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Toughening of epoxy resins using particles prepared by emulsion polymerization effects of particle surface functionality size and morphology on impact fracture properties

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Polymer International 44 (1997) 277È282
Thermoreversible Behaviour in Water of
Chemically Crosslinked
Poly(2-methoxyethylacrylateco -N ,N -dimethylacrylamide)¤
Ali A. S. El-Ejmi & Malcolm B. Huglin*
Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, UK
(Received 24 February 1997 ; revised version received 1 April 1997 ; accepted 12 June 1997
Abstract : Hydrogels
of
poly(N,N-dimethylacrylamide-co-2-methoxyethylacrylate) have been synthesized by crosslinking copolymerization, and the dimensions of swollen Ðlms at swelling equilibrium in water have been measured over a
range of temperature (T ). At 4¡C, transparency and maximum water content are
exhibited. On heating, deswelling occurs and loss of transparency develops at
35¡C, opacity and deswelling being complete at 50¡C. The process is reversible
and the lower critical swelling temperature (T ) is found to be 38 ^ 2¡C.
c with decreasing content of
Although water content at any temperature increases
crosslinker, the latter has no discernible inÑuence on T . Microgels of the same
c
copolymer have also been prepared by dispersion polymerization
in 2-propanol.
After dialysis in water, the diameters (D) of the microbeads in dilute aqueous
suspension were measured by dynamic light scattering. D decreased from 2É3 km
at 10¡C to 0É2 km at T º 50¡C, and T was located at 36 ^ 2¡C. Optical microsc polydispersity of the beads, the dimencopy revealed the spherical nature and
sions of which lay in the range 7 km [ D [ 0É8 km. These copolymers were sticky
in both the dry and the swollen states, which may be advantageous for coating to
an inert substrate for application as a possible separation device based on thermoreversible swelling.
Polym. Int. 44, 277È282 (1997)
No. of Figures : 4. No. of Tables : 1. No. of References : 12
Key words : poly(N,N-dimethylacrylamide-co-2-methoxyethylacrylate), hydrogel,
thermoreversible swelling, dynamic light scattering, crosslinking.
gel, denoted here by T . The hydrophilic/hydrophobic
c
balance is operative in several N-substituted acrylamido
polymers, the most widely studied of which is poly(Nisopropylacrylamide), which has the hydrophobic isopropyl group and which has a T of about 32È34¡C.
c
Groups other than the isopropyl one a†ord values of T
c
both higher and lower than 32È34¡C.4
The decrease in swelling on heating is normally continuous and only in a very few instances (incorporation
of a small ionic content) is there a drastic change in
swelling over an extremely short span in temperature,
i.e. discontinuous.5 Attempts have been made by Lele et
INTRODUCTION
Thermoreversible behaviour is exhibited in water by
several linear and crosslinked polymers. This e†ect
arises from a critical balance between hydrogen
bonding and hydrophobic interaction.1h4 The analogue
of the lower critical solution temperature (LCST) for a
solution is the lower critical swelling temperature for a
¤ Dedicated to Professor Bob Stepto on the occasion of his
60th birthday.
* To whom all correspondence should be addressed.
277
( 1997 SCI. Polymer International 0959-8103/97/$17.50
Printed in Great Britain
278
A. A. S. El-Ejmi, M. B. Huglin
EXPERIMENTAL
which 2 ml of 5% (w/v) aqueous ammonium persulphate
had been added. The dispersion was stirred at ambient
temperature and was rendered clear and homogeneous
by the stepwise addition with stirring of 8 ml 2propanol. The weights of MBAM used for the three sets
of experiments were 0É160 g, 0É081 g and 0É061 g, which
correspond to crosslinker concentrations of 2, 1 and
0É75 wt%, respectively, based on the total weight of the
two main comonomers.
