Nondestructive testing of defects in adhesive joints

et al [4] reported the swelling behaviour of hydrogels prepared from polyethylene glycol (PEG)
and poly (acrylamide-co-styrene) (PAMS). The water uptake of this hydrogel varied sensitively
with the variation of the contents of hydrophilic polymer, crosslinking agent and temperature of
the swelling medium. Karaky et al [14] synthesized a new type of pH switchable suparamolecular
sliding gels based on polyrotaxanes of polyethyleneimine-block-poly (ethylene oxide)-blockpolyethyleneimine
block copolymer and α - cyclodextrin.
In the present work, transport behaviour of water through NR/PEG block copolymer with varying
NCO/OH ratios has been investigated. The shape of diffusion profile, mechanism of sorption,
sorption kinetics and the transport coefficients including diffusion coefficient, sorption coefficient
and permeation coefficient are described here. Effect of NCO/OH ratio and pH on swelling
behaviour of these block copolymers is also described.
Experimental
Materials
Natural crumb rubber (ISNR-5): Weight average molecular weight = 8,20,000; Intrinsic viscosity
in benzene at 30 0 C= 4.45 dL/g; Wallace Plasticity, P0=39 was supplied by Rubber Research
Institute of India, Kottayam, Kerala. Hydrogen peroxide (30% w/v) supplied by E. Merck, India
was used without further purification. Toluene (reagent grade) obtained from E. Merck; India was
used as solvent without further purification. Methanol (reagent grade) supplied by E. Merck,
India was used without further purification. Toluene diisocyanate (TDI) (80/20 mixture of 2,4and
2,6- isomers) was supplied by Aldrich, Germany and was used as received. Dibutyl tin
dilaurate (DBTDL) supplied by Fluka, Switzerland, was used as catalyst without further
purification. Chloroform (laboratory reagent grade) was dried with anhydrous calcium oxide and then
distilled before use. It was supplied by E. Merck, India. Polyethylene glycol (PEG) of molecular
weight 4000 supplied by Aldrich, Germany was used as received. Water used for the swelling
studies was double distilled and deionized.
Hydroxyl terminated liquid natural rubber (HTNR) of number average molecular weight 4000
was prepared in the laboratory by the photochemical degradation of natural rubber as per reported
procedure [15]. It was reprecipitated thrice from toluene using methanol and dried at 70 o C – 80 o C
in vacuum.
Synthesis of the block copolymer
HTNR was dissolved in chloroform to get a 35% solution and taken in a flat-bottomed flask
equipped with a magnetic stirrer, nitrogen inlet and outlet, water condenser and a dropping funnel.
Dibutyl tin dilaurate (DBTDL) catalyst (6.1% by weight of HTNR) was added and the solution
brought to reflux with vigorous stirring. Variation in crosslink density was brought about by varying
the NCO/OH ratio. TDI was added drop wise over a period of 30 min followed by 2 h of the
reaction to end cap HTNR. This was accompanied by the addition of the required amount of PEG as
a solution in chloroform (35% w/v) drop wise during 1.5 h followed by 2 h of the reaction. The
excess chloroform was distilled off and the viscous polymer was cast in tray treated with silicone
release agent. The sheet removed from the tray after 24 h was kept in vacuum oven at 60 0 C to
remove traces of the solvent present and then cured at 70 0 C for 24 h followed by one week ageing
at room temperature in a moisture-free atmosphere [16].
Polymer designation
The block copolymer studied in this work is designated as NR/PEO (4000/4000)-1 which
indicates that the sample contain HTNR and PEG where HTNR molecular weight is 4000 and
PEG molecular weight is 4000. The number indicates the order of synthesis with the variation in
NCO/OH ratio (Table I).