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Microstructure changes of extruded ultra high

molecular weight polyethylene after gamma irradiation

and shelf-aging

F.J. Medela, F. Garca-A lvarezb, E. Go mez-Barrenac, J.A. Pue rtolasa,)

aDepartment of Materials Science and Technology, Centro Politecnico Superior de Ingenieros, Universidad de Zaragoza,

Calle Mar a de Luna 3, 50018 Zaragoza, SpainbDepartment of Orthopaedic Surgery, Hospital Lozano Blesa, Zaragoza, Spain

cDepartment of Orthopaedic Surgery, Hospital Fundacion Jimenez Daz, Universidad Autonoma de Madrid, Madrid, Spain

Received 16 September 2004; accepted 27 November 2004

Available online 2 February 2005

Abstract

Ultra high molecular weight polyethylene (UHMWPE) components in joint prostheses may undergo oxidative degradation in the

long term. This leads to material embrittlement, associated with a subsurface defect named white band that favours surface

delamination and premature implant failure. The polymer microstructure, which affects the mechanical properties of the material, is

altered by oxidative degradation. In this study, the microstructure of bar-extruded UHMWPE (GUR 1050) was compared before

irradiation, one week after 25 kGy gamma irradiation in air (GUR 1050), and 7 years after commercial (25e40 kGy) gamma

irradiation in air and shelf-aging (GUR 415). This was performed using transmission electron microscopy (TEM) and differential

scanning calorimetry (DSC) which help to characterize the changes in material microstructure and crystallinity during degradation.

Quantitative (lamellar thickness), semiquantitative (lamellar twisting, orientation, lamellar density), and qualitative (lamellarborder, stacking) parameters were recorded in the surface, subsurface and inner material. There was an evolution in the degradation

in the bar-extruded UHMWPE microstructure during irradiation and shelf-aging. Changes in the in-depth microstructure indicate

the progression of an oxidation front. The stacking or high lamellar concentration in the subsurface in shelf-aged samples was

coherent with higher crystallinity and with a degradation process controlled by oxygen diffusion and free radical distribution after

gamma irradiation.

2005 Elsevier Ltd. All rights reserved.

Keywords: UHMWPE; Microstructure; TEM; DSC; Degradation

1. Introduction

In the past decades, ultra high molecular weight

polyethylene (UHMWPE) has been the preferred

interposition material for joint prostheses. Its micro-

structure, based on the extremely high molecular weight

(4e6! 106 g/mol), confers optimal mechanical proper-

ties such as high wear and fatigue resistance and highfracture toughness. Those properties, along with a low

friction coefficient and high biocompatibility, make

UHMWPE the perfect candidate to articulate against

metallic alloys or ceramics [1]. However, wear and

degradation can limit the in vivo service and duration of

the reconstructed joint system [1,2].

The UHMWPE microstructure includes an amor-

phous region, with entangled long molecular chains, and

a crystalline region, with thin lamellar crystals or

) Corresponding author. Tel.: C34 976762521; fax: C34

976761957.

E-mail address: [email protected] (J.A. Pue rtolas).

0141-3910/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2004.11.015

Polymer Degradation and Stability 88 (2005) 435e443

www.elsevier.com/locate/polydegstab
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lamellae. Both regions are separated by a narrow border

which is considered a part of the lamella. Tie molecules

interconnect the regions by crossing the amorphous

region from one lamella to another. The tie molecules

and entanglements in the amorphous phase are what

help to provide the excellent mechanical properties of

this material [3,4].The material has usually been irradiated for sterili-

zation (between 25 and 40 kGy), but this has a negative

effect on the entanglement density of long molecular

chains, as well as the concentration of tie molecules [5].

Molecular chains undergo scission and free radicals

appear in the amorphous region and on the crystal

surfaces. The free radicals can take part in the cross-

linking of the material or oxidative degradation in the

presence of oxygen [5,6]. In the amorphous region, free

radicals disappear by recombination or by reacting with

oxygen since the molecular chain is more mobile and

oxygen easily diffuses in this phase. However, in the

crystalline regions of UHMWPE, radicals are long

lasting [7]. They diffuse towards the amorphous region

at a slow rate and participate in different stages of

oxidation. Finally, UHMWPE also contains degrada-

tion products such as ketones, acid groups, and esters,

as shown by FTIR [8].

