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Polymer International
Polym Int 48:1141±1146 (1999)
Surface modification of poly(vinyl chloride)
using high intensity ultrasound
Gareth J Price* and Andrew A Clifton
School of Chemistry, University of Bath, Bath, BA2 7AY, UK
Abstract: The use of high intensity ultrasound to promote a number of reactions at the surface of solid
poly(vinyl chloride) is reported. Substitution reactions involving a range of compounds, including
dyes, can rapidly be carried out from aqueous solution under labile conditions. Sonochemically
enhanced treatment with strong aqueous base produces dehydrochlorination even at room
temperature to produce a thin layer of conjugated material at the surface which can then be grafted
with a number of compounds. Some speculation as to the mechanistic features of the process as well as
its potential utility is made.
# 1999 Society of Chemical Industry
Keywords: Sonochemistry; surface modi®cation; ultrasound; PVC; dyeing; dehydrochlorination
INTRODUCTION
While the search for new polymers with novel and
improved properties continues, there is great interest
in altering the properties of cheaper, commodity
materials to achieve greater functionality. In particular, the way that a polymer interacts with its
surroundings is largely determined by the nature of
its surface so that surface modi®cation is a particular
target.1,2 In this way, the good mechanical properties
of commodity materials can be combined with
enhanced functionality such as adhesion, printing,
barrier properties or, in more recent work, biocompatibility.
PVC is a widely used commodity polymer. However, there is great interest3,4 in reducing its environmental degradation, for example by increasing its
surface hardness and resistance to oxidation as well as
reducing its harmful emissions on combustion. A
number of workers have succeeded in introducing
functional groups onto the surface by nucleophilic
substitution of chlorine on the backbone.5,6 For
example, Takeishi and co-workers7 reported the
replacement of Cl with azide and dithiocarbamate in
aqueous solution although signi®cant reaction occurred only in the presence of surfactants. In similar
work, Gilbert8 reported that it was possible to replace
up to 60% of the surface chlorine atoms with azide.
Surface modi®cation has recently been used to prevent
plasticiser migration through the surface.9 Surface
substitution with chains of a different polymer,
effectively yielding graft copolymers, is also possible,10
this approach recently having been used by Rios and
Bertorello to produce PVC ®lms grafted with sucrose
acetate.11
As long ago as 1939, it was realised that extended
reaction could lead to dehydrochlorination and the
production of conjugated sequences.12 This has
usually been regarded as a nuisance and an unfavourable side reaction. In heterogeneous systems, the rate
is usually very slow but can be speeded up by the
addition of a phase transfer catalyst in conjunction
with a strong base.13 An alternative approach has
involved electrochemical dehydrochlorination.14 It is
known15 that ultrasound can accelerate this type of
reaction so that it was of interest to determine whether
it could be achieved under facile conditions and also if
the generated double bonds could be used as sites for
further chemical modi®cation.
The use of ultrasound to promote modi®cation of
molecular weight distributions of polymers due to
controlled chain cleavage around cavitation bubbles
has been known for many years.16,17 There has been a
large number of examples of its application to
chemical synthesis18 as well as to the preparation of
polymers and other related processes.19 Surface cleaning is a major commercial application of ultrasonics.
Cavitational collapse causes microjets of liquids
around the bubbles near a surface to impinge at high
speeds, dislodging any oil, grease or other contaminant
adhering to the surface. In the main, this has been
studied as a purely physical effect with little interest in
promoting surface chemistry although the removal of
oxide layers or other adherents is at least in part
responsible for the remarkable rate accelerations seen
* Correspondence to: GJ Price, School of Chemistry, University of Bath, Bath BA2 7AY, UK
E-mail: G.J.Price@bath.ac.uk
Contract/grant sponsor: EPSRC
Contract/grant sponsor: BICC Cables Ltd
(Received 4 December 1998; accepted 14 June 1999)
# 1999 Society of Chemical Industry. Polym Int 0959±8103/99/$17.50
1141
GJ Price, AA Clifton
in heterogeneous reactions carried out under sonication.20
Previous work employing ultrasound for polymer
surface modi®cation includes that by Urban and coworkers21,22 who found that the rate of base-catalysed
dehydrohalogenation of PVF2 was faster under ultrasound and that lower temperatures could be used,
although the depth of modi®cation was less. The
current authors have also reported23,24 that ultrasonic
enhancement can allow the oxidation of polyethylene
surfaces using relatively mild reagents and so modify
its surface composition and properties in a controlled
manner.
