Degradation and Stability of Polymers:
The Role of Aggregation Structure
and a New Stabilization Method
Guangxian Li
College of polymer science and engineering, State key laboratory of polymer
materials engineering, Sichuan University, Chengdu 610065, China
NIST/UL Workshop on Photovoltaic Materials Durability Dec 12-13, 2019
Outline
Introduction
Effects of aggregation structures on the degradation:
▪ Crystallization
▪ Orientation
▪ Phase domain
New stabilization method
Outdoor degradation behaviors of different
polymers under typical climatic regions in China5
Degradation of polymers
⚫ There are two critical factors governing the degradation of polymers:
(1) Permeability of environmental medium (Light, oxygen, solvent etc.)
(2) formation of free radicals
6
Introduction
1. Diffusion of environmentalmedium
2. Formation of free radicals
Degradation(Free radical chain reactions)
UV, Heat, O2, Solvent…
Polymer chains Free radicals
4
Major efforts so far focused on the elimination of free radicals by using anti-aging additives
Smith, Accounts of Chemical Research, 2018, 51, 2006; Gijsman, Polymer Degradation and Stability, 2017, 145, 2
⚫ To prevent the degradation of polymers, small-molecule stabilizers are
widely used which act by interrupting in the free radicals reaction cycle
✓ Alkyl radicals
• Hindered Amine Stabilizers (HAS)
O O✓ Peroxy radicals
• Primary antioxidant (phenolic antioxidant)• HAS
➢ Scavenge radicals
➢ Forestall hydroperoxides
• Secondary antioxidant(trivalent phosphorous, thioethers)
• HAS
O OH✓ Hydroperoxide
Is there any way else to control the degradation?block the channel of permeability of environment factors
5
➢ Structuring of aggregation of polymers
⚫ Crosslinking⚫ Crystallization
Lin, Macromolecules 2005; S. Zekriardehani, Macromolecules 50(7) (2017) 2845-2855. K. Bandzierz, Materials 9(7) (2016) 607.
⚫ Orientation of polymer chains
Orientated
0.95
0.85
Density (g/cm3)
Crystallinity(%)
Free volume radius (nm)
5 0.35
27 0.33
Crosslinking density x 104
(mol/cm3)
Free volume radius (nm)
0 0.34
3.4 0.32
6
⚫ Adding barring fillers in polymer matrix ; or producing multiphase materials
✓Phase separated structure
Dispersed Co-continuous
✓Nano sheet filler (Graphene)
O2 size: 0.30 nm; Mesh size: 0.28 nm
⚫ Aggregation structures might be used for governing the diffusion of
environmental factors, by changing the available space for diffusing
channel
Phase size: 100 nm – 100 μm
Adv. Mater. 2010, 22, 4759; Nanoscale, 2010, 2, 373
Case 1:Crystallized polymers
7
8
Controlling the permeability by adjusting crystallization (crystallinity, lamellae size and crystal forms)
⚫ Crystalline structures would decline the diffusion probability of environmental
factors, which play critical roles in the degradation of polymers
⚫ Crystallinity Highercrystallinity
Diffusion channel
⚫ Crystal forms✓ α spherulites (~100 um) ✓ β sheet (~10 um)
⚫ Lamellae sizeThicker and
denser of lamellae
dc dc
60 80 100 120 140 160 180 200
Tempreature (oC)
12 h
24 h
36 h
0 h
0.5 h
2 h
6 h80 oC
En
do
therm
ic
60 80 100 120 140 160 180 200
100 oC
Tempreature (oC)
12 h
24 h
36 h
0 h
0.5 h
2 h
6 h
En
do
therm
ic
60 80 100 120 140 160 180 200
140 oC
12 h
24 h
36 h
0 h
0.5 h
2 h
6 h
Tempreature (oC)
En
do
the
rmic
60 80 100 120 140 160 180 200
120 oC
12 h
24 h
36 h
0 h
0.5 h
2 h
6 h
Tempreature (oC)
En
do
the
rmic
120 140 160 180
150 oC
Tempreature (oC)
En
do
the
rmic
0 h
0.5 h
2 h
6 h
12 h
24 h
36 h
⚫ Annealing induced secondary crystallization: increase in crystallinity; thickening of crystals
Fig. 1 DSC melting curves at different annealing temperature and time
1. iPP: Annealing changes the crystallinity and lamellae size
➢ Effects of annealing time on crystal structures at differenttemperatures
Q Liu, GX Li, Polymer Degradation and Stability, 2019, 162, 18012
0 6 12 18 24 30 36
0.35
0.40
0.45
0.50
0.55
Cry
sta
llin
ity
Annealing time (h)
80 oC
100 oC
120 oC
140 oC
150 oC
⚫ Crystallinity increases with increasing annealing time due to the
secondary crystallization, which becomes stable after 24 h
⚫ Primary melting temperature does not change too much
