Factors Controlling the Rate of Photodegradation in Polymers

Chapter 23

Factors Controlling the Rate of Photodegradation in Polymers

Downloaded by STANFORD UNIV GREEN LIBR on August 10, 2012 | http://pubs.acs.org Publication Date: September 23, 2006 | doi: 10.1021/bk-2006-0939.ch023

Bevin C. Daglen and David R. Tyler* Department of Chemistry, University of Oregon, Eugene, OR 97403

The effects of the glass transition temperature and of radical trap concentration on the quantum yields of polymer photochemical degradation were studied. Special polymers with metal-metal bonded units incorporated into the polymer backbone were synthesized in order to investigate these effects because these polymers photodegrade in a relatively straightforward reaction involving metal-metal bond photolysis without complicating side-reactions. Using these polymers, it was shown that when polymers are irradiated above their glass transition temperatures (Tg) their quantum yields of degradation are similar to their quantum yields in solution. When irradiated below their glass transition temperatures, the photochemical degradation reactions are much less efficient. When irradiation takes place above the glass transition temperature there is no dependence of the quantum yields on the radical trap concentration. However, when irradiation occurs below the glass transition temperature, the quantum yields are dependent on the concentration of radical trap. These results are explained in terms of polymer chain mobility. It is suggested that, when irradiation takes place above T , chain mobility is facile enough that a metalradical trap is encountered before metal radical - metal radical coupling occurs. In contrast, when irradiation takes place below T , chain mobility is limited and metal radical - metal radical coupling occurs in many instances before a metal radical encounters a trap. Chain mobility also explains the affect of radical trap concentration on the efficiency of photodegradation. When the irradiation takes place above T g

g

g,

384

© 2006 American Chemical Society

In Degradable Polymers and Materials; Khemani, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

385


the reaction of the metal radicals with radical traps is kinetically saturated with trap at the concentrations of trap used in these experiments. In contrast, when irradiation occurs below the glass transition temperature then, because of limited chain mobility, the reaction kinetics are not saturated in trap concentration and the quantum yields are dependent on the concentration of radical trap.

Introduction Considerable research is being devoted to devising new photodegradable polymers with improved performance because there are compelling economic and social reasons for using degradable plastics in certain applications (1-4). The biggest use for photodegradable plastics is in agriculture, specifically in the burgeoning subdiscipline called plasticulture. In plasticulture, the ground is covered with plastic sheeting (typically a polyolefin), which acts as a mulch to prevent the growth of weeds (thus requiring the use of fewer herbicides), to decrease water demand, and to extend the growing season by keeping the ground warmer. By making these agricultural films out of degradable plastics, considerable labor and money can be saved in the plastics recovery phase of the technique. In the environmental area, photodegradable plastics are finding increased use as packaging materials in items that have a high probability of becoming litter. The idea is that if such materials should end up as litter they will degrade rather quickly and not be an eyesore. There are two basic methods for making polymer materials photochemically degradable (2,3). One method is to chemically incorporate a chromophore into the polymer chains. Although numerous chromophores have been evaluated, the most commercially successful chromophore is the carbonyl group (2,3,5). Absorption of UV radiation leads to degradation by the Norrish Type I and II processes or by an atom abstraction process (Scheme 1), all of which are typical photoreactions of the carbonyl chromophore. Note that once radicals are introduced into the system, chain degradation can occur by the autooxidation mechanism (Scheme 2). The second general method for making polymer materials photochemically degradable is to mix a radical initiator into the polymer. Once carbon-based radicals have formed, the chains degrade by the autooxidation cycle (Scheme 2). Numerous radical initiators have been investigated, and a partial list includes metal oxides (e.g., Ti0 , ZnO, CuO), metal chlorides (e.g., LiCl, FeCl ), M(acac) complexes, M(stearate) complexes, benzophenone, quinones, and peroxides (2,3). 2

n

3

n


386


ο

Scheme L Photochemical Degradation Pathways for Polymers Containing Carbonyl Groups

