Download - Application of Controlled Degradation
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Journal of Applied Polymer Science (2013) vol.129, Page 3079-3088
Current application of controlled degradation processes
in polymer modification and functionalization
Ahmad Mohamad Gumel1, M. Suffian M. Annuar1* and Thorsten Heidelberg2
1Institute of Biological Sciences, 2Department of Chemistry
Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Published: Journal of Applied Polymer Science (2013) 129:3079-3088
(ISI cited publication)
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Abstract
The ecological concerns over accumulation of polymeric material wastes and the demand for
functionalized polymers in specialty applications have promoted intensive research on the different
controlled degradation processes and their applications. Production of functionalized or modified
polymers by conventional synthetic routes is expensive and time consuming. The advancement in
the degradation technology is becoming an enabling factor in the production of modified polymers
and their functionalization. Mild irradiation, ozonization and enzymatic routes were among the
processes that have been explored for polymer modification. Biopolymers such as chitosan,
hyaluronic acids and polyhydroxyalkanoates are known to have diverse applications ranging from
biomedical to organo-electronics. However, their high molecular weight, crystallinity and shelf
degradability issues limit their applications. Controlled degradation processes can be used to
prepare these types of polymers in reasonably low molecular weights oligomers or radicals with
superior properties. Application of controlled degradation processes for polymer modification and
functionalization is reviewed.
Keywords: biodegradable; biopolymers; biocompatibility; controlled; degradation; enzymatic;
radiation.
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1. Introduction
Polymers are increasingly being used in diverse applications. They are especially important in
various food, cosmetics and biomedical applications.1-3 The increasing utilization of these polymers
has health and ecological limitations especially in biomedical and environmental perspectives. The
severe environmental pollution as a result of poorly degradable polymeric waste disposal and the
increasing demand of controlling the precise target delivery of a therapeutic drugs as well as the
capability of the polymeric drug carrier to release a drug at independent time scale warrant an
intense research exploration in the controlled degradation processes for polymer functionalization
and modification. The recent momentum in the use of biodegradable polymers over their non-
degradable counterpart is among the measures taken towards being environmental friendly and
sophistication in biomedical applications. However, most of these biodegradable polymers lack the
superiority of types and quality as compared to their non-degradable counterparts. These issues
are further compounded by the fact that the production of tailormade or functionalized
biodegradable polymer may be costly due to difficult synthetic steps that are often needed in the
processes. Although several approaches in devising simpler and cost effective means to improve the
quality of these biodegradable polymers have been reported, which include dual biosynthesis4 and
blending5, wider options need to be explored. In view of this, researchers have turned to utilizing an
opposite route of degradation processes to either improve the degradability of these important
polymers or to customize the process for the production of specialty polymers for niche
applications. Among the recently reported processes are the use of high energy radiation,6
ozonization,7 ultrasound irradiation,8 microwave irradiation,9 oxidation,10, 11 biodegradation12 and
photo-degradation.13
This paper reviews the current research approaches in the application of controlled degradation
processes as alternative and viable routes towards enhanced polymer degradation and/or
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modification/functionalization. In most cases, the mechanisms and biochemistry of the degradation
process are also highlighted.
2. Use of biodegradation and organomodifiers
Biodegradation is known to be an effective method of completely removing the degradable polymer
and its constituent from the environment.14 Several approaches were developed that either
employed the used of enzymes 12, 15, 16 or microbial consortia 17-19 to effect the polymer degradation
and modification. The metabolic degradation mechanism of polymers has been reported to utilize
several enzymes such as dehydrogenases, hydrolases and oxidases, while the process is mostly
based on either hydroxyl group oxidation to yield ketones or by hydrolysis of carbonyl structure
that are followed by final mineralization of the components. The hydroxyl group oxidation is
reported to be based on two steps; either by oxidation of one adjacent hydroxyl group to yield
mono-ketone structure or by oxidizing two adjacent hydroxyl groups to form -diketones
structures.20
Extensive research on a plethora of microbial enzymes that are normally involved in
polymer degradation has been reported recently.16-18 For example, controlling the rate of silk-based
polymeric material degradation is vital to its potential use in biomedical applications such as drug
delivery and tissue engineering scaffolding. Recently, Pritchard, et al. 21 reported the use of protease
type XIV and ethylenediamine tetraacetic acid (EDTA) as bio-controlling switch to control the in
vitro degradation of silk-based drug carrier devices. The researchers observed the effect of protease
concentration on accelerating degradation, and the use of EDTA on reducing the rates of
degradation and on controlling the drug release from silk-based biomaterials. They reported an
increased rate of proteolysis with increasing protease concentration, which resulted in an increased
dye release from silk carriers. On the other hand, the release of EDTA from the silk carriers
inhibited proteolysis, which in turn controlling the proteolytic rate and hence the drug release.
