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A: Spectroscopy, Photochemistry, and Excited States

Photoinduced Reactions of Benzophenone in Biaxially Oriented Polypropylene Peter P. Levin, Alexei F. Efremkin, Alexey Vladimir Krivandin, Sergei Michael Lomakin, Olga V. Shatalova, and Igor Vladimir Khudyakov J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01483 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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The Journal of Physical Chemistry

Photoinduced Reactions of Benzophenone in Biaxially Oriented Polypropylene

Peter P. Levin,a,b Alexei F. Efremkin,b Alexei V. Krivandin,a Sergei M. Lomakin,a Olga V. Shatalova,a and Igor V. Khudyakovc†* a

Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, 119334, Russia

b

Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991, Russia

c

Department of Chemistry, Columbia University, New York, NY 10027, USA

ABSTRACT: The photoinduced reactions of benzophenone (B) in biaxially oriented polypropylene (BOPP) were studied with nanosecond laser photolysis (N2 laser, λ337.1 nм). The first observed transient was a triplet state 3B*. Decay of 3B* led to formation of a radical pair (RP) of BH• and R•, where R• is a radical formed by hydrogen abstraction from BOPP (RH) by 3B*. We studied BOPP after the pre-heating for a short time in a temperature range 298 - 423 K, which is essentially lower than its melting point of 453К. All measurements with not-heated and with pre-heated (annealed) BOPP were made at 298 K. A radical pair (RP) apparently decays as a contact pair 3[BH•, R•] in non-heated BOPP. A critical phenomenon takes place: dissociation of RP with a formation of free radicals in the polymer bulk is observed at pre-heating temperature Tcrit ≈ 403 K and at a higher T. The physical process of heating and cooling of BOPP apparently resulted in the restructuring of crystallites, their agglomeration, shrinking of the distribution of crystallites according to their sizes in BOPP. Overall BOPP becomes softer which manifests itself in the radical kinetics. The decay kinetics of 3B* and RP in the cage fits well the first-order law. Rate constants were obtained. Radicals BH•, which exits into the polymer bulk at temperatures of pre-heating T ≥ 403 K decay ACS Paragon Plus Environment

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by cross-termination according the second-order law. A relatively high rate constant ~ 108 M-1.s-1 for this reaction was obtained due to diffusion of BH• enclosed in the soft amorphous phase of BOPP. Properties of BOPP containing B were studied with ESR, DSC, IR and WAXD.

n INTRODUCTION Time-resolved techniques allowed essential progress in understanding of cage effect in photoinduced free-radical reactions in liquids.1,2 Cage effect is an inalienable property of the condensed phase. Our efforts have been devoted to the study of cage effect in polymers and recombination of free radicals in the polymer bulk, see for review ref. 3. Simultaneous study of properties of a polymer by different methods allows understanding of the effect of polymer matrix on the fast radical reactions. In the present work we studied biaxially oriented polypropylene (BOPP) with a benzophenone (B) probe.3-5 BOPP is very important polymer. It is widely used in as a packaging material instead of cellophane, waxed paper, and aluminium foil. “Biaxially oriented” means that the polypropylene (PP) film was stretched in two perpendicular directions. Because of its superior strength at low gauges, flatness, clarity, light weight and excellent printability, and some other useful properties BOPP has become an indispensable packaging material for different consumer products. Much information on the properties of BOPP of different origin can be found in publications and technical data sheets of BOPP manufacturers.6-12 BOPP has oriented chains, high modulus, high stiffness, certain crystallinity due to stretching. That makes a semicrystalline polymer BOPP a challenging object for study with B photoprobe. Study of photoinduced reactions in BOPP pre-heated at high temperatures up till its melting point (453 K) will shed light on BOPP stability towards free-radical degradation under such conditions. It is important to learn how the properties of BOPP change during its thermal treatment. Thus, we studied in this work photochemistry probe B in BOPP pre2 ACS Paragon Plus Environment

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heated (annealed) at different temperatures. Kinetics of photoinduced reactions of B in a number of polyolefins (RH) was investigated in details.3-5 The studied reactions are described by the following Scheme 1:3 3

H trans fe r

B* + RH

3

k1

cage escape

BH.

+

BH.

... R. cage reaction

k esc

R.

kr kb

Products

recombination in the polymer bulk

Scheme 1 3

[BH• … R•] in the Scheme 1 and in the text below stands for a triplet radical pair (RP). It was demonstrated that cage escape and a cage reaction can be nicely described as the first-

order reaction with kesc and kr, respectively (dimension s-1) in many “soft” polymers.3,4 In addition, Scheme 1 presents a second-order reaction in the polymer bulk with kb, M-1.s-1. The Scheme 1 is described in more detail in the Results Section below. We used several physical methods of analysis applied to BOPP in order to understand the reasons of kinetic observations. These methods are described in the Experimental Section below.

n EXPERIMENTAL SECTION Materials and preparation of samples. We used BOPP PropafilmTM RGP120 of Innovia Films in our experiments.6 Commercial BOPP RGP120 is a laminate covered at both sides with polyolefinic copolymers.6 We intended to use the pristine BOPP in this work. For that purpose we extracted commercial BOPP for 48 h in chloroform at room temperature and dried out at room 3 ACS Paragon Plus Environment