➢ Effects of annealing time on crystal structures
0 6 12 18 24 30 36155
160
165
170
Tm (
oC
)
Annealing time (h)
80 oC
100 oC
120 oC
140 oC
150 oC
Melting temperatures of the primary melting peaks
13
Fig. 2 Crystallinity and Tm vs. annealing time
⚫ Increasing temperature will enhance crystallinity
remarkably (up to 55%)
11
➢ Effects of annealing temperature on crystal structure
120 140 160 180
Tempreature (oC)
En
do
the
rmic
Unannealed
80 oC
100 oC
120 oC140 oC150 oC
Annealing at differenttemperatures for 24 hours
0.40
0.45
0.50
0.55
Cry
sta
llin
ity
Unannealed 80 oC 100 oC 120 oC 140 oC 150 oC
Annealing for 24 hours
Annealing temperature
Fig. 3 DSC melting curves at different annealing temperature, and crystallinity vs. annealing temperature
➢ Effects of annealing temperature on lamellae size
0 5 10 15 20 25 30-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0 unannealed
80 oC-24 h
100 oC-24 h
120 oC-24 h
140 oC-24 h
150 oC-24 h
unaged
K(z)
z (nm)0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
I(q
)q2
q (nm-1)
Annealed for 24 h
Fresh
80 oC
100 oC120 oC
140 oC
150 oC
2
2
( ) cos( z)(z)
( )
I q q q dqK
I q q dq=
⚫ Annealing induced SAXS curves shift to lower q, the thickening
of lamellae
Fig. 4 SAXS curves and corresponding correlation function at different annealing temperature
15
Long period L = 2π/q
⚫ Annealing leads to the thickening of lamellae up to ~ 16 μm
13
4
6
8
10
12
14
16
L (
nm
)
Unannealed 80 oC 100 oC 120 oC 140 oC 150 oC
➢ Effects of annealing temperature on lamellae size
Fig. 5 Lamellae size calculated based on SAXS data vs. annealing temperature
14
➢ Effects of annealing on storage modulus
⚫ Annealing enhances the storage modulus, resisting deterioration in
mechanical properties induced by polymer degradation
Fig. 6 (a) Storage modulus vs. annealing temperature measured by DMA
2200
2300
2400
2500
2600
2700
2800
2900
Sto
rag
e m
od
ulu
s G
' (M
Pa
)
G
Polynomial fitting
Unannealed 100 oC 120 oC 140 oC 150 oC80 oC-120 -80 -40 0 40 80 120
0
500
1000
1500
2000
2500
3000
neat
80 oC
100 oC
120 oC
140 oC
150 oC
S
tora
ge
mo
du
lus
G' (M
Pa
)
Temperature (o
C)
⚫ Annealing reduces the entanglement of polymer chains and the
concentration of polymer segments in amorphous phase, leading
to the decrease of Tg15
-120 -80 -40 0 40 80 1200.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Temperature (o
C)
Tan
neat
80 oC
100 oC
120 oC
140 oC
150 oC
Tg (
oC
)
22
24
26
Tg
Temperature (o
C)15014012010080neat
Tg
➢ Effects of annealing on glass transition temperature
Fig. 6 (b) Glass transition temperature vs. annealing temperature measured by DMA
Polynomial fitting
16
2
4
6
Pre
-ex
po
ne
nti
al
fac
tor
P: Permeability (10-14
cm3 cm/cm
2s Pa)
S: Solubility (10-7
cm3/cm
3 Pa)
D: Diffusivity (10-7cm
2/s)
Permeability reduced by 53 %
P = S x D
⚫ Oxygen permeability of annealed samples is reduced significantly by
the increase of crystallinity and lamellae thickness
➢ Effects of crystallization on the oxygen permeability
Fig. 7 Permeability, solubility and diffusivity vs. annealing temperature, measured by gas permeability tester using film samples with thickness of 500 μm
17
0 10 20 30 40 500
3
6
9
12
15
18
21
24
27
To
tal
rad
ica
l c
on
ce
ntr
ati
on
UV exposure time (min)
Unannealed
Annealed at 150 oC for 24 h
➢ Effects of crystallization on the concentration of free radicals
⚫ Concentration of free radicals decreased significantly after annealing
Fig. 8 Concentration of free radicals measured by Electron Paramagnetic Resonance (EPR) under UV light
18
⚫ Annealing reduced the concentration of carbonyl group by up to 70 %
• Formation of C=O during UV degradation
➢ Effect of crystallization on the oxidation of PP
0 30 60 90 120 150 180 210 240
0
5
10
15
C
=O in
dex
Aging time (h)
Unannealed
80 oC
100 oC
120 oC
140 oC
150 oC
Reduced by 70 %when annealed at 150 oC
Fig. 9 C=O index vs. photooxidation time measured by FTIR
➢ UV caused damage of surface morphology
⚫ Annealing suppressed the formation of microcracks after UV exposure
19
Unexposed(0 h)
Photodegraded(192 h)
UnannealedAnnealed at 120 oC (24 h)
Annealed at 150 oC (24 h)