To assess reproducibility and overall homogeneity,
each of the three sets was conducted in
poly(tetraÑuoroethylene) (PTFE) moulds of di†erent
thickness (0É6, 1É1 and 1É9 mm), the remaining mould
dimensions being Ðxed at 2 ] 12 cm. The feed mixtures
were outgassed for 10 min with nitrogen and introduced
into the moulds with a disposable pipette, nitrogen gas
being passed through the moulds beforehand. The
moulds were Ðtted with a PTFE lid and the assembly
was closed tightly with clips and a rubber band. After
an appropriate time in an oven at 50¡C, the moulds
were removed and disconnected. Times were 2 h, 3 h
and 4 h for the highest, intermediate and lowest contents of MBAM, respectively. Circular discs of hydrogel
were cut with a corkborer and placed in a bottle containing deionized water for 24 h at ambient temperature,
the water being changed regularly. Swelling to equilibrium at 4¡C was then attained by placing the vials in
a refrigerator.
Diameters D of hydrogels at various temperatures T
were measured by an image magniÐcation method11 in
which the number of millimetre squares on a graph
paper positioned below the disc was counted. The
average diameter was calculated from di†erent positions. At each temperature, the time to attain constancy
of D depended mainly on the thickness of disc, but in all
cases a time of 2 h was found to be adequate. Measurements were made at about thirteen temperatures on
heating from 4 to 60¡C and again at the same temperatures on cooling back to 4¡C.
Materials
Synthesis of hydrogels as microgel particles
DMA, MOEA, and initiator azobis-isobutyronitrile
(AIBN) were obtained and puriÐed as described previously.9 Sorbitan monolaurate (Span-20) (Aldrich)
methylene-bis-acrylamide (MBAM) (99% purity)
(Aldrich), ammonium persulphate (Wilkinson-Vickers
Ltd., Wharfdale Laboratories, UK) and poly(N-vinyl-2pyrrolidone)(nominal molecular mass 4 ] 104 g mol~1,
Sigma) were used as received. 2-Propanol (Fisons ScientiÐc Equipment, UK) was distilled at atmospheric pressure after drying over anhydrous magnesium sulphate.
Table 1 lists the ingredients for the synthesis of microgel
particles in 2-propanol by dispersion polymerization.
The composition of components and reaction temperatures for four other attempted syntheses are not
given here. In these rather unsuccessful syntheses it was
found that the higher concentrations of total monomers,
initiator and surfactant as well as temperatures higher
or lower than 80¡C all tended to promote coagulation
and polydispersity. The procedure adopted Ðnally was
similar to that reported elsewhere12 for the preparation
of uniform polystyrene particles in ethanol by dispersion polymerization.
All dispersions in the present study were prepared by
using a 2-litre three-necked round-bottomed Ñask
equipped with a long condenser, thermometer, and
al.6 to predict the thermoreversible discontinuous
volume
change
transition
in
poly(N-isopropylacrylamide) in water. Copolymers, in general, a†ord
the possibility of exploiting the hydrogen bonding propensity of one monomer unit with the hydrophobic
interaction imparted by the co-monomer. With
dimethylacrylamide (DMA) as hydrophilic component
and several alkyl, alkoxyalkyl acrylates and methacrylates, Mueller has reported on several features of
the dissolved linear and swollen crosslinked copolymers
in water7,8 DMA is a versatile hydrophilic co-monomer
but its homopolymer does not have a T in water.8,9
c
2-Methoxyethylacrylate (MOEA) is hydrophobic and
its homopolymer is entirely insoluble in water. Rough
estimates of the LCSTs for a series of linear poly(DMAco-MOEA) samples of various composition have been
made by Mueller.7 Subsequently the present authors
reÐned the values by making use of entire cloud point
curves9 (instead of a single concentration) and later by
making allowance for molecular weight.10 The values of
the LCST ranged from 7 to 78¡C over the span of
copolymer composition examined.