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The important step in polymer degradation, especially that of polyhydroxyalkanoate is
attributed to the degradation of the polymeric lamellar crystal.12, 22 It has been reported that in
most cases the extracellular enzymes such as polymerases and hydrolases secreted by the
microbial consortia are responsible for the polymer biodegradation. Kulkarni, et al. 23 reported the
selective enzymatic degradation of block co-polymer (polycaprolactone-b-poly-p-dioxanone) using
Pseudomonas lipase (Table 1). The researchers reported that after 200 hours of subjecting the
material to enzymatic degradation, the poly-p-dioxanone copolymer part was completely stable and
not tampered with, while the degradation affects the polycaprolactone (PCL) part. They
demonstrated that degradation properties of multifunctional polymers can be manipulated and
controlled using selective enzymatic degradation resulting in unique polymers with specific
properties for specialized applications. They further reported that the degree of enzymatic
degradation relies heavily on the apparent enzyme penetration depth and the initial molecular
weight of the block copolymer; a suggestion substantiated further by Numata, et al. 12 and Tanuma,
et al..24 In addition, chemical structure,12, 25 molecular branches 12 and degree of acetylation 26 are
among the parameters suggested to exert influence on enzymatic degradation of polymeric
materials.
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Table 1 Application of controlled degradation processes towards polymer degradation and/or modification
Degradation Degradation polymer Application Reference
method agent
Selective Pseudomonas
Multifunctional 23
enzymatic lipase polymers
degradation
Radiation gamma
Agricultural 50
induced partial (from Mw 10 x103 kDa to 2 x103 kDa) growth
degradation promoters
Radiation gamma
Pulsatile protein 54
induced graft release
polymerization
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ozonization ozone
Antioxidant and 7
(from Mw 1535 kDa to 87 kDa) wound healing
activities
ozonization ozone
Improved 61,62
electro
conductivity
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Recently, controlled enzymatic degradation of polycaprolactone was reported using
polymer embedded Candida antarctica lipase B solution (1.6%) in both continuous fluid exchange
flow and controlled humidity chamber processes.27 Using 20 mM potassium phosphate buffer at pH
7.1 and a flow rate of 0.2 ml min-1, the researchers were able to achieve polymer weight loss of
about 85% in three days, an increase that is more than in process without the flow where bulk
weight loss of 70% within nine days incubation was observed. The researchers suggested that the
increase in degradation rate under the flow condition could be attributed to the more efficient
removal of degradation products that can act as competitive inhibitors. However, they observed a
slower decrease in polymer weight loss (70% in seven days) as the flow rate was further increased
to 0.5 ml min-1, which they attributed to the negative influence of the increase flow rate on enzyme
stability.27 When studying the rate of enzymatic degradation in controlled relative humidities (RH)
of 20, 75 and 95% using the same polymer embedded 1.6% C. antarctica lipase B, the researchers
observed an insignificance polymer weight loss at 20% relative humidity condition.27 However, at
75 and 95% RH, the polymer film was observed to exhibit weight loss of 25% and 58% after
28 days of incubation, respectively. The application of this controlled enzymatic degradation of
polymeric materials under controlled humidity condition could be an advantage in applications
where the unique degradation properties of enzyme-embedded bioresorbable films will be
exploited for release of active materials ranging from fragrances, flavors and therapeutic agents.