This paper describes a number of processes leading
to surface modi®cation of PVC. A large number of
reactions could potentially be used for this but, in view
of present commercial and environmental considerations, interest was con®ned in the main to aqueous
systems.
EXPERIMENTAL
Sonication procedures
Two sources of ultrasound were used. For initial trials,
the polymer was suspended in the reagent contained in
a conical ¯ask and immersed in a Kerry PUL325
Ultrasonic Cleaning Bath which operated at 35 kHz.
The ¯ask was positioned so as to give the maximum
intensity judged by the agitation of its contents. For
more quantitative work, a Sonics and Materials
VC600 `horn' system was used in the con®guration
described previously.23,25 The sound frequency was
23 kHz and, for the work reported here, a constant
intensity of 26.2( 1.2) W cmÿ2 (measured calorimetrically) was used. Thermostatting to approximately 0.5 °C was achieved by circulating
water through a jacket surrounding the reaction vessel.
Where powdered PVC samples were used the streaming caused by sonication was suf®cient to keep the
powder in suspension; other experiments were conducted on ®lms (approx 1 mm in thickness) of PVC
mounted in a stainless steel holder. Details of the
reagents and precise conditions such as the time and
temperature used are included in the following
sections as appropriate.
that sonication of powders has a marked effect on their
particle sizes.24 Energy dispersive X-ray mapping was
carried out using a JEOL35C scanning electron
microscope.
Materials
The PVC used was a medium molecular weight
sample with inherent viscosity 0.92 supplied by
Aldrich Ltd. Other reagents used were of reagent
grade or better and were also obtained from Aldrich
Ltd. Films were obtained by melt pressing the assupplied polymer powder.
RESULTS AND DISCUSSION
Modification with dyes and other chromophores
A target with obvious commercial relevance would be
the rapid and permanent attachment, under mild
conditions, of dyes. Initial studies focused on the
possibility of a straightforward mass transfer acceleration of reactions at the surface and also on whether the
radicals formed due to sonolysis of the aqueous solvent
would promote grafting reactions. To give easily
characterisable systems, model compounds containing
aromatic chromophores were selected for initial study.
Figure 1 shows the dispersion UV spectra of PVC
powder that had been sonicated for 1 h in 1 wt%
solutions in propanol of naphthalene-or anthracenecontaining compounds. Exhausive extractions were
performed to remove any material that had not been
covalently attached to the surface.
While quantitative comparison is dif®cult due to the
nature of the spectra, it is clear that attachment of the
aromatics has occurred. Further consideration of the
results for 2-vinyl naphthalene, 2-VN, is instructive.
The lmax for the peaks in the spectra are c10 nm lower
than those in 2-VN or in experiments carried out in the
Spectroscopic Analysis
Infra-red spectra were recorded on a Nicolet 510P
instrument. For Attenuated Total Re¯ection (ATR)
spectra, a ZnSe 45 ° crystal was used in a Specac ATR
accessory. Diffuse Re¯ectance (DRIFT) spectra were
also recorded with a Specac accessory. Ultra-violet
spectra were measured against reference samples on a
Perkin-Elmer 330 spectrophotometer ®tted with a
Hitachi integrating sphere. Quantitative application of
this method is made dif®cult since changes in particle
size and sample homogeneity will affect the results.
Wherever possible, the reference samples were therefore sonicated under similar conditions with the
omission of the appropriate reagents since it is known
1142
Figure 1. Diffuse reflectance UV spectra of sonicated PVC powders
ultrasound bath, 1 h. A: 2-VN; B: 4-bromonaphthalene; C: anthracene.
Polym Int 48:1141±1146 (1999)
Surface modi®cation of PVC using ultrasound
Figure 4. DRIFT IR spectra of PVC powder (A) before and after reaction
with (B) PMMA in toluene and (C) MMA in propanol.
absence of ultrasound, implying a loss of conjugation.
