I think the meaning of the word "bulk" — as it is used in English language — is preserved here despite chemical context.
I suspect that it's been used to stress some aspects or features of the surface chemistry: thin film is a 2D object, whereas bulk polymer is a 3D object (e.g. continuous in three directions) possessing a significant volume.
From IUPAC's Compendium of polymer terminology and nomenclature [1, p. 199]
3.12 continuous phase domain
matrix phase domain
Phase domain consisting of a single phase in a heterogeneous mixture through which a continuous path to all phase domain boundaries may be drawn without crossing a phase domain boundary.
Note: In a polymer blend, the continuous phase domain is sometimes referred to as the host polymer, bulk substance, or matrix.
More general from McGraw-Hill dictionary of chemistry [2, p. 171]:
gross sample [ANALY CHEM] One or more increments taken from a larger quantity of a material that is to be analyzed. Also known as bulk sample […]"
Gooch's Encyclopedic Dictionary of Polymers [3, p. 98] keeps it even simpler:
Bulk Material n A material or product in large quantity such as a drum or sack.
- IUPAC “Purple Book” Compendium of Polymer Terminology and Nomenclature; International Union of Pure and Applied Chemistry, Jones, R. G., Eds.; RSC Pub.: Cambridge, 2009. ISBN 978-0-85404-491-7.
- McGraw-Hill Dictionary of Chemistry, 2nd ed.; McGraw-Hill: New York, 2003. ISBN 978-0-07-141046-5.
- Encyclopedic Dictionary of Polymers; Gooch, J. W., Ed.; Springer New York: New York, NY, 2011. https://doi.org/10.1007/978-1-4419-6247-8.