200 μm
APC(%)
unannealed 27.1
120 oC 24.0
150 oC 21.0
Fig. 10 Surface morphology of annealed samples after photooxidation measured by SEM
Table 1 Area percentage of cracks (APC)
➢ Effect of UV irradiation on the melting point after annealing
⚫ The better crystal structure had the better effects in preventing
the UV degradation
20
0 50 100 150 200 250
130
140
150
160
170
Aging time (h)
Tm (
oC
)
Unannealed80 oC
100 oC120 oC
140 oC150 oC
The drop of Tm during photo-oxidationis reduced by 50 % when annealed at 150 oC
Fig. 11 Tm vs. photooxidation time for unannealed and annealed samples
21
Annealing dc
➢Increases crystallinity
➢Increases lamellae
dimension
dc
Permeable channel of oxygen, H2O, UV, … Block the permeable channel of oxygen
➢Lower oxygen permeability
➢Lower oxygen solubility
cry
⚫ Higher crystallinity and thicker lamellae size of annealed PP lowers the
oxygen permeability, and enhances its resistance to photo-oxidation
22
2. iPP: Using β-nucleating agent crystal forms
➢ α β
α-iPP,crystallinity = 48 %
β-nucleated iPP (β-iPP) (0.05 wt.% β-NA), crystallinity = 49 %
⚫ β-sheet crystals have a better fine structure and higher density,
which reduced the oxygen permeability
Large spherulites with an average diameter of ~100 μm
Size of β-crystal reduced significantly to ~10 µm
50 µm 50 µm
Fig. 12 Polarized optical images of α-iPP and β-PP
YD Lv, GX Li, Polymer Testing, 2013, 32, 179
23
⚫ With a similar crystallinity (48 - 49%), β-PP has much lower
oxygen permeability; it is very effective in controlling the
diffusion channel of oxygen
➢ Effects of crystal form types on oxygen permeability
0
2
4
6
8
10
12
P: Permeability (10-14
cm3 cm/cm
2s Pa)
S: Solubility (10-7
cm3/cm
3 Pa)
D: Diffusivity (10-7cm
2/s)
Pre
-exp
on
en
tia
l fa
cto
r
Reduced by 68 %
P = S x D
α-PP β-PPFig. 13 Permeability, solubility and diffusivity for α-PP and β-PP, measured by gas permeability tester using film samples with thickness of 150 μm
24
⚫ β-iPP has much lower carbonyl concentration ascribed to its
better oxygen barrier ability
➢ Effects of crystal form types on oxidation (C=O)
Similar degradation products
Thermal oxidation, 100 oC
Fig. 14 C=O index for neat PP and β-PP during thermal oxidation
Fig. 15 FTIR curves for neat PP and β-PP during thermal oxidation
α-iPP
β-iPP
25
➢ Mechanisms of the size of crystals on thedegradation(the higher density and lower permeability)
✓ α spherulites ✓ β sheet
➢Lower oxygen permeability
and solubility
β-nucleating
agent
➢Formation of β
crystal form
Reduce the diffusion channels of oxygenDiffusion channel of oxygen
⚫ Tiny and small diffusion channels of β-PP lower the oxygen
permeability, which enhances its resistance to thermal-oxidation
26
3. PC: Solvent induced surface crystallization
➢ Preparation of surface-crystallized PC
➢ Condensed states on surface to hinder diffusion channels
O2, H2O, UV, etc. O2, H2O, UV, etc.
➢ How to construct a denser barrier layer: crystallization?
YL Zhou, GX Li, Polymer Degradation and Stability, 2013, 98, 1465
Soaked in acetonefor 1 min
Acetone, 25 oC
to remove residual acetone on the surface 80 oC
• Amorphous PC • Surface crystallized PC
Zoom in
27
⚫ Hydrophobicity of PC is enhanced obviously with the
surface crystallization
➢ Effects of surface-crystallization on the hydrophobicity
Fig. 14 Cross-sectional images and contact angle for neat PC and surface-crystallized PC
Crystallinity=4.76 %
28
➢ Effects of surface-crystallization on the hydrothermaloxidation stability
⚫ Enhancement in the hydrophobicity of surface crystallized PC
inhibits the hydrolysis
0 10 20 30 400.80
0.85
0.90
0.95
1.00 Amorphous
Surface crystallized
Mw/M
w0
hydrolytic time (day)
Retention of molecular weight
0 10 20 30 400.0
0.2
0.4
0.6
0.8
1.0
1.2 Amorphous
Surface crystallized
Elo
ng
ati
on
at
bre
ak
re
ten
tio
n
hydrolytic time (day)
Retention of elongation at break
Neat PC Surface crystallized PC
Reduction in C=O
(Hydrolysis for 36 days)33.9% 23.9%
Fig. 15 Retention of molecular weight and elongation at break vs. hydrolytic time for neat PC and surface-crystallized PC