The general observations of Mueller on crosslinked
poly(DMA-co-MOEA) have motivated us to explore
these potentially useful materials in somewhat more
detail. Our previous results9,10 on the LCST of the
linear copolymers form the basis of a comparison with
the corresponding crosslinked copolymer. A single composition a†ording an experimentally convenient LCST
of 38¡C is selected here, and it is our intention to
examine whether the value of the T is the same. Further
c
aspects include, (a) an assessment of the e†ect of the
nature and concentration of crosslinker on T , (b) the
c
e†ect of using di†erent synthetic modes of preparing the
hydrogels, and (c) improving data analysis to obtain the
value of T .
c
Synthesis of hydrogels as swollen thin films
DMA (2É02 g), MOEA (6É01 g) and MBAM (three di†erent quantities) were mixed in 15 ml deionized water to
POLYMER INTERNATIONAL VOL. 44, NO. 3, 1997
T hermoreversible behaviour of poly(DMA-co-MOEA)
TABLE 1. Experimental data for dispersion polymerization
synthesis
of
DMA/MOEA/MBAM/
stabilizer/co-stabilizer organogel using 2-propanol
as a solvent
DMA (mol dmÉ3)
MOEA (mol dmÉ3)
MBAM (mol dmÉ3)
AIBN (mol dmÉ3)
PVP (g dmÉ3)
SPAN–20 (mol dmÉ3)
2-propanol (mol dmÉ3)
Reaction time (h)
Temperature (¡C)
Total monomer (mol dmÉ3)
0·064
0·144
5·80 Ã 10É3
1·55 Ã 10É3
3·68
2·94 Ã 10É3
13·47
24
80
0·208
mechanical stirrer. The ingredients were weighed
separately, mixed and homogenized by addition of 2propanol. The polymerization mixture was purged with
a constant stream of nitrogen gas and stirred at
450 rev min~1 followed by a speed of 375 rev min~1 for
the remaining reaction time at constant temperature. A
test of polymerization was made by withdrawing a drop
of the dispersion and assessing the size and shape of the
particles in an optical microscope. The mixture was left
to cool and then Ðltered through glasswool to remove
any agglomerates, or coagulated materials. The Ðltrate
was concentrated on a rotary evaporator and then dialysed in visking tubing against deionized water. The
water was changed daily for at least 2 weeks. The resultant aqueous dispersion was slightly white, which might
indicate the presence of some Span-20 left undialysed.
The dialysate was left for about 2È3 months and then
Ðltered through glasswool to yield a clear aqueous dispersion, which was then concentrated by gentle rotary
evaporation to about 5 ml.
An aliquot (1 ml) of the concentrated dispersion was
transferred to a dust-free dynamic light scattering (DLS)
cell. After dilution with another 2 ml of deionized water,
the particle size was measured by DLS at various temperatures. The conditions were similar to those reported
in a previous communication.10 Other conditions such
as accumulation times of 200, channels (varied from 64
to 1024) and sample time (8È40 ks) were followed by
using a DLS-700S Otsuka instrument. The wavelength
of the argon-ion laser used was 488 nm. The built-in
software treated the autocorrelation curves to yield the
di†usion coefficient. The diameter D was determined via
the StokesÈEinstein equation using a single exponential
Ðt ; the value was close to the average in the histogram
distribution method.
Microscopy
The aqueous dispersion of microgel was examined with
a Zeiss Axioskop compound optical microscope Ðtted
with a MC80 microscope camera, having a magniÐPOLYMER INTERNATIONAL VOL. 44, NO. 3, 1997
279
cation range of ]25 to ]250. One drop of pure sample
was placed on a microscope slide, and an optical micrograph was taken using the above optical system in conjunction with black-white and coloured Ðlms. The
prints produced were used to estimate the diameter D.
All measurements of D using the optical microscope
were made at room temperature.
Determination of the gel composition
The gel compositions of the three dried thin Ðlms of
di†erent content of MBAM were determined by nitrogen analysis, because only the DMA units of the
copolymer contain nitrogen. A Kjeltec Auto 1030
Analyzer employing the Kjeldahl procedure was used.
Glass transition temperature (T )
g
The glass transition temperatures of the three dry
copolymeric gels (xerogels) were determined by DSC on
a Mettler TA300 instrument, employing conditions
adopted previously.9
RESULTS AND DISCUSSION
Dry copolymer
Kjeldahl analysis using the measured percentage nitrogen of pure poly(DMA)homopolymer to e†ect a correction factor, yielded the weight fraction of DMA in the
xerogels as 0É330, 0É311 and 0É321 for feed contents of 2,
1 and 0É75 wt% MBAM, respectively. For a linear
copolymer prepared from the same feed composition,
but in the absence of MBAM, the measured weight fraction of DMA in the copolymer was 0É315. For the xerogels containing 0É75, 1, and 2 wt% MBAM, the values
of T displayed a small, but detectable, increase with
g
crosslinker content : T \ [3É3, [1É4 and 0É4¡C,
g
respectively.