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temperature in the air. The thickness of the treated and studied BOPP film is 30 µм measured with a micrometer. We used benzophenone В, stable nitroxyl radical benzoyl ester of 4-hydroxy-2,2,6,6tetramethylpiperidine-1-oxyl benzoate (BzONO), and solvents, all chemicals from Sigma-Aldrich. В and BzONO were re-crystallized from ethanol. We used propane-2-ol as a solvent for penetrants into BOPP. В and BzONO were introduced in the BOPP film by keeping the propane-2-ol solution with BOPP and one or another penetrant for five days at ambient temperature. In a number of cases BOPP films were pre-heated (see this Section below) before penetration of the probes. Solution had either 10 wt.% of B or 3x10-3 M of BzONO. After that BOPP was removed from propane-2-ol and was dried out at ambient temperature in the air and in a dark room. The concentration of B in film of BOPP was evaluated with a spectrophotometer by measuring its absorption at 337 nm (ε = 125 M−1. cm−1). Penetrants were dissolved in BOPP after its pre-heating as described above. Non-heated or pre-heated BOPP samples with a penetrant were studied in 24 h after of preparation at 298 K in these and all other experiments performed in this work. Absorption spectra of the samples were measured in the UV- wavelength range with a Shimadzu UV-3101PC spectrophotometer using a small sample adapter. Optical density (OD) of the studied samples at excitation wavelength was 0.1. In other words, the concentration of B in BOPP was low, less than 0.1 M in the all studied films. When necessary, we used several layers of the film in order to get such an OD, see the Results Section below. B dissolved in BOPP we denote below as B@BOPP. The thickness of the studied layers l was measured with micrometer. l was especially important for calculation of the second-order rate constants. Heating of BOPP films (a piece of size 210х50 мм) was conducted in a thermostat in the presence of air. We avoided contacts of a film with metal parts of the thermostat. For that purpose we covered BOPP film by a Teflon film at one side. A loose roll was prepared from BOPP – Teflon pair of 4 ACS Paragon Plus Environment

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films. The roll was put in the thermostat at the edge. BOPP was clipped to Teflon at one point. All these actions allowed a free and reproducible contraction of BOPP upon heating. BOPP films were heated for 2 min in all experiments with pre-heating at several temperatures up till 423 K. Use of the temperatures higher than 423 K leads to a formation of uneven samples and to melting of BOPP at 453 K. We measured thickness of films after heating. The thickness increased up till 35, 50 and 60 µm after pre-heating at temperatures 407, 413 and 423 K, respectively. Special examination demonstrated that such pre-heating in the air does not lead to autoxidation of BOPP. We were concerned with the uniformity of distribution of B and BzONO across the thickness of a very thin film l . Individual films had l ≤ 60 µm in all our experiments, see this Section above. It is hard to expect a gradient of the probe concentration in such a thin film upfront. The probes penetrated BOPP film from both sides leading obviously to a uniform distribution. Concentration of the probe in a film was measured by UV-, IR- and ESR spectroscopy. (The latter evidently was used for BzONO.) A concentration of a probe reached 99% of its maximum achievable concentration in 24 h, and a saturation of BOPP with a probe was completed in five days, see this Section above. In addition, ESR spectra do not demonstrate that BzONO is constrained in a thin surface layer. Devices. A nanosecond laser flash photolysis (LFP) apparatus described in detail elsewhere was used.4 The device was adapted for measuring of thin films. It consists of a PRA LN 1000 N2 laser with pulse duration of 1 ns and emission wavelength of 337.1 nm. Time resolution of a device is 10 ns; it operates at frequency ≤10 Hz. The data presented in this paper are average values obtained by processing at least ten kinetic curves under the same selected conditions. All kinetic measurements were made in thermostated fused silica cell at 298 K. B@BOPP film was placed in the cell. The air was removed from the samples by prolonged evacuation. In experiments with magnetic films, the cell was placed between the poles of a permanent magnet (magnetic flux density B = 0.2 T). The 5 ACS Paragon Plus Environment


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laser flash irradiated area was only 1 mm x 2 mm. After each experiment, the cell of 1 cm size was slightly moved up or down to enable irradiation of a fresh portion of the film. ESR spectra of BzONO@BOPP were taken with a Bruker EMX X-band spectrometer with the following settings: microwave power 0.2 mW, modulation amplitude 1.0 G. Thermogramms of B@BOPPwere obtained with DSC device NETZSCH DSC 204F1; temperature ramp 10 K/min. WAXD study of B@BOPP films was carried out with a HZG4 X-ray diffractometer (Freiberger Präzisionsmechanik) in Bragg-Brentano geometry with scintillation counter and diffracted beam graphite monochromator (Cu Kα radiation). Diffraction patterns were recorded at 2θ angle step 0.02o. No correction for the experimental intensity was applied. Thermo Scientific Nicolet TM FT-IR 6700 was used to take IR spectra of films. The resolution of a device is 0.5-2.0 cm-1.

n RESULTS Laser flash photolysis. LPF of samples of B in the studied BOPP leads to the appearance of a triplet 3B* immediately after a flash with its characteristic absorption maximum at λ525 nm and a shoulder at λ630 nm,3-5,13 see Figure 1, curve 1:

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OD 8x10

-3

1 6 4 2

2 0 500

550

600

650

700 λ, nm

Figure 1. Absorption spectra of transients in BOPP film (no pre-heating) obtained under LFP photolysis of B@BOPP at 298 K with a delay after laser pulse of: 20 ns – 1; 5 µs – 2. OD stands here and below for optical density.