The biodegradation of water-borne polyurethane is reported to be enhanced by the
incorporation of vinyltrimethoxysilane modied starch.28 Incubating the chemically hybridized
polymer with 10% modified starch in -amylase solution for 10 days, a maximum weight loss of
15% and a decrease in tensile strength of 60% were observed. This result was reported to be much
bigger than that of the polymer containing the unmodified starch (5% weight loss and about 17%
tensile strength decrease). Extracellular polyhydroxybutyrate depolymerase puried from
Ralstonia pickettii T1 is used to degrade the film of poly[(R)-3-hydroxybutyrate-co-4-
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hydroxybutyrate] that was prepared by uniaxial cold-drawing from an amorphous polymer at a
temperature just below the glass transition.29 In this type of polymer, the researchers observed the
degradation rate to range from 0.14-0.67 mg cm-2 h-1 depending on the polymer mechanical
structure. The degree of enzymatic degradation was observed to increase with increasing draw
ratio and 4HB content, a reason mostly attributed to the decrease in the polymer crystallinity. They
further reported that the enzyme preferably attacked the -form than the -form which was
attributed to be due to less steric hindrance against the ester bonds in the planner zigzag
conformation of the -form as compared to -form helical conformation.29 The controlled
enzymatic degradation of poly (3-hydroxybutyrate-co-4-hydroxybutyrate) was studied using
commercial lipases16 where the researchers employed the use of non-regiospecific Amano lipase AK
and 1,3-regiospecific Novozym lipopan BG to control the degradation of P3HB-co-4HB from 400
kDa to low molecular weight polymers of 1 to 5 kDa within 72 hours, suitable for drug release
device. In another study, the use of non-specific protease (pronase) to catalyze the controlled
degradation of CaCO3-templated capsules that were prepared via layer-by-layer (LbL) deposition
techniques has been reported.30 The researchers showed that either by increasing the number of
biodegradable layers in the capsules, or by inserting synthetic polyelectrolyte of poly-allylamine
hydrochloride (PAH) and poly-sodium-4-styrene sulfonate (PSS) forming a multicompartment
polyelectrolyte multilayer capsules (Figure 1), the pronase-induced degradation of capsules can be
slowed down on the order of hours resulting in controlled detachment of subcompartments of
multicompartment capsules, with potential for intracellular delivery or in vivo applications.
Furthermore, the degradation rate is observed to increase with an increase in the pronase
concentration.30
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Figure 1 Controlled enzymatic degradation of multilayer capsule in drug delivery devices
Beside extracellular or in vitro enzymatic degradation, whole cell microbial polymer
degradation has been reported.20,31 Polyvinyl alcohol is considered as an excellent compatible
polymer blend with other polyhydroxyalkanoates due to its water solubility, biodegradability and
diverse applications. Jecu, et al.32 observed the degradation of polyvinyl alcohol by fungal strains
belonging to genera of Aspergillus, Monillia, Penicillium, Aureobasidium and Trichoderma. The
researchers observed that of all the species tested, A. niger came out to be the best in degrading the
PVA composite with the degree of degradation largely depending on the media and polymer
compositions. Higher degradation ( 60%) of copolymer of sucrose polyesters was also reported
using A. niger.33 The increasing demand of soft wood especially in current infrastructural
developments has been a point of ecological concern. It has been suggested that a polymer
composite of fast growing herbs such as kenaf, grass, palm oil leaves and bamboo could serve as an
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alternative to wood.34 A biocomposite polymer of polylactide and Hibiscus cannabinus (kenaf) is
said to have a similar properties as that of particleboard and as such been considered as a softwood
alternative.35 Recently, mycelia of Pleurotus ostreatus immobilized on calcium alginate beads was
used by Hidayat and Tachibana35 to degrade a composite polymer of polylactide and kenaf fiber,
achieving 48% degradation after six months as compared to the non-composite fiber degradation of
84%. They reported that the degradation of PLA/kenaf composite by P. ostreatus mycelia occurred
via oxidation causing the rupture of hydroxyl groups and the formation of carboxylic acids groups.
In contrast to eukaryotic fungal species, several bacterial genera such as Bacillus,
Comamonas, Pseudomonas, Staphylococcus and Streptococcus are among those reported to be
employed in polymer biodegradation.31 Schneider, et al.36 observed the effects of oleic acid
concentration on poly(3-hydroxybutyrate) (PHB) biodegradation produced by Cupriavidus necator.
