Golf ball having a thermoplastic positive hardness gradient inner core layer and a thermoset shallow positive hardness gradient outer core layer

1. A golf ball comprising:

an inner core layer comprising a thermoplastic highly-neutralized ionomer comprising a copolymer of ethylene and an α,β-unsaturated carboxylic acid, an organic acid or salt thereof, and sufficient cation source to neutralize the acid groups of the copolymer by 80% or greater, and having a geometric center hardness less than a surface hardness to define a first positive hardness gradient;

an outer core layer disposed about the inner core, the outer core being formed from a homogenous thermoset composition and having an interior hardness less than an outer surface hardness to define a second positive hardness gradient;

an inner cover layer disposed about outer core layer; and

an outer cover layer disposed about the inner cover layer,

wherein the first and second hardness gradients each have a slope and the slope of the second hardness gradient is less than the slope of the first hardness gradient such that a ratio of the first and second slopes is 0.8 to 0.85.

2. The golf ball of claim 1 , wherein the ratio of the first and second slopes is 0.8 to 0.83.

3. The golf ball of claim 2 , wherein the ratio of the first and second slopes is 0.81 to 0.82.

4. The golf ball of claim 1 , wherein the slope of the first hardness gradient is greater than about 1 and the slope of the second hardness ratio is less than about 1.

5. The golf ball of claim 4 , wherein the slope of the first hardness gradient is greater than about 1.15 and the slope of the second hardness ratio is less than about 0.98.

6. The golf ball of claim 1 , wherein the inner core layer has an outer diameter of about 0.5 inches to about 1.13 inches.

7. The golf ball of claim 1 , wherein the acid groups of the copolymer are neutralized by 90% or greater.

8. The golf ball of claim 7 , wherein the acid groups of the copolymer are neutralized by about 100%.

9. The golf ball of claim 1 , wherein the organic acid or salt thereof comprises barium, lithium, sodium, zinc, bismuth, chromium, cobalt, copper, potassium, strontium, titanium, tungsten, magnesium, cesium, iron, nickel, silver, aluminum, tin, or calcium salts, or salts of fatty acids.

10. The golf ball of claim 9 , wherein the fatty acid salt comprises stearic acid, behenic acid, erucic acid, oleic acid, linoelic acid or dimerized derivatives thereof.

11. The golf ball of claim 9 , wherein the organic acid or salt thereof comprises a magnesium salt of oleic acid.

12. The golf ball of claim 1 , wherein the outer core layer comprises a soft and fast agent.

13. The golf ball of claim 12 , wherein the soft and fast agent comprises a halogenated thiophenol.

14. A golf ball comprising:

an inner core layer consisting of a thermoplastic material and having a geometric center hardness less than a surface hardness to define a first positive hardness gradient having a slope of about 1.15 or greater;

an outer core layer disposed about the inner core, the outer core being formed from a substantially homogenous thermoset composition comprising a diene rubber and having a second positive hardness gradient having a slope of about 0.98 or less;

a cover layer disposed outer core layer, the cover layer comprising an inner cover layer comprising an ionomer and an outer cover layer comprising a castable polyurethane or polyurea material.

15. The golf ball of claim 14 , wherein a ratio of the second positive hardness gradient to the first positive hardness gradient is about 0.8 to 0.85.