Development of Bi-Layer Hydrogels for Horticultural Applications

Abstract
Hydrogels are hydrophilic polymers that have the ability to absorb large amounts of water and nutrients while still retaining the solid state.  This has made them attractive as growth media to provide optimal control of the growth environment of plants, particularly in horticulture.  However, hydrogels have several drawbacks.  While they have excellent water absorption properties, they are also prone to rapid dehydration when exposed to air or dry soil.  They are also extremely fragile and breakup easily which further accelerates their dehydration.  Plant growth studies with commercially available hydrogels suggest little if any benefit from their addition to soils.
In this study we are developing a novel bi-layer hydrogel to address these problems.  The larger, inner core of this bi-layer materials is made up of the highly water absorbent, but quite fragile, polyacrylamide (PAAm) hydrogel.  This core is encased by a thin layer of polyurethane (PU) polymer which provides the mechanical strength for the material.  More significantly, the outer polyurethane layer provides a low permeability aqueous barrier for the water-laden polyacrylamide core.  We have also developed methods to adjust the porosity of the polyurethane layer by varying the synthesis conditions for it.  This has allowed for the preparation of various PU-PAAm materials with different dehydration rates lasting from several days to well over a week.  However, characterization of this material shows that the dehydration rate strongly depends on temperature and humidity, which means that the properties of the material needs to be adjusted based on the growth conditions.  Additionally, strength measurements done on the material show that it has excellent mechanical properties, making it suitable for deployment as growth media.