Nondestructive testing of defects in adhesive joints

Abstract
Swelling behaviour of hydrogels based on crosslinked
NR/PEG block copolymers.
Christina George and M.R. Gopinathan Nair
School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills
Kottayam 686560, India.
E-mail: christeena.george@gmail.com
A series of NR/PEG block copolymers were prepared from polyethylene glycol (PEG) and
hydroxyl terminated liquid natural rubber (HTNR) by solution polymerization and dynamics of
the water sorption process were studied. The effect of NCO/OH ratio and pH of the swelling
medium were investigated with respect to the water sorption characteristics of the hydrogels.
Results indicated that the hydrogels exhibited overshooting effect in their dynamic swelling
behaviour. Mechanism of sorption, sorption kinetics and transport coefficients including diffusion
coefficient, sorption coefficient and permeation coefficient were also determined. The sorption
behaviour was found to vary with the variation in NCO/OH ratio of the block copolymers.
Introduction
Hydrogels are multi – component systems comprising of hydrophilic polymeric networks capable
of absorbing enough water (> 20% of its dry weight) causing macroscopic changes in the sample
dimension while maintaining its structural integrity. Due to the relatively high water content of
these materials, they have found a plethora of applications in biomedical, pharmaceutical, food
and environmental fields. Their high water content lends them high biocompatibility and makes
them well tolerated when implanted in vivo. Consequently, they have emerged as promising
candidates for a variety of biomedical applications such as drug delivery systems [1], wound
dressings [2], contact lenses [3], artificial implants [4] etc. However, on increasing the water
content, the mechanical strength of the hydrogel becomes lower and the gel transparency can
decrease as a consequence of a macrophase separation between water and polymer. Hence to
achieve a gel with high mechanical strength, transparency and water content, a composite
molecular structure is needed. This structure will comprise of a hydrophilic component that
absorbs large amounts of water and a hydrophobic component, which improves the mechanical
strength [5]. It is according to this idea that for our study we synthesized amphiphilic NR/PEG
block copolymers with hydrophobic natural rubber (NR) and hydrophilic polyethylene glycol
(PEG).
The multitude of hydrogels available leaves numerous choices for polymeric formulations. The
best approach for development of a hydrogel with desired characteristics is to correlate the
macromolecular structure of the polymer with desired swelling characteristics. The swelling
studies not only describes the amount of water contained within the hydrogel at equilibrium but
also give ample insight into the network structure of the gels and the transport mechanisms of the
water uptake process 4 . During these studies controllable volume change can be brought about in
the hydrogels by changing the surrounding conditions such as pH [6], temperature [7], salt
concentrations [8], chemistry [9], electric field [10], photo irradiation [11] and solvent
composition [12].
Polyethylene glycol (PEG) with its desired set of properties has always been a front-runner
among synthetic polymers used for designing hydrogels primarily because of its nontoxicity and
biocompatibility [4]. The dynamic swelling behaviour of amphiphilic block copolymer based on
polyethylene glycol (PEG) and polybutyl acrylate (PBA) was reported by Wang et al [13]. Bajpai