They observed the polymer crystallinity decreases with increasing oleic acid concentration feed,
thus, increasing the degradability of the produced polymer.
The presence of impurities, organomodifiers and plasticizers were reported to affect the
biodegradability of the polymers. Researchers reported the use of organomodifiers such as clay to
manipulate polymer degradability and stability. Clay nanoparticles were reported to modulate the
biodegradability of gluten-based agromaterials37 resulting in a degree of degradation as high as
92%. Heteroaromatic ring derivatives were used to control the degree of degradation in
polypropylene.38 The researchers reported that the degradation rate is highly influenced by the
electron density of the C=C bond in the heteroaromatic derivatives. Low electron density
heteroaromatic derivatives like 2-(furan-2-ylmethylene)-malononitrile was found to restrict the -
scission of the polypropyline macroradicals by converting it to stable resonance macroradicals. In
comparison, changing the aromatic rings with high electron density derivatives such as 2-cyano-3-
(pyrrole-2-yl)-2-propionic acid ethyl ester results in high degradation rate.38
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3. High energy radiation
High energy radiation has been proven to be a useful tool in polymer degradation and/or
manipulating the physicochemical structure of many industrially important polymers.39,40 For
instance, high energy radiation has been applied to form chain scission and branching in
polypropylene.41 High melt strength polypropylene (HMS-PP) grains were synthesized by Oliani, et
al.42 using gamma radiation. The same radiation was also shown to degrade polylactide-co-
glycolide.40 It has also been used to enhance the electric conductivity of polymer electrolyte43 and
polyaniline-[poly(vinylidene chloride)-co-(vinyl acetate)] blends.44 Electron beam was employed to
degrade polylactide-co-glycolide by chain scission,45 it was also reported to be used to achieve
controlled surface degradation in bioresorbable polymers.39,46 Carbon ion beams were reported to
modify the physicochemical structures of both polyallyl diglycol carbonate and polyethylene
terepthalate polymer lms.47 An informative review on the effects of irradiation on controlled drug
delivery and release systems was compiled by Raem and Katuin-Raem.48
Galovic, et al.49 studied the effect of gamma radiation on polyethylenes of different densities
using temperature modulated differential scanning calorimetry. The researchers observed an
increase in crystallinity and stability of the polymer at radiation doses up to 200 kGy; beyond this
value, a decrease in these parameters was observed. They suggested that the low radiation favors
polymer macromolecular breakage than cross linking, which in turn leads to an increased
perfection of the crystals due to the alleviation of tension at the sites of lamellae surfaces where the
molecules enter the lattice. On the other hand, the high radiation doses favor the increase in
macromolecular surface free energy that induced crosslinking at the lateral grain boundaries,
resulting in lower crystallinity because of lattice distortion and expansion.49 El-Sawy, et al.50
employed gamma radiation to degrade chitosan of molecular weight about 10 x103 kDa into water
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soluble chitosan of average molecular weight of less than 2 x 103 kDa that are suitable for use as
growth promoters in agricultural fields (Table 1). Using initiators such as ammonium persulfate
and hydrogen peroxide, the researchers showed that the degree of degradation depends not only
on radiation dose but also on the concentration of the initiator. Technically by modulating the
radiation dose and initiator concentration, a specific oligomeric chitosan polymer will be achieved.