Natural aging of UHMWPE on the shelf and in vivo

cause maximum subsurface degradation (between 1 and

2 mm under the surface) [9]. The oxidation causes

embrittlement of the material and loss of mechanical

properties. As a result, research in orthopaedics has

focused on the physico-chemical characterization of the

degradation. Many studies have been performed usingdensity measurements, calorimetry, and FTIR [10e22].

However, it is still unclear how aging changes the

microstructure and how it affects degradation profiles.

Advances in this area could help to explain how

UHMWPE loses its mechanical properties.

We analysed the microstructural variations due to

irradiation and aging in bar-extruded UHMWPE.

Differential scanning calorimetry (DSC) and transmis-

sion electron microscopy (TEM) studies were performed

to determine depth-dependent changes in microstruc-

tural characteristics of non-irradiated material, freshly

gamma irradiated in air, and shelf-aged gamma

irradiated in air commercial components. We propose

a model of microstructural changes based on our

findings and previous reports in the literature.

2. Materials and methods

2.1. Materials

The polyethylene resins were bar-extruded Ticona

GUR 1050 processed by a commercial converter (Perplas

Medical Ltd.), and components manufactured from

bar-extruded Hoechst GUR 415 or 4150 according to

the new nomenclature [2]. The experimental samples

made from the resins were classified into three different

groups.

The first group was from three pucks (10 mm thick),

machined from a bar of extruded GUR 1050. Three

sections were cut from the centre of each puck witha microtome, at 0.1e0.3 mm, 1 mm, and 3 mm from the

surface. Samples were prepared for DSC and TEM tests

from every 200 mm thick section.

The second group consisted of three pucks machined

from extrusion bars of GUR 1050 UHMWPE, and then

gamma irradiated in air at conventional sterilization

dosage (25 KGy; Aragogamma, Spain). Again, three

sections were extracted from the centre of each puck

(0.1e0.3 mm, 1 mm, and 3 mm from the puck surface).

Those sections were used for DSC analysis and TEM,

and tested one week after the irradiation process.

The third group corresponded to naturally shelf-aged

Hoechst GUR 415 material, obtained from three tibial

components of knee prostheses that had never been

implanted (Natural Knee system, manufactured by

Intermedics Orthopedics, Austin, TX, in 1991). They

were discarded for clinical use after being stored for 7

years on the shelf after sterilization. All prostheses had

a wide subsurface white band in sections. On the back of

the component we cut 200 mm sections at different

depths to obtain a surface group (0.1e0.3 mm deep),

a white band or subsurface group (1 mm deep), and an

inner material (below white band, 3 mm deep). The

samples of each section were divided in two parts, one

for DSC measurements and another for TEM.

2.2. Transmission electron microscopy

Subsequent sample preparation for TEM included

different phases. First, 200 mm sections of UHMWPE

were stained with 99% chlorosulphonic acid at 60 C

for 5 h, which is thought to stabilize the amorphous

regions. Samples were washed with acetone (at 0 C)

and rinsed with distilled water. Stained samples were

dried at 60 C for 1 h and later embedded in epoxy and

cured at 60 C for 2 days. Ultrathin sections (w60 nm

thick) were cut with a diamond knife and collected on

carbon grids. Then the sections were post-stained with

uranyl acetate in 1% methanol for 4 min. A Jeol 100CX

TEM (operating at 100 kV) was used to produce

micrographs at 20,000! and 60,000! magnifications.

The best 5 TEM preparation samples were chosen from

each group and analysed using the Digital Micrograph

3.3.1 software package (Gatan Inc., Pleasanton, CA,

USA).

Morphological variables were selected by image

analysis and evaluated quantitatively or semiquantita-

tively. These variables assessed lamellar properties

(thickness, boundaries, kinking) and interlamellar

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features (distance or spacing, chaotic distribution,

concentration, stacking). Some were ranged on a

semiquantitative scale of one dot (minimum) to three

dots (maximum). Lamellar thickness and interlamellar

distance in stacked lamellae were measured at least five

times per 60,000! images using the software.