Only ketyl radical with its maximum absorption at λ545 nm is observed at t ≥5 µs after a laser pulse, see Figure 1, curve 2.3-5 Estimation of BH• yield (YBH) from the comparison of the initial 3B* concentration and a concentration of BH• after 5 µs after a laser pulse leads to a value close to 0.3. We used extinction coefficients of these transients in benzene: ε525 = 7220 and ε545 = 4600 M-1.cm-1 of 3B* and BH•, respectively.14-16 Decay kinetics of 3B* was measured at λ630 nm where only 3B* absorbs, see Figure 1 and also at λ545 nm where both transients absorb light.3-5,13 The decay of 3B* (Figure 2, curve 1) followed the first-order kinetic law which is in fact two concurrent reactions (1,2):

RH 3B*

k1

3

.... R.

BH

(1)

k2

B

(2)



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Here RH is BOPP with its C-H bonds, 3[BH• … R•] is a triplet radical pair (RP), see the Introduction Section above. We got rate constants of a triplet decay by two paths as k1 and k2, see Table 1: Table 1

Rate constants of 3B* decay according to reactions (1) и (2) (k1 and k2, respectively), of dissociation (kesc) and recombination (kr) of RP, rate constants of cross-termination of BH• in the polymer bulk (kb), rotational correlation times of a BzONO (τrot), heats of fusion (∆hf0), percent crystallinity (Xc), and average size of crystallites Lhkl in B@BOPP at different pre-heating temperaturesa

Parameter

298 K

407 K

423 K

k1 x 10-5, s-1 b

4.9

4.9

4.8

k2 x 10-5, s-1 b

9.5

9.4

9.4

kesc x 10-4, s-1 b

≤0.2

1.7

2.2

kr x 10-4, s-1 b

7.8c

5.4

4.5

kb x 10-8, M-1.s-1 b

-

1.45

1.36

τrot, ns

11.9±0.1

∆hf0, J/g

98.3±0.3

-

88.1±0.3

Xc, %d

47.0±0.5

-

42.0±0.5

L110, nme

13±1

16±1

21±1

L040, nme

13±1

16±1

21±1

10.9±0.1

a

See Scheme 1 and eqns. 1-7. The presented data after pre-heating was also obtained at 298 K; see the Experimental Section above b Determination error 15% c In this case kr = k4 d See eq. (6) e See eq. (7) and Figure 7 below


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The yield of BH•, k1- and k2 - values practically do not depend upon the history of a sample (whether it was pre-heated or not, see Table 1). In our previous experiments with relatively soft elastomers 3B* was surrounded by –CH2–CH2– accessible fragments. YBH was close to 1.0 and the corresponding k1 was an order of magnitude higher in elastomers.3,4 The studied polymer was a relatively hard stretched BOPP.6,7,9 3B* was surrounded by –CH2–CH2– accessible fragments in elastomers. YBH in B@BOPP is essentially lower than in elastomers due to relatively low k1 (Table 1). 3B* in B@BOPP is surrounded by fragments –CH(CH3) –CH2–. Methyl substituent should easily accessible to 3B* but bond dissociation energy (BDE) in C–H of methyl is higher than that in –CH2–. (BDE of C–H at the tertiary C atom of BOPP is lower than that of –CH2–. However, the tertiary C is screen of by CH3 –group, see above. So, probability of H abstraction from the tertiary C–H is low.) We assume that the lower number of –CH2– groups in B@BOPP surrounding 3B* and screening of a “weak” tertiary H compared to the studied elastomers3 leads to a much lower k1 and lower YBH in B@BOPP. The rest of 3B* undergoes a radiationless intersystem crossing with the formation of initial B.13 The absorption spectra BH• and 3B* are also independent of the history of BOPP as well. The thermal history of B@BOPP manifests itself in the kinetics of BH• decay. Figures 2-4 below demonstrate traces of 3B* and BH• decay a different time scales and different pre-heating temperatures:



10x10 OD

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-3

8 6 4 2

3 2

0

1

0

10

20

30

t, µ s

40

Figure 2. Decay kinetics of transients: 3B* monitored λ630 nm, no pre-heating-1; 3B* and BH• in a sample, no pre-heating; thickness of a film 120 µm - 2; 3B* and BH• in a sample pre-heated at 423 K; thickness of a film 120 µm – 3. Decay kinetics of 3B* and BH• was monitored at λ545 in this Figure and in Figures 3,4 below. Curves 2,3 correspond to the caged and to the free radicals. Kinetic curves here and in the Figures 3,4 were obtained under LFP of B@BOPP at 298 K. The thin solid lines coinciding with “hairy” experimental curves are computer-simulated kinetic curves here and in Figures 3,4.

Figure 2 demonstrates that the very fast decay of 3B* is followed by a fast recombination of a triplet RP in the cage and by fast exit of radicals from the cage (Scheme 1) as it was observed earlier.3


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2.0x10

-3

OD

1.5 1.0 3

0.5

1

2

0.0 0

50

100

150

t, µs µ

200

Figure 3. Decay kinetics of BH• monitored at t ≥ 5 µs: no pre-heating of a sample of B@BOPP – 1; after pre-heating of the sample at 407 K – 2; after pre-heating of a sample at 423 K – 3. The thickness of films is 120, 105, and 120 µm for cases 1,2,3, respectively. Curves 2,3 correspond to the caged and to the free radicals.