Swelling behaviour of hydrogel films
The overall change in size and transparency of swollen
hydrogel Ðlms is shown schematically in Fig. 1, where it
is seen that onset of the loss of transparency occurs at
about 35¡C and there is full opacity at T º 50¡C. The
water content of hydrogel is expressed as its volume
fraction / , where
1
/ \ 1 [ (D /D)3
(1)
1
0
In eqn (1), / and the measured diameter D relate to the
1
particular temperature of measurement T , and D is the
0
diameter of dry unswollen xerogel (essentially uninÑuenced by temperature). The stickiness of these particular copolymers, rendered it impossible to cut a dry
disc ; on drying a perfectly cut circular disc in its swollen
280
A. A. S. El-Ejmi, M. B. Huglin
Fig. 1. Schematic representation of dimensional changes in the thermoreversible swelling/deswelling of a poly(DMA-co-MOEA)
hydrogel at the temperatures (¡C) indicated. Transparency and opacity are shown by open and shaded symbols respectively.
Directions of heating and cooling are shown by ( ] ) and (O) respectively.
Fig. 2. Variation of volume fraction of water in hydrogel with swelling temperature for copolymer discs prepared in the presence of
2 wt% MBAM and in moulds of thickness 0É6 mm (L), 1É1 mm (K) and 1É9 mm (|).
POLYMER INTERNATIONAL VOL. 44, NO. 3, 1997
T hermoreversible behaviour of poly(DMA-co-MOEA)
281
form, the resultant dry xerogel became distorted. It was
found experimentally that changes in D ceased at
T º 50¡C, i.e. at complete deswelling, and hence D was
0
taken to be equal to the measured diameter at 50¡C.
Figure 2 illustrates the variation of / with tem1
perature for samples all having the same composition,
but prepared in moulds of di†erent thickness. Independence of thickness and reproducibility are conÐrmed
over the whole range of temperature.
Figure 3(A) shows the decrease in / with tem1
perature for gels of one thickness but di†erent contents
of crosslinker. At any value of T the water content
increases with decreasing content of MBAM, but the
critical temperature T for swelling/deswelling appears
c
to be independent of crosslinker concentration, as is
also the temperature at which complete deswelling
occurs. By presenting the data in a di†erential form, i.e.
d/ /dT versus T , in Fig. 3(B), the transition tem1
perature can be located more easily. The minimum for
each crosslinker concentration occurs at T \ 38 ^ 2¡C.
Swelling behaviour of microgel particles
Fig. 3. (A) Variation of / with temperature for hydrogels
1
prepared in a mould of thickness 1É9 mm ; concentrations of
crosslinker are : (a) 2 wt%, (b) 1 wt% and (c) 0É75 wt%. (B)
Data for systems in (A) plotted in the di†erential form of
d/ /dT versus T .
1
Dilute aqueous dispersions, when examined by optical
microscopy, were revealed as uniformly spherical but of
a high polydispersity of size. From the prints the diameter D of the swollen beads ranged from about 0É8È7 km
at ambient temperature (20¡C). Treatment of the DLS
data of the same aqueous dispersion yields an equivalent StokesÈEinstein diameter, denoted for convenience
also by D. At T \ 20¡C, the value of D is about 2 km.
The temperature dependence of D is shown in Fig. 4,
where the initial maximum value of 2É3 km at 10¡C
Fig. 4. Decrease with temperature of StokesÈEinstein diameter determined by DLS for aqueous dispersions of poly(DMA-coMOEA) microgels (curve a). Also shown are the same data plotted in the di†erential form of dD/dT versus T (curve b).
