Nondestructive testing of defects in adhesive joints

them. The nanocomposites prepared using C20A organoclay exhibited minimum intercalation of 33
A 0 as compared to the other two modified organoclays C30B and B109 respectively. The lower
amount of intercalation possibly occurs due to less favourable intercalation between the ditallow
intercalant in C20A containing more no. hydrophobic –CH2 groups in the clay surface, with the
polymer chain segments. In case of B109, a smectite organoclay, modified with hydrogenated tallow,
provides more no. of clay platelets per surface area, which might have been the primary reason for
enhancement of d spacing as compared with PMMA/C20A nanocomposites. However grafting of
PMMA with MA could not show any appreciable increase in the d spacing as compared with the
ungrafted nanocomposites .
Transmission Electron Microscopy: (TEM)
Bright field TEM images of PMMA with and without MA are represented in the fig.2. Dark lines
represent silicate layers whereas bright region corresponds to PMMA matrix. This also indicated
intercalated clay layers along with some individual clay layers for untreated PMMA/C30B
nanocomposites. In case of grafted nanocomposites, PMMA-g-MA/C30B hybrid the smaller amount
of stack plates appear in broad and obscure region. TEM results confirmed that PMMA based grafted
and ungrafted nanocomposites mainly display an intercalated nanomorphology which is in agreement
to the WAXD technique.
Thermal Analysis (DSC/TGA) :
Fig.3 shows the TGA curves of PMMA, PMMA/ Na+ MMT, PMMA/C20A, PMMA/C30B,
PMMA/B109 nanocomposites and PMMA-g-MA/C30B and PMMA-g-MA/B109 grafted
nanocomposites respectively. It is observed that onset of temperature increased in all the cases is
around 25 0 C for PMMA/C30B, 6 0 C for PMMA/B109 and 24 0 C for PMMA/C20A as compared with
virgin PMMA. The temperature at 50% mass loss (T0.5) also increases from 365 0 C in case of PMMA
to 412 0 C in PMMA/C20A nanocomposites. This improvement in thermal stability of PMMA
nanocomposites is mainly due to the intercalation of polymer matrix into the clay galleries, which act
as a barrier for thermal degradation as well as nucleating effect of organoclays. The effect of maleic
anhydride did not show a considerable improvement in thermal stability of nanocomposites. However
in all the nanocomposite system the confinement of polymer chains into the clay galleries delayed the
degradation process & diffusion of volatile products thereby increasing the flame retardancy.
The DSC thermograms of PMMA virgin and the nanocomposites systems are illustrated in fig .4.
DSC thermograms represented the presence of second order transition corresponding to the Tg of the
virgin PMMA matrix around 121.90 0 C. Absence of 2 nd order transition or Tm indicated amorphous
characteristics of the matrix polymer. DSC isotherms revealed marginal decrease in the Tg of PMMA
in the nanocomposites system. PMMA/C30B nanocomposites exhibited Tg around 118.83 0 C with
PMMA/B109 at 120.01 0 C and PMMA/C20A around 114.31 0 C respectively .The grafted samples also
exhibited glass transition in the similar range indicating no appreciable effect of grafting on the
segmental mobility of the matrix polymer. The depression Tg is probably due to reduction in density,
which caused chain end localization and reduced chain entanglements as compared with the bulk
matrix. PMMA/C20A nanocomposites system exhibits maximum depression in Tg to the tune of
114.31 0 C which reveals less compatibility of the ditallow organic modifier with the hydrophobic
matrix.
Mechanical Properties:
Mechanical properties such as Young’s modulus (E), stress at break and % strain at break and tensile
strength is represented in the table1. It is evident that with the increase in clay loading from 1 to 5 wt
%, there is a linear increase in the young’s modulus of PMMA nanocomposites system. Incorporation
of organically modified nanoclays to the tune of 5wt% increases (E) of PMMA from 1980 MPa to
2681 MPa in PMMA/C30B, 2469 MPa in PMMA/B109 and 2168 MPa in PMMA/C20A
nanocomposites hybrid respectively. The nanocomposites prepared using C30B nanoclay exhibited an
optimum increase in E as compared with virgin matrix which is probably due to polar interactions
between ester groups of PMMA with the hydroxyl group of C30B. The grafted nanocomposites
exhibited a higher modulus as compared with the ungrafted hybrids in all the cases PMMA-g-
MA/C30B nanocomposites exhibited a maximum increase in Youngs modulus to the tune of 38.22 %
as compared with the virgin matrix. This behaviour is probably due to formation of interfacial bonds
between the anhydride groups of MA with the –OH groups in C30B as well as polar interactions of
the ammonium cations with the MA. The tensile strength also increases with the increase in clay