Thermoplastic core having a negative hardness gradient formed from a plasticizer-based gradient-initiating solution

1. A golf ball comprising:

a core consisting essentially of a thermoplastic material, the core having an outer diameter of 1.51 inches to 1.59 inches and having an outer surface and a geometric center, each having a hardness;

an outer cover layer; and

an inner cover layer disposed between the core and the outer cover layer;

wherein the hardness of the outer surface is less than the hardness of the geometric center to define a negative hardness gradient of 5 Shore C or greater; and

wherein the thermoplastic material comprises an ionomer, a highly-neutralized ionomer, a thermoplastic polyurethane, a thermoplastic polyurea, a styrene block copolymer, a polyester amide, polyester ether, a polyethylene acrylic acid copolymer or terpolymer, or a polyethylene methacrylic acid copolymer or terpolymer.

2. The golf ball of claim 1 , wherein the hardness gradient is 10 Shore C or greater.

3. The golf ball of claim 1 , wherein the inner cover comprises an ionomer or a partially- or fully-neutralized ionomer and the outer cover comprises a polyurethane or a polyurea material.

4. The golf ball of claim 1 , wherein the inner cover layer has a hardness of 60 Shore D or greater.

5. The golf ball of claim 4 , wherein the inner cover layer hardness is 65 Shore D or greater.

6. The golf ball of claim 1 , wherein the inner cover layer has a thickness of 0.015 inches to 0.060 inches.

7. The golf ball of claim 6 , wherein the inner cover layer thickness is 0.02 inches to 0.045 inches.

8. The golf ball of claim 1 , wherein the outer cover layer has a hardness of 60 Shore D or less and is softer than the hardness of the inner cover layer.

9. The golf ball of claim 1 , wherein the outer cover layer has a thickness of 0.015 inches to 0.040 inches.

10. The golf ball of claim 9 , wherein the outer cover layer has a thickness of 0.020 inches to 0.030 inches.