The reaction was more ef®cient when carried out
under nitrogen than in air, suggestive of a mechanism
involving radical intermediates. The effect of these
gases on the sonication process would be negligibly
different. Hence, the species on the surface probably
consists of grafted vinyl naphthalene or short chains of
poly(vinyl naphthalene). In these reactions, there is the
possibility of homopolymerisation of 2-VN. However,
sonication of 2-VN alone produced no polymer under
these conditions. Thus, we are con®dent that any
polymer produced is grafted onto the surface. While
this process offers the possibility of surface modi®cation by grafting (see below), these results also show
that the vinyl functionality is not a prerequisite for
attaching compounds to the surface.
Experiments were then performed using 1% (w/v)
solutions of some commercially available dyes. Figure
2 shows dispersion UV spectra for PVC sonicated for
1 h on the ultrasound bath in solutions of bromophenol blue under varying conditions. Spectra recorded under `silent' conditions showed little
signi®cant difference from the starting materials.
Again, exhausive extractions with hot water and
alcohol solvents were performed to remove any
unattached dye. Prolonged sonication caused little
increase in the strength of absorption as would be
expected once the complete surface is covered.
Although not shown here, varying the dye concentration, the addition of a surfactant or carrying out the
reaction in the absence of oxygen had no signi®cant
effect on the results. This suggests that neither the
Figure 3. Diffuse reflectance UV spectra of PVC powders after sonication
with dyes (5 min reaction with ultrasound horn apparatus). A: Crystal Violet;
B: Rose Bengal; C: Methylene Blue; D: Brilliant Green.
Figure 5. Diffuse reflectance UV spectra of PVC powders after sonication
with NaOH/TBAB. A: powder, ultrasound bath, 1 h; B: powder, ultrasound
horn, 5 min; C: PVC film, ultrasound bath, h.
Figure 2. Diffuse reflectance UV spectra of PVC powders after sonication
ultrasound bath) with 1% aqueous bromophenol blue. A: 1 h, water solvent;
B: 2 h, water solvent; C: 1 h, 0.1 mol dmÿ3 H‡; D: 1 h; 0.1 mol dmÿ3 OHÿ.
Polym Int 48:1141±1146 (1999)
1143
GJ Price, AA Clifton
Surface modification by direct polymer grafting
Figure 6. Diffuse reflectance UV spectra of PVC powders after sonication
with NaOH/TBAB and addition of metal salts. A: CuSO4; B: FeCl3; C:
KCrO4; D: Cr2O3.
transport of reactants to the surface nor the production
of radical species in solution is rate-limiting. One
parameter of importance, given the ionic character of
the dye, is the pH of the reaction medium. Figure 2
also indicates that variation of the pH can be used to
determine which form of the dye is attached to the
surface. This effect was also clearly evident from visual
observation of the powders where a change from a
bright blue colour to pale yellow was seen.
A range of other dyes (including neutral and cationic
examples such as Brilliant Green, Crystal Violet,
Methylene Blue, Rose Bengal and Rhodamine B)
were used under similar conditions and intense
coloration could be introduced to the surfaces.
Microscopic inspection of fractured particles showed
that the coloration was present at the surface but did
not penetrate deeply into the particle, being con®ned
to the outermost 1±10 mm.
The reaction times involved with using the cleaning
bath are too long to be useful so that we also
conducted experiments using a high intensity sonic
horn. Signi®cant attachment of the dyes could be
achieved with reaction times as short as 5 min, some
results being shown in Figure 3.
The spectra indicate that reaction has occurred but
it was even more apparent from the highly visible
coloration which was introduced onto the powders.
This was reduced somewhat on extended extraction
(24 h) with boiling water although a deep coloration
persisted, suggesting that the method could be viable
under these higher intensity conditions. The use of
higher acoustic intensities should enable further
reductions in the reaction times.
1144
The possibility of grafting achieved with 2- vinyl
naphthalene described above led us to investigate the
possibility of utilising other monomers. It is well known
that carbon backbone polymers in solution cleave when
subjected to sonication yielding two macromolecular
radicals. A number of experiments were attempted by
sonicating PVC powders suspended in dilute solutions
of polymers, for example polyacrylamide and poly(acrylic acid) in the expectation that the radical centres
would attach to the surface by an abstraction mechanism. However, no indication of grafting was seen. More
success was achieved when an organic solvent was used.
