Porous polymer oil sorbents based on PET fibers with crosslinked copolymer coatings

Article abstract of DOI:10.1039/c3ra44759f

Oil sorbents - namely materials that can be used to extract oil after a spill - were fabricated from non-woven polyethylene terephthalate (NWPET) fibers modified by the adherence of crosslinked polymer coatings to the fiber surface. The NWPET fibers, which serve as a structural support for the applied functional coatings, were produced from recycled PET (polyethylene terephthalate) bottles. The oil absorbing coatings were comprised of crosslinked homopolymers and copolymers based on octadecyl acrylate (ODA), maleic anhydride (MA), and related esters of MA. The crosslinked polymer networks were synthesized by both suspension and bulk polymerization techniques using divinylbenzene (DVB) as the crosslinking agent. Efficacy of the coated NWPET fibers as oil sorbers was determined by oil absorption tests in toluene and in 10% crude oil in toluene. Rigidity, porosity and swelling of the crosslinked polymers were evaluated and correlated to the chemical structures, composition, and reaction media. Suspension polymerization yielded the desired morphology and function, providing higher porosity and in consequence a high absorption capacity.

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Porous polymer oil sorbents based on PET fibers with
crosslinked copolymer coatings†
Cite this: RSC Adv., 2013, 3, 25849
bd
Ayman M. Atta,abc Witold Brostow,
Haley E. Hagg Lobland,bd
*
Abdul-Raheim M. Hasanab and Jose M. Perezd
Oil sorbents—namely materials that can be used to extract oil after a spill—were fabricated from non-
woven polyethylene terephthalate (NWPET) fibers modified by the adherence of crosslinked polymer
coatings to the fiber surface. The NWPET fibers, which serve as a structural support for the applied
functional coatings, were produced from recycled PET (polyethylene terephthalate) bottles. The oil
absorbing coatings were comprised of crosslinked homopolymers and copolymers based on octadecyl
acrylate (ODA), maleic anhydride (MA), and related esters of MA. The crosslinked polymer networks
were synthesized by both suspension and bulk polymerization techniques using divinylbenzene (DVB) as
the crosslinking agent. Efficacy of the coated NWPET fibers as oil sorbers was determined by oil
absorption tests in toluene and in 10% crude oil in toluene. Rigidity, porosity and swelling of the
crosslinked polymers were evaluated and correlated to the chemical structures, composition, and
reaction media. Suspension polymerization yielded the desired morphology and function, providing
higher porosity and in consequence a high absorption capacity.
Received 29th August 2013
Accepted 10th October 2013
DOI: 10.1039/c3ra44759f
www.rsc.org/advances
industry.5–8 We have tested the capability to absorb petroleum
as well as toluene; toluene has the advantage of being a good
1. Introduction
solvent for asphaltenes which are contained in crude oil.9 These
facts are the starting point for the present work, plus the nding
of Kim and coworkers10 that non-woven polyethylene tere-
phthalate (NWPET) based sorbents might be useful for oil-spill
recovery in marine environments, but only aer modication
with hydrophobic polymers. Furthermore, the sustainability of
our approach is increased by our use of recycled PET.
The recent Gulf of Mexico oil-spill reminds us again of the
importance of oil spill cleanup and environmental remedia-
tion.1 The typical methods of dealing with oil spills are
mechanical extraction, in situ burning, and bioremediation.2
One of the most economical and efficient methods for oil spill
cleanup is mechanical extraction by sorbents.3 Continuous
sorbent belts constructed of polymers hold great promise since
oil is removed from the water surface by the oleophilic material
from which the belt is made. Oil sorbents are able to concen-
trate and transform liquid oil to the semi-solid or solid phase,
which can then be removed from the spilled area in a conve-
nient manner.4 Such sorbents are typically made of polymers,
while polymers also have other applications in the petroleum
Polymers based on crosslinked or graed hydrophobic
polymers like alkyl acrylates, rubber, and alkyl styrene have
proved useful in the absorption of oil or oil-like solvents. Porous
gels based on these materials have been intensively researched
to evaluate the inuence of the synthesis conditions on the
structural characteristics and oil absorbency.11–13 However,
these gels alone lack the mechanical strength and structural
integrity needed for the intended application. Similarly, emul-
sion templating offers an alternative route for preparation of
functional porous polymers; however the materials produced by
this method are not necessarily ideal for operation in the
intended environmental conditions.14 To overcome the disad-
vantageous mechanical properties of swollen polymeric gels,
Ceylan and Okay designed tough macroporous gels based on
butyl rubber.15 We sought to achieve a similar effect by
designing a composite material, thereby drawing on the
strengths and minimizing the weaknesses of two materials to
obtain the desired properties in the nal product. Polyester
terephthalate (PET) bers offer suitable mechanical and
thermal properties for application as oil sorbers, but their use is
aPetroleum Applications Department, Egyptian Petroleum Research Institute, Ahmed
El-Zomor St., Nasr City, 11727 Cairo, Egypt. E-mail: khaled_00atta@yahoo.com;
araheim@hotmail.com
bLaboratory of Advanced Polymers & Optimized Materials (LAPOM), Department of
Materials Science and Engineering and Center for Advanced Research & Technology
(CART), University of North Texas, 1155 Union Circle #305310, Denton, TX 76203-
5017, USA. E-mail: brostow@unt.edu; haleylobland@gmail.com; Web: http://www.