Figure 3 demonstrates kinetics of geminate recombination/cage escape of a triplet RP. The main route of RP decay in the non-preheated B@BOPP is their cage recombination (curve 1). The preheated samples demonstrate partial decay in the cage (curves 2,3) is followed by a slow secondorder reaction ascribed to a cross-termination in the polymer bulk, see Scheme 1.

1.0x10

-3

OD 0.8

0.6 0.4

3 2

0.2 1

0.0 0

2

4


6

t, ms

8


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Figure 4. Decay kinetics of BH•: no pre-heating of a sample – 1; after pre-heating at 407 K – 2; after pre-heating at 423 K – 3. The thickness of films is 120, 105 and 120 µm for cases 1,2,3, respectively.

Figure 4 demonstrates cross-termination kinetics of BH• predominantly in the polymer bulk (curves 2,3). See also Scheme 1 above and eq. 3 below:

.

BH

.

+ R

kb

Products

(3)

Therefore the kinetic curves 1-3 on Figures 3,4 can be separated into two parts: the curve which reflects decay of RP up till ~ 50 µs, and into the rest of the curve corresponding to crosstermination of radicals in the solvent bulk (see eq. 3 and Scheme 1 above). Computer simulation of the studied reactions allowed getting rate constants of reactions (14) and reactions of the Scheme 1, see this Section above. All curves presented in Figures 2-4 demonstrate a good fit of computer simulated curves to the corresponding experimental data. The obtained rate constants presented in the Table 1 above. Individual simulations of 3B* decay and of RP decay as the first-order reactions and decay BH• in the polymer bulk in the corresponding time scales as a second-order reaction led to the same rate constants within their determination error as presented in the Table 1. Analysis of kinetic traces of cross-termination (3) leads to the experimental values of kb/(ε545.l ), where l is the thickness of a B@BOPP layer. Kinetic measurements of reaction (3) with different l demonstrate the expected dependence upon l , which additionally confirms that we deal with a second-order reaction. There is no additional long-lasting component in the case of non-heated B@BOPP (Figure 3, curve 1; Figure 4, curve 1). That means that all RPs decay in the cage. There is no effect of external magnetic field (MF, see the Experimental Section above) of the rate constant within 12 ACS Paragon Plus Environment

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determination error. In such a case a probable situation is that BH• and R• are produced and stay as a contact RP.5,17,18 We denote a contact RP as 3[BH•, R•]:

3

. . BH , R

k4 (4)

Products

Contact RP is characterized by a large absolute value of the exchange integral J, larger than an effective hyperfine coupling constant Aeff, which provides S-T interconversion of RP in a moderate magnetic field (MF), |J| > Aeff.19 It is believed that in such cases S-T interconversion (ic) of RP occurs primarily due to action of spin-orbit coupling, which can be characterized by kic= k4. Another possible explanation is that RP lives in the not pre-heated BOPP long enough, and paramagnetic spin-lattice and spin-spin relaxation leads to S-T interconversion, to a large cage effect value, and to the lack of MF effect.13,19 (The further study would allow identifying a reason for the complete RP decay in the cage. Experiments with B-d10, B-13C, experiments under high MF B >> 0.2 T will be very useful in this system.) Exit of radicals in the polymer bulk (e) can be calculated by a simple formula (5):2,3 e = kesc/(kesc + kr),

(5)

see Scheme 1. Figure 5 demonstrates changes of the measured e vs. pre-heating temperatures:



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0.30 e 0.25 0.20 0.15 0.10 0.05 0.00 380

390

400

410 420 T, deg K

Figure 5. Dependence of cage escape values e measured at 298 K in B@BOPP samples vs. pre-heating temperature.

Application of MF increased exit from the cage e by 7±3% at T ≥ 432 K. This MF effect is very small compared to our previous observations in elastomers3 and indicates probably that a significant fraction of RPs decay as magneto-insensitive contact pairs (eq. 4). ESR measurements. Slow-motion rotational correlation time τrot for BzONO@BOPP was determined at 293 K. Obtained values of τrot for non-heated and pre-heated at BOPP are presented in Table 1. For calculation τrot we used a model of random rotational large angle jumps described in details in ref. 20. Two obtained values of τrot are rather close to each other indicating that preheating feebly affects rotational diffusion of a low MW molecule in BOPP. (Molecular dimensions of BzONO are close to those of B.3) At the same time, these τrot are thrice higher than in the elastomers studied earlier.4


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DSC measurements. Specific equilibrium heat of fusion of polymer crystals, ∆hf0 [J/g] is an essential thermal property of polymers which allows obtain the percent of crystallinity (Xc) from thermal measurements.21 The width of a thermogramm (DSC curve) reflects a width of distribution of crystallites according to their melting points.21 The heating (annealing) of BOPP shed light on the BOPP partially crystal structure and should be useful for understanding of transients kinetics (see the Discussion Section below). Figure 6 below presents results of DSC measurements of B@BOPP:

Figure 6. DSC traces obtained for non-heated B@BOPP – 1; pre-heated at 423 K B@BOPP – 2. Heat is absorbed.