The production of high performance carbon fibers mostly depends on precursors, among which
polyacrylonitrile (PAN) is the most important. Unfortunately, using PAN as a precursor involves
peroxidation to achieve the oligomerization of the nitrile groups to form a ladder chain structure
which improves the thermal stability of the bers. The conventional method is time consuming,
required use of chemicals and a highly exothermic process.6 Gamma radiation has recently been
reported to induce the formation of free radicals and crosslinking in polyacrylonitrile fibers
improving both the thermal stability and cyclization (oligomerization) efficiency of the fibers.6,51
Polymer ion beam irradiation liberates hydrogen and other volatile gasses which are
suggested to influence the formation of free radicals and unsaturation; this in turn improves the
polymer dielectric constant and conductivity. Singh, et al.52 employed this phenomenon to improve
the electrical conductivity of copper-doped polymethyl methacrylate for applications in organo-
electronic components. High energy radiation has been used to induce graft polymerization (Table
1) due to its simplicity and requires no use of catalyst or additives over the conventional methods,
hence resulting in almost pure product.53 Recently, the mild condition of gamma radiation was used
to crosslink poly(N-2-hydroxyethyl)-DL-aspartamide with maleic anhydride producing a
functionalized hydrogel which was used as drug delivery device by encapsulation and pulsatile
release of proteins.54
Lotfy55 studied the control degradation of low molecular weight dextrin in presence gamma
irradiation (5-100 kGy). Using both electron spin resonance (ESR) and X-ray diffraction (XRD)
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spectra, the researchers reported the dextrin to undergo oxidative degradation that takes place at
the crystalline regions of the amylopectin chains. Furthermore, they revealed the polymer weight
loss of the irradiated sample occurred at lower temperatures as compared to un-irradiated
samples, resulting in oligomeric dextrin components with melting temperatures that decreases
with increasing irradiation dose.55
Flocculation is an efficient and cost effective process for water treatment, polymers are
popularly used as flocculating agents due to their ability to destabilize colloidal suspensions.56
Ironically, the biodegradability of the natural polymers reduces their shelf life in this process, while
the synthetic polymers are costly and nonbiodegradable. In view of this, gamma radiation was used
a cost effective route to produce a novel flocculant by grafting 2-methacryloyloxyethyl trimethyl
ammonium chloride onto chitosan resulting in copolymer with high cationic properties and able to
treat water over a wide range of pH.56
Utilization of gamma radiation graft polymerization was also applied in the preparation of
thermo- and pH sensitive copolymer of polypropylene that was prepared by grafting N-
isopropylacrylamide and acrylic acid onto polypropylene films.53 The researchers observed the
degree of grafting to increase with increasing radiation dose to 20 kGy; above this value the
increase is insignificant, which they reported to be due to increased free radical concentration
resulting in high probability of radical recombination.53 Previously, gamma-induced control release
of clonazepam by radiolysis of poly-D,L-lactide-co-glycolide (PLGA) loaded microspheres was
investigated using matrix electron paramagnetic resonance (EPR) spectroscopy at a vacuum
temperature range of 77-298 K.57 The researchers observed that increasing the radiation resulted in
an increase in the generation of the drug free radicals, while reporting a stabilization of the polymer
matrix in the mixed system with respect to the radiation damage.57
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The tensile strength and stability of starch based thermoplastic was enhanced by
crosslinking with aromatic cinnamyl alcohol using electron beam irradiation.58 The macromolecular
chain scission as a result of the electron beam irradiation was observed to be counterbalanced by
the induced inter-chain covalent linkage bridged by the cinnamyl alcohol leading to a grafted
polymer of superior properties.
4. Ozonization
Ozonization is among the most important aspects in polymer degradation as a result of atmospheric
exposure. Under strained condition, a non-resistance elastomer can be attacked by ozone
concentration as little as 1 ppb.59 Polymer degradation by ozonolysis has been reported to occur
mostly by the cleavage of bonds between sp2 or sp carbon atoms of olefinic polymers.59,60 However,
sp3 carbon-hydrogen bonds of polymers containing labile hydrogen atoms are also attacked but at
much slower rates.59 The ozonolysis mechanism is said to involve such steps like cycloaddition of
ozone to the olenic double bond forming unstable molozonide (Figure 2) which is decomposed to
carbonyl compounds and a carbonyl oxide moieties via cycloreversion process. Lastly, a stereo-
selective cycloaddition occurs as a result of the carbonyl oxide which ips over with the
nucleophilic oxyanion attacking the carbon atom of the carbonyl group resulting in the formation of
peroxidic ozonolysis product.59
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Figure 2 Mechanism of polymer ozonization
Though reactive and degradative to polymer, ozone still finds applications in polymer
