Thermoplastic core having a hardness gradient formed from a gradient-initiating solution

A method of forming a golf ball comprising the steps of:

• providing a thermoplastic material comprising 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;
• forming the thermoplastic material into an inner core having a surface and a geometric center;
• combining a solvent, a free radical initiator, a reactive co-agent comprising a metal salt of an acrylic or methacrylic acid, a mono- or multi-functional acrylate or methacrylate, or having the chemical structure
• where R=linear, branched, or aromatic C1-C14 and n=1-5 to form a gradient-initiating solution;
• soaking the thermoplastic core in the gradient-initiating solution for a pre-determined time to create a positive or negative hardness gradient of 5 Shore C or greater between the surface and the geometric center;
• forming an outer core layer comprising a rubber-based composition such that the dual core has an outer diameter of 1.51 inches to 1.59 inches;
• forming an inner cover layer, the inner cover layer comprising an ionomer or a highly-neutralized ionomer;
• and
• forming an outer cover layer, the outer cover layer comprising a polyurea or a polyurethane.