Sonications were carried out on PVC powder suspended in a solution of PMMA in toluene. Figure 4
shows the DRIFT-IR spectra before and after treatment. It is apparent that there was a small amount of
carbonyl-containing material on the surface prior to
modi®cation but that this was greatly increased due to
grafting of the methacrylate. A corresponding absorption peak was seen in the UV spectrum.
The reasons for the reactivity in this system in
contrast to that in aqueous solution are unclear but
may be related to the wetting of the hydrophilic PVC
surface. Further discussion of the mechanistic aspects
of the process will be given below.
Dehydrochlorination
The unreliability of the direct grafting approach to
Figure 7. Transmission IR spectra (as KBr disc) of PVC after reaction with
1% aqueous acrylic acid. A: untreated; B: after sonication with NaOH/
TBAB/acrylic acid.
Polym Int 48:1141±1146 (1999)
Surface modi®cation of PVC using ultrasound
PVC surface modi®cation led us to explore in more
detail the methodology used by Urban and coworkers21 with PVF2. It was felt that if unsaturation
could be introduced on the surface without extensive
unzipping of chains in the bulk polymer occurring to
modify the mechanical properties, it would allow a
wider range of further chemistry to be performed. The
base-catalysed dehydrochlorination of PVC using a
phase transfer catalyst such as tetra-butyl ammonium
bromide (TBAB) has been reported13 to occur via an
unzipping mechanism to produce conjugated sequences consisting of 7±12 double bonds. Measurement by GPC of the molecular weight distributions
before and after sonochemical treatment in this work
indicated no signi®cant change in the bulk material.
Sonications were carried out using 25% sodium
hydroxide solution containing 0.2% TBAB. The
re¯ectance UV spectra of the resulting powders are
shown in Fig 5. The extremely broad absorptions show
that extended sequences of conjugation have been
introduced. This is also supported by the appearance
of peaks at c1650±1750 cmÿ1 in the IR spectrum. As a
result, the initially white powders became a deep
brown colour even after less than 5 min treatment with
the ultrasonic horn. Analogous reactions were carried
out on solution-cast ®lms of PVC and, while the
coloration and broad absorption in the UV spectrum
were observed, no information could be gleaned from
the IR spectrum. This suggests that the surface
modi®cation is very surface-speci®c and does not
penetrate very far into the bulk polymer. Also of
signi®cance is that experiments performed using no
TBAB gave signi®cantly less reaction.
The potential use of these surface double bonds to
perform further chemistry is illustrated in Fig 6 which
demonstrates the chelation of metal ions onto the
surface. After treatment as above to produce the
unsaturated surface, a solution of each of the
compounds indicated was added to give a
0.02 mol dmÿ3 solution. After 1 h sonication (cleaning
bath) the resulting powders were Soxhlet extracted
with water for 24 h. In blank experiments, omission of
the ®rst reaction step resulted in no chelation of the
ions so that the prior generation of the double bonds is
apparently a necessary step.
To investigate the distribution of the metal ions,
PVC ®lms cast from solution were treated after
dehydrochlorination with a solution of Fe3‡ on the
ultrasound horn for 60 min and EDAX spectra
recorded. It was evident that a substantial amount of
iron had been incorporated after sonication, shown by
the changes in the relative intensities of the Cl and Fe
peaks. Elemental `mapping' using EDAX of a number
of particles and ®lms showed a relatively even
distribution of iron across the surface suggesting that
the reaction sites were not clustered at particular
points.
Carty et al recently suggested26 that Fe3‡ could have
a catalytic effect in promoting crosslinking via dehydrochlorination of PVC. To investigate this, treated
Polym Int 48:1141±1146 (1999)
samples of PVC powder were exhaustively extracted
with tetrahydrofuran, a good solvent for PVC. Some
material remained visibly undissolved. While extensively conjugated polymer may also be insoluble, this
result also suggests that some crosslinking may be
occurring in these reactions.