modification. Hyaluronic acids especially those of low molecular weight have electron scavenging
and antioxidant activities as well as promotion of excisional wound healing.7 Recently, ozone
treatment was used to prepare low molecular weight hyaluronic acid.7 Using ozonization (Table 1),
the researchers reported a reduction in the native hyaluronic acid molecular weight as high as
94.3% (from 1535 to 87 kDa within 120 min at 40 oC), and they further observed that the
heterogeneous reaction between the gaseous phase ozone and the hyaluronic acid solution affect
the polydispersity of the polymer. Experimental parameters such as reaction temperature, ozone
concentration, media pH, ionic strength and agitation speed are among the factors reported to
influence the ozonization process.7
Ozonization is further reported to be used in the production of organo-conductive
polymers. It has been reported to influence the electrical conductivity of poly[3-pentylthiophene]
lm due to the formation of charge transfer complex.61 However, the conductivity was observed to
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drop significantly with time as a result of oxidative degradation of the polymer. In contrast to this
observation, Nowaczyk, et al.62 reported the use of ozonization to induce both permanent and
temporary increase in electrical specific conductivity of gold sandwich poly(3-pentylthiophene)
polymer. The researchers observed that a permanent increase in conductivity can be induced by
ozonization effect on the polymer morphology caused by grain boundary resistance when the
polymer expands resulting in aggregates of polymer grains, while the temporary increase in
conductivity was observed to be as a result of induced p-doping by ozonization, which takes place
because of a highly electronegativity of the ozone readily captures electron from the delocalized
-orbitals in the polymer backbone forming charge-transfer complexes which operate as excess
charge carriers.62
Polyethylene has been widely used in a number of industrial applications. Unfortunately,
the polymer is characterized by poor dye adhesion to its surface especially in high density
polyethylene. The currently employed chemical surface modification methods such as thermal
oxidation and the use of strong electrical field are costly and time consuming. Surface
functionalization of ultra-high molecular weight polyethylene with oxygen-bearing moieties was
successfully achieved by ozonization process using 10% ozone in oxygen under mild conditions.63
In this process the researchers reported that the ozonization mostly affect the amorphous phase of
the polymer while preserving the crystalline phase. Similar report of polymer surface modification
by ozonization has been reported in styrenic tri block copolymers of elastomeric poly(ethylene-
butylene) capped by polystyrene.64 The researchers observed that prolonged exposure to ozone
induces crystallization that confers higher oxidative thermal stability to the polymer resulting in a
qualitative polymer with a wide range of applications from organo-electronics to bio-patterning.64
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5. Photodegradation The diverse applications of polymers in almost all aspects of human endeavors ranging from
spacecraft down to agricultural irrigation materials exposed the applied polymers to adverse
environmental conditions that warrant research concerning the polymers degradation and
stability. Although some of these polymers are biodegradable, environmental plastic wastes are
usually thermally decomposed. The thermal degradation process is known to be costly and
releases carcinogenic volatile gasses, as such the process is normally discouraged. The basis of
polymer photolysis arises due to the presence of repeating carbonyl groups in polyesters that result
in photochemical cleavage by Norrish-type reaction to degrade the polymer.65-67 The ability of light
photons such as UV and IR to degrade polymers has previously been reported.65, 68, 69
Previously, Tsuji, et al.69 studied the photodegradation of polylactide and polycaprolactone for 200
hours. They reported that UV radiation penetrates the polymer sample without reduction in
intensity irrespective of crystallinity or chemical structure, degrading the polymer via bulk erosion
mechanism. However, they reported the chemical structure adjacent to carbonyl oxygen to play a
role in the photodegradability of the polymer. The effect of UV radiation on PHB has been
reported.70 It is observed that in PHB, UV irradiation degrade the polymer by predominant chain
scission into oligomers that can easily be functionalized (Figure 3) with lesser cross-linking
reactions. They further reported when operating the UV irradiation at temperature higher than that
of polymers glass transition, the crystallinity increases as a result of degraded molecules mobility
in the amorphous region that tends to rearrange themselves by crystallization.70 Klinger and
Landfester71 observed the effect of UV induced degradation on dual stimuli poly(2-hydroxyethyl
methacrylate-co-methacrylic acid) microgels. They reported the degradation rate to depend on
parameters like media pH, the intensity and wave length of the applied irradiation, molecular
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structure of the crosslinking molecules and the overall molecular weight of the copolymer.