unt.edu/LAPOM/
cSurfactant Research Chair, Department of Chemistry, College of Science, King Saud
University, P.O. Box-2455, Riyadh-11451, Kingdom of Saudi Arabia. E-mail:
khaled_00atta@yahoo.com
dDepartment of Physics, University of North Texas, Denton, TX 76203, USA. E-mail:
jperez@unt.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra44759f
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restricted by low hygroscopicity and undesirable oil swelling Petroleum crude oil was obtained from Petrobel Egypt, with the
capacities. To modify properties of PET in these disadvanta- specications listed in Table 1.
geous points, radiation-induced graing of acrylic acid has
been carried out with satisfactory results.16 Modication of PET
is not limited, though, to only radiation induced graing. More
generally, application of any type of monomer or polymer as a
3. Experimental methods
3.1 Synthesis of maleic anhydride esters
coating onto PET bers depends on diffusion of the monomer
or polymer solutions into the bers.16,17 Additionally, there are
other challenges associated with modifying the surface of PET;
these are well described by Chen and McCarthy.18
Maleic anhydride was esteried using octanol (for synthesis of
dioctyl maleate) or dodecanol (for synthesis of dilauryl mal-
eate) in the presence of p-toluene sulphonic acid (1% per
weight of reactants) and xylene as a solvent (comprising
60 wt% of the reactants. MA (1 mol)) and octanol or dodecanol
(2 mol) were mixed together (constituting 40 wt%) in xylene
using a three-necked ask tted with condenser and Dean
Sturk apparatus (to separate the water and excess of xylene)
and reuxed at 140 ꢁC for 5 hours and then at 170 ꢁC for
30 minutes. The reaction products were evaporated to half and
mixed with isopropanol followed by mixing with hot salty
water (supersaturated with NaCl) to separate the aqueous and
organic layers. The organic layer was evaporated using a
vacuum rotary evaporator. The prepared dioctyl maleate
(designated DOM) has the following properties: density (0.94 g
cmꢀ3), boiling point at 10 mm Hg (235 ꢁC), ash point (closed
cup, 160 ꢁC), refractive index (1.45) and freezing point
(ꢀ40 ꢁC). The prepared dilauryl maleate (designated DLM,
synonym didodecyl maleate) has the following properties:
density (0.921 g cmꢀ3), ash point (closed cup, 240 ꢁC), and
refractive index (1.46). Fig. 1 shows schematically the esteri-
cation reaction yielding DOM and DLM monomers. The
The oil sorption capacities of the sorbents typically used to
coat NWPET are generally only ten grams of oil per gram of
sorbent, while water is also absorbed. Given the extent of oil
associated with spills and the importance of timeliness in
cleaning up oil spills, it is clearly desirable to enhance the
absorption capacity of the synthetic sorbents applied to
NWPET substrates. Ono and coauthors report superabsorbent
copolymers with not only improved mechanical properties
and but also increased absorbency of non-polar compounds;
however their materials did not absorb toluene to a signi-
cant degree.19 Our aim was to design a novel oil sorbent at low
cost, tailoring it to efficiently absorb oil in preference to water.
We therefore selected NWPET bers that were produced from
recycled PET bottles as the substrate for our oil sorbers. These
were coated with octadecyl acrylate maleic anhydride copol-
ymers via suspension and bulk polymerization techniques.
The sorption capacity and oil/water selectivity of the modied
NWPET sorbents were evaluated through a variety of tests.