2.3. Differential scanning calorimetry

A PerkineElmer DSC-7 was used for the calorimetric

analysis. Samples were heated from 25 to 180 C at a rate

of 10 C/min. Their mass ranged from 3e4 mg. Fusion

heat was obtained by integrating the area under the

endothermic peak from 80 C to 160 C. Melting

temperature was registered as the maximum of the

endothermic melting peak. In addition to the direct

parameters, sample crystallinity was calculated by com-

paring the fusion heat of the sample to the fusion heat for

a fully crystalline polyethylene material (290 J/g) [17]. An

estimation of the lamellar thickness, Lc, was obtained

from DSC data using the ThomsoneGibbs equation:

TmZTm0

1 2s=LcrcDH0m

1

where Tm is the melting point of the polymer, Tm0 is the

equilibrium melting point of a perfect crystalline poly-

ethylene, s the specific surface energy, rc the crystallinity

phase density, and DH0m the enthalpy of melting of

a perfect crystalline polyethylene. Lamellar thickness

was estimated by considering DH0mZ 290 J/g, Tm0Z

145.7 C, rcZ 1.005 g/cm3, and taking a constant value

for sZ

93! 10

7

J/cm

2

[18].

3. Results

3.1. Calorimetric analysis

The thermogram of the ram-extruded material was

fairly typical, with a strong endothermic peak (Fig. 1a).

As seen in Table 1, non-irradiated specimens had

a melting temperature of 135.5G 1.1 C, an enthalpy

content of 140G 12 J/g, a crystallinity percentage of

49G 4%, and an estimated lamellar thickness (using the

ThomsoneGibbs equation) of 27G 3 nm. In the first

group, there were no significant differences between

specimens from different depths.

Gamma-irradiated, one week shelf-aged UHMWPE

samples had a higher melting temperature (137.3G

0.4 C) and crystallinity (53G 1%; Table 1). Lamellar

thickness was also higher (32G 2 nm) than virgin

UHMWPE. As in non-irradiated samples, there were

no significant differences with depth in the DSC results.

A typical thermogram of irradiated UHMWPE is shown

in Fig. 1a, along with the thermogram of virgin

UHMWPE.

Finally, the thermal values of samples from aged

tibial components varied with depth. Fig. 1b shows the

typical thermograms of the three regions. Samples from

the subsurface (where the white band was present), had

the highest crystallinity (71G 3%), and an increased

melting temperature (139.8G 1.2 C), as well as higher

lamellar thickness (48G 9 nm). In contrast, surface

specimens had the lowest enthalpy content, melting

temperature (137.1G 0.1 C), and lamellar thickness

(32G 1 nm). Inner samples, which were already near the

wide white bands of the prostheses, had an enthalpy of

190G 1 J/g, crystallinity percentage of 65G 1, melting

temperature of 140.1G 1.0 C and lamellar thickness of

50G 9 nm. The direct parameters obtained from the

Fig. 1. (a) Thermograms of virgin, non-irradiated, UHMWPE, and

gamma-irradiated (25 kGy) plus one week shelf-aging UHMWPE. (b)

Thermograms of the three regions (surface, subsurface or white band,

and inner or below white band) on gamma-irradiated (25 kGy) plus 7

years of shelf-aging prostheses.

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DSC experiments and the crystallinity percentages, as

well as the estimated Lc values for these three material

groups are also presented in Table 1.

3.2. Transmission electron microscopy

The structure of extruded, non-irradiated GUR 1050

UHMWPE was a semicrystalline polymer. The crystal-

line region included lamellae that appeared as narrow

white ribbons that were more or less curved, with sharp

boundaries (Fig. 2a). Zones among the lamellae denoted

the amorphous region. The boundary between lamellae

and amorphous region was so thin that it was hard to

perceive. Lamellar thickness was between 25e30 nm. All

these key variables are shown in Table 2.

The lamellae in the TEM images of gamma-irradi-

ated, one week shelf-aged UHMWPE (Fig. 2b) were less

contrasted than non-irradiated polymer (Fig. 2a).Lamellar thickness averaged 35 nm, which is clearly

thicker than virgin UHMWPE. A slight decrease in the

lamellar concentration could be deduced from the

20,000! images. The sharpness of the lamellar bound-

aries also decreased. There were no significant changes

in the other variables.

There were marked differences between the irradi-

ated, 7-year shelf-aged samples and the non-irradiated

material. As shown in Fig. 3, the lamellae were thicker in

the irradiated, 7-year shelf-aged group than in the virgin

specimens, and the boundaries were broader and clearly

visible. On the other hand, there were some differences

in the depth of the shelf-aged samples. In the surface

specimens, lamellae were kinked and their distribution

was chaotic. In the white band specimens, lamellae

appeared straightened and formed stacks in many areas.