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The obtained ∆hf0 (Table 1, Figure 6) allows calculating percent crystallinity Xc of the two studied samples:

Xc = ∆hf0/ ∆hm0,

(6)

where ∆hm0 = 209 J/g is the heat of fusion of fully crystallized PP.8,9 The Xc data are presented in the Table 1. Non-heated BOPP and pre-heated BOPP have ∆hf0 values which differ only by 10% (Table 1). Non-heated B@BOPP plot (Figure 6, curve 1) demonstrates a shoulder observed elsewhere for BOPP.8,12 It is quite possible that the peak and the shoulder reflect melting of two types of crystals. It is also important, that two DSC curves have significantly different width (Figure 6), see Discussion Section below. WAXD measurements. WAXD patterns of non-heated and pre-heated B@BOPP are presented in the Figure7 below:


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Figure 7. WAXD pattern of B@BOPP taken at 298 K. The green line (a sample was not preheated), the red line (a sample was pre-heated at 407 K), the blue line (a sample was pre-heated 423 K). Insert: A blown up peak (040). The angular position of diffraction lines (Figure 7) are consistent with PP crystalline α-form.22 Diffraction lines (111), (041), and (-131) at 2θ=21–22o which are inherent to this crystalline form, were not observed in our study (Figure 7) due to the preferred orientation of crystallites (texture) in the [email protected] No diffraction lines characteristic for polypropylene crystalline β– or γ–forms22 were detected for non-heated or pre-heated B@BOPP in our study. Due to a low concentration of B in BOPP (see the Experimental Section above), we did not observed diffraction on B even if some B in BOPP agglomerated into crystals. The change of relative intensities of B@BOPP diffraction lines after preheating (Figure 7) may be explained it terms of a partial misalignment of initial crystallite orientation under the heat. The average crystallite size (Table 1) was calculated according to the Scherrer23 formula:

Lhkl = λ/(β⋅cosθhkl),

(7)

where Lhkl is the average crystallite size in the direction normal to the hkl diffraction planes, λ is an X-ray wavelength, θhkl is the Bragg angle, β is the integral width of the hkl diffraction line (2θ scale, in radians) corrected for instrumental broadening. IR measurements. We took mid IR spectra of B@BOPP (not shown). They were almost identical to those reported in the literature.7,24 . We did not observe any changes in IR spectra in the studied pre-heating temperature interval (298-423 K). In particular, we focused on the known bands at 809 and 841 cm-1, which belong to a crystalline part of BOPP.7,24 (We will note in parenthesis that



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a change of the ratio of intensities of these two bands was observed only in the temperature range close to a melting point of BOPP (453 K).) A general statement in the conclusion of this part is that we did not observe any differences in physical and mechanical properties of B@BOPP and neat BOPP.

n DISCUSSION Benzophenone B is a convenient probe of polymeric media.3 LFP of B dissolved in a polymer allows measuring kinetics three consecutive processes: decay of a triplet state 3B*, decay of radical pairs 3[BH• … R•], cross-termination of ketyl free radical BH• in the polymer bulk. Rate constants of these processes as well as cage escape are presented in the Table 1. We found that the measured reactions nicely followed the first- and the second-order law. It is known that the kinetics of many elementary reactions in polymers is described by dispersive kinetics, where kinetic nonequivalence of chemically identical radicals is accepted for the analysis of the decay kinetics, see e.g., refs 4,5,25. However, “simple” first- and second-order kinetic laws describe relevant elementary processes quite satisfactory in BOPP and in elastomers, see Figures 2-4 and ref 3. The e –value (Figure 5) gives useful info on molecular mobility in the polymer cage. In this work we studied with different techniques a very important commercial polymer BOPP only at 298 K. We will remind, that BOPP was pre-heated at different elevated temperatures for 2 min and cooled down to 298 K and kept for 24 h prior experiments (see the Experimental Section above). We observed an unexpected critical phenomenon on the exit of free radicals from a polymer cage. BH• starts to escape the polymer cage (both e and kesc > 0 ) at a critical temperature of pre-heating Tcrit ≈ 403 K and at higher temperatures of pre-heating (see Figure 5). Moreover, BH• crossterminates with a macroradical in the polymer bulk with a high rate constant of k3 ~ 108 M-1.s-1 (Table 1). BH• apparently exists in the amorphous phase of B@BOPP. That justifies a relatively 18 ACS Paragon Plus Environment

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high cross-termination constant k3. τrot of a nitroxyl, which is probably located in the amorphous phase as well, only slightly decreased after a pre-heating of BzONO@BOPP (Table 1). It is often observed that changes in a viscous drag of a media have much smaller effect on rotational then on translational diffusion.2 It is worthwhile mentioning that kb (eq. 3) value in B@BOPP (Table 1) is marginally higher than kb values in the “soft” elastomers.3 It is not trivial, because kesc is almost two orders of magnitude smaller and τrot is three times smaller in BOPP than the corresponding values in elastomers, see the Table 1 and refs 3,4. A surprisingly large kb in B@BOPP can be attributed to the location of BH• in the confined parts of the amorphous phase of B@BOPP. In particular, BH• could be in the vicinity of crystallite – amorphous interfaces. Such possible location of BH• should result in significantly smaller actual volume, where a reaction takes place, and an apparent l value smaller compared to the measured l . The existence of interfacial layers of long-chain organic molecules at the interfaces lead to an apparent acceleration of bimolecular reaction on going from homogeneous media to the polycrystalline ones. This phenomenon which been observed in other systems, e.g., polycrystallite frozen solvents and thin films made out of these solvents.26 Obviously such reactions of photoexcited B are related to the properties of BOPP. Semicrystalline polymer BOPP has a complex structure which changes with temperature and with stretching/contraction of the poymer.7-12,27,28 Briefly, commercial BOPP at room temperature consists mainly of amorphous phase and of monoclinic α-crystallites or α- form, see Figure 7.8,11,12, 22