Table 2 Reduction in concentration of C=O group (hot water 65 oC )
Case 2:Orientated polymer
29
30
➢ Effect of drawing orientation on crystal structures of PA6
⚫ Polymer drawing increases the orientation
degree and crystallinity of PA6
Drawing ratio (%) Crystallinity (%) Orientation factor
0 39.2 -
200 45.0 0.17
400 45.4 0.18
600 53.6 0.21
Crystalline morphology of etched surface
DR = 0%
DR = 600%
PA6: Orientation prevented the hydrothermal oxidation
Fig. 16 DSC melting curves and crystal morphology of PA6 and orientated PA6
Table 3 Draw ratio against crystallinity
KH Shi, GX Li, Journal of Thermal Analysis and Calorimetry, 2016, 126, 795
31
DR
(%)
D
(mm2 s-1)
0 9.89 x 10-5
200 2.40 x 10-5
400 1.02 x 10-5
600 0.71 x 10-5
⚫ With increase of drawing ratio, water absorption decreases ;
leading to lower coefficient of water diffusion
Contact angle
DR = 0 %
DR = 600 %
➢ Effect of drawing orientation on the hydrophobicity of PA6
Fig. 17 Water absorption curves of PA6 in hot water at 85 oC
Table 4 Draw ratio against coefficient of water diffusion
32
⚫ Polymer orientation suppresses the cracking and yellowing of
PA6 during hydrothermal oxidation
Fig. 18 SEM and photographs of PA6 samples after immersing in hot water (85 oC) for 60 days
0 d
60d
0 day --> 60 day
Orientated0% 600%400% 200%
➢ Effect of drawing orientation on surface cracking
33
➢ Effect of drawing orientation on tensile strength
⚫ The orientation increases the retention of mechanical
properties during hydrothermal oxidation
hot water (85oC)
Fig. 19 Retention of tensile strength vs. hydrolytic time
34
➢ Effect of drawing orientation on molecular weight
⚫ Polymer orientation suppresses the decline of molecular weight
during hydrothermal oxidation
hot water (85oC)
DR(%)k1
(103· day-1)
0 10.04
200 8.94
400 6.36
600 3.98
• Auto-catalyzed hydrolytic
Fig. 20 Intrinsic viscosity vs. hydrolytic time
Table 5 Draw ratio against Hydrolytic rate k1
35
➢Lower water absorption
➢Higher hydrophobicity
Mechanical drawing
➢Orientation
➢Increase in the
crystallinity
Diffusion channel of waterBlock the diffusion channel of water
⚫ Higher crystallinity and orientation degree of orientated PA6 lowers its
water absorption, produced better resistance to hydrothermal oxidation
Case 3:Adding barrier filler into polymer matrix
36
Compton, et al. Advanced Materials, 2010, 22, 4759-4763; Galano. Nanoscale, 2010, 2, 373–38037
1. Adding oxygen-barrier fillers in polymer matrix
Two dimensional plane graphene nanosheet
Mesh size: 0.28 nm, O2 size: 0.296 nm Carbon materials with conjugated double bond
CNT, C60, et al.
⚫ Graphene sheets possess great potential in improving the
thermal-oxidative stability of polymers
➢ Graphene hindered two critical factors of degradation:
(1) blocking oxygen diffusion (2) scavenging free radical
38Yang JL, Li GX. Journal of Materials Chemistry A, 2013, 37, 11184
(1) Preparation of chemically reduced graphene oxide (crGO)
(2) Preparation of composites
K2MnO4
H2SO4, NaNO3
crGOGraphite powders
N2H4·H2O, NH3·H2O
Graphene oxide (GO)
98ᵒC, 3 hours
(a modified Hummers method)
180ᵒC, 10 min
Melt blending
PPR02 (crGO, 0.2 wt.%)
Ultrasonic dispersing Solution mixing
Vacuum drying
Neat PP
crGO + iPP (T30s)
Master batch(crGO, 5 wt.%)
PPR05 (crGO, 0.5 wt.%)
PPR10 (crGO, 1.0 wt.%)
PPR20 (crGO, 2.0 wt.%)
PPR36 (crGO, 3.6 wt.%)
130 oC
➢ Preparation of crGO/polymer composites
➢ Characterization of crGO
⚫ crGO is successfully synthesized
TEM
XRD
39
AFM
XPS
GO rGO
Fig. 21 The AFM, XRD and XPS results of GO and crGO
➢ Oxygen permeability and radical scavenging ability of GO
40
7.9
5.1
Po
2 (1
0-1
4 c
m3
cm
/cm
2s
Pa
)
7.9
5.1
0 40 80 120 160
0
10
20
30
40
50
R2= 0.98308
y = 0.16479x - 0.3098
Scaven
gin
g A
cti
vit
y (
%)
Concentration (g/ml)
crGO
Barrier effect Radical scavenging (DPPH)
⚫ PP/crGO composites have much lower oxygen permeability and
higher radical scavenging ability
Reduced by 35%
Neat PP PP/crGO
Fig. 22 Oxygen permeability and radicals scavenging ability of GO and crGO. Radical scavenging ability was measured using DPPH (2,2-diphenyl-1-picrylhydrazyl), which is a stable free-radical molecules.