Figure 3 Schematic diagram of UV induced progressive chain scission in PHB
The control of protein adsorption onto a polymer surface is known to be a difficult challenge
in biotechnological applications, because of the strong adsorption that is irreversible causing
protein patterning almost impossible to be done72 It has been reported that polymer brushes of
oligo (ethylene glycol) methacrylate showed an exceptional resistance to protein adsorption.
Ahmad, et al.72 used UV radiation of 244 nm to modify poly-oligo (ethylene glycol) methacrylate
under mild condition to break the oligo (ethyline glycol) chain to aldehyde that covalently binds
protein enhancing the patterning process.
Low molecular weight chitosan is reported to increase post-harvest quality of citrus, exhibit
anti-mycotic, antibacterial and anti-carcinogenic activities 73-75. The deacetylation of chitin gives a
chitosan of high molecular weight that has low solubility in aqueous media, and this limits its
industrial applications in many fields 76. Recently, Yue, et al. 76 reported the use of UV to induce
accelerated degradation of chitosan during ozonolization to produce low molecular weight chitosan
that can be used in agro-base, biomedical, cosmetics and food applications.
Silk fiber is reported to have a tensile strength of up to 4.5 GPa and elasticity of about 35%
placing it to be the toughest fiber known to man.77 Silk fibroins were known to be surprisingly
soluble in salt-containing aqueous, aqueous-organic, and organic solvents; unfortunately the
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process is said to be time consuming. Ultra violet radiation was used to generate the silk fibroin in
water within a shorter time yielding fibroin that is biocompatible while at the same time posseses
remarkable physico-mechanical properties that can be used in diverse applications like surgical
sutures, 3D porous sponges and microcapsules.77
6. Thermo-mechanical and oxidative degradation
The polymeric physical and chemical structures are known to influence the thermo-mechanical
properties of the polymer. It has been reviewed earlier that mechanical action induced chemical
changes in polymers.78 Moreover, fermentation substrates especially ammonium cations in
polyhydroxyalkanoates were reported to highly influence the thermo-mechanical degradation of
the polymer.79,80 Hydroxyalkanoic acids (HAs) are reported to have a wide range of industrial
applications such as metal corrosion inhibitors81 and antibacterial agents.82 These hydroxyalkanoic
acids are said to be a chiral building blocks of antibiotic like elaiophylin, pharmaceuticals such as -
lactam, captopril, the hydroxyacly hydrazine in visconsin as well as fungicide like vermuculin and
norpyrenophorin.83 Conventionally, HAs are produced by acid/base hydrolysis or methanolysis of
high molecular weight polyhydroxyalkanoates, a process that mostly results in traces of impurities
within the final products. Thermal degradation is seen as an alternative process due to the
generation of pure hydroxyalkanoic acids. Sin, et al.84 reported the use of moderate high
temperature (160o 190oC) to produce oligomeric hydroxyalkanoic acids from medium-chain-
length polyhydroxyalkanoates. They proposed the degradation mechanism to occur via hydrolytic
chain cleavage initiated at COO-CH-alkyl group.85 The researchers observed a loss of crystallinity
when the PHA is heated at 180o 190oC, attributing the effects to be due to the degradation of the
polymer crystalline phase which causes an increase in the mobility of the degraded polymeric units
as the temperature approaches decomposition point 84. In contrast to this observation, Sadi, et al.70
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reported that increased mobility of the degraded polymeric molecules causes increased
crystallinity because the molecules tends to rearrange themselves by crystallization.
Thermal degradation using microwave irradiation (Table 2) to induce controlled
production of oligoesters (Mn 1000 g mol-1) in polyhydroxyalkanoates has recently been
reported.9 The researchers observed the process to be 100 times faster than the conventional
thermal degradation process and within a very short time (< 15 min). They attributed the faster
rate of degradation to be due to the generated high-efficient internal heating as a result of direct
coupling of microwave energy with the polar molecules leading to carboxyl-ester linkage.9 The
researchers suggested that for effective control of the degradation rate, microwave power should
be modulated while applying simultaneous cooling.