Scanning electron microscopy was used to characterize the
copolymer structures and help elucidate the oil sorption
mechanisms.
esterication reactions were carried out in
a nitrogen
atmosphere to prevent oxidation of the double bonds and
conversion to fumarate bonds. The isomerization of maleic
2. Materials
Octadecyl acrylate (ODA), 2,2-azobis isobutyronitrile (ABIN),
were obtained from Aldrich Chem. Co. (Germany) and used
without purication. Maleic anhydride (MA), n-octanol,
n-dodecyl alcohol (i.e. dodecanol), cyclohexane, polyvinyl
alcohol (PVA) having average molecular weight 25000 g molꢀ1
,
toluene, p-toluene sulfonic acid (pTSA), xylene, and divinyl-
benzene (DVB) were obtained from Fluka. White polyethylene
terephthalate nonwoven bers, produced from recycled PET
bottles, were purchased from Zhejiang Waysun Chemical Fiber
co., Ltd. (China). The NWPET is fabricated from PET that has
been cut into small pieces, cleaned in a hot water process to
remove dirt, then sorted and extruded to polyester bers.
Table 1 Physicochemical properties of the petroleum crude oil
Test
Method
Value
0.875
21.70
14.00
762.8
2.45
Specic gravity at 60 ꢁ
API gravity
F
IP 160/87
Calculated
IP 15/67(86)
IP 71
UOP 46/64
IP 143/84
Pour point (ꢁC)
Kinematic viscosity (60 ꢁF) (Cst)
Wax content (wt%)
Fig. 1 Copolymerization reaction schemes. (a) Preparation of monomers, (b)
Asphaltene content (wt%)
8.80
preparation of homopolymers and (c) preparation of copolymers.
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Table 2 Soluble fraction percentages for crosslinked ODA, DOM, and DLM
homopolymers and for ODA/MA, ODA/DOM, and ODA/DLM copolymers as a
function of DVB crosslinker concentration
The synthesis reaction for DOM and DLM homopolymers is
shown schematically in Fig. 1b, the copolymerization of DOM
and DLM with ODA in Fig. 1c.
SF/%
3.3 Coating of NWPET bers
Polymers
ODA
DVB/wt%
Cyclohexane
Bulk
For the coating of crosslinked homopolymers and copolymers
onto PET bers, the NWPET bers were added in a 3 : 1 weight
ratio (ber : monomers) to the reaction mixtures of the
suspension technique as described in the previous sub-section
b. The mixtures were bubbled with nitrogen. The copolymeri-
zation and crosslinking reactions were performed at 60 ꢁC for 5
hours to ensure complete reaction of all monomers. The poly-
mer-coated PET ber was washed with methanol and dried at 40
ꢁC until a constant weight was reached. The coated NWPET
1
4
1
4
1
4
1
4
1
4
1
4
12a
9a
8
6
14
10
10
8
12
8
19
15
14
11
DOM
16
14
13
11
14
11
21
18
17
14
DLM
ODA/MA
ODA/DOM
ODA/DLM
ꢁ
bers were then post-cured at 105 C for 24 hours.
Polymers prepared by bulk polymerization were cast on the
PET bers prior to thermal crosslinking. According to this
method, the mixture was bubbled with nitrogen, then cast over
rolled PET bers. The weight ratio between PET bers and
monomers was 3 : 1, respectively. The copolymerization and
a
Polymerization completed in isopropanol rather than cyclohexane
solvent.
anhydride to the fumarate was not obtained in the present crosslinking reactions were performed in siliconized test tubes
reaction, while it has long been known that the isomerization at 60 ꢁC for a total of 9 hours to ensure complete reaction of all
of maleic anhydride to fumarate in polycondensation reac- monomers. The coated NWPET bers were then post-cured at
ꢁ
tions depends on the temperature and the glycol chemical 105 C for 24 hours.
structures.20 On the other hand, the isomerization to the
fumarate can be carried out at high temperature in the pres-
ence of catalysts such as mineral acid or thiourea and under
applied pressure, which further prevents the esterication
reaction.21
3.4 Characterization of polymers
Infrared (IR) spectra were recorded in KBr pellets using a Bruker
FT-IR model IF 566 spectrometer. Scanning electron microscopy
(SEM) was used to study the morphology of the crosslinked
polymers. Dynamic mechanical analysis (DMA) was carried out
on the puried crosslinked copolymers (i.e. aer extraction of
soluble, uncrosslinked monomers and polymers as described in
Section 3.2). The glass transition temperature Tg, melting
temperature Tm, and crystallization temperature Tc of the
crosslinked copolymers were determined by DMA as well as by
differential scanning calorimetry (DSC). Electron microscopy,
DMA and DSC methods are described in the ESI.†
3.2 Synthesis of crosslinked homopolymers and copolymers
Three series of crosslinked homopolymers and copolymers
based on MA, ODA, DOM and DLM were prepared using DVB as
crosslinker either in the presence of cyclohexane as solvent
(suspension technique) or in absence of a solvent (bulk tech-
nique). Rod-shaped specimens of crosslinked homopolymers
and copolymers were prepared.