In the latter, interlamellar distances ranged between

25e30 nm (measured between adjacent lamellae).

Lamellar concentration or density in the white band

(Fig. 3b) was much higher than at the surface. Finally,

some features of the inner samples (Fig. 3c) were similar

to the subsurface, but with no significant stacking. Table

2 includes the results of the quantitative and qualitative

microstructural parameters for all the materials.

4. Discussion

Many studies have analysed the changes in UHMWPE

after irradiation and natural aging [10e22,24e26].

Table 1

Enthalpic contents, % crystallinity, melting temperature data and lamellar thickness estimations based on the ThomsoneGibbs equation of non-

irradiated (virgin), gamma-irradiated and one week aged, and gamma-irradiated and 7 years aged extruded UHMWPE

UHMWPE Enthalpic

contents

(J/g)

% Crystallinity

(ref. 290 J/g)

Melting

temperature

( C)

Lamellar

thickness

(nm)a

Virgin 140G 12 49G 4 135.5G 1.1 27G 3

GammaC 1 week 154G 4 53G 1 137.3G 0.4 32G 2GammaC 7 years

Surface 169G 5 57G 2 137.1G 0.1 32G 1

Subsurface 206G 8 71G 3 139.8G 1.1 48G 9

Inner region 190G 1 65G 1 140.1G 1.0 50G 9

a The values for s and rc are mentioned in the text.

Fig. 2. TEM images (60,000!) of virgin ram-extruded UHMWPE (a),

and gamma-irradiated (25 kGy) UHMWPE after one week aging (b).


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However, although variation in-depth profiles in terms of

physico-chemical properties (density, oxidation index,

crystallinity, etc.) are well documented, little is known

about microstructural depth changes and its relationship

with physico-chemical changes. On the other hand, DSC

thermograms and TEM images suggest an in-depth

material distribution in the 7-year shelf-aged UHMWPE

but not in the freshly irradiated UHMWPE or virgin

material. In view of these findings, a global model of

structural evolution after gamma irradiation in air can be

proposed to discuss our results in relation with the

literature. This model would rely on three consecutive

stages.

4.1. First stage (virgin UHMWPE)

This stage relates to virgin UHMWPE. Its micro-

structure can be considered a semicrystalline polymer

with crystalline and amorphous phases. The crystalline

phase consists of folded long molecules added to thin

orthorhombic crystals, named lamellae. Those lamellae

are randomly oriented and separated by highly en-

tangled polymer molecules, which form the amorphous

phase. Some tie molecules connect different lamellae

through the amorphous phase and improve the me-

chanical properties of UHMWPE quite remarkably

[4,25,28]. These well-known features are represented in

Fig. 4a.

In the TEM images, typical white ribbons are easily

identified as randomly oriented lamellae, immersed

within a dark region corresponding to the amorphous

phase. Lamellar boundaries appeared to be sharp,

probably because there was only primary crystallization

during the processing of the extruded bars, and fold

surfaces of the lamellae lie almost parallel to the electron

beam of the TEM [29]. Several studies support our

results [4,23,25,29e32]. Regarding the physico-chemical

properties at this stage, crystallinity values are

reportedly around 50% [27], depending on the process-

ing route, resin, molecular weight, etc. There is no

detectable oxidation and the density is about 0.93 g/cm3

[1,14]. According to the DSC thermograms, crystallinity

was 49G 4% and melting temperature was

135.5G 1.1 C, as found in previous studies [1,2].

Kurtz and Schmitt point out that the crystallinephase of UHMWPE consists of folded long molecules,

added to thin orthorhombic crystals (10e50 nm thick,

and 10e50 mm long). Lin and Argon [33] also report

similar features for polyethylene single crystals obtained

from a solution, with an orthorhombic structure

(5e25 nm thick, and 1e50 mm long). Our estimated

Lc, using the melting temperature data and the

ThomsoneGibbs equation was 27G 3 nm, in agree-

ment with measured Lc values on TEM images of virgin

UHMWPE (compare Tables 1 and 2).