Mechanical ultrasonic degradation of commercially important polymers such as
polystyrene, polybutadiene, polystyrenebutadiene, polyacrylonitrile-butadiene and polystyrene
acrylonitrile has been reported.86 Pinheiro, et al.87 proposed that chain scission is a starting point in
thermo-mechanical degradation of the longer chains of high density polyethylene where their
higher probability of entanglements resulted in macroradicals that can be functionalized. However,
they noticed that the chain scission mechanism to be less prominent in the shorter chains as a
result of their mobility, but rather useful for grafting the macroradicals, thus increasing the
molecular weight.87
Recently, Klukovich, et al.88 studied the pulsed ultrasound mechanical effect on
perfluorocyclobutane polymers (Table 2), which leads to mechanically induced chain scission and
molecular weight degradation via a stepwise mechanism with a 1,4-diradical intermediate yielding
a polymer for localized functionalization and cross linking.88 The high crystallinity and extensive
hydrogen bond within the cellulosic polymer backbone confers to it an advantage of being natural
fiber composite.
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Table 2 Thermo-mechanical and oxidative degradation in polymer modifications
Degradation Polymer Application Reference
method
Pulse ultrasonication
1,4-diradical intermediate for 88
polymer functionalization
Ultrasound fragmentation
Improve crystallinity for 90
biocomposite polymer
Microwave irradiation
Oligoesters for functionalization 9
TEMPO chemo-oxidative
Surface oxidized cellulosic nanofibril 11
degradation
macrofibril
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However, due to the presence of an amorphous region in these naturally occurring
celluloses, their tensile strength is highly limited. Hence, for an efficient application of these
materials, a modification in the polymeric properties especially crystallinity is required. Goodwin,
et al.89 reported the used of ultrasound irradiation to produce a customized molecular weight of
pharmaceutical cellulosic ethers. The rate of the degradation was observed to depend on the type of
polymeric material and the irradiation time.89 Recently, prolonged ultrasound fragmentation is
reported to alter the crystallinity and molecular weight of cellulosic materials.90 Using plant and
bacterial cellulosic samples having a Mw of about 100 and 200 kDa respectively, the researchers
observed a continuous increase in crystallinity index and the reduction in the molecular weight of
the materials within 60 min of ultrasound irradiation to 46 and 47 kDa, respectively. Pectin,
which is a complex heteropolysaccharide commonly found in plant cell walls and middle lamella,
has a wide application in food and pharmaceutical industries as gelling, thickening, texturizing,
stabilizing and emulsifying agents.91 However, the strong resistance of pectin to degradation by
other physico-mechanical processes such as ultrasound irradiation and some mechanical
degradation methods limits their optimal utilization. Chen, et al.91 reported the use of dynamic high
pressure microfluidization (DHPM) to induce a controlled degradation of high-methoxyl pectin. The
researchers observed about 50% reduction in molecular weight by applying a DHPM of 80 MPa at
pH 3.7, whereas increasing the DHPM to 200 MPa at the same pH resulted in the molecular weight
reduction of about 74%. However, changing the pH to a more acidic condition appeared to highly
influence the degradation. For instance, applying 160 MPa of DHPM at pH 1.0 resulted in about 89%
degradation of the methoxyl pectin.91 This reduction in the pectins average molecular weight with
the treatments is attributed to the breakdown of the covalent bond inside the polymer chain.
Chemo-oxidative degradation of cellulose microfibril using 2,2,6,6 -tetramethylpiperidinyl-
1-oxy radical (TEMPO) derivatives and its analogous compound were employed by Iwamoto, et al.11
to produce surface-oxidized cellulosic nanofibrils via hydro-mechanical treatment. On comparing
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the oxidative effect of the TEMPO and its derivatives on wood cellulose surface oxidation, the
researchers observed that both the TEMPO and those analogous compounds of 4-acetamide and 4-
methoxy derivatives showed efficient catalytic surface-oxidation of nanofibrils (>56%) as
compared to those of 4-hydroxy and 4-oxo derivatives (
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The authors acknowledged University of Malaya for grant research PV036/2012A, RG165-11AFR,
UM.C/625/1/HIR/MOHE/05 and RP024-2012A.
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