The suspension homopolymerization and copolymerization
of the monomers of MA, ODA, DOM and DLM were carried out 3.5 Evaluation of absorption behavior
in a 250 ml three-necked ask tted with a thermometer,
The Soxhlet extraction technique was used to determine the
nitrogen gas, and reux condenser. The monomers were mixed
with 0.02 wt% of ABIN initiator, 1 wt% of PVA (170 ml for each
100 g of monomers), and different weight ratios of DVB cross-
linker (1% or 4%) in isopropanol or cyclohexane solvents.
Copolymerization reactions were all performed at a monomer
mole ratio of 50/50; the different homopolymers and copoly-
mers prepared are identied in the rst column of Table 2. The
soluble fraction of the crosslinked polymer coatings. Dried
xerogel discs of the polymer rods and coated bers were
transferred into an extraction thimble and extracted with
chloroform for 24 h. Aer extraction, the samples were dried at
atmospheric pressure for several hours and then dried to a
constant weight in a vacuum oven at 35 ꢁC. The soluble fraction
(SF) was expressed as the fractional loss in weight of xerogel. SF
values were calculated according to the following equation:
ꢁ
polymerization reactions were performed at 60 C for 3 hours.
Aer the reaction was completed, the resulting precipitate was
washed with methanol and dried in a vacuum oven at 40 ꢁC
until a constant weight was reached.
SF ¼ ((w1 ꢀ w2) ꢂ 100%)/w1
(1)
By the bulk technique, crosslinked homopolymers and where w1 is the dry weight before extraction and w2 is the dry
copolymers were prepared in the same fashion as by suspension weight of the sample aer extraction. As discussed later, SF is
but without the PVA or solvent (cyclohexane or isopropanol). The indicative of the crosslinking efficiency of monomers, which is
amount of DVB crosslinker used varied between 1 and 10 wt%.
affected by reactivity ratios between monomers. Higher SF
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values indicate low reactivity of homopolymers or copolymers completed without interference by impurities. FTIR and
with the chemical crosslinker DVB.
1HNMR were used to conrm the chemical structures of DOM,
Data representing the swelling equilibrium reect the nature DLM, MA, and ODA, but for brevity only 3 spectra are shown
of the polymer + solvent system. Practically, one places a sample in gures.
of known density in the chosen solvent until weight measure-
The IR spectrum in Fig. S1† of DLM monomer showed
ments indicate the saturation of the polymer by liquid. characteristic absorption bands at 3100 cmꢀ1 (–CH stretching
Assuming that no extractable components are present and that vibration of the C]C bond), 2859 cmꢀ1 (stretching vibration of
the entirety of swelling is the result of absorbed solvent, the the aliphatic C–H bond), 1731 cmꢀ1 (stretching vibration of C]
polymer volume fraction Fp of the swollen polymers is:
O), 1460 cmꢀ1 (bending vibration of CH2), 1381 cmꢀ1 (bending
vibration of CH3), 1164 cmꢀ1 (stretching vibration of C–O) and
950 cmꢀ1 (bending vibration of C]C bond). An important
feature of the prepared homopolymers and copolymers, based
on ODA and MA, is their crosslinking. Crosslinking is respon-
sible for formation of the three-dimensional network structure
that is necessary for oil sorbers to swell rather than to dissolve.
DVB (divinylbenzene) was used as a chemical crosslinker at 1%
and 4% weight ratios. Recall the crosslinking reactions—of
homopolymers and copolymers—are represented in Fig. S1b
and c.† The IR spectrum of ODA homopolymer, Fig. S1b,† shows
characteristic absorption bands at 2926 cmꢀ1 (for stretching
vibration of the aliphatic C–H bond), 1750 cmꢀ1 (assigned for
stretching vibration of the C]O ester group of ODA), 1143 cmꢀ1
(assigned for stretching of C–O of ester group of ODA). The
absence (compared to Fig. S1a†) of vinyl group C]C stretching
and bending vibrations at 3100 and 950 cmꢀ1, respectively,
suggests complete polymerization and crosslinking of ODA
polymers; we recall that DVB concentrations were small. The
same absorption bands were detected for all prepared copoly-
mers, which indicated the crosslinking and copolymerization of
ODA with DOM or DLM monomer. Similarly, the 1HNMR
spectrum (Fig. S2†) of DOM conrmed expectations. The
absence of COOH peak at 10–14 ppm of MA indicated the
formation of dialkyl maleate esters.