Of greater potential use in the controlled modi®cation of the surface properties would be the grafting of
polymers onto these unsaturated sites. Grafting of
PMMA from toluene solutions was carried out under
conditions similar to those described above. However,
in view of environmental and application considerations, an aqueous system would be preferred. In
contrast to the earlier work, the pre-treatment to
produce surface unsaturation allowed this reaction to
proceed under aqueous conditions. Figure 7 shows the
IR spectra of PVC that had been modi®ed after
dehydrochlorination by the grafting of acrylic acid.
Reaction on both powdered and ®lm samples clearly
occurred. While this is a preliminary result, it opens
the way for a considerable number of other modi®cation regimes in order to precisely tailor the surface
properties of PVC.
Further discussion
While several types of surface modi®cation have been
shown to be feasible, determination of the precise
mechanism by which they occur requires further
investigation. The application of ultrasound to heterogeneous systems has a number of effects. The major of
these is to markedly accelerate mass transfer of
reagents to the surface. This effect has been used to
explain the enhanced activity in electrochemical
systems27 under sonication and is certainly at least
partly responsible for the acceleration in the reactions
observed in this work. In addition, the microjets29
caused by cavitation bubble collapse would force the
reactants to the surface, overcoming the hydrophobicity to increase wetting of the surface and hence allow
the reagents greater access to surface sites on which to
react. This greater accessibility of the surface to the
reactants is the major part that ultrasound plays in
these reactions.
Sonication is known to affect the particle size of
PVC powders25 and so some increase in surface area
may be expected. However, in samples examined
microscopically, the coloration (where appropriate)
was uniform so that the creation of fresh surface
appears not to play a signi®cant part in the process;
indeed, this could not be the case when PVC ®lms
were used.
Currently, a number of surface modi®cation techniques rely on radical initiators such as AIBN to start
the reaction. Sonication even of pure water produces
signi®cant concentrations of radicals28 and so, particularly in the case of grafting monomers, this provides
another source of enhanced sonochemical activity.
There have been other approaches to the surface
modi®cation of PVC which have used approaches
similar to those involved here, although without the
1145
GJ Price, AA Clifton
sonochemical steps. For example, Bergbreiter and
Xu30 reported ef®cient surface dehydrochlorination
using lithiated diaminopoly(alkylene oxides) to give
sites for further reaction. However, those processes
required anaerobic, anhydrous conditions. The use of
ultrasound in our work has allowed the reactions to
proceed under milder, more commercially appropriate
conditions.
The timescales which were involved in this laboratory study limit the potential utility of this technique.
However, ultrasound sources with relatively low
powers were used. The use of high intensity sources,
several of which are now commercially available to
operate on a large scale, should allow us to tailor
precisely the surface properties of the polymer.
Further possibilities are opened up if spatial control
of the ultrasound ®eld can be achieved, for example in
the use of cylindrical reactors for the treatment of
®bres etc. The use of small, highly directed sources to
achieve patterning of the modi®cations can also be
envisaged.
CONCLUSIONS
This preliminary report has demonstrated that the use
of ultrasound allows a number of chemical modi®cations to PVC surfaces to proceed under mild conditions. Signi®cant results can be achieved using a
simple cleaning bath type apparatus although much
faster results can be obtained by using higher sound
intensities such as those available from a horn
apparatus.
Of particular signi®cance is the facile dehydrochlorination which gives readily accessible sites for further
reaction such as chelation of metal ions or, likely to be
of more commercial signi®cance, grafting of other
monomers or polymers. The penetration and attachment of dyes can also be markedly accelerated. While
considerable work remains, particularly to identify
unambiguously the surface species and to measure the
process kinetics to allow an estimation of its commercial viability, there is clear potential in the application
of sonochemistry to polymer surface modi®cation.
ACKNOWLEDGEMENTS
The award of an EPSRC Research Studentship (to
1146
AAC) is acknowledged. We are also grateful for useful
contributions to the work from Drs J Toynbee and F
Keen of the Wrexham Technology Centre of BICC
Ltd and to BICC Cables Ltd for providing additional
funding.
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Polym Int 48:1141±1146 (1999)
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