200 250 300 350 400 450 500 550
0
20
40
60
80
100
120
We
igh
t (%
)
Temperture (C)
neat PP
PPR02
PPR05
PPR10
PPR20
PPR36
Ti (C)(b)
➢ Thermal-oxidative stability of PP/crGO composites
⚫ crGO suppresses the oxidation of PP in air atmosphere
41
Air
Thermal degradation curves
240
270
300
330
T
i (o
C)
Onset thermal degradation temperature (Ti)
Fig. 23 Thermal degradation curves and thermal degradation temperature of PP/GO and PP/crGO composites
Neat PP PPR02 PPR05 PPR10 PPR20 PPR36
42
➢ Physical mixture of GO/antioxidant (AO) in PP
Preparation of iPP/GO/AO composites
Phenolic antioxidant (3,5-di-tertbutyl-4-hydroxycinnamic acid)
(To avoid the complexity of system , no surface modification or compatibilizer is used)
(a) Preparation of GO-coated PP pellets (b) Preparation of GO/iPP composites
Ultrasonic dispersing Solution mixing
Vacuum drying
GO or GO/AO
Coated iPP
iPP (free of AO)
180ᵒC, 10 min
Melt blending
Neat PP
PP/AO (100/0.5)
PP/GO (100/1.0)
PP/AO/GO (100/0.5/1.0)
Coated iPP
AO:
⚫ Interaction between AO and GO caused a better dispersion
⚫ Oxygen barrier of GO and radical scavenging ability of both GO
and AO could generate much better resistance to the degradation
Yang JL, Li GX. Carbon 2015, 89, 340-349; Journal of Materials Chemistry A, 2013, 1: 11184
Po
2 (1
0-1
4 c
m3
·cm
/cm
2·s
·Pa
)
43
⚫ Interaction between AO and GO caused better dispersion of
GO, which leads to synergistic effects: better oxygen barrier
+AO
➢ Dispersion and oxygen permeability
Better dispersion of GO in iPP Lower oxygen permeability of PP/GO
Reduced by 47 %
Neat PP PP/AO PP/AO/GO
Fig. 24 Dispersion status of GO and oxygen permeability of PP/GO and PP/AO/GO composites
PP/GO
7.97.1
5.1
4.2
44
10 15 20 25 30 35 40 45 50 55
Heat
Flo
w (
a.u
) E
nd
o
Time (min)
OIT(min)
⚫ By incorporating mixture of AO/GO: (0.5 wt. % / 1.0 wt. %),
OIT and Ti of PP were improved significantly.
Thermal weight loss curvesOxidation induction time (OIT) measurements
100 150 200 250 300 350 400 450
0
20
40
60
80
100
Under Air
We
igh
t (%
)
Temperature (C)
Neat PP 241
PP/GO 258
PP/AO 252
PP/AO/GO 278
Samples Ti (oC)
Increased by 37 oC
➢ Thermal-oxidative stability of PP/AO/GO composites
Fig. 25 Oxidation induction time curves and thermal degradation curves of of PP/GO and PP/AO/GO nanocomposites
PP0.8 min
PP/GO1.2 min
PP/AO18.4 min
PP/GO/AO38.4 min
Three-point bending clamp
T=50oC
Strain=0-4%
⚫ Aging at constant strain
⚫ Aging at constant stress
Component phr
PVC 100
Lead sulfate tribasic 2
Lead phosphite dibasic 1
Stearic acid (HSt) 0.5
Photo-oxidation experiment
⚫ High-pressure mercury lamp
⚫ 500 W (55W/m2 @365nm)
⚫ T=50±5oC
⚫ Aging time: 20 d, sampling for every 5 d
Formulation of PVC material
45
2. Opposite example: to produce a diffusion channel by making surface microcracks
46
➢ Stress caused microcracks on the surface of PVC
Without stress With stress
⚫ Deeper microcracks are formed in stressed samples, facilitating
the diffusion of oxygen
300x
3000x
Strain=2%
Aging time: 20 days
Fig. 26 Surface morphology of PVC before and after degradation
50 μm50 μm
5 μm 5 μm
47
⚫ Without UV irradiation, there is no mass loss in stressed sample
⚫ After stress induceed cracks, UV leads to more rapid mass loss than
the case only UV is involved
0 2 4 6 8 10
0
2
4
6
8
10
12
aged+strain
aged+non-strain
unaged+strain
Mass lo
se (
%)
Time (h)
➢ Mass loss of PVC sample under UV exposure
Fig. 27 Mass loss vs. photodegradation time
Without UV
UV/without microcracks
UV/microcracks
Fig. 28 Color of sliced sample at different depths (irradiated for 20 days)
⚫ The color change caused by chromophore is depth dependent
⚫ The stressed samples show a photobleaching effect: darkening
color happened only in the surface layer
➢ Appearance of sliced samples
48
⚫ Stress promotes the oxidation
of C=C bonds and lowers the
concentration of double bonds
0 5 10 15 20
3.0
3.5
4.0
4.5
To
tal
do
ub
le c
arb
on
bo
nd
s (
10
-5m
ol/g)
Irridiation time (day)
no strain
2% strain
49
➢ Total C=C bonds (n=3~8) by UV-Vis analysis
⚫ C=C bonds reach a maximum at
subsurface layer due to the
competition between oxidation
and generation of double bonds
0 100 200 300 400 500
2
3
4
5
6
7
To
tal d
ou
ble
ca
rbo
n b
on
ds
(10
-5m
ol/g)
Depth from the exposed surface (m)
unstrain
2% strain20 days
Fig. 29 Total double carbon bonds vs. irradiation time and depth (irradiated for 20 days)
50
⚫ Stressed samples have higher content of carbonyl groups
0 100 200 300 400 500
0
2
4
6
8
10
C=
O in
dex
Depth from exposured surface (m)
no strain
2% strain
20 days
➢ C=O index by FTIR
Fig. 30 C=O vs. depth (irradiated for 20 days)
0 5 10 15 20
1.5
2.0
2.5
3.0
3.5
Im
pact
str
en
gth
(KJ/m
2)
Irridiation time (day)
no strain
2% strain
0 1 2 3 418
20
22
24
26
28
Imp
ac
t s
tren
gth
(k
J/m
2)
Strain (%)
51
⚫ stressed samples have high oxidation-induced degradation, resulting
in lower impact strength
➢ Impact strength
Fig. 31 Notched impact strength vs. irradiation time Fig. 32 Notched impact strength vs. strain after 20 days
Case 4:New stabilization methods:
novel antioxidant
52
53
❑ Synthesis of chemically grafted hindered phenol (HP) with GO
(1) GO + SOCl2 Activated GO
(2) Activated GO + HP GO-HP
• Two steps for the synthesis
(1) Grafting of HP improves thedispersion of GO, which is muchbetter than the physical mixtureof GO and AO
(2) GO can immobilize HP, whichsuppresses the migration of HP(This is a urgent problem for thereliability of long-term service ofpolymer )
Synergistic effects?
