Modeling the Hydrolytic Degradation and Distribution of Polymer Species In Biopolymers

Modeling the hydrolytic degradation and distribution of polymer species in biopolymers

Justin Kaffenberger, Huajiang Huang, Ulrike Tschirner, Ben Schroeder, Waleed Al-dajani, and Shri Ramaswamy

Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, MN 55108

Abstract:

The US Department of Energy in its Plant/Crop-Based Renewable Resources Report 2020 has put forth an initiative to have 10 percent of chemical feedstock coming from plant-based renewable sources by 2020 and a further increase to 50 percent by 2050.^[1]  Meeting these targets will require production of current chemical feedstocks from renewable sources, such as the dehydration of plant-based ethanol to produce ethylene.  It will also require the development of alternative feedstocks that can supplant the petroleum-based ones that currently predominate.  One example of a chemical feedstock of increasing importance is lactic acid. 

Lactic acid is a metabolite formed during glucose fermentation by many organisms.  Due to its versatility, lactic acid has the potential to be a major renewable chemical feedstock.  It can be esterified to form lactate esters, reduced to propylene glycol, or dehydrated to acrylic acid.^[2]  It can also be homopolymerized to form polylactic acid (PLA) via condensation reaction.  The resulting polymer exhibits mechanical properties similar to many petroleum-based polymers, including polyethylene teraphthalate, polyethylene, and polystyrene. 

The degradability of PLA is critical to its current commercial success.  In disposable food packaging and service ware, the degradability and bio-absorptivity of the degradation product of PLA makes it marketable as a green and environmentally-friendly product.  In biomedical applications, the degradability and biocompatibility of PLA are essential.  As PLA's ability to degrade is a major factor in its application functionality, understanding the fundamentals of this process is critical to all stages of PLA manufacture and end-use application. 

To this end, a mathematical model has been developed that predicts the time dependent change in the average molecular weight of polylactic acid (PLA) of varying crystallinity as it hydrolytically degrades at a given temperature. This model includes three possible hydrolysis reaction routes: random chain scission, end scission (monomer removal from the acid end group, and monomer removal from the alcohol end group).  Apparent rate constants for these reaction pathways were determined from experimental data.  The resulting values suggest that, independent of the crystallinity of the starting PLA, acid end groups are hydrolyzed more rapidly than either alcohol end groups or random chain scission.  This may help develop suitable approaches to control the polymer properties and degradation and help develop wider application for PLA products.