degradation and stability of polymers: the role of ... · 1/15/2020 · degradation and stability...

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.%)





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


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


⚫ 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


0 200 400 600 800 1000

0

40

80

120

160

Carb

onyl in

dex

Aging time (day)

Outdoor- RQ


0 200 400 600 800 1000

0

40

80

120

160

Ca

rbo

nyl in

dex

Aging time (day)

Outdoor-LS


0 200 400 600 800 1000

0

40

80

120

160

Ca

rbo

nyl in

dex

Aging time (day)

Outdoor-QD


0 200 400 600 800 1000

0

40

80

120

160

Carb

onyl in

dex

Aging time (day)

Outdoor-HLR


⚫ 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.

degradation and stability of polymers: the role of ... · 1/15/2020 · degradation and stability...

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