All obtained diffraction peaks correspond to α- form.22 .Quite similar WAXD patterns of BOPP

were observed elsewhere.7,27 The usual analysis of WAXD patterns7,27 demonstrates that pre-heating does not change Xc by more than 5-10%. DSC data also demonstrates a small change of Xc after pre-heating (Table 1). 19 ACS Paragon Plus Environment


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WAXD patterns give a valuable info on a pre-heating effect on B@BOPP (see Figure 7). In particular, the narrowing of WAXD peaks (Figures 7, insert) occurs due to the increase of the average crystallite size.22 Table 1 above presents average size of crystallites Lhkl in B@BOPP films calculated using integral width of two diffraction peaks at different temperature of pre-heating. It follows from the data of Table 1, that increase of pre-heating temperature up to 423 K leads to increase of crystallites in initially non-heated samples in 1.5 times. However, it was demonstrated eslewhere10 that under cooling of BOPP from high temperatures down, the β−form of BOPP is formed at T ≤ 406 K. The latter is very close to our Tcrit. Melting points of α- and β-forms are 437-438 and 413-423 K, respectively.7,10,12 However, our WAXD data (Figure 7) definitely demonstrates a lack of β−form. Pre-heating up till 423 K leads to softening of α-form. Cooling down of pre-heated BOPP should lead to further agglomeration of initially softened α-form.12 α- Form aggregates in a lamellae inside BOPP.7,8,11,12 Pre-heated B@BOPP demonstrates narrowing of a thermogramm (Figure 6, curve 2). Such changes in the curve width (Figure 6) signifies that pre-heating of B@BOPP leads to a narrower distribution of crystals according to their melting points. Pre-heating apparently leads to a formation of larger crystallites with a higher melting point and melting of small defective crystallites. Information on production of BOPP is useful for understanding of its properties. BOPP is produced at 400-410 K with a several times extension in the two perpendicular directions.6-12 The starting PP with a large molecular weight distribution and multiple crystallites defects is taken in order to stretch it. Certainly, structure of the produced BOPP essentially depends upon properties of the starting PP and the conditions of stretching. In general, BOPP forms fiber-like structure.29 Small crystallites and other defects, voids in particular, are inside the amorphous phase.8,12,30 ESR data 20 ACS Paragon Plus Environment

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demonstrates that the viscosity of a pre-heated B@BOPP is slightly lower than in the non-heated B@BOPP (Table 1). Non-heated B@BOPP is in the strained state; BOPP is fixed in such a state like a strained rope. The heating leads to an increase of mobility of polymer segment. Segments escape from the hampered points. Polymer chains between crystallites collapse into coils because coils are their equilibrium (or close to the equilibrium) configuration. Several publications are devoted to properties of BOPP produced by stretching PP at elevated temperatures.11,12, 28 We were interested in a quite different task: What are the properties of the initially stretched BOPP which underwent free shrinkage at elevated temperatures. Scheme 2 below summarizes our considerations at a molecular level:

Scheme 2 Pictorial presentation of cage reactions in BOPP at 298 K (case 1) and at T ≥ 403 K 21 ACS Paragon Plus Environment


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(case 2). BH• is presented as a blue circle with a red dot. Red dot stands for unpaired electron of BH• and R•. Macroradical R• is formed in the proximity to BH• in both cases. Wavy lines depict individual macromolecules. BH• may escape the polymer cage in the case 2 (red wavy arrows). See the text for more detail.

The large flat crystallites are located parallel to each other. Crystallites are bound by fibrils (not shown on the Scheme 2), which penetrate crystallites perpendicular along the X-axis. The edges of crystallites are directed towards the surface of a film along the Z-axis. There is an amorphous phase of BOPP in the room between crystallites. The amorphous phase in the absence of pre-heating (1) has many relatively small crystallites according to our DSC data. The small crystallites are formed most probably by the mechanical destruction of the crystal phase of PP during production of BOPP. According to the ESR data (Table 1), the amorphous phase (case 1) is in a strained and less mobile state compared to the case 2 (Scheme 2). BOPP (case 1) is a typical polymer with barriers for a retarded diffusion. Such condition limits mobility of BH•. In the case 2 the amorphous phase relaxed due to pre-heating31 and allows faster translational diffusion of BH• compared to case 1. In the case 2 BH• is surrounded predominantly by recrystallized and agglomerated α−crystallites. Almost all small crystallites join large crystallites at temperatures of about 423 K. That leads to an increase of the average crystallite size 1.5 times in our estimation. The total percent crystallinity Xc is approximately the same in the both cases 1, 2, see Table 1 and Scheme 2. The Scheme 2 case 1 demonstrates that the surroundings of BH• hinder its escape into the polymer bulk (e=0), whereas in the case 2 such escape is possible (e > 0). Red wavy lines on the Scheme 2, case 2 demonstrate possible directions of the BH• cage escape.