4.2. Second stage (low degradation)

At low doses, gamma or electron beam irradiation

causes the radiolytic scission of molecular chains by

cleavage of CeC and CeH bonds, producing different

free radicals. Since irradiation affects both crystalline

and amorphous regions, the mobility of the free radicals

is different. Depending on their location and mobility,

they can undergo recombination (leading to cross-

linking, branching, and the formation of double bonds),

stabilize, or react with incoming oxygen [5]. Irradiating

in the absence of oxygen will increase cross-linking and

decrease crystallinity [10,18,26]. It also decreases above

100 Mrad since the crystals are seriously damaged [24].However, at low and medium doses in air, crystallinity

increases after gamma [10,25,26] or electron beam

[16,22,24] irradiation. In those cases, the degradation

process (via oxygen) is more probable, favouring

oxidative chain scission as oxygen molecules react with

free radicals. Recrystallization processes are favoured by

oxidative reactions, and crystallinity will increase.

In the proposed microstructure for this stage, the

crystallinity and lamellar thickness are higher than in the

virgin samples. In addition, lamellar boundaries are less

sharp, darker and wider in TEM images (see Fig. 4b).

Our DSC and TEM results on gamma-irradiated

UHMWPE confirm the increase in crystallinity. One

week after irradiation, the estimation for UHMWPE

crystallinity, melting temperature and lamellar thick-

ness increased by 8%, 2 C, and 5 nm, respectively

(Table 1). According to the TEM images, the lamellae

were thicker (32e38 nm), than virgin UHMWPE.

Although our thermodynamic results agree with

other reports, there was an apparent contradiction in

the lamellar size. Bhateja et al. [16] and Premnath et al.

[22] found that in the first hours after beta irradiation,

UHMWPE crystallinity and melting temperature in-

creased but there was no change in lamellar thickness, as

Table 2

The results of a semiquantitative analysis of lamellar variables

Key features Virgin

UHMWPE

Gamma

C 1

week

GammaC 7 years

Surface Subsurface

(white band)

Inner

region

Thickness (nm) 25e30 32e38 32e38 30e35 32e38

Border/frontier No Yes Yes Yes YesTwisting Spacing (nm) 30e35 35e40 45e50 25e30 35e40

Chaotic

appearance

Orientation Density of

lamellae

Stacking No No No Yes No

In the qualitative variables the scale ranges from one dot () minimum,to three dots () maximum.


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detected by SAXS measurements. In our samples,

lamellar thickness increased one week after gamma

irradiation in air.

These findings can be explained by the Thomsone

Gibbs equation (Eq. (1)) and the relation provided by

Bershtein and Egorov [34] for the surface enthalpy of the

lamellae qi, (Eq. (2)), which relates this thermodynamicproperty to lamellar thickness and total melting

enthalpy, DHm:

qiZDH0m DHm

Lc=2:5 2

where DH0m is again the melting enthalpy of a perfect

crystalline polyethylene. The surface enthalpy, s, and qiare related by the expression sZ qi Tmsi, where Tm isthe melting point of the polymer and si the entropy of

the polymer [34].

In the first hours after irradiation, the increase of

DHm in the virgin sample does not correspond to an

increase in the degree of crystallinity, since the lamellarsize remains constant. According to Eq. (2), the increase

is associated with a decrease in surface enthalpy, qi. On

the other hand, the positive shift of the melting

temperature can only be related to a decrease in the

free energy surface, s. Both changes are attributed to the

radiolytic scission of tie molecules, which subsequently

rearrange into the crystals improving the surface of the

original lamellae. However, the increase in melting

temperature, enthalpy, and lamellar size could be related

to the beginning of a secondary crystallization at the

surface. An indication of this process is the change in

the lamellar boundary. As seen in the TEM images, theboundaries are less sharp. After the secondary crystalli-

zation, the surface folds of the lamellae are less parallel to

the TEM electron beam than in the virgin samples [13].

The low degradation level associated with this stage

can be explained within the general framework of

UHMWPE degradation, which is controlled by the

diffusion of oxygen and free radicals after irradiation.

The oxidation process requires that oxygen and radicals

generated during irradiation coincide in space and time.

The theoretical simulation of oxygen diffusion into

UHMWPE, along with the time evolution of the spatial

radical concentration [35] implies that the surface

degradation of up to 0.5e1 mm deep remains basically

constant (not time dependent). However, the subsurface

is strongly degraded with physical and chemical changes

after more than 5 years post-irradiation [13,19,23,35].

This is why the thermodynamic parameters and the

microscopic features in the 7-year old samples were

similar to the ones after one week of natural aging.