54
⚫ PP/GO-HP has much lower oxygen
permeability
⚫ GO-HP has excellent radical
scavenging ability
➢ Oxygen permeability of GO-HP
Lowermagnification
Highermagnification
GO-HPGO
GO-HPGO
1 μm 1 μm
200 nm200 nm0
5
10
2.95
4.91
6.53
8.10
9.48
Po
2 (
×10
-14 c
m3·c
m/c
m2·s
·Pa)
PP
PP/HP
PP/GO
PP/GO/H
P
PP/GO-H
P
Reduced by 70%
Fig. 34 TEM images, oxygen permeability and EPR curves for PP, PP/GO, PP/HP and PP/GO-HP nanocomposites
Disappear
➢ Photo-oxidation of PP/GO-HP composites
⚫ GO-HP is much more effective to suppressed photo-oxidation than
that of the physical mixture
0 25 50 75 100 125 150 175
0
5
10
15
20
25
30
35
40
Raw PP
PP/GO/HP
PP/GO-HP
ca
rbo
ny
l in
de
x
Aging time (h)
(a)
0.0
0.4
0.8
1.2
1.6
(b)
Mw (10
5 g
/mo
l)
Before aging
After aging for 168 h
Raw
PP
PP/GO
/HP
PP/GO
-HP
Molecular weightCarbonyl index
Reduced by 84%
PP/GO-HP
Fig. 35 Carbonyl index and molecular weight for PP, PP/GO, PP/HP and PP/GO-HP nanocomposites experiencingthe same thermal processing procedure after UV degradation.
58
➢ Thermal oxidation of PP/GO-HP composites
5 10 15 20 25 30 35
8.54 min
19.96 min
4.71 min
1.33 min
0.37 min
Time (min)
Heat
Flo
w (
a.u
) E
nd
o
PP
PP/HP
PP/GO
PP/GO/HP
PP/GO-HP
Oxidation induction period (OIT) at 180 oC
⚫ Addition of HP-GO improves the thermal-oxidative
stability remarkably
Onset thermal degradation temperature (Ti)
0 100 200 300 400 500
0
20
40
60
80
100
PP
PP/HP
PP/GO
PP/GO/HP
PP/GO-HP
(a)
Ma
ss
(%
)
Temperature (oC)
Air
239Ti (oC)
Increase 51 oC
246246258290
Fig. 36 Oxidation induction period and thermal degradation curves for raw PP, PP/GO, PP/HP and PP/GO-HPnanocomposites
59
➢ Extraction resistance of GO-HP
⚫ PP/GO-HP effectively reduces the migration and loss of small-
molecule antioxidant (long-term stable application)
0
5
10
15
20
OIt
(m
in)
Before extraction Extracted for 276 h
PP/H
P
PP/G
O/H
P
PP/G
O-H
P
4.71
1.64
8.54
5.51
19.96 19.83(a)
OIT before and after extraction
60
Fig. 37 Oxidation induction period and retention rate for PP, PP/GO/HP and PP/GO-HP nanocomposites after extraction
➢ Summary of the novel anti-oxidant of GO-HP
60
⚫ .
⚫ The grafting HP with GO possessed a high scavenging- radicals
ability, strong oxygen barrier property , and excellent anti
migration effect which was a major and urgent problem for small
molecular antioxidants.