Initiation Initiator

Ri*

R, ·

+ 0

RiOO-

*

2

+

RjOO'

R-H

RjOOH

+ R-

RjOOH

+ R-

Propagation

R* + ROO ·

0

+-

2

+

R-H

ROO ·

Termination various radical-radical coupling or disproportionation reactions Scheme 2. The Autooxidation Mechanism for Hydrocarbon Materials



387 The ideal photodegradable polymer has (at least) three ideal properties. First, the onset of degradation should be reliably predictable. Although it is obvious why this property is desirable for practical applications, it is noted that it is difficult to predict polymer lifetimes in practice because light intensities vary, as do temperatures and a host of other mechanistic variables that control degradation rates and degradation onsets. Second, the onset of degradation should be tunable. Photodegradable polymers have different applications and each application will generally require different polymer lifetimes. Methods must be found for manipulating polymer lifetimes. Third, the polymer should degrade completely and quickly once degradation starts. This characteristic is important for practical reasons because most polymer mechanical properties are related to molecular weight (6). Small amounts of degradation can drastically decrease the molecular weight (and thus mechanical properties) of a plastic, yet to all appearances the plastic piece is visually unchanged. In essence, the plastic is still present but it is not structurally sound - and hence useless and perhaps dangerous. Under such circumstances, it may as well be completely degraded. In order to predict polymer lifetimes, to control when a polymer starts to degrade, and to control the rate of degradation, it is necessary to identify the experimental parameters that affect polymer degradation rates and to understand how these parameters affect degradation. Among the parameters that have been identified as affecting polymer lifetime are temperature, exposure to ultraviolet radiation, light intensity and wavelength, oxygen diffusion rates in the polymer, tensile stress, compressive stress, chromophore concentration, molecular weight, humidity, and polymer morphology (2-4,7). In this manuscript, we add to this list by reporting that the glass transition temperature and the radical trap concentration also affect polymer photodegradation rates.

Results and Discussion Experimental Approach to the Problem Several challenging experimental problems hinder the rigorous experimental mechanistic exploration of polymer photodegradation. One of the difficulties is that polymer degradations are mechanistically complicated (8). This is not to say that the mechanisms are not understood; in fact, they are understood in detail (8). Rather, the mechanisms are intricate, often involving multiple steps, cross-linking, and side-reactions; this makes pinpointing the effects of stress difficult. Another complication is that oxygen diffusion is the rate-limiting step in photooxidative degradations, the primary degradation mechanism in most polymers (9,10). This can add to the intricacy of the kinetics analysis because cracks and fissures develop in the polymer as



388 degradation proceeds; these fractures provide pathways for direct contact of the polymers with oxygen, which will then no longer degrade at a rate controlled by oxygen diffusion. To circumvent these experimental and mechanistic complexities and therefore make it less difficult to interpret data and obtain fundamental insights, we use three key experimental strategies in our investigations. First, we study the problem using special photodegradable polymers of our own design that contain metal-metal bonds along the backbone (11-16). These polymers are photodegradable because the metal-metal bonds can be cleaved with visible light (eq 1) and the resulting metal radicals captured with an appropriate radical trap, typically an organic halide or molecular oxygen; Scheme 3 (17,18).