Fig. 3. TEM image (60,000!) of gamma-irradiated (25 kGy) plus 7

years of shelf-aging prostheses. Surface zone (a), subsurface or strong

degradation zone corresponding to the white band (b), and inner or

medium degradation zone below the white band (c).


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Both situations can be considered as two imagescorresponding to a stage of low degradation. There was

an apparent increase in lamellar tortuosity at the surface

after long shelf-aging (see Fig. 4c), which is mainly due to

oxygen accumulation at the lamellar boundaries [32].

4.3. Third stage (strong degradation)

The microstructure in the third stage is linked to the

oxidative degradation process that generates the white

band defect. This introduces important changes in

material properties such as density, molecular weight,

crystallinity degree or toughness. As widely accepted,

the changes only occur after irradiation in air followed

by an incubation period of more than 4e5 years.

Our microscopic model for this stage includes

lamellar thickening. The concentration is high with

predominant stacking of lamellae that almost lie straight

and parallel (see Fig. 4d). There are also small

crystallites within the amorphous region. The model is

based on the results from DSC and TEM from highly

degraded material (see Fig. 1b and Fig. 3b) and data

from the literature.

The DSC results in the white band region imply

an increase in crystallinity and melting temperature after

7 years of shelf-aging. The shoulder in the DSCthermograms (Fig. 1b) may reflect the presence of new

crystallites produced by chain scission processes that

generate shorter molecular chains. As a result, new

crystallization can occur in the amorphous region with

crystals that are shorter than the original lamellae.

Generalized chain scission also produces an overall

decrease in tie molecules, allowing for lamellar rap-

prochement and reordering [31], which increases me-

chanical brittleness [21,36] and melting temperature, as

shown by DSC (see Fig. 1b).

This small crystallite production is supported by

previous findings of a decrease in the interlamellar space

at SAXS and associated with the shoulder of the DSC

thermograms [22]. Partial recrystallization in the amor-

phous region has also been proposed [17]. Other studies

of crystallinity and melting temperatures confirm an

increase in fusion heat after some months of aging,

mostly with high radiation dosage [22]. The conven-

tional dosage of 25 kGy produces a mild increase in

fusion heat in the early months, which proceeds at

a lower rate with aging time [10e11]. Later on, fusion

heat continues to increase, almost inadvertently in the

first year [10,31,32], but quite noticeably after 5 years

[9e11,21].

Fig. 4. Schematic drawings of UHMWPE microstructure evolution after irradiation and shelf-aging: (a) first stage in virgin UHMWPE, (b) freshly

irradiated UHMWPE, (c) second stage in the surface region after 7 years shelf-aging UHMWPE, and (d) third stage in the white band area.


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Finally, the findings in the subsurface region (related

to high degradation), were basically similar to the deeper

region, with some differences. Whereas the melting

transition occurred at the same temperature (140 C),

neither the enthalpy content nor the TEM images were

similar. According to the micrographs, the lamellae were

more tortuous, less oriented, and not clustered. On theother hand, the incipient shoulder in the DSC thermo-

grams and the lower value of crystallinity could indicate

the lack of new crystallization, which would have

produced the small crystals in the subsurface region.

However, both thermal aspects suggest that the inner

region is in a medium degradation stage. This behavior

is supported by Blanchet and Burroughs [35], who found

that the combination of oxygen diffusion and radicals

concentration explains the presence of carbonyl groups

in the inner region (close to the subsurface). Their

density is higher than at the surface but lower than at

the white band, which corresponds to the highest

degradation region.

5. Conclusions

Both irradiation and post-irradiative degradation of

UHMWPE cause thermal and microstructural changes.

There was an increase in crystallinity in UHMWPE just

after gamma irradiation in air. As expected, lamellar

crystals appeared to thicken soon after gamma irradi-

ation. Lamellar thickness seemed to remain almost

constant after irradiation and during shelf-aging, as

shown by TEM. As degradation proceeded, crystallinityincreased at the subsurface region, whereas the surface

regions had relatively low crystallinity contents due to

the oxygen diffusion front and the specific spatial radical

concentration at the surface. Subsurface areas had high

crystallinity contents and lamellar stacking. In addition,

shoulders in thermograms of subsurface regions denote

the crystallization of new small crystals in highly

degraded zones. The lower enthalpy content and the

absence of stacking at the inner region indicate that this

region is immersed in a low degradation stage,

compared to the subsurface.

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