A. Eisenberg, B. Hird, R.B. Moore, Macromolecules, 23, 4098 (1990).