⚫ It has much better anti-oxidation effect than that of GO or HP
used individually, or that of the physical mixture of GO/HP used
Case 5:Outdoor degradation behaviors of different polymers under typical
climatic regions in China
59
◼ QH (Hot-wet)
◼ GZ (sub-humid hot)
◼ RQ (Hot-dry)
◼ QD (Warm temperate)
◼ HLR (Cold-temperate I)
◼ LS (Cold-temperate II)
plateau: Lowest oxygen
concentration and highest
irradiation
*The classification of climatic types is according to China national standard GB4797 and GB4796-198460
➢ The 6 typical climate regions in China
Guangzhou (Warm damp II) Hailar (Cold I) Qingdao (Warm)
Qionghai (Warm damp I) Lhasa (Cold II, plateau) Ruoqiang (Warm dry)
➢ Outdoor weathering stations
6110
20
30
RQ QH GZ LS QD HLR
Temperature (oC)
0.01
0.02
Oxygen pressure (MPa)
RQ QH GZ LS QD HLR 0
2500
5000
7500
10000
Irradiation (MJ/m2)
RQ QH GZ LS QD HLR0
20
40
60
80
100
Humidity (%) Rainfall (mm)
RQ QH GZ LS QD HLR0
1000
2000
3000
62
Irradiation (MJ/m2) Relative humidity (%) Temperature (oC) Oxygen content (g/m3)
En
vir
on
me
nta
l m
ed
ium
QH GZ QD HLR RQ LS
Solar irrdiation
Relative humidity
Temperature
Oxygen content
➢ Weathering features for different outdoor stations
63
⚫ Experimental samples: PE, PP, PVC, PS
⚫ Engineering plastic: PA6, PC, ABS
⚫ Coating: Acrylic ester coating
⚫ Rubber: SBR
➢ Samples for field exposure
➢ Characterization techniques:
UV-Vis for yellowness index, FTIR for chemical changes, GPC for
molecular weight, DSC, WAXS and SAXS for crystal structures, SEM for
surface morphology, tensile tests for mechanical properties
⚫ Yellowing and darkening of PVC samples occur during aging
➢ Color
64
Fig. 38 Digital images showing the color changes vs. aging time for PVC aged under different outdoor climatic conditions
➢ Appearance by SEM
⚫ Microcracks perpendicular to the injection direction develop during aging
65
Fig. 39 SEM morphology images vs. aging time for iPP aged under different outdoor climatic conditions
66
0
10
20
30
40
50
Tensile
str
en
gth
(M
Pa
)
Aging time (month)
QH
GZ
RQ
LS
QD
HLR
0 1 3 6 12 18
(a)
➢ Mechanical properties
⚫ The mechanical properties of iPP deteriorate with exposure time
Fig. 41 Tensile strength, flexural modulus and elongation at break vs. aging time for iPP
67
0.0
0.5
1.0
1.5
2.0
2.5
(a)
MW
(×1
0-5 g
/mo
l)
Aging time (month)
QH
GZ
RQ
LS
QD
HLR
0 1 3 6 12 18
➢ Chemical structure
⚫ Molecular weight of iPP drops gradually with increasing aging time
Fig. 43 Molecular weight vs. aging time for iPP
Predict the lifetime of polymers in different regions
Establish a standard method to evaluate the aging
degree of polymer materials at different regions
Two key issues
72
1. Establish the measuring standard and map the regional distribution
10.80.60.40.20
Regional distribution
Establishing the relationship between normalized physical quantity Pand aging time t.
Quantitatively converting aging index to intuitive map
P/P0
t
1
tend
73
➢ The aging degree of PE with time at different regions Elongation at break data of PE at different regions
0 month 6 months 12 months
18 months 24 months30 months
⚫ With increase of aging time, the aging degree of PE aggravated; aging
of PE under heat–dry and humid heat is the severest74
➢ Comparison of elongation at break with yellowness index of PE
⚫ The sensitivity of different aging indicators of PE is quite different
Elongation at break mapping Yellowness index mapping
(Aging time: 18 months)
75
72
(Aging time: 18 months)
PP
PC PA6
SBR
PVC
➢ The aging degree maps of different raw polymers
⚫ The aging degree of different types of raw polymers
varies from sites to sites
Processed with thermal stabilizer and plasticizer
• Elongation at break maps
73
0 200 400 600 800
0
40
80
120
160
QH
GZ
RQ
LS
QD
HLR
CI3000+
Carb
onyl in
dex
Aging time (day)0 200 400 600 800
0
1
2
3
QH
GZ
RQ
LS
QD
HLR
CI3000+
Mw(،1ء0
-5 g
/mol)
Aging time (day)
2. Predict the lifetime of iPP based on correlation betweenindoor and outdoor exposure➢ Comparison of degradation mechanism between indoor and outdoor
conditions
Fig. 44 Carbonyl index and molecular weight vs. aging time for iPP aged underaccelerated condition and outdoor conditions
YD Lv, GX Li. Polymer Degradation and Stability, 2015, 112, 145
74
0.0 0.1 0.2 0.3 0.4 0.5
0.0
0.3
0.6 QH
GZ
RQ
LS
QD
HLR
CI3000+
Absorb
ance a
t 1713 c
m-1
Absorbance at 1720 cm-1
0.0 0.1 0.2 0.3 0.4 0.5
0.0
0.3
0.6
0.9
Absorb
ance a
t 1735 c
m-1
Absorbance at 1720 cm-1
QH
GZ
RQ
LS
QD
HLR
CI3000+
0.0 0.1 0.2 0.3 0.4 0.5
0.0
0.4
0.8
QH
GZ
RQ
LS
QD
HLR
CI3000+
Absorb
ance a
t 1780 c
m-1
Absorbance at 1720 cm-1
⚫ Accelerated weathering and outdoor weathering show similar carbonyl
product concentration ratio, thus a similar degradation mechanism
➢ Comparison of the ratio between different degradationproducts
1713 cm-1: Carboxylic acid1720 cm-1: Ketone1735 cm-1: Ester1780 cm-1: Lactone
Fig. 45 Correlation between different degradationproducts including carboxylic acids, esters as welllactones ketones for all exposure conditions.