Fp ¼ (rp(W1 ꢀ W0))/W0rs
(2)
here W0 is the sample weight before swelling, W1 is the sample
weight aer swelling, rp is the density of polymer sample and rs
is the density of the solvent.
Oil absorbency of the homopolymers and copolymers
crosslinked with DVB and prepared either in solution or by bulk
polymerization were determined at 25 ꢁC according to ASTM
(F726-81): 0.1 g polymer (as coated onto NWPET) was put in a
pure stainless steel mesh (4 ꢂ 4 ꢂ 2 cm) that had been
immersed in pure toluene or with crude oil diluted with toluene
(10% oil) and weighted beforehand. The sample and the mesh
were together picked up from oil, drained for 30 s, tapped with
lter paper to remove excess oil from the bottom of the mesh,
and then weighed on a balance. Oil sorption capacity (Q) in both
toluene (QT) and crude oil (Qcrude) was calculated as follows:
Q (g gꢀ1) ¼ wabs/wdry
(3)
where wabs is the weight of oil (or toluene) that has been
absorbed by the sample and wdry is the weight of the sample
before immersion in and absorption of oil (or toluene). Equi-
librium toluene content (ETC) and crude content (ECC) were
thus calculated as:
ETC (or ECC) ¼ [(wabs ꢀ wdry)/wabs)] ꢂ 100%.
(4)
4.2 Structure and composition of crosslinked polymers and
coated NWPET bers
The preparation of composite materials is frequently used to
increase strength of a material. Fibers, sponges and non-woven
fabrics are sometimes used as reinforcements in composite
materials. In the present work, non-woven PET bers, derived
4. Results and discussion
4.1 Physicochemical characterization
It is well known that oil sorbents should exhibit hydrophobicity from recycled PET bottles, were used as reinforcement and
in order to swell in an oil medium. The primary aim of the work essentially a support structure for crosslinked ODA, DOM and
presented here was to synthesize new crosslinked polymers DLM homopolymers and for crosslinked ODA/MA, ODA/DOM
incorporating hydrophobic moieties such as alkyl or phenyl and ODA/DLM copolymers. As described earlier, the monomers
groups. MA (maleic anhydride) contains both hydrophilic and were crosslinked onto NWPET bers at 70 ꢁC by radical bulk and
hydrophobic moieties. Consequently, it is important to convert solution suspension crosslinking copolymerization reactions.
it, by reaction of its anhydride group, into hydrophobic mono-
A wide variety of vinyl crosslinking agents are used to form
mers. This was carried out by esterication reactions of MA with crosslinked polymer networks, and the available choices of a
octanol or dodecanol (Fig. 1a) to add the alkyl moiety as a crosslinker for bulk and solution polymerization are numerous.
hydrophobic group. The prepared esters are not by themselves Crosslinker concentration is usually about 0.05–1 wt%, and
new materials, but the copolymerization of the esters to form crosslinking can be used to produce superabsorbent polymers
crosslinked polymers that function as oil sorbers is indeed with high swelling capacity and low soluble polymer content.
novel. For this purpose, it is necessary to use puried mono-
However, many side reactions, such as intrachain cycliza-
mers, which was conrmed by measuring the physical proper- tion, decrease the efficiency of the crosslinker and result in a
ties (and comparing them to tabulated reference data) of the later gel point in the polymerization than would be predicted by
monomers to ensure that the copolymerization could be theory and in networks less crosslinked than would be
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Table 3 Soluble fraction percentages for crosslinked ODA, DOM, and DLM
homopolymers and for ODA/MA, ODA/DOM, and ODA/DLM copolymers, all
coated onto NWPET fibers, as a function of DVB crosslinker concentration
predicted from the number of potential crosslink sites. This
deviation from theory can be signicant at higher crosslinker
concentrations, such as those used in styrene DVB copolymers,
where intramolecular cyclization is believed to occur. Intra-
molecular cyclization increases with low monomer content and
high crosslinker concentration. Because of the high monomer
content and low crosslinker levels used in typical gel processes
(such as what we have conducted), there is a reduced probability
of this inefficient side reaction occurring, especially during the
initial part of the polymerization process.