L M-ML n

n

—!=->

L M - + *ML n

n

(1) 5

• M L = CpMo(CO) (Cp = η - C H ) , CpW(CO) , n

3

5

5

Mn(CO) , Re(CO) , CpFe(CO) 5

5

3

2

By studying these "model" systems, we are able to extract information without the mechanistic complications inherent in the degradation mechanisms of organic radicals. (For example, metal radicals do not lead to crosslinking, so we can avoid this complicating feature found with organic radicals.) The second key experimental strategy is to use polymers that have built-in radical traps, namely C-Cl bonds (19,20). By eliminating the need for external oxygen to act as a trap, we excluded the complicating kinetic features of rate limiting oxygen diffusion. The third experimental strategy is to use the distinctive M - M bond chromophore to spectroscopically monitor the photodegradation reactions of the polymers. This allows us to compare the efficiencies of the photodegradations by measuring the quantum yields of the reactions. (The quantum yield, Φ, is defined as the rate of a photoreaction divided by the absorbed light intensity; i.e., Φ = rate/absorbed intensity.) The use of quantum yields to quantify and compare the various degradation rates is a crucial advance because polymer degradation reactions have typically been monitored by stress testing, molecular weight measurements, or attenuated total reflection (ATR) spectroscopy (27), all of which can be laborious and time consuming. Relative to these techniques, quantum yield measurements are straightforward. (Note that quantum yields in regular carbon-chain polymers cannot be measured conveniently by UV-vis spectroscopy because there are generally no suitable chromophores.) To further expedite our quantum yield measurements, we use a computerized apparatus that automatically measures the quantum yields on thin film polymer samples (22).



389

Λ Λ Λ Λ

'Μ~Μ

Γ ν Λ Λ

Μ-Χ

+

χ-Μ

Α / ν υ

Μ-Μ'

ν Λ Λ / υ

Scheme 3. Photochemical reaction of a polymer with metal-metal bonds along its backbone.

Polymer synthesis strategy Our general synthetic route for incorporating metal-metal bonds into polymer backbones is based on the step polymerization techniques for incorporating ferrocene into polymer backbones (6,23-28). Step polymers of ferrocene can be made by substituting the cyclopentadienyl (Cp) rings with appropriate functional groups, followed by reaction with appropriate difiinctional organic monomers (e.g., eq 2) (29-31).

Ο Ο Il II CIC—R—CCI

CHoCH^OH

ι

Fe

Ql li

^^^—CH CH 0-R--C 2

2

1

Fe (2) n


390


The analogous strategy for synthesizing metal-metal bond-containing polymers also uses difunctional, cyclopentadienyl-substituted metal dimers. A sample polymerization reaction is shown in eq 3, which illustrates the reaction of a metal-metal bonded "diol" with hexamethylene diisocyanate (HMDI) to form a polyurethane (13). This step polymerization strategy is quite general, and a number of metal-metal bond-containing polymers have been made from monomers containing functionalized Cp ligands (11,32).

g . ι C°

Q, C

HOCH CH 2

Ν

2

0

Μ ο

_ 4 -/

/y C *t)

HMDI dibutyltin diacetate (cat.) p-dioxane,26 C

\ ^^-CH,CH,OH

0

Ο C -hOCH CH 2

2

\

Ο it CH CH OCNH(CH ) NHC-

V, __' Mo -Mo 0

2

2

2

6

Jn

(3)

Synthesis of the PU-XX Polymers Using the synthetic strategy in eqs 2 and 3, the polymers for this study were synthesized by the route shown in Scheme 4. Note that the amount of Clcontaining aromatic diisocyanate was varied, which gave polymers with different glass transition temperatures as well as polymers that have different metal-radical trap to metal atom ratios. For example, PU-90 has a T of 35 °C and a 9:1 [C-Cl]:[Mo] ratio, and PU-70 has a T^of -44 °C and a 7:1 [C-Cl]:[Mo] ratio. (The X X number in the PU-XX nomenclature indicates the mole fraction of aromatic diisocyanate in the overall amount of diisocyanate used in the formulation.) g

Photodegradation Occurs in the Absence of 0 . 2

As expected, polymers containing C-Cl bonds photochemically degraded in the absence of oxygen. Spectroscopic monitoring of the reactions showed the disappearance of the Cp Mo (CO) chromophore (λ^χ = 390 and 510 nm; 2

2

6


391

CH C\ 2

Ο CHi

Ο CH

• N - C -f O C H C H 4 O - C - Ν 2

CH

OCN


3

3

NCO

NCO

90% 70%

10% 30%

PU-90 PU-70

O H H O Il I I II -C-N-R'—N-C—O-R-04-

-

°cc°

o

CH C\ 2

Ο R* -

CH —ά 3

Ο

^N~C4-OCH CH4O-C-N~

Factors Controlling the Rate of Photodegradation in Polymers

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