75
Ea activation energy, R gas constant, p and q are coefficients related with the materials, p=0.5~1, q=~1
➢ A three-parameter equation: considering temperature,irradiation and oxygen concentration
Schwarzschild law(Irradiance)
pk BI=
Temperature (T) + Irradiation (I) /a TE Rpk a eI−
=
Linear law( oxygen concentration)
qk CO=
Temperature (T) + Oxygen content (O) /a TE Rqk b eO−
=
Arrhenius equation(Temperature): /aE TRk Ae
−=
( ) ( )aE
q
ITOT
pRk a eI O−
=Temperature (T) + Irradiation (I) + Oxygen content (O)
2
1
2 12 2
1 1
1 1a
T T
p qE
ITO R
ITO
ITO
I O
I O
ke
k
−−
= =
Acceleration factor:
76
0 200 400 600 800 1000
0
40
80
120
160
Carb
onyl in
dex
Aging time (day)
Outdoor-QH
Prediction based on accelerated results (x8.2)
0 200 400 600 800 1000
0
40
80
120
160
Carb
onyl in
dex
Aging time (day)
Outdoor-GZ
Prediction based on accelerated results (x10.3)
0 200 400 600 800 1000
0
40
80
120
160
Carb
onyl in
dex
Aging time (day)
Outdoor- RQ
Prediction based on accelerated results (x10.6)
0 200 400 600 800 1000
0
40
80
120
160
Ca
rbo
nyl in
dex
Aging time (day)
Outdoor-LS
Prediction based on accelerated results (x20.5)
0 200 400 600 800 1000
0
40
80
120
160
Ca
rbo
nyl in
dex
Aging time (day)
Outdoor-QD
Prediction based on accelerated results (x16.4)
0 200 400 600 800 1000
0
40
80
120
160
Carb
onyl in
dex
Aging time (day)
Outdoor-HLR
Prediction based on accelerated results (x30.0)
⚫ Satisfactory predictions based on three-parameter Arrhenius
equation are obtained
➢ Lifetime prediction at 6 different sites: carbonyl index
Fig. 46 Comparisons between the outdoor results and the predictions based onthe accelerated laboratory results for carbonyl index.
77
0 200 400 600 800
0
1
2
3
Outdoor-QH
Predcition (x8.2)
Mw(x
10
-5 g
/mol)
Aging time (day)
0 200 400 600 800
0
1
2
3
Outdoor-GZ
Predcition (x10.3)
Mw(x
10
-5 g
/mol)
Aging time (day)
0 200 400 600 800
0
1
2
3
Outdoor-RQ
Predcition (x10.6)
Mw(x
10
-5 g
/mol)
Aging time (day)
0 200 400 600 800
0
1
2
3
Outdoor-HLR
Predcition (x30.0)
Mw(x
10
-5 g
/mol)
Aging time (day)0 200 400 600 800
0
1
2
3
Outdoor-QD
Predcition (x16.4)
Mw(x
10
-5 g
/mol)
Aging time (day)
0 200 400 600 800
0
1
2
3
Outdoor-LS
Predcition (x20.5)
Mw(x
10
-5 g
/mol)
Aging time (day)
➢ Lifetime prediction at 6 different sites : molecular weight
Fig. 47 Comparisons between the outdoor results and the predictions based onthe accelerated laboratory results for molecular weight
⚫ Predictions based on three-parameter Arrhenius equation are satisfactory
78
Conclusions
⚫A method to map the regional distribution of aging degree of different polymers
was established, and a three-parameter lifetime prediction model based on
Arrhenius equation was successfully constructed, which was well verified by the
outdoor data
⚫Controlling the diffusion channel by structuring the aggregations is an efficient
way to prevent degradation of polymers. It can effectively block the diffusion
channels for aging factors (O2, H2O, solvent etc.), which resist the photo- and
hydrothermal oxidation process.
⚫A new stabilization route for polymers was developed by grafting antioxidants
onto functionalized graphene, which is better than traditional anti-aging agents
in preventing polymer from degradation.
Acknowledgement
• National Natural Science Foundation of China (NSFC,
51133005, 50533080)
• Foundation for Innovative Research Groups of the NSFC
(51421061)
• Dr. Yajiang Huang, Yadong Lv, Junlong Yang
• Graduated students Qiang Liu, Shixiang Liu, Jialu Yao
79
Thanks for your attention !
80
Prediction with and without considering the oxygen
effects
81
1000 1500 2000 2500 3000 3500 4000
C-O (ester)
-C(CH3)3
COOH
C=O (ester)
-C(CH3)3
Ab
so
rba
nce
(a.u
.)
Wavenumber (cm-1)
GO
GO-HP
HP
1.277 nm0
1.6 μm0.996 nm0 1.6 μm
(a) (b)
FTIR
AFM
⚫ Successfully prepared
GO-HP
282 284 286 288 290
Inte
nsit
y (
a.u
.)
(b) GO-HP
Binding Energy (eV)
Raw
Peak sum
C-C, C=C
C-O
C-S
C=O
COO
➢ Characterization of GO-HP
Fig. 33 FTIR curves, XPS curves and AFM heightimages for GO-HP.