SF/%
Polymers, as coatings
on NWPET bers
DVB/wt%
Cyclohexane
Bulk
ODA
1
4
1
4
1
4
1
4
1
4
1
4
24a
18a
25
17
22
13
15
12
35
29
25
18
20
16
33
29
30
25
13
10
40
33
30
22
DOM
DLM
Furthermore, to understand the distribution of crosslinks in
the network, reactivity of the various double bonds in the
system should be assessed. This includes—in the present
study—the reactions between double bonds of DOM, DLM, MA
and ODA, the initial double bonds of the DVB crosslinker, and
the various double bonds that are pendant from the polymer
chain aer incorporation of the crosslinker. Compared with
ODA and MA, DOM and DLM monomers are expected to
provide a certain level of hydrophobicity that improves oil
affinity of the resultant polymer. As the reaction proceeds, the
pace of the crosslinking reaction increases very rapidly at some
point, and the reaction product begins to form an innite
molecular weight network—this is called the gel point. In the
gel state, the chemical reaction can proceed and chains form
the network by crosslinking, and the crosslink density or degree
of crosslinking is a measure of the total links between chains in
a given mass of substance. Typically, aer the reaction there
remain some polymer chains not attached to the innite
network; these can be extracted from the gel fraction. Thus, in a
crosslinked system, there are soluble portions and insoluble
portions: the former can be extracted with suitable solvents,
whereas the latter cannot be extracted with any solvent owing to
the crosslinking. This feature is quantied as the soluble frac-
tion (SF) and contributes to the swelling behavior of the
specimen.
ODA/MA
ODA/DOM
ODA/DLM
a
Polymerization completed in isopropanol rather than cyclohexane
solvent.
for the polymers as coatings on NWPET ber supports. This
data and the physicochemical characterization show that it is
possible to efficiently prepare crosslinked copolymers from
octadecyl acrylate and maleic anhydride by both suspension
and bulk polymerization techniques using DVB as the cross-
linking agent. The crosslinking efficiency decreases in the order
ODA > DLM > DOM > ODA/DLM > ODA/DOM > ODA/MA.
Crosslinking is desirable to create a network structure suitable
for oil absorption. However, too low or too high crosslinking
efficiency limits the absorption. As a consequence of the former,
the oil uptake of ODA/MA was quite low. Evaluation of the
morphology of these specimens provides more insight.
4.3 Morphology of coated NWPET bers
Again, the swelling behavior of crosslinked polymer
networks is critical to our aim of developing better oil sorbers. ESEM micrographs of the specimens are shown in Fig. 2. (Note
According to Flory's swelling theory,22 swelling behavior is that the images were taken of samples aer extraction of the
affected by rubber elasticity, affinity to solution and cross- soluble fraction.) Crosslinked polymers on NWPET—by the
linking density. Thus the soluble fraction and conversion of our bulk polymerization technique—are seen in Fig. 2b–d. They can
polymer gels, prepared with a crosslinker concentration of 1 or be compared to the uncoated NWPET shown in Fig. 2a. The
4 wt%, were determined. Note that SF measurements are crosslinked polymers appear homogeneous on the surface
indicative of the reactivity of the crosslinker towards the texture. The surface changes from smooth to channel-like pores
monomers. The insoluble material (w2 in eqn (1)) is considered with increasing chain lengths of side groups, from MA to DOM
as the gel fraction, while the soluble materials are calculated as to DLM. Fig. 2e–h shows homopolymers and copolymers on
the soluble fraction (SF/%).
NWPET prepared by suspension technique.
The soluble fraction data for crosslinked DOM, DLM, and
On average, the size of NWPET ber increased from ca. 12
ODA homopolymers (using the crosslinker DVB) and their mm for uncoated NWPET to ca. 20 mm for coated NWPET. Both
copolymers were determined and calculated according to eqn synthesis methods utilized a similar weight of reactants. Fig. 2
(1); values for the polymers alone are listed in Table 2, for the shows that the coating layers obtained by the bulk polymeri-
polymers coated onto NWPET bers in Table 3. Careful zation technique are less porous than those obtained by
inspection of the data in Tables 2 and 3 shows that with and suspension polymerization.
without the NWPET bers the SF% was lowest at the high
The variations in coating morphology and thickness suggest
crosslinker concentration. For the plain (unsupported) poly- that different formation mechanisms for the skin layers of ODA,
mers (Table 2), bulk preparation resulted in a smaller SF (i.e. DOM, DLM, ODA/MA, ODA/DOM and ODA/DLM polymers onto
greater extent of crosslinking) than solution preparation (in NWPET bers may be involved. On the basis of the above
cyclohexane or isopropanol). However, that effect was not seen results, a ber skin layer formation mechanism onto NWPET is
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Fig. 2 ESEM micrographs. (a) Untreated NWPET fibers. Crosslinked copolymers on NWPET, prepared by bulk technique: (b) ODA/MA (4% DVB); (c) ODA/DOM (1%
DVB); and (d) ODA/DLM (1% DVB). Crosslinked homopolymers and copolymers on NWPET, prepared by suspension technique: (e) DOM (4% DVB); (f) DLM (4% DVB);
(g) DLM (1% DVB); and (h) ODA/DLM (1% DVB).