n CONCLUSIONS


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We studied with LPF photoinduced reactions of benzophenone (B) in a stretched PP (BOPP) at room temperature. B@BOPP was pre-heated at elevated temperatures and cooled down to the room temperature. This is obviously a “physical” process of heating-cooling. However, after pre-heating properties B@BOPP (or BOPP) undergoes changes. These changes are obviously the relaxation of metastable stretched BOPP.31 Such methods as WAXD and DSC demonstrate some changes in BOPP after pre-heating. Percent crystallinity of BOPP does not change upon pre-heating with an accuracy 5-10% and is Xc = 40-50%. The BOPP property which apparently changes upon preheating is melting of small and defect crystallites in BOPP, relaxation of amorphous phase, agglomeration of lamellae. That explains a critical temperature at which benzophenone radical escapes the polymer cage into the polymer bulk. It happens at Tcrit ≈ 403 K and higher. “Soft” amorphous phase and large agglomerates of crystallites formed after re-crystallization and amalgamation of crystallites during cooling allow radical escape (Scheme 2). Interestingly, such widely used in polymer and analytical chemistry method as WAXD demonstrates only increase of crystallites size in BOPP after preheating, whereas kinetic measurements manifest a critical phenomenon related to BOPP cage. In conclusion, a photoprobe B alongside with other physical methods listed in the Results Section above provides a powerful arsenal to investigate structural transformation in polymers. In general, B has been used as a kinetic probe for a long time in many areas – from material science to biochemistry.32-34

n AUTHOR INFORMATION Corresponding author

*E-mail: [email protected] †

(I.V.K.).

Present address Performance Coatings International, LLC, Bangor, Pennsylvania, 18013, USA. 23 ACS Paragon Plus Environment


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ORCID Peter Levin: 0000-0002-7197-5775 Aleksei Efremkin: 0000-0002-3010-0550 Aleksey Krivandin: 0000-0003-4595-3449 Sergei Lomakin: 0000-0003-3191-8415 Olga Shatalova: 0000-0002-0975-5712 Igor Khudyakov: 0000-0002-0217-1167 Notes The authors declare no competing financial interest.

n ACNOWLEGEMENT The work was supported by the grant of Russian Fund for Basic Research (Project No. 16-0300275a).

n REFERENCES (1) Barry, J.T; Berg, D.J.; Tyler, D.R. Radical Cage Effects: The Prediction of Radical Cage Pair Recombination Efficiencies Using Microviscosity Across a Range of Solvent Types. J. Am. Chem. Soc. 2017, 139, 14399-14405. (2) Khudyakov, I.V. Transient Free Radicals in Viscous Solvents. Res. Chem. Interm. 2013, 39, 781-804. (3) Levin, P.P.; Efremkin, A.F.; Khudyakov, I.V. Benzophenone as a Photoprobe of Polymer Films. Chem. Phys. 2017, 495, 23-28. (4) Levin, P.P; Efremkin, A.F.; Sultimova, N.B.; Kasparov, V.V.; Khudyakov, I.V. Decay Kinetics of Benzophenone Triplets and of Corresponding Free Radicals in the Soft and Rigid Polymers Studied by Laser Flash Photolysis. Photochem. Photobiol. 2014, 90, 369-373. (5) Levin, P.P; Khudyakov, I.V. Laser Flash Photolysis of Benzophenone in Polymer Films. J. Phys. Chem. A 2011, 115, 10996-11000. (6) http://www.innoviafilms.com/support/datasheets 24 ACS Paragon Plus Environment

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(7) Türkҁü, H.N. Investigation of the Crystallinity and Orientation of Polypropylene with Respect to Temperature Changes Using FT-IR, XRD, and Raman Techniques. M.S. Thesis. Ankara, 2004, 83 pp. (8) Rozanski, A.; Galeski, A. Crystalline Lamellae Fragmentation during Drawing of Polypropylene. Macromolecules 2015, 48, 5310-5322. (9) Kanai, T.; Egoshi, K.; Ohno, S.; Takebe, T. The Evaluation of Stretchability and Its Applications for Biaxially Oriented Polypropylene Film. Adv. Polym. Technol. 2017, 00, 1-8. (10) Sun, X.; Li, H.; Zhang, X.; Wang, D.; Schultz, J.M.; Yan. S. Effect of Matrix Molecular Mass on the Crystallization of β-Form isotactic Polypropylene around an Oriented Polypropylene Fiber. Macromolecules 2010, 43, 561-564. (11) Lu, Y.; Chen, R.; Zhao, J.; Jiang, Z.; Men, Y. Stretching Temperature Dependency of Fibrillation Process in Isotactic Polypropylene. J. Phys. Chem. B 2017, 121, 6969-6978. (12) Elias, M.B.; Machado, R.; Canevarolo, S.V. Thermal and Dynamic-Mechanical Characterization of Uni- and Biaxially Oriented Polypropylene Films. J. Therm. Anal. Cal. 2000, 59, 143-155. (13) Turro, N.J.; Ramamurthy, V; Scaiano, J.C. Modern Molecular Photochemistry of Organic Molecules, University Science Books, Sausalito, 2010, 1084 pp., see p. 583. (14) Hurley, J.K.; Sinai, N.; Linschitz, H. Actinometry in Monochromatic Flash Photolysis: The Extinction Coefficient of Triplet Benzophenone and Quantum Yield of Triplet Zinc Tetraphenylporphyrin. Photochem. Photobiol. 1983, 38, 9-14. (15) Inbar, S.; Linschitz, H; Cohen, S.G. Nanosecond Flash Studies of Reduction of Benzophenone by Aliphatic Amines. Quantum Yields and Kinetic Isotope Effects. J. Amer. Chem. Soc. 1981, 103, 1048-1054.