POLYMER INTERNATIONAL VOL. 44, NO. 3, 1997
A. A. S. El-Ejmi, M. B. Huglin
282
decreases to about one-tenth of this value when deswelling is complete at T º 55¡C. The value of T is estic
mated to be 36 ^ 2¡C.
CONCLUSIONS
As already indicated in the Introduction, the present
Ðndings are most pertinent to previous ones by
Mueller7,8 and, as far as the authors are aware, there
are no other reports on hydrogels of poly(DMA-coMOEA). The syntheses of Mueller involved a di†erent
crosslinker, a di†erent crosslinker concentration, and a
di†erent (UV) mode of initiation. With regard to the
Ðrst two of these aspects, it is not possible to make a
quantitative comparison with present results. For
example, Fig. 3 shows the large e†ect of crosslinker concentration. For a hydrogel of similar (but not the same)
composition as that used here, MuellerÏs gravimetric
swelling ratios show a continuous decrease with T , but
the data do not allow one to isolate the value of T .
c
Moreover, even at the highest temperature (60¡C) used,
the swelling ratio is of a quite considerable magnitude
(about 2), and clearly the hydrogels have not deswelled
completely. This Ðnding is completely di†erent from
what has been observed experimentally here both for
thin discs and dispersions of microgels. For the former
the volumetric swelling and for the latter the StokesÈ
Einstein diameter have both attained constancy in their
minimum value at T º 55¡C. Although the present
system is not an ionic one and the deswelling is not
discontinuous, it has been possible to estimate with reasonable accuracy the value of T for the two types of
c
hydrogel. The Ðnal value of 37 ^ 3¡C encompasses the
results for hydrogel discs, microgel dispersions and
linear copolymer of the same composition.
Finally,
it
is
noteworthy
that
these
alkoxyalkylacrylate/DMA copolymers are tacky in the
swollen state and especially sticky as dry xerogels. It is
possible that this property, combined with their thermotropic swelling behaviour, may lead to applications in
which the copolymer is affixed to an inert non-swellable
substance for use as drug release matrix, or as a separation device for concentrating aqueous solutions.
Poly(MOEA) has a low T (about [35¡C) which is
g
probably a partial contributory factor for the stickiness
imparted by it to the copolymer.
ACKNOWLEDGEMENTS
One of us would like to thank the Libyan Secretary of
Higher Education and Al-Fatah University for their
Ðnancial support. Our deep thanks are due to Dr. B. R.
Heywood and Mr. S. Champ for their help in the
optical microscopy and to Dr. J. L. Velada for his computer program used to plot data in a di†erential form
(Figs 3B and 4).
DEDICATION
Dedicated, with personal friendship and professional
admiration, to Bob on this momentous milestone in his
mid-career.
REFERENCES
1 Otake, K., Inomata, H., Kanno, M. & Saito, S., Macromolecules,
23 (1990) 283.
2 Hirokawa, Y., Tanaka, T. & Matsuo, E. S., J. Chem. Phys., 81
(1984) 6379.
3 Feil, H., Bae, Y. H., Feijen, J. & Kim, S. W., Macromolecules, 26
(1993) 2496.
4 Inomata, H., Goto, S. & Saito, S., Macromolecules, 23 (1990) 4887.
5 Hirosa, Y., Amiya, T., Hirokawa, Y. & Tanaka, T., Macromolecules, 20 (1987) 1342.
6 Lele, A. K., Badiger, M. V., Hirve, M. M. & Mashelkar, R. A.,
Chem. Eng. Sci., 50 (1995) 3535.
7 Mueller, K. F., Polymer, 33 (1992) 3470.
8 Mueller, K. F., US Patent No. 5 057 560, NY, 1991.
9 El-Ejmi, A. S. & Huglin, M. B., Polym. Int., 39 (1996) 113.
10 El-Ejmi, A. S. & Huglin, M. B. Eur. Polym. J., 33 (1997) 1281.
11 Huglin, M. B., Rehab, M. M. A.-M. & Zakaria, M. B., Macromolecules, 19 (1986) 2986.
12 Tseng, C. M., Lu, Y. Y., El-AAsser, M. S. & Vanderho†, J. W.,
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