plausible. It is believed that most of the homopolymer and or by the addition of plasticizer.26 The storage modulus
copolymer particles might have initiated and continued their E0 (representing solid-like behavior) and tan delta, obtained by
growth in the solution phase, and only those particles (or the DMA for ODA/MA are shown in Fig. S3.† We see a single tran-
growing polymer chains) at or close to the liquid/solid inter- sition at 30 ꢁC for ODA/MA (50/50 mol%) crosslinked with
faces had deposited onto the PET bers via chemisorption 1 wt% DVB. This while ODA/MA (50/50 mol%) crosslinked wꢁith
ꢁ
and/or physical adsorption. The comparison of the results from 4 wt% DVB shows two transitions, namely at 10 C and 42 C.
the two coating systems also indicates that the bulk polymeri- The lower and higher transition temperatures should, respec-
zation formed a dense mat inhibiting the diffusion of mono- tively, correspond to the Tg and the melting temperature Tm of
mers around the PET bers, making it difficult to create porous so segments. The high crystallinity of ODA/MA with 1 wt%
polymers. The wet suspension polymerization technique was DVB is reected in a single transition corresponding to melting
developed to prevent the collapse of the crosslinked homopol- of so segments. On the other hand, less perfect crystalline
ymers and copolymers onto the NWPET ber network. There are structures of ODA/MA with 4% DVB show both Tg for the
indications in the literature that a moist ber substrate provides amorphous domains and Tm for the crystalline domains. These
a more porous network support and thus greater inltration of
adhesive monomers and polymers.14,23 When one uses the wet
suspension technique, the channels between the NWPET bers
are lled with solvent or monomer uids. The main mechanism
available for coating of PET bers with homopolymers and
copolymers is thus diffusion of the resin into whatever uid is
in the spaces of the substrate as well as along the NWPET bers.
Our results conrm those in14,23 showing, as already discussed
above, higher porosity in materials obtained by suspension
polymerization.
4.4 Thermal and mechanical properties
The elasticity and rigidity of the crosslinked polymer networks
were determined by measuring the characteristic thermal
changes of the polymers. Using dynamic mechanical analysis
(DMA) and differential scanning calorimetry (DSC), we identi-
ed the glass transition temperature Tg, which represents the
temperature range at which amorphous polymers undergo a
transition from a rubbery, viscous amorphous liquid to a brittle,
glassy amorphous solid.24 Both DMA and DSC techniques are
well explained by Menard.25 The Tg may be engineered by
Fig. 3 Swelling uptake at 25 ꢁC of NWPET fibers coated with crosslinked ODA/
MA, ODA/DOM and ODA/DLM copolymers using (a) suspension technique, and
altering the degree of branching or crosslinking in the polymer (b) bulk technique.
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technique has some impact on the transition temperatures.
Thus, glass transition temperatures of materials obtained by the
bulk process are higher than those obtained by suspension
polymerization. At a crosslink, adjacent polymer chains are
pulled closer together, and the free volume is decreased, so
raising Tg.31,32 Apparently we have a higher crosslink density per
unit volume in the copolymers created in the bulk.
Melting temperatures in Table S1† are also higher for
products of bulk polymerization. Since the polymers are cross-
linked, we are talking here only about melting of side chains.
Overall the DSC data indicate that the crosslinked polymers
based on the DVB crosslinker formed elastic amorphous and
semicrystalline polymer networks. The apparent rigidity of the
crosslinked networks generally increased with increasing DVB
content, while less rigidity results in higher liquid uptakes.
4.5 Oil absorbency of coated NWPET
Fig. 4 Swelling uptake at 25 ꢁC of NWPET fibers coated with crosslinked ODA/
MA, ODA/DOM and ODA/DLM copolymers using (a) suspension technique, and
(b) bulk technique.
The crude oil used in this experiment (Table 1) was diluted (to
10% oil) with toluene. For real applications in cleaning up oil
spills, the oil absorption test has to be operated using not only
light or medium oil but also heavy oil, because crude oil has a
high viscosity. Heavy oil with high viscosity, such as Belayium
crude oil, cannot easily diffuse from the surface of the samples
into the internal space of the network. However, and as already
noted, toluene is the most applicable solvent that is used to
dissolve asphaltene of crude oil.9 For these reasons, we have
used crude oil diluted with toluene in this experiment so that
the swelling behavior of the samples could be easily evaluated.