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(16) Kawai, A; Hirakawa, M; Abe, T.; Obi, K.; Shibuya, K. Specific Solvent Effects on the Structure and Reaction Dynamics of Benzophenone Ketyl Radical. J. Phys. Chem. A 2001, 105, 9628-9636. (17) Khudyakov, I.V.; Serebrennikov, Yu. A; Turro, N.J. Spin-Orbit Orbit Coupling in Freeradical Reactions: On the Way to Heavy Elements. Chem. Rev. 1993, 93, 537-570. (18) Levin, P.P.; Kuzmin, V.A.; Khudyakov, I.V. Comparative Analysis of Geminate Recombination Kinetics of Triplet Radical Pairs in Viscous and Micellar Solutions. Russian J. Chem. Phys. 1989, 8, 902-910. (19) Hayashi, H. Introduction to Dynamic Spin Chemistry: Magnetic Field Effects on Chemical and Biochemical Reactions, World Scientific, New Jersey, 2004, 254 pp. (20) Goldman, S.A.; Bruno, G.V.; Freed, J.H. Estimating slow-motional rotational correlation times for nitroxides by electron spin resonance. J. Phys. Chem. 1972 , 76, 1858-1860. (21) Cebe, P; Thomas, D.; Merffeld, J.; Partlow, B.P.; Kaplan, D.L.; Alamo, R.G.; Wurm, A.; Zhuravlev, E.; Schick, C. Heat of Fusion of Polymer Crystals by Fast Scanning Calorimetry. Polymer 2017, 126, 240-247. (22) Turner, J.A.; Aizlewood, J.M.; Beckett, D.R. Crystalline forms of isotactic polypropylene. Makromol. Chem. 1964, 75, 134–158. (23) Scherrer P. Bestimmung der Grösse und der inneren Struktur von Kolliodteilchen mittels Rönthenstrhalen. Nachrichten von der Gesellschaft der Wissenschaften, Göttingen. 1918, 98100. (24) Andreassen, E. Infrared and Raman Spectroscopy of Polypropylene. In: Polypropylene. An A-Z Reference, Kluwer, Dordrecht; Karger-Kocis, J., Ed. 1999, pp. 320-328. (25) Caspar, J.V.; Khudyakov, I.V.; Turro, N.J.; Weed, G.C. ESR Study of Lophyl Free Radicals in Dry Films. Macromolecules 1995, 28, 636-641. 26 ACS Paragon Plus Environment

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(26) Levin, P. P; Costa, S. M. B. ; Nunes, T. G.; Vieira Ferreira, L. F. ; Ilharco, L. M.;| Botelho do Rego, A. M. Kinetics of Triplet-Triplet Annihilation of Tetraphenylporphyrin in Liquid and Frozen Films of Decanol on the External Surface of Zeolite. Fast Probe Diffusion in Monolayers and Polycrystals. J. Phys. Chem. A 2003, 107, 328-336. (27) Diez, F.J.; Alvarino, C.; Lopez, J.; Ramirez, C; Abad, M.J.; Cao, J.; Garcia-Garabal, S.; Barral, L. Influence of the Stretching in the Crystrallinity of Biaxially Oriented Polypropylene (BOPP) Films. J. Therm. Anal. Cal. 2005, 81, 21-25. (28) Meng, L.-P.; Chen, X.-W.; Lin Y.-F.; Li, L.-B. Improving of Softness of BOPP Films: From Laboratory Investigation to Industrial Processing. Chinese J. Polym. Sci. 2017, 35, 1122-1131. (29) Nie, H.-Y., Walzak, M.J, McIntyre, N.S. Draw-ratio-dependent morphology of biaxially oriented polypropylene films as determined by atomic force microscopy. Polymer, 2000, 41, 2213-2218. (30) Spieckermann, F., Polt, P., Wilhelm, H., Kerber, M.B., Shafler, E., Reinecker, M., Soprunyuk, V., Bernstorff, S., Zehetbauer, M. Dislocation Movement Induced by Molecular Relaxations in Isotactic Polypropylene. Macromolecules 2017, 50, 6362−6368. (31) Leroux, B.; Elmes, T.; Mills, P. Study of Shrinkage in Biaxially Oriented Isotactic Polypropylene. J. Mater. Sci. 1992, 27, 1475-1478. (32) Dormán, G.; Nakamura, H.; Pulsipher, A.; Prestwich, G.D. The life of Pi Star: Exploring the Exciting and Forbidden Worlds of the Benzophenone Photophore. Chem. Rev. 2016, 116, 15284-15398. (33) Levin, P.P.; Ferreira, L.F.V; Costa, S.M.B. Diffuse-Reflectance Laser Photolysis Studies of Geminate Recombination Kinetics of Triplet Radical Pairs Adsorbed on Microcrystalline Cellulose. Chem. Phys. Lett. 1990, 173, 277- 281.



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(34) Jiao, C-Y.; Sachon, E.; Alves, I.D.; Chassaing, G.; Bolbach, G.; Sagan, S. Exploiting Benzophenone Photoreactivity To Probe the Phospholipid Environment and Insertion Depth of the Cell-Penetrating Peptide Penetratin in Model Membranes. Angew. Chem. Intern. Ed. 2017, 56, 8226-8230.


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0.30 Polymer cage escape 0.25 0.20 0.15

RP

Products

0.10 0.05

RP

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Radicals in polymer bulk 0.00 380

390

400 410 420 Pre-heating temperature, deg K

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