Swelling uptake by the homopolymer coatings on NWPET is
shown in Fig. 3. Corresponding diagrams for the copolymers
coated on NWPET are given in Fig. 4. Immersion times were
from 0 to 120 minutes, and the oil and toluene absorbency
increased with immersion time up to a maximum. In all cases a
maximum absorbency was attained followed by a leveling off
with no further absorption occurring with the increased
immersion time.
large changes in mechanical properties observed are reversible
and cyclically reproducible in repeated temperature changes.
This can be attributed to the order–disorder transition of the
crystalline aggregates formed through side-by-side interactions
of octadecyl groups; the aggregates can reversibly “lock” the
original as well as deformed shape on cooling.27,28
Differential scanning calorimetry (DSC) provides additional
information related to crystallization; both melting and crys-
tallization temperatures (Tm and Tc) are observed. The transi-
tion temperatures are listed in Table S1.† The accuracy of
ꢃ0.1 ꢁC comes from the equipment used and should not be
taken literally. We do not include the DSC diagrams for brevity.
Analysis of the glass transition behavior of crosslinked copoly-
mers is typically more complicated than that of uncrosslinked
copolymers.29,30 Single glass transition temperatures are
observed, indicating full miscibility. The polymerization
Values of the swelling parameters such as QT, ETC, and Fp
were calculated and are listed in Table 4. Based on their higher
Table 4 Swelling parameters of crosslinked homopolymers and copolymers based on DVB crosslinker in toluene at 25 ꢁC
ETC%
QT g gꢀ1
Fp
Polymers
ODA
DVB wt%
Cyclohexane
Bulk
Cyclohexane
Bulk
Cyclohexane
Bulk
1
4
1
4
1
4
1
4
1
4
1
4
98.66a
98.34a
95.56
94.62
96.17
95.43
94.22
93.06
96.35
95.82
97.01
96.75
94.25
93.01
93.51
92.42
95.53
95.07
93.55
92.38
96.05
95.48
96.91
96.44
75.1a
60.1a
22.5
18.6
26.1
21.9
17.3
14.4
27.4
23.9
34.5
30.8
17.4
14.3
15.4
13.2
22.4
20.3
15.5
13.12
25.3
22.1
32.3
28.1
0.0075a
0.011a
0.0350
0.0412
0.0214
0.0297
0.0462
0.0582
0.0182
0.0220
0.0130
0.0156
0.0286
0.0699
0.0650
0.0758
0.0451
0.0490
0.0657
0.0721
0.0395
0.0452
0.0310
0.0356
DOM
DLM
ODA/MA
ODA/DOM
ODA/DLM
a
Polymerization completed in isopropanol rather than cyclohexane solvent.
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equilibrium toluene contents, higher toluene sorption capac- can be calculated as follows: regeneration efficiency ¼ (total
ities, and lower polymer volume fractions, the materials absorption capacity in the nth run) ꢂ 100%/(total absorption
prepared by solution polymerization (in cyclohexane or iso- capacity in the rst run). Regeneration of the prepared copoly-
propanol) exhibit higher toluene uptake than their counterparts mers was calculated over 7 cycles; the data of regeneration show
prepared by bulk polymerization. The low absorption capacity that the efficiency decreases slightly and in nearly all polymers
of the bulk polymerized specimens was attributed to their reaches a constant value aer the second cycle. The prepared
higher extent of crosslinking, a feature discussed earlier in polymers are glassy in the dry state and rubbery in the swollen
relation to the soluble fraction data. While some crosslinking is state. The copolymers adhered onto PET bers both before
needed, a very large extent of crosslinking limits the diffusion of and aer swelling as seen in SEM micrographs. This in
solvent into the material and lowers its absorption capacity. conjunction with repeated swelling and deswelling without
Along with this, the higher porosity of specimens prepared in effect on the absorption rate suggests good stability of the
suspension contributes to their good swelling behavior.
We also see from these results that greater hydrophobicity of
DLM and ODA/DLM increases absorption capacity. Addition-
ally, Fig. 4 shows that of the copolymers, ODA/MA displayed the
least absorption by far compared to ODA/DLM and ODA/DOM,
while oil and toluene uptake of the latter two were similar to one
another. These results agree with the ndings of others who
have looked at swelling in crosslinked copolymers systems
containing maleic anhydride.33
formed copolymers.
Acknowledgements
Financial support for this project was provided by the US
National Science Foundation (NSF Award # OISE 1135101). We
appreciate the guidance provided by Dr Osman Shinaishin of
NSF.
5. Conclusions
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Fig. 3 and 4 indicate also that the prepared polymers on NWPET
bers absorb a large quantity of toluene—from 35 to 90 g of
toluene per g of polymer—in